Author name code: mathioudakis
ADS astronomy entries on 2022-09-14
author:"Mathioudakis, Mihalis"
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Title: Radiative Hydrodynamic Modelling Of The Lyman Continuum During
Solar Flares
Authors: Mclaughlin, Shaun; Simões, Paulo; Milligan, Ryan;
Mathioudakis, Mihalis; Monson, Aaron; Kerr, Graham
Bibcode: 2022cosp...44..845M
Altcode:
Increases in solar irradiance associated with solar flares are known
to drive dynamic and compositional changes in Earth's ionosphere,
which can have adverse implications for modern technology on which
society has become dependent. The EUV part of the solar spectrum can
increase dramatically during the largest events. The 850-1027\AA\
range, in particular, is absorbed at an altitude of around 105--120km
in the ionosphere (E-layer), where it drives the partial dissociation
of molecular oxygen. This part of the spectrum is dominated by the
free-bound Lyman continuum of neutral hydrogen (<912\AA; LyC),
along with higher-order Lyman emission lines. As LyC is formed
in the solar chromosphere, it is highly sensitive to changes in
the energy balance associated with solar flares. To understand the
mechanism(s) responsible for driving the increases in LyC emission
associated with solar flares, we generated synthetic LyC profiles
using the 1D radiative hydrodynamic code, RADYN. The grid of RADYN
models hosted at Queen's University Belfast simulates the response of
the solar chromosphere to flare heating, assumed to be in the form
of accelerated electrons. The specific intensity at the head of the
continuum was found to be highly sensitive to the energy flux density of
the nonthermal electrons, increasing by 3-4 orders of magnitude. This
translates to a factor of five increase in irradiance during the
largest events. The increase in LyC emission was found to be less
sensitive to changes in the spectral index and low-energy cutoff of
the nonthermal electron distribution. By fitting the LyC profile with
a blackbody function, the plasma conditions at the formation height
were determined. This revealed LyC to be optically thick, and formed
at a deeper layer during flares compared to quiescent periods. Plots
of the contribution function also suggest optically thin components
of LyC emission formed higher in the solar atmosphere, deemed to be
associated with chromospheric evaporation. These findings will help
to diagnose the physical conditions in the flaring chromosphere that
lead to increased geoeffective EUV emission under different heating
regimes. They will also help guide the interpretation of spectral
irradiance observations such as those provided by SDO/EVE and flare
irradiance models such as FISM.
Title: Photospheric Spectral Line Velocity Diagnostics in Solar and
Stellar Flares.
Authors: Monson, Aaron; Milligan, Ryan; Kowalski, Adam; Mathioudakis,
Mihalis
Bibcode: 2022cosp...44.2449M
Altcode:
We present radiative-hydrodynamic simulations of solar flares generated
by the RADYN and RH codes to study the perturbations induced in
photospheric Fe I lines by electron beam heating. We investigate the
induced line-of-sight velocities by various electron beam parameter
combinations, and the primary energy transport mechanisms responsible
for heating the lower solar atmosphere. From these models, we synthesize
several deep forming Fe I spectral lines and study the Doppler velocity
information retrievable during the flare. It is shown that throughout
the period of beam heating a significant proportion of the line
intensity is contributed from the chromosphere, leading to erroneous
Doppler shifts not reflective in the photospheric LOS velocities. The
apparent m/s Doppler shifts can even indicate false downflows in the
photosphere, making their study vital for correctly considering momentum
transfer throughout the lower solar atmosphere. We have expanded our
analysis to stellar flare scenarios, where the sensitivity of these deep
forming spectral lines provides a powerful diagnostic tool for analysing
extreme stellar flare effects and features in the chromosphere.
Title: The many faces of small-scale solar magnetic features
Authors: Mathioudakis, Mihalis
Bibcode: 2022cosp...44.2518M
Altcode:
From impulsive events to quasi-stable structures, small-scale magnetic
fields are ubiquitous in the quiet solar atmosphere and dominate its
structure and dynamics. The processes and timescales that underpin
their evolutionary characteristics depend on their size; the smaller
the structures the faster they evolve. Their magnetic fields constitute
the fundamental blocks of solar magnetism, provide a perpetual source of
flux to the boundaries of the supergranular cells and play an important
role to the energy budget of the outer solar atmosphere. Even areas
which appear devoid of magnetic flux are likely to harbour weak magnetic
fields that are below the detection limits of current instruments. We
will review some of the latest results on this topic and highlight
how our understanding of the physical parameters of small-scale solar
structures can benefit other areas of astrophysics.
Title: Exploring dynamic, small-scale quiet Sun magnetism at high
S/N with the GREGOR/GRIS-IFU
Authors: Campbell, Ryan; Collados, Manuel; Quintero Noda, Carlos;
Mathioudakis, Mihalis; Gafeira, Ricardo
Bibcode: 2022cosp...44.2510C
Altcode:
We have taken advantage of the improvements to GREGOR (Kleint et
al. 2020, A&A, 641, A27), Europe's largest solar telescope,
to reveal small-scale magnetism at the solar internetwork (IN) in
unprecedented detail. The observations were carried out at solar disk
centre with the highly magnetically sensitive Fe I line at 1565nm. Our
observations suggest that GREGOR's overhaul has helped achieve a
higher effective spatial resolution while our synthetic profiles
produced from MHD simulations suggests this data have been obtained
at the telescope diffraction limit in the near infrared. By observing
with high signal-to-noise (S/N), and exceptional seeing conditions, we
reveal that as much as 35% of the IN shows linear polarisation signal
at the 5$\sigma$ level, the highest fraction of linear polarization
ever recorded in the quiet Sun IN, while as much as 70% shows circular
polarization. We use the Stokes inversion based on response functions
(SIR) inversion code to retrieve the thermodynamic, kinematic and
magnetic properties of the atmosphere. We statistically compare our
results to previous GRIS-IFU observations (Campbell et al. 2021,
647, A182) obtained in 2019, prior to GREGOR's overhaul, focusing on
controversies surrounding the impact of noise on the retrieval of the
magnetic inclination angle. We employ the new open-source SIR Explorer
(SIRE) application to easily and efficiently study several dynamic,
small-scale magnetic features. We see evidence for weak transverse
and complex small-scale 'loop-like' structures, with transverse fields
flanked between opposite polarity longitudinal fields. In the last part
of the presentation, SIRE will be demonstrated live for the audience.
Title: The Solar Activity Monitor Network - SAMNet
Authors: Erdélyi, Robertus; Korsós, Marianna B.; Huang, Xin; Yang,
Yong; Pizzey, Danielle; Wrathmall, Steven A.; Hughes, Ifan G.;
Dyer, Martin J.; Dhillon, Vikram S.; Belucz, Bernadett; Brajša,
Roman; Chatterjee, Piyali; Cheng, Xuewu; Deng, Yuanyong; Domínguez,
Santiago Vargas; Joya, Raúl; Gömöry, Peter; Gyenge, Norbert G.;
Hanslmeier, Arnold; Kucera, Ales; Kuridze, David; Li, Faquan; Liu,
Zhong; Xu, Long; Mathioudakis, Mihalis; Matthews, Sarah; McAteer,
James R. T.; Pevtsov, Alexei A.; Pötzi, Werner; Romano, Paolo; Shen,
Jinhua; Temesváry, János; Tlatov, Andrey G.; Triana, Charles; Utz,
Dominik; Veronig, Astrid M.; Wang, Yuming; Yan, Yihua; Zaqarashvili,
Teimuraz; Zuccarello, Francesca
Bibcode: 2022JSWSC..12....2E
Altcode:
The Solar Activity Magnetic Monitor (SAMM) Network (SAMNet) is a
future UK-led international network of ground-based solar telescope
stations. SAMNet, at its full capacity, will continuously monitor
the Sun's intensity, magnetic, and Doppler velocity fields at
multiple heights in the solar atmosphere (from photosphere to upper
chromosphere). Each SAMM sentinel will be equipped with a cluster of
identical telescopes each with a different magneto-optical filter (MOFs)
to take observations in K I, Na D, and Ca I spectral bands. A subset
of SAMM stations will have white-light coronagraphs and emission line
coronal spectropolarimeters. The objectives of SAMNet are to provide
observational data for space weather research and forecast. The goal
is to achieve an operationally sufficient lead time of e.g., flare
warning of 2-8 h and provide many sought-after continuous synoptic
maps (e.g., LoS magnetic and velocity fields, intensity) of the lower
solar atmosphere with a spatial resolution limited only by seeing or
diffraction limit, and with a cadence of 10 min. The individual SAMM
sentinels will be connected to their master HQ hub where data received
from all the slave stations will be automatically processed and flare
warning issued up to 26 h in advance.
Title: HiRISE - High-Resolution Imaging and Spectroscopy Explorer
- Ultrahigh resolution, interferometric and external occulting
coronagraphic science
Authors: Erdélyi, Robertus; Damé, Luc; Fludra, Andrzej; Mathioudakis,
Mihalis; Amari, T.; Belucz, B.; Berrilli, F.; Bogachev, S.; Bolsée,
D.; Bothmer, V.; Brun, S.; Dewitte, S.; de Wit, T. Dudok; Faurobert,
M.; Gizon, L.; Gyenge, N.; Korsós, M. B.; Labrosse, N.; Matthews,
S.; Meftah, M.; Morgan, H.; Pallé, P.; Rochus, P.; Rozanov, E.;
Schmieder, B.; Tsinganos, K.; Verwichte, E.; Zharkov, S.; Zuccarello,
F.; Wimmer-Schweingruber, R.
Bibcode: 2022ExA...tmp...21E
Altcode:
Recent solar physics missions have shown the definite role of waves and
magnetic fields deep in the inner corona, at the chromosphere-corona
interface, where dramatic and physically dominant changes occur. HiRISE
(High Resolution Imaging and Spectroscopy Explorer), the ambitious new
generation ultra-high resolution, interferometric, and coronagraphic,
solar physics mission, proposed in response to the ESA Voyage 2050
Call, would address these issues and provide the best-ever and most
complete solar observatory, capable of ultra-high spatial, spectral,
and temporal resolution observations of the solar atmosphere, from the
photosphere to the corona, and of new insights of the solar interior
from the core to the photosphere. HiRISE, at the L1 Lagrangian
point, would provide meter class FUV imaging and spectro-imaging,
EUV and XUV imaging and spectroscopy, magnetic fields measurements,
and ambitious and comprehensive coronagraphy by a remote external
occulter (two satellites formation flying 375 m apart, with a
coronagraph on a chaser satellite). This major and state-of-the-art
payload would allow us to characterize temperatures, densities, and
velocities in the solar upper chromosphere, transition zone, and inner
corona with, in particular, 2D very high resolution multi-spectral
imaging-spectroscopy, and, direct coronal magnetic field measurement,
thus providing a unique set of tools to understand the structure and
onset of coronal heating. HiRISE's objectives are natural complements
to the Parker Solar Probe and Solar Orbiter-type missions. We present
the science case for HiRISE which will address: i) the fine structure
of the chromosphere-corona interface by 2D spectroscopy in FUV at
very high resolution; ii) coronal heating roots in the inner corona by
ambitious externally-occulted coronagraphy; iii) resolved and global
helioseismology thanks to continuity and stability of observing at the
L1 Lagrange point; and iv) solar variability and space climate with,
in addition, a global comprehensive view of UV variability.
Title: Implications of spicule activity on coronal loop heating and
catastrophic cooling
Authors: Nived, V. N.; Scullion, E.; Doyle, J. G.; Susino, R.; Antolin,
P.; Spadaro, D.; Sasso, C.; Sahin, S.; Mathioudakis, M.
Bibcode: 2022MNRAS.509.5523N
Altcode: 2021arXiv211107967N; 2021MNRAS.tmp.3004N
We report on the properties of coronal loop foot-point heating
with observations at the highest resolution, from the CRisp Imaging
Spectro-Polarimeter located at the Swedish 1-m Solar Telescope and
co-aligned NASA Solar Dynamics Observatory observations, of Type II
spicules in the chromosphere and their signatures in the extreme
ultraviolet (EUV) corona. Here, we address one important issue,
as to why there is not always a one-to-one correspondence, between
Type II spicules and hot coronal plasma signatures, i.e. beyond
TR temperatures. We do not detect any difference in their spectral
properties in a quiet Sun region compared to a region dominated by
coronal loops. On the other hand, the number density close to the
foot-points in the active region is found to be an order of magnitude
higher than in the quiet Sun case. A differential emission measure
analysis reveals a peak at ~5 × 105 K of the order of
1022 cm-5 K-1. Using this result as
a constraint, we conduct numerical simulations and show that with an
energy input of 1.25 × 1024 erg (corresponding to ~10 RBEs
contributing to the burst) we manage to reproduce the observation very
closely. However, simulation runs with lower thermal energy input do not
reproduce the synthetic AIA 171 Å signatures, indicating that there
is a critical number of spicules required in order to account for the
AIA 171 Å signatures in the simulation. Furthermore, the higher energy
(1.25 × 1024 erg) simulations reproduce catastrophic cooling
with a cycle duration of ~5 h, matching a periodicity we observe in
the EUV observations.
Title: The high-energy Sun - probing the origins of particle
acceleration on our nearest star
Authors: Matthews, S. A.; Reid, H. A. S.; Baker, D.; Bloomfield, D. S.;
Browning, P. K.; Calcines, A.; Del Zanna, G.; Erdelyi, R.; Fletcher,
L.; Hannah, I. G.; Jeffrey, N.; Klein, L.; Krucker, S.; Kontar, E.;
Long, D. M.; MacKinnon, A.; Mann, G.; Mathioudakis, M.; Milligan,
R.; Nakariakov, V. M.; Pesce-Rollins, M.; Shih, A. Y.; Smith, D.;
Veronig, A.; Vilmer, N.
Bibcode: 2021ExA...tmp..135M
Altcode:
As a frequent and energetic particle accelerator, our Sun provides
us with an excellent astrophysical laboratory for understanding
the fundamental process of particle acceleration. The exploitation
of radiative diagnostics from electrons has shown that acceleration
operates on sub-second time scales in a complex magnetic environment,
where direct electric fields, wave turbulence, and shock waves all
must contribute, although precise details are severely lacking. Ions
were assumed to be accelerated in a similar manner to electrons, but
γ-ray imaging confirmed that emission sources are spatially separated
from X-ray sources, suggesting distinctly different acceleration
mechanisms. Current X-ray and γ-ray spectroscopy provides only a basic
understanding of accelerated particle spectra and the total energy
budgets are therefore poorly constrained. Additionally, the recent
detection of relativistic ion signatures lasting many hours, without
an electron counterpart, is an enigma. We propose a single platform
to directly measure the physical conditions present in the energy
release sites and the environment in which the particles propagate and
deposit their energy. To address this fundamental issue, we set out
a suite of dedicated instruments that will probe both electrons and
ions simultaneously to observe; high (seconds) temporal resolution
photon spectra (4 keV - 150 MeV) with simultaneous imaging (1 keV -
30 MeV), polarization measurements (5-1000 keV) and high spatial and
temporal resolution imaging spectroscopy in the UV/EUV/SXR (soft X-ray)
regimes. These instruments will observe the broad range of radiative
signatures produced in the solar atmosphere by accelerated particles.
Title: Constraining the magnetic vector in the quiet solar photosphere
and the impact of instrumental degradation
Authors: Campbell, R. J.; Shelyag, S.; Quintero Noda, C.; Mathioudakis,
M.; Keys, P. H.; Reid, A.
Bibcode: 2021A&A...654A..11C
Altcode: 2021arXiv210701519C
Context. With the advent of next generation high resolution telescopes,
our understanding of how the magnetic field is organized in the
internetwork (IN) photosphere is likely to advance significantly.
Aims: We aim to evaluate the extent to which we can retrieve accurate
information about the magnetic vector in the IN photosphere using
inversion techniques.
Methods: We use a snapshot produced from
high resolution three-dimensional magnetohydrodynamic (MHD) simulations
and employ the Stokes Inversions based on Response functions (SIR) code
to produce synthetic observables in the same near infrared spectral
window as observed by the GREGOR Infrared Spectrograph (GRIS), which
contains the highly magnetically sensitive photospheric Fe I line pair
at 15 648.52 Å and 15 652.87 Å. We then use a parallelized wrapper to
SIR to perform nearly 14 million inversions of the synthetic spectra to
test how well the `true' MHD atmospheric parameters can be constrained
statistically. Finally, we degrade the synthetic Stokes vector
spectrally and spatially to GREGOR resolutions and examine how this
influences real observations, considering the impact of stray light,
spatial resolution and signal-to-noise (S/N) in particular.
Results: We find that the depth-averaged parameters can be recovered
by the inversions of the undegraded profiles, and by adding simple
gradients to magnetic field strength, inclination, and line of sight
velocity we show that an improvement in the χ2 value
is achieved. We also evaluate the extent to which we can constrain
these parameters at various optical depths, with the kinematic and
thermodynamic parameters sensitive deeper in the atmosphere than
the magnetic parameters. We find the S/N and spatial resolution both
play a significant role in determining how the degraded atmosphere
appears. At the same time, we find that the magnetic and kinematic
parameters are invariant upon inclusion of an unpolarized stray
light. We compare our results to recent IN observations obtained
by GREGOR. We studied a linear polarization feature which resembles
those recently observed by GRIS in terms of appearing as `loop-like'
structures and exhibiting very similar magnetic flux density. Thus,
we demonstrate that realistic MHD simulations are capable of showing
close agreement with real observations, and the symbiosis between them
and observations continues to prove essential. We finally discuss the
considerations that must be made for DKIST-era observations.
Title: Oscillations in the line-of-sight magnetic field strength in
a pore observed by the GREGOR Infrared Spectrograph (GRIS)
Authors: Nelson, C. J.; Campbell, R. J.; Mathioudakis, M.
Bibcode: 2021A&A...654A..50N
Altcode: 2021arXiv210710183N
Context. Numerous magnetohydrodynamic oscillations have been reported
within solar pores over the past few decades, including in line-of-sight
(LOS) velocities, intensities, and magnetic field strengths.
Aims: Our aim is to identify whether high-amplitude oscillations in the
LOS magnetic field strength can be detected within a pore located in
Active Region 12748 and to investigate which physical mechanisms could
be responsible for them.
Methods: A solar pore was observed on
1 September 2019 using the GREGOR Infrared Spectrograph instrument for
around one hour. Full-Stokes vectors were sampled in a 37 Å window
containing the Fe I 15 648.52 Å line (effective Landé g factor of
3). The LOS magnetic field strength was inferred using the strong-field
approximation. Additionally, the Stokes Inversion based on Response
functions code was used to gain a more complete understanding of the
physical properties of the solar atmosphere at the locations of these
oscillations.
Results: Oscillations of more than 100 G are
observed in the LOS magnetic field in the period window between 600
and 1272 s at three localised (> 1″2) regions. These
oscillations have coherence across individual regions, indicating that
jitter cannot account for their occurrence. Longer-period amplitude
variations, amplitudes over 200 G, are also detected, but they
have periods outside of the cone-of-influence. Numerical inversions
confirm both oscillations in the LOS magnetic field strength at optical
depths of around log τ5000 = −0.5 (potentially caused by
compression) and other effects (e.g. changes in the optical depth or the
inclination of the magnetic field) may account for these changes.
Conclusions: The oscillations in the separations of the Stokes-V lobes
of the 15 648.52 Å line appear to be solar in nature. Future work will
be required to understand whether these are truly oscillations in the
magnetic field strength at a specific depth in the solar atmosphere
or whether other effects are responsible for these signatures.
Title: Flare-induced Sunquake Signatures in the Ultraviolet as
Observed by the Atmospheric Imaging Assembly
Authors: Quinn, Sean; Mathioudakis, Mihalis; Nelson, Christopher J.;
Milligan, Ryan O.; Reid, Aaron; Jess, David B.
Bibcode: 2021ApJ...920...25Q
Altcode: 2021arXiv210505704Q
Sunquakes (SQs) have been routinely observed in the solar photosphere,
but it is only recently that signatures of these events have been
detected in the chromosphere. We investigate whether signatures of
SQs are common in ultraviolet (UV) continua that sample the solar
plasma several hundred km above where SQs are typically detected. We
analyze observations from the Solar Dynamics Observatory's Atmospheric
Imaging Assembly (SDO/AIA) 1600 and 1700 Å passbands, for SQ signatures
induced by the flares of Solar Cycle 24. We base our analysis on the
62 SQs detected in the recent statistical study presented by Sharykin
& Kosovichev. We find that 9 out of 62 SQ candidates produced a
response that is clearly detected in running-difference images from
the AIA 1600 and 1700 Å channels. A binary frequency filter with a
width of 2 mHz, centered on 6 mHz, was applied to the data. The first
signature of each SQ was detected at distances between 5.2 and 25.7
Mm from the associated flare ribbon. Time-distance and regression
analysis allowed us to calculate the apparent transverse velocities
of the SQs in the UV data sets and found maximum velocities as high
as 41 km s-1, 87 Mm away from the SQ source. Our analysis
shows that flare-induced SQ signatures can be detected in the SDO/AIA
1600 and 1700 Å passbands, hinting at their presence in the lower
chromosphere. There was no apparent correlation between GOES flare
classification and the appearance of the SQ at these heights.
Title: Temporal evolution of small-scale internetwork magnetic fields
in the solar photosphere (Corrigendum)
Authors: Campbell, R. J.; Mathioudakis, M.; Collados, M.; Keys, P. H.;
Asensio Ramos, A.; Nelson, C. J.; Kuridze, D.; Reid, A.
Bibcode: 2021A&A...652C...2C
Altcode:
No abstract at ADS
Title: Flare-induced Photospheric Velocity Diagnostics
Authors: Monson, Aaron J.; Mathioudakis, Mihalis; Reid, Aaron;
Milligan, Ryan; Kuridze, David
Bibcode: 2021ApJ...915...16M
Altcode: 2021arXiv210502199M
We present radiative-hydrodynamic simulations of solar flares generated
by the RADYN and RH codes to study the perturbations induced in
photospheric Fe I lines by electron beam heating. We investigate how
variations in the beam parameters result in discernible differences
in the induced photospheric velocities. Line synthesis revealed a
significant chromospheric contribution to the line profiles resulting
in an apparent red asymmetry by as much as 40 m s-1 close
to the time of maximum beam heating, which was not reflective of the
upflow velocities that arose from the radiative-hydrodynamic simulations
at those times. The apparent redshift to the overall line profile was
produced by significant chromospheric emission that was blueshifted
by as much as 400 m s-1 and fills in the blue side of
the near-stationary photospheric absorption profile. The velocity
information that can be retrieved from photospheric line profiles during
flares must therefore be treated with care to mitigate the effects of
higher parts of the atmosphere providing an erroneous velocity signal.
Title: The National Science Foundation's Daniel K. Inouye Solar
Telescope — Status Update
Authors: Rimmele, T.; Woeger, F.; Tritschler, A.; Casini, R.; de Wijn,
A.; Fehlmann, A.; Harrington, D.; Jaeggli, S.; Anan, T.; Beck, C.;
Cauzzi, G.; Schad, T.; Criscuoli, S.; Davey, A.; Lin, H.; Kuhn, J.;
Rast, M.; Goode, P.; Knoelker, M.; Rosner, R.; von der Luehe, O.;
Mathioudakis, M.; Dkist Team
Bibcode: 2021AAS...23810601R
Altcode:
The National Science Foundation's 4m Daniel K. Inouye Solar Telescope
(DKIST) on Haleakala, Maui is now the largest solar telescope in the
world. DKIST's superb resolution and polarimetric sensitivity will
enable astronomers to unravel many of the mysteries the Sun presents,
including the origin of solar magnetism, the mechanisms of coronal
heating and drivers of flares and coronal mass ejections. Five
instruments, four of which provide highly sensitive measurements
of solar magnetic fields, including the illusive magnetic field of
the faint solar corona. The DKIST instruments will produce large and
complex data sets, which will be distributed through the NSO/DKIST Data
Center. DKIST has achieved first engineering solar light in December
of 2019. Due to COVID the start of the operations commissioning phase
is delayed and is now expected for fall of 2021. We present a status
update for the construction effort and progress with the operations
commissioning phase.
Title: Chromospheric Heating by Magnetohydrodynamic Waves and
Instabilities
Authors: Srivastava, A. K.; Ballester, J. L.; Cally, P. S.; Carlsson,
M.; Goossens, M.; Jess, D. B.; Khomenko, E.; Mathioudakis, M.;
Murawski, K.; Zaqarashvili, T. V.
Bibcode: 2021JGRA..12629097S
Altcode: 2021arXiv210402010S
The importance of the chromosphere in the mass and energy transport
within the solar atmosphere is now widely recognized. This review
discusses the physics of magnetohydrodynamic waves and instabilities
in large-scale chromospheric structures as well as in magnetic flux
tubes. We highlight a number of key observational aspects that have
helped our understanding of the role of the solar chromosphere
in various dynamic processes and wave phenomena, and the heating
scenario of the solar chromosphere is also discussed. The review
focuses on the physics of waves and invokes the basics of plasma
instabilities in the context of this important layer of the solar
atmosphere. Potential implications, future trends and outstanding
questions are also delineated.
Title: Critical Science Plan for the Daniel K. Inouye Solar Telescope
(DKIST)
Authors: Rast, Mark P.; Bello González, Nazaret; Bellot Rubio,
Luis; Cao, Wenda; Cauzzi, Gianna; Deluca, Edward; de Pontieu, Bart;
Fletcher, Lyndsay; Gibson, Sarah E.; Judge, Philip G.; Katsukawa,
Yukio; Kazachenko, Maria D.; Khomenko, Elena; Landi, Enrico; Martínez
Pillet, Valentín; Petrie, Gordon J. D.; Qiu, Jiong; Rachmeler,
Laurel A.; Rempel, Matthias; Schmidt, Wolfgang; Scullion, Eamon; Sun,
Xudong; Welsch, Brian T.; Andretta, Vincenzo; Antolin, Patrick; Ayres,
Thomas R.; Balasubramaniam, K. S.; Ballai, Istvan; Berger, Thomas E.;
Bradshaw, Stephen J.; Campbell, Ryan J.; Carlsson, Mats; Casini,
Roberto; Centeno, Rebecca; Cranmer, Steven R.; Criscuoli, Serena;
Deforest, Craig; Deng, Yuanyong; Erdélyi, Robertus; Fedun, Viktor;
Fischer, Catherine E.; González Manrique, Sergio J.; Hahn, Michael;
Harra, Louise; Henriques, Vasco M. J.; Hurlburt, Neal E.; Jaeggli,
Sarah; Jafarzadeh, Shahin; Jain, Rekha; Jefferies, Stuart M.; Keys,
Peter H.; Kowalski, Adam F.; Kuckein, Christoph; Kuhn, Jeffrey R.;
Kuridze, David; Liu, Jiajia; Liu, Wei; Longcope, Dana; Mathioudakis,
Mihalis; McAteer, R. T. James; McIntosh, Scott W.; McKenzie, David
E.; Miralles, Mari Paz; Morton, Richard J.; Muglach, Karin; Nelson,
Chris J.; Panesar, Navdeep K.; Parenti, Susanna; Parnell, Clare E.;
Poduval, Bala; Reardon, Kevin P.; Reep, Jeffrey W.; Schad, Thomas A.;
Schmit, Donald; Sharma, Rahul; Socas-Navarro, Hector; Srivastava,
Abhishek K.; Sterling, Alphonse C.; Suematsu, Yoshinori; Tarr, Lucas
A.; Tiwari, Sanjiv; Tritschler, Alexandra; Verth, Gary; Vourlidas,
Angelos; Wang, Haimin; Wang, Yi-Ming; NSO and DKIST Project; DKIST
Instrument Scientists; DKIST Science Working Group; DKIST Critical
Science Plan Community
Bibcode: 2021SoPh..296...70R
Altcode: 2020arXiv200808203R
The National Science Foundation's Daniel K. Inouye Solar Telescope
(DKIST) will revolutionize our ability to measure, understand,
and model the basic physical processes that control the structure
and dynamics of the Sun and its atmosphere. The first-light DKIST
images, released publicly on 29 January 2020, only hint at the
extraordinary capabilities that will accompany full commissioning of
the five facility instruments. With this Critical Science Plan (CSP)
we attempt to anticipate some of what those capabilities will enable,
providing a snapshot of some of the scientific pursuits that the DKIST
hopes to engage as start-of-operations nears. The work builds on the
combined contributions of the DKIST Science Working Group (SWG) and
CSP Community members, who generously shared their experiences, plans,
knowledge, and dreams. Discussion is primarily focused on those issues
to which DKIST will uniquely contribute.
Title: Temporal evolution of small-scale internetwork magnetic fields
in the solar photosphere
Authors: Campbell, R. J.; Mathioudakis, M.; Collados, M.; Keys, P. H.;
Asensio Ramos, A.; Nelson, C. J.; Kuridze, D.; Reid, A.
Bibcode: 2021A&A...647A.182C
Altcode: 2021arXiv210200942C
Context. While the longitudinal field that dominates in photospheric
network regions has been studied extensively, small-scale transverse
fields have recently been found to be ubiquitous in the quiet
internetwork photosphere and this merits further study. Furthermore,
few observations have been able to capture how this field evolves.
Aims: We aim to statistically characterize the magnetic vector in
a quiet Sun internetwork region and observe the temporal evolution of
specific small-scale magnetic features.
Methods: We present
two high spatio-temporal resolution observations that reveal the
dynamics of two disk-centre internetwork regions taken by the new GREGOR
Infrared Spectrograph Integral Field Unit with the highly magnetically
sensitive photospheric Fe I line pair at 15648.52 Å and 15652.87
Å. We record the full Stokes vector and apply inversions with the
Stokes inversions based on response functions code to retrieve the
parameters characterizing the atmosphere. We consider two inversion
schemes: scheme 1 (S1), where a magnetic atmosphere is embedded in
a field free medium, and scheme 2 (S2), with two magnetic models
and a fixed 30% stray light component.
Results: The magnetic
properties produced from S1 inversions returned a median magnetic
field strength of 200 and 240 G for the two datasets, respectively. We
consider the median transverse (horizontal) component, among pixels
with Stokes Q or U, and the median unsigned longitudinal (vertical)
component, among pixels with Stokes V, above a noise threshold. We
determined the former to be 263 G and 267 G, and the latter to be 131
G and 145 G, for the two datasets, respectively. Finally, we present
three regions of interest, tracking the dynamics of small-scale magnetic
features. We apply S1 and S2 inversions to specific profiles of interest
and find that the latter produces better approximations when there is
evidence of mixed polarities. We find patches of linear polarization
with magnetic flux density of the order of 130−150 G and find that
linear polarization appears preferentially at granule-intergranular
lane boundaries. The weak magnetic field appears to be organized in
terms of complex `loop-like' structures, with transverse fields often
flanked by opposite polarity longitudinal fields.
Title: Mapping the Magnetic Field of Flare Coronal Loops
Authors: Kuridze, David; Morgan, Huw; Oliver, Ramon; Mathioudakis,
Mihalis; Koza, Julius
Bibcode: 2021cosp...43E1791K
Altcode:
The magnetic field is key to the dynamics, evolution, and heating
of the solar atmosphere, yet direct measurements are rare and highly
uncertain. We report on the unique observation of flaring coronal loops
at the solar limb using high-resolution imaging spectropolarimetry from
the Swedish 1-m Solar Telescope. The vantage position, orientation,
and nature of the chromospheric material that filled the flare loops
allowed us to determine their magnetic field with unprecedented accuracy
using the weak-field approximation method. Our analysis reveals coronal
magnetic field strengths as high as 350 G at heights up to 25 Mm above
the solar limb. These measurements are substantially higher than a
number of previous estimates and may have considerable implications
for our current understanding of the extended solar atmosphere.
Title: High-Cadence DST/ROSA Observations of the NUV/Blue Continuum
Radiation in a Solar Flare
Authors: Kowalski, A. F.; Keys, P.; Mathioudakis, M.
Bibcode: 2020AGUFMSH0500011K
Altcode:
Radiative-hydrodynamic models of solar flares provide sophisticated
predictions of the ultraviolet and optical continuum shape and
strength on shorter than 1 s timescales covering a broad wavelength
range. However, most optical observations of solar flares in the
modern era are not obtained at fast cadence and are not optimized
at blue continuum wavelengths, which provide a critical constraint
on the heating properties at large column mass. To rectify this
major gap in our knowledge of solar flare spectra, custom Balmer
jump filters were designed for the ROSA instrument at the Dunn Solar
Telescope. Unprecedented observations at 7.5 - 30 frames per second
were obtained covering the C9.7 flare SOL20141025T15:52 in NOAA AR
12192 during the NSO's Service Mode operations (and provided by the
F-CHROMA solar flare database). We report on how the flare response in
the 350 nm and 417 nm ROSA filters constrains several common assumptions
employed in modern flare modeling, such as the duration of individual
heating bursts. We also report on the Balmer jump properties in this
flare and compare to several M dwarf flares, which have been observed
in identical filters using the ULTRACAM and ARCTIC instruments on the
4.2m WHT, the 3.6m NTT, and the 3.5m ARC telescope. We compare the
light curves to radiative-hydrodynamic model predictions of the 350
nm brightness evolution on short timescales.
Title: An Introduction to Photospheric Flare Line Diagnostics
Authors: Monson, A.; Mathioudakis, M.; Milligan, R.; Reid, A.;
Kuridze, D.
Bibcode: 2020AGUFMSH057..04M
Altcode:
In preparation for solar cycle 25, we present radiative hydrodynamic
flare models from the F-CHROMA archive constructed using the RADYN and
RH codes. We simulate potential observable perturbations seen in the
line-of-sight velocity and heating induced in the photosphere during
a flare. Many works have focused on the recreation of chromospheric
spectral line profiles as the atmosphere rapidly heats and expands
due to the injection of energy from magnetic reconnection. This
extreme heating mechanism can permeate through the upper atmosphere
and affect the lowest levels of the solar atmosphere. This results
in an observable brightening and Doppler shifting of spectral lines
formed in the deepest regions, though the extent of this photospheric
reaction is still not fully characterised. We investigate how variations
in the properties of a solar flare-accelerated electron beam result in
discernible asymmetries in the flaring profiles of several deep forming
Fe I spectral lines. Through analysis of the contribution functions
and response functions of these lines during a flare, we have found
an unprecedented level of high velocity chromospheric contribution
which significantly alters the emergent profile of these lines. This
comparatively weak, high velocity surplus contributing region of
the chromosphere can result in false red shifts and/or significantly
adds to the flaring Doppler shifted profile, resulting in a greater
blue shift of the line core, which had previously been assumed as a
purely photospheric velocity diagnostic. Based on this evidence, it
is concluded that any future work using these photospheric lines to
diagnose properties of the photosphere in a flaring atmosphere must
be carefully analysed to mitigate effect of higher forming regions
providing a surplus velocity signal.
Title: The Daniel K. Inouye Solar Telescope - Observatory Overview
Authors: Rimmele, Thomas R.; Warner, Mark; Keil, Stephen L.; Goode,
Philip R.; Knölker, Michael; Kuhn, Jeffrey R.; Rosner, Robert R.;
McMullin, Joseph P.; Casini, Roberto; Lin, Haosheng; Wöger, Friedrich;
von der Lühe, Oskar; Tritschler, Alexandra; Davey, Alisdair; de Wijn,
Alfred; Elmore, David F.; Fehlmann, André; Harrington, David M.;
Jaeggli, Sarah A.; Rast, Mark P.; Schad, Thomas A.; Schmidt, Wolfgang;
Mathioudakis, Mihalis; Mickey, Donald L.; Anan, Tetsu; Beck, Christian;
Marshall, Heather K.; Jeffers, Paul F.; Oschmann, Jacobus M.; Beard,
Andrew; Berst, David C.; Cowan, Bruce A.; Craig, Simon C.; Cross,
Eric; Cummings, Bryan K.; Donnelly, Colleen; de Vanssay, Jean-Benoit;
Eigenbrot, Arthur D.; Ferayorni, Andrew; Foster, Christopher; Galapon,
Chriselle Ann; Gedrites, Christopher; Gonzales, Kerry; Goodrich, Bret
D.; Gregory, Brian S.; Guzman, Stephanie S.; Guzzo, Stephen; Hegwer,
Steve; Hubbard, Robert P.; Hubbard, John R.; Johansson, Erik M.;
Johnson, Luke C.; Liang, Chen; Liang, Mary; McQuillen, Isaac; Mayer,
Christopher; Newman, Karl; Onodera, Brialyn; Phelps, LeEllen; Puentes,
Myles M.; Richards, Christopher; Rimmele, Lukas M.; Sekulic, Predrag;
Shimko, Stephan R.; Simison, Brett E.; Smith, Brett; Starman, Erik;
Sueoka, Stacey R.; Summers, Richard T.; Szabo, Aimee; Szabo, Louis;
Wampler, Stephen B.; Williams, Timothy R.; White, Charles
Bibcode: 2020SoPh..295..172R
Altcode:
We present an overview of the National Science Foundation's Daniel
K. Inouye Solar Telescope (DKIST), its instruments, and support
facilities. The 4 m aperture DKIST provides the highest-resolution
observations of the Sun ever achieved. The large aperture of
DKIST combined with state-of-the-art instrumentation provide the
sensitivity to measure the vector magnetic field in the chromosphere
and in the faint corona, i.e. for the first time with DKIST we will
be able to measure and study the most important free-energy source
in the outer solar atmosphere - the coronal magnetic field. Over its
operational lifetime DKIST will advance our knowledge of fundamental
astronomical processes, including highly dynamic solar eruptions
that are at the source of space-weather events that impact our
technological society. Design and construction of DKIST took over two
decades. DKIST implements a fast (f/2), off-axis Gregorian optical
design. The maximum available field-of-view is 5 arcmin. A complex
thermal-control system was implemented in order to remove at prime
focus the majority of the 13 kW collected by the primary mirror and
to keep optical surfaces and structures at ambient temperature, thus
avoiding self-induced local seeing. A high-order adaptive-optics
system with 1600 actuators corrects atmospheric seeing enabling
diffraction limited imaging and spectroscopy. Five instruments, four
of which are polarimeters, provide powerful diagnostic capability
over a broad wavelength range covering the visible, near-infrared,
and mid-infrared spectrum. New polarization-calibration strategies
were developed to achieve the stringent polarization accuracy
requirement of 5×10−4. Instruments can be combined and
operated simultaneously in order to obtain a maximum of observational
information. Observing time on DKIST is allocated through an open,
merit-based proposal process. DKIST will be operated primarily in
"service mode" and is expected to on average produce 3 PB of raw
data per year. A newly developed data center located at the NSO
Headquarters in Boulder will initially serve fully calibrated data to
the international users community. Higher-level data products, such as
physical parameters obtained from inversions of spectro-polarimetric
data will be added as resources allow.
Title: Statistical Signatures of Nanoflare Activity. II. A Nanoflare
Explanation for Periodic Brightenings in Flare Stars Observed by NGTS
Authors: Dillon, C. J.; Jess, D. B.; Mathioudakis, M.; Watson, C. A.;
Jackman, J. A. G.; Wheatley, P. J.; Goad, M. R.; Casewell, S. L.;
Anderson, D. R.; Burleigh, M. R.; Raynard, L.; West, R. G.
Bibcode: 2020ApJ...904..109D
Altcode: 2020arXiv201004167D
Several studies have documented periodic and quasi-periodic signals
from the time series of dMe flare stars and other stellar sources. Such
periodic signals, observed within quiescent phases (i.e., devoid of
larger-scale microflare or flare activity), range in a period from
1 to 1000 s and hence have been tentatively linked to ubiquitous
p-mode oscillations generated in the convective layers of the star. As
such, most interpretations for the observed periodicities have been
framed in terms of magnetohydrodynamic wave behavior. However,
we propose that a series of continuous nanoflares, based upon a
power-law distribution, can provide a similar periodic signal in the
associated time series. Adapting previous statistical analyses of
solar nanoflare signals, we find the first statistical evidence for
stellar nanoflare signals embedded within the noise envelope of M-type
stellar lightcurves. Employing data collected by the Next Generation
Transit Survey (NGTS), we find evidence for stellar nanoflare activity
demonstrating a flaring power-law index of 3.25 ± 0.20, alongside
a decay timescale of 200 ± 100 s. We also find that synthetic time
series, consistent with the observations of dMe flare star lightcurves,
are capable of producing quasi-periodic signals in the same frequency
range as p-mode signals, despite being purely composed of impulsive
signatures. Phenomena traditionally considered a consequence of wave
behavior may be described by a number of high-frequency but discrete
nanoflare energy events. This new physical interpretation presents a
novel diagnostic capability, by linking observed periodic signals to
given nanoflare model conditions.
Title: Power-law energy distributions of small-scale impulsive events
on the active Sun: results from IRIS
Authors: Vilangot Nhalil, Nived; Nelson, Chris J.; Mathioudakis,
Mihalis; Doyle, J. Gerry; Ramsay, Gavin
Bibcode: 2020MNRAS.499.1385V
Altcode: 2020MNRAS.tmp.2719N; 2020arXiv200903123V
Numerous studies have analysed inferred power-law distributions
between frequency and energy of impulsive events in the outer solar
atmosphere in an attempt to understand the predominant energy supply
mechanism in the corona. Here, we apply a burst detection algorithm to
high-resolution imaging data obtained by the Interface Region Imaging
Spectrograph to further investigate the derived power-law index, γ,
of bright impulsive events in the transition region. Applying the
algorithm with a constant minimum event lifetime (of either 60 s or
110 s) indicated that the target under investigation, such as Plage
and Sunspot, has an influence on the observed power-law index. For
regions dominated by sunspots, we always find γ < 2; however,
for data sets where the target is a plage region, we often find that
γ > 2 in the energy range (~1023, ~1026)
erg. Applying the algorithm with a minimum event lifetime of three
time-steps indicated that cadence was another important factor, with
the highest cadence data sets returning γ > 2 values. The estimated
total radiative power obtained for the observed energy distributions
is typically 10-25 per cent of what would be required to sustain the
corona indicating that impulsive events in this energy range are not
sufficient to solve coronal heating. If we were to extend the power-law
distribution down to an energy of 1021 erg, and assume parity
between radiative energy release and the deposition of thermal energy,
then such bursts could provide 25-50 per cent of the required energy
to account for the coronal heating problem.
Title: Umbral chromospheric fine structure and umbral flashes modelled
as one: The corrugated umbra
Authors: Henriques, Vasco M. J.; Nelson, Chris J.; Rouppe van der
Voort, Luc H. M.; Mathioudakis, Mihalis
Bibcode: 2020A&A...642A.215H
Altcode: 2020arXiv200805482H
Context. The chromosphere of the umbra of sunspots features an
assortment of dynamic fine structures that are poorly understood and
often studied separately. Small-scale umbral brightenings (SSUBs),
umbral microjets, spikes or short dynamic fibrils (SDFs), and umbral
dark fibrils are found in any observation of the chromosphere with
sufficient spatial resolution performed at the correct umbral flash
stage and passband. Understanding these features means understanding
the dynamics of the umbral chromosphere.
Aims: We aim to fully
understand the dynamics of umbral chromosphere through analysis of the
relationships between distinct observed fine features and to produce
complete models that explain both spectral profiles and the temporal
evolution of the features. We seek to relate such understanding to
umbral flashes.
Methods: We studied the spatial and spectral
co-evolution of SDFs, SSUBs, and umbral flashes in Ca II 8542 Å
spectral profiles. We produced models that generate the spectral
profiles for all classes of features using non-local thermodynamic
equilibrium radiative transfer with a recent version of the NICOLE
inversion code.
Results: We find that both bright SSUBs and dark
SDF structures are described with a continuous feature in the parameter
space that is distinct from the surroundings even in pixel-by-pixel
inversions. We find a phase difference between such features and
umbral flashes in both inverted line-of-sight velocities and timing
of the brightenings. For umbral flashes themselves we resolve,
for the first time in inversion-based semi-empirical modelling,
the pre-flash downflows, post-flash upflows, and the counter-flows
present during the umbral flash phase. We further present a simple
time-dependent cartoon model that explains the dynamics and spectral
profiles of both fine structure, dark and bright, and umbral flashes
in umbral chromospheres.
Conclusions: The similarity of the
profiles between the brightenings and umbral flashes, the pattern of
velocities obtained from the inversions, and the phase relationships
between the structures all lead us to put forward that all dynamic
umbral chromospheric structures observed to this date are a locally
delayed or locally early portion of the oscillatory flow pattern that
generates flashes, secondary to the steepening large-scale acoustic
waves at its source. Essentially, SSUBs are part of the same shock or
merely compression front responsible for the spatially larger umbral
flash phenomenon, but out of phase with the broader oscillation.
Title: Evolution of downflows in the transition region above a
sunspot over short time-scales
Authors: Nelson, C. J.; Krishna Prasad, S.; Mathioudakis, M.
Bibcode: 2020A&A...640A.120N
Altcode: 2020arXiv200613617N
Context. Downflows with potentially super-sonic velocities have been
reported to occur in the transition region above many sunspots; however,
how these signatures evolve over short time-scales in both spatial and
spectral terms is still unknown and requires further research.
Aims: In this article, we investigate the evolution of downflows
detected within spectral lines sampling the transition region on
time-scales of the order of minutes and we search for clues as to
the formation mechanisms of these features in co-temporal imaging
data.
Methods: For the purposes of this article, we used
high-resolution spectral and imaging data sampled by the Interface
Region Imaging Spectrograph on the 20 and 21 May 2015 to identify and
analyse downflows. Additionally, photospheric and coronal imaging data
from the Hinode and Solar Dynamics Observatory satellites were studied
to provide context about the wider solar atmosphere.
Results:
Four downflows were identified and analysed through time. The potential
super-sonic components of these downflows had widths of around 2″ and
were observed to evolve over time-scales of the order of minutes. The
measured apparent downflow velocities were structured both in time and
space, with the highest apparent velocities occurring above a bright
region detected in Si IV 1400 Å images. Downflows with apparent
velocities below the super-sonic threshold that was assumed here
were observed to extend a few arcseconds away from the foot-points,
suggesting that the potential super-sonic components are linked to
larger-scale flows. The electron density and mass flux for these events
were found to be within the ranges of 109.6-1010.2
cm-3 and 10-6.81-10-7.48 g
cm-2 s-1, respectively. Finally, each downflow
formed at the foot-point of thin "fingers", extending out around
3-5″ in Si IV 1400 Å data with smaller widths (< 1″) than
the super-sonic downflow components.
Conclusions: Downflows
can appear, disappear, and recur within time-scales of less than one
hour in sunspots. As the potential super-sonic downflow signatures
were detected at the foot-points of both extended fingers in Si IV
1400 SJI data and sub-sonic downflows in Si IV 1394 Å spectra, it
is likely that these events are linked to larger-scale flows within
structures such as coronal loops.
Title: Can active late-type stars contribute to the Galactic lithium
abundance?
Authors: Kelly, Darnell E.; Christian, Damian J.; Mathioudakis,
Mihalis; Jevremović, Darko
Bibcode: 2020RAA....20..104K
Altcode:
Lithium abundances in our Galaxy and especially 6Li
abundances provide important constraints on our understanding of
Big Bang Nucleosynthesis (BBNS), stellar evolution and the creation
of light elements by cosmic rays in the ISM. 6Li has been
detected in energetic solar events, one chromospherically active binary
and several dwarf halo stars. Continuing our work on active late-type
stars with high lithium abundances, we expand our study to consider
if the flare origin of lithium created by spallation can contribute
significantly to the Galactic abundance of lithium. We previously
derived $\displaystyle \frac{{}^{6}{\rm{Li}}}{{}^{7}{\rm{Li}}}=0.030\pm
0.010$?--> for active K dwarf GJ 117 using VLT UVES observations. We
find $\displaystyle \frac{{}^{6}{\rm{Li}}}{{}^{7}{\rm{Li}}}$?-->
ratios of 0.02 and 0.10 for two other stars in our sample, GJ 182
and EUVE J1145-55.3A, respectively. Considering that these later
type, active stars have significant flare rates and stellar winds,
we have estimated the contribution of these stars to the Galactic
lithium abundance. Given that K and M stars comprise over 84% of our
Galaxy and that many of these can have significant stellar winds,
we conclude that spallation in stellar flares can contribute 1% and
up to 5% of the Galactic lithium abundance.
Title: Spectral Characteristics and Formation Height of Off-limb
Flare Ribbons
Authors: Kuridze, David; Mathioudakis, Mihalis; Heinzel, Petr; Koza,
Július; Morgan, Huw; Oliver, Ramon; Kowalski, Adam F.; Allred, Joel C.
Bibcode: 2020ApJ...896..120K
Altcode: 2020arXiv200510924K
Flare ribbons are bright manifestations of flare energy dissipation
in the lower solar atmosphere. For the first time, we report on
high-resolution imaging spectroscopy observations of flare ribbons
situated off limb in the Hβ and Ca II 8542 Å lines and make a detailed
comparison with radiative hydrodynamic simulations. Observations of
the X8.2 class solar flare SOL 2017-09-10T16:06 UT obtained with the
Swedish Solar Telescope reveal bright horizontal emission layers
in Hβ line-wing images located near the footpoints of the flare
loops. The apparent separation between the ribbon observed in the Hβ
wing and the nominal photospheric limb is about 300-500 km. The Ca II
8542 Å line-wing images show much fainter ribbon emissions located
right on the edge of the limb, without clear separation from the
limb. RADYN models are used to investigate synthetic spectral line
profiles for the flaring atmosphere, and good agreement is found
with the observations. The simulations show that, toward the limb,
where the line of sight is substantially oblique with respect to the
vertical direction, the flaring atmosphere model reproduces the high
contrast of the off-limb Hβ ribbons and their significant elevation
above the photosphere. The ribbons in the Ca II 8542 Å line-wing
images are located deeper in the lower solar atmosphere with a lower
contrast. A comparison of the height deposition of electron beam energy
and the intensity contribution function shows that the Hβ line-wing
intensities can be a useful tracer of flare energy deposition in the
lower solar atmosphere.
Title: Chromospheric Bubbles in Solar Flares
Authors: Reid, A.; Zhigulin, B.; Carlsson, M.; Mathioudakis, M.
Bibcode: 2020ApJ...894L..21R
Altcode: 2020arXiv200510586R
We analyze a grid of radiative hydrodynamic simulations of solar flares
to study the energy balance and response of the atmosphere to nonthermal
electron beam heating. The appearance of chromospheric bubbles is one
of the most notable features that we find in the simulations. These
pockets of chromospheric plasma get trapped between the transition
region and the lower atmosphere as it is superheated by the particle
beam. The chromospheric bubbles are seen in the synthetic spectra,
appearing as an additional component to Balmer line profiles with high
Doppler velocities as high as 200 km s-1. Their signatures
are also visible in the wings of Ca II 8542 Å line profiles. These
bubbles of chromospheric plasma are driven upward by a wave front that
is induced by the shock of energy deposition, and require a specific
heating rate and atmospheric location to manifest.
Title: Velocity Response of the Observed Explosive Events in the
Lower Solar Atmosphere. I. Formation of the Flowing Cool-loop System
Authors: Srivastava, A. K.; Rao, Yamini K.; Konkol, P.; Murawski,
K.; Mathioudakis, M.; Tiwari, Sanjiv K.; Scullion, E.; Doyle, J. G.;
Dwivedi, B. N.
Bibcode: 2020ApJ...894..155S
Altcode: 2020arXiv200402775S
We observe plasma flows in cool loops using the Slit-Jaw Imager on board
the Interface Region Imaging Spectrometer (IRIS). Huang et al. observed
unusually broadened Si IV 1403 Šline profiles at the footpoints of
such loops that were attributed to signatures of explosive events
(EEs). We have chosen one such unidirectional flowing cool-loop
system observed by IRIS where one of the footpoints is associated
with significantly broadened Si IV line profiles. The line-profile
broadening indirectly indicates the occurrence of numerous EEs below
the transition region (TR), while it directly infers a large velocity
enhancement/perturbation, further causing the plasma flows in the
observed loop system. The observed features are implemented in a
model atmosphere in which a low-lying bipolar magnetic field system
is perturbed in the chromosphere by a velocity pulse with a maximum
amplitude of 200 km s-1. The data-driven 2D numerical
simulation shows that the plasma motions evolve in a similar manner
as observed by IRIS in the form of flowing plasma filling the skeleton
of a cool-loop system. We compare the spatio-temporal evolution of the
cool-loop system in the framework of our model with the observations,
and conclude that their formation is mostly associated with the velocity
response of the transient energy release above their footpoints in
the chromosphere/TR. Our observations and modeling results suggest
that the velocity responses most likely associated to the EEs could
be one of the main candidates for the dynamics and energetics of the
flowing cool-loop systems in the lower solar atmosphere.
Title: Evolution of supersonic downflows in a sunspot
Authors: Nelson, C. J.; Krishna Prasad, S.; Mathioudakis, M.
Bibcode: 2020A&A...636A..35N
Altcode: 2020arXiv200302489N
Context. Supersonic downflows have been observed in transition
region spectra above numerous sunspots, however, little research
has been conducted to date into how persistent these signatures
are within sunspots on timescales longer than a few hours.
Aims: We aim to analyse the lead sunspot of AR 12526 to infer the
properties and evolution of supersonic downflows occurring within
it using high-spatial and spectral resolution data.
Methods:
We analysed 16 large, dense raster scans sampled by the Interface
Region Imaging Spectrograph. These rasters tracked the lead sunspot
of AR 12526 across the solar disc at discrete times between 27 March
2016 and 2 April 2016, providing spectral profiles from the Si IV, O
IV, Mg II, and C II lines. Additionally, we studied one sit-and-stare
observation acquired on 1 April 2016 centred on the sunspot to analyse
the evolution of supersonic downflows on shorter timescales.
Results: Supersonic downflows are variable within this sunspot both
in terms of spatial structuring and velocities. Thirteen of the 16
raster scans display some evidence of supersonic downflows in the Si
IV 1394 Å line co-spatial to a sustained bright structure detected
in the 1400 Å slit-jaw imaging channel; a peak velocity of 112 km
s-1 is recorded on 29 March 2016. Evidence for supersonic
downflows in the O IV 1401 Å line is found in 14 of these rasters; the
spatial structuring in this line often differs from that inferred from
the Si IV 1394 Å line. Only one example of a supersonic downflow is
detected in the C II 1335 Å line and no downflows are found in the Mg
II 2796 Å lines at these locations. In the sit-and-stare observations,
no dual flow is initially detected, however, a supersonic downflow
develops after approximately 60 min. This downflow accelerates from 73
km s-1 to close to 80 km s-1 in both the Si IV
1394 Å and O IV 1401 Å lines over the course of 20 min before the
end of the observation.
Conclusions: Supersonic downflows are
found in the Si IV 1394 Å line in 13 of the 16 rasters studied in this
work. The morphology of these downflows evolved over the course of both
hours and days and was often different in the Si IV 1394 Å and O IV
1401 Å lines. These events were found co-spatial to a bright region
in the core of the Si IV 1394 Å line, which appeared to form at the
footpoints of coronal fan loops. Our results indicate that one raster
is not enough to conclusively draw inferences about the properties of
supersonic downflows within a sunspot during its lifetime.
Title: Propagation of waves above a plage as observed by IRIS and SDO
Authors: Kayshap, P.; Srivastava, A. K.; Tiwari, S. K.; Jelínek,
P.; Mathioudakis, M.
Bibcode: 2020A&A...634A..63K
Altcode: 2019arXiv191011557K
Context. Magnetohydrodynamic waves are proposed as the mechanism
that transport sufficient energy from the photosphere to heat the
transition region (TR) and corona. However, various aspects of these
waves, such as their nature, propagation characteristics, and role
in the atmospheric heating process, remain poorly understood and
require further investigation.
Aims: We aim to investigate
wave propagation within an active-region plage using IRIS and AIA
observations. The main motivation is to understand the relationship
between photospheric and TR oscillations. We identify the locations in
the plage region where magnetic flux tubes are essentially vertical,
and further we discuss the propagation and nature of these waves.
Methods: We used photospheric observations from AIA (i.e., AIA
1700 Å) as well as TR imaging observations (IRIS SJI Si IV 1400.0
Å). We investigated the propagation of the waves into the TR from the
photosphere using wavelet analysis (e.g., cross power, coherence, and
phase difference) with the inclusion of a customized noise model.
Results: A fast Fourier transform algorithm shows the distribution of
wave power at photospheric and TR heights. Waves with periods between
2.0 and 9.0 min appear to be correlated between the photosphere and
TR. We exploited a customized noise model to estimate the 95% confidence
levels for the IRIS observations. On the basis of the sound speed in the
TR and estimated propagation speed, these waves are best interpreted
as slow magneto acoustic waves (SMAWs). It is found that almost all
locations show correlation and propagation of waves over a broad range
of periods from the photosphere to the TR. Our observations suggest
that the SMAWs spatial occurrence frequency is stronly correlated
between the photosphere and transition region within plage areas.
Title: High-resolution spectropolarimetric observations of the
temporal evolution of magnetic fields in photospheric bright points
Authors: Keys, P. H.; Reid, A.; Mathioudakis, M.; Shelyag, S.;
Henriques, V. M. J.; Hewitt, R. L.; Del Moro, D.; Jafarzadeh, S.;
Jess, D. B.; Stangalini, M.
Bibcode: 2020A&A...633A..60K
Altcode: 2019arXiv191108436K
Context. Magnetic bright points (MBPs) are dynamic, small-scale
magnetic elements often found with field strengths of the order of a
kilogauss within intergranular lanes in the photosphere.
Aims:
Here we study the evolution of various physical properties inferred from
inverting high-resolution full Stokes spectropolarimetry data obtained
from ground-based observations of the quiet Sun at disc centre.
Methods: Using automated feature-tracking algorithms, we studied
300 MBPs and analysed their temporal evolution as they evolved to
kilogauss field strengths. These properties were inferred using
both the NICOLE and SIR Stokes inversion codes. We employ similar
techniques to study radiative magnetohydrodynamical simulations
for comparison with our observations.
Results: Evidence was
found for fast (∼30-100 s) amplification of magnetic field strength
(by a factor of 2 on average) in MBPs during their evolution in our
observations. Similar evidence for the amplification of fields is seen
in our simulated data.
Conclusions: Several reasons for the
amplifications were established, namely, strong downflows preceding
the amplification (convective collapse), compression due to granular
expansion and mergers with neighbouring MBPs. Similar amplification of
the fields and interpretations were found in our simulations, as well
as amplification due to vorticity. Such a fast amplification will have
implications for a wide array of topics related to small-scale fields
in the lower atmosphere, particularly with regard to propagating wave
phenomena in MBPs.
Title: Continuum Enhancements, Line Profiles, and Magnetic Field
Evolution during Consecutive Flares
Authors: Zuccarello, Francesca; Guglielmino, Salvo L.; Capparelli,
Vincenzo; Mathioudakis, Mihalis; Keys, Peter H.; Criscuoli, Serena;
Falco, Mariachiara; Murabito, Mariarita
Bibcode: 2020ApJ...889...65Z
Altcode: 2019arXiv191200859Z
During solar flares, magnetic energy can be converted into
electromagnetic radiation from radio waves to γ-rays. Enhancements in
the continuum at visible wavelengths, as well as continuum enhancements
in the FUV and NUV passbands, give rise to white-light flares. In
addition, the strong energy release in these events can lead to
the rearrangement of the magnetic field at the photospheric level,
causing morphological changes in large and stable magnetic structures
like sunspots. In this context, we describe observations acquired
by satellite instruments (Interface Region Imaging Spectrograph
(IRIS), Solar Dynamics Observatory/Helioseismic and Magnetic Imager,
Hinode/Solar Optical Telescope) and ground-based telescopes (Rapid
Oscillations in the Solar Atmosphere (ROSA)/Dunn Solar Telescope)
during two consecutive C7.0 and X1.6 flares that occurred in active
region NOAA 12205 on 2014 November 7. The flare was accompanied by an
eruption. The results of the analysis show the presence of continuum
enhancements during the evolution of the events, observed both in ROSA
images and in IRIS spectra. In the latter, a prominent blueshifted
component is observed at the onset of the eruption. We investigate
the role played by the evolution of the δ sunspots of the active
region in the flare triggering, and finally we discuss the changes in
the penumbrae surrounding these sunspots as a further consequence of
these flares.
Title: Science Requirement Document (SRD) for the European Solar
Telescope (EST) (2nd edition, December 2019)
Authors: Schlichenmaier, R.; Bellot Rubio, L. R.; Collados, M.;
Erdelyi, R.; Feller, A.; Fletcher, L.; Jurcak, J.; Khomenko, E.;
Leenaarts, J.; Matthews, S.; Belluzzi, L.; Carlsson, M.; Dalmasse,
K.; Danilovic, S.; Gömöry, P.; Kuckein, C.; Manso Sainz, R.;
Martinez Gonzalez, M.; Mathioudakis, M.; Ortiz, A.; Riethmüller,
T. L.; Rouppe van der Voort, L.; Simoes, P. J. A.; Trujillo Bueno,
J.; Utz, D.; Zuccarello, F.
Bibcode: 2019arXiv191208650S
Altcode:
The European Solar Telescope (EST) is a research infrastructure
for solar physics. It is planned to be an on-axis solar telescope
with an aperture of 4 m and equipped with an innovative suite of
spectro-polarimetric and imaging post-focus instrumentation. The EST
project was initiated and is driven by EAST, the European Association
for Solar Telescopes. EAST was founded in 2006 as an association
of 14 European countries. Today, as of December 2019, EAST consists
of 26 European research institutes from 18 European countries. The
Preliminary Design Phase of EST was accomplished between 2008 and
2011. During this phase, in 2010, the first version of the EST Science
Requirement Document (SRD) was published. After EST became a project
on the ESFRI roadmap 2016, the preparatory phase started. The goal
of the preparatory phase is to accomplish a final design for the
telescope and the legal governance structure of EST. A major milestone
on this path is to revisit and update the Science Requirement Document
(SRD). The EST Science Advisory Group (SAG) has been constituted by
EAST and the Board of the PRE-EST EU project in November 2017 and has
been charged with the task of providing with a final statement on the
science requirements for EST. Based on the conceptual design, the SRD
update takes into account recent technical and scientific developments,
to ensure that EST provides significant advancement beyond the current
state-of-the-art. The present update of the EST SRD has been developed
and discussed during a series of EST SAG meetings. The SRD develops
the top-level science objectives of EST into individual science
cases. Identifying critical science requirements is one of its main
goals. Those requirements will define the capabilities of EST and the
post-focus instrument suite. The technical requirements for the final
design of EST will be derived from the SRD.
Title: Spatially Resolved Signatures of Bidirectional Flows Observed
in Inverted-Y Shaped Jets
Authors: Nelson, C. J.; Freij, N.; Bennett, S.; Erdélyi, R.;
Mathioudakis, M.
Bibcode: 2019ApJ...883..115N
Altcode: 2019arXiv190805132N
Numerous apparent signatures of magnetic reconnection have been
reported in the solar photosphere, including inverted-Y shaped jets. The
reconnection at these sites is expected to cause localized bidirectional
flows and extended shock waves; however, these signatures are rarely
observed as extremely high spatial-resolution data are required. Here,
we use Hα imaging data sampled by the Swedish Solar Telescope’s CRisp
Imaging SpectroPolarimeter to investigate whether bidirectional flows
can be detected within inverted-Y shaped jets near the solar limb. These
jets are apparent in the Hα line wings, while no signature of either
jet is observed in the Hα line core, implying reconnection took place
below the chromospheric canopy. Asymmetries in the Hα line profiles
along the legs of the jets indicate the presence of bidirectional flows,
consistent with cartoon models of reconnection in chromospheric anemone
jets. These asymmetries are present for over two minutes, longer than
the lifetimes of Rapid Blue Excursions, and beyond ±1 Å into the wings
of the line indicating that flows within the inverted-Y shaped jets
are responsible for the imbalance in the profiles, rather than motions
in the foreground. Additionally, surges form following the occurrence
of the inverted-Y shaped jets. This surge formation is consistent with
models, which suggests such events could be caused by the propagation
of shock waves from reconnection sites in the photosphere to the
upper atmosphere. Overall, our results provide evidence that magnetic
reconnection in the photosphere can cause bidirectional flows within
inverted-Y shaped jets and could be the driver of surges.
Title: Hydrogen Emission in Type II White-light Solar Flares
Authors: Procházka, Ondřej; Reid, Aaron; Mathioudakis, Mihalis
Bibcode: 2019ApJ...882...97P
Altcode: 2019arXiv190710888P
Type II white-light flares (WLFs) have weak Balmer line emission
and no Balmer jump. We carried out a set of radiative hydrodynamic
simulations to understand how the hydrogen radiative losses vary with
the electron-beam parameters and more specifically with the low-energy
cutoff. Our results have revealed that for low-energy beams, the excess
flare Lyman emission diminishes with increasing low-energy cutoff as
the energy deposited into the top chromosphere is low compared to the
energy deposited into the deeper layers. Some Balmer excess emission is
always present and is driven primarily by direct heating from the beam
with a minor contribution from Lyman continuum backwarming. The absence
of Lyman excess emission in electron-beam driven models with high
low-energy cutoff is a prominent spectral signature of type II WLFs.
Title: The magnetic properties of photospheric magnetic bright points
with high-resolution spectropolarimetry
Authors: Keys, Peter H.; Reid, Aaron; Mathioudakis, Mihalis; Shelyag,
Sergiy; Henriques, Vasco M. J.; Hewitt, Rebecca L.; Del Moro, Dario;
Jafarzadeh, Shahin; Jess, David B.; Stangalini, Marco
Bibcode: 2019MNRAS.488L..53K
Altcode: 2019MNRAS.tmpL..98K; 2019MNRAS.tmpL..95K; 2019arXiv190607687K
Magnetic bright points (MBPs) are small-scale magnetic elements
ubiquitous across the solar disc, with the prevailing theory suggesting
that they form due to the process of convective collapse. Employing a
unique full Stokes spectropolarimetric data set of a quiet Sun region
close to disc centre obtained with the Swedish Solar Telescope, we look
at general trends in the properties of magnetic bright points. In total
we track 300 MBPs in the data set and we employ NICOLE inversions to
ascertain various parameters for the bright points such as line-of-sight
magnetic field strength and line-of-sight velocity, for comparison. We
observe a bimodal distribution in terms of maximum magnetic field
strength in the bright points with peaks at ∼480 G and ∼1700 G,
although we cannot attribute the kilogauss fields in this distribution
solely to the process of convective collapse. Analysis of MURAM
simulations does not return the same bimodal distribution. However,
the simulations provide strong evidence that the emergence of new flux
and diffusion of this new flux play a significant role in generating
the weak bright point distribution seen in our observations.
Title: The Chromospheric Response to the Sunquake Generated by the
X9.3 Flare of NOAA 12673
Authors: Quinn, Sean; Reid, Aaron; Mathioudakis, Mihalis; Nelson,
Christoper; Krishna Prasad, S.; Zharkov, Sergei
Bibcode: 2019ApJ...881...82Q
Altcode: 2019arXiv190608545Q
Active region NOAA 12673 was extremely volatile in 2017 September,
producing many solar flares, including the largest of solar cycle 24,
an X9.3 flare of 2017 September 06. It has been reported that this flare
produced a number of sunquakes along the flare ribbon. We have used
cotemporal and cospatial Helioseismic and Magnetic Imager (HMI) line of
sight (LOS) and Swedish 1 m Solar Telescope (SST) observations to show
evidence of the chromospheric response to these sunquakes. Analysis
of the Ca II 8542 Å line profiles of the wavefronts revealed that
the crests produced a strong blue asymmetry, whereas the troughs
produced at most a very slight red asymmetry. We used the combined
HMI, SST data sets to create time-distance diagrams and derive the
apparent transverse velocity and acceleration of the response. These
velocities ranged from 4.5 to 29.5 km s-1 with a constant
acceleration of 8.6 × 10-3 km s-2. We employed
NICOLE inversions, in addition to the center-of-gravity method to
derive LOS velocities ranging from 2.4 km s-1-3.2 km
s-1. Both techniques show that the crests are created by
upflows. We believe that this is the first chromospheric signature of
a flare induced sunquake.
Title: Stellar Surface Magnetoconvection as a Source of Astrophysical
Noise. III. Sun-as-a-Star Simulations and Optimal Noise Diagnostics
Authors: Cegla, H. M.; Watson, C. A.; Shelyag, S.; Mathioudakis, M.;
Moutari, S.
Bibcode: 2019ApJ...879...55C
Altcode: 2019arXiv190308446C
Stellar surface magnetoconvection (granulation) creates asymmetries in
the observed stellar absorption lines that can subsequently manifest
themselves as spurious radial velocities (RVs) shifts. In turn, this
can then mask the Doppler reflex motion induced by orbiting planets on
their host stars and represents a particular challenge for determining
the masses of low-mass, long-period planets. Herein, we study this
impact by creating Sun-as-a-star observations that encapsulate
the granulation variability expected from 3D magnetohydrodynamic
simulations. These Sun-as-a-star model observations are in good
agreement with empirical observations of the Sun but may underestimate
the total variability relative to the quiet Sun due to the increased
magnetic field strength in our models. We find numerous line profile
characteristics that linearly correlate with the disk-integrated
convection-induced velocities. Removing the various correlations
with the line bisector, equivalent width, and the V asy
indicator may reduce ∼50%-60% of the granulation noise in the measured
velocities. We also find that simultaneous photometry may be a key
diagnostic, as our proxy for photometric brightness also allowed us to
remove ∼50% of the granulation-induced RV noise. These correlations
and granulation-noise mitigations break down in the presence of low
instrumental resolution and/or increased stellar rotation, as both
act to smooth the observed line profile asymmetries.
Title: Multi-wavelength observations of the 2014 June 11 M3.9 flare:
temporal and spatial characteristics
Authors: Christian, Damian J.; Kuridze, David; Jess, David B.; Yousefi,
Menoa; Mathioudakis, Mihalis
Bibcode: 2019RAA....19..101C
Altcode: 2018arXiv181107077C
We present multi-wavelength observations of an M-class flare (M3.9)
that occurred on 2014 June 11. Our observations were conducted
with the Dunn Solar Telescope (DST), employing adaptive optics,
the multi-camera system Rapid Oscillations in Solar Atmosphere
(ROSA), the new Hydrogen-Alpha Rapid Dynamics camera (HARDcam) in
various wavelengths, such as Ca II K, Mg I b2 (at 5172.7
Å), and Hα narrow band and G-band continuum filters. Images were
re-constructed using the Kiepenheuer-Institut Speckle Interferometry
Package (KISIP) code, to improve our image resolution. We observed
intensity increases of ≈120%-150% in the Mg, Ca K and Hα narrow band
filters during the flare. Intensity increases for the flare observed
in the SDO EUV channels were several times larger, and the X-rays,
as recorded by GOES, increased over a factor of 30 for the harder
band. Only a modest delay was found between the onset of flare ribbons
of a nearby sympathetic flare and the main flare ribbons observed in
these narrow band filters. The peak flare emission occurred within a
few seconds for the Ca K, Mg and Hα bands. Time-distance techniques
indicate propagation velocities of ≈60 km s-1 for the
main flare ribbon and as high as 300 km s-1 for smaller
regions, which we attribute to filament eruptions. This result and
delays and velocities observed with SDO (≈100 km s-1)
for different coronal heights agree well with the simple model of
energy propagation versus height, although a more detailed model for
the flaring solar atmosphere is needed. Finally, we detected marginal
quasi-periodic pulsations (QPPs) in the 40-60 s range for the Ca K,
Mg and Hα bands, and such measurements are important for disentangling
the detailed flare-physics.
Title: The Effect Of Cooling On Driven Kink Oscillations Of Coronal
Loops
Authors: Nelson, Chris J.; Shukhobodskiy, Alexander A.; Erdélyi,
Robertus; Mathioudakis, Mihalis
Bibcode: 2019FrASS...6...45N
Altcode: 2019arXiv190513137N
Ever since their detection two decades ago, standing kink oscillations
in coronal loops have been extensively studied both observationally
and theoretically. Almost all driven coronal loop oscillations
(e.g., by flares) are observed to damp through time often with
Gaussian or exponential profiles. Intriguingly, however, it has
been shown theoretically that the amplitudes of some oscillations
could be modified from Gaussian or exponential profiles if cooling
is present in the coronal loop systems. Indeed, in some cases the
oscillation amplitude can even increase through time. In this article,
we analyse a flare-driven coronal loop oscillation observed by the
Solar Dynamics Observatory's Atmospheric Imaging Assembly (SDO/AIA)
in order to investigate whether models of cooling can explain the
amplitude profile of the oscillation and whether hints of cooling can
be found in the intensity evolution of several SDO/AIA filters. During
the oscillation of this loop system, the kink mode amplitude appears to
differ from a typical Gaussian or exponential profile with some hints
being present that the amplitude increases. The application of cooling
coronal loop modelling allowed us to estimate the density ratio between
the loop and the background plasma, with a ratio of between 2.05-2.35
being returned. Overall, our results indicate that consideration of
the thermal evolution of coronal loop systems can allow us to better
describe oscillations in these structures and return more accurate
estimates of the physical properties of the loops (e.g., density,
scale height, magnetic field strength).
Title: Mapping the Magnetic Field of Flare Coronal Loops
Authors: Kuridze, D.; Mathioudakis, M.; Morgan, H.; Oliver, R.; Kleint,
L.; Zaqarashvili, T. V.; Reid, A.; Koza, J.; Löfdahl, M. G.; Hillberg,
T.; Kukhianidze, V.; Hanslmeier, A.
Bibcode: 2019ApJ...874..126K
Altcode: 2019arXiv190207514K
Here, we report on the unique observation of flaring coronal loops at
the solar limb using high-resolution imaging spectropolarimetry from
the Swedish 1 m Solar Telescope. The vantage position, orientation,
and nature of the chromospheric material that filled the flare loops
allowed us to determine their magnetic field with unprecedented accuracy
using the weak-field approximation method. Our analysis reveals coronal
magnetic field strengths as high as 350 G at heights up to 25 Mm above
the solar limb. These measurements are substantially higher than a
number of previous estimates and may have considerable implications
for our current understanding of the extended solar atmosphere.
Title: Statistical Signatures of Nanoflare Activity. I. Monte Carlo
Simulations and Parameter-space Exploration
Authors: Jess, D. B.; Dillon, C. J.; Kirk, M. S.; Reale, F.;
Mathioudakis, M.; Grant, S. D. T.; Christian, D. J.; Keys, P. H.;
Krishna Prasad, S.; Houston, S. J.
Bibcode: 2019ApJ...871..133J
Altcode: 2018arXiv181206978J
Small-scale magnetic reconnection processes in the form of nanoflares
have become increasingly hypothesized as important mechanisms for the
heating of the solar atmosphere, driving propagating disturbances along
magnetic field lines in the Sun’s corona, and instigating rapid
jetlike bursts in the chromosphere. Unfortunately, the relatively
weak signatures associated with nanoflares place them below the
sensitivities of current observational instrumentation. Here we
employ Monte Carlo techniques to synthesize realistic nanoflare
intensity time series from a dense grid of power-law indices and
decay timescales. Employing statistical techniques, which examine
the modeled intensity fluctuations with more than 107
discrete measurements, we show how it is possible to extract and
quantify nanoflare characteristics throughout the solar atmosphere,
even in the presence of significant photon noise. A comparison
between the statistical parameters (derived through examination of the
associated intensity fluctuation histograms) extracted from the Monte
Carlo simulations and Solar Dynamics Observatory (SDO)/Atmospheric
Imaging Assembly (AIA) 171 and 94 Å observations of active region
NOAA 11366 reveals evidence for a flaring power-law index within the
range of 1.82 ≤ α ≤ 1.90, combined with e-folding timescales
of 385 ± 26 and 262 ± 17 s for the SDO/AIA 171 and 94 Å channels,
respectively. These results suggest that nanoflare activity is not the
dominant heating source for the active region under investigation. This
opens the door for future dedicated observational campaigns to not only
unequivocally search for the presence of small-scale reconnection in
solar and stellar environments but also quantify key characteristics
related to such nanoflare activity.
Title: Continuum emission enhancements and penumbral changes observed
during flares by IRIS, ROSA, and Hinode
Authors: Zuccarello, F.; Guglielmino, S. L.; Capparelli, V.;
Mathioudakis, M.; Keys, P.; Fletcher, L.; Criscuoli, S.; Falco, M.;
Murabito, M.
Bibcode: 2019NCimC..42...13Z
Altcode: 2019arXiv190101732Z
In this paper we describe observations acquired by satellite instruments
( Hinode/SOT and IRIS) and ground-based telescopes (ROSA@DST) during
two consecutive C7.0 and X1.6 flares occurred in active region NOAA
12205 on 2014 November 7. The analysis of these data show the presence
of continuum enhancements during the evolution of the events, observed
both in ROSA images and in IRIS spectra. Moreover, we analyze the role
played by the evolution of the δ sunspots of the active region in the
flare triggering, indicating the disappearance of a large portion of
penumbra around these sunspots.
Title: Coronal and chromospheric observations of pre- and post-flare
plasma evolution
Authors: Long, David; Reid, Aaron; Harra, Louise; Mathioudakis, Mihalis
Bibcode: 2018csc..confE..16L
Altcode:
Solar flares are among the most energetic and spectacular events
occurring in our solar system, produced by the release of stored
magnetic energy in the solar atmosphere through the reconnection
of twisted magnetic fields. Although the magnetic field itself is
difficult to observe in the solar atmosphere, we can gain vital
insights into the reconnection process by studying the evolution
of solar plasma prior to and following the flare. Here we present
contemporaneous spectroscopic and imaging observations of an X9.3 solar
flare from 2017-September-6. This was the largest flare of the current
solar cycle to date, and was well observed in the corona by SDO/AIA
and Hinode/EIS as well as in the chromosphere by instruments at the
ground-based Swedish Solar Telescope. This combination of observations
provides spectroscopic information throughout the solar atmosphere,
giving a unique insight into the evolution of plasma in the lead-up
to and following the flare.
Title: Stellar Surface Magneto-convection as a Source of Astrophysical
Noise. II. Center-to-limb Parameterization of Absorption Line Profiles
and Comparison to Observations
Authors: Cegla, H. M.; Watson, C. A.; Shelyag, S.; Chaplin, W. J.;
Davies, G. R.; Mathioudakis, M.; Palumbo, M. L., III; Saar, S. H.;
Haywood, R. D.
Bibcode: 2018ApJ...866...55C
Altcode: 2018arXiv180711423C
Manifestations of stellar activity (such as star-spots, plage/faculae,
and convective flows) are well-known to induce spectroscopic
signals often referred to as astrophysical noise by exoplanet
hunters. For example, setting an ultimate goal of detecting true
Earth analogs demands reaching radial velocity (RV) precisions of
∼9 cm s-1. While this is becoming technically feasible
with the latest generation of highly stabilized spectrographs, it
is astrophysical noise that sets the true fundamental barrier on
attainable RV precisions. In this paper, we parameterize the impact
of solar surface magneto-convection on absorption line profiles, and
extend the analysis from the solar disk center (Paper I) to the solar
limb. Off disk-center, the plasma flows orthogonal to the granule tops
begin to lie along the line of sight, and those parallel to the granule
tops are no longer completely aligned with the observer. Moreover, the
granulation is corrugated and the granules can block other granules, as
well as the intergranular lane components. Overall, the visible plasma
flows and geometry of the corrugated surface significantly impact the
resultant line profiles and induce center-to-limb variations in shape
and net position. We detail these herein, and compare to various solar
observations. We find our granulation parameterization can recreate
realistic line profiles and induced radial velocity shifts, across
the stellar disk, indicative of both those found in computationally
heavy radiative 3D magnetohydrodynamical simulations and empirical
solar observations.
Title: Construction update of the Daniel K. Inouye Solar Telescope
project
Authors: Warner, Mark; Rimmele, Thomas R.; Martinez Pillet, Valentin;
Casini, Roberto; Berukoff, Steve; Craig, Simon C.; Ferayorni, Andrew;
Goodrich, Bret D.; Hubbard, Robert P.; Harrington, David; Jeffers,
Paul; Johansson, Erik M.; Kneale, Ruth; Kuhn, Jeff; Liang, Chen; Lin,
Haosheng; Marshall, Heather; Mathioudakis, Mihalis; McBride, William
R.; McMullin, Joseph; McVeigh, William; Sekulic, Predrag; Schmidt,
Wolfgang; Shimko, Steve; Sueoka, Stacey; Summers, Rich; Tritschler,
Alexandra; Williams, Timothy R.; Wöger, Friedrich
Bibcode: 2018SPIE10700E..0VW
Altcode:
Construction of the Daniel K. Inouye Solar Telescope (DKIST) is
well underway on the Haleakalā summit on the Hawaiian island of
Maui. Featuring a 4-m aperture and an off-axis Gregorian configuration,
the DKIST will be the world's largest solar telescope. It is designed
to make high-precision measurements of fundamental astrophysical
processes and produce large amounts of spectropolarimetric and
imaging data. These data will support research on solar magnetism
and its influence on solar wind, flares, coronal mass ejections,
and solar irradiance variability. Because of its large aperture, the
DKIST will be able to sense the corona's magnetic field—a goal that
has previously eluded scientists—enabling observations that will
provide answers about the heating of stellar coronae and the origins
of space weather and exo-weather. The telescope will cover a broad
wavelength range (0.35 to 28 microns) and operate as a coronagraph
at infrared (IR) wavelengths. Achieving the diffraction limit of
the 4-m aperture, even at visible wavelengths, is paramount to these
science goals. The DKIST's state-of-the-art adaptive optics systems
will provide diffraction-limited imaging, resolving features that are
approximately 20 km in size on the Sun. At the start of operations,
five instruments will be deployed: a visible broadband imager (VTF),
a visible spectropolarimeter (ViSP), a visible tunable filter (VTF),
a diffraction-limited near-IR spectropolarimeter (DLNIRSP), and a
cryogenic near-IR spectropolarimeter (cryo-NIRSP). At the end of
2017, the project finished its fifth year of construction and eighth
year overall. Major milestones included delivery of the commissioning
blank, the completed primary mirror (M1), and its cell. Commissioning
and testing of the coudé rotator is complete and the installation
of the coudé cleanroom is underway; likewise, commissioning of the
telescope mount assembly (TMA) has also begun. Various other systems and
equipment are also being installed and tested. Finally, the observatory
integration, testing, and commissioning (IT&C) activities have
begun, including the first coating of the M1 commissioning blank and
its integration within its cell assembly. Science mirror coating and
initial on-sky activities are both anticipated in 2018.
Title: Reproducing Type II White-light Solar Flare Observations with
Electron and Proton Beam Simulations
Authors: Procházka, Ondřej; Reid, Aaron; Milligan, Ryan O.; Simões,
Paulo J. A.; Allred, Joel C.; Mathioudakis, Mihalis
Bibcode: 2018ApJ...862...76P
Altcode: 2018arXiv180600249P
We investigate the cause of the suppressed Balmer series and the origin
of the white-light continuum emission in the X1.0 class solar flare on
2014 June 11. We use radiative hydrodynamic simulations to model the
response of the flaring atmosphere to both electron and proton beams,
which are energetically constrained using Ramaty High Energy Solar
Spectroscopic Imager and Fermi observations. A comparison of synthetic
spectra with the observations allows us to narrow the range of beam
fluxes and low energy cutoff that may be applicable to this event. We
conclude that the electron and proton beams that can reproduce the
observed spectral features are those that have relatively low fluxes
and high values for the low energy cutoff. While electron beams shift
the upper chromosphere and transition region to greater geometrical
heights, proton beams with a similar flux leave these areas of the
atmosphere relatively undisturbed. It is easier for proton beams to
penetrate to the deeper layers and not deposit their energy in the
upper chromosphere where the Balmer lines are formed. The relatively
weak particle beams that are applicable to this flare do not cause
a significant shift of the τ = 1 surface and the observed excess WL
emission is optically thin.
Title: The Evolution of T = 10,000 K Blackbody-Like Continuum
Radiation in the Impulsive Phase of dMe Flares
Authors: Kowalski, Adam F.; Mathioudakis, Mihalis; Hawley, Suzanne L.
Bibcode: 2018csss.confE..42K
Altcode: 2018arXiv181007226K
The near-ultraviolet and optical (white-light) continuum radiation
in M dwarf flares exhibits a range of observed characteristics,
suggesting that the amount of heating at large optical depth varies
among impulsive-type and gradual-type flares. Specific flux ratios
from high-time cadence spectra and narrowband continuum photometry
have also shown that these characteristics vary from the peak to the
gradual decay phases of flares. In these proceedings, we present the
highest-time cadence ( 1 s), highest signal-to-noise ( 100) constraints
on the optical color temperature evolution during the rise phase of a
large, impulsive-type dMe flare event. The flare exhibits compelling
evidence of a hot, color temperature (T 10,000 K), but the Balmer jump
ratios show that the flare cannot be explained by isothermal slabs or
blackbody surfaces at any time in the evolution. The new data analysis
establish these properties as critical challenges for any flare model,
and we discuss 1D radiative-hydrodynamic modeling that will be compared
to the evolution of the flare colors in this intriguing event.
Title: Status of the Daniel K. Inouye Solar Telescope: unraveling
the mysteries the Sun.
Authors: Rimmele, Thomas R.; Martinez Pillet, Valentin; Goode, Philip
R.; Knoelker, Michael; Kuhn, Jeffrey Richard; Rosner, Robert; Casini,
Roberto; Lin, Haosheng; von der Luehe, Oskar; Woeger, Friedrich;
Tritschler, Alexandra; Fehlmann, Andre; Jaeggli, Sarah A.; Schmidt,
Wolfgang; De Wijn, Alfred; Rast, Mark; Harrington, David M.; Sueoka,
Stacey R.; Beck, Christian; Schad, Thomas A.; Warner, Mark; McMullin,
Joseph P.; Berukoff, Steven J.; Mathioudakis, Mihalis; DKIST Team
Bibcode: 2018AAS...23231601R
Altcode:
The 4m Daniel K. Inouye Solar Telescope (DKIST) currently under
construction on Haleakala, Maui will be the world’s largest solar
telescope. Designed to meet the needs of critical high resolution and
high sensitivity spectral and polarimetric observations of the sun,
this facility will perform key observations of our nearest star that
matters most to humankind. DKIST’s superb resolution and sensitivity
will enable astronomers to address many of the fundamental problems
in solar and stellar astrophysics, including the origin of stellar
magnetism, the mechanisms of coronal heating and drivers of the
solar wind, flares, coronal mass ejections and variability in solar
and stellar output. DKIST will also address basic research aspects of
Space Weather and help improve predictive capabilities. In combination
with synoptic observations and theoretical modeling DKIST will unravel
the many remaining mysteries of the Sun.The construction of DKIST is
progressing on schedule with 80% of the facility complete. Operations
are scheduled to begin early 2020. DKIST will replace the NSO
facilities on Kitt Peak and Sac Peak with a national facility with
worldwide unique capabilities. The design allows DKIST to operate as
a coronagraph. Taking advantage of its large aperture and infrared
polarimeters DKIST will be capable to routinely measure the currently
illusive coronal magnetic fields. The state-of-the-art adaptive optics
system provides diffraction limited imaging and the ability to resolve
features approximately 20 km on the Sun. Achieving this resolution
is critical for the ability to observe magnetic structures at their
intrinsic, fundamental scales. Five instruments will be available at
the start of operations, four of which will provide highly sensitive
measurements of solar magnetic fields throughout the solar atmosphere
- from the photosphere to the corona. The data from these instruments
will be distributed to the world wide community via the NSO/DKIST data
center located in Boulder. We present examples of science objectives
and provide an overview of the facility and project status, including
the ongoing efforts of the community to develop the critical science
plan for the first 2-3 years of operations.
Title: Spectropolarimetric Inversions of the Ca II 8542 Å Line in
an M-class Solar Flare
Authors: Kuridze, D.; Henriques, V. M. J.; Mathioudakis, M.; Rouppe
van der Voort, L.; de la Cruz Rodríguez, J.; Carlsson, M.
Bibcode: 2018ApJ...860...10K
Altcode: 2018arXiv180500487K
We study the M1.9-class solar flare SOL2015-09-27T10:40 UT using
high-resolution full Stokes imaging spectropolarimetry of the Ca II
8542 Å line obtained with the CRISP imaging spectropolarimeter at the
Swedish 1-m Solar Telescope. Spectropolarimetric inversions using the
non-LTE code NICOLE are used to construct semiempirical models of the
flaring atmosphere to investigate the structure and evolution of the
flare temperature and magnetic field. A comparison of the temperature
stratification in flaring and nonflaring areas reveals strong heating
of the flare ribbon during the flare peak. The polarization signals
of the ribbon in the chromosphere during the flare maximum become
stronger when compared to its surroundings and to pre- and post-flare
profiles. Furthermore, a comparison of the response functions to
perturbations in the line-of-sight magnetic field and temperature in
flaring and nonflaring atmospheres shows that during the flare, the
Ca II 8542 Å line is more sensitive to the lower atmosphere where the
magnetic field is expected to be stronger. The chromospheric magnetic
field was also determined with the weak-field approximation, which
led to results similar to those obtained with the NICOLE inversions.
Title: First Evidence of the Frequency Filtering of Magnetoacoustic
Waves in the Flaring Star EK Dra
Authors: Srivastava, A. K.; Pandey, J. C.; Karmakar, Subhajeet;
Chowdhury, Partha; Moon, Y. -J.; Goossens, Marcel; Jelínek, P.;
Mathioudakis, M.; Doyle, J. G.; Dwivedi, B. N.
Bibcode: 2018arXiv180408858S
Altcode:
Using the data obtained from XMM-Newton, we show the gradual evolution
of two periodicities of ~4500 s and ~2200 s in the decay phase of the
flare observed in a solar analog EK Dra. The longer period evolves
firstly for first 14 ks, while the shorter period evolves for next
10 ks in the decay phase. We find that these two periodicities are
associated with the magnetoacoustic waves triggered in the flaring
region. The flaring loop system shows cooling and thus it is subjected
to the change in the scale height and the acoustic cut-off period. This
serves to filter the longer period magnetoacoustic waves and enables
the propagation of the shorter period waves in the later phase of
the flare. We provide the first clues of the dynamic behaviour of
EK Dra's corona which affects the propagation of waves and causes
their filtering.
Title: Stellar flare oscillations: evidence for oscillatory
reconnection and evolution of MHD modes
Authors: Doyle, J. G.; Shetye, J.; Antonova, A. E.; Kolotkov, D. Y.;
Srivastava, A. K.; Stangalini, M.; Gupta, G. R.; Avramova, A.;
Mathioudakis, M.
Bibcode: 2018MNRAS.475.2842D
Altcode: 2018MNRAS.tmp...77D
Here, we report on the detection of a range of quasi-periodic
pulsations (20-120 s; QPPs) observed during flaring activity of several
magnetically active dMe stars, namely AF Psc, CR Dra, GJ 3685A,
Gl 65, SDSS J084425.9+513830, and SDSS J144738.47+035312.1 in the
GALEX NUV filter. Based on a solar analogy, this work suggests that
many of these flares may be triggered by external drivers creating
a periodic reconnection in the flare current sheet or an impulsive
energy release giving rise to an avalanche of periodic bursts that
occur at time intervals that correspond to the detected periods, thus
generating QPPs in their rising and peak phases. Some of these flares
also show fast QPPs in their decay phase, indicating the presence of
fast sausage mode oscillations either driven externally by periodic
reconnection or intrinsically in the post-flare loop system during
the flare energy release.
Title: Photospheric Observations of Surface and Body Modes in Solar
Magnetic Pores
Authors: Keys, Peter H.; Morton, Richard J.; Jess, David B.; Verth,
Gary; Grant, Samuel D. T.; Mathioudakis, Mihalis; Mackay, Duncan H.;
Doyle, John G.; Christian, Damian J.; Keenan, Francis P.; Erdélyi,
Robertus
Bibcode: 2018ApJ...857...28K
Altcode: 2018arXiv180301859K
Over the past number of years, great strides have been made in
identifying the various low-order magnetohydrodynamic wave modes
observable in a number of magnetic structures found within the solar
atmosphere. However, one aspect of these modes that has remained
elusive, until now, is their designation as either surface or body
modes. This property has significant implications for how these modes
transfer energy from the waveguide to the surrounding plasma. Here, for
the first time to our knowledge, we present conclusive, direct evidence
of these wave characteristics in numerous pores that were observed to
support sausage modes. As well as outlining methods to detect these
modes in observations, we make estimates of the energies associated
with each mode. We find surface modes more frequently in the data,
as well as that surface modes appear to carry more energy than those
displaying signatures of body modes. We find frequencies in the range
of ∼2-12 mHz, with body modes as high as 11 mHz, but we do not find
surface modes above 10 mHz. It is expected that the techniques we have
applied will help researchers search for surface and body signatures
in other modes and in differing structures from those presented here.
Title: X-ray line coincidence photopumping in a solar flare
Authors: Keenan, F. P.; Poppenhaeger, K.; Mathioudakis, M.; Rose,
S. J.; Flowerdew, J.; Hynes, D.; Christian, D. J.; Nilsen, J.; Johnson,
W. R.
Bibcode: 2018MNRAS.474.3782K
Altcode: 2017arXiv171107761K
Line coincidence photopumping is a process where the electrons of
an atomic or molecular species are radiatively excited through the
absorption of line emission from another species at a coincident
wavelength. There are many instances of line coincidence photopumping
in astrophysical sources at optical and ultraviolet wavelengths,
with the most famous example being Bowen fluorescence (pumping of O
III 303.80 Å by He II), but none to our knowledge in X-rays. However,
here we report on a scheme where a He-like line of Ne IX at 11.000 Å
is photopumped by He-like Na X at 11.003 Å, which predicts significant
intensity enhancement in the Ne IX 82.76 Å transition under physical
conditions found in solar flare plasmas. A comparison of our theoretical
models with published X-ray observations of a solar flare obtained
during a rocket flight provides evidence for line enhancement, with the
measured degree of enhancement being consistent with that expected from
theory, a truly surprising result. Observations of this enhancement
during flares on stars other than the Sun would provide a powerful
new diagnostic tool for determining the sizes of flare loops in these
distant, spatially unresolved, astronomical sources.
Title: Penumbral Waves Driving Solar Fan-shaped Chromospheric Jets
Authors: Reid, A.; Henriques, V. M. J.; Mathioudakis, M.; Samanta, T.
Bibcode: 2018ApJ...855L..19R
Altcode:
We use Hα imaging spectroscopy taken via the Swedish 1 m Solar
Telescope to investigate the occurrence of fan-shaped jets at the solar
limb. We show evidence for near-simultaneous photospheric reconnection
at a sunspot edge leading to the jets appearance, with upward velocities
of 30 km s-1, and extensions up to 8 Mm. The brightening at
the base of the jets appears recurrent, with a periodicity matching that
of the nearby sunspot penumbra, implying running penumbral waves could
be the driver of the jets. The jets’ constant extension velocity
implies that a driver counteracting solar gravity exists, possibly
as a result of the recurrent reconnection erupting material into the
chromosphere. These jets also show signatures in higher temperature
lines captured from the Solar Dynamics Observatory, indicating a very
hot jet front, leaving behind optically thick cool plasma in its wake.
Title: Penumbral Waves driving Solar chromospheric fan-shaped jets
Authors: Reid, A.; Henriques, V. M. J.; Mathioudakis, M.; Samanta, T.
Bibcode: 2018arXiv180207537R
Altcode:
We use H$\alpha$ imaging spectroscopy taken via the Swedish 1-m Solar
Telescope (SST) to investigate the occurrence of fan-shaped jets at
the solar limb. We show evidence for near-simultaneous photospheric
reconnection at a sunspot edge leading to the jets appearance, with
upward velocities of 30\ks, and extensions up to 8~Mm. The brightening
at the base of the jets appears recurrent, with a periodicity matching
that of the nearby sunspot penumbra, implying running penumbral waves
could be the driver of the jets. The jets' constant extension velocity
implies that a driver counteracting solar gravity exists, possibly
as a result of the recurrent reconnection erupting material into the
chromosphere. These jets also show signatures in higher temperature
lines captured from the Solar Dynamics Observatory (SDO), indicating
a very hot jet front, leaving behind optically thick cool plasma in
its wake.
Title: Hα and Hβ emission in a C3.3 solar flare: comparison between
observations and simulations
Authors: Zuccarello, F.; Simoes, P. J. D. A.; Capparelli, V.; Fletcher,
L.; Romano, P.; Mathioudakis, M.; Cauzzi, G.; Carlsson, M.; Kuridze,
D.; Keys, P.
Bibcode: 2017AGUFMSH41A2742Z
Altcode:
This work is based on the analysis of an extremely rare set of
simultaneous observations of a C3.3 solar flare in the Hα and Hβ lines
at high spatial and temporal resolution, which were acquired at the
Dunn Solar Telescope. Images of the C3.3 flare (SOL2014-04-22T15:22)
made at various wavelengths along the Hα line profile by the
Interferometric Bidimensional Spectrometer (IBIS) and in the Hβ
with the Rapid Oscillations in the Solar Atmosphere (ROSA) broadband
imager are analyzed to obtain the intensity evolution. The analysis
shows that Hα and Hβ intensity excesses in three identified flare
footpoints are well correlated in time. In the stronger footpoints,
the typical value of the the Hα/Hβ intensity ratio observed is ∼
0.4 - 0.5, in broad agreement with values obtained from a RADYN non-LTE
simulation driven by an electron beam with parameters constrained by
observations. The weaker footpoint has a larger Hα/Hβ ratio, again
consistent with a RADYN simulation but with a smaller energy flux.
Title: Modelling of Electron and Proton Beams in a White-light
Solar Flare
Authors: Procházka, O.; Reid, A.; Milligan, R. O.; Allred, J. C.;
Mathioudakis, M.
Bibcode: 2017AGUFMSH41A2756P
Altcode: 2017AGUFMSH41A2756M
Observations of an X1 class WL solar flare on 2014 June 11 showed a
surprisingly weak emission in both higher order Balmer and Lyman
lines and continua. The flare was observed by RHESSI but low
energy cut-off of non-thermal component was indeterminable due to
the unusually hard electron spectrum (delta = 3). An estimate of
power in non-thermal electron beams together with an area of WL
emission observed by HMI yielded to an upper and lower estimate of
flux 1E9 and 3E10 erg/cm2/s, respectively. We performed a grid of
models using a radiative hydrodynamic code RADYN in order to compare
synthetic spectra with observations. For low energy cut-off we chose
a range from 20 to 120 keV with a step of 20 keV and delta parameter
equal to 3. Electron beam-driven models show that higher low energy
cut-off is more likely to produce an absorption Balmer line profile,
if the total energy flux remains relatively low. On the other hand a
detectable rise of HMI continuum (617 nm) lays a lower limit on the
beam flux. Proton beam-driven models with equivalent fluxes indicate a
greater penetration depth, while the Balmer lines reveal significantly
weaker emission. Atmospheric temperature profiles show that for higher
values of low energy cut-off the energy of the beam is deposited lower
in chromosphere or even in temperature minimum region. This finding
suggests, that suppressed hydrogen emission can indicate a formation
of white-light continuum below chromosphere.
Title: Non-thermal distributions and energy transport in the solar
flares
Authors: Matthews, Sarah; del Zanna, Guilio; Calcines, Ariadna;
Mason, Helen; Mathioudakis, Mihalis; Culhane, Len; Harra, Louise;
van Driel-Gesztelyi, Lidia; Green, Lucie; Long, David; Baker, Deb;
Valori, Gherardo
Bibcode: 2017arXiv171200773M
Altcode:
Determining the energy transport mechanisms in flares remains a central
goal in solar flares physics that is still not adequately answered
by the 'standard flare model'. In particular, the relative roles of
particles and/or waves as transport mechanisms, the contributions of low
energy protons and ions to the overall flare budget, and the limits of
low energy non-thermal electron distribution are questions that still
cannot be adequately reconciled with current instrumentation. In this
'White Paper' submitted in response to the call for inputs to the Next
Generation Solar Physics Mission review process initiated by JAXA,
NASA and ESA in 2016, we outline the open questions in this area and
possible instrumentation that could provide the required observations
to help answer these and other flare-related questions.
Title: Hα and Hβ Emission in a C3.3 Solar Flare: Comparison between
Observations and Simulations
Authors: Capparelli, Vincenzo; Zuccarello, Francesca; Romano, Paolo;
Simões, Paulo J. A.; Fletcher, Lyndsay; Kuridze, David; Mathioudakis,
Mihalis; Keys, Peter H.; Cauzzi, Gianna; Carlsson, Mats
Bibcode: 2017ApJ...850...36C
Altcode: 2017arXiv171004067C
The hydrogen Balmer series is a basic radiative loss channel from
the flaring solar chromosphere. We report here on the analysis of an
extremely rare set of simultaneous observations of a solar flare in the
{{H}}α and {{H}}β lines, at high spatial and temporal resolutions,
that were acquired at the Dunn Solar Telescope. Images of the C3.3 flare
(SOL2014-04-22T15:22) made at various wavelengths along the {{H}}α line
profile by the Interferometric Bidimensional Spectrometer (IBIS) and in
the {{H}}β with the Rapid Oscillations in the Solar Atmosphere (ROSA)
broadband imager are analyzed to obtain the intensity evolution. The
{{H}}α and {{H}}β intensity excesses in three identified flare
footpoints are well-correlated in time. We examine the ratio of {{H}}α
to {{H}}β flare excess, which was proposed by previous authors as
a possible diagnostic of the level of electron-beam energy input. In
the stronger footpoints, the typical value of the the {{H}}α /H β
intensity ratio observed is ∼0.4-0.5, in broad agreement with values
obtained from a RADYN non-LTE simulation driven by an electron beam
with parameters constrained (as far as possible) by observation. The
weaker footpoint has a larger {{H}}α /H β ratio, again consistent
with a RADYN simulation, but with a smaller energy flux. The {{H}}α
line profiles observed have a less prominent central reversal than is
predicted by the RADYN results, but can be brought into agreement if
the {{H}}α -emitting material has a filling factor of around 0.2-0.3.
Title: The formation of small-scale umbral brightenings in sunspot
atmospheres
Authors: Nelson, C. J.; Henriques, V. M. J.; Mathioudakis, M.; Keenan,
F. P.
Bibcode: 2017A&A...605A..14N
Altcode: 2017arXiv170505617N
Context. Sunspot atmospheres have been shown to be highly inhomogeneous
hosting both quasi-stable and transient features, such as small-scale
umbral brightenings (previously named "umbral micro-jets") and dark
fibril-like events.
Aims: We seek to understand the morphological
properties and formation mechanisms of small-scale umbral brightenings
(analogous to umbral micro-jets). In addition, we aim to understand
whether links between these events and short dynamic fibrils,
umbral flashes, and umbral dots can be established.
Methods:
A Swedish 1 m Solar Telescope (SST) filtergram time-series sampling
the Ca II H line and a CRisp Imaging Spectro-Polarimeter (SST/CRISP)
full-Stokes 15-point Ca II 8542 Å line scan dataset were used. The
spatial resolutions of these datasets are close to 0.1'' and 0.18''
with cadences of 1.4 s and 29 s, respectively. These data allowed
us to construct light-curves, plot line profiles, and to perform
a weak-field approximation in order to infer the magnetic field
strength.
Results: The average lifetime and lengths of the 54
small-scale brightenings identified in the sunspot umbra are found
to be 44.2 s (σ = 20 s) and 0.56'' (σ = 0.14''), respectively. The
spatial positioning and morphological evolution of these events in
Ca II H filtergrams was investigated finding no evidence of parabolic
or ballistic profiles nor a preference for co-spatial formation with
umbral flashes. Line scans in Ca II 8542 Å and the presence of Stokes
V profile reversals provided evidence that these events could form in a
similar manner to umbral flashes in the chromosphere (I.e. through the
formation of shocks either due to the steepening of localised wavefronts
or due to the impact of returning material from short dynamic fibrils,
a scenario we find evidence for). The application of the weak-field
approximation indicated that changes in the line-of-sight magnetic
field were not responsible for the modifications to the line profile
and suggested that thermodynamic effects are, in fact, the actual
cause of the increased emission. Finally, a sub-set of small-scale
brightenings were observed to form at the foot-points of short dynamic
fibrils.
Conclusions: The small-scale umbral brightenings studied
here do not appear to be jet-like in nature. Instead they appear to be
evidence of shock formation in the lower solar atmosphere. We found
no correlation between the spatial locations where these events were
observed and the occurrence of umbral dots and umbral flashes. These
events have lifetimes and spectral signatures comparable to umbral
flashes and are located at the footpoints of short dynamic fibrils,
during or at the end of the red-shifted stage. It is possible that
these features form due to the shocking of fibrilar material in the
lower atmosphere upon its return under gravity.
Title: Spectroscopic Inversions of the Ca II 8542 Å Line in a
C-class Solar Flare
Authors: Kuridze, D.; Henriques, V.; Mathioudakis, M.; Koza, J.;
Zaqarashvili, T. V.; Rybák, J.; Hanslmeier, A.; Keenan, F. P.
Bibcode: 2017ApJ...846....9K
Altcode: 2017arXiv170800472K
We study the C8.4-class solar flare SOL2016-05-14T11:34 UT using
high-resolution spectral imaging in the Ca II 8542 Å line obtained
with the CRISP imaging spectropolarimeter on the Swedish 1 m Solar
Telescope. Spectroscopic inversions of the Ca II 8542 Å line using
the non-LTE code NICOLE are used to investigate the evolution of
the temperature and velocity structure in the flaring chromosphere. A
comparison of the temperature stratification in flaring and non-flaring
areas reveals strong footpoint heating during the flare peak in the
lower atmosphere. The temperature of the flaring footpoints between
{log} {τ }500 ≈ -2.5 {and} -3.5, where τ 500
is the continuum optical depth at 500 nm, is ∼ 5{--}6.5 {kK} close
to the flare peak, reducing gradually to ∼ 5 {kK}. The temperature in
the middle and upper chromosphere, between {log} {τ }500≈
-3.5 and -5.5, is estimated to be ∼6.5-20 kK, decreasing to preflare
temperatures, ∼5-10 kK, after approximately 15 minutes. However, the
temperature stratification of the non-flaring areas is unchanged. The
inverted velocity fields show that the flaring chromosphere is dominated
by weak downflowing condensations at the formation height of Ca II
8542 Å.
Title: A Hot Downflowing Model Atmosphere for Umbral Flashes and
the Physical Properties of Their Dark Fibrils
Authors: Henriques, V. M. J.; Mathioudakis, M.; Socas-Navarro, H.;
de la Cruz Rodríguez, J.
Bibcode: 2017ApJ...845..102H
Altcode: 2017arXiv170605311H
We perform non-LTE inversions in a large set of umbral flashes,
including the dark fibrils visible within them, and in the quiescent
umbra by using the inversion code NICOLE on a set of full Stokes
high-resolution Ca II λ8542 observations of a sunspot at disk
center. We find that the dark structures have Stokes profiles that are
distinct from those of the quiescent and flashed regions. They are
best reproduced by atmospheres that are more similar to the flashed
atmosphere in terms of velocities, even if with reduced amplitudes. We
also find two sets of solutions that finely fit the flashed profiles:
a set that is upflowing, featuring a transition region that is deeper
than in the quiescent case and preceded by a slight dip in temperature,
and a second solution with a hotter atmosphere in the chromosphere but
featuring downflows close to the speed of sound at such heights. Such
downflows may be related, or even dependent, on the presence of coronal
loops, rooted in the umbra of sunspots, as is the case in the region
analyzed. Similar loops have been recently observed to have supersonic
downflows in the transition region and are consistent with the earlier
“sunspot plumes,” which were invariably found to display strong
downflows in sunspots. Finally, we find, on average, a magnetic field
reduction in the flashed areas, suggesting that the shock pressure is
moving field lines in the upper layers.
Title: IRIS Burst Spectra Co-spatial to a Quiet-Sun Ellerman-like
Brightening
Authors: Nelson, C. J.; Freij, N.; Reid, A.; Oliver, R.; Mathioudakis,
M.; Erdélyi, R.
Bibcode: 2017ApJ...845...16N
Altcode: 2017arXiv170705080N
Ellerman bombs (EBs) have been widely studied over the past two
decades; however, only recently have the counterparts of these events
been observed in the quiet-Sun. The aim of this article is to further
understand small-scale quiet-Sun Ellerman-like brightenings (QSEBs)
through research into their spectral signatures, including investigating
whether the hot signatures associated with some EBs are also visible
co-spatial to any QSEBs. We combine Hα and Ca II 8542 Å line scans at
the solar limb with spectral and imaging data sampled by the Interface
Region Imaging Spectrograph (IRIS). Twenty-one QSEBs were identified
with average lifetimes, lengths, and widths measured to be around 120 s,
0.″63, and 0.″35, respectively. Three of these QSEBs displayed clear
repetitive flaring through their lifetimes, comparable to the behavior
of EBs in active regions. Two QSEBs in this sample occurred co-spatial
to increased emission in SDO/AIA 1600 Å and IRIS slit-jaw imager 1400
Å data; however, these intensity increases were smaller than those
reported co-spatially with EBs. One QSEB was also sampled by the IRIS
slit during its lifetime, displaying increases in intensity in the
Si IV 1393 Å and Si IV 1403 Å cores, as well as the C II and Mg II
line wings, analogous to IRIS bursts (IBs). Using RADYN simulations,
we are unable to reproduce the observed QSEB Hα and Ca II 8542 Å
line profiles, leaving the question of the temperature stratification
of QSEBs open. Our results imply that some QSEBs could be heated to
transition region temperatures, suggesting that IB profiles should be
observed throughout the quiet-Sun.
Title: Chromospheric Inversions of a Micro-flaring Region
Authors: Reid, A.; Henriques, V.; Mathioudakis, M.; Doyle, J. G.;
Ray, T.
Bibcode: 2017ApJ...845..100R
Altcode: 2017arXiv170700029R
We use spectropolarimetric observations of the Ca II 8542 Å line, taken
from the Swedish 1 m Solar Telescope, in an attempt to recover dynamic
activity in a micro-flaring region near a sunspot via inversions. These
inversions show localized mean temperature enhancements of ∼1000
K in the chromosphere and upper photosphere, along with co-spatial
bi-directional Doppler shifting of 5-10 km s-1. This heating
also extends along a nearby chromospheric fibril, which is co-spatial
to 10-15 km s-1 downflows. Strong magnetic flux cancellation
is also apparent in one of the footpoints, and is concentrated in the
chromosphere. This event more closely resembles that of an Ellerman
Bomb, though placed slightly higher in the atmosphere than what is
typically observed.
Title: An assessment of Fe xx-Fe xxii emission lines in SDO/EVE
data as diagnostics for high-density solar flare plasmas using EUVE
stellar observations
Authors: Keenan, F. P.; Milligan, R. O.; Mathioudakis, M.; Christian,
D. J.
Bibcode: 2017MNRAS.468.1117K
Altcode: 2017arXiv170208714K
The Extreme Ultraviolet Variability Experiment (EVE) on the Solar
Dynamics Observatory obtains extreme-ultraviolet (EUV) spectra of
the full-disc Sun at a spectral resolution of ∼1 Å and cadence
of 10 s. Such a spectral resolution would normally be considered
to be too low for the reliable determination of electron density
(Ne) sensitive emission line intensity ratios, due
to blending. However, previous work has shown that a limited
number of Fe xxi features in the 90-160 Å wavelength region of
EVE do provide useful Ne-diagnostics at relatively low
flare densities (Ne ≃ 1011-1012
cm-3). Here, we investigate if additional highly ionized
Fe line ratios in the EVE 90-160 Å range may be reliably employed
as Ne-diagnostics. In particular, the potential for such
diagnostics to provide density estimates for high Ne
(∼1013 cm-3) flare plasmas is assessed. Our
study employs EVE spectra for X-class flares, combined with
observations of highly active late-type stars from the EUVE satellite
plus experimental data for well-diagnosed tokamak plasmas, both of
which are similar in wavelength coverage and spectral resolution
to those from EVE. Several ratios are identified in EVE data,
which yield consistent values of electron density, including Fe xx
113.35/121.85 and Fe xxii 114.41/135.79, with confidence in their
reliability as Ne-diagnostics provided by the EUVE and
tokamak results. These ratios also allow the determination of density in
solar flare plasmas up to values of ∼1013 cm-3.
Title: Suppression of Hydrogen Emission in an X-class White-light
Solar Flare
Authors: Procházka, Ondřej; Milligan, Ryan O.; Allred, Joel C.;
Kowalski, Adam F.; Kotrč, Pavel; Mathioudakis, Mihalis
Bibcode: 2017ApJ...837...46P
Altcode: 2017arXiv170200638P
We present unique NUV observations of a well-observed X-class flare
from NOAA 12087 obtained at the Ondřejov Observatory. The flare
shows a strong white-light continuum but no detectable emission in
the higher Balmer and Lyman lines. Reuven Ramaty High-Energy Solar
Spectroscopic Imager and Fermi observations indicate an extremely
hard X-ray spectrum and γ-ray emission. We use the RADYN radiative
hydrodynamic code to perform two types of simulations: one where an
energy of 3 × 1011 erg cm-2 s-1
is deposited by an electron beam with a spectral index of ≈3, and a
second where the same energy is applied directly to the photosphere. The
combination of observations and simulations allows us to conclude
that the white-light emission and the suppression or complete lack
of hydrogen emission lines is best explained by a model where the
dominant energy deposition layer is located in the lower layers of
the solar atmosphere, rather than the chromosphere.
Title: Solar Ellerman Bombs in 1D Radiative Hydrodynamics
Authors: Reid, A.; Mathioudakis, M.; Kowalski, A.; Doyle, J. G.;
Allred, J. C.
Bibcode: 2017ApJ...835L..37R
Altcode: 2017arXiv170104213R
Recent observations from the Interface Region Imaging Spectrograph
appear to show impulsive brightenings in high temperature lines,
which when combined with simultaneous ground-based observations in
Hα, appear co-spatial to Ellerman Bombs (EBs). We use the RADYN
one-dimensional radiative transfer code in an attempt to try and
reproduce the observed line profiles and simulate the atmospheric
conditions of these events. Combined with the MULTI/RH line synthesis
codes, we compute the Hα, Ca II 8542 Å, and Mg II h and k lines for
these simulated events and compare them to previous observations. Our
findings hint that the presence of superheated regions in the
photosphere (>10,000 K) is not a plausible explanation for the
production of EB signatures. While we are able to recreate EB-like
line profiles in Hα, Ca II 8542 Å, and Mg II h and k, we cannot
achieve agreement with all of these simultaneously.
Title: A Chromospheric Flare Model Consisting of Two Dynamical Layers:
Critical Tests from IRIS Data of Solar Flares
Authors: Kowalski, Adam; Allred, Joel C.; Daw, Adrian N.; Cauzzi,
Gianna; Carlsson, Mats; Inglis, Andrew; O'Neill, Aaron; Mathioudakis,
Mihalis; Uitenbroek, Han
Bibcode: 2017AAS...22933902K
Altcode:
Recent 1D radiative-hydrodynamic simulations of flares have shown that
a heated, chromospheric compression layer and a stationary layer, just
below the compression, are produced in response to high flux electron
beam heating. The hot blackbody-like continuum and redshifted intensity
in singly ionized chromospheric lines in these model predictions are
generally consistent with broad wavelength coverage spectra of M dwarf
flares and with high spectral resolution observations of solar flares,
respectively. We critically test this two-component chromospheric
flare model against the Fe II profiles and NUV continuum brightness
for several X-class solar flares observed with the Interface Region
Imaging Spectrograph (IRIS). We present several new predictions for
the Daniel K. Inoue Solar Telescope (DKIST).
Title: Observations and Simulations of the Na I D1 Line
Profiles in an M-class Solar Flare
Authors: Kuridze, D.; Mathioudakis, M.; Christian, D. J.; Kowalski,
A. F.; Jess, D. B.; Grant, S. D. T.; Kawate, T.; Simões, P. J. A.;
Allred, J. C.; Keenan, F. P.
Bibcode: 2016ApJ...832..147K
Altcode: 2016arXiv160908120K
We study the temporal evolution of the Na I D1 line profiles
in the M3.9 flare SOL2014-06-11T21:03 UT, using observations at high
spectral resolution obtained with the Interferometric Bidimensional
Spectrometer instrument on the Dunn Solar Telescope combined with
radiative hydrodynamic simulations. Our results show a significant
increase in the intensities of the line core and wings during the
flare. The analysis of the line profiles from the flare ribbons
reveals that the Na I D1 line has a central reversal
with excess emission in the blue wing (blue asymmetry). We combine
RADYN and RH simulations to synthesize Na I D1 line
profiles of the flaring atmosphere and find good agreement with
the observations. Heating with a beam of electrons modifies the
radiation field in the flaring atmosphere and excites electrons
from the ground state 3s 2S to the first excited state 3p
2P, which in turn modifies the relative population of the
two states. The change in temperature and the population density of
the energy states make the sodium line profile revert from absorption
into emission. Furthermore, the rapid changes in temperature break the
pressure balance between the different layers of the lower atmosphere,
generating upflow/downflow patterns. Analysis of the simulated spectra
reveals that the asymmetries of the Na I D1 flare profile
are produced by the velocity gradients in the lower solar atmosphere.
Title: Suppression of Hydrogen Emission in a White-light Solar Flare
Authors: Milligan, R. O.; Procházka, O.; Mathioudakis, M.; Allred,
J. C.; Kowalski, A. F.
Bibcode: 2016AGUFMSH31B2561P
Altcode: 2016AGUFMSH31B2561M
We present an analysis of an X-class flare that occurred on 11
June 2014 in active region NOAA 12087 using a newly developed high
cadence Image Selector operated by Astronomical Institute in Ondrejov,
Czech Republic. This instrument provides spectra in the 350-440 nm
wavelength range, which covers the higher order Balmer lines as well
as the Balmer jump at 364 nm. However, no detectable increase in these
emissions were detected during the flare, and support observations from
SDO/EVE MEGS-B also show that the Lyman line series and recombination
continuum were also suppressed, particularly when compared to two
other X-class flares on the preceding day. The X-class flare under
investigation also showed strong white light emission in SDO/HMI
data, as well as an extremely hard electron spectrum (delta 3.6),
and gamma-ray emission, from RHESSI data. This unique combination
of datasets was amended with radiative hydrodynamic model of direct
minimum temperature region heating. The results allow us to conclude
that the white light emission from this flare corresponds to a black
body heated by high-energy electrons (and/or ions), as opposed to
optical chromospheric emission from hydrogen.
Title: Kelvin-Helmholtz Instability in Solar Chromospheric Jets:
Theory and Observation
Authors: Kuridze, D.; Zaqarashvili, T. V.; Henriques, V.; Mathioudakis,
M.; Keenan, F. P.; Hanslmeier, A.
Bibcode: 2016ApJ...830..133K
Altcode: 2016arXiv160801497K
Using data obtained by the high-resolution CRisp Imaging
SpectroPolarimeter instrument on the Swedish 1 m Solar Telescope,
we investigate the dynamics and stability of quiet-Sun chromospheric
jets observed at the disk center. Small-scale features, such as rapid
redshifted and blueshifted excursions, appearing as high-speed jets
in the wings of the Hα line, are characterized by short lifetimes and
rapid fading without any descending behavior. To study the theoretical
aspects of their stability without considering their formation
mechanism, we model chromospheric jets as twisted magnetic flux tubes
moving along their axis, and use the ideal linear incompressible
magnetohydrodynamic approximation to derive the governing dispersion
equation. Analytical solutions of the dispersion equation indicate
that this type of jet is unstable to Kelvin-Helmholtz instability
(KHI), with a very short (few seconds) instability growth time at
high upflow speeds. The generated vortices and unresolved turbulent
flows associated with the KHI could be observed as a broadening of
chromospheric spectral lines. Analysis of the Hα line profiles shows
that the detected structures have enhanced line widths with respect to
the background. We also investigate the stability of a larger-scale Hα
jet that was ejected along the line of sight. Vortex-like features,
rapidly developing around the jet’s boundary, are considered as
evidence of the KHI. The analysis of the energy equation in the
partially ionized plasma shows that ion-neutral collisions may lead
to fast heating of the KH vortices over timescales comparable to the
lifetime of chromospheric jets.
Title: The Effects of Transients on Photospheric and Chromospheric
Power Distributions
Authors: Samanta, T.; Henriques, V. M. J.; Banerjee, D.; Krishna
Prasad, S.; Mathioudakis, M.; Jess, D.; Pant, V.
Bibcode: 2016ApJ...828...23S
Altcode: 2016arXiv160406289S
We have observed a quiet-Sun region with the Swedish 1 m Solar Telescope
equipped with the CRISP Imaging SpectroPolarimeter. High-resolution,
high-cadence, Hα line scanning images were taken to observe
different layers of the solar atmosphere from the photosphere to upper
chromosphere. We study the distribution of power in different period
bands at different heights. Power maps of the upper photosphere
and the lower chromosphere show suppressed power surrounding the
magnetic-network elements, known as “magnetic shadows.” These also
show enhanced power close to the photosphere, traditionally referred
to as “power halos.” The interaction between acoustic waves and
inclined magnetic fields is generally believed to be responsible
for these two effects. In this study we explore whether small-scale
transients can influence the distribution of power at different
heights. We show that the presence of transients, like mottles, Rapid
Blueshifted Excursions (RBEs), and Rapid Redshifted Excursions (RREs),
can strongly influence the power maps. The short and finite lifetime of
these events strongly affects all power maps, potentially influencing
the observed power distribution. We show that Doppler-shifted transients
like RBEs and RREs that occur ubiquitously can have a dominant effect
on the formation of the power halos in the quiet Sun. For magnetic
shadows, transients like mottles do not seem to have a significant
effect on the power suppression around 3 minutes, and wave interaction
may play a key role here. Our high-cadence observations reveal that
flows, waves, and shocks manifest in the presence of magnetic fields
to form a nonlinear magnetohydrodynamic system.
Title: Construction status of the Daniel K. Inouye solar telescope
Authors: McMullin, Joseph P.; Rimmele, Thomas R.; Warner, Mark;
Martinez Pillet, Valentin; Casini, Roberto; Berukoff, Steve; Craig,
Simon C.; Elmore, David; Ferayorni, Andrew; Goodrich, Bret D.;
Hubbard, Robert P.; Harrington, David; Hegwer, Steve; Jeffers, Paul;
Johansson, Erik M.; Kuhn, Jeff; Lin, Haosheng; Marshall, Heather;
Mathioudakis, Mihalis; McBride, William R.; McVeigh, William; Phelps,
LeEllen; Schmidt, Wolfgang; Shimko, Steve; Sueoka, Stacey; Tritschler,
Alexandra; Williams, Timothy R.; Wöger, Friedrich
Bibcode: 2016SPIE.9906E..1BM
Altcode:
We provide an update on the construction status of the Daniel
K. Inouye Solar Telescope. This 4-m diameter facility is designed to
enable detection and spatial/temporal resolution of the predicted,
fundamental astrophysical processes driving solar magnetism at
their intrinsic scales throughout the solar atmosphere. These data
will drive key research on solar magnetism and its influence on
solar winds, flares, coronal mass ejections and solar irradiance
variability. The facility is developed to support a broad wavelength
range (0.35 to 28 microns) and will employ state-of-the-art adaptive
optics systems to provide diffraction limited imaging, resolving
features approximately 20 km on the Sun. At the start of operations,
there will be five instruments initially deployed: Visible Broadband
Imager (VBI; National Solar Observatory), Visible SpectroPolarimeter
(ViSP; NCAR High Altitude Observatory), Visible Tunable Filter (VTF
(a Fabry-Perot tunable spectropolarimeter); Kiepenheuer Institute for
Solarphysics), Diffraction Limited NIR Spectropolarimeter (DL-NIRSP;
University of Hawaii, Institute for Astronomy) and the Cryogenic NIR
Spectropolarimeter (Cryo-NIRSP; University of Hawaii, Institute for
Astronomy). As of mid-2016, the project construction is in its 4th
year of site construction and 7th year overall. Major milestones in
the off-site development include the conclusion of the polishing of
the M1 mirror by University of Arizona, College of Optical Sciences,
the delivery of the Top End Optical Assembly (L3), the acceptance of
the Deformable Mirror System (Xinetics); all optical systems have been
contracted and are either accepted or in fabrication. The Enclosure
and Telescope Mount Assembly passed through their factory acceptance
in 2014 and 2015, respectively. The enclosure site construction
is currently concluding while the Telescope Mount Assembly site
erection is underway. The facility buildings (Utility and Support
and Operations) have been completed with ongoing work on the thermal
systems to support the challenging imaging requirements needed for the
solar research. Finally, we present the construction phase performance
(schedule, budget) with projections for the start of early operations.
Title: The European Solar Telescope (EST)
Authors: Matthews, Sarah A.; Collados, Manuel; Mathioudakis, Mihalis;
Erdelyi, Robertus
Bibcode: 2016SPIE.9908E..09M
Altcode:
The European Solar Telescope (EST) is being designed to optimize
studies of the magnetic coupling between the lower layers of the solar
atmosphere (the photosphere and chromosphere) in order to investigate
the origins and evolution of the solar magnetic field and its role
in driving solar activity. In order to achieve this, the thermal,
dynamic and magnetic properties of the solar plasma must be probed
over many scale heights and at intrinsic scales, requiring the use of
multi wavelength spectroscopy and spectropolarimetry at high spatial,
spectral and temporal resolution. In this paper we describe some of
the over-arching science questions that EST will address and briefly
outline the main features of the proposed telescope design and the
associated instrumentation package.
Title: Magnetic Flux Cancellation in Ellerman Bombs
Authors: Reid, A.; Mathioudakis, M.; Doyle, J. G.; Scullion, E.;
Nelson, C. J.; Henriques, V.; Ray, T.
Bibcode: 2016ApJ...823..110R
Altcode: 2016arXiv160307100R
Ellerman Bombs (EBs) are often found to be co-spatial with bipolar
photospheric magnetic fields. We use Hα imaging spectroscopy along with
Fe I 6302.5 Å spectropolarimetry from the Swedish 1 m Solar Telescope
(SST), combined with data from the Solar Dynamic Observatory, to study
EBs and the evolution of the local magnetic fields at EB locations. EBs
are found via an EB detection and tracking algorithm. Using NICOLE
inversions of the spectropolarimetric data, we find that, on average,
(3.43 ± 0.49) × 1024 erg of stored magnetic energy
disappears from the bipolar region during EB burning. The inversions
also show flux cancellation rates of 1014-1015
Mx s-1 and temperature enhancements of 200 K at the detection
footpoints. We investigate the near-simultaneous flaring of EBs due to
co-temporal flux emergence from a sunspot, which shows a decrease in
transverse velocity when interacting with an existing, stationary area
of opposite polarity magnetic flux, resulting in the formation of the
EBs. We also show that these EBs can be fueled further by additional,
faster moving, negative magnetic flux regions.
Title: Construction Status and Early Science with the Daniel K. Inouye
Solar Telescope
Authors: McMullin, Joseph P.; Rimmele, Thomas R.; Warner, Mark;
Martinez Pillet, Valentin; Craig, Simon; Woeger, Friedrich; Tritschler,
Alexandra; Berukoff, Steven J.; Casini, Roberto; Goode, Philip R.;
Knoelker, Michael; Kuhn, Jeffrey Richard; Lin, Haosheng; Mathioudakis,
Mihalis; Reardon, Kevin P.; Rosner, Robert; Schmidt, Wolfgang
Bibcode: 2016SPD....4720101M
Altcode:
The 4-m Daniel K. Inouye Solar Telescope (DKIST) is in its seventh
year of overall development and its fourth year of site construction
on the summit of Haleakala, Maui. The Site Facilities (Utility
Building and Support & Operations Building) are in place with
ongoing construction of the Telescope Mount Assembly within. Off-site
the fabrication of the component systems is completing with early
integration testing and verification starting.Once complete this
facility will provide the highest sensitivity and resolution for study
of solar magnetism and the drivers of key processes impacting Earth
(solar wind, flares, coronal mass ejections, and variability in solar
output). The DKIST will be equipped initially with a battery of first
light instruments which cover a spectral range from the UV (380 nm)
to the near IR (5000 nm), and capable of providing both imaging and
spectro-polarimetric measurements throughout the solar atmosphere
(photosphere, chromosphere, and corona); these instruments are being
developed by the National Solar Observatory (Visible Broadband Imager),
High Altitude Observatory (Visible Spectro-Polarimeter), Kiepenheuer
Institute (Visible Tunable Filter) and the University of Hawaii
(Cryogenic Near-Infrared Spectro-Polarimeter and the Diffraction-Limited
Near-Infrared Spectro-Polarimeter). Further, a United Kingdom consortium
led by Queen's University Belfast is driving the development of high
speed cameras essential for capturing the highly dynamic processes
measured by these instruments. Finally, a state-of-the-art adaptive
optics system will support diffraction limited imaging capable of
resolving features approximately 20 km in scale on the Sun.We present
the overall status of the construction phase along with the current
challenges as well as a review of the planned science testing and the
transition into early science operations.
Title: Quiet-Sun Hα Transients and Corresponding Small-scale
Transition Region and Coronal Heating
Authors: Henriques, V. M. J.; Kuridze, D.; Mathioudakis, M.; Keenan,
F. P.
Bibcode: 2016ApJ...820..124H
Altcode: 2016arXiv160204820H
Rapid blue- and redshifted excursions (RBEs and RREs) are likely to
be the on-disk counterparts of Type II spicules. Recently, heating
signatures from RBEs/RREs have been detected in IRIS slit-jaw
images dominated by transition region (TR) lines around network
patches. Additionally, signatures of Type II spicules have been observed
in Atmospheric Imaging Assembly (AIA) diagnostics. The full-disk,
ever-present nature of the AIA diagnostics should provide us with
sufficient statistics to directly determine how important RBEs and RREs
are to the heating of the TR and corona. We find, with high statistical
significance, that at least 11% of the low coronal brightenings detected
in a quiet-Sun region in He II 304 Å can be attributed to either RBEs
or RREs as observed in Hα, and a 6% match of Fe IX 171 Å detected
events to RBEs or RREs with very similar statistics for both types
of Hα features. We took a statistical approach that allows for noisy
detections in the coronal channels and provides us with a lower, but
statistical significant, bound. Further, we consider matches based on
overlapping features in both time and space, and find strong visual
indications of further correspondence between coronal events and
co-evolving but non-overlapping, RBEs and RREs.
Title: M Dwarf Flare Continuum Variations on One-second Timescales:
Calibrating and Modeling of ULTRACAM Flare Color Indices
Authors: Kowalski, Adam F.; Mathioudakis, Mihalis; Hawley, Suzanne L.;
Wisniewski, John P.; Dhillon, Vik S.; Marsh, Tom R.; Hilton, Eric J.;
Brown, Benjamin P.
Bibcode: 2016ApJ...820...95K
Altcode: 2016arXiv160204879K
We present a large data set of high-cadence dMe flare light curves
obtained with custom continuum filters on the triple-beam, high-speed
camera system ULTRACAM. The measurements provide constraints for models
of the near-ultraviolet (NUV) and optical continuum spectral evolution
on timescales of ≈1 s. We provide a robust interpretation of the
flare emission in the ULTRACAM filters using simultaneously obtained
low-resolution spectra during two moderate-sized flares in the dM4.5e
star YZ CMi. By avoiding the spectral complexity within the broadband
Johnson filters, the ULTRACAM filters are shown to characterize
bona fide continuum emission in the NUV, blue, and red wavelength
regimes. The NUV/blue flux ratio in flares is equivalent to a Balmer
jump ratio, and the blue/red flux ratio provides an estimate for the
color temperature of the optical continuum emission. We present a new
“color-color” relationship for these continuum flux ratios at the
peaks of the flares. Using the RADYN and RH codes, we interpret the
ULTRACAM filter emission using the dominant emission processes from a
radiative-hydrodynamic flare model with a high nonthermal electron beam
flux, which explains a hot, T ≈ 104 K, color temperature
at blue-to-red optical wavelengths and a small Balmer jump ratio as
observed in moderate-sized and large flares alike. We also discuss the
high time resolution, high signal-to-noise continuum color variations
observed in YZ CMi during a giant flare, which increased the NUV flux
from this star by over a factor of 100. Based on observations
obtained with the Apache Point Observatory 3.5 m telescope, which is
owned and operated by the Astrophysical Research Consortium, based on
observations made with the William Herschel Telescope operated on the
island of La Palma by the Isaac Newton Group in the Spanish Observatorio
del Roque de los Muchachos of the Instituto de Astrofsica de Canarias,
and observations, and based on observations made with the ESO Telescopes
at the La Silla Paranal Observatory under programme ID 085.D-0501(A).
Title: Solar coronal magnetic fields derived using seismology
techniques applied to omnipresent sunspot waves
Authors: Jess, David B.; Reznikova, Veronika E.; Ryans, Robert S. I.;
Christian, Damian J.; Keys, Peter H.; Mathioudakis, Mihalis; Mackay,
Duncan H.; Krishna Prasad, S.; Banerjee, Dipankar; Grant, Samuel D. T.;
Yau, Sean; Diamond, Conor
Bibcode: 2016NatPh..12..179J
Altcode: 2016arXiv160506112J
Sunspots on the surface of the Sun are the observational signatures of
intense manifestations of tightly packed magnetic field lines, with
near-vertical field strengths exceeding 6,000 G in extreme cases. It
is well accepted that both the plasma density and the magnitude of the
magnetic field strength decrease rapidly away from the solar surface,
making high-cadence coronal measurements through traditional Zeeman and
Hanle effects difficult as the observational signatures are fraught
with low-amplitude signals that can become swamped with instrumental
noise. Magneto-hydrodynamic (MHD) techniques have previously been
applied to coronal structures, with single and spatially isolated
magnetic field strengths estimated as 9-55 G (refs ,,,). A drawback
with previous MHD approaches is that they rely on particular wave modes
alongside the detectability of harmonic overtones. Here we show, for
the first time, how omnipresent magneto-acoustic waves, originating
from within the underlying sunspot and propagating radially outwards,
allow the spatial variation of the local coronal magnetic field to be
mapped with high precision. We find coronal magnetic field strengths
of 32 +/- 5 G above the sunspot, which decrease rapidly to values of
approximately 1 G over a lateral distance of 7,000 km, consistent with
previous isolated and unresolved estimations. Our results demonstrate
a new, powerful technique that harnesses the omnipresent nature of
sunspot oscillations to provide magnetic field mapping capabilities
close to a magnetic source in the solar corona.
Title: Hα Line Profile Asymmetries and the Chromospheric Flare
Velocity Field
Authors: Kuridze, D.; Mathioudakis, M.; Simões, P. J. A.; Rouppe van
der Voort, L.; Carlsson, M.; Jafarzadeh, S.; Allred, J. C.; Kowalski,
A. F.; Kennedy, M.; Fletcher, L.; Graham, D.; Keenan, F. P.
Bibcode: 2015ApJ...813..125K
Altcode: 2015arXiv151001877K
The asymmetries observed in the line profiles of solar flares can
provide important diagnostics of the properties and dynamics of the
flaring atmosphere. In this paper the evolution of the Hα and Ca ii
λ8542 lines are studied using high spatial, temporal, and spectral
resolution ground-based observations of an M1.1 flare obtained with
the Swedish 1 m Solar Telescope. The temporal evolution of the Hα
line profiles from the flare kernel shows excess emission in the red
wing (red asymmetry) before flare maximum and excess in the blue wing
(blue asymmetry) after maximum. However, the Ca ii λ8542 line does
not follow the same pattern, showing only a weak red asymmetry during
the flare. RADYN simulations are used to synthesize spectral line
profiles for the flaring atmosphere, and good agreement is found
with the observations. We show that the red asymmetry observed in
Hα is not necessarily associated with plasma downflows, and the blue
asymmetry may not be related to plasma upflows. Indeed, we conclude
that the steep velocity gradients in the flaring chromosphere modify
the wavelength of the central reversal in the Hα line profile. The
shift in the wavelength of maximum opacity to shorter and longer
wavelengths generates the red and blue asymmetries, respectively.
Title: Predicting and Disentangling Stellar Variability in Quiet
Stars: Implications for Low-mass Planet Confirmation
Authors: Cegla, Heather; Watson, Chris; Stassun, Keivan; Shelyag,
Sergiy; Mathioudakis, Mihalis; Bastien, Fabienne; Pepper, Josh
Bibcode: 2015IAUGA..2256546C
Altcode:
Stellar variability is one of the main limiting factors in the detection
and confirmation of low-mass planets. Even for magnetically quiet stars,
astrophysical noise could be present at the 10m/s level. For these stars
we demonstrate, using Kepler and GALEX data, that it may be possible to
use photometric variability to predict the RV noise. Such a relationship
creates a filtering mechanism to prioritize planetary candidates in
transit surveys that are ideal for RV follow-up. Going beyond this, we
use 3D magnetohydrodynamical (MHD) simulations to create Sun-as-a-star
observations, focusing on stellar surface magneto-convection. Using
these model observations we have identified correlations between the
stellar line shape, brightness, and RV variability. We demonstrate
that photometric observations may be key in disentangling stellar
and planetary induced RV signals. Disentangling these signals allows
us to reach the 10 cm/s noise level in our model star, indicating
significant promise for the confirmation of low-mass planet candidates
in the future.
Title: Daniel K. Inouye Solar Telescope: Overview and Status
Authors: Rimmele, Thomas; McMullin, Joseph; Warner, Mark; Craig,
Simon; Woeger, Friedrich; Tritschler, Alexandra; Cassini, Roberto;
Kuhn, Jeff; Lin, Haosheng; Schmidt, Wolfgang; Berukoff, Steve; Reardon,
Kevin; Goode, Phil; Knoelker, Michael; Rosner, Robert; Mathioudakis,
Mihalis; DKIST TEAM
Bibcode: 2015IAUGA..2255176R
Altcode:
The 4m Daniel K. Inouye Solar Telescope (DKIST) currently under
construction on Haleakala, Maui will be the world’s largest solar
telescope. Designed to meet the needs of critical high resolution and
high sensitivity spectral and polarimetric observations of the sun,
this facility will perform key observations of our nearest star that
matters most to humankind. DKIST’s superb resolution and sensitivity
will enable astronomers to unravel many of the mysteries the Sun
presents, including the origin of solar magnetism, the mechanisms of
coronal heating and drivers of the solar wind, flares, coronal mass
ejections and variability in solar output. The all-reflecting, off-axis
design allows the facility to observe over a broad wavelength range and
enables DKIST to operate as a coronagraph. In addition, the photon flux
provided by its large aperture will be capable of routine and precise
measurements of the currently elusive coronal magnetic fields. The
state-of-the-art adaptive optics system provides diffraction limited
imaging and the ability to resolve features approximately 20 km on
the Sun. Five first light instruments, representing a broad community
effort, will be available at the start of operations: Visible Broadband
Imager (National Solar Observatory), Visible Spectro-Polarimeter (High
Altitude Observatory), Visible Tunable Filter (Kiepenheuer Institute,
Germany), Diffraction Limited NIR Spectro-Polarimeter (University
of Hawaii) and the Cryogenic NIR Spectro-Polarimeter (University of
Hawaii). High speed cameras for capturing highly dynamic processes
in the solar atmosphere are being developed by a UK consortium. Site
construction on Haleakala began in December 2012 and is progressing
on schedule. Operations are scheduled to begin in 2019. We provide an
overview of the facility, discuss the construction status, and present
progress with DKIST operations planning.
Title: Radiative hydrodynamic modelling and observations of the
X-class solar flare on 2011 March 9
Authors: Kennedy, Michael B.; Milligan, Ryan O.; Allred, Joel C.;
Mathioudakis, Mihalis; Keenan, Francis P.
Bibcode: 2015A&A...578A..72K
Altcode: 2015arXiv150407541K
Aims: We investigated the response of the solar atmosphere to
non-thermal electron beam heating using the radiative transfer and
hydrodynamics modelling code RADYN. The temporal evolution of the
parameters that describe the non-thermal electron energy distribution
were derived from hard X-ray observations of a particular flare, and
we compared the modelled and observed parameters.
Methods:
The evolution of the non-thermal electron beam parameters during
the X1.5 solar flare on 2011 March 9 were obtained from analysis of
RHESSI X-ray spectra. The RADYN flare model was allowed to evolve for
110 s, after which the electron beam heating was ended, and was then
allowed to continue evolving for a further 300 s. The modelled flare
parameters were compared to the observed parameters determined from
extreme-ultraviolet spectroscopy.
Results: The model produced
a hotter and denser flare loop than that observed and also cooled more
rapidly, suggesting that additional energy input in the decay phase of
the flare is required. In the explosive evaporation phase a region of
high-density cool material propagated upward through the corona. This
material underwent a rapid increase in temperature as it was unable to
radiate away all of the energy deposited across it by the non-thermal
electron beam and via thermal conduction. A narrow and high-density
(ne ≤ 1015 cm-3) region at the
base of the flare transition region was the source of optical line
emission in the model atmosphere. The collision-stopping depth of
electrons was calculated throughout the evolution of the flare, and
it was found that the compression of the lower atmosphere may permit
electrons to penetrate farther into a flaring atmosphere compared to
a quiet Sun atmosphere.
Title: Ellerman Bombs with Jets: Cause and Effect
Authors: Reid, A.; Mathioudakis, M.; Scullion, E.; Doyle, J. G.;
Shelyag, S.; Gallagher, P.
Bibcode: 2015ApJ...805...64R
Altcode: 2015arXiv150305359R
Ellerman Bombs (EBs) are thought to arise as a result of photospheric
magnetic reconnection. We use data from the Swedish 1 m Solar
Telescope to study EB events on the solar disk and at the limb. Both
data sets show that EBs are connected to the foot points of forming
chromospheric jets. The limb observations show that a bright structure
in the Hα blue wing connects to the EB initially fueling it, leading
to the ejection of material upwards. The material moves along a loop
structure where a newly formed jet is subsequently observed in the
red wing of Hα. In the disk data set, an EB initiates a jet which
propagates away from the apparent reconnection site within the EB
flame. The EB then splits into two, with associated brightenings in
the inter-granular lanes. Micro-jets are then observed, extending
to 500 km with a lifetime of a few minutes. Observed velocities of
the micro-jets are approximately 5-10 km s-1, while their
chromospheric counterparts range from 50 to 80 km s-1. MURaM
simulations of quiet Sun reconnection show that micro-jets with
properties similar to those of the observations follow the line of
reconnection in the photosphere, with associated Hα brightening at
the location of increased temperature.
Title: Hα and EUV Observations of a Partial CME
Authors: Christian, Damian J.; Jess, David B.; Antolin, Patrick;
Mathioudakis, Mihalis
Bibcode: 2015ApJ...804..147C
Altcode: 2015arXiv150303982C
We have obtained Hα high spatial and time resolution observations
of the upper solar chromosphere and supplemented these with
multi-wavelength observations from the Solar Dynamics Observatory
(SDO) and the Hinode Extreme-ultraviolet Imaging Spectrometer. The Hα
observations were conducted on 2012 February 11 with the Hydrogen-Alpha
Rapid Dynamics Camera instrument at the National Solar Observatory’s
Dunn Solar Telescope. Our Hα observations found large downflows
of chromospheric material returning from coronal heights following a
failed prominence eruption. We have detected several large condensations
(“blobs”) returning to the solar surface at velocities of ≈200 km
s-1 in both Hα and several SDO Atmospheric Imaging Assembly
band passes. The average derived size of these “blobs” in Hα is 500
by 3000 km2 in the directions perpendicular and parallel to
the direction of travel, respectively. A comparison of our “blob”
widths to those found from coronal rain, indicate that there are
additional, smaller, unresolved “blobs” in agreement with previous
studies and recent numerical simulations. Our observed velocities and
decelerations of the “blobs” in both Hα and SDO bands are less
than those expected for gravitational free-fall and imply additional
magnetic or gas pressure impeding the flow. We derived a kinetic energy
of ≈2 orders of magnitude lower for the main eruption than a typical
coronal mass ejection, which may explain its partial nature.
Title: The Radiated Energy Budget Of Chromospheric Plasma In A Major
Solar Flare Deduced From Multi-Wavelength Observations
Authors: Milligan, Ryan; Kerr, Graham Stewart; Dennis, Brian; Hudson,
Hugh; Fletcher, Lyndsay; Allred, Joel; Chamberlin, Phillip; Ireland,
Jack; Mathioudakis, Mihalis; Keenan, Francis
Bibcode: 2015TESS....130209M
Altcode:
The response of the lower solar atmosphere is an important diagnostic
tool for understanding energy transport during solar flares. The 15
February 2011 X-class flare was fortuitously observed by a host of
space-based instruments that sampled the chromospheric response over
a range of lines and continua at <20s cadence. These include the
free-bound EUV continua of H I (Lyman), He I, and He II, plus the
emission lines of He II at 304Å and H I (Lyα) at 1216Å by SDO/EVE,
the UV continua at 1600Å and 1700Å by SDO/AIA, and the white light
continuum at 4504Å, 5550Å, and 6684Å, along with the Ca II H line
at 3968Å using Hinode/SOT. RHESSI also observed the entire event at
energies up to ~100keV, making it possible to determine the properties
of the nonthermal electrons deemed to be responsible for driving the
enhanced chromospheric emission under the assumption of thick-target
collisions. Integrating over the duration of the impulsive phase,
the total energy contained in the nonthermal electrons was found to be
>2×1031 erg. By comparison, the summed energy detected by
instruments onboard SDO and Hinode amounted to ~3×1030 erg;
about 15% of the total nonthermal energy. The Lyα line was found to
dominate the measured radiative losses in contrast to the predictions
of numerical simulations. Parameters of both the driving electron
distribution and the resulting chromospheric response are presented
in detail to encourage the numerical modeling of flare heating for
this event to determine the depth of the solar atmosphere at which
these line and continuum processes originate, and the mechanism(s)
responsible for their generation.
Title: The Dynamics of Rapid Redshifted and Blueshifted Excursions
in the Solar Hα Line
Authors: Kuridze, D.; Henriques, V.; Mathioudakis, M.; Erdélyi, R.;
Zaqarashvili, T. V.; Shelyag, S.; Keys, P. H.; Keenan, F. P.
Bibcode: 2015ApJ...802...26K
Altcode: 2015arXiv150106205K
We analyze high temporal and spatial resolution time-series of spectral
scans of the Hα line obtained with the CRisp Imaging SpectroPolarimeter
instrument mounted on the Swedish Solar Telescope. The data reveal
highly dynamic, dark, short-lived structures known as Rapid Redshifted
and Blueshifted Excursions (RREs, RBEs) that are on-disk absorption
features observed in the red and blue wings of spectral lines formed
in the chromosphere. We study the dynamics of RREs and RBEs by tracking
their evolution in space and time, measuring the speed of the apparent
motion, line of sight (LOS) Doppler velocity, and transverse velocity
of individual structures. A statistical study of their measured
properties shows that RREs and RBEs have similar occurrence rates,
lifetimes, lengths, and widths. They also display non-periodic,
nonlinear transverse motions perpendicular to their axes at speeds of
4-31 km s-1. Furthermore, both types of structures either
appear as high speed jets and blobs that are directed outwardly from
a magnetic bright point with speeds of 50-150 km s-1,
or emerge within a few seconds. A study of the different velocity
components suggests that the transverse motions along the LOS of
the chromospheric flux tubes are responsible for the formation and
appearance of these redshifted/blueshifted structures. The short
lifetime and fast disappearance of the RREs/RBEs suggests that, similar
to type II spicules, they are rapidly heated to transition region
or even coronal temperatures. We speculate that the Kelvin-Helmholtz
instability triggered by observed transverse motions of these structures
may be a viable mechanism for their heating.
Title: Stable umbral chromospheric structures
Authors: Henriques, V. M. J.; Scullion, E.; Mathioudakis, M.; Kiselman,
D.; Gallagher, P. T.; Keenan, F. P.
Bibcode: 2015A&A...574A.131H
Altcode: 2014arXiv1412.6100H
Aims: We seek to understand the morphology of the chromosphere in
sunspot umbra. We investigate if the horizontal structures observed
in the spectral core of the Ca II H line are ephemeral visuals
caused by the shock dynamics of more stable structures, and examine
their relationship with observables in the H-alpha line.
Methods: Filtergrams in the core of the Ca II H and H-alpha lines
as observed with the Swedish 1-m Solar Telescope are employed. We
utilise a technique that creates composite images and tracks the
flash propagation horizontally.
Results: We find 0.̋15 wide
horizontal structures, in all of the three target sunspots, for every
flash where the seeing is moderate to good. Discrete dark structures
are identified that are stable for at least two umbral flashes, as well
as systems of structures that live for up to 24 min. We find cases of
extremely extended structures with similar stability, with one such
structure showing an extent of 5''. Some of these structures have a
correspondence in H-alpha, but we were unable to find a one-to-one
correspondence for every occurrence. If the dark streaks are formed at
the same heights as umbral flashes, there are systems of structures
with strong departures from the vertical for all three analysed
sunspots.
Conclusions: Long-lived Ca II H filamentary horizontal
structures are a common and likely ever-present feature in the umbra
of sunspots. If the magnetic field in the chromosphere of the umbra
is indeed aligned with the structures, then the present theoretical
understanding of the typical umbra needs to be revisited. Movies
associated to Figs. 3 and 4 are available in electronic form at http://www.aanda.org
Title: Time-Resolved Near-Ultraviolet Flare Spectra with the Hubble
Space Telescope / Cosmic Origins Spectrograph
Authors: Kowalski, Adam F.; Hawley, Suzanne L.; Johns-Krull,
Christopher M.; Schmidt, Sarah J.; Brown, Alexander; Wisniewski, John
P.; Davenport, James R. A.; Farina, Cecilia; Pietro Gentile Fusillo,
Nicola; Xilouris, Manolis; Mathioudakis, Mihalis; Osten, Rachel A.;
Holtzman, Jon A.; Phan-Bao, Ngoc; Valenti, Jeff A.; Walkowicz, Lucianne
Bibcode: 2015AAS...22544903K
Altcode:
A large amount of the radiated energy during solar and stellar flares
is emitted as white-light continuum emission, extending through the
ultraviolet and optical wavelength regimes. Broadband photometry
and optical spectral observations of M dwarf flares suggest that the
white-light peak is located in the near-ultraviolet wavelength regime
similar to a blackbody with T ~ 10,000 K, whereas radiative-hydrodynamic
models using a solar-type flare heating mechanism (nonthermal electrons
with a flux ~ 1011 erg / s / cm2 accelerated in
the corona) predict that the peak lies at redder wavelengths at the
head of the Balmer continuum. We have completed a successful flare
monitoring campaign on the dM4e star GJ 1243, in order to constrain
the time-evolution of the peak of the white-light continuum. The
campaign took place over 12 hours on Aug 31/Sept 1, 2014, and included
optical monitoring from nine ground-based telescopes as the Hubble
Space Telescope recorded time-tagged spectra in the near-ultraviolet
(2450-2840 Å) with the Cosmic Origins Spectrograph. Two flares occurred
during the HST observations, and we show preliminary results relating
the continuum and line (Fe II and Mg II) emission to the simultaneous
ground-based optical spectra and photometry. This dataset provides new
constraints for radiative-hydrodynamic modeling of the lower flaring
atmosphere in addition to input for models of the effects of flares
on biomarkers and habitability around M dwarfs.
Title: Understanding Astrophysical Noise from Stellar Surface
Magneto-Convection
Authors: Cegla, H. M.; Watson, C. A.; Shelyag, S.; Mathioudakis, M.
Bibcode: 2015csss...18..567C
Altcode: 2014arXiv1408.2301C
To obtain cm s^{-1} precision, stellar surface magneto-convection
must be disentangled from observed radial velocities (RVs). In
order to understand and remove the convective signature, we create
Sun-as-a-star model observations based off a 3D magnetohydrodynamic
solar simulation. From these Sun-as-a-star model observations, we
find several line characteristics are correlated with the induced RV
shifts. The aim of this campaign is to feed directly into future high
precision RV studies, such as the search for habitable, rocky worlds,
with forthcoming spectrographs such as ESPRESSO.
Title: Nanoflare Activity in the Solar Chromosphere
Authors: Jess, D. B.; Mathioudakis, M.; Keys, P. H.
Bibcode: 2014ApJ...795..172J
Altcode: 2014arXiv1409.6726J
We use ground-based images of high spatial and temporal
resolution to search for evidence of nanoflare activity in the solar
chromosphere. Through close examination of more than 1 × 109
pixels in the immediate vicinity of an active region, we show that
the distributions of observed intensity fluctuations have subtle
asymmetries. A negative excess in the intensity fluctuations indicates
that more pixels have fainter-than-average intensities compared with
those that appear brighter than average. By employing Monte Carlo
simulations, we reveal how the negative excess can be explained by a
series of impulsive events, coupled with exponential decays, that are
fractionally below the current resolving limits of low-noise equipment
on high-resolution ground-based observatories. Importantly, our Monte
Carlo simulations provide clear evidence that the intensity asymmetries
cannot be explained by photon-counting statistics alone. A comparison
to the coronal work of Terzo et al. suggests that nanoflare activity
in the chromosphere is more readily occurring, with an impulsive
event occurring every ~360 s in a 10,000 km2 area of the
chromosphere, some 50 times more events than a comparably sized region
of the corona. As a result, nanoflare activity in the chromosphere
is likely to play an important role in providing heat energy to this
layer of the solar atmosphere.
Title: The Radiated Energy Budget of Chromospheric Plasma in a Major
Solar Flare Deduced from Multi-wavelength Observations
Authors: Milligan, Ryan O.; Kerr, Graham S.; Dennis, Brian R.; Hudson,
Hugh S.; Fletcher, Lyndsay; Allred, Joel C.; Chamberlin, Phillip C.;
Ireland, Jack; Mathioudakis, Mihalis; Keenan, Francis P.
Bibcode: 2014ApJ...793...70M
Altcode: 2014arXiv1406.7657M
This paper presents measurements of the energy radiated by the lower
solar atmosphere, at optical, UV, and EUV wavelengths, during an
X-class solar flare (SOL2011-02-15T01:56) in response to an injection
of energy assumed to be in the form of nonthermal electrons. Hard
X-ray observations from RHESSI were used to track the evolution of
the parameters of the nonthermal electron distribution to reveal the
total power contained in flare accelerated electrons. By integrating
over the duration of the impulsive phase, the total energy contained
in the nonthermal electrons was found to be >2 × 1031
erg. The response of the lower solar atmosphere was measured in
the free-bound EUV continua of H I (Lyman), He I, and He II, plus
the emission lines of He II at 304 Å and H I (Lyα) at 1216 Å by
SDO/EVE, the UV continua at 1600 Å and 1700 Å by SDO/AIA, and the
white light continuum at 4504 Å, 5550 Å, and 6684 Å, along with the
Ca II H line at 3968 Å using Hinode/SOT. The summed energy detected
by these instruments amounted to ~3 × 1030 erg about 15%
of the total nonthermal energy. The Lyα line was found to dominate
the measured radiative losses. Parameters of both the driving electron
distribution and the resulting chromospheric response are presented
in detail to encourage the numerical modeling of flare heating for
this event, to determine the depth of the solar atmosphere at which
these line and continuum processes originate, and the mechanism(s)
responsible for their generation.
Title: F-CHROMA.Flare Chromospheres: Observations, Models and Archives
Authors: Cauzzi, Gianna; Fletcher, Lyndsay; Mathioudakis, Mihalis;
Carlsson, Mats; Heinzel, Petr; Berlicki, Arek; Zuccarello, Francesca
Bibcode: 2014AAS...22412339C
Altcode:
F-CHROMA is a collaborative project newly funded under the EU-Framework
Programme 7 "FP7-SPACE-2013-1", involving seven different European
research Institutes and Universities. The goal of F-CHROMA is to
substantially advance our understanding of the physics of energy
dissipation and radiation in the flaring solar atmosphere, with a
particular focus on the flares' chromosphere. A major outcome of the
F-CHROMA project will be the creation of an archive of chromospheric
flare observations and models to be made available to the community
for further research.In this poster we describe the structure and
milestones of the project, the different activities planned, as well
as early results. Emphasis will be given to the dissemination efforts
of the project to make results of these activities available to and
usable by the community.
Title: Dynamic properties of bright points in an active region
Authors: Keys, P. H.; Mathioudakis, M.; Jess, D. B.; Mackay, D. H.;
Keenan, F. P.
Bibcode: 2014A&A...566A..99K
Altcode: 2014arXiv1405.3923K
Context. Bright points (BPs) are small-scale, magnetic features
ubiquitous across the solar surface. Previously, we have observed and
noted their properties for quiet Sun regions. Here, we determine the
dynamic properties of BPs using simultaneous quiet Sun and active region
data.
Aims: The aim of this paper is to compare the properties
of BPs in both active and quiet Sun regions and to determine any
difference in the dynamics and general properties of BPs as a result of
the varying magnetic activity within these two regions.
Methods:
High spatial and temporal resolution G-band observations of active
region AR11372 were obtained with the Rapid Oscillations in the Solar
Atmosphere instrument at the Dunn Solar Telescope. Three subfields of
varying polarity and magnetic flux density were selected with the aid of
magnetograms obtained from the Helioseismic and Magnetic Imager on board
the Solar Dynamics Observatory. Bright points within these subfields
were subsequently tracked and analysed.
Results: It is found that
BPs within active regions display attenuated velocity distributions
with an average horizontal velocity of ~0.6 km s-1,
compared to the quiet region which had an average velocity of 0.9 km
s-1. Active region BPs are also ~21% larger than quiet
region BPs and have longer average lifetimes (~132 s) than their
quiet region counterparts (88 s). No preferential flow directions are
observed within the active region subfields. The diffusion index (γ) is
estimated at ~1.2 for the three regions.
Conclusions: We confirm
that the dynamic properties of BPs arise predominately from convective
motions. The presence of stronger field strengths within active regions
is the likely reason behind the varying properties observed. We believe
that larger amounts of magnetic flux will attenuate BP velocities by a
combination of restricting motion within the intergranular lanes and
by increasing the number of stagnation points produced by inhibited
convection. Larger BPs are found in regions of higher magnetic flux
density and we believe that lifetimes increase in active regions as
the magnetic flux stabilises the BPs.
Title: Plasma properties and Stokes profiles during the lifetime of
a photospheric magnetic bright point
Authors: Hewitt, R. L.; Shelyag, S.; Mathioudakis, M.; Keenan, F. P.
Bibcode: 2014A&A...565A..84H
Altcode: 2014arXiv1404.0132H
Aims: In this paper we aim to investigate the evolution of plasma
properties and Stokes parameters in photospheric magnetic bright points
using 3D magneto-hydrodynamical simulations and radiative diagnostics of
solar granulation.
Methods: Simulated time-dependent radiation
parameters and plasma properties were investigated throughout the
evolution of a bright point. Synthetic Stokes profiles for the FeI
630.25 nm line were calculated, which also allowed the evolution of the
Stokes-I line strength and Stokes-V area and amplitude asymmetries
to be investigated.
Results: Our results are consistent
with theoretical predictions and published observations describing
convective collapse, and confirm this as the bright point formation
process. Through degradation of the simulated data to match the spatial
resolution of SOT, we show that high spatial resolution is crucial for
the detection of changing spectro-polarimetric signatures throughout
a magnetic bright point's lifetime. We also show that the signature
downflow associated with the convective collapse process tends towards
zero as the radiation intensity in the bright point peaks, because
of the magnetic forces present restricting the flow of material in
the flux tube. Appendix A is available in electronic form at http://www.aanda.org
Title: Intensity Enhancement of O VI Ultraviolet Emission Lines in
Solar Spectra due to Opacity
Authors: Keenan, F. P.; Doyle, J. G.; Madjarska, M. S.; Rose, S. J.;
Bowler, L. A.; Britton, J.; McCrink, L.; Mathioudakis, M.
Bibcode: 2014ApJ...784L..39K
Altcode: 2014arXiv1403.1470K
Opacity is a property of many plasmas. It is normally expected that if
an emission line in a plasma becomes optically thick, then its intensity
ratio to that of another transition that remains optically thin
should decrease. However, radiative transfer calculations undertaken
both by ourselves and others predict that under certain conditions the
intensity ratio of an optically thick to an optically thin line can show
an increase over the optically thin value, indicating an enhancement
in the former. These conditions include the geometry of the emitting
plasma and its orientation to the observer. A similar effect can
take place between lines of differing optical depths. While previous
observational studies have focused on stellar point sources, here we
investigate the spatially resolved solar atmosphere using measurements
of the I(1032 Å)/I(1038 Å) intensity ratio of O VI in several regions
obtained with the Solar Ultraviolet Measurements of Emitted Radiation
instrument on board the Solar and Heliospheric Observatory satellite. We
find several I(1032 Å)/I(1038 Å) ratios observed on the disk to be
significantly larger than the optically thin value of 2.0, providing
the first detection (to our knowledge) of intensity enhancement in
the ratio arising from opacity effects in the solar atmosphere. The
agreement between observation and theory is excellent and confirms
that the O VI emission originates from a slab-like geometry in the
solar atmosphere, rather than from cylindrical structures.
Title: The Atmospheric Response to High Fluxes of Nonthermal Electrons
during M Dwarf Flares
Authors: Kowalski, Adam; Allred, J. C.; Carlsson, M.; Hawley, S. L.;
Holman, G. D.; Mathioudakis, M.; Osten, R. A.; Uitenbroek, H.
Bibcode: 2014AAS...22315117K
Altcode:
Flares are thought to be the result of magnetic fields in the stellar
corona that undergo reconnection and accelerate charged particles
into the lower atmosphere. Spectra of M dwarf flares in the optical
and near-ultraviolet wavelength regimes can be used to constrain the
heating mechanism of the lower stellar atmosphere. These observations
show several ubiquitous properties of the continuum emission, which
is not reproduced by models that use typical “solar-type” heating
functions. We present results from a grid of new flare models using the
RADYN code, which simultaneously calculates the radiative transfer and
hydrodynamics on short timescales. We explore the atmospheric response
to a short ~2 second burst of a very high heating rate from nonthermal
electrons using a solar-type heating function, and we propose a new
“M dwarf-type” heating variation that explains a range of observed
spectral properties, such as ~10,000 K blackbody emission and a smooth
continuum across the Balmer jump wavelength (3646A).
Title: Comet C/2012 S1 (ISON): Observations of the Dust Grains from
SOFIA and of the Atomic Gas from NSO Dunn and McMath-Pierce Solar
Telescopes (Invited)
Authors: Wooden, D. H.; Woodward, C. E.; Harker, D. E.; Kelley, M. S.;
Sitko, M.; Reach, W. T.; De Pater, I.; Gehrz, R. D.; Kolokolova,
L.; Cochran, A. L.; McKay, A. J.; Reardon, K.; Cauzzi, G.; Tozzi,
G.; Christian, D. J.; Jess, D. B.; Mathioudakis, M.; Lisse, C. M.;
Morgenthaler, J. P.; Knight, M. M.
Bibcode: 2013AGUFM.P24A..07W
Altcode:
Comet C/2012 S1 (ISON) is unique in that it is a dynamically new comet
derived from the Oort cloud reservoir of comets with a sun-grazing
orbit. Infrared (IR) and visible wavelength observing campaigns were
planned on NASA's Stratospheric Observatory For Infrared Astronomy
(SOFIA) and on National Solar Observatory Dunn (DST) and McMath-Pierce
Solar Telescopes, respectively. We highlight our early results. SOFIA
(+FORCAST [1]) mid- to far-IR images and spectroscopy (~5-35 μm)
of the dust in the coma of ISON are to be obtained by the ISON-SOFIA
Team during a flight window 2013 Oct 21-23 UT (r_h≈1.18 AU). Dust
characteristics, identified through the 10 μm silicate emission
feature and its strength [2], as well as spectral features from
cometary crystalline silicates (Forsterite) at 11.05-11.2 μm, and
near 16, 19, 23.5, 27.5, and 33 μm are compared with other Oort cloud
comets that span the range of small and/or highly porous grains (e.g.,
C/1995 O1 (Hale-Bopp) [3,4,5] and C/2001 Q4 (NEAT) [6]) to large and/or
compact grains (e.g., C/2007 N4 (Lulin) [7] and C/2006 P1 (McNaught)
[8]). Measurement of the crystalline peaks in contrast to the broad
10 and 20 μm amorphous silicate features yields the cometary silicate
crystalline mass fraction [9], which is a benchmark for radial transport
in our protoplanetary disk [10]. The central wavelength positions,
relative intensities, and feature asymmetries for the crystalline
peaks may constrain the shapes of the crystals [11]. Only SOFIA can
look for cometary organics in the 5-8 μm region. Spatially resolved
measurements of atoms and simple molecules from when comet ISON is
near the Sun (r_h< 0.4 AU, near Nov-20--Dec-03 UT) were proposed
for by the ISON-DST Team. Comet ISON is the first comet since comet
Ikeya-Seki (1965f) [12,13] suitable for studying the alkalai metals Na
and K and the atoms specifically attributed to dust grains including
Mg, Si, Fe, as well as Ca. DST's Horizontal Grating Spectrometer
(HGS) measures 4 settings: Na I, K, C2 to sample cometary organics
(along with Mg I), and [O I] as a proxy for activity from water [14]
(along with Si I and Fe I). State-of-the-art instruments that will also
be employed include IBIS [15], which is a Fabry-Perot spectral imaging
system that concurrently measures lines of Na, K, Ca II, or Fe, and ROSA
(CSUN/QUB) [16], which is a rapid imager that simultaneously monitors Ca
II or CN. From McMath-Pierce, the Solar-Stellar Spectrograph also will
target ISON (320-900 nm, R~21,000, r_h<0.3 AU). Assuming survival,
the intent is to target ISON over r_h<0.4 AU, characteristic of
prior Na detections [12,13,17,18,19]. References: [1] Adams, J.D., et
al. 2012, SPIE, 8446, 16; [2] Kelley, M.S., Wooden, D.H. 2009, PSS, 57,
1133; [3] Harker et al. 2002, ApJ, 580, 579; [4] Hayward et al. 2000,
ApJ, 538, 428; [5] Hadamcik, E., Levasseur-Regourd, A.C. 2003, JQSRT,
79-80, 661; [6] Wooden, D.H. 2004, ApJL, 612, L77; [7] Woodward et
al. 2011, AJ, 141, 181; [8] Kelley et al. 2010, LPSC, 41, #2375;
[9] Kelley, M.S. et al. 2011, AAS, 211, 560; [10] Wooden, D.H. 2008,
SSRv, 138, 75; [11] Lindsay et al. 2013, ApJ, 766, 54; [12] Preston,
G. W. 1967, ApJ, 147, 718; [13] Slaughter, C.D. 1969, AJ, 74, 929;
[14] McKay et al. 2012, Icarus, 222, 684; [15] Cavallini, F., 2006,
Solar Phys., 236, 415; [16] Jess et al., 2010, Solar Phys, 261, 363;
[17] Watanabe, J-I. et al. 2003, ApJ, 585, L159; [18] Leblanc, F. et
al. 2008, A&A, 482, 293; [19] Fulle, M. et al. 2013, ApJL, 771, L21
Title: Characteristics of Transverse Waves in Chromospheric Mottles
Authors: Kuridze, D.; Verth, G.; Mathioudakis, M.; Erdélyi, R.;
Jess, D. B.; Morton, R. J.; Christian, D. J.; Keenan, F. P.
Bibcode: 2013ApJ...779...82K
Altcode: 2013arXiv1310.3628K
Using data obtained by the high temporal and spatial resolution
Rapid Oscillations in the Solar Atmosphere instrument on the Dunn
Solar Telescope, we investigate at an unprecedented level of detail
transverse oscillations in chromospheric fine structures near the solar
disk center. The oscillations are interpreted in terms of propagating
and standing magnetohydrodynamic kink waves. Wave characteristics
including the maximum transverse velocity amplitude and the phase
speed are measured as a function of distance along the structure's
length. Solar magnetoseismology is applied to these measured parameters
to obtain diagnostic information on key plasma parameters (e.g.,
magnetic field, density, temperature, flow speed) of these localized
waveguides. The magnetic field strength of the mottle along the ~2 Mm
length is found to decrease by a factor of 12, while the local plasma
density scale height is ~280 ± 80 km.
Title: Solar Flare Impulsive Phase Emission Observed with SDO/EVE
Authors: Kennedy, Michael B.; Milligan, Ryan O.; Mathioudakis, Mihalis;
Keenan, Francis P.
Bibcode: 2013ApJ...779...84K
Altcode: 2013arXiv1310.4649K
Differential emission measures (DEMs) during the impulsive phase
of solar flares were constructed using observations from the
EUV Variability Experiment (EVE) and the Markov-Chain Monte Carlo
method. Emission lines from ions formed over the temperature range log
Te = 5.8-7.2 allow the evolution of the DEM to be studied
over a wide temperature range at 10 s cadence. The technique was applied
to several M- and X-class flares, where impulsive phase EUV emission
is observable in the disk-integrated EVE spectra from emission lines
formed up to 3-4 MK and we use spatially unresolved EVE observations
to infer the thermal structure of the emitting region. For the nine
events studied, the DEMs exhibited a two-component distribution during
the impulsive phase, a low-temperature component with peak temperature
of 1-2 MK, and a broad high-temperature component from 7 to 30 MK. A
bimodal high-temperature component is also found for several events,
with peaks at 8 and 25 MK during the impulsive phase. The origin of the
emission was verified using Atmospheric Imaging Assembly images to be
the flare ribbons and footpoints, indicating that the constructed DEMs
represent the spatially average thermal structure of the chromospheric
flare emission during the impulsive phase.
Title: Ellerman Bombs—Evidence for Magnetic Reconnection in the
Lower Solar Atmosphere
Authors: Nelson, C. J.; Shelyag, S.; Mathioudakis, M.; Doyle, J. G.;
Madjarska, M. S.; Uitenbroek, H.; Erdélyi, R.
Bibcode: 2013ApJ...779..125N
Altcode: 2013arXiv1310.7756N
The presence of photospheric magnetic reconnection has long been thought
to give rise to short and impulsive events, such as Ellerman bombs (EBs)
and Type II spicules. In this article, we combine high-resolution,
high-cadence observations from the Interferometric BIdimensional
Spectrometer and Rapid Oscillations in the Solar Atmosphere instruments
at the Dunn Solar Telescope, National Solar Observatory, New Mexico,
with co-aligned Solar Dynamics Observatory Atmospheric Imaging Assembly
and Hinode Solar Optical Telescope (SOT) data to observe small-scale
events situated within an active region. These data are then compared
with state-of-the-art numerical simulations of the lower atmosphere
made using the MURaM code. It is found that brightenings, in both the
observations and the simulations, of the wings of the Hα line profile,
interpreted as EBs, are often spatially correlated with increases in
the intensity of the Fe I λ6302.5 line core. Bipolar regions inferred
from Hinode/SOT magnetic field data show evidence of flux cancellation
associated, co-spatially, with these EBs, suggesting that magnetic
reconnection could be a driver of these high-energy events. Through
the analysis of similar events in the simulated lower atmosphere,
we are able to infer that line profiles analogous to the observations
occur co-spatially with regions of strong opposite-polarity magnetic
flux. These observed events and their simulated counterparts are
interpreted as evidence of photospheric magnetic reconnection at scales
observable using current observational instrumentation.
Title: Alfvén Waves in Simulations of Solar Photospheric Vortices
Authors: Shelyag, S.; Cally, P. S.; Reid, A.; Mathioudakis, M.
Bibcode: 2013ApJ...776L...4S
Altcode: 2013arXiv1309.2019S
Using advanced numerical magneto-hydrodynamic simulations of the
magnetized solar photosphere, including non-gray radiative transport and
a non-ideal equation of state, we analyze plasma motions in photospheric
magnetic vortices. We demonstrate that apparent vortex-like motions in
photospheric magnetic field concentrations do not exhibit "tornado"-like
behavior or a "bath-tub" effect. While at each time instance the
velocity field lines in the upper layers of the solar photosphere show
swirls, the test particles moving with the time-dependent velocity
field do not demonstrate such structures. Instead, they move in a
wave-like fashion with rapidly changing and oscillating velocity field,
determined mainly by magnetic tension in the magnetized intergranular
downflows. Using time-distance diagrams, we identify horizontal
motions in the magnetic flux tubes as torsional Alfvén perturbations
propagating along the nearly vertical magnetic field lines with local
Alfvén speed.
Title: The Decaying Long-period Oscillation of a Stellar Megaflare
Authors: Anfinogentov, S.; Nakariakov, V. M.; Mathioudakis, M.;
Van Doorsselaere, T.; Kowalski, A. F.
Bibcode: 2013ApJ...773..156A
Altcode:
We analyze and interpret the oscillatory signal in the decay phase of
the U-band light curve of a stellar megaflare observed on 2009 January
16 on the dM4.5e star YZ CMi. The oscillation is well approximated
by an exponentially decaying harmonic function. The period of the
oscillation is found to be 32 minutes, the decay time about 46 minutes,
and the relative amplitude 15%. As this observational signature is
typical of the longitudinal oscillations observed in solar flares at
extreme ultraviolet and radio wavelengths, associated with standing
slow magnetoacoustic waves, we suggest that this megaflare may be of a
similar nature. In this scenario, macroscopic variations of the plasma
parameters in the oscillations modulate the ejection of non-thermal
electrons. The phase speed of the longitudinal (slow magnetoacoustic)
waves in the flaring loop or arcade, the tube speed, of about 230
km s-1 would require a loop length of about 200 Mm. Other
mechanisms, such as standing kink oscillations, are also considered.
Title: Solar Flare Impulsive Phase Footpoint Emission Observed
with SDO/EVE
Authors: Kennedy, Michael; Milligan, R. O.; Mathioudakis, M.
Bibcode: 2013SPD....44...72K
Altcode:
The differential emission measure of solar flare plasmas was constructed
using observations from the EUV Variability Experiment (EVE) and the
Markov-Chain Monte Carlo method. Emission lines from ions formed over
the temperature range Log T = 5.8 - 7.4 allow for the evolution of the
DEM to be studied over a wide temperature range at 10s cadence. The DEM
construction technique is applied to several M and X-class flares where
impulsive phase EUV emission is observable in the disk-integrated EVE
spectra. The emission is verified using AIA images to be originating
from the flare ribbons and footpoints and EVE observations are used to
infer the thermal structure of the EUV emitting flare chromosphere. For
the nine events studied the constructed differential emission measures
have a two component distribution during the impulsive phase. The low
temperature component has peak temperatures of 1 - 2 MK, and a high
temperature component peaking at 10 MK.
Title: A Pathway to Earth-like Worlds: Overcoming Astrophysical
Noise due to Convection
Authors: Cegla, Heather M.; Watson, C.; Shelyag, S.; Mathioudakis, M.
Bibcode: 2013AAS...22230402C
Altcode:
One of the consequences of the plasma motions within the convective
envelopes of low-mass stars (i.e. potential planet hosting stars)
are the radial velocity (RV) shifts due to variable stellar
line profile asymmetries, known as astrophysical noise (or
stellar jitter). This can pose a major problem for planet hunters
because RV follow-up is mandatory for most planet confirmation and
characterization. Furthermore, as the net RV shifts produced from
these photospheric convective motions are around the m/s level this is
especially troublesome for confirmation of Earth-analogs that induce
Doppler-wobbles on the cm/s level. The currently implemented noise
removal technique for granulation rests on adapting observational
strategies to average out such noise. However, this technique is
extremely observationally intensive and does not provide information on
the nature of jitter. We aim to go beyond these previous techniques
by understanding the physical processes involved in granulation
and removing the actual RV signature from granulation. We outline
our techniques to characterize photospheric granulation as an
astrophysical noise source. The backbone of this characterization is
a state-of-the-art 3D magnetohydrodynamic solar simulation, coupled
with detailed wavelength-dependent radiative transfer. Due to the
time-intensive nature of these simulations, we use a short time-series
to parameterize the granulation signal. This parameterization is then
used to create full Sun-as-a-star observations from which we examine
the convective noise. We present the results of this study, as well as
the identification of variable gravitational redshift as a potential
source of stellar jitter, both of which could impact the RV follow-up
and confirmation of low-mass terrestrial planets and Earth-like worlds.
Title: Alfvén Waves in the Solar Atmosphere. From Theory to
Observations
Authors: Mathioudakis, M.; Jess, D. B.; Erdélyi, R.
Bibcode: 2013SSRv..175....1M
Altcode: 2012SSRv..tmp...94M; 2012arXiv1210.3625M
Alfvén waves are considered to be viable transporters of the
non-thermal energy required to heat the Sun's quiescent atmosphere. An
abundance of recent observations, from state-of-the-art facilities, have
reported the existence of Alfvén waves in a range of chromospheric
and coronal structures. Here, we review the progress made in
disentangling the characteristics of transverse kink and torsional
linear magnetohydrodynamic (MHD) waves. We outline the simple, yet
powerful theory describing their basic properties in (non-)uniform
magnetic structures, which closely resemble the building blocks of
the real solar atmosphere.
Title: Failed filament eruption inside a coronal mass ejection in
active region 11121
Authors: Kuridze, D.; Mathioudakis, M.; Kowalski, A. F.; Keys, P. H.;
Jess, D. B.; Balasubramaniam, K. S.; Keenan, F. P.
Bibcode: 2013A&A...552A..55K
Altcode: 2013arXiv1302.5931K
Aims: We study the formation and evolution of a failed filament
eruption observed in NOAA active region 11121 near the southeast limb on
November 6, 2010.
Methods: We used a time series of SDO/AIA 304,
171, 131, 193, 335, and 94 Å images, SDO/HMI magnetograms, as well as
ROSA and ISOON Hα images to study the erupting active region.
Results: We identify coronal loop arcades associated with a quadrupolar
magnetic configuration, and show that the expansion and cancellation
of the central loop arcade system over the filament is followed by
the eruption of the filament. The erupting filament reveals a clear
helical twist and develops the same sign of writhe in the form of
inverse γ-shape.
Conclusions: The observations support the
"magnetic breakout" process in which the eruption is triggered by
quadrupolar reconnection in the corona. We propose that the formation
mechanism of the inverse γ-shape flux rope is the magnetohydrodynamic
helical kink instability. The eruption has failed because of the
large-scale, closed, overlying magnetic loop arcade that encloses
the active region. Movies are available in electronic form at
http://www.aanda.org
Title: Statistical Analysis of Small Ellerman Bomb Events
Authors: Nelson, C. J.; Doyle, J. G.; Erdélyi, R.; Huang, Z.;
Madjarska, M. S.; Mathioudakis, M.; Mumford, S. J.; Reardon, K.
Bibcode: 2013SoPh..283..307N
Altcode: 2013arXiv1301.1351N
The properties of Ellerman bombs (EBs), small-scale brightenings in
the Hα line wings, have proved difficult to establish because their
size is close to the spatial resolution of even the most advanced
telescopes. Here, we aim to infer the size and lifetime of EBs using
high-resolution data of an emerging active region collected using
the Interferometric BIdimensional Spectrometer (IBIS) and Rapid
Oscillations of the Solar Atmosphere (ROSA) instruments as well as
the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics
Observatory (SDO). We develop an algorithm to track EBs through their
evolution, finding that EBs can often be much smaller (around 0.3″)
and shorter-lived (less than one minute) than previous estimates. A
correlation between G-band magnetic bright points and EBs is also
found. Combining SDO/HMI and G-band data gives a good proxy of the
polarity for the vertical magnetic field. It is found that EBs often
occur both over regions of opposite polarity flux and strong unipolar
fields, possibly hinting at magnetic reconnection as a driver of these
events.The energetics of EB events is found to follow a power-law
distribution in the range of a nanoflare (1022−25 ergs).
Title: Tracking magnetic bright point motions through the solar
atmosphere
Authors: Keys, P. H.; Mathioudakis, M.; Jess, D. B.; Shelyag, S.;
Christian, D. J.; Keenan, F. P.
Bibcode: 2013MNRAS.428.3220K
Altcode: 2012MNRAS.tmp..182K; 2012arXiv1210.5904K
High-cadence, multiwavelength observations and simulations are
employed for the analysis of solar photospheric magnetic bright
points (MBPs) in the quiet Sun. The observations were obtained with
the Rapid Oscillations in the Solar Atmosphere (ROSA) imager and
the Interferometric Bidimensional Spectrometer at the Dunn Solar
Telescope. Our analysis reveals that photospheric MBPs have an average
transverse velocity of approximately 1 km s-1, whereas their
chromospheric counterparts have a slightly higher average velocity of
1.4 km s-1. Additionally, chromospheric MBPs were found to be
around 63 per cent larger than the equivalent photospheric MBPs. These
velocity values were compared with the output of numerical simulations
generated using the muram code. The simulated results were similar,
but slightly elevated, when compared to the observed data. An average
velocity of 1.3 km s-1 was found in the simulated G-band
images and an average of 1.8 km s-1 seen in the velocity
domain at a height of 500 km above the continuum formation layer. Delays
in the change of velocities were also analysed. Average delays of ∼4
s between layers of the simulated data set were established and values
of ∼29 s observed between G-band and Ca ii K ROSA observations. The
delays in the simulations are likely to be the result of oblique
granular shock waves, whereas those found in the observations are
possibly the result of a semi-rigid flux tube.
Title: Stellar Surface Magneto-convection as a Source of Astrophysical
Noise. I. Multi-component Parameterization of Absorption Line Profiles
Authors: Cegla, H. M.; Shelyag, S.; Watson, C. A.; Mathioudakis, M.
Bibcode: 2013ApJ...763...95C
Altcode: 2012arXiv1212.0236C
We outline our techniques to characterize photospheric granulation
as an astrophysical noise source. A four-component parameterization
of granulation is developed that can be used to reconstruct stellar
line asymmetries and radial velocity shifts due to photospheric
convective motions. The four components are made up of absorption
line profiles calculated for granules, magnetic intergranular lanes,
non-magnetic intergranular lanes, and magnetic bright points at disk
center. These components are constructed by averaging Fe I 6302 Å
magnetically sensitive absorption line profiles output from detailed
radiative transport calculations of the solar photosphere. Each
of the four categories adopted is based on magnetic field and
continuum intensity limits determined from examining three-dimensional
magnetohydrodynamic simulations with an average magnetic flux of 200
G. Using these four-component line profiles we accurately reconstruct
granulation profiles, produced from modeling 12 × 12 Mm2
areas on the solar surface, to within ~ ±20 cm s-1 on a
~100 m s-1 granulation signal. We have also successfully
reconstructed granulation profiles from a 50 G simulation using the
parameterized line profiles from the 200 G average magnetic field
simulation. This test demonstrates applicability of the characterization
to a range of magnetic stellar activity levels.
Title: Observations of ubiquitous compressive waves in the Sun's
chromosphere
Authors: Morton, Richard J.; Verth, Gary; Jess, David B.; Kuridze,
David; Ruderman, Michael S.; Mathioudakis, Mihalis; Erdélyi, Robertus
Bibcode: 2012NatCo...3.1315M
Altcode: 2012NatCo...3E1315M; 2013arXiv1306.4124M
The details of the mechanism(s) responsible for the observed
heating and dynamics of the solar atmosphere still remain a
mystery. Magnetohydrodynamic waves are thought to have a vital role
in this process. Although it has been shown that incompressible
waves are ubiquitous in off-limb solar atmospheric observations,
their energy cannot be readily dissipated. Here we provide, for the
first time, on-disk observation and identification of concurrent
magnetohydrodynamic wave modes, both compressible and incompressible,
in the solar chromosphere. The observed ubiquity and estimated energy
flux associated with the detected magnetohydrodynamic waves suggest
the chromosphere is a vast reservoir of wave energy with the potential
to meet chromospheric and coronal heating requirements. We are also
able to propose an upper bound on the flux of the observed wave energy
that is able to reach the corona based on observational constraints,
which has important implications for the suggested mechanism(s) for
quiescent coronal heating.
Title: Coupled Effects of a Perturbation in a Complex Structure
Observed with SDO/AIA, SDO/HMI and ROSA/HARDcam
Authors: Cadavid, A. C.; Lawrence, J. K.; Christian, D. J.; Jess,
D. B.; Mathioudakis, M.
Bibcode: 2012AGUFMSH51A2197C
Altcode:
We study properties of intensity fluctuations in NOAA Active Region
11250 observed on 13 July 2011 starting at UT 13:32. Included are
data obtained in the EUV bands of the Atmospheric Imaging Assembly
on board the Solar Dynamics Observatory (SDO/AIA) as well as nearly
simultaneous observations of the chromosphere made, at much higher
spatial and temporal resolution, with the Rapid Oscillations in the
Solar Atmosphere (ROSA) and Hydrogen-Alpha Rapid Dynamics camera
(HARDcam) systems at the Dunn Solar Telescope. A complex structure
seen in both the ROSA/HARDcam and SDO data sets comprises a system of
loops extending outward from near the boundary of the leading sunspot
umbra. It is visible in the ROSA Ca II K and HARDcam Hα images, as
well as the SDO 304 Å, 171 Å and 193 Å channels, and it thus couples
the chromosphere, transition region and corona. In the ground-based
images the loop structure is 4.1 Mm long. Some 17.5 Mm, can be traced
in the SDO/AIA data. The chromospheric emissions observed by ROSA and
HARDcam appear to occupy the inner, and apparently cooler and lower,
quarter of the loop. We compare the intensity fluctuations of two points
within the structure. From alignment with SDO/HMI images we identify a
point "A" near the loop structure, which sits directly above a bipolar
magnetic feature in the photosphere. Point "B" is characteristic of
locations within the loops that are visible in both the ROSA/HARDcam
and the SDO/AIA data. The intensity traces for point A are quiet
during the first part of the data string. At time ~ 19 min they
suddenly begin a series of impulsive brightenings. In the 171 Å and
193 Å coronal lines the brightenings are localized impulses in time,
but in the transition region line at 304 Å they are more extended
in time. The intensity traces in the 304 Å line for point B shows
a quasi-periodic signal that changes properties at about 19 min. The
wavelet power spectra are characterized by two periodicities. A 6.7 min
period extends from the beginning of the series until about 25 minutes,
and another signal with period ~3 min starts at about 20 min. The 193
Å power spectrum has a characteristic period of 5 min, before the
20 min transition and a 2.5 min periodicity afterward. In the case of
HARDcam Hα data a localized 4 min periodicity can be found until about
7 min, followed by a quiet regime. After ~20 min a 2.3 min periodicity
appears. Interestingly a coronal loop visible in the 94 Å line that
is centrally located in the AR, running from the leading umbra to the
following polarity, at about time 20 min undergoes a strong brightening
beginning at the same moment all along 15 Mm of its length. The fact
that these different signals all experience a clear-cut change at time
about 20 min suggests an underlying organizing mechanism. Given that
point A has a direct connection to the photospheric magnetic bipole,
we conjecture that the whole extended structure is connected in a
complex manner to the underlying magnetic field. The periodicities
in these features may favor the wave nature rather than upflows and
interpretations will be discussed.
Title: Vortices in the Solar Photosphere
Authors: Shelyag, S.; Fedun, V.; Erdélyi, R.; Keenan, F. P.;
Mathioudakis, M.
Bibcode: 2012ASPC..463..107S
Altcode: 2012arXiv1202.1966S
Using numerical simulations of the magnetized solar photosphere
and radiative diagnostics of the simulated photospheric models,
we further analyse the physical nature of magnetic photospheric
intergranular vortices. We confirm the magnetic nature of the vortices
and find that most MHD Umov-Poynting flux is produced by horizontal
vortex motions in the magnetised intergranular lanes. In addition,
we consider possible ways to directly observe photospheric magnetic
vortices using spectropolarimetry. Although horizontal plasma motions
cannot be detected in the spectropolarimetric observations of solar disk
center, we find an observational signature of photospheric vortices in
simulated observations of Stokes-V amplitude asymmetry close to the
solar limb. Potential ways to find the vortices in the observations
are discussed.
Title: Turbulent Fluctuations in G-band and K-line Intensities
Observed with the Rapid Oscillations in the Solar Atmosphere (ROSA)
Instrument
Authors: Cadavid, A. C.; Lawrence, J. K.; Christian, D. J.; Jess,
D. B.; Mathioudakis, M.
Bibcode: 2012ASPC..463...75C
Altcode:
Using the Rapid Oscillation in the Solar Atmosphere (ROSA) instrument at
the Dunn Solar Telescope we have found that the spectra of fluctuations
of the G-band (cadence 1.05 s) and Ca II K-line (cadence 4.2 s)
intensities show correlated fluctuations above white noise out
to frequencies beyond 300 mHz and up to 70 mHz, respectively. The
noise-corrected G-band spectrum presents a scaling range (Ultra High
Frequency “UHF”) for f = 25-100 mHz, with an exponent consistent
with the presence of turbulent motions. The UHF power, is concentrated
at the locations of magnetic bright points in the intergranular lanes,
it is highly intermittent in time and characterized by a positive
kurtosis κ. Combining values of G-band and K-line intensities, the UHF
power, and κ, reveals two distinct “states” of the internetwork
solar atmosphere. State 1, with κ ≍ 6, which includes almost all
the data, is characterized by low intensities and low UHF power. State
2, with κ ≍ 3, including a very small fraction of the data, is
characterized by high intensities and high UHF power. Superposed epoch
analysis shows that for State 1, the K-line intensity presents 3.5
min chromospheric oscillations with maxima occurring 21 s after G-band
intensity maxima implying a 150-210 km effective height difference. For
State 2, the G-band and K-line intensity maxima are simultaneous,
suggesting that in the highly magnetized environment sites of G-band
and K-line emission may be spatially close together. Analysis of
observations obtained with Hinode/SOT confirm a scaling range in the
G-band spectrum up to 53 mHz also consistent with turbulent motions
as well as the identification of two distinct states in terms of the
H-line intensity and G-band power as functions of G-band intensity.
Title: The Source of 3 Minute Magnetoacoustic Oscillations in
Coronal Fans
Authors: Jess, D. B.; De Moortel, I.; Mathioudakis, M.; Christian,
D. J.; Reardon, K. P.; Keys, P. H.; Keenan, F. P.
Bibcode: 2012ApJ...757..160J
Altcode: 2012arXiv1208.3194J
We use images of high spatial, spectral, and temporal resolution,
obtained using both ground- and space-based instrumentation, to
investigate the coupling between wave phenomena observed at numerous
heights in the solar atmosphere. Analysis of 4170 Å continuum images
reveals small-scale umbral intensity enhancements, with diameters
~0farcs6, lasting in excess of 30 minutes. Intensity oscillations
of ≈3 minutes are observed to encompass these photospheric
structures, with power at least three orders of magnitude higher
than the surrounding umbra. Simultaneous chromospheric velocity and
intensity time series reveal an 87° ± 8° out-of-phase behavior,
implying the presence of standing modes created as a result of
partial wave reflection at the transition region boundary. We find a
maximum waveguide inclination angle of ≈40° between photospheric
and chromospheric heights, combined with a radial expansion factor
of <76%. An average blueshifted Doppler velocity of ≈1.5 km
s-1, in addition to a time lag between photospheric and
chromospheric oscillatory phenomena, confirms the presence of upwardly
propagating slow-mode waves in the lower solar atmosphere. Propagating
oscillations in EUV intensity are detected in simultaneous coronal
fan structures, with a periodicity of 172 ± 17 s and a propagation
velocity of 45 ± 7 km s-1. Numerical simulations reveal that
the damping of the magnetoacoustic wave trains is dominated by thermal
conduction. The coronal fans are seen to anchor into the photosphere
in locations where large-amplitude umbral dot (UD) oscillations
manifest. Derived kinetic temperature and emission measure time series
display prominent out-of-phase characteristics, and when combined with
the previously established sub-sonic wave speeds, we conclude that
the observed EUV waves are the coronal counterparts of the upwardly
propagating magnetoacoustic slow modes detected in the lower solar
atmosphere. Thus, for the first time, we reveal how the propagation
of 3 minute magnetoacoustic waves in solar coronal structures is a
direct result of amplitude enhancements occurring in photospheric UDs.
Title: Time-dependent Density Diagnostics of Solar Flare Plasmas
Using SDO/EVE
Authors: Milligan, Ryan O.; Kennedy, Michael B.; Mathioudakis, Mihalis;
Keenan, Francis P.
Bibcode: 2012ApJ...755L..16M
Altcode: 2012arXiv1207.1990M
Temporally resolved electron density measurements of solar flare plasmas
are presented using data from the EUV Variability Experiment (EVE) on
board the Solar Dynamics Observatory. The EVE spectral range contains
emission lines formed between 104 and 107 K,
including transitions from highly ionized iron (gsim10 MK). Using
three density-sensitive Fe XXI ratios, peak electron densities of
1011.2-1012.1 cm-3 were found during
four X-class flares. While previous measurements of densities at such
high temperatures were made at only one point during a flaring event,
EVE now allows the temporal evolution of these high-temperature
densities to be determined at 10 s cadence. A comparison with GOES
data revealed that the peak of the density time profiles for each line
ratio correlated well with that of the emission measure time profile
for each of the events studied.
Title: Mechanisms for MHD Poynting Flux Generation in Simulations
of Solar Photospheric Magnetoconvection
Authors: Shelyag, S.; Mathioudakis, M.; Keenan, F. P.
Bibcode: 2012ApJ...753L..22S
Altcode: 2012arXiv1206.0030S
We investigate the generation mechanisms of MHD Poynting flux in the
magnetized solar photosphere. Using radiative MHD modeling of the
solar photosphere with initial magnetic configurations that differ
in their field strength and geometry, we show the presence of two
different mechanisms for MHD Poynting flux generation in simulations
of solar photospheric magnetoconvection. The weaker mechanism is
connected to vertical transport of weak horizontal magnetic fields in
the convectively stable layers of the upper photosphere, while the
stronger is the production of Poynting flux in strongly magnetized
intergranular lanes experiencing horizontal vortex motions. These
mechanisms may be responsible for the energy transport from the solar
convection zone to the higher layers of the solar atmosphere.
Title: Solar Flare Observations of the EUV Continua
Authors: Milligan, Ryan O.; Chamberlin, P.; Hudson, H.; Woods, T.;
Mathioudakis, M.; Fletcher, L.; Kowalski, A.; Keenan, F.
Bibcode: 2012AAS...22052105M
Altcode:
Recent solar flare simulations suggest that the energy deposited in the
chromosphere by nonthermal electrons during a flare's impulsive phase
is re-emitted in the form of recombination (free-bound) continua, in
particular, the Lyman, Balmer, and Paschen continua of hydrogen, and
the He I and He II continua (Allred et al. 2005). However, definitive
observations of free-bound emission during solar flares have been scarce
in recent years as many modern, space-based instruments do not have
the required sensitivity, wavelength coverage, or duty cycle. With
the launch of SDO, these observations are now routinely available
thanks to the EUV Variability Experiment (EVE) instrument. Here we
present unambiguous, spectrally and temporally resolved detections of
enhanced free-free and free-bound continua during the first X-class
solar flare of Solar Cycle 24. While we find that the flare energy
in the EVE spectral range amounts to at most a few percent of the
total flare energy, these findings highlight the capability of EVE
in giving us the first comprehensive look at these diagnostically
important continuum components.
Title: Time-resolved NUV And Optical Spectra Of A Stellar Megaflare
On YZ CMi With SALT/RSS
Authors: Brown, Benjamin; Kowalski, A. F.; Mathioudakis, M.; Hooper,
E. J.; Hawley, S. L.; Osten, R. A.; Wisniewski, J. P.
Bibcode: 2012AAS...22020452B
Altcode:
The primary mode of radiative energy release in stellar flares is
in the optical and near-ultraviolet (NUV) continuum. Active M-dwarf
stares flare more frequently than the Sun, and their flares can be
substantially more energetic. The dominant component in solar flare
white light is thought to be Hydrogen recombination, whereas for stellar
M dwarf flares, the dominant component is thought to be T 10,000 K
blackbody emission. Recently we have obtained very high time-cadence
spectral observations of the flaring M-dwarf YZ CMi (3200-6000A) using
the Robert Stobie Spectrograph on the 11-meter South African Large
Telescope (SALT/RSS), achieving 100x better temporal resolution than
has previously been possible at the atmospheric limit. We observed
a megaflare of over 100x flux enhancement in the NUV emission. Here
we discuss the evolution of the stellar flare spectrum during the
rapid impulsive phase of the flare and the implications for stellar
flare models.
Title: Transverse Oscillations in Chromospheric Mottles
Authors: Kuridze, D.; Morton, R. J.; Erdélyi, R.; Dorrian, G. D.;
Mathioudakis, M.; Jess, D. B.; Keenan, F. P.
Bibcode: 2012ApJ...750...51K
Altcode: 2012arXiv1202.5697K
A number of recent investigations have revealed that transverse
waves are ubiquitous in the solar chromosphere. The vast majority
of these have been reported in limb spicules and active region
fibrils. We investigate long-lived, quiet-Sun, on-disk features such as
chromospheric mottles (jet-like features located at the boundaries of
supergranular cells) and their transverse motions. The observations
were obtained with the Rapid Oscillations in the Solar Atmosphere
instrument at the Dunn Solar Telescope. The data set is comprised
of simultaneous imaging in the Hα core, Ca II K, and G band of an
on-disk quiet-Sun region. Time-distance techniques are used to study
the characteristics of the transverse oscillations. We detect over
40 transverse oscillations in both bright and dark mottles, with
periods ranging from 70 to 280 s, with the most frequent occurrence
at ~165 s. The velocity amplitudes and transverse displacements
exhibit characteristics similar to limb spicules. Neighboring mottles
oscillating in-phase are also observed. The transverse oscillations
of individual mottles are interpreted in terms of magnetohydrodynamic
kink waves. Their estimated periods and damping times are consistent
with phase mixing and resonant mode conversion.
Title: Solar Particle Acceleration Radiation and Kinetics (SPARK). A
mission to understand the nature of particle acceleration
Authors: Matthews, Sarah A.; Williams, David R.; Klein, Karl-Ludwig;
Kontar, Eduard P.; Smith, David M.; Lagg, Andreas; Krucker, Sam;
Hurford, Gordon J.; Vilmer, Nicole; MacKinnon, Alexander L.; Zharkova,
Valentina V.; Fletcher, Lyndsay; Hannah, Iain G.; Browning, Philippa
K.; Innes, Davina E.; Trottet, Gerard; Foullon, Clare; Nakariakov,
Valery M.; Green, Lucie M.; Lamoureux, Herve; Forsyth, Colin; Walton,
David M.; Mathioudakis, Mihalis; Gandorfer, Achim; Martinez-Pillet,
Valentin; Limousin, Olivier; Verwichte, Erwin; Dalla, Silvia; Mann,
Gottfried; Aurass, Henri; Neukirch, Thomas
Bibcode: 2012ExA....33..237M
Altcode: 2011ExA...tmp..124M
Energetic particles are critical components of plasma populations
found throughout the universe. In many cases particles are accelerated
to relativistic energies and represent a substantial fraction of
the total energy of the system, thus requiring extremely efficient
acceleration processes. The production of accelerated particles
also appears coupled to magnetic field evolution in astrophysical
plasmas through the turbulent magnetic fields produced by diffusive
shock acceleration. Particle acceleration is thus a key component
in helping to understand the origin and evolution of magnetic
structures in, e.g. galaxies. The proximity of the Sun and the range
of high-resolution diagnostics available within the solar atmosphere
offers unique opportunities to study the processes involved in particle
acceleration through the use of a combination of remote sensing
observations of the radiative signatures of accelerated particles, and
of their plasma and magnetic environment. The SPARK concept targets the
broad range of energy, spatial and temporal scales over which particle
acceleration occurs in the solar atmosphere, in order to determine how
and where energetic particles are accelerated. SPARK combines highly
complementary imaging and spectroscopic observations of radiation from
energetic electrons, protons and ions set in their plasma and magnetic
context. The payload comprises focusing-optics X-ray imaging covering
the range from 1 to 60 keV; indirect HXR imaging and spectroscopy
from 5 to 200 keV, γ-ray spectroscopic imaging with high-resolution
LaBr3 scintillators, and photometry and source localisation
at far-infrared wavelengths. The plasma environment of the regions
of acceleration and interaction will be probed using soft X-ray
imaging of the corona and vector magnetography of the photosphere
and chromosphere. SPARK is designed for solar research. However,
in addition it will be able to provide exciting new insights into the
origin of particle acceleration in other regimes, including terrestrial
gamma-ray flashes (TGF), the origin of γ-ray bursts, and the possible
existence of axions.
Title: Observations of Enhanced Extreme Ultraviolet Continua during
an X-Class Solar Flare Using SDO/EVE
Authors: Milligan, Ryan O.; Chamberlin, Phillip C.; Hudson, Hugh S.;
Woods, Thomas N.; Mathioudakis, Mihalis; Fletcher, Lyndsay; Kowalski,
Adam F.; Keenan, Francis P.
Bibcode: 2012ApJ...748L..14M
Altcode: 2012arXiv1202.1731M
Observations of extreme ultraviolet (EUV) emission from an X-class solar
flare that occurred on 2011 February 15 at 01:44 UT are presented,
obtained using the EUV Variability Experiment (EVE) on board the
Solar Dynamics Observatory. The complete EVE spectral range covers
the free-bound continua of H I (Lyman continuum), He I, and He II,
with recombination edges at 91.2, 50.4, and 22.8 nm, respectively. By
fitting the wavelength ranges blueward of each recombination edge
with an exponential function, light curves of each of the integrated
continua were generated over the course of the flare, as was emission
from the free-free continuum (6.5-37 nm). The He II 30.4 nm and Lyα
121.6 nm lines, and soft X-ray (SXR; 0.1-0.8 nm) emission from GOES are
also included for comparison. Each free-bound continuum was found to
have a rapid rise phase at the flare onset similar to that seen in the
25-50 keV light curves from RHESSI, suggesting that they were formed
by recombination with free electrons in the chromosphere. However,
the free-free emission exhibited a slower rise phase seen also in the
SXR emission from GOES, implying a predominantly coronal origin. By
integrating over the entire flare the total energy emitted via
each process was determined. We find that the flare energy in the
EVE spectral range amounts to at most a few percent of the total
flare energy, but EVE gives us a first comprehensive look at these
diagnostically important continuum components.
Title: Stellar jitter from variable gravitational redshift:
implications for radial velocity confirmation of habitable exoplanets
Authors: Cegla, H. M.; Watson, C. A.; Marsh, T. R.; Shelyag, S.;
Moulds, V.; Littlefair, S.; Mathioudakis, M.; Pollacco, D.; Bonfils, X.
Bibcode: 2012MNRAS.421L..54C
Altcode: 2011arXiv1112.1553C
A variation of gravitational redshift, arising from stellar radius
fluctuations, will introduce astrophysical noise into radial velocity
measurements by shifting the centroid of the observed spectral
lines. Shifting the centroid does not necessarily introduce line
asymmetries. This is fundamentally different from other types
of stellar jitter so far identified, which do result from line
asymmetries. Furthermore, only a very small change in stellar radius,
∼0.01 per cent, is necessary to generate a gravitational redshift
variation large enough to mask or mimic an Earth-twin. We explore
possible mechanisms for stellar radius fluctuations in low-mass
stars. Convective inhibition due to varying magnetic field strengths and
the Wilson depression of starspots are both found to induce substantial
gravitational redshift variations. Finally, we investigate a possible
method for monitoring/correcting this newly identified potential source
of jitter and comment on its impact for future exoplanet searches.
Title: Propagating Wave Phenomena Detected in Observations and
Simulations of the Lower Solar Atmosphere
Authors: Jess, D. B.; Shelyag, S.; Mathioudakis, M.; Keys, P. H.;
Christian, D. J.; Keenan, F. P.
Bibcode: 2012ApJ...746..183J
Altcode: 2012arXiv1201.1981J
We present high-cadence observations and simulations of the solar
photosphere, obtained using the Rapid Oscillations in the Solar
Atmosphere imaging system and the MuRAM magnetohydrodynamic (MHD) code,
respectively. Each data set demonstrates a wealth of magnetoacoustic
oscillatory behavior, visible as periodic intensity fluctuations
with periods in the range 110-600 s. Almost no propagating waves with
periods less than 140 s and 110 s are detected in the observational
and simulated data sets, respectively. High concentrations of power
are found in highly magnetized regions, such as magnetic bright points
and intergranular lanes. Radiative diagnostics of the photospheric
simulations replicate our observational results, confirming that the
current breed of MHD simulations are able to accurately represent the
lower solar atmosphere. All observed oscillations are generated as
a result of naturally occurring magnetoconvective processes, with no
specific input driver present. Using contribution functions extracted
from our numerical simulations, we estimate minimum G-band and 4170 Å
continuum formation heights of 100 km and 25 km, respectively. Detected
magnetoacoustic oscillations exhibit a dominant phase delay of -8°
between the G-band and 4170 Å continuum observations, suggesting
the presence of upwardly propagating waves. More than 73% of MBPs
(73% from observations and 96% from simulations) display upwardly
propagating wave phenomena, suggesting the abundant nature of
oscillatory behavior detected higher in the solar atmosphere may be
traced back to magnetoconvective processes occurring in the upper
layers of the Sun's convection zone.
Title: The Origin of Type I Spicule Oscillations
Authors: Jess, D. B.; Pascoe, D. J.; Christian, D. J.; Mathioudakis,
M.; Keys, P. H.; Keenan, F. P.
Bibcode: 2012ApJ...744L...5J
Altcode: 2011arXiv1111.5464J
We use images of high spatial and temporal resolution, obtained with
the Rapid Oscillations in the Solar Atmosphere instrument at the Dunn
Solar Telescope, to reveal how the generation of transverse waves
in Type I spicules is a direct result of longitudinal oscillations
occurring in the photosphere. Here we show how pressure oscillations,
with periodicities in the range of 130-440 s, manifest in small-scale
photospheric magnetic bright points, and generate kink waves in
the Sun's outer atmosphere with transverse velocities approaching
the local sound speed. Through comparison of our observations with
advanced two-dimensional magnetohydrodynamic simulations, we provide
evidence for how magnetoacoustic oscillations, generated at the
solar surface, funnel upward along Type I spicule structures, before
undergoing longitudinal-to-transverse mode conversion into waves at
twice the initial driving frequency. The resulting kink modes are
visible in chromospheric plasma, with periodicities of 65-220 s, and
amplitudes often exceeding 400 km. A sausage mode oscillation also
arises as a consequence of the photospheric driver, which is visible
in both simulated and observational time series. We conclude that the
mode conversion and period modification is a direct consequence of
the 90° phase shift encompassing opposite sides of the photospheric
driver. The chromospheric energy flux of these waves are estimated to
be ≈3 × 105 W m-2, which indicates that they
are sufficiently energetic to accelerate the solar wind and heat the
localized corona to its multi-million degree temperatures.
Title: Towards Earth-like Worlds: Identifying and Removing Stellar
Jitter
Authors: Cegla, Heather M.; Watson, C.; Marsh, T.; Shelyag, S.;
Moulds, V.; Littlefair, S.; Mathioudakis, M.; Pollacco, D.; Bonfils, X.
Bibcode: 2012AAS...21943203C
Altcode:
Space-based, photometric surveys have moved us into a new era
of exoplanet discovery. In order to confirm the masses and hence
the planetary nature of exoplanet candidates from such surveys,
radial velocity (RV) follow up is mandatory. To do this for low-mass
planets typically requires cm/s RV precision. However, astrophysical
noise sources (or stellar jitter) due to spots, plages, granulation
and stellar oscillations, for example, become an issue at the m/s
level. These phenomena alter the shape of the stellar absorption lines,
injecting spurious or systematic RV signals that may mask or mimic
planetary signals. As such, `quiet’ stars (those with little activity)
are the most likely candidates for the detection of low-mass planets,
but even these stars will still exhibit some stellar jitter. We present
our techniques to explore the stellar jitter due to granulation through
the use of sophisticated 3D magnetohydrodynamical simulations of the
Sun. In addition, we also present the identification of an entirely new
source of stellar jitter that has been hitherto unrecognized and that
could impact the RV follow-up and confirmation of low-mass terrestrial
planets and Earth-like worlds.
Title: White Light Flare Continuum Observations with ULTRACAM
Authors: Kowalski, A. F.; Mathioudakis, M.; Hawley, S. L.; Hilton,
E. J.; Dhillon, V. S.; Marsh, T. R.; Copperwheat, C. M.
Bibcode: 2011ASPC..448.1157K
Altcode: 2011arXiv1103.0822K; 2011csss...16.1157K
We present sub-second, continuous-coverage photometry of three flares
on the dM3.5e star, EQ Peg A, using custom continuum filters with
WHT/ULTRACAM. These data provide a new view of flare continuum emission,
with each flare exhibiting a very distinct light curve morphology. The
spectral shape of flare emission for the two large-amplitude flares
is compared with synthetic ULTRACAM measurements taken from the
spectra during the large 'megaflare' event on a similar type flare
star. The white light shape during the impulsive phase of the EQ
Peg flares is consistent with the range of colors derived from the
megaflare continuum, which is known to contain a Hydrogen recombination
component and compact, blackbody-like components. Tentative evidence
in the ULTRACAM photometry is found for an anti-correlation between
the emission of these components.
Title: Rapid Fluctuations in the Lower Solar Atmosphere
Authors: Lawrence, J. K.; Cadavid, A. C.; Christian, D. J.; Jess,
D. B.; Mathioudakis, M.
Bibcode: 2011ApJ...743L..24L
Altcode: 2011arXiv1111.4253L
The Rapid Oscillations in the Solar Atmosphere instrument reveals solar
atmospheric fluctuations at high frequencies. Spectra of variations
of the G-band intensity (IG ) and Ca II K-line intensity
(IK ) show correlated fluctuations above white noise to
frequencies beyond 300 mHz and 50 mHz, respectively. The noise-corrected
G-band spectrum for f = 28-326 mHz shows a power law with exponent -1.21
± 0.02, consistent with the presence of turbulent motions. G-band
spectral power in the 25-100 mHz ("UHF") range is concentrated at
the locations of magnetic bright points in the intergranular lanes
and is highly intermittent in time. The intermittence of the UHF
G-band fluctuations, shown by a positive kurtosis κ, also suggests
turbulence. Combining values of IG , IK , UHF
power, and κ reveals two distinct states of the solar atmosphere. State
1, including almost all the data, is characterized by low IG
, IK , and UHF power and κ ≈ 6. State 2, including only a
very small fraction of the data, is characterized by high IG
, IK , and UHF power and κ ≈ 3. Superposed epoch analysis
shows that the UHF power peaks simultaneously with spatio-temporal
IG maxima in either state. For State 1, IK
shows 3.5 minute chromospheric oscillations with maxima occurring 21
s after IG maxima implying a 150-210 km effective height
difference. However, for State 2 the IK and IG
maxima are simultaneous; in this highly magnetized environment sites
of G-band and K-line emission may be spatially close together.
Title: The Velocity Distribution of Solar Photospheric Magnetic
Bright Points
Authors: Keys, P. H.; Mathioudakis, M.; Jess, D. B.; Shelyag, S.;
Crockett, P. J.; Christian, D. J.; Keenan, F. P.
Bibcode: 2011ApJ...740L..40K
Altcode: 2011arXiv1109.3565K
We use high spatial resolution observations and numerical simulations to
study the velocity distribution of solar photospheric magnetic bright
points. The observations were obtained with the Rapid Oscillations in
the Solar Atmosphere instrument at the Dunn Solar Telescope, while the
numerical simulations were undertaken with the MURaM code for average
magnetic fields of 200 G and 400 G. We implemented an automated bright
point detection and tracking algorithm on the data set and studied
the subsequent velocity characteristics of over 6000 structures,
finding an average velocity of approximately 1 km s-1, with
maximum values of 7 km s-1. Furthermore, merging magnetic
bright points were found to have considerably higher velocities,
and significantly longer lifetimes, than isolated structures. By
implementing a new and novel technique, we were able to estimate the
background magnetic flux of our observational data, which is consistent
with a field strength of 400 G.
Title: A search for line intensity enhancements in the far-UV spectra
of active late-type stars arising from opacity
Authors: Keenan, F. P.; Christian, D. J.; Rose, S. J.; Mathioudakis, M.
Bibcode: 2011A&A...534A..71K
Altcode: 2011arXiv1109.0513K
Context. Radiative transfer calculations have predicted intensity
enhancements for optically thick emission lines, as opposed to the
normal intensity reductions, for astrophysical plasmas under certain
conditions. In particular, the results are predicted to be dependent
both on the geometry of the emitting plasma and the orientation of the
observer. Hence in principle the detection of intensity enhancement
may provide a way of determining the geometry of an unresolved
astronomical source.
Aims: To investigate such enhancements
we have analysed a sample of active late-type stars observed in the
far ultraviolet spectral region.
Methods: Emission lines of O
vi in the FUSE satellite spectra of ɛ Eri, II Peg and Prox Cen were
searched for intensity enhancements due to opacity.
Results:
We have found strong evidence for line intensity enhancements due to
opacity during active or flare-like activity for all three stars. The
O vi 1032/1038 line intensity ratios, predicted to have a value of
2.0 in the optically thin case, are found to be up to ~30% larger
during several orbital phases.
Conclusions: Our measurements,
combined with radiative transfer models, allow us to constrain both
the geometry of the O vi emitting regions in our stellar sources and
the orientation of the observer. A spherical emitting plasma can be
ruled out, as this would lead to no intensity enhancement. In addition,
the theory tells us that the line-of-sight to the plasma must be close
to perpendicular to its surface, as observations at small angles to
the surface lead to either no intensity enhancement or the usual line
intensity decrease over the optically thin value. For the future, we
outline a laboratory experiment, that could be undertaken with current
facilities, which would provide an unequivocal test of predictions of
line intensity enhancement due to opacity, in particular the dependence
on plasma geometry.
Title: The Search for Super-saturation in Chromospheric Emission
Authors: Christian, Damian J.; Mathioudakis, Mihalis; Arias, Tersi;
Jardine, Moira; Jess, David B.
Bibcode: 2011ApJ...738..164C
Altcode: 2011arXiv1107.1786C
We investigate if the super-saturation phenomenon observed at X-ray
wavelengths for the corona exists in the chromosphere for rapidly
rotating late-type stars. Moderate resolution optical spectra
of fast-rotating EUV- and X-ray-selected late-type stars were
obtained. Stars in α Per were observed in the northern hemisphere
with the Isaac Newton 2.5 m telescope and Intermediate Dispersion
Spectrograph. Selected objects from IC 2391 and IC 2602 were observed
in the southern hemisphere with the Blanco 4 m telescope and R-C
spectrograph at CTIO. Ca II H and K fluxes were measured for all
stars in our sample. We find the saturation level for Ca II K at log
(L CaK/L bol) = -4.08. The Ca II K flux does
not show a decrease as a function of increased rotational velocity
or smaller Rossby number as observed in the X-ray. This lack of
"super-saturation" supports the idea of coronal stripping as the cause
of saturation and super-saturation in stellar chromospheres and coronae,
but the detailed underlying mechanism is still under investigation.
Title: Small-scale Hα jets in the solar chromosphere
Authors: Kuridze, D.; Mathioudakis, M.; Jess, D. B.; Shelyag, S.;
Christian, D. J.; Keenan, F. P.; Balasubramaniam, K. S.
Bibcode: 2011A&A...533A..76K
Altcode: 2011arXiv1108.1043K
Aims: High temporal and spatial resolution observations from
the Rapid Oscillations in the Solar Atmosphere (ROSA) multiwavelength
imager on the Dunn Solar Telescope are used to study the velocities
of small-scale Hα jets in an emerging solar active region.
Methods: The dataset comprises simultaneous imaging in the Hα
core, Ca ii K, and G band, together with photospheric line-of-sight
magnetograms. Time-distance techniques are employed to determine
projected plane-of-sky velocities.
Results: The Hα images
are highly dynamic in nature, with estimated jet velocities as high
as 45 km s-1. These jets are one-directional, with their
origin seemingly linked to underlying Ca ii K brightenings and G-band
magnetic bright points.
Conclusions: It is suggested that the
siphon flow model of cool coronal loops is suitable for interpreting our
observations. The jets are associated with small-scale explosive events,
and may provide a mass outflow from the photosphere to the corona.
Title: Rapid Oscillations in the Solar Atmosphere: Spectra and
Physical Effects
Authors: Lawrence, John K.; Christian, D. J.; Cadavid, A. C.; Jess,
D. B.; Mathioudakis, M.
Bibcode: 2011SPD....42.1727L
Altcode: 2011BAAS..43S.1727L
High-frequency fluctuations are observed with the Rapid Oscillations in
the Solar Atmosphere (ROSA) instrument (Jess et al. 2010, Solar Phys,
261, 363) at the Dunn Solar Telescope. This can produce simultaneous
observations in up to six channels, at different heights in the
photosphere and chromosphere, at an unprecedentedly high cadence of 0.5
seconds, and at a spatial resolution of 100 km after photometrically
correct speckle reconstruction. Here we concentrate on observations
at two levels. The first is in the G-band of the CH radical at 4305.5Å,
bandpass 9.2Å, with height of formation z < 250 km at a cadence
of 0.525 sec corresponding to Nyquist frequency 950 mHz. The second
is in the Ca II K-line core at 3933.7Å, bandpass 1.0Å, with height
of formation z < 1300 km, and cadence 4.2 sec giving Nyquist
frequency 120 mHz. The data span 53 min, and the maximum field of
view is 45 Mm. The data were taken on 28 May 2009 in internetwork
and network near disk center. Using both Fourier and Morlet
wavelet methods we find evidence in the G-band spectra for intensity
fluctuations above noise out to frequencies f >> 100 mHz. The
K-line signal is noisier and is seen only for f < 50 mHz. With
wavelet techniques we find that G-band spectral power with 20 <
f < 100 mHz is clearly concentrated in the intergranular lanes and
especially at the locations of magnetic elements indicated by G-band
bright points. This wavelet power is highly intermittent in time. By
cross-correlating the data we find that pulses of high-frequency G-band
power in the photosphere tend to be followed by increases in K-line
emission in the chromosphere with a time lag of about 2 min.
Title: Chromospheric velocities of a C-class flare
Authors: Keys, P. H.; Jess, D. B.; Mathioudakis, M.; Keenan, F. P.
Bibcode: 2011A&A...529A.127K
Altcode: 2011arXiv1103.4252K
Aims: We use high spatial and temporal resolution observations
from the Swedish Solar Telescope to study the chromospheric velocities
of a C-class flare originating from active region NOAA 10969.
Methods: A time-distance analysis is employed to estimate directional
velocity components in Hα and Ca ii K image sequences. Also, imaging
spectroscopy has allowed us to determine flare-induced line-of-sight
velocities. A wavelet analysis is used to analyse the periodic nature
of associated flare bursts.
Results: Time-distance analysis
reveals velocities as high as 64 km s-1 along the flare
ribbon and 15 km s-1 perpendicular to it. The velocities are
very similar in both the Hα and Ca ii K time series. Line-of-sight Hα
velocities are red-shifted with values up to 17 km s-1. The
high spatial and temporal resolution of the observations have allowed
us to detect velocities significantly higher than those found in
earlier studies. Flare bursts with a periodicity of ≈60 s are also
detected. These bursts are similar to the quasi-periodic oscillations
observed at hard X-ray and radio wavelength data.
Conclusions:
Some of the highest velocities detected in the solar atmosphere are
presented. Line-of-sight velocity maps show considerable mixing of
both the magnitude and direction of velocities along the flare path. A
change in direction of the velocities at the flare kernel has also been
detected which may be a signature of chromospheric evaporation. Movies associated to Fig. 5 are only available in electronic form
at http://www.aanda.org
Title: Observations of Sausage Modes in Magnetic Pores
Authors: Morton, R. J.; Erdélyi, R.; Jess, D. B.; Mathioudakis, M.
Bibcode: 2011ApJ...729L..18M
Altcode: 2010arXiv1011.2375M
We present here evidence for the observation of the magnetohydrodynamic
(MHD) sausage modes in magnetic pores in the solar photosphere. Further
evidence for the omnipresent nature of acoustic global modes is also
found. The empirical decomposition method of wave analysis is used to
identify the oscillations detected through a 4170 Å "blue continuum"
filter observed with the Rapid Oscillations in the Solar Atmosphere
(ROSA) instrument. Out of phase, periodic behavior in pore size and
intensity is used as an indicator of the presence of magnetoacoustic
sausage oscillations. Multiple signatures of the magnetoacoustic
sausage mode are found in a number of pores. The periods range from
as short as 30 s up to 450 s. A number of the magnetoacoustic sausage
mode oscillations found have periods of 3 and 5 minutes, similar to
the acoustic global modes of the solar interior. It is proposed that
these global oscillations could be the driver of the sausage-type
magnetoacoustic MHD wave modes in pores.
Title: Vorticity in the solar photosphere
Authors: Shelyag, S.; Keys, P.; Mathioudakis, M.; Keenan, F. P.
Bibcode: 2011A&A...526A...5S
Altcode: 2010arXiv1010.5604S
Aims: We use magnetic and non-magnetic 3D numerical simulations
of solar granulation and G-band radiative diagnostics from the resulting
models to analyse the generation of small-scale vortex motions in
the solar photosphere.
Methods: Radiative MHD simulations of
magnetoconvection are used to produce photospheric models. Our starting
point is a non-magnetic model of solar convection, where we introduce
a uniform magnetic field and follow the evolution of the field in the
simulated photosphere. We find two different types of photospheric
vortices, and provide a link between the vorticity generation and the
presence of the intergranular magnetic field. A detailed analysis of
the vorticity equation, combined with the G-band radiative diagnostics,
allows us to identify the sources and observational signatures of
photospheric vorticity in the simulated photosphere.
Results: Two
different types of photospheric vorticity, magnetic and non-magnetic,
are generated in the domain. Non-magnetic vortices are generated by the
baroclinic motions of the plasma in the photosphere, while magnetic
vortices are produced by the magnetic tension in the intergranular
magnetic flux concentrations. The two types of vortices have different
shapes. We find that the vorticity is generated more efficiently in
the magnetised model. Simulated G-band images show a direct connection
between magnetic vortices and rotary motions of photospheric bright
points, and suggest that there may be a connection between the magnetic
bright point rotation and small-scale swirl motions observed higher
in the atmosphere.
Title: The Area Distribution of Solar Magnetic Bright Points
Authors: Crockett, P. J.; Mathioudakis, M.; Jess, D. B.; Shelyag,
S.; Keenan, F. P.; Christian, D. J.
Bibcode: 2010ApJ...722L.188C
Altcode: 2010arXiv1009.2410C
Magnetic bright points (MBPs) are among the smallest observable objects
on the solar photosphere. A combination of G-band observations and
numerical simulations is used to determine their area distribution. An
automatic detection algorithm, employing one-dimensional intensity
profiling, is utilized to identify these structures in the observed and
simulated data sets. Both distributions peak at an area of ≈45,000
km2, with a sharp decrease toward smaller areas. The
distributions conform with log-normal statistics, which suggests
that flux fragmentation dominates over flux convergence. Radiative
magneto-convection simulations indicate an independence in the MBP
area distribution for differing magnetic flux densities. The most
commonly occurring bright point size corresponds to the typical width
of inter-granular lanes.
Title: A Study of Magnetic Bright Points in the Na I D1
Line
Authors: Jess, D. B.; Mathioudakis, M.; Christian, D. J.; Crockett,
P. J.; Keenan, F. P.
Bibcode: 2010ApJ...719L.134J
Altcode: 2010arXiv1007.2515J
High-cadence, multiwavelength, optical observations of solar magnetic
bright points (MBPs), captured at the disk center using the ROSA and
IBIS imaging systems on the Dunn Solar Telescope, are presented. MBPs
manifesting in the Na I D1 core are found to preferentially
exist in regions containing strong downflows, in addition to cospatial
underlying photospheric magnetic field concentrations. Downdrafts
within Na I D1 bright points exhibit speeds of up
to 7 km s-1, with preferred structural symmetry in
intensity, magnetic field, and velocity profiles about the bright
point center. Excess intensities associated with G-band and Ca II K
observations of MBPs reveal a power-law trend when plotted as a function
of the magnetic flux density. However, Na I D1 observations
of the same magnetic features indicate an intensity plateau at weak
magnetic field strengths below ≈150 G, suggesting the presence of a
two-component heating process: one which is primarily acoustic and the
other predominantly magnetic. We suggest that this finding is related to
the physical expansion of magnetic flux tubes, with weak field strengths
(≈50 G) expanding by ~76%, compared to a ~44% expansion when higher
field strengths (≈150 G) are present. These observations provide the
first experimental evidence of rapid downdrafts in Na I D1
MBPs and reveal the nature of a previously unresolved intensity plateau
associated with these structures.
Title: A photospheric bright point model
Authors: Shelyag, S.; Mathioudakis, M.; Keenan, F. P.; Jess, D. B.
Bibcode: 2010A&A...515A.107S
Altcode: 2010arXiv1003.1653S
Aims: A magneto-hydrostatic model is constructed with
spectropolarimetric properties close to those of solar photospheric
magnetic bright points.
Methods: Results of solar radiative
magneto-convection simulations are used to produce the spatial
structure of the vertical component of the magnetic field. The
horizontal component of magnetic field is reconstructed using the
self-similarity condition, while the magneto-hydrostatic equilibrium
condition is applied to the standard photospheric model with the
magnetic field embedded. Partial ionisation processes are found to be
necessary for reconstructing the correct temperature structure of the
model.
Results: The structures obtained are in good agreement
with observational data. By combining the realistic structure of
the magnetic field with the temperature structure of the quiet solar
photosphere, the continuum formation level above the equipartition
layer can be found. Preliminary results are shown of wave propagation
through this magnetic structure. The observational consequences of the
oscillations are examined in continuum intensity and in the Fe I 6302
Å magnetically sensitive line.
Title: Emission lines of FeXI in the 257-407Å wavelength region
observed in solar spectra from EIS/Hinode and SERTS
Authors: Keenan, F. P.; Milligan, R. O.; Jess, D. B.; Aggarwal, K. M.;
Mathioudakis, M.; Thomas, R. J.; Brosius, J. W.; Davila, J. M.
Bibcode: 2010MNRAS.404.1617K
Altcode: 2010MNRAS.tmp..299K; 2010arXiv1001.3627K
Theoretical emission-line ratios involving FeXI transitions in the
257-407Å wavelength range are derived using fully relativistic
calculations of radiative rates and electron impact excitation
cross-sections. These are subsequently compared with both long
wavelength channel Extreme-Ultraviolet Imaging Spectrometer (EIS)
spectra from the Hinode satellite (covering 245-291Å) and first-order
observations (~235-449Å) obtained by the Solar Extreme-ultraviolet
Research Telescope and Spectrograph (SERTS). The 266.39, 266.60 and
276.36Å lines of FeXI are detected in two EIS spectra, confirming
earlier identifications of these features, and 276.36Å is found to
provide an electron density (Ne) diagnostic when ratioed
against the 257.55Å transition. Agreement between theory and
observation is found to be generally good for the SERTS data sets,
with discrepancies normally being due to known line blends, while the
257.55Å feature is detected for the first time in SERTS spectra. The
most useful FeXI electron density diagnostic is found to be the
308.54/352.67 intensity ratio, which varies by a factor of 8.4 between
Ne = 108 and 1011cm-3,
while showing little temperature sensitivity. However, the
349.04/352.67 ratio potentially provides a superior diagnostic,
as it involves lines which are closer in wavelength, and varies
by a factor of 14.7 between Ne = 108 and
1011cm-3. Unfortunately, the 349.04Å line is
relatively weak, and also blended with the second-order FeX 174.52Å
feature, unless the first-order instrument response is enhanced.
Title: Microflare Activity Driven by Forced Magnetic Reconnection
Authors: Jess, D. B.; Mathioudakis, M.; Browning, P. K.; Crockett,
P. J.; Keenan, F. P.
Bibcode: 2010ApJ...712L.111J
Altcode: 2010arXiv1002.3792J
High cadence, multiwavelength, optical observations of a solar active
region, obtained with the Swedish Solar Telescope, are presented. Two
magnetic bright points are seen to separate in opposite directions
at a constant velocity of 2.8 km s-1. After a separation
distance of ≈4400 km is reached, multiple Ellerman bombs are
observed in both Hα and Ca-K images. As a result of the Ellerman
bombs, periodic velocity perturbations in the vicinity of the magnetic
neutral line, derived from simultaneous Michelson Doppler Imager data,
are generated with amplitude ±6 km s-1 and wavelength
≈1000 km. The velocity oscillations are followed by an impulsive
brightening visible in Hα and Ca-K, with a peak intensity enhancement
of 63%. We interpret these velocity perturbations as the magnetic field
deformation necessary to trigger forced reconnection. A time delay of
≈3 minutes between the Hα-wing and Ca-K observations indicates that
the observed magnetic reconnection occurs at a height of ~200 km above
the solar surface. These observations are consistent with theoretical
predictions and provide the first observational evidence of microflare
activity driven by forced magnetic reconnection.
Title: ROSA: A High-cadence, Synchronized Multi-camera Solar Imaging
System
Authors: Jess, D. B.; Mathioudakis, M.; Christian, D. J.; Keenan,
F. P.; Ryans, R. S. I.; Crockett, P. J.
Bibcode: 2010SoPh..261..363J
Altcode: 2009arXiv0912.4118J; 2010SoPh..tmp...12J
The Rapid Oscillations in the Solar Atmosphere (ROSA) instrument
is a synchronized, six-camera high-cadence solar imaging instrument
developed by Queen's University Belfast. The system is available on
the Dunn Solar Telescope at the National Solar Observatory in Sunspot,
New Mexico, USA, as a common-user instrument. Consisting of six 1k
× 1k Peltier-cooled frame-transfer CCD cameras with very low noise
(0.02 - 15 e s−1 pixel−1), each ROSA camera
is capable of full-chip readout speeds in excess of 30 Hz, or 200 Hz
when the CCD is windowed. Combining multiple cameras and fast readout
rates, ROSA will accumulate approximately 12 TB of data per 8 hours
observing. Following successful commissioning during August 2008,
ROSA will allow for multi-wavelength studies of the solar atmosphere
at a high temporal resolution.
Title: Automated detection and tracking of solar magnetic bright
points
Authors: Crockett, P. J.; Jess, D. B.; Mathioudakis, M.; Keenan, F. P.
Bibcode: 2009MNRAS.397.1852C
Altcode: 2009MNRAS.tmp..999C; 2009arXiv0905.3138C
Magnetic bright points (MBPs) in the internetwork are among the
smallest objects in the solar photosphere and appear bright against the
ambient environment. An algorithm is presented that can be used for the
automated detection of the MBPs in the spatial and temporal domains. The
algorithm works by mapping the lanes through intensity thresholding. A
compass search, combined with a study of the intensity gradient across
the detected objects, allows the disentanglement of MBPs from bright
pixels within the granules. Object growing is implemented to account
for any pixels that might have been removed when mapping the lanes. The
images are stabilized by locating long-lived objects that may have
been missed due to variable light levels and seeing quality. Tests of
the algorithm, employing data taken with the Swedish Solar Telescope,
reveal that ~90 per cent of MBPs within a 75 × 75 arcsec2
field of view are detected.
Title: Possible Connection Between Umbral and Bright Point
Oscillations
Authors: Andic, Aleksandra; Jess, D. B.; Mathioudakis, M.; Keenan,
F. P.; Bloomfield, D. S.
Bibcode: 2009SPD....40.0909A
Altcode:
Active region NOAA 10794 was observed using the Rapid Dual Imager
(RDI) instrument on the Dunn Solar Telescope at the National Solar
Observatory, Sacramento Peak. G-band and H-alpha filters were employed
for the observations, and the resulting data were subjected to both
speckle reconstruction and wavelet analyses. Intensity oscillations
in the period range 8--584 s were detected. Those with frequencies
near the Nyquist value of 125 mHz had a similar behavior to other
oscillations detected in the period range of 8 - 60 s. However,
oscillations in the range around 180s observed above bright points
show a temporal coincidence in the light curve shape and the number
of the emitted frequencies in an umbral flash, indicating a possible
connection.
Title: Alfvén Waves in the Lower Solar Atmosphere
Authors: Jess, David B.; Mathioudakis, Mihalis; Erdélyi, Robert;
Crockett, Philip J.; Keenan, Francis P.; Christian, Damian J.
Bibcode: 2009Sci...323.1582J
Altcode: 2009arXiv0903.3546J
The flow of energy through the solar atmosphere and the heating of the
Sun’s outer regions are still not understood. Here, we report the
detection of oscillatory phenomena associated with a large bright-point
group that is 430,000 square kilometers in area and located near the
solar disk center. Wavelet analysis reveals full-width half-maximum
oscillations with periodicities ranging from 126 to 700 seconds
originating above the bright point and significance levels exceeding
99%. These oscillations, 2.6 kilometers per second in amplitude,
are coupled with chromospheric line-of-sight Doppler velocities
with an average blue shift of 23 kilometers per second. A lack of
cospatial intensity oscillations and transversal displacements rules
out the presence of magneto-acoustic wave modes. The oscillations
are a signature of Alfvén waves produced by a torsional twist of
±22 degrees. A phase shift of 180 degrees across the diameter of the
bright point suggests that these torsional Alfvén oscillations are
induced globally throughout the entire brightening. The energy flux
associated with this wave mode is sufficient to heat the solar corona.
Title: Ultraviolet and extreme-ultraviolet line ratio diagnostics
for O IV
Authors: Keenan, F. P.; Crockett, P. J.; Aggarwal, K. M.; Jess, D. B.;
Mathioudakis, M.
Bibcode: 2009A&A...495..359K
Altcode: 2009arXiv0901.2222K
Aims: We generate theoretical ultraviolet and extreme-ultraviolet
emission line ratios for O IV and show their strong versatility
as electron temperature and density diagnostics for astrophysical
plasmas.
Methods: Recent fully relativistic calculations of
radiative rates and electron impact excitation cross sections for O
IV, supplemented with earlier data for A-values and proton excitation
rates, are used to derive theoretical O IV line intensity ratios for
a wide range of electron temperatures and densities.
Results:
Diagnostic line ratios involving ultraviolet or extreme-ultraviolet
transitions in O IV are presented, that are applicable to a wide variety
of astrophysical plasmas ranging from low density gaseous nebulae to the
densest solar and stellar flares. Comparisons with observational data,
where available, show good agreement between theory and experiment,
providing support for the accuracy of the diagnostics. However,
diagnostics are also presented involving lines that are blended in
existing astronomical spectra, in the hope this might encourage further
observational studies at higher spectral resolution.
Title: Do All Flares Have White-Light Emission?
Authors: Jess, D. B.; Mathioudakis, M.; Crockett, P. J.; Keenan, F. P.
Bibcode: 2008ApJ...688L.119J
Altcode: 2008arXiv0810.1443J
High-cadence, multiwavelength optical observations of a solar active
region (NOAA AR 10969), obtained with the Swedish Solar Telescope,
are presented. Difference imaging of white light continuum data
reveals a white-light brightening, 2 minutes in duration, linked to a
cotemporal and cospatial C2.0 flare event. The flare kernel observed
in the white-light images has a diameter of 300 km, thus rendering it
below the resolution limit of most space-based telescopes. Continuum
emission is present only during the impulsive stage of the flare,
with the effects of chromospheric emission subsequently delayed
by ≈2 minutes. The localized flare emission peaks at 300% above
the quiescent flux. This large, yet tightly confined, increase in
emission is only resolvable due to the high spatial resolution of
the Swedish Solar Telescope. An investigation of the line-of-sight
magnetic field derived from simultaneous MDI data shows that the
continuum brightening is located very close to a magnetic polarity
inversion line. In addition, an Hα flare ribbon is directed along
a region of rapid magnetic energy change, with the footpoints of the
ribbon remaining cospatial with the observed white-light brightening
throughout the duration of the flare. The observed flare parameters
are compared with current observations and theoretical models for M-
and X-class events and we determine the observed white-light emission
is caused by radiative back-warming. We suggest that the creation of
white-light emission is a common feature of all solar flares.
Title: 6Li in the Atmosphere of GJ 117 Revisited
Authors: Christian, D. J.; Mathioudakis, M.; Jevremović, D.
Bibcode: 2008ApJ...686..542C
Altcode: 2008arXiv0807.3693C
Detection of 6Li has been shown for energetic solar events,
one chromospherically active binary, and several dwarf halo stars. We
had previously found a 6Li/7Li = 0.03 +/- 0.01
for active K dwarf GJ 117 using VLT UVES observations. Here we present
high signal-to-noise (>1000) high spectral resolution observations
taken with the McDonald Observatory's 2.7 m and echelle spectrometer
of GJ 117. We have used the solar spectrum and template stars to
eliminate possible blends, such as Ti I, in the 6Li spectral
region. Our new analysis, using an updated PHOENIX model atmosphere,
finds 6Li/7Li = 0.05 +/- 0.02. In addition,
bisector analysis showed no significant red asymmetries that would
affect the lithium line profile. No changes above the statistical
uncertainties are found between the VLT and McDonald data. The amount
of 6Li derived for GJ 117 is consistent with creation
in spallation reactions on the stellar surface, but we caution that
uncertainties in the continuum level may cause additional uncertainty
in the 6Li fraction.
Title: Transition Region Velocity Oscillations Observed by EUNIS-06
Authors: Jess, D. B.; Rabin, D. M.; Thomas, R. J.; Brosius, J. W.;
Mathioudakis, M.; Keenan, F. P.
Bibcode: 2008ApJ...682.1363J
Altcode: 2008arXiv0804.1629J
Spectroscopic measurements of NOAA AR 10871, obtained with the Extreme
Ultraviolet Normal Incidence Spectrograph (EUNIS) sounding rocket
instrument on 2006 April 12, reveal velocity oscillations in the He
II 303.8 Å emission line formed at T ≈ 5 × 104 K. The
oscillations appear to arise in a bright active region loop arcade
about 25'' wide which crosses the EUNIS slit. The period
of these transition region oscillations is 26 +/- 4 s, coupled with
a velocity amplitude of ±10 km s-1, detected over four
complete cycles. Similar oscillations are observed in lines formed at
temperatures up to T ≈ 4 × 105 K, but we find no evidence
for the coupling of these velocity oscillations with corresponding
phenomena in the corona. We interpret the detected oscillations as
originating from an almost purely adiabatic plasma, and infer that
they are generated by the resonant transmission of MHD waves through
the lower active region atmospheres. Through the use of seismological
techniques, we establish that the observed velocity oscillations display
wave properties most characteristic of fast body global sausage modes.
Title: The 2008 RAS National Astronomy Meeting
Authors: Fitzsimmons, Alan; Mathioudakis, Mihalis
Bibcode: 2008A&G....49c..11F
Altcode:
This year's RAS National Astronomy Meeting was held at Queen's
University Belfast from 31 March to 4 April. Also joining NAM 2008 were
the meetings of the UK Solar Physics (UKSP) and the Magnetospheric,
Ionospheric and Solar-Terrestrial (MIST) special interest groups. The
event was organized by QUB scientists within the Astrophysics Research
Centre and the Centre for Theoretical Atomic, Molecular and Optical
Physics. Alan Fitzsimmons, Mihalis Mathioudakis and Andrew Kavanagh
report.
Title: Discovery of Spatial Periodicities in a Coronal Loop Using
Automated Edge-Tracking Algorithms
Authors: Jess, D. B.; Mathioudakis, M.; Erdélyi, R.; Verth, G.;
McAteer, R. T. J.; Keenan, F. P.
Bibcode: 2008ApJ...680.1523J
Altcode: 2008arXiv0802.1971J
A new method for automated coronal loop tracking, in both spatial
and temporal domains, is presented. Applying this technique to TRACE
data, obtained using the 171 Å filter on 1998 July 14, we detect a
coronal loop undergoing a 270 s kink-mode oscillation, as previously
found by Aschwanden et al. However, we also detect flare-induced,
and previously unnoticed, spatial periodicities on a scale of 3500 km,
which occur along the coronal loop edge. Furthermore, we establish a
reduction in oscillatory power for these spatial periodicities of 45%
over a 222 s interval. We relate the reduction in detected oscillatory
power to the physical damping of these loop-top oscillations.
Title: Line intensity enhancements in stellar coronal X-ray spectra
due to opacity effects
Authors: Rose, S. J.; Matranga, M.; Mathioudakis, M.; Keenan, F. P.;
Wark, J. S.
Bibcode: 2008A&A...483..887R
Altcode: 2008arXiv0803.3184R
Context: The I(15.01 Å)/I(16.78 Å) emission line intensity ratio
in Fe xvii has been reported to deviate from its theoretical value
in solar and stellar X-ray spectra. This is attributed to opacity in
the 15.01 Å line, leading to a reduction in its intensity, and was
interpreted in terms of a geometry in which the emitters and absorbers
are spatially distinct.
Aims: We study the I(15.01 Å)/I(16.78
Å) intensity ratio for the active cool dwarf EV Lac, in both flare and
quiescent spectra.
Methods: The observations were obtained with
the Reflection Grating Spectrometer on the XMM-Newton satellite. The
emission measure distribution versus temperature reconstruction
technique is used for our analysis.
Results: We find that the
15.01 Å line exhibits a significant enhancement in intensity over the
optically thin value. To our knowledge, this is the first time that such
an enhancement has been detected on such a sound statistical basis. We
interpret this enhancement in terms of a geometry in which the emitters
and absorbers are not spatially distinct, and where the geometry is such
that resonant pumping of the upper level has a greater effect on the
observed line intensity than resonant absorption in the line-of-sight.
Title: High frequency oscillations in the solar chromosphere and
their connection with heating
Authors: Andic, Aleksandra; Mathioudakis, M.; Keenan, F. P.; Jess,
D. B.; Bloomfield, D. S.
Bibcode: 2008IAUS..247..312A
Altcode: 2007IAUS..247..312A
High frequency acoustic waves have been suggested as a source of
mechanical heating in the quiet solar chromosphere. To investigate
this, we have observed intensity oscillations of several lines in the
frequency interval 1.64-70mHz using data from the VTT Tenerife and the
Dunn Solar Telescope at the National Solar Observatory. Our analysis
of Fe i 543.45 nm, Fe i 543.29 nm and the G-band, indicate that the
majority of oscillations are connected with the magnetic fields and
do not provide sufficient mechanical flux for the heating of the
chromosphere. This correlation is also observed in quiet Sun areas.
Title: Solar feature tracking in both spatial and temporal domains
Authors: Jess, D. B.; Mathioudakis, M.; Erdélyi, R.; Verth, G.;
McAteer, R. T. J.; Keenan, F. P.
Bibcode: 2008IAUS..247..288J
Altcode: 2007IAUS..247..288J
A new method for automated coronal loop tracking, in both spatial and
temporal domains, is presented. The reliability of this technique
was tested with TRACE 171 Å observations. The application of this
technique to a flare-induced kink-mode oscillation, revealed a 3500
km spatial periodicity which occur along the loop edge. We establish
a reduction in oscillatory power, for these spatial periodicities,
of 45% over a 322 s interval. We relate the reduction in oscillatory
power to the physical damping of these loop-top oscillations.
Title: Twisting flux tubes as a cause of micro-flaring activity
Authors: Jess, D. B.; McAteer, R. T. J.; Mathioudakis, M.; Keenan,
F. P.; Andic, A.; Bloomfield, D. S.
Bibcode: 2008IAUS..247..360J
Altcode: 2007IAUS..247..360J
High-cadence optical observations of an H-α blue-wing bright point
near solar AR NOAA 10794 are presented. The data were obtained with the
Dunn Solar Telescope at the National Solar Observatory/Sacramento Peak
using a newly developed camera system, the rapid dual imager. Wavelet
analysis is undertaken to search for intensity-related oscillatory
signatures, and periodicities ranging from 15 to 370 s are found with
significance levels exceeding 95%. During two separate microflaring
events, oscillation sites surrounding the bright point are observed to
twist. We relate the twisting of the oscillation sites to the twisting
of physical flux tubes, thus giving rise to reconnection phenomena. We
derive an average twist velocity of 8.1 km/s and detect a peak in the
emitted flux between twist angles of 180° and 230°.
Title: Twisting flux tubes as a cause of micro-flaring activity
Authors: Jess, D. B.; McAteer, R. T. J.; Mathioudakis, M.; Keenan,
F. P.; Andic, A.; Bloomfield, D. S.
Bibcode: 2007A&A...476..971J
Altcode: 2007arXiv0709.2268J
High-cadence optical observations of an H-α blue-wing bright point
near solar AR NOAA 10794 are presented. The data were obtained with the
Dunn Solar Telescope at the National Solar Observatory/Sacramento Peak
using a newly developed camera system, the rapid dual imager. Wavelet
analysis is undertaken to search for intensity-related oscillatory
signatures, and periodicities ranging from 15 to 370 s are found with
significance levels exceeding 95%. During two separate microflaring
events, oscillation sites surrounding the bright point are observed to
twist. We relate the twisting of the oscillation sites to the twisting
of physical flux tubes, thus giving rise to reconnection phenomena. We
derive an average twist velocity of 8.1 km s-1 and detect
a peak in the emitted flux between twist angles of 180° and 230°. Figure 4 is available as a movie at http://www.aanda.org
Title: High-frequency oscillations in a solar active region observed
with the RAPID DUAL IMAGER
Authors: Jess, D. B.; Andić, A.; Mathioudakis, M.; Bloomfield, D. S.;
Keenan, F. P.
Bibcode: 2007A&A...473..943J
Altcode: 2007arXiv0707.2716J
High-cadence, synchronized, multiwavelength optical observations
of a solar active region (NOAA 10794) are presented. The data
were obtained with the Dunn Solar Telescope at the National Solar
Observatory/Sacramento Peak using a newly developed camera system:
the rapid dual imager. Wavelet analysis is undertaken to search
for intensity related oscillatory signatures, and periodicities
ranging from 20 to 370 s are found with significance levels exceeding
95%. Observations in the H-α blue wing show more penumbral oscillatory
phenomena when compared to simultaneous G-band observations. The H-α
oscillations are interpreted as the signatures of plasma motions with
a mean velocity of 20 km s-1. The strong oscillatory power
over H-α blue-wing and G-band penumbral bright grains is an indication
of the Evershed flow with frequencies higher than previously reported.
Title: Detection of MHD waves in the solar chromosphere
Authors: Kontogiannis, Giannis; Bloomfield, D. Shaun; McAteer, James
R. T.; Mathioudakis, Mihalis; Antonopoulou, E.
Bibcode: 2007AIPC..895...80K
Altcode:
One of the main features of the quiet solar chromosphere is the Network
Bright Points (NBP), formed by emerging magnetic flux at the boundaries
of supergranular cells. Using SSW IDL routines and wavelet analysis
of series of images in four bandpasses (CaIIK3, Mgb1, Mgb2 and Hα
core) we have detected MHD wave modes in the Network Bright Points
of the solar chromosphere. The observations have been analyzed using
cross-correlation techniques and we have drawn conclusions on wave
propagation and mode-coupling.
Title: The Influence of Magnetic Field on Oscillations in the Solar
Chromosphere
Authors: Bloomfield, D. Shaun; McAteer, R. T. James; Mathioudakis,
Mihalis; Keenan, Francis P.
Bibcode: 2006ApJ...652..812B
Altcode: 2006astro.ph..8314B
Two sequences of solar images obtained by the Transition Region and
Coronal Explorer in three UV passbands are studied using wavelet
and Fourier analysis and compared to the photospheric magnetic
flux measured by the Michelson Doppler Interferometer on the Solar
Heliospheric Observatory to study wave behavior in differing magnetic
environments. Wavelet periods show deviations from the theoretical
cutoff value and are interpreted in terms of inclined fields. The
variation of wave speeds indicates that a transition from dominant
fast-magnetoacoustic waves to slow modes is observed when moving from
network into plages and umbrae. This implies preferential transmission
of slow modes into the upper atmosphere, where they may lead to heating
or be detected in coronal loops and plumes.
Title: The periodic variations of a white-light flare observed
with ULTRACAM
Authors: Mathioudakis, M.; Bloomfield, D. S.; Jess, D. B.; Dhillon,
V. S.; Marsh, T. R.
Bibcode: 2006A&A...456..323M
Altcode: 2006astro.ph..5196M
High time resolution observations of a white-light flare on the active
star EQ PegB show evidence of intensity variations with a period of
≈10 s. The period drifts to longer values during the decay phase of
the flare. If the oscillation is interpreted as an impulsively-excited,
standing-acoustic wave in a flare loop, the period implies a loop
length of ≈3.4 Mm and ≈6.8 Mm for the case of the fundamental mode
and the second harmonic, respectively. However, the small loop lengths
imply a very high modulation depth making the acoustic interpretation
unlikely. A more realistic interpretation may be that of a fast-MHD
wave, with the modulation of the emission being due to the magnetic
field. Alternatively, the variations could be due to a series of
reconnection events. The periodic signature may then arise as a result
of the lateral separation of individual flare loops or current sheets
with oscillatory dynamics (i.e., periodic reconnection).
Title: Opacity in the upper atmospheres of active stars. II. AD Leonis
Authors: Christian, D. J.; Mathioudakis, M.; Bloomfield, D. S.;
Dupuis, J.; Keenan, F. P.; Pollacco, D. L.; Malina, R. F.
Bibcode: 2006A&A...454..889C
Altcode: 2006astro.ph..2447C
We present FUV and UV spectroscopic observations of AD Leonis, with
the aim of investigating opacity effects in the transition regions
of late-type stars. The C III lines in FUSE spectra show significant
opacity during both the quiescent and flaring states of AD Leonis, with
up to 30% of the expected flux being lost during the latter. Other FUSE
emission lines tested for opacity include those of O VI, while C IV, Si
IV and N V transitions observed with stis are also investigated. These
lines only reveal modest amounts of opacity with losses during flaring
of up to 20%. Optical depths have been calculated for homogeneous
and inhomogeneous geometries, giving path lengths of ≈20-60 km and
≈10-30 km, respectively, under quiescent conditions. However path
lengths derived during flaring are ≈2-3 times larger. These values
are in excellent agreement with both estimates of the small-scale
structure observed in the solar transition region, and path lengths
derived previously for several other active late-type stars.
Title: Traveling Waves In Network Bright Points
Authors: Kontogiannis, G.; Bloomfield, D. Shaun; McAteer, J. R. T.;
Mathioudakis, M.; Antonopoulou, E.
Bibcode: 2006AIPC..848..229K
Altcode:
One of the main features of the quiet solar chromosphere is the
Network Bright Points (NBP), formed by the emerging magnetic flux,
at the boundaries of supergranular cells. Triggered by the motions
of magnetic loop foot-points, at the top of the convection zone, MHD
waves propagate inside the NBP's. Using SSW IDL routines and wavelet
analysis of series of images in four bandpasses (CaII K3, Mgb1-0.4,
Mgb2 and Ha core) we detected these MHD wave modes. The observations
have been analyzed using cross-correlation techniques and we have
drawn conclusions on wave propagation and mode coupling.
Title: Observational Evidence of Gentle and Explosive Chromospheric
Evaporation
Authors: Milligan, R. O.; Gallagher, P. T.; Mathioudakis, M.; Keenan,
F. P.
Bibcode: 2006ESASP.617E.138M
Altcode: 2006soho...17E.138M
No abstract at ADS
Title: Observational Evidence of Chromospheric Evaporation
Authors: Milligan, Ryan O.; Gallagher, P. T.; Mathioudakis, M.;
Keenan, F. P.
Bibcode: 2006SPD....37.0828M
Altcode: 2006BAAS...38R.662M
Observational evidence for chromospheric evaporation during the
impulsive phase of two solar flares is presented using data from the
Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) and the
Coronal Diagnostic Spectrometer (CDS) on board SOHO. For the first
time, co-spatial imaging and spectroscopy have been used to observe
both gentle and explosive evaporation processes within HXR emitting
regions. For a GOES C9.1 flare, a low flux of non-thermal electrons
was found to produce low-velocity upflows in the cool He I and O V
emission lines and moderate upflows in the 8 MK Fe XIX line indicative
of gentle evaporation. An M2.2 flare, on the other hand, showed
low-velocity downflows in the He I and O V lines and high-velocity
upflows in the Fe XIX line, for an electron flux value which was an
order of magnitude higher, indicative of an explosive process. These
findings confirm that the dynamic response of the solar atmosphere is
sensitively dependent on the flux of incident electrons as predicted
by current hydrodynamical simulations.
Title: Opacity in the transition region of cool dwarfs: A powerful
diagnostic
Authors: Mathioudakis, M.; Christian, D. J.; Bloomfield, D. S.;
Keenan, F. P.; Linsky, J. L.; Dupuis, J.
Bibcode: 2006ASPC..348..159M
Altcode:
A detailed study of the C III 1176Å multiplet in active cool stars
has shown significant deviations of the line flux from the optically
thin approximation. These deviations become more pronounced during
flares. Opacity can be used as a powerful diagnostic to estimate path
lengths in the transition region. Our analysis reveals that stellar
transition regions have very small spatial characteristics typically
in the range of 10 - 100 km. These path lengths are in agreement with
the small scale structure seen in the solar transition region.
Title: A Comparative Study of Flaring Loops in Active Stars
Authors: Mullan, D. J.; Mathioudakis, M.; Bloomfield, D. S.; Christian,
D. J.
Bibcode: 2006ApJS..164..173M
Altcode:
Dynamo activity in stars of different types is expected to generate
magnetic fields with different characteristics. As a result, a
differential study of the characteristics of magnetic loops in a broad
sample of stars may yield information about dynamo systematics. In the
absence of direct imaging, certain physical parameters of a stellar
magnetic loop can be extracted if a flare occurs in that loop. In
this paper we employ a simple nonhydrodynamic approach introduced by
Haisch, to analyze a homogeneous sample of all of the flares we could
identify in the EUVE DS database: a total of 134 flares that occurred
on 44 stars ranging in spectral type from F to M and in luminosity
class from V to III. All of the flare light curves that have been
used in the present study were obtained by a single instrument (EUVE
DS). For each flare, we have applied Haisch's simplified approach
(HSA) in order to determine loop length, temperature, electron density,
and magnetic field. For each of our target stars, a literature survey
has been performed to determine quantitatively the extent to which our
results are consistent with independent studies. The results obtained
by HSA are found to be well supported by results obtained by other
methods. Our survey suggests that, on the main sequence, short loops
(with lengths <=0.5R*) may be found in stars of all
classes, while the largest loops (with lengths up to 2R*)
appear to be confined to M dwarfs. Based on EUVE data, the transition
from small to large loops on the main sequence appears to occur between
spectral types K2 and M0. We discuss the implications of this result
for dynamo theories.
Title: Observational Evidence of Gentle Chromospheric Evaporation
during the Impulsive Phase of a Solar Flare
Authors: Milligan, Ryan O.; Gallagher, Peter T.; Mathioudakis, Mihalis;
Keenan, Francis P.
Bibcode: 2006ApJ...642L.169M
Altcode: 2006astro.ph..3652M
Observational evidence of gentle chromospheric evaporation during
the impulsive phase of a C9.1 solar flare is presented using data
from the Reuven Ramaty High-Energy Solar Spectroscopic Imager and the
Coronal Diagnostic Spectrometer on board the Solar and Heliospheric
Observatory. Until now, evidence of gentle evaporation has often
been reported during the decay phase of solar flares, where thermal
conduction is thought to be the driving mechanism. Here we show that
the chromospheric response to a low flux of nonthermal electrons
(>=5×109 ergs cm-2 s-1) results
in plasma upflows of 13+/-16, 16+/-18, and 110+/-58 km s-1
in the cool He I and O V emission lines and the 8 MK Fe XIX line,
respectively. These findings, in conjunction with other recently
reported work, now confirm that the dynamic response of the solar
atmosphere is sensitively dependent on the flux of incident electrons.
Title: RHESSI and SOHO CDS Observations of Explosive Chromospheric
Evaporation
Authors: Milligan, Ryan O.; Gallagher, Peter T.; Mathioudakis, Mihalis;
Bloomfield, D. Shaun; Keenan, Francis P.; Schwartz, Richard A.
Bibcode: 2006ApJ...638L.117M
Altcode: 2005astro.ph..9664M
Simultaneous observations of explosive chromospheric evaporation
are presented using data from the Reuven Ramaty High-Energy Solar
Spectroscopic Imager (RHESSI) and the Coronal Diagnostic Spectrometer
(CDS) on board the Solar and Heliospheric Observatory. For the first
time, cospatial imaging and spectroscopy have been used to observe
explosive evaporation within a hard X-ray emitting region. RHESSI
X-ray images and spectra were used to determine the flux of nonthermal
electrons accelerated during the impulsive phase of an M2.2 flare. When
we assumed a thick-target model, the injected electron spectrum was
found to have a spectral index of ~7.3, a low-energy cutoff of ~20 keV,
and a resulting flux of >=4×1010 ergs cm-2
s-1. The dynamic response of the atmosphere was determined
using CDS spectra; we found a mean upflow velocity of 230+/-38 km
s-1 in Fe XIX (592.23 Å) and associated downflows of
36+/-16 and 43+/-22 km s-1 at chromospheric and transition
region temperatures, respectively, relative to an averaged quiet-Sun
spectra. The errors represent a 1 σ dispersion. The properties of
the accelerated electron spectrum and the corresponding evaporative
velocities were found to be consistent with the predictions of theory.
Title: Plasma diagnostics of active-region evolution and implications
for coronal heating
Authors: Milligan, R. O.; Gallagher, P. T.; Mathioudakis, M.; Keenan,
F. P.; Bloomfield, D. S.
Bibcode: 2005MNRAS.363..259M
Altcode: 2005MNRAS.tmp..764M; 2005astro.ph..9219M
A detailed study is presented of the decaying solar-active region NOAA
10103 observed with the Coronal Diagnostic Spectrometer (CDS), the
Michelson Doppler Imager (MDI) and the Extreme-ultraviolet Imaging
Telescope (EIT) onboard the Solar and Heliospheric Observatory
(SOHO). Electron-density maps formed using SiX (356.03 Å/347.41 Å)
show that the density varies from ~1010cm-3
in the active-region core to ~7 × 108cm-3
at the region boundaries. Over the 5d of observations, the average
electron density fell by ~30 per cent. Temperature maps formed using
FeXVI (335.41 Å)/FeXIV (334.18 Å) show electron temperatures of
~2.34 × 106 K in the active-region core and ~2.10 ×
106 K at the region boundaries. Similarly to the electron
density, there was a small decrease in the average electron temperature
over the 5-d period. The radiative, conductive and mass-flow losses
were calculated and used to determine the resultant heating rate
(PH). Radiative losses were found to dominate the
active-region cooling process. As the region decayed, the heating
rate decreased by almost a factor of 5 between the first and last
day of observations. The heating rate was then compared to the
total unsigned magnetic flux , yielding a power law of the form
PH~Φ0.81+/-0.32tot. This result
suggests that waves rather than nanoflares may be the dominant heating
mechanism in this active region.
Title: 6Li in the Atmosphere of GJ 117
Authors: Christian, D. J.; Mathioudakis, M.; Jevremović, D.;
Hauschildt, P. H.; Baron, E.
Bibcode: 2005ApJ...632L.127C
Altcode: 2005astro.ph..9247C
We present high-resolution VLT UVES observations of the active K dwarf
GJ 117. 6Li enhancement has been shown for energetic solar
events, one chromospherically active binary, and several dwarf halo
stars. Our analysis reveals the detection of 6Li on this
source with 6Li/7Li = 0.030 +/- 0.010. We found
no significant contribution from other lines, including Ti I, in the
Li profile of GJ 117 and a template star of similar spectral type and
metallicity. We discuss the possibility for 6Li production
by spallation and find it to be consistent with the activity levels
of the object.
Title: Chromospheric Response to Non-Thermal Electrons During Flares
Using RHESSI and SOHO/CDS
Authors: Milligan, R. O.; Gallagher, P. T.; Mathioudakis, M.; Keenan,
F. P.
Bibcode: 2005AGUSMSP52A..04M
Altcode:
Many current solar flare models predict a relationship between the
amount of energy deposited in the chromospheric and transition region
layers of the solar atmosphere by non-thermal electrons, and the
velocity at which this superheated material rises. For the first time,
we present findings from simultaneous observations of loop footpoints
using RHESSI and SOHO/CDS. RHESSI HXR images and spectra are used to
determine the flux of non-thermal electrons using the thick-target
bremsstrahlung model, while upflow velocities in the Fe XIX emission
line are observed using CDS.
Title: AB Dor: Evidence for stellar coronal opacity in Fe XVII
Authors: Matranga, M.; Mathioudakis, M.; Kay, H. R. M.; Keenan, F. P.
Bibcode: 2005ESASP.560..807M
Altcode: 2005csss...13..807M
No abstract at ADS
Title: Intensity oscillations during a flare on EQ Peg
Authors: Mathioudakis, M.; Bloomfield, D. S.; Dhillon, V. S.; Marsh,
T. R.
Bibcode: 2005ESASP.560..803M
Altcode: 2005csss...13..803M
No abstract at ADS
Title: Magnetohydrodynamic mode coupling in the quiet-Sun network
Authors: Bloomfield, D. S.; McAteer, R. T. J.; Mathioudakis, M.;
Williams, D. R.; Keenan, F. P.
Bibcode: 2005ESASP.560..449B
Altcode: 2005csss...13..449B
No abstract at ADS
Title: Flare X-Ray Observations of AB Doradus: Evidence of Stellar
Coronal Opacity
Authors: Matranga, M.; Mathioudakis, M.; Kay, H. R. M.; Keenan, F. P.
Bibcode: 2005ApJ...621L.125M
Altcode: 2005astro.ph..2063M
X-ray spectra of the late-type star AB Dor obtained with the
XMM-Newton satellite are analyzed. AB Dor was particularly active
during the observations. An emission measure reconstruction technique
is employed to analyze flare and quiescent spectra, with emphasis on
the Fe XVII 15-17 Å wavelength region. The Fe XVII 16.78 Å/15.01 Å
line ratio increases significantly in the hotter flare plasma. This
change in the ratio is opposite to the theoretical predictions
and is attributed to the scattering of 15.01 Å line photons from
the line of sight. The escape probability technique indicates an
optical depth of ~0.4 for the 15.01 Å line. During the flare, the
electron density is 4.4+2.7-1.6×1010
cm-3, and the fractional Fe abundance is 0.5+/-0.1 of the
solar photospheric value. Using these parameters, a path length of
~8000 km is derived. There is no evidence of opacity in the quiescent
X-ray spectrum of the star.
Title: Observations of Hα Intensity Oscillations in a Flare Ribbon
Authors: McAteer, R. T. James; Gallagher, Peter T.; Brown, Daniel S.;
Bloomfield, D. Shaun; Moore, Ruth; Williams, David R.; Mathioudakis,
Mihalis; Katsiyannis, A.; Keenan, Francis P.
Bibcode: 2005ApJ...620.1101M
Altcode:
High-cadence Hα blue wing observations of a C9.6 solar flare
obtained at Big Bear Solar Observatory using the Rapid Dual Imager
are presented. Wavelet and time-distance methods were used to study
oscillatory power along the ribbon, finding periods of 40-80 s during
the impulsive phase of the flare. A parametric study found statistically
significant intensity oscillations with amplitudes of 3% of the peak
flare amplitude, periods of 69 s (14.5 mHz) and oscillation decay times
of 500 s. These measured properties are consistent with the existence
of flare-induced acoustic waves within the overlying loops.
Title: Wavelet Phase Coherence Analysis: Application to a Quiet-Sun
Magnetic Element
Authors: Bloomfield, D. Shaun; McAteer, R. T. James; Lites, Bruce W.;
Judge, Philip G.; Mathioudakis, Mihalis; Keenan, Francis P.
Bibcode: 2004ApJ...617..623B
Altcode:
A new application of wavelet analysis is presented that utilizes
the inherent phase information residing within the complex Morlet
transform. The technique is applied to a weak solar magnetic network
region, and the temporal variation of phase difference between TRACE
1700 Å and SOHO/SUMER C II 1037 Å intensities is shown. We present,
for the first time in an astrophysical setting, the application of
wavelet phase coherence, including a comparison between two methods
of testing real wavelet phase coherence against that of noise. The
example highlights the advantage of wavelet analysis over more
classical techniques, such as Fourier analysis, and the effectiveness
of the former to identify wave packets of similar frequencies but
with differing phase relations is emphasized. Using cotemporal,
ground-based Advanced Stokes Polarimeter measurements, changes in the
observed phase differences are shown to result from alterations in
the magnetic topology.
Title: A Detailed Study of Opacity in the Upper Atmosphere of
Proxima Centauri
Authors: Christian, D. J.; Mathioudakis, M.; Bloomfield, D. S.;
Dupuis, J.; Keenan, F. P.
Bibcode: 2004ApJ...612.1140C
Altcode:
We present far-UV and UV spectroscopic observations of Proxima Centauri
obtained as part of our continued investigation into the optically
thin approximation assumed for the transition regions of late-type
stars. Significant opacity is found in the C III lines during both
the quiescent and flaring states of Proxima Cen, with up to 70% of the
expected flux being lost in the latter. Our findings cast some doubt
on the suitability of the C III λ977 line for estimating the electron
density in stellar atmospheres. However, the opacity has no significant
effect on the observed line widths. We calculate optical depths for
homogeneous and inhomogeneous geometries and estimate an electron
density of 6×1010 cm-3 for the transition region
using the O IV line ratios at 1400 Å. The combination of electron
density and optical depth indicates path lengths as low as ~10 km,
which are in excellent agreement with estimates of the small-scale
structure seen in the solar transition region.
Title: Erratum: ``Propagating Waves and Magnetohydrodynamic
Mode Coupling in the Quiet-Sun Network'' (ApJ, 604, 936 [2004])
Authors: Bloomfield, D. Shaun; McAteer, R. T. James; Mathioudakis,
Mihalis; Williams, David R.; Keenan, Francis P.
Bibcode: 2004ApJ...609..465B
Altcode:
Proof corrections were not carried correctly to the
final printed version of this paper, which resulted
in an extra set of overbars appearing in both terms
of the denominator of equation (3). The correct form of this equation
is:C(Δt,ν)=(Σ[Pλ1(t,ν)-Pλ1(t,ν)][Pλ2(t+Δt,ν)-Pλ2(t+Δt,ν)])/(sqrt(Σ[Pλ1(t,ν)-Pλ1(t,ν)]2Σ[Pλ2(t,ν)-Pλ2(t,ν)]2)).
(3)As noted in footnote 3, this correlation equation was stated
incorrectly in our previous paper (eq. [4] in R. T. J. McAteer et al.,
ApJ, 604, 936 [2004]). However,
the correct form of equation (3), now given here, was used for the
analysis in both papers. The press sincerely regrets this error.
Title: Plasma Diagnostics of Active Region Evolution and Implications
for Coronal Heating
Authors: Milligan, R. O.; Gallagher, P. T.; Mathioudakis, M.;
Bloomfield, D. S.; Keenan, F. P.
Bibcode: 2004AAS...204.9803M
Altcode: 2004BAAS...36..984M
The decaying solar active region NOAA 10103 was observed during 2002
September 10--14 using the Coronal Diagnostic Spectrometer (CDS) onboard
the Solar and Heliospheric Observatory (SOHO). Electron density maps
were created using the Si X (356Å/347Å) ratio, which show densities
of ∼1010cm-3 in the active region core, and
∼7x108cm-3 in the region boundaries. Over
the five days of observations, the average electron density fell by
approximately a factor of two. Temperature maps were created using
the Fe XVI (335.4Å)/Fe XIV (334.2Å) ratio, which show electron
temperatures of ∼2.2x106K in the active region core,
and ∼2.0x106K in the region boundaries. Similarly to the
electron density, the average electron temperature decreased over the
five days, but by only ∼3%. The radiative and conductive losses, and
resultant heating rate, were then calculated and compared to the total
unsigned magnetic flux (Φ tot = ∫ dA |Bz|)
from Michelson Doppler Interferometer (MDI) magnetograms. Losses due to
radiation were found to exceed conductive losses by a factor of ten. As
the region decayed, the heating rate decreased by close to an order of
magnitiude between the first and last day of observations. In line with
several coronal heating theories, a power-law relationship of the form
Ptot} ∼Φ {tot0.95±0.07 was found
between the heating rate and the unsigned magnetic flux. This result
provides further observational evidence for wave rather than nanoflare
heating of solar active regions. Ryan Milligan would like to
thank the Solar Physics Division for being awarded an SPD Studentship.
Title: Propagating Waves and Magnetohydrodynamic Mode Coupling in
the Quiet-Sun Network
Authors: Bloomfield, D. Shaun; McAteer, R. T. James; Mathioudakis,
Mihalis; Williams, David R.; Keenan, Francis P.
Bibcode: 2004ApJ...604..936B
Altcode:
High-cadence multiwavelength optical observations were taken with the
Dunn Solar Telescope at the National Solar Observatory, Sacramento Peak,
accompanied by Advanced Stokes Polarimeter vector magnetograms. A total
of 11 network bright points (NBPs) have been studied at different
atmospheric heights using images taken in wave bands centered on
Mg I b1 - 0.4 Å, Hα, and Ca II K3. Wavelet
analysis was used to study wave packets and identify traveling
magnetohydrodynamic waves. Wave speeds were estimated through the
temporal cross-correlation of signals, in selected frequency bands
of wavelet power, in each wavelength. Four mode-coupling cases were
identified, one in each of four of the NBPs, and the variation of the
associated Fourier power with height was studied. Three of the detected
mode-coupling, transverse-mode frequencies were observed in the 1.2-1.6
mHz range (mean NBP apparent flux density magnitudes over 99-111 Mx
cm-2), with the final case showing 2.0-2.2 mHz (with 142 Mx
cm-2). Following this, longitudinal-mode frequencies were
detected in the range 2.6-3.2 mHz for three of our cases, with 3.9-4.1
mHz for the remaining case. After mode coupling, two cases displayed a
decrease in longitudinal-mode Fourier power in the higher chromosphere.
Title: Ultraviolet Oscillations in the Chromosphere of the Quiet Sun
Authors: McAteer, R. T. James; Gallagher, Peter T.; Bloomfield,
D. Shaun; Williams, David R.; Mathioudakis, Mihalis; Keenan, Francis P.
Bibcode: 2004ApJ...602..436M
Altcode:
Quiet-Sun oscillations in the four Transition Region and Coronal
Explorer (TRACE) ultraviolet passbands centered on 1700, 1600,
1216, and 1550 Å are studied using a wavelet-based technique. Both
network and internetwork regions show oscillations with a variety of
periods and lifetimes in all passbands. The most frequent network
oscillation has a period of 283 s, with a lifetime of 2-3 cycles
in all passbands. These oscillations are discussed in terms of
upwardly propagating magnetohydrodynamic wave models. The most
frequent internetwork oscillation has a period of 252 s, again with
a lifetime of 2-3 cycles, in all passbands. The tendency for these
oscillations to recur in the same position is discussed in terms of
``persistent flashers.'' The network contains greater oscillatory
power than the internetwork at periods longer than 300 s in the
low chromosphere. This value is shown to decrease to 250 s in the
high chromosphere. The internetwork also displays a larger number of
short-lifetime, long-period oscillations than the network, especially
in the low chromosphere. Both network and internetwork regions contain
a small number of nonrecurring long-lifetime oscillations.
Title: Intensity Oscillations During a flare on II Peg
Authors: Avgoloupis, S.; Seiradakis, J. H.; Mathioudakis, M.;
Bloomfield, D. Shaun; McAteer, J.
Bibcode: 2004hell.conf..120A
Altcode:
No abstract at ADS
Title: Oscillatory Signatures above Quiet Sun Magnetic Elements
Authors: Bloomfield, D. S.; McAteer, R. T. J.; Lites, B. W.; Judge,
P. G.; Mathioudakis, M.; Keenan, F. P.
Bibcode: 2004ESASP.547...51B
Altcode: 2004soho...13...51B
A new application of wavelet analysis is presented. The data used are
part of Joint Observing Proposal 72 between SoHO and TRACE, obtained on
1998 May 16 with accompanying groundbased data taken with the Dunn Solar
Telescope at Sacramento Peak, New Mexico. A weak magnetic network region
is studied and the temporal variation of phase difference between TRACE
1700 Å and SoHO/SUMER C II intensities is shown. The example clearly
highlights the advantages of wavelet analysis over more classical
techniques such as Fourier analysis, where the effectiveness of the
technique to identify wavepackets with differing phase difference
relations is emphasised.
Title: An Automated Wavelet Analysis Approach to TRACE Quiet Sun
Oscillations
Authors: McAteer, R. T. J.; Gallagher, P. T.; Williams, D. R. Williams
D. R.; Bloomfield, D. S.; Mathioudakis, M.; Keenan, F. P.
Bibcode: 2004ESASP.547..139M
Altcode: 2004soho...13..139M
An automated wavelet analysis approach to TRACE UV quiet Sun datasets
is discussed. Periodicity and lifetime of oscillations present in the
network and internetwork are compared and contrasted. This provides
a means of extending previous Fourier results into the time-localised
domain. The longest lifetime oscillations occur around the acoustic band
and the network tends to dominate over the internetwork at periods 4
mins. However, it is shown that the internetwork can dominate over the
network at long periods (7 - 20 mins), but only for short lifetimes
( 3 complete oscillations). These results are discussed in terms of
chromospheric heating theories.
Title: Coronal opacity variation during a flare on AB Doradus
Authors: Kay, H.; Mathioudakis, M.; Matranga, M.
Bibcode: 2004cosp...35.1745K
Altcode: 2004cosp.meet.1745K
One of the fundamental assumptions in the study of stellar transition
regions and coronae is that the emission is optically thin. However,
opacity effects have been seen in the solar corona for many years
and there is evidence from FUSE observations of cool stars that they
may also be important when considering stellar transition regions and
coronae. The active star AB Doradus has been observed frequently by
XMM-Newton as part of the instrument calibration programme. We present
detailed analysis of the corona of AB Doradus and investigate possible
changes in opacity, by means of the Fe XVII emission lines. The use
of opacity as a diagnostic tool for constraining the scale-height and
emitting volume are outlined
Title: A New EUVE-detected Flare Star (EUVE J0613-23.9B)
Authors: Christian, D. J.; Mathioudakis, M.; Jevremovic, D.; Dupuis,
J.; Vennes, S.; Kawka, A.
Bibcode: 2003IBVS.5447....1C
Altcode:
We present observations of a strong flare from EUVE J0613-23.9B,
a source serendipitously detected with the EUVE Deep Survey Telescope
2.5' south of HD 43162. Optical spectroscopy revealed EUVE J0613-23.9B
to be an active dM3.5e star. A dM4e star was found 23'' south of HD
43162 that we associate with 1RXS J061345.1235205.
Title: The Extreme-Ultraviolet Continuum of a Strong Stellar Flare
Authors: Christian, D. J.; Mathioudakis, M.; Jevremović, D.; Dupuis,
J.; Vennes, S.; Kawka, A.
Bibcode: 2003ApJ...593L.105C
Altcode:
We present the serendipitous detection of an extreme-ultraviolet flare
on EUVE J0613-23.9B. The flare showed over a 200-fold increase above the
quiescent emission in the DS/Lexan 60-200 Å wavelength band. Optical
spectroscopy revealed that the event was associated with an active
dM3.5e star. The EUVE spectra are dominated by emission lines formed at
temperatures in excess of 107 K. The observation is unique
as we have detected, for the first time, a strong Lyman continuum in
the EUVE long-wavelength range (320-650 Å). The flare in the continuum
(T~20,000-30,000 K) was extremely short, lasting for less than 500 s,
while in the DS (T~107 K) its duration was ~28 ks. The
total energy of the flare in the DS is ~3×1034 ergs. We
have made a fit to the continuum using semiempirical model atmospheres
and derived the time-averaged temperature and density structures.
Title: White-light oscillations during a flare on II Peg.
Authors: Mathioudakis, M.; Seiradakis, J. H.; Williams, D. R.;
Avgoloupis, S.; Bloomfield, D. S.; McAteer, R. T. J.
Bibcode: 2003A&A...403.1101M
Altcode:
We analyse the intensity oscillations observed in the gradual phase
of a white-light flare on the RS CVn binary II Peg. Fast Fourier
Transform power spectra and Wavelet analysis reveal a period of
220 s. The reliability of the oscillation is tested using several
criteria. Oscillating coronal loop models are used to derive physical
parameters such as temperature, electron density and magnetic field
strength associated with the coronal loop. The derived parameters
are consistent with the near-simultaneous X-ray observations of the
flare. There is no evidence for oscillations in the quiescent state
of the binary.
Title: Observational Evidence for Mode Coupling in the Chromospheric
Network
Authors: McAteer, R. T. James; Gallagher, Peter T.; Williams, David R.;
Mathioudakis, Mihalis; Bloomfield, D. Shaun; Phillips, Kenneth J. H.;
Keenan, Francis P.
Bibcode: 2003ApJ...587..806M
Altcode:
Oscillations in network bright points (NBPs) are studied at a variety
of chromospheric heights. In particular, the three-dimensional
variation of NBP oscillations is studied using image segmentation
and cross-correlation analysis between images taken in light of Ca II
K3, Hα core, Mg I b2, and Mg I b1-0.4
Å. Wavelet analysis is used to isolate wave packets in time and
to search for height-dependent time delays that result from upward-
or downward-directed traveling waves. In each NBP studied, we find
evidence for kink-mode waves (1.3, 1.9 mHz), traveling up through the
chromosphere and coupling with sausage-mode waves (2.6, 3.8 mHz). This
provides a means for depositing energy in the upper chromosphere. We
also find evidence for other upward- and downward-propagating waves in
the 1.3-4.6 mHz range. Some oscillations do not correspond to traveling
waves, and we attribute these to waves generated in neighboring regions.
Title: Si IX Emission Lines in Spectra Obtained with the Solar euv
Research Telescope and Spectrograph (Serts)
Authors: Keenan, F. P.; Katsiyannis, A. C.; Aggarwal, K. M.;
Mathioudakis, M.; Brosius, J. W.; Davila, J. M.; Thomas, R. J.
Bibcode: 2003SoPh..212...65K
Altcode:
Theoretical electron-density-sensitive emission line ratios involving
2s22p2-2s2p3 transitions in Si ix
between 223 and 350 Å are presented. A comparison of these with
an extensive dataset of solar-active-region, quiet-Sun, subflare
and off-limb observations, obtained during rocket flights by the
Solar EUV Research Telescope and Spectrograph (SERTS), reveals
generally very good agreement between theory and experiment. This
provides support for the accuracy of the line-ratio diagnostics,
and hence the atomic data on which they are based. In particular,
the density-sensitive intensity ratio I(258.10 Å)/I(349.87 Å)
offers an especially promising diagnostic for studies of coronal
plasmas, as it involves two reasonably strong emission lines and
varies by more than an order of magnitude over the useful density
range of 109-1011 cm−3. The
2s22p2 1S0-2s2p3
1P1 transition at 259.77 Å is very marginally
identified for the first time in the SERTS database, although it has
previously been detected in solar flare observations.
Title: Late-type Stars with Coronae but No Chromospheres?
Authors: Christian, D. J.; Mathioudakis, M.; Jevremovic, D.; Mullan,
D. J.; Lehner, N.
Bibcode: 2002AAS...201.1206C
Altcode: 2002BAAS...34.1113C
We present high resolution optical observations of several late-type
stars selected from ROSAT and EUVE surveys. These particular stars
show no Balmer or Ca II emission typical of late-type stars with X-ray
evidence for coronae. Since our sample shows no evidence for binarity
in their optical spectra, these stars may have no chromospheres. We
present preliminary results from simulated spectra using the MULTI
code of Carlsson for solving multi-level non-LTE radiative transfer
problems in plane parallel atmospheres. These simulations can reproduce
the observed Ca II line profiles. Stars without chromospheres are
expected to have high density coronae and transition regions, which
we will investigate with future FUV and X-ray observations.
Title: A Study Of Extreme Ultraviolet Flare Activity On Late-Type
Stars
Authors: Bloomfield, D. S.; Mathioudakis, M.
Bibcode: 2002ASPC..277..425B
Altcode: 2002sccx.conf..425B
No abstract at ADS
Title: The OIV and SIV intercombination lines in the ultraviolet
spectra of astrophysical sources
Authors: Keenan, F. P.; Ahmed, S.; Brage, T.; Doyle, J. G.; Espey,
B. R.; Exter, K. M.; Hibbert, A.; Keenan, M. T. C.; Madjarska, M. S.;
Mathioudakis, M.; Pollacco, D. L.
Bibcode: 2002MNRAS.337..901K
Altcode:
New electron density diagnostic line ratios are presented for
the OIV 2s22p 2P-2s2p24P and SIV
3s23p 2P-3s3p24P intercombination
lines around 1400Å. A comparison of these with observational data
for the symbiotic star RR Telescopii (RR Tel), obtained with the
Space Telescope Imaging Spectrograph (STIS), reveals generally
very good agreement between theory and observation. However the
SIV2P3/2-4P1/2 transition
at 1423.824Å is found to be blended with an unknown feature at
1423.774Å. The linewidth for the latter indicates that the feature
arises from a species with a large ionization potential. In addition,
the SIV2P1/2-4P3/2
transition at 1398.044Å is identified for the first time (to
our knowledge) in an astrophysical source other than the Sun,
and an improved wavelength of 1397.166 Å is measured for the
OIV2P1/2-4P3/2 line. The
OIV and SIV line ratios in a sunspot plume spectrum, obtained with
the Solar Ultraviolet Measurements of the Emitted Radiation (SUMER)
instrument on the Solar and Heliospheric Observatory, are found to be
consistent, and remove discrepancies noted in previous comparisons of
these two ions.
Title: An observational study of a magneto-acoustic wave in the
solar corona
Authors: Williams, D. R.; Mathioudakis, M.; Gallagher, P. T.; Phillips,
K. J. H.; McAteer, R. T. J.; Keenan, F. P.; Rudawy, P.; Katsiyannis,
A. C.
Bibcode: 2002MNRAS.336..747W
Altcode:
The Solar Eclipse Corona Imaging System (SECIS) observed a strong 6-s
oscillation in an active region coronal loop, during the 1999 August 11
total solar eclipse. In the present paper we show that this oscillation
is associated with a fast-mode magneto-acoustic wave that travels
through the loop apex with a velocity of 2100 km s-1. We use
near-simultaneous SOHO observations to calculate the parameters of the
loop and its surroundings such as density, temperature and their spatial
variation. We find that the temporal evolution of the intensity is in
agreement with the model of an impulsively generated, fast-mode wave.
Title: Detections of high-frequency oscillations in solar active
region coronal loops
Authors: Katsiyannis, Athanassios C.; Williams, David R.; McAteer,
R. T. James; Gallagher, Peter T.; Mathioudakis, Mihalis; Keenan,
Francis P.
Bibcode: 2002ESASP.505..441K
Altcode: 2002IAUCo.188..441K; 2002solm.conf..441K
One of the mechanisms proposed as a possible solution to the
Sun's coronal heating problem is the damping of energy carried by
magnetohydrodynamic (MHD) waves that are expected to be present
in active regions. Continuing previous work on total solar eclipse
data, we provide further obervational evidence for the existence of
high-frequency MHD waves in coronal loops. Wavelet analysis is used to
identify 21 areas of 4×4 arcsec2 with periodic intensity
oscillations. The frequency range of these detections was 0.2-0.3 Hz
and all last for at least 3 periods at a confidence level of more than
99%. All of the above detections are made just outside known coronal
loops, leading us to suggest a possible, unconventional mechanism.
Title: Detection of propagating waves throughout the chromosphere
in network bright points
Authors: McAteer, R. T. James; Gallagher, Peter T.; Williams, David
R.; Mathioudakis, Mihalis; Phillips, Kenneth J. H.; Keenan, Francis P.
Bibcode: 2002ESASP.505..305M
Altcode: 2002IAUCo.188..305M; 2002solm.conf..305M
We analysed oscillations in individual Network Bright Points (NBPs)
in Ca II K3, Hα core, Mg I b2, and Mg I
b1-0.4 Å giving us a range of heights from the upper
to the lower chromosphere. Lightcurves, and hence power spectra,
were created by isolating distinct regions of the NBP via a simple
intensity thresholding technique. Using this technique, it was
possible to identify peaks in the power spectra with particular spatial
positions within the NBPs. This was extended into the time domain by
means of wavelet analysis. We track the temporal evolution of power
in particular frequency bands by creating power curves. These are
then cross-correlated across all observed wavelengths to search for
propagating waves. In particular, long-period waves with periods of
4-15 minutes (1-4 mHz) were found in the central portion of each NBP,
indicating that these waves are certainly not acoustic, but possibly
due to magneto-acoustic or magneto-gravity wave modes. We note the
possible existence of fast-mode MHD waves in the lower chromosphere,
coupling and transferring power top higher-frequency slow-mode MHD
waves in the upper chromosphere.
Title: Observations of a high-frequency, fast-mode wave in a
coronal loop
Authors: Williams, David R.; Mathioudakis, Mihalis; Gallagher, Peter
T.; Phillips, Kenneth J. H.; McAteer, R. T. James; Keenan, Francis P.;
Katsiyannis, Athanassios C.
Bibcode: 2002ESASP.505..615W
Altcode: 2002IAUCo.188..615W; 2002solm.conf..615W
The high-cadence Solar Eclipse Corona Imaging System (SECIS) observed
a strong 6-second oscillation in an active region coronal loop, during
the 1999 August 11 total solar eclipse. In the present paper we show
that this oscillation is associated with a fast-mode wave that travels
through the loop apex with a velocity of ~2000 km s-1. We
use near-simultaneous SoHO/CDS obervations to calculate the parameters
of the loop and its surroundings such as density, temperature and their
spatial variation. We also calculate radiative losses from the loop in
the temperature range 105.8 - 106.4K, and compare
these losses with the wave energy density. Although the wave travels a
distance greater than λ/4π and therefore meets a necessary criterion
for slow dissipation, the dissipation length is well in excess of the
loop length. The temporal evolution of the intensity is found to be
in agreement with the model of an impulsively generated, fast-mode wave.
Title: Opacity in the upper atmosphere of AU Mic
Authors: Bloomfield, D. S.; Mathioudakis, M.; Christian, D. J.;
Keenan, F. P.; Linsky, J. L.
Bibcode: 2002A&A...390..219B
Altcode:
In this paper we investigate the validity of the optically thin
assumption in the transition region of the late-type star AU Mic. We use
Far-Ultraviolet Spectroscopic Explorer (FUSE) observations of the C Iii
multiplet and O Vi resonance lines, hence yielding information at two
different levels within the atmosphere. Significant deviations from the
optically thin fluxes are found for C Iii in both quiescent and flare
spectra, where only 60% of the flux is actually observed. This could
explain the apparent deviation of C Iii observed in emission measure
distributions. We utilize escape probabilities for both homogeneous and
inhomogeneous geometries and calculate optical depths as high as 10 for
the C Iii 1175.71 Åcomponent of the multiplet. Using a lower limit
to the electron density (1011 cm-3) we derive
an effective thickness of <100 km for the scattering layer. The
emission originates from very small and compact regions, consistent
with a filling factor of 10-5 derived for the flare plasma.
Title: EUVE J2244-15.9: A New Spectroscopic Binary
Authors: Christian, Damian J.; Mathioudakis, Mihalis; Vennes, Stephane
Bibcode: 2002IBVS.5281....1C
Altcode:
We report the detection of a new late-type spectroscopic binary, EUVE
J224415.9 (HD 215341) using the 0.6 meter Coude auxiliary telescope
and Hamilton echelle spectrograph at Lick observatory.
Title: High-Resolution Optical Observations of
Extreme-Ultraviolet-selected Active Late-type Stars
Authors: Christian, Damian J.; Mathioudakis, Mihalis
Bibcode: 2002AJ....123.2796C
Altcode:
We present high-resolution optical spectra of EUV-selected late-type
stars. We have detected seven new spectroscopic binaries, including
a pair of F stars and an RS Canum Venaticorum candidate. Many of the
stars in our sample show remarkable Hα emission with equivalent widths
up to 8 Å. Several sources show strong He I 5876 Å emission. We use
cross-correlation techniques to estimate line-of-sight rotational
velocities and find that they range from less than 5 to over 80 km
s-1. Several of the stars show a strong Li 6707 Å line,
which we use to determine their Li abundances. The EUV-to-bolometric
luminosity, a measure of the coronal heating efficiency, is as high as
10-3. The present sample extends the EUV-selected late-type
stars to lower Rossby numbers and makes the coronal saturation boundary
better pronounced. Our results indicate that there is a drop in the
EUV emission from the corona in late M dwarfs possibly because of a
change in the nature of the magnetic dynamo in fully convective stars.
Title: Long-Period Chromospheric Oscillations in Network Bright Points
Authors: McAteer, R. T. James; Gallagher, Peter T.; Williams, David
R.; Mathioudakis, Mihalis; Phillips, Kenneth J. H.; Keenan, Francis P.
Bibcode: 2002ApJ...567L.165M
Altcode:
The spatial variation of chromospheric oscillations in network bright
points (NBPs) is studied using high-resolution observations in Ca II
K3. Light curves and hence power spectra were created by isolating
distinct regions of the NBP via a simple intensity thresholding
technique. Using this technique, it was possible to identify peaks in
the power spectra with particular spatial positions within the NBPs. In
particular, long-period waves with periods of 4-15 minutes (1-4 mHz)
were found in the central portions of each NBP, indicating that these
waves are certainly not acoustic but possibly due to magnetoacoustic
or magnetogravity wave modes. We also show that spatially averaged
or low spatial resolution power spectra can lead to an inability to
detect such long-period waves.
Title: A Comparison of Theoretical mg vi Emission Line Strengths
with Active-Region Observations From Serts
Authors: Keenan, F. P.; Mathioudakis, M.; Katsiyannis, A. C.;
Ramsbottom, C. A.; Bell, K. L.; Thomas, R. J.; Brosius, J. W.
Bibcode: 2002SoPh..205..265K
Altcode:
R-matrix calculations of electron impact excitation
rates in N-like Mg vi are used to derive theoretical
electron-density-sensitive emission line ratios involving
2s22p3−2s2p4transitions in
the 269-403 Å wavelength range. A comparison of these with
observations of a solar active region, obtained during the
1989 flight of the Solar EUV Rocket Telescope and Spectrograph
(SERTS), reveals good agreement between theory and observation for the
2s22p34S−2s2p44Ptransitions
at 399.28, 400.67, and 403.30 Å, and the
2s22p32P−2s2p42Dlines
at 387.77 and 387.97 Å. However, intensities for the other lines
attributed to Mg vi in this spectrum by various authors do not match the
present theoretical predictions. We argue that these discrepancies are
not due to errors in the adopted atomic data, as previously suggested,
but rather to observational uncertainties or mis-identifications. Some
of the features previously identified as Mg vi lines in the SERTS
spectrum, such as 291.36 and 293.15 Å, are judged to be noise, while
others (including 349.16 Å) appear to be blended.
Title: A search for supersaturation in stellar chromospheres
Authors: Mathioudakis, Mihalis; Christian, Damian
Bibcode: 2002noao.prop..225M
Altcode:
The rotation-activity relations of late-type stars exhibit the effects
of saturation in the chromospheric and coronal emission. Recent
X-ray observations have shown that ultra fast rotators (vsini >
30 km sec^-1) show a decline in their coronal emission. This effect
is called super-saturation. Theoretical models predict that as the
increased rotational velocity decreases the apparent surface gravity,
the hot coronal plasma becomes unstable and cools down to chromospheric
temperatures. More cool loops emitting in chromospheric lines can exist
but not in X-rays. We propose to observe the chromospheric lines of
Ca II H& K and hydrogen Balmer, in a group of EUV/X-ray selected
active late-type stars. The rotational periods of these objects are
in excess of 10 km s^-1. The requested observations will allow us to
test the model predictions.
Title: Solar coronal observations at high frequencies
Authors: Katsiyannis, A. C.; Mathioudakis, M.; Phillips, K. J. H.;
Williams, D. R.; Keenan, F. P.
Bibcode: 2001astro.ph.11447K
Altcode:
The Solar Eclipse Coronal Imaging System (SECIS) is a simple and
extremely fast, high-resolution imaging instrument designed for studies
of the solar corona. Light from the corona (during, for example,
a total solar eclipse) is reflected off a heliostat and passes via
a Schmidt-Cassegrain telescope and beam splitter to two CCD cameras
capable of imaging at 60 frames a second. The cameras are attached via
SCSI connections to a purpose-built PC that acts as the data acquisition
and storage system. Each optical channel has a different filter allowing
observations of the same events in both white light and in the green
line (Fe XIV at 5303 A). Wavelet analysis of the stabilized images
has revealed high frequency oscillations which may make a significant
contribution on the coronal heating process. In this presentation we
give an outline of the instrument and its future development.
Title: Electron Densities in the Coronae of the Sun and Procyon from
Extreme-Ultraviolet Emission Line Ratios in Fe XI
Authors: Pinfield, D. J.; Keenan, F. P.; Mathioudakis, M.; Widing,
K. G.; Gallagher, P. T.; Gupta, G. P.; Tayal, S. S.; Thomas, R. J.;
Brosius, J. W.
Bibcode: 2001ApJ...562..566P
Altcode:
New R-matrix calculations of electron impact excitation rates for Fe
XI are used to determine theoretical emission line ratios applicable
to solar and stellar coronal observations. These are subsequently
compared to solar spectra of the quiet Sun and an active region made
by the Solar EUV Rocket Telescope and Spectrograph (SERTS-95), as well
as Skylab observations of two flares. Line blending is identified,
and electron densities of 109.3, 109.7,
>=1010.8, and >=1011.3 cm-3
are found for the quiet Sun, active region, and the two flares,
respectively. Observations of the F5 IV-V star Procyon, made with
the Extreme Ultraviolet Explorer (EUVE) satellite, are compared and
contrasted with the solar observations. It is confirmed that Procyon's
average coronal conditions are very similar to those seen in the quiet
Sun, with Ne=109.4 cm-3. In addition,
although the quiet Sun is the closest solar analog to Procyon, we
conclude that Procyon's coronal temperatures are slightly hotter than
solar. A filling factor of 25+38-12% was derived
for the corona of Procyon.
Title: Extreme ultraviolet transitions of FeXXI in solar, stellar
and laboratory spectra
Authors: Keenan, F. P.; Aggarwal, K. M.; Williams, D. R.; Mathioudakis,
M.; Phillips, K. J. H.
Bibcode: 2001MNRAS.326.1387K
Altcode:
Recent <b><sansserif>R</sansserif></b>-matrix
calculations of electron impact excitation rates for transitions
among the 2s22p2, 2s2p3 and
2p4 levels of FeXXI are used to derive theoretical electron
density (Ne) sensitive emission-line ratios involving
2s22p2-2s2p3 transitions in the
~98-146Å wavelength range. A comparison of these with observations
from the PLT tokamak plasma, for which the electron density has been
independently determined, reveals generally very good agreement between
theory and experiment, and in some instances removes discrepancies
found previously. The observed FeXXI ratios for a solar flare,
obtained with the OSO-5 satellite, imply electron densities which are
consistent, with discrepancies that do not exceed 0.2dex. In addition,
the derived values of Ne are similar to those estimated for
the high-temperature regions of other solar flares. The good agreement
between theory and observation, in particular for the tokamak spectra,
provides experimental support for the accuracy of the present line-ratio
calculations, and hence for the atomic data on which they are based.
Title: High-frequency oscillations in a solar active region coronal
loop
Authors: Williams, D. R.; Phillips, K. J. H.; Rudawy, P.; Mathioudakis,
M.; Gallagher, P. T.; O'Shea, E.; Keenan, F. P.; Read, P.; Rompolt, B.
Bibcode: 2001MNRAS.326..428W
Altcode:
The Solar Eclipse Corona Imaging System (SECIS) was used to record
high-cadence observations of the solar corona during the total solar
eclipse of 1999 August 11. During the 2min 23.5s of totality, 6364
images were recorded simultaneously in each of the two channels:
a white light channel, and the Fexiv (5303Å) `green line' channel
(T~2MK). Here we report initial results from the SECIS experiment,
including the discovery of a 6-s intensity oscillation in an active
region coronal loop.
Title: The solar-stellar connection
Authors: Mathioudakis, M.
Bibcode: 2001hell.confE..59M
Altcode:
No abstract at ADS
Title: Solar coronal observations in high frequencies
Authors: Katsiyannis, A. C.; Mathioudakis, M.; Phillips, K. J. H.;
Williams, D. R.; Keenan, F. P.
Bibcode: 2001hell.confE.174K
Altcode:
No abstract at ADS
Title: X-ray and extreme-ultraviolet emission from the coronae
of Capella
Authors: Phillips, K. J. H.; Mathioudakis, M.; Huenemoerder, D. P.;
Williams, D. R.; Phillips, M. E.; Keenan, F. P.
Bibcode: 2001MNRAS.325.1500P
Altcode:
The primary objective of this work is the analysis and interpretation
of coronal observations of Capella obtained in 1999 September
with the High Energy Transmission Grating Spectrometer on the
Chandra X-ray Observatory and the Extreme Ultraviolet Explorer
(EUVE). He-like lines of O (Ovii) are used to derive a density of
1.7×1010cm-3 for the coronae of the binary,
consistent with the upper limits derived from Fexxi, Neix and Mgxi
line ratios. Previous estimates of the electron density based on Fexxi
should be considered as upper limits. We construct emission measure
distributions and compare the theoretical and observed spectra to
conclude that the coronal material has a temperature distribution
that peaks around 4-6MK, implying that the coronae of Capella were
significantly cooler than in the previous years. In addition, we
present an extended line list with over 100 features in the 5-24Å
wavelength range, and find that the X-ray spectrum is very similar to
that of a solar flare observed with SMM. The observed to theoretical
Fexvii 15.012-Å line intensity reveals that opacity has no significant
effect on the line flux. We derive an upper limit to the optical depth,
which we combine with the electron density to derive an upper limit of
3000km for the size of the Fexvii emitting region. In the same context,
we use the Siiv transition region lines of Capella from HST/Goddard
High-Resolution Spectrometer observations to show that opacity can be
significant at T=105K, and derive a path-length of ~75km
for the transition region. Both the coronal and transition region
observations are consistent with very small emitting regions, which
could be explained by small loops over the stellar surfaces.
Title: Opacity Effects in the Transition Region of Active Stars
(CD-ROM Directory: contribs/williams)
Authors: Williams, D. R.; Mathioudakis, M.; McKenny, J.; Keenan, F. P.
Bibcode: 2001ASPC..223..889W
Altcode: 2001csss...11..889W
No abstract at ADS
Title: Coordinated Optical, Radio and EUV Observations of a Flare
on YZ CMi (CD-ROM Directory: contribs/robinson)
Authors: Robinson, R. D.; Airapetian, V.; Slee, O. B.; Mathioudakis,
M.; Carpenter, K. G.
Bibcode: 2001ASPC..223.1151R
Altcode: 2001csss...11.1151R
No abstract at ADS
Title: S xi Emission Lines in Active Region Spectra Obtained with
the Solar euv Rocket Telescope and Spectrograph (Serts)
Authors: Keenan, F. P.; Pinfield, D. J.; Mathioudakis, M.; Aggarwal,
K. M.; Thomas, R. J.; Brosius, J. W.
Bibcode: 2000SoPh..197..253K
Altcode:
Theoretical electron density sensitive emission line ratios involving
a total of eleven 2s22p2-2s2p3
transitions in S xi between 187 and 292 Å are presented. A
comparison of these with solar active region observations obtained
during rocket flights by the Solar EUV Rocket Telescope and
Spectrograph (SERTS) reveals generally good agreement between
theory and experiment. However, the 186.87 Å line is masked by
fairly strong Fe xii emission at the same wavelength, while 239.83
Å is blended with an unknown feature, and 285.58 Å is blended with
possibly N iv 285.56 Å. In addition, the 191.23 Å line appears to
be more seriously blended with an Fe xiii feature than previously
believed. The presence of several new S xi lines is confirmed in the
SERTS spectra, at wavelengths of 188.66, 247.14 and 291.59 Å, in
excellent agreement with laboratory measurements. In particular, the
detection of the 2s22p23P1
-2s2p33P0,1 transitions at 242.91
Å is the first time (to our knowledge) that this feature has been
identified in the solar spectrum. The potential usefulness of the S xi
line ratios as electron density diagnostics for the solar transition
region and corona is briefly discussed.
Title: Extreme-Ultraviolet Flares in an F2 Star
Authors: Mullan, D. J.; Mathioudakis, M.
Bibcode: 2000ApJ...544..475M
Altcode:
We report on the coronal properties of two early F dwarfs. For
both stars, spectroscopic data reveal a hot corona with material at
temperatures as high as 107.1-107.2 K. The EUV
photometric observations of one of the stars (HR 120: F2) reveal two
large flares with rapid decay times. The rapid decays are in striking
contrast with previously published slowly decaying flares on a late
type F star (HR 1817: F8/9). Analysis of decay times suggests that the
loop sizes which flared in the F2 star are significantly shorter than
those which flared in the F8/9 star. We discuss these results in the
context of a proposal made by Giampapa & Rosner.
Title: The Radial and Angular Variation of Electron Density in the
Solar Corona.
Authors: Gallagher, P. T.; Mathioudakis, M.; Phillips, K. J. H.;
Tsinganos, K.; Keenan, F. P.
Bibcode: 2000SPD....31.0234G
Altcode: 2000BAAS...32.1290G
We derive, for the first time, electron densities as a function of
both radius (R) and position angle (θ ) for the south-west quadrant of
the off-limb corona, using the density-sensitive Si ix (349.9 Å/341.9
Å) and Si x (356.0 Å/347.7 Å) extreme ultraviolet line ratios. The
observations were made with the Coronal Diagnostic Spectrometer (cds)
on board the Solar and Heliospheric Observatory (soho), over the range
1.00R⊙ < R < 1.20R⊙ and 180° <
θ < 270° . Within the south polar coronal hole, the density varies
from 2.3 x 108 cm-3 at 1.0R⊙ to 8.3x
107 cm-3 at 1.20R⊙ , while at the
equator the density varies from 6.3 x 108 cm-3
at 1.0R⊙ to 1.6x 108 cm-3 at
1.20R⊙ . The density falloff with height is therefore
faster in the equatorial region. We also find that electron densities
are, on average, a factor of 2.7 larger in the equatorial regions than
in the polar coronal hole at a given radial distance. Finally, we find
remarkable agreement between our measured densities as a function of
radius and position angle and those predicted by a recent analytic
MHD model of the solar wind, strongly supporting its basic premises.
Title: Is Lithium produced in active stellar atmospheres?
Authors: Christian, Damian J.; Mathioudakis, Mihalis
Bibcode: 2000noao.prop..165C
Altcode:
Current mixing models predict that fast rotation leads to increased
Li depletion in late-type stars. This is in sharp contrast with
observations of stellar clusters where the largest Li abundances are
found in the most active stars. One of the key assumptions in these
models is that Li can not be produced on the stellar surface and its
abundance will decrease as a function of time. The present proposal will
investigate whether the unusually high Li abundance of very active flare
stars, could be attributed to spallation reactions. Lithium production
by particle induced spallation reactions will produce an isotope ratio
of ^7Li/^6Li ~ 2. This isotope ratio will create a clear asymmetry in
the red wing of the Li I 6707.8Aline profile. This asymmetry combined
with any temporal variation of the Li equivalent width, will provide
the observational signatures required to test the spallation theory.
Title: Multi-wavelength observations of the 1998 September 27
flare spray
Authors: Gallagher, Peter T.; Williams, David R.; Phillips, Kenneth
J. H.; Mathioudakis, Mihalis; Smartt, Raymond N.; Keenan, Francis P.
Bibcode: 2000SoPh..195..367G
Altcode:
We report on observations of a large eruptive event associated with a
flare that occurred on 27 September 1998 made with the Richard B. Dunn
Solar Telescope at Sacramento Peak Observatory (several wave bands
including off-line-center Hα), in soft and hard X-rays (GOES and
BATSE), and in several TRACE wave bands (including Fe ix/x 171 Å,
Fe xii 195 Å, and C iv 1550 Å). The flare initiation is signaled by
two Hα foot-point brightenings which are closely followed by a hard
X-ray burst and a subsequent gradual increase in other wavelengths. The
flare light curves show a complicated, three-component structure which
includes two minor maxima before the main GOES class C5.2 peak after
which there is a characteristic exponential decline. During the initial
stages, a large spray event is observed within seconds of the hard
X-ray burst which can be directly associated with a two-ribbon flare
in Hα. The emission returns to pre-flare levels after about 35 min,
by which time a set of bright post-flare loops have begun to form at
temperatures of about 1.0-1.5 MK. Part of the flare plasma also intrudes
into the penumbra of a large sunspot, generally a characteristic of
very powerful flares, but the flare importance in GOES soft X-rays is in
fact relatively modest. Much of the energy appears to be in the form of
a second ejection which is observed in optical and ultraviolet bands,
traveling out via several magnetic flux tubes from the main flare site
(about 60° from Sun center) to beyond the limb.
Title: Rotational modulation and flares on RS CVn and BY Dra
stars. XX. Photometry and spectroscopy of CC Eri in late 1989
Authors: Amado, P. J.; Doyle, J. G.; Byrne, P. B.; Cutispoto, G.;
Kilkenny, D.; Mathioudakis, M.; Neff, J. E.
Bibcode: 2000A&A...359..159A
Altcode:
The active flaring binary CC Eri was studied via multi-wavelength
observations involving multi-based photometry and ground- and
space-based spectroscopy. Combining early spectroscopic data with
the present implies an orbital period of P=1.5615 days. Furthermore,
the spectroscopic data suggests spectral types of K7 and M3 for the
system. The optical photometry indicated a small spot coverage in
late 1989, consistent with data taken a year later which showed CC Eri
entering its brightest-to-date phase. Two flares were detected in the
ultraviolet spectral data. These flares radiated 2.7 1031
erg and 1.6 1031 erg in the C Iv line alone, each with a
total estimated radiative energy budget of ~ 1036 erg. For
the higher-temperature lines, such as C Iv, there was no systematic
variability with phase. The lower-temperature lines show a slight
indication of rotational modulation. However, there is a much larger
scatter in the individual measurements of the Mg Ii and C Iv fluxes
than would be expected from measurement errors alone, consistent with
an atmosphere showing continual small-scale activity.
Title: Transition region and coronal structuring
Authors: O'Shea, E.; Gallagher, P. T.; Mathioudakis, M.; Phillips,
K. J. H.; Keenan, F. P.; Katsiyannis, A. C.
Bibcode: 2000A&A...358..741O
Altcode:
In this paper we examine regions of internetwork, network and bright
network emission, observed in the quiet Sun with the Coronal Diagnostic
Spectrometer (cds) onboard SoHO. The slopes of the emission measure
distributions, between 5.4 <= log Te <= 6.0, are
found to differ in each region, suggesting the presence of different
atmospheric structures. From an analysis of emission area the network
is shown to have two populations of structures, a low transition region
group and a coronal group. Using mdi magnetograms the bright network
emission is shown to originate from regions of strong magnetic field
composed of bipolar loops and unipolar funnels, that extend from the
low transition region up to the corona. Up to 30% of all radiative
losses between 5.7 <= log Te <= 6.3 are found to come
from these continuous bright network structures. Cross-sectional areas
calculated from redshift values suggest that the area expansion seen
in the bright network emission is the result of flux tube expansion
into the corona, accompanied by either a downflow or upflow of material.
Title: Identification of the [Al II] Forbidden Line at 2661 Å in
the Spectrum of RR Telescopii
Authors: Crawford, F. L.; Keenan, F. P.; Mathioudakis, M.; Aggarwal,
K. M.; McKenna, F. C.; Feibelman, W. A.; Espey, B. R.
Bibcode: 2000ASPC..204..387C
Altcode: 2000tiaf.conf..387C
No abstract at ADS
Title: Evidence for Non-Maxwellian Electron Energy Distributions in
the Solar Transition Region: Si III Line Ratios from SUMER
Authors: Pinfield, D. J.; Keenan, F. P.; Mathioudakis, M.; Phillips,
K. J. H.; Curdt, W.; Wilhelm, K.
Bibcode: 1999ApJ...527.1000P
Altcode:
Recent calculations of Si III emission-line strengths are compared with
SUMER observations from a quiet solar region, a coronal hole, and an
active region. Diagnostic line ratios are used to derive Te
and Ne for the emitting plasma in each region, and good
agreement between theory and observations is found. A major enhancement
in the intensity of the 1313 Å emission line is observed, as well
as some evidence of a small decrease in the temperature of maximum
ionization fraction, as one moves from the coronal hole to the active
region. Possible explanations for these effects are discussed, and
it is concluded that they may be caused by the presence of nonthermal
electrons in the transition region.
Title: The effects of opacity in the transition region of YZ CMi
Authors: Mathioudakis, M.; McKenny, J.; Keenan, F. P.; Williams,
D. R.; Phillips, K. J. H.
Bibcode: 1999A&A...351L..23M
Altcode:
It has generally been assumed that the emission from the upper
atmosphere of late-type stars is optically thin. In the present paper we
use the Si IV and C IV resonance lines to investigate this assumption
for the active dMe star YZ CMi. The significant deviations of the line
ratios from their optically thin values, demonstrate that opacity can
be quite important particularly in the case of stellar flares. These
deviations are combined with a method of escape probabilities to
derive optical depths of approximately unity for the lines under
consideration. We demonstrate that, if the electron density in the
atmosphere is known, opacity can provide important information on the
linear dimensions of the scattering layer. Using this technique, we
have estimated path lengths of a few kilometers for one of the flares
under consideration. Based on observations made with the NASA/ESA Hubble
Space Telescope, obtained from the data archive at the Space Telescope
Science Institute. STScI is operated by the Association of Universities
for Research in Astronomy, Inc. under NASA contract NAS 5-26555.
Title: A New Study of the Quiet Sun
Authors: O'Shea, E.; Gallagher, P. T.; Mathioudakis, M.; Phillips,
K. J. H.; Keenan, F. P.
Bibcode: 1999ESASP.446..519O
Altcode: 1999soho....8..519O
In this paper we present results of an emission measure analysis
performed on regions of cell, network and bright network emission
which were observed in the quiet Sun at Sun centre with the Coronal
Diagnostic Spectrometer (CDS) on board SOHO. Seperate emission from
the three regions was obtained by a method of image segmentation
using a histogram modification technique. From the averaged emission
it was possible to produce emission measures over a broad temperature
range. These emission measures were then used to produce estimates of
radiative and conductive losses in the three regions considered. Using
a magnetogram obtained from the MDI instrument on SOHO we show that
the bright network emission originates from bipolar magnetic loops
and from what appear to be monopole regions, which could be the
remains of footpoints of larger loops. The bright network emission
shows significantly higher mean magnetic field strength as well as
significantly larger radiative energy losses than either of the other
two regions. We find some evidence suggesting the presence of cool
loops. The process of image segmentation also enabled us to obtain
the relative areas of the cell, network and bright network emission
as a function of temperature. We found that only the bright network
emission showed an expansion in area with temperature and therefore
height. Fitting this area curve with the functional form suggested
by Rabin (1991),i.e A(T)/A(Th) = [1 + (Gamma2
- 1)(T/Th)nu]1/2/Gamma , for the
cross-sectional area of a flux tube, we obtain a value of Gamma=7.1
and nu=2.5 for the constriction and shape factors respectively.
Title: The Radial and Angular Variation of the Electron Density in
the Solar Corona
Authors: Gallagher, P. T.; Mathioudakis, M.; Keenan, F. P.; Phillips,
K. J. H.; Tsinganos, K.
Bibcode: 1999ApJ...524L.133G
Altcode:
We derive, for the first time, electron densities as a function
of both radius (R) and position angle (θ) for the southwest
quadrant of the off-limb corona, using the density-sensitive Si IX
λ349.9/λ341.9 and Si X λ356.0/λ347.7 extreme-ultraviolet line
ratios. The observations were made with the coronal diagnostic
spectrometer on board the Solar and Heliospheric Observatory over
the ranges of 1.00 Rsolar<R<1.20 Rsolar
and 180deg<θ<270deg. Within the south
polar coronal hole, the density varies from 2.3×108
cm-3 at 1.0 Rsolar to 8.3×107
cm-3 at 1.20 Rsolar, while at the equator,
the density varies from 6.3×108 cm-3 at 1.0
Rsolar to 1.6×108 cm-3 at 1.20
Rsolar. The density falloff with height is therefore faster
in the equatorial region. We also find that electron densities are,
on average, a factor of 2.7 larger in the equatorial regions than in
the polar coronal hole at a given radial distance. Finally, we find
remarkable agreement between our measured densities as a function of
radius and position angle and those predicted by a recent analytic
MHD model of the solar wind, strongly supporting its basic premises.
Title: [Alii] in the ultraviolet spectrum of the symbiotic star
RR Telescopii
Authors: Keenan, F. P.; Espey, B. R.; Mathioudakis, M.; Aggarwal,
K. M.; Crawford, F. L.; Feibelman, W. A.; McKenna, F. C.
Bibcode: 1999MNRAS.309..195K
Altcode:
An inspection of a GHRS/HST spectrum of the symbiotic star RR Telescopii
reveals the presence of the [Alii] 3s^2 ^1S - 3s3p ^3P_2 line at a
vacuum wavelength of 2661.06+/-0.08Å, 8.89+/-0.08Å away from the Alii]
3s^2 ^1S - 3s3p ^3P_1 intercombination transition at 2669.95Å, in good
agreement with the theoretical prediction of Δλ=8.80Å. We also find
that the Alii] line profile is asymmetric, showing a strong low-density
component with a weak high-density wing, redshifted by 30kms^-1, in
agreement with the findings of Schild & Schmid, which were based
on optical observations. Our measurement of the emission-line ratio
RI(2661.06Å)/I(2669.95Å)=0.027+/-0.003 implies logN_e=5.8+/-0.2,
in good agreement with the densities found from other ions, such as
Siiii. These results provide strong evidence that we have detected the
[Alii] line, the first time (to our knowledge) that this feature has
been reliably identified in an astrophysical or laboratory spectrum.
Title: HR 1099 - A multi-wavelength study of a short period RS
CVn binary
Authors: Mathioudakis, Mihalis; Christian, Damian; Keenan, Francis
Bibcode: 1999noao.prop...50M
Altcode:
The launch of AXAF in the summer of 1999 will open a new era in the
study of stellar coronae. The active RS CVn binary HR 1099 is one of
the AXAF primary calibration targets that will be observed in the
first 3-4 months of the mission. We propose to carry out a ground
based spectroscopic study of HR 1099. With the combination of optical
and X- ray observations we will be able to : (i) Study the effects
of rotational modulation in photospheric (TiO bands), chromospheric
(H(alpha), Ca II H& K) and coronal lines (e.g. Fe XVII) (ii)
Carry out a near simultaneous estimate of photospheric and coronal
abundances (iii) Investigate whether the absorption features that
cross the H(alpha) profile are due to cool hydrogen prominences or mass
transfer between the components of the binary. The requested observing
time will allow us to cover ~ 3 rotational revolutions with an ~ 80%
phase coverage.
Title: Electron temperature diagnostics for the quiet Sun using SI
BT IV lines
Authors: Ahmed, S.; Pinfield, D. J.; Mathioudakis, M.; Keenan, F. P.;
Phillips, K. J. H.; Curdt, W.
Bibcode: 1999A&A...346L..69A
Altcode:
We compare SUMER observations of six Si riptsize IV emission lines
detected at the quiet Sun disk centre with recent theoretical line ratio
calculations. Good agreement is found between theory and observation
for ratios involving the 1394, 1403 and 818 Ä line intensities. This
agreement supports the theoretical prediction that the temperature
where Si riptsize IV has its maximum ionisation fraction in ionisation
equilibrium is T_max =~ 10(4.8) K, as well as showing that Lyman
continuum absorption does not significantly effect line intensities
for transitions with wavelengths below 912 Ä. We find that the 815,
1122 and 1128 Ä lines are blended by approximately 30, 55 and 45%,
respectively, in the SUMER transitions.
Title: Soft X-ray emission lines of Ni XVIII in the solar spectrum
Authors: Keenan, F. P.; Mathioudakis, M.; Pinfield, D. J.; Brown,
W. A.; Bruner, M. E.
Bibcode: 1999SoPh..185..289K
Altcode:
R-matrix calculations of electron impact excitation rates in Ni xviii
are used to derive theoretical electron-temperature-sensitive emission
line ratios involving 3s−4p,3p−4d,3p −4s, and 3d−4f transitions
in the 41-53 Å wavelength range. A comparison of these with solar
flare observations from a rocket-borne X-ray spectrograph (XSST) reveals
generally excellent agreement between theory and experiment (within the
experimental and theoretical uncertainties), which provides support
for the atomic data adopted in the analysis. However the 3s 2S−4p
2P1/2 line of Ni xviii at 41.22 Å appears to be blended with the Fe
xix 13.74 Å feature observed by XSST in third order. In addition, the
measured Ni xviii intensity ratio I(3p 2P3/2− 4s 2S)/I(3p 2P1/2−4s
2S)=I(51.02 Å)/I(50.26 Å)=0.56, a factor of ∼3.8 smaller than the
theoretical (temperature and density-insensitive) value of 2.1. The
reason for this discrepancy is currently unexplained, but is unlikely
to be due to blending of the 50.26 Å line, as the intensity of this
feature is consistent with that expected from the other Ni xviii lines
in the XSST spectrum. Future observations of the Ni xviii lines by
the Advanced X-ray Astrophysics Facility (AXAF) should allow this
problem to be resolved, and may also permit the use of the lines as
electron-temperature diagnostics.
Title: Magnetic activity in the extreme ultraviolet.
Authors: Mathioudakis, M.
Bibcode: 1999TJPh...23..399M
Altcode:
The wealth of astronomical observations obtained in the extreme
ultraviolet in recent years, have allowed comprehensive studies of the
stellar transition regions and coronae to be carried out. For the first
time the author has been able to resolve individual coronal lines of
various elements formed over a large temperature range (105 -
107.2K). The temperature, densities, abundances and magnetic
field strengths of the stellar coronae can now be determined. The
author reviews some of the observations in the field of cool stars
and discusses the implications on atmospheric heating requirements.
Title: HR 1817: the EUV properties of an active F dwarf
Authors: Mathioudakis, M.; Mullan, D. J.
Bibcode: 1999A&A...342..524M
Altcode:
We examine the coronal properties of the active F dwarf HR
1817. Photometric observations with EUVE show that the source is
in a near-continuous state of flare-like activity. Using IUE and
EUVE spectroscopic observations we construct the emission measure
distribution in the 10(4) -10(7.2) K temperature range. These
observations reveal a hot corona and activity levels similar to those
of RS CVn binaries. Based on Fe XXI line ratios we derive an upper
limit of 10(11.7) cm(-3) for the coronal density of HR 1817 and a
magnetic field strength of B<= 160 Gauss. A comparison of the EUVE
spectroscopic observations with synthetic spectra derived from ASCA
and ROSAT fits, shows that optimal agreement is obtained for fits with
sub-solar metal abundances. The reduced metal abundances increase the
radiative losses significantly at temperatures above 10(6.5) K where
Fe is no longer the dominant radiative cooling agent.
Title: Magnetic Activity and Variability in Stellar Coronae (EUV)
Authors: Mathioudakis, M.
Bibcode: 1999ASPC..158...38M
Altcode: 1999ssa..conf...38M
No abstract at ADS
Title: The O V 1213.9 Angstroms forbidden line in the quiet Sun
Authors: Pinfield, D. J.; Mathioudakis, M.; Keenan, F. P.; Phillips,
K. J. H.; Curdt, W.
Bibcode: 1998A&A...340L..15P
Altcode:
We present the first unambiguous detection of the O v 1213.9 Angstrom
(2s(2) (1) S_0 -> 2s2p (3) P_2) forbidden line in the solar spectrum,
using observations obtained with the SUMER instrument on board SOHO. The
wavelength separation of the forbidden to the (2s(2) (1) S_0 ->
2s2p (3) P_1) intercombination line at 1218.35 Angstroms is 4.5+/-0.1
Angstroms in excellent agreement with theoretical predictions. The
observed line ratio, combined with the latest calculations, allows us
to derive an electron density of log {N_e/cm(-3}) = 8.5 +/- 0.15 for
the quiet Sun. However, the O v 1213.9 Angstroms line was not detected
in the HST spectra of Procyon and we can only place a lower limit of
log {N_e/cm(-3}) >= 7.8 to the electron density in this object.
Title: Extreme Ultraviolet Explorer Right Angle Program Observations
of Cool Stars
Authors: Christian, D. J.; Drake, J. J.; Mathioudakis, M.
Bibcode: 1998AJ....115..316C
Altcode:
The Extreme Ultraviolet Explorer (EUVE) Right Angle Program (RAP)
obtains photometric data in four bands centered at ~100 Å (Lexan/B),
~200 Å (Al/Ti/C), ~400 Å (Ti/Sb/Al), and ~550 Å (Sn/SiO) during
pointed spectroscopic observations. RAP observations are up to 20 times
more sensitive than those in the EUVE all-sky survey. We present RAP
observations of two dozen late-type stars: BD +03 deg301, BD +05 deg300,
HR 1262, BD +23 deg635, BD +22 deg669, Melotte 25 VA 334, Melotte 25
1366, Melotte 25 59, Melotte 25 65, theta^1 Tau, V834 Tau, GJ 2037,
BD -21 deg1074, GJ 205, RE J0532-030, GJ 9287A, HT Vir, BD +46 deg1944,
Proxima Cen, alpha Cen A/B, HR 6094, HR 8883, CPD -48 deg10901, and HR
8964. We derive surface fluxes from the Lexan/B and Al/Ti/C count rates
and cataloged ROSAT Position Sensitive Proportional Counter (PSPC)
data. The EUVE surface fluxes are reasonably correlated with surface
fluxes calculated from PSPC measurements. The time variability of the
sources has been examined. Most of the sources show no significant
variability at the 99% confidence level. Flares were detected from the
K7 V star Melotte 25 VA 334, the K3 V star V834 Tau (HD 29697), and the
K3 + K8 Hyades binary BD +22 deg669. The BD +22 deg669 count rate at
the peak of the flare is a factor of 6 higher than the quiescent count
rate, with a peak Lexan/B luminosity of 7.9 x 10^29 ergs s^-1. The
V834 Tau flare was detected in both Lexan/B and Al/Ti/C bands. The
peak luminosity of the flare is 1.6 x 10^29 and 8 x 10^28 ergs s^-1
for Lexan/B and Al/Ti/C, respectively.
Title: The Extreme Ultraviolet Explorer Stellar Spectral Atlas
Authors: Craig, N.; Abbott, M.; Finley, D.; Jessop, H.; Howell, S. B.;
Mathioudakis, M.; Sommers, J.; Vallerga, J. V.; Malina, R. F.
Bibcode: 1997ApJS..113..131C
Altcode:
We present an atlas of extreme ultraviolet (EUV) spectra of 95 bright
stellar sources observed between 1992 July and 1996 June with the
Extreme Ultraviolet Explorer (EUVE) spectrometers. These data are
taken in the short- (SW; 70-190 Å), medium- (MW; 140-380 Å), and
long-wavelength bandpasses (LW; 280-760 Å) at roughly 0.5, 1, and 2
Å resolution, respectively. We describe the spectrometers and detail
the procedure used to reduce the observational data to spectra. The
atlas is grouped by the type of source: O-A stars, F-M stars, white
dwarfs, and cataclysmic variables. We present a brief overview of
the general nature and EUV spectral distribution of each group and
present accompanying notes and individual spectra for each source. We
show selected F-M sources in more detail with identifications of the
brightest spectral lines illustrating the characteristics of the EUV
spectra of stars of various temperatures. The current study is the
most complete compilation to date of aggregate spectra of bright EUV
stellar sources.
Title: Faint Sources in the EUVE Survey: Identification of White
Dwarfs, Active Late-Type Stars, and Galactic Nuclei
Authors: Polomski, Elisha; Vennes, Stéphane; Thorstensen, John R.;
Mathioudakis, Mihalis; Falco, Emilio E.
Bibcode: 1997ApJ...486..179P
Altcode:
We report the classification of 21 new extreme-ultraviolet sources
from the recent catalog of Lampton et al. The optical spectra presented
identify the objects as 14 active late-type stars (including two double
active stars and a possible T Tauri star), three white dwarfs, and
six active galactic nuclei (a Seyfert galaxy, the BL Lac object 1ES
1028+511 [=EUVE J1031+508], and four quasi-stellar objects). We have
detected Ca II absorption lines in the BL Lac object and measured its
redshift. Two of the white dwarfs are unusually massive (M > 1.1
M⊙). Our sample of late-type stars includes five previously
known high proper motion objects (EUVE J1004+503, J2244-332A,B,
J1802+642, and J1131-346), of which one is the well-known flare star
TX PsA (EUVE J2244-332B). We report an unusually high level of activity
for the primary component of the TX PsA system (EUVE J2244-332A), which
may indicate flare activity. The group of late-type stars is on average
almost 3 mag fainter (<m> ~ 13) than the typical member of the
Extreme Ultraviolet Explorer (EUVE) all-sky survey catalog. All Galactic
and extragalactic objects were also detected in the ROSAT Position
Sensitive Proportional Counter survey, and most are at the faint limit
of the EUVE detectors. These new identifications substantially increase
the total number of EUV-selected extragalactic sources.
Title: Radio and extreme-ultraviolet observations of CF Tucanae
Authors: Gunn, A. G.; Migenes, V.; Doyle, J. G.; Spencer, R. E.;
Mathioudakis, M.
Bibcode: 1997MNRAS.287..199G
Altcode:
We present contemporaneous EUV and radio observations obtained for
the totally eclipsing chromospherically active RS CVn-type binary CF
Tucanae. Observations were made with the ATNF Compact Array at 6 and
3.6cm over one complete orbital cycle of the system, and with the EUVE
Deep Survey/Spectrometer telescope over five orbital cycles. We obtained
a strong detection of the source (1.9mJy at 6cm and 1.1mJy at 3.6cm)
in the radio. Both EUV and radio flux levels appear to show a clear
modulation at the orbital period with a maximum at phase 0.5 which
indicates the existence of an intra-binary region of activity. This
has important consequences for the details of coronal formation and
field interaction in active close binary stars. We interpret the EUV
emission as an isotropically radiating thermal source; on the other
hand, the radio emission shows evidence of intrinsic gyromagnetic
directivity. We also demonstrate that contemporaneous EUV and radio
observations are not sufficient to determine the characteristics of
the gyro-emission electron population.
Title: The intrinsic LY α to H α ratio in M dwarf stars.
Authors: Doyle, J. G.; Mathioudakis, M.; Andretta, V.; Short, C. I.;
Jelinsky, P.
Bibcode: 1997A&A...318..835D
Altcode:
Using Ly α line profiles generated from a grid of M
dwarf model atmospheres we calculate the attenuation factor
[i.e. Lyα(obs)/Lyα(intrin)] due to the interstellar medium as
a function of hydrogen column density. Then using selected model
atmospheres, attenuation factors were calculated for those M dwarfs with
available Ly α observations. The prime motivation in this work has been
to look afresh at the intrinsic Ly α to H α flux ratio, an important
constraint in the radiative transfer modelling of M dwarfs. For those
active dMe stars where both lines were observed, the intrinsic ratio
is ~3-5 (with ~50% variation). The major uncertainty in this work has
been the interstellar hydrogen column density, emphasizing the need
for further work in this area and in particular an accurate model of
its variation in all directions.
Title: An active K0 IV-V star and a hot white dwarf (EUVE J0702+129)
in a wide binary.
Authors: Vennes, S.; Christian, D. J.; Mathioudakis, M.; Doyle, J. G.
Bibcode: 1997A&A...318L...9V
Altcode:
We present far ultraviolet and optical spectroscopy of the Extreme
Ultraviolet Explorer (EUVE) survey source EUVE J0702+129 revealing
a composite K0 star plus DA white dwarf spectrum. The International
Ultraviolet Explorer spectra show continuum emission from a hot white
dwarf (Teff=30-40,000K) and a rising contribution from the K0 star
at λ>2500Å. High resolution optical spectroscopy uncovers a high
level of activity with strong Hα and Ca H&K emission; application
of the Wilson-Bappu relation indicates that the secondary star is
slightly above the main sequence (K0 IV-V). Both objects are found at
a distance of ~130pc and they likely constitute a physical pair. The
EUV emission is dominated by the white dwarf, but the late-type star
certainly contributes at higher energy. An interesting parallel is
drawn with other DA+K0 pairs with moderately active secondaries such
as HD 18131 and HR 1608. The present discovery as well as other recent
ones demonstrate the existence of a large population of white dwarfs
hidden by evolved companions (III-IV).
Title: EUV Emission from RS Canum Venaticorum binaries.
Authors: Mitrou, C. K.; Mathioudakis, M.; Doyle, J. G.; Antonopoulou,
E.
Bibcode: 1997A&A...317..776M
Altcode:
We performed a study of 104 RS CVn systems in the extreme ultraviolet
(EUV) using the all-sky survey data obtained by the Extreme Ultraviolet
Explorer (EUVE). The present sample includes several new RS CVn
detections; 11 more than in the published EUVE catalogs, and 8 more
than in the ROSAT Wide Field Camera catalog. The ratio of detections to
non-detections remained constant throughout the sky, implying that our
detections are not limited by the exposure time but are most likely
limited by absorption from the interstellar medium. A general trend
of increasing Lex/B (50-180Å) flux with decreasing rotational period
is clear. The dwarf systems exhibit a leveling-off for the faster
rotators. In contrast, the evolved systems exhibit no such effect. For
the RS CVn systems the losses in the EUV represent a smaller fraction
of the coronal radiative losses, as compared to active late-type dwarfs.
Title: The Stellar Astrophysical Laboratory
Authors: Mathioudakis, M.
Bibcode: 1997eaun.book..123M
Altcode:
No abstract at ADS
Title: UV and EUV observations of the active binary UX Ari
Authors: Antonopoulou, E.; Mathioudakis, M.; Ferentinos, E.
Bibcode: 1997jena.confE.104A
Altcode:
We present a study in the UV and EUV region of the electromagnetic
spectrum (60-3200AA), of UX Ari, an active RS CVn--type of binary
system. Combining the UV and EUV observations we have been able to
cover the atmosphere from the chromosphere all the way up into the lower
corona, ie the temperature range between 10{^4} and 10^{7.2}K. The EUV
spectra show several hot coronal lines of highly ionized Fe, formed at
the higher end of this range. We construct the Emission Measure (EM)
distribution, which is then used to generate synthetic spectra. A timing
analysis of the EUV observations, shows the existence of a distinct
"active phase" for this system. The spectrum of the active interval,
contains the FeXXIV line which is formed by material at temperatures
of logT = 7.2K. This line is not present in the quiescent spectrum. We
have also found, in this system's active spectrum, a line at 114.39AA,
which is not mentioned in the existing lists. This unidentified line
was also present in the EUV spectrum of HR1099, a similarly active
system. A discussion of the errors involved in obtaining the EM
distribution and the synthetic spectra, is also given. The dominant
error arises from the values of the atomic parameters.
Title: Coronal variability of RS CVn binaries in the EUV
Authors: Antonopoulou, E.; Ferentinos, E.; Mathioudakis, M.; Mitrou,
C. K.
Bibcode: 1997jena.confE.221A
Altcode:
No abstract at ADS
Title: Fe X Emission Lines in Solar and Stellar Spectra
Authors: Foster, V. J.; Mathioudakis, M.; Keenan, F. P.; Drake, J. J.;
Widing, K. G.
Bibcode: 1996ApJ...473..560F
Altcode:
Theoretical electron density sensitive emission line ratios involving
Fe X 3s23p5-3s23p43d
transitions in the 170-190 Å wavelength range are compared with
observational data for a solar active region and flares, obtained
during the Skylab mission, and Cen and Procyon observations from the
Extreme Ultraviolet Explorer (EUVE) satellite. Electron densities
derived from the majority of the ratios are consistent for the events
but are in poor agreement with the values of Ne estimated
from diagnostic lines in other species observed in the spectra, casting
doubt on the accuracy of the theoretical line ratio calculations and,
hence, the atomic data of Mohan et al. used in their derivation. At
low Ne, the present ratios are significantly different
from those of Young et al., while the latter imply densities that
are in somewhat better agreement with densities derived from other
diagnostics. This would appear to indicate that the electron impact
excitation rates of Bhatia & Doschek adopted by Young et al. are
to be preferred over the Mohan et al. results.
Title: A New K0 IV--V Plus White Dwarf Binary in The EUVE Survey
Authors: Christian, D. J.; Vennes, S.; Mathioudakis, M.
Bibcode: 1996AAS...189.7902C
Altcode: 1996BAAS...28.1380C
We present far ultraviolet and optical spectroscopy of the Extreme
Ultraviolet Explorer (EUVE) survey source EUVE J0702+129 showing
a composite K0 star plus DA white dwarf spectrum. International
Ultraviolet Explorer spectra show continuum emission from a hot white
dwarf (T_eff =30-40,000 K) and a rising contribution from the K0 star
at lambda >= 2500 Angstroms. High resolution optical spectroscopy
reveals a moderate level of activity with Ca II H & K emission;
application of the Wilson-Bappu relation indicates that the secondary
star is slightly above the main-sequence (K0 IV--V). Both objects are
found at a distance of ~ 100 pc and probably constitute a physical
pair. The EUV emission is probably dominated by the white dwarf, but
the late-type star possibly contributes at higher energy. Interesting
parallels are drawn with other DA+K0 pairs with moderately active
secondaries such as HD 18131 and HR 1608. The present discovery as well
as other recent ones demonstrate the existence of a large population
of white dwarfs hidden by evolved companions (III--IV). This work is
supported by NASA grant NAG-2405 and contract NAS5-29298.
Title: Detection of Quiescent Extreme Ultraviolet Emission from
the Very Low Mass Dwarf van Biesbroeck 8: Evidence for a Turbulent
Field Dynamo
Authors: Drake, Jeremy J.; Stern, Robert A.; Stringfellow, Guy;
Mathioudakis, Mihalis; Laming, J. Martin; Lambert, David L.
Bibcode: 1996ApJ...469..828D
Altcode:
We report the detection of quiescent EUV emission from the very low
mass dwarf VB 8 by the Extreme Ultraviolet Explorer (EUVE) in the
Lexan/B band (65-190 Å). We interpret this emission in terms of a
hot coronal plasma and combine this information with previous X-ray
detections to estimate the quiescent plasma temperature and emission
measure. The combined observations made by Einstein, ROSAT, and EUVE
between 1979 and 1994 are consistent with a quiescent coronal plasma
temperature of (2-6) x 106 degrees and indicate the same
emission measure to within a factor of about 2. The nonflaring corona
of VB 8 then appears relatively constant over timescales of more than
10 yr. Our results are consistent with the picture of a turbulently
driven or distributive dynamo for VB 8, rather than with a large-scale
field dynamo which appears to dominate the solar corona. Evidence from
X-ray and optical data concerning the long-term coronal variability of
the more active stars of higher mass also points toward the idea that
active late-type stars in general are dominated by a turbulent dynamo.
Title: Discovery of a White Dwarf Companion (MS0354.6-3650 = EUVE
J0356-366) to a G2V Star
Authors: Christian, Damian J.; Vennes, Stephane; Thorstensen, John R.;
Mathioudakis, Mihalis
Bibcode: 1996AJ....112..258C
Altcode:
We present x-ray, ultraviolet, and optical observations
of the mysterious EUV/soft x-ray source EUVE J0356-336 (=
MS0354.6-3650). Initial Einstein observations identified this source
with a cluster of galaxies, but the relatively high source count rate
in the Extreme Ultraviolet Explorer (EUVE) 100 Å band and the lack
of variability hinted that EUVE J0356-3650 might be a white dwarf;
the UK Schmidt plate of the field surrounding this object found a 12.45
magnitude G2V star that could hide a compact companion. This hypothesis
was confirmed in an IUE ultraviolet spectrum that shows the definite
signature of a hydrogen-rich white dwarf (DA). A model atmosphere
analysis shows that the DA star is hot (Teff≥52,000 K)
and may have a low abundance of heavy elements. We show that the G2V
star and the DA star form a physical pair at a distance of 400 pc and
therefore add to the list of white dwarf plus luminous main sequence
star binaries discovered in EUV surveys.
Title: Optical Identifications of Fainter Late-type Stars from EUVE
and ROSAT Detections
Authors: Christian, D. J.; Craig, N.; Roberts, B.; Lampton, M. L.;
Mathioudakis, M.; Drake, J. J.
Bibcode: 1996AAS...188.7102C
Altcode: 1996BAAS...28..941C
We present results of the continuing optical campaign to identify
sources detected in the 100 Angstroms Extreme Ultraviolet Explorer
(EUVE ) all-sky survey that are also detected in the ROSAT 0.25
keV band. This paper concentrates on a dozen sources for which the
optical counterparts are identified as Me stars. Data were obtained
in 1995 December and 1996 February using the CTIO 1.5 m Cassegrain
with resolution of 8 km s(-1) and covering a bandpass of 3500--6700
Angstroms. All spectra show the Balmer series and Ca H & K lines
in emission. We present source identifications, spectral classes,
distances, visual magnitudes, estimated hydrogen column densities,
and the EUVE and ROSAT count rates. Implications for the fainter end
of the EUV log N-log S distribution will be discussed. This work was
supported by NASA contract NAS5-29298.
Title: An EUVE Stellar Spectral Atlas
Authors: Abbott, M.; Craig, N.; Jessop, H.; Sommers, J.; Finley, D.;
Vallerga, J. V.; Malina, R. F.; Howell, S. B.; Mathioudakis, M.
Bibcode: 1996AAS...188.5509A
Altcode: 1996BAAS...28..909A
We present an atlas of stellar Extreme Ultraviolet Explorer (EUVE )
spectra. The spectra of approximately 80 stars acquired between 1992
June and 1995 June have been uniformly processed from data in the EUVE
public archive. Sources were chosen for inclusion in the atlas based
on their brightness and the quality of their spectra. We have divided
the atlas into four sections, representing classes of stellar sources
seen by EUVE : early (O--F) stars, late (G--M) stars, white dwarfs,
and cataclysmic variables. Other stellar and non-stellar classes will
be presented in future atlases. For each section, we summarize the EUV
spectral properties of the class and then show the spectra along with
detailed comments on the individual sources. The spectra contained
in the atlas will also be made available through the CEA WWW site at
http://www.cea.berkeley.edu. This work was supported by NASA contract
NAS5-29298 and NASA grant NAG5-2902.
Title: The Secrets of EUVE J2056-17.1
Authors: Mathioudakis, M.; Drake, J. J.; Craig, N.; Kilkenny, D.;
Doyle, J. C.; Sirk, M.; Dupuis, J.; Fruscione, A.; Christian, C. A.;
Abbott, M. J.
Bibcode: 1996aeu..conf..475M
Altcode: 1996IAUCo.152..475M
No abstract at ADS
Title: Observation and modelling of main sequence star
chromospheres. V. Ultraviolet excess emission in active M dwarfs.
Authors: Houdebine, E. R.; Mathioudakis, M.; Doyle, J. G.; Foing, B. H.
Bibcode: 1996A&A...305..209H
Altcode:
The variation in the continuum intensity (50-5000nm) of late-type
M dwarfs is investigated via two grids of model atmospheres with
different temperature minima. It is shown that the (E)UV intensity
is mostly dependent on the transition region pressure, although the
temperature minimum also plays an important role. We also observe
a significant frequency redistribution of the photospheric flux when
changing the minimum temperature, and a black-body type of emission from
the lower chromosphere. We examine the formation of the continuum and
point out that, although some differences appear for very low or very
high activity levels, in general the global picture is much alike the
Sun. We show that the UV continua are very good diagnostics of cool
dwarf atmospheres, from the temperature minimum to the transition
region. Our calculations give a good overview of the domains where
physical parameters and spectral signatures are most likely observed. We
compute the UBV(RIJKL)_J_ broad band fluxes for our models and conclude
that the chromospheric contribution should be detectable in the U
band and possibly also in the B band. We compare our calculations to
recent high resolution observations for selected stars in a narrow
spectral range ((R-I)_K_=0.875+/-0.05); we show that Hα line profiles
behave as expected, with a tight correlation between the line width
and equivalent width. Hα emission line stars show an excess in U-B
color but not in B-V. They are also more luminous than their less
active absorption line counterparts, which indicates that active dwarfs
have not yet reached the main sequence and are intermediate between T
Tauri stars and main sequence stars. The anomalously large proportion
of active stars towards late spectral types is attributed to the very
slow contraction phase for low mass stars. We calculate the fluxes in
the Extreme Ultraviolet Explorer Sn/SiO band (500-740A) and show that
they are compatible with observed upper limits. We further compare
our results for the upper activity range to pre-main sequence models
and observations. They strongly support the case for a chromospheric
contribution to Hα and the blue/UV excess for those objects (T
Tauris, naked T Tauris, YSOs). An important conclusion is that at
high pressures, corresponding to active dMe stellar atmospheres,
the chromosphere becomes a very efficient radiator at continuum
wavelengths. From log(M)~-5 (column mass), radiative losses in the
continuum rise exponentially and faster than in Hydrogen spectral lines
because of their larger optical depths. As a consequence, Hi spectral
lines have a small or negligible contribution to the total Hi (lines and
continua) and white light radiative budget. We show that the hydrogen
series dominate the radiative cooling in spectral lines (from 40% to
90%) for Hα emission line stars. The cooling in the (E)UV continuum
overwhelms the total radiative budget and is much larger than that
in outstanding chromospheric and transition region lines (e.g. CaII,
MgII, HI Lyman and Balmer). We propose this as a possible contribution
for the apparent saturation observed in some spectral lines, therefore
questioning the suggestion of saturation in magnetic activity levels.
Title: EUVE right angle program observations of cool stars
Authors: Christian, D. J.; Drake, J. J.; Mathioudakis, M.
Bibcode: 1996ASPC..109..593C
Altcode: 1996csss....9..593C
No abstract at ADS
Title: The RS CVn binaries seen through the Eye of IRAS
Authors: Mitrou, C. K.; Doyle, J. G.; Mathioudakis, M.; Antonopoulou,
E.
Bibcode: 1996hell.conf..301M
Altcode:
No abstract at ADS
Title: EM distribution and radiative losses of II Peg and lambda And
Authors: Mitrou, C. K.; Doyle, J. G.; Mathioudakis, M.; Antonopoulou,
B.
Bibcode: 1996ASPC..109..275M
Altcode: 1996csss....9..275M
No abstract at ADS
Title: Searching for a Fundamental Heating Mechanism in Low-activity
Dwarfs
Authors: Mathioudakis, M.
Bibcode: 1996ApL&C..34..187M
Altcode:
No abstract at ADS
Title: Searching EUVE Data for Transient/Flaring Extreme Ultraviolet
Sources
Authors: Lewis, J.; Bowyer, S.; Lampton, M.; Wu, X.; Mathioudakis, M.
Bibcode: 1996aeu..conf..481L
Altcode: 1996IAUCo.152..481L
No abstract at ADS
Title: The dependence of II Pegasi flare activity on the Phase of
the short-term periodic light Variations
Authors: Mathioudakis, M.; Avgoloupis, S. I.; Mavridis, L. N.;
Seiradakis, J. H.
Bibcode: 1996hell.conf..234M
Altcode:
No abstract at ADS
Title: The EUVE Optical Identification Campaign II: Late-Type and
White Dwarf Stars
Authors: Craig, N.; Fruscione, A.; Dupuis, J.; Mathioudakis, M.; Drake,
J. J.; Abbott, M.; Christian, C.; Green, R.; Boroson, T.; Howell, S. B.
Bibcode: 1996aeu..conf..491C
Altcode: 1996IAUCo.152..491C
No abstract at ADS
Title: EUV Constraints on Models of Low Mass X-ray Binaries
Authors: Christian, D. J.; Edelstein, J. E.; Mathioudakis, M.;
McDonald, K.; Sirk, M. M.
Bibcode: 1996aeu..conf..425C
Altcode: 1996IAUCo.152..425C
No abstract at ADS
Title: The emission of the RS CVn binaries in the IRAS passbands.
Authors: Mitrou, C. K.; Doyle, J. G.; Mathioudakis, M.; Antonopoulou,
E.
Bibcode: 1996A&AS..115...61M
Altcode:
In the literature, there is an ambiguity pertaining to the existence
of a far-IR excess in RS CVn systems. In the current paper we undertook
a study of the behaviour of 103 such systems in the IRAS passbands. We
found 72 acceptable detections in the 12μm band, and 40 in the 25μm
band (50% more than the IRAS Point Source Catalog). Although our
findings may be interpreted as indicating towards the existence of an
excess beyond 12μm for some systems, the evidence is not conclusive
in all but two cases. These are systems GX Lib and HR 7428, with
Capella being the only system where the IRAS fluxes in all four bands
originate from the stellar photosphere. Given the accuracy of the data
we did not find 12μm excess for any system. We argue that the IRAS
data alone cannot settle the issue, as their uncertainty is, in many
cases, higher than what the Signal-to-Noise ratio of each observation
implies. Furthermore, at the higher wavelength bands the IRAS angular
resolution drops from 0.5' at 12μm to 2' at 100μm and the background
becomes very complex, so one cannot be certain about the origin of
the observed flux. The existence of IR excess in the RS CVn stars is
important as far as the evolutionary scenarios for these systems are
concerned. Future missions such as the Infrared Space Observatory will
return more sensitive and accurate measurements and the ambiguity can
be removed. The fluxes we quote will be helpful when planning these
future observations, as we provide more accurate photometry and for
a larger selection of sources than the IRAS Point Source Catalog.
Title: EUVE Right Angle Program Observations of Late-Type Stars
Authors: Christian, D. J.; Mathioudakis, M.; Drake, J. J.
Bibcode: 1995AAS...18710320C
Altcode: 1995BAAS...27.1433C
The EUVE Right Angle Program (RAP) obtains photometric data in four
bands centered at ~ 100 Angstroms (Lexan/B), ~ 200 Angstroms (Al/Ti/C),
~ 400 Angstroms (Ti/Sb/Al), and ~ 550 Angstroms (Sn/SiO). RAP
observations are up to 20 times more sensitive than the all-sky
survey. We present RAP observations of the late-type stars: BD+03
301, BD+05 300, HR 1262, BD+23 635, BD+22 669, Melotte 25 VA 334,
Melotte 25 1366, Melotte 25 59, Melotte 25 65, theta (1) Tau, V834
Tau, GJ 2037, BD-21 1074, GJ 205, RE J0532-030, GJ 9287A, HT Vir,
BD+46 1944, Proxima Cen, alpha Cen A/B, HR 6094, CPD-48 10901, and
HR 8883. We derive fluxes and emission measures from Lexan/B and
Al/Ti/C count rates. The time variability of the sources has been
examined. Most of the sources show no significant variability at the
99% confidence level. Flares were detected from the K3V star V834 Tau
(HD 29697) and the K0 star BD+22 669. The BD+22 669 count rate at the
peak of the flare is a factor of 10 higher than the quiescent count
rate with a peak Lexan/B luminosity of 7.9 x 10(29) erg s(-1) . The
V834 Tau flare was detected in both Lexan/B and Al/Ti/C bands. The
peak luminosity of the flare is 1.6 x 10(29) erg s(-1) and 8 x 10(28)
ergs s(-1) for Lexan/B and Al/Ti/C, respectively. This is a factor of
4.3 higher than the quiescent luminosity in Lexan/B, and a factor of
4.6 in Al/Ti/C\@. This work is supported by NASA contract NAS5-29298.
Title: Optical identification of EUV sources: the secrets of EUVE
J2056-17.1.
Authors: Mathioudakis, M.; Drake, J. J.; Craig, N.; Kilkenny, D.;
Doyle, J. G.; Sirk, M. M.; Dupuis, J.; Fruscione, A.; Christian,
C. A.; Abbott, M. J.
Bibcode: 1995A&A...302..422M
Altcode:
We present optical, ultraviolet (UV), and extreme ultraviolet (EUV)
results for a new active late-type dwarf star discovered by the Extreme
Ultraviolet Explorer (EUVE). A large flare with an energy in excess of
10^35^erg and duration of ~1.1 days was detected in the EUVE Lex/B band
(60-200A). The energetics of the event indicate that radiative losses in
the EUV are a significant contributor to the energy budget in stellar
flares. The "quiescent" EUV emission of the source is similar to the
most active flare stars with the "quiescent" optical spectrum showing
strong Hα, Ca II H & K, and Mg II h & k emission. A strong
Li I 6707.8A absorption line is also present in the spectrum. We have
estimated a Li abundance of log N(Li)=2.5+/-0.4. Although the high
Li abundance suggests that EUVE J2056-17.1 is a young object having
recently arrived on the main sequence; the high flare activity favours
an interpretation where the enhanced Li is sustained by spallation
reactions.
Title: The EUVE Optical Identification Campaign II: Late-Type and
White Dwarf Stars
Authors: Craig, N.; Fruscione, A.; Dupuis, J.; Mathioudakis, M.; Drake,
J. J.; Abbott, M.; Christian, C.; Green, R.; Boroson, T.; Howell, S. B.
Bibcode: 1995AJ....110.1304C
Altcode:
We present optical identifications of nine previously unidentified
extreme ultraviolet (EUV) sources discovered during the Extreme
Ultraviolet Explorer (EUVE) satellite surveys. The all-sky survey
detected four of the sources and the more sensitive deep survey
detected the other five sources. Three of the four all-sky survey
sources, EUVE_J1918+599, EUVE_J2249+585, and EUVE_J2329+414, are
listed in present catalogs as having possible associations with
optical counterparts but without spectral class. The first two of these
sources are hot DA white dwarfs showing an optical spectrum with broad
Balmer lines. The source EUVE_J2329+414 is listed as having a possible
association with an unclassified M star. We show that a pair of dMe
stars are actually optical counterparts located within the error circle
of the EUVE source position. The EUVE_J2114+503 remains unidentified
even though all the possible candidates have been studied. Based on the
count rates we predict a fainter white dwarf or a cataclysmic variable
counterpart for this candidate. All five sources discovered with the
EUVE deep survey, EUVE_J03 18+184, EUVE_J0419+217, EUVE_J2053-175,
EUVE_J2056-171, and EUVE_J2233-096, have been identified as late-type
stars. The spectral classes, distances, visual magnitudes, and estimated
hydrogen column densities for these EUVE sources are presented.
Title: Activity versus rotation in the extreme ultraviolet.
Authors: Mathioudakis, M.; Fruscione, A.; Drake, J. J.; McDonald,
K.; Bowyer, S.; Malina, R. F.
Bibcode: 1995A&A...300..775M
Altcode:
This paper presents extreme ultraviolet (EUV) fluxes for 74
main-sequence stars observed with the Extreme Ultraviolet Explorer
(EUVE). The sample of stars was chosen on the basis of rotational
period and includes 23 new EUV detections. These detections are in
addition to the ROSAT WFC and EUVE catalogs and have increased the
number of main-sequence stars detected in the extreme ultraviolet by
~12%. We present an activity-rotation relation in the EUV and show
that in a sample of stars with different effective temperatures the
Rossby number is a better parameter for describing the levels of EUV
emission as compared to the rotational period. This is also known
to be the case for the chromospheric emission. A comparison with the
emission in the Mg II h and K lines shows that saturation in the EUV
occurs in lower Rossby numbers than does chromospheric saturation,
where saturation in X-rays occurs in the same Rossby numbers as in the
EUV. The radiative losses in the EUV are comparable to the radiative
losses in X-rays. The EUVE observations indicate that the low activity
dM star Gl 685 has a cool corona with a characteristic temperature of
<2.5x10^6^K. A large amount of the coronal radiative losses in low
activity dwarfs is expected to emerge in EUV wavelengths.
Title: Discovery of a white dwarf companion (EUVE J0254-053) to the
K0 IV star HD18131
Authors: Vennes, S.; Mathioudakis, M.; Doyle, J. G.; Thorstensen,
J. R.; Byrne, P. B.
Bibcode: 1995A&A...299L..29V
Altcode:
New ultraviolet (UV) observations of late-type stars detected in
the Extreme Ultraviolet Explorer (EUVE) all-sky survey revealed
an unsuspected white dwarf companion to the K0 star HD18131. The
International Ultraviolet Explorer (IUE) spectrum show a composite of a
white dwarf and a late-type star. The white dwarf dominates the emission
below 2000A while the K0 star prevails at longer wavelengths. A model
atmosphere analysis of the new ultraviolet spectrophotometry and of
the extreme ultraviolet (EUV) photometry reveals a hot, hydrogen-rich
(DA) white dwarf (Teff~30000K) that is the most likely source of the
EUV emission (EUVE J0254-053). We estimate a distance to the white
dwarf of 70-90pc. The K0 star shows a modest level of chromospheric
activity with the detection of Mg II h and k emission in the IUE
spectrum. Optical spectroscopy revealed that the K0 star is a subgiant
(K0 IV). The star is located at a distance of ~70pc, consistent with
the estimated distance of the white dwarf. Therefore, it most likely
constitutes a physical pair with the white dwarf. Until results of a
radial velocity study are made available we cannot establish whether
the pair is wide or close. In earlier works the strong EUV emission was
attributed to the K0 star; however, our multiwavelength observations
show the white dwarf as the most likely source. This discovery has
important implications for the EUV white dwarf population survey and,
in particular, for the binary frequency.
Title: Erratum - a Preliminary Review of EUVE Science
Authors: Vennes, S.; Drake, J. J.; Mathioudakis, M.; Welsh, B.;
Fruscione, A.; Hall, D. T.; Warren, J.; Howell, S. B.
Bibcode: 1995IrAJ...22..228V
Altcode:
No abstract at ADS
Title: Dissecting stellar coronae with EUVE.
Authors: Drake, J. J.; Mathioudakis, M.
Bibcode: 1995IrAJ...22...10D
Altcode:
No abstract at ADS
Title: A Preliminary Review of EUVE Science
Authors: Vennes, S.; Drake, J. J.; Mathioudakis, M.; Welsh, B.;
Fruscione, A.; Hall, D. T.; Warren, J.; Howell, S. B.
Bibcode: 1995IrAJ...22....7V
Altcode:
No abstract at ADS
Title: Optical Identification and Spectroscopy of EUV Sources
Authors: Craig, N.; Fruscione, A.; Abbott, M.; Christian, C. A.;
Drake, J. J.; Dupuis, J.; Mathioudakis, M.; Green, R.; Boroson, T.
Bibcode: 1994AAS...185.8014C
Altcode: 1994BAAS...26.1448C
We present optical identification and analysis of previously
unidentified extreme ultraviolet (EUV) sources (60--200 Angstroms)
discovered with the Extreme Ultraviolet Explorer (EUVE) during the
all-sky and deep surveys. Two EUVE deep survey sources, EUVE_J0318+184
and EUVE_J2056-171, are identified as active, young, late-type
stars. They display strong Hα , Ca ii H and K emission, and Li
6707.8 Angstroms absorption suggesting spectral types of dM0e. A very
strong flare with energy in excess of 10(35) erg was also observed
on EUVE_J2056-171 (Mathioudakis et al. 1994, in prep.). We also
identified four previously cataloged EUVE sources: EUVE_J1918+59.9,
EUVE_J2249+58.5, EUVE_J0419+21.7, and EUVE_J2329+41.4. The first
two sources are identified in both the ROSAT Wide Field Camera and
the EUVE catalogs as unclassified ``stars.'' They are in fact hot
DA white dwarfs showing a clean optical spectrum with broad Balmer
lines. EUVE_J0419+21.7 was previously assigned a possible identification
with 56 Tau, an A0sp star. Optical spectra of other candidates within
the error circle, in addition to the bright A star, show three emission
line dMe stars in the field. We attribute the EUV emission to one or
more of this collection of dMe stars. The source EUVE_J2329+41.4 is
currently identified as probably being G 190--28 with spectral type
M. However we show that both G 190--28 and G 190--27 are dMe stars
and are located within the error circle of the EUVE position of this
source, and therefore both of these stars probably contribute to the
EUV emission. This work has been supported by NASA contract NAS5-29298.
Title: Detection of EUV emission from the low activity dwarf HD 4628:
evidence for a cool corona.
Authors: Mathioudakis, M.; Drake, J. J.; Vedder, P. W.; Schmitt,
J. H. M. M.; Bowyer, S.
Bibcode: 1994A&A...291..517M
Altcode:
We present observations of low activity late-type stars obtained with
the Extreme Ultraviolet Explorer (EUVE). These stars are the slowest
rotators, and acoustic heating may dominate their outer atmospheric
heating process. We report detection of EUV emission from the low
activity K dwarf HD 4628 during the EUVE Deep Survey in the Lexan/boran
band. This detection, in conjunction with the non-detection of this
object in the ROSAT PSPC all-sky survey, suggests the existence of a
cool corona with a characteristic temperature of less than 10^6^K. The
flux and spectral signature are consistent with current theories of
acoustic heating.
Title: Surface inhomogeneities on SZ Piscium.
Authors: Doyle, J. G.; Mitrou, C. K.; Mathioudakis, M.; Avgoloupis,
S.; Mavridis, L. N.; Varvoglis, P. P.; Graos, F.; Antonopoulou, E.
Bibcode: 1994A&A...291..135D
Altcode:
Ultraviolet and optical data are used to look at surface inhomogeneities
on the active close binary system SZ Piscium. Lower chromospheric
emission lines such as Mg II h&k, and to a lesser extend the
upper chromospheric/lower transition region lines of O I 1305A and C
II 1335A show a pronounced decrease when the F star is eclipsing the
K sub-giant. However, higher temperature lines such as the resonance
line of C IV 1550A do not show any change in flux. This suggests the
presence of a plage with a high contrast at lower temperatures with
a Mg II plage-to-K-star ratio of 5. Alternative explanations are
explored. Monitoring of this system in the optical U-band and the
ultraviolet lines such as C IV 1550A, indicates frequent high energy
(10^35^-10^36^erg) flares.
Title: Lower chromospheric activity in low activity M dwarfs
Authors: Doyle, J. G.; Houdebine, E. R.; Mathioudakis, M.; Panagi,
P. M.
Bibcode: 1994A&A...285..233D
Altcode:
Four low activity dM(e) stars Gl 105B, Gl 643, Gl 813 and Gl 821
were observed spectroscopically in the optical and ultraviolet
regions. Despite relatively long integrations, only one of the four
had detectable Ca II K&H emission, Gl 821, which had an observed
flux of 9 10^-15^ erg/cm2/s, giving a surface flux of only 0.8 10^4^
erg/cm2/s. Similarly, the IUE observations implied very weak Mg II
emission. In one of these stars, Gl 813, the Mg II h&k surface
flux was less than 0.4 10^4^ erg/cm2/s, which is a factor of two
less than the previous known weakest dM(e) star, Gl 105B. Using
radiative transfer techniques the Ca II H&K fluxes have been
derived for a range of model atmospheres. The zero Hα flux can be
produced by a range of models, this line is therefore a rather poor
constraint for the lower atmosphere. On the other hand, the Ca II K
flux can be dramatically effected by simply varying the temperature
minimum. Comparing our calculations to the observational data, we find
that the Ca II H&K surface fluxes cannot be reproduced with T_min_
3000K. However, decreasing the temperature minimum by less than 400K
produces Ca II H&K fluxes of the order of 10^3^ erg/cm2/s, in very
good agreement with the weakest dM(e) stars.
Title: High-velocity evaporation during a flare on AT Microscopii
Authors: Gunn, A. G.; Doyle, J. G.; Mathioudakis, M.; Houdebine,
E. R.; Avgoloupis, S.
Bibcode: 1994A&A...285..489G
Altcode:
We present and discuss observations of a flare event on the dM4.5e star
AT Mic (Gl 799AB). These data show evidence of an energetic flare with
a radiated energy of 3 10^31^erg in the range 3600-4200A. The Balmer
emission lines display a prominent blue asymmetry which has been used
to constrain the velocity of the associated bulk plasma motion. Simple
models were applied to derive order of magnitude estimates for the mass
and kinetic energy budget of the flare for assumed electron density,
ionization fraction and temperature values. It is found that the
kinetic energy of the flare plasma is two orders of magnitude less
than the radiative energy release. The chromospheric Ca II H &
K lines are also observed to have a blue-shifted component which has
not been observed prior to these results. We suggest that this is
evidence of a very energetic particle beam penetrating to the cooler
atmospheric layers where Ca II H & K is formed.
Title: The Extreme Ultraviolet (EUV) Rotation-activity Relation for
Late-Type Stars
Authors: Mathioudakis, M.; Fruscione, A.; Drake, J. J.; McDonald,
K.; Bowyer, S.; Malina, R. F.
Bibcode: 1994AAS...184.0520M
Altcode: 1994BAAS...26Q.866M
We present results on an optically selected sample of 44 main sequence
stars, including 18 EUV sources not previously reported, detected
with the Extreme Ultraviolet Explorer (EUVE) satellite. The sample
covers a broad range of spectral types (early F to late M) with known
rotation periods from 0.4 days to 50 days. A comparison of the EUV
fluxes with activity related parameters shows that the dynamo related
Rossby number is the best parameter for describing the levels of coronal
activity. This is in agreement with the conclusion of Noyes et al. (ApJ,
279,763) for the chromosphere. The saturated levels of coronal activity
are discussed. This work has been supported by NASA contract NAS5-30180.
Title: An optical flare on YZ Canis Minoris
Authors: Gunn, A. G.; Doyle, J. G.; Mathioudakis, M.; Avgoloupis, S.
Bibcode: 1994A&A...285..157G
Altcode:
We present and discuss observations of an energetic optical flare on
the dM4.5e star YZ CMi (Gl 285). The flux-time variations and energy
release of the flare are fully consistent with previous observations
of this star. The observations have also revealed a symmetric excess
emission in the bases of the chromospheric Balmer lines. If due to a
bulk plasma motion event, the maximum extension in the Balmer wings
represents a Doppler velocity of ~ 250 km/s.
Title: Ultraviolet flare activity on the eclipsing binary SZ Piscium.
Authors: Doyle, J. G.; Mitrou, C. K.; Mathioudakis, M.; Antonopoulou,
E.
Bibcode: 1994A&A...283..522D
Altcode:
The estimated total chromospheric, transition region and coronal
losses for a flare observed on the eclipsing binary SZ Psc with the
International Ultraviolet Explorer (IUE) satellite on 6 July 1990 is
of the order of 4.5 1036 ergs. This estimate is similar to
that derived by Doyle et al. (1989) for an ultraviolet flare on another
RS CVn star, II Peg, and is a substantial fraction of the total amount
that is possible to store on these stars without having to evoke the
presence of kiloGauss fields.
Title: EUVE Observations of Late Type Stars with Minimum Levels
of Activity
Authors: Mathioudakis, M.; Drake, J. J.; Vedder, P. W.; Patterer,
R. J.; Bowyer, S.
Bibcode: 1994ASPC...64...38M
Altcode: 1994csss....8...38M
No abstract at ADS
Title: H alpha Variability on HK Aqr: Coronal Condensations or Plages?
Authors: Byrne, P. B.; Mathioudakis, M.; Young, A.; Skumanich, A.
Bibcode: 1994ASPC...64..375B
Altcode: 1994csss....8..375B
No abstract at ADS
Title: EUVE Photometry Observations of Cool Stars (Invited Review)
Authors: Vedder, P. W.; Brown, A.; Drake, J. J.; Patterer, R. J.;
Mathioudakis, M.; Gully, S.; Pettersen, B. R.
Bibcode: 1994ASPC...64...13V
Altcode: 1994csss....8...13V
No abstract at ADS
Title: The extreme ultraviolet (EUV) rotation-activity relation for
late-type stars.
Authors: Mathioudakis, M.; Fruscione, A.; Drake, J. J.; McDonald,
K.; Bowyer, S.; Malina, R. F.
Bibcode: 1994BAAS...26R.866M
Altcode:
No abstract at ADS
Title: Low Activity M Dwarfs
Authors: Doyle, J. G.; Houdebine, E. R.; Mathioudakis, M.
Bibcode: 1994ASPC...64..557D
Altcode: 1994csss....8..557D
No abstract at ADS
Title: Coronal Variability in the Extreme Ultraviolet
Authors: Drake, J. J.; Mathioudakis, M.; Pye, J. P.; Fruscione, A.;
Bowyer, S.; Vedder, P. W.; Patterer, R. J.
Bibcode: 1993AAS...183.1506D
Altcode: 1993BAAS...25.1314D
Late-type stars are the most numerous among the many different
categories of object detected in the first all-sky survey of the 60--760
Angstroms region recently completed by the Extreme Ultraviolet Explorer
satellite (EUVE). To date, this survey has yielded extreme ultraviolet
(EUV) photometric data for nearly 200 late-type stars. Two years
earlier, photometry was obtained by the ROSAT Wide Field Camera (WFC)
in the 60--200 Angstroms band. Using both data sets, we have looked
for changes over this two year period in the coronal emission from
the late-type stars that have been detected by both EUVE and the ROSAT
WFC. The results are discussed with reference to stellar activity cycles
and flaring. This work has been supported by NASA contract NAS5--30180.
Title: EUVE Observations of Planetary Nebulae
Authors: Fruscione, A.; Abbott, M.; Drake, J. J.; Dupuis, J.; Malina,
R. F.; Mathioudakis, M.; McDonald, K.; Chu, K. C.
Bibcode: 1993AAS...183.5112F
Altcode: 1993BAAS...25.1371F
We present preliminary results of extreme ultraviolet observations
(60--740 Angstroms) of a complete sample of planetary nebulae
(PN) selected from the Strasbourg-ESO Catalog (Acker et al. 1992)
and observed during the Extreme Ultraviolet Explorer (EUVE) all-sky
survey. Our selection criteria are focused on minimizing the impact of
the interstellar absorption on the extreme ultraviolet observations, and
our sample includes the 26 closest PN (distances <= 1 kpc) at high
galactic latitude (|b|>= 20deg ). During the EUVE all-sky survey at
least five PN in our sample (NGC 246, NGC 1360, K1-16, Lo Tr5, NGC 4361)
were detected in the shortest band (58--174 Angstroms). The extreme
ultraviolet emission originates in the central star. Upper limits
in the four EUVE bands (centered at ~ 100, 200, 400, 600 Angstroms)
are given for all the sources. This work has been supported by NASA
contract NAS5-30180.
Title: Far infrared properties of late type dwarfs. Infrared fluxes
of K and M dwarfs.
Authors: Mathioudakis, M.; Doyle, J. G.
Bibcode: 1993A&A...280..181M
Altcode:
IRAS fluxes/upper limits are presented for a large sample of K
and M dwarfs. Good agreement is found between the 12 micrometer
fluxes and those derived from the photospheric models of Mould
(1976). Relationships between the optical and infrared colors are
derived. The active dMe/dKe stars appear systematically brighter
in the infrared compared with the less active dM/dK stars, which
could be attributed to more efficient nonradiative heating in their
atmosphere. Any systematic differences found in our results when
compared with those obtained from previous studies are attributed to
the different analysis packages used.
Title: Rotational modulation and flares on the RS Canum Venaticorum
binary II Pagasi in July/September 1990 : spots and flares on II Peg.
Authors: Doyle, J. G.; Mathioudakis, M.; Murphy, H. M.; Avgoloupis,
S.; Mavridis, L. N.; Seiradakis, J. H.
Bibcode: 1993A&A...278..499D
Altcode:
During ultraviolet spectroscopic observations of the RS CVn star II
Peg in September 1990 a long duration (greater than or = 3 hrs.) flare
was observed. During the early stage of the event, a feature at 1354
A was present, however, within the spectral resolution of the data
it is not possible to identify this line. A contribution from the
hot coronal ion Fe XXI is suspected. From line diagnostic ratios, the
electron pressure at flare peak was estimated to be 1017/cu
cm K, decreasing to 1016/cu cm K towards the end of the
flare. One other flare was observed with IUE, and three optical flares
(unfortunately none of these were observed simultaneously). The
chromospheric and transition region losses from the larger of
the two IUE flares was approx. 3 x 1031 erg/s at flare
maximum, with total chromospheric/transition region radiative losses
over the duration of the event being approx. 1.5 x 1035
erg. Continuum radiative losses over the wavelength region 1150A to
1950A were approximately 3% of the above figure. At flare maximum,
the N V 1240 A line showed an enhancement factor of approx. 3 over the
preflare value compared to 9 for the C IV 1550 A line. We interpret this
difference as due to an underabundance of nitrogen during the flare,
possibly related to photoionization of lower chromospheric material
by soft X-ray photons sometime prior to the flare. No evidence of
rotational modulation was present in any of the transition region lines,
although the chromospheric lines did show a phase variation. However,
these lines (H-alpha, Ca II K and Mg II h&k) were not consistent
with one another although it is clear that the H-alpha equivalent
width showed variations faster than the star's rotation period, being
perhaps related to the decay/activation of individual active regions.
Title: Controversy on the Infrared Excess of GL:735
Authors: Mathioudakis, M.
Bibcode: 1993IrAJ...21..152M
Altcode:
No abstract at ADS
Title: The importance of surface inhomogeneities for K and M
dwarfchromospheric fluxes.
Authors: Panagi, P. M.; Mathioudakis, M.
Bibcode: 1993A&AS..100..343P
Altcode:
We present published and archived spectroscopic and spectrophotometric
data of H-alpha, Ca II, Mg II, and X-rays for a large sample of K
and M dwarfs. The data set points to the importance that surface
inhomogeneities have in the flux luminosity diagrams in these
late-type dwarfs, irrespective of whether the Balmer lines are in
emission or absorption. Although supporting the fact that cooler
stars exhibit increasing levels of surface activity, evident through
an increasing incidence of Balmer emission, surface inhomogeneities,
or variations in the local temperature and density structure, at the
chromospheric level, dominate the total Ca II and Mg II fluxes. We
show that the flux-flux and luminosity-luminosity relations indicate
differing extents of inhomogeneity from the chromosphere through to the
corona. A good correlation between Ca II and Mg II fluxes indicates
that they are formed in overlapping regions of the chromosphere,
so that the contribution of surface inhomogeneities is not evident
from this particular flux-flux diagram. In the region of the upper
chromosphere through to the transition and corona, the correlation
between Ly-alpha and X-ray fluxes indicates regions with similar levels
of areal inhomogeneity. This appears to be uncorrelated with that at
the chromospheric level.
Title: Imaging the Photosphere &Chromosphere of the Rapidly
Rotating dMe Star, HK Aqr
Authors: Byrne, P. B.; Mathioudakis, M.
Bibcode: 1993ASSL..183..435B
Altcode: 1993pssc.symp..435B
No abstract at ADS
Title: Chromospheric Heating in K &M Dwarfs
Authors: Doyle, J. G.; Mathioudakis, M.
Bibcode: 1993ASSL..183..471D
Altcode: 1993pssc.symp..471D
No abstract at ADS
Title: Dynamic phenomena on the RS Canum Venaticorum binary II Pegasi
in August 1989. I. Observational data.
Authors: Doyle, J. G.; Kellett, B. J.; Butler, C. J.; Byrne, P. B.;
Neff, J. E.; Brown, A.; Fox, D.; Linsky, J. L.; Bromage, G. E.;
Avgoloupis, S.; Mavridis, L. N.; Seiradakis, J. H.; Mathioudakis,
M.; Murphy, H. M.; Krzesinski, J.; Pajdosz, G.; Dadonas, V.;
Sperauskas, J.; van Wyk, F.; Marang, F.; Olah, K.; Collier Cameron,
A.; Antonomoulos, E.; Rovithis, P.; Rovithis-Livaniou, H.
Bibcode: 1992A&AS...96..351D
Altcode:
Observational results are given for two flares in Pi Pegasi that were
detected by the GINGA and IUE satellites, as well as by ground-based
Johnson U-band photometry and optical spectroscopy. The best-fitting
results were obtained for a combination of a single-temperature
plasma together with a power law; for a two-temperature model, the
temperature of the second component was very high due to the tail in
the photon distribution.
Title: Limits on Detectability of Mass Loss from Cool Dwarfs
Authors: Mullan, D. J.; Doyle, J. G.; Redman, R. O.; Mathioudakis, M.
Bibcode: 1992ApJ...397..225M
Altcode:
Recent spectroscopic evidence supports the theoretical expectation
that certain cool dwarfs may have stellar winds with M-dot values
several orders of magnitude larger than the solar rate. For large
enough values of M-dot, the emission from the wind is expected to have
a spectrum which, at low enough frequencies, becomes a power law,
S(v) about v exp alpha with alpha about 0.7. Data from IRAS and VLA
suggest that such a spectrum may in fact occur in certain M dwarfs:
a key test of the wind spectrum would be provided if the stars could
be detected at lambda about 1 mm. We show that the M-dot required to
ensure power-law emission is a few times 10 exp -10 solar mass/yr. With
M-dot of this order, fluxes at lambda about 1 mm would be tens of
mJy. Using the James Clerk Maxwell Telescope, we have tested this
prediction on several stars: the data are suggestive but are near the
limits of detection. Confirmation of our estimates will be important
for evolution and for interstellar medium (ISM) physics: if even a
few percent of all M dwarfs are losing mass at the above rates, the
mass balance of the ISM will be dominated by M dwarfs.
Title: Chromspheric diagnostics in M dwarfs
Authors: Mathioudakis, M.; Doyle, J. G.
Bibcode: 1992IrAJ...20..272M
Altcode:
No abstract at ADS
Title: Chromospheric heating in late-type dwarfs : acoustic or
magnetic ?
Authors: Mathioudakis, M.; Doyle, J. G.
Bibcode: 1992A&A...262..523M
Altcode:
Mg II h and k fluxes for 69 K and 88 M dwarfs have been analyzed. An
empirical lower limit is found in the Mg II flux, extended down to
the latest spectral types. Based on the data presented here it is the
magnetic component that dominates the emission from the atmosphere of
cool dwarfs, especially the M dwarfs. It, however, coexists with an
acoustically heated component, which can only be identified in the
stars with the lowest fluxes in the flux-color diagram, these stars
possibly being the slowest rotators. Data for the dM(e) stars, i.e.,
those stars with zero H-alpha, show that these are divided into two
classes: (1) an inactive M dwarf star with very weak chromospheric
heating, perhaps only by acoustic waves; and (2) an intermediate
chromospheric activity star dominated by magnetic heating.
Title: Limits on detectability of mass loss from cool dwarfs
Authors: Mullan, D. J.; Doyle, J. G.; Mathioudakis, M.; Redman, R. O.
Bibcode: 1992AAS...180.6006M
Altcode: 1992BAAS...24..826M
Recent spectroscopic evidence supports the theoretical expectation
that certain cool dwarfs may have stellar winds with dot {M} values
several orders of magnitude larger than the solar rate. For large
enough values of dot {M}, the emission from the wind is expected to
have a spectrum which, at low enough frequencies, becomes a power law,
S_ν ~ nu (alpha ) with alpha ~ 0.7. Data from IRAS and VLA suggest
that such a spectrum may in fact occur in certain M dwarfs: a key
test of the wind spectrum would be provided if the stars could be
detected at lambda ~ 1 mm. We show that the dot {M} required to ensure
power law emission is a few times 10(-10) Msun \ yr(-1)
. With dot {M} of this order, fluxes at lambda ~ 1 mm would be tens
of mJy. Using the James Clerk Maxwell Telescope, we have tested this
prediction on several stars: the data are suggestive but are near the
limits of detection. Confirmation of our estimates will be important
for studies of evolution of low mass stars and for interstellar medium
(ISM) physics: if even a few percent of all M dwarfs are losing mass
at the above rates, the mass balance of the ISM will be dominated by
M dwarfs. Moreover, if flares on these stars are accompanied by mass
ejecta, the ISM may become contaminated with deuterium produced in
the surface layers of the star by energetic protons.
Title: Optical flares on the RS CVn binary II Peg.
Authors: Mathioudakis, M.; Doyle, J. G.; Avgoloupis, V.; Mavridis,
L. N.; Seiradakis, J. H.
Bibcode: 1992MNRAS.255...48M
Altcode:
During 57.4 hr of optical monitoring in the Johnson U- and B-bands of
the RS CVn binary II Peg, 10 flares were detected. The absolute energies
of these events are in the range 1-180 x 10 exp 33 erg. This implies
a flare activity in the U-band of 0.17 flares per hour, compared with
0.12 flares per hour from UV C IV data. A double power-law distribution
is required to fit the cumulative flare-frequency distribution of the
U-band flares.
Title: Millimeter; Sub-Millimeter Emission from Flare Stars
Authors: Mullan, D. J.; Doyle, J. G.; Mathioudakis, M.; Redman, R. O.
Bibcode: 1992ASPC...26..328M
Altcode: 1992csss....7..328M
No abstract at ADS
Title: GL 890 - The Magnetic Brake?
Authors: Byrne, P. B.; Doyle, J. G.; Mathioudakis, M.
Bibcode: 1992ASPC...26..438B
Altcode: 1992csss....7..438B
No abstract at ADS
Title: The UV Spectrum of the RS CVn Binary SZ PSc
Authors: Danezis, E.; Antonopoulou, E.; Mathioudakis, M.; Theodossiou,
E.
Bibcode: 1992LNP...397..273D
Altcode: 1992sils.conf..273D
No abstract at ADS
Title: Magnetic activity phenomena in dwarf K and M stars
Authors: Mathioudakis, Mihalis
Bibcode: 1992PhDT.......295M
Altcode:
No abstract at ADS
Title: Optical Flares on II Peg
Authors: Mathioudakis, M.; Doyle, J. G.; Avgoloupis, S.; Mavridis,
L. N.; Seiradakis, J. H.
Bibcode: 1992ASPC...26..303M
Altcode: 1992csss....7..303M
No abstract at ADS
Title: The Ultraviolet Spectrum of the Binary System Sz-Piscium
Authors: Danezis, E.; Antonopoulou, E.; Theodossiou, E.; Mathioudakis,
M.
Bibcode: 1992Ap&SS.187..307D
Altcode:
In this paper we study the far-UV as well as the UV spectrum of the
spectroscopic binary system SZ Psc in the wavelength ranges λλ1235
1950 Å and λλ2710 3090 Å, respectively, from spectra obtained with
the International Ultraviolet Explorer (IUE). The UV spectrum of SZ Psc
is mainly an emission spectrum. The short wavelength region includes
emission lines formed from the low chromosphere to the transition region
(e.g., Si iv, Civ, and Nv) and also a deep and broad absorption line
of Fe ii. The Mg ii[1] resonance doublet at about λ2800 Å presents
a P Cygni profile and a multiple structure with two emission and two
absorption satellite components. We also present the emission measure
diagram in the temperature region 4.4<logT e <53.
Title: The August 1989 Spot Lightcurve of II Peg and Related Flare
Activity
Authors: Avgoloupis, S.; Doyle, J. G.; Mavridis, L. N.; Seiradakis,
J. H.; Mathioudakis, M.
Bibcode: 1992LNP...397..247A
Altcode: 1992sils.conf..247A
No abstract at ADS
Title: Rotationalmodulation and flares on RS Canum Venaticorum and BY
Draconis stars. XVI. IUE spectroscopy and VLA observations of GL 182
(=V 1005 Orionis) in October 1983.
Authors: Mathioudakis, M.; Doyle, J. G.; Rodono, M.; Gibson, D. M.;
Byrne, P. B.; Avgoloupis, S.; Linsky, J. L.; Gary, D.; Mavridis,
L. N.; Varvoglis, P.
Bibcode: 1991A&A...244..155M
Altcode:
A large flare was detected simultaneously with IUE and VLA on Gl 182 on
October 5, 1983, this event showing the largest C IV flare enhancement
yet observed by IUE. A smaller flare was also detected on October 4,
although only with the IUE satellite. Line ratio and emission measure
techniques are used to derive various physical parameters of the
flares. The radiative losses in the temperature region log T(e) =
4.3-5.4 in the two flares are 2.9 x 10 to the 33rd and 8.4 x 10 to
the 32nd ergs, respectively. Total radiative losses over the whole
temperature range log T(e) = 4.0-8.0 are estimated to be of the order
of 6.4 x 10 to the 34th and 1.1 x 10 to the 34th ergs, respectively. In
the October 5, flare, a very strong ultraviolet continuum is present
with a total energy of 1.9 x 10 to the 33rd ergs over the wavelength
range 1250-1950 A.
Title: Active M-type stars from the ultraviolet to the infrared.
Authors: Mathioudakis, M.; Doyle, J. G.
Bibcode: 1991A&A...244..433M
Altcode:
Fluxes at 12 microns are presented for a group of seven active dMe
stars. Five of these stars are considered as significant detections
at 25 microns, while only two stars were detected at 60 microns and
only one at 100 microns. Comparing with previous results it is found
that the fluxes are systematically lower by 7-50 percent depending on
the band. Above 60 microns there is a definite excess over a blackbody
radiation curve.
Title: Chromospheric diagnostics in late-type stars.
Authors: Mathioudakis, M.; Doyle, J. G.
Bibcode: 1991A&A...244..409M
Altcode:
Moderate and low resolution spectroscopic observations of late type
dwarfs covering a broad range of activity are presented. The strength of
the TiO band in 4762 A is used for spectral classification. Calibrated
fluxes for the Balmer and the Ca II H and K lines are derived. H-alpha
equivalent widths are given for those stars where the line is in
absorption as well as those in emission. It is confirmed that stars
with no detectable or very weak H-alpha do exist. It is also shown
that stars with very weak or no detectable emission in Ca II H and K
also exist, this occurring preferentially in late M dwarfs. Although
this can be explained in terms of reduced nonradiative heating in
the chromosphere, the results cannot be conclusive due to the limited
resolution of the data.
Title: Millimeter continuum emission from flare stars.
Authors: Doyle, J. G.; Mathioudakis, M.
Bibcode: 1991A&A...241L..41D
Altcode:
The first detection of continuum emission in the millimeter region
for two dwarf M flare stars is reported. The observed flux at 1.1 and
2 mm is weak, being just above background, but above that based on a
black-body curve. This confirms the impression gained from infrared
fluxes at 60 and 100 microns, which indicated excess emission at
these wavelengths.
Title: Circumstellar Material in the RS CVn System SZ Psc
Authors: Mathioudakis, M.
Bibcode: 1991iue..prop.4081M
Altcode:
No abstract at ADS
Title: Chromospheric/Coronal Emission Correlations in 'Quiescent'
and Eruptive Phenomena in M-Dwarf Stars (With 1 Figure)
Authors: Mathioudakis, M.; Doyle, J. G.
Bibcode: 1991mcch.conf..279M
Altcode:
No abstract at ADS
Title: Discovery of flare activity on the dwarf M stars, GL 375 and
GL 431.
Authors: Doyle, J. G.; Mathioudakis, M.; Panagi, P. M.; Butler, C. J.
Bibcode: 1990A&AS...86..403D
Altcode:
Optical and infrared photometry plus spectroscopic data is present
for two new flare stars, Gl 375 and Gl 431. Both of these stars have
the hydrogen Balmer lines strongly in emission. Several flares were
detected on Gl 375 implying a high level of flare activity. The H-alpha
surface flux of 1.0 x 10 to the 6th erg/sq cm per sec for both stars
is similar to that of other active flare stars. Fluxes are given for
several of the higher Balmer lines.
Title: Chromospheric/coronal emission correlations in "quiscent"
and eruptive phenomena in M-dwarf stars.
Authors: Mathioudakis, M.; Doyle, J. G.
Bibcode: 1990A&A...240..357M
Altcode:
A power correlation between 'quiescent' and flaring H-gamma and
X-ray emission is presented for a group of active dwarf M stars. The
relationship holds for both 'quiescent' and flaring conditions for
several orders of magnitude change in energy. Explanations in terms
of X-ray back-heating and microflaring are discussed.
Title: IUE observations of GL 644 AB (=Wolf 630) in the wavelength
region 1150-1950 A, in June 1981.
Authors: Mathioudakis, M.; Doyle, J. G.
Bibcode: 1990A&A...232..114M
Altcode:
IUE short wavelength observations of the flare star Gl 644, obtained
in June 1981 are presented. A flare was detected in one of the spectra
as a strong increase in C IV, He II, and continuum fluxes. The electron
density of the flaring plasma, determined using line ratio techniques,
is 4 x 10 to the 10th/cu cm, which is an increase by a factor of
2.7 above the quiescent. The radiative losses of the flare are at
least 1.3 x 10 to the 32nd erg and 4.0 x 10 to the 33rd erg over the
temperature range log Te between 4.3 and 5.4 and log Te between 4.0
and 8.0, respectively. This compares with continuum losses of 8.1 x
10 to the 31st erg over the wavelength region 1250-1950 A.
Title: Flare activity and orbital rotation of YY Geminorum.
Authors: Doyle, J. G.; Mathioudakis, M.
Bibcode: 1990A&A...227..130D
Altcode:
The observed flare activity on the eclipsing binary star YY Geminorum,
as determined by the time-average flare energy, is more than as order
of magnitude greater out-of-eclipse than that during eclipses. The
out-of-eclipse value is however, heavily biased by four large
flares. In the cumulative distribution diagram these larger flares
have a substantially different statistical occurrence rate than the
remainder of the flares. It is suggested that the small separation
between the two stars leads to significantly enhanced magnetic field
between the two components and thus to the occurrence of energetic
flares, observable when the stars are out of eclipse.
Title: Rotational Modulation of Plages on II Peg
Authors: Mathioudakis, M.
Bibcode: 1990iue..prop.3874M
Altcode:
No abstract at ADS
Title: Flux-flux relation : MG II H and K versus X-rays in dwarf M
and K stars.
Authors: Mathioudakis, M.; Doyle, J. G.
Bibcode: 1989A&A...224..179M
Altcode:
Surface X-ray and Mg II h and k fluxes are presented for a group of
dMe, dKe, dM, and dK stars. No correlation is apparent between the
fluxes for the group of stars as a whole, although a good correlation
is present for a subgroup, i.e., the dMe/dKe's. The Mg II and X-ray
luminosities for these active stars define an upper boundary, which
is dependent upon the stellar radii. The Mg II luminosity from the
less active dM/dK stars are also radii-dependent, but this is not the
case for the X-ray luminosity. This may indicate a difference in the
heating mechanisms for the chromosphere and corona, with acoustic wave
heating being an important contributor in the chromosphere and/or a
difference in the plage filling factors in the two regions.