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Author name code: nindos
ADS astronomy entries on 2022-09-14
author:"Nindos, Aexander"
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Title: Magnetic helicity and energy of emerging solar active regions
and their erruptivity
Authors: Liokati, E.; Nindos, A.; Liu, Y.
2022A&A...662A...6L Altcode: 2022arXiv220204353L
<BR /> Aims: We investigate the role of the accumulation of magnetic
helicity and magnetic energy in the generation of coronal mass ejections
(CMEs) from emerging solar active regions (ARs). <BR /> Methods: Using
vector magnetic field data obtained by the Helioseismic and Magnetic
Imager on board the Solar Dynamics Observatory, we calculated the
magnetic helicity and magnetic energy injection rates as well as the
resulting accumulated budgets in 52 emerging ARs from the start time
of magnetic flux emergence until they reached a heliographic longitude
of 45° West (W45). <BR /> Results: Seven of the ARs produced CMEs, but
45 did not. In a statistical sense, the eruptive ARs accumulate larger
budgets of magnetic helicity and energy than the noneruptive ARs over
intervals that start from the flux emergence start time and end (i)
at the end of the flux emergence phase and (ii) when the AR produces
its first CME or crosses W45, whichever occurs first. We found magnetic
helicity and energy thresholds of 9 × 10<SUP>41</SUP> Mx<SUP>2</SUP>
and 2 × 10<SUP>32</SUP> erg. When these thresholds were crossed,
ARs are likely to erupt. In terms of accumulated magnetic helicity and
energy budgets, the segregation of the eruptive from the noneruptive
ARs is violated in one case when an AR erupts early in its emergence
phase and in six cases in which noneruptive ARs exhibit large magnetic
helicity and energy budgets. Decay index calculations may indicate that
these ARs did not erupt because the overlying magnetic field provided
a stronger or more extended confinement than in eruptive ARs. <BR />
Conclusions: Our results indicate that emerging ARs tend to produce CMEs
when they accumulate significant budgets of both magnetic helicity and
energy. Any study of their eruptive potential should consider magnetic
helicity together with magnetic energy.
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Title: First detection of metric emission from a solar surge
Authors: Alissandrakis, C. E.; Patsourakos, S.; Nindos, A.; Bouratzis,
C.; Hillaris, A.
2022A&A...662A..14A Altcode: 2022arXiv220301043A
We report the first detection of metric radio emission from a surge,
observed with the Nançay Radioheliograph (NRH), STEREO, and other
instruments. The emission was observed during the late phase of the
M9 complex event SOL2010-02-012T11:25:00, described in a previous
publication. It was associated with a secondary energy release,
also observed in STEREO 304 Å images, and there was no detectable
soft X-ray emission. The triangulation of the STEREO images allowed
for the identification of the surge with NRH sources near the central
meridian. The radio emission of the surge occurred in two phases and
consisted of two sources, one located near the base of the surge,
apparently at or near the site of energy release, and another in the
upper part of the surge; these were best visible in the frequency
range of 445.0 to about 300 MHz, whereas a spectral component of
a different nature was observed at lower frequencies. Sub-second
time variations were detected in both sources during both phases,
with a 0.2-0.3 s delay of the upper source with respect to the lower,
suggesting superluminal velocities. This effect can be explained if
the emission of the upper source was due to scattering of radiation
from the source at the base of the surge. In addition, the radio
emission showed signs of pulsations and spikes. We discuss possible
emission mechanisms for the slow time variability component of
the lower radio source. Gyrosynchrotron emission reproduced the
characteristics of the observed total intensity spectrum at the
start of the second phase of the event fairly well, but failed to
reproduce the high degree of the observed circular polarization or the
spectra at other instances. On the other hand, type IV-like plasma
emission from the fundamental could explain the high polarization
and the fine structure in the dynamic spectrum; moreover, it gives
projected radio source positions on the plane of the sky, as seen from
STEREO-A, near the base of the surge. Taking all the properties into
consideration, we suggest that type IV-like plasma emission with
a low-intensity gyrosynchrotron component is the most plausible
mechanism. <P />Movie associated to Fig. A.2 is available at <A
href="https://www.aanda.org/10.1051/0004-6361/202243169/olm">https://www.aanda.org</A>
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Title: A first look at the submillimeter Sun with ALMA
Authors: Alissandrakis, C. E.; Bastian, T. S.; Nindos, A.
2022A&A...661L...4A Altcode: 2022arXiv220501008A
We present the first full-disk solar images obtained with the Atacama
Large Millimeter/submillimeter Array (ALMA) in Band 7 (0.86 mm;
347 GHz). In spite of the low spatial resolution (21″), several
interesting results were obtained. During our observation, the sun
was practically devoid of active regions. Quiet Sun structures on
the disk are similar to those in Atmospheric Imaging Assembly images
at 1600 Å and 304 Å, after the latter are smoothed to the ALMA
resolution, as noted previously for Band 6 (1.26 mm) and Band 3 (3 mm)
images; they are also similar to negative Hα images of equivalent
resolution. Polar coronal holes, which are clearly seen in the 304
Å band and small Hα filaments, are not detectable at 0.86 mm. We
computed the center-to-limb variation of the brightness temperature,
T<SUB>b</SUB>, in Band 7, as well as in Bands 6 and 3, which were
obtained during the same campaign, and we combined them to a unique
curve of T<SUB>b</SUB>(log μ<SUB>100</SUB>), where μ<SUB>100</SUB>
is the cosine of the heliocentric angle reduced to 100 GHz. Assuming
that the absolute calibration of the Band 3 commissioning observations
is accurate, we deduced a brightness temperature at the center of the
disk of 6085 K for Band 7, instead of the value of 5500 K, extrapolated
from the recommended values for Bands 3 and 6. More importantly, the
T<SUB>b</SUB>(log μ<SUB>100</SUB>) curve flattens at large values
of μ<SUB>100</SUB>, and so does the corresponding T<SUB>e</SUB>(log
τ<SUB>100</SUB>) at large τ<SUB>100</SUB>. This is probably an
indication that we are approaching the temperature minimum.
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Title: Multiwavelength observations of a metric type-II event
Authors: Alissandrakis, C. E.; Nindos, A.; Patsourakos, S.; Hillaris,
A.
2021A&A...654A.112A Altcode: 2021arXiv210802855A
We have studied a complex metric radio event that originated in a
compact flare, observed with the ARTEMIS-JLS radiospectrograph on
February 12, 2010. The event was associated with a surge observed at 195
and 304 Å and with a coronal mass ejection observed by instruments on
board STEREO A and B near the eastern and western limbs respectively. On
the disk the event was observed at ten frequencies by the Nançay
Radioheliograph (NRH), in Hα by the Catania observatory, in soft
X-rays by GOES SXI and Hinode XRT, and in hard X-rays by RHESSI. We
combined these data, together with MDI longitudinal magnetograms,
to get as complete a picture of the event as possible. Our emphasis
is on two type-II bursts that occurred near respective maxima in
the GOES light curves. The first, associated with the main peak
of the event, showed an impressive fundamental-harmonic structure,
while the emission of the second consisted of three well-separated
bands with superposed pulsations. Using positional information for the
type-IIs from the NRH and triangulation from STEREO A and B, we found
that the type-IIs were associated neither with the surge nor with the
disruption of a nearby streamer, but rather with an extreme ultraviolet
(EUV) wave probably initiated by the surge. The fundamental-harmonic
structure of the first type-II showed a band split corresponding to
a magnetic field strength of 18 G, a frequency ratio of 1.95 and a
delay of 0.23−0.65 s of the fundamental with respect to the harmonic;
moreover it became stationary shortly after its start and then drifted
again. The pulsations superposed on the second type-II were broadband
and had started before the burst. In addition, we detected another
pulsating source, also before the second type-II, polarized in the
opposite sense; the pulsations in the two sources were out of phase
and hence hardly detectable in the dynamic spectrum. The pulsations
had a measurable reverse frequency drift of about 2 s<SUP>−1</SUP>.
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Title: ALMA observations of the variability of the quiet Sun at
millimeter wavelengths
Authors: Nindos, A.; Patsourakos, S.; Alissandrakis, C. E.; Bastian,
T. S.
2021A&A...652A..92N Altcode: 2021arXiv210604220N
<BR /> Aims: We address the variability of the quiet solar chromosphere
at 1.26 mm and 3 mm with a focus on the study of spatially resolved
oscillations and transient brightenings, which are small, weak
events of energy release. Both phenomena may have a bearing on the
heating of the chromosphere. <BR /> Methods: We used Atacama Large
Millimeter/submillimeter Array (ALMA) observations of the quiet Sun
at 1.26 mm and 3 mm. The spatial and temporal resolution of the data
were 1 − 2″ and 1 s, respectively. The concatenation of light
curves from different scans yielded a frequency resolution in spectral
power of 0.5−0.6 mHz. At 1.26 mm, in addition to power spectra of
the original data, we degraded the images to the spatial resolution
of the 3 mm images and used fields of view that were equal in area
for both data sets. The detection of transient brightenings was made
after the effect of oscillations was removed. <BR /> Results: At both
frequencies, we detected p-mode oscillations in the range 3.6−4.4
mHz. The corrections for spatial resolution and field of view at 1.26 mm
decreased the root mean square (rms) of the oscillations by a factor of
1.6 and 1.1, respectively. In the corrected data sets, the oscillations
at 1.26 mm and 3 mm showed brightness temperature fluctuations of
∼1.7 − 1.8% with respect to the average quiet Sun, corresponding to
137 and 107 K, respectively. We detected 77 transient brightenings at
1.26 mm and 115 at 3 mm. Although their majority occurred in the cell
interior, the occurrence rate per unit area of the 1.26 mm events was
higher than that of the 3 mm events; this conclusion does not change
if we take into account differences in spatial resolution and noise
levels. The energy associated with the transient brightenings ranged
from 1.8 × 10<SUP>23</SUP> to 1.1 × 10<SUP>26</SUP> erg and from 7.2
× 10<SUP>23</SUP> to 1.7 × 10<SUP>26</SUP> erg for the 1.26 mm and
3 mm events, respectively. The corresponding power-law indices of the
energy distribution were 1.64 and 1.73. We also found that ALMA bright
network structures corresponded to dark mottles or spicules that can be
seen in broadband Hα images from the GONG network. <BR /> Conclusions:
The fluctuations associated with the p-mode oscillations represent
a fraction of 0.55−0.68 of the full power spectrum. Their energy
density at 1.26 mm is 3 × 10<SUP>−2</SUP> erg cm<SUP>−3</SUP>. The
computed low-end energy of the 1.26 mm transient brightenings is among
the smallest ever reported, irrespective of the wavelength of the
observation. Although the occurrence rate per unit area of the 1.26
mm transient brightenings was higher than that of the 3 mm events,
their power per unit area is smaller likely due to the detection of
many weak 1.26 mm events.
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Title: Tracking solar wind flows from rapidly varying viewpoints by
the Wide-field Imager for Parker Solar Probe
Authors: Nindos, A.; Patsourakos, S.; Vourlidas, A.; Liewer, P. C.;
Penteado, P.; Hall, J. R.
2021A&A...650A..30N Altcode: 2020arXiv201013140N
<BR /> Aims: Our goal is to develop methodologies to seamlessly track
transient solar wind flows viewed by coronagraphs or heliospheric
imagers from rapidly varying viewpoints. <BR /> Methods: We
constructed maps of intensity versus time and elongation (J-maps)
from Parker Solar Probe (PSP) Wide-field Imager (WISPR) observations
during the fourth encounter of PSP. From the J-map, we built an
intensity on impact-radius-on-Thomson-surface map (R-map). Finally,
we constructed a latitudinal intensity versus time map (Lat-map). Our
methodology satisfactorily addresses the challenges associated with
the construction of such maps from data taken from rapidly varying
viewpoint observations. <BR /> Results: Our WISPR J-map exhibits
several tracks, corresponding to transient solar wind flows ranging
from a coronal mass ejection down to streamer blobs. The latter
occurrence rate is about 4-5 per day, which is similar to the
occurrence rate in a J-map made from ~1 AU data obtained with the
Heliospheric Imager-1 (HI-1) on board the Solar Terrestrial Relations
Observatory Ahead spacecraft (STEREO-A). STEREO-A was radially aligned
with PSP during the study period. The WISPR J-map tracks correspond
to angular speeds of 2.28 ± 0.7°/h (2.49 ± 0.95°/h), for linear
(quadratic) time-elongation fittings, and radial speeds of about
150-300 km s<SUP>−1</SUP>. The analysis of the Lat-map reveals
a bifurcating streamer, which implies that PSP was flying through
a slightly folded streamer during perihelion. <BR /> Conclusions:
We developed a framework to systematically capture and characterize
transient solar wind flows from space platforms with rapidly varying
vantage points. The methodology can be applied to PSP WISPR observations
as well as to upcoming observations from instruments on board the Solar
Orbiter mission. <P />Movie associated to Fig. 8 is available at <A
href="https://www.aanda.org/10.1051/0004-6361/202039414/olm">https://www.aanda.org</A>
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Title: Relative field line helicity of a large eruptive solar
active region
Authors: Moraitis, K.; Patsourakos, S.; Nindos, A.
2021A&A...649A.107M Altcode: 2021arXiv210303643M
Context. Magnetic helicity is a physical quantity of great importance
in the study of astrophysical and natural plasmas. Although a density
for helicity cannot be defined, a good proxy for this quantity is field
line helicity. The appropriate quantity for use in solar conditions
is relative field line helicity (RFLH). <BR /> Aims: This work aims
to study in detail the behaviour of RFLH, for the first time, in a
solar active region (AR). <BR /> Methods: The target AR is the large,
eruptive AR 11158. In order to compute RFLH and all other quantities
of interest, we used a non-linear force-free reconstruction of the AR
coronal magnetic field of excellent quality. <BR /> Results: We find
that the photospheric morphology of RFLH is very different than that
of the magnetic field or electrical current, and this morphology is
not sensitive to the chosen gauge in the computation of RFLH. The
value of helicity experiences a large decrease, that is ∼25% of
its pre-flare value, during an X-class flare of the AR; this change
is also depicted in the photospheric morphology of RFLH. Moreover,
the area of this change coincides with the area that encompasses the
flux rope, which is the magnetic structure that later erupted. <BR />
Conclusions: The use of RFLH can provide important information about
the value and location of the magnetic helicity expelled from the
solar atmosphere during eruptive events.
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Title: Observations of Solar Spicules at Millimeter and Ultraviolet
Wavelengths
Authors: Bastian, T.; De Pontieu, B.; Shimojo, M.; Iwai, K.;
Alissandrakis, C.; Nindos, A.; Vial, J. C.; White, S. M.
2020AGUFMSH004..08B Altcode:
Solar spicules are a ubiquitous chromospheric phenomenon in which
multitudes of dynamic jets with temperatures of order 10<SUP>4</SUP>
K extend thousands of kilometers into the solar atmosphere. Recent
progress has been made refining the observational characteristics
of spicules using the Hinode Solar Optical Telescope (SOT) and the
Interface Region Imaging Spectrograph (IRIS) observations at optical
and ultraviolet wavelengths, respectively. Two types of spicule
have been identified. Type I spicules, prevalent in solar active
regions, have upward speeds of order 25 km/s and lifetimes of 3-7
min. They may be the limb counterpart to shock-wave-driven fibrils
commonly seen against the solar disk in active regions. In contrast,
type II spicules, more common in quiet regions and coronal holes,
display upward speeds of 50-150 km/s, lifetimes of 30-110 s, and
appear to be partially heated to temperatures of 10<SUP>5 </SUP>K and
higher. These observations have provoked intense interest in spicules
and have led to proposals that type II spicules play a central role
as a source of hot plasma in the corona. Nevertheless, their role in
mass and energy transport between the lower and upper layers of the
solar atmosphere remains an outstanding problem. <P />Here, we report
imaging observations of solar spicules at millimeter wavelengths using
the Atacama Large Millimeter-submillimeter Array (ALMA) with arcsecond
angular resolution. Continuum millimeter wavelength radiation forms
under conditions of local thermodynamic equilibrium, thereby providing a
complementary tool to UV lines, which form under non-LTE conditions. The
observations were made on 2018 December 24-25 at λ=1.25 mm and λ=3
mm. The ALMA observations pose special challenges, particularly at
1.25 mm, where the limited field of view of the instrument motivated
us to use a novel mosaic imaging technique: multiple pointings were
assembled to form a single map with an angular resolution of 1" x 0.7"
on a cadence of roughly 2 min. In contrast, we were able to image at 3
mm continuously, with a map cadence of 2 s and an angular resolution of
2.3" x 1.3". <P />We compare and contrast the morphology and dynamics
of mm-λ observations of spicules with those obtained by IRIS at UV
wavelengths and place constraints on spicule temperatures and masses
using the joint millimeter-wavelength observations.
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Title: Decoding the Pre-Eruptive Magnetic Field Configurations of
Coronal Mass Ejections
Authors: Patsourakos, S.; Vourlidas, A.; Török, T.; Kliem, B.;
Antiochos, S. K.; Archontis, V.; Aulanier, G.; Cheng, X.; Chintzoglou,
G.; Georgoulis, M. K.; Green, L. M.; Leake, J. E.; Moore, R.; Nindos,
A.; Syntelis, P.; Yardley, S. L.; Yurchyshyn, V.; Zhang, J.
2020SSRv..216..131P Altcode: 2020arXiv201010186P
A clear understanding of the nature of the pre-eruptive magnetic
field configurations of Coronal Mass Ejections (CMEs) is required
for understanding and eventually predicting solar eruptions. Only
two, but seemingly disparate, magnetic configurations are considered
viable; namely, sheared magnetic arcades (SMA) and magnetic flux ropes
(MFR). They can form via three physical mechanisms (flux emergence,
flux cancellation, helicity condensation). Whether the CME culprit
is an SMA or an MFR, however, has been strongly debated for thirty
years. We formed an International Space Science Institute (ISSI) team to
address and resolve this issue and report the outcome here. We review
the status of the field across modeling and observations, identify
the open and closed issues, compile lists of SMA and MFR observables
to be tested against observations and outline research activities
to close the gaps in our current understanding. We propose that the
combination of multi-viewpoint multi-thermal coronal observations
and multi-height vector magnetic field measurements is the optimal
approach for resolving the issue conclusively. We demonstrate the
approach using MHD simulations and synthetic coronal images.
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Title: When do solar erupting hot magnetic flux ropes form?
Authors: Nindos, A.; Patsourakos, S.; Vourlidas, A.; Cheng, X.;
Zhang, J.
2020A&A...642A.109N Altcode: 2020arXiv200804380N
<BR /> Aims: We investigate the formation times of eruptive magnetic
flux ropes relative to the onset of solar eruptions, which is
important for constraining models of coronal mass ejection (CME)
initiation. <BR /> Methods: We inspected uninterrupted sequences of
131 Å images that spanned more than eight hours and were obtained
by the Atmospheric Imaging Assembly on board the Solar Dynamics
Observatory to identify the formation times of hot flux ropes that
erupted in CMEs from locations close to the limb. The appearance
of the flux ropes as well as their evolution toward eruptions were
determined using morphological criteria. <BR /> Results: Two-thirds
(20/30) of the flux ropes were formed well before the onset of the
eruption (from 51 min to more than eight hours), and their formation
was associated with the occurrence of a confined flare. We also found
four events with preexisting hot flux ropes whose formations occurred
a matter of minutes (from three to 39) prior to the eruptions without
any association with distinct confined flare activity. Six flux ropes
were formed once the eruptions were underway. However, in three of
them, prominence material could be seen in 131 Å images, which may
indicate the presence of preexisting flux ropes that were not hot. The
formation patterns of the last three groups of hot flux ropes did not
show significant differences. For the whole population of events, the
mean and median values of the time difference between the onset of the
eruptive flare and the appearance of the hot flux rope were 151 and 98
min, respectively. <BR /> Conclusions: Our results provide, on average,
indirect support for CME models that involve preexisting flux ropes; on
the other hand, for a third of the events, models in which the ejected
flux rope is formed during the eruption appear more appropriate. <P
/>Movies attached to Figs. 2, 5, 8, and 10 are available at <A
href="https://www.aanda.org/10.1051/0004-6361/202038832/olm">http://www.aanda.org</A>
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Title: The Solar Orbiter Science Activity Plan. Translating solar
and heliospheric physics questions into action
Authors: Zouganelis, I.; De Groof, A.; Walsh, A. P.; Williams, D. R.;
Müller, D.; St Cyr, O. C.; Auchère, F.; Berghmans, D.; Fludra,
A.; Horbury, T. S.; Howard, R. A.; Krucker, S.; Maksimovic, M.;
Owen, C. J.; Rodríguez-Pacheco, J.; Romoli, M.; Solanki, S. K.;
Watson, C.; Sanchez, L.; Lefort, J.; Osuna, P.; Gilbert, H. R.;
Nieves-Chinchilla, T.; Abbo, L.; Alexandrova, O.; Anastasiadis, A.;
Andretta, V.; Antonucci, E.; Appourchaux, T.; Aran, A.; Arge, C. N.;
Aulanier, G.; Baker, D.; Bale, S. D.; Battaglia, M.; Bellot Rubio,
L.; Bemporad, A.; Berthomier, M.; Bocchialini, K.; Bonnin, X.; Brun,
A. S.; Bruno, R.; Buchlin, E.; Büchner, J.; Bucik, R.; Carcaboso,
F.; Carr, R.; Carrasco-Blázquez, I.; Cecconi, B.; Cernuda Cangas, I.;
Chen, C. H. K.; Chitta, L. P.; Chust, T.; Dalmasse, K.; D'Amicis, R.;
Da Deppo, V.; De Marco, R.; Dolei, S.; Dolla, L.; Dudok de Wit, T.;
van Driel-Gesztelyi, L.; Eastwood, J. P.; Espinosa Lara, F.; Etesi,
L.; Fedorov, A.; Félix-Redondo, F.; Fineschi, S.; Fleck, B.; Fontaine,
D.; Fox, N. J.; Gandorfer, A.; Génot, V.; Georgoulis, M. K.; Gissot,
S.; Giunta, A.; Gizon, L.; Gómez-Herrero, R.; Gontikakis, C.; Graham,
G.; Green, L.; Grundy, T.; Haberreiter, M.; Harra, L. K.; Hassler,
D. M.; Hirzberger, J.; Ho, G. C.; Hurford, G.; Innes, D.; Issautier,
K.; James, A. W.; Janitzek, N.; Janvier, M.; Jeffrey, N.; Jenkins,
J.; Khotyaintsev, Y.; Klein, K. -L.; Kontar, E. P.; Kontogiannis,
I.; Krafft, C.; Krasnoselskikh, V.; Kretzschmar, M.; Labrosse, N.;
Lagg, A.; Landini, F.; Lavraud, B.; Leon, I.; Lepri, S. T.; Lewis,
G. R.; Liewer, P.; Linker, J.; Livi, S.; Long, D. M.; Louarn, P.;
Malandraki, O.; Maloney, S.; Martinez-Pillet, V.; Martinovic, M.;
Masson, A.; Matthews, S.; Matteini, L.; Meyer-Vernet, N.; Moraitis,
K.; Morton, R. J.; Musset, S.; Nicolaou, G.; Nindos, A.; O'Brien,
H.; Orozco Suarez, D.; Owens, M.; Pancrazzi, M.; Papaioannou, A.;
Parenti, S.; Pariat, E.; Patsourakos, S.; Perrone, D.; Peter, H.;
Pinto, R. F.; Plainaki, C.; Plettemeier, D.; Plunkett, S. P.; Raines,
J. M.; Raouafi, N.; Reid, H.; Retino, A.; Rezeau, L.; Rochus, P.;
Rodriguez, L.; Rodriguez-Garcia, L.; Roth, M.; Rouillard, A. P.;
Sahraoui, F.; Sasso, C.; Schou, J.; Schühle, U.; Sorriso-Valvo, L.;
Soucek, J.; Spadaro, D.; Stangalini, M.; Stansby, D.; Steller, M.;
Strugarek, A.; Štverák, Š.; Susino, R.; Telloni, D.; Terasa, C.;
Teriaca, L.; Toledo-Redondo, S.; del Toro Iniesta, J. C.; Tsiropoula,
G.; Tsounis, A.; Tziotziou, K.; Valentini, F.; Vaivads, A.; Vecchio,
A.; Velli, M.; Verbeeck, C.; Verdini, A.; Verscharen, D.; Vilmer, N.;
Vourlidas, A.; Wicks, R.; Wimmer-Schweingruber, R. F.; Wiegelmann,
T.; Young, P. R.; Zhukov, A. N.
2020A&A...642A...3Z Altcode: 2020arXiv200910772Z
Solar Orbiter is the first space mission observing the solar plasma
both in situ and remotely, from a close distance, in and out of the
ecliptic. The ultimate goal is to understand how the Sun produces
and controls the heliosphere, filling the Solar System and driving
the planetary environments. With six remote-sensing and four in-situ
instrument suites, the coordination and planning of the operations are
essential to address the following four top-level science questions:
(1) What drives the solar wind and where does the coronal magnetic field
originate?; (2) How do solar transients drive heliospheric variability?;
(3) How do solar eruptions produce energetic particle radiation that
fills the heliosphere?; (4) How does the solar dynamo work and drive
connections between the Sun and the heliosphere? Maximising the
mission's science return requires considering the characteristics
of each orbit, including the relative position of the spacecraft
to Earth (affecting downlink rates), trajectory events (such
as gravitational assist manoeuvres), and the phase of the solar
activity cycle. Furthermore, since each orbit's science telemetry
will be downloaded over the course of the following orbit, science
operations must be planned at mission level, rather than at the level
of individual orbits. It is important to explore the way in which those
science questions are translated into an actual plan of observations
that fits into the mission, thus ensuring that no opportunities are
missed. First, the overarching goals are broken down into specific,
answerable questions along with the required observations and the
so-called Science Activity Plan (SAP) is developed to achieve this. The
SAP groups objectives that require similar observations into Solar
Orbiter Observing Plans, resulting in a strategic, top-level view of
the optimal opportunities for science observations during the mission
lifetime. This allows for all four mission goals to be addressed. In
this paper, we introduce Solar Orbiter's SAP through a series of
examples and the strategy being followed.
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Title: Modeling the quiet Sun cell and network emission with ALMA
Authors: Alissandrakis, C. E.; Nindos, A.; Bastian, T. S.; Patsourakos,
S.
2020A&A...640A..57A Altcode: 2020arXiv200609886A
Observations of the Sun at millimeter wavelengths with the Atacama
Large Millimeter/submillimeter Array (ALMA) offer a unique opportunity
to investigate the temperature structure of the solar chromosphere. In
this article we expand our previous work on modeling the chromospheric
temperature of the quiet Sun, by including measurements of the
brightness temperature in the network and cell interiors, from
high-resolution ALMA images at 3 mm (Band 3) and 1.26 mm (Band 6). We
also examine the absolute calibration of ALMA full-disk images. We
suggest that the brightness temperature at the center of the solar disk
in Band 6 is ∼440 K above the value recommended by White et al. (2017,
Sol. Phys., 292, 88). In addition, we give improved results for the
electron temperature variation of the average quiet Sun with optical
depth and the derived spectrum at the center of the disk. We found
that the electron temperature in the network is considerably lower
than predicted by model F of Fontenla et al. (1993, ApJ, 406, 319)
and that of the cell interior considerably higher than predicted by
model A. Depending on the network/cell segregation scheme, the electron
temperature difference between network and cell at τ = 1 (100 GHz)
ranges from ∼660 K to ∼1550 K, compared to ∼3280 K predicted
by the models; similarly, the electron temperature, T<SUB>e</SUB>
ratio ranges from ∼1.10 to 1.24, compared to ∼1.55 of the model
prediction. We also found that the network/cell T<SUB>e</SUB>(τ)
curves diverge as τ decreases, indicating an increase of contrast
with height and possibly a steeper temperature rise in the network
than in the cell interior.
---------------------------------------------------------
Title: Transient brightenings in the quiet Sun detected by ALMA at
3 mm
Authors: Nindos, A.; Alissandrakis, C. E.; Patsourakos, S.; Bastian,
T. S.
2020A&A...638A..62N Altcode: 2020arXiv200407591N
<BR /> Aims: We investigate transient brightenings, that is, weak,
small-scale episodes of energy release, in the quiet solar chromosphere;
these episodes can provide insights into the heating mechanism of the
outer layers of the solar atmosphere. <BR /> Methods: Using Atacama
Large Millimeter/submillimeter Array (ALMA) observations, we performed
the first systematic survey for quiet Sun transient brightenings at 3
mm. Our dataset included images of six 87″ × 87″ fields of view
of the quiet Sun obtained with angular resolution of a few arcsec at
a cadence of 2 s. The transient brightenings were detected as weak
enhancements above the average intensity after we removed the effect
of the p-mode oscillations. A similar analysis, over the same fields
of view, was performed for simultaneous 304 and 1600 Å data obtained
with the Atmospheric Imaging Assembly. <BR /> Results: We detected 184
3 mm transient brightening events with brightness temperatures from
70 K to more than 500 K above backgrounds of ∼7200 - 7450 K. All
events showed light curves with a gradual rise and fall, strongly
suggesting a thermal origin. Their mean duration and maximum area were
51.1 s and 12.3 Mm<SUP>2</SUP>, respectively, with a weak preference
of appearing at network boundaries rather than in cell interiors. Both
parameters exhibited power-law behavior with indices of 2.35 and 2.71,
respectively. Only a small fraction of ALMA events had either 304
or 1600 Å counterparts but the properties of these events were not
significantly different from those of the general population except
that they lacked their low-end energy values. The total thermal
energies of the ALMA transient brightenings were between 1.5 ×
10<SUP>24</SUP> and 9.9 × 10<SUP>25</SUP> erg and their frequency
distribution versus energy was a power law with an index of 1.67 ±
0.05. We found that the power per unit area provided by the ALMA events
could account for only 1% of the chromospheric radiative losses (10%
of the coronal ones). <BR /> Conclusions: We were able to detect, for
the first time, a significant number of weak 3 mm quiet Sun transient
brightenings. However, their energy budget falls short of meeting the
requirements for the heating of the upper layers of the solar atmosphere
and this conclusion does not change even if we use the least restrictive
criteria possible for the detection of transient brightenings.
---------------------------------------------------------
Title: Modeling of the Brightness of the Chromospheric Network Based
on ALMA High Resolution Observations of the Quiet Sun
Authors: Alissandrakis, C. E.; Nindos, A.; Bastian, T.; Patsourakos, S.
2020AAS...23610607A Altcode:
ALMA observations of the Sun at mm-λ offer a unique opportunity
to investigate the temperature/density structure of the solar
chromosphere. In a previous work (Alissandrakis et al 2017, A&A
605, A78) we measured the center-to-limb variation of the brightness
temperature, T<SUB>b</SUB>, using low resolution ALMA full-disk
observations in Band 3 (3mm) and Band 6 (1.26 mm), together with data
at 0.85 mm from Bastian et al. 1993 (ApJ, 415, 364). Combining all
data and inverting the solution of the transfer equation we found
that the electron temperature, T<SUB>e</SUB>, in the range of 0.34
< τ<SUB>100</SUB> < 12, where τ<SUB>100</SUB> is the optical
depth at 100 GHz, was ~5% (~300 K) below the one predicted by model C
(average quiet sun) of FAL93 (Fontenla, Avrett, & Loeser, 1993,
ApJ, 406, 319). Here we expand that work by including measurements
of the brightness temperature in the network and cell interiors,
from high resolution ALMA images in Bands 3 and 6. We found that
the observed T<SUB>b</SUB> in the network is considerably lower
than predicted by the FAL93 model F and that of the cell interior
considerably higher than predicted by the FAL93 model A. The observed
network/cell difference of brightness temperature at the center of the
disk, at 100 GHz is about 920 K, compared to ~3250 K predicted by the
FAL93 models; similarly, the T<SUB>b</SUB>, ratio is ~1.14, against
~1.51 of the model prediction. After inversion of the observed data,
the electron temperature of cell interior at τ<SUB>100</SUB>=1 is
~390 K below the average (~600 K above model A) and of the network
~400 K above the average (~1800 K below model A). The implications of
these results will be discussed. We will also discuss the question of
the normalization of brightness temperature observed by ALMA.
---------------------------------------------------------
Title: Observations of solar chromospheric oscillations at 3 mm
with ALMA
Authors: Patsourakos, S.; Alissandrakis, C. E.; Nindos, A.; Bastian,
T. S.
2020A&A...634A..86P Altcode: 2019arXiv191203480P
<BR /> Aims: We aim to study spatially resolved chromospheric
oscillations of the quiet Sun (QS) in the mm-domain at a resolution
of a few arcsec, typically 2.4″ × 4.5″. <BR /> Methods: We used
Atacama Large millimeter and submillimeter Array (ALMA) time series
of interferometric observations of the QS obtained at 3 mm with a 2-s
cadence and a spatial resolution of a few arcsec. The observations were
performed on March 16, 2017 and seven 80″ × 80″ fields of view
(FoV) going from disk center to limb were covered, each one observed for
10 min, therefore limiting the frequency resolution of the power spectra
to 1.7 mHz. For each FoV, masks for cell and network were derived,
and the averaged power spectral densities (PSDs) for the entire FoV,
cell, and network were computed. The resulting power spectra were
fit with an analytical function in order to derive the frequency
and the root-mean-square (rms) power associated with the peaks. The
same analysis, over the same FoVs and for the same intervals, was
performed for simultaneous Atmospheric Imaging Assembly (AIA) image
sequences in 1600 Å. <BR /> Results: Spatially resolved chromospheric
oscillations at 3 mm, with frequencies of 4.2 ± 1.7 mHz are observed
in the QS, in both cell and network. The coherence length-scale of
the oscillations is commensurate with the spatial resolution of our
ALMA observations. Brightness-temperature fluctuations in individual
pixels could reach up to a few hundred K, while the spatially averaged
PSDs yield rms in the range ≈55-75 K, i.e., up to ≈1% of the
averaged brightness temperatures and exhibit a moderate increase
towards the limb. For AIA 1600 Å, the oscillation frequency is 3.7
± 1.7 mHz. The relative rms is up to 6% of the background intensity,
with a weak increase towards the disk center (cell, average). ALMA
3 mm time-series lag AIA 1600 Å by ≈100 s, which corresponds to
a formation-height difference of ≈1200 km, representing a novel
determination of this important parameter. <BR /> Conclusions: The
ALMA oscillations that we detected exhibit higher amplitudes than those
derived from previous lower (≈10″) resolution observations at 3.5 mm
by the Berkeley-Illinois-Maryland Array. Chromospheric oscillations are,
therefore, not fully resolved at the length-scale of the chromospheric
network, and possibly not even at the spatial resolution of our ALMA
observations. Any study of transient brightenings in the mm-domain
should take into account the oscillations.
---------------------------------------------------------
Title: Sheared Magnetic Arcades and the Pre-eruptive Magnetic
Configuration of Coronal Mass Ejections: Diagnostics, Challenges
and Future Observables
Authors: Patsourakos, Spiros; Vourlidas, A.; Anthiochos, S. K.;
Archontis, V.; Aulanier, G.; Cheng, X.; Chintzoglou, G.; Georgoulis,
M. K.; Green, L. M.; Kliem, B.; Leake, J.; Moore, R. L.; Nindos, A.;
Syntelis, P.; Torok, T.; Yardley, S. L.; Yurchyshyn, V.; Zhang, J.
2019shin.confE.194P Altcode:
Our thinking about the pre-eruptive magnetic configuration of Coronal
Mass Ejections has been effectively dichotomized into two opposing
and often fiercely contested views: namely, sheared magnetic arcades
and magnetic flux ropes. Finding a solution to this issue will have
important implications for our understanding of CME initiation. We
first discuss the very value of embarking into the arcade vs. flux rope
dilemma and illustrate the corresponding challenges and difficulties to
address it. Next, we are compiling several observational diagnostics of
pre-eruptive sheared magnetic arcades stemming from theory/modeling,
discuss their merits, and highlight potential ambiguities that could
arise in their interpretation. We finally conclude with a discussion
of possible new observables, in the frame of upcoming or proposed
instrumentation, that could help to circumvent the issues we are
currently facing.
---------------------------------------------------------
Title: Solar physics with the Square Kilometre Array
Authors: Nindos, A.; Kontar, E. P.; Oberoi, D.
2019AdSpR..63.1404N Altcode: 2018arXiv181004951N
The Square Kilometre Array (SKA) will be the largest radio
telescope ever built, aiming to provide collecting area larger than 1
km<SUP>2</SUP>. The SKA will have two independent instruments, SKA-LOW
comprising of dipoles organized as aperture arrays in Australia and
SKA-MID comprising of dishes in South Africa. Currently the phase-1 of
SKA, referred to as SKA1, is in its late design stage and construction
is expected to start in 2020. Both SKA1-LOW (frequency range of 50-350
MHz) and SKA1-MID Bands 1, 2, and 5 (frequency ranges of 350-1050,
950-1760, and 4600-15,300 MHz, respectively) are important for solar
observations. In this paper we present SKA's unique capabilities
in terms of spatial, spectral, and temporal resolution, as well as
sensitivity and show that they have the potential to provide major new
insights in solar physics topics of capital importance including (i) the
structure and evolution of the solar corona, (ii) coronal heating, (iii)
solar flare dynamics including particle acceleration and transport,
(iv) the dynamics and structure of coronal mass ejections, and (v)
the solar aspects of space weather. Observations of the Sun jointly
with the new generation of ground-based and space-borne instruments
promise unprecedented discoveries.
---------------------------------------------------------
Title: First high-resolution look at the quiet Sun with ALMA at 3mm
Authors: Nindos, A.; Alissandrakis, C. E.; Bastian, T. S.; Patsourakos,
S.; De Pontieu, B.; Warren, H.; Ayres, T.; Hudson, H. S.; Shimizu,
T.; Vial, J. -C.; Wedemeyer, S.; Yurchyshyn, V.
2018A&A...619L...6N Altcode: 2018arXiv181005223N
We present an overview of high-resolution quiet Sun observations,
from disk center to the limb, obtained with the Atacama Large
millimeter and sub-millimeter Array (ALMA) at 3 mm. Seven quiet-Sun
regions were observed at a resolution of up to 2.5″ by 4.5″. We
produced both average and snapshot images by self-calibrating the ALMA
visibilities and combining the interferometric images with full-disk
solar images. The images show well the chromospheric network, which,
based on the unique segregation method we used, is brighter than the
average over the fields of view of the observed regions by ∼305
K while the intranetwork is less bright by ∼280 K, with a slight
decrease of the network/intranetwork contrast toward the limb. At 3
mm the network is very similar to the 1600 Å images, with somewhat
larger size. We detect, for the first time, spicular structures,
rising up to 15″ above the limb with a width down to the image
resolution and brightness temperature of ∼1800 K above the local
background. No trace of spicules, either in emission or absorption,
is found on the disk. Our results highlight the potential of ALMA for
the study of the quiet chromosphere.
---------------------------------------------------------
Title: Center-to-limb observations of the Sun with ALMA . Implications
for solar atmospheric models
Authors: Alissandrakis, C. E.; Patsourakos, S.; Nindos, A.; Bastian,
T. S.
2017A&A...605A..78A Altcode:
<BR /> Aims: We seek to derive information on the temperature structure
of the solar chromosphere and compare these results with existing
models. <BR /> Methods: We measured the center-to-limb variation of
the brightness temperature, T<SUB>b</SUB>, from ALMA full-disk images
at two frequencies and inverted the solution of the transfer equation
to obtain the electron temperature, T<SUB>e</SUB> as a function of
optical depth, τ. <BR /> Results: The ALMA images are very similar
to AIA images at 1600 Å. The brightness temperature at the center
of the disk is 6180 and 7250 K at 239 and 100 GHz, respectively,
with dispersions of 100 and 170 K. Plage regions stand out clearly in
the 239/100 GHz intensity ratio, while faculae and filament lanes do
not. The solar disk radius, reduced to 1 AU, is 961.1 ± 2.5” and 964.1
± 4.5” at 239 and 100 GHz, respectively. A slight but statistically
significant limb brightening is observed at both frequencies. <BR />
Conclusions: The inversion of the center-to-limb curves shows that
T<SUB>e</SUB> varies linearly with the logarithm of optical depth for
0.34 <τ<SUB>100 GHz</SUB>< 12, with a slope dT<SUB>e</SUB>/
dlnτ = -608 K. Our results are 5% lower than predicted by the average
quiet Sun model C of Fontenla et al. (1993, ApJ. 406, 319), but do
not confirm previous reports that the mm-λ solar spectrum is better
fitted with models of the cell interior.
---------------------------------------------------------
Title: Evidence for two-loop interaction from IRIS and SDO
observations of penumbral brightenings
Authors: Alissandrakis, C. E.; Koukras, A.; Patsourakos, S.; Nindos, A.
2017A&A...603A..95A Altcode: 2017arXiv170407344A
<BR /> Aims: We investigate small scale energy release events which
can provide clues on the heating mechanism of the solar corona. <BR />
Methods: We analyzed spectral and imaging data from the Interface
Region Imaging Spectrograph (IRIS), images from the Atmospheric
Imaging Assembly (AIA) aboard the Solar Dynamics Observatoty (SDO),
and magnetograms from the Helioseismic and Magnetic Imager (HMI)
aboard SDO. <BR /> Results: We report observations of small flaring
loops in the penumbra of a large sunspot on July 19, 2013. Our main
event consisted of a loop spanning 15”, from the umbral-penumbral
boundary to an opposite polarity region outside the penumbra. It lasted
approximately 10 min with a two minute impulsive peak and was observed
in all AIA/SDO channels, while the IRIS slit was located near its
penumbral footpoint. Mass motions with an apparent velocity of 100 km
s<SUP>-1</SUP> were detected beyond the brightening, starting in the
rise phase of the impulsive peak; these were apparently associated
with a higher-lying loop. We interpret these motions in terms of
two-loop interaction. IRIS spectra in both the C II and Si iv lines
showed very extended wings, up to about 400 km s<SUP>-1</SUP>, first
in the blue (upflows) and subsequently in the red wing. In addition
to the strong lines, emission was detected in the weak lines of Cl
I, O I and C I, as well as in the Mg II triplet lines. Absorption
features in the profiles of the C II doublet, the Si iv doublet and
the Mg II h and k lines indicate the existence of material with a
lower source function between the brightening and the observer. We
attribute this absorption to the higher loop and this adds further
credibility to the two-loop interaction hypothesis. Tilts were
detected in the absorption spectra, as well as in the spectra of Cl I,
O I, and C I lines, possibly indicating rotational motions from the
untwisting of magnetic flux tubes. <BR /> Conclusions: We conclude
that the absorption features in the C II, Si iv and Mg II profiles
originate in a higher-lying, descending loop; as this approached
the already activated lower-lying loop, their interaction gave rise
to the impulsive peak, the very broad line profiles and the mass
motions. <P />Movies associated to Figs. A.1-A.3 are available at <A
href="http://www.aanda.org/10.1051/0004-6361/201730643/olm">http://www.aanda.org</A>
---------------------------------------------------------
Title: Center-to-limb observations of the Sun with ALMA
Authors: Alissandrakis, C. E.; Patsourakos, S.; Nindos, A.; Bastian,
T. S.
2017arXiv170509008A Altcode:
We measured the center-to-limb variation of the brightness temperature,
$T_b$, from ALMA full-disk images at two frequencies and inverted the
solution of the transfer equation to obtain the electron temperature,
$T_e$ as a function of optical depth, $\tau$. The ALMA images are very
similar to AIA images at 1600Å. The brightness temperature at the
center of the disk is 6180 and 7250 K at 239 and 100 GHz respectively,
with dispersions of 100 and 170 K. Plage regions stand out clearly
in the 239/100 GHz intensity ratio, while faculae and filament lanes
do not. The solar disk radius, reduced to 1 AU, is $961.1\pm2.5$
arcsec and $964.1\pm4.5$ arcsec at 239 and 100 GHz respectively. A
slight but statistically significant limb brightening is observed at
both frequencies. The inversion of the center-to-limb curves shows
that $T_e$ varies linearly with the logarithm of optical depth for
$0.34<\tau_{100\,GHz}<12$, with a slope $d\ln T_e/d\tau=-608$
K. Our results are 5% lower than predicted by the average quiet sun
model C of Fontenla et al. (1993), but do not confirm previous reports
that the mm-$\lambda$ solar spectrum is better fitted with models of
the cell interior.
---------------------------------------------------------
Title: Interplanetary Type IV Bursts
Authors: Hillaris, A.; Bouratzis, C.; Nindos, A.
2016SoPh..291.2049H Altcode: 2016arXiv160407677H
We study the characteristics of moving type IV radio bursts that extend
to hectometric wavelengths (interplanetary type IV or type IVIP bursts)
and their relationship with energetic phenomena on the Sun. Our dataset
comprises 48 interplanetary type IV bursts observed with the Radio
and Plasma Wave Investigation (WAVES) instrument onboard Wind in the
13.825 MHz - 20 kHz frequency range. The dynamic spectra of the Radio
Solar Telescope Network (RSTN), the Nançay Decametric Array (DAM),
the Appareil de Routine pour le Traitement et l' Enregistrement
Magnetique de l' Information Spectral (ARTEMIS-IV), the Culgoora,
Hiraso, and the Institute of Terrestrial Magnetism, Ionosphere and Radio
Wave Propagation (IZMIRAN) Radio Spectrographs were used to track the
evolution of the events in the low corona. These were supplemented with
soft X-ray (SXR) flux-measurements from the Geostationary Operational
Environmental Satellite (GOES) and coronal mass ejections (CME) data
from the Large Angle and Spectroscopic Coronagraph (LASCO) onboard
the Solar and Heliospheric Observatory (SOHO). Positional information
of the coronal bursts was obtained by the Nançay Radioheliograph
(NRH). We examined the relationship of the type IV events with coronal
radio bursts, CMEs, and SXR flares. The majority of the events (45)
were characterized as compact, their duration was on average 106
minutes. This type of events was, mostly, associated with M- and
X-class flares (40 out of 45) and fast CMEs, 32 of these events had
CMEs faster than 1000 kms−<SUP>1</SUP>. Furthermore, in 43 compact
events the CME was possibly subjected to reduced aerodynamic drag as
it was propagating in the wake of a previous CME. A minority (three) of
long-lived type <SUB>IVIP</SUB> bursts was detected, with durations from
960 minutes to 115 hours. These events are referred to as extended or
long duration and appear to replenish their energetic electron content,
possibly from electrons escaping from the corresponding coronal type
IV bursts. The latter were found to persist on the disk, for tens of
hours to days. Prominent among them was the unusual interplanetary
type IV burst of 18 - 23 May 2002, which is the longest event in the
Wind/WAVES catalog. The three extended events were typically accompanied
by a number of flares, of GOES class C in their majority, and of CMEs,
many of which were slow and narrow.
---------------------------------------------------------
Title: Multi-viewpoint Observations of a Widely distributed Solar
Energetic Particle Event: The Role of EUV Waves and White-light
Shock Signatures
Authors: Kouloumvakos, A.; Patsourakos, S.; Nindos, A.; Vourlidas,
A.; Anastasiadis, A.; Hillaris, A.; Sandberg, I.
2016ApJ...821...31K Altcode:
On 2012 March 7, two large eruptive events occurred in the same active
region within 1 hr from each other. Each consisted of an X-class flare,
a coronal mass ejection (CME), an extreme-ultraviolet (EUV) wave,
and a shock wave. The eruptions gave rise to a major solar energetic
particle (SEP) event observed at widely separated (∼120°) points
in the heliosphere. From multi-viewpoint energetic proton recordings
we determine the proton release times at STEREO B and A (STB, STA)
and the first Lagrange point (L1) of the Sun-Earth system. Using EUV
and white-light data, we determine the evolution of the EUV waves in
the low corona and reconstruct the global structure and kinematics of
the first CME’s shock, respectively. We compare the energetic proton
release time at each spacecraft with the EUV waves’ arrival times
at the magnetically connected regions and the timing and location
of the CME shock. We find that the first flare/CME is responsible
for the SEP event at all three locations. The proton release at STB
is consistent with arrival of the EUV wave and CME shock at the STB
footpoint. The proton release time at L1 was significantly delayed
compared to STB. Three-dimensional modeling of the CME shock shows
that the particle release at L1 is consistent with the timing and
location of the shock’s western flank. This indicates that at L1
the proton release did not occur in low corona but farther away from
the Sun. However, the extent of the CME shock fails to explain the
SEP event observed at STA. A transport process or a significantly
distorted interplanetary magnetic field may be responsible.
---------------------------------------------------------
Title: Solar Science with the Atacama Large Millimeter/Submillimeter
Array—A New View of Our Sun
Authors: Wedemeyer, S.; Bastian, T.; Brajša, R.; Hudson, H.;
Fleishman, G.; Loukitcheva, M.; Fleck, B.; Kontar, E. P.; De Pontieu,
B.; Yagoubov, P.; Tiwari, S. K.; Soler, R.; Black, J. H.; Antolin,
P.; Scullion, E.; Gunár, S.; Labrosse, N.; Ludwig, H. -G.; Benz,
A. O.; White, S. M.; Hauschildt, P.; Doyle, J. G.; Nakariakov, V. M.;
Ayres, T.; Heinzel, P.; Karlicky, M.; Van Doorsselaere, T.; Gary,
D.; Alissandrakis, C. E.; Nindos, A.; Solanki, S. K.; Rouppe van
der Voort, L.; Shimojo, M.; Kato, Y.; Zaqarashvili, T.; Perez, E.;
Selhorst, C. L.; Barta, M.
2016SSRv..200....1W Altcode: 2015SSRv..tmp..118W; 2015arXiv150406887W
The Atacama Large Millimeter/submillimeter Array (ALMA) is a new
powerful tool for observing the Sun at high spatial, temporal, and
spectral resolution. These capabilities can address a broad range
of fundamental scientific questions in solar physics. The radiation
observed by ALMA originates mostly from the chromosphere—a complex
and dynamic region between the photosphere and corona, which plays a
crucial role in the transport of energy and matter and, ultimately,
the heating of the outer layers of the solar atmosphere. Based on
first solar test observations, strategies for regular solar campaigns
are currently being developed. State-of-the-art numerical simulations
of the solar atmosphere and modeling of instrumental effects can help
constrain and optimize future observing modes for ALMA. Here we present
a short technical description of ALMA and an overview of past efforts
and future possibilities for solar observations at submillimeter and
millimeter wavelengths. In addition, selected numerical simulations
and observations at other wavelengths demonstrate ALMA's scientific
potential for studying the Sun for a large range of science cases.
---------------------------------------------------------
Title: The Major Geoeffective Solar Eruptions of 2012 March 7:
Comprehensive Sun-to-Earth Analysis
Authors: Patsourakos, S.; Georgoulis, M. K.; Vourlidas, A.; Nindos,
A.; Sarris, T.; Anagnostopoulos, G.; Anastasiadis, A.; Chintzoglou,
G.; Daglis, I. A.; Gontikakis, C.; Hatzigeorgiu, N.; Iliopoulos, A. C.;
Katsavrias, C.; Kouloumvakos, A.; Moraitis, K.; Nieves-Chinchilla, T.;
Pavlos, G.; Sarafopoulos, D.; Syntelis, P.; Tsironis, C.; Tziotziou,
K.; Vogiatzis, I. I.; Balasis, G.; Georgiou, M.; Karakatsanis, L. P.;
Malandraki, O. E.; Papadimitriou, C.; Odstrčil, D.; Pavlos, E. G.;
Podlachikova, O.; Sandberg, I.; Turner, D. L.; Xenakis, M. N.; Sarris,
E.; Tsinganos, K.; Vlahos, L.
2016ApJ...817...14P Altcode:
During the interval 2012 March 7-11 the geospace experienced a
barrage of intense space weather phenomena including the second
largest geomagnetic storm of solar cycle 24 so far. Significant
ultra-low-frequency wave enhancements and relativistic-electron dropouts
in the radiation belts, as well as strong energetic-electron injection
events in the magnetosphere were observed. These phenomena were
ultimately associated with two ultra-fast (>2000 km s<SUP>-1</SUP>)
coronal mass ejections (CMEs), linked to two X-class flares launched
on early 2012 March 7. Given that both powerful events originated from
solar active region NOAA 11429 and their onsets were separated by less
than an hour, the analysis of the two events and the determination
of solar causes and geospace effects are rather challenging. Using
satellite data from a flotilla of solar, heliospheric and magnetospheric
missions a synergistic Sun-to-Earth study of diverse observational
solar, interplanetary and magnetospheric data sets was performed. It was
found that only the second CME was Earth-directed. Using a novel method,
we estimated its near-Sun magnetic field at 13 R<SUB>⊙</SUB> to be
in the range [0.01, 0.16] G. Steep radial fall-offs of the near-Sun
CME magnetic field are required to match the magnetic fields of the
corresponding interplanetary CME (ICME) at 1 AU. Perturbed upstream
solar-wind conditions, as resulting from the shock associated with the
Earth-directed CME, offer a decent description of its kinematics. The
magnetospheric compression caused by the arrival at 1 AU of the shock
associated with the ICME was a key factor for radiation-belt dynamics.
---------------------------------------------------------
Title: SSALMON - The Solar Simulations for the Atacama Large
Millimeter Observatory Network
Authors: Wedemeyer, S.; Bastian, T.; Brajša, R.; Barta, M.; Hudson,
H.; Fleishman, G.; Loukitcheva, M.; Fleck, B.; Kontar, E.; De Pontieu,
B.; Tiwari, S.; Kato, Y.; Soler, R.; Yagoubov, P.; Black, J. H.;
Antolin, P.; Gunár, S.; Labrosse, N.; Benz, A. O.; Nindos, A.;
Steffen, M.; Scullion, E.; Doyle, J. G.; Zaqarashvili, T.; Hanslmeier,
A.; Nakariakov, V. M.; Heinzel, P.; Ayres, T.; Karlicky, M.
2015AdSpR..56.2679W Altcode: 2015arXiv150205601W
The Solar Simulations for the Atacama Large Millimeter Observatory
Network (SSALMON) was initiated in 2014 in connection with two ALMA
development studies. The Atacama Large Millimeter/submillimeter Array
(ALMA) is a powerful new tool, which can also observe the Sun at
high spatial, temporal, and spectral resolution. The international
SSALMONetwork aims at co-ordinating the further development of solar
observing modes for ALMA and at promoting scientific opportunities
for solar physics with particular focus on numerical simulations,
which can provide important constraints for the observing modes and
can aid the interpretation of future observations. The radiation
detected by ALMA originates mostly in the solar chromosphere - a
complex and dynamic layer between the photosphere and corona, which
plays an important role in the transport of energy and matter and the
heating of the outer layers of the solar atmosphere. Potential targets
include active regions, prominences, quiet Sun regions, flares. Here,
we give a brief overview over the network and potential science cases
for future solar observations with ALMA.
---------------------------------------------------------
Title: A tiny event producing an interplanetary type III burst
Authors: Alissandrakis, C. E.; Nindos, A.; Patsourakos, S.;
Kontogeorgos, A.; Tsitsipis, P.
2015A&A...582A..52A Altcode: 2015arXiv150708423A
<BR /> Aims: We investigate the conditions under which small-scale
energy release events in the low corona gave rise to strong
interplanetary (IP) type III bursts. <BR /> Methods: We analyzed
observations of three tiny events, detected by the Nançay Radio
Heliograph (NRH), two of which produced IP type III bursts. We
took advantage of the NRH positioning information and of the high
cadence of AIA/SDO data to identify the associated extreme-UV (EUV)
emissions. We measured positions and time profiles of the metric and
EUV sources. <BR /> Results: We found that the EUV events that produced
IP type III bursts were located near a coronal hole boundary, while the
one that did not was located in a closed magnetic field region. In all
three cases tiny flaring loops were involved, without any associated
mass eruption. In the best observed case, the radio emission at the
highest frequency (435 MHz) was displaced by ~55” with respect to
the small flaring loop. The metric type III emission shows a complex
structure in space and in time, indicative of multiple electron
beams, despite the low intensity of the events. From the combined
analysis of dynamic spectra and NRH images, we derived the electron
beam velocity as well as the height, ambient plasma temperature, and
density at the level of formation of the 160 MHz emission. From the
analysis of the differential emission measure derived from the AIA
images, we found that the first evidence of energy release was at the
footpoints, and this was followed by the development of flaring loops
and subsequent cooling. <BR /> Conclusions: Even small energy release
events can accelerate enough electrons to give rise to powerful IP
type III bursts. The proximity of the electron acceleration site to
open magnetic field lines facilitates the escape of the electrons
into the interplanetary space. The offset between the site of
energy release and the metric type III location warrants further
investigation. <P />The movie is available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201526265/olm">http://www.aanda.org</A>
---------------------------------------------------------
Title: Properties of solar energetic particle events inferred from
their associated radio emission
Authors: Kouloumvakos, A.; Nindos, A.; Valtonen, E.; Alissandrakis,
C. E.; Malandraki, O.; Tsitsipis, P.; Kontogeorgos, A.; Moussas, X.;
Hillaris, A.
2015A&A...580A..80K Altcode: 2015arXiv150703776K
<BR /> Aims: We study selected properties of solar energetic particle
(SEP) events as inferred from their associated radio emissions. <BR
/> Methods: We used a catalogue of 115 SEP events, which consists
of entries of proton intensity enhancements at one AU, with complete
coverage over solar cycle 23 based on high-energy (~68 MeV) protons from
SOHO/ERNE. We also calculated the proton release time at the Sun using
velocity dispersion analysis (VDA). After an initial rejection of cases
with unrealistic VDA path lengths, we assembled composite radio spectra
for the remaining events using data from ground-based and space-borne
radio spectrographs. We registered the associated radio emissions for
every event, and we divided the events in groups according to their
associated radio emissions. In cases of type III-associated events, we
extended our study to the timings between the type III radio emission,
the proton release, and the electron release as inferred from VDA
based on Wind/3DP 20-646 keV data. <BR /> Results: The proton release
was found to be most often accompanied by both type III and II radio
bursts, but a good association percentage was also registered in cases
accompanied by type IIIs only. The worst association was found for the
cases only associated with type II. In the type III-associated cases,
we usually found systematic delays of both the proton and electron
release times as inferred by the particles' VDAs, with respect to the
start of the associated type III burst. The comparison of the proton
and electron release times revealed that, in more than half of the
cases, the protons and electrons were simultaneously released within
the statistical uncertainty of our analysis. For the cases with type
II radio association, we found that the distribution of the proton
release heights had a maximum at ~2.5 R<SUB>⊙</SUB>. Most (69%) of
the flares associated with our SEP events were located in the western
hemisphere, with a peak within the well-connected region of 50°-60°
western longitude. <BR /> Conclusions: Both flare- and shock-related
particle release processes are observed in major proton events at
>50 MeV. Typically, the protons are released after the start of the
associated type III bursts and simultaneously or before the release of
energetic electrons. Our study indicates that a clear-cut distinction
between flare-related and CME-related SEP events is difficult to
establish. <P />Appendices are available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201424397/olm">http://www.aanda.org</A>
---------------------------------------------------------
Title: How Common Are Hot Magnetic Flux Ropes in the Low Solar
Corona? A Statistical Study of EUV Observations
Authors: Nindos, A.; Patsourakos, S.; Vourlidas, A.; Tagikas, C.
2015ApJ...808..117N Altcode: 2015arXiv150703766N
We use data at 131, 171, and 304 Å from the Atmospheric Imaging
Assembly on board the Solar Dynamics Observatory to search for hot
flux ropes in 141 M-class and X-class solar flares that occurred at
solar longitudes equal to or larger than 50°. Half of the flares were
associated with coronal mass ejections. The goal of our survey is to
assess the frequency of hot flux ropes in large flares irrespective
of their formation time relative to the onset of eruptions. The flux
ropes were identified in 131 Å images using morphological criteria and
their high temperatures were confirmed by their absence in the cooler
171 and 304 Å passbands. We found hot flux ropes in 45 of our events
(32% of the flares); 11 of them were associated with confined flares
while the remaining 34 were associated with eruptive flares. Therefore
almost half (49%) of the eruptive events involved a hot flux rope
configuration. The use of supplementary Hinode X-Ray Telescope data
indicates that these percentages should be considered as lower limits
of the actual rates of occurrence of hot flux ropes in large flares.
---------------------------------------------------------
Title: Release timescales of solar energetic particles in the
low corona
Authors: Agueda, N.; Klein, K. -L.; Vilmer, N.; Rodríguez-Gasén,
R.; Malandraki, O. E.; Papaioannou, A.; Subirà, M.; Sanahuja, B.;
Valtonen, E.; Dröge, W.; Nindos, A.; Heber, B.; Braune, S.; Usoskin,
I. G.; Heynderickx, D.; Talew, E.; Vainio, R.
2014A&A...570A...5A Altcode:
<BR /> Aims: We present a systematic study of the timing and duration of
the release processes of near-relativistic (NR; >50 keV) electrons
in the low corona. <BR /> Methods: We analyze seven well-observed
events using in situ measurements by both the ACE and Wind spacecraft
and context electromagnetic observations in soft X-rays, radio, hard
X-rays and white light. We make use of velocity dispersion analysis to
estimate the release time of the first arriving electrons and compare
with the results obtained by using a simulation-based approach,
taking interplanetary transport effects into account to unfold the
NR electron release time history from in situ measurements. <BR />
Results: The NR electrons observed in interplanetary space appear
to be released during either short (<30 min) or long (>2
h) periods. The observation of NR electron events showing beamed
pitch-angle distributions (PADs) during several hours is the clearest
observational signature of sustained release in the corona. On the
other hand, the in situ observation of PADs isotropizing in less
than a couple of hours is a clear signature of a prompt release of
electrons in the low corona. Short release episodes appear to originate
in solar flares, in coincidence with the timing of the observed type
III radio bursts. Magnetic connectivity plays an important role. Only
type III radio bursts reaching the local plasma line measured at 1 AU
are found to be related with an associated release episode in the low
corona. Other type III bursts may also have a release of NR electrons
associated with them, but these electrons do not reach L1. Long
release episodes appear associated with signatures of long acceleration
processes in the low corona (long decay of the soft X-ray emission,
type IV radio bursts, and time-extended microwave emission). Type II
radio bursts are reported for most of the events and do not provide
a clear discrimination between short and long release timescales.
---------------------------------------------------------
Title: SEPServer catalogues of solar energetic particle events at
1 AU based on STEREO recordings: 2007-2012
Authors: Papaioannou, A.; Malandraki, O. E.; Dresing, N.; Heber, B.;
Klein, K. -L.; Vainio, R.; Rodríguez-Gasén, R.; Klassen, A.; Nindos,
A.; Heynderickx, D.; Mewaldt, R. A.; Gómez-Herrero, R.; Vilmer, N.;
Kouloumvakos, A.; Tziotziou, K.; Tsiropoula, G.
2014A&A...569A..96P Altcode:
The Solar Terrestrial Relations Observatory (STEREO) recordings
provide an unprecedented opportunity to study the evolution of
solar energetic particle (SEP) events from different observation
points in the heliosphere, allowing one to identify the effects of
the properties of the interplanetary magnetic field (IMF) and solar
wind structures on the interplanetary transport and acceleration of
SEPs. Two catalogues based on STEREO recordings, have been compiled as
a part of the SEPServer project, a three-year collaborative effort of
eleven European partners funded under the Seventh Framework Programme
of the European Union (FP7/SPACE). In particular, two instruments on
board STEREO have been used to identify all SEP events observed within
the descending phase of solar cycle 23 and the rising phase of solar
cycle 24 from 2007 to 2012, namely: the Low Energy Telescope (LET)
and the Solar Electron Proton Telescope (SEPT). A scan of STEREO/LET
protons within the energy range 6-10 MeV has been performed for each of
the two STEREO spacecraft. We have tracked all enhancements that have
been observed above the background level of this particular channel
and cross-checked with available lists of interplanetary coronal mass
ejections (ICMEs), stream interaction regions (SIRs), and shocks, as
well as with the reported events in literature. Furthermore, parallel
scanning of the STEREO near relativistic electrons has been performed
in order to pinpoint the presence (or absence) of an electron event in
the energy range of 55-85 keV, for all of the aforementioned proton
events included in our lists. We provide the onset and peak time as
well as the peak value of all events for both protons and electrons,
the relevant solar associations in terms of electromagnetic emissions,
soft and hard X-rays (SXRs and HXRs). Finally, a subset of events with
clear recordings at both STEREO spacecraft is presented together with
the parent solar events of these multispacecraft SEP events.
---------------------------------------------------------
Title: Parametric study of drag force acting on interplanetary CME
Authors: Podladchikova, O.; Patsourakos, S.; Nindos, A.
2013hell.confR..22P Altcode:
The interaction of an interplanetary coronal mass ejection (ICME)
with the solar wind leads to an equalisation of the ICME and solar
wind velocities at 1 AU. The forces acting on ICMEs have been
evaluated so far in terms of an empirical drag coefficient C_D ~
1 that describes the aerodynamic drag experienced by a typical ICME
due to its interaction with the ambient solar wind. The consideration
of viscous drag coefficients due to proton-magnetic kink encounters
is more realistic for solar wind turbulence. We compare aerodynamic
and viscous drag description and their impact on ICME propagation in
solar wind. We also consider the impact of ICME distortions to their
kinematics as they propagate in the inner heliosphere.
---------------------------------------------------------
Title: The spatial relationship between coronal mass ejections and
solar flares
Authors: Nikou, E.; Nindos, A.; Patsourakos, S.
2013hell.conf...21N Altcode:
Using 19 well-observed eruptions that gave both coronal mass ejections
(CMEs) and flares, we quantified the spatial relationship between pairs
of CMEs and associated flares. The flare and CME source locations were
identified using images obtained at 174 A by the SWAP instrument aboard
PROBA 2 satellite. The SWAP data are suitable for this study because
flare emission does not saturate much. To reduce saturation even more,
our database did not contain any M-class or X-class flare events. We
selected eruptions that occurred close to disk center, as viewed from
Earth, whereas they appeared as limb events in images obtained by the
EUV Imagers (EUVI) aboard the SECCHI/STEREO spacecraft. The centroids
of the CME-associated EUV dimmings in the SWAP images were used as
proxies for the CME source locations. For each event, we compared the
location of the flare brightenings with the location of the dimmings'
centroid at the time of CME initiation which was determined from
the EUVI data. In six cases the CME location was cospatial with flare
brightenings while in the remaining cases the distance between each pair
of flare-CME locations varied from 4 to 191 arcsecs with a median value
of 71 arcsecs. Furthermore, we investigated the CME source locations
with respect to the underlying magnetic field structures.
---------------------------------------------------------
Title: Sun-to-Earth Analysis of a Major Geoeffective Solar Eruption
within the Framework of the
Authors: Patsourakos, S.; Vlahos, L.; Georgoulis, M.; Tziotziou,
K.; Nindos, A.; Podladchikova, O.; Vourlidas, A.; Anastasiadis, A.;
Sandberg, I.; Tsinganos, K.; Daglis, I.; Hillaris, A.; Preka-Papadema,
P.; Sarris, M.; Sarris, T.
2013hell.conf...10P Altcode:
Transient expulsions of gigantic clouds of solar coronal plasma into
the interplanetary space in the form of Coronal Mass Ejections (CMEs)
and sudden, intense flashes of electromagnetic radiation, solar flares,
are well-established drivers of the variable Space Weather. Given the
innate, intricate links and connections between the solar drivers and
their geomagnetic effects, synergistic efforts assembling all pieces
of the puzzle along the Sun-Earth line are required to advance our
understanding of the physics of Space Weather. This is precisely the
focal point of the Hellenic National Space Weather Research Network
(HNSWRN) under the THALIS Programme. Within the HNSWRN framework,
we present here the first results from a coordinated multi-instrument
case study of a major solar eruption (X5.4 and X1.3 flares associated
with two ultra-fast (>2000 km/s) CMEs) which were launched early
on 7 March 2012 and triggered an intense geomagnetic storm (min Dst
=-147 nT) approximately two days afterwards. Several elements of
the associated phenomena, such as the flare and CME, EUV wave, WL
shock, proton and electron event, interplanetary type II radio burst,
ICME and magnetic cloud and their spatiotemporal relationships and
connections are studied all way from Sun to Earth. To this end, we
make use of satellite data from a flotilla of solar, heliospheric and
magnetospheric missions and monitors (e.g., SDO, STEREO, WIND, ACE,
Herschel, Planck and INTEGRAL). We also present our first steps toward
formulating a cohesive physical scenario to explain the string of the
observables and to assess the various physical mechanisms than enabled
and gave rise to the significant geoeffectiveness of the eruption.
---------------------------------------------------------
Title: Magnetic helicity ejections and coronal activity
Authors: Nindos, A.
2013IAUS..294..519N Altcode:
Magnetic helicity quantifies the degree of linkage and/or twistedness
in the magnetic field. It is probably the only physical quantity which
is approximately conserved even in resistive MHD. This makes it an
ideal tool for the exploration of the physics of solar eruptions. In
this article, I discuss the sources of magnetic helicity injected
into active regions and I point out that coronal mass ejections (CMEs)
are probably necessary to remove at least part of the excess helicity
produced in the Sun. I also discuss the importance of magnetic helicity
in the overall coronal evolution that may lead to eruptions.
---------------------------------------------------------
Title: Radio Signatures of Solar Energetic Particles During the
23^rd Solar Cycle
Authors: Miteva, R.; Klein, K. -L.; Samwel, S. W.; Nindos, A.;
Kouloumvakos, A.; Reid, H.
2013CEAB...37..541M Altcode: 2014arXiv1402.6442M
We present the association rates between solar energetic particles
(SEPs) and the radio emission signatures in the corona and IP space
during the entire solar cycle 23. We selected SEPs associated with X
and M-class flares from the visible solar hemisphere. All SEP events
are also accompanied by coronal mass ejections. Here, we focus on
the correlation between the SEP events and the appearance of radio
type II, III and IV bursts on dynamic spectra. For this we used the
available radio data from ground-based stations and the Wind/WAVES
spacecraft. The associations are presented separately for SEP events
accompanying activity in the eastern and western solar hemisphere. We
find the highest association rate of SEP events to be with type III
bursts, followed by types II and IV. Whereas for types III and IV no
longitudinal dependence is noticed, these is a tendency for a higher
SEP-association rate with type II bursts in the eastern hemisphere. A
comparison with reports from previous studies is briefly discussed.
---------------------------------------------------------
Title: Scientific Analysis within SEPServer - New Perspectives in
Solar Energetic Particle Research: The Case Study of the 13 July
2005 Event
Authors: Malandraki, O. E.; Agueda, N.; Papaioannou, A.; Klein, K. -L.;
Valtonen, E.; Heber, B.; Dröge, W.; Aurass, H.; Nindos, A.; Vilmer,
N.; Sanahuja, B.; Kouloumvakos, A.; Braune, S.; Preka-Papadema, P.;
Tziotziou, K.; Hamadache, C.; Kiener, J.; Tatischeff, V.; Riihonen,
E.; Kartavykh, Y.; Rodríguez-Gasén, R.; Vainio, R.
2012SoPh..281..333M Altcode: 2012SoPh..tmp..287M; 2012SoPh..tmp..267M
Solar energetic particle (SEP) events are a key ingredient of
solar-terrestrial physics both for fundamental research and space
weather applications. Multi-satellite observations are an important
and incompletely exploited tool for studying the acceleration and the
coronal and interplanetary propagation of the particles. While STEREO
uses for this diagnostic two identical sets of instrumentation, there
are many earlier observations carried out with different spacecraft. It
is the aim of the SEPServer project to make these data and analysis
tools available to a broad user community. The consortium will carry
out data-driven analysis and simulation-based data analysis capable
of deconvolving the effects of interplanetary transport and solar
injection from SEP observations, and will compare the results with the
electromagnetic signatures. The tools and results will be provided on
the web server of the project in order to facilitate further analysis
by the research community. This paper describes the data products and
analysis strategies with one specific event, the case study of 13 July
2005. The release time of protons and electrons are derived using
data-driven and simulation-based analyses, and compared with hard
X-ray and radio signatures. The interconnection of the experimental
and the simulation-based results are discussed in detail.
---------------------------------------------------------
Title: Scientific analysis within SEPServer: the 13 July 2005 SEP
event case study
Authors: Malandraki, O. E.; Valtonen, E.; Agueda, N.; Papaioannou, A.;
Klein, K. -L.; Heber, B.; Droege, W.; Aurass, H.; Nindos, A.; Vilmer,
N.; Sanahuja, B.; Kouloumvakos, A.; Braune, S.; Preka-Papadema, P.;
Tziotziou, K.; Hamadache, C.; Kiener, J.; Tatischeff, V.; Kartavykh,
J.; Vainio, R.
2012EGUGA..14.9001M Altcode:
SEPServer is set out to make the first database of particle and
corresponding EM observations of solar energetic particle (SEP) events
over roughly three solar cycles. It will also provide users with results
from the scientific analysis of multiple datasets using different
observational and simulation based methods. Therefore, SEPServer
will lead to new perspectives of scientific analysis and will serve
as a new asset valuable for SEP and Space Weather research. In this
contribution, the event of 13 July 2005 has been used as a case study,
which is a proxy for the overall information that the SEPServer will
include and at the same time it reveals the capabilities offered to
the future users of SEPServer. The analysis of the 13 July 2005 event
- focusing on the data driven analysis, i.e., onset and release time
determination from SOHO/ERNE, SOHO/EPHIN and ACE/EPAM together with
pitch angle distributions from ACE/EPAM, simulations based on WIND/3DP
and ACE/EPAM electrons as well as direct comparison of the observed SEP
fluxes with the associated electromagnetic emissions - is performed. The
physical interpretation and the interconnection of the experimental and
the simulation based results are discussed in detail. The 13 July 2005
case study exemplifies the future usage of SEPServer, which will provide
a comprehensive and up to date SEP analysis service. Acknowledgements:
The research leading to these results has received funding from the
European Union's Seventh Framework Programme (FP7/2007-2013) under
grant agreement No 262773 (SEPServer).
---------------------------------------------------------
Title: On the Role of the Background Overlying Magnetic Field in
Solar Eruptions
Authors: Nindos, A.; Patsourakos, S.; Wiegelmann, T.
2012ApJ...748L...6N Altcode:
The primary constraining force that inhibits global solar eruptions is
provided by the overlying background magnetic field. Using magnetic
field data from both the Helioseismic and Magnetic Imager aboard the
Solar Dynamics Observatory and the spectropolarimeter of the Solar
Optical Telescope aboard Hinode, we study the long-term evolution of
the background field in active region AR11158 that produced three major
coronal mass ejections (CMEs). The CME formation heights were determined
using EUV data. We calculated the decay index -(z/B)(∂B/∂z) of the
magnetic field B (i.e., how fast the field decreases with height, z)
related to each event from the time of the active region emergence until
well after the CMEs. At the heights of CME formation, the decay indices
were 1.1-2.1. Prior to two of the events, there were extended periods
(of more than 23 hr) where the related decay indices at heights above
the CME formation heights either decreased (up to -15%) or exhibited
small changes. The decay index related to the third event increased (up
to 118%) at heights above 20 Mm within an interval that started 64 hr
prior to the CME. The magnetic free energy and the accumulated helicity
into the corona contributed the most to the eruptions by their increase
throughout the flux emergence phase (by factors of more than five and
more than two orders of magnitude, respectively). Our results indicate
that the initiation of eruptions does not depend critically on the
temporal evolution of the variation of the background field with height.
---------------------------------------------------------
Title: Preface
Authors: Marqué, C.; Nindos, A.; van Driel-Gesztelyi, L.; Mandrini,
C. H.
2012esrs.book....1M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Radio Emission Associated with Solar Energetic Particle Events
Authors: Kouloumvakos, A.; Nindos, A.; Preka-Papadema, P.; Hillaris,
A.; Caroubalos, C.; Moussas, X.; Alissandrakis, C.; Tsitsipis, P.;
Kontogeorgos, A.
2012hell.confR..12K Altcode:
The corona probed at meter and decimeter wavelengths is a crucial region
for the acceleration and propagation of solar energetic particles
(SEPs), and radio diagnostics in this plasma plays a major role in
assessing the origin of SEP events. Using data from the ARTEMIS IV solar
radio spectro-graph, we report the properties of the radio emission
associated with several major SEP events. The association of the radio
emission with the related flares and CMEs is also investigated.
---------------------------------------------------------
Title: Multi-wavelength Observations of a Metric Type-II Event
Authors: Alissandrakis, C.; Nindos, A.; Patsourakos, S.; Hillaris,
Al.; Artemis Group
2012hell.conf....6A Altcode:
We have studied a complex metric radio event,observed with the ARTEMIS
radiospectrogarph on February 12, 2010. The event was associated with
a surge observed at 195 and 304 A and with a coronal mass ejection
observed by STEREO A and B instruments near the East wnd West limbs
respectively. On the disk the event was observed at 10 frequencies by
the Nancay Radioheliograph (NRH), in H-alpha by the Catania observatory
and in soft x-rays by GOES SXI. We combined these data, together with
MDI longitudinal magnetic field, to get as complete a picture of the
event as possible. Our emphasis is on two type-II bursts that occured
near respective maxima in the GOES light curves. The first, associated
with the main peak of the event, showed a clear foundamental-harmonic
structure, while the emission of the second consisted of three
well-separated bands. Using positional infornation for the type-IIs
from the NRH we explore their possible association with the surge,
the coronal front and the CME. We also studied fine structured and
foundamental harmonic structure in the metric dynamic spectrum.
---------------------------------------------------------
Title: Preface
Authors: Marqué, C.; Nindos, A.; van Driel-Gesztelyi, L.; Mandrini,
C. H.
2011SoPh..273..307M Altcode: 2011SoPh..tmp..385M
No abstract at ADS
---------------------------------------------------------
Title: On the relationship of shock waves to flares and coronal
mass ejections
Authors: Nindos, A.; Alissandrakis, C. E.; Hillaris, A.;
Preka-Papadema, P.
2011A&A...531A..31N Altcode: 2011arXiv1105.1268N
Context. Metric type II bursts are the most direct diagnostic
of shock waves in the solar corona. <BR /> Aims: There are two
main competing views about the origin of coronal shocks: that they
originate in either blast waves ignited by the pressure pulse of a
flare or piston-driven shocks due to coronal mass ejections (CMEs). We
studied three well-observed type II bursts in an attempt to place
tighter constraints on their origins. <BR /> Methods: The type II
bursts were observed by the ARTEMIS radio spectrograph and imaged by
the Nançay Radioheliograph (NRH) at least at two frequencies. To take
advantage of projection effects, we selected events that occurred away
from disk center. <BR /> Results: In all events, both flares and CMEs
were observed. In the first event, the speed of the shock was about
4200 km s<SUP>-1</SUP>, while the speed of the CME was about 850 km
s<SUP>-1</SUP>. This discrepancy ruled out the CME as the primary shock
driver. The CME may have played a role in the ignition of another shock
that occurred just after the high speed one. A CME driver was excluded
from the second event as well because the CMEs that appeared in the
coronagraph data were not synchronized with the type II burst. In
the third event, the kinematics of the CME which was determined by
combining EUV and white light data was broadly consistent with the
kinematics of the type II burst, and, therefore, the shock was probably
CME-driven. <BR /> Conclusions: Our study demonstrates the diversity
of conditions that may lead to the generation of coronal shocks.
---------------------------------------------------------
Title: Magnetic helicity of solar active regions
Authors: Nindos, A.
2009IAUS..257..133N Altcode:
Magnetic helicity is a quantity that describes the linkage and
twistedness/shear in the magnetic field. It has the unique feature
that it is probably the only physical quantity which is approximately
conserved even in resistive MHD. This makes magnetic helicity an ideal
tool for the exploration of the physics of eruptive events. The concept
of magnetic helicity can be used to monitor the whole history of a CME
event from the emergence of twisted magnetic flux from the convective
zone to the eruption and propagation of the CME into interplanetary
space. In this article, I discuss the sources of the magnetic helicity
injected into active regions and the role of magnetic helicity in the
initiation of solar eruptions.
---------------------------------------------------------
Title: Radio Emission of Flares and Coronal Mass Ejections. Invited
Review
Authors: Nindos, A.; Aurass, H.; Klein, K. -L.; Trottet, G.
2008SoPh..253....3N Altcode:
We review recent progress on our understanding of radio emission from
solar flares and coronal mass ejections (CMEs) with emphasis on those
aspects of the subject that help us address questions about energy
release and its properties, the configuration of flare - CME source
regions, coronal shocks, particle acceleration and transport, and the
origin of solar energetic particle (SEP) events. Radio emission from
electron beams can provide information about the electron acceleration
process, the location of injection of electrons in the corona, and
the properties of the ambient coronal structures. Mildly relativistic
electrons gyrating in the magnetic fields of flaring loops produce
radio emission via the gyrosynchrotron mechanism, which provides
constraints on the magnetic field and the properties of energetic
electrons. CME detection at radio wavelengths tracks the eruption from
its early phase and reveals the participation of a multitude of loops
of widely differing scale. Both flares and CMEs can ignite shock waves
and radio observations offer the most robust tool to study them. The
incorporation of radio data into the study of SEP events reveals that a
clear-cut distinction between flare-related and CME-related SEP events
is difficult to establish.
---------------------------------------------------------
Title: A Statistical Study of Microwave Flare Morphologies
Authors: Tzatzakis, V.; Nindos, A.; Alissandrakis, C. E.
2008SoPh..253...79T Altcode: 2008SoPh..tmp..166T
This study has been motivated by the detection of a small number of
optically thin microwave bursts with maximum emission near the loop
top, which is contrary to the prediction of isotropic gyrosynchrotron
models. Using Nobeyama Radioheliograph (NoRH) high-spatial-resolution
images at 17 and 34 GHz, we study the morphology at the radio peak of
104 flares that occurred relatively close to the limb. Using data from
the Nobeyama Polarimeter we were able to determine whether the 17-
and 34-GHz emissions came from optically thin or thick sources. We
identified single-loop events, taking into account supplementary
information from EUV and soft X-ray (SXR) images. We found optically
thin emission from the top of the loop in 36% of single-loop events. In
agreement with standard models, in this sample 46% and 18% of the events
showed optically thin emission from the footpoints and optically thick
emission from the entire loop, respectively. The derived percentage
of events with gyrosynchrotron emission from isotropic populations
of energetic electrons is possibly an upper limit. This point is
illustrated by the analysis of an optically thin event that shows
footpoint emission during the rise phase and loop-top emission during
the decay phase. A model that takes into account both anisotropies in
the distribution function of nonthermal electrons and time evolution
can reproduce the observed transition from footpoint to loop-top
morphology, if electrons with pitch-angle anisotropy are injected near
one of the footpoints.
---------------------------------------------------------
Title: Studying The Magnetic Origins Of Solar Eruptions Using
"Solar Orbiter"
Authors: Nindos, A.
2007ESASP.641E..20N Altcode:
Recent studies have provided observational support for the paradigm
that solar eruptions result from the inter- play between magnetic
reconnection and the approximate magnetic helicity conservation
in the corona: confined flares are regarded as reconnection events
while CMEs are the valves through which the Sun gets rid of excess
helicity. However, several aspects of the paradigm need clarification
and possibly modification. The instruments onboard the "Solar Orbiter"
mission will provide a unique opportunity for such work. In this
article, I propose how to exploit the capabilities of the "Solar
Orbiter" remote sensing instruments in order to shed more light on
the processes leading to flares and CMEs.
---------------------------------------------------------
Title: Flare Physics with the Nobeyama Radioheliograph and RHESSIs
Authors: Nindos, A.
2006spnr.conf...39N Altcode:
In this article I review the present status of knowledge in areas of
special interest for further study using solar flare observations
at microwaves from the Nobeyama Radioheliograph and hard X-ray
(HXR) observations from RHESSI. The most direct tracers of the
electrons accelerated in a flare are the microwave and HXR emissions
they produce. The combined microwave and hard X-ray data provide
powerful diagnostics of the physical conditions in flaring regions
and particularly the magnetic field configuration of the flare and
the properties of the energetic electrons. I point out that in order
to tackle better the fundamental problem of particle acceleration in
solar flares, detailed comparison of the spatial and spectral data
with models is needed.
---------------------------------------------------------
Title: Magnetic Helicity and Coronal Mass Ejections
Authors: Nindos, A.
2006GMS...165...59N Altcode:
Magnetic helicity is a quantity that descibes the chiral properties
of magnetic structures. It has the unique feature that it is probably
the only physical quantity which is approximately conserved even in
resistive MHD. This makes magnetic helicity an ideal tool for the
exploration of the physics of coronal mass ejections (CMEs). CMEs
carry away from the Sun twisted magnetic fields and the concept of
helicity can be used to monitor the whole history of a CME event from
the emergence of twisted magnetic flux from the convective zone to
the eruption and propagation of the CME into interplanetary space. I
discuss the sources of the helicity shed by CMEs and the role of
magnetic helicity in the initiation of CMEs.
---------------------------------------------------------
Title: A Statistical Study of Microwave Flare Morphologies
Authors: Tzatzakis, V.; Nindos, A.; Alissandrakis, C. E.; Shibasaki, K.
2006AIPC..848..248T Altcode:
Using Nobeyama Radioheliograph (NoRH) high spatial resolution images
at 17 and 34 GHz, we study the morphology of several flare events that
occured relatively close to the limb. Our study has been motivated by
the recent detection of a small number of optically thin flares whose
maximum emission peaks close to the loop top. These events may show
significant anisotropy of the density and pitch angle distribution
of the nonthermal electrons. However, we do not know how often they
occur. Our sample consists of 104 flare events. Using data from the
Nobeyama Polarimeter we were able to determine whether the 17 and 34
GHz emissions are optically thin or thick. Almost half of our events
appear unresolved in the NoRH images. Among the resolved events,
special attention is paid to those with circular polarization (V)
showing two distinct sources. Such V sources are considered proxies to
the flaring loop footpoints if the total intensity (I) morphology is
consistent with a single flaring loop configuration. A small number of
events, as expected at such high frequencies, are optically thick with
I maximum between the loop footpoints. The number of optically thin
events showing I maxima co-spatial with the V sources is approximately
the same as the number of those optically thin resolved events showing
emission peak between the V sources. Our future plans concerning the
study of our database are also outlined.
---------------------------------------------------------
Title: Ten Years of the Solar Radiospectrograph ARTEMIS-IV
Authors: Caroubalos, C.; Alissandrakis, C. E.; Hillaris, A.;
Preka-Papadema, P.; Polygiannakis, J.; Moussas, X.; Tsitsipis, P.;
Kontogeorgos, A.; Petoussis, V.; Bouratzis, C.; Bougeret, J. -L.;
Dumas, G.; Nindos, A.
2006AIPC..848..864C Altcode: 2010arXiv1009.4150C
The Solar Radiospectrograph of the University of Athens (ARTEMIS-IV)
is in operation at the Thermopylae Satellite Communication Station
since 1996. The observations extend from the base of the Solar Corona
(650 MHz) to about 2 Solar Radii (20 MHz) with time resolution
1/10-1/100 sec. The instruments recordings, being in the form of
dynamic spectra, measure radio flux as a function of height in the
corona; our observations are combined with spatial data from the
Nancay Radioheliograph whenever the need for 3D positional information
arises. The ARTEMIS-IV contribution in the study of solar radio bursts
is two fold- Firstly, in investigating new spectral characteristics
since its high sampling rate facilitates the study of fine structures in
radio events. On the other hand it is used in studying the association
of solar bursts with interplanetary phenomena because of its extended
frequency range which is, furthermore, complementary to the range of the
WIND/WAVES receivers and the observations may be readily combined. This
reports serves as a brief account of this operation. Joint observations
with STEREO/WAVES and LOFAR low frequency receivers are envisaged in
the future.
---------------------------------------------------------
Title: The Role of Magnetic Helicity in Solar Eruptions
Authors: Nindos, A.
2006AIPC..848...64N Altcode:
Magnetic helicity is a quantity that descibes the chiral properties
of magnetic structures. It has the unique feature that it is probably
the only physical quantity which is approximately conserved even in
resistive MHD. This makes magnetic helicity an ideal tool for the
exploration of the physics of coronal mass ejections (CMEs). CMEs
carry away from the Sun twisted magnetic fields and the concept of
helicity can be used to monitor the whole history of a CME event from
the emergence of twisted magnetic flux from the convective zone to
the eruption and propagation of the CME into interplanetary space. I
discuss the sources of the helicity shed by CMEs and the role of
magnetic helicity in the initiation of CMEs.
---------------------------------------------------------
Title: What is the spatial distribution of magnetic helicity injected
in a solar active region?
Authors: Pariat, E.; Nindos, A.; Démoulin, P.; Berger, M. A.
2006A&A...452..623P Altcode:
Context: .Magnetic helicity is suspected to play a key role in
solar phenomena such as flares and coronal mass ejections. Several
investigations have recently computed the photospheric flux of
magnetic helicity in active regions. The derived spatial maps of the
helicity flux density, called G_A, have an intrinsic mixed-sign patchy
distribution. <BR /> Aims: . Pariat et al. (2005) recently showed
that G<SUB>A</SUB> is only a proxy of the helicity flux density,
which tends to create spurious polarities. They proposed a better
proxy, G<SUB>θ</SUB>. We investigate here the implications of this
new approach on observed active regions.<BR /> Methods: . The magnetic
data are from MDI/SoHO instrument and the photospheric velocities are
computed by local correlation tracking. Maps and temporal evolution of
G<SUB>A</SUB> and G<SUB>θ</SUB> are compared using the same data set
for 5 active regions.<BR /> Results: . Unlike the usual G<SUB>A</SUB>
maps, most of our G<SUB>θ</SUB> maps show almost unipolar spatial
structures because the nondominant helicity flux densities are
significantly suppressed. In a few cases, the G<SUB>θ</SUB> maps still
contain spurious bipolar signals. With further modelling we infer that
the real helicity flux density is again unipolar. On time-scales larger
than their transient temporal variations, the time evolution of the
total helicity fluxes derived from G<SUB>A</SUB> and G<SUB>θ</SUB>
show small differences. However, unlike G_A, with G<SUB>θ</SUB>
the time evolution of the total flux is determined primarily by the
predominant-signed flux while the nondominant-signed flux is roughly
stable and probably mostly due to noise. <BR /> Conclusions: .Our
results strongly support the conclusion that the spatial distribution
of helicity injected into active regions is much more coherent than
previously thought: on the active region scale the sign of the injected
helicity is predominantly uniform. These results have implications for
the generation of the magnetic field (dynamo) and for the physics of
both flares and coronal mass ejections.
---------------------------------------------------------
Title: Multiwavelength Analysis of an Active Region
Authors: Gontikakis, C.; Dara, H. C.; Zachariadis, Th. G.;
Alissandrakis, C. E.; Nindos, A.; Vial, J. -C.; Tsiropoula, G.
2006SoPh..233...57G Altcode:
We study active region NOAA 8541, observed with instruments on board
SOHO, as well as with TRACE. The data set mainly covers the transition
region and the low corona. In selected loops studied with SUMER on SOHO,
the VIII 770 Å line is systematically redshifted. In order to estimate
the plasma velocity, we combine the Doppler shifts with proper motions
(TRACE) along these loops. In the case of an ejection, apparently
caused by the emergence of a parasitic polarity, proper motions and
Doppler shifts give consistent results for the velocity. A cooler loop,
observed in the same active region with CDS, shows a unidirectional
motion reminiscent of a siphon flow. The derived electron temperature
and density along a large steady loop confirm that it cannot be
described by hydrostatic models.
---------------------------------------------------------
Title: When does the first CME from an emerging active region occur?
Authors: Nindos, A.
2006cosp...36.1270N Altcode: 2006cosp.meet.1270N
Recently we have found that in a statistical sense the preflare
value of the coronal magnetic helicity of active regions producing
CME-associated big flares is larger than the value of magnetic
helicity of those producing big flares that do not have associated
CMEs The obvious question arising from the above result is whether CME
initiation is controlled by the amount of the magnetic helicity stored
in the corona This problem can be investigated by studying whether the
active region itself that forms after a bipole emergence erupts or not
and what magnetic helicity has to do with the eruption To this end we
have studied several active regions that emerge on the visible side of
the solar disk For each active region its magnetic helicity content is
monitored from its emergence until it reaches a heliographic longitude
of 45-50 degrees or until the occurrence of the first CME associated
with it whichever happens first The main statistical result of our
study suggests that when CMEs occur the coronal magnetic helicity is
larger than the coronal magnetic helicity of those active regions that
do not produce any CMEs This result is independent of the strength of
CME-associated flares As a byproduct of this study we have devoped
a new technique that is able to recover a significant part of the
magnetic helicity injected into the corona which is not detectable
using conventional techniques The new method has been tested against
synthetic data and it will be briefly presented
---------------------------------------------------------
Title: How to derive the real pattern of magnetic helicity injection
in an active region?
Authors: Pariat, E.; Nindos, A.; Démoulin, P.; Berger, M.
2006cosp...36..851P Altcode: 2006cosp.meet..851P
Magnetic helicity a topological quantity which measures the twist the
writhe and the shear of a magnetic field has recently appeared as
a key quantity to understand some mechanisms of the solar activity
such as Coronal Mass Ejections and flare onset It is thus becoming
of major importance to be able to compute magnetic helicity in active
regions Looking at the pattern of the photospheric injection of magnetic
helicity may provide new useful pieces of information to understand the
basic properties of solar activity If several helicity flux density
maps were published no one yet wondered if helicity flux density is
a correct physical quantity Unfortunately the classical helicity flux
density do induce spurious signal fake polarities which mask the real
injection of helicity To map the real helicity injection the knowledge
of the complete connectivity of the field lines is fundamental Even
without the connectivity improved helicity flux density maps can be
derived which present strong differences with the previous incorrect
maps This leads to a complete new way of understanding the dynamics
of the active region in the frame of the magnetic helicity study
---------------------------------------------------------
Title: Complex Solar Events Observed with the Artemis-Iv
Radio-Spectrograph in October/november 2003
Authors: Alissandrakis, C. E.; Nindos, A.; Hilaris, A.; Caroubalos,
C.; Artemis Team
2005ESASP.600E.106A Altcode: 2005dysu.confE.106A; 2005ESPM...11..106A
No abstract at ADS
---------------------------------------------------------
Title: Coronal Magnetography of Solar Active Region 8365 with the
SSRT and NoRH Radio Heliographs
Authors: Ryabov, B. I.; Maksimov, V. P.; Lesovoi, S. V.; Shibasaki,
K.; Nindos, A.; Pevtsov, A.
2005SoPh..226..223R Altcode:
Microwave maps of solar active region NOAA 8365 are used to derive
the coronal magnetograms of this region. The technique is based on
the fact that the circular polarization of a radio source is modified
when microwaves pass through the coronal magnetic field transverse to
the line of sight. The observations were taken with the Siberian Solar
Radio Telescope (SSRT) on October 21 - 23 and with the Nobeyama Radio
Heliograph (NoRH) on October 22 - 24, 1998. The known theory of wave
mode coupling in quasi-transverse (QT) region is employed to evaluate
the coronal magnetograms in the range of 10 - 30 G at the wavelength 5.2
cm and 50 - 110 G at 1.76 cm, taking the product of electron density and
the scale of coronal field divergence to be constant of 10<SUP>18</SUP>
cm<SUP>−2</SUP>. The height of the QT-region is estimated from the
force-free field extrapolations as 6.2 × 10<SUP>9</SUP> cm for the
20 G and 2.3 × 10<SUP>9</SUP> cm for 85 G levels. We find that on
large spatial scale, the coronal magnetograms derived from the radio
observations show similarity with the magnetic fields extrapolated
from the photosphere.
---------------------------------------------------------
Title: The Association of Big Flares and Coronal Mass Ejections:
What is the Role of Magnetic Helicity?
Authors: Nindos, A.; Andrews, M. D.
2005IAUS..226..194N Altcode:
In a recent study Andrews found that approximately 40% of M-class flares
between 1996 and 1999, classified according to GOES X-ray flux, are not
associated with Coronal Mass Ejections (CMEs). Using 133 events from
his dataset for which suitable photospheric magnetograms and coronal
images were available, we studied the pre-flare coronal helicity of the
active regions that produced big flares. The coronal magnetic field of
78 active regions was modeled under the "constant α" linear force-free
field assumption. We find that in a statistical sense the pre-flare
value of α and coronal helicity of the active regions producing big
flares that do not have associated CMEs is smaller than the coronal
helicity of those producing CME-associated big flares. A further
argument supporting this conclusion is that for the active regions
whose coronal magnetic field deviates from the force-free model, the
change of the coronal sign of α within an active region is twice more
likely to occur when the active region is about to produce a confined
flare than a CME-associated flare. Our study indicates that the amount
of the stored pre-flare coronal helicity may determine whether a big
flare will be eruptive or confined.
---------------------------------------------------------
Title: The Association of Big Flares and Coronal Mass Ejections:
What Is the Role of Magnetic Helicity?
Authors: Nindos, A.; Andrews, M. D.
2004ApJ...616L.175N Altcode:
Recently, M. D. Andrews found that approximately 40% of M-class flares
between 1996 and 1999, classified according to GOES X-ray flux, are not
associated with coronal mass ejections (CMEs). Using 133 events from
his data set for which suitable photospheric magnetograms and coronal
images were available, we studied the preflare coronal helicity of the
active regions that produced big flares. The coronal magnetic field
of 78 active regions was modeled under the “constant α” linear
force-free field assumption. We find that in a statistical sense
the preflare value of α and coronal helicity of the active regions
producing big flares that do not have associated CMEs is smaller than
the coronal helicity of those producing CME-associated big flares. A
further argument supporting this conclusion is that for the active
regions whose coronal magnetic field deviates from the force-free model,
the change of the coronal sign of α within an active region is twice as
likely to occur when the active region is about to produce a confined
flare than a CME-associated flare. Our study indicates that the amount
of the stored preflare coronal helicity may determine whether a big
flare will be eruptive or confined.
---------------------------------------------------------
Title: The configuration of simple short-duration solar microwave
bursts
Authors: Kundu, M. R.; Nindos, A.; Grechnev, V. V.
2004A&A...420..351K Altcode:
Using data from the Nobeyama Radioheliograph (NoRH) we study the
source configuration of four simple short-duration 17 and 34 GHz
bursts which have also been observed partially by the Yohkoh Soft X-ray
Telescope (SXT). Two events are consistent with a single flaring loop
configuration. In one of them the flaring loop is resolved in the SXT
images. We derive a self-consistent model for this event by comparing
the radio observations with gyrosynchrotron model loop calculations. Our
best-fit model is able to reproduce both the observed flaring loop
shape as well as the fluxes and structures of the radio emission at
the peak of the event. The flaring loop is relatively small having a
footpoint separation of 16 arcsec and maximum height of 7.7 arcsec. The
variation of the magnetic field along the loop is small (800 G at
the footpoints and 665 G at loop top) and the loop is filled with
electrons with energies up to 10 MeV. The other two bursts show two
radio sources; one source being cospatial with a compact bright soft
X-ray loop associated with a patch of parasitic magnetic polarity whose
photospheric magnetic flux increases before the flares while the other
source is not prominent at any other wavelength range. The two sources
are connected with diffuse loop-like soft X-ray emission. We infer that
these bursts originate from the interaction of two sets of loops with
different sizes. Therefore the simple short duration microwave bursts
we studied do not always appear in the same configuration. Contrary
to previous results not all of them appear as single-loop events. It
is possible that some events are caused by two interacting loops.
---------------------------------------------------------
Title: The Association of Big Flares and CMEs: What is the Role of
Magnetic Helicity?
Authors: Nindos, A.; Andrews, M. D.
2004AAS...204.2703N Altcode: 2004BAAS...36..693N
In a recent study Andrews (2003) found that approximately 40% of the
GOES soft X-ray M-class flares observed during the years 1996-1999
are not associated with CMEs. In this paper, we investigate the role
of magnetic helicity in the association of big flares and CMEs. We
compute the coronal relative magnetic helicity of all active regions
which were located within ±50 degr from the central meridian and
produced M-class flares during the years 1996-1999. The computations
use SOHO/MDI magnetograms taken close to the initiation of each event
as boundary conditions for "constant alpha" linear force-free field
magnetic extrapolations. The extrapolated field lines are fitted with
the corresponding active region's coronal loops observed with EIT at
195 A. Then the value of alpha giving the best fit between the models
and the coronal observations is adopted for the computation of the
coronal magnetic helicity. Within the uncertainties involved in our
computations, we find that the coronal magnetic helicity of about
65-70% of the active regions which gave M-class flares without CMEs
is smaller than the coronal magnetic helicity of all active regions
which gave M-class flares associated with CMEs.
---------------------------------------------------------
Title: Coronal Radio Magnetography of Solar Active Region 8365
Authors: Ryabov, B. I.; Nindos, A.; Shibasaki, K.; Maksimov, V. P.;
Lesovoi, S. V.; Pevtsov, A. A.
2003AGUFMSH42B0508R Altcode:
Using the microwave radio observations with Siberian Solar Radio
Telescope (SSRT) and Nobeyama Radio Heliograph (NoRH) we derived 2D
coronal magnetograms of solar active region NOAA 8365. The circular
polarization (CP) of radio source is modified, when the radiation
passes through the overlying magnetic field transverse to the
line-of-sight. This change in CP was used to study the properties of
coronal fields. We employed the known theory of wave-mode coupling
in quasi-transverse (QT) region to evaluate the distribution of the
field strength at the level of transformation of 5.2 cm radiation
(SSRT, field strength ∼ 10-30 G) and 1.76 cm (NoRH, ∼ 50-110
G). The magnetic field strength was derived under the assumption
N~L<SUB>α </SUB> = 10<SUP>18</SUP> cm<SUP>-2</SUP>, where N is electron
density and L<SUB>α </SUB> is the scale of coronal field divergence
along line-of-sight. The height of QT-region (H<SUB>QTR</SUB>) was
estimated using force free field extrapolations, H<SUB>QRT</SUB> =
6.3 x 10<SUP>9</SUP> cm (2.3 x 10<SUP>9</SUP> cm) for 20 G (85 G). We
then compared the coronal radio magnetograms and the force free field
extrapolation of photospheric magnetic field, and we found close
similarity between them on large spatial scale.
---------------------------------------------------------
Title: The Magnetic Helicity Budget of Solar Active Regions and
Coronal Mass Ejections
Authors: Nindos, A.; Zhang, J.; Zhang, H.
2003ApJ...594.1033N Altcode:
We compute the magnetic helicity injected by transient photospheric
horizontal flows in six solar active regions associated with halo
coronal mass ejections (CMEs) that produced major geomagnetic storms and
magnetic clouds (MCs) at 1 AU. The velocities are computed using the
local correlation tracking (LCT) method. Our computations cover time
intervals of 110-150 hr, and in four active regions the accumulated
helicities due to transient flows are factors of 8-12 larger than the
accumulated helicities due to differential rotation. As was first
pointed out by Démoulin and Berger, we suggest that the helicity
computed with the LCT method yields not only the helicity injected from
shearing motions but also the helicity coming from flux emergence. We
compare the computed helicities injected into the corona with the
helicities carried away by the CMEs using the MC helicity computations
as proxies to the CME helicities. If we assume that the length of the
MC flux tubes is l=2 AU, then the total helicities injected into the
corona are a factor of 2.9-4 lower than the total CME helicities. If
we use the values of l determined by the condition for the initiation
of the kink instability in the coronal flux rope or l=0.5 AU then the
total CME helicities and the total helicities injected into the corona
are broadly consistent. Our study, at least partially, clears up some
of the discrepancies in the helicity budget of active regions because
the discrepancies appearing in our paper are much smaller than the ones
reported in previous studies. However, they point out the uncertainties
in the MC/CME helicity calculations and also the limitations of the
LCT method, which underestimates the computed helicities.
---------------------------------------------------------
Title: Analysis of polarized microwave emission of Flare-Productive
Active Region 9415
Authors: Bogod, V. M.; Gelfreikh, G. B.; Drago, F. Ch.; Maximov,
V. P.; Nindos, A.; Kaltman, T. I.; Ryabov, B. I.; Tokhchukova, S. Kh.
2003astro.ph..9444B Altcode:
The results of the microwave observations of the Sun made with
the RATAN-600 have shown the existence of many types of spectral
peculiarities in polarized emission of active regions, which produce
powerful flares. These phenomena happen at microwaves and reflect
inhomogeneous structure of magnetic field in magnetospheres of flaring
active regions in wide range of heights above the photosphere. In
this presentation we demonstrate an analysis of the AR 9415 during all
the period of its passage across the solar disk. Results of the study
point out to existence of different scenarios of circular polarization
variations in the radio wave band. Here, we separated the phenomenon of
the cyclotron emission passage through the quasi-transverse magnetic
field (QT-region) and several effects connected with flare activity
of active region. New observational data are presented and compared
with the data of several observatories: SSRT, NoRH, MDI SOHO, GOES
and MEES. The preliminary interpretation of the phenomena are given.
---------------------------------------------------------
Title: The origin of CME helicity
Authors: Nindos, A.; Zhang, H.
2002ESASP.506...99N Altcode: 2002svco.conf...99N; 2002ESPM...10...99N
We investigate whether the bulk of magnetic helicity carried away
from the Sun by CMEs comes from helicity injected to the corona by
shearing motions or by emerging magnetic flux. We use photospheric
magnetic field observations of active region NOAA 9165 (AR 9165)
which is an ideal candidate for such study because (1) it is the site
of both new flux emergence and intense horizontal shearing flows;
(2) it shows rapid development and rapid decay and for a few days it
is the site of violent activity; (3) the horizontal motions occur when
it is close to disk center, thus minimizing the errors involved in the
relevant computations; (4) observations of a magnetic cloud associated
with one of the CMEs linked to the active region are available. The
computed helicity change due to horizontal shearing motions is probably
the largest ever reported; it amounts to about the total helicity that
the active region's differential rotation would have injected within
3 solar rotations. But the CMEs linked to the active regions remove
at least a factor of 4-64 more helicity than the helicity injected
by horizontal shearing motions. Consequently the main source of the
helicity carried away by the CMEs is the new magnetic flux that emerges
twisted from the convective zone.
---------------------------------------------------------
Title: Study of coronal loops using TRACE and SOHO
Authors: Gontikakis, C.; Dara, H. C.; Zachariadis, Th.; Nindos, A.;
Alissandrakis, C.; Tsiropoula, G.; Vial, J. -C.
2002ESASP.505..417G Altcode: 2002solm.conf..417G; 2002IAUCo.188..417G
We analysed coronal loops observed in many spectral lines from TRACE and
SUMER, at the active region NOAA 8541, on May 15, 1999. For the loops,
which are identified in a number of wavelengths, we try to combine
the good temporal and spatial resolution of TRACE with the spectral
information obtained by the SUMER rasters, in order to determine some
physical parameters such as temperature, flow velocity and electron
density. The morphology of the magnetic field of the loops is also
approximated by a force-free extrapolation of the photospheric magnetic
field, measured with MDI, and is compared to the loop morphology.
---------------------------------------------------------
Title: Photospheric Motions and Coronal Mass Ejection Productivity
Authors: Nindos, A.; Zhang, H.
2002ApJ...573L.133N Altcode:
Shearing motions have been frequently used in MHD simulations of
coronal mass ejection (CME) initiation but have hardly been reported
from observations of CME-producing regions. In this Letter, we
investigate whether the bulk of magnetic helicity carried away from
the Sun by CMEs comes from helicity injected to the corona by such
motions or by emerging magnetic flux. We use photospheric magnetic field
observations of NOAA Active Region 9165, which is an ideal candidate for
such study because (1) it is the site of both new flux emergence and
intense horizontal shearing flows; (2) it shows rapid development and
rapid decay, and for a few days it is the site of violent activity;
(3) the horizontal motions occur when it is close to disk center,
thus minimizing the errors involved in the relevant computations;
and (4) observations of a magnetic cloud associated with one of the
CMEs linked to the active region are available. The computed helicity
change due to horizontal shearing motions is probably the largest
ever reported; it amounts to about the total helicity that the active
region's differential rotation would have injected within three solar
rotations. But the CMEs linked to the active region remove at least a
factor of 4-64 more helicity than the helicity injected by horizontal
shearing motions. Consequently, the main source of the helicity carried
away by the CMEs is the new magnetic flux that emerges twisted from
the convective zone. Our study implies that shearing motions, even
when they are strong, have little effect in the process of buildup of
magnetic free energy that leads to the initiation of CMEs.
---------------------------------------------------------
Title: Photospheric Motions and CME Productivity
Authors: Nindos, A.; Zhang, H.
2002AGUSMSH22D..05N Altcode:
We investigate whether the bulk of magnetic helicity carried away from
the Sun by CMEs comes from photospheric horizontal shear motions or
from emerging magnetic flux. Our study is focused on active region
NOAA 9165 which is an ideal candidate for such study because (1) both
new flux emergence and intense horizontal flows are observed during
its short lifetime; (2) it shows rapid development and rapid decay and
for a few days it is the site of violent activity; (3) the horizontal
motions occur when it is close to disk center, thus minimizing the
errors involved in the relevant computations; (4) observations of a
magnetic cloud associated with one of the CMEs linked to the active
region are available. The computed helicity change due to horizontal
shear motions is probably the largest ever reported; it amounts to
about the total helicity injected by the active region's differential
rotation within more than 2 solar rotations. But the CMEs linked to the
active region remove a factor of 8--64 more helicity than the helicity
injected by horizontal shear motions. Consequently the main source of
the helicity carried away by the CMEs is the new magnetic flux that
emerges twisted from the convective zone. Our study implies that shear
motions, even when they are strong, have little effect in the process
of buildup of magnetic free energy that leads to the initiation of CMEs.
---------------------------------------------------------
Title: Spatially resolved microwave oscillations above a sunspot
Authors: Nindos, A.; Alissandrakis, C. E.; Gelfreikh, G. B.; Bogod,
V. M.; Gontikakis, C.
2002A&A...386..658N Altcode:
Using high quality VLA observations, we detected for the first time
spatially resolved oscillations in the microwave total intensity
(I) and circular polarization (V) emission of a sunspot-associated
gyroresonance (g-r) source. Oscillations were detected at 8.5 and 5
GHz during several time intervals of our 10-hour-long dataset. The
oscillations are intermittent: they start suddenly and are damped
somehow more gradually. Despite their transient nature when they
are observed they show significant positional, amplitude and phase
stability. The spatial distribution of intensity variations is
patchy and the location of the patches of strong oscillatory power
is not the same at both frequencies. The strongest oscillations are
associated with a small region where the 8.5 GHz emission comes from
the second harmonic of the gyrofrequency while distinct peaks of weaker
oscillatory power appear close to the outer boundaries of the 8.5 and 5
GHz g-r sources, where the emissions come from the third harmonic of the
gyrofrequency. Overall, the 5 GHz oscillations are weaker than the 8.5
GHz oscillations (the rms amplitudes of the I oscillations are 1.3-2.5 x
10<SUP>4</SUP> K and 0.2-1.5 x 10<SUP>5</SUP> K, respectively). At both
frequencies the oscillations have periods in the three-minute range: the
power spectra show two prominent peaks at 6.25-6.45 mHz and 4.49-5.47
mHz. Our models show that the microwave oscillations are caused by
variations of the location of the third and/or second harmonic surfaces
with respect to the base of the chromosphere-corona transition region
(TR), i.e. either the magnetic field strength or/and the height of the
base of the TR oscillates. The best-fit model to the observed microwave
oscillations can be derived from photospheric magnetic field strength
oscillations with an rms amplitude of 40 G or oscillations of the height
of the base of the TR with an rms amplitude of 25 km. Furthermore,
small variations of the orientation of the magnetic field vector yield
radio oscillations consistent with the observed oscillations. Tables
1-3 are only available in electronic form at http://www.edpsciences.org
---------------------------------------------------------
Title: Photospheric Motions and CME Productivity
Authors: Nindos, A.; Zhang, H.
2002AAS...200.2003N Altcode: 2002BAAS...34..673N
Shearing motions have been frequently used in MHD simulations of
CME initiation but have hardly been reported from observations of
CME--producing regions. In this paper we investigate whether the bulk of
magnetic helicity carried away from the Sun by CMEs comes from helicity
injected to the corona by such motions or by emerging magnetic flux. We
use photospheric magnetic field observations of active region NOAA
9165 which is an ideal candidate for such study because (1) it is the
site of both new flux emergence and intense horizontal shearing flows;
(2) it shows rapid development and rapid decay and for a few days it
is the site of violent activity; (3) the horizontal motions occur when
it is close to disk center, thus minimizing the errors involved in the
relevant computations; (4) observations of a magnetic cloud associated
with one of the CMEs linked to the active region are available. The
computed helicity change due to horizontal shearing motions is probably
the largest ever reported; it amounts to about the total helicity that
the active region's differential rotation would have injected within
3 solar rotations. But the CMEs linked to the active region remove
at least a factor of 4--64 more helicity than the helicity injected
by horizontal shearing motions. Consequently the main source of the
helicity carried away by the CMEs is the new magnetic flux that emerges
twisted from the convective zone. Our study implies that shearing
motions, even when they are strong, have little effect in the process
of buildup of magnetic free energy that leads to the initiation of CMEs.
---------------------------------------------------------
Title: ARTEMIS IV Radio Observations of the 14 July 2000 Large
Solar Event
Authors: Caroubalos, C.; Alissandrakis, C. E.; Hillaris, A.; Nindos,
A.; Tsitsipis, P.; Moussas, X.; Bougeret, J. -L.; Bouratzis, K.;
Dumas, G.; Kanellakis, G.; Kontogeorgos, A.; Maroulis, D.; Patavalis,
N.; Perche, C.; Polygiannakis, J.; Preka-Papadema, P.
2001SoPh..204..165C Altcode: 2010arXiv1009.3654C
In this report we present a complex metric burst, associated with
the 14 July 2000 major solar event, recorded by the ARTEMIS-IV radio
spectrograph at Thermopylae. Additional space-borne and Earth-bound
observational data are used, in order to identify and analyze the
diverse, yet associated, processes during this event. The emission at
metric wavelengths consisted of broad-band continua including a moving
and a stationary type IV, impulsive bursts and pulsating structures. The
principal release of energetic electrons in the corona was 15-20 min
after the start of the flare, in a period when the flare emission spread
rapidly eastwards and a hard X-ray peak occurred. Backward extrapolation
of the CME also puts its origin in the same time interval, however,
the uncertainty of the extrapolation does not allow us to associate
the CME with any particular radio or X-ray signature. Finally, we
present high time and spectral resolution observations of pulsations
and fiber bursts, together with a preliminary statistical analysis.
---------------------------------------------------------
Title: Metric Radio Emission Associated with X-Ray Plasmoid Ejections
Authors: Kundu, M. R.; Nindos, A.; Vilmer, N.; Klein, K. -L.; Shibata,
K.; Ohyama, M.
2001ApJ...559..443K Altcode:
In this paper we report the first detection of metric/decimetric
radio emission associated with two soft X-ray plasmoid ejecta events
that occurred during two limb flares observed by the Yohkoh SXT. In
the first event a loop started to rise slowly (~10 km s<SUP>-1</SUP>)
before the beginning of the hard X-ray impulsive phase of the flare. At
about the onset of the impulsive flare, there was acceleration of the
ejecta, resulting in a speed of 130 km s<SUP>-1</SUP> and finally to
~200 km s<SUP>-1</SUP>. The associated radio emission was observed with
the Nançay radioheliograph (NRH) in the frequency range of 230-450
MHz. It was an unpolarized continuum that lasted 8-10 minutes. The 410
MHz source was located close to the height where the plasmoid was last
identified in the SXT images. In the second event an eruption resulted
in the expansion of a large-scale, looplike feature and the development
of two plasmoid ejecta which moved in different directions. The speed of
the ejecta was 60-100 km s<SUP>-1</SUP>. In this event, the associated
radio emission was a long-lasting (about 2 hr) continuum observed from
450 to 164 MHz. The onset of the low-frequency emission was delayed
with respect to the onset of the high-frequency emission. In both
cases the radio sources were located above the soft X-ray ejecta in
the general direction of the prolongation of the ejecta movement. In
both cases the radio emission comes from nonthermal electrons which
are accelerated in close relationship with the propagation of the X-ray
plasmoid: as the plasmoid reaches higher altitudes, it interacts with
increasingly more extended magnetic field lines and new coronal sites
of production of nonthermal electrons are created.
---------------------------------------------------------
Title: A Multiwavelength Study of Three Solar Flares
Authors: Kundu, M. R.; Nindos, A.; White, S. M.; Grechnev, V. V.
2001ApJ...557..880K Altcode:
In this paper we seek a self-consistent model for three strong limb
flares observed at 17 and 34 GHz by the Nobeyama radioheliograph and
also in soft X-rays and hard X-rays by the Yohkoh SXT (Soft X-Ray
Telescope) and HXT (Hard X-Ray Telescope) instruments. Additional
radio spectral data were provided by the Nobeyama polarimeter. The
flare geometry is simple, with one well-defined flaring loop in each
event. The 17 and 34 GHz emissions are optically thin gyrosynchrotron
radiation from energetic electrons that outlines the flaring loops and
peaks close to the loop tops. We infer that the variation of magnetic
field along the loops is very small. We try to reproduce the observed
radio morphologies and fluxes using a model gyrosynchrotron loop. The
results of our modeling rely on the model magnetic field geometry that
we choose. Although the exact loop geometry cannot be constrained from
a two-dimensional snapshot, we choose for simplicity a line-dipole
magnetic field, and the model field lines are circular. The SXT/HXT
images are used to provide the physical parameters of the model
loops. The high-frequency polarimeter data give the energy spectral
index of the radio-emitting electrons. We could not reconcile the
observed radio morphologies and fluxes using classic dipole magnetic
field models. The best-fit model that uses the same input parameters
for both frequencies and partly reconciles the observed 17 and 34 GHz
morphologies and fluxes is produced when we invoke a magnetic field with
constant strength along the model loop. These model loops have uniform
thickness. The derived densities of the radio-emitting electrons are
(1-6)×10<SUP>4</SUP> cm<SUP>-3</SUP> with energy limits between 60
and 5000 keV. These models are the best fits we can get under the best
assumptions we can justify, but they do not in fact match the radio
morphologies very well; their problems and limitations are discussed.
---------------------------------------------------------
Title: Soft X-Ray and Gyroresonance Emission above Sunspots
Authors: Nindos, A.; Kundu, M. R.; White, S. M.; Shibasaki, K.;
Gopalswamy, N.
2000ApJS..130..485N Altcode:
Using Yohkoh SXT and Nobeyama 17 GHz data, we have studied the soft
X-ray and microwave emission above several stable, large sunspots
near central meridian passage. Our study confirms the well-known fact
that soft X-ray emission is depressed above sunspots. It also shows
that the distribution of their soft X-ray intensity is not uniform;
usually the darkest pixels are associated with the umbra or the far
edges of the leading part of the penumbra while the following part
of the penumbra may contain higher intensity pixels associated with
brighter loops. For the first time, we present a systematic survey
of the temperatures and emission measures of the soft X-ray material
above sunspots. Sunspots always contain the lowest temperatures and
emission measures in the active regions. The mean umbral temperature
is 1.8×10<SUP>6</SUP> K, and the mean penumbral temperature is
2.4×10<SUP>6</SUP> K. The mean umbral and penumbral emission measures
are logEM=26.60 cm<SUP>-5</SUP> and logEM=27.00 cm<SUP>-5</SUP>,
respectively. The differences between the umbral and penumbral plasma
temperatures are physically significant. The higher penumbral values
imply that the loops associated with the penumbrae are generally hotter
and denser than the loops associated with the umbrae. The highest
sunspot temperatures and emission measures are still lower than the
average active region parameters but higher than the quiet-Sun plasma
parameters. The coronal radiative energy loss rate above the umbrae
is 15% higher than the radiative loss rate of the quiet-Sun plasma
but a factor of 8.3 lower than the typical active region radiative
loss rate. The radio emission comes from the gyroresonance mechanism,
and, as expected, it is sensitive to the magnetic field rather than
the soft X-ray-emitting plasma.
---------------------------------------------------------
Title: Observations and models of a flaring loop.
Authors: Nindos, A.; White, S. M.; Kundu, M. R.; Gary, D. E.
2000BAAS...32..818N Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Observations and Models of a Flaring Loop
Authors: Nindos, A.; White, S. M.; Kundu, M. R.; Gary, D. E.
2000SPD....31.0243N Altcode:
Simultaneous images of a flaring loop at two frequencies are used to
model the magnetic structure of the loop and the energy distribution of
the radiating electrons. The imaging data were obtained with the VLA at
5 and 15 GHz. Additional spectral data were provided by the OVRO Solar
Array at several frequencies between 2 GHz and 15 GHz. At 15 GHz, the
flare emission was optically thin and came from the footpoints of the
flaring loop, while at 5 GHz the loop itself was outlined. Most of the
5 GHz emission was optically thick and its spatial maximum was close
to the loop top. A striking feature of the observations is that the 5
GHz emission does not reach down to the 15 GHz footpoints. We compare
the observations with calculations of gyrosynchrotron emission from an
inhomogeneous magnetic loop in order to determine the conditions in the
flaring loop. The best fit to the OVRO fluxes was reached with a model
flaring loop with photospheric footpoint magnetic field strength of 870
G. The thickness of the model loop was small compared to its footpoint
separation. The energy spectral index of the energetic electrons was 3.7
and their number density was 7.9 x 10<SUP>7</SUP> cm<SUP>-3</SUP>. The
low and high energy cutoffs of the nonthermal electrons were 8 and
210 keV. The 5 GHz emission in this model is at low harmonics (3--7)
and harmonic effects are responsible for the weak 5 GHz emission at
the footpoints. The absence of electrons above 210 keV is necessary in
this model to explain why no emission is observed from the loop top at
15 GHz. That model reproduced well the high frequency part of the OVRO
flux spectrum as well as the VLA spatial structure. Thus comparisons
between the spatially--resolved observations and models reveal the
three-dimensional structure of the loop geometry.
---------------------------------------------------------
Title: Observations and Models of a Flaring Loop
Authors: Nindos, A.; White, S. M.; Kundu, M. R.; Gary, D. E.
2000ApJ...533.1053N Altcode:
Simultaneous images of a flaring loop at two frequencies are used to
model the magnetic structure of the loop and the energy distribution of
the radiating electrons. The imaging data were obtained with the VLA at
5 and 15 GHz. Additional spectral data were provided by the Owens Valley
Radio Observatory (OVRO) solar array at several frequencies between
2 and 15 GHz. At 15 GHz, the flare emission was optically thin and
came from the footpoints of the flaring loop, while at 5 GHz the loop
itself was outlined. Most of the 5 GHz emission was optically thick,
and its spatial maximum was close to the loop top. A striking feature
of the observations is that the 5 GHz emission does not reach down to
the 15 GHz footpoints. We compare the observations with calculations of
gyrosynchrotron emission from an inhomogeneous magnetic loop in order
to determine the conditions in the flaring loop. The best fit to the
OVRO fluxes was reached with a model flaring loop with photospheric
footpoint magnetic field strength of 870 G. The thickness of the
model loop was small compared with its footpoint separation. The
energy spectral index of the energetic electrons was 3.7, and their
number density was 7.9×10<SUP>7</SUP> cm<SUP>-3</SUP>. The low- and
high-energy cutoffs of the nonthermal electrons were 8 and 210 keV,
respectively. The 5 GHz emission in this model is at low harmonics
(3-7), and harmonic effects are responsible for the weak 5 GHz emission
at the footpoints. The absence of electrons above 210 keV is necessary
in this model to explain why no emission is observed from the loop
top at 15 GHz. That model reproduced well the high-frequency part of
the OVRO flux spectrum as well as the VLA spatial structure. Thus,
comparisons between the spatially resolved observations and models
reveal the three-dimensional structure of the loop geometry.
---------------------------------------------------------
Title: Observations and Models of a Flaring Loop
Authors: Nindos, A.; White, S. M.; Kundu, M. R.; Gary, D. E.
2000ASPC..206..359N Altcode: 2000hesp.conf..359N
No abstract at ADS
---------------------------------------------------------
Title: Microwave and Extreme Ultraviolet Observations of Solar
Polar Regions
Authors: Nindos, A.; Kundu, M. R.; White, S. M.; Gary, D. E.;
Shibasaki, K.; Dere, K. P.
1999ApJ...527..415N Altcode:
The poles of the Sun are brighter than the rest of the quiet Sun's
emission in a limited range of radio frequencies from 17 GHz to 87
GHz. We have studied microwave images of the quiet Sun made with
the Nobeyama radioheliograph at 17 GHz. They show that the so-called
polar-cap brightening consists of two components: a diffuse component
of 1500 K excess brightness and patchy compact sources with localized
excess brightness of about 3500 K. We test the reality of the compact
sources using the maximum entropy method deconvolution. The total flux
and the number of compact polar sources as well as the north-south
extent of the diffuse polar emission are larger in the pole that is
closest to the Earth. We compared the microwave polar emission with
nearly simultaneous SOHO EIT images taken in the lines of He II at 304
Å and Fe XII at 195 Å. No one-to-one correlation between the compact
radio sources and the bright EUV features was found: most of the radio
emission arises between the plumes visible to EIT. The boundaries of
the polar-cap brightenings did not match exactly the boundaries of the
coronal holes as seen in either the Fe XII 195 Å images or the He II
304 Å images. The temporal variations of the compact microwave sources
did not correspond to any significant changes in EUV emission. On the
other hand, most He II 304 Å changing features were associated with the
diffuse polar microwave emission, which was practically constant. Our
data suggest that the origin of the polar brightening is not coronal;
it seems that the bulk of the patchy radio emission comes from heights
below the 80,000 K layer.
---------------------------------------------------------
Title: A Microwave Study of Coronal and Chromospheric Ejecta
Authors: Nindos, A.; Kundu, M. R.; Raulin, J. -P.; Shibasaki, K.;
White, S. M.; Nitta, N.; Shibata, K.; Shimojo, M.
1999spro.proc..135N Altcode:
We have studied the radio properties of 18 X-ray coronal jets (observed
by the Yohkoh SXT) using Nobeyama 17 GHz data. We also searched for
chromospheric ejecta (Hα surges) during the time intervals that the
X-ray images were available. Microwave emission was associated with the
majority of the X-ray jets. The radio emission came from the base or
the lower part of the jets. We detected radio emission from almost all
jets which showed flare-like activity at their footpoints. The 17 GHz
time profiles were gradual and unpolarized, implying that the emission
was thermal. When possible, we computed the physical properties of the
X-ray-emitting ejected plasma. In one two-sided-loop type jet and one
anemone-type jet, the observed microwave fluxes from the lower part of
the jets were well above the fluxes predicted from the computed electron
temperatures and emission measures of the soft X-ray-emitting material
on the basis of thermal free-free emission. We interpreted the large
discrepancies in terms of the presence of lower temperature material
which cannot be detected by the SXT but produces strong microwave
free-free emission. This is the first time that such material is
observed in two-sided-loop type jets. Thus our observations confirm the
theoretical prediction by Yokoyama and Shibata (1996). We detected no
cool material at the base of the jets. We also observed an Hα surge
which was not associated with an X-ray jet and showed no signatures on
the SXT images but was detected with the Nobeyama Radioheliograph. The
emission of the microwave surge-associated source was free-free from the
chromospheric plasma. Constraints for the surge density were derived.
---------------------------------------------------------
Title: A Microwave Study of Coronal Ejecta
Authors: Kundu, M. R.; Nindos, A.; Raulin, J. -P.; Shibasaki, K.;
White, S. M.; Nitta, N.; Shibata, K.; Shimojo, M.
1999ApJ...520..391K Altcode:
Using Nobeyama 17 GHz data, we have studied the radio properties of
19 coronal jets identified in Yohkoh soft X-ray imaging telescope
(SXT) X-ray observations. The radio data provide information on the
physical conditions in the jets, which complements the data from the
X-ray surveys. Microwave emission was associated with the majority of
the X-ray jets in our sample. The radio emission typically came from
the base or the base and lower part of the jets. We detected radio
emission from almost all jets that showed flarelike activity at their
bases. The jets that were not associated with radio emission did not
show any significant increase in X-ray emission at their bases. The
strongest radio emission came from two of the largest jets in our
sample. Our data show a general correlation between the X-ray jet
fluxes and the associated radio fluxes. The 17 GHz time profiles were
gradual and unpolarized, implying that the emission was thermal. In a
two-sided-loop jet (1992 July 22 event) and one anemone-type jet (1993
February 9 event), the observed microwave fluxes from the lower part
of the jets were well above the fluxes calculated from the computed
physical parameters of the soft X-ray-emitting material on the basis
of thermal free-free emission. We interpret the large discrepancies
in terms of the presence of lower temperature material, which cannot
be detected by the SXT (the SXT is most sensitive to hot plasma above
2×10<SUP>6</SUP> K), but which produces strong microwave free-free
emission. This is the first time that such material has been observed
in two-sided-loop-type jets. We also observed motion of a jet-associated
microwave source with a velocity of 55 km s<SUP>-1</SUP>. The microwave
motion occurred after the appearance of the X-ray jet. There is clear
evidence that the microwave emission of that source was associated
with the jet and not with the associated small flare.
---------------------------------------------------------
Title: Microwave and Extreme Ultraviolet Observations of Solar
Polar Regions
Authors: Nindos, A.; Kundu, M. R.; White, S. M.; Gary, D. E.;
Shibasaki, K.; Dere, K. P.
1999AAS...194.3207N Altcode: 1999BAAS...31..871N
The radio emission of solar poles is brighter than the rest of the
quiet Sun's emission in a limited range of frequencies from 17 GHz to
87 GHz. We have studied microwave images of the quiet Sun made with
the Nobeyama Radioheliograph at 17 GHz. They show that the so-called
polar-cap brightening consists of two components: a diffuse component
of 1500 K excess brightness, and patchy compact sources with localized
excess brightness of about 3500 K. The total flux and the number of
compact polar sources as well as the North-South extent of the diffuse
polar emission are larger in the pole which is closest to the Earth. We
compared the microwave polar emission with nearly simultaneous SoHO EIT
images taken in the lines of He ii at 304 Angstroms and Fe xii at 195
Angstroms. No one-to-one correlation between the compact radio sources
and the bright EUV features was found: most of the radio emission arises
between the plumes visible to EIT. The boundaries of the polar-cap
brightenings did not match exactly the boundaries of the coronal holes
as seen in the Fe xii 195 Angstroms images. The temporal variations of
the compact microwave sources did not correspond to any significant
changes in EUV emission. On the other hand, most He ii 304 Angstroms
changing features were associated with the diffuse polar microwave
emission which was practically constant. Our data suggest that the
origin of the polar brightening is not coronal; it seems that the bulk
of the patchy radio emission comes from heights below the 80000 K layer.
---------------------------------------------------------
Title: A Microwave Study of Coronal Ejecta
Authors: Kundu, M. R.; Nindos, A.; Raulin, J. -P.; Shibasaki, K.;
White, S. M.; Nitta, N.; Shibata, K.; Shimojo, M.
1999AAS...194.1704K Altcode: 1999BAAS...31..853K
Using Nobeyama 17 GHz data, we have studied the radio properties
of 19 coronal jets identified in Yohkoh SXT X-ray observations. The
radio data provide information on the physical conditions in the jets
which complements the data from the X-ray surveys. Microwave emission
was associated with the majority of the X-ray jets in our sample. The
radio emission typically came from the base or the base and lower part
of the jets. We detected radio emission from almost all jets which
showed flare-like activity at their bases. The jets which were not
associated with radio emission did not show any significant increase
in X-ray emission at their bases. The strongest radio emission came
from two of the largest jets in our sample. Our data show a general
correlation between the X-ray jet fluxes and the associated radio
fluxes. The 17 GHz time profiles were gradual and unpolarized, implying
that the emission was thermal. In a two-sided-loop jet (July 22, 1992
event) and one anemone-type jet (February 9, 1993 event), the observed
microwave fluxes from the lower part of the jets were well above the
fluxes calculated from the computed physical parameters of the soft
X-ray-emitting material on the basis of thermal free-free emission. We
interpret the large discrepancies in terms of the presence of lower
temperature material which cannot be detected by the SXT (the SXT is
most sensitive to hot plasma above 2 x 10(6) K) but which produces
strong microwave free-free emission. This is the first time that
such material has been observed in two-sided-loop type jets. We also
observed motion of a jet-associated microwave source with a velocity
of 55 km/sec. The microwave motion occurred after the appearance of
the X-ray jet. There is clear evidence that the microwave emission of
that source was associated with the jet and not with the associated
small flare.
---------------------------------------------------------
Title: A Study of Microwave-selected Coronal Transient Brightenings
Authors: Nindos, A.; Kundu, M. R.; White, S. M.
1999ApJ...513..983N Altcode:
We present the results of a search for radio-selected transient
brightenings (TBs) in the solar atmosphere as a complement to the
more common X-ray-selected surveys. The Sun was generally quiet during
the observations, making these data sensitive to weak TBs both in and
outside active regions. Five small impulsive events were identified
in a set of VLA observations at 4.5, 1.5, and 0.33 GHz and compared
with soft X-ray images from Yohkoh and EUV images from SOHO/EIT. Four
of the events were located at the edges of an active region, but one
was located 100" away in a quiet region of the atmosphere. Possible
emission mechanisms for these brightenings are investigated. The time
profiles of the radio TBs show impulsive peaks, while the corresponding
soft X-ray profiles are gradual. The impulsive radio peaks were
up to 35% polarized. Our data favor an interpretation in terms of
gyrosynchrotron radiation from mildly relativistic electrons. A small
number of nonthermal electrons with spectral index 3 can explain the
observed properties of the TBs. Thus, nonthermal TBs can be found away
from active regions. Two of the microwave TBs also show evidence for
type III radio emission at 327 MHz.
---------------------------------------------------------
Title: The relation of CaII K line features to magnetic field
intensity
Authors: Nindos, A.; Zirin, H.
1997BAAS...29.1119N Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A Model for Active Region Emission at Centimeter Wavelengths
Authors: Nindos, A.; Alissandrakis, C. E.; Gelfreikh, G. B.; Kundu,
M. R.; Dere, K. P.; Korzhavin, A. N.; Bogod, V. M.
1996SoPh..166...55N Altcode:
We present multi-frequency observations and model computations of the
microwave emission of a solar active region. The radio observations
were obtained with the RATAN-600 at several wavelengths between 0.8
and 31.6 cm and with the VLA at 6 and 20 cm. The active region was
also observed in the EUV O Iv lines by the HRTS instrument aboard
the Space Shuttle Spacelab-2 mission. These lines are formed in the
chromosphere-corona transition region and their intensity ratio is
sensitive to pressure. Photospheric magnetograms provided both the
longitudinal and the transverse component of the magnetic field. The
microwave observations were checked against model computations
taking into account both the free-free and the gyro-resonance emission
mechanisms and using the pressure data from the O IV lines. The magnetic
field was computed through constant-α force-free extrapolations of
the longitudinal photospheric field. We computed both the flux from
2 to 20 cm and the spatial structure of the microwave emission at 6
and 20 cm. The comparison of the computed and observed flux spectra
allowed us to estimate the magnetic field strength at the base of the
transition region and in the low corona, as well as the values of the
conductive flux and the height of the base of the transition region. The
model maps at 6 cm and 20 cm showed that α was not constant above the
active region; the same conclusion was reached on the basis of the
photospheric observations. The use of pressure measurements allowed
us to identify microwave structures which were determined by pressure
enhancements. At 6 cm the computations confirmed the fact that the
magnetic field is the principal factor that determines the structure
of sunspot-associated sources and showed that the effect of pressure
variations was small. Pressure variations were more important at 20 cm,
where the peak of the emission was associated with the sunspot and a
diffuse component was associated with the plage which had an average
pressure higher by a factor of 1.54 than the sunspot.
---------------------------------------------------------
Title: Two-Dimensional Mapping of the Sun with the RATAN-600
Authors: Nindos, A.; Alissandrakis, C. E.; Gelfreikh, G. B.; Borovik,
V. N.; Korzhavin, A. N.; Bogod, V. M.
1996SoPh..165...41N Altcode:
We present two-dimensional solar maps at 2.7, 3.2, 4, and 8.2 cm
computed from one-dimensional observations with the RATAN-600, using
Earth rotation aperture synthesis techniques. Before the calculation
of maps, the position of each scan was corrected with respect to the
center of the solar disk and the scans were calibrated. The circular
polarization scans were corrected for polarization cross-talk between
the I and V channels. Subsequently, the quiet-Sun background emission
was subtracted. After all corrections, a dirty map was computed by
combining the scans at different position angles. The last step of
the processing was an attempt to free the dirty map of the sidelobes,
using the standard CLEAN procedure. The resolution of the clean maps
at 2.7 cm was 0.5' by 6'. Both active regions which were present on
the solar disk were mapped. We studied the flux spectra of different
types of sources: one was associated with a sunspot, the second was
located over the neutral line of an active region, and the other was
associated with the plage. The emission mechanism of the former was
attributed to the gyroresonance process, while the short wavelength
emission of the others was attributed to the free-free process. For
the sunspot-associated source we estimated the magnetic field strengths
at the base of the transition region and in the low corona.
---------------------------------------------------------
Title: Evidence for Ordinary Mode Emission from Microwave Bursts
Authors: Alissandrakis, C. E.; Nindos, A.; Kundu, M. R.
1993SoPh..147..343A Altcode:
We analyze high-resolution, one-dimensional observations of simple
microwave bursts, obtained at 4.9 GHz with the Westerbork Synthesis
Radio Telescope in 1980, together with Hα photographs of the associated
flares from the Observatories of Athens and Meudon. In most cases the
polarization structure can be interpreted in terms of extraordinary mode
emission, taking into account the polarity of the underlying magnetic
field and propagation effects, which may lead to inversion of the sense
of polarization in the limbward part of the flaring loop. We found
evidence for ordinary mode emission in two classes of events. In one
class theo-mode comes from regions overlying strong magnetic field,
which we interpret in terms of thermal gyroresonance absorption of
the extraordinary mode at the third harmonic of the gyrofrequency. In
the other class the entire burst emits in theo-mode, which may be
attributed to high gyrosynchrotron optical depth.
---------------------------------------------------------
Title: Spectral observations of active region sources with RATAN-600
and WSRT
Authors: Alissandrakis, C. E.; Gel'Frejkh, G. B.; Borovik, V. N.;
Korzhavin, A. N.; Bogod, V. M.; Nindos, A.; Kundu, M. R.
1993A&A...270..509A Altcode:
We present spectral observations of neutral line and sunspot associated
sources obtained with the RATAN-600 radio telescope and the WSRT
in the wavelength range of 2 to 6 cm. Sources associated with large
sunspots have flat spectra, while neutral line sources have very steep
spectra. In the case of a large spot we estimated the magnetic field
to be at least 2700 G at the base of the transition region and 1800 G
in the low corona. We consider possible interpretations of the radio
emission above the neutral lines. Gyroresonance emission at the fourth
harmonic is inadequate, whereas emission from a small population of
nonthermal electrons (total number 10 exp 30 to 10 exp 31) with a
delta = 3 power law distribution seems to be sufficient.
