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Author name code: bastian
ADS astronomy entries on 2022-09-14
author:"Bastian, Timothy S." OR author:"Bastian, Tim"
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Title: Solar Observing with the Atacama Large Millimeter-Submillimeter
Array
Authors: Bastian, Timothy; Shimojo, Masumi; Barta, Miroslav; White,
Stephen; Iwai, Kazumasa
2022arXiv220901659B Altcode:
The Atacama Large Millimeter-submillimeter Array (ALMA), sited on the
high desert plains of Chajnantor in Chile, has opened a new window onto
solar physics in 2016 by providing continuum observations at millimeter
and sub-millimeter wavelengths with an angular resolution comparable
to that available at optical (O), ultraviolet (UV), extreme ultraviolet
(EUV), and X-ray wavelengths, and with superior time resolution. In the
intervening years, progress has been made testing and commissioning
new observing modes and capabilities, in developing data calibration
strategies, and in data imaging and restoration techniques. Here we
review ALMA current solar observing capabilities, the process by which
a user may propose to use the instrument, and summarize the observing
process and work flow. We then discuss some of the challenges users
may encounter in imaging and analyzing their data. We conclude with a
discussion of additional solar observing capabilities and modes under
consideration that are intended to further exploit the unique spectral
coverage provided by ALMA.
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Title: The quiet Sun at mm Wavelengths as Seen by ALMA
Authors: Alissandrakis, Costas; Bastian, Timothy; Brajša, Roman
2022arXiv220902569A Altcode:
Solar observations at sub-mm, mm and cm wavelengths offer a
straightforward diagnostic of physical conditions in the solar
atmosphere because they yield measurement of brightness temperature
which, for optically thick features, equals intrinsic temperature -
much unlike solar diagnostics in other spectral ranges. The Atacama
Large Millimeter and sub-millimeter Array (ALMA) has therefore opened a
new, hitherto underexplored, spectral window for studying the enigmatic
solar chromosphere. In this review we discuss initial ALMA studies of
the quiet chromosphere that used both single-dish and compact-array
interferometric observing modes. We present results on the temperature
structure of the chromosphere, comparison with classic empirical models
of the chromosphere, and observations of the chromospheric network and
spicules. Furthermore, we discuss what may be expected in the future,
since the ALMA capabilities continuously expand and improve towards
higher angular resolution, wavelength coverage, and polarization
measurement for magnetometry.
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Title: Radio Emission from UV Cet: Auroral Emission from a Stellar
Magnetosphere
Authors: Bastian, T. S.; Cotton, W. D.; Hallinan, G.
2022ApJ...935...99B Altcode: 2022arXiv220614099B
The archetypical flare star UV Cet was observed by MeerKAT on 2021
October 5-6. A large radio outburst with a duration of ~2 hr was
observed between 886 and 1682 MHz, with a time resolution of 8 s
and a frequency resolution of 0.84 MHz, enabling sensitive dynamic
spectra to be formed. The emission is characterized by three peaks
containing a multitude of broadband arcs or partial arcs in the
time-frequency domain. In general, the arcs are highly right-hand
circularly polarized. At the end of the third peak, brief bursts occur
that are significantly elliptically polarized. We present a simple
model that appears to be broadly consistent with the characteristics of
the radio emission from UV Cet. Briefly, the stellar magnetic field is
modeled as a dipole aligned with the rotational axis of the star. The
radio emission mechanism is assumed to be due to the cyclotron maser
instability, where x-mode radiation near the electron gyrofrequency is
amplified. While the elliptically polarized bursts may be intrinsic to
the source, rather stringent limits are imposed on the plasma density
in the source and along the propagation path. We suggest that the
elliptically polarized radiation may instead be the result of reflection
on an overdense plasma structure at some distance from the source. The
radio emission from UV Cet shares both stellar and planetary attributes.
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Title: Understanding the Correlation between Solar Coronal Abundances
and F10.7 Radio Emission
Authors: To, Andy S. H.; Baker, Deborah; Long, David; James, Alexander;
Brooks, David; van Driel-Gesztelyi, Lidia; Valori, Gherardo; Bastian,
Tim; Lomuscio, Samantha; Stansby, David
2022cosp...44.2592T Altcode:
Solar corona plasma composition, derived from full-Sun spectra, and
the F10.7 radio flux (2.8 GHz) have been shown to be highly correlated
(r = 0.88) during the recent weak solar cycle. However, this correlation
becomes nonlinear at times of increased solar magnetic activity. We used
co-temporal, high spatial resolution, radio (JVLA), and EUV (Hinode/EIS)
images of the Sun taken on the 3 and 7 April 2020 to understand the
underlying causes of the non-linearity of the FIP bias-F10.7 solar
index correlation. We then calculated differential emission measures
from AIA images, and paired them with the observed FIP bias to predict
the bremsstrahlung component of F10.7 radio emission. Results of this
study provide constraints on the amplitude of composition variability
related to solar cycle amplitude, and provide an alternative method
to calculate coronal composition.
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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: Novel Magnetic Field and Electron Density Measurements of
CMEs (within AU) with the Proposed Multiview Observatory for Solar
Terrestrial Science (MOST) Mission
Authors: Jensen, P. E., C. S. P., Elizabeth; Manchester, Ward; Fung,
Shing; Gopalswamy, Nat; Jian, Lan; Kenny, Megan; Kooi, Jason; Lazio,
Joseph; Li, Lihua; Nieves-Chinchilla, Teresa; Pevtsov, Alexei; Wexler,
David; Wilson, Lynn; Wood, Brian; Bale, Stuart; Bastian, Tim
2021AGUFMSH33A..08J Altcode:
The Multiview Observatory for Solar Terrestrial Science (MOST) mission
concept will be the most advanced solar observatory to date (Gopalswamy
et al, SH0001, 2021). Comprising four spacecraft, two located in the L4
and ahead of L4 position and two located in the L5 and behind of the L5
position, the four lines-of-sight (LOSs) form the basis for the unique
Faraday Effect Tracker of Coronal and Heliospheric Structures (FETCH)
instrument (Wexler et al, SH0019, 2021). We report on our modeling
into the expected Faraday rotation (FR) caused by an Earth-directed
CME crossing the MOST/FETCH radio-sensing paths using a heliospheric
3-D MHD model to obtain the necessary LOS data on electron density
and magnetic field components (see example image). Specifically, we
utilized simulation data of the 2005 May 13 CME (Manchester IV et al.,
2014, Plasma Phys. Control. Fusion), which erupted from the north-south
polarity inversion line of AR 10759 at 16:03 UT, reaching speeds around
2000 km/s in the corona. The trajectory of the CME at an acute angle
to the Earth-Sun line crosses each FETCH LOS at a different time. Two
LOSs are at different viewing angles with little overlap between
the CME sheath and magnetic flux rope core. A blind test fitting of
the Faraday rotation functions (Figures 6 and 7 in Jensen et al.,
2010, Sol. Phys.) to the simulated FETCH observations reproduced the
orientation of the CME for its handedness as well as its associated
complementary degenerate solution. In conclusion, one of the four
LOSs will be more sensitive to observing CME flux rope structure of
Earthward CMEs, depending on their trajectory. We find that two of the
four LOSs enable analyzing CME evolution, whereas the other two LOSs
enable analyzing the average magnetic field vector in the corresponding
high density regions dominating the measurements at that time. For
example, the average sheath magnetic field vector can be partially
measured in the plane of the ecliptic due to the angular differences
between 2 LOSs. We discuss future work as this effort develops.
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Title: FETCH Concept: Investigating Quiescent and Transient Magnetic
Structures in the Inner Heliosphere using Faraday Rotation of
Spacecraft Radio Signals
Authors: Wexler, David; Jensen, Elizabeth; Gopalswamy, Nat; Wilson,
Lynn; Fung, Shing; Nieves-Chinchilla, Teresa; Jian, Lan; Bastian,
Tim; Pevtsov, Alexei; Manchester, Ward; Kenny, Megan; Lazio, Joseph;
Wood, Brian; Kooi, Jason
2021AGUFMSH31A..05W Altcode:
The Faraday Effect Tracker of Coronal and Heliospheric structures
(FETCH) is a new instrument concept being developed to probe coronal
and interplanetary magnetic field structures in the ambient solar wind,
corotating interaction regions and coronal mass ejections (CMEs) as
they evolve in the inner heliosphere. FETCH is one of the instruments
that constitute the Multiview Observatory for Solar Terrestrial (MOST)
science mission. FETCH will measure Faraday rotation (FR) of linearly
polarized spacecraft radio signals transmitted along four lines of sight
provided by the four MOST spacecraft: two large spacecraft deployed
at Sun-Earth Lagrange points 4 and 5 and two smaller spacecraft, one
ahead of L4 and the other behind L5. FETCH will transmit and receive at
selected radio frequencies in the 1-100 MHz range for lines of sight
with solar impact parameters < 0.5 AU. FR yields the line-of-sight
(LOS) integrated product of electron number density and LOS-projected
magnetic field strengths. The FR measurements will be obtained from
the Stokes polarization parameters while additional plasma parameters,
such as electron column density, will be extracted from other signal
diagnostics. The multifrequency FR data and four lines-of-sight
will be used to constrain the magnetic field topology and dynamics of
interplanetary plasma structures upstream from Earth. Unique to this FR
experiment, the FETCH transmitter-receiver instrumentation is positioned
such that the entire sensing path remains in interplanetary space, thus
avoiding the complications of trans-ionospheric FR observations. The
FETCH key science objectives include: (1) characterizing CME magnetic
field structure and flux rope orientation, (2) tracking CME propagation
and shock signatures, (3) understanding the magnetic field features
of corotating interaction regions in the extended corona and inner
heliosphere, and (4) determination of large-scale MHD wave organization
in regions of developed ambient solar wind and its evolution during
perturbed flows. The MOST mission will build upon the achievements of
the Solar Heliospheric Observatory (SOHO) and the Solar Terrestrial
Relations Observatory (STEREO) missions during the last couple of
decades. FETCH will help fill the long-standing measurement gap of
magnetic field data in the inner heliosphere.
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Title: The Multiview Observatory for Solar Terrestrial Science (MOST)
Authors: Gopalswamy, Nat; Kucera, Therese; Leake, James; MacDowall,
Robert; Wilson, Lynn; Kanekal, Shrikanth; Shih, Albert; Christe,
Steven; Gong, Qian; Viall, Nicholeen; Tadikonda, Sivakumar; Fung,
Shing; Yashiro, Seiji; Makela, Pertti; Golub, Leon; DeLuca, Edward;
Reeves, Katharine; Seaton, Daniel; Savage, Sabrina; Winebarger, Amy;
DeForest, Craig; Desai, Mihir; Bastian, Tim; Lazio, Joseph; Jensen,
P. E., C. S. P., Elizabeth; Manchester, Ward; Wood, Brian; Kooi,
Jason; Wexler, David; Bale, Stuart; Krucker, Sam; Hurlburt, Neal;
DeRosa, Marc; Pevtsov, Alexei; Tripathy, Sushanta; Jain, Kiran;
Gosain, Sanjay; Petrie, Gordon; Kholikov, Shukirjon; Zhao, Junwei;
Scherrer, Philip; Woods, Thomas; Chamberlin, Philip; Kenny, Megan
2021AGUFMSH12A..07G Altcode:
The Multiview Observatory for Solar Terrestrial Science (MOST) is a
comprehensive mission concept targeting the magnetic coupling between
the solar interior and the heliosphere. The wide-ranging imagery and
time series data from MOST will help understand the solar drivers and
the heliospheric responses as a system, discerning and tracking 3D
magnetic field structures, both transient and quiescent in the inner
heliosphere. MOST will have seven remote-sensing and three in-situ
instruments: (1) Magnetic and Doppler Imager (MaDI) to investigate
surface and subsurface magnetism by exploiting the combination of
helioseismic and magnetic-field measurements in the photosphere; (2)
Inner Coronal Imager in EUV (ICIE) to study large-scale structures
such as active regions, coronal holes and eruptive structures by
capturing the magnetic connection between the photosphere and the
corona to about 3 solar radii; (3) Hard X-ray Imager (HXI) to image
the non-thermal flare structure; (4) White-light Coronagraph (WCOR) to
seamlessly study transient and quiescent large-scale coronal structures
extending from the ICIE field of view (FOV); (5) Faraday Effect
Tracker of Coronal and Heliospheric structures (FETCH), a novel radio
package to determine the magnetic field structure and plasma column
density, and their evolution within 0.5 au; (6) Heliospheric Imager
with Polarization (HIP) to track solar features beyond the WCOR FOV,
study their impact on Earth, and provide important context for FETCH;
(7) Radio and Plasma Wave instrument (M/WAVES) to study electron beams
and shocks propagating into the heliosphere via passive radio emission;
(8) Solar High-energy Ion Velocity Analyzer (SHIVA) to determine spectra
of electrons, and ions from H to Fe at multiple spatial locations
and use energetic particles as tracers of magnetic connectivity; (9)
Solar Wind Magnetometer (MAG) to characterize magnetic structures at
1 au; (10) Solar Wind Plasma Instrument (SWPI) to characterize plasma
structures at 1 au. MOST will have two large spacecraft with identical
payloads deployed at L4 and L5 and two smaller spacecraft ahead of L4
and behind L5 to carry additional FETCH elements. MOST will build upon
SOHO and STEREO achievements to expand the multiview observational
approach into the first half of the 21st Century.
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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: Enhancing ALMA's Future Observing Capabilities
Authors: Maud, L.; Villard, E.; Takahashi, S.; Asaki, Y.; Bastian, T.;
Cortes, P.; Crew, G.; Fomalont, E.; Hales, A.; Ishii, S.; Matthews,
L.; Messias, H.; Nagai, H.; Sawada, T.; Schieven, G.; Shimojo, M.;
Vila-Vilaro, B.; Biggs, A.; Petry, D.; Phillips, N.; Paladino, R.
2021Msngr.183...13M Altcode:
With each observing cycle at the Atacama Large Millimeter/submillimeter
Array (ALMA) new features and observing modes are offered. Here we
provide some background about how these new capabilities are tested
and then made available to ALMA users. These activities help to drive
the cutting-edge science conducted with ALMA and to maintain ALMA's
position as the foremost interferometric array operating at millimetre
and submillimetre wavelengths. We focus in particular on opening up
high-frequency observing using ALMA's longest baselines, which offers
the highest possible angular resolution.
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Title: Solar Radio Burst Effects on Radio- and Radar-Based Systems
Authors: Gary, Dale E.; Bastian, Timothy S.
2021GMS...262..141G Altcode:
Radio emission from solar flares can attain such high flux density that
the Sun becomes the dominant source of broadband radio noise in the
terrestrial environment. The effects of this radio noise on wireless
communication and navigation systems can take many forms, depending on
the design and operation of the affected system. These effects can be of
special concern for regional or global systems, since the effects can
occur simultaneously over the entire sunlit hemisphere of Earth. This
chapter reviews the origin of solar radio bursts, the threat they pose
based on statistics of the flux-density distribution of such events
vs. frequency, and some of the effects that have been documented in the
literature. The chapter concludes with a discussion of the potential
impacts on current and future technology and how these impacts can
be mitigated (1) through improved radio monitoring of the Sun in
both circular polarizations to supply meaningful real-time warnings,
(2) through improved scientific understanding of the solar phenomena
underlying the radio bursts, and (3) through improved system design
that takes account of solar radio noise.
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Title: The ObsMode 2020 Process
Authors: Takahashi, Satoko; Fomalont, Edward B.; Asaki, Yoshiharu;
Crew, Geoff; Matthews, Lynn D.; Cortes, Paulo; Vila-Vilaro, Baltasar;
Bastian, Tim; Shimojo, Masumi; Biggs, Andy; Messias, Hugo; Hales,
Antonio; Villard, Eric; Humphreys, Elizabeth
2021arXiv210412681T Altcode:
ObsMode is a yearly process which aims at preparing capabilities
for future ALMA Observing cycles. The process has been running for
a number of years tied to each ALMA observing cycle, with various
leaderships. This document specifically summarizes the ObsMode2020
process (April- October 2020) with a new scheme led by the Joint
ALMA Observatory. In the ObsMode2020 process, seven capabilities are
identified as high priority items, for which it was originally aimed
to be ready for Cycle 9. However, because of the observatory shutdown
due to the covid-19 pandemic, we were forced to delay the test plan by
one year. While no new data sets were obtained during the observatory
shutdown, verifications using the existing data allowed us to offer
the 7m-array polarization capability (in ACA standalone mode, single
field) for Cycle 8 starting from October, 2021. In addition, subsystem
readiness and policy-side preparations for the phased array observing
mode were improved for Cycle 8. Other high priority items were decided
to be carried over to the ObsMode2021 process.
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Title: Radio Spectral Imaging of an M8.4 Eruptive Solar Flare:
Possible Evidence of a Termination Shock
Authors: Luo, Yingjie; Chen, Bin; Yu, Sijie; Bastian, T. S.; Krucker,
Säm
2021ApJ...911....4L Altcode: 2021arXiv210206259L
Solar flare termination shocks have been suggested as one of the
viable mechanisms for accelerating electrons and ions to high
energies. Observational evidence of such shocks, however, remains
rare. Using radio dynamic spectroscopic imaging of a long-duration
C1.9 flare obtained by the Karl G. Jansky Very Large Array (VLA),
Chen et al. suggested that a type of coherent radio bursts, referred
to as "stochastic spike bursts," were radio signatures of nonthermal
electrons interacting with myriad density fluctuations at the front
of a flare termination shock. Here we report another stochastic spike
burst event recorded during the extended energy release phase of a
long-duration M8.4-class eruptive flare on 2012 March 10. VLA radio
spectroscopic imaging of the spikes in 1.0-1.6 GHz shows that, similar
to the case of Chen et al., the burst centroids form an extended,
∼10″-long structure in the corona. By combining extreme-ultraviolet
imaging observations of the flare from two vantage points with hard
X-ray and ultraviolet observations of the flare ribbon brightenings,
we reconstruct the flare arcade in three dimensions. The results show
that the spike source is located at ∼60 Mm above the flare arcade,
where a diffuse supra-arcade fan and multitudes of plasma downflows are
present. Although the flare arcade and ribbons seen during the impulsive
phase do not allow us to clearly understand how the observed spike
source location is connected to the flare geometry, the cooling flare
arcade observed 2 hr later suggests that the spikes are located in the
above-the-loop-top region, where a termination shock presumably forms.
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Title: Multiwavelength Observations of the Formation and Eruption
of a Complex Filament
Authors: Zhang, Y.; Bastian, T. S.; Liu, J. H.; Yu, S. J.; Feng, S.;
Chen, J.; Yan, Y. H.
2021ApJ...910...40Z Altcode:
We present an analysis of the formation and eruption of a filament
and fast coronal mass ejection associated with a flare that occurred
in active region 11429 using observations in the ultraviolet, extreme
ultraviolet, X-ray, and radio wavelength bands. Precursor activity began
as an interaction between two filaments, F1 and F2, that are identified
as having twisted magnetic flux ropes (MFRs). Transient brightenings in
all wavelengths are observed as a result of this interaction, likely
the result of magnetic reconnection between the two filaments. This
interaction results in a reconfiguration of the two filaments into a
long overlying filament and a shorter low-lying filament. The upper
filament subsequently undergoes a partial confined eruption. Plasma
flows originating near the east footpoint of F1 lead to an extension
of the upper filament into the filament channel to the west, resulting
in a new active region filament (ARF). This new filament begins a slow
rise and expansion. During its slowly rising phase, the MFR in which the
filament is embedded becomes visible, with both the filament and flux
rope rising and expanding simultaneously. The twist of the magnetic
rope is determined as four turns. The erupting configuration changes
from a twisted arch shape to a reversed γ shape within ∼75 s at the
beginning of the fast-rise phase, representing a transformation from
twist to writhe. The observations provide a clear example of filament
formation via the tether-cutting reconnection of two nearby filaments. A
helical kink instability may be the trigger of the ARF eruption.
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Title: ALMA and IRIS Observations of the Solar
Chromosphere. II. Structure and Dynamics of Chromospheric Plages
Authors: Chintzoglou, Georgios; De Pontieu, Bart; Martínez-Sykora,
Juan; Hansteen, Viggo; de la Cruz Rodríguez, Jaime; Szydlarski,
Mikolaj; Jafarzadeh, Shahin; Wedemeyer, Sven; Bastian, Timothy S.;
Sainz Dalda, Alberto
2021ApJ...906...83C Altcode: 2020arXiv201205970C
We propose and employ a novel empirical method for determining
chromospheric plage regions, which seems to better isolate a plage from
its surrounding regions than other methods commonly used. We caution
that isolating a plage from its immediate surroundings must be done
with care in order to successfully mitigate statistical biases that,
for instance, can impact quantitative comparisons between different
chromospheric observables. Using this methodology, our analysis suggests
that λ = 1.25 mm free-free emission in plage regions observed with
the Atacama Large Millimeter/submillimeter Array (ALMA)/Band6 may
not form in the low chromosphere as previously thought, but rather
in the upper chromospheric parts of dynamic plage features (such as
spicules and other bright structures), i.e., near geometric heights
of transition-region temperatures. We investigate the high degree of
similarity between chromospheric plage features observed in ALMA/Band6
(at 1.25 mm wavelengths) and the Interface Region Imaging Spectrograph
(IRIS)/Si IV at 1393 Å. We also show that IRIS/Mg II h and k are
not as well correlated with ALMA/Band6 as was previously thought,
and we discuss discrepancies with previous works. Lastly, we report
indications of chromospheric heating due to propagating shocks supported
by the ALMA/Band6 observations.
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Title: ALMA and IRIS Observations of the Solar Chromosphere. I. An
On-disk Type II Spicule
Authors: Chintzoglou, Georgios; De Pontieu, Bart; Martínez-Sykora,
Juan; Hansteen, Viggo; de la Cruz Rodríguez, Jaime; Szydlarski,
Mikolaj; Jafarzadeh, Shahin; Wedemeyer, Sven; Bastian, Timothy S.;
Sainz Dalda, Alberto
2021ApJ...906...82C Altcode: 2020arXiv200512717C
We present observations of the solar chromosphere obtained
simultaneously with the Atacama Large Millimeter/submillimeter Array
(ALMA) and the Interface Region Imaging Spectrograph. The observatories
targeted a chromospheric plage region of which the spatial distribution
(split between strongly and weakly magnetized regions) allowed the
study of linear-like structures in isolation, free of contamination
from background emission. Using these observations in conjunction with
a radiative magnetohydrodynamic 2.5D model covering the upper convection
zone all the way to the corona that considers nonequilibrium ionization
effects, we report the detection of an on-disk chromospheric spicule
with ALMA and confirm its multithermal nature.
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Title: Next-Generation Solar Radio Imaging Spectroscopy
Authors: Gary, D. E.; Bastian, T.; Chen, B.; Saint-Hilaire, P.; White,
S. M.
2020AGUFMSH056..07G Altcode:
Recent progress using instruments such as the Expanded Owens Valley
Solar Array (EOVSA) and the Jansky Very Large Array (JVLA) have amply
demonstrated the power of radio imaging spectroscopy at centimeter and
decimeter wavelengths for quantitative diagnostics of both flaring
and non-flaring solar plasma. For example, the unique sensitivity
of radio emission to the flaring coronal magnetic field has been
dramatically shown in a series of recent EOVSA papers, along with
accelerated electron diagnostics in the same volume. JVLA observations
have been used to probe particle acceleration in a termination shock
during a flare. In addition, multi-frequency full-disk imaging of
the non-flaring Sun with EOVSA is showing promise for quantitative
diagnostics of electron-based emission measure and active region
magnetic field and temperature structure. But EOVSA and the JVLA are
mere demonstrators for a far more advanced solar radio instrument,
the Frequency Agile Solar Radiotelescope (FASR), that is designed to
address a much more comprehensive science program with much greater
precision than is possible with EOVSA or non-solar-dedicated instruments
like the JVLA. This includes direct imaging of Coronal Mass Ejections
(CMEs) and their associated energetic particles both on and off
the solar disk, routine coronal magnetic field measurements at high
cadence, and continuous sampling of the full-Sun coronal temperature
and emission measure. Here we use examples from EOVSA and the JVLA to
envision what FASR's advanced design will be capable of. When paired
with highly complementary new X-ray-based diagnostics from space,
the remote sensing of high-energy particles through radio imaging
spectroscopy from the ground provides a far more complete picture of
the broad range of energetic phenomena that occur on the Sun than
either alone. Scientists interested in high-energy solar phenomena
have ample incentive to coordinate their efforts to ensure that space-
and ground-based capabilities advance together.
---------------------------------------------------------
Title: Detection of 2-4 GHz Continuum Emission from ɛ Eridani
Authors: Suresh, A.; Chatterjee, S.; Cordes, J. M.; Bastian, T. S.;
Hallinan, G.
2020ApJ...904..138S Altcode: 2020arXiv201005929S
The nearby star ɛ Eridani has been a frequent target of radio surveys
for stellar emission and extraterrestrial intelligence. Using deep
2-4 GHz observations with the Very Large Array, we have uncovered
a 29 μJy compact, steady continuum radio source coincident with
ɛ Eridani to within 0"06 (≲2σ; 0.2 au at the distance of the
star). Combining our data with previous high-frequency continuum
detections of ɛ Eridani, our observations reveal a spectral turnover
at 6 GHz. We ascribe the 2-6 GHz emission to optically thick, thermal
gyroresonance radiation from the stellar corona, with thermal free-free
opacity likely becoming relevant at frequencies below 1 GHz. The steep
spectral index (α ≃ 2) of the 2-6 GHz spectrum strongly disfavors
its interpretation as stellar-wind-associated thermal bremsstrahlung
(α ≃ 0.6). Attributing the entire observed 2-4 GHz flux density
to thermal free-free wind emission, we thus derive a stringent upper
limit of 3 × 10<SUP>-11</SUP> M<SUB>⊙</SUB> yr<SUP>-1</SUP> on the
mass-loss rate from ɛ Eridani. Finally, we report the nondetection
of flares in our data above a 5σ threshold of 95 μJy. Together with
the optical nondetection of the most recent stellar maximum expected
in 2019, our observations postulate a likely evolution of the internal
dynamo of ɛ Eridani.
---------------------------------------------------------
Title: ALMA and IRIS Observations Highlighting the Dynamics and
Structure of Chromospheric Plage
Authors: Chintzoglou, G.; De Pontieu, B.; Martinez-Sykora, J.;
Hansteen, V. H.; de la Cruz Rodriguez, J.; Szydlarski, M.; Jafarzadeh,
S.; Wedemeyer, S.; Bastian, T.; Sainz Dalda, A.
2020AGUFMSH0010009C Altcode:
We present observations of the solar chromosphere obtained
simultaneously with the Atacama Large Millimeter/submillimeter Array
(ALMA) and the Interface Region Imaging Spectrograph (IRIS). The
observatories targeted a chromospheric plage region of which the spatial
distribution (split between strongly and weakly magnetized regions)
allowed the study of linear-like structures in isolation, free of
contamination from background emission. Using these observations
in conjunction with a radiative magnetohydrodynamic 2.5D model
covering the upper convection zone all the way to the corona
that considers non-equilibrium ionization effects, we report the
detection of an on-disk chromospheric spicule with ALMA and confirm
its multithermal nature. In addition, we discuss the strikingly high
degree of similarity between chromospheric plage features observed
in ALMA/Band6 and IRIS/\ion{Si}{4} (also reproduced in our model)
suggesting that ALMA/Band6 does not observe in the low chromosphere as
previously thought but rather observes the upper chromospheric parts
of structures such as spicules and other bright structures above plage
at geometric heights near transition region temperatures. We also show
that IRIS/\ion{Mg}{2} is not as well correlated with ALMA/Band6 as was
previously thought. For these comparisons, we propose and employ a novel
empirical method for the determination of plage regions, which seems
to better isolate plage from its surrounding regions as compared to
other methods commonly used. We caution that isolating plage from its
immediate surroundings must be done with care to mitigate statistical
bias in quantitative comparisons between different chromospheric
observables. Lastly, we report indications for chromospheric heating
due to traveling shocks supported by the ALMA/Band6 observations.
---------------------------------------------------------
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.
---------------------------------------------------------
Title: Radio Spectral Imaging of Another Likely Solar Flare
Termination Shock Event
Authors: Luo, Y.; Chen, B.; Yu, S.; Bastian, T.
2020SPD....5121118L Altcode:
Solar flare termination shocks are believed to be produced by
super-magnetosonic reconnection outflows impinging upon dense,
reconnected flare loops. They have been suggested as one of
the viable mechanisms for accelerating electrons and ions to high
energies. Although such shocks have been indicated to exist in numerical
experiments, observational evidence remains rare. Using radio dynamic
spectroscopic imaging of a long-duration C1.9 flare obtained by the
Karl G. Jansky Very Large Array (VLA), Chen et al. 2015 (Science,
350, 1238) found that a type of decimetric coherent bursts, referred
to as "stochastic spike bursts", were likely the radio signature of
nonthermal electrons interacting with myriad density fluctuations at
the front of a flare termination shock. Here we report another spike
burst event recorded during the gradual phase of a long-duration
M8.4-class eruptive flare on 2012 March 10 by VLA in 1—2 GHz. VLA
radio spectroscopic imaging of the spikes shows that, similar to the
case of Chen et al. 2015, the burst centroids form an extended structure
in the corona located well above the flare arcade. We combine RHESSI
X-ray imaging, extreme ultraviolet imaging observations of the flare
from two vantage points, one from the Solar Dynamics Observatory (SDO)
against the disk and another from the Solar Terrestrial Relations
Observatory Ahead (STEREO-A) from the limb, as well as radio imaging
to elucidate the location of spike bursts in 3 dimensions. The results
show that this structure is located in the above-the-looptop region
where a termination shock presumably forms. In the close vicinity of
this structure, we also find the presence of a diffuse supra-arcade fan
structure where multitudes of plasma downflows are observed from both
SDO and STEREO-A. Our observations provide another piece of evidence
that supports the interpretation of such stochastic spike bursts as
radio emission from solar flare termination shocks.
---------------------------------------------------------
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: Machine Learning in Heliophysics and Space Weather Forecasting:
A White Paper of Findings and Recommendations
Authors: Nita, Gelu; Georgoulis, Manolis; Kitiashvili, Irina; Sadykov,
Viacheslav; Camporeale, Enrico; Kosovichev, Alexander; Wang, Haimin;
Oria, Vincent; Wang, Jason; Angryk, Rafal; Aydin, Berkay; Ahmadzadeh,
Azim; Bai, Xiaoli; Bastian, Timothy; Filali Boubrahimi, Soukaina; Chen,
Bin; Davey, Alisdair; Fereira, Sheldon; Fleishman, Gregory; Gary, Dale;
Gerrard, Andrew; Hellbourg, Gregory; Herbert, Katherine; Ireland,
Jack; Illarionov, Egor; Kuroda, Natsuha; Li, Qin; Liu, Chang; Liu,
Yuexin; Kim, Hyomin; Kempton, Dustin; Ma, Ruizhe; Martens, Petrus;
McGranaghan, Ryan; Semones, Edward; Stefan, John; Stejko, Andrey;
Collado-Vega, Yaireska; Wang, Meiqi; Xu, Yan; Yu, Sijie
2020arXiv200612224N Altcode:
The authors of this white paper met on 16-17 January 2020 at the New
Jersey Institute of Technology, Newark, NJ, for a 2-day workshop that
brought together a group of heliophysicists, data providers, expert
modelers, and computer/data scientists. Their objective was to discuss
critical developments and prospects of the application of machine and/or
deep learning techniques for data analysis, modeling and forecasting
in Heliophysics, and to shape a strategy for further developments in
the field. The workshop combined a set of plenary sessions featuring
invited introductory talks interleaved with a set of open discussion
sessions. The outcome of the discussion is encapsulated in this white
paper that also features a top-level list of recommendations agreed
by participants.
---------------------------------------------------------
Title: The Sun at millimeter wavelengths. I. Introduction to ALMA
Band 3 observations
Authors: Wedemeyer, Sven; Szydlarski, Mikolaj; Jafarzadeh, Shahin;
Eklund, Henrik; Guevara Gomez, Juan Camilo; Bastian, Tim; Fleck,
Bernhard; de la Cruz Rodriguez, Jaime; Rodger, Andrew; Carlsson, Mats
2020A&A...635A..71W Altcode: 2020arXiv200102185W
Context. The Atacama Large Millimeter/submillimeter Array (ALMA) started
regular observations of the Sun in 2016, first offering receiver Band
3 at wavelengths near 3 mm (100 GHz) and Band 6 at wavelengths around
1.25 mm (239 GHz). <BR /> Aims: Here we present an initial study
of one of the first ALMA Band 3 observations of the Sun. Our aim is
to characterise the diagnostic potential of brightness temperatures
measured with ALMA on the Sun. <BR /> Methods: The observation covers
a duration of 48 min at a cadence of 2 s targeting a quiet Sun region
at disc-centre. Corresponding time series of brightness temperature
maps are constructed with the first version of the Solar ALMA Pipeline
and compared to simultaneous observations with the Solar Dynamics
Observatory (SDO). <BR /> Results: The angular resolution of the
observations is set by the synthesised beam, an elliptical Gaussian
that is approximately 1.4″ × 2.1″ in size. The ALMA maps exhibit
network patches, internetwork regions, and elongated thin features
that are connected to large-scale magnetic loops, as confirmed by a
comparison with SDO maps. The ALMA Band 3 maps correlate best with
the SDO/AIA 171 Å, 131 Å, and 304 Å channels in that they exhibit
network features and, although very weak in the ALMA maps, imprints
of large-scale loops. A group of compact magnetic loops is very
clearly visible in ALMA Band 3. The brightness temperatures in the
loop tops reach values of about 8000-9000 K and in extreme moments
up to 10 000 K. <BR /> Conclusions: ALMA Band 3 interferometric
observations from early observing cycles already reveal temperature
differences in the solar chromosphere. The weak imprint of magnetic
loops and the correlation with the 171, 131, and 304 SDO channels
suggests, however, that the radiation mapped in ALMA Band 3 might
have contributions from a wider range of atmospheric heights than
previously assumed, but the exact formation height of Band 3 needs to
be investigated in more detail. The absolute brightness temperature
scale as set by total power measurements remains less certain and
must be improved in the future. Despite these complications and the
limited angular resolution, ALMA Band 3 observations have a large
potential for quantitative studies of the small-scale structure and
dynamics of the solar chromosphere. <P />Movies are available at <A
href="https://www.aanda.org/10.1051/0004-6361/201937122/olm">https://www.aanda.org</A>
---------------------------------------------------------
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: Implementation of the Sun Radio Interferometer Space Experiment
(SunRISE) Mission Concept
Authors: Lazio, J.; Kasper, J. C.; Romero-Wolf, A.; Bastian, T.; Cohen,
C.; Landi, E.; Manchester, W.; Hegedus, A. M.; Schwadron, N.; Sokolov,
I.; Bain, H. M.; Cecconi, B.; Hallinan, G.; Krupar, V.; Maksimovic,
M.; Moschou, S. P.; Zaslavsky, A.; Lux, J. P.; Neilsen, T. L.
2019AGUFMSH31C3328L Altcode:
The Sun Radio Interferometer Space Experiment (SunRISE) would provide
an entirely new view on particle acceleration and transport in the
heliosphere by obtaining spatially and temporally resolved observations
of Decametric-Hectometric (DH, < 15 MHz) Type II and Type III
radio bursts. <P />In order to obtain the required angular resolution,
SunRISE would be a free-flying interferometer. Building on more than 50
years of experience from ground-based very long baseline interferometry
(VLBI), SunRISE would fly six small spacecraft in a supersynchronous
geosynchronous orbit (GEO) in a passive formation. Their orbits
are designed to keep them within approximately 6 km of each other. A
space-based interferometer is required because most of the DH band does
not penetrate the Earth's ionosphere, due to ionospheric absorption. <P
/>Each 6U spacecraft would carry only a single science radio designed
to operate in the DH band. The radio would form spectra on-board,
with pre-selected sub-bands identified for downlink. This science
payload radio would be integrated into a Global Positioning System
(GPS) receiver, allowing precise time to be measured on board the
spacecraft as well. The spacecraft would be independent of each other,
as is the practice for ground-based VLBI arrays. <P />On a regular
basis, both science data and GPS timing would be downlinked. NASA's
Deep Space Network antennas would be used for the downlink, with an
efficient multiple spacecraft per aperture (MSPA) mode enabling the
data from three spacecraft to be downlinked simultaneously. After orbit
determination, the interferometric data processing would form images of
Type II and Type III solar radio bursts and identify the locations of
radio emission relative to the structures of CMEs. <P />SunRISE would
leverage advances in software-defined radios, GPS navigation and timing,
and small spacecraft technologies that have been demonstrated over
the past few years. An Extended Phase A study of the SunRISE mission
concept is scheduled to be completed in 2019 September. <P />Part
of this research was carried out at the Jet Propulsion Laboratory,
California Institute of Technology, under a contract with the National
Aeronautics and Space Administration. Some of the information presented
is pre-decisional and for planning and discussion purposes only.
---------------------------------------------------------
Title: Towards Next Generation Radio Imaging Spectroscopy: a Path
Forward to FASR
Authors: Bastian, T.; Chen, B.; Gary, D. E.
2019AGUFMSH31C3327B Altcode:
A long term goal of the solar physics community, one that has been
endorsed by several NRC decadal surveys, is the Frequency Agile
Solar Radiotelescope (FASR). FASR is designed to address a broad
program of fundamental solar physics, including coronal magnetic
fields, magnetic energy release, particle acceleration and transport,
coronal and chromospheric heating, and drivers of space weather such as
coronal mass ejections. Fundamental to the FASR concept is observing the
chromosphere and corona as a system using innovative measurements —
ultra-broadband imaging spectropolarimetry — that will yield unique
science data products that are complementary to those produced in,
e.g., the O/IR and EUV/X-ray wavelength regimes. The potential of the
instrument is already being demonstrated by pathfinder observations made
by the Expanded Owens Valley Solar Array (EOVSA) and by the Jansky Very
Large Array (JVLA). This talk discusses a step-by-step Implementation of
FASR to spread out the cost (the full FASR concept would be in excess
of $75M) and achieve some of the most compelling science in time for
the upcoming solar maximum. This "stepping stone" approach would first
build out the high frequency subsystem at Owens Valley (~1-20 GHz)
and rely on other initiatives such as the OVRO Long Wavelength Array
to cover the metric wavelength range, deferring the mid-frequency
subsystem (~0.2-2 GHz) for the following decade. Such an approach
allows a timely deployment of critical science infrastructure that
can meet the demands of the wider community while training students
and researchers to exploit this new asset. The talk will emphasize
the great gain in image quality and science capability of the larger
array relative to EOVSA and JVLA.
---------------------------------------------------------
Title: Measuring Coronal Magnetic Fields with the Jansky Very Large
Array and RATAN Telescopes
Authors: Bastian, T.; Gary, D. E.; Fleishman, G. D.; Nita, G. M.;
Chen, B.; Kaltman, T.; Bogod, V.
2019AGUFMSH41B..05B Altcode:
Quantitative knowledge of coronal magnetic fields is fundamental to
understanding essentially all solar phenomena above the photosphere,
including the structure and evolution of solar active regions,
magnetic energy release, charged particle acceleration, flares,
coronal mass ejections (CMEs), coronal heating, the solar wind and,
ultimately, space weather and its impact on Earth. Characterized as
the solar and space physics community's "dark energy" problem, useful
quantitative measurements of the coronal magnetic field have been
largely unavailable until recently. Although understood in principle for
many years, suitable instrumentation at radio wavelengths - requiring
the ability to perform wideband imaging spectropolarimetry - has not
been available in practice for making quantitative maps of coronal
magnetic fields. This has changed in recent years, with the advent of
the Jansky Very Large Array (JVLA) and the Expanded Owens Valley Solar
Array (EOVSA) which are being exploited to demonstrate the utility
of radio observations for measuring coronal magnetic fields. <P />We
report radio observations of a large solar active region by the Jansky
Very Large Array (JVLA). The active region, AR 12209, was mapped on
four days: 18, 20, 22, and 24 November 2014 in 56 spectral windows
spanning 1-8 GHz. At the lowest frequencies the emission is dominated
by thermal free-free emission but at frequencies >1.5 GHz thermal
gyroresonance (GR) emission at the second or third harmonic of the
electron gyrofrequency dominates. GR emission enables nested coronal
isogauss surfaces ranging from approximately 180 G to as high as 1400
G to be mapped. At the time these observations were obtained, the JVLA
was not yet fully commissioned for solar observing. In particular,
the switched-power flux calibration system was not yet implemented. We
therefore cross-calibrated the JVLA observations against well-calibrated
one-dimensional observations obtained by the RATAN 600 telescope in
Zelenchukskaya, Russia. We present coronal magnetograms obtained by
the JVLA and compare them to model calculations.
---------------------------------------------------------
Title: The Sun Radio Interferometer Space Experiment (SunRISE)
Mission Concept
Authors: Kasper, J. C.; Lazio, J.; Romero-Wolf, A.; Bain, H. M.;
Bastian, T.; Cohen, C.; Landi, E.; Manchester, W.; Hegedus, A. M.;
Schwadron, N.; Sokolov, I.; Cecconi, B.; Hallinan, G.; Krupar, V.;
Maksimovic, M.; Moschou, S. P.; Zaslavsky, A.; Lux, J. P.; Neilsen,
T. L.
2019AGUFMSH33A..02K Altcode:
The Sun Radio Interferometer Space Experiment (SunRISE) would provide
an entirely new view on particle acceleration and transport in the
inner heliosphere by obtaining spatially and temporally resolved
observations of solar Decametric-Hectometric (DH, < 15 MHz) radio
bursts. These bursts are produced by electrons energized near expanding
CMEs (Type II) and released by solar flares (Type III). SunRISE would
track DH bursts from 2 R<SUB>S</SUB> to 20 R<SUB>S</SUB> in order to
achieve two science objectives. The first objective is to discriminate
competing hypotheses for the source mechanism of CME-associated SEPs by
measuring the location of Type II bursts relative to expanding CMEs. By
locating Type II emission relative to the overall structure of CMEs,
SunRISE would reveal where particle acceleration occurs and determine
if specific properties of CMEs lead to DH bursts. The second objective
is to determine if a broad magnetic connection between active regions
and interplanetary space is responsible for the wide longitudinal extent
of some SEPs by imaging the field lines traced by Type III bursts from
active regions through the corona. By tracing the radio emission from
energetic electrons as they travel along magnetic field lines, SunRISE
would reveal the field line topology, and its time variation, from
active regions into interplanetary space. <P />SunRISE would consist
of six 6U small spacecraft in a supersynchronous geosynchronous orbit
(GEO) in a passive formation. Forming a synthetic aperture and observing
at frequencies that cannot be observed on Earth due to ionospheric
absorption, SunRISE would leverage advances in software-defined radios,
GPS navigation and timing, and small spacecraft technologies. These
advances have been flown over the past few years, making this concept
finally affordable and low-risk. <P />An Extended Phase A study
of the SunRISE mission concept is scheduled to be completed in 2019
September. This paper presents a summary of the concept study. <P />Part
of this research was carried out at the Jet Propulsion Laboratory,
California Institute of Technology, under a contract with the National
Aeronautics and Space Administration. Some of the information presented
is pre-decisional and for planning and discussion purposes only.
---------------------------------------------------------
Title: Imaging Spectroscopic Observations of Type I Noise Storms
with Ultrahigh Temporal and Spectral Resolution
Authors: Yu, S.; Chen, B.; Bastian, T.; Gary, D. E.
2019AGUFMSH23C3336Y Altcode:
Type I noise storms are the most common, but perhaps the least
understood type of solar radio bursts in the decimeter-meter wavelength
range. Noise storms are non-flare-related radio phenomena. Their
existence indicates that energy release and particle energization
can continue in the corona without notable solar activities such as
flares or coronal mass ejections. Noise storms manifest themselves as
intermittent short-lived narrow-band bursts superposed on a broad-band,
long-lasting continuum (sometimes referred to as the noise storm
continuum). However, previous studies were either based on total-power
dynamic spectroscopy (without spatial resolution), or on imaging
observations at one or few frequency channels. During the recent Karl
G. Jansky Very Large Array (VLA) solar observing campaign, we performed
dynamic imaging spectroscopic observations with unprecedentedly high
temporal resolution,10 milliseconds, along with fine spectral resolution
(250 kHz) in the 290-450 MHz P band. We recorded several noise storm
events associated with quiescent, non-flaring solar active regions. For
the first time, we are able to image these type I bursts and fully
resolve them in the frequency-time domain, which are used to better
elucidate the origin of these bursts. We discuss the implications of
our results for understanding energy release and particle energization
in the seemly quiescent solar corona.
---------------------------------------------------------
Title: Principles Of Heliophysics: a textbook on the universal
processes behind planetary habitability
Authors: Schrijver, Karel; Bagenal, Fran; Bastian, Tim; Beer,
Juerg; Bisi, Mario; Bogdan, Tom; Bougher, Steve; Boteler, David;
Brain, Dave; Brasseur, Guy; Brownlee, Don; Charbonneau, Paul; Cohen,
Ofer; Christensen, Uli; Crowley, Tom; Fischer, Debrah; Forbes, Terry;
Fuller-Rowell, Tim; Galand, Marina; Giacalone, Joe; Gloeckler, George;
Gosling, Jack; Green, Janet; Guetersloh, Steve; Hansteen, Viggo;
Hartmann, Lee; Horanyi, Mihaly; Hudson, Hugh; Jakowski, Norbert;
Jokipii, Randy; Kivelson, Margaret; Krauss-Varban, Dietmar; Krupp,
Norbert; Lean, Judith; Linsky, Jeff; Longcope, Dana; Marsh, Daniel;
Miesch, Mark; Moldwin, Mark; Moore, Luke; Odenwald, Sten; Opher, Merav;
Osten, Rachel; Rempel, Matthias; Schmidt, Hauke; Siscoe, George;
Siskind, Dave; Smith, Chuck; Solomon, Stan; Stallard, Tom; Stanley,
Sabine; Sojka, Jan; Tobiska, Kent; Toffoletto, Frank; Tribble, Alan;
Vasyliunas, Vytenis; Walterscheid, Richard; Wang, Ji; Wood, Brian;
Woods, Tom; Zapp, Neal
2019arXiv191014022S Altcode:
This textbook gives a perspective of heliophysics in a way that
emphasizes universal processes from a perspective that draws attention
to what provides Earth (and similar (exo-)planets) with a relatively
stable setting in which life as we know it can thrive. The book is
intended for students in physical sciences in later years of their
university training and for beginning graduate students in fields of
solar, stellar, (exo-)planetary, and planetary-system sciences.
---------------------------------------------------------
Title: Frequency Agile Solar Radiotelescope
Authors: Bastian, Tim; Bain, H.; Bradley, R.; Chen, B.; Dahlin, J.;
DeLuca, E.; Drake, J.; Fleishman, G.; Gary, D.; Glesener, L.; Guo,
Fan; Hallinan, G.; Hurford, G.; Kasper, J.; Ji, Hantao; Klimchuk,
J.; Kobelski, A.; Krucker, S.; Kuroda, N.; Loncope, D.; Lonsdale,
C.; McTiernan, J.; Nita, G.; Qiu, J.; Reeves, K.; Saint-Hilaire, P.;
Schonfeld, S.; Shen, Chengcai; Tun, S.; Wertheimer, D.; White, S.
2019astro2020U..56B Altcode:
We describe the science objectives and technical requirements for a
re-scoped Frequency Agile Solar Radiotelescope (FASR). FASR fulfills
a long term community need for a ground-based, solar-dedicated, radio
telescope - a next-generation radioheliograph - designed to perform
ultra-broadband imaging spectropolarimetry.
---------------------------------------------------------
Title: Radio Spectroscopic Imaging of Solar Flare Termination Shocks:
Split-band Feature and A Second Possible Event
Authors: Chen, Bin; Luo, Yingjie; Yu, Sijie; Krucker, Sam; Reeves,
Kathy; Shen, Chengcai; Bastian, Timothy S.
2019AAS...23421003C Altcode:
Solar termination shocks (TSs) can form above the looptop when
reconnection outflows that impinge upon newly reconnected flare
arcades exceed the local fast-mode magnetosonic speed. TSs have been
suggested as one of the promising drivers for particle acceleration in
solar flares, yet observational evidence remains rare. By utilizing
radio dynamic spectral imaging of decimetric stochastic spike bursts
(SSBs) observed during a C1.9 eruptive flare on 2012 March 3, Chen et
al. (2015) found that the bursts were associated with a dynamic TS-like
feature above the looptop. They also showed evidence for the TS as
an electron accelerator. One piece of observational evidence that
strongly supports the TS interpretation is the split-band feature
- a phenomenon well-known in type II radio bursts associated with
CME-driven shocks, one interpretation for which attributes to radio
emission from both the upstream and downstream side of the shock. We
perform detailed spectral imaging analysis of the split-band feature in
the 2012 March 3 SSB event, and find evidence that supports the shock
upstream-downstream interpretation. We also report another SSB event
observed during an M8.4 eruptive flare on 2012 March 10, and show that
the radio centroids of the SSBs form a similar shock-surface-shaped
structure to the earlier event, located above the reconnected flare
arcades and below supra-arcade plasma downflows.
---------------------------------------------------------
Title: Radio Propagation Diagnostics of the Inner Heliosphere in
the Era of the Parker Solar Probe
Authors: Kobelski, Adam; Bastian, Timothy S.; Vourlidas, Angelos
2019AAS...23410706K Altcode:
The solar wind offers and extraordinary laboratory for studying
turbulence, turbulent dissipation, and heating. The Parker Solar Probe
(PSP) was launched in August 2018 to study these and other important
processes in the inner heliosphere. One type of observation that will
complement those of PSP are radio propagation measurements of solar
wind turbulence in the outer corona and the inner heliosphere. This
type of observation can provide measurements of the angular broadening
of distant spatially coherent background sources that transilluminate
the foreground solar wind plasma. This well-known technique can be used
to measure the spatial spectrum of electron density inhomogeneities in
the solar wind on scales of 100s of meters to 10s of kilometers inside
of 10-15 solar radii over a wide range of position angles. <P />Here
we report the results of a pilot study of background sources using the
Jansky Very Large Array (JVLA) in summer 2015. Unlike previous studies
of this kind, the JVLA's much greater sensitivity allows fainter
and more numerous sources to be used as probes of the foreground
medium. We observed 11 background sources in 16 sessions at apparent
radial distances of 2-7 solar radii. We confirm previous findings:
that the spectrum is flatter than Kolmogorov and that is highly
anisotropic. Unlike previous studies we find breaks into steeper spectra
for some sources on short spatial scales, suggestive of a transition to
dissipation. <P />Looking forward, we describe observations planned in
August 2019 in support of the third PSP perihelion passage (35.7 solar
radii). The VLA will be used to observe the corona and inner heliosphere
along 70 pierce points <10 solar radii. These observations will not
only provide global context about the state of the inner heliosphere
at time of perihelion passage, they will also baseline key solar wind
parameters that can be compared directly with PSP measurements. These
include turbulence level, spectral index, degree of anisotropy, and
the orientation of the magnetic field. The PSP measurements will, in
turn, provide measurements that will validate key assumptions made in
interpreting the radio data.
---------------------------------------------------------
Title: Advancing Understanding of Star-Planet Ecosystems in the Next
Decade: The Radio Wavelength Perspective
Authors: Osten, Rachel; Bastian, Tim; Bower, Geoff; Forbrich, Jan;
Gudel, Manuel; Kao, Melodie M.; Lazio, Joseph; Linsky, Jeffrey;
MacGregor, Meredith; Moschou, Sofia P.; Pineda, J. Sebastian; Rupen,
Michael P.; Villadsen, Jackie; White, Stephen; Williams, Peter K. G.;
Wolk, Scott J.
2019BAAS...51c.434O Altcode: 2019astro2020T.434O
In this white paper we advocate for stellar radio observations as a way
to advance understanding of stars in service of a better understanding
of star- planet ecosystems. Specific key advances needed are sensitivity
and access to a broader range of frequency space to make progress in
understanding the space weather environments of exoplanets.
---------------------------------------------------------
Title: Particle Acceleration and Transport, New Perspectives from
Radio, X-ray, and Gamma-Ray Observations
Authors: Gary, Dale; Bastian, Timothy S.; Chen, Bin; Drake, James F.;
Fleishman, Gregory; Glesener, Lindsay; Saint-Hilaire, Pascal; White,
Stephen M.
2019BAAS...51c.371G Altcode: 2019astro2020T.371G
Particle acceleration and particle transport are ubiquitous in
astrophysics. The Sun offers an astrophysical laboratory to study these
in minute detail, using radio dynamic imaging spectroscopy to measure
coronal magnetic fields, time and space evolution of the electron
distribution function.
---------------------------------------------------------
Title: Reconstructing Extreme Space Weather From Planet Hosting Stars
Authors: Airapetian, Vladimir; Adibekyan, V.; Ansdell, M.; Alexander,
D.; Barklay, T.; Bastian, T.; Boro Saikia, S.; Cohen, O.; Cuntz,
M.; Danchi, W.; Davenport, J.; DeNolfo, G.; DeVore, R.; Dong, C. F.;
Drake, J. J.; France, K.; Fraschetti, F.; Herbst, K.; Garcia-Sage,
K.; Gillon, M.; Glocer, A.; Grenfell, J. L.; Gronoff, G.; Gopalswamy,
N.; Guedel, M.; Hartnett, H.; Harutyunyan, H.; Hinkel, N. R.; Jensen,
A. G.; Jin, M.; Johnstone, C.; Kahler, S.; Kalas, P.; Kane, S. R.;
Kay, C.; Kitiashvili, I. N.; Kochukhov, O.; Kondrashov, D.; Lazio, J.;
Leake, J.; Li, G.; Linsky, J.; Lueftinger, T.; Lynch, B.; Lyra, W.;
Mandell, A. M.; Mandt, K. E.; Maehara, H.; Miesch, M. S.; Mickaelian,
A. M.; Mouschou, S.; Notsu, Y.; Ofman, L.; Oman, L. D.; Osten, R. A.;
Oran, R.; Petre, R.; Ramirez, R. M.; Rau, G.; Redfield, S.; Réville,
V.; Rugheimer, S.; Scheucher, M.; Schlieder, J. E.; Shibata, K.;
Schnittman, J. D.; Soderblom, David; Strugarek, A.; Turner, J. D.;
Usmanov, A.; Van Der Holst, B.; Vidotto, A.; Vourlidas, A.; Way, M. J.;
Wolk, Scott J.; Zank, G. P.; Zarka, P.; Kopparapu, R.; Babakhanova,
S.; Pevtsov, A. A.; Lee, Y.; Henning, W.; Colón, K. D.; Wolf, E. T.
2019BAAS...51c.564A Altcode: 2019astro2020T.564A; 2019arXiv190306853A
The goal of this white paper is to identify and describe promising key
research goals to aid the theoretical characterization and observational
detection of ionizing radiation from quiescent and flaring upper
atmospheres of planet hosts as well as properties of stellar coronal
mass ejections (CMEs) and stellar energetic particle (SEP) events.
---------------------------------------------------------
Title: Diagnostics of Space Weather Drivers Enabled by Radio
Observations
Authors: Bastian, Tim; Bain, Hazel; Chen, Bin; Gary, Dale E.;
Fleishman, Gregory D.; Glesener, Lindsay; Saint-Hilaire, Pascal;
Lonsdale, Colin; White, Stephen M.
2019BAAS...51c.323B Altcode: 2019astro2020T.323B; 2019arXiv190405817B
The Sun is an active star that can impact the Earth, its magnetosphere,
and the technological infrastructure on which modern society
depends. Radio emission from space weather drivers offers unique
diagnostics that complement those available at other wavelengths. We
discuss the requirements for an instrument to enable such diagnostics.
---------------------------------------------------------
Title: Radio, Millimeter, Submillimeter Observations of the Quiet Sun
Authors: Bastian, Tim; Chen, Bin; Gary, Dale E.; Fleishman, Gregory
D.; Glesener, Lindsay; Lonsdale, Colin; Saint-Hilaire, Pascal; White,
Stephen M.
2019BAAS...51c.493B Altcode: 2019arXiv190405826B; 2019astro2020T.493B
We point out the lack of suitable radio observations of the quiet Sun
chromosphere and corona and outline requirements for next generation
instrumentation to address the gap.
---------------------------------------------------------
Title: Probing Magnetic Reconnection in Solar Flares: New Perspectives
from Radio Dynamic Imaging Spectroscopy
Authors: Chen, Bin; Bastian, Tim; Dahlin, Joel; Drake, James F.;
Fleishman, Gregory; Gary, Dale; Glesener, Lindsay; Guo, Fan; Ji,
Hantao; Saint-Hilaire, Pascal; Shen, Chengcai; White, Stephen M.
2019BAAS...51c.507C Altcode: 2019astro2020T.507C; 2019arXiv190311192C
Magnetic reconnection is a fundamental physical process in many
laboratory, space, and astrophysical plasma contexts. In this white
paper we emphasize the unique power of remote-sensing observations
of solar flares at radio wavelengths in probing fundamental physical
processes in magnetic reconnection.
---------------------------------------------------------
Title: Community Input Solicited for Heliophysics Decadal Survey
Midterm Assessment Committee
Authors: Woods, Thomas; Millan, Robyn; Charo, Art; Bastian, Tim;
Bobra, Monica; Coster, Anthea; DeLuca, Ed; England, Scott; Fuselier,
Stephen; Lopez, Ramon; Luhmann, Janet; Nykyri, Katariina; Oberheide,
Jens; Opher, Merav; Schrijver, Karel; Semeter, Josh; Thayer, Jeff;
Title, Alan
2019shin.confE...6W Altcode:
The National Academies of Sciences, Engineering, and Medicine has
convened a committee to review the progress towards implementing the
2013 Heliophysics Decadal Survey, titled Solar and Space Physics: a
Science for a Technological Society. This review serves as a midterm
assessment before the next Heliophysics Decadal Survey committee would
begin its formulation. This committee is interested to receive input
from the heliophysics and space weather communities about the 2013-2018
progress realizing the 15 recommendations and applications specified in
the 2013 Heliophysics Decadal Survey, about any suggested actions to
optimize the science value during 2019-2023, about any suggestions to
improve the process for the next Heliophysics Decadal Survey, and about
any suggested actions to enhance all stages of careers for scientists
and engineers in the solar and space physics community. This poster
outlines the Heliophysics Decadal Survey recommendations and recent
progress, and it also summarizes the tasks for this midterm assessment
committee. There will be an opportunity to discuss your inputs with
a couple of the Committee members during the SHINE meeting.
---------------------------------------------------------
Title: Radio Observational Constraints on Turbulent Astrophysical
Plasmas
Authors: Bastian, Tim; Cordes, James; Kasper, Justin; Kobelski,
Adam; Korreck, Kelly; Howes, Gregory; Salem, Chadi; Spangler, Steve;
Vourlidas, Angelos
2019astro2020T.307B Altcode: 2019arXiv190405807B
Using radio observations of background sources, scattering phenomena
may be used to characterize the properties of foreground turbulent
plasma. We discuss the potential of such techniques to explore
turbulence in the solar wind and interstellar medium. The Next
Generation VLA will be an ideal instrument to exploit these techniques.
---------------------------------------------------------
Title: Solar Coronal Magnetic Fields: Quantitative Measurements at
Radio Wavelengths
Authors: Fleishman, Gregory; Bastian, Timothy S.; Chen, Bin; Gary,
Dale E.; Glesener, Lindsay; Nita, Gelu; Saint-Hilaire, Pascal; White,
Stephen M.
2019BAAS...51c.426F Altcode: 2019astro2020T.426F
Quantitative measurements of coronal and chromospheric magnetic field is
currently in its infancy. We describe a foundation of such observations,
which is a key input for MHD numerical models of the solar atmosphere
and eruptive processes, and a key link between lower layers of the
solar atmosphere and the heliosphere.
---------------------------------------------------------
Title: Flat-spectrum Radio Continuum Emission Associated with
ɛ Eridani
Authors: Rodríguez, Luis F.; Lizano, Susana; Loinard, Laurent;
Chávez-Dagostino, Miguel; Bastian, Timothy S.; Beasley, Anthony J.
2019ApJ...871..172R Altcode:
We present Very Large Array observations at 33.0 GHz that detect
emission coincident with ɛ Eridani to within 0.″07 (0.2 au at the
distance of this star), with a positional accuracy of 0.″05. This
result strongly supports the suggestion of previous authors that
the quiescent centimeter emission comes from the star and not from
a proposed giant exoplanet with a semimajor axis of ∼1.″0 (3.4
au). The centimeter emission is remarkably flat and is consistent
with optically thin free-free emission. In particular, it can be
modeled as a stellar wind with a mass-loss rate of the order of 6.6
× 10<SUP>-11</SUP> {M}<SUB>⊙ </SUB> yr<SUP>-1</SUP>, which is 3300
times the solar value, exceeding other estimates of this star’s
wind. However, interpretation of the emission in terms of other thermal
mechanisms like coronal free-free and gyroresonance emission cannot
be discarded.
---------------------------------------------------------
Title: Flat Spectrum Radio Continuum Emission Associated with
$\epsilon$ Eridani
Authors: Rodriguez, Luis F.; Lizano, Susana; Loinard, Laurent;
Chávez-Dagostino, Miguel; Bastian, Timothy S.; Beasley, Anthony J.
2019arXiv190100903R Altcode:
We present Very Large Array observations at 33.0 GHz that detect
emission coincident with $\epsilon$ Eridani to within $0\rlap.{"}07$
(0.2 AU at the distance of this star), with a positional accuracy
of $0\rlap.{"}05$. This result strongly supports the suggestion of
previous authors that the quiescent centimeter emission comes from the
star and not from a proposed giant exoplanet with a semi-major axis of
$\sim1\rlap.{"}0$ (3.4 AU). The centimeter emission is remarkably flat
and is consistent with optically thin free-free emission. In particular,
it can be modeled as a stellar wind with a mass loss rate of the order
of $6.6 \times 10^{-11}~ M_\odot ~yr^{-1}$, which is 3,300 times the
solar value, exceeding other estimates of this star's wind. However,
interpretation of the emission in terms of other thermal mechanisms
like coronal free-free and gyroresonance emission cannot be discarded.
---------------------------------------------------------
Title: Resolved imaging of the quiet and flaring radio corona of
active M dwarfs
Authors: Villadsen, Jacqueline; Hallinan, Gregg; Mioduszewski, Amy;
Bourke, Stephen; Bastian, Timothy
2019AAS...23320405V Altcode:
Magnetically active M dwarfs produce non-thermal quiescent radio
emission, along with coherent and incoherent flares. Very long
baseline imaging (VLBI) has revealed that these luminous phenomena
can occur on spatial scales significantly larger than the stellar
photosphere, in processes which have no clear analog in the solar
corona. This non-thermal radio emission originates directly from
energetic electrons, making radio the only direct observational
probe of accelerated particles in stellar coronae. I will present
VLBI observations of the quiescent and flaring radio emission from
two nearby active M dwarfs. These two stars with saturated coronal
activity have distinctly different structure in their radio coronae,
perhaps indicative of different underlying electron acceleration
mechanisms responsible for the quiescent radio emission.
---------------------------------------------------------
Title: ngVLA Observations of Coronal Magnetic Fields
Authors: Fleishman, G. D.; Nita, G. M.; White, S. M.; Gary, D. E.;
Bastian, T. S.
2018ASPC..517..125F Altcode:
Energy stored in the magnetic field in the solar atmosphere above
active regions is a key driver of all solar activity (e.g., solar
flares and coronal mass ejections), some of which can affect life
on Earth. Radio observations provide a unique diagnostic of the
coronal magnetic fields that make them a critical tool for the
study of these phenomena, using the technique of broadband radio
imaging spectropolarimetry. Observations with the ngVLA will provide
unique observations of coronal magnetic fields and their evolution,
key inputs and constraints for MHD numerical models of the solar
atmosphere and eruptive processes, and a key link between lower layers
of the solar atmosphere and the heliosphere. In doing so they will
also provide practical "research to operations" guidance for space
weather forecasting.
---------------------------------------------------------
Title: Radio Observations of Solar Flares
Authors: Gary, D. E.; Bastian, T. S.; Chen, B.; Fleishman, G. D.;
Glesener, L.
2018ASPC..517...99G Altcode:
Solar flares are due to the catastrophic release of magnetic energy
in the Sun's corona, resulting in plasma heating, mass motions,
particle acceleration, and radiation emitted from radio to γ-ray
wavelengths. They are associated with global coronal eruptions of plasma
into the interplanetary medium—coronal mass ejections—that can
result in a variety of “space weather” phenomena. Flares release
energy over a vast range of energies, from ∼10<SUP>23</SUP> ergs
(nanoflares) to more than 10<SUP>32</SUP> ergs. Solar flares are
a phenomenon of general astrophysical interest, allowing detailed
study of magnetic energy release, eruptive processes, shock formation
and propagation, particle acceleration and transport, and radiative
processes. Observations at radio wavelengths offer unique diagnostics of
the physics of flares. To fully exploit these diagnostics requires the
means of performing time-resolved imaging spectropolarimetry. Recent
observations with the Jansky Very Large Array (JVLA) and the Expanded
Owens Valley Solar Array (EOVSA), supported by extensive development
in forward modeling, have demonstrated the power of the approach. The
ngVLA has the potential to bring our understanding of flare processes
to a new level through its combination of high spatial resolution,
broad frequency range, and imaging dynamic range—especially when
used in concert with multi-wavelength observations and data at hard
X-ray energies.
---------------------------------------------------------
Title: ngVLA Observations of the Solar Wind
Authors: Bastian, T. S.
2018ASPC..517...87B Altcode:
The ngVLA has the potential to play a significant role in characterizing
properties of the outer corona and the heating and acceleration of the
solar wind into the inner heliosphere. In particular, using distant
background sources to transilluminate the foreground corona and
solar wind, a variety of radio propagation phenomena can be used to
map plasma properties as a function of solar elongation and position
angle throughout the solar cycle. These include angular broadening,
interplanetary scintillations, and differential Faraday rotation,
which can be used to map the solar wind velocity, determine properties
of solar wind turbulence, and constrain the solar wind magnetic
field. These observations will provide a global characterization of the
solar wind that will be highly complementary to in situ observations
made by various spacecraft. In addition, such observations can be
used to probe disturbances in the solar wind - coronal mass ejections,
for example - that may impact the near-Earth environment.
---------------------------------------------------------
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: Science with an ngVLA: Observations of the Solar Wind
Authors: Bastian, T. S.
2018arXiv181006633B Altcode:
The ngVLA has the potential to play a significant role in characterizing
properties of the outer corona and the heating and acceleration of the
solar wind into the inner heliosphere. In particular, using distant
background sources to transilluminate the foreground corona and
solar wind, a variety of radio propagation phenomena can be used to
map plasma properties as a function of solar elongation and position
angle throughout the solar cycle. These include angular broadening,
interplanetary scintillations, and differential Faraday rotation,
which can be used to map the solar wind velocity, determine properties
of solar wind turbulence, and constrain the solar wind magnetic
field. These observations will provide a global characterization of the
solar wind that will be highly complementary to in situ observations
made by various spacecraft. In addition, such observations can be used
to probe disturbances in the solar wind -- coronal mass ejections,
for example -- that may impact the near-Earth environment.
---------------------------------------------------------
Title: Magnetic Reconnection Null Points as the Origin of
Semirelativistic Electron Beams in a Solar Jet
Authors: Chen, Bin; Yu, Sijie; Battaglia, Marina; Farid, Samaiyah;
Savcheva, Antonia; Reeves, Katharine K.; Krucker, Säm; Bastian,
T. S.; Guo, Fan; Tassev, Svetlin
2018ApJ...866...62C Altcode: 2018arXiv180805951C
Magnetic reconnection, the central engine that powers explosive
phenomena throughout the universe, is also perceived to be one
of the principal mechanisms for accelerating particles to high
energies. Although various signatures of magnetic reconnection
have been frequently reported, observational evidence that links
particle acceleration directly to the reconnection site has been rare,
especially for space plasma environments currently inaccessible to in
situ measurements. Here we utilize broadband radio dynamic imaging
spectroscopy available from the Karl G. Jansky Very Large Array to
observe decimetric type III radio bursts in a solar jet with high
angular (∼20″), spectral (∼1%), and temporal resolution (50
ms). These observations allow us to derive detailed trajectories of
semirelativistic (tens of keV) electron beams in the low solar corona
with unprecedentedly high angular precision (<0.″65). We found that
each group of electron beams, which corresponds to a cluster of type III
bursts with 1-2 s duration, diverges from an extremely compact region
(∼600 km<SUP>2</SUP>) in the low solar corona. The beam-diverging
sites are located behind the erupting jet spire and above the closed
arcades, coinciding with the presumed location of magnetic reconnection
in the jet eruption picture supported by extreme ultraviolet/X-ray
data and magnetic modeling. We interpret each beam-diverging site as a
reconnection null point where multitudes of magnetic flux tubes join
and reconnect. Our data suggest that the null points likely consist
of a high level of density inhomogeneities possibly down to 10 km
scales. These results, at least in the present case, strongly favor
a reconnection-driven electron-acceleration scenario.
---------------------------------------------------------
Title: Science with an ngVLA: Radio Observations of Solar Flares
Authors: Gary, Dale E.; Bastian, Timothy S.; Chen, Bin; Fleishman,
Gregory D.; Glesener, Lindsay
2018arXiv181006336G Altcode:
Solar flares are due to the catastrophic release of magnetic energy
in the Sun's corona, resulting in plasma heating, mass motions,
particle acceleration, and radiation emitted from radio to $\gamma$-ray
wavelengths. They are associated with global coronal eruptions of plasma
into the interplanetary medium---coronal mass ejections---that can
result in a variety of "space weather" phenomena. Flares release energy
over a vast range of energies, from $\sim\!10^{23}$ ergs (nanoflares)
to more than $10^{32}$ ergs. Solar flares are a phenomenon of general
astrophysical interest, allowing detailed study of magnetic energy
release, eruptive processes, shock formation and propagation, particle
acceleration and transport, and radiative processes. Observations at
radio wavelengths offer unique diagnostics of the physics of flares. To
fully exploit these diagnostics requires the means of performing
time-resolved imaging spectropolarimetry. Recent observations with the
Jansky Very Large Array (JVLA) and the Expanded Owens Valley Solar Array
(EOVSA), supported by extensive development in forward modeling, have
demonstrated the power of the approach. The ngVLA has the potential
to bring our understanding of flare processes to a new level through
its combination of high spatial resolution, broad frequency range,
and imaging dynamic range---especially when used in concert with
multi-wavelength observations and data at hard X-ray energies.
---------------------------------------------------------
Title: Roadmap for Reliable Ensemble Forecasting of the Sun-Earth
System
Authors: Nita, Gelu; Angryk, Rafal; Aydin, Berkay; Banda, Juan;
Bastian, Tim; Berger, Tom; Bindi, Veronica; Boucheron, Laura; Cao,
Wenda; Christian, Eric; de Nolfo, Georgia; DeLuca, Edward; DeRosa,
Marc; Downs, Cooper; Fleishman, Gregory; Fuentes, Olac; Gary, Dale;
Hill, Frank; Hoeksema, Todd; Hu, Qiang; Ilie, Raluca; Ireland,
Jack; Kamalabadi, Farzad; Korreck, Kelly; Kosovichev, Alexander;
Lin, Jessica; Lugaz, Noe; Mannucci, Anthony; Mansour, Nagi; Martens,
Petrus; Mays, Leila; McAteer, James; McIntosh, Scott W.; Oria, Vincent;
Pan, David; Panesi, Marco; Pesnell, W. Dean; Pevtsov, Alexei; Pillet,
Valentin; Rachmeler, Laurel; Ridley, Aaron; Scherliess, Ludger; Toth,
Gabor; Velli, Marco; White, Stephen; Zhang, Jie; Zou, Shasha
2018arXiv181008728N Altcode:
The authors of this report met on 28-30 March 2018 at the New Jersey
Institute of Technology, Newark, New Jersey, for a 3-day workshop
that brought together a group of data providers, expert modelers, and
computer and data scientists, in the solar discipline. Their objective
was to identify challenges in the path towards building an effective
framework to achieve transformative advances in the understanding
and forecasting of the Sun-Earth system from the upper convection
zone of the Sun to the Earth's magnetosphere. The workshop aimed to
develop a research roadmap that targets the scientific challenge
of coupling observations and modeling with emerging data-science
research to extract knowledge from the large volumes of data (observed
and simulated) while stimulating computer science with new research
applications. The desire among the attendees was to promote future
trans-disciplinary collaborations and identify areas of convergence
across disciplines. The workshop combined a set of plenary sessions
featuring invited introductory talks and workshop progress reports,
interleaved with a set of breakout sessions focused on specific topics
of interest. Each breakout group generated short documents, listing
the challenges identified during their discussions in addition to
possible ways of attacking them collectively. These documents were
combined into this report-wherein a list of prioritized activities
have been collated, shared and endorsed.
---------------------------------------------------------
Title: Science with an ngVLA: ngVLA Observations of Coronal Magnetic
Fields
Authors: Fleishman, Gregory D.; Nita, Gelu M.; White, Stephen M.;
Gary, Dale E.; Bastian, Tim S.
2018arXiv181006622F Altcode:
Energy stored in the magnetic field in the solar atmosphere above
active regions is a key driver of all solar activity (e.g., solar
flares and coronal mass ejections), some of which can affect life
on Earth. Radio observations provide a unique diagnostic of the
coronal magnetic fields that make them a critical tool for the
study of these phenomena, using the technique of broadband radio
imaging spectropolarimetry. Observations with the ngVLA will provide
unique observations of coronal magnetic fields and their evolution,
key inputs and constraints for MHD numerical models of the solar
atmosphere and eruptive processes, and a key link between lower layers
of the solar atmosphere and the heliosphere. In doing so they will
also provide practical "research to operations" guidance for space
weather forecasting.
---------------------------------------------------------
Title: Radio Emission from the Exoplanetary System Epsilon Eridani
Authors: Bastian, Tim; Villadsen, J.; Maps, A.; Hallinan, G.; Beasley,
A. J.
2018shin.confE...3B Altcode:
After many years of trying, radio emission from the nearby system
Epsilon Eridani (3.2 pc) - containing a
---------------------------------------------------------
Title: Probing the Inner Heliosphere Using Radio Diagnostic Techniques
Authors: Bastian, Tim; Kobelski, Adam
2018shin.confE..38B Altcode:
A variety of radio propagation techniques - angular and spectral
broadening, scintillation, and Faraday rotation - can be used to probe
regions in the corona and solar wind that are otherwise inaccessible to
direct observation. We present some recent pilot observations made by
the Jansky Very Large Array to illustrate their potential for deducing
key properties of solar wind turbulence in the inner heliosphere. We
discuss prospects for exploiting these techniques in a more systematic
way in the era of the Parker Solar Probe and the Solar Obiter.
---------------------------------------------------------
Title: Erratum: “A First Comparison of Millimeter Continuum and
Mg II Ultraviolet Line Emission from the Solar Chromosphere”
(<A href="http://doi.org/10.3847/2041-8213/aa844c">2017, ApJL,
845, L19</A>)
Authors: Bastian, T. S.; Chintzoglou, G.; De Pontieu, B.; Shimojo,
M.; Schmit, D.; Leenaarts, J.; Loukitcheva, M.
2018ApJ...860L..16B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Radio Emission from the Exoplanetary System ɛ Eridani
Authors: Bastian, T. S.; Villadsen, J.; Maps, A.; Hallinan, G.;
Beasley, A. J.
2018ApJ...857..133B Altcode: 2017arXiv170607012B
As part of a wider search for radio emission from nearby systems known
or suspected to contain extrasolar planets, ɛ Eridani was observed by
the Jansky Very Large Array (VLA) in the 2-4 GHz and 4-8 GHz frequency
bands. In addition, as part of a separate survey of thermal emission
from solar-like stars, ɛ Eri was observed in the 8-12 GHz and the 12-18
GHz bands of the VLA. Quasi-steady continuum radio emission from ɛ Eri
was detected in the three high-frequency bands at levels ranging from
67 to 83 μJy. No significant variability is seen in the quasi-steady
emission. The emission in the 2-4 GHz emission, however, is shown
to be the result of a circularly polarized (up to 50%) radio pulse
or flare of a few minutes in duration that occurred at the beginning
of the observation. We consider the astrometric position of the radio
source in each frequency band relative to the expected position of the
K2V star and the purported planet. The quasi-steady radio emission at
frequencies ≥8 GHz is consistent with a stellar origin. The quality of
the 4-8 GHz astrometry provides no meaningful constraint on the origin
of the emission. The location of the 2-4 GHz radio pulse is >2.5σ
from the star; however, based on the ephemeris of Benedict et al., it
is not consistent with the expected location of the planet either. If
the radio pulse has a planetary origin, then either the planetary
ephemeris is incorrect or the emission originates from another planet.
---------------------------------------------------------
Title: Exploring the Sun with ALMA
Authors: Bastian, T. S.; Bárta, M.; Brajša, R.; Chen, B.; Pontieu,
B. D.; Gary, D. E.; Fleishman, G. D.; Hales, A. S.; Iwai, K.; Hudson,
H.; Kim, S.; Kobelski, A.; Loukitcheva, M.; Shimojo, M.; Skokić,
I.; Wedemeyer, S.; White, S. M.; Yan, Y.
2018Msngr.171...25B Altcode:
The Atacama Large Millimeter/submillimeter Array (ALMA) Observatory
opens a new window onto the Universe. The ability to perform continuum
imaging and spectroscopy of astrophysical phenomena at millimetre and
submillimetre wavelengths with unprecedented sensitivity opens up new
avenues for the study of cosmology and the evolution of galaxies, the
formation of stars and planets, and astrochemistry. ALMA also allows
fundamentally new observations to be made of objects much closer
to home, including the Sun. The Sun has long served as a touchstone
for our understanding of astrophysical processes, from the nature of
stellar interiors, to magnetic dynamos, non-radiative heating, stellar
mass loss, and energetic phenomena such as solar flares. ALMA offers
new insights into all of these processes.
---------------------------------------------------------
Title: Solar and Heliospheric Physics with the ngVLA
Authors: Bastian, Timothy; ngVLA Solar Science Working Group
2018AAS...23134219B Altcode:
The ngVLA offers unprecedented sensitivity, angular resolution,
and frequency bandwidth for studies of astrophysical phenomena from
millimeter to decimeter wavelengths, including the Sun and solar
wind. This paper summarizes key solar and heliospheric science
objectives that can be addressed by the ngVLA. For the Sun these
include: 1) the quantitative measurement of coronal magnetic fields;
2) magnetic energy release; 3) particle acceration and transport;
4) drivers of space weather; and 5) the structure and dynamics of
the solar chromosphere and corona. For the outer corona and the inner
heliosphere the science objectives include 1) solar wind acceleration;
2) the development of solar wind turbulence; 3) transient disturbances
(e.g., coronal mass ejections) in the solar wind. The techniques that
will be exploited for these studies will be briefly reviewed and the
necessary capabilities of the ngVLA in support of these techniques
will be discussed.
---------------------------------------------------------
Title: The First ALMA Observation of a Solar Plasmoid Ejection from
an X-Ray Bright Point
Authors: Shimojo, M.; Hudson, H. S.; White, S. M.; Bastian, T.;
Iwai, K.
2017AGUFMSH41A2754S Altcode:
Eruptive phenomena are important features of energy releases events,
such solar flares, and have the potential to improve our understanding
of the dynamics of the solar atmosphere. The 304 A EUV line of helium,
formed at around 10^5 K, is found to be a reliable tracer of such
phenomena, but the determination of physical parameters from such
observations is not straightforward. We have observed a plasmoid
ejection from an X-ray bright point simultaneously with ALMA, SDO/AIA,
and Hinode/XRT. This paper reports the physical parameters of the
plasmoid obtained by combining the radio, EUV, and X-ray data. As
a result, we conclude that the plasmoid can consist either of
(approximately) isothermal ∼10^5 K plasma that is optically thin
at 100 GHz, or a ∼10^4 K core with a hot envelope. The analysis
demonstrates the value of the additional temperature and density
constraints that ALMA provides, and future science observations with
ALMA will be able to match the spatial resolution of space-borne and
other high-resolution telescopes.
---------------------------------------------------------
Title: The Sun Radio Imaging Space Experiment (SunRISE) Mission
Authors: Kasper, J. C.; Lazio, J.; Alibay, F.; Amiri, N.; Bastian,
T.; Cohen, C.; Landi, E.; Hegedus, A. M.; Maksimovic, M.; Manchester,
W.; Reinard, A.; Schwadron, N.; Cecconi, B.; Hallinan, G.; Krupar, V.
2017AGUFMSH41B2760K Altcode:
Radio emission from coronal mass ejections (CMEs) is a direct tracer
of particle acceleration in the inner heliosphere and potential
magnetic connections from the lower solar corona to the larger
heliosphere. Energized electrons excite Langmuir waves, which then
convert into intense radio emission at the local plasma frequency,
with the most intense acceleration thought to occur within 20 R_S. The
radio emission from CMEs is quite strong such that only a relatively
small number of antennas is required to detect and map it, but many
aspects of this particle acceleration and transport remain poorly
constrained. Ground-based arrays would be quite capable of tracking
the radio emission associated with CMEs, but absorption by the Earth's
ionosphere limits the frequency coverage of ground-based arrays (nu >
15 MHz), which in turn limits the range of solar distances over which
they can track the radio emission (< 3 R_S). The state-of-the-art
for tracking such emission from space is defined by single antennas
(Wind/WAVES, Stereo/SWAVES), in which the tracking is accomplished by
assuming a frequency-to-density mapping; there has been some success
in triangulating the emission between the spacecraft, but considerable
uncertainties remain. We describe the Sun Radio Imaging Space Experiment
(SunRISE) mission concept: A constellation of small spacecraft in a
geostationary graveyard orbit designed to localize and track radio
emissions in the inner heliosphere. Each spacecraft would carry a
receiving system for observations below 25 MHz, and SunRISE would
produce the first images of CMEs more than a few solar radii from
the Sun. Part of this research was carried out at the Jet Propulsion
Laboratory, California Institute of Technology, under a contract with
the National Aeronautics and Space Administration.
---------------------------------------------------------
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: A First Comparison of Millimeter Continuum and Mg II
Ultraviolet Line Emission from the Solar Chromosphere
Authors: Bastian, T. S.; Chintzoglou, G.; De Pontieu, B.; Shimojo,
M.; Schmit, D.; Leenaarts, J.; Loukitcheva, M.
2017ApJ...845L..19B Altcode: 2017arXiv170604532B
We present joint observations of the Sun by the Atacama Large
Millimeter/submillimeter Array (ALMA) and the Interface Region Imaging
Spectrograph (IRIS). Both millimeter/submillimeter-λ continuum emission
and ultraviolet (UV) line emission originate from the solar chromosphere
and both have the potential to serve as powerful and complementary
diagnostics of physical conditions in this enigmatic region of the solar
atmosphere. The observations were made of a solar active region on 2015
December 18 as part of the ALMA science verification effort. A map of
the Sun’s continuum emission was obtained by ALMA at a wavelength of
1.25 mm (239 GHz). A contemporaneous map was obtained by IRIS in the
Mg II h doublet line at 2803.5 Å. While a clear correlation between
the 1.25 mm brightness temperature T<SUB>B</SUB> and the Mg II h
line radiation temperature T<SUB>rad</SUB> is observed, the slope
is <1, perhaps as a result of the fact that these diagnostics
are sensitive to different parts of the chromosphere and that the
Mg II h line source function includes a scattering component. There
is a significant difference (35%) between the mean T<SUB>B</SUB>
(1.25 mm) and mean T<SUB>rad</SUB> (Mg II). Partitioning the maps
into “sunspot,” “quiet areas,” and “plage regions” we
find the relation between the IRIS Mg II h line T<SUB>rad</SUB> and
the ALMA T<SUB>B</SUB> region-dependent. We suggest this may be the
result of regional dependences of the formation heights of the IRIS
and ALMA diagnostics and/or the increased degree of coupling between
the UV source function and the local gas temperature in the hotter,
denser gas in plage regions.
---------------------------------------------------------
Title: Solar Commissioning Observations of the Sun with ALMA
Authors: White, Stephen M.; Shimojo, Masumi; Bastian, Timothy S.;
Iwai, Kazumasa; Hales, Antonio; Brajsa, Roman; Skokic, Ivica; Kim,
Sujin; Hudson, Hugh S.; Loukitcheva, Maria; Wedemeyer, Sven
2017SPD....4820402W Altcode:
PI-led science observations have commenced with the Atacama
Large Millimeter-submillimeter Array (ALMA) following an extensive
commissioning effort. This talk will summarize that effort and discuss
some of the scientific results derived from the commissioning data. As
the solar cycle declines, ALMA observations will mainly address
chromospheric science topics. Examples of data obtained during
commissioning, both from the interferometer and from single-dish
observations, will be presented. The temperatures of the layers that
ALMA is most sensitive to have been determined for the two frequency
bands currently used for solar observations. Curious behavior in a
sunspot umbra and an observations of a small chromospheric ejection
will be discussed.
---------------------------------------------------------
Title: Observing the Sun with the Atacama Large
Millimeter/submillimeter Array (ALMA): High-Resolution Interferometric
Imaging
Authors: Shimojo, M.; Bastian, T. S.; Hales, A. S.; White, S. M.;
Iwai, K.; Hills, R. E.; Hirota, A.; Phillips, N. M.; Sawada, T.;
Yagoubov, P.; Siringo, G.; Asayama, S.; Sugimoto, M.; Brajša, R.;
Skokić, I.; Bárta, M.; Kim, S.; de Gregorio-Monsalvo, I.; Corder,
S. A.; Hudson, H. S.; Wedemeyer, S.; Gary, D. E.; De Pontieu, B.;
Loukitcheva, M.; Fleishman, G. D.; Chen, B.; Kobelski, A.; Yan, Y.
2017SoPh..292...87S Altcode: 2017arXiv170403236S
Observations of the Sun at millimeter and submillimeter wavelengths
offer a unique probe into the structure, dynamics, and heating of the
chromosphere; the structure of sunspots; the formation and eruption
of prominences and filaments; and energetic phenomena such as jets
and flares. High-resolution observations of the Sun at millimeter and
submillimeter wavelengths are challenging due to the intense, extended,
low-contrast, and dynamic nature of emission from the quiet Sun,
and the extremely intense and variable nature of emissions associated
with energetic phenomena. The Atacama Large Millimeter/submillimeter
Array (ALMA) was designed with solar observations in mind. The
requirements for solar observations are significantly different from
observations of sidereal sources and special measures are necessary
to successfully carry out this type of observations. We describe the
commissioning efforts that enable the use of two frequency bands,
the 3-mm band (Band 3) and the 1.25-mm band (Band 6), for continuum
interferometric-imaging observations of the Sun with ALMA. Examples of
high-resolution synthesized images obtained using the newly commissioned
modes during the solar-commissioning campaign held in December 2015
are presented. Although only 30 of the eventual 66 ALMA antennas
were used for the campaign, the solar images synthesized from the
ALMA commissioning data reveal new features of the solar atmosphere
that demonstrate the potential power of ALMA solar observations. The
ongoing expansion of ALMA and solar-commissioning efforts will continue
to enable new and unique solar observing capabilities.
---------------------------------------------------------
Title: Observing the Sun with the Atacama Large
Millimeter/submillimeter Array (ALMA): Fast-Scan Single-Dish Mapping
Authors: White, S. M.; Iwai, K.; Phillips, N. M.; Hills, R. E.; Hirota,
A.; Yagoubov, P.; Siringo, G.; Shimojo, M.; Bastian, T. S.; Hales,
A. S.; Sawada, T.; Asayama, S.; Sugimoto, M.; Marson, R. G.; Kawasaki,
W.; Muller, E.; Nakazato, T.; Sugimoto, K.; Brajša, R.; Skokić, I.;
Bárta, M.; Kim, S.; Remijan, A. J.; de Gregorio, I.; Corder, S. A.;
Hudson, H. S.; Loukitcheva, M.; Chen, B.; De Pontieu, B.; Fleishmann,
G. D.; Gary, D. E.; Kobelski, A.; Wedemeyer, S.; Yan, Y.
2017SoPh..292...88W Altcode: 2017arXiv170504766W
The Atacama Large Millimeter/submillimeter Array (ALMA) radio
telescope has commenced science observations of the Sun starting
in late 2016. Since the Sun is much larger than the field of view
of individual ALMA dishes, the ALMA interferometer is unable to
measure the background level of solar emission when observing the
solar disk. The absolute temperature scale is a critical measurement
for much of ALMA solar science, including the understanding of energy
transfer through the solar atmosphere, the properties of prominences,
and the study of shock heating in the chromosphere. In order to provide
an absolute temperature scale, ALMA solar observing will take advantage
of the remarkable fast-scanning capabilities of the ALMA 12 m dishes
to make single-dish maps of the full Sun. This article reports on the
results of an extensive commissioning effort to optimize the mapping
procedure, and it describes the nature of the resulting data. Amplitude
calibration is discussed in detail: a path that uses the two loads in
the ALMA calibration system as well as sky measurements is described
and applied to commissioning data. Inspection of a large number of
single-dish datasets shows significant variation in the resulting
temperatures, and based on the temperature distributions, we derive
quiet-Sun values at disk center of 7300 K at λ =3 mm and 5900 K at
λ =1.3 mm. These values have statistical uncertainties of about 100
K, but systematic uncertainties in the temperature scale that may be
significantly larger. Example images are presented from two periods
with very different levels of solar activity. At a resolution of about
25<SUP>″</SUP>, the 1.3 mm wavelength images show temperatures on
the disk that vary over about a 2000 K range. Active regions and plages
are among the hotter features, while a large sunspot umbra shows up as
a depression, and filament channels are relatively cool. Prominences
above the solar limb are a common feature of the single-dish images.
---------------------------------------------------------
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: The First ALMA Observation of a Solar Plasmoid Ejection from
an X-Ray Bright Point
Authors: Shimojo, Masumi; Hudson, Hugh S.; White, Stephen M.; Bastian,
Timothy S.; Iwai, Kazumasa
2017ApJ...841L...5S Altcode: 2017arXiv170404881S
Eruptive phenomena such as plasmoid ejections or jets are important
features of solar activity and have the potential to improve our
understanding of the dynamics of the solar atmosphere. Such ejections
are often thought to be signatures of the outflows expected in
regions of fast magnetic reconnection. The 304 Å EUV line of helium,
formed at around 10<SUP>5</SUP> K, is found to be a reliable tracer
of such phenomena, but the determination of physical parameters
from such observations is not straightforward. We have observed
a plasmoid ejection from an X-ray bright point simultaneously at
millimeter wavelengths with ALMA, at EUV wavelengths with SDO/AIA,
and in soft X-rays with Hinode/XRT. This paper reports the physical
parameters of the plasmoid obtained by combining the radio, EUV, and
X-ray data. As a result, we conclude that the plasmoid can consist
either of (approximately) isothermal ∼10<SUP>5</SUP> K plasma that
is optically thin at 100 GHz, or a ∼10<SUP>4</SUP> K core with a
hot envelope. The analysis demonstrates the value of the additional
temperature and density constraints that ALMA provides, and future
science observations with ALMA will be able to match the spatial
resolution of space-borne and other high-resolution telescopes.
---------------------------------------------------------
Title: Getting to know the nearest stars: Intermittent radio bursts
from Ross 614
Authors: Winterhalter, Daniel; Knapp, Mary; Bastian, Tim
2017EGUGA..1918385W Altcode:
Radio observations have been used as a search tool for exoplanets since
before the confirmed discovery of the first extrasolar planet. To
date, there have been no definitive detections of exoplanets in the
radio regime. We are engaged in an ongoing blind radio survey of the
nearest star systems for exoplanetary radio emission. The goal of this
survey is to obtain meaningful upper limits on radio emission from (or
modulated by) sub-stellar companions of the nearest stars. Nearby stars
are strongly preferred because they suffer the least from the dilution
of potential radio signals by distance. Targets are selected by distance
and observability (both LOFAR and VLA) only. Other properties of target
stars, such as stellar type, are not considered to avoid biasing the
search. Five survey targets, Procyon, GJ 1111, GJ 725, Ross 614, and
UGPSJ072227.51, have been observed with the VLA telescope L- and S-band
receivers. P-band observations are ongoing. Of particular interest are,
at this time, our observation of the Ross 614 System. Ross 614 is an
M-dwarf binary system at a distance of about 13 Ly, with an orbital
period of 16.6 years. The binary companions are classified as flare
stars because strong radio emission has been detected from the location
of the system in previous work. Analyses are in progress to determine
if the intermittent burst are similar to solar-type burst, and/or if
there is any evidence for emissions from sub-stellar companions.
---------------------------------------------------------
Title: The Sun Radio Imaging Space Experiment (SunRISE) Mission
Authors: Lazio, Joseph; Kasper, Justin; Maksimovic, Milan; Alibay,
Farah; Amiri, Nikta; Bastian, Tim; Cohen, Christina; Landi, Enrico;
Manchester, Ward; Reinard, Alysha; Schwadron, Nathan; Cecconi,
Baptiste; Hallinan, Gregg; Hegedus, Alex; Krupar, Vratislav; Zaslavsky,
Arnaud
2017EGUGA..19.5580L Altcode:
Radio emission from coronal mass ejections (CMEs) is a direct tracer
of particle acceleration in the inner heliosphere and potential
magnetic connections from the lower solar corona to the larger
heliosphere. Energized electrons excite Langmuir waves, which then
convert into intense radio emission at the local plasma frequency,
with the most intense acceleration thought to occur within 20 RS. The
radio emission from CMEs is quite strong such that only a relatively
small number of antennas is required to detect and map it, but many
aspects of this particle acceleration and transport remain poorly
constrained. Ground-based arrays would be quite capable of tracking
the radio emission associated with CMEs, but absorption by the Earth's
ionosphere limits the frequency coverage of ground-based arrays (ν
≳ 15 MHz), which in turn limits the range of solar distances over
which they can track the radio emission (≲ 3RS). The state-of-the-art
for tracking such emission from space is defined by single antennas
(Wind/WAVES, Stereo/SWAVES), in which the tracking is accomplished by
assuming a frequency-to-density mapping; there has been some success
in triangulating the emission between the spacecraft, but considerable
uncertainties remain. We describe the Sun Radio Imaging Space Experiment
(SunRISE) mission concept: A constellation of small spacecraft in a
geostationary graveyard orbit designed to localize and track radio
emissions in the inner heliosphere. Each spacecraft would carry a
receiving system for observations below 25 MHz, and SunRISE would
produce the first images of CMEs more than a few solar radii from
the Sun. Part of this research was carried out at the Jet Propulsion
Laboratory, California Institute of Technology, under a contract with
the National Aeronautics and Space Administration.
---------------------------------------------------------
Title: A Search for Radio Emission from Nearby Exoplanets
Authors: Maps, Amethyst D.; Bastian, Timothy S.; Beasley, Anthony J.
2017AAS...22914635M Altcode:
Since the discovery of the first extrasolar planet orbiting a main
sequence star more than 20 years ago, the study of exoplanets has become
a burgeoning field with more than 3300 confirmed extrasolar planets now
known. A variety of techniques has been used to discover exoplanets
orbiting main sequence stars and to deduce their properties: timing,
radial velocities, direct imaging, microlensing, and transits in the
optical/IR bands. Absent from this list so far is the detection of
exoplanets at radio wavelengths, but not for lack of trying. Searches
for radio emission from exoplanets predate their discovery (Winglee et
al. 1986) and have continued sporadically to this day. The majority of
searches for radio emission from exoplanets has searched for coherent
radio emission. It is indeed the case that in our own solar system,
all magnetized planets are powerful radio emitters, the likely emission
mechanism being the cyclotron maser instability. The outstanding example
is Jupiter, which emits 10<SUP>10</SUP>-10<SUP>11</SUP> W at decameter
wavelengths (frequencies <40 MHz). If there are Jupiter-like planets
in other solar systems, many must surely emit CMI radiation. The emitted
radiation could be orders of magnitude more intense than Jupiter’s
if the interaction between the magnetized planet and the wind from
the primary star is stronger than the Sun/Jupiter interaction - due,
for example, to a more powerful wind and/or the planet being closer
to the star.We have initiated a new search for radio emission from
exoplanets, focusing on all known exoplanetary systems within 20 pc -
more than 50 systems containing nearly 100 planets using the Jansky
Very Large Array (JVLA) in three frequency bands: 1-2 GHz, 2-4 GHz,
and 4-8 GHz with a target sensitivity of ~10 microJy. We have completed
the 2-4 GHz survey and report our preliminary results, which include
the detection of two systems. We discuss whether the emission is from
a planet or from the star and the implications of our conclusions for
habitability of exoplanets.
---------------------------------------------------------
Title: The Brightness Temperature of the Quiet Solar Chromosphere
at 2.6 mm
Authors: Iwai, Kazumasa; Shimojo, Masumi; Asayama, Shinichiro;
Minamidani, Tetsuhiro; White, Stephen; Bastian, Timothy; Saito, Masao
2017SoPh..292...22I Altcode: 2016arXiv161208241I
The absolute brightness temperature of the Sun at millimeter wavelengths
is an important diagnostic of the solar chromosphere. Because the
Sun is so bright, measurement of this property usually involves
the operation of telescopes under extreme conditions and requires
a rigorous performance assessment of the telescope. In this study,
we establish solar observation and calibration techniques at 2.6
mm wavelength for the Nobeyama 45 m telescope and accurately
derive the absolute solar brightness temperature. We tune the
superconductor-insulator-superconductor (SIS) receiver by inducing
different bias voltages onto the SIS mixer to prevent saturation. Then,
we examine the linearity of the receiver system by comparing outputs
derived from different tuning conditions. Furthermore, we measure
the lunar filled beam efficiency of the telescope using the New Moon,
and then derive the absolute brightness temperature of the Sun. The
derived solar brightness temperature is 7700 ±310 K at 115 GHz. The
telescope beam pattern is modeled as a summation of three Gaussian
functions and derived using the solar limb. The real shape of the Sun
is determined via deconvolution of the beam pattern from the observed
map. Such well-calibrated single-dish observations are important for
high-resolution chromospheric studies because they provide the absolute
temperature scale that is lacking from interferometer observations.
---------------------------------------------------------
Title: MWA Observations of Solar Radio Bursts and the Quiet Sun
Authors: Cairns, I.; Oberoi, D.; Morgan, J.; Bastian, T.; Bhatnagar,
S.; Bisi, M.; Benkevitch, L.; Bowman, J.; Donea, A.; Giersch, O.;
Jackson, B.; Chat, G. L.; Golub, L.; Hariharan, K.; Herne, D.; Kasper,
J.; Kennewell, J.; Lonsdale, C.; Lobzin, V.; Matthews, L.; Mohan, A.;
Padmanabhan, J.; Pankratius, V.; Pick, M.; Subramanian, P.; Ramesh,
R.; Raymond, J.; Reeves, K.; Rogers, A.; Sharma, R.; Tingay, S.;
Tremblay, S.; Tripathi, D.; Webb, D.; White, S.; Abidin, Z. B. Z.
2017mwa..prop..A06C Altcode:
A hundred hours of observing time for solar observations is requested
during the 2017-A observing semester. These data will be used to address
science objectives for solar burst science (Goal A), studies of weak
non-thermal radiation (Goal B) and quiet sun science (Goal C). Goal
A will focus on detailed investigations of individual events seen in
the MWA data, using the unsurpassed spectroscopic imaging ability
of the MWA to address some key solar physics questions. Detailed
observations of type II bursts, of which MWA has observed two, will
be one focus, with MWA polarimetric imaging observations of type III
bursts another focus. Goal B will address studies of the numerous
short lived and narrow band emission features, significantly weaker
than those seen by most other instruments revealed by the MWA. These
emission features do not resemble any known types of solar bursts, but
are possible signatures of "nanoflares" which have long been suspected
to play a role in coronal heating. A large database of these events is
needed to be able to reliably estimate their contribution to coronal
heating. These observations will contribute to this database. Goal C
will focus on characterizing the Sun's background thermal emission,
their short and long term variability and looking for evidence of a
scattering disc around the Sun.
---------------------------------------------------------
Title: Systematic Search of the Nearest Stars for Exoplanetary Radio
Emission: VLA observations in L and S Bands
Authors: Winterhalter, D.; Knapp, M.; Bastian, T.
2016AGUFM.P13C..07W Altcode:
Radio observations have been used as a search tool for exoplanets since
before the confirmed discovery of the first extrasolar planet. To
date, there have been no definitive detections of exoplanets in the
radio regime. We are engaged in an ongoing blind radio survey of the
nearest star systems for exoplanetary radio emission. The goal of this
survey is to obtain meaningful upper limits on radio emission from
(or modulated by) sub-stellar companions of the nearest stars. Nearby
stars are strongly preferred because they suffer the least from
the dilution of potential radio signals by distance. Targets are
selected by distance and observability (both LOFAR and VLA) only. Other
properties of target stars, such as stellar type, are not considered
to avoid biasing the search. Five survey targets, Procyon, GJ 1111,
GJ 725, Ross 614, and UGPSJ072227.51, have been observed with the VLA
telescope L- and S-band receivers. P-band observations are ongoing. All
targets have multiple L-band observations, while Procyon and Ross 614
also have S-band observations. GJ 725 has been observed with LOFAR
at lower frequencies (30-75 MHz) as well. Upper limits on planetary
radio flux (and implied magnetic field strength) derived from these
VLA observations are presented.
---------------------------------------------------------
Title: The Sun Radio Interferometer Space Experiment (SunRISE)
Authors: Alibay, F.; Lazio, J.; Kasper, J. C.; Amiri, N.; Bastian,
T.; Cohen, C.; Landi, E.; Manchester, W.; Reinard, A.; Schwadron, N.;
Hegedus, A. M.; Maksimovic, M.; Zaslavsky, A.; Cecconi, B.; Hallinan,
G.; Krupar, V.
2016AGUFMSH41B2540A Altcode:
Radio emission from coronal mass ejections (CMEs) is a direct tracer
of the particle acceleration in the inner heliosphere and potential
magnetic connections from the lower solar corona to the larger
heliosphere. However, many aspects of this particle acceleration
remain poorly constrained. The radio emission from CMEs is quite
strong such that only a relatively small number of antennas is
required to map it. However, the state-of-the-art for tracking
such emission is only defined by single antennas (Wind/WAVES,
Stereo/SWAVES) in which the tracking is accomplished by assuming
a frequency-to-density mapping. These are limited to tracking CMEs
to only a few solar radii before the frequencies of radio emission
drop below the Earth's ionospheric cutoff. Triangulation between the
STEREO/SWAVES and Wind/WAVES instruments have provided some initial
constraints on particle acceleration sites at larger distances (lower
frequencies), but the uncertainties remain considerable. We present
the Sun Radio Imaging Space Experiment (SunRISE) mission concept:
a space-based array designed to localize such radio emissions. This
low-cost constellation is composed of small spacecraft placed in a
geostationary graveyard orbit, each carrying an HF radio receiver. In
this concept, each spacecraft would perform concurrent observations
below 25 MHz, which would then be correlated on the ground to produce
the first images of CMEs more than a few solar radii from the Sun. Part
of this research was carried out at the Jet Propulsion Laboratory,
California Institute of Technology, under a contract with the National
Aeronautics and Space Administration.
---------------------------------------------------------
Title: Solar Observations with the Atacama Large
Millimeter/submillimeter Array (ALMA)
Authors: Kobelski, A.; Bastian, T. S.; Bárta, M.; Brajša, R.; Chen,
B.; De Pontieu, B.; Fleishman, G.; Gary, D.; Hales, A.; Hills, R.;
Hudson, H.; Hurford, G.; Loukitcheva, M.; Iwai, K.; Krucker, S.;
Shimojo, M.; Skokić, I.; Wedemeyer, S.; White, S.; Yan, Y.; ALMA
Solar Development Team
2016ASPC..504..327K Altcode:
The Atacama Large Millimeter/Submillimeter Array (ALMA) is a
joint North American, European, and East Asian project that opens
the mm-sub mm wavelength part of the electromagnetic spectrum for
general astrophysical exploration, providing high-resolution imaging
in frequency bands currently ranging from 84 GHz to 950 GHz (300
microns to 3 mm). It is located in the Atacama desert in northern
Chile at an elevation of 5000 m. Despite being a general purpose
instrument, provisions have been made to enable solar observations
with ALMA. Radiation emitted at ALMA wavelengths originates mostly
from the chromosphere, which plays an important role in the transport
of matter and energy, and the in heating the outer layers of the solar
atmosphere. Despite decades of research, the solar chromosphere remains
a significant challenge: both to observe, owing to the complicated
formation mechanisms of currently available diagnostics; and to
understand, as a result of the complex nature of the structure and
dynamics of the chromosphere. ALMA has the potential to change the
scene substantially as it serves as a nearly linear thermometer at
high spatial and temporal resolution, enabling us to study the complex
interaction of magnetic fields and shock waves and yet-to-be-discovered
dynamical processes. Moreover, ALMA will play an important role in
the study of energetic emissions associated with solar flares at
sub-THz frequencies.
---------------------------------------------------------
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: Probing Solar Wind Turbulence with the Jansky Very Large Array
Authors: Kobelski, A.; Bastian, T. S.; Betti, S.
2016ASPC..504...97K Altcode:
The solar wind offers an extraordinary laboratory for studying MHD
turbulence, turbulent dissipation, and heating. Radio propagation
phenomena can be exploited as probes of the solar wind in regions that
are generally inaccessible to in situ spacecraft measurements. Here,
we have undertaken a study with the Jansky Very Large Array (VLA)
to observe point-like sources drawn from the JVAS catalog, and 3 VLA
calibrator sources, to trans-illuminate the outer corona/inner solar
wind. In doing so, we will exploit angular broadening and refractive
scintillation to deduce properties of the solar wind along ≍23 lines
of sight within 7 solar radii of the Sun and a wide range of position
angles. By fitting the complex visibilities using well-known techniques
we can deduce or constrain a number of key parameters. In particular,
we fit the visibilities to a function of the known source flux,
displacement of the source due to refraction, source broadening due to
an elliptical structure function, spectral slope of the turbulence, and
the coherence scale. Of particular interest is α, the spectral slope of
the turbulence which we probe at both small (km to 10s of km) and large
(thousands of km) scales. This will help us determine the presence
and evolution of an inner scale, measure the degree of anisotropy,
and constrain the topology of the global coronal magnetic field. The
inner scale is of particular interest for constraining current theories
of turbulence dissipation and heating. Initial analysis show the
visibilities vary notably on timescales of individual integrations
(2 seconds) and that the source is not uniformly broadened. All
sources appear to preferentially broaden perpendicular to the magnetic
field, consistent with theories of kinetic Alfvén waves. This type
of observation will also help to interpret data from the upcoming
Solar Probe Plus and Solar Orbiter missions. A full set of results
and analysis is forthcoming. More details on previous results can be
found in Bastian (1999), which used the previous generation VLA.
---------------------------------------------------------
Title: ALMA Observations of the Sun in Cycle 4 and Beyond
Authors: Wedemeyer, S.; Fleck, B.; Battaglia, M.; Labrosse, N.;
Fleishman, G.; Hudson, H.; Antolin, P.; Alissandrakis, C.; Ayres, T.;
Ballester, J.; Bastian, T.; Black, J.; Benz, A.; Brajsa, R.; Carlsson,
M.; Costa, J.; DePontieu, B.; Doyle, G.; Gimenez de Castro, G.;
Gunár, S.; Harper, G.; Jafarzadeh, S.; Loukitcheva, M.; Nakariakov,
V.; Oliver, R.; Schmieder, B.; Selhorst, C.; Shimojo, M.; Simões,
P.; Soler, R.; Temmer, M.; Tiwari, S.; Van Doorsselaere, T.; Veronig,
A.; White, S.; Yagoubov, P.; Zaqarashvili, T.
2016arXiv160100587W Altcode:
This document was created by the Solar Simulations for the Atacama
Large Millimeter Observatory Network (SSALMON) in preparation of
the first regular observations of the Sun with the Atacama Large
Millimeter/submillimeter Array (ALMA), which are anticipated to start
in ALMA Cycle 4 in October 2016. The science cases presented here
demonstrate that a large number of scientifically highly interesting
observations could be made already with the still limited solar
observing modes foreseen for Cycle 4 and that ALMA has the potential
to make important contributions to answering long-standing scientific
questions in solar physics. With the proposal deadline for ALMA Cycle
4 in April 2016 and the Commissioning and Science Verification campaign
in December 2015 in sight, several of the SSALMON Expert Teams composed
strategic documents in which they outlined potential solar observations
that could be feasible given the anticipated technical capabilities
in Cycle 4. These documents have been combined and supplemented
with an analysis, resulting in recommendations for solar observing
with ALMA in Cycle 4. In addition, the detailed science cases also
demonstrate the scientific priorities of the solar physics community
and which capabilities are wanted for the next observing cycles. The
work on this White Paper effort was coordinated in close cooperation
with the two international solar ALMA development studies led by
T. Bastian (NRAO, USA) and R. Brajsa, (ESO). This document will be
further updated until the beginning of Cycle 4 in October 2016. In
particular, we plan to adjust the technical capabilities of the solar
observing modes once finally decided and to further demonstrate the
feasibility and scientific potential of the included science cases by
means of numerical simulations of the solar atmosphere and corresponding
simulated ALMA observations.
---------------------------------------------------------
Title: Solar Simulations for the Atacama Large Millimeter Observatory
Network
Authors: Wedemeyer, S.; Bastian, T.; Brajša, R.; Barta, M.;
Shimojo, M.
2015ASPC..499..341W Altcode: 2015arXiv150206379W
The Atacama Large Millimeter/submillimeter Array (ALMA) will
be a valuable tool for observing the chromosphere of our Sun at
(sub-)millimeter wavelengths at high spatial, temporal and spectral
resolution and as such has great potential to address long-standing
scientific questions in solar physics. In order to make the best use
of this scientific opportunity, the Solar Simulations for the Atacama
Large Millimeter Observatory Network has been initiated. A key goal
of this international collaboration is to support the preparation and
interpretation of future observations of the Sun with ALMA.
---------------------------------------------------------
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: Particle acceleration by a solar flare termination shock
Authors: Chen, Bin; Bastian, Timothy S.; Shen, Chengcai; Gary, Dale
E.; Krucker, Säm; Glesener, Lindsay
2015Sci...350.1238C Altcode: 2015arXiv151202237C
Solar flares—the most powerful explosions in the solar system—are
also efficient particle accelerators, capable of energizing a large
number of charged particles to relativistic speeds. A termination
shock is often invoked in the standard model of solar flares as a
possible driver for particle acceleration, yet its existence and
role have remained controversial. We present observations of a solar
flare termination shock and trace its morphology and dynamics using
high-cadence radio imaging spectroscopy. We show that a disruption of
the shock coincides with an abrupt reduction of the energetic electron
population. The observed properties of the shock are well reproduced
by simulations. These results strongly suggest that a termination
shock is responsible, at least in part, for accelerating energetic
electrons in solar flares.
---------------------------------------------------------
Title: Observations and Simulations of a Termination Shock in an
Eruptive Solar Flare as a Possible Particle Accelerator
Authors: Chen, B.; Bastian, T.; Shen, C.; Gary, D. E.; Krucker, S.;
Glesener, L.
2015AGUFMSH11F..05C Altcode:
A termination shock has been often invoked in the standard model for
eruptive solar flares as a possible driver for particle acceleration. It
is hypothesized as a standing shock wave generated by super-magnetosonic
reconnection outflows impinging upon dense, newly-reconnected magnetic
loops during the flare energy release process. However, such shock
wave has largely remained a theoretical concept inferred from model
predictions due to the lack of observational evidence. Here we present
observations of a termination shock in a solar flare and trace its
morphology and dynamics using high-cadence radio imaging spectroscopy
enabled by the upgraded Karl G. Jansky Very Large Array. The observed
properties of the shock, including its location, morphology, and
dynamics, are well-reproduced by magnetohydrodynamics simulations
in a standard Kopp-Pneuman-type reconnection geometry for two-ribbon
flares. We further show that a disruption of the shock coincides with
an abrupt reduction of the energetic electron population. These results
strongly suggest that a termination shock is responsible, at least in
part, for accelerating energetic electrons in solar flares.
---------------------------------------------------------
Title: Solar ALMA Observations - A New View of Our Host Star
Authors: Wedemeyer, S.; Bastian, T.; Brajša, R.; Barta, M.; Shimojo,
M.; Hales, A.; Yagoubov, P.; Hudson, H.
2015ASPC..499..345W Altcode: 2015arXiv150206397W
ALMA provides the necessary spatial, temporal and spectral resolution to
explore central questions in contemporary solar physics with potentially
far-reaching implications for stellar atmospheres and plasma physics. It
can uniquely constraint the thermal and magnetic field structure in
the solar chromosphere with measurements that are highly complementary
to simultaneous observations with other ground-based and space-borne
instruments. Here, we highlight selected science cases.
---------------------------------------------------------
Title: Fast Single-Dish Scans of the Sun Using ALMA
Authors: Phillips, N.; Hills, R.; Bastian, T.; Hudson, H.; Marson,
R.; Wedemeyer, S.
2015ASPC..499..347P Altcode: 2015arXiv150206122P
We have implemented control and data-taking software that makes it
possible to scan the beams of individual ALMA antennas to perform
quite complex patterns while recording the signals at high rates. We
conducted test observations of the Sun in September and December,
2014. The data returned have excellent quality; in particular they
allow us to characterize the noise and signal fluctuations present
in this kind of observation. The fast-scan experiments included both
Lissajous patterns covering rectangular areas, and “double-circle”
patterns of the whole disk of the Sun and smaller repeated maps of
specific disk-shaped targets. With the latter we find that we can
achieve roughly Nyquist sampling of the Band 6 (230 GHz) beam in 60
s over a region 300” in diameter. These maps show a peak-to-peak
brightness-temperature range of up to 1000 K, while the time-series
variability at any given point appears to be of order 0.5% RMS over
times of a few minutes. We thus expect to be able to separate the
noise contributions due to transparency fluctuations from variations in
the Sun itself. Such timeseries have many advantages, in spite of the
non-interferometric observations. In particular such data should make
it possible to observe microflares in active regions and nanoflares
in any part of the solar disk and low corona.
---------------------------------------------------------
Title: The Atacama Large Millimeter/Submillimeter Array: a New Asset
for Solar and Heliospheric Physics
Authors: Bastian, Timothy S.; Barta, Miroslav; Brajsa, Roman; Chen,
Bin; De Pontieu, Bart; Fleishman, Gregory; Gary, Dale; Hales, Antonio;
Hills, Richard; Hudson, Hugh; Iwai, Kazamasu; Shimojo, Masumi; White,
Stephen; Wedemeyer, Sven; Yan, Yihua
2015IAUGA..2257295B Altcode:
The Atacama Large Millimeter/Submillimeter Array (ALMA) is a joint
North American, European, and East Asian interferometric array that
opens the mm-submm wavelength part of the electromagnetic spectrum
for general astrophysical exploration, providing high-resolution
imaging in frequency bands ranging from 86 to 950 GHz. Despite being
a general purpose instrument, provisions have been made to enable
solar observations with ALMA. Radiation emitted at ALMA wavelengths
originates mostly from the chromosphere, which plays an important
role in the transport of energy and matter and the heating of the
outer layers of the solar atmosphere. In this paper we describe
recent efforts to ensure that ALMA can be usefully exploited by
the scientific community to address outstanding questions in solar
physics. We summarize activities under North American and European
ALMA development studies, including instrument testing, calibration
and imaging strategies, a science simulations. With the support of
solar observations, ALMA joins next-generation groundbased instruments
that can be used alone or in combination with other ground-based and
space-based instruments to address outstanding questions in solar
and heliospheric physics. Opportunities for the wider community to
contribute to these efforts will be highlighted.
---------------------------------------------------------
Title: SSALMON - The Solar Simulations for the Atacama Large
Millimeter Observatory Network
Authors: Wedemeyer, Sven; Bastian, Timothy S.; Brajsa, Roman; Barta,
Miroslav
2015IAUGA..2257466W Altcode:
The Atacama Large Millimeter/submillimeter Array (ALMA) provides
a new powerful tool for observing the solar chromosphere at high
spatial, temporal, and spectral resolution, which will allow for
addressing fundamental scientific questions. Based on first solar test
observations, observing strategies for regular solar campaigns are
currently under development. State-of-the-art numerical simulations of
the solar atmosphere and modeling of instrumental effects can help in
this respect, constraining and optimizing future observing modes for
ALMA. On September 1st, 2014, the Solar Simulations for the Atacama
Large Millimeter Observatory Network (SSALMON) has been initiated
with the aim to co-ordinate related activities and to promote the
scientific potential of ALMA observations of the Sun. The network
is connected to two currently ongoing ALMA development studies. As
of March 18th, 57 scientists from 15 countries have joined the
international SSALMONetwork. Among the affiliations are NRAO, ESO,
NAOJ, the Czech ALMA ARC node at Ondrejov, ESA and many more. Since
March 2015, we are building up expert teams, which work on specific
tasks in preparation of future regular ALMA observations (expected to
start in late 2016) and their interpretation. Registration and more
information at http://www.ssalmon.uio.no.
---------------------------------------------------------
Title: Solar ALMA observations - A revolutionizing new view at our
host star
Authors: Wedemeyer, Sven; Brajsa, Roman; Bastian, Timothy S.; Barta,
Miroslav; Hales, Antonio; Yagoubov, Pavel; Hudson, Hugh; Loukitcheva,
Maria; Fleishman, Gregory
2015IAUGA..2256732W Altcode:
Observations of the Sun with the Atacama Large Millimeter/submillimeter
Array (ALMA) have a large potential for revolutionizing our
understanding of our host star with far reaching implications
for stars in general. The radiation emitted at ALMA wavelengths
originates mostly from the chromosphere - a complex and dynamic layer
between the photosphere and the corona, which plays an important
role in the transport of energy and matter and the heating of the
outer layers of the solar atmosphere.Despite decades of intensive
research, the chromosphere is still elusive and challenging to
observe owing to the complicated formation mechanisms of currently
available diagnostics. ALMA will change the scene substantially as
it serves as a nearly linear thermometer at high spatial, temporal,
and spectral resolution, enabling us to study the complex interaction
of magnetic fields and shock waves and yet-to-be-discovered dynamical
processes. Furthermore, radio recombination and molecular lines
may have great diagnostic potential but need to be investigated
first. These unprecedented capabilities promise important new findings
for a large range of topics in solar physics including the structure,
dynamics and energy balance of quiet Sun regions, active regions and
sunspots, flares and prominences. As a part of ongoing development
studies, an international network has been initiated, which aims at
defining and preparing key solar science with ALMA through simulation
studies: SSALMON -- Solar Simulations for the Atacama Large Millimeter
Observatory Network (http://ssalmon.uio.no). Here, we give an overview
of potential science cases.
---------------------------------------------------------
Title: Radio Coronal Magnetography of a Large Active Region
Authors: Bastian, Timothy S.; Gary, Dale E.; White, Stephen; Fleishman,
Gregory; Chen, Bin
2015TESS....111301B Altcode:
Quantitative knowledge of coronal magnetic fields is fundamental to
understanding energetic phenomena such as solar flares. Flares occur
in solar active regions where strong, non-potential magnetic fields
provide free energy. While constraints on the coronal magnetic field
topology are readily available through high resolution SXR and EUV
imaging of solar active regions, useful quantitative measurements of
coronal magnetic fields have thus far been elusive. Recent progress has
been made at infrared (IR) wavelengths in exploiting both the Zeeman
and Hanle effects to infer the line-of-sight magnetic field strength
or the orientation of the magnetic field vector in the plane of the
sky above the solar limb. However, no measurements of coronal magnetic
fields against the solar disk are possible using IR observations. Radio
observations of gyroresonance emission from active regions offer the
means of measuring coronal magnetic fields above the limb and on the
solar disk. In particular, for plasma plasma conditions in the solar
corona, active regions typically become optically thick to emission
over a range of radio frequencies through gyroresonance absorption
at a low harmonic of the electron gyrofrequency. The specific range
of resonant frequencies depends on the range of coronal magnetic
field strengths present in the active region.The Karl G. Jansky Very
Large Array was used in November 2014 to image NOAA/USAF active region
AR12209 over a continuous frequency range of 1-8 GHz, corresponding to
a wavelength range of 3.75-30 cm. This frequency range is sensitive to
coronal magnetic field strengths ranging from ~120-1400G. The active
region was observed on four different dates - November 18, 20, 22,
and 24 - during which the active region longitude ranged from -15 to
+70 degrees, providing a wide range of aspect angles. In this paper
we provide a preliminary description of the coronal magnetic field
measurements derived from the radio observations.
---------------------------------------------------------
Title: Observation and Modeling of a Termination Shock in a Solar
Eruption as a Possible Particle Accelerator
Authors: Gary, Dale E.; Chen, Bin; Bastian, Timothy S.; Shen, Chengcai;
Krucker, Sam
2015TESS....130402G Altcode:
Solar eruptions and their associated solar flares are the most energetic
particle accelerators in our solar system. Yet the acceleration
mechanism remains uncertain. A possible candidate often invoked in the
standard picture of solar eruptions is a termination shock, produced
by fast reconnection outflows impinging upon dense, closed loops in
a helmet-type geometry. However, the importance of termination shocks
in solar particle acceleration remains controversial, mainly because
there has been no direct detection of such shocks. Here we report direct
imaging of the location and evolution of a termination shock during the
rise phase of a solar eruption. The shock appears at radio wavelengths
as a narrow surface sandwiched between multitudes of downward-moving
plasma blobs and the underlying, newly-reconnected flaring loops,
and evolves coherently with a loop-top hard X-ray source in the shock
downstream region. The shock produces many short-lived, point-like
radio sources, each interpreted as emission from a turbulence cell
interacting with fast (nonthermal) electrons. These point-like radio
sources clearly outline the termination shock front and their positions
change in reaction to the arrival of the fast plasma blobs, which are
well-reproduced by our numerical simulations based on a resistive
magnetohydrodynamics reconnection model in a standard two-ribbon
flare geometry. We further show that a temporary disruption of the
shock coincides with a reduction of radio and hard X-ray emission
associated with the energetic electron population. Our observations
strongly favor a scenario in which the termination shock is responsible
for accelerating electrons to high energies.
---------------------------------------------------------
Title: Solar Observations with the Atacama Large
Millimeter/submillimeter Array (ALMA)
Authors: Bastian, Timothy S.
2015TESS....120323B Altcode:
The Atacama Large Millimeter/Submillimeter Array (ALMA) is a joint
North American, European, and East Asian project that opens the
mm-submm wavelength part of the electromagnetic spectrum for general
astrophysical exploration, providing high-resolution imaging in
frequency bands currently ranging from 84 GHz to 950 GHz (300 microns
to 3 mm). Despite being a general purpose instrument, provisions have
been made to enable solar observations with ALMA. Radiation emitted at
ALMA wavelengths originates mostly from the chromosphere, which plays
an important role in the transport of matter and energy, and the in
heating the outer layers of the solar atmosphere. Despite decades of
research, the solar chromosphere remains a significant challenge:
both to observe, owing to the complicated formation mechanisms of
currently available diagnostics; and to understand, as a result of the
complex nature of the structure and dynamics of the chromosphere. ALMA
has the potential to change the scene substantially as it serves as
a nearly linear thermometer at high spatial and temporal resolution,
enabling us to study the complex interaction of magnetic fields and
shock waves and yet-to-be-discovered dynamical processes. Moreover,
ALMA will play an important role in the study of energetic emissions
associated with solar flares at sub-THz frequencies.In this paper we
describe recent efforts to ensure that ALMA can be usefully exploited
by the scientific community to address outstanding questions in
solar physics. We summarize activities by the ALMA solar development
team comprised of scientists from the East Asia, North America, and
Europe. These activities include instrument testing, development of
calibration and imaging strategies, software requirements development,
and science simulations. Opportunities for the wider community to
contribute to these efforts will be highlighted.
---------------------------------------------------------
Title: Observing the Sun with ALMA: A New Window into Solar Physics
Authors: Bastian, Timothy S.; Shimojo, Masumi; Wedemeyer-Bohm, Sven;
ALMA North American Solar Development Team
2015AAS...22541301B Altcode:
The Atacama Large Millimeter/Submillimeter Array (ALMA) is a joint
North American, European, and East Asian interferometric array that
opens the mm-submm wavelength part of the electromagnetic spectrum for
general astrophysical exploration, providing high resolution imaging
in frequency bands. Despite being a general purpose instrument,
provisions have been made to enable solar observations with ALMA,
thereby offering a new window into solar physics. Radiation emitted
at ALMA wavelengths originates mostly from the chromosphere, which
plays an important role in the transport of energy and matter and the
heating of the outer layers of the solar atmosphere. Despite decades
of intensive research, an understanding of the chromosphere is still
elusive, and challenging to observe owing to the complicated formation
mechanisms of currently available diagnostics. ALMA will change
the scene substantially as it serves as a nearly linear thermometer
at high spatial, temporal, and spectral resolution, enabling us to
study the complex interaction of magnetic fields and shock waves and
yet-to-be-discovered dynamical processes.Moreover, ALMA will play an
important role in the study of energetic emissions associated with
solar flares at sub-THz frequencies.This presentations introduces
ALMA to the solar physcis community and motivates the science that
can be addressed by ALMA using a number of examples based on 3D MHD
simulations. In addition, the means by which ALMA is used to acquire and
calibrate solar observations will be discussed. Finally, we encourage
potential users to join us in further defining and articulating the
exciting science to be explored with this fundamentally new instrument.
---------------------------------------------------------
Title: Constraining Solar Coronal Magnetic Fields with New Radio
Observing Techniques
Authors: Chen, B.; Gary, D. E.; Bastian, T.
2014AGUFMSH23C..07C Altcode:
Solar radio emission, produced by energetic electrons in the low-beta
solar corona, is highly dependent on coronal magnetic field strength
and direction, hence offering a rich potential for constraining coronal
magnetic fields. However, the observed radio intensity is contributed
by several different emission mechanisms and moreover, is known to
be sensitive to parameters other than the magnetic field, such as the
electron distribution function. Collectively, they introduce ambiguities
that are difficult to resolve in the absence of high-quality, broadband
radio dynamic imaging spectroscopic observations with sufficiently high
spatial, spectral, and temporal resolution. Such observations have
not been possible until very recently, thanks to the newly developed
radio instruments such as the Karl G. Jansky Very Large Array (VLA)
and the Expanded Owens Valley Solar Array (EOVSA). In this talk, we
will present first results obtained from the Jansky VLA demonstrating
their ability of constraining the coronal magnetic fields in active
regions. We will also introduce a forward-modeling tool currently under
development, which can be used to reconstruct the 3D coronal magnetic
fields based on the newly available radio observations.
---------------------------------------------------------
Title: VLA Observations of Solar Decimetric Spike Bursts: Direct
Signature of Accelerated Electrons in Reconnection Outflow Region
Authors: Chen, B.; Bastian, T.; Gary, D. E.
2014AGUFMSH23A4149C Altcode:
Solar decimetric spike bursts, which appear in a radio dynamic
spectrum as a cluster of short-lived and narrowband brightenings,
have been suggested as a possible signature of many, "elementary"
particle accelerations at or near a magnetic reconnection site. Their
dynamic spectral feature can be potentially used to diagnose important
parameters of the reconnection site such as plasma density and
spatial size of the fragmentation. Yet direct observational evidence
supporting this scenario has been elusive mainly due to the lack of
imaging observations. The upgraded Karl G. Jansky Very Large Array
(VLA) provides the first opportunity of performing simultaneous radio
imaging and dynamic spectroscopy, which allows radio sources to be
imaged at every spatio-temporal pixel in the dynamic spectrum. Here
we report Jansky VLA observations of decimetric spike bursts recorded
during an eruptive solar limb flare. Combined with EUV and X-ray data
from SDO and RHESSI, we show that the spike bursts coincide spatially
with a loop-top hard X-ray source, which are located in a region where
supra-arcade downflows meet the underlying hot, EUV/X-ray loops. We
interpret the observed spike bursts as a direct signature of non-thermal
electrons accelerated by turbulences and/or shocks in the reconnection
outflow region.
---------------------------------------------------------
Title: Direct Evidence of an Eruptive, Filament-hosting Magnetic
Flux Rope Leading to a Fast Solar Coronal Mass Ejection
Authors: Chen, Bin; Bastian, T. S.; Gary, D. E.
2014ApJ...794..149C Altcode: 2014arXiv1408.6473C
Magnetic flux ropes (MFRs) are believed to be at the heart of solar
coronal mass ejections (CMEs). A well-known example is the prominence
cavity in the low corona that sometimes makes up a three-part
white-light (WL) CME upon its eruption. Such a system, which is
usually observed in quiet-Sun regions, has long been suggested to
be the manifestation of an MFR with relatively cool filament material
collecting near its bottom. However, observational evidence of eruptive,
filament-hosting MFR systems has been elusive for those originating
in active regions. By utilizing multi-passband extreme-ultraviolet
(EUV) observations from Solar Dynamics Observatory/Atmospheric Imaging
Assembly, we present direct evidence of an eruptive MFR in the low
corona that exhibits a hot envelope and a cooler core; the latter is
likely the upper part of a filament that undergoes a partial eruption,
which is later observed in the upper corona as the coiled kernel of a
fast, WL CME. This MFR-like structure exists more than 1 hr prior to
its eruption, and displays successive stages of dynamical evolution, in
which both ideal and non-ideal physical processes may be involved. The
timing of the MFR kinematics is found to be well correlated with the
energy release of the associated long-duration C1.9 flare. We suggest
that the long-duration flare is the result of prolonged energy release
associated with the vertical current sheet induced by the erupting MFR.
---------------------------------------------------------
Title: Probing Magnetic Energy Release in a Solar Flare with Radio
Dynamic Imaging Spectroscopy
Authors: Chen, Bin; Bastian, T. S.; Gary, D. E.; White, S. M.
2014AAS...22311804C Altcode:
Solar flares involve sudden release of magnetic energy that is
previously stored in the Sun's corona. Yet details of the flare
energy release processes are still poorly understood. Solar radio
bursts are intense and short-lived radio emissions that occur in
solar flares. They are believed to be intimately related to flare
energy release processes. However, their potential in diagnosing flare
energy release has been greatly limited by the lack of simultaneous
spatial information. The upgraded Karl G. Jansky Very Large Array (VLA)
provides the first opportunity of radio synthesis imaging along with
high spectral and temporal resolution, making the new technique of radio
dynamic imaging spectroscopy possible. We report VLA observations of a
solar flare event using this new technique, during which a rich variety
of radio bursts are recorded. With the help of concurrent data in
extreme ultra-violet and X-ray wavelengths, these observations allow us
to establish the relation between the bursts and flare energy release,
and use them to probe physical properties of the energy release site.
---------------------------------------------------------
Title: Preface
Authors: Tomczyk, Steven; Zhang, Jie; Bastian, Timothy; Leibacher,
John W.
2013SoPh..288..463T Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A view from the ground: Next generation instrumentation for
solar and heliospheric physics
Authors: Bastian, T. S.
2013AIPC.1539..442B Altcode:
The solar and space physics community has recently completed its second
decadal survey under the auspices of the National Research Council. An
integrated strategy for ground and space based studies of the Sun and
space physics has been recommended, with specific recommendations made
regarding new instrumentation, programs, and facilities. The ground
based component of these recommendations is briefly reviewed here:
the Advanced Technology Solar Telescope (ATST), the Frequency Agile
Solar Radiotelescope (FASR), and the Coronal Solar Magnetism Observatory
(COSMO). Although not considered as part of the decadal portfolio, but
of which the community should nevertheless be aware, are the Atacama
Large Millimeter/submillimeter Array (ALMA) and the Jansky Very Large
Array (VLA). Several additional instruments are briefly mentioned as
pathfinders for those instruments recommended by the decadal survey,
including the Coronal Multichannel Polarimeter (CoMP) and the Expanded
Owens Valley Solar Array (EOVSA). The planned instruments discussed
offer uniquely powerful observations of emissions that originate from
the photosphere to well out into the solar wind. As such, they provide
observations that are highly complementary to space based missions
such as Solar Probe Plus and Solar Orbiter. The status and science
goals of the recommended instruments are briefly reviewed.
---------------------------------------------------------
Title: Solar flares at submillimeter wavelengths
Authors: Krucker, Säm; Giménez de Castro, C. G.; Hudson, H. S.;
Trottet, G.; Bastian, T. S.; Hales, A. S.; Kašparová, J.; Klein,
K. -L.; Kretzschmar, M.; Lüthi, T.; Mackinnon, A.; Pohjolainen, S.;
White, S. M.
2013A&ARv..21...58K Altcode:
We discuss the implications of the first systematic observations of
solar flares at submillimeter wavelengths, defined here as observing
wavelengths shorter than 3 mm (frequencies higher than 0.1 THz). The
events observed thus far show that this wave band requires a new
understanding of high-energy processes in solar flares. Several events,
including observations from two different observatories, show during
the impulsive phase of the flare a spectral component with a positive
(increasing) slope at the highest observable frequencies (up to 405
GHz). To emphasize the increasing spectra and the possibility that
these events could be even more prominent in the THz range, we term
this spectral feature a "THz component". Here we review the data and
methods, and critically assess the observational evidence for such
distinct component(s). This evidence is convincing. We also review the
several proposed explanations for these feature(s), which have been
reported in three distinct flare phases. These data contain important
clues to flare development and particle acceleration as a whole, but
many of the theoretical issues remain open. We generally have lacked
systematic observations in the millimeter-wave to far-infrared range
that are needed to complete our picture of these events, and encourage
observations with new facilities.
---------------------------------------------------------
Title: Tracing Electron Beams in the Sun's Corona with Radio Dynamic
Imaging Spectroscopy
Authors: Chen, Bin; Bastian, T. S.; White, S. M.; Gary, D. E.; Perley,
R.; Rupen, M.; Carlson, B.
2013ApJ...763L..21C Altcode: 2012arXiv1211.3058C
We report observations of type III radio bursts at decimeter wavelengths
(type IIIdm bursts)—signatures of suprathermal electron beams
propagating in the low corona—using the new technique of radio dynamic
imaging spectroscopy provided by the recently upgraded Karl G. Jansky
Very Large Array. For the first time, type IIIdm bursts were imaged with
high time and frequency resolution over a broad frequency band, allowing
electron beam trajectories in the corona to be deduced. Together with
simultaneous hard X-ray and extreme ultraviolet observations, we show
that these beams emanate from an energy release site located in the
low corona at a height below ~15 Mm, and propagate along a bundle of
discrete magnetic loops upward into the corona. Our observations enable
direct measurements of the plasma density along the magnetic loops,
and allow us to constrain the diameter of these loops to be less than
100 km. These overdense and ultra-thin loops reveal the fundamentally
fibrous structure of the Sun's corona. The impulsive nature of the
electron beams, their accessibility to different magnetic field lines,
and the detailed structure of the magnetic release site revealed by
the radio observations indicate that the localized energy release is
highly fragmentary in time and space, supporting a bursty reconnection
model that involves secondary magnetic structures for magnetic energy
release and particle acceleration.
---------------------------------------------------------
Title: Observing the Sun at Radio Wavelengths: Current Status and
Future Prospects
Authors: Bastian, T.; Gary, D.
2012IAUSS...6E.215B Altcode:
Radio emission from the Sun offers the means of probing thermal
and non-thermal processes in the chromosphere and corona using
diagnostics that are largely complementary to those available at optical
wavelengths. This talk briefly reviews radio diagnostics and observing
techniques and several of the science objectives motivating new and
planned instrumentation, including coronal magnetography, magnetic
energy release and particle acceleration, drivers of space weather, and
the quiet solar atmosphere. New instrumentation includes ALMA, opening
a new spectral window at millimeter and submillimeter wavelengths; and
the Jansky Very Large Array (JVLA), a powerful new instrument operating
at centimeter and decimeter wavelengths. A solar-dedicated instrument
under construction is the Expanded Owens Valley Solar Array (EOVSA);
and a planned, new, solar-dedicated facility is the Frequency Agile
Solar Radiotelescope (FASR). EOVSA will soon pioneer dynamic imaging
spectroscopy techniques. FASR will fully exploit these techniques as
a general purpose radioheliograph designed to perform ultra-wideband
dynamic imaging spectroscopy.
---------------------------------------------------------
Title: The Role of Inverse Compton Scattering in Solar Coronal Hard
X-Ray and γ-Ray Sources
Authors: Chen, Bin; Bastian, T. S.
2012ApJ...750...35C Altcode: 2011arXiv1108.0131C
Coronal hard X-ray (HXR) and continuum γ-ray sources associated with
the impulsive phase of solar flares have been the subject of renewed
interest in recent years. They have been interpreted in terms of
thin-target, non-thermal bremsstrahlung emission. This interpretation
has led to rather extreme physical requirements in some cases. For
example, in one case, essentially all of the electrons in the source
must be accelerated to non-thermal energies to account for the coronal
HXR source. In other cases, the extremely hard photon spectra of
the coronal continuum γ-ray emission suggest that the low-energy
cutoff of the electron energy distribution lies in the MeV energy
range. Here, we consider the role of inverse Compton scattering (ICS)
as an alternate emission mechanism in both the ultra- and mildly
relativistic regimes. It is known that relativistic electrons are
produced during powerful flares; these are capable of upscattering
soft photospheric photons to HXR and γ-ray energies. Previously
overlooked is the fact that mildly relativistic electrons, generally
produced in much greater numbers in flares of all sizes, can upscatter
extreme-ultraviolet/soft X-ray photons to HXR energies. We also
explore ICS on anisotropic electron distributions and show that the
resulting emission can be significantly enhanced over an isotropic
electron distribution for favorable viewing geometries. We briefly
review results from bremsstrahlung emission and reconsider circumstances
under which non-thermal bremsstrahlung or ICS would be favored. Finally,
we consider a selection of coronal HXR and γ-ray events and find that
in some cases the ICS is a viable alternative emission mechanism.
---------------------------------------------------------
Title: Radio Spectroscopic Imaging of Electron Beams in the Solar
Corona
Authors: Bastian, Timothy S.; Chen, B.
2012AAS...22020443B Altcode:
The recently upgraded Jansky Very Large Array was used to observe the
radio emission from a C class solar flare. Observations were performed
from 1-2 GHz with a spectral resolution of 1 MHz and time resolution
of 100 ms. A number of fast-drift, type-III-like radio bursts was
observed, the result of nonthermal electron beams propagating from the
flare site, guided by the coronal magnetic field. Using these dynamic,
imaging, spectroscopic observations, the electron beam trajectories
are deduced.Implications are briefly discussed.
---------------------------------------------------------
Title: The Role of Inverse Compton Scattering in Solar Coronal Hard
X-ray and γ-ray Sources
Authors: Chen, B.; Bastian, T. S.
2011AGUFMSH44A..05C Altcode:
We consider the role of inverse Compton scattering (ICS) as a means of
producing coronal hard X-ray (HXR) and continuum γ-ray sources during
solar flares. Coronal HXR and continuum γ-ray emission observed during
solar flares has been interpreted in terms of thin-target bremsstrahlung
emission. In one case, this interpretation leads to the conclusion that
the number of energetic electrons required to account for the coronal
HXR source must be large, implying that essentially all electrons in the
source must be accelerated to energies ≥ 16 keV. In other cases, the
spectral index of the photon spectrum of γ-ray sources approaches the
theoretical limit for bremsstrahlung emission (α ~ 1.5 - 2). Here we
investigate ICS in both the fully relativistic and mildly relativistic
regimes as an alternative to non-thermal bremsstrahlung. It is known
that relativistic electrons are produced during powerful flares;
these are capable of up-scattering soft photospheric photons to HXR
energies. Previously overlooked is the fact that mildly relativistic
electrons, generally produced in much greater numbers in flares of
all sizes, can up-scatter EUV/SXR photons to HXR energies. We also
explore ICS on anisotropic electron distributions and show that
the resulting emission can be significantly enhanced over the case
of scattering on an isotropic electron distribution. We apply our
results to a selection of coronal HXR and γ-ray events and find that
in some cases the ICS mechanism is a viable alternative to non-thermal
bremsstrahlung emission, particularly if the energetic population of
electrons responsible for the emission is anisotropic.
---------------------------------------------------------
Title: Spatially and Spectrally Resolved Observations of a Zebra
Pattern in a Solar Decimetric Radio Burst
Authors: Chen, Bin; Bastian, T. S.; Gary, D. E.; Jing, Ju
2011ApJ...736...64C Altcode: 2011arXiv1105.0715C
We present the first interferometric observation of a zebra-pattern
radio burst with simultaneous high spectral (≈1 MHz) and high time
(20 ms) resolution. The Frequency-Agile Solar Radiotelescope Subsystem
Testbed (FST) and the Owens Valley Solar Array (OVSA) were used in
parallel to observe the X1.5 flare on 2006 December 14. By using
OVSA to calibrate the FST, the source position of the zebra pattern
can be located on the solar disk. With the help of multi-wavelength
observations and a nonlinear force-free field extrapolation, the zebra
source is explored in relation to the magnetic field configuration. New
constraints are placed on the source size and position as a function
of frequency and time. We conclude that the zebra burst is consistent
with a double-plasma resonance model in which the radio emission occurs
in resonance layers where the upper-hybrid frequency is harmonically
related to the electron cyclotron frequency in a coronal magnetic loop.
---------------------------------------------------------
Title: Uncovering Mechanisms of Coronal Magnetism via Advanced 3D
Modeling of Flares and Active Regions
Authors: Fleishman, Gregory; Gary, Dale; Nita, Gelu; Alexander,
David; Aschwanden, Markus; Bastian, Tim; Hudson, Hugh; Hurford,
Gordon; Kontar, Eduard; Longcope, Dana; Mikic, Zoran; DeRosa, Marc;
Ryan, James; White, Stephen
2010arXiv1011.2800F Altcode:
The coming decade will see the routine use of solar data of
unprecedented spatial and spectral resolution, time cadence, and
completeness. To capitalize on the new (or soon to be available)
facilities such as SDO, ATST and FASR, and the challenges they present
in the visualization and synthesis of multi-wavelength datasets,
we propose that realistic, sophisticated, 3D active region and flare
modeling is timely and critical, and will be a forefront of coronal
studies over the coming decade. To make such modeling a reality, a
broad, concerted effort is needed to capture the wealth of information
resulting from the data, develop a synergistic modeling effort, and
generate the necessary visualization, interpretation and model-data
comparison tools to accurately extract the key physics.
---------------------------------------------------------
Title: Radiative signatures of energetic particles
Authors: Bastian, Tim
2010hssr.book...79B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Spatially and Spectrally Resolved Observations of a "Zebra”
Solar Radio Burst
Authors: Bastian, Timothy S.; Chen, B.; Gary, D. E.
2010AAS...21542201B Altcode: 2010BAAS...42..291B
The FASR Subsystems Testbed (FST) is a frequency-agile three-element
interferometer located at the Owens Valley Radio Observatory in
California. A frequency band of 500 MHz can be dynamically selected
within the 1-9 GHz frequency FST operating range. The signal from each
antenna is sampled at 1 Gsps and written to disk. The full-resolution
time-domain data are then correlated offline to produce amplitude
and phase spectra on three interferometric baselines. The FST was
used on 14 December 2006 to observe the GOES X1.5 soft X-ray flare in
NOAA/USAF active region 10930 at S06W46. The FST observed the event
between 1.0-1.5 GHz with a time resolution of 20 ms and a frequency
resolution of approximately 1 MHz, time sharing between observations
sensitive to right- and left-circularly polarized radiation. A variety
of coherent radio bursts was observed, including a highly circularly
polarized "Zebra” burst characterized by 7-10 regularly spaced bands
of emission in the dynamic spectrum. With new constraints available
on the source size and the relative source position as a function
of frequency, the double-plasma resonance model is explored, wherein
emission in a given band occurs at the upper hybrid frequency that is,
in turn, harmonically related to the local electron cyclotron frequency.
---------------------------------------------------------
Title: Interplanetary Type II Radio Bursts and the Role of Synchrotron
Radiation
Authors: Chen, Bin; Bastian, T. S.
2009SPD....41.3709C Altcode:
Type II radio bursts are a type of slow drift radio emission associated
with shocks in the solar corona and in the interplanetary medium. It
is widely assumed that interplanetary (IP) type II radio bursts are the
result of plasma radiation from Langmuir waves excited by suprathermal
electrons in the upstream region of a shock driven by a fast coronal
mass ejection (CME). Alternatively, some IP type II events may instead
be the result of synchrotron radiation from 1 MeV electrons. We present
a sample of fast-CME/IP-type-II events characterized by smoothly varying
emission and large frequency bandwidths and consider the implications
of both the plasma radiation hypothesis and the synchrotron emission
hypothesis.
---------------------------------------------------------
Title: Digital Instrumentation for the Radio Astronomy Community
Authors: Parsons, Aaron; Werthimer, Dan; Backer, Donald; Bastian, Tim;
Bower, Geoffrey; Brisken, Walter; Chen, Henry; Deller, Adam; Filiba,
Terry; Gary, Dale; Greenhill, Lincoln; Hawkins, David; Jones, Glenn;
Langston, Glen; Lasio, Joseph; Van Leeuwen, Joeri; Mitchell, Daniel;
Manley, Jason; Siemion, Andrew; So, Hayden Kwok-Hay; Whitney, Alan;
Woody, Dave; Wright, Melvyn; Zarb-Adami, Kristian
2009astro2010T..21P Altcode: 2009arXiv0904.1181P
Time-to-science is an important figure of merit for digital
instrumentation serving the astronomical community. A digital
signal processing (DSP) community is forming that uses shared
hardware development, signal processing libraries, and instrument
architectures to reduce development time of digital instrumentation and
to improve time-to-science for a wide variety of projects. We suggest
prioritizing technological development supporting the needs of this
nascent DSP community. After outlining several instrument classes
that are relying on digital instrumentation development to achieve
new science objectives, we identify key areas where technologies
pertaining to interoperability and processing flexibility will reduce
the time, risk, and cost of developing the digital instrumentation for
radio astronomy. These areas represent focus points where support of
general-purpose, open-source development for a DSP community should
be prioritized in the next decade. Contributors to such technological
development may be centers of support for this DSP community, science
groups that contribute general-purpose DSP solutions as part of their
own instrumentation needs, or engineering groups engaging in research
that may be applied to next-generation DSP instrumentation.
---------------------------------------------------------
Title: Magnetospheric Emissions from Extrasolar Planets
Authors: Lazio, J.; Bastian, T.; Bryden, G.; Farrell, W. M.;
Griessmeier, J. -M.; Hallinan, G.; Kasper, J.; Kuiper, T.; Lecacheux,
A.; Majid, W.; Osten, R.; Shklonik, E.; Stevens, I.; Winterhalter,
D.; Zarka, P.
2009astro2010S.177L Altcode: 2009arXiv0903.0873L
The magnetospheric emissions from extrasolar planets represent a science
frontier for the next decade. All of the solar system giant planets and
the Earth produce radio emissions as a result of interactions between
their magnetic fields and the solar wind. In the case of the Earth,
its magnetic field may contribute to its habitability by protecting
its atmosphere from solar wind erosion and by preventing energetic
particles from reaching its surface. Indirect evidence for at least
some extrasolar giant planets also having magnetic fields includes
the modulation of emission lines of their host stars phased with the
planetary orbits, likely due to interactions between the stellar and
planetary magnetic fields. If magnetic fields are a generic property
of giant planets, then extrasolar giant planets should emit at radio
wavelengths allowing for their direct detection. Existing observations
place limits comparable to the flux densities expected from the
strongest emissions. Additional sensitivity at low radio frequencies
coupled with algorithmic improvements likely will enable a new means
of detection and characterization of extrasolar planets within the
next decade.
---------------------------------------------------------
Title: Particle Acceleration and Transport on the Sun
Authors: Bastian, T. S.; Emslie, G.; Fleishman, G.; Gary, D. E.;
Holman, G.; Hudson, H.; Hurford, G.; Krucker, S.; Lee, J.; Miller,
J.; White, S.
2009astro2010S..13B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Magnetically driven activity in the solar corona: a path to
understanding the energetics of astrophysical plasmas
Authors: Gibson, Sarah; Bastian, Tim; Lin, Haoscheng; Low, B. C.;
Tomczyk
2009astro2010S..94G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Coronal "Dark Energy" and Solar/Stellar Activity
Authors: White, Stephen; Bastian, Tim; Judge, Phil; Lin, Haosheng
2009astro2010S.315W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Broadband Quasi-periodic Radio and X-Ray Pulsations in a
Solar Flare
Authors: Fleishman, Gregory D.; Bastian, T. S.; Gary, Dale E.
2008ApJ...684.1433F Altcode: 2008arXiv0804.4037F
We describe microwave and hard X-ray observations of strong
quasi-periodic pulsations from the GOES X1.3 solar flare on 2003
June 15. The radio observations were made jointly by the Owens
Valley Solar Array (OVSA), the Nobeyama Polarimeter (NoRP), and
the Nobeyama Radioheliograph (NoRH). Hard X-ray observations were
made by RHESSI. Using Fourier analysis, we study the frequency-
and energy-dependent oscillation periods, differential phase, and
modulation amplitudes of the radio and X-ray pulsations. Focusing on
the more complete radio observations, we also examine the modulation
of the degree of circular polarization and of the radio spectral
index. The observed properties of the oscillations are compared
with those derived from two simple models for the radio emission. In
particular, we explicitly fit the observed modulation amplitude data
to the two competing models. The first model considers the effects of
MHD oscillations on the radio emission. The second model considers
the quasi-periodic injection of fast electrons. We demonstrate that
quasi-periodic acceleration and injection of fast electrons is the
more likely cause of the quasi-periodic oscillations observed in the
radio and hard X-ray emission, which has important implications for
particle acceleration and transport in the flaring sources.
---------------------------------------------------------
Title: Ultrahigh Time Resolution Observations of Radio Bursts on
AD Leonis
Authors: Osten, Rachel A.; Bastian, T. S.
2008ApJ...674.1078O Altcode: 2007arXiv0710.5881O
We report observations of a radio burst that occurred on the flare
star AD Leonis over a frequency range of 1120-1620 MHz (λ ≈ 18-27
cm). These observations, made by the 305 m telescope of the Arecibo
Observatory, are unique in providing the highest time resolution
(1 ms) and broadest spectral coverage (Δ ν/ν = 0.36) of a stellar
radio burst yet obtained. The burst was observed on 2005 April 9. It
produced a peak flux density of ~500 mJy, and it was essentially
100% right-circularly polarized. The dynamic spectrum shows a rich
variety of structure: patchy emission, diffuse bands, and narrowband,
fast-drift striae. Focusing our attention on the fast-drift striae,
we consider the possible role of dispersion, and find that it requires
rather special conditions in the source to be a significant factor. We
suggest that the emission may be due to the cyclotron maser instability,
a mechanism known to occur in planetary magnetospheres. We briefly
explore possible implications of this possibility.
---------------------------------------------------------
Title: Radio Spectral Evolution of an X-Ray-poor Impulsive Solar
Flare: Implications for Plasma Heating and Electron Acceleration
Authors: Bastian, T. S.; Fleishman, G. D.; Gary, D. E.
2007ApJ...666.1256B Altcode: 2007arXiv0704.2413B
We present radio and X-ray observations of an impulsive solar flare
that was moderately intense in microwaves, yet showed very meager EUV
and X-ray emission. The flare occurred on 2001 October 24 and was well
observed at radio wavelengths by the Nobeyama Radioheliograph (NoRH),
the Nobeyama Radio Polarimeters (NoRP), and the Owens Valley Solar Array
(OVSA). It was also observed in EUV and X-ray wavelength bands by the
TRACE, GOES, and Yohkoh satellites. We find that the impulsive onset of
the radio emission is progressively delayed with increasing frequency
relative to the onset of hard X-ray emission. In contrast, the time
of flux density maximum is progressively delayed with decreasing
frequency. The decay phase is independent of radio frequency. The
simple source morphology and the excellent spectral coverage at radio
wavelengths allowed us to employ a nonlinear χ<SUP>2</SUP>-minimization
scheme to fit the time series of radio spectra to a source model that
accounts for the observed radio emission in terms of gyrosynchrotron
radiation from MeV-energy electrons in a relatively dense thermal
plasma. We discuss plasma heating and electron acceleration in view
of the parametric trends implied by the model fitting. We suggest
that stochastic acceleration likely plays a role in accelerating the
radio-emitting electrons.
---------------------------------------------------------
Title: Radio emission from the Sun, planets, and the interplanetary
medium
Authors: Bastian, Timothy S.
2007HiA....14..362B Altcode:
A brief review is given of radio phenomena on the Sun, the planet
Jupiter, and the interplanetary medium and its outer boundary. A brief
aside is made to draw parallels between radio emission from Jupiter
and extrasolar planets.
---------------------------------------------------------
Title: Synchrotron Radio Emission from a Fast Halo Coronal Mass
Ejection
Authors: Bastian, T. S.
2007ApJ...665..805B Altcode: 2007arXiv0704.3108B
An interplanetary (IP) type II-like radio burst is analyzed. It
occurred on 2003 June 17-18 in association with a fast halo coronal
mass ejection (CME), an M6.8 soft-X-ray (SXR) flare, and it produced
a solar proton event. Unlike coronal type II bursts and the majority
of IP type II radio emissions, the IP type-II-like event associated
with the fast halo CME on June 17-18 varies smoothly in time and
frequency and has a frequency bandwidth that is several times larger
than is typical for coronal and IP type II emissions. Moreover, the
frequency change with time is inconsistent with that expected from
plasma radiation associated with a CME-driven shock. I suggest that
this IP type-II-like event, referred to here as an IP type II-S event,
is not due to plasma radiation, but instead to incoherent synchrotron
radiation from near-relativistic electrons entrained in the CME magnetic
field or in the sheath region between the shock and the CME driver. This
event may be an example of a new and distinct class of interplanetary
radio phenomenon.
---------------------------------------------------------
Title: Synchrotron Radiation From A Fast Halo CME
Authors: Bastian, Timothy S.
2007AAS...210.2922B Altcode: 2007BAAS...39..141B
An interplanetary (IP) type-II-like radio burst observed by WIND/WAVES
is analyzed. It occurred on 2003 June 17 in association with a fast halo
coronal mass ejection (CME), an M6.8 soft-X-ray (SXR) flare, and a solar
proton event. Unlike coronal type II radio bursts and majority of IP
type II radio emissions, the IP type-II-like event associated with the
fast halo CME varies smoothly in time and frequency and has a frequency
bandwidth that is several times larger than is typical for coronal
and IP type II emissions. Moreover, the frequency change with time is
inconsistent with that expected from plasma radiation associated with a
CME-driven shock. I suggest that this IP type-II-like event, referred
to here as an IP type II-S event, is not due to plasma radiation but,
rather, incoherent synchrotron radiation from relativistic electrons
entrained in the CME magnetic field, or in the sheath region between
the shock and the CME driver. This event may be an example of a new
and distinct class of interplanetary radio phenomenon.
---------------------------------------------------------
Title: The FASR Reference Instrument
Authors: Bastian, Timothy S.; FASR Design Team
2007AAS...210.8203B Altcode: 2007BAAS...39..191B
The Frequency Agile Solar Radiotelescope (FASR) is a next-generation
radioheliograph designed to perform dynamic broadband imaging
spectroscopy over a frequency range of 50 MHz to 20 GHz. It will do
so with high time, frequency, and angular resolution. Currently in
the design stage, it will address an extremely broad science program,
including coronal magnetography, energetic phenomena such as flares
and coronal mass ejections, the corona and inner heliosphere, and
space weather. This paper introduces the FASR reference instrument
and presents the operations plan. The current project status is also
discussed.
---------------------------------------------------------
Title: Progress in Understanding Radio Emission from Solar Flares:
Observations with the Nobeyama Radioheliograph 1999-2004
Authors: Bastian, T. S.
2006spnr.conf....3B Altcode:
Studies of solar flares based on observations by the Nobeyama
Radioheliograph and related instruments such as the Owens Valley
Solar Array, the Nançay Radioheliograph, and the Solar Submillimeter
Telescope, as well as supporting instruments such as the RHESSI, TRACE,
and SOHO missions, are reviewed for the period 1999-2004. Work on
the classification of solar radio bursts is briefly discussed. Recent
observational work on loop-top radio sources and rapidly propagating
emission signatures is summarized and its interpretation in terms of
electron anisotropies is described. Possible acceleration processes
yielding such anisotropies are also briefly discussed. Recent work
on electron acceleration and transport in a relatively dense plasma
environment is summarized. Finally, recent work at millimeter and
submillimeter wavelengths is described.
---------------------------------------------------------
Title: Magnetic Field Measurements in Flare Loops and CMEs
Authors: Bastian, T. S.
2006ASPC..358..173B Altcode:
Quantitative measurements of magnetic fields in flares and CMEs are
critical to understanding both phenomena. In this paper we briefly
explore the use of radio observations to measure coronal fields. While
several radio diagnostics exist, we focus on gyrosynchrotron radiation
from energetic electrons in the source. It is shown that with broadband
imaging spectroscopy at decimeter and centimeter wavelengths, the
magnetic field can be extracted from the data along with a number
of other important physical parameters. It is expected that coronal
magnetography will be a key new capability of the Frequency Agile
Solar Radiotelescope (FASR), an instrument currently under development.
---------------------------------------------------------
Title: Fast-Drifting Radio Bursts Seen on the Flare Star AD Leo with
the Arecibo Observatory
Authors: Osten, Rachel A.; Bastian, T.
2006AAS...209.2914O Altcode: 2006BAAS...38..943O
We report on observations of two radio bursts at wavelengths 18--27
cm seen on the flare star AD Leonis with the Arecibo Observatory in
April 2005. These observations are unique in providing the highest
time resolution (1 ms) and largest instantaneous contiguous frequency
coverage (Δ ν 400 MHz) of stellar radio bursts. The second radio
burst on April 9, 2005 near 01:14:50 UT, exhibited a combination
of fast-drifting radio bursts and diffuse emission. A comparison
of this data degraded to 10 ms time resolution with previous radio
bursts (reported in Osten & Bastian 2006) reveals numerous
similarities. However, when examined at the highest time resolution,
the drifting radio bursts have predominantly negative slopes (the
bursts appear at high frequency and drift to lower frequencies), with
a characteristic drift rate of 3 GHz/s. We discuss this new finding
in light of possible coherent emission mechanisms, and suggest that
these properties are consistent with plasma radiation from beams of
accelerated particles traversing AD Leo's low corona.
---------------------------------------------------------
Title: Radio Diagnostics of Magnetic Fields in the Solar Corona
Authors: Bastian, T. S.
2006IAUJD...3E..93B Altcode:
Measurement of the coronal magnetic field has been an elusive goal
for many years. In recent years, however, progress has been made
in developing techniques at radio and IR wavelengths to measure or
constrain the magnetic field in the quiet and active corona. In this
talk, radio diagnostic techniques are reviewed in a variety of contexts,
including the quiet corona, active regions, flares, and coronal mass
ejections. Techniques that exploit emission intrinsic to the phenomena
themselves - e.g., gyrosynchrotron radiation - and those which exploit
external probes such as spacecraft beacons or extragalactic background
sources are both discussed. Prospects for exploiting these techniques
with future generations of radio instrumentation are also considered.
---------------------------------------------------------
Title: Low Frequency Radiophysics of the Sun and Heliosphere
Authors: Bastian, T. S.
2006IAUJD..12E...1B Altcode:
The solar corona and heliosphere offer rich environments for
exploration via low frequency radio techniques. Recent progress in
observing and understanding both direct and indirect radio phenomena
associated with flares and coronal mass ejections is discussed. In
the case of flares, radio diagnostics of coronal energy release and
particle acceleration are discussed. In the case of coronal mass
ejections (CMEs), direct and indirect signatures of their initiation
and acceleration are discussed. Magnetic field measurements of CMEs
using direct measurements of their incoherent synchrotron radiation,
or using observations of Faraday rotation via trans-illumination of
the CME using background sidereal sources or spacecraft beacons, is
also discussed. Finally, recent work on theory and observations of type
II radio bursts, signatures of interplanetary shocks driven by CMEs ,
is briefly reviewed.
---------------------------------------------------------
Title: Quasi-periodic Radio And X-ray Pulsations In A Solar Flare
Authors: Bastian, Timothy S.; Fleishman, G.; Gary, D. E.
2006SPD....37.1309B Altcode: 2006BAAS...38..242B
We describe microwave and hard X-ray observations of strong
quasiperiodic pulsations from the GOES X1.3 solar flare on 15
June 2003. Using Fourier analysis, we study the frequency- and
energy-dependent oscillation periods, phase differences, and modulation
depths of the radio and X-ray pulsations. Focusing on the more complete
radio observations, the observational properties of the oscillations
are compared with those derived from models for the radio emission. Two
simple models are considered: i) gyrosynchrotron emission modulated
by the quasiperiodic injection of fast electrons; ii) gyrosynchrotron
emission modulated by an MHD oscillation of the magnetic field. We
demonstrate that quasiperiodic injection of fast electrons is the more
likely cause of the observed quasiperiodic oscillations observed in
the radio and hard X-ray emission. We discuss the implications of this
finding for particle acceleration and transport in the flaring sources.
---------------------------------------------------------
Title: Imaging Observations of a Very High Frequency Type II Burst
Authors: White, S. M.; Mercier, C.; Bradley, R.; Bastian, T.; Kerdraon,
A.; Pick, M.
2006AGUSMSH24A..05W Altcode:
A remarkable Type II burst was detected by the high-frequency system of
the Green Bank Solar Radio Burst Spectrometer on 2005 November 14. The
harmonic branch of the Type II extended up to 800 MHz, making it one
of the highest frequency Type II bursts ever detected, but it failed
to propagate to heights corresponding to frequencies below 100 MHz. At
such high frequencies, it implies the formation of a shock relatively
low in the corona. No coronal mass ejection was evident in the LASCO
data for this east limb event. It is one of the few Type II bursts to
be observable at every frequency of observation of the Nancay Radio
Heliograph (164-432 MHz). Here we present analysis of images of the
event, including simultaneous imaging of the fundamental and harmonic
branches.
---------------------------------------------------------
Title: The FASR Reference Instrument
Authors: Bastian, T. S.; Gary, D. E.; Hurford, G. J.; Kawakubo, H.;
Ruf, C.; White, S. M.; Zurbuchen, T.
2006AGUSMSH33A..08B Altcode:
The Frequency Agile Solar Radiotelescope (FASR) is a unique,
solar-dedicated radio facility slated for completion by 2012. The
instrument will address an extremely broad range of solar and
space weather science, including routine measurement of coronal
magnetic fields, imaging coronal mass ejections near the solar
surface, quantitative diagnostics of energy release and particle
acceleration in flares, and the extension of the solar corona into
the heliosphere. Although the precise details of the FASR design
are still being developed, we present for the first time a complete
high-level design referred to as the FASR Reference Instrument. The
Reference Instrument meets the science requirements and will serve
as the basis for cost estimates for construction and operation of
the instrument. This paper gives an overview of the FASR Reference
Instrument, describes the science goals and objectives, and gives the
flowdown of science goals to engineering specifications. The innovative
aspects of the FASR design are highlighted, and a complete, end-to-end
description of the instrument is given. The instrument operations plan
is also discussed.
---------------------------------------------------------
Title: Coronal Mass Ejections and their Associated Radio Emissions
Authors: Bastian, T.
2006AGUSMSH24A..02B Altcode:
Coronal mass ejections (CMEs) are important drivers of processes in
the interplanetary medium and the near- Earth environment, including
shocks, CMEs, and geomagnetic disturbances. Radio observations of CMEs
and associated phenomena offer a number of unique diagnostics of CMEs
and their environment in the nascent stages of their development. These
include thermal free-free radiation, nonthermal synchrotron radiation,
and plasma radiation. These emissions can be used to constrain
the plasma properties of CMEs - including the plasma density and
magnetic field - as they propagate outward from the Sun. Under certain
conditions, they can probe the response of the corona to the CME. Some
recent and suggestive observations are presented of radio emission
from CMEs, as well as of associated radio bursts.
---------------------------------------------------------
Title: Wide-Band Spectroscopy of Two Radio Bursts on AD Leonis
Authors: Osten, Rachel A.; Bastian, T. S.
2006ApJ...637.1016O Altcode: 2005astro.ph..9815O
We report high time-resolution broadband spectroscopic observations of
two radio bursts on the classical flare star AD Leonis. The observations
were acquired by the 305 m telescope at Arecibo Observatory on 2003 June
13-14. Using the Wideband Arecibo Pulsar Processor, these observations
sampled a total bandwidth of 400 MHz, distributed over a 500 MHz
frequency range, 1120-1620 MHz, with a frequency resolution of 0.78 MHz
and a time resolution of 10 ms. The radio burst observed on June 13 is
characterized by the presence of multitudes of short-duration (Δt~30
ms), high brightness temperature (T<SUB>b</SUB>>10<SUP>14</SUP> K),
highly circularly polarized, fast-drift radio sub-bursts, with median
bandwidths Δν/ν~5%. The inverse drift rates are small and have a
symmetric distribution (both positive and negative frequency drifts),
with a Gaussian FWHM inverse drift rate of 4.5×10<SUP>-4</SUP> s
MHz<SUP>-1</SUP>. The fast-drift sub-bursts occur at a mean rate of
13 s<SUP>-1</SUP> and show no evidence for periodic recurrence. The
fast-drift radio events on AD Leo are highly reminiscent of solar
decimetric spike bursts. We suggest that the emission is due to
fundamental plasma radiation. A second highly circularly polarized
radio burst, recorded June 14, has markedly different properties: a
smoothly varying intensity profile characterized by a slow drift in
frequency with time (-52 MHz s<SUP>-1</SUP>). Under the assumption
that the source is due to a disturbance propagating through the low
corona, a source size of 0.1-1 R<SUB>*</SUB> is inferred, implying a
brightness temperature range 6×10<SUP>11</SUP>-6×10<SUP>13</SUP> K;
another example of a coherent radio burst.
---------------------------------------------------------
Title: The Radio Spectrum of TVLM 513-46546: Constraints on the
Coronal Properties of a Late M Dwarf
Authors: Osten, Rachel A.; Hawley, Suzanne L.; Bastian, Timothy S.;
Reid, I. Neill
2006ApJ...637..518O Altcode: 2005astro.ph..9762O
We explore the radio emission from the M9 dwarf TVLM 513-46546 at
multiple radio frequencies, determining the flux spectrum of persistent
radio emission, as well as constraining the levels of circular
polarization. Detections at both 3.6 and 6 cm provide a spectral index
measurement α (where S<SUB>ν</SUB>~ν<SUP>α</SUP>) of -0.4+/-0.1. A
detection at 20 cm suggests that the spectral peak is between 1.4 and
5 GHz. The most stringent upper limits on circular polarization are
at 3.6 and 6 cm, with V/I<15%. These characteristics agree well
with those of typical parameters for early- to mid-type M dwarfs,
confirming that magnetic activity is present at levels comparable with
those extrapolated from earlier M dwarfs. We apply analytic models to
investigate the coronal properties under simple assumptions of dipole
magnetic field geometry and radially varying nonthermal electron
density distributions. Requiring the spectrum to be optically thin
at frequencies higher than 5 GHz and reproducing the observed 3.6 cm
fluxes constrains the magnetic field at the base to be less than about
500 G. There is no statistically significant periodicity in the 3.6
cm light curve, but it is consistent with low-level variability.
---------------------------------------------------------
Title: NoRH and RHESSI Observations of Quasiperiodic Radio and X-ray
Oscillations in a Solar Flare
Authors: Bastian, T.; Fleishman, G.; Gary, D.
2006cosp...36.3251B Altcode: 2006cosp.meet.3251B
We describe microwave and hard X-ray observations of strong
quasiperiodic pulsations from the GOES X1 3 solar flare of 15 June 2003
Using Fourier analysis we study the frequency- and energy-dependent
oscillation periods phase differences and modulation depths of the radio
and X-ray pulsations Focusing on the more complete radio observations
the observational properties of the oscillations are compared with
those derived from models for the radio emission Two simple models are
considered i gyrosynchrotron emission modulated by the quasiperiodic
injection of fast electrons ii gyrosynchrotron emission modulated
by an MHD oscillation of the magnetic field We demonstrate that
quasiperiodic injection of fast electrons is the more likely cause
of the observed quasiperiodic oscillations observed in the radio and
hard X-ray emission We discuss the implications of this finding for
particle acceleration and transport in the flaring sources
---------------------------------------------------------
Title: Low Frequency Solar Radiophysics and Next Generation
Instrumentation
Authors: Bastian, T. S.; Gary, D. E.
2005ASPC..345..142B Altcode:
Radio astronomy and solar radio astronomy developed rapidly together
in the years following the Second World War. Much of this development
occurred at low frequencies. In more recent decades, the emphasis has
been on centimeter and millimeter wavelengths. However, motivated
by both computational advances and new science drivers, there is
a strong desire on the part of both the solar and astronomical
communities to build modern low frequency radio telescopes. Here,
some of the early work in solar radio astronomy is briefly reviewed
and recent developments in solar and heliospheric physics are
noted. Two next-generation radio telescopes, the Frequency Agile Solar
Radiotelescope and the Long Wavelength Array, are described. Possible
synergies between the two projects are discussed.
---------------------------------------------------------
Title: Low-Frequency Solar Radio Bursts from Green Bank
Authors: White, S. M.; Bastian, T. S.; Bradley, R.; Parashare, C.;
Wye, L.
2005ASPC..345..176W Altcode:
A low-frequency spectrometer for the study of solar radio bursts is
under development at Green Bank. Since January 2004 an 18-70 MHz system
has been operating daily. The system is described and examples of data
from the low-frequency system are shown..
---------------------------------------------------------
Title: An RFI Survey at the Site of the Long Wavelength Deomonstration
Array (LWDA)
Authors: Stewart, K. P.; Crane, P. C.; Paravastu, N.; Hicks, B. C.;
Theodorou, A.; Price, R. M.; Pihlstrom, Y. M.; Bastian, T. S.; Gary,
D. E.
2005AAS...20713604S Altcode: 2005BAAS...37.1389S
An initial survey of the radio-frequency environment at the site of the
Long Wavelength Demonstration Array (LWDA) has been performed using a
measurement protocol developed for both the Long Wavelength Array (LWA)
and the Frequency Agile Solar Radiotelescope (FASR). The measurements
cover the frequency range from 25 MHz to 18 GHz to include the LWA
(23-80 MHz) and FASR (30 MHz-30 GHz). Measurements were obtained
nearly continuously for a week to characterize the day/night and
weekday/weekend variations expected for many sources of RFI. The
equipment, antennas, and protocols are suitable for measuring strong
RFI that potentially threatens the linearity of radio-astronomical
receivers and may therefore rule out possible sites or influence the
design of the receivers. (Weak RFI which may obscure weak signals
of interest is beyond the capabilities of these measurements.) The
protocol seeks only to identify RFI originating from terrestrial
sources; RFI from satellites and astrophysical sources is assumed to
be site independent and not a factor in site selection. Therefore,
the receiving antennas provide azimuthal coverage in the direction of
the horizon. This first survey, conducted prior to the start of LWDA
construction, establishes a baseline for the later identification of
any self-generated interference from the LWDA and its mitigation to
ensure no adverse effect on the operations of the VLA.
---------------------------------------------------------
Title: Instrument Development for the Green Bank Solar Radio Burst
Spectrometer (GB/SRBS)
Authors: Bradley, R.; Parashare, C.; White, S. M.; Bastian, T. S.
2005ASPC..345..357B Altcode:
Details of two on-going technical development projects for the Green
Bank Solar Radio Burst Spectrometer (GB/SRBS) are presented. One is
a high dynamic range active balun covering 20-350 MHz. The other is
a log periodic array feed for the Green Bank 45-Foot radio telescope
covering 300-3000 MHz. Basic design, fabrication, and evaluation
information are included for both projects.
---------------------------------------------------------
Title: Quasiperiodic Electron Acceleration in the 15 June 2003
Solar Flare
Authors: Fleishman, G. D.; Bastian, T. S.; Gary, D. E.
2005AGUSMSP41C..04F Altcode:
We report a comprehensive analysis of strong quasiperiodic radio-
and X-ray pulsations observed from the X1.3-class flare which
occurred at S06E78 on 23:42-23:50 UT, 15 June 2003. Because of the
favorable time of the flare, it was jointly observed by the Owens
Valley Solar Array (OVSA) and the Nobeyama Solar Radio Observatory,
which allowed us the advantage of combining high spectral, temporal,
and spatial resolution radio observations. In addition, the part of
this event displaying the strongest pulsations was also observed
in hard X-rays with RHESSI. We study the frequency dependence of
quantitative measures of the pulsations, including Fourier spectra,
phase differences, modulation depth, as well as the degree of (radio)
polarization. We compare these measures with the expectations of
competing radio pulsation models, i.e., MHD loop oscillations and
quasiperiodic electron injection. Although the Fourier spectra display
a few significant peaks at each observing frequency, we found that
none of them can be explained by the MHD-oscillations. In contrast,
the model of quasiperiodic particle acceleration/injection is capable
of explaining all quantitative measures observed for this event in a
natural way. We discuss implications of these findings for electron
acceleration and transport in solar flares.
---------------------------------------------------------
Title: The Green Bank Solar Radio Burst Spectrometer
Authors: Bastian, T. S.; Bradley, R.; White, S.; Mastrantonio, E.
2005AGUSMSH43A..16B Altcode:
The Solar Radio Burst Spectrometer (SRBS) is a project designed to
1) provide high quality radio dynamic spectra to the wider solar,
heliospheric, and space weather communities; 2) serve as a development
platform for ultra-wideband feeds and receivers. Dynamic spectroscopy is
a powerful tool for observing radio bursts in the Sun's corona. These
bursts are associated with solar flares and/or coronal mass ejections
and result from coronal shocks (type II radio bursts), electron beams
(type III radio bursts), and other forms of energy release in the
corona. The community has been hampered by a lack of readily available
dynamic spectra in the 12-24 hr UT time range, a shortcoming that
has now been remedied. The instrument is located at the Green Bank
Site of the National Radio Astronomy Observatory in the National Radio
Quiet Zone, where the effects of radio frequency interference are much
reduced compared with unprotected sites. The spectrometer is composed
of two swept-frequency systems that together support observations from
18 MHz to 2 GHz with a time resolution of approximately 1 sec. The
low frequency system, operating from 18-70 MHz, is a standalone dipole
antenna. The high frequency system is fed by an antenna mounted at the
vertex of a 13.7 m telescope and operates from 70-300 MHz; a broadband
feed at the prime focus of the telescope provides frequency coverage
from 300-2500 MHz. The data are available daily through a web-based
interface. Both raw and background-subtracted data are available in a
variety of formats. Users are encouraged to view and download selected
data for research or forecasting purposes.
---------------------------------------------------------
Title: Radio observations of brown dwarfs
Authors: Osten, R. A.; Hawley, S. L.; Bastian, T. S.; Reid, I. N.
2005ESASP.560..857O Altcode: 2005csss...13..857O
No abstract at ADS
---------------------------------------------------------
Title: Radio Observations of Brown Dwarfs
Authors: Quick, L. C.; Osten, R.; Bastian, T.; Hawley, S.
2004AAS...205.1101Q Altcode: 2004BAAS...36.1351Q
Radio emission commonly signifies the presence of magnetic
activity in the coronae of late-type stars, while emission from
the H-alpha transition is a sign of magnetic activity in their
chromospheres. Surveys of H-alpha emission from ultracool dwarfs of
spectral types late M, L, and T have not shown this indicator of
magnetic activity. As a result, it was believed that all magnetic
activity declined in these cool bodies. Although sparse, the most
recent detections of radio emission contradict this behavior. As
part of a large VLA survey of nearby ultracool dwarfs out to 13pc,
the aim of my research was to determine how common radio emission is
in these late-type stars. Data from 9 L and T dwarfs was reduced, and
evidence for emission was detected from three of these objects. This
research was conducted as a part of the NRAO Summer Student program,
with partial funding from the National Science Foundation.
---------------------------------------------------------
Title: Low-frequency solar radiophysics with LOFAR and FASR
Authors: Bastian, T. S.
2004P&SS...52.1381B Altcode:
Low-frequency radio observations offer unique diagnostics of the solar
corona and solar wind. After a prolongued hiatus, there is renewed
interest in this important frequency regime. Two new ground-based
instruments will provide critical new low-frequency observations: the
low-frequency array (LOFAR) and the frequency agile solar radiotelescope
(FASR). This brief topical review summarizes low-frequency radio
phenomena that will be accessible to detailed study by LOFAR and
FASR in the coming decade. Energy release, drivers of space weather,
and studies of the solar wind are emphasized. Both instruments are
expected to play important roles in both basic research problems and
national and international space weather capabilities. While FASR is a
solar-dedicated instrument, LOFAR is not. Solar observing requirements
for LOFAR are briefly discussed.
---------------------------------------------------------
Title: The radio search for extrasolar planets with LOFAR
Authors: Farrell, W. M.; Lazio, T. J. W.; Zarka, P.; Bastian, T. J.;
Desch, M. D.; Ryabov, B. P.
2004P&SS...52.1469F Altcode:
The Low Frequency Array (LOFAR) will come on line with unprecedented
radio sensitivity and resolution between 10 and 240 MHz. Such a system
will provide a factor of 10-30 improvement in sensitivity in the
pursuit of the weak radio emission from extrasolar planets. To date,
previous examinations of extrasolar planetary systems with the most
advanced radio telescopes have yielded a negative result. However, the
improvement in sensitivity by LOFAR over current systems will increase
the likelihood of extrasolar planet detection in the radio. We apply
radiometric models derived previously from the study of planets in our
solar system to the known extrasolar planets, and demonstrate that
approximately 3-5 of them should emit in the proper frequency range
and with enough power to possibly become detectable at Earth with LOFAR.
---------------------------------------------------------
Title: The Frequency Agile Solar Radiotelescope
Authors: Bastian, T. S.
2004ASSL..314...47B Altcode:
The Frequency Agile Solar Radiotelescope (FASR) will be a ground based
solar-dedicated radio telescope designed and optimized to produce high
resolution, high-fidelity, and high-dynamic-range images over a broad
range of radio frequencies ~ 0.05-24 GHz). That is, FASR will perform
broadband imaging spectroscopy, producing unique data and enabling a
wide variety of radio-diagnostic tools to be exploited to study the
Sun from the mid-chromosphere to coronal heights. FASR will address an
extremely broad science program, including the nature and evolution
of coronal magnetic fields, the physics of flares, drivers of space
weather, the quiet Sun, and synoptic studies. FASR may also play an
important role in forecasting solar activity and space weather. An
important goal is to mainstream solar radio observations by providing
a number of standard data products for use by the wider solar physics
and space weather communities.
---------------------------------------------------------
Title: Radio Emission from Extrasolar Planets
Authors: Farrell, W. M.; Lazio, T. Joseph W.; Desch, M. D.; Bastian,
T. S.; Zarka, P.
2004IAUS..213...73F Altcode:
By virtue of their planetary-scale magnetic fields, the Earth and
all of the gas giants in our solar system possess solar-wind deformed
magnetospheres. The magnetic polar regions of these “magnetic planets”
produce intense, aurora-related radio emission from solar-wind powered
electron currents. Simple scaling laws suggest that Jovian-mass
planets close to their host stars should produce radio emission;
detecting such emission would be the first direct detection of many of
these planets. We describe searches using the Very Large Array (VLA)
for radio emission from the planets orbiting HD 114762, 70 Vir, and
τ Boo. Our limits are just above those predicted for the planetary
emissions. We discuss the possibilities for more stringent limits and
the implications that the existing observations have for the planets'
radio emissions, and hence on the planetary magnetic fields and stellar
wind environments.
---------------------------------------------------------
Title: Radio Emission from the Sun and Stars: New Insights into
Energetic Phenomena
Authors: Bastian, T. S.
2004IAUS..219..145B Altcode: 2003IAUS..219E..20B
Energetic phenomena on the Sun pose a number of fascinating puzzles
for solar physicists and more generally astrophysicists. These include
solar flares involving the catastrophic release of magnetic energy
in the low corona and coronal mass ejections which result from the
destabilization and expulsion of a significant portion of the corona. <P
/>Radio radiation is a sensitive tracer of a variety of energetic
processes on the Sun and stars. Radio observations of the Sun over the
past decade have produced new insights into the physics of magnetic
energy release particle acceleration and coronal and interplanetary
shocks. <P />Energetic phenomena on stars are similar to the extent
that solar flare analogs appear to be relevant in some cases. However
certain phenomena are seen on stars which have no solar counterpart. <P
/>Observations from the Very Large Array the Nobeyama Radioheliograph
the Nancay Radioheliograph and the space-based WIND/WAVES experiment
are discussed as well as observations of stars from Arecibo and the
Very Long Baseline Array. The next generation of radio telescopes that
will yield new progress on these puzzles will be briefly discussed.
---------------------------------------------------------
Title: Radio Bursts on the Flare Star AD Leo from the Arecibo
Observatory
Authors: Osten, R. A.; Bastian, T. S.
2003AAS...203.4802O Altcode: 2003BAAS...35.1283O
We discuss recent observations of the M dwarf flare star AD Leo taken
with the Arecibo Observatory. The wide bandwidths currently available
(ν <SUB></SUB> eff ∼ 500 MHz) are a factor of >10 improvement
over previous investigations of highly circularly polarized, high
brightness temperature bursting phenomena. The large degrees of circular
polarization (π <SUB>c</SUB> -> ±100%), coupled with large inferred
brightness temperatures (T<SUB>b</SUB> ≥10<SUP>16</SUP>K), suggest
a coherent emission mechanism, but interpretation has been hindered
by narrow bandwidths (Δ ν /nu ≤ few percent). We find evidence
of drifting structures, whose drift rates are compatible with the
range from solar type III bursts. We present a summary of the burst
characteristics, and interpret the data in the framework of different
coherent emission mechanisms. <P />RAO gratefully acknowledges support
from a Jansky fellowship at NRAO.
---------------------------------------------------------
Title: Future Capabilities for Solar/Stellar Observations
Authors: Carpenter, K.; Hassler, D.; Berger, T.; Bastian, T.;
Pallavicini, R.; Balachandran, S.
2003csss...12..359C Altcode:
This session outlines the potential instrumental capabilities for the
observation of cool stars and the Sun. The individual contributions
cover space-based solar missions (Hassler), ground-based solar
optical/IR instruments (Berger), solar-stellar radio capabilities
(Bastian), space-based stellar missions (Carpenter), ground-based
optical (Pallavicini) and infrared facilities (Balachandran) for
stellar observations.
---------------------------------------------------------
Title: Low Frequency Solar Radiophysics
Authors: Bastian, T.
2003EAEJA.....7841B Altcode:
Low frequency radio observations of the Sun - defined here to be those
that comprise roughly decimeter to decameter wavelengths - offer a
rich phenomenology and a number of important diagnostics of physical
processes in the solar corona and the heliosphere. Of particular
interest are active solar phenomena such as flares and drivers of space
weather: coronal mass ejections and fast solar solar wind streams. This
talk will briefly review the relevant emission mechanisms at these
wavelengths and present a number of examples of recent low-frequency
observations, including: 1) radio tracers of energy release in the
solar corona; 2) radio precursor activity of coronal mass ejections;
3) radio signatures of coronal mass ejections; 4) radio diagnostics
of the solar wind, disturbances in the solar wind, and turbulence
in the solar wind. These observations motivate the need for the next
generation of radio imaging instrumentation, the Low Frequency Array
(LOFAR) and the Frequency Agile Solar Radiotelescope (FASR). LOFAR is
a general purpose instrument designed to operate between 15-240 MHz. A
key component of its science program is solar radiophysics. FASR is
a solar-dedicated instrument designed to perform broadband imaging
spectroscopy between ~100 MHz and 30 GHz. These two instruments are
therefore complementary in their frequency coverage and will together
address an outstanding science program.
---------------------------------------------------------
Title: The Frequency Agile Solar Radiotelescope (FASR)
Authors: White, S. M.; Gary, D. E.; Bastian, T. S.; Hurford, G. J.;
Lanzerotti, L. J.
2003EAEJA....11021W Altcode:
The Frequency Agile Solar Radiotelescope (FASR) is a radio
interferometer designed to make high spatial resolution images of the
Sun across a broad range of radio wavelengths simultaneously, allowing
the technique of imaging spectroscopy to be exploited on a routine
basis. The telescope will cover the frequency range 0.1-30 GHz using
several sets of receiving elements that provide full-disk imaging,
with of order 100 antennas at highest frequency range. FASR will be
optimized for solar radio phenomena and will be the most powerful
and versatile radioheliograph ever built, providing an improvement of
orders of magnitude in image quality over existing instruments. FASR
recently received the top ranking amongst all small projects considered
by the decadal survey of the National Academy of Science Committee on
Solar and Space Physics. FASR will probe all phenomena in the solar
atmosphere from the mid-chromosphere outwards. In particular, FASR
will provide direct measurement of coronal magnetic field strengths,
will image the nonthermal solar atmosphere and show directly the
locations of electrons accelerated by solar flares, will provide
images of coronal mass ejections travelling outwwards through the
solar corona, and supply extensive data products for forecasting and
synoptic studies. A major emphasis in the project is to make FASR data
as widely and easily used as possible, i.e., providing the general
user with processed, fully-calibrated high-quality images that do not
need particular knowledge of radio astronomy for interpretation. This
paper will describe the telescope and its science goals, and summarize
its current status.
---------------------------------------------------------
Title: Microwave observations of jupiter's synchrotron emission
during the galileo flyby of amalthea in 2002.
Authors: Klein, M. J.; Bolton, S. J.; Bastian, T. S.; Blanc, M.; Levin,
S. M.; McLeod, R. J.; MacLaren, D.; Roller, J. P.; Santos-Costa, D.;
Sault, R.
2003EAEJA....14635K Altcode:
In November, 2002, the Galileo spacecraft trajectory provided a close
flyby of Amalthea, one of Jupiter's inner most moons (∼2.4 RJ). During
this pass, Galileo entered into a region rarely explored by spacecraft,
the inner radiation belts of Jupiter. We present preliminary results
from a campaign of microwave observations of Jovian synchrotron emission
over a six month interval centered around the flyby. The observations
were made with NASA's Deep Space Network (DSN) antennas at Goldstone,
California, and the NRAO Very Large Array. We report preliminary
measurements of the flux density of the synchrotron emission and the
rotational beaming curves and a compare them with the long term history
of Jupiter's microwave emission which varies significantly on timescales
of months to years. The new data are also being examined to search for
evidence of short-term variations and to compare single aperture beaming
curves with the spatially resolved images obtained with the VLA. These
radio astronomy data will be combined with in-situ measurements from
Galileo (see companion paper by Bolton et al) to improve models of the
synchrotron emission from Jupiter's radiation belts. A large percentage
of the Goldstone observations were conducted by middle- and high school
students from classrooms across the nation. The students and their
teachers are participants in the Goldstone-Apple Valley Radio Telescope
(GAVRT) science education project, which is a partnership involving
NASA, the Jet Propulsion Laboratory and the Lewis Center for Educational
Research (LCER) in Apple Valley, CA. Working with the Lewis Center over
the Internet, GAVRT students conduct remotely controlled radio astronomy
observations using 34-m antennas at Goldstone. The JPL contribution to
this paper was performed at the Jet Propulsion Laboratory, California
Institute of Technology, under contract with the National Aeronautics
and Space Administration 2756 Planetary magnetospheres (5443, 5737,
6030) 6218 Jovian satellites 6220 Jupiter Planetary Sciences
---------------------------------------------------------
Title: Radio Emission from Extrasolar Planetary Systems
Authors: Farrell, W. M.; Lazio, T. J.; Desch, M. D.; Zarka, P.;
Bastian, T.
2003EAEJA.....3834F Altcode:
Each of the gas giants in our solar system and the Earth possess
planetary-scale magnetic fields, generated by internal dynamo
currents. The magnetic polar regions of these solar system 'magnetic
planets' produce intense, aurora-related radio emission, generated
via coherent cyclotron radiation from solar-wind powered electron
currents. Simple scaling laws suggest that Jovian-mass planets close to
their host stars should produce radio emission; detecting such emission
would be the first direct detection of these planetary systems. We
describe a series of Very Large Array (VLA) observations of extrasolar
planets, largely using its 74 MHz observing system. The predicted
emission frequencies of extrasolar planet radio emission depend upon the
(unknown) magnetic moments and rotation rates of the planets. Scaling
from Jovian properties, it seems likely that many extrasolar planets
will emit below ~100 MHz, making observations at 74 MHz a useful first
effort. Our current limits are close to, but generally just above that
predicted for the radiative power from planetary cyclotron emission. We
discuss the implications that the existing observations have for the
planet's radio emission, and hence on the planetary magnetic field
and stellar wind environment, as well as the possibility for far more
stringent observational limits with the Low Frequency Array (LOFAR).
---------------------------------------------------------
Title: Imaging capabilities of the Frequency Agile Solar
Radiotelescope (FASR)
Authors: White, Stephen; Lee, Jeongwoo; Aschwanden, Markus A.; Bastian,
Tim S.
2003SPIE.4853..531W Altcode:
The Frequency Agile Solar Radiotelescope (FASR) will observe the Sun
over a wide range of radio frequencies and make high spatial resolution
images at many frequencies nearly simultaneously. FASR will need to
be able to observe both the very bright, usually compact emission
from solar flares as well as much fainter fluctuations in the solar
chromosphere across a broad range of spatial scales (from 1 arcsec to
1 degree) at high time resolution, and these constraints impose severe
requirements on telescope design. We discuss the problem of imaging
the Sun at radio wavelengths and present simulations of imaging the
thermal free-free emission from the Sun's atmosphere using models
based on EUV data.
---------------------------------------------------------
Title: Frequency agile solar radiotelescope
Authors: Bastian, Tim S.
2003SPIE.4853...98B Altcode:
The Frequency Agile Solar Radiotelescope (FASR) is a solar-dedicated,
ground based, interferometric array optimized to perform broadband
imaging spectroscopy from ~ 0.1-30+ GHz. It will do so with the
angular, spectral, and temporal resolution required to exploit
radio emission from the Sun as a diagnostic of the wide variety of
astrophysical processes that occur there. FASR represents a major
advance over existing radioheliographs, and is expected to remain the
world's premier solar radio instrument for two decades or more after
completion. FASR will be a versatile and powerful instrument, providing
unique data to a broad users community. Solar, solar-terrestrial, and
space physicists will exploit FASR to attack a broad science program,
including problems of fundamental interest: coronal magnetography,
solar flares and particle acceleration, drivers of space weather, and
the thermal structure and dynamics of the solar atmosphere. A design
study and implementation planning are underway. Recent progress is
reviewed here.
---------------------------------------------------------
Title: The Frequency Agile Solar Radiotelescope
Authors: Bastian, T. S.
2003AdSpR..32.2705B Altcode:
The Frequency Agile Solar Radiotelescope (FASR), a telescope concept
currently under study, will be a ground based solar-dedicated
radio telescope designed and optimized to produce high resolution,
high-fidelity, and high-dynamic-range images over a broad frequency
range (∼0.1-24 GHz). As such, FASR will address an extremely broad
science program, including the nature and evolution of coronal magnetic
fields, the physics of flares, drivers of space weather, and the quiet
Sun. An important goal is to mainstream solar radio observations by
providing a number of standard data products for use by the wider
solar physics community. The instrument specifications and the key
science elements that FASR will address are briefly discussed, as well
as several operational issues.
---------------------------------------------------------
Title: Limits on the Magnetosphere/Stellar Wind Interactions for
the Extrasolar Planet about Tau Bootis
Authors: Farrell, W. M.; Desch, M. D.; Lazio, T. J.; Bastian, T.;
Zarka, P.
2003ASPC..294..151F Altcode:
Among the most impressive astronomical discoveries in the past
decade are the observations of Jupiter-like planets in orbit around
stars similar to our sun (Mayor and Queloz 1995; Marcy 1998). These
extrasolar planets are detected primarily from optical signatures
of the star's orbital perturbation about the star/planet center of
mass. To date, over 80 massive planets have been discovered about
sun-like stars, these stars located in the near-vicinity of our
own solar system (< 100 pc). By analogy with the sun's gas giant
planets, it has been predicted that these extrasolar planets will
have electrically-active stellar-wind driven planetary magnetospheres
possibly capable of emitting long-wavelength radio emission (Burke
1992; Farrell et al. 1999; Bastian et al. 2000; Zarka et al. 2001)
consistent with radiometric Bode's laws known to apply in our solar
system. In 1999 and in 2002, the Very Large Array (VLA) surveyed the
region near Tau Bootes for long-wavelength radio emission from its
extrasolar planet. This planet had been previously predicted to be
a good candidate for coherent electron cyclotron radio emission in
the 10's of MHz. While no obvious signal was detected at 74 MHz to a
sensitivity of <0.12 Janskys, the results can be applied to place
upper limits on the stellar winds and planetary magnetic field.
---------------------------------------------------------
Title: A Search for Cyclotron Emission from known Extra-Solar Planets
Authors: Langston, G. I.; Orban, C. M.; Bastian, T. S.
2002AAS...201.4610L Altcode: 2002BAAS...34.1176L
All magnetized planets in our solar system emit intense cyclotron
maser radiation. Like Jupiter, the recently discovered extra-solar
planets are probably magnetized. If in addition, there is a source of
energetic (keV) electrons in their magnetospheres, it is likely that
extra-solar planets are also cyclotron-maser emitters. We present Green
Bank 100m Radio Telescope observations at 330 MHz of 20 Extra-solar
planets around nearby stars. Using the GBT spectrometer, we produced
high time and frequency resolution observations, searching for flaring
events. The calibration, radio interference and sensitivity limits are
presented. Since cyclotron maser emission from Jupiter is episodic,
long observations may be required to detect Extra-solar planets. The
National Radio Astronomy Observatory (NRAO) is a facillity of the
National Science Foundation operated under cooperative agreement by
Associated Universities, Inc.
---------------------------------------------------------
Title: ALMA and the Sun
Authors: Bastian, T. S.
2002AN....323..271B Altcode:
The Atacama Large Millimeter Array (ALMA) is a large Fourier synthesis
imaging telescope designed to operate at millimeter and sub-millimeter
wavelengths, a wavelength regime that remains, thus far, largely
unexplored. Yet this important wavelength range offers a number of
unique and important diagnostics of astrophysical processes. ALMA is
a general purpose instrument designed to address an extremely broad
science program, including cosmology; formation of clusters, galaxies,
stars, and planets; synthesis of the elements; and low temperature
thermal science. ALMA is also designed to support solar observing in
order to address outstanding issues in solar physics. After reviewing
ALMA's instrument specifications, the problem of solar observing in the
mm/submm-lambda band is discussed. The relevant emission mechanisms on
the Sun are introduced and several examples of previous observations
in the mm/submm-lambda range are presented. The use of ALMA for solar
observing is then considered.
---------------------------------------------------------
Title: Radio Diagnostics of Magnetic Fields in CMEs
Authors: Bastian, T. S.
2002AAS...200.6507B Altcode: 2002BAAS...34Q.752B
A variety of remote sensing techniques are available at radio
wavelengths for measuring or constraining the magnetic field in CMEs
in the corona and inner heliosphere. Some have been exploited; for
others the “proof of concept" has been established; still others are
purely speculative at this point and rely on the development of future
instrumentation. They can be grouped into three distinct classes: 1)
radio emission intrinsic to CMEs (synchrotron emission); 2) Faraday
rotation of background radio sources viewed through CMEs; 3) studies
of radio bursts associated with CMEs (type II, SA type III, and type
IV). This talk will review the specific techniques available in each of
these classes and describe what they can tell us about magnetic fields
in CMEs. The requirements on instrumentation required to exploit these
techniques will be emphasized.
---------------------------------------------------------
Title: The Frequency Agile Solar Radiotelescope
Authors: Bastian, T. S.
2002AAS...200.4902B Altcode: 2002BAAS...34..721B
The Frequency Agile Solar Radiotelescope (FASR) is a Fourier synthesis
telescope designed to perform broadband imaging spectroscopy over an
extremely broad frequency range ( 0.1-30 GHz). The frequency, temporal,
and angular resolution of the instrument will be optimized for the many
and varied radio phenomena produced by the Sun. Consequently, FASR will
the most powerful and versatile radioheliograph ever built. FASR was
recommended by the NAS/NRC Astronomy and Astrophysics Survey Committee
for construction in the coming decade and is currently under review
by the decadal survey of Solar and Space Physics. An NSF-funded study
of the instrument concept is currently under way. FASR will probe all
phenomena in the solar atmosphere from the mid-chromosphere to the outer
corona. The range of science that FASR will address is correspondingly
broad. The design and function of FASR offer several unique
capabilities, to which several key science goals are well-matched: 1-
The nature and evolution of coronal magnetic fields, including direct
measurement of coronal magnetic fields; the temporal and spatial
evolution of coronal fields; inference of coronal electric currents;
the storage and release of magnetic energy. 2- Transient energetic
phenomena such as energy release in flares; plasma heating; particle
acceleration; electron transport; the formation and destabilization
of large scale structures (filaments, coronal mass ejections). 3-
Quantitative diagnostics of the three-dimensional solar atmosphere; the
quiet Sun and coronal holes; origin of the solar wind; coronal heating;
formation of filaments. In addition, FASR will be a powerful tool for
synoptic programs and for forecasting activities. The operational model
for FASR will make the data widely available for immediate use by the
wider scientific community. This talk will introduce the instrument,
the science drivers, and the current status and plans for the project.
---------------------------------------------------------
Title: Ultra-relativistic electrons in Jupiter's radiation belts
Authors: Bolton, S. J.; Janssen, M.; Thorne, R.; Levin, S.; Klein,
M.; Gulkis, S.; Bastian, T.; Sault, R.; Elachi, C.; Hofstadter, M.;
Bunker, A.; Dulk, G.; Gudim, E.; Hamilton, G.; Johnson, W. T. K.;
Leblanc, Y.; Liepack, O.; McLeod, R.; Roller, J.; Roth, L.; West, R.
2002Natur.415..987B Altcode:
Ground-based observations have shown that Jupiter is a two-component
source of microwave radio emission: thermal atmospheric emission and
synchrotron emission from energetic electrons spiralling in Jupiter's
magnetic field. Later in situ measurements confirmed the existence
of Jupiter's high-energy electron-radiation belts, with evidence for
electrons at energies up to 20MeV. Although most radiation belt models
predict electrons at higher energies, adiabatic diffusion theory can
account only for energies up to around 20MeV. Unambiguous evidence for
more energetic electrons is lacking. Here we report observations of
13.8GHz synchrotron emission that confirm the presence of electrons with
energies up to 50MeV the data were collected during the Cassini fly-by
of Jupiter. These energetic electrons may be repeatedly accelerated
through an interaction with plasma waves, which can transfer energy
into the electrons. Preliminary comparison of our data with model
results suggests that electrons with energies of less than 20MeV are
more numerous than previously believed.
---------------------------------------------------------
Title: A radio survey of fast CMEs accompanied by interplanetary
type II bursts
Authors: Bastian, T.; Payne, J.; Pick, M.; Kerdraon, A.
2002cosp...34E1872B Altcode: 2002cosp.meetE1872B
Following the detection of synchrotron radiation from a fast CME at
radio wavelengths by Bastian et al. (2001; ApJ 558, L65), a sample of
coronal mass ejections (CMEs) was selected between 1998-2000. Using
data compiled by SOHO/LASCO, the WIND/WAVES experiment, and the Nancay
radioheliograph, white light CMEs were selected on the basis of three
criteria: 1) that the projected speed reported by SOHO/LASCO exceeded
1200 km/s; that it was accompanied by an interplanetary type II radio
burst with a start frequency >1 MHz; that it was observed by the
Nancay Radioheliograph in France. The resulting sample numbers 34
events. The motivation for analyzing these events is to: 1) detect
additional examples of synchrotron emission from CMEs ("radio CMEs")
and determine their incidence rate; 2) constrain the conditions under
which they occur.A preliminary report of our results is presented.
---------------------------------------------------------
Title: The frequency agile solar telescope (FASR)
Authors: Bastian, T.; Gary, D.; White, S.; Hurford, G.
2002cosp...34E1870B Altcode: 2002cosp.meetE1870B
The Frequency Agile Solar Radiotelescope (FASR) will be a large-N,
solar- dedicated, Fourier synthesis array designed to perform broadband
imaging spectroscopy across a frequency range of approximately 0.1-30
GHz with an angular resolution as high as 1" (>20 GHz). This
paper reviews the FASR science drivers -- including coronal magnetic
fields, energetic phenomena, and thermal physics -- and the associated
instrument requirements and specifications. Applications for synoptic
studies and space weather are also discussed. Ongoing activities are
briefly described, including site surveys, configuration studies,
science simulations, and system design.
---------------------------------------------------------
Title: Cassini, VLA and DSN Observations of Synchrotron Emission
from Jupiter's Radiation Belts
Authors: Bolton, S. J.; Janssen, M.; Levin, S.; Klein, M.; Gulkis,
S.; Elachi, C.; Thorne, R.; Bastian, T.; Sault, R.; Dulk, G.; Leblanc,
Y.; McLeod, R.; Roller, J.
2001DPS....33.0504B Altcode: 2001BAAS...33.1029B
We report on new measurements of Jupiter's synchrotron emission from
Cassini, the Very Large Array (VLA) and the NASA Deep Space Network
(DSN) obtained during Cassini's flyby of Jupiter. On January 2-3, 2001,
on route to Saturn, measurements of Jupiter's synchrotron emission
were carried out using the radiometer subsystem of the Cassini Radar
Instrument operating at 2.2 cm (13.8 GHz), the VLA operating at 20
cm (1400 MHz) and 90 cm (333 MHz) and the DSN operating at 13 cm,
3.5 cm, and 2.2 cm. The data provide new information on a wide range
of energetic electrons trapped in Jupiter's magnetosphere ( ~5 to
>50 MeV). At frequencies above 100 MHz, Jupiter's radio emission is
comprised of both thermal emission from the atmosphere, and non-thermal
synchrotron emission generated by relativistic electrons trapped
in Jupiter's radiation belts. Earth-based radio telescopes cannot
accurately measure and map the synchrotron radiation at wavelengths
shorter than about 6 cm due to the difficulty of separating the thermal
atmospheric emission from the non-thermal synchrotron emission. Because
in-situ measurements of the electrons are limited, investigations are
largely dependent on ground-based observations. The observations provide
the first accurate measurement of Jupiter's synchrotron emission at 2.2
cm and the first high resolution maps depicting the distribution of
ultra-relativistic electrons (> 50 MeV) near Jupiter. The results
suggest that the relativistic electrons have a softer energy spectrum
than expected, and as a result, electrons between 5-20 MeV may be
more abundant than previously realized. Acknowledgements: The research
reported in this paper was performed at the Jet Propulsion Laboratory,
California Institute of Technology, under contract with the National
Aeronautics and Space Administration.
---------------------------------------------------------
Title: The Coronal Mass Ejection of 1998 April 20: Direct Imaging
at Radio Wavelengths
Authors: Bastian, T. S.; Pick, M.; Kerdraon, A.; Maia, D.; Vourlidas,
A.
2001ApJ...558L..65B Altcode:
We observed the fast coronal mass ejection (CME) of 1998 April 20
with the radioheliograph at Nançay, France, between 164 and 432
MHz. Spectroscopic data were obtained between 40 and 800 MHz by the
spectrometer at Tremsdorf, Germany, and between 20 kHz and 14 MHz
with the WAVES instrument on board the Wind spacecraft. Energetic
particle data were obtained from the Wind 3D Plasma and Energetic
Particle experiment. The CME was observed in white light by the
Large-Angle Spectrometric COronagraph experiment on board the Solar and
Heliospheric Observatory spacecraft. For the first time, the expanding
CME loops are imaged directly at radio wavelengths. We show that the
radio-emitting CME loops are the result of nonthermal synchrotron
emission from electrons with energies of ~0.5-5 MeV interacting
with magnetic fields of ~0.1 to a few gauss. They appear nearly
simultaneously with the onset of an associated type II radio burst,
shock-accelerated type III radio bursts, and the initiation of a solar
energetic particle event. We suggest possible sources of the energetic
electrons responsible for this “radio CME” and point out diagnostic
uses for synchrotron emission from CME loops.
---------------------------------------------------------
Title: Radio Wave Propagation in the Corona and the Interplanetary
Medium
Authors: Bastian, T. S.
2001Ap&SS.277..107B Altcode:
Radio wave propagation through an inhomogeneous, random plasma produces
a variety of observable phenomena - group delay, Faraday rotation,
refraction, angular broadening, spectral broadening, and scintillations
in phase, amplitude, and frequency. These may be exploited to constrain
the mean and fluctuating properties of the medium through a variety
of remote sensing techniques. In the case of the solar corona and the
solar wind, the mean density, magnetic field, solar wind speed, and
the spatial spectrum of the density fluctuation scan all be constrained
in regions that are inaccessible to in situmeasurements.
---------------------------------------------------------
Title: Observations of Jovian Synchrotron Emission
Authors: Bolton, S. J.; Janssen, M.; Levin, S.; Sault, R.; Bastian,
T.; Klein, M.; Gulkis, S.; Hofstadter, M.; Elachi, C.; Johnston, W.;
Bunker, A.; Hamilton, G.; Liepack, O.; Roth, L.; West, R.; Dulk, G.;
Leblanc, Y.; Thorne, R.; Roller, J.; McLeod, R.
2001AGUSM...P52A08B Altcode:
On route to Saturn, the Cassini spacecraft flew past Jupiter and
provided the first opportunity to observe the Jovian synchrotron
radiation at a wavelength of 2.2 cm. Measurements were successfully
carried out shortly after Jupiter closest approach using the
radiometer subsystem of the Cassini Radar Instrument. The resulting
data provide unique information on the highest energy electrons in
Jupiter's magnetosphere. Earth-based radio telescopes have difficulty
measuring the synchrotron radiation at wavelengths this short because
of the difficulty in separating atmospheric thermal emission from
the synchrotron radiation. The 2.2 cm Cassini radiometer was used to
produce 20 maps covering two complete rotations of Jupiter in both
horizontal and vertical linear polarization. Synchrotron emission was
clearly detected distinct from the thermal emission as evidenced by
its polarization and spatial distribution. A ground based campaign
involving the VLA (operating at 20 and 90 cm) and the NASA Deep Space
Network antennas (operating at 2.3, 8.5, 13.8 and 32 GHz) observed
simultaneously with Cassini. The combined data set provides a complete
picture of the electron energy spectrum and distribution in the Jovian
inner radiation belts. Preliminary results from the observations and
modeling efforts will be presented. The JPL contribution to this paper
was performed at the Jet Propulsion Laboratory, California Institute
of Technology, under contract with the National Aeronautics and Space
Administration.
---------------------------------------------------------
Title: Joint EUV/Radio Observations of a Solar Filament
Authors: Chiuderi Drago, F.; Alissandrakis, C. E.; Bastian, T.;
Bocchialini, K.; Harrison, R. A.
2001SoPh..199..115C Altcode:
In this paper we compare simultaneous extreme ultraviolet (EUV) line
intensity and microwave observations of a filament on the disk. The
EUV line intensities were observed by the CDS and SUMER instruments on
board SOHO and the radio data by the Very Large Array and the Nobeyama
radioheliograph. The main results of this study are the following: (1)
The Lyman continuum absorption is responsible for the lower intensity
observed above the filament in the EUV lines formed in the transition
region (TR) at short wavelengths. In the TR lines at long wavelengths
the filament is not visible. This indicates that the proper emission of
the TR at the filament top is negligible. (2) The lower intensity of
coronal lines and at radio wave lengths is due to the lack of coronal
emission: the radio data supply the height of the prominence, while EUV
coronal lines supply the missing hot matter emission measure (EM). (3)
Our observations support a prominence model of cool threads embedded
in the hot coronal plasma, with a sheath-like TR around them. From the
missing EM we deduce the TR thickness and from the neutral hydrogen
column density, derived from the Lyman continuum and He i absorption,
we estimate the hydrogen density in the cool threads.
---------------------------------------------------------
Title: The Frequency-Agile Solar Radiotelescope (FASR) (invited)
Authors: Gary, D. E.; Bastian, T. S.; White, S. M.; Hurford, G. J.
2001aprs.conf..236G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Coordinated BeppoSAX and VLA observations of UX Arietis
(CD-ROM Directory: contribs/francio1)
Authors: Franciosini, E.; Pallavicini, R.; Bastian, T.; Chiuderi-Drago,
F.; Randich, S.; Tagliaferri, G.; Massi, M.; Neidhöfer, J.
2001ASPC..223..930F Altcode: 2001csss...11..930F
No abstract at ADS
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Title: Cassini RADAR/Radiometer and VLA Observations of Jupiter's
Synchrotron Emission
Authors: Janssen, M. A.; Bolton, S. J.; Levin, S. M.; Sault, R.; Klein,
M. J.; Gulkis, S.; Hofstadter, M. D.; Elachi, C.; Johnson, W. T. K.;
Bunker, A.; Gudim, E. J.; Hamilton, G. A.; Liepack, O.; Roth, L. E.;
West, R. D.; Bastian, T.; Dulk, G.; Leblanc, Y.; Thorne, R.; Roller,
J. P.; McLeod, R. K.
2001pre5.conf..229J Altcode:
We present observations of Jupiter's synchrotron emission made jointly
from the Earth and from the Cassini spacecraft as it recently flew
past Jupiter. These include observations at the lowest and highest
frequencies ever used to image this emission, which thereby provide
new and unique information on the Jovian electron energy spectrum. In
particular, the Cassini Radar instrument includes a passive radiometer
operating at 2.2-cm wavelength that clearly detected synchrotron
emission as evidenced by its polarization and spatial distribution,
even though it amounted to only about 1.1 percent of the total emission
from Jupiter. We conclude from this result that the population of
electrons with energies in excess of about 20 MeV is several times
less than expected based on the best current model.
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Title: Acceleration of electrons at type II shock fronts and
production of shock-accelerated type III bursts
Authors: Dulk, George A.; Leblanc, Yolande; Bastian, T. S.; Bougeret,
Jean-Louis
2000JGR...10527343D Altcode:
We present evidence of electron acceleration by type II-burst-emitting
shocks in the corona. Some of the electrons travel outward along open
magnetic field lines and produce “shock-accelerated type III bursts”
(or SA type III bursts) along their paths. The SA type III bursts are
evident in dynamic spectra that cover part or all of the range from
metric to kilometric wavelengths. The unique feature of our observations
is the complete or near-complete frequency coverage from about 2 GHz to
<0.1 MHz, that is, <~1.01 Ro to 1 AU. A sample of eight events
is presented. All would be classified as “shock accelerated events”
at hectometric wavelengths, as first defined by Cane et al. [1981]. Our
complete spectra frequently show several to many type III-like bursts
emanating from near the type II burst toward low frequencies, with
no trace of emission at frequencies higher than that of the type
II burst. The drift rates of these SA type III bursts are similar
to those of normal type III bursts, and the exciting electrons have
speeds of order 0.1c to 0.2c, or energies of 3-10 keV and higher. Their
intensity at hectometer wavelengths is similar to that of normal type
III bursts. They often persist to the lowest frequencies observable,
near the local plasma frequency at 1 AU. In most of the events examined,
there were no microwave bursts from the low corona whose intensity
profiles were similar to the hectometric profiles. We therefore conclude
that these SA type III bursts originate in type II shocks and are caused
by energetic electrons accelerated at the shocks. Of the eight events
analyzed, three contain only SA type III bursts. For the remainder,
normal type III bursts predominate initially, followed by SA type III
bursts later in the event. We emphasize the need for spectra with
near-continuous coverage, especially from decametric to kilometric
wavelengths, to identify SA type III bursts unambiguously and to
distinguish between the contributions of normal and SA type III bursts.
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Title: A Search for Radio Emission from Extrasolar Planets
Authors: Bastian, T. S.; Dulk, G. A.; Leblanc, Y.
2000ApJ...545.1058B Altcode:
All magnetized planets in the solar system emit intense cyclotron
maser radiation. Like Jupiter, extrasolar giant planets are probably
magnetized. If, in addition, there is a source of energetic (keV)
electrons in their magnetospheres, from auroral processes or as a result
of magnetic coupling between the planet and a satellite, it is likely
that extrasolar planets are cyclotron-maser emitters. Detection and
follow-up observations of cyclotron maser radiation from an exoplanet
would reveal the presence, strength, and complexity of the planetary
magnetic field, the planet's rotation rate, and possibly the presence of
an Io-like moon within the planet's magnetosphere. Magnetic fields may
be necessary for life to exist on the surface of planets because they
provide protection from the nefarious effects of energetic particles
of stellar winds, stellar flares, and cosmic rays. We have conducted a
search for radio emission from extrasolar planets and brown dwarfs at
decimeter and meter wavelengths using the Very Large Array (VLA). We
have observed seven extrasolar planets and two brown dwarfs at 333
and 1465 MHz, and one extrasolar planet and one brown dwarf at 74
MHz. Typical (1 σ) sensitivities were 0.02-0.07 mJy at 1465 MHz,
1-10 mJy at 333 MHz, and ~50 mJy at 74 MHz. To date, no detections
have been made.
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Title: Solar Flares: Radio Bursts
Authors: Bastian, T.
2000eaa..bookE2293B Altcode:
Energy release in solar flares heats plasma and accelerates electrons
and ions to high energies. Radio bursts of various types are produced
by energetic electrons interacting with the ambient plasma or with the
magnetic field. They may be divided into two broad classes according
to frequency range and the dominant emission mechanisms. (1) Coherent
plasma radiation plays a dominant role for burst ...
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Title: Solar Wind: Radio Techniques for Probing
Authors: Bastian, T.
2000eaa..bookE2599B Altcode:
The solar wind is a complex magnetized plasma containing large-scale
magnetohydrodynamic (MHD) structures, waves and turbulence (see SOLAR
WIND PLASMA WAVES and SOLAR WIND TURBULENCE). The structure of the solar
wind is modulated in both time and space by solar variability. The
solar activity cycle modulates the structure of the solar wind over
a time scale of years while transient energetic phen...
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Title: Space VLBI at Low Frequencies
Authors: Jones, D. L.; Allen, R.; Basart, J.; Bastian, T.; Blume, W.;
Bougeret, J. -L.; Dennison, B.; Desch, M.; Dwarakanath, K.; Erickson,
W.; Farrell, W.; Finley, D.; Gopalswamy, N.; Howard, R.; Kaiser,
M.; Kassim, N.; Kuiper, T.; MacDowall, R.; Mahoney, M.; Perley, R.;
Preston, R.; Reiner, M.; Rodriguez, P.; Stone, R.; Unwin, S.; Weiler,
K.; Woan, G.; Woo, R.
2000aprs.conf..265J Altcode: 2000astro.ph..3120J
At sufficiently low frequencies, no ground-based radio array will
be able to produce high resolution images while looking through
the ionosphere. A space-based array will be needed to explore the
objects and processes which dominate the sky at the lowest radio
frequencies. An imaging radio interferometer based on a large number
of small, inexpensive satellites would be able to track solar radio
bursts associated with coronal mass ejections out to the distance
of Earth, determine the frequency and duration of early epochs of
nonthermal activity in galaxies, and provide unique information about
the interstellar medium. This would be a “space-space" VLBI mission,
as only baselines between satellites would be used. Angular resolution
would be limited only by interstellar and interplanetary scattering.
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Title: The ALFA Medium Explorer Mission
Authors: Jones, D. L.; Allen, R. J.; Basart, J. P.; Bastian, T.;
Blume, W. H.; Bougeret, J. -L.; Dennison, B. K.; Desch, M. D.;
Dwarakanath, K. S.; Erickson, W. C.; Farrell, W.; Finley, D. G.;
Gopalswamy, N.; Howard, R. E.; Kaiser, M. L.; Kassim, N. E.; Kuiper,
T. B. H.; MacDowall, R. J.; Mahoney, M. J.; Perley, R. A.; Preston,
R. A.; Reiner, M. J.; Rodriguez, P.; Stone, R. G.; Unwin, S. C.;
Weiler, K. W.; Woan, G.; Woo, R.
2000AdSpR..26..743J Altcode:
The frequency range below a few tens of MHz is unexplored with high
angular resolution due to the opacity of Earth's ionosphere. An
interferometer array in space providing arcminute angular resolution
images at frequencies of a few MHz would allow a wide range of
problems in solar, planetary, galactic, and extragalactic astronomy
to be attacked. These include the evolution of solar radio emissions
associated with shocks driven by coronal mass ejections and searches
for coherent radio emission from supernova remnants and relativistic
jets. In addition, it is likely that unexpected objects or emission
processes will be discovered by such an instrument, as has always
happened when high resolution astronomical observations first
become possible in a new region of the electromagnetic spectrum. The
Astronomical Low Frequency Array (ALFA) mission will consist of 16
identical small satellites forming an aperture synthesis array. The
satellites will cover the surface of a spherical region. ~100 km
in diameter, thus providing good aperture plane coverage in all
directions simultaneously. The array will operate in two modes: 1)
“snapshot” imaging of strong, rapidly changing sources such as
solar radio bursts and 2) long-term aperture synthesis observations
for maximum sensitivity, high dynamic range imaging. In both cases a
large number of array elements is needed
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Title: Solar Radio Emission
Authors: Bastian, T.
2000asqu.book..375B Altcode:
No abstract at ADS
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Title: Propagation of Radio Waves in the Corona and Solar Wind
Authors: Bastian, T. S.
2000GMS...119...85B Altcode: 2000ralw.conf...85B
The solar corona and solar wind are plasmas characterized by large
scale MHD structures, waves, and turbulence. These introduce both
systematic and random variations in the refractive index which affect
the propagation of radio waves. A variety of propagation phenomena occur
-- regular refraction; angular, temporal, and spectral broadening;
scintillations in amplitude and phase -- widely referred to as
scattering phenomena. In this tutorial I review the physical basis
of these phenomena and describe a variety of techniques designed to
exploit observations of scattering phenomena to deduce properties of
the corona and solar wind plasma.
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Title: The Astronomical Low Frequency Array: A Proposed Explorer
Mission for Radio Astronomy
Authors: Jones, D.; Allen, R.; Basart, J.; Bastian, T.; Blume, W.;
Bougeret, J. -L.; Dennison, B.; Desch, M.; Dwarakanath, K.; Erickson,
W.; Finley, D.; Gopalswamy, N.; Howard, R.; Kaiser, M.; Kassim, N.;
Kuiper, T.; MacDowall, R.; Mahoney, M.; Perley, R.; Preston, R.;
Reiner, M.; Rodriguez, P.; Stone, R.; Unwin, S.; Weiler, K.; Woan,
G.; Woo, R.
2000GMS...119..339J Altcode: 2000ralw.conf..339J
A radio interferometer array in space providing high dynamic range
images with unprecedented angular resolution over the broad frequency
range from 0.03 - 30 MHz will open new vistas in solar, terrestrial,
galactic, and extragalactic astrophysics. The ALFA interferometer
will image and track transient disturbances in the solar corona
and interplanetary medium - a new capability which is crucial
for understanding many aspects of solar-terrestrial interaction
and space weather. ALFA will also produce the first sensitive,
high-angular-resolution radio surveys of the entire sky at low
frequencies. The radio sky will look entirely different below about
30 MHz. As a result, ALFA will provide a fundamentally new view of
the universe and an extraordinarily large and varied science return.
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Title: Radio Interferometric Observations of Scattering Phenomena
in the Outer Solar Corona
Authors: Bastian, T. S.
1999ESASP.448.1131B Altcode: 1999mfsp.conf.1131B; 1999ESPM....9.1131B
No abstract at ADS
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Title: Solar Physics with Radio Observations
Authors: Bastian, T. S.; Gopalswamy, N.; Shibasaki, K.
1999spro.proc.....B Altcode:
Radio observations contribute a unique perspective on the many physical
phenomena, which occur on the Sun. From thermal bremsstrahlung emission
in the quiet solar atmosphere and filaments, to thermal gyroresonance
emission in strongly magnetized solar active regions, to the nonthermal
emission from MeV electrons accelerated in flares, observations of radio
emission provide a powerful probe of physical conditions on the Sun and
provide an additional means of understanding the myriad phenomena which
occur there. Moreover, radio observing techniques have led the way in
developing and exploiting Fourier synthesis imaging techniques. The
Nobeyama Radioheliograph, commissioned in June, 1992, soon after the
launch of Yohkoh satellite in August, 1991, is the most powerful,
solar-dedicated Fourier synthesis in the world, now capable of imaging
the full disk of the Sun simultaneously at frequencies of 17 and 34
GHz, with an angular resolution as much as 10" and 5", respectively,
and with a time resolution as fine as 100 msec. Between 27-30 October,
1998, the Nobeyama Radio Observatory and the National Astronomical
Observatory of Japan hosted the Nobeyama Symposium on Solar Physics
with Radio Observations, an international meeting bringing more than
sixty participants together at the Seisenryo Hotel in Kiyosato, for a
meeting devoted to reviewing recent progress in outstanding problems
in solar physics. Emphasis was placed on radio observations and,
in particular, radio observations from the very successful Nobeyama
Radioheliograph. These results were compared and contrasted with those
that have emerged from the Yohkoh mission. In addition, looking forward
to the next solar maximum, new instruments, upgrades, and collaborative
efforts were discussed. The result is the more than seventy invited
and contributed papers that appear in this volume.
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Title: Impulsive Flares: A Microwave Perspective
Authors: Bastian, T. S.
1999spro.proc..211B Altcode:
Impulsive flares, by far the most common flares on the sun, impose
strong demands on any theory of energy release and of particle
acceleration. For a large flare, ~ 10<SUP>37</SUP> electrons with
energies >20 keV must be accelerated each second. These electrons
produce hard X-rays by nonthermal bremsstrahlung emission and radio
waves by plasma radiation processes and gyrosynchrotron emission. The
hard X-ray emission is dominated by thick-target bremsstrahlung from
fast electrons streaming directly from the acceleration site to the low
corona and upper chromosphere, and by fast electrons precipitating from
the magnetic trap. By contrast, microwave emission may be dominated by
trapped electrons. In this paper, I briefly discuss the basic properties
of microwave emission from impulsive flares: its morphology, source
kinematics, and its relation to emissions at other wavelengths. I then
summarize diagnostic uses of microwave emission and conclude with
a description of recent work on the problem of the relative timing
of HXR and microwave emission. It is shown that magnetic loops act
as dispersive elements, allowing fixed-frequency observations to be
used to probe electrons of differing energy. Spatially and temporally
resolved observations of gyrosynchrotron emission from solar flares,
such as those available from the Nobeyama Radioheliograph, enable one
to constrain the evolution of the electron distribution function in
time. I discuss some results of a comparative timing analysis of a
sample of flares observed by the Nobeyama Radioheliograph and by the
CGRO BATSE instrument.
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Title: Structure and Dynamics in the Transition Region
Authors: Gontikakis, C. P.; Dara, H. C.; Alissandrakis, C. E.;
Zachariadis, Th. G.; Vial, J. -C.; Bastian, T.; Chiuderi Drago, F.
1999ESASP.448..297G Altcode: 1999mfsp.conf..297G; 1999ESPM....9..297G
No abstract at ADS
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Title: A search for radio emission from extrasolar planets.
Authors: Bastian, T. S.; Dulk, G. A.; Leblanc, Y.; Sault, R. J.
1999BAAS...31.1086B Altcode:
No abstract at ADS
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Title: A Search for Radio Emission from Extrasolar Planets
Authors: Bastian, T. S.; Dulk, G. A.; Leblanc, Y.; Sault, R.
1999DPS....31.0901B Altcode:
Fifteen giant planets of mass about 1--13 M_J and eleven brown dwarfs
of mass 13--50 M_J have now been discovered by their gravitational
effects on solar-like stars. There is reason to believe that
these objects have magnetic fields and that they emit meter- or
decimeter-wavelength radio radiation through the electron cyclotron
maser mechanism. Although the visible and infrared radiation from
exoplanets is much weaker than that of the parent stars, the radio
emission need not necessarily be. Extremely intense radio emission can
be generated by the electron-cyclotron maser instability. For Jupiter
the cyclotron-maser radiation is 10(5-10^6) times more intense than
its synchrotron emission from the radiation belts and its thermal
emission from the disk. If detected from a planet, several important
parameters can be deduced, e.g. its rotation period and magnetic
field strength. We report the results of searchs for radio emission
from a sample of confirmed extrasolar planets and brown dwarfs. The
Very Large Array (VLA) was used in late 1996 to observe seven planets
and two brown dwarfs at 1400 and 330 MHz. In early 1998, both 47 UMa
(planet) and HD98239 (brown dwarf) were observed at 330 and the new 74
MHz system. The National Radio Astronomy Observatory is a facility of
the National Science Foundation operated under cooperative agreement
by Associated Universities, Inc.
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Title: FASR - A Frequency-Agile Solar Radiotelescope
Authors: Hurford, G. J.; Gary, D. E.; Bastian, T. S.; White, S. M.
1999AAS...194.7603H Altcode: 1999BAAS...31..956H
The Frequency-Agile Solar Radiotelescope (FASR) is a multi-frequency
imaging array designed specifically for imaging spectroscopy of the
Sun. Using < 100 antennas, it will combine high-quality/high spatial
resolution imaging (1" resolution at 20 GHz) with spectroscopy (dnu
/nu 0.01-0.03) and high time resolution (<1 s) across two decades in
frequency from 0.3-30 GHz. In so doing, it will produce a continuous,
dynamic, three-dimensional picture of the solar atmosphere from the
chromosphere through the mid-corona. These capabilities represent a
quantum leap beyond existing solar radio instruments, yet are well
within reach of emerging technologies. The range of science that
can be addressed by such an instrument is as broad as solar physics
itself. Virtually every solar feature from within a few hundred km
of the visible surface of the Sun to high in the solar corona can be
studied in detail with the unique diagnostics available in the radio
regime. Particular diagnostics include measuring the properties of
both thermal and nonthermal electrons accelerated in solar flares
from the largest events to the tiniest microflares/nanoflares,
measuring coronal magnetic field strengths in active regions and
elsewhere (coronal magnetography), and mapping kinetic electron
temperatures throughout the chromosphere and corona. In addition,
FASR's far-reaching exploration of the Sun in the radio regime gives
the instrument tremendous potential for new discoveries beyond those
that we can now anticipate. FASR is expected to be one of the major
new ground-based solar instruments of the next decade, and can be
operational by 2006, well before the decade is out. It will play a
major role in supporting NASA space missions with the unique diagnostics
and perspective provided by high-resolution radio imaging/spectroscopy.
---------------------------------------------------------
Title: Solar System Objects
Authors: Butler, B. J.; Bastian, T. S.
1999ASPC..180..625B Altcode: 1999sira.conf..625B
Long wavelength (λ ~> 350 microns) interferometric observations
of solar system objects can yield important information on the nature
of these bodies, including information on orbits, spins, surfaces,
atmospheres, magnetic fields, rings, and plasma processes. This lecture
will describe some of the peculiarities involved with observing of
solar system bodies with synthesis arrays. These include planning,
scheduling, calibration, and imaging.
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Title: Broadband microwave imaging spectroscopy with a solar-dedicated
array
Authors: Bastian, Tim S.; Gary, D. E.; White, S. M.; Hurford, Gordon J.
1998SPIE.3357..609B Altcode:
For many years, ground-based radio observations of the Sun have
proceeded into two directions: (1) high resolution imaging at a few
discrete wavelengths; (2) spectroscopy with limited or no spatial
resolution at centimeter, decimeter, and meter wavelengths. Full
exploitation of the radio spectrum to measure coronal magnetic fields
in both quiescent active regions and flares, to probe the thermal
structure of the solar atmosphere, and to study energy release and
particle energization in transient events, requires a solar-dedicated,
frequency-agile solar radiotelescope, capable of high-time, - spatial,
and -spectral resolution imaging spectroscopy. In this paper we
summarize the science program and instrument requirements for such a
telescope, and present a strawman interferometric array composed of
many (greater than 40), small (2 m) antenna elements, each equipped
with a frequency- agile receiver operating over the range 1 - 26.5 GHz.
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Title: Coordinated Observations with SOHO, YOHKOH and VLA
Authors: Aschwanden, Markus J.; Bastian, Tim S.; Nitta, Nariaki;
Newmark, Jeff; Thompson, Barbara J.; Harrison, Richard A.
1998ASPC..155..311A Altcode: 1998sasp.conf..311A
No abstract at ADS
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Title: 3-Dimensional Models of Active Region Loops
Authors: Aschwanden, M. J.; Neupert, W. M.; Newmark, J.; Thompson,
B. J.; Brosius, J. W.; Holman, G. D.; Harrison, R. A.; Bastian, T. S.;
Nitta, N.; Hudson, H. S.; Zucker, A.
1998ASPC..155..145A Altcode: 1998sasp.conf..145A
No abstract at ADS
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Title: VLBA Imaging of UX Ari
Authors: Beasley, A. J.; Bastian, T. S.
1998ASPC..144..321B Altcode: 1998rege.conf..321B; 1998IAUCo.164..321B
We have used the Very Long Baseline Array (VLBA) and the Very Large
Array (VLA) to perform multi-wavelength radio imaging of the RS CVn
active binary UX Arietis during the period November 16-21 1995. A
series of strong flaring events exhibiting optically-thick spectra
peaking above 100 GHz and extended source structures consistent with
inter-binary emission were detected.
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Title: Radio Emission from Solar Flares
Authors: Bastian, T. S.; Benz, A. O.; Gary, D. E.
1998ARA&A..36..131B Altcode:
Radio emission from solar flares offers a number of unique diagnostic
tools to address long-standing questions about energy release,
plasma heating, particle acceleration, and particle transport
in magnetized plasmas. At millimeter and centimeter wavelengths,
incoherent gyrosynchrotron emission from electrons with energies of
tens of kilo electron volts (keV) to several mega electron volts (MeV)
plays a dominant role. These electrons carry a significant fraction
of the energy released during the impulsive phase of flares. At
decimeter and meter wavelengths, coherent plasma radiation can play a
dominant role. Particularly important are type III and type III-like
radio bursts, which are due to upward- and downward-directed beams
of nonthermal electrons, presumed to originate in the energy release
site. With the launch of Yohkoh and the Compton Gamma-Ray Observatory,
the relationship between radio emission and energetic photon emissions
has been clarified. In this review, recent progress on our understanding
of radio emission from impulsive flares and its relation to X-ray
emission is discussed, as well as energy release in flare-like phenomena
(microflares, nanoflares) and their bearing on coronal heating.
---------------------------------------------------------
Title: EUV and Microwave Observations of a Filament
Authors: Chiuderi-Drago, F.; Bocchialini, K.; Lamartinie, S.;
Vial, J. -C.; Bastian, T.; Alissandrakis, C. E.; Harrison, R.;
Delaboudiniere, J. -P.; Lemaire, P.
1998ASPC..150...55C Altcode: 1998IAUCo.167...55C; 1998npsp.conf...55C
No abstract at ADS
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Title: Toward a Frequency-Agile Solar Radiotelescope
Authors: Bastian, T. S.; Gary, D. E.; White, S. M.; Hurford, G. J.
1998ASPC..140..563B Altcode: 1998ssp..conf..563B
No abstract at ADS
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Title: Search for radio emission of extrasolar planets
Authors: Dulk, George A.; Leblanc, Yolande; Bastian, Timothy S.
1998pslv.conf..421D Altcode:
No abstract at ADS
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Title: The Structure of the Solar Corona above Sunspots as Inferred
from Radio, X-Ray, and Magnetic Field Observations
Authors: Vourlidas, A.; Bastian, T. S.; Aschwanden, M. J.
1997ApJ...489..403V Altcode:
We present observations of a solar active region, NOAA/USAF no. 7123,
during 1992 April 3-10. The database includes high-angular-resolution
radio, soft X-ray, magnetograph, and Hα observations. The radio
observations include VLA maps in the Stokes I and V parameters at 4.7
and 8.4 GHz. The soft X-ray observations were obtained by the Soft
X-Ray Telescope on board the Yohkoh satellite, the magnetograms were
obtained at Kitt Peak, Mt. Wilson, and Big Bear, and the Hα data were
obtained at Big Bear. <P />The lead sunspot in the active region is
studied here. In particular, the polarization properties and brightness
temperature spectrum are used to constrain the thermal structure of the
corona over the sunspot. It is found that the 4.7 GHz emission of the
sunspot is polarized in the sense of the ordinary mode, in contradiction
with simple gyroresonance models that predict that the spot should be
polarized in the sense of the extraordinary mode. We model the spectral
and temporal evolution of the polarization structure in two frequencies,
4.7 and 8.4 GHz, using gyroresonance models to fit one-dimensional
brightness temperature profiles across the spot in each polarization
and frequency. The constraints provided by the X-ray and magnetic
field observations help us to derive a qualitatively self-consistent
picture for the daily evolution of the spot. We attribute the excess
of the o-mode emission to the magnetic field configuration and to the
temperature inhomogeneities across the spot. Namely, we find that (1)
the umbral and penumbral environments are distinct, with the X-rays and
the o-mode radio emission coming from the hotter penumbral loops, while
the observed x-mode emission originates from the cooler umbral loops;
(2) there exist temperature inhomogeneities in both the radial and
vertical direction over the spot; and (3) the umbral magnetic field
remains more confined in the corona than that predicted by a dipole
model. Instead, a field configuration based on the magnetohydrostatic
equilibrium model of Low gives a better agreement with the observations.
---------------------------------------------------------
Title: X-Ray Network Flares of the Quiet Sun
Authors: Krucker, Säm; Benz, Arnold O.; Bastian, T. S.; Acton,
Loren W.
1997ApJ...488..499K Altcode:
Temporal variations in the soft X-ray (SXR) emission and the radio
emission above the solar magnetic network of the quiet corona are
investigated using Yohkoh SXR images with deep exposure and VLA
observations in the centimeter radio range. The SXR data show several
brightenings, with an extrapolated occurrence probability of one
brightening per 3 seconds on the total solar surface. During the roughly
10 minutes of enhanced flux, total radiative losses of the observed
plasma are around 10<SUP>25</SUP> ergs per event. These events are more
than an order of magnitude smaller than previously reported X-ray bright
points or active region transient brightenings. For all of the four
SXR events with simultaneous radio observations, a corresponding radio
source correlating in space and time can be found. There are several
similarities between solar flares and the SXR/radio events presented in
this paper. (1) Variations in temperature and emission measure during
the SXR enhancements are consistent with evaporation of cooler material
from the transition region and the chromosphere. (2) The ratio of the
total energies radiated in SXR and radio frequencies is similar to
that observed in flares. (3) At least one radio event shows a degree
of polarization as high as 35%. (4) In three out of four substructures
the centimeter radio emission peaks several tens of seconds earlier
than in the SXR emission. (5) The associated radio emission tends to
be more structured and to have faster rise times. These events thus
appear to be flare-like and are called network flares.
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Title: On the feasibility of imaging coronal mass ejections at
radio wavelengths
Authors: Bastian, T. S.; Gary, D. E.
1997JGR...10214031B Altcode:
Coronal mass ejections (CMEs) can have a profound impact on the
interplanetary medium and the near-Earth environment. We discuss the
feasibility of detecting coronal mass ejections at radio wavelengths
with a ground-based instrument. In particular, we explore the
possibility that a radio telescope employing Fourier synthesis imaging
techniques can detect thermal bremsstrahlung emission from CMEs. Using
a simulated database from such a telescope, we explore three detection
schemes: direct detection, an approximate differential detection scheme,
and an “exact” differential detection scheme. We conclude that thermal
bremsstrahlung emission from CMEs can be detected by such a telescope
provided differential techniques are employed. While the approximate
differential detection scheme may be sufficient for CMEs viewed near
the solar limb, detection of CMEs against the solar disk may require
the more sensitive exact differential scheme. The detection and imaging
of nonthermal radio emissions from CMEs is also discussed.
---------------------------------------------------------
Title: Search for Cyclotron-maser Radio Emission from Extrasolar
Planets
Authors: Dulk, G. A.; Leblanc, Y.; Bastian, T. S.
1997DPS....29.2803D Altcode: 1997BAAS...29.1025D
There is reason to believe that extrasolar planets and/or brown dwarfs
of mass about 1 to 50 M_J have magnetic fields, that they emit extremely
intense cyclotron-maser radiation at metric wavelengths, and that this
radiation may be detectable with sensitive radio telescopes like the
VLA. The radiation is emitted at the electron cyclotron frequency,
and has been detected from Earth, Jupiter, Saturn, Uranus and Neptune,
from the Sun, from flare stars, and close binaries. The frequency range
of cyclotron maser radiation is fixed by the magnetic field strength
on the object. To be detected at, say 0.33 GHz, the required field
strength is 118 G, which is intermediate between the 14 G field of
Jupiter and the ~ 1000 G field of stellar active regions. An estimation
of the flux density of the expected radiation can be made from an
interpolation between Jupiter's radio emission (10(10) mJy at 5 AU)
and that of nearby red dwarf stars ( ~ 100 mJy at 3 pc). Thus the flux
from a planet or brown dwarf 3 to 30 times massive than Jupiter is
plausibly 1 to 10 mJy, easily detectable with the VLA. If emission is
detected, several important parameters about the planet can be deduced:
the strength of its magnetic field, the period of its rotation, and the
possible existence of a moon such as Jupiter's Io. Possible means of
distinguishing stellar maser emissions from those of planets include:
1) Temporal variations and spectra differ from stars to planets. 2) The
polarization is likely to be 100% circular or elliptical for planets,
but not for stars. We have searched for exoplanet radio emission with 60
hours of observations with the VLA during November 1996. The observed
stars with giant planets or brown dwarfs included 51 Peg, 70 Vir, 47
UMa, 55 CnC, Tau Boo, Gl 229, and HD 114762. We will present the method
of observation, the limitations due to confusion and background noise,
and the results.
---------------------------------------------------------
Title: Relative Timing of Microwave and HXR Bursts
Authors: Bastian, T. S.; Aschwanden, M. J.
1997SPD....28.1803B Altcode: 1997BAAS...29..922B
The close correlation between microwave and hard X-ray (HXR) emission
during flares has often been cited as evidence that the same population
of energetic electrons is responsible for both types of emission. The
two emissions differ in detail, however. Imaging observations
have demonstrated that the two are not necessarily cospatial and
timing observations have demonstrated that the microwaves are often
significantly delayed with respect to HXR emission, typically by
several seconds, but occassionally by much longer times. Such delays
are in seeming conflict with the thick target model for HXR emission in
its simplest form, and with the idea that microwave and HXR emissions
result from essentially the same population of electrons. One way to
reconcile the delay between microwaves and HXRs in the thick target
model is to suppose that that electron trapping is significant (e.g.,
Cornell et al., ApJ, 279, 875). For a magnetic trap containing a
plasma of constant density, high energy electrons have a longer
lifetime against collisions than low energy electrons (tau_ {def} ~
E(3/2) ). Hence the energetic electrons responsible for the microwaves
remain in the trap longer and the microwave emission they emit peaks
later than the HXR emission. Another possibility is that higher energy
electrons are accelerated later than lower energy electrons (so-called
“second-step” acceleration models; e.g., Bai and Dennis 1985, ApJ,
292, 699). To explore the question in detail we have assembled a sample
of 16 flares observed simultaneously in microwaves by the Nobeyama
radioheliograph and in HXRs by the BATSE instrument on board the CGRO
in burst trigger mode. The former imaged the flares at 17 GHz with
an angular resolution of ~ 10” and a time resolution of either 50
msec or 1 sec. The latter obtained medium energy resolution spectra
(16 channels) between 20-200 keV with a time resolution of 16 or 64
msec. We present preliminary results of our analysis.
---------------------------------------------------------
Title: Chromospheric Events in the Quiet Network
Authors: Keller, C.; Bastian, T.; Benz, A.; Krucker, S.
1997SPD....28.1304K Altcode: 1997BAAS...29..917K
Time sequences of a quiet network region close to disk center have
been simultaneously recorded with the VLA, various instruments on SOHO,
and the solar telescopes on Kitt Peak. The analysis of the Hα spectra
obtained at the McMath-Pierce telescope revealed down-flows with
apparent velocities of more than 2.5 km/s associated with magnetic
field structures in the quiet network. During such events, the Hα
spectra show a pronounced asymmetry. The photospheric magnetic field was
determined from rapid scans in three iron lines with the Zurich Imaging
Stokes Polarimeter. Up- and down-flow velocity excursions outside of
magnetic field regions are compatible with chromospheric waves. We
describe the properties of these events as seen in the observations
of the visible part of the spectrum and their signatures at radio
and UV wavelengths. The final goal of this study is the construction
of a time-dependent 3-D picture of the quiet solar atmosphere and the
understanding of the dynamical coupling of photospheric magnetic fields
with the chromosphere and the corona.
---------------------------------------------------------
Title: Fine structure of the X-ray and radio emissions of the quiet
solar corona.
Authors: Benz, A. O.; Krucker, S.; Acton, L. W.; Bastian, T. S.
1997A&A...320..993B Altcode:
Two deep soft X-ray exposures of a quiet region on the Sun were made
with the SXT telescope on board the Yohkoh satellite on 20 Feb 1995. We
report on the spatial X-ray fine structure. Regions of enhanced X-ray
emission, more than two orders of magnitude fainter than previously
reported X-ray bright points, are loosely associated with bipolar
regions in the magnetic network. The power spectrum of quiet X-ray
images at small spatial scales is similar to that of active regions,
but exhibits a kink at a scale of =~25,000km, possibly connected to
the supergranular structure. The spatial X-ray structures in the
time averaged image amount to an rms amplitude which is 6% of the
mean value. The X-ray structures correlate with contemporaneous radio
maps obtained by the VLA at wavelengths of 1.3, 2.0, and 3.6cm. The
amplitude of the brightness variations in the images increases with
radio wavelength, i.e., with increasing height. The cross-correlation
coefficient with the absolute magnetic field strength, however,
generally decreases with height, consistent with the idea of bipolar
regions in the network and of the magnetic field deviating from
vertical in the upper chromosphere. The X-ray observations require an
enhanced pressure in the corona above the magnetic network, but suggest
similar temperatures. Model calculations show that, under a constant
temperature, an rms density increase (relative to that in the cell
interior) ranging from about 20% in the chromosphere to 60% in the low
corona is sufficient to explain the observed standard deviations due
to the spatial structures in radio waves and soft X-rays, respectively.
---------------------------------------------------------
Title: High Resolution Millimeter and Submillimeter Observations of
Active Stars
Authors: Bastian, Tim S.
1997msma.conf...27B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Prospects for the Solar Radio Telescope
Authors: Bastian, Timothy S.; Gary, Dale E.
1997LNP...483..218B Altcode: 1997cprs.conf..218B
The Solar Radio Telescope (SRT) is an instrument concept for a powerful
solar-dedicated radio telescope. As presently conceived. it would
combine a high-resolution imaging capability (2″ at 20 GHz) with a
broadband spectroscopic capability (0.3-26 GHz). In other words, the SRT
would perform broadband imaging spectroscopy on a wide range of quiet-
and active-Sun phenomena. On 17-20 April, 1995, a workshop was held in
San Juan Capistrano, California. The purpose of the workshop, which was
attended by more than 40 scientists from the US and around the world,
was to discuss the science that could be done with a solar-dedicated
radio synthesis telescope, and to discuss the design constraints
imposed by the science envisioned. Special attention was also given
to nighttime uses for the instrument. We summarize the "strawman"
concept for the instrument here.
---------------------------------------------------------
Title: Energy Release in the Solar Corona
Authors: Bastian, Timothy S.; Vlahos, Loukas
1997LNP...483...68B Altcode: 1997cprs.conf...68B
Energy release in the solar corona drives a wide variety of phenomena,
including flares, filament/prominence eruptions, coronal mass ejections,
solar particle events, as well as coronal heating and the solar
wind. The basic physics of these phenomena and their relationship
to each other remains a vigorous area of inquiry. The Working Group
on Energy Release at Mont Evray directed its attention to recent
observational and theoretical developments relevant to flares and
coronal heating. Particular attention was given to the "fragmentation"
of energy release in solar flares and its interpretation; to the
statistics of the flare phenomenon and whether they can be understood in
terms of "driven dissipative systems"; to quasisteady energy release
and the problem of coronal heating; and to recent observations of
flares and related phenomena.
---------------------------------------------------------
Title: X-Ray/Radio Network Flares of the Quiet Sun
Authors: Benz, Arnold O.; Krucker, Sam; Acton, Loren W.; Bastian, T. S.
1997IAUJD..19E...1B Altcode:
The temporal variations in the soft X-ray (SXR) emission and the
radio emission above the solar magnetic network of the quiet corona
have been investigated using Yohkoh SXR images with deep exposure and
VLA observations in the centimetric radio range. The SXR data show
several brightenings with an extrapolated occurrence probability of
one brightening per 3 seconds on the total solar surface. During the
roughly 10 minutes of enhanced flux, the total radiative losses of
the observed plasma are betwee () n 0.6 and 2.4 cdot 10<SUP>26</SUP>
erg per event. These events are more than an order of magnitude smaller
than previously reported X-ray bright points or active region transient
brightenings. For all of the four SXR events with simultaneous radio
observations, a corresponding radio source correlating in space
and time can be found. There are several similarities between these
SXR/radio events and regular solar flares. These events thus appear
to be flare-like and are called network flares. We will report also
on very recent work using SOHO's EIT and CDS experiments combined with
VLA and Kitt Peak observations.
---------------------------------------------------------
Title: Microwave and UV observations of filaments with SOHO and
the VLA
Authors: Alissandrakis, C. E.; Drago, F.; Bastian, T.; Bocchialini,
K.; Delaboudiniere, J. -P.; Lemaire, P.; Vial, J. -C.; Harrison,
R. A.; Thompson, B.
1997ASPC..118..289A Altcode: 1997fasp.conf..289A
Observations performed in coordination between SOHO instruments and
ground-based observatories offer the unique possibility to derive
information simultaneously in several wavelengths formed at different
altitudes and/or temperatures in the solar atmosphere. The SUMER and
CDS spectrometers, the imaging telescope EIT aboard SOHO, and the VLA
provide complementary information in the UV and the radio ranges. We
illustrate such a coordination with observations of filaments in the
transition region, performed in July 1996. The observations in the UV
between 10(4) and 10(6) K provide the differential emission measure
as a function of temperature; this can be used to compute the expected
brightness temperature in the microwave range and check models of the
filament-corona transition region.
---------------------------------------------------------
Title: High-Resolution Microwave Observations of the Quiet Solar
Chromosphere
Authors: Bastian, T. S.; Dulk, G. A.; Leblanc, Y.
1996ApJ...473..539B Altcode:
The VLA was used to observe a quiet region of the Sun on 1992 September
23 at 1.3 and 2 cm. Unlike previous interferometric microwave
observations of the quiet Sun, we have used the total power data
to calibrate the brightness temperature distribution in an absolute
sense. We find a good correlation between the time-averaged 1.3 and 2 cm
brightness distributions, and, in agreement with past studies at 3.6 and
6 cm, we find that both the 1.3 and 2 cm brightness distributions are
closely correlated with the network magnetic field. The mean brightness
at 1.3 and 2 cm was 10,400±1230 K and 12890±1415 K, respectively. The
width of the σ<SUB>1.3</SUB> cm brightness distribution function
is σ<SUB>1.3</SUB> = 270 K, while that at 2 cm is σ<SUB>2</SUB> =
460 K. <P />We have examined the time variability of the 1.3 and 2 cm
emission on a timescale of 2 hr. The correlation between the 1.3 and
2 cm brightness distributions is maintained, as is the correlation
with the underlying magnetic field. However, considerable variability
in the details of the brightness distribution is evident during the
course of the day. <P />We compare our mean brightness measurements
with those of Zirin, Baumert, & Hurford and compare the ensemble
of observations with semi-empirical models of the chromosphere and
transition region. The M<SUB>CO</SUB> model proposed by Avrett,
which is in agreement with carbon monoxide observations, yields
a microwave brightness temperature spectrum that is in excellent
agreement with the microwave observations of Zirin et al. and those
reported in this paper. The need for a model that reconciles all
chromospheric observations optical, UV, infrared, and radio remains
however. Inhomogeneous and/or dynamic chromospheric models are likely
required.
---------------------------------------------------------
Title: Multiband VLA Observations of Solar Active Regions:
Implications for the Distribution of Coronal Plasma
Authors: Vourlidas, A.; Bastian, T. S.
1996ApJ...466.1039V Altcode:
We present high-quality radio observations of a solar active region
(NOAA/USAF region 5131) made by the Very Large Array in the 0.33, 1.5,
4.7, and 8.4 0Hz frequency bands. The observations were made during
the IAU-sanctioned International Solar Month on 1988 September 1 and
4. In addition to the radio maps, the database includes images from the
Solar Maximum Mission Soft X-ray Polychromator in the Fe XVII line,
photo spheric magnetograms, and Hα filtergrams. Because coverage in
only a single wavelength is available, the soft X-ray data are used for
qualitative comparisons. <P />We identify those areas of the active
region from which the radio emission is predominantly due to thermal
bremsstrahlung emission. To account for the observed four-point
radio spectra in these areas, we examine three different models,
which characterize the corona in terms of a single plasma component,
two components, and multiple components. The latter differs somewhat
from past models. In particular, we take explicit account of the
highly inhomogeneous structure of active regions through a multilayer
slab model in which the thermal distribution is constrained by the
generic form of the differential emission measure. While multicomponent
models provide the best agreement with the observations, we find that a
complete model must include both free-free and gyroresonance opacity. We
compare our radio data and model results with those resulting from
previous studies.
---------------------------------------------------------
Title: On the Peculiar Radio Polarization of a Sunspot and the
Distibution of the Coronal Plasma
Authors: Vourlidas, A.; Bastian, T. S.; Aschwanden, M. J.
1996AAS...188.3602V Altcode: 1996BAAS...28..873V
A comprehensive set of microwave, soft x-ray and magnetic observations
of solar active region NOAA/USAF 7123 was obtained during 4--9 April,
1992. The observations show an unusual 4.7 GHz source which is polarized
in the sense of the ordinary mode over the umbra of the leading spot. A
detailed analysis of the daily radio and soft X-ray maps of the spot
indicates that (i) the umbral magnetic field is strongly confined,
(ii) the umbral and penumbral loops have distinct atmospheres and (iii)
temperature gradients may exist both radially and vertically over the
spot. A possible scenario for the temporal evolution of the spot is
also presented.
---------------------------------------------------------
Title: A Search for Radio Pulsations from AE Aquarii
Authors: Bastian, T. S.; Beasley, A. J.; Bookbinder, J. A.
1996ApJ...461.1016B Altcode:
AE Aquarii is classified as a DQ Herculis-type cataclysmic
variable, comprising a magnetized white dwarf primary and a KS dwarf
secondary. Its orbital period is 9.88 hr while the rotational period of
the magnetized primary is 33.08 s. AE Aqr is characterized by coherent
pulsations and quasi-periodic oscillations (QPOs) in the optical,
UV, and soft X-ray wavelength bands. It also emits TeV γ-rays, which
may themselves be modulated on a period near the rotation period of
the primary. In addition, AE Aqr displays violent flaring activity at
optical, soft X-ray, and radio wavelengths. <P />We have searched for
coherent pulsations and QPOs in the radio emission from AE Aqr. The
Very Large Array (VLA) was used on 1994 January 29 to observe AE Aqr
continuously for ∼10 hr at 8.4 GHz (λ = 3.6 cm). We find no evidence
for the presence of coherent pulsations or QPOs. An upper limit of 1.6%
of the mean flux density is placed on the amplitude of pulsations on
frequencies greater than 0.01 Hz. Over bandwidths of 1% on the known
fundamental and harmonic periods, the upper limits are 0.8%. The power
spectrum is characterized by red noise on frequencies less than 0.01
Hz. It is well fitted by a power law with an index of 1.8. Furthermore,
comparing the present observations with data obtained at seven previous
epochs, we find no evidence of a relationship between flaring activity
and the orbital phase.
---------------------------------------------------------
Title: Broadband Imaging Spectroscopy with the Solar Radio Telescope
Authors: Bastian, T. S.; Gary, D. E.; Hurford, G. J.; Hudson, H. S.;
Klimchuk, J. A.; Petrosian, V.; White, S. M.
1996ASPC...93..430B Altcode: 1996ress.conf..430B
No abstract at ADS
---------------------------------------------------------
Title: The Time Variability of Radio Emission from AE Aquarii
Authors: Bastian, T. S.; Beasley, A. J.; Bookbinder, J. A.
1996ASPC...93..185B Altcode: 1996ress.conf..185B
No abstract at ADS
---------------------------------------------------------
Title: Radio Emission from AE Aquarii
Authors: Abada-Simon, M.; Bastian, T. S.; Fletcher, L.; Horne, K.;
Kuijpers, J.; Steeghs, D.; Bookbinder, J. A.
1996ASPC...93..182A Altcode: 1996ress.conf..182A
No abstract at ADS
---------------------------------------------------------
Title: Radio Flares on Stars: Possible Solar Analogs?
Authors: Bastian, T. J.
1996ASPC...93..447B Altcode: 1996ress.conf..447B
No abstract at ADS
---------------------------------------------------------
Title: VLBA imaging of RS CVn systems: UX Ari
Authors: Beasley, A. J.; Bastian, T. S.
1996ASPC..109..639B Altcode: 1996csss....9..639B
No abstract at ADS
---------------------------------------------------------
Title: VLA Observations of Decimetric Microbuzsts in the Solar Corona
Authors: Kliem, B.; Bastian, T. S.
1996ASPC...93..372K Altcode: 1996ress.conf..372K
No abstract at ADS
---------------------------------------------------------
Title: Joint Radio and Soft X-Ray Imaging of an `Anemone' Active
Region
Authors: Vourlidas, A.; Bastian, T. S.; Nitta, N.; Aschwanden, M. J.
1996SoPh..163...99V Altcode:
The Very Large Array and the Soft X-ray Telescope (SXT) aboard the
Yohkoh satellite jointly observed the rapid growth and decay of a
so-called `anemone' active region on 3-6 April, 1992 (AR 7124). The
VLA obtained maps of the AR 7124 at 1.5, 4.7, and 8.4 GHz. In general,
discrete coronal loop systems are rarely resolved at 1.5 GHz wavelengths
because of limited brightness contrast due to optical depth effects and
wave scattering. Due to its unusual anemone-like morphology, however,
several discrete loops or loop systems are resolved by both the VLA
at 1.5 GHz and the SXT in AR 7124.
---------------------------------------------------------
Title: Radio Emission from Solar and Stellar Flares
Authors: Bastian, T. S.
1996mpsa.conf..259B Altcode: 1996IAUCo.153..259B
No abstract at ADS
---------------------------------------------------------
Title: Location of Type I Radio Continuum and Bursts on YOHKOH Soft
X-ray Maps
Authors: Krucker, S.; Benz, A. O.; Aschwanden, M. J.; Bastian, T. S.
1996mpsa.conf..441K Altcode: 1996IAUCo.153..441K
No abstract at ADS
---------------------------------------------------------
Title: Active Region 7123: Its Peculiar Radio Polarization and the
Distribution of the Umbral Coronal Plasma
Authors: Vourlidas, A.; Bastian, T. S.; Aschwanden, M. J.
1995AAS...18710105V Altcode: 1995BAAS...27.1426V
A comprehensive set of microwave and soft x-ray observations of solar
active region 7123 was obtained during 3--10 April, 1992. Here, we
present a detailed analysis of the polarized radio emission from the
sunspot-associated component of AR 7123. We use the VLA observations
at 1.5, 4.7 and 8.4 GHz, supported by the available x-ray and magnetic
data. We concentrate on understanding the 4.7 GHz o-mode polarization
over the umbra of the leading spot of AR 7123 and its variation
with aspect angle within the physical context provided by the x-ray
observations (SXT/Yohkoh) and past work on umbral atmospheres.
---------------------------------------------------------
Title: First VLA observation of a solar narrowband, millisecond
spike event.
Authors: Krucker, S.; Aschwanden, M. J.; Bastian, T. S.; Benz, A. O.
1995A&A...302..551K Altcode:
The first spatially resolved observation of solar, narrowband spikes
in two dimensions is presented. The 'metric' spikes around 333MHz
are classified by the broadband spectrometer Phoenix (ETH Zuerich),
whereas the simultaneously observing Very Large Array (VLA) provides
high angular resolution images of the solar disc. At lower frequencies,
a group of associated type III bursts is detected. The spikes occur
at high altitude (=~4.5x10^10^cm above the photosphere), and at least
3 separated locations of emission can be identified. The different
spike sources are separated by up to 130" and show different degrees of
polarization. Spikes and type III bursts have the same sense of circular
polarization, and according to the extrapolated potential field lines,
the polarization of the different spike sources is in x-mode. With
a delay of 42s, a thermal source appears on the same extrapolated
potential field lines as the spikes at the second frequency of the
VLA (1446MHz). The location of the energy release relative to the
spikes source is discussed. A scenario is proposed where the energy
is released in or near the spike source, and in which the spikes,
the type III bursts and the thermal source originate from the same
energy release. Hot electrons expanding along the field lines generate
a type III burst (upward direction) and heat the underlying dense plasma
(thermal source).
---------------------------------------------------------
Title: Location of Type I Radio Continuum and Bursts on YOHKOH Soft
X-Ray Maps
Authors: Krucker, S.; Benz, A. O.; Aschwanden, M. J.; Bastian, T. S.
1995SoPh..160..151K Altcode:
A solar type I noise storm was observed on 30 July, 1992 with the
radio spectrometer Phoenix of ETH Zürich, the Very Large Array (VLA)
and the soft X-ray (SXR) telescope on board theYohkoh satellite. The
spectrogram was used to identify the type I noise storm. In the VLA
images at 333 MHz a fully left circular polarized (100% LCP) continuum
source and several highly polarized (70% to 100% LCP) burst sources
have been located. The continuum and the bursts are spatially separated
by about 100″ and apparently lie on different loops as outlined
by the SXR. Continuum and bursts are separated in the perpendicular
direction to the magnetic field configuration. Between the periods of
strong burst activities, burst-like emissions are also superimposed
on the continuum source. There is no obvious correlation between the
flux density of the continuum and the bursts. The burst sources have
no systematic motion, whereas the the continuum source shows a small
drift of ≈ 0.2″ min<SUP>−1</SUP> along the X-ray loop in the
long-time evolution. The VLA maps at higher frequency (1446 MHz) show
no source corresponding to the type I event. The soft X-ray emission
measure and temperature were calculated. The type I continuum source
is located (in projection) in a region with enhanced SXR emission,
a loop having a mean density of «n<SUB>e</SUB>» = (1.5 ± 0.4) ×
10<SUP>9</SUP> cm<SUP>−3</SUP> and a temperature ofT = (2.1 ± 0.1)
× 10<SUP>6</SUP> K. The centroid positions of the left and right
circularly polarized components of the burst sources are separated
by 15″-50″ and seem to be on different loops. These observations
contradict the predictions of existing type I theories.
---------------------------------------------------------
Title: A Solar Radio Telescope for the Future: Science Summary from
the SRT Workshop
Authors: Gary, D. E.; Bastian, T. S.; Hudson, H. S.; Hurford, G. J.;
Klimchuk, J. A.; Petrosian, V.; White, S. M.
1995SPD....26..801G Altcode: 1995BAAS...27..971G
No abstract at ADS
---------------------------------------------------------
Title: Aspect Angle Dependence of the Polarized Radio Emission from
AR 7123
Authors: Vourlidas, A.; Bastian, T. S.; Aschwanden, M. J.; Nitta, N.
1995SPD....26..701V Altcode: 1995BAAS...27..965V
No abstract at ADS
---------------------------------------------------------
Title: A Solar Radio Telescope for the Future: Strawman Concept from
the SRT Workshop
Authors: Hurford, G. J.; Bastian, T. S.; Gary, D. E.; Hudson, H. S.;
Klimchuk, J. A.; Petrosian, V.; White, S. M.
1995SPD....26..802H Altcode: 1995BAAS...27..971H
No abstract at ADS
---------------------------------------------------------
Title: Angular Scattering of Radio Waves: Implications for Mode
Coupling in the Solar Corona
Authors: Bastian, T. S.
1995ApJ...439..494B Altcode:
The solar corona is highly inhomogeneous. The magnetionic modes may
therefore couple under certain circumstances, and the polarization
properties of solar radio sources may, as a consequence, be modified. An
application of the simple theory of mode coupling to the propagation
of radio waves in the solar corona leads to the expectation that
mode coupling in quasi-transverse (QT) magnetic field regions
should always be weak (coupling ratio Q much less than 1) at meter
wavelengths. Observations to date suggest that, on the contrary,
mode coupling is moderate (Q approximately 1) to strong (Q much
greater than 1) in QT regions at meter wavelengths. I suggest that
observations and theory can be reconciled by including the effect
of turbulence on mode coupling. Specifically, angular scattering on
turbulent inhomogeneities greatly reduces the spatial scale relevant
to the theory at low frequencies. As a result, the coupling ratio is
increased by a corresponding factor and mode coupling may be moderate
to strong in QT regions at meter wavelengths.
---------------------------------------------------------
Title: Extension of the Radio Spectrum of AE Aquarii to the
Sub-millimetric Range
Authors: Abada-Simon, Meil; Bastian, Tim S.; Bookbinder, Jay A.;
Aubier, Monique; Bromage, Gordon; Dulk, George A.; Lecacheux, Alain
1995LNP...454..268A Altcode: 1995IAUCo.151..268A; 1995flfl.conf..268A
No abstract at ADS
---------------------------------------------------------
Title: The VLA (Very Large Array) development plan
Authors: Bastian, T. S.; Bridle, Alan H.
1995vvla.conf.....B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Radio and Optical Flares of A.E Aquarii
Authors: Abada-Simon, M.; Bastian, T. S.; Horne, K.; Robinson, E. L.;
Bookbinder, J. A.
1995ASPC...85..355A Altcode: 1995mcv..conf..355A
No abstract at ADS
---------------------------------------------------------
Title: Radio Diagnostics of Conditions in the Solar Atmosphere
Authors: Bastian, T. S.
1995itsa.conf..115B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Joint Radio and Soft X-ray Imaging of an “Anemone” Active
Region
Authors: Vourlidas, A.; Bastian, T. S.; Aschwanden, M.; Nitta, N.
1994AAS...185.8609V Altcode: 1994BAAS...26Q1465V
The Very Large Array and the Soft X-ray Telescope aboard the Yohkoh
satellite observed the rapid growth and decay of a so-called “anemone”
active region on 3-6 April (AR 7124). In general, discrete coronal
loop systems are rarely resolved at radio wavelengths due to optical
depth effects and scattering. In the case of AR 7124, however, several
discrete loops or loop systems are resolved by both the VLA and the SXT,
probably due to its unusual “anemone”-like morphology. Furthermore,
the region exhibited a significant amount of variability, especially
on April 3, marked by many subflares and intensity changes as has been
reported in flare patrol observations. In the present study, physical
parameters derived from multiband radio and soft x-ray techniques for
discrete loops are compared. The temporal evolution of these parameters
is also examined.
---------------------------------------------------------
Title: A Microwave Survey of Magnetic Cataclysmic Variables
Authors: Beasley, A. J.; Bastian, T. S.; Ball, Lewis; Wu, K.
1994AJ....108.2207B Altcode:
We present the results of a sensitive radio survey of 22 previously
unobserved magnetic cataclysmic variable stars (MCVs), made by the
Very Large Array (VLA) and the Australia Telescope Compact Array. In
addition, we summarize the results of previous observations of 15
MCVs made with the VLA. None of the 37 MCVs were detected. Extensive
observations of the star V834 Cen have failed to detect significant
emission, suggesting that either the high flux densities previously
reported by Wright et al. (1988) for this source are incorrect, or
that it produces occasional transient emissions.
---------------------------------------------------------
Title: A Multisource Limb Flare Observed at Multiple Radio Wavelengths
Authors: Kucera, T. A.; Dulk, G. A.; Gary, D. E.; Bastian, T. S.
1994ApJ...433..875K Altcode:
A flare with several radio sources occurred on the solar limb at 2155
UT on 1989 June 20. It was observed by the Very Large Array (VLA) and
the Owens Valley Radio Observatory (OVRO). The VLA data consisted of
images at 1.4 GHz, while OVRO provided spectral and spatial information
over the range 1-15 GHz. We develop a new gyrosynchrotron model to
analyze the sources observed at flare peak. This model differs from
many previous ones in that it contains spatial variations of both
the magnetic field and accelerated particle density. It uses a new
gyrosynchrotron approximation which is valid at very low harmonics of
the gyrofrequency. For the first time we find that the cause of the
change of microwave source size with frequency in the event studied
was due primarily to the spatial variation of the accelerated electron
density. This is contrary to the common assumption that the variation
in size is due to a nonuniform magnetic field. We also investigate a
polarized source which brightened later in the flare, finding that it
could have been due to plasma radiation or gyrosynchrotron emission.
---------------------------------------------------------
Title: Propagation of Radio Waves in the Sun's Corona: Angular
Broadening in the Limit of Small-Angle Scattering
Authors: Bastian, T. S.
1994kofu.symp..321B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: VLA Stereoscopy of Solar Active Regions
Authors: Aschwanden, M. J.; Bastian, T. S.
1994kofu.symp..357A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Energy Transport During a Solar Flare: VLA Observations of
the M1.9 Flare of 20 Aug 1992
Authors: Bastian, T. S.; Nitta, N.; Kiplinger, A. L.; Dulk, G. A.
1994kofu.symp..199B Altcode:
The GOES M1.9 flare of 20 August 1992 was observed by a large complement
of instruments including the VLA, Yohkoh, and a high-speed H-alpha
camera. We present a brief overview of the VLA data here. The VLA
acquired maps of the evolving microwave emission at 8.4 and 15 GHz
with a time resolution of 0.2 s. The main observational results are
as follows: i) the microwave sources consist of two, parallel, sheared
loops or loop systems; these two loop systems flared sequentially; ii)
the second microwave source is clearly associated with two magnetic
footpoints; iii) the microwave source shows a disturbance which
propagates from the initial footpoint and over the magnetic neutral
line; iv) the speed of the disturbance is roughly 3000 kms^(-1). Points
along the loop show a brightening that is delayed relative to that at
the primary footpoint, and the two footpoints spread apart over the
course of a few minutes.
---------------------------------------------------------
Title: Rotation in the Ionized Envelope of MWC 349A
Authors: Rodriguez, Luis F.; Bastian, Timothy S.
1994ApJ...428..324R Altcode:
Using the Very Large Array (VLA) in the A configuration we mapped the
H92 alpha recombination line and the adjacent continuum emission at 3.6
cm from the peculiar emission-line star MWC 349A. The recombination
line emission is consistent with local thermodynamic equilibrium
conditions. Our data resolves spatially for the first time the kinematic
structure of this envelope. The observed motions can be explained
in terms of a bipolar outflow undergoing significant rotation. The
inclination angle of the outflow axis with respect to the plane of
the sky is estimated to be about 15 deg. Angular momentum conservation
arguments indicate that this rotating gas cannot have originated from
the stellar surface and most probably is photoevaporating from a disk
around the star. Assuming Keplerian rotation, we estimate the mass
of the star to be approximately 30 solar masses. The presence of a
rotating disk in association with MWC 349A favors a pre-main-sequence
nature for this object.
---------------------------------------------------------
Title: First millimetric detections of AE Aquarii
Authors: Abada-Simon, M.; Lecacheux, A.; Bastian, T. S.; Bookbinder,
J. A.; Dulk, G. A.
1994SSRv...68..291A Altcode:
We report the first millimetric detections of the magnetic cataclysmic
variable AE Aquarii, accompanied by contemporaneous microwave
observations. These data show that the time-averaged spectrum is well
fit by a power-law which extends to mm wavelengths. We suggest that
the spectrum is consistent with that expected from a superposition of
flare-like events.
---------------------------------------------------------
Title: Stellar flares
Authors: Bastian, T. S.
1994SSRv...68..261B Altcode:
Radio and X-ray observations of stellar flares provide the most direct
probes of energy relaase particle acceleration, and energy transport
on stars other than the Sun. In this review, the observational
basis for our understanding of the flare phenomenon on other stars
is briefly described and outstanding interpretive and theoretical
issues are discussed. I shall confine my attention to objects which
are “solar-like”, to the extent that they possess deep convective
envelopes and display activity which is presumed to be magnetic in
origin. These include pre-main sequence objects, classical flare stars,
and close binaries. Future directions are briefly discussed.
---------------------------------------------------------
Title: VLA Stereoscopy of Solar Active Regions. I. Method and Tests
Authors: Aschwanden, Markus J.; Bastian, T. S.
1994ApJ...426..425A Altcode:
We develop a new technique for extracting three-dimensional information
from multiday solar Very Large Array (VLA) observations. While standard
stereoscopic methods provide a three-dimensional view of an object by
combining simultaneous observations from two different aspect angles,
we relax the condition of simultaneity and exploit solar rotation
to vary the aspect angle. The solar radio images are decomposed into
Gaussian source components, which are then cross-correlated in maps
from preceding and following days. This provides measurements of the
three-dimensional position of correlated source centroids. In this
first paper, we describe the stereoscopic method and perform tests
with simulated and real radio maps (from the VLA at 20 cm), in order
to study the accuracy of altitude measurements, and the limitations
introduced by (i) source confusion, (ii) source motion, and (iii)
the assumed differential rotation rate. The tests demonstrate that
(i) the information content of a VLA map relevant for stereoscopic
correlation can be conveniently represented in terms of a small number
of Gaussian components; (ii) the fitting of the three-dimensional
source position is stable within a numerical accuracy of less than or
approximately equal to 0.02 map pixels, (iii) the relative accuracy
of the altitude determination is uniform over the solar disk, and (iv)
source confusion does not affect the accuracy of stereoscopic position
measurements for sources with a signal-to-noise ratio of greater than
or approximately equal to 36.
---------------------------------------------------------
Title: VLA Stereoscopy of Solar Active Regions. II. Altitude, Relative
Motion, and Center-to-Limb Darkening of 20 Centimeter Emission
Authors: Aschwanden, Markus J.; Bastian, T. S.
1994ApJ...426..434A Altcode:
A newly developed method of stereoscopic correlation is applied to solar
radio maps at a wavelength of 20 cm. On the basis of stereoscopic
correlations between radio maps obtained on 6 different days we
establish the presence of 66 radio source components associated with
22 active regions. We find the following statistical results for
active region source structures at 20 cm: 1. The observed lifetime
is consistent with an exponential distribution having an e-folding
time scale of greater than 18 days. 2. The average altitude of 20 cm
sources is 25 +/- 15 Mm; 90% of the sources are found in heights less
than 40 Mm. 3. The average diameter of discrete source structures
is 48 +/- 15 Mm, implying a vertical/horizontal aspect ratio of
q<SUB>A</SUB> approx. = 0.5. 4. No significant source motion has
been found with respect to the standard differential rotation rate
of Omega = 13.45 deg - ((3 deg (sin<SUP>2</SUP>B)). 5. We find a
statistical limb darkening, which can be described by the relation
(T<SUB>B</SUB>(alpha))/T<SUB>B</SUB>(0)) = 0.4 + ((0.6(cos<SUP>2</SUP>
alpha)) for sources with T<SUB>B</SUB> greater than 0.5 MK. 6. The
degree of source polarization is 15% +/- 10% and is independent of
source location. 7. Bright sources (greater than or approximately
equal to 0.5 MK), or equivalently, long-lived sources (greater than
or approximately equal to 5 days) show a systematic variation of their
altitude as function of the center-limb distance.
---------------------------------------------------------
Title: Angular Scattering of Solar Radio Emission by Coronal
Turbulence
Authors: Bastian, T. S.
1994ApJ...426..774B Altcode:
With the advent of large synthesis imaging radio telescopes (e.g., the
Westerbrook Synthesis Radio Telescope (WSRT) and the very large array
(VLA), many solar observations have been made with a high degree of
angular resolution at microwave and decimeter wavelengths. These have
revealed a distinct absence of structure on small angular scales. I
show that scattering on a turbulent spectrum of fluctuations in the
coronal electron number density offers a plausible explanation for the
absence of fine structure at radio wavelengths. I review the theory
of scattering in an inhomogeneous medium as it pertains to angular
broadening and describe recent work on the nature of turbulence in the
Sun's outer corona and the solar wind. I then reassess the problem of
scattering as it applies to the angular broadening of radio sources
embedded in the solar corona in the limit of small-angle scattering. It
is pointed out that angular broadening due to scattering represents a
fundamental limit on the degree of angular resolution with which solar
radio emission can be mapped. The techniques of speckle interferometry
are of no help, with the possible exception of observations of
millisecond spike bursts. Implications are briefly discussed.
---------------------------------------------------------
Title: A Multiband Study of Radio Emission from Solar Active Regions
Authors: Vourlidas, Angelos; Bastian, T. S.
1994ASPC...68..369V Altcode: 1994sare.conf..369V
No abstract at ADS
---------------------------------------------------------
Title: 850 MU M Observations of the 11 July 1991 Total Solar Eclipse
Authors: Ewell, M. W., Jr.; Zirin, H.; Jensen, J. B.; Bastian, T. S.
1994IAUS..154..161E Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A Study of Solar Prominences near lambda = 1 Millimeter
Authors: Bastian, T. S.; Ewell, M. W., Jr.; Zirin, H.
1993ApJ...418..510B Altcode:
The 10.4 m Leighton telescope at the Caltech Submillimeter Observatory
was used to produce full-disk and partial area raster maps of the Sun
during 1991 July 9-11 in advance of the total eclipse of the Sun on
July 11. Maps were made at a wavelength of 850 microns with an angular
resolution of 20".6 and at 1250 microns with an angular resolution
of 30". <P />We have analyzed the 850 microns brightness associated
with Hα filaments present on the disk and find that (1) they are all
associated with regions possessing a brightness comparable to or below
that of the quiet Sun at the center of the disk; and (2) because of
their lack of contrast with the background brightness distribution,
Hα filaments are optically thin at 850 μm. We have also analyzed
contemporaneous observations of a prominence above the solar limb
at both 850 and 1250 mum. We show that (1) the optical depth of
the prominence is τ ≍ 0.12 at 850 μm, confirming the conclusion
drawn from the analysis of Hα filaments; and (2) the line-of- sight
emission measure is <n<SUB>e</SUB><SUP>2</SUP>L> = 1.3-2.0 ×
10<SUP>29</SUP> cm<SUP>-5</SUP>, and the electron number density is
n<SUB>e</SUB> ∼ 1-3 × 10<SUP>10</SUP> cm<SUP>-3</SUP>. <P />We
argue that the reduced 850 microns brightness associated with Hα
filaments and, more generally, magnetic neutral lines is due neither
to the presence of an overlying coronal cavity nor to the presence
of absorbing material, as has been suggested previously. We instead
suggest that the reduced brightness is due to the structure of the
underlying chromosphere where, in the case of filament channels,
the predominantly horizontal magnetic field leads to a reduction of
heating and/or a smaller density scale height.
---------------------------------------------------------
Title: The Center-to-Limb Brightness Variation of the Sun at lambda =
850 Microns
Authors: Bastian, T. S.; Ewell, M. W., Jr.; Zirin, H.
1993ApJ...415..364B Altcode:
The 10.4 m submillimeter telescope of the Caltech Submillimeter
Observatory was used to map the full disk of the sun on July 9-10,
1991 at a wavelength 850 microns with an angular resolution of 20.6
arcsec. A maximum entropy-type deconvolution was performed to correct
the map for the broad wings of the telescope beam. The brightness
distribution of the resulting map shows (1) a high degree of correlation
with corresponding images in H-alpha, the Ca II K line, and with the
photospheric magnetogram; (2) a systematic center-to-limb brightening
of the quiet sun (about 12 percent at an offset of 0.94 solar radius)
with no apparent difference in the limb brightening between the EW and
NS quadrants; (3) an average contrast of about 11 percent between active
region plages and the quiet sun near disk center; and (4) a smaller
degree of center-to-limb brightening of active region plages (about 5
percent at an offset of 0.94 solar radius), leading to a decrease in
contrast between active region plages and the quiet sun from the center
to limb. We compare the observed center-to-limb brightness variation of
the quiet sun and plages with that expected for several chromospheric
models, including those which include a 'spicular' component. We find
that although the observed center-to-limb variation of the quiet sun
at 850 microns is broadly consistent with the chromospheric models of
Vernazza et al. (1981), it differs in detail.
---------------------------------------------------------
Title: Dynamic spectra of radio sources from 4.5 to 5.0 GHz.
Authors: Lecacheux, A.; Rosolen, C.; Davis, M.; Bookbinder, J.;
Bastian, T. S.; Dulk, G. A.
1993A&A...275..670L Altcode:
We report on the first observations of astronomical sources made with
the mini-Gregorian feed of the 305 m Arecibo telescope together with an
acousto-optic spectrometer (AOS). Both the mini-Gregorian and the AOS
have a bandwidth of 500 MHz, permitting wide bandwidth dynamic spectra
of both celestial and solar sources to be obtained. In the examples
reported herein, we used the 5 GHz receiver, operating between 4.5 and
5.0 GHz with a spectral resolution of 1 MHz and temporal resolution
of 20 ms. <P />We represent observations of the flare star AD Leo,
of the Sun, and of interplanetary scintillations of the quasar 0735 +
178 which, at the time it was observed, was about 8° from the Sun.
---------------------------------------------------------
Title: Multiple Wavelength Observations of an Off-Limb Eruptive
Solar Flare
Authors: Kucera, T. A.; Dulk, G. A.; Kiplinger, A. L.; Winglee, R. M.;
Bastian, T. S.; Graeter, M.
1993ApJ...412..853K Altcode:
The eruptive prominence and limb flare which occurred at 1454 UT on
June 20, 1989 is described and analyzed. This event was observed by
many different instruments providing an unusual amount and variety of
data: images at 1.4 GHz, 37 GHz, and H-alpha, and spectra in hard X-ray,
soft X-ray, and radio frequencies. This array of data makes it possible
to explore the relationships between flare and eruptive prominence
emissions at different wavelengths. VLA images at 1.4 GHz show changing
sources in a set of high (about 10 exp 10 cm) coronal loops associated
with the erupting prominence. We use a full gyrosynchrotron code to
model a 1.4 GHz source early in the flare as a large coronal loop. The
model results lead us to conclude that the initial acceleration occurs
in smaller, denser loops which also produce the flare's hard X-ray
emission. We also present evidence that a source at 1.4 GHz later in
the event is due to second-harmonic plasma emission. This source is
adjacent to a leg of the prominence and comes from a dense column of
material in the magnetic structure supporting the prominence.
---------------------------------------------------------
Title: VLA Stereoscopy of Solar Active Region 7123
Authors: Aschwanden, M. J.; Bastian, T. S.; Nitta, N.
1993BAAS...25.1224A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The M1.9 Flare of 20 August 1992: Joint Imaging with the VLA,
Yohkoh, and a High Speed Hα Camera
Authors: Bastian, T. S.; Nitta, N.; Kiplinger, A.
1993BAAS...25.1222B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Multitemperature Observations of an Emerging Flux Region
Authors: Bruner, M. E.; Acton, L. W.; Brown, W. A.; Lemen, J. R.;
Shine, R.; Strong, K. T.; Tarbell, T.; Dulk, G.; Tsuneta, S.; Bastian,
T.; Dame, L.
1993BAAS...25.1179B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Observations of an Emerging Flux Region
Authors: Brown, W. A.; Acton, L. W.; Bruner, M. E.; Lemen, J. R.;
Shine, R.; Strong, K. T.; Tarbell, T.; Dulk, G.; Tsuneta, S.; Bastian,
T.; Dame, L.
1993BAAS...25Q1214B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: OVRO Microwave Observations During the Max'91/CoMStOC'92
Campaign
Authors: Gary, D. E.; Hurford, G. J.; Bastian, T.
1993BAAS...25.1224G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Spectrum and Variability of Radio Emission from AE Aquarii
Authors: Abada-Simon, Meil; Lecacheux, Alain; Bastian, Tim S.;
Bookbinder, Jay A.; Dulk, George A.
1993ApJ...406..692A Altcode:
The first detections of the magnetic cataclysmic variable AE Aquarii at
millimeter wavelengths are reported. AE Aqr was detected at wavelengths
of 3.4 and 1.25 mm. These data are used to show that the time-averaged
spectrum is generally well fitted by a power law S(nu) varies as nu
exp alpha, where alpha is approximately equal to 0.35-0.60, and that
the power law extends to millimeter wavelengths, i.e., the spectral
turnover is at a frequency higher than 240 GHz. It is suggested that
the spectrum is consistent with that expected from a superposition of
flarelike events where the frequency distribution of the initial flux
density is a power law f (S0) varies as S0 exp -epsilon, with index
epsilon approximately equal to 1.8. Within the context of this model,
the high turnover frequency of the radio spectrum implies magnetic
field strengths in excess of 250 G in the source.
---------------------------------------------------------
Title: The Kinematics of the Ionized Envelope of MWC349A
Authors: Rodríguez, L. F.; Bastian, T. S.
1993ESOC...46..468R Altcode: 1993mlab.conf..468R
No abstract at ADS
---------------------------------------------------------
Title: Submillimeter Observations of the 1991 July 11 Total Solar
Eclipse
Authors: Ewell, M. W., Jr.; Zirin, H.; Jensen, J. B.; Bastian, T. S.
1993ApJ...403..426E Altcode:
We present observations of the 1991 July 11 total solar eclipse at
850 microns made with the Caltech Submillimeter Observatory on Mauna
Kea. We find that the 850-micron limb is 3380 +/- 140 km above the
visible limb. We also find that there is a 10 percent limb brightening
in the outer 7 arcsec of the solar disk, and we measure a central
brightness temperature of 6400 +/- 700 K. These results require that
the upper chromosphere not be in hydrostatic equilibrium, with a higher
electron density than is predicted by the standard (VAL) model. We show
that the dependence of limb height on wavelength is well fitted by a
one-parameter model with an electron density scale height of 1200 km;
there is no need to invoke complex spicule geometry to explain the
observations.
---------------------------------------------------------
Title: 3D reconstruction methods of coronal structures by radio
observations.
Authors: Aschwanden, M. J.; White, S. M.; Bastian, T. S.
1992ESASP.348..217A Altcode: 1992cscl.work..217A
The ability to carry out a three-dimensional reconstruction of
structures in the solar corona would represent a major advance
in our study of the physical properties in active regions and
in flares. The authors describe several new methods which allow
a geometric reconstruction of quasi-stationary coronal structures
(e.g. active region loops) or dynamic structures (e.g. flaring loops):
(1) steroscopy of multi-day imaging observations by the VLA. (2)
Tomography of optically thin emission (in radio or soft X-rays). (3)
Multi-frequency band imaging by the VLA. (4) Tracing of magnetic field
lines by propagating electron beams.
---------------------------------------------------------
Title: Simultaneous Observations of Coronal Bright Points in X-Ray
and Radio Wavelengths
Authors: Nitta, Nariaki; Bastian, Timothy S.; Aschwanden, Markus J.;
Harvey, Karen L.; Strong, Keith T.
1992PASJ...44L.167N Altcode:
We present a first explicit comparison of coronal bright points in
soft X-ray and radio wavelengths, using the Soft X-ray Telescope
aboard the Yohkoh spacecraft and the Very Large Array. About half of
the 33 compact sources indentified in a 20-cm full-disk map appear
as X-ray bright points in the X-ray data. The other half apparently
corresponds to unipolar regions with enhanced magnetic fields. Thus,
the identification of radio bright points alone cannot reliably serve
as a proxy for X-ray bright points. A preliminary analysis reveals that
bright points commonly observed at 20 cm and in X-rays have temperatures
of (1.4--2.9) times 10(6) K and emission measures of (0.4--2.5) times
10(45) cm(-3) . The observed brightness temperatures at 20 cm [(1--2.5)
times 10(5) K] can be explained in terms of optically thin free-free
emission from a plasma with these parameters.
---------------------------------------------------------
Title: Radio Emission from Chemically Peculiar Stars
Authors: Linsky, Jeffrey L.; Drake, Stephen A.; Bastian, T. S.
1992ApJ...393..341L Altcode:
In five VLA observing runs the initial survey of radio emission from
magnetic Bp-Ap stars by Drake et al. is extended to include a total of
16 sources detected at 6 cm out of 61 observed, giving a detection rate
of 26 percent. Of these stars, three are also detected at 2 cm, four at
3.6 cm, and five at 20 cm. The 11 new stars detected as radio sources
have spectral types B5-A0 and are He-weak and Si-strong. No classical
(SrCrEu-type) Ap stars have yet been detected. The 16 detected sources
show a wide range of radio luminosities with the early-B He-S stars on
average 20 times more radio luminous than the late-B He-W stars and 1000
times more luminous than Theta Aurigae. Multifrequency observations
indicate flat spectra in all cases. Four stars have a detectable
degree of circular polarization at one or more frequencies. It is
argued that the radio-emitting CP (chemically peculiar) stars form a
distinct class of radio stars that differs from both the hot star wind
sources and the active late-type stars. The observed properties of radio
emission from these stars may be understood in terms of optically thick
gyrosynchrotron emission from a nonthermal distribution of electrons
produced in a current sheet far from the star. In this model the
electrons travel along magnetic fields to smaller radii and higher
magnetic latitudes where they mirror and radiate microwave radiation.
---------------------------------------------------------
Title: Radio observations of the M8.1 solar flare of 23 June, 1988:
Evidence for energy transport by thermal processes
Authors: Bastian, T. S.; Gary, D. E.
1992SoPh..139..357B Altcode:
The Very Large Array (VLA) and the frequency agile interferometer at
the Owens Valley Radio Observatory (OVRO) were used to observe the
M8.1 flare of 23 June, 1988. The VLA obtained images prior to and
during the flare at 333 MHz, and at 1.5 and 4.7 GHz. The frequency
agile interferometer at Owens Valley obtained interferometer amplitude
and total power spectra of the flare at 45 frequencies between 1 and
18 GHz. The observations were supplemented by radiometer measurements
made by the USAF RSTN network site at Palehua, HI, by GOES soft X-ray
observations, by USAF SOON Hα filtergrams, and by a KPNO photospheric
magnetogram.
---------------------------------------------------------
Title: Decimetric Solar Type U Bursts: VLA and PHOENIX Observations
Authors: Aschwanden, Markus J.; Bastian, T. S.; Benz, A. O.; Brosius,
J. W.
1992ApJ...391..380A Altcode:
Observations of type U bursts, simultaneously detected by the VLA at
1.446 GHz and by the broadband spectrometer Phoenix in the 1.1-1.7
GHz frequency band on August 13, 1989 are reported. Extrapolations
of the coronal magnetic field, assuming a potential configuration,
indicate that the VLA 20 cm source demarcates an isodensity level. The
source covers a wide angle of diverging magnetic field lines whose
footpoints originate close to a magnetic intrusion of negative polarity
into the main sunspot group of the active region with dominant positive
polarity. The centroid of the 20-cm U-burst emission, which corresponds
to the turnover frequency of the type U bursts and remains stationary
during all U bursts, coincides with the apex of extrapolated potential
field lines at a height of about 130,000 km. It is demonstrated
that the combination of radio imaging and broadband dynamic spectra,
combined with the magnetic field reconstruction from magnetograms,
can constrain all physical parameters of a magnetic loop system.
---------------------------------------------------------
Title: Model of Gyrosynchrotron Emission from a High Coronal Loop
Authors: Kucera, T. A.; Dulk, G. A.; Bastian, T. S.
1992AAS...180.4205K Altcode: 1992BAAS...24..796K
Gyrosynchrotron emission is a main source of microwave emission from
solar flares. We have developed a model of a high, face-on, coronal
flare loop with a dipole magnetic field. The loop is divided into
homogeneous segments to which are applied a general gyrosynchrotron
code. We compare the model with observations made of an off-limb
flare taken in June of 1989 during the first Max '91 campaign. These
observations force a strong set of constraints upon the model. VLA
imaging data at 1.4 GHz dictate the source position, shape and
brightness at that frequency. Radiometer data from RSTN and Bern
provide microwave spectra to compare to model-produced spectra, and
there are constraints from SMM/HXRBS hard X-ray spectra and derived
emission measures. The model is also compared with general results
from previous measurements of source parameters at other frequencies.
---------------------------------------------------------
Title: Scattering of Radio Waves in the Solar Corona
Authors: Bastian, T. S.
1992AAS...180.1107B Altcode: 1992BAAS...24..747B
Over the past two decades, our understanding of turbulence in the solar
wind and the outer corona has progressed significantly. Coupled with
this have come many important developments in the theory of radiation
transport in random media. While the importance of scattering of
radio waves at meter wavelengths emitted by sources embedded in the
solar corona has long been recognized, the formalism used to describe
the scattering has not been brought up to date. In this paper I point
out several developments which modify and extend our understanding of
scattering of radio waves in the solar corona. Specifically, I show
i) the importance of scattering of radio waves emitted by sources
embedded in the solar corona extends to microwaves; ii) the potential
importance of refractive scintillation at meter wavelengths. The
practical consequences of these developments are briefly explored.
---------------------------------------------------------
Title: The Structure of Solar Active Regions
Authors: Vourlidas, A.; Bastian, T. S.
1992AAS...180.4204V Altcode: 1992BAAS...24..796V
In past years, x-ray observations of solar active regions have
lead to the expectation of greater brightness temperatures at radio
wavelengths than those typically observed. It has been suggested that
cool plasma in the corona along the line of sight attenuates radio
emission via free-free absorption. If such plasma is present, it has
consequences for both the microwave spectrum and its polarization
properties. In order to test these ideas, high quality radio and x-ray
maps are required. We present a comprehensive set of observations of
a large solar active region (NOAO/USAF number 5131) made during the
IAU sanctioned International Solar Month in September, 1988. The VLA
was used to image the Sun in the 90, 20, 6 and 3.6 cm bands between
1--4 September. To improve the image quality we used the technique of
frequency synthesis at 3.6, 6 and 20 cm. The final maps are among the
best in dynamic range yet obtained. In addition to the radio maps, the
data base includes images from the SMM XRP in Fe XVII, magnetograms,
and Hα observations. We reconcile the x-ray and radio observations
with a simple model which differs somewhat from past models. Rather than
relying on a screen of cool plasma between the source and the observer,
we take explicit account of the highly inhomogeneous structure of solar
active regions. We briefly compare and contrast the consequences of
this model with existing models.
---------------------------------------------------------
Title: Solar Radio Pulsation Event Observed by the VLA and OVRO
Authors: Aschwanden, M. J.; Bastian, T. S.; Gary, D. E.
1992AAS...180.4505A Altcode: 1992BAAS...24..802A
We investigate interpretational aspects of the radio pulsation
event which has been observed by the Very Large Array (VLA) and
the Owens Valley Radio Observatory (OVRO) on December 21, 1990,
1930 UT, during the second MAX'91 observing campaign. The VLA was
observing with a time resolution of 0.4 s at 0.33, 1.4 and 4.9 GHz,
while OVRO used a time resolution of 0.2 s at 1.2, 1.4, 1.6, 1.8
and 2.0 GHz. The radio pulsation event was triggered by a C3.3 class
flare in soft X-rays in active region 6412 (NOAA). The time profile
of the quasi-periodic radio emission exhibits a period of 8.8 s and
shows indications of secondary periodicities with faster periods. We
investigate whether these periodicities can be explained in terms of
(i) MHD eigen-modes in cylindrical fluxtubes (MHD surface modes and
harmonic modes), or (ii) relaxational oscillations of a nonlinear
dissipative system. We test these two options by means of Fourier
spectra and by reconstruction of the “strange attractor dimension”
of nonlinear systems. We attempt a three-dimensional reconstruction of
the pulsating source by means of magnetic field extrapolation from the
photospheric magnetogram and by using the constraints of the radio
maps at multiple frequencies. Preliminary analysis indicates that
plasma emission as well as gyrosynchrotron emission is coherently
modulated in spatially diverging magnetic structures. We investigate
whether the pulsating radio emission originates from plasma confined
in a pulsating loop structure or whether the radio emission is driven
by electron beams which are produced in a pulsating acceleration
mechanism. The latter possibility would support a model where the
primary energy release itself is governed by a nonlinear dissipative
system, e.g. by oscillatory magnetic reconnection.
---------------------------------------------------------
Title: Detection of the H92 alpha Radio Recombination Line from the
Symbiotic Star H1-36 Arae
Authors: Bastian, T. S.
1992ApJ...387L..77B Altcode:
The H92-alpha radio recombination line at 8309.383 MHz has been
detected from the symbiotic star H1-36 Arae by the VLA. The continuum
flux density is 73.5 mJy and the observed line-to-continuum ratio is
Tl/Tc greater than about 1 percent. The line width is 46 km/s. Since
the photoionized nebula may account for as little as one-half of
the continuum radio flux, the true line-to-continuum ratio may be as
high as 2 percent. If this is the case, and if pressure broadening
is negligible, the electron temperature is between 10 exp 4 and 2
x 10 exp 4 K. If pressure broadening is marginally significant, the
expansion velocity of the wind is consistent with v<SUB>exp</SUB> =
10 km/s, and the electron temperature drops to a range comparable to
that found for certain planetary nebulae.
---------------------------------------------------------
Title: High Sensitivity; Dynamic Spectral Search for Flare Star
Radio Bursts with the Arecibo Single Dish
Authors: Lecacheux, A.; Abada-Simon, M.; Belkora, L.; Dulk, G. A.;
Bookbinder, J.; Bastian, T. S.; Louarn, P.
1992ASPC...26..319L Altcode: 1992csss....7..319L
No abstract at ADS
---------------------------------------------------------
Title: The Class of RadioEmitting Magnetic B Stars; a Wind-Fed
Magnetospheric Model
Authors: Linsky, J. L.; Drake, S. A.; Bastian, T. S.
1992ASPC...26..325L Altcode: 1992csss....7..325L
No abstract at ADS
---------------------------------------------------------
Title: Multifrequency Observations of a Remarkable Solar Radio Burst
Authors: White, S. M.; Kundu, M. R.; Bastian, T. S.; Gary, D. E.;
Hurford, G. J.; Kucera, T.; Bieging, J. H.
1992ApJ...384..656W Altcode:
Observations of an impulsive solar-radio burst from three observatories
are presented. The striking observational aspects of this flare are
that the time profile was identical throughout at 8.6, 15, and 86
GHz, that the spectrum was apparently flat from 15 to 86 GHz, and
that there was a sharp cutoff in the spectrum between 5.0 and 8.6
GHz. The simplest interpretation of the cutoff, namely as a plasma
frequency effect, leads to the conclusion that there was exceptionally
high-density material in the solar corona (of about 5 x 10 exp 11/cu
cm). Very Large Array images at 15 GHz show a single-loop structure
which brightened uniformly and showed little change in size during the
whole impulsive phase. The flat spectrum is consistent with optically
thin thermal bremsstrahlung emission, but the lack of observed soft
X-ray emission and other properties of the flare cannot easily be
accommodated by this mechanism. The possibility is explored that the
emission is optically thick due to thermal absorption of nonthermal
gyrosynchrotron emission, or optically thin gyrosynchrotron emission
absorbed by high-density material intervening along the line of
sight. Both of these explanations also face difficulties.
---------------------------------------------------------
Title: The Distribution of Magnetic Fields and Related Activity on
A'D Leo
Authors: Saar, S. H.; Bookbinder, J. A.; Neff, J.; Bromage, G.;
Bastian, T.
1991BAAS...23.1383S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Imaging the Sun and the Planets with a Radio Schmidt Telescope
Authors: Dulk, G. A.; Bastian, T. S.
1991rst..work..123D Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Improved Solar Capabilities of the VLA
Authors: Bastian, T. S.
1991BAAS...23R1072B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Observations of Two Large Off Limb Solar Flares in the Radio,
X-Rays, and Hα
Authors: Kucera, T. A.; Dulk, G. A.; Winglee, R. M.; Kiplinger, A. L.;
Bastian, T. S.; Gary, D. E.
1991BAAS...23R1065K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Multifrequency Observations of a Remarkable Solar Radio Burst
Authors: White, S. M.; Kundu, M. R.; Bastian, T. S.; Gary, D. E.;
Hurford, G. J.; Kucera, T.; Bieging, J. H.
1991BAAS...23.1043W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Coordinated VLA-PHOENIX-SMM Observations: Microwave Type
U-Bursts
Authors: Aschwanden, M. J.; Bastian, T. S.; Benz, A. O.; Dennis, B. R.
1991BAAS...23.1065A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Temporally, Spectrally, and Spatially Resolved Observations
of a Pulsating Solar Radio Burst
Authors: Bastian, T. S.; Aschwanden, M. J.; Gary, D. E.
1991BAAS...23Q1072B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Solar Radio Microbursts at 1.4 GHz
Authors: Bastian, T. S.
1991ApJ...370L..49B Altcode:
The Very Large Array was used for a period of about 80 minutes
on September 11, 1989 to observe the sun at two frequencies in
the 1.4 GHz band. In addition to a strong radio burst (700 sfu
at maximum) of a duration of roughly 15 minutes, a multitude of
small-amplitude, transient bursts occurred during the course of the
observation. Specifically, some tens of microbursts occurred during a
period of 65 minutes, ranging in amplitude from less than about 0.05 to
4 sfu, and ranging in duration from less than 10 to 40 s. A sample of
these microbursts studied in detail shows that they occurred at five
distinct locations in four different active regions. The microbursts
are probably due to plasma radiation, although the microburst mechanism
remains unknown. Their relation to known radio burst phenomena and
hard X-ray microflares is unclear.
---------------------------------------------------------
Title: Observations of Two Large Limb Flares on 20 June 1989
Authors: Kucera, T. A.; Winglee, R. M.; Dulk, G. A.; Bastian, T. S.;
Gary, D. E.
1991max..conf..172K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: VLA and Hα Observations of the M8.7 Flare of 17 June 1989:
A Preliminary Report
Authors: Bastian, T. S.; Kiplinger, A. L.
1991max..conf..153B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Radio Emission from Flare Stars
Authors: Bastian, T. S.
1990SoPh..130..265B Altcode:
Observations of radio emission from flare stars are reviewed,
including surveys of flare stars in the solar neighborhood and in
stellar associations, studies of quiescent emission, and continuum
and spectral studies of radio burst emission. The radio observations
are placed in an observational context provided by soft X-ray, UV,
and optical observations. It is stressed that, as is the case for the
latter wavelength regimes, observations of rado bursts on flare stars
are qualitatively similar to those on the Sun, albeit in a dramatically
scaled-up fashion.
---------------------------------------------------------
Title: Dynamic Spectra of Radio Bursts from Flare Stars
Authors: Bastian, T. S.; Bookbinder, J.; Dulk, G. A.; Davis, M.
1990ApJ...353..265B Altcode:
The Arecibo 305 m telescope has been used to observe radio bursts from
flare stars at 430 and 1415 MHz. Dynamic spectra of the emission with
bandwidths of 10 MHz in the former case and 40 MHz in the latter are
recorded. For AD Leo, the microwave burst emission was 100 percent right
circularly polarized, achieved brightness temperatures near 10 to the
16th K, was generally broadband in character, but was superposed with
finite structures in both frequency and time. Quasi-periodic pulsations
were clearly present as well as a sudden reduction feature. For YZ
CMi, the emission was 100 percent left circularly polarized and was
relatively broadband with fine structures. Instabilities driven by
anisotropies in the electron distribution, particularly the loss-cone
distribution, are considered to account for the coherent radiation.
---------------------------------------------------------
Title: Direct Imaging of a Coherent Solar Radio Burst
Authors: Bastian, T. S.
1990BAAS...22..824B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Broadband Spectral Radio Observations of Flare Stars
Authors: Benz, A. O.; Guedel, M.; Bastian, T. S.; Fuerst, E.; Simnett,
G. M.; Pointon, L.
1990IAUS..137..139B Altcode:
The first broadband spectrometer observation of a stellar flare
event is reanalyzed and discussed. Rough estimates of the stellar
source parameters are given, including the source size, exciter drift
velocity, and source magnetic field. An interpretation in terms of
coherent cyclotron emission is presented.
---------------------------------------------------------
Title: Extension of the Class of Magnetic B-Star Nonthermal Radio
Sources
Authors: Linsky, Jeffrey L.; Drake, Stephen A.; Bastian, Timothy S.
1990ASPC....9..189L Altcode: 1990csss....6..189L
A large VLA survey of radio emission from magnetic B stars is conducted
at 2, 3.6, 6, and 20 cm to identify and classify the stars and check
for the existence of classical Ap stars. Observations of 30 previously
unidentified stars in the Sco-Cen association are reported, and 10
stars are found which have spectral types B5-A0, measured magnetic
fields, and are He-weak and Si-strong. No radio emission is detected
from stars later than spectral type A1, and the extreme magnetic field
strength and large magnetosphere of GL Lac make it a good test for
magnetosphere theories.
---------------------------------------------------------
Title: Broadband spectral observations of a dMe star radio flare.
Authors: Gudel, M.; Benz, A. O.; Bastian, T. S.; Furst, E.; Simnett,
G. M.; Davis, R. J.
1989A&A...220L...5G Altcode:
A flare on the dMe star AD Leonis was simultaneously observed with
the radio telescopes in Effelsberg, Jodrell Bank, and Arecibo using
spectrometers at 1665, 1666, and 1415 MHz with bandwidths of 25,
100, and 40 MHz, respectively. The time coincidence confirms the
stellar origin of the radiation. The flare emission was resolved into a
multitude of broadband pulsations. Their e-folding rise and decay times
were of the order of the time resolution of the Effelsberg data (125
ms), or less. The circular polarization was about 100 percent. Similar
bursts, but 4 orders of magnitude less powerful, have been observed
from the sun at lower frequencies.
---------------------------------------------------------
Title: Extension of the Class of Magnetic B Star Nonthermal Radio
Sources
Authors: Linsky, J. L.; Drake, S. A.; Bastian, T. S.
1989BAAS...21Q.742L Altcode: 1989BAAS...21..742L
No abstract at ADS
---------------------------------------------------------
Title: Solar Imaging with a Synthesis Telescope
Authors: Bastian, Tim
1989ASPC....6..395B Altcode: 1989sira.conf..395B
Solar Imaging with the VLA, Hardware modifications, Solar data
calibration, Topics in Solar Synthesis Imaging (or: "Why Does My
Solar Image Look So Awful?"), Sensitivity, The problem of calibration,
The problem of confusion, The problem of u-v coverage, The problem of
source variability, Solar Observing Strategies, Imaging the quiet Sun,
Microwave bursts
---------------------------------------------------------
Title: Microwave Spectra and Polarization of Active Stars
Authors: Slee, O. B.; Stewart, R. T.; Nelson, G. J.; Wright, Alan E.;
Dulk, George A.; Bastian, T. S.; McKean, M.
1988ApL&C..27..247S Altcode: 1988ApL....27..247S
Almost-simultaneous observations of 24 active-chromosphere stars with
the VLA and the Parkes 64-m telescope at 4.9, 8.4 and 15 GHz over a
24-h interval on September 14, 1985 are reported. In all, 14 stars
were detected. There is a striking difference in the power outputs and
brightness temperatures of the higher-optical-luminosity stars (RS CVn,
Algol-like and CAII emitters) and the low-luminosity dMe stars; this can
be explained by differences in the surface areas and rotation velocities
of the two groups of stars. Two three-point spectra show a broad peak in
the region near 8.4 GHz, indicating a transition from optically thick
to optically thin radio sources. For another three stars, two-point
spectral indices are negative, indicating that their spectra peak
probably well below 15 GHz; two stars with two-point positive spectral
indices have higher-frequency turnovers, although perhaps below 15 GHz.
---------------------------------------------------------
Title: Radio Flares from AE Aquarii: A Low-Power Analog to Cygnus
X-3?: Erratum
Authors: Bastian, T. S.; Dulk, G. A.; Chanmugam, G.
1988ApJ...330..518B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A Search for Radio Emission from Flare Stars in the Pleiades
Authors: Bastian, T. S.; Dulk, G. A.; Slee, O. B.
1988AJ.....95..794B Altcode:
The authors have used the VLA to search for radio emission from
flare stars in the Pleiades. Two observational strategies were
employed. First, about 1/2 deg<SUP>2</SUP> of the cluster, containing
≈40 known flare stars, was mapped at 1.4 GHz at two epochs. More than
120 sources with flux densities >0.3 mJy exist on the maps. Detailed
analysis shows that all but two of these sources are probably
extragalactic. The two sources identified as stellar are probably not
Pleiades members as judged by their proper motions; rather, based on
their colors and magnitudes, they seem to be foreground G stars. The
second observational strategy, where five rapidly rotating flare stars
were observed at three frequencies, yielded no detections. The 0.3 mJy
flux-density limit of this survey is such that only the most intense
outbursts of flare stars in the solar neighborhood could have been
detected if those stars were at the distance of the Pleiades. Hence,
while the activity on flare stars in the Pleiades may be somewhat
in excess of that on flare stars in the solar neighborhood, the
"superflares" that have been occasionally reported in the Pleiades
and in Orion must be very rare or nonexistent.
---------------------------------------------------------
Title: Imaging the Sun at 1.4 GHz.
Authors: Bastian, T. S.; Dulk, G. A.
1988NRAOW..15..125B Altcode: 1988usoa.conf..125B
The authors briefly describe the maximum entropy-like image
reconstruction techniques employed in reconstructing images of the
quiet Sun and present a representative example.
---------------------------------------------------------
Title: Variation of the quiet sun at 21 cm: 1981 - 1987.
Authors: Bastian, T. S.; Dulk, G. A.
1988sscd.conf..386B Altcode:
Using the Very Large Array (VLA), the Green Bank 91 m telescope,
the Arecibo 305 m telescope, and powerful maximum entropy image
reconstruction techniques, the authors have imaged the Sun at λ ≍
21 cm during the years 1981 - 1987. While the 21 cm radio brightness
distribution is in qualitative agreement with past mapping efforts,
several new results have emerged. Among these, the authors have noted a
systematic decrease in the quiet Sun's brightness temperature at 21 cm
as the Sun declined from sunspot maximum to sunspot minumum accompanied
by systematic decrease in the Sun's radius at 21 cm. Simple modeling
has shown that the observed variation in the quiet Sun's brightness
temperature and radius at 21 cm could have resulted from a factor of
≍2 decrease in the electron number density in the solar transition
region and low corona.
---------------------------------------------------------
Title: Radio Flares from AE Aquarii: A Low-Power Analog to Cygnus X-3?
Authors: Bastian, T. S.; Dulk, G. A.; Chanmugam, G.
1988ApJ...324..431B Altcode:
The magnetic cataclysmic variable AE Aquarii has been observed at 1.5,
4.9, 15, and 22.5 GHz. The source shows temporal variations in the
flux density, often with a flarelike morphology. The flux variations
are at times extreme, with values ranging from 1-12 mJy at 15 GHz;
the degree of variability increases with frequency. The spectrum shows
a variety of short-term behavior, at times increasing as nu, at other
times decreasing with nu, and at still others flat. The long-term
average of the spectrum increases as nu exp 0.3-0.4. No significant
degree of circular polarization was observed. The source properties
can be explained as a superposition of flare events in which electrons
are accelerated to relativistic energies which then emit synchrotron
radiation in discrete, expanding sources. The origin of the flare
events may be connected to the disruption of the accretion disk some
ditance from the white dwarf primary. The radio emission of AE Aqr
shows similarities to the low-level flares from Cygnus X-3.
---------------------------------------------------------
Title: The Discovery of Nonthermal Radio Emission from Magnetic
Bp--Ap Stars
Authors: Drake, Stephen A.; Abbott, David C.; Bastian, T. S.; Bieging,
J. H.; Churchwell, E.; Dulk, G.; Linsky, Jeffrey L.
1987ApJ...322..902D Altcode:
In a VLA survey of chemically peculiar B- and A-type stars with strong
magnetic fields, five of the 34 stars observed have been identified
as 6 cm continuum sources. Three of the detections are helium-strong
early Bp stars (Sigma Ori E, HR 1890, and Delta Ori C), and two are
helium weak, silicon-strong stars with spectral types near A0p (IQ
Aur = HD 34452, Babcock's star = HD 215441). The 6 cm luminosities L6
(ergs/s Hz) range from log L6 = 16.2 to 17.9, somewhat less than the
OB supergiants and W-R stars. Three-frequency observations indicate
that the helium-strong Bp stars are variable nonthermal sources.
---------------------------------------------------------
Title: Aperture Synthesis Observations of Solar and Stellar Radio
Emission.
Authors: Bastian, Timothy Stephen
1987PhDT........19B Altcode:
The work presented in this thesis relied upon the radio astronomical
instrument, The Very Large Array. The thesis is divided into three
major sections. In the first I have applied maximum entropy-type image
reconstruction techniques, using both single dish and iterferometer
data, to generate full disk images of the Sun at a wavelength lambda
~ 21 cm. Using a set of six such images obtained during the Sun's
decline from sunspot maximum to minimum, we have noted a number of
previously unreported phenomena. Among these: (1) a systematic decrease
in quiet Sun's brightness temperature as it declined to minimum; (2)
a systematic decrease in the Sun's radius at 21 cm; (3) evidence for
the evolution of polar coronal holes during the course of the solar
cycle. The observed variation, though not noted previously at radio
wavelengths, is entirely consistent with white light K coronagraph
data. The results reported here explain the conflicting nature of
a number of past observations. In the second section of the thesis,
I present the results of a long term survey of magnetic cataclysmic
variables (CVs). Cataclysmic variables are close binary systems which
contain a white dwarf accreting mass from a late-type secondary,
typically a dwarf of spectral type G, K, or M. Our survey resulted in
the detection of two out of the eighteen systems observed. In the third
section of the thesis, I present new results on flare stars in the
solar neighborhood and in the Pleiades. We have successfully employed
the technique of dynamic spectroscopy to constrain the mechanism(s)
for radio flaring on other stars. In particular, we used the Very Large
Array in spectral line mode to observe two intense flares on the nearby
flare star UV Ceti. The second part of section three is devoted to a
search for radio emission from flare stars in the Pleiades which was
motivated by the evolutionary questions raised by flare stars and the
Pleiades lower main sequence. Of the ~170 sources found in the Pleiades'
fields, all but two were determined to be extragalactic. Neither of
the two stellar radio sources is a known flare star or a Pleiades
member. (Abstract shortened with permission of author.).
---------------------------------------------------------
Title: First dynamic spectra of stellar microwave flares
Authors: Bastian, T. S.; Bookbinder, J. A.
1987Natur.326..678B Altcode:
Records of radiation intensity as a function of time and frequency
(dynamic spectra) have been profitably used as a probe of the Sun's
corona for many years. Motivated by the possibility of using the
frequency domain to constrain theoretical models of coherent microwave
emission from red dwarf (dMe) flare stars, we have used the Very Large
Array in spectral-line mode at 1.4 GHz to obtain the first dynamic
spectra of stellar sources other than the Sun. Two very intense, highly
circularly polarized, microwave outbursts were observed on the dMe flare
star UV Get (L726-8B), in addition to a slowly varying, unpolarized
component. One outburst was purely left circularly polarized and showed
no variations as a function of frequency across the 41 MHz band,
whereas the other was as much as 70% right-circularly polarized and
showed distinct variations with frequency. Although the slowly varying
emission is probably due to incoherent gyrosynchrotron emission, the
two flaring events are the result of coherent mechanisms. We interpret
the coherent emission in terms of plasma radiation and the cyclotron
maser instability.
---------------------------------------------------------
Title: Stellar dynamic spectroscopy
Authors: Bastian, T. S.; Dulk, G. A.; Bookbinder, J. A.
1987SoPh..113..289B Altcode: 1982SoPh..113..289B
The dynamic spectrum, a three dimensional record of the radio intensity
as a function both of time and frequency, has long been used as a probe
of plasma processes in the solar corona. Beginning with the work of Wild
and McCready (1950) dynamic spectroscopy has been used to distinguish
between the multitude of radio wave emitting phenomena which occur in
the solar corona and to infer the physical mechanisms responsible.
---------------------------------------------------------
Title: A Search for Cyclotron Maser Radiation from Substellar and
Planet-like Companions of Nearby Stars
Authors: Winglee, Robert M.; Dulk, George A.; Bastian, Timothy S.
1986ApJ...309L..59W Altcode:
Several stars within 5 pc have variations in their rectilinear or
Keplerian motion that suggest that they have substellar or planetlike
companions. However, direct, unambiguous evidence of such companions is
difficult to obtain. It is argued that such objects should commonly emit
cyclotron maser radiation and that this radiation should be observable
with modern radio telescopes. Its detection would confirm the presence
of the companions and permit many of the properties of the stars and
companions to be derived. A search was made for the cyclotron maser
emission from six nearby stars with suspected companions using the
VLA at frequencies of 0.33 and 1.4 GHz. None was detected. Typical
observing times were 3.5 hr, and typical upper limits were 30 mJy at
0.33 GHz and 0.3 mJy at 1.4 GHz.
---------------------------------------------------------
Title: Dynamic Spectra of Microwave Outbursts on dMe Flare Stars
Authors: Bastian, T. S.; Bookbinder, J. A.
1986BAAS...18..985B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Two-Frequency Imaging of Microwave Impulsive Flares near the
Solar Limb
Authors: Dulk, G. A.; Bastian, T. S.; Kane, S. R.
1986ApJ...300..438D Altcode:
Using the VLA, the authors observed two impulsive microwave and hard
X-ray flares that occurred close to the solar limb on 1981 November 21
and 22. Images were obtained simultaneously at 4.9 and 15 GHz every
10 s during the 5 minutes durations of the flares. The apparently
simple impulsive flares were unexpectedly complicated when imaged
with arcsecond resolution: the 4.9 and 15 GHz radiation came from
spatially separated sources in one flare, and there was no 4.9 GHz
radiation at all in the other. The authors interpret the observations
in terms of an inhomogeneous flare volume with the magnetic field
strength and orientation varying with position, both transverse to
and along the line of sight. The 4.9 GHz radiation of the November 22
flare probably arose mainly from thermal electrons with temperatures
between 5 and 20×10<SUP>6</SUP>K. The 15 GHz radiation of the flares
on both days probably arose from electrons of E ≈ 300 keV in a weak,
nonthermal tail.
---------------------------------------------------------
Title: Radio Observations of Flare Stars in the Pleiades
Authors: Bastian, T. S.; Dulk, G. A.
1986LNP...254..256B Altcode: 1986csss....4..256B
No abstract at ADS
---------------------------------------------------------
Title: Radio Emission from AM Herculis
Authors: Bastian, T. S.; Dulk, G. A.; Chanmugam, G.
1985ASSL..116..225B Altcode: 1985rst..conf..225B
Observations of the quiescent microwave emission of the magnetic
cataclysmic variable AM Herculis are presented. The emission,
which declined from a mean value of 0.58 mJy at 4.9 GHz to about
0.3 mJy, in rough coincidence with the entry of AM Herculis into
an optical low state (mid-1983), is explained in terms of optically
thick gyrosynchrotron emission. It is noted that the observation of a
coherent outburst at 4.9 GHz, interpreted as the result of a cyclotron
maser on the red dwarf secondary, indicates that the secondary is
magnetized. Possible implications are briefly explored. Comparisons
between this system and other stellar continuum radio sources are made.
---------------------------------------------------------
Title: Full disk maps of the sun at 1.4 GHz.
Authors: Bastian, T. S.; Dulk, G. A.
1985BAAS...17R.632B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Full Disk Maps of the Sun at 1.4 GHz
Authors: Bastian, T. S.; Dulk, G. A.
1985BAAS...17..632B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: AM Herculis: An Outburst at 4.9 GHz
Authors: Bastian, T. S.; Dulk, G. A.; Chanmugam, G.
1985ASSL..113..231B Altcode: 1985cvlm.proc..231B
The authors report the results of radio observations of AM Her with
the Very Large Array (VLA). The quiescent emission first discovered
by Chanmugam and Dulk (1982) at 4.9 GHz is confirmed and upper limits
to the flux density at 1.5 GHz and 15 GHz obtained. The authors also
report the discovery of a remarkable outburst at 4.9 GHz which was
essentially 100% circularly polarized. The outburst is probably due to
an electron-cyclotron maser which operates near the red-dwarf companion
in a region where the magnetic field is ≈1000 gauss.
---------------------------------------------------------
Title: Radio emission from AM Herculis : the quiescent component
and an outburst.
Authors: Dulk, G. A.; Bastian, T. S.; Chanmugam, G.
1983ApJ...273..249D Altcode:
The VLA has been used to search for radio emission from the AM Her-type
binaries VV Pup, EF Eri, PG 1550 + 191, CW 1103 + 354, and AN UMa,
at 4.9 GHz. A remarkable 10-min outburst was detected from AM Her
at 4.9 GHz, which was about 20 times more intense than the quiescent
emission and was essentially 100 percent circularly polarized. It is
suggested that the quiescent emission of AM Her can be accounted for
by 500-keV electrons trapped in the magnetosphere of the white dwarf,
provided that the electron energy spectrum is quite hard and that the
spectral hardness or number density of energetic electrons increases
with radius, while the outburst is probably due to an electron-cyclotron
maser operating near the surface of the red dwarf companion. The implied
existence of a 1000-gauss localized magnetic field and a corona on the
red dwarf has consequences for mass transfer, field line interactions,
and variable activity.
---------------------------------------------------------
Title: Dual Frequency Observations of Flares with the VLA
Authors: Dulk, G. A.; Bastian, T. S.; Hurford, G. J.
1983SoPh...86..219D Altcode:
We describe observations of three flares made at 5 and 15 GHz with
the VLA, two subflares near the limb on 1981 November 21 and 22, and
an M7.7 flare on 1981 May 8. Even though the time histories of the
November flares indicated simple impulsive bursts, the VLA observed
no 5 GHz radiation at all from one flare, and from the other, the
15 GHz radiation emanated from a source which was smaller, lower and
displaced from the 5 GHz source. Without the spatial information, we
would have derived incorrect results from the assumption that photons
of different energy (both at X-ray and radio wavelengths) arose from
one homogeneous volume.
---------------------------------------------------------
Title: Two-frequency imaging of impulsive microwave flares.
Authors: Bastian, T. S.; Dulk, G. A.
1983BAAS...15..874B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Gradients of galactic cosmic rays and anomalous helium to
more than 23 A.U. during the increase of solar modulation in 1978-80
Authors: Bastian, T. S.; McKibben, R. B.; Pyle, K. R.; Simpson, J. A.
1981ICRC...10...88B Altcode: 1982ICRC...10...88B; 1981ICRC...17j..88B
During the transition to solar maximum (1978-81) the radial gradient of
high energy galactic cosmic rays increased slightly, while the gradients
of lower energy cosmic rays and the anomalous helium decreased. It
is concluded that (1) Pioneer-10 at about 23 AU was still deep in a
modulation region extending to more than 50 AU for both the galactic
cosmic rays and the anomalous helium component, and (2) the modulation
is controlled by changes in the size/geometry of heliospheric magnetic
fields and/or propagation parameters, changes associated with large
solar-flare shocks in 1978 and later.
---------------------------------------------------------
Title: The Trapped Radiations of Saturn and Their Absorption by
Satellites and Rings
Authors: Simpson, J. A.; Bastian, T. S.; Chenette, D. L.; McKibben,
R. B.; Pyle, K. R.
1980JGR....85.5731S Altcode:
The Pioneer 11 spacecraft encounter with Saturn (closest approach
September 1,1979) has resulted in the discovery of a fully developed
magnetosphere with high-energy trapped radiation around Saturn,
as reported in Science, 207, 400-453, 1980, by several investigators
with charged-particle instruments on the spacecraft. The present paper
contains in detail the final energetic charged-particle measurements and
new observations obtained from the University of Chicago instrumentation
on Pioneer 11, including the overall characteristics of the trapped
electron, proton, and helium radiation, which was found to lie inside
~20 Saturn radii (R<SUB>s</SUB>) from the planet, and the regions
extending outward to beyond the planetary bow shocks and into the
interplanetary medium. For analytical purposes we divided that
magnetosphere into an inner magnetosphere (<5R<SUB>s</SUB>),
where the intensity profiles displayed the near-axial symmetry
characteriscis of the sipole magnetic field alignment with the spin
axis, and an outer magnetosphere whose characteristic on the sunward
side inbound were significantly different from the dawn side out-bound,
indicative of a possible magnetotial but with no dramatic evidence
in the charged-particle data for an equatorial current sheet, as
observed at Jupiter. The intensities and energy ranges of the protons
and electrons were intermediate between the levels found previously at
Jupiter by Pioneer 10 and 11 and at earth. Each spectra for protons
and electrons and relative abundances of protons and helium nuclei
are presented along with the average characteristics of particle
anisotropies. At the time of encounter the magnetosphere was immersed
in intense fluxes of electrons, protons, and helium nuclei of solar
flare origin which are shown to penetrate from 1 R<SUB>s</SUB> to 1 10
R<SUB>s</SUB> into the magnetosphere, where they dominated the flux
levels in the far outer magnetosphere. A corotation anisotropy has
been measured at the proton energy ~1 MeV in the rotating magnetosphere
after correcting the observed unidirectional anisotropy for the radial
gradient of the proton flux. The principal focus of the paper is on
the analysis of the trapped radiation in the inner magnetosphere,
where the radiation reaches high intensity, and has a high degree of
symmetry in the L shells around Saturn. Consequently, the absorption
signatures in the radiation intensity profiles produced by rings and
moons of Saturn can be analyzed quantitatively. Among other results
the observation of the charged-particle absorption features have led
to the discovery of satellite 1979 S2 at L=2.53, which corresponds
with the optically detected 1979 S1. A concentration of matter
probably located at a Lagrangian point in the orbital range of
Mimas, and the identification of narrow rings of matter and one or
more satellites inside the radiation range of the F ring dicovered
by the optical-imaging investigators. It is pointed out that these
discoveries will provide important tests for models of accretion of
matter, satellite formation and the stability of narrow rings near
planets. From the discrete character of the absorption symmetry it
is shown that except for two extremely narrow intervals of Saturn's
longitude, the equatorial offset of the dipole magnetic moment must be
<~0.01 R<SUB>s</SUB>. The inward diffusion coefficients for protons
and electrons have been determined from the above absorption regions,
especially at Mimas. Some questions are considered which may be resolved
by the forthcoming Voyager encounters with Saturn.
---------------------------------------------------------
Title: Charged Particle Anisotropies in Saturn's Magnetosphere
Authors: Bastian, T. S.; Chenette, D. L.; Simpson, J. A.
1980JGR....85.5763B Altcode:
We report observations of anisotropies and pitch angle distributions
for 0.5-1.8 MeV protons, 7-17 MeV electrons and >3.4 MeV electrons in
Saturn's magnetosphere made with the University of Chicago experiments
on Pioneer 11. In the outer magnetosphere (L>6) there is clear
evidence for corotation of the proton flux, and the proton pitch
angle distribution shows maximum flux perpendicular to the magnetic
field ('pancake' distribution). Observed changes in the amplitude
and shape of the pitch angle distributions suggest the existence of
significant temporal variations in the outer magnetosphere. From L=6
to L=4, the proton intensity decreased by more than two orders of
magnitude, and the pitch angle distribution shifted to a 'dumbbell'
form (maximum flux parallel to magnetic field). The shift in pitch
angle distribution most likely results from preferential absorption
of large pitch angle particles by the tenuous E- ring found in the
equatorial plane out to at least R=5 R<SUB>s</SUB>. For L<4, the
proton intensity increased inwards, implying an inner edge for the
E ring at R?4 R<SUB>s</SUB>. Except for regions where the flux was
reduced by satellite absorption, the pitch angle distributions remained
dumbbell. In absorption regions, pancake distributions were found. The
observations are consistent with the suggestion by McKibben and
Simpson (this issue) that inward diffusion and acceleration at Saturn
may proceed primarily via large, infrequent disturbances. Electron
anisotropies were measurable only for L<~4.5, and the pitch angle
distributions were found to be pancake for the entire region L<4.5,
suggesting that the absorbing particles in the E ring have radii that
lie between the range of ~10 MeV proton and ~10 MeV electrons, or of
the order of millimeters.
---------------------------------------------------------
Title: Saturnian Trapped Radiation and its Absorption by Satellites
and Rings: The First Results from Pioneer 11
Authors: Simpson, J. A.; Bastian, T. S.; Chenette, D. L.; Lentz,
G. A.; McKibben, R. B.; Pyle, K. R.; Tuzzolino, A. J.
1980Sci...207..411S Altcode:
Electrons and protons accelerated and trapped in a Saturnian magnetic
field have been found by the University of Chicago experiments on
Pioneer 11 within 20 Saturn radii (R<SUB>S</SUB>) of the planet. In
the innermost regions, strong absorption effects due to satellites and
ring material were observed, and from ~ 4 R<SUB>S</SUB> inwards to the
outer edge of the A ring at 2.30 R<SUB>S</SUB> (where the radiation is
absorbed), the intensity distributions of protons (>= 0.5 million
electron volts) and electrons (2 to 20 million electron volts) were
axially symmetric, consistent with a centered dipole aligned with
the planetary rotation axis. The maximum fluxes observed for protons
(>35 million electron volts and for electrons >3.4 million
electron volts) were 3 × 10<SUP>4</SUP> and 3 × 10<SUP>6</SUP> per
square centimeter per second, respectively. Absorption of radiation by
Mimas provides a means of estimating the radial diffusion coefficient
for charged particle transport. However, the rapid flux increases
observed between absorption features raise new questions concerning the
physics of charged particle transport and acceleration. An absorption
feature near 2.5 R<SUB>S</SUB> has led to the discovery of a previously
unknown satellite with a diameter of [Note: Equation omitted. See the
image of page 411 for this equation.] kilometers, semimajor axis of 2.51
R<SUB>S</SUB>, and eccentricity of 0.013. Radiation absorption features
that suggest a nonuniform distribution of matter around Saturn have
also been found from 2.34 to 2.36 R<SUB>S</SUB>, near the position of
the F ring discovered by the Pioneer imaging experiment. Beneath the
A,B, and C rings we continued to observe a low flux of high-energy
electrons. We conclude that the inner Saturn magnetosphere, because
of its near-axial symmetry and the many discrete radiation absorption
regions, offers a unique opportunity to study the acceleration and
transport of charged particles in a planetary magnetic field.
---------------------------------------------------------
Title: The Radial Intensity Gradients for Cosmic Rays and the
Anomalous Helium from Pioneer 10 and 11
Authors: Bastian, T. S.; McKibben, R. B.; Pyle, K. R.; Simpson, J. A.
1979ICRC...13..405B Altcode: 1979ICRC...16m.405B
No abstract at ADS
---------------------------------------------------------
Title: Recurrent Modulation of Galactic Cosmic Rays and the Anomalous
Helium
Authors: Bastian, T. S.; McKibben, R. B.; Simpson, J. A.
1979ICRC...12..324B Altcode: 1980ICRC...12..324B; 1979ICRC...16l.324B
The observations of 27 day variations in the intensity of low energy
cosmic ray protons (29-67 MeV) and anomalous helium (11-20 MeV/nucleon)
are reported. These observations were made with instruments on IMP-8
in the earth orbit on Pioneer 11 at heliographic latitudes up to 16
deg and on Pioneer 10 at heliocentric distances of from 4 to 14 AU. 27
day variations are observed at all locations, most strongly in the
anomalous helium, and reflect the very large scale structure of the
interplanetary medium. Intensity minima are most often associated with
high speed solar wind streams, but the physical mechanism producing
the variations is not clear.
---------------------------------------------------------
Title: The Radial Intensity Gradients for Cosmic Rays and the
Anomalous Helium from Pioneer 10 and 11
Authors: Bastian, T. S.; McKibben, R. B.; Pyle, K. R.; Simpson, J. A.
1979ICRC....5..338B Altcode: 1979ICRC...16e.338B; 1980ICRC....5..338B
This report extends and updates previous studies of the heliocentric
radial gradients of the integral flux of cosmic rays (E greater than
70 MeV) and of differential fluxes of protons and helium nuclei in the
approximate energy windows 11-20 and 30-70 MeV/n to include data taken
through the end of 1978 when Pioneer 10 had reached a distance of 18
AU from the sun, and Pioneer 11 was at a distance of 8.4 AU. Small
positive gradients are found for all species considered, although the
actual values of the gradients may vary as a function of time. The data
suggest that the modulation region has a radial extent large compared
to 18 AU. The anomalous helium component was observed at all radial
distances, and is found to have a much higher flux in the outer solar
system than near 1 AU.
---------------------------------------------------------
Title: Variations in the Intensity of Galactic Cosmic Rays and the
Anomalous Helium as a Function of Solar Latitude
Authors: Bastian, T. S.; McKibben, R. B.; Pyle, K. R.; Simpson, J. A.
1979ICRC...12..318B Altcode: 1979ICRC...16l.318B; 1980ICRC...12..318B
Pioneer 11 rose to a heliographic latitude of 16 deg in the course of
its trajectory from Jupiter to Saturn. From analysis of the fluxes of
low energy protons and anomalous helium measured by Pioneer 11 as a
function of latitude, it was found that the data are consistent with
latitude gradients of 2% per degree for the anomalous helium and 1%
per degree for 29-67 MeV protons.
