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Proposal ID Willson_46

Date of Proposal

Robert F.


Department of Physics, Tufts University

Department of Physics and Astronomy
Tufts University
Medford, MA 02155 USA


(617) 636-6683

(617) 636-6536

Multiple-wavelength VLA observations of the Sun may be used to
detect thermal plasmas at different heights above solar active
regions and to provide information about magnetic interactions
and energy release during transient events such as flares. Observations at 2 and 3.5 cm wavelength,
for example, have shown that the quiescent emission from active regi
ons originates in compact (size
less than 5") highly circularly polarized up to 90%) sources in regions of
strong magnetic fields near
sunspots (Willson and Lang 1986). The observed brightness temperature=
s of Tb=F7 1-2 x 10**5K
suggests that this emission occurs in the transition region or low
corona. VLA observations have
also revealed a different class of 2 cm sources which are time variab=
le (minutes to hours), compact,
and sometimes circularly polarized but which are located away from
sunspots in regions of apparently
weak (B less than 100 G) photospheric fields (Willson and Lang 1987).

These microwave sources are enigmatic because magnetic fields of =
1500-2000 G are required if the observed brightness temperatures
and circular polarization are to be attributed to thermal
gyroresonance radiation or propagation effects (thermal bremsstrahlun=
g). Alternatively, the 2 cm
emission could be due to the gyrosynchrotron radiation of mildly rel=
ativistic electrons accelerated in
regions of changing magnetic flux (Willson and Lang 1986). The observ=
ed variability could then be
caused by fluctuations in magnetic field strength or to variations i=
n electron density.

On June 14, 15 and 16, 1996 the Very Large Array was used to
observe the Sun (between
1530-2030 UT) in coordination with SUMER, CDS and EIT. The primary go=
al of this Joint
Observing Program was to study the dynamics and physical parameters =
of these evolving magnetic
structures in the solar transition region and low corona. On all
three days, the VLA was
used to track an active region located on the west limb at 3.5 and 6=
.2 cm and to image the full solar
disk at 20 and 91 cm. Radio observations at these wavelengths
detect emission from different heights corresponding to the temperature
range of thermal plasmas detected by the EUV
instruments on board SOHO. SUMER and CDS tracked the west-limb region=
and produced 4' x 4'
spectroheliograms in 18 different ionic species with formation temper=
atures ranging between 2 x
10**4 K and 3 x 10**6 K. EIT provided daily full-disk images at 195, =
284 and 304 A as well as smaller field-of-view (9.7' x 9.7')
images at these wavelengthss at a cadence of about 90 minutes.

One aspect of our data analysis will be to compare VLA snapshot map=
s (on intervals as short as 3.3 seconds) with sequences of EIT
images in order to correlate source variations in the radio and EUV
bands. A preliminary examination of the EIT data at the SOHO EOF,
for example, reveals low-level
changes in the brightness of EUV structures associated with the target active region on timescales of
tens of minutes. These source variations may reflect changes in the
temperature or magnetic field structure in the underlying
active region and we have reason to believe that similiar changes may also
be detected by the VLA. The EIT synoptic images have already
been downloaded to the Tufts
computer, but here we request that the full set of images (between 1=
530-2030 UT on June 14, 15 and
16, 1996) be made available ( preferably in FITS or GIF format)
so that they can be more thoroughly examined and compared with
the VLA data.

Specific questions that we hope to answer include:

What radiation mechanism can account for the brightness and circular =
polarization? VLA
observations at 3.5 and 6.2 cm should be able to determine whether
it is thermal or nonthermal.

What is the spatial and temporal distribution of the UV and radio
sources and what does this comparison tell us about their
magnetic structure and particle acceleration?

IS the variability due to intermittent heatingm continued low-level f=
lare activity, vhanging
magnetic field topology or any combination of these? VLA and EIT imag=
es at different wavelengths
be used to study source variations at different heights and to monito=
r spectral and polarization
variations that give clues to the changing thermal and magnetic


Willson, R.F., and Lang, 1986, Ap. J., 308, 443.
Willson, R.F., and Lang, K.R. 1987, Solar Phys., 114, 93.=20

Barbara Thompson


Transition Region


VLA-EIT Observations of Evolving Sources in the Transition Region and Low Corona

update added on 1998/03/04 at 10:41:30
To date, our analysis of EIT observations taken in conjunction with
the VLA in April, June and November 1997 has resulted in the following
presentations and publications:

"VLA-SOHO Observations of Evolving Coronal Structures on the Sun"
R.F. Willson, K.R.Lang, B. Thompson, U. Schuehle and D.M. Zarro,
(to appear in the Proceedings of the 10th Workshop on Cool Stars,
Stellar Systems and the Sun, July, 1997, Boston, MA.

"The Radio and EIV SIgnatures of Small-Scale Coronal Magnetic
Reconnection Events" R.F. Willson (to appear in the Proceedings
of the meeting on Solar Jets and Coronal Plumes, Point a-Pitre
Guadeloupe, February, 1998

update added on 2005/01/11 at 03:49:44

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