The Very Large Array (VLA) was used to observe the active region AT 4508 in the C-configuration between 1530 and 2330 UT on June 4, 1984. The position of this region was N06 E57 at 1300 UT on this day. Followup observations were made between 1500 and 2300 UT on January 17, 1986 in the D-configuration. Observations were compared with Mt. Wilson magnetograms. Results are discussed

Lang, Kenneth R.

Willson, Robert F.

English

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COMPACT,VARIABLE,MOVINGSOURCES
OBSERVEDONTHESUNAT2 CENTIMETERSWAVELENGTH
KENNETHR. LANG
ROBERTF. WILLSON
Department of Physics and Astronomy
Tufts University
Medford, MA 02155
INTRODUCTION
The high angular resolution provided by the Very Large Array (VLA) has
permitted the spatial resolution of solar microwave sources and opened the way for
comparisons with observations of similar angular resolution at optical and X-ray
wavelengths. High-resolution VLA observations of solar active regions at
relatively long wavelengths of 6 cm and 20 cm have, for example, led to the
discovery of the microwave counterpart of the ubiquitous coronal loops that had
previously only been observed by X-ray telescopes lofted above the Earth's
atmosphere. The microwave emission of the coronal loops is attributed to the
gyroresonant radiation and/or the bremsstrahlung of million-degree, thermal
electrons trapped within the loops by strong magnetic fields; observations of this
emission have provided valuable new insights into the nature of solar active
regions and eruptions from the Sun and nearby stars [ _ndu and Lang (1985); Lang
(1986 a,b - this proceedings)].
In contrast, the short wavelength 2 cm emission of solar active regions is
poorly understood. In spite of numerous VLA solar observations at 2 cm, there are
only two published results [Lang, Willson and Gaizauskas (1983); Shevgaonkar and
Kundu (1984)]. In both instances, compact (angular sizes 8 _ 15"), highly
polarized (degrees of circular polarization Pc = 80% to 90%) sources were found in
regions of strong magnetic field (_trength H _ 2,000 G) above sunspots. The
brightness temperatures of T B _ i0- K were characteristic of the electron
temperature in the transition region.
Subsequent examination of the compact 2 cm sources in active regions indicated
that they are variable over time scales of an hour or shorter. This probably
explains the paucity of VLA results; synthesis maps averaged over ii or 12 hours
would not reveal several relatively-weak, time-variable sources.
To further complicate the matter, _e have recently discovered compact,
variable, highly-polarized 2 cm sources in regions of apparently-weak, photospheric
magnetic field [Willson and Lang (1986)]. Our subsequent VLA observations have
confirmed the existence of compact, variable 2 cm sources that are not associated
with active regions, but these sources had no detectable circular polarization. In
addition, both the unpolarized and polarized 2 cm sources were found to move
laterally across the solar surface with velocities V _ 1 km s-I. In the next
section we present observations of these compact, variable, moving sources. The
concluding discussion mentions possible radiation mechanisms and implications for
studies of the quiet Sun.
PRECEDINC--, pAGE BLANK NOT F1LMED
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OBSERVATIONS
The VLA was used to observe the active region AR 4508 in the C configuration
between 1530 and 2330 UT on June 4, 1984. The position of this region was NO6 E57
at 1300 UT on this day. Follow-up observations were made between 1500 and 2300 UT
on January 17, 1986 in the D configuration. In this case, a region of bright plage
and relatively-weak magnetic fields (no sunspots) was observed; its position was
SI0 W62 at 1300 UT on this day.
,June 4, 1984
X=2cm
V maps
1525-1527 UT
:':z:?.:.
"9:?
1732" 1752 UT
1547-1602 UT
,,.}_ r
i i
/
@
1820" 1835 UT
1618-1658 UT
@
/
@
1845-1900 UT
@
1658"1713 UT
,/-},':
'-;;"
l t
/
1928-1943
Figure I. VLA synthesis maps of left circularly polarized (solid contours) and
right circularly polarized (dashed contours) radiation at 2 cm wavelength.
Here each box refers to the same area on the surface of the Sun, and the
fiducial marks on the axes are separated by 10 arc-seconds. The northern
source (top) varied over time scales of 30 minutes and moved laterally
across the solar surface in the northwest direction a velocity of
NI km s -I . The southernmost source varied over a time scale of about 60
minutes, and moved laterally towards the southwest at a velocity of
_2 km s -I . Here the contours mark levels of equal brightness with an
outermost contour of 6.1 x 104 and a contour interval of 3.1 x 104 K.
354
As illustrated in Figure I, the 2 cm maps on June 4 showed two compact
(O - 5"), highly circularly polarized (Pc = 80 to 90%) sources that vary on time
scales of 30 to 60 minutes. The left circularly polarized _ource (solid contours)
varied in maximum brightness temperature from TB = 2.0 x i0- K to T B < 0.5 x i0 v K.
Here each box refers to the same area of the Sun, and the arrows illustrate
systematic motion to the northwest with a total motion of about 15" in three
hours. The left circularly polarized source was therefore moving laterally across
the surface of the Sun with a velocity of V - 1 km s -I. The right circularly
polarized source (dashed contours) apparently moved towards the southwest at about
twice this speed, but the motion is confused by the presence of more than one
source.
Comparisons with Mt. Wilson magnetograms indicate that the two compact,
variable, moving sources were located in regions of apprently-weak photospheric
magnetic field (H < 80 G), and that they did not overlie sunspots. The high
polarization of these sources is therefore somewhat enigmatic, for the polarization
of thermal radiation requires strong magnetic fields of H - 2,000 G. We will
return to this paradox in the discussion.
January 17, 1986
X=2cm
[ maps
1500" 1600 UT
0
/#
1600-1700 UT
0
o
/
17OO" 1900 UT
0
1900-:)000 UT ZOOO-2100 UT
/
J
2100-2200 UT
Figure 2. VLA synthesis map of the total intensity of the radiation at 2 cm
wavelength. Here each box refers to the same area on the surface of the
Sun, and the fiducial marks on the axes are separated by 60 arc-seconds.
The southern source (bottom) increased slowly in brightness over a seven
hour period while moving laterally across the solar surface in the
southeast direction at a velocity of _ I km s-I . The northern source
(top) varied over a time scale of 180 minutes, but showed no detectable
lateral motion. Here the contours mark levels of equal brightness with
an outermost contour of 5.2 x 104 K and a contour interval of 1.8 x 104 K.
355
Our confirming observations on January 17, 1986 revealed compact 2 cm sources
with angular sizes @ _ 25" and maximum brightness temperatures of T B = 2.0 x 105 K
These sources were observed in plage regions with apparently-weak photospheric
magnetic field and no sunspots, but in this case the compact sources had no
detectable circular polarization (Pc 4 15%).
As illustrated in Figure 2, the compact unpolarized sources either varied
substantially in intensity over time scales of hours (top sources) or slowly
increased in brightness over a seven hour period (bottom source). Here each box
refers to the same area on the surface of the Sun, and the arrows indicate a
lateral motion at a velocity of V _ 1 km s-I . Curiously, the relatively-unvarying
source exhibited this motion, but the variable one showed no detectable motion.
DISCUSSION
We have discovered previously-unobserved sources at 2 cm wavelength in regions
of apparently-weak photospheric magnetic field. The brightness temperatures of
T B ~ l0 K are characteristic of the transition region. The angular sizes are
8 ~ 5" to 25", and they vary in intensity over time-scales of 30 minutes to more
than 180 minutes. We have observed at least two of these compact, variable 2-cm
sources within the 3' field of view every time we have observed the Sun;
extrapolating to the 30' - wide Sun, we would expect hundreds of them on the
visible surface of the quiet Sun. The compact, variable sources can either be
highly circularly polarized (Pc = 90%) or they can exhibit no detectable circular
polarization (Pc 4 15%).
The enigmatic presence of highly polarized sources in regions of apparently-
weak photospheric magnetic field may be explained by any one of three hypothesis.
First, the photospheric field may have strengths of up to 2,000 G in compact
regions that are not readily detected by the photospheric magnetograms.
Alternatively, the magnetic field in the transition region or the low corona may
be amplified by currents to a strength above that in the underlying photosphere.
If either of these hypothesis is true, then the high circular polarization of the
2 cm sources can be attributed to either thermal gyroradiation or the propagation
of thermal bremsstrahlung in the presence of a magnetic field of strength H _ 2,000
G. A third hypothesis, developed by Willson and Lang(1985), is that the compact
2 cm sources are due to nonthermal gyrosynchrotron radiation of mildly relativistic
electrons in relatively weak magnetic fields of strength H _ 50 G.
But what accounts for the variability and lateral motion of both the polarized
and the unpolarized sources? The source variability might be due to a variable
magnetic field that comes and goes within the transition region and low solar
corona. Alternatively, the variations could be interpreted in terms of thermal
electron density variations related to heating changes or to non-thermal electron
density variations resulting from a variable acceleration mechanism. The lateral
motion can be attributed to an upward expansion of dipolar loops; the 2-cm
observations detect the apparent lateral motion of the loop legs.
Finally, we would like to point out certain resemblances between the compact,
variable 2-cm sources and other phenomena reported in this proceeding. These
sources are resolved (they are not points) with angular sizes comparable to those
of small erupting filaments [Martin (1986 - this proceedings)] and the 20 cm
observations of so-called coronal bright points [Habbal (1986 - this proceedings)].
The time scale of the variations and the lateral motions of the 2-cm sources are
356
comparable to those of the small erupting filaments. The brightness temperatures
of the 2-cm sources are the same as those of the 20-cm ones. Comparisons with
features seem at the He I, I 10830 transition are very misleading, for there are so
many of these features that the statistical significance of a correlation has to be
very low.
ACKNOWLEDGEMENTS
Radio astronomical studies of the Sun at Tufts University are supported under
grant AFOSR-83-0019 with the Air Force Office of Scientific Research (AFOSR) and
contract N0014-86-K-0068 with the Office of Naval Research (ONR).
REFERENCES
Habbal, S., 1986, " Spatial and Temporal Observations of Coronal Bright Points" -
this Proceedings.
Kundu, M.R. and K.R. Lang, 1985, "The Sun and Nearby Stars: Microwave Observations
at High Resolution, " Science 228, 9-15
Kundu, M.R., 1986 a, "Coronal Plasmas on the Sun and Nearby Stars" - this
Proceedings.
Lang, K.R., 1986a "Coronal Plasmas on the Sun and Nearby Stars"- this Proceedings.
Lang, _R., 1986b, "Coronal Diagnostics - this proceedings.
Lang, _R., R.F. Willson and V. Gaizauskas, 1983, " Very Large Array Observations
of Solar Active Regions III. Multiple Wavelength Observations," Astrophysical
J., 267, 455-464.
Martin, S., 1986, : Small Eruptive Filamentary Sources" - this Proceedings.
Shevgaonkar, R.K. and M.R. D/ndu, 1984 "Three Dimensional Structures of two Solar
Active Regions from VLA Observations at 2,6 and 20 cm. Centimeter Wavelength",
Astrophy. J., 283, 413-420.
Willson, R.F. and K.R. Lang, 1986, "VLA Observations of Compact, Variable Sources
on the Sun," Astrophysical Journal - to be published.
357


Compact, variable, moving sources observed on the sun at 2 centimeters wavelength

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