JOP060: Joint VLA-SOHO Observations of Transient Sources in the Solar Corona Proposers: Robert Willson and Kenneth R. Lang Department of Physics and Astronomy=20 Tufts University Medford, MA 02155 (rwillson@pearl.tufts.edu) PROJECT DESCRIPTION VLA observations at 2 and 3.5 cm wavelength have shown that the quiescent emission from active regions originates in compact (size~5") highly circularly polarized (up to 90%) sources in regions of strong magnetic fields near sunspots (Willson and Lang 1986). The observed brightness temperatures of Tb 1-2 x 10**5 K suggests that this emission occurs in the transition region or low corona. Other VLA observations have revealed a different class of 2 cm sources which are also compact and sometimes circularly polarized but which are located away from sunspots in regions of apparently weak (B 100 G) photospheric fields (Willson and Lang 1987). These sources are often time variable, changing on timescales of minutes to hours. 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 bremsstrahlung). Alternatively, the 2 cm emission could be due to the gyrosynchrotron radiation of mildly relativistic electrons accelerated in regions of changing magnetic flux (Willson and Lang 1986). The observed variability could then be caused by fluctuations in magnetic field strength or to variations in electron density. Similiar conclusions were reached by Gopalswamy et al. (1994) who detected transient microwave brightenings (2 cm) in association with soft X-ray brightenings from active regions. The timescale of the fastest 2 cm variations is similiar to those of small-scale transient events that have been observed at optical, ultraviolet or X-wavelengths. Examples of such transient brightenings include H alpha surges (Schmieder et al. 1993), small-scale UV explosive events (Dere et al. 1989), high velocity UV jets (Brueckner and Bartoe, 1983), X-ray and UV bright points and X-ray jets (Hanaoka, 1994). These eruptive or explosive events are believed to be triggered by the magnetic reconnection of oppositely-directed magnetic fields, perhaps in response to motions at lower levels in the photosphere. The process of magnetic reconnection may produce energetic particles that escape from the reconnection site and produce characteristic signatures at different heights or locations in the active region. The high spatial resolution of SUMER, CDS and EIT can be used to image the intensities and profiles of temperature and density-sensitive lines, thereby establishing the physical conditions in which these evolving sources arise. Here we propose to use the VLA in conjunction with SOHO to produce images of total intensity and circular polarization at 2 and 3.5 cm that can provide additional information about the magnetic field strength within the regions of energy release in the transition region and corona. The combined VLA/SOHO data will be used to investigate transient phenomena in the chromosphere and transition region, magnetic changes in the underlying photosphere and thermal and nonthermal particle acceleration in the corona. Specific questions which we hope to answer include: 1. What radiation mechanism can account for the brightness and circular polarization? observations of the transition region/low corona at 2 and 3.5 cm should be able to determine whether it is thermal or nonthermal. It is important to note that the UV data alone cannot determine whether the transient energetic sources produce nonthermal emission; it is only the microwave data, through measurements of brightness temperature and circular polarization, that can distinguish between a thermal or nonthermal radiation mechanism. 2. 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? 3. Is the variability due to intermittent heating, continued low-level flare activity, changing magnetic field topology or any combination of these? VLA snapshot maps at 2 and 3.5 cm will be able to study source variations at these wavelengths and to monitor spectral and polarization variations that give clues to magnetic field changes. PROPOSED VLA_SOHO OBSERVATIONS OF TRANSIENT EVENTS We have been given three periods of VLA observing time to observe the Sun on April 15, 17 and 19 from approximately 1500-1930 UT on each day. It is also possible that we will have up to three hours of VLA time on April 10 and 12 - the VLA will let us know within the next month about these additional observations. The entire array will be used to switch between 2 and 3.5 cm every few minutes while also observing continuously at 91 cm The requested time intervals will allow us to study the temporal evolution of transition region sources while also allowing investigations of different types of photospheric magnetic configurations including active regions and areas of "quiet" Sun. Snapshot maps of total intensity and circular polarization at 2 and 3.5 cm (on time intervals of 1.7 seconds) will be used to study magnetic and/or temperature changes in the transition region and low corona that may be produced during transient events that originate in the same or lower levels. Images made simultaneously at 91 cm may reveal decimetric Type III-like bursts if energetic electrons are produced and escape along open magnetic field lines anchored to the sites of reconnection events. Here we propose that our VLA observations be part of a Joint Observing Program (JOP) involving the SOHO CDS, EIT and SUMER. The objective of these SOHO observations would be to study the transition region and low-corona counterparts of transient brightenings and evolving 2 cm and 3.5 cm sources. CDS might be used to make raster scans of one or two spectral lines (over a 120" x 120" field of view every 5 - 10 minutes) with peak formation temperatures (T == 1 - 10 x 10**5 K) that are comparable to the expected brightness temperatures of the evolving 2 and 3.5 cm sources. Temporal changes in source structure could also be studied by having EIT obtain partial field-of-view images (!9') at 171, 195, 284 and 304 Angstroms on timescales of a few minutes or less. Exxplosive events lasting only a few tens of seconds will be identified from their broadened line profiles seen in the one-dimensional SUMER and CDS slits. Finally, the VLA and SOHO data will be compared with Yohkoh soft X-ray images in order to study changes in source structure or plasma parameters that might accompany explosive events, energetic jets or flaring bright points. Last June 14, 15 and 16, CDS cycled through seqnences of the following lines: Si XII 520 A, Fe XIV 334 A, Ca X, 557 A, Si X 347 A, He I, 584 A, Fe XIV 353 A, Si X 356 A, O III 599 A, Fe XVI 360 A, Fe XII 364 A, Mg IX 368 A and OV 629 A. SUMER, at the same time, went through something like the following sequence: O IV 1407.38, 1404.81 A OV 629.73 A MG X 624.940 Fe XII 1242.01 N V 1242.80 Ne VIII 770.409 780.324 C IV 1548.20 C IV 1550.77 EIT, in additionto taking its normal sequence of full-disk images at 171, 195, 284 and 304 A, also took partial-frame (9') images of a region near the west limb at these wavelengths. Basically, if CDS could image a couple of lines with formation temperatures of ~5 x 10**5 K and 1.5 x 10**6 K,as often as possible over a field of view of 4' x 4', that would be very useful.