JOP 122, Micro-scale heating blocks of the solar atmosphere R. Erdelyi, R. Willson, J.G. Doyle Date: 16:30-21:30 UT 6--8 May 2000 Received: 11 April 2000 Background ========== It is widely recognized that coronal heating for the Sun and solar-like stars is tied to magnetic activity. Explanations for the high temperature of the solar corona have given rise to many ideas, including Alfven wave dissipation, turbulent cascades, currents sheet dissipation or nano-flaring and anomalous interruption of field-aligned currents. As of yet, none of these suggestions have proven to be entirely satisfactory. Perhaps one of the most interesting of these is the idea that the corona is heated by numerous small localised events which has being termed `nano-flaring'. Although individual nano-flares may be undetectable with current instrumentation, observations of high resolution UV and EUV spectra taken with HRTS and later with instruments on-board SOHO and TRACE have revealed large transient brightenings. Some of these compact sources (sizes of a few arcseconds) show sudden enhancements of line intensities, others have small enhancements in their line intensities but strongly broadened non-Gaussian Doppler shifted line profiles. The fact that micro-flares/explosive events appear to be concentrated in areas of emerging and cancelling flux suggests that these events are triggered and driven by magnetic reconnection when photospheric fields emerge, interact and reconnect with pre-existing fields at coronal levels. Here we propose to study the spatial and temporal evolution of micro-flares/ explosive events and associated structures by combining VLA observations with EUV and X-ray data taken by SOHO/CDS, SUMER and EIT, TRACE and YOHKOH. The combined data sets will be used to delineate thermal and non-thermal plasmas at different heights and temperatures and to study the changing magnetic configurations. These results will then be compared with the predictions of 3D MHD codes that we have developed to model micro-scale heating events. Aims of the proposal ==================== Our observations will be a Joint Observing Program (JOP) involving VLA, YOHKOH, TRACE and the SOHO/SUMER, CDS, EIT and MDI. The objective will be to study the transition region and low-corona counterparts of transient brightenings. TRACE has the unique ability to acquire images in chromospheric, transition region and coronal wave-bands over relatively large fields of view with a high cadence and an angular resolution of about 1 arcsec. Simultaneous imaging in C IV (1550 A), Fe IX (171 A), Fe XII (195 A) and Fe XV (284 A) wave-bands will complement snapshot maps obtained with the VLA by providing additional information on source variability at different temperatures with similar spatial and temporal resolution to the VLA, while also allowing us to study connections between fine-scale features at different heights and locations. CDS observations will also be used to image both hot (T > 2 x 10E6 K) and cool (T < 3 x 10E5 K) loops, while SUMER and CDS can provide spectral line information for line shifts and density diagnostics. YOHKOH and EIT data will be used to provide both imaging and temperature information. The VLA, TRACE, YOHKOH and SOHO/EIT and CDS images will also be compared with simultaneous high time resolution magnetograms taken by the SOHO/MDI. Using these data, we will investigate how changes in the brightness of microwave and EUV/X-ray sources are correlated with changes in the underlying photospheric magnetic fields and how the magnetic evolution may determine the rate of micro-flaring/explosive events itself. Specific questions that we hope to address include: 1, What is the time-scale of temperature variations in coronal loops? Is the variability due to intermittent heating, continued low-level flare activity, changing magnetic topology or any combination of these? 2, Are micro-flares associated with non-thermal emission processes? This is an important but yet unsolved question. 3, Are transient brightenings associated with specific photospheric or coronal magnetic configurations? The VLA maps will be used to localize time-variable sources within specific coronal loop structures whose magnetic geometry is described by the TRACE, YOHKOH and SOHO EUV and magnetograph data, thereby providing a better understanding of the magnetic environment in which transient brightenings occur. Proposed Observations Here we propose a JOP involving simultaneous VLA, YOHKOH, TRACE, SUMER, CDS, MDI & EIT. We propose to observe a quiet Sun region (initial pointing to be disk center although will be up-dated late) using a high cadence mode in each instrument. The programme runs from 16:30 - 21:30 UT on the 6, 7 & 8 May 2000. VLA === We propose to observe the Sun from 16:30-21:30 UT on 6,7 and 8 May when the VLA is in the C configuration. This configuration is well-suited for studies of small-scale solar transient events because problems with source confusion are reduced in comparison with more extended configurations. The requested time intervals will allow us to study impulsive micro-flares/explosive events and monitor longer term (hours) changes in nearby loop structures that might be influenced by persistent micro-flaring. The entire array will be used to observe a quiet Sun continuously at 3.5 and 6 cm (using sub-arrays or frequent switching). Snapshot maps (on time intervals of 1.7 seconds) of total intensity and circular polarization at these wavelengths will be used to study magnetic and/or temperature changes in the low corona that may be produced by small-scale heating events such as micro-flares. YOHKOH ====== a high cadence mode with SXT TRACE ===== cadence ~20sec image size lines 171A & 1600A SUMER ===== FULLSCAN .. use 1x120 slit, obtain a 1024 pixel spectrum centered at Si IV 1402 with 100 sec integration TEMPORAL .. use 1x120 slit, 3 windows of 50 pixels each centered at O IV 1399, O IV 1401 & Si IV 1402 with 20 sec integration .. sit-and-stare mode with rotation comp. off Use the following pointing FULLSCAN X0, Y0 total duration 100 sec TEMPORAL X0, Y0 4400 sec FULLSCAN X0, Y0 100 sec TEMPORAL X0, Y0 4400 sec FULLSCAN X0, Y0 100 sec FULLSCAN X1, Y0 100 sec TEMPORAL X1, Y0 4400 sec FULLSCAN X1, Y0 100 sec TEMPORAL X1, Y0 4400 sec FULLSCAN X1, Y0 100 sec where X0, Y0 are the initial pointing and X1 is moved 60 arcsec east after 2.5 hrs CDS === Use CHROM_N5 where we have Ca X 574, He I 584, O III 599, Mg IX 368 & O V 629 in a raster mode using the 2x240 slit .. change existing CHROM_N5 sequence to 35 sec. integration with 60 mirror step thus doing a 120x240 raster .. total duration 2535 sec Use existing CHROM_N6 sequence where we have He I 584, O III 599 & O V 629 in a sit-and-stare type mode (rotation comp. off) .. total duration 5076 sec Sequence as CHROM_N5 pointing X0, Y0 CHROM_N6 X0, Y0 CHROM_N5 X0, Y0 CHROM_N6 X1, Y0 CHROM_N5 X0, Y0 where X0, Y0 are the initial pointing & X1 is moved 60 arcsec East. MDI === Require MDI in the high resolution mode. EIT === Require EIT in its high cadence mode observing one of the Fe lines from 16:30 - 21:30 UT on 6, 7 & 8 May. POINTING ======== The pointing on all three days will be QUIET SUN. As a default it will be initially disk center (0,0) although this may change depending on solar activity. The final pointing for the 6 & 7 May will be decided on 5 May (in order to upload the weekend tasks), while that for 8 May will be confirmed on 7 May 2000. R. Erdelyi, J.G. Doyle will be present at MEDOC from the 4 - 8 May 2000. R. Willson will be at the VLA emails: R.von.Fay-Siebenburgen@sheffield.ac.uk jgd@star.arm.ac.uk rwillson@emerald.tufts.edu bdp@blackadder.lmsal.com