Proposal for a joint observing program including SUMER, TRACE, MDI, CDS, and EIT SEARCH FOR A RELATIONSHIP BETWEEN CORONAL MICROFLARES AND TRANSITION REGION EVENTS Bernhard Kliem^1, Markus J. Aschwanden^2, Ted D. Tarbell^2, Richard A. Shine^2, Andrzej Fludra^3, Dan Moses^4, Klaus Wilhelm^5 1: Astrophysical Institute Potsdam, Germany; bkliem@aip.de 2: Lockheed-Martin ATC, Palo Alto, CA; aschwanden@lmsal.com; tarbell@lmsal.com; shine@lmsal.com 3: NASA/GSFC, Greenbelt, MD; fludra@cds8.nascom.nasa.gov 4: NRL, Washington, DC; moses@perseus.nrl.navy.mil 5: MPAE, Katlenburg-Lindau, Germany; wilhelm@Linmpi.mpg.de Version: September 20, 1999 We propose a joint observing run of SUMER, TRACE, MDI, CDS, and EIT to investigate whether and how the occurrence of microflares in the corona is related to the occurrence of localized energy release events in the transition region. Scientific Justification: ========================= Coronal microflares in the ``quiet sun'' have recently been detected with unprecedented sensitivity in the EUV emission lines of Fe IX and Fe XII, using the EIT and TRACE instruments (Benz & Krucker 1998, 1999; Aschwanden et al. 1999a,b; Berghmans et al. 1998, 1999). The number of microflares seen in Fe IX is of order 100 events per second on the whole sun. There is an active debate about the importance of these events for the heating of the corona. The frequency distribution of microflare peak fluxes or energy content appears to be a (possibly broken) power law: a power-law index near 2.5 pointed to an energetically dominant role of the smallest events and supported the hypothesis of a nanoflare-heated corona (Krucker & Benz 1998), while a recent study, which employed a more restrictive flare criterion, obtained a power-law index near 1.8 (Aschwanden et al. 1999a,b), which does not support the nanoflare-heating hypothesis. Localized energy release events in the transition region have been observed in the EUV in a variety of forms. Best known are perhaps the explosive events, seen as jets in the wings of spectral lines, which occur at a rate of about 600 events per second on the whole sun (Dere et al. 1989). Turbulent line broadenings, line shifts, and EUV blinkers are other forms. Explosive events can best be seen in EUV lines originating near 0.1 MK, but have also been traced to higher temperatures (Wilhelm et al. 1998; Mendoza-Torres et al. 1999; Brekke et al. 1999) [and to lower temperatures as well (Chae et al. 1999)]. A statistical investigation to establish a frequency distribution of transition region events as function of their peak velocity, broadening, or energy content is still missing (although the observations required are in all likelihood available in the SOHO archives). The interrelation between small-scale events in the corona and those in the transition region has not yet been clarified. Moses et al. (1994) did not find an association between explosive events and coronal X-ray bright points. Porter et al. (1995) observed a correlation between brightenings in the 3.5-5.5 keV band (small flares or microflares) and brightenings in the C IV line. Benz & Krucker (1999) also observed correlated brightenings in transition region lines (O V) and coronal lines (Fe IX/X, Fe XII). The former two of these studies may have been severely limited by the available sensitivity and by short time coverage. These studies seem to indicate that the association of coronal microflares with transition region brightenings is stronger than the association with jet-like activities. On the other hand, explosive event studies have shown that the jets are often accompanied by a brightening in the line center (Innes et al. 1997). Thus, the relation between transition region brightenings, jets, and coronal microflares requires further study. >From a physical point of view, the question of cause and effect also appears to be unclear. Explosive events are usually regarded as reconnection events originating in the transition region. Their blue-shifted outflow may last for several minutes but does not appear to move on the sun, hence the material probably ``disappears'' from EUV lines at about 0.1 MK by heating up to coronal temperatures. Conversely, the time delays between brightenings at different temperatures have led Benz & Krucker (1999) to conclude that electron bombardments into the transition region, resulting from a flare-like coronal energy release, presumably coronal reconnection, causes the transition region brightenings. The investigation of coronal microflares and correlated transition region brightenings, which may indicate heating in the upper chromosphere, could help resolve the discrepancy between observed and calculated conductive cooling times in these flares (Aschwanden et al. 1999b). We aim at a more sensitive and systematic investigation of the possible correlation between coronal microflares and transition region events than previously possible by performing a coordinated observation involving TRACE, SUMER, MDI, CDS and EIT. Participating Instruments: ========================== (1) TRACE is the most sensitive instrument for coronal microflare detection. The methodology of microflare detection using TRACE images has recently been improved considerably (Aschwanden et al. 1999a,b). An observing mode similar to the one used in that investigation appears to be well suited: full-resolution images interleaved at 171 and 195 Angstrom using the highest cadence possible (~ 2 min). Every ~ 4 hours one image in C IV 1550, which shows the network most clearly, shall be taken to support alignment with the SUMER, CDS, and EIT images in the subsequent analysis. Contact: M.J. Aschwanden, T.D. Tarbell, R.A. Shine (2) SUMER is ideally suited to detect small-scale transition region events of virtually all types due to its high spatial and spectral resolution. A coordinated observation using both instruments has the potential to make significant progress into the direction discussed above. A line providing a high detection probability of transition region events, such as Si IV or O V, shall be selected. SUMER shall obtain one raster image of the area covered by the other instruments (or part of it) every ~ 4 hours and shall perform the usual ``sit and stare'' observation in between, using the long slit (1"x300") and the highest cadence possible (~ 15 s). One line with a 50 px spectral window shall be used. An image will be formed by solar rotation. Contact: B. Kliem (3) In addition, simultaneous MDI observations in high resolution mode are highly useful for investigating the physical origins of the observed activity. Not only will the photospheric magnetic field be provided with high resolution, but also the horizontal photospheric velocity field can be derived using correlation tracking. This velocity field is often regarded as the driver of magnetic reconnection events in higher layers. The MDI campain envisioned is cam_hr_[t_]ve_fe_me. Contact: T.D. Tarbell, R.A. Shine (4) CDS will study the interrelation of the activity in the whole temperature range using multi-line observations with somewhat smaller spatial and spectral resolutions than SUMER. CDS will mainly be able to detect brightenings in a number of lines. One block of CDS observation would consist of an overview raster like EJECT_V3 (240"x240" in 17 min), followed by a raster (e.g., BLINK_ST) with a high cadence (2.5 min) in a sub-area (40"x124") centered on the SUMER pointing; several such blocks will build the whole observing sequence. Contact: A. Fludra (5) EIT will provide context images in addition to the TRACE images. Both the coronal and the transition region structure shall be imaged, using the Fe IX/X and the He II lines. Contact: D. Moses Pointing: ========= The pointing should be chosen within the MDI high-resolution FOV. Main target is the quiet sun, where most of the studies discussed above have been performed. Including an area in proximity to an active region may be optimal. Observing Schedule: =================== The duration of the coordinated observation should be of order 8 (...10) hours to obtain a large field of view for SUMER by solar rotation and to support the MDI correlation tracking technique. References: =========== M.J. Aschwanden, R. Nightingale, T. Tarbell, S. Krucker, A. Benz, subm. to ApJ (1999a) M.J. Aschwanden, T. Tarbell, R. Nightingale, C.J. Schrijver, A. Title, C.C. Kankelborg, P. Martens, H.P. Warren, S. Krucker, subm. to ApJ (1999b) A.O. Benz, S. Krucker, Solar Phys. 182, 349, 1998 A.O. Benz, S. Krucker, A&A 341, 286, 1999 D. Berghmans, F. Clette, D. Moses, A&A 336, 1039, 1998 D. Berghmans, F. Clette, Solar Phys. 186, 207, 1999 P. Brekke, N. Brynildsen, O. Kjeldseth-Moe, P. Maltby, K. Wilhelm, Adv. Space Res., in press, 1999 J. Chae, J. Qiu, H. Wang, P.R. Goode, ApJ Lett. 513, L75, 1999 K.P. Dere, J.-D.F. Bartoe, G. Brueckner, Solar Phys. 123, 41, 1989 D.E. Innes, P. Brekke, D. Germerott, K. Wilhelm, Solar Phys. 175, 341, 1997 S. Krucker, A.O. Benz, ApJ Lett. 501, L213, 1998 D. Moses, J.W. Cook, J.-D.F. Bartoe, G. Brueckner, K.P. Dere et al. ApJ 430, 913, 1994 J.G. Porter, J.M. Fontenla, G.M. Simnett, ApJ 438, 472, 1995 K. Wilhelm, D.E. Innes, W. Curdt, B. Kliem, P. Brekke, ESA SP-421, 103, 1998 J.E. Mendoza-Torres, K. Wilhelm, D.E. Innes, W. Curdt, B. Kliem, P. Brekke, A&A in preparation (1999)