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RHESSI Microflares: II. Implications for Loop Structure and Evolution  

Sigrid Stoiser   Submitted: 2008-07-14 02:55

We present simple analytic models which predict the peak X-ray emission measure and temperature attained in flares in which the chromospheric evaporation process takes place either in a single `monolithic' loop or in a loop consisting of several filaments that are created successively as the energy release process proceeds in time. As possible mechanisms driving chromospheric evaporation we consider both classical heat conduction from the loop top and non-thermal electron beams. The model predictions are tested for a set of 18 well studied RHESSI microflares. The results suggest beam driven evaporation in filamented loops as being capable of accounting for the observed emission measures and temperatures though there are issues with the very high beam densities needed. On the other hand, estimates of the emission measures achieved by conductive evaporation which are derived by using the Rosner-Tucker-Vaiana (RTV) scaling law are much larger than the observed ones. Possible reasons for this discrepancy are discussed.

Authors: Stoiser, S., Brown, J.C., Veronig, A.M.
Projects: RHESSI

Publication Status: Solar Physics (in press)
Last Modified: 2008-09-23 20:57
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Sigrid Stoiser   Submitted: 2007-11-21 07:17

We study the general X-ray and multiwavelength characteristics of microflares of GOES class A0.7 to B7.4 (background subtracted) detected by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) on 26 September 2003 comparing them with the properties of regular flares. All the events for which X-ray imaging was feasible originated in one active region and were accumulated in areas with intermixed magnetic polarities. During the events? rise and peak phase, the RHESSI X-ray spectra show a steep nonthermal power-law component (4≲γ≲10) for energies ≳ 10 keV. Further evidence for the presence of electron beams is provided by the association with radio type III bursts in 5 out of 11 events where AIP radio spectra were available. The strongest event in our sample shows radio signatures of a type II precursor. The thermally emitting flare plasma observed by RHESSI is found to be hot, 11≲T≲15 MK, with small emission measures, 1046≲EM≲1047 cm-3, concentrated in the flare loop. In the EUV (TRACE 171 Å), the UV (TRACE 1600 Å) and Kanzelhöhe Solar Observatory Hα, impulsive brightenings at both ends of the RHESSI 3 ? 6 keV X-ray loop source are observed, situated in opposite magnetic polarity fields. During the decay phase, a postflare loop at the location of the RHESSI loop source is observed in the TRACE 171 Å  channel showing plasma that is cooled from ≳ 10 MK to ≈ 1 MK. Correlations between various thermal and nonthermal parameters derived from the RHESSI microflare spectra compared to the same correlations obtained for a set of small and large flares by Battaglia et al. (Astron. Astrophys. 439, 737, 2005) indicate that the RHESSI instrument gives us a spectrally biased view since it detects only hot (T≳10 MK) microflares, and thus the correlations between RHESSI microflare parameters have to be interpreted with caution. The thermal and nonthermal energies derived for the RHESSI microflares are Ē_th=7*1027 ergs and Ē_nth=2*1029 ergs, respectively. Possible reasons for the order-of-magnitude difference between the thermal and nonthermal microflare energies, which was also found in previous studies, are discussed. The determined event rate of 3.7 h^-1 together with the average microflare energies indicate that the total energy in the observed RHESSI microflares is far too small to account for the heating of the active region corona in which they occur.

Authors: Stoiser, S., Veronig, A.M., Aurass, H., Hanslmeier, A.

Publication Status: Solar Physics (in press)
Last Modified: 2007-11-21 11:20
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