Multi-wavelength fine structure and mass flows in solar microflares

Abstract

Aims. We study the multi-wavelength characteristics at high spatial resolution, as well as chromospheric evaporation signatures of solar microflares. To this end, we analyze the fine structure and mass flow dynamics in the chromosphere, transition region and corona of three homologous microflares (GOES class Methods. A multi-wavelength analysis using temporally and spatially highly resolved imaging data from the Dutch open telescope (H α , Ca ii H), the transition region and coronal explorer (17.1 nm), the extreme-ultraviolet imaging telescope (19.5 nm), and the Reuven Ramaty high energy solar spectroscopic imager ( ≳ 3 keV) was carried out. EUV line spectra provided by the coronal diagnostic spectrometer are searched for Doppler shifts in order to study associated plasma flows at chromospheric (He i, $T\sim3.9\times 10^4$ K), transition region (e.g. O v, $T\sim 2.6\times 10^5$ K), and coronal temperatures (Si xii, $T\sim 2\times 10^6$ K). RHESSI X-ray spectra provide information about non-thermal electrons. Results. The multi-wavelength appearance of the microflares is in basic agreement with the characteristics of large flares. For the first event, a complex flare sequence is observed in TRACE 17.1 nm images ($T\approx 1$ MK), which show several brightenings, narrow loops of enhanced emission, and an EUV jet. EIT 19.5 nm data ($T\approx 1.5$ MK) exhibit similar features for the third event. DOT measurements show finely structured chromospheric flare brightenings for all three events, loop-shaped fibrils of increased emission between H α brightenings, as well as a similar feature in Ca ii. For all three events, a RHESSI X-ray source (3–8 keV, $T \gtrsim 10$ MK) is located in between two chromospheric brightenings situated in magnetic flux of opposite polarity. We find the flow dynamics associated with the events to be very complex. In the chromosphere and transition region, CDS observed downflows for the first ($v \lesssim 40$ km s -1 ), and upflows for the second event ($v \lesssim 40$ km s -1 ). During the third microflare, we find upflows of $\lesssim $20 km s -1 and also weak downflows of ≲ 20 km s -1 in two separate brightenings. For all three microflares, multi-component fitting is needed for several profiles of He i, O v, and Ne vi lines observed at the flare peaks, which indicate spatially unresolved, oppositely directed flows of ≲ 180 km s -1 . We interpret these flows as twisting motions of the flare loops. Loop-shaped fibrils in between H α brightenings showing opposite flow directions ($v\approx5$ km s -1 ) are also observed in DOT H α Dopplergrams. RHESSI X-ray spectra show evidence of non-thermal bremsstrahlung for two of the three microflares. The electron beam flux density deposited in the chromosphere for these events is estimated to straddle the threshold heating flux between gentle and explosive evaporation.