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 <A9/0.7 with/without background), which occurred on July 4, 2006 in AR 10898. Methods. A multi-wavelength analysis using temporally and spatially highly resolved imaging data from the Dutch open telescope (Hα, 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 ∼ 3.9 × 10 4 K), transition region (e.g. O v, T ∼ 2.6 × 10 5 K), and coronal temperatures (Si xii, T ∼ 2 × 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 ≈ 1 MK), which show several brightenings, narrow loops of enhanced emission, and an EUV jet. EIT 19.5 nm data (T ≈ 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 Hα brightenings, as well as a similar feature in Ca ii. For all three events, a RHESSI X-ray source (3-8 keV, T 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 40 km s −1 ), and upflows for the second event (v 40 km s −1 ). During the third microflare, we find upflows of 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 Hα brightenings showing opposite flow directions (v ≈ 5 km s −1 ) are also observed in DOT Hα 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.

A Hot Cusp-shaped Confined Solar Flare

“…On the other hand, although the jet-like feature is composed mostly of hot flaring plasma, the low temperature component at transition region temperatures around 0.3-0.5 MK indicates that it could consist of plasma from both the reconnection outflow and chromospheric evaporation from the flare footpoints along the newly reconnected large scale fields (due to the impact of high energy particles once the connectivity between the lower and higher corona was established). The complex dynamics of chromospheric evaporation flows during flares, microflares and jets, in particular in transition region lines, has, e.g., been reported in Veronig et al (2010) and Berkebile-Stoiser et al (2009).…”

Chromospheric Inversions of a Micro-flaring Region

“…Due to their appearance in coronal lines, this has indicated upper temperatures for these events of the order of 1 × 10 7 K (Chifor et al 2008). Bi-directional flows at micro-flaring locations are also commonly reported, with typical values ranging from ±40-80 km s −1 (Berkebile-Stoiser et al 2009;Chen & Ding 2010;Archontis & Hansteen 2014;Leiko & Kondrashova 2015;Hong et al 2016). Numerical simulations of chromospheric micro-flares due to magnetic reconnection appear to also show bi-directional flows either side of the X-point, with the upflow appearing with a higher velocity than the downflow (Jiang et al 2010).…”

Chromospheric evaporation flows and density changes deduced from Hinode/EIS during an M1.6 flare

“…Since the slit covers only a very limited and fixed field of view, it is difficult to obtain data across entire flare kernels, however. Thus, flare studies based on sit-and-stare observations are still rare (e.g., Brosius 2001Brosius , 2013aBerkebile-Stoiser et al 2009;Veronig et al 2010;Brosius & Daw 2015).…”