Observations of the Thermal and Dynamic Evolution of a Solar Microflare

Abstract

We observed a solar microflare over a wide temperature range with three instruments aboard the SOHO spacecraft (Coronal Diagnostic Spectrometer (CDS), Extreme-ultraviolet Imaging Telescope (EIT), and Michelson Doppler Imager (MDI)), TRACE (1600 Å), GOES, and RHESSI. The microflare's properties and behavior are those of a miniature flare undergoing gentle chromospheric evaporation, likely driven by nonthermal electrons. Extreme-ultraviolet spectra were obtained at a rapid cadence (9.8 s) with CDS in stare mode that included emission lines originating from the chromosphere (temperature of formation T_m ≈ 1 × 10⁴ K) and transition region (TR), to coronal and flare (T_m ≈ 8 × 10⁶ K) temperatures. Light curves derived from the CDS spectra and TRACE images (obtained with a variable cadence ≈34 s) reveal two precursor brightenings before the microflare. After the precursors, chromospheric and TR emission are the first to increase, consistent with energy deposition by nonthermal electrons. The initial slow rise is followed by a brief (20 s) impulsive EUV burst in the chromospheric and TR lines, during which the coronal and hot flare emission gradually begin to increase. Relative Doppler velocities measured with CDS are directed upward with maximum values ≈20 km s^-1 during the second precursor and shortly before the impulsive peak, indicating gentle chromospheric evaporation. Electron densities derived from an O IV line intensity ratio (T_m ≈ 1.6 × 10⁵ K) increased from 2.6 × 10¹⁰ cm^-3 during quiescent times to 5.2 × 10¹¹ cm^-3 at the impulsive peak. The X-ray emission observed by RHESSI peaked after the impulsive peak at chromospheric and TR temperatures and revealed no evidence of emission from nonthermal electrons. Spectral fits to the RHESSI data indicate a maximum temperature of ≈13 MK, consistent with a slightly lower temperature deduced from the GOES data. Magnetograms from MDI show that the microflare occurred in and around a growing island of negative magnetic polarity embedded in a large area of positive magnetic field. The microflare was compact, covering an area of 4 × 10⁷ km² in the EIT image at 195 Å, and appearing as a point source located 7'' west of the EIT source in the RHESSI image. TRACE images suggest that the microflare filled small loops.