Spectral diagnostic of a microflare. Evidences of resonant scattering in C IV 1548 Å, 1550 Å lines

Abstract

Aims. We study a microflare, classified as a GOES-A1 after background subtraction, which was observed in active region NOAA 8541 on May 15, 1999. Methods. We used TRACE filtergrams to study the morphology and time evolution. SUMER spectral lines were used to diagnose the chromospheric plasma (Si ii 1533 Å), transition region plasma (C iv 1548, 1550 Å), and coronal plasma (Ne viii 770 Å).Results. In the 171 Å and 195 Å filtergrams, we measure apparent mass motions along two small loops that compose the microflare from the eastern toward the western footpoints. In SUMER, the microflare is detected as a small (47 Mm 2 ), bright area at the western footpoints of the TRACE loops. The spectral profiles recorded over the bright area are complex. The Si ii 1533 Å line is self-reversed owing to opacity, and the coronal Ne viii line profile is composed of two Gaussian components, one of them systematically redshifted. The C iv 1548 Å and 1550 Å profiles are badly distorted because of the temporary depression of the detector local gain caused by the very high count rates reached in the flaring region and we can only confirm the presence of strong blueshifts of -200 km s −1 .Few, unaffected C iv profiles show two spectral components. In the northern part of the bright area, all SUMER spectral lines have at least one blueshifted spectral component. In the southern region of the bright area the spectral lines are redshifted. Adjacent to the microflare we measure, for the first time on the solar disk, an intensity ratio of the 1548 Å line to 1550 Å line with values of three to four, indicating that resonance scattering prevails in the lines formation. Moreover, the scattering region is found to be cospatial to a solar pore. Conclusions. The blueshifts in the footpoints of the microflare and the apparent mass motions observed with TRACE can be explained by a gentle chromospheric evaporation triggered by the microflare. The redshifted spectral components can be explained as cooling material that is falling back on the solar surface. The presence of resonant scattering can be explained by the low electron density expected in the transition region of a solar pore, combined with the high photon flux coming from the nearby microflare. We estimate that the lower limit of the electron density in the pore lies in the range 10 8 cm −3 to 10 9 cm −3 .

Evidence of scattering effects influenced by plasma flows in C VI 1548 Å, 1550 Å spectral lines emitted from the Sun

“…The characteristics of these data sets are detailed in Table 1. The data from 15 May 1999 are recorded on active region NOAA 8541 (Winebarger et al 2002;Gontikakis et al 2013). On 15 May, SUMER performed two rasters that recorded five spectral lines:…”

“…The dispersion axis of the spectra recorded during the raster on NOAA 8541, has been calibrated using the single exposure data sets in the lines 1 to 3 of Table 1 (see Gontikakis et al 2013). …”

“…We took care to avoid profiles with count rates higher than, or of the order of, 50 counts/s/pixel because such profiles have complex spectral features caused by the saturation of the detector (see Gontikakis et al 2013). A good way to track such unusual events in the two rasters is to compute a map of the spectral line width using a single Gaussian fit.…”

“…In active regions and also in impulsive events, such as flares, bright points, or jets, the resonance scattering of the radiation coming from these bright regions has a chance to compete with thermal emission (Judge & Pietarila 2004). Gontikakis et al (2013) showed that in active region NOAA 8541, the C iv 1548 Å and 1550 Å spectral lines' emission was caused by resonant scattering, in the vicinity of a microflare.…”

Chromospheric Inversions of a Micro-flaring Region

“…Most micro-flares recorded in the literature appear to be rooted in the chromosphere while showing strong responses in higher energy coronal lines (Chifor et al 2008;Brosius & Holman 2010;Chen & Ding 2010;Hannah et al 2011;Gontikakis et al 2013), which are most similar in appearance to flares, only on a smaller scale. 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).…”