Objectives:
1: To observe multi-spectral Doppler velocities during the flare impulsive phase.
2: To further understand the nature of chromospheric evaporation and heating mechanisms.
Scientific Background: Chromospheric evaporation is largely accepted to be the dynamic responses of solar flare energy release and heating of the chromosphere. Current flare models predict that evaporation occurs when the chromospheric material is heated either by non-thermal electrons or by thermal conduction. The overpressure of evaporated material drives both upward (at coronal levels) and downward (at chromospheric levels) motions simultaneously. These motions can be detected through Doppler velocity measurements in chromospheric and coronal lines. Antonucci & Dennis (1983), Zarro & Lemen (1988), Canfield et al. (1987), Acton et al. (1982), and Doschek & Warren (2005) obtained blueshifts of 300-400 km/s in the Ca XIX line using SMM/BCS (Acton et al. 1980) and Yohkoh (Culhane et al. 1991), respectively. Similar measurements using data from SOHO/CDS (Harrison et al. 1995) revealed upflow velocities of 150-300 km/s in the Fe XIX line (Brosius & Phillips 2004, Teriaca et al. 2003, Milligan et al. 2006a,,b ). On the other hand, in chromospheric and transition region lines, redshifts were observed, which are the observational signature of downward motions (known as chromospheric condensation). With the launch of Hinode (Kosugi et al. 2007) in September 2006, it is now possible to investigate the process of chromospheric evaporation by observing several emission lines simultaneously with the EUV Imaging Spectrometer (EIS; Culhane et al. 2007). EIS performs high spatial, spectral, and temporal resolution to determine the Doppler velocities. Using EIS spectral data, Milligan & Dennis (2009) measured Doppler shifts in 15 emission lines covering the temperature range T= 0.05-16 MK during the flare impulsive phase. Blueshifts indicative of the evaporation were observed in six lines from Fe XIV - XXIV (2-16 MK) at the loop footpoints, and scaled with temperature as V (km/s) ≈ 5 - 17T (MK). Some O VI - Fe XIII lines (0.5-1.5 MK) were found to be redshifted by V (km/s) ≈ 60 - 17T (MK). Though there have been many researches in this respect, many problems still remain to be solved, such as how to derive velocity-dependent emission measure from the asymmetric profiles and why redshifts appear in lines of higher temperatures than previously expected or reported (Milligam & Dennis 2009). We therefore submit a request for EIS multi-spectral observations in combination with ground-telescope observation to study these unsolved problems. The Solar Tower of Nanjing University can observe chromospheric lines. Moreover, we are trying to theoretically calculate the line profiles from flares using the radiative-hydrodynamic simulation. The calculated results can be compared with the observed ones of high cadence and multi-lines, which provide a diagnostic of solar flare energetics and dynamics. |
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