High resolution EUV structure of the chromosphere-corona transition region above a sunspot

Abstract

Ion emission line intensities between 1170 and 1700 ,~ allow one to determine the differential emission measure (DEM) and electron pressure of the plasma in the solar transition region (TR). These line intensities together with their Doppler shifts and line widths are measured simultaneously for the first time above a sunsport from data obtained with the NRL High Resolution Telescope and Spectrograph with 0.06 spectral and 1" spatial resolution.The Doppler shifts show both subsonic and supersonic flow in the same line of sight over the umbra. The temperature structure for 40 resolution elements in the sunspot umbra and penumbra is derived from the DEM and the observed electron pressures.Extrapolation of the emission measure curves supports the previous EUV and X-ray observations that coronal plasma above sunspots with T e > 106 K is reduced while emission from TR plasma between 2 x l0 s and 106 K is greatly enhanced relative to quiet or active regions. This enhancement shifts the minimum of the DEM to lower temperatures and increases the slope at 2 x 105 K by a factor of two.New pressure diagnostics using the emission line intensity ratios of C Iv to N Iv are presented, and applied to the data.The energy balance in the TR for the sunspot umbra is dominated by radiative losses from the large amount of TR plasma.An estimate of the energy budget shows that an energy input is required to balance the radiative energy losses above the umbra. The observed divergence of the enthalpy flux for the umbral downflows can balance these radiative losses for T~ between 30000 and 200000 K.A typical umbral model of T~ versus reduced mass column density is compared with one for chromospheric temperatures determined from the Ca H and K lines.

Microscopic Nonthermal Plasma Motions of Coronal Loops in a Solar Active Region

“…Nonthermal velocities in the corona with temperature above 3 MK estimated by Saba & Strong (1991a) seem to be located on the extrapolated line of the relation between the nonthermal velocity and temperature in the transition region and are di †erent from (m P T e 1@4) those in the corona with temperature of 1È2 MK. As for the relation between the nonthermal velocity and temperature in the transition region, there is a report of by Nicolas et al (1982). This relation is consistent with ScudderÏs model of m P T e 1@2 formation of the stellar corona (Scudder 1992a(Scudder , 1992b.…”

The transition region above sunspots

“…18 of Tian et al (2010). This scenario is consistent with the temperature structure of sunspots proposed by Nicolas et al (1982), in which the temperature in the TR above sunspots is lower than the surrounding temperature at the same height.…”

Observations of the Sun at Vacuum-Ultraviolet Wavelengths from Space. Part II: Results and Interpretations

“…The square box of 19 × 19 contains bright moss structures with a brightness distribution given in the histogram inset. Active-region loops can be seen in the neighbourhood (Martens et al 2000) supersonic flows along the same LOS (Nicolas et al 1982). A Doppler-shifted Mg IX line was observed by SERTS over a large sunspot indicating an outflow speed of ≈14 km s −1 relative to the surrounding plasma in the low corona (Neupert et al 1992).…”

“…Active regions are very dynamic at transition-region temperatures, but show less Doppler shift at coronal temperatures (Nicolas et al 1982;Feldman et al 1982;Kjeldseth-Moe and Brekke 1999;Spadaro et al 2000;Brekke et al 2000;. In addition, rotational motions around the loop axes were found with speeds up to 50 km s −1 .…”