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Register a new userFlarelike brightenings of active region loops observed with SUMER
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Coronal loops on the east limb of the Sun were observed by SUMER on SOHO for several days. Small flare-like brightenings are detected very frequently in the hot flare line Fe~{small XIX}. We find that the relatively intense events are in good coincidence with the transient brightenings seen by Yohkoh/SXT. A statistical analysis shows that these brightenings have durations of 5-84 min and extensions along the slit of 2-67 Mm. The integrated energy observed in Fe~{small XIX} for each event is in the range of $3times10^{18}-5times10^{23}$ ergs, and the estimated thermal energy ranges from $10^{26}-10^{29}$ ergs. Application of the statistical method proposed by Parnell & Jupp (2000) yields a value of 1.5 to 1.8 for the index of a power law relation between the frequency of the events and the radiated energy in Fe~{small XIX}, and a value of 1.7 to 1.8 for the index of the frequency distribution of the thermal energy in the energy range $>10^{27}$ ergs. We examine the possibility that these small brightenings give a big contribution to heating of the active region corona.
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Using a full spectral scan of an active region from the Extreme-Ultraviolet Imaging Spectrometer (EIS) we have obtained Emission Measure EM$(T)$ distributions in two different moss regions within the same active region. We have compared these with theoretical transition region EMs derived for three limiting cases, namely textit{static equilibrium}, textit{strong condensation} and textit{strong evaporation} from cite{ebtel}. The EM distributions in both the moss regions are strikingly similar and show a monotonically increasing trend from $log T[mathrm{K}]=5.15 -6.3$. Using photospheric abundances we obtain a consistent EM distribution for all ions. Comparing the observed and theoretical EM distributions, we find that the observed EM distribution is best explained by the textit{strong condensation} case (EM$_{con}$), suggesting that a downward enthalpy flux plays an important and possibly dominant role in powering the transition region moss emission. The downflows could be due to unresolved coronal plasma that is cooling and draining after having been impulsively heated. This supports the idea that the hot loops (with temperatures of 3{-}5 MK) seen in the core of active regions are heated by nanoflares.
A class X1.5 flare started on the solar limb at 00:43 UT on 21 April 2002, which was associated with a CME observed at 01:27 UT by LASCO C2. The coordinated analyses of this flare include TRACE 195 {AA} images and SUMER spectra in lines of Fe XXI, Fe XII, and C II. We find that: 1) The flare began with a jet seen by TRACE, which was detected by SUMER in the C II line as a strong brightening with blue shifts up to 170 km s$^{-1}$. At that time only weak emission was detected in Fe XII and Fe XXI. 2) Subsequently, a weak looplike brightening started south of the jet, moving outwards with an average speed of about 150 km s$^{-1}$. The SUMER spectra responded this moving loop as separatingly brightenings, visible only in the Fe XXI line. The southwards moving component contains red- and blue-shifted emission features and has an apparent speed of $sim$120 km s$^{-1}$. The absence of signatures in Fe XII and C II lines indicates that the moving weak loop seen by TRACE corresponds to the emission from very hot plasma, in a blend line in the 195 {AA} bandpass due to Fe XXIV formed at T > 10 MK. 3) The trigger mechanism of the flare and associated CME can be interpreted in the same way as that proposed by Wang et al. (2002) for an event with similar initial features.
We study the formation of transient loops in the core of the AR 11890. For this purpose, we have used the observations recorded by the Atmospheric Imaging Assembly (AIA) and the Interface Region Imaging Spectrograph (IRIS). For photospheric field configuration, we have used the line-of-sight (LOS) magnetograms obtained from the Helioseismic and Magnetic Imager (HMI). The transient is simultaneously observed in all the UV and EUV channels of AIA and the three slit-jaw images from IRIS. The co-existence of the transient in all AIA and IRIS SJI channels suggests the transients multi-thermal nature. The transient consists of short loops located at the base of the transient as well as longe loops. A differential emission measure (DEM) analysis shows that the transient has a clumpy structure. The highest emission observed at the base is within the temperature bin of $log, T = 6.65 - 6.95$. We observe the longer loops at a similar temperature, albeit very feeble. Using LOS magnetograms, we conclude that the magnetic reconnection may have caused the transient. Our observations further suggest that the physics of the formation of such transients may be similar to those of typical coronal jets, albeit in different topological configurations. Such multi-wavelength observations shed light on the formation of hot plasma in the solar corona and provide further essential constraints on modeling the thermodynamics of such transients.
The characteristic electron densities, temperatures, and thermal distributions of 1MK active region loops are now fairly well established, but their coronal magnetic field strengths remain undetermined. Here we present measurements from a sample of coronal loops observed by the Extreme-ultraviolet Imaging Spectrometer (EIS) on Hinode. We use a recently developed diagnostic technique that involves atomic radiation modeling of the contribution of a magnetically induced transition (MIT) to the Fe X 257.262A spectral line intensity. We find coronal magnetic field strengths in the range of 60--150G. We discuss some aspects of these new results in the context of previous measurements using different spectropolarimetric techniques, and their influence on the derived Alfv{e}n speeds and plasma $beta$ in coronal loops.
The physical conditions leading the sunspot penumbra decay are poorly understood so far. We investigate the photospheric magnetic and velocity properties of a sunspot penumbra during the decay phase to advance the current knowledge of the conditions leading to this process. A penumbral decay was observed with the CRISP instrument at the Swedish 1m Solar Telescope on 2016 September 4 and 5 in active region NOAA 12585. During these days, full-Stokes spectropolarimetric scans along the Fe I 630 nm line pair were acquired over more than one hour. We inverted these observations with the VFISV code in order to obtain the evolution of the magnetic and velocity properties. We complement the study with data from instruments onboard the Solar Dynamics Observatory and Hinode space missions. The studied penumbra disappears progressively in both time and space. The magnetic flux evolution seems to be linked to the presence of Moving Magnetic Features (MMFs). Decreasing Stokes V signals are observed. Evershed flows and horizontal fields were detected even after the disappearance of the penumbral sector. The analyzed penumbral decay seems to result from the interaction between opposite polarity fields in type III MMFs and penumbra, while the presence of overlying canopies rules the evolution in the different penumbral sectors.
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