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Solar System Research, Vol. 39, No. 6, 2005, pp. 508­512. Translated from Astronomicheskii Vestnik, Vol. 39, No. 6, 2005, pp. 571­576. Original Russian Text Copyright © 2005 by Bogachev, Kuzin, Zhitnik, Urnov, Grechnev.

The Dynamics of High-Temperature Plasma in the Solar Corona according to SPIRIT Observations in Mg XII 8.42 е Line
S. A. Bogachev1, S. V. Kuzin1, I. A. Zhitnik1, A. M. Urnov1, and V. V. Grechnev2
1Lebedev

Institute of Physics, Russian Academy of Sciences, Leninskii pr. 53, Moscow, 117924 Russia 2Institute for Solar-Terrestrial Physics, Russian Academy of Sciences, Siberian Branch, ul. Lermontova 126, Irkutsk, 664033 Russia
Received March 31, 2005

Abstract--The spatial-distribution dynamics of the hot coronal plasma with T ~ 10 MK during a period of high solar activity is studied. We analyze images of the NOAA 9830 active region and its surroundings obtained during the second half of February 2002 with the SPIRIT spectroheliograph in the Mg XII 8.42-е line and simultaneously on the SOHO satellite with the EIT instrument and on the TRACE satellite in the 195-е channel. As shown by a multiwavelength analysis, a high-temperature plasma is concentrated in the corona near the apices of magnetic loops, it has long lifetimes (up to several days), and its dynamics is complex and bears no direct relation to flare activity. During the flares, conspicuous increases are observed in the X-ray flux and the emission measure for temperatures of ~5­15 MK. Our analyses of the time variations in emission during a flare suggest that hot plasma is heated by fluxes of accelerated electrons.

INTRODUCTION During a solar flare, the impulsive release of energy stored in the magnetic field efficiently accelerates particles and heats the plasma. Observations carried out using the SXT telescope onboard the Yohkoh satellite (Masuda et al., 1994; Tsuneta et al., 1997) have shown that a hot plasma can be produced during flares in both the chromosphere and the solar corona. Currently, the spatial and temporal parameters of the high-temperature coronal plasma are studied with the RES-K X-ray spectroheliograph, which is part of the SPIRIT complex operated onboard the CORONAS-F satellite (Oraevskii et al., 2002). The RES-K spectroheliograph provides monochromatic images of the full solar disk in the Mg XII 8.42-е line with a temperature of T = 5­ 15 MK (Zhitnik et al., 2003). This makes it possible to directly observe the hot coronal plasma, whereas the data of the Yohkoh satellite can be used to reveal the high-temperature plasma only indirectly, by comparing SXT images obtained with various filters. We study here the structure and dynamics of the hot coronal plasma during solar flares and between them based on RES-K observations in the Mg XII line. OBSERVATIONS AND EMISSION PROFILES The SPIRIT instrumentation onboard the CORONAS-F satellite is designed to obtain full-disk images of the Sun in soft-X-ray and extreme-UV spectral channels and lines (for a technical description of the SPIRIT instrumentation, see Zhitnik et al., 2003). We analyze the series of successive images of the NOAA 9830 active region that was obtained using the RES-K spectroheliograph in the Mg XII 8.42-е chan-

nel from 21:40 UT on February 21, 2002, to 08:00 UT on February 22, 2002, during a period approximately coincident with a flare of class M4.4, according to the GOES (Geostationary Operational Environmental Satellite) scale. The series contains 351 images with an exposure time of 37 s, being a subset of the data obtained during a long session of continuous SPIRIT observations in the second half of February 2002. Fragments measuring 50 в 50 pixels and covering the active region were cut out of the full-disk solar images; they were then aligned with one another using a cross-correlation technique to compensate for the solar rotation over the period of observation and to remove the frame jitter. Figure 1 shows a time profile of the Mg XII emission obtained by integrating the emission of all fragments, together with the simultaneous emission curves obtained in the 25­50-keV range onboard the RHESSI (Ramaty High Energy Solar Spectroscopic Imager) satellite and in the 1- to 8-е range onboard the GOES satellite. The profile of hard X-ray emission should correlate with the injection rate of accelerated electrons. By comparing this profile with the Mg XII emission curve, we can conclude that the hot coronal emission sources observable in the Mg XII line could be heated by accelerated electrons during the flare. This suggestion is based on the fact that the Mg XII emission starts growing nearly simultaneously with the beginning of the flare-induced acceleration of electrons (RHESSI missed the onset of the flare). After the cessation of the electron injection, the Mg XII emission weakens to its original level within almost four hours, which can be interpreted as a long conductive cooling of the electronheated plasma. The time lag of the Mg XII emission rel-

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THE DYNAMICS OF HIGH-TEMPERATURE PLASMA Intensity (a) 10
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Fig. 1. Synchronous time profiles of emission: (a) integrated solar emission in the 1­8 е range according to GOES data; (b) MgXII 8.42 е emission of the high-temperature plasma in the core; and (c) hard X-ray emission of the flare in the 25­50-keV range according to RHESSI data.

ative to the hard-X-ray outburst could be a manifestation of the Neupert law (Neupert, 1968). STRUCTURE AND DYNAMICS OF HIGH-TEMPERTURE PLASMA The structure of the active region in the photosphere and corona is shown in Fig. 2 by images in various emission ranges. All these images are reduced to the same scale and, in addition, compensation for their displacement due to solar rotation is applied. Upon analyzing the SOHO/EIT and TRACE images, we selected four groups of magnetic loops, which are denoted in the figure as A1­A4. In the bottom panels, these loop systems are superimposed onto the pattern of high-temperature emission sources that we detected in
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the Mg XII SPIRIT images. A comparison between the panels in Fig. 2 shows that the location of hot-plasma regions observed in the Mg XII line corresponds to systems of coronal loops, and the plasma-emission intensity is maximum near the apices of the loops. To determine the lifetimes of the observed sources, we additionally examined the SPIRIT images obtained over several preceding days. On this basis, we can state that all four sources existed in the corona for at least three days. This substantially exceeds the characteristic time of conducting cooling of the plasma at a temperature of 10 MK, which ranges from one to several hours (depending on the electron density). We can thus conjecture that the energy release in the "quiet" solar corona heats high-temperature sources even without flares. We believe that, most likely, this energy release is due to magnetic reconnection that occurs high in the


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Fig. 2. Active region NOAA 9830: (a) SOHO/EIT image in the 195-е line; (b) SOHO/MDI magnetogram and the schematized neutral line of the photospheric magnetic field; (c) location of high-temperature-plasma sources (R1­R4) before the flare (SPIRIT image in the MgXII 8.42 е line); and (d) distribution of high-temperature plasma after the flare. The loop systems observed on the TRACE satellite are marked with dotted curves and A1­A4 symbols.

corona, at separators of the large-scale magnetic field of the active region. TEMPORAL DYNAMICS AND OSCILLATIONS OF EMISSION SOURCES During the considered series of SPIRIT observations, a class-M4.4 flare occurred. Its onset in the Mg XII line was observed at about 23:45 UT in core R2, whose position is shown in Fig. 2. At the time of the flare, cores R2­R4 merged into one in the SPIRIT

images, which prevented separate study of the dynamics of each of them. Core R1 was at a large distance from the flare center and could be identified in the images as a separate emission source even at the flare maximum. Figure 3 shows time profiles in the R2­R4 flare area (dashed) and in the distant core R1 (solid). It is remarkable that the time profiles exhibit a correlation, which can be noted at many intervals. In particular, some time after the onset of the flare (dashed curve), an emission outburst was also observed in the distant core R1, although it was weaker. Based on the
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Fig. 3. Comparison of the emission profiles of high-temperature plasma in the core of the flare (dotted curve) and in the distant source R1 (solid curve).

time delay of the second outburst, ~5 min, and the distance between the cores, ~2 в 105 km, the speed of the disturbance propagation from the center of the flare can be estimated to be 650 km/s. This speed can be even higher if the flare-induced disturbance propagates in a curved rather than straight path (e.g., along magnetic field lines). However, we do not see reasons for such a supposition, since magnetoacoustic disturbances can propagate in any direction with respect to the magnetic field, in contrast to the plasma, which, to a strong-field approximation, can flow only along the magnetic field lines. Note that the propagation speed thus estimated by us for a flare-induced disturbance in the solar corona agrees with the result obtained by Aschwanden et al. (1999), who studied coronal-loop oscillations and demonstrated that the disturbance triggering these oscillations propagated radially from the flare center at a speed of 700 km/s. The traveling flare-induced disturbances interact with loop systems and can generate in them oscillations of various periods (see, e.g., Wang et al., 2002). During the declining phase of the outburst that occurred in source R1 after the flare, we also detected quasiperiodic intensity pulsations with an amplitude of 10­20% and a period of about 10 min. We do not identify these pulsations with coronal-loop oscillations but suggest that free MHD plasma oscillations could be excited inside the high-temperature sources after the flare. The condiSOLAR SYSTEM RESEARCH Vol. 39 No. 6 2005

tions necessary for such oscillations are described by Roberts et al. (1982, 1984). The MHD plasma oscillations inside the source should manifest themselves observationally as (a) emission oscillations in the source and (b) transverse oscillations of the source, which can be detected by the Doppler shift of spectral lines. CONCLUSION Using the series of 351 images obtained with the SPIRIT X-ray spectroheliograph in the Mg XII 8.42-е line, we have studied the active region NOAA 9830 in the second half of February 2002 and analyzed the structure and dynamics of the high-temperature coronal plasma observed at that time. We have found that (a) a plasma with a temperature of T ~ 10 MK can be observed in the corona near the apices of magnetic loops and (b) the lifetime of the high-temperature emission sources is several days, which far exceeds the characteristic times of plasma cooling via radiation and electron thermal conduction. We suggest that energy release (possibly related to magnetic reconnection) occurs in the solar corona, which heats the high-temperature sources and compensates losses to conductive cooling. We have analyzed the time variation of the emission in the core of the flare. The soft X-ray emission in the Mg XII line starts growing in the flare simultaneously with the onset of electron acceleration and then


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BOGACHEV et al. Edwin, P.M. and Roberts, B., Wave Propagation in a Magnetically Structured Atmosphere. III. The Slab in a Magnetic Environment, Solar Phys., 1982, vol. 76, p. 239. Masuda, S., Kosugi, T., Hara, H., et al., A Loop-Top Hard X-ray Source in a Compact Solar Flare as Evidence for Magnetic Reconnection, Nature, 1994, vol. 371, p. 495. Neupert, W.M., Comparison of Solar X-ray Line Emission with Microwave Emission during Flares, Astrophys. J. 1968, vol. 153, p. L59. Oraevsky, V.N. and Sobelman, I.I., Comprehensive Studies of Solar Activity on the CORONAS-F Satellite, Pis'ma Astron. Zh., 2002, vol. 28, no. 6, p. 457 [Astron. Lett. (Engl. Transl.), 2002, vol. 28, no. 6, p. 401]. Robert, B., Edwin, P.M., and Benz, A.O., On Coronal Oscillations, Astrophys. J., 1984, vol. 279, p. 857. Tsuneta, S., Masuda, S., and Kosugi, T., Hot and Superhot Plasmas above an Impulsive Flare Loop, Astrophys. J., 1997, vol. 478, p. 787. Wang, T.J., Solanki, S.K., Curdt, W., et al., Doppler Shift Oscillations of Hot Solar Coronal Plasma Seen by SUMER: A Signature of Loop Oscillations? Astrophys. J., 2002a, vol. 574, p. L101. Wang, T.J., Solanki, S.K., Curdt, W., et al., Hot Loop Oscillations Seen by SUMER: Examples and Statistics, Proc. Euro Conf. and IAU Coll. 188, Santorini, Greece, 2002b ESA SP-505. Zhitnik, I.A., Bugaenko, O.I., Ignatiev, A.P., et al., Dynamic 10 MK Plasma Structures Observed in Monochromatic Full-Sun Images by the SPIRIT Spectroheliograph on the CORONAS-F Mission, Mon. Not. R. Astron. Soc., 2003, vol. 338, p. 67.

decreases to its original level within four hours. We regard this as evidence for plasma heating by fluxes of accelerated electrons. We revealed decaying post-flare oscillations in the emission of one high-temperature source with a period of about 10 min. The presence of quasiperiodic oscillations in the brightness of hot coronal structures could be used at a later time to diagnose plasma parameters in the corona. ACKNOWLEDGMENTS We are grateful to the instrumentation teams of the SOHO/EIT projects (ESA and NASA), TRACE, and RHESSI, whose data we used. This work was supported by the Russian Foundation for Basic Research (project nos. 03-02-16591, 05-0217105, and 05-02-17415), the Ministry of Education and Science of the Russian Federation (grant NSh 477.2003.2), and the research programs of the Russian Academy of Sciences "Nonstationary Phenomena in Astronomy" and "Solar Activity and Physical Processes in the Sun­Earth System." REFERENCES
Aschwanden, ander, D., Transition 1999, vol. M.J., Fletcher, L., Schrijver, C., and AlexCoronal Loop Oscillations Observed with the Region and Coronal Explorer, Astrophys. J., 520, p. 880.

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2005