RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 9, ES3004, doi:10.2205/2007ES000276, 2007

2. Observations of the Sun and Its Activity

2.1. Global Oscillations of the Sun

2007ES000276-fig01
Figure 1
[4]  Global oscillations of the Sun registered by methods of the helioseismology make it possible to diagnose the inner solar layers. The DIFOS device on board the CORONAS-F satellite registered low ( l = 0, 1, and 2) eigenmodes of the global oscillations of the Sun (oscillations of an acoustic type called p -modes and having a period of about 5 min) within the broad wavelength range (350-1500 nm). Very small changes (10-5-10-6 ) of the total solar radiation flux related to these oscillations ( Lebedev et al. [2004]) were measured. Establishing of principal possibility of helioseismological studies in observations at near-Earth satellites (when the light reflected from the Earth's surface reduces the measurement accuracy) was an important result of these observations. In the power spectrum (Figure 1), the peaks corresponding to harmonic global oscillations ( p -modes) with a particular period and the values of l and n numbers (the number of oscillation meshes in azimuth and solar radius, respectively) are distinctly pronounced. From 10 to 15 separate harmonics which in the coarse of time were changing and substituted by other harmonics were registered simultaneously, the latter fact manifesting the dynamics of the inner layers of the Sun caused by the interaction of the outer convective shell having a broad noise spectrum to eigen oscillations of the Sun.

2.2. X-ray Solar Images

[5]  The entire complex of unique studies was carried out [Zhitnik et al., 2005] using the multi-channel X-ray telescope. Numerous images of the Sun with high spatial resolution in various spectral lines corresponding to different temperature layers of the solar atmosphere made it possible to localize and study the morphology of various active phenomena.

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Figure 2
[6]  Solar activity level unusually high for a few recent decades was observed on the Sun in the period of October-November 2003. The whole series of flares was prominent by their power. The flares were accompanied by powerful ejections with velocities up to a few hundreds of km s-1. The sun was covered by active regions, numerous magnetic loops, and strongly heated regions. The global longitudinal (almost hemispherical) asymmetry of the location of active regions on the solar surface was the case of the very powerful flares and ejections. The images of the Sun (see Figure 2) obtained with the interval of a half of solar rotation show that there is almost no active regions on one hemisphere, whereas on the other hemisphere there are more than 30 such regions. Strong gradients of the magnetic field were the very factor what provided such powerful flares what caused strong magnetic storms on the Earth and enhanced cosmic ray fluxes in space.

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Figure 3
[7]  The morphology and dynamics of development of these flares, ejections, and related to them dimmings (darkening) (see Figure 3) were studied in these events on the basis of the X-ray observations [Chertok et al., 2005; Slemzin et al., 2005]. As a results, it is found that the observed dimmings were mainly formed as a result of a complete or partial opening of magnetic fields in the process of coronal mass ejections and (related to this process) flowing of the matter out of the large-scale magnetic structures of the intermediate layer and corona and corresponding lowering of the emission measure. It is found also that the global character of the dimmings in these and other events means that a considerable part of the solar atmosphere mass was involved into the process of eruption of coronal mass ejection. A repeatability of eruptive events and the dimming picture was observed from one event to another, and each time the coronal ejections eruption involved approximately the same structures which succeeded in recovering their magnetic field and luminosity during the time interval between the events.

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Figure 4
[8]  On the basis of observations by the spectroheliograph RES-K (the SPIRIT experiment) in the resonance MgXII (8.42 Å) line in the solar corona, a complete class of new events (rapid-dynamic plasma formations with the temperatures up to 20 millions of degrees [Grechnev et al., 2006; Zhitnik et al., 2003]) was discovered. Images and dynamics of these high-temperature formations are studied for the first time. They are formed, apparently, when the hot plasma formed during solar flares fill in the magnetic configuration in the corona, and the emission of this hot plasma contours it. As a result the formations have a variable form (see Figure 4) in a shape of "hot clouds", "spiders", loops, propagating wave fronts, and igniting in sequence magnetic arches. The hot plasma formations registered in the corona present a manifestation of one of the heating mechanisms of the solar corona as a result of energy release in magnetic configurations and its transformation into the plasma energy. It is found that the mass ejections from the solar atmosphere often observed by the coronagraph on board the SOHO European satellite are often related to the found hot plasma formations in the corona.

[9]  Observations by the X-ray telescope operating in the coronagraph regime made it possible for the first time to obtain data on the solar corona dynamics at distances up to three solar radii. This region is important for understanding of the nature of many phenomena, but is not observed by other devices, because it is an intermediate region between the near-limb region observed by telescopes and the far corona observed in the white light by coronagraphs. It this region of the corona, loops and magnetic field arches, aligned streamers, and jets of the elapsing solar wind are distinctly seen, whereas mass ejections and eruptive filaments at these altitudes were observed for the first time. The X-ray coronagraph obtained also for the first time maps of the distribution of the hard X-ray radiation for quiet and disturbed corona in the presence at the limb of bright active regions which illustrate visually the magnetic heating of the corona over active regions.

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Figure 5
[10]  The magnetic heating of the corona was confirmed also by the spectra of the solar X-ray radiation obtained by the X-ray spectrometer RPS-1 for various numbers of sunspots: the more sunspots, the more hard spectrum of the X-ray radiation was registered (see Figure 5) and, respectively, the stronger was the impact on the Earth's atmosphere.

2007ES000276-fig06
Figure 6
[11]  In the period 6-17 September 2005, a series of extreme events occurred on the Sun related to the passage over the disk of the AR10808 active region. The series included 10 X-ray flares of the X class and 27 flares of the M class accompanied by powerful ejections of the coronal substance and strong geomagnetic storms. During that period, the SPIRIT/CORONAS-F equipment registered 13 flares. The analysis of the images in the coronal lines in the 175 Å range with the temperature of 1.2 MK showed that during all flare events there occurred a strong restructurization of the magnetic field in the region of the flare (see Figure 6). The AR10808 active region had an area of > 1400 m. p. of the visible hemisphere and numerous delta-structures. Å series of flares within this active region caused on the Earth an increased magnetic activity during a durable time interval. The magnetic storm started on 10 September 2005 passed its maximum on 11 September 2005. Magnetic storms and considerable disturbances were observed 6 days in a row (10-15 September 2005) and 8 days in a row (9-16 September 2005), respectively. On the night from 10 to 11 September 2005, a Forbush effect with a value of 13% for the 10 GeV energy was registered. It was one of the largest (the forth by its magnitude) Forbush effect in the current solar cycle. Later, after 19 September 2005, the AR10808 group decreased considerably and was simplified, but still had a delta-configuration.

2.3. Solar Flares

[12]  During the period of the observations by the CORONAS-F devices with high time and spectral resolution in the broad energy range, a vast amount of new information on various physical processes in flares was obtained. The information concerns spectral, energetic, polarization, and dynamical characteristics of flare emissions, spectra of accelerated particles, gamma lines etc.

[13]  On the basis of the observations with high time resolution in the X-ray range (3-40 keV) using the flare spectrometer IRIS and spectral analysis, characteristic periods of plasma oscillations in active regions before the flare, during the flare, and after the flare were determined [Dmitriev et al., 2002, 2006]. These periods differ considerably (from a few seconds to tens of seconds), this fact manifesting changes in the resonant properties of the magnetic configuration of active regions. Such signs could be used for forecasting of solar flares on the basis of the observed X-ray emission and for creation of a model of the flare process.

2.4. Impulsive Phase of a Flare

[14]  The impulsive phase of a flare is characterized by an energy release which (as we see it in the X-ray radiation) has an oscillation (saw-like) character [Dmitriev et al., 2002]. The beams of charged particles accelerated in the flare inject into the dense layers of the solar atmosphere and form peaks of the X-ray emission with characteristic intervals of 20 s. Quite similar picture of the energy release in the form of saw-like oscillations is observed also in tokomaks during an instability break related to reconnection of magnetic field lines [Prist and Forbes, 2000].

[15]  The hard X-ray emission generated at the interaction of directed beams of accelerated particles with the dense solar atmosphere should be linearly polarized. Attempts to measure this polarization for a long time were failing. A considerable linear polarization in the flare maximum was for the first time measured by the SPR-N specropolarimeter at the CORONAS-F satellite for one of the most powerful flares on 29 November 2003 (class X-10) [Zhitnik et al., 2006]. This is a direct proof not only of the existence of the accelerated particle beams themselves, but a confirmation of the fact that these particles are accelerated by the pulse electric field during the magnetic reconnection, but not by some stochastic mechanism.

[16]  The impulsive character of the energy release and particle acceleration was registered also in the time profiles and dynamic spectra of the hard X-ray emission of flares in 8 energetic channels (26-380 keV) of the HELICON gamma spectrometer. On the basis of these observations, the dynamics of hard emission spectrum (characteristic times and values of the spectrum slope change) was determined. The most hard spectrum is realized in the flare maximum.

2.5. Atomic Processes in Flares

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Figure 7
[17]  The detailed spectroscopic diagnostics of the flare plasma and atomic processes in the flares were performed with the help of DIAGENESS spectrophotometer and RESIK X-ray spectrometer [Sylwester et al., 2005a, 2005b]. For the first time the absolute Doppler shifts of the X-ray spectral lines in solar flares and the absolute content of K (potassium) and CL (chlorine) in the solar corona were determined, and spectral lines of solar plasma ions for high values of the quantum numbers n (see Figure 7) opening new possibilities for a new method of the temperature diagnostics of the coronal plasma were detected. New spectral lines were also detected and their identification was performed, hundreds of spectra of helium-like ions Ca XIX, S XV, and Si XIII were measured. More than one million of linear spectra were obtained during the period of observations, a catalog was created (http://www.cbk.pan.wroc.pl/resik_catalogue.htm), and an atlas of spectral lines of the Sun in the range 3.4 Å -6.1 Å was prepared.

2.6. Nuclear Processes in Flares

[18]  In powerful flares, the accelerated protons and nuclei with an energy above a few MeV cause numerous nuclear reactions at collisions with nuclei of the medium. The gamma radiation, gamma lines of excited nuclei, annihilation electron-positron lines, and neutrons formed in the reactions bring important diagnostic information on the acceleration process and the composition of the solar atmosphere. These processes were registered by the devices of the gamma range: the amplitude-time spectrometer AVS-F and the spectrometer of solar neutrons and gamma radiation, SONG.

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Figure 8
[19]  Figure 8 presents an example of registration of the gamma lines in the flare on 29 October 2003 showing the presence in the solar atmosphere of various chemical elements and their isotopes [Arkhangel'skaya et al., 2006]. In this case they are iron, magnesium, silicon, neon, oxygen, and carbon. A peculiarity in the spectrum corresponding to the line caused by the capture of the neutrons born in the flare is also seen. As far as the gamma radiation of the flare presents a sum of separate gamma lines excited by the accelerated protons, the presented gamma radiation spectrum simultaneously characterize the initial spectrum of the protons accelerated in the flare, this spectrum being one of the most important characteristics of the acceleration process.

[20]  The line of the annihilation of electrons and positrons was observed in the energy range of 0.5 MeV in the spectrum of the gamma radiation of the flare by the amplitude-time spectrometer AVS-F. The line appears only in powerful flares when sufficient number of positrons is formed from the radioactive nuclei born in the flare.

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Figure 9
[21]  At the near-Earth orbit, the neutrons born in solar flares could be registered only at high energies (higher than 30 MeV), because at lower energies they decay to proton, electron, and electron antineutrino (it is the beta-decay of a neutron, the decay time of neutrons in vacuum being 16 min), not reaching the Earth. The gamma radiation and neutron flux for powerful flares of October 2003 registered by the SONG device are shown in Figure 9 [Kuznetsov et al., 2006]. This data compared with other observations make it possible to determine the moment of the exit of accelerated protons and energetic neutrons from the solar corona.

2.7. Ultraviolet Emission of Solar Flares

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Figure 10
[22]  On the basis of the observations by the solar ultraviolet radiometer SUFR-Sp-K and ultraviolet solar spectrophotometer VUSS-L, the fluxes of ultraviolet emission from solar flares were measured by T. V. Kazachevskaya et al. (in press, 2008) and Nusinov et al., [2005]. Figure 10 shows an example of such measurements in the La line (in the wavelength range shorter than 130 nm) by the VUSS device for the 21 January 2003 flare. The typical value of the changes of the ultraviolet emission in the band in the vicinity of the La line for the flares of the X-ray class C9-M1 is approximately 0.5%. For the most powerful flares, the increase in the ultraviolet emission in the 130 nm band does not exceed a few percents.


RJES

Citation: Kuznetsov, V. D. (2007), From the geophysical to heliophysical year: The results of the CORONAS-F project, Russ. J. Earth Sci., 9, ES3004, doi:10.2205/2007ES000276.

Copyright 2007 by the Russian Journal of Earth Sciences

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