arXiv subjects

I. V. Zimovets

Publications and source records attributed to I. V. Zimovets.

27 records · Page 2Linked to original sources

arXiv2016

Relationship between chromospheric evaporation and magnetic field topology in M-class solar flare

Chromospheric evaporation is observed as Doppler blueshift during solar flares. It plays one of key roles in dynamics and energetics of solar flares, however, its mechanism is still unknown. In this paper we present a detailed analysis of spatially-resolved multi-wavelength observations of chromospheric evaporation during an M 1.0 class solar flare (SOL2014-06-12T21:12) using data from the NASA's IRIS (Interface Region Imaging Spectrograph) and HMI/SDO (Helioseismic and Magnetic Imager onboard Solar Dynamics Observatory) telescopes, and VIS/NST (Visible Imaging Spectrometer at New Solar Telescope) high-resolution observations, covering the temperature range from 10^4 K to 10^7 K. The results show that the averaged over the region Fe XXI blueshift of the hot evaporating plasma is delayed relative to the C II redshift of the relatively cold chromospheric plasma by about 1 min. The spatial distribution of the delays is not uniform across the region and can be as long as 2 min in several zones. Using vector magnetograms from HMI we reconstruct the magnetic field topology and the quasi-separatrix layer (QSL) and find that the blueshift delay regions as well as the H-alpha flare ribbons are connected to the region of magnetic polarity inversion line (PIL) and an expanding flux rope via a system of low-lying loop arcades with height < ~4.5 Mm. This allows us to propose an interpretation of the chromospheric evaporation based on the geometry of local magnetic fields, and the primary energy source associated with the PIL.

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arXiv2015

A statistical study of decaying kink oscillations detected using SDO/AIA

Despite intensive studies of kink oscillations of coronal loops in the last decade, a large scale statistically significant investigation of the oscillation parameters has not been made using data from the Solar Dynamics Observatory (SDO). We carry out a statistical study of kink oscillations using Extreme Ultra-Violet (EUV) imaging data from a previously compiled catalogue. We analysed 58 kink oscillation events observed by the Atmospheric Imaging Assembly (AIA) onboard SDO during its first four years of operation (2010-2014). Parameters of the oscillations, including the initial apparent amplitude, period, length of the oscillating loop, and damping are studied for 120 individual loop oscillations. Analysis of the initial loop displacement and oscillation amplitude leads to the conclusion that the initial loop displacement prescribes the initial amplitude of oscillation in general. The period is found to scale with the loop length, and a linear fit of the data cloud gives a kink speed of Ck =(1330+/-50) km s-1 . The main body of the data corresponds to kink speeds in the range Ck =(800-3300) km s-1. Measurements of 52 exponential damping times were made, and it was noted that at least 22 of the damping profiles may be better approximated by a combination of non-exponential and exponential profiles, rather than a purely exponential damping envelope. There are an additional 10 cases where the profile appears to be purely non-exponential, and no damping time was measured. A scaling of the exponential damping time with the period is found, following the previously established linear scaling between these two parameters.

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arXiv2015

Plasma Heating to Super-Hot Temperatures (>30 MK) in the August 9, 2011 Solar Flare

We investigate the August 9, 2011 solar flare of X-ray class X6.9, the "hottest" flare from 2000 to 2012, with a peak plasma temperature according to GOES data of 32.5 MK. Our goal is to determine the cause of such an anomalously high plasma temperature and to investigate the energy balance in the flare region with allowance made for the presence of a super-hot plasma (>30 MK). We analyze the RHESSI, GOES, AIA/SDO, and EVE/SDO data and discuss the spatial structure of the flare region and the results of our spectral analysis of its X-ray emission. Our analysis of the RHESSI X-ray spectra is performed in the one-temperature and two-temperature approximations by taking into account the emission of hot (20 MK) and super-hot (45 MK) plasmas. The hard X-ray spectrum in both models is fitted by power laws. The observed peculiarities of the flare are shown to be better explained in terms of the two-temperature model, in which the super-hot plasma is located at the flare loop tops (or in the magnetic cusp region). The formation of the super-hot plasma can be associated with its heating through primary energy release and with the suppression of thermal conduction. The anomalously high temperature (32.5 MK according to GOES) is most likely to be an artefact of the method for calculating the temperature based on two-channel GOES measurements in the one-temperature approximation applied to the emission of a multi-temperature flare plasma with a minor contribution from the low-temperature part of the differential emission measure.

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arXiv2014

Energy Release and Initiation of Sunquake in C-class Flare

We present analysis of C7.0 solar flare of Febrary 17, 2013, revealing a strong helioseismic response (sunquake) caused by a very compact impact in the photosphere. This is the weakest known C-class flare generating a sunquake event. To investigate possible mechanisms of this event, and to understand the role of accelerated charged particles and photospheric electric currents, we use data from three space observatories: Ramaty High Energy Solar Spectroscopic Imager (RHESSI), Solar Dynamics Observatory (SDO) and Geostationary Operational Environmental Satellite (GOES). We find that the photospheric flare impact does not spatially correspond to the strongest HXR emission source, but both of these events are parts of the same energy release. Our analysis reveals a close association of the flare energy release with a rapid increase of the electric currents, and suggests that the sunquake initiation is unlikely to be explained by the impact of high-energy electrons but may be associated with a rapid current dissipation or a localized impulsive Lorentz force.

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arXiv2012

Non-Thermal "Burst-on-Tail" of Long-Duration Solar Event on 26 October 2003

Observations of a rare long-duration solar event of GOES class X1.2 from 26 October 2003 are presented. This event showed a pronounced burst of hard X-ray and microwave emission, which was extremely delayed (>60 min) with respect to the main impulsive phase and did not have any significant response visible in soft X-ray emission. We refer to this phenomenon as a "burst-on-tail". Based on TRACE observations of the growing flare arcade and some simplified estimation, we explain why a reaction of active region plasma to accelerated electrons may change drastically over time. We suggest that, during the "burst-on-tail", non-thermal electrons were injected into magnetic loops of larger spatial scale than during the impulsive phase bursts, thus resulting in much smaller values of plasma temperature and emission measure in their coronal volume, and hence little soft X-ray flux. The nature of the long gap between the main impulsive phase and the "burst-on-tail" is, however, still an open question.

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arXiv2011

Instrumental oscillations in RHESSI count rates during solar flares

Aims: We seek to illustrate the analysis problems posed by RHESSI spacecraft motion by studying persistent instrumental oscillations found in the lightcurves measured by RHESSI's X-ray detectors in the 6-12 keV and 12-25 keV energy range during the decay phase of the flares of 2004 November 4 and 6. Methods: The various motions of the RHESSI spacecraft which may contribute to the manifestation of oscillations are studied. The response of each detector in turn is also investigated. Results: We find that on 2004 November 6 the observed oscillations correspond to the nutation period of the RHESSI instrument. These oscillations are also of greatest amplitude for detector 5, while in the lightcurves of many other detectors the oscillations are small or undetectable. We also find that the variation in detector pointing is much larger during this flare than the counterexample of 2004 November 4. Conclusions: Sufficiently large nutation motions of the RHESSI spacecraft lead to clearly observable oscillations in count rates, posing a significant hazard for data analysis. This issue is particularly problematic for detector 5 due to its design characteristics. Dynamic correction of the RHESSI counts, accounting for the livetime, data gaps, and the transmission of the bi-grid collimator of each detector, is required to overcome this issue. These corrections should be applied to all future oscillation studies.

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arXiv2010

Oscillatory processes in solar flares

Electromagnetic (radio, visible-light, UV, EUV, X-ray and gamma-ray) emission generated by solar and stellar flares often contains pronounced quasi-periodic pulsations (QPP). Physical mechanisms responsible for the generation of long-period QPP (with the periods longer than one second) are likely to be associated with MHD processes. The observed modulation depths, periods and anharmonicity of QPP suggest that they can be linked with some kind of MHD auto-oscillations, e.g. an oscillatory regime of magnetic reconnection. Such regimes, of both spontaneous and induced nature, have been observed in resistive-MHD numerical simulations. The oscillations are essentially nonlinear and non-stationary. We demonstrate that a promising novel method for their analysis is the Empirical Mode Decomposition technique.

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arXiv2009

Observations of quasi-periodic solar X-ray emission as a result of MHD oscillations in a system of multiple flare loops

We investigate the solar flare of 20 October 2002. The flare was accompanied by quasi-periodic pulsations (QPP) of both thermal and nonthermal hard X-ray emissions (HXR) observed by RHESSI in the 3-50 keV energy range. Analysis of the HXR time profiles in different energy channels made with the Lomb periodogram indicates two statistically significant time periods of about 16 and 36 seconds. The 36-second QPP were observed only in the nonthermal HXR emission in the impulsive phase of the flare. The 16-second QPP were more pronounced in the thermal HXR emission and were observed both in the impulsive and in the decay phases of the flare. Imaging analysis of the flare region, the determined time periods of the QPP and the estimated physical parameters of magnetic loops in the flare region allow us to interpret the observations as follows. 1) In the impulsive phase energy was released and electrons were accelerated by successive acts with the average time period of about 36 seconds in different parts of two spatially separated, but interacting loop systems of the flare region. 2) The 36-second periodicity of energy release could be caused by the action of fast MHD oscillations in the loops connecting these flaring sites. 3) During the first explosive acts of energy release the MHD oscillations (most probably the sausage mode) with time period of 16 seconds were excited in one system of the flare loops. 4) These oscillations were maintained by the subsequent explosive acts of energy release in the impulsive phase and were completely damped in the decay phase of the flare.

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arXiv2008

Imaging Observations of Quasi-Periodic Pulsatory Non-Thermal Emission in Ribbon Solar Flares

Using RHESSI and some auxiliary observations we examine possible connections between spatial and temporal morphology of the sources of non-thermal hard X-ray (HXR) emission which revealed minute quasi-periodic pulsations (QPPs) during the two-ribbon flares on 2003 May 29 and 2005 January 19. Microwave emission also reveals the same quasi-periodicity. The sources of non-thermal HXR emission are situated mainly inside the footpoints of the flare arcade loops observed by the TRACE and SOHO instruments. At least one of the sources moves systematically both during the QPP-phase and after it in each flare that allows to examine the sources velocities and the energy release rate via the process of magnetic reconnection. The sources move predominantly parallel to the magnetic inversion line or the appropriate flare ribbon during the QPP-phase whereas the movement slightly changes to more perpendicular regime after the QPPs. Each QPP is emitted from its own position. It is also seen that the velocity and the energy release rate don't correlate well with the flux of the HXR emission calculated from the sources. The sources of microwaves and thermal HXRs are situated near the apex of the loop arcade and are not stationary either. Almost all QPPs and some spikes of HXR emission during the post-QPP-phase reveal the soft-hard-soft spectral behavior indicating separate acts of electrons acceleration and injection, rather than modulation of emission flux by some kinds of magnetohydrodynamic (MHD) oscillations of coronal loops. In all likelihood, the flare scenarios based on the successively firing arcade loops are more preferable to interpret the observations, although we can not conclude exactly what mechanism forces these loops to flare up.

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