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Long- and Mid-Term Variations of the Soft X-ray Flare Type in Solar Cycles  

Ilya Chertok   Submitted: 2017-09-14 05:43

Using data from the GOES spacecraft in the 1?8 ? wavelength range for Solar Cycles 23, 24, and part of Cycles 21 and 22, we compare mean temporal parameters (rising, decay times, duration) and the proportion of impulsive short-duration events (SDE) and gradual long-duration events (LDE) among C- and ≥M1.0-class flares. It is found that the fraction of the SDE ≥M1.0-class flares (including spikes) in Cycle 24 exceeds that in Cycle 23 in all three temporal parameters at the maximum phase and in the decay time during the ascending cycle phase. However, Cycles 23 and 24 barely differ in the fraction of the SDE C-class flares. The temporal parameters of SDEs, their fraction, and consequently the relationship between the SDE and LDE flares do not remain constant, but they reveal regular changes within individual cycles and during the transition from one cycle to another. In all phases of all four cycles, these changes have the character of pronounced, large-amplitude ''quasi-biennial'' oscillations (QBOs). In different cycles and at the separate phases of individual cycles, such QBOs are superimposed on various systematic trends displayed by the analyzed temporal flare parameters. In Cycle 24, the fraction of the SDE ≥M1.0-class flares from the N- and S-hemispheres displays the most pronounced synchronous QBOs. The QBO amplitude and general variability of the intense ≥M1.0-class flares almost always markedly exceeds those of the moderate C-class flares. The ordered quantitative and qualitative variations of the flare type revealed in the course of the solar cycles are discussed within the framework of the concept that the SDE flares are associated mainly with small sunspots (including those in developed active regions) and that small and large sunspots behave differently during cycles and form two distinct populations.

Authors: I.M. Chertok and A.V. Belov
Projects: GOES X-rays

Publication Status: Solar Physics, October 2017, 292:144
Last Modified: 2017-10-21 18:40
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An Early Diagnostics of the Geoeffectiveness of Solar Eruptions from Photospheric Magnetic Flux Observations: The Transition from SOHO to SDO  

Ilya Chertok   Submitted: 2017-02-20 23:55

In our previous articles (Chertok et al.: 2013, Solar Phys. 282, 175, and 2015, Solar Phys. 290, 627), we presented a preliminary tool for the early diagnostics of the geoeffectiveness of solar eruptions based on the estimate of the total unsigned line-of-sight photospheric magnetic flux in accompanying extreme-ultraviolet arcades and dimmings. This tool was based on the analysis of eruptions observed in 1996-2005 with SOHO/EIT and MDI. Empirical relationships were obtained to estimate the probable importance of upcoming space weather disturbances caused by an eruption, which just occurred, without data on the associated coronal mass ejections. It was possible to estimate the intensity of a non-recurrent geomagnetic storm (GMS) and Forbush decrease (FD), as well as their onset and peak times. After 2010-2011, data on solar eruptions are obtained with SDO/AIA and HMI. We use relatively short intervals of overlapping EIT-AIA and MDI-HMI detailed observations and a number of large eruptions over the next five years with the 12-hour cadence EIT images to adapt the SOHO diagnostic tool to SDO data. The adopted brightness thresholds select from the EIT 195 Å and AIA 193 Å image practically the same areas of arcades and dimmings with a cross-calibration factor of 3.6-5.8 (5.0-8.2) for the AIA exposure time of 2.0 s (2.9 s). For the same photospheric areas, the MDI magnetic flux systematically exceeds the HMI flux by a factor of 1.4. Based on these results, the empirical diagnostic relationships obtained from SOHO data are adjusted to SDO instruments. Examples of a post-diagnostics based on SDO data are presented. As before, the tool is applicable to non-recurrent GMSs and FDs caused by nearly central eruptions from active regions, provided that the southern component of the interplanetary magnetic field near the Earth is predominantly negative, which is not predicted by this tool.

Authors: I.M. Chertok, V.V. Grechnev, A.A. Abunin
Projects: SDO-AIA,SDO-HMI,SoHO-EIT,SoHO-MDI

Publication Status: Solar Physics, accepted
Last Modified: 2017-02-21 15:43
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A Simple Way to Estimate the Soft X-ray Class of Far-Side Solar Flares Observed with STEREO/EUVI  

Ilya Chertok   Submitted: 2015-05-08 00:32

Around the peaks of substantial flares, bright artifact nearly horizontal saturation streaks (B-streaks) corresponding to the brightest parts of the flare sources appear in the STEREO/EUVI 195 Å images. We show that the length of such B-streaks can be used for the solution of an actual problem of evaluating the soft X-ray flux and class of far-side flares registered with double STEREO spacecraft but invisible from Earth. For this purpose from data on about 350 flares observed from January 2007 to July 2014 (mainly exceeding the GOES M1.0 level) both with GOES and STEREO, an empirical relation is established correlating the GOES 1-8 Å peak flux and the B-streak length. This allowed us for the same years to estimate the soft X-ray classes for approximately 65 strong far-side flares observed by STEREO. The results of this simple and prompt method are consistent with the estimations of Nitta et al. (Solar Phys., 288, 241, 2013) based on the calculations of the EUVI full-disk digital number output. In addition, we studied some features of the B-streaks in impulsive and long-duration flares and demonstrated that B-streaks in several consecutive EUVI images can be used to reconstruct a probable time history of strong far-side flares.

Authors: Chertok, I. M., Belov, A. V., Grechnev, V. V.
Projects: GOES X-rays ,STEREO

Publication Status: Solar Physics, submitted
Last Modified: 2015-05-12 14:02
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Relationship between the Magnetic Flux of Solar Eruptions and the Ap Index of Geomagnetic Storms  

Ilya Chertok   Submitted: 2014-10-07 22:50

Solar coronal mass ejections (CMEs) are main drivers of the most powerful non-recurrent geomagnetic storms. In the extreme-ultraviolet range, CMEs are accompanied by bright post-eruption arcades and dark dimmings. The analysis of events of the Solar Cycle 23 (Chertok et al., 2013, Solar Phys. 282, 175) revealed that the summarized unsigned magnetic flux in the arcades and dimming regions at the photospheric level, Φ, is significantly related to the intensity (Dst index) of geomagnetic storms. This provides the basis for the earliest diagnosis of geoefficiency of solar eruptions. In the present article, using the same data set, we find that a noticeable correlation exists also between the eruptive magnetic flux, Φ, and another geomagnetic index, Ap. As the magnetic flux increases from tens to ≈500 (in units of 1020 Mx), the geomagnetic storm intensity measured by the 3-hour Ap index, enhances in average from Ap ≈ 50 to a formally maximum value of 400 (in units of 2 nT). The established relationship shows that in fact the real value of the Ap index is not limited and during the most severe magnetic storms may significantly exceed 400.

Authors: I. M. Chertok, M. A. Abunina, A. A. Abunin, A. V. Belov, V. V. Grechnev
Projects: SoHO-EIT,SoHO-MDI,SoHO-LASCO

Publication Status: Solar Phys., accepted
Last Modified: 2014-10-08 10:10
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Magnetic Flux of EUV Arcade and Dimming Regions as a Relevant Parameter for Early Diagnostics of Solar Eruptions - Sources of Non-Recurrent Geomagnetic Storms and Forbush Decreases  

Ilya Chertok   Submitted: 2012-09-11 22:43

This study aims at the early diagnostics of geoeffectiveness of coronal mass ejections (CMEs) from quantitative parameters of the accompanying EUV dimming and arcade events. We study events of the 23th solar cycle, in which major non-recurrent geomagnetic storms (GMS) with Dst < -100 nT are sufficiently reliably identified with their solar sources in the central part of the disk. Using the SOHO/EIT 195 ÅA images and MDI magnetograms, we select significant dimming and arcade areas and calculate summarized unsigned magnetic fluxes in these regions at the photospheric level. The high relevance of this eruption parameter is displayed by its pronounced correlation with the Forbush decrease (FD) magnitude, which, unlike GMSs, does not depend on the sign of the Bz component but is determined by global characteristics of ICMEs. Correlations with the same magnetic flux in the solar source region are found for the GMS intensity (at the first step, without taking into account factors determining the Bz component near the Earth), as well as for the temporal intervals between the solar eruptions and the GMS onset and peak times. The larger the magnetic flux, the stronger the FD and GMS intensities are and the shorter the ICME transit time is. The revealed correlations indicate that the main quantitative characteristics of major non-recurrent space weather disturbances are largely determined by measurable parameters of solar eruptions, in particular, by the magnetic flux in dimming areas and arcades, and can be tentatively estimated in advance with a lead time from 1 to 4 days. For GMS intensity, the revealed dependencies allow one to estimate a possible value, which can be expected if the Bz component is negative.

Authors: I. M. Chertok, V. V. Grechnev, A. V. Belov, A. A. Abunin
Projects: GOES X-rays ,SoHO-EIT,SoHO-MDI,SoHO-LASCO

Publication Status: Accepted for publication in Solar Physics
Last Modified: 2012-09-13 10:13
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On the Correlation between Spectra of Solar Microwave Bursts and Proton Fluxes near the Earth  

Ilya Chertok   Submitted: 2009-11-11 06:26

Studies of the extreme solar proton event of January 20, 2005 intensified the contest over of a long-standing problem: are solar cosmic rays arriving at the Earth accelerated by solar flares or by shocks preceding rapidly moving coronal mass ejections? Among the most important questions is the relationship between the energy spectra of the solar cosmic rays and the frequency spectra of flare microwave bursts. Some studies of previous solar-activity cycles have shown that such a relationship does exist, in particular, for protons with energies of tens of MeV. The present work analyzes this relation using data for 1987-2008. For flare events observed in the western half of the disk, there is a significant correlation between the index d=log(J10/J100), (J10 and J100 are the peak proton fluxes at E > 10 and 100 MeV, respectively; d is equivalent to the power-law index of the integrated energy spectrum of 10-100 MeV protons detected near the Earth's orbit) and radio burst parameters such as a ratio of peak fluxes S at two frequencies (for example, at 9 and 15 GHz) and a microwave peak frequency fm. Proton fluxes with hard (flat) energy spectra (d~<1.5) correspond to hard microwave frequency spectra (S9/S15~<1 and fm>15 GHz), while flares with soft radio spectra (S9/S15>~1.5 and fm~<5 GHz) result in proton fluxes with soft (steep) energy spectra (d>~1.5-2). It is also shown that powerful high-frequency bursts with the hardest radio spectra (fm~30 GHz) can point at acceleration of significant proton fluxes in flares occurring in strong magnetic fields. These results argue that solar cosmic rays (or at least their initial impulses) are mainly accelerated in flares associated with impulsive and post-eruptive energy release, rather than in shocks driven by coronal mass ejections.

Authors: I. M. Chertok, V. V. Grechnev, and N. S. Meshalkina
Projects: None

Publication Status: Astronomy Reports, 2009, Vol. 53, No. 11, pp. 1059-1069.
Last Modified: 2009-11-12 07:48
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Large-Scale Phenomena at the Sun Associated with Filament Eruption Ouside Active Regions: the 12.09.1999 Event  

Ilya Chertok   Submitted: 2009-08-31 23:44

The event of September 12, 1999 is used to analyze large-scale disturbances associated with coronal mass ejections during the eruption of filaments outside active regions. The analysis is based on Hα filtergrams, EUV and soft X-ray images, and coronograph data. The filament eruption occurred in relatively weak magnetic fields, but was accompanied by larger-scale phenomena than flare events. During several hours after the eruption, a large-scale arcade developed, whose bases formed diverging flare-like ribbons. The volume of the event was bounded by an ''EIT wave'', which was quasi-stationary at the solar surface and expanded above the limb. The event did not have an impulsive component; therefore the ''EIT wave'' above the limb was a magnetic structure, identified as the front of a coronal mass ejection by virtue of its shape, structural features, and kinematics. Three types of dimmings were observed within the areal of the event, cause by (a) the evacuation of plasma, (b) heating of plasma with its subsequent evacuation, and (c) the absorption of radiation in a system of filaments activated by the eruption. The fact that a dimming appeared due to plasma heating was revealed by its presence in soft X-rays, whereas the four EIT channels did not demonstrate this. This brings into question the correctness of certain conclusions drawn earlier based purely on EIT data. A transformation of magnetic fields brought about by the eruption also occurred in a stationary coronal hole adjacent to the areal of the event. The expansion of the coronal mass ejection was self-similar and characterized by a rapidly decreasing acceleration, which is not taken into account in the widely used polynomial approximation.

Authors: I.M. Chertok, V.V. Grechnev, and A.M. Uralov
Projects: SoHO-EIT,SoHO-LASCO,Yohkoh-SXT

Publication Status: Astronomy Reports, 2009, Vol. 53, No. 4, pp. 355-368
Last Modified: 2009-09-10 08:15
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Large-Scale Activity in Solar Eruptive Events of October-November 2003 by SOHO/EIT Data  

Ilya Chertok   Submitted: 0000-00-00 00:00

We analyze large-scale disturbances associated with extremely powerful flares and coronal mass ejections (CMEs) occurred during two passages across the solar disk of a grandiose complex of three active regions in October?November 2003 using SOHO/EIT imaging data. In particular, we study dimmings (i.e. transient coronal holes or regions of temporary depressions of the EUV emission) and, in a lesser degree, coronal waves (propagating emitting fronts). Our analysis is based on derotated fixed-difference heliograms, i.e., difference images in which the solar rotation is previously compensated with the same base pre-event frame being subtracted from all heliograms. Such a method allows us to consider both the 12-min interval heliograms at 195 and images observed in different-temperature bands of 171, 195, 284, and 304

Authors: Chertok, I.M. and Grechnev, V.V.
Projects:

Publication Status:
Last Modified: 0000-00-00 00:00
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Abstracts by Author
Long- and Mid-Term Variations of the Soft X-ray Flare Type in Solar Cycles
An Early Diagnostics of the Geoeffectiveness of Solar Eruptions from Photospheric Magnetic Flux Observations: The Transition from SOHO to SDO
A Simple Way to Estimate the Soft X-ray Class of Far-Side Solar Flares Observed with STEREO/EUVI
Relationship between the Magnetic Flux of Solar Eruptions and the Ap Index of Geomagnetic Storms
Magnetic Flux of EUV Arcade and Dimming Regions as a Relevant Parameter for Early Diagnostics of Solar Eruptions - Sources of Non-Recurrent Geomagnetic Storms and Forbush Decreases
On the Correlation between Spectra of Solar Microwave Bursts and Proton Fluxes near the Earth
Large-Scale Phenomena at the Sun Associated with Filament Eruption Ouside Active Regions: the 12.09.1999 Event
Large-Scale Activity in Solar Eruptive Events of October-November 2003 by SOHO/EIT Data

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