Soft X-ray Flare Research Articles

Coronal Loops Heating in the Atmosphere of the Ad Leo Red Dwarf

We study the possible origin of long-lasting soft X-ray flares on the AD Leo star, which were observed onboard the Extreme Ultraviolet Explorer (EUVE) spacecraft for the period of 1993-2000 [1]. These flares have relatively long rise and decay times of the radiation intensity (τ R ≈ 104 s and τ d ≈ 5 · 103, respectively), as well as a relatively large emission measure EM ≈ 1051cm−3, which exceeds by 1–3 orders of magnitude the emission measure of soft X-ray flares on the Sun. Assuming that the radiation appears in magnetic loops and basing on the observed values of the emission measure and radiation decay time, the authors of [1] determined the typical length $$ \overline{l}\approx 1.5\cdot {10}^{10} cm $$ , electron number density $$ \overline{n}\approx 3\cdot {10}^{11}c{m}^{-3} $$ , and plasma temperature $$ \overline{T}\approx 2.5\cdot 107 $$ K of the loops. This paper considers plasma heating due to dissipation of the electric currents in the coronal magnetic loops of the star induced by the photospheric convection. The large inductance of the loop as an equivalent electric circuit determines the long time of the current rise in the source and explains the observed time of plasma heating and the rise time of the X-ray radiation intensity. It is shown that the parameters of the X-ray sources in the AD Leo atmosphere agree with the parameters calculated under the assumption of simultaneous emission of a great number of loops (about 50) with electric currents greater than 1013 A, which exceeds the electric currents in the solar coronal magnetic loops by 1–3 orders of magnitude. Such an exceeding can be related to the higher photospheric convection velocities on the late-type stars compared with the Sun.

The statistical properties of the solar soft X-ray fluence during 1997–2008

The statistical properties of the solar soft X-ray (SXR) flare fluence, i.e. the time integral of SXR flux of a flare, $F_{\mathit{SXR}}$ , and sum of the $F_{\mathit{SXR}}$ of all flares produced by a solar active region (AR), $F_{\mathit{AR}}$ , during 1997–2008 have been investigated. The results show that $F_{\mathit{SXR}}$ has moderate correlation with the area of the associated AR, while the correlation between $F_{\mathit{AR}}$ and the largest area of the associated AR is also moderate. The total number of ARs that can produce at least one SXR flare during 1997–2008, $N_{t}$ , is 1408. The sum of $F_{\mathit{AR}}$ produced by 1408 ARs, $\sum F_{\mathit{AR}}$ , is 89 585.81 (erg s cm−2) and the average value of $F_{\mathit{AR}}$ is 63.6 (erg s cm−2). 34 ARs ( $F_{\mathit{AR}}\ge 500\ \mbox{erg}\,\mbox{s}\,\mbox{cm}^{-2}$ ) contributed 55.72 % of $\sum F_{\mathit{AR}}$ . 111 ARs $(100\ (\mbox{erg}\,\mbox{s}\,\mbox{cm}^{-2})\le F_{\mathit{AR}}<500\ (\mbox{erg}\,\mbox{s}\,\mbox{cm}^{-2}))$ contributed 24.33 % of $\sum F_{\mathit{AR}}$ . 437 ARs $(10\ (\mbox{erg}\,\mbox{s}\,\mbox{cm}^{-2} )\le F_{\mathit{AR}}< 100\ (\mbox{erg}\,\mbox{s}\,\mbox{cm}^{-2}))$ contributed 17.48 % of $\sum F_{\mathit{AR}}$ . The rest 826 ARs only contributed 2.52 % of $\sum F_{\mathit{AR}}$ . The number of ARs decreases dramatically with $F_{\mathit{AR}}$ and the distribution function of $F_{\mathit{AR}}$ is $N(F_{\mathit{AR}})=2840e^{-0.1286F_{\mathit{AR}}}$ . Finally, the north-south asymmetry of $F_{\mathit{AR}}$ has been investigated. The results show that the solar flare activities dominated in the northern hemisphere during 1997–2000 and the southern hemisphere during 2001–2008 respectively. During 1997–2008, the solar flare activities are stronger in the southern hemisphere than in the northern hemisphere.

Was the soft X-ray flare in NGC 3599 due to an AGN disc instability or a delayed tidal disruption event?

We present unpublished data from a tidal disruption candidate in NGC 3599 which show that the galaxy was already X-ray bright 18 months before the measurement which led to its classification. This removes the possibility that the flare was caused by a classical, fast-rising, short-peaked, tidal disruption event. Recent relativistic simulations indicate that the majority of disruptions will actually take months or years to rise to a peak, which will then be maintained for longer than previously thought. NGC 3599 could be one of the first identified examples of such an event. The optical spectra of NGC 3599 indicate that it is a low-luminosity Seyfert/LINER with L_bol~10^40 ergs/s The flare may alternatively be explained by a thermal instability in the accretion disc, which propagates through the inner region at the sound speed, causing an increase of the disc scale height and local accretion rate. This can explain the <9 years rise time of the flare. If this mechanism is correct then the flare may repeat on a timescale of several decades as the inner disc is emptied and refilled.

Evolutionary aspects and north-south asymmetry of soft X-ray flare index during solar cycles 21, 22, and 23

Aims. In this paper, we investigate the temporal evolution and north-south (N-S) asymmetry in the occurrence of solar flares during cycle 21, 22, and 23, and compare the results with traditional solar activity indices. Methods. The flare activity is characterized by a soft X-ray (SXR) flare index, which incorporates information about flare occurrences during a selected interval along with the peak intensity of individual events. Results. The SXR flare index correlates well with other conventional parameters of solar activity. Further, it exhibits a significantly higher correlation with sunspot area over sunspot number, which suggests the variations in sunspot area to be more closely linked with the transient energy release in the solar corona. The cumulative plots of the flare index indicate a slight excess of activity in the northern hemisphere during cycle 21, while a southern excess clearly prevails for cycles 22 and 23. The study reveals a significant N-S asymmetry, which exhibits variations with the phases of solar cycle. The reliability and persistency of this asymmetry significantly increases when the data is averaged over longer periods, while an optimal level is achieved when data is binned for 13 Carrington rotations. The time evolution of the flare index further confirms evolution of dual peaks in solar cycles during the solar maxima and violation of Gnevyshev-Ohl rule for the pair of solar cycles 22 and 23. Conclusions. The SXR flare index in the northern and the southern hemispheres of the Sun exhibits significant asymmetry during the evolutionary phases of the solar cycle, which implies that N-S asymmetry of solar flares is manifested in terms of the flare counts as well as the intensity of flare events.

Towards understanding the frequency distribution of solar flares—part 1: a Stochastic-Diffusive model of solar flares

Systematic relationships of the subclass-averaged rise, decay times and durations of flare events as a function of the logarithmic peak flux ( $\operatorname{ln}f$ ) are investigated, employing the soft X-ray flares observed by GOES during the period from September 1975 to October 2014. Different behaviors are found before and after 1997. Since 1997 they all vary linearly with $\operatorname{ln}f$ , obeying the RV model. However, prior to 1997 they vary quadratically with $\operatorname{ln}f$ , implying a different energy storage/release process of flaring. The discrepancy may be related to the variation in the turbulence in the corona caused by the weakening magnetic field strength in the recent two decades. This motivates us to propose a Stochastic-Diffusive model for explaining the above result, by assuming that the temporal rate of flare energy resulted by external forces is proportional to the total energy already stored in the flare system and inversely proportional to the size scale of diffusion.

Solar flare soft X-ray spectra from Diogeness observations

AbstractDiogeness was an uncollimated scanning flat crystal spectrometer observing solar flare X-ray spectra in four narrow wavelength bands in the vicinity of Ca xix, S xv and Si xiii He-like line ‘triplets’ around 3.18 Å, 5.04 Å and 6.65 Å. In two of the spectral channels, emission lines around the Ca xix 3.178 Å resonance line were scanned in opposite directions, being diffracted from precisely adjusted identical Quartz crystals mounted on a common shaft in a so-called Dopplerometer (tachometer) configuration. Observations of solar X-ray spectra made by Diogeness provide a direct diagnostic information on plasma characteristics during the impulsive flare energy release. We present a sample of events which occurred during the Diogeness operation time from August 16, 2001 to September 17, 2001.

Abnormal rotation rates of sunspots and durations of associated flares

Context. Short-duration and long-duration flares are important in terms of their association with coronal heating and coronal mass ejections, respectively. Sunspot motions in the photosphere have been known to be associated with flare occurrences. Aims. We study the association between the abnormal rotation rates (longitudinal displacement in a given latitude in contrast with the rotation of spots around their umbral centre) of sunspots and flare duration. Methods. We compute the rotation rates of sunspots for different days during their evolution. We consider rotation rates that are in excess of one standard deviation as abnormal rotation rates. Also, the duration of time between the initial and final stages of the flares are computed. Results. Using Kodaikanal Observatory white light picture and GOES soft X-ray flare data, we find that a good association with a high significance exists between abnormal rotation rates of sunspots and flare durations. In contrast, we find that duration of flare is independent of sunspot area. Conclusions. The present study suggests that sub-surface dynamics plays a dominant role in determining the duration and rate of dissipation of energy during flares.

Statistical Properties of Soft X-ray Fluxes of Solar Flares

In order to quantitatively study the statistical properties of the soft X-ray emission in solar flares, an algorithm has been developed to automatically detect flares in a given range of peak fluxes, and to analyze the flares observed by the GOESs (Geostationary Operational Environmental Satellites) from 1980 to 2013 in two soft X-ray bands. This study indicates that the statistical characteristics of the variation of flare soft X-ray flux near the peak time are independent to the magnitude of peak flux: on the average, the rising time of flare's soft X-ray flux is about half of the decay time, and the rising and decay times in the high-energy channel are shorter than the corresponding times in the low-energy channel, however, all these times will increase with the variation amplitude of flare's soft X-ray flux.

A year of operation of Melibea e-Callisto Solar Radio Telescope

The e-CALLISTO (Compound Astronomical Low-cost Low-frequency Instrument for Spectroscopy and Transportable Observatory) is a worldwide radio-spectrograph network with 24 hours a day solar radio burst monitoring. The e-CALLISTO network is led by the Swiss Federal Institute of Technology Zurich (ETHZ Zurich), which work up collaborations with local host institutions. In 2013 the University of Alcalá joined the e-CALLISTO network with the installation of two Solar Radio Telescopes (SRT): the EA4RKU-SRT that was located at the University of Alcalá from January 2013 till June 2013 and the Melibea-SRT that is located at Peralejos de las Truchas (Guadalajara) in operation from June 2013. The Spanish e-Callisto SRTs provide routine data to the network. We present examples of type III and type II radio-bursts observed by Melibea during its first year of operation and study their relation with soft X-ray flares observed by GOES and Coronal Mass Ejections (CMEs) and Solar Energetic Particle (SEP) events observed by space-borne instrumentation.

THE PECULIAR BEHAVIOR OF HALO CORONAL MASS EJECTIONS IN SOLAR CYCLE 24

We report on a remarkable finding that the halo coronal mass ejections (CMEs) in cycle 24 are more abundant than in cycle 23, although the sunspot number in cycle 24 has dropped by about 40%. We also find that the distribution of halo-CME source locations is different in cycle 24: the longitude distribution of halos is much flatter with the number of halos originating at central meridian distance >/=60 degrees twice as large as that in cycle 23. On the other hand, the average speed and the associated soft X-ray flare size are the same in the two cycles, suggesting that the ambient medium into which the CMEs are ejected is significantly different. We suggest that both the higher abundance and larger central meridian longitudes of halo CMEs can be explained as a consequence of the diminished total pressure in the heliosphere in cycle 24 (Gopalswamy et al. 2014). The reduced total pressure allows CMEs expand more than usual making them appear as halos.

Analysis of the Major Parameters in Solar Active Regions Based on the PCA Method

The occurrence of solar flares and solar proton events has a close relation with the solar active regions. A deep analysis on this relation is helpful to build a concrete model of forecasting solar flares and solar proton events. In this paper the Principal Component Analysis (PCA) method is adopted to analyze the main parameters of the solar active regions with solar proton events happened in 1997∼2010, the selected parameters include the magnetic classification of sunspot, the McIntosh classification of sunspot group, the area of sunspot group, the 10.7cm radio flux, the flare index, the proton flare position, and the soft X-ray flare intensity. The order of principal component scores for 81 solar activities has been obtained (the score represents the strength of each event), which is compared with the peak flux of solar proton event, the annual mean values of sunspot area and 10.7cm radio flux, it is found that the similarity is very high, and this shows that the principal component scores can represent the strength of solar activity to some extent.

Latitudinal distribution of soft X-ray flares and dispairty in butterfly diagram

We present statistical analysis of about 63000 soft X-ray flare (class≥C) observed by geostationary operational environmental satellite (GOES) during the period 1976–2008. Class wise occurrence of soft X-ray (SXR) flare is in declining trend since cycle 21. The distribution pattern of cycle 21 shows the transit of hemispheric dominance of flare activity from northern to southern hemisphere and remains there during cycle 22 and 23. During the three cycles, 0–100, 21–300 latitude belts in southern hemisphere (SH) and 31–400 latitude belt in northern hemisphere (NH) are mightier. The 11–200 latitude belt of both hemisphere is mightiest. Correlation coefficient between consecutive latitude appears to be increasing from equator to poleward in northern hemisphere whereas pole to equatorward in southern hemisphere. Slope of the regression line fitted with asymmetry time series of daily flare counts is negative in all three cycles for different classes of flares. The yearly asymmetry curve fitted by a sinusoidal function varies from 5.6 to 11 years period and depends upon the intensity of flare. Variation, of curve fitted with wings of butterfly diagram, from first to second order polynomial suggests that latitudinal migration of flare activity varies from cycle to cycle, northern to southern hemisphere. The variation in slope of the butterfly wing of different flare class indicates the non uniform migration of flare activity.

Performance assessment of GPS receivers during the September 24, 2011 solar radio burst event

The sudden outburst of in-band solar radio noise from the Sun is recognized as one of the potential Radio Frequency Interference (RFI) sources that directly impact the performance of Global Navigation Satellite System (GNSS) receivers. On September 24, 2011, the solar active region 1302 unleashed a moderate M7.1 soft X-ray flare associated with a very powerful radio burst at 1415 MHz. The Solar Radio Burst (SRB) event spanned over three distinct episodes of solar radio noise emission that reached the maximum radio flux density of 114,144 Solar Flux Units (SFU) at 13:04:46 UTC. This paper analyzes the impact of September 24, 2011 SRB event on the performance of a significant subset of NAVSTAR Global Positioning System (GPS) receivers located in the sunlit hemisphere. The performance assessment is carried out in terms of Carrier-to-Noise power spectral density ratio (C /N 0 ) degradation, dual-frequency pseudorange measurements availability, pseudorange residual errors, and dual-frequency positioning errors in the horizontal and vertical dimensions. We observed that during the SRB event the GPS C /N 0 is reduced at most by 13 dB on L1 and 24 dB on L2. The C /N 0 degradation caused the loss of lock on GPS L1 and L2 signals and significant code-tracking errors. We noticed that many stations experienced less than four satellite measurements, which are the minimum required number of measurements for position estimation. The deteriorated satellite-receiver geometry due to loss of signal lock and significant code-tracking errors during the solar radio burst event introduced large positioning errors in both the horizontal and vertical dimensions. Rise in vertical positioning error of 303 m and rise in horizontal positioning of 55 m could be noticed during the solar radio burst event.

Major solar eruptions and high-energy particle events during solar cycle 24

We report on a study of all major solar eruptions that occurred on the front-side of the Sun during the rise to peak phase of cycle 24 (first 62 months) in order to understand the key factors affecting the occurrence of large solar energetic particle events (SEPs) and the ground levels enhancement (GLE) events. The eruptions involve major flares with soft X-ray peak flux >/= 5.0 x10-5 Wm-2 (i.e., flare size >/= M5.0) and accompanying coronal mass ejections (CMEs). The selection criterion was based on the fact that the only front-side GLE in cycle 24 (GLE 71) had a flare size of M5.1. Only ~37% of the major eruptions from the western hemisphere resulted in large SEP events. Almost the same number of large SEP events was produced in weaker eruptions (flare size <M5.0), suggesting that the soft X-ray flare is not a good indicator of SEP or GLE events. On the other hand, the CME speed is a better indicator of SEP and GLE events because it is consistently high supporting the shock acceleration mechanism for SEPs and GLEs. We found the CME speed, magnetic connectivity to Earth, and ambient conditions as the main factors that contribute to the lack of high energy particle events during cycle 24. Several eruptions poorly connected to Earth (eastern-hemisphere or behind-the-west-limb events) resulted in very large SEP events detected by the STEREO spacecraft. Some very fast CMEs, likely to have accelerated particles to GeV energies, did not result in a GLE event because of poor latitudinal connectivity. The stringent latitudinal requirement suggests that the highest energy particles are likely accelerated in the nose part of shocks. There were also well-connected fast CMEs, which did not seem to have accelerated high energy particles due to possible unfavorable ambient conditions (high Alfven speed, overall reduction in acceleration efficiency in cycle 24).

TEMPERATURE EVOLUTION OF A MAGNETIC FLUX ROPE IN A FAILED SOLAR ERUPTION

In this presentation, we report for the first time the detailed temperature evolution process of the magnetic flux rope in a failed solar eruption. Occurred on January 05, 2013, the flux rope was impulsively accelerated to a speed of ~ 400 km/s in the first minute, then decelerated and came to a complete stop in two minutes. The failed eruption resulted in a large-size high-lying (~ 100 Mm above the surface) high-temperature "fire ball" sitting in the corona for more than two hours. The time evolution of the thermal structure of the flux rope was revealed through the differential emission measure analysis technique, which produced temperature maps using observations of the Atmospheric Imaging Assembly on board Solar Dynamic Observatory. The average temperature of the flux rope steadily increased from ~ 5 MK to ~ 10 MK during the first nine minutes of the evolution, which was much longer than the rise time (about three minutes) of the associated soft X-ray flare. We suggest that the flux rope be heated by the energy release of the continuing magnetic reconnection, different from the heating of the low-lying flare loops, which is mainly produced by the chromospheric plasma evaporation. The loop arcade overlying the flux rope was pushed up by ~ 10 Mm during the attempted eruption. The pattern of the velocity variation of the loop arcade strongly suggests that the failure of the eruption be caused by the strapping effect of the overlying loop arcade.

CME Expansion as the Driver of Metric Type II Shock Emission as Revealed by Self-consistent Analysis of High-Cadence EUV Images and Radio Spectrograms

On 13 June 2010, an eruptive event occurred near the solar limb. It included a small filament eruption and the onset of a relatively narrow coronal mass ejection (CME) surrounded by an extreme ultraviolet wave front recorded by the Solar Dynamics Observatory's (SDO) Atmospheric Imaging Assembly (AIA) at high cadence. The ejection was accompanied by a GOES M1.0 soft X-ray flare and a Type-II radio burst; high-resolution dynamic spectra of the latter were obtained by the ARTEMIS IV radio spectrograph. The combined observations enabled a study of the evolution of the ejecta and the EUV wavefront and its relationship with the coronal shock manifesting itself as metric Type-II burst. By introducing a novel technique, which deduces a proxy of the EUV compression ratio from AIA imaging data and compares it with the compression ratio deduced from the band-split of the Type-II metric radio burst, we are able to infer the potential source locations of the radio emission of the shock on that AIA images. Our results indicate that the expansion of the CME ejecta is the source for both EUV and radio shock emissions. Early in the CME expansion phase, the Type-II burst seems to originate in the sheath region between the EUV bubble and the EUV shock front in both radial and lateral directions. This suggests that both the nose and the flanks of the expanding bubble could have driven the shock.

CHROMOSPHERIC AND CORONAL OBSERVATIONS OF SOLAR FLARES WITH THE HELIOSEISMIC AND MAGNETIC IMAGER

We report observations of white-light ejecta in the low corona, for two X-class flares on the 2013 May 13, using data from the Helioseismic and Magnetic Imager (HMI) of the Solar Dynamics Observatory. At least two distinct kinds of sources appeared (chromospheric and coronal), in the early and later phases of flare development, in addition to the white-light footpoint sources commonly observed in the lower atmosphere. The gradual emissions have a clear identification with the classical loop-prominence system, but are brighter than expected and possibly seen here in the continuum rather than line emission. We find the HMI flux exceeds the radio/X-ray interpolation of the bremsstrahlung produced in the flare soft X-ray sources by at least one order of magnitude. This implies the participation of cooler sources that can produce free-bound continua and possibly line emission detectable by HMI. One of the early sources dynamically resembles "coronal rain", appearing at a maximum apparent height and moving toward the photosphere at an apparent constant projected speed of 134 $\pm$ 8 $\mathrm{km s^{-1}}$. Not much literature exists on the detection of optical continuum sources above the limb of the Sun by non-coronagraphic instruments, and these observations have potential implications for our basic understanding of flare development, since visible observations can in principle provide high spatial and temporal resolution.

LONG-TERM SPECTRAL EVOLUTION OF TIDAL DISRUPTION CANDIDATES SELECTED BY STRONG CORONAL LINES

We present results of follow-up optical spectroscopic observations of seven rare, extreme coronal line emitting galaxies reported by Wang et al. (2012) with Multi-Mirror Telescope (MMT). Large variations in coronal lines are found in four objects, making them strong candidates of tidal disruption events (TDE). For the four TDE candidates, all the coronal lines with ionization status higher than [Fe VII] disappear within 5-9 years. The [Fe VII] faded by a factor of about five in one object (J0952+2143) within 4 years, whereas emerged in other two without them previously. A strong increment in the [O III] flux is observed, shifting the line ratios towards the loci of active galactic nucleus on the BPT diagrams. Surprisingly, we detect a non-canonical [O III]5007/[O III]4959 2 in two objects, indicating a large column density of O$^{2+}$ and thus probably optical thick gas. This also requires a very large ionization parameter and relatively soft ionizing spectral energy distribution (e.g. blackbody with $T < 5\times 10^4$ K). Our observations can be explained as echoing of a strong ultraviolet to soft X-ray flare caused by tidal disruption events, on molecular clouds in the inner parsecs of the galactic nuclei. Re-analyzing the SDSS spectra reveals double-peaked or strongly blue-shouldered broad lines in three of the objects, which disappeared in the MMT spectra in two objects, and faded by a factor of ten in 8 years in the remaining object with a decrease in both the line width and centroid offset. We interpret these broad lines as arising from decelerating biconical outflows. Our results demonstrate that the signatures of echoing can persist for as long as ten years, and can be used to probe the gas environment in the quiescent galactic nuclei.

The Solar Connection of Enhanced Heavy Ion Charge States in the Interplanetary Medium: Implications for the Flux-Rope Structure of CMEs

We investigated a set of 54 interplanetary coronal mass ejection (ICME) events whose solar sources are very close to the disk center (within 15 degrees from the central meridian). The ICMEs consisted of 23 magnetic cloud (MC) events and 31 non-MC events. Our analyses suggest that the MC and non-MC ICMEs have more or less the same eruption characteristics at the Sun in terms of soft X-ray flares and CMEs. Both types have significant enhancements in charge states, although the non-MC structures have slightly lower levels of enhancement. The overall duration of charge state enhancement is also considerably smaller than that than that in MCs as derived from solar wind plasma and magnetic signatures. We find very good correlation between the Fe and O charge state measurements and the flare properties such as soft X-ray flare intensity and flare temperature for both MCs and non-MCs. These observations suggest that both MC and non-MC ICMEs are likely to have a flux-rope structure and the unfavorable observational geometry may be responsible for the appearance of non-MC structures at 1 AU. We do not find any evidence for active region expansion resulting in ICMEs lacking a flux rope structure because the mechanism of producing high charge states and the flux rope structure at the Sun is the same for MC and non-MC events.

The 1859 space weather event revisited: limits of extreme activity

The solar flare on 1 September 1859 and its associated geomagnetic storm remain the standard for an extreme solar-terrestrial event. The most recent estimates of the flare soft X-ray (SXR) peak intensity and D st magnetic storm index for this event are: SXR class = X45 (±5) (vs. X35 (±5) for the 4 November 2003 flare) and minimum D st = −900 (+50, −150) nT (vs. −825 to −900 nT for the great storm of May 1921). We have no direct evidence of an associated solar energetic proton (SEP) event but a correlation between >30 MeV SEP fluence (F30 ) and flare size based on modern data yields a best guess F30 value of ~1.1 × 1010 pr cm−2 (with the ±1σ uncertainty spanning a range from ~109 –1011 pr cm−2 ) for a composite (multi-flare plus shock) 1859 event. This value is approximately twice that of estimates/measurements – ranging from ~5–7 × 109 pr cm−2 – for the largest SEP episodes (July 1959, November 1960, August 1972) in the modern era.