Flare Peak Research Articles

High-resolution Observations of an X-1.0 White-light Flare with Moving Flare Ribbons

Abstract We analyze high-resolution observations of an X-1.0 white-light flare, triggered by a filament eruption, on 2022 October 2. The full process of filament formation and subsequent eruption was captured in the Hα passband by the Visible Imaging Spectrograph (VIS) on board the Goode Solar Telescope (GST) within its center field of view. White-light emissions appear in flare ribbons following the filament eruption and Hα ribbon brightening. GST Broadband Filter Imager data show that the continuum intensity, as compared to the nearby quiet-Sun area, has increased by up to 20% in the photospheric TiO band around 7057 Å. The Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory reported 10% contrast enhancement in the continuum near Fe i 6173 Å line. The separation motion of two white-light kernels is recorded by the high-cadence GST/TiO images and is well accompanied by the motion of the VIS Hα flare ribbon leading edge. One kernel, located in a 150 Gauss field within a granulation area, exhibited an average apparent motion speed of 55 km s−1, which is the highest average speed ever reported. The other kernel drifted at 9 km s−1 in an 800 Gauss magnetic field area. Hard X-ray (HXR) emissions reaching up to 300 keV have been observed for this flare. The simultaneous occurrence of high-cadence HXR, microwave, and white-light emissions strongly suggests that the energetic particles from the flare directly contribute to the heating. The inverted HXR energy flux density corresponding to 10% TiO brightening is 2.07 ± 0.23 × 1011 erg cm−2 s−1 during the flare peak.

Multi-spacecraft observations of the decay phase of solar energetic particle events

The parameters of solar energetic particle (SEP) event profiles such as the onset time and peak time have been researched extensively to obtain information on the acceleration and transport of SEPs. The corotation of particle-filled magnetic flux tubes with the Sun is generally thought to play a minor role in determining intensity profiles. However recent simulations have suggested that corotation affects the SEP decay phases and depends on the location of the observer with respect to the active region associated with the event. We aim to determine whether signatures of corotation are present in observations of the decay phases of SEP events, and we study the dependence of the parameters of the decay phase on the properties of the flares and coronal mass ejections associated with the events. We analysed multi-spacecraft observations of SEP intensity profiles from 11 events between 2020 and 2022 using data from Solar Orbiter, PSP, STEREO-A, and SOHO. We determined the decay-time constant, τ, in three energy channels; electrons ∼1 MeV, protons ∼25 MeV, and protons ∼60 MeV. We studied the dependence of τ on the longitudinal separation, Δ ϕ, between the source of the active region and the spacecraft magnetic footpoint on the Sun. Individual events show a tendency for the decay-time constant to decrease with increasing Δ ϕ. This agrees with test particle simulations. The magnitude of the event as measured through the intensity of the associated flare and SEP peak flux affects the measured τ values and likely is the cause of the observed large inter-event variability together with the varying solar wind and the conditions in the interplanetary magnetic field. We conclude that corotation affects decay phase of an SEP event and should be included in future simulations and interpretations of these events.

Characterizing Superflares in HR 1099 Using Temporal and Spectral Analysis of XMM-Newton Observations

Abstract In the present paper, we analyze three energetic X-ray flares from the active RS CVn binary HR 1099 using data obtained from XMM-Newton. The flare duration ranges from 2.8 to 4.1 hr, with e-folding rise and decay times in the range of 27–38 minutes and 1.3–2.4 hr, respectively, indicating rapid rise and slower decay phases. The flare frequency for HR 1099 is one flare per rotation period. Time-resolved spectroscopy reveals peak flare temperatures of 39.44, 35.96, and 32.48 MK, emission measures of 7 × 1053–8 × 1054 cm−3, global abundances of 0.250, 0.299, and 0.362 Z ⊙, and peak X-ray luminosities of 1031.21−32.29 erg s−1. The quiescent state is modeled with a three-temperature plasma maintained at 3.02, 6.96, and 12.53 MK. Elemental abundances during quiescent and flaring states exhibit the inverse-first ionization potential (i-FIP) effect. We have conducted a comparative analysis of coronal abundances with previous studies and found evidence supporting the i-FIP effect. The derived flare semi-loop lengths of 6–8.9 × 1010 cm were found to be comparable to the other flares detected on HR 1099; however, they are significantly larger than typical solar flare loops. The estimated flare energies, ranging from 1035.83−37.03 erg, classify these flares as super-flares. The magnetic field strengths of the loops are found to be in the range of 350–450 G. We diagnose the physical conditions of the flaring corona in HR 1099 through the observations of superflares and provide inference on the plasma processes.

Enhanced Peak and Extended Cooling of the Extreme-ultraviolet Late Phase in a Confined Solar Flare

We present observations and analysis of an X1.8 noneruptive solar flare on 2012 October 23, which is characterized by an extremely large late-phase peak seen in the warm coronal extreme-ultraviolet (EUV) emissions (∼3 MK), with the peak intensity over 1.4 times that of the main flare peak. The flare is driven by a failed eruption of a magnetic flux rope, whose strong squeeze force acting on the overlying magnetic structures gives rise to an intense early heating of the late-phase loops. Based on differential emission measure analysis, it is found that the late-phase loops experience a “longer-than-expected” cooling, without the presence of any obvious additional heating, while their volume emission measure maintains a plateau for a long time before turning into an evident decay. Without the need for an additional heating, we propose that the special thermodynamic evolution of the late-phase loops revealed in this flare might arise from loop cross-sectional expansions with height, which are evidenced by both direct measurements from EUV images and by magnetic field extrapolation. By blocking the losses of both heat flux and mass from the corona, such an upward cross-sectional expansion not only elongates the loop-cooling time, but also more effectively sustains the loop density, therefore leading to a later-than-expected occurrence of the warm coronal late phase in combination with a sufficiently high late-phase peak. We further verify such a scenario by analytically solving the cooling process of a late-phase loop characterized by a variable cross section.

Recurring tidal disruption events a decade apart in IRAS F01004-2237

In theory, recurring tidal disruption events (TDEs) may occur when a close stellar binary encounters a supermassive black hole, if one star is captured and undergoes repeating partial TDEs, or if both stars are tidally disrupted (double TDEs). In addition, independent TDEs may be observed over decades in some special galaxies where the TDE rate is extremely high. Exploring the diversity of recurring TDEs and probing their natures with rich observational data helps us to understand these mechanisms. We report the discovery of a second optical flare that occurred in September 2021 in IRAS F01004-2237, where a first flare that occurred in 2010 had already been reported. We also present a detailed analysis of multi-band data. We aim to understand the nature of the flare and explore the possible causes of the recurring flares. We describe our analysis of the position of the flare, the multi-band light curves (LCs), the optical and ultraviolet (UV) spectra, and the X-ray LC and spectra. The position of the flare coincides with the galaxy centre with a precision of 650 pc. The flare peaks in $ days with an absolute magnitude of $ and fades in two years, roughly following $L $. It maintains a nearly constant blackbody temperature of sim 22,000 K in later stages. Its optical and UV spectra show hydrogen and helium broad emission lines with full width at half maxima of 7,000--21,000 km s$^ $ and a He II/Halpha ratio of 0.3--2.3. It shows weak X-ray emission relative to UV emission, with X-ray flares lasting for $<2-3$ weeks, during which the spectrum is soft with a power-law index of $ $. These characters are consistent with a TDE, ruling out the possibilities of a supernova or an active galactic nucleus flare. With a TDE model, we infer a peak UV luminosity of $3.3 $ erg s$^ $ and an energy budget of $4.5 $ erg. A TDE caused the flare that occurred in 2021. The two optical flares separated by $10.3 years can be interpreted as repeating partial TDEs, double TDEs, or two independent TDEs. Although no definitive conclusion can be drawn, the partial TDEs interpretation predicts a third flare around 2033, and the independent TDEs interpretation predicts a high TDE rate of $ $ yr$^ $ in F01004-2237, both of which can be tested by future observations.

An X-ray flaring event and a variable soft X-ray excess in the Seyfert LCRS B040659.9–385922 as detected with eROSITA

Context. Extreme continuum variability in extragalactic nuclear sources can indicate extreme changes in accretion flows onto supermassive black holes. Aims. We explore the multiwavelength nature of a continuum flare in the Seyfert LCRS B040659.9−385922. The all-sky X-ray surveys conducted by the Spectrum Roentgen Gamma (SRG)/extended ROentgen Survey with an Imaging Telescope Array (eROSITA) showed that its X-ray flux increased by a factor of roughly five over six months, and concurrent optical photometric monitoring with the Asteroid Terrestrial-impact Last Alert System (ATLAS) showed a simultaneous increase. Methods. We complemented the eROSITA and ATLAS data by triggering a multiwavelength follow-up monitoring program (X-ray Multi-Mirror Mission: XMM-Newton, Neutron star Interior Composition Explorer: NICER; optical spectroscopy) to study the evolution of the accretion disk, broad-line region, and X-ray corona. During the campaign, X-ray and optical continuum flux subsided over roughly six months. Our campaign includes two XMM-Newton observations, one taken near the peak of this flare and the other taken when the flare had subsided. Results. The soft X-ray excess in both XMM-Newton observations was power law-like (distinctly nonthermal). Using a simple power law, we observed that the photon index of the soft excess varies from a steep value of Γ ∼ 2.7 at the flare peak to a relatively flatter value of Γ ∼ 2.2 as the flare subsided. We successfully modeled the broadband optical/UV/X-ray spectral energy distribution at both the flare peak and post-flare times with the AGNSED model, incorporating thermal disk emission into the optical/UV and warm thermal Comptonization in the soft X-rays. The accretion rate falls by roughly 2.5, and the radius of the hot Comptonizing region increases from the flaring state to the post-flare state. Additionally, from the optical spectral observations, we find that the broad He IIλ4686 emission line fades significantly as the optical/UV/X-ray continuum fades, which could indicate a substantial flare of disk emission above 54 eV. We also observed a redshifted broad component in the Hβ emission line that is present during the high flux state of the source and disappears in subsequent observations. Conclusions. A sudden strong increase in the local accretion rate in this source manifested itself via an increase in accretion disk emission and in thermal Comptonized emission in the soft X-rays, which subsequently faded. The redshifted broad Balmer component could be associated with a transient kinematic component distinct from that comprising the rest of the broad-line region.

Short-wave fadeout on mars: Radio absorption in the dayside martian ionosphere enhanced by solar flares

Solar flares can cause radio absorption in the D region of the Earth’s ionosphere and consequently interrupt high-frequency radio communication systems, also known as short-wave fadeout or the Dellinger effect. We present an analogous radio absorption event observed at Mars during a solar flare. In this event, the Mars Express Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument fortuitously operated at low altitudes on the dayside around the terminator in a favorable configuration for surface echo measurements at the flare peak. The surface echo power during the flare is abnormally weak compared to nominal echo powers at corresponding solar zenith angles, suggesting flare-induced radio absorption in the dayside lower ionosphere of Mars. Additionally, long-term MARSIS data statistically demonstrate the radio absorption dependence on solar soft X-ray fluxes. Our results point to the need for Martian space weather prediction including ionospheric effects on radio waves.

Photospheric Pore Rotation Associated with a C-class Flare from Spectropolarimetric Observations with DKIST

We present high-resolution observations of a C4.1-class solar flare (SOL2023-05-03T20:53) in AR 13293 from the Visible Spectro-Polarimeter (ViSP) and Visible Broadband Imager (VBI) instruments at the DKIST. The fast cadence, good resolution, and high polarimetric sensitivity of ViSP data provide a unique opportunity to explore the photospheric magnetic fields before and during the flare. We infer the magnetic field vector in the photosphere from the Fe i 6302 Å line using Milne–Eddington inversions. Combined analysis of the inverted data and VBI images reveals the presence of two opposite polarity pores exhibiting rotational motion both prior to and throughout the flare event. Data-driven simulations further reveal a complex magnetic field topology above the rotating pores, including a null-point-like configuration. We observed a 30% relative change in the horizontal component (δ F h ) of Lorentz force at the flare peak time and roughly no change in the radial component. We find that the changes in δ F h are the most likely driver of the observed pore rotation. These findings collectively suggest that the back reaction of magnetic field line reconfiguration in the corona may influence the magnetic morphology and rotation of pores in the photosphere on a significantly smaller scale.

Modeling Time-variable Elemental Abundances in Coronal Loop Simulations

Numerous recent X-ray observations of coronal loops in both active regions and solar flares have shown clearly that elemental abundances vary with time. Over the course of a flare, they have been found to move from coronal values toward photospheric values near the flare peak, before slowly returning to coronal values during the gradual phase. Coronal loop models typically assume that the elemental abundances are fixed, however. In this work, we introduce a time-variable abundance factor into the 0D ebtel++ code that models the changes due to chromospheric evaporation in order to understand how this affects coronal loop cooling. We find that for strong heating events (≳1 erg s−1 cm−3), the abundances quickly tend towards photospheric values. For smaller heating rates, the abundances fall somewhere between coronal and photospheric values, causing the loop to cool more quickly than the time-fixed photospheric cases (typical flare simulations) and more slowly than time-fixed coronal cases (typical AR simulations). This suggests heating rates in quiescent AR loops no larger than ≈0.1 erg s−1 cm−3 to be consistent with recent measurements of abundance factors f ≳ 2.

Numerous bidirectionally propagating plasma blobs near the reconnection site of a solar eruption

A current sheet is a common structure involved in solar eruptions. However, it is observed in a minority of the events, and the physical properties of its fine structures during a solar eruption are rarely investigated. Here, we report an on-disk observation that displays 108 compact, circular, or elliptic bright structures, presumably plasma blobs, propagating bidirectionally along a flare current sheet during a period of ∼24 min. Using extreme ultraviolet images, we investigated the temporal variation of the blob number around the flare’s peak time. The current sheet connects the flare loops and the erupting filament. The width, duration, projected velocity, temperature, and density of these blobs are ∼1.7 ± 0.5 Mm, ∼79 ± 57 s, ∼191 ± 81 km s−1, ∼106.4 ± 0.1 K, and ∼1010.1 ± 0.3 cm−3, respectively. The reconnection site rises with a velocity of ≤69 km s−1. The observational results suggest that plasmoid instability plays an important role in the energy-release process of solar eruptions.

The impact of moderate solar flare activity on ionospheric response from august 5th to 7th, 2023

This study investigates the ionospheric response to a period of heightened solar flare activity from August 5th to August 7th, 2023, by analysing ground-based observations of various ionospheric parameters. The data includes detailed information on solar flare events, including their class, start time, maximum time, and end time, along with solar activity indicators such as sunspot numbers, solar radio flux, and solar zenith angles. On August 5th, multiple C-class, M-class, and an X1.63 flare were recorded, with the X-class flare being the most intense event. Ionospheric measurements revealed enhancements in electron temperatures, reaching a peak of 2104K at 6 UT, coinciding with the onset of an M1.6 flare. Additionally, a pronounced increase in total electron content (TEC) was observed, peaking at 34 TEC units at 6 UT, suggesting increased ionization due to the flare's influence. On August 6th, an M5.51 flare was the most significant event. Notably, the peak electron temperature of 2097K and the TEC maximum of 35.9 units were recorded at 6 UT, several hours before the flare's maximum phase, indicating potential preconditioning effects from the flare's preparatory stages. August 7th witnessed an X1.51 flare, along with multiple M-class flares. The peak electron temperature of 2088K and the TEC maximum of 35.3 units were observed at 7 UT, again preceding the X1.51 flare's peak, suggesting the influence of precursor effects. Throughout the observation period, the data exhibited typical diurnal patterns in ion temperatures and TEC, consistent with regular ionospheric behavior driven by solar radiation intensity variations. The study's findings provide evidence of the ionosphere's responsive nature to intense solar flare activity, with significant enhancements in electron temperatures and TEC observed during flare events. Notably, the effects were evident not only during the peak flare phases but also several hours prior, potentially due to the arrival of energetic particles or precursor electromagnetic radiation, indicating the influence of preconditioning processes. Furthermore, the data suggests that solar flares can impact the ionosphere's chemistry and dynamics, as inferred from changes in the "Ion Percentage" parameter, representing relative ionospheric compositions. Continuous monitoring and analysis of such events across different geophysical conditions are crucial for advancing the understanding of solar-terrestrial interactions and their impacts on space-based technologies, contributing to the development of improved space weather prediction and mitigation strategies.

Evolution of the Ratio of Mg ii Intensities during Solar Flares

The Mg ii k and h line intensity ratios can be used to probe the characteristics of the plasma in the solar atmosphere. In this study, using the observations recorded by the Interface Region Imaging Spectrometer, we study the variation of the Mg ii k and h intensity ratio for three flares belonging to X-class, M-class, and C-class, throughout their evolution. We also study the k-to-h intensity ratio as a function of magnetic flux density obtained from the line-of-sight magnetograms recorded by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. Our results reveal that, while the intensity ratios are independent of magnetic flux density, they show significant changes during the evolution of the C-class and M-class flares. The intensity ratios start to increase at the start of the flare and peak during the impulsive phase before the flare peak and decrease rapidly thereafter. The values of the ratios fall even below the preflare level during the peak and decline phases of the flare. These results are important in light of heating and cooling of localized plasma and provide further constraint on the understanding of flare physics.

Observations of Geomagnetic Crochet at High‐Latitudes Due To X1.5 Class Solar Flare on 3 July 2021

AbstractOn 3 July 2021, an X1.5 solar flare from the National Oceanic and Atmospheric Administration solar Active Region AR12838 (24°N, 88°W) occurred at 14:18 UT, peaked at 14:29 UT, and decayed at 14:34 UT. The study of this X1.5 solar flare is significant due to its unique geomagnetic crochet feature at high latitudes and its effective signature on Earth. The study examined X‐rays, the extreme ultraviolet spectrum, ionospheric equivalent current (IEC), and geomagnetic field components. The study reveals a sudden increase in IEC during the X1.5 flare episode, forming a zonal current region and producing a geomagnetic crochet signature in geomagnetic field components at high latitudes (50°–80°N) along the 11°–26°E longitude sector during the flare peak time. All three geomagnetic field components show different sensitivity to the solar flare effect (sfe), and the amplitude and phase of the geomagnetic crochet across latitudes (for a given longitude) are consistent with the variations in the IEC. The present study is the first to appraise geomagnetic crochets of low magnitude (8–40 nT) and short duration (10–15 min) at high latitudes, particularly in the polar cusp region, during the X‐class limb flare.

Thermal instability in the impulsive phase of solar flares with sub-THz component

AbstractCoronal rain is formed in the post-impulsive phase of solar flares due to the thermal instability of coronal plasma in EUV loops. As a result, the sub-terahertz (sub-THz) emission flux in the post-impulsive phase of solar flares can be increased due to the increasing of the optical thickness of the thermal source. This suggests that sub-THz observations can be used as a diagnostic tool for coronal rain.This work is aimed to analyse the relationship between the sub-THz radiation and variations of the temperature and the emission measure of the EUV coronal plasma during the post-impulsive phase of the SOL2022-05-04T08:45 solar flare.Based on the two-dimensional temperature and emission measure distributions obtained from the AIA/SDO EUV intensity data, it was found that the temperature decreases whereas the emission measure reaches the maximum near the sub-THz flare peak. This circumstance and peculiarities of the radiation time profiles in different wave ranges show evidence in favor of the significant contribution of the thermal coronal loop plasma to the flare sub-THz radiation at least for some flare events. The sub-THz emission may be associated with a coronal condensation, accompanied by the formation of coronal rain.

Multiwavelength Observations of a B-class Flare Using XSM, AIA, and XRT

We present multiwavelength observations by Chandrayaan-2/Solar X-ray Monitor, Solar Dynamics Observatory/Atmospheric Imaging Assembly, and Hinode/X-Ray Telescope (XRT) of a B-class flare observed on 2021 February 25, originating from an active region (AR 12804) near the northwest limb. The microflare lasts for ∼30 minutes and is composed of hot loops reaching temperatures of 10 MK. We report excellent agreement (within 20%) for the average effective temperatures obtained at the flare peak from all the three instruments, which have different temperature sensitivities. The XRT filter combination of Be-thin and Be-med provides an excellent opportunity to measure the high temperatures in such microflare events. The elemental abundances during the evolution of the microflare are also studied and observed to drop toward photospheric values at the flare peak time, compared to coronal values during the rise and decay phase. This is consistent with previous XSM studies.

Intensity and velocity oscillations in a flaring active region

ABSTRACT Chromospheric oscillations can give us insight into the physical environment in the solar atmosphere, both in quiet Sun and flaring conditions. Many authors have reported increases in the prevalence of 3-minute oscillations which are thought to be excited by events which impact the chromosphere such as flares. In this study, we utilized the Ca ii 8542 Å line to study the oscillatory behaviour of the chromosphere in an active region which underwent two B-class flares. We analysed oscillations in both intensity and velocity, and found different behaviours in both. Intensity oscillations were most prevalent over the umbrae of sunspots and magnetic pores in the active region, and the extent of the area which contained significant oscillations was found to decrease when comparing times after the flares to before. By measuring the evolution of the magnetic field, we found that this could be because the areas surrounding the umbrae were becoming more ‘penumbral’ with an increase to the magnetic field inclination. Velocity oscillations were found across the active region both before and after the flares but were observed clearly in areas which were brightened by the second flare. By comparing to EUV imaging, it was seen that strong chromospheric velocity oscillations with 3–4-minute periods occurred at the same time and location as a flare loop cooling 30 min after the second flare peak. This could be evidence of disturbances in the loop exciting a response from the chromosphere at its acoustic cut-off frequency.

Eruption of prominence initiated by loss of equilibrium: multipoint observations

ABSTRACT Using the SDO/AIA, SOHO/LASCO, STEREO/SECCHI, and ground-based H α, radio observations, we studied a prominence eruption (PE) from the western limb that occurred on 2013 December 4. PE is associated with a moderate coronal mass ejection (CME) and GOES class C4.7 flare. Before a couple of days, the prominence pre-existed as an inverse-S shaped filament lying above fragmented opposite polarities between two active regions. Initially, the prominence appears as kinked or writhed as observed from different vantage points. From a careful study of magnetic field observations, we infer that the flux emergence at one leg of the prominence causes the loss of equilibrium which then initiates the slow upward motion of the prominence followed by onset of the eruption at a projected height of 35 Mm. The fast rise motion is also in synchronization with the flare impulsive phase but the average acceleration is quite small (150 ms−2) compared to strong flare cases. In the LASCO field of view (FOV), the CME continues to accelerate at 3 ms−2 attaining a speed of 450 km s−1 at 16 R⊙. In the extended STEREO-A FOV upto 38 R⊙, the CME decelerates 0.82 m s−2. The PE launched type III bursts delayed by 14 min with respect to the flare peak time (04:58 UT). Since the prominence is lying in the fragmented polarities, it is likely that the sheared arcade has little contribution to the poloidal flux of the rising magnetic flux rope and subsequently weak flare is recorded. This study of PE emphasizes the influence of the magnetic reconnection on the CME speed, launch of type II, III burst, and the CME propagation distance farther away from the Sun.

The Impulsive Acceleration of a Solar Filament Eruption Associated with a B-class Flare

The eruption of magnetic flux ropes (MFRs), often taking filaments together, leads to coronal mass ejections (CMEs). Theoretical studies propose that both the resistive magnetic reconnection and the ideal instability of an MFR system can release magnetic-free energy and accelerate CMEs (i.e., MFRs or filaments) during eruptions. Observations find that the full kinematic evolution of CMEs usually undergoes three phases: the initiation phase, impulsive acceleration phase, and propagation phase. The impulsive acceleration phase often starts and ceases simultaneously with the flare onset time and peak time, respectively. This synchronization can be explained by the positive feedback relationship between the acceleration of CMEs and flare magnetic reconnection, and suggests that the reconnection has the dominant contribution to the acceleration of CMEs. It is rare to see strong evidence that supports the dominant contribution of ideal instability to the acceleration. In this paper, we report an intriguing filament eruption that occurred on 2011 May 11. Its complete acceleration is well recorded by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory. The kinematic analysis shows that the impulsive acceleration phase starts and ceases obviously earlier than the flare onset time and peak time, respectively, which means a complete asynchronization between the impulsive acceleration phase and flare rise phase, and strongly supports that the ideal instability plays a dominant role in this impulsive acceleration. Furthermore, the accompanied flare is a B-class one, also implying that the contribution of reconnection is negligible in the energy release process.

Study of the energetic X-ray superflares from the active fast rotator AB doradus

ABSTRACT We present the analyses of intense X-ray flares detected on the active fast rotator AB Dor using observations from the XMM–Newton. A total of 21 flares are detected, and 13 flares are analysed in detail. The total X-ray energy of these flares is found to be in the range of 1034−36 erg, in which the peak flare flux increased up to 34 times from the pre-/post-flaring states for the strongest observed flare. The duration of these flaring events is found to be 0.7 to 5.8 h. The quiescent state X-ray spectra are found to be explained by a three-temperature plasma with average temperatures of 0.29, 0.95, and 1.9 keV, respectively. The temperatures, emission measures, and abundances are found to be varying during the flares. The peak flare temperature was found in the 31–89 MK range, whereas the peak emission measure was 1052.5–54.7 cm−3 . The abundances vary during the flares and increase by a factor of ∼3 from the quiescent value for the strongest detected flare. The variation in individual abundances follows the inverse-FIP effect in quiescent and flare phases. The X-ray light curves of AB Dor are found to exhibit rotational modulation. The semi-loop lengths of the flaring events are derived in the range of 109.9−10.7 cm, whereas the minimum magnetic field to confine the plasma in the flaring loop is estimated between 200 and 700 G.

Estimations of Elemental Abundances during Solar Flares Observed in Soft X-Rays by the MinXSS-1 CubeSat Mission

Solar flares are complex phenomena emitting all types of electromagnetic radiation and accelerating particles on timescales of minutes, converting magnetic energy to thermal, radiative, and kinetic energy through magnetic reconnections. As a result, local plasma can be heated to temperatures in excess of 20 MK. During the soft X-ray (SXR) solar flare peak, the elemental abundance of low first ionization potential elements are typically observed to be depleted from coronal values. We explored the abundance variations using disk-integrated solar spectra from the Miniature X-ray Solar Spectrometer CubeSat-1 (MinXSS-1). MinXSS-1 is sensitive to the 1–12 keV energy range with an effective 0.25 keV FWHM resolution at 5.9 keV. During the year-long mission of MinXSS-1, between 2016 May and 2017 May, 21 flares with intermittent data downlinks were observed ranging from C to M class. We examine the time evolution of temperature, volume emission measure, and elemental abundances of Fe, Ca, Si, S, and Ar with CHIANTI spectral models near the peak SXR emission times observed in the MinXSS-1 data. We determined the average absolute abundance of A(Fe) = 7.81, A(Ca) = 6.84, A(S) = 7.28, A(Si) = 7.90, and A(Ar) = 6.56. These abundances are depleted from coronal values during the SXR peak compared to nonflaring times. The elemental abundance values that are depleted from their coronal values are consistent with the process of chromospheric evaporation, in which the lower atmospheric plasma fills the coronal loops.