Phase Of Flare Research Articles

The Atmospheric Response to High Nonthermal Electron-beam Fluxes in Solar Flares. II. Hydrogen-broadening Predictions for Solar Flare Observations with the Daniel K. Inouye Solar Telescope

Abstract Redshifted components of chromospheric emission lines in the hard X-ray impulsive phase of solar flares have recently been studied through their 30 s evolution with the high resolution of the Interface Region Imaging Spectrograph. Radiative-hydrodynamic flare models show that these redshifts are generally reproduced by electron-beam-generated chromospheric condensations. The models produce large ambient electron densities, and the pressure broadening of the hydrogen Balmer series should be readily detected in observations. To accurately interpret the upcoming spectral data of flares with the DKIST, we incorporate nonideal, nonadiabatic line-broadening profiles of hydrogen into the RADYN code. These improvements allow time-dependent predictions for the extreme Balmer line wing enhancements in solar flares. We study two chromospheric condensation models, which cover a range of electron-beam fluxes (1 − 5 × 1011 erg s−1 cm−2) and ambient electron densities (1 − 60 × 1013 cm−3) in the flare chromosphere. Both models produce broadening and redshift variations within 10 s of the onset of beam heating. In the chromospheric condensations, there is enhanced spectral broadening due to large optical depths at Hα, Hβ, and Hγ, while the much lower optical depth of the Balmer series H12−H16 provides a translucent window into the smaller electron densities in the beam-heated layers below the condensation. The wavelength ranges of typical DKIST/ViSP spectra of solar flares will be sufficient to test the predictions of extreme hydrogen wing broadening and accurately constrain large densities in chromospheric condensations.

No Such Thing as a Simple Flare: Substructure and Quasi-periodic Pulsations Observed in a Statistical Sample of 20 s Cadence TESS Flares

Abstract A 20 s cadence Transiting Exoplanet Survey Satellite monitoring campaign of 226 low-mass flare stars during Cycle 3 recorded 3792 stellar flares of ≥1032 erg. We explore the time-resolved emission and substructure in 440 of the largest flares observed at high signal-to-noise, 97% of which released energies of ≥1033 erg. We discover degeneracy present at 2 minute cadence between sharply peaked and weakly peaked flares is common, although 20 s cadence breaks these degeneracies. We better resolve the rise phases and find 46% of large flares exhibit substructure during the rise phase. We observe 49 candidate quasi-periodic pulsations (QPP) and confirm 17 at ≥3σ. Most of our QPPs have periods less than 10 minutes, suggesting short-period optical QPPs are common. We find QPPs in both the rise and decay phases of flares, including a rise-phase QPP in a large flare from Proxima Cen. We confirm that the Davenport et al. template provides a good fit to most classical flares observed at high cadence, although 9% favor Gaussian peaks instead. We characterize the properties of complex flares, finding 17% of complex flares exhibit “peak-bump” morphologies composed of a large, highly impulsive peak followed by a second, more gradual Gaussian peak. We also estimate the UVC surface fluences of temperate planets at flare peak and find one-third of 1034 erg flares reach the D90 dose of Deinococcus radiodurans in just 20 s in the absence of an atmosphere.

Statistical Analyses of the Energies of X-Ray Plateaus and Flares in Gamma-Ray Bursts

Abstract Distinct X-ray plateau and flare phases have been observed in the afterglows of gamma-ray bursts (GRBs), and most of them should be related to central engine activities. In this paper, we collect 174 GRBs with X-ray plateau phases and 106 GRBs with X-ray flares. There are 51 GRBs that overlap in the two selected samples. We analyze the distributions of the proportions of the plateau energy E plateau and the flare energy E flare relative to the isotropic prompt emission energy E γ,iso. The results indicate that they well meet the Gaussian distributions and the medians of the logarithmic ratios are ∼−0.96 and −1.39 in the two cases. Moreover, strong positive correlations between E plateau (or E flare ) and E γ,iso with slopes of ∼0.95 (or ∼0.80) are presented. For the overlapping sample, the slope is ∼0.80. We argue that most of X-ray plateaus and flares might have the same physical origin but appear with different features because of the different circumstances and radiation mechanisms. We also test the applicabilities of two models, i.e., black holes surrounded by fractured hyperaccretion disks and millisecond magnetars, on the origins of X-ray plateaus and flares.

The Precursor Phase of an X-class Flare: Magnetic Reconnection, Powering and Non-thermal Electrons

In this paper, we report three interesting phenomena that occurred during the precursor phase of the X1.6 class flare on 2014 September 10. (1) The magnetic reconnection initiating the flare occurs between one of the two J-shaped magnetic flux ropes that constitute a sigmoidal structure and the overlying sheared magnetic arcade that runs across the sigmoid over its middle part. The reconnection formed an erupting structure that ultimately leads to flare onset. Another J-shaped magnetic flux rope remains unaffected during the whole eruption. The phenomenon is revealed by the observation made by the Atmospheric Imaging Assembly on board the Solar Dynamic Observatory (SDO) at 94 and 131 Å. (2) Being simultaneously with starting time of the precursor, photospheric vertical electric current (VEC) around the footpoint region of the overlying magnetic arcade underwent an obvious increase, as observed by the Helioseismic and Magnetic Imager (HMI) on board SDO. By only taking into account the VEC with current density over 3σ value (1σ: 10 mA m−2), we are able to pick out precursor-associated VEC increase starting from nearly the level of zero. We regard it as a kind of powering process for the magnetic reconnection between the two magnetic loops. (3) With high-resolution narrow-band Helium 10830 Å images taken by Goode Solar Telescope at Big Bear Solar Observatory (BBSO), we observe a narrow absorption (dark) front that runs along the erupting magnetic structure (or the erupting hot channel) and moves in the direction of the eruption during the precursor phase. Assuming the excitation mechanism of Helium atoms along the absorption front by non-thermal electrons, the phenomenon shows that the interaction between the erupted hot channel and the overlying (or surrounding) magnetic field has yielded electron acceleration.

Pitch-Angle Anisotropy Controls Particle Acceleration and Cooling in Radiative Relativistic Plasma Turbulence.

Nature's most powerful high-energy sources are capable of accelerating particles to high energy and radiating it away on extremely short timescales, even shorter than the light crossing time of the system. It is yet unclear what physical processes can produce such an efficient acceleration, despite the copious radiative losses. By means of radiative particle-in-cell simulations, we show that magnetically dominated turbulence in pair plasmas subject to strong synchrotron cooling generates a nonthermal particle spectrum with a hard power-law range (slope p∼1) within a few eddy turnover times. Low pitch-angle particles can significantly exceed the nominal radiation-reaction limit, before abruptly cooling down. The particle spectrum becomes even harder (p<1) over time owing to particle cooling with an energy-dependent pitch-angle anisotropy. The resulting synchrotron spectrum is hard (νF_{ν}∝ν^{s} with s∼1). Our findings have important implications for understanding the nonthermal emission from high-energy astrophysical sources, most notably the prompt phase of gamma-ray bursts and gamma-ray flares from the Crab nebula.

Reconstructing and quantifying methane emissions from the full duration of a 38-day natural gas well blowout using space-based observations

We quantify and characterize methane emissions from a natural gas well blowout in Louisiana, USA in 2019 using TROPOMI (TROPOspheric Monitoring Instrument) methane observations and VIIRS (Visible Infrared Imaging Radiometer Suite) gas flare detections. We use TROPOMI data in a Bayesian inversion to optimize daily methane emissions while VIIRS-based radiant heat data are used to estimate the amount of gas flared. This hybrid approach allows for the characterization of methane emissions during both the flaring and venting phases of the blowout, where more than half of the blowout duration was associated with flaring. We estimate overall emissions of this 38-day event at 49 (21–63) Gg, which is lower than reported by the gas company but exceeds all known US point sources emitting at the time in terms of emission rate. We estimate that over 80% of total emissions come from the venting phase, leaving the flaring phase responsible for only a small fraction of total emissions. We show that routine satellite operations from global-scale observing instruments such as TROPOMI and VIIRS when combined can provide a detailed assessment of quantitative greenhouse gas emissions from large point sources including stochastic events and thus these data could be used for regulatory enforcement.

A Search for Planetary Metastable Helium Absorption in the V1298 Tau System

Abstract Early in their lives, planets endure extreme amounts of ionizing radiation from their host stars. For planets with primordial hydrogen and helium-rich envelopes, this can lead to substantial mass loss. Direct observations of atmospheric escape in young planetary systems can help elucidate this critical stage of planetary evolution. In this work, we search for metastable helium absorption—a tracer of tenuous gas in escaping atmospheres—during transits of three planets orbiting the young solar analog V1298 Tau. We characterize the stellar helium line using HET/HPF, and find that it evolves substantially on timescales of days to months. The line is stable on hour-long timescales except for one set of spectra taken during the decay phase of a stellar flare, where absoprtion increased with time. Utilizing a beam-shaping diffuser and a narrowband filter centered on the helium feature, we observe four transits with Palomar/WIRC: two partial transits of planet d (P = 12.4 days), one partial transit of planet b (P = 24.1 days), and one full transit of planet c (P = 8.2 days). We do not detect the transit of planet c, and we find no evidence of excess absorption for planet b, with ΔR b/R ⋆ &lt; 0.019 in our bandpass. We find a tentative absorption signal for planet d with ΔR d/R ⋆ = 0.0205 ± 0.054, but the best-fit model requires a substantial (−100 ± 14 minutes) transit-timing offset on a two-month timescale. Nevertheless, our data suggest that V1298 Tau d may have a high present-day mass-loss rate, making it a priority target for follow-up observations.

Detection of Flare Multiperiodic Pulsations in Mid-ultraviolet Balmer Continuum, Lyα, Hard X-Ray, and Radio Emissions Simultaneously

Abstract Quasi-periodic pulsations (QPPs), which usually appear as temporal pulsations of the total flux, are frequently detected in the light curves of solar/stellar flares. In this study, we present the investigation of nonstationary QPPs with multiple periods during the impulsive phase of a powerful flare on 2017 September 6, which were simultaneously measured by the Hard X-ray Modulation Telescope (Insight-HXMT), as well as the ground-based BLENSW. The multiple periods, detected by applying a wavelet transform and Lomb–Scargle periodogram to the detrended light curves, are found to be ∼20–55 s in the Lyα and mid-ultraviolet Balmer continuum emissions during the flare impulsive phase. Similar QPPs with multiple periods are also found in the hard X-ray emission and low-frequency radio emission. Our observations suggest that the flare QPPs could be related to nonthermal electrons accelerated by the repeated energy release process, i.e., triggering of repetitive magnetic reconnection, while the multiple periods might be modulated by the sausage oscillation of hot plasma loops. For the multiperiodic pulsations, other generation mechanisms could not be completely ruled out.

Identification and Mapping of HBsAg Loss-Related B-Cell Linear Epitopes in Chronic HBV Patients by Peptide Array.

Identification of immunogenic targets against hepatitis B virus (HBV)-encoded proteins will provide crucial advances in developing potential antibody therapies. In this study, 63 treatment-naïve patients with chronic HBV infection and 46 patients who achieved hepatitis B surface antigen loss (sAg loss) following antiviral treatment were recruited. Moreover, six patients who transitioned from the hepatitis B e antigen-positive chronic infection phase (eAg+CInf) to the hepatitis phase (eAg+CHep) were enrolled from real-life clinical practice. Additionally, telbivudine-treated eAg+CHep patients and relapsers or responders from an off-treatment cohort were longitudinally studied. The frequencies and function of B cells were assessed by flow cytometry. We devised a peptide array composed of 15-mer overlapping peptides of HBV-encoded surface (S), core (C), and polymerase (P) proteins and performed a screening on B-cell linear epitopes with sera. Naïve B cells and plasmablasts were increased, whereas total memory, activated memory (AM), and atypical memory (AtM) B cells were reduced in sAg- patients compared with sAg+ patients. Importantly, longitudinal observations found that AtM B cells were associated with successful treatment withdrawal. Interestingly, we identified six S-specific dominant epitopes (S33, S34, S45, S76, S78, and S89) and one C-specific dominant epitope (C37) that reacted with the majority of sera from sAg- patients. Of note, more B-cell linear epitopes were detected in CHep patients with alanine aminotransferase (ALT) flares than in nonflare CInf patients, and five B-cell linear epitopes (S4, S5, S10, S11, and S68) were overwhelmingly recognized by ALT flare patients. The recognition rates of epitopes on C and P proteins were significantly increased in CHep patients relative to CInf patients. Strikingly, a statistically significant elevation in the number of positive epitopes was observed when ALT nonflare patients shifted into the flare phase. Moreover, S76 identified at baseline was confirmed to be associated with a complete response after 48 weeks of telbivudine therapy. Taken together, we identified several functional cure-related B-cell linear epitopes of chronic HBV infection, and these epitopes may serve as vaccine candidates to elicit neutralizing antibodies to treat HBV infection.

Multi-Wavelength Study of 4C+28.07

Abstract 4C+28.07 is a γ-ray flat-spectrum-radio-quasar-type source. It is often monitored at different frequencies, though long-term multi-wavelength data of this source have not been modeled in detail before. We have analyzed ∼12 yr (2008 August–2020 May) of Fermi-LAT data with a binning of 10 day timescale and observed three distinctive flaring states. Each flaring state consists of different phases of activity, namely, pre-flare, flare, and post-flare regions. γ-ray spectral analysis of these different activity phases has been performed and the best-fit model for its spectra is found to be a log-parabola model. We have also studied the correlation of simultaneous γ-ray light curves with the optical &amp; radio counterparts in these flaring states and report the DCF with 95% significance level. A large time delay is found between radio and gamma-ray data for two flares, indicating two zones of emission. We have fitted the multi-wavelength data with a two-zone leptonic model. In our two-zone leptonic model the maximum required power in the jet is 9.64 × 1046 erg s−1, which is lower than its Eddington luminosity 2.29 × 1047 erg s−1.

The Relationship between Platelet to Lymphocyte Ratio and Platelet Indices with Disease Severity Level of Systemic Lupus Erythematosus

Systemic Lupus Erythematosus (SLE) is an episodic, chronic autoimmune inflammatory disease characterized by remission and flare phases. Laboratory parameters required to assess the severity of disease activity in SLE include platelet count and platelet indices. Several studies regarding the Platelet to Lymphocyte Ratio (PLR) and platelet indices on the severity of SLE patients remain inconsistent. This study aimed to evaluate the relationship between PLR value and platelet index with the degree of disease severity in SLE patients. This study used a retrospective analytic observational design in SLE patients from January 2016 to December 2019 at Dr. Sardjito Central Hospital. Disease severity was assessed using the Systemic Lupus Erythematosus Disease Activity Index (SLEDAI) score. Platelet to Lymphocyte Ratio (PLR) values and platelet indices were measured with a hematology analyzer. The data were analyzed using correlation, bivariate, multiple regression tests, and the ROC curve to determine the PLR cut-off. There were 55 SLE patients with high activity (SLEDAI 11-19; n=30(54,54%)) and very high activity (SLEDAI 20; n=25(45.45%)). There was a significant correlation (p &lt;0.05) between the PLR value, platelet count, plateletcrit, and Mean Platelet Volume (MPV) with SLEDAI scores (p &lt;0.05), but only the MPV variable was significant as an independent variable (p=0.0357). In the ROC curve, a cut-off PLR value of 124 was obtained with a sensitivity of 68.0%, specificity of 66.7%, likelihood ratio=2.04 (AUC=0.659 with p-value=0.035) to detect very high disease activity. Based on the PLR value, platelet count and plateletcrit negatively correlated with SLEDAI score but were related to the very high degree of thrombocytopenia in disease activity. The MPV value reflected the high platelet turnover, which had a positive correlation with the SLEDAI score. Patients with a PLR value ≤124 were 2.04 times more likely to have a SLEDAI score of 20, indicating potential use as a predictor of disease activity. The PLR value and platelet indices were significantly related to the degree of SLE activity.

Analysis of Type II radio burst relationship with CME driven shocks

Type II radio bursts are known to be the signatures of coronal shocks. In this paper we examine the relationship between 129 type II bursts in the frequency range 35 – 450 MHz observed at Culgooora observatory during May 2002 – October 2015 and the associated CMEs. We apply Newkirk (1961) density model to determine the formation height of type IIs. We find that in 109/129 cases, type II bursts were preceded/ succeeded by CMEs. The CME associated type II events in which the CME height is above the type II burst source are categorized as group I events (91/129 cases). 91% of the bursts in this group are also associated with flares and 58% of these bursts originate during decaying phase of the flare. The correlation between CME speed and type II shock speed for limb events in this group is 0.33.The CME associated type IIs in which the CME height is below the type II source are categorized as group II (18/129 cases). CME driven shock could have been the exciter of these type II bursts.88% of this group events are associated with flares and 62% of these bursts originate during the rising phase of the flare. The correlation between CME speed and type II shock speed for limb events in this group is 0.96. In 20/129 cases of our data set, type II bursts are not associated with CME and are categorized as group III. 90% of the bursts in this group are associated with flares. 77% of the bursts in the group are originating in the decaying phase of flares. Poor temporal association (9/69 cases) between type IIs and flares of X class during this period. Our results suggest that inspite of temporal association with metric type II bursts, majority of the CME driven shocks (84%) are not successful in exciting type II bursts in 35-450 MHz domain. The type II bursts temporally correlated with CMEs and likely to have been excited by CMEs (type II height &gt; CME height) are originating during the rising phase of the flares in majority of the events. In case of type II bursts temporally correlated with CMEs supposedly not excited by the CMEs (type II height &lt; CME height) ,majority of them are originating in the decaying phase of flares.

The lived experiences of people with inflammatory bowel diseases: A phenomenological hermeneutic study.

The aim of this research was to provide an in-depth study about the life experiences of patients with inflammatory bowel diseases. The phenomenological (hermeneutic) method was used to analyse data. The data were collected between March 25 and July 1, 2019. A homogeneous group of 25 patients were included as participants by purposeful sampling methods. Interviews were conducted in the hospital patient-education room. The results of this study emphasize the importance of understanding the fears, worries, difficulties and life experiences of patients with inflammatory bowel diseases. Three main themes were identified: a flare phase of the disease, a remission period and coping behaviours. Study findings included the experiences of social isolation; inability to perform daily activities of living and to work; stress in the flare phase of the disease; concerns about the future in the remission phase and physical and spiritual strategies used for coping. Coping with the physical symptoms and emotional difficulties of inflammatory bowel diseases can prevent patients from living a normal life. The results of this article highlight the importance of understanding the fear, anxiety, difficulties and life experiences of these patients.

Solar flares and geomagnetic storms of September 2017: Their impacts on the TEC over 75°E longitude sector

• Solar flare effect on TEC at a specific longitude sector is studied. • A special case of co-occurrence of geomagnetic storm and solar flare is examined. • A delayed TEC response is observed during recovery phase of solar flare. This study investigates the ionospheric Total Electron Content (TEC) responses over 75°E longitude to the solar flares and geomagnetic storms of September 6–9, 2017. The results of this study provide the impacts of solely solar flares on the ionosphere and such impact when the effects of solar flares and geomagnetic storm are combined. On September 6, two X class solar flares, namely X2.2 at 0857 UT and X9.3 at 1153 UT, were recorded with quiet geomagnetic conditions. The EUV/X-ray intensity of X9.3 flare was significantly greater than that of X2.2 flare, and the recovery phase of both the flares was slower than their respective impulsive phase. The slower recovery rate in EUV/X-ray intensity is reflected as a delayed TEC response. A nearly 8% higher crest to trough TEC change on flare day than the pre-flare day suggests an enhanced level of the equatorial electrojet. The overall weak TEC response to X9.3 solar flare is attributed to solar zenith angle dependency and shifting of solar flare location from disk center to west limb. The solar flares on September 7–8 were co-occurred with geomagnetic storms and observed large increments in TEC are additionally induced by prompt penetration electric field and the enhanced level of thermospheric compositional changes. On September 9, an increase in TEC is observed during M class solar flares under effect of solar flares and disturbed dynamo electric field.

The effect of a solar flare on chromospheric oscillations

ABSTRACT Oscillations in the solar atmosphere have long been observed both in quiet conditions and during solar flares. The chromosphere is known for its 3-min signals, which are strong over sunspot umbrae, and have periods determined by the chromosphere’s acoustic cut-off frequency. A small number of observations have shown the chromospheric signals to be affected by energetic events such as solar flares, however the link between flare activity and these oscillations remains unclear. In this work, we present evidence of changes to the oscillatory structure of the chromosphere over a sunspot which occurs during the impulsive phase of an M1 flare. Using imaging data from the CRISP instrument across the H α and Ca ii 8542 Å spectral lines, we employed a method of fitting models to power spectra to produce maps of where there is evidence of oscillatory signals above a red-noise background. Comparing results taken before and after the impulsive phase of the flare, we found that the oscillatory signals taken after the start of the flare differ in two ways: the locations of oscillatory signals had changed and the typical periods of the oscillations had tended to increase (in some cases increasing from &amp;lt;100 s to ∼200 s). Both of these results can be explained by a restructuring of the magnetic field in the chromosphere during the flare activity, which is backed up by images of coronal loops showing clear changes to magnetic connectivity. These results represent one of the many ways that active regions can be affected by solar flares.

Comparison of physical conditions in two phases of the solar flare of July 19, 2000 of M6.4/3N class

We study the solar flare of July 19, 2000 of M6.4/3N class, which occurred in the active region NOAA 9087. Echelle Zeeman spectrograms of this flare were obtained with the horizontal solar telescope of the Astronomical Observatory of Taras Shevchenko National University of Kyiv. Physical conditions were compared for two times corresponding to the pre-peak and peak phases of the flare in area of a small sunspot. It was found that the effective magnetic field $B$ eff by Fe I 6301.5 and Fe I 6302.5 lines was almost the same for both times and both spectral lines. However, $B$ eff in peak phase measured by the splitting of the emission peaks of Fe II and H $\upbeta $ lines turned out to be 300–500 G stronger than those by the above-mentioned Fe I lines, whereas by D3 He I line it was found to be close to zero. This may indicate a local magnetic field increase in height range from temperature minimum region to the lower chromosphere. A semi-empirical model of the photospheric layers of the flare, constructed on a base of observations of Fe I 5123.7 and Fe I 5434.5 lines by solving the inverse problem for non-equilibrium radiative transfer using Tikhonov stabilizers, shows the moderate increasing of temperature (∼300 K) in altitude range 250–500 km. The radial velocities were decreased during peak phase. The micro-turbulent velocity was increased in the photosphere layers before the flare peak, while during the peak phase it was significantly decreased. The obtained results indicate that in the pre-peak phase the altitude perturbations in the photosphere were larger than in the peak phase.

The morphology of average solar flare time profiles from observations of the Sun’s lower atmosphere

ABSTRACT We study the decay phase of solar flares in several spectral bands using a method based on that successfully applied to white light flares observed on an M4 dwarf. We selected and processed 102 events detected in the Sun-as-a-star flux obtained with SDO/AIA images in the 1600 and 304 Å channels and 54 events detected in the 1700 Å channel. The main criterion for the selection of time profiles was a slow, continuous flux decay without significant new bursts. The obtained averaged time profiles were fitted with analytical templates, using different time intervals, that consisted of a combination of two independent exponents or a broken power law. The average flare profile observed in the 1700 Å channel decayed more slowly than the average flare profile observed on the M4 dwarf. As the 1700 Å emission is associated with a similar temperature to that usually ascribed to M dwarf flares, this implies that the M dwarf flare emission comes from a more dense layer than solar flare emission in the 1700 Å band. The cooling processes in solar flares were best described by the two exponents model, fitted over the intervals t1 = [0, 0.5]t1/2 and t2 = [3, 10]t1/2, where t1/2 is time taken for the profile to decay to half the maximum value. The broken power-law model provided a good fit to the first decay phase, as it was able to account for the impact of chromospheric plasma evaporation, but it did not successfully fit the second decay phase.

Study of Long-Duration Flares and Associated Chromospheric and Coronal Ejections in Solar Cycles 23 and 24

A statistical study of long-duration (long-duration event, LDE) flares in the soft X-ray (SXR) range for solar cycles 23 and 24 is carried out. Graphs of the dependence of the strength of LDE flares and coronal mass ejections (CMEs) on the duration of these flares are constructed. As a result, we find that a significant proportion of LDE flares is not associated with large mass CMEs. This contradicts the hypothesis that the energy required to maintain the post-eruptive phase of LDE flares is supplied by the energy of settling matter after CME eruption. However, most flares are accompanied by mini-eruptions, which may have a significance in the energy balance of LDEs. The diagram of the strength–duration of flares shows that there is a limit of energy and maximum strength of an LDE flare in the SXR range in LDE events. Apparently, this limit differs for cycles 23 and 24. This fact may indicate that there is a limit to the maximum flare energy that can be observed in a particular solar cycle.

Hot X-ray onsets of solar flares

ABSTRACT The study of the localized plasma conditions before the impulsive phase of a solar flare can help us understand the physical processes that occur leading up to the main flare energy release. Here, we present evidence of a hot X-ray ‘onset’ interval of enhanced isothermal plasma temperatures in the range of 10–15 MK over a period of time prior to the flare’s impulsive phase. This ‘hot onset’ interval occurs during the initial soft X-ray increase and definitely before any detectable hard X-ray emission. The isothermal temperatures, estimated by the Geostationary Operational Environmental Satellite X-ray sensor, and confirmed with data from the Reuven Ramaty High Energy Solar Spectroscopic Imager, show no signs of gradual increase, and the ‘hot onset’ phenomenon occurs regardless of flare classification or configuration. In a small sample of four representative flare events, we tentatively identify this early hot onset soft X-ray emission to occur within footpoint and low-lying loop regions, rather than in coronal structures, based on images from the Atmospheric Imaging Assembly. We confirm this via limb occultation of a flaring region. These hot X-ray onsets appear before there is evidence of collisional heating by non-thermal electrons, and hence challenge the standard modelling techniques.

FEATURES OF THE DEVELOPMENT OF A CIRCULAR SOLAR FLARE

We present the results of the analysis ofmorphology and evolution of the circular solar flare usingH-alpha images. H-alpha filtergrams were obtained withthe Meudon spectroheliograph. The active region NOAA9087 had a complex multipolar magnetic field configura-tion. New magnetic fluxes emerged during the evolution ofthis flare-productive active region. The high flare and surgeactivity was observed in the active region.According to Solar Geophysical Data (SGD) the3N/M6.4 class solar flare occurred on July 19, 2000 at06:37 UT, peaked at 07:23 UT and lasted 2.5 hours. Twobright kernels appeared near large positive-polarity sunspotat the beginning of the flare. In a few minutes bright kernelsoccurred in the center of the active region near polarity in-version line. Space solar observatory Yohkoh detected ahard X-ray (HXR) coronal source in the 13.9-22.7 keV and22.7-32.7 keV energy bands in this location.New kernels appeared in the southern and eastern partsof the active region at the boundaries of the chromosphericnetwork. They brightened sequentially clockwise, whichmay indicate a slipping reconnection. Magnetic reconnec-tion was observed in the main phase of the flare in the east-ern part of the active region. In the late flare phase arcadeof post-reconnection EUV loops connected the main flareribbon with the place of repeated reconnection. Additionalheating may be required for the explanation of the longflare decay phase.Flare ribbons of the circular shape were formed. Thecomplex magnetic configuration of the studied active re-gion and circular shape of the ribbons suggest that it had afan-spine magnetic topology with null points. Possibly,flare ribbons are the locations of intersections of the fanquasi-separatrix layer with the chromosphere. They ap-peared as a result of heating or particle beam moving alonga quasi-separatrix layer from a source in the corona.