Solar Limb Flare Research Articles

Spatial and Spectral Evolution of Microwave and X-Ray Sources During the Solar Limb Flare on February 5, 2023

Spatial and Spectral Evolution of Microwave and X-Ray Sources During the Solar Limb Flare on February 5, 2023

X-Ray Fine Structure of a Limb Solar Flare Revealed by Insight-HXMT, RHESSI and Fermi

Abstract We conduct a detailed analysis of an M1.3 limb flare occurring on 2017 July 3, which have the X-ray observations recorded by multiple hard X-ray telescopes, including Hard X-ray Modulation Telescope (Insight-HXMT), Ramaty High Energy Solar Spectroscopic Imager (RHESSI), and the Fermi Gamma-ray Space Telescope (Fermi). Joint analysis has also used the extreme ultraviolet (EUV) imaging data from the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamic Observatory. The hard X-ray spectral and imaging evolution suggest a lower corona source, and the non-thermal broken power law distribution has a rather low break energy ∼15 keV. The EUV imaging shows a rather stable plasma configuration before the hard X-ray peak phase, and accompanied by a filament eruption during the hard X-ray flare peak phase. Hard X-ray image reconstruction from RHESSI data only shows one foot point source. We also determined the DEM for the peak phase by SDO/AIA data. The integrated EM beyond 10 MK at foot point onset after the peak phase, while the >10 MK source around reconnection site began to fade. The evolution of EM and hard X-ray source supports lower corona plasma heating after non-thermal energy dissipation. The combination of hard X-ray spectra and images during the limb flare provides the understanding on the interchange of non-thermal and thermal energies, and relation between lower corona heating and the upper corona instability.

Signatures of superstrong magnetic fields in a limb solar flare from observations of the Hα line

We present signatures of the super-strong magnetic fields of almost 105 Gauss range in the limb solar flare of 14 July 2005 at the altitudes of 5 – 30 Mm during the post-peak phase of the flare. These features follow from the study of the Stokes V profile in the Hα line at large distances of 1.5 up to 4 Å from its center. A reliable circular polarization (till 1%) was found that changes its sign when crossing the line center. The profile of the polarization differs significantly from the intensity gradient profile dI/dλ, which indicates the strong magnetic field mode. If these features are interpreted as manifestations of the Zeeman effect, the corresponding magnetic field strength reaches ∼ 90 kG. Similar spectral manifestations were not found in another bright emission formation on the solar limb, namely in the active prominence of 24 July 1999. Theoretical analysis of the picture of the Zeeman effect for the case of such giant fields shows that the σ components of the Hα line should be quite narrow (∼0.2 Å) due to the Pashen-Back effect, despite the complex picture of the splitting of this line. Since the observed widths of the peaks of the Stokes V profiles are much wider, this may mean a certain dispersion of the magnetic field strengths in the flare, at which the maximum strengths can be even some greater than those indicated above.

ОСОБЛИВОСТІ МАГНІТНОГО ПОЛЯ У ЛІМБОВОМУ СОНЯЧНОМУ СПАЛАХУ 12 липня 2012 року НА ВИСОТІ БЛИЗЬКО 40 мегаметрів

We present results of magnetic field measurements in limb solar flare of 12 July 2012 of M7.7 class based of analysis of I ± V profiles of Нα line. Our results relate to moment 06:45:50 UT when top of flare arcade reached ≈ 40 Mm according to observations of space observatory SDO. Observed Нα profiles have specific signs of magnetic field inhomogeneity, in particular, unequal splitting of bisectors of I ± V profiles on various distances from their centers. In general, splitting of bisectors increases from line wings to its core, and value of this splitting corresponds to magnetic field in range 400–1200 G. In addition to this trend, local peak of bisector splitting was found on distance about 420 mÅ which may indicate local magnetic field with strength about 20 kG. As the authors know, observational indications of such very strong magnetic fields at a height of 40 mm have been detected for the first time. Obviously, such magnetic fields can exist in the rarefied solar corona only with a certain special structure of the magnetic field, obviously of the force-free type, which implies strong twisting of the lines of force.

Nonequilibrium Ionization Plasma during a Large Solar Limb Flare Observed by Hinode/EIS

Abstract This study on plasma heating considers the time-dependent ionization process during a large solar flare on 2017 September 10, observed by Hinode/EUV Imaging Spectrometer (EIS). The observed Fe xxiv/Fe xxiii ratios increase downstream of the reconnection outflow, and they are consistent with the time-dependent ionization effect at a constant electron temperature T e = 25 MK. Moreover, this study also shows that the nonthermal velocity, which can be related to the turbulent velocity, reduces significantly along the downstream of the reconnection outflow, even when considering the time-dependent ionization process.

Magnetic field measurements in a limb solar flare by hydrogen, helium and ionized calcium lines

We present simultaneous magnetic field measurements for the limb solar flare of 1981 July 17 using of the Ca II K, H δ , He I 4471.5 Å and Hβ lines. For two moments during the flare, which differ in time by 16 min, we analyzed Stokes I ± V profiles of these lines from observations made on the Echelle spectrograph of the horizontal solar telescope of the Astronomical Observatory of Taras Shevchenko National University of Kiev. At the time step that was close to the peak phase of the flare, all the spectral lines under study showed very wide emissions, with a full width at half maximum (FWHM) of 3.5–4 Å. An interesting feature was observed in the blue wings of these lines, namely, narrow emission peaks with a FWHM of only 0.25–0.35 Å. For heights of 10–18 Mm above the level of the photosphere, we found that (a) very strong kG magnetic fields (up to about 3 kG) existed at both moments of the flare, (b) the locations with strongest fields, in general, do not coincide for different spectral lines, (c) the polarities of the magnetic field for different spectral lines are in most cases identical, but sometimes they do not coincide. The data obtained indicate a significant inhomogeneity of the magnetic field in the flaring corona and the probable presence of the conditions necessary for magnetic reconnection of field lines.

ПОРІВНЯННЯ РІЗНИХ МЕТОДІВ ВИМІРЮВАННЯ МАГНІТНИХ ПОЛІВ У СОНЯЧНИХ СПАЛАХАХ

We present a comparison of possibilities of three methods of magnetic field measurements using the Zeeman effect, namely, method of ”center of gravity”, splitting of Stokes V peaks and analysis of bisectors of I ± V profiles. We note that first method gives very averaged data in form of effective magnetic field Beff which presents the lower limit of local magnetic field in spatially unresolved structures. Splitting of Stokes V peaks DlV can present local magnetic field B, but only in a case when this splitting exceeds considerably the splitting of peaks of the Stokes I gradient dI/dl. Analysis of bisectors of I ± V profiles presents simple method for rapid diagnostics of magnetic field inhomogeneity. In particular, in case of really weak and homogeneous magnetic field, bisectors of I + V and I – V profiles should be parallel to each other. If these bisectors are non-parallel having some extrema, this could be an evidence of hidden presence of strong spatially unresolved magnetic field. Last method is very subtle but needs careful accounting of instrumental effects in line profiles. In this paper, we present also new data related to magnetic fields in limb solar flare of 14 July 2005. Spectral observations of this flare were carried out with the Echelle spectrograph of the Horizontal Solar Telescope of the Astronomical Observatory of Taras Shevchenko National University of Kyiv. In order to measure the magnetic fields in this flare, I ± V profiles of Нa line were studied. It was found that effective magnetic field Вeff in the flare reached 850 ± 100 G on height 16 Mm. However, the spectral evidences to yet stronger local fields of 104 – 105 G range were found. In particular, for several places of the flare, a weak circular polarization of opposite signs was found in line wings on distances of 1.8-2.2 Å from line center. If this polarization to interpret as manifestations of the Zeeman effect, the corresponding magnetic field is » 85–100 kG.

ON THE POSSIBLE EXISTENCE OF SUPERSTRONG MAGNETIC FIELDS IN A LIMB SOLAR FLARE

We present the simultaneousobservations of the K Ca II 3933.7 A, H δ 4101.7 A andHe I 4471.5 A lines in the limb solar flare of July 17,1981. For two moments of the flare we analyzed StokesI ± V and V profiles of these lines from observationsmade on the Echelle spectrograph of the horizontal solartelescope of the Astronomical Observatory of TarasShevchenko National University of Kiev. In the flashphase of the flare, all named lines had very wideemissions, with a wing length of 6–8 A. An interestingfeature was observed in the violet wings of these lines,namely, narrow emission peaks with a width of only 0.25–0.35 A. The Zeeman splitting of these emission peakscorrespond to magnetic field strength in range 1300-2900G at altitudes of 10–15 Mm above the level of thephotosphere. Magnetic fields of ‘kilogauss’ range (up to3200 G) were found for some locations also in post-peakphase of the flare. Likely, the true local magnetic fields inthe flare could be even larger, since the obtained resultsrepresent a longitudinal component of the magnetic fieldassuming that the filling factor equals unity. A newindication of the existence of superstrong magnetic fieldsfollows from a comparison of the kinetic temperatures andturbulent velocities found from the narrow emissioncomponent in the flash phase of the flare. Considering thisemission component to be optically thin, we found thatthere is an anti-correlation between temperature andturbulent velocity. Such a dependence seems unlikely and,possibly, the widths of the line profiles reflect, in our case,not turbulent velocities, but very strong magnetic fields.The corresponding estimates of the magnetic field by theK Ca II line lead to the value B = 8.3 kG, and by the He I4471.5 line - to the value B = 6.7 kG. Considering that theHe I line is clearly closer to the case of an optically thinlayer, the closeness of these estimates is very encouraging.Our results are apparently the first indications ofsuperstrong magnetic fields of 6-7 kG at an altitude ofabout 10 Mm in a solar flare

Line profiles and magnetic fields in the limb solar flare of July 17, 1981. Preliminary results

Line profiles and magnetic fields in the limb solar flare of July 17, 1981. Preliminary results

Broken-up spectra of the loop-top hard X-ray source during a solar limb flare

Solar hard X-rays (HXRs) appear in the form of either footpoint sources or coronal sources. Each individual source provides its own critical information on acceleration of nonthermal electrons and plasma heating. Earlier studies found that the HXR emission in some events manifests a broken-up power-law spectrum, with the break energy around a few hundred keV based on spatially-integrated spectral analysis, and it does not distinguish the contributions from individual sources. In this paper, we report on the broken-up spectra of a coronal source studied using HXR data recorded by Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) during the SOL2017–09–10T16:06 (GOES class X8.2) flare. The flare occurred behind the western limb and its footpoint sources were mostly occulted by the disk. We could clearly identify such broken-up spectra pertaining solely to the coronal source during the flare peak time and after. Since a significant pileup effect on the RHESSI spectra is expected for this intense solar flare, we have selected the pileup correction factor, p = 2. In this case, we found the resulting RHESSI temperature (∼30 MK) to be similar to the GOES soft X-ray temperature and break energies of 45–60 keV. Above the break energy, the spectrum hardens with time from spectral index of 3.4 to 2.7, and the difference in spectral indices below and above the break energy increases from 1.5 to 5 with time. However, we note that when p = 2 is assumed, a single power-law fitting is also possible with the RHESSI temperature higher than the GOES temperature by ∼10 MK. Possible scenarios for the broken-up spectra of the loop-top HXR source are briefly discussed.

X-ray of the 2017 September 10 Solar Flare

The analysis of soft and hard X-ray radiation of behind the limb solar flare SOL2017-09-10T15:35 according to the RHESSI data was carried out. The plasma parameters were determined: emission measure, temperature, density. Sources of hard X-ray radiation >30 keV are observed for several hours after the maximum of the flare. It indicates the long-term processes of acceleration of electrons in the solar corona. The rise of the X-ray source during 6.5 hours was ∼70 arcsec. It was proposed also an alternative method for determining the plasma density, based on cross-correlation analysis of time series (time delay method).

Microwave and Hard X-Ray Observations of the 2017 September 10 Solar Limb Flare

Abstract We report the first science results from the newly completed Expanded Owens Valley Solar Array (EOVSA), which obtained excellent microwave (MW) imaging spectroscopy observations of SOL2017-09-10, a classic partially occulted solar limb flare associated with an erupting flux rope. This event is also well-covered by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) in hard X-rays (HXRs). We present an overview of this event focusing on MW and HXR data, both associated with high-energy nonthermal electrons, and we discuss them within the context of the flare geometry and evolution revealed by extreme ultraviolet observations from the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatory. The EOVSA and RHESSI data reveal the evolving spatial and energy distribution of high-energy electrons throughout the entire flaring region. The results suggest that the MW and HXR sources largely arise from a common nonthermal electron population, although the MW imaging spectroscopy provides information over a much larger volume of the corona.

Hard X-ray morphology of the X1.3 April 25, 2014 partially occulted limb solar flare

At hard X-ray energies, the bright footpoint emission from solar flare loops often prevents a detailed analysis of the weaker loop-top source morphology due to the limited dynamic range available for X-ray imaging. Here, we study the X1.3 April 25, 2014 flare with the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI). This partially occulted limb flare allows the analysis of the loop-top emission in isolation. We present results on the flare light curve at different energies, the source morphology from X-ray imaging and a detailed spectral analysis of the different source components by imaging spectroscopy. The loop-top source, a likely site of particle acceleration, shows a clear composition of different emission components. The results indicate the opportunities that detailed imaging of hard X-rays can provide to learn about particle acceleration, transport and heating processes in solar flares.

Magnetic fields in a limb flare on July 19, 2012

The magnetic field intensity in the limb solar flare of July 19, 2012, was measured by the Zeeman bisector splitting in the Hα line. An average magnetic field is established to attain 200 G at a measurement error of ±100 G in the top of a radiant flare loop at a height of about 40 Mm. The confinement of a strong magnetic field in the high coronal arcade at a relatively weak external field of the corona (about 1–2 G) is considered using a model of a force-free magnetic flux rope with a fine magnetic structure at a scale of about 300 km.

The emission in the region [formula omitted] during disk and limb faint solar flares

Hard X-ray and gamma-ray emission in energy band E>50keV was first observed by AVS-F apparatus onboard CORONAS-F satellite (detector SONG-D) during some solar flares with classes B and C by GOES classification. Such component registered in flares with duration less than 30min. However γ-emission up to several tens of MeV was observed during some classes B and C events, which temporal profiles were not corresponded to Neupert effect. For example, during class B2.3 limb solar flare January 7, 2005 maximum observed energy was Emax∼36MeV and during class B4.6 disk solar event January 12, 2005 maximum observed energy was Emax∼7MeV. Properties of temporal profiles and energy spectra of faint solar flares, during which emission in the energy band of E>0.1MeV were registered are discussed in the presented work. There is not any strong correlation between presence or absence of hard X-ray and γ-ray emission and the intensity of soft X-ray emission during solar flares. The one of illustration of this fact is the absence of any observed statistically significant count rate exceed above background level during some class M flares in the energy band E>0.1MeV. The typical example of such flares is event November 8, 2001 (class M4.2, lasts from 14:59 UT up to 16:00 UT, maximum of soft X-ray emission was at 15:35 UT on GOES data).

Chromospheric magnetic field and density structure measurements using hard X-rays in a flaring coronal loop

Aims. A novel method of using hard X-rays as a diagnostic for chromospheric density and magnetic structures is developed to inf er sub-arcsecond vertical variation of magnetic flux tube size and neutral gas density. Methods. Using Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) X-ray data and the newly developed X-ray visibilities forward fitting technique we find the FWHM and ce ntroid positions of hard X-ray sources with sub-arcsecond r esolution (∼ 0.2 ′′ ) for a solar limb flare. We show that the height variations of t he chromospheric density and the magnetic flux densities can be found with unprecedented vertical resolution of∼ 150 km by mapping 18-250 keV X-ray emission of energetic electrons propagating in the loop at chromospheric heights of 400-1500 km. Results. Our observations suggest that the density of the neutral gas is in good agreement with hydrostatic models with a scale height of around 140± 30 km. FWHM sizes of the X-ray sources decrease with energy suggesting the expansion (fanning out) of magnetic flux tube in the chromosphere with height. The magnetic scale height B(z) (dB/dz) −1 is found to be of the order of 300 km and strong horizontal magnetic field is associated with noticeable flux tube expansion at a height of∼ 900 km.

Direct Observation of High-Speed Plasma Outflows Produced by Magnetic Reconnection in Solar Impulsive Events

Spectroscopic observations of a solar limb flare recorded by SUMER on SOHO reveal, for the first time, hot fast magnetic reconnection outflows in the corona. As the reconnection site rises across the SUMER spectrometer slit, significant blue- and red-shift signatures are observed in sequence in the Fe XIX line, reflecting upflows and downflows of hot plasma jets, respectively. With the projection effect corrected, the measured outflow speed is between 900-3500 km/s, consistent with theoretical predictions of the Alfvenic outflows in magnetic reconnection region in solar impulsive events. Based on theoretic models, the magnetic field strength near the reconnection region is estimated to be 19-37 Gauss.

Multiwavelength Analysis of a Solar Flare on 2002 April 15

We carried out a multiwavelength analysis of the solar limb flare on 2002 April 15. The observations all indicate that the flare occurred in an active region with an asymmetric dipole magnetic configuration. The earlier conclusion that magnetic reconnection is occurring in a large-scale current sheet in this flare is M e r supported by these observations: (1) Several bloblike sources, seen in RHESSI 12-25 keV X-ray images later in the flare, appeared along a line above the flare loops. These indicate the continued presence of the current sheet and are likely to be magnetic islands in the stretched sheet produced by the tearing-mode instability. (2) A cusplike structure is seen in Nobeyama Radioheliogiaph (NoRH) 34 GHz microwave images around the time of the peak flare emission. We quantitatively demonstrate that the X-ray-emitting thermal plasma seen with RHESSI had a higher temperature than the microwave-emitting plasma seen with NoRH. Since the radio data preferentially see cooler thermal plasma, this result is consistent with the picture in which energy release occurs at progressively greater heights and the hard X-rays see hot new loops while the radio sees older cooling loops. The kinetic energy of the coronal mass ejection (CME) associated with this flare was found to be about 1 order of magnitude less than both the thermal energy in the hot plasma and the nonthermal energy carried by the accelerated electrons in the flare, as deduced from the RHESSI observations. This contrasts with the higher CME kinetic energies typically deduced for large flares.

Multi-wavelength study of a strong impulsive solar limb flare on 2002 August 3

We made a detailed study of the impulsive solar flare of GOES class X1.0 which occurred near the west limb on 2002 August 3, peak time 19:07 UT. There is particularly good data coverage of this event, with simultaneous observations in EUV, soft and hard X-rays available. We used TRACE 171 Å images to study the morphology and evolution of this event. Soft X-ray spectra in the wavelength range 3.34–6.05 Å measured by the RESIK Bragg crystal spectrometer on CORONAS-F were used for determination of the evolution of the flare plasma temperature. Data from the RHESSI instrument were used to investigate properties of the higher-temperature plasma during the flare.

Mass Motions and Plasma Properties in the 107K Flare Solar Corona

In the present work, we analyze Solar Ultraviolet Measurement of Emitted Radiation (SUMER) observations of a solar limb flare that occurred on 1999 May 9. The analyzed data cover a time span of around 6.4 hr, during which an M-7.6 flare erupted and decayed in the field of view. Two selected regions along the SUMER slit have been considered for quantitative analysis. The main purpose of the present analysis is to measure the mass motions and the nonthermal velocities of the postflare plasmas and their temporal evolution. To achieve this we use lines having formation temperatures in the 2.5 × 106 to 2 × 107 K range from which we derive net mass motions and nonthermal velocities and compare them with the properties of the surrounding plasma not affected by the flare activity. To understand the physical conditions of the flaring plasma and of the surrounding material, we derive electron temperature, electron density, and emission measures of the emitting plasma. We find that bulk motions, initially of the order of several hundreds of kilometers per second in both directions, decay within 10 minutes from the flare onset; nonthermal velocities decay to preflare values of around 30 km s-1 in less than 2 hr from the maximum value of around 100 km s-1 at flare onset. The measured electron density does not seem to change during activity, while the flare plasma temperature steadily decays to preflare values. The temperature evolution is consistent with a radiatively cooling plasma, although the uncertainties associated to the measurement of the variation of thermal energy of the flare plasma prevent a definitive conclusion on possible continuous heating of the flaring plasma.