arXiv subjects

Gregory D. Fleishman

Publications and source records attributed to Gregory D. Fleishman.

At least 37 records · Page 2Linked to original sources

arXiv2021

Ultimate Fast Gyrosynchrotron Codes

The past decade has seen a dramatic increase of practical applications of the microwave gyrosynchrotron emission for plasma diagnostics and three-dimensional modeling of solar flares and other astrophysical objects. This break-through turned out to become possible due to apparently minor, technical development of Fast Gyrosynchrotron Codes, which enormously reduced the computation time needed to calculate a single spectrum, while preserving accuracy of the computation. However, the available fast codes are limited in that they could only be used for a factorized distribution over the energy and pitch-angle, while the distributions of electrons over energy or pitch-angle are limited to a number of predefined analytical functions. In realistic simulations, these assumptions do not hold; thus, the codes free from the mentioned limitations are called for. To remedy this situation, we extended our fast codes to work with an arbitrary input distribution function of radiating electrons. We accomplished this by implementing fast codes for a distribution function described by an arbitrary numerically-defined array. In addition, we removed several other limitations of the available fast codes and improved treatment of the free-free component. The Ultimate Fast Codes presented here allow for an arbitrary combination of the analytically and numerically defined distributions, which offers the most flexible use of the fast codes. We illustrate the code with a few simple examples.

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arXiv2021

Gyroresonance and free-free radio emissions from multi-thermal multi-component plasma

Thermal plasma of solar atmosphere includes a wide range of temperatures. This plasma is often quantified, both in observations and models, by a differential emission measure (DEM). DEM is a distribution of the thermal electron density square over temperature. In observations, the DEM is computed along a line of sight, while in the modeling -- over an elementary volume element (voxel). This description of the multi-thermal plasma is convenient and widely used in the analysis and modeling of extreme ultraviolet emission (EUV), which has an optically thin character. However, there is no corresponding treatment in the radio domain, where optical depth of emission can be large, more than one emission mechanism are involved, and plasma effects are important. Here, we extend the theory of the thermal gyroresonance and free-free radio emissions in the classical mono-temperature Maxwellian plasma to the case of a multi-temperature plasma. The free-free component is computed using the DEM and temperature-dependent ionization states of coronal ions, contributions from collisions of electrons with neutral atoms, exact Gaunt factor, and the magnetic field effect. For the gyroresonant component, another measure of the multi-temperature plasma is used which describes the distribution of the thermal electron density over temperature. We give representative examples demonstrating important changes in the emission intensity and polarization due to considered effects. The theory is implemented in available computer code.

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arXiv2021

Energy budget of plasma motions, heating, and electron acceleration in a three-loop solar flare

Non-potential magnetic energy promptly released in solar flares is converted to other forms of energy. This may include nonthermal energy of flare-accelerated particles, thermal energy of heated flaring plasma, and kinetic energy of eruptions, jets, up/down flows, and stochastic (turbulent) plasma motions. The processes or parameters governing partitioning of the released energy between these components is an open question. How these components are distributed between distinct flaring loops and what controls these spatial distributions is also unclear. Here, based on multi-wavelength data and 3D modeling, we quantify the energy partitioning and spatial distribution in the well observed SOL2014-02-16T064620 solar flare of class C1.5. Nonthermal emissions of this flare displayed a simple impulsive single-spike light curves lasting about 20\,s. In contrast, the thermal emission demonstrated at least three distinct heating episodes, only one of which was associated with the nonthermal component. The flare was accompanied by up and down flows and substantial turbulent velocities. The results of our analysis suggest that (i) the flare occurs in a multi-loop system that included at least three distinct flux tubes; (ii) the released magnetic energy is divided unevenly between the thermal and nonthermal components in these loops; (iii) only one of these three flaring loops contains an energetically important amount of nonthermal electrons, while two other loops remain thermal; (iv) the amounts of direct plasma heating and that due to nonthermal electron loss are comparable; (v) the kinetic energy in the flare footpoints constitute only a minor fraction compared with the thermal and nonthermal energies.

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arXiv2021

Coronal Heating Law Constrained by Microwave Gyroresonant Emission

The question why the solar corona is much hotter than the visible solar surface still puzzles solar researchers. Most theories of the coronal heating involve a tight coupling between the coronal magnetic field and the associated thermal structure. This coupling is based on two facts: (i) the magnetic field is the main source of the energy in the corona and (ii) the heat transfer preferentially happens along the magnetic field, while is suppressed across it. However, most of the information about the coronal heating is derived from analysis of EUV or soft X-ray emissions, which are not explicitly sensitive to the magnetic field. This paper employs another electromagnetic channel -- the sunspot-associated microwave gyroresonant emission, which is explicitly sensitive to both the magnetic field and thermal plasma. We use nonlinear force-free field reconstructions of the magnetic skeleton dressed with a thermal structure as prescribed by a field-aligned hydrodynamics to constrain the coronal heating model. We demonstrate that the microwave gyroresonant emission is extraordinarily sensitive to details of the coronal heating. We infer heating model parameters consistent with observations.

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arXiv2020

Magnetic Reconnection During the Post-Impulsive Phase of a Long-Duration Solar Flare: Bi-Directional Outflows as a Cause of Microwave and X-ray Bursts

Magnetic reconnection plays a crucial role in powering solar flares, production of energetic particles, and plasma heating. However, where the magnetic reconnections occur, how and where the released magnetic energy is transported, and how it is converted to other forms remain unclear. Here we report recurring bi-directional plasma outflows located within a large-scale plasma sheet observed in extreme ultraviolet emission and scattered white light during the post-impulsive gradual phase of the X8.2 solar flare on 2017 September 10. Each of the bi-directional outflows originates in the plasma sheet from a discrete site, identified as a magnetic reconnection site. These reconnection sites reside at very low altitudes ($< 180$ Mm, or 0.26 $R_{\odot}$) above the top of the flare arcade, a distance only $<3\%$ of the total length of a plasma sheet that extends to at least 10 $R_{\odot}$. Each arrival of sunward outflows at the looptop region appears to coincide with an impulsive microwave and X-ray burst dominated by a hot source (10-20 MK) at the looptop, which is immediately followed by a nonthermal microwave burst located in the loopleg region. We propose that the reconnection outflows transport the magnetic energy released at localized magnetic reconnection sites outward in the form of kinetic energy flux and/or electromagnetic Poynting flux. The sunward-directed energy flux induces particle acceleration and plasma heating in the post-flare arcades, observed as the hot and nonthermal flare emissions.

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arXiv2020

Measurement of magnetic field and relativistic electrons along a solar flare current sheet

In the standard model of solar flares, a large-scale reconnection current sheet is postulated as the central engine for powering the flare energy release and accelerating particles. However, where and how the energy release and particle acceleration occur remain unclear due to the lack of measurements for the magnetic properties of the current sheet. Here we report the measurement of spatially-resolved magnetic field and flare-accelerated relativistic electrons along a current-sheet feature in a solar flare. The measured magnetic field profile shows a local maximum where the reconnecting field lines of opposite polarities closely approach each other, known as the reconnection X point. The measurements also reveal a local minimum near the bottom of the current sheet above the flare loop-top, referred to as a "magnetic bottle". This spatial structure agrees with theoretical predictions and numerical modeling results. A strong reconnection electric field of ~4000 V/m is inferred near the X point. This location, however, shows a local depletion of microwave-emitting relativistic electrons. These electrons concentrate instead at or near the magnetic bottle structure, where more than 99% of them reside at each instant. Our observations suggest that the loop-top magnetic bottle is likely the primary site for accelerating and/or confining the relativistic electrons.

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arXiv2020

Evolution of Flare-accelerated Electrons Quantified by Spatially Resolved Analysis

Nonthermal electrons accelerated in solar flares produce electromagnetic emission in two distinct, highly complementary domains - hard X-rays (HXRs) and microwaves (MWs). This paper reports MW imaging spectroscopy observations from the Expanded Owens Valley Solar Array of an M1.2 flare that occurred on 2017 September 9, from which we deduce evolving coronal parameter maps. We analyze these data jointly with the complementary Reuven Ramaty High-Energy Solar Spectroscopic Imager HXR data to reveal the spatially-resolved evolution of the nonthermal electrons in the flaring volume. We find that the high-energy portion of the nonthermal electron distribution, responsible for the MW emission, displays a much more prominent evolution (in the form of strong spectral hardening) than the low-energy portion, responsible for the HXR emission. We show that the revealed trends are consistent with a single electron population evolving according to a simplified trap-plus-precipitation model with sustained injection/acceleration of nonthermal electrons, which produces a double-powerlaw with steadily increasing break energy.

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arXiv2020

Spatio-temporal energy partitioning in a non-thermally dominated two-loop solar flare

Solar flares show remarkable variety of the energy partitioning between thermal and nonthermal components. Those with a prominent nonthermal component but only a modest thermal one are particularly well suited to study the direct effect of the nonthermal electrons on plasma heating. Here, we analyze such a well observed, impulsive single-spike nonthermal event, a SOL2013-11-05T035054 solar flare, where the plasma heating can be entirely attributed to the energy losses of these impulsively accelerated electrons. Evolution of the energy budget of thermal and nonthermal components during the flare is analysed using X-ray, microwave, and EUV observations and three-dimensional modeling. The results suggest that (i) the flare geometry is consistent with a two-loop morphology and the magnetic energy is likely released due to interaction between these two loops; (ii) the released magnetic energy converted to the nonthermal energy of accelerated electrons only, which is subsequently converted to the thermal energy of the plasma; (iii) the energy is partitioned in these two flaring loops in comparable amounts; (iv) one of these flaring loops remained relatively tenuous but rather hot, while the other remained relatively cool but denser than the first one. Therefore, this solar flare demonstrates an extreme efficiency of conversion of the free magnetic energy to the nonthermal energy of particle acceleration and the energy flow into two loops from the non-thermal to thermal one with a negligible direct heating.

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arXiv2019

Estimating the temperature and density of a spicule from 100 GHz data obtained with ALMA

We succeeded in observing two large spicules simultaneously with the Atacama Large Millimeter/submillimeter Array (ALMA), the Interface Region Imaging Spectrograph (IRIS), and the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory. One is a spicule seen in the IRIS Mg II slit-jaw images and AIA 304Å images (MgII/304A spicule). The other one is a spicule seen in the 100GHz images obtained with ALMA (100GHz spicule). Although the 100GHz spicule overlapped with the MgII/304A spicule in the early phase, it did not show any corresponding structures in the IRIS Mg II and AIA 304A images after the early phase. It suggests that the spicules are individual events and do not have a physical relationship. To obtain the physical parameters of the 100GHz spicule, we estimate the optical depths as a function of temperature and density using two different methods. One is using the observed brightness temperature by assuming a filling factor, and the other is using an emission model for the optical depth. As a result of comparing them, the kinetic temperature of the plasma and the number density of ionized hydrogens in the 100GHz spicule are ~6800 K and 2.2 x 10^10 cm^-3. The estimated values can explain the absorbing structure in the 193A image, which appear as a counterpart of the 100GHz spicule. These results suggest that the 100GHz spicule presented in this paper is classified to a macrospicule without a hot sheath in former terminology.

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arXiv2019

Rapid Variability in the SOL2011-08-04 Flare: Implications for Electron Acceleration

Particle acceleration in solar flares remains an outstanding problem in solar physics. It is yet unclear which of the acceleration mechanisms dominates and how exactly is the excessive magnetic energy transferred to the nonthermal and other forms of energy. We emphasize, that the ultimate acceleration mechanism must be capable of efficiently working in the most extreme conditions, such as the shortest detected time scales and the highest acceleration efficiency. Here we focus on detailed multiwavelength analysis of a very initial phase of the SOL2011-08-04 flare, which demonstrated prominent short subpeaks of nonthermal emission during filament eruption associated with the flare. We demonstrate that the three-dimensional configuration of the flare, combined with timing and spectral behavior of the rapidly varying component, put very stringent constraints on the acceleration regime. Specifically, the rapid subpeaks are generated by short injections of nonthermal electrons with a reasonably hard, single power-law spectrum and a relatively narrow spread of pitch-angles along the mean magnetic field. The acceleration site is a compact volume located near the top of extended coronal loop(s). The electrons are accelerated up to several hundreds of keV promptly, with the characteristic acceleration time shorter than 50 ms. We show, that these properties are difficult to reconcile with widely adopted stochastic acceleration models, while the data inescapably require acceleration by a super-Dreicer electric field, whether regular or random.

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arXiv2019

Record-breaking coronal magnetic field in solar active region 12673

At the Sun, the strongest magnetic fields are routinely detected at dark sunspots. The magnitude of the field is typically about 3000 G, with only a few exceptions that reported the magnetic field in excess of 5000 G. Given that the magnetic field decreases with height in the solar atmosphere, no coronal magnetic field above ~2000 G was ever reported. Here, we present imaging microwave observations of an anomalously strong magnetic field of about 4000 G at the base of the corona in solar active region NOAA 12673 on 06 September 2017. Combining the photospheric vector measurements of the magnetic field and the coronal probing, we created and validated a nonlinear force-free field coronal model, with which we quantify the record-breaking coronal magnetic field at various coronal heights.

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arXiv2019

Gamma-ray emission from the impulsive phase of the 2017 September 06 X9.3 flare

We report hard X-ray and gamma-ray observations of the impulsive phase of the SOL2017-09-06T11:55 X9.3 solar flare. We focus on a high-energy part of the spectrum, >100 keV, and perform time resolved spectral analysis for a portion of the impulsive phase, recorded by the Konus-Wind experiment, that displayed prominent gamma-ray emission. Given a variety of possible emission components contributing to the gamma-ray emission, we employ a Bayesian inference to build the most probable fitting model. The analysis confidently revealed contributions from nuclear deexcitation lines, electron-positron annihilation line at 511 keV, and a neutron capture line at 2.223 MeV along with two components of the bremsstrahlung continuum. The revealed time evolution of the spectral components is particularly interesting. The low-energy bremsstrahlung continuum shows a soft-hard-soft pattern typical for impulsive flares, while the high-energy one shows a persistent hardening at the course of the flare. The neutron capture line emission shows an unusually short time delay relative to the nuclear deexcitation line component, which implies that the production of neutrons was significantly reduced soon after the event onset. This in turn may imply a prominent softening of the accelerated proton spectrum at the course of the flare, similar to the observed softening of the low-energy component of the accelerated electrons responsible for the low-energy bremsstrahlung continuum. We discuss possible physical scenarios, which might result in the obtained relationships between these gamma-ray components.

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arXiv2019

Astro2020 Science White Paper: Probing Magnetic Reconnection in Solar Flares - New Perspectives from Radio Dynamic Imaging Spectroscopy

Magnetic reconnection is a fundamental physical process in many laboratory, space, and astrophysical plasma contexts. Solar flares serve as an outstanding laboratory to study the magnetic reconnection and the associated energy release and conversion processes under plasma conditions difficult to reproduce in the laboratory, and with considerable spatiotemporal details not possible elsewhere in astrophysics. Here we emphasize the unique power of remote-sensing observations of solar flares at radio wavelengths. In particular, we discuss the transformative technique of broadband radio dynamic imaging spectroscopy in making significant contributions to addressing several outstanding challenges in magnetic reconnection, including the capability of pinpointing magnetic reconnection sites, measuring the time-evolving reconnecting magnetic fields, and deriving the spatially and temporally resolved distribution function of flare-accelerated electrons.

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arXiv2018

Electron Acceleration and Jet-Facilitated Escape in an M Class Solar Flare on 2002 August 19

Sudden jets of collimated plasma arise from many locations on the Sun, including active regions. The magnetic field along which a jet emerges is often open to interplanetary space, offering a clear "escape route" for any flare-accelerated electrons and making jets lucrative targets for studying particle acceleration and the solar sources of transient heliospheric events. Bremsstrahlung hard X-rays (HXRs) could, in principle, trace the accelerated electrons that escape along the paths of the jets, but measurements of the escaping electron beams are customarily difficult due to the low densities of the corona. In this work, we augment HXR observations with gyrosynchrotron emission observed in microwaves, as well as extreme ultraviolet (EUV) emission and modeling to investigate flare-accelerated electrons in a coronal jet. HXR and microwave data from RHESSI and OVSA, respectively, give complementary insight into electron spectra and locations, including the presence of accelerated electrons in the jet itself. High-time-resolution HXR data from the Konus-Wind instrument suggest electron acceleration timescales on the order of 1 second or shorter. We model the energetic electron distributions in the GX Simulator framework using SoHO/MDI, RHESSI, TRACE, and OVSA data as constraints. The result is a modeled distribution, informed and constrained by measurements, of accelerated electrons as they escape the Sun. Combining the detection of microwave gyrosynchrotron emission from an open, rather than closed, magnetic configuration, with realistic 3D modeling constrained by magnetograms, EUV, and X-ray emission, we obtain the most stringent constraints to date on the accelerated electrons within a solar jet.

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arXiv2018

Science with an ngVLA: Radio Observations of Solar Flares

Solar flares are due to the catastrophic release of magnetic energy in the Sun's corona, resulting in plasma heating, mass motions, particle acceleration, and radiation emitted from radio to $γ$-ray wavelengths. They are associated with global coronal eruptions of plasma into the interplanetary medium---coronal mass ejections---that can result in a variety of "space weather" phenomena. Flares release energy over a vast range of energies, from $\sim\!10^{23}$ ergs (nanoflares) to more than $10^{32}$ ergs. Solar flares are a phenomenon of general astrophysical interest, allowing detailed study of magnetic energy release, eruptive processes, shock formation and propagation, particle acceleration and transport, and radiative processes. Observations at radio wavelengths offer unique diagnostics of the physics of flares. To fully exploit these diagnostics requires the means of performing time-resolved imaging spectropolarimetry. Recent observations with the Jansky Very Large Array (JVLA) and the Expanded Owens Valley Solar Array (EOVSA), supported by extensive development in forward modeling, have demonstrated the power of the approach. The ngVLA has the potential to bring our understanding of flare processes to a new level through its combination of high spatial resolution, broad frequency range, and imaging dynamic range---especially when used in concert with multi-wavelength observations and data at hard X-ray energies.

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arXiv2018

Science with an ngVLA: ngVLA Observations of Coronal Magnetic Fields

Energy stored in the magnetic field in the solar atmosphere above active regions is a key driver of all solar activity (e.g., solar flares and coronal mass ejections), some of which can affect life on Earth. Radio observations provide a unique diagnostic of the coronal magnetic fields that make them a critical tool for the study of these phenomena, using the technique of broadband radio imaging spectropolarimetry. Observations with the ngVLA will provide unique observations of coronal magnetic fields and their evolution, key inputs and constraints for MHD numerical models of the solar atmosphere and eruptive processes, and a key link between lower layers of the solar atmosphere and the heliosphere. In doing so they will also provide practical "research to operations" guidance for space weather forecasting.

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arXiv2018

Casting the Coronal Magnetic Field Reconstruction Tools in 3D Using MHD Bifrost Model

Quantifying coronal magnetic field remains a central problem in solar physics. Nowadays the coronal magnetic field is often modelled using nonlinear force-free field (NLFFF) reconstructions, whose accuracy has not yet been comprehensively assessed. Here we perform a detailed casting of the NLFFF reconstruction tools, such as pi-disambiguation, photospheric field preprocessing, and volume reconstruction methods using a 3D snapshot of the publicly available full-fledged radiative MHD model. Specifically, from the MHD model we know the magnetic field vector in the entire 3D domain, which enables us to perform "voxel-by-voxel" comparison of the restored and the true magnetic field in the 3D model volume. Our tests show that the available pi-disambiguation methods often fail at the quiet sun areas dominated by small-scale magnetic elements, while they work well at the AR photosphere and (even better) chromosphere. The preprocessing of the photospheric magnetic field, although does produce a more force-free boundary condition, also results in some effective `elevation' of the magnetic field components. This `elevation' height is different for the longitudinal and transverse components, which results in a systematic error in absolute heights in the reconstructed magnetic data cube. The extrapolations performed starting from actual AR photospheric magnetogram are free from this systematic error, while have other metrics comparable with those for extrapolations from the preprocessed magnetograms. This finding favors the use of extrapolations from the original photospheric magnetogram without preprocessing. Our tests further suggest that extrapolations from a force-free chromospheric boundary produce measurably better results, than those from the photospheric boundary.

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arXiv2018

The coronal volume of energetic particles in solar flares as revealed by microwave imaging

The spectrum of gyrosynchrotron emission from solar flares generally peaks in the microwave range. Its optically-thin, high-frequency component, above the spectral peak, is often used for diagnostics of the nonthermal electrons and the magnetic field in the radio source. Under favorable conditions, its low-frequency counterpart brings additional, complementary information about these parameters as well as thermal plasma diagnostics, either through gyrosynchrotron self-absorption, free-free absorption by the thermal plasma, or the suppression of emission through the so-called Razin effect. However, their effects on the low-frequency spectrum are often masked by spatial nonuniformity. To disentangle the various contributions to low-frequency gyrosynchrotron emission, a combination of spectral and imaging data is needed. To this end, we have investigated Owens Valley Solar Array (OVSA) multi-frequency images for 26 solar bursts observed jointly with Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) during the first half of 2002. For each, we examined dynamic spectra, time- and frequency-synthesis maps, RHESSI images with overlaid OVSA contours, and a few representative single-frequency snapshot OVSA images. We focus on the frequency dependence of microwave source sizes derived from the OVSA images and their effect on the low-frequency microwave spectral slope. We succeed in categorizing 18 analyzed events into several groups. Four events demonstrate clear evidence of being dominated by gyrosynchrotron self-absorption, with an inferred brightness temperature of $\geq10^8$~K. The low-frequency spectra in the remaining events are affected to varying degree by Razin suppression. We find that many radio sources are rather large at low frequencies, which can have important implications for solar energetic particle production and escape.

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