RHESSI Research Articles

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.

MEM_GE: A New Maximum Entropy Method for Image Reconstruction from Solar X-Ray Visibilities

Abstract Maximum Entropy is an image reconstruction method conceived to image a sparsely occupied field of view, therefore it is particularly effective at achieving super-resolution effects. Although widely used in image deconvolution, this method has been formulated in radio astronomy for the analysis of observations in the spatial frequency domain, and an Interactive Data Language code has been implemented for image reconstruction from solar X-ray Fourier data. However, this code relies on a non-convex formulation of the constrained optimization problem addressed by the Maximum Entropy approach, and this sometimes results in unreliable reconstructions characterized by unphysical shrinking effects. This paper introduces a new approach to Maximum Entropy based on the constrained minimization of a convex functional. In the case of observations recorded by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), the resulting code provides the same super-resolution effects of the previous algorithm, while also working properly when that code produces unphysical reconstructions. We also obtain results via testing the algorithm with synthetic data simulating observations of the Spectrometer/Telescope for Imaging X-rays (STIX) in Solar Orbiter. The new code is available in the HESSI folder of the Solar SoftWare (SSW) tree.

Microwave and Hard X-Ray Flare Observations by NoRH/NoRP and RHESSI: Peak-flux Correlations

Abstract This paper presents initial results from a statistical study of solar microwave and hard X-ray flares jointly observed over the past two solar cycles by the Nobeyama Radio Polarimeters, the Nobeyama Radio Heliograph, and the Reuven Ramaty High Energy Solar Spectroscopic Imager. As has been previously demonstrated, the microwave (17 GHz and 34 GHz) peak flux shows a linear correlation with the nonthermal hard X-ray bremsstrahlung peak emission seen above 50 keV. The correlation holds for the entire rise phase of each individual burst, while the decay phases tend to show more extended emission at microwaves than is generally attributed to particle trapping. While the correlation is highly significant (coefficient of 0.92) and holds over more than four orders of magnitude, individual flares can be above or below the fitted line by an average factor of about 2. By restricting the flare selection to source morphologies with the radio emission from the top of the flare loop, the correlation tightens significantly, with a correlation coefficient increasing to 0.99 and the scatter reduced to a factor of 1.3. These findings corroborate the assumption that gyrosynchrotron microwave and hard X-ray bremsstrahlung emissions are produced by the same flare-accelerated electron population. The extent of the linear correlation over four orders of magnitude suggests that magnetic field strengths within nonthermal 17 GHz sources are surprisingly similar over a wide range of flare sizes.

A Solar Magnetic-fan Flaring Arch Heated by Nonthermal Particles and Hot Plasma from an X-Ray Jet Eruption

Abstract We have investigated an M1.3 limb flare, which develops as a magnetic loop/arch that fans out from an X-ray jet. Using Hinode/EIS, we found that the temperature increases with height to a value of over 107 K at the loop top during the flare. The measured Doppler velocity (redshifts of 100–500 km s−1) and the nonthermal velocity (≥100 km s−1) from Fe xxiv also increase with loop height. The electron density increases from 0.3 × 109 cm−3 early in the flare rise to 1.3 × 109 cm−3 after the flare peak. The 3D structure of the loop derived with Solar TErrestrial RElations Observatory/EUV Imager indicates that the strong redshift in the loop-top region is due to upflowing plasma originating from the jet. Both hard X-ray and soft X-ray emission from the Reuven Ramaty High Energy Solar Spectroscopic Imager were only seen as footpoint brightenings during the impulsive phase of the flare, then, soft X-ray emission moved to the loop top in the decay phase. Based on the temperature and density measurements and theoretical cooling models, the temperature evolution of the flare arch is consistent with impulsive heating during the jet eruption followed by conductive cooling via evaporation and minor prolonged heating in the top of the fan loop. Investigating the magnetic field topology and squashing factor map from Solar Dynamics Observatory/HMI, we conclude that the observed magnetic-fan flaring arch is mostly heated from low atmospheric reconnection accompanying the jet ejection, instead of from reconnection above the arch as expected in the standard flare model.

Relationships between Photospheric Vertical Electric Currents and Hard X-Ray Sources in Solar Flares: Statistical Study

Abstract There are still debates over whether particle acceleration in solar flares may occur due to interruption of electric currents flowing along magnetic loops. To contribute to this debate, we performed the first statistical study of relationships between flare hard X-ray (HXR; 50–100 keV) sources observed by the Ramaty High-Energy Solar Spectroscopic Imager and photospheric vertical electric currents (PVECs, j r ) calculated using vector magnetograms obtained with the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. We analyzed a sample of 48 flares from the C3.0 to X3.1 class and were observed in the central part of the solar disk by both instruments in 2010–2015. We found that ≈70% of all HXR sources overlapped with islands or ribbons of enhanced ( statampere cm−2) PVECs. However, less than ≈40% of the HXR sources overlapped with PVEC maxima, with an accuracy of ±3″. In more than half of the flares, there were HXR sources outside of regions of enhanced PVECs. We found no correlation between the intensity of the HXR sources and PVEC density or total PVEC under them. No systematic dissipation of PVECs under the HXR sources was found during the flares. Collectively, the results do not support the current-interruption flare models. However, the results indicate the importance of the presence of longitudinal currents in flare regions. To understand their specific role in the processes of energy release, plasma heating, and acceleration of particles requires further investigation.

Accelerated Electrons Observed Down to

We report the detection of emission from a nonthermal electron distribution in a small solar microflare (GOES class A5.7) observed by the Nuclear Spectroscopic Telescope Array, with supporting observation by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). The flaring plasma is well accounted for by a thick-target model of accelerated electrons collisionally thermalizing within the loop, akin to the "coronal thick-target" behavior occasionally observed in larger flares. This is the first positive detection of nonthermal hard X-rays from the Sun using a direct imager (as opposed to indirectly imaging instruments). The accelerated electron distribution has a spectral index of 6.3 ± 0.7, extends down to at least 6.5 keV, and deposits energy at a rate of ~2 × 1027 erg s-1, heating the flare loop to at least 10 MK. The existence of dominant nonthermal emission in X-rays down to <5 keV means that RHESSI emission is almost entirely nonthermal, contrary to what is usually assumed in RHESSI spectroscopy. The ratio of nonthermal to thermal energies is similar to that of large flares, in contrast to what has been found in previous studies of small RHESSI flares. We suggest that a coronal thick target may be a common property of many small microflares based on the average electron energy and collisional mean free path. Future observations of this kind will enable understanding of how flare particle acceleration changes across energy scales, and will aid the push toward the observational regime of nanoflares, which are a possible source of significant coronal heating.

A Multiwavelength Analysis of the Long-duration Flare Observed on 15 April 2002

We present a multiwavelength analysis of the long-duration flare observed on 15 April 2002 (soft X-ray peak time at 03:55 UT, SOL2002-04-15T03:55). This flare occurred on the disk (S15W01) in NOAA 9906 and was observed by a number of space instruments including the Extreme-Ultraviolet Imaging Telescope on the Solar and Heliospheric Observatory (SOHO/EIT), the RESIK spectrometer onboard the Coronas-F spacecraft, and the Ramaty High Energy Solar Spectroscopic Imager (RHESSI). We have performed a complex analysis of these measurements and studied the morphology and physical parameters characterizing the conditions in flaring plasmas. The 195 Å SOHO/EIT images have been used to study evolution of flaring loops. Analysis of RHESSI data provided the opportunity for a detailed analysis of hard X-ray emission with 1 keV energy resolution. We have used Geostationary Operational Environmental Satellite (GOES) observations for isothermal interpretation of the X-ray measurements. Temperature diagnostics of the flaring plasma have been carried out by means of a differential emission measure (DEM) analysis based on RESIK X-ray spectra. The DEM distributions were calculated based on two methods: Withbroe–Sylwester (WS) and differential evolution (DE). Both of the approaches provided similar results. We obtained two-component DEM distributions independent of the evolutionary flare phase. We found that the amount of energy of thermal plasma for this flare is of the order of 1030 ergs. The values obtained by assuming an isothermal plasma model are lower than those determined from the differential emission measure distributions.

In-flight Low Energy X-ray Calibration of POLAR Detector on TianGong2

POLAR is a γ-ray burst polarimeter on board the TianGong2 space lab, which was launched into a near earth orbit in 2016 September 15. It is designed to measure the linear polarization of hard X-ray in the 50–500keV energy range. The detector has 25 modules, and each module has 64 plastic scintillator bars. With a total number of 1600 plastic scintillator bars, POLAR has a very large effective detection area and field of view. During its on-orbit operation, POLAR has detected multiple small flares. Their hard X-ray photon energies are usually less than 50keV, and can not be analyzed directly by using the on-orbit and ground-based high-energy calibration results. By combining the RHESSI (Reuven Ramaty High Energy Solar Spectroscopic Imager) observation of the flare SOL2016112907 with the Monte Carlo simulation, this paper calibrates the response of POLAR in the energy band lower than 50keV. We find that the energy threshold (∼ 10keV) and conversion factor obtained by calibration are relatively stable, but they are quite different from the values obtained via calibration in the high-energy range, and that the differences for the different scintillator bars have no evident regularity.

A Remarkably Narrow RHESSI X-Ray Flare on 2011 September 25

Abstract The unusually narrow X-ray source imaged with the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) during an impulsive spike lasting for ∼10 s during the Geostationary Operational Environmental Satellite C7.9 flare on 2011 September 25 (SOL2011-09-25T03:32) was only ∼2″ wide and ∼10″ long. Comparison with Helioseismsic and Magnetic Imager magnetograms and Atmospheric Imaging Assembly images at 1700 Å shows that the X-ray emission was primarily from a long ribbon in the region of positive polarity with little if any emission from the negative polarity ribbon. However, a thermal plasma source density of ∼1012 cm−3 estimated from the RHESSI-derived emission measure and source area showed that this could best be interpreted as a coronal hard X-ray source in which the accelerated electrons with energies less than ∼50 keV were stopped by Coulomb collisions in the corona, thus explaining the lack of the more usual bright X-ray footpoints. Analysis of RHESSI spectra shows greater consistency with a multi-temperature distribution and a low-energy cutoff to the accelerated electron spectrum of 22 keV compared to 12 keV if a single-temperature distribution is assumed. This leads to a change in the lower limit on the total energy in electrons by an order of magnitude, given the steepness of the best-fit electron spectrum with a power-law index of ∼6.

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.

Accelerated Electron Propagation Model for the Flare Arcade of the September 23, 2014, Event from RHESSI, SDO, and Nobeyama Radioheliograph Observations

A multi-wavelength study and simulation is conducted for the solar flare SOL2014-09-23T23:11. GOES-class M2.5. Radio (Nobeyama Radioheliograph) and X-ray (Reuven Ramaty High Energy Solar Spectroscopic Imager) maps and energy spectra are studied, including their temporal and spatial evolution, with the superposition of radiation sources on the magnetic field structure. For the first time, a simulation of accelerated electrons transport in several characteristic magnetic structures of a flare arcade simultaneously is conducted. Accelerated electron injection functions, which depend on the space along the magnetic loop, pitch angle, and energy, are fitted to reconcile observations with the simulation results such that the calculated radiation parameters are consistent with the observed brightness distributions of hard X‑rays in the range of 28–135 keV and gyrosynchrotron radiation of 17 and 35 GHz. The energy spectrum of the electron beam corresponds in this case to one- and two-power law functions with spectral indices in the ranges δ1 ≈ 4–5 and δ2 ≈ 2.0–2.4 and spectrum break energy Ebr ≈ 240–320 keV, depending on the specific magnetic structure. The pitch-angle distribution of accelerated electrons is essentially an anisotropic electron flux with an divergence angle of ~80° from the magnetic looptops towards the footpoint with a dominant hard X-ray source.

Preflare Processes, Flux Rope Activation, Large-scale Eruption, and Associated X-class Flare from the Active Region NOAA 11875

Abstract We present a multiwavelength analysis of the eruption of a hot coronal channel associated with an X1.0 flare (SOL2013-10-28T02:03) from the active region NOAA 11875 by combining observations from Atmospheric Imaging Assembly/Solar Dynamics Observatory (SDO), Helioseismic and Magnetic Imager/SDO, Reuven Ramaty High Energy Solar Spectroscopic Imager, and Hiraiso Radio Spectrograph. EUV images at high coronal temperatures indicated the presence of a hot channel at the core of the active region from the early preflare phase evidencing the preexistence of a quasi-stable magnetic flux rope. The hot channel underwent an activation phase after a localized and prolonged preflare event occurring adjacent to one of its footpoints. Subsequently, the flux rope continued to rise slowly for ≈16 minutes during which soft X-ray flux gradually built-up characterizing a distinct precursor phase. The flux rope transitioned from the state of slow rise to the eruptive motion with the onset of the impulsive phase of the X1.0 flare. The eruptive expansion of the hot channel is accompanied by a series of type III radio bursts in association with the impulsive rise of strong hard X-ray nonthermal emissions that included explicit hard X-ray sources of energies up to ≈50 keV from the coronal loops and ≈100 keV from their footpoint locations. Our study contains evidence that preflare activity occurring within the spatial extent of a stable flux rope can destabilize it toward eruption. Moreover, sudden transition of the flux rope from the state of slow rise to fast acceleration precisely bifurcated the precursor and the impulsive phases of the flare, which points toward a feedback relationship between early coronal mass ejection dynamics and the strength of the large-scale magnetic reconnection.

Statistical Study of Hard X-Ray Spectral Breaks in Solar Flares

The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) provides hard X-ray spectral observations with ${\approx}\,1~\mbox{keV}$ resolution to study flare-accelerated ( ${>}\,10~\mbox{keV}$ ) electrons through their bremsstrahlung emission. Here we report on a statistical study of RHESSI flares with emission above 150 keV, focusing on the spectral shape at the hard X-ray peak. Spectral parameters are derived by fitting the photon spectrum with a broken power law and by the standard thick-target fit. Consistent with previous studies, the most common spectral shape of the photon spectrum (52 out of 65 events) is a double power law with a downward break (“knee”), with ten events showing a single power law and three events having an upward break (“ankle”). The spectral breaks occur typically around 55 keV and the difference of the spectral index above and below the break, $\gamma _{2}$ and $\gamma _{1}$ , is typically between 0.3 and 1. We show correlations between the downward break parameters. The most prominent correlation, with a rank order coefficient of $\rho =0.92$ , is between the power-law indices above and below the break: $\gamma _{1} = (0.74\pm 0.04)\gamma _{2} + (0.34 \pm 0.14)$ . Applying a thick target fit to the photon spectrum, a similar correlation is also found for the flare-accelerated electron spectra with $\delta _{1} =(0.85\pm 0.08)\delta _{2} - (0.3\pm 0.3)$ ( $\rho =0.67$ ). Spectral breaks could be a property of the acceleration mechanism itself or they could be a secondary effect produced by particle transport or wave-particle interactions. Any theoretical models should be consistent with these correlations. In addition, we find that one upward and 23 (49%) downward breaks are consistent with nonuniform ionization within the thick target.

Hot Plasma in a Quiescent Solar Active Region as Measured by RHESSI, XRT, and AIA

Abstract This paper investigates a quiescent (nonflaring) active region observed on 2010 July 13 in extreme ultraviolet (EUV), soft X-ray (SXR), and hard X-rays to search for a hot component that is speculated to be a key signature of coronal heating. We use a combination of Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) imaging and long-duration time integration (up to 40 minutes) to detect the active regions in the 3–8 keV range during apparently nonflaring times. The RHESSI imaging reveals a hot component that originates from the entire active region, as speculated for a nanoflare scenario where the entire active region is filled with a large number of unresolved small energy releases. An isothermal fit to the RHESSI data gives temperatures around ∼7 MK with an emission measure of several times 1046 cm−3. Adding EUV and SXR observations taken by AIA and the X-ray Telescope, respectively, we derive a differential emission measure (DEM) that shows a peak between 2 and 3 MK with a steeply decreasing high-temperature tail, similar to what has been previously reported. The derived DEM reveals that a wide range of temperatures contributes to the RHESSI flux (e.g., 40% of the 4 keV emission being produced by plasma below 5 MK, while emission at 7 keV is almost exclusively from plasmas above 5 MK) indicating that the RHESSI spectrum should not be fitted with an isothermal. The hot component has a rather small emission measure (∼0.1% of the total EM is above 5 MK), and the derived thermal energy content is of the order of 10% for a filling factor of unity, or potentially below 1% for smaller filling factors.

Oscillations Accompanying a He I 10830 Å Negative Flare in a Solar Facula II. Response of the Transition Region and Corona

We studied oscillations related to flare SOL2012-09-21T02:19 in the transition region and chromosphere based on Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and Solar Dynamics Observatory (SDO) data, as well as data from a ground-based observatory. We found that 2-minute oscillations triggered by the flare first appeared in the RHESSI channels then subsequently showed up in the SDO 171 {\AA} and 304 {\AA} channels and after that in the chromospheric He I 10830 {\AA} line. The delay of the chromospheric signal compared to the RHESSI signal is 7 minutes, which indicates that the wave perturbation propagated from the corona to the chromosphere. We used the sharp increase in 3- and 5-minute oscillations during the flare in the lower atmosphere to trace the propagation of the oscillation trains to the transition region and corona. The results show that the 171 {\AA} channel signals lagged behind the photospheric and chromospheric signals by 200 s on average. We suggest that we observed slow magnetoacoustic waves both in the case of 2-minute oscillations propagating downwards from the corona and in the case of 3- and 5-minute oscillations leaking to the corona from beneath.

Persistent Quasi-periodic Pulsations during a Large X-class Solar Flare

Abstract Solar flares often display pulsating and oscillatory signatures in the emission, known as quasi-periodic pulsations (QPP). QPP are typically identified during the impulsive phase of flares, yet in some cases, their presence is detected late into the decay phase. Here, we report extensive fine structure QPP that are detected throughout the large X8.2 flare from 2017 September 10. Following the analysis of the thermal pulsations observed in the Geostationary Operational Environmental Satellite/X-ray sensor and the 131 Å channel of Solar Dynamics Observatory/Atmospheric Imaging Assembly, we find a pulsation period of ∼65 s during the impulsive phase followed by lower amplitude QPP with a period of ∼150 s in the decay phase, up to three hours after the peak of the flare. We find that during the time of the impulsive QPP, the soft X-ray source observed with the Reuven Ramaty High Energy Solar Spectroscopic Imager rapidly rises at a velocity of approximately 17 km s−1 following the plasmoid/coronal mass ejection eruption. We interpret these QPP in terms of a manifestation of the reconnection dynamics in the eruptive event. During the long-duration decay phase lasting several hours, extended downward contractions of collapsing loops/plasmoids that reach the top of the flare arcade are observed in EUV. We note that the existence of persistent QPP into the decay phase of this flare are most likely related to these features. The QPP during this phase are discussed in terms of magnetohydrodynamic wave modes triggered in the post-flaring loops.

Count-based imaging model for the Spectrometer/Telescope for Imaging X-rays (STIX) in Solar Orbiter

The Spectrometer/Telescope for Imaging X-rays (STIX) will study solar flares across the hard X-ray window provided by the Solar Orbiter cluster. Similarly to the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), STIX is a visibility-based imaging instrument that will require Fourier-based image reconstruction methods. However, in this paper we show that as for RHESSI, count-based imaging is also possible for STIX. Specifically, we introduce and illustrate a mathematical model that mimics the STIX data formation process as a projection from the incoming photon flux into a vector consisting of 120 count components. Then we test the reliability of expectation maximization for image reconstruction in the case of several simulated configurations that are typical of flare morphology.

Characteristics of solar microflares as seen in soft X-ray emission

In this paper, we present the thermal and non-thermal characteristics of solar plasma producing microflares in 4–12 keV energy range. The X-ray spectra of 10 B-class solar microflares observed by the silicon (Si) detector (4–25 keV) on-board solar X-ray spectrometer (SOXS) mission were analysed in 4–12 keV energy range. We employed forward fitting for the spectral modelling of thermal and non-thermal components of X-ray spectra with isothermal, multithermal and single power-law functions in order to determine flare parameters. The fit results obtained from the combination of isothermal and single power-law functions yield a weighted mean value of emission measure \((\mathrm{EM})\approx 0.0203 \times 10^{49} \mathrm{cm^{-3}} \), plasma temperature [Case (1)] \( {T(1)} \approx 10.24\,{\mathrm{MK}}\) and non-thermal spectral index \( \gamma (1) \approx 3.90 \). The fit results obtained from the combination of multithermal and single power-law functions yield a weighted mean value of differential emission measure, \((\mathrm{DEM}) \approx 0.00116 \times 10^{49}\,\mathrm{cm^{-3}}\, \mathrm{keV^{-1}} \), plasma temperature [Case (2)], \({T(2)}\approx 12.90\,{\mathrm{MK}}\), thermal spectral index, \(\delta \approx 4.06\) and non-thermal spectral index, \( \gamma (2) \approx 3.81 \). Further, we obtained the mean value of conduction cooling time, \( \tau _{\mathrm{c}}(T)\approx 283\,{\mathrm{s}}\) at 11.6 MK, thermal energy, \({E_{\mathrm{th}}} \approx 0.50 \times 10^{29}{\mathrm{erg}}\) and thermal–non-thermal cross-over energy, \( \epsilon _{\mathrm{th}} \approx 9.23\) keV. In this analysis, the obtained results were found to be compatible with the earlier analysis carried out for the microflares through Reuven Ramety High Energy Solar Spectroscopic Imager (RHESSI), Solar Dynamics Observatory / Atmospheric Imaging Assembly (SDO/AIA) and NuSTAR observations. Here, we observed that EM decreases with increasing plasma temperature (T). We find that \(\tau _{\mathrm{c}}\)(T) scale with plasma temperature (T) with an inverse gradient exhibits time delay characteristic of the cooling process of plasma. The correlation of \( E_{\mathrm{th}} \) and temperature (T) shows moderate anticorrelation. The present analysis demonstrates the multithermal plasma model and conduction cooling process during high temperature of microflares (similar to large flares) followed by radiative cooling in post-flare.

Observationally Weak TGFs in the RHESSI Data.

Terrestrial gamma ray flashes (TGFs) are sub‐millisecond bursts of high energetic gamma radiation associated with intracloud flashes in thunderstorms. In this paper we use the simultaneity of lightning detections by World Wide Lightning Location Network to find TGFs in the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) data that are too faint to be identified by standard search algorithms. A similar approach has been used in an earlier paper, but here we expand the data set to include all years of RHESSI + World Wide Lightning Location Network data and show that there is a population of observationally weak TGFs all the way down to 0.22 of the RHESSI detection threshold (three counts in the detector). One should note that the majority of these are “normal” TGFs that are produced further away from the subsatellite point (and experience a 1/r 2 effect) or produced at higher latitudes with a lower tropoause and thus experience increased atmospheric attenuation. This supports the idea that the TGF production rate is higher than currently reported. We also show that compared to lightning flashes, TGFs are more partial to ocean and coastal regions than over land.

A comparative study on the lightning sferics associated with terrestrial gamma-ray flashes observed in Americas and Asia

We examine the broadband (<1 Hz to 400 kHz) electromagnetic lightning signals associated with Terrestrial Gamma-ray Flashes (TGFs) detected by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and the Gamma-ray Burst Monitor (GBM) on the Fermi Gamma-ray Space Telescope during 2010/2011. The TGF-associated lightning sferics are mainly recorded at two stations located near National Cheng-Kung University in Taiwan and near Duke University in the United States, respectively. The general features exhibited by the TGF-associated lightning sferics are consistent with previous findings that gamma-rays in TGFs are typically produced during a slow process that creates a considerable (but not necessarily) charge moment change within several milliseconds. In some cases, this slow process can be attributed to the upward negative leader during the initial stage of normal intra-cloud (IC) lightning, and it is usually punctuated by one or several fast discharges, the major one of which (i.e. TGF-related discharge) is closely involved in the gamma-ray production. The equivalent peak current of TGF-related discharges could be as high as >+500 kA, and the associated charge transfer is also considerable (typically >+20 C km). The observed complexity of TGF-associated lightning emissions can also be interpreted in the context of the initial development of normal IC lightning flashes, where the upward negative leader drives the millisecond-scale current and may also provide the seed electrons for avalanche multiplication in the upper part of active thunderstorms. Our analyses show that the thunderstorms in the land area of South China produce TGFs that can be readily observed by Fermi/GBM, and the future ground-based coordinated observations in this area would be rather promising to gain more insights into the physical mechanism of TGFs.