Reuven Ramaty High Energy Solar Spectroscopic Imager Data Research Articles

Visibility Interpolation in Solar Hard X-Ray Imaging: Application to RHESSI and STIX

Abstract Space telescopes for solar hard X-ray imaging provide observations made of sampled Fourier components of the incoming photon flux. The aim of this study is to design an image reconstruction method relying on enhanced visibility interpolation in the Fourier domain. The interpolation-based method is applied to synthetic visibilities generated by means of the simulation software implemented within the framework of the Spectrometer/Telescope for Imaging X-rays (STIX) mission on board Solar Orbiter. An application to experimental visibilities observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) is also considered. In order to interpolate these visibility data, we have utilized an approach based on Variably Scaled Kernels (VSKs), which are able to realize feature augmentation by exploiting prior information on the flaring source and which are used here, for the first time, in the context of inverse problems. When compared to an interpolation-based reconstruction algorithm previously introduced for RHESSI, VSKs offer significantly better performance, particularly in the case of STIX imaging, which is characterized by a notably sparse sampling of the Fourier domain. In the case of RHESSI data, this novel approach is particularly reliable when the flaring sources are either characterized by narrow, ribbon-like shapes or high-resolution detectors are utilized for observations. The use of VSKs for interpolating hard X-ray visibilities allows remarkable image reconstruction accuracy when the information on the flaring source is encoded by a small set of scattered Fourier data and when the visibility surface is affected by significant oscillations in the frequency domain.

Automatic Detection of Occulted Hard X-Ray Flares Using Deep-Learning Methods

We present a concept for a machine-learning classification of hard X-ray (HXR) emissions from solar flares observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), identifying flares that are either occulted by the solar limb or located on the solar disk. Although HXR observations of occulted flares are important for particle-acceleration studies, HXR data analyses for past observations were time consuming and required specialized expertise. Machine-learning techniques are promising for this situation, and we constructed a sample model to demonstrate the concept using a deep-learning technique. Input data to the model are HXR spectrograms that are easily produced from RHESSI data. The model can detect occulted flares without the need for image reconstruction nor for visual inspection by experts. A technique of convolutional neural networks was used in this model by regarding the input data as images. Our model achieved a classification accuracy better than 90 %, and the ability for the application of the method to either event screening or for an event alert for occulted flares was successfully demonstrated.

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.

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.

Electron Acceleration and Jet-facilitated Escape in an M-class Solar Flare on 2002 August 19

Abstract 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, 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 the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and the Owens Valley Solar Array (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 s or shorter. We model the energetic electron distributions in the GX Simulator framework using the Solar and Heliospheric Observatory (SOHO)/Michelson Doppler Imager (MDI), the Transition Region and Coronal Explorer (TRACE), RHESSI, 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.

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.

Solar hard X-ray imaging by means of compressed sensing and finite isotropic wavelet transform

Aims. Compressed sensing realized by means of regularized deconvolution and the finite isotropic wavelet transform is effective and reliable in hard X-ray solar imaging. Methods. The method uses the finite isotropic wavelet transform with the Meyer function as the mother wavelet. Furthermore, compressed sensing is realized by optimizing a sparsity-promoting regularized objective function by means of the fast iterative shrinkage-thresholding algorithm. Eventually, the regularization parameter is selected by means of the Miller criterion. Results. The method is applied against both synthetic data mimicking measurements made with the Spectrometer/Telescope Imaging X-rays (STIX) and experimental observations provided by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). The performances of the method are qualitatively validated by comparing some morphological properties of the reconstructed sources with those of the corresponding synthetic configurations. Furthermore, the results concerning experimental data are compared with those obtained by applying other visibility-based reconstruction methods. Conclusions. The results show that when the new method is applied to synthetic STIX visibility sets, it provides reconstructions with a spatial accuracy comparable to the accuracy provided by the most popular method in hard X-ray solar imaging and with a higher spatial resolution. Furthermore, when it is applied to experimental RHESSI data, the reconstructions are characterized by reliable photometry and by a notable reduction of the ringing effects caused by the instrument point spread function.

Periodic Recurrence Patterns In X-Ray Solar Flare Appearances

Abstract The temporal recurrence of micro-flare events is studied for a time interval before and after of major solar flares. Our sample is based on the X-ray flare observations by the Geostationary Operational Environmental Satellite (GOES) and Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). The analyzed data contain 1330/301 M-class and X-class GOES/RHESSI energetic solar flares and 4062/4119 GOES/RHESSI micro-flares covering the period elapse since 2002. The temporal analysis of recurrence, by Fast Fourier Transform, of the micro-flares, shows multiple significant periods. Based on the GOES and RHESSI data, the temporal analysis also demonstrates that multiple periods manifest simultaneously in both statistical samples without any significant shift over time. In the GOES sample, the detected significant periods are: 11.33, 5.61, 3.75, 2.80, and 2.24 minutes. The RHESSI data show similar significant periods at 8.54, 5.28, 3.66, 2.88, and 2.19 minutes. The periods are interpreted as signatures of standing oscillations, with the longest period (P 1) being the fundamental and others being higher harmonic modes. The period ratio of the fundamental and higher harmonics (P 1/P N ) is also analyzed. The standing modes may be signatures of global oscillations of the entire solar atmosphere encompassing magnetized plasma from the photosphere to the corona in active regions.

A new population of terrestrial gamma‐ray flashes in the RHESSI data

AbstractTerrestrial gamma‐ray flashes (TGFs) are the most energetic photon phenomenon occurring naturally on Earth. An outstanding question is as follows: Are these flashes just a rare exotic phenomenon or are they an intrinsic part of lightning discharges and therefore occurring more frequently than previously thought? All measurements of TGFs so far have been limited by the dynamic range and sensitivity of spaceborne instruments. In this paper we show that there is a new population of weak TGFs that has not been identified by search algorithms. We use the World Wide Lightning Location Network (WWLLN) to identify lightning that occurred in 2006 and 2012 within the 800 km field of view of Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). By superposing 740,210 100 ms RHESSI data intervals, centered at the time of the WWLLN detected lightning, we identify at least 141 and probably as many as 191 weak TGFs that were not part of the second RHESSI data catalogue. This supports the suggestion that the global TGF production rate is larger than previously reported.

Connection between the appearance of γ-ray sources and Hα line impact linear polarization in the July 23, 2002 flare

During our spectropolarimetric observations of the 2B/4.8X flare on July 23, 2002, with the Large Solar Vacuum Telescope (LSVT), we detected impact linear polarization in the southern ribbon. The maximum polarization exceeded 10%. On the whole, the polarization was observed only for ~6 min out of almost 2 h of observations of this flare. At this time, the Hα line profiles had a deep central self-reversal. We have compared the LSVT observations with the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) data, where the locations of γ-ray emission were obtained for the first time. A careful comparison has revealed that the effects in the Hα line were observed in a region <10″ located between two X-ray sources in the southern ribbon. According to the RHESSI data, a γ-ray source attributable to a beam of high-energy (~1 MeV) electrons was observed at the same place. The events in the corona and the upper chromosphere also coincide in time with the observed effects in the Hα line. The two X-ray sources are assumed to represent a common footpoint of the southern foot of the loop bifurcated due to the precipitation of high-energy electrons. It is suggested that the penetration of these electrons into the dense layers of the chromosphere could lead to impact polarization and a decrease of the Hα line intensity in the southern ribbon of the flare, in contrast to the northern ribbon where a typical hard X-ray source with an energy ~20–120 keV was observed; there were no self-reversal at the Hα line center and polarization.

STATISTICAL PROPERTIES OF SUPER-HOT SOLAR FLARES

We use Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) high-resolution imaging and spectroscopy observations from ∼6 to 100 keV to determine the statistical relationships between measured parameters (temperature, emission measure, etc.) of hot, thermal plasma in 37 intense (GOES M- and X-class) solar flares. The RHESSI data, most sensitive to the hottest flare plasmas, reveal a strong correlation between the maximum achieved temperature and the flare GOES class, such that "super-hot" temperatures >30 MK are achieved almost exclusively by X-class events; the observed correlation differs significantly from that of GOES-derived temperatures, and from previous studies. A nearly ubiquitous association with high emission measures, electron densities, and instantaneous thermal energies suggests that super-hot plasmas are physically distinct from cooler, ∼10–20 MK GOES plasmas, and that they require substantially greater energy input during the flare. High thermal energy densities suggest that super-hot flares require strong coronal magnetic fields, exceeding ∼100 G, and that both the plasma β and volume filling factor f cannot be much less than unity in the super-hot region.

A new method reveals more TGFs in the RHESSI data

This letter presents a new search algorithm for identifying Terrestrial Gamma ray Flashes (TGFs) in the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) data. The algorithm has been applied to data from the period 2004–2006 and we have found more than twice as many TGFs as previously reported. The new TGFs follow the same geographical and seasonal variations as the previously reported TGFs. The match percentage between the new TGFs and World Wide Lightning Location Network (WWLLN) data is comparable to the RHESSI catalog TGFs. Our results shows that previous searches only identified the most intense events, and that there might be a large population of faint TGFs.

A STATISTICAL STUDY OF SPECTRAL HARDENING IN SOLAR FLARES AND RELATED SOLAR ENERGETIC PARTICLE EVENTS

Using hard X-ray observations from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), we investigate the reliability of spectral hardening during solar flares as an indicator of related solar energetic particle (SEP) events at Earth. All RHESSI data are analyzed, from 2002 February through the end of Solar Cycle 23, thereby expanding upon recent work on a smaller sample of flares. Previous investigations have found very high success when associating soft-hard-harder (SHH) spectral behavior with energetic proton events, and confirmation of this link would suggest a correlation between electron acceleration in solar flares and SEPs seen in interplanetary space. In agreement with these past findings, we find that of 37 magnetically well-connected flares (W30–W90), 12 of 18 flares with SHH behavior produced SEP events and none of 19 flares without SHH behavior produced SEPs. This demonstrates a statistically significant dependence of SHH and SEP observations, a link that is unexplained in the standard scenario of SEP acceleration at the shock front of coronal mass ejections and encourages further investigation of the mechanisms which could be responsible.

Different Behaviors between Microwave and Hard X-Ray Spectral Hardness in Two Solar Flares

We explore the spectral behaviors of microwave and hard X-ray emissions in two solar flares observed by the Nobeyama Radio Polarimeters (NoRP) and the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) on 2003 May 29 and 2004 January 6. The microwave spectral index γMW is derived from the emissions at two frequency channels of 17 and 35 GHz, and the standard RHESSI data analysis is used to derive the hard X-ray spectral index γHXR. The 2003 May 29 flare displays a soft-hard-soft spectral behavior in the hard X-ray rise-peak-decay phases, but a soft-hard-harder pattern in the microwave emission. The 2004 January 6 flare shows several soft-hard-soft spectral behaviors in its hard X-ray subpeaks, but only one soft-hard-soft pattern in the microwave emission from start to end. The imaging observations show that the thermal emission could be an important contribution to microwave spectral hardening with the time.

Re-analysis of polarization in the γ-ray flux of GRB 021206

A previous analysis of the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) observation of GRB 021206 found that the γ-ray flux was 80 ± 20 per cent polarized. We re-examine these data and find no signal that can be interpreted as due to polarization. First, we find that the number of scattering events suitable for measuring polarization – having been scattered from one detector to another, with a count produced in both – is considerably lower than estimated by Coburn & Boggs, by a factor of 10 (830 ± 150, versus 9840 ± 96). The signal-to-noise ratio of the data set is thus too low to produce a detection, even from a 100 per cent polarized source. None the less, we develop a polarization-detection analysis limited in sensitivity only by Poisson noise, which does not require a spacecraft mass model to detect polarization, as in Coburn & Boggs. We find no signal which might be interpreted as due to polarization of GRB 021206. Separately, we reproduce the Coburn & Boggs signal and show that it is not due to polarization. Rather, the Coburn & Boggs signal is consistent with the previously neglected systematic uncertainty in the ‘null light curve’ used for detection. Due to the low signal-to-noise ratio of the RHESSI data, our Poisson noise-limited analysis results in an upper limit consistent with 100 per cent polarization of the γ-ray flux from GRB 021206. Thus, no observational constraint on the polarization of GRB 021206 can be derived from these data.