Reuven Ramaty High Energy Spectroscopic Imager Research Articles

An overview of HMI off-disk flare observations

Context. White-light continuum observations of solar flares often have coronal counterparts, including the classical ‘white-light prominence’ (WLP) phenomenon. Aims. Coronal emissions by flares, seen in the white-light continuum, have only rarely been reported. We seek to use modern data to understand the morphology of WLP events. Methods. We have identified a set of 14 examples of WLPs detected by the Heliospheric and Magnetic Imager (HMI) experiment on board the Solar Dynamics Observatory (SDO) satellite using a new online catalogue that covers 2011–2017. These WLPs invariably accompanied white-light flare (WLF) emission from the lower atmosphere from flares near the limb, as identified by hard X-ray images from the Reuven Ramaty High Energy Spectroscopic Imager (RHESSI). HMI provides full Stokes information, and we have used the linear polarisations (Q and U) to distinguish Thomson scattering from cool material. Results. The event morphologies fit roughly into three categories – ejection, loop, and spike – but many events show multiple phenomena. Conclusions. The coronal white-light continuum, observed by HMI analogously to the observations made by a coronagraph, reveals many examples of coronal emission and dynamics. Using the Stokes linear polarisation, we estimate the masses of hot coronal plasma in 11 of the 14 events and find them to be similar to typical coronal-mass-ejection masses, but without exceeding 1015 g. We note that the HMI observations do not occult the bright solar disk and were not designed for coronal observations, resulting in relatively low signal-to-noise ratios. We therefore believe that future such observations with better optimisation will be even more fruitful.

The Solar X-Ray Limb

Abstract We describe a new technique to measure the height of the X-ray limb with observations from occulted X-ray flare sources as observed by the RHESSI (the Reuven Ramaty High-Energy Spectroscopic Imager) satellite. This method has model dependencies different from those present in traditional observations at optical wavelengths, which depend upon detailed modeling involving radiative transfer in a medium with complicated geometry and flows. It thus provides an independent and more rigorous measurement of the “true” solar radius, which means that of the mass distribution. RHESSI’s measurement makes use of the flare X-ray source’s spatial Fourier components (the visibilities), which are sensitive to the presence of the sharp edge at the lower boundary of the occulted source. We have found a suitable flare event for analysis, SOL2011-10-20T03:25 (M1.7), and report a first result from this novel technique here. Using a four-minute integration over the 3–25 keV photon energy range, we find R X ‐ ray = 960.11 ± 0.15 ± 0.29 arcsec, at 1 au, where the uncertainties include statistical uncertainties from the method and a systematic error. The standard VAL-C model predicts a value of 959.94 arcsec, which is about 1σ below our value.

THE HEIGHT OF A WHITE-LIGHT FLARE AND ITS HARD X-RAY SOURCES

We describe observations of a white-light flare (SOL2011-02-24T07:35:00, M3.5) close to the limb of the Sun, from which we obtain estimates of the heights of the optical continuum sources and those of the associated hard X-ray sources.For this purpose we use hard X-ray images from the Reuven Ramaty High Energy Spectroscopic Imager (RHESSI), and optical images at 6173 \AA from the Solar Dynamics Observatory (SDO). We find that the centroids of the impulsive-phase emissions in white light and hard X-rays (30-80 keV) match closely in central distance (angular displacement from Sun center), within uncertainties of order 0.2". This directly implies a common source height for these radiations, strengthening the connection between visible flare continuum formation and the accelerated electrons. We also estimate the absolute heights of these emissions, as vertical distances from Sun center. Such a direct estimation has not been done previously, to our knowledge. Using a simultaneous 195 \AA image from the Solar-Terrestrial RElations Observatory (STEREO-B) spacecraft to identify the heliographic coordinates of the flare footpoints, we determine mean heights above the photosphere (as normally defined; \tau = 1 at 5000 \AA) of 305 \pm 170 km and 195 \pm 70 km, respectively, for the centroids of the hard X-ray (HXR) and white light (WL) footpoint sources of the flare. These heights are unexpectedly low in the atmosphere, and are consistent with the expected locations of \tau = 1 for the 6173 \AA and the ~40 keV photons observed, respectively.

Hard X-ray emission from a flare-related jet

Aims. We aim to understand the physical conditions in a jet event which occurred on the 22nd of August 2002, paying particular attention to evidence for non-thermal electrons in the jet material. Methods. We investigate the flare impulsive phase using multiwavelength observations from the Transition Region and Coronal Explorer (TRACE) and the Reuven Ramaty High Energy Spectroscopic Imager (RHESSI) satellite missions, and the ground-based Nobeyama Radioheliograph (NoRH) and Radio Polarimeters (NoRP). Results. We report what we believe to be the first observation of hard X-ray emission formed in a coronal jet. We present radio observations which confirm the presence of non-thermal electrons present in the jet at this time. The evolution of the event is best compared with the magnetic reconnection jet model in which emerging magnetic field interacts with the pre-existing coronal field. We calculate an apparent jet velocity of ~500 km s -1 which is consistent with model predictions for jet material accelerated by the $\vec{J}\times\vec{B}$ force resulting in a jet velocity of the order of the Alfven speed (~100–1000 km s -1 ).

Response to Comment on “A Large Excess in Apparent Solar Oblateness Due to Surface Magnetism”

Our analysis of satellite data from the Reuven Ramaty High-Energy Spectroscopic Imager revealed a strong correlation with a magnetic proxy, leading to an anomalous increase of the solar oblateness. We modeled this effect to yield an oblateness measurement of an assumed underlying nonmagnetic Sun. Kuhn et al . note that this measurement could be confused with a systematic spatial brightness variation. The existing data on this are ambiguous.

Magnetic flux cancellation associated with a recurring solar jet observed withHinode,RHESSI, andSTEREO/EUVI

Aims. We study the physical properties of a recurring solar active region jet observed in X-rays and extreme-ultraviolet (EUV).Methods. Multi-wavelength data from all three instruments on board Hinode were analysed. X-ray imaging and spectroscopy of the microflaring emission associated with the jets was performed with the Reuven Ramaty High Energy Spectroscopic Imager (RHESSI ). Associated EUV jets were observed with the Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI)/Extreme Ultraviolet Imager (EUVI) on board STEREO .Results. We found a correlation between recurring magnetic flux cancellation close to a pore, the X-ray jet emission, and associated Ca II H ribbon brightenings. We estimated the lower limit for the decrease in magnetic energy associated with the X-ray jet emission at 3 1029 erg. The recurring plasma ejection was observed simultaneously at EUV and X-ray temperatures, associated with type III radio bursts and microflaring activity at the jet footpoint.Conclusions. The recurring jet (EUV and X-ray) emissions can be attributed to chromospheric evaporation flows due to recurring coronal magnetic reconnection. In this process, the estimated minimum loss in the magnetic energy is sufficient to account for the total energy required to launch the jet.

Coronal γ-Ray Bremsstrahlung from Solar Flare-accelerated Electrons

The Reuven Ramaty High Energy Spectroscopic Imager (RHESSI) provides for the first time imaging spectroscopy of solar flares up to the γ-ray range. The three RHESSI flares with best counting statistics are analyzed in the 200-800 keV range revealing γ-ray emission produced by electron bremsstrahlung from footpoints of flare loops, but also from the corona. Footpoint emission dominates during the γ-ray peak, but as the γ-ray emission decreases the coronal source becomes more and more prominent. Furthermore, the coronal source shows a much harder spectrum (with power-law indices γ between 1.5 and 2) than the footpoints (with γ between 3 and 4). These observations suggest that flare-accelerated high-energy (~MeV) electrons stay long enough in the corona to lose their energy by collisions producing γ-ray emission, while lower energetic electrons precipitate more rapidly to the footpoints.

X-Ray Polarization of Solar Flares Measured with Rhessi

The degree of linear polarization in solar flares has not yet been precisely determined despite multiple attempts to measure it with different missions. The high energy range, in particular, has very rarely been explored, due to its greater instrumental difficulties. We approached the subject using the Reuven Ramaty High Energy Spectroscopic Imager (RHESSI) satellite to study six X-class and 1 M-class flares in the energy range between 100 and 350 keV. Using RHESSI as a polarimeter requires the application of strict cuts to the event list in order to extract those photons that are Compton scattered between two detectors. Our measurements show polarization values between 2 and 54%, with errors ranging from 10 to 26% in 1σ level. In view of the large uncertainties in both the magnitude and direction of the polarization vector, the results can only reject source models with extreme properties.

Rhessi Images and Spectra of Two Small Flares

We studied the evolution of two small flares (GOES class C2 and C1) that developed in the same active region with different morphological characteristics: one is extended and the other is compact. We analyzed the accuracy and the consistency of different algorithms implemented in Reuven Ramaty High-Energy Spectroscopic Imager (RHESSI) software to reconstruct the image of the emitting sources, for energies between 3 and 12 keV. We found that all tested algorithms give consistent results for the peak position, while the other parameters can differ at most by a factor 2. Pixon and Forward-fit generally converge to similar results but Pixon is more reliable for reconstructing a complex source. We investigated the spectral characteristics of the two flares during their evolution in the 3–25 keV energy band. We found that a single thermal model of the photon spectrum is inadequate to fit the observations and we needed to add either a non-thermal model or a hot thermal one. The non-thermal and the double thermal fits are comparable. If we assume a non-thermal model, the non-thermal energy is always higher than the thermal one. Only during the very final decay phase a single thermal model fits the observed spectrum fairly well.