Reuven Ramaty High Energy Solar Spectroscopic Imager Images Research Articles

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.

FLARE FOOTPOINT REGIONS AND A SURGE OBSERVED BYHINODE/EIS,RHESSI, ANDSDO/AIA

The Extreme-ultraviolet Imaging Spectrometer (EIS) on the Hinode spacecraft observed flare footpoint regions coincident with a surge for a M3.7 flare observed on 25 September 2011 at N12 E33 in active region 11302. The flare was observed in spectral lines of O VI, Fe X, Fe XII, Fe XIV, Fe XV, Fe XVI, Fe XVII, Fe XXIII and Fe XXIV. The EIS observations were made coincident with hard X-ray bursts observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). Overlays of the RHESSI images on the EIS raster images at different wavelengths show a spatial coincidence of features in the RHESSI images with the EIS upflow and downflow regions, as well as loop-top or near-loop-top regions. A complex array of phenomena was observed including multiple evaporation regions and the surge, which was also observed by the Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly (AIA) telescopes. The slit of the EIS spectrometer covered several flare footpoint regions from which evaporative upflows in Fe XXIII and Fe XXIV lines were observed with Doppler speeds greater than 500 km s$^{-1}$. For ions such as Fe XV both evaporative outflows (~200 km s$^{-1}$) and downflows (~30-50 km s$^{-1}$) were observed. Non-thermal motions from 120 to 300 km s$^{-1}$ were measured in flare lines. In the surge, Doppler speeds are found from about 0 to over 250 km s$^{-1}$ in lines from ions such as Fe XIV. The non-thermal motions could be due to multiple sources slightly Doppler-shifted from each other or turbulence in the evaporating plasma. We estimate the energetics of the hard X-ray burst and obtain a total flare energy in accelerated electrons of $\geq7\times10^{28}$ ergs. This is a lower limit because only an upper limit can be determined for the low energy cutoff to the electron spectrum. We find that detailed modeling of this event would require a multi-threaded model due to its complexity.

Impulsive Heating of Solar Flare Ribbons Above 10 MK

The chromospheric response to the input of flare energy is marked by extended extreme ultraviolet (EUV) ribbons and hard X-ray (HXR) footpoints. These are usually explained as the result of heating and bremsstrahlung emission from accelerated electrons colliding in the dense chromospheric plasma. We present evidence of impulsive heating of flare ribbons above 10 MK in a two-ribbon flare. We analyse the impulsive phase of SOL2013-11-09T06:38, a C2.6 class event using data from Atmospheric Imaging Assembly (AIA) on board of Solar Dynamics Observatory (SDO) and the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) to derive the temperature, emission measure and differential emission measure of the flaring regions and investigate the evolution of the plasma in the flaring ribbons. The ribbons were visible at all SDO/AIA EUV/UV wavelengths, in particular, at 94 and 131 \AA\ filters, sensitive to temperatures of 8 MK and 12 MK. Time evolution of the emission measure of the plasma above 10 MK at the ribbons has a peak near the HXR peak time. The presence of hot plasma in the lower atmosphere is further confirmed by RHESSI imaging spectroscopy analysis, which shows resolved sources at 11-13 MK associated with at least one ribbon. We found that collisional beam heating can only marginally explain the necessary power to heat the 10 MK plasma at the ribbons.

Spatially Resolved Energetic Electron Properties for the 21 May 2004 Flare from Radio Observations and 3D Simulations

We investigate in detail the 21 May 2004 flare using simultaneous observations of the Nobeyama Radioheliograph, Nobeyama Radiopolarimeters, Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and Solar and Heliospheric Observatory (SOHO). The flare images in different spectral ranges reveal the presence of a well-defined single flaring loop in this event. We have simulated the gyrosynchrotron microwave emission using the recently developed interactive IDL tool GX Simulator. By comparing the simulation results with the observations, we have deduced the spatial and spectral properties of the non-thermal electron distribution. The microwave emission has been found to be produced by the high-energy electrons ($>100$ keV) with a relatively hard spectrum ($\delta\simeq 2$); the electrons were strongly concentrated near the loop top. At the same time, the number of high-energy electrons near the footpoints was too low to be detected in the RHESSI images and spatially unresolved data. The SOHO Extreme-ultraviolet Imaging Telescope images and the low-frequency microwave spectra suggest the presence of an extended "envelope" of the loop with lower magnetic field. Most likely, the energetic electron distribution in the considered flare reflects the localized (near the loop top) particle acceleration (injection) process accompanied by trapping and scattering.

OBSERVATION OF HEATING BY FLARE-ACCELERATED ELECTRONS IN A SOLAR CORONAL MASS EJECTION

We report a Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) observation of flare-accelerated electrons in the core of a coronal mass ejection (CME) and examine their role in heating the CME. Previous CME observations have revealed remarkably high thermal energies that can far surpass the CME's kinetic energy. A joint observation by RHESSI and the Atmospheric Imaging Assembly of a partly occulted flare on 2010 November 3 allows us to test the hypothesis that this excess energy is collisionally deposited by flare-accelerated electrons. Extreme ultraviolet (EUV) images show an ejection forming the CME core and sheath, with isothermal multifilter analysis revealing temperatures of ~11 MK in the core. RHESSI images reveal a large (~100 × 50 arcsec2) hard X-ray (HXR) source matching the location, shape, and evolution of the EUV plasma, indicating that the emerging CME is filled with flare-accelerated electrons. The time derivative of the EUV emission matches the HXR light curve (similar to the Neupert effect observed in soft and HXR time profiles), directly linking the CME temperature increase with the nonthermal electron energy loss, while HXR spectroscopy demonstrates that the nonthermal electrons contain enough energy to heat the CME. This is the most direct observation to date of flare-accelerated electrons heating a CME, emphasizing the close relationship of the two in solar eruptive events.

X-ray source motion along the loop in two solar flares

We explore the 3-8 keV X-ray source motion along the loop legs in two solar flares observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) on August 12 and November 28, 2002. Firstly, an artificial loop is constructed to have an outline with a fixed width wide enough to cover the X-ray sources at an energy band between 3-60 keV and at various times. Secondly, RHESSI images are reconstructed at 15 energy bands with an 8 s integration window but 1 s cadence. Thirdly, the X-ray source motions are traced from the brightness distribution along the flare loop. We find that these two events tend to start as a single source at 3-8 keV around the loop top, and then separate into two which move downward along the loop legs. These two almost reach the feet of the loop at the hard X-ray (i.e. at 25-50 keV) peak. After that, the two sources move back upward to the loop top and merge together at the same position where they began. The typical timescale is about similar to 70 s, and the maximum speed can reach 1000 km s(-1). Such a downward-to-upward motion along the loop is rarely seen in the observations, and it seems to be consistent with the density evolution at the loop top, first decreasing after heating and then increasing due to evaporation.

Thermal and non-thermal effects driven by magnetic reconnections observed in a confined flare

In order to better understand the energy processes occurring during the impulsive phase of solar flares we compare observations with our magnetic model calculations. We study the 1N/M1.9 confined flare of 20 October 2003 observed during a Joint Observation Program (JOP157), and concentrate on the spectral analysis of the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). These X-ray observations are combined with those from the Solar and Heliospheric Observatory (SOHO) instruments, the French Italian magnetograph (THEMIS), and the Multi-channel Infrared Solar Spectrograph (MISS). The flare occurred in a complex active region, NOAA 10484, with a delta configuration. For model calculations we extrapolate magnetic field lines, which allows us to understand the magnetic topology of the region. This topology and the long impulsive phase of the flare with numerous peaks (GOES, RHESSI) suggest multiple magnetic field reconnection processes. The RHESSI images show a bright structure in hard X-rays (HXR) that could be the tops of the loops. We measure a significant displacement of this structure between the two main maxima of emission, which infers different sites for the reconnection process. The energy release processes can be understood by analyzing the RHESSI spectra using different models. First, using the thermal plus broken power law non-thermal component, we derive the low energy cutoff for the power law distribution of the high-energy electrons (approximate to 25 keV). Then, we apply two models (thermal plus thick- target and thermal plus thin-target non-thermal component) that allow us to fit the observations. These two models lead to similar results; non-thermal energy contributes a significant amount (approximately 20%) of the total flare energy only during the first peak of the impulsive phase. This suggests that the energy that heats the chromosphere is transported mainly by thermal conduction. The temperature of the thermal plasma is 34 MK and 20 MK at the first and second peaks, respectively.

Multi-Wavelength Observation Results of the C5.6 Limb Flare of 1 August 2003

We obtained a complete set of Hα, Ca Π 8542 A and He I 10830 A spectra and slit-jaw Hα images of the C5.6 limb flare of 1 August 2003 using the Multi-channel Infrared Solar Spectrograph (MISS) at Purple Mountain Observatory. This flare was also observed by the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) and partially by the Extreme-ultraviolet Imaging Telescope (EIT) on SOHO. This flare underwent a rapid rising and expanding episode in the impulsive phase. All the Hα, Ca Π 8542 A and He I 10830 A profiles of the flare are rather wide and the widest profiles were observed in the middle bright part of the flare instead of at the flare loop top near the flare maximum. The flare manifested obvious rotation in the flare loop and the decrease of the rotation angular speed with time at the loop-top may imply a de-twisting process of the magnetic field. The significant increases of the Doppler widths of these lines in the impulsive phase reflect quick heating of the chromosphere, and rapid rising and expanding of the flare loop. The RHESSI observations give a thermal energy spectrum for this flare, and two thermal sources and no non-thermal source are found in the reconstructed RHESSI images. This presumably indicates that the energy transfer in this flare is mainly by heat conduction. The stronger thermal source is located near the solar limb with its position unchanged in the flare process and spatially coincident with the intense EUV and Hα emissions. The weaker one moved during the flare process and is located in the Hα dark cavities. This flare may support the theory of the magnetic reconnections in the lower solar atmosphere.

The PMTRAS Roll Aspect System on RHESSI

High-resolution solar hard X-ray imaging on the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) spacecraft is achieved by a set of rotating modulation collimators. The interpretation of the observed time-modulated X-ray flux in terms of high-resolution, accurately located images requires continuous, arc-minute roll aspect, which at present is provided by the `Photo-Multiplier Tube Roll Aspect System' (PMTRAS). This paper describes the PMTRAS operating principles, hardware implementation, calibration, performance and data analysis approach, with emphasis on its effect on RHESSI imaging.

The RHESSI Imaging Concept

The Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) observes solar hard X-rays and gamma-rays from 3 keV to 17 MeV with spatial resolution as high as 2.3 arc sec. Instead of focusing optics, imaging is based on nine rotating modulation collimators that time-modulate the incident flux as the spacecraft rotates. Starting from the arrival time of individual photons, ground-based software then uses the modulated signals to reconstruct images of the source. The purpose of this paper is to convey both an intuitive feel and the mathematical basis for this imaging process. Following a review of the relevant hardware, the imaging principles and the basic back-projection method are described, along with their relation to Fourier transforms. Several specific algorithms (Clean, MEM, Pixons and Forward-Fitting) applicable to RHESSI imaging are briefly described. The characteristic strengths and weaknesses of this type of imaging are summarized.

RHESSI and Trace Observations of the 21 April 2002 X1.5 Flare

Observations of the X1.5 flare on 21 April 2002 are reviewed using the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) and the Transition Region and Coronal Explorer (TRACE). The major findings are as follows: (1) The 3–25 keV X-rays started < 4 min before the EUV (195 A) emission suggesting that the initial energy release heated plasma directly to ≳20 MK, well above the 1.6 MK needed to produce the Fexu (195 A) line. (2) Using coaligned 12–25 keV RHESSI and TRACE images, further evidence is found for the existence of hot (15–20 MK) plasma in the 195 A passband. This hot, diffuse emission is attributed to the presence of the Fe xxiv (192 A) line within the TRACE 195 A passband. (3) The 12–25 keV source centroid moves away from the limb with an apparent velocity of ~ 9.9 km s−1, slowing to ~ 1.7 km s−1 after 3 hours, its final altitude being ~ 140 Mm after ~ 12 hours. This suggests that the energy release site moves to higher altitudes in agreement with classical flare models. (4) The 50–100 keV emission correlates well with EUV flare ribbons, suggesting thick-target interactions at the footpoints of the magnetic arcade. The 50–100 keV time profile matches the time derivative of the GOES light curve (Neupert effect), which suggests that the same electrons that produced the thick-target hard X-ray emission also heat the plasma seen in soft X-rays. (5) X-ray footpoint emission has an E−3 spectrum down to ~ 10 keV suggesting a lower electron cutoff energy than previously thought. (6) The hard X-ray (25–200 keV) peaks have FWHM durations of ~ 1 min suggesting a more gradual energy release process than expected. (7) The TRACE images reveal a bright symmetric front propagating away from the main flare site at speeds of ≥ 120 km s−1. This may be associated with the fast CME observed several minutes later by LASCO. (8) Dark sinuous lanes are observed in the TRACE images that extend almost radially from the post-flare loop system. This ‘fan of spines’ becomes visible well into the decay phase of the flare and shows evidence for both lateral and downward motions.