Gamma-rays and neutrons are the only sources of information on energetic ions present during solar flares and on properties of these ions when they interact in the solar atmosphere. The production of
...γ
-rays and neutrons results from convolution of the nuclear cross-sections with the ion distribution functions in the atmosphere. The observed
γ
-ray and neutron fluxes thus provide useful diagnostics for the properties of energetic ions, yielding strong constraints on acceleration mechanisms as well as properties of the interaction sites. The problem of ion transport between the accelerating and interaction sites must also be addressed to infer as much information as possible on the properties of the primary ion accelerator. In the last couple of decades, both theoretical and observational developments have led to substantial progress in understanding the origin of solar
γ
-rays and neutrons. This chapter reviews recent developments in the study of solar
γ
-rays and of solar neutrons at the time of the
RHESSI
era. The unprecedented quality of the
RHESSI
data reveals
γ
-ray line shapes for the first time and provides
γ
-ray images. Our previous understanding of the properties of energetic ions based on measurements from the former solar cycles is also summarized. The new results—obtained owing both to the gain in spectral resolution (both with
RHESSI
and with the non solar-dedicated
INTEGRAL
/SPI instrument) and to the pioneering imaging technique in the
γ
-ray domain—are presented in the context of this previous knowledge. Still open questions are emphasized in the last section of the chapter and future perspectives on this field are briefly discussed.
We present the first statistical analysis of the thermal and nonthermal X-ray emission of all 25,705 microflares (RHESSI) observed between 2002 March and 2007 March. These events were found by ...searching the 6-12 keV energy range (see Paper I) and are small active region flares, from low (GOES) C class to below A class. Each microflare is automatically analyzed at the peak time of the 6-12 keV emission: the thermal source size is found by forward-fitting the complex visibilities for 4-8 keV, and the spectral parameters (temperature, emission measure, power-law index) are found by forward-fitting a thermal plus nonthermal model. The resulting wealth of information we determine about the events allows a range of the thermal and nonthermal properties to be investigated. In particular, we find that there is no correlation between the thermal loop size and the flare magnitude, indicating that microflares are not necessarily spatially small. We present the first thermal energy distribution of RHESSI flares and compare it to previous thermal energy distributions of transient events. We also present the first nonthermal power distribution of RHESSI flares and find that a few microflares have unexpectedly large nonthermal powers up to image erg s super(-1). The total microflare nonthermal energy, however, is still small compared to that of large flares as it occurs for shorter durations. These large energies and difficulties in analyzing the steep nonthermal spectra suggest that a sharp broken power law and thick-target bremsstrahlung model may not be appropriate for microflares.
X-radiation from energetic electrons is the prime diagnostic of flare-accelerated electrons. The observed X-ray flux (and polarization state) is fundamentally a convolution of the cross-section for ...the hard X-ray emission process(es) in question with the electron distribution function, which is in turn a function of energy, direction, spatial location and time. To address the problems of particle propagation and acceleration one needs to infer as much information as possible on this electron distribution function, through a deconvolution of this fundamental relationship. This review presents recent progress toward this goal using spectroscopic, imaging and polarization measurements, primarily from the
Reuven Ramaty High Energy Solar Spectroscopic Imager
(
RHESSI
). Previous conclusions regarding the energy, angular (pitch angle) and spatial distributions of energetic electrons in solar flares are critically reviewed. We discuss the role and the observational evidence of several radiation processes: free-free electron-ion, free-free electron-electron, free-bound electron-ion, photoelectric absorption and Compton backscatter (albedo), using both spectroscopic and imaging techniques. This unprecedented quality of data allows for the first time inference of the angular distributions of the X-ray-emitting electrons and improved model-independent inference of electron energy spectra and emission measures of thermal plasma. Moreover, imaging spectroscopy has revealed hitherto unknown details of solar flare morphology and detailed spectroscopy of coronal, footpoint and extended sources in flaring regions. Additional attempts to measure hard X-ray polarization were not sufficient to put constraints on the degree of anisotropy of electrons, but point to the importance of obtaining good quality polarization data in the future.
We present RHESSI imaging of three flares (2003 October 28 and 29 and November 2) in the 2.223 MeV neutron-capture gamma-ray line with angular resolution as high as 35". Comparisons of imaged and ...spatially integrated fluences show that in all cases most, if not all, of the emission was confined to compact sources with size scales of tens of arcseconds or smaller that are located within the flare active region. Thus, the gamma-ray-producing ions appear to be accelerated by the flare process and not by a widespread shock driven by a fast coronal mass ejection. The 28 October event yielded the first such image to show double-footpoint gamma-ray line sources. These footpoint sources straddled the flaring loop arcade but were displaced from the corresponding 0.2-0.3 MeV electron-bremsstrahlung emission footpoints by 14" and 17" c 5". As with the previously studied 2002 July 23 event, this implies spatial differences in acceleration and/or propagation between the flare-accelerated ions and electrons.
In this study we present the results of a new approach to studying the acceleration and propagation of bremsstrahlung-producing electrons in solar flares. The method involves an analysis of the size ...of extended solar flare structures as a function of photon energy. Hard X-ray images from 10 M-class limb events, observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) to have the general form of a single extended source, were analyzed by forward fitting to the source visibilities In each energy band. On average the source sizes sigma increased slowly with photon energy epsilon as sigma similar to epsilon super(mh). This behavior is consistent neither with the predictions of a single-loop thermal model nor with a model in which nonthermai electrons are injected into a constant-density structure from a compact acceleration region. While a nonuniform density distribution along the flare loop can in principle reconcile the data with a nonthermal collisional model with point-source injection, the resulting density profiles are highly questionable. On the other hand, the data are consistent with a nonthermal collisional model that incorporates an extended acceleration region, perhaps in combination with a localized thermal source. We present best-fit results on the density and length of this acceleration region. To our knowledge, this is the first quantitative empirical analysis of the physical characteristics of electron acceleration regions in solar flares.
RHESSI produces solar flare images with the finest angular and spectral resolutions ever achieved at hard X-ray energies. Because this instrument uses indirect, collimator-based imaging techniques, ...the 'native' output of which is in the form of 'visibilities' (two-dimensional spatial Fourier components of the image), the development and application of robust, accurate, visibility-based image reconstruction techniques is required. Recognizing that the density of spatial-frequency (u, v) coverage by RHESSI is much sparser than that normally encountered in radio astronomy, we therefore introduce a method for image reconstruction from a relatively sparse distribution of sampled visibilities. The method involves spline interpolation at spatial frequencies less than the largest sampled frequency and the imposition of a positivity constraint on the image to reduce the ringing effects resulting from an unconstrained Fourier transform inversion procedure. Using simulated images consisting both of assumed mathematical forms and of the type of structure typically associated with solar flares, we validate the fidelity, accuracy, and robustness with which the new procedure recovers input images. The method faithfully recovers both single and multiple sources, both compact and extended, over a dynamic range of ~10:1. The performance of the method, which we term as uv_smooth, is compared with other RHESSI image reconstruction algorithms currently in use and its advantages summarized. We also illustrate the application of the method using RHESSI observations of four solar flares.
Soft-gamma-ray repeaters (SGRs) are galactic X-ray stars that emit numerous short-duration (about 0.1 s) bursts of hard X-rays during sporadic active periods. They are thought to be magnetars: ...strongly magnetized neutron stars with emissions powered by the dissipation of magnetic energy. Here we report the detection of a long (380 s) giant flare from SGR 1806-20, which was much more luminous than any previous transient event observed in our Galaxy. (In the first 0.2 s, the flare released as much energy as the Sun radiates in a quarter of a million years.) Its power can be explained by a catastrophic instability involving global crust failure and magnetic reconnection on a magnetar, with possible large-scale untwisting of magnetic field lines outside the star. From a great distance this event would appear to be a short-duration, hard-spectrum cosmic gamma-ray burst. At least a significant fraction of the mysterious short-duration gamma-ray bursts may therefore come from extragalactic magnetars.
The Spectrometer/Telescope for Imaging X-rays (STIX) is a remote sensing instrument on Solar Orbiter that observes the hard X-ray bremsstrahlung emission of solar flares. This paper describes the ...STIX Aspect System (SAS), a subunit that measures the pointing of STIX relative to the Sun with a precision of
±
4
″
, which is required to accurately localize the reconstructed X-ray images on the Sun. The operating principle of the SAS is based on an optical lens that images the Sun onto a plate that is perforated by small apertures arranged in a cross-shaped configuration of four radial arms. The light passing through the apertures of each arm is detected by a photodiode. Variations of spacecraft pointing and of distance from the Sun cause the solar image to move over different apertures, leading to a modulation of the measured lightcurves. These signals are used by ground analysis to calculate the locations of the solar limb, and hence the pointing of the telescope.
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