Pascal Saint-Hilaire

Implications of X-Ray Observations for Electron Acceleration and Propagation in Solar Flares

High-energy X-rays and γ-rays from solar flares were discovered just over fifty years ago. Since that time, the standard for the interpretation of spatially integrated flare X-ray spectra at energies above several tens of keV has been the collisional thick-target model. After the launch of the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) in early 2002, X-ray spectra and images have been of sufficient quality to allow a greater focus on the energetic electrons responsible for the X-ray emission, including their origin and their interactions with the flare plasma and magnetic field. The result has been new insights into the flaring process, as well as more quantitative models for both electron acceleration and propagation, and for the flare environment with which the electrons interact. In this article we review our current understanding of electron acceleration, energy loss, and propagation in flares. Implications of these new results for the collisional thick-target model, for general flare models, and for future flare studies are discussed.

Thermal and non-thermal energies of solar flares

The energy of the thermal flare plasma and the kinetic energy of the non-thermal electrons in 14 hard X-ray peaks from 9 medium-sized solar flares have been determined from RHESSI observations. The emissions have been carefully separated in the spectrum. The turnover or cutoff in the low-energy distribution of electrons has been studied by simulation and fitting, yielding a reliable lower limit to the non-thermal energy. It remains the largest contribution to the error budget. Other effects, such as albedo, non-uniform target ionization, hot target, and cross-sections on the spectrum have been studied. The errors of the thermal energy are about equally as large. They are due to the estimate of the flare volume, the assumption of the filling factor, and energy losses. Within a flare, the non-thermal/thermal ratio increases with accumulation time, as expected from loss of thermal energy due to radiative cooling or heat conduction. Our analysis suggests that the thermal and non-thermal energies are of the same magnitude. This surprising result may be interpreted by an efficient conversion of non-thermal energy to hot flare plasma.

Survey on Solar X-ray Flares and Associated Coherent Radio Emissions

The radio emission during 201 selected X-ray solar flares was surveyed from 100 MHz to 4 GHz with the Phoenix-2 spectrometer of ETH Zürich. The selection includes all RHESSI flares larger than C5.0 jointly observed from launch until June 30, 2003. Detailed association rates of radio emission during X-ray flares are reported. In the decimeter wavelength range, type III bursts and the genuinely decimetric emissions (pulsations, continua, and narrowband spikes) were found equally frequently. Both occur predominantly in the peak phase of hard X-ray (HXR) emission, but are less in tune with HXRs than the high-frequency continuum exceeding 4 GHz, attributed to gyrosynchrotron radiation. In 10% of the HXR flares, an intense radiation of the above genuine decimetric types followed in the decay phase or later. Classic meter-wave type III bursts are associated in 33% of all HXR flares, but only in 4% are they the exclusive radio emission. Noise storms were the only radio emission in 5% of the HXR flares, some of them with extended duration. Despite the spatial association (same active region), the noise storm variations are found to be only loosely correlated in time with the X-ray flux. In a surprising 17% of the HXR flares, no coherent radio emission was found in the extremely broad band surveyed. The association but loose correlation between HXR and coherent radio emission is interpreted by multiple reconnection sites connected by common field lines.

Probing the solar magnetic field with a Sun-grazing comet.

A Comet in the Sun In 2011, comet Lovejoy plunged into the solar atmosphere and survived its flight through a region of the Sun that has never been visited by spacecraft. Downs et al. (p. 1196) used spacecraft observations of this Sun-grazing comet, combined with advanced magnetohydrodynamic simulations, to constrain the magnetic field of the solar atmosphere—a quantity that has been very difficult to measure directly. Observations of a comets motion through the solar corona constrain this regions magnetic field and plasma properties. On 15 and 16 December 2011, Sun-grazing comet C/2011 W3 (Lovejoy) passed deep within the solar corona, effectively probing a region that has never been visited by spacecraft. Imaged from multiple perspectives, extreme ultraviolet observations of Lovejoys tail showed substantial changes in direction, intensity, magnitude, and persistence. To understand this unique signature, we combined a state-of-the-art magnetohydrodynamic model of the solar corona and a model for the motion of emitting cometary tail ions in an embedded plasma. The observed tail motions reveal the inhomogeneous magnetic field of the solar corona. We show how these motions constrain field and plasma properties along the trajectory, and how they can be used to meaningfully distinguish between two classes of magnetic field models.

Destruction of Sun-Grazing Comet C/2011 N3 (SOHO) Within the Low Solar Corona

Star Grazing Some comets come perilously close to the Sun. Schrijver et al. (p. 324; see the Perspective by Lisse) describe observations made by the Solar Dynamics Observatory of one such Sun-grazing comet, which penetrated into, fragmented, and completely sublimated within the solar atmosphere. More than 2000 Sun-grazing comets have been observed in the past 15 years but none could be followed into the Suns atmosphere. By showing that comets can be observed at such small distances from the Sun, this study opens up new ways to study comets and also the solar atmosphere. The NASA Solar Dynamics Observatory detected and tracked a comet as it penetrated the solar atmosphere. Observations of comets in Sun-grazing orbits that survive solar insolation long enough to penetrate into the Sun’s inner corona provide information on the solar atmosphere and magnetic field as well as on the makeup of the comet. On 6 July 2011, the Solar Dynamics Observatory (SDO) observed the demise of comet C/2011 N3 (SOHO) within the low solar corona in five wavelength bands in the extreme ultraviolet (EUV). The comet penetrated to within 0.146 solar radius (~100,000 kilometers) of the solar surface before its EUV signal disappeared. Before that, material released into the coma—at first seen in absorption—formed a variable EUV-bright tail. During the final 10 minutes of observation by SDO’s Atmospheric Imaging Assembly, ~6 × 108 to 6 × 1010 grams of total mass was lost (corresponding to an effective nucleus diameter of ~10 to 50 meters), as estimated from the tail’s deceleration due to interaction with the surrounding coronal material; the EUV absorption by the comet and the brightness of the tail suggest that the mass was at the high end of this range. These observations provide evidence that the nucleus had broken up into a family of fragments, resulting in accelerated sublimation in the Sun’s intense radiation field.

Extreme-ultraviolet and X-Ray Observations of Comet Lovejoy (C/2011 W3) in the Lower Corona

We present an analysis of EUV and soft X-ray emission detected toward Comet Lovejoy (C/2011 W3) during its post-perihelion traverse of the solar corona on December 16, 2011. Observations were recorded by the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatory and the X-Ray Telescope (XRT) aboard Hinode. A single set of contemporaneous images is explored in detail, along with prefatory consideration for time evolution using only the 171 u data. For each of the eight passbands, we characterize the emission and derive outgassing rates where applicable. As material sublimates from the nucleus and is immersed in coronal plasma, it rapidly ionizes through charge states seldom seen in this environment. The AIA data show four stages of oxygen ionization (O III - O VI) along with C IV, while XRT likely captured emission from O VII, a line typical of the corona. With a nucleus of at least several hundred meters upon approach to a perihelion that brought the comet to within 0.2 R⊙ of the photosphere, Lovejoy was the most significant sungrazer in recent history. Correspondingly high outgassing rates on the order of 10 32.5 oxygen atoms per second are estimated. Assuming that the neutral oxygen comes from water, this translates to a mass-loss rate of �9.5×10 9 g s -1 , and based only on the 171 u observations, we find a total mass loss of �10 13 g over the AIA egress. Additional and supporting analyses include a differential emission measure to characterize the coronal environment, consideration for the opening angle, and a comparison of the emission’s leading edge with the expected position of the nucleus. Subject headings: Comets: general — Comets: individual: C/2011 W3 — Sun: corona

The Gamma-Ray Imager/Polarimeter for Solar Flares (GRIPS)

The balloon-borne Gamma-Ray Imager/Polarimeter for Solar flares (GRIPS) instrument will provide a near-optimal combination of high-resolution imaging, spectroscopy, and polarimetry of solar-flare gamma-ray/hard X-ray emissions from ~20 keV to >~10 MeV. GRIPS will address questions raised by recent solar flare observations regarding particle acceleration and energy release, such as: What causes the spatial separation between energetic electrons producing hard X-rays and energetic ions producing gamma-ray lines? How anisotropic are the relativistic electrons, and why can they dominate in the corona? How do the compositions of accelerated and ambient material vary with space and time, and why? The spectrometer/polarimeter consists of sixteen 3D position-sensitive germanium detectors (3D-GeDs), where each energy deposition is individually recorded with an energy resolution of a few keV FWHM and a spatial resolution of <0.1 mm3. Imaging is accomplished by a single multi-pitch rotating modulator (MPRM), a 2.5-cm thick tungstenalloy slit/slat grid with pitches that range quasi-continuously from 1 to 13 mm. The MPRM is situated 8 meters from the spectrometer to provide excellent image quality and unparalleled angular resolution at gamma-ray energies (12.5 arcsec FWHM), sufficient to separate 2.2 MeV footpoint sources for almost all flares. Polarimetry is accomplished by analyzing the anisotropy of reconstructed Compton scattering in the 3D-GeDs (i.e., as an active scatterer), with an estimated minimum detectable polarization of a few percent at 150–650 keV in an X-class flare. GRIPS is scheduled for a continental-US engineering test flight in fall 2013, followed by long or ultra-long duration balloon flights in Antarctica.

CORONAL HARD X-RAY EMISSION ASSOCIATED WITH RADIO TYPE III BURSTS

We report on a purely coronal hard X-ray source detected in a partially disk-occulted solar flare by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) that is associated with radio type III bursts and a suprathermalelectron eventdetectednear1AUbytheWIND3-DPlasmaand Energetic Particle(3DP)instrument. Several observational characteristics suggest that the coronal hard X-ray source is thin target bremsstrahlung emission from the escaping electrons that produce the radio type III bursts. The hard X-ray emission correlates in time with the radio type IIIbursts and originates from aradially elongated source in the corona with alength (� 65 Mm) similar to typical coronaldensityscaleheights.Furthermore,thedifferencebetweenthehardX-rayphotonspectralindex(� ¼ 4:1 � 0:4) and the electron spectral index of the in situ observed event (� in situ ¼ 2:9 � 0:3) is around 1, consistent with the thin targetinterpretation.Afurthertestforthethintargetscenarioistocomparethenumberofelectronsneededtoproduce the observed hard X-ray emission with the number of in situ observed electrons. However, the number of escaping electrons derived from the single-spacecraft WIND measurement is in the best case an order of magnitude estimate and could easily underestimate the actual number of escaping electrons. Using the WIND observations, the estimated number of escaping electrons is about an order of magnitude too low. Thus, the thin target interpretation only holds if the WIND measurements are significantly underestimating the actual number of escaping electrons. Future multispacecraft observations with STEREO, Solar Orbiter, and Sentinels will resolve this uncertainty.

The Focusing Optics X-ray Solar Imager (FOXSI)

The Focusing Optics x-ray Solar Imager (FOXSI) is a sounding rocket payload funded under the NASA Low Cost Access to Space program to test hard x-ray focusing optics and position-sensitive solid state detectors for solar observations. Todays leading solar hard x-ray instrument, the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) provides excellent spatial (2 arcseconds) and spectral (1 keV) resolution. Yet, due to its use of indirect imaging, the derived images have a low dynamic range (<30) and sensitivity. These limitations make it difficult to study faint x-ray sources in the solar corona which are crucial for understanding the solar flare acceleration process. Grazing-incidence x-ray focusing optics combined with position-sensitive solid state detectors can overcome both of these limitations enabling the next breakthrough in understanding particle acceleration in solar flares. The FOXSI project is led by the Space Science Laboratory at the University of California. The NASA Marshall Space Flight Center, with experience from the HERO balloon project, is responsible for the grazing-incidence optics, while the Astro H team (JAXA/ISAS) will provide double-sided silicon strip detectors. FOXSI will be a pathfinder for the next generation of solar hard x-ray spectroscopic imagers. Such observatories will be able to image the non-thermal electrons within the solar flare acceleration region, trace their paths through the corona, and provide essential quantitative measurements such as energy spectra, density, and energy content in accelerated electrons.

THE X-RAY DETECTABILITY OF ELECTRON BEAMS ESCAPING FROM THE SUN

We study the detectability and characterization of electron beams as they leave their acceleration site in the low corona toward interplanetary space through their nonthermal X-ray bremsstrahlung emission. We demonstrate that the largest interplanetary electron beams (1035 electrons above 10 keV) can be detected in X-rays with current and future instrumentation, such as RHESSI or the X-Ray Telescope (XRT) onboard Hinode. We make a list of optimal observing conditions and beam characteristics. Amongst others, good imaging (as opposed to mere localization or detection in spatially integrated data) is required for proper characterization, putting the requirement on the number of escaping electrons (above 10 keV) to 3 × 1036 for RHESSI, 3 × 1035 for Hinode/XRT, and 1033 electrons for the FOXSI sounding rocket scheduled to fly in 2011. Moreover, we have found that simple modeling hints at the possibility that coronal soft X-ray jets could be the result of local heating by propagating electron beams.