X-Ray and Gamma-Ray Polarimetry of Solar Flares

Solar and Heliophysics Decadal Survey!!X-Ray and Gamma-Ray Polarimetry of Solar Flares X-Ray and Gamma-Ray Polarimetry of Solar Flares White paper Submitted to the Solar and Heliospheric Physics Decadal Survey November 12, 2010 prepared by Mark L. McConnell (UNH) with support from Peter F. Bloser (UNH) Brian Dennis (NASA/GSFC) A. Gordon Emslie (WKU) Robert P. Lin (UCB) James M. Ryan (UNH) Albert Y. Shih (NASA/GSFC) David M. Smith (UCSC) Solar and Heliophysics Decadal Survey!!Hard X-Ray and Gamma-Ray Solar Flare Polarization BRIEF DESCRIPTION A determination of the extent to which flare accelerated electrons are beamed constitutes an essential step towards a greater understanding of particle acceleration in solar flares. Ro- bust high energy polarization measurements offer the most effective means of measuring the electron beaming. INTRODUCTION Solar flares represent a process of explosive energy release in a magnetized plasma, a process which is believed to take place at many other sites in the universe. Solar flares accelerate ions up to tens of GeV and electrons to hundreds of MeV, releasing as much as 1033 ergs in the process (see, e.g., Lin & Hudson 1976). The accelerated 10–100 keV electrons appear to contain a significant fraction, perhaps the bulk, of this energy, indicating that the particle acceleration and energy release processes are intimately linked. The details of how the Sun releases this energy, presumably stored in the magnetic fields of the corona, and how it rap- idly accelerates electrons and ions with such high efficiency, and to such high energies, is presently unknown. Furthermore, the electrical currents associated with the accelerated electrons impose formidable constraints on the global electrodynamics of the system (Emslie & Hénoux 1995). These problems can be alleviated somewhat if the particle acceleration is nearly isotropic, such as in stochastic acceleration models (e.g., Miller et al. 1996). Further- more, solar flare radiation spectra are dependent not only on the energy distribution, but also on the angular distribution of the energetic particles. Consequently, independent measurements of the angular distribution of the energetic particles may be important for proper interpretation of the radiation spectra. A determination of the extent to which the accelerated electrons are beamed (anisotropic) therefore constitutes an essential step towards a greater understanding of particle acceleration in solar flares and, more generally, throughout the cosmos. The angular distribution of accelerated ions can be studied by measuring the energies and widths of broad γ-ray lines. There are several ways in which the anisotropy of the accelerated electrons can be ascertained from the high-energy photon emissions. However, statistical directivity studies (e.g., Kašparová et al. 2007) are inconclusive and multi-spacecraft stereoscopic observations (e.g., Kane et al. 1980) are sparse. High energy polarimetry is the most effective means of measuring the anisotropy of the accelerated electrons. It provides the abil- ity to derive the anisotropy of the accelerated electrons from measurements by a single instrument and measurements with sufficient temporal resolution could even measure variations of the electron anisotropy during the evolution of the flare. Unfortunately, reliable measurements of the polarization of solar flare X-ray and gamma-ray radiation lag well be- hind theoretical predictions of this key diagnostic. We are finally seeing the onset of observations capable of testing the numerous theoretical predictions of high energy polarization that have been in the literature for decades. The few existing polarization measurements are intriguing and inconclusive, yet collectively they suggest that the magnitude of the polarization vector is of the order predicted by models that have a strong anisotropy of the emitting electrons. However, the measured orientation of this vector may be in a direction substantially different from the local solar radial, as predicted by most solar models. This raises the fascinating possibility that significant refinements of our models for particle acceleration and transport in solar flares may be required. page 1 Solar and Heliophysics Decadal Survey!!Hard X-Ray and Gamma-Ray Solar Flare Polarization The importance of polarization measurements at high energies has often been recognized. For example, the High Energy from Space Panel of the Astronomy and Astrophysics Survey Committee (1991) pointed out the possibilities for polarization measurements and suggested that the design of “various types of polarimeters” be considered as one useful area for basic technology development. In April of 1997, NASA's Gamma Ray Astronomy Working Group (GRAPWG), in their report titled Recommended Priorities for NASA’s Gamma-Ray Astron- omy Program, 1996-2010, states that one “scientifically interesting” solar mission would be “to design an instrument to attack a specific problem such as the polarization of the electron bremsstrahlung radiation.” 1.0 SCIENTIFIC MOTIVATION Studies of γ-ray line data from the SMM Gamma Ray Spectrometer (GRS) suggest that protons and α-particles are likely being accelerated in a rather broad angular distribution (Share & Murphy, 1997; Share et al., 2002). There is no reason to expect, however, that electrons are being accelerated in a similar fashion. Here we review two possible means of measuring the accelerated electron angular distribution: 1) by measuring photon directivity at X-ray and gamma-ray energies; and 2) by measuring X-ray and gamma-ray polarization. We argue that high energy polarimetry is the preferred approach. 1.1 Photon Directivity Measurements as a Probe of Electron Beaming An anisotropic ensemble of bremsstrahlung-producing electrons will produce a radiation field that is not only polarized, but anisotropic. Measurements of the high energy photon directivity can therefore provide a probe of the extent to which the accelerated electrons are beamed. One technique for studying directivity on a statistical basis is to look for center-to- limb variations. Correlations between flare longitude and flare intensity or spectrum reflect the anisotropy of the X-ray emission and hence any directivity of the energetic electrons. For example, if the radiation is preferably emitted in a direction parallel to the surface of the Sun, then a flare located near the limb will look brighter than the same flare near the disk center. Analysis of SMM GRS data collected during cycle 21 (for E > 300 keV) provided the first clear evidence for directed emission, with a tendency for the high energy events to be located near the limb (Vestrand et al., 1987; Bai, 1988). Observations from SMM GRS during cycle 22 provided further support for directivity (Vestrand et al., 1991). However, several high energy events were also observed near the disk center by a number of different experi- ments during cycle 22 (e.g., on GRANAT and CGRO; see Vilmer 1994 for a summary), perhaps suggesting a more complex pattern. Significant differences from one flare to the next in terms of geometry, energy release, time variability, etc., make quantifying the magnitude of the directivity from these statistical measurements a difficult task. Another method for studying the directivity in individual flares is the stereoscopic technique (Catalano & van Allen 1973). This method compares simultaneous observations made on two spacecraft that view the same flare from different directions. Several efforts to make stereoscopic measurements (using, for example, simultaneous data from PVO and ISEE-3) have failed to produce clear evidence for directivity (Kane et al. 1988; Li et al. 1994; Kane et al. 1998). A potential problem with these data is that stereoscopic observations tend to suf- fer from cross-calibration issues between different instruments. page 2 Solar and Heliophysics Decadal Survey!!Hard X-Ray and Gamma-Ray Solar Flare Polarization The difficulties of statistical and stereoscopic observations for measuring photon directivity suggest the need for an alternative technique that can measure time-dependent particle ani- sotropies in individual flares. Polarization is a diagnostic that can meet these requirements. 1.2 Polarization Measurements as a Probe of Electron Beaming Using polarization measurements at high energies for determining the accelerated electron angular distribution is possible because the emission from any bremsstrahlung source (such as a solar flare) will be polarized if the phase-space distribution of the emitting electrons is anisotropic. Electrons accelerated in the flare spiral around guiding magnetic field lines, emitting high energy photons from collisions with ambient protons and heavier ions. In con- ventional models of a solar flare, the magnetic field forms a loop structure that penetrates the chromosphere in a vertical direction (Figure 1). Since most of the high energy emission is emitted in the dense chromospheric regions of the loop, the direction of the magnetic field in these layers represents a preferred direction in the source. Hence, one expects the emission to be linearly polarized either in, or perpendicular to, the plane defined by this preferred (vertical) direction and the direction to the observer. This plane intersects the visible solar disk in the radial direction from the center of the disk to the source. Extensive modeling by many authors (e.g., Brown 1972; Hénoux 1975; Langer & Petrosian 1977; Bai & Ramaty 1978; Leach & Petrosian 1983; Zharkova et al., 1995; Charikov et

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