X-Ray and Gamma-Ray Polarimetry of Solar Flares

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 rapidly 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 ability 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.

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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 experiments 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.

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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 al., 1996) shows that, for the generally-accepted “thick-target” model, in which the accelerated electrons are preferen- tially accelerated downward, the polarization vector is parallel to this reference plane, i.e., radial on the solar disk. Emslie & Brown (1980) and Emslie, Bradsher, & McConnell (2008) have shown, however, that non-radial polarization orientations are possible both for thermal models of the high energy emission (in which non-vertical source regions are involved), or for deviations of the chromospheric magnetic field direction from the local vertical. Model predictions for the polarization fraction range from a few percent (in cases where, for example, a rapidly converging magnetic field changes the pitch angle of the electrons to a more hori- zontal, “pancake-type,” distribution, or where the accelerated electrons have a wide range of initial pitch angles), to some tens of percent (e.g., for highly-directed acceleration in near- uniform guiding fields). Thus, measurement of the magnitude and direction of the polarization vector provide key diagnostic evidence to dis- tinguish between particle acceleration scenarios. Some fraction of the flux at energies below 100 keV will be flux that is backscattered from the solar photosphere. For spatially-integrated (non-imaging) measurements, the reflected component will influence polarization results, since backscattering will tend to introduce polarization fractions of a few percent at energies below 100 keV (e.g., Langer & Petrosian, 1977; Bai & Ramaty, 1978). Even thermal models of the X-ray source predict a finite polarization of Figure 1 - High energy polarimetry offers a unique means of probing the angular distribution of the elec- order a few percent (Emslie & Brown, 1980). trons that are accelerated in solar flares.

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The thermal component, with its rather low polarization, tends to dominate the emission from all flares at energies below about ~25 keV. Imaging measurements will be required at these energies to ensure separation of the thermal and non-thermal components At non-thermal energies, predictions for solar flare polarization are often criticized on the grounds that the modeling assumptions they contain are overly simplified. For example, most models typically assume a single, simple magnetic field structure. It could be argued that any real flare, particularly one sufficiently large to produce a signal of sufficient strength to en- able a polarization measurement, will likely contain a mix of structures that would average out any polarization signal (see also Hudson et al., 2003), suggesting the need for imaging polarimetry. However, hard X-ray imaging observations in the impulsive phase generally show a fairly simple geometry, consisting of two footpoint sources and perhaps a loop-top source (e.g., Sakao et al., 1992; Masuda et al., 1995). These observations suggest that simple magnetic structures are responsible for the energetic emissions and give support to the possibility that a statistically significant polarization signal could be produced in a large event, even in the case of non-imaging measurements. 1.3 Efforts to Measure Polarization The earliest efforts to measure X-ray polarization from solar flares were all obtained at energies below ~15 keV (Tindo et al. 1970, 1972a, 1972b; Nakada et al., 1974; Tindo et al. 1976; Tramiel et al., 1984). Some of these observations have been met with considerable skepticism (e.g., Brown, McClymont, & McLean 1974). Collectively, these measurements do not present a consistent picture of solar flare polarization. Recent efforts to measure polarization at higher energies (> 20 keV) have been made using data from the Ramaty High Energy Solar Spectroscopic Imager (RHESSI; Lin et al. 2002; Smith et al. 2002). Although designed primarily as a hard X-ray imager and spectrometer, RHESSI is also capable of measuring the polarization of hard X-rays and γ-rays from solar flares using two distinct methods. Polarimetry of low and medium energies (20-100 keV) is made possible by the inclusion of a small unobstructed Be scattering element that is strate- gically located within the array of nine segmented germanium detectors (McConnell et al. 2002). Solar flare photons below ~100 keV can reach a rear segment of a Ge detector only indirectly, by scattering. The azimuthal distribution of photons scattered from the Be carries with it a signature of the linear polarization of the incident flux. Sensitivity estimates suggest that a polarization sensitivity of less than a few percent can be achieved for X-class flares. Initial results from the X4.3 flare of 23-July-2003 show some significant modulation of the Be-scattered flux, but it is not yet clear whether this modulation results from polarization of the incident flux or from instrumental systematics (McConnell et al. 2003). A second approach to doing polarimetry with RHESSI (at higher energies, above ~200 keV) uses the scattering of photons between the Ge detectors. Boggs et al. (2005) reported marginal evidence for polarization in two X-class flares using this technique. Suarez-Garcia et al. (2006) also used Ge-Ge scatter events to study six X-class and one M-class flare in the energy range from 100-350 keV, but obtained no significant detection of polarization. To date, none of the RHESSI results provide unambiguous evidence for polarization. Additional attempts to measure polarization have recently been made using data from the SPR-N instrument on the Coronas-F spacecraft (e.g., Veselovsky et al. 2004; Bogomolov et al. 2003a, 2003b; Zhitnik et al. 2006). The SPR-N instrument is sensitive to polarization

page 4 Solar and Heliophysics Decadal Survey!!Hard X-Ray and Gamma-Ray Solar Flare Polarization from 10-100 keV, using passive Be scattering elements, coupled with active scintillators, to detect and measure scattered photons. The effective area ranges from ~0.5 cm2 (20 keV) to ~1.5 cm2 (100 keV). Zhitnik et al. (2006) reported the detection of hard X-rays from more than 90 flares, only one of which (that of of October 29, 2003) showed significant levels of polarization, at levels in excess of 50% (with large uncertainties). Upper limits for 25 other events were in the range of 8% to 40%.

2.0 INSTRUMENTATION Various techniques have been developed for measuring the polarization of hard X-rays and γ-rays. Here we review three different approaches that collectively cover the full range of X- ray and gamma-ray energies from a few keV up to energies above 1 MeV. All three approaches are currently being considered for use in solar flare measurements. In addition, all three techniques could be applied to the development of an imaging instrument. With sufficient angular resolution, spatially-resolved polarization measurements could study the particle distribution at different points along a flaring loop, providing further insight into the evolution of the particle distribution as it propagates along the field lines. 2.1 Low-Energy Techniques (< 50 keV) Although Compton (Thomson) scattering has been used as a means to measure polarization at these energies, in recent years the photoelectric effect has been exploited for a number of polarimeter designs. In the photoelectric effect, the ejected photoelectron tends to be scattered parallel to the incident electric field (polarization). Photoelectric photometry relies on the ability to track the direction of the photoelectron and thus providing a measurement of the incident photon’s polarization. Polarization measurements that exploit this process have recently become practical with the development of a highly sensitive measurement technique based on imaging the photoelectron track using a micropattern gas detector (MPGD). The sensitivity of MPGD polarimeters can be optimized by implementing a time projection readout scheme (Black et al. 2003). These time- 2000 projection chamber (TPC) polarimeters have been demonstrated C-max as sensitive polarimeters in the 2– 1500 10 keV band and have also been optimized for higher energies. 1000 polarization angle polarization This technology is also the basis Counts of the instrument on GEMS 500 SMEX mission, currently under C-min development for studying the 2- 0 10 keV polarization of astrophysi- 0 50 100 150 200 250 300 350 cal sources. The Imaging X-ray Azimuthal Scatter Angle (degs) Polarimeter for Solar flares Figure 2 - The azimuthal scatter angle distribution of Compton scat- (IXPS) represents one concept for tered photons that are initially polarized. This represents the polariza- performing polarization measure- tion signature of a Compton scatter polarimeter, where the photons ments in the 20-50 keV energy tend to scatter perpendicular to the incident polarization direction. In a range, utilizing a TPC for track- photoelectric polarimeter, a similar distribution is obtained for the distribution of emitted photoelectrons, except that the photoelectrons ing the photoelectron (Dennis et tend to scatter parallel to the incident polarization direction.

page 5 Solar and Heliophysics Decadal Survey!!Hard X-Ray and Gamma-Ray Solar Flare Polarization al. 2010). Imaging capability will be achieved by measuring the spatial Fourier components of the X-ray flare as is done with RHESSI. 2.2 Medium Energy Techniques (50-500 keV) The physical process used to measure polarization in the 50-500 keV energy range is Compton scattering. Compton scattered photons tend to be scattered at right angles with respect to the incident electric field. In the case of an unpolarized beam of incident photons, there will be no preferred azimuthal scattering angle; the azimuthal distribution of scattered photon angles will therefore be uniform. However, in the polarized case, the incident photons will exhibit an asymmetric azimuthal distribution. Polarimetry in the 50-500 keV energy band requires low-Z scattering elements coupled with high-Z photon absorbers for achieving the best result. The Gamma-RAy Polarimeter Experiment (GRAPE; McConnell et al. 2009) is a scintillator-based Compton polarimeter designed to observe polarized astrophysical phenomena in the hard X-ray energy band (50 – 500 keV). The basic instrument concept has been validated in laboratory experiments and during a calibration campaign at a polarized X-ray beam (Bloser et al. 2006), and the hardware has been flight-tested on an engineering balloon flight. A large array of polarimeter modules is currently being prepared for a balloon flight in the fall of 2011, with the goal of measuring the polarization of the Crab (Connor et al. 2010). Subsequent long-duration balloon flights will permit observations of both solar flares and gamma-ray bursts. The GRAPE design lends itself readily to imaging with Rotation Modulation Collimators, as is currently done with RHESSI. 2.3 High Energy Techniques (>300 keV) Compton scattering is also an effective way to measure polarization in the gamma-ray range (at energies extending up to 1 MeV or more). Newly developed 3D position-sensitive germanium detectors (3D-GeDs) combine the high spectral resolution of germanium with the capability of spatially resolving each energy deposition to within <0.1 mm3. Using this information, it is possible to reconstruct the path of each gamma ray as it Compton scatters around the volume, and thus precisely measure the anisotropic scattering from a polarized source. Furthermore, the track reconstruction allows for the rejection of background events and the identification of incomplete depositions that both can reduce the polarization sensitivity. The Gamma-Ray Imager/Polarimeter for Solar flares (GRIPS) is a balloon-borne instrument that combines 3D-GeDs with a novel imaging technique using a multi-pitch rotat- ing modulator (MRPM) to produce gamma-ray images with an unprecedented resolution of 12.5 arcsec (Shih et al. 2009). With the spectrometer acting as its own active scatterer, GRIPS thus has the capability for imaging polarimetry with sufficient fluxes. GRIPS is currently being built and is slated for a one-day test flight in the spring of 2012. Long-duration Antarctic balloon flights in following years are anticipated to fully test out the instrument and return science. Future instruments based on the GRIPS design can have increased angular resolution and sensitivity.

3.0 SUMMARY X-ray and gamma-ray emission from solar ﬂares, like any other form of electromagnetic radiation, has four, and only four properties. Each photon can be completely characterized by its time of arrival, its energy, its direction of arrival, and its polarization state. To date, our understanding of high energy solar ﬂare emissions has come from measurements of the

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first three of these quantities. Efforts to study the fourth quantity (polarization) have so far met with only very limited success. The study of polarization at X-ray and gamma-ray energies is especially appealing in that the high-energy emission from any bremsstrahlung source (such as a solar flare) will be polarized if the phase-space distribution of the emitting electrons is anisotropic. Polarization measurements therefore provide a direct handle on the extent to which the accelerated electrons are beamed, which, in turn, has important implica- tions for particle acceleration models. Observations to date are highly inconclusive, in no small part because they have been made using instruments that were not optimized for the measurement of polarization. With the increasing realization that polarization measurements provide uniquely insightful information on the physical processes at work in particle acceleration sites throughout astrophysics, the time is propitious to develop instruments specifically tailored to the measurement of solar flare polarization at high energies. This will constitute a quantum leap in our capabilities to resolve many of the current ambiguities and outstanding questions in solar flare physics. The primary goal of any future high energy polarimetry experiment must be to improve upon the sensitivity of current instrumentation. This can be achieved either by incorporating high energy polarimetry as one aspect of an instrument with a broad range of capabilities (such as GRIPS) and/or through the development of specialized instrumentation (like GRAPE or IXPS) that could be included as part of a suite of instruments on a major obser- vatory. In either case, high quality data collected during periods of maximum solar activity promise to provide important insights into the solar flare acceleration process. Polarimetry would yield direct information on the directivity (i.e., anisotropy) of the emitting electron population and so provide critical constraints on the acceleration mechanism. Imaging polarimetry would reveal the characteristic polarimetric signature of the photospheric albedo patch, clearly distinguishing it from the direct X-ray flux, which, in general has a significantly different spatial distribution of the magnitude and orientation of the polarization vector.

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