Planned improvements to the Owens Valley frequency-agile interferometer

Three small antennas will be added to the OVRO interferometer to form a five-element solar-dedicated array. This would provide up to 7 or 10 baselines (compared to the present 1 or 3). This would be sufficient to apply microwave diagnostics to most active region and burst sources. By using frequency-synthesis it would also provide an imaging capability comparable to that of an approximately 100 baseline interferometer. Expansion of the array is discussed

The Solar X-ray Limb

We describe a new technique to measure the height of the X-ray limb with observations from occulted X-ray flare sources as observed by the RHESSI (the Reuven Ramaty High-Energy Spectroscopic Imager) satellite. This method has model dependencies different from those present in traditional observations at optical wavelengths, which depend upon detailed modeling involving radiative transfer in a medium with complicated geometry and flows. It thus provides an independent and more rigorous measurement of the "true" solar radius, meaning that of the mass distribution. RHESSI's measurement makes use of the flare X-ray source's spatial Fourier components (the visibilities), which are sensitive to the presence of the sharp edge at the lower boundary of the occulted source. We have found a suitable flare event for analysis, SOL2011-10-20T03:25 (M1.7), and report a first result from this novel technique here. Using a 4-minute integration over the 3-25 keV photon energy range, we find $R_{\mathrm{X-ray}} = 960.11\ \pm\ 0.15 \pm 0.29$ arcsec, at 1 AU, where the uncertainties include statistical uncertainties from the method and a systematic error. The standard VAL-C model predicts a value of 959.94 arcsec, about 1$\sigma$ below our value.Comment: 12 pages, 5 figures, accepted for publication in Ap

Properties of Energetic Ions in the Solar Atmosphere from {\gamma} -Ray and Neutron Observations

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 {\gamma}-rays and neutrons results from convolution of the nuclear cross-sections with the ion distribution functions in the atmosphere. The observed {\gamma}-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 {\gamma}-rays and neutrons. This chapter reviews recent developments in the study of solar {\gamma}-rays and of solar neutrons at the time of the RHESSI era. The unprecedented quality of the RHESSI data reveals {\gamma}-ray line shapes for the first time and provides {\gamma}-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 {\gamma}-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.Comment: This is a chapter in a monograph on the physics of solar flares, inspired by RHESSI observations. The individual articles are to appear in Space Science Reviews (2011

A new method of observing weak extended x-ray sources with RHESSI

We present a new method, fan-beam modulation, for observing weak extended x-ray sources with the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI). This space-based solar x-ray and gamma-ray telescope has much greater sensitivity than previous experiments in the 3-25 keV range, but is normally not well suited to detecting extended sources since their signal is not modulated by RHESSI's rotating grids. When the spacecraft is offpointed from the target source, however, the fan-beam modulation time-modulates the transmission by shadowing resulting from exploiting the finite thickness of the grids. In this paper we detail how the technique is implemented and verify its consistency with sources with clear known signals that have occurred during RHESSI offpointing: microflares and the Crab Nebula. In both cases the results are consistent with previous and complementary measurements. Preliminary work indicates that this new technique allows RHESSI to observe the integrated hard x-ray spectrum of weak extended sources on the quiet Sun.Comment: Publishe

On the Photometric Accuracy of RHESSI Imaging and Spectrosocopy

We compare the photometric accuracy of spectra and images in flares observed with the Ramaty High Energy Solar Spectroscopic Imager (RHESSI)}spacecraft. We test the accuracy of the photometry by comparing the photon fluxes obtained in different energy ranges from the spectral-fitting software SPEX with those fluxes contained in the images reconstructed with the Clean, MEM, MEM-Vis, Pixon, and Forward-fit algorithms. We quantify also the background fluxes, the fidelity of source geometries, and spatial spectra reconstructed with the five image reconstruction algorithms. We investigate the effects of grid selection, pixel size, field-of-view, and time intervals on the quality of image reconstruction. The detailed parameters and statistics are provided in an accompanying CD-ROM and web page. We find that Forward-fit, Pixon, and Clean have a robust convergence behavior and a photometric accuracy in the order of a few percents, while MEM does not converge optimally for large degrees of freedom (for large field-of-views and/or small pixel sizes), and MEM-Vis suffers in the case of time-variable sources. This comparative study documents the current status of the RHESSI spectral and imaging software, one year after launch.Comment: 2 Figures, full version on http://www.lmsal.com/~aschwand/eprints/2003_photo/index.htm

The Fourier Imaging X-ray Spectrometer (FIXS) for the Argentinian, Scout-launched satelite de Aplicaciones Cienficas-1 (SAC-1)

The Fourier Imaging X-ray Spectrometer (FIXS) is one of four instruments on SAC-1, the Argentinian satellite being proposed for launch by NASA on a Scout rocket in 1992/3. The FIXS is designed to provide solar flare images at X-ray energies between 5 and 35 keV. Observations will be made on arcsecond size scales and subsecond time scales of the processes that modify the electron spectrum and the thermal distribution in flaring magnetic structures

Cyclone Hard X-Ray Observatory

In response to the recent NASA-SMEX Announcement of Opportunity, our collaboration proposed Cyclone, the Cyclotron/Nuclear Explorer. Cyclone is a broadband pointed astrophysical observatory, combining the highest spectral resolutions (E/(Delta) E approximately 30 - 300) and angular resolutions (15') achieved in the optimized hard X-ray range (10 - 200 keV). The instrument consists of 19 co-aligned rotation modulation collimator (RMC) telescopes, each with a high spectral resolution, 6-cm diameter germanium detector (GeD) covering energies from 3 keV to 600 keV. Both the optics and detectors are actively shielded with 15-mm BGO to gain low background an high sensitivity to astrophysical sources. A 550-km altitude, circular equatorial orbit also minimizes background. Building strongly upon instrumental heritage from the High-Energy Solar Spectroscopic Imager (HESSI) program, Cyclone would be ready for launch by September 2003. The instrument design and expected performance are discussed, as well as a brief overview of scientific goals

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 approximately 20 keV to greater than approximately 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 less than 0.1 cubic millimeter. Imaging is accomplished by a single multi-pitch rotating modulator (MPRM), a 2.5-centimeter thick tungsten alloy slit/slat grid with pitches that range quasi-continuously from 1 to 13 millimeters. 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

Cyclone Hard X-Ray Observatory

In response to the recent NASA-SMEX Announcement of Opportunity, our collaboration proposed Cyclone, the Cyclotron/Nuclear Explorer. Cyclone is a broadband pointed astrophysical observatory, combining the highest spectral resolutions (E/(Delta) E approximately 30 - 300) and angular resolutions (15') achieved in the optimized hard X-ray range (10 - 200 keV). The instrument consists of 19 co-aligned rotation modulation collimator (RMC) telescopes, each with a high spectral resolution, 6-cm diameter germanium detector (GeD) covering energies from 3 keV to 600 keV. Both the optics and detectors are actively shielded with 15-mm BGO to gain low background an high sensitivity to astrophysical sources. A 550-km altitude, circular equatorial orbit also minimizes background. Building strongly upon instrumental heritage from the High-Energy Solar Spectroscopic Imager (HESSI) program, Cyclone would be ready for launch by September 2003. The instrument design and expected performance are discussed, as well as a brief overview of scientific goals