Albert Y. Shih

Global Energetics of Thirty-Eight Large Solar Eruptive Events

We have evaluated the energetics of 38 solar eruptive events observed by a variety of spacecraft instruments between 2002 February and 2006 December, as accurately as the observations allow. The measured energetic components include: (1) the radiated energy in the Geostationary Operational Environmental Satellite 1-8 A band, (2) the total energy radiated from the soft X-ray (SXR) emitting plasma, (3) the peak energy in the SXR-emitting plasma, (4) the bolometric radiated energy over the full duration of the event, (5) the energy in flare-accelerated electrons above 20 keV and in flare-accelerated ions above 1 MeV, (6) the kinetic and potential energies of the coronal mass ejection (CME), (7) the energy in solar energetic particles (SEPs) observed in interplanetary space, and (8) the amount of free (non-potential) magnetic energy estimated to be available in the pertinent active region. Major conclusions include: (1) the energy radiated by the SXR-emitting plasma exceeds, by about half an order of magnitude, the peak energy content of the thermal plasma that produces this radiation; (2) the energy content in flare-accelerated electrons and ions is sufficient to supply the bolometric energy radiated across all wavelengths throughout the event; (3) the energy contents of flare-accelerated electrons and ions are comparable; (4) the energy in SEPs is typically a few percent of the CME kinetic energy (measured in the rest frame of the solar wind); and (5) the available magnetic energy is sufficient to power the CME, the flare-accelerated particles, and the hot thermal plasma.

High-Resolution Spectroscopy of Gamma-Ray Lines from the X-Class Solar Flare of 2002 July 23

The Reuven Ramaty High Energy Solar Spectroscopy Imager (RHESSI) has obtained the first high-resolution measurements of nuclear de-excitation lines produced by energetic ions accelerated in a solar flare, a GOES X4.8 event occurring on 2002 July 23 at a heliocentric angle of ~73°. Lines of neon, magnesium, silicon, iron, carbon, and oxygen were resolved for the first time. They exhibit Doppler redshifts of 0.1%-0.8% and broadening of 0.1%-2.1% (FWHM), generally decreasing with mass. The measured redshifts are larger than expected for a model of an interacting ion distribution isotropic in the downward hemisphere in a radial magnetic field. Possible interpretations of the large redshifts include (1) an inclination of the loop magnetic field to the solar surface so that the ion distribution is oriented more directly away from the observer and (2) extreme beaming of the ions downward along a magnetic field normal to the solar surface. Bulk downward motion of the plasma in which the accelerated ions interact can be ruled out.

Coronal γ-Ray Bremsstrahlung from Solar Flare-accelerated Electrons

The Reuven Ramaty High Energy Spectroscopic Imager (RHESSI) provides for the first time imaging spectroscopy of solar flares up to the γ-ray range. The three RHESSI flares with best counting statistics are analyzed in the 200-800 keV range revealing γ-ray emission produced by electron bremsstrahlung from footpoints of flare loops, but also from the corona. Footpoint emission dominates during the γ-ray peak, but as the γ-ray emission decreases the coronal source becomes more and more prominent. Furthermore, the coronal source shows a much harder spectrum (with power-law indices γ between 1.5 and 2) than the footpoints (with γ between 3 and 4). These observations suggest that flare-accelerated high-energy (~MeV) electrons stay long enough in the corona to lose their energy by collisions producing γ-ray emission, while lower energetic electrons precipitate more rapidly to the footpoints.

SunPy - Python for Solar Physics

SunPy is a data analysis toolkit which provides the necessary software for analyzing solar and heliospheric datasets in Python. SunPy aims to provide a free and open-source alternative to the current standard, an IDL- based solar data analysis environment known as SolarSoft (SSW). We present the latest release of SunPy, version 0.3. Though still in active development, SunPy already provides important functionality for solar data analysis. SunPy provides data structures for representing the most common solar data types: images, lightcurves, and spectra. To enable the acquisition of scientific data, SunPy provides integration with the Virtual Solar Observatory (VSO), a single source for accessing most solar data sets, and integration with the Heliophysics Event Knowledgebase (HEK), a database of transient solar events such as solar flares or coronal mass ejections. SunPy utilizes many packages from the greater scientific Python community, including NumPy and SciPy for core data types and analysis routines, PyFITS for opening image files, in FITS format, from major solar missions (e.g., SDO/AIA, SOHO/EIT, SOHO/LASCO, and STEREO) into WCS-aware map objects, and pandas for advanced time-series analysis tools for data from missions such as GOES, SDO/EVE, and Proba2/LYRA, as well as support for radio spectra (e.g., e-Callisto). Future releases will build upon and integrate with current work in the Astropy project and the rest of the scientific python community, to bring greater functionality to SunPy users.

High-Resolution Observation of the Solar Positron-Electron Annihilation Line

The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) has observed the positron-electron annihilation line at 511 keV produced during the 2002 July 23 solar flare. The shape of the line is consistent with annihilation in two vastly different solar environments. It can be produced by formation of positronium by charge exchange in flight with hydrogen in a quiet solar atmosphere at a temperature of ~6000 K. However, the measured upper limit to the 3γ/2γ ratio (ratio of annihilation photons in the positronium continuum to the number in the line) is only marginally consistent with what is calculated for this environment. The annihilation line can also be fitted by a thermal Gaussian having a width of 8.1 ± 1.1 keV (FWHM), indicating temperatures of ~(4-7) × 105 K. The measured 3γ/2γ ratio does not constrain the density when the annihilation takes place in such an ionized medium, although the density must be high enough to slow down the positrons. This would require the formation of a substantial mass of atmosphere at transition-region temperatures during the flare.

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.

THE FIRST FOCUSED HARD X-RAY IMAGES OF THE SUN WITH NuSTAR

We present results from the the first campaign of dedicated solar observations undertaken by the \textit{Nuclear Spectroscopic Telescope ARray} ({\em NuSTAR}) hard X-ray telescope. Designed as an astrophysics mission, {\em NuSTAR} nonetheless has the capability of directly imaging the Sun at hard X-ray energies (

THE FIRST X-RAY IMAGING SPECTROSCOPY OF QUIESCENT SOLAR ACTIVE REGIONS WITH NuSTAR

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Earth-Affecting Solar Causes Observatory (EASCO): A mission at the Sun-Earth L5

3~keV) with an increase in sensitivity of at least two magnitude compared to current non-focusing telescopes. In this paper we describe the scientific areas where \textit{NuSTAR} will make major improvements on existing solar measurements. We report on the techniques used to observe the Sun with \textit{NuSTAR}, their limitations and complications, and the procedures developed to optimize solar data quality derived from our experience with the initial solar observations. These first observations are briefly described, including the measurement of the Fe K-shell lines in a decaying X-class flare, hard X-ray emission from high in the solar corona, and full-disk hard X-ray images of the Sun.

HARD X-RAY IMAGING OF INDIVIDUAL SPECTRAL COMPONENTS IN SOLAR FLARES

We present the first observations of quiescent active regions (ARs) using NuSTAR, a focusing hard X-ray telescope capable of studying faint solar emission from high temperature and non-thermal sources. We analyze the first directly imaged and spectrally resolved X-rays above 2~keV from non-flaring ARs, observed near the west limb on 2014 November 1. The NuSTAR X-ray images match bright features seen in extreme ultraviolet and soft X-rays. The NuSTAR imaging spectroscopy is consistent with isothermal emission of temperatures 3.1 − 4.4~MK and emission measures 1 – 8 × 10^(46)~cm^(−3). We do not observe emission above 5~MK but our short effective exposure times restrict the spectral dynamic range. With few counts above 6~keV, we can place constraints on the presence of an additional hotter component between 5 and 12~MK of ∼ 10^(46)cm^(−3) and ∼10^(43) cm^(−3), respectively, at least an order of magnitude stricter than previous limits. With longer duration observations and a weakening solar cycle (resulting in an increased livetime), future NuSTAR observations will have sensitivity to a wider range of temperatures as well as possible non-thermal emission.