656 research outputs found
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
Multiple energetic injections in a strong spike-like solar burst
An intense and fast spike-like solar burst was built up of short time scale structures superimposed on an underlying gradual emission, the time evolution of which shows remarkable proportionality between hard X-ray and microwave fluxes. The finer time structure were best defined at mm-microwaves. At the peak of the event, the finer structures repeat every 30x60ms. The more slowly varying component with a time scale of about 1 second was identified in microwave hard X-rays throughout the burst duration. It is suggested that X-ray fluxes might also be proportional to the repetition rate of basic units of energy injection (quasi-quantized). The relevant parameters of one primary energy release site are estimated both in the case where hard X-rays are produced primarily by thick-target bremsstrahlung, and when they are purely thermal. The relation of this figure to global energy considerations is discussed
A frequency-agile interferometer for solar microwave spectroscopy
A high-resolution microwave spectrometer has been developed by converting the Owens Valley solar interferometer to frequency-agile operation. The system uses 27 m antennas equipped with phase-locked receivers which can change their observing frequency in 25 or 50 ms. Microwave spectra between 1 and 18 GHz are obtained in a few seconds by successive observations at up to 86 discrete frequencies. At each frequency the data are equivalent to the total power from each antenna and the interferometric amplitude and phase. All data are fully calibrated with respect to cosmic sources.
The instrument was motivated by the need for better microwave spectral resolution for the study of plasma parameters, non-thermal electrons and coronal magnetic field strengths in solar flares and active regions. Early observations with the system are illustrated by a sequence of flare spectra featuring cases with exceptionally narrow continuum bandwidths
Observations of Hydrogen and Helium Isotopes in Solar Cosmic Rays
Hydrogen and helium isotopes in solar cosmic rays between 1.2
and 15.0 MeV/nuc have been observed with the Ca1tech Electron/Isotope Spectrometer on IMP-7. During 1973 three "^3He rich
events", containing more ^3He than ^2H or ^3H, were observed on
14 February, 29 June, and 5 September. The latter event was
particularly interesting in that (^3He/^4He) ~ 6 and (^3He/^1H)
~ 1. Excluding these three events, flare-averaged ratios
for ^2H/^1H and ^3H/^1H have been obtained for energies below
8.6 MeV/nuc. When compared with the ratios at higher energies,
the observed energy dependence is consistent with the thin
target model of Ramaty and Kozlovsky with a relativistic pathlength
of ~ 1 g/cm^2. Flare-averaged ^3He results reported here
might suggest a somewhat longer pathlength
A spatial and spectral maximum entropy method as applied to OVRO solar data
We present first results of applying a Maximum Entropy Method (MEM) algorithm that acts in both the spatial and spectral domains to data obtained with the frequency-agile solar interferometer at Owens Valley Radio Observatory (OVRO) taken at 45 frequencies in the range 1 – 18 GHz. The traditional MEM algorithm does not exploit the spatial information available at adjacent frequencies in the OVRO data, but rather applies separately to each frequency. We seek an algorithm that obtains a global solution to the visibilities in both the spatial and spectral domains. To simplify the development process, the algorithm is at present limited to the one-dimensional spatial case. We apply our 1-d algorithm to observations taken with the OVRO frequency-agile interferometer of active region AR 5417 near the solar limb on March 20, 1989 (vernal equinox). The interferometer's two 27 m antennas and 40 m antenna were arranged in a linear east-west array, which at the vernal equinox gives a good match to the 1-d algorithm. Our results show that including the spectral MEM term greatly improves the dynamic range of the reconstructed image compared with a reconstruction without using this information. The derived brightness temperature spectra show that for AR 5417 the dominant radio emission mechanism is thermal gyroresonance and we use this information to deduce the spatial variation of electron temperature and magnetic field strength in the corona above the active region
Measurements of the Flux of Low-Energy Solar-Flare Positrons
We report new upper limits to the flux of solar-flare positrons in
the energy range 0.2 - 2 MeV. The observations were made during
four solar-particle events in late 1972, with the Caltech Electron/
Isotope Spectrometer on IMP-7. The 0.2 - 2 MeV positron flux is
compared directly to the solar-flare electron (0.2 - 2 MeV) and
proton (1.2 - 27.5 MeV) fluxes measured in the same detector system.
Summing over four solar events, we find e^+/(e^+ + e^-) < 6 x 10^(- 3).
Calculated fluxes of solar-flare positrons for these four events
are well below our upper limits
Observations of Low Energy Hydrogen and Helium Isotopes during Solar Quiet Times
We report a new quiet-time measurement of the relative abundance
of cosmic-ray ^2H and ^4He. The observations were made in selected
time intervals between September 1972 and February 1973 with the
Caltech Electron/Isotope Spectrometer on IMP-7. In the energy
interval 13-29 MeV/nucleon, we find an upper limit to the ^2H to ^4He
ratio of Γ(^2H/^4He)< 0.06. This new upper limit is significantly
lower than finite ^2H/^4He ratios measured in earlier years by other
workers. Possible implications of this new result are discussed
Quantitative characterization of the x-ray imaging capability of rotating modulation collimators with laser light
We developed a method for making quantitative characterizations of bi-grid rotating modulation collimators (RMC ’s) that are used in a Fourier transform x-ray imager. With appropriate choices of the collimator spacings, this technique can be implemented with a beam-expanded He -Ne laser to simulate the plane wave produced by a point source at infinity even though the RMC ’s are diffraction limited at the He -Ne wavelength of 632.8 nm. The expanded beam passes through the grid pairs at a small angle with respect to their axis of rotation, and the modulated transmission through the grids as the RMC ’s rotate is detected with a photomultiplier tube. In addition to providing a quantitative characterization of the RMC ’s, the method also produces a measured point response function and provides an end-to-end check of the imaging system. We applied our method to the RMC ’s on the high-energy imaging device (HEIDI) balloon payload in its preflight configuration. We computed the harmonic ratios of the modulation time profile from the laser measurements and compared them with theoretical calculations, including the diffraction effects on irregular grids. Our results indicate the 25-in. (64-cm) x-ray imaging optics on HEIDI are capable of achieving images near the theoretical limit and are not seriously compromised by imperfections in the grids
First limits on the 3-200 keV X-ray spectrum of the quiet Sun using RHESSI
We present the first results using the Reuven Ramaty High-Energy Solar
Spectroscopic Imager, RHESSI, to observe solar X-ray emission not associated
with active regions, sunspots or flares (the quiet Sun). Using a newly
developed chopping technique (fan-beam modulation) during seven periods of
offpointing between June 2005 to October 2006, we obtained upper limits over
3-200 keV for the quietest times when the GOES12 1-8A flux fell below
Wm. These values are smaller than previous limits in the 17-120 keV
range and extend them to both lower and higher energies. The limit in 3-6 keV
is consistent with a coronal temperature MK. For quiet Sun periods
when the GOES12 1-8A background flux was between Wm and
Wm, the RHESSI 3-6 keV flux correlates to this as a power-law,
with an index of . The power-law correlation for microflares has
a steeper index of . We also discuss the possibility of
observing quiet Sun X-rays due to solar axions and use the RHESSI quiet Sun
limits to estimate the axion-to-photon coupling constant for two different
axion emission scenarios.Comment: 4 pages, 3 figures, Accepted by ApJ letter
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