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We present an analysis of hard X-ray imaging observations from one of the first solar flares observed with the Reuven Ramaty
High-Energy Solar Spectroscopic Imager (RHESSI) spacecraft, launched on 5 February 2002. The data were obtained from the 22
February 2002, 11:06 UT flare, which occurred close to the northwest limb. Thanks to the high energy resolution of the germanium-cooled
hard X-ray detectors on RHESSI we can measure the flare source positions with a high accuracy as a function of energy. Using
a forward-fitting algorithm for image reconstruction, we find a systematic decrease in the altitudes of the source centroids
z(ε) as a function of increasing hard X-ray energy ε, as expected in the thick-target bremsstrahlung model of Brown. The altitude
of hard X-ray emission as a function of photon energy ε can be characterized by a power-law function in the ε=15–50 keV energy
range, viz., z(ε)≈2.3(ε/20 keV)−1.3 Mm. Based on a purely collisional 1-D thick-target model, this height dependence can be inverted into a chromospheric density
model n(z), as derived in Paper I, which follows the power-law function n
e(z)=1.25×1013(z/1 Mm)−2.5 cm−3. This density is comparable with models based on optical/UV spectrometry in the chromospheric height range of h≲1000 km, suggesting that the collisional thick-target model is a reasonable first approximation to hard X-ray footpoint sources.
At h≈1000–2500 km, the hard X-ray based density model, however, is more consistent with the `spicular extended-chromosphere model' inferred from radio sub-mm observations, than with standard models based on hydrostatic equilibrium. At coronal heights,
h≈2.5–12.4 Mm, the average flare loop density inferred from RHESSI is comparable with values from hydrodynamic simulations
of flare chromospheric evaporation, soft X-ray, and radio-based measurements, but below the upper limits set by filling-factor
insensitive iron line pairs. 相似文献
45.
Aschwanden Markus J. De Pontieu Bart Schrijver Carolus J. Title Alan M. 《Solar physics》2002,206(1):99-132
Solar Physics - We measure geometric and physical parameters oftransverse oscillations in 26 coronal loops, out of the 17 events described in Paper I by Schrijver, Aschwanden, and Title (2002).... 相似文献
46.
Claudia Maraston Markus Kissler-Patig Jean Brodie Pauline Barmby John Huchra 《Astrophysics and Space Science》2002,281(1-2):137-138
A new method to detect and study young star clusters is presented. This is based on the knowledge that the light of stellar
populations with ages between ∼ 200 Myr and ∼ 1/2 Gyr is dominated by very red, bright AGB stars. Star clusters undergoing this so-called ‘AGB phase transition’ are featured by very red V-K colours, like those of Magellanic
Clouds clusters, while optical colours like B-V remain blue typical for young populations. The best channel for detecting
star clusters in this age range is therefore the near IR. From the theoretical side, SSP models including properly the contribution
of the bright AGB are required (Maraston, 1998). Using this strategy, we succesfully detected the AGB phase transition among
the clusters of the merger remnant galaxy NGC 7252 (see Maraston etal., 2001).
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
47.
Geotechnical and Geological Engineering - This paper proposes a new design method for the axial capacity of driven piles in glacial deposits with the standard penetration test (SPT) based on a... 相似文献
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