Springer Online Journal Archives 1860-2000
Abstract The HIgh-REsolution Gamma-ray and hard X-ray Spectrometer (HIREGS) consists of an actively shielded array of twelve liquid-nitrogen-cooled germanium detectors designed to provide unprecedented spectral resolution and narrow-line sensitivity for solar gamma-ray line observations. Two long-duration, circumpolar balloon flights of HIREGS in Antarctica (10–24 January, 1992 and 31 December, 1992–10 January, 1993) provided 90.9 and 20.4 hours of solar observations, respectively. During the observations, eleven soft X-ray bursts at C levels and above (largest M1.7) occurred, and three small solar hard X-ray bursts were detected by the Compton Gamma-Ray Observatory. HIREGS detected a significant increase above 30 keV in one. No solar gamma-ray line emission was detected. Limits on the 2.223-MeV line and the hard X-ray emission are used to estimate the relative contribution of protons and electrons to the energy in flares, and to coronal heating. For the 2.223-MeV line, the upper limit fluence is ≲ 0.8 ph cm-2 in the flares, and the upper limit flux is 1.8 × 10-4 ph s-1 cm-2 in the absence of flares. These limits imply that ≲ 6 × 1030 (2σ) protons above 30 MeV were accelerated in the flares, assuming standard photospheric abundances and a thick target model. The total energy contained in the accelerated protons 〉30 MeV is ≲ 4 × 1026 ergs, but this limit can be more than 1030 ergs if the spectrum extends down to ∽1 MeV. The upper limit on the total energy in accelerated electrons during the observed flares can also exceed 1030 ergs if the spectrum goes down to ∽ 7 keV. Quiet-Sun observations indicate that ≲ 1026erg s-1 are deposited by energetic protons 〉1 MeV, well below the1027 –1028 erg s-1 required for coronal heating, while 〈3 × 1027 erg s-1 are deposited by energetic electrons, which does not exclude the possibility of coronal heating by quiet-time accelerated electrons. The quiet-Sun observations also suggest that if protons stored in the corona are to supply the energy for flares, as suggested by Elliot (1964), the proton spectrum must extend down to at least ∽2 MeV. However, collisional losses at typical coronal-loop densities prevent those low-energy protons from being stored for ≳ 104 s. It therefore seems unlikely that the energy for flares could come from energetic protons stored over long periods.
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