Hard X-ray Spectral Evolution and SEP Events. Gerry Share, Allan Tylka, and Ron Murphy. HISTORY.
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Gerry Share, Allan Tylka, and Ron Murphy
Kiplinger (1995) found that SEP events almost always occur when 40-200 keV X-ray spectra either harden over flux peaks or during flux decays; these events seldom occur when such hardening is not observed.
Share et al. (2001) studied hard X-ray emission from the 2000 July 14 (Bastille Day) flare using the Yohkoh HXS detector and did not find evidence for spectral hardening even though the SEP event was the 3rd largest event between 1976 and 2000.
Saldanha, Krucker, and Lin (2008) found progressive spectral hardening in peaks in four flares in January 2005 that were associated with SEP events and one event without this hardening that did not have an associated SEP event.
7.6 cm (diam.) X 2.5 cm (thick) NaI detector coupled to a PMT
Energy range ~30 keV to ~780 keV in 32 channels
1 s time resolution
Accumulate data at 1 s resolution in two energy bands: ~47 – 103 keV and 103 keV – 210 keV.
Subtract background taken both before and after the flare where available.
Spectral hardness is defined by the ratio: counts (103 –210 keV )/counts (47-103 keV)
We study 6 flares occurring from 1991 to 2001; four of these were GLEs. All of these flares emitted nuclear gamma-ray lines.
Flare location 15 E. Listed as an SEP but mostly ESP were observed (poor connection?). Sparse Yohkoh; background taken 25 hrs earlier. Each peak appears to exhibit S-H-S and the overall trend is S-H-S.
W 63 producing a large SEP/GLE with particle injection likely >12:10. Hardness ratio doesn’t follow flux peaks but no clear S-H-H. Perhaps weak hardening when flux is barely detectable (just due to harder background?)
Location W 63. Relatively weak GLE. Hardness ratio for the residual background is ~0.6. Spectrum softens up to the peak of the flare then gradually hardens as flux approaches bckgrd. Next peak shows the same behavior. Typically higher-energy X-ray are more susceptible to background variations.
Location W07 produced large GLE (atmospheric gamma-ray observed). Overall the spectral ratio is significantly softer than previous flares. Each peak shows soft-hard-soft evolution. Particle injection appears to have occurred between ~10:15 and 10:25.
Location W5. Observed as an SEP but not as a GLE. No obvious S-H-H behavior.
Location W85. Strong SEP and GLE. No obvious S-H-H evolution. Particle injection time ~13:50 – 13:57 UT
Flare SEP S-H-H
1991 Oct. 27 Delayed No
1997 Nov. 6 GLE No
1998 May 6 Weak GLE ? Due to bckgrd?
2000 July 14 GLE No.
2000 Nov. 14 SEP No
2001 April 15 GLE No
2005 January 20 flare (Salhanha et al. 2008). Strong delayed nuclear line emission. Note the ratio of brems/nuclear line emission early in flare. Could effects propagate into front detector? Pion emission stays at relatively high level throughout producing 511 keV line – Compton scattering.