SEPs Link not Confirmed

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Introduction   Solar and space scientists have endeavored for years to predict the occurrence of solar energetic particle (SEP) events. These events have the potential for damaging satellite electronics and providing significant radiation doses to astronauts in space. In Nugget 46 entitled “Soft-Hard-Harder” Sam Krucker and Hugh Hudson describe RHESSI observations of five solar flares that occurred in 2005 January. These observations appear to confirm an earlier study done by Alan Kiplinger using Solar Maximum Mission (SMM HXRBS) data from the 1980’s indicating that soft-hard-hard spectral evolution in hard X-ray bursts and soft-hard-harder evolution throughout a flare are indicators of an ensuing SEP event. Both studies have also been published in the Astrophysical Journal. It is important to note that the definition of this spectral hardening is somewhat vague and the analyses have been subjective. If one can find flares associated with SEP events that have no soft-hard-hard characteristics of any kind then the premise can be ruled out.   In this Nugget, we discuss hard X-ray observations of six solar flares between 1991 and 2001 made by Yohkoh that were accompanied by SEP events. We find no evidence for Soft-Hard-Hard evolution in these flares even though four of the SEP events had intensities of >400 MeV protons, sufficiently high to be detected at ground level on Earth (by neutron monitors). A similar conclusion was reached in a Yohkoh Nugget (2001 August 17) by Nariaki Nitta based on much more limited analysis.

Observations   The Yohkoh hard X-ray spectrometer (HXS) consisted of a 7.6 cm (diam) by 2.5 cm (thick) NaI scintillator coupled to a photomultiplier tube. It covered the energy range for ~35 keV to 800 keV that varied over the mission. Incident X-ray photons create energetic electrons in NaI that produce light pulses proportional to the amount of energy that they lose. A spectrum of the incident X-ray flux can then be created.   In order to search for spectral hardening signature we accumulated data at 1 sec temporal resolution in two energy ranges ~50-100 keV and ~100 – 200 keV. In doing this we corrected for gain variations during the mission. We subtracted the background using data taken just before and after the flare, where possible. For all these flares we plot the number of counts/sec observed in the ~100 – 200 keV range in the top panel and the 100-200 keV/50-100 keV count ratio in the bottom panel. Thus harder spectra will have larger ratios.   1991 October 27 flare

This flare occurred in an active region at 15E and produced what was listed as an SEP event although the particles that were observed came from an ESP event associated with a local shock. The Yohkoh data at this point in the mission was rather sparse; we therefore obtained backgrounds about 25 hours earlier. There were several peaks in the flare that all appear to follow a soft-hard-soft evolution.   1997 November 6 Flare   This flare occurred near W63 and produced a large SEP/GLE with a likely particle injection time no earlier than 12:10 UT. The hardness ratio does not follow the peaks in the hard x-ray range closely but there is no clear evidence for soft-hard-hard evolution. In

1998 May 6 Solar Flare   The flare came from an active region at ~W63 and the GLE was relatively weak. The peak flux was about 25 times weaker than the 1997 Nov. 6 flare. The spectrum is considerably softer than that found for the 1997 Nov. 6 flare. The flux in many parts of the flare is close to background leading to large uncertainties in the hardness ratio. There is hardening through the second peak but it softens on the trailing edge. The particle injection likely occurred between 8 hr 9min and 8 hr 12 min, after the second peak.

2000 July 14 Solar Flare   The flare arose from an active region at W 07 and produced a large GLE. The peak flux was about a factor of two below that observed in the 2000 July 14 flare. The overall spectrum appears relatively soft. All of the spectral evolution appears to be of the soft-hard-soft variety. A study of this flare has already been published and this lack of a soft-hard-hard evolution was noted. The particle injection appeared to occur between 10:18 and 10:24 during the early phase of the observations.

2000 November 24 Solar Flare   The flare occurred in an active region at W5 and was observed as an SEP but not a GLE. Its peak hard X-ray intensity was as strong as that observed in the 1997 November 6 flare but it was softer. Only soft-hard-soft spectral evolution appears to have been observed.

2001 April 15 Flare   The flare occurred at W85. The peak hard X-ray flux was about the same as observed from the 2000 July 14 flare. Only soft-hard-soft evolution appears to have been observed. Particle injection is likely to have occurred between 13:50 and 13:56, i.e. late in the flare.

Conclusion   The Yohkoh HXS detector observed six gamma-ray line flares associated with SEP and GLE events. We cannot find any evidence for the soft-hard-hard spectral evolution between 50 and 200 keV that has been suggested in flares associated with SEP events. Thus soft-hard-hard spectral evolution is not a necessary condition for production of an SEP event and is not an appropriate predictor for such events.

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