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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-harder (SHH) spectral evolution “either over flux peaks or during flux decays” 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 can be subjective. If one can find SEP events in which the associated flares have no soft-hard-harder characteristics, then the premise can be ruled out.

In this Nugget, we discuss hard X-ray observations of five solar flares between 1998 and 2003 made by Yohkoh and RHESSI that were accompanied by SEP events. In two of the flares we observe progressive spectral hardening, which may be interpreted as SHH evolution in an X-ray peak. We cannot find evidence for SHH spectral evolution in the other flares even though they were associated with SEP having intensities in >400 MeV protons sufficiently high to be detected at ground level on Earth (by neutron monitors).

Method

  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. We use the front detectors in analysis of the two RHESSI flares.

In order to search for spectral hardening signature we accumulated data in two energy ranges ~50-100 keV and ~100 – 200 keV. In doing this we corrected for gain variations during the Yohkoh HXS mission. We subtracted the background using data taken just before and after the flare, where possible. We determined the RHESSI background using data accumulated under the same environmental conditions 15 orbits earlier and/or later.

For all five flares we plot the number of counts/time interval observed in the ~100 – 200 keV range in the top panel and the ~100-200 keV/~50-100 keV count ratio (hardness ratio) in the bottom panel.  Thus harder spectra will have larger ratios.

==Two Flares Appearing to Exhibit SHH Behavior== 

The 1998 May 6 solar flare observed by Yohkoh occurred at ~W63. The accompanied ground level event (GLE) for this flare was one of the weakest observed to date. The hard x-ray emission occurred primarily in two peaks. In Figure 1 we plot the 100 – 200 keV rates for the second peak in the top panel and the ~100 – 200 keV/~50 – 100 keV hardness ratio in the bottom panel. The hard X-ray spectrum appears to harden through the second peak and into its decay phase before it softens at the trailing edge.

Figure 1. ~100-200 keV counting rate (top panel) and ~100 – 200 keV/~50 – 100 keV hardness ratio (bottom panel) for the second peak of the 1998 May 6 solar flare.

The 2002 April 21 flare observed by RHESSI occurred at W84 and was accompanied by a solar energetic particle (SEP) that was too soft to be detected at ground level. The X-ray emission came in two peaks. The first one showed no clear spectral evolution but the second one exhibited SHH behavior from onset to the falling edge of the burst, as can be seen in Figure 2.

Figure 2. 100-200 keV counting rate (top panel) and 100 – 200 keV/50 – 100 keV hardness ratio (bottom panel) for the second peak of the 2002 April 21 solar flare

Three Solar Flares with No Apparent SHH Spectral Evolution=

The 2000 July 14 ‘Bastille Day solar flare observed by Yohkoh occurred at W07 and gave rise to one of the largest SEP events ever observed. It was also observed at ground level. All of the spectral evolution appears to be of the soft-hard-soft variety as can be seen in Figure 3. A study of this flare has already been published and this lack of a soft-hard-hard evolution was noted. This flare was also the subject of a Yohkoh Nugget (author Hugh Hudson) in June 2001 that also found only SHS evolution based on observations with the HXT experiment on Yohkoh.

Figure 3. ~100-200 keV counting rate (top panel) and ~100 – 200 keV/~50 – 100 keV hardness ratio (bottom panel) for the 2000 July 14 solar flare.

The 2001 April 15 flare observed by Yohkoh occurred at W85. It was associated with one of the largest GLEs seen in Cycle 23.. Once again only soft-hard-soft evolution appears to have been observed as can be seen in Figure 4

Figure 4. ~100-200 keV counting rate (top panel) and ~100 – 200 keV/~50 – 100 keV hardness ratio for the 2001 April 15 solar flare.

The 2003 November 2 flare observed by RHESSI occurred at W56 and was also associated with a GLE. The X-ray time history and hardness ratio are plotted in Figure 5. Once again there is no clear evidence for SHH spectral evolution.

Figure 5. 100-200 keV counting rate (top panel) and 100 – 200 keV/50 – 100 keV hardness ratio for the 2003 November 02 solar flare.

Conclusion

  Based on our inspection of five solar flares, all associated with SEP events we find two (one observed by Yohkoh and one by RHESSI) that may be interpreted as soft-hard-harder (SHH) spectral evolution. However, the remaining three flares (two observed by Yohkoh and one observed by RHESSI) only appear to exhibit SHS spectral evolution, at least to our ‘untrained eyes’ (we welcome a dialog on this subject). Thus soft-hard-hard (SHH) spectral evolution is not a necessary condition for production of an SEP event and is not an appropriate predictor for such events.