Revisiting the SHH and SEP Link

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Contents

Introduction

Two previous Nuggets have debated the possibility of a link between solar energetic particle (SEP) events and "soft-hard-harder" (SHH) spectral behavior in solar flares. In "Soft-Hard-Harder", Sam Krucker and Hugh Hudson discussed five RHESSI flares that confirmed this relation, but Gerry Share and Allan Tylka later questioned the link when considering five more RHESSI and Yohkoh events in "SEPs Link not Confirmed".

This correlation was originally studied by Alan Kiplinger, using 10 years of data from the Hard X-ray Burst Spectrometer aboard the Solar Maximum Mission. Kiplinger found that 96% of SEP-producing flares were predicted by non-thermal, hard X-ray (HXR) SHH behavior, suggesting that the two processes are somehow linked. This strong statistical correlation alone is interesting for space-weather prediction applications, but confirming or discrediting the link is also important for motivating further investigation into the actual dynamics that could relate the two phenomena.

Here we present the results of our recent study on the subject. Our findings appear to ratify the statistical correlation between SHH and SEP occurrence, in agreement with Kiplinger's work.

Online Catalog of Events

For this study, we initially gathered RHESSI observations of all 661 X- and M-class flares from 2002 February 12 through the end of Solar Cycle 23 . Flares were then limited to solar locations between West 30o and West 90o, as these western longitudes are more likely to connect SEPs to Earth by interplanetary magnetic field lines. After eliminating flares without sufficient observational coverage by RHESSI and those without non-thermal HXR emission above background levels, we were left with 84 flares that were further analyzed for SHH behavior. SEP events were determined by particle flux observations at Earth. The data we used included GOES proton flux measurements and both proton and electron data from the 3DP instrument aboard WIND.

A full catalog of all our flares, along with SEP event data and SHH analysis is available online. The online catalog also serves as a quick general reference of all RHESSI observations of major flares, complete with spectrograms and accompanying GOES soft X-ray lightcurves.

Spectral Analysis

In order to search for SHH behavior, we are required to analyze the HXR spectra at multiple time steps during each flaring event. A "harder" spectrum implies that there is a higher ratio of higher-energy emission, and is typical during periods of increased flare emission. The majority of events then return to "softer" spectra after emission lessens, and are therefore referred to as soft-hard-soft (SHS) flares. Soft-hard-harder (SHH) events instead show increasing ratios of higher-energy emission, even after emission subsides. Since a power-law spectra is typically assumed for the HXR Bremsstrahlung emission of interest, plotting the spectral-index (γ) as a function of time is a good way to visualize spectral hardening and softening during a flaring event; lower γ values correspond to a flatter slope of the power-law on a log-log plot, and therefore a harder spectrum (the power-law form is

I \propto E^{-\gamma},

where I is the count intensity and E is the emission energy). We took this approach to finding the spectral behavior of each flare by performing linear fits of the log-log spectra to find the slope (γ) at 4s intervals (see Figure 1 for an example of our fitting). We should mention that this is only one of many possible methods for quantifying spectral hardening and softening; for example, a ratio of counts from two energy ranges was used in "SEPs Link not Confirmed".

Figure 1: Spectra and power-law fits during two time steps of the 2004 September 19 flare. The later time step, on the right, shows a "harder" spectrum (and correspondingly lower γ value).

Samples of our analysis are shown for two flare events in Figures 2 and 3. Spectrograms (top panels) were useful for determining appropriate power-law energy ranges, which had sufficient non-thermal HXR emission for fitting. Lightcurves (middle panels) were compared to spectral-index (γ) progressions (bottom panels) in order to search for SHH events.

Figure 2: The 2004 September 19 M1.9 flare, displaying distinct SHH spectral evolution. This is clear from the continuously decreasing spectral-index (γ) in the bottom panel.


Figure 3: The 2002 August 03 X1.0 flare, showing the more typical SHS spectral behavior. The soft-hard-soft signature is seen in the spectral-index (γ) time-progression (bottom panel) as the inverse of the lightcurve (middle panel).

Conclusion

After discarding flare observations whose spectral behavior or energetic particle occurrence were indeterminable, and those without visible SHH behavior but only partial RHESSI observational coverage, we were left with 37 events in our final results.

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