Research in the Department "Space Weather"

Solar energetic particle events and associated solar phenomena:
statistical studies in solar cycle 23
Presentation on 29^th January 2014, Space Research and Technology Institute
Rositsa Miteva

The presentation outlines some results on the topic of solar energetic particles (SEPs) during a postdoctoral stay in Observatory of Paris, France (2011-2013).

Solar energetic particles are electrons, protons and ions (above 10 keV up to 100s MeV) observed onboard spacecraft. The particles travel along the interplanetary magnetic field (IMF) lines and in order to be detected around Earth, these IMF lines need to be connected to the observer. The long-standing debate in the field is on the solar origin of these charged particles, where the main candidates are the solar flares and coronal mass ejections (CMEs). Timing arguments are usually employed to identify the solar source, but previous and also this study show that, both, a flare and a CME candidate are present for a given SEP event. In addition, statistical studies on the correlation between the particle intensity and the coronal source parameters (flare SXR flux and CME projected speed) are used to discriminate in favor of one accelerator over the other.

Here we present the results from a recent work, where we re-evaluated this statistical relationship taking for first time the uncertainties on the correlation coefficients (using the bootstrap method). The main result, contrary to some earlier results, is that the same correlation between SEPs and flares and between SEPs and CMEs is present. This is when a large sample of SEP events is considered (covering the entire solar cycle 23, 1997‑2006). A way out of this is searching for new ordering parameters and we test the influence of the IP conditions on the correlations. It is known that particles may propagate in quiet solar wind conditions, inside interplanetary CMEs (ICMEs) or in a mixed IMF configuration. The separation of the initial SEP sample into these three sub-samples shows that the particles inside ICMEs correlate stronger, within the uncertainties, with the flare than the sample of particles propagating in the solar wind, both for protons and electrons. The correlation with the CME properties is the same for both subsamples. In order to minimize the observational bias, we limited these subsamples to limb events. The stronger correlation is lost for the proton subsample but is still kept for the electrons. In addition, the electrons propagating inside ICMEs now correlated better also with the CME speed than the electrons propagating in quiet solar wind. So far, there is no clear interpretation found for this difference.

In addition to the statistical correlations, we utilize another type of observation, the solar radio signatures, in order to quantify the contribution of a flare vs. CME to the escaping particle flux. Here we implicitly assume that the radio signature of energetic electrons provide a proxy to the observed in-situ particle flux. We use the timing of the microwave emission (exclusively flare dominated) and the timing of the escaping electron beams in order to test the flare hypothesis. We find that in about half of the event sample (about 180 events in total), the microwave emission coincides in time with the escaping electrons and we expect flare contribution to the particle flux released into the IP space. In about 35% of the cases, the flare emission starts too late to be able to contribute to the first escaping particles and we expect predominantly CME-related particles. In less than 20% of the sample, the flare emission happens too early or is already declining in intensity, compared to the first radio signatures of escaping electrons. CME is still the most plausible accelerator for these particles, but for some events flare contribution is not excluded. Detailed analysis on the duration of the microwave emission with respect to the electron beam timing is expected to finalize the above percentages.

In summary, statistical studies cannot provide the final answer to the question on the SEP origin. A detailed timing analysis of different radio emission signatures, as proposed here, can provide some constrains on the flare vs. CME contribution to SEP events.

Look at the presentation