Solar energetic particle event lists

SEP flux enhancements within a short space of time (hours to days) are called SEP events. There are various definitions of SEP events depending on the energy level, the flux threshold, the lagtime (the time after the flux drops below the threshold), the sampling time or the minimum event characteristics (e.g. lowest fluence or lowest peak flux). Statistical models of the SEP Event environment are usually based upon statistical techniques and the constructed list of past events.

In the JPL model, the events were defined separately in the >10 MeV energy channel using a threshold of 1 pfu, a lagtime of 2 days, a sampling time of 24 hours and a minimum event fluence of 106cm2 [1]. It is evident that the use of daily averaged fluxes limits the precision at which the start and end times of the SEP events are calculated.

In the ESP model by Xapsos [2], the begining of a period of interest is defined as when the flux exceeds the background for the first time and the end as when the flux returns to the background for the first time. For the construction of the associated list (PSYCHIC list) events were excluded if the peak differential flux in the 1.15–1.43 MeV channel did not exceed >4 cm-2s-1sr-1MeV-1 and the peak flux in the 42.9–51.0 MeV channel did not exceed >0.001 cm-2s-1sr-1MeV-1. Due to its manual construction, the PSYCHIC list is difficult to reproduce. It should be noted that as the PSYCHIC model does not apply statistical analysis to SEP events but rather to the yearly fluence value that this is not a SEP event list by the orthodox definition but some event definition was required to highlight the time periods where fluxes were significant for the yearly fluence.

The definition used for the construction of the NOAA event list [3] considers the beginning of a proton event to be the first of three consecutive (5-min averaged) data points with fluxes of energy >10 MeV greater or equal to 10 pfu, while the end of the event is defined by the last time the flux was greater than 10 pfu. In this definition there is no lagtime, while the high temporal resolution of 5 minutes and the requirement of 3 consecutive data points at the start of the event points serves to exclude 'non-events' detected by data errors.

SEPEM and event lists

Using the event list manager of the SEPEM application server, the SEPEM user can generate event lists by selecting available (raw or cleaned) flux data and by choosing values for the following event parameters:

Furthermore, SEPEM provides results of the statistical description of SEP radiation environment based on the application of standard and new statistical models to the event list constructed by the Southampton Astronautics Research Group. With a continuous, homogenous, flux time series covering 35.66 years established in the reference proton data set the SEP events for the SEPEM Reference Event List (REL) were extracted. Events were defined as beginning when the flux in the 7.23 - 10.46 MeV SEPEM energy channel went above 0.01  cm-2s-1sr-1MeV-1. The event nominally ends when the flux drops below the 0.01  cm-2s-1sr-1MeV-1 threshold again.

However, to ensure that consecutive flux enhancements were independent in time a minimum dwell time of 24 hours was introduced in our study such that if one enhancement occurred within 24 hours of the previous enhancement then they were combined into the same event. This prevents the the model from systematically under-predicting the likelihood of a sequence of enhancements resulting from the same active region and therefore inter-dependent in time and/or enhanced flux levels resulting from reacceleration of seed particles resulting from an earlier CME. To eliminate erroenous/insignificant events a minimum event duration of 24 hours was established as it is known that significant SEP events last from days to weeks and a minimum peak flux (intensity) of 0.5  cm-2s-1sr-1MeV-1 was introduced. The final SEPEM REL includes 225 events at an average of 6.3095 per year of which 194 are in active years (9.2381 per year) and 31 are at solar minimum (2.1145 per year).


[1] J. Feynman, G. Spitale, J. Wang, and S. B. Gabriel, JGR 98 (A8), 13281 (1993).
[2] M. A. Xapsos et al, IEEE Transactions on Nuclear Science 51 (6), 3394 (2004).

Last modified: 27.04.2011 by PJ