The decay phase of solar energetic particle events

Abstract

[1] The shape of the particle flux profile in the decay phase of solar energetic particle (SEP) events is of particular importance in understanding the propagation of energetic particles in the interplanetary medium. The majority of few-MeV proton events (>90%) exhibit exponential law declines, suggesting the dominant role of convection transport and adiabatic deceleration. In this case, the characteristic decay time t should depend on the spectral index, the solar wind speed, and the distance from the Sun. More than 600 decays observed in the fluxes of 0.5-48 MeV protons by IMP 8 in 1973-2001 were selected and analyzed in detail. We consider the dependence of t on environmental plasma parameters. The observed decay times, t obs , are compared with those considering convection transport and adiabatic deceleration, t theor , and fair agreement is found (within 25%) in nearly 50% of all declines with constant values of V. Although high-energy proton profiles at various radial distances (Helios, IMP 8, Ulysses) are surprisingly identical, MeV protons in the same events decay more slowly, with increasing r, in agreement with the model. The solar cycle variation and the distribution of the t values as a function of energy are also examined. The dependence on the heliographic longitude suggests that events with significantly different t obs and t theor can be explained by the variation of magnetic connection between the observer and a flare site. The observational results are compared with numerical simulations in the frame of a simple particle propagation model involving scattering, adiabatic cooling, and a propagating shock.

Statistical Study on the Decay Phase of Solar Near-Relativistic Electron Events

“…Statistical studies of the decay phase of SEP events have been mostly focused on proton observations (e.g., Daibog et al 2003;Kecskeméty et al 2009, and references therein). Daibog et al (2003), on a sample of ∼700 events collected during almost three solar cycles, concluded that in nearly 90% of the cases for 1-5 MeV proton fluxes, an exponential decay gives the best approximation to the time-intensity profiles.…”

“…The most recent study on the decay rate of SEP events was performed by Kecskeméty et al (2009). These authors studied the decay phase of 0.5-48 MeV proton events observed by the Interplanetary Monitor Platform (IMP-8) near the ecliptic at ∼1 AU from the Sun between 1973 and 2001.…”

“…These authors studied the decay phase of 0.5-48 MeV proton events observed by the Interplanetary Monitor Platform (IMP-8) near the ecliptic at ∼1 AU from the Sun between 1973 and 2001. Following Daibog et al (2003), Kecskeméty et al (2009) selected time intervals during the decay of the events that could "reasonably" be approximated by an exponential. Within the declines of the particle intensities, Kecskeméty et al (2009) distinguished various time intervals (no shorter than 12 hr) that could be fitted by exponential decays of the same or different decay rate within the same event.…”

“…Following Daibog et al (2003), Kecskeméty et al (2009) selected time intervals during the decay of the events that could "reasonably" be approximated by an exponential. Within the declines of the particle intensities, Kecskeméty et al (2009) distinguished various time intervals (no shorter than 12 hr) that could be fitted by exponential decays of the same or different decay rate within the same event. Sometimes these exponential decays could last from the maximum of the event to the background intensity, other times they were seen during a part of an event only, were followed by large fluctuations, and then they could become exponential again with the same or different decay rate.…”

Prediction and warning system of SEP events and solar flares for risk estimation in space launch operations

“…Regarding SEP duration prediction, few studies (e.g. Kahler 2005;Kecskemety et al 2009) have been carried out on the downstream (post-shock) region, mostly discussing the dependences between the characteristic decay time and several solar and interplanetary factors (e.g. heliolongitude of the solar parent event, CME properties, particle energy of accelerated particles, solar wind conditions).…”