Rate of proton intensity decay in solar cosmic ray events as a generalized characteristic of the interplanetary space

K. Kecskeméty
Central Institute of Physical Research of Hungarian Academy of Sciences, Budapest, Hungary

Abstract

[1]  The transport of solar energetic particles in interplanetary space is determined by the structure and dynamics of the solar wind between the source and the observer. The intensity time profile, in particular the decay phase, is determined by the transport processes. In this paper we discuss the decay phases of solar energetic proton events in the energy range 1-48 MeV for the period 1974-2001. For events with exponential shape of decay, the dependence of the characteristic time τ on the exponent of the energetic spectrum γ, the solar wind velocity V, and the proton energy E assumed in a form τ(E)= CE^-n is given. Such presentation allows us to consider the action of three main mechanisms of propagation (diffusion, convection, and adiabatic cooling) that determine the τ value. It is shown that approximately half of decays with the constant value of V is described quite satisfactorily within the frame of the model with predominant convection and adiabatic deceleration in comparison with particle diffusion. The dependence of n on heliolongitude of the parent flare (the source of particles) is investigated.

Received 11 April 2006; revised 8 May 2007; accepted 13 August 2007; published 20 November 2007.

Keywords: solar cosmic ray; proton intensity decline; interplanetary space.

Index Terms: 2104 Interplanetary Physics: Cosmic rays; 2114 Interplanetary Physics: Energetic particles; 2194 Interplanetary Physics: Instruments and techniques.