Introduction

Solar eruptive events sometimes result in large fluxes of high-energy solar protons at the Earth. This period of time, wherein the solar proton flux is generally elevated for a few days, is known as a solar proton event (SPE) and primarily occurs near solar maximum. Solar cycle 23 experienced several very large SPEs, which occurred in July and November 2000, September and November 2001, October/November 2003, and January 2005. The twelve largest SPEs in the past 40 years are given in Table I and over half of them (seven) occurred in the past solar maximum period.

The Earth's magnetic field guides the solar protons into the northern and southern polar cap regions (>60° geomagnetic latitude), e.g., see Jackman and McPeters (2004). The protons interact with the neutral middle atmosphere (stratosphere and mesosphere) and produce ionizations, dissociations, dissociative ionizations, and excitations.

Both HOx (H, OH, HO2) and NOy (N, NO, NO2, NO3, N2O5, HNO3, HO2NO2, BrONO2, ClONO2) can be enhanced either directly by the protons and their

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