Effects of solar particle events on Geospace

Abstract

The Earth receives all of its external energy input from the Sun (except for galactic sources), which consists of both particles of various energies and electromagnetic radiation of various wavelengths. The particles emitted from the Sun constitute the solar wind, from which the interplanetary magnetic field originates. These particles transport magnetic, thermal, and kinetic energy from the Sun to the Earth. The solar wind has two modes: slow (typical speed 400 km.r 1 ), and fast (typical speed 800 km.r'). Particles of various energies are released and accelerated into the heliosphere during (i) sporadic events involving both flares and coronal mass ejections, and (ii) recurrent events driven by co-rotating interaction regions associated with fast solar wind streams from trans-equatorial coronal holes. A small proportion of these particles, of which protons are by far the most significant for our purposes, deposit their magnetic and kinetic energy within geospace (defined herein as 'that which is within the magnetopause').

A survey of the solar energetic particle (SEP) events and trans-equatorial coronal holes occurring during solar cycles 19 to 23 confirmed that the cumulative frequency of SEP occurrence and the cumulative monthly averages of sunspot number exhibit a strong correlation which improves with the inclusion of successive solar cycles. Furthermore, there is good evidence that the emergence of low-latitude, open, coronal magnetic flux is also synchronised with the solar cycle. Four SEPs and one trans-equatorial coronal hole were then selected from cycle 23 for a detailed investigation of their effects on geospace. Specifically, the effects on the magnetosphere and the ionosphere are studied in relation to geomagnetic storms, cutoff latitude depression, substorms and polar cap absorption.

The energetic proton populations of three selected SEP events from 2001 are used as 'tracers' to quantify the compression of the geomagnetic cavity during storm conditions. Empirical relations between the cutoff latitude and the ring current indices D,t and SYM-H are defined, and the compliance of the observations with the Tsyganenko TO] geomagnetic field model is tested during storm conditions ranging from quiet to severe. The TOl model was found to be limited in its utility during severe storm conditions (D8 < — lOOnT).

Another selected SEP event from 2003, the 'Halloween flare', resulted in a polar cap absorption event for which empirical altitude-dependent relations have been derived between proton flux and D-region electron density. Values of effective recombination coefficient have been determined for both day and night. It was found that, though chemical models predict that the production and recombination process should deviate from a square law, there is actually no evidence for this in the observations.

The D- and E-region effects of a co-rotating interaction region associated with the meridional crossing of a trans-equatorial coronal hole during late June 2005 have been investigated during periods of substorm activity in relation to electron density, cosmic-radio-noise absorption, and
energetic electron fluxes. The hardening of the spectrum between the evening and morning periods is noted, and the movements of plasma during those periods are studied in detail, the motion being consistent with observations from previous related studies. The height and thickness of the absorption layer are estimated and the calculated total radio absorption is
compared with measurement.

The same co-rotating interaction region provided the opportunity to investigate the spatial and temporal variations of the trapped and precipitated electrons in the auroral regions in terms of flux, spectra, and pitch angle. Differences of spectral hardness between the trapped and precipitated electrons, with the time of day, and with the intensity, are noted. It is observed that, for the population on the day-side, the precipitated/trapped flux ratio is highly variable and is energy dependent in a manner consistent with the theory of pitch angle scattering.Another population, observed mainly on the night-side, shows near-isotropy at all energies observed. Diagrams of precipitated against trapped flux ('pitch-o-grams') are found to be helpful in identifying various electron populations.

A method of determining the solar wind velocity from the delay in the B. is presented. The development of the event within the magnetosphere is described in detail, with particular reference to the results of the LISCAT observations. Comparisons are then drawn between a selection of fast solar wind streams and CMEs in relation to the Li environment, the magnetospheric cutoff latitudes and the energy deposited within the magnetosphere. Finally, the relative contributions of SEPs and fast solar wind streams to geomagnetic storms, cutoff latitude depression, substorms and polar cap absorption are compared.