SOLAR PARTICLE ARRIVAL NEAR EARTH

Can A Single Upstream Monitor At L1 Detect Every Solar Particle Onset?

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Inspect the Geoeffective SEP events measured by GOES-8 at geosynchronous orbit (~6.6 Re).

Inspect the Energetic Ion Fluxes measured by EPIC/ICS near Earth (~30 to ~8-9 Re).

Background

This project is supported by a NASA grant under the Living with a Star (LWS) Geospace program. Our goal is to help determine how effective a single spacecraft (S/C) monitor at the sunward L1 libration region is in being the first respondent to detect the onset of Solar Energetic Particles (SEPs) at Earth with arrival energies in the range 0.5-30 MeV at the onset of prompt solar events. The sunward L1 libration region is about 220 Re from Earth along the Earth-Sun line. At L1 the competing gravitational attractions of the Earth and Sun are approximately equal, so a spacecraft can be placed in an easily maintained orbit about this "gravitational well".

The most effective and costly way to ensure detection of all SEPs would be to launch a large number of spacecraft to orbit the Earth at about the distance of L1. However, the cost of monitoring and maintaining a large fleet of spacecraft would be prohibitive. Therefore, this study is aiding the search for the least number of spacecraft needed to effectively identify approaching SEP fluxes and to determine the nature of the onset front of outflowing SEP flux. The most common assumption in solar particle transport in interplanetary space is that of spherical symmetry, which ignores spatial variations on the order of the Earth-L1 distance.

In order to ensure that the particle events have a simple transport history between the Sun and the Earth, we seek SEP onsets in which we can identify electron and proton fluxes which arrive in the approximately expected time sequence. That is, when a solar flare generates a burst of accelerated particles, x-rays are typically emitted at the Sun from locations near the flare site. These x-rays travel at the speed of light and reach the Earth in about 7 minutes. Electrons and protons with energies of 100 keV and 5 MeV streaming to the Earth from the same flaresite along the average interplanetary Magnetic Field (IMF) will take about 10-18 minutes and 1.6-11 hours, respectively. We select events observed at ACE by looking for electron and proton onsets separated by about 84 to 180 minutes with anisotropic flux arrival angular distributions.

The Solar Cycle

The Sun has an approximately 11-year activity cycle during which its luminosity and energetic particle production vary significantly. The accompanying figure shows the solar radio emission from the sun at a wavelength of 10.7 cm (generally called "the 10 cm flux", or simply "F10.7") for about one-half of a century. F10.7 correlates well with the number of sunspots, a general measure of solar activity. The F10.7 data has been compiled by the NGDC . "Solar Minimum" is the phase when solar activity is low, as in 1986 and 1996, for example, and "Solar Maximum" is the phase when solar activity is high, as in 1959 and 2001.