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ACE News #115 - Sep 26, 2008

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Solar Injection Histories of Near-Relativistic Solar Electrons

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Figure 1 (left): Solar energetic particles observed near 1 AU (solid dot) contain both an injected and a mirrored component. The mirrored component has backscattered beyond 1 AU, mirrored inside 1 AU, and returned to 1 AU. The equation/algorithm (lower right) describes this process. Figure 2 (right) shows an application of our algorithm to extract the actual solar injection history from a near-relativistic (175-300keV) electron event.

The near-light speed and nearly scatter-free propagation out to 1 AU of 175-300 keV electrons preserves the essential details of their injection onto the interplanetary magnetic field. We have developed a quantitative method that uses the pitch-angle anisotropies to extract the electron injection history for near-relativistic beam-like solar electron events well past the rise-to-maximum phase. As sketched in Fig. 1, this is accomplished by subtracting from the outgoing electrons at 1AU (j+) those that have already back-scattered from beyond 1AU, re-crossed 1AU (j-), mirrored and finally returned to 1 AU (jm). The difference between the outgoing intensity (j-) and that of this mirrored component, delayed by its mirroring time 2τ inside 1AU, jm(t) = (j-)(t-2τ), gives the intensity j0(t) = js(t-z/v) of the "first-crossing" electrons that are still arriving directly from the Sun (js) after traveling a distance (z) along the field line at velocity (v). The implementation of the time-shifted algorithm (Fig. 2) starts from the spin-sectored intensities (different colors) in the three EPAM heads (top 3 panels). These sectored data are fit with exponential functions of pitch-angle jFIT = jnormexp(αμ) independently for both the outward (μ>0) and inward (μ<0) hemispheres. The best fit to the data (jFIT) is shown in the fourth panel for three pitch angles (0°,90°,180°). Referring to the algorithm given in Fig. 1, we express the outgoing intensity j+ = jFIT(0°) and the ingoing intensity j- = jFIT(180°) along the field-line in terms of the "first-crossing" intensity (j0) and the mirroring intensity (jm). The simplicity of this algorithm yields the solar injection history (well past the intensity maximum) directly without recourse to models for acceleration/release or propagation. We are finding that it is not unusual for the injection history to last >1/2 day (bottom panel), implying that the acceleration/release process for near-relativistic electrons is extended much longer than the flash phase of solar flares.

This item was contributed by Dennis K Haggerty and Edmond C Roelof (JHUAPL). Address questions and comments to or

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Last modified 19 Mar 2008.