Variability of the solar wind suprathermal electron strahl

At suprathermal energies, interplanetary electrons commonly exhibit a magnetic field-aligned beam referred to as the strahl. The finite strahl width is a consequence of competition between magnetic focusing as the interplanetary field weakens with distance from the Sun, and particle scattering acting to broaden the strahl along its propagation path. We present a statistical survey of the strahl using ACE Solar Wind Electron, Proton, and Alpha Monitor (SWEPAM) measurements from 1998 to 2002. A systematic fitting algorithm was applied to pitch angle distributions to identify unidirectional or counterstreaming strahl features and to quantify beam widths and intensities. The analysis indicated that a strahl is present >= 75% of the time, while counterstreaming strahls were observed about 10% of the time. The strahl width ranges from 5 degrees to 90 degrees; importantly, the strahl cannot be characterized by any typical width. Within counterstreaming intervals the strahl peak intensity anticorrelates with beam width while the integrated fluxes of the two simultaneously observed strahls are similar, within a factor of 2 for 75% of counterstreaming periods. Observations are consistent with a model in which integrated strahl flux leaving the corona varies over a limited range, but the degree of beam scattering along the propagation path to 1 AU varies widely. The difference between two concurrent counterstreaming strahls is likely due to different scattering profiles along the different legs of a closed field line loop, both rooted in similar coronal regions. Narrow strahls (<20 degrees) are strongly associated with counterstreaming intervals, as well as high-speed streams. Strahl width can either increase or decrease with energy.