SOLAR WIND similar to 20-200 keV SUPERHALO ELECTRONS AT QUIET TIMES

摘要

High-energy superhalo electrons are present in the interplanetary medium (IPM) even in the absence of any significant solar activity, carrying important information on electron acceleration in the solar wind. We present a statistical survey of similar to 20-200 keV superhalo electrons measured at 1 AU by the WIND. 3D Plasma & Energetic Particle instrument during quiet-time periods from 1995 January through 2013 December. The selected 242 quiettime samples mostly occur during the rising, maximum and decay phases of solar cycles. The observed omnidirectional differential flux of these quiet-time superhalo electrons generally fits to a power-law spectrum J = A x (E/m(e)C(2)))(-beta), with beta ranging from similar to 1.6 to similar to 3.7 and the integrated density n(sup) ranging from 10(-8) to 10(-5) cm(-3). In solar cycle 23 (24), the distribution of beta has a broad maximum between 2.4 and 2.8 (2.0 and 2.4). Both beta and the logarithm of n(sup) show no obvious correlation with sunspot number, solar flares, solar wind core population, etc. These superhalo electrons may form a quiet-time energetic electron background/reservoir in the IPM. We propose that they may originate from nonthermal processes related to the acceleration of the solar wind such as nanoflares, or could be formed in the IPM due to further acceleration and/or long-distance propagation effects.