Interplanetary fast shocks and associated drivers observed through the 23rd solar minimum by Wind over its first 2.5 years

摘要

A list of the interplanetary shocks observed by Wind from its launch tin Nov 1994) to May 1997 is presented. The magnetohydrodynamic nature of the shocks is investigated, and the associated shock parameters and their uncertainties are accurately computed using two techniques. These are: 1) a combination of the "preaveraged" magnetic-coplanarity, velocity-coplanarity, and the Abraham-Schrauner-mixed methods, and 2) the Vinas and Scudder [1986] technique for solving the nonlinear least squares Rankine-Hugoniot equations. Within acceptable limits these two techniques generally gave the same results, with some exceptions. The reasons for the exceptions are discussed. The mean strength and rate of occurrence of the shocks appear to correlate with the solar cycle. Both showed a decrease in 1996 coincident with the time of the lowest ultraviolet solar radiance, indicative of solar minimum and the beginning of solar cycle 23. Eighteen shocks appeared to be associated with corotating interaction regions (CIRs). The shock normal distribution showed a mean direction peaking in the ecliptic plane and with a longitude of similar to 200 degrees (GSE coordinates). Another 16 shocks were determined to be driven by solar transients, including magnetic clouds. These had a broader distribution of normal directions than those of the CLR cases with a mean direction close to the Sun-Earth line. Eight shocks of unknown origin had normal orientations far off the ecliptic plane. No shock propagated with longitude phi (n) greater than or equal to 220+/-10 degrees, i.e. against the average Parker spiral direction. Examination of the obliquity angle theta (Bn) (i.e., between the shock normal and the upstream interplanetary magnetic field) for the full set of shocks revealed that about 58% were quasiperpendicular, and about 32% of the shocks oblique, and the rest quasi-parallel. Small uncertainty in the estimated angle theta (Bn) was obtained for about 10 shocks with magnetosonic Mach numbers between 1 and 2. [References: 76]