Magnetic decrease formation from <1 AU to ∼5 AU: Corotating interaction region reverse shocks

Abstract

Magnetic decreases (MDs) have been identified and studied throughout a Ulysses fast latitude scan that lasted from 29 February 1992 to 14 September 1993. Ulysses' distance was ∼5 AU from the Sun. MDs were unbiasedly selected by application of the Interplanetary Magnetic Decrease Automatic Detection code. MDs were found to occur in high‐occurrence‐frequency “clusters” with the top 10 peak events varying in magnitude from 116 MDs per day to 36 MDs per day. For comparative purposes, quiet, nonpeak intervals had an occurrence rate of 4.3 MDs per day. Each of the 10 MD clusters was analyzed in detail to determine their solar wind dependences. MD clusters were often found to occur within corotating interaction regions (CIRs), mainly localized in the trailing portions of CIRs between the interface (IF) and the reverse shock (RS). The MD clusters were divided into smaller subclusters. Within the limits of this study, MD subclusters were always found to occur in high‐β (1 < β < 102) regions (HBRs). Small MD subclusters were detected in HBRs downstream of forward shocks (FSs) but less frequently than for the trailing portion of the CIR. The FS to IF region is generally a low‐β region (LBR), where β ≤ 1.0. The 3920 MDs were identified in the study. The temporal thickness distribution of MDs is given by N = 2173 e−(t/17.3), where t is in seconds. The magnetic field angular changes were calculated across MDs. The angular dependence is %MDs = 2 + 48e−(ΔΘ°/18.8°). Only 13.5% of MDs were “linear” with angular changes <10° across the structures. Because MDs are found in abundance in the region spanning the CIR RS to close to the IF, it is argued that MDs must be formed continuously from close to the Sun (r < 0.5 AU) to ∼5 AU. The older MDs (those found close to the IF) are “fossils” that have been convected radially outward to ∼5 AU. A scenario is presented to explain the HBRs downstream of both CIR RSs and quasi‐parallel FSs. The location of MD clusters in the trailing parts of CIRs and the paucity of linear MDs indicate that MD generation by mirror mode instability is unlikely. More promising candidates are shock compression of phase‐steepened Alfvén waves, shock–directional discontinuity interactions, and downstream turbulence. We emphasize the phase‐steepened Alfvén wave mechanism.