A Statistical Analysis of the Fluctuations in the Upstream and Downstream Plasmas of 109 Strong‐Compression Interplanetary Shocks at 1 AU

Abstract

The upstream and downstream plasmas of 109 strong-compression forward interplanetary shocks are statistically analyzed using 3-s measurements from the WIND spacecraft. The goal is a comparison of the fluctuation properties of downstream plasmas in comparison with the fluctuation properties of upstream plasmas in the inertial range of frequencies and the magnetic-structure range of spatial scales. The shocks all have density compression rations of ~2 of more. When possible, each shock is categorized according to the type of solar wind plasma it propagates through: 15 shocks are in coronal-hole-origin plasma, 42 shocks are in streamer-belt-origin plasma, 36 shocks are in sector-reversal-region plasmas, and 11 shocks are in ejecta plasma. The statistical study examines magnetic-field and velocity spectral indices, the Alfvénicity, the fluctuation amplitudes, Alfvén ratios, the degree of plasma inhomogeneity, and Taylor microscales, looking in particular at (1) fluctuation values downstream that are related to fluctuation values upstream and (2) systematic differences in fluctuation values associated with the type of plasma. It is argued that inhomogeneity of the downstream plasma can be caused by spatial variations in the shock normal angle Bn caused by field-direction variations in the upstream magnetic structure. The importance of determining the type of plasma that the shock propagates through is established. Key Points: 1. Spectral indices, amplitudes, Alfvénicity, Alfvén ratios, the degree of plasma inhomogeneity, and Taylor microscales are examined. 2. Statistically, the properties of downstream fluctuations are correlated with the properties of upstream fluctuations. 3. Because of variations in the shock normal angle, upstream magnetic structure can lead to downstream plasma structure.

The Compression of the Heliospheric Magnetic Structure by Interplanetary Shocks: Is the Structure at 1AU a Manifestation of Solar-Wind Turbulence or Is It Fossil Structure From the Sun?

“…Each element of the downstream plasma had been shock-compressed for a time considerably greater than the time between the observation and the shock crossing. Using the methodology of Pitna et al (2016), Borovsky (2020d) provided an estimate for the strong-compression shocks of the duration of time τ age since the plasma was shocked: τ age ∼ 5.5(t-t shock ), where t is the time that the spacecraft samples the element of downstream plasma and t shock is the time that the spacecraft crossed the shock. Hence an element of plasma that is seen 1 h after the shock crossing had been shocked ∼5.5 h earlier than the spacecraft measured it.…”

“…In Table 1 of Borovsky (2020d) 104 of the 109 interplanetary shocks were categorized [using the Xu and Borovsky (2015) plasma-categorization scheme] according to the type of solarwind plasma that is being shocked. The scheme categorizes the solar wind into four plasma types: coronal-hole-origin plasma, streamer-belt-origin plasma, sector-reversal-region plasma, and ejecta.…”

“…Shock compressing the upstream plasma decreases L eddy and increases δv, and so τ nl < 4-44 h in the downstream plasma. Further, the downstream plasma is systematically less Alfvénic than the upstream plasma (Borovsky, 2020d) so the lengthening of τ nl by cross-helicity effects is not as applicable downstream. In an MHD-turbulence simulation based on solar-wind parameters at 1 AU, Yang et al (2017) examined the birth-to-death lifetimes of 228 current sheets; analyzing the distribution of lifetimes in Figure 9 of Yang et al (2017) finds the median lifetime to be ∼1.5 h and the average lifetime to be ∼1.9 h.…”

“…In this study the evolution of compressed magnetic structure is studied downstream of 109 strong-compression forward interplanetary shocks observed with the WIND spacecraft at 1 AU. These 109 shocks are cataloged in Table 1 of Borovsky (2020d). The shocks were chosen to have density compression rations of ∼2 of more.…”

“…For each of the 109 forward shocks, the driver for each shock was assessed by examining the temporal profile of the solar-wind speed v sw , the magnetic-field strength B mag , the proton temperature T p in comparison with the solarwind speed, the orientation of the magnetic field, the fluctuation level of the magnetic field, the presence or absence of a bidirectional electron strahl, and the presence of magnetic sector reversals. Only 7 of the 107 shocks were consistent with the shock being driven by the interaction of a high-speed stream with slower plasma [shock numbers 7, 11, 26, 35, 38, 42, and 105 in Table 1 of Borovsky (2020d)]; 100 shocks were not consistent with this. Of the 100 shocks, 42 had drivers that were identifiable as coronal mass ejections and 58 had drivers that were not seen or that could not be clearly identified as a coronal mass ejection.…”