Dynamics of He++ Ions at Interplanetary and Earth's Bow Shocks

Experimental investigations of the fine plasma structure of interplanetary shocks are extremely difficult to conduct due to their small thickness and high speed relative to the spacecraft. We studied the variations in the parameters of twice-ionized helium ions (4He(++) ions or alpha-particles) in the solar wind plasma during the passage of interplanetary shocks and Earth's bow shock. We used data with high time resolution gathered by the BMSW (Bright Monitor of Solar Wind) instrument installed on the SPEKTR-R satellite, which operated between August 2011 and 2019. The MHD parameters of He++ ions (the bulk velocity V alpha, temperature T alpha, absolute density N alpha, and helium abundance N alpha/Np) are analyzed for 20 interplanetary shocks and compared with similar parameters for 25 Earth bow shock crossings. Measurements from the WIND, Cluster, and THEMIS satellites were also analyzed. The correlations in the changes in helium abundance N alpha/Np with the parameters beta(i), theta(Bn), and M-MS were investigated. The following correlation between N alpha/Np and the angle theta(Bn) was found: the lower the value of theta(Bn), the greater the drop in helium abundance (N alpha/Np) falls behind the IP shock front. For Earth's bow shock crossings, we found a significant increase in the helium abundance (N alpha/Np) in quasi-perpendicular events.