Systematics of ion Walén analysis of rotational discontinuities using E/Z measurements

The assignment of the ion counts from an E/Z detector to be exclusively protons is a routine approximation. In the process, minor ion species possessing charges and masses that differ from protons are inadvertently misassigned. We have studied the quantitative effects this has on traditional Walén analysis (using ion measurements) at rotational discontinuities. The minor ions studied are alphas (He++). This study was undertaken to explain the observationally prevalent subunity values for the Walén slope at the subsolar magnetopause, high‐latitude cusp and in the solar wind. The simulation parameter spaces used were drawn from observations. The overall effect of the misassignment is to lower the Walén slope (from the theoretical expectation of 1) as the alpha concentration increases and as the relative motion between the protons and alphas approaches the local Alfvén speed. The most profound reduction of the Walén slope occurs for a small corner of parameter space which is infrequently observed. We conclude that the misassignment surely contributes to subunity Walén slopes but is probably not the main cause.