Fig. 3

Upper panel: solar wind speed, proton density, and the parameter
derived from the He2+
VDF. Four time periods are marked that correspond to quiet solar wind (blue and red), compressed wind after the
stream interface (gray), and compressed fast wind (cyan). Time periods that are displayed with the red solid line in the
panel are excluded from our analysis. Lower left panel: He+
count histograms measured in the marked time periods. Lower right panel:
He2+
VDF measured by the sensor’s main channel in one 5 min interval within the considered time periods.
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