Comparison of the bubble and blob distributions during the solar minimum

Abstract

Plasma blobs, local plasma density enhancements with respect to the background, in low to middle latitudes are understood as byproducts of equatorial plasma bubbles, but this hypothesis is challenged by observations of blobs in the absence of equatorial bubbles in the same magnetic meridian. This study investigates the causal linkage between bubbles and blobs using their occurrence statistics during the solar minimum. The bubble and blob distributions were obtained by analyzing the ion density data acquired during the period August 2008–April 2010 by the Coupled Ion‐Neutral Dynamics Investigation instrument on board the Communication/Navigation Outage Forecasting System satellite. The seasonal, longitudinal, and latitudinal distributions of bubbles during the solar minimum are similar to those during other periods of the solar cycle. The distinctive bubble distribution during the solar minimum is the high occurrence rate of bubbles after midnight. The blob distribution shows the peak occurrence rate near midnight in the altitude range 400–450 km and around ±25° magnetic latitudes. The bubble occurrence rate is minimal during the June solstice, but the blob occurrence rate is maximal during that season. The blob occurrence rate has a peak value at about a 1700 km magnetic apex height, where the bubble occurrence rate is negligibly small. The difference in the bubble and blob distributions and the frequent occurrence of blobs beyond the bubble apex height suggest that bubbles are not a prerequisite for the creation of blobs.