Latitudinal distribution of the equatorial plasma bubbles: Altitude variability

In this paper, the latitudinal distributions of the equatorial plasma bubbles (EPBs) and their variability with the observation heights were studied. For this purpose, a detailed comparative analysis of the latitudinal histograms of the EPB occurrence probability obtained at the different altitudes was performed. The data from the ISS-b (∼ 972− 1220km) ROCSAT-1 (∼ 600km) and AE-E (∼ 350− 475km) spacecrafts were used for this purpose. The EPB structures were identified from the ISS-b data as the He density depletions, and from the ROCSAT-1 and AE-E data as the ion density (Ni) depletions. Different seasons during the years of the high solar activity conditions were under consideration. It was obtained that as the observation height increases from ∼ 600km to ∼ 1000km, the mid-latitude maxima in addition to the equatorial maximum develop in the histograms of the EPB occurrence probability. This feature was most pronounced in both the hemispheres of the equinox periods and in the winter hemispheres. At the altitudes of the upper topside ionosphere (∼ 972 − 1220km, ISS-b), the values of the mid-latitude maxima of the EPB occurrence probability become comparable or begin to dominate in relation to the equatorial maximum. This feature is typical for all seasons of the observations. Moreover, it was obtained that the latitudinal positions of the mid-latitude maxima of the EPB occurrence probability shift to the equator with increase in the observation heights. The latter is in good accordance with the latitudinal courses of the magnetic field tubes along which the plasma bubbles that have already reached their ceiling heights are stretched.