Occurrence probability of light ion trough and subtrough in He+ density on season and local time

This paper based on He + density data gathered between 1000 and 1200 km from ∼1100 orbits of the ISS-b satellite. Its goal is to obtain of the occurrence probability, P , of mid-latitude troughs and subtroughs in He + density as function of local time and season. Mid-latitude He + (H + ) density trough, LIT, is the prominent feature of the upper ionosphere. Other distinct structure of the upper ionosphere as He + (H + ) density subtroughs, located equatoward than LIT, is not so unique and seldom structure, as it was supposed early, since we revealed them in ∼570 cases in ∼1100 orbits. Obtained trough features differ in many respects from qualitative regularities, detected in earlier publications. So, mid-latitude He + troughs are not so frequent by day; they occur more often in summer ( P ≅30%) than in winter ( P =5 to 10%). Subtroughs are also observed by day rather seldom but stable in summer ( P =7 to 12%) and only in exclusive cases in winter. At night the mid-latitude troughs do quite often occur ( P =40 to 60% in winter and 35 to 55% in summer). The subtroughs are more often observed under nighttime conditions ( P ∼30% in winter till 5–10% in summer). It was also further revealed that the occurrence probability of LIT and subtroughs is associated with corresponding variations of the He + density. Our findings might be helpful for the IRI and for theories of trough formation as well.