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Van Allen Probe observations indicate that whistler‐mode hiss waves below 1 kHz are absorbed at low altitudes near magnetic equator. The lowest cutoff frequency of equatorial hiss is close to the gyrofrequency of hydrogen ions. The lowest cutoff altitude of global hiss is extracted when its occurrence rate is equal to 0.005 on the plane of altitude (L in RE) and magnetic local time (MLT). By fitting the lowest cutoff altitude of global hiss, we constructed the empirical model of the lowest cutoff altitude of equatorial hiss under geomagnetically quiet (AE < 200 nT) and active (AE ≥ 200 nT) conditions. The enhanced substorm activities reduce the lowest cutoff altitude of hiss waves on the dawnside (MLT ∼ 1–5 hr), whereas the lowest cutoff altitude of the dayside hiss is nearly fixed at ∼1.1 RE (MLT ∼ 6–20 hr). From the dayside to the nightside (MLT ∼ 0–6 hr and 20–24 hr), the lowest cutoff altitude of equatorial hiss raises gradually from 1.1 RE to 1.4 RE. Plain Language Summary: Plasmaspheric hiss can cause the scattering loss of Earth's radiation belt electrons via wave‐particle resonant interactions and creates a slot region relatively safe for satellites. Thus, their spatial distribution is a key parameter controlling the dynamics of radiation belts. Although the outer boundary of hiss waves can be estimated with the empirical plasmapause location, there is no any empirical model to quantificationally describe their inner boundary. Here, based on the Van Allen Probe observations, we constructed empirical models to estimate the lowest cutoff altitude of global hiss under different geomagnetic conditions. The new empirical models provide an inner boundary for simulating the resonant interactions between hiss waves and high‐energy electrons. Key Points: The lowest cutoff altitude of global hiss is quantificationally examined with Van Allen Probes observations near the magnetic equatorThe lowest cutoff altitude of equatorial hiss is nearly fixed at L ∼ 1.1 on the dayside, but changes between L ∼ 1.1–1.4 on the nightsideNew empirical models are constructed to estimate the lowest cutoff altitude of global hiss under different geomagnetic conditions [ABSTRACT FROM AUTHOR]

By analyzing the latitudinal distributions of ionospheric ions observed by the Detection of Electro‐Magnetic Emissions Transmitted from Earthquake Regions satellite at 670 km altitude in different seasons, we found that the large‐scale depletion of nighttime oxygen ions is prevalent at the low and middle latitudes of winter hemisphere (center ∼ 30°) under different geomagnetic conditions. The latitudinal width of the winter oxygen ion depletion (WOD) region is about 20°–60° at different longitudes, and its upper boundary latitudes are mostly lower than the midlatitude trough (|λ| ≥ 50°) of nighttime hydrogen ions (H+) near the plasmapause. In the WOD region, the density ratio of oxygen and hydrogen ions and that of their neutral densities (O and H) approximately agree with the chemical equilibrium relationship of charge exchange reactions (H + O+ ↔ H+ + O) predicted by previous theory, indicating that the charge exchange reactions are mainly responsible for the large‐scale oxygen ion depletion at the low and middle latitudes in winter hemisphere. Plain Language Summary: Previous studies focused on the nighttime midlatitude trough of ionospheric electrons (e−) and light ions (H+ and He+). Although the percentage of nighttime oxygen ions was found to decrease in winter hemisphere at 400 km altitude, it is not clear whether the winter oxygen ion depletion (WOD) is a common phenomenon in the topside ionosphere. Moreover, the mechanism of the WOD has not been understood clearly. Here, our statistical results indicate that the large‐scale depletion of nighttime oxygen ions is prevalent at the low and middle latitudes of winter hemisphere under different geomagnetic conditions. By analyzing the relationship among the latitudinal changes in the ion bulk velocity, densities and their neutral densities, we found that the charge exchange reactions mainly cause the nighttime oxygen ion depletion at the low and middle latitudes in winter hemisphere in the topside ionosphere. Key Points: The nighttime oxygen ions decrease remarkably at the low and middle latitudes of winter hemisphere in the topside ionosphereThe latitudes of large‐scale oxygen ion depletion (center ∼ 30°) are mostly lower than the midlatitude trough of nighttime hydrogen ionsThe winter depletion of nighttime oxygen ions is largely due to the charge exchange reactions between oxygen ions and atomic hydrogen [ABSTRACT FROM AUTHOR]

Thermal ion measurements from the French Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions micro-satellite have been used to study the climatology of nighttime Medium-scale traveling ionospheric disturbances (MSTIDs) in the topside ionosphere at 650 km altitude during the 6 years of the satellite operation, from 2005 to 2010. This period encompasses the declining phase of solar cycle 23, the deep solar minimum of 2008–2009 and the early rise of solar cycle 24. MSTIDs were detected from the quasi-periodic variations of the density of the dominant ionospheric O+ ions. Mostly present between ∼15° and ∼40° invariant latitudes with a small number of events in equatorial regions, the MSTIDs exhibit, on the average, larger occurrence rates in the Southern hemisphere with a peak in the Eastern Pacific-South America longitude sector. The dependence of MSTID activity on solar activity appears more complex than the previously reported simple anti-correlation. In addition to year-to-year variations, our study has, in particular, put in evidence a noticeable hemispheric asymmetry in the seasonal variations during the deep solar minimum of 2008–2009. These statistical observations provide a remarkable observational support to the key role of sporadic Es layers and the conjugate mapping of the associated electric fields. Yet, improved theoretical and numerical models, taking into account the actual inter-hemispheric differences of the seasonal and solar activity variations of thermospheric and ionospheric processes, are needed for a better understanding of the highly complex MSTID phenomena. [ABSTRACT FROM AUTHOR]

Utilizing the DEMETER observations at 670 km, we examined the day‐night difference of energetic electrons (100–800 keV) in the South Atlantic Anomaly (SAA) region and their dependence on geomagnetic activities in different seasons. Under geomagnetically quiet conditions, the fluxes of higher‐energy electrons (>200 keV) in the dusk and midnight (MLT ~ 19–24 hr) are usually larger than those in the morning (MLT ~ 8–12 hr) in the core region of the SAA (−50 ≤ λ ≤ − 20deg) during the northern and southern summers (21 March 2007 to 23 September 2007 and 23 September 2007 to 21 March 2008). The day‐night difference of energetic electrons in SAA region depends not only on the electron energy but also on the geomagnetic activity levels. Enhanced geomagnetic activities increase the energetic electrons in morning and hence weaken their day‐night difference in the SAA region. Key Points: Fluxes of energetic electrons (>200 keV) in SAA region are larger in the nighttime than the morning during geomagnetically quiet timesThe day‐night difference of energetic electrons in SAA region depends not only on the electron energy but also on the geomagnetic conditionsEnhanced geomagnetic activities increase the energetic electrons in morning and hence weaken their day‐night difference in the SAA region [ABSTRACT FROM AUTHOR]