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1. Oxygen torus and warm plasma cloak: a review

   https://doi.org/10.3389/fspas.2026.1817245  

   Nosé, Masahito; Maruyama, Naomi (April 2026, Frontiers in Astronomy and Space Sciences)

   This paper reviews past observational studies of low-energy ion populations in the Earth’s magnetosphere, known as the oxygen torus and the warm plasma cloak. These populations have been investigated since the early 1980s and have recently regained attention. The oxygen torus is characterized by enhanced O⁺ions at energies below several tens of eV near the plasmapause (L= 3–5), with a magnetic local time distribution skewed toward the dawn sector, whereas the warm plasma cloak consists primarily of field-aligned H⁺and O⁺ions with energies from ∼10 eV to ∼3 keV, extending from the nightside through dawn to the dayside atL= 4–12. Several formation mechanisms have been proposed for the oxygen torus, including ionospheric heating due to ring current–plasmasphere interactions, the geomagnetic mass spectrometer effect, and direct supply of low-energy O⁺ions from the nightside ionosphere followed by eastward drift. Recent observations favor the latter mechanism, although the relative importance of each process remains uncertain. The formation of the warm plasma cloak is generally attributed to ionospheric outflow transported through the magnetotail and subsequently convected earthward. Considering their energy and spatial distributions as well as plausible generation mechanisms, the oxygen torus and the warm plasma cloak appear to be distinct plasma populations. Their mutual relationship and the dominant formation process of the oxygen torus remain open questions, motivating future observations with improved ion composition measurements across a wide energy range. 

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2. Gradient drift instability and decameter ionospheric irregularities at the edge of polar holes

   Thaller, S; Hughes, J; Crowley, G; Noto, J; Blay, R; Wilson, J (May 2023, Ionospheric Effects Symposium)

   The polar and high latitude regions of the ionosphere are host to complex plasma processes involving Magnetosphere-Ionosphere (MI) coupling, plasma convection, and auroral dynamics. The magnetic field lines from the polar cusp down through the auroral region map out to the magnetosphere and project the footprint of the large-scale convective processes driven by the solar wind onto the ionosphere. This region is also a unique environment where the magnetic field is oriented nearly vertical, resulting in horizontal drifts along closed, localized, convection patterns, and where prolonged periods of darkness during the winter result in the absence of significant photoionization. This set of conditions results in unique ionospheric structures which can set the stage for the generation of the gradient drift instability (GDI). The GDI occurs when the density gradient and ExB plasma drift are in the same direction. The GDI is a source of structuring at density gradients and may give rise to ionospheric irregularities that impact over-the-horizon radars and GPS signals. While the plasma ExB drifts are supplied by magnetospheric convection and MI coupling, sharp density gradients in the polar regions will be present at polar holes. Since the GDI occurs where the density gradient and plasma drift are parallel, the ionospheric irregularities caused by the GDI should occur at the leading edge of the polar hole. If so, the resulting production of small-scale density irregularities may, if the density is high enough, give rise to scintillation of GNSS signals and backscatter on HF radars. In this study, we investigate whether these irregularities can occur at the edges of polar holes as detected by the HF radar scatter. We use the Ionospheric Data Assimilation 4-Dimentional (IDA4D) and Assimilative Mapping of Ionospheric Electrodynamics (AMIE) models to characterize the high latitude ionospheric density and ExB drift convective structures, respectively, for one of nine polar hole events identified using RISR-N incoherent scatter radar in Forsythe et al [2021]. The combined IDA4D and AMIE assimilative outputs indicate where the GDI could be triggered, e.g., locations where the density gradient and ExB drift velocity have parallel components and the growth rate is smaller than the characteristic time over which the convective pattern changes, in this case, ~1/15 min. The presence of decameter ionospheric plasma irregularities is detected using the Super Dual Auroral Radar Network (SuperDARN). SuperDARN radars are HF coherent scatter radars. The presence of ionospheric radar returns in regions unstable to GDI grown strongly suggest the GDI is producing decameter scale plasma irregularities. The statistical analyses conducted in the above investigation do not show a clear pattern of enhanced scatter with larger computed GDI growth rates. Further investigation must be conducted before concluding that the GDI does not cause irregularities detectable with HF radar at polar holes. 

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3. The Significance of Magnetospheric Precipitation for the Coupling of Magnetosphere-Ionosphere-Thermosphere Systems: Sources and Properties

   https://doi.org/10.3847/25c2cfeb.be32d93d  

   Ozturk, Dogacan; Lin, Dong; Gabrielse, Christine; Chen, Margaret; Grandin, Maxime; Sivadas, Nithin; Robinson, Robert; Garcia-Sage, Katherine; Damiano, Peter; Blum, Lauren; et al (August 2023, Bulletin of the AAS)

   Magnetospheric precipitation plays an important role for the coupling of Magnetosphere, Ionosphere, and Thermosphere (M-I-T) systems. Particles from different origins could be energized through various physical mechanisms and in turn disturb the Ionosphere, the ionized region of the Earth’s atmosphere that is important for telecommunication and spacecraft operations. Known to cause aurora, bright displays of light across the night sky, magnetospheric particle precipitation, modifies ionospheric conductance further affecting the plasma convection, field-aligned (FAC) and ionospheric currents, and ionospheric/thermospheric temperature and densities. Therefore, understanding the properties of different sources of magnetospheric precipitation and their relative roles on electrodynamic coupling of M-I across a broad range of spatiotemporal scales is crucial. In this paper, we detail some of the important open questions regarding the origins of magnetospheric particle precipitation and how precipitation affects ionospheric conductance. In a companion paper titled “The Significance of Magnetospheric Precipitation for the Coupling of Magnetosphere-Ionosphere-Thermosphere Systems: Effects on Ionospheric Conductance”, we describe how particle precipitation affects the vertical structure of the ionospheric conductivity and provide recommendations to improve its modelling. 

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4. Ionospheric Plasma Density Gradients Associated With Night‐Side Energetic Electron Precipitation

   https://doi.org/10.1029/2023GL105630  

   Zhang, Xiao‐Jia; Meng, Xing; Artemyev, Anton V.; Zou, Ying; Mourenas, Didier (November 2023, Geophysical Research Letters)

   Abstract Energetic electron precipitation from the equatorial magnetosphere into the atmosphere plays an important role in magnetosphere‐ionosphere coupling: precipitating electrons alter ionospheric properties, whereas ionospheric outflows modify equatorial plasma conditions affecting electromagnetic wave generation and energetic electron scattering. However, ionospheric measurements cannot be directly related to wave and energetic electron properties measured by high‐altitude, near‐equatorial spacecraft, due to large mapping uncertainties. We aim to resolve this by projecting low‐altitude measurements of energetic electron precipitation by ELFIN CubeSats onto total electron content (TEC) maps serving as a proxy for ionospheric density structures. We examine three types of precipitation on the nightside: precipitation of <200 keV electrons in the plasma sheet, bursty precipitation of <500 keV electrons by whistler‐mode waves, and relativistic (>500 keV) electron precipitation by EMIC waves. All three types of precipitation show distinct features in TEC horizontal gradients, and we discuss possible implications of these features. 

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5. Observations and modeling of scintillation in the vicinity of a polar cap patch

   https://doi.org/10.1051/swsc/2022023  

   Lamarche, Leslie J.; Deshpande, Kshitija B.; Zettergren, Matthew D. (January 2022, Journal of Space Weather and Space Climate)

   Small-scale ionospheric plasma structures can cause scintillation in radio signals passing through the ionosphere. The relationship between the scintillated signal and how plasma structuring develops is complex. We model the development of small-scale plasma structuring in and around an idealized polar cap patch observed by the Resolute Bay Incoherent Scatter Radars (RISR) with the Geospace Environment Model for Ion-Neutral Interactions (GEMINI). Then, we simulate a signal passing through the resulting small-scale structuring with the Satellite-beacon Ionospheric-scintillation Global Model of the upper Atmosphere (SIGMA) to predict the scintillation characteristics that will be observed by a ground receiver at different stages of instability development. Finally, we compare the predicted signal characteristics with actual observations of scintillation from ground receivers in the vicinity of Resolute Bay. We interpret the results in terms of the nature of the small-scale plasma structuring in the ionosphere and how it impacts signals of different frequencies and attempt to infer information about the ionospheric plasma irregularity spectrum. 

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