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1. 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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2. Turbulence Transitions in Kelvin–Helmholtz Instability “Tube” and “Knot” Dynamics: Vorticity, Helicity, and Twist Waves

   https://doi.org/10.3390/atmos14121770  

   Fritts, David C; Lund, Thomas S; Lund, Adam C; Wang, Ling (November 2023, Atmosphere)

   We address the sources and dynamics of vorticity and helicity and their relations in transitions to turbulence arising due to Kelvin–Helmholtz instability (KHI) “Tube” and “Knot” (T&K) events. Such events are common in the atmosphere and oceans, and initial numerical simulations reveal that T&K dynamics significantly accelerate turbulence transitions and enhance KHI peak and mean energy dissipation rates. KHI T&K events arise where emerging KH billows exhibit varying wavelengths, phases, amplitudes, and/or discontinuities along their axes. As the KH billows intensify, these regions evolve roughly orthogonal billow cores and induced vortex tubes in close proximity. Their mutual advection as they intensify induces large-amplitude Kelvin vortex waves, or “twist waves”, that arise where locally uniform vortices are distorted by axial or radial advection. The twist waves propagate along, and fragment, the vortex tubes and billow cores, thus accounting for the emergence of helicity and the down-scale energy, enstrophy, and helicity fluxes within the turbulence inertial range. We describe the results of four direct numerical simulations (DNS) addressing KHI T&K dynamics in large and idealized small domains. The large-domain vorticity fields reveal the character and diversity of KHI T&K dynamics, the emergence of twist waves at larger and smaller scales, and their driving of turbulence transitions. Two small-domain DNS exhibit idealized KHI T&K events arising from KH billows that are mis-aligned and that exhibit phase variability along their axes. A third examines the interactions of two vortex tubes in close proximity. These reveal that twist waves drive the character and evolutions of the vorticity and helicity fields. 

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3. Severe L-band scintillation over low-to-mid latitudes caused by an extreme equatorial plasma bubble: joint observations from ground-based monitors and GOLD

   https://doi.org/10.1186/s40623-023-01797-5  

   Sousasantos, Jonas; Gomez Socola, Josemaria; Rodrigues, Fabiano S.; Eastes, Richard W.; Brum, Christiano G.; Terra, Pedrina (December 2023, Earth, Planets and Space)

   Abstract The occurrence of plasma irregularities and ionospheric scintillation over the Caribbean region have been reported in previous studies, but a better understanding of the source and conditions leading to these events is still needed. In December 2021, three ground-based ionospheric scintillation and Total Electron Content monitors were installed at different locations over Puerto Rico to better understand the occurrence of ionospheric irregularities in the region and to quantify their impact on transionospheric signals. Here, the findings for an event that occurred on March 13–14, 2022 are reported. The measurements made by the ground-based instrumentation indicated that ionospheric irregularities and scintillation originated at low latitudes and propagated, subsequently, to mid-latitudes. Imaging of the ionospheric F-region over a wide range of latitudes provided by the GOLD mission confirmed, unequivocally, that the observed irregularities and the scintillation were indeed caused by extreme equatorial plasma bubbles, that is, bubbles that reach abnormally high apex heights. The joint ground- and space-based observations show that plasma bubbles reached apex heights exceeding 2600 km and magnetic dip latitudes beyond 28 ° . In addition to the identification of extreme plasma bubbles as the source of the ionospheric perturbations over low-to-mid latitudes, GOLD observations also provided experimental evidence of the background ionospheric conditions leading to the abnormally high rise of the plasma bubbles and to severe L-band scintillation. These conditions are in good agreement with the theoretical hypothesis previously proposed. Graphical Abstract 

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

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

   Lamarche, L. J. (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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