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  1. Abstract

    Total electron content (TEC) and L‐band scintillations measured by several networks of GPS and GNSS receivers that operate in South and Central America and the Caribbean region are used to observe the morphology of the equatorial ionization anomaly (EIA), examine the evolution of plasma bubbles, and investigate the enhancement of L‐band scintillations that occurred on February 12 and 13, 2016. A few weak and short magnetic storms developed these days, and a minor sudden stratospheric warming (SSW) event was initiated a few days before. During these unusual conditions, TEC maps reported a split of the otherwise continuous crests of the EIA and the formation of a large‐scale (thousands of kilometers) almost‐circular structure. The western part of the southern crest faded, and a north‐south aligned segment developed near the center of the South American continent, joining the north and south crests of the EIA, forming an anomaly that resembled a closed loop on the eastern side of the continent. Concurrently with the anomaly events, several GPS stations reported increases in the L‐band scintillation index from 0.4 to values greater than 1. We analyzed TEC values from receivers between ±6° from the magnetic equator to identify and follow TEC depletions associated with plasma bubbles when they reach different stations. Although the magnetic activity was moderate (Kp = 3°), we believe that the anomaly redistribution and the scintillation enhancements are not related to a prompt penetration electric field but to enhancing the semidiurnal lunar tide propitiated by the onset of the minor SSW event. We found that depending on the lunar tide phase cycle, the neutral wind's meridional component can augment sub‐km scale irregularities and enhance L‐band scintillations through the wind gradient instability when U·n < 0 or the action of wind gradients (U) within the bubbles. Our observations imply that the SSW event enables prominent changes in the thermosphere wind system at F‐region altitudes.

     
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  3. Abstract

    We propose a mechanism for the formation of the horse‐collar auroral configuration during periods of strongly northward interplanetary magnetic field (IMF), invoking the action of dual‐lobe reconnection (DLR). Auroral observations are provided by the Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) satellite and spacecraft of the Defense Meteorological Satellite Program (DMSP). We also use ionospheric flow measurements from DMSP and polar maps of field‐aligned currents (FACs) derived from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE). Sunward convection is observed within the dark polar cap, with antisunward flows within the horse‐collar auroral region, together with the NBZ FAC distribution expected to be associated with DLR. We suggest that newly closed flux is transported antisunward and to dawn and dusk within the reverse lobe cell convection pattern associated with DLR, causing the polar cap to acquire a teardrop shape and weak auroras to form at high latitudes. Horse‐collar auroras are a common feature of the quiet magnetosphere, and this model provides a first understanding of their formation, resolving several outstanding questions regarding the nature of DLR and the magnetospheric structure and dynamics during northward IMF. The model can also provide insights into the trapping of solar wind plasma by the magnetosphere and the formation of a low‐latitude boundary layer and cold, dense plasma sheet. We speculate that prolonged DLR could lead to a fully closed magnetosphere, with the formation of horse‐collar auroras being an intermediate step.

     
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