A “tongue” of neutral composition

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Both the thermosphere and the ionosphere react to forcing from the magnetospherically imposed ion convection pattern. A twin-celled, neutral convection pattern is set up that normally follows the ion convection pattern; however, unlike the ionosphere, the thermosphere takes a considerable time to react to geomagnetic forcing. In solar maximum conditions this reaction time can take 1–3 h. The close relationship between the ion and neutral winds suggests that other features may exist in the high-latitude thermosphere that have counterparts in the high-latitude ionosphere. Two possible examples of these counterparts are tongues and patches. In this study, we use a thermosphere–ionosphere nested grid (TING) model simulation of the Bastille Day storm to determine whether such features can occur and whether they can, in turn, affect the ionosphere. We draw the following conclusions: (1) there is a “tongue” of neutral composition, which is formed in much the same way that the tongue of ionization is; (2) the neutral “tongue” is formed when parcels of air, which are rich in O/N 2, are drawn from the dayside by the antisunward winds associated with the neutral convection pattern and transported across the polar cap towards the night side auroral oval; (3) this O/N 2-rich air can only be transported from a small region on the dayside due to the geometry of the transport, so the neutral “tongue” is narrow like the ion tongue; (4) this neutral “tongue” tends to be weaker than the ion tongue, it extends a shorter distance across the polar cap and it takes longer to form; (5) the formation of the tongue of ionization is affected by the “leakage” of air that contains high concentrations of molecular species into the daytime middle latitudes, which, through recombination, results in a region of low electron densities in the dayside middle latitudes; (6) this region of the ionosphere, which is ion-poor, cannot produce flux tubes that can advect across the polar cap with the high ion densities that are needed to produce a strong tongue of ionization; (7) when enough time has elapsed to allow a neutral “tongue” to form in conjunction with the tongue of ionization, the latter becomes both stronger and longer, but causality in this relationship has not been established.