The importance of neutral hydrogen for the maintenance of the midlatitude winter nighttime ionosphere: Evidence from IS observations at Kharkiv, Ukraine, and field line interhemispheric plasma model simulations

Abstract

This study investigates the causes of nighttime enhancements in ionospheric density that are observed in winter by the incoherent scatter radar at Kharkiv, Ukraine. Calculations with a comprehensive physical model reveal that large downward ion fluxes from the plasmasphere are the main cause of the enhancements. These large fluxes are enabled by large upward H+ fluxes into the plasmasphere from the conjugate summer hemisphere during the daytime. The nighttime downward H+ flux at Kharkiv is sensitive to the thermosphere model H density, which had to be increased by factors of 2 to 3 to obtain model‐data agreement for the topside H+ density. Other studies support the need for increasing the thermosphere model H density for all seasons at solar minimum. It was found that neutral winds are less effective than plasmaspheric fluxes for maintaining the nighttime ionosphere. This is partly because increased equatorward winds simultaneously oppose the downward H+ flux. The model calculations also reveal the need for a modest additional heat flow from the plasmasphere in the afternoon. This source could be the quiet time ring current.

Coincident Observations by the Kharkiv IS Radar and Ionosonde, DMSP and Arase (ERG) Satellites, and FLIP Model Simulations: Implications for the NRLMSISE‐00 Hydrogen Density, Plasmasphere, and Ionosphere

“…This conclusion is supported by the observations with all used facilities. Similar result (2 to 3 times higher H density) was obtained by us for the last solar minimum 24 and for winter of solar minimum 23 (Kotov et al, , ). This means that (a) the H density is similar for three solar minima (the same was pointed out by Nossal et al () but for the cycles 21–23) and (b) NRLMSISE‐00 thermospheric H density is systematically underestimated by ~100% at low solar activity. For the first time, using Kharkiv IS radar data, the plasmasphere electron density measured by Arase satellite, and observational based simulation with FLIP model, we showed that actual plasmaspheric density at L ≈ 2.1 can be reproduced with doubled NRLMSISE‐00 density only. A significant (by a factor of 2) decrease of plasmaspheric density was observed by the Arase satellite at the relatively low L shell (≈2.1) at the end of the night of 25 December 2017.…”

“…It should be noted that doubling the H density does not significantly affect the FLIP N m F 2 values for the September, March, and June periods but does improve the model N m F 2 during the night of 24 and 25 December very much (by ~50%). Similar results were obtained for the winter nights of the years of 2007–2010 when the solar and magnetic activities were similar (Kotov et al, ). N m F 2 enhancements at midlatitudes on the long winter nights are a well‐known phenomenon (e.g., Bertin & Papet‐Lepine, ) and are due to strong H + flow from the plasmasphere to ionosphere.…”

“…As a check on the reliability of the NRLMSISE‐00 neutral temperature and O and N 2 densities, the FLIP model ion temperature T i and N m F 2 are compared to the observed values. This study and our previous studies (Kotov et al, , ) for the equinoxes of 2006–2010, winter season of 2007–2010, and summer of 2010 confirm the consistency of the NRLMSISE‐00 model and the IS radar/FLIP model for all of the standard IS plasma parameters.…”

“…The integrated nature of the H emission measurements and consequent lack of altitude information is a disadvantage of this method. Nevertheless, the Nossal et al () results are supported by the IS radar measurements of Kotov et al (, ). By combining observations of the H + ion density and other key parameters of the ionosphere from the Kharkiv incoherent scatter (IS) radar (49.6°N, 36.3°E) with the field line interhemispheric plasma (FLIP) model, it was found that the NRLMSISE‐00 model H density had to be increased by a factor of 2 to 3 to bring the ionosphere model H + density into agreement with the data in all seasons during the 2006–2010.…”

“…It is important to note that the higher H density not only brought the FLIP topside H + ion density into very good agreement with the IS radar observations but also helped to resolve other long‐standing model‐data discrepancies. For example, the higher H density brings the model into better agreement with (1) IS observations of the heavy to light ion transition height, (2) IS and ionosonde measurements of high values of N m F 2 at night, and (3) the satellite measurements of high plasmasphere refilling rates (Kotov et al, , , and references therein). These results provide a high level of confidence in the validity of our estimation of thermospheric H density.…”