Time-dependent responses of the neutral mass density to fixed magnetospheric energy inputs into the cusp region in the thermosphere during a period of large IMF BY: a high-resolution two-dimensional local modeling

Tomokazu Oigawa,Satoshi Taguchi,Hiroyuki Shinagawa

doi:10.1186/s40623-021-01535-9

Abstract

Thermospheric mass density values around the 400-km altitude in the cusp can be significantly enhanced as compared to regions around the cusp. To gain insights into the extent to which the magnitude of the cusp mass density enhancements can be explained by the static distributions of moderate electric field and electron precipitation typical for a period of large IMF BY, we employed a high-resolution two-dimensional local model that can represent the plasma features that are characteristic of the cusp: azimuthal ion flow and low-energy electron precipitation. We also calculated the thermospheric dynamics with and without neutral–ion drag. We found that in the calculation with this drag the obtained mass density enhancement is 10% at most, indicating that the thermospheric dynamics imposing the moderate static electric field and electron precipitation can only explain about one-third of the typical magnitude of cusp thermospheric mass density, i.e., 33%. We also found that in the calculation without neutral–ion drag the magnitude of the mass density enhancement is slightly larger than the one with the neutral–ion drag. To explain the average magnitude of the cusp mass density enhancements completely, other energy inputs such as Alfvén waves, in addition to the static distributions of electric field and electron precipitation, are needed.Graphical

Highlights

The cusp is typically located around 75° magnetic latitude between ~ 1000 and ~ 1400 magnetic local time (MLT) in the altitudes of the ionosphere–thermosphere, where low-energy electrons almost directly come from the dayside magnetosheath
The results indicate that both neutral–ion drags and chemical reactions in this model reduce Joule heating rates, and that the enhancement of the neutral mass density is overestimated when the fixed ionosphere condition is adopted
We have focused on what extent the static electric field can explain the mass density anomaly, and clarified that other energy inputs different from those related to the static distributions of electric field and electron precipitation are absolutely needed

Summary

Introduction

The cusp is typically located around 75° magnetic latitude between ~ 1000 and ~ 1400 magnetic local time (MLT) in the altitudes of the ionosphere–thermosphere, where low-energy electrons almost directly come from the dayside magnetosheath. The ionosphere–thermosphere dynamics in the cusp region are strongly dependent on the condition of interplanetary magnetic fields (IMFs) and the condition of solar extreme ultraviolet (EUV) radiation as well as the inputs from the dayside magnetosheath (e.g., solar wind particles). Recent CHAllenging Minisatellite Payload (CHAMP) satellite observations have shown that the neutral mass density around the 400-km altitude in the cusp is remarkably larger than that of ambient regions. After Lühr et al.’s (2004) discovery, the anomalous mass density structure and other related phenomena have been extensively investigated in observational and modeling studies. Kervalishvili and Lühr (2013) have shown that the mass density enhancement is, on average, 33%. After Lühr et al.’s (2004) discovery, the anomalous mass density structure and other related phenomena have been extensively investigated in observational and modeling studies. Kervalishvili and Lühr (2013) have shown that the mass density enhancement is, on average, 33%.

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Conclusion