A two-dimensional simulation of thermospheric vertical winds in the vicinity of an auroral arc

Abstract

The observations made by Fabry-Perot interferometers (FPIs), radars, and satellites have indicated that large vertical motion in the polar region is occasionally generated in the thermosphere associated with auroral activities. However, the behavior of the vertical wind is often very complicated, and the cause of the vertical wind has not been explained by auroral heating or by ion-neutral drag alone. It has been pointed out that a background horizontal flow is likely to significantly alter the dynamics of the neutral atmosphere near an auroral arc. Recent observations have also suggested that strong downward motion is generated in the vicinity of an auroral arc. To study the thermospheric dynamics near a local heating region embedded in a large-scale horizontal flow, a twodimensional numerical simulation of the thermospheric dynamics has been performed. It is found that interaction of local heating and strong horizontal flow could play an important role in generating vertical motion near an auroral arc. The simulation results indicate that for a horizontal wind speed larger than about 300 m/s, a steady wave-like structure of the neutral wind is formed within and downstream of the heated region. For a horizontal wind speed less than about 300 m/s, on the other hand, no significant vertical motion is generated outside the heated region. This process might account for at least some of the observed features of vertical motion within and outside an auroral arc.

Generation of the lower-thermospheric vertical wind estimated with the EISCAT KST radar at high latitudes during periods of moderate geomagnetic disturbance

“…Sun et al, 1995;Shinagawa and Oyama, 2006) are inconsistent with the observation results shown here and from the FPIs. Most thermospheric-wind simulations show no significant variations in either vertical or horizontal winds below 120 km when the heating rate is at the same level as observed.…”

Lower-thermospheric wind fluctuations measured with an FPI during pulsating aurora at Tromsø, Norway

“…We compared with the ion temperature measured with the EISCAT radar, but clear increases were not identified for the time interval of the vertical-wind fluctuations. This may be because the measurement uncertainty of the EISCAT ion temperature at that time is almost equivalent to 20-30 K. We should note that a neutral temperature increase by a few degrees is enough to change the wind dynamics in the lower thermosphere (Sun et al, 1995;Shinagawa and Oyama, 2006). However, we are not sure if the increased thermal energy of ions is enough to elevate that of neutral particles or the neutral temperature.…”

“…1. The Joule heating process increases the thermospheric temperature even at the lower thermospheric heights (Shinagawa and Oyama, 2006), and the temperature increase can directly affect the wind dynamics due to modification of the pressure gradient (Oyama et al, 2008;Tsuda et al, 2009). Therefore we investigate the temperature variations induced by the electric-field perturbations.…”

A numerical simulation of ionospheric and atmospheric variations associated with the Sumatra earthquake on December 26, 2004

“…The neutral atmosphere is treated as a nonhydrostatic compressible fluid, and the ionosphere is treated as a single-fluid collisional plasma. The modeling method of the neutral atmosphere is basically the same as that of Shinagawa and Oyama (2006). The simulation domain and basic configuration of the model are illustrated schematically in Fig.…”