| The thermospheric atmosphere is an important part of the Earth's middle and upper atmospheric environment,and magnetic storms,as a typical catastrophic event,have a significant impact on the thermospheric atmosphere.Therefore,the study of the response of the thermospheric atmosphere to a magnetic storm has important scientific significance.However,due to the lack of observations,there are few studies on the magnetic storm response of the vertical wind field in the thermosphere,especially the physical mechanism of the response is still unclear.This paper uses the Thermosphere Ionosphere Mesosphere Electrodynamics General Circulation Model(TIMEGCM)to simulate the geomagnetic storm time caused by the Coronal Mass Ejection in September 2005,and study the response process of the mid-latitude thermosphere vertical wind field to the geomagnetic storm event.And the diagnostic analysis module of the dynamic equation and the continuity equation in TIMEGCM is developed to reveal the potential connection between the vertical wind field and the horizontal wind field during a storm,and used to analyze and study the physical mechanism of this horizontal wind field.The main work and conclusion of this article are as follows:(1)The change of the mid-latitude horizontal wind field during a storm above 250 km determines the change of the vertical wind field and the vertical wind of 100-250 km is pulled by the high-altitude wind field,so the vertical wind can be explained by analyzing the change of the horizontal wind field during the storm.By analyzing the contribution of the zonal wind field and the meridional wind field to the change of the horizontal wind field,we can obtain the influence of the zonal wind field and the meridional wind field on the vertical wind change.The vertical wind field on September 10 was mainly driven by the change of the zonal wind field,and since the change in the intensity of the geomagnetic disturbance on September 11 was much greater than the previous day,we come to a completely different conclusion.At the beginning of the September 10 geomagnetic disturbance(0600UT-0800UT),the changes in the meridional wind along the meridian play a leading role in the changes of the vertical wind,followed by the development of geomagnetic storms.Between 0800 UT and 2400 UT,the changes in the zonal wind field along the latitude have a greater impact on the changes in the vertical wind.On September 11,at 0000UT-0400 UT there is a similar conclusion as the periods of 0800UT-2400 UT on September 10,that is,the change of the zonal wind field along the latitude has a greater impact on the change of the vertical wind.However,the change of the meridional wind field along the meridian plays a leading role in the change of the vertical wind at 0400UT-2400 UT.Moreover,when the intensity of local magnetic disturbance increases sharply,the change of the meridional wind field will increase,and when the local magnetic disturbance decreases,the change of the zonal wind field will increase accordingly.Therefore,the change of Kp within 48 hours is analyzed.This can be seen that the increase of Kp on September 11 is much larger than that in the previous day,so the change of meridional wind field is much larger than that of zonal wind field.On the contrary,the change of zonal wind field on September 10 is much larger than the meridional wind field.(2)When the geomagnetic disturbance increases sharply,the gradient of temperature propagates along the meridian.During this time,the gradient of the meridional wind field along the meridian is more obvious than the latitude.The meridional wind field plays a leading role in the change of the vertical wind;while the local magnetic disturbance weakens or increases slowly,the gradient of temperature travels along the latitude line,the gradient of the zonal wind field is more obvious,so the zonal wind field has a greater influence on the vertical wind.By comparing the temperature diffusion process along the meridian line and latitude line,it can be found that when the intensity of geomagnetic disturbance increases sharply,through the action of Joule heating,the atmosphere of the polar thermosphere will warm up rapidly,and this increase will be transferred from high to low latitudes along the longitude.And the temperature gradient along the meridian will increase the change of the meridional wind field.When the intensity of the geomagnetic disturbance increases slowly or decreases,the Joule heating in the polar region weakens,the highlatitude-lower latitude temperature gradient generated by the Joule heating weakens,and the temperature starts to travel laterally along the latitude,and the change in the zonal wind field increases.The temperature propagation within 48 hours has a good correlation with the change of the zonal wind field,and the change of September 11 meridional wind field has a low correlation with the temperature propagation.When the change of the September 10 zonal wind field has the main effect on the vertical wind,the meridional wind field always maintains toward the west;when the change of September 11 meridional wind field has a greater impact on the vertical wind,the zonal wind field always keep southward.(3)When the geomagnetic disturbance increases sharply,the pressure gradient force and the viscous force become the main dynamic terms of the total meridional acceleration,and the change of the meridional wind field increases accordingly.Therefore,the meridional wind field plays a leading role in the change of the vertical wind;When the disturbance weakens or grows slowly,the pressure gradient force from the temperature change and the horizontal advection term provide the main driving force for the total zonal acceleration.During this time,the zonal wind changes greatly,so the zonal wind field has a more obvious influence on the vertical wind.For the zonal wind field within 48 hours,the pressure gradient force and horizontal advection term caused by temperature changes are the main physical mechanisms that cause the total zonal acceleration change,and Coriolis force,viscous force and ion drag force are the reaction forces.It is always a secondary physical mechanism,and neither the centrifugal force nor the vertical convection term contributes significantly to the total zonal acceleration during a magnetic storm.The meridional wind field within 48 hours needs to be discussed separately.On September 10,the pressure gradient force caused by the temperature change is the main physical mechanism that causes the meridional acceleration change,and on September 11,the temperature no longer affects the pressure gradient force.The only factor is that the viscous force has become one of the main physical mechanisms due to the substantial increase in geomagnetic disturbance.And the centrifugal force also affects the change of the total meridional acceleration.Vertical convection has no contribution to the meridional wind field change within 48 hours. |
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