Global features of the disturbance winds during storm time deduced from CHAMP observations

Abstract

A wind‐driven disturbance dynamo has been postulated many decades ago. But due to the sparseness of thermospheric wind measurements, details of the phenomena could not be investigated. In this study we use the CHAMP zonal wind observations from 2001 to 2005 to investigate the global features of the disturbance winds during magnetically disturbed periods. The disturbance zonal wind is mainly westward, which increases with magnetic activity and latitude. At subauroral region, the westward zonal wind is strongly enhanced in the magnetic local time (MLT) sector from afternoon to midnight, which we relate to the plasma drift within the subauroral polarization streams. At middle and low latitudes, the disturbance zonal wind is largely independent of season. Peak values of the disturbance zonal wind occur at different MLTs for different latitudes. That is around 1800 MLT at subauroal region, with average values of about 200 m/s; around 2300 MLT at middle latitudes, with average values of about 80 m/s; and around 0300 MLT at low latitudes, with average values up to 50 m/s. The shift of the peak values of the westward disturbance zonal wind in local time at different latitudes could be considered as a response of the disturbance wind when it propagates from high to low latitudes. Further by applying for the first time a superposed epoch analysis, we show that the disturbance zonal wind responds with a delay to the sudden changes of solar wind input, which is different for the various latitudinal ranges. The propagation time of disturbance wind from the auroral region to the equator is about 3–4 h. This is consistent with the speed of traveling atmospheric disturbances. Based on CHAMP observations, we try to illustrate the whole chain of processes from the solar wind driving to the ionospheric effects at lower latitudes.

The response of equatorial electrojet, vertical plasma drift, and thermospheric zonal wind to enhanced solar wind input

“…According to previous studies, the time‐integrated merging electric field ( E m ) can be expressed as where is treated as a continuous function of time , t 1 is chosen 3 h before the actual epoch, and τ is the e ‐folding time of the weighting function in the integrands, with a value τ = 0.5 h. Richmond et al [] found that it is appropriate to consider the past 3 h of solar wind variations. An e ‐folding time of 0.5 h was also found suitable for calculating the merging electric field in previous ionospheric studies [ Xiong et al , ; Xiong and Lühr , ; Xiong et al , ].…”

“…During active periods enhanced energy input into the thermosphere at high latitudes on the nightside will cause density bulges, which propagate to middle and low latitudes, via traveling atmospheric disturbance, and or enhanced equatorward wind. The equatorward winds, increasingly turning westward toward middle and low latitudes owing to the action of Coriolis force, need 3–4 h to propagate to equatorial regions [ Fujiwara et al , ; Ritter et al , ; Xiong et al , ]. As shown in Figure c, the westward disturbance wind is most prominent during 00–06 MLT after Δ t = 4.5 h. In fact, this enhanced westward disturbance wind explains well the eastward disturbance electric field on the nightside, as shown in Figure b.…”

“…To check the response of EEJ and vertical plasma drift to the enhanced solar wind input, we use the superposed epoch analysis. Similar to Xiong et al [], we choose the merging electric field, as defined by Newell et al [], for representing the energy input into the ionosphere‐thermosphere system, where V sw denotes the solar wind speed, B y and B z denote the y and z components of the interplanetary magnetic field (IMF) in geocentric solar magnetospheric coordinates, and θ is the clock angle of the IMF . To make the numerical value of the merging electric field (in mV/m) comparable with the one defined by Kan and Lee [], we apply V sw in units of km/s, B y and B z in nT, and then divided the result by a factor of 3000.…”

Responses in the polar and equatorial ionosphere to the March 2015 St. Patrick Day storm

“…If the DD is indeed the explanation for these observed enhanced flows, then there are two things to note. First, it does not appear until 11 h after the onset, a longer time than the 3 to 6 h delay reported by other researchers [ Balan et al ., ; Tulasi Ram et al ., ; Xiong et al ., ]. It is possible, as argued by Tulasi Ram et al .…”

The Influence of Geomagnetic Storm of 7‐8 September 2017 on the Swarm Precise Orbit Determination

“…The input of enhanced solar wind and magnetospheric energy can alter physical and chemical characters of the ionosphere-thermosphere system due to their dynamical coupling, further leads to an ionosphere and thermosphere storm (Buonsanto, 1999). Meanwhile, the energy input at the auroral or subauroral regions heats the thermosphere and drives equatorward neutral winds and traveling atmospheric/ionosheric disturbances (e.g., Huang et al, 2012;Ritter et al, 2010;Xiong et al, 2015). Meanwhile, the energy input at the auroral or subauroral regions heats the thermosphere and drives equatorward neutral winds and traveling atmospheric/ionosheric disturbances (e.g., Huang et al, 2012;Ritter et al, 2010;Xiong et al, 2015).…”