2:30 PM - 2:45 PM
[PEM15-14] Ionospheric Electric Field Response in the Dayside Equatorial Region During Isolated Substorms: Assessing the Combined Effects of Substorms and IMF Bz
Keywords:substorm, ionosphere, magnetosphere
This study investigates the ionospheric electric field response in the dayside equatorial region during isolated substorms, focusing on the combined effects of substorms and the interplanetary magnetic field (IMF) Bz.
Substorms are triggered by plasma injections in the nightside, leading to the formation of the substorm current wedge (SCW) due to plasma vortices at the flow edge. Previous studies have suggested that the electromagnetic response associated with the SCW extends not only to the nightside high-latitude region but also to the dayside equatorial region. On the dayside, the Pi2 pulsations observed at the substorm onset are produced by ionospheric currents in the equatorial region and the terminator, implying the global ionospheric current system connecting the nightside SCW to the equatorial electrojet (EEJ) via dawn and dusk sectors. Studies on the global magnetic field response during substorms have reported a decrease in the dayside H component. However, this decrease has been attributed to the remote effect of the SCW rather than to ionospheric current. These findings highlight significant questions regarding the dayside ionospheric response to substorm onset.
In Hayashi et al. (2025), a statistical analysis of the equatorial ionospheric electric field response during isolated substorms revealed no significant changes in EEJ on average. However, small increase and decrease variations were observed in individual events. The polarity of these variations likely depends on additional conditions, resulting in a decrease and an increase, with no significant change observed on average.
One key factor influencing the equatorial ionospheric response is the IMF Bz. Numerous studies have reported a correlation between geomagnetic disturbances at the magnetic equator and the IMF Bz component, with a northward IMF Bz leading to a weakened EEJ. Since IMF Bz tends to turn northward during substorms, it is necessary to consider its influence as an additional factor in the electric field response.
In this study, we classified isolated substorm events that occurred between 2009 and 2011 based on post-onset IMF Bz variations and compared the differences in dayside equatorial electric field responses across the classified groups. The results indicate that EEJ decreases when IMF Bz turns northward, whereas little change is observed when IMF Bz turns southward. These findings suggest that IMF Bz plays a dominant role in modulating EEJ responses during substorms, whereas substorms have a limited direct impact on EEJ variations.
Substorms are triggered by plasma injections in the nightside, leading to the formation of the substorm current wedge (SCW) due to plasma vortices at the flow edge. Previous studies have suggested that the electromagnetic response associated with the SCW extends not only to the nightside high-latitude region but also to the dayside equatorial region. On the dayside, the Pi2 pulsations observed at the substorm onset are produced by ionospheric currents in the equatorial region and the terminator, implying the global ionospheric current system connecting the nightside SCW to the equatorial electrojet (EEJ) via dawn and dusk sectors. Studies on the global magnetic field response during substorms have reported a decrease in the dayside H component. However, this decrease has been attributed to the remote effect of the SCW rather than to ionospheric current. These findings highlight significant questions regarding the dayside ionospheric response to substorm onset.
In Hayashi et al. (2025), a statistical analysis of the equatorial ionospheric electric field response during isolated substorms revealed no significant changes in EEJ on average. However, small increase and decrease variations were observed in individual events. The polarity of these variations likely depends on additional conditions, resulting in a decrease and an increase, with no significant change observed on average.
One key factor influencing the equatorial ionospheric response is the IMF Bz. Numerous studies have reported a correlation between geomagnetic disturbances at the magnetic equator and the IMF Bz component, with a northward IMF Bz leading to a weakened EEJ. Since IMF Bz tends to turn northward during substorms, it is necessary to consider its influence as an additional factor in the electric field response.
In this study, we classified isolated substorm events that occurred between 2009 and 2011 based on post-onset IMF Bz variations and compared the differences in dayside equatorial electric field responses across the classified groups. The results indicate that EEJ decreases when IMF Bz turns northward, whereas little change is observed when IMF Bz turns southward. These findings suggest that IMF Bz plays a dominant role in modulating EEJ responses during substorms, whereas substorms have a limited direct impact on EEJ variations.