Introduction
Penetrating electric field is a high latitude dawn-dusk field applied to the mid and low latitudes at a much short time scale than thermosphere-ionosphere interaction process such as ion neutral collision. Penetration electric fields have been intensively studied in the past [e.g., Nishida, 1968; Kikuchi et al., 2008; W. Wang et al., 2008; Huang, 2019; Huang et al., 2005; Maruyama et al., 2005; Kelley et al., 2003; Fejer et al., 2007; Lu et al, 2012].
While the basic underlying physics is known, many aspects of the penetrating electric field (PEF) remain to be addressed. For example, it is difficult to simulate the penetrating electric field, which changes rapidly, using a first principle thermosphere and ionosphere model driven by empirical high latitude convection models such as Heelis et al. [1982] and Weimer [2005]. Lu et al. [2012] used the AMIE (Assimilative Mapping of Ionospheric Electrodynamics) [Richmond and Kamide, 1988] driven TIMEGCM (Thermosphere-Ionosphere-Mesosphere-Electrodynamic General Circulation Model) model to simulate penetrating electric field effects. While AMIE uses real data as inputs, the derived convection pattern at high latitudes still relies heavily on empirical data information. The case examined by Lu et al. [2012] was a very strong storm event (November 9-10, 2004), which produced 120 m/s vertical ion drift at the equator. High latitude model inputs with a high time resolution will enable the examination of the penetrating electric field in great details and improve the understanding of observed results, especially under weak to moderate geomagnetic activity conditions. One of the outstanding questions is how the penetration electric field is related to changes in the IMF Bz and the cross polar cap potential (CPCP). Penetrating electric field effect becomes complicated when the IMF Bz changes back and forth between northward and southward.
Towards the goal of better understanding the penetrating electric field, we use a coupled magnetosphere ionosphere model (MAGE, Multiscale Atmosphere-Geospace Environment) to simulate the penetrating electric field effect on the equatorial ionosphere [e.g., Lin et al., 2021; Pham et al., 2022], specifically on the vertical ion drift. The MAGE model provides much higher and more dynamic high latitude inputs of convection pattern and auroral precipitation by coupling with a magnetospheric model. The objective is to show how the IMF Bz component controls the equatorial vertical ion drift, through the CPCP and penetrating electric field. In addition to the latest modeling tools, we also will use the NASA ICON mission ion drift measurement to observe the effect of the penetrating electric field from an equatorial orbiting satellite platform. MAGE simulations are used to facilitate interpretation of the observational results.
We selected two geomagnetically active intervals during September 2020. The first interval is from September 24, 5-6 UT when the IMF Bz turned northward first and then turned southward within one hour, which is highlighted (blue shaded) in the IMF Bz and By subplot of Figure 1. The figure also includes solar wind speed and density, and the interplanetary electric field (IEF) dawn-dusk component. The IMF Bz variations led to changes in the IEF, which affects the dawn-dusk electric potential and penetrating electric field. The second interval is from September 26, 9-10 UT with the interval highlighted in Figure 2. In this case, IMF Bz turned southward within the highlighted interval. Moreover, we have ICON observations of the equatorial ExB meridional ion drift, which is upward at the magnetic dip equator, for comparison. The paper is organized by (1) a brief description of the model setup, (2) discussion of the simulation results for the two intervals, (3) comparison between the model results and ICON ion drift observations, and (4) additional discussion on the ICON ion drift data.