The transition to overshielding after sharp and gradual interplanetary magnetic field northward turning

Abstract

[1] Overshielding is referred to a shielding status, during which the dawnward shielding electric field dominates over the duskward penetration electric field in the inner magnetosphere, typically appearing when the interplanetary magnetic field (IMF) suddenly turns northward after a prolonged southward orientation. It is expected that the transition to overshielding after IMF northward turning can be affected by the shape of northward turning (sharp or gradual). Moreover, the initial shielding status (undershielding or goodshielding) prior to the transition may also have influence on the transition. Here we analyze two groups of cases, in which the transitions appear after sharp (duration less than 5 min) and gradual (duration more than 30 min) northward turning. Each group includes two cases, in which the transition initiated from undershielding and goodshielding. These cases show that (1) the beginning of the transition to overshielding coincides with sharp IMF northward turning but appears in the midst of gradual IMF northward turning; (2) the transition from goodshielding to overshielding is always associated with convection electric field drop and/or polar cap shrinkage, regardless of the shape of IMF northward turning; and (3) the typical solar wind condition in which the IMF suddenly turns northward after a prolonged southward orientation is neither a necessary condition nor a sufficient condition for overshielding. Furthermore, we will discuss the effect of substorm processes on overshielding.

High‐speed stream impacts on the equatorial ionization anomaly region during the deep solar minimum year 2008

“…One of these important factors that modulate the storm‐time low‐equatorial ionosphere is the perturbed electric fields, which can be broadly grouped into two categories: prompt penetration electric fields (PPEFs) and disturbance dynamo electric fields (DDEFs). The short‐lived PPEF, ranging from few minutes to several hours, is highly correlated with interplanetary magnetic field (IMF) Bz component [e.g., Fejer , 1997; Huang et al , 2005; Wolf et al , 2007; Wei et al , 2011]. In general, on the dayside ionosphere, a southward turning of IMF Bz leads to eastward electric field disturbance (undershielding), while an abrupt northward turning after prolonged southward IMF may result in westward electric field disturbance (overshielding).…”

Interhemispheric Asymmetry in Response of Low‐Latitude Ionosphere to Perturbation Electric Fields in the Main Phase of Geomagnetic Storms

“…It is quite likely (following previous studies on oscillating IMF Bz, e.g., Wang et al, 2008, andWei et al, 2011) that the instances of southward (northward) turning of IMF Bz may not be associated with under (over) shielding conditions at high latitudes. It is quite likely (following previous studies on oscillating IMF Bz, e.g., Wang et al, 2008, andWei et al, 2011) that the instances of southward (northward) turning of IMF Bz may not be associated with under (over) shielding conditions at high latitudes.…”

“…Such ASYM-H variations are possibly related to continuous decrease in R2 field-aligned current (FAC) producing a condition of imbalance between R1 and R2 leading to penetration of R1 associated undershielding electric field. The works of Wei et al (2008Wei et al ( , 2009Wei et al ( , 2011 have emphasized the effect of oscillating IMF Bz and penetration of westward (overshielding) electric field during daytime. In fact, a sudden impulse like southward turning of IMF Bz at about 15 UT on 24 August (see Figure 9a resulted in a simultaneous positive spike in E × B drift over equator evidently proving the above mentioned argument (Kil et al, 2007).…”

The long‐duration positive storm effects in the equatorial ionosphere over Jicamarca

“…Under the prompt penetration effect, the interplanetary electric field penetrates from high to low latitudes during geomagnetic storms. The PPEF is short lived and highly correlated with interplanetary magnetic field (IMF) Bz [e.g., Fejer , ; Huang et al ., ; Wei et al ., ]. In general, on the dayside ionosphere, the southward turning of IMF B z will result in eastward electric field disturbance, and a northward turning after prolonged southward IMF may lead to westward electric field disturbance.…”