M–I coupling across the auroral oval at dusk and midnight: repetitive substorm activity driven by interplanetary coronal mass ejections (CMEs)

Abstract

Abstract. We study substorms from two perspectives, i.e., magnetosphere-ionosphere coupling across the auroral oval at dusk and at midnight magnetic local times. By this approach we monitor the activations/expansions of basic elements of the substorm current system (Bostrøm type I centered at midnight and Bostrøm type II maximizing at dawn and dusk) during the evolution of the substorm activity. Emphasis is placed on the R1 and R2 types of field-aligned current (FAC) coupling across the Harang reversal at dusk. We distinguish between two distinct activity levels in the substorm expansion phase, i.e., an initial transient phase and a persistent phase. These activities/phases are discussed in relation to polar cap convection which is continuously monitored by the polar cap north (PCN) index. The substorm activity we selected occurred during a long interval of continuously strong solar wind forcing at the interplanetary coronal mass ejection passage on 18 August 2003. The advantage of our scientific approach lies in the combination of (i) continuous ground observations of the ionospheric signatures within wide latitude ranges across the auroral oval at dusk and midnight by meridian chain magnetometer data, (ii) "snapshot" satellite (DMSP F13) observations of FAC/precipitation/ion drift profiles, and (iii) observations of current disruption/near-Earth magnetic field dipolarizations at geostationary altitude. Under the prevailing fortunate circumstances we are able to discriminate between the roles of the dayside and nightside sources of polar cap convection. For the nightside source we distinguish between the roles of inductive and potential electric fields in the two substages of the substorm expansion phase. According to our estimates the observed dipolarization rate (δB z /δt) and the inferred large spatial scales (in radial and azimuthal dimensions) of the dipolarization process in these strong substorm expansions may lead to 50-100 kV enhancements of the cross-polar-cap potential due to inductive electric field coupling.

Strong induction effects during the substorm on 27 August 2001

“…Indeed, the induction EMF U ind = −70 kV is much larger than the observed decrease in voltage ΔU PC = − 13 kV in the second half of the loading phase. This is consistent with the earlier results (Pellinen and Heikkila 1984;Gordeev et al 2011;Sandholt et al 2014). On the contrary, their values are sufficiently close during the active phase (see the "U PC variation and the induction effect" section).…”

“…The dependence U PC ∝ dΨ/dt, which is consistent with the induction effect, during the substorm expansion phase was suggested by Shelomentsev et al (1988), Gordeev et al (2011), andMishin et al (2012). According to Sandholt et al (2014), the dipolarization process in substorm expansions could lead to enhancements of the cross polar cap potential by 50-100 kV due to inductive electric fields. The absence of the growth of I R1 until the substorm expansion onset at 04:08 UT implies that the ionospheric load is almost disconnected from the generator in the geomagnetic tail lobes (Akasofu 2013).…”

Aspects of magnetosphere–ionosphere coupling in sawtooth substorms: a case study

“…The clock angle increase (from 140 to 170 • ) and E KL enhancement (from 7.2 to 8.2 mV m −1 ) at 09:55-10:00 UT led to a convection increase at the dayside polar cap boundary at ∼ 11:00 UT, as inferred from ground magnetometer data (Sandholt et al, 2014). This is consistent with a propagation delay from ACE of 65 min.…”

“…A central feature of this sector is the boundary between the EEJ and WEJ in the evening to premidnight sector -also delimiting the southern and northern branches of the aurora, marked by a dashed curved line -is the Harang reversal boundary (HRB). The northern auroral branch in this sector (HR-N) is characterized by equatorward-moving auroral forms (streamers) which are coupled to earthwardmoving plasma-depleted flux tubes (Chen and Wolf, 1993) and associated bursty bulk flows (BBFs) in the PS (Sergeev et al, 2004(Sergeev et al, , 2012Sandholt et al, 2014). During "classical" substorms the HR is typically found in the 19:00-24:00 MLT sector (Nielsen and Greenwald, 1979).…”

“…In a recent study we documented the activation of magnetospheric current systems (Bostrøm types I and II;see Bostrøm, 1964see Bostrøm, , 1967 with associated auroral electrojet events on both sides of the Harang reversal boundary at dusk during the ICME passage at Earth on 18 August 2003 substorms (Sandholt et al, 2014) (hereafter referred to as Paper 1). The WEJ was observed to expand repeatedly into the 17:00-18:00 MLT sector during substorm activations.…”