Polar cap flow channel events: spontaneous and driven responses

P E Sandholt,Y Andalsvik,C J Farrugia

doi:10.5194/angeo-28-2015-2010

Abstract

Abstract. We present two case studies of specific flow channel events appearing at the dusk and/or dawn polar cap boundary during passage at Earth of interplanetary (IP) coronal mass ejections (ICMEs) on 10 January and 25 July 2004. The channels of enhanced (>1 km/s) antisunward convection are documented by SuperDARN radars and dawn-dusk crossings of the polar cap by the DMSP F13 satellite. The relationship with Birkeland currents (C1–C2) located poleward of the traditional R1–R2 currents is demonstrated. The convection events are manifest in ground magnetic deflections obtained from the IMAGE (International Monitor for Auroral Geomagnetic Effects) Svalbard chain of ground magnetometer stations located within 71–76° MLAT. By combining the ionospheric convection data and the ground magnetograms we are able to study the temporal behaviour of the convection events. In the two ICME case studies the convection events belong to two different categories, i.e., directly driven and spontaneous events. In the 10 January case two sharp southward turnings of the ICME magnetic field excited corresponding convection events as detected by IMAGE and SuperDARN. We use this case to determine the ground magnetic signature of enhanced flow channel events (the NH-dusk/By<0 variant). In the 25 July case a several-hour-long interval of steady southwest ICME field (Bz<0; By<0) gave rise to a long series of spontaneous convection events as detected by IMAGE when the ground stations swept through the 12:00–18:00 MLT sector. From the ground-satellite conjunction on 25 July we infer the pulsed nature of the polar cap ionospheric flow channel events in this case. The typical duration of these convection enhancements in the polar cap is 10 min.

Highlights

In previous studies we documented the presence of a channel of enhanced (1–3 km/s) antisunward ionospheric convection appearing at the dawn and/or dusk sides of the polar cap, its location depending on the interplanetary magnetic field (IMF) orientation
We reported ion drift features and associated ground magnetic deflections in the postnoon-dusk sector of the Northern Hemisphere polar cap during Earth passage of two interplanetary coronal mass ejections (ICME) in 2004
The flow channel variant we study here is that which appears on the dusk side of the polar cap in the Northern Hemisphere during IMF By < 0 conditions (NH-dusk/By < 0)

Summary

Introduction

In previous studies we documented the presence of a channel of enhanced (1–3 km/s) antisunward ionospheric convection appearing at the dawn and/or dusk sides of the polar cap, its location depending on the IMF orientation (see Sandholt and Farrugia, 2009, and references therein). In Sandholt et al (2010) we reported one case showing the presence of a series of 5–10-min-long negative X-component magnetic deflections detected at ground stations located underneath the FC 2 flow channel of type NH-dusk/By < 0. This led us to speculate that the ionospheric ion flow was pulsed in this case and possibly related to flux transfer events (FTEs), i.e., momentum transfer in a later stage of the evolution of field lines previously opened by pulsed magnetopause reconnection. In our case 1 we shall discuss the variant of flow channel FC 2 which is found on old open field lines in the dusk side convection cell of the Northern Hemisphere during negative IMF By conditions, which we refer to as NH-dusk/By < 0 (Sandholt and Farrugia, 2009). The dusk-to-dawn crossing of satellite DMSP F13 marked in the figure documented the presence of flow channel FC 2 (data not shown)

Case 1

Case 2

70 HO8R0 90 80 70 60

Summary and conclusions