Interplanetary Magnetic Field Control of Polar Ionospheric Equivalent Current System Modes

R M Shore,J W Gjerloev,M P Freeman

doi:10.1029/2019sw002161

Abstract

AbstractWe analyze the response of different ionospheric equivalent current modes to variations in the interplanetary magnetic field (IMF) components By and Bz. Each mode comprises a fixed spatial pattern whose amplitude varies in time, identified by a month‐by‐month empirical orthogonal function separation of surface measured magnetic field variance. Here we focus on four sets of modes that have been previously identified as DPY, DP2, NBZ, and DP1. We derive the cross‐correlation function of each mode set with either IMF By or Bz for lags ranging from −10 to +600 mins with respect to the IMF state at the bow shock nose. For all four sets of modes, the average correlation can be reproduced by a sum of up to three linear responses to the IMF component, each centered on a different lag. These are interpreted as the statistical ionospheric responses to magnetopause merging (15‐ to 20‐min lag) and magnetotail reconnection (60‐min lag) and to IMF persistence. Of the mode sets, NBZ and DPY are the most predictable from a given IMF component, with DP1 (the substorm component) the least predictable. The proportion of mode variability explained by the IMF increases for the longer lags, thought to indicate conductivity feedbacks from substorms. In summary, we confirm the postulated physical basis of these modes and quantify their multiple reconfiguration timescales.

Highlights

Electrodynamical coupling between the magnetosphere and the interplanetary magnetic field (IMF) drives current systems which span near Earth space and which are highly variable on a wide range of spatial and temporal scales (Dungey, 1961; Schunk & Nagy, 2009)
We focus on both the time delay of the ionospheric response and the proportion of the ionospheric equivalent current system variability described by the IMF
We find that IMF By accounts for 46% of the DPY mode variability and that IMF Bz accounts for, respectively, 34%, 73%, and 17% of the DP2, NBZ, and DP1 modes

Summary

Introduction

Electrodynamical coupling between the magnetosphere and the interplanetary magnetic field (IMF) drives current systems which span near Earth space and which are highly variable on a wide range of spatial and temporal scales (Dungey, 1961; Schunk & Nagy, 2009). At present, it is only possible to predict the ionospheric currents (or their associated magnetic perturbation at ground) with rather low levels of accuracy from measurements of the IMF, on small spatial scales. This is because while the ionospheric current systems are driven by disturbances happening on the Sun, these systems vary in the extent to which they are driven either directly by the IMF or by internal magnetospheric processes, and the entire set of current systems is subject to feedbacks operating within and between the ionosphere and magnetosphere. These feedbacks include the inertia of neutral winds, changes in ionospheric conductivity from particle precipitation, and the acceleration of electrons by parallel electric fields in the upper ionosphere caused by intense field-aligned currents (Knight & Parallel electric fields, 1973)

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