Seasonal and diurnal variations in AMPERE observations of the Birkeland currents compared to modeled results

J C Coxon,B J Anderson,L B N Clausen,H Korth,S E Milan,J A Carter

doi:10.1002/2015ja022050

Abstract

AbstractWe reduce measurements made by the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) to give the total Birkeland (field‐aligned) current flowing in both hemispheres in monthly and hourly bins. We analyze these totals using 6 years of data (2010–2015) to examine solar zenith angle‐driven variations in the total Birkeland current flowing in both hemispheres, simultaneously, for the first time. A diurnal variation is identified in the total Birkeland current flowing, consistent with variations in the solar zenith angle. A seasonal variation is also identified, with more current flowing in the Northern (Southern) Hemisphere during Bartels rotations in northern (southern) summer. For months close to equinox, more current is found to flow in the Northern Hemisphere, contrary to our expectations. We also conduct the first test of the Milan (2013) model for estimating Birkeland current magnitudes, with modifications made to account for solar contributions to ionospheric conductance based on the observed variation of the Birkeland currents with season and time of day. The modified model, using the value of ΦD averaged by Bartels rotation (scaled by 1.7), is found to agree with the observed AMPERE currents, with a correlation of 0.87 in the Northern Hemisphere and 0.86 in the Southern Hemisphere. The improvement over the correlation with dayside reconnection rate is demonstrated to be a significant improvement to the model. The correlation of the residuals is found to be consistent with more current flowing in the Northern Hemisphere. This new observation of systematically larger current flowing in the Northern Hemisphere is discussed in the context of previous results which suggest that the Northern Hemisphere may react more strongly to dayside reconnection than the Southern Hemisphere.

Highlights

IntroductionThese field-aligned currents are known as Birkeland currents, since they were first proposed by Birkeland [1908, 1913]
Field-aligned currents electrodynamically connect the ionosphere to the magnetopause and the inner magnetosphere, transmitting stresses through the magnetosphere
We evaluate the solar contribution to conductance using the expressions of Robinson and Vondrak [1984] and Moen and Brekke [1993], which both depend on F10.7 flux and solar zenith angle; we find that the two models give near-identical results, and so we use the model of Moen and Brekke [1993]; where we have not included the F10.7 parameter we set F10.7 = 100 solar flux units

Summary

Introduction

These field-aligned currents are known as Birkeland currents, since they were first proposed by Birkeland [1908, 1913]. R2 currents flow in the opposite sense in the dawn and dusk sectors and were shown to map to the partial ring current in the inner magnetosphere [e.g., Cowley, 2000, and references therein] It was shown by Fujii et al [1981] that the currents show a seasonal dependence, which is connected to seasonal variations in ionospheric conductance, and more recently, Coxon et al [2014a, 2014b] showed that the currents are consistent with driving by magnetic reconnection on the dayside of Earth

Objectives

Findings

Discussion

Conclusion