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

TL;DR

This study analyzes 6 years of AMPERE data to reveal seasonal and diurnal variations in Birkeland currents, and tests a model that accurately predicts these currents by incorporating solar and seasonal effects, showing significant improvements over previous models.

Contribution

First comprehensive analysis of seasonal and diurnal variations in Birkeland currents using AMPERE data, and validation of a modified Milan model with improved correlation to observations.

Findings

Diurnal variation in Birkeland currents consistent with solar zenith angle changes.

Seasonal variation shows more current in respective hemispheres during summer.

Modified Milan model with solar contributions predicts currents with high correlation.

Abstract

We 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 $\Phi_D$ 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.