Abstract
We analyze the auroral boundary corrected geomagnetic latitude provided by the Auroral Boundary Index (ABI) database to estimate long-term changes of core origin in the area enclosed by this boundary during 1983–2016. We design a four-step filtering process to minimize the solar contribution to the auroral boundary temporal variation for the northern and southern hemispheres. This process includes filtering geomagnetic and solar activity effects, removal of high-frequency signal, and additional removal of a ~20–30-year dominant solar periodicity. Comparison of our results with the secular change of auroral plus polar cap areas obtained using a simple model of the magnetosphere and a geomagnetic core field model reveals a decent agreement, with area increase/decrease in the southern/northern hemisphere respectively for both observations and model. This encouraging agreement provides observational evidence for the surprising recent decrease of the auroral zone area.
Highlights
Auroral oval regions, where charged particles accelerate along magnetic field lines from the magnetosphere down to the upper atmosphere [1,2], are of special interest to space weather due to the broad consequences of particle precipitation on technological systems
The auroral oval boundary is almost a circle whose center can be reasonably approximated by the eccentric geomagnetic dipole [9,10,11]
Four successive filtering steps are applied to the auroral boundary data to minimize geomagnetic activity and solar activity effects and obtain its long-term variation, which originates from the geomagnetic core field secular variation
Summary
Auroral oval regions, where charged particles accelerate along magnetic field lines from the magnetosphere down to the upper atmosphere [1,2], are of special interest to space weather due to the broad consequences of particle precipitation on technological systems. While the sun induces short-term variability, both core and solar origin magnetic forcings present additional long-term temporal changes, leading to secular variation of the auroral oval areas. Under the self-similarity hypothesis for a pure dipole magnetic field and a steady solar wind under quiet conditions, which is reasonable for a long-term average condition, polar cap boundary latitude, λp , scaling laws imply a variation in terms of the Earth’s dipole moment, M, according to cos(λp )∝(1/M)1/6 [22,23,24]. Four successive filtering steps are applied to the auroral boundary data to minimize geomagnetic activity and solar activity effects and obtain its long-term variation, which originates from the geomagnetic core field secular variation
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