Geomagnetic Field Model Indicates Shrinking Northern Auroral Oval

Abstract

Polar cap and auroral oval areas are presented for the period 1900‐2015 using a model of the magnetosphere and the IGRF‐12 model for the geomagnetic field. The effect of the main field long‐term changes is addressed considering steady interplanetary conditions. Until ~1940 both northern and southern polar cap areas increased with time, consistent with the geomagnetic dipole moment decrease during this period. Thereafter, while the southern polar cap area continued to increase steadily, the northern polar cap area has been decreasing rapidly. This hemispherical dichotomy is related to asymmetry in the core field and its secular variation. In the southern polar region the surface field intensity has been decreasing throughout the entire period, whereas in the northern polar region an intensity decrease turned to an increase from 1940 to present day. This recent surface intensity increase stems from intensification of the flux patch below Siberia and cessation of weakening of the flux patch below North America, both on the core‐mantle boundary. The surprising temporal decrease in the area of the northern polar cap has important implications for space weather and communication systems. While the decreasing dipole intensity allows more particles to penetrate the atmosphere, a shrinking northern auroral oval implies an increase in particle precipitation density, resulting in more severe damage to airplanes and ships positioning systems, spacecraft electronic systems and airline passengers when passing above this region. Further study is needed to observationally establish that the northern auroral oval, as defined by optical occurrences of aurora, is indeed shrinking.