Regular Article

The importance of geomagnetic field changes versus rising CO₂ levels for long-term change in the upper atmosphere

British Antarctic Survey, High Cross, Madingley Road, Cambridge, CB3 0ET, United Kingdom

^* Corresponding author: e-mail: inos@bas.ac.uk

Received: 30 October 2013
Accepted: 7 May 2014

Abstract

The Earth’s upper atmosphere has shown signs of cooling and contraction over the past decades. This is generally attributed to the increasing level of atmospheric CO₂, a coolant in the upper atmosphere. However, especially the charged part of the upper atmosphere, the ionosphere, also responds to the Earth’s magnetic field, which has been weakening considerably over the past century, as well as changing in structure. The relative importance of the changing geomagnetic field compared to enhanced CO₂ levels for long-term change in the upper atmosphere is still a matter of debate. Here we present a quantitative comparison of the effects of the increase in CO₂ concentration and changes in the magnetic field from 1908 to 2008, based on simulations with the Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIE-GCM). This demonstrates that magnetic field changes contribute at least as much as the increase in CO₂ concentration to changes in the height of the maximum electron density in the ionosphere, and much more to changes in the maximum electron density itself and to low-/mid-latitude ionospheric currents. Changes in the magnetic field even contribute to cooling of the thermosphere at ~300 km altitude, although the increase in CO₂ concentration is still the dominant factor here. Both processes are roughly equally important for long-term changes in ion temperature.