North‐south asymmetries in the polar thermosphere‐ionosphere system: Solar cycle and seasonal influences

• Published in: Journal of geophysical research. Space physics 2016-01, Vol.121 (1), p.612-627
• Main Authors: Cnossen, Ingrid, Förster, Matthias
• Format: Article
• Language: English
• Subjects: Asymmetry; Cluster Mission; Clusters; Computer simulation; Drag; Drift; Fluid flow; Geomagnetic activity; Geomagnetic field; Geomagnetic fields; Geomagnetism; high latitude; Ion drag; Ionosphere; Latitude; magnetic field; Magnetic fields; Magnetosphere; Magnetospheres; neutral wind; Northern Hemisphere; Plasma density; plasma drift; Polar caps; Polar thermosphere; Seasons; Solar activity; Solar cycle; Solar cycles; Solar physics; Solstices; Southern Hemisphere; Spacecraft; Summer; Superposition (mathematics); Thermosphere; Vortices; Vorticity; Wind speed
• ISSN: 2169-9380; 2169-9402
• DOI: 10.1002/2015JA021750

Previous studies have revealed that ion drift and neutral wind speeds at ~400 km in the polar cap (>80° magnetic latitude) are on average larger in the Northern Hemisphere (NH) than in the Southern Hemisphere, which is at least partly due to asymmetry in the geomagnetic field. Here we investigate for the first time how these asymmetries depend on season and on solar/geomagnetic activity levels. Ion drift measurements from the Cluster mission show little seasonal dependence in their north‐south asymmetry when all data (February 2001–December 2013) are used, but the asymmetry disappears around June solstice for high solar activity and around December solstice for low solar activity. Neutral wind speeds in the polar cap obtained from the Challenging Minisatellite Payload spacecraft (January 2002–December 2008) are always larger in the summer hemisphere, regardless of solar activity, but the high‐latitude neutral wind vortices at dawn and dusk tend to be stronger in the NH, except around December solstice, in particular, when solar activity is low. Simulations with the Coupled Magnetosphere‐Ionosphere‐Thermosphere (CMIT) more or less capture the behavior of the ion drift speeds, which can be explained as a superposition of seasonal and geomagnetic field effects, with the former being stronger for higher solar activity. The behavior of the neutral wind speed and vorticity is not accurately captured by the model. This is probably due to an incorrect seasonal cycle in plasma density around ~400 km in CMIT, which affects the ion drag force. This must be addressed in future work. Key Points North‐south asymmetries in high‐latitude ion drift speed depend on season and solar activity High‐latitude neutral wind speeds are always larger in summer, regardless of solar activity The CMIT model can explain the ion drift results but does not capture the neutral wind behavior