Seasonal dependence and solar wind control of transpolar arc luminosity

Abstract

[1] The influence of the solar wind and the interplanetary magnetic field (IMF) on the luminosity of transpolar arcs (TPAs) is examined by taking into account seasonal effects. The study focuses on those transpolar arcs that appear after an IMF B y sign change during steady northward IMF. It includes 21 northern hemisphere events identified in a previous study from global UV images taken by the Polar spacecraft between 1996 and 2000. Sorting the TPA events by sign of the Earth dipole tilt we find that the TPAs which appear in the dark hemisphere are on average much weaker than TPAs in the sunlit hemisphere. For the dark hemisphere events, no clear correlation between solar wind parameters and TPA luminosity is found. However, in the sunlit hemisphere, a clear dependence on solar wind and IMF conditions is seen. The TPA brightness is strongly influenced by IMF magnitude, northward IMF B z and solar wind speed. A weak, negative correlation with the ion density is found. The TPA luminosity in the sunlit hemisphere is much more strongly controlled by the magnetic energy flux than by the kinetic energy flux of the solar wind. This explains the absence of transpolar arcs for the two B y sign change cases for positive dipole tilts with lowest magnetic energy flux values. The strong influence of the Earth dipole tilt on the transpolar arc luminosity appears due to the dependence of the ionospheric conductivity on solar EUV emissions.Citation: Kullen, A., J. A. Cumnock, and T. Karlsson (2008), Seasonal dependence and solar wind control of transpolar arc luminosity,

Cluster observations and numerical modeling of energy‐dispersed ionospheric H+ ions bouncing at the plasma sheet boundary layer

“…Thus the flow direction is in agreement with the SuperDARN detection of a localized vortex. Kullen et al [2008] have recently shown that the plasma flows measured in the ionosphere are a good approximation for the global plasma convection in the magnetosphere, even in regions of strong, field‐aligned parallel E fields.…”

Small-scale characteristics of extremely high latitude aurora

“…The Cumnock (2005) study was expanded by Kullen et al (2008) who found that the brightness of theta aurorae which occur in the summer hemisphere after an IMF B y sign change during steady northward IMF shows the same dependence on season and solar wind conditions as the main auroral oval except in the nightside sector. Increasing TPA brightness is associated with high solar wind speed, strongly northward IMF, high IMF magnitude, and low solar wind density (which correspond to high solar wind Alfvén velocity).…”

“…In contrast to large-scale arcs, small-scale Sun-aligned arcs are seen about half the time during northward IMF (e.g., Lassen and Danielsen, 1978;Gussenhoven, 1982;Ismail and Meng, 1982). In a statistical study, Valladares et al (1994) examined stable narrow Sun-aligned optical polar cap arcs viewed by all-sky imaging photometers.…”

GPS TEC variations in the polar cap ionosphere: Solar wind and IMF dependence

“…Kullen et al . [] reported a greater IMF B Y dependence on the formation of transpolar arcs in the summer months compared to winter but attributed this to lower arc luminosity in the winter months due to lower ionospheric conductivity.…”

“…Past studies have shown that the solar wind dependence of polar ionosphere structure and dynamics varies with season. For our purposes, most notable is the seasonal variability of the IMF BY influence on the IMF B Y influence on the polar cap convection pattern [Papitashvili and Rich, 2002;Ruohoniemi and Greenwald, 2005], formation of transpolar arcs [Kullen et al, 2008], and field-aligned currents [Green et al, 2009]. Figure 14 shows the occurrence of F region TEC variations observed in the three months surrounding (a) winter and (b) summer solstice, during periods of dawnward (B Y < À2 nT) and duskward (B Y > 2 nT) IMF.…”