Characteristics of the solar wind controlled auroral emissions

Abstract

We performed a high‐time resolution (5 min) correlative study of the energy deposition rate in the northern auroral zone with the concurrent solar wind plasma and interplanetary magnetic field (IMF) observations for a 4 month period from March 30 to July 29, 1996. Auroral power, inferred by auroral emissions, was derived from images acquired by the ultraviolet imager (UVI) on board the Polar satellite, and the interplanetary parameters were based on Wind observations. It is found that dayside aurorae in the afternoon sector (65°–80° magnetic latitude (MLAT) and 1300–1800 magnetic local time (MLT)) are more active for large IMF cone angles and large solar wind electric fields. This result can be attributed to the manifestation of the antiparallel magnetic field merging in different locations and the partial “penetration” of the IMF on the dayside magnetopause. The integrated nightside (60°–75° MLAT and 2000–0100 MLT) auroral brightness is moderately correlated with the north–south component of the IMF and the solar wind speed with correlation coefficients of 0.49 and 0.35, respectively. The mean nightside auroral power is found to be approximately linearly proportional to the IMF Bz with a constant slope of 2 GW/nT. The solar wind speed, however, affects the nightside auroral power for both polarities of IMF Bz. Interestingly, the solar wind dynamic pressure shows no effect on the nightside auroral brightness. All these findings indicate that both reconnection and viscous‐like interaction mechanisms play an important role in producing auroral emissions in the night sector. It is also found that the nightside auroral brightness responds to the southward turning of the IMF with a peak delay time of ∼60 min. This result favors the model of loading‐unloading magnetosphere. We also found that a negative IMF By condition favors the nightside auroral activity, and we attributed this effect to the partial penetration of the IMF By. Finally, the response function for nightside aurora is given as ∼VB4Tsin4θc2) with a median correlation coefficient of 0.63, indicating that there may be other factors other than the solar wind and IMF responsible for lightening up the northern–southern hemispheric sky.