On the motion of dayside auroras caused by a solar wind pressure pulse

Abstract

Abstract. Global ultraviolet auroral images from the IM-AGE satellite were used to investigate the dynamics of the dayside auroral oval responding to a sudden impulse (SI) in the solar wind pressure. At the same time, the TV all-sky camera and the EISCAT radar on Svalbard (in the pre-noon sector) allowed for detailed investigation of the auroral forms and the ionospheric plasma flow. After the SI, new discrete auroral forms appeared in the poleward part of the auroral oval so that the middle of the dayside oval moved poleward from about 70 • to about 73 • of the AACGM latitude. This poleward shift first occurred in the 15 MLT sector, then similar shifts were observed in the MLT sectors located more westerly, and eventually the shift was seen in the 6 MLT sector. Thus, the auroral disturbance "propagated" westward (from 15 MLT to 6 MLT) at an apparent speed of the order of 7 km/s. This motion of the middle of the auroral oval was caused by the redistribution of the luminosity within the oval and was not associated with the corresponding motion of the poleward boundary of the oval. The SI was followed by an increase in the northward plasma convection velocity. Individual auroral forms showed poleward progressions with velocities close to the velocity of the northward plasma convection. The observations indicate firstly a pressure disturbance propagation through the magnetosphere at a velocity of the order of 200 km/s which is essentially slower than the velocity of the fast Alfvén (magnetosonic) wave, and secondly a potential (curl-free) electric field generation behind the front of the propagating disturbance, causing the motion of the auroras. We suggest a physical explanation for the slow propagation of the disturbance through the magnetosphere and a model for the electric field generation. Predictions of the model are supported by the global convection maps produced by the SuperDARN HF radars. Finally, the interchange instability and the eigenmode toroidal Alfvén oscillations areCorrespondence to: A. Kozlovsky (alexander.kozlovsky@oulu.fi) discussed as possible generation mechanisms for the dayside auroral forms launched by the SI.

Changes of dayside auroral distribution caused by a solar wind pressure pulse and associated interplanetary magnetic field disturbances

“…Indeed, the PAB position strongly depends on the selected threshold value. For an individual case, if there is a sharp difference between the Polar Cap (PC) and auroral oval intensities, the threshold may be carefully selected in such a way that the obtained contour curve more or less corresponds to the boundary of diffuse luminosity (e.g., Kozlovsky et al, 2005). In this case, the curve can be defined as the Poleward auroral Oval Boundary (POB).…”

“…), I P C , with the standard deviation ( I P C ) was calculated. Then, for a quantitative analysis, using a method similar to one suggested by Kozlovsky et al (2005), two characteristics of the large-scale auroral distribution were determined.…”

“…One case of simultaneous IMAGE satellite and groundbased optical observations during the SI was studied by Kozlovsky et al (2005). They have shown that after the SI, new discrete auroral forms appeared in the poleward part of the auroral oval so that the dayside oval moved poleward.…”

“…In particular, the motion of dayside auroral 930 A. Kozlovsky et al: Changes of dayside auroral distribution oval and its boundaries can be an important indicator of the magnetospheric disturbances caused by the SI (see, e.g., Kozlovsky et al, 2005).…”

“…Owning to that, auroral imagers are very useful to study global magnetospheric response to interplanetary changes. In particular, UV satellite images were used to investigate the auroral oval response to Solar Wind Pressure (SWP) changes (e.g., Zhou and Tsurutani, 1999;Brittnacher et al, 2000;Boudouridis et al, 2003;Meurant et al, 2003Meurant et al, , 2004Kozlovsky et al, 2005;Lyons et al, 2005).…”