Morphological features of bipolar magnetic impulsive events and associated interplanetary medium signatures

Abstract

Morphological features of magnetic impulsive events and the dynamics of associated ionospheric perturbances are examined from the magnetic records of high‐latitude stations. Most impulses chosen for investigation display a bipolar variation in all the magnetic field components. Event occurrence patterns versus magnetic local time for different latitudes reveal the existence of large‐scale and small‐scale events. The large‐scale magnetic events were observed typically throughout the wide latitude range from 67° to 77° corrected geomagnetic latitude (CGL). The occurrence pattern of events versus magnetic local time (MLT) shows a single maximum near 0900 MLT. Only about 12% of the impulsive perturbations were generated in the afternoon sector from 1200 to 1500 MLT. These events first moved westward as well as sunward up to noon and then tailward. The small‐scale impulsive magnetic perturbations are generally registered in the midday sector from 70° to 74° CGL. The small‐scale events, like the large‐scale traveling convectin vortices (TCVs), move westward with an average velocity of about 3.7 km/s. IMP‐8 observations of interplanetary medium conditions were investigated for the large‐scale events. The events occur mainly during periods when the Bz component of the interplanetary magnetic field (IMF) was northward. The events do not tend to occur for the highspeed solar wind flows or the flows with unusually high plasma densities. The IMF was frequently oriented close to the ecliptic plane and was almost radial along the Sun‐Earth direction. Magnetic events were caused generally by the solar wind dynamic pressure variations with increases in amplitude from 20 to 140%. We suggest that pressure impulses may be amplified by a factor of 2 by the simultaneous change of the foreshock geometry from lying upstream of the prenoon magnetosphere toward dawn, when the IMF cone angle changes from <30° to >30°.