A statistical study on characteristics of high latitude Pi 3 pulsations

Abstract

Using magnetic data from the high‐latitude North American IMS network, Pi 3 pulsations having periods 6–25 min are analyzed. The latitude and magnetic local time distributions of the sense of polarization and the position of maximum intensity for Pi 3 pulsations are studied. These characteristics are compared with those of other pulsations such as Pc 5 and Pi 2. The following characteristics are found: (1) In the Xm‐Ym plane in dipole coordinates, regions of right‐handed (R) and left‐handed (L) polarizations are clearly separated during daytime; in the morning the R region is at higher latitudes than the L region, and in the afternoon the relative positions are reversed. The dividing line of the two regions is approximately at 70° dipole latitude at dawn and dusk and is about 75° latitude near noon. Between 2100 and 0500 MLT there are small R or L regions and a large mixed region. (2) For the polarization in the Xm‐Z plane the distribution is rather simple; namely it shows predominantly L polarization on the dayside and R polarization on the nightside, where the sense of polarization is defined by viewing the Xm‐Z plane from its west side. (3) The gross daytime distributions of the Pi 3 pulsations are similar to those for Pc 5, while the nightside distributions resemble those for Pi 2. (4) The maximum amplitude of Pi 3 occurs near the auroral electrojet region around midnight, while it is at much higher latitudes during late morning hours. For each of the three components, Xm, Ym, and Z, the locus of maximum amplitude forms an oval shape, but the oval does not coincide with Feldstein's auroral oval. On the basis of the morphological characteristics of Pi 3 pulsations found by the present analysis, we believe that dayside Pi 3 pulsations have a different generation mechanism from that for nightside Pi 3 events. The Kelvin‐Helmholtz instability on the magnetopause is likely to be the primary excitation source for the daytime Pi 3 events. For the cause of the nightside Pi 3 pulsations, oscillations of a magnetospheric‐ionospheric current system appear to be a more probable source.