Proton aurora related to intervals of pulsations of diminishing periods

Geomagnetic pulsations in the Pc1 frequency range are believed to be an indicator of electromagnetic ion cyclotron waves arriving from the equatorial magnetosphere, where the waves are generated because of a cyclotron instability of the anisotropic distribution of ring current ions. Proton precipitation produced by the cyclotron instability can be responsible for proton aurora. Indeed, the relationship between some types of proton aurora (proton spots and proton flashes) and pulsations in the Pc1 range (quasi‐monochromatic Pc1 and Pc1 bursts) has already been found. The aim of this study is to find the proton aurora pattern, which relates to the kind of geomagnetic pulsations in the Pc1 range called intervals of pulsation of diminishing periods (IPDP). This is done on the basis of 2 year observations of geomagnetic pulsations at the Finnish meridional network of search coil magnetometers and proton aurora from the IMAGE spacecraft. We found that during IPDP the proton arcs appear equatorward of the proton oval at the meridian of the ground magnetometers. The maximum intensity of the pulsations is observed at the ground station, which is closest to the proton arc. The proton arcs tend to appear at lower latitudes at later magnetic local times (MLTs). This agrees with the facts that the IPDP occurrence exhibits a similar behavior and that the IPDP end frequency tends to increase with increasing MLT. In addition, data from geosynchronous spacecraft showed that IPDP occur when clouds of energy‐dispersed energetic protons pass through the meridian of the ground magnetometers. The spatial‐temporal correlation of IPDP with proton aurora arcs confirms the expectation that the proton arcs, like the proton spots and flashes, are the ionospheric image of the region where the ion cyclotron instability develops in the equatorial magnetosphere. In the case of IPDP the instability develops when drifting proton clouds resulting from particle injections in the night sector contact the plasmaspheric plume onto which the proton arcs map.