Daytime high‐latitude auroral pulsations: Some morphological features and the region of the magnetospheric source

We present the results of a statistical study of auroral pulsations observed at cusp and cleft latitudes as a function of solar wind and geomagnetic conditions. Pulsation periods range from 10 to 40 s but are modulated by longer‐period variations ranging from 3 to 5 min. The auroral pulsations are generally accompanied by geomagnetic pulsations, which occur almost every day, whereas the auroral pulsations can only be seen on 20 to 30% of all days studied. The auroral pulsations show a distinct tendency to occur during geomagnetically quiet periods when the interplanetary magnetic field (IMF) has dawnward and northward or only slightly southward components. The auroral pulsations are most common from 0800 to 1100 MLT at 74°–75° geomagnetic latitude. The shorter‐period pulsations have latitudinal extents ranging from several 10 s to 100 km, and longitudinal extents of several hundred kilometers. The auroral pulsations show no tendency to occur during intervals of enhanced solar wind velocities or densities, or during intervals of near‐radial IMF orientation. Case and statistical studies comparing the latitude at which the pulsations occur with observations by low‐altitude DMSP F7 and NOAA 7 spacecraft demonstrate that they occur deep within the magnetosphere on closed magnetic field lines immediately equatorward of the high‐latitude trapping boundary for energetic particles. Consequently, we conclude that they do not result from magnetic merging on the dayside magnetopause, the Kelvin‐Helmholtz instability, or the transmission of pulsations generated within the Earth's foreshock through the magnetosheath and into the magnetosphere. The origin of the auroral pulsations remains unknown, although it may ultimately be possible to explain them in terms of instabilities at the outer edge of the radiation belt.