A quantitative description of the spatial distribution and dynamics of the energy flux in the continuous aurora

Abstract

Energy flux in the continuous (or diffuse) aurora is found to be constant instantaneously along contours that are defined analytically as a function of magnetic latitude and UT, coordinates that are approximately circles in magnetic latitude‐magnetic local time centered at a pole offset from the magnetic pole. As a consequence, the latitudinal distribution in these coordinates is the same instantaneously at all magnetic local times (the observed extent of which is 10 hours); changes that take place in the distribution do so simultaneously (within 15 min of UT resolution) throughout this entire extent. These properties are observed in a case study of 8 hours duration by an array of ground and air based ionospheric sounders that span the 10 hours of magnetic local time at auroral latitudes. Energy flux in the continuous aurora is derived from the resulting ionized auroral E layer critical frequency that is measured simultaneously at 15 min intervals by all members of the array. Two latitudinal distributions are identified. The principal one is a Gaussian distribution at lower latitude; a separate distribution that is of lower intensity occurs at higher latitude. The two correspond respectively to the central plasma sheet and boundary plasma sheet regions of particle precipitation. The central plasma sheet distribution is found in all 32 determinations to be well described by a single Gaussian function, 3.2° full width at half maximum. Accordingly, the dynamics of the entire latitude‐local time distribution can be described quantitatively by the time history of two parameters, the Gaussian maximum energy flux (which varies between extremes of 0.25 and 7 ergs/cm²s) and the latitude of the maximum (which varies throughout a range of 5°). The determination of these parameters every 15 min resolves such rapidly varying features as substorm onset; the 8 hour duration encompasses such long term features as substorm lifetime and UT dependent latitude variation. A number of areas of agreement are found between results reported here and those reported by others. In particular, evidence that the continuous aurora is highly uniform exists not only in this case study but also in statistical studies that find iso‐energy flux contours to be offset circles.