Midday Auroral Breakup

Abstract

Ground-based observations of the midday aurora by all-sky TV and meridian scanning photometers reveal the intermittent occurrence of discrete auroral displays within the cusp/cleft. A typical sequence includes the following features: Auroral brightening, near the equatorward boundary of the persistent cusp/cleft arc and subsequent poleward motion of discrete forms through the cusp/cleft region. A strong westward component of auroral motion, both of the individual forms and internal ray structures within these forms, is often observed. At maximum brightness, green line intensities of -10 kilo-rayleighs are observed, even within the interval which is otherwise characterized as the midday gap. The duration of the whole sequence is normally less than 10 minutes. During this period the auroral activity moves poleward, usually by a few degrees, in some cases by 3-5 degrees, say from 71° up to 75° MLAT. Characteristic ground magnetic signatures are observed, including a 50-100nT positive deflection in the H-component and a negative Z-component at stations located poleward of the initial brightening. A poleward propagating filamentary Hall current belt associated with the discrete aurora is inferred from the optical and magnetic data. A quantitative estimate shows that the conductivity enhancement due to electron precipitation in conjunction with northward electric field roughly accounts for the magnetic deflection on the ground. Series of such events are often observed when the cusp is located at rather low latitudes, say south of 75° MLAT, associated with negative IMF B_z.