Observations and Analysis of Ionospheric Electrodynamics during a Substorm Recovery Phase Using the Miracle Network

The luminous tongues tend to be narrower and having We present ground-based electromagnetic data from the may occasionally develop into auroral torches which are MIRACLE and BEAR networks and satellite optical narrow, finger-like auroral forms that extend several observations from the UVI and PIXIE instruments on the degrees of latitude poleward from the auroral oval. Inside Polar satellite of an omega band event over Northern the tongues which are primarily deformations of the Scandinavia on June 26, 1998, which occurs close to the diffuse auroral boundary, also discrete aurora can be morning side edge of a substorm auroral bulge. Our found, especially at the tongues' poleward boundary. analysis of the data concentrates on one omega band Pulsating aurora has been observed inside torches. The period from 0318-0327 UT, for which we use the method typical longitudinal as well as latitudinal extent of the of characteristics combined with an analysis of the UVI tongues amounts to about 400-500 km. Omega bands and PIXIE data to derive a time series of instantaneous, have been found to grow simultaneously over a solely data-based distributions of the mesoscale longitudinal range of several magnetic local time (MLT) ionospheric electrodynamic parameters with a 1-min time hours, and have also been observed simultaneously in resolution. Our results show that zonally alternating both hemispheres (Mravlag et al., 1991). regions of enhanced ionospheric conductances The auroral forms are drifting eastward with velocities (“tongues”) up to ~ 60 S and low conductance regions are ranging between 400 and 2000 ms , with a tendency of associated with the omega bands. The tongues have a increasing drift speed with time. These velocities have poleward extension of ~ 400 km from their base and a been found to agree closely with the local drift zonal extension of ~ 380 km. While they are moving velocity. During their eastward drift, the shape of the coherently eastward with a velocity of ~ 770 ms , the auroral forms is essentially preserved, thus indicating a -1 structures are not strictly stationary. The current system of mostly stationary structure in the frame of reference the omega band can be described as a superposition of comoving with the forms. A ground magnetometer two parts: One consists of anticlockwise rotating Hall positioned below the periodic, stationary and moving currents around the tongues, and Pedersen currents with a auroral omega bands measures Ps6 pulsations, i.e., negative divergence in their centers. The sign of this magnetic disturbances with periods of 5-40 min, and system is reversing in the low conductance areas. It amplitudes that may vary from 10 to over 1000 nT. The causes the characteristic ground magnetic signature. The causal relation between the auroral and the magnetic second part consists of zonally aligned current wedges of phenomena was first pointed out by Saito (1978). The westward flowing Hall currents and is mostly ground magnetic pulsations are most prominent in the magnetically invisible below the ionosphere. This system magnetic Y (eastward) component, in which the maxima dominates the field-aligned current (FAC) pattern and are more pronounced than the minima. The maxima in Y causes alternating upward and downward FAC at the occur between respective minima and maxima in the Z flanks of the tongues with maximum upward FAC of ~ 25 (vertical downward) component related to the same μA m . The total FAC of ~ 2 MA are comparable to the omega band, where the Z minima are leading the Y -2 ones diverted inside a westward traveling surge (WTS). maxima by a phase shift of approximately 90 degrees. In Throughout the event, the overwhelming part of the FAC the X (northward) component, the pulsations are also are associated with gradients of the ionospheric often visible, but they are typically quite disturbed due to conductances, and 66-84% of the FAC are connected with the temporal variation of the substorm electrojet intensity ionospheric Hall currents. during the recovery phase. For more details, see the recent