R. J. Pellinen

JOINT TWO-DIMENSIONAL OBSERVATIONS OF GROUND MAGNETIC AND IONOSPHERIC ELECTRIC FIELDS ASSOCIATED WITH AURORAL ZONE CURRENTS: CURRENT SYSTEMS ASSOCIATED WITH LOCAL AURORAL BREAK-UPS

Abstract On 15 February, 1977, ground magnetic, ionospheric electric and auroral signatures of a multiple onset substorm were observed simultaneously by the Scandinavian Magnetometer Array (SMA), the Scandinavian Twin Auroral Radar Experiment (STARE) and the Finnish all-sky camera chain. Between 21:00 and 21:30 U.T., i.e. around local magnetic midnight, three consecutive local auroral break-ups were observed over Scandinavia. Each of these break-ups was preceded by a clear fading of the aurora and magnetic fields (while the electric fields remained unaffected), and occurred slightly south of the Harang discontinuity in the region of north-westward-directed electric fields. They were associated with a sudden change in direction of the electric field from north-west to south-west and the appearance of a westward equivalent current in the localized active region (about 1200 × 300 km 2 ). These observations matched the features to be expected during the generation of a Cowling channel by a strong increase of the ionospheric conductivities due to precipitating auroral electrons. Numerical model calculations, based on the observations during the initial brightening and peak development of the second, most conspicuous break-up, show that the field-aligned currents at the northern and southern border of the active region are indeed very weak. However, highly localized and intense upward field-aligned currents at the western edge of the active region and more widespread and less intense downward currents in the eastern half preserve current continuity of the westward Cowling current and complete the substorm current wedge.

Ionospheric convection response to slow, strong variations in a northward interplanetary magnetic field: A case study for January 14, 1988

We analyze ionospheric convection patterns over the polar regions during the passage of an interplanetary magnetic cloud on January 14, 1988, when the interplanetary magnetic field (IMF) rotated slowly in direction and had a large amplitude. Using the assimilative mapping of ionospheric electrodynamics (AMIE) procedure, we combine simultaneous observations of ionospheric drifts and magnetic perturbations from many different instruments into consistent patterns of high-latitude electrodynamics, focusing on the period of northward IMF. By combining satellite data with ground-based observations, we have generated one of the most comprehensive data sets yet assembled and used it to produce convection maps for both hemispheres. We present evidence that a lobe convection cell was embedded within normal merging convection during a period when the IMF By and Bz components were large and positive. As the IMF became predominantly northward, a strong reversed convection pattern (afternoon-to-morning potential drop of around 100 kV) appeared in the southern (summer) polar cap, while convection in the northern (winter) hemisphere became weak and disordered with a dawn-to-dusk potential drop of the order of 30 kV. These patterns persisted for about 3 hours, until the IMF rotated significantly toward the west. We interpret this behavior in terms of a recently proposed merging model for northward IMF under solstice conditions, for which lobe field lines from the hemisphere tilted toward the Sun (summer hemisphere) drape over the dayside magnetosphere, producing reverse convection in the summer hemisphere and impeding direct contact between the solar wind and field lines connected to the winter polar cap. The positive IMF Bx component present at this time could have contributed to the observed hemispheric asymmetry. Reverse convection in the summer hemisphere broke down rapidly after the ratio |By/Bz| exceeded unity, while convection in the winter hemisphere strengthened. A dominant dawn-to-dusk potential drop was established in both hemispheres when the magnitude of By exceeded that of Bz, with potential drops of the order of 100 kV, even while Bz remained northward. The later transition to southward Bz produced a gradual intensification of the convection, but a greater qualitative change occurred at the transition through |By/Bz| = 1 than at the transition through Bz = 0. The various convection patterns we derive under northward IMF conditions illustrate all possibilities previously discussed in the literature: nearly single-cell and multicell, distorted and symmetric, ordered and unordered, and sunward and antisunward.

Event study on pre-substorm phases and their relation to the energy coupling between solar wind and magnetosphere

Abstract On 11 November 1976, after a magnetically quiet period with the interplanetary magnetic field (IMF) directed northward, a sudden southward turning of the IMF immediately led to a world-wide intensification of convection which was observed to start almost simultaneously at stations within the auroral zone and polar cap. The two-dimensional equivalent current system over the northern hemisphere had a typical two-cell convection pattern with a maximum disturbance of ΔH = −300 nT observed on the morningside in the westward electrojet region. This enhancement of activity ended after 35 min in a localized substorm onset in the midnight sector over Scandinavia. The recordings made in this area indicate large fluctuations of various ionospheric parameters starting several minutes before the substorm onset. Two subsequent stages can be resolved: (1) high-energy particle precipitation recorded by balloon X-ray detectors and maximum ionospheric current density increase, while the electrojet halfwidth shrinks and the total electrojet current becomes weaker; (2) the maximum ionospheric current density stays constant and the high-energy particle precipitation decreases, while the auroral brightness increases and the total electrojet current and its half-width show a growing trend prior to the final breakup. A suggestion is made that the time interval of these two stages should be called “trigger phase”. A short discussion explains the trigger phase observations in a magnetospheric scale. The energy coupling between solar wind and magnetosphere during the pre-substorm phases is discussed by utilizing the energy coupling function ϵ defined by Perreault and Akasofu ( Geophys. J. R. Astr. Soc. 54 , 547, 1978). The ϵ values appear to be on substorm level during the period of enhanced convection. A good correlation between ϵ and the growth of the Joule heating rate (estimated from the AE data) is found in the beginning, but during the last 20 min before substorm triggering ϵ is high while the Joule heating rate decreases. The behaviour of ϵ during the two stages of the trigger phase suggests that the start of the trigger phase is purely internally controlled while the length of the trigger phase and the final substorm onset may be influenced by the variation in ϵ.

Inductive Electric Fields in the Magnetotail and Their Relation to Auroral and Substorm Phenomena

The paper reviews the importance of inductive electric fields in explaining different magnetospheric and auroral phenomena during moderately and highly disturbed conditions. Quiet-time particle energization and temporal development of the tail structure during the substorm growth phase are explained by the presence of a large-scale electrostatic field directed from dawn to dusk over the magnetotail. Conservation of the first adiabatic invariant in the neutral sheet with a small value of the gradient in the magnetic field implies that the longitudinal energy increases at each crossing of the neutral sheet. At a certain moment, this may result in a rapid local growth of the current and in an instability that triggers the onset. During the growth phase energy is stored mainly in the magnetic field, since the energy density in the electric field is negligible compared to that of the magnetic field (ratio 1: 107). An analytical model is described in which the characteristic observations of a substorm onset are taken into account. One major feature is that the triggering is confined to a small local time sector. During moderate disturbances, the induction fields in the magnetotail are stronger by at least one order of magnitude than the average cross-tail field. Temporal development of the disturbed area results in X- and O-type neutral lines. Particles near to these neutral lines are energized to over 1 MeV energies within a few seconds, due to an effective combination of linear and betatron acceleration. The rotational property of the induction field promotes energization in a restricted area with dimensions equivalent to a few Earths radii. The model also predicts the existence of highly localized cable-type field-aligned currents appearing on the eastern and western edges of the expanding auroral bulge. It is shown that the predictions agree with satellite observations and with the data obtained from the two-dimensional instrument networks operated in Northern Europe during the International Magnetospheric Study (IMS, 1976–79).