Polar Cap Potential Drop Research Articles

Theoretical magnetograms based on quantitative simulation of a magnetospheric substorm

Using substorm currents derived from the Rice computer simulation of the substorm event of September 19, 1976, we have computed theoretical magnetograms as a function of universal time for various stations. A theoretical Dst has also been computed. Our computed magnetograms were obtained by integrating the Biot‐Savart law over a maze of approximately 2700 wires and bands that carry the ring currents, the Birkeland currents, and the horizontal ionospheric currents. Ground currents and dynamo currents were neglected. Computed contributions to the magnetic field perturbation from eleven different kinds of currents are displayed (e.g., ring currents, northern hemisphere Birkeland currents). On the basis of comparison of theoretical results with corresponding observations, we make the following remarks. First, overall agreement of theory and data is generally satisfactory, especially for stations at high and mid‐magnetic latitudes. Second, model results suggest that the ground magnetic field perturbations arise from very complicated combinations of different kinds of currents and that the magnetic field disturbances due to different but related currents often cancel each other, despite the fact that complicated inhomogeneous conductivities in our model prevent rigorous application of Fukushima's theorem. Third, both the theoretical and observed Dst decrease during the expansion phase of the substorm, but data indicate that Dst relaxes back toward its initial value within about an hour after the peak of the substorm. This effect exists qualitatively in the computer simulation if we sharply reduce the assumed polar cap potential drop and conductivity at the end of the substorm. Fourth, the dawn‐dusk asymmetry in the horizontal component of magnetic field disturbance at low latitudes in a substorm is essentially due to a net downward Birkeland current at noon, net upward current at midnight, and generally antisunward flowing electrojets; it is not due to a physical partial ring current injected into the duskside of the inner magnetosphere.

Correction to ‘Dependence of polar cap potential drop on interplanetary parameters’

Correction to ‘Dependence of polar cap potential drop on interplanetary parameters’

Dependence of polar cap potential drop on interplanetary parameters

We have computed the convection potential drop across the polar cap from data obtained on high‐inclination low‐altitude satellites (AE‐C, AE‐D, S3‐3) and correlated these potential measurements with various combinations of parameters measured simultaneously in the upstream solar wind. These combinations of solar wind parameters consist of predictions based on magnetic merging theory and suggestions based on earlier empirical work. We find that the bulk of the potential drop, and its variation with interplanetary magnetic field (IMF) parameters, are successfully predicted by merging theory (to the accuracy with which they can presently be measured), but that a significant ‘background’ potential drop (∼35 kV) does not depend on IMF parameters and may thus be attributed to an unknown process other than merging. Our results indicate that small values of the IMF are amplified by a factor of 5–10 at the dayside magnetopause as a combined effect of bow shock compression and the Zwan‐Wolf depletion layer effect; correlations between IMF parameters and the polar cap potential drop are dramatically improved when this amplification is taken into account. The potential drop is better correlated with IMF parameters than with geomagnetic activity indices, presumably because the latter are affected by nonlinear reponses of the magnetosphere to the polar cap input.

High-latitude electric fields and the three-dimensional interaction between the interplanetary and terrestrial magnetic fields

More than 200 hours of ionospheric electric field measurements taken on balloons flown from three polar cap sites have been analyzed to determine average properties of the large-scale polar cap electric field and its dependencies on the interplanetary magnetic field. The major component of this electric field was directed from dawn to dusk and produced an average polar cap potential drop of about 55 kV. The magnitude of this potential provides an upper limit of about 700 RE for the length of the magnetospheric tail and implies an energy input from the solar wind to the magnetosphere of about 5 × 1019 ergs/sec. The dawn to dusk component of the high-latitude polar cap electric field responds to Bz, the northward component of the interplanetary magnetic field, on a time scale ≲1 hour and with an average increase of about 3 mV/m for each 1 γ decrease of Bz. The hourly averages of the electric field data at each of the three sites are well described by a two-cell convection pattern whose location depends on the y component of the interplanetary magnetic field. When By is positive (negative), the two-cell convection pattern shifts toward dawn (dusk) in the northern hemisphere with the following consequences: the maximum intensity of the northern polar cap dawn to dusk electric field component occurs at local morning (evening), and the auroral zone return flow reaches higher latitudes in the evening (morning). Evidence of the vector nature of the interaction between interplanetary and terrestrial magnetic fields is provided by the observation that the above By dependent signatures are most evident when Bz is most negative.

Electric field variations during substorms: OGO-6 measurements

The OGO-6 electric field measurements make it clear that the general pattern of high latitude electric fields in magnetic time-invariant latitude coordinates is not highly variable and that when unusual variations, or field distributions, occur they are relatively isolated in time and spatial extent. Thus, electric field changes on a global scale cannot, in general, be invoked as a direct cause of substorms. Polar traverses along the 1800 to 0600 magnetic time meridian show that the sum of potential drops across the evening auroral belt (poleward E) and morning auroral belt (equatorward E) approximately equals the potential drop across the polar cap (dawn-dusk E). The integrated polar cap potential drop ranges from 20 to 100 keV and values in the center of this range (e.g. 40–70 keV) are most common under conditions of moderate magnetic disturbance (e.g. Kp = 3). Roughly near 1800 magnetic local time, a latitudinally narrow strip at the transition between auroral belt and polar cap fields exhibits unusually large field fluctuations immediately following the sudden onset of a negative bay at later magnetic local times. It appears likely that this spatially isolated correlation is related to an effect rather than a cause of substorm enhancement. Other indications of direct correlation between electric field behavior and stages of magnetic bay development have not been found to be repeatable for multiple cases. These preliminary studies do not, however, rule out the possibility that more subtle relationships will be found in future studies.