Showing papers by "Arthur D. Richmond published in 1982"

Global distribution of ionospheric and field‐aligned currents during substorms as determined from six IMS meridian chains of magnetometers: Initial results

Abstract: As a part of the joint efforts of operating six meridian chains of magnetometers during the IMS, magnetic records from 70 stations are used to deduce patterns of electric fields and currents in high latitudes on March 17, 18, and 19, 1978. First of all this data set is used to examine the fidelity of the AE(12) index by comparing it with the AE(70) index and also the fidelity of the AL(70) index as a measure of the total westward electrojet intensity. The coefficients for the two correlations are found to be more than 0.8. Then the distribution of both ionospheric currents and field-aligned currents, as well as the electric field, are for the first time determined with a time resolution of 5 min by using an appropriate conductivity distribution model. Although much improvement is still needed for better accuracy, especially in the estimation of ionospheric conductivity, it has now become possible to study the growth and decay of the three-dimensional current system over the north polar region during individual magnetospheric substorms with sufficient time resolution. Our initial results show that the gross features of the instantaneous distributions of the ionospheric and field-aligned currents are remarkably similar to the daily average pattern during a very weak activity and at different substorm epochs and that the large-scale current pattern grows and decays systematically as a whole. There are, however, significant changes in local scales.

Ionospheric conductivity dependence of electric fields and currents estimated from ground magnetic observations

Abstract: Recent efforts in estimating the global distribution of the electric potential as well as of ionospheric and field-aligned currents on the basis of ground magnetic records are reevaluated. For this purpose, we have repeated crucial tests for the algorithms by using a data base from the IMS Alaska chain of magnetic observatories along with completely different ionospheric conductivity models. It is found that the ionospheric current patterns are only weakly dependent on the choice of the conductivity, while the calculated field-aligned currents are somewhat more sensitive and the electric field is quite sensitive to the assumed conductivity, as expected. A test is also conducted by using the calculated field-aligned currents as inputs to an inverse calculation to attempt to reproduce the original ground magnetic perturbations. The results were quite satisfactory, indicating that the numerical accuracy of the algorithms is adequate. These tests increase our confidence that ground magnetic records from a close network can be used to study the extent to which the magnetosphere and ionosphere are electrically coupled.

In‐situ generated gravity waves as a possible seeding mechanism for equatorial spread‐F

Abstract: Recent observations have shown that, prior to the onset of equatorial spread-F, there exists an altitude modulation in bottomside F-region electron density contours. It has generally been accepted that such a modulation is caused by internal-gravity waves propagating upward from below. In this brief note we suggest that gravity waves may be generated in situ, via the wind shear mechanism. Theoretical calculations and some observations indicate that large wind shears can occur during early evening hours in the low latitude thermosphere. We show that the wind shear may possibly generate gravity waves with horizontal wavelengths on the order of a few hundred kilometers, which is the scale of the observed electron density modulation.