Millstone Hill incoherent scatter observations of auroral convection over 60° < Λ < 75°: 3. Average patterns versus Kp

Abstract

The Millstone Hill radar, Westford, Massachusetts (Λ = 56°), has been upgraded for observations in support of the International Magnetospheric Study (IMS) by means of the addition of a 150‐ft. diameter fully steerable antenna. This antenna allows measurements of F‐region ion drifts, from which F‐region electric fields can be derived, using the existing UHF (68‐cm wavelength) radar. This paper describes analytical models of the mean electric field behavior in the auroral zone over the interval 60° < Λ < 75° as determined from our measurements made during the year 1978. There was a greater preponderance of summer data in our 1978 observations and also more periods in which the inferred interplanetary magnetic field was directed toward the sun. A separate model is obtained for each of the three Kp ranges (< 2−, 2o to 3−, > 3o). Each model incorporates a polynomial description of intensity with invariant latitude and a sinusoidal harmonic variation in local time with 102 degrees of freedom for each of the perpendicular electric field components. Coefficients of this model are provided for numerical computation by users. Tabulations of these fields and a computer program to calculate fields and potentials are available on request from the authors. The results are compared with models published by Heppner [1977]. Each of these average field models is consistent with a two‐cell convection pattern having antisunward plasma flow over the pole and return paths eastward and westward through the dawn and dusk meridians. The centers of the vortices are near the far limit of observation (Λ = 75°). The entire convection pattern appears to be rotated by as much as two hours toward earlier local times from the nominal noon‐midnight over‐the‐pole flow direction, though this conclusion is influenced strongly by our least reliable (highest latitude) data. The dusk cell is dominant in size and strength; however, both the pattern rotation and cell dissimilarity lessen as Kp increases. The Harang discontinuity, which was recognizable on individual days, is largely absent in the average patterns, presumably as a result of the variability in its position. Nevertheless, we do see a residual, low‐speed, late‐evening protrusion of the dusk cell extending toward lower latitudes with increasing local time, reaching as far as 0300 LT for low Kp but only to midnight for high Kp. For low Kp, we observe a region of very small average flow from dawn to noon, apparently disconnected from the overall convection pattern. The dawn cell encroaches into this region as Kp increases, but the 1000–1200 LT sector maintains its low‐speed character even for high Kp. The cross‐polar‐cap potential varies linearly with Kp as V = 10.06 + 14.44 Kp kV, but the smoothing in our observations may have caused this to be an underestimate. Penetration of auroral electric fields to mid‐latitudes is most likely to occur and to be strongest in the evening sector.