EMPIRICAL POLAR-CAP POTENTIALS

DMSP satellite plasma flow data from 1987-1990 are used to derive empi rical models of the polar cap potential for quasi-steady interplanetar y magnetic field (IMF) conditions. The large data set, due to the high duty cycle and nearly Sun synchronous DMSP orbits, allowed very strin gent data selection criteria. The analysis indicates that a good descr iption of the unskewed (Heppner Maynard pattern A) steady state polar cap potential is Phi(A) = 10(-4)nu(2)+11.7B sin(3) (theta/2) kV, where nu is the solar wind velocity in kilometers per second, B is the magn itude of the interplanetary magnetic field in nanoteslas, and theta = arccos (B-Z/\B\)(GMS). The IMF-dependent contribution to the cross pol ar cap potential does not depend significantly on solar wind pressure. Functional forms for the potential do benefit from inclusion of an IM F independent term proportional to the solar wind flow energy. Best fi ts to IMF-independent contributions to the steady state polar cap pote ntial yield similar to 16 kV for nu(SW) = 400 kilometers per second. D uring steady IMF the total unskewed polar cap potential drop is shown to be approximately Phi(A) = 16.5 + 15.5 Kp kV. The distribution of po tential around the polar cap is examined as a function of magnetic loc al time. A sinusoidal distribution is an excellent description of the distribution, and more complex forms are not justified by this data se t. Analysis of this data set shows no evidence of saturation of the po lar cap potential for large \IMF\. A simple unified description of the polar cap potential at all magnetic local times (MLT) and IMF, Phi(IM F, MLT) = -4.1 + 0.5 sin ((2 pi/24) MLT + 0.056 + 0.015 B-Y(eff))(1.1 x 10-4 nu(2) + 11.1 B sin(3) (theta/2)) kV, is generated, where B-Y(ef f) is B-Y (-B-y) in the northern (southern) hemisphere. If IMF data is unavailable, the polar cap potential is well described by Phi(A)(Kp, MLT) = -4.1 +1/2 sin ((2 pi/24) MLT +phi(HM))(16.4 + 15.2 Kp) kV, wher e phi(HM) is a small phase correction of (-0.054, -0.031, 0.040) for H eppner-Maynard convection patterns (BC, A, DE), respectively.