Penetration of Hydromagnetic Energy Deep into the Magnetosphere

Magnetometer data, acquired simultaneously at high and low latitudes on spacecraft and on the ground, are compared in order to study the propagation characteristics of hydromagnetic energy deep into the magnetosphere. Single events, such as one on October 9, 1985, show that the wave amplitudes at Tuckerton and Green Hill (L-3) may be only one order of magnitude lower than at South Pole (L-14). In addition, statistical analyses of the H-component data obtained during local daytime hours for July 17 to August 3, 1985 show that wave amplitudes at Green Hill (L-3) are generally 1-2 orders of magnitude lower than at Iqaluit (L-14). The L-dependence of near-equator magnetic field fluctuations measured on ISEE-2 also shows gradual fall-off of energy inside the magnetosphere. Such high levels of wave power deep in the magnetosphere have not been previously understood. Our initial attempt is to calculate the decay length of an evanescent wave generated at a thick magnetopause boundary. Numerical calculations show that a fast magnetosonic mode can be generated under very restrictive conditions for the field and plasma parameters. This fast or inner mode may have its amplitude reduced by only one order of magnitude over a penetration depth of about 8 R sub E. More realistic numerical simulations need to be carried out to attain better agreement with the data.