Power spectra of the magnetospheric electric field

Abstract

Fluctuations of the magnetospheric electric field with periods between 1 minute and 1 day have been studied by analyses of more than 200 power spectra generated from about 700 hours of electric field data obtained by flight of 44 balloons at sites ranging in L from 2.8 to 23. Arguments that include the variations of these power spectra with magnetic activity, local time, and latitude are advanced to show that fluctuations of ionospheric and not weather-associated electric fields have been measured. The variations of the ionospheric power spectra with latitude and local time are consistent with a uniform power (independent of local time and L for 8≥L≥3) in the equatorial electric field of magnitude Pi(ν) = (100−70+200)(exp 0.4 Kp) (ν−1.6±0.3), where Pi (ν) is the power (millivolts2/meter2 hertz) of the ith perpendicular component of the equatorial electric field at a frequency of ν cycles/hour. The uncertainties in this expression are standard deviations in single determinations of power spectra from 6-hour segments of data and represent real variations in the electric field power. The fluctuations in the two perpendicular components are anticorrelated such that the power in the total field is generally less than that of the larger component. The broadband root mean square equatorial electric field strength of either component obtained from the above equation is about 0.6 mvolts/m for Kp=2, a value indicating that a significant fraction of the quasistatic magnetospheric electric field arises from long-period fluctuations and that the field is therefore extremely turbulent. Since the equatorial power does not decrease inside the plasmapause, magnetospheric electric field fluctuations must penetrate efficiently into the outer plasmasphere. The measured power in the electric field fluctuations is at least an order of magnitude larger than that which would be obtained in a completely open magnetosphere from fluctuations of the interplanetary electric field if it convected freely through the magnetosphere. The measured power spectra are adequate to drive the radial diffusion of trapped energetic charged particles in the Van Allen belts although detailed comparisons between electric field power spectra and particle diffusion rates require further information on scale lengths over which the fluctuating fields are correlated in longitude.