The Hidden Ion Population - Revisited

Satellite potentials in the outer plasmasphere range from near zero to +5 to +10 V. Under such conditions ion measurements may not include the low energy core of the plasma population. In eclipse, the photoelectron current drops to zero, and the spacecraft potential can drop to near zero volts. In regions where the ambient plasma density is below 100 cm, previously unobserved portions of the ambient plasma distribution function can become visible in eclipse. A survey of the data obtained from the Retarding Ion Mass Spectrometer (RIMS) on Dynamics Explorer 1 shows that the RIMS detector generally measured the isotropic background in both sunlight and eclipse in the plasmasphere. Absolute density measurements for the "hidden" ion population are obtained for the first time using the Plasma Wave Instrument observations of the upper hybrid resonance. Agreement in total density is found in sunlight and eclipse measurements at densities above 80 cm. In eclipse, agreement is found at densities as low as 20 cm. The isotropic plasma composition is primarily H+, with ~10% He+, and 0.1 to 1.0% 0+. A low energy field-aligned ion population appears in eclipse measurements outside the plasmasphere, which is obscured in sunlight. These field-aligned ions can be interpreted as fieldaligned flows with densities of a few particles per cm3, flowing at 5-20 km/s. The problem in measuring these field-aligned flows in sunlight is the masking of the high-energy tail of the field-aligned distribution by the isotropic background. Effective measurement of the core of the magnetospheric plasma distributions awaits satellites with active means of controlling the satellite potential.