Plasma observations at the Earth's magnetic equator

Abstract

The magnetic equator provides a unique location for thermal plasma and plasma wave measurements. Plasma populations are found to be confined within a few degrees latitude of the equator, particularly the ions. The equatorially trapped ion population is found to be primarily hydrogen, and we find little evidence for preferential heating of heavier ions. Helium is occasionally found to be heated along with the protons, and forms about 10% of the equatorially trapped populations at such times, similar to the percentage of He+ in the cold, core plasma of the plasmasphere. One case of a heated O+ component was found; at the 0.1% level it generally comprises in the outer plasmasphere core plasma. The heated H+ ions can be characterized by a bi‐Maxwellian with kT∥ = 0.5–1.0 eV, and kT = 5–50 eV, with a density of 10–100 cm−3. The total plasma density, as inferred from the plasma wave instrument measurements of the upper hybrid resonance (UHR), is relatively constant with latitude, occasionally showing a local minimum at the magnetic equator, even though the ion flux has increased substantially. The first measurements of the equatorially trapped plasma and coincident UHR measurements show that the trapped plasma is a feature of the plasmapause region, found at total plasma densities of 20–200 cm−3. The warm, trapped plasma is found in conjunction with equatorial noise, a plasma wave feature found at frequencies near 100 Hz, with a broad spectrum generally found between the proton gyrofrequency at the low frequency edge and the geometric mean gyrofrequency at the high frequency edge. This latter frequency is generally the lower hybrid resonance (LHR) for a proton‐electron plasma. Sharp spatial boundaries are occasionally found with latitude, delimiting the equatorially trapped plasma. In such cases, the equator is a region with a relative minimum in density, and it appears that field‐aligned ions found at higher latitudes are “bounced” from these boundaries, indicating a positive plasma potential of a volt or two in the equatorial region.