Neptune's polar cusp region: Observations and magnetic field analysis

Abstract

Upon entering Neptune's magnetosphere, Voyager 2 penetrated its distant polar cusp on August 24, 1989. At that time the planet's magnetic dipole axis was approximately parallel to the Sun‐planet line. Broadly speaking, the spacecraft on the day before closest approach traversed four plasma regions. These (and the “boundaries” separating them) are (1) solar wind (bow shock at 1438 UT), (2) magnetosheath (inner boundary of the magnetosheath, the magnetopause (MP), at 1800 UT), (3) cusp region (outer boundary of magnetosphere at 1930 UT), and (4) the magnetosphere. The MP is identified primarily by a 45° directional discontinuity in the magnetic field but also by a decrease in the flux of antisunward flowing plasma, and a change in flow direction toward the corotation direction; it is approximately where the global MP would be expected based on upstream conditions. It is shown that the MP boundary was a rotational discontinuity whose normal nMP was a severe ≈77° from the planet‐Sun line. Its thickness was 5.1×104 km (i.e., 24 proton gyroradii, consistent with MPs of the other magnetic planets). The boundary was moving at U = 148 km/s, relative to the Sun‐planet line, along nMP, in a sense opposite to the approximate rigid motion of the outer magnetosphere and being dominated by the speed of an Alfven wave moving into the magnetosphere. Plasma β drops from 1.21 to 0.73 across the MP, going from outside to inside. Total pressure balance was shown to exist across the boundary, and the normal momentum flux change was apparently balanced by forces related to the change in thermal anisotropy and the normal field according to MHD theory. Energetic (⪞28 keV) ion intensities peaked at ≈1800 UT and then decreased prior to a second onset at ≈2000 UT, while the electrons (⪞22 keV) showed only a marginal increase. The field in the cusp changed direction slowly and smoothly, approaching the magnetospheric field direction at its inner boundary. The plasma velocity and density changed very moderately as the spacecraft entered the cusp region. However, the quasi‐steadiness of these quantities in the sheath (on average) disappears as the cusp is entered, just as was true of the magnetic field. Similar magnetic field and plasma profiles in the vicinity of the Earth's distant polar cusp have been observed by Earth orbiting spacecraft.