Meridional plasma flow in the outer heliosphere

Abstract

Voyager 2 observations made in the outer heliosphere near 25 AU and within 2° of the heliographic equatorial plane show periodic variations in the meridional (North/South) flow velocities that are much more prominent than the East/West variations. An autocorrelation analysis shows that the flow variation has an ∼25.5 day period in the latter half of 1986, in approximate agreement with the solar rotation period. The turning of the flow from northward to southward is often accompanied by abrupt increases in solar wind speed and pressure consistent with changes expected at forward shocks, suggesting origins of non‐radial flow in stream dynamics. Approximately 80% of the time, the magnetic field data indicate that the northward flow is seen when Voyager is above the current sheet and southward flow when it is below, but the transitions from northward to southward and vice versa do not occur at the crossings of the current sheet. Thus increased pressure in interaction regions remains the best candidate for the driver of the non‐radial flows.