The relationship between interplanetary discontinuities and Alfvén waves: Ulysses observations

Abstract

The rate of occurrence of interplanetary discontinuities (ROID) is examined using Ulysses magnetic field and plasma data from 1 to 5 AU radial distance from the Sun and at high heliographic latitudes. We find two regions where the ROID is high: in stream-stream interaction regions and in Alfvén wave trains. This latter feature is particularly obvious at high latitudes when Ulysses enters a high speed stream associated with a polar coronal hole. These streams are characterized by the presence of continuous, large-amplitude (ΔB→/|B|∼1-2)Alfvén waves and an extraordinarily high ROID value (∼150 discontinuities/day). In a number of intervals examined, it is found that (rotational) discontinuities are an integral part of the Alfvén waves. The nonlinear Alfvén waves are spherically polarized, i.e., the tip of the perturbation vector resides on the surface of a sphere (a consequence of constant |B|). The slowly rotating part of the wave rotates ∼270° in phase. There is a slight arc in the B₁-B₂ hodogram, suggesting an almost linear polarization. The phase rotation associated with the discontinuity is ∼90°, lies in the same plane as the slowly rotating part of the Alfvén wave, and therefore completes the 360° phase rotation. The best description of the overall Alfvén wave plus discontinuity is a spherical, arc-polarized, phase-steepened wave.