Radial evolution of corotating merged interaction regions and flows between ≈14 AU and ≈43 AU

Abstract

During 1993 and 1994 the solar coronal holes and the heliospheric current sheet were relatively stationary, and recurrent streams and interaction regions with periods of the order of the solar rotation period were present within 5 AU. One expects that during 1994 Voyager 2 (V2) (located at ≈43 AU and at ≈12°S latitude, in the sector zone) would have observed some evolutionary form of corotating streams and interaction regions. The “sector zone” is the latitude band in which a spacecraft observes both positive and negative sectors [Burlaga and Ness, 1996]. We present the observations of the magnetic field strength (B), the speed (V), density (N), and proton temperature (T) made by V2‐94, and for reference we also discuss the observations made by V2 at ≈14 AU during 1983 (V2‐83) a solar cycle earlier. Correlated, quasiperiodic variations in B and N with a period of ≈26 days (corotating merged interaction regions) were observed at ≈14 AU but not at ≈43 AU. The speed and temperature profiles were irregular at ≈14 AU but quasiperiodic at ≈43 AU. An ƒ−2 spectrum of the magnetic field strength B (indicating the dominance of shocks) was observed at ≈14 AU, but an ƒ−5/3 spectrum (indicating the dominance of Kolmogorov turbulence) was observed at ≈43 AU in the range (2.7 × 10−6 to 2.3 × 10−5)Hz. An ƒ−2.5 spectrum of the speed fluctuations was observed at ≈14 AU, but an ƒ−2 spectrum was observed at ≈43 AU in the range (8.8 × 10−7 to 2.3 × 10−5)Hz. We suggest the hypothesis that the qualitative differences between the observations at ≈14 AU and ≈43 AU represent a change in the state of the solar wind as it moves between these two positions. This change involves a transition from a quasiperiodic (ordered) state in B and N at ≈14 AU to a disordered state at ≈43 AU and from an aperiodic state in V and T at ≈14 AU to a quasiperiodic state at ≈43 AU. The standard MHD models for the radial evolution of corotating streams and interaction regions have not predicted such a transition. Our results suggest that it would be fruitful to develop a new MHD model of such flows, which should include (1) three‐dimensional effects, (2) the intermediate‐scale fluctuations, and (3) the interstellar pickup ion pressure.