The radial evolution of corotating merged interaction regions and flows between
» 14 AU and » 43 AUL. F. Burlaga, N. F. Ness, J. W. Belcher
Laboratory for Extraterrestrial Physics, NASA-Goddard Space Flight Center, Greenbelt, MD 20771
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 (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. 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. Quasi-periodic 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 quasi-periodic at » 43 AU. An f -2 spectrum of the magnetic field strength B (indicating the dominance of shocks) was observed at » 14 AU, but a f -5/3 spectrum (indicating the dominance of Kolmogorov turbulence) was observed at » 43 AU in the range (2.7 ´ 10-6 - 2.3 ´ 10-5 )Hz. An f -2.5 spectrum of the speed fluctuations was observed at » 14 AU but a f -2 spectrum was observed at » 43 AU in the range (8.8 ´ 10-7 - 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 of state has the nature of a phase transition from an ordered (quasi-periodic) state in B and N at » 14 AU to a disordered state at » 43 AU and from an irregular state in V and T at » 14 AU to an ordered state at » 43 AU. The standard MHD models for the radial evolution of corotating streams and interaction regions do not predict such a transition. Our results suggest the need for a new MHD model of such flows, which should include 1) 3-D effects, 2) the intermediate-scale fluctuations, and 3) the interstellar pickup ion pressure.
J. Geophys. Res., 102, 4661, 1997.