Large‐Scale Magnetic Field Fluctuations and Development of the 1999–2000 Global Merged Interaction Region: 1–60 AU

Abstract

We use a multifluid, spherically symmetric, MHD model with neutral atoms and pickup protons, with 1999 solar wind data at 1 AU as input, to calculate the magnetic field strength (B) profiles that would be observed at various points between 1 and 60 AU with a resolution of 1 day over an interval of ~1 yr. These temporal profiles show radial evolution of the multiscale fluctuations in B near solar maximum. From the daily points in these profiles, one finds the following statistical results for the radial evolution of daily averages of B: (1) the distribution functions of B are approximately lognormal at all distances from 1 to 60 AU; (2) the standard deviation of B divided by the mean value of B, B, for the magnetic field profile at a given distance is approximately a constant, independent of distance between 10 and 60 AU; and (3) the power spectrum of B/B evolves such that (a) at small scales the power spectral density decreases with increasing distance from the Sun, (b) at large scales the power spectral density increases with distance, and (c) there is a range of frequencies in which the power spectrum is a power law, the power law extending to ever lower frequencies with increasing distance. All three of these results have been observed by the Voyager 1 and 2 spacecraft over the last 25 yr, between 1 and ~60 AU. The radial evolution of the multiscale changes in B/B is described by analyzing the normalized changes in B at different scales, dBn. The multiscale structure of the probability distributions of dBn changes qualitatively with increasing distance from the Sun. The standard deviation surface, SDn(n, R), shows (1) a ridge at 5 AU (which is a function of scale n), corresponding to the development of shocks and interaction regions at large and intermediate scales; (2) a second ridge at a scale of 64 days (which is a function of R), corresponding to the formation, growth, and initial decay of a large-scale, global merged interaction region; and (3) a valley corresponding to the decrease in the amplitude of the fluctuations in B/B over an increasing range of scales with increasing distance from the Sun.