Evolution of Magnetic Fields in Corotating Interaction Regions from 1 to 95 AU: Order to Chaos

Abstract

We discuss the large-scale heliospheric magnetic field strength fluctuations as a function of distance from the Sun during the declining phase of a solar cycle, based on a one-dimensional, MHD, three-fluid model with observations made at 1 AU during 1995 as input. We consider daily averages of the magnetic field strength, B, as a function of time for a approximate to1 year interval. The model predicts that B(t) is quasi-periodic, and that the amplitudes of fluctuations in B relative to the yearly average of B ({[}B]) are relatively large between 5 and 20 AU ({''}the corotating merged interaction region zone,'' or CMIR zone). The model predicts that the fluctuations are aperiodic and that their amplitudes are relatively small between 30 and 95 AU ( the ``wave interaction region zone `'). It predicts a transition between these two zones at approximate to25 AU. These results are consistent with a conceptual model proposed by Burlaga in 1983 for the declining phase of the solar cycle. In the CMIR zone, neighboring CMIRs merge in a sequence of events that defines a topological tree. The model predicts the following statistical properties of the fluctuations in B/{[}B] for the declining phase of a solar cycle in the CMIR zone: ( 1) the power spectrum of B/{[}B] has a prominent peak at 26 days and a secondary peak at 13 days; ( 2) the distribution of B/{[}B] has no simple form; and ( 3) the standard deviation (SD) of B/{[}B] is relatively large and has a maximum of 1.2 at 10 AU. In the wave interaction zone, ( 1) the spectrum has no significant peak, and the power level at 26 days is an order of magnitude smaller than in the CMIR zone; ( 2) the distribution of B/{[}B] is approximately lognormal; and ( 3) the SD(B/{[}B]) is nearly constant, approximate to0.48. The SD(B/{[}B]) versus R shows that the transition between the CMIR and the wave interaction zones is at approximate to25 +/- 5 AU. The results of the model are consistent with the Voyager 1 (V1) observations near 15 and 55 AU during 1983 and 1994, respectively. During the declining phase of solar cycle 23, in approximate to2003 - 2005, Voyager 2 (V2) will move from 69 to 76 AU while V1 moves from 87 to 95 AU. The model makes the following predictions for the properties of B between 65 and 95 AU: ( 1) B( t) will be aperiodic, with relatively low amplitude fluctuations similar to those observed at 55 AU; ( 2) the power spectra will resemble those observed near 55 AU, and there will be little radial variation in the power at the solar rotation period, 26 days; ( 3) the distribution of B will be approximately lognormal; (4) the SD(B) of the daily averages of B will be approximate to0.48 {[}B]; and (5) the tail of the distribution of B will be exponential.