The Amplitude of Magnetohydrodynamic Turbulence in the Inner Solar Wind

Abstract

Recent radio wave interplanetary scintillation observations have improved our knowledge of the level of turbulent fluctuations of plasma density in the inner solar wind at heliocentric distances of 16-26 R and less accurately outside this range. This information relates to fluctuations in the inertial subrange of the turbulence. Of particular concern in this paper is the heliocentric distance dependence of the power-law normalization parameter C 2 N . This measurement of C 2 N , when combined with independent measurements of the mean plasma density in the inner solar wind, indicates that the dimensionless amplitude of the turbulence n =n n is low, of the order of 6%-15%, where n is the standard deviation of the density fluctuations and n n is the mean value. The deduced value for depends on an assumed value for the outer scale of the turbulence, l o . I adopt a fiducial value of l o ¼ 1 R . A comparison is made with independent estimates of , from both radio remote sensing in the same part of space and in situ measurements at greater distances. Satisfactory agreement occurs. I discuss the possibility that this dimensionless density amplitude might be a proxy for the corresponding amplitude of the magnetic field fluctuations. This is a controversial assertion. However, if it is the case, the measurements suggest that the turbulence in the inner solar wind is weak, in the sense of being dynamically and thermodynamically important.

A review of solar type III radio bursts

“…deviation of the density turbulence in the fast solar wind at wavenumber k = 1.4 × 10 6 km −1 . The results for the slow solar wind density turbulence (Marsch & Tu, 1990;Woo et al, 1995) showed a constant level around 10 % which was also found in the later study from Spangler (2002). More recently Chen et al (2012Chen et al ( , 2013b have examined the slope of the density fluctuations power spectral density for 17 intervals of solar wind.…”

Monte Carlo Simulations in Solar Radio Astronomy

“…F o rt h es o l a rw i n d ,α ∼ 11/3 agrees with that of Kolmogorov derived for fluid turbulence (Spangler & Sakurai, 1995;Spangler et al , 2002;Spangler , 2002;Tu & Marsch , 1994;Wohlmuth et al , 2001;Woo et al , 1995). The inner scale l i (which is also known as the dissipative scale) increases linearly with heliocentric distance as l i =(…”

“…For a Kolmogorov spectrum with α = 11/3, this expression takes the form Spangler (2002). For the flat spectrum with α = 3, this becomes…”

Constraints on coronal turbulence models from source sizes of noise storms at 327 MHz

“…Lee and Jokipii, 1975;Rickett, 1977;Coles and Harmon, 1989;Bastian, 1994;Cairns, 1998;Spangler, 2002] …”

“…[9] The empirical model we use for C N 2 (R) was originally mooted by Armstrong and Woo [1980] and later refined, based on VLBI observations between 10 and 50 R , by, for example, Spangler and Sakurai [1995] and Spangler [2002]:…”