6 Compressive Turbulence
Interplanetary medium is slightly compressive, magnetic field intensity and proton number
density experience fluctuations over all scales and the compression depends on both the scale
and the nature of the wind. As a matter of fact, slow wind is generally more compressive than
fast wind, as shown in Figure 63 where, following Bavassano et al. (1982a) and Bruno and
Bavassano (1991), we report the ratio between the power density associated with magnetic field
intensity fluctuations and that associated with the fluctuations of the three components. In
addition, as already shown by Bavassano et al. (1982a), this parameter increases with heliocentric
distance for both fast and slow wind as shown in the bottom panel, where the ratio between the
compression at and that at is generally greater than . It is also interesting to
notice that within the Alfvénic fast wind, the lowest compression is observed in the middle
frequency range, roughly between . On the other hand, this frequency range
has already been recognized as the most Alfvénic one, within the inner heliosphere (Bruno
et al., 1996).
As a matter of fact, it seems that high Alfvénicity is correlated with low compressibility of the medium
(Bruno and Bavassano, 1991; Klein et al., 1993; Bruno and Bavassano, 1993) although compressibility is
not the only cause for a low Alfvénicity (Roberts et al., 1991, 1992; Roberts, 1992).
||The first two rows show magnetic field compression (see text for definition) for fast (left
column) and slow (right column) wind at (upper row) and (middle row). The
bottom panels show the ratio between compression at and compression at . This
ratio is generally greater than for both fast and slow wind.
The radial dependence of the normalized number density fluctuations for the inner and outer
heliosphere were studied by Grappin et al. (1990) and Roberts et al. (1987b) for the hourly frequency
range, but no clear radial trend emerged from these studies. However, interesting enough, Grappin
et al. (1990) found that values of were closely associated with enhancements of on scales longer
On the other hand, a spectral analysis of proton number density, magnetic field intensity, and proton
temperature performed by Marsch and Tu (1990b) and Tu et al. (1991) in the inner heliosphere, separately
for fast and slow wind (see Figure 64), showed that normalized spectra of the above parameters within slow
wind were only marginally dependent on the radial distance. On the contrary, within fast wind, magnetic
field and proton density normalized spectra showed not only a clear radial dependence but also similar level
of power for . For larger these spectra show a flattening that becomes steeper for
increasing distance, as was already found by Bavassano et al. (1982b) for magnetic field intensity.
Normalized temperature spectra does not suffer any radial dependence neither in slow wind nor in fast
Spectral index is around for all the spectra in slow wind while, fast wind spectral index is
around for and slightly less steep for larger wave numbers.
||From left to right: normalized spectra of proton temperature (adopted from Tu
et al., 1991), number density, and magnetic field intensity fluctuations (adopted from Marsch and
Tu, 1990b, © 1990 American Geophysical Union, reproduced by permission of American Geophysical
Union) Different lines refer to different heliocentric distances for both slow and fast wind.