Cross‐helicity and residual energy in solar wind turbulence: Radial evolution and latitudinal dependence in the region from 1 to 5 AU

Abstract

Abstract. Solar wind plasma and magnetic field measurements by Ulysses have been used to study magnetohydrodynamic turbulence in different hehospheric regions. Four intervals of six solar rotations have been analyzed. Two of them are on the ecliptic around 2 and 5 AU, respectively, one is at midlatitude near 5 AU, and the last one is at high latitude around 3 AU. Conditions on the ecliptic are those typical of high solar activity periods. The midlatitude interval is characterized by very strong gradients in the wind speed, due to an intermittent appearance of the wind coming from the polar coronal hole. In the high-latitude interval, fully inside the polar wind, the speed is steadily high. We investigated at three different scales (1, 4, and 12 hours) the level of correlation between velocity and magnetic field fluctuations, as given by the normalized cross-helicity, and the sharing of the fluctuation energy between its kinetic and magnetic component, as measured by the normalized residual energy. The observations on the ecliptic, while confirming previous findings based on Voyagers data, clearly indicate that the normalized crosshellcity is well different from zero also at distances as large as 5 AU. The midlatitude turbulence, when compared to that at low and high hellographic latitudes, appears much more evolved, with a remarkably lower normalized cross-helicity (in absolute value). This unambiguously highlights that processes at velocity gradients are an important factor in the turbulence evolution. For all the analyzed intervals the residual energy values indicate an imbalance in favor of magnetic fluctuations, in agreement with previous results. The strongest imbalance is observed for the high-latitude sample, where the turbulence is comparatively the least evolved. This is a quite unexpected result, probably related to the presence of interstellar pickup ion populations. In conclusion, our analysis indicates that (1) velocity gradients play a dominant role in driving the turbulence evolution in the solar wind and (2) pickup ion effects might be significant.

A Stringent Limit on the Amplitude of Alfvénic Perturbations in High-Beta Low-Collisionality Plasmas

“…An interesting observational feature that may be related to this is the appearance of a distinct, magnetically dominated, population of fluctuations at increasing solar radii (Tu & Marsch 1991;Bruno et al 2007;Bruno & Carbone 2013). This population's sudden appearance across a range of latitudes (Bavassano et al 1998) suggests it does not arise through continuous evolution of turbulence (see Figure 2 of Bruno et al 2007). Such characteristics would be expected from SA wave interruption in regions where  b 1, a scenario that is also consistent with the observed excess of magnetic energy (Goldstein et al 1995;Roberts 2010;Chen et al 2013;Oughton et al 2015).…”

Magnetically dominated structures as an important component of the solar wind turbulence

“…Successively, we followed a type of analysis similar to that of Bavassano et al (1998). We computed the normalized cross helicity σ C and the residual energy σ R within a window of 1 h that was repeatedly shifted by 81 s across the whole data set.…”

“…Although Tu and Marsch (1991) concluded their paper with the recommendation to carry out more case studies and statistical studies on the existing data sets to establish the real importance of these structures in the solar wind turbulence, the first studies, to our knowledge, which statistically located a whole family of magnetically dominated structures, within the zoo of interplanetary MHD fluctuations, were presented by Bavassano et al (1998. These authors studied the distributions of the events characterized by given values of normalized cross-helicity σ C and normalized residual energy…”

“…Following this recommendation, we performed a study similar to that of Bavassano et al (1998) in the ecliptic, between 0.3 and 1 AU, and within the southernmost latitudinal excursion of Ulysses, not analyzed by these authors. Results relative to this analysis will be reported and discussed in the following sections.…”

“…Besides the fact that the most Alfvénic region resulted to be the one at high latitude and at shorter heliocentric distance, in all the analyzed regions they clearly found a relative peak of the distribution at σ C 0 and σ R −1 strongly reminiscent of those magnetic structures found by Tu and Marsch (1991) in the ecliptic, namely the MFDTs. Bavassano et al (1998) stressed the necessity to perform a systematic study in the inner heliosphere "to get a term of comparison with Ulysses results and hence improve our knowledge about their features and their dependence on wind speed regime and radial distance. "…”