Multipoint analysis of compressive fluctuations in the fast and slow solar wind

Abstract

Compressible turbulence in the solar wind is a topic of much recent debate. To understand the various compressive fluctuations at scales comparable to proton characteristic lengths, we use multipoint magnetic field and density data (derived from spacecraft potential which allows higher time resolution than is typically possible than with particle instruments) from the Cluster spacecraft when they were in undisturbed intervals of slow and fast solar wind. The application of the multipoint signal resonator technique is performed for the first time, to the analysis of scalar time series. Fluctuations in the measurements of electron density and in magnitude of the magnetic field (used as a proxy for compressible magnetic fluctuations) are used as inputs in addition to the traditional vector components of the magnetic field. This analysis is performed on two streams of solar wind, one which can be classed as a slow stream and one which can be classed as a fast stream to investigate the difference in the compressible components in the two types of wind. The recovered plasma frame frequencies for the incompressible component show low propagation speed in the plasma frame consistent with previous applications of the method, while the compressible components are more scattered, some with very high phase speeds. Based on these results, we propose a picture of compressive solar wind turbulence as a mixed state of (1) linear modes such as kinetic Alfvén, kinetic slow, and ion Bernstein modes; (2) coherent structures with very small intrinsic frequencies; and (3) nonlinear or sideband modes.

Multi-scale analysis of compressible fluctuations in the solar wind

“…Moreover, even though some points in Fig. 1c and d, may agree better with the curves for IBWs as there is no significant signature in the cross-correlation, this is most likely due to wave-wave interactions between individual KAW packets or KAWs and KSWs (Narita and Motschmann, 2017;Roberts et al, 2017). It is also interesting to note that while the KAW dominates both other wave modes, the majority of the fluctuations do not fall into the thresholds in Table 1.…”

“…At the highest frequencies there is a small increase in both of these wave modes; however, it is unclear whether this is true or related to the instrumental noise. It should be noted that a sunwardpropagating KSW would in this case produce the same signature as the KAW; therefore, in interpreting the data in Roberts et al (2017) and Fig. 1c and d this mixture of two waves may explain the scatter seen.…”

“…The spacecraft potential is subject to a strong spin effect at 0.25 Hz and charging effects due to different parts of the spacecraft being illuminated. To remove these fluctuations we use the method presented in Roberts et al (2017) to construct an empirical model of the spacecraft charging as a function of the phase angle. This allows the fluctuations in the spacecraft potential due to solar illumination to be removed, leaving only fluctuations due to density fluctuations.…”

“…1c and d. These are derived by the application of the multi-point signal resonator technique (Narita et al, 2011), which allows the most energetic wave vector k to be identified at each spacecraft frequency ω sc . The corresponding plasma frame frequency can be obtained from the equation ω pla = ω sc − k · v. Further details can be obtained in Roberts et al (2017). Figure 1c and d also show the theoretical linear dispersion curves for the KAWs, IBWs and KSWs for plasma β = 0.85 (ratio of thermal to magnetic pressure).…”

“…Several multi-spacecraft observations of the solar wind have revealed that the fluctuations at proton kinetic scales typically have low intrinsic propagation speeds in the plasma frame. This result has been interpreted as evidence of kinetic Alfvén waves (KAWs) (Sahraoui et al, 2010), as coherent structures which are predominantly advected with the bulk velocity (Perrone et al, 2017), as a combination of KAW turbulence and coherent structures (Roberts et al, 2013(Roberts et al, , 2015, or as nonlinear modes where wave-wave interactions have broadened the dispersion relation diagram (Narita and Motschmann, 2017;Roberts et al, 2017). Kinetic slow waves (KSWs) are the kinetic counterpart of the magnetohydrodynamic (MHD) slow mode when it develops a large perpendicular wave number, and it shares many properties with the KAW, such as a similar dispersion relation and anticorrelated fluctuations in magnetic field and density Narita and Marsch, 2015).…”