Density Fluctuations in the Solar Wind Driven by Alfvén Wave Parametric Decay

Abstract

Measurements and simulations of inertial compressive turbulence in the solar wind are characterized by anticorrelated magnetic fluctuations parallel to the mean field and density structures. This signature has been interpreted as observational evidence for non-propagating pressure balanced structures (PBS), kinetic ion acoustic waves, as well as the MHD slow-mode. Given the high damping rates of parallel propagating compressive fluctuations, their ubiquity in satellite observations is surprising, and suggestive of a local driving process. One possible candidate for the generation of compressive fluctuations in the solar wind is Alfvén wave parametric instability. Here we test the parametric decay process as a source of compressive waves in the solar wind by comparing the collisionless damping rates of compressive fluctuations with growth rates of the parametric decay instability daughter waves. Our results suggest that generation of compressive waves through parametric decay is overdamped at 1 AU, but that the presence of slow-mode like density fluctuations is correlated with the parametric decay of Alfvén waves.

A Quarter Century ofWindSpacecraft Discoveries

“…Boldyrev et al (2011); Boldyrev and Perez (2012), based on previous work by Grappin et al (1983), proposed that the difference between magnetic and velocity fluctuation spectra is due to turbulence-generated residual energy, which is predicted to scale as k ⊥ −2 and this steep scaling was confirmed by Chen et al (2013). The large dataset provided by Wind allows conditional statistics to be used to separate solar wind with different properties and this has allowed the measurement of the impact of cross helicity and residual energy on the turbulent cascade to be measured simultaneously (Bowen et al, 2018;Bruno et al, 2007;following Bavassano et al, 1998). The current state of knowledge is summarized in Figure 10 which shows the inertial range spectral indices of the MHD fields as functions of cross-helicity, |σ c |, which is a quantitative measure of imbalance -the different fluxes of turbulent fluctuations propagating toward or away from the sun.…”

Solar Wind Electron Parameters Determination on Wind Spacecraft Using Quasi‐Thermal Noise Spectroscopy

“…Within a 4.4‐s measurement cadence in the ACE regime, the sampling time for each channel is determined by the time needed to produce statistical variations of less than 0.3 dB (∼7.2%). Considering typical plasma VDF fluctuations during the acquisition time interval at 1 au are reaching up to few percent (Chen et al., 2014; Bowen et al., 2018; Kellogg et al., 2009), we further on assume that the measurement uncertainties are ∼10%. The time reserved for each band is 1.472 s, while the actual acquisition time within this interval varies for each band (Bougeret et al., 1995), and the time left after the acquisition is used for performing on board fast Fourier transforms and data storage.…”

A Hamiltonian gyrofluid model based on a quasi-static closure

“…This effect contributes to the generation of counter-propagating waves required for the development of a turbulent dynamics at these scales. Interestingly, this parametric instability, proposed by Bowen et al (2018) for the generation of the solar wind compressive fluctuations, turns out to occur in a wider range of beta parameters in the presence of temperature anisotropy (Tenerani, Velli & Hellinger 2017). A useful feature of a new gyrofluid model for space plasma studies would thus be its capability to account for equilibrium temperature anisotropies and the coupling to ion acoustic waves.…”