A STATISTICAL STUDY OF THE SOLAR WIND TURBULENCE AT ION KINETIC SCALES USING THEK-FILTERING TECHNIQUE AND CLUSTER DATA

Abstract

Plasma turbulence at ion kinetic scales in the solar wind is investigated using the multi-point magnetometer data from the Cluster spacecraft. By applying the k-filtering method, we are able to estimate the full three-dimensional power spectral density P (ω sc , k) at a certain spacecraft frequency ω sc in wavevector (k) space. By using the wavevector at the maximum power in P (ω sc , k) at each sampling frequency ω sc and the Doppler shifted frequency ω pla in the solar wind frame, the dispersion plot ω pla = ω pla (k) is found. Previous studies have been limited to very few intervals and have been hampered by large errors, which motivates a statistical study of 52 intervals of solar wind. We find that the turbulence is predominantly highly oblique to the magnetic field k ⊥ k , and propagates slowly in the plasma frame with most points having frequencies smaller than the proton gyrofrequency ω pla < Ω p . Weak agreement is found that turbulence at the ion kinetic scales consists of kinetic Alfvén waves and coherent structures advected with plasma bulk velocity plus some minor more compressible components. The results suggest that anti-sunward and sunward propagating magnetic fluctuations are of similar nature in both the fast and slow solar wind at ion kinetic scales. The fast wind has significantly more anti-sunward flux than sunward flux and the slow wind appears to be more balanced.

Multi-scale analysis of compressible fluctuations in the solar wind

“…The mean bulk speed of the solar wind is 655 ± 15 km s −1 , the mean proton density is low 2.8 ± 0.2 cm −3 , and plasma β = 0.85 ± 0.29. There are no interplanetary coronal mass ejections or stream interaction regions recorded at the time of the interval, and the large scale from Active Composition Explorer (ACE) data (not shown) suggests that this interval can be regarded as typical of the fast solar wind (see Roberts et al, 2015).…”

“…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).…”

Coherent Structures at Ion Scales in Fast Solar Wind: Cluster Observations

“…This has variously been interpreted as evidence of quasi-linear waves that propagate slowly in the plasma frame (such as the quasi-perpendicular Kinetic Alfvén Wave (Sahraoui et al 2010)) or coherent structures with k ⊥ ≫ k which are advected by the plasma bulk flow or a combination of these two phenomena (Roberts et al 2013(Roberts et al , 2015. However, in the present work, by looking directly at the magnetic fluctuations around ion scales in the region of high intermittency, it has been clearly shown that the considered time interval is covered by coherent structures (making up ∼ 30% of the interval).…”

Kinetic Alfvén turbulence below and above ion cyclotron frequency

“…Its validity has been supported by the gyrokinetic simulations (i.e., Howes et al 2011b). The quasilinear premise has been widely used to distinguish the wave modes in the dissipation range of the solar wind turbulence (e.g., Sahraoui et al 2010;He et al 2011;Salem et al 2012;Podesta 2013;Roberts et al 2013Roberts et al , 2015, as well in modeling the kinetic Alfvén turbulence (i.e., Schekochihin et al 2009;Howes et al 2011a;Voitenko and De Keyser 2011;Boldyrev & Perez 2012;Zhao et al 2013). The related assumption of the critical balance between the linear Alfvén propagation timescale and the nonlinear turnover timescales (Goldreich & Sridhar 1995) is also regularly used in modeling the strong Alfvénic turbulence.…”

“…Alfvénic turbulence, measured in situ by satellites in the Earth's space environment (Chaston et al 2008(Chaston et al , 2009Huang et al 2012Huang et al , 2014 and in the solar wind (Sahraoui et al 2010;He et al 2011He et al , 2013Alexandrova et al 2012;Horbury et al 2012;Salem et al 2012;Podesta 2013;Roberts et al 2013Roberts et al , 2015, extends from magnetohydrodynamic (MHD) scales down to ion and even electron kinetic scales. Turbulence at kinetic scales is often referred to as kinetic Alfvén turbulence.…”