We perform a statistical assessment of solar wind stability at 1 AU against ion sources of free energy using Nyquist's instability criterion. In contrast to typically employed threshold models which consider a single free-energy source, this method includes the effects of proton and He^{2+} temperature anisotropy with respect to the background magnetic field as well as relative drifts between the proton core, proton beam, and He^{2+} components on stability. Of 309 randomly selected spectra from the Wind spacecraft, 53.7% are unstable when the ion components are modeled as drifting bi-Maxwellians; only 4.5% of the spectra are unstable to long-wavelength instabilities. A majority of the instabilities occur for spectra where a proton beam is resolved. Nearly all observed instabilities have growth rates γ slower than instrumental and ion-kinetic-scale timescales. Unstable spectra are associated with relatively large He^{2+} drift speeds and/or a departure of the core proton temperature from isotropy; other parametric dependencies of unstable spectra are also identified.

The solar wind shows periods of highly Alfvénic activity, where velocity fluctuations and magnetic fluctuations are aligned or anti-aligned with each other. It is generally agreed that solar wind plasma velocity and magnetic field fluctuations observed by Parker Solar Probe (PSP) during the first encounter are mostly highly Alfvénic. However, quantitative measures of Alfvénicity are needed to understand how the characterization of these fluctuations compares with standard measures from prior missions in the inner and outer heliosphere, in fast wind and slow wind, and at high and low latitudes. To investigate this issue, we employ several measures to quantify the extent of Alfvénicity -the Alfvén ratio r A , normalized cross helicity σ c , normalized residual energy σ r , and the cosine of angle between velocity and magnetic fluctuations cos θ vb . We show that despite the overall impression that the Alfvénicity is large in the solar wind sampled by PSP during the first encounter, during some intervals the cross helicity starts decreasing at very large scales. These length-scales (often > 1000d i ) are well inside inertial range, and therefore, the suppression of cross helicity at these scales cannot be attributed to kinetic physics. This drop at large scales could potentially be explained by large-scale shears present in the inner heliosphere sampled by PSP. In some cases, despite the cross helicity being constant down to the noise floor, the residual energy decreases with scale in the inertial range. These results suggest that it is important to consider all these measures to quantify Alfvénicity. arXiv:1912.07181v1 [physics.space-ph]

The power spectral density of magnetic fluctuations in the solar wind exhibits several power-law-like frequency ranges with a well defined break between approximately 0.1 and 1 Hz in the spacecraft frame. The exact dependence of this break scale on solar wind parameters has been extensively studied but is not yet fully understood. Recent studies have suggested that reconnection may induce a break in the spectrum at a "disruption scale" λ D , which may be larger than the fundamental ion kinetic scales, producing an unusually steep spectrum just below the break. We present a statistical investigation of the dependence of the break scale on the proton gyroradius ρ i , ion inertial length d i , ion sound radius ρ s , proton-cyclotron resonance scale ρ c and disruption scale λ D as a function of β ⊥i . We find that the steepest spectral indices of the dissipation range occur when β e is in the range of 0.1-1 and the break scale is only slightly larger than the ion sound scale (a situation occurring 41% of the time at 1 AU), in qualitative agreement with the reconnection model. In this range the break scale shows remarkably good correlation with λ D . Our findings suggest that, at least at low β e , reconnection may play an important role in the development of the dissipation range turbulent cascade and causes unusually steep (steeper than -3) spectral indices.