Discontinuities and Alfvénic fluctuations in the solar wind

Abstract

Abstract. We examine the Alfvénicity of a set of 188 solar wind directional discontinuities (DDs) identified in the Cluster data from 2003 by Knetter (2005), with the objective of separating rotational discontinuities (RDs) from tangential ones (TDs). The DDs occurred over the full range of solar wind velocities and magnetic shear angles. By performing the Walén test in the de Hoffmann–Teller (HT) frame, we show that 77 of the 127 crossings for which a good HT frame was found had plasma flow speeds exceeding 80% of the Alfvén speed at an average angular deviation of 7.7°; 33 cases had speeds exceeding 90% of the Alfvén speed at an average angle of 6.4°. We show that the angular deviation between flow velocity (in the HT frame) and the Alfvén velocity can be obtained from a reduced form of the Walén correlation coefficient. The corresponding results from the Walén test expressed in terms of jumps in flow speed and corresponding jumps in Alfvén speed are similar: 66 of the same 127 cases had velocity jumps exceeding 80% with average angular deviation of 5.8°, and 22 exceeding 90% of the jump in Alfvén speed, with average angular deviation 6.2°. We conclude that a substantial fraction of the 127 events can be identified as RDs. We present further evidence for coupling across the DDs by showing that, for most of the 127 crossings, the HT frame velocities, evaluated separately on the two sides of the DD, are nearly the same – a result required for RDs but not for TDs. We also show that the degree of Alfvénicity is nearly the same for the DDs and fluctuations in which the DDs are embedded. Whatever process causes deviations from ideal Alfvénicity appears to operate equally for the DDs as for the surrounding fluctuations. Finally, our study has established a unique relation between the strahl electron pitch angle and the sign of the Walén slope, implying antisunward propagation in the plasma frame for all 127 cases.

Intense Cross‐Tail Field‐Aligned Currents in the Plasma Sheet at Lunar Distances

“…(The magnitude of this current varies from ∼1 to 10 nA/m 2 , comparable to or larger than typical cross-tail currents at lunar distances.) Similar, largely field-aligned currents have been observed in the solar wind (e.g., Paschmann et al, 2013) and in planetary magnetotails, where the plasma pressure is so small that it is insufficient to maintain the pressure balance (e.g., Jupiter, Venus, and Mars; see Artemyev et al, 2014Artemyev et al, , 2017Rong et al, 2015).…”

The Influence of Intermittency on the Spectral Anisotropy of Solar Wind Turbulence

“…The magnetic intermittent fluctuations were identified (Wang et al 2013) as being due to mostly rotational discontinuities and rarely tangential discontinuities. Also the recent work by Paschmann et al (2013) supports such an identification. More intermittency around θ RB ≈ 90…”

Near‐Earth Solar Wind: Plasma Characteristics From ARTEMIS Measurements

“…Third, simultaneous measurements of solar wind plasma and magnetic field characteristics by the two ARTEMIS probes open many opportunities for correlation analysis, including reconstruction of the magnetic field orientation in solar wind discontinuities (see discussion of this problem in Horbury et al, 2001;Neugebauer, 2006) and investigation of magnetic fluctuation propagation along/across the slow solar wind flow (see discussion of this problem in Dasso et al, 2005;Weygand et al, 2009). Previous investigations of such correlations were mostly performed using observations from four Cluster spacecraft e.g., Knetter et al, 2004;Mistry et al, 2015;Paschmann et al, 2013; that mostly measure the solar wind close to the Earth's bow shock) or from spacecraft separated by tens of Earth radii (see examples in Horbury et al, 2001;Němeček et al, 2013;Phan et al, 2006). Therefore, the two ARTEMIS probes allow for the first-time investigation of the inner structure of small-scale magnetic field/plasma variations (see an example in Dorfman et al, 2017).…”