On the Motion of the Heliospheric Magnetic Structure Through the Solar Wind Plasma

Abstract

A reference frame in the solar wind can often be found wherein the flow vector v is everywhere approximately parallel to the magnetic field vector B. This is the frame of the heliospheric magnetic structure moving relative to the plasma. Since v⊥ is very small in this reference frame, the magnetic structure appears to have little temporal evolution. The structure moves outward away from the Sun faster than the plasma flow. Even for highly Alfvénic plasma, the structure does not move at the Alfvén speed relative to the proton plasma, rather it moves at ~0.7 vA. This may be caused by the spread in the vector directions of the local magnetic field. The degree of inhomogeneity of the plasma is hypothesized to control the motion of the magnetic structure through the plasma. If so, non‐Alfvénicity may be owed to an inability of Alfvénic perturbations to coherently propagate from Alfvénic injections at the Sun. Schemes that mathematically advect magnetic and plasma structure from upstream solar wind monitors to the Earth may be improved by calculating and including the motion of the structure relative to the solar wind velocity vector.

Nine Outstanding Questions of Solar Wind Physics

“…In the highly Alfvénic solar wind, variations in the proton flow vector and the magnetic field vector seen by a spacecraft are highly time correlated with Δv ≈ ±ΔB/(4πρ) 1/2 : The changes in v are parallel (or antiparallel) to the changes in B. This means that if you shift into the reference frame that moves with the magnetic structure, you see a network of flux tubes, with each flux tube having a parallel proton plasma flow inside of it at a fraction of the Alfvén speed (Borovsky, 2020a). The correlations are such that the plasma flow relative to the structure is toward the Sun, hence the structure is moving outward along the Parker spiral relative to the proton plasma at a fraction of the Alfvén speed.…”

Some Similarities and Differences Between the Observed Alfvénic Fluctuations in the Fast Solar Wind and Navier–Stokes Turbulence

“…It is anisotropic globally in that the magnetic structure of the plasma can propagate without evolution in the direction of the global mean magneticfield vector (cf. Figure 7.1 of Parker, 1979;Borovsky J. E., 2020a;Nemecek et al, 2020), and it is anisotropic locally in that the nature of the forces perpendicular and parallel to the local magnetic-field vector B differs. In the MHD description of plasmas, the momentum transport is given by…”

“…If there is turbulence acting in the fast solar wind, it is related to the inward (toward the Sun) propagating Alfvénic fluctuations, which, if they exist, are in the noise of the measurements (Wang et al, 2018). Relatedly, in the reference frame that moves outward away from the Sun at the velocity of the Alfvénic fluctuations, the plasma flow velocity component locally perpendicular to B is in the noise of the measurement, indicating little or no evolution of the magnetic structure as it propagates outward (Borovsky J. E., 2020a). This study will make a systematic comparison of the properties of Alfvénic solar wind fluctuations with true Navier-Stokes Kolmogorov active turbulence, asking what is similar, what is different, and for the properties that are similar asking why they are similar.…”

The Compression of the Heliospheric Magnetic Structure by Interplanetary Shocks: Is the Structure at 1AU a Manifestation of Solar-Wind Turbulence or Is It Fossil Structure From the Sun?

“…Among the arguments that there is an active MHD turbulence operating in the inner heliosphere are: (1) a matching of the magnetic and velocity Fourier spectra of the solar wind (with an inertial-range, a high-frequency breakpoint, and a low-frequency breakpoint) to the spectra expected for MHD turbulence (e.g., Goldstein et al, 1995;Tu and Marsch, 1995;Podesta, 2010;Carbone, 2012;Bruno and Carbone, 2016); (2) third-order-moment analysis of the solar-wind fluctuations indicating an energy cascade operating in the solar wind (Sorriso-Valvo et al, 2007;MacBride et al, 2008;Stawarz et al, 2010;Podesta, 2011); and (3) the solar-wind plasma having an extremely high Reynolds number for its flow structures (Borovsky and Gary, 2009). Some of the arguments against an active MHD turbulence operating in the inner heliosphere are: (1) the absence of turbulent mixing in the slow solar wind (Borovsky, 2012); (2) the absence of discernable time-evolution of the magnetic fluctuations of the fast solar wind (Borovsky, 2020b); (3) the lack of discernable isotropization of the magnetic structure in coroating-interactionregion compressions and trailing-edge rarefactions (Borovsky and Denton, 2016); (4) the strong current sheets of the solar wind being co-located with ion-composition boundaries, which can only be formed in the corona (Borovsky, 2020c); (5) the strong current sheets of the solar wind being co-located with electronstrahl-intensity boundaries, implying that the current sheets are coherent all the way back to the corona (Borovsky, 2020c); (6) the co-location of strong current sheets with intensity boundaries of solar energetic particles, again implying a coherence of the flux tubes back to near-Sun region (Trenchi et al, 2013a,b); (7) the survival of some inertial-range-sized solar-wind structures from the corona to the Earth (Viall and Vourlidas, 2015;Kepko and Viall, 2019). The absence of turbulent mixing (Paul et al, 2003;Borovsky, 2012) means (a) an absence of evidence for the action of turbulent stretching and folding reducing (macromixing) the sizes of plasma chunks as the solar wind advects from 0.3 to 1 AU and (b) an absence of homogenization of plasma properties (micromixing) as the solar wind advects from 0.3 to 1 AU.…”

“…The absence of turbulent mixing (Paul et al, 2003;Borovsky, 2012) means (a) an absence of evidence for the action of turbulent stretching and folding reducing (macromixing) the sizes of plasma chunks as the solar wind advects from 0.3 to 1 AU and (b) an absence of homogenization of plasma properties (micromixing) as the solar wind advects from 0.3 to 1 AU. The absence of time evolution of the structure (Borovsky, 2020b) come from the fact that in the Alfvenic fast solar wind a reference frame can be found that moves outward from the Sun with the magnetic structure; in this frame all flow velocities are parallel to the local magnetic field and, to within measurement error, v ⊥ is everywhere zero. With v ⊥ = 0 everywhere in the frame of the magnetic structure, there is no time evolution to the propagating structure [cf.…”