Kinetic Range Spectral Features of Cross Helicity Using the Magnetospheric Multiscale Spacecraft

We study spectral features of ion velocity and magnetic field correlations in the magnetosheath and in the solar wind using data from the Magnetospheric Multiscale (MMS) spacecraft. High-resolution MMS observations enable the study of the transition of these correlations between their magnetofluid character at larger scales into the subproton kinetic range, previously unstudied in spacecraft data. Cross-helicity, angular alignment, and energy partitioning is examined over a suitable range of scales, employing measurements based on the Taylor frozen-in approximation as well as direct two-spacecraft correlation measurements. The results demonstrate signatures of alignment at large scales. As kinetic scales are approached, the alignment between $\mathbf{v}$ and $\mathbf{b}$ is destroyed by demagnetization of protons.

Figure 1

Equivalent spectra for $B$, $V_i$, $V_e$. Lines are single spacecraft values using Taylor’s hypothesis, symbols are multispacecraft estimates.

Figure 2

Magnetosheath equivalent spectra of (top panel) ${\sigma }_c$ and ${\sigma }_r$, and (bottom panel) the alignment cosines. Lines are single spacecraft estimates using Taylor’s hypothesis and symbols are multispacecraft estimates.

Figure 3

Solar wind equivalent spectra for ${\sigma }_c$, ${\sigma }_r$, and alignment cosines.

Figure 4

Solar wind interval: PDFs of alignment angles for various increment lags (top panel) as well as multispacecraft lags (bottom panel) with number pairs representing pairs of spacecraft.

Figure 5

Cross-helicity spectrum, alignment PDFs for small increments, and probability density at maximal alignment (stars in middle panel) as a function of lag for the PIC simulation of [48]. See text for details.