The impact of Alfvenic shear flow on magnetic reconnection and turbulence

TL;DR

The paper addresses why magnetic reconnection is frequently absent in near-Sun solar wind by testing the influence of Alfvénic shear on the collisionless tearing mode using Parker Solar Probe data. It computes the shear-modified growth rate ratio $\gamma_{tr}/\gamma_{0tr}$ from analytic theory and correlates it with wind type, finding reconnection-prone periods have larger ratios, while highly Alfvénic winds suppress reconnection, consistent with theory. The study also finds that most reconnection events occur in slow, non-Alfvénic wind and that reconnection periods exhibit steeper magnetic-field spectra and signs of generated compressible fluctuations, linking reconnection to turbulence dynamics. These results highlight flow shear as a key factor in reconnection onset and have implications for energy partitioning and heating in turbulence-rich astrophysical plasmas. Future work should quantify absolute growth rates and relevant timescales to further constrain reconnection onset in the solar wind.

Abstract

Magnetic reconnection is a fundamental and omnipresent energy conversion process in plasma physics. Novel observations of fields and particles from Parker Solar Probe (PSP) have shown the absence of reconnection in a large number of current sheets in the near-Sun solar wind. Using near-Sun observations from PSP Encounters 4 to 11 (Jan 2020 to March 2022), we investigate whether reconnection onset might be suppressed by velocity shear. We compare estimates of the tearing mode growth rate in the presence of shear flow for time periods identified as containing reconnecting current sheets versus non-reconnecting times, finding systematically larger growth rates for reconnection periods. Upon examination of the parameters associated with reconnection onset, we find that 85% of the reconnection events are embedded in slow, non-Alfvenic wind streams. We compare with fast, slow non-Alfvenic, and slow Alfvenic streams, finding that the growth rate is suppressed in highly Alfvenic fast and slow wind and reconnection is not seen in these wind types, as would be expected from our theoretical expressions. These wind streams have strong Alfvenic flow shear, consistent with the idea of reconnection suppression by such flows. This could help explain the frequent absence of reconnection events in the highly Alfvenic, near-Sun solar wind observed by PSP. Finally, we find a steepening of both the trace and magnitude magnetic field spectra within reconnection periods in comparison to ambient wind. We tie this to the dynamics of relatively balanced turbulence within these reconnection periods and the potential generation of compressible fluctuations.

Figures (5)

• Figure 1: Overview of the fields and plasma parameters used in this study for the time period surrounding a reconnection event identified by Eriksson-2024 (highlighted in purple). Panel (a) shows the trace magnetic field spectrum and associated fit for the time period associated with the reconnection event (highlighted in panels (b-d)). The right side panels show the (b) SPANi proton velocity, (c) FIELDS fluxgate magnetometer magnetic field measurements, and (d) ion and electron temperatures measured by the SPANi and SPANe instruments. $v_{R, T, N}$ and $B_{R, T, N}$ are the radial, tangential, and normal components of the proton velocity (b) and magnetic field (c) respectively where R is the direction from the Sun to the spacecraft, T is the cross product of the Sun’s rotation vector with R, and N = R $\times$ T Hapgood-1992. All data has been filtered to remove time periods where the bulk of the distribution moves out of the SPANi FOV.
• Figure 2: Comparison of (a) $\gamma_{tr} / \gamma_{0tr}$, (b) $\tau$, and (c) $\sqrt{r_A} = \alpha = \langle \delta u \rangle / \langle \delta b \rangle$ for time periods associated with reconnection (black) versus non-reconnection (pink) times. Reconnection periods are those identified by Eriksson-2024.
• Figure 3: Comparison of $\gamma_{tr} / \gamma_{0tr}$ (blue) and normalized flow shear (black) for (a) fast wind streams, (b) slow Alfvénic wind streams, and (c) slow wind streams. Wind is identified as fast, slow Alfvénic, or slow non-Alfvénic based on the categorization scheme of Ervin-2024c.
• Figure 4: Comparison of (a) $v_R$, (b) $\sigma_C$, and (c) $\tau$ for time periods associated with reconnection (black) and $\pm$10-minutes near-reconnecting current sheet (NRCS) wind surrounding these events (purple).
• Figure 5: Comparison of magnetic field spectral index for reconnecting periods (black) identified by Eriksson-2024 and ambient wind (green). Panel (a) shows the spectral index associated with the trace spectra ($\alpha_B$) and (b) is the spectral index for the spectra of $|B|$ ($\alpha_{|B|}$). The legend reports the mean and standard deviation of spectral indices.