Search

Downstream of the Earth's bow shock, the magnetosheath can exhibit strong turbulence. This turbulent cascade is associated with electron‐scale current sheets where magnetic reconnection can occur. We present data from NASA's Magnetospheric Multiscale mission during a turbulent magnetosheath interval that includes two electron‐scale reconnecting current sheets, one with higher guide field and one that is closer to antiparallel. We examine the characteristics of energy conversion and the role of parallel electric fields and waves in that energy conversion. The high‐guide field reconnection event shows energy conversion dominated by electric fields and currents parallel to the background magnetic field, while the lower guide field event is perpendicular dominated, consistent with observations of laminar reconnecting current sheets in the magnetosheath. Both current sheets exhibit signs of a parallel electric field acceleration channel, and the higher guide field event shows whistler‐mode waves at the x‐line. Both current sheets show variation in dissipation along the n‐direction, with perpendicular dissipation on one side and parallel on the other. Between the two is a dispersion of pitch angles from 90° to 0° or 180°. This dispersion is likely caused by the acceleration channel and rapidly suppresses whistler waves, suggesting whistler waves do not contribute to the dissipation in guide field reconnection. Key Points: Two magnetic reconnection events are observed in the turbulent magnetosheath with different guide fieldBoth current sheets exhibit a parallel electric field acceleration channel that changes the anisotropy of the electronsThis acceleration channel is sufficient to suppress whistler waves near the diffusion region [ABSTRACT FROM AUTHOR]

We compare the relative importance of two competing mechanisms (Alfvén wing vs. magnetosheath force balance) in determining the saturation of the reverse convection potential under northward interplanetary magnetic field. We run the Lyon‐Fedder‐Mobarry magnetohydrodynamic model under strong northward interplanetary magnetic field (20 nT) twice: once with a solar wind number density of 20 cm−3 and once with a density of 2 cm−3. We find that the reverse convection potential is reduced for the low‐density run, as predicted by both models. We also find that despite the formation of Alfvén wings, the sunward flows in the magnetosphere are governed by the local reconnection physics. Further, the length of the x‐line poleward of the cusp is reduced by a factor of 2 for the low‐density run. This suggests that the reverse convection potential is governed by the local reconnection physics at the x‐line and the geoeffective length. Plain Language Summary: One major challenge in understanding solar wind‐magnetosphere‐ionosphere coupling is understanding why the electric potential across the polar ionosphere saturates in response to increasing solar wind driving. At present, there are two competing hypotheses for why this happens, and phenomena associated with both are found in simulations of the magnetosphere. In the present study, we simulate a special magnetospheric case where we can separate out the contributions of each mechanism. We find that only one of the hypotheses is necessary to explain the saturation of the polar ionospheric potential. This is a new finding with respect to the phenomena of polar cap potential saturation, and may help explain why it occurs. Key Points: Reverse convection potential reduced at lower solar wind densityAlfven wings also form at low density, but they mostly are connected to the interplanetary magnetic fieldReverse convection potential reduction likely due to reduced geoeffective length [ABSTRACT FROM AUTHOR]

We report observations from the Magnetospheric Multiscale satellites of reconnecting current sheets in the magnetosheath over a range of out‐of‐plane "guide" magnetic field strengths. The currents exhibit nonideal energy conversion in the electron frame of reference, and the events are within the ion diffusion region within close proximity (a few electron skin depths) to the electron diffusion region. The study focuses on energy conversion on the electron scale only. At low guide field (antiparallel reconnection), electric fields and currents perpendicular to the magnetic field dominate the energy conversion. Additionally, electron distributions exhibit significant nongyrotropy. As the guide field increases, the electric field parallel to the background magnetic field becomes increasingly strong, and the electron nongyrotropy becomes less apparent. We find that even with a guide field less than half the reconnecting field, the parallel electric field and currents dominate the dissipation. This suggests that parallel electric fields are more important to energy conversion in reconnection than previously thought and that at high guide field, the physics governing magnetic reconnection are significantly different from antiparallel reconnection. Key Points: We observe ion diffusion regions and electron jets in the magnetosheath for multiple eventsThe dissipation due to the parallel electric field becomes more significant at increasing guide magnetic fieldThis may be associated with a dissipative electric field along the direction of the electron jet [ABSTRACT FROM AUTHOR]

Abstract: We report multispacecraft observations by the Magnetospheric MultiScale (MMS) mission of large (80–155 λDe parallel to B) electrostatic solitary waves (ESWs). Observations of the same ESWs at different positions and times enable unprecedented accuracy in determining the size, velocity, and evolution of these nonlinear structures. This particular event is a short series of negative potential solitary waves in the magnetosheath. The observed structures differ in amplitude and speed, merging or colliding with one another. A Vlasov simulation supports initial wave growth by a cold feature in the electron distribution function. Such cold electrons are likely the result of mixing between the cold magnetospheric and warm magnetosheath plasma along a reconnection separatrix. ESW speed and perpendicular size are inconsistent with ion phase space holes. Solving Poisson's equation for the measured potential, we find a reduction in electron density rather than an enhancement expected from an electron soliton. These ESWs have an unusual combination of properties on both ion and electron scales, likely supported by a complex electron distribution in the nonlinear state. [ABSTRACT FROM AUTHOR]