Whistler waves are often observed in magnetopause reconnection associated with electron beams. We analyze seven MMS crossings surrounding the electron diffusion region (EDR) to study the role of electron beams in whistler excitation. Waves have two major types: (1) Narrow-band waves with high ellipticities and (2) broad-band waves that are more electrostatic with significant variations in ellipticities and wave normal angles. While both types of waves are associated with electron beams, the key difference is the anisotropy of the background population, with perpendicular and parallel anisotropies, respectively. The linear instability analysis suggests that the first type of wave is mainly due to the background anisotropy, with the beam contributing additional cyclotron resonance to enhance the wave growth. The second type of distribution excites broadband waves via Landau resonance, and as seen in one event, the beam anisotropy induces an additional cyclotron mode. The results are supported by particle-in-cell simulations. We infer that the first type occurs downstream of the central EDR, where background electrons experience Betatron acceleration to form the perpendicular anisotropy; the second type occurs in the central EDR of guide field reconnection. A parametric study is conducted with linear instability analysis. A beam anisotropy alone of above ~3 likely excites the cyclotron mode waves. Large beam drifts cause Doppler shifts and may lead to left-hand polarizations in the ion frame. Future studies are needed to determine whether the observation covers a broader parameter regime and to understand the competition between whistler and other instabilities.

Generation and propagation of lower hybrid drift wave (LHDW) within and near the electron diffusion region (EDR) during guide field reconnection at the magnetopause is studied with data from the Magnetospheric Multiscale mission and a theoretical model. Inside the EDR where the electron beta is high (β ~ 5), the long-wavelength electromagnetic LHDW propagating obliquely to the local magnetic field is observed. In contrast, the short-wavelength electrostatic LHDW propagating nearly perpendicular to the local magnetic field is observed slightly away from the EDR, where β is small (~0.6). These observed LHDW features are explained by a local theoretical model only after including effects from the electron temperature anisotropy, finite electron heat flux and parallel current. The short-wavelength LHDW is capable of generating significant drag force between electrons and ions.

We perform a statistical study of 3-s ultra-low frequency (ULF) waves using MMS observations in the Earth’s foreshock region. The average phase velocity in the plasma rest frame is determined to be anti-sunward, and the intrinsic polarization is right-handed. We further examine the linear instability conditions based on drift-bi-Maxwellian distribution functions according to observed plasma conditions. The resulting instability is a solution to the common dispersion equation of the ion/ion right-hand non-resonant and left-hand resonant instabilities. The predicted wave propagation is also predominantly anti-sunward. The cyclotron resonant conditions of the solar wind and backstreaming beam ions are evaluated, and we find that in some cases, the anti-sunward propagating waves can be resonant with beam ions, which was overlooked in previous studies. The result suggests that the dispersion equation provides the 3-s ULF waves a fundamental explanation that unifies a rich variety of resonant conditions. In the later stage, the 3s ULF waves could further develop into Short Large Amplitude Magnetic Structures, contributing to the turbulence in the foreshock region.

Widely employed to model collisionless plasma phenomena occurring naturally in Earth’s magnetic environment, throughout the heliosphere, and in laboratory fusion devices, the Vlasov equation self-consistently describes the fundamental kinetic dynamics of plasma particles as they are accelerated through phase space via electric and magnetic forces. The Fast Plasma Investigation (FPI) onboard NASA’s Magnetospheric Multiscale (MMS) four-spacecraft mission sufficiently resolves the seven spatial, temporal, and velocity-space dimensions of phase space needed to directly observe terms in the Vlasov equation, as recently demonstrated by Shuster et al. [2021] in the context of electron-scale current layers at the reconnecting magnetopause. These results motivate novel exploration of the types of distinct kinetic signatures in ∂fe/∂t, v⋅∇fe, and (F/me)⋅∇vfe which are associated with the magnetic reconnection process, where F = −e(E + v×B) represents the Lorentz force on an electron, and fe specifies the electron phase space density. We apply this approach to characterize the structure of the velocity-space gradient terms in the electron Vlasov equation measured by MMS. Discussion of the uncertainties which arise when computing the velocity-space gradients of the FPI phase space densities is presented, along with initial validation of the (F/me)⋅∇vfe measurements by comparison to the ∂fe/∂t and v⋅∇fe terms. Successful measurement of the force term (F/me)⋅∇vfe in the Vlasov equation suggests a new technique for inferring spatial gradients from single spacecraft measurements which may be applied to improve the spatial resolution of the electron pressure divergence ∇⋅Pe necessary to understand the microphysics of the electron diffusion region of magnetic reconnection. Reference: Shuster, J. R., et al. (2021), Structures in the terms of the Vlasov equation observed at Earth’s magnetosphere, Nature Physics, doi:10.1038/s41567-021-01280-6.

The unprecedented spatiotemporal resolution of the sixty-four dual electron and ion spectrometers comprising the Fast Plasma Investigation (FPI) onboard the four Magnetospheric Multiscale (MMS) spacecraft enables us to compute terms of the Vlasov equation and thus compare MMS measurements directly to kinetic theory. Here we discuss our techniques for determining spatial and velocity-space gradients of the ion and electron distribution function from the skymaps provided by FPI. We present initial results validating and comparing the contributions from ∂f/∂t, v·∇f, and a·∇vf for a variety of kinetic plasma contexts including both reconnection and non-reconnection events, such as electron diffusion regions (EDRs), magnetic holes, and thin electron-scale current sheets. The ability to resolve gradients of the distribution function motivates comparison of MMS observations to predictions from gyro-kinetic theory and particle-in-cell (PIC) simulations as an approach for determining which physical mechanism is responsible for generating ion and electron crescent distributions observed in association with EDRs and thin boundary layers.