arXiv subjects

James L. Burch

Publications and source records attributed to James L. Burch.

29 records · Page 2Linked to original sources

arXiv2020

Direct Measurement of the Solar-Wind Taylor Microscale using MMS Turbulence Campaign Data

Using the novel Magnetospheric Multiscale (MMS) mission data accumulated during the 2019 MMS Solar Wind Turbulence Campaign, we calculate the Taylor microscale $(λ_{\mathrm{T}})$ of the turbulent magnetic field in the solar wind. The Taylor microscale represents the onset of dissipative processes in classical turbulence theory. An accurate estimation of Taylor scale from spacecraft data is, however, usually difficult due to low time cadence, the effect of time decorrelation, and other factors. Previous reports were based either entirely on the Taylor frozen-in approximation, which conflates time dependence, or that were obtained using multiple datasets, which introduces sample-to-sample variation of plasma parameters, or where inter-spacecraft distance were larger than the present study. The unique configuration of linear formation with logarithmic spacing of the 4 MMS spacecraft, during the campaign, enables a direct evaluation of the $λ_{\mathrm{T}}$ from a single dataset, independent of the Taylor frozen-in approximation. A value of $λ_{\mathrm{T}} \approx 7000 \, \mathrm{km}$ is obtained, which is about 3 times larger than the previous estimates.

physics.space-ph↗

arXiv2020

Statistics of Kinetic Dissipation in Earth's Magnetosheath -- MMS Observations

A familiar problem in space and astrophysical plasmas is to understand how dissipation and heating occurs. These effects are often attributed to the cascade of broadband turbulence which transports energy from large scale reservoirs to small scale kinetic degrees of freedom. When collisions are infrequent, local thermodynamic equilibrium is not established. In this case the final stage of energy conversion becomes more complex than in the fluid case, and both pressure-dilatation and pressure strain interactions (Pi-D $\equiv -Π_{ij} D_{ij}$) become relevant and potentially important. Pi-D in plasma turbulence has been studied so far primarily using simulations. The present study provides a statistical analysis of Pi-D in the Earth's magnetosheath using the unique measurement capabilities of the Magnetospheric Multiscale (MMS) mission. We find that the statistics of Pi-D in this naturally occurring plasma environment exhibit strong resemblance to previously established fully kinetic simulations results. The conversion of energy is concentrated in space and occurs near intense current sheets, but not within them. This supports recent suggestions that the chain of energy transfer channels involves regional, rather than pointwise, correlations.

physics.plasm-ph↗

arXiv2020

In-situ observation of Hall Magnetohydrodynamic Cascade in Space Plasma

We present estimates of the turbulent energy cascade rate, derived from a Hall-MHD third-order law. We compute the contribution from the Hall term and the MHD term to the energy flux. We use MMS data accumulated in the magnetosheath and the solar wind, and compare the results with previously established simulation results. We find that in observation, the MHD contribution is dominant at inertial scales, as in the simulations, but the Hall term becomes significant in observations at larger scales than in the simulations. Possible reasons are offered for this unanticipated result.

physics.space-ph↗

arXiv2020

Intermittency and Ion Temperature-Anisotropy Instabilities: Simulation and Magnetosheath Observation

Weakly collisional space plasmas are rarely in local thermal equilibrium and often exhibit non-Maxwellian electron and ion velocity distributions that lead to the growth of microinstabilities, that is, enhanced electric and magnetic fields at relatively short wavelengths. These instabilities play an active role in the evolution of space plasmas, as does ubiquitous broadband turbulence induced by turbulent structures. This study compares certain properties of a 2.5 dimensional Particle-In-Cell (PIC) simulation for the forward cascade of Alfvenic turbulence in a collisionless plasma against the same properties of turbulence observed by the Magnetospheric Multiscale Mission spacecraft in the terrestrial magnetosheath. The PIC simulation is of decaying turbulence which develops both coherent structures and anisotropic ion velocity distributions with the potential to drive kinetic scale instabilities. The uniform background magnetic field points perpendicular to the plane of the simulation. Growth rates are computed from linear theory using the ion temperature anisotropies and ion beta values for both the simulation and the observations. Both the simulation and the observations show that strong anisotropies and growth rates occur highly intermittently in the plasma, and the simulation further shows that such anisotropies preferentially occur near current sheets. This suggests that, though microinstabilities may affect the plasma globally , they act locally and develop in response to extreme temperature anisotropies generated by turbulent structures. Further studies will be necessary to understand why there is an apparent correlation between linear instability theory and strongly intermittent turbulence.

physics.space-ph↗

arXiv2020

In situ Measurement of Curvature of Magnetic Field in Turbulent Space Plasmas: A Statistical Study

Using in situ data, accumulated in the turbulent magnetosheath by the Magnetospheric Multiscale (MMS) Mission, we report a statistical study of magnetic field curvature and discuss its role in the turbulent space plasmas. Consistent with previous simulation results, the Probability Distribution Function (PDF) of the curvature is shown to have distinct power-law tails for both high and low value limits. We find that the magnetic-field-line curvature is intermittently distributed in space. High curvature values reside near weak magnetic-field regions, while low curvature values are correlated with small magnitude of the force acting normal to the field lines. A simple statistical treatment provides an explanation for the observed curvature distribution. This novel statistical characterization of magnetic curvature in space plasma provides a starting point for assessing, in a turbulence context, the applicability and impact of particle energization processes, such as curvature drift, that rely on this fundamental quantity.

physics.space-ph↗

arXiv2019

Sign singularity of the local energy transfer in space plasma turbulence

In weakly collisional space plasmas, the turbulent cascade provides most of the energy that is dissipated at small scales by various kinetic processes. Understanding the characteristics of such dissipative mechanisms requires the accurate knowledge of the fluctuations that make energy available for conversion at small scales, as different dissipation processes are triggered by fluctuations of a different nature. The scaling properties of different energy channels are estimated here using a proxy of the local energy transfer, based on the third-order moment scaling law for magnetohydrodynamic turbulence. In particular, the sign-singularity analysis was used to explore the scaling properties of the alternating positive-negative energy fluxes, thus providing information on the structure and topology of such fluxes for each of the different type of fluctuations. The results show the highly complex geometrical nature of the flux, and that the local contributions associated with energy and cross-helicity nonlinear transfer have similar scaling properties. Consequently, the fractal properties of current and vorticity structures are similar to those of the Alfvénic fluctuations.

physics.space-ph↗

arXiv2019

In situ spacecraft observations of a structured electron diffusion region during magnetopause reconnection

The Electron Diffusion Region (EDR) is the region where magnetic reconnection is initiated and electrons are energized. Because of experimental difficulties, the structure of the EDR is still poorly understood. A key question is whether the EDR has a homogeneous or patchy structure. Here we report Magnetospheric MultiScale (MMS) novel spacecraft observations providing evidence of inhomogeneous current densities and energy conversion over a few electron inertial lengths within an EDR at the terrestrial magnetopause, suggesting that the EDR can be rather structured. These inhomogenenities are revealed through multi-point measurements because the spacecraft separation is comparable to a few electron inertial lengths, allowing the entire MMS tetrahedron to be within the EDR most of the time. These observations are consistent with recent high-resolution and low-noise kinetic simulations.

physics.plasm-ph↗

arXiv2018

Observational evidence of magnetic reconnection in the terrestrial bow shock transition region

We report evidence of magnetic reconnection in the transition region of the terrestrial bow shock when the angle between the shock normal and the immediate upstream magnetic field is 65 degrees. An ion-skin-depth-scale current sheet exhibits the Hall current and field pattern, electron outflow jet, and enhanced energy conversion rate through the nonideal electric field, all consistent with a reconnection diffusion region close to the X-line. In the diffusion region, electrons are modulated by electromagnetic waves. An ion exhaust with energized field-aligned ions and electron parallel heating are observed in the same shock transition region. The energized ions are more separated from the inflowing ions in velocity above the current sheet than below, possibly due to the shear flow between the two inflow regions. The observation suggests that magnetic reconnection may contribute to shock energy dissipation.

physics.space-ph↗

arXiv2018

A nearly continuous observation of the equatorial plasmasphere from the inner radiation belt to near a magnetopause reconnection site

On 22 October 2015, VAP and MMS obtained near-continuous observations of the full radial extent of the duskside equatorial plasmasphere and plume. The plume is evident in in situ plasma data and an equatorial mapping of the ionospheric total electron content. The properties of the equatorial plasmasphere change dramatically from its the inner radiation belt to its outermost boundary (the magnetopause, near a reconnection site). The density decreases by a factor of $\sim$1000 over this range and scales with $L$-shell as $L^{-4.3\pm0.4}$, in good agreement with with theoretical expectations of the expansion of a flux tube volume during outward radial transport. The proton temperature increases by a factor of $\sim$100 over this same range, with the most pronounced heating occurring at $L>7$, which was covered by the orbit of MMS.

physics.space-ph↗

arXiv2018

How accurately can we measure the reconnection rate $E_M$ for the MMS diffusion region event of 2017-07-11?

We investigate the accuracy with which the reconnection electric field $E_M$ can be determined from in-situ plasma data. We study the magnetotail electron diffusion region observed by NASA's Magnetospheric Multiscale (MMS) on 2017-07-11 at 22:34 UT and focus on the very large errors in $E_M$ that result from errors in an $LMN$ boundary-normal coordinate system. We determine several $LMN$ coordinates for this MMS event using several different methods. We use these $M$ axes to estimate $E_M$. We find some consensus that the reconnection rate was roughly $E_M$=3.2 mV/m $\pm$ 0.06 mV/m, which corresponds to a normalized reconnection rate of $0.18\pm0.035$. Minimum variance analysis of the electron velocity (MVA-$v_e$), MVA of $E$, minimization of Faraday residue, and an adjusted version of the maximum directional derivative of the magnetic field (MDD-$B$) technique all produce {reasonably} similar coordinate axes. We use virtual MMS data from a particle-in-cell simulation of this event to estimate the errors in the coordinate axes and reconnection rate associated with MVA-$v_e$ and MDD-$B$. The $L$ and $M$ directions are most reliably determined by MVA-$v_e$ when the spacecraft observes a clear electron jet reversal. When the magnetic field data has errors as small as 0.5\% of the background field strength, the $M$ direction obtained by MDD-$B$ technique may be off by as much as 35$^\circ$. The normal direction is most accurately obtained by MDD-$B$. Overall, we find that these techniques were able to identify $E_M$ from the virtual data within error bars $\geq$20\%.

physics.space-ph↗

arXiv2018

Assessing the time dependence of reconnection with Poynting's theorem: MMS observations

We investigate the time dependence of electromagnetic-field-to-plasma energy conversion in the electron diffusion region of asymmetric magnetic reconnection. To do so, we consider the terms in Poynting's theorem. In a steady state there is a perfect balance between the divergence of the electromagnetic energy flux $\nabla \cdot \vec{S}$ and the conversion between electromagnetic field and particle energy $\vec{J} \cdot \vec{E}$. This energy balance is demonstrated with a particle-in-cell simulation of reconnection. We also evaluate each of the terms in Poynting's theorem during an observation of a magnetopause reconnection region by Magnetospheric Multiscale (MMS). We take the equivalence of both sides of Poynting's theorem as an indication that the errors associated with the approximation of each term with MMS data are small. We find that, for this event, balance between $\vec{J}\cdot\vec{E}=-\nabla\cdot\vec{S}$ is only achieved for a small fraction of the energy conversion region at/near the X-point. Magnetic energy was rapidly accumulating on either side of the current sheet at roughly three times the predicted energy conversion rate. Furthermore, we find that while $\vec{J}\cdot\vec{E}>0$ and $\nabla\cdot\vec{S}<0$ are observed, as is expected for reconnection, the energy accumulation is driven by the overcompensation for $\vec{J}\cdot\vec{E}$ by $-\nabla\cdot\vec{S}>\vec{J}\cdot\vec{E}$. We note that due to the assumptions necessary to do this calculation, the accurate evaluation of $\nabla\cdot\vec{S}$ may not be possible for every MMS-observed reconnection event; but if possible, this is a simple approach to determine if reconnection is or is not in a steady-state.

physics.space-ph↗