Engineering topics

Pollock, C. J.

Publications and source records attributed to Pollock, C. J..

At least 19 records

DOE OSTI2022

Spin-orbit coupling controlled ground states in the double perovskite iridates $\textit{A}_2\textit{B}$IrO$_6$ ($\textit{A}$ = Ba, Sr; $\textit{B}$ = Lu, Sc)

Iridates with the 5$d^4$ electronic configuration have attracted recent interest due to reports of magnetically ordered ground states despite longstanding expectations that their strong spin-orbit coupling would generate a $\textit{J}$ = 0 electronic ground state for each Ir 5+ ion. The major focus of prior research has been on the double perovskite iridates Ba 2 YIrO 6 and Sr 2 YIrO 6 , where the nature of the ground states (i.e., ordered vs nonmagnetic) is still controversial. Here, in this work, we present neutron powder diffraction, high-energy-resolution fluorescence-detected x-ray absorption spectroscopy (HERFD-XAS), resonant inelastic x-ray scattering (RIXS), magnetic susceptibility, and muon spin relaxation data on the related double perovskite iridates Ba 2 LuIrO 6 , Sr 2 LuIrO 6 , Ba 2 ScIrO 6 , and Sr 2 ScIrO 6 that enable us to gain a general understanding of the electronic and magnetic properties for this family of materials. Our HERFD-XAS and RIXS measurements establish $\textit{J}$ = 0 electronic ground states for the Ir 5+ ions in all cases, with similar values for Hund's coupling $J_H$ and the spin-orbit coupling constant $λ_{\text{SOC}}$. Our bulk susceptibility and muon spin relaxation data find no evidence for long-range magnetic order or spin freezing, but they do exhibit weak magnetic signals that are consistent with extrinsic local moments. Our results indicate that the large $λ_{\text{SOC}}$ is the key driving force behind the electronic and magnetic ground states realized in the $5d^4$ double perovskite iridates, which agrees well with conventional wisdom.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

NASA NTRS2018

Small-Scale Flux Transfer Events Formed in the Reconnection Exhaust Region Between Two X Lines

We report MMS observations of the ion-scale flux transfer events (FTEs) that may involve two main X lines and tearing instability between the two X lines. The four spacecraft detected multiple isolated regions with enhanced magnetic field strength and bipolar B(sub )n signatures normal to the nominal magnetopause, indicating FTEs. The currents within the FTEs flow mostly parallel to B, and the magnetic tension force is balanced by the total pressure gradient force. During these events, the plasma bulk flow velocity was directed southward. Detailed analysis of the magnetic and electric field and plasma moments variations suggests that the FTEs were initially embedded within the exhaust region north of an X line but were later located southward/downstream of a subsequent X line. The cross sections of the individual FTEs are in the range of ~2.5-6.8 ion inertial lengths. The observations suggest the formation of multiple secondary FTEs. The presence of an X line in the exhaust region southward of a second X line results from the southward drift of an old X line and the reformation of a new X line. The current layer between the two X lines is unstable to the tearing instability, generating multiple ion-scale flux-rope-type secondary islands.

Hwang, K.-J.↗

NASA NTRS2018

Magnetospheric Multiscale Observations of Turbulent Magnetic and Electron Velocity Fluctuations in Earth's Magnetosheath Downstream of a Quasi-Parallel Bow Shock

We present statistical single-spacecraft observations of magnetic and electron velocity fluctuations in Earth's magnetosheath, likely in the vicinity of the magnetopause, downstream of a bow shock immersed in quasi-parallel interplanetary magnetic field conditions, a situation conducive to plasma turbulence in the downstream flow. These fluctuations exhibit scale-dependent behavior, wherein histograms of their Partial Variance of Increments (PVIB or PVIV(sub e)) demonstrate highly non-Gaussian forms at small scales and are reasonably well-described by kappa distributions, albeit with fitted values of the kappa parameter only slightly larger than 1.5, exemplifying their power law nature at large values of PVI. At larger scales, the PVI histograms lose their non-Gaussian nature and are well described by both Gaussian and kappa distributions with large values of the kappa parameter. The PVI histograms furthermore exhibit kurtosis that increases with decreasing scale, a characteristic that is much more prominent in the magnetic fluctuations than in the electron velocity fluctuations. This feature that is not yet explained. In both cases, the results are characteristic of turbulent intermittency.

Pollock, C. J.↗

NASA NTRS2018

Electron Crescent Distributions as a Manifestation of Diamagnetic Drift in an Electron-Scale Current Sheet: Magnetospheric Multiscale Observations Using New 7.5 ms Fast Plasma Investigation Moments

We report Magnetospheric Multiscale observations of electron pressure gradient electric fields near a magnetic reconnection diffusion region using a new technique for extracting 7.5 ms electron moments from the Fast Plasma Investigation. We find that the deviation of the perpendicular electron bulk velocity from E × B drift in the interval where the out-of-plane current density is increasing can be explained by the diamagnetic drift. In the interval where the out-of-plane current is transitioning to in-plane current, the electron momentum equation is not satisfied at 7.5 ms resolution.

fast plasma↗

NASA NTRS2017

Magnetospheric Multiscale Mission Observations of the Outer Electron Diffusion Region

This paper presents Magnetospheric Multiscale mission (MMS) observations of the exhaust region in the vicinity of the central reconnection site in Earth's magnetopause current sheet. High-time-resolution measurements of field and particle distributions enable us to explore the fine structure of the diffusion region near the X line. Ions are decoupled from the magnetic field throughout the entire current sheet crossing. Electron jets flow downstream from the X line at speeds greater than the E by B drift velocity. At or around the magnetospheric separatrix, large-amplitude electric fields containing field-aligned components accelerate electrons along the magnetic field toward the X line. Near the neutral sheet, crescent-shaped electron distributions appear coincident with (1) an out-of-plane electric field whose polarity is opposite to that of the reconnection electric field and (2) the energy transfer from bulk kinetic to field energy. The observations indicate that MMS passed through the edge of an elongated electron diffusion region (EDR) or the outer EDR in the exhaust region.

Hwang, K.-J.↗

NASA NTRS2017

Electron Heating at Kinetic Scales in Magnetosheath Turbulence

We present a statistical study of coherent structures at kinetic scales, using data from the Magnetospheric Multiscale mission in the Earths magnetosheath. We implemented the multi-spacecraft partial variance of increments (PVI) technique to detect these structures, which are associated with intermittency at kinetic scales. We examine the properties of the electron heating occurring within such structures. We find that, statistically, structures with a high PVI index are regions of significant electron heating. We also focus on one such structure, a current sheet, which shows some signatures consistent with magnetic reconnection. Strong parallel electron heating coincides with whistler emissions at the edges of the current sheet.

Chasapis, Alexandros↗

NASA NTRS2016

Observations of Turbulence in a Kelvin-Helmholtz Event on 8 September 2015 by the Magnetospheric Multiscale Mission

Spatial and high-time-resolution properties of the velocities, magnetic field, and 3-D electric field within plasma turbulence are examined observationally using data from the Magnetospheric Multiscale mission. Observations from a Kelvin-Helmholtz instability (KHI) on the Earth's magnetopause are examined, which both provides a series of repeatable intervals to analyze, giving better statistics, and provides a first look at the properties of turbulence in the KHI. For the first time direct observations of both the high-frequency ion and electron velocity spectra are examined, showing differing ion and electron behavior at kinetic scales. Temporal spectra exhibit power law behavior with changes in slope near the ion gyrofrequency and lower hybrid frequency. The work provides the first observational evidence for turbulent intermittency and anisotropy consistent with quasi two-dimensional turbulence in association with the KHI. The behavior of kinetic-scale intermittency is found to have differences from previous studies of solar wind turbulence, leading to novel insights on the turbulent dynamics in the KHI.

Stawarz, J. E.↗

NASA NTRS2016

Rippled Quasiperpendicular Shock Observed by the Magnetospheric Multiscale Spacecraft

Collisionless shock nonstationarity arising from microscale physics influences shock structure and particle acceleration mechanisms. Nonstationarity has been difficult to quantify due to the small spatial and temporal scales. We use the closely spaced (subgyroscale), high-time-resolution measurements from one rapid crossing of Earths quasiperpendicular bow shock by the Magnetospheric Multiscale (MMS) spacecraft to compare competing nonstationarity processes. Using MMSs high-cadence kinetic plasma measurements, we show that the shock exhibits nonstationarity in the form of ripples.

Johlander, A.↗

NASA NTRS2016

The Substructure of a Flux Transfer Event Observed by the MMS Spacecraft

On 15 August 2015, MMS (Magnetospheric Multiscale mission), skimming the dusk magnetopause, detected an isolated region of an increased magnetic strength and bipolar Bn, indicating a flux transfer event (FTE). The four spacecraft in a tetrahedron allowed for investigations of the shape and motion of the FTE. In particular, high-resolution particle data facilitated our exploration of FTE substructures and their magnetic connectivity inside and surrounding the FTE. Combined field and plasma observations suggest that the core fields are open, magnetically connected to the northern magnetosphere from which high-energy particles leak; ion "D" distributions characterize the axis of flux ropes that carry old-opened field lines; counter streaming electrons superposed by parallel-heated components populate the periphery surrounding the FTE; and the interface between the core and draped regions contains a separatrix of newlyopened magnetic field lines that emanate from the X line above the FTE.

Hwang, K.-J.↗

NASA NTRS2016

Observations of Large-Amplitude, Parallel, Electrostatic Waves Associated with the Kelvin-Helmholtz Instability by the Magnetospheric Multiscale Mission

On 8 September 2015, the four Magnetospheric Multiscale spacecraft encountered a Kelvin-Helmholtz unstable magnetopause near the dusk flank. The spacecraft observed periodic compressed current sheets, between which the plasma was turbulent. We present observations of large-amplitude (up to 100 mVm) oscillations in the electric field. Because these oscillations are purely parallel to the background magnetic field, electrostatic, and below the ion plasma frequency, they are likely to be ion acoustic-like waves. These waves are observed in a turbulent plasma where multiple particle populations are intermittently mixed, including cold electrons with energies less than 10 eV. Stability analysis suggests a cold electron component is necessary for wave growth.

Wilder, F. D.↗

NASA NTRS2016

MMS Observations of Ion-Scale Magnetic Island in the Magnetosheath Turbulent Plasma

In this letter, first observations of ion-scale magnetic island from the Magnetospheric Multiscale mission in the magnetosheath turbulent plasma are presented. The magnetic island is characterized by bipolar variation of magnetic fields with magnetic field compression, strong core field, density depletion, and strong currents dominated by the parallel component to the local magnetic field. The estimated size of magnetic island is about 8 di, where di is the ion inertial length. Distinct particle behaviors and wave activities inside and at the edges of the magnetic island are observed: parallel electron beam accompanied with electrostatic solitary waves and strong electromagnetic lower hybrid drift waves inside the magnetic island and bidirectional electron beams, whistler waves, weak electromagnetic lower hybrid drift waves, and strong broadband electrostatic noise at the edges of the magnetic island. Our observations demonstrate that highly dynamical, strong wave activities and electron-scale physics occur within ion-scale magnetic islands in the magnetosheath turbulent plasma..

Huang, S. Y.↗

NASA NTRS2016

Finite Gyroradius Effects in the Electron Outflow of Asymmetric Magnetic Reconnection

We present observations of asymmetric magnetic reconnection showing evidence of electron demagnetization in the electron outflow. The observations were made at the magnetopause by the four Magnetospheric Multiscale (MMS) spacecraft, separated by approximately 15 km. The reconnecting current sheet has negligible guide field, and all four spacecraft likely pass close to the electron diffusion region just south of the X line. In the electron outflow near the X line, all four spacecraft observe highly structured electron distributions in a region comparable to a few electron gyroradii. The distributions consist of a core with T(sub parallel) greater than T(sub perpendicular) and a nongyrotropic crescent perpendicular to the magnetic field. The crescents are associated with finite gyroradius effects of partly demagnetized electrons. These observations clearly demonstrate the manifestation of finite gyroradius effects in an electron-scale reconnection current sheet.

Norgren, C.↗

NASA NTRS2016

Magnetospheric Multiscale Observations of the Electron Diffusion Region of Large Guide Field Magnetic Reconnection

We report observations from the Magnetospheric Multiscale (MMS) satellites of a large guide field magnetic reconnection event. The observations suggest that two of the four MMS spacecraft sampled the electron diffusion region, whereas the other two spacecraft detected the exhaust jet from the event. The guide magnetic field amplitude is approximately 4 times that of the reconnecting field. The event is accompanied by a significant parallel electric field (E(sub parallel lines) that is larger than predicted by simulations. The high-speed (approximately 300 km/s) crossing of the electron diffusion region limited the data set to one complete electron distribution inside of the electron diffusion region, which shows significant parallel heating. The data suggest that E(sub parallel lines) is balanced by a combination of electron inertia and a parallel gradient of the gyrotropic electron pressure.

Eriksson, S.↗

NASA NTRS2016

Comparison of Magnetospheric Multiscale Ion Jet Signatures with Predicted Reconnection Site Locations at the Magnetopause

Magnetic reconnection at the Earths magnetopause is the primary process by which solar wind plasma and energy gains access to the magnetosphere. One indication that magnetic reconnection is occurring is the observation of accelerated plasma as a jet tangential to the magnetopause. The direction of ion jets along the magnetopause surface as observed by the Fast Plasma Instrument (FPI) and the Hot Plasma Composition Analyzer (HPCA) instrument on board the recently launched Magnetospheric Multiscale (MMS) set of spacecraft is examined. For those cases where ion jets are clearly discerned, the direction of origin compares well statistically with the predicted location of magnetic reconnection using convected solar wind observations in conjunction with the Maximum Magnetic Shear model.

Petrinec, S. M.↗

NASA NTRS2016

Observations of Energetic Particle Escape at the Magnetopause: Early Results from the MMS Energetic Ion Spectrometer (EIS)

Energetic (greater than tens of keV) magnetospheric particle escape into the magnetosheath occurs commonly, irrespective of conditions that engender reconnection and boundary-normal magnetic fields. A signature observed by the Magnetospheric Multiscale (MMS) mission, simultaneous monohemispheric streaming of multiple species (electrons, H+, Hen+), is reported here as unexpectedly common in the dayside, dusk quadrant of the magnetosheath even though that region is thought to be drift-shadowed from energetic electrons. This signature is sometimes part of a pitch angle distribution evolving from symmetric in the magnetosphere, to asymmetric approaching the magnetopause, to monohemispheric streaming in the magnetosheath. While monohemispheric streaming in the magnetosheath may be possible without a boundary-normal magnetic field, the additional pitch angle depletion, particularly of electrons, on the magnetospheric side requires one. Observations of this signature in the dayside dusk sector imply that the static picture of magnetospheric drift-shadowing is inappropriate for energetic particle dynamics in the outer magnetosphere.

Cohen, I. J.↗

NASA NTRS2016

Estimates of Terms in Ohm's Law During an Encounter with an Electron Diffusion Region

We present measurements from the Magnetospheric Multiscale (MMS) mission taken during a reconnection event on the dayside magnetopause which includes a passage through an electron diffusion region (EDR). The four MMS satellites were separated by about 10 km such that estimates of gradients and divergences allow a reasonable estimate of terms in the generalized Ohm's law, which is key to investigating the energy dissipation during reconnection. The strength and character of dissipation mechanisms determines how magnetic energy is released. We show that both electron pressure gradients and electron inertial effects are important, but not the only participants in reconnection near EDRs, since there are residuals of a few mVm (approximately 30-50%) of E+ U(sub e) x B (from the sum of these two terms) during the encounters. These results are compared to a simulation, which exhibits many of the observed features, but where relatively little residual is present.

Torbert, R. B.↗

NASA NTRS2016

Magnetospheric Multiscale Observations of Large-Amplitude Parallel, Electrostatic Waves Associated with Magnetic Reconnection at the Magnetopause

We report observations from the Magnetospheric Multiscale satellites of large-amplitude, parallel, electrostatic waves associated with magnetic reconnection at the Earth's magnetopause. The observed waves have parallel electric fields (E(sub parallel)) with amplitudes on the order of 100 mV/m and display nonlinear characteristics that suggest a possible net E(sub parallel). These waves are observed within the ion diffusion region and adjacent to (within several electron skin depths) the electron diffusion region. They are in or near the magnetosphere side current layer. Simulation results support that the strong electrostatic linear and nonlinear wave activities appear to be driven by a two stream instability, which is a consequence of mixing cold (less than 10eV) plasma in the magnetosphere with warm (approximately 100eV) plasma from the magnetosheath on a freshly reconnected magnetic field line. The frequent observation of these waves suggests that cold plasma is often present near the magnetopause.

Ergun, R. E.↗

NASA NTRS2016

Magnetospheric Multiscale Satellites Observations of Parallel Electric Fields Associated with Magnetic Reconnection

We report observations from the Magnetospheric Multiscale satellites of parallel electric fields (E (sub parallel)) associated with magnetic reconnection in the subsolar region of the Earth's magnetopause. E (sub parallel) events near the electron diffusion region have amplitudes on the order of 100 millivolts per meter, which are significantly larger than those predicted for an antiparallel reconnection electric field. This Letter addresses specific types of E (sub parallel) events, which appear as large-amplitude, near unipolar spikes that are associated with tangled, reconnected magnetic fields. These E (sub parallel) events are primarily in or near a current layer near the separatrix and are interpreted to be double layers that may be responsible for secondary reconnection in tangled magnetic fields or flux ropes. These results are telling of the three-dimensional nature of magnetopause reconnection and indicate that magnetopause reconnection may be often patchy and/or drive turbulence along the separatrix that results in flux ropes and/or tangled magnetic fields.

Ergun, R. E.↗