ORCID Profile
0000-0002-3096-8579
Current Organisation
University of Colorado at Boulder
Does something not look right? The information on this page has been harvested from data sources that may not be up to date. We continue to work with information providers to improve coverage and quality. To report an issue, use the Feedback Form.
Publisher: American Geophysical Union (AGU)
Date: 11-08-2021
DOI: 10.1029/2021GL093164
Abstract: Magnetic reconnection is a fundamental process in collisionless space plasma environment, and plasma waves relevant to the kinetic interactions can have a significant impact on the multiscale behavior of reconnection. Here, we present Magnetospheric Multiscale (MMS) observations during an encounter of an X line of symmetric magnetic reconnection in the magnetotail. The X line is characterized by reversals of ion and electron jets and electromagnetic fields, agyrotropic electron velocity distribution functions (VDFs), and an electron‐scale current sheet. MMS observe large‐ litude nonlinear upper‐hybrid (UH) waves on both sides of the neutral line, and the wave litudes have highly localized distribution along the normal direction. The inbound meandering electrons drive the UH waves, releasing the free energy stored from the reconnection electric field along the meandering trajectories. The interaction between the meandering electrons and the UH waves may modify the balance of the reconnection electric field around the X line.
Publisher: American Geophysical Union (AGU)
Date: 12-2019
DOI: 10.1029/2019JA027228
Abstract: Magnetic reconnection at the subsolar magnetopause is persistently accompanied by strong fluctuations of the magnetic field ( B ), plasma density ( n ), and all components of the electric field ( E ) and current ( J ). The strongest fluctuations are at frequencies below the lower hybrid frequency and observed in a thin, intense current sheet adjacent to the electron diffusion region. In this current sheet, the background magnitudes of B and n are changing considerably, creating an inhomogeneous plasma environment. We show that the fluctuations in B and n are consistent with an oscillatory displacement of the current sheet in the surface normal direction. The displacement is propagating parallel to the magnetic reconnection X line. Wavelengths are on the order of or longer than the thickness of the current sheet to which they are confined, so we label these waves electromagnetic drift waves. E and J fluctuations are more complex than a simple displacement. They have significant variations in the component along B , which suggest that the drift waves also may be confined along B . The current sheet is supported by an electron drift driven by normal electric field, which, in turn, is balanced by an ion pressure gradient. We suggest that wave growth comes from an instability related to the drift between the electrons and ions. We discuss the possibility that drift waves can displace or penetrate into the electron diffusion region giving magnetic reconnection three‐dimensional structure. Drift waves may corrugate the X line, possibly breaking the X line and generating turbulence.
Publisher: American Geophysical Union (AGU)
Date: 03-2018
DOI: 10.1002/2017JA024517
Publisher: American Geophysical Union (AGU)
Date: 06-2018
DOI: 10.1029/2018JA025245
Publisher: American Astronomical Society
Date: 06-2021
Publisher: American Geophysical Union (AGU)
Date: 15-10-2020
DOI: 10.1029/2020GL089880
Publisher: American Geophysical Union (AGU)
Date: 04-2020
DOI: 10.1029/2019JA027665
Publisher: American Geophysical Union (AGU)
Date: 04-03-2021
DOI: 10.1029/2020GL091437
Abstract: We investigate agyrotropic electron distributions from two magnetopause events observed by magnetospheric multiscale (MMS) spacecraft. Agyrotropic electron distributions can be generated by the finite electron gyration at an electron‐scale boundary, and the electric field normal to this boundary usually contributes to the electron acceleration to make the agyrotropic distributions more apparent. The effect of the electric field becomes important only when it is sufficiently strong and local, meaning its electrostatic potential is comparable to or larger than the electron temperature, and its width is smaller than the electron thermal gyroradius, so that this electric field can directly accelerate part of the electrons out of the original core to form agyrotropic electron distributions. Also, we reproduce the measured electron “finger” structures from test particle simulations, which can be effectively suppressed by increasing the s ling rate of the electron measurement.
Publisher: American Geophysical Union (AGU)
Date: 21-07-2020
DOI: 10.1029/2020GL087159
Abstract: We investigate an exhaust crossing of asymmetric guide field reconnection observed by Magnetospheric Multiscale (MMS) mission at Earth's dayside magnetopause. One MMS spacecraft (MMS 4) observes multicomponent electron distributions, including two counterstreaming electron beams, while the other three MMS spacecraft, with a separation of ∼30 km, record nearly isotropic electron distributions. As counterstreaming electrons are unstable for the electron two‐stream instability, our observations suggest that the electrostatic waves generated by the fast‐growing electron two‐stream instability can contribute to the rapid isotropization of electron distributions in the reconnection exhaust, indicating that wave‐particle interactions play an important role in electron dynamics.
Publisher: American Geophysical Union (AGU)
Date: 2021
DOI: 10.1029/2020JA028650
Abstract: An extended interval of perturbed magnetospheric conditions in November 2016 supported increased convection and sunward transport of plasmaspheric material. During this period of time the Magnetospheric Multiscale satellites, with their apogees along Earth's dusk‐side outer magnetosphere, encountered several cold plasma density structures at the same time as plasma bulk flows capable of accelerating hidden cold plasma occurred. Investigating the charged particle and fields data during two subintervals showed that the satellites made direct measurements of cold plasmaspheric ions embedded within multicomponent hot plasmas as well as electromagnetic emissions consistent with electromagnetic ion cyclotron (EMIC) waves. The complex in situ ion composition measurements were applied to linear wave modeling to interpret the impacts of cold and hot ion species on wave growth and band structure. Although the waves for both intervals were predicted to have peak growth rate below Ω He+ , substantial differences were observed among all other dispersive properties. The modeling also showed EMIC waves generated in the presence of heavy ions had growth rates and unstable wave numbers always smaller than predicted for a pure proton‐electron plasma. The results provide implications for future investigation of EMIC wave generation with and without direct measurements of the cold and hot plasma composition as well as of subsequent wave‐particle interactions.
Publisher: American Geophysical Union (AGU)
Date: 05-2017
DOI: 10.1002/2017JA024062
Publisher: American Geophysical Union (AGU)
Date: 10-02-2018
DOI: 10.1002/2017GL076809
Publisher: Wiley
Date: 30-07-2020
Publisher: American Geophysical Union (AGU)
Date: 09-05-2017
DOI: 10.1002/2017GL072703
Publisher: American Geophysical Union (AGU)
Date: 2017
DOI: 10.1002/2016JA023572
Publisher: American Geophysical Union (AGU)
Date: 05-2019
DOI: 10.1002/2017JA024024
Publisher: American Geophysical Union (AGU)
Date: 10-2017
DOI: 10.1002/2017JA024287
Publisher: American Geophysical Union (AGU)
Date: 23-11-2018
DOI: 10.1029/2018GL080826
Publisher: Wiley
Date: 04-2018
DOI: 10.1002/2017JA025034
Publisher: American Geophysical Union (AGU)
Date: 09-04-2017
DOI: 10.1002/2016GL072493
Publisher: American Geophysical Union (AGU)
Date: 23-04-2019
DOI: 10.1029/2019GL082471
Abstract: Plasma and wave measurements from the NASA Magnetospheric Multiscale mission are presented for magnetotail reconnection events on 3 July and 11 July 2017. Linear dispersion analyses were performed using distribution functions comprising up to six drifting bi‐Maxwellian distributions. In both events electron crescent‐shaped distributions are shown to be responsible for upper hybrid waves near the X‐line. In an adjacent location within the 3 July event a monodirectional field‐aligned electron beam drove parallel‐propagating beam‐mode waves. In the 11 July event an electron distribution consisting of a drifting core and two crescents was shown to generate upper‐hybrid and beam‐mode waves at three different frequencies, explaining the observed broadband waves. Multiple harmonics of the upper hybrid waves were observed but cannot be explained by the linear dispersion analysis since they result from nonlinear beam interactions.
Publisher: American Geophysical Union (AGU)
Date: 19-03-2019
DOI: 10.1029/2019GL082231
Publisher: American Geophysical Union (AGU)
Date: 02-2018
DOI: 10.1002/2017JA024789
Publisher: American Geophysical Union (AGU)
Date: 11-2019
DOI: 10.1029/2019JA027155
Abstract: Waves around the lower hybrid frequency are frequently observed at Earth's magnetopause and readily reach very large litudes. Determining the properties of lower hybrid waves is crucial because they are thought to contribute to electron and ion heating, cross‐field particle diffusion, anomalous resistivity, and energy transfer between electrons and ions. All these processes could play an important role in magnetic reconnection at the magnetopause and the evolution of the boundary layer. In this paper, the properties of lower hybrid waves at Earth's magnetopause are investigated using the Magnetospheric Multiscale mission. For the first time, the properties of the waves are investigated using fields and direct particle measurements. The highest‐resolution electron moments resolve the velocity and density fluctuations of lower hybrid waves, confirming that electrons remain approximately frozen in at lower hybrid wave frequencies. Using fields and particle moments, the dispersion relation is constructed and the wave‐normal angle is estimated to be close to 90° to the background magnetic field. The waves are shown to have a finite parallel wave vector, suggesting that they can interact with parallel propagating electrons. The observed wave properties are shown to agree with theoretical predictions, the previously used single‐spacecraft method, and four‐spacecraft timing analyses. These results show that single‐spacecraft methods can accurately determine lower hybrid wave properties.
Publisher: American Geophysical Union (AGU)
Date: 12-2019
DOI: 10.1029/2019JA027275
Abstract: We present a model of electromagnetic drift waves in the current sheet adjacent to magnetic reconnection at the subsolar magnetopause. These drift waves are potentially important in governing 3‐D structure of subsolar magnetic reconnection and in generating turbulence. The drift waves propagate nearly parallel to the X line and are confined to a thin current sheet. The scale size normal to the current sheet is significantly less than the ion gyroradius and can be less than or on the order of the wavelength. The waves also have a limited extent along the magnetic field ( B ), making them a three‐dimensional eigenmode structure. In the current sheet, the background magnitudes of B and plasma density change significantly, calling for a treatment that incorporates an inhomogeneous plasma environment. Using detailed examination of Magnetospheric Multiscale observations, we find that the waves are best represented by series of electron vortices, superimposed on a primary electron drift, that propagate along the current sheet (parallel to the X line). The waves displace or corrugate the current sheet, which also potentially displaces the electron diffusion region. The model is based on fluid behavior of electrons, but ion motion must be treated kinetically. The strong electron drift along the X line is likely responsible for wave growth, similar to a lower hybrid drift instability. Contrary to a classical lower hybrid drift instability, however, the strong changes in the background B and n o , the normal confinement to the current sheet, and the confinement along B are critical to the wave description.
Publisher: American Geophysical Union (AGU)
Date: 09-2020
DOI: 10.1029/2020JA027854
Abstract: Spacecraft potential has often been used to infer electron density with much higher time resolution than is typically possible with plasma instruments. However, recently, two studies by Torkar et al. (2017, 0.1002/2017JA024724 ) and Graham, Vaivads, Khotyaintsev, Eriksson, et al. (2018, 0.1029/2018JA025874 ) have shown that external electric fields can also have an effect on the spacecraft potential by enhancing photoelectron escape from the surface. Consequently, should the electron density derived from the spacecraft potential be used during an event with a large electric field, the estimation would be contaminated and the user would see the effects of the electric field rather than density perturbations. The goal of this paper is to propose a method to remove the electric field effects to allow the density derived from spacecraft potential to be used even during large‐ litude wave events such as Langmuir waves or upper hybrid waves.
Publisher: American Geophysical Union (AGU)
Date: 11-2017
DOI: 10.1002/2017JA024535
Abstract: In this paper we use the full armament of the MMS (Magnetospheric Multiscale) spacecraft to study magnetic reconnection in the turbulent magnetosheath downstream of a quasi‐parallel bow shock. Contrarily to the magnetopause and magnetotail cases, only a few observations of reconnection in the magnetosheath have been reported. The case study in this paper presents, for the first time, both fluid‐scale and kinetic‐scale signatures of an ongoing reconnection in the turbulent magnetosheath. The spacecraft are crossing the reconnection inflow and outflow regions and the ion diffusion region (IDR). Inside the reconnection outflows D shape ion distributions are observed. Inside the IDR mixing of ion populations, crescent‐like velocity distributions and ion accelerations are observed. One of the spacecraft skims the outer region of the electron diffusion region, where parallel electric fields, energy dissipation/conversion, electron pressure tensor agyrotropy, electron temperature anisotropy, and electron accelerations are observed. Some of the difficulties of the observations of magnetic reconnection in turbulent plasma are also outlined.
Publisher: American Geophysical Union (AGU)
Date: 06-2017
DOI: 10.1002/2016JA023666
Publisher: American Geophysical Union (AGU)
Date: 09-2017
DOI: 10.1002/2017JA024553
Publisher: American Geophysical Union (AGU)
Date: 2018
DOI: 10.1002/2017JA024551
Abstract: Mirror mode waves are ubiquitous in the Earth's magnetosheath, in particular behind the quasi‐perpendicular shock. Embedded in these nonlinear structures, intense lion roars are often observed. Lion roars are characterized by whistler wave packets at a frequency ∼100 Hz, which are thought to be generated in the magnetic field minima. In this study, we make use of the high time resolution instruments on board the Magnetospheric MultiScale mission to investigate these waves and the associated electron dynamics in the quasi‐perpendicular magnetosheath on 22 January 2016. We show that despite a core electron parallel anisotropy, lion roars can be generated locally in the range 0.05–0.2 f ce by the perpendicular anisotropy of electrons in a particular energy range. We also show that intense lion roars can be observed up to higher frequencies due to the sharp nonlinear peaks of the signal, which appear as sharp spikes in the dynamic spectra. As a result, a high s ling rate is needed to estimate correctly their litude, and the latter might have been underestimated in previous studies using lower time resolution instruments. We also present for the first‐time 3‐D high time resolution electron velocity distribution functions in mirror modes. We demonstrate that the dynamics of electrons trapped in the mirror mode structures are consistent with the Kivelson and Southwood (1996) model. However, these electrons can also interact with the embedded lion roars: first signatures of electron quasi‐linear pitch angle diffusion and possible signatures of nonlinear interaction with high‐ litude wave packets are presented. These processes can lead to electron untrapping from mirror modes.
Publisher: American Geophysical Union (AGU)
Date: 28-08-2018
DOI: 10.1029/2018GL078660
Publisher: American Geophysical Union (AGU)
Date: 12-2017
DOI: 10.1002/2017JA024550
Location: United States of America
No related grants have been discovered for ROBERT ERGUN.