M.-c. Fok - Academia.edu (original) (raw)

Papers by M.-c. Fok

Research paper thumbnail of Geospace storm processes coupling the ring current, radiation belt and plasmasphere

Inner Magnetosphere Interactions: New Perspectives From Imaging, 2005

The plasmasphere/ring-current/radiation-belt are interacting systems. The magnetic field generate... more The plasmasphere/ring-current/radiation-belt are interacting systems. The magnetic field generated by the ring current changes the drift paths of energetic particles. Pressure gradients in the ring current produce the region 2 field aligned currents, which close in the ionosphere and create an electric field that acts to shield the lower-latitude region from the full force of convection. In turn, this shielding field alters the transport of the ring current and plasmaspheric plasmas. Furthermore, the anisotropy in the ring current plasmas excites waves that cause pitch-angle and energy diffusion of radiation belt and ring current particles. On the other hand, the precipitation of energetic electrons modifies the ionospheric conductances, and thus the electric field configuration in the magnetosphere-ionosphere system. A number of models of the plasmasphere, ring current and the radiation belt have been developed to study the behaviors of the inner magnetosphere during geospace storms. However, the majority of these models are designed to study a particular plasma population, without the consideration of interactions from others. In this paper, we briefly describe state-of-the-art models of the plasmasphere, ring current, and radiation belt, and present results from a preliminary coupling effort. The coupled models are shown to produce certain observed features of the inner magnetosphere: the post-midnight peak of storm main phase ring current ion flux; the plasmaspheric disturbance produced by impulsive substorm plasma injections, and the slow ramp-up of geosynchronous fluxes associated with energy diffusion. We conclude by presenting a framework on coupling these models together interactively to make significant progress toward a realistic plasmasphere/ring-current/radiation-belt interaction model.

Research paper thumbnail of Ring Current Asymmetry and the Love‐Gannon Relation

Research paper thumbnail of Modeling the Inner-Magnetosphere Ionosphere with the CIMI Model

Research paper thumbnail of A Case Study on the Origin of Near‐Earth Plasma

Journal of Geophysical Research: Space Physics, 2020

Summary of data files. This is the dataset for a paper "A case study on the origin of near-E... more Summary of data files. This is the dataset for a paper "A case study on the origin of near-Earth Plasma"<br> submitted to JGR-space by Glocer et al The paper conducts multifluid MHD simulations of the magnetosphere by the<br> BATSRUS code, with solar wind input, ionospheric outflow by the PWOM code,<br> and ionospheric potential solver, and inner magnetospehre by the CIMI code.<br> Solutions with and without plasmasphere are considered. All plots in the paper are made with spacepy (specific fork:<br> https://github.com/aglocer/spacepy) or with tecplot The data is organized as follows Top directory:<br> imf.dat: Has the solar wind input as a simple time series. Can be read and<br> plotted with the "ImfInput" tool in spacepy tared Directories with "PS" and "noPS" tags have simulation<br> output with and without plasmasphere. Each has three subdirectories described<br> as follows: GM: Has the "Global Magnetosphere" output from BATSRUS for the images shown.<br> y=0*out are cuts in the y=0 GSM plane and the *log file is the log output<br> containing Dst. Both can be plotted with spacepy pybats. The 3d files are<br> used in a few images and are tecplot binary files and read and plotted with<br> tecplot. PW: Has the "Polar Wind" output from the PWOM code for images shown. The<br> *out files are time dependent binary output for each field line. They can be<br> read and plotted with the pybats.pwom tool in spacepy. North and South<br> indicte northern and southern hemisphere respectively IE: Has the "Ionosphere Electrodynamics" output from the potential solver for<br> plots shown. the *log files have the CPCP data as a function of time. They<br> can be read and plotted with pybats in spacepy IM: Has the "Inner Magnetosphere" output from the CIMI code. The CIMIeq.out<br> file has time dependent snapshots of the solution on the min B surface for<br> plots shown. The *log files have the total energy as a function of time for<br> each species (among other variables). Both files are read and plotted with<br> pybats and pybats. [...]

Research paper thumbnail of Contribution of ULF Wave Activity to the Global Recovery of the Outer Radiation Belt During the Passage of a High‐Speed Solar Wind Stream Observed in September 2014

Journal of Geophysical Research: Space Physics, 2019

Energy coupling between the solar wind and the Earth's magnetosphere can affect the electron popu... more Energy coupling between the solar wind and the Earth's magnetosphere can affect the electron population in the outer radiation belt. However, the precise role of different internal and external mechanisms that leads to changes of the relativistic electron population is not entirely known. This paper describes how ultralow frequency (ULF) wave activity during the passage of Alfvénic solar wind streams contributes to the global recovery of the relativistic electron population in the outer radiation belt. To investigate the contribution of the ULF waves, we searched the Van Allen Probes data for a period in which we can clearly distinguish the enhancement of electron fluxes from the background. We found that the global recovery that started on 22 September 2014, which coincides with the corotating interaction region preceding a high-speed stream and the occurrence of persistent substorm activity, provides an excellent scenario to explore the contribution of ULF waves. To support our analyses, we employed ground-and space-based observational data and global magnetohydrodynamic simulations and calculated the ULF wave radial diffusion coefficients employing an empirical model. Observations show a gradual increase of electron fluxes in the outer radiation belt and a concomitant enhancement of ULF activity that spreads from higher to lower L-shells. Magnetohydrodynamic simulation results agree with observed ULF wave activity in the magnetotail, which leads to both fast and Alfvén modes in the magnetospheric nightside sector. The observations agree with the empirical model and are confirmed by phase space density calculations for this global recovery period.

Research paper thumbnail of Magnetospheric boundary perturbations on MHD and kinetic scales

Journal of Geophysical Research: Space Physics, 2015

ABSTRACT To study the magnetopause on both MHD and kinetic scales, we have analyzed two THEMIS/AR... more ABSTRACT To study the magnetopause on both MHD and kinetic scales, we have analyzed two THEMIS/ARTEMIS magnetopause crossings under steady slow-solar wind and minimum magnetic shear conditions. These events approximate a ground state of the magnetospheric boundary with minimum influences from large solar wind disturbances and magnetic reconnection. Our observations reveal evidence for the Kelvin-Helmholtz instability, the quasi-periodicity of magnetopause surface waves accompanied by highly asymmetrical plasma signatures between the inbound (from magnetosheath to LLBL) and the outbound (from LLBL to magnetosheath) magnetopause crossings. Stronger plasma and magnetic gradients were observed during the outbound crossings but more gradual and volatile variations at higher frequencies during the inbounds. The scale lengths of the magnetic and plasma gradients were comparable or less than the proton gyro-radius. Enhancements of lower-hybrid waves occurred at the locations of strong gradients or variations. We interpreted the collocations of the lower-hybrid waves and plasma gradients and their variations in terms of (1) lower hybrid instabilities that directly convert solar wind flow energy into lower-hybrid waves and other wave modes in the LLBL, or (2) Kelvin-Helmholtz instability and magnetic reconnection which produce the conditions for the lower-hybrid instabilities to grow. The rate of ion diffusion across the magnetopause caused by the lower hybrid instability is marginally sufficient to populate the LLBL. The diffusion coefficient of O+ is ~30 times larger than that of H+. The lower hybrid waves could contribute to the energy dissipation at plasma gradients in magnetopause surface wave structures and limit KHI growth further downstream.

Research paper thumbnail of A scheme for forecasting severe space weather

Journal of Geophysical Research: Space Physics, 2017

Research paper thumbnail of TWINS stereoscopic imaging of multiple peaks in the ring current

Journal of Geophysical Research: Space Physics, 2015

ABSTRACT Global, ion equatorial flux distributions and energy spectra are presented from stereosc... more ABSTRACT Global, ion equatorial flux distributions and energy spectra are presented from stereoscopic TWINS 1 and TWINS 2 ENA images for two time periods, 29 May 2010, 1330–1430 UT and 26 May 2011, 1645–1715 UT. The first is just after the main phase of a weak (minimum SYM/H ≈ −70 to −80 nT) CIR (Co-rotating Interaction Region) driven geomagnetic storm. The second is during a relatively quiet period. The global ion distributions show multiple spatial peaks that are coincident with peaks in the AE index. The energy spectra have a primary maximum in the 15–20 keV range. Below the energy maximum, the flux is Maxwellian. Above the main maximum, the flux is either significantly below that of a Maxwellian or has a second component with a maximum in the 40–50 keV range. For the 29 May 2010, 1330–1430 UT time period, the flux from the TWINS stereoscopic images is compared to the results from TWINS 1 and TWINS 2 alone illustrating the advantage of stereoscopic viewing. The flux deconvolved from the TWINS images also show spatial and temporal correlations with THEMIS in-situ measurements. Magnetic field dipolarizations observed by GOES support the existence of a peak in the ion flux in the midnight/dawn sector. In summary, increased spatial resolution from TWINS stereoscopic ENA images is demonstrated. Multiple peaks in the ion flux of trapped particles in the ring current are observed. THEMIS ESA in-situ ion flux measurements and GOES geosynchronous magnetic field measurements are consistent with the spatial and temporal structure obtained.

Research paper thumbnail of Comparative analysis of low-altitude ENA emissions in two substorms

Journal of Geophysical Research: Space Physics, 2013

Research paper thumbnail of Using Plasma Convection to Observe Global Electric Fields in the Inner Magnetosphere

Solar wind driven large scale electric fields play an important role in the dynamics of the plasm... more Solar wind driven large scale electric fields play an important role in the dynamics of the plasmasphere, the storm-time ring current, and the plasma sheet access to the inner magnetosphere. Local in situ measurements of the electric field provide a detailed yet highly localized picture. Global field maps, often derived by mapping out ionospheric measurements or models, may only inadequately

Research paper thumbnail of Evolution of low-altitude and ring current ENA emissions from a moderate magnetospheric storm: Continuous and simultaneous TWINS observations

Journal of Geophysical Research, 2010

Research paper thumbnail of A method for estimating the ring current structure and the electric potential distribution using energetic neutral atom data assimilation

Journal of Geophysical Research, 2008

Research paper thumbnail of Generation of plasmaspheric undulations

Geophysical Research Letters, 2008

Research paper thumbnail of On the effect of IMF turning on ion dynamics at Mercury

Annales Geophysicae, 2011

We investigate the effect of a rotation of the Interplanetary Magnetic Field (IMF) on the transpo... more We investigate the effect of a rotation of the Interplanetary Magnetic Field (IMF) on the transport of magnetospheric ion populations at Mercury. We focus on ions of planetary origin and investigate their large-scale circulation using three-dimensional single-particle simulations. We show that a nonzero B X component of the IMF leads to a pronounced asymmetry in the overall circulation pattern. In particular, we demonstrate that the centrifugal acceleration due to curvature of the E × B drift paths is more pronounced in one hemisphere than the other, leading to filling of the magnetospheric lobes and plasma sheet with more or less energetic material depending upon the hemisphere of origin. Using a time-varying electric and magnetic field model, we investigate the response of ions to rapid (a few tens of seconds) reorientation of the IMF. We show that, for ions with gyroperiods comparable to the field variation time scale, the inductive electric field should lead to significant nonadiabatic energization, up to several hundreds of eVs or a few keVs. It thus appears that IMF turning at Mercury should lead to localized loading of the magnetosphere with energetic material of planetary origin (e.g., Na +).

Research paper thumbnail of LENA observations on March 31, 2001: Magnetosheath remote sensing

We discuss observations from the IMAGE Low Energy Neutral Atom (LENA) imager on March 31, 2001 wh... more We discuss observations from the IMAGE Low Energy Neutral Atom (LENA) imager on March 31, 2001 when the solar wind flux, as measured by ACE/SWEPAM, was over a factor of ten higher than typical solar wind conditions. Observations from LANL-94 on this day indicate that over the time period of interest, about 0330-0600 UT, the magnetopause was inside of geosynchronous

Research paper thumbnail of Large magnetic storms as viewed by TWINS: A study of the differences in the medium energy ENA composition

Journal of Geophysical Research: Space Physics, 2014

ABSTRACT During large geomagnetic storms (Dst ≤ -100 nT) oxygen can become a significant componen... more ABSTRACT During large geomagnetic storms (Dst ≤ -100 nT) oxygen can become a significant component of the energetic particles of the inner magnetosphere. Until recently, there were no available global observations of the medium energy (&lt;50 keV) oxygen populations. Using observations from the Two Wide-angle Imaging Neutral-atom Spectrometers (TWINS) Energetic Neutral Atom (ENA) imagers we present a study of nine large storms of solar cycle 24 as a function of storm phase. For these storms we observe that the H and O ENA fluxes and their temperatures increase in tandem during the storm&#39;s initial phase. However, there is no increase in the O+/H+ ratio in the inner magnetosphere until the storm main phase. Also seen during the main phase is an energy dispersion with higher energy (32 keV) H ENAs seen before the arrival of O ENAs of the same energy. The O ENAs take longer to return to pre-storm levels during the recovery phases. This longer recovery time is likely because of the large difference between the storm-time and pre-storm O populations compared to H (i.e. there is always some pre-storm H in the inner magnetosphere, but effectively no O pre-storm). These results imply that medium-energy O ENAs evolve over long time scales (hours to days) as opposed to the shorter substorm time-scales of the higher energy (&gt; 52 keV) O ENAs.

Research paper thumbnail of Seasonal variations in the subauroral electron temperature enhancement

Journal of Geophysical Research, 1991

A statistical study of the seasonal variations of the subauroral electron temperature enhancement... more A statistical study of the seasonal variations of the subauroral electron temperature enhancement was undertaken using data from the Langmuir probe experiment on the DE 2 satellite throughout most of the mission (1981-1982). In the winter hemisphere the nighttime background electron temperature is the highest and the magnitude of the peak Te responds most weakly to the geomagnetic activity. This behavior can be explained by seasonal trends in the nighttime downward heat flux due to conjugate photoelectrons. Moreover, model results indicate that a factor of about three increase in heat inflow during equinox relative to solstice is required to raise the electron temperature to a given level. This is a consequence of the higher electron densities at the Te peak near equinox. The Te peak occurs on field lines which thread the outer plasmasphere in the vicinity if the plasmapause and thus can be used as a tracer of the plasmapause position.

Research paper thumbnail of New Developments in the Comprehensive Inner Magnetosphere‐Ionosphere Model

Journal of Geophysical Research: Space Physics

Research paper thumbnail of Imaging Plasma Density Structures in the Soft X-Rays Generated by Solar Wind Charge Exchange with Neutrals

Space Science Reviews

Both heliophysics and planetary physics seek to understand the complex nature of the solar wind's... more Both heliophysics and planetary physics seek to understand the complex nature of the solar wind's interaction with solar system obstacles like Earth's magnetosphere, the ionospheres of Venus and Mars, and comets. Studies with this objective are frequently conducted with the help of single or multipoint in situ electromagnetic field and particle observations, guided by the predictions of both local and global numerical simulations, and placed in con

Research paper thumbnail of On the Contribution of EMIC Waves to the Reconfiguration of the Relativistic Electron Butterfly Pitch Angle Distribution Shape on 2014 September 12—A Case Study

The Astrophysical Journal

Following the arrival of two interplanetary coronal mass ejections on 2014 September 12, the Rela... more Following the arrival of two interplanetary coronal mass ejections on 2014 September 12, the Relativistic Electron-Proton Telescope instrument on board the twin Van Allen Probes observed a long-term dropout in the outer belt electron fluxes. The interplanetary shocks compressed the magnetopause, thereby enabling the loss of relativistic electrons in the outer radiation belt to the magnetosheath region via the magnetopause shadowing. Previous studies have invoked enhanced radial transport associated with ultra-low-frequency waves activity and/or scattering into the atmosphere by whistler mode chorus waves to explain electron losses deep within the magnetosphere (L<5.5). We show that energetic electron pitch angle distributions (PADs) provide strong evidence for precipitation also via interaction with electromagnetic ion cyclotron (EMIC) waves. High-resolution magnetic field observations on Van Allen Probe B confirm the sporadic presence of EMIC waves during the most intense dropout phase on September 12. Observational results suggest that magnetopause shadowing and EMIC waves together were responsible for reconfiguring the relativistic electron PADs into peculiar butterfly PAD shapes a few hours after an interplanetary shock arrived at Earth.

Research paper thumbnail of Geospace storm processes coupling the ring current, radiation belt and plasmasphere

Inner Magnetosphere Interactions: New Perspectives From Imaging, 2005

The plasmasphere/ring-current/radiation-belt are interacting systems. The magnetic field generate... more The plasmasphere/ring-current/radiation-belt are interacting systems. The magnetic field generated by the ring current changes the drift paths of energetic particles. Pressure gradients in the ring current produce the region 2 field aligned currents, which close in the ionosphere and create an electric field that acts to shield the lower-latitude region from the full force of convection. In turn, this shielding field alters the transport of the ring current and plasmaspheric plasmas. Furthermore, the anisotropy in the ring current plasmas excites waves that cause pitch-angle and energy diffusion of radiation belt and ring current particles. On the other hand, the precipitation of energetic electrons modifies the ionospheric conductances, and thus the electric field configuration in the magnetosphere-ionosphere system. A number of models of the plasmasphere, ring current and the radiation belt have been developed to study the behaviors of the inner magnetosphere during geospace storms. However, the majority of these models are designed to study a particular plasma population, without the consideration of interactions from others. In this paper, we briefly describe state-of-the-art models of the plasmasphere, ring current, and radiation belt, and present results from a preliminary coupling effort. The coupled models are shown to produce certain observed features of the inner magnetosphere: the post-midnight peak of storm main phase ring current ion flux; the plasmaspheric disturbance produced by impulsive substorm plasma injections, and the slow ramp-up of geosynchronous fluxes associated with energy diffusion. We conclude by presenting a framework on coupling these models together interactively to make significant progress toward a realistic plasmasphere/ring-current/radiation-belt interaction model.

Research paper thumbnail of Ring Current Asymmetry and the Love‐Gannon Relation

Research paper thumbnail of Modeling the Inner-Magnetosphere Ionosphere with the CIMI Model

Research paper thumbnail of A Case Study on the Origin of Near‐Earth Plasma

Journal of Geophysical Research: Space Physics, 2020

Summary of data files. This is the dataset for a paper "A case study on the origin of near-E... more Summary of data files. This is the dataset for a paper "A case study on the origin of near-Earth Plasma"<br> submitted to JGR-space by Glocer et al The paper conducts multifluid MHD simulations of the magnetosphere by the<br> BATSRUS code, with solar wind input, ionospheric outflow by the PWOM code,<br> and ionospheric potential solver, and inner magnetospehre by the CIMI code.<br> Solutions with and without plasmasphere are considered. All plots in the paper are made with spacepy (specific fork:<br> https://github.com/aglocer/spacepy) or with tecplot The data is organized as follows Top directory:<br> imf.dat: Has the solar wind input as a simple time series. Can be read and<br> plotted with the "ImfInput" tool in spacepy tared Directories with "PS" and "noPS" tags have simulation<br> output with and without plasmasphere. Each has three subdirectories described<br> as follows: GM: Has the "Global Magnetosphere" output from BATSRUS for the images shown.<br> y=0*out are cuts in the y=0 GSM plane and the *log file is the log output<br> containing Dst. Both can be plotted with spacepy pybats. The 3d files are<br> used in a few images and are tecplot binary files and read and plotted with<br> tecplot. PW: Has the "Polar Wind" output from the PWOM code for images shown. The<br> *out files are time dependent binary output for each field line. They can be<br> read and plotted with the pybats.pwom tool in spacepy. North and South<br> indicte northern and southern hemisphere respectively IE: Has the "Ionosphere Electrodynamics" output from the potential solver for<br> plots shown. the *log files have the CPCP data as a function of time. They<br> can be read and plotted with pybats in spacepy IM: Has the "Inner Magnetosphere" output from the CIMI code. The CIMIeq.out<br> file has time dependent snapshots of the solution on the min B surface for<br> plots shown. The *log files have the total energy as a function of time for<br> each species (among other variables). Both files are read and plotted with<br> pybats and pybats. [...]

Research paper thumbnail of Contribution of ULF Wave Activity to the Global Recovery of the Outer Radiation Belt During the Passage of a High‐Speed Solar Wind Stream Observed in September 2014

Journal of Geophysical Research: Space Physics, 2019

Energy coupling between the solar wind and the Earth's magnetosphere can affect the electron popu... more Energy coupling between the solar wind and the Earth's magnetosphere can affect the electron population in the outer radiation belt. However, the precise role of different internal and external mechanisms that leads to changes of the relativistic electron population is not entirely known. This paper describes how ultralow frequency (ULF) wave activity during the passage of Alfvénic solar wind streams contributes to the global recovery of the relativistic electron population in the outer radiation belt. To investigate the contribution of the ULF waves, we searched the Van Allen Probes data for a period in which we can clearly distinguish the enhancement of electron fluxes from the background. We found that the global recovery that started on 22 September 2014, which coincides with the corotating interaction region preceding a high-speed stream and the occurrence of persistent substorm activity, provides an excellent scenario to explore the contribution of ULF waves. To support our analyses, we employed ground-and space-based observational data and global magnetohydrodynamic simulations and calculated the ULF wave radial diffusion coefficients employing an empirical model. Observations show a gradual increase of electron fluxes in the outer radiation belt and a concomitant enhancement of ULF activity that spreads from higher to lower L-shells. Magnetohydrodynamic simulation results agree with observed ULF wave activity in the magnetotail, which leads to both fast and Alfvén modes in the magnetospheric nightside sector. The observations agree with the empirical model and are confirmed by phase space density calculations for this global recovery period.

Research paper thumbnail of Magnetospheric boundary perturbations on MHD and kinetic scales

Journal of Geophysical Research: Space Physics, 2015

ABSTRACT To study the magnetopause on both MHD and kinetic scales, we have analyzed two THEMIS/AR... more ABSTRACT To study the magnetopause on both MHD and kinetic scales, we have analyzed two THEMIS/ARTEMIS magnetopause crossings under steady slow-solar wind and minimum magnetic shear conditions. These events approximate a ground state of the magnetospheric boundary with minimum influences from large solar wind disturbances and magnetic reconnection. Our observations reveal evidence for the Kelvin-Helmholtz instability, the quasi-periodicity of magnetopause surface waves accompanied by highly asymmetrical plasma signatures between the inbound (from magnetosheath to LLBL) and the outbound (from LLBL to magnetosheath) magnetopause crossings. Stronger plasma and magnetic gradients were observed during the outbound crossings but more gradual and volatile variations at higher frequencies during the inbounds. The scale lengths of the magnetic and plasma gradients were comparable or less than the proton gyro-radius. Enhancements of lower-hybrid waves occurred at the locations of strong gradients or variations. We interpreted the collocations of the lower-hybrid waves and plasma gradients and their variations in terms of (1) lower hybrid instabilities that directly convert solar wind flow energy into lower-hybrid waves and other wave modes in the LLBL, or (2) Kelvin-Helmholtz instability and magnetic reconnection which produce the conditions for the lower-hybrid instabilities to grow. The rate of ion diffusion across the magnetopause caused by the lower hybrid instability is marginally sufficient to populate the LLBL. The diffusion coefficient of O+ is ~30 times larger than that of H+. The lower hybrid waves could contribute to the energy dissipation at plasma gradients in magnetopause surface wave structures and limit KHI growth further downstream.

Research paper thumbnail of A scheme for forecasting severe space weather

Journal of Geophysical Research: Space Physics, 2017

Research paper thumbnail of TWINS stereoscopic imaging of multiple peaks in the ring current

Journal of Geophysical Research: Space Physics, 2015

ABSTRACT Global, ion equatorial flux distributions and energy spectra are presented from stereosc... more ABSTRACT Global, ion equatorial flux distributions and energy spectra are presented from stereoscopic TWINS 1 and TWINS 2 ENA images for two time periods, 29 May 2010, 1330–1430 UT and 26 May 2011, 1645–1715 UT. The first is just after the main phase of a weak (minimum SYM/H ≈ −70 to −80 nT) CIR (Co-rotating Interaction Region) driven geomagnetic storm. The second is during a relatively quiet period. The global ion distributions show multiple spatial peaks that are coincident with peaks in the AE index. The energy spectra have a primary maximum in the 15–20 keV range. Below the energy maximum, the flux is Maxwellian. Above the main maximum, the flux is either significantly below that of a Maxwellian or has a second component with a maximum in the 40–50 keV range. For the 29 May 2010, 1330–1430 UT time period, the flux from the TWINS stereoscopic images is compared to the results from TWINS 1 and TWINS 2 alone illustrating the advantage of stereoscopic viewing. The flux deconvolved from the TWINS images also show spatial and temporal correlations with THEMIS in-situ measurements. Magnetic field dipolarizations observed by GOES support the existence of a peak in the ion flux in the midnight/dawn sector. In summary, increased spatial resolution from TWINS stereoscopic ENA images is demonstrated. Multiple peaks in the ion flux of trapped particles in the ring current are observed. THEMIS ESA in-situ ion flux measurements and GOES geosynchronous magnetic field measurements are consistent with the spatial and temporal structure obtained.

Research paper thumbnail of Comparative analysis of low-altitude ENA emissions in two substorms

Journal of Geophysical Research: Space Physics, 2013

Research paper thumbnail of Using Plasma Convection to Observe Global Electric Fields in the Inner Magnetosphere

Solar wind driven large scale electric fields play an important role in the dynamics of the plasm... more Solar wind driven large scale electric fields play an important role in the dynamics of the plasmasphere, the storm-time ring current, and the plasma sheet access to the inner magnetosphere. Local in situ measurements of the electric field provide a detailed yet highly localized picture. Global field maps, often derived by mapping out ionospheric measurements or models, may only inadequately

Research paper thumbnail of Evolution of low-altitude and ring current ENA emissions from a moderate magnetospheric storm: Continuous and simultaneous TWINS observations

Journal of Geophysical Research, 2010

Research paper thumbnail of A method for estimating the ring current structure and the electric potential distribution using energetic neutral atom data assimilation

Journal of Geophysical Research, 2008

Research paper thumbnail of Generation of plasmaspheric undulations

Geophysical Research Letters, 2008

Research paper thumbnail of On the effect of IMF turning on ion dynamics at Mercury

Annales Geophysicae, 2011

We investigate the effect of a rotation of the Interplanetary Magnetic Field (IMF) on the transpo... more We investigate the effect of a rotation of the Interplanetary Magnetic Field (IMF) on the transport of magnetospheric ion populations at Mercury. We focus on ions of planetary origin and investigate their large-scale circulation using three-dimensional single-particle simulations. We show that a nonzero B X component of the IMF leads to a pronounced asymmetry in the overall circulation pattern. In particular, we demonstrate that the centrifugal acceleration due to curvature of the E × B drift paths is more pronounced in one hemisphere than the other, leading to filling of the magnetospheric lobes and plasma sheet with more or less energetic material depending upon the hemisphere of origin. Using a time-varying electric and magnetic field model, we investigate the response of ions to rapid (a few tens of seconds) reorientation of the IMF. We show that, for ions with gyroperiods comparable to the field variation time scale, the inductive electric field should lead to significant nonadiabatic energization, up to several hundreds of eVs or a few keVs. It thus appears that IMF turning at Mercury should lead to localized loading of the magnetosphere with energetic material of planetary origin (e.g., Na +).

Research paper thumbnail of LENA observations on March 31, 2001: Magnetosheath remote sensing

We discuss observations from the IMAGE Low Energy Neutral Atom (LENA) imager on March 31, 2001 wh... more We discuss observations from the IMAGE Low Energy Neutral Atom (LENA) imager on March 31, 2001 when the solar wind flux, as measured by ACE/SWEPAM, was over a factor of ten higher than typical solar wind conditions. Observations from LANL-94 on this day indicate that over the time period of interest, about 0330-0600 UT, the magnetopause was inside of geosynchronous

Research paper thumbnail of Large magnetic storms as viewed by TWINS: A study of the differences in the medium energy ENA composition

Journal of Geophysical Research: Space Physics, 2014

ABSTRACT During large geomagnetic storms (Dst ≤ -100 nT) oxygen can become a significant componen... more ABSTRACT During large geomagnetic storms (Dst ≤ -100 nT) oxygen can become a significant component of the energetic particles of the inner magnetosphere. Until recently, there were no available global observations of the medium energy (&lt;50 keV) oxygen populations. Using observations from the Two Wide-angle Imaging Neutral-atom Spectrometers (TWINS) Energetic Neutral Atom (ENA) imagers we present a study of nine large storms of solar cycle 24 as a function of storm phase. For these storms we observe that the H and O ENA fluxes and their temperatures increase in tandem during the storm&#39;s initial phase. However, there is no increase in the O+/H+ ratio in the inner magnetosphere until the storm main phase. Also seen during the main phase is an energy dispersion with higher energy (32 keV) H ENAs seen before the arrival of O ENAs of the same energy. The O ENAs take longer to return to pre-storm levels during the recovery phases. This longer recovery time is likely because of the large difference between the storm-time and pre-storm O populations compared to H (i.e. there is always some pre-storm H in the inner magnetosphere, but effectively no O pre-storm). These results imply that medium-energy O ENAs evolve over long time scales (hours to days) as opposed to the shorter substorm time-scales of the higher energy (&gt; 52 keV) O ENAs.

Research paper thumbnail of Seasonal variations in the subauroral electron temperature enhancement

Journal of Geophysical Research, 1991

A statistical study of the seasonal variations of the subauroral electron temperature enhancement... more A statistical study of the seasonal variations of the subauroral electron temperature enhancement was undertaken using data from the Langmuir probe experiment on the DE 2 satellite throughout most of the mission (1981-1982). In the winter hemisphere the nighttime background electron temperature is the highest and the magnitude of the peak Te responds most weakly to the geomagnetic activity. This behavior can be explained by seasonal trends in the nighttime downward heat flux due to conjugate photoelectrons. Moreover, model results indicate that a factor of about three increase in heat inflow during equinox relative to solstice is required to raise the electron temperature to a given level. This is a consequence of the higher electron densities at the Te peak near equinox. The Te peak occurs on field lines which thread the outer plasmasphere in the vicinity if the plasmapause and thus can be used as a tracer of the plasmapause position.

Research paper thumbnail of New Developments in the Comprehensive Inner Magnetosphere‐Ionosphere Model

Journal of Geophysical Research: Space Physics

Research paper thumbnail of Imaging Plasma Density Structures in the Soft X-Rays Generated by Solar Wind Charge Exchange with Neutrals

Space Science Reviews

Both heliophysics and planetary physics seek to understand the complex nature of the solar wind's... more Both heliophysics and planetary physics seek to understand the complex nature of the solar wind's interaction with solar system obstacles like Earth's magnetosphere, the ionospheres of Venus and Mars, and comets. Studies with this objective are frequently conducted with the help of single or multipoint in situ electromagnetic field and particle observations, guided by the predictions of both local and global numerical simulations, and placed in con

Research paper thumbnail of On the Contribution of EMIC Waves to the Reconfiguration of the Relativistic Electron Butterfly Pitch Angle Distribution Shape on 2014 September 12—A Case Study

The Astrophysical Journal

Following the arrival of two interplanetary coronal mass ejections on 2014 September 12, the Rela... more Following the arrival of two interplanetary coronal mass ejections on 2014 September 12, the Relativistic Electron-Proton Telescope instrument on board the twin Van Allen Probes observed a long-term dropout in the outer belt electron fluxes. The interplanetary shocks compressed the magnetopause, thereby enabling the loss of relativistic electrons in the outer radiation belt to the magnetosheath region via the magnetopause shadowing. Previous studies have invoked enhanced radial transport associated with ultra-low-frequency waves activity and/or scattering into the atmosphere by whistler mode chorus waves to explain electron losses deep within the magnetosphere (L<5.5). We show that energetic electron pitch angle distributions (PADs) provide strong evidence for precipitation also via interaction with electromagnetic ion cyclotron (EMIC) waves. High-resolution magnetic field observations on Van Allen Probe B confirm the sporadic presence of EMIC waves during the most intense dropout phase on September 12. Observational results suggest that magnetopause shadowing and EMIC waves together were responsible for reconfiguring the relativistic electron PADs into peculiar butterfly PAD shapes a few hours after an interplanetary shock arrived at Earth.