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

Papers by M.-c. Fok

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.

Ring Current Asymmetry and the Love‐Gannon Relation

Modeling the Inner-Magnetosphere Ionosphere with the CIMI Model

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

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.

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.

Journal of Geophysical Research: Space Physics, 2017

We have developed and tested a scheme for forecasting severe space weather (SvSW) that caused all... more We have developed and tested a scheme for forecasting severe space weather (SvSW) that caused all known electric power outages and telecommunication system failures since 1957 and the Carrington event of 1859. The SvSW events of 04 August 1972 has puzzled the scientific community as it occurred during a moderate storm (DstMin = -124 nT) while all other SvSW events occurred during super storms (DstMin ≤ -250 nT). The solar wind velocity V and IMF Bz measured by ACE satellite at the L1 point since 1998 are used. For the earlier SvSW events such as the Carrington event of 1859, Quebec event of 1989, and the events in February 1958 and August 1972 we used the information from the literature. The coincidence of high ICME front (or shock) velocity ΔV (sudden increase in V over the background by over 275 km/s) and sufficiently large Bz southward at the time of the ΔV increase is associated with SvSW; and their product (ΔV×Bz) is found to exhibit a large negative spike at the speed increase. Such a product (ΔV×Bz) exceeding a threshold seems suitable for forecasting SvSW, with a maximum forecasting time of 35 minutes using ACE data. However, the coincidence of high V (not containing ΔV) and large Bz southward does not correspond to SvSW, indicating the importance of the impulsive action of high ΔV and large Bz southward coming through when they coincide. The need for the coincidence is verified using the CRCM.

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.

Journal of Geophysical Research: Space Physics, 2013

In this article, we introduce the Bioinspired Neuroprosthetic Design Environment (BNDE) as a prac... more In this article, we introduce the Bioinspired Neuroprosthetic Design Environment (BNDE) as a practical platform for the development of novel brain-machine interface (BMI) controllers, which are based on spiking model neurons. We built the BNDE around a hard real-time system so that it is capable of creating simulated synapses from extracellularly recorded neurons to model neurons. In order to evaluate the practicality of the BNDE for neuroprosthetic control experiments, a novel, adaptive BMI controller was developed and tested using real-time closed-loop simulations. The present controller consists of two in silico medium spiny neurons, which receive simulated synaptic inputs from recorded motor cortical neurons. In the closed-loop simulations, the recordings from the cortical neurons were imitated using an external, hardware-based neural signal synthesizer. By implementing a reward-modulated spike timing-dependent plasticity rule, the controller achieved perfect target reach accuracy for a two-target reaching task in one-dimensional space. The BNDE combines the flexibility of software-based spiking neural network (SNN) simulations with powerful online data visualization tools and is a low-cost, PC-based, and all-in-one solution for developing neurally inspired BMI controllers. We believe that the BNDE is the first implementation, which is capable of creating hybrid biological/in silico neural networks for motor neuroprosthetic control and utilizes multiple CPU cores for computationally intensive real-time SNN simulations.

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

Journal of Geophysical Research, 2010

is the largest measured during the extended solar minimum between December 2006 and March 2010. W... more is the largest measured during the extended solar minimum between December 2006 and March 2010. We present observations of this storm made by the two wide-angle imaging neutral-atom spectrometers (TWINS) mission. The TWINS mission measures energetic neutral atoms (ENAs) using sensors mounted on two separate spacecrafts. Because the two spacecrafts' orbital planes are significantly offset, the pair provides a nearly optimal combination of continuous magnetospheric observations from at least one of the TWINS platforms with several hours of simultaneous, dual-platform viewing over each orbit. The ENA imaging study presented in this paper is the first reported magnetospheric storm for which both continuous coverage and stereoscopic imaging were available. Two populations of ENAs are observed during this storm. The first are emissions from the ring current and come from a parent population of trapped ions in the inner magnetosphere. The second, low-altitude emissions (LAEs), are the result of precipitating ions which undergo multiple charge exchange and stripping collisions with the oxygen exosphere. The temporal evolution of this storm shows that the LAEs begin earlier and are the brightest emissions seen during the main phase, while later, during the recovery, the LAE is only as bright as the bulk ring current emissions.

Journal of Geophysical Research, 2008

We present a technique for estimating ring current ion distributions and electric potential in th... more We present a technique for estimating ring current ion distributions and electric potential in the inner magnetosphere by incorporating data from the High-Energy Neutral Atom (HENA) imager on the IMAGE satellite into a kinetic ring current model in the context of data assimilation. Data assimilation is an approach which adjusts a physicsbased model according to differences with observations. We perform the data assimilation using the particle filter (PF) which is applicable to high-dimensional systems and observations with relatively low computational cost. In the present technique, the magnetospheric electric potential distribution is represented by the sum of a Volland-Stern field and a deviation, and the deviation is improved in the assimilation process. The ring current ion distribution is then determined from the electric potential distribution. The method is tested by assimilating artificial data generated by another simulation. The results demonstrate that the ring current ion distribution is successfully reconstructed by the proposed algorithm and that the distorted structures of the electric potential distribution are also well reproduced. An example to illustrate how the present technique could be applied for assimilating a real IMAGE/HENA data set is also demonstrated. This technique provides a useful tool for investigating the global dynamic structure of ring current and electric potential.

Geophysical Research Letters, 2008

We have modeled the plasmaspheric plume region using the Comprehensive Ring Current model (CRCM) ... more We have modeled the plasmaspheric plume region using the Comprehensive Ring Current model (CRCM) and the Dynamical Global Core Plasma model (DGCPM), for an event that exhibited substantial undulations or ripples as observed by the IMAGE EUV imager during 17 April 2002. We drove the simulated electric field using the Weimer cross polar cap potential. We specified the magnetic field to vary in response to solar wind conditions according to the T96 model. As a control, we performed a run with a fixed magnetic field and a run with a low ring current pressure. The results show that particle injections into the inner magnetosphere and ring current-ionosphere-plasmasphere interaction are an essential part of the undulation response. We also conclude that the undulations are stronger in the case of magnetic field variations because associated induction electric field causes more pronounced injections.

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 +).

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

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 (<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'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 (> 52 keV) O ENAs.

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.

New Developments in the Comprehensive Inner Magnetosphere‐Ionosphere Model

Journal of Geophysical Research: Space Physics

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

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.

Geospace storm processes coupling the ring current, radiation belt and plasmasphere

Inner Magnetosphere Interactions: New Perspectives From Imaging, 2005

Ring Current Asymmetry and the Love‐Gannon Relation

Modeling the Inner-Magnetosphere Ionosphere with the CIMI Model

A Case Study on the Origin of Near‐Earth Plasma

Journal of Geophysical Research: Space Physics, 2020

Journal of Geophysical Research: Space Physics, 2019

Magnetospheric boundary perturbations on MHD and kinetic scales

Journal of Geophysical Research: Space Physics, 2015

Journal of Geophysical Research: Space Physics, 2017

TWINS stereoscopic imaging of multiple peaks in the ring current

Journal of Geophysical Research: Space Physics, 2015

Journal of Geophysical Research: Space Physics, 2013

Using Plasma Convection to Observe Global Electric Fields in the Inner Magnetosphere

Journal of Geophysical Research, 2010

Journal of Geophysical Research, 2008

Geophysical Research Letters, 2008

Annales Geophysicae, 2011

LENA observations on March 31, 2001: Magnetosheath remote sensing

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

Journal of Geophysical Research, 1991

New Developments in the Comprehensive Inner Magnetosphere‐Ionosphere Model

Journal of Geophysical Research: Space Physics

Space Science Reviews

The Astrophysical Journal