1,004 research outputs found
Dynamics of the Innermost Accretion Flows Around Compact Objects: Magnetosphere-Disc Interface, Global Oscillations and Instabilities
We study global non-axisymmetric oscillation modes and instabilities in
magnetosphere- disc systems, as expected in neutron star X-ray binaries and
possibly also in accreting black hole systems. Our two-dimensional
magnetosphere-disc model consists of a Keplerian disc in contact with an
uniformly rotating magnetosphere with low plasma density. Two types of global
overstable modes exist in such systems, the interface modes and the disc
inertial-acoustic modes. We examine various physical effects and parameters
that influence the properties of these oscillation modes, particularly their
growth rates, including the magnetosphere field configuration, the velocity and
density contrasts across the magnetosphere-disc interface, the rotation profile
(with Newtonian or General Relativistic potential), the sound speed and
magnetic field of the disc. The interface modes are driven unstable by
Rayleigh-Taylor and Kelvin-Helmholtz in- stabilities, but can be stabilized by
the toroidal field (through magnetic tension) and disc differential rotation
(through finite vorticity). General relativity increases their growth rates by
modifying the disc vorticity outside the magnetosphere boundary. The interface
modes may also be affected by wave absorption associated with corotation
resonance in the disc. In the presence of a magnetosphere, the
inertial-acoustic modes are effectively trapped at the innermost region of the
relativistic disc just outside the interface. They are driven unstable by wave
absorption at the corotation resonance, but can be stabilized by modest disc
magnetic fields. The overstable oscillation modes studied in this paper have
characteristic properties that make them possible candidates for the
quasi-periodic oscillations observed in X-ray binaries.Comment: 18 pages, 9 figures, MNRAS accepte
Particle orbits at the magnetopause: Kelvin-Helmholtz induced trapping
The Kelvin-Helmholtz instability (KHI) is a known mechanism for penetration
of solar wind matter into the magnetosphere. Using three-dimensional, resistive
magnetohydrodynamic simulations, the double mid-latitude reconnection (DMLR)
process was shown to efficiently exchange solar wind matter into the
magnetosphere, through mixing and reconnection. Here, we compute test particle
orbits through DMLR configurations. In the instantaneous electromagnetic
fields, charged particle trajectories are integrated using the guiding centre
approximation. The mechanisms involved in the electron particle orbits and
their kinetic energy evolutions are studied in detail, to identify specific
signatures of the DMLR through particle characteristics. The charged particle
orbits are influenced mainly by magnetic curvature drifts. We identify complex,
temporarily trapped, trajectories where the combined electric field and
(reconnected) magnetic field variations realize local cavities where particles
gain energy before escaping. By comparing the orbits in strongly deformed
fields due to the KHI development, with the textbook mirror-drift orbits
resulting from our initial configuration, we identify effects due to current
sheets formed in the DMLR process. We do this in various representative stages
during the DMLR development.Comment: Matching accepted version in AGU JGR: Space Physic
Shear flow-interchange instability in nightside magnetotail causes auroral beads as a signature of substorm onset
A geometric wedge model of the near-earth nightside plasma sheet is used to
derive a wave equation for low frequency shear flow-interchange waves which
transmit sheared zonal flows along magnetic flux tubes
towards the ionosphere. Discrepancies with the wave equation result used in
Kalmoni et al. (2015) for shear flow-ballooning instability are discussed. The
shear flow-interchange instability appears to be responsible for substorm
onset. The wedge wave equation is used to compute rough expressions for
dispersion relations and local growth rates in the midnight region of the
nightside magnetotail where the instability develops, forming the auroral beads
characteristic of geomagnetic substorm onset. Stability analysis for the shear
flow-interchange modes demonstrates that nonlinear analysis is necessary for
quantitatively accurate results and determines the spatial scale on which the
instability varies
The role of the Hall effect in the global structure and dynamics of planetary magnetospheres: Ganymede as a case study
We present high resolution Hall MHD simulations of Ganymede's magnetosphere
demonstrating that Hall electric fields in ion-scale magnetic reconnection
layers have significant global effects not captured in resistive MHD
simulations. Consistent with local kinetic simulations of magnetic
reconnection, our global simulations show the development of intense
field-aligned currents along the magnetic separatrices. These currents extend
all the way down to the moon's surface, where they may contribute to Ganymede's
aurora. Within the magnetopause and magnetotail current sheets, Hall currents
in the reconnection plane accelerate ions to the local Alfv\'en speed in the
out-of-plane direction, producing a global system of ion drift belts that
circulates Jovian magnetospheric plasma throughout Ganymede's magnetosphere. We
discuss some observable consequences of these Hall-induced currents and ion
drifts: the appearance of a sub-Jovian "double magnetopause" structure, an
Alfv\'enic ion jet extending across the upstream magnetopause and an asymmetric
pattern of magnetopause Kelvin-Helmholtz waves.Comment: 14 pages, 12 figures; presented at Geospace Environment Modeling
(GEM) workshop (June, 2014) and Fall American Geophysical Union (AGU) meeting
(December, 2014); submitted to Journal of Geophysical Research, December 201
Asymmetric Kelvin-Helmholtz Instability at Jupiter's Magnetopause Boundary: Implications for Corotation-Dominated Systems
Ă©2018. American Geophysical Union. The multifluid Lyon-Fedder-Mobarry (MFLFM) global magnetosphere model is used to study the interactions between solar wind and rapidly rotating, internally driven Jupiter magnetosphere. The MFLFM model is the first global simulation of Jupiter magnetosphere that captures the Kelvin-Helmholtz instability (KHI) in the critically important subsolar region. Observations indicate that Kelvin-Helmholtz vortices are found predominantly in the dusk sector. Our simulations explain that this distribution is driven by the growth of KHI modes in the prenoon and subsolar region (e.g., > 10 local time) that are advected by magnetospheric flows to the dusk sector. The period of density fluctuations at the dusk terminator flank (18 magnetic local time, MLT) is roughly 1.4 h compared with 7.2 h at the dawn flank (6 MLT). Although the simulations are only performed using parameters of the Jupiter's magnetosphere, the results may also have implications for solar wind-magnetosphere interactions at other corotation-dominated systems such as Saturn. For instance, the simulated average azimuthal speed of magnetosheath flows exhibit significant dawn-dusk asymmetry, consistent with recent observations at Saturn. The results are particularly relevant for the ongoing Juno mission and the analysis of dawnside magnetopause boundary crossings for other planetary missions.Link_to_subscribed_fulltex
Asymmetric Kelvin-Helmholtz Propagation at Saturn\u27s Dayside Magnetopause
At Saturn\u27s magnetopause, the shear flows are maximized (minimized) in the prenoon (postnoon) sector due to the rapid planetary rotation and the corotating magnetodisc. As such, the prenoon sector is expected to be more KelvinâHelmholtz (KH) unstable than the postnoon sector; however, in situ Cassini data analyses showed that the evidence of KH activity favors the postnoon sector. In this study, we use a twoâdimensional MHD simulation to demonstrate that fastâgrowing KH modes strongly deform and diffuse the boundary layer on a time scale of a few minutes in the prenoon sector. Therefore, the KH observational signature is difficult to identify by spacecraft in the diffused boundary layer. KH vortices originating in the subsolar region (roughly from 10 to 14 local times) are transported to the postnoon sector and the wavelength is enlarged due to the gradient of shear flow, which is a plausible reason why KH events are more often observed in the postnoon sector. The prediction of the local boundary normal direction distribution as a function of spacecraft inward/outward crossing in the postnoon sector suggested by our simulation is qualitatively consistent with Cassini in situ observational results. We also discuss the impact of this dawnâdusk asymmetric KelvinâHelmholtz evolution on magnetic reconnection at Saturn\u27s magnetopause boundary
Nonlinear evolution of the magnetized Kelvin-Helmholtz instability: from fluid to kinetic modeling
The nonlinear evolution of collisionless plasmas is typically a multi-scale
process where the energy is injected at large, fluid scales and dissipated at
small, kinetic scales. Accurately modelling the global evolution requires to
take into account the main micro-scale physical processes of interest. This is
why comparison of different plasma models is today an imperative task aiming at
understanding cross-scale processes in plasmas. We report here the first
comparative study of the evolution of a magnetized shear flow, through a
variety of different plasma models by using magnetohydrodynamic, Hall-MHD,
two-fluid, hybrid kinetic and full kinetic codes. Kinetic relaxation effects
are discussed to emphasize the need for kinetic equilibriums to study the
dynamics of collisionless plasmas in non trivial configurations. Discrepancies
between models are studied both in the linear and in the nonlinear regime of
the magnetized Kelvin-Helmholtz instability, to highlight the effects of small
scale processes on the nonlinear evolution of collisionless plasmas. We
illustrate how the evolution of a magnetized shear flow depends on the relative
orientation of the fluid vorticity with respect to the magnetic field direction
during the linear evolution when kinetic effects are taken into account. Even
if we found that small scale processes differ between the different models, we
show that the feedback from small, kinetic scales to large, fluid scales is
negligable in the nonlinear regime. This study show that the kinetic modeling
validates the use of a fluid approach at large scales, which encourages the
development and use of fluid codes to study the nonlinear evolution of
magnetized fluid flows, even in the colisionless regime
Modeling radiation belt radial diffusion in ULF wave fields: 1. Quantifying ULF wave power at geosynchronous orbit in observations and in global MHD model
[1] To provide critical ULF wave field information for radial diffusion studies in the radiation belts, we quantify ULF wave power (f = 0.5â8.3 mHz) in GOES observations and magnetic field predictions from a global magnetospheric model. A statistical study of 9 years of GOES data reveals the wave local time distribution and power at geosynchronous orbit in field-aligned coordinates as functions of wave frequency, solar wind conditions (Vx, ÎPd and IMF Bz) and geomagnetic activity levels (Kp, Dst and AE). ULF wave power grows monotonically with increasing solar wind Vx, dynamic pressure variations ÎPd and geomagnetic indices in a highly correlated way. During intervals of northward and southward IMF Bz, wave activity concentrates on the dayside and nightside sectors, respectively, due to different wave generation mechanisms in primarily open and closed magnetospheric configurations. Since global magnetospheric models have recently been used to trace particles in radiation belt studies, it is important to quantify the wave predictions of these models at frequencies relevant to electron dynamics (mHz range). Using 27 days of real interplanetary conditions as model inputs, we examine the ULF wave predictions modeled by the Lyon-Fedder-Mobarry magnetohydrodynamic code. The LFM code does well at reproducing, in a statistical sense, the ULF waves observed by GOES. This suggests that the LFM code is capable of modeling variability in the magnetosphere on ULF time scales during typical conditions. The code provides a long-missing wave field model needed to quantify the interaction of radiation belt electrons with realistic, global ULF waves throughout the inner magnetosphere
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