251 research outputs found
Ambipolar Electric Field, Photoelectrons, and their Role in Atmospheric Escape From Hot-jupiters
Atmospheric mass-loss from Hot-jupiters can be large due to the close
proximity of these planets to their host star and the strong radiation the
planetary atmosphere receives. On Earth, a major contribution to the
acceleration of atmospheric ions comes from the vertical separation of ions and
electrons, and the generation of the ambipolar electric field. This process,
known as the "polar wind", is responsible for the transport of ionospheric
constituents to the Earth's magnetosphere, where they are well observed. The
polar wind can also be enhanced by a relatively small fraction of super-thermal
electrons (photoelectrons) generated by photoionization. We formulate a
simplified calculation of the effect of the ambipolar electric field and the
photoelectrons on the ion scale-height in a generalized manner. We find that
the ion scale-height can be increased by a factor of 2-15 due to the polar wind
effects. We also estimate a lower limit of an order of magnitude increase of
the ion density and the atmospheric mass-loss rate when polar wind effects are
included.Comment: 7 pages, 3 figures, accepted to ApJ Letter
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
Modeling the Quiet Time Outflow Solution in the Polar Cap
We use the Polar Wind Outflow Model (PWOM) to study the geomagnetically quiet conditions in the polar cap during solar maximum, The PWOM solves the gyrotropic transport equations for O(+), H(+), and He(+) along several magnetic field lines in the polar region in order to reconstruct the full 3D solution. We directly compare our simulation results to the data based empirical model of Kitamura et al. [2011] of electron density, which is based on 63 months of Akebono satellite observations. The modeled ion and electron temperatures are also compared with a statistical compilation of quiet time data obtained by the EISCAT Svalbard Radar (ESR) and Intercosmos Satellites (Kitamura et al. [2011]). The data and model agree reasonably well. This study shows that photoelectrons play an important role in explaining the differences between sunlit and dark results, ion composition, as well as ion and electron temperatures of the quiet time polar wind solution. Moreover, these results provide validation of the PWOM's ability to model the quiet time ((background" solution
Including Kinetic Ion Effects in the Coupled Global Ionospheric Outflow Solution
We present a new expansion of the Polar Wind Outflow Model to include kinetic ions using the particleâ inâ cell (PIC) approach with Monte Carlo collisions. This implementation uses the original hydrodynamic solution at low altitudes for efficiency and couples to the kinetic solution at higher altitudes to account for kinetic effects important for ionospheric outflow. The modeling approach also includes waveâ particle interactions, suprathermal electrons, and a hybrid parallel computing approach combining shared and distributed memory paralellization. The resulting model is thus a comprehensive, global, model of ionospheric outflow that can be run efficiently on large supercomputing clusters. We demonstrate the model’s capability to study a range of problems starting with the comparison of kinetic and hydrodynamic solutions along a single field line in the sunlit polar cap, and progressing to the altitude evolution of the ion conic distribution in the cusp region. The interplay between convection and the cusp on the global outflow solution is also examined. Finally, we demonstrate the impact of these new model features on the magnetosphere by presenting the first twoâ way coupled ionospheric outflowâ magnetosphere calculation including kinetic ion effects.Key PointsWe present a global ionospheric outflow model with kinetic ions and waveâ particle interactionsThe code uses a hybrid parallelization scheme to achieve fast executionIt is the first twoâ way coupled global kinetic outflow code coupled to a multifluid MHD magnetospherePeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/144220/1/jgra54182_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/144220/2/jgra54182.pd
Photoelectrons in the quiet polar wind
This study presents a newly coupled model capable of treating the superthermal electron population in the global polar wind solution. The model combines the hydrodynamic Polar Wind Outflow Model (PWOM) with the kinetic SuperThermal Electron Transport (STET) code. The resulting PWOM‐STET coupled model is described and then used to investigate the role of photoelectrons in the polar wind. We present polar wind results along single stationary field lines under dayside and nightside conditions, as well as the global solution reconstructed from nearly 1000 moving field lines. The model results show significant day‐night asymmetries in the polar wind solution owing to the higher ionization and photoelectron fluxes on the dayside compared to the nightside. Field line motion is found to modify this dependence and create global structure by transporting field lines through different conditions of illumination and through the localized effects of Joule heating.Key PointsStudy presents a newly coupled model capable of treating the superthermal electron population in the global polar wind solutionSingle stationary field line solutions under sunlit and dark conditions are presented as is the global solution from ∼1000 moving linesField line motion creates global structure by transporting field lines through different conditions of illumination and Joule heatingPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/137691/1/jgra53574.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/137691/2/jgra53574_am.pd
Tracing magnetic separators and their dependence on IMF clock angle in global magnetospheric simulations
A new, efficient, and highly accurate method for tracing magnetic separators
in global magnetospheric simulations with arbitrary clock angle is presented.
The technique is to begin at a magnetic null and iteratively march along the
separator by finding where four magnetic topologies meet on a spherical
surface. The technique is verified using exact solutions for separators
resulting from an analytic magnetic field model that superposes dipolar and
uniform magnetic fields. Global resistive magnetohydrodynamic simulations are
performed using the three-dimensional BATS-R-US code with a uniform
resistivity, in eight distinct simulations with interplanetary magnetic field
(IMF) clock angles ranging from 0 (parallel) to 180 degrees (anti-parallel).
Magnetic nulls and separators are found in the simulations, and it is shown
that separators traced here are accurate for any clock angle, unlike the last
closed field line on the Sun-Earth line that fails for southward IMF. Trends in
magnetic null locations and the structure of magnetic separators as a function
of clock angle are presented and compared with those from the analytic field
model. There are many qualitative similarities between the two models, but
quantitative differences are also noted. Dependence on solar wind density is
briefly investigated.Comment: 10 pages, 10 figures, Presented at 2012 AGU Fall Meeting and 2013
Geospace Environment Modeling (GEM) Worksho
- …