35 research outputs found
Ionospheric control of the magnetosphere: conductance
It is well known that the ionosphere plays a role in determining the global state of the magnetosphere. The ionosphere allows magnetospheric currents to close, thereby allowing magnetospheric convection to occur. The amount of current which can be carried through the ionosphere is mainly determined by the ionospheric conductivity. This paper starts to quantify the nonlinear relationship between the ionospheric conductivity and the global state of the magnetosphere. It is found that the steady-state magnetosphere acts neither as a current nor as a voltage generator; a uniform Hall conductance can influence the potential pattern at low latitudes, but not at high latitude; the EUV generated conductance forces the currents to close in the sunlight, while the potential is large on the nightside; the solar generated Hall conductances cause a large asymmetry between the dawn and dusk potential, which effects the pressure distribution in the magnetosphere; a uniform polar cap potential removes some of this asymmetry; the potential difference between solar minimum and maximum is &#x223C;11%; and the auroral precipitation can be related to the local field-aligned current through an exponential function.<br><br> <b>Key words.</b> Ionosphere (ionosphere-magnetosphere interactions; modelling and forecasting; polar ionosphere
Magnetic Effects Change Our View of the Heliosheath
There is currently a controversy as to whether Voyager 1 has already crossed
the Termination Shock, the first boundary of the Heliosphere. The region
between the Termination Shock and the Heliopause, the Helisheath, is one of the
most unknown regions theoretically. In the Heliosheath magnetic effects are
crucial, as the solar magnetic field is compressed at the Termination Shock by
the slowing flow. Recently, our simulations showed that the Heliosheath
presents remarkable dynamics, with turbulent flows and the presence of a jet
flow at the current sheet that is unstable due to magnetohydrodynamic
instabilities \cite{opher,opher1}. In this paper we review these recent
results, and present an additional simulation with constant neutral atom
background. In this case the jet is still present but with reduced intensity.
Further study, e.g., including neutrals and the tilt of the solar rotation from
the magnetic axis, is required before we can definitively address how the
Heliosheath behaves. Already we can say that this region presents remarkable
dynamics, with turbulent flows, indicating that the Heliosheath might be very
different from what we previously thought.Comment: 6 pages, 5 figures, to appear in IGPP 3rd Annual International
Astrophysics Conference, "PHYSICS OF THE OUTER HELIOSPHERE
Low frequency m=1 normal mode oscillations of a self-gravitating disc
A continuous system such as a galactic disc is shown to be well approximated
by an N-ring differentially rotating self-gravitating system. Lowest order
(m=1) non-axisymmetric features such as lopsidedness and warps are global in
nature and quite common in the discs of spiral galaxies. Apparently these two
features of the galactic discs have been treated like two completely disjoint
phenomena. The present analysis based on an eigenvalue approach brings out
clearly that these two features are fundamentally similar in nature and they
are shown to be very Low frequency Normal Mode (LNM) oscillations manifested in
different symmetry planes of the galactic disc. Our analysis also show that
these features are actually long-lived oscillating pattern of the N-ring
self-gravitating system.Comment: 5 figures. Accepted for publication in MNRAS Letter
Magnetic field structure at the diamagnetic cavity boundary (Numerical simulations)
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95513/1/grl13492.pd
Correction to “Interchange instability in the inner magnetosphere associated with geosynchronous particle flux decreases”
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94678/1/grl17909.pd
Magnetic Effects at the Edge of the Solar System: MHD Instabilities, the de Laval nozzle Effect and an Extended Jet
To model the interaction between the solar wind and the interstellar wind,
magnetic fields must be included. Recently Opher et al. 2003 found that, by
including the solar magnetic field in a 3D high resolution simulation using the
University of Michigan BATS-R-US code, a jet-sheet structure forms beyond the
solar wind Termination Shock. Here we present an even higher resolution
three-dimensional case where the jet extends for beyond the Termination
Shock. We discuss the formation of the jet due to a de Laval nozzle effect and
it's su bsequent large period oscillation due to magnetohydrodynamic
instabilities. To verify the source of the instability, we also perform a
simplified two dimensional-geometry magnetohydrodynamic calculation of a plane
fluid jet embedded in a neutral sheet with the profiles taken from our 3D
simulation. We find remarkable agreement with the full three-dimensional
evolution. We compare both simulations and the temporal evolution of the jet
showing that the sinuous mode is the dominant mode that develops into a
velocity-shear-instability with a growth rate of . As a result, the outer edge of the heliosphere
presents remarkable dynamics, such as turbulent flows caused by the motion of
the jet. Further study, e.g., including neutrals and the tilt of the solar
rotation from the magnetic axis, is required before we can definitively address
how this outer boundary behaves. Already, however, we can say that the magnetic
field effects are a major player in this region changing our previous notion of
how the solar system ends.Comment: 24 pages, 13 figures, accepted for publication in Astrophysical
Journal (2004
Multi-Scale Modeling of Global of Magnetospheric Dynamics
To understand the role of magnetic reconnection in global evolution of magnetosphere and to place spacecraft observations into global context it is essential to perform global simulations with physically motivated model of dissipation that is capable to reproduce reconnection rates predicted by kinetic models. In our efforts to bridge the gap between small scale kinetic modeling and global simulations we introduced an approach that allows to quantify the interaction between large-scale global magnetospheric dynamics and microphysical processes in diffusion regions near reconnection sites. We utilized the high resolution global MHD code BATSRUS and incorporate primary mechanism controlling the dissipation in the vicinity of reconnection sites in terms of kinetic corrections to induction and energy equations. One of the key elements of the multiscale modeling of magnetic reconnection is identification of reconnection sites and boundaries of surrounding diffusion regions where non-MHD corrections are required. Reconnection site search in the equatorial plane implemented in our previous studies is extended to cusp and magnetopause reconnection, as well as for magnetotail reconnection in realistic asymmetric configurations. The role of feedback between the non-ideal effects in diffusion regions and global magnetosphere structure and dynamics will be discussed
Magnetic field topology during July 14–16 2000 (Bastille Day) solar CME event
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95295/1/grl15382.pd
A model of solar wind-magnetosphere-ionosphere coupling for due northward IMF, Planet
Abstract A solar wind±magnetosphere±ionosphere coupling model for due northward IMF is proposed. The magnetosphere couples with the solar wind through reconnection nightside of the cusps. Other than the two small regions where reconnection takes place, the magnetosphere is closed. There are three plasma regions in the magnetosphere. The inner core is dominated by corotation. The outer magnetosphere contains two convection cells, and maps to the ionospheric viscous cells and Region I currents. The boundary layer and magnetotail region consists of a pair of¯ow channels, and maps to the ionospheric reverse cells. The three regions are separated by separatrix surfaces. Energy coupling across the surfaces can be facilitated by non-ideal-MHD processes such as ionospheric coupling, viscous and diusive interactions, and waves and instabilities, although only the ionospheric coupling is essential to the model. This model is consistent with most established characteristics from observations and MHD computer simulations in both the ionosphere and magnetosphere. There are two speci®c features that need to be further con®rmed from observations. The model expects that the ionospheric NBZ currents and reverse cells are maximized in the region sunward of the pole near the dayside cusps, and that in the tail there is a region which separates the earthward and tailward¯ows although the ®eld and plasma characteristics are magnetospheric on both sides.