Global Modeling: Global simulations of Earth magnetosphere#

The top panel shows a snapshot of the magnetospheric topology computed using the global MHD model "BATS-R-US". The lower panel is identical except that the adaptive grid is visible. The grid refines as the model progresses in time.

Global magnetohydrodynamic (MHD) modeling is an essential tool for analyzing the response of Earth's magnetosphere to ever-changing solar wind and interplanetary magnetic field (IMF) conditions. Simulating the terrestrial magnetosphere as a whole has the advantage of allowing the study of complicated inter-relationships between different regions of Earth's space environment. It is also necessary in providing unified background conditions that facilitate more detailed and specific studies.

The workhorse for global MHD modeling in FDAM is the "Space Weather Modeling Framework" (SWMF) developed at the University of Michigan. The SWMF consists of various modular blocks, including a core MHD solver "BATS-R-US" that is applicable to the outer magnetosphere. Other modules include the Rice Convection Model (RCM) for the inner magnetosphere, and an ionospheric model. The SWMF is an advanced MPI-based parallel code designed to perform best on modern supercomputers with a large number of processors, such as those provided by Westgrid (www.westgrid.ca). Magnetospheric convection

Magnetospheric convection#

The interplanetary magnetic field (IMF) carried by the solar wind interacts in a complicated fashion with the geomagnetic field. The details of this interaction depend on many parameters but are primarily controlled by the orientation of the IMF (southward or northward). In each case, reconnection between the IMF and Earth internal magnetic field stirs the terrestrial magnetosphere and drives magnetospheric convection. The two basic magnetopsheric convection patterns are the two -cell convection (typical for southward IMF as illustrated in the top-left panel on each of our web-pages) and four-cell convection pattern (for northward IMF) as illustrated below panel of this page. Motion of the field lines in the magnetosphere drives convection in the ionosphere as well, where it can be observed by SuperDARN radars, DMSP satellites and other instruments. Thus, studies of magnetospheric convection have very important applications for multiple aspects of space science. In addition, magnetospheric convection can affect radio transmissions over the polar regions and GPS systems that transmit signals through the ionosphere.

Four-cell convection pattern for northward IMF.

Steady-state configurations#

The terrestrial magnetosphere is very dynamic, but may sometimes be approximated by a steady state global MHD simulation under northward IMF conditions or during Steady Magnetospheric Convection (SMC). Steady state simulations provide valuable insights into various magnetospheric phenomena, and can generally be analyzed in more detail than time-dependent runs. The example below shows the results of a steady-state simulation for northward IMF and large dipole tilt. The simulation demonstrates that the magnetic field topology in this case is consistent with the classical "null-separator model" and restrictions of "kinematic reconnection theory".

Analysis of the ionospheric convection for the simulation reveals the existence of lobe cells in the summer hemisphere (convection cells circulating entirely in the open field line region) and reciprocal cells in the winter hemisphere (convection cells in the closed field line region). These features were later identified in SuperDARN observations for northward IMF conditions, indicating that global MHD models are very useful for analyzing real events.

References:

1. Internal reconnection for northward interplanetary magnetic field, M. Watanabe, K. Kabin, G. J. Sofko, R. Rankin, T.I. Gombosi, A. J. Ridley, C. R. Clauer, Journal of Geophysical Research, 110(A6), A06210, doi:10.1029/2004JA010832, 2005.
2. Ionospheric signatures of internal reconnection for northward interplanetary magnetic field: Observation of reciprocal cells and magnetosheath ion precipitation, M. Watanabe, G. J. Sofko, D. A. Andre, J. M. Ruohoniemi, M. R. Hairston, K. Kabin, Journal of Geophysical Research, 111(A6), A06201, doi:10.1029/2005JA011446, 2006

Fig. 1. A three dimensional view of the last closed field line surface. Red lines represent the field lines emanating from the northern polar cap boundary and reconnecting at a null point in the southern hemisphere; blue line originate from the southern ionosphere and reconnect at the null in the northern hemisphere.

Fig. 2. Selected streamlines (white lines) in the simulated magnetosphere in the noon-midnight meridian plane. Pink lines are the magnetic field separatrices. Plasma pressure is shown by the color code in the background.

Open-closed field line boundary#

Earth is surrounded by closed geomagnetic field lines that closely resemble the field produced by a bar magnet. The solar wind, on the other hand, carries with it an IMF field that connects with Earth's magnetic field. The boundary between open (to the solar wind) and closed field lines (OCB) is an important topological separatrix that can be extracted from global modeling simulations. The OCB projection into the ionosphere is usually within one or two degrees of the poleward edge of the auroral oval, and very close to the edge of energetic particle precipitation measured by DMSP satellites. As well as using satellite measurements, the OCB can be extracted from SuperDARN measurements of convection in the ionosphere.

Comparisons between simulations and measurements of the OCB have been made in FDAM, and found to be in a very good agreement for cases when the magnetosphere is close to a steady state. This agreement is demonstrated by figures below.

The location of the OCB also depends strongly on the IMF orientation, and for pure northward IMF (if Earth's dipole tilt is zero) it collapses to a single point (cusp). For southward IMF the OCB in the ionosphere reaches its furthest equatorward extent. FDAM scientists are undertaking studies to investigate the IMF By and Bz dependence of the ionospheric area and amount of open magnetic flux inside the OCB. Global MHD models used in conjunction with observations are critical in making this evaluation.

References:

1. Comparison of Photometer and Global MHD determination of the Open-Closed Field Line Boundary, I.J. Rae, K. Kabin, R. Rankin, F.R. Fenrich, W. Liu, J.A. Wanliss, A.J. Ridley, T.I. Gombosi, and D.L. DeZeeuw, Journal of Geophysical Research, 109, A01204, doi:10.1029/2003JA009968, 2004.
2. Open-closed field line boundary position: A parametric study using an MHD model, K. Kabin, R. Rankin, G. Rostoker, R. Marchand, I. J. Rae, T. I. Gombosi, C. R. Clauer, A. J. Ridley, D. L. DeZeeuw, Journal of Geophysical Research, 109, A05222, doi:10.1029/2003JA010168, 2004.

Latitude vs time intensities from the Rankin Inlet (RAN) MSP. Day and start times are (a) 961129 0100 UT, (b) 960314 0300 UT, (c) 971218 0200 UT, (d) 950224 0500 UT, and (e) 950306 0600 UT. The white line indicates the poleward border of red-line emissions. Black and white crosses denote the OCB at the the photometer location determined from BATS-R-US

Magnetic local time: magnetic latitude (MLT: MLAT) plot of the ionospheric projection of the OCBs. Crosses denote observed OCB points, and circles BATS-R-US OCBs. Each observed point has a corresponding simulated point, and each simulated interval is denoted by a color code (see left panel). Solid lines denote average position of the auroral oval for Kp = 2.

Magnetospheric response to interplanetary shocks#

Interplanetary shocks which are often associated with energetic disruptive events on the Sun (such as Coronal Mass Ejections or solar flares) can cause major geomagnetic disturbances on the ground. Sometimes these disturbances are large enough to damage power lines, satellites in near Earth orbits or other infrastructure. Such disturbances may also interfere with radio communications, navigation equipment, and GPS systems. Responding to an interplanetary shock Earth magnetosphere contracts in size as the solar wind dynamic pressure increases behind the shock (often by a factor of 10 or more). This is illustrated by the two movies below.

A simulation of a particular event (June 8th, 2000) showing the evolution of the last closed field line surface during a magnetic storm.