2008 THEMIS SCIENCE NUGGETS

FLUX ROPES AT THE EARTH'S MAGNETOPAUSE

by David G. Sibeck

Introduction

The Earth’s magnetic field carves out a cavity known as the magnetosphere in the oncoming supersonic solar wind flow (see figure below). Variations in the solar wind pressure constantly batter the magnetosphere, driving antisunward-propagating waves on the outer boundary of the Earth’s magnetic field (‘the magnetopause’). Other antisunward-propagating waves are generated by the motion of the solar wind flow past the magnetopause, just as waves on a lake are generated by gail force winds.

A Depiction of the Earth's magnetosphere.
Figure 1. The Earth's magnetic field carves out a cavity in the
oncoming solar wind known as the magnetosphere. Because the
solar wind flow is supersonic, a bow shock wave stands
upstream from the cavity.
Click here to enlarge the image.

Waves aren’t the only transient features observed in the vicinity of the magnetopause. Transient reconnection, the process by which solar and magnetospheric magnetic field lines become interconnected, should create flux ropes of intertwined magnetosheath and magnetospheric magnetic fields. Numerical simulations (see figure below) indicate that magnetic field lines spiral around strong core magnetic fields within these flux ropes. Because it can be difficult to distinguish the signatures of flux ropes from those of waves with only one or two spacecraft, multispacecraft missions are essential.

Model predictions of a flux rope.
Figure 2. A view of the dayside magnetosphere from the Sun.
A magnetic flux rope, or flux transfer event (FTE), stretches
across the dayside magnetosphere from dawn to dusk. The rope
contains magnetospheric magnetic field lines (red),
interplanetary magnetic field lines (yellow), and lines that pass
from the magnetosphere into interplanetary space (grey).
Results are from the University of Michigan's BATS-R-US model
with enhanced resolution in the vicinity of the dayside
magnetopause.
Click here to enlarge the image.

THEMIS

The five THEMIS spacecraft were launched on February 17, 2007. Each carries a full complement of plasma, particle, and field instruments appropriate to studies of the Earth’s magnetosheath, magnetosphere, and magnetopause. During the coast phase of the mission, from launch until the Fall of 2007, the spacecraft followed common orbits with 14.7 Earth radii apogees between the Earth and Sun. Since the outer boundary of the Earth’s magnetic field lines is located some 11 Earth radii from Earth along the Sun-Earth line, the spacecraft crossed the magnetopause at least once each outbound and inbound pass. Inter-spacecraft distances during this phase of the mission were small (on the order of one Earth radius), just right to provide multiple vantage points of waves and flux ropes with similar dimensions at the magnetopause.

Science

At 2200 UT on May 20, 2007, the five THEMIS spacecraft were moving inbound towards the Earth across the afternoon magnetopause. THEMIS-B and –C had already entered the magnetosphere, THEMIS-D was right at the magnetopause, and THEMIS-E and –A were still outside the magnetosphere. Consequently, THEMIS-B and –C observed the strong steady northward (Bz > 0) magnetic fields that characterize our equatorial magnetosphere, THEMIS-E and –A observed disturbed and highly variable southward magnetosheath magnetic fields, and THEMIS-D observed field orientations that lay in between those of the magnetosheath and magnetosphere.

THEMIS observations.
Figure 3. At 22:00 UT on May 20, 2007, the five THEMIS
spacecraft were on the inbound leg of a post-noon orbit.
THEMIS-B and -C had already entered the magnetosphere,
THEMIS-D was at the magnetopause, while THEMIS-E
and -A were trailing in the magnetosheath.
Click here to enlarge the image.

The spacecraft observed a transient event from 2201:30 to 2202:45 UT. THEMIS-D (red) observed the strongest magnetic field strength enhancement and a strong bipolar magnetic field signature (Bn) normal to the nominal magnetopause (at 2202:05 UT). THEMIS-E and –A (blue) observed crater-like structures in the total magnetic field strength and bipolar magnetic field signatures normal to the nominal magnetopause (-, + Bn). THEMIS-B and –C observed only a slight magnetic field strength increase and an outward pointing magnetic field component normal to the magnetopause (+Bn).

With an assumption that the structure moved past the spacecraft at a steady speed, we can transform time history of the observations into a picture of the structure. The observations provide evidence for a region of enhanced magnetic field strengths (the core of the flux rope) detached from the magnetosphere. Magnetic field lines spiral about this core region, just as predicted by the model described above. The entire structure lies embedded in the weak magnetic field strength region of the magnetopause current layer.

THEMIS Observations.
Figure 4. THEMIS-A (solid black), -B (solid blue),
-C (dashed blue), -D (red), and -E (dashed black).
Magnetic field observations in boundary normal
coordinates, where L lies in the plane of the
magnetopause and points northward, N points
outwards along the normal to the nominal
magnetopause, and M completes the triad. THEMIS-D,
located within the magnetopause current layer,
observed magnetic field orientations between
those of the magnetosphere and magnetosheath,
and the largest amplitude magnetic field perturbations
during the passage of an FTE from 2201:30 to 2202:30 UT.
Click here to enlarge the image.

Schematic
Figure 5. Under an assumption that the FTE moves past the five
THEMIS spacecraft with a constant velocity, the structure of
the event can be inferred from their observations. This figure
demonstrates that the FTE is an island of enhanced magnetic
field strengths detached from the magnetosphere. Magnetic
field lines spiral about the enhanced magnetic field strengths
within the core region.
Click here to enlarge the image.

Conclusions

The THEMIS observations on May 20, 2007 offered a remarkable opportunity to simultaneously observe a flux rope from multiple vantage points inside, outside, and at the magnetopause. They decisively demonstrate that the rope exhibited a strong core field encircled by spiraling magnetic field lines. Observations like these are common throughout the course of the coast phase of the mission, and offer innumerable opportunities to understand the response of the magnetosphere to varying solar wind conditions and instabilities at the magnetopause boundary. The observations validate the output from numerical simulations, thereby paving the way towards a comprehensive understanding of the solar wind-magnetosphere interaction.

Biographical Note

David G. Sibeck is the THEMIS Project Scientist at NASA’s Goddard Space Flight Center in Greenbelt, MD. His research interests focus on the interaction of the solar wind with the Earth’s magnetosphere.


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