2008 THEMIS SCIENCE NUGGETS

Breach in Earth's Magnetosphere Discovered

by D.G. Sibeck, Marit Øieroset, Jimmy Raeder, and Wenhui Li

Introduction

Earth's magnetic field deflects highly charged particles emitted by the sun, known as solar wind, which speed towards Earth at a million miles per hour. However, these particles are not fully deflected by the magnetosphere, but instead penetrate through two areas. The extent to which these breaches allow solar wind particles to enter through the magnetopshere is dependent on the orientation of the sun's magnetic field. Previously, it was thought that when the sun's magnetic field aligned with that of Earth, the transfer of solar wind particles into Earth's magnetosphere was minimal. However, THEMIS team scientists recently discovered that contrary to longstanding views on how and when solar plasma enters the Earth's magnetosphere, 20 times more solar wind plasma penetrates Earth's magnetosphere when the sun's magnetic field is aligned with that of the Earth.

The Earth's magnetic field
Movie 1. The Earth's magnetosphere deflects solar wind
energy and particles, with the exception of the two
breaches that occur during geomagnetic storms.
Click here to play the animation.

Southward Interplanetary Magnetic Field (IMF) orientations enable solar wind energy to enter the Earth's magnetopshere, while northward IMF orientations allow solar wind plasma to enter the magnetosphere. Eruptions on the sun, known as Coronal Mass Ejections (CMEs), cause geomagnetic storms on Earth. The sun has an eleven-year activity cycle, at the end of which its magnetic field change its polarity, or direction. The next cycle is due to bring mostly north-south CMEs, which will cause plasma to enter into the northern hemisphere of Earth's magnetosphere, fueling the storm and energizing the southern hemisphere, creating larger geomagnetic storms than in the current solar cycle.

THEMIS Observations

The THEMIS spacecraft detected a thick layer of solar particles inside of Earth's magnetosphere. This solar particle layer is 1RE thick, or approximately 4,000 miles, and is growing rapidly. This solar particle layer is much thicker when the sun's and Earth's magnetic fields are aligned than when they are not aligned. Øieroset et al. reports that twenty times more particles enter the magnetosphere when the fields are aligned.

Solar wind magnetic field not aligned with the Earth'sSolar wind magnetic field not aligned with the Earth's
Figure 1. The left-hand figure represents the solar wind's magnetic field not aligned with the Earth's.
The right-hand figure represents the solar wind's magnetic field aligned with the Earth's, which allows
for a 2000% increase in solar wind particles that enter the Earth's magnetosphere during a geomagnetic storm.

Simulation Results

Simulation results from Li et al. show that the solar particle layer extends over the entire sunward side of the magnetosphere. As the solar wind carries a magnetic field from the sun to the Earth's magnetosphere, it drapes over the Earth's magnetosphere and connects to the Earth's field above the north pole. Approximately one minute thereafter, the solar wind's magnetic field connects with the Earth's field over the south pole, thus melding the solar wind field line with the Earth's and propelling the solar plasma into the Earth's magnetosphere.

Solar wind magnetic field line Plasma density on equatorial plane
Figure 2. The left-hand figure depicts a solar wind magnetic field line in yellow. The middle figure shows the sun's
magnetic field draping over the Earth's. The plasma density is shown on the equatorial plane. In the right-hand figure,
the solar wind's field line has connected with the Earth's magnetosphere over the south pole; the plasma density is
on a cross plane behind the Earth.

Implications of the Results

The realization that the solar magnetic field and the Earth's magnetic field alignment creates an intense surge of solar particles into the magnetosphere gives us a basic predictive capability for the severity of geomagnetic storms. This is important because large storms can cause power lines to overload with excess current, which can cause widespread blackouts. Large surges of solar particles can also be hazardous to spacecraft in high orbits and astronauts passing through the storms. The more we can predict the intensity of the storms that are forming in the magnetosphere, the more we can take measures to mitigate their effects.

Biographical note

David G. Sibeck is the THEMIS Project Scientist at NASA’s Goddard Space Flight Center in Greenbelt, MD. Marit Øieroset is the lead author of one of two papers on this research, entitled "THEMIS multi-spacecraft observations of magnetosheath plasma penetration deep into the dayside low-latitude magnetosphere for northward and strong By IMF," published in May 2008 in Geophysical Research Letters and is a THEMIS researcher at the University of California, Berkeley. Jimmy Raeder is a THEMIS co-investigator at the University of New Hampshire. Wenhui Li is a THEMIS researcher at the University of New Hampshire and author of a paper on this research, entitled "Cold dense magnetopause boundary layer under northward IMF: Results from THEMIS and MHD simulations," in the Journal of Geophysical Research.

Credit

Credit for the animation and the photos: NASA
http://www.nasa.gov/mission_pages/themis/main/index.html



Please send comments/suggestions to
Emmanuel Masongsong / emasongsong@igpp.ucla.edu