. The Earthís magnetic field looks like that which would be produced by a bar magnet at the center of the Earth, with the North Magnetic Pole corresponding to the South Geographic Pole and vice versa. It originates in swirling currents of molten iron deep in the Earthís core, and extends more than 20 Earth radii, or 126,000 meters out into space. Magnetic field lines loop out of the South Geographic Pole and into the North Geographic Pole. A compass needle will always point along a field line. The lines are close together near the magnetic poles where the magnetic force is strong, and spread out where it is weak. The magnetic axis is tilted at an angle of 11.7 degrees with respect to the Earthís rotational axis. Notice that the poles of the magnet are inverted with respect to the geographic poles, following the custom of defining positive, north magnetic polarity as the one in which magnetic fields point out, and negative, south magnetic polarity as the place where magnetic fields point in. This dipolar (two poles) configuration applies near the surface of the Earth, but further out the magnetic field is distorted by the solar wind.
. The Earthís magnetic field carves out a hollow in the solar wind, creating a protective cavity, called the magnetosphere. A bow shock forms at about ten Earth radii on the sunlit side of our planet. Its location is highly variable since it is pushed in and out by the gusty solar wind. The magnetopause marks the outer boundary of the magnetosphere, at the place where the solar wind takes control of the motions of charged particles. The solar wind is deflected around the Earth, pulling the terrestrial magnetic field into a long magnetotail on the night side. Plasma in the solar wind is deflected at the bow shock (left), flows along the magnetopause into the magnetic tail (right), and is then injected back toward the Earth and Sun within the plasma sheet (center). The Earth, its auroras, atmosphere and ionosphere, and the two Van Allen radiation belts all lie within this magnetic cocoon.
. The Sunís wind brings solar and terrestrial magnetic fields together on the night side of Earthís magnetosphere, in its magnetotail. Magnetic fields that point in opposite directions (thin arrows), or roughly toward and away from the Earth, are brought together and merge, reconnecting and pinching off the magnetotail close to Earth. Material in the plasma sheet is accelerated away from this disturbance (thick arrows). Some of the plasma is ejected down the magnetotail and away from the Earth, while other charged particles follow magnetic field lines back toward Earth.
. Electrons and protons encircle the Earth within two donut-shaped, or torus-shaped, regions near the equator, trapped by the terrestrial magnetic field. These regions are now called the inner and outer Van Allen radiation belts, named after James A. Van Allen who first observed them with the Explorer 1 and 3 satellites in 1958. The inner beltís charged particles tend to have higher energies than those in the outer belt. The trapped particles can damage the microcircuits, solar arrays and other materials of spacecraft that pass through them.
. Charged particles can be trapped by Earthís magnetic field. They bounce back and fourth between polar mirror points in either hemisphere at intervals of seconds to minutes, and they also drift around the planet on time scales of hours. As shown by Carl StÝrmer in 1907, with the trajectories shown here, the motion is turned around by the stronger magnetic fields near the Earthís magnetic poles. Because of their positive and negative charge, the protons and electrons drift in opposite directions.
. As it moves away from the Sun (top left) a fast coronal mass ejection (CME, top right) pushes an interplanetary shock wave before it, amplifying the solar wind speed, V, and magnetic field strength, B (bottom). The CME produces a speed increase all the way to the shock front, where the windís motion then slows down precipitously to its steady, unperturbed speed. Compression, resulting from the relative motion between the fast CME and its surroundings, produces strong magnetic fields in a broad region extending sunward from the shock. The strong magnetic fields and high flow speeds commonly associated with interplanetary disturbances driven by fast CMEs are what make such events effective in stimulating geomagnetic activity.
. When a coronal mass ejection travels into interplanetary space, it can create a huge magnetic cloud containing bidirectional, or counter-streaming, beams of electrons that flow in opposite directions within the magnetic loops that are rooted at both ends in the Sun. The magnetic cloud also drives an upstream shock ahead of it. Magnetic clouds are only present in a subset of observed interplanetary coronal mass ejections. (Courtesy of Deborah Eddy and Thomas Zurbuchen.)
. When fast solar-wind streams, emanating from coronal holes, interact with slow streams, they can produce Co-rotating Interaction Regions in interplanetary space. The magnetic fields of the slow streams in the solar wind are more curved due to the lower speeds, and the fields of the fast streams are more radial because of their higher speeds. Intense magnetic fields can be produced at the interface (IF) between the fast and slow streams in the solar wind. The Co-rotating Interaction Regions are bounded by a forward shock (FS) and a reverse shock (RS).
. POLAR looks down on an aurora from high above the Earth's north polar region on 22 October 1999, showing the northern lights in their entirety. The glowing oval, imaged in ultra violet light, is 4.5 million meters across. The most intense aurora activity appears in bright red or yellow. It is typically produced by magnetic reconnection events in Earth's magnetotail, on the night side of the Earth. (Courtesy of the Visible Imaging System, University of Iowa and NASA.)
. A powerful solar flare (left), occurring at 10 hours 24 minutes Universal Time on Bastille day 14 July 2000, unleashed high-energy protons that began striking the SOHO spacecraft near Earth about 8 minutes later, continuing for many hours, as shown in the image taken on 22 hours 43 minutes Universal Time on the same day (right). Both images were taken at a wavelength of 19.5 nanometers, emitted at the Sun by eleven times ionized iron, denoted Fe XII, at a temperature of about 1.5 million kelvin, using the Extreme Ultraviolet Imaging Telescope, abbreviated EIT, on the SOlar and Heliospheric Observatory, or SOHO for short. (Courtesy of the SOHO EIT consortium. SOHO is a project of international cooperation between ESA and NASA.)
. Astronaut Donald Peterson, on a 50-foot tether line during his 4-hour, 3-orbit space walk, moving toward the tail of the Space Shuttle Challenger as it glides around the Earth. Hundreds of miles above the Earth, there is no air and an astronaut must wear a space-suit. It supplies the oxygen he needs and insulates his body from extreme heat or cold. However, a space-suit cannot protect an astronaut from energetic particles hurled out from explosions on the Sun. He or she must then be within the protective shielding of a spacecraft or other shelter to avoid the danger. (Courtesy of NASA.).
. The first untethered walk in space, on 7 February 1984, where there is no place to hide from inclement Sun-driven storms. Bruce McCandless II, a mission specialist, wears a 300-pound Manned Maneuvering Unit (MMU) with 24 nitrogen gas thrusters and a 35 mm camera. The MMU permits motion in space where the sensation of gravity has vanished, but it does not protect the astronaut from solar flares or coronal mass ejections. High-energy particles resulting from these explosions on the Sun could kill the astronaut. (Courtesy of NASA.)
. Intense radiation, or photons, generated during solar flares passes right through the interplanetary magnetic field and arrives at the Earth just 8 minutes after being emitted from the Sun. In contrast, solar flares beam energetic charged particles across a narrow trajectory that follows the interplanetary magnetic spiral, and the time for the particles to reach Earth depends on their energy and velocity, taking roughly an hour for a particle energy of about 10 MeV. A coronal mass ejection, or CME, with an average speed of 450 thousand meters per second takes about 4 days to travel from the Sun to Earthís orbit. The CME can energize particles across a wide swath in interplanetary space. The heliospheric current sheet separates magnetic fields of opposite polarities, or directions, denoted by the arrows on the spiral lines. (Courtesy of Frances Bagenal.)
. When a coronal mass ejection hits the Earth, electrons in the Earthís magnetosphere cascade into the polar regions, creating a current that flows along the auroral oval at an altitude of about 100 thousand meters. The magnetic field from this current induces a voltage potential on the surface of the Earth of up to 6 volts per kilometer. A strong pulse of direct current enters long conductors like power lines through their ground connection. This can throw circuit breakers, destroy transformers, and shut down power grid systems, sometimes turning off the lights in entire cities.
. The X-ray activity of the Sun is monitored by the Geostationary Operations Environmental Satellites, or GOES. The GOES data shown here includes three exceptionally intense, X-class flares emitted from the same active region on 6 and 7 June 2000. The first two flares were also associated with a powerful coronal mass ejection observed with an instrument aboard the SOHO spacecraft (Fig. 8.15). The GOES data are collected by the National Oceanic and Atmospheric Administrationís Space Environment Center and distributed to all interested persons though its space weather forecasts.
. A coronal mass ejection is observed billowing out from the Sun on 6 June 2000, using the Large Angle Spectrometric COronagraph, or LASCO, on the Solar and Heliospheric Observatory, SOHO. A central occulting disk blocks out the Sunís intense light to reveal the faint corona, along with background stars and planets. The white circle in the disk denotes the outer edge of the Sunís photosphere. Venus is next to the disk on the right side, while Mars is located at the far left center of the image. This event was a halo mass ejection that grew larger as it expanded, forming a halo around our star, indicating that it was headed toward the Earth. The velocity of the ejected material was at least 900 thousand meters per second. Although coronal mass ejections can occur without a solar flare, this one was accompanied by two intense solar flares (Fig. 8.14). (Courtesy of the SOHO LASCO consortium. SOHO is a project of international collaboration between ESA and NASA.)
. Satellites monitor a solar storm from its beginning on the Sun to its interaction with the Earth. The Yohkoh satellite observes the X-ray emission from tightly coiled magnetic loops, that release their pent-up energy as a coronal mass ejection, detected by SOHOís LASCO. Other satellites track the bubble of magnetized gas on its way to Earth, and then record the collision with our magnetosphere. For instance, a radio experiment on board the WIND spacecraft can track the shocks driven by the mass ejection through the interplanetary medium, and the GEOTAIL satellite can observe the magnetic connections when the solar ejection collides with the terrestrial field. The resultant auroras are seen in images obtained with the POLAR or IMAGE satellites.