3. The invisible buffer zone with space - atmospheres, magnetospheres and the solar wind


A thin colored line

A thin colored line

. Brilliant red and blue mark the thin atmosphere that warms and protects us, as viewed from space at sunrise over the Pacific Ocean. Without this atmospheric membrane we could not breathe and water would freeze. (Courtesy of NASA.)


Earth's weather patterns

Earth

. Trade winds blow from east to west along the Earth's equatorial regions, in the same direction that the planet rotates. At higher latitudes, there are high-altitude jet streams that move at speeds of up to 40 meters per second in the opposite direction to the trade winds. In the northern hemisphere there are high-pressure cyclones, denoted by H, and low pressure anti-cyclones, L, rotating in the clockwise and counter-clockwise directions, respectively; in the southern hemisphere the cyclones and anti-cyclones rotate in the opposite direction. The prevailing winds are given in Annie Dillard's poem, "the windy planet".


Raging winds

Raging winds

. A high-velocity wind whips the upper layer of Venus's cloud deck around the planet's equator once every four Earth days, moving at speeds of up to 100 meters per second. This is illustrated by these photographs taken at ultraviolet wavelengths on consecutive days by the Pioneer Venus spacecraft in 1979. The Y-shaped clouds move towards the west (left). The winds of Venus are dominated by a zonal (east-to-west) circulation. The low atmosphere and the planet's surface also rotate westward, but with the much slower period of 243 days. (Courtesy of Larry Travis and NASA.)


Martian surface pressure

Martian surface pressure

. Daily mean surface pressures at the two Viking Lander (VL) sites for one Martian year, showing that the red planet periodically removes and replaces carbon dioxide in its atmosphere. The seasons are for the northern hemisphere, and the pressure is given in millibars, abbreviated, mb, where 1 mb = 0.01 bar and 1 bar is the sea-level pressure of the Earthís atmosphere. At the Viking 1 site (bottom curve), the pressure ranged from 6.7 mb during the northern summer to 8.89 mb at the commencement of northern winter. At the Viking 2 site (top curve) the equivalent data were 7.4 mb and 10 mb. The higher values are probably due to the lower elevation; there was an approximate 1,100 meter difference in the elevation of the two landing sites. In the southern winter, and northern summer, carbon dioxide is condensed out of the atmosphere, enlarging the southern polar cap and reducing the total surface pressure of the planet. In southern summer, and northern winter, the carbon dioxide has been released back into the atmosphere, with an increase in the total surface pressure.


Molecules in the atmospheres of Jupiter and Saturn

Molecules in the atmospheres of Jupiter and Saturn

. The infrared radiation from the thin, cold upper atmospheres of Jupiter and Saturn exhibit numerous features that have no counterpart in the spectrum of sunlight. Strong features are seen in Jupiter for molecular hydrogen, H2, ammonia, NH3, methane, CH4, and water vapor, H2O. Saturnís outer atmosphere is also abundant in hydrogen and methane, but the ammonia features are missing and those of acetylene, C2H2, and ethane, C2H6, are enhanced. These spectra were taken with instruments aboard Voyager 1 and 2 during their Jupiter and Saturn flybys in 1979 and 1980, respectively. (Courtesy of Rudolf A. Hanel.)


Jupiter's counter-flowing winds

Jupiter

. Fig. 3.6. Jupiter's counter-flowing winds. The rapid rotation of Jupiter has pulled its winds into bands that flow east to west and west to east, shown in this image taken from the Cassini spacecraft in 2001. The windswept clouds therefore move in alternating light-colored, high-pressure zones and dark-colored, low-pressure belts. The Great Red Spot swirls in the counter-clockwise direction, like a high-pressure cyclone in the Earth's southern hemisphere (Fig. 3.2), but it has lasted for more than 300 years, much longer than terrestrial storms. (Courtesy of JPL and NASA.)


Winds on the giant planets

Winds on the giant planets

. Variation of wind speed and direction as a function of latitude. Since the giant planets have no solid surfaces, the winds are measured relative to the internal rotation speeds; the rotation period is determined from observations of the planet's periodic radio emission. Positive velocities correspond to winds blowing in the same direction but faster than the internal rotation; negative velocities are winds moving more slowly than the rotation. The winds are faster on Saturn than any other planet. (Courtesy of Andrew P. Ingersoll.)


Titan's dense, smoggy atmosphere

Titan

. The surface of Titan is hidden from view by a hazy layer of smog, giving it a fuzzy, tennis-ball appearance in a Voyager 1 image (left) taken on 4 November 1980. When illuminated from behind, the dense atmosphere forms a crescent several hundred thousand meters above the satellite's surface (right) seen by Voyager 2 on 25 August 1981. The extension of blue light around the moon's night side is due to scattering from smog particles in the sunlit portion. The Cassini spacecraft is expected to drop its Huygens Probe into Titan's atmosphere in 2004. (Courtesy of JPL and NASA.)


Comet tails

Comet tails

. Telescopic photograph of Comet Mrkos taken in August 1957, showing the straight, well-defined ion tail and the more diffuse, slightly curved dust tail. Both comet tails point away from the Sun. The electrified solar wind deflects the charged ions and accelerates them to high velocities, creating the relatively straight ion tails. The radiation pressure of sunlight suffices to blow away the unionized comet dust particles, forming a broad arc that can resemble a scimitar. (Courtesy of Lick Observatory.)


The heliosphere

The heliosphere

. With its solar wind going out in all directions, the Sun blows a huge bubble in space called the heliosphere. The heliopause is the name for the boundary between the heliosphere and the interstellar gas outside the solar system. Interstellar winds mold the heliosphere into a non-spherical shape, creating a bow shock where they first encounter it. The orbits of the planets are shown near the center of the drawing.


Earthís magnetic dipole

Earthís magnetic dipole

. 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. The Earth's magnetic dipole originates in swirling currents of molten iron deep in the Earthís core, and extends more than 10 Earth radii , or 63.7 million meters out into space on the side facing the Sun, and all the way to the Moon's orbit, at 384.4 million meters on the opposite side. Magnetic field lines loop out of the South Geographic Pole and into the North Geographic Pole. The lines are close together near the magnetic poles where the magnetic force is strong, and spread out where it is relatively weak. The magnetic axis is tilted at an angle of 11.7 degrees with respect to the Earthís rotational axis. This dipolar (two poles) configuration applies near the surface of the Earth, but further out the magnetic field is distorted by the solar wind (Fig. 3.12).


The Earth's magnetosphere

The Earth

. The Earthís magnetic field carves out a hollow in the solar wind, creating a protective cavity called the magnetosphere. The Earth, its auroras, atmosphere and ionosphere, and the two Van Allen radiation belts all lie within this magnetic cocoon. Similar magnetospheres are found around other magnetized planets. 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. Electrons and protons in the solar wind are deflected at the bow shock (left), and flow along the magnetopause into the magnetic tail (right). Electrified particles can be injected back toward the Earth and Sun within the plasma sheet (center).


Magnetic trap

Magnetic trap

. 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.


Magnetic connection on the back side

Magnetic connection on the back side

. 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. Electrified material is accelerated away from this disturbance (thick arrows), both away from the Earth and back toward it.


Jupiter's magnetosphere

Jupiter

. High-speed electrons that are trapped in the Jovian magnetosphere emit steady radio radiation (top) by the synchrotron process; it is detected by ground-based radio telescopes such as the Very Large Array. An instrument aboard Cassini measures energetic atoms (bottom) created when fast-moving ions within the magnetosphere pick up electrons to become neutral atoms. The two open circles denote Io's orbital position on each side of the planet; Jupiter is denoted by the central black disk. [Courtesy of Imke de Pater, University of California at Berkeley (top) and NASA, JPL, and the Johns Hopkins University Applied Physics Laboratory (bottom).]


Satellites within Jupiter's magnetic field

Satellites within Jupiter

. This cross section shows that the four Galilean satellites, Io, Europa, Ganymede and Callisto, are all embedded within Jupiter's magnetosphere. Small satellites orbit Jupiter within the orbit of Io, the innermost Galilean satellite. The outermost Galilean satellite, Callisto, orbits Jupiter near its bow shock. All of these satellites are being continuously bombarded with energetic charged particles that are trapped within Jupiter's magnetosphere. The distance from Jupiter to Callisto is 1.88 billion (1.88 x 109) meters, while the radius of the Sun is 0.6955 billion meters, so Jupiter's bow shock is bigger than the Sun.


Tilted magnetic fields

Tilted magnetic fields

. The magnetic fields of Uranus and Neptune can be represented by a simple bar magnet, or dipole, embedded in the planet, but with a magnetic axis that is tilted with respect to the rotation axis. For Uranus the tilt is about 60 degrees; Neptune has a tilt of 47 degrees. In contrast, the magnetic axes of Jupiter, Saturn and Earth are much more nearly aligned with their rotation axes. The arrow of the rotation axis points from the geographic south towards geographic north, and the magnetic axis similarly points from magnetic south to magnetic north. On Uranus and Neptune a terrestrial compass would point toward the southern hemisphere of the planet, while on Earth it points toward the geographic north pole. In addition to dipole part of their magnetic field, Uranus and Neptune have a large additional component known as the quadrupole one. A method of visualizing this is to imagine that the dipole has a magnetic center that is offset radially from the center of the planet. As shown here, the equivalent offset for Uranus is almost a third of the planet's radius, and there is a larger offset for Neptune of nearly half its radius. But such off-center dipoles are only useful as a picture of what the external field looks like and do not help in understanding how it is produced deep down.


Aurora Borealis

Aurora Borealis

. Spectacular curtains of multi-colored light are found in these photographs of the fluorescent Northern Lights, or Aurora Borealis, taken by Forrest Baldwin in Alaska. (Courtesy of Kathi and Forrest Baldwin, Palmer, Alaska).


The aurora oval

The aurora oval

. The POLAR spacecraft looks down on an aurora from high above the Earth's north polar region in February 2000, showing the northern lights in their entirety. The glowing oval is 4.5 million meters across. The most intense aurora activity appears in bright red or yellow. The Earth's aurora is typically initiated by magnetic reconnection events in the Earth's magnetotail (Fig. 3.14), on the night side of the Earth. (Courtesy of the University of Iowa and NASA.)


Aurora Austalis

Aurora Austalis

. The eerie, beautiful glow of auroras can be detected from space, as shown in this image of the Aurora Australis, or Southern Lights, taken from the Space Shuttle Discovery. The colored emission of atomic oxygen extends upward to between 200 thousand and 300 thousand meters above the Earth's surface. (Courtesy of NASA.)


Jupiter's auroras

Jupiter

. High-energy electrons and ions cascade into Jupiter's upper atmosphere and create bright auroras at ultraviolet wavelengths. This composite image, taken by the Hubble Space Telescope in September 1997, shows ovals in both the northern and southern regions. Elongated trails outside the ovals, starting from the far left below the ovals and moving to the right, are believed to mark the locations where powerful electrical currents from the volcanic moon Io enter the Jovian atmosphere. (Courtesy of John Clarke, University of Michigan, the Space Telescope Science Institute, and NASA.)


Auroras on Saturn

Auroras on Saturn

. High-energy electrons and ions are captured from the solar wind and funneled down into Saturn's upper atmosphere, creating aurora ovals at its northern (upper left) and southern (lower right) magnetic poles. This ultraviolet image was recorded by the Hubble Space Telescope in October 1997. The bright red aurora features are dominated by emission from atomic hydrogen, while the white regions within them are emitted by molecular hydrogen. (Courtesy of John T. Trauger, Jet Propulsion Laboratory, the Space Telescope Science Institute, and NASA.)