3. The invisible buffer zone with space - atmospheres, magnetospheres and the solar wind
Curtains of green and red light dance and shimmer across the night sky in the Earth's polar regions, far above the highest clouds. This light is called the aurora after the Roman goddess of the rosy-fingered dawn, a designation that has been traced back to Galileo Galilei (1564-1642). The auroras seen near the north and south poles have been given the Latin names aurora borealis, for northern lights, and aurora australis, for southern lights.
Nowadays we can use spacecraft to view both the northern and southern lights from space. The Space Shuttle has even flown right through the northern lights. While inside the display, astronauts could close their eyes and see flashes of light caused by the charged aurora particles speeding through their eyeballs.
When viewed from above, the auroras form a luminous oval centered at each magnetic pole, resembling a fiery halo. The aurora oval is constantly in motion, expanding toward the equator or contacting toward the pole, and constantly changing in brightness. Such ever-changing aurora ovals are created simultaneously in both hemispheres and can be viewed at the same time from the Moon.
The auroras are themselves caused by energetic electrons bombarding the upper atmosphere. The reason that auroras are usually located near the polar regions is that the Earth's magnetic fields guide the energetic electrons there. Electrical currents as great as a million amperes can be produced along the aurora oval, and the electric power generated during the discharge is truly awesome - about ten times the annual consumption of electricity in the United States.
When the electrons slam into the upper atmosphere, at speeds of about 50 thousand meters per second, they collide with the oxygen and nitrogen atoms there and excite them to energy states unattainable in the denser air below. The pumped-up atoms quickly give up the energy they acquired from the electrons, emitting a burst of color in a process called fluorescence. It is something like electricity making the gas in a neon light shine or a fluorescent lamp glow. The process also resembles the beam of electrons that strikes the screen of your color television set, making it glow in different colors depending on the type of chemicals, or phosphors, that coat the screen.
Even though changing conditions on the Sun may trigger the northern and southern lights, we now know that the electrons that cause the auroras arrive indirectly at the polar regions, from the Earth's magnetic tail, and that these electrons can be energized locally within the magnetosphere. Changing solar wind conditions can temporarily pinch off the Earth’s magnetotail, opening a valve that lets the solar-wind energy cross into the magnetosphere and additionally shoot energy stored in the magnetic tail back toward the aurora zones near the poles. During this magnetic reconnection process, the magnetic fields heading in opposite direction - having opposite north and south polarities – break and reconnect at 140 to 160 million meters downwind of Earth on its night side. Electrons are pushed up and down the tail, and can be accelerated within the magnetosphere as they travel back toward the Earth and then down into the upper atmosphere at the poles.
The brightest auroras in the solar system are those of Jupiter; they are one thousand times more powerful than Earth's, at about 1014 Watts. Like their terrestrial counterparts, the curtains of light on Jupiter are found in two oval-shaped regions circling the magnetic poles of the planet, just above the clouds. The aurora glows are produced in these high-latitude regions because that is were the magnetic field directs electrically-charged particles, electrons, protons and other ions. When these particles hit the planet's upper atmosphere, they collide with atoms and molecules there, leaving them in an excited state. As on Earth, the atoms and molecules release the extra energy in the form of light, and return to their normal state.
Unlike Earth's colored light show, Jupiter's aurora ovals were first observed from space at ultraviolet wavelengths because that is where most of the atoms and molecules radiate the most intense light. More recently, the Galileo spacecraft has obtained visible light images of thin, patchy aurora arcs originating at about 500 thousand meters above the cloud tops. Both internal and Sun-driven processes probably account for the brilliant curtains of light detected in Jupiter's upper atmosphere, just above the clouds
Scientists speculate that one reason that Jupiter's aurora is so powerful is that they are driven by both internal processes and the solar wind. Electron and ions spewed out by volcanoes on Jupiter's satellite Io are captured by the intense, rapidly rotating magnetic field and spiral inward at high energies toward the planet's polar regions. As the rotating magnetic field sweeps past Io, an invisible current of charged particles, equal to about 1 million amperes, is generated (Section 9.4). The current flows along Jupiter's magnetic field lines into the polar regions, bolting in and out of the planet's upper atmosphere, and producing bright trails in the ultraviolet images.
Saturn's ultraviolet auroras are most likely caused when the gusty solar wind sweeps over the planet, perhaps like the Earth's aurora. But unlike the Earth, Saturn's aurora oval has only been seen from spacecraft in ultraviolet light, at least so far. It could not be detected from beneath the Earth's atmosphere that absorbs the ultraviolet. We now turn our attention to our home planet, Earth, third rock from the Sun.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University