For centuries navigators knew that lodestones when floated would point North, but it was William Gilbert in his 1600 book de Magnete who figured out why — the Earth itself is a great magnet. The end (or pole) of a bar magnet which points towards to Earth’s north pole is called a north-seeking pole. Since opposite ends of magnets attract, all north-seeking poles are attracted to south-seeking poles. Since the north-seeking ends of all magnets are attracted to the Earth’s North pole, the Earth’s North pole must in fact overlie a the south-seeking magnet pole. He noticed that a magnetized needle suspended by a string only points horizontally north at Earth’s middle latitudes; near the north pole it also points down (dips), and it points up near the south pole (Figure 1). This procedure permits a determination of the direction of the magnetic force at every location on Earth. The resulting patterns are very similar to the directions one gets when placing a sheet of iron filings above a bar magnet and tapping it gently (teachers can do this by putting a magnet on an overhead projector, covering the magnet with a transparency and and putting the filings on it). The filings line themselves up along the direction of the local magnetic force, and make long chains from one end of the magnet to the other. The general shape is called a dipole. On the Earth, you could do this by using the suspended magnetized needle to trace out the direction of the magnetic force, moving it its length each time and placing the tail of the needle (the south-seeking end) where the north-seeking end had been the previous step. This procedure will trace out lines of force, imaginary continuous lines showing at every location in space the direction of the magnetic force. These lines of force are also called magnetic field lines, and are a very useful concept for explaining many effects in space physics. The Earth’s field lines leave the surface in the southern hemisphere, travel outwards and northwards, being farthest from the surface near the equator, and crossing the surface again in the northern hemisphere. If you could continue these lines under the surface, they would pass through the Earth’s core and connect back to one another. In fact, magnetic field lines have no beginning nor end.

Furthermore, a compass doesn’t point accurately towards the geographic North pole: there is a persistent directional difference, called the magnetic deviation. In the continental United States, compasses point consistently East of true north; in Europe, they point West of true North. The place to which they point is called the magnetic north pole, which is actually in northern Canada. If you are in the arctic sea north of Greenland, a compass would actually point south! You can think of a bar magnet located deep within the Earth, but tilted slightly, so that the magnet is not precisely aligned with the Earth’s spin axis, but tipped over about 11 degrees and moved away from the center of the Earth slightly. The place where the line containing the bar magnet, the magnetic axis, crosses the surface is called the magnetic pole. The location of the magnetic pole actually moves, although slowly, and the strength of the Earth’s magnetic field also is slowly changing. The magnetic field is created by electrical currents deep in the Earth’s molten core, and as that liquid slowly changes, the magnetic field changes too. We know from studying magnetized rocks on the ocean floor that every hundred thousand years or so the magnetic field actually reverses direction, and in fact we are due for a reversal within the next two thousand years.

But it is the more rapid changes that are more interesting (and potentially harmful) to our generation. At times the Earth’s magnetic field can temporarily change direction (by a few degrees) and strength (by a few percent). These changes can be measured all over the Earth, but the most dramatic changes are seen in a ring around the pole, the auroral zone. And the most spectacular changes occur when the majestic northern lights, or aurora borealis, blaze overhead. These gorgeous shimmering colors in the sky are the signature of electrical currents not in the deep core, but in the charged (ionized) part of the Earth’s upper atmosphere, called the ionosphere. And it is those same electrical currents that distort the local magnetic field.

The sun is the ultimate energy source for the auroral electrical circuit. The electrical current connecting the Earth to the Sun varies in strength from one to ten million amps, generating up to tens of terawatts of power (one terawatt is a million megawatts). The current changes from year to year (and minute to minute) both in intensity and in location. It is unfortunate that we can’t tap into that current, because it could supply the entire United States with power. It is too high up and too spread out in space.

There are two reasons why the power in the circuit changes. The first is that the power source itself changes. The ultimate source of energy is the sun, but that power is transmitted to the Earth via the solar wind, the very fast stream of electrons and ions given off from the sun’s corona. (Figure 3). The corona (which is Latin for "crown") is the ghostly halo around the sun seen during total eclipses. Until the space age, it was only observable during total eclipses, only by the lucky few in the path of totality, and only for a few minutes at a time. Now we can photograph the corona any time we like, if we put our cameras in space.

The corona is bright because the sun’s magnetic field acts like a magnetic bottle, trapping the plasma (a combination of electrons and ions) on it. The bottle isn’t perfect, though – some leaks out the top, creating the solar wind. But in the gaps between the bright coronal streamers, the solar wind can flow out much easier, creating a very fast stream. Think of water coming from a garden hose – if you just hold the hose, the flow comes out slowly, but if you hold your thumb over the opening, it comes out fast from the gap. The solar wind then varies from being fast but thin (coming from the gaps, called coronal holes) or being slow but dense (coming from the streamers). Even in its "dense" state, it’s very very thin - only a few particles per cubic centimeter by the time it reaches Earth. Compare that to a cubic centimeter of water, which has 6 x 10²³ particles in it! That’s nearly a million, million, million, million times more dense! So this wind, even though it is very very fast (300 to 1000 km per second, or approximately a million miles per hour), its breeze wouldn’t even ruffle your hair.

And although it does bring a great deal of energy to the Earth’s vicinity (enough to power the motion of the magnetosphere and the ionosphere), that energy is very spread out and would be impossible to harness. The energy in sunlight at the top of the atmosphere, for example, is 1.4 kilowatts per square meter, which reduces to about 1 kW/m² by the time it reaches the Earth’s surface. The size of the target is p RE², or about 1.2 x 10⁸ km² (1 RE is defined as one Earth radius, or about 6378 km). That means the total solar energy hitting Earth is about 10¹⁷ Watts! This the the energy that heats the Earth and provides the energy for photosynthesis. Most of the life forms on Earth depend on that radiant energy to survive. In contrast, the energy flux in the solar wind is only about a half a milliWatt per square meter – roughly three million times less energy delivered per square meter than sunlight! The effective target for absorbing this energy is larger, though – the size of the magnetosphere is about 30 RE across, making the total energy delivered "only" a thousand times less than sunlight.

Even though the energy flux in the solar wind is far less intense than sunlight, it is important because charged particles respond to the electric and magnetic fields in the solar wind. Consider a comet – Figure 4 shows Comet Hale-Bopp from 1997. The brightest tail of the comet is the dust tail. The yellowish dust tail points away from the Sun because the pressure of sunlight bouncing off its dust particles. It curves slowly because the dust particles have a significant mass. The other comet tail is the ion tail – here the blue tail seen above the dust tail. This tail is caused by ionized particles being pushed away from the sun not by sunlight but by the pressure of the solar wind. The comet’s neutral atmosphere, the coma, is ionized by sunlight (the neutral atoms are stripped of one or more electrons by absorbing energetic particles of light, or photons). The coma is also ionized by electron impact - electrons in the solar wind striking the comet’s atoms or molecules, knocking off additional electrons in the process and leaving the initial atom or molecule as a positively charged ion. The resulting ions and electrons are "picked up" by the solar wind and join with the flow, which also points away from the Sun but at a slightly different angle. The blue color is caused by sunlight scattering off of OH+ ions.

Just as the nucleus of a comet is tiny compared to the visible parts of the comet (the central coma and extended tails), the Earth is small compared to the size of the magnetosphere. The inner part of the magnetic field looks like an offset, tilted dipole; but the outermost shape looks like a comet, and for the same reason. The pressure of the solar wind compresses the part of the magnetic field that is in the direction of the sun. Friction with the solar wind drags out the extended tail, which is well formed to over 250 RE and a wake-like region has been observed out to 1000 RE - many times farther than the distance of the Moon (60 RE). A diagram of the magnetosphere is shown in Figure 5. The boundary of the magnetosphere is called the magnetopause. The distance to the dayside magnetopause is about 10 RE; the distance to the magnetopause perpendicular to the Earth-Sun line is about 15 RE. Both those numbers increase or decrease as the solar wind pressure decreases or increases, respectively.

The innermost part of the magnetosphere has two components. The best known is the Van Allen radiation belts, which are very tenuous (low density) but very energetic trapped electrons and ions. First discovered at the dawn of the space age by James Van Allen’s "Explorer 1" satellite, these particles can damage spacecraft electronics and injure people. It is important to keep at a minimum the total radiation fluence (the flux, or rate, times the time exposed to that flux) that people or spacecraft endure. Unmanned spacecraft missions are designed so that they pass through the belts relatively quickly and do not linger there. Manned missions (e.g. the Shuttle and the International Space Station) are designed to fly below the Van Allen belts, inside the protective cover of the Earth’s atmosphere. The place where most of the Van Allen belts are absorbed by the atmosphere is where the magnetic field is the weakest, in the South Atlantic Anomaly between South America and Africa. The most likely place for low-altitude spacecraft to experience electrical malfunctions from radiation damage is in this area (Figure 6).

The other inner-magnetosphere particle region is the plasmasphere, the very cold but fairly dense (100’s of particles per cubic centimeter) reservoir of plasma that evaporates off of the ionosphere. This region is not harmful to spacecraft or people; in fact, since it acts as a reservoir, it helps prevent the permanent loss of Hydrogen from Earth’s atmosphere. In Venus and Mars, such evaporated Hydrogen gets ionized by sunlight or electron impact and gets swept away by the solar wind. The equivalent amount of water that would make an ocean ten meters deep over the entire planet has been lost by these processes on Venus and Mars over the course of geologic time (4 billion years). Considering the nature of the magnetic field to protect us from energetic particle fluxes, and to keep our Hydrogen (and thus water) from evaporating away, it may well be that no planet can have higher forms of life if it doesn't have a significant magnetic field. (Europa, for example, could have simple life in an ocean under the ice cover, but the creatures there are not likely to develop interplanetary travel if they can't see the stars!)

The magnetosphere is not steady – as the solar wind varies in speed and direction, the magnetosphere waves like a flag in the wind, and changes in shape and size with the pressure of the solar wind. More importantly, the direction of the Interplanetary Magnetic Field (IMF) has an extremely important effect on the Earth’s magnetosphere. When the IMF is southward directed, it is aligned opposite to the Earth’s magnetic field near the boundary. These fields can then interconnect, allowing particles and electrical currents to flow into the Earth’s vicinity from the solar wind. When the IMF is strongly southward, and the pressure of the solar wind is high, the amount of energy transferred to the magnetosphere from the solar wind can be extremely large. One of the easiest ways to observe these changes in energy input is to monitor the aurora.

The aurora is caused by charged particles trapped in the magnetosphere that carry the electrical current linking the Earth to the Sun. The accelerated electrons strike the atoms in the upper atmosphere, which glow as they release the energy, in a process very similar to what happens inside a TV screen. Only specific frequencies of light are given off, as line emission, similar to that of a neon or mercury vapor light. The brightest visible line in the aurora is green (557.7 nm), from impact of primary or secondary electrons on Oxygen atoms. Less intense (and higher in altitude) is the red line (630 nm), also from electron impact on atomic Oxygen. The principal blue line is from electrons impacting N2 (319.4 nm), which also has crimson (427.8 nm) emission at very low altitudes (Figure 8). Since the aurora includes all the primary colors, the combination of the various lines can yield an apparent color anywhere in the rainbow. In addition, the aurora emits non-visible light, in the ultraviolet and x-ray portions of the spectrum, which are visible from spacecraft but do not penetrate to the surface.

The aurora makes a ring around both poles: the northern aurora is called the aurora borealis, and the southern aurora is called the aurora australis. In fact, auroral rings have now been observed at Jupiter and Saturn as well as Earth. When the IMF is southward (the z-component is negative), or the pressure of the solar wind is large, the aurora moves to very low latitudes and gets very bright. The North-South component (Bz) affects the brightness and size of the aurora (brighter and larger aurora for Bz negative). The East-West component of the IMF (By) causes a dawn-dusk shift in the aurora location. Figure 9 shows the statistical position of the aurora for two conditions of the IMF (in the Space Update software, you can adjust both the By and Bz to get an auroral prediction). The case shown in Figure 7 occurred after a major series of flares in March of 1989, the last solar maximum. It was that aurora, which appeared to follow the US-Canadian border, which caused a major blackout of power from Hydro Quebec. In addition, power outrages or trippages of electrical equipment were observed as far south as New Mexico. A weak aurora was even observed in Houston! A similar event is very likely for this next solar maximum time. The power companies have installed better control mechanisms to reduce the probability of electrical disruptions, but a major problem still might happen. Scientists now attempt to understand, and yes, to attempt to predict "space weather".

Space Weather

Conditions in space change both slowly (e.g. the 11-year solar cycle) and rapidly (hours to minutes). The study of these changes can be compared to the study of Earth’s lower atmosphere. We call slow changes to the atmosphere as "climate" and short-term changes as "weather". Generally, climate is easier to predict than the weather. The same is true for conditions in space – we are just now able to predict the conditions on Earth (space weather) by studying the Sun. A large disruption to the magnetosphere is called a geomagnetic storm. Until recently we couldn’t predict their occurrence – we could only know that one was happening right now. The situation was like trying to predict a hurricane at the turn of the century, when all they had to use was a barometer. When they saw the barometric pressure fall to very low levels, they knew that a major hurricane was imminent, but by then it was too late to run. Atmospheric scientists now use a network of ground measurements, and views of clouds from space, to enable them to predict hurricanes a day or two ahead (although the precise location where the eye will come onshore is often not known until the last few hours). A similar situation is true for space weather. We can now observe massive clouds of plasma leave the sun, and get a few days' worth of warning.

A coronal mass ejection (CME) is an event in which a large mass of the corona becomes disconnected and accelerated away from the Sun. They expand as they travel, becoming nearly 1/3 AU in length by the time they reach Earth’s vicinity: even traveling nearly a thousand km per second, the Earth can be engulfed in these clouds for an entire day or more. (1 AU is the average distance from the Sun to the Earth). These CME’s can bring high densities, high speeds, and high magnetic fields, all at the same time – just what it takes for a one-two-three punch to the magnetosphere. A CME can occur even in times of solar minimum, and if the imbedded magnetic field is southward, can create a spectacular low-latitude aurora. Figure 10 shows an image of an aurora with Comet Hale-Bopp, taken from Gloucester, Massachusetts during the major storm of April 1997 (photo courtesy Frank F. Sienkiewicz).

For some storms, a day or two after the maximum pressure, the energetic particles build up deep inside the magnetosphere. The electrons can reach energies of a Million electron-volts or more (equivalent to a temperature of ten billion degrees!). These very energetic electrons can do great damage to spacecraft systems, causing memory shifts, spacecraft charging events, and other damage. Although it’s difficult to prove a direct connection, many communication spacecraft failures occur when the energetic electron flux is very high – that’s why many scientists call these "killer electrons" and are attempting to determine why they build up to such incredible energies.

The upcoming solar maximum will be extremely interesting, both for the scientists and the public alike. The public will be able to observe aurora at much lower latitudes than are typical for solar minimum conditions. These low latitude aurorae are quite rare, so our new predictive capabilities are critical to seeing these elusive events. In addition, a number of web sites now have "real-time aurora" images, so that if you see a light in the sky which you think might be an aurora, you can check out the image from space to be sure.

For the scientists, this solar maximum will herald a quantum increase in our understanding and ability to observe the Sun’s activity and predict and observe its results on Earth. Many spacecraft are in place or will be launched in the next two years that will participate either as solar observers, solar wind detectors, or as monitors of the magnetosphere’s response. More and more spacecraft have real-time data streams, web-based interfaces and open data policies, so that the public can see the data in near real time.

One of the most exciting spacecraft to be launched for this solar maximum is IMAGE, which will for the first time "see" the energetic ions trapped deep within the magnetosphere. This will be the first spacecraft totally devoted to remote sensing of the magnetosphere, with several innovative techniques to "see the invisible". Energetic ions will be detected by means of Energetic Neutral Atom (ENA) imaging, a process whereby neutral atoms from charge exchange with the Earth’s upper atmosphere can be detected remotely. IMAGE will also have the first magnetospheric radio sounder, which can remotely sense density structures, such as the magnetopause and the plasmapause (the outer boundary of the plasmasphere). In addition, it will have auroral imagers and imagers than can see solar far-ultraviolet light scattered from the plasmasphere, all in near real time.

Far from being a boring empty space, the near-Earth region (geospace) is filled with dynamic plasmas and fields. When the Sun gets active, so does the magnetosphere. And with the myriad of real-time links now available, you truly can be "live from the magnetosphere" as well as "live from the Sun"!

This research which underlies this chapter was supported at Rice University by NASA under the ISTP (NAG5-3216) and IMAGE programs. Thanks to Jan Curtis and Frank Sienkiewicz for ground-based auroral images; to L. A. Frank for the DE auroral image; to Charles Botkin for the Hale-Bopp image; to the High Altitude Observatory at NOAA for the February eclipse image, and to Colin Law for the images used from the "Space Update" CD, which was funded by the NASA Digital Library Technology Program and the Office of Space Science.

Last updated: January 11, 1999