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by Kris Sigsbee
The operator on duty at the Lulea, Sweden telephone office probably never expected the shock she got at work on September 25, 1909. When the operator picked up a microphone, she received a sudden electrical shock that partially paralyzed her hand. "Both the instrument and the hand were surrounded by an intense, diffuse light, casting out sparks and causing blisters," wrote David Stenquist, an early 20th century scholar at the University of Stockholm, in his 1914 Inaugural Dissertation called "The Magnetic Storm of September 25, 1909." Stenquist reported that "a strong, crackling, thunder-like sound" was heard on the telephone lines to Lulea, Sweden that made communications impossible at times. Observers in both the northern and southern hemispheres saw incredibly spectacular aurora on September 25, 1909. Stenquist believed the cause of the strange events of the strange events of that day was "the sun and the dust it sends out." He was not too far from the truth. The magnetic storm that disrupted communications throughout Scandinavia in 1909 was most likely the result of a disturbance that began on the sun and traveled through interplanetary space toward Earth.
Magnetic plasma storms in outer space probably sound like something you saw on Star Trek last night, but they aren't science fiction. Magnetic storms occur in the space environment near Earth and still cause problems with communications and other terrestrial systems in spite of the advances in technology since 1909.
The 1909 magnetic storm was an early example of space weather -- events in space that interfere with communications, power systems on Earth, and satellites. Space physicists and engineers at the University of Minnesota and around the world are trying to understand why geomagnetic storms occur and how space weather affects life on Earth. Geomagnetic storms are related to particles from the sun, just as Stenquist believed. The uppermost layer of the sun's atmosphere, called the corona, constantly emits streams of plasma, known as the solar wind, into space. Dr. Barbara Thompson is a 1996 University of Minnesota graduate who now studies the structure of the solar corona using data from the Extreme Ultraviolet Imaging Telescope on board the SOHO (Solar and Heliospheric Observatory) spacecraft, a joint mission of NASA and the European Space Agency. She explained the solar wind by comparing the atmospheres of Earth and the sun. "Imagine if the Earth's atmosphere was really hot - you would expect that a lot of the gas could reach escape velocity."
When a particle in Earth's atmosphere or the sun's corona reaches escape velocity, it is moving fast enough to escape from the gravitational field pulling it down towards Earth or the sun. Solar wind electrons and ions escaping from the sun's corona travel to the most distant regions of our solar system.
The solar wind plasma also carries the sun's magnetic field far away into space, creating an interplanetary magnetic field throughout the solar system. On an average day, the solar wind blows at a blustery 400 km/sec (894,816 miles/hour). The solar wind was predicted by astronomers who observed that long cometary tails were always directed radially away from the sun. Its existence was verified by instruments on board the Soviet Lunik 1 and Lunik 2 spacecraft in 1960.
Although the solar wind blows continuously, the level of activity on the sun varies with a period of about 22 years. During solar maximum, the peak in the cycle of solar activity, the number of sunspots increases. Solar wind velocities and temperatures are higher than usual and the solar wind is accompanied by large fluctuations of the interplanetary magnetic field. Violent activity such as solar flares and coronal mass ejections is more common. "A coronal mass ejection is a sudden eruption of millions of tons of solar material, and a large portion of the sun can participate in the eruption," explained Thompson.
Fortunately for life on Earth, when the high speed solar wind plasma encounters a magnetized planet, such as Earth, the interaction between the solar wind and the planetary magnetic field forms a protective magnetic cocoon called a magnetosphere (diagram below).
The sunward location of Earth's magnetopause is about 10 times the radius of Earth (39,630 miles) away. However, increases in the solar wind speed can compress the day-side location of the magnetopause to within the distance of 6 times the radius of Earth (23,778 miles), close to the orbits of communications satellites in geosynchronous orbit.
On the night side of Earth, the interaction between the solar wind and Earth's magnetic field drags the magnetic field lines backwards, creating a long magnetic tail, or magnetotail, extending beyond 200 times the radius of Earth (792,600 miles). Professor John Wygant, a space physicist studying Earth's magnetosphere at the University of Minnesota, describes the magnetotail as a sort of magnetic "wind sock" blowing in the solar wind.
The interplanetary magnetic field embedded in the solar wind has a strong effect on the configuration of the magnetosphere. When the solar wind magnetic field points northward (same direction as Earth's magnetic field) the solar wind is deflected harmlessly around the magnetosphere since like magnets repel each other. If the solar wind magnetic field turns southward (opposite Earth's magnetic field), magnetic field lines on the day-side magnetopause can merge with the solar wind magnetic field in a process called magnetic reconnection.
Magnetic reconnection opens up the magnetosphere and allows solar wind particles to enter. Newly reconnected field lines convect over Earth's poles and eventually pile up in the magnetotail. Vast amounts of energy can be stored in the magnetotail by this process. During a geomagnetic storm, the energy stored in the magnetotail is explosively released towards Earth. The Earth's magnetic field pulsates and huge electrical current systems reach down into Earth's ionosphere producing brilliant displays of aurora.
The auroral light is produced when energetic particles traveling along Earth's magnetic field hit the ionosphere, exciting atoms of oxygen and nitrogen at altitudes of around 100 km (60 miles). Aurora occur in a ring around Earth's north and south poles called the auroral oval. To an observer on Earth, the aurora can appear as undulating green and red ribbons of light, or as a diffuse glowing cloud in the northern sky. In North America, the aurora borealis or northern lights are normally only visible at high latitudes in Alaska and Canada. However, during periods of high geomagnetic activity the auroral oval expands southward, making the northern lights visible at lower latitudes. Sometimes the northern lights can actually be seen in Minnesota.
Because the solar wind magnetic field undergoes daily and sometimes hourly variations, small magnetic storms occur often even during solar minimum. The most severe geomagnetic storms are related to solar events such as coronal mass ejections and occur more frequently near solar maximum.
The physical processes involved in geomagnetic storms and how they affect the magnetosphere are not well understood. According to Wygant, the outstanding problems of space physics research today are the creation of the aurora, the processes that cause the release of stored energy in the magnetotail and initiate major geomagnetic storms, how particles in the magnetosphere are accelerated, and how the radiation belts are produced.
"The United States, Europe, Russia and Japan have put up a fleet of spacecraft that are monitoring electric fields and particles in different regions of space," said Wygant. He has assisted with the design, construction, and testing of electric field instruments on the CRRES (Chemical Release and Radiation Effects Satellite), Polar, and Cluster spacecraft.
In space, electric fields are measured by long pairs of wire booms held out by centripetal force on board a spinning satellite. Spherical probes at the ends of the booms contain electronics and sensors. The voltage difference measured between the spherical probes tells scientists the value of the local electric field. Designing this type of instrument has many unique problems. On the Polar spacecraft, which spins at a rate of 6 revolutions/second and has 100 meter booms, the spheres at the ends of the wire booms rotate at a speed of 150 miles/hour. The signals space physicists want to measure are small and typically have values of only 0.1 to 1 volt. "The spacecraft body itself can charge up to 1000 volts," explained Wygant. This makes it difficult to distinguish the signal from the background. In spite of these problems, Wygant believes building electric field instruments for spacecraft is worth the effort.
Studying electric fields in Earth's magnetosphere helps space physicists understand how particles are accelerated in auroral processes. When a charged particle, such as an electron or proton, encounters an electric field it feels a force that changes its speed and direction. Similar processes may accelerate particles in other astrophysical environments light years away.
"There aren't many situations where we can study the acceleration of particles in space directly," said Wygant. Understanding the acceleration of charged particles in the magnetosphere may give scientists valuable insight into the production of cosmic rays, extremely high energy charged particles traveling through the universe at close to the speed of light. Some cosmic rays may have been accelerated by solar flares, while others come from pulsars and supernova explosions in distant regions of the galaxy where it is impossible to send a spacecraft.
"The purpose of flying these spacecraft is largely scientific in nature," said Wygant, but space physics research has important applications. The same phenomena that produce beautiful displays of aurora have the potential to severely disrupt communications and damage satellites in low-Earth orbit. During geomagnetic storms, ions and electrons striking a spacecraft can cause different portions of the satellite to become electrically charged. If enough charge builds up, an electrical discharge will arc across the spacecraft, damaging electronic components. Other kinds of malfunctions, such as single particle upsets can also damage satellites. A single particle upset occurs when an individual particle penetrating the spacecraft creates enough free electrons, negatively-charged particles that are not permanently attached to a specific atom or molecule, to produce a logic state change in a microelectronic device.
Logic circuits generally operate between two discrete voltages, a high voltage that represents a 1 and a low voltage that represents a 0 in a binary system. This kind of circuit stores information, performs arithmetic, and carries out instructions in a computer. When errors occur in circuits critical to spacecraft operation, control systems can latch-up and switch into undesirable modes, causing burnout of electrical systems.
Potentially worse effects could occur if systems controlling propulsion or the orientation of the spacecraft, also known as the spacecraft attitude, are involved. Safeguards against malfunctions can be built into spacecraft, but accidents still happen. The great geomagnetic storm of March 13, 1989, caused one such accident. The Navy had to take four navigational satellites out of service for up to a week.
In addition to causing problems with electronics on satellites, solar and geomagnetic activity can actually influence satellite orbits. The level of short wavelength solar radiation and geomagnetic activity changes the scale height of Earth's upper atmosphere, which affects the drag on satellites in low-Earth orbit. Understanding the changes that will occur during the next solar cycle will help scientists and engineers plan how to boost the orbit of the Hubble Space Telescope and the assembly of the International Space Station.
Here on Earth, geomagnetic storms can wreak havoc with electrical power systems. Electrical and computer engineering Professor Emeritus Vernon Albertson first became interested in the effects of geomagnetic storms on power grids while working for Otter Tail Power Company in Fergus Falls, Minnesota in 1958. On a clear, winter night a transformer station "tripped out" and the aurora borealis was suspected to be the cause. When Albertson came to the University of Minnesota in 1963, he began doing his own research on how geomagnetic storms affect power systems.
During geomagnetic storms, the auroral current systems produce fluctuating magnetic fields. "The Earth is a conducting sphere that finds itself in a changing magnetic field," said Albertson. Magnetic fields that change with time produce electric fields that can drive currents in an electrical conductor. Because of this physical principle, "you are going to induce currents in the conducting sphere," said Albertson.
Geomagnetically induced currents flowing through the Earth cause problems in power systems because transformers have grounded neutral points connected to the Earth. When geomagnetically induced electrical currents flowing through Earth come near a transformer, they always follow the path of least resistance.
"Zip. Up goes the current through the transformer and into the transmission line," said Albertson. The presence of geomagnetically induced currents in a transformer causes the magnetic flux, or number of magnetic field lines crossing a unit area, inside the transformer to go to abnormally high levels during half-cycle saturation. This can cause heating and generate oscillations at multiples of the frequency of the alternating voltage, called harmonics, elsewhere on the power system. During the great geomagnetic storm of March 13, 1989, the entire Hydro Quebec system and its more than 6 million customers plunged into a blackout for 9 hours because of this kind of malfunction. A large transformer at a nuclear power plant in New Jersey overheated and failed as a result of the same storm.
Currently, the sun is in a period of relatively low activity, and the risk of major geomagnetic disturbances is small. However, the level of solar activity is increasing and the number and severity of violent solar disturbances, such as coronal mass ejections, is growing.
"For a while there was only one [mass ejection] per month which had a strong impact on Earth, but as we head toward solar maximum we could be dealing with almost daily events," said Thompson.
The next solar cycle is predicted to near its peak in late 1999 or early 2000. Severe geomagnetic storms are likely to occur during the period from 1999 to 2005.
The threat posed by geomagnetic storms during the next solar cycle has sparked a renewed interest in space physics and has spawned the field of space weather forecasting. In the not so distant future, space weather forecasters will be able to predict geomagnetic storms by carefully monitoring activity on the sun, in much the same way meteorologists predict weather here on Earth. Satellites such as SOHO and Wind, which carries an instrument built by University of Minnesota scientists, are already providing enormous amounts of data about the sun and the solar wind. Missions such as ACE (Advanced Composition Explorer), launched in late summer 1997, will send valuable new information to scientists and space weather forecasters on Earth. During the next millennium, space physics research and space weather forecasts will be crucial to the protection of spacecraft and electrical power networks against costly malfunctions and damage.
