The long held view that the northern lights, which last for days to weeks following magnetic storms, are part of the magnetic storms themselves is not correct, two scientists from NASA's Jet Propulsion Laboratory said.
In paper recently presented before the American Geophysical Union spring meeting in Baltimore, Md., Drs. Bruce Tsurutani and Walter Gonzalez described more complex and continuous process as the source of the Aurora Borealis and the similar south pole phenomenon, the Aurora Australis.
It is well known that magnetic storms are caused by solar flares and other solar disturbances which blast out into the interplanetary medium. Tsurutani and Gonzalez have shown that the cause of major magnetic storms are intense interplanetary electric fields that last for more than three hours. The effects of these fields, however, are short-lived, they said.
The northern and southern lights which last for days to weeks in the trailing portion of the storm are not part of the storm decay, as has been the long-held view of the scientific community, but are caused by waves, called Alfven waves.
Tsurutani and Gonzalez, working with data provided for 500 consecutive days by the International Sun Earth Explorer-3 satellite (ISEE-3), have investigated the process causing the long-lasting auroras.
The process begins with wave action propagating through the plasma, the interplanetary medium, between the sun and Earth, Tsurutani said. The waves originate near the sun, but their cause is presently not known, he said.
The waves cause directional changes in the interplanetary magnetic field. When the wave field becomes opposite to the northward direction of Earth's magnetic field, magnetic interaction occurs and the two fields are coupled.
"In short, the interplanetary magnetic field which originates with the sun, connects with the Earth's magnetic field and pulls the field in the downstream direction away from the sun," Tsurutani said.
"This magnetic field reconnects behind the Earth and causes charged atomic particles to slingshot toward the night side of Earth."
When the plasma gets close to the planet an instability occurs and causes the particles to scatter. Then, the ions and electrons follow the magnetic field lines of Earth toward its surface at both poles. They strike atoms in the upper atmosphere and excite them to higher energy.
These excited atoms radiate this excess energy by emitting light which is characteristically seen as the wavy red and green lights of the auroras.
The scientists said that magnetic storms are continuous process, but differ in intensity. Very large storms are rare and occur about once in every three or four years. Large storms as discussed happen once every month or two and substorms occur every three hours.
They said the solar wind, the constant stream of particles emitted by the sun, may be initially accelerated by Alfven waves near the sun.
"What we may be seeing," Tsurutani said, "are the remnants of the solar wind acceleration process striking the Earth's magnetic field and causing injection of the solar wind energy into the Earth's atmosphere."
The ISEE-3 satellite, later named ICE for International Cometary Explorer to examine the comet Giacobini-Zinner in 1985, is under the control of NASA's Goddard Space Flight Center in Greenbelt, Md. It was in orbit about 850,000 miles from Earth when the data were acquired in 1978 and 1979 for the Tsurutani-Gonzalez study.
Tsurutani is JPL scientist conducting research in magnetic fields. Gonzalez spent one year visiting JPL on leave from the Instituto de Pesquisas Espaciais in Sao Jose dos Campos, Brazil.
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