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High Solar Latitudes and the

Spiral Interplanetary Magnetic Field

Last updated - November 9, 1996

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    New Ulysses Radio Observations

Ever since Professor Eugene Parker (Univ. of Chicago) predicted in 1958 that the interplanetary extension of the solar magnetic field would be drawn out into Archimedian spirals due to the rotation of the sun (once every 25 days) and the outward flow of the solar wind, interplanetary spacecraft been sent into space to verify this hypothesis. Measurements of the average angle of the interplanetary magnetic field at various points in space have proved to be consistent with the Parker theory. These single point in-situ measurements, however, could not reveal the global spiral structure of the magnetic field lines threading through interplanetary space.

In the early 70's Drs. J. Fainberg and R. G. Stone (GSFC) pioneered the idea of obtaining a global picture of the magnetic field topology by using solar type III radio bursts as tracers. However, this methodology had to wait more than 20 years to realize its full potential with the passage of

This is a dynamic spectral representation


showing several solar type III radio bursts. These radio data were measured by Ulysses as it was passing far above the south pole of the sun, at a distance of about 2.2 AU below the ecliptic plane, on October 25 of 1994.

From an analysis of these radio data, Drs. M. J. Reiner, J. Fainberg, and R. G. Stone (GSFC) were able to track these type III radio bursts through interplanetary space, clearly revealing, for the first time, the global spiral magnetic field topology.

A summary plot of the results of the radio tracking (long-time exposure) for the intense radio burst beginning at 10:15 UT on October 25, 1994 are presented below.

To download a higher resolution color image click on figure
To download a high resolution black-on-white image click here

The viewpoint shown is looking down on our solar system from the northern ecliptic pole, but the results shown are based on Ulysses radio measurements made as it passed far above the south pole of the sun. The white symbols show the "radio" track resulting from the propagation of energetic electrons through the interplanetary medium. These electrons were injected into interplanetary space by a flare that occurred at 10:08 UT on the sun. The yellow arrow indicates the location of the flare site on the sun.

The radio track follows very closely the spiral field line originating at this flare site on the sun and corresponding to a solar wind speed of 400 km/s. The numbers to the right of the symbols indicate the radio frequency at which the radio measurement was made, e.g., "940" means 940 kHz. The radio frequency decreases as the radio source moves farther from the sun due to the decrease in the solar wind density with increasing heliocentric distance. The approximate locations of the planets Mercury, Venus and the Earth are also shown in the figure.

Five days later, at about 13:35 UT on October 30, 1994, another intense type III radio burst was observed by the Ulysses radio instrument. Its radio track is also shown on the figure. It follows very closely a spiral field line corresponding to a solar wind speed of 300 km/s.

These radio observations made by Ulysses as it passed over the south pole of the sun constitute the first direct observation of the spiral structure of the interplanetary space between the sun and earth. Because of the short time during which these observations were made, they represent a radio "snapshot" of the interplanetary magnetic field topology.

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Computer Simulated Movie of the Ulysses Tracking two Type III Radio Bursts

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Publicaton:

These new sciencific results were published in a recent issue of

For reprint requests, e-mail to any author

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NASA Press Release:

For the NASA press release associated with these results click here and look under 10/19/95:

"First 'Snapshot' taken of the Shape of Interplanetary Field"

Related articles:

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Dr. Michael J. Reiner/reiner@urap.gsfc.nasa.gov/(301)286-2595, NASA/GSFC, Code 690.2,Greenbelt, MD 20771
Dr. Joseph Fainberg/fainberg@urap.gsfc.nasa.gov/(301)286-6940, NASA/GSFC, Code 692.0,Greenbelt, MD 20771
Dr. Robert G. Stone/stone@urap.gsfc.nasa.gov/(301)286-8631, NASA/GSFC, Code 690.0,Greenbelt, MD 20771

Web homepage by
Dr. M. J. Reiner/reiner@urap.gsfc.nasa.gov/(301)286-2595