Volume 16, Number 1, March 2000
By Joseph King
The Sun spins at approximately 360 deg per 27 days, but sunspots rotate faster near the Sun's equator than at higher latitudes. The Sun's effective magnetic dipole flips over once per 11-year solar activity cycle. The solar wind emanates from a solar atmosphere seething with activity. For all these reasons it might be expected that no long-term patterns dependent on solar longitude would be observed in the solar wind.
However, quite the opposite has just been found by Dr. Marcia Neugebauer of NASA JPL and collaborators. They used mainly data from NSSDC's 1963-1999 OMNI data set of near-Earth solar wind field and plasma observations and data from various other solar wind measuring spacecraft away from the Earth in NSSDC's COHOWeb system. By using the times of interplanetary observations and the observed solar wind speeds (which vary between 300 km/s and 700+ km/s for varying plasma elements but are assumed constant versus solar distance for any given element for this analysis), they were able to time stamp each observation with the time the observed plasma left the Sun.
They then assumed a zero point for solar longitude at a time in 1962, prior to any data availability. For each of many assumed solar rotation rates, they were able to assign solar surface longitudes to each plasma element whose solar surface departure time had previously been determined. Then for each assumed rotation rate, they took averages of the flow speeds and magnetic field radial components, observed from many spacecraft over nearly 40 years, in each of many longitude bins. The expectation was that any shorter-lived longitudinal variations would be washed out by averaging such a long run of data.
However, for an assumed solar rotation period of 27.03 (+/-.02) days, significant longitudinal variations were obtained in both flow speed (amplitude ~ 30 km/s) and magnetic field radial component (amplitude ~ 0.2 nT). In the analysis published in the February 2000 issue of the Journal of Geophysical Research, the authors conclude that the solar magnetic dipole re-establishes the same longitude after each 11-year flip, which is an unexpected and significant result on solar processes.
The authors used some 1960's data from Mariner 2 (which initially confirmed the continued existence of the solar wind) and from Pioneers 6 and 7. It was, however, the longevity of the OMNI data set as well as the uniformity of the OMNI and COHOWeb data that enabled and greatly facilitated, respectively, this significant study.
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