Astronomy and Astrophysics – Astrophysics
Scientific paper
Aug 1987
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1987a%26a...182..329s&link_type=abstract
Astronomy and Astrophysics (ISSN 0004-6361), vol. 182, no. 2, Aug. 1987, p. 329-336.
Astronomy and Astrophysics
Astrophysics
15
Geomagnetism, Interplanetary Medium, Magnetic Fields, Solar Cycles, Solar Terrestrial Interactions, Solar Wind, Coronas, Equatorial Regions
Scientific paper
The quiet-days form the largest set of geomagnetic activity indices while the fluctuating activity is the most frequent form of storminess. Both categories of geomagnetic phenomena are associated with the flow, in the ecliptic plane, of low velocity wind for the quiet-days and moderate velocity wind for the fluctuating activity, respectively. Therefore their solar origin takes place in the narrow "equatorial belt" scanned twice a year by the ecliptic plane. We write down a series of conclusions coming from experimental evidences relating the occurrence in the "equatorial belt" of bipolar structures of high density, identified either with coronal streamers or with the heliosheet, with the flow in the ecliptic plane of a slow wind. These conclusions concern the distribution of the wind velocity on both sides of the associated neutral sheet as well as the cyclical evolution of its shape, size and stability. Therefore we can show that the cyclical behaviour of the annual number of quiet days as well as that of the level of fluctuating activity are linked to the cyclical evolution of both the shape and the thickness of the neutral sheet. We study the various properties of these slow wind sources both from their patterns in the Bartels diagram and from the annual distribution of the related geomagnetic indices. Those specific properties explain that, in our data set, the annual sum of the quiet-day activity remains nearly constant, without any link with the quiet-day number itself which, however, varies from 125 to 354 quiet days a year. Finally we discover an inverse ratio relation in between the thickness of the heliosheet at the sunspot minimum and the following peak of the sunspot activity (sunspot maximum number). This new result confirms our previous finding of a phase/intensity relationship between the poloidal and the toroidal components of the solar field. Therefore the solar cycle phenomenon and its amplitude do not concern only the cyclical behaviour of the sunspot number but also the cyclical evolution of the solar dipole during the 5 year long interval preceding the cyclical sunspot occurrence.
Legrand Jean-Pierre
Simon P. A.
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