Solar and lunar daily geomagnetic variations in New Zealand

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SR_1995-033-pdf
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McKnight, J.D. 1995 Solar and lunar daily geomagnetic variations in New Zealand . Lower Hutt: Institute of Geological & Nuclear Sciences Institute of Geological & Nuclear Sciences science report 95/33 57 p.

Abstract: Regular geomagnetic variations with a fundamental period of one day are due to electric current systems located in the ionosphere and consequent induced currents in the outer layers of the Earth. The current systems arise from the interaction between Sun-driven ionospheric phenomena and the geomagnetic field. Regular geomagnetic variations with a fundamental lunar period are about an order of magnitude smaller in amplitude than those of solar origin and arise from the interaction between tidal motions in the ionosphere and oceans and the geomagnetic field. These solar (S) and lunar (L) daily geomagnetic variations are examined for New Zealand using selected areas of data from the Amberley Magnetic Observatory, a 15-year data set from the Eyrewell Magnetic Observatory and a three- to four-month data set from an array comprising 22 magnetometers spread throughout New Zealand. Coefficients for the first four harmonics of S and L are determined using the Chapman-Miller method of harmonic analysis, and the second harmonic of L is separated into parts of ionospheric and of oceanic origin. Strong seasonal variation is apparent in S, the magnitude of S being greater and the daily pattern of S occurring earlier in summer than in winter. The northward horizontal component of S increases in amplitude from north to south. The vertical component of S varies in both amplitude and phase throughout the region in line with proximity to induced currents in the oceans. The amplitude of S varies in line with sunspot number throughout the sunspot cycle. The horizontal components of L are predominantly ionospheric in origin and exhibit variations with season and sunspot number similar to those in S. The vertical component of L is highly variable in both amplitude and phase throughout the region, more so than for the vertical component of S, and this can be explained in large part by the influence of electric currents produced directly in the oceans surrounding New Zealand by tidal streams. Two spherical harmonic models of S are tested against data from Amberley for the years upon which the models are based. The models fit reasonably well for the particular phase of the sunspot cycle and, where appropriate, the season. The extent of the change of S through the sunspot cycle and the seasons is such that the models are limited in their application. They are not suitable for comparison with the data set from the magnetometer array. (auth)