Lunar daily geomagnetic variations in New Zealand

Lunar daily geomagnetic variations ( L) exhibit distinctive behaviour at many sites near coasts. In this study, data from an array of three-component magnetometers that operated for a four-month summer period at 22 sites throughout New Zealand are used to examine L. The Chapman-Miller method is used to determine the four main luni-solar harmonics of L at each site. Analysis of 3.5 years of data from the geomagnetic observatory at Eyrewell indicates that the array data produce rep-resentative values of L. Separation of the dominant lunar semidiurnal harmonic (L2) into parts of oceanic and ionospheric origin indicates near-uniform ionospheric contributions to the X and Y field components at all sites, with peak values occurring within 3 hours of lunar transit. The ionospheric contribution to the 2 field component is smaller, and suggests secondary electric currents flowing in the deep water to the north-east and south-west of New Zealand. The oceanic contribution to L, tends not to be significant for X and Y. Its amplitude for 2 is greatest in the eastern North Island and in the south-western South Island, and its phase angle for Z is locally consistent throughout the North Island and in the south-western South Island. The difference between the phase angles that characterize these northern and southern regions is about 180". This pattern suggests near-parallel, in-phase electric currents flowing in the deep water to the north-east and south-west of New Zealand in response to tidal flows. This is in agreement with both the general pattern seen in the United Kingdom and lunar daily geomagnetic variations observed under the Tasman Sea and in eastern Australia, where the magnetic effects of tidal flows in shallow coastal waters are secondary to those from more distant, deep waters. The oceanic contribution to L2 might be used to estimate the poorly known tidal flows in the deep water off New Zealand. Key words: geomagnetism, lunar geomagnetic variations, tides