Magnetic field fluctuations at the Eyrewell Observatory, Christchurch, New Zealand

Abstract

Four months of magnetic records from the Eyrewell Observatory near Christchurch, New Zealand, have been digitised, and analysed to produce Parkinson response arrows. The magnitudes and azimuths of both the real and quadrature arrows are smooth functions of period. Real arrows at three particular periods are similar to arrows published previously for the earlier observatory at Amberley, some 43 km northeast of Eyrewell. Differences between the Eyrewell and Amberley arrows are consistent with effects due to the closer proximity of Amberley to the ocean, the different analysis techniques employed in the various studies, and possibly with some local geological effects.

The magnitude of the real arrow at Eyrewell has a narrow peak at a period of about 1000s. At longer periods, the Eyrewell real response decreases sharply, a characteristic which is interpreted as being due to the limited width of the New Zealand landmass, and the balancing effect of the Tasman Sea on its western side. At intermediate periods (500–4500 s) the Eyrewell quadrature arrow has a northerly component. Part of this northerly component could be associated with the strong electric current through Cook Strait, some 320km to the north.

Keywords:

• electromagnetic induction
• Parkinson arrows
• Earth-current surveys
• Eyrewell Observatory