McKnight, J.D. 2000 New Zealand first-order geomagnetic repeat observations, 1996-1998. Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences science report 2000/01 36 p.
Abstract: A network of stations is maintained throughout the New Zealand region and in Antarctica at which periodic (or repeat) observations of the geomagnetic field are made for the purpose of monitoring the secular variation. This report describes the stations in the network and presents the results from the third round of first-order reoccupations, conducted between 1996 and 1998. A first-order reoccupation of a repeat station involves running a portable three-component variometer near the station for a period spanning at least two nights, and making absolute observations on the station during the intervening day(s). The absolute observations are used to calibrate the variometer. An ''undisturbed'' field value for the station is determined in a two-step process; taking night-time values from the calibrated variometer record removes short period variations, and referencing these to records from the nearest observatory removes long period variation. The difference between field values from separate reoccupations of a station is taken as the measure of the secular variation at that station. The results of the 23 repeat station reoccupations made between October 1996 and November 1998 are presented. They include details of the absolute observations, the mean hourly values recorded by the variometer, and the ''undisturbed'' or adopted field values for each reoccupation. Descriptions of the reoccupations and information about the location of the repeat stations and access to them are also given. Plots of the results from the full set of repeat observations made since 1940 show trends in the secular variation throughout the region, as well as the effects of the introduction of new technology to the repeat station programme in the 1980s and 1990s. (auth)