Daily Variation and Secular Variation of the Geomagnetic Field from Shipboard Observations in the Gulf of Aden

Summary The problems of reducing geomagnetic observations from ships at sea in areas influenced by the effect of the equatorial electrojet are discussed. In particular, observations within the Gulf of Aden have been corrected for daily variation and secular variation for the purposes of constructing a contoured magnetic anomaly chart.
An empirical formula is given with which the range of daily variation at different latitudes within the Gulf was estimated for the purpose of correcting the data for daily variation. The observed secular variation, which was used to correct the data, is—11 γ/yr. which differs from the secular variation of +19 γ/yr. in the Gulf of Aden given by the recently adopted International Geomagnetic Reference Field (Zmuda 1969).     

Palaeosecular Variation of the Geomagnetic Field in the Aleutian Islands, Alaska

Sequences of late Tertiary to Quaternary lava flows at six Aleutian Island sites were sampled for palaeosecular variation (PSV) measurement using palaeomagnetic methods. Due to limitations in probable time span represented by each sequence of flows and statistical constraints, data from only two sites are considered in themselves to be reliable indicators of PSV. When all the data are combined to give greater statistical significance, an angular standard deviation of palaeofield directions of = 10.8° is obtained. This is compatible with other Pacific Ocean PSV data and points to the persistence of the present secular variation low in this region at least into Quaternary time.