| Popis: |
The magnetic field from Earth's core (the main field) is a global phenomena with measurable temporal variations with periods ranging from one year to millennia. Geomagnetic studies are thus heavily dependent on the availability of data well distributed over the globe and acquired over long periods of time. Satellite data provide the best geographic coverage, but are unlikely to be available except possibly at intervals of 10 to 30 years. Accurate mapping of the main field over long periods of time is mostly dependent upon a network of geomagnetic observatories, each of which contributes continuous, three-component, data of high accuracy. The overall accuracy of knowledge of the main field depends both upon the adequacy of the geographic distribution of those observatories and on the existence of periodic surveys by satellite. Analysis of models based on the existing observatory distribution reveals large geographic regions in which their accuracy is degraded such that studies of the field, its source dynamo, etc. are seriously limited. Model accuracy is studied for three distributions of 92, 162, and 252 equally spaced observatory sites and for degradation of those distributions by a large area with no data. The 92-site distribution is the most economically realistic. Expansion of the existing network so that a subset of observatories approximates this 92-site distribution can be accomplished by a phased program of collocating magnetometers at 20 sites already established, or now planned, for other geophysical networks such as FLINN, GEOSCOPE, IDA, and IRIS, at 10 additional land or island sites, and at 8 sea bottom sites. Specific locations for these sites are proposed. While not meeting all of the needs for study of current problems in geomagnetism, if implemented, this extension of the current observatory network would form a firm foundation for most such studies. Such implementation will only be accomplished if the burden for doing so is partially shouldered by most or all of the national agencies and organizations representing data users and if such users unit in expressing their own need. |
