Geology topics

J.C. Cain

Publications and source records attributed to J.C. Cain.

4 recordsLinked to original sources

USGS2012

The use of MAGSAT data to determine secular variation

A combined spatial and secular variation model of the geomagnetic field, labeled M061581, is derived from a selection of Magsat data. Secular variation (SV) data computed from linear fits to midnight hourly values from 19 magnetic observatories were also included in the analysis but were seen to have little effect on the model. The SV patterns from this new model are compared with those from the 1980 IGRF and with those for 1970 computed by the DGRF and with the 1960 patterns computed using the GSFC(12/66) model. Most of the features of the M061581 are identical in location and level with those of the 1980 IGRF. Together they confirm that the reversals in sign of field change seen over Asia and North America between 1965 and 1975 are reverting to the pre-1965 states. The M061581 model gives −32 nT/yr for the dipole decay rate, larger than the 70% increase already reported since 1965. This abnormally high value is interpreted as being a defect of the model because it appears to result from a much larger (−100 nT/yr) drop in field over the polar regions not indicated by the 1980 IGRF. This north polar decrease is shown to be of external origin as the result of a combination of the seasonal effect of the north polar ionospheric (counterclockwise) afternoon Sq p cell increasing in intensity from the beginning (November 1979) to the end (June 1980) of the Magsat data collection period, coupled with an enhancement of its effect as the orbit lowered from the 350– 550 km initial altitudes to near 200 km just prior to burnup. This experiment indicates that secular variation can be obtained from satellite data for intervals of less than a full year if corrections can be made for seasonal effects and that ‘annual snapshots’ of the field by a satellite would allow easy and accurate models of secular change without the use of any surface data.

Journal of Geophysical Research Solid Earth↗

USGS2012

Small-scale features in the Earth's magnetic field observed by Magsat

A spherical harmonic expansion to degree and order 29 is derived using a selected magnetically quiet sample of Magsat data. Global maps representing the contribution due to terms of the expansion above n = 13 at 400 km altitude are compared with previously published residual anomaly maps and shown to be similar, even in polar regions. An expansion with such a high degree and order displays all but the sharpest features seen by the satellite and gives a more consistent picture of the high-order field structure at a constant altitude than do component maps derived independently.

Journal of Geophysical Research Solid Earth↗

USGS2011

Geomagnetic Workshop

A workshop on geomagnetism, sponsored by the Geologic Division of the U.S. Geological Survey, was held in the Denver West Office Complex in Golden, Colorado, April 13–15, 1982. There were 90 registered participants from government agencies, academic institutions, and industry. This effort stemmed from the realization that geomagnetism, once a small but coherent discipline, has now expanded into numerous areas of the geosciences, yet those doing research in these specialties seldom make contact outside their area of immediate interest. The impetus for this event came from the members of a committee formed to review the geomagnetic activities within the U.S. Geological Survey. They observed that the level of communication between the various elements of this now diverse discipline was inadequate, not only within their organization but also between federal agencies, academia, and the private sector. While the desire was to cover as much of geomagnetism as possible, it was necessary for a workshop of reasonable size and length to exclude some important areas of the subject: magnetic reversal chronology, studies of the externally produced variations, and most aspects of internal induction. The plan was to give emphasis to some of the newer areas: those which have recently seen a high level of activity and those with increasing activity abroad compared to that in the United States. The purpose was to evaluate the status and problems in selected areas with an eye to those whose emphasis might produce fruitful results in the next decade.

Eos, Transactions, American Geophysical Union↗

USGS1987

Evaluation of the 1985-1990 IGRF secular variation candidates

The IGRF secular variation model for 1985-1990 was adopted by the International Association of Geomagnetism and Aeronomy at its Prague meeting in August 1985 as an average of the three candidate models submitted to the committee. We compared the three models at epoch 1985.0 against each other and against a new model based on observatory data available as of July 1, 1985. These comparisons showed that one of the three candidate models disagreed more with the other two and our model, especially in the eastern Pacific. None of the candidate models was seen to respond to a change in the secular variation of the vertical component that appears to have taken place most strongly in the western Pacific area since 1982. The lack of satellite data was seen to be a significant handicap towards prediction of the field change over most of the Earth's surface, especially the southern oceans. Maximum errors of any model are estimated to be of the order of 80 nT a-1. ?? 1987.

Physics of the Earth and Planetary Interiors↗