The use of Magsat data to determine secular variation

Abstract

The appendix is available with the entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009. Document B83‐004; $2.50. Payment must accompany order. 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 Sqp 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.

Numerical experiments in geomagnetic modeling.

“…that from the core and crust of the Earth), one must also simultaneously evaluate or eliminate the field due to the ionospheric currents below and near the spacecraft as well as that contributed from induced currents. Although it was clear from such results as those reported by CAIN et al (1983) that variations in the ionospheric currents below the spacecraft orbit significantly alter the observed field, later work has not adequately come to grips with the problem. For example, in processing the data to be averaged for integrating, CAIN et al (1989a) made adjustments for average differences observed between dawn and dusk orbits, and for estimated symmetric ring current effects after correcting for estimated Dst.…”

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

“…ANALYSISCain et al [1983] discussed the selection of a Magsat data set and the calculation of a model (M061581) using spherical harmonic coefficients to n* = 24 and h* = 7 in an effort to determine the secular variation (SV) over the 7-month data collection interval of Magsat. For the present numerical experiment the same data set was used as in the M061581 derivation.…”

Observing, Modeling, and Interpreting Magnetic Fields of the Solid Earth

“…However, as MAGSAT lasted only 7 months, it is difficult to compute the secular variation over such a short period. A few studies dealing with secular variation models computed using only satellite data have been published (Cain et al, 1983;Barraclough, 1985). Opposing views on using satellite data for secular variation have been expressed.…”