Goals for the Decade

Michael Purucker and John LaBrecque, NASA

The first five years of the 'Decade of Geopotential Research' has a full complement of geomagnetic missions, but only the proposed French mission 'Ampere' combines absolute geomagnetic field measurement with precision attitude determination for the second half of the decade. Additional magnetics missions will need to be developed for that time period if we are to meet the stated goal of the 'Decade', a continuous record of geomagnetic field variability. We believe that collaborations between Sun-Earth Interactions and the Solid Earth community will yield strong synergies based on the old saw "One person's noise is another's signal."

We propose a goal for the Decade should be a measurement of the geomagnetic field variability to better than .5 nT/yr at wavelengths of 300 km or longer. Thus far our measurements of geomagnetic field variability have been sparse and noisy. The Decade now promises continuous high quality main field measurements with its attendant secular field measurements.

We should also focus upon the intermediate wavelength field and its time varying components. We should look for the higher order secular variation components beyond n=13 that we might correlate with geodetic measurements of Earth rotation and low wavelength surface deformation, seismic probing of the core and the recent advances in magnetohydrodynamic models, to better understand the internal dynamics of the Earth. Modern geodesy provides us with extremely accurate meaurements of the crustal strain, but perhaps geopotential field measurements might provide the all important remote measurement of lithospheric stress change.

Magnetotelluric response could be exploited from spaceborne arrays to study heterogeneous petrology of the lithosphere and mantle as well as temperature and current structure of the oceans.

As we set out to explore the planets, let us use the Earth to refine our techniques and knowledge of planetary evolution. The magnetic field experiment on Mars Global Surveyor (MGS) is the most successful example of a mission to map a planetary magnetic field. Because MGS was able to dip to 90 km above the Martian surface during aerobraking, it captured in unprecedented detail a recording of tectonic events from the earliest epoch of Martian history.

Missions which dip to very low altitudes are capable of resolving intermediate wavelength, time-variable sources. The satellite (or satellites) should be in a strongly elliptical (200 to 2000 km) orbit and capable of dipping to as low as 130 km in preselected regions. Of course these missions will require propulsion to insure sufficient mission life.

An alternate or complementary approach is to develop dense polar orbiting geomagnetic nano-satellite constellations at two or more altitudes. The resulting dense geomagnetic measurements in radial and angular dimensions will increase the measurement accuracy of the static and time varying geomagnetic components while allowing the measurement of gradients to estimate magnetospheric current flow.

Accurate knowledge of the external geomagnetic field component is key to the seperation of internal field components from the observations as per the comprehensive field modelling techniques. From a measurement strategy and modelling perspective our collaboration with the space physics community is essential.

Recent advances in the stability and the miniaturization of magnetometers need to be accelerated. Magnetometer designs should be integrated with GPS and star imager capabilities to develop an absolute vector geomagnetic measurement package suitable for main field studies. The challenge is to develop an accurate measurement instrument that can be placed aboard nano-satellites or satellites of opportunity.