global electromagnetic induction in the moon and planets - MTNet

264 P. Dyal and C.W. Parkin, Induction in theMoon
TABLE II
Apollo surfacemagnetometer characteristics
Apollo stationary Apollo portable
Parameter magnetometer magnetometer
Ranges(7) 0 —±200 0—i 256
(each sensor) 0 — ±100
0—i 50
Resolution (~‘) 0.1 1.0
Frequency response (Hz) dc — 3 dc — 0.05
Angular response Cosine of angle Cosine of angle
betweenfield between field
and sensor and sensor
Sensor geometry 3 orthogonal 3 orthogonal
sensors at ends of sensors in
100-cm booms 6-cm cube
Analog zero determination 180°flip of sensor 180°flip of sensor
Power OW) 3.5 1.5
Weight (kg) 8.9 4.6
Size(cm) 63x28X25 56x15x14
Operating temperature (‘C) —50 — +85 0 — +50
most of the solar wind magnetic driving waves with fluctuations should envelop the body and induce a
periods less than minutes and the dominant field poloidal responsewhich can be used to study the inchange
occurring over the entire globe will be due to ternal properties. The bodies possessing inductive time
the daily formation of the ionosphere whose period constants comparable to driving field periods (e.g.,
wifi be the axial rotation period of the planet. Higher- the Moon and 10) may be better explored using electrofrequency
fluctuations reaching the planetary surface
will be considerably lowerin amplitude than the diur-
magnetic induction techniques than by seismic methods.
nal variations (on Earth the daily variations are approximately
50 y whereas the shorter-period solar wind
Appendix: magnetic field measurement technique
changes are less than 10 ‘y); these diurnal ionospheric
magnetic field variations can induce a global response
Magnetic fields at the lunar surface have been meas-
uredwith magnetometers emplaced by astronauts on
in the planet’s outer layers. Since in this case the domi- the Apollo 12, 14, 15, and 16 missions. The three ornant
driving source for the planet is geometrically in thogonal vector components are measured as a function
the shape of a spherical shell, many simultaneous meas- of time and position and telemetered to Earth. Simulurements
are required to describe the total inductive taneously a magnetometer in the lunar orbiting Explorresponse
and separate it into external driving and inter- er 35 spacecraft measures the ambient solar or terresnal
response fields. This formidable task has been ac- -trial driving field and transmits this information to
complished to study the electromagneticproperties of Earth. Properties of the lunar surface magnetometers
Earth’s crust and mantle (Chapman, 1919; Lahiri and
Price, 1939; Ashour and Price, 1948; Rikitake, 1950,
1966; McDonald, 1957;Eckhardt et al., 1963). For the
are given in Table Ill.
core region of the Earth, however, seismic techniques
have provento be more useful than electromagnetic
Acknowledgements
induction techniques. We thank Richard Hartin of Philco-Ford, Ray
For the planets and satellites in group 3 of Table II Stepi~iensof the University of Santa Clara, and Jeffrey
which have neither a global permanent magnetic field Hammill and Roy Horinouchi of DeAnza College for
nor an ionosphere and which do not lie within the mag- their valuable contributions to the laboratory superconnetosphere
of a larger body, solar wind magnetic field ducting experiments.