The distant geomagnetic field: 1. Infinitesimal hydromagnetic waves

The presence of magnetoacoustic, mixed (oblique), and transverse hydromagnetic waves is demonstrated at geocentric distances of 3.7 to 7 earth radii on the sunlit hemisphere. The frequencies present extend to 13 rad/sec, which is the upper cutoff for the passband of the magnetometer used in this experiment. The mixed mode is identified with the ordinary transverse (Alfven) wave, and the pure transverse mode with the extraordinary wave. The power spectra display discrete lines from which coupling of modes can be deduced. Typical wave energy densities vary from 10−13 to 10−12 erg/cm3 over 1 cps. On the assumption that the available sample typifies the magnetospheric volume to 7 Re, the hydromagnetic radiation energy appears to be of the order of 1018 ergs over 1 cps. At a mean hydromagnetic velocity of 3×107 cm/sec, and assuming that all this radiation was entrant upon the ionosphere, the energy input would be some 10−5 erg/cm2 sec between 0 and 1 cps. In addition to many discrete lines in the spectra, there is some continuum radiation. Finite line width provides a measure of broadening possibly due to positive ion Landau dumping. The spectra suggest excitation of the far magnetosphere.