[Comment to] “Polar auroral, geomagnetic, and ionospheric disturbances” [by Vestine and Sibley]

In a recent letter Vestine and Sibley [1959] noted that the integral invariant of auroral particle motion appears to predict successfully features associ- ated with auroral isochasms, when the geo-magnetic field in space can be computed. In equation 1, dl is an element of length of a geo-magnetic line of force along which the charged particle is spiraling while slowly drifting to adjacent lines. In the integral, F is the geo- magnetic field strength, and Fm the field value at which the particle is reflected (the mirror point). In fact is regarded as the approximate component of the velocity parallel to the field line, where v is the total velocity of the particle.