High frequency sky wave propagation during geomagnetic field reversals

Abstract

The ionosphere, a plasma embedded in the Earth's magnetic field, affects propagation of electromagnetic waves in the high frequency range since the refractive index at these frequencies depends on a combination of plasma density and magnetic field intensity and direction. In particular, the ground range of high frequency waves that reflect in the ionosphere, or sky waves, presents detectable Earth's magnetic field effects. This field varies greatly, with the most drastic scenario being a polarity reversal. The spatial variability of the ground range during possible reversal scenarios is analyzed in the present work using numerical ray tracing. In order to isolate the magnetic field effect we exclude the effect of changing ionospheric conditions by considering a uniform ionosphere. Our results show that the ground range increases with increasing ray alignment with the field direction as well as with increasing magnetic field intensity. For the present field that is dominated by an axial dipole, the ground range is greatest for northward propagation at the magnetic equator. A similar situation occurs for a prevailing equatorial dipole with eastward propagation at the corresponding magnetic equator that here runs almost perpendicular to the geographic equator. For less dipolar configurations the ground range spatial variability becomes smaller. Although a reversal is foreseeable only in a very distant future, studying potential consequences during a reversal may highlight possible effects of dipole decrease which is already ongoing at present. In addition to the geophysical insight, our results may have applications for communication and radar systems.