Effects of Earth's magnetic field variation on high frequency wave propagation in the ionosphere (original) (raw)
Annales Geophysicae Discussions
The ionosphere is an anisotropic, dispersive medium for the propagation of radio frequency electromagnetic waves due to the presence of the Earth's intrinsic magnetic field and free charges. The detailed physics of electromagnetic wave propagation through a plasma is more complex when it is embedded in a magnetic field. In particular, the ground range of waves reflecting in the ionosphere presents detectable magnetic field effects. Earth's magnetic field varies greatly, with the most drastic scenario being a polarity reversal. Here the spatial variability of the ground range is analyzed using numerical ray tracing under possible reversal scenarios. Pattern changes of the "spitze", a cusp in the ray path closely related to the geomagnetic field, are also assessed. The ground range increases with magnetic field intensity and ray alignment with the field direction. For the present field, which is almost axial dipolar, this happens for Northward propagation at the magnetic equator, peaking in Indonesia where the intensity is least weak along the equator. A similar situation occurs for a prevailing equatorial dipole with Eastward ray paths at the corresponding magnetic equator that here runs almost perpendicular to the geographic equator. Larger spitze angles occur for smaller magnetic inclinations, and higher intensities. This is clearly observed for the present field and the dipole rotation scenario along the corresponding magnetic equators. For less dipolar configurations the ground range and spitze spatial variabilities become smaller scale. Overall, studying ionospheric dynamics during a reversal may highlight possible effects of dipole decrease which is currently ongoing. 1 Introduction Radio frequency electromagnetic waves between 3 and 30 MHz, designated as high frequency (HF) waves by the International Telecommunication Union (ITU), are used in long-distance communications and detection, and have been of interest since the 1920's from a geophysical point of view as well as for practical reasons. Since the advent of telecommunication systems it has been a challenge to establish radio links as well as exact positions with radar systems using the ionosphere as a reflector due to the theoretical complexity of electromagnetic wave propagation through the ionospheric plasma that is embedded in the Earth's magnetic field. The ray tracing technique is commonly employed to solve this problem and to estimate the ray path
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