Spitze Angle Changes during Rapid Geomagnetic Core Field Variation

Abstract

The spitze phenomenon is an outstanding feature of electromagnetic wave propagation through the ionosphere. It consists in a ray path cusp that occurs between vertical propagation and a critical angle called the spitze angle. This angle that depends on the propagating wave frequency and on the geomagnetic field intensity and inclination, has gained renewed importance for its role in ionospheric modification by powerful radio wave experiments. Here we consider Earth?s magnetic field secular variation on two distinctive timescales. First, the most drastic scenario considered is a polarity reversal. We present consequent changes in the spitze angle for three plausible transitional scenarios. Second, we track changes during the past 115 years based on geomagnetic field observations. Globally, we find that the spitze spatial variation is determined mainly by geomagnetic inclination with larger values for smaller inclinations. However, locally the spitze time variation differs from site to site and may depend on the evolution of either field intensity or inclination. Despite the long-term timescales considered, understanding the origin of the spitze spatio-temporal variations on various timescales is important from a physical point of view to unravel magnetized plasma phenomena.