Understanding Maximum Geomagnetic Latitude and Auroral Ovals
The Max textbox in the GeoMag panel of DXView's Main window displays the maximum magnetic latitude encountered by a signal traversing the short or long path from your QTH to the selected position. This parameter is relevant because it indicates the likelihood of auroral interaction -- the higher the maximum geomagnetic latitude, the more likely a signal is to traverse the auroral zones.
The earth's magnetic poles do not align with its geographic poles, and are in fact moving. In 2005, the geomagnetic north pole was estimated to be at 82 18' N, 113 24' W. Interaction between the stream of charged particles emitted by our sun and the earth's magnetic field produces auroral zones over the earth's north and south geomagnetic poles. The auroral zones expand and contract as a function of the intensity and orientation of this particle stream. KN4LF provides a nice discussion of the impact of these zones on propagation, excerpted below:
- The aurora ovals generally have a negative impact on medium-frequency propagation. If the path over which you are communicating lies along or inside one of the Aurora Ovals, you will experience degraded propagation in one of several different forms; strong signal absorption, brief periods of strong signal enhancement, which is mainly caused by tilts in the ionosphere that allow signals to become focused at your location or very erratic signal behavior in the form of strong and rapid fading, etc., caused by a variety of effects such as multi-pathing, anomalous and rapid variations in absorption, non-great-circle propagation, horizontal or side refraction and/or scatter (skewing) due to changes in electron density and polarization changes.
- When the Aurora Oval zones are contracted and latitudinally-thin coinciding with low geomagnetic activity, it is possible for a medium-frequency frequency transmitted signal to propagate through the Aurora Oval zone without being heavily absorbed by skirting underneath it.
- During periods of very low geomagnetic activity, areas of the Aurora Oval zones may only have a latitudinal thickness of approximately 300 miles. But radio signals reflected from the E-layer can travel over distances of as much as 300 to 1250 miles at heights below the ionosphere for low take-off angles of between 10 and 25 degrees. When the geometry is just right, the medium-frequency transmitted signal can literally propagate underneath and through the Aurora Oval zones into the polar ionosphere which is less disturbed and from the polar ionosphere back into the middle latitude ionosphere, without ever coming in contact with the highly absorptive Aurora Ionosphere. This type of propagation is not as rare as you might think and it can provide unusually stable polar region path openings to Transatlantic and Transpacific regions. But because the Aurora Oval zone expands and contracts constantly, such conditions often do not last very long.
To configure DXView to display the estimated position of the auroral zone boundaries (based on the K-index and time-of-day) on its world map, check the Auroral zones box in the World Map window's Map panel; then you'll directly see the likely interaction experienced by a paricularly signal.