Geomagnetic Latitude Boundaries

This maps shows lines of constant geomagnetic latitude (350km apex coordinates) for various latitudes of general interest for ionospheric effects. The heavy black dashed line indicates the geomagnetic equator, and the red dashed lines to the immediate north and south indicate nominal locations of the crests of the equatorial (Appleton) anomalies, regions of potentially enhanced ionization during the afternoon and early evening hours.

The dot-dash green lines at high latitudes indicate the 50th-percentile latitude of the auroral oval at local (geomagnetic) midnight and the 3dot-dash red lines indicate the 90th-precentile latitudes. These latitudes were determined from analysis of >120,000 observations (1983-1991) of the location of the auroral boundary from the USAF DMSP SSJ/4 precipitating-particle sensor. These data were provided by Dr. Fred Rich of the USAF Research Laboratory.

A note about the definition of "geomagnetic midnight." Geomagnetic "time" is defined in a manner similar to local solar time, except that geomagnetic longitude is used in the calculation rather than geographic longitude. Every step of 15 degrees of geomagnetic longitude away from the sub-solar point (where it is geomagnetic "noon") in an geomagnetic eastward direction is an increase in geomagnetic local time of one hour. Thus, it is geomagnetic "midnight" along the geomagnetic longitude meridian that is 180deg of geomagnetic longitude away from the sub-solar point. As with local solar time, this can be calculated from either the fictitious location of the "mean" sun or the true location of the sub-solar point.