The Configuration of the Geomagnetic Field

Abstract

Spherical harmonic representations of the geomagnetic field based on ground-based and low-altitude spacecraft measurements adequately describe the field within several Earth radii of the Earth’s surface. As the internal field decreases with increasing distance from the Earth, external field sources become relatively more important. In the region 3–6 R _E magnetospheric plasma inflates the field lines and decreases the field strength by an amount which is typically 10’s of gammas and occasionally 100’s of gammas. At greater distances on the day side of the Earth the solar wind compresses the field and produces equatorial field strengths of approximately 60 γ at 10 R _E. Field lines near the magnetopause intersect the Earth at approximately 78° latitude in the sunward hemisphere. The solar wind drags high latitude field lines away from the Earth in the night hemisphere forming the geomagnetic tail and neutral sheet. The average equatorial field at 10 R _E near midnight is approximately 8γ and the best estimate for the last closed line of force near midnight in an average magnetosphere is 69°. Magnetic tail field lines away from the neutral sheet diverge in both the east-west and north-south directions and the average quiet time magnitude decreases from 16 γ at 20 R _E to 7 γ at 80 R _E. Within 6 R _E of the equatorial plane the field strength is depressed by several gammas. Asymmetric field inflation in the magnetosphere occurs during magnetic disturbances with the largest effects concentrated in the evening quadrant. The tail configuration can deviate substantially from the average configuration during magnetic disturbances. Prior to a substorm the configuration is characterized by a maximum number of lines extending far into the tail and relatively little flux crossing the equatorial plane. After a substorm or during quiet times the tail is characterized by an increased number of field lines crossing the equatorial plane in cislunar space.