Superconducting magnetometer measurements of geomagnetic activity in the 0.1- to 14-Hz frequency range

A high-sensitivity superconducting magnetometer has been used to measure geomagnetic activity at Stanford, California, in the 0.1- to 14-Hz frequency range. The measurements, which covered the 2-month interval January 26 to March 26, 1974, consistently show a minimum of activity in the interval 3–7 Hz, where conventional measurement systems are likely to be limited by their internal noise. At frequencies, below the minimum, in the range 0.1–3 Hz, there is typically a monotonic decrease of background activity with frequency f. The variation is approximately of the form f−1 25 for the Pc 1 frequency range (0.2–5 Hz). At frequencies above the minimum, in the range 7–14 Hz, the activity is dominated by the first Schumann resonance. The peak amplitude of this activity is usually low in comparison with the amplitude of the background activity at 1 Hz. No splitting of the first Schumann resonance peak into a triplet could be detected, but a doublet structure was observed on 7 days in the 2-month measurement interval.