From superchrons to secular variation: A broadband dynamo frequency spectrum for the geomagnetic dipole

We build a broadband frequency spectrum for geomagnetic dipole intensity variations originating in the core using results from numerical dynamos. Scaling dipole intensity spectra from numerical dynamos with diverse input parameters (Ekman, Rayleigh, and magnetic Prandtl numbers) and magnetic Reynolds numbers from 170 to 1985 yields a composite spectrum ranging from ultra-low frequencies (∼ 100 Myr period) to very high frequencies (∼ 1 yr period). The amplitude and shape of our composite spectrum compare favorably with estimates of the geomagnetic and paleomagnetic dipole moment spectra. Significant power is found in an ultra-low frequency band f < 0.1 Myr− 1 that corresponds to geomagnetic superchrons and also in a low frequency band f = 0.1 − 5 Myr− 1 that corresponds to the time scales of geomagnetic polarity chrons. In higher frequency bands our composite spectrum varies with frequency approximately like f− n. In a transitional frequency band from f = 5 Myr− 1 to f = 2 kyr− 1, characteristic of relative paleointensity variations in sediments, we find n ≃ 1.8. In a high frequency band with f > 2 kyr− 1, representing archaeomagnetic and historical geomagnetic secular variation, we find n ≃ 4 on average, with spectral slope increasing at very high frequencies to near n ≃ 6 at f = 1 yr− 1. The secular variation and the secular acceleration spectra for the dipole field reach their peak values in the high frequency band, close to frequencies where spectra for the dynamo fluid velocity decrease sharply.

► We present a broad-band spectrum for the geomagnetic dipole from numerical dynamos. ► Our spectrum shows power-law behavior that agrees with measured frequency spectra. ► Most of the geomagnetic dipole power lies at low frequencies. ► We identify a convective time scale in the outer core.