Comparison of ULF Wave Indices from Conjugate Magnetometer Arrays in Antarctica and Greenland

ULF (Ultra-Low-Frequency) wave index, characterizing the level of geomagnetic field fluctuations in the frequency band of most intense Pc5 pulsations (1.7–6.0 MHz), was introduced earlier for the analysis of space weather. This global hourly index was produced from available 1-min magnetometer data, but from stations in the Northern hemisphere only. If ULF wave powers in conjugate hemispheres were strongly asymmetric, an index derived from one hemisphere data only may be misleading to characterize the global magnetospheric ULF activity. Therefore, it is important to examine how strong is the interhemispheric asymmetry of the ULF power. This issue is relevant to important problem: how strongly does the ionosphere control interhemispheric properties of Pc5 pulsations? To resolve this problem, we use data from the conjugate Antarctica–Greenland autonomous magnetometer arrays along the 40° magnetic meridian. Regional ULF wave power indices and latitudes with a maximal ULF power have been calculated from pairs of conjugate stations in both hemispheres. Examination of daytime ULF indices shows that even under contrasting ionospheric conductances during winter/summer seasons the ULF wave powers in both hemispheres are nearly the same and differ not more that 10%. This result indicates that the excitation of geomagnetic pulsations in the Pc5 band corresponds the current generator regime and justify the usage of the Northern hemisphere index for characterization of an entire magnetosphere.