Relationship between the IMF magnitude and Pc 3 magnetic pulsations in the magnetosphere

Abstract

Compressional waves in the Pc 3–4 frequency range (f = 16–100 mHz) detected at the synchronous orbit (GOES 2, L = 6.67) and low‐latitude pulsations detected on the ground (L = 1.3–1.8) are correlated with the strength of the interplanetary magnetic field (BIMF) observed by ISEE 3 and are expressed as f (mHz) ∼6.0 × BIMF (nT). The correlation between f and BIMF is better in the outer magnetosphere than at low latitudes on the ground, i.e., γ = 0.70 at GOES 2, 0.60 at L = 1.8, and 0.54 at L = 1.3. The correlation study suggests that the solar wind energy in the Pc 3–4 range, associated with the upstream waves of which frequencies are related to the ion cyclotron frequency and thus with the IMF magnitude, is ultimately transmitted across the ambient geomagnetic field into the deep magnetosphere. Scatter plots of pulsation frequencies observed in the magnetosphere versus the IMF magnitude show a remarkable distribution restricted within the forms f = 4.5 × BIMF and f = 7.5 × BIMF. The results are theoretically examined. If the restricted Pc 3–4 frequencies of the compressional waves at the synchronous orbit correspond to the Doppler‐shifted frequencies of magnetosonic upstream waves excited by the well‐known ion cyclotron resonance with reflected ion beams in the earth's foreshock, the resonant kinetic energy of the reflected ion beams can be inferred to be restricted within the 1–5 keV range in the spacecraft frame. The inference is consistent with the observational distributions of the reflected ion beams by the ISEE satellites in the earth's foreshock. It is likely that the magnetosonic upstream waves in the Pc 3–4 range in the earth's foreshock are convected through the magnetosheath to the magnetopause, transmitted into the deep magnetosphere without significant changes in spectra, and then couple with various hydromagnetic waves in the magnetosphere.