Relation of compressional HM waves at Goes 2 to low‐latitude Pc 3 magnetic pulsations

Abstract

Daytime magnetic pulsations observed at synchronous orbit by GOES 2 are analyzed to determine whether there is a candidate for the source of low‐latitude Pc 3's in the outer magnetosphere. Compressional, radially transverse and azimuthally transverse modes of magnetic pulsations in a wide frequency range exist simultaneously in the outer daytime magnetosphere. The compressional and the transverse modes of the daytime magnetic pulsations observed at GOES 2 are statistically dominant in the Pc 3 and in the Pc 4 frequency ranges, respectively. Some 70% of the compressional Pc 3 pulsations at GOES 2 have periods similar to the low‐latitude Pc 3's observed at San Gabriel Canyon, which is located ∼11° west of GOES 2's longitude at L = 1.8. Amplitudes of the compressional. Pc 3 pulsations at GOES 2 (L = 6.67) are relatively larger than those of the low‐latitude Pc 3's at SGC (L = 1.8). The transverse pulsations with larger amplitude (δB ≳ 1.0 nT) at GOES 2 can propagate only into the high‐latitude ionosphere. The compressional Pc 3 pulsations with smaller amplitude (δB∼ 0.5 nT) at GOES 2 are theoretically expected to propagate across the ambient magnetic field to very low latitudes to couple with one or more the following: (1) a surface wave at the plasmapause (Lpp); (2) a trapped oscillation of the fast magnetosonic wave in the plasmasphere (L = 1.7 ‐ Lpp); (3) a higher‐harmonic standing oscillation of a local field line at mid‐latitude (L = 2.0 ‐ Lpp); or (4) a fundamental standing oscillation at very low‐latitudes (L = 1.1 and 1.7–2.6). Although further research is needed to clarify the wave and propagation characteristics of the low‐latitude Pc 3 pulsations, we believe that the compressional Pc 3 magnetic pulsation in the outer magnetosphere is one candidate for the source of the low‐latitude Pc 3 magnetic pulsation.