Harmonically structured ULF pulsations observed by the AMPTE CCE Magnetic Field Experiment

Abstract

Spectrograms of ULF waves in the 0 to 80 mHz frequency range have been prepared from magnetic field data obtained by the elliptically orbiting AMPTE/CCE satellite (with an apogee of approximately 8.8 Re). The most prominent feature of these spectrograms (which cover a full 15.6 hour orbit) is the presence of harmonically structured, azimuthally polarized pulsations in the outer magnetosphere during daytime hours. The frequencies of these pulsations decrease with increasing radial distance from the earth, indicating that they represent independent resonances of local magnetic flux tubes. The latitudinal structure of these harmonic pulsations, observed as AMPTE/CCE traveled to + 16 ø magnetic latitude, is consistent with accepted field line resonance models. Correlations with IMP 8 satellite data confirm a close connection between appearance of these harmonic pulsations in the dayside magnetosphere and interplanetary magnetic field conditions favorable for the generation of upstream waves. Although we also observe pulsations associated with ring current injections, the monochromatic pulsations predicted by some theoretical models are extremely rare in the dayside AMPTE/CCE data set.

Spatio-temporal structure of a poloidal Alfvén wave detected by Cluster adjacent to the dayside plasmapause

“…Singer and Kivelson, 1979;Singer et al, 1982;Engebretson and Cahill, 1981;Engebretson et al, 1986). However, single spacecraft observations have a limitation as they exhibit an ambiguity between spatial and temporal variations.…”

ULF waves: 1997 IAGA division 3 reporter review

“…Previous spectral analyses of the same data set had indicated that discrete-frequency compressional pulsations are not evident in the satellite data (Engebretson et al, 1986). In the new study, Anderson and Engebretson (1995) con®rm the absence of discrete compressional oscillations, but they also ®nd high positive correlation between the amplitudes of the toroidal and compressional components.…”

Spectral structure of Pc3–4 pulsations: possible signatures of cavity modes

“…Yumoto (1985) used ISEE 3 (International Sun/Earth Explorer) and GOES 2 (Geostationary Operational Environmental Satellites) observations and measurements from two ground stations to relate solar wind data to Pc3s on the ground and suggested that compressional waves from a broadband source near the magnetopause couples to FLRs near the Earth. Engebretson et al (1986) cited the same mechanism to explain simultaneous observations of multiple harmonics of toroidal mode FLR's and an increase in broadband compressional wave power. Heilig et al (2007) computed the latitudinal distribution of dayside Pc3 amplitudes and concluded that UWs enter near the subsolar point as compressional waves where they couple to the Alfvén mode resulting in field line oscillations.…”

“…Yumoto et al, 1984;Le and Russell, 1992). Engebretson et al (1986) noted the IMF control of Pc3-4 pulsations when activity at low L shells (L ∼ 3) suddenly resumed upon IMF orientation changing from quasi-perpendicular (large cone angle) to quasi-parallel (small cone angle). Heilig et al (2010) showed that the intensity of UW-related Pc3s is dependent on the density of the solar wind, and density-related parameters like the dynamic pressure and distance from the magnetopause to the Earth.…”

“…It is also commonly suggested that the energy source which drives the FLRs are upstream waves (UWs) generated in the Earth's foreshock (e.g. Russel and Hoppe, 1981;Yumoto et al, 1984;Takahashi et al, 1984;Engebretson et al, 1986Engebretson et al, , 1987. Contrarily, a number of investigations using numerical modelling have suggested that cavity or waveguide modes in the magnetosphere are the drivers of the FLRs (Allan et al, 1985;Samson et al, 1995;Waters et al, 2000).…”