Statistical study of geomagnetic field line resonances observed by CHAMP and on the ground

Abstract

[1] Studies of single events showing the signatures of Pc3 pulsation field line resonances (FLRs) observed in magnetic field data by CHAMP above the E-region ionosphere and at conjugate ground stations at L < 2 have previously been reported. Notable features of FLRs observed in these investigations were a Doppler shift in the satellite azimuthal component and a 90 rotation of the polarization ellipse between CHAMP and the ground. In this report, we extend the previous work by means of the first statistical investigation of the same features for a large number of events using CHAMP and ground magnetic field data. Our statistical study confirms the Doppler shifts to higher or lower frequencies relative to the ground FLR frequencies for equatorward or poleward passes of CHAMP over the ground station. Our statistical investigation shows the amount of rotation of the FLR polarization ellipses between the ionosphere and the ground to be~100 on average, rather than the theoretically predicted 90 . This is explained by the polarization ellipses on the ground being oriented close to N-S irrespective of the orientation above the ionosphere, which is À13 on average. We find no evidence of ionospheric conductivity gradients associated with sunrise and sunset affecting the amount of rotation of the polarization ellipses as predicted by more recent theoretical advances. We are not able to provide conclusive evidence on the relationship between the polarization ellipse rotation and azimuthal wave number as predicted in recent numerical modeling.

Nightside ULF Waves Observed in the Topside Ionosphere by the DEMETER Satellite

“…Previous studies have shown that Pc3–4 waves propagate into the low‐ L ( L = 2–3) dayside magnetosphere and low‐latitude ground stations as fast mode waves (Kim & Takahashi, ). The conjunction of observations from LEO satellites and ground magnetometers demonstrated that compressional Pc3–4 waves observed during daytime propagate to midlatitudes and become the driver of field line resonances (Heilig et al, ; Ndiitwani & Sutcliffe, ; Vellante et al, ). Field line resonance is impeded at near‐equatorial latitudes since the damping rate increases sharply there (Yumoto et al, ).…”

Near-equatorial Pi2 and Pc3 waves observed by CHAMP and on SAMBA/MAGDAS stations

“…The coordinated low-altitude satellite (CHAMP) and ground observations of Pc3-4 waves indicated on a significant contribution of compressional component into the wave structure in the upper ionosphere (Heilig et al, 2007). Pc3-4 waves demonstrated a localized enhancement of transverse components at midlatitudes attributed to the field line resonance, where the frequency of compressional mode transporting the wave energy from extra-magnetospheric source matches the local frequency of Alfven eigenoscillations (Vellante et al, 2004;Ndiitwani and Sutcliffe, 2009;Heilig et al, 2013). Away from the resonant region, the structure of Pc3-4 waves in the upper ionosphere was dominated by the compressional mode.…”

“…However, a mechanism of such amplification has not been unambiguously identified yet. Possible mechanisms can involve enhancement of the ionospheric currents induced by mid-latitude Alfven wave which spread into the equatorial region with elevated ionospheric Cowling conductance (Shinohara et al, 1997); or direct penetration of compressional wave towards the near-equatorial region without conversion into Alfven field line oscillations (Yumoto and Saito, 1983;Heilig et al, 2007Heilig et al, , 2013. Most probably, combination of all these mechanisms takes place, and to resolve them is a challenging goal.…”

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

“…Vellante et al (2004), Ndiitwani and Sutcliffe (2009), and more recently Heilig et al (2013) demonstrated the apparent shift in frequency of the shear Alfvén wave component of the pulsations due to rapid motion of CHAMP with respect to the ground stations. Waters et al (1991) introduced the cross-phase method to identify field line resonant frequencies from phase spectra from a pair of meridional ground stations.…”