A long‐duration narrowband Pc5 pulsation

Abstract

[1] We report on a rare event of a narrowband ULF magnetospheric pulsation that is observed at geosynchronous orbit over five consecutive days in the recovery period of a magnetospheric storm, with frequencies ranging from 5 to 9 mHz. The pulsation displays distinctive localization, being observed over several hours of local time in the noon and postnoon sectors during successive passes of GOES-8 and GOES-9. Geotail, which is traversing the outer magnetosphere a couple of R E farther out from geosynchronous orbit at the time the pulsations are observed, shows no distinct narrowband fluctuation in the magnetic field, excluding oscillations in the solar wind as a possible source of the pulsations. On the ground, several of the IMAGE magnetometers observe pulsations with similar spectral characteristics. By performing a phase cross-spectrogram analysis on measurements from adjacent and latitudinally aligned ground stations we estimate the azimuthal mode number m of the pulsation, which we find to range between 20 and 55. Phase difference calculations indicate an eastward phase propagation of these pulsations. Density measurements from LANL spacecraft at geosynchronous orbit show evidence of a plasmasphere refilling process that is slower than usual, possibly associated with the observed slow frequency decrease and the unusually long lasting of the pulsations.

Observations and analysis of Alfvén wave phase mixing in the Earth's magnetosphere

“…In rotating the field components into MFA coordinates, the 30‐min average acts as a high pass filter removing frequencies below 0.55 mHz. On this day pulsations were also observed on the ground; from these observations mode number m was calculated to range between 20 and 55, as shown by Sarris et al [2009]. In Figure 1b the Dynamic Power Spectra (DPS) of B r , the radial component of the magnetic field data are plotted.…”

“…In the event reported herein the pulsations last for days, much longer than the theoretically calculated damping times; thus we find it most reasonable to assume that the pulsations are continuously driven for the duration of the event. It is possible that the slow decay of the ring current ions during the recovery phase after the storm as described above and also by Sarris et al [2009] provided the energy for the sustained ULF waves, via drift‐bounce resonance.…”

Geomagnetic activity and local time dependence of the distribution of ultra low-frequency wave power in azimuthal wavenumbers, <i>m</i>

“…In reality, well-coherent Pc5 waves at two spacecraft azimuthally separated by as much as 60 • for a prolonged period are not a very common phenomenon (for an exception to this, see, e.g., the prolonged event described in Sarris et al, 2009a). However, for the purposes of investigating the effects of ULF waves on the radial diffusion of energetic electrons, the broadband (in azimuthal wavelengths) spatial structure of ULF waves is decomposed in these prevailing individual azimuthal wavenumbers in the same way as the spectrally broadband ULF waves are decomposed into a series of frequencies in order to identify the components which are resonant with each drift period.…”

GPS TEC response to Pc4 “giant pulsations”

“…Magnetic field measurements of the GOES 13 satellite were converted to a mean field‐aligned coordinate system, similar to measurements presented by Sarris et al . [] and Watson et al . [].…”