Conjugate observations of a remarkable quasiperiodic event by the low-altitude DEMETER spacecraft and ground-based instruments

We present a detailed analysis of a long-lasting quasiperiodic (QP) event observed simultaneously by the low-altitude DEMETER spacecraft and on the ground by the instrumentation of the Sodankyla Geophysical Observatory, Finland. The event was observed on 26 February 2008. It lasted for several hours, and it was detected both in the Northern and Southern Hemispheres. The time intervals when the event was observed on board the satellite and/or on the ground provide us with an estimate of the event dimensions. When the event is detected simultaneously by the satellite and on the ground, the observed frequency-time structure is generally the same. However, the ratio of detected intensities varies significantly as a function of the spacecraft latitude, indicating the wave guiding along the plasmapause. Moreover, there is a delay as large as about 13 s between the times when individual QP elements are detected by the spacecraft and on the ground. This appears to be related to the azimuthal separation of the instruments, and it is highly relevant to the identification of a possible source mechanism. We suggest that it is due to an azimuthally propagating ULF wave which periodically modulates the azimuthally extended source region. Finally, we find that at the times when the intensity of the QP event suddenly increases, there is a distinct increase of the amplitude of Alfvenic ULF pulsations measured on the ground at high latitudes. This might indicate that the source region is located at L shells larger than about 7.1.