Whistler-Mode Waves Observed by the DEMETER Spacecraft and the Kannuslehto Station: Spatial Extent and Propagation to the Ground

We use conjugate observations of magnetospheric whistler-mode waves at frequencies up to 16 kHz by the DEMETER spacecraft (at an altitude of approximately 660 km) and the ground-based Kannuslehto station in Finland $\left(L\approx 5.38\right)$ to investigate the wave propagation to the ground and their characteristic spatial scales. For this purpose, we evaluate correlations between the wave intensities measured by the spacecraft and the ground-based station at various frequencies as a function of their longitudinal and L-shell separations. Two different approaches are used: (a) direct correlation of wave intensities measured at the same times and (b) correlation of wave intensities within corresponding frequency-time windows, focusing on the identification of the same frequency-time wave signatures. We show that the characteristic longitudinal scales of the investigated waves are between about $60°$ and $90°$. We further demonstrate that, while the wave intensities measured by DEMETER are generally larger during periods of enhanced geomagnetic activity, wave intensities measured on the ground during increased activity are only slightly larger during the daytime and decrease during the nighttime.