Comparison of Very Low Frequency Wave Intensities Measured by a Low-Altitude Spacecraft and on the Ground

We evaluate average wave intensities at frequencies up to 10 kHz measured by two ground stations in Canada and two others in Finland at auroral and subauroral latitudes over a full year, as well as by the low-altitude DEMETER spacecraft during the years 2004–2010. Lightning location and energy data obtained by the World Wide Lightning Location Network, along with geomagnetic activity characterized by the Kp index, are further used. Latitudinal, diurnal, and annual variations are analyzed, and the global intensities measured on the ground and by the spacecraft are systematically compared for the first time. We show that lightning-generated waves often dominate the measured wave intensities, particularly during the night, in summer, and at higher frequencies. DEMETER observations, supported by ray-tracing analysis, reveal a significant role of nonducted lightning-generated whistler propagation between the hemispheres. Finally, the wave intensity response to geomagnetic activity variations is quite different on the ground compared to in space. While spacecraft-measured wave intensities are considerably larger during periods of enhanced geomagnetic activity, the ground-based intensities are only sporadically enhanced during geomagnetically active periods.