Hayakawa, M.; Hobara, Y.; Yasuda, Y.; Yamaguchi, H.; Ohta, K.; Izutsu, J.; Nakamura, T. Possible Precursor to the March 11, 2011, Japan Earthquake: Ionospheric Perturbations As Seen by Subionospheric Very Low frequency/Low Frequency Propagation. Ann. Geophys. 2012, 55.