The Relationship Between “Large” Early/Fast VLF Events and Modal Propagation Nulls

Early/fast very low-frequency (VLF) events evidence the direct impulsive coupling of lightning energy to the overlying mesosphere and lower ionosphere, and they are readily detected as rapid (<20 ms) changes to the amplitude and phase of VLF transmitter signals propagating within the Earth-ionosphere waveguide. Typical early/fast VLF events exhibit 0.1–1.0 dB amplitude changes, but can be smaller (<0.1 dB) or significantly larger (>10 dB). Large (>6 dB) early/fast events have not yet been satisfactorily simulated, and past work has suggested that large amplitude early/fast VLF events may only be detected when the receiver is located in a VLF propagation “null.” VLF propagation “nulls” result from the destructive interference of propagating waveguide modes, and they would result in deceptively large relative changes in field amplitude by reducing the magnitude of the ambient field, rather than by increasing the magnitude of the scattered field. In this article, we present a new method to quantify the degree of destructive modal interference at the location of the VLF receiver, and we apply this method to analyze the impact of modal interference on the detected amplitude changes associated with 235 early/fast VLF events occurring over the Midwestern United States on 21 September 2016. No meaningful dependence on destructive modal interference is detected. We conclude that contemporary models of lightning-ionosphere interactions and/or VLF scattering require modification to accurately reproduce observations of large early/fast VLF events.