VLF Transmitters Signal Dynamics from the Radio Paths across and along the Totality Path during the April 8, 2024 Total Solar Eclipse

During the total solar eclipse in the United States on April 8, 2024, we observed the amplitude and phase of VLF signals from five U.S. Navy VLF transmitters using a novel geometry of VLF receivers deployed along and across the totality path. Nine receiver sites (four of them inside the totality path) were deployed to collect the data in different transmitter-receiver configurations relative to the totality path, which intersected with the radio propagation paths from the Cutler, MA (NAA, 24 kHz) and the  LaMoure, ND (NML, 25.2 kHz) Navy transmitters. The transmitters themselves experienced  98.7%  (NAA) and  68\% (NML) solar obscuration at 75 km altitude. The novelty of the observations is the near-total obscuration of one of the transmitters and observations of several radio propagation paths closely aligned to the path of totality. This configuration enabled observations of the effects of the Moon’s shadow progression from Texas to Maine for over three hours on a total of 27 radio paths (with coverage from 55% to 100%) with transmitter-to-receiver distances ranging from 780 km to 7700 km.  Both positive and negative (5- 10 dB) amplitude changes were observed by receivers throughout the eclipse period. The observed phase changes were all negative. The unique observations of VLF propagation along the totality path produced a temporally dynamic quasiperiodic response in amplitude that can be used to determine the gradients and spatial scales of the eclipse effect on the lower ionosphere.  For observations near NAA, the eclipse produced a 13 dB surge in amplitude at Ithaca, NY (780 km from NAA). 

Two different types of VLF receivers were used to make VLF observations.  The University of Colorado Denver (CU Denver) deployed receivers of the type described by {Cohen et al. 2009) in Denver, CO; Antlers, OK; and Ithaca, NY. The CU Denver receivers have automatic demodulation to extract the carrier phase (Gross et al. 2018).  The University of California, Berkeley (UCB) hardware was originally developed to support the NASA-guided VIPER rocket (Bonnell et al. 2024), and these systems were deployed in San Antonio, TX, Austin, TX, Antlers, OK, Cleveland, OH, College Park, MD, and Ann Arbor, MI. The specific locations of all deployed VLF receivers are listed in Table 1. Both receiver types record two orthogonal components of the horizontal magnetic fields of  VLF waves.