Total solar eclipse effects on VLF signals: observations and modeling

During the total solar eclipse observed in Europe on August 11, 1999, measurements were made of the amplitude and phase of four VLF transmitters in the frequency range 16–24 kHz. Five receiver sites were set up, and significant variations in phase and amplitude are reported for 17 paths, more than any previously during an eclipse. Distances from transmitter to receiver ranged from 90 to 14,510 km, although the majority were <2000 km. Typically, positive amplitude changes were observed throughout the whole eclipse period on path lengths 10,000 km. Negative phase changes were observed on most paths, independent of path length. Although there was significant variation from path to path, the typical changes observed were ∼3 dB and ∼50°. The changes observed were modeled using the Long Wave Propagation Capability waveguide code. Maximum eclipse effects occurred when the Wait inverse scale height parameter β was 0.5 km−1 and the effective ionospheric height parameter H′ was 79 km, compared with β=0.43 km−1 and H′=71 km for normal daytime conditions. The resulting changes in modeled amplitude and phase show good agreement with the majority of the observations. The modeling undertaken provides an interpretation of why previous estimates of height change during eclipses have shown such a range of values. A D region gas-chemistry model was compared with electron concentration estimates inferred from the observations made during the solar eclipse. Quiet-day H′ and β parameters were used to define the initial ionospheric profile. The gas-chemistry model was then driven only by eclipse-related solar radiation levels. The calculated electron concentration values at 77 km altitude throughout the period of the solar eclipse show good agreement with the values determined from observations at all times, which suggests that a linear variation in electron production rate with solar ionizing radiation is reasonable. At times of minimum electron concentration the chemical model predicts that the D region profile would be parameterized by the same β and H′ as the LWPC model values, and rocket profiles, during totality and can be considered a validation of the chemical processes defined within the model.

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