Daytime midlatitude D region parameters at solar minimum from short-path VLF phase and amplitude

[1] Observed phases and amplitudes of VLF radio signals propagating on a short (∼360 km) path are used to find improved parameters for the lowest edge of the (D region of the) Earth's ionosphere at a geomagnetic latitude of ∼53.5° in midsummer near solar minimum. The phases, relative to GPS 1 s pulses, and the amplitudes were measured both near (∼110 km from) the transmitter, where the direct ground wave is very dominant, and at distances of ∼360 km near where the ionospherically reflected waves form a (modal) minimum with the (direct) ground wave. The signals came from the 24.0 kHz transmitter, NAA, on the coast of Maine near the U.S.-Canada border, propagating ∼360 km E-NE, mainly over the sea, to Saint John and Prince Edward Island. The bottom edge of the midday, midsummer, ionosphere at ∼53.5° geomagnetic latitude was thus found to be well modeled by H′ = 71.8 ± 0.6 km and β = 0.335 ± 0.025 km⁻¹ where H′ and β are Wait's traditional height and sharpness parameters used by the U.S. Navy in their Earth-ionosphere VLF radio waveguide programs. The variation of β with latitude is also estimated with the aid of interpolation using measured galactic cosmic ray fluxes.