A new approach for monitoring the 27-day solar rotation using VLF radio signals on the Earths surface

Typical solar parameters such as sunspot number, Lyman alpha radiation, and 10.7 cm radio flux exhibit 27-day variations associated with the rotation of the Sun. We have discovered a robust indicator of this 27-day rotation from measurements of VLF radio signals produced by lightning around the globe. The solar rotation signal is found only at VLF frequencies close to the Earth-ionosphere waveguide cutoff frequency (∼2 kHz). Furthermore, the 27-day solar rotation is detected only during daylight hours, implying a 27-day periodicity in the daytime collision frequency between free electrons and neutral air molecules in the lower ionosphere (∼80 km). We propose that continuous monitoring of VLF radio noise at frequencies close to the waveguide cutoff could provide a new method of continuously monitoring changes in the solar rotation rate.