A new approach for monitoring the 27‐day solar rotation using VLF radio signals on the Earth's 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.