Rapid fluctuations in the signal strength of the 18.6-kHz transmission from NLK, Jim Creek, Washington, along the 2100-km path to St. Cloud, Minnesota, were monitored continuously for several months. A specical band-pass filter, with center period of 120 s, was used to separate relatively small amplitude fluctuations from the overall signal level. Raw data strip chart recordings for 216 consecutive day show that nighttime amplitude fluctuations tend to repeat at 27-day intervals corresponding to the solar rotation period. A strong correlation also appears between the amount of nighttime amplitude fluctuation and the amount of daily geomagnetic activity as indicated by magnetic indices. This rapid fluctuation behavior of the signal strength, which is most likely caused by varying particle precipitation along the radio propagation path, displays a definiteness of character which makes it especially suitable for detecting and monitoring changes of the lower ionosphere related to geomagnetic disturbances.