Recurrent VLF amplitude fluctuations: Detection using a filter and correspondence with geomagnetic disturbances

Abstract

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 special 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 days 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.

D‐Region High‐Latitude Forcing Factors

“…However, limited studies have been done using VLF data recorded at high‐latitude regions, where long‐term geomagnetic activity might be expected to strongly influence the VLF signals (e.g., Clilverd et al, ). There are also only a few reports on short‐term solar variations, that is, associated with the 27‐day solar rotation, in the lower ionosphere (Demirkol et al, ; McWilliams & Strait, ). These studies used nighttime VLF data recorded at high‐latitude and midlatitude regions to show solar related short‐term variations.…”

“…This upper boundary fluctuates in the altitude range ~60–90 km from day to night and is known as the D‐region (Hargreaves, ; Samanes et al, ). The lower ionosphere can be affected by sporadic (Bracewell & Straker, ) or periodic (McWilliams & Strait, ) external forces. As a result, they can be detected using subionospheric VLF wave analysis techniques.…”