Processing of VLF amplitude measurements: Deduction of a quiet time
The amplitude of Very Low Frequency (VLF) transmissions propagating from
transmitter to receiver between the Earth’s surface and the ionospheric
D-region is a useful measurement to detect changes in the ionization
within the D-region ranging from 60-90 km. The VLF signal amplitude is
disturbed by geomagnetic, solar, and atmospheric phenomena. To be able
to identify perturbations in the VLF signal amplitude, we determine its
averaged seasonal variation under quiet solar and geomagnetic
conditions. Here it is challenging, that long time series of the VLF
signal amplitude show significant jumps and outliers, which are caused
artificially by technical adjustments/maintenance work.
This paper presents a new approach for processing long VLF data time
series over multiple years resulting in level 2 data. The new level 2
data enables the consideration of time series with artificial jumps
since the jumps are leveled. Moreover, the outliers are removed by a
robust and systematic 2-step outlier filtering.
The average seasonal and diurnal variation for different
transmitter-receiver combinations can be computed with the new level 2
data by applying a composite analysis. A subsequently applied polynomial
fit obtains the quiet time lines for daytime and nighttime, representing
the typical seasonal variation under undisturbed conditions of the VLF
signal amplitude for each considered link.
The developed quiet time lines may serve as a tool to determine
perturbations of the VLF signal amplitude with solar and geomagnetic as
well as atmospheric origin. Also, they allow comparison of the VLF
signal amplitude variation for different transmitter-receiver links.