| We present examples and statistical analysis of the events with
statistically significant correlation between the cosmic noise
absorption (CNA) and the signal-to-noise ratio (SNR) of the VHF
coherent echo intensity in the area monitored simultaneously by an
imaging riometer and two oblique-sounding coherent VHF radars in
Northern Scandinavia. By only considering the observations from
the narrow riometer beams comparable (in terms of the intersection
with the ionosphere) with the VHF radar cells, we identify ~200
one-hour high correlation periods (HCPs) for 2 years near the
solar cycle maximum, 2000–2001. The HCP occurrence is maximized
in the afternoon (12:00–17:00 UT, MLT≅UT+3), with the secondary
peak near the midnight (21:00–02:00 UT). Relative to the VHF echo
occurrence, HCPs occur more frequently from 11:00 to 20:00 UT. The
diurnal variation of HCP occurrence is similar to that of the
1-h intervals with the lowest mean absorption A<0.25dB.
The HCPs are observed more frequently during the winter months,
which, combined with the fact that VHF echoes observed during HCPs
exhibit features typical for field-aligned E-region
irregularities, makes their association with the polar mesospheric
echoes (for which some positive CNA/SNR correlation has been
reported in the past) very unlikely. Instead, we attribute the
high positive CNA/SNR correlation to the synchronous, to a first
approximation, variation of the particle fluxes for two different
but close sets of energies.
By considering the dependence of the CNA/SNR correlation
coefficients for both VHF radars (CA1 and CA2) upon the
correlation between SNRs for two radars (C12), we show that
both coefficients, CA1 and CA2, and the agreement between
them decrease drastically with a C12 decrease, which we
interpreted through the progressively increasing role of the
spatial inhomogeneity of the processes leading to the enhanced CNA
and SNR. In this situation, a similarity between the radio signal
collection areas should become important, and we demonstrate that
the HCP occurrence and mean correlation coefficient decrease as
the riometer beams and radar cells become less comparable in terms
of mutual orientation and closeness between the points of maximum
sensitivity.
Keywords. Ionosphere (Auroral ionosphere; Particle precipitation;
Instruments and techniques) |