LDI DIGISONDE 4D - Bulk Plasma Drift Velocity; Data Products Derived from Drift Measurement; Doppler Skymap; Ionospheric Tilt

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DIGISONDE-4D
SYSTEM MANUAL
VERSION 1.2.11
3-24 SECTION 3 - OPERATING INSTRUCTIONS
Data Products Derived from Drift Measurement
Doppler Skymap
3:40. The Doppler skymap is a presentation technique for the drift measurement data in which a large num-
ber of individual echoes from the ionosphere (“sources”) are resolved by their time of arrival and further by
their Doppler shift that corresponds to the line-of-sight velocity of the reflecting area of plasma. Then, for each
range/Doppler bin that is now assumed to contain a single ionospheric echo, four channel phases are used for
the interferometry calculation of the azimuth and zenith angles of the source. DFT2Sky software is used to ex-
tract sources from the drift data for multiple frequencies and ranges and then save them in the SKY output file.
SKY2PNG software is then used to plot sources as a skymap (Figure 3-16) where the azimuth and zenith an-
gles of each source are used to place it on the skymap plane using a symbol whose shape and color indicate the
Doppler velocity (positive Doppler uses + symbol and blue shades of color, negative Doppler uses o symbol
and red shades of color). Further description of the skymap technique can be found in Section 1 of the manual,
chapter “Drift Mode – Super-Resolution Direction Finding”.
Bulk Plasma Drift Velocity
3:41. The first order approach of describing plasma movement in the ionosphere is to assume its uniform
drift over the station as a fixed 3D bulk pattern moving in the same direction with one velocity. In this case the
off-vertical sources show the appropriate Doppler velocities (positive for the sources corresponding to the
plasma drifting towards the station and negative for sources drifting away from the station). It turns out that
such approximation works well in many situations, reflecting the large scale dynamics of the ionosphere at the
sounder location. The bulk drift velocity is calculated using a least-squared fit to the skymap sources data in
SKY files and output in DVL files. Daily plots of bulk drift velocities are produced for the Digisonde
®
webpage
(see paragraph 3:44 below).
Ionospheric Tilt
3:42. Skymap sources can also be used to estimate local tilt of the ionosphere by calculating offset of the
“gravity center” of skymap sources from the nadir. Zenith and azimuth angles of the local tilt are used to appro-
priately morph the 3D plasma distribution in the area surrounding the station, thus improving the accuracy of
raytracing applications. TILT software is used to calculate zenith and azimuth angles and store them in .TLT
file.
Skymap Display for WWW Homepage
3:43. The skymap display (Figure 3-16) combines the skymap of ionospheric echoes calculated by the
DFT2Sky software with bulk drift velocity obtained by DDAV software and ionospheric tilt parameters derived
by the TILT software. The bulk Doppler velocity is shown in Figure 3-16 as the triple tick marks indicating the
drift azimuth and horizontal velocity magnitude, as well as two labels that show the horizontal and vertical ve-
locity components in m/s. The ionospheric tilt is described on the skymap display in terms of the zenith and
azimuth angles of the source center.