1. HF radio sounding the horizontally inhomogeneous ionosphere
O.A.Laryunin
HF radio sounding the horizontally inhomogeneous ionosphere

2. A schematic illustration of vertical and oblique ionospheric sounding

3. Digisonde vertical incidence ionogram, Irkutsk.
Near-vertical incidence ionogram
The layout of the experiment
Three sounders are available. However, triangulation is hardly possible due to low repetition rate of the Digisonde (5 to 15 min). The other two sounders record ionograms once each minute.
Digisonde vertical incidence ionogram, Irkutsk.
Reconstruction of electron density profile using Huang-Reinisch method

4. The succession of the complex TID features in the ionograms, indicating reflection from the same structure.
Since we have, in fact, two VIS ionosondes spaced 60 km apart, and the time delay between the stations is 6 min, this provides a basis for the estimate of the apparent southward TID velocity.

5. Satellite traces ascending between second hop and third hop traces and schematic representation of the corresponding TID
From [Lynn et al., 2013]
Lynn et al., (2013) investigated the so called "satellite" traces in ionograms. The phenomenon was found to be associated with off-vertical echoes from a moving electron density gradients. A consistent explanation has been given [Lynn et al., 2013] based on the model for the tilted ionospheric reflector.

6. Cusp is descending in height as the perturbation proceeds.
A series of quasi-vertical ionograms observed on 7 November 2011 displaying the passage of a TID.
Cusp is descending in height as the perturbation proceeds.

7. Ray tracing on the basis of Gaussian model
where N0 (z) is the background electron density; is the amplitude of electron density perturbation; is
a tilt of the disturbance with respect to the horizontal; Zb, Xb - scale sizes of the disturbance.
The background plasma frequency profile
Isoelectronic contours for
Gaussian disturbance
Off-angle reflection
Schematic representation of TID progression and the corresponding ray paths for near-vertical sounding
(120 km)

8. Simplified ray tracing on the basis of combined parabolic layer model
Fitting the ionogram in the proximity of critical
frequency with a function like:

9. Varying TID characteristics to fit the experimental cusp

10. An example of simultaneous reception of two different rays
Approaching/retreating satellite traces and descending/ascending cusps can also be identified by using Doppler measurements. Studying off-angle reflections require angle-of-arrival measurements. This would allow us to minimize ambiguity in reconstructing the space-time TID structure and to reveal its additional characteristics. In particular, the above data can be insufficient to allow full velocity vector measurements. At least, three sounders are required to perform triangulation.
Simultaneous reception of three different rays (S-shaped signature in Doppler shift spectrogram). From J.Chum et al., Cospar-2014
An example of simultaneous reception of two different rays

11. OBSERVING RAPID QUASI-WAVE IONOSPHERIC DISTURBANCES
Focusing and defocusing TIDs.
After W.A.Bristow et al. 1995
If one replaces the "group delay" coordinate with the "time" on the ionogram, and transposes the frequency from the abscissa axis onto the ordinate axis, then one can obtain a visual picture of the ionosphere diurnal condition (A-map).
An alternation of (quasi)-vertical stripes is often observed on A-maps.
A-maps, 15 September 2013.
a) NVIS over the Usolie-Tory path;
b) VIS in the Tory end of the path.
A pronounced succession takes place for the two closely spaced ionosondes.

12. Each scaled ionogram is processed to provide a histogram, where pronounced peaks correspond to quasi-vertical parts, i.e. cusps and critical frequencies. Then, a time dependence is constructed to form the diurnal map. Additional tilted traces represent descending cusps in the ionograms.
f-t plot,
1 March 2015
f-t plot,
15 February 2015

13. CONCLUDING REMARKS AND FUTURE WORK
For many years complex traces on ionograms have been explained by the presence of off-vertical reflections from tilted isoelectronic contours. The echoes recorded at more than one receiver can show similar features, with a time delay between stations, indicating the traveling nature of the disturbance. This, in turn, requires Doppler and angle-of-arrival measurement to supplement the study.
Study of rapid quasi-wave ionospheric disturbances is also to be developed based on A-maps and f-t plots by using simultaneous data from several sounders.

14. Thank you for your attention