For the detection of Travelling Ionospheric disturbances, eight complementary methodologies are applied in the TechTIDE project with real-time and historical data from Digisonde DPS4D ionospheric sounders, from the Continuous Doppler Sounding System and from GNSS receivers.
- HF-TID method
- CDSS-MSTID detection method
- GNSS TEC gradient algorithms
- Spatial and Temporal GNSS analysis
- The AATR indicator
- HF Interferometry method
- HTI technique to monitor wave activity
- TaD 3D mapping of the electron density
2. CDSS-MSTID detection method
(Lastovicka and Chum, 2017)
The multipoint continuous Doppler sounding system (CDSS) is currently operating at three frequencies (f=3.59, 4.65 and 7.04 MHz) in the Czech Republic and in South Africa. There are at least three sounding paths (transmitter – receiver pairs) at each frequency. An interesting aspect is that the CDSS method is suitable for the monitoring of MSTIDs and not of LSTIDs. There are two reasons. First, the triangle of measuring points has a size of the order of 100 km, which is suitable for the monitoring of MSTIDs. Second, temporal changes at LSTIDs are slower, i.e. CDSS is less sensitive to them.
The basic principles of the method have been tested (Lastovicka and Chum, 2017). Figure 2 shows the Doppler shift spectrogram recorded during the Tohoku earthquake in 2011.
Based on the experience with CDSS, it is anticipated that the detection of TIDs will run as follows: (a) First, it will be recognized at which frequencies the signals reflected from the ionosphere during the latest, e.g., 90 minutes. For this recognition, information about signal power received by CDSS and information about critical frequency foF2 obtained from DPS4 will be used. (b) The highest frequency fM at which the ionospheric signals were received will be selected for the next analysis. The selection of the highest frequency will more or less ensure that only signals which reflected from the F2 layer will be processed. (c) Doppler shifts fDi corresponding to maxima of spectral densities will be found for subsequent specific time intervals (probably 1 minute intervals) for each sounding path at the selected frequency fM. Thus, the Doppler shifts fDi will be obtained as single valued functions of time during the latest, e,g., 90 minutes. At the same time, the signal to noise ratio will be evaluated by comparing the spectral intensities in a narrow frequency band around fDi with spectral intensities outside this band. If the signal to noise ratio will be low, a mark “cannot be decided” will be set. This situation is expected, e.g., during spread F events. (d) The fDi values will be recalculated to velocities wi of the reflecting level motions to avoid dependence on the selected sounding frequency fM. (e) Root Mean Square (RMS) values of wi fluctuations during the latest, e.g, 90 minutes will be evaluated for each available sounding path and averaged to get one RMS value, RMS(wA). This RMS value will be compared with a threshold vT. If RMS(wA) > vT, then it will be decided that significant TIDs are observed. The threshold will be set by experience. (f) The obtained information will be transferred to the dedicated website.
A possible future extension is the determination of propagation velocities and directions from the time delays between the wi fluctuations on different sounding paths. Also, providing information about the spectra of wi fluctuations is a possible extension. However, it is expected that running these analyses automatically will not be reliable for all the intervals and could be sometimes misleading without visual inspection. A way how to mark the reliability will be searched.
Lastovicka, J., Chum, J., 2017, A review of results of the international ionospheric Doppler sounder network, Adv. Space Res. 60, 8, 1629-1643, https://doi.org/10.1016/j.asr.2017.01.032.