The power lines
The English term used to point out theses emissions due to man-made activities is PLHR (Power Line Harmonic Radiation), i.e. the radiations emitted by the power lines at the harmonics of 50 Hz (or 60 Hz in USA). But theses lines are not alone to radiate harmonics. One also must consider some heavy industries which can generate numerous harmonics when they convert alternative current to direct current. A systematic research of these lines has been performed using all burst Demeter data and the result is shown in Figure 11. One well find at the satellite altitude, lines separated by 50 Hz in Europe and by 60 Hz in USA (Nemec et al., 2006, 2007a, 2007b).
During a ground based campaign done in common with Demeter, a remarkable event have been simultaneously registered on ground and on board the satellite (Parrot et al., 2007b). Emissions as PLHR and MLR (Magnetospheric Line Radiation) have been observed during 2 hours and the satellite records allowed to show that these emissions stretched away a vast area including the opposite hemisphere. The MLR are lines which are not at the exact harmonics of 50 Hz (or of 60 Hz) and whose frequencies shift as function of time. The Figure 12 shows the ground registration. The Figure 13 shows the respective positions of Demeter and of the ground-based experiment. A Demeter spectrogram is represented in Figure 14. The Figure 15 shows a comparison between the ground and the satellite emissions. One can observe a perfect fit between the frequencies of the lines on the ground and onboard DEMETER. This shows that the crossing of the ionosphere do not affect these emissions.
|Figure 11: The points in bold indicates the geographic locations of PLHR with a frequency separation of 50 Hz (left panel) and of 60 Hz (right panel). The magnetic field lines going through these dots are indicated.|
Figure 12: Spectrogram of the signal received at Kannuslehto (Finland) between 18.00 and 19.00 UT. The frequency range is between 1 and 3 kHz. The signal intensity is color-coded according to the scale on the right. The horizontal lines observed just above 1 kHz and between 2 and 3 kHz are the PLHR at exact harmonics of 50 Hz. A second set of lines shifting in frequency is observed between 1.2 and 2.4 kHz. They are called MLR (Magnetospheric Line Radiation).
Figure 13: Map showing the location where the event has been registered. The star indicates the position of Kannuslehto (67.74°N, 26.27°E) in Finland. The line represents the projection of the Demeter orbit (12842.1) on November 28, 2006. The thick part indicates the place where DEMETER is in burst mode and the dash the place where we stop to record data.
Figure 14: Spectrogram of the signal received onboard Demeter between 18.23.30 and 18.24.30 UT at the end of the burst area shown in Figure 13. The signal intensity is color coded according to the scale on the right. MLR are observed between 1.4 and 2.2 kHz.
Figure 15: Comparison between the frequencies of the lines observed at Kannuslehto (in blue) and onboard the satellite (in red). The spectra are between 1200 and 2400 Hz and correspond to the time interval 18.24.20 – 18.24.30 UT. In addition to MLR, the ground Kannuslehto spectrum shows PLHR at 1250, 1450, 1650, 1950, 2050, 2150, and 2350 Hz.
A very complete statistical study on the MLR has been done by Nemec et al. (2009) in order to show their main characteristics (frequency range, occurrence,…). They have shown that these emissions are more common that it was expected, and that they occur during sustained magnetic activity, more often at the USA longitude. A typical example is shown in figure 16.
Figure 16: Typical example of MLR around 3 kHz. These emissions practically spread along the complete half orbit.
Other examples of PLHR and MLR have been published by Parrot and Nemec (2009). The figure 17 shows an example of PLHR meanwhile the figure 18 shows an event related to MLR.
Figure 17: VLF spectrogram of an electric field component recorded on 13 August 2006 during one minute between 21:42:30 and 21:43:30 UT. The frequency range is between 2 and 5 kHz. The intensity is color-coded according to the scale on the right. A set of three lines can be observed just above 3.5 kHz. The frequencies of lines are close to 3603, 3711, and 3808 Hz, which means that the frequency interval is approximately equal to 100 Hz. There is no apparent frequency shift of the lines during the observation. The event was measured close to the New Zealand and from 21:41:30 UT until 21:46:00 UT when the satellite stops the registration at high latitudes. Relatively thin lines forming the event and frequency spacing close to the multiple of base power system frequency (50 Hz at New Zealand) represent a good indication that the event is caused by PLHR.
Figure 18: Spectrogram showing a MLR event around 3 kHz. The data are recorded on 4 April 2007. The MLR event is recorded during an orbit which is not very far from the east coast of Japan and ends over Kamchatka. It is observed in a frequency band between 2.6 and 3.7 kHz and consists of periodic falling elements with a time period of ~ 3.4 seconds. Different sets of elements with a negative slope appear because they are not synchronized. It depends on the frequency as it is with the periodic elements in Figure 3. In fact in Figures 3 and 4 MLR only appear due to the pattern displayed on the spectrogram by the periodic elements. These periodic emissions are very similar to the emissions previously observed by Helliwell (1965) on ground. In this case the MLR have a natural origin.
The VLF transmitters
The ground-based VLF transmitters are mainly used for communications by the army. They emit at fixed frequencies and their waves are propagated and bouncing in the Earth-ionosphere waveguide. But the ionosphere is not regular and these waves can also cross the ionosphere and be observed by a satellite. Demeter has shown that the most powerful transmitter NWC in Australia can perturb and heat the ionosphere on a vast scale (Parrot et al., 2007). The Figure 19 shows an example of these ionospheric modifications which are observed at the satellite altitude, and the area of these perturbations is represented on the map of Figure 20.
The waves which cross the ionosphere and which are propagated in the opposite hemisphere can also perturb the particles of the radiation belts as it has been studied by Sauvaud et al. (2008). A map showing a global view of the particle flux measured by the IDP instrument around the Earth is shown in Figure 21. One can see that the effect of the transmitter NWC is particularly important.
Figure 19: Data recorded on September 22, 2006 between 14.49.00 and 14.56.00 UT. From the top to the bottom the panels show: - the HF spectrogram of an electric component up to 3.33 MHz, - the VLF spectrogram of the same component up to 20 kHz (the white line represents the lower hybrid frequency), - the electron density, - the electron temperature, and – the ion temperature as function of the time. A large perturbation is observed in the North of the transmitter NWC (21°47'S, 114°09'E).
Figure 20: Map of the North-West coast of Australia. The lines represent the projections of parts of DEMETER orbits where ionospheric perturbations identical to those of Figure 19 are observed. The square shows the position of the conjugate point of NWC at the satellite altitude. The position of NWC is indicated by a star. The dimension of the perturbed area is ~ 500000 km².
Figure 21: Geographic distribution of quasi-trapped 200 keV electron flux. The L=1.7 calculated at an altitude of 700 km is shown by two dashed lines. One can see a large increase of the flux at the South Atlantic anomaly and its counter part in the North hemisphere where a decrease is observed. The outer radiation belt is detected at all longitudes and for latitudes from -45° (-180° longitude) to -60° (-90° longitude). Oppositely, the structure associated to NWC in Australia is only detected from the west coast and follows the line L=1.7 as it is expected with the electron drift.