High-power minimum shift keying (MSK) VLF transmitters operate at various locations around the world. The signals from these transmitters are extremely stable in both amplitude and phase. By monitoring them with narrowband VLF receivers, perturbations in the signal from a given transmitter can then be associated with ionospheric modification somewhere along the propagation path.

VLF radio signals can thus be used to probe the lower ionosphere. As this region is too high for atmospheric balloons and too low for satellites, it is one of the least studied regions of the Earth’s atmosphere, and VLF waves are the only practical probes available. Estimates of D-region electron density have been determined via VLF propagation measurements. ^2,3,4 Further information on the response of the D-region during ionospheric disturbances, such as solar flares and gamma ray bursts, can be retrieved by constantly monitoring single-frequency signals and their prompt response to changing conditions.

UltraMSK is a VLF narrowband receiver designed to measure both the amplitude and phase of modulated VLF signals. This paper describes the installation of UltraMSK on a VLF system at the SANAE IV base in Antarctica (72°40′S, 2°51′W). Data from several M-class and C-class solar flares illustrate the response of the instrument, and from these measurements the characteristics of the D-region can be modelled.

Figure 1 shows a schematic of the UltraMSK system. A preamplifier at the base of the antenna boosts the signal, which is transmitted to the laboratory via a lengthy coaxial cable. The signal is passed through a second amplifier before reaching the service unit. From here the signal is transferred to the sound card on a personal computer. The service unit also integrates GPS timing. The phase of the signal is determined by a quadrature-phase mixer, where the received signal is split and mixed with an in-phase component on one hand and a quadrature-phase component on the other. After the signals have been demodulated, the two components can be compared and information regarding the phase and amplitude of the signal can be extracted. In order to achieve a reliable centre frequency for the quadrature-phase mixer, the precise sampling frequency of the sound card is calibrated using the pulse-per-second signal from the GPS. The system runs at a sampling rate of 96 kHz, and the signal intensity and phase are recorded at 20 Hz.

In early 2009, the UltraMSK system was installed on a smaller (~1 m) omnidirectional whip antenna, but tests showed that this antenna did not provide an adequate signal-to-noise ratio (SNR), especially as it was positioned on top of the SANAE IV base, where disturbances from adjacent instruments and the base infrastructure were prominent. For this reason, the system was moved to the larger magnetic loop antenna. After testing and verification, the north–south loop was selected.

FIGURE 1: Block diagram showing the structure of the UltraMSK system.

A number of VLF transmitter signals were identified at SANAE IV. Figure 3 shows a 5-min averaged spectrum for the north–south loop, where the MSK frequencies are marked by vertical dotted lines. Figure 4 illustrates the great circle paths between the corresponding transmitters and SANAE IV. An overview of transmitters is given in Table 1.⁶ The 18.3-kHz HWV transmitter is currently the strongest signal in the spectrum. Several weaker signals of European origin are also present. Note, however, that these signals are subject to change, and the 16.4-kHz JXN transmitter in particular follows a discontinuous transmission pattern, generally transmitting for only a few hours daily.

In addition to the narrowband MSK signals in Figure 3, there is also a broader hump visible in the 4 kHz – 25 kHz range as a result of the radiation from lightning strokes. Lightning discharges around the world radiate most of their energy in the VLF band and thus constitute the main source of VLF waves propagating in the EIWG. When arriving at a receiver, they are seen as an impulsive broadband signal termed a ‘sferic’. A network of VLF lightning location sensors, the World Wide Lightning Location Network (WWLLN) has been set up to map global lightning activity by combining the sferics detected at different locations around Earth.⁷ SANAE IV also operates as a node in this network.

Several non-MSK peaks can also be seen in the spectrum, with the most prominent ones located at above 30 kHz and below 2 kHz. These disturbances likely originate from the electrical system at SANAE IV, and do not interfere with the monitored MSK signals.

In general, north–south paths are preferred as they allow a single local time to be studied. East–west paths are to a greater extent disturbed by dawn and dusk terminator effects. The main advantage of the east–west facing loop is the strong 1-MW 19.8-kHz NWC signal from Western Australia, although the signal strength is reduced as a result of extensive propagation over the Antarctic ice sheet. Because of SANAE IV’s position relative to the global VLF transmitters, signals originating from the east or west travel a great distance over the Antarctic ice mass, as illustrated in Figure 4. The thickness of the ice layer can be a significant fraction of a wavelength, which implies increased attenuation as a result of the low conductivity of ice. This attenuation is typically 20 dB/Mm – 40 dB/Mm, whereas the attenuation over the sea is about 1 dB – 3 dB/Mm. Experimental data has shown even greater attenuation over ice (Barr⁸ and references therein).⁸

FIGURE 2: A typical diurnal power profile, measured at SANAE IV, for the 18.3-kHz HWV transmitter in France. Dips in the signal power as a result of the terminator effect can be seen at the dusk and dawn transitions (~06:00 and ~18:00).

FIGURE 3: Averaged spectrum for 07 February 2010. The vertical dotted lines mark the frequencies of some known minimum shift keying transmitters. The broadband peak between 4 kHz and 25 kHz is as a result of sferics generated by global lightning activity.

FIGURE 4: Great circle paths from various minimum shift keying transmitters to SANAE IV. The shading indicates the location of the day–night terminator at midday on 12 February 2010.

N_e(z, H’, β) = 1.43 x 10¹³ · e^-0.15H’ · e ^{(β –0.15)(z–H’)} [m^-3]. [Eqn 1]

The Long-Wave Propagation Capability (LWPC) code was developed by the Naval Oceans Systems Center (San Diego, USA) to model the propagation of VLF signals. LWPC uses the electron density profile described by [Eqn 1] to model the ionosphere. A change in H’ or β would lead to a modification in the EIWG, thus causing a change in the amplitude and phase of the signal. The modelled values of H’ and β are modified iteratively to find a signal amplitude and phase which matches the VLF measurements. The unperturbed values for H’ and β depend on the solar zenith angle¹⁰ and are derived from the empirical equations presented by McRae and Thomson¹¹. In this study, estimates of H’ and β were calculated using the average solar zenith angle along the north–south transmission path.

On 08 February 2010, several X-ray bursts associated with solar flares were detected by the Geostationary Operational Environmental Satellite (GOES) 14 satellite. The VLF signal from the 18.3-kHz HWV transmitter on 08 February 2010 is shown in Figure 5. As a comparison to illustrate a quiescent curve, the signal from 11 February 2010 is also shown. The corresponding GOES 14 X-ray (0.1 nm – 0.8 nm) flux data is plotted in the bottom panel of Figure 5. Four M-class flares (peak flux between 10^-5 W/m² and 10^-4 W/m²) and a number of C-class flares (peak flux between 10^-6 W/m² and 10^-5 W/m²) were detected during the day. The increases in X-ray flux are well correlated with the disturbances in the VLF signal. Two artificial spikes are present in Figure 5 at 07:50 and 11:38 (UTC), the first is as a result of a data dropout where the signal is temporarily lost during the solar flare event, and the second is a result of saturation in the received signal. These dropouts have no physical significance and can be disregarded in further analysis of the event.

Four days later, on 12 February 2010, two other M-class flares were detected in the GOES 14 data. The UltraMSK and GOES 14 data for this day are shown in Figure 6. The strongest flare had a flux of almost 10^-4 W/m². As the second flare occurred around dusk, the associated VLF disturbance was superimposed onto terminator effects, which led to a general increase of the signal strength for the duration of the event.

Because of terminator effects, only events that took place close to midday (between 11:00 and 15:00) were analysed further. Two M-class flares and one C-class flare fulfilled this criterion in the 08 February data, shown in Figure 7. The times of the peaks of these flares were registered at 11:14, 12:03 and 13:47. The stronger M-class flare on 12 February had a peak flux at 11:26, as shown in Figure 7.

For all three M-class flares, the HWV signal strength experienced a rapid increase of 3 dB – 4 dB over a period of a few minutes, after which the intensity fell off again and returned to the unperturbed value about 30 min after the peak. For two of these events, the signal strength dipped after the peak, approximately 4 dB below the initial value, after which the amplitude slowly recovered during the next hour. The third flare also had a dip associated with it, but of smaller amplitude. This dip is believed to be a result of modal interference because the ionosphere gradually recovered to normal levels. Although the flux was significantly larger for the 12 February event, the increase in VLF signal strength was about the same as for the weaker M-class flare at 13:47 on 08 February.

Deviations in signal amplitude in nine of the monitored MSK signals for the 12 February 2010 event are shown in Figure 8. Because the event occurred near midday, the disturbance pattern was clearly visible in the signals of central European origin, at a similar longitude to SANAE IV. For these transmitters the signal increased in power by 4 dB – 6 dB. The main increase was more prominent in the stations located at other longitudes, that is, NPM on Hawaii, NAA in Maine and TBB in Turkey, with an increase of 8 dB – 14 dB. However, the subsequent decrease was partly drowned out as a result of the diurnal variations in the signals, so the power did not drop below the quiescent value after the flare.

Because of its stable phase during quiet conditions, the 23.4-kHz DHO signal was selected for the LWPC modelling of the H’ and β parameters for the events. An instantaneous increase in the phase was detected at the times of the flares, which then fell back to the quiescent state with a relaxation time in the order of 1 h. The unperturbed values for H’ and β just before the 11:26 flare on 12 February were found to be 73.74 km and 0.334 km^-1, respectively. At the peak times of the flares, the reflection height was lowered by several kilometres, and the rate of electron density variation with height increased. Table 2 lists the four flares and the associated changes in ionospheric parameters. The electron density height profiles for all four flares, as well as the unperturbed profile, are shown in Figure 9. Not surprisingly, the stronger flares resulted in larger increases of D-region electron density. These results compare well to those of McRae and Thomson¹³, who modelled a similar M-class flare where H’ was found to decrease by ~8 km and β to increase by ~0.075 km^-1, as well as to the results presented in a larger study of stronger solar flares.¹⁴

FIGURE 5: (a) Signal strength from the 18.3-kHz HWV transmitter in France on 08 February 2010 (thick line), showing fluctuations as a result of solar activity. The signal strength from a quiet day (11 February 2010, thin line) is plotted for comparison. (b) GOES 14 X-ray (0.1 nm – 0.8 nm) flux data from 08 February 2010, showing a number of C-class and M-class flares.

FIGURE 6: (a) Signal strength from the 18.3-kHz HWV transmitter in France on 12 February 2010 (thick line), showing fluctuations as a result of solar activity. The signal strength from a quiet day (11 February 2010, thin line) is plotted for comparison. (b) GOES 14 X-ray (0.1 nm – 0.8 nm) flux data from 12 February 2010, showing a number of C-class and M-class flares.

FIGURE 7: (a) X-ray flux (0.1 nm – 0.8 nm) for 11:00 – 15:00 on 08 February 2010 (thick grey line) and 12 February 2010 (thick black line) from the Geostationary Operational Environmental Satellite (GOES) 14. (b) 18.3-kHz HWV transmitter power from France for the same periods.

FIGURE 8: Power of nine signals monitored at SANAE IV. The solar flare at 11:27 UT on 12 February 2010 appears as a rapid increase, followed by a drop in the signal amplitudes.

FIGURE 9: Unperturbed (solid line) and perturbed (a–d) electron density profiles during the solar flares that occurred on 08 February and 12 February 2010.

For a weak flare (ΔH’ = –2 km), the interference pattern shifts (as shown in Figure 12) and then reverts to the unperturbed pattern again. For Station A, located approximately 500 km before a maximum, this means the receiver effectively rides up the slope towards the peak as the interference pattern is shifted towards the transmitter and back down as the height returns to the quiet-time value. This increase and then decrease is shown in the temporal response in Figure 13. An analogous response is observed for Station B, located approximately 400 km before a minimum, except that a decrease is experienced instead of an increase. For a larger flare (ΔH’= –6 km), the response can be slightly more complicated and the temporal profile of ionospheric height plays a major role (Figure 12). As a result of the very fast rise time of the perturbation, a large flare would result in the receiver ’skipping’ over the maximum (A) or minimum (B) and this initial change in signal strength is not observed. Once the perturbation has reached its maximum value, it starts to decay slowly back to unperturbed conditions. The receiver now spends a fair amount of time on the minimum or maximum it skipped during the rise and fully displays this change in signal strength shown in Figure 14.

The modelling results could be used to explain the initial increase and subsequent decrease in the measured amplitude for the M-class flare shown in Figure 6. Consider Receiver B in the modelling results for comparison with the real event (Figures 12 and 14). For a large flare, the ionosphere is ionised at such a great rate that the receiver does not resolve the initial decrease in signal as a result of the lowering of H’ to between 74 km and 71 km (Figure 14, right panel). After the maximum perturbation in the ionosphere was reached at the time of peak flux (resulting in an increase of 4 dB in the signal amplitude), the ionosphere started to slowly recover and return to its initial condition. At this time, the ionosphere was fully experiencing the lowering of H’ between 74 km and 71 km, leading to a decrease of 4 dB in the signal amplitude. The recovery phase of the ionosphere was much slower than the onset phase and led to a gradual change in H’, reflecting a gradual return of the amplitude to unperturbed levels.

FIGURE 10: Modal interference pattern for different ionospheric heights (H’). As the reflection height of the ionosphere decreases, the modal interference pattern becomes more compressed, moving towards the transmitter.

FIGURE 11: Modelled temporal variation in height showing the response of a typical solar flare, with a sharp rise and a slower decay.

FIGURE 12: The interference pattern for different ionospheric heights (H’), as shown in Figure 10. The signal strength is dependent on the distance between the transmitter and the receiver. A and B mark the positions of two stations and the signal responses during a weak and a strong solar flare are shown in Figures 13 and 14, respectively.

FIGURE 13: Temporal response of the signal at two stations: (a) Station A at 11 500 km and (b) Station B at 13 300 km, as a result of a weak flare. The ionospheric height is reduced by 2 km, causing the signal to change from the unperturbed thick black curve to the thin black curve in Figure 12.

FIGURE 14: Temporal response of the signal at the two stations: (a) Station A at 11 500 km and (b) Station B at 13 300 km, as a result of a stronger flare The ionospheric height is reduced by 6 km, causing the signal to change from the unperturbed thick black curve to the thin grey curve in Figure 12.

The functionality of the system was illustrated by the analysis of several M-class solar flares on 08 February and 12 February 2010. By using LWPC to model the ionospheric parameters at the time of the disturbances, it could be shown that the reflection height of the EIWG was lowered by between 2 km and 9 km during the flares, with the largest decrease occurring at the time of the strongest solar flare. The sharpness of the electron density gradient also increased, with a larger increase for the stronger events.

The form of the recorded signal could be investigated by comparing it with signals produced by a waveguide propagation model. It was shown that the distance between the transmitting and receiving stations, as well as the temporal variation of ionospheric parameters, affects the shape of the signal as a result of shifts in the modal interference pattern, and such shifts can account for the decrease below the unperturbed values that followed some of the strong increases in the MSK signal amplitudes.

The results from the data and modelling are in accordance with typical ionospheric parameters during perturbed conditions and illustrate the system’s capability in measuring ionospheric properties. With the new UltraMSK system, the signal was sufficiently elevated above the noise level compared to the previous system, mostly as a result of the larger antenna and moving the system away from the interfering infrastructure at the base. The presented data show that the UltraMSK system thus appears to be working in a satisfactory manner, and the data can be used to remotely probe the lower part of the ionosphere.

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