Observations of Ionospheric Disturbances Produced by a Powerful Very-Low-Frequency Radio Signal in the Magnetic Conjugated Region Respect the Transmitter

Abstract

We investigate four observational cases over NWC magnetically conjugate region by DEMETER spacecraft and CSES satellite. The cases of DEMETER on 23 February 2008 and CSES on 22 March 2020 show the evident ionospheric heating effects, in which the electron density and electron temperature suggest simultaneous enhancements associated with the intense spectra of the VLF electric and magnetic field above the conjugate region. This indicates that the heating effects associated with ionospheric modification are indeed triggered by the VLF signal transmitted by the NWC transmitter. In other words, the strong disturbances induced directly above the transmitter propagate along the magnetic field lines and extend into the magnetic conjugated region. Differently, the cases on 11 February 2010 (DEMETER) and 15 February 2019 (CSES) show obvious increases in electron densities while having no significant elevation in electron temperatures. The presence of enhanced energetic electron spectra at higher L-values, rather than directly above the conjugate region, suggests precipitation events induced by the VLF transmitter.

1. Introduction

Terrestrial very-low-frequency (VLF) transmitters have been observed to induce notable perturbations in electron density and temperature [1], particle precipitations [2], as well as ion heating in the ionosphere [3]. The initial proposition regarding the potential for VLF-induced ionospheric heating effects was made by Galejs [4], while the first experimental evidence of nighttime lower ionospheric heating phenomena through VLF radiation was reported by Inan [5]. Subsequently, Rodriguez and Inan [6] proposed a model for VLF wave ionospheric modification and presented results that demonstrated a 250% increase in electron temperature and a 26% depletion in electron density within the nighttime D region. In 2007, Parrot et al. [3] presented the first in situ observations of substantial ionospheric fluctuations induced by VLF signals using DEMETER observations. Later, Mishin et al. [7] showed that the DEMETER observations could be attributed to nonlinear interactions, which lead to the energy losses of high-power VLF signals in the ionosphere. Nevertheless, Bell et al. [1] provided evidence of significant VLF-induced ionospheric heating phenomena observed by DEMETER and indicated that the mechanism behind these VLF heating effects remained an open question.

Another significant effect of VLF transmitter emissions is their ability to induce particle precipitation in space [8,9]. The latitude of the NWC transmitter corresponds to magnetic parameter L ≈ 1.45, which is within the range where conspicuous disturbances of the electrons in the inner radiation belt (1.1 < L < 1.35) are predicted to occur [10]. In recent studies, the first investigations of electron precipitation characteristics have been conducted using high-energy particle spectra from the CSES [11]. These studies have revealed a strong association between the magnetic parameter L values ranging from 1.44 to 1.74 and the precipitation belt. Furthermore, Zhao et al. [11] provided additional evidence of ionospheric precipitation induced by the NWC transmitter, as observed by the Chinese CSES satellite. They observed that the precipitation belt is shifted to the east of the transmitter, attributed to the eastward drift of electrons within the radiation belts.

The substantial electric conductivity of the Earth’s geomagnetic field lines facilitates a robust interconnection between the two hemispheres, enabling the mapping of anomalous electric fields and the generation of perturbations at the conjugate point along the geomagnetic field lines [12]. Hence, the intense electric and electromagnetic field responses of VLF transmitters are not limited to wide geographical areas surrounding the transmitters but also extend to their geomagnetic conjugated regions in the opposite hemisphere. This phenomenon has been observed and confirmed by satellites [1,11,13]. The propagation of plasma waves along the magnetic field lines accounts for this observed behavior [13]. Bell et al. [1] analyzed 50 cases of DEMETER data and concluded that the fluctuations in electron density in the NWC magnetic conjugated region were less than 10%. Hence, they suggested that the strong perturbations in electron density and temperature over the transmitter location did not extend into the magnetic conjugated region. However, in our study, we specifically concentrate on the magnetic conjugated region with respect to the NWC transmitter and aim to examine the VLF heating effects and precipitation events. We achieve this by analyzing two cases observed by DEMETER and comparing them with similar cases observed by CSES.

2. Data

Our data set is based on six-year DEMETER data from 2005 to 2010 and CSES data from January to March 2019 and March 2020. We analyze the ionospheric fluctuations in the vicinity of the conjugate region of the VLF transmitter NWC, which is one of the most powerful VLF transmitters in the world with high power of 1 MW [3]. The NWC transmitter located at Northwest Cape (NWC), Western Australia (21.82°S, 114.17°E), radiates at a nominal frequency of 19.8 kHz with a bandwidth of ~200 Hz.

Detection of electromagnetic emissions transmitted from earthquake regions (DEMETER) is a sun-synchronous French satellite operating at a low altitude of ~660 km (after December 2005). There are two working modes of DEMETER, burst and survey. The data we used here is with survey mode (all-weather observations), where the electric and magnetic spectra are measured up to 20 kHz with a good frequency resolution of 19.25 Hz. The instruments of DEMETER consist of an electric field instrument (ICE), magnetic field instrument (IMSC), Langmuir probe instrument (ISL), particle spectrometer instrument (IDP), and so on. These instruments provide measurements of plasma, energetic particles, and waves in the vicinity of the satellite.

To compare with the DEMETER satellite observations, the China Seismo-Electromagnetic Satellite (CSES, which is also called ZH-1) data is used in this study. The CSES, launched on 2 February 2018, is the first Chinese satellite for geophysical field environment measurement and earthquake research [14]. The CSES is designed to operate for 5 years at ~507 km with an orbit inclination of 97° [2]. The data we used here are from an electric field detector (EFD), search coil magnetometer (SCM), Langmuir probe (LAP), and high energy particle package (HEPP) in survey mode. The HEPP contains three sub-detectors: low energy band (0.1~3 MeV of electrons), high energy band (2~50 MeV of electrons), and solar X-ray monitor (1~20 keV). We use the HEPP data with a low energy band captured by the No. 4 tube, as it offers the most distinct observation of electron precipitation characteristics [2].

3. Results

Figure 1 shows the DEMETER observations of ionospheric heating effects over the magnetic conjugated region of the NWC transmitter on 23 February 2008. There are enhancements in the VLF electric field and magnetic field spectra in Figure 1b,c at the nominal frequency of the NWC transmitter, 19.8 kHz, which suggests that the ionospheric modification is attributed to the VLF signal from the NWC transmitter. During the time interval 14:0:2~14:6:56 UT, the electron density increases by ~10%, and the electron temperature increases by ~200 K. Meanwhile, there are obvious spectrum-broadening effects of the VLF electric field and magnetic field spectra in Figure 1b,c. Figure 2 presents similar designations but from CSES data on 22 March 2020. There is an evident increase in electron density of ~60% during 17:34:14~17:35:34 UT, and the electron temperature increases by ~200 K in the meantime. Bell et al. [1] reported larger perturbations in the vicinity of the NWC transmitter, with a depletion of approximately 50% in electron density and an increase of about 50% in electron temperature. It is evident that the heating effects in the magnetic conjugated region are comparatively weaker. This can be attributed to the loss of power due to Landau damping as the electromagnetic wave propagates from the transmitter location to the magnetic conjugated region [15]. In Figure 1b and Figure 2b, the frequency range of the quasi-electrostatic waves spans from 12 to 20 kHz, with the lower cutoff frequency roughly corresponding to the local lower hybrid resonance frequency [1]. In these panels, the extension of frequency spectra observed over the magnetic conjugated region of the transmitter is a typical characteristic of the ionospheric modification induced by powerful VLF signals.

Figure 1a and Figure 2a display an absence of notable MF whistler impulses within the conjugate region, despite the presence of strong electromagnetic field enhancements and severe ionospheric perturbations. Parrot et al. [3] established that plasma density irregularities are essential for VLF waves to propagate to satellite altitudes above the transmitter location. However, it is important to note that the direction of wave propagation in the magnetic conjugated region differs from that directly overhead the transmitter. Hence, it is reasonable to expect the lack of MF irregularities in Figure 1a and Figure 2a.

In the conjugate region, another notable phenomenon is the occurrence of energetic electron precipitation induced by the VLF wave emitted by the NWC transmitter. Figure 3 presents a representative DEMETER observation of the electron density enhancement. Figure 3b–e clearly illustrates that the electric and magnetic field spectra broaden in the presence of a significant increase in electron density. Conversely, the electron temperature exhibits no apparent fluctuations during this process. The analogous results are also observed in the CSES data, as shown in Figure 4. To gain further insight into the mechanism behind the electron density enhancement, Figure 5a–d presents the distributions of energetic electron flux corresponding to Figure 1, Figure 2, Figure 3 and Figure 4. This analysis provides additional information on the behavior of energetic electrons during the observed phenomena. The low energy band electron precipitation observed by DEMETER has also been observed in CSES (100 to 600 keV) results. In Figure 5a,b, marked by black dashed rectangles, the electron energy spectra in the vicinity of the NWC magnetic conjugated region show no enhancement. However, in Figure 5c,d, energy-dispersed wisps [2,16] are observed in both the northern and southern hemispheres, indicated by red rectangles. Notably, these precipitations occur at higher L values rather than at the exact location or its magnetic conjugated area where the strong VLF electric and magnetic field spectra are obtained. These findings are consistent with observations reported in both the NWC transmitter region [3,17] and its conjugate regions [2]. Parrot et al. [3] determined that the precipitation area is situated to the south of the transmitter at a larger L value of approximately 1.9. It was observed that the electron energy generally increases in the southern direction of the transmitter, corresponding to larger L-shells [17]. Additionally, Wang et al. [2] concluded that electron precipitations between L = 1.4~1.8 in both hemispheres are likely triggered by the NWC transmitter. Hence, the cases illustrated in Figure 3 and Figure 4 provide evidence of electron precipitation in the magnetic conjugated region caused by the VLF signal emitted by the NWC transmitter, while the electron temperatures do not exhibit obvious enhancements.

4. Discussion

Ground-based VLF transmitters have a dual impact on the ionosphere, directly through ionospheric heating and indirectly through energetic electron precipitation [18]. The generation mechanism of electron precipitation is well-known as cyclotron resonance [19], where the wave interacts with particles when the Doppler-shifted frequency of the particle closely matches the electron gyrofrequency. In the case of transmitters with a low L value (NWC at L~1.45), electron precipitation is observed at higher L values due to the balance between the distance from the source and the increase in particle flux with L [1]. As shown in Figure 3 and Figure 4, there are enhancements in electron densities, while electron temperatures do not show any significant increase. The corresponding Figure 5c,d further illustrate that the presence of electron energy wisps is observed at higher L values rather than directly over the transmitter or the magnetic conjugated region. These results manifest that the ionospheric disturbances above the magnetic conjugated area of the transmitter are induced by the electron precipitation events.

The ionospheric heating effects induced by VLF waves from the NWC transmitter location are believed to be primarily caused by three mechanisms: Joule heating [20], triggered emissions [21], and ionospheric parametric instability [22,23]. In the case of Joule heating, as the VLF wave propagates through the ionosphere, its energy is gradually transferred to the electrons, leading to an increase in electron temperature. Simultaneously, the wave itself experiences attenuation due to self-absorption effects in the lower ionosphere [20]. Triggered emissions represent another mechanism. As VLF transmissions are typically observed near the magnetically conjugate region, it is postulated that pulses from the transmitter can initiate self-sustaining VLF emissions, resulting in in-situ disturbances. Parametric instability is a third mechanism that can explain the observed triggered emissions [19]. Powerful VLF waves can interact with the ionospheric plasma and parametrically excite Stokes and anti-Stokes lower hybrid waves, as well as zero-frequency field-aligned density irregularities, in a rapid four-wave interaction process [23,24]. The ionosphere perturbances owing to the NWC transmitter will extend along the magnetic field into the conjugate region [1]. However, the mechanism of ionospheric modifications over the magnetic conjugated region of the transmitter caused by powerful VLF signals remains an open question.

The electron densities and temperatures exhibit a simultaneous increase over the magnetic conjugated region of the NWC transmitter, as shown in Figure 1 and Figure 2. This finding contrasts with the observations reported by Bell et al. [1], where a depletion in electron density was observed over the NWC transmitter. As mentioned above, one possible explanation for the increased electron density above the magnetically conjugate region is the fact that the direction of particle velocity is reversed from that overhead the transmitter. Specifically, above the transmitter, particles move in an opposite direction as the velocity of the VLF wave, leading to a depletion of electron density at the satellite altitude. Likely, over the magnetic conjugate region, waves propagate in a downward direction, and particles exhibit an upward direction, resulting in an increase in electron density at the satellite altitude. In addition, the extensions of the VLF frequency spectra clearly shown in Figure 1b and Figure 2b are more possibly attributed to the parametric instability that is excited during the modifications of the VLF signals.

5. Summary

In this study, we have analyzed four cases over the conjugate region of the NWC transmitter using data from the DEMETER and CSES satellites. The first two cases demonstrate simultaneous enhancements of electron density and electron temperature, accompanied by intense VLF electric and magnetic fields. These observations provide clear evidence of ionosphere modifications caused by the NWC transmitter. In contrast, the other two cases show only electron density enhancements with negligible changes in electron temperature. Notably, the electron energy wisps appear at higher L values in the energy spectra rather than in the magnetic conjugated region. This characteristic helps to distinguish these phenomena from heating effects and electron precipitation events induced by the NWC signals. Furthermore, the electron density is enhanced over the conjugate region instead of depleted above the transmitter, which might be because the directions of particle velocities are opposite above these two areas. Although the mechanism of VLF ionosphere heating over the magnetic conjugated region remains an open question, our observational findings contribute to a better understanding of wave-particle interactions in this region.