The Typical ELF/VLF Electromagnetic Wave Activities in the Upper Ionosphere Recorded by the China Seismo-Electromagnetic Satellite

Abstract

Driven by the scientific objective of geophysical field detection and natural hazard monitoring from space, China launched an electromagnetic satellite, which is known as the China Seismo-Electromagnetic Satellite (CSES-01), on 2 February 2018, into a circular sun-synchronous orbit with an altitude of about 507 km in the ionosphere. The CSES-01 has been in orbit for over 6 years, successfully exceeding its designed 5-year lifespan, and will continually operate as long as possible. A second identical successor (CSES-02) will be launched in December 2024 in the same orbit space. The ionosphere is a highly dynamic and complicated system, and it is necessary to comprehensively understand the electromagnetic environment and the physical effects caused by various disturbance sources. The motivation of this report is to introduce the typical electromagnetic waves, mainly in the ELF/VLF band (i.e., ~100 Hz to 25 kHz), recorded by the CSES-01 in order to call the international community for deep research on EM wave activities and geophysical sphere coupling mechanisms. The wave spectral properties and the wave propagation parameters of those typical EM wave activities in the upper ionosphere are demonstrated in this study based on wave vector analysis using the singular value decomposition (SVD) method. The analysis shows that those typical and common natural EM waves in the upper ionosphere mainly include the ionospheric hiss and proton whistlers in the ELF band (below 1 kHz), the quasiperiodic (QP) emissions, magnetospheric line radiations (MLR), the falling-tone lightning whistlers, and V-shaped streaks in the ELF/VLF band (below 20 kHz). The typical artificial EM waves in the ELF/VLF band, such as power line harmonic radiation (PLHR) and radio waves in the VLF band, are also well recorded in the ionosphere.

1. Introduction

The ionosphere acts like a passive screen to reflect those physical effects originating either from the lithosphere, atmosphere, magnetosphere, or the solar wind. The disturbances induced by solar activity (e.g., solar flares, coronal mass ejections, associated solar wind, etc.) and the magnetic storms/substorms mix with those induced by natural hazards or artificial events from the lithosphere or atmosphere (e.g., earthquakes, volcanoes, thunderstorms, communication transmitters, electric power system, etc.), making the ionosphere a highly dynamic and complicated system [1,2].

With the development of aerospace technology, there are a growing number of LEO (Low Earth Orbit) satellites operating in the upper ionosphere which provide all kinds of routine services such as natural hazards monitoring, agriculture observations, communication, and navigation. Therefore, whether from the perspective of ensuring the safe flight of the LEO satellites or natural hazards monitoring, it is necessary to comprehensively understand the electromagnetic environment and the physical effects caused by various disturbance sources in the upper ionosphere.

It is widely accepted that electromagnetic (EM) wave activity is the most important factor in demonstrating the disturbed status of the ionosphere. Previous studies reported that in the ionosphere, there are several kinds of EM waves originating locally or propagating downward from the magnetosphere/plasmasphere. Examples include the whistler-mode chorus waves [3,4], the equatorial noise [5], or the plasmaspheric hiss waves [6]. Especially during storm times, the wave activities in the magnetosphere can more easily penetrate the electron density gradient of plasmapause, propagate downward into the ionosphere, and even reach the lithosphere. After propagation down into the ionosphere, the magnetospheric/plasmaspheric waves still keep the most of wave properties, but some unique features are also displayed in the ionosphere. For example, chorus waves which are intense electromagnetic whistler-mode waves in the magnetosphere, occasionally parts of the lower band chorus propagate into the ionosphere [4], and at certain times, the downward propagating chorus waves evolve as the hiss wave structures [7]; the ionospheric hiss, which originates from plasmaspheric hiss waves, commonly appears at frequencies lower than the plasmaspheric hiss [8] and mostly often appears in the high-latitude ionosphere, and mainly propagates along the local proton cyclotron frequency band [9].

Another important EM wave source is lightning activities in the atmosphere. It is known that a lightning strike produces a short pulse of intense EM emissions in the Extremely-Low-Frequency (ELF) and Very-Low-Frequency (VLF) bands, and although most of its energy is trapped in the waveguide of the lithosphere–ionosphere, some portions of their wave energy can leak into the ionosphere, even penetrating the plasmapause and reaching up to the magnetosphere [10]. Normally, the phase velocity of the high-frequency parts of lightning EM pulses is faster than the lower-frequency parts; the high-frequency parts arrive in the satellite orbit space earlier than the lower ones, resulting in a falling tone structure, or the “L” shaped dispersion characteristics of wave spectrum [10,11], and thus the lightning-induced waves are also known as lightning whistlers [12]. Occasionally, the lower = frequency part of lightning whistlers usually appears as ion cyclotron waves (ICW) (below a few hundred Hz), showing a rising tone structure at some times.

Besides the magnetosphere and atmosphere, the lithosphere is also an important source for the ELF/VLF wave activity in the ionosphere. The EM waves propagating from the lithosphere mainly include both natural and artificial electromagnetic radiation. For example, VLF radio waves emitted from the ground-based VLF transmitters are common wave phenomena in the ionosphere [13], and the harmonic emissions (50/60 Hz) radiated by electric power systems in the lithosphere are also frequently recorded by satellites when flying over industrial regions [14]. Additionally, the abnormal EM emissions over the epicenter have also been identified by previous studies [15,16].

Identifying all kinds of EM waves and their generation sources in the ionosphere is the first step before extracting the ionospheric disturbances associated with natural hazards [17]. Thus, the motivation of this report is to introduce the main EM waves in the ELF/VLF band (i.e., ~100 Hz to 25 kHz) recorded by the CSES-01 satellite in order to call the international community for deep research on EM wave activities and geophysical sphere coupling mechanism utilizing CSES-01’s observations.

In this report, the typical and commonly known EM activities in the upper ionosphere observed by CSES-01 are introduced. The satellite data and main methods are presented in Section 2. The typical natural EM waves are introduced in Section 3 (including waves below 1 kHz, below 2.5 kHz, and below 25 kHz). The artificial EM waves are briefly introduced in Section 4. Section 5 and Section 6 are the discussion and conclusion.

2. Satellite and Data Preprocessing

The CSES, also known as Zhangheng-1 (ZH-1), named after the ancient Chinese scientist Zhangheng who invented the first seismo-scope in the second century CE, is a low-Earth orbit (LEO) satellite launched by China with the aim of geophysical field measurement and natural hazard monitoring. It was successfully launched on 2 February 2018 into a circular sun-synchronous orbit with an altitude of about 507 km and an inclination of about 97.4° [18].

Detailed information about payloads and the standard data products was introduced by Zhima et al. [17]. This work mainly utilized the standard L2 data products from HPM, SCM, and EFD. The HPM (High Precision Magnetometer) measures the total geomagnetic field in the frequency range from DC to 15 Hz [19,20]; the SCM (Search Coil Magnetometer) detects the fluctuating magnetic field from 10 Hz to 20 kHz [21]; and the EFD (Electric Field Detector) collects the spatial electric field within the frequency range of DC to 3.5 MHz [22]. The frequency band information of three payloads is listed in

Table 1. The frequency band definition of the EM field detection payloads onboard CSES.

Frequency Band		SCM	EFD	HPM
ULF	Frequency range Sampling rate	10 Hz–200 Hz	DC-16 Hz	DC-15 Hz
		1024 Hz	125 Hz	1 Hz or 60 Hz
ELF	Frequency range Sampling rate	200 Hz–2.2 kHz	6 Hz–2.2 kHz	/
		10.24 kHz	5 kHz	/
VLF	Frequency range Sampling rate	1.8 kHz–20 kHz	1.8 kHz–20 kHz	/
		50 kHz	51.2 kHz	/
HF	Frequency range Sampling rate	/	18 kHz–3.5 MHz	/
		/	10 MHz	/

.

The raw data can be downlinked from the satellite through two working modes: survey mode along the whole orbit trajectory (lower sampling rate) and burst mode (higher sampling rate) over the potential seismic zone within several minutes.

It is noted that Table 1 lists the frequency information of the standard calibration Level 2 data product which is shared with the public via the website (https://leos.ac.cn, accessed on 30 July 2024). One advantage is that EFD and SCM can provide continuous waveform data in the survey mode, allowing us to compute the wave propagation parameters with the six-component waveform data below 2.5 kHz at any time of interest [23]. The cross-calibration among the SCM, EFD, and HPM [23,24] confirmed good consistency between HPM and SCM in the Ultra Low Frequency (ULF) band and between SCM and EFD in the ELF/VLF bands.

The wave propagation parameters such as the polarization, ellipticity, the wave normal angle, azimuthal angle, planarity, etc., were computed by the singular value decomposition (SVD) method [25], which has been widely applied to study wave propagation feature [6,10,11,26,27]. The SVD algorithm was introduced in detail by Santolík et al. [25]; our works [28,29] further introduced how to build the Magnetic Field Aligned Coordinate (MFAC) coordinate system for CSES-01. The total geomagnetic field data from HPM were used to calculate the local proton cyclotron frequency and build the MFAC coordinate system for the six components of the EM field waveform data. Detailed information for wave propagation parameters can be found in Section 2.0 of Lv et al. [28] and Hu et al. [29]. Here, we briefly introduce the values of wave propagation parameters.

The wave normal angle θ_k (or polar angle) and the azimuthal angle ϕ_k are defined by the relations between the wave vector k and the background magnetic field B0 (see Figure 1 in [28]). The wave normal angles θ_k vary from 0° to 90°, through which we can determine whether the wave vector is parallel or perpendicular to the ambient magnetic field B0. The ϕ_k = ±180° means the wave is propagating towards the decreasing L shell direction in the meridian plane (toward the Earth), while ϕ_k = 0° means the wave is propagating along the local magnetic meridian plane (outward from the Earth), and ϕ_k = 90° suggests azimuthal direction propagation [29].

The ellipticity is defined as the ratio of the minor to the major axis of the polarization ellipse of the wave magnetic field. The positive, negative, and zero values of ellipticity mean the right-handed (+1), left-handed (−1), and linear polarization (0), respectively. The planarity indicates the observed waves are coming toward the spacecraft as a plane wave propagation (1: plane wave) or a spherical propagation (0: spherical wave).

It should be noted that the calculation of the wave normal angle solely relies on magnetic field data, thereby making it impossible to distinguish between two antiparallel directions of k within this particular panel [25]. We further used the Poynting vector ($S=E\times {\displaystyle \frac{B}{{\mu }_0}}$) to resolve this ambiguity. The parallel (S_//) and perpendicular (S_⊥) components of the Poynting flux mean the wave propagates parallel or perpendicularly to B₀.

3. The Natural EM Waves Recorded by CSES-01

3.1. The Waves below 1 kHz

3.1.1. Ionospheric Hiss Waves

The hiss wave is an incoherent and structureless whistler-mode EM emission in the ELF frequency band (~ below 3 kHz) that predominantly appears in the high plasma density area, such as the plasmasphere [30] or the ionosphere [6]. The hiss wave can induce energetic electrons captured along Earth’s magnetic field lines precipitation into the upper atmosphere through pitch angle scattering across a broad energy spectrum, significantly influencing radiation belt electron dynamics [30].

The LEO satellites such as DEMETER, Freja, and CSES-01 have recorded abundant ionospheric hiss waves in the ionosphere and demonstrate that the ionospheric hiss waves appear in a narrower frequency range (i.e., ~100 Hz to 1 kHz) than the ones in the plasmasphere/magnetosphere (i.e., ~several hundred Hz to ~3 kHz). And most importantly, the ionospheric hiss waves prefer to appear close to the local proton cyclotron frequency [6,9] and also during times of intense storms. Xia et al. [31] statistically analyzed DEMETER’s six-year observations, obtained the distribution characteristics of the ionospheric hiss wave, and concluded that both on the dayside and in local summer, the wave power of the ionospheric hiss is stronger than that on the nightside and in local winter.

Figure 1 shows one example of the ionospheric hiss event recorded by the CSES-01 on 4 January 2020, when the CSES-01 was operating in the dayside ionosphere. Figure 1a,b show the power spectral density (PSD) values of the magnetic field and electric field. It can be seen that the hiss waves closely propagate along the local proton cyclotron frequency (f_cp, denoted by the black dashed lines in Figure 1), and the frequency width of the ionospheric hiss varies mainly from ~300 to 800 Hz (denoted by the black rectangles). It can be seen from Figure 1 that these hiss waves exhibit apparent cutoff effects: the lower cutoff effect is clear, and the upper one is relatively diffuse. Our previous study [9] confirms that the wave properties of ionospheric hiss observed by CSES-01 are consistent with the previous missions [6,7,31], unexceptionally demonstrating a similar wave spectral property, that is: the wave intensity descends with the latitude and disappears near the equatorial area due to damping effects by the high-density equatorial plasma peak.

Previous studies [6,8] suggest that the ionospheric hiss waves mainly originate from plasmaspheric hiss in the plasmasphere, or some of them evolve from the downward magnetospheric chorus waves. Chen et al. [6] revealed that ionospheric hiss exhibits a vertical downward propagation direction in both hemispheres at high latitudes, while as the latitudes decrease, the propagation direction gradually shifts slightly towards the equatorward direction in the lower latitudes. Our previous wave vector analysis results [28] for the ionospheric hiss waves recorded by CSES-01 also confirm similar propagation features.

3.1.2. Proton Whistler Waves

Proton whistlers are types of ion cyclotron whistler-mode waves mainly generated by lightning discharge in the atmosphere [32]. Observations from the previous missions in LEO space, such as Injun 3, Alouette satellites [32], Interkosmos 5 satellite [33], and DEMETER [11], indicated that the dominant characteristic frequencies of proton whistlers mainly fall within the local proton cyclotron frequency range, which means they are significantly damped by the local proton cyclotron effects. Commonly, the upper cutoff frequency is close to the proton cyclotron frequency due to the resonance of the left-handed polarized ion cyclotron wave [34]. According to the previous observations, proton whistles can be divided into two types. The first type only appears in the same hemisphere as the lightning source and does not cross the equator [32], with an upward propagation direction [32], and is effectively cut off by the local proton gyrofrequency effects [35]. The second type propagates across the geomagnetic equator to conjugate points of the opposite hemisphere, which is defined as trans-equatorial or downgoing proton whistlers [34], mainly cut off by the magnetic equatorial proton cyclotron frequency [11].

An example of two types of proton whistler waves recorded by CSES-01 is shown in Figure 2. Figure 2a,b show the PSD values of the magnetic and electric fields, respectively. The black and red dashed lines in Figure 2 represent the local (f_cp) and equatorial (f_{cp_eq}) proton cyclotron frequencies. The f_{cp_eq} is computed by mapping the L value of the observation point to the magnetic equatorial plane using the geomagnetic field and L values obtained from the IGRF model. The discrete proton whistler structures with varying upper cutoff frequencies are visible (Figure 2a,b). This event mainly appeared in the equatorial area at a latitude from 14°S to 25°N. It is seen from Figure 2 that the first type of proton whistler (denoted by black arrows) is mainly cut off by the f_cp (black dashed line), roughly varying between 500 to 800 Hz; while the second type of proton whistlers (denoted by the red arrows) is mainly cut off by f_{cp_eq} (red dashed line). It is noted that in the region where the second type of wave was observed, there are some artificial noises from the mixed EFD payload [23].

To further study the propagation characteristics of proton whistlers, we selected a period from Figure 2 (denoted by the two vertical lines) where both two types of proton whistlers coexist to carry out Singular Value Decomposition (SVD) computation. The results are shown in Figure 3. Figure 3a,b show the frequency-time spectrograms of the magnetic and electric fields, which are computed by the waveform data in the ELF band. We removed points with PSD values smaller than 10⁻⁸ nT²/Hz to highlight the proton whistler structure.

Figure 3c shows the ellipticity, which is around 0, meaning these waves are predominately linear-polarized. Figure 3d presents the wave-normal angles that vary from 60° to 90° from the southern to the northern direction. The variation of the azimuthal angle is given in Figure 3e, which dominates around 80°–90°. Figure 3f exhibits the planarity of the waves with values approaching +1, implying that the observed waves are coming toward the spacecraft as a plane wave. Figure 3g,h show the Poynting flux’s parallel (S_//) and perpendicular (S_⊥) components. These results are consistent with the oblique propagation feature revealed by the polar and azimuthal angles. From these propagation parameters, it can be said that the two types of proton whistlers exhibit similar propagation characteristics. However, the first type of proton whistler has a near-normal angle close to 90°, which is significantly larger than the second type. Moreover, the ellipticity of the first type is slightly smaller than that of the second type.

3.2. The Waves below 2.5 kHz

3.2.1. Quasi-Periodic Emissions

Quasiperiodic (QP) emissions are whistler-mode EM emissions observed both in space [27,36,37] and on the ground [38] in the ELF/VLF frequency range of approximately several hundred Hz to 4 kHz. This type of emission usually exhibits discrete spectral elements with a slightly varying periodic time modulation from a few seconds up to 10 minutes [37]. The dispersion characteristics are quite similar to the whistler-mode chorus wave with discrete rising/falling tones [39], but the frequency is not related to the electron cyclotron frequency as Chorus is.

Figure 4 is a typical example of a QP event recorded by CSES-01 on 31 January 2020. The clear, discrete quasiperiodic wave structure in high-latitude regions of both the northern and southern hemispheres is seen in Figure 4a, which shows dayside half-orbit magnetic field spectral data. The frequency range of this event varies between the f_cp and approximately 2.5 kHz. The intensity of the QP structure gradually decreased from high to low latitudes, but the modulation period remained relatively stable. Similar structures are also exhibited in the electric field.

We calculated the typical modulation period within the red rectangle (08:44 to 08:52 UT, Frequency: 1000 Hz to 1300 Hz in Figure 4a), as shown in Figure 4b. Results show that the modulation period changes from 25 to 33 s, with a typical modulation period of about 30s. According to previous studies [27], the QP events recorded by CSES-01 in the ionosphere show the common feature of the cutoff frequencies: the lower cutoff frequency typically corresponds to the local proton cyclotron frequency f_cp, and all the events appear to have a rising or falling spectral structure.

We further computed the wave propagation parameters of the QP event in Figure 4, and the results are depicted in Figure 5. We can see distinct quasiperiodic and rising-tone spectra from the power spectral density values of the magnetic field (Figure 5a) and the electric field (Figure 5b). Figure 5c indicates that QP emissions are right-hand polarized. Figure 5d,e display wave normal angles between 40° and 50° and azimuth angles between 120° and 180°. These propagation angles indicate that the QP emissions propagate obliquely toward Earth from outer space.

CSES-01’s observations of the QP events demonstrate that different QP events often have different modulation periods and frequency drifts, indicating variations in the regions through which the waves propagate and possibly differences in their source mechanisms. The CSES-01 has captured a series of quasiperiodic emissions in the upper ionosphere with different spectral structures [27], which provide additional observational evidence for a better understanding of ELF/VLF whistler-mode waves in the low-altitude space. Notably, these QP emissions are primarily observed on the dayside ionosphere or magnetosphere [27,37].

Previous space and ground observations confirmed that QP emissions occur under both geomagnetically quiet and disturbed conditions [36]. Two types of QP waves were classified according to their association with (Type I) or without (Type II) ultra-low frequency (ULF) magnetic field pulsations [40]. Thus, there are mainly two generation mechanisms proposed. One suggests that QP waves are generated by ULF magnetic field pulsations modulating the ELF/VLF whistler-mode waves into discrete structures. Particularly for Type I waves, their discrete QP structures are believed to be produced by modulations of co-variant ULF mode waves [38,40].

Another generation mechanism of QP waves involves energetic electrons [41], as the occurrence of QP emissions often coincides with energetic electron precipitation [27,36]. Thus, it is suggested that energetic particles can modulate whistler-mode waves into QP structures through cyclotron instability of interactions between waves and particles [42]. The cyclotron instability is likely caused by the electron beam or pitch angle anisotropy of plasma electrons [42].

Although they have been recognized for decades, their generation locations are still not fully understood. Simultaneous conjugation observations of quasiperiodic (QP) waves in both the upper ionosphere and the inner magnetosphere have been reported in previous studies [37]. These observations suggest that the source of QP waves is located near the equatorial region, specifically in the plasmapause boundary. However, the precise origin of QP waves remains a puzzle, whether they are locally generated in the high-latitude plasmapause region or propagate from the equatorial region in the inner magnetosphere.

3.2.2. Magnetospheric Line Radiation

Magnetospheric Line Radiation (MLR) is whistler-mode radiation characterized by a series of nearly parallel and equidistant intense spectral lines. The frequency of this type of emission ranges from a few hundred Hz to 8 kHz, and the frequency drift occurs along the L shell or latitude [43,44]. Hu et al. [44] studied the MLR events recorded by CSES-01 and suggested that the lower cutoff frequency of MLR typically approximates the local proton cyclotron frequency, with frequency spacings between adjacent spectral lines ranging from tens of Hz to over 100 Hz. MLR may play an important role in triggering whistler-mode emission, thereby affecting energetic particles in the radiation belts [45].

Figure 6 shows an MLR event observed by CSES-01 on the dayside ionosphere. Figure 6a displays the PSD values of the magnetic field, indicating that this MLR spans a large spatial range, extending from approximately 56°N to 56°S in latitude. It exhibits a distinct parallel spectral structure within a frequency range from the local proton cyclotron frequency to ~1.6 kHz. The radiation intensity of MLR gradually decreases from higher to lower latitudes.

To gain a better understanding of how the frequency spacing varies with latitude, we calculated the mean values and applied the smoothing method to three latitude regions, as shown in Figure 6b–d. Figure 6b shows that the frequency spacing varies from 75 to 85 Hz in the high-latitude Northern Hemisphere, Figure 6c shows the frequency spacing ranging from 50 to 80 Hz in the equatorial region, and Figure 6d shows that it falls between 40 and 100 Hz in the high-latitude Southern Hemisphere. It can be seen that the frequency spacing in the high-latitude areas is more significant than that of low-latitude regions.

We further computed the wave propagation parameters of MLR events occurring in the high-latitude regions of the Southern Hemisphere by using the six-component waveform data, and the results are presented in Figure 7. Both the magnetic field (Figure 7a) and the electric field (Figure 7b) exhibit distinct parallel spectral structures, clearly indicating that this event is a typical EM emission characterized by right-hand polarization (Figure 7c, with ellipticity close to +1). The normal angle ranges between 20° and 40° (Figure 7d), while the azimuthal angle ranges from −180° to −90° (Figure 7e), suggesting that the MLR propagation direction is oblique toward Earth. A planarity value of 1 confirms the reliability of the calculations (Figure 7f), and the components of the Poynting vector in both the parallel and perpendicular directions to the magnetic field (Figure 7g,h) validate the calculated propagation direction.

MLR events significantly depend on space weather conditions [43]. Observations from the CSES-01 satellite indicate that large-scale MLR events primarily occur on the dayside of the ionosphere during storm time [44].

The MLR was initially thought to be triggered by the interaction between energetic particles and power line harmonic radiation (PLHR) emitted by the electric power system of industrial areas on the lithosphere [46], just because the frequency spacings of MLR coincidently equal the fundamental frequency of PLHR (50/60 Hz or the harmonic frequencies). After more LEO satellite observations accumulated, it was finally confirmed that there is no definite relationship between MLR and PLHR [47], as the frequency spacings of MLR randomly vary without any fixed relation with PLHR [45]. Later, Rodger et al. [48] found that the MLR is associated with a combination of chorus waves and midlatitude hissing waves based on the Antarctic Halley ground-based observatory (75.5°S, 26.9°W, L shell ∼ 4.3).

Due to limitations in observational data, previous research on MLR has had several shortcomings, especially in the case of large-scale MLR events, which have been rarely observed. Consequently, there is limited understanding of their sources and generation mechanisms. However, CSES-01’s comprehensive full-orbit waveform data provides strong support for further investigating the mechanisms behind MLR generation and the overall propagation processes.

3.3. The Waves below 20 kHz

3.3.1. VLF Lightning Whistlers

The powerful lighting discharges in the atmosphere usually induce intensive whistler-mode emissions in the VLF band. When the lightning-induced VLF whistlers propagate within the Earth–ionosphere waveguide, some portions leak out to the ionosphere (even up to the magnetosphere) along with the magnetic field line [49]. As they propagate through the plasma surrounding the Earth, they exhibit a distinct time-frequency profile due to the dispersive effect of the medium [50]. Due to the dispersion effect during the propagation process, the higher frequency arrives at the satellite position early, and the lower frequency comes later, making the wave spectrogram of satellite observations look like a falling tone structure. Despite extensive research on lightning whistlers, numerous unresolved questions remain, such as the still disputed origins of plasmaspheric hiss [39,51], and how whistlers penetrate the ionosphere.

Figure 8 shows a typical example of the lightning whistler wave event recorded by the CSES-01 on 7 January 2020. Multiple discrete and intense lightning whistlers with falling tone structures can be clearly distinguished in the magnetic and electric fields. CSES-01 recorded this event for about two minutes. It can be seen from Figure 8c,d that the wave is right-handed polarized, with the wave-normal angle varying between 50° and 60° and the azimuth angle changing between 90° and 180°. These wave propagation parameters indicate that these lightning-induced whistlers propagate obliquely from the atmosphere to outer space. Although SVD methods cannot solve the wave-vector k ambiguity (the two antiparallel directions), to determine the direction, we can examine both the Poynting parallel component (Figure 8h) and the wave spectral properties (Figure 8a,b). It can be determined that the direction of propagation is parallel and opposite to the directions obtained in Figure 8d,e, propagating obliquely downward in the direction opposite to the geomagnetic field. Such falling tone whistler-mode waves are ubiquitous in CSES-01’s observations.

Interestingly, these strong whistler waves exhibit almost the same and narrow dispersion features. Smith et al. [52] explained that these waves may be trapped in field-aligned density plasma irregularities or ducts extending between the two hemispheres; thus, the waves are possibly reflected by the opposite hemisphere and propagate back and forth in the two hemispheres. Smith et al. [53] presented evidence of ducts’ existence and described the propagation of whistlers within such structures: these ducts act as guides, directing the whistlers toward the magnetic conjugate point, where, under favorable conditions, they may re-enter through the ionosphere and become detectable on the ground in the conjugate hemisphere [53]. However, it is worth noting that a significant portion of the signals remain nonducted. Based on the early ray-tracing results of Kimura [54], Smith and Angerami [55] introduced a terminology to describe lightning whistlers observed in space: the whistler wave that propagates directly to the satellite without crossing the magnetic equator is referred to as a “0+” whistler or, occasionally, a short fractional-hop whistler. It is widely accepted that these fractional-hop whistlers propagate through the ionosphere without being ducted until they potentially enter ducts at higher altitudes ranging from 1000 to 2000 km. The fractional-hop whistlers’ wave vector directions are very close to vertical just after they penetrate upward into the ionosphere [10].

According to the wave spectral property and the wave propagation parameters, we suggest that this event shows the propagation characteristics of a “0+” whistler, propagating from the lightning source to the satellite in a nonducted mode. The wave vector analysis results suggest that up-going whistlers have traversed a relatively short distance of a few hundred kilometers, reaching the CSES-01 primarily through the dispersive ionosphere. As a result, they exhibit minimal frequency dispersions. On the other hand, down-going whistlers have completed nearly a full path through the magnetosphere, resulting in dispersions that closely resemble those observed on the ground at the corresponding L-value. Unfortunately, no noticeable down-going whistlers were observed in this event.

3.3.2. V-Shaped Streaks

There is a very interesting and rarely occurring EM emission in the VLF band which is called V-shaped streaks [56], as its wave spectral property generally exhibits a set of V-character shapes. Figure 9 shows two examples of V-shaped streaks recorded on 9 October 2019 and 8 December 2019, respectively, denoted by the PSD values of the magnetic field in the VLF band. It can be seen that intense oblique spectral lines are symmetrically distributed along the central points (i.e., ~23:36 UT in Figure 9a; ~18:57 in Figure 9b) in the frequency band from 0.2 kHz to 15 kHz. Parrot et al. [56] reported the presence of unusual V-shaped streaks observed in DEMETER VLF spectra above intense thunderstorms during nighttime. We confirm that CSES-01 observations also show that this kind of V-shaped streak most commonly appears in the nightside ionosphere.

These V-shaped streaks were previously observed on low-altitude satellites [57] and were interpreted as indications of upward-propagating whistler-mode waves originating from a source located below the satellite’s altitude. The propagation and effects of these waves have been investigated by numerous researchers following the pioneering works of Storey [49] and Helliwell [50]. The V-shaped pattern generally exhibits either a single V feature or multiple V features. The multiple V shape indicates the potential existence of multiple source regions distributed at varying altitudes. Parrot [56] provided an explanation that can be applied to the observed V-shaped streaks by mapping the frequency-dependent positions of mode interference nulls (and crests) within the Earth–ionosphere waveguide to higher altitudes. The crucial aspect of this explanation lies in recognizing that the occurrence rate of 0+ whistlers intensifies as the spacecraft approaches the center of the V shape, coinciding with the intersection of the geocentric footprint of an active thunderstorm. In this scenario, the thunderstorms exhibit a high lightning rate, effectively acting as a nearly continuous broadband transmitter. Consequently, the spatial nulls/crests remain continuously visible as the spacecraft approaches the storm’s center. To test the validity of this concept, a new full-wave model of VLF propagation within the Earth–ionosphere waveguide, as well as in the overlying stratified ionosphere, was employed [56].

The source region of V-shaped emissions associated with thunderstorm activity, where these V-shaped emissions are generated, does not exceed a few tens of kilometers. Therefore, we can consider this source region as a point. These V-shaped emissions may be similar to emissions from terrestrial VLF transmitters in the Earth-ionospheric guide at multiple frequencies.

V-shaped emissions associated with intense and numerous whistlers in the VLF range have also been observed in the auroral zones. The existence of these V-shaped emissions suggests a possible analogy with the emissions observed solely in the auroral zones. Studies investigating the Poynting flux indicate that the auroral emissions, also known as VLF saucers, originate from whistler-mode waves originating below the satellite [58]. Similar events have also been observed by the Viking [59] and FAST [60] satellites.

4. The Typical Artificial EM Waves Recorded by CSES-01

In addition to the typical natural EM wave phenomena recorded during CSES-01’s more than five-year operational period (2018–2023), some ELF/VLF waves generated by the artificial sources were also clearly recorded by CSES-01. The most common artificial waves in the ELF/VLF band include power line harmonic radiation (PLHR) and radio waves emitted by the ground-based broadcasting transmitters.

4.1. Power Line Harmonic Radiation/

PLHR (power line harmonic radiation) is radiated by electric power systems at the harmonic frequencies of 50/60 Hz and can penetrate the atmosphere and reach the ionosphere [14,61]. The LEO satellite observations show that its wave spectral properties appear in several narrow horizontal lines without apparent frequency drift. The spacing between these horizontal spectral lines depends on the fundamental frequency of the power system in the source region, most commonly at 50/100 Hz or 60/120 Hz. Němec et al. [14] reported that the PHLR frequency closely matches the fundamental frequency of the ground electric power system directly beneath the satellite position. Due to the absorption effect of the ionosphere, the spectra typically exhibit a lower intensity and can be easily masked by emissions generated by other strong emissions (e.g., lightning discharges) or the background EM field of the ionosphere that become intensified during the storm time. Although the intensity of PLHR events is typically low, they can induce new electromagnetic emissions [62] and may cause electron precipitation through wave–particle interactions.

Figure 10 shows a PLHR event recorded by the CSES-01 when it flew over the China–Vietnam region on 20 October 2019. From the PSD values of the electric field in Figure 10, several relatively weak horizontal lines can be identified at frequencies 1650 Hz, 1950 Hz, 2000 Hz, 2050 Hz, 2100 Hz, and 2350 Hz. This corresponds to an exact multiple of the fundamental frequency of 50 Hz for the Chinese and Vietnamese power systems. PLHR is also present at other frequencies (multiples of 50 Hz), annotated with numbers in Figure 10. However, due to their extremely weak intensity, they appear to be hardly noticeable.

4.2. Transmitter-Emitted VLF Radio Waves

VLF transmitters are one of the major artificial sources that can cause significant EM field disturbances in near-Earth space. These radio waves emitted by transmitters appear as the narrowband enhancement along the central frequency of the transmitter, both on the electric and magnetic field, in a broad frequency range below ~60 kHz [63], usually showing as an intense horizontal spectral line from the satellite’s observations. Given a transmission power and a constant frequency band, the transmitter’s signal can be identified over the location of the transmitter and its conjugate points [64]. Transmitter-emitted VLF radio waves play a significant impact on the electromagnetic environment of the ionosphere. For example, transmitter radiation can cause ionospheric heating and precipitation of the energetic particles into the atmosphere [65,66].

The NWC transmitter, which is one of the best-known transmitters, located in northwestern Australia (~21.82°S, ~114.17°E) generates VLF radio waves at a central frequency of 19.8 kHz that can propagate over considerable distances within the waveguide of the Earth–ionosphere with minimal attenuation. However, under certain conditions, some wave power still can leak into the ionosphere, although they experience significant attenuation when penetrating the lower ionosphere [13], but they eventually reach higher altitudes.

Figure 11 shows the VLF radio wave recorded by CSES-01 in the nightside ionosphere when it passed near the NWC transmitter. It can be seen that the magnetic field (Figure 11a) and electric field (Figure 11b) disturbances are simultaneously intensified at 19.8 kHz. The shaded area in Figure 11a is due to the SCM switching to burst mode, which increases the sampling rate and results in variations in data resolution. Due to the conjugate effect of NWC, near its operating frequency (19.8 kHz), significant enhancement of the electric and magnetic fields occurs simultaneously in the source hemisphere and the conjugate hemisphere. A simulation work [64] explained this as due to the fact that the radiation from NWC propagates in nonducted mode, while the D-layer of the ionosphere strongly absorbs it during the day. Conversely, the D-layer disappears at night, leading to much greater nighttime disturbance intensity than in the daytime.

5. Discussion

Based on the above introduction, we have gained a better understanding of the general wave activities in the upper ionosphere based on CSES-01’s observations. Although some theoretical models and observations from the previous mission have been proposed to explain the generation and propagation mechanisms of these phenomena [6], there are still many aspects that require further exploration and understanding. For example, some studies have found that whistler-mode EM waves originating from the plasmasphere can form various types of emissions in the ionosphere. These research results still need further observational data and theoretical research to verify.

By analyzing the propagation characteristics of ionospheric EM waves, we can infer whether they originate from the magnetosphere or the bottom of the ionosphere, but it remains challenging to determine their specific sources. For example, despite it being a relatively common EM type of radiation, the origin and propagation mechanism of MLR are still not well understood. The propagation characteristics, including frequency, amplitude, and direction of propagation, are influenced by many factors, such as geomagnetic activity, season variation (sunlight-ionization), geographical location, and so on. However, current studies from the international scientific community are still unable to fully explain how these factors affect the propagation characteristics of EM wave activities in space.

For example, the propagation characteristics of ionospheric hiss waves are a typical example [31]. There are certain challenges in the classification and recognition of these phenomena; the QP emissions have been divided into two types by previous studies (Type I and Type II) based on their association with the same period of geomagnetic pulsations [38]; however, recent studies suggest that the distinction between these two types is not as clear as previously thought, especially when using satellite observations, as both types may have the same generation mechanism.

The study of lightning has always been a hot topic because lightning is an essential source of EM waves in the ionosphere. In the case of this emission propagating as the whistler mode, for some waves with frequencies ω > ωc/2, the higher frequencies correspond to larger group velocities when propagating in the plasma in cab form as falling-tone whistlers, while the lower frequency part of this emission (below a few hundred Hz) usually appear as the ion cyclotron waves (ICW) or the proton whistler, which is the most common wave phenomenon associated with lightning discharge. For the falling-tone lightning whistlers, the ducted and nonducted modes have different spectrum and propagation characteristics. Unfortunately, the falling-tone lightning whistler given in the paper does not include the whistler formed in the ducted mode. During the propagation of lightning emission, due to characteristics such as refraction and damping, a variety of EM waves with interesting spectral structures, such as a V shape, will also be formed. The process by which lightning can penetrate the ionosphere has long been a subject of fascination. Furthermore, the complexity of propagation is heightened by the various modes of propagation of electromagnetic waves induced by lightning, such as ducted and non-ducted modes.

In the near-Earth space environment, man-made EM waves or noises, such as PLHR and transmitter-emitted VLF radio waves, are also important wave sources impacting the ionosphere. Although PLHR is a man-made electromagnetic pollution source in the near-Earth space environment, so far, there has been little quantitative research on its formation mechanism, and the intensity of PLHR is usually quite weak, easily overshadowed by the strong emissions produced by lightning; therefore, improving the accuracy of identification methods is still challenging. The propagation model of VLF radio waves in stratified anisotropic ionospheres has reasonably explained the electromagnetic disturbances caused by VLF transmitters in the ionosphere [64]. However, to continually refine this model, it is still necessary to compare it with a large amount of observational data.

According to the wave vector analysis, it is interesting that during seismic periods, we found some portions of those EM waves showing upward directions (i.e., propagating from the Earth direction to outer space). For example, we compared the wave propagation parameter of the ionospheric hiss waves recorded during an earthquake and during non-earthquake time over the epicenter area. Results show that during the non-earthquake time, the ionospheric hiss is purely propagating downward to the Earth’s direction (See Figure 4 in [28]). However, during the seismic time, we found that there are both upward and downward propagation mixed together (see Figure 6 in [28]). The statistical analysis of the shallow strong earthquakes (M ≥ 6.0, depth below 30 km) that occurred in mainland China from 2019 to 2022 suggests that the majority of the upward ionospheric hiss waves mainly appear at the frequency band from 300 to 800 Hz during the seismic time, with wave normal angles vary from 40° to 60°.

It is important to note that, in addition to the above-mentioned mechanisms of QP waves, there exists a potential theoretical mechanism related to seismo-ionospheric coupling that could lead to the generation of QP emissions. This mechanism involves ELF waves generated by the modulation of ULF waves in the Earth’s crust or lower atmosphere caused by earthquakes. These ELF waves can penetrate the ionosphere from below, forming a combined ELF/VLF-ULF disturbance. As acoustic wave beams propagate through the atmosphere and ionosphere, they can interact with the plasma and produce new frequency components through nonlinear frequency down-conversion. Different modulation methods can lead to different frequency conversion effects, which might explain the complex characteristics of seismic electromagnetic signals caused by seismic events, including the generation of QP radiation [67].

However, the relation between the upward propagating EM waves and the seismic activity remains a puzzle to us and needs further research in the future. The scientific objective of the CSES mission is to study the ionospheric disturbances induced by natural or artificial sources in the ionosphere and search for observational evidence for the lithosphere–atmosphere–ionosphere coupling mechanism, then explore the possibility of short-term earthquake forecasting. ELF and VLF emissions can be used to monitor and investigate various geophysical phenomena, such as thunderstorm activity and ionospheric disturbances. Moreover, variations in ELF/VLF emissions may be associated with natural disasters such as earthquakes and volcanic eruptions. ELF emissions are typically generated by lightning and natural disturbances in the Earth’s magnetic field. ELF emissions can penetrate the Earth’s crust, mantle, and seawater, allowing them to propagate over long distances. VLF emissions are primarily generated by artificial sources (e.g., transmitters and PLHR) and natural sources (e.g., lightning). VLF radiation can propagate through the ionosphere and magnetosphere, but it is subject to attenuation. The CSES-01 and CSES-02 satellites are equipped with high-resolution instruments to observe ELF and VLF radiation at an altitude of 507 km. However, the electromagnetic environment of the ionosphere is very complicated, with various EM wave activities and different scales of plasma structures. Thus, before searching for the precursor or the abnormal signals associated with natural hazards, it is necessary to comprehensively understand the regular features of the electromagnetic environment and all the disturbance sources that impact the ionosphere.

6. Conclusions and Outlook

This report briefly introduces the natural and artificial electromagnetic activities in the ELF/VLF frequency band in the ionosphere recorded by CSES-01 at an orbital altitude of 507 km. Based on the observations during the operation of CSES-01, the propagation characteristics and generation mechanisms of some typical EM waves are briefly analyzed by using the SVD algorithm. Those typical and commonly appearing EM wave activities from CSES-01’s observations mainly include the ionospheric hiss and proton whistles in the ELF band (below 1 kHz), the QP emissions, MLR in the ELF/VLF band (below 2.5 kHz), and the falling-tone lightning whistlers and V-shaped streaks in the ELF/VLF band (below 20 kHz). In addition, CSES-01 also captured typical man-made emissions in the ELF/VLF band, such as PLHR and VLF transmitter-emitted VLF radio waves. Through these observations, we have gained a better understanding of the EM environment in the upper ionosphere.

Although some theoretical models and observations from the previous mission have been proposed to explain the generation and propagation mechanisms of these phenomena [6], there are still many aspects that require further exploration and understanding. The resolution of these issues requires more observational data and theoretical research, as well as more advanced measurement and analysis techniques.

Following the successful operation (2005 to 2010) of the DEMETER (Detection of Electromagnetic Emissions Transmitted from Earthquake Regions) satellite, which is the first satellite aimed at studying the ionospheric perturbations associated with seismic, volcanic, and anthropogenic activities [43], China launched the CSES-01 on 2 February 2018, into a synchronous orbit at an altitude of 507 km [18]. Up to now, the CSES-01 has been in orbit for over 6 years, successfully exceeding its designed 5-year lifespan. Around December 2023, we evaluated the status of the satellite platform and payloads and confirmed its still-stable performance. A longer-term operation in the next several years can be expected. The second probe (CSES-02), which is almost an identical successor, will be launched in December 2024 with the same orbit configuration but on the opposite side of the Earth, forming a twin constellation flying on the day and nightside ionosphere synchronously.

A large amount of ionospheric electromagnetic waves have been observed by CSES-01, bringing us a new chance to deeply study their formation mechanisms. Thus, we call on the international scientific community to utilize the observations from CSES mission to deeply explore EM wave activities and geophysical sphere coupling mechanism.