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Satellite observations of Schumann resonances in the Earth's ionosphere



[1] Using electric field measurements gathered on the C/NOFS satellite, we report, Schumann resonance signatures detected in space, well beyond the upper boundary of the resonant cavity formed by the earth's surface and the lower edge of the ionosphere. The resonances are routinely observed in the satellite ELF data during nighttime conditions within the altitude region of 400–850 km sampled by the satellite. They exhibit the distinctive frequency patterns predicted for Schumann resonances and are consistent with the corresponding frequency characteristics of ground-based observations of this phenomenon. The observations of Schumann resonances in space support a leaky cavity interpretation of the ionosphere and call for revisions of models of extremely low frequency wave propagation in the ionosphere. They suggest new remote sensing capabilities for investigating atmospheric electricity on Earth and other planets.

1. Introduction

[2] The Earth can be regarded as a nearly conducting sphere, wrapped in a thin dielectric atmosphere that extends up to the ionosphere, where the conductivity is also substantial. Atmospheric electric discharges generate broadband electromagnetic waves that propagate between the surface and the ionosphere. These two layers define the surface-ionosphere cavity, which supports two types of electromagnetic modes: (i) longitudinal modes corresponding to global, quasi-horizontal wave propagation around the globe and (ii) transverse modes related to local, quasi-vertical propagation between the surface and the ionosphere, as described by Nickolaenko and Hayakawa [2002]. In this work, we discuss the longitudinal modes only. Random lightning strokes with spatial probability distribution peaking over the continents, particularly in the low latitude regions, induce development of standing waves whose wavelength is related to the radius of the cavity. This phenomenon, known as Schumann resonance, develops when the average equatorial circumference is approximately equal to an integral number of wavelengths of the electromagnetic waves.

[3] For a thin, lossless cavity, the Schumann resonance eigenfrequencies, ωn, are approximately given by

equation image

where c is the velocity of light in free space, R is the Earth radius, and n = 1, 2, 3,… is the corresponding eigenmode [Schumann, 1952]. When more elaborate conditions are considered, namely losses in the cavity and variability of the upper boundary due to atmospheric conductivity and ionospheric dynamics, the eigenfrequencies are slightly lower [Balser and Wagner, 1960]. The average frequencies of the five lowest eigenmodes are, approximately, 7.8, 14.3, 20.8, 27.3, and 33.8 Hz, which fall in the Extremely Low Frequency (ELF) range [e.g., Nickolaenko and Hayakawa, 2002]. The corresponding Q-factors are Q ∼ 5 and provide estimates of wave propagation conditions in the cavity. The Q-factor is commonly defined as the ratio of the accumulated field power to the power lost in the oscillation period. The ELF wave attenuation can be computed from the Q-factor of the cavity, which can be approximated by Qnfnfn, where Δfn is the full width at half maximum of peak n.

[4] Schumann resonances have been used to investigate multiple phenomena related to the surface-ionosphere cavity, namely electromagnetic sources, properties of the medium, and boundary conditions. Since lightning is the major source of electromagnetic radiation in the ELF range, Schumann resonances are used to study the daily and seasonal variability of lightning in the cavity [e.g., Balser and Wagner, 1960; Sentman, 1995; Nickolaenko and Hayakawa, 2002] as well as other phenomena such as tropospheric water vapor, aerosol distributions, transient luminous events, and solar flares and geomagnetic storms [e.g., Reid, 1986; Williams, 1992; Boccippio et al., 1995; Price, 2000].

2. Electric Field Measurements of Schumann Resonances Gathered on the C/NOFS Satellite

[5] The Communications/Navigation Outage Forecasting System (C/NOFS) satellite was launched in April 2008 to study the ionospheric conditions that create low latitude irregularities and scintillations [de la Beaujardiere et al., 2004]. C/NOFS was inserted into an elliptical orbit of 401 km perigee, 852 km apogee, and 13° inclination, and includes instrumentation for measuring the electron and ion densities and temperatures, DC electric and magnetic fields, the ion velocity and lightning flash rates, and low frequency electric field waves. C/NOFS is equipped with a vector double probe experiment with three, orthogonal pairs of 20 m tip-to-tip booms as shown in Figure 1 [Pfaff et al., 2010]. This experiment provides vector measurements of both DC and AC (or wave) electric fields. The ELF electric field data reported here are digitized on-board at 1024 samples/s with 16 bit resolution and include a gain of 10. Successive data points are then averaged by 2 within the instrument and treated with a low pass, dual octave Butterworth filter with 3dB frequency near 192 Hz. The resulting 512 sample/s waveforms are then telemetered to the ground, where they may be rotated into different coordinate systems prior to subsequent spectral processing. The sensitivity of the electric field measurements is ∼10 nVm−1Hz−1/2 in the ELF range.

Figure 1.

A sketch of the C/NOFS satellite showing the three orthogonal pairs of 20 m tip-to-tip electric field double probes.

[6] An ELF spectrogram from 0–80 Hz of the total electric field component perpendicular to the magnetic field recorded during orbit 666 gathered on 31 May 2008 is presented in Figure 2. The Schumann resonances are the horizontal lines clearly evident below ∼50 Hz with peaks at about 7.8, 14.0, 20.4, 26.7, and 33.0 Hz, as indicated by the white arrows. The spectrogram in Figure 2 also reveals the rich variety of ELF waves, mostly forms of whistler mode ELF hiss, observed by probes on a satellite experiencing a changing magnetic field and plasma conditions along its orbit [e.g., Smith and Brice, 1964]. Temporally narrow regions of broadband electrostatic irregularities, strongest within the DC/ELF frequency range, can also be seen. In this letter, we focus on the faint, regularly-spaced horizontal spectral emissions indicative of the Schumann resonances.

Figure 2.

Spectrogram of ELF electric field data for one complete orbit of the C/NOFS satellite. See text for details.

[7] Below the spectrogram in Figure 2 is a bar which indicates when the satellite was in local eclipse. The lowest panel shows the satellite's path and altitude. Gray-shaded regions show where the earth's surface was in shadow.

[8] The resonances reported here are typical of those observed during essentially every C/NOFS orbit. They are primarily observed during nighttime conditions, suggesting the resonant wave energy cannot efficiently penetrate the more dense daytime plasma. The resonances generally appear when the satellite is over the night side of the Earth and do not seem to depend on when the satellite itself is precisely in shadow. The resonances observed by C/NOFS (not shown here) reveal somewhat larger amplitudes at the lower altitudes within the orbital confines of the satellite. Some variations with season have also been observed. Investigation of these effects requires a large number of orbits and is currently underway.

[9] Figure 3 provides the average spectra of the entire ELF data ensemble corresponding to the nighttime observations in Figure 2. Definitive peaks are shown for the lowest seven modes. The spectral components are shown separately, along with smooth curves representing the estimated background (upper panel), and then (lower panel) with the background subtracted and Gaussian fits computed (and the square root taken). The values of the peaks derived from these fits correspond quite well to average ground measurements. In addition to the peak frequencies, the fits also provide the Q–factor for each eigenmode that characterizes each resonator mode's center frequency relative to its bandwidth. These Q values are ∼3.5, 4.5, 6.2, 7.7, and 8.2 for the first five modes, as determined from the full width at half maximum analysis of the fits shown in Figure 3.

Figure 3.

Average spectra of the ELF electric fields for the nighttime portion of the data in Figure 2: (top) the complete spectra including the background levels and (bottom) the same data with the background contributions removed and superimposed Gaussian fits of each mode.

[10] No electric field wave component along the magnetic field direction has been observed. The electric field power perpendicular to the magnetic field direction in the meridional component (i.e., the near outward component, at low latitudes) has been observed to be comparable to that in the zonal (i.e., east-west) component, supporting either a whistler mode or extraordinary mode interpretation. Detailed polarization analysis is currently underway.

3. Discussion

[11] The Schumann resonances detected by the electric field probes on the C/NOFS satellite have been identified by their distinctive frequency pattern. This pattern agrees well with that of typical ground-based observations of Schumann resonances [Balser and Wagner, 1960; Sentman, 1995]. Though generally weak in amplitude, such waves are a common phenomena in the C/NOFS electric field data, identified throughout the ∼3 year satellite lifetime. Although sometimes faintly observable during the day, Schumann resonance signatures are generally detected during the nighttime throughout the altitude range covered by C/NOFS, i.e., between 400 and 850 km. Multiple peaks (resonances) are also common in the data set. For example, in orbit 666 we can easily observe the lowest seven peaks (Figure 3). In some cases, at least 10 Schumann resonance peaks have been observed on a single orbit. Averaging data from multiple orbits extends the number of observed peaks even further.

[12] The electric field of the first peak is ∼0.25 μVm−1Hz−1/2, about three orders of magnitude lower than typical ground-based measurements [e.g., Sentman, 1995; Nickolaenko and Hayakawa, 2002]. That the resonances are more difficult to observe during the day on the space-based platform may be due to the fact that the daytime plasma density is an order of magnitude higher than at night, and the ionospheric layer, the roof of the resonant cavity, is lower during the day [Madden and Thompson, 1965]. Indeed, the C/NOFS perigee of 401 km is well above the lowest altitudes of the daytime ionosphere lower ledge (∼90 km). These factors would both contribute to a dampening of the resonant wave energy reaching the satellite. Stratospheric Schumann resonance measurements gathered onboard balloons show a scale height of ∼50 km at 25 km altitude [Ogawa et al., 1979], emphasizing that Schumann resonance signatures indeed decrease with altitude. These authors showed that at 25 km during fair weather conditions, the electric field associated with the Schumann resonance is about half that observed on the ground despite a quite small atmospheric conductivity in the troposphere.

[13] A larger question is why wave energy associated with the Schumann resonances is detected at all in the ionosphere, particularly above the F-peak. As the spherical cavity waveguide would be bounded by conductors on each side, this energy would not be expected to penetrate into the ionosphere, and certainly not to altitudes above 400 km, if the walls were perfectly conducting [Greifinger and Greifinger, 1978; Sentman, 1990]. The Schumann resonance modes, on the other hand, like other low-frequency modes, are able to leak into the ionosphere, particularly at night when the plasma density is lower and a leaky cavity interpretation of the ionosphere is thus deemed necessary to explain the presence of Schumann resonances and their observed peak frequencies and Q values. Anisotropy and other factors are believed to play key roles in proposed leakage mechanisms [Madden and Thompson, 1965; Grimalsky et al., 2005]. Analytical models, although useful for providing perceptive, approximate solutions of wave propagation in the cavity, neglect anisotropy and consequently assume that waves are confined to the cavity, i.e., below ∼100 km [e.g., Greifinger and Greifinger, 1978; Sentman, 1990]. Numerical models include anisotropy and more reliable conductivity profiles but are not fully compatible with C/NOFS findings because they consider wave propagation only up to ∼300 km and significantly overestimate wave damping [e.g., Madden and Thompson, 1965; Grimalsky et al., 2005; Simões et al., 2009]. Nevertheless, it is clear that anisotropy plays a major role in ELF wave propagation in the ionosphere and the geomagnetic field must be taken into account to explain the C/NOFS measurements. Both analytical and numerical models must be improved to embrace these new findings.

[14] Ultimately, the source of energy of the Schumann resonances is tropospheric lightning. Indeed, multi-point ground-based observations of Schumann resonances can be used to locate the lightning sources [e.g., Shvets et al., 2010], even though the resonant wave energy fills the global waveguide. The strength of the Schumann resonances observed in space should depend on the collective strength of the lightning activity in the troposphere. Both seasonal dependencies and correlations with actual lightning indicators have been noted in the Schumann resonance data observed by the C/NOFS satellite and these results will be reported in a subsequent communication.

[15] The absence of previous space-based observations of such waves may be due to the continuous measurements afforded by the C/NOFS satellite, to the long antennae and highly sensitive electric field receiver that returned the broadband time series ELF data, and to the low inclination orbit that permits long observing periods at night at low latitudes. An additional reason may be that the measurements were gathered during an exceptionally low solar minimum, which has created lower than typical plasma density values [Heelis et al., 2009].

[16] The discovery of ELF waves leaking from the Earth surface-ionosphere cavity prompts a new approach to the investigation of ELF waves in the space environments of other planets. Schumann resonances have been proposed to exist in planets with atmospheres, from Venus to Neptune, and even in Saturn's moon, Titan [Simões et al., 2008a]. Moreover, inconclusive measurements have been reported for Mars and Titan [Béghin et al., 2007; Simões et al., 2007; Ruf et al., 2009]. Therefore, the present “leaky cavity” model provides a means of studying the longitudinal modes of planetary cavities from orbit. Remote monitoring can also be combined with in situ measurements gathered by descent probes, buoyant vessels, or landers. The discovery of Schumann resonances in the Earth's ionosphere has important implications for the detection of similar phenomena at other planets and moons with ionospheres, providing a means to remotely infer the permittivities and conductivities of their surfaces, atmospheres, and charged layers (i.e., ionospheres) as well as evidence of lightning activity within their respective surface-ionosphere cavities [Simões et al., 2008b].

4. Conclusions

[17] Using electric field data gathered on the low earth orbiting C/NOFS satellite, Schumann resonances have been detected in the ionosphere. The major findings are as follows:

[18] (i) Identified by their distinctive frequency patterns, Schumann resonances have been routinely detected by a satellite-borne, low frequency electric field wave detector within the low latitude ionosphere in the altitude range of 400–850 km.

[19] (ii) The Schumann resonances are generally only detected during the night, when the plasma density below the satellite is considerably reduced.

[20] (iii) The observed Schumann resonance frequencies and Q-factors detected by the C/NOFS electric field probe are consistent with those characteristics of Schumann resonances typically measured on the ground.

[21] (iv) In some cases, at least 10 Schumann resonance peaks have been observed within data gathered in a single orbit. Averaging data from multiple orbits extends the number of observed peaks even further.

[22] (v) Typical amplitudes of the first peak are ∼0.25 μVm−1Hz−1/2, which is about 3 orders of magnitude lower than observed on the ground.

[23] (vi) Evidence of Schumann resonances has not been found along the geomagnetic field direction. The electric field power perpendicular to the magnetic field direction in the meridional component has been shown to be comparable to that in the zonal component, supporting either a whistler mode or extraordinary mode interpretation for the wave propagation characteristics.

[24] The detection of Schumann resonances in the ionsophere strongly supports the conclusion that the earth's ionosphere might be described as a leaky cavity for ELF radiowave propagation, particularly since the waves are generally observed at night, when the plasma density below the satellite location is considerably lower. The observations thus call for revisions of models of extremely low frequency wave propagation in the ionosphere. Detection of Schumann resonances in space offers new remote sensing capabilities for investigating atmospheric electricity and tropospheric weather effects from orbit, not only on Earth but also within other planetary environments.


[25] The Communication/Navigation Outage Forecast System (C/NOFS) mission, conceived and developed by the US Air Force Research Laboratory, is sponsored and executed by the USAF Space Test Program. We acknowledge support from the Air Force Office of Scientific Research. One of us (FS) acknowledges the NASA Postdoctoral Program that is administered by the Oak Ridge Associated Universities. We thank K. Bromund, C. Liebrecht, and S. Martin for assistance with the data processing.

[26] The Editor thanks Davis Sentman and an anonymous reviewer for their assistance in evaluating this paper.