Cold Plasma Density Research Articles

Electron precipitation coincident with ELF/VLF wave bursts

On 15 October 1999, charged particle and plasma wave detectors on the Polar satellite observed an enhancement in precipitating energetic electrons coincident with an ELF/VLF electromagnetic emission. Assuming that the electromagnetic wave was field aligned and scattered trapped electrons by first‐order cyclotron resonance near the equatorial plane, the resulting pitch angle distribution of precipitating electrons was calculated. For reasonable values of the cold plasma density and the wave intensity at the equatorial plane, the calculated precipitation agrees with measurements for an equatorial interaction region approximately 1000 km in length.

Precipitation of radiation belt electrons induced by obliquely propagating lightning‐generated whistlers

A new combined ray tracing and test particle formulation is presented which calculates the spatiotemporal electron precipitation flux signatures at ionospheric altitudes induced by obliquely propagating lightning‐generated whistler waves. The formulation accounts for the variation in wave characteristics (frequency‐time dispersion, wave normal angle, and power density) as the whistler generated by an individual lightning discharge propagates through vast volumes of the magnetosphere in the absence of field‐aligned cold plasma density enhancements, and calculates the detailed gyroresonance scattering of trapped energetic electrons into the atmospheric loss cone over a broad range of field lines (L shells) to determine the precipitation flux over extensive ionospheric regions. Results show that peak precipitation flux patches (hotspots) several tens of degrees of latitude and longitude in size develop at locations ∼7° to ∼20° poleward of the discharge as a consequence of propagation paths which convey wave energy from lower to higher L shells. For oblique whistler intensities matching satellite observations (10 to 30 pT) peak flux levels of several milli‐ergs cm−2 s−1 are indicated, arriving as early as ∼ 1/4 s and lasting ∼1/2 s at lower observing latitudes (e.g., ∼32° for North American longitudes), while being delayed to ∼l s or more and lasting up to ∼2 s at higher latitudes (∼47°), creating a sense of poleward hotspot motion. Summary profiles integrated over time, latitude and L shell suggest that lightning‐generated oblique whistler‐induced electron precipitation deposits appreciable energy to the upper atmosphere at midlatitudes and contributes significantly to the loss of energetic (>100 keV) radiation belt electrons, particularly over 2.2 ≤ L ≤ 3.5 where the slot region forms.

Dynamics of the plasmasphere and plasmapause under the action of geomagnetic storms

Number density and temperature of cold H + ion fluxes in the plasmasphere measured by the Auroral probe/Alpha 3 experiment during two geomagnetic storms are analyzed. It was found that after the onset of geomagnetic storms the plasmapause both on the night and day sides starts moving toward the Earth almost simultaneously. The delay of the beginning of plasmapause movement on the day side relative to the night side is at least much less than the corotation time of magnetic tube with cold plasma from the night sector into the day sector of the plasmasphere. It was found that the number density of cold plasma inside the daytime plasmasphere could significantly change, either decrease or increase, during moderate geomagnetic storms. Now we cannot definitely determine the reason for these variations, but we assume they are caused by variations of parameters (e.g., n max F 2, h max F 2 ) in the underlying ionosphere initiated by geomagnetic storms.

Cyclotron Wave-Particle Interactions in the Whistler-Mode Waveguide

We study the cyclotron interaction of energetic electrons and whistler waves in plasma waveguides formed by inhomogeneous distribution of cold plasma. Such waveguides can be formed in the Earth's magnetosphere, e.g., by the plasmapause or by ducts with enhanced background-plasma density. In this paper, we consider a cylindrically symmetric model of a magnetospheric duct with enhanced cold-plasma density in a homogeneous magnetic field. The spatial structure of the eigenmodes of such a waveguide is found. We obtain a set of self-consistent quasilinear equations for cyclotron instability with eigenmode structure taken into account, thus generalizing the quasilinear theory of a magnetospheric cyclotron maser.

A technique to investigate plasma mass density in the topside ionosphere using ULF waves

The relatively cold core plasma density in the region where the ionosphere mixes with the plasmasphere is difficult to probe experimentally. It is shown that resonances of ULF magnetohydrodynamic waves can provide unique information on plasma mass density in the region where the ionosphere and plasmasphere merge. The technique depends on the detection of harmonics of ULF field line resonances and allows the determination of plasma mass densities at discrete locations along a magnetic field line without assuming any function for plasma distribution along the field. The plasma mass densities obtained from ULF harmonics at L=1.8 are compared with several plasma mass density models of this region. In addition to describing and illustrating the technique, the method is critically examined for its limitations. These include the finite number of ULF harmonics that can be detected, the complexity of the nonlinear solving scheme, the consequences of using the method at larger L values, and approximating the system of equations with a finite difference scheme. It is shown that the technique can provide independent estimates of plasma mass densities at discrete, magnetic field‐aligned altitudes, which gives an additional experimental check on theoretical models that predict the dynamics of plasma from the topside ionosphere to the inner plasmasphere.

A generation mechanism for chorus emission

Abstract. A chorus generation mechanism is discussed, which is based on interrelation of ELF/VLF noise-like and discrete emissions under the cyclotron wave-particle interactions. A natural ELF/VLF noise radiation is excited by the cyclotron instability mechanism in ducts with enhanced cold plasma density or at the plasmapause. This process is accompanied by a step-like deformation of the energetic electron distribution function in the velocity space, which is situated at the boundary between resonant and nonresonant particles. The step leads to the strong phase correlation of interacting particles and waves and to a new backward wave oscillator (BWO) regime of wave generation, when an absolute cyclotron instability arises at the central cross section of the geomagnetic trap, in the form of a succession of discrete signals with growing frequency inside each element. The dynamical spectrum of a separate element is formed similar to triggered ELF/VLF emission, when the strong wavelet starts from the equatorial plane. The comparison is given of the model developed using some satellite and ground-based data. In particular, the appearance of separate groups of chorus signals with a duration 2-10 s can be connected with the preliminary stage of the step formation. BWO regime gives a succession period smaller than the bounce period of energetic electrons between the magnetic mirrors and can explain the observed intervals between chorus elements.Key words. Magnetospheric physics (Energetic particles · trapped). Space plasma physics (wave-particle interactions; waves and instabilities)

Observational test of local proton cyclotron instability in the Earth's magnetosphere

We present a study of the proton cyclotron instability in the Earth's outer magnetosphere, L > 7, using Active Magnetosphere Particle Tracer Explorers/Charge Composition Explorer (AMPTE/CCE) magnetic field, ion, and plasma wave data. The analysis addresses the energy of protons that generate the waves, the ability of linear theory to predict both instability and stability, comparison of the predicted wave properties with the observed wave polarization and frequency, and the temperature anisotropy/parallel beta relation. The data were obtained during 24 intervals of electromagnetic ion cyclotron (EMIC) wave activity (active) and 24 intervals from orbits without EMIC waves (quiet). This is the same set of events used by Anderson and Fuselier [1994]. The active events are drawn from noon and dawn local times for which the wave properties are significantly different. For instability analysis, magnetospheric hot proton distributions required the use of multiple populations to analytically represent the data. Cyclotron waves are expected to limit the proton temperature anisotropy, Ap = T⊥p/T‖p − 1, according to Ap < aβ‖pc with a ∼ 1 and c ∼ 0.5, where T⊥p, T‖p, and β‖p are the perpendicular and parallel proton temperatures and the proton parallel beta, respectively. During cyclotron wave events, Ap should be close to aβ‖pc whereas in the absence of waves Ap should be below aβ‖pc. The active dawn cases yielded instability in 9 of 12 cases using the measured plasma data with an average growth rate γ/Ωp = 0.025 and followed the relation Ap = 0.85β‖p−0.52. The active noon events gave instability in 10 of 12 cases, but only when an additional ∼2 cm−3 cold plasma was assumed. The noon wave events fell well below the dawn events in Ap‐β‖p space, slightly above the Ap = 0.2β‖p−0.5 curve. The lower Ap limit for the noon cases is attributed to the presence of unmeasured cold plasma. The quiet events were all stable even for additional assumed cold ion densities of up to 10 cm−3, the upper limit implied by the plasma wave data. The quiet events gave Ap < 0.2β‖p−0.5. At noon, the unstable component has T⊥p ∼ 20 keV and Ap ∼0.8. At dawn the unstable component has T⊥p ∼ 4 keV and Ap ∼ 2.3. Observed wave frequencies agree with the frequencies of positive growth, and the difference in frequency between noon and dawn is attributable to the combined effects of the different hot proton T⊥p and Ap and the inferred higher cold plasma density at noon. The dawn events had significant growth for highly oblique waves, suggesting that the linear polarization of the dawn waves may be due to domination of the wave spectrum by waves generated with oblique wave vectors.

Ponderomotive wave forces in longitudinal MHD waveguides

By using a general expression for the ponderomotive wave force, we have evaluated the action of ion cyclotron Alfvén waves on the plasma density in a longitudinal waveguide. The main effect is the condensation of the plasma in the vicinity of the waveguide equator. According to observations onboard the GEOS 1 and Viking satellites, the amplitudes of such geomagnetic pulsations can reach up to several nanoteslas. This is sufficient to cause a strong modification of the cold plasma density in the magnetosphere under the action of ion cyclotron Alfvén pulsations. Furthermore, we have also speculated on the possible effects of the ponderomotive wave force on the problems connected with the so called “wave‐induced substorms” in the neutral sheet of the geomagnetic tail as well as with the phenomenon of “ponderomotive plasma filling,” which determines the relaxation of the plasmasphere after the geomagnetic perturbations.

Simultaneous measurements of waves and precipitating electrons near the equator in the outer radiation belt

An investigation of wave‐particle interactions is made using several simultaneous electron and wave measurements performed at near‐equatorial positions from the CRRES satellite. Bursts of electron precipitation were observed, most frequently at local times near dawn. Examples of bursts are presented in which the fluxes of the precipitating electrons and the wave intensities are correlated with coefficients as high as 0.7. During bursts the frequencies of the enhanced waves spanned a wide range from 311 Hz to 3.11 kHz, and the energies of the enhanced electrons were in the range 1.7 keV to 288 keV. The changes of the precipitating fluxes were generally less pronounced at the lowest energies. On the basis of electron‐cyclotron resonant calculations using the cold plasma densities and ambient magnetic fields taken from the CRRES measurements it was found that the wave frequencies and precipitating electron energies were generally consistent with those expected from electron resonance with parallel propagating whistler waves. The electron data of principal concern here were acquired in and about the loss cone with narrow angular resolution spectrometers covering the energy range 340 eV to 5 MeV. The wave data included electric field measurements spanning frequencies from 5 Hz to 400 kHz and magnetic field measurements from 5 Hz to 10 kHz.

Midlatitude detection of ELF whistlers

Narrow‐band, whistlerlike magnetic events distinguished by nearly monochromatic signals decreasing in frequency with time have been observed for the first time at midlatitudes in the ELF band. Measurements performed during September 3 to October 5, 1985 at Table Mountain, California (34.4°N, 117.7°W), show that the frequency and dispersion characteristics of these events are similar to events detected at auroral latitudes (Heacock, 1974), including a narrow‐band magnetic signal monotonically decreasing in frequency from 120 to 60 Hz over a 40 s interval with a mean center frequency of approximately 90 Hz. No echoes were observed. Maximum amplitudes of the magnetic signals ranged from just above the approximately 1 pT Hz−1/2 floor of the ambient background to roughly 20 pT Hz−1/2. The polarization was predominantly linear in the geographic east‐west direction. The midlatitude ELF whistlers reported here have a significantly lower average daily rate of occurrence than those reported for auroral latitudes. However, as with the high‐latitude events, they displayed an occurrence rate that is maximum during local daytime. Following Heacock (1974), it is suggested that a possible source for these events is whistler mode lion roars occurring in field‐aligned ducts of enhanced cold plasma densities in the distant magnetosheath. For favorable configurations of these ducts extending from the magnetosheath into the polar cusp, the waves may propagate to the Earth through the cusp acting as a waveguide. Although lightning is usually considered to be the dominant source of ELF noise in the Earth ionosphere cavity, magnetosheath ELF noise coupled into the cavity at high latitudes may represent an additional source. The fractional intensities of the natural ELF noise power within the cavity that are generated by this mechanism are presently unknown. However, a determination of these fractional intensities would be useful in the interpretation of the Schumann resonances as a measure of the totality of global lightning, particularly in the case of measurements made at high latitudes in the vicinity of the cusp‐ionosphere coupling region. Similarly, high‐latitude detection of very weak, artificially generated ELF signals, such as by HF heating of the polar electrojet, may be affected by the proximity of such a local ionospheric ELF noise source.

Electromagnetic ion cyclotron wave amplification and source regions in the magnetosphere

The amplification of electromagnetic ion cyclotron (EMIC) waves in the magnetosphere is studied by integrating local convective growth rates along the wave path. The integrated wave amplification is used to investigate the radial extent of wave source regions. Typical heavy ion concentrations (He+ ∼10‐20%, O+ ∼1‐10%) are considered in the calculation. In contrast to previous studies, the results presented indicate that the radial structure of the integrated wave amplification is controlled by the ion composition in the magnetosphere, as well as by the cold (or thermal) plasma density profile. It is found that the source region of the lowest‐frequency wave branch (i.e., the O+ wave branch here) is confined to the plasmapause and inside the plasmasphere, whereas the other wave branches (i.e., the He+ and H+ wave branches here) can be amplified over most of the magnetosphere. It is suggested that the structured Pc 1‐2 waves observed mainly on the ground are due to the lowest‐frequency wave branch, and the unstructured Pc 1‐2 waves observed both on the ground and in space result from the other wave branches.

Seasonal variations of energetic electron precipitation by cyclotron resonance with VLF waves from a ground‐based transmitter

We report here for the first time the detection of a seasonal variation in the occurrence of peaks in the spectra of energetic ∼ 100‐keV electrons precipitating from the inner radiation belt as the result of cyclotron resonance between the waves and the electrons. The spectra reported here were observed from the low‐altitude polar orbiting satellite P78‐1 at midnight local time. The data, acquired over the 5‐year period from March 1979 to July 1984, compose the first multiyear data set used to study cyclotron resonance peaks. The geographical distribution of the events is strongly clustered around the 22.3‐kHz VLF transmitter NWC in Australia. Binning the data into seasons, we find that the events occur 2.5 times more often during the 6 months from day 80 to 263 than during the remainder of the year and that the seasonal difference is statistically significant at a high level. Since the observed change is in the frequency of occurrence of the peaks and not in the mean energy of the peaks, we attribute the effect to a seasonal change in ionospheric transmission of VLF waves rather than a seasonal change in the equatorial cold plasma density.

Probe measurements of low plasma densities with applications to the auroral acceleration region and auroral kilometric radiation sources

When the Viking satellite crossed the auroral acceleration region, where the electron density is supposed to be less than a few particles per cubic centimeter, a strong spin modulation of the signal of a positively biased Langmuir probe was observed. This spin modulation is better correlated with the angle of the antenna to the geomagnetic field than with the angle to the Sun. A simple model based on the photoelectron orbits in a spherically symmetric electric field, in the presence of a uniform magnetic field, gives the right phase and amplitude of the extrema. In addition, it shows that the perturbation could be negligible when the antenna is perpendicular to the magnetic field, provided that the photoelectron sources are far enough from the magnetic field lines connected to the probe (about 2 Larmor radii). As a consequence, we deduce important information for the design of experiments devoted to very low density measurements, and we show that the magnetic field may be a crucial parameter for the study of the exchange of photoelectrons between parts of a satellite. The most important result of this study is to facilitate the probe density measurements inside the acceleration structures. It is shown that in these regions the cold plasma density is depleted, with a density of thermal electrons below 5 cm−3 at altitudes between 6500 km and 8500 km. When auroral kilometric radiation (AKR) is generated inside such structures, the density of thermal electrons can be below 1 cm−3. The edges of the structures are sharply defined, and the gradient scale can be as low as 1.4 km. This is the first time that density measurements of thermal electrons have been realized inside clearly identified AKR sources, with appropriate resolution.

Effects of a lightning discharge detected by the DE 2 satellite over Hurricane Debbie

We report the satellite observation of a large, ∼40 mV m−1, transient electric field disturbance over Hurricane Debbie in September 1982. The event lasted less than a second and correlated closely with a burst of highly field‐aligned, upward moving electrons with nearly 1 keV of energy. The electric field event is viewed as a spheric disturbance from a lightning discharge in the active weather system located beneath the satellite. A spheric interpretation of the observed electric field transient is consistent with a subsequent observation of energetic electrons precipitating from the radiation belts. Measured quasi‐dc electric fields and cold plasma density variations are only roughly consistent with model predictions for ULF wave propagation from a storm system to the ionosphere. To understand this first observation of upward moving electrons in the ionosphere associated with a lightning event, we compare the several mechanisms for electron acceleration by electric fields with components (E∥) along the magnetic field. In our scenario, “runaway” electrons were accelerated in ∼1 ms by a downward directed E∥ pulse of ∼1 V m−1 magnitude. Such fields can result from rapidly exposed, negative space charges near the tops of clouds during positive cloud‐to‐ground discharges. High‐frequency Fourier components of the E∥ pulse must propagate through the low‐conducting nighttime atmosphere to the ionosphere with little dissipation.

A statistical study of Pc 1–2 magnetic pulsations in the equatorial magnetosphere: 2. Wave properties

A statistical study of narrowband transverse 0.1‐ to 4.0‐Hz magnetic pulsations, essentially Pc 1–2 (0.1–5.0 Hz), occurring from L=3.5 to L=9, |MLAT| < 16°, and all local times, has been made using data from the AMPTE CCE satellite. This work is reported in a pair of papers of which this is the second. In the first paper (Anderson et al., this issue) the occurrence distributions of Pc 1–2 were reported, and here we present statistical distributions of normalized frequency, X = F/FH+ (F and FH+ are the wave frequency and proton gyrofrequency, respectively); ellipticity, ε, and spectral power. Events occurring from 1000 to 1800 MLT are left‐hand to linearly polarized and exhibited properties corresponding to previous reports. The normalized frequency was found to depend systematically on MLT and L, decreasing from 0.5–0.45 at 1000 MLT to 0.30 at 1800 MLT and increasing from 0.28 at L = 3–5 to 0.39 at L = 8–9 (for 12–15 MLT). The average X dependence on MLT and L can be accounted for by several mechanisms including variations in cold or hot plasma density, He+ concentration, and hot proton parameters T⊥ and T⊥/T∥. A decrease (increase) in T⊥/T∥ with MLT (L) is shown to be necessary, and increases (decreases) in either T⊥ or plasma densities with MLT (L) are also required to reproduce the observed X variation. The role of He+ in causing the X variations is unclear. Events occurring at dawn, 0300–0900 MLT, and L > 7 have not previously been discussed and are found to exhibit remarkable polarization behavior. The A.M. events have the highest X observed in the data base, with X averaging 0.4 to 0.5, but most remarkably, they are linearly polarized at all magnetic latitudes sampled. Given the high normalized frequency, the equatorial linear polarization of the A.M. events cannot be explained in terms of a crossover from left‐ to right‐hand polarization occurring during propagation from low to high magnetic field strengths. Oblique propagation or the effects of multiple reflection through the wave growth region might lead to linear polarizations. The results suggest that a new examination of EMIC wave generation specifically addressing the properties of this A.M. population may be needed.

A statistical study of Pc 1–2 magnetic pulsations in the equatorial magnetosphere: 1. Equatorial occurrence distributions

A study of AMPTE CCE magnetic field data covering the frequency range 0.1–4.0 Hz using all data obtained during the first complete local time precession of the satellite orbit major axis (7500 hours of observations) has been made to evaluate the occurrence of transverse, narrowband Pc 1–2 emissions, identified as electromagnetic ion cyclotron (EMIC) waves, in the equatorial magnetosphere from L = 3.5 to L = 9 at all local times. We report this work in a pair of papers of which this is the first. A set of example events illustrates the pattern of Pc 1–2 occurrence: events occur primarily for L > 7, and a radial separation of several RE between low (<6) and high L (>6) pulsations is observed. Statistically, the highest concentration of events occurred near apogee in the afternoon. The L > 6 and L < 6 event populations appear to be radially separated in the morning but merge together in the afternoon. We construct a normalized occurrence distribution throughout the equatorial plane from L = 3.5 to L = 9 of Pc 1–2 with peak to peak amplitudes greater than 0.8 nT. The occurrence distribution exhibits a number of properties: for L > 7, Pc 1–2 occur at any given place in the early afternoon (1200–1500 MLT) with 10–20% probability and in the morning (0300–0900 MLT) with ≈3% probability; the L = 6–7 local time distribution reproduces results obtained previously from data at geostationary orbit; L < 5 events occur with a probability of ≤1% and a relatively uniform local time distribution. The predominance of L > 7 events implies that plasma sheet ions develop sufficient temperature anisotropy to generate EMIC waves on a routine basis in their drift from the nightside to the dayside and that plasma sheet ions on open drift paths rather than ring current ions on closed drift paths present the greatest source of equatorially generated EMIC waves. Consideration of the maximum convective growth rate of EMIC waves in a pure proton plasma versus cold plasma density and magnetic field strength illustrates how the predominance of L > 7 events and radial gap structure can be understood in terms of radial profiles of linear convective growth rates.

Detailed analysis of broadband electrostatic noise in the dayside auroral zone

The great number of bursts of broadband electrostatic noise (BEN) recorded during crossings of the dayside auroral zone by the Viking satellite enables their statistical study. The angular distribution of BEN with respect to the Earth's magnetic field is shown to be most likely isotropic, implying that it cannot consist of a unique linear plasma mode. Most of the bursts evidence a power law spectrum from the lower hybrid and ion plasma frequencies up to frequencies sometimes much higher than the electron plasma frequency, suggesting the presence of nonlinear effects. This is confirmed by their high intensity, and by the correlation between their amplitude and their frequency extension. BEN emissions are associated with ion conical distributions and with field‐aligned electron beams. Although most of the power is concentrated at very low frequencies and around the lower hybrid and ion plasma frequencies, electron acoustic and beam mode waves may contribute to the high‐frequency extension of BEN. The most intense BEN emissions are also correlated with sharp cold plasma density gradients and probably involve drift instabilities. Strong quasi‐static perpendicular electric fields, which induce high‐speed plasma flows, are also measured, so that the Doppler effect may contribute to the broadening of the BEN spectrum.

Energy source and generation mechanism for auroral kilometric radiation

The orbit of the Swedish satellite Viking was optimized for observations of plasma phenomena in the auroral acceleration regions, including auroral kilometric radiation. The auroral kilometric radiation (AKR) is the strongest electromagnetic radiation produced on auroral field lines, and the Viking satellite passed through the source region of this radiation on many orbits, as evidenced by the very strong signals observed at the electron cyclotron frequency. For the first time, electron distribution functions were observed inside the AKR source region and immediately adjacent to the source region. The particle data show the presence of energetic electrons in and around the source region and also of upgoing ion beams of several keV energy in and only in the source region. Thus the particle data show clear signatures of parallel electric potentials both above and below the AKR source. The effect of a parallel potential below the source is to decrease the density of low‐energy electrons and expand the upward loss cone, particularly at small parallel velocities. The reduction in the cold plasma density is clearly observed. Adjacent to the AKR source a clear loss cone is observed in the upgoing electron population. However, inside the source region the loss cone is filled or nearly filled. Our observations are in agreement with the cyclotron maser instability theory including the effect of a parallel electric field as discussed by Wu et al. (1982) and Pritchett and Strangeway (1985).

First measurements of plasma waves near Mars

HERE we report preliminary results from electic field measurements in the environment of Mars using the plasma-wave system on board the Soviet spacecraft Phobos 2. It also includes a Lang-muir probe which measured plasma densities. Electron-plasma oscillations observed upstream of the bow shock correspond to a solar-wind density of 2 cm-3. The shock-foot boundary was crossed up to three times on each orbit. The shock ramp was detected at altitudes between 0.45 and 0.75 Mars radii (RM) above the planetary surface. The density increased by about a factor of two at the ramp. The shock position, although variable, seemed to be consistent with previous measurements. The downstream mag-netosheath contained broadband electric-field noise below the plasma frequency. The boundary of the obstacle, or planetopause, was crossed at altitudes of the order of 0.28 RM; the cold plasma density was highly variable within the planetopause and reached the unexpected value of 700 cm-3 on the third orbit, at 0.25 Rm altitude. Bursts of waves with frequencies below the electron cyclotron frequency, possibly in the whistler mode, occur within the planetopause.

Theoretical model of Saturn's kilometric radiation spectrum

A theoretical model is developed here in order to determine an envelope for the average spectrum of the Saturnian kilometric radiation (SKR). The microscopic generation mechanism is supposed to be the so‐called synchrotron (or cyclotron) maser instability. As in recent works on the terrestrial kilometric radiation, the effect of the magnetic field inhomogeneity on the generation process must be taken into account. Then, assuming that the emission is nonlinearly saturated by trapping, our calculation allows us to put an upper limit on the SKR spectral intensity very simply: the maximum level of the wave electric field within the source region is indeed linked to a few macroscopic plasma parameters, which can be derived from the observations (the structure of the magnetospheric magnetic field, the cold plasma density, and the characteristic energy of the hot emitting electrons). We have used a dipolar magnetic field model, while the plasma distribution results from the superposition of two components: an ionospheric population and a plasma disc, whose scale heights have been roughly determined from Voyager measurements. The energetic electrons responsible for the emission are supposed to precipitate along the high‐latitude magnetic field lines where SKR emission is known to take place. The width of the source region can be self‐consistently estimated from the model. A very good agreement is obtained between the theoretical spectrum and the observational radio data. The calculated spectral intensities exceed the most intense observed intensities by up to 1 order of magnitude, suggesting that the SKR emission is only marginally saturated by nonlinear processes.