VLF wave-injection experiments from Siple Station, Antarctica

Geophysical investigation of some aeromagnetic lineaments in North Finland

Energetic (>6 keV) electron data from the SEEP payload on the low altitude (∼200 km) polar orbiting S81‐1 satellite indicate a high rate of occurrence of short duration (<0.6 s) electron precipitation bursts. Characteristics of events observed at night (2230 MLT) versus daytime (1030 MLT) and at midlatitudes (2<L<3) versus higher latitudes (L>3) were distinctly different in several ways. For 2<L<3 the daytime bursts occurred approximately uniformly in longitude and were equally distributed between the northern and southern hemispheres. The nighttime bursts in the same L shell range occurred approximately twice as often on a worldwide basis and were observed predominantly in the northern hemisphere and at longitudes of 260°E to 320°E. In a significant number of the nighttime events at 2<L<3 the median electron energy increased with time during the burst, but most of the other spectra showed no well‐defined trend. During some of the nighttime bursts broad peaks were observed in the energy spectra, but these peaks were not so evident in the daytime bursts. On higher L shells, L>3, narrow electron precipitation bursts (<0.3 s duration) were frequently observed poleward of the plasmapause, more often near noon than near midnight and much more frequently than at 2<L<3. Comparison of the electron data with simultaneous VLF wave data from Palmer (L≃2.4) and Siple (L≃4.3) stations in Antarctica indicated a varying degree of association of electron bursts with whistlers and chorus emissions. Several of the electron bursts observed at nighttime and at 2<L<3 were correlated with lightning‐generated whistlers observed at Palmer Station. When daytime bursts at higher latitudes (L>3) were observed on satellite passes within ±50° of the Siple meridian, chorus was invariably detected at Siple, but correlation of electron bursts with individual chorus spectral elements was not evident. The lack of such correlation may be due to the limited spatial extent of flux tubes excited by individual chorus elements which are possibly generated without mutual coherence in multiple high altitude magnetospheric locations. Within one hour of all four of the satellite passes within 400 km of Siple with the highest rate (≥10 individual bursts per pass) of burst occurrence, overhead electron precipitation was clearly detected by ground based sensors at Siple Station. Quasi‐periodic bursts of several seconds duration were observed on one or more of the photometer, riometer, and magnetic pulsation sensors and many of these bursts were correlated with clustered VLF chorus bursts. Hence, there is at least association of chorus with electron precipitation.

A combination of simulated VLF noise, 200 Hz wide, superimposed on a variable amplitude constant frequency test signal (≈ 3 kHz) is transmitted from Siple Station, Antarctica, and received at Lake Mistissini, Quebec. As the test signal is slowly ramped up in power a “threshold” level is reached at which growth and triggering of emissions begin (coherent wave instability). Results show that sufficiently strong simulated noise surpresses the coherent wave instability, which corresponds to increasing the threshold level. This experiment is repeated at progressively lower levels of the simulated noise, until the threshold level for growth and triggering on the test signal no longer changes. At this point the simulated noise power is estimated to equal typical background noise levels due to magnetospheric hiss in the interaction region (near the equatorial plane in a duct at L ≈ 4). These results suggest that unducted magnetospheric hiss is responsible for the threshold effect.

Growth of electrostatic waves around the lower hybrid resonance (LHR) frequency has been discussed. Expressions for growth rate of the excited electrostatic wave around the LHR frequency have been derived and evaluated for ionospheric plasma parameters. The results are in good agreement with the experimental observations reported by earlier workers.

During 1986, a series of special VLF transmissions at ∼3 kHz and ∼5 kHz were made from the crossed dipole antenna at Siple Station, Antarctica, which simulated transmissions from a single horizontal dipole at a number of different orientations. The subionospheric signals thus excited were recorded at four Antarctic stations: Faraday, Halley, South Pole and Arrival Heights (McMurdo Sound). The signals excited broadside to the dipole were seen to exhibit characteristics notably different from those of signals excited along the axis of the antenna, showing a minimum in received power (the depth of the minimum decreasing for the more highly attenuating paths) and increases in apparent arrival azimuth error and elevation angle. A simple computer model for mode propagation close to 5 kHz in the Antarctic Earth‐ionosphere waveguide showed that these variations were the consequence of two effects: the preferential excitation of quasi‐transverse magnetic (QTM) modes along the axis of the antenna and the lower attenuation of quasi‐transverse electric (QTE) modes (with respect to QTM modes) over the Antarctic ice sheet. These results have important implications for studies involving the subionospheric propagation of signals with frequencies at the lower end of the VLF band and over highly attenuating surfaces, showing that QTE modes play a significant and sometimes dominant role. The propagation characteristics derived from the model may be applied to studies of the effect of burst energetic electron precipitation on subionospheric VLF signals (the Trimpi effect). Furthermore, the mode structure of subionospheric VLF signals radiated from the Siple transmitter, or a similar facility in Antarctica, may be controlled through the appropriate choice of signal frequency and antenna arrangement.

The parametric excitation of the ordinary mode and the extraordinary mode propagating perpendicular to the magnetostatic field is investigated for the case of a cold uniform plasma. The pump wave is assumed to be another ordinary mode having its electric field oriented parallel to the magnetostatic field and its frequency exceeding the plasma frequency of the electrons. The extraordinary mode has three branches, namely, a high-frequency branch, an intermediate-frequency branch having a resonance at the upper hybrid resonant frequency, and a low-frequency branch which in the regime of hydromagnetic frequencies is the Alfvén wave and which also has a resonance at the lower hybrid resonant frequency. The ordinary mode does not parametrically couple with the high-frequency branch of the extraordinary mode but such a coupling takes place with the other two branches. This parametric coupling leads to instabilities of the extraordinary mode. Illustrative numerical results are presented for the parametric instabilities of the extraordinary mode near the upper hybrid resonant frequency, the lower hybrid resonant frequency, and the Alfvén wave frequency. Compared to the intermediate-frequency branch, the requirements for the parametric interaction are found to be met easily for the low-frequency branch. The dependence of the maximum growth rates of the parametric instabilities on the strength of the magnetostatic field is also examined.

Equatorial noise in the frequency range below the lower hybrid resonance frequency, whose structure is shaped by high proton cyclotron harmonics, has been observed by the Cluster spacecraft. We develop a model of this wave phenomenon which assumes (as, in general, has been suggested long ago) that the observed spectrum is excited due to loss cone instability of energetic ions in the equatorial region of the magnetosphere. The wavefield is represented as a sum of constant frequency wave packets which cross a number of cyclotron resonances while propagating in a highly oblique mode along quite specific trajectories. The growth (damping) rate of these wave packets varies both in sign and magnitude along the raypath, making the wave net amplification, but not the growth rate, the main characteristic of the wave generation process. The growth rates and the wave amplitudes along the ray paths, determined by the equations of geometrical optics, have been calculated for a 3‐D set of wave packets with various frequencies, initial L shells, and initial wave normal angles at the equator. It is shown that the dynamical spectrum resulting from the proposed model qualitatively matches observations.

Three sounding rocket measurements of energetic electron precipitation in the 65 to 80 km altitude range above Siple Station, Antarctica suggest a possible diurnal modulation of the precipitation. Flights made on January 10 and 11, 1981, to investigate this possibility showed that no electrons were present after local noon, while a significant flux of penetrating electrons was observed the following day near local dawn, even though the level of magnetic activity was higher during the postnoon flight. A previous flight from Siple Station also observed a large flux of penetrating electrons near local dawn. Calculations of electron drift trajectories in a model magnetic field indicate that moderate values of the convection electric field could cause significant local time variations in the penetrating electron flux at the western edge of the South Atlantic anomaly. These calculations indicate that Siple Station is shadowed by eastern Siberia when it is at local noon, but not when it is at local dawn.

Waves near the lower hybrid resonance frequency are electrostatically generated with plates just outside a cylindrical plasma column.These waves penetrate radially and, above a threshold power, are observed to undergo a parametric decay into two lower frequency waves.

Radial density profiles are measured with various frequencies, powers, pressures, and gas masses in m=0 helicon wave plasmas. The radial density profiles show clear mode changes for the E mode (capacitive coupling), the H mode (inductive coupling), and the W mode (wave coupling) as the rf (radio frequency) power increases. The radial density profile hardly changes as the frequency varies in the E and the H modes but changes substantially in the W mode; it changes from a central peak profile to a hollow density profile near the lower hybrid resonance frequency. The change of radial density profile near the lower hybrid resonance frequency occurs only at pressures below 20 mTorr, and the resonance effects become very weak at higher pressures. The change of the radial density profile may be explained by the density enhancement near the lower hybrid resonance layer. The experiments with various magnetic fields and gas masses show a close correlation between the lower hybrid frequencies and the density profile changes.

Using a one-dimensional electromagnetic particle code which simulates self-consistently the full ion and electron dynamics, we study instabilities driven by energetic ions injected continuously in a plasma where the density and the lower-hybrid resonance frequency ωLH increase with time. The simulation shows that the ion cyclotron harmonic wave with ω ≃ lΩi, where l is an integer and Ωi is the ion cyclotron frequency, is excited when ωLH becomes close to lΩi. When ωLH is greater than lΩi, this wave couples with the ion Bernstein mode that has the dispersion curve connecting to ωLH. It is also found that as a result of the instabilities and the wave-wave coupling, the stair-like frequency chirping with the riser Ωi appears in the magnetic fluctuations in the frequency range of ωLH. The frequency chirping has characteristics similar to the frequency chirping observed in the RF radiations at the plasma start-up phase of the LHD experiments.

The derivation and the accuracy of an expression relating the fractional concentration of H+, α1 = n(H+)/ne, to the crossover frequency ω12 for a VLF proton whistler are discussed. To an accuracy of ±3½% it is found that α1 = (264/255) (1 - Λ122), where Λ12 = ω12/Ω1 and Ω1 is the proton gyrofrequency. Values of α1 have been deduced from measurement of proton whistler spectrograms for some of the satellite Injun 3 VLF data and plotted against altitude (400–2600 km) and against invariant latitude (20°–63°) for summer daytime and winter nighttime of 1963. The fractional concentration of H+ is found to be higher for winter nighttime than for summer daytime at all altitudes and latitudes; the ratio was approximately 3:1 at 1000 km. At 1000 km the value of α1 dropped from 0.43 in the 20°–30° invariant latitude range to 0.27 in the 40°–45° range for summer daytime. For winter nighttime α1 was nearly constant at 0.82 for 1000 km from 30° to 50° latitude, but dropped to 0.75 in the 50°–63° latitude range. Near 2400 km for summer daytime, α1 drops from 0.65 at 46° latitude to 0.20 at 56° latitude. This same tendency is observed for winter nighttime, apparently being due to auroral-zone heating. These observations are consistent with the assumption that the heavier ions tend to predominate with increasing latitude for a given altitude. Comparison of these VLF proton whistler results for α1 is made with reasonably good agreement to rocket ion mass spectrometer results of NASA 8.23 and to Alouette 1 VLF results deduced from lower hybrid resonance frequency. This general agreement establishes the VLF radio technique as an independent method for determining ion concentrations in the ionosphere. The general equations for uniquely determining the concentration of O+, He+, and H+ from observation of the critical frequencies (crossovers, hybrid resonances, and cutoffs) due to the presence of ions are presented, and their usage is discussed.

Some factors are considered which contribute to asymmetry between the premidnight and postmidnight sectors of the plasma both in the equatorial plane and at low altitudes. In particular the greater probability of occurrence of extended auroral arcs in the premidnight sector is noted. The formation of these quiet arcs, their relationship to ‘inverted V’ structures and their effect on energetic particle trapping are considered. It is concluded that the energetic electron ‘trapping boundary’ is not necessarily the limit of closed field lines but instead is caused by wave turbulence in the plasma sheet at the most equatorward of the extended arcs. Various implications of this conclusion for plasma sheet structure are discussed.KeywordsPitch AnglePlasma SheetAuroral OvalEvening SectorPitch Angle DistributionThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Doppler‐shifted cyclotron resonance between electrons and narrowband whistler mode waves is employed in a new feedback model to account for observed signal growth and emission triggering in controlled experiments using the Siple Station, Antarctica, VLF transmitter. The interaction region (IR) is centered on the magnetic equator and is treated like an unstable feedback amplifier with a delay line. The temporal growth rate is given by γ = (G−1)/T, where G is the loop gain and T is the effective loop delay. For G<1 the system acts like an amplifier, while for G>1 the system is unstable and can generate self‐excited oscillations. The self‐excited oscillations reach saturation when G falls to unity, which occurs when the electron transit time through the interaction region is comparable with the phase bunching time. At signal levels well below saturation the model predicts exponential temporal growth. Saturation fields just inside and outside the plasmapause are estimated to be 1 mγ and 12 mγ, respectively, consistent with measurements. Because of a progressive loop phase shift the stimulated radiation can cancel the input signal at a certain point (located near the input to the IR) in time and space, giving rise to transient conditions resembling those at the tail‐end of the triggering signal. It is postulated that this temporary null may be the cause of emission triggering, and hence the region around this point is called the ‘triggering window.’

VLF wave growth and emission triggering in the magnetosphere are known to depend on the coherence of the input signal. In these experiments the input signal may take the form of a constant frequency pulse (1 s duration) or a linearly chirped pulse (also 1 s duration) with a frequency ramp (1000 Hz/s) of either positive or negative slope. When a frequency ramp of 1000 Hz/s is approximated by a contiguous sequence of short, constant frequency steps, the growth behavior is unchanged for step durations less than 25 ms. For a step duration of 25 ms, the behavior more nearly resembles that of a pure ramp, but some evidence of individual step growth can be seen. For step durations of 50 ms and 100 ms, growth is relatively small and is confined to each individual step, with little coupling between steps. Rising ramps generally show more growth than falling ramps of similar step size and average slope, confirming earlier observations. An equivalent “sawtooth” frequency variation with an average slope of zero and a step size of 25 ms behaves much like a constant frequency pulse. An interpretation of the response to step size can be found in the theory of second‐order resonance, where the spatial rates of change of the electron gyrofrequency and the Doppler‐shifted wave frequency are equal. This theory also accounts for the previously observed “dot‐dash” anomaly, where a long constant frequency pulse shows much more growth and emission triggering than a short pulse.