Magnetospheric Hiss Research Articles

Unique concurrent observations of whistler mode hiss, chorus, and triggered emissions

AbstractWe present a unique 2 h ground‐based observation of concurrent magnetospheric hiss, chorus, VLF triggered emissions as well as ELF/VLF signals generated locally by the High Frequency Active Auroral Research Program (HAARP) facility. Eccentricity of observed wave polarization is used as a criteria to identify magnetospheric emissions and estimate their ionospheric exit points. The observations of hiss and chorus in the unique background of coherent HAARP ELF/VLF waves and triggered emissions allow for more accurate characterization of hiss and chorus properties than in typical ground‐based observations. Eccentricity and azimuth results suggest a moving ionospheric exit point associated with a single ducted path at L ~ 5. The emissions exhibit dynamics in time suggesting an evolution of a magnetospheric source from hiss generation to chorus generation or a moving plasmapause location. We introduce a frequency band‐limited autocorrelation method to quantify the relative coherency of the emissions. A range of coherency was observed from high order of coherency in local HAARP transmissions and their echoes to lower coherency in natural chorus and hiss emissions.

Excitation of Chirping Whistler Waves in a Laboratory Plasma.

Whistler mode chorus emissions with a characteristic frequency chirp are important magnetospheric waves, responsible for the acceleration of outer radiation belt electrons to relativistic energies and also for the scattering loss of these electrons into the atmosphere. Here, we report on the first laboratory experiment where whistler waves exhibiting fast frequency chirping have been artificially produced using a beam of energetic electrons launched into a cold plasma. Frequency chirps are only observed for a narrow range of plasma and beam parameters, and show a strong dependence on beam density, plasma density, and magnetic field gradient. Broadband whistler waves similar to magnetospheric hiss are also observed, and the parameter ranges for each emission are quantified.

Ray tracing of whistler-mode chorus elements: implications for generation mechanisms of rising and falling tone emissions

Abstract. Using a well-established magnetospheric very-low-frequency (VLF) ray tracing method, in this work we trace the propagation of individual rising- and falling-frequency elements of VLF chorus from their generation point in the equatorial region of the magnetosphere through to at least one reflection at the lower-hybrid resonance point. Unlike recent work by Bortnik and co-workers, whose emphasis was on demonstrating that magnetospheric hiss has its origins in chorus, we here track the motion in the equatorial plane of the whole chorus element, paying particular regard to movement across field lines, rotation, and compression or expansion of the wave pulse. With a generation point for rising chorus at the equator, it was found the element wave pulse remained largely field aligned in the generation region. However, for a falling tone generation point at 4000 km upstream from the equator, by the time the pulse crosses the equator the wavefield had substantial obliquity, displacement, and compression, which has substantial implications for the theory of falling chorus generation.

VLF electromagnetic noise bursts observed in a borehole and their relation with low-latitude hiss

In an attempt to monitor subsurface VLF electric field changes associated with earthquakes, a borehole antenna has been installed at Bichpuri, Agra (Geograph. Lat. 27.2°N, Geograph. Long. 78°E, Geomag. Lat. 17.1° N, L=1.1 ) in India and observations have been taken since February, 1998 using an analog system. The electric field changes have been found to occur in the form of noise bursts of varying amplitudes and durations. A thorough analysis of the data of the year 1999 has shown occasional occurrence of large amplitudes, long duration (∼5– 6 h or more) noise bursts which show seasonal, stormtime, and diurnal variations similar to those of low latitude magnetospheric hiss observed on the ground. Similar noise bursts have also been recorded in the vertical terrestrial antenna operated in conjunction with the borehole antenna. This result indicates penetration of ionospheric/magnetospheric VLF signals to large depths in the crustal region and cautions for careful identification of potential seismogenic signals in VLF data.

An experiment on the threshold effect in the coherent wave instability

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.

Magnetospherically reflected whistlers as a source of plasmaspheric hiss

Ray‐tracing simulations and estimates of whistler wave damping show that magnetospherically reflected whistlers can persist for ∼102 s in a low frequency band (ƒ ∼1 kHz). The combined contribution from whistler rays produced by a single lightning flash but entering the magnetosphere at different points form a continuous hiss‐like signal, as observed at a fixed point. Estimates indicate that the total whistler wave energy input into the magnetosphere from lightning discharges may maintain experimentally observed levels of magnetospheric hiss.

Lightning as an embryonic source of VLF hiss

Data from the DE 1 satellite show that lightning‐generated whistlers often trigger hiss emissions that endure for up to 10‐ to 20‐s periods. The data consist of the measured electric and magnetic fields in the frequency range of 1.5 kHz to 6.0 kHz, during 22 DE 1 passes during the period December 28, 1986, to January 18, 1987. The 22 passes were nearly identical in terms of their projections on a magnetic meridional plane, and they covered L shells of 3.4 to 5.1 and geomagnetic latitude of 20°N to 40°S in the afternoon (∼1400–1500 MLT) sector. The geographic longitudes of the 22 passes were within ±50° of 80° W, and the geomagnetic activity during the period covered was relatively quiet (ΣKp<20 for most of the days). The whistler‐triggered hiss emissions were observed on 16 of the passes, and they generally exhibited the following characteristics: (1) emission spectra were wide band (1–2 kHz) and rather structureless, (2) well‐defined and sustained fading patterns were observed at twice the spin frequency over 10‐ to 20‐s periods, (3) the spin fading characteristics of the triggered hiss bursts were similar to those reported for background plasmaspheric hiss (Sonwalkar and Inan, 1988), indicating a large wave normal angle with respect to the ambient magnetic field. Our results indicate that lightning‐generated whistlers may be an important embryonic source for magnetospheric hiss and that whistlers and emissions triggered by them often constitute the dominant wave activity in the ∼1.5‐ to 6‐kHz range on L shells of 3.5 to 5 in the afternoon sector during geomagnetically quiet periods. Through cyclotron and Landau resonant scattering, it is likely that these lightning‐generated waves play a dominant role in the loss of ∼0.5‐ to 50‐keV electrons trapped on these field lines in the afternoon sector. Through anisotropic proton instability, these waves can also interact with ring current protons in the range of several tens of keV leading to a loss mechanism for ring current protons.

Wave normal direction and spectral properties of whistler mode hiss observed on the DE 1 satellite

A new study of magnetospheric hiss as a spatially and temporally enduring phenomenon is undertaken using a recently developed formalism that allows the representation of hiss by a field distribution function (FDF). This formalism explicitly takes into account the whistler mode relationships and the linear and spin motion of the satellite, so that on a spin‐stabilized satellite, it becomes possible to measure the wave propagation direction(s) from the observed fading patterns in the received electromagnetic field data. We have analyzed hiss signals received by electric and magnetic field antennae aboard the DE 1 satellite during a ∼3‐hour period on September 23, 1983. A band of hiss at frequencies <2 kHz was observed continuously from 0236 UT to 0539 UT over a range of geomagnetic latitudes from λm = 45°N to λm = 20°S and L shells of L = 4.3 to L = 5.3. Electron density deduced from in situ and remote measurements indicates that during this time the DE 1 satellite was near the boundary of the plasmasphere. Observations can be summarized as follows: (1) The general character of the electric and magnetic field spectrum remained the same throughout the 3‐hour‐long period, exhibiting an intensity peak at 1550 Hz. (2) The intensities of both the electric and the magnetic field decreased during this interval by ∼30 dB. (3) Well‐defined fading patterns at half the spin period (3.04 s) were observed throughout this period. These patterns were stable over a time scale of ∼1 min. (4) In addition, a slow fading pattern with a time scale of 30 s was observed approximately 30% of the time. (5) The hiss intensity also showed fading over a time scale of ∼10‐15 min. The spin fading patterns led to the measurement of wave propagation direction(s). The results of our analysis can be summarized as follows: (1) Near the geomagnetic equator (within ∼ ±3° latitude) we observe a wave normal angle of 60° ± 5° with respect to the local geomagnetic field. (2) Away from the equator, wave normal directions range from 30° to 80° with respect to the local geomagnetic field. (3) All wave normal directions observed over the 3‐hour‐long period were within ∼ ±45° of the plane normal to the local magnetic meridional plane. Results indicate that the hiss source radiates with initial wave normal angles in the range 30° < θ0 < θg (Gendrin angle) and 90° < ϕ0 < 270°, contrary to the common assumption that the source emits with wave normal angles closely aligned with the geomagnetic field. The FDF formulation has also permitted elucidation of the statistical nature of hiss. The slow time fading (∼30 s) is interpreted in terms of a coherence bandwidth Δω of about 0.2 rad/s.

VLF wave-injection experiments from Siple Station, Antarctica

The background of VLF wave-particle experiments from Siple Station, Antarctica, is briefly reviewed. Most of these results are interpreted in terms of Doppler shifted cyclotron interaction between energetic electrons and the VLF waves near the equatorial plane. Single frequency ducted signals that exceed a certain ‘threshold’ intensity are observed at the conjugate point (Roberval, or Lake Mistissini, Quebec) to be amplified 30–50 dB, with temporal growth rates of 30–200 dB/s. Following saturation, variable frequency emissions are triggered. With the addition of a second signal, separated in frequency from the first by Δƒ < 100 Hz, signal growth at both frequencies is reduced and sidebands are generated at frequencies separated from the carriers by integer multiples (up to seven) of Δƒ. The sidebands are attributed to short emissions triggered by the beats between the two input carriers. Midlatitude magnetospheric hiss is crudely simulated by a sequence of 10 ms pulses whose frequencies are chosen randomly within a 400 Hz band. Results show that certain combinations of 10 ms pulses link together to form chorus-like elements, suggesting a common origin for hiss and chorus. In contrast to the equatorial region phenomena are low altitude (1000–8000 km) satellite observations of narrowband Siple pulses showing increases in bandwidth of up to 1000 Hz with impulsive triggered emissions of similar bandwidth; these phenomena are interpreted in terms of a new type of quasi-electrostatic plasma instability in the subauroral region of the magnetosphere, involving 200 m wide field aligned irregularities and occurring at frequencies near the lower hybrid resonance frequency. A new crossed dipole antenna at Siple Station that generates left-hand, right-hand, or linear polarization is used to increase the effective radiated power and to perform polarization experiments.

Generation of band‐limited VLF noise using the Siple transmitter: A model for magnetospheric hiss

Band‐limited VLF noise generated by the experimental transmitter at Siple Station, Antarctica, is used to simulate the interaction of natural magnetospheric hiss with energetic radiation belt particles. While the observed spectrum of the generated noise at times closely resembles that of natural VLF hiss, at other times discrete emissions are found to be generated from the incoherent noise signal. Results imply that magnetospheric chorus can be triggered by hiss signals, indicating the similarity of generation mechanisms for coherent and incoherent VLF emissions. A model based on second‐order cyclotron resonance can explain the conversion of the relatively short duration wavelets of a hiss spectrum into the longer, semicoherent discrete emissions that are typical of chorus.

VLF transmitter‐induced quiet bands: A quantitative interpretation

Raghuram et al. (1977) have shown that a long‐duration monochromatic wave generated by a high‐power VLF transmitter may quench natural magnetospheric hiss emissions over a frequency range Δf of the order of 50–150 Hz below the transmitter frequency. We show that quasi‐linear diffusion in an inhomogeneous medium (Ashour‐Abdalla, 1972; Welti et al., 1973) cannot be invoked to explain these results. On the contrary, by using a formalism similar to the one introduced by Roux and Pellat (1976), i.e., nonlinear trapping and detrapping of natural particles, we are able to interpret the variation of Δf with both the field intensity and the transmitter frequency. The time constants for the establishment and disappearance of the quiet band are also qualitatively interpreted. Numerical agreement with experimental data is obtained if the field intensity induced by the Siple transmitter in the equatorial region at L = 3.5 is of the order of 4 mγ (1 mγ=1 pT (picotesla)).