The first results of observations of auroral hiss during the «North Pole–41» expedition

During the North Pole–41 expedition, three components of the VLF electromagnetic field were simultaneously measured on a drifting ice-resistant platform and at the Lovozero and Barentsburg observatories. We consider three VLF events that occurred in magnetically quiet time. During two of them (the events on January 24, 2023 and March 12, 2024), auroral hiss bursts were recorded at three stations located in the auroral and circumpolar regions and spaced up to 2.600 km apart. The spectral and temporal characteristics of the bursts at all the stations were almost the same. The fact that hiss was recorded with the same properties at such large distances can be explained under the assumption of a homogeneous flow of auroral electrons with energies from 0.1 to 10 keV throughout the precipitation area, which generate quasi-electrostatic waves at altitudes 10–20 thousand km, along with the simultaneous presence of small-scale ionospheric irregularities in the vicinity of all three stations, where these waves are scattered into the propagation cone to the Earth surface. We examine the case of hiss recording (the January 25, 2023 event) demonstrating the locality of the hiss recording area during one day — a hiss burst is first observed at one station, then at another. This is probably due to the appearance/disappearance of local areas of small-scale irregularities, where quasi-electrostatic waves are scattered providing propagation to the Earth surface.

On 16 October 2001, the Galileo spacecraft made a close flyby of Jupiter's moon Io. During the flyby the plasma wave instrument detected an electric field emission with spectral characteristics very similar to a type of whistler‐mode noise called “auroral hiss” that is commonly observed in Earth's auroral region. This paper gives a detailed analysis of the “auroral hiss” observed near Io. The frequency‐time spectrum of the emission has a sharp high‐frequency cutoff near the electron cyclotron frequency and a V‐shaped low‐frequency cutoff. On a frequency‐time spectrogram these cutoffs give the emission a characteristic funnel shape that is very similar to the spectrum of terrestrial auroral hiss. Strong magnetic field perturbations occurred near the vertex of the funnel indicating the presence of a field‐aligned current. To explain the origin of the emission, a brief review is given of whistler‐mode wave propagation and the unipolar inductor model of Io's interaction with the magnetosphere of Jupiter. Assuming propagation near the whistler‐mode resonance cone, ray‐tracing analyses show that the radiation originates from a source very close to the surface of Io. The source is located in the same region where field‐aligned currents are believed to originate in the ionosphere of Io. Since terrestrial auroral hiss is known to be produced by beams of low‐energy auroral electrons, these observations suggest that the auroral hiss at Io is generated by an electron beam that is part of the field‐aligned current system induced by the interaction of Io with the rapidly rotating magnetosphere of Jupiter.

The polar‐orbiting DE 1 spacecraft is now providing the first measurements of high‐latitude auroral phenomena in the radial distance range between about 2 and 5 RE, where several important types of auroral plasma wave emissions are believed to be generated. This paper describes the initial observations from the DE 1 plasma wave instrument in this interesting region. Three principal types of plasma wave emissions are discussed: auroral hiss, Z mode radiation, and auroral kilometric radiation. Whistler mode auroral hiss emissions are observed on essentially every pass over the auroral zone. The auroral hiss usually has a characteristic ‘funnel‐shaped’ frequency‐time spectrum which can be explained by a simple whistler mode propagation effect if the radiation is emitted from a spatially localized source below the spacecraft. Ray path studies show that the auroral hiss is propagating upward from a source, the lower boundary of which is an altitude of about 0.7–0.9 RE. The DE 1 observations have identified broadband Z mode emissions in the low‐density region over the auroral zone and polar cap. This radiation usually has a sharply defined upper cutoff near the electron gyrofrequency and extends downward in frequency to a cutoff called the fL = 0 cutoff. The Z mode radiation is sometimes difficult to distinguish from the auroral hiss, which occurs in the same general frequency range with about the same intensity. The Z mode radiation probably corresponds to the noise previously identified as ‘continuum radiation’ in the Hawkeye polar region data. The auroral hiss can usually be distinguished from the Z mode radiation by the sharp upper cutoff of the whistler mode at the local electron plasma frequency. Broadband emissions identified as auroral kilometric radiation are frequently observed over the evening auroral regions at frequencies between about 100 kHz and 400 kHz. These emissions are very intense, usually 30–50 dB above the intensity of the auroral hiss and Z mode radiation, and are highly variable, sometimes disappearing completely. The auroral kilometric radiation usually occurs at frequencies above the electron gyrofrequency, consistent with earlier measurements which indicate that this radiation is propagating in the free‐space R‐X mode.

Saturn auroral hiss is intense whistler mode emission similar in morphology to terrestrial auroral hiss, and is observed at high latitude very often in quasiperiodic episodes with a period of approximately 1 hr. Bader et al. (2019) report auroral pulsations that may be due to duskside magnetodisk reconnection. The source of the 1‐hr period is not definitively known but has been purported to be due to second harmonic Alfven waves standing along near planet magnetic field lines (Yates et al., 2016). Observations of auroral hiss at high latitude along Cassini proximal orbits are often excellent, and we have focused on an event for which we have concurrent ultraviolet auroral images as well as electron flux data. A series of repeating auroral hiss episodes is observed to initiate near the magnetic field line that traverses a Saturn kilometric radiation source region in each hemisphere, with periodic episodes of hiss recurring at higher L‐shells. Magnetic field lines centered on individual hiss episodes have auroral footprints that lie near and within a region of intense auroral ultraviolet emissions. These observations have a parallel in terrestrial return current electron beam‐generated auroral hiss seen near magnetic field lines supporting auroral kilometric radiation source regions. Recent findings link periodic plasma injections with Saturn reconnection sites observed preferentially on the duskside. These injections may spawn Saturn kilometric radiation source regions and periodic auroral hiss emission in nearby return current regions.

Observations from the Cassini spacecraft have shown that Saturn's small icy moon Enceladus ejects a plume of water vapor and small ice particles into Saturn's rapidly co-rotating magnetosphere. In this paper we show that the interaction of the moon with the magnetospheric plasma produces a number of electrodynamics effects that are remarkably similar to those observed in Earth's auroral regions and near Jupiter's moon Io. These include whistler-mode emissions similar to terrestrial auroral hiss, magnetic-field-aligned electron beams, and currents associated with a standing Alfven wave excited by the moon. Ray path analyses of the auroral hiss show that the electron beams responsible for the emissions are accelerated very close to the moon, most likely by parallel electric fields associated with the Alfven wave. However, other possibilities such as electric fields due to electrostatic charging of the moon's surface or of particles in the water vapor plume should be considered. Citation: Gurnett, D. A., et al. (2011), Auroral hiss, electron beams and standing Alfven wave currents near Saturn's moon Enceladus, Geophys. Res. Lett., 38, L06102, doi:10.1029/2011GL046854.

It has been known for many years that several types of electromagnetic plasma wave emissions are generated in the Earth’s polar regions in association with auroras. As long ago as 1933, Burton and Boardman (1933) detected bursts of very-low-frequency (VLF) “static” at high latitudes that occurred simultaneously with flashes of auroral light. Later a variety of investigations using ground VLF radio receivers at high latitudes firmly established that broadband bursts of radio noise are produced in the auroral regions during periods of enhanced auroral activity (Ellis, 1957; Martin et al., 1960; Jorgensen and Ungstrup, 1962; Harang and Larsen, 1964). This type of VLF radio emission came to be known as “auroral hiss,” following the classification scheme of Helliwell (1965). Because of the low frequencies involved, it was realized relatively early that the auroral hiss must be propagating in the whistler mode. The first satellite investigation of auroral hiss was reported by Gurnett (1966) who showed that auroral hiss is closely correlated with intense fluxes of precipitating auroral electrons. This relationship was subsequently confirmed and refined by a number of low-altitude satellite studies.

We present the results of the auroral hiss propagation modeling from the source region to the ground and its comparison with the observed results. We analyzed propagation conditions explaining locality of the auroral hiss illuminated area observed on the ground. It is shown that quasielectrostatic waves with random amplitudes, phases and wavenormals directions representing the auroral hiss propagating to the ground form two wave beams in the meridional plane. The scattering of these waves at altitudes of 800–1300 km can cause the observed locality of the illuminated area.

Small-scale ionospheric irregularities affect navigation and radio telecommunications. We studied small-scale irregularities observed during the 22 June 2015 geomagnetic storm and used experimental facilities at the Institute of Solar-Terrestrial Physics of the Siberian Branch of the Russian Academy of Sciences (ISTP SB RAS) located near Irkutsk, Russia (~52°N, 104°E). The facilities used were the DPS-4 ionosonde (spread-F width), receivers of the Irkutsk Incoherent Scatter Radar (Cygnus A signal amplitude scintillations), and GPS/GLONASS receivers (amplitude and phase scintillations), while 150 MHz Cygnus A signal recording provides a unique data set on ionosphere small-scale structure. We observed increased spread-F, Cygnus A signal amplitude scintillations, and GPS phase scintillations near 20 UT on 22 June 2015 at mid-latitudes. GPS/GLONASS amplitude scintillations were at a quiet time level. By using global total electron content (TEC) maps, we conclude that small-scale irregularities are most likely caused by the auroral oval expansion. In the small-scale irregularity region, we recorded an increase in the precise point positioning (PPP) error. Even at mid-latitudes, the mean PPP error is at least five times that of the quiet level and reaches 0.5 m.

The auroral oval is a region of footprints where configuration of the Earth’s magnetic field lines is such that energetic particles could penetrate into the denser part of the ionosphere. The auroral oval features the most complex ionospheric processes. In this region intensive small-scale ionospheric irregularities exist during both calm and disturbed geomagnetic conditions. Such irregularities could result in radio wave scattering, GNSS (global navigation satellite system) positioning quality deterioration, failures in radio communication, etc. During the last decades scientists have been using GNSS ROTI (rate of total electron content index) data along with other tools to study small-scale irregularities. This data can help to study the oval dynamics. The current report provides data on the auroral oval dynamics, based on GNSS receiver global network data, coherent radars data, and satellite data. To calculate ROTI data we used SIMuRG system (https://simurg.iszf.irk.ru/). The auroral oval regions are assumed to correspond to high values of ROTI. Therefore, we can keep track of the oval’s boundary using these data Coherent scatter radars record signals scattered from plasma irregularities which intensively appear at the oval boundary. We used SuperDARN-like radars located in Russia. Satellite data show sharp variations in field-aligned currents. During magnetic storms the oval expands equatorward, and small-scale irregularities generation shifts to mid-latitudes. All instruments are in good agreement when positioning the oval’s boundary. That allows us to use different data to estimate the oval boundary. Some advance was achieved with computer vision techniques to find the auroral oval boundary in the Northern hemisphere. The techniques implemented mathematical morphology to expand data and decrease data gaps, and K-means and Otsu techniques to cluster image data.

We present first results from a study of the plasma instability mechanism responsible for the small-scale (∼10 m) ionospheric density irregularities commonly observed by the Super Dual Auroral Radar Network (SuperDARN) HF radars in the vicinity of Sub Auroral Polarization Streams (SAPS) during periods of geomagnetic disturbance. A focus is placed on the mid-latitude region of the ionosphere over North America where recent expansion of the SuperDARN network allows for extensive direct comparisons with total electron content (TEC) measurements from a dense network of ground-based GPS receivers. The TEC observations indicate that high-speed SAPS channels and the associated small-scale irregularities are typically located within the mid-latitude ionospheric trough. The Millstone Hill Incoherent Scatter Radar (ISR), operating in campaign mode in support of the NASA Van Allen Probes mission, provided measurements of F region ion/electron density, velocity, and temperature suitable for identifying potential mechanisms of plasma instability during a SAPS event that extended over 12 hours of magnetic local time (MLT) on 2 February 2013. Previous work has indicated that the density gradients associated with the poleward wall of the mid-latitude trough can produce small-scale irregularities due to the gradient drift instability during quiet periods by cascade from larger-scale structures. In this study we demonstrate that the gradient drift instability is a viable source for the direct generation of the small-scale irregularities observed by SuperDARN radars in the mid-latitude ionosphere during geomagnetically disturbed conditions.

An 8 year database of VLF auroral hiss observations from South Pole station (invariant latitude of −74° with magnetic local time (MLT) = UT −3.5 h) is analyzed. There are three peaks in hiss occurrence as a function of MLT in the evening sector (19–23 MLT), afternoon sector (13–17 MLT), and morning sector (7–11 MLT). The geomagnetic and interplanetary magnetic field (IMF) drivers of hiss are examined in the three MLT sectors, and the results are interpreted using an empirical model of auroral boundaries and an empirical model of field‐aligned current patterns. Auroral hiss on the dayside occurs when the auroral oval is centered near the latitude of the station, and in the afternoon sector higher disturbance levels are required. The afternoon sector favors positive By when Bz is positive and negative By when Bz is strongly negative, while the morning sector favors the complementary conditions. In each case the preference for hiss occurrence follows the pattern of upward field‐aligned currents, and hiss is more likely in the configuration where the peak in the upward current is closer to the latitude of the station. IMF By does not play a role on the nightside where hiss is most likely to occur during moderately weak driving conditions as higher disturbance levels are expected to move the auroral oval and upward current systems to latitudes well equatorward of South Pole.

Experimental observations on the ISIS 1, ISIS 2, ISEE 1, and DE 1 spacecraft demonstrate that strong lower hybrid (LH) waves can be excited by VLF electromagnetic (em) whistler mode waves as the em waves propagate through regions of the ionosphere and magnetosphere where small‐scale magnetic‐field‐aligned irregularities exist in the mean plasma density. There is strong evidence that the LH waves are excited by linear mode coupling as the em waves scatter from the irregularities. The present paper considers two related aspects of the linear mode conversion mechanism: (1) the controlled heating of suprathermal ions in the ionosphere and magnetosphere over a powerful VLF/ELF transmitter using LH waves excited by the em transmitter signals through linear mode conversion; (2) the excitation of intense LH waves in the auroral regions through the linear conversion of VLF/ELF em auroral hiss, and the subsequent heating of ions by the excited LH waves. In addressing both aspects a critical feature is the behavior of the mode coupling mechanism at frequencies less than 10.2 kHz, the lowest value observed in earlier experiments. Using the results of controlled experiments carried out at Siple Station, Antarctica, it is demonstrated that strong LH waves can be excited by em waves down to frequencies as low as 2 kHz in the subauroral low‐altitude magnetosphere. Extrapolating from these observations and making use of ISEE 1 satellite wave amplitude data, we conclude that a ∼250 kW VLF/ELF transmitter operating in the subauroral region at 2 kHz could excite sufficiently intense LH waves to heat suprathermal 1 eV H+ ions and 16 eV O+ ions to roughly 50 eV in a region extending in altitude from 1000 to 5000 km above the transmitter with a horizontal scale of ∼500 km. Furthermore, if coherent wave stochastic heating occurs, the energy gain of O+ ions could be as large as 200 eV. This effect would be readily measurable with available satellite instrumentation. Using a recently developed model of the linear mode coupling mechanism, we furthermore conclude that a significant portion of the LH waves observed in the auroral zone in regions of ion conic development may be excited by em VLF/ELF auroral hiss as the hiss propagates through the irregular background plasma commonly observed in the auroral regions.

Observations of auroral hiss obtained from the Voyager 1 encounter with Jupiter have been reanalyzed. The Jovian auroral hiss was observed near the inner boundary of the warm Io torus and has a low‐frequency cutoff caused by propagation near the resonance cone. A simple ray tracing procedure using an offset tilted dipole of the Jovian magnetic field is used to determine possible source locations. The results obtained are consistent with two sources located symmetrically with respect to the centrifugal equator along an L shell (L ≃ 5.59) that is coincident with the boundary between the hot and cold regions of the Io torus and is located just inward of the ribbon feature observed from Earth. The distance of the sources from the centrifugal equator is approximately 0.58 ± 0.01 RJ. Based on the similarity to terrestrial auroral hiss, the Jovian auroral hiss is believed to be generated by beams of low energy (∼tens to thousands of eV) electrons. The low‐frequency cutoff of the auroral hiss suggests that the electrons are accelerated near the inferred source region, possibly by parallel electric fields similar to those existing in the terrestrial auroral regions. A field‐aligned current is inferred to exist at L shells just inward of the plasma ribbon. A possible mechanism for driving this current is discussed.

Rocket observations of electron densities in the night-time auroral E-region at Fort Churchill, Canada

: This report results from a contract tasking Semiconductor Physics Institute as follows: To achieve the proposed goals, the modeling of the circular waveguide section containing semiconductor obstacle will be performed. To calculate the average electric field in the semiconductor obstacle the finite difference time domain method will be used. Although, the lowest critical frequency in the circular waveguide is characteristic to H11 mode higher modes such as E01 and H01 are also sometimes used. Depending on mode some components of electromagnetic field are suppressed, nevertheless in the vicinity of the obstacle all six components of electromagnetic field might be excited and they should be taken into account when determining averaged electric field in the semiconductor obstacle. Since the resistive sensor actually feels the amplitude of electric field, the distribution of electric field component within the waveguide should have a crucial influence on the performance of the sensor and this fact should be taken into account in sensors design. Therefore it is also planning to investigate the behavior of the averaged electric field in the semiconductor obstacle when different modes are excited in the waveguide. To perform investigation it is planned to use internal programs for the modeling of electromagnetic wave propagating in the circular waveguide with the obstacle. Such modeling has already been done in rectangular waveguide with semiconductor obstacle placed on a wide wall of the waveguide and in the obstacle under the thin metal diaphragm. In both cases all six components of electromagnetic field in Cartesian coordinate system has been determined. It is planned to modify the program so that it should be able to calculate the electromagnetic field components in cylindrical coordinate system.