Hiss Intensity Research Articles

Observation of pulsing hiss at low latitudes

In this paper we report pulsing hiss emissions observed at low latitude Indian station Varanasi (geomag. Lat.=14°55′N, Long.=153°55′E) in which intensity seems to decrease with increase in frequency. The bandwidth of the pulsing hiss also decreases with increase in frequency. The entire dynamic spectra has irregular structure and varied from pulse to pulse. It is proposed that pulsing hiss is generated through wave particle interaction near the geomagnetic equator and propagated to the earth in the whistler mode. Further, the micropulsations propagating along the geomagnetic field lines could modulate the growth rate of the wave resulting in the modulation of hiss intensity.

Latitudinal changes of polar hiss and plasmapause hiss associated with magnetospheric processes

The magnetospheric VLF (very low frequency, 3–30 kHz) hiss observed by a low altitude polar orbiter represents integrated spectra of whistler mode VLF hiss waves propagating along geomagnetic field lines from wide-altitude sources in the magnetosphere. We investigate latitudinal changes of the magnetospheric VLF hiss associated with magnetospheric processes in geomagnetic quiet and disturbed periods by using VLF hiss intensity data at six frequency bands processed from VLF electric field (50 Hz–30 kHz) data of ISIS-2 (circular polar orbit at 1400 km in altitude). The magnetospheric VLF hiss observed by the ISIS-2 mainly consists of a broad-band polar hiss and narrow-band plasmapause hiss. The polar hiss is the whistler mode Cerenkov emissions generated in the polar magnetosphere by inverted- V electrons (100 eV–40 keV) precipitating from the plasmasheet boundary layer. The plasmapause hiss is also whistler-mode waves excited by the cyclotron instability of energetic electrons convected inward from the magnetotail. In a geomagnetically very quiet period, only a plasmaspheric ELF (extremely low frequency, 0.3–3 kHz) hiss and diffused whistlers appear at invariant latitudes below 73°. The broad-band polar hiss and narrow-band plasmapause hiss occur, respectively, at night-side invariant latitudes from the middle of the auroral zone to the polar cap and for nighttime invariant latitudes of 50–64° in a quiet or weakly disturbed period. In a substorm period, the polar hiss region shifts to lower latitudes due to an inward movement of the plasmasheet inner edge associated with an intensified substorm westward electric field in the magnetotail. Also, an intensity hump of the plasmapause hiss appears between regions of the polar hiss and plasmapause hiss on the night side in a lingering substorm period. This seems to be generated in a plasmaspheric plume outside the night-time plasmapause. As the substorm develops in the expansion phase, the polar hiss region finally joins to the plasmapause hiss region. These show that the invariant latitudinal changes of magnetospheric VLF hiss generated by the micro-scale mechanism are greatly affected by the macro-scale dynamical processes in the magnetosphere.

On the origin of whistler mode radiation in the plasmasphere

The origin of whistler mode radiation in the plasmasphere is examined from 3 years of plasma wave observations from the Dynamics Explorer and the Imager for Magnetopause‐to‐Aurora Global Exploration spacecraft. These data are used to construct plasma wave intensity maps of whistler mode radiation in the plasmasphere. The highest average intensities of the radiation in the wave maps show source locations and/or sites of wave amplification. Each type of wave is classified on the basis of its magnetic latitude and longitude rather than any spectral feature. Equatorial electromagnetic (EM) emissions (∼30–330 Hz), plasmaspheric hiss (∼330 Hz to 3.3 kHz), chorus (∼2–6 kHz), and VLF transmitters (∼10–50 kHz) are the main types of waves that are clearly delineated in the plasma wave maps. Observations of the equatorial EM emissions show that the most intense region is on or near the magnetic equator in the afternoon sector and that during times of negative Bz (interplanetary magnetic field) the maximum intensity moves from L values of 3 to <2. These observations are consistent with the origin of this emission being particle‐wave interactions in or near the magnetic equator. Plasmaspheric hiss shows high intensity at high latitudes and low altitudes (L shells from 2 to 4) and in the magnetic equator with L values from 2 to 3 in the early afternoon sector. The longitudinal distribution of the hiss intensity (excluding the enhancement at the equator) is similar to the distribution of lightning: stronger over continents than over the ocean, stronger in the summer than in the winter, and stronger on the dayside than on the nightside. These observations strongly support lightning as the dominant source for plasmaspheric hiss, which, through particle‐wave interactions, maintains the slot region in the radiation belts. The enhancement of hiss at the magnetic equator is consistent with particle‐wave interactions. The chorus emissions are most intense on the morningside as previously reported. At frequencies from ∼10 to ∼50 kHz, VLF transmitters dominate the spectrum. The maximum intensity of the VLF transmitters is in the late evening or early morning with enhancements all along L shells from 1.8 to 3.

VLF effects in the outer ionosphere from the underground nuclear explosion on Novaya Zemlya Island on 24 October, 1990 (intercosmos 24 satellite data)

The results of the digital processing of auroral hiss and whistler data registered by INTERCOSMOS 24 satellite at the height about 1000 km after 17 minutes passage over Novaya Zemlya Island were investigated since the moment of the underground nuclear explosion. The outcome of power acoustic influence on the magnetosphere is abnormal gain (∼200dB) of the VLF hiss intensity arising in a narrow band of invariant latitudes (Δφ ∼3°), including the source of possible disturbances. Power density spectrum was modulated in the space with intervals about 100 and 23 km. At the same time, in plasmasphere several groups whistlers with extraordinary number of echoes (∼37 and more) have been registered within a wide latmide interval (front 30° to 50°). The corresponding one hop whistler from these groups gave the same dispersion and the time interval between the emerging echoes was constant, namely, 2.9 ± 0.2 s.

Generation mechanism of plasmaspheric ELF/VLF hiss: A statistical study from GEOS 1 data

ELF/VLF hiss is whistler mode waves that interact with energetic electrons in the magnetosphere. One of the consequences of these interactions is the precipitation of energetic electrons into the atmosphere due to pitch angle diffusion. One related question that is still controversial is how these waves can reach the intensities that are measured. A study of ELF/VLF hiss in the plasmasphere (Solomon et al., 1988) has shown that the observed plasmaspheric hiss intensities can be explained by the presence of a relatively high level of anisotropic energetic electron fluxes, with the wave growth rate γ being directly proportional to the flux level. Sonwalkar and Inan (1989) have argued, however, that this result was based on a limited number of cases only and that lightning‐generated whistlers may be a more important source for plasmaspheric hiss. In order to clear up this problem we have performed a statistical study using 75 orbits of GEOS 1, providing 48 events. All of them occurred on the day side at 4 < L < 5, where L is the McIlwain parameter, in the vicinity of the geomagnetic equator during relatively quiet geomagnetic conditions. These events have the usual characteristics of plasmaspheric hiss (frequency range and intensity). Simultaneous measurements of hiss and electron fluxes show on a statistical basis that (1) there is some trend for the strongest wave events to correspond to the largest electron fluxes, (2) the observed fluxes for the 48 events are generally much above the “typical” values used in previous theoretical studies, and (3) in agreement with the suggestion of Solomon et al. (1988) the statistical results are consistent with wave generation in the equatorial region, the plasmaspheric hiss being amplified during a single crossing of the waves through this region.

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.

Full wave analysis of altitude profiles of auroral hiss observed by antarctic rocket.

Electromagnetic waves in the VLF range associated with auroral hiss were observed by the S-310JA-11 and -12 rockets launched from Syowa Station into quiet auroral arcs and breakup auroras, respectively. The altitude profile of electric field intensity observed by the S-310JA-12 rocket in the frequency range less than 5kHz showed a distinct peak at an altitude of 90km. The source location of the auroral hiss has been estimated so that the observed altitude profile of the hiss intensity, particularly around the altitude of 90km, coincides with the altitude profile calculated using the full wave analysis. It was found that the auroral hiss is most likely to be generated at an altitude about 700km. The intensity of auroral hiss around the altitude of 200km at a frequency of 4kHz observed by the rocket was 4.3×10-5V/m/√Hz, which is much stronger, by about 50dB, than that of auroral hiss observed on the ground at Syowa Station. This means that the k-vector direction of auroral hiss observed on the rocket is outside the transmission cone.

The plasma wave environment of an auroral arc: 3. VLF hiss

Intense auroral hiss was observed by a sounding rocket payload under circumstances that have allowed detailed comparison of the hiss spectrum with the predictions of a convective beam amplification hiss production model. The payload was launched from Poker Flat, Alaska at 0813 UT on March 9, 1978; it carried an electric field detector and electron detectors covering the energy range from 30 eV to 25 keV. A complete pitch angle scan was made every 0.2 s. The trajectory was close to the magnetic meridian and crossed the diffuse aurora, a dark region, and a quiet 40 kR auroral arc. Auroral hiss was first encountered at the poleward edge of the diffuse aurora, and was observed for the rest of the flight. A maximum intensity of 10−8(V/m)²/Hz at 30 kHz was seen in the dark region. Electron distribution functions were seen with δf/δv∥>0 and a large enough slope to have produced appreciable VLF amplification. These distributions were found in a region a few kilometers wide on the equatorward boundary of the arc. A model distribution function has been fitted to one of the observed distributions and used as input into an auroral beam noise generation model. Reasonable assumptions about the distribution of cold plasma and width of the unstable region yields a predicted VLF spectrum in good agreement with observation. Several secondary results were also obtained. The observed VLF hiss does not appear to have been produced by a payload‐plasma interaction. The intensity of the hiss measured in situ, above the ionosphere, was greater equatorward of the arc than within the arc, thus ruling out the ionospheric transmission model as an explanation for this intensity pattern. Quasi‐linear back reaction on the electron distribution was not found to be significant in this case. The correlation between 0.1 – 0.7 keV electron flux and hiss intensity was not better than the correlation with the flux of 5‐keV electrons. The data did not suggest the occurence of a 3 wave parametric decay. Finally, the possibility of a secondary source at ∼100 km altitude is not ruled out by these data.

Rocket‐borne wave measurements in the dayside auroral oval

VLF and ELF waves were measured on two rocket payloads launched into the dayside auroral oval on December 18, 1974, and on January 11, 1975. The correlation of the VLF hiss intensity with the fluxes of precipitating electrons above 500 eV on short spatial and time scales was often poor, even when a positive slope existed in the electron phase space density. During the flight of January 11, 1975, distinct features were observed at the lower hybrid frequency. The frequency of this emission was used together with an independent density measurement to determine the relative concentration of O+ to the heavier ions NO+ and O2+. It was found that the density of NO+ or O2+, which could not be distinguished, is enhanced over expected values between 150 and 400 km altitude. This may be due to conversion of O+ to NO+ by large dc electric fields as observed in the experiments. Broadband electrostatic and electromagnetic waves were observed shortly after the barium‐shaped charged explosion on January 11, 1975. The observed lowering of the auroral VLF hiss cutoff due to the barium is consistent with either ducting of VLF hiss emissions along the enhanced density duct produced by the barium or with local generation VLF hiss during the barium injection. ELF hiss observations were consistent with previous results, indicating that it is limited to latitudes equatorward of the dayside auroral oval.

Hiss emitting auroral activity

Real-time auroral records on a video tape obtained by use of a highly sensitive TV camera, and simultaneous records of VLF waves on the sound track of the same video tape were analyzed and specific small scale auroral activities were found to be associated with specific burst-like hiss enhancements with durations of 0.1–1 s. The hiss emitting auroral activities were identified by cross correlation analysis between the temporal variations in luminosity of auroral structures and the temporal variations of hiss intensities. The hiss emitting aurora were long-rayed sheet fragments or folds of a few km to several tens of km in horizontal scale which rapidly changed in luminosity, and displayed rapid motions such as splittings and rotations. The enhancements of local auroral electron precipitations responsible for the local auroral activity and of hiss emissions are concluded to be simultaneous within a time accuracy of 200 ms at worst and within a few tens of ms at best.

A calculation of auroral hiss with improved models for geoplasma and magnetic field

Intensities of auroral hiss generated by the Cerenkov radiation process by electrons in the lower magnetosphere are calculated with respect to a realistic model of the Earth's magnetosphere. In this calculation, the magnetic field is expressed by the “Mead-Fairfield Model” (1975), and a static model of the iono-magnetospheric plasma distribution is constructed with data accumulated by recent satellites (Alouette-I, -II, ISIS-I, OGO-4, -6 and Explorer 22). The energy range of hiss producing electrons and the frequency range of the calculated VLF are 100–200 keV, and 2–200 kHz, respectively. Intensities with a maximum around 20 kHz, of the order of 10 −14 W/m 2/Hz 1 at the ground seem to be ascribable to the incoherent Cerenkov emission from soft electrons with a differential energy spectrum E −2 having an intensity of the order of 10 8 cm −2/sec/sr/eV at 100 eV. It is shown that the frequency of the maximum hiss spectral density at geomagnetic latitudes 80° on the day-side and 70° on the night-side is around 20 kHz for the soft spectrum (∼ E −2) electrons, which shifts toward lower frequency (∼10 kHz) for a hard spectrum (∼ E −1·2) electrons. The maximum hiss intensity produced by soft electrons is more than one order higher than that of hard electron produced hiss. The higher rate of hiss occurrence in the daytime side, particularly in the soft electron precipitation zone in the morning sector, and the lesser occurrence of auroral hiss in night-time sectors must be, therefore, due to the local time dependence of the energy spectra of precipiating electrons rather than the difference in the geomagnetic field and in the geoplasma distributions.

Plasmaspheric hiss intensity variations during magnetic storms

The storm time intensity variations of ELF electromagnetic emissions have been studied by using the Ogo 6 search coil magnetometer. Low-latitude signals exhibit a sharp low-frequency cutoff and are identified as plasmaspheric hiss. Such waves show pronounced intensification during the recovery phase of magnetic storms but remain close to background levels during the storm main phase. This behavior is consistent with cyclotron resonant generation within the plasmasphere as the latter expands into the intensified belt of outer zone electrons during the storm recovery.

An evaluation of the intensity of cerenkov radiation from auroral electrons with energies down to 100 ev

The intensity of incoherent Cerenkov radiation in the dayside polar-cusp region has been estimated by including the very soft (100 ev to 1 kev) energy range, which recent experimental results have shown to be the source of broad-band auroral hiss at least on the dayside of the earth. Our results show that, for an electron flux spectrum of the approximate form dJ/dE ∝ E−2, electrons in the range 100 ev to 1 kev contribute about 2 orders of magnitude more to the intensity of Cerenkov hiss than those in the range 1–10 kev. If the electron flux is assumed to have the value of 5 × 105 el cm−2 sec−1 ster−1 ev−1 at 700 ev, a peak Cerenkov hiss intensity of ≃10−13 w m−2 Hz−1 results at ionospheric heights at ≃70 kHz and decreases in intensity toward both lower and higher frequencies. A limitation of the present theories is pointed out, and the possible role of anomalous cyclotron radiation in the auroral hiss generation is discussed. It is concluded that, even if the generation mechanism is not totally incoherent, as has been assumed in the calculations, it almost certainly involves electrons that are moving faster than the wave. If the calculations are repeated at a lower latitude, appropriate to the nightside auroral hiss zone, essentially identical results are obtained if the dayside model ionosphere and the dayside electron flux spectrum are used. These results indicate that, if incoherent Cerenkov radiation is the source of auroral hiss, the generally weaker hiss on the nightside of the earth must be due to lower electron fluxes, harder spectra, lower magnetospheric electron densities, or a combination of these factors.

Comparison of very-low-frequency auroral hiss with precipitating low-energy electrons by the use of simultaneous data from two Ogo 4 experiments

Determination of the origin of auroral hiss by comparing the records of a vlf experiment (0.3 to 18 kHz) with simultaneous data obtained by an auroral-particle experiment having detectors for precipitating electrons at 0.7, 2.3, and 7.3 keV. It is found that, on the dayside of the earth, the occurrence of vlf hiss correlates well with precipitation events at 0.7 keV, but in general very poorly with activity in the higher-energy channels. Exact correlation between variations in vlf hiss intensity and in electron fluxes is rare even at 0.7 keV. In addition, vlf hiss tends to be observed over a somewhat larger spatial region than precipitating 0.7-keV electrons. It is concluded that, on the dayside, auroral hiss is generated by soft (E less than 1 keV) 'cusp region' electrons and that the lack of detailed correlation between the two phenomena is caused by propagation effects as the hiss travels downward and spreads from the generation region.

VLF hiss and related plasma observations in the polar magnetosphere

This paper presents a study of auroral-zone VLF hiss and low-energy charged-particle observations with the Injun 5 satellite. The results of this study provide a direct verification of the association between auroral-zone VLF hiss and intense fluxes, 104 to 107 electrons (cm² sec ster ev)−1, of low-energy electrons with energies on the order of 100 ev to several kev. On the dayside of the magnetosphere, these low-energy electrons are identified with the dayside polar-cusp region observed at higher latitudes with the Imp 5 satellite. At other local times, through the dawn and dusk regions and into the nightside of the magnetosphere, the VLF hiss and low-energy electron precipitation regions are believed to correspond to the extension of the dayside polar cusp into the distant plasma sheet and downstream magnetosheath on the nightside of the magnetosphere. Intense fluxes of upgoing electrons are often observed in a narrow latitudinal band near the low-energy electron precipitation bands. These upgoing electrons are believed to be associated with another type of VLF emission called a saucer, which is frequently observed with Injun 5. On the basis of present models, the observed VLF hiss intensities cannot be accounted for by incoherent Cerenkov radiation from the observed electron fluxes, thus indicating that a coherent plasma instability mechanism is involved in some, if not all, of the VLF hiss generation. A model for the generation regions of VLF hiss and saucer emissions is discussed.

Rocket observation of VLF hiss in aurora

Three rocket payloads with VLF receivers have been launched during auroral disturbances to study VLF hiss in the ionosphere. One payload was launched during relatively quiet conditions and penetrated a quiet auroral arc. Another payload was launched during the break up phase of the aurora at the start of a magnetic substorm. The third payload was launched in a situation that was similar to an after break up auroral glow situation. From the 7.35 kHz hiss observations on these three payloads it is concluded that in some circumstances auroral hiss is generated in the auroral arcs at altitudes of 105–110 km. The observations also show that auroral hiss often is present in the ionosphere even if it is not observed on the ground. The observed variation in intensity of auroral hiss is interpreted as being caused by absorption as the hiss propagates away from a generation region at 105–110 km altitude or propagates down from greater altitudes. This interpretation together with the occasional spin modulation of the observed signals indicates that the wave normal direction for auroral hiss often is within a few degrees of the resonance cone (89.7°).

Rocket observations of the intensity of very low frequency noise above the ionosphere

Abstract The intensity of radio signals in the frequency range 0.5–12 kc s was measured during two rocket flights, one near midnight and one near noon. On both flights, whistlers were observed—22 at night and 122 in the day. Hiss also was recorded on both flights—at 2.8 kc s at night and at 800 c s in the day. No hiss or whistlers were observed on the ground on either occasion. At night the intensity of whistlers in the ionosphere was at least 750 times that on the ground and the hiss intensity was at least 180 times that on the ground. During the day the corresponding factors were 8000 and 10,000. These ratios are much greater than could be caused by absorption. They are probably due to total internal reflection in the low ionosphere, since VLF waves penetrate to the ground only if their wave normals are within a small cone about the vertical. It is concluded that it is impossible to deduce the intensity of VLF waves in the ionosphere from measurements made on the ground.

An Artificial Talker Driven from a Phonetic Input

This paper describes a method of producing artificial speech from a phonetic input, i.e., symbols representing the names of phonemes corresponding to a given text are fed into a machine and the acoustic waveforms of connected speech emerge. The experimental work was accomplished on an electronic computer (IBM 7090), but the scheme is simple enough to permit realization with analog hardware. The talking machine program is divided into two parts. The first part simulates a more or less conventional resonance synthesizer of the tandem variety, requiring nine control signals; buzz intensity, hiss intensity, pitch, plus the center frequencies and bandwidths of three formants. Initially, this part of the program was used alone in experiments for which the inputs were detailed specifications of the control signals derived from spectrograms and physiological data, sampled at approximately three times the phonemic rate. Results from this phase were later combined with known results in speech perception to produce the rules used by the second program. That program accepts for its input the names of phonemes punched on IBM cards and produces the control signals to drive the resonant synthesizer.

Formants as a Voicing Cue in the Perception of /z/

In earlier synthetic speech studies by researchers at Haskins Laboratories of the cues for the voiced-voiceless distinction of fricatives in initial position, hiss duration, intensity of the hiss relative to the vowel, separation of hiss from the vowel by a short period of silence, and the consonant-vowel transitions all had been shown to have an effect. In the present study, the addition of voiced formants to the hiss portion has been investigated. These formants are a rather obvious device for signaling voicing since a voice bar and often higher resonances as well can be seen clearly on spectrograms. In this experiment versions of the syllables /si/ and /zi/ were synthesized on PAT using the following variables: hiss amplitude: −5, −10, −15, and −20 db from the peak intensity of the vowel: voiced formant amplitude during fricative portion: −10, −20, and −30 db from the peak intensity of the vowel, as well as hiss without these formants; duration of the fricative portion: 85, 124, 170, and 213 msec; a 20-msec separation of the fricative portion from the onset of vowel transitions vs no separation; and vowel portions copied from spectrograms of spoken /si/ and /zi/. Low hiss intensity, short duration, no separation, and vowel formants from spoken /zi/ all tended to produce z judgments, but, as anticipated, the addition of voiced formants was the overriding cue.