VLF Electric Field Research Articles

Separation of spin synchronized signals

In the measurements of VLF electric fields with the Pioneer Venus spacecraft in sunlight, spin synchronized signals often dominate over the naturally generated emissions. We present a method to separate natural emissions from the several possible sources of noise. Our major objective by this method is not to remove all spin modulation, but to effectively subtract the background noise caused by the identifiable noise sources. Examination of the data shows that the background spin synchronized noise is quite sensitive to ϑ(n), the angle between the sense axis and the solar direction. We model the observed data asy(n)=w(n)t(n)f(ϑ(n))+x(n), wheref(ϑ) represents the phase response of the background noise andx(n) is the estimated natural emissions.t(n) andw(n) are the long-term trend component and time- and phase-independent component of the intensity of the background noise, respectively. The method to decomposey(n) is based on the Bayesian approach which has been recently applied to various inversion problems such as nonstationary time series modeling and image reconstruction. In this procedure, the estimated parametersw(n),t(n),f(ϑ), andx(n) can be determined automatically. We will describe the Bayesian scheme and its application to the Pioneer Venus VLF electric field data.

Narrow‐band plasmapause hiss observed by ISIS satellites

Typical frequency‐time spectra of narrow‐band hiss obtained near plasmapause latitude from ISIS VLF electric field data (50 Hz to 30 kHz) telemetered at Syowa Station, Antarctica, under quiet conditions, are similar to those of narrow‐band 5‐kHz hiss observed at mid‐latitudes and low‐latitudes on the ground. This hiss in the topside ionosphere first appeared as a thin, bar‐shaped emission around 5 kHz at geomagnetic invariant latitude of about 56° and grew into a wider band between 3 and 6 kHz at invariant latitudes around 60° which is the average plasmapause latitude. Then, at higher latitudes, the band narrowed somewhat and disappeared around 65°. The hiss had no lower‐frequency cutoff, and its center frequency remained approximately constant at 5 kHz, between about 56° and 65°. This narrow‐band hiss is completely different from the electrostatic LHR hiss which is also observed above the ionosphere with an electric antenna, but which has latitude dependent lower‐frequency cutoff. Since the narrow‐band 5‐kHz hiss occurs often around plasmapause latitude (60°), hereafter we will call this hiss narrow‐band plasmapause hiss. Latitude and local time distributions of the occurrence rate for narrow‐band plasmapause hiss were obtained by analyzing six‐frequency narrow‐band data processed from the wideband VLF signals received at Syowa Station from ISIS 1 and ISIS 2 during 507 passes between December 1976 and January 1983. Although when taken over the whole latitude range, 55° to 64°, the occurrence rate of narrow‐band plasmapause hiss under various geomagnetic conditions (Kp = 0–6) is 0.83, the rate at any given latitude is below 0.35. Thus it is rather a rare phenomenon as compared with LHR hiss. The peak occurrence rate lies at 58° to 59°, under geomagnetically quiet and moderate conditions (Kp = 0–3) and extends over the somewhat broader range of 56° to 60° under disturbed conditions (Kp = 4–6). The occurrence rate under disturbed conditions is about one third that under quiet and moderate conditions. The magnetic local time (MLT) distribution of the occurrence rate has a significant maximum at 1800–2100 hours MLT under quiet and moderate conditions, and a maximum at 0000–0300 hours MLT under disturbed conditions. The similarity between frequency‐time spectra of narrow‐band hiss observed by the ISIS satellites and on the ground suggests that the narrow‐band hiss generated in the magnetosphere propagates toward the Earth in the whistler mode. The peak occurrence rates of the hiss at invariant latitude 60° (average plasmapause latitude) in the late evening sector under quiet and moderate conditions, and in the midnight sector under disturbed conditions, suggest that narrow‐band plasmapause hiss is generated by cyclotron resonant instability of energetic electrons which are injected from the magnetotail and drift eastward around the equatorial plasmapause.

A coordinated study of a storm system over the South American continent: 2. Lightning‐related data

On December 13, 1989, a coordinated campaign was conducted in Brazil to study the electrical signatures associated with a large storm system over the South American continent. Balloon‐borne VLF electric field data showed more than a hundred lightning‐related stratospheric vertical electric field (sferics) signatures during approximately 6 hours. The sferic signatures were characterized by a rapid transient with peak amplitudes exceeding 7 V/m which would be expected by the removal of positive charge below followed by a slow recovery. This signature indicates that the lightning strokes experienced by the balloon were either cloud discharges or positive ground discharges. The peak amplitudes indicate that the positively charged region is of the order of tens of coulombs. The recovery curves are quite variable, reflecting the complex nature of the propagating phenomena in the atmosphere. A lightning position and tracking system (LPATS) network recorded 326 ground strokes at distances less than 100 km of the balloon. In contrast with previous observations during the summer in the northern hemisphere it was found that most lightning flashes (60%) were positive. In addition, only a small fraction of them occurred simultaneously with a balloon sferic signature, indicating that most balloon sferics were a result of cloud discharges. An analysis of wind and temperature data during the event showed that the predominance of positive ground flashes is apparently not supported by the tilted dipole hypothesis, being necessary to search for other mechanisms to explain it. Although this observation cannot be considered as usual in this region, it clearly indicates that the flash polarity should depend on many factors besides the season. These factors should be probably related to the geographical location and the meteorological environment.

Venus lightning: An update

The evidence for Venus lightning comes from three distinct sources: the Venera 11–14 landers, the Venera 9 orbiter and the Pioneer Venus orbiter. The largest data set and the one being analyzed most thoroughly at present comes from the VLF electric field experiment on PVO spacecraft. Recent results show that the Poynting flux out of the atmosphere is consistent with that expected from a terrestrial sized source of lightning or possibly one much greater. Estimates of the flash rate from the statistics of impulsive signals also points to a source that is stronger than the terrestrial source. On-going studies of the polarization of the signals and their properties relative to their direction of propagation as well as a recalibration of the Pioneer Venus star sensor promise to provide continuing information on this phenomenon.

Broad-band auroral VLF hiss and inverted-V electron precipitation in the polar magnetosphere

A polar map of the occurrence rate of broad-band auroral VLF hiss in the topside ionosphere was made by a criterion of simultaneous intensity increases more than 5 dB above the quiet level at 5, 8, 16 and 20 kHz bands, using narrow-band intensity data processed from VLF electric field (50 Hz–30 kHz) tapes of 347 ISIS passes received at Syowa Station, Antarctica, between June 1976 and January 1983. The low-latitude contour of occurrence rate of 0.3 is approximately symmetric with respect to the 10–22 MLT (geomagnetic local time) meridian. It lies at 74° around 10 MLT, and extends down to 67° around 22 MLT. The high-latitude contour of 0.3 lies at invariant latitude of about 82° for all geomagnetic local times. The polar occurrence map of broad-band auroral VLF hiss is qualitatively similar to that of inverted-V electron precipitation observed by Atmospheric Explorer.(AE-D) ( Huffman and Lin, 1981, American Geophys. Union, Geophysics Monograph, No. 25, p. 80), especially concerning the low-latitude boundary and axial symmetry of the 10–22 h MLT meridian. The frequency range of the broad-band auroral VLF hiss is discussed in terms of whistler Aode Cerenkov radiation by inverted-V electrons (1–30 keV) precipitated from the boundary plasma sheet. High-frequency components, above 12 kHz of whistler mode Cerenkov radiation from inverted-V electrons with energy below 40 keV, may be generated at altitudes below 3200 km along geomagnetic field lines at invariant latitudes between 70 and 77°. Low-frequency components below 2 kHz may be generated over a wide region at altitudes below 6400 km along the same field lines. Thus, the frequency range of the downgoing broad-band auroral hiss seems to be explained by the whistler mode Cerenkov radiation generated from inverted-V electrons at geocentric distances below about 2 R E (Earth's radius) along polar geomagnetic field lines of invariant latitude from 70 to 77°, since the whistler mode condition for all frequencies above 1 kHz of the downgoing hiss is not satisfied at geocentric distance of 3 r e on the same field lines.

Whistler‐triggered emissions observed by ISIS satellites

VLF emissions triggered by whistlers are often observed at middle and high latitudes in the topside ionosphere by ISIS satellites. Most of them are so‐called LHR emissions lasting for a few seconds. Latitudinal distributions of the occurrence rate for the whistler‐triggered emissions in the topside ionosphere have been obtained by using VLF electric field data (50 Hz to 30 kHz) received from the ISIS 1 and 2 satellites at Kashima station, Communications Research Laboratory, Japan. These VLF emissions are classified into two groups according to the type of whistlers, i.e., ducted whistlers with a continuous trace over the full frequency range of the spectrum and nonducted whistlers without a complete trace below fLHR. The latitudinal distribution of the occurrence rate for emissions triggered by ducted whistlers is considerably different from that for emissions triggered by nonducted whistlers, especially at high latitudes. The occurrence rate for the emissions by nonducted whistlers is distributed rather randomly in latitude between L = 2.0 and L = 4.2. The occurrence rate for emissions by ducted whistlers increases with latitudes between L = 1.5 and L = 2.9, and it attains a maximum of 0.33 at L = 2.7. It then abruptly drops to 0.1 at L = 3.0, and it remains below 0.1 between L = 3.0 and L = 4.0. The decrease of the occurrence rate for emissions by ducted whistlers at L = 3.0 seems to be caused by the decrease of the radiation belt electron flux near the slot region. These results suggest that the VLF emissions triggered by ducted whistlers in the topside ionosphere are generated by the cyclotron resonant interaction of ducted whistlers with the magnetospheric electrons near the geomagnetic equatorial plane. Most VLF emissions triggered by nonducted whistlers seem to be observed as the LHR hiss produced by nonducted whistlers in the vicinity of the satellite in the topside ionosphere.

Simultaneous observations of E-region irregularities by ground-based and rocket-borne techniques

During the Energy Budget Campaign (November–December 1980) E-region irregularities were observed under various geomagnetic conditions. Under disturbed conditions on the night of 30 Nov./1 Dec. the SAFARI auroral radar detected 10m wavelength irregularities north of Lycksele and measured their south to north phase velocity. Simultaneous observations of 1 m irregularities using one of the STARE auroral radars provided measurements of their SW-NE phase velocity. Associated rocket-borne measurements of electron densities over Kiruna using Langmuir probes showed vertical/temporal structure indicating irregularities in ionisation between 90 and 110 km, having a wide range of scale sizes. The power spectra of electron current variability were ‘white’ above about 90 km. 100 min later irregularities in the electric field were detected below 110 km by an E-field probe and an associated TMA release indicated the presence of turbulent regions in the neutral atmosphere between 90 and 105 km. A separate set of rocket-borne measurements under moderately disturbed conditions on the night of 15–16 November 1980 showed the presence of VLF electric fields in the frequency bands 168–5519 Hz at altitudes between 103 and 145 km at Kiruna. On the same occasion measurements of ion density fluctuations were obtained using a rocket-borne positive ion probe at Andøya. Power spectra derived from both these experiments have spectral indices near − 5 3 at those heights where the irregularities were strong. Although this would be consistent with the generation of plasma inhomogeneities by neutral air turbulence below about 95 km, these observations were at too great an altitude for this mechanism to be effective. The results are therefore tentatively interpreted as due to the decay of ion acoustic waves generated by plasma instabilities in the auroral electrojet.

Comment on “Are equatorial electron cyclotron waves responsible for diffuse auroral electron precipitation?” by G. Belmont, D. Fontaine, and P. Canu

Journal of Geophysical Research: Space PhysicsVolume 89, Issue A9 p. 7589-7589 CommentariesFree Access Comment on “Are equatorial electron cyclotron waves responsible for diffuse auroral electron precipitation?” by G. Belmont, D. Fontaine, and P. Canu L. R. Lyons, L. R. LyonsSearch for more papers by this author L. R. Lyons, L. R. LyonsSearch for more papers by this author First published: 1 September 1984 https://doi.org/10.1029/JA089iA09p07589Citations: 5AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL No abstract is available for this article. References Belmont, G., D. Fontaine, P. Canu, Are equatorial electron cyclotron waves responsible for diffuse auroral electron precipitation ?, J. Geophys. Res., 88, 9163, 1983. DeForest, S. E., C. E. McIlwain, Plasma clouds in the magnetosphere, J. Geophys. Res., 76, 3587, 1971. Kennel, C. F., F. L. Scarf, R. W. Fredricks, J. H. McGehee, F. V. Coroniti, “VLF electric field observations in the magnetosphere, J. Geophys. Res., 75, 6136, 1970. Lyons, L. R., Electron diffusion driven by magnetospheric electrostatic waves, J. Geophys. Res., 79, 575, 1974. Mauk, B. H., C. I. Meng, Characterization of geostationary particle signatures based on the “injection boundary” model, J. Geophys. Res., 88, 3055, 1983. Citing Literature Volume89, IssueA91 September 1984Pages 7589-7589 ReferencesRelatedInformation

Standing wave patterns in VLF Hiss

Observations have been made of systematic patterns in VLF hiss, which can be interpreted as a standing wave pattern formed by reflection in the lower ionosphere. Multicomponent VLF electric and magnetic field experiments were flown on three sounding rockets (Nike‐Tomahawk 18.203‐205) from Siple Station. Antarctica during December 1980 to January 1981. One feature of the natural emissions was observed in a very similar form during each flight. A band of hiss, typically from 1.5 to 3 kHz, was seen on the upleg to form a series of closely spaced stripes with whistler‐like dispersion. These first appeared at an altitude of 90–95 km and extended for as much as 40 km in altitude. On the downleg the stripes were observed at the same altitude with the pattern reversed in time. No such patterns were observed by the VLF receivers operating at the same time on the ground at Siple or at its conjugate point. It is suggested that the patterns are interference effects due to downcoming waves reflecting from a layer in the E region and forming a standing wave pattern. The observed stripes are then due to the rocket traversing the standing wave pattern in opposite directions on upleg and downleg. If this interpretation is correct, the fringe spacing should be related to the wavelength and permits a calculation of the refractive index. In one example we calculate n = 43.7 in good agreement with two independent determinations of the refractive index.

Narrow-band 5 kHz hiss observed in the vicinity of the plasmapause

Latitudinal distributions of narrow-band 5 kHz hisses have been statistically obtained by using VLF electric field data received from the ISIS-1 and -2 at Syowa station, Antartica and Kashima station, Japan, in order to study an origin of the narrow-band 5 kHz hisses which are often observed on the ground in mid- and low-latitudes. The result shows that the narrow-band 5 kHz hiss occurs most frequently at geomagnetically invariant latitudes from 55° to 63°, that are roughly the plasmapause latitudes at various geomagnetic activities, both in the northern and southern hemispheres. The narrow-band 5 kHz hiss seems to be generated by the cyclotron instabilities of several keV to a few ten keV electrons for the most feasible electron density of 10 cm −3−10 3 cm −3 in the vicinity of the equatorial plasmapause since the hotter electrons with energy of 10–100 keV are dominant just outside the plasmapause. This will be the origin of the narrow-band 5 kHz hiss observed frequently in mid- and low-latitudes.

Deuteron whistler and trans-equatorial propagation of the ion cyclotron whistler

New ion cyclotron whistlers which have the asymptotic frequency of one half the local proton gyrofrequency, G p 2 , and the minimum (or equatorial) proton gyrofrequency, G pm , along the geomagnetic field line passing through the satellite have been found in the low-latitude topside ionosphere from the spectrum analysis of ISIS VLF electric field data received at Kashima, Japan. Ion cyclotron whistlers with asymptotic frequency of G pm or G pm 2 are observed only in the region of B m > B 2 or rarely B m > B 4 , where B is the local magnetic field and B m is the mini magnetic field along the geomagnetic field line passing through the satellite. The particles with one half the proton gyrofrequency may be the deuteron or alpha particle. Theoretical spectrograms of the electron whistlers ( R-mode) and the ion cyclotron whistlers ( L-mode) propagating along the geomagnetic field lines are computed for the appropriate distributions of the electron density and the ionic composition, and compared with the observed spectrograms. The result shows that the ion cyclotron whistler with the asymptotic frequency of G p 2 is the deuteron whistler, and that the ion cyclotron whistlers with the asymptotic frequency of G pm or G pm 2 are caused by the trans-equatorial propagation of the proton or deuteron whistler from the other hemisphere.

ELF and VLF electric field measurements with the Interkosmos 10 satellite

The ELF-VLF experiment onboard the Interkosmos 10 satellite consisted mainly of a broadband receiver covering the frequency range from 20 Hz to 22 kHz. The signal level in this broad band has also been measured, as well as the level in two narrow bands with centre frequencies of 720 Hz and 4 kHz. The electric component of the ELF-VLF fields has been measured by using an electric dipole antenna, 2.35 m in length. The purpose of this paper is to characterise the data obtained by the broadband RTT transmissions at the Panska Ves station during the seven-months active period of the satellite. The spectral analysis of all broad-band ELF-VLF recordings has been used. Examples of some typical phenomena, most frequently observed at different invariant latitudes are given.

Plasma flow hypothesis in the magnetosphere relating to frequency shift of electrostatic plasma waves

The frequency dynamic spectrum indicating a monotonic frequency shift of the electrostatic electron cyclotron harmonic wave emissions in the data of VLF electric field observations by Ogo 5 has been detected in the midnight and dawn meridians outside the plasmapause; the emissions are produced from turbulent areas in the plasma states that include the temperature anisotropy, loss cone velocity distribution, or the double hump in the velocity distribution function. The frequency shift is also observed for the case of the emissions in the magnetosheath near the subsolar point. These frequency shifts are interpreted on a hypothesis of the Doppler effects of the plasma waves due to the plasma flow with respect to the satellite frame. The speeds of the plasma flow are obtained and have a range from 150 to 300 km/s. Disruption of the plasma flow due to the magnetic field bumps may produce the turbulent source generating the Harris type or beam type instabilities that generate the electrostatic electron cyclotron harmonic waves.

Plasma waves in the dayside polar cusp: 2. Magnetopause and polar magnetosheath

During the outbound pass of Nov. 1, 1968, Ogo 5 sporadically encountered the low-altitude polar cusp at low magnetic latitudes. The spacecraft remained in the cusp beyond six earth radii, and it then traversed the interface region between the magnetospheric cusp and the magnetosheath. Two large scale discontinuities were detected in this sheath-cusp transition region, and several possible interpretations are evaluated here. At 1427 UT, local changes in magnetic field orientation and the variation in ULF magnetic power spectral density were typical of shifts detected at the magnetopause, although the spacecraft did not traverse a true boundary of warm plasma at this point. The second discontinuity, detected at 1456 UT, resembled a collisionless shock, and it was characterized by observations of intense, impulsive VLF electric field bursts and rapid local variations in both total ion flux and differential electron flux. The simplest interpretation is that Ogo 5 had traversed a standing shock within the sheath.

Dependence of the polar cusp on the north-south component of the interplanetary magnetic field

Ogo 5 observations of the polar cusp on November 1, 1968, show that the north-south component of the interplanetary field exhibits control over both the location of and the physical processes occurring in the polar cusp. When the interplanetary field turned from north to south, the polar cusp moved equatorward. During intervals when the interplanetary field was southward, the electron temperature in the polar cusp was lower and the currents were stronger than when the interplanetary field was northward. Also during these intervals of southward field, regions of apparently rapidly varying currents were encountered within the cusp. Associated with these regions were enhanced VLF electric field levels. When the interplanetary field was northward, quasi-monochromatic Pc 1 waves close to but below the proton gyrofrequency and energetic electrons (E > 50 kev) were observed. Most of these observations are consistent with the existence of merging of the interplanetary magnetic field with the dayside magnetospheric field when the interplanetary field is southward and the absence of merging when it is northward. The dependence of electron temperature on field direction remains unexplained.

The Pioneer 9 electric field experiment

We present a detailed analysis of the Pioneer 9 VLF electric field observations for 20 selected storm periods covering a heliocentric range extending from 0.754 AU to 0.99 AU. Although data from only two low frequency channels are available, the results of the present study tend to confirm the preliminary speculation by Scarf and Siscoe (1971) that the turbulentE-field spectrum in the disturbed solar wind has a significant radial gradient.

Recent studies of magnetospheric electric field emissions above the electron gyrofrequency

In a previous paper Kennel et al [1970] presented a survey of VLF electric field observations in the magnetosphere made with the short electric dipoles aboard Ogo 5. Particular emphasis was placed on the geomagnetic equatorial region between L = 4 ,and L = 10. The preliminary findings in that paper suggested that the predominant emissions were those at odd half-integral multiples of the local electron gyrofrequency ƒce and that, among these multiples, emissions at ƒ ≈ 3ƒce/2 were by far the most common, although other multiples, including ƒce/2, were also observed at times. Kennel et al. [1970] noted that the waves have large electric field amplitudes (1–10 mv/m), and they suggested that 3ƒce/2 emissions could be important sources of turbulent energization and pitch angle diffusion for auroral zone electrons.

Color spectrograms of very-low-frequency Poynting flux data

Color frequency-time spectrograms of VLF electric and magnetic field Poynting flux data from Injun 5 satellite

Very-low-frequency electric fields in the interplanetary medium: Pioneer 8

We present Pioneer 8 observations of magnetosheath and interplanetary VLF electric fields, consisting of hourly ranges of the potential amplitude in a broadband channel (0.1 to 100 kHz) and in a 15% bandpass channel centered on 400 Hz. Significant signals are correlated with position with respect to earth, and with solar wind plasma parameters obtained from the lunar orbiting Explorer 35 spacecraft. We detect two principal features: noise bursts or spikes with duration less than approximately 10 sec, and persistent signals with durations typically 1 day or more. The noise bursts coincide with plasma and magnetic field discontinuities where these data are available for comparison. The persistent signals correlate loosely with solar wind density, and this correlation holds whether the density increases are due to interplanetary shocks, the ‘snow plow’ effect, or other processes. Although the experiment is too limited to provide unambiguous determination of the wave modes, at present it appears most likely that ion acoustic waves have been detected.

VLF electric field observations in the magnetosphere

Magnetospheric VLF electric field emissions above electron cyclotron frequency from OGO 5 observation at magnetic equator