Relations of VLF emissions to impulsive electron precipitation measured by EISCAT radar in the morning sector of auroral oval (original) (raw)
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Annales Geophysicae, 2006
Using the data from satellite CRRES and three geostationary LANL spacecraft, the propagation of an electron cloud from midnight to the evening sector is investigated. An electron cloud was injected during a weak isolated substorm that developed on a quiet geomagnetic background. It is found that within the local time sector from 03:00 until at least 08:00 MLT, the propagation of electrons at perpendicular pitch-angles is well described by a simple model of drift in the dipole magnetic field. The flux levels in the field-aligned electrons increase simultaneously with the flux at perpendicular pitch angles, which is attributed to the pitch angle diffusion by the whistler mode. This pitch-angle diffusion leads to precipitation of electrons from a drifting cloud and an increase in the ionospheric electron density, simultaneously observed above Tromsø, Norway, by the EISCAT UHF radar in the morning sector (04:40-05:25 MLT). The precipitation develops as quasi-periodic pulses with a period of about 100 s. We discuss the models of pulsating precipitation due to the whistler cyclotron instability and show that our observations can be explained by such a model.
Auroral pulsations and accompanying VLF emissions
Annales Geophysicae, 1999
Results of simultaneous TV observations of pulsating auroral patches and ELF-VLF-emissions in the morning sector carried out in SodankylaÈ (Finland) on February 15, 1991 are presented. Auroral pulsating activity was typical having pulsating patches with characteristic periods of about 7 s. Narrow-band hiss emissions and chorus elements at intervals of 0.3±0.4 s formed the main ELF-VLF activity in the frequency range 1.0±2.5 kHz at the same time. The analysis of auroral images with time resolution of 0.04 s allowed perfectly separate analysis of spatial and temporal variations in the auroral luminosity. Mutual correspondence between the behaviour of the luminous auroral patches and the appearance of ELF noise type hiss emissions and VLF chorus trains was found in two intervals chosen for analysis. While the hiss emissions were associated with the appearance of luminosity inside a limited area close to the zenith, the structured VLF emissions were accompanied by rapid motion of luminosity inside the area. The spatial dimension of the pulsating area was about 45±50 km and luminosity propagated inside it with velocity of about 10±12 kms. We discuss a new approach to explain the 5±15 s auroral pulsation based on the theory of¯owing cyclotron maser and relaxation characteristics of ionosphere.
Journal of Geophysical Research, 1997
Subionospheric very low frequency and low-frequency (VLF/LF) transmitter signals received at middle-latitude ground stations at nighttime were found to exhibit pulsating behavior with periods that were typically in the •5-60 s range but sometimes reached •100 s. The amplitude versus time shape of the pulsations was often triangular or zigzag-like, hence the term "zigzag effect." Variations in the envelope shape were usually in the direction of faster development than recovery. Episodes of zigzag activity at Siple, Antarctica (L-4.3), and Saskatoon, Canada (L •4.2), were found to occur widely during the predawn hours and were not observed during geomagnetically quiet periods. The fluctuations appeared to be caused by ionospheric perturbations at the-• 85 km nighttime VLF reflection height in regions poleward of the plasmapause. We infer that in the case of the Saskatoon and Siple data, the perturbations were centered within-•500 km of the stations and within •100-200 km of the affected signal paths. Their horizontal extent is inferred to have been in the range •50-200 km. The assembled evidence, supported by Corcuffs [1996] recent research at Kerguelen (L-3.7), suggests that the underlying cause of the effect was pulsating auroral precipitation. The means by which that precipitation produces ionospheric perturbations at 85 km is not yet clear. Candidate mechanisms include (1) acoustic waves that propagate downward from precipitation regions above the-• 85 km VLF reflection level; (2) quasi-static perturbation electric fields that give rise to ExB drifts of the bottomside ionosphere; (3) secondary ionization production and subsequent decay at or below 85 km. Those zigzag fluctuations exhibiting notably faster development than recovery probably originated in secondary ionization produced near 85 km by the more energetic (E ;>40 keV) electrons in the incident electron spectrum. 1. Introduction In recent years, subionospherically propagating •-20-50 kHz signals from very low frequency and low-frequency (VLF/LF) communication transmitters have increasingly been used as probes of ionospheric processes that occur near •-85 km, the inferred nighttime upper re
VLF electric field observations in the magnetosphere
Journal of Geophysical Research, 1970
New types of magnetospheric electric field emissions have been detected by Ogo 5 with wave frequencies ([) above the local electron cyclotron frequency ([c). The most common emissions occur between the lowest electron cyclotron harmonics ([ • 3•c/2), followed by emissions at high frequencies ([ >> [c). A few signals have been observed slightly above the local [c. The strong emissions with [ >• 3[c/2 appear to be localized within a few degrees of t. he geomagnetic equator. More than 60% of the equator crossings between 4 < L < 10 in the 0000-1200 LT sector have such activity. The waves have large electric field amplitudes (1-10 my/m), which suggests they may be sources of pitch angle diffusion and turbulent energization of auroral zone electrons.
New type of ensemble of quasi-periodic, long-lasting VLF emissions at the auroral zone
Annales Geophysicae, 2012
A new type of the series of quasi-periodic (QP) very low frequency (VLF) emissions in frequency range of 1-5 kHz, and not associated with geomagnetic pulsations, has been discovered at auroral latitudes (L = 5.3) during the Finnish VLF campaign (held in December 2011). At least five unusually spectacular events, each with a duration of several hours, have been observed during the night under conditions of quiet geomagnetic activity (Kp = 0-1), although QPs usually occur during the daytime. Contrary to the QP emissions typically occurring during the day, the spectral structure of these QP events represented an extended, complicated sequence of repeated discrete rising VLF signals. Their duration was about 2-3 min each, with the repetition periods ranging from ∼1 min to ∼10 min. Two such nighttime nontypical events are reported in this paper. The fine structure of the separated QP elements may represent a mixture of the different frequency band signals, which seem to have independent origins. It was found that the periodic signals with lower frequency appear to trigger the strong dispersive upper frequency signals. The temporal dynamics of the spectral structure of the QPs studied were significantly controlled by some disturbances in the solar wind and interplanetary magnetic field (IMF). This finding is very important for future theoretical investigations because the generation mechanism of this new type of QP emissions is not yet understood.
VLF emissions associated with enhanced magnetospheric electrons
Journal of Geophysical Research, 1977
During periods of geomagnetic disturbances, VLF emissions and enhancements of low-energy electrons are simultaneously observed by the equatorial orbiting Sa-A (Explorer 45) satellite. These events are characterized by the following features. (1) The VLF emissions occur outside the plasmasphere in the nightside of the magnetosphere. (2) The VLF emissions consist of two frequency regimes, one below the local electron gyrofrequency fg and the other above fg. (3) The VLF emissions below fg are relatively broadband whistler mode waves characteristic of chorus and frequently have a conspicuous band of 'missing emissions' near fs/2. (4) The emissions above fs are electrostatic and typically have components near 3f•/2. Occasionally, higher-frequency components are also observed. (5) The onset of the emissions coincides with abrupt increases outside the plasmasphere (L >• 4) in 1-to 10-keV electrons to intensities of the order of 11Y el cm -•' s -• sr -• keV -•. Less pronounced enhancements sometimes occur for electrons with energies as high as 70 keV. (6) The cessation of the emissions coincides with a drop in the electron intensities to their preenhancement levels, which are of the order of 11Y el cm -2 s -• sr -• keV-• or less. This drop in low-energy electron intensities occurs before or when the satellite crosses the plasmapause back into the plasmasphere. These observed features indicate that the ¾LF emissions are produced by lowenergy (1-to 10-keY) electrons which penetrate into the dusk-midnight sector of the magnetosphere from the geomagnetic tail during magnetic storms and substorms and drift eastward outside the plasmasphere. In this paper, events observed during geomagnetically disturbed periods in December 1971 and January 1972 are discussed.
A new type of daytime high-frequency VLF emissions at auroral latitudes (“bird emissions”)
Geomagnetism and Aeronomy, 2017
This paper is concerned with a new, previously unknown type of high-frequency (above 4 kHz) VLF emissions that were detected during winter VLF campaigns in Kannuslehto (L ~ 5.5), Finland. These previously unknown emissions have been discovered as a result of the application of special digital filtering: it clears the VLF records from pulse signals of intensive atmospherics, which prevent other kinds of VLF emissions in the same frequency range from being seen on spectrograms. As it appears, aside from wellknown bursts of auroral hisses and discrete quasiperiodic emissions, a previously unknown type of daytime right-hand polarized VLF waves is also present at frequencies above 4 kHz. These emissions can persist for several hours as series of separate short discrete wideband (from 4 to 10 kHz and higher) signals, each with a duration between one and several minutes. It has been found that such signals can be observed almost daily in winter. These emissions sound like bird's chirping to a human ear; for that reason, they were called "bird emissions." The dynamic spectra of individual signals often resemble flying birds. The signals are observed during daytime, more often in magnetically quiet conditions preceded by geomagnetic disturbances. As a rule, the occurrence of these bird emissions is accompanied by a slight increase in electron density in the lower ionosphere, which is evidence of the precipitation of energetic (>30 keV) electrons. This raises a number of questions as to where and how the VLF bird emissions are generated and how such emissions, at frequencies greatly exceeding half the electron equatorial gyrofrequency at L ~ 5.5, can reach the Earth's surface.
Ground-based Auroral Hiss Recorded at Northern Finland with Reference to Magnetic Substorms
Very low frequency (VLF) auroral hiss at Kannuslehto (KAN), Finland, was analyzed with reference to the progress of 98 isolated magnetic substorms measured during the winter months of 2015-2018. Of these, 91 were accompanied by auroral hiss during the substorm growth phase. No auroral hiss was recorded during the expansion and recovery phases. We found that auroral hiss was observed under rising polar cap (PC) index, showing an increased solar wind energy input into the magnetosphere during the substorm growth phase. We also found that in 58 of the 65 events studied, KAN was located in the vicinity of enhanced field-aligned currents (FACs) during auroral hiss occurrence. For the first time, it was established that auroral VLF hiss generation in the equatorial part of the auroral oval is a typical signature of a substorm growth phase. Plain Language Summary Auroral hiss is a well-known type of nighttime natural VLF emission with a noise-like structure generated by the Cherenkov instability of precipitating soft electrons above the ionosphere. Auroral hiss occurrence up to 39 kHz was studied in the equatorward region of the auroral oval at the Finnish station Kannuslehto (KAN, MLAT = 64.2°N) during 11 winter months in 2015-2018. During this time interval, 98 isolated and rather powerful magnetic substorms were recorded over Scandinavia. In 93% of the substorms studied, an auroral VLF hiss was recorded at the same time as enhancement of field-aligned currents (FACs). FACs are caused by soft electron precipitation which could be a plausible source of the auroral VLF hiss generation. For the first time, it was found that auroral VLF hiss occurrence in the equatorward region of the auroral oval is a typical signature of the substorm growth phase.
The behaviour of the auroral emissions 5577 Å and 6300 Å and the ratio I 6300 /I 5577 during substorms occurred at the time of recurrent streams (RS) has been examined. The development of the substorm bulge is followed up. The variations of the emissions depending on the different locations of the substorm bulge with respect to the point of observation have been studied. Estimations of the particle precipitation spectra at the polar edge of the auroral bulge and inside it have been obtained. For the study, data from the All-Sky Imagers at Andøya Rocket Range (ARR), Andenes, Norway (69.3°N, 16.03°E) and at the Auroral Observatory, Longyearbyen, Svalbard (78.20°N, 15.83°E) from the observational season 2005-2006 have been used. Data access has been provided under the Project "ALOMAR eARI" (RITA-CT-2003-506208), Andenes, Norway. This Project received research funding from the European Community's 6th Framework Program. Additional data concerning the solar wind parameters, IMF and the magnetic field are used from the WIND satellite and the IMAGE magnetometer network to determine the recurrent streams and the substorms during RS.