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1.
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?103 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.  相似文献   

2.
Extremely low frequency (ELF)/Very low frequency (VLF) hiss is whistler mode wave that interacts with energetic electrons in the magnetosphere. The characteristics features of ELF/VLF hiss observed at low latitude ground station Jammu (Geomag. lat. 22°16′ N, L=1.17) are reported. It is observed that most of hiss events first propagate in ducted mode along higher L-values (L = 4–5), after reaching lower edge of ionosphere excite the Earth-ionosphere waveguide and propagate towards equator to be received at low-latitude station Jammu. To understand the generation mechanism of ELF/VLF hiss, incoherent Cerenkov radiated power from the low-latitude and mid-latitude plasmasphere are evaluated. Considering this estimated power as an input for wave amplification through wave–particle interaction, the growth rate and amplification factor is evaluated which is too small to explain the observed wave intensity. It is suggested that some non-linear mechanism is responsible for the generation of ELF/VLF hiss.  相似文献   

3.
Observations and analyses of hiss events, recorded at College (dp. lat. 64.62°N) and Bar 1 (dp. lat. 70.20°N) during periods of varying auroral and geomagnetic activity, reveal three different types of events. These are (1) auroral substorm events with associated hiss bursts during disturbed period, (2) quiet-time hiss events accompanying stationary quiet auroral arcs and (3) hissless events at times of auroral and magnetic activity. Quiet-time observations seem to suggest that the substorm activity is not a necessary requirement for generating wideband hiss. On the other hand, examples of auroral and magnetic activity with complete absence of VLF hiss indicate that the ground reception of VLF/ELF natural emissions is largely controlled by propagation conditions in the ionosphere. There is either little or no correlation found between hiss observations at the two stations separated by about 600 km.  相似文献   

4.
Synoptic observations made on magnetic recording tape at Huancayo, Peru, at the magnetic dip equator, during the International Geophysical Year 1957–1958, were aurally reviewed at that time and no whistlers, hiss, or other emissions were heard. In view of the more recent observation of whistlers at geomagnetic latitudes as low as 12°, and in conjunction with a study of equatorial hiss observed in the topside ionosphere, these recordings have recently been reassessed by reducing them with modern real-time, digital spectrographic equipment. Although the observations were found to be of high quality, and to show the classical features of ground-wave and sky-wave propagation of sferics and VLF transmissions, again no evidence whatsoever of whistlers, hiss, or other emissions is found. Thus it is concluded that the whistlers observed at very low latitudes do not propagate subionospherically to the equator and it is confirmed that “hybrid” whistlers must be due to subionospheric propagation across the equator of the causative sferic rather than of the short whistler.  相似文献   

5.
The purpose of the paper is to present the statistical characterictics of mid-latitude VLF emissions (both unstructured hiss and structured emissions) based on the VLF data obtained at Moshiri in Japan (geomag. lat. 35°; L = 1.6) during the period January 1974–March 1984. Local time dependence of occurrence rate and the association with geomagnetic disturbances have been studied for both types of emissions. Both types (unstructured and structured) of mid-latitude VLF emissions are found to have definite correlations with geomagnetic disturbances. Then, the time delay of the emission event behind the associated geomagnetic disturbance has enabled us to estimate the resonant electron energy for VLF hiss to be 5 keV at L = 3–4 and that for structured VLF emissions to be considerably larger, such as 20 keV at L 4. Combined considerations of these estimated resonant energies, theoretical electron drift orbits and the local time dependences, allow us to construct the following model to explain the experimental results in a reasonable way. Electrons in a wide energy range are injected during disturbances around the midnight sector, followed by the eastward drift. Lower energy ( 5 keV) electrons tend to drift closer to the Earth, resulting in the dawnside enhancement of VLF hiss within the plasmasphere. Further, these lower energy electrons are allowed to enter the duskside asymmetric plasmaspheric bulge and to generate VLF hiss there. On the other hand, higher energy (20 keV) electrons tend to drift at L shells farther away from the Earth and those substorm electrons are responsible for the generation of structured VLF emissions around dawn due to an increase of plasma density from the sunlit ionosphere. However, such higher energy electrons are forbidden from entering the duskside of the magnetosphere and so we cannot expect a duskside peak in the occurrence of structured VLF emissions, which is in agreement with the experimental result.  相似文献   

6.
A unique night-time natural electromagnetic disturbances in the VLF/ELF range received during a magnetically quite period at a low latitude Indian ground station, Jammu (geomag. lat. 19°26′ N, L=1.17) has been reported. During the routine observation of VLF waves at Jammu, whistlers and different types of VLF/ELF emissions such as whistlers of varying dispersion confined to a small band limited frequency range, hisslers, pulsing hiss, discrete chorus emissions of rising and falling tones with multiple bands, oscillating tone discrete emission, whistler-triggered hook and discrete chorus risers emissions, etc. have been observed simultaneously during the quiet period on a single night. Such type of unique simultaneous observations has never been reported from any of the low latitude ground stations and this is the first observation of its kind. The results are discussed in the light of recorded features of whistlers and emissions. Generation and propagation mechanism are discussed briefly. Plasma parameters are further derived from the dispersion analysis of nighttime whistlers and emissions recorded simultaneously during magnetically quiet periods.  相似文献   

7.
Extreme low frequency electromagnetic waves have been observed below the F peak in the equatorial ionosphere by instruments onboard OGO-6. Electrostatic wave observations indicate that the steep gradient was unstable to the process which causes equatorial spread F above the region where the electromagnetic waves were observed. The data are very similar to observations near the polar cusp and give further evidence that ELF waves are excluded from regions of rapid and irregular density increases. Low level electromagnetic waves with similar properties were occasionally observed on the nightside by the OVI-17 electric field sensor and may be plasmaspheric hiss which has propagated to low altitude.  相似文献   

8.
Comparison of the low altitude polar orbiting Injun 5 Satellite data with the ground VLF data has revealed that there is a definite scarcity of VLF/ELF emissions at the ground level compared with the extent to which they are present at or above the auroral altitudes. Reasons for this have been investigated by performing ray path computations for whistler mode VLF propagation in an inhomogeneous and anisotropic medium, such as the magnetosphere and the ionosphere. Based on wave normal computations in the lower ionosphere, it has been found that many of the near-auroral zone VLF/ELF events are frequently either reflected from, or heavily attenuated in, the lower ionosphere. Besides collisional loss, severe attenuation of VLF signals in the lower ionosphere is also caused by the divergence of ray paths from the vertical (spatial attenuation). Cone of wave normal angles for the wave, within which VLF/ ELF signals are permitted to reach the ground, has been established. Wave normals lying outside this transmission cone are reflected from the lower ionosphere and do not find exit to the Earth-ionosphere cavity. Computations for VLF signals produced at auroral zone distances in the equatorial plane of the magnetosphere indicates that these signals are more or less trapped in the magnetosphere at altitudes > 1RE.  相似文献   

9.
During August 1972, Explorer 45 orbiting near the equatorial plane with an apogee of ～5.2 Re traversed magnetic field lines in close proximity to those simultaneously traversed by the topside ionospheric satellite ISIS 2 near dusk in the L range 2.0–5.4. The locations of the Explorer 45 plasmapause crossings (determined by the saturation of the d.c. electric field double probe) during this month were compared to the latitudinal decreases of the H+ density observed on ISIS 2 (by the magnetic ion mass spectrometer) near the same magnetic field lines. The equatorially determined plasmapause field lines typically passed through or poleward of the minimum of the ionospheric light ion trough, with coincident satellite passes occurring for which the L separation between the plasmapause and trough field lines was between 1 and 2. Hence, the abruptly decreasing H+ density on the low latitude side of the ionospheric trough is not a near earth signature of the equatorial plasmapause. Vertical flows of the H+ ions in the light ion trough as detected by the magnetic ion mass spectrometer on ISIS were directed upward with velocities between 1 and 2 km s?1 near dusk on these passes. These velocities decreased to lower values on the low latitude side of the H+ trough but did not show any noticeable change across the field lines corresponding to the magnetospheric plasmapause. The existence of upward accelerated H+ flows to possibly supersonic speeds during the refilling of magnetic flux tubes in the outer plasmasphere could produce an equatorial plasmapause whose field lines map into the ionosphere at latitudes which are poleward of the H+ density decrease.  相似文献   

10.
In order to investigate Pc3-4 geomagnetic pulsations at very low and equatorial latitudes, L=1.0 to 1.2, we analyzed simultaneous geomagnetic data from Brazilian stations for 26 days during October-November 1994. The multitaper spectral method based on Fourier transform and singular value decomposition was used to obtain pulsation power spectra, polarization parameters and phase. Eighty-one (81) simultaneous highly polarized Pc3-4 events occurring mainly during daytime were selected for the study. The diurnal events showed enhancement in the polarized power density of about 3.2 times for pulsations observed at stations close to the magnetic equator in comparison to the more distant ones. The phase of pulsation observed at stations near the magnetic equator showed a delay of 48-62° in relation to the most distant one. The peculiarities shown by these Pc3-4 pulsations close to the dip equator are attributed to the increase of the ionospheric conductivity and the intensification of the equatorial electrojet during daytime that regulates the propagation of compressional waves generated in the foreshock region and transmitted to the magnetosphere and ionosphere at low latitudes. The source mechanism of these compressional Pc3-4 modes may be the compressional global mode or the trapped fast mode in the plasmasphere driving forced field line oscillations at very low and equatorial latitudes.  相似文献   

11.
This paper presents some features of the ionospheric response observed in equatorial and mid-latitudes region to two strong geomagnetic storms, occurring during Oct. 19–23, 2001 and May 13–17, 2005 and to understand the phenomena of pre-storm that lead to very intense geomagnetic storms. The result point to the fact that pre-storm phenomena that leads to intense ionospheric storm are; large southward turning of interplanetary magnetic field Bz, high electric field, increase in flow speed stream, increase in proton number density, high pressure ram and high plasma beta. The magnitude of Bz turning into southward direction from northward highly depends upon the severity of the storm and the variation in F2 layer parameter at the time of geomagnetic storm are strongly dependent upon the storm intensity. A detailed analysis of the responses of the ionosphere shows that during the storm periods, foF2 values depleted simultaneously both in the equatorial and mid latitude. Observation also shows that low to moderate variations in ionospheric F2 at the pre-storm period may signal the upcoming of large ionospheric disturbances at the main phase. The ionospheric F2response for low and mid latitude does not show any significant differences during the storm main phase and the pre-storm period. The ionospheric response during the pre-storm period is thought very puzzling. The period is observed to be depleted throughout with low-moderate effect across all the stations in the low and mid latitude.  相似文献   

12.
A detailed analysis of rapid-run magnetograms from Guam (geomagnetic latitude = 4.2°) revealed that there are two kinds of geomagnetic sudden commencement (SC) observed in nighttime. One is the ordinary SC consisting of a main impulse only which has a smooth rise of the H-component. The other is a superposition by a small positive impulse on the very beginning part of the smooth rise of the main impulse and consequently the SC starts with a small stepwise increase of the H-component. The latter type of SC occurs between 20 and 08 h L.T. and its occurrence rate takes the maximum value of about 50% around 03 h L.T. Corresponding magnetograms from a dayside equatorial station (Huancayo, geomagnetic latitude = ?0.7°) were examined and a good correlation was found between the stepwise SC at the nightside (Guam) and SC1 with a preliminary reverse impulse (PRI) at the dayside (Huancayo). Since PRI observed at the dayside equator may be interpreted as an extension of an ionospheric current due to an dusk-to-dawn electric field impressed on the polar ionosphere, our results show that a polar originating ionospheric current can extend to the nightside equator and produce a small but observable magnetic effect in spite of much reduced nighttime ionospheric conductivity.  相似文献   

13.
It is assumed that whistler ducts are formed by electric fields interchanging magnetospheric flux tubes of ionization. It is found that such ducts end several hundred kilometres above the transition level, that is usually in the altitude range of 1000–1500 km. Further, the enhancement factor is found to increase towards the equator if the background density has little latitudinal variation. Both of the above properties make such ducts ideal for trapping whistlers.The half-life of whistler ducts is estimated to be of the order of one day. During quiet times ducts decay through enhanced plasma flow into the underlying ionosphere, whereas during storm times, when the plasmasphere is depleted of ionization, large upward plasma flows reduce the enhancement factors of ducts.  相似文献   

14.
VLF whistler mode signals have previously been used to infer radial plasma drifts in the equatorial plane of the plasmasphere and the field-aligned ionosphere-protonosphere coupling fluxes. Physical models of the plasmasphere consisting of O+ and H+ ions along dipole magnetic field lines, and including radial Ez × B drifts, are applied to a mid-latitude flux tube appropriate to whistler mode signals received at Wellington, New Zealand, from the fixed frequency VLF transmitter NLK (18.6 kHz) in Seattle, U.S.A. These models are first shown to provide a good representation of the recorded Doppler shift and group delay data. They are then used to simulate the process of deducing the drifts and fluxes from the recorded data. Provided the initial whistler mode duct latitude and the ionospheric contributions are known, the drifts at the equatorial plane can be estimated to about ± 20 ms?1 (～10–15%), and the two hemisphere ionosphere-protonosphere coupling fluxes to about ± 1012 m?2 s?1 (～40%).  相似文献   

15.
Dynamic behavior of the coupled ionosphere-protonosphere system in the magnetospheric convection electric field has been theoretically studied for two plasmasphere models. In the first model, it is assumed that the whole plasmasphere is in equilibrium with the underlying ionosphere in a diurnal average sense. The result for this model shows that the plasma flow between the ionosphere and the protonosphere is strongly affected by the convection electric field as a result of changes in the volume of magnetic flux tubes associated with the convective cross-L motion. Since the convection electric field is assumed to be directed from dawn to dusk, magnetic flux tubes expand on the dusk side and contract on the dawn side when rotating around the earth. The expansion of magnetic flux tubes on the dusk side causes the enhancement of the upward H+ flow, whereas the contraction on the dawn side causes the enhancement of the downward H+ flow. Consequently, the H+ density decreases on the dusk side and increases on the dawn side. It is also found that significant latitudinal variations in the ionospheric structures result from the L-dependency of these effects. In particular, the H+ density at 1000 km level becomes very low in the region of the plasmasphere bulge on the dusk side. In the second model, it is assumed that the outer portion of the plasmasphere is in the recovery state after depletions during geomagnetically disturbed periods. The result for this model shows that the upward H+ flux increases with latitude and consequently the H+ density decreases with latitude in the region of the outer plasmasphere. In summary, the present theoretical study provides a basis for comparison between the equatorial plasmapause and the trough features in the topside ionosphere.  相似文献   

16.
A method of measuring the radial component of plasma velocity (or azimuthal electric field) in the equatorial plane of the plasmasphere, and of measuring the flux of plasma between the plasmasphere and the ionosphere is presented. The method uses measured rates of change of phase path and group delay of whistler-mode VLF signals transmitted between geomagnetically conjugate points.Results from four nights are in general agreement with results published by others—equatorial radial velocities of tens of metres per second (azimuthal electrical fields of one or two tenths of a mV/m), fluxes a few times 1012 m?2 s?1-and illustrates the potential and limitations of the method.  相似文献   

17.
PL whistlers     
Simultaneous ground and satellite VLF observations together with raytracing studies clearly establishes the existence of ground observed PL whistlers. The dynamic spectrum (?-ν-t shape) of observed PL whistlers may be reproduced exactly by raytracing in TLG magnetospheric models consistent with lower ionosphere, topside ionosphere and equatorial density measurements. The Transition Level Gradient (TLG) model is based on the observation that the transition level altitude increases towards the plasmapause (Titheridge, 1976). PL ground whistlers (i) are observed downgoing over large latitudinal ranges, for up to 2000 km of satellite travel, by ISIS II at 1400 km altitude, (ii) have almost the same dynamic spectrum over the entire latitudinal range observed by ISIS II, (iii) are indistinguishable from ducted whistlers over the observed frequency range (i.e. linear Q for ? < 10 kHz), (iv) have nose frequencies > 16 kHz, (v) at 1400 km altitude have a lower latitudinal cutoff at L ～ 2 and a higher latitudinal cutoff between L ～ 3 and L ～ 4 and (vi) probably only occur at night-time during or immediately following disturbed magnetic activity.  相似文献   

18.
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/m2/Hz1 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 108cm?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.  相似文献   

19.
Cross-spectral analysis of ULF wave measurements recorded at ground magnetometer stations closely spaced in latitude allows accurate determinations of magnetospheric field line resonance (FLR) frequencies. This is a useful tool for remote sensing temporal and spatial variations of the magnetospheric plasma mass density. The spatial configuration of the South European GeoMagnetic Array (SEGMA, 1.56 <  L <  1.89) offers the possibility to perform such studies at low latitudes allowing to monitor the dynamical coupling between the ionosphere and the inner plasmasphere. As an example of this capability we present the results of a cross-correlation analysis between FLR frequencies and solar EUV irradiance (as monitored by the 10.7-cm solar radio flux F10.7) suggesting that changes in the inner plasmasphere density follow the short-term (27-day) variations of the solar irradiance with a time delay of 1–2 days. As an additional example we present the results of a comparative analysis of FLR measurements, ionospheric vertical soundings and vertical TEC measurements during the development of a geomagnetic storm.  相似文献   

20.
The association between VLF hiss and auroral-light intensity has been studied for pulsating auroras by coordinated observations with a broad band VLF receiver and a low light level TV system viewing the N2+ ING emissions. Power spectral analyses of the VLF hiss and auroral-light intensity fluctuations display a common peak at 1.3 ± 0.3 Hz. Cross-spectral analysis shows that the times of the peaks in the auroral-light intensity fluctuations differ from those of the VLF hiss by times ranging between zero and 0.2 s. This result is shown to be compatible with a cyclotron resonance interaction in the vicinity of the equatorial plane. The periodicity of the intensity fluctuations can be accounted for by assuming the process is driven by echoing VLF hiss, which may be single-phase or three-phase.  相似文献   

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