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1.
The non-ducted whistler propagation in the inner magnetosphere is discussed using the broad-band VLF measurement on board the K-9M-26 rocket launched at 1703 hr JST on 24 August 1969 from Kagoshima Space Center (geomagnetic lat 20°N). A large number of whistlers which seemed to be two-hop whistlers originating in the northern hemisphere were observed. The main features of these whistlers are summarized: (1) their dispersion value is widely scattered in the range 55–75 sec12, (2) their frequency spectra show a broad maximum in the frequency range 2–5 kHz and higher frequency components are likely to disappear. Attempts are made to interpret these properties in terms of ducted or non-ducted propagation. It is then found from the ray tracing studies that the measurements are satisfactorily explained by non-ducted propagation in the inner magnetospheric model with latitudinal density gradient such as the equatorial anomaly.  相似文献   

2.
The propagationmechanism of low latitude daytime whistlers is investigated on the basis of ground measurements made continuously during daytime in North India at Jammu (geomag. lat. 22°26°N;L = 1.17). On February 14, 1998 extremely small dispersion (ESD) whistlers with dispersion varying from 5–10 sec1/2 in surprisingly large numbers were recorded at Jammu during daytime in the late afternoon. The results of a study of the characteristics of ESD whistlers are presented and the discussion indicates that ESD whistlers recorded are the VLF waves radiated from the return stroke of the lightning discharge launched at the ionosphere with different initial wave normal angles, propagated upwards under eitherquasi-longitudinal conditions or pro-longitudinal whistler mode, turned around at different heights due to quasi-transverse propagation and received at Jammu with the dispersion of the order of 5–10 sec1/2. The validity of this suggestion has been tested by performing actualray-tracing computations in thepresence of equatorial anomaly model. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

3.
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   

4.
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5.
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   

6.
《Planetary and Space Science》1987,35(8):1053-1059
Whistler-mode signals were measured at the conjugate point (L = 1.93) in South Australia of the VLF transmitter in Eastern U.S.S.R. The intensity of whistler-mode signals increased usually at sunset and occasionally at sunrise, and was moderate during night-time. By means of the calculation of the wave attenuation in the lower ionosphere, the result may be interpreted in terms of different mechanisms of propagation in the magnetosphere and transmission out of the ionosphere: spatially coherent propagation in a field-aligned duct and transmission out of the duet onto the ground at sunset and sunrise; and spatially incoherent propagation and transmission at night. A high correlation was found between the occurrence probability of the 14.881 kHz whistler-mode signals and the occurrence number of whistlers, which may suggest that the occurrence of whistlers is controlled predominantly by propagation conditions.  相似文献   

7.
During the flight of a Petrel rocket, instrumented by the SRC Radio and Space Research Station with Geiger counters and launched westwards from South Uist, Outer Hebrides, Scotland (L=3.38), a transient increase was observed in the intensity of energetic electrons having pitch angles between 60 and 120°. The increase, by a factor of 20 above the quasi-steady intensity observed throughout the remainder of the flight, occurred in 0.8 sec and was simultaneous for both >45 keV and >110 keV electrons. Recorded ～0.5 sec later, on the ground, was a two-hop whistler. During the enhanced electron intensity event, the entire duration of which was ～6 sec, the four-, six- and eight-hop whistlers were also received. From an analysis of the whistlers' spectrogram, it is concluded that the whistlers were ducted through the magnetosphere along the L=3.3 ±0.1 field line; the electron density in the equatorial plane is found to be 330 ±10 cm?3, a value characteristic of conditions within the plasmapause. It is suggested that these temporally and/or spatially associated phenomena, rather than arising by a chance coincidence, were the result of a gyroresonant interaction between energetic electrons and whistler mode waves moving in opposite directions. For gyroresonance on this field line at the equator, the parallel component of energy of the electrons is 25 keV at 3 kHz in the whistler band, or 100 keV at 1 kHz below it. It is suggested that a magnetospheric event occurred, causing both sudden enhanced electron precipitation and favourable conditions for the propagation and/or amplification of whistlers. A possible explanation is that energetic electrons, having a sufficiently anisotropic distribution function and associated with those injected during an earlier auroral substorm, become unstable via the transverse resonance instability when they drift into the plasmasphere, a region of high density thermal plasma.  相似文献   

8.
Employing the Haselgrove ray tracing equations and a diffusive equilibrium model of the ionosphere, the propagation characteristics of hook whistlers recorded at low-latitude ground station Varanasi (geomag. lat., 16°6′.N) are discussed. It is shown that the two traces of the hook whistlers are caused by the VLF waves radiated from the return stroke of a lightning discharge which after penetrating the ionosphere at two different entry points, propagated to the opposite hemisphere in the whistler mode and were received at 16 geomagnetic latitude. Further the crossing of ray paths for the same frequency leads to the explanation of the hook whistler. The lower and higher cut-off frequencies are explained in terms of their deviating away from the bunch of the recorded whistler waves and crossing of ray paths for the same frequency.  相似文献   

9.
The propagation features of extremely low frequency electromagnetic waves through the multicomponent ionospheric plasma are studied. It is shown that at relatively lower frequencies refractive index for right hand mode is higher than the left-hand mode, which is reversed at higher frequencies. The thermal temperature of plasma particle causes decrease in phase and group velocities of both right and left-hand modes. The crossover frequencies for different plasma models are computed and variation with ion concentration and thermal velocity is studied. Explicit expression for group velocity and travel time has been derived and studied numerically. Finally, we have presented simulation of the ion whistler spectrograms for Hydrogen, Helium and Oxygen ions present in the ionospheric plasma. The results are compared with the experimentally detected hydrogen and helium ion whistlers. The importance of the present study in the exploration of ionospheric plasma is illustrated.  相似文献   

10.
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   

11.
In this study, the physical structure for the propagation of whistler waves within a duct in the Earth's magnetosphere is investigated by means of magnetohydrodynamic (MHD) theory. Expressions for the current density and induced magnetic field are determined analytically and evaluated in terms of two models for the duct plasma density distribution. It is found that once the duct is formed, forces associated with the current structure will maintain it. MHD instabilities are examined briefly and found to be unlikely to threaten duct maintenance in regions where whistlers are typically observed. Examination of some effects of field-aligned currents suggest that this may be a viable mechanism for duct formation.  相似文献   

12.
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   

13.
An attempt has been made to estimate the east-west component (Ew) of the magnetospheric equatorial electric field near L = 1.12 during a magnetic storm period from the whistlers observed at our low latitude ground station, Nainital (geomag.lat. 19°1'N), on March 25, 1971 in the 0130–0500 IST sector. The method of measuring Ew from the observed cross L-motions of whistler ducts within the plasmasphere, indicated by changes in nose frequency of whistlers, has been outlined. The nose frequencies of non-nose whistlers under consideration have been deduced from Dowden-Allcock linear Q-technique. The variation of (?n)23 with local time has been shown, the slope of which can be directly related to the convection electric field. The estimated equatorial electric field at L? 1.12 is in the range 0.1–0.5 mV m?1 (in the 0130–0500 IST sector) during a storm period, which is in agreement with the results reported by earlier workers. The departure from a dipole field and the contribution of an induced electric field from the temporal changes have been discussed. The importance of an electric field study has been indicated.  相似文献   

14.
The magnetosphere of Jupiter has been the subject of extensive research in recent years due to its detectable radio emissions. Observations in the decimetric radio band have been particular helpful in ascertaining the general shape of the Jovian magnetic field, which is currently believed to be a dipole with minor perturbations. Although there is no direct evidence for thermal plasma in the magnetosphere of Jupiter, theoretical considerations about the physical processes that must occur in the ionosphere and magnetosphere surrounding Jupiter have lead to estimates of the thermal plasma distribution. These models of the Jovian magnetic field and thermal plasma distribution, specify the characteristic plasma and cyclotron frequencies in the magnetosplasma and thereby provide a basis for estimating thelocal electromagnetic and hydromagnetic noise around Jupiter. Spatial analogs of the well-known Clemmow-Mullaly-Allis (CMA) diagrams have been constructed to identify the loci of electron and ion resonances and cutoffs for the different field and plasma models. Regions of reflection, mode coupling, and probable amplification are readily identified. The corresponding radio noise properties may be estimated qualitatively on the basis of these various electromagnetic and hydromagnetic wave mode regions. Frequency bands and regions of intense natural noise may be estimated. On the basis of the models considered, the radio noise properties around Jupiter are quite different from those encountered in the magnetosphere around the Earth. Wave particle interactions are largely confined to the immediate vicinity of the zenographic equatorial plane and guided propagation from one hemisphere to the other apparently does not occur, except for hydromagnetic modes of propagation. The characteristics of these local signals are indicative of the physical processes occurring in the Jovian magnetosphere. Thus, as a remote sensing tool, their observation will be a vital asset in the exploration of Jupiter.  相似文献   

15.
We explore the detailed polarization behaviour of pulsar 0823 + 26 using the technique of constructing partial ‘mode-separated’ profiles corresponding to the primary and secondary polarization modes. The characteristics of the two polarization modes in this pulsar are particularly interesting, both because they are anything but orthogonal and because the secondary mode exhibits a structure seen neither in the primary mode nor in the total profile. The new leading and trailing features in the secondary mode, which appear to represent a conal component pair, are interpreted geometrically on the basis of their width and the associated polarization-angle traverse as an outer cone. If the secondary-mode features are, indeed, an outer cone, then questions about the significance of the pulsar’s postcursor component become more pressing. It seems that 0823 + 26 has a very nearly equatorial geometry, in that both magnetic poles and the sightline all fall close to the rotational equator of the star. We thus associate the postcursor component with emission along those bundles of field lines which are also equatorial and which continue to have a tangent in the direction of our sight line for a significant portion of the star’s rotation cycle. It seems that in all pulsars with postcursor components, this emission follows the core component, and all may thus have equatorial emission geometries. No pulsars with precursors in this sense — including the Crab pulsar — are known. The distribution of power between the primary and secondary modes is very similar at both 430 and 1400 MHz. Our analysis shows that in this pulsar considerable depolarization must be occurring on time scales that are short compared to the time resolution of our observations, which is here some 0.5–1.0 milliseconds. One of the most interesting features of the modeseparated partial profiles is a phase offset between the primary and secondary modes. The secondary-mode ‘main pulse’ arrives some 1.5 ± 0.1‡ before the primary-mode one at 430 MHz and some 1.3 +0.1 ‡ at 21 cm. Given that the polar cap has an angular diameter of 3.36‡, we consider whether this is a geometric effect or an effect of differential propagation of the two modes in the inner magnetosphere of the pulsar.  相似文献   

16.
This study explores the relationship between the sunspot number (SSN) and the ionospheric foF2. It is of interest to locate the SSN value at which the foF2 values are saturated. A regression model is built based on the data of the strictly rise period of the 21st solar cycle recorded by eight ionosonde stations scattering roughly between 40°N and 40°S geomagnetic latitude. Results show that clear saturation features appear around the equatorial anomaly crest region.  相似文献   

17.
In recent years with the advancement in satellite based navigational applications, study of Total Electron Content (TEC) has gained significant importance. It is well known that due to dynamical behaviour of equatorial and low latitude ionosphere, the levels of ionization is relatively high herein. The sustained decrease in solar extreme ultraviolet radiations during the current minimum is greater than any in recent history. This gives us the opportunity to study the observations of global positioning system total electron content (GPS-TEC) dual frequency signals from the GPS satellites continuously recorded at Trivandrum (an equatorial station) and Delhi (a low latitude station) during the extremely low solar activity period from January 2007 to June 2009. This study illustrates the diurnal, seasonal and annual variations of TEC during the extended solar minimum period. This study also investigates the behaviour of daytime ionosphere around spring and autumn equinoxes at low solar activity period. The results clearly reveal the presence of equinoctial asymmetry which is more pronounced at equatorial station Trivandrum. The diurnal variation of TEC shows a short-lived day minimum which occurs between 0500 to 0600 LT at both the stations. Delhi TEC values show its steep increase and reach at its peak value between 1200 and 1400 LT, while at the equator the peak is broad and occurs around 1600 LT. Further, the daily maximum TEC ranges from about 5 to 40 TEC units at Trivandrum and about 10 to 40 TEC units at Delhi, which correspond to range delay variations of about 1 to 8 m at the GPS L1 frequency of 1.575 GHz. The Maximum values of TEC were observed during spring equinox rather than autumn equinox, showing presence of semi annual variation at both the locations. The minimum values of TEC were observed during the summer solstice at Trivandrum indicating the presence of winter anomaly at equatorial region while Delhi TEC values were minimum during winter solstice showing absence of winter anomaly. Also the TEC values at both the locations have been decreasing since 2007 onwards exhibit good positive correlation with solar activity.  相似文献   

18.
Voyager flybys of Saturn in 1980-1981 revealed a circumpolar wave at ≈78° north planetographic latitude. The feature had a dominant wavenumber 6 mode, and has been termed the Hexagon from its geometric appearance in polar-projected mosaics. It was also noted for being stationary with respect to Saturn’s Kilometric Radiation (SKR) rotation rate. The Hexagon has persisted for over 30 years since the Voyager observations until now. It has been observed from ground based telescopes, Hubble Space Telescope and multiple instruments onboard Cassini in orbit around Saturn. Measurements of cloud motions in the region reveal the presence of a jet stream whose path closely follows the Hexagon’s outline. Why the jet stream takes the characteristic six-sided shape and how it is stably maintained across multiple saturnian seasons are yet to be explained. We present numerical simulations of the 78.3°N jet using the Explicit Planetary Isentropic-Coordinate (EPIC) model and demonstrate that a stable hexagonal structure can emerge without forcing when dynamic instabilities in the zonal jet nonlinearly equilibrate. For a given amplitude of the jet, the dominant zonal wavenumber is most strongly dependent on the peak curvature of the jet, i.e., the second north-south spatial derivative of the zonal wind profile at the center of the jet. The stable polygonal shape of the jet in our simulations is formed by a vortex street with cyclonic and anticyclonic vortices lining up towards the polar and equatorial side of the jet, respectively. Our result is analogous to laboratory experiments of fluid motions in rotating tanks that develop polygonal flows out of vortex streets. However, our results also show that a vortex street model of the Hexagon cannot reproduce the observed propagation speed unless the zonal jet’s speed is modified beyond the uncertainties in the observed zonal wind speed, which suggests that a vortex street model of the Hexagon and the observed zonal wind profile may not be mutually compatible.  相似文献   

19.
A transistorized wide-band (0.5–11 kHz) VLF goniometer has been developed for the study of whistlers and ELF/VLF emissions. It consists of two crossed vertical loops from which a single loop aerial, rotating about a vertical axis at a frequency of 25 sec?1, is synthesized electronically. During periods of high whistler activity, when the same propagation paths may be identified in successive whistler groups, it is possible to determine the bearing of the exit point of such a magnetospheric path with an error, typically, of ± 10–20°.  相似文献   

20.
A theoretical study is made of the whistler mode cyclotron instability both in linear and nonlinear regimes in conjunction with the generation of VLF emissions in the magnetosphere. For the nonlinear treatment, a well-established quasilinear method is used and some physical processes of the cyclotron instability viz. energy conservation, mechanism of instability and frequency change of the excited emissions are clarified. The results are applied to some types of the triggered VLF emissions; whistler triggered emissions and artificially stimulated emissions (ASE). It is found that whistler triggered emissions excited around the upper cutoff frequencies of whistlers may be explained by the whistler mode cyclotron instability by a model distribution function inferred from satellite data. In order to see a nonlinear evolution of the whistler mode cyclotron instability, computer simulations were carried out and it is shown that the change of frequency with time of whistler triggered emissions as well as characteristics of ASE are well explained by resonant nonlinear behaviour of whistler mode cyclotron instability considered in the present paper.  相似文献   

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