A theory of VLF emissions

The work attempts to give a theoretical explanation of the triggering of VLF emissions by magnetospheric whistler morse pulses. First studied is the behaviour of resonant particles in a whistler wave train in an inhomogeneous medium. It is found that second order resonant particles become stably trapped in the wave. After 1–2 trapping periods such particles dominate the resonant particle distribution function, and produce large currents that are readily estimated.     

A model of quasiperiodic VLF emissions

A self-consistent quasilinear model of the interaction between VLF emissions and geomagnetic pulsation is set forth. As a result an explicit expression of a modulation frequency dependence can be obtained.     

Low-frequency upstream wave as a probable source of low-latitude Pc 3–4 magnetic pulsations

Daytime Pc 3–4 pulsation activities observed at globally coordinated low-latitude stations [SGC (L = 1.8,λ = 118.0°W), EWA(1.15,158.1°W), ONW(1.3,141.5°E)] are evidently controlled by the cone angle θ_XB of the IMF observed at ISEE 3. Moreover, the Pc 3–4 frequencies ($\text{?}$) at the low latitudes and high latitude (COL; L = 5.6 and λ = 147.9°W) on the ground and that of compressional waves at geosynchronous orbit (GOES 2; L = 6.67 and λ = 106.7°W) are also correlated with the IMFmagnitude(B_IMF).The correlation of $\text{?}$ of the compressional Pc 3–4 waves at GOES 2 against B_IMF is higher than those of the Pc 3–4 pulsations at the globally coordinated ground stations, i.e., γ = 0.70 at GOES 2, and (0.36,0.60,0.66,0.54) at (COL, SGC, EWA, ONW), respectively. The standard deviation ($\text{σ}{}_{\mathrm{n}}\text{= ± Δ?}\text{mHz}$) of the observed frequencies from the form $\text{?}$ (mHz) = 6.0 × B_IMF (nT) is larger at the ground stations than at GOES 2, i.e., $\text{Δ? = ± 6.6}\text{mHz at}$GOES 2, and ±(13.9, 9.1, 10.7, 12.1) mHz at (COL, SGC, EWA, ONW), respectively. The correlations between the IMF magnitude B_IMF and Pc 3–4 frequencies at the low latitudes are higher than that at the high latitude on the ground, which can be interpreted by a “filtering action” of the magnetosphere for daytime Pc 3–4 magnetic pulsations. The scatter plots of pulsation frequency $\text{?}$ against the IMF magnitude B_IMF for the compressional Pc 3–4 waves at GOES 2 are restricted within the forms $\text{? = 4.5 × B}{}_{\mathrm{<mtext>IMF</mtext>}}\text{and}\text{? = 7.5 × B}{}_{\mathrm{<mtext>IMF</mtext>}}$. The frequency distribution is in excellent agreement with the speculation ($\text{<ω}{}_{\mathrm{sc}}\text{Ω}{}_{\mathrm{i}}\text{= 0.3 ～ 0.5}$) of the spacecraft frame frequency of the magnetosonic right-hand waves excited by the anomalous ion cyclotron resonance with reflected ion beams with V₆ = 650 ～ 1150 km s^?1 in the solar wind frame observed by the ISEE satellite in the Earth's foreshock. These observational results suggest that the magnetosonic right-handed waves excited by the reflected ion beams in the Earth's foreshock are convected through the magnetosheath to the magnetopause, transmitted into the magnetosphere without significant changes in spectra, and then couple with various HM waves in the Pc 3–4 frequency range at various locations in the magnetosphere.     

Simultaneous spaced direction-finding measurements of medium-latitude VLF/ELF emissions

Simultaneous spaced measurements of medium-latitude VLF/ELF emissions were carried out during the three northern winters from 1976 to 1979. The experiment was making use of two different kinds of direction-finding systems (a field-analysis method and a goniometer network) at two stations in Europe, namely Brorfelde in Denmark (L = 2.9) and Chambron-la-Foret in France (L = 1.9); this enabled us to locate the ionospheric exit regions of emissions over a wide range of L-values up to and beyond 4.0, the average plasmapause location. In order to study the time delay in the temporal evolution of VLF emissions or the longitudinal drift of the emissions, observations from the Moshiri Observatory in Japan, widely separated in longitude, are also used. The overall system of the VLF equipment installed at the three stations is described. Then we present the VLF/ELF data of good quality obtained during the final year's campaign (Nov. 1978–Feb. 1979). By making use of the direction-finding data, we were able to classify the observed emissions into several categories, and some early results for some of the emissions are presented.     

Characteristics of VLF emissions manifested by their cross-spectral phase information

Natural VLF emissions received by a single antenna can be characterised at each point in the emissions' frequency-time domain by a power and a phase. Emissions received at a single point by two antennae with a fixed relative orientation in space can be similarly described by the cross-spectral power and relative phase. It is shown that the cross-spectral phase contains information on the propagation characteristics of the waves which is better utilised in wave analysis than the power. In fact, the phase information allows weak signals to be identified more readily than is possible from a power spectrogram. It also allows the recognition of waves propagating with different wave normal directions. Data from the Geos-1 electric and magnetic antennae, pre-processed by the on-board correlator, are used to study the cross-spectral characteristics of VLF hiss and chorus in the Earth's magnetosphere.     

Propagation of VLF emissions in the magnetosphere and the ionosphere

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 > 1R_E.     

Properties of storm-time magnetospheric VLF emissions as deduced from the Ariel 3 satellite and ground-based observations

The characteristics of VLF emissions during the two magnetic storms on 5–7 June and 19–21 September, 1967 are investigated based on the Ariel 3 satellite data and ground observation at a low-latitude station in Japan. During the study of the June event, the satellite lay in the dawn-dusk plane. Soon after the onset of the main phase there appeared VLF emissions on the morning side of the magnetosphere on the ground as well as on the satellite, but the emissions were at very low level in the evening. It was not until the recovery phase that we could recognize the stationary occurrence of intense emissions in the evening. Furthermore, it is found that the chorus-type morning emissions are observed outside the plasmapause, while the narrow-banded hiss-type evening emissions lay within the plasmapause. It is shown that the cyclotron instability of ring current electrons can, on the whole, account for the many properties of the storm-like emissions. Similarly, we discuss the characteristics of the September event.     

On a mechanism for the formation of VLF electrostatic emissions in the high latitude F-region

The presence of highly anisotropic ion velocity distributions in the weakly-ionized plasma of strongly convecting areas of the high latitude F-region leads to the excitation of electrostatic microinstabilities (λ ～ 50 cm) at frequencies of the order of the lower hybrid frequency and smaller. We have estimated the threshold conditions for the excitation of the unstable waves under various physical circumstances. For some representative conditions we have also calculated the frequencies, growth rates, and wavelengths for the fastest growing modes using the linear approximation. We stress that the present theory breaks down in regions where the plasma cannot be treated as locally homogeneous. The altitude range over which the theory is applicable also varies with conditions. For highly disturbed conditions the upper altitude limit may be as high as 400 km.     

Peculiarities of 3C 454.3 as a superluminal source

This paper begins with a brief summary of the superluminal sources discovered so far. For one of the sources, the quasar 3C 454.3, the main observed results with VLBI at many frequencies and many epochs are then presented. The peculiarities of 3C 454.3 in relation to the general properties of the other superluminal sources are highlighted by means of a comparative table. Lastly, a preliminary discussion on possible explanations for the peculiarities is given.

This paper can be regarded as a supplement to the reports [12, 13] on the observations of 3C 454.3     

The main belt as a source of near-Earth asteroids

We investigate the flux of main-belt asteroid fragments into resonant orbits converting them into near-Earth asteroids (NEAs), and the variability of this flux due to chance interasteroidal collisions. A numerical model is used, based on collisional physics consistent with the results of laboratory impact experiments. The assumed main-belt asteroid size distribution is derived from that of known asteroids extrapolated down to sizes of 40 cm, modified in such a way to yield a quasi-stationary fragment production rate over times 100 Myr. The results show that the asteroid belt can supply a few hundred km-sized NEAs per year, well enough to sustain the current population of such bodies. On the other hand, if our collisional physics is correct, the number of existing 10-km objects implies that these objects either have very long-lived orbits, or must come from a different source (i.e., comets). Our model predicts that the fragments supplied from the asteroid belt have initially a power-law size distribution somewhat steeper than the observed one, suggesting preferential removal of small objects. The component of the NEA population with dynamical lifetimes shorter than or of the order of 1 Myr can vary by a factor reaching up to a few tens, due to single large-scale collisions in the main belt; these fluctuations are enhanced for smaller bodies and faster evolutionary time scales. As a consequence, the Earth's cratering rate can also change by about an order of magnitude over the 0.1 to 1 Myr time scales. Despite these sporadic spikes, when averaged over times of 10 Myr or longer the fluctuations are unlikely to exceed a factor two.     

The main belt as a source of near-Earth asteroids

We investigate the flux of main-belt asteroid fragments into resonant orbits converting them into near-Earth asteroids (NEAs), and the variability of this flux due to chance interasteroidal collisions. A numerical model is used, based on collisional physics consistent with the results of laboratory impact experiments. The assumed main-belt asteroid size distribution is derived from that of known asteroids extrapolated down to sizes of ≈ 40 cm, modified in such a way to yield a quasi-stationary fragment production rate over times ≈ 100 Myr. The results show that the asteroid belt can supply a few hundred km-sized NEAs per year, well enough to sustain the current population of such bodies. On the other hand, if our collisional physics is correct, the number of existing 10-km objects implies that these objects either have very long-lived orbits, or must come from a different source (i.e., comets). Our model predicts that the fragments supplied from the asteroid belt have initially a power-law size distribution somewhat steeper than the observed one, suggesting preferential removal of small objects. The component of the NEA population with dynamical lifetimes shorter than or of the order of 1 Myr can vary by a factor reaching up to a few tens, due to single large-scale collisions in the main belt; these fluctuations are enhanced for smaller bodies and faster evolutionary time scales. As a consequence, the Earth's cratering rate can also change by about an order of magnitude over the 0.1 to 1 Myr time scales. Despite these sporadic spikes, when averaged over times of 10 Myr or longer the fluctuations are unlikely to exceed a factor two.     

The scattered disk as a source of Halley-type comets

We investigate a new dynamical mechanism for producing Halley-type comets from the scattered disk of comets. Levison and Duncan [Levison, H., Duncan, M., 1997. Icarus 127, 13-32] and Duncan and Levison [Duncan, M., Levison, H., 1997. Science 276, 1670-1672] showed that a significant number of objects leave the scattered disk by evolving to semi-major axes greater than 1000 AU. We find that once these objects reach semi-major axes on the order of ¹⁰⁴ AU, a significant fraction immediately have their perihelia driven inward by the galactic tides. Approximately 0.01% of the objects that reach ¹⁰⁴ AU then evolve onto orbits similar to the observed Halley-like comets due to gravitational interactions with the giant planets. The orbital element distribution resulting from this process is statistically consistent with observations. We discuss the implications of this model for the number of objects in the scattered disk in the text. The model predicts a temporal variation in the influx of HTCs with a period of ∼118 Myr. At the peak, the model predicts that there should be roughly 10 times as many HTCs as currently observed (i.e., there should be weak HTC showers). However, the model may inflate the importance of these showers because it does not include the effects of passing stars and giant molecular clouds.     

Statistical characteristics of medium-latitude VLF emissions (unstructured and structured): Local time dependence and the association with geomagnetic disturbances

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.     

Asteroid 2002NY40 as a source of meteorite-dropping bolides

The existence of asteroidal meteoroid streams capable of producing meteorite-dropping bolides has long being invoked, but evidence is scarce. Recent modelling of previously reported associations suggests that the time-scales to keep the orbital coherence of these streams producing meteorites are too short. We present an unequivocal association between near earth object (NEO) 2002NY40 and at least one bright fireball detected over Finland in 2006 August. Another two additional fireballs recorded from Spain and Finland seem to be related, together producing a fireball-producing stream (β Aquarids). On the basis of historical data, the 2006 finding suggests the existence of a meteoroid complex capable of producing meteorites. Taking into account present time-scales for orbital decoherence, if 2002NY40 has large meteoroids associated with it, such behaviour would be the consequence of a relatively recent asteroidal fragmentation. Supporting our claim, the heliocentric orbits of two recently discovered NEOs, 2004NL8 and 2002NY40, were found to exhibit a good similarity to each other and also to the orbits of the three bolides. The fireball spectra of the two Finish bolides showed that the chemical abundances of these objects are consistent with the main elements found in chondrites. This result is consistent with the probable Low iron, Low metal (LL) chondritic mineralogy of asteroid 2002NY40. Consequently, this asteroid may be delivering LL chondrites to the Earth. Additional fireball reports found in the literature suggest that the associated β Aquarid complex may have been delivering meteorites to the Earth during, at least, the last millennium.     

Heterogeneous chemical processes as a source of persistent meteor trains

Abstract— Observations of long‐lasting persistent trains following the entry of some meteoroids into the Earth's atmosphere are suggested to arise in part from the interaction between meteoroid components and the atmosphere and in the heterogeneous recombination reaction of atmospheric O atoms with NO. The latter occurs on the surfaces of dust left by the explosive fragmentation of larger meteoroids. A strong role is attributed to reactions of troilite (FeS), a meteorite component, with the atmosphere at elevated temperatures. The suggestions made in this paper complement previous work that suggested that long‐lived emissions results from a variety of species made in the shock of larger meteoroids.     

Transneptunian object 2003 UB 313 as a source of comets

The possibility of interrelation between long-period comets and 2003 UB 313, a recently discovered large Kuiper Belt body, is investigated. For this purpose, 78 objects crossing the plane of motion of this body at distances from 37.8 to 97.6 AU have been selected from 860 long-period comets. The overpopulation of comets with this characteristic is also considered. The plane of motion of 2003 UB 313 is compared with the orbital planes of other objects in number of comet crossings in the specified distance interval or in some parts of it. A statistically significant overpopulation of elliptic and intermediate comets with the corresponding orbital nodes has been established. Recently discovered and absolutely faint comets show the best effect in this sense. The same is also true for comets with osculating eccentricities e < 1. A similar result is also obtained for comets with “original” a ^?1 > 0.010000. It is hypothesized that the 2003 UB 313 family is present among the 78 comets. Four of them have aphelion distances from 37.8 to 97.6 AU. An ellipticity is traceable in the distribution of some of the 78 distant nodes. This may be considered as a further argument for the suggested hypothesis. Generally, the body 2003 UB 313 may be assumed to play a prominent role in injecting observable comets from the transneptunian region