摘要:

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 belowfLHR. 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 betweenL= 2.0 andL= 4.2. The occurrence rate for emissions by ducted whistlers increases with latitudes betweenL= 1.5 andL= 2.9, and it attains a maximum of 0.33 atL= 2.7. It then abruptly drops to 0.1 atL= 3.0, and it remains below 0.1 betweenL= 3.0 andL= 4.0. The decrease of the occurrence rate for emissions by ducted whistlers atL= 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 ionosph

ISSN:0148-0227    

DOI:10.1029/JA094iA01p00089

年代:1989

数据来源: WILEY

摘要:

Large‐amplitude (5‐30 nT) Pc 1 micropulsations were detected near the plasmapause by a low‐altitude satellite, Magsat, in the ionospheric F region. Both left‐ and right‐hand‐polarized waves were observed in a latitudinally narrow (<100 km) region. The sense of polarization tends to switch around the amplitude peak, suggesting the coupling of fast mode and transverse mode waves and the propagation of the incident wave not only along the ambient magnetic field but also in the east‐west direction across the magnetic field. On the ground, periodic hm emissions having nearly the same frequencies as those at Magsat were observed at the stations 1000‐3000 km from Magsat orbit, and the amplitude on the ground was more than 2 orders of magnitude smaller than that obs

ISSN:0148-0227    

DOI:10.1029/JA094iA01p00093

年代:1989

数据来源: WILEY

摘要:

The European Space Agency GEOS 2 spacecraft happened to cross the magnetopause several times, at various local times. Intense electric and magnetic fluctuations, in the ultralow‐frequency (ULF) range (0‐10 Hz) have been detected during each such crossing, with a peak at the magnetopause and still large amplitudes in the adjacent magnetosheath and magnetopause boundary layer. By applying spectral analysis and correlations to the electric and magnetic fluctuations, and a minimum variance analysis to the magnetic fluctuations, we investigate the nature of these fluctuations which appear as short‐lasting bursts in the spacecraft frame. Having reviewed possible interpretations, we show that the observed electric and magnetic signatures are consistent with small‐scale (L≈ ion Larmor radius) Alfvenic field‐aligned structures passing by the spacecraft at high speed. It is suggested that these structures correspond to nonlinear Alfvenic

ISSN:0148-0227    

DOI:10.1029/JA094iA01p00101

年代:1989

数据来源: WILEY

摘要:

Two different mechanisms for field‐aligned current propagation, i.e., the Alfvén wave and the ballistic motion of energetic electrons associated with a localized source, are examined with linear kinetic theory. The characteristics of these two mechanisms and the relationship between them are investigated as an initial value problem. It is shown that in addition to the shear Alfvén waves, the ballistic motion of electrons can also carry currents with the perpendicular scale comparable to the electron inertia length. The propagation speeds of these currents are independent of the Alfvén s

ISSN:0148-0227    

DOI:10.1029/JA094iA01p00111

年代:1989

数据来源: WILEY

摘要:

Observations of a Jovian decametric non‐Io‐A noise storm made from Voyager 1, the University of Florida Radio Observatory, the University of Texas Radio Astronomy Observatory, and the Jupiter station at Goddard Space Flight Center at frequencies of 26.3, 22.2, 20.0, and 18.0 MHz were found to be correlated. The activity observed at the ground stations occurred 68 min after the corresponding activity at Voyager 1. After correction is made for propagation time differences, this delay is reduced to 34 min. It is demonstrated that at each frequency the envelope of the individual‐event beams occurring during the storm (some or all of which are associated with dynamic spectral arcs) is a quasi‐constant structure that corotates with the inner Jovian magnetosphere, and that the width of this envelope beam is frequency dependent. The width increases as frequency is decreased, mainly because of the change in position of the trailing‐edge beam boundary. Evidence for a relatively slow temporal change in beam geometry is also

ISSN:0148-0227    

DOI:10.1029/JA094iA01p00115

年代:1989

数据来源: WILEY

摘要:

Results of a study comparing long‐term time variations (years) in Jupiter's synchrotron radio emission with a variety of solar wind parameters and the 10.7‐cm solar flux are reported. Data from 1963 through 1985 were analyzed, and the results suggest that many solar wind parameters are correlated with the intensity of the synchrotron emission produced by the relativistic electrons in the Jovian Van Allen radiation belts. Significant nonzero correlation coefficients appear to be associated with solar wind ion density, ram pressure, thermal pressure, flow velocity, momentum, and ion temperature. The highest correlation coefficients are obtained for solar wind ram pressure (NV²) and thermal pressure (NT). The correlation analysis suggests that the delay time between fluctuations in the solar wind and changes in the Jovian synchrotron emission is typically about 2 years. The delay time of the correlation places important constraints on the theoretical models describing the radiation belts. The implication of these results, if the correlations are real, is that the solar wind is influencing the supply and/or loss of electrons to Jupiter's inner magnetosphere. We note that the data for this work spans only about two periods of the solar activity cycle, and because of the long time scales of the observed variations, it is important to confirm these results with additional observat

ISSN:0148-0227    

DOI:10.1029/JA094iA01p00121

年代:1989

数据来源: WILEY

摘要:

Two‐dimensional particle simulations and three‐dimensional ray‐tracing calculations are used to investigate the generation and propagation of electromagnetic radiation produced by the electron cyclotron maser instability under conditions that prevail in the auroral plasma cavity. The simulations incorporate a continual flow of primary energetic electrons along the magnetic field. In such a driven system the maser instability leads to a quasi‐steady state in which as much as 7% of the incident flux of electron energy is converted into radiation. The maximum radiation intensity occurs within 20‐30 km from the injection point of the primary electrons, and the radiation consists of discrete wave packets with a spatial extent of 4‐6 km parallel to the field. The magnetic field gradient is shown to be unimportant for these quasi‐local properties of the maser instability. The ray path calculations are performed in a dipolar field geometry and retain the relativistic corrections to the wave dispersion. For the case where the energetic electrons are the dominant species, amplifications of the order ofe10are achieved within path lengths of less than 100 km. This indicates that the maser instability can produce the observed amplification of auroral kilometric radiation above cosmic background levels within a distance smaller than the dimensions of the a

ISSN:0148-0227    

DOI:10.1029/JA094iA01p00129

年代:1989

数据来源: WILEY

摘要:

Geomagnetically trapped protons in the 0.6‐ to 9‐MeV energy range were measured at latitudes near the geomagnetic equator by the Phoenix 1 experiment on board the S81‐1 mission from May to November 1982. The protons show a distribution in latitude along the line of minimum magnetic field strength with a full width at half maximum of ∼10° but with no appreciable longitudinal variation. Between 170 and 290 km the peak proton flux shows a fifth‐power altitude dependence, in contrast to previous measurements at higher altitudes, possibly demonstrating source attenuation. The efficiency of the telescope is calculated as a function of particle pitch angle and used to investigate the time dependence (1969‐1982) of t

ISSN:0148-0227    

DOI:10.1029/JA094iA01p00145

年代:1989

数据来源: WILEY

摘要:

Recently reported occurrence rates of coronal mass ejections (CMEs) are compared with the time scale for the long‐term evolution of the global white light coronal density distribution. This time scale is estimated from the synoptic observations of the corona made from Mauna Loa, Hawaii, by a series of K‐coronameters. The data span a period of more than 20 years and show evolution rates which vary with time roughly in phase with the solar activity cycle. However, there are detailed differences between the sunspot number curve and the long‐term behavior of this quantity. When the occurrence rates of CMEs observed from orbiting coronagraphs, available mainly during the descending phase of the activity cycle, are compared with this evolution time, it is found that the two quantities are inversely proportional. From energy considerations, it is unlikely that there is a causal relationship between CMEs and this coronal evolution. Rather, the result indicates that the processes which lead to the global evolution are intimately related to those which give rise to CMEs, a hypothesis consistent with current theories that CMEs arise from preexisting magnetic structures which become stressed by the global magnetic field rearrangement to the point of instab

ISSN:0148-0227    

DOI:10.1029/JA094iA01p00151

年代:1989

数据来源: WILEY

摘要:

We present the first detailed study of the properties of the radio emission of interplanetary type II bursts, based on a large sample of type II events. From an analysis of 33 bursts, a data base of more than 600 flux density versus frequency spectra has been developed and used to determine statistical properties of type II bursts, as well as to study the evolution of the emission of individual events. From this analysis the following results have been obtained: (1) The type II spectra appear to be composed of fundamental and harmonic components of plasma emission. The intensity of the fundamental component increases relative to the harmonic as the burst evolves with heliocentric distance. (2) The average flux density of a type II burst increases as a power of the average velocity of the associated shock. (3) As the shock propagates outward, the observed flux density at the peak of a type II spectrum remains relatively constant. The flux density integrated over frequency, however, generally decreases with distance because of a decrease in the bandwidth of the type II emission. (4) It is likely that solar wind density structures has a significant influence on type II bandwidths, and it may be responsible for differences between coronal and interplanetary type II spectral features.

ISSN:0148-0227    

DOI:10.1029/JA094iA01p00159

年代:1989

数据来源: WILEY