摘要:

Cosmic ray measurements obtained with integral detectors on Voyager 1 and 2 (Ep≳ 70 MeV) moving toward the outer solar system and the earth‐orbiting IMP 8 satellite (Ep≳ 35 MeV) over the period late 1977 through mid‐1982 are presented. During this period, Voyager 1 and 2 traversed the region from 1 to ∼13 AU and ∼10 AU, respectively, with little separation in heliolongitude; separation in heliolatitude was also small (≲2°) through the end of 1980, at which time the trajectory of Voyager 1 changed toward higher ecliptic latitudes. The following results have been obtained from comparisons of 27‐day averages of the data: (1) there exists a positive radial intensity gradientgr∼ 2–4%/AU, on the average, during this period; (2) the decrease in cosmic ray intensity toward solar maximum in 1980–1981 for both ground‐based and spacecraft detectors proceeds in a stepwise fashion, with alternating periods of intensity “plateau” and “steep decrease” during which the gradient changes from ∼0%/AU to ∼8%/AU, respectively; (3) the cosmic ray minimum reached in late 1980/early 1981 appears nearly simultaneously (to within one solar rotation) at 1 AU and at ∼10 AU, with and without correction for propagation time delay effects between IMP 8 and the Voyager spacecraft; (4) recurrent intensity enhancements appearing at both Voyager 1 and 2 at 6–10 AU are generally absent at 1 AU; (5) the data are generally consistent with a heliolatitudinal gradient of 0±1%/deg, obtained from the periodic excursions of IMP 8 about the heliographic equator and the separation in latitude between Voyager 1 and IMP 8 of ∼15° in 1982. The value of the intensity gradient is found to be generally consistent with that obtained by Pioneer 10 at ∼23 AU around the same time but just prior to solar maximum. The implications of the results in the overall context of cosmic

ISSN:0148-0227    

DOI:10.1029/JA089iA06p03735

年代:1984

数据来源: WILEY

摘要:

Recent studies of the time behavior of the galactic cosmic ray intensity have concluded that long term decreases in the intensity are generally associated with systems of interplanetary flows that contain flare‐generated shock waves, magnetic clouds, and other transient phenomena. In this paper the magnetic field power spectral signatures of such flow systems are compared to power spectra obtained during times when the solar wind is dominated by stable corotating streams that do not usually produce long‐lived reductions in the cosmic ray intensity. We find that the spectral signatures of these two types of regimes (transient and corotating) are distinct. However, the distinguishing features are not the same throughout the heliosphere. The transient flows at 1 AU tend to have smaller correlation lengths and larger magnetic helicity scale lengths than do the corotating flows. In data collected beyond 1 AU, the primary differences are in the power spectra of the magnitude of the magnetic field rather than in the power in the field components. Consequently, decreases in cosmic ray intensity are very likely due to magnetic mirror forces and gradient drifts rather than to pitch angle scatter

ISSN:0148-0227    

DOI:10.1029/JA089iA06p03747

年代:1984

数据来源: WILEY

摘要:

Evidence for the presence of relatively large amplitude right‐hand elliptically polarized MHD waves upstream and downstream of quasi‐parallel (θBn<45°) forward and reverse interplanetary shocks is presented. The mode is observed with frequencies in the range of 0.025–0.17 Hz (in the spacecraft frame) and propagates along the magnetic field. The MHD modes have rest frame frequencies in the range 3.8–31 mHz, with typical wavelengths about 1.5×108to 4.7×108cm. The magnetic field power spectrum in the vicinity of these interplanetary shocks is much steeper thanf−5/3at high frequencies. The observed spectra have a high frequency dependence off−2.5tof−4. A peculiar feature of the fast mode identification in one event is the large correlation observed between |B| and proton density ρ for field‐aligned propagation. This appears to be a nonlinear effect, second order in the wave amplitude. The properties of the MHD waves can be understood in terms of the electromagnetic ion‐cyclotron instability. Both resonant and nonresonant interactions need be considered to account for the polarization and spectral content of the observed fluctuations. In association with one event, a distinct suprathermal component was observed in the solar wind proton distribution. The parameters of this distribution are adequate to drive a resonant instability with a maximum growth rate that coincides with the observe

ISSN:0148-0227    

DOI:10.1029/JA089iA06p03762

年代:1984

数据来源: WILEY

摘要:

Using Voyager 1 and 2 energetic (≳ 30 keV) ion measurements near the magnetopause, in the magnetosheath, and immediately upstream of the Jovian bow shock, the available ion compositional patterns have been examined together with typical energy spectra in each of these regions. Field‐aligned, highly anisotropic ion bursts occurring early in most upstream events have relatively low fluxes of electrons and heavy ions associated with them. Characteristic spectral changes are found late in long‐lived ion events at the same time that heavy ion and energetic electron fluxes are enhanced. The interplanetary magnetic field is always connected toward the dawn side of the planet late in this subset of upstream events at the time that relativistic electrons and heavy ions are high. A model emphasizing energetic particle escape in the prenoon part of the Jovian magnetosphere late in events is presented to explain many of the upstream region fea

ISSN:0148-0227    

DOI:10.1029/JA089iA06p03775

年代:1984

数据来源: WILEY

摘要:

A theoretical model and analysis of the acceleration of high‐energy ∼ MeV/nuc ions in Jupiter's magnetosphere are presented. All measurements conducted in this energy range have consistently pointed to the probable occurrence of local nonadiabatic acceleration above that obtainable by radial diffusion alone. We consider first the creation of fast 75 km/s neutral sulfur and oxygen atoms in the Io torus by charge exchange. This process will eject upward of 2×1028neutrals per second which are recaptured in the magnetosphere by subsequent electron impact and charge exchange with the ambient plasma outside the torus as well as by solar UV photoionization. All processes together lead to an injection rate of greater than 4×1025heavy ions recaptured per second with a gyrospeed comparable to the local plasma flow speed. We show that subsequent adiabatic radial diffusion cannot produce the observed spectrum or high energies required by observations. We then consider the stochastic acceleration of this seed population by magnetohydrodynamic (MHD) waves. It is demonstrated how the properties of the observed particle spectrum are related to those of the power spectrum of MHD fluctuations measured by the magneto

ISSN:0148-0227    

DOI:10.1029/JA089iA06p03789

年代:1984

数据来源: WILEY

摘要:

On the Voyager 1 inbound pass through the Jovian magnetosphere, the frequent acquisition of wide band data from the plasma wave instrument has permitted a quasi‐continuous survey of the waves which occur in the frequency band below the electron cyclotron frequency, the chorus band. Structured, rising frequency chorus was observed just outside the dense Io plasma torus but inside the low‐density middle magnetosphere. A quasi‐continuous, very narrow band emission occurred just above one‐half the electron cyclotron frequency. Power spectra, which were constructed from the wide band data, showed that the half‐cyclotron emission is unaffected by the simultaneous occurrence of chorus at lower frequencies and is separated from the chorus by a deep spectral gap just below one‐half the cyclotron frequency. Within the rising chorus band, a hisslike emission was observed which consisted of two quasi‐continuous, very narrow band tones located near the upper and lower frequency limits of the rising chorus band. Sequential power spectra showed that these twin‐frequency tones persisted during rising chorus bursts. Chorus occurs in the spatial regions where whistlers have cyclotron resonant interactions with the suprathermal population of ∼keV electrons. A possible interpretation of the twin tones is that these signals are electrostatic or resonance cone whistlers. Growth rate calculations show that the electrostatic whistler is destabilized by a loss cone distribution with the same pitch angle anisotropy which is needed to excite the electromagnetic chorus whistlers. A speculative hypothesis is presented to explain the half‐cyclotron emission. The Landau absorption of oblique chorus whistlers results in the spectral gap just below one‐half the cyclotron frequency and in the formation of a quasi‐linear plateau in the parallel velocity distribution of the suprathermal electrons. The plateau alters the Landau and cyclotron resonant interactions and permits the unstable growth of electrostatic and electromagnetic whistlers in a narrow range just above one‐hal

ISSN:0148-0227    

DOI:10.1029/JA089iA06p03801

年代:1984

数据来源: WILEY

摘要:

The spatial and compositional distribution of the thermal plasma in the magnetosphere of Saturn is described in the light of the Voyager encounters. Theoretical considerations are applied to the elucidation of the structure, including two external and two internal boundaries. The outer boundary is a magnetohydrodynamic entity while the inner boundary of locally created thermal plasma is a result of the dissociative recombination of corotating molecular ions. The internal boundaries which we have reason to conclude separate plasmas of different composition are explained as a charge exchange quasi‐resonance phenomeno

ISSN:0148-0227    

DOI:10.1029/JA089iA06p03821

年代:1984

数据来源: WILEY

摘要:

Diurnal variation of the electrostatic potential of the rings of Saturn is considered by examining a wide range of possible combinations of plasma sources and sinks. It is shown that because of the presence of the ring absorption effect, the ambient thermal plasma density is kept very low . If no trapping or thermalization of the photoelectrons, ionospheric plasma, and impact plasma generated by meteoroid impact occurs, the ring surface potential could vary between 4–8 V (dayside and nightside) and −7 V (shadow cone). The operation of trapping the field‐aligned plasma flow changes the time variation of the ring surface potential only slightly. It is the variations of the peak ionospheric plasma flux and of the impact plasma which have more significant effect. The impact plasma, in particular, can change the ring surface potential from a value of −7.3 V to a small but positive value if its flux reaches ∼106ion

ISSN:0148-0227    

DOI:10.1029/JA089iA06p03829

年代:1984

数据来源: WILEY

摘要:

The energetics of the nightside ionosphere of Venus are still not well understood. Rather large ion temperatures have been observed on the nightside. We have constructed a two‐dimensional theoretical model of the energetics of the nightside ionosphere which takes into account the effects of horizontal and vertical bulk transport of heat. We demonstrate that the observed maintenance of the ion temperature can be explained in terms of these transport terms for solar zenith angles less than 150°. Simple heat transport cannot explain the observed high temperatures for angles greater than 15

ISSN:0148-0227    

DOI:10.1029/JA089iA06p03837

年代:1984

数据来源: WILEY

摘要:

A survey of the locations of flux transfer events at the dayside magnetopause has been undertaken with emphasis on the direction of the energetic ion anisotropy along the magnetic field. The instrument used is the medium energy particle spectrometer on board ISEE 2. The events studied are from 1977 and 1978, giving coverage over the morning and afternoon sides within latitude range −10° to 40°. The anisotropy is an unambiguous indicator of the hemisphere from which the observed particles are escaping. Magnetic data from the flux gate magnetometer and plasma data from the fast plasma experiment have also been studied. The magnetic signature can be used to decide the direction of motion of the event with respect to the ambient field, while the change in plasma bulk velocity allows one to estimate the direction of magnetic tension in the reconnected field line. Apparent disagreements between the magnetic signature (positive‐negative indicating northward motion) and particle anisotropy (parallel to the field indicating southern connection) are resolved by the plasma data, which show that the bulk (northward) plasma speed exceeds the Alfvén speed, and thus these south connected events are blown northward faster than magnetic tension pulls them southward. The magnetic signature is therefore an unreliable indication of the hemisphere of connection. With this effect taken into consideration the overall pattern is that of an equatorial rather than a high‐latitude source for flux transfe

ISSN:0148-0227    

DOI:10.1029/JA089iA06p03843

年代:1984

数据来源: WILEY