摘要:

Magnetic field observations by Pioneer 11 and ISEE 3/ICE are used to investigate the large scale temporal behavior and spatial gradients in the heliospheric magnetic field. We extend and refine previous analyses by expanding the data set to include observations out to a radial distance of 24 AU and over a longer time interval which includes the recent solar minimum. The spatial gradients are investigated by removing the solar cycle variations with data obtained at 1 AU at the corresponding time. To first order, the gradients in the magnetic field magnitude and in the azimuthal component are found to agree with the Parker model. Furthermore, this analysis of the Pioneer data confirms previous reports of a magnetic field flux deficit of approximately 1%/AU, so that at 20 AU the relative deficit is of the order of 20%. The solar wind speed measured at 1 AU as well as at Pioneer 11 has been analyzed, but it is found that the small variations observed between the two locations affect this result only minimally. The average field strength at Pioneer 11 at a distance of 16 AU has been compared with a Voyager 2 average obtained at nearly the same radial distance and time interval but at a significantly different latitude. They agree very closely and both exhibit a deficit of ≈ 20% relative to the 1‐AU baseline. This agreement demonstrates thatBdid not vary significantly with latitude at this time (1984–1985 near solar minimum). Furthermore, the 1‐AU baseline has been adjusted to allow for an observed dependence of the field on latitude near solar maximum (1978–1982) inferred from a comparison of measurements in the inner heliosphere at 0.7 AU (PVO) and 1.0 AU (ISEE 3). This adjustment of the 1‐AU baseline did not significantly affect the deficit, again confirming that the Pioneer measurements were not strongly influenced by the latitude dependences. We thus argue that most of the magnetic field deficit is in the radi

ISSN:0148-0227    

DOI:10.1029/JA095iA01p00001

年代:1990

数据来源: WILEY

摘要:

The nonlinear theory of the steepening of Alfvén waves into solitons and then into rotational discontinuities (RDs) predicts correlations between the β of the plasma, the sense of polarization of the discontinuity, and changes of magnetic field strength and plasma density across the discontinuity. The predicted correlations have been searched for in ISEE 3 field and plasma data, but no statistically significant evidence for the evolution of RDs from Alfvén solitons could be found. Approximately equal numbers of right‐ and left‐handed discontinuities were observed. Some of the rotations approached 180°, but none exceeded that value. Nearly all the RDs in the 33‐day data set were propagating outward from the Sun, which implies that sunward propagating Alfvénic fluctuations produced by stream interactions in the interplanetary medium seldom steepen into discontinuities before reaching 1 AU. The only unambiguously sunward propagating RD in the data set showed all of the expected relations for a steepened Alfvén soliton. Interplanetary processes, such as changing wave speeds and β and refraction of the discontinuities, may have changed the properties of the RDs created close to the Sun to such an extent that any original correlations are no longer detectable at 1 AU. Another possible explanation of the basically negative result is that some of the RDs produced near the Sun may have steepened from fast or slow Korteweg‐de

ISSN:0148-0227    

DOI:10.1029/JA095iA01p00013

年代:1990

数据来源: WILEY

摘要:

We set out to fit the velocity profile of the solar wind during Giotto's approach to comet Halley with the mass loading produced by a simple model of the neutral particle distribution. The model assumes a constant gas production rate,Q, ionization rate, ν, and a radial expansion velocity,Ve, from the comet. It is used to calculate the implanted ion flux at Giotto for any given time and position along the spacecraft trajectory. Comparing the added flux with Giotto solar wind proton data from the inbound leg outside the bow shock, we compute the ratio of the total mass‐loaded ion flux to the solar wind flux at the spacecraft. Hence using equations given by Galeev et al. (1985) we infer values of the solar wind velocity,u∞, far upstream from the comet. Imposing the condition thatu∞should be as nearly as possible constant in time, we deduce values ofQand the ratioVe/ν required to fit the model to our data. The values obtained are consistent with those derived by more direct methods. Finally, we use ourQ, Veand ν values to predict a possible innermost shock

ISSN:0148-0227    

DOI:10.1029/JA095iA01p00021

年代:1990

数据来源: WILEY

摘要:

Ion composition measurements made by the orbiter ion mass spectrometer on the Pioneer Venus Orbiter are used to determine the ion flow characteristics within two ionospheric holes on the nightside of Venus. A comparison of the altitude profiles of the observed ion densities with those expected under diffusive equilibrium conditions indicates that the major ions O+, NO+, and O2+and the minor ions H+and He+flow upward, away from Venus, along the axis of the holes. The result is substantiated by a quantitative evaluation of the ion flow speeds appearing in expressions derived from the equations for conservation of mass and momentum of the ions and electrons. The resulting equations for the ion flow speeds are functions of the observed ion and neutral species densities and the ion and electron temperatures. The analysis reveals that all ion species flow upward in the holes because the upward force produced by the plasma pressure gradient exceeds all downward forces. Furthermore, the ion flow speeds are found to increase with altitude as a result of the continual acceleration caused by the net upward force. In the acceleration region, where the observing altitudes were limited to the range extending from the [O+] peak (∼170 km) to 250 km, none of the ion components attain their acoustic speeds, although the light ions come within about a factor of 2. The light ions H+and He+attain the greatest speeds because as minor ions they experience the largest polarization electric fields produced in the ion‐electron gas. Although upward ion flow is inferred from the ion composition measurements, the nature of the ion source required to maintain such flow remains a puzzle. For example, the classical diffusion rate of O+across the magnetic field, into the base of the holes, is about an order of magnitude less than what is needed to feed the upward O+f

ISSN:0148-0227    

DOI:10.1029/JA095iA01p00031

年代:1990

数据来源: WILEY

摘要:

The results of a computer code modeling the radial diffusion of equatorially confined energetic electrons in Jupiter's inner magnetosphere are compared with spacecraft as well as ground‐based radio (synchrotron radiation) data. We find that the synchrotron radiation spectrum cannot be reproduced without a significant hardening of the electron spectrum betweenL= 3 andL= 1.5. This hardening may be due to energy degradation by Jupiter's ring particles. Our calculations also suggest that there may be larger‐sized material outside Jupiter's ring up to L ∼ 4 or Io's

ISSN:0148-0227    

DOI:10.1029/JA095iA01p00039

年代:1990

数据来源: WILEY

摘要:

Assuming the cyclotron maser instability is the source mechanism of the smooth high‐frequency, nightside component of Uranian kilometric radiation, a plasma density model is developed for the nightside atr<4.2rubased on radio wave observations of the Planetary Radio Astronomy instrument on board the Voyager 2 spacecraft. Using this plasma model and the Q3 magnetic field model (Connerney et al., 1987), a systematic ray tracing of right‐hand extraordinary (RX) emission is performed from a grid of nightside sources along magnetic field lines at frequencies just above the RX cutoff. By requiring that the rays satisfy several observational constraints, we are able to specify the source region with more accuracy than previously prescribed. Our results indicate a source region extended in longitude compared to those previously published. In addition, the emission at higher frequencies that exhibits a periodic “bite‐out” appears to be observed by the satellite at wave normal angles in the range 90°<Ψ<140°. The emission observed near closest approach has a different source region and may have different radiation lobe characteristics. These results are consistent with the cyclotron maser instability for moderately low densities in the source region (fp/fce<0.25, wherefpandfceare the plasma and gyrofrequency, r

ISSN:0148-0227    

DOI:10.1029/JA095iA01p00051

年代:1990

数据来源: WILEY

摘要:

This paper gives the first explicit test of the rotational discontinuity‐slow expansion fan structure predicted by MHD models of the open tail boundary. We analyzed the magnetic field and plasma data collected by IMP 8 during its magnetotail boundary crossings at geocentric distances near 25Reand high latitudes. Many crossings show the particle and field signatures of the paired MHD tail boundary structure. The openness of the tail is revealed by rotational discontinuities. They often show relatively large normal magnetic field components. Other crossings show properties that suggest a tangential discontinuity and reveal regions where or times when the tail is closed. We give examples of each her

ISSN:0148-0227    

DOI:10.1029/JA095iA01p00061

年代:1990

数据来源: WILEY

摘要:

A new three‐dimensional magnetohyrodynamic (MHD) simulation code based on the fourth‐order Runge‐Kutta‐Gill time advance and the direct finite space difference is applied to the detailed study of the solar wind interaction with the Earth's dipole field with special emphasis on its numerical soundness. The dynamic formation processes of the bow shock and the magnetopause are revealed, where the transient propagation characteristics of fast waves play the leading role. The plasma sheet formation process is also revealed. The high temperature, high density, and hence high pressure in the plasma sheet are caused by the fast waves which compress the dipole field. The study of the effects of the interplanetary magnetic field (IMF) on the plasma sheet finds that when the IMF is directed south, the plasma sheet is strongly compressed by the reconnected magnetic flux which is carried from the day side. The peak plasma sheet pressure that appears around 8RE(RE∶ Earth's radius) on the nightside is almost exactly as large as the solar wind dynamic pressure in the absence of the IMF. When the IMF is north, the nightside lobe magnetic field is peeled off, so that the plasma sheet pressure becomes weakly depressed in comparison with the no‐IMF case. This indicates that the direction of the IMF largely influences the pressure of the plasma sheet. More interestingly and importantly, no Kelvin‐Helmholtz instability occurs along the magnetopause in an ideal MHD, contrary to the conventional understanding. Furthermore, no entry of mass, momentum, and energy through the magnetopause is observed at all in our simulation. This strongly ensures enough numerical accuracy and validity of the present work. Thus the simulation results of earlier works where a considerable amount of solar wind momentum is transported into the magnetosphere must be due to a numerical inaccuracy. As further evidence showing the numerical validity of the present code, the relationship between the standoff distance and the adiabatic constant is compared with the strong shock theory, and a fairly good agreement is obtained. On the other hand, the standoff distance is found to be weakly dependent upon the dynamic pressure of the solar wind in the realistic pa

ISSN:0148-0227    

DOI:10.1029/JA095iA01p00075

年代:1990

数据来源: WILEY

摘要:

In this paper we study the resonant coupling of fast magnetosonic waves to the Alfven or slow magnetosonic waves in general geometries. If the magnetic field lines are straight and the Alfven velocity does not vary along them, the linearized magnetohydrodynamic (MHD) equations exhibit a singularity at a given line which manifests the resonant coupling on that line. However, if the lines are not straight, or if the Alfven velocity does vary along them (or both) no singularity in the linearized MHD equations is apparent. Nonetheless, we show here that the solutions of the linearized MHD equations behave singularly in the neighborhood of an entire field line. This gives rise to the resonant coupling of the fast to the Alfven or slow waves along the entire line. We calculate the solutions near the resonant magnetic field line as well as the nonzero net energy flux across it. The method used is applicable to general geometries and plasma pressures.

ISSN:0148-0227    

DOI:10.1029/JA095iA01p00089

年代:1990

数据来源: WILEY

摘要:

Magnetic field data acquired at high‐latitude, near‐conjugate stations (Iqaluit, Northwest Territories, Canada, and South Pole Station, Antarctica) are studied in order to examine in more detail the nature of magnetic “impulse” signatures that occur in the data and that are produced by ionosphere currents which are caused by magnetopause processes. An examination of the data, both visually and with a computer algorithm “detector,” from the 5‐month interval July–November 1985 found many such magnetic “impulse” events which could be interpreted in terms of intense field‐aligned currents above the observing stations. All the events have a half‐width in time of a few minutes, and most are reasonably conjugate. The majority of the events studied have magnetic field perturbations in the vertical component which can be interpreted in terms of field‐aligned currents directed in the same direction (either into or out of the ionosphere) in both hemispheres. The perturbations in the horizontal plane are consistent with an interpretation in terms of a single cycle of an odd mode Alfvén wave. For the events shown in detail in this paper, the impulsive magnetic signatures are found to occur for the interplanetary magnetic fieldBZcomponent positive, negative, or variable. The observations are discussed in the context of some contemporary ideas on the generation of ionospheric disturbances by magnetopause processes such as sporadic reconnection (“flux transfer events”), plasma injections into the low‐latitude boundary layer, Kelvin‐Helmholtz instability, and solar wind “pressure pulses.” Near‐equatorial data from a location in the same local time section as the high‐latitude data are used to show the gross differences in global ionospheric currents stimulated by sudden commen

ISSN:0148-0227    

DOI:10.1029/JA095iA01p00097

年代:1990

数据来源: WILEY