ISSN:0148-0227    

DOI:10.1029/92JA02658

年代:1992

数据来源: WILEY

摘要:

The problem of predicting future cycles of the sunspot number is physically significant. Recently, a number of authors have made complex systems analyses (“chaos”) of the set of monthly Wolf sunspot numbers. Each of these analyses revealed the presence of low‐dimensional deterministic behavior, and some of the papers applied the techniques of nonlinear prediction to predict future sunspot numbers. All of these papers analyzed filtered or smoothed sunspot number sets derived from the monthly Wolf sunspot numbers. Here, we have performed the same type of analysis on the raw (e.g., unfiltered) monthly sunspot number data set and on data sets derived from it, in particular, an “unrectified” set with a 22‐year period. We find no evidence that the sunspot numbers are generated by a low‐dimensional deterministic nonlinear process; further, by considering suitably constructed surrogate data sets (Theiler et al., 1991), we show that filtering techniques can give some spurious evidence for the presence of deterministic nonlinear behavior. Consequently, any predictions based on the assumption of such a process are not significantly better than those from linear stochastic models (Ca

ISSN:0148-0227    

DOI:10.1029/92JA01459

年代:1992

数据来源: WILEY

摘要:

Measurements from the ISEE 1 and ISEE 2 spacecraft are used to examine the terrestrial bow shock under high beta conditions. On November 4, 1979, the large separation between the spacecraft, over 2500 km along the shock normal, was such to allow simultaneous upstream and downstream measurements of the solar wind plasma for a period of about 8 min. These measurements are compared with and found to be in agreement with the predicted values of the Rankine‐Hugoniot relations using the simple adiabatic approximation and a ratio of specific heats, γ, of 5/3. No additional terms, such as the heat flux, are needed to bring the measured and predicted values into agreement. Large magnetic field and density fluctuations were observed, but average downstream plasma conditions well away from the shock were relatively steady, near the predicted Rankine‐Hugoniot values. The magnetic disturbances persisted well downstream and a hot, dense ion beam was detected leaking from the downstream region of the shock. Both of these phenomena can be attributed to the excitation of the Alfvén ion cyclotron instability, which acts quickly to isotropize the ions close to the ramp of the shock. Additional shock crossings under high beta conditions having smaller separations allow us to confirm many of these observa

ISSN:0148-0227    

DOI:10.1029/92JA01976

年代:1992

数据来源: WILEY

摘要:

The Alfvénic turbulence observed in the interplanetary space is mainly made up of outward traveling waves of solar origin. Inward waves, although a minor component of the turbulence, play an important role in transferring energy between different scales through nonlinear interactions with outward modes. Inward waves can only have an interplanetary origin. A good understanding of how inward waves are generated in the interplanetary space is crucial in modeling the Alfvénic turbulence evolution. Solar wind regions with high differential kinetic energy, which can be converted into turbulent energy, are the best places to search for local sources of Alfvénic turbulence. We have studied one of these regions at different heliocentric distances inside 1 AU, focusing on large‐scale fluctuations (periods approximately between 1 and 12 hours). Such a region was indicated by previous investigations as one of the best candidates for turbulence generation. In fact, fluctuations with an inward sense of correlation are observed, but a detailed analysis leads us to favor the hypothesis that they are mainly due to variations in plasma velocity, density, and magnetic field related to structures convected by the solar wind and not to real inward propagating waves. At least in the inner heliosphere, a view in which local sources of waves play a relevant role in driving the Alfvénic turbulence evolution does not seem realistic. We propose that such evolution, especially at the larger scales, is mainly governed not by interplanetary sources but rather by interactions of the outward waves of solar origin with the wind stru

ISSN:0148-0227    

DOI:10.1029/92JA01510

年代:1992

数据来源: WILEY

摘要:

Measurements of angular scattering of cosmic radio sources observed near the solar limb indicate that the density microstructure can be highly anisotropic (Armstrong et al., 1990). Here we point out that Alfvén waves, which are known to be present in the solar wind, will tend to isotropize the angular scattering measurements. Thus such observations set an upper bound on the amplitude of Alfvén waves in the scattering region. Recently measurements of velocity versus distance were made near the Sun (using interplanetary scintillations). In about half of the observed streams the velocity profiles were found to be consistent with an MHD model of the solar wind in which Alfvén waves are used to increase the flow velocities (Coles et al., 1991). In this paper we demonstrate that the Alfvén wave amplitudes necessary to match the measured velocities are consistent with the bounds derived here from angular scattering observati

ISSN:0148-0227    

DOI:10.1029/92JA01572

年代:1992

数据来源: WILEY

摘要:

We use data from 1971 to 1989 from the Nagoya cosmic ray muon telescope to look at waves in the cosmic ray north‐south anisotropy. TheGGcomponent of the Nagoya data is used because it is particularly sensitive to the north‐south anisotropy. The presence of waves with a period of 27 days is demonstrated through most of the period. For each 27‐day solar rotation the 27 daily values for theGGcomponent have been harmonically analyzed to determine the amplitude of the best fit sine wave to the data. When theseGG27‐day amplitudes are plotted for each solar rotation during the 19‐year period, it becomes apparent that there is a very marked annual modulation of these amplitudes; the annual variation is greatest at solar maximum, and is also an 11‐year modulation of the effect. The cosmic ray north‐south anisotropy arises from theBy × ▽NR, anisotropy resulting from the interaction of theycomponent of the interplanetary magnetic field (IMF),Byand ▽NR, the radial heliocentric cosmic ray density gradient. We have also determined the daily average values forByfrom 1971 to 1987, and for each 27‐day solar rotation these daily values have also been harmonically analyzed to obtain the best fit sine wave of period 27 days. When the amplitudes of the 27‐dayBywaves are plotted for each solar rotation, an annual wave and an 11‐year wave are also found; the behavior is very similar in nature to the behavior of the waves in the NagoyaGGdata. These two sets of data have been compared with the values of α, the tilt of the heliospheric neutral sheet, which is available for most of the solar rotations between 1971 and 1989. The largest annual waves in the amplitude of the 27‐dayGGwaves appear to coincide with relatively rapid excursions in the tilt of the neutral sheet. The annual mean of the amplitudes as well as the range (highest to lowest) in the amplitudes for the 27‐day waves in theGGcomponent and in theBycomponent have been determined for each year; in each case there is a periodicity of about 11 years comparable to t

ISSN:0148-0227    

DOI:10.1029/92JA01663

年代:1992

数据来源: WILEY

摘要:

High‐time‐resolution spectra of plasma wave emissions detected in the interaction region of comet Giacobini‐Zinner with the solar wind reveal a wave phenomenology much more complicated than first reported. Spectra often exhibit three or more independent peaks, which become more prominent the deeper into the interaction region the spacecraft traversed. The main peaks correspond to whistler emissions below the electron cyclotron frequency, a midfrequency peak near the maximum Doppler shift frequency for waves withkλD= 1, a high‐frequency peak above the Doppler shift maximum frequency, and electron plasma oscillations at the plasma frequency. Similar multipeaked spectra are also observed downstream from weak shocks at Earth, which suggests that the plasma wave generation mechanisms responsible need not require particle populations created by photoio

ISSN:0148-0227    

DOI:10.1029/92JA01601

年代:1992

数据来源: WILEY

摘要:

We consider an ion transport equation describing the source, convection, adiabatic acceleration and quasi‐linear velocity diffusion of cometary pickup ions. The equation is solved numerically along solar wind plasma flow lines for the environment of comet Halley, to obtain distributions at positions on the Giotto spacecraft trajectory which may be compared with observations. We use realistic input models for the observed spectrum of upstream and downstream propagating wave turbulence and for the ion source and mass loading of the solar wind. The ion resonance condition is not approximated in the diffusion term and we obtain full two‐dimensional (pitch angle and velocity) numerical distributions which show pitch angle scattering about the wave scattering centers at ±VAwhereVAis the Alfvén wave speed in the solar wind frame. Peak ion phase‐space densities approximately follow the bispherical shell geometry. The energy distributionsF(υ) agree well with Giotto observations at lower υ but in the high‐energy tail there is evidence possibly for the first‐order Fermi acceleration mechanism taking place in the comet Halley for

ISSN:0148-0227    

DOI:10.1029/92JA01873

年代:1992

数据来源: WILEY

摘要:

We have considered vertical density profiles of dust rings with a dust size distribution. The density profiles are determined by random vertical velocities of the dust, the vertical component of gravity, and the electrostatic lifting force on the charged dust due to the self‐screening electric field of the dust ring. We reconsider the case of electrostatically supported dust rings with monosized dust and no random vertical velocity (Havnes and Morfill, 1984). We thereafter look at a pure electrostatically supported dust ring with dust size distribution, which we find will be perfectly separated according to dust size with the largest dust particles in the ring plane and the smallest and lightest furthest above the plane. When the dust particles are given a random velocity, this leads to an increasing mixing of the different dust sizes as the random velocity is increased. The electrostatic effect will still lead to separation of dust particles according to size, and we find that a general trend in dust rings will be the existence of a halo of small dust particles with a degree of mixing of dust sizes near the ring plane which is dependent on the dust random kinetic energy and on the dust and plasma parameter

ISSN:0148-0227    

DOI:10.1029/92JA01402

年代:1992

数据来源: WILEY

摘要:

The magnetic field data returned from the Voyager 1 and 2 transit through the upstream region of Saturn have been examined from 0.1 mHz to 0.25 Hz to search for the signatures of waves associated with both the electron and ion foreshocks. In contrast to the Earth, in the ion foreshock of Saturn we find two distinct bands of wave activity at frequencies around 0.5 and 2‐mHz in the spacecraft frame. These two waves differ considerably in their properties. The 0.5‐mHz waves are right handed, strongly elliptically polarized, while the 2‐mHz waves are elliptically or circularly polarized in the left‐handed sense. These two different waves may be associated with two different backstreaming particle distributions, possibly those reflected from the shock and those leaking from the hot magnetosheath. We also observe right‐hand elliptically polarized upstream waves propagating obliquely to the magnetic field at frequencies between 90 and 150 mHz, which are above the proton gyrofrequency in the spacecraft frame. The frequency of these waves appears to be dependent on the interplanetary magnetic field orientation. Such dependence is characteristic of the 1‐Hz waves at Earth. Discrete wave packets associated with shocklets are for the first time observed in the foreshock of an o

ISSN:0148-0227    

DOI:10.1029/92JA01461

年代:1992

数据来源: WILEY