摘要:

We have reexamined the relationship between “U‐shaped” peak flux density microwave spectra and solar proton events for ∼200 large (Sp(≥2 GHz) ≥ 800 solar flux units (sfu)) microwave bursts (1965–1979). The radio spectra fell into two basic classes: U‐shaped, with two maxima (≥ 800 sfu) in the range from 200 MHz to ≥ 10 GHz (59% of all events), and cutoff spectra, with a maximum ≥800 sfu atf≥ 2 GHz andSp(200 MHz)<100 sfu (18%). Nine percent of the events had “intermediate” spectra with a maximum ≥800 sfu atf≥ 2 GHz and 100 sfu ≤Sp(200 MHz)10 protons cm−2s−1sr−1observed from 1965 to 19

ISSN:0148-0227    

DOI:10.1029/JA090iA07p06251

年代:1985

数据来源: WILEY

摘要:

Temporal variations of the Nimbus 7 measurements of solar UV flux, important for their stratospheric effects, are compared with ground‐based measurements of the solar infrared He I absorption line at 1083 nm. The close similarity of their temporal characteristics shows that the 1083‐nm line is a better estimator of the UV flux than the classical indices of solar activity, the 10.7‐cm radio flux and the sunspot number, for short time scales (days, weeks). The power spectrum of the He I line intensity matches that of the Nimbus 7 205‐nm flux at the 27‐day period peak but is weaker at the peak near 13 to 14 days period. The 27‐day peak is caused by the combination of solar rotation of active regions with one major concentration in their solar longitude distribution, and the 13‐day case involves two concentrations with solar longitude roughly 180° apart. The 13‐day periodicity is not simply a second harmonic of the 27‐day periodicity, because some episodes of activity are dominated by the 13‐day periodicity with very weak 27‐day periodicity while other episodes are dominated by 27‐day periodicity with weak 13‐day periods. These episodes of activity, which last typically 4 to 8 months, are caused mainly by groups of strong active regions that dominate the solar‐rotational variations for several months. In addition to the enhanced short‐term modulation during these episodes, the valleys in the solar‐rotational modulation also slowly rise and decay.F10 andRtend to rise more steeply and peak earlier during these episodes th

ISSN:0148-0227    

DOI:10.1029/JA090iA07p06267

年代:1985

数据来源: WILEY

摘要:

Pioneer 10 and 11 and Voyager 1 and 2 observations are used to study global aspects of the solar wind interaction with Jupiter and Saturn. Solar wind measurements before and after the encounters are used to determine average upstream flow parameters at 5 and 9 AU. Bow shock and magnetopause position are found to vary as the fourth root of dynamic pressure at Jupiter and the sixth root at Saturn. The average distances to the nose of the magnetopause based upon the Pioneer and Voyager boundary crossings for Jupiter and Saturn are 68RJand 19RS, respectively, after correction for varying solar wind pressure. In shape, the Jovian bow shock and magnetopause surfaces are similar to their terrestrial counterparts, but the width of the magnetosheath is 45% less than predicted by axisymmetric gas dynamic theory. This result is interpreted as evidence for strong polar flattening of the Jovian magnetosphere. The Saturnian magnetopause and bow shock boundaries are significantly more flared than at the earth with a subsolar magnetosheath that is 20% thinner than predicted by gas dynamic theory. On the basis of these results it is suggested that Saturn is intermediate between the earth and Jupiter in terms of polar flattening, with the unusual flaring of the Saturn magnetopause being due to the low ratio of static to dynamic pressure in the distant heliosphere.

ISSN:0148-0227    

DOI:10.1029/JA090iA07p06275

年代:1985

数据来源: WILEY

摘要:

We present hydromagnetic equilibrium configurations for the nonrotating “pole‐on” magnetosphere of Uranus. According to a plasma supply mechanism proposed by Cheng (1984) we assume that charged particle sputtering of the water‐ice covered moons of Uranus provides a continuous internal plasma source to the Uranian magnetosphere. We assume further that (1) dynamical changes of the magnetosphere are quasi‐static, (2) the internal plasma source fills the Uranian magnetosphere in a homologous manner, and (3) the plasma residence time within the magnetospheric cavity is long enough to allow the Uranian magnetosphere to evolve from a vacuum configuration toward a plasma‐dominated equilibrium. With these assumptions, we calculated configurational changes of Uranus's magnetosphere, given the amount of the thermal plasma pressure as a free parameter. In order to include Uranus's dipole in the equilibrium calculations the homogeneous Grad‐Shafranov equation for cylindrically symmetric magnetospheres has been complemented by an inhomogeneous source term. We derived linear analytic solutions to the inhomogeneous problem. We found that a plasma ring forms near the equatorial plane of the planet when the thermal pressure reaches about 75% of the maximum amount that would lead to a Harris sheet magnetotail configuration. The appearance of a plasma ring is due to the particular pole‐on orientation of Uranus's dipole; such a ring would not exist in an earth‐type equilibr

ISSN:0148-0227    

DOI:10.1029/JA090iA07p06287

年代:1985

数据来源: WILEY

摘要:

Phase space densities for low‐energy (80 MeV/G) ions in Saturn's inner magnetosphere are analyzed using solutions of the time‐averaged radial diffusion equation for charged particle transport in a dipolar magnetic field. A series of distributed loss models ranging from satellite absorption only to satellite and maximum estimated Ring E absorption losses plus pitch angle scattering losses occurring at the strong diffusion limit in the inner magnetosphere are assumed. In each case the corresponding form of the magnetospheric radial diffusion coefficient (assumed to be expressible asD(L)=DoLn, whereDois a constant andnis an integer) which yields a minimum rms residual between model and data is determined. Independent constraints on the diffusion rate at specificLvalues derivable from satellite microsignatures in low‐energy ions and electrons are then considered. Estimates previously derived from the Dione microsignature in Voyager 1 low‐energy electrons are supplemented by additional analyses of the Tethys microsignature in Voyager 2 low‐energy ions and the Rhea microsignature in Voyager 1 low‐energy electrons. The resulting diffusion rate estimates ofD(4.9) ∼4 ± 2×10−8Rs² s−1andD(8.7)>6 ± 5×10−8Rs² s−1are consistent with those derived from the Dione microsignature ifD(L)increases outward. A comparison with the phase space density modeling results shows that satellite and maximum Ring E absorption losses alone are insufficient to yield diffusion rates at Tethys and Dione that are in agreement with the microsignature estimates. Models containing weak pitch angle scattering losses of low‐energy ions in the inner magnetosphere occurring at a rate less than one‐tenth that of the strong diffusion limit produce diffusion rates that are compatible with microsignature estimates and additionally result in an improved fit to the radial variation of the experimental phase space densities. The preferred radial diffusion coefficient for these low‐energy ions is characterized by a relatively high amplitude and low‐orderLdependence that is most consistent with Jovian‐type diffusion mechanisms including the centrifugal interchange instability

ISSN:0148-0227    

DOI:10.1029/JA090iA07p06295

年代:1985

数据来源: WILEY

摘要:

We show that the clocklike modulation of the spectral index of energetic electrons (>2 MeV) in the outer Jovian magnetosphere is due to a periodic shift of the particle energy spectrum toward higher and lower energies. This shift results in a modulation of the spectral index when the spectrum is not a pure power law in energy. We suggest that the periodic energization is due to a periodic modulation of the magnetic field in the outer magnetosphere. This modulation is caused by a variation of the longitudinally averaged Pedersen conductivity due to the asymmetric solar illumination of the trace of the magnetodisc in the high‐latitude ionospheres. Such a modulation requires the presence of a surface magnetic anomal

ISSN:0148-0227    

DOI:10.1029/JA090iA07p06304

年代:1985

数据来源: WILEY

摘要:

The propagation characteristics of electromagnetic waves below the proton gyrofrequency are strongly influenced by the changing ion composition in the Io plasma torus. Obliquely propagating waves experience a natural reversal of their dominant sense of polarization when their frequency becomes equal to the crossover frequency. This dramatically modifies both the wave growth characteristics and their effect on scattering resonant particles. A numerical simulation of the path‐integrated gain of unducted waves in the torus has confirmed the importance of L mode ion cyclotron instability as the most likely mechanism for wave excitation. Whistler (R mode) instability is insignificant and it should be totally quenched by strong ion cyclotron damping. The favored region for wave excitation is at intermediate latitudes (λ ≥ 12°) on field lines passing through the outer torus (L ≥ 6). Unstable waves can subsequently propagate toward lower latitude following the natural polarization reversal (to R mode waves) at the wave crossover frequency. But the wave amplitudes observed in the low‐latitude region sampled by Voyager 1 are expected to be significantly lower those that in the source region due to strong ion cyclotron damping. This is consistent with recent observational evidence on the power spectral intensity of low‐frequency waves i

ISSN:0148-0227    

DOI:10.1029/JA090iA07p06311

年代:1985

数据来源: WILEY

摘要:

Winds measured by the two Viking landers have been filtered and then compared with predictions from the general circulation model and to orbiter observations of clouds and surface phenomena that indicate wind direction. This was done to determine the degree to which filtered winds may represent aspects of the general circulation. Excellent agreement was found between wind direction data from Lander 1 and the model predictions and orbiter observations. For Lander 2, agreement was generally good, but there were periods of disagreement which indicate that the filtering did not remove other extraneous effects. It is concluded that Lander 1 gives a good representation of the general circulation at 22.5°N latitude but that Lander 2 is suspect. Most wind data from Lander 1 have yet to be analyzed. It appears that when analyzed these Lander 1 data (covering 3.5 Mars years) can provide information about interannual variations in the general circulation at the lander latitude

ISSN:0148-0227    

DOI:10.1029/JA090iA07p06319

年代:1985

数据来源: WILEY

摘要:

Intense electrostatic waves of short wavelength are frequently observed upstream of the earth's bow shock. These waves are very impulsive, and their peak amplitudes correlate strongly with the temporal flux profiles of simultaneously observed energetic ions and electrons which are reflected from the shock. Our paper suggests an explanation of this broadband electrostatic noise in terms of electron acoustic waves driven unstable by the ion and/or electron beams. These waves do not exist in thermodynamic equilibrium plasma and therefore require nonthermal distributions for their occurrence. A simple model is developed for finite temperature beam and background particles with Maxwellian or Lorentzian distributions. The beam‐associated instability can be classified as being due to Landau resonance or negative wave energy. Depending on the plasma parameters the mode discussed here naturally extends and links to the ion acoustic wave or electron plasma oscillation. A numerical parameter study is provided for various beam plasma situations. The frequency theoretically obtained ranges between the ion and the electron plasma frequency, and the wavelength amounts to several Debye lengths for maximum growth. Since the beam mode has a linear dispersion relation, the Doppler‐shifted frequency in the spacecraft frame should depend linearly on the beam velocity and on the wave vector as well. It is anticipated that these characteristics may allow parts of the observed broadband high‐frequency noise to be identified as beam‐driven electron acousti

ISSN:0148-0227    

DOI:10.1029/JA090iA07p06327

年代:1985

数据来源: WILEY

摘要:

Magnetic field short‐period fluctuations δBare observed throughout the earth's magnetosheath. We report a study of these fluctuations with frequencies 20

ISSN:0148-0227    

DOI:10.1029/JA090iA07p06337

年代:1985

数据来源: WILEY