摘要:

ISEE 1, 2, 3, IMP 8, and Prognoz 7 observations of interplanetary shocks in 1978 and 1979 provide five instances where a single shock is observed by four spacecraft. These observations are used to determine best‐fit normals for these five shocks. In addition to providing well‐documented shocks for future investigations these data allow us to evaluate the accuracy of several shock normal determination techniques. When the angle between upstream and downstream magnetic field is greater than 20°, magnetic coplanarity can be an accurate single spacecraft method. However, no technique based solely on the magnetic measurements at one or multiple sites was universally accurate. Thus, we recommend using overdetermined shock normal solutions whenever possible, utilizing plasma measurements, separation vectors, and time delays together with magnetic constra

ISSN:0148-0227    

DOI:10.1029/JA088iA06p04739

年代:1983

数据来源: WILEY

摘要:

The nature of the interactions between the Jovian magnetosphere and Europa, Ganymede, and Callisto is examined. Effects of plasma and micrometeoroid bombardment of each satellite are considered in an attempt to understand the influences of these exogenic processes on the surface properties of each of the three moons. Earth‐based infrared observations of satellite surface temperatures are used as a basis to calculate (sublimated) H2O vapor pressures for each of the three satellites as a function of satellite latitude and local time. Sublimated and sputtered H2O leads to a net O2atmosphere on each of the three moons, and the effectiveness of such an atmosphere in preventing the Jovian plasma from reaching each satellite surface is considered under a variety of magnetospheric conditions. The effects of the magnitude and orientation of possible satellite magnetic fields were also studied and used to estimate the properties of the satellite magnetospheres under conditions appropriate to each moon being alternately inside and outside the Jovian plasma sheet. In addition, the orbits of charged micrometeoroids in Jupiter's magnetosphere were examined to determine the relative flux of these particles over the surfaces of each of the three satellites. Finally, possible observational effects of continual (over geological time periods) plasma and micrometeoroid bombardment of the satellite surfaces are considered and compared with available laboratory simulations and earth‐ and space‐based observations of Europa, Ganymede, and Callisto. These efforts suggest that plasma and micrometeoroid bombardment of the icy Galilean satellites could act to substantially modify the surface properties of each of the three moons, resulting in ‘leading‐trailing’ asymmetries of the satellite surfaces not unlike those observed in the satellite spectral and polarization

ISSN:0148-0227    

DOI:10.1029/JA088iA06p04749

年代:1983

数据来源: WILEY

摘要:

A new model for the basal pressure of Io's atmosphere is developed. This model takes into account the previously ignored fact that much of Io's surface has very high porosity, typically ∼90%. Such porosity allows efficient subsurface cold trapping of atmospheric gases which tends to keep ambient surface pressures very low. SO2is the only gas identified on Io, and the basal pressures for atmospheric models are usually pegged to local surface temperature via the SO2vapor pressure equilibrium curve. Near Io's subsolar point the pressure in equilibrium with a surface SO2frost deposit is ∼10−7bar. Porous surface models of the type we develop invoke equilibrium with the colder, subsurface permafrost (at ∼3‐cm depth) and yield pressures of ∼10−12bar. The subsurface cold trapping model explains many but not all observations relevant to Io's atmosphere. The new subsurface cold trapping model and the earlier surface frost equilibrium model, when taken together, provide lower and upper limits, respectively, on the basal SO2pressure of Io

ISSN:0148-0227    

DOI:10.1029/JA088iA06p04771

年代:1983

数据来源: WILEY

摘要:

Absorption and reemission by N2of the Lyman‐Birge‐Hopfield (LBH) and Birge‐Hopfield (BH) band systems in the earth aurora and Titan dayglow has been calculated by using multiple scattering in a plane‐parallel radiative transfer model and a line‐by‐line synthesis of individual bands. The effects of rotational structure are included through the use of temperature‐dependent band escape functions and band transmission functions. Iteration through successive scatterings shows that fewer than ten scatterings are important, even for the thickest bands. The excitation rate caused by multiple scattering is sensitive to the vibrational population distribution of the ambient N2, especially for the BH system in the earth aurora. For a vibrational temperature of 2000 K, the fluorescent scattering contribution at 110 km is 15 times that of a vibrational temperature of 300 K. Comparison of theoretical intensities to rocket observations of the BH (1, 10) band indicates a vibrational temperature of 1000 K, but the corresponding (1, 3) band intensity is brighter than the observation. Scattering on Titan was modeled for both a high‐ and low‐altitude source and for various vibrational temperatures. The LBH system has an enhanced sensitivity to vibrational temperature on Titan because of the rapidly changing photoabsorption cross section of CH4around 1400 Å. It is argued that thec4′ ¹Σu+Rydberg bands should have an intensity profile similar to the BH bands. Thec4′ (0, 0) band limb brightening inferred from the full‐disk and bright limb spectra observed by Voyager indicates a source around 4000 km, but this is in conflict with the observed intensity peak at 366

ISSN:0148-0227    

DOI:10.1029/JA088iA06p04784

年代:1983

数据来源: WILEY

摘要:

In a large laboratory plasma a current sheet is generated in the process of magnetic field line reconnection. The stability of the sheet with respect to local current increases is investigated. When the current density in the center of the sheet exceeds a critical value, spontaneous local current disruptions are observed. The current from the center of the sheet moves out to the sides. Magnetic flux variations in regions remote from the current sheet generate an inductive voltage in the current loop that drops off inside the plasma in the form of a potential double layer. This leads to particle acceleration with velocities much larger than those expected from the steady state electric fields in the plasma. The particle beams acquire their energy at the expense of the stored magnetic field energy of the current system. Beam‐plasma instabilities are generated that dissipate some of the directed kinetic energy and heat the background plasma. A model for the mechanism of the current disruptions is formulated. The potential structure leads to ion expulsion creating a localized density drop. The associated current drop in an inductive circuit drives the potential structure, thereby providing feedback for the disruptive instability. It saturates at a total current loss upon which the current system recovers, and the process repeats randomly. Similarities and differences to magnetospheric substorm phenomena are pointed ou

ISSN:0148-0227    

DOI:10.1029/JA088iA06p04793

年代:1983

数据来源: WILEY

摘要:

A preliminary kinetic analysis is presented of the effects of variation along the magnetic field lines on Kelvin Helmholtz instabilities driven by transverse velocity shear. These develop provided the parallel variation is much less than the transverse,k∥ls<(m/M)1/2k⊥rL, whererLis the ion gyroradius andlsthe shear length. They produce small parallel electric fields, but substantial potential drops along field li

ISSN:0148-0227    

DOI:10.1029/JA088iA06p04805

年代:1983

数据来源: WILEY

摘要:

VLF/ELF electric field wave data acquired on the ISIS 1, ISIS 2, and ISEE 1 satellites demonstrate the existence of a new phenomenon in which initially narrowband (∼1 Hz) upgoing signals from ground‐based VLF transmitters undergo a significant spectral broadening as they propagate through the ionosphere and protonosphere up to altitudes in the range 600–3800 km. For transmitter signals in the range 10–20 kHz, the spectral broadening can be as high as 10% of the nominal frequency of the input signal. Spectral broadening occurs only in the presence of impulsive VLF hiss and/or a lower hybrid resonance (LHR) noise band with an irregular lower cutoff frequency, and only for signals whose frequency exceeds the LHR frequency at the satellite location. It is often observed in association with a band of impulsive ELF hiss below 700 Hz. In many cases, the bandwidth of the spectrally broadened signals is a strong function of the electric dipole antenna orientation with respect to the local direction of the earth's magnetic field. Unusual dispersion in the components of the spectrally broadened pulses suggests that the spectral broadening may be due to a doppler shift effect in which the initial signals scatter from irregularities in the F region and couple into quasi‐electrostatic modes of short wave length. The large doppler shift associated with these short wavelength modes produces a significant increase in the bandwidth of the signal, as observed on a moving satellite. Since impulsive VLF hiss and irregular LHR noise bands have been linked to energetic (<1 keV) electron precipitation in the past, it is conjectured that the spectral broadening effect may be driven by precipitating

ISSN:0148-0227    

DOI:10.1029/JA088iA06p04813

年代:1983

数据来源: WILEY

摘要:

Pi 1 magnetic pulsations observed at Siple Station, Antarctica, contain an unusual class of fluctuations. They are large amplitude, 0.2–1 nT/s, one‐sided pulsations in dB/dt with variable direction. The pulsations are from 2 to 10 s wide and occur in an irregular train of pulses that lasts, sporadically, up to an hour. An earlier study (Arnoldy et al., 1982) reported that in 1979 data these pulses were often closely associated with riometer and auroral pulsations. The present report extends this work by using 1980 data and describes the variety in observation of these pulsations. The pulsations are observed from 2200 local time until noon with most frequent occurrence near dawn. They usually occur during or shortly after a burst of high amplitude pulsation activity within a long duration (several hours) interval of large or moderate amplitude Pi. Although the direction of the pulsations is typically west or southwest other directions are also observed, with occasionally a shift of direction (for example, from north to south) occurring in a few minutes. We interpret these Pi 1 pulsations to be the result of local enhancements in ionospheric and field‐aligned currents associated with precipitating electrons responsible for the associated pulsations in auroral luminosity. No evidence was found for a sinusoidal component of these magnetic pulsations, travelling as a wave in the magnetos

ISSN:0148-0227    

DOI:10.1029/JA088iA06p04841

年代:1983

数据来源: WILEY

摘要:

A nonlinear unified theory of type I and II irregularities is presented that explains their principal observed characteristics. The power spectrum is predicted by using Kolmogoroff‐type conservation law for the power flow in cascading eddie

ISSN:0148-0227    

DOI:10.1029/JA088iA06p04853

年代:1983

数据来源: WILEY

摘要:

A comparative study has been carried out of the spread F occurrence characteristics at the magnetic equatorial station, Fortaleza (4°S, 38°W, dip latitude 1.8°S) and the low‐latitude station, Cachoeira Paulista (22°S, 45°W, dip latitude 14°S) located with a relatively small longitudinal difference, in Brazil. The results show strong correlation between the occurrences of spread F events at the two stations for most of the time. While all the spread F events over Cachoeira Paulista are accompained by such events over Fortaleza, the converse is not true. Also, the spread F onset times over the equator are systematically earlier than those over the low latitude. Another interesting result is that while the spread F durations at the two stations are approximately linearly related, an event over Cachoeira Paulista is observed only for those that have durations more than 4–5 hours over Fortaleza. Possible implications of these results on the plasma bubble rise velocities in the equatorial ionosphere are discussed. It seems possible also to predict the approximate duration of a spread F event over the equator by observations carried out at a low‐latitude station situated close to a common magnetic merid

ISSN:0148-0227    

DOI:10.1029/JA088iA06p04861

年代:1983

数据来源: WILEY