摘要:

Around Venus the planetary ionosphere is directly exposed to the shocked solar wind. The interaction takes place in a broad region surrounding the dayside ionosphere, called the mantle, where the shocked solar wind plasma and the plasma of planetary origin have equally important roles. In this paper both the experimentally determined characteristics and the microphysics of the mantle are discussed in detail. It is shown that as a result of the interaction between the two plasma populations, a modified two‐stream instability develops, and waves are excited with a frequency of a few times the lower hybrid frequency. The polarization of the waves is almost perpendicular to the magnetic field. The stabilization of the higher‐frequency part of the wave spectrum is the result of transverse wave convection in the particular sheet‐like geometry of the mantle. The interaction of these waves with planetary ions and electrons is described within the framework of a nonlinear model in which the saturation of the modified two‐stream instability is due to induced scattering of the waves on cold planetary ions. The effective collision frequency between the shocked solar wind protons and planetary ions is also calculated; it is shown how this leads to ion pick up and heating. Other macroscopically observable effects of these processes are electron acceleration along the magnetic field and ionospheric heating. The experimental data collected in the dayside mantle of Venus by the instruments carried onboard the Pioneer Venus Orbiter are compared to our model. It is believed that the observations support the scenario pr

ISSN:0148-0227    

DOI:10.1029/95JA01831

年代:1995

数据来源: WILEY

摘要:

The automatic space plasma experiment with a rotating analyzer (ASPERA) onboard the Phobos 2 spacecraft has recently revealed the presence of planetary He+ions at Mars (Barabash and Norberg, 1994). In the present work the analysis is continued in order to estimate the total outflow of the He+ions which are swept away by the solar wind. For the Phobos epoch the total He+outflow rate was found to be (1.2±0.6)×1024ions/s. The escape occurs mainly near the Martian magnetopause. Considering extreme errors in the measurements, the maximum helium outflow could range up to 2.4×1024s−1. From a scaling of the helium profile suggested by Moroz et al. (1990) to obtain the measured loss rate, one can deduce the helium abundance in the Martian upper atmosphere. It turns out that helium is a dominant gas in the Martian exosphere at altitudes between 500 and 1250 km. However, recently reported observations of the weak EUV emissions (108 photons) from the Martian He I suggest an abundance that is 18.5 times lower (Krasnopolsky et al., 1994). Possible reasons for this disagreement are discussed. The helium production rate near Mars can, in turn, be roughly estimated from the production rate for the Earth by using a scaling argument, since the only source of helium in the atmospheres of the terrestrial planets is radioactive decay of certain isotopes of uranium and thorium. Present estimates suggest a degassing rate of 8×1022atoms/s only (Krasnopolsky et al., 1993). However, under steady state conditions one would expect the production and loss rates to be equal. The discrepancy leads us to the conclusion that either the helium degassing rate should be corrected (or the amount of uranium is higher on Mars than anticipated) or helium may also be delivered on Mars by other sources, for example, as solar wind α particles. The observed high total outflow of ions which are 4 times heavier than protons may result in an effective mass loading. Thus helium may play an important role in the solar wind‐Mars int

ISSN:0148-0227    

DOI:10.1029/95JA01914

年代:1995

数据来源: WILEY

摘要:

At Venus H+is expected to be under diffusive equilibrium above about 400 km, but it is difficult to verify this under normal circumstances, because solar wind interaction terminates the subsolar ions around 400 km. However, under conditions of extremely low solar wind pressure, the terminal height, (called the ionopause) can move up to very high altitudes. Such events are rare, but after searching through the Pioneer Venus data, we have been able to identify some orbits with ionopause altitudes above 1000 km. Altitude distributions of O+and H+, measured by the ion mass spectrometer experiment, have been studied for these orbits. It is found that while O+distribution is consistent with diffusive equilibrium, H+shows large departures providing evidence of upward flow of H+. Approximate calculations indicate that the flow is subsonic.

ISSN:0148-0227    

DOI:10.1029/95JA01933

年代:1995

数据来源: WILEY

摘要:

The magnetic meridional component of the thermospheric neutral wind is derived from theF2layer height measured from ionograms collected at Kokubunji, Japan and compared with the winds measured by the middle and upper atmosphere radar at Shigaraki, Japan and the winds predicted by the empirical Horizontal Wind Model 1990 (HWM90). Good agreement is found between the ionosonde and the radar winds. This agreement supports the previous finding of the radar study that the winds over Japan are smaller in amplitude than the winds reported elsewhere by a factor of 1.5–2. On the other hand, disagreement is found at some local times between the ionosonde and HWM90 winds. The ionosonde winds in the period from 1986 to 1988 are, on average, poleward in the daytime and equatorward in the nighttime. The daytime‐nighttime difference remains at about 100 m/s summer or winter, but this diurnal pattern is superposed upon a daily average drift of about 10 m/s southward in summer and 10 m/s northward in winter, suggesting a consistent summer‐to‐winter circulation pattern. This limited study validates the ionosonde wind method as applied at Japanese longitudes and opens the way for comprehensive studies of the thermospheric wind over Japan from the long and continuous ionogram libraries from the Japanese ionosond

ISSN:0148-0227    

DOI:10.1029/95JA02387

年代:1995

数据来源: WILEY

摘要:

Ground‐based Fabry‐Perot interferometers located at Thule, Greenland (76.5°N, 69.0°W, Λ=86°) and at Søndre Strømfjord, Greenland (67.0°N, 50.9°W, Λ=74°) have monitored the upper thermospheric (∼240‐km altitude) neutral wind and temperature over the northern hemisphere geomagnetic polar cap since 1983 and 1985, respectively. The thermospheric observations are obtained by determining the Doppler characteristics of the (O I) 15,867‐K (630.0‐nm) emission of atomic oxygen. The instruments operate on a routine, automatic, (mostly) untended basis during the winter observing seasons, with data coverage limited only by cloud cover and (occasional) instrument failures. This unique database of geomagnetic polar cap measurements now extends over the complete range of solar activity. We present an analysis of the measurements made between 1985 (near solar minimum) and 1991 (near solar maximum), as part of a long‐term study of geomagnetic polar cap thermospheric climatology. The measurements from a total of 902 nights of observations are compared with the predictions of two semiempirical models: the vector spherical harmonic (VSH) model of Killeen et al. (1987) and the horizontal wind model (HWM) of Hedin et al. (1991). The results are also analyzed using calculations of thermospheric momentum forcing terms from the thermosphere‐ionosphere general circulation model (TIGCM) of the National Center for Atmospheric Research (NCAR). The experimental results show that upper thermospheric winds in the geomagnetic polar cap have a fundamental diurnal character, with typical wind speeds of about 200 m s−1at solar minimum, rising to up to about 800 m s−1at solar maximum, depending on geomagnetic activity level. These winds generally blow in the antisunward direction, but are interrupted by episodes of modified wind velocity and altered direction often associated with changes in the orientation of the interplanetary magnetic field (IMF). The central polar cap (>∼80 magnetic latitude) antisunward wind speed is found to be a strong function of both solar and geomagnetic activity. The polar cap temperatures show variations in both solar and geomagnetic activity, with temperatures near 800 K for lowKpandF10.7and greater than about 2000 K for highKpandF10.7. The observed temperatures are significantly greater than those predicted by the mass spectrometer/incoherent scatter model for high activity conditions. Theoretical analysis based on the NCAR TIGCM indicates that the antisunward upper thermospheric winds, driven by upstream ion drag, basically “coast” across the polar cap. The relatively small changes in wind velocity and direction within the polar cap are induced by a combination of forcing terms of commensurate magnitude, including the nonlinear advection term, the Coriolis term, and the pressure gradient force term. The polar cap thermospheric thermal balance is dominated by horizontal advection, and adi

ISSN:0148-0227    

DOI:10.1029/95JA01208

年代:1995

数据来源: WILEY

摘要:

Observations of incoherent‐scatter plasma‐line intensities, measured in theE‐region with the European Incoherent Scatter UHF radar, during auroral precipitation, are presented. Intensities up to 200 times the thermal level were observed in a restricted frequency interval from 5.5 to 6.5 MHz. Intensities were lower at both higher and lower frequencies. The intensities are compared with quantitative estimates based on model suprathermal electron fluxes for the prevailing conditions. The minimum in suprathermal electron fluxes between 2 and 4 eV, which is caused by the excitation of vibrational levels in N2, is found to result in a minimum in expected plasma‐line damping and a maximum in intensity between 5.5 and 6.5 MHz, for the EISCAT UHF wavelength. Good agreement is found between the calculated and observed inte

ISSN:0148-0227    

DOI:10.1029/95JA00765

年代:1995

数据来源: WILEY

摘要:

In this paper we present and discuss the cryogenic infrared radiance instrumentation for shuttle (CIRRIS) 15‐µm CO2and 5.3‐µm NO data with respect to limb emission variability and within the context of latitudinal, diurnal, and geomagnetic variations during two days of observations onboard shuttle flight STS 39, April 29–30, 1991. About 50 limb emission profiles were examined for the two emissions. Enhancements were observed at high latitudes relative to midlatitudes and low latitudes at 140 km altitude for the 15‐µm CO2emission (factor of 2–5). The high‐latitude enhancement in the 5.3‐µm NO emission was larger (factor of 11–14). The high‐latitude nighttime data were collected in the auroral zone during a class III aurora. Diurnal variations are examined at midlatitudes. A significant enhancement in the 15‐µm emission was observed between 0500 and 0700 LT at 140 and 160 km. This effect was modeled by the SHARC atmospheric generator (SAG) which uses the mass spectrometer incoherent scatter (MSIS) model. Species concentrations from the thermosphere‐ionosphere‐mesosphere electrodynamics general circulation model (TIME‐GCM) and SAG models were input to the SHARC radiance code to simulate the CIRRIS limb emission data. The TIME‐GCM predicted the 15‐µm CIRRIS radiances generally well for 100 km

ISSN:0148-0227    

DOI:10.1029/95JA02053

年代:1995

数据来源: WILEY

摘要:

A three‐dimensional model of very low frequency (VLF) radio wave propagation in the Earth‐ionosphere waveguide in the presence of lower ionospheric disturbances is used to quantitatively interpret VLF signatures of lightning‐induced electron precipitation (LEP) events observed in two previously reported cases. One case is that of a 28.5‐kHz signal originating in Puerto Rico and propagating to a receiver in Lake Mistissini, Quebec. The other case involves a 24.0‐kHz signal originating in Cutler, Maine, and received at Stanford, California. In both cases, high‐resolution measurements of the VLF signals were made to accurately document characteristic signatures of LEP events (Inan et al., 1988b, 1990). The comparison of the model calculations with the data yields information about the altitude profiles of electron density of both the extra ionization produced by the LEP events and of the ambient ionosphericDregion. The comparisons are carried out using generally accepted values of the spatial extent of the disturbed regions and the intensity of the particle flux constituting th

ISSN:0148-0227    

DOI:10.1029/95JA01615

年代:1995

数据来源: WILEY

摘要:

We use continuum (white light) and filtered (427.8 nm) images from the Atmospheric Emissions Photometric Imaging (AEPI) experiment on the Atmospheric Laboratory for Applications and Science (ATLAS) 1 shuttle mission to investigate the shape and evolution of artificial auroral patches generated by the Space Experiments with Particle Accelerators (SEPAC) electron beam experiment. Auroral patches generated by this beam experiment are complex and differ in the white light and filtered images. In the white light images, the auroral patch consists of a relatively large, diffuse, and somewhat symmetric head and a tail that is directed approximately opposite the spacecraft velocity vector. From the growth of the tail during a beam pulse, the distance from the imager to the emissions is estimated to be about 200 km, consistent with expectations from a simple model of auroral emissions in the atmosphere. In addition to the auroral patch, an intense, diffuse, and variable background glow filling essentially the entire field of view of the white light imager is seen during the beam pulses. This background glow may be caused by low‐energy electrons very near the shuttle. This glow is absent in the filtered images, in which the shape of the auroral patch differed, consisting of a relatively large, diffuse, but more asymmetric head, a tail, and a smaller and less intense spot below the head. Curvature of the magnetic field and spacecraft motion during the 1‐s filtered images allows an estimate of the relative distance from the shuttle to the head, tail, and small spot. This shape is consistent with head emissions generated relatively near the spacecraft (nearest few kilometers), tail emissions somewhat farther away, and finally the small spot emissions generated the farthest away in the lower atmosphere where the natural auroras would be created. In addition, the shape of these patches in the filtered images suggests that the risetime for the head and tail emissions in the filtered images appears to be longer than that for the small spot emission. The differences between the white light images and the filtered images are consistent with the difference between the emission parent state lifetimes and the energy requirement for the emission production. The white light images contain emissions with long lifetimes, and these emissions which are produced near the observer are swept out of the field of view because they are left behind by the shuttle. This gives a bias toward emissions generated farther from the orbiter. The filtered images contain only the very fast produced N2+emission with a substantial component generated near the orbiter and with an inverse square law attenuated auroral spot in the lower atmosphere. The presence of the tail and the apparent risetime in the near‐field emission suggest that there is a buildup and decay time associated with the hot plasma created by the electron

ISSN:0148-0227    

DOI:10.1029/95JA01105

年代:1995

数据来源: WILEY

摘要:

Simulations from the National Center for Atmospheric Research thermosphere‐ionosphere general circulation model constitute controlled numerical experiments which may be used to assess current understanding of Earth's upper atmosphere. Comparisons with a long term data base are particularly valuable in this regard. Accordingly model simulations of geomagnetically quiet and active periods are compared with an observational database from Saint Santin. The simulations and observations are for equinox and northern summer and winter during solar cycle minimum. The observations consist of the diurnal variation of the meridional neutral winds near 300 km; harmonic analysis yielded the mean components and the 24‐, 12‐, 8‐, and 6‐hour waves. The model/data comparisons for the diurnal variations are good to excellent: differences are generally ≤ 25 m/s with largest differences typically occurring between 1800 and 0600 UT. In the observations, the diurnal component is approximately 60 m/s in amplitude and 12 hours in phase during quiet periods. These values persist during active periods except in summer when the diurnal amplitude falls to 47 m/s. The model predicts a weaker diurnal amplitude in winter than the observations indicate; it also does not predict the observed decrease of the diurnal amplitude in summer with increasing activity. Harmonic analysis of the data indicated that (1) 12‐r and 8‐hour waves are important in summer and equinox; in winter the variation is largely diurnal; (2) the semidiurnal and terdiurnal waves vary with season during quiet periods; and (3) the largest effect of geomagnetic activity is in the 12‐ and 8‐hour waves. In the model, the semidiurnal and terdiurnal tides do not vary with season. Further, the effects of varying geomagnetic activity are predominantly in the diurnal component; the semidiurnal and terdiurnal tides in the TIGCM are relatively unaffected in marked contrast to the observations. The differences between the modeled and observed winds illustrate the pervasiveness and importance of variability in the atmosphere: in the high‐latitude energy and momentum sources, in the solar forcing, and in the waves that originate in the lower atmosphere and penet

ISSN:0148-0227    

DOI:10.1029/95JA01211

年代:1995

数据来源: WILEY