During the 4‐day period when the moon is in the geomagnetic tail, the principal constituents of the lunar atmosphere are neon and argon. The surface concentrations of neon and argon are calculated from a theoretical model to be 3.9×103 and 1.7×103, respectively. The lunar atmosphere is ionized by solar ultraviolet radiation, resulting in electrons at a temperature of about 1.5×105 °K and ions at about 370°K. We investigated dynamic properties of the lunar ionosphere in the high‐latitude tail lobes during quiescent times when plasma energy density from external sources is below the sensitivity threshold of the suprathermal ion detector at the lunar surface. We found that a hydrostatic model of the ionospheric plasma is inadequate because the gravitational potential energy of the plasma is considerably smaller than its thermal energy. A hydrodynamic model, comparable to that used to describe the solar wind, is developed to obtain plasma densities and flow velocities as functions of altitude. The hydrodynamic flow of the ionospheric particles is away from the sunlit hemisphere, in a direction parallel to the magnetic field, and forms a cylinder whose base is the lunar diameter. At 100‐km altitude the calculated ionospheric density is 1.2×10−2 cm−3, with a flow velocity of 4–7 km/s. The corresponding energy density is 2.5×10−13 erg/cm3. Flow under these quiescent conditions exists approximately one third of the time in the geotail. During other times when cross‐tail electric fields are present, the steady flow away from the moon is disrupted by drift velocity components perpendicular to the geomagnetic field lines; also, sporadic occurrences of plasma sheet or lobe plasma temporarily dominate the plasma environment during nonquiescent times. The electromagnetic properties of the quiescent ionosphere are investigated, and it is concluded that plasma effects on lunar induction studies can be neglected for quiescent conditions in the geomagnetic tail lobes.
Neutral upper thermospheric wind and temperature measurements obtained at South Pole, Antarctica (90°S, 75° invariant latitude (INV)), and Mt John, New Zealand (44°S, 171°E, 52° INV) during the storm period June 11‐13 1991 are presented. Winds peaking at about 800 m/s and typical temperatures reaching up to 2000 K were found in the polar cap. Peak winds above Mount John reached 300 m/s in response to the strong high‐latitude forcing which had spread equatorward to midlatitudes. The temperature predictions of the MSIS 86 model were in broad agreement with the observations but were overestimates by several hundred degrees near 0800 UT on June 12 and underestimates by a similar amount near 0800 UT on June 13. The observed meridional winds at South Pole were less uniform and a few hours different in phase than indicated by the vector spherical harmonic (VSH) model predictions. Although the wind magnitudes were similar to VSH at most times, there was a 12 hour difference in the phase in the zonal component. For the Mount John observations the VSH model predictions exaggerated the equatorward penetration of the polar thermospheric circulation relative to the observations. Predicted zonal winds exceeded observations at almost all times. The observed wind pattern at Mount John differed from normal midlatitude quiescent behavior according to the usual pattern but not in simple proportionality with the variations of geomagnetic activity, expressed as Ap or Kp. It is suggested that the sign of the interplanetary magnetic field Y component is an important factor which determines how momentum is coupled between high and middle latitudes during storms and that it will be needed in the specifications for models such as VSH.
A normal modes model that includes realistic temperature and electron density profiles, and the effects of viscosity, heat conduction, and Lorentz forcing has been used to study the response of the high‐latitude thermosphere to temporal variations in the forcing due to the plasma convection pattern. In particular, the rotation rate of the two‐cell convection pattern was varied from the usual 2π in 24 hours to produce a crude simulation of the effects of temporal variations in the forcing. The model calculation proceeds from an initial steady state at 48 hours through four 6 hour integrations, where the rotation rate is alternately decreased and increased, corresponding to decelerated and accelerated flow. The results show that the rotational wind component is only slightly modified when the plasma rotation has been varied. The divergent wind component responds more strongly to variation of the rotation rate, an effect which is greatest in the E region. Variation of the rotation rate then results in greater magnitude and phase variation with height of the divergent wind component in the E region, indicating that vertically propagating gravity waves are of importance in that height range. To the extent to which varying the rotation rate can simulate variations in the configuration of the plasma convection, as are due to substorms or variations in the interplanetary magnetic field, the model indicates that detailed consideration of such geophysical effects is necessary if the transient response of the neutral wind is to be better understood.
Observations have been made of Lyman‐alpha auroral emissions during January and February 1967. The observations were made with a narrow band sky‐scanning photometer mounted on an earth‐oriented satellite in polar orbit. Auroras in the southern hemisphere were usually single in structure, whereas those in the north were complex. The single auroral emissions show a typical latitude spread of about 6.5°. Intensities up to 60 kR were observed at times of high magnetic activity, implying energy deposition up to 20 ergs cm−2 sec−1. The intensities show a correlation with high‐latitude geomagnetic activity, with a breakdown during extremely active periods.
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