The measurements of electron density made by the Plasma Wave Subsystem instruments on Galileo during its pass through the torus on December 7th, 1995 are compared with a model based on Voyager 1 measurements made in March 1979. Outside Io's orbit, the plasma densities observed by Galileo are approximately a factor of two higher than the Voyager values. Shortly after crossing Io's orbit, the Galileo density profile dropped sharply and remained at low values for the rest of the inbound leg, suggesting that the ‘ribbon‧ region was either absent or much farther from Jupiter than usual. The peak density on the outbound leg is consistent with Voyager‐based predictions for the cold torus in both location (5.1 Rj) and magnitude (950 cm −3 ). Inside 5 Rj the density dropped sharply to less than 3 cm −3 .
Images of the nightside of Venus in the (0,1)δ band of nitric oxide have been obtained by the Pioneer Venus orbiter ultraviolet spectrometer (OUVS). The emission, which is produced by radiative association of N and O, shows a bright spot reaching ∼5 kR and located at 2 a.m. local solar time just south of the equator. The emitting layer is at 111±7‐km altitude. A one dimensional vertical transport model shows that the hemispheric average brightness of 0.8 kR is consistent with the orbiter neutral mass spectrometer (ONMS) measurements of N and O near 167 km, and that the altitude of the emitting layer is consistent with the eddy mixing model proposed to explain the dayside helium profile measured by the bus neutral mass spectrometer. In the model, N reaches a peak of 7 × 10 8 cm −3 at 114 km, and O reaches a peak of 2.6 × 10 11 cm −3 at 106 km. There is a fair degree of consistency between the ONMS, OUVS, and other airglow measurements, except as regards the local time dependence.
Abstract We report results from a study of two consecutive Martian years of imaging observations of nitric oxide ultraviolet nightglow by the Imaging Ultraviolet Spectrograph (IUVS) on the Mars Atmosphere and Volatile Evolution (MAVEN) mission spacecraft. The emission arises from recombination of N and O atoms in Mars' nightside mesosphere. The brightness traces the reaction rate as opposed to the abundance of constituents, revealing where circulation patterns concentrate N and O and enhance recombination. Emissions are brightest around the winter poles, with equatorial regions brightening around the equinoxes. These changes offer clear evidence of circulation patterns transitioning from a single cross‐equatorial cell operating during solstice periods to more symmetric equator‐to‐poles circulation around the equinoxes. Prominent atmospheric tides intensify the emissions at different longitudes, latitude ranges, and seasons. We find a strong eastward‐propagating diurnal tide (DE2) near the equator during the equinoxes, with a remarkably bright spot narrowly confined near (0°, 0°). Wave features at the opposite winter poles are dissimilar, reflecting different circulation patterns at perihelion versus aphelion. LMD‐MGCM simulations agree with the patterns of most observed phenomena, confirming that the model captures the dominant physical processes. At the south winter pole, however, the model fails to match a strong wave‐1 spiral feature. Observed brightnesses exceed model predictions by a factor of 1.9 globally, probably due to an underestimation of the dayside production of N and O atoms. Further study of discrepancies between the model and observations offers opportunities to improve our understanding of chemical and transport processes controlling the emission.
The ultraviolet nitric oxide spectrometer (UVNO) experiment on the Atmosphere Explorer D (AE‐D) satellite measured thermospheric nitric oxide during the winter of 1974–1975 using resonant fluorescence from the 1–0 gamma band of the molecule. Almost complete latitude coverage was obtained, but the observations were confined to morning local times close to 0900. The 1–0 gamma band intensity profiles measured by the instrument were inverted to provide vertical profiles of the NO number density between about 90 and 200 km. Typically, the measured NO concentrations reached a maximum between altitudes of 100 and 110 km, and more NO was observed at higher latitudes than at low latitudes, in agreement with previous observational studies. The shape of the NO profile was also found to be a function of latitude, with a plateau appearing in the profile near 130 km for low latitudes and mid‐latitudes in the winter hemisphere.
Abstract Recent results from the MAVEN Langmuir Probe and Waves instrument suggest higher than predicted electron temperatures ( T e ) in Mars' dayside ionosphere above ~180 km in altitude. Correspondingly, measurements from Neutral Gas and Ion Mass Spectrometer indicate significant abundances of O 2 + up to ~500 km in altitude, suggesting that O 2 + may be a principal ion loss mechanism of oxygen. In this article, we investigate the effects of the higher T e (which results from electron heating) and ion heating on ion outflow and loss. Numerical solutions show that plasma processes including ion heating and higher T e may greatly increase O 2 + loss at Mars. In particular, enhanced T e in Mars' ionosphere just above the exobase creates a substantial ambipolar electric field with a potential ( e Φ) of several k B T e , which draws ions out of the region allowing for enhanced escape. With active solar wind, electron, and ion heating, direct O 2 + loss could match or exceed loss via dissociative recombination of O 2 + . These results suggest that direct loss of O 2 + may have played a significant role in the loss of oxygen at Mars over time.
The Pioneer Venus Orbiter ultraviolet spectrometer (PVOUVS) routinely obtained interplanetary hydrogen Lyman α data while viewing ecliptic latitudes near 30°S from 1978 to 1992 (during solar cycles 21 and 22). We describe “hot” models for this interplanetary Lyman α data that include the solar cycle variation of (1) the solar flux, as a function of latitude and longitude; (2) the radiation pressure on hydrogen atoms; (3) the solar wind flux; (4) the solar EUV flux; and (5) the multiple scattering correction to an optically thin radiative transfer model. These models make use of solar radiation flux parameters (solar wind, solar EUV, and solar Lyman α) from spacecraft and ground‐based solar proxy observations. Comparison of the upwind data and model indicates that the ratio of the solar Lyman α line center flux (responsible for the interplanetary signal) to the observed solar Lyman α integrated flux is constant to within ∼20%, with an effective line width near 1.1 Å. Averaging the solar radiation pressure and hydrogen atom lifetime over 1 year before the observation reproduces the upwind intensity time variation but not the downwind. A better fit to the downwind time series is found using the 1 year average appropriate for the time that the atoms passed closest to the sun. Solar Lyman α measurements from two satellites are used in our models. Upper Atmosphere Research Satellite (UARS) solar Lyman α measurements are systematically higher than Solar Mesosphere Explorer (SME) values and have a larger solar maximum to solar minimum ratio. UARS‐based models work better than SME‐based models in fitting the PVOUVS downwind time series Lyman α data.
