R. F. Theis

Plasma clouds above the ionopause of Venus and their implications

Abstract Early Pioneer Venus orbiter measurements by the Electron Temperature Probe (OETP) have revealed wavelike structures at the ionopause and clouds of plasma above the ionopause, features which may represent ionospheric plasma at different stages in its removal by solar wind-ionosphere interaction processes. Continuing operation of the orbiter through three Venus years has now provided enough additional examples of these features to permit their morphologies to be examined in some detail. The global distribution of the clouds suggests that they originate at the dayside ionopause as wavelike structures which may become detached and swept downstream in the ionosheath flow. Alternatively the clouds may actually be attached streamers analogous to cometary structure. Estimates of the total ion escape rate from Venus by this process yields values up to 7 × 10 26 ions s −1 , based on their measured transit times, their probability of occurrence, their statistical distribution and their average electron density. Preliminary analysis shows that such an excape flux could be supplied by the upward diffusion limited flow of 0 + from the entire dayside ionosphere. Observed distortions of dayside ionosphere height profiles suggest that such flows may be present much of the time. If such an escape flux were to continue over the entire lifetime of Venus, the effects upon the evolution of its primitive atmosphere may have been significant.

Electron temperatures and densities in the Venus ionosphere - Pioneer Venus orbiter electron temperature probe results

Altitude profiles of electron temperature and density in the ionosphere of Venus have been obtained by the Pioneer Venus orbiter electron temperatutre probe. Elevated temperatutres observed at times of low solar wind flux exhibit height profiles that are consistent with a model in which less than 5 percent of the solar wind energy is deposited at the ionopause and is conducted downward through an unmagnetized ionosphere to the region below 200 kilomneters where electron cooling to the neutral atmosphere proceeds rapidly. When solar wind fluxes are higher, the electron temperatures and densities are highly structured and the ionopause moves to lower altitudes. The ionopause height in the late afternoon sector observed thus far varies so widely from day to (day that any height variation with solar zenith angle is not apparent in the observations. In the neighborhood of the ionopause, measuremnents of plasma temperatures and densities and magnetic field strength indicate that an induced magnetic barrier plays an important role in the pressure transfer between the solar wind and the ionosphere. The bow, shock is marked by a distinct increase in electron current collected by the instrument, a featutre that provides a convenient identification of the bow shock location.

Empirical Models of the Electron Temperature and Density in the Nightside Venus Ionosphere

Empirical models of the electron temperature and electron density of the late afternoon and nightside Venus ionosphere have been derived from Pioneer Venus measurements acquired between 10 December 1978 and 23 March 1979. The models describe the average ionosphere conditions near 18�N latitude between 150 and 700 kilometers altitude for solar zenith angles of 80� to 180�. The average index of solar flux was 200. A major feature of the density model is the factor of 10 decrease beyond 90� followed by a very gradual decrease between 120� and 180�. The density at 150� is about five times greater than observed by Venera 9 and 10 at solar minimum (solar flux ≈80), a difference that is probably related to the effects of increased solar activity on the processes that maintain the nightside ionosphere. The nightside electron density profile from the model (above 150 kilometers) can be reproduced theoretically either by transport of 0+ ions from the dayside or by precipitation of low-energy electrons. The ion transport process would require a horizontal flow velocity of about 300 meters per second, a value that is consistent with other Pioneer Venus observations. Although currently available energetic electron data do not yet permit the role of precipitation to be evaluated quantitatively, this process is clearly involved to some extent in the formation of the nightside ionosphere. Perhaps the most surprising feature of the temperature model is that the electron temperature remains high throughout the nightside ionosphere. These high nocturnal temperatures and the existence of a well-defined nightside ionopause suggest that energetic processes occur across the top of the entire nightside ionosphere, maintaining elevated temperatures. A heat flux of 2 x 1010 electron volts per square centimeter per second, introduced at the ionopause, is consistent with the average electron temperature profile on the nightside at a solar zenith angle of 140�.

Ionospheric electron temperature at solar maximum

Abstract Langmuir probe measurements made at solar maximum from the Dynamics Explorer-2 satellite in 1981 and 1982 are employed to examine the latitudinal variation of electron temperature, Te, at altitudes between 300 and 400 km and its response to 27 day variations of solar EUV. Comparison of these data with Te models based on the solar minimum measurements from Atmosphere Explorer-C suggest that the daytime Te does not change very much during the solar cycle, except at low latitudes where an especially large 27 day variation occurs. The 27 day component decreases from about 7°/F10.7 unit at the equator to 3°/F10.7 unit at 851V 3 middle and higher latitudes. From these DE-2 measurements, and those from AE-C, we conclude that the daytime Te near the F2 peak is more responsive to short-term (daily) variations in F10.7 than to any longer term changes that may occur between solar minimum and solar maximum. To investigate this sensitivity of the dayside ionosphere to solar activity we employ the inverse relationship of Te and Ne, that was found at solar minimum, to see if it can be used to order the Te behaviour at solar maximum. We introduce a simple quadratic correction for the F10.7 influence on Te based on the entire daytime AE-C and DE-2 data base between 300 and 400 km. Although this equation may be found useful, the systematic deviations of the DE-2 data suggest that the solar minimum model does not accurately describe the Te-Ne relationships at solar maximum, at least above 300 km where the DE-2 measurements were made. Future work with this data base should attempt to see if such a relationship exists.

The nightward ion flow scenario at Venus revisited

Abstract In this paper, we indicate how existing Pioneer Venus Orbiter (PVO) data might be used to gain a better understanding of nightward ion flow in the Venusian ionosphere. Calculations based on PVO measurements made at solar maximum suggest that the global nightward flow of O+ may be significantly greater than is required to maintain the observed nightside ionosphere densities. The validity of this conclusion depends upon (1) the accuracy with which the flow can be determined from the PVO ion density and velocity measurements and (2) the validity of the ionosphere theory used to estimate the required downward O+ flux on the night side. If the measurements and theory are assumed to be accurate, the excess nightward flow implies a significant rate of ion escape from the planet, particularly at times of low solar wind dynamic pressure, Psw, when the ionopause rises to allow increased nightward flow. To illustrate a potentially important mechanism for ion escape from Venus, we present Orbiter Electron Temperature Probe (OETP) observations of plasma clouds and scavenged ionospheric plasma observed above the ionopause. We then employ OETP and Orbiter Retarding Potential Analyzer (ORPA) data to reexamine the global ion flow for average Psw conditions, and we find the net flow to be in agreement with previous estimates. We also find that the net transterminator flow at average Psw approaches the limiting upward ion flow available from the dayside ionosphere, a limit which is imposed by O+ collisions with O2+. OETP measurements in the terminator ionosphere show that Ne declines at times of low Psw. This suggests that the dayside ionosphere is unable to supply the increased demand when the ionopause rises above its average height, thus requiring an upward diversion of some of the nightward flow to populate the overlying, newly-created ionosphere. We predict that the ion velocities will also be found to decrease at times of low Psw, in part due to the source-limited flow, and in part because the expansion of the ionosphere diminishes the “nozzle effect” that has been identified as a source of nightward ion acceleration. Collectively, these effects act to limit the increase in the net nightward ion flow at times of low Psw. Since O+-O2+ collisions will impart an upward velocity to the O2+ ions, the O+ diffusion limit may be a soft one, so the nightward flow can continue to increase somewhat as Psw declines. The collision process should also enhance the O2+ concentration in the dayside upper ionosphere and, to the extent that the O2+ participates in the nightward flow, should produce O2+ enhancements on the night side as well. These expectations could be verified by an examination of the orbit-to-orbit changes in ion composition and velocity in response to Psw variations.

Evaluation of the international reference ionosphere with the large AE-C and DE2 data bases

Abstract Empirical models such as the International Reference Ionosphere (IRI) are synthesized from large data bases. They can be viewed as analytical tools to facilitate accessing information stored in the data banks. However, in establishing the models, one has to apply smoothing and averaging procedures that in effect reduce the original information content. Our study evaluates the agreement between the data base and the model at two opposite extremes of time resolution. We compare electron densities and temperatures in the altitude range of 300 to 400 km predicted by the IRI and measured by the AE-C and DE 2 satellites on the level of individual orbits as well as on the level of mission averages. Whereas the averages show excellent agreement, the comparison for individual measurements indicates the limitations of empirical models.

Global models of Ne and Te at solar maximum based on DE-2 measurements

Abstract Newly developed global models of Ne and Te in the F-region at solar maximum, based on Langmuir probe measurements from the Dynamics Explorer-2 satellite, are compared with solar minimum models that were developed earlier from Atmosphere Explorer data. Spherical harmonics are used in both models to describe the variations with geomagnetic latitude and local time, but the solar maximum model also includes longitudinal variations. Temporal variations associated with season, UT, Kp and F10.7 were obtained by analysis of the deviations of the measurements from the global models, but these effects will not be presented here. The solar minimum models were for the fixed altitudes of 300 km and 400 km, while the solar maximum model covers all altitudes between 300 and 1000 km. In this paper we compare the global patterns of Ne and Te at 400 km at solar maximum and minimum with the IRI model for the corresponding parts of the solar cycle. In most respects the IRI model describes the empirical models quite well at both solar maximum and solar minimum.

An extension of the VIRA electron temperature and density models to include solar cycle variations

Abstract The original VIRA ionosphere model was based primarily on the Pioneer Venus Orbiter (PVO) data obtained at solar maximum (F10.7∼200) in 1979 and 1980 when periapsis was being maintained deep in the Venusian ionosphere. In situ measurements provided data on temperature, composition, density, and drift velocity, while the radio occultation method provided height profiles of electron density, N e . The solar cycle variation was deduced by comparison with the Venera 9 and 10 occultation data from the previous solar minimum. No data were available on the solar cycle variations of other ionospheric parameters, because periapsis had already risen out of the ionosphere by the time solar activity began to decline early in 1983. During the Entry period in the Fall of 1992, however, PVO got a brief glimpse of the nightside ionosphere at lower solar activity (F10.7∼120). During the intervening decade important in situ data were obtained on the upper nightside ionosphere that extends far down stream from the planet. This region was found to be highly sensitive to solar wind interactions and solar activity. In this paper, we discuss ways in which the later PVO data can be used to extend the VIRA model to higher altitudes and to include the solar cycle variations. As an example, we present some pre-entry Orbiter Electron Temperature Probe measurements that provide new clues as to the dayside T e behavior at low solar activity.

Empirical models of the latitudinal variations of Te and Ne in the ionosphere at solar maximum

Abstract Global spectral models of electron temperature (T e ) and density (N e ) were produced earlier using Dynamics Explorer-2 (DE-2) Langmuir probe measurements made between 1981 and 1983. The models were derived for the fixed altitudes of 300, 500, and 850 km. The variables in those models included spatial structure (geomagnetic latitude and longitude) and temporal variations (local time, season, UT, F 10.7 , and kp). Unfortunately, the DE-2 data base is not adequate to fully define all of these variables because of the eccentricity of the orbit, the slow evolution of the orbit, and the brevity of the mission (19 months). Legendre polynomials beyond order 5 could not be well enough defined to resolve small scale structures like the auroral oval, the midlatitude trough, and the equatorial anomaly, though these feature are clearly evident in the profiles from individual orbits. In the present work we take a different approach by modelling only the latitudinal variations observed in narrow altitude and local time bands and using measurements obtained during intervals short compared to a season. This approach allows the data to accurately define latitudinal terms up to 17 th order. To illustrate this improved latitudinal resolution, selected models of T e and N e at 500 km are compared with latitude profiles from the earlier global models and with the DE-2 measurements for the same local times and altitudes.