L. H. Brace

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.

Measurements of the ambient photoelectron spectrum from atmosphere explorer: II. AE-E measurements from 300 to 1000 km during solar minimum conditions

Abstract The ambient photoelectron spectrum above 300 km has been measured for a sample of 500 AE-E orbits during the period 13 December 1975 to 24 February 1976 corresponding to solar minimum conditions. The 24 h average and maximum ΣKp were 19 and 35, respectively. The photoelectron flux above 300 km was found to have an intensity and energy spectrum characteristic of the 250–300 km production region only when there was a low plasma density at the satellite altitude. Data taken at local times up to 3 h after sunrise were of this type and the escaping flux was observed to extend to altitudes above 900 km with very little modification, as predicted by several theoretical calculations. The flux at high altitudes was found to be extremely variable throughout the rest of the day, probably as a result of attenuation and energy loss to thermal plasma along the path of the escaping photoelectrons. This attenuation was most pronounced where the photoelectrons passed through regions of high plasma density associated with the equatorial anomaly. At altitudes of 600 km, the photoelectron fluxes ranged from severely attenuated to essentially unaltered—depending on the specific conditions, Photoelectron fluxes from conjugate regions were often less attenuated than those observed arriving from the high density regions immediately below. Comparison of the observed attenuations, photoelectron line broadening, and energy loss due to coulomb scattering from the thermal plasma with rough calculations based on stopping power and transmission coefficients of thermal plasma for fast electrons yielded order of magnitude agreement—satisfactory in view of the large number of assumptions necessary for the calculations. Overall, the impression of the high altitude photoelectron flux which emerges from this work is that the fluxes are extremely variable as a consequence of interactions with the thermal plasma whose density is in turn affected by electrodynamic and neutral wind processes in the underlying F region.

Global empirical models of ionospheric electron temperature in the upper F-region and plasmasphere based on in situ measurements from the atmosphere explorer-c, isis-1 and isis-2 satellites

Abstract Langmuir probe measurements of electron temperature, Te, in the vicinity of 300, 400, 1400 and 3000 km from the Atmosphere Explorer-C and the ISIS satellites have been employed to construct empirical models of the global distribution of Te at each of these altitudes. Legendre polynomials are employed to describe the observations at solstice and equinox in terms of dip latitude and local time. Sources of Te variations, such as solar activity, magnetic activity and longitude are found to be of second order importance, although they are resolvable in some cases by comparisons of the data with the model. The behavior of Te at the altitudes of these models is discussed in terms of its implications for our understanding of the energy exchange between the F-region and the plasmasphere.

Ionospheric storm effects at subauroral latitudes: A case study

An attempt is made to classify ionospheric storm effects at subauroral latitudes according to their presumed origin. The storm of December 7/8, 1982, serves as an example. It is investigated using ionosonde, electron content, and DE 2 satellite data. The following effects are distinguished: (1) positive storm effects caused by traveling atmospheric disturbances, (2) positive storm effects caused by changes in the large-scale thermospheric wind circulation, (3) positive storm effects caused by the expansion of the polar ionization enhancement, (4) negative storm effects caused by perturbations of the neutral gas composition, and (5) negative storm effects caused by the equatorward displacement of the trough region.

An equatorial temperature and wind anomaly (ETWA)

Data obtained from the WATS (Wind and Temperature Spectrometer) and LP (Langmuir Probe) experiments on board DE-2 (Dynamic Explorer) during high solar activity show evidence of anomalous latitudinal variations in the zonal winds and temperature at low latitudes. The zonal winds exhibit a broad maximum centered around the dip equator, flanked by minima on either side around 25 degrees; while the temperature exhibits a pronounced bowl-shaped minimum at the dip equator which is flanked by maxima. The two minima in the zonal winds and the corresponding maxima in the temperature are nearly collocated with the crests of the well known Equatorial Ionization Anomaly (EIA). The maximum in the zonal winds and the minimum in the gas temperature are collocated with the trough of the EIA. The differences between the maxima and minima in temperature and zonal winds, on many occasions, are observed to exceed 100 K and 100 m/s, respectively. The characteristics of this new phenomenon have eluded present day empirical models of thermospheric temperature and winds. The connection among these variables can be understood from the ion-neutral drag effect on the motions of the neutrals that in turn affect their energy balance.

Conjugate occurrence of the electric field fluctuations in the nighttime midlatitude ionosphere

The DE 2 satellite observed electric field fluctuations on the topside of the nighttime midlatitude ionosphere. They extended several hundred kilometers in the latitudinal direction with wavelengths of several tens of kilometers, and their amplitudes were a few millivolts per meter. Such fluctuations were often observed at magnetically conjugate points in the northern and southern hemispheres. These electric field fluctuations are perpendicular to the geomagnetic field. They are not accompanied by any significant plasma depletion or electron temperature variations. Magnetic field fluctuations are sometimes observed simultaneously with electric field fluctuations. We interpret that these fluctuations are caused by field-aligned currents which flow from the ionosphere in one hemisphere to the conjugate point in the other hemisphere. The power spectrum of these midlatitude electric field fluctuations follows a power law of the form Power α ƒ −n, with the spectral index n of 3.5 to 4.5, which is steeper than that of the electric field fluctuations in the high-latitude ionosphere or in the equatorial ionosphere. This phenomenon may be related to other ionospheric phenomena, for example, the F region field-aligned irregularities or spread-F, observed by ground-based methods such as the MU radar, but the relationship is not clear.

Detailed behaviour of the midlatitude ionosphere from the explorer XVII satellite

Abstract Measurements of electron temperature and ion density by electrostatic probes on the Explorer XVII satellite have revealed the detailed diurnal and latitudinal behaviour of the summer ionosphere near the altitude of the F 2 maximum over the eastern United States at the time of solar minimum. The electron temperature at 40° north magnetic latitude is observed to rise rapidly from a nighttime value of about 1100°K to a mid-morning maximum of 2700°K followed by an afternoon plateau of 2000°K. The electron temperature always exceeds accepted values of neutral gas temperature and thus reflects the existence of heat sources in both the daytime and nocturnal ionosphere. The ion density displays a maximum value about three hours after local noon. A strong degree of latitude control, evident near the F 2 maximum, causes the temperature to increase and the density to decrease with increasing latitude. The electron temperature is relatively independent of altitude between 260 and 450 km in the forenoon but displays a slight increase with altitude at night. The diurnal variations found over stations at 10 and 60° north magnetic latitude display general characteristics similar to those found at 40°N and also reveal the inverse gradients of temperature and density with latitude. Calculations based on the measurements show that the ion temperature begins to exceed the neutral temperature significantly above 300 km and approaches the electron temperature near 600 km. The ratio of electron to ion temperature at 300 km is between 2 and 3 in the daytime and about 1.5, but highly variable, at night. The measurements are also employed to calculate the amount of electron heating in the F -region. It appears that heating by solar ultraviolet radiation is adequate to explain the daytime results below 300 km, and that the available flux of fast photoelectrons is adequate to account for the heating at higher altitudes. The heating at night is consistent with a corpuscular flux of soft electrons with a total energy of about 1 × 10 −2 ergs cm −2 sec −1 .

Disappearing ionospheres on the nightside of Venus

Abstract Instruments on the Pioneer Venus Orbiter have detected a substantial ionosphere on the nightside of Venus during most orbits. However, during some orbits the nightside ionosphere seems to have almost disappeared, existing only as irregular patches of low-density plasma. The solar wind dynamic pressure on these occasions is greater than average. We have correlated data from several instruments (Langmuir probe, ion mass spectrometer, retarding potential analyzer, magnetometer, and plasma analyzer) for a number of orbits during which the nightside ionosphere had disappeared. The magnetic field tends to be coherent, horizontal, and larger than usual, and the electron and ion temperatures are much larger than they usually are on the nightside. We suggest mechanisms which might explain the reasons for the disappearance of the ionosphere when the solar wind dynamic pressure is large.

A theoretical model of the ionosphere dynamics with interhemispheric coupling

Abstract The ionospheric plasma is described by means of the momentum and continuity equations for O + , He + , H + and the energy equations for T e and T i . The ion production rate profiles and the photoelectron heating rates to the plasma are computed separately and serve as source functions in the continuity and energy equations respectively. The neutral atmosphere (composition, winds, temperature) as well as the incident photon spectrum are part of the inputs. Charge transfer, collisional energy loss processes, ion-neutral drag, diffusion and electron heat conduction are among the physical processes included in the calculations. Ion heat conduction is neglected. Assuming steady state, the equations are transformed into integral equations and then solved iteratively along geomagnetic field lines from the region of chemical equilibrium up to the equatorial plane. In this scheme non-local heating is included in a self-consistent way. The lower boundary conditions are obtained by satisfying photochemical and local energy equilibrium. Under conditions of asymmetry with respect to the equatorial plane the solutions are carried out for both hemispheres, and the upper boundary conditions are determined by requiring continuity of the physical parameters across the equatorial plane. In this way inter hemispheric plasma transport is introduced in a natural way. For symmetric conditions the transport fluxes are assumed zero at the Equator. An extension of this model to include time-dependent phenomena is presented.

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�.