W. R. Hoegy

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.

Neutral temperature anomaly in the equatorial thermosphere - A source of vertical winds

Data obtained from the WATS (Wind and Temperature Spectrometer) instrument on DE-2 (Dynamics Explorer) during high solar activity, show new evidence for the presence of vertical winds of a significant magnitude in the equatorial thermosphere. They reveal a latitudinal structure that can be related to the recently discovered phenomena of the Equatorial Temperature and Wind Anomaly (ETWA). In the local evening hours, the vertical winds usually are downward around the dip equator and collocated with the temperature minimum of ETWA. In general, they are upward at about 24° dip latitude away from the dip equator and are collocated with the ETWA temperature crests. The magnitude of the vertical winds is in the 10–40 m/s range. It is proposed that the temperature and pressure ridges, formed by the excess ion drag on the zonal winds around the two crests and ordered by the relatively lower ion drag at the trough of the well known Equatorial Ionization Anomaly (EIA), drive a new wind system in the meridional plane and that the measured vertical winds form part of this wind system.

Momentum transfer collision frequency of O^+-O

The interaction of the thermosphere and ionosphere is largely governed by collisions between ions and neutral particles. On Venus and the Earth, O+ is a dominant ion, and atomic O dominates throughout much of the thermosphere; therefore an accurate O+-O cross section is an important prerequisite for understanding the dynamics of planetary upper atmospheres. The cross section and momentum, transfer collision frequency are calculated with a quantum mechanical code which includes resonance charge exchange, polarization, and charge-quadrupole effects. Our results agree well with earlier calculations of Stubbe [1968] and Stallcop et al [1991].

E and F region study of the evening sector auroral oval: A Chatanika/Dynamics Explorer 2/NOAA 6 comparison

Simultaneous data obtained with the Chatanika incoherent scatter radar and the Dynamics Explorer 2 (DE 2) and NOAA 6 satellites are used to relate the locations of the precipitating particles, field-aligned currents, and E and F region ionization structures in the evening-sector auroral oval. The auroral E layer observed by the radar extends about 2° equatorward of the electron precipitation region, and its equatorward edge coincides with the equatorward edges of the region 2 field-aligned current and intense convection region (E ≃ 50 mV/m). It is shown that precipitating protons are responsible for part of the E region ionization within the electron precipitation region as well as south of it. E region density profiles calculated from ion spectra measured by the DE 2 and NOAA 6 satellites are in fairly good agreement with the Chatanika data. In the F region, a channel of enhanced ionization density, elongated along the east-west direction and having a width of about 100 km, marks the poleward edge of the main trough. It is colocated with the equatorward boundary of the electron precipitation from the central plasma sheet. Although enhanced fluxes of soft electrons are observed at this boundary, the energy input to the ionospheric electron gas, calculated from the radar data, shows that this ionization channel is not locally produced by this soft precipitation, but that it is rather a convected feature. In fact, both the trough and the ionization channel are located in a region where the plasma flows sunward at high speed, but the flux tubes associated with these two features have different convective time histories. Keeping in mind that several processes operate together in the F region, our data set is consistent with the following trough and ionization channel formation mechanisms. (1) The mid-latitude trough, located equatorward of the electron precipitation region, is mainly the result of transport and enhanced recombination due to large electric fields. Flux tubes on the low-latitude edge of the trough have most probably corotated eastward before flowing sunward at higher latitudes where magnetospheric convection predominates. The trough thus forms by recombination during the long time the flux tubes stagnate in the region where the flow reverses. (2) Flux tubes associated with the ionization channel have drifted antisunward in the polar cap before drifting sunward in the auroral zone. Its formation results from the distortion of polar cap F region ionization structures, due to the incompressibility of the flow.

Effects of a lightning discharge detected by the DE 2 satellite over Hurricane Debbie

We report the satellite observation of a large, ∼40 mV m−1, transient electric field disturbance over Hurricane Debbie in September 1982. The event lasted less than a second and correlated closely with a burst of highly field-aligned, upward moving electrons with nearly 1 keV of energy. The electric field event is viewed as a spheric disturbance from a lightning discharge in the active weather system located beneath the satellite. A spheric interpretation of the observed electric field transient is consistent with a subsequent observation of energetic electrons precipitating from the radiation belts. Measured quasi-dc electric fields and cold plasma density variations are only roughly consistent with model predictions for ULF wave propagation from a storm system to the ionosphere. To understand this first observation of upward moving electrons in the ionosphere associated with a lightning event, we compare the several mechanisms for electron acceleration by electric fields with components (E∥) along the magnetic field. In our scenario, “runaway” electrons were accelerated in ∼1 ms by a downward directed E∥ pulse of ∼1 V m−1 magnitude. Such fields can result from rapidly exposed, negative space charges near the tops of clouds during positive cloud-to-ground discharges. High-frequency Fourier components of the E∥ pulse must propagate through the low-conducting nighttime atmosphere to the ionosphere with little dissipation.

The poleward edge of the mid-latitude trough—its formation, orientation and dynamics

Abstract Data from the Advanced Ionospheric Sounder (AIS) deployed at Halley, Antarctica (76°S, 27°W; L = 4.2) and the Dynamics Explorer-2 spacecraft (DE-2) are used to investigate several aspects of the formation processes and dynamics of the poleward edge of the mid-latitude electron density trough. These include a study of the flux and energy of charged particles precipitating into the F -region as a function of Magnetic Local Time. It is found that local energetic electron precipitation is a major source of ionisation of the poleward edge in the evening sector, but after magnetic midnight transport processes become more important. Occasionally a significant increase in the flux of conjugate photo-electrons is co-located with the poleward edge of the trough in the morning sector. Some possible mechanisms are discussed but no firm conclusions are drawn. The combination of AIS and DE-2 data has allowed identification of significant longitudinal structure on the poleward edge of the trough that may be the result of substorm activity. It is found that the orientation of the poleward edge of the trough and the locus of the plasmapause predicted from the ‘tear-drop’ model vary in rather a similar manner with local time, though no close physical link between the two features is inferred from this coincidence. A comparison of the equatorward motion of the poleward edge on many nights is used to show that Kp is a poor index to use in any empirical model for predicting the temporal variations of the location of the trough. It is suggested that a more thorough understanding of the processes controlling the variability of the magnetospheric convection electric field is required before any significant improvement to the empirical models is likely to occur.

Solar activity variation of ionospheric plasma temperatures

Abstract We review the present status of understanding and modelling of the variation of electron and ion temperatures with solar activity. The observed solar activity dependence is a function of latitude, time of day, altitude, and season. The ion temperature and its dependence on solar activity are determined by the close coupling with the neutrals and their temperatures at low altitudes and with electrons at high altitudes. The ionospheric electron gas exhibits a rather complex thermal response pattern to changing solar activity. Satellite data have indicated a positive correlation between electron temperature and solar activity at high altitudes. Incoherent scatter measurements have shown that the response of electron temperature to changes in solar activity depends on season and is different for day and night. By combining the results from the separate studies we attempt to describe the average variation of the plasma temperatures with solar activity and to assess its importance for ionospheric modelling. We have investigated temperature variations at low latitudes with the help of incoherent scatter measurements from the Arecibo radar facility and satellite probe measurements from AE-C and DE 2.

Local time variation of equatorial temperature and zonal wind anomaly (ETWA)

Abstract We describe the average and apparent local (solar) time (LST) variations in the zonal winds (Z) and temperatures (T) as measured with the Wind and Temperature Spectrometer (WATS) on-board the polar orbiting DE-2 satellite in the altitude range of 300–450 km during a near solar maximum period of 1981–1982. During this time period, the variations in the solar flux (F10.7) and magnetic activity (Ap) contribute significantly to the apparent LST variations in T; while those effects on the LST variations in Z are small. The observations are related to the equatorial ionization anomaly (EIA) as seen in the electron density data obtained from the same satellite with the LANG instrument. The latitudinal variations in Z always reveal a maximum at the dip equator where the EIA trough forms and minima (with velocities reduced by a factor of two) are seen on either side of the equator where the EIA crests form. At 20:00 LST, the largest wind velocities are observed, directed eastward, after changing direction at around 17:00 LST where the largest accelerations occur. We delineate the diurnal variations in the strength of the Equatorial Temperature and Wind Anomaly (ETWA) defined by the differences in the wind velocities (DZ) and temperatures (DT) at the crests and troughs of the ionization. The diurnal variations in DZ are similar to those in Z at the trough. The diurnal variations in DT differ from those apparent in T. Excess temperatures, DT, at the crests show up with the development of the EIA as early as 09:30 LST by which time the zonal wind has attained its westward maximum. But DT continues to increase with the EIA crest until 14:00 LST, followed by a dip at around 16:00 LST in phase with the zero crossing of Z. The highest value of DT is only reached at 20:00 LST when both Z and the crest intensification reach their respective maxima. This demonstrates that the development of the equatorial temperature anomaly critically depends both on the development of the EIA crests and the zonal winds, which clearly establishes the primary role of ion drag in generating the ETWA phenomenon.

Spacecraft potential effects on the Dynamics Explorer 2 satellite

The relationship between the plasma environment and spacecraft potential is examined for the Dynamics Explorer 2 (DE 2) spacecraft in an attempt to improve the accuracy of ion drift measurements by the retarding potential analyzer (RPA). Because of the DE 2 orbit characteristics (apogee near 1000 km and perigee near 300 km) and the configuration of conducting surfaces on the spacecraft, thermal electrons and ions constituted the only significant contributions to the charging currents to the spacecraft surface for the majority of geophysical conditions encountered. The geomagnetic field had considerable effect on the spacecraft potential due to magnetic field confinement of the electrons as well as to the V × B electric field resulting from the movement of the spacecraft across magnetic field lines. Using a database of inferred spacecraft potentials from the RPA, measured electron temperatures from the Langmuir probe (LANG), and calculated V × B electric fields, we derive an algorithm for determining the spacecraft potential (at the location of the RPA on the spacecraft) for any point of the DE 2 orbit. Knowledge of the spacecraft potential subsequently allows us to retrieve relatively accurate ion drifts from the RPA data.

Model and Observation Comparison of the Universal Time and IMF By Dependence of the Ionospheric Polar Hole

The polar ionospheric F-region often exhibits regions of marked density depletion. These depletions have been observed by a variety of polar orbiting ionospheric satellites over a full range of solar cycle, season, magnetic activity, and universal time (UT). An empirical model of these observations has recently been developed to describe the polar depletion dependence on these parameters. Specifically, the dependence has been defined as a function of F10.7 (solar), summer or winter, Kp (magnetic), and UT. Polar cap depletions have also been predicted /1, 2/ and are, hence, present in physical models of the high latitude ionosphere. Using the Utah State University Time Dependent Ionospheric Model (TDIM) the predicted polar depletion characteristics are compared with those described by the above empirical model. In addition, the TDIM is used to predict the IMF By dependence of the polar hole feature.