Nightside Ionosphere Of Venus Research Articles

Measurement of plasma channels in the Venus wake

Plasma channels have been suggested to account for the observation of ionospheric holes in the Venus nightside ionosphere. They are produced by the erosion of the solar wind on the magnetic polar regions of the Venus ionosphere, and are related to a sharp plasma transition that extends along the flanks of the wake and that is located downstream from the bow shock. The sharp plasma (intermediate) transition is a feature that has been detected with instruments onboard all the main spacecraft that have probed the Venus wake. From the early flyby transit of the Mariner 5 spacecraft to the later orbit crossings of the Venera 10, the Pioneer Venus and more recently the Venus Express there has been accumulated evidence of the presence of that transition by the flanks of the Venus wake. We report here measurements made with the ASPERA-4 instrument of the Venus Express that were conducted in orbit crossings by the midnight plane over a 4 year period (between 2006 and 2009), and that show different positions of the intermediate transition in the Venus wake at locations closer to the sun-Venus axis near solar minimum conditions by 2009. Measurements conducted in this latter time period also indicate that the intermediate transition by the midnight plane derives from the plasma channels and that is replaced by a broad velocity layer in orbits that probed farther away from that plane. It is argued that changes in the position of the intermediate transition with respect to the plasma channels are related to processes produced by the transport of solar wind momentum to the Venus polar ionosphere. The energy spectra of planetary ions measured in VEX orbits that probed by the midnight plane show momentum transport from the solar wind protons. This result is contrary to the conclusions reported by Collinson et al. (2014) in the sense that there is ¨no difference in the peak energy of the ionospheric outflow¨ when measured inside and outside an ionospheric hole.

The ionospheric source of the red and green lines of atomic oxygen in the Venus nightglow

We have modeled the nightside ionosphere of Venus for 11 solar zenith angles between 90° and 165° with a view toward determining the intensities of the atomic oxygen red, green and 2972Å features in the nightglow. The ionospheric source of these emissions is mainly dissociative recombination of O2+, which may proceed via four channels, which produce two O atoms in various combinations of the ground O(3P) state and the electronically excited O(1D) and O(1S) states. These excited states are the precursors to the nightglow features. We have modeled the O2+(v) vibrational distribution for each of the models, and we have used it to determine branching ratios for the DR channels. We have assumed that the ionosphere is produced by transport of atomic ions from the dayside, and that the ratios of the atomic ions are the same as the model values of the upward ion fluxes on the dayside. For each model, we have normalized the downward atomic ion fluxes to the peak O+ densities that were measured by the Pioneer Venus Orbiter Ion Mass Spectrometer. For the non-sunlit models, the O2+ densities peak in the altitude range 152–159km. We have computed the density profiles of O(1D) and O(1S), and the integrated overhead intensities of the transitions for each of the models. We find that the average intensities, weighted by the area of the solar zenith angle bins, for the red, green and 2972Å emissions are 26R, 4.6R, and 0.46R, respectively. Thus the average high solar activity nightside ionosphere cannot produce green line intensities that are as large as the maximum intensity of about 150R as measured from the ground. Our predicted red line intensities are, however, larger than the upper limits determined by ground based and spacecraft measurements. Possibilities for increasing the green line intensities and decreasing the red line intensities are discussed.

Venus nightside ionospheric holes

Pioneer Venus Orbiter (PVO) measured the ionosphere and atmosphere of Venus for 13 years 10 months yielding a rich data set of archived data from a complement of instruments. One particularly striking feature seen was the occurrence of deep localized nightside ionization depletions commonly called holes. A number of theories have been put forth to explain their observed characteristics, but there is still no consensus on their source. A possibly related phenomenon in the Venus nightside ionosphere is the occurrence of “disappearing” or severely disturbed ionospheres characterized by deep, widespread plasma depletions in almost the entire nightside. This paper reexamines the holes and “disappearing ionospheres” and other characteristics of the nightside ionosphere during solar maximum using a more extensive database than earlier studies. The hole locations, occurrences, and dependencies on solar wind dynamic pressure (Psw) are analyzed, and a comparison is made with earlier studies. It is shown that there is no Psw threshold for holes to occur and at Psw values greater than ∼9 nPa, hole occurrence decreases while the occurrence of severely disturbed orbits increases, suggesting that holes may evolve into severely disturbed orbits. Other characteristics of the nightside are shown to be influenced by solar wind pressure to varying degrees; for example, the density integrated along the orbit path below the ionopause, and the median density at low altitudes exhibit strong inverse correlation with Psw, while the peak density is nearly independent of Psw.

Morphology of the dayside ionosphere of Venus: Implications for ion outflows

The nightside ionosphere of Venus is formed mostly by day‐to‐night transport of ions below the ionopause, with a small contribution from precipitation of energetic electrons from the wake. This nightward flux of ions should result in dayside ionospheres that are characterized by smaller electron density scale heights at high altitudes than those that are characteristic of diffusive equilibrium. In order to determine both the maximum possible values of the upward fluxes and the upward fluxes implied by comparison of our computed ion and electron density profiles to measurements, we have constructed more than 60 models of the high solar activity Venus ionosphere for upward velocity boundary conditions ranging from 0 to 3 × 105 cm s−1. We show that as the upward velocity at the top of the model is increased, the high‐altitude densities of the atomic ions decrease, as do the total ion and electron densities. The maximum upward O+ flux that we derive is ∼5.3 × 108 cm−2 s−1. By comparing our model electron or total ion density profiles to those from Pioneer Venus data, we have derived upward fluxes of O+ that range from 1.8 × 108 to 2.5 × 108 cm−2 s−1 with an average of 2.1 × 108 cm−2 s−1. This value is roughly equal to estimates of the transterminator ion flux and to estimates of the downward fluxes of ions over the nightside. From this, we infer that the escape flux due to ion outflow is small. We conclude that the upward fluxes of ions that we have inferred from the morphology of the ionosphere contribute mostly to the day‐to‐night flow and to the formation of the nightside ionosphere and are much less than their source‐limited values.

Asymmetry of the Venus nightside ionosphere: Magnus force effects

A study of the dawn–dusk asymmetry of the Venus nightside ionosphere is conducted by examining the configuration of the ionospheric trans-terminator flow around Venus and also the dawn-ward displacement of the region where most of the ionospheric holes and the electron density plateau profiles are observed (dawn meaning the west in the retrograde rotation of Venus and that corresponds to the trailing side in its orbital motion). The study describes the position of the holes and the density plateau profiles which occur at neighboring locations in a region that is scanned as the trajectory of the Pioneer Venus Orbiter (PVO) sweeps through the nightside hemisphere with increasing orbit number. The holes are interpreted as crossings through plasma channels that extend downstream from the magnetic polar regions of the Venus ionosphere and the plateau profiles represent cases in which the electron density maintains nearly constant values in the upper ionosphere along the PVO trajectory. From a collection of PVO passes in which these profiles were observed it is found that they appear at neighboring positions of the ionospheric holes in a local solar time (LST) map including cases where only a density plateau profile or an ionospheric hole was detected. It is argued that the ionospheric holes and the density plateau profiles have a common origin at the magnetic polar regions where plasma channels are formed and that the density plateau profiles represent crossings through a friction layer that is adjacent to the plasma channels. It is further suggested that the dawn–dusk asymmetry in the position of both features in the nightside ionosphere results from a fluid dynamic force (Magnus force) that is produced by the combined effects of the trans-terminator flow and the rotational motion of the ionosphere that have been inferred from the PVO measurements.

Viscous Magnus Force for the Rotating Venus Ionosphere

The Magnus force applicable to the Venus ionosphere is calculated for the high-altitude transterminator flow that streams around the low-altitude ionospheric plasma that is subject to rotating motion. The Magnus force can account for the observed dawn-dusk (east-west) asymmetry of the Venus nightside ionosphere and is suitable to describe conditions in astrophysical problems in which the plasma of a rotating body interacts with a directional plasma flow. The Magnus force equation is derived including the viscous force term in the momentum equation of the transterminator flow in the Venus upper ionosphere and is examined by comparing the expected dawnward-directed displacement of that flow with the Pioneer Venus Orbiter measurements. Values of the coefficient of the Magnus force equation suitable for the dawnward deflection of the transterminator flow are comparable to those of a rotating sphere subject to a streaming flow in fluid dynamics problems and imply Re ~ 200-300 values of the Reynolds number for that flow using the ~600 km width of the region where it is measured. Such values of the Reynolds number are 1 order of magnitude larger than those inferred from the viscous transport of solar wind momentum to the Venus upper ionosphere that is implied by the velocity boundary layer observed along the flanks of the Venus ionosheath. With values of the coefficient of the Magnus force equation for the Venus ionosphere that are similar to those applicable to ordinary fluid dynamic problems it is possible to suggest a common quality in the response of rotating obstacles subject to a streaming flow in fluid dynamics and in astrophysical problems.

Loss of hydrogen and oxygen from the upper atmosphere of Venus

Atmospheric escape from the upper atmosphere of Venus is mainly influenced by the loss of hydrogen and oxygen caused by the interaction of solar radiation and particle flux with the unprotected planetary environment. Because one main aim of the ASPERA-4 particle/plasma and VEX-MAG magnetic field experiments on board of ESA's forthcoming Venus Express mission is the investigation of atmospheric erosion processes from the planet's ionosphere–exosphere environment, we study the total loss of hydrogen and oxygen and identified the efficiency of several escape mechanisms involved. For the estimation of pick up loss rates we use a gas dynamic test particle model and obtained average loss rates for H + , and O + pick up ions of about 1 × 10 25 s - 1 and about 1.6 × 10 25 s - 1 , respectively. Further, we estimate ion loss rates due to detached plasma clouds, which were observed by the pioneer Venus orbiter and may be triggered by the Kelvin–Helmholtz instability of about 0.5 – 1 × 10 25 s - 1 . Thermal atmospheric escape processes and atmospheric loss by photo-chemically produced oxygen atoms yield negligible loss rates. Sputtering by incident pick up O + ions give O atom loss rates in the order of about 6 × 10 24 s - 1 . On the other hand, photo-chemically produced hot hydrogen atoms are a very efficient loss mechanism for hydrogen on Venus with a global average total loss rate of about 3.8 × 10 25 s - 1 , which is in agreement with Donahue and Hartle [1992. Solar cycle variations in H + and D + densities in the Venus ionosphere: implications for escape. Geophys. Res. Lett. 12, 2449–2452] and of the same order but less than the estimated H + ion outflow on the Venus nightside of about 7.0 × 10 25 s - 1 due to acceleration by an outward electric polarization force related to ionospheric holes by Hartle and Grebowsky [1993. Light ion flow in the nightside ionosphere of Venus. J. Geophys. Res. 98, 7437–7445]. Our study indicates that on Venus, due to its larger mass and size compared to Mars, the most relevant atmospheric escape processes of oxygen involve ions and are caused by the interaction with the solar wind. The obtained results indicate that the ratio between H/O escape to space from the Venusian upper atmosphere is about 4, and is in a much better agreement with the stoichiometrically H/O escape ratio of 2:1, which is not the case on Mars. However, a detailed analysis of the outflow of ions from the Venus upper atmosphere by the ASPERA-4 and VEX-MAG instruments aboard Venus Express will lead to more accurate atmospheric loss estimations and a better understanding of the planet's water inventory.

Effect of the motional electric field on the Venus nightside ionopause

The purpose of this paper is to clarify how the average structure of the Venus nightside ionopause for solar zenith angles (SZA) greater than 90° depends on (1) the direction and (2) the magnitude of the motional electric field of the solar wind. Plasma density structure in the Venus nightside ionosphere has been investigated by using data sets of the Pioneer Venus Orbiter observations. It is found that the distribution of the nightside ionopause locations is asymmetric with respect to the direction of the solar wind electric field, leaning opposite to the electric field vector. It is also found that the asymmetry is increasingly prominent as the magnitude of the motional electric field increases, while not so prominent for small field magnitude. This result suggests that the asymmetric ionopause location in the nightside is related to the acceleration of pickup ions.

Magnetic field configuration in the Venus ionosphere

We investigated the magnetic field configuration in the Venus ionosphere, from the data (orbit 1–600) obtained by Pioneer Venus Orbiter (PVO). The magnetic field lines were draping the Venus ionosphere, but the magnetic field lines were opened to the interplanetary space in the nightside ionosphere. We found the closed magnetic field line around Venus near the magnetic equator at 600–800 km altitudes during the high solar wind dynamic pressure. The result shows that magnetic reconnection occurs in the Venus nightside ionosphere.

Friction layer along the sides of the plasma channels in the Venus nightside ionosphere

Electron density profiles of the Venus nightside ionosphere measured with the Pioneer Venus Orbiter (PVO) reveal cases in which there is a plateau signature that extends across the upper ionosphere. Such cases are not seen throughout the nightside hemisphere but occur mostly near the midnight plane. The distribution in the local solar time (LST) position of passes in which this peculiarity is seen was derived from a study of 40 PV orbits that scanned the nightside ionosphere in the first and third season of that spacecraft operation. The dominant contribution of passes with a density plateau is slightly shifted in LST from the midnight plane towards the dawn terminator and thus suggest a displacement in the same direction as that of the distribution of ionospheric holes reported from the early PVO observations. The peak of the distribution occurs in the dawn side vicinity of the midnight plane indicating a displacement that is slightly smaller than that of the ionospheric holes. It is argued that density profiles with a plateau signature represent passes through a friction layer that is formed along the sides of the plasma channels. This result is viewed as implying that the ionospheric material located at the sides of the plasma channels, which provide a useful interpretation of the ionospheric holes, is strongly eroded by the solar wind that flows within the channels. Momentum transport from the solar wind to the ionospheric plasma is assumed to be responsible for eroding the sides of the plasma channels and thus producing density profiles with a plateau signature.

Plasma channels and electron density profiles near the midnight plane in the Venus nightside ionosphere

Pioneer Venus Orbiter (PVO) data with information on the electron densities in the Venus nightside ionosphere are examined in a study of the plasma channels that extend downstream from the magnetic polar regions. The plasma channels are produced by the solar wind that erodes the polar upper ionosphere and provide a useful interpretation of the ionospheric holes as regions of depleted plasma density that are observed in the nightside hemisphere. The plasma channels can also account for the observed distribution of ionospheric holes in the nightside hemisphere as a function of the solar wind dynamic pressure. Wide plasma channels that result from the enhanced erosion of the polar upper ionosphere under high dynamic pressures lead to the ionospheric holes that are observed far from the midnight plane. Data obtained from the PVO show that in addition to the ionospheric holes there are orbits traced near the midnight plane with measurements that are also related to the plasma channels. In some cases the electron density does not change significantly through the upper ionosphere but exhibits a density plateau that extends from a high‐altitude nightside ionopause crossing to a low‐altitude location where a sharp change in the density is observed near periapsis. In addition, there are PVO passes in which the nightside ionopause detected near the midnight plane is located at very low altitudes, implying that the PVO may have remained within the plasma channels as it approached or moved away from periapsis. Transit through the bottom of the plasma channels can account for the low‐altitude ionopause crossings seen in these orbits and also for the sharp change at the low‐altitude edge of the density plateau measured in orbits that probed near the midnight plane. Unlike PVO passes in which ionospheric holes are detected near the midnight plane, the magnetic field measured in the solar wind during these latter orbits is not oriented near the ecliptic plane, and thus the magnetic polar regions in the Venus ionosphere and the plasma channels that extend behind them are not encountered along the near polar PVO trajectory. In such cases the spacecraft may not move across the plasma channels but remain along their sides as it travels through the nightside ionosphere. The orientation of the magnetic field with respect to the ecliptic plane can thus account for the observation of ionospheric holes or a density plateau in profiles traced near the midnight plane.

The central nightside venus ionosphere: dependence of ion concentration and plasma holes location on solar wind dynamic pressure

The aeronomy experiments on the Pioneer Venus Orbiter (PVO) have established three characteristic features of the nightside ionosphere of Venus: (1) disappearing ionospheres, (2) large spatial/temporal variability of ion densities and (3) plasma holes/troughs. As the nightside ionosphere is essentially maintained by the transterminator flow of O + ions from the dayside, this flow should then be the major parameter controlling these features. Disappearing ionospheres, for example, have indeed been found to occur during episodes of high solar wind dynamic pressure (Psw), when the height of the terminator ionopause is greatly reduced and thus the transterminator flow severely diminished. In this paper, we study the other two features, viz the temporal/spatial ion density variability and the location of plasma holes/troughs by analyzing O + density profiles measured by the ion mass spectrometer experiment on the PVO. We find that most of the variability in the central nightside ionosphere is related to Psw, which is seen to control the O + peak density. Further, we observe that the altitude of the plasma holes/troughs is also dependant on Psw.

Solar wind erosion of the Venus polar ionosphere

The geometry of the Venus nightside ionosphere is reexamined on the basis of plasma and magnetic field data obtained with the Pioneer Venus Orbiter (PVO) and the Mariner 5 spacecraft. There is evidence indicating that the shocked solar wind that streams over the magnetic polar regions of the Venus ionosphere experiences a sharp expansion and also a strong deceleration to speeds much smaller than those present in the outer ionosheath. It is argued that the deficiency of solar wind momentum that is inferred from the PVO and Mariner 5 measurements along the flanks of the Venus ionosheath may be responsible for the erosion of the polar upper ionosphere in plasma channels that extend downstream from the magnetic polar regions. The erosion of the Venus ionosphere is mostly produced within those channels and represents the main source of material delivered to the Venus wake. The configuration that is proposed accounts for the PVO observation of plasma clouds and ionospheric holes in the nightside hemisphere and leads to a mass loss smaller than that expected from a global erosion. The formation and evolution of the ionospheric polar channels should also be important in the solar wind interaction and erosion of the Mars ionosphere.

Whistler-mode propagation in the collisional ionosphere of Venus

Waves identified as whistler-mode waves have been observed in the nightside ionosphere of Venus by the Pioneer Venus Orbiter. These waves are propagating in a collisional, weakly magnetized ionosphere, and it has been argued that as such they should be damped through collisions, and the wave should not retain the characteristics of the whistler-mode because of non-linear modifications to the dispersion relation. We show that non-linearities do not modify the wave dispersion since the non-linear force causes a longitudinal current, and the associated charge separation electric field acts to balance the non-linear force. The quasi-longitudinal approximation is therefore appropriate for whistler-mode waves in the nightside ionosphere of Venus. Further, we find that while the waves will heat the bottomside ionosphere, little effect is found at altitudes where in situ observations were made by the Pioneer Venus Orbiter.

Suprathermal electron fluxes in the Venus nightside ionosphere at moderate and high solar activity

Suprathermal electron fluxes up to energies of 50 eV measured at moderate solar activity (MSA) during the Pioneer Venus Orbiter entry mission are presented and compared with measurements made during high solar activity (HSA). The median height profiles measured at these two seasons do not significantly differ from each other in the Venus nightside ionosphere. At HSA the fluxes decrease significantly with increasing solar zenith angel (SZA). At MSA, only a small decrease in flux is suggested from individual orbits and a statistical study. Flux changes between different orbits and locations during MSA are usually less than an order of magnitude, a relatively small value compared with the spread of the thermal plasma density of more than 2 orders of magnitude at altitudes above 200‐km altitude. Independent of altitude and SZA, the shape of the spectrum of the suprathermal electrons is similar to the spectrum of the dayside photoelectrons with a characteristic energy of about 7 eV in many cases, but the flux is an order of magnitude smaller. In other cases the suprathermal electrons are approximately twice as energetic as photoelectrons of the dayside, comparable with the characteristic energy of solar wind electrons and assumed to originate there. The results indicate that the source of high thermal plasma densities, which occur especially at HSA, is mainly plasma transport from the dayside ionosphere. The median total plasma density above 200‐km altitude in the central nightside sector at MSA is depleted by a factor of 5 compared with that at HSA. Between HSA and MSA, plasma transport is typically reduced by a factor of 6. At MSA, transport and particle precipitation contribute about equally to the nightside ionization.

Electron density fluctuations in the nightside Venus ionosphere: Role of gravity waves

A one‐dimensional, two‐ion (O+, O2+) model is described for studying fluctuations in electron density, electron temperature, and magnetic field produced by atmospheric gravity waves in the low‐altitude (<200 km) ionosphere of Venus just past the terminator (solar zenith angle 100°). Plasma velocity and self‐consistent horizontal magnetic field are calculated utilizing the observed data on plasma densities and temperatures. Effect of the Hall and Pedersen conductivity on the gravity wave dispersion relation is investigated. A gravity wave of time period 33 min and horizontal wavelength 700 km is considered for the case study. Amplitude and phase of electron density, electron temperature, and magnetic field fluctuations relative to the fluctuations in neutral atomic oxygen density are calculated. Results are discussed in the light of observed wave structure and correlations in these parameters. It is found that the fluctuations observed in the ionospheric and magnetic parameters in the nightside ionosphere of Venus can be produced by internal gravity waves.

Time delays in the solar wind flow past Venus: Galileo‐Pioneer Venus correlations

The Galileo and Pioneer Venus Orbiter (PVO) spacecraft obtained simultaneous measurements at Venus during the Galileo flyby on February 9 and 10, 1990. The magnetic field data sets from the two spacecraft show corresponding convected features, even when Pioneer Venus is near periapsis on the nightside of the planet. Comparison of the observation times of these corresponding features reveals substantial delays at PVO associated with the deceleration of the flow carrying the draped magnetic field past Venus. Convection lags of up to 10 min are identified near PVO periapsis at an altitude of 1/4 Venus radii (1500 km) above the planet surface. A three‐dimensional convected field gas dynamic model is used to calculate expected lag times by tracing field lines from PVO to the open solar wind. Expected lags from the model are relatively insensitive to changes in field orientation and Mach number. This simple gas dynamic model with the standard ionopause obstacle size gives convection lags too small to explain the Galileo‐PVO observations. A likely explanation is that mass loading of the solar wind by the Venus atmosphere contributes to the deceleration of the flow near the planet. The observation of similar magnetic structures in the solar wind and deep in the Venus nightside ionosphere suggests that the impenetrable obstacle model is inappropriate in this region and the solar wind magnetic field is convected into the nightside ionosphere on timescales of 10 min or more.

Morphology of O++ ions and their maintenance in the nightside Venus ionosphere

In this paper, measurements made by the Pioneer Venus orbiter ion mass spectrometer are used to study the temporal and spatial distribution of O++ ion concentrations in the Venus ionosphere. The diurnal analysis shows an enhancement of O++ around midnight. On the nightside the O++ densities are highly variable but show a positive correlation with variations in the O+ densities. Model calculations of O++ concentrations in the nightside are made assuming that impact ionization by precipitating electrons and transport of ions from the dayside were the sources for O++ ions. We find that in situ ionization by precipitating electrons contributes very little, whereas a downward O++ flux in the range of 105 to 106 cm−2 s−1 can account for the measured values of O++ densities near midnight. The high abundance of O++ near midnight is related to the special characteristics of multiply charged ions. Because of the double charge, thermal diffusion and the polarization electric field are particularly found to be signifiicant factors in the momentum equations for O++, and therefore the downward velocity for O++ is small compared with that of other ions. High O++ densities are observed to occur under low solar wind dynamic pressure conditions, consistent with the transterminator transport mechanism as the source of O++ ions in the antisolar region.

A theoretical study concerning the solar cycle dependence of the nightside ionosphere of Venus

We modeled the chemical and physical processes taking place in the nightside ionosphere of Venus by solving the one dimensional coupled continuity and momentum equations for 12 ion species [CO2+, O2+, O+, H+, NO+, CO+, N2+, N+, He+, C+, O+ (²D) and O+ (²P)]. We investigated the relative importance of the two major processes responsible for maintaining the nightside plasma densities: atomic ion transport from the dayside and impact ionization due to energetic electron precipitation. We compared our model calculations with electron density and ion composition observations obtained by instruments aboard the Pioneer Venus Orbiter during both high and moderate solar cycle conditions. These studies lead us to conclude that day‐to‐night atomic ion transport is dominant during high solar activity, while during moderate solar activity conditions the combined effects of the electron precipitation and reduced day‐to‐night ion transport are responsible for maintaining the nightside ionosphere of Venus.

Ion density, temperature, and composition of the Venus nightside ionosphere during a period of moderate solar activity: Implications for maintaining the central nightside

Ion density, temperature, and the partial densities of the main constituents of the Venusian nightside ionosphere during moderate solar activity are presented. The data were measured by the retarding potential analyzer of the Pioneer Venus Orbiter during the entry phase of the mission in 1992. The highly variable plasma density is, on average, clearly reduced relative to that measured at high solar activity. A significant dawn‐dusk asymmetry, an excess of H+ at dawn and O+ at dusk, occurs in the postterminator sector. In the central nightside sector, beyond 150° solar zenith angle, the ionosphere is strongly depleted. The ion temperature distribution is similar to that measured during high solar activity, but the values are slightly smaller. The measurements indicate that the ion flux across the terminator, which is the dominant maintenance source for the nightside ionosphere at high solar activity, decreases with decreasing solar activity. At moderate solar activity we find that plasma transport and particle precipitation contribute approximately equally to the ionization of the central sector.