Ionopause Altitude Research Articles

The Dayside Ionopause of Mars: Solar Wind Interaction, Pressure Balance, and Comparisons With Venus

AbstractDue to the lower ionospheric thermal pressure and existence of the crustal magnetism at Mars, the Martian ionopause is expected to behave differently from the ionopause at Venus. We study the solar wind interaction and pressure balance at the ionopause of Mars using both in situ and remote sounding measurements from the Mars Advanced Radar for Subsurface and Ionosphere Sounding instrument on the Mars Express orbiter. We show that the magnetic pressure usually dominates the thermal pressure to hold off the solar wind at the ionopause at Mars, with only 13% of the cases where the ionospheric thermal pressure plays a more important role in pressure balance. This percentage at Venus, however, is up to 65%. We also find that the ionopause altitude at Mars decreases as the normal component of the solar wind dynamic pressure increases, similar to the altitude variation of the ionopauses at Venus. Moreover, our results show that the ionopause thickness at Mars and Venus is mainly determined by the ion gyromotion and is equivalent to about five ion gyroradii.

EUV-dependence of Venusian dayside ionopause altitude: VEX and PVO observations

The Venusian dayside ionosphere, similar to other planetary ionospheres, is produced primarily by ionization of its neutral upper atmosphere due to solar extreme ultraviolet (EUV) radiation. It has become clear that the expansion of the ionosphere may be strongly controlled by the EUV level, as exhibited in data collected by the Pioneer Venus Orbiter (PVO) during one solar cycle (1978–1992). However, the EUV-dependence of the Venusian dayside ionopause altitude, which defines the outer boundary of the ionosphere, remains obscure because the PVO crossed the dayside ionopause only during the solar maximum; its periapsis lifted too high during the solar minimum. Recently, during the period 2006–2014, which included the longest and quietest solar minimum of the past several decades, Venus Express (VEX) provided measurements of the photoelectron boundary (PEB) over the northern high-latitude region. Since the photoelectron boundary is closely related to the ionopause, we have an opportunity to analyze the EUV effect on the dayside ionopause by combining PVO and VEX observations. We have evaluated and then reduced the orbit bias effect in data from both PVO and VEX, and then used the results to derive a relationship between solar EUV level and the dayside ionopause altitude. We find that the dayside ionopause altitude increases as the solar EUV level increases, which is consistent with theoretical expectations.

The Effects of Crustal Magnetic Fields and Solar EUV Flux on Ionopause Formation at Mars

AbstractWe study the ionopause of Mars using a database of 6,893 ionopause detections obtained over 11 years by the MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) experiment. The ionopause, in this work, is defined as a steep density gradient that appears in MARSIS remote sounding ionograms as a horizontal line at frequencies below 0.4 MHz. We find that the ionopause is located on average at an altitude of 363±65 km. We also find that the ionopause altitude has a weak dependence on solar zenith angle and varies with the solar extreme ultraviolet (EUV) flux on annual and solar cycle time scales. Furthermore, our results show that very few ionopauses are observed when the crustal field strength at 400 km is greater than 40 nT. The strong crustal fields act as minimagnetospheres that alter the solar wind interaction and prevent the ionopause from forming.

Giant vortices lead to ion escape from Venus and re‐distribution of plasma in the ionosphere

The interaction of the solar wind with Venus has a significant influence on the evolution of its atmosphere. Due to the lack of an intrinsic planetary magnetic field, there is direct contact between the fast flowing solar wind and the Venusian ionosphere. This leads to a number of different types of atmospheric escape process. Using Venus Express observations, we show that such contact leads to the formation of global vortices downstream of the Venusian bow shock. These vortices accelerate heavy ionospheric ions such as oxygen, leading to their escape. We argue that these vortices are the result of the Kelvin‐Helmholtz instability excited by the shear velocity profile at the boundary between the solar wind and ionospheric plasma. These vortices also help to repopulate the night‐side ionosphere during solar minimum, when the ionospheric flow from day to night is restricted by the lowered ionopause altitude at the terminator.

Time variation of nonthermal escape of oxygen from Mars after solar wind dynamic pressure enhancement

We investigate the nonthermal escape of oxygen from Mars due to the dissociative recombination of O2+. By combining a time‐dependent two‐stream model for the nonthermal oxygen with an MHD model of the ionosphere‐magnetosheath interaction, the response of the oxygen escape rate to time‐varying solar wind conditions is examined. Over the last few decades a number of model calculations of expected nonthermal oxygen escape have been performed, but all assumed steady state conditions. In this study we calculate the time variation of the nonthermal escape rate after enhancement of the solar wind dynamic pressure. We find that the escape rate of nonthermal oxygen increases by a factor of about two to four as the ionosphere is compressed in response to solar wind dynamic pressure enhancement. We also find that in steady states the escape rate for high ionopause altitude cases (h ∼ 600 km) is twice as large as that for low ionopause altitude cases (h ∼ 250 km) due to non‐negligible dissociative recombination above the exobase.

Magnus force in the Venus ionosphere

The retrograde superrotation motion of the Venus ionosphere is included in a study of the configuration of the ionospheric transterminator flow by using a fluid‐dynamic interpretation of the interaction that takes place between both flow motions. It is argued that the interaction leads to a fluid dynamic force (Magnus force) that produces an overall (∼10°) dawnward directed deflection of the transterminator flow which is comparable to that implied from the PVO observations. The Magnus force results from a pressure difference that is set across the planet by the difference in the speed of the plasma when both the rotation motion and the transterminator flow are in the same sense (dusk side terminator) to that where both motions are opposite to each other (dawn side terminator). The dawnward deflection of the ionospheric plasma in the transterminator flow also deviates the plasma channels (which serve to account for the ionospheric holes) that extend downstream from the magnetic polar regions and is responsible as well for that of the peak of the histogram of the electron density profiles with an upper ionospheric plateau that are seen near the midnight plane. The effects of the Magnus force are also related to the strong difference in the ionopause altitude observed between the dusk and the dawn side terminator (with higher altitudes seen in the dawn side) and to the larger ionospheric transterminator flow speeds measured at high altitudes by the dawn side terminator.

Planetary ENA Imaging: Venus and a comparison with Mars

We present simulated images of energetic neutral atoms (ENAs) produced in charge exchange collisions between solar wind protons and neutral atoms in the exosphere of Venus, and make a comparison with earlier results for Mars. The images are found to be dominated by two local maxima. One produced by charge exchange collisions in the solar wind, upstream of the bow shock, and the other close to the dayside ionopause. The simulated ENA fluxes at Venus are lower than those obtained in similar simulations of ENA images at Mars at solar minimum conditions, and close to the fluxes at Mars at solar maximum. Our numerical study shows that the ENA flux decreases with an increasing ionopause altitude. The influence of the Venus nighttime hydrogen bulge on the ENA emission is small.

Probing Mars' crustal magnetic field and ionosphere with the MGS Electron Reflectometer

The Electron Reflectometer (ER) on board Mars Global Surveyor measures the energy and angular distributions of solar wind electrons and ionospheric photoelectrons. These data can be used in conjunction with magnetometer data to probe Mars' crustal magnetic field and to study Mars' ionosphere and solar wind interaction. During aerobraking, ionospheric measurements were obtained in the northern hemisphere at high solar zenith angles (SZAs, typically ∼78°). The ionopause was crossed at altitudes ranging from 180 km to over 800 km, with a median of 380 km. The 400‐km‐altitude polar mapping orbit allows observations at SZAs from 25° to 155° in both the northern and southern hemispheres. The near‐planet ionosphere and magnetotail structure of the night hemisphere is dominated by the presence of intense crustal magnetic fields, which can exceed 200 nT at the spacecraft altitude. Closed field lines anchored to highly elongated crustal sources form “magnetic cylinders,” which exclude solar wind plasma traveling up the magnetotail. When the spacecraft passes through one of these structures, the ER count rate falls to the instrumental background, representing an electron flux drop of at least two orders of magnitude. A map of these flux dropouts in longitude and latitude closely resembles a map of the crustal magnetic sources. When the crustal magnetic cylinders rotate into sunlight, they fill with ionospheric plasma. Since many of these crustal fields are locally strong enough to stand off the solar wind to altitudes well above 400 km, the ionosphere can extend much higher than would otherwise be possible in the absence of crustal fields. Even weak crustal fields may locally bias the median ionopause altitude, which provides an indirect method of detecting crustal fields using ER observations.

Response of Venus ions to solar wind dynamic pressure

Orbiter ion mass spectrometer measurements, as available in the UADS data files are used to study the response of dayside Venus ions at various altitudes to solar wind dynamic pressure, Psw. Ion densities below about 200 km are not affected by changes in Psw. At altitudes above 200 km the ions get abruptly depleted with increase in Psw, and this abrupt depletion occurs at lower altitudes when Psw is high. At lower Psw, the depletion occurs at higher altitudes. The effect is similar for all ions. These results are also compared with the empirical relationship observed by Brace et al. (1980) between the ionopause altitude and Psw from electron density measurements on orbiter electron temperature probe.

Evidence of upward H+ flow in the Venus dayside ionosphere

At Venus H+ is expected to be under diffusive equilibrium above about 400 km, but it is difficult to verify this under normal circumstances, because solar wind interaction terminates the subsolar ions around 400 km. However, under conditions of extremely low solar wind pressure, the terminal height, (called the ionopause) can move up to very high altitudes. Such events are rare, but after searching through the Pioneer Venus data, we have been able to identify some orbits with ionopause altitudes above 1000 km. Altitude distributions of O+ and H+, measured by the ion mass spectrometer experiment, have been studied for these orbits. It is found that while O+ distribution is consistent with diffusive equilibrium, H+ shows large departures providing evidence of upward flow of H+. Approximate calculations indicate that the flow is subsonic.

The ancient oxygen exosphere of Mars: Implications for atmosphere evolution

The evolution of the Martian atmosphere, particularly with regard to water, is influenced by (1) “nonthermal” escape of oxygen atoms created by dissociative recombination and (2) by oxygen ion pickup by the solar wind. Both processes depend on the intensity of solar EUV radiation, which affects atmosphere temperatures (scale heights) and photoionization rates, and thereby the exosphere and the fluxes of escaping atoms and ions. This study involves the calculation, by the two‐stream model method of Nagy and Cravens (1988), of the exospheric hot oxygen densities for “ancient” atmospheres and ionospheres (e.g., for different EUV fluxes), and the associated neutral escape fluxes. The ion production rates above nominal ionopause altitudes are also computed and are considered to be the upper limits to losses by direct solar wind pickup. Since we do not consider the pickup ion precipitation and additional neutral escape due to the sputtering process described by Luhmann and Kozyra (1991), the results presented here represent conservative estimates of the neutral escape fluxes, but somewhat generous estimates of ion loss rates. We find that when the inferred increased solar EUV fluxes of the past are taken into account, oxygen equivalent to that in several tens of meters of water, planet‐wide, should have escaped to space over the last 3 Gyr.

Comment on “Ionospheric evidence of hot oxygen in the upper atmosphere of Venus”

The conclusion of Mahajan et al. (1992) that 'the existence of O(+) as dominant at (Venusian) ionopause altitudes in excess of 500-1000 km can only be explained if atomic oxygen is the major neutral constituent' is argued to be incorrect. It is suggested that at a transition region of about 200 km, thermal atomic oxygen is the dominant neutral gas, and hot oxygen is a minor species; thus the O(+) to H(+) ratio at high altitudes is not an indicator of the presence of hot oxygen at these altitudes. A 1D model for H(+) and O(+) appropriate for the dayside ionosphere of Venus shows that within hot atomic oxygen density values from 1000 to 10 exp 6/ cu cm at 150 km, the calculated H(+) and O(+) densities did not change in any meaningful way, because the hot oxygen population remained a minor neutral constituent below 200 km, which is the approximate height of the transition between chemical and diffusive equilibrium conditions for the ions.

The Venus transterminator ion flux at solar maximum

The median Venus transterminator ion flux nv(h) derived from Pioneer Venus retarding potential analyzer measurements of the ion velocity and ion density during a period of solar maximum is presented as a function of altitude h from 150 km to the solar maximum median ionopause altitude of 800 km. This flux is integrated with altitude to provide the integrated total ion flux S(h) crossing the terminator plane as a function of altitude. S(h) is nearly a linear function of h, reflecting the nearly constant value of nv with altitude. The value of S at 800 km altitude, called F, is 5×1026 s−1, a value equal to the total nightside hemispheric molecular ion recombination rate L calculated previously. The loss of ions down the wake of Venus is smaller than F or L by at least a factor of 5. Thus the equality of F and L is evidence that the dominant Venus nightside ionization source at solar maximum is transterminator ion transport.

A reconsideration of the effects of terminator ionopause height on the nightward ion transport at Venus

The ionopause altitude near the terminator is a crucial parameter for studies dealing with the maintenance of the nightside ionosphere of Venus. It is generally thought that during high solar wind dynamic pressures (Psw) or during solar minimum conditions the ionopause comes down to very low altitudes so that the dayside ionosphere is not able to supply sufficient plasma to maintain the observed nightside densities. However, there are a number of workable definitions of the ionopause. Near the terminator, the altitude of the ionopause differs considerably depending upon the definition. The ionopause deduced from the radio occultation experiment (used to study the nightward transport for solar minimum conditions) as well as the pressure ionopause can be significantly lower than the density ionopause deduced from the Langmuir probe at these locations. The latter refers to the altitude where the electron density falls to 100 cm−3. Using in situ data from the Pioneer Venus Orbiter, it is shown that the density ionopause remains fairly high even for high Psw conditions. Simple quantitative estimates indicate that significant flow of plasma is still possible under these conditions. Thus nightward transport of plasma during high Psw conditions may be more efficient than has been assumed so far. Since such conditions are more prevalent during solar cycle minimum, it is argued that transport may be relevant in the maintenance of nightside ionosphere at that time also.

On the lower altitude limit of the Venusian ionopause

It has been observed from the plasma experiments on the Pioneer Venus Orbiter that the altitude of the upper boundary of the ionosphere (viz ionopause) decreases in response to increasing solar wind dynamic pressure. However, at pressures above about 2.5 × 10−8 dynes cm−2, the further decrease in the ionopause height is rather small. Following the model of Cloutier et al., [1969] we suggest that during high solar wind conditions, when the ionopause is formed at lower altitudes, the solar wind induces vertical and horizontal flows which sweep away the ionospheric plasma that is produced locally by photoionization. As a result a disturbed photo‐dynamical ionosphere is formed which has the scale height of the ionizable neutral constituent. We show that such a photo‐dynamical ionosphere is observed at the subsolar ionopause under these conditions. As a consequence of this interaction, the ionopause altitude is observed to follow the small scale height of the ionizable species, atomic oxygen, showing only small changes with solar wind pressure.

Solar cycle changes in the morphology of the Venus ionosphere

I present two median plasma density curves for the central nightside Venus ionosphere, one for solar cycle maximum (SCmax) conditions and one for solar cycle minimum (SCmin) conditions. The curves are constructed from Pioneer Venus retarding potential analyzer total ion density data. Although the two curves do not overlap in altitude because of orbit evolution, they constitute strong evidence that the nightside ionospheric density during SCmin is typically a factor of 10 smaller between 200 and 2000 km altitude than it is at SCmax. From these data, data presented previously (Knudsen et al., 1987), and Venera radio occultation data, I conclude that the Venus SCmin ionosphere is typically confined to an altitude below approximately 250 km at all solar zenith angles (SZA) with the possible exception of an SZA interval near 90°. The layer is composed primarily of molecular ions, O2+ being the most abundant. At SCmax an extensive O+ layer exists on top of the O2+ layer with the typical ionopause altitude changing from 300 km in the subsolar region to 900 km at the terminator and to over 1800 km altitude in the antisolar region.

The Venus ionosphere

Physical properties of the Venus ionosphere obtained by experiments on the US Pioneer Venus and the Soviet Venera missions are presented in the form of models suitable for inclusion in the Venus International Reference Atmosphere. The models comprise electron density (from 120 km), electron and ion temperatures, and relative ion abundance in the altitude range from 150 km to 1000 km for solar zenith angles from 0° to 180°. In addition, information on ion transport velocities, ionopause altitudes, and magnetic field characteristics of the Venus ionosphere, are presented in tabular or graphical form. Also discussed is the solar control of the physical properties of the Venus ionosphere.

Dependence of Venus ionopause altitude and ionospheric magnetic field on solar wind dynamic pressure

The shape of the dayside Venus ionopause, and its dependence on solar wind parameters, is examined using Pioneer Venus Orbiter field and particle data. The ionopause is defined here as the altitude of pressure equality between magnetosheath magnetic pressure and ionospheric thermal pressure; its typical altitudes range from ∼300 km near the subsolar point to ∼900 km near the terminator. A strong correlation between ionopause altitude and magnetosheath magnetic pressure is demonstrated; correlation between magnetic pressure and the normally incident component of solar wind dynamic pressure is also evident. The data support the hypothesis of control of the ionopause altitude by solar wind dynamic pressure, manifested in the sheath as magnetic pressure. The presence of large scale magnetic fields in the ionosphere is observed primarily when dynamic pressure is high and the ionopause is low.

Improved Venus ionopause altitude calculation and comparison with measurement

We calculate the altitude of the Venus ionopause following the inviscid fluid approach initially introduced by Spreiter et al. (1970) and later modified by Spreiter and Stahara (1980) but incorporate several improved approximations. The calculated altitude is compared with median altitudes measured by the Pioneer Venus retarding potential analyzer. The calculated ionopause shape approximates the measured median shape closely in the solar zenith angle (SZA) range 0°–135° and is a definite improvement over previous calculations. Improved approximations include use of an ionospheric pressure field which varies with SZA in a manner consistent with measurements, use of the modified Newtonian approximation for the pressure exerted on the ionosphere by the ionosheath, use of the Prandtl‐Meyer expansion approximation for a short distance downstream from the terminator, and separation of the solar wind terminator from the solar extreme ultraviolet radiation terminator. The calculation is not expected to be accurate for solar zenith angles in excess of approximately 135°. Use of the improved approximations lowers the terminator ionopause altitude to approximately half that obtained with use of the usual Spreiter approximations. The calculated dawn ionopause is approximately 300 km higher in altitude than the dusk ionopause. Both ionopauses are close to their respective measured median values. The value of the upstream solar wind momentum flux required to fit the measured median ionopause altitudes is within 15% of the measured median solar wind momentum flux. We conclude that the median altitude of the ionopause between 0° and approximately 135° SZA may be adequately calculated without including a viscous interaction in the theory.

The Venus ionopause current sheet: Thickness length scale and controlling factors

Data from the Pioneer Venus orbiter magnetometer, plasma wave detector and Langmuir probe are used to investigate the scale thickness and controlling factors associated with the current sheet at the Venus ionopause, the boundary between the ionosphere and the solar wind. The current sheet is found to have two principal scale sizes: (1) a few ion gyroradii when the external compressed solar wind field is less than 80 nT and the ionopause altitude is above ∼300 km; (2) several tens of ion gyroradii when high external fields occur and ionopause altitudes are below ∼300 km. The current sheet thickness may be significantly affected by collisional diffusion at low altitudes, and wave turbulence may also play a role in controlling thickness. The high‐altitude boundaries are consistent with the expected thickness of the thinnest charge‐neutral, collisionless current sheet.