Ionospheric Pressure Research Articles

Effects of the 2007 Martian Global Dust Storm on Boundary Positions in the Induced Magnetosphere

Mars's magnetosphere is a sensitive system, varying due to external and internal factors, such as solar wind conditions and crustal magnetic fields. A signature of this influence can be seen in the position of two boundaries; the bow shock and the induced magnetospheric boundary (IMB). The bow shock moves closer to Mars during times of high solar activity, and both the bow shock and IMB bulge away from Mars over crustal magnetic fields in the southern hemisphere. This study investigates whether large-scale atmospheric events at Mars have any signature in these two magnetic boundaries, by investigating the 2007 storm. The 2007 global storm lasted for several months and increased atmospheric temperatures and densities of both water vapor and carbon dioxide in the atmosphere, leading to an increase in atmospheric escape. Using Mars Express, we identified boundary locations before, during, and after the event, and compared these to modeled boundary locations and areographical locations on Mars. We find that, while it is unclear whether the bow shock position is impacted by the storm, the IMB location does change significantly, despite the orbital bias introduced by Mars Express. The terminator distance for the IMB peaks at longitudes 0°–40° and 310°–360°, leaving a depression around 180° longitude, where the boundary usually extends to higher altitudes due to the crustal magnetic fields. We suggest this may be due to the confinement of ionospheric plasma over crustal fields preventing mixing with the dust, creating a dip in ionospheric pressure here.

Development, Implementation and Testing of Algorithm to Detect Ionopause-Like Structure Using Parallel Processing in the Martian Atmosphere

Abstract: This study assesses the Martian ionopause using MAVEN datasets between periapsis and 150-600 km. Ionopause is an abrupt reduction of the electron density with increasing altitude. It is also required to verify the simultaneous increase of the electron temperature and variability below 400 km. To address this issue, we have adopted a computational approach in determining the ionopause-like density structure of the ionospheric profile. From computing thermal & magnetic pressures, radial magnetic field components, ionopause-like density gradient are detected and stored. The ionopause (theoretically) is formed where the total ionospheric pressure equals solar wind dynamic pressure. The present algorithm consists of a comprehensive set of conditions to be performed on the dataset sequentially. These include datasets from various instruments simultaneously observed. The primary objective of the present study is to describe the implementation and testing of this algorithm for big datasets of the Martian ionosphere and extract ionopause-like density gradient using automation. Keywords: Ionopause, Mars, Remote sensing, MAVEN dataset, Parallel-processing

Mars' Ionopause: A Matter of Pressures

AbstractThis study assesses under what circumstances the Martian ionopause is formed on the dayside, both in regions where there are strong crustal magnetic fields and areas where these fields are small (<30 nT). Multiple data sets from three MAVEN dayside deep dip campaigns are utilized between periapsis and 600–1,000 km, as well as solar wind observations from Mars Express. The ionopause is identified as a sudden decrease of the electron density with increasing altitude and a simultaneous increase of the electron temperature and variability below 400 km. This is a physically robust approach as the electron temperature is a key parameter in determining the structure of the ionospheric profile, and, therefore, also a strong indicator of the ionopause location. We find that 36% (54%) of the electron density profiles over strong (weak) crustal magnetic field regions had an ionopause event. We also evaluate the roles of ionospheric thermal and magnetic pressures on the ionopause formation as well as the presence of solar wind particles, H+, down to the location of the ionopause. We found that the topside ionosphere is typically magnetized at mostly all altitudes. The ionopause, if formed, occurs where the total ionospheric pressure (magnetic + thermal) equals the upstream solar wind dynamic pressure. Moreover, the lower edge of the ionopause coincides with the altitude where the solar wind flow stops: The thermal pressure suffers a significant reduction with increasing altitude and the solar wind proton density has a prominent increase.

Cold ion escape from the Martian ionosphere

We here report on new measurements of the escape flux of oxygen ions from Mars by combining the observations of the ASPERA-3 and MARSIS experiments on board the European Mars Express spacecraft. We show that in previous estimates of the total heavy ion escape flow the contribution of the cold ionospheric outflow with energies below 10eV has been underestimated. Both case studies and the derived flow pattern indicate that the cold plasma observed by MARSIS and the superthermal plasma observed by ASPERA-3 move with the same bulk speed in most regions of the Martian tail. We determine maps of the tailside heavy ion flux distribution derived from mean ion velocity distributions sampled over 7 years. If we assume that the superthermal bulk speed derived from these long time averages of the ion distribution function represent the total plasma bulk speed we derive the total tailside plasma flux. Assuming cylindrical symmetry we determine the mean total escape rate for the years 2007–2014 at 2.8±0.4×1025atoms/s which is in good agreement with model estimates. A possible mechanism to generate this flux can be the ionospheric pressure gradient between dayside and nightside.

Transterminator ion flow in the Martian ionosphere

The upper ionospheres of Mars and Venus are permeated by the magnetic fields induced by the solar wind. It is a long-standing question whether these fields can put the dense ionospheric plasma into motion. If so, the transterminator flow of the upper ionosphere could explain a significant part of the ion escape from the planets atmospheres. But it has been technically very challenging to measure the ion flow at energies below 20 eV. The only such measurements have been made by the ORPA instrument of the Pioneer Venus Orbiter reporting speeds of 1–5 km/s for O + ions at Venus above 300 km altitude at the terminator ( Knudsen et al., 1980, 1982). At Venus the transterminator flow is sufficient to sustain a permanent nightside ionosphere, at Mars a nightside ionosphere is observed only sporadically. We here report on new measurements of the transterminator ion flow at Mars by the ASPERA-3 experiment on board Mars Express with support from the MARSIS radar experiment for some orbits with fortunate observation geometry. We observe a transterminator flow of O + and O 2 + ions with a super-sonic velocity of around 5 km/s and fluxes of 0.8×10 9/cm 2 s. If we assume a symmetric flux around the terminator this corresponds to an ion flow of 3.1±0.5×10 25/s half of which is expected to escape from the planet. This escape flux is significantly higher than previously observed on the tailside of Mars. A possible mechanism to generate this flux can be the ionospheric pressure gradient between dayside and nightside or momentum transfer from the solar wind via the induced magnetic field since the flow velocity is in the Alfvénic regime. We discuss the implication of these new observations for ion escape and possible extensions of the analysis to dayside observations which may allow us to infer the flow structure imposed by the induced magnetic field.

Determining the mass loss limit for close-in exoplanets: what can we learn from transit observations?

<i>Aims. <i/>We study the possible atmospheric mass loss from 57 known transiting exoplanets around F, G, K, and M-type stars over evolutionary timescales. For stellar wind induced mass loss studies, we estimate the position of the pressure balance boundary between Coronal Mass Ejection (CME) and stellar wind ram pressures and the planetary ionosphere pressure for non- or weakly magnetized gas giants at close orbits.<i>Methods. <i/>The thermal mass loss of atomic hydrogen is calculated by a mass loss equation where we consider a realistic heating efficiency, a radius-scaling law and a mass loss enhancement factor due to stellar tidal forces. The model takes into account the temporal evolution of the stellar EUV flux by applying power laws for F, G, K, and M-type stars. The planetary ionopause obstacle, which is an important factor for ion pick-up escape from non- or weakly magnetized gas giants is estimated by applying empirical power-laws.<i>Results. <i/>By assuming a realistic heating efficiency of about 10–25% we found that WASP-12b may have lost about 6–12% of its mass during its lifetime. A few transiting low density gas giants at similar orbital location, like WASP-13b, WASP-15b, CoRoT-1b or CoRoT-5b may have lost up to 1–4% of their initial mass. All other transiting exoplanets in our sample experience negligible thermal loss (<i>≤<i/>1%) during their lifetime. We found that the ionospheric pressure can balance the impinging dense stellar wind and average CME plasma flows at distances which are above the visual radius of “Hot Jupiters”, resulting in mass losses <2% over evolutionary timescales. The ram pressure of fast CMEs cannot be balanced by the ionospheric plasma pressure for orbital distances between 0.02–0.1 AU. Therefore, collisions of fast CMEs with hot gas giants should result in large atmospheric losses which may influence the mass evolution of gas giants with masses <<i>M<i/><sub>Jup<sub/>. Depending on the stellar luminosity spectral type, planetary density, heating efficiency, orbital distance, and the related Roche lobe effect, we expect that at distances between 0.015–0.02 AU, Jupiter-class and sub-Jupiter-class exoplanets can lose several percent of their initial mass. At orbital distances <i>≤<i/>0.015 AU, low density hot gas giants in orbits around solar type stars may even evaporate down to their coresize, while low density Neptune-class objects can lose their hydrogen envelopes at orbital distances <i>≤<i/>0.02 AU.

The solar wind interaction with Venus through the eyes of the Pioneer Venus Orbiter

Venus has no internal magnetic dynamo and thus its ionosphere and hot oxygen exosphere dominate the interaction with the solar wind. The solar wind at 0.72 AU has a dynamic pressure that ranges from 4.5 nPa (at solar max) to 6.6 nPa (at solar min), and its flow past the planet produces a shock of typical magnetosonic Mach number 5 at the subsolar point. At solar maximum the pressure in the ionospheric plasma is sufficient to hold off the solar wind at an altitude of 400 km above the surface at the subsolar point, and 1000 km above the terminators. The deflection of the solar wind occurs through the formation of a magnetic barrier on the inner edge of the magnetosheath, or shocked solar wind. Under typical solar wind conditions the time scale for diffusion of the magnetic field into the ionosphere is so long that the ionosphere remains field free and the barrier deflects almost all the incoming solar wind. Any neutral atoms of the hot oxygen exosphere that reach the altitude of the magnetosheath are accelerated by the electric field of the flowing magnetized plasma and swept along cycloidal paths in the antisolar direction. This pickup process, while important for the loss of the Venus atmosphere, plays a minor role in the deceleration and deflection of the solar wind. Like at magnetized planets, the Venus shock and magnetosheath generate hot electrons and ions that flow back along magnetic field lines into the solar wind to form a foreshock. A magnetic tail is created by the magnetic flux that is slowed in the interaction and becomes mass-loaded with thermal ions. The structure of the ionosphere is very much dependent on solar activity and the dynamic pressure of the solar wind. At solar maximum under typical solar wind conditions, the ionosphere is unmagnetized except for the presence of thin magnetic flux ropes. The ionospheric plasma flows freely to the nightside forming a well-developed night ionosphere. When the solar wind pressure dominates over the ionospheric pressure the ionosphere becomes completely magnetized, the flow to the nightside diminishes, and the night ionosphere weakens. Even at solar maximum the night ionosphere has a very irregular density structure. The electromagnetic environment of Venus has not been well surveyed. At ELF and VLF frequencies there is noise generated in the foreshock and shock. At low altitude in the night ionosphere noise, presumably generated by lightning, can be detected. This paper reviews the plasma environment at Venus and the physics of the solar wind interaction on the threshold of a new series of Venus exploration missions.

Effects of decreasing ionospheric pressure and the plasma mixing recess on the solar wind interaction with non-magnetized planets

Abstract The solar wind interaction with the ionosphere of non-magnetized planets is numerically simulated in the framework of 3-D magnetohydrodynamics (MHD) with two-component plasma. In cases 1 and 2, solar extreme ultraviolet (EUV) flux values is set so the peak ionospheric plasma pressure is below and above the incident solar wind dynamic pressure. While the formation of the bow shock and the magnetic barrier in the upstream region is seen in both cases, a clear formation of the ionopause is seen only in case 2. In case 1, the interplanetary magnetic field (IMF) penetrates from the magnetosheath to the dayside ionosphere so as to adjust the ionospheric total pressure. Penetrating IMF affects the vertical motion of the ionospheric plasma. However, formation process of the ionotail is little affected by the penetrating IMF. In bith cases 1 and 2, partial penetration of the IMF occurs from the magnetic barrier to the terminator ionosphere. This nonideal MHD process characterized by the penetration of flowing magnetized plasma into non-magnetized plasma plays a principal role in the mixing interaction between the solar wind and the planetary ionosphere.

Effects of decreasing ionospheric pressure on the solar wind interaction with non-magnetized planets

The large-scale solar wind interaction with the ionosphere of non-magnetized planets is numerically simulated in the framework of three-dimensional (3-D) magnetohydrodynamics (MHD) with a two-component plasma. The finite-volume total variation diminishing (TVD) scheme is used to solve this problem. Numerical results are given for two cases of different solar extreme ultraviolet (EUV) flux values. In case 1, solar EUV ionization is set so the peak ionospheric plasma pressure is below the incident solar wind dynamic pressure. In case 2, on the other hand, it is set so the peak ionospheric pressure exceeds the solar wind dynamic pressure. While the formation of the bow shock and the magnetic barrier in the upstream region is seen in both cases, a clear formation of the ionopause is seen only in case 2. In case 1, the interplanetary magnetic field (IMF) penetrates from the magnetosheath to the dayside ionosphere so as to adjust the ionospheric total pressure. Penetrating IMF affects the vertical motion of the ionospheric plasma to cause anomalous stratifications of the terminator ionosphere. However, formation process of the ionotail is little affected by the penetrating IMF. Another important process predicted from the present study is partial penetration of the IMF from the magnetic barrier to the terminator ionosphere. This nonideal MHD process characterized by the penetration of flowing magnetized plasma into non-magnetized plasma plays a principal role in the mixing interaction between the solar wind and the planetary ionosphere.

Quantitative model of the Martian magnetopause shape and its variation with the solar wind ram pressure based on Phobos 2 observations

A model of the Martian magnetopause is developed for the period of maximum solar activity which simultaneously describes (1) the observed relation between the solar wind ram pressure ρ V2 and the magnetopause position in the magnetotail, (2) the observed relation between ρ V2 and the flaring angle, and (3) a few magnetopause crossing observations above the day side of the planet. The shape of the magnetopause is determined from the equation of pressure balance across this boundary when both the magnetic pressure (with a planetary magnetic moment of (0.8–1.0)×l022 G cm3) and the ionospheric pressure are taken into account in the planetary magnetosphere. The specific feature of the model is the “stagnation” of the subsolar magnetopause when the ram pressure increases to higher values (≥ 6×10−9 dyn cm−2).

Comparisons of peak ionosphere pressures at Mars and Venus with incident solar wind dynamic Pressure

Previous calculations of the potential for pressure balance between the solar wind and the ionospheres of the weakly magnetized planets, Mars and Venus, indicated that the maximum or peak ionospheric thermal pressure is sufficient to stand off the solar wind at Venus but not at Mars. In this study we used radio occultation measurements of electron density profiles from Mariner 6 and 7, the Mariner 9 extended mission, and the U.S. Viking orbiters, together with model ion and electron temperature profiles, to derive thermal pressure profiles in the Mars ionosphere. Similarly, Pioneer Venus Orbiter (PVO) radio occultation data and temperature models were used to obtain ionospheric pressure profiles at Venus. Because the radio occultation data give information for both active and quiet phases of the solar cycle, this method allows one to consider how the balance changes between solar minimum and maximum. The comparison of the Mars peak ionosphere pressures with the incident solar wind dynamic pressure suggests that at solar maximum the Mars ionosphere, like that of Venus, should generally be sufficient to balance the incident solar wind pressure. At solar minimum, when the ionosphere is weakest and the solar wind dynamic pressure is highest, only the peak pressures at high solar zenith angles (SZAs) at Mars appear to be strong enough to balance the incident solar wind pressure. This is similar to the situation at Venus at solar minimum. However, due to the lack of radio occultation data for Mars for SZAs less than 45°, we can only infer what happens near the subsolar point. Nevertheless, our results are somewhat contrary to the frequent assumption that the Mars ionosphere is everywhere and always too weak to withstand the incident solar wind dynamic pressure. When one takes into account the SZA dependence of both the incident solar wind pressure and the peak electron density, and the solar cycle variation of the ionosphere, the situations at Mars and Venus appear to be quite similar. Our previous perceptions for Mars may have been colored by the fact that most of the available Mars ionospheric data are from low or moderate solar activity levels.

Solar cycle changes in the ionization sources of the nightside Venus ionosphere

We argue, using new and previous experimental evidence that the height‐integrated flux of O+ ions transported from the dayside Venus ionosphere into the nightside ionosphere is strongly reduced (shut off) at solar cycle minimum (SCmin) and that the electron impact source becomes the predominant nightside ionization source during this phase of the solar cycle. The height‐integrated transterminator O+ flux, which is probably the dominant nightside ionization source at solar cycle maximum (SCmax), is shut off at SCmin by compression of the dayside ionopause to low altitude. We suggest that the ionopause is depressed because the dayside ionospheric kinetic pressure field is reduced by a factor of approximately 3 from its SCmax value and that the typical SCmin solar wind dynamic pressure, which is not less than the typical SCmax value, cannot be balanced by the ionospheric pressure field at altitudes above approximately 250 km.

Characteristics of the Marslike limit of the Venus‐solar wind interaction

Many authors have already noted that Mars' interaction with the solar wind may be like Venus', e.g., that of a supermagnetosonic plasma flowing past an effectively unmagnetized body having an ionosphere. However, at Mars, the incident solar wind dynamic pressure usually exceeds the peak ionospheric plasma pressure, while at Venus this condition prevails only when the incident dynamic pressure is extraordinarily high. With the aim of predicting what might be expected at Mars, this study examines the subset of Pioneer Venus Orbiter observations obtained during intervals of extremely high solar wind dynamic pressure. The characteristic features of this limit of the Venus‐solar wind interaction include a bow shock position that is not notably different from the norm, altitude profiles of the dayside upper ionosphere density without a sharp increase in gradient at the ionopause, and dayside electron temperatures that rapidly increase with altitude to consistently exceed the temperatures above 200 km which are present for lower solar wind pressures. Depleted nightside ionosphere densities, and a large‐scale horizontal magnetic field in both the dayside and nightside ionospheres, are among the previously identified responses to high dynamic pressure. Here, emphasis is placed on the dayside ionosphere because some data are available for the dayside Martian ionosphere from the Viking mission. The density and ion temperature trends are found to be similar to those seen in the Viking data. Overall, the Venus observations at high dynamic pressure provide a framework for reassessing the available Mars observations. In particular, this study shows that the observed absence of a distinct ionopause “cutoff” in the dayside plasma density gradient cannot be construed as evidence for an intrinsic magnetic field which stops the solar wind at higher altitudes. Similarly, the rules of thumb that the electron temperature is given by twice the ion temperature and that the ionospheric magnetic field pressure can only double the total ionospheric pressure, which have been applied to assess Mars' ability to standoff the solar wind, are not justified. Observations at Venus thus illustrate the expected modification of the common pressure balance picture of the solar wind interaction at an unmagnetized planet with a weak ionosphere. The available Mars observations appear to be consistent with this modified picture.

Solar wind mass‐Loading at comet Halley: A lesson from Venus?

Recent observations at comet Halley show that the region within which cometary ions become the dominant component lies outside of the magnetic field‐free cavity. This behavior resembles that found at Venus under conditions where the incident solar wind dynamic pressure exceeds the ionospheric pressure. On these occasions the magnetosheath magnetic field is found well inside of the region where planetary ions are observed. Although scaling and the details of formation of the inner boundary of the magnetic field are different for these two objects, the processes by which the interplanetary magnetic field penetrates into the ionospheres at Venus and at Comet Halley are in many ways analogous.

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.

An empirical model of the Venusian outer environment 1. The shape of the dayside solar wind‐atmosphere interface

Solar wind plasma and magnetic field data and ionospheric data obtained from the Pioneer Venus orbiter are considered. It is shown that the variation of the magnetic field pressure within the magnetic barrier is similar to that expected for the solar wind pressure variations along an obstacle's boundary when a more realistic approximation of the shape of the ionosphere is included. Simultaneous solar wind pressure, ionospheric pressure, and magnetic barrier pressure data show that the ionospheric pressure below the ionopause is approximately equal to the solar wind pressure. The magnetic barrier pressure is equal to approximately 2/3 to 3/4 of both the solar wind pressure and the ionopsheric pressure. Estimates of the hot plasma pressure contribution to the total pressure within the magnetic barrier vary from 1/4 to 1/3. Just below the ionopause the ionospheric pressure deviations from the mean ionospheric pressure are significant especially below ∼400 km and appear to be indicative of the adjustment of the ionospheric structure to changing solar wind conditions. A first‐order model of the ionopause pressure variations as a function of height and solar‐zenith angle is suggested. This model provides an estimate of an ‘instant’ ionopause profile for the given solar wind conditions. The mean and ‘instant’ shapes of the ionopause do not appear to correspond to the estimates obtained from ionospheric pressure equilibrium for a given H/r0.

Pressure balance and pressure distribution along the dayside ionopause of Venus

The pressure distribution within the magnetic barrier just outside the ionopause of Venus is shown here to be similar to expected solar wind pressure variation provided the more realistic shape of the ionopause is taken into account. The ionospheric pressure just below the magnetic barrier is about equal to the solar wind pressure while magnetic field pressure within the magnetic barrier amounts to about two-thirds of both ionospheric and solar wind pressures. So we can expect that one-third of magnetic barrier pressure is accounted for by the hot plasma contribution.

Plasma diffusion into the wake of Venus

A model of the diffusion of ionosheath plasma into the wake region of Venus is presented. It is shown that particle diffusion, which is assumed to be a consequence of the fluctuating magnetic field observed in the wake of Venus by Pioneer Venus and Veneras 9 and 10, can explain the plasma observations made in the wake by Veneras 9 and 10. The pressure due to these diffusing particles when balanced against the ionospheric pressure yield ionopause heights less than 1000 km for zenith angles less than about 135°. The model also predicts significant fluxes of low energy electrons and ions for zenith angles less than 135°, which are capable of producing the observed nightside ionosphere.

Ionospheric Storms and Small Pressure Fluctuations at Ground Level

IN the past a number of attempts have been made to establish relationships between events occurring in the troposphere and in the ionosphere1–3. However, none of them has as yet been decisive. Recent work4,5 has shown that it may be possible to link the two regions by the passage of gravity waves from one region to the other. Infrasonic waves, recorded at ground level near polar regions6, indicate a possible relationship between aurora and these infrasonic waves. Earlier7, infrasonic waves had been recorded at Washington, D.C., at times of magnetic activity travelling at speeds somewhat higher than the local speed of sound. Maeda and Watanabe8 and, more recently, Iyengar9 have discussed the possibility of pressure waves being generated at auroral heights. Also, Maeda and Young10 have discussed the periods of pressure waves from aurorae. Bowman11 has suggested that pressure waves generated near polar regions may be responsible for ionospheric storms at the times of magnetic storms. This report gives some results of possible troposphere–ionosphere relations from the analysis of microbarograph records, which have been made at Brisbane since July 1963. The type of microbarograph used is described by Jones and Forbes12.

A Correlation between Ionospheric Phenomena and Surface Pressure

A Correlation between Ionospheric Phenomena and Surface Pressure