We use in situ plasma observations made by the Pioneer Venus Orbiter spacecraft to show for the first time that magnetosonic waves can couple the solar wind to the upper ionosphere and deposit energy ...there. The waves are generated upstream of Venus, are advected into the shock and propagate across the draped magnetic field, through the magnetosheath and into the dayside upper ionosphere. The magnetosonic waves damp in the upper ionosphere in a region where physical collisions are rare, and electromagnetic forces must control this damping. The waves damp when the ionospheric heavy ion density is a few thousand cm−3 and wave‐particle interactions with the dominant O+ ions are postulated as the damping mechanism. Estimates of ion heating rates show that 1%–5% of the O+ ion distribution function could be heated to escape energy in 10–40 s.
Plain Language Summary
Our Sun emits a stream of charged particles radially outward into our Solar system, known as the solar wind. When the solar wind encounters obstacles such as planets and comets, a variety of forces may act to divert the flow around the obstacle, much like when flowing water in a stream encounters a rock and is diverted around it. This study uses measurements made by a spacecraft that orbited Venus, known as Pioneer Venus Orbiter, to investigate some of the side effects that can arise when the solar wind flow is diverted around Venus. We show for the first time how a particular pathway allows energy to be deposited from the flowing solar wind into the Venusian atmosphere, and that this energy can be deposited quickly enough to significantly impact the particles in the atmosphere. The characteristics observed in this study at Venus are similar to those at Mars where this process has also been observed, suggesting that the solar wind can interact with the two planets in similar ways in this respect.
Key Points
Magnetosonic waves propagate from upstream into the dayside upper ionosphere of Venus
Magnetosonic waves are damped by wave‐particle interactions with heavy ionospheric ions
Subsequent ion heating rates could heat 1%–5% of the ion distribution function to escape energy in 10–40 s
The Pioneer and Voyager spacecraft made close-up measurements of Saturn's ionosphere and upper atmosphere in the 1970s and 1980s that suggested a chemical interaction between the rings and ...atmosphere. Exploring this interaction provides information on ring composition and the influence on Saturn's atmosphere from infalling material. The Cassini Ion Neutral Mass Spectrometer sampled in situ the region between the D ring and Saturn during the spacecraft's Grand Finale phase. We used these measurements to characterize the atmospheric structure and material influx from the rings. The atmospheric He/H
ratio is 10 to 16%. Volatile compounds from the rings (methane; carbon monoxide and/or molecular nitrogen), as well as larger organic-bearing grains, are flowing inward at a rate of 4800 to 45,000 kilograms per second.
The two main sources of the magnetic field in the Martian ionosphere are the solar wind interaction with the planet, and, mainly in the southern hemisphere, remnant crustal magnetization. The ...magnetic fields measured by the Mars Atmosphere and Volatile EvolutioN (MAVEN) and Mars Global Surveyor spacecraft displayed a wide range of spatial scales, from the global (i.e., L ≈ 103 km) to mesoscale (L ≈ 102 km) to small‐scale (L < 10 km). Hamil et al. (2022) used MAVEN magnetometer and Langmuir Probe data to study these structures and suggested that they might be advected into the ionosphere from the solar wind and magnetosheath. In the current study, we apply a Fourier analysis to the fields and interpret the resulting power spectral density profiles versus frequency. The power spectral density function found from MAVEN data resembles that of the solar wind magnetic field (or interplanetary magnetic field) (i.e., power law with an index of about −2), but shifted upward in frequency by a factor of about 100. From a comparison of ionospheric power spectra with solar wind power spectra, we deduce that plasma, carrying a magnetic field with it moves from the magnetic pile‐up region downward into the ionosphere with speeds of roughly tens of meters per second. The derived power spectra in the ionosphere, in addition to the basic power law shape, show hints of extra power at a spatial scale of about 10 km, and this might be due to the creation of a magnetic structure within the ionosphere itself.
Plain Language Summary
The two main sources of magnetic field for the Martian ionosphere are the solar wind magnetic fields (i.e., the interplanetary magnetic field), and, particularly in the southern hemisphere, remnant crustal magnetization. The magnetic fields measured in the ionosphere by the magnetometers onboard the Mars Atmosphere and Volatile EvolutioN spacecraft and the Mars Global Surveyor display a wide range of spatial scales. In this study, we use a Fourier analysis of the magnetic field to quantify the field strength as a function of scale‐size. The fourier analysis represents a function of time (or some other variable) as the sum over a wide range of sinusoidal functions of different frequencies. We find that the results of the Fourier analysis for the ionospheric magnetic field have much in common with the Fourier analysis of the interplanetary magnetic field. The study exploits this similarity to probe the sources of the ionosphere's magnetic structure.
Key Points
A wide range of spatial scales of magnetic structure has been observed in the dayside ionosphere by the Mars Atmosphere and Volatile EvolutioN magnetometer
A Fourier analysis of the ionospheric magnetic structure suggests that the power spectrum is like that of the interplanetary magnetic field
Simple magnetohydrodynamical theory shows that plasma in the Martian dayside ionosphere flows downward with speeds of tens of meters per second
This paper evaluates the role of magnetic reconnection in the dayside ionosphere of Mars in the collisional regime and presents some relevant data from the MAVEN (Mars Atmosphere and Volatile ...EvolutioN) mission. Magnetic reconnection is an important process operating in the solar corona, planetary magnetospheres, and astrophysical plasmas, but most of the literature focuses on collisionless plasmas. However, at Mars the reconnection should often occur in regions where collisions are important. Mars does not have a large‐scale global magnetic field; however, Mars has locally large magnetic fields associated with remnant crustal magnetization, particularly in the highland regions of the southern hemisphere. The crustal fields provide a “target” for reconnection both in the ionosphere and in the magnetotail of Mars. The current paper emphasizes the dayside ionosphere. Magnetic reconnection is associated with topological changes in the magnetic field and is also a source of energy for the plasma and can thus affect the ionospheric dynamics and ion loss at Mars. Both theoretical concepts and MAVEN particle and field data from several instruments are considered in this paper.
Plain Language Summary
Magnetic reconnection is an important physical process in solar system and astrophysical plasmas (i.e., magnetized charged particle gases). During this process, the topology of the field lines is generally altered. For example, two sets of field lines—one connected at both ends to the Sun (i.e., interplanetary field lines advected into the ionosphere by the solar wind flow) and a second set connecting at both ends to the interior of Mars, “reconnect” in the relatively small “diffusion region.” The resulting field lines reach from the Sun into the surface of Mars (i.e., “open” field lines). Energy stored in the magnetic field is released during the reconnection process, thus providing kinetic energy to the electrons and ions in the plasma. The purpose of this paper is to explore some theoretical ideas of how collisions could affect magnetic reconnection in the Martian ionosphere and then to present one observational example using MAVEN (Mars Atmosphere and Volatile EvolutioN) data.
Key Points
Magnetic reconnection in the Martian ionosphere affects the transition between solar wind and crustal magnetic fields
The efficiency of magnetic reconnection is reduced by collisions at lower altitudes in the Martian ionosphere
Magnetic reconnection in the ionosphere is evident in data collected by the MAVEN spacecraft orbiting Mars
The large-scale, steady-state magnetic field configuration of the solar corona is typically computed using boundary conditions derived from photospheric observations. Two approaches are typically ...used: (1) potential field source surface (PFSS) models, and (2) the magnetohydrodynamic (MHD) models. The former have the advantage that they are simple to develop and implement, require relatively modest computer resources, and can resolve structure on scales beyond those that can be handled by current MHD models. However, they have been criticized because their basic assumptions are seldom met. Moreover, PFSS models cannot directly incorporate time-dependent phenomena, such as magnetic reconnection, and do not include plasma or its effects. In this study, we assess how well PFSS models can reproduce the large-scale magnetic structure of the corona by making detailed comparisons with MHD solutions at different phases in the solar activity cycle. In particular, we (1) compute the shape of the source surface as inferred from the MHD solutions to assess deviations from sphericity, (2) compare the coronal hole boundaries as determined from the two models, and (3) estimate the effects of nonpotentiality. Our results demonstrate that PFSS solutions often closely match MHD results for configurations based on untwisted coronal fields (i.e., when driven by line-of-sight magnetograms). It remains an open question whether MHD solutions will differ more substantially from PFSS solutions when vector magnetograms are used as boundary conditions. This will be addressed in the near future when vector data from SOLIS, the Solar Dynamics Observatory, and Solar-B become incorporated into the MHD models.
Estimates of Ionospheric Transport and Ion Loss at Mars Cravens, T. E.; Hamil, O.; Houston, S. ...
Journal of geophysical research. Space physics,
October 2017, 2017-10-00, 20171001, Letnik:
122, Številka:
10
Journal Article
Recenzirano
Odprti dostop
Ion loss from the topside ionosphere of Mars associated with the solar wind interaction makes an important contribution to the loss of volatiles from this planet. Data from NASA's Mars Atmosphere and ...Volatile Evolution mission combined with theoretical modeling are now helping us to understand the processes involved in the ion loss process. Given the complexity of the solar wind interaction, motivation exists for considering a simple approach to this problem and for understanding how the loss rates might scale with solar wind conditions and solar extreme ultraviolet irradiance. This paper reviews the processes involved in the ionospheric dynamics. Simple analytical and semiempirical expressions for ion flow speeds and ion loss are derived. In agreement with more sophisticated models and with purely empirical studies, it is found that the oxygen loss rate from ion transport is about 5% (i.e., global O ion loss rate of Qion ≈ 4 × 1024 s−1) of the total oxygen loss rate. The ion loss is found to approximately scale as the square root of the solar ionizing photon flux and also as the square root of the solar wind dynamic pressure. Typical ion flow speeds are found to be about 1 km/s in the topside ionosphere near an altitude of 300 km on the dayside. Not surprisingly, the plasma flow speed is found to increase with altitude due to the decreasing ion‐neutral collision frequency.
Key Points
Oxygen ion loss from the ionosphere of Mars is mainly driven by magnetic forces generated by the solar wind interaction
Global ion loss from Mars scales approximately as the square root of both the upstream solar wind pressure and solar ionizing photon flux
Ion flow speeds in the ionosphere increase with altitude and with solar wind pressure
Energetic ion precipitation at Titan Cravens, T. E.; Robertson, I. P.; Ledvina, S. A. ...
Geophysical research letters,
February 2008, Letnik:
35, Številka:
3
Journal Article
Recenzirano
Odprti dostop
Energetic protons and oxygen ions have been observed in Saturn's outer magnetosphere and can precipitate into Titan's atmosphere where they deposit energy, ionize, and drive ionospheric chemistry. ...Ion production rates caused by this precipitation are calculated using fluxes of incident 27 keV to 4 MeV protons measured by the Cassini MIMI instrument. We find that significant ion production rates exist in the 500 km to 1000 km altitude range and estimate associated electron densities of about 200–2000 cm−3 in reasonable agreement with measured densities. We demonstrate that energetic oxygen ions do not penetrate below about 650 km, but they can also generate significant ionization. We suggest that a few percent of the oxygen flux is converted to negative O ions as a consequence of charge exchange collisions, which might help explain the negative ions observed near 960 km by the Cassini CAPS instrument.
Simulation has become a valuable tool that compliments more traditional methods used to understand solar system plasmas and their interactions with planets, moons and comets. The three popular ...simulation approaches to studying these interactions are presented. Each approach provides valuable insight to these interactions. To date no one approach is capable of simulating the whole interaction region from the collisionless to the collisional regimes. All three approaches are therefore needed. Each approach has several implicit physical assumptions as well as several numerical assumptions depending on the scheme used. The magnetohydrodynamic (MHD), test-particle/Monte-Carlo and hybrid models used in simulating flowing plasmas are described. Special consideration is given to the implicit assumptions underlying each model. Some of the more common numerical methods used to implement each model, the implications of these numerical methods and the resulting limitations of each simulation approach are also discussed.
A key process populating the oxygen exosphere at Mars is the dissociative recombination of ionospheric O2+, which produces fast oxygen atoms, some of which have speeds exceeding the escape speed and ...thus contribute to atmospheric loss. Theoretical studies of this escape process have been carried out and predictions made of the loss rate; however, directly measuring the escaping neutral oxygen is difficult but essential. This paper describes how energetic pickup ion measurements to be made near Mars by the SEP (Solar Energetic Particle) instrument on board the MAVEN (Mars Atmosphere and Volatile EvolutioN) spacecraft can be used to constrain models of photochemical oxygen escape. In certain solar wind conditions, neutral oxygen atoms in the distant Martian exosphere that are ionized and picked up by the solar wind can reach energies high enough to be detected near Mars by SEP.
Key Points
Photochemical hot oxygen escape rate at Mars is predicted
Martian exospheric neutral oxygen model is constructed
Pickup ion fluxes measured by SEP will constrain neutral oxygen escape from Mars
Composition of Titan's ionosphere Cravens, T. E.; Robertson, I. P.; Waite, J. H. ...
Geophysical research letters,
April 2006, Letnik:
33, Številka:
7
Journal Article
Recenzirano
Odprti dostop
We present Cassini Ion and Neutral Mass Spectrometer (INMS) measurements of ion densities on the nightside of Titan from April 16, 2005, and show that a substantial ionosphere exists on the nightside ...and that complex ion chemistry is operating there. The total ionospheric densities measured both by the INMS and the Cassini Radio and Plasma Wave (RPWS) experiments on Cassini suggest that precipitation from the magnetosphere into the atmosphere of electrons with energies ranging from 25 eV up to about 2 keV is taking place. The absence of ionospheric composition measurements has been a major obstacle to understanding the ionosphere. Seven “families” of ion species, separated in mass‐to‐charge ratio by 12 Daltons (i.e., the mass of carbon), were observed and establish the importance of hydrocarbon and nitrile chains in the upper atmosphere. Several of the ion species measured by the INMS were predicted by models (e.g., HCNH+ and C2H5+). But the INMS also saw high densities at mass numbers not predicted by models, including mass 18, which we suggest will be ammonium ions (NH4+) produced by reaction of other ion species with neutral ammonia.
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