The Martian bow shock distance has previously been shown to be anticorrelated with solar wind dynamic pressure but correlated with solar extreme ultraviolet (EUV) irradiance. Since both of these ...solar parameters reduce with the square of the distance from the Sun, and Mars' orbit about the Sun increases by ∼0.3 AU from perihelion to aphelion, it is not clear how the bow shock location will respond to variations in these solar parameters, if at all, throughout its orbit. In order to characterize such a response, we use more than 5 Martian years of Mars Express Analyser of Space Plasma and EneRgetic Atoms (ASPERA‐3) Electron Spectrometer measurements to automatically identify 11,861 bow shock crossings. We have discovered that the bow shock distance as a function of solar longitude has a minimum of 2.39RM around aphelion and proceeds to a maximum of 2.65RM around perihelion, presenting an overall variation of ∼11% throughout the Martian orbit. We have verified previous findings that the bow shock in southern hemisphere is on average located farther away from Mars than in the northern hemisphere. However, this hemispherical asymmetry is small (total distance variation of ∼2.4%), and the same annual variations occur irrespective of the hemisphere. We have identified that the bow shock location is more sensitive to variations in the solar EUV irradiance than to solar wind dynamic pressure variations. We have proposed possible interaction mechanisms between the solar EUV flux and Martian plasma environment that could explain this annual variation in bow shock location.
Key Points
Automatically identified bow shock crossings by Mars Express over 5 Martian years of data
The bow shock distance at the terminator increases by 11% from near aphelion to near perihelion
Functional forms produced for bow shock response to solar EUV and dynamic pressure
We study atmospheric escape from Mars during solar wind pressure pulses. During the solar minimum of 2007–08 we have observed 41 high pressure events, which are predominantly identified as corotating ...interaction regions (CIR) while a few are coronal mass ejections (CME), in data from the Advanced Composition Explorer (ACE) upstream of the Earth. 36 of these events are also identified using Mars Express (MEX) data at Mars. We use MEX measurements at Mars to compare the antisunward fluxes of heavy planetary ions during the passage of these pulses to the fluxes during quiet solar wind conditions. The ion fluxes are observed to increase by a factor of ∼2.5, on average. Hence, a third of the total outflow from Mars takes place during ∼15% of the time, when a solar wind pressure pulse impacts on the planet. This can have important consequences for the total time‐integrated outflow of plasma from Mars.
Saturn's Dusty Ionosphere Morooka, M. W.; Wahlund, J.‐E.; Hadid, L. Z. ...
Journal of geophysical research. Space physics,
March 2019, Volume:
124, Issue:
3
Journal Article
Peer reviewed
Open access
Measurements of electrons and ions in Saturn's ionosphere down to 1,500‐km altitudes as well as the ring crossing region above the ionosphere obtained by the Langmuir probe onboard the Cassini ...spacecraft are presented. Five nearly identical deep ionosphere flybys during the Grand Finale orbits and the Final plunge orbit revealed a rapid increase in the plasma densities and discrepancies between the electrons and ions densities (Ne and Ni) near the closest approach. The small Ne/Ni ratio indicates the presence of a dusty plasma, a plasma which charge carrier is dominated by negatively charged heavy particles. Comparison of the Langmuir probe obtained density with the light ion density obtained by the Ion and Neutral Mass Spectrometer confirmed the presence of heavy ions. An unexpected positive floating potential of the probe was also observed when Ne/Ni ≪ 1. This suggests that Saturn's ionosphere near the density peak is in a dusty plasma state consisting of negatively and positively charged heavy cluster ions. The electron temperature (Te) characteristics in the ionosphere are also investigated and unexpectedly high electron temperature value, up to 5000 K, has been observed below 2,500‐km altitude in a region where electron‐neutral collisions should be prominent. A well‐defined relationship between Te and Ne/Ni ratio was found, implying that the electron heating at low altitudes is related to the dusty plasma state of the ionosphere.
Plain Language Summary
Cassini Langmuir probe measurements revealed ion densities in excess of the electron densities, indicative of a dusty plasma, in Saturn's ionosphere below 2,500‐km altitude. Comparison of the Langmuir probe measurements with those of the Ion and Neutral Mass Spectrometer, sensitive to only lighter ions during this period, showed that heavy ions dominate in this region. Positive spacecraft potentials were also found, suggesting that Saturn's ionosphere contains dusty plasma of negatively and positively charged heavy ions.
Key Points
In situ measurements of Saturn's ionospheric plasma densities down to 1,500 km and the ring above the ionosphere is presented
Charge imbalance in the ions and electrons, evidence of the negatively charged heavy particles, has been observed below 2,500 km
Observations suggest that Saturn's ionosphere consists of a significant amount of negatively and positively charged heavy ions
We study the evolution of the plasma environment of comet 67P using measurements of the spacecraft potential from early September 2014 (heliocentric distance 3.5 AU) to late March 2015 (2.1 AU) ...obtained by the Langmuir probe instrument. The low collision rate keeps the electron temperature high (∼5 eV), resulting in a negative spacecraft potential whose magnitude depends on the electron density. This potential is more negative in the northern (summer) hemisphere, particularly over sunlit parts of the neck region on the nucleus, consistent with neutral gas measurements by the Cometary Pressure Sensor of the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis. Assuming constant electron temperature, the spacecraft potential traces the electron density. This increases as the comet approaches the Sun, most clearly in the southern hemisphere by a factor possibly as high as 20–44 between September 2014 and January 2015. The northern hemisphere plasma density increase stays around or below a factor of 8–12, consistent with seasonal insolation change.
Key Points
Plasma density, deduced from spacecraft potential, traces neutral density, implying local ionization
We determine the plasma density increase due to decreased heliocentric distance and seasonal effects
Low collision rate keeps the electron temperature high (∼5 eV), giving a negative spacecraft potential
Cassini discovered a plethora of neutral and ionized molecules in Titan's ionosphere including, surprisingly, anions and negatively charged molecules extending up to 13,800 u q−1. In this Letter, we ...forward model the Cassini electron spectrometer response function to this unexpected ionospheric component to achieve an increased mass resolving capability for negatively charged species observed at Titan altitudes of 950-1300 km. We report on detections consistently centered between 25.8 and 26.0 u q−1 and between 49.0-50.1 u q−1 which are identified as belonging to the carbon chain anions, CN−/C3N− and/or C2H−/C4H−, in agreement with chemical model predictions. At higher ionospheric altitudes, detections at 73-74 u q−1 could be attributed to the further carbon chain anions C5N−/C6H− but at lower altitudes and during further encounters extend over a higher mass/charge range. This, as well as further intermediary anions detected at >100 u, provide the first evidence for efficient anion chemistry in space involving structures other than linear chains. Furthermore, at altitudes below <1100 km, the low-mass anions (<150 u q−1) were found to deplete at a rate proportional to the growth of the larger molecules, a correlation that indicates the anions are tightly coupled to the growth process. This study adds Titan to an increasing list of astrophysical environments where chain anions have been observed and shows that anion chemistry plays a role in the formation of complex organics within a planetary atmosphere as well as in the interstellar medium.
Scale size of cometary bow shocks Edberg, N.J.T.; Eriksson, A.I.; Vigren, E. ...
Astronomy and astrophysics (Berlin),
02/2024, Volume:
682
Journal Article
Peer reviewed
Open access
Context. In past decades, several spacecraft have visited comets to investigate their plasma environments. In the coming years, Comet Interceptor will make yet another attempt. This time, the target ...comet and its outgassing activity are unknown and may not be known before the spacecraft has been launched into its parking orbit, where it will await a possible interception. If the approximate outgassing rate can be estimated remotely when a target has been identified, it is desirable to also be able to estimate the scale size of the plasma environment, defined here as the region bound by the bow shock. Aims. This study aims to combine previous measurements and simulations of cometary bow shock locations to gain a better understanding of how the scale size of cometary plasma environments varies. We compare these data with models of the bow shock size, and we furthermore provide an outgassing rate-dependent shape model of the bow shock. We then use this to predict a range of times and cometocentric distances for the crossing of the bow shock by Comet Interceptor, together with expected plasma density measurements along the spacecraft track. Methods. We used data of the location of cometary bow shocks from previous spacecraft missions, together with simulation results from previously published studies. We compared these results with an existing model of the bow shock stand-off distance and expand on this to provide a shape model of cometary bow shocks. The model in particular includes the cometary outgassing rate, but also upstream solar wind conditions, ionisation rates, and the neutral flow velocity. Results. The agreement between the gas-dynamic model and the data and simulation results is good in terms of the stand-off distance of the bow shock as a function of the outgassing rate. For outgassing rates in the range of 1027–1031–s-1, the scale size of cometary bow shocks can vary by four orders of magnitude, from about 102 km to 106 km, for an ionisation rate, flow velocity, and upstream solar wind conditions typical of those at 1 AU. The proposed bow shock shape model shows that a comet plasma environment can range in scale size from the plasma environment of Mars to about half of that of Saturn. Conclusions. The model-data agreement allows for the planning of upcoming spacecraft comet encounters, such as that of Comet Interceptor, when a target has been identified and its outgassing rate is determined. We conclude that the time a spacecraft can spend within the plasma environment during a flyby can range from minutes to days, depending on the comet that is visited and on the flyby speed. However, to capture most of the comet plasma environment, including pick-up ions and upstream plasma waves, and to ensure the highest possible scientific return, measurements should still start well upstream of the expected bow shock location. From the plasma perspective, the selected target should preferably be an active comet with the lowest possible flyby velocity.
The ionized upper layer of Saturn's atmosphere, its ionosphere, provides a closure of currents mediated by the magnetic field to other electrically charged regions (for example, rings) and hosts ...ion-molecule chemistry. In 2017, the Cassini spacecraft passed inside the planet's rings, allowing in situ measurements of the ionosphere. The Radio and Plasma Wave Science instrument detected a cold, dense, and dynamic ionosphere at Saturn that interacts with the rings. Plasma densities reached up to 1000 cubic centimeters, and electron temperatures were below 1160 kelvin near closest approach. The density varied between orbits by up to two orders of magnitude. Saturn's A- and B-rings cast a shadow on the planet that reduced ionization in the upper atmosphere, causing a north-south asymmetry.
We present the electron density (ne) altitude profiles of Saturn's ionosphere at near‐equatorial latitudes from all 23 orbits of Cassini's Grand Finale. The data are collected by the Langmuir probe ...part of the Radio and Plasma Wave Science investigation. A high degree of variability in the electron density profiles is observed. However, organizing them by consecutive altitude ranges revealed clear differences between the southern and northern hemispheres. The ne profiles are shown to be more variable and connected to the D‐ring below 5,000 km in the southern hemisphere compared to the northern hemisphere. This observed variability is explained to be a consequence of an electrodynamic interaction with the D‐ring. Moreover, a density altitude profile is constructed for the northern hemisphere indicating the presence of three different ionospheric layers. Similar properties were observed during Cassini's final plunge, where the main ionospheric peak is crossed at ∼1,550‐km altitude.
Plain Language Summary
The Cassini Langmuir probe measured directly the uppermost layer of Saturn's atmosphere, the ionosphere, during its Grand Finale. The observations revealed a layered electron density altitude profile with evidence in the southern hemisphere of an electrodynamic type of interaction with the planet innermost D‐ring. Moreover, the main peak of the ionosphere is observed for the first time in the final plunge around 1,550 km.
Key Points
Cassini RPWS observations during the Grand Finale show an electrodynamic type of interaction between the topside ionosphere and the D‐ring in the southern hemisphere
A layered electron density profile is observed, characterized by at least a diffusive and a chemical equilibrium region
The main ionospheric peak is observed around 1,550 km in the final plunge
The importance of the heavy ions and dust grains for the chemistry and aerosol formation in Titan's ionosphere has been well established in the recent years of the Cassini mission. In this study we ...combine independent in situ plasma (Radio Plasma and Wave Science Langmuir Probe (RPWS/LP)) and particle (Cassini Plasma Science Electron Spectrometer, Cassini Plasma Science Ion Beam Spectrometer, and Ion and Neutral Mass Spectrometer) measurements of Titan's ionosphere for selected flybys (T16, T29, T40, and T56) to produce altitude profiles of mean ion masses including heavy ions and develop a Titan‐specific method for detailed analysis of the RPWS/LP measurements (applicable to all flybys) to further constrain ion charge densities and produce the first empirical estimate of the average charge of negative ions and/or dust grains. Our results reveal the presence of an ion‐ion (dusty) plasma below ~1100 km altitude, with charge densities exceeding the primary ionization peak densities by a factor ≥2 in the terminator and nightside ionosphere (ne/ni ≤ 0.1). We suggest that ion‐ion (dusty) plasma may also be present in the dayside ionosphere below 900 km (ne/ni < 0.5 at 1000 km altitude). The average charge of the dust grains (≥1000 amu) is estimated to be between −2.5 and −1.5 elementary charges, increasing toward lower altitudes.
Key Points
Detection of electron‐depleted dusty ion‐ion plasma in lower ionosphere with enhanced densities
First empirical estimate of the negative ion and dust grain charge
We present radio and plasma wave science (RPWS) Langmuir probe (LP) observations that give evidence for a population of heavy, negative ions at altitudes below 900 km in Titan's ionosphere during the ...Cassini T70 flyby. The negative ion density in this region is comparable to, or higher than, the electron density of 760 cm−3. Both positive and negative ions are moving with a velocity of at least a few hundred m s−1 relative to Titan. We show two limiting cases where we have analysed RPWS/LP ion measurements. The data can be interpreted as either that a population of negative ions with density comparable to the electron density is present, moving at a very high (>2 km s−1) velocity, or that the ion population is moving at a few hundred m s−1, but with a density an order of magnitude larger than the electron density in the same region.
Key Points
We have found negative ions in the deep (< 900 km) ionosphere of Titan
The ions have a density in the range of around 1000 to more than 10 000 cm3/Z
The ions are moving at high velocities
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