Context.
Solar wind charge-changing reactions are of paramount importance to the physico-chemistry of the atmosphere of a comet. The ESA/Rosetta mission to comet 67P/Churyumov-Gerasimenko (67P) ...provides a unique opportunity to study charge-changing processes in situ.
Aims.
To understand the role of these reactions in the evolution of the solar wind plasma and interpret the complex in situ measurements made by Rosetta, numerical or analytical models are necessary.
Methods.
We used an extended analytical formalism describing solar wind charge-changing processes at comets along solar wind streamlines. The model is driven by solar wind ion measurements from the Rosetta Plasma Consortium-Ion Composition Analyser (RPC-ICA) and neutral density observations from the Rosetta Spectrometer for Ion and Neutral Analysis-Comet Pressure Sensor (ROSINA-COPS), as well as by charge-changing cross sections of hydrogen and helium particles in a water gas.
Results.
A mission-wide overview of charge-changing efficiencies at comet 67P is presented. Electron capture cross sections dominate and favor the production of He and H energetic neutral atoms (ENAs), with fluxes expected to rival those of H
+
and He
2+
ions.
Conclusions.
Neutral outgassing rates are retrieved from local RPC-ICA flux measurements and match ROSINA estimates very well throughout the mission. From the model, we find that solar wind charge exchange is unable to fully explain the magnitude of the sharp drop in solar wind ion fluxes observed by Rosetta for heliocentric distances below 2.5 AU. This is likely because the model does not take the relative ion dynamics into account and to a lesser extent because it ignores the formation of bow-shock-like structures upstream of the nucleus. This work also shows that the ionization by solar extreme-ultraviolet radiation and energetic electrons dominates the source of cometary ions, although solar wind contributions may be significant during isolated events.
We use data from the first two dayside seasons of the Magnetospheric Multiscale (MMS) mission to study current systems associated with quasi‐perpendicular bow shocks of generator type. We have ...analyzed 154 MMS bow shock crossings near the equatorial plane. We compute the current density during the crossings and conclude that the component perpendicular to the shock normal (J⊥) is consistent with a pileup of the interplanetary magnetic field (IMF) inside the magnetosheath. For predominantly southward IMF, we observe a component Jn parallel (antiparallel) to the normal for GSM Y > 0 (<0), and oppositely directed for northward IMF. This indicates current closure across the equatorial magnetosheath, and it is observed for IMF clock angles near 0∘ and 180∘. To our knowledge, these are the first observational evidence for bow shock current closure across the magnetosheath. Since we observe no clear signatures of |J⊥| decreasing toward large |Y| we suggest that the main region of current closure is further tailward, outside MMS probing region. For IMF clock angles near 90∘, we find indications of the current system being tilted toward the north‐south direction, obtaining a significant Jz component, and we suggest that the current closes off the equatorial plane at higher latitudes where the spacecraft are not probing. The observations are complicated for several reasons. For example, variations in the solar wind and the magnetospheric currents and loads affect the closure, and Jn is distributed over large regions, making it difficult to resolve inside the magnetosheath proper.
Key Points
These are the first observations of bow shock current closure across the magnetosheath and toward the magnetosphere
For large IMF |Bz|, there are indications of current closure in the equatorial plane
For large IMF |By|, the current system is tilted and closure may occur at higher latitudes
The plasma around comet 67P/Churyumov–Gerasimenko showed remarkable variability throughout the entire Rosetta mission. Plasma boundaries such as the diamagnetic cavity, solar wind ion cavity and ...infant bow shock separate regions with distinct plasma parameters from each other. Here, we focus on a particular feature in the plasma: warm, slow solar wind protons. We investigate this particular proton population further by focusing on the proton behaviour and surveying all of the Rosetta comet phase data. We find over 300 events where Rosetta transited from a region with fast, cold protons into a region with warm, slow protons. We investigate the properties of the plasma and magnetic field at this boundary and the location where it can be found. We find that the protons are preferentially detected at intermediate gas production rates with a slight trend towards larger cometocentric distances for higher gas production rates. The events can mostly be found in the positive convective electric field hemisphere. These results agree well with simulations of the infant bow shock (IBS), an asymmetric structure in the plasma environment previously detected on only 2 d during the comet phase. The properties of the plasma on both sides of this structure are harder to constrain, but there is a trend towards higher electron flux, lower magnetic field, higher magnetic field power spectral density and higher density in the region that contains the warm protons. This is in partial agreement with the previous IBS definitions; however, it also indicates that the plasma and this structure are highly non-stationary. For future research, Comet Interceptor, with its multi-point measurements, can help to disentangle the spatial and temporal effects and give more clarity on the influence of changing upstream conditions on the movement of boundaries in this unusual environment.
Plasma dynamics in Earth's magnetotail is often studied using moments of the distribution function, which results in losing information on the kinetic properties of the plasma. To better understand ...oscillatory flows observed in the midtail plasma sheet, we investigate two events, one in each hemisphere, in the transition region between the central plasma sheet and the lobes using the 2‐D ion distribution function from the Cluster 4 spacecraft. In this case study, the oscillatory flows are a manifestation of repeated ion flux enhancements with pitch angle changing from 0° to 180° in the Northern Hemisphere and from 180° to 0° in the Southern Hemisphere. Similar pitch angle signatures are observed seven times in about 80 min for the Southern Hemisphere event and three times in about 80 min for the Northern Hemisphere event. The ion flux enhancements observed for both events are slightly shifted in time between different energy channels, indicating a possible time‐of‐flight effect from which we estimate that the source of particle is located ∼5–25RE and ∼40–107RE tailward of the spacecraft for the Southern and Northern Hemisphere event, respectively. Using a test particle simulation, we obtain ∼21–46 RE for the Southern Hemisphere event and tailward of X∼ − 65RE (outside the validity region of the model) for the Northern Hemisphere event. We discuss possible sources that could cause the enhancements of ion flux.
Key Points
Oscillatory flows can be due to repeated particle flux enhancements with clear pitch angle patterns
The pitch angle of the flux enhancements changes from 0 to 180 degrees in the Northern Hemisphere and from 180 to 0 degrees in the Southern Hemisphere
The particles associated with the flux enhancements originate from a source tailward of the spacecraft
Abstract
We report the first in situ detection of halogens in a cometary coma, that of 67P/Churyumov-Gerasimenko. Neutral gas mass spectra collected by the European Space Agency's Rosetta spacecraft ...during four periods of interest from the first comet encounter up to perihelion indicate that the main halogen-bearing compounds are HF, HCl and HBr. The bulk elemental abundances relative to oxygen are ∼8.9 × 10−5 for F/O, ∼1.2 × 10−4 for Cl/O and ∼2.5 × 10−6 for Br/O, for the volatile fraction of the comet. The cometary isotopic ratios for 37Cl/35Cl and 81Br/79Br match the Solar system values within the error margins. The observations point to an origin of the hydrogen halides in molecular cloud chemistry, with frozen hydrogen halides on dust grains, and a subsequent incorporation into comets as the cloud condensed and the Solar system formed.
Magnetosheath jets at Mars Gunell, Herbert; Hamrin, Maria; Nesbit-Östman, Sara ...
Science advances,
06/2023, Volume:
9, Issue:
22
Journal Article
Peer reviewed
Open access
Plasma entities, known as magnetosheath jets, with higher dynamic pressure than the surrounding plasma, are often seen at Earth. They generate waves and contribute to energy transfer in the ...magnetosheath. Affecting the magnetopause, they cause surface waves and transfer energy into the magnetosphere, causing throat auroras and magnetic signatures detectable on the ground. We show that jets exist also beyond Earth's environment in the magnetosheath of Mars, using data obtained by the MAVEN spacecraft. Thus, jets can be created also at Mars, which differs from Earth by its smaller bow shock, and they are associated with an increased level of magnetic field fluctuations. Jets couple large and small scales in magnetosheaths in the solar system and can play a similar part in astrophysical plasmas.
Aims.
We wish to investigate the energy conversion between particles and electromagnetic fields and determine the location where it occurs in the plasma environment of comets.
Methods.
We used a ...hybrid plasma model that included photoionization, and we considered two cases of the solar extreme ultraviolet flux. Other parameters corresponded to the conditions of comet 67P/Churyumov–Gerasimenko at a heliocentric distance of 1.5 AU.
Results.
We find that a shock-like structure is formed upstream of the comet and acts as an electromagnetic generator, similar to the bow shock at Earth that slows down the solar wind. The Poynting flux transports electromagnetic energy toward the inner coma, where newly born cometary ions are accelerated. Upstream of the shock-like structure, we find local energy transfer from solar wind ions to cometary ions. We show that mass loading can be a local process with a direct transfer of energy, but also part of a dynamo system with electromagnetic generators and loads.
Conclusions.
The energization of cometary ions is governed by a dynamo system for weak ionization, but changes into a large conversion region with local transfer of energy directly from solar wind protons for high ionization.
Ion escape is of particular interest for studying the evolution of the atmosphere on geological timescales. Previously, using Cluster-CODIF data, we investigated the oxygen ion outflow from the ...plasma mantle for different solar wind conditions and geomagnetic activity. We found significant correlations between solar wind parameters, geomagnetic activity (K.sub.p index), and the O.sup.+ outflow. From these studies, we suggested that O.sup.+ ions observed in the plasma mantle and cusp have enough energy and velocity to escape the magnetosphere and be lost into the solar wind or in the distant magnetotail. Thus, this study aims to investigate where the ions observed in the plasma mantle end up. In order to answer this question, we numerically calculate the trajectories of O.sup.+ ions using a tracing code to further test this assumption and determine the fate of the observed ions. Our code consists of a magnetic field model (Tsyganenko T96) and an ionospheric potential model (Weimer 2001) in which particles initiated in the plasma mantle region are launched and traced forward in time. We analysed 131 observations of plasma mantle events in Cluster data between 2001 and 2007, and for each event 200 O.sup.+ particles were launched with an initial thermal and parallel bulk velocity corresponding to the velocities observed by Cluster. After the tracing, we found that 98 % of the particles are lost into the solar wind or in the distant tail. Out of these 98 %, 20 % escape via the dayside magnetosphere.
Context. Solar wind charge-changing reactions are of paramount importance to the physico-chemistry of the atmosphere of a comet because they mass-load the solar wind through an effective conversion ...of fast, light solar wind ions into slow, heavy cometary ions. The ESA/Rosetta mission to comet 67P/Churyumov-Gerasimenko (67P) provided a unique opportunity to study charge-changing processes in situ. Aims. To understand the role of charge-changing reactions in the evolution of the solar wind plasma and to interpret the complex in situ measurements made by Rosetta, numerical or analytical models are necessary. Methods. An extended analytical formalism describing solar wind charge-changing processes at comets along solar wind streamlines is presented. It is based on a thorough book-keeping of available charge-changing cross sections of hydrogen and helium particles in a water gas. Results. After presenting a general 1D solution of charge exchange at comets, we study the theoretical dependence of charge-state distributions of (He2+, He+, He0) and (H+, H0, H−) on solar wind parameters at comet 67P. We show that double charge exchange for the He2+−H2O system plays an important role below a solar wind bulk speed of 200 km s−1, resulting in the production of He energetic neutral atoms, whereas stripping reactions can in general be neglected. Retrievals of outgassing rates and solar wind upstream fluxes from local Rosetta measurements deep in the coma are discussed. Solar wind ion temperature effects at 400 km s−1 solar wind speed are well contained during the Rosetta mission. Conclusions. As the comet approaches perihelion, the model predicts a sharp decrease of solar wind ion fluxes by almost one order of magnitude at the location of Rosetta, forming in effect a solar wind ion cavity. This study is the second part of a series of three on solar wind charge-exchange and ionization processes at comets, with a specific application to comet 67P and the Rosetta mission.
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