Charlotte Goetz

In the cometary plasma environment, the solar wind interacts with the expanding coma of heavy cometary ions. At high gas production rates, it can have plasma environment characteristics similar to the environment of Mars, with a bow shock and a cometosheath. Magnetosheath jets are transient enhancements in the dynamic pressure of the magnetosheath plasma. They can be generated at a disturbed bow shock surface and have been confirmed at Mars, Earth, and possibly Jupiter and Mercury. We have adapted commonly used methods to find magnetosheath jets to the Rosetta data at 67P/Churyumov–Gerasimenko and find candidate events throughout the postperihelion mission. We show that the identified density enhancements have similarities to magnetosheath jets. The events are of typical duration and density enhancement, and occur at a time when the bow shock is thought to be close to the spacecraft. We found a cometary ion velocity enhancement, although no conclusion on the solar wind velocity can be reached due to lack of data. These jet-like features are embedded in the heavy-ion population. We hypothesize that jets are formed in the solar wind ion population in the cometosheath, and then, as the two plasmas interact, are transferred to the cometary ion population. While we cannot rule out other phenomena that could cause these jet-like enhancements, they bear a remarkable resemblance to magnetosheath jets at Mars and Earth. If they are indeed jets, then this is the first time jets have been shown to exist in a plasma environment dominated by heavy ions.

Introduction: Comet Interceptor [1] is a joint space mission of the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA). Its primary goals are to provide the first-ever in-situ characterisation of a Long Period Comet (LPC), which could be a dynamically-new comet or an interstellar object, and to perform the first simultaneous multi-point exploration of a cometary coma and nucleus.Comet Interceptor is the first rapid response mission [2]. The mission will be launched in piggy back mode between the second semester of 2028 and the first semester of 2029 on Ariane 6, towards the Sun-Earth Lagrange point L2, where it will wait for its target comet passage.Comet Interceptor consists of one main spacecraft (S/C A), and two deployable probes, named Probe B1 and Probe B2, which are provided by JAXA and ESA, respectively, allowing unique and simultaneous observations of the target from different geometries. The S/C A is devoted to the physical characterisation of the cometary nucleus and coma via remote sensing and in situ observations during the selected target fly-by.Science Objectives: The mission will investigate the processes of planetesimal formation and disentangle primordial versus evolutionary processes by comparing the composition and physical properties of a pristine LPC with those of the Short Period Comets (SPCs) previously investigated by space missions, in particular by the Rosetta mission which explored comet 67P for more than two years. Specifically, the objectives of Comet Interceptor are:1) Comet Nucleus Science: What is the surface composition, shape, morphology, and structure of the target object?2) Comet Environment Science: What is the composition of the coma, its connection to the nucleus (activity) and the nature of its interaction with the solar wind?Science Activities: The search for the target comet is underway, and preparations are being made for the scientific exploitation of the data from the mission’s three spacecraft.The selection and scientific investigations of the target comet, as well as the development of the mission instruments and science operation areas, are supported by Working Groups (WGs). These are the Science Operation WG, Target Identification WG, and Comet Environment WG. The latter comprises three sub-WGs, covering the Comet Nucleus, Near-Environment (inner dust and gas coma), and Far-Environment (outer dust and gas coma and tails) topics. The specific tasks of the Comet Environment WG are to develop and provide results from scientific models addressing questions relevant to the implementation and operation of the mission and to achieve its scientific objectives.Here, we provide a brief overview of the mission, and we present and describe the aims and activities of the working groups over the past year. One of the recent activities of all the Comet Environment sub-working groups is the preparation of a large repository of numerical models to favour the successful planning for scientific operations and data interpretation. Other activities include the analysis of available data on comets to better understand the environment expected during the encounter and establish base line parameters for target selection.A list of virtual LPC targets has been established and these targets are under evaluation. Additional LPCs are expected to be detected in the near future thanks to ground-based survey, notably from the Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST) [3]. These are going to be considered as proxy to inquire on the:1) Nucleus science: (i) measure the size, shape, and rotation rate of the target comet nucleus; (ii) return resolved images of the surface that reveal its morphology: (iii) constrain the nucleus composition directly via remote sensing observations (imaging, spectroscopy, and polarimetry).2) Environment science: (i) map the bulk neutral composition of the coma and determine any local structure and connection to the nucleus ices distributions and active areas; (ii) derive coma isotopic composition by mapping the distribution of neutral gasses and their relation to nucleus inhomogeneity; (iii) characterise the structure of the coma dust environment and determine any connection to the nucleus; (iv) characterise coma dust properties, and determine dust fluxes; (v) analyse motion and evolution of ion rays and other coma and tail features (i.e., dust and gas); (vi) characterise the plasma environment around the target, determine any resulting boundaries and assess energy, mass, and momentum transfer. References: [1] Jones, G.H. et al. (2024) Space Sci. Rev. 220, 9. [2] Snodgrass, C. & Jones, G.H. (2019) Nat. Comm. 10, 5418. [3] Inno, L., et al. (2025) Icarus 429, 116443.

Context. Rosetta followed comet 67P at heliocentric distances from 1.25 to 3.6 au. Close to perihelion, Rosetta was located in the solar wind ion cavity, with only sporadic observations of solar wind ions. Just outside the solar wind ion cavity, the solar wind ion-flow direction was mainly sunward. Aims. We aim to study the evolution of solar wind interaction with a comet as its scale size increases from sub-ion gyroradius to above the proton gyroradius. The former was observed at 67P during most of the Rosetta mission. The latter was observed during sporadic solar wind encounters around perihelion. Methods. We analysed particle data from the mass-resolving ion spectrometer ICA, searching for the weak and sporadic occurrences of solar wind ions close to perihelion. All Rosetta plasma data were used to understand the plasma parameter regime of the observed interaction. Results. At comet 67P, we observe the transition from solar wind interaction during the first partial gyration of the solar wind ions in the slowed-down plasma around the comet to multiple gyrations in an interaction region larger than the proton gyroradius. During the multiple-gyration stage, the protons show a consistent pattern of motion ordered by the direction of the solar wind electric field. In the hemisphere where the electric field points away from the nucleus, the protons move sunward. They move anti-sunward in the opposite hemisphere. This can be explained by simple trochoid trajectories and a strong density gradient. Under the same conditions, the water ions also show a consistent flow pattern relative to the electric field determined by the point in their gyration at which they pass closest to the nucleus. Protons are slowed down and heated, which is consistent with a shock upstream. The plasma beta is close to one, and the magnetosonic Mach number was typically less than unity.

Wave activity in the cometary plasma environment occurs across various levels of cometary activity, from large heliocentric distances to perihelion. These waves play a key role in the thermalisation of cometary pick-up ions and energy redistribution. Starting thousands of kilometres upstream of the nucleus, the gyrating motion of solar wind and cometary ions produces highly anisotropic velocity distributions, which can drive various wave phenomena.In this study, we employ the 3D hybrid particle simulation code Amitis to model a cometary magnetosphere at approximately Mars’ distance in the solar wind, assuming an outgassing rate of Q ≈ 1027 s-1. The simulations reveal large-scale wave structures extending from far upstream of the comet nucleus to downstream of the bow shock. Wave signatures are most pronounced in the +E hemisphere and near the quasi-parallel bow shock, while the −E hemisphere is dominated by magnetic field pile-up. In the inner magnetosphere, where cometary ions dominate, waves are absent. Magnetic field peaks and solar wind density enhancements are out of phase—characteristics consistent with slow magnetosonic waves.These waves amplify solar wind density, increasing dynamic pressure and potentially contributing to the formation of magnetosheath jets. Our simulations indicate that magnetosheath jets—similar to those observed at Earth—can also occur at comets. We explore the role of waves for the generation of magnetosheath jets within cometary magnetospheres.

The MESSENGER magnetometer data for the years 2012 through 2015 are used to study the presence of mirror modes in Mercury's environment. Using magnetic field only detection of these structures, 10715 intervals of 20 s and longer are found in the solar wind, and in the magnetosheath and magnetosphere of Mercury. The distribution of mirror modes around the planet is similar to that found at Venus and Mars, however, the occurrence rate of 1.0E-4 per second is up to 2 orders of magnitude smaller than at the other two planets. The mirror modes are more prone to be excited near Mercury's aphelion. A comparison shows that mirror modes and magnetic holes have a similar behaviour around Mercury, and that about 40% of the mirror modes can also be identified as magnetic holes. Ion cyclotron waves show a lesser dependence on the true anomaly angle.

As comets journey through the solar system, ices on the surface sublimate and the released neutral gas is ionized. Thus, the comet nucleus is surrounded by a cloud of heavy ions and electrons that interact with the solar wind. Their environment therefore is a unique laboratory to study plasma pick-up processes and multi-ion plasmas. In turn, remote observations of comets with imaging telescopes on ground and in space provide information about the solar wind properties at the comet. Comets are therefore laboratory and measurement at the same time. Studying the impact of solar wind structures like corotating interaction regions and interplanetary coronal mass ejections can therefore inform our knowledge of the processes in a collisional plasma. Active comets have a highly disturbed bow shock that can give rise to a number of cometosheath structures, one of which are magnetospheric jets: regions of enhanced dynamic pressure in the magnetosheath. They are usually associated to the region behind a quasi-parallel shock and have been well documented at Earth.However, in recent years there have been efforts to identify these structures in other magnetosheaths as well. For example, recently it has been shown that they exist in the Martian magnetosheath. As the cometary plasma environment has very similar characteristics as the Martian one, it stands to reason that these structures also exist at Comets.We present a study that uses Rosetta magnetic field, density and ion measurements to identify possible jet structures.We find that they are ubiquitous especially in the plasma environment where Rosetta is probably in the cometosheath. Their occurrence matches what was found for similar events at Mars. The magnetic field can correlate or anti-correlate with the density enhancement, just like it is observed at Earth and Mars. Although velocity data is limited, at least some of the events show a velocity increase in the cometary ion population. This could be the first detection of a magnetosheath structure in the non-solar wind component of the plasma.

Many different kinds of waves occur in the ionised coma of a comet, and these waves span a wide range of temporal and spatial scales. For example, ESA's Rosetta spacecraft, which spent to years near comet 67P/Churyumov-Gerasimenko from 2014 to 2016, detected singing comet waves, steepened magnetosonic waves, ion Bernstein waves, ion acoustic waves, and lower hybrid waves.Using data from the Rosetta Plasma Consortium of the Rosetta mission, we present a study of waves near the electron plasma frequency that we identify as Langmuir waves (Gunell et al. 2025, doi:10.1051/0004-6361/202555043). These Langmuir waves were observed when the comet was near perihelion. During this period a diamagnetic cavity had developed around the nucleus, and outside this cavity steepened magnetosonic waves were observed. Significant Langmuir wave activity was detected only in the environment outside the diamagnetic cavity, where simultaneously the steepened magnetosonic waves were observed. We suggest a possible generation mechanism for the Langmuir waves and a scenario through which energy can be transferred from the large and slow scale of the steepened waves to the small and fast scales of the Langmuir waves.

Europe hosts a large and highly active community of scientists working in the broad domain of Heliophysics. This broad discipline addresses plasmas in the regions of space and atmosphere influenced by the Sun and solar wind. However, this community has historically been fragmented, both geographically and thematically, which has limited the potential for strategic coordination, collaboration, and growth. This has recently prompted a grass-roots community-building effort to foster communication and interactions within the European Heliophysics Community (EHC). This white paper outlines the motivation, priorities, and initial steps towards establishing the EHC, and presents a vision for the future of Heliophysics in Europe. As a crucial first step of this endeavour, a dedicated EHC website is now available: https://www.heliophysics.eu/ (last access: November 2025).

In the plasma environment of a comet, waves are generated on vastly different temporal and spatial scales. Wave observations were carried out during the cometary flybys in the 1980s and 1990s as well as by the Rosetta spacecraft which accompanied comet 67P/Churyumov-Gerasimenko between 2014 and 2016. Waves are thought to contribute to the transfer of energy in the ionised coma. One of the fundamental plasma waves observed in space is the Langmuir wave, which appears at or above the electron plasma frequency. The Mutual Impedance Probe of the Rosetta Plasma Consortium (RPC-MIP) recorded frequency spectra of electric field fluctuations in the cometary plasma, and we used these spectra in order to detect and identify Langmuir waves. Langmuir waves were found during the part of the Rosetta mission when the comet was less than 2.65-2.8 AU from the Sun. The Langmuir waves appear near, but always outside, the diamagnetic cavity boundary, in a region where, at much lower frequencies, steepened magnetosonic waves also are present.

We describe how the ESA Comet Interceptor mission, which is due to launch in 2028/29 to a yet-to-be-discovered target, can provide a conceptual basis for a future mission to visit an Interstellar Object. Comet Interceptor will wait in space until a suitable long period comet is discovered, allowing rapid response to perform a fast flyby of an object that will be in the inner Solar System for only a few years; an enhanced version of this concept could realistically provide the first in situ investigation of a visitor from another star system.