Herbert Gunell

The present study reconstructs the evolution of Earth's oxygen escape into space since the Great Oxidation Event, approximately 2450 Myr ago. We use a semi‐empirical model based on Gunell et al. (2018, https://doi.org/10.1051/0004‐6361/201832934 ) and further develop it to account for the evolution of solar wind and solar extreme ultraviolet (EUV) radiation. Seven distinct oxygen escape mechanisms are considered, and a time‐dependent escape rate is computed for each. The results show that Earth remained in a magnetized escape regime throughout the last 2450 Myr. Oxygen loss has remained dominated by nonthermal ion escape, mainly through cusp escape and polar wind. Presently, cusp escape dominates. Earlier than ∼1300 Myr ago, polar wind was the primary contributor. Oxygen ion escape is mainly driven by the solar wind and EUV radiation, limited by the ionospheric supply, and moderated by the Earth's magnetic field. Since the solar wind and solar EUV radiation were more intense in the past, the oxygen escape rate was more than an order of magnitude higher than the current value, exceeding . Over the last 2450 Myr this corresponds to a loss of ∼60% of the Earth's current atmospheric oxygen mass to space and only 0.06% of the present ocean mass. Over geological timescales, the persistence of Earth's atmospheric oxygen reservoir against escape to space is maintained by the combined effects of gravity, a strong planetary magnetic field, and a limited ionospheric supply.

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.

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.

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.

The magnetopause marks the boundary where the pressure of the magnetosheath balances the magnetic pressure of Earth's magnetic field. Localized increases in magnetosheath pressure can therefore displace the magnetopause. One known plasma structure that exhibits such a local pressure increase is magnetosheath jets defined as transient dynamic pressure enhancements. These jets are particularly prominent downstream of the quasi-parallel bow shock, where they frequently interact with the magnetopause. To investigate the spatial and temporal characteristics of these interactions, we employ the 3D hybrid-kinetic plasma model Amitis with a steady plasma inflow. Our results show that jets downstream of the quasi-parallel bow shock can generate magnetopause surface waves that propagate flankward. Interestingly, while jets appear to have a similar velocity as the surface waves and continuously drive them, the jets apparent flankward motion does not align with the ion bulk velocity measured within the jets. This discrepancy may suggest ongoing jet generation at the bow shock, potentially linked to foreshock compressive structures. Additionally, we find that jets might also contribute to triggering the development of the Kelvin-Helmholtz instability, leading to the formation of vortices that extend further tailward.

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.

Context . The solar wind affects the plasma environment around all Solar System bodies. A strong solar wind dynamic pressure pushes plasma boundaries closer to these objects. For small objects, kinetic effects on scales smaller than an ion gyroradius play an important role and species with various mass-per-charge may act differently. In this case, the solar wind composition can be important. Aims . Protons are the dominant ion species in the solar wind, however, the density of alpha particles can sometimes increase significantly. We analyse the effect of different solar wind alpha-to-proton ratios on the plasma boundaries of a weakly outgassing comet ( Q ≈ 1 − 2 × 10 ²⁶ s ⁻¹ ). In addition, we investigate the energy transfer between the solar wind ions, the cometary ions, and the electromagnetic fields. Methods . Using the hybrid model Amitis, we simulated two different alpha-to-proton ratios and analysed the resulting plasma structures. We calculated the power density ( E ∙ J ) of all three ion species (solar wind protons, alphas, and cometary ions) to identify load and generator regions. The integrated 1D power density shows the evolution of the power density from the upstream solar wind to downstream of the nucleus. Results . A higher alpha-to-proton ratio leads to a larger comet magnetosphere, but weaker magnetic field pile-up. The protons transfer energy to the fields and the cometary ions in the entire upstream region and the pile-up layer. Upstream of the nucleus, alphas are inefficient in transferring energy and can act as a load, especially for low alpha-to-proton ratios. The transfer of energy from alphas to cometary ions happens further downstream due to their larger inertia. Conclusions . For a multi-species solar wind, the mass loading and energy transfer upstream of the pile-up layer will be most efficient for the species with the lowest inertia. Protons represent such a species, and different gyroradii of the ions result in distinct flow patterns for each individual species.

Magnetosheath jets, plasma structures with enhanced dynamic pressure, are frequently observed in the terrestrial magnetosheath. However, their mass, momentum, and energy content are still unknown. We utilize Amitis, a 3D hybrid‐kinetic plasma simulation, to study the mass, momentum, and energy content of jets in the subsolar magnetosheath. We also analyze the kinetic, thermal, and electromagnetic energy flux associated with jets. Jets comprise up to 21% of the quasi‐parallel magnetosheath and can carry up to half of the kinetic energy. Furthermore, jets convert kinetic energy to thermal energy. Our hybrid simulations also suggest that while jets can form downstream of the quasi‐perpendicular shock, their volume and energy content are much small compared to jets downstream of the quasi‐parallel bow shock. We conclude that magnetosheath jets play a vital role in heating up the magnetosheath and significantly influence the dynamics of the quasi‐parallel magnetosheath.