Undisturbed Solar Wind Research Articles

Turbulence development behind the bow shock during disturbed and undisturbed solar wind

Magnetosheath is a transition layer between the solar wind and the magnetosphere and may contribute to the geoeffectiveness of various large-scale interplanetary phenomena. In this paper, we examine the dynamics of the turbulent fluctuation spectra behind the bow shock during undisturbed solar wind and when interplanetary coronal mass ejections and corotation interaction regions interact with the magnetosphere. The study is based on statistical analysis of the turbulence features inside the magnetosheath at different distances from the bow shock. We demonstrate that the turbulence features change when plasma crosses the bow shock for the solar wind of all types and they usually recover when plasma moves away from the bow shock. However, peculiarities in the turbulence development occur during interplanetary coronal mass ejections. Moreover, during disturbed solar wind there are relations between the turbulence features at the sub-ion scales and background plasma parameters such as plasma parameter β, the angle θBN between the interplanetary magnetic field and the local bow shock normal, solar wind bulk velocity, and the distance to the magnetosheath boundaries.

Развитие турбулентности за околоземной ударной волной в периоды спокойного и возмущенного солнечного ветра

Magnetosheath is a transition layer between the solar wind and the magnetosphere and may contribute to the geoeffectiveness of various large-scale interplanetary phenomena. In this paper, we examine the dynamics of the turbulent fluctuation spectra behind the bow shock during undisturbed solar wind and when interplanetary coronal mass ejections and corotation interaction regions interact with the magnetosphere. The study is based on statistical analysis of the turbulence features inside the magnetosheath at different distances from the bow shock. We demonstrate that the turbulence features change when plasma crosses the bow shock for the solar wind of all types and they usually recover when plasma moves away from the bow shock. However, peculiarities in the turbulence development occur during interplanetary coronal mass ejections. Moreover, during disturbed solar wind there are relations between the turbulence features at the sub-ion scales and background plasma parameters such as plasma parameter β, the angle θBN between the interplanetary magnetic field and the local bow shock normal, solar wind bulk velocity, and the distance to the magnetosheath boundaries.

Helios 2 observations of solar wind turbulence decay in the inner heliosphere

Aims. A linear scaling of the mixed third-order moment of the magnetohydrodynamic (MHD) fluctuations is used to estimate the energy transfer rate of the turbulent cascade in the expanding solar wind. Methods. In 1976, the Helios 2 spacecraft measured three samples of fast solar wind originating from the same coronal hole, at different distances from the Sun. Along with the adjacent slow solar wind streams, these intervals represent a unique database for studying the radial evolution of turbulence in samples of undisturbed solar wind. A set of direct numerical simulations of the MHD equations performed with the Lattice-Boltzmann code FLAME was also used for interpretation. Results. We show that the turbulence energy transfer rate decays approximately as a power law of the distance and that both the amplitude and decay law correspond to the observed radial temperature profile in the fast wind case. Results from MHD numerical simulations of decaying MHD turbulence show a similar trend for the total dissipation, suggesting an interpretation of the observed dynamics in terms of decaying turbulence and that multi-spacecraft studies of the solar wind radial evolution may help clarify the nature of the evolution of the turbulent fluctuations in the ecliptic solar wind.

Large-Scale Solar Wind Phenomena Affecting the Turbulent Cascade Evolution behind the Quasi-Perpendicular Bow Shock

The Earth’s magnetosphere is permanently influenced by the solar wind. When supersonic and superalfvenic plasma flow interacts with the magnetosphere, the magnetosheath region is formed, which is filled with shocked turbulent plasma. Varying SW parameters influence the mechanisms of formation of this boundary layer, including the dynamics of turbulence behind the bow shock. The effect of the solar wind on the development of turbulence in the magnetosheath was demonstrated recently based on broad statistics of spacecraft measurements. The present study considers the multipoint observations of turbulent fluctuations in the solar wind, in the dayside magnetosheath and at the flanks, to analyze the evolution of the turbulent cascade while the solar wind plasma enters the magnetosheath. Observations of the magnetosheath behind the quasi-perpendicular bow shock are analyzed to exclude the influence of the bow shock topology from consideration. Three basic types of solar wind flows are considered: slow undisturbed solar wind, compressed regions, and interplanetary manifestations of coronal mass ejections. The results show surviving Kolmogorov scaling behind the bow shock for steady solar wind flow and amplification of the compressive fluctuations at the kinetic scales at the magnetosheath flanks for the solar wind associated with compressed plasma streams. During interplanetary manifestations of the coronal mass ejection, the spectra in the dayside magnetosheath substantially deviate from those observed in the solar wind (including the absence of Kolmogorov scaling and steepening at the kinetic scales) and restore at the flanks.

The Relationship Between Solar Wind Dynamic Pressure Pulses and Solar Wind Turbulence

Solar wind dynamic pressure pulses (DPPs) are small-scale plasma structures with abrupt and large-amplitude plasma dynamic pressure changes on timescales of seconds to several minutes. Overwhelming majority of DPP events (around 79.13%) reside in large-scale solar wind transients, i.e., coronal mass ejections, stream interaction regions, and complex ejecta. In this study, the intermittency, which is a typical feature of solar wind turbulence, is determined and compared during the time intervals in the undisturbed solar wind and in large-scale solar wind transients with clustered DPP events, respectively, as well as in the undisturbed solar wind without DPPs. The probability distribution functions (PDFs) of the fluctuations of proton density increments normalized to the standard deviation at different time lags in the three types of distinct regions are calculated. The PDFs in the undisturbed solar wind without DPPs are near-Gaussian distributions. However, the PDFs in the solar wind with clustered DPPs are obviously non-Gaussian distributions, and the intermittency is much stronger in the large-scale solar wind transients than that in the undisturbed solar wind. The major components of the DPPs are tangential discontinuities (TDs) and rotational discontinuities (RDs), which are suggested to be formed by compressive magnetohydrodynamic (MHD) turbulence. There are far more TD-type DPPs than RD-type DPPs both in the undisturbed solar wind and large-scale solar wind transients. The results imply that the formation of solar wind DPPs could be associated with solar wind turbulence, and much stronger intermittency may be responsible for the high occurrence rate of DPPs in the large-scale solar wind transients.

Mars’ plasma system. Scientific potential of coordinated multipoint missions: “The next generation”

The objective of this White Paper, submitted to ESA’s Voyage 2050 call, is to get a more holistic knowledge of the dynamics of the Martian plasma system, from its surface up to the undisturbed solar wind outside of the induced magnetosphere. This can only be achieved with coordinated multi-point observations with high temporal resolution as they have the scientific potential to track the whole dynamics of the system (from small to large scales), and they constitute the next generation of the exploration of Mars analogous to what happened at Earth a few decades ago. This White Paper discusses the key science questions that are still open at Mars and how they could be addressed with coordinated multipoint missions. The main science questions are: (i) How does solar wind driving impact the dynamics of the magnetosphere and ionosphere? (ii) What is the structure and nature of the tail of Mars’ magnetosphere at all scales? (iii) How does the lower atmosphere couple to the upper atmosphere? (iv) Why should we have a permanent in-situ Space Weather monitor at Mars? Each science question is devoted to a specific plasma region, and includes several specific scientific objectives to study in the coming decades. In addition, two mission concepts are also proposed based on coordinated multi-point science from a constellation of orbiting and ground-based platforms, which focus on understanding and solving the current science gaps.

Atmospheric Loss of Atomic Oxygen during Proton Aurorae on Mars

—For the first time, the calculations of the penetration of protons of the undisturbed solar wind into the daytime atmosphere of Mars due to charge exchange in the extended hydrogen corona (Shematovich et al., 2021) are used allowing us to determine self-consistently the sources of suprathermal oxygen atoms, as well as their kinetics and transport. An additional source of hot oxygen atoms—collisions accompanied by the momentum and energy transfer from the flux of precipitating high-energy hydrogen atoms to atomic oxygen in the upper atmosphere of Mars—was included in the Boltzmann kinetic equation, which was solved with the Monte-Carlo kinetic model. As a result, the population of the hot oxygen corona of Mars has been estimated; and it has been shown that the proton aurorae are accompanied by the atmospheric loss of atomic oxygen, which is evaluated within a range of (3.5–5.8) × 107 cm–2 s–1. It has been shown that the exosphere becomes populated with a substantial amount of suprathermal oxygen atoms with kinetic energies up to the escape energy, 2 eV. The atomic oxygen loss rate caused by a sporadic source in the Martian atmosphere—the precipitation of energetic neutral atoms of hydrogen (H‑ENAs) during proton aurorae at Mars—was estimated by the self-consistent calculations according to a set of the Monte-Carlo kinetic models. These values turned out be comparable to the atomic oxygen loss supported by a regular source—the exothermic photochemical reactions (Groeller et al., 2014; Jakosky et al., 2018). It is currently supposed that the atmospheric loss of Mars due to the impact of the solar wind plasma and, in particular, the fluxes of precipitating high-energy protons and hydrogen atoms during solar flares and coronal mass ejections may play an important role in the loss of the neutral atmosphere on astronomic time scales (Jakosky et al., 2018).

Foreshock as a Source Region of Electron-scale Magnetic Holes in the Solar Wind at 1 au

Abstract Electron-scale magnetic holes (ESMHs) can dissipate energy and transport electrons in astrophysical plasmas. They are often observed in the solar wind at 1 au, but whether they are locally generated remains unclear. Here we investigate the ESMHs in the solar wind at 1 au based on observations of the Magnetospheric Multiscale (MMS) and Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon’s Interaction with the Sun (ARTEMIS) P1 spacecraft. There are 47 and 18 ESMH events observed by MMS1 and ARTEMIS P1, respectively, where an isolated ESMH or a train of ESMHs is regarded as an ESMH event. Our findings are as follows: (1) the occurrence rate of the ESMH events observed by MMS1 is much larger than that by ARTEMIS P1, which is located in the upstream solar wind; and (2) the proportion of the train of ESMHs in the ESMH events observed by each spacecraft is much higher in the ion foreshock than that in the undisturbed solar wind. These observations suggest that the terrestrial foreshock is an important source region of the ESMHs at 1 au, while some ESMHs come from the upstream undisturbed solar wind. The generation of these ESMHs can be explained by the electron vortex magnetic hole. Finding out the origin of the electron vortex may help to shed light on the whole chain of their generation and evolution in the foreshock.

Effect of Variations in the Extended Hydrogen Corona of Mars on the Efficiency of Charge Exchange with Solar Wind Protons

This paper presents the results of calculating the efficiency of solar wind proton charge exchange as a function of variations in the column density of hydrogen atoms in the extended corona of Mars. The presence of regular density variations has been known for a long time and has been associated with the change of seasons on Mars. However, sporadic density variations in the upper Martian atmosphere were detected recently as well. They were caused by various processes in the underlying atmosphere, such as ejection of water vapor and ice particles at altitudes up to 100 km due to global dust storms, which was discovered in the Mars Express and MAVEN observations. Taking into account the variations in the column density with the observed amplitude up to one order of magnitude is absolutely necessary to correctly consider the interaction of solar wind protons with the Martian atmosphere. The calculations using the kinetic Monte Carlo model revealed that with a 2- and 5-fold increase in the column density of H atoms in the corona of Mars, the charge exchange efficiency also increases and reaches 6% and 8%, respectively, as compared to the base value of 4%. The energy spectrum of hydrogen atoms penetrating into the Martian atmosphere does not change and remains identical in structure to the spectrum of undisturbed solar wind protons. These estimates, combined with the previously developed kinetic model of the proton and hydrogen atom precipitation into the planetary atmosphere, make it possible to do the following: to trace all the stages of the penetration of the undisturbed solar wind protons into the dense layers of the atmosphere, as well as interpret the observed characteristics of proton auroras depending on the variations of atomic hydrogen content in the Martian corona.

On the direction of the velocity and magnetic field fluctuations in undisturbed solar wind

Abstract Measurements of velocity and magnetic field in near-Earth heliosphere is analized in order to investigate systematical deflection from transversality of the velocity and magnetic field fluctuations in undisturbed solar wind. Fluctuations occurred in the meridional plain of heliosphere (RN plain of the RTN reference system) are transversal with respect to mean magnetic field during periods of high solar activity, but they become non-transversal close to solar cycle minima. This phenomenon is investigated focusing on a role of Alfvén waves. It is shown that deflections from transversality is mostly expressed by fluctuations in slow solar wind streams with low contribution of Alfvén waves, whereas strongly Alfvénic turbulence undergo such deflection in a less degree. In addition, we consider orientation of velocity fluctuations in the azimuthal (RT) plain of heliosphere, which also indicates some interesting features.

Characteristics of Turbulent Solar Wind Flow in Plasma Compression Regions

This work is devoted to the study of properties of spectra of turbulent solar wind fluctuations in plasma compression regions, such as CIR (the plasma compression regions in front of high-speed streams from coronal holes) and SHEATH (the compression regions in front of interplanetary manifestations of coronal mass ejections EJECTA and magnetic clouds MC). The spectra of ion flux fluctuations, on both magnetohydrodynamic and ion-kinetic scales, are considered on the basis of data from the BMSW spectrometer on the SPEKTR-R spacecraft with a high (up to 31 ms) time resolution. The comparison of turbulent characteristics in the plasma compression regions and in the undisturbed solar wind is carried out both on a separate example and using extensive statistical material. It is shown in the paper that the turbulent cascade characteristics on the kinetic interval can significantly change in the plasma compression regions, and the signatures of a change in the main processes that determine the energy dissipation are revealed in them, which may cause increased heating in the regions under consideration.

Features of the Dynamics of a Shock Excited by Fast Coronal Mass Ejections

The event on January 27, 2012 is an example of the features of the dynamics of formation of a shock, excited by a fast coronal mass ejection (CME) at a velocity higher than 2000 km/s, are investigated. The following data were used: (a) images of the Sun in the UV channel of 13.1 nm from the AIA instrument at distances of 1.0–1.4 solar radii from the Sun’s center, (b) the images of the white corona from the LASCO C2 and C3 coronographs at R ≈ 2.1–30 solar radii. Investigations indicated the validity of regularities, established earlier for the cases of relatively slow CMEs (at velocities <1500 km/s): (1) The shock front is formed, when the velocity of a leading part of a CME relative to the undisturbed solar wind becomes greater than the local Alfven velocity of wind, which corresponds to the phenomenon of “transition through the sound velocity” for magnetized plasma. (2) The change in the width of a shock front and, accordingly, in the energy dissipation mechanism occurs in a shock front from a “collisional” one, at distances from the Sun’s center lower than 10 solar radii, to a “collisionless” one, at distances greater than ten solar radii. During the event of January 27, 2012, it was possible to investigate in more detail the process of transition from the collisional to collisionless shock front. It was found that the longitudinal length of a shock front, excited by a fast CME at distances lower than 6 solar radii, increased almost 10 times as compared to a slower CME. One of the main causes for this increase, along with a high velocity, is the fact that the motion of CME has occurred in the plane of the belt of coronal streamers. It is concluded that, at distances lower than 6 solar radii, the shock front structure ahead of the CME is of “parallel” type; that is, the angle between the vector of an undisturbed magnetic field and the normal to the front was close to 0°.

Features of the Spectral Characteristics of Plasma Fluctuations in Different Large-Scale Streams of the Solar Wind

The turbulent characteristics of plasma fluctuations in the solar wind (SW) may substantially change depending on the SW conditions. Large-scale streams of different types, such as undisturbed slow solar wind, fast solar wind, interplanetary coronal mass ejection (EJECTA), magnetic cloud (MC), the compression region at the boundary between fast and slow streams (CIR), and the compression region in front of the EJECTA/MC (SHEATH), are usually characterized by specific plasma parameter values, which may influence turbulent cascade formation. In this paper, we analyze the properties of the spectra of ion flux fluctuations in the SW in the region of transition from the magnetohydrodynamic (MHD) scale to the kinetic one. The analysis is based on measurements carried out with the BMSW plasma spectrometer onboard the SPEKTR-R spacecraft with a high time resolution. The observational intervals inside the SW streams of different large-scale types are considered, and the main turbulence characteristics in these streams are compared. It was shown that the properties of the fluctuation spectra may strongly depend on the SW type; in particular, the spectrum of kinetic-scale fluctuations usually becomes much steeper inside the MC regions and the compression regions in front of them, as well as in the CIR. The characteristics of the fluctuation spectra on MHD scale are less dependent on the type of large-scale SW structures, and, on average, they correspond to the Kolmogorov spectra. However, it can be noted that the smallest spectral slopes are observed in the slow undisturbed solar wind, which is indicative of the differences from the traditional Kolmogorov model of the developed turbulence.

Some Problems of Identifying Types of Large-Scale Solar Wind and Their Role in the Physics of the Magnetosphere: 2

This work is a continuation of paper [1], in which we discussed some incorrect approaches to the identification of large-scale types of solar wind and associated incorrect conclusions in the analysis of solar–terrestrial physics data. In this paper, we analyze the lists of 28 events of coronal mass ejection (CME) and 31 events of corotating interaction region (CIR) in 2013–2016 used by Shen et al. [2] to compare the responses of the Earth’s radiation belts to various interplanetary drivers. The interpretation of solar wind types in these lists differs both from our catalog [3] for Sheath, ICME, and CIR and from the catalog by Richardson and Cane [4] for CME. In addition, the authors of paper [2] do not distinguish the Sheath- and ICME-induced magnetic storms and include them in the general type of CME-induced storms. Our analysis has shown that, among the 28 events of CME-induced storms mentioned, 16 events belong to Sheath, 2 events to MC, 4 events to Ejecta, 2 events to CIR, and 4 events to undisturbed solar wind with shocks. The catalog [4], which also does not distinguish the Sheath and ICME, contains 18 of the 28 events presented in paper [2]. Among 31 CIR events presented by the authors of paper [2], according to our analysis, 25 events belong to CIR, 2 events to Sheath, and 4 events to undisturbed solar wind. In catalog [4], one of the 31 CIR events from paper [2] is presented as CME. Since the properties of CIR and Sheath compression regions are close, the conclusions of the authors of paper [2] on the properties of CIR-induced storms are only slightly distorted by the incorrect identification of stream types, whereas the conclusions concerning the CME-induced storms, which more than by a half represent the Sheath-induced storms, seem incorrect to us.

Spectral variation of suprathermal protons associated with stream interaction regions: STEREO A/PLASTIC observations

Context. The observation of power-law spectra of suprathermal particles is typically associated with the occurrence of stream interaction regions (SIRs), indicating that these particles are accelerated close to the observer. However, recent observations have identified the existence of sunwards streaming particles at low suprathermal energies following SIRs. In addition, the observational evidence for turnover spectra in the low suprathermal energies has also been presented, suggesting that these particles might be accelerated at remote shocks and travel back to the Sun along the interplanetary magnetic field lines. Aims. We investigate the spectral evolution and variation of suprathermal protons from SIR to SIR as the observer moves from inside the compression regions of SIRs to the outside undisturbed solar wind regions away from the reverse shocks. Methods. The spectral analysis in the range from solar wind to suprathermal energies was based on proton data, which are obtained by the Plasma and Suprathermal Ion Composition instrument (PLASTIC) on the Solar Terrestrial Relations Observatory mission (STEREO). Results. All spectra in the compressed fast wind regions (F′ regions) of twelve SIRs exhibit power-law suprathermal tails. Six of them show clear turnover spectra at velocities below 2500 km s−1 in the undisturbed fast solar wind regions (F regions) following the compression regions, while the remaining six events exhibit continuous power-law spectra. Overall, the spectra at velocities higher than 2500 km s−1 harden in the F regions, consistent with previous observations.

Identifying Ultra Low Frequency Waves in the Lunar Plasma Environment Using Trajectory Analysisand Resonance Conditions.

Recent studies show that localized crustal magnetic fields on the lunar surface can reflect a significant portion of the incoming solar wind protons. These reflected ions can drive a wide range of plasma waves. It is difficult to determine the intrinsic properties of low-frequency waves with single-spacecraft observations, which can be heavily Doppler shifted. We describe a technique to combine trajectory analysis of reflected protons with the Doppler shift and resonance conditions to identify ultralow-frequency waves at the Moon. On 31 January 2014 plasma waves were detected by one of the Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) probes as it approached the lunar wake; these waves were not detected by the second ARTEMIS probe located upstream in the undisturbed solar wind. The observed waves had a frequency below the local ion cyclotron frequency and had right-hand circular polarization in the reference frame of the Moon. By solving the Doppler shift and the cyclotron resonance equations, we determined the conditions for reflected ions to excite the observed waves. Simulated trajectories of reflected ions correspond to ARTEMIS ion observations and support the hypothesis that reflected ions are the primary driver of the waves. By combining trajectory analysis with the resonance conditions, we identify scenarios where ions that satisfy the resonance conditions are present in the right location to generate the observed waves. Using this method, we can uniquely identify the observed waves as upstream propagating right-hand polarized waves, subject to the assumption that they are generated by cyclotron resonance with ions.

Energy–angle dispersion of accelerated heavy ions at 67P/Churyumov–Gerasimenko: implication in the mass-loading mechanism

The Rosetta spacecraft studied the comet 67P/Churyumov–Gerasimenko for nearly two years. The Ion Composition Analyzer instrument on board Rosetta observed the positive ion distributions in the environment of the comet during the mission. A portion of the comet's neutral coma is expected to get ionized, depending on the comet's activity and position relative to the Sun, and the newly created ions are picked up and accelerated by the solar wind electric field, while the solar wind flow is deflected in the opposite direction. This interaction, known as the mass-loading mechanism, was previously studied by comparing the bulk flow direction of both the solar wind protons and the accelerated cometary ions with respect to the direction of the magnetic and the convective solar wind electric field. In this study, we show that energy–angle dispersion is occasionally observed. We report two types of dispersion: one where the observed motion is consistent with ions gyrating in the local magnetic field and another where the energy–angle dispersion is opposite to that expected from gyration in the local magnetic field. Given that the cometary ion gyro-radius in the undisturbed solar wind magnetic and electric field is expected to be too large to be detected in this way, our observations indicate that the local electric field might be significantly smaller than that of the undisturbed solar wind. We also discuss how the energy–angle dispersion, which is not consistent with gyration, may occur due to spatially inhomogeneous densities and electric fields.

MAVEN measured oxygen and hydrogen pickup ions: Probing the Martian exosphere and neutral escape

AbstractSoon after the MAVEN (Mars Atmosphere and Volatile EvolutioN) spacecraft started orbiting Mars, the SEP (Solar Energetic Particle), SWIA (Solar Wind Ion Analyzer), and STATIC (Supra‐Thermal and Thermal Ion Composition) instruments on board the spacecraft detected planetary pickup ions. SEP can measure energetic (&gt;60 keV) oxygen pickup ions, the source of which is the extended hot oxygen exosphere of Mars. Model results show that these pickup ions originate from tens of Martian radii upstream of Mars and are energized by the solar wind motional electric field as they gyrate back toward Mars. SWIA and STATIC can detect both pickup oxygen and pickup hydrogen with energies below ~30 keV and created closer to Mars. In this study, data from the SEP, SWIA, and STATIC instruments containing pickup ion signatures are provided and model‐data comparisons are shown. During the times when MAVEN is outside the Martian bow shock and in the upstream undisturbed solar wind, the solar wind velocity measured by SWIA and the solar wind (or interplanetary) magnetic field measured by the MAG (magnetometer) instrument can be used to model pickup oxygen and hydrogen fluxes. By comparing measured pickup ion fluxes with model results, the Martian thermal hydrogen and hot oxygen neutral densities can be probed outside the bow shock, providing a helpful tool in constraining estimates of neutral oxygen and hydrogen escape rates. Our analysis reveals an order of magnitude density change with Mars season in the hydrogen exosphere, whereas the hot oxygen exosphere was found to remain steadier.

The atmosphere of comet 67P/Churyumov-Gerasimenko diagnosed by charge-exchanged solar wind alpha particles

Context. The ESA/Rosetta mission has been orbiting comet 67P/Churyumov-Gerasimenko since August 2014, measuring its dayside plasma environment. The ion spectrometer onboard Rosetta has detected two ion populations, one energetic with a solar wind origin (H+, He2+, He+), the other at lower energies with a cometary origin (water group ions such as H2O+). He+ ions arise mainly from charge-exchange between solar wind alpha particles and cometary neutrals such as H2O. Aims. The He+ and He2+ ion fluxes measured by the Rosetta Plasma Consortium Ion Composition Analyser (RPC-ICA) give insight into the composition of the dayside neutral coma, into the importance of charge-exchange processes between the solar wind and cometary neutrals, and into the way these evolve when the comet draws closer to the Sun. Methods. We combine observations by the ion spectrometer RPC-ICA onboard Rosetta with calculations from an analytical model based on a collisionless neutral Haser atmosphere and nearly undisturbed solar wind conditions. Results. Equivalent neutral outgassing rates Q can be derived using the observed RPC-ICA He+/He2+ particle flux ratios as input into the analytical model in inverse mode. A revised dependence of Q on heliocentric distance Rh in AU is found to be Rh -7.06Rh-7.06 between 1.8 and 3.3 AU, suggesting that the activity in 2015 differed from that of the 2008 perihelion passage. Conversely, using an outgassing rate determined from optical remote sensing measurements from Earth, the forward analytical model results are in relatively good agreement with the measured RPC-ICA flux ratios. Modelled ratios in a 2D spherically-symmetric plane are also presented, showing that charge exchange is most efficient with solar wind protons. Detailed cometocentric profiles of these ratios are also presented. Conclusions. In conclusion, we show that, with the help of a simple analytical model of charge-exchange processes, a mass-capable ion spectrometer such as RPC-ICA can be used as a remote-sensing instrument for the neutral cometary atmosphere. © 2016 ESO.

Scattering characteristics and imaging of energetic neutral atoms from the Moon in the terrestrial magnetosheath

AbstractWe study hydrogen energetic neutral atom (ENA) emissions from the lunar surface, when the Moon is inside the terrestrial magnetosheath. The ENAs are generated by neutralization and backscattering of incident protons of solar wind origin. First, we model the effect of the increased ion temperature in the magnetosheath (&gt;10 times larger than that in the undisturbed solar wind) on the ENA scattering characteristics. Then, we apply these models to ENA measurements by Chandrayaan‐1 and simultaneous ion measurements by Kaguya at the Moon, in the magnetosheath. We produce maps of the ENA scattering fraction, covering a region at the lunar near‐side that includes mare and highland surfaces and several lunar magnetic anomalies. We see clear signatures of plasma shielding by the magnetic anomalies. The maps are made at different lunar local times, and the results indicate an extended influence and altered morphology of the magnetic anomalies at shallower incidence angles of the magnetosheath protons. The scattering fraction from the unmagnetized regions remains consistent with that in the undisturbed solar wind (10%–20%). Moreover, the observed ENA energy spectra are well reproduced by our temperature‐dependent model. We conclude that the ENA scattering process is unchanged in the magnetosheath. Similarly to the undisturbed solar wind case, it is only magnetic anomalies that provide contrast in the ENA maps, not any selenomorphological features such as mare and highland regions.