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At Mercury, several processes can release ions and neutrals out of the planet's surface. Here we present enhancements of dayside planetary ions in the solar wind entry layer during flux transfer event (FTE) "showers" near Mercury's northern magnetospheric cusp. The FTE showers correspond to the intervals of intense magnetopause reconnection of Mercury's magnetosphere, which form a solar wind entry layer equatorward of the magnetospheric cusps. In this entry layer, solar wind ions are accelerated and move downward (i.e. planetward) toward the cusps, which sputter upward-moving planetary ions within 1 minute. The precipitation rate is enhanced by an order of magnitude during FTE showers and the neutral density of the exosphere can vary by >10% due to this FTE-driven sputtering. These in situ observations of enhanced planetary ions in the entry layer likely correspond to an escape channel of Mercury's planetary ions, and the large-scale variations of the exosphere observed on minute-timescales by Earth observatories. Comprehensive, future multi-point measurements made by BepiColombo will greatly enhance our understanding of the processes contributing to Mercury's dynamic exosphere and magnetosphere., 34 pages, 10 figures, 1 table

Context. Solar wind electrons play an important role in the energy balance of the solar wind acceleration by carrying energy into interplanetary space in the form of electron heat flux. The heat flux is stored in the complex electron velocity distribution functions (VDFs) shaped by expansion, Coulomb collisions, and field-particle interactions. Aims. We investigate how the suprathermal electron deficit in the anti-strahl direction, which was recently discovered in the near-Sun solar wind, drives a kinetic instability and creates whistler waves with wave vectors that are quasi-parallel to the direction of the background magnetic field. Methods. We combined high-cadence measurements of electron pitch-angle distribution functions and electromagnetic waves provided by Solar Orbiter during its first orbit. Our case study is based on a burst-mode data interval from the Electrostatic Analyser System (SWA-EAS) at a distance of 112 RS (0.52 au) from the Sun, during which several whistler wave packets were detected by Solar Orbiter’s Radio and Plasma Waves (RPW) instrument. Results. The sunward deficit creates kinetic conditions under which the quasi-parallel whistler wave becomes unstable. We directly test our predictions for the existence of these waves through solar wind observations. We find whistler waves that are quasi-parallel and almost circularly polarised, propagating away from the Sun, coinciding with a pronounced sunward deficit in the electron VDF. The cyclotron-resonance condition is fulfilled for electrons moving in the direction opposite to the direction of wave propagation, with energies corresponding to those associated with the sunward deficit. Conclusions. We conclude that the sunward deficit acts as a source of quasi-parallel whistler waves in the solar wind. The quasilinear diffusion of the resonant electrons tends to fill the deficit, leading to a reduction in the total electron heat flux.

Small-scale kinetic processes associated with turbulence, plasma instabilities, magnetic reconnection, etc., play a major role in dissipating energy and governing the large-scale evolution of the solar wind. However, a general impediment to improving the understanding of the kinetic physics of the solar wind is in the limitations on measurement cadences of particle instruments, which are usually several orders of magnitude below the equivalent cadences of field instruments. Nevertheless, knowledge of the details of the particle velocity distribution functions (VDFs) at sub-second cadence is required to make progress in this area. This is particularly true for the electron VDFs, which play a significant role in the overall energetics of the solar wind through their transmission of heat flux from the Sun. In this paper, we detail and illustrate a novel measurement scheme deployed on Solar Orbiter’s Solar Wind Analyser Electron Analyser System (SWA-EAS), which allows for 2D pitch angle distributions (PAD) to be returned over short periods (5–10 min) at a cadence of 0.125 s. This is achieved through the use of a B-field vector shared by the magnetometer (MAG) instrument to steer the SWA-EAS system to record only that part of the full SWA-EAS field-of-view needed to construct the PAD. We provide an example of early observations using this scheme to illustrate that it is working well. Given that the electrons are usually gyrotropic, these measurements provide a new tool with which to derive details of the electron VDFs at high cadence for the study of the solar wind’s kinetic processes.

The first radial alignment between Parker Solar Probe and Solar Orbiter spacecraft is used to investigate the evolution of solar wind turbulence in the inner heliosphere. Assuming ballistic propagation, two 1.5 hr intervals are tentatively identified as providing measurements of the same plasma parcels traveling from 0.1 to 1 au. Using magnetic field measurements from both spacecraft, the properties of turbulence in the two intervals are assessed. Magnetic spectral density, flatness, and high-order moment scaling laws are calculated. The Hilbert–Huang transform is additionally used to mitigate short sample and poor stationarity effects. Results show that the plasma evolves from a highly Alfvénic, less-developed turbulence state near the Sun, to fully developed and intermittent turbulence at 1 au. These observations provide strong evidence for the radial evolution of solar wind turbulence.

The Solar Orbiter mission is currently in its cruise phase, during which the spacecraft's in-situ instrumentation measures the solar wind and the electromagnetic fields at different heliocentric distances. We evaluate the solar wind angular-momentum flux by combining proton data from the Solar Wind Analyser (SWA) Proton-Alpha Sensor (PAS) and magnetic-field data from the Magnetometer (MAG) instruments on board Solar Orbiter during its first orbit. This allows us to evaluate the angular momentum in the protons in addition to that stored in magnetic-field stresses, and compare these to previous observations from other spacecraft. We discuss the statistical properties of the angular-momentum flux and its dependence on solar-wind properties. Our results largely agree with previous measurements of the solar wind’s angular-momentum flux in the inner heliosphere and demonstrate the potential for future detailed studies of large-scale properties of the solar wind with the data from Solar Orbiter.

The ESA-JAXA BepiColombo mission to Mercury will provide simultaneous measurements from two spacecraft, offering an unprecedented opportunity to investigate magnetospheric and exospheric particle dynamics at Mercury as well as their interactions with solar wind, solar radiation, and interplanetary dust. The particle instrument suite SERENA (Search for Exospheric Refilling and Emitted Natural Abundances) is flying in space on-board the BepiColombo Mercury Planetary Orbiter (MPO) and is the only instrument for ion and neutral particle detection aboard the MPO. It comprises four independent sensors: ELENA for neutral particle flow detection, Strofio for neutral gas detection, PICAM for planetary ions observations, and MIPA, mostly for solar wind ion measurements. SERENA is managed by a System Control Unit located inside the ELENA box. In the present paper the scientific goals of this suite are described, and then the four units are detailed, as well as their major features and calibration results. Finally, the SERENA operational activities are shown during the orbital path around Mercury, with also some reference to the activities planned during the long cruise phase.

International audience; Solar Orbiter's in situ coordination working group met frequently during the development of the mission with the goal of ensuring that its in situ payload has the necessary level of coordination to maximise science return. Here we present the results of that work, namely how the design of each of the in situ instruments (EPD, MAG, RPW, SWA) was guided by the need for coordination, the importance of time synchronisation, and how science operations will be conducted in a coordinated way. We discuss the mechanisms by which instrument sampling schemes are aligned such that complementary measurements will be made simultaneously by different instruments, and how burst modes are scheduled to allow a maximum overlap of burst intervals between the four instruments (telemetry constraints mean different instruments can spend different amounts of time in burst mode). We also explain how onboard autonomy, inter-instrument communication, and selective data downlink will be used to maximise the number of transient events that will be studied using high-resolution modes of all the instruments. Finally, we briefly address coordination between Solar Orbiter's in situ payload and other missions.

All authors: Zouganelis, I.; De Groof, A.; Walsh, A. P.; Williams, D. R.; Müller, D.; St Cyr, O. C.; Auchère, F.; Berghmans, D.; Fludra, A.; Horbury, T. S.; Howard, R. A.; Krucker, S.; Maksimovic, M.; Owen, C. J.; Rodríguez-Pacheco, J.; Romoli, M.; Solanki, S. K.; Watson, C.; Sanchez, L.; Lefort, J.; Osuna, P.; Gilbert, H. R.; Nieves-Chinchilla, T.; Abbo, L.; Alexandrova, O.; Anastasiadis, A.; Andretta, V.; Antonucci, E.; Appourchaux, T.; Aran, A.; Arge, C. N.; Aulanier, G.; Baker, D.; Bale, S. D.; Battaglia, M.; Bellot Rubio, L.; Bemporad, A.; Berthomier, M.; Bocchialini, K.; Bonnin, X.; Brun, A. S.; Bruno, R.; Buchlin, E.; Büchner, J.; Bucik, R.; Carcaboso, F.; Carr, R.; Carrasco-Blázquez, I.; Cecconi, B.; Cernuda Cangas, I.; Chen, C. H. K.; Chitta, L. P.; Chust, T.; Dalmasse, K.; D'Amicis, R.; Da Deppo, V.; De Marco, R.; Dolei, S.; Dolla, L.; Dudok de Wit, T.; van Driel-Gesztelyi, L.; Eastwood, J. P.; Espinosa Lara, F.; Etesi, L.; Fedorov, A.; Félix-Redondo, F.; Fineschi, S.; Fleck, B.; Fontaine, D.; Fox, N. J.; Gandorfer, A.; Génot, V.; Georgoulis, M. K.; Gissot, S.; Giunta, A.; Gizon, L.; Gómez-Herrero, R.; Gontikakis, C.; Graham, G.; Green, L.; Grundy, T.; Haberreiter, M.; Harra, L. K.; Hassler, D. M.; Hirzberger, J.; Ho, G. C.; Hurford, G.; Innes, D.; Issautier, K.; James, A. W.; Janitzek, N.; Janvier, M.; Jeffrey, N.; Jenkins, J.; Khotyaintsev, Y.; Klein, K. -L.; Kontar, E. P.; Kontogiannis, I.; Krafft, C.; Krasnoselskikh, V.; Kretzschmar, M.; Labrosse, N.; Lagg, A.; Landini, F.; Lavraud, B.; Leon, I.; Lepri, S. T.; Lewis, G. R.; Liewer, P.; Linker, J.; Livi, S.; Long, D. M.; Louarn, P.; Malandraki, O.; Maloney, S.; Martinez-Pillet, V.; Martinovic, M.; Masson, A.; Matthews, S.; Matteini, L.; Meyer-Vernet, N.; Moraitis, K.; Morton, R. J.; Musset, S.; Nicolaou, G.; Nindos, A.; O'Brien, H.; Orozco Suarez, D.; Owens, M.; Pancrazzi, M.; Papaioannou, A.; Parenti, S.; Pariat, E.; Patsourakos, S.; Perrone, D.; Peter, H.; Pinto, R. F.; Plainaki, C.; Plettemeier, D.; Plunkett, S. P.; Raines, J. M.; Raouafi, N.; Reid, H.; Retino, A.; Rezeau, L.; Rochus, P.; Rodriguez, L.; Rodriguez-Garcia, L.; Roth, M.; Rouillard, A. P.; Sahraoui, F.; Sasso, C.; Schou, J.; Schühle, U.; Sorriso-Valvo, L.; Soucek, J.; Spadaro, D.; Stangalini, M.; Stansby, D.; Steller, M.; Strugarek, A.; Štverák, Š.; Susino, R.; Telloni, D.; Terasa, C.; Teriaca, L.; Toledo-Redondo, S.; del Toro Iniesta, J. C.; Tsiropoula, G.; Tsounis, A.; Tziotziou, K.; Valentini, F.; Vaivads, A.; Vecchio, A.; Velli, M.; Verbeeck, C.; Verdini, A.; Verscharen, D.; Vilmer, N.; Vourlidas, A.; Wicks, R.; Wimmer-Schweingruber, R. F.; Wiegelmann, T.; Young, P. R.; Zhukov, A. N., Solar Orbiter is the first space mission observing the solar plasma both in situ and remotely, from a close distance, in and out of the ecliptic. The ultimate goal is to understand how the Sun produces and controls the heliosphere, filling the Solar System and driving the planetary environments. With six remote-sensing and four in-situ instrument suites, the coordination and planning of the operations are essential to address the following four top-level science questions: (1) What drives the solar wind and where does the coronal magnetic field originate?; (2) How do solar transients drive heliospheric variability?; (3) How do solar eruptions produce energetic particle radiation that fills the heliosphere?; (4) How does the solar dynamo work and drive connections between the Sun and the heliosphere? Maximising the mission's science return requires considering the characteristics of each orbit, including the relative position of the spacecraft to Earth (affecting downlink rates), trajectory events (such as gravitational assist manoeuvres), and the phase of the solar activity cycle. Furthermore, since each orbit's science telemetry will be downloaded over the course of the following orbit, science operations must be planned at mission level, rather than at the level of individual orbits. It is important to explore the way in which those science questions are translated into an actual plan of observations that fits into the mission, thus ensuring that no opportunities are missed. First, the overarching goals are broken down into specific, answerable questions along with the required observations and the so-called Science Activity Plan (SAP) is developed to achieve this. The SAP groups objectives that require similar observations into Solar Orbiter Observing Plans, resulting in a strategic, top-level view of the optimal opportunities for science observations during the mission lifetime. This allows for all four mission goals to be addressed. In this paper, we introduce Solar Orbiter's SAP through a series of examples and the strategy being followed. © 2020 ESO., Solar Orbiter is a space mission of international collaboration between ESA and NASA with contributions from national agencies of ESA member states. The spacecraft has been developed by Airbus and is being operated by ESA from the European Space Operations Centre (ESOC) in Darmstadt, Germany. Science operations are carried out at ESA's European Space Astronomy Centre (ESAC) in Villafranca del Castillo, Spain. SWA is an international collaboration which has been funded by the UKSA, CNES, ASI, NASA and the Czech contribution to the ESA PRODEX programme. UAH authors want to thanks the Spanish MINECO-FPI-2016 predoctoral grant with FSE, and its project FEDER/MCIU-AEEI/Proyecto ESP2017-88436-R. The Spanish contribution to SO/PHI has been funded by the Spanish Ministry of Science and Innovation through several projects, the last one being RTI2018-096886-B-C5, and by "Centro de Excelencia Severo Ochoa" programme under grant SEV-2017-0709. RAH, RCC, DMM, SPP, and AV acknowledge the support of the NASA Heliophysics Division, Solar Orbiter Collaboration Office under IAT NNG09EK11I. JEP acknowledges grant UKRI/STFC ST/N000692/1. All French involvements are supported by CNES and CNRS. DV is supported by the STFC Ernest Rutherford Fellowship ST/P003826/1 and STFC Consolidated Grant ST/S000240/1. The authors thank the referee for her constructive comments and suggestions, which led to the substantial improvement of this paper.

The Solar Orbiter mission seeks to make connections between the physical processes occurring at the Sun or in the solar corona and the nature of the solar wind created by those processes which is subsequently observed at the spacecraft. The mission also targets physical processes occurring in the solar wind itself during its journey from its source to the spacecraft. To meet the specific mission science goals, Solar Orbiter will be equipped with both remote-sensing and in-situ instruments which will make unprecedented measurements of the solar atmosphere and the inner heliosphere. A crucial set of measurements will be provided by the Solar Wind Analyser (SWA) suite of instruments. This suite consists of an Electron Analyser System (SWA-EAS), a Proton and Alpha particle Sensor (SWA-PAS), and a Heavy Ion Sensor (SWA-HIS) which are jointly served by a central control and data processing unit (SWA-DPU). Together these sensors will measure and categorise the vast majority of thermal and suprathermal ions and electrons in the solar wind and determine the abundances and charge states of the heavy ion populations. The three sensors in the SWA suite are each based on the top hat electrostatic analyser concept, which has been deployed on numerous space plasma missions. The SWA-EAS uses two such heads, each of which have 360° azimuth acceptance angles and ±45° aperture deflection plates. Together these two sensors, which are mounted on the end of the boom, will cover a full sky field-of-view (FoV) (except for blockages by the spacecraft and its appendages) and measure the full 3D velocity distribution function (VDF) of solar wind electrons in the energy range of a few eV to ∼5 keV. The SWA-PAS instrument also uses an electrostatic analyser with a more confined FoV (−24° to +42° × ±22.5° around the expected solar wind arrival direction), which nevertheless is capable of measuring the full 3D VDF of the protons and alpha particles arriving at the instrument in the energy range from 200 eV/q to 20 keV/e. Finally, SWA-HIS measures the composition and 3D VDFs of heavy ions in the bulk solar wind as well as those of the major constituents in the suprathermal energy range and those of pick-up ions. The sensor resolves the full 3D VDFs of the prominent heavy ions at a resolution of 5 min in normal mode and 30 s in burst mode. Additionally, SWA-HIS measures 3D VDFs of alpha particles at a 4 s resolution in burst mode. Measurements are over a FoV of −33° to +66° × ±20° around the expected solar wind arrival direction and at energies up to 80 keV/e. The mass resolution (m/Δm) is > 5. This paper describes how the three SWA scientific sensors, as delivered to the spacecraft, meet or exceed the performance requirements originally set out to achieve the mission’s science goals. We describe the motivation and specific requirements for each of the three sensors within the SWA suite, their expected science results, their main characteristics, and their operation through the central SWA-DPU. We describe the combined data products that we expect to return from the suite and provide to the Solar Orbiter Archive for use in scientific analyses by members of the wider solar and heliospheric communities. These unique data products will help reveal the nature of the solar wind as a function of both heliocentric distance and solar latitude. Indeed, SWA-HIS measurements of solar wind composition will be the first such measurements made in the inner heliosphere. The SWA data are crucial to efforts to link the in situ measurements of the solar wind made at the spacecraft with remote observations of candidate source regions. This is a novel aspect of the mission which will lead to significant advances in our understanding of the mechanisms accelerating and heating the solar wind, driving eruptions and other transient phenomena on the Sun, and controlling the injection, acceleration, and transport of the energetic particles in the heliosphere.

Aims. Solar Orbiter, the first mission of ESA's Cosmic Vision 2015-2025 programme and a mission of international collaboration between ESA and NASA, will explore the Sun and heliosphere from close up and out of the ecliptic plane. It was launched on 10 February 2020 04:03 UTC from Cape Canaveral and aims to address key questions of solar and heliospheric physics pertaining to how the Sun creates and controls the Heliosphere, and why solar activity changes with time. To answer these, the mission carries six remote-sensing instruments to observe the Sun and the solar corona, and four in-situ instruments to measure the solar wind, energetic particles, and electromagnetic fields. In this paper, we describe the science objectives of the mission, and how these will be addressed by the joint observations of the instruments onboard. Methods. The paper first summarises the mission-level science objectives, followed by an overview of the spacecraft and payload. We report the observables and performance figures of each instrument, as well as the trajectory design. This is followed by a summary of the science operations concept. The paper concludes with a more detailed description of the science objectives. Results. Solar Orbiter will combine in-situ measurements in the heliosphere with high-resolution remote-sensing observations of the Sun to address fundamental questions of solar and heliospheric physics. The performance of the Solar Orbiter payload meets the requirements derived from the mission's science objectives. Its science return will be augmented further by coordinated observations with other space missions and ground-based observatories. © 2020 ESO., Solar Orbiter is a space mission of international collaboration between ESA and NASA. The spacecraft has been developed by Airbus and is being operated by ESA from the European Space Operations Centre (ESOC) in Darmstadt, Germany. Science operations are carried out at ESA's European Space Astronomy Centre (ESAC) in Villafranca del Castillo, Spain. Conceiving, designing and building Solar Orbiter has been an international team e ffort of many people. In particular, the authors would like to thank ESA's Mission Operations Centre (MOC) and Science Operations Centre (SOC) teams, Yves Langevin and Jose-Manuel Sanchez Perez for their skillful optimisation of mission trajectories, the ESA and NASA Project o ffices, Airbus, IABG, NASA-LSP, ULA, and all national funding agencies that have enabled Solar Orbiter. The German contribution to SO/PHI is funded by the Bundesministerium fur Wirtschaft und Technologie through Deutsches Zentrum fur Luftund Raumfahrt e.V. (DLR), Grants No. 50 OT 1001/1201/1901 as well as 50 OT 0801/1003/1203/1703, and by the President of the Max Planck Society (MPG). The Spanish contribution has been partially funded by Ministerio de Ciencia, Innovacion y Universidades through projects ESP2014-56169-C6 and ESP2016-77548-C5. IAA-CSIC acknowledges financial support from the Spanish Research Agency (AEI/MCIU) through the "Center of Excellence Severo Ochoa" award for the Instituto de Astrofisica de Andalucia (SEV-2017-0709). The French contribution is funded by the Centre National d'Etudes Spatiales. Further detailed acknowledgements regarding each instrument can be found in the individual instrument papers of this special issue. R.A.H. is supported by the NASA Solar Orbiter Collaboration Office, under contract NNG09EK11I. The Spanish contribution to SO/PHI has been funded by the Spanish Ministry of Science and Innovation through several projects, the last one of which being RTI2018-096886-B-C5, and by "Centro de Excelencia Severo Ochoa" Programme under grant SEV-2017-0709. The authors would like to highlight Rainer Schwenn's (1941-2017) important and enthusiastic contribution to the Solar Orbiter mission in its early phase. Portions of the text have been reproduced with permission from Muller & Marsden (2013) copyright by Springer. The authors would like to thank John Leibacher and Bernhard Fleck for their support, and the referee for providing helpful suggestions.

Context.Turbulence dominated by large-amplitude, nonlinear Alfvén-like fluctuations mainly propagating away from the Sun is ubiquitous in high-speed solar wind streams. Recent studies have demontrated that slow wind streams may also show strong Alfvénic signatures, especially in the inner heliosphere.Aims.The present study focuses on the characterisation of an Alfvénic slow solar wind interval observed by Solar Orbiter between 14 and 18 July 2020 at a heliocentric distance of 0.64 AU.Methods.Our analysis is based on plasma moments and magnetic field measurements from the Solar Wind Analyser (SWA) and Magnetometer (MAG) instruments, respectively. We compared the behaviour of different parameters to characterise the stream in terms of the Alfvénic content and magnetic properties. We also performed a spectral analysis to highlight spectral features and waves signature using power spectral density and magnetic helicity spectrograms, respectively. Moreover, we reconstruct the Solar Orbiter magnetic connectivity to the solar sources both via a ballistic and a potential field source surface (PFSS) model.Results.The Alfvénic slow wind stream described in this paper resembles, in many respects, a fast wind stream. Indeed, at large scales, the time series of the speed profile shows a compression region, a main portion of the stream, and a rarefaction region, characterised by different features. Moreover, before the rarefaction region, we pinpoint several structures at different scales recalling the spaghetti-like flux-tube texture of the interplanetary magnetic field. Finally, we identify the connections between Solar Orbiter in situ measurements, tracing them down to coronal streamer and pseudostreamer configurations.Conclusions.The characterisation of the Alfvénic slow wind stream observed by Solar Orbiter and the identification of its solar source are extremely important aspects for improving the understanding of future observations of the same solar wind regime, especially as solar activity is increasing toward a maximum, where a higher incidence of this solar wind regime is expected.

Recent studies have utilized different charge states of oxygen ions as a tracer for the origins of plasma populations in the magnetosphere of Earth, using O+ as an indicator of ionospheric-originating plasma and O6+ as an indicator of solar wind-originating plasma. These studies have correlated enhancements in O6+ to various solar wind and geomagnetic conditions to characterize the dominant solar wind injection mechanisms into the magnetosphere, but did not include analysis of the temporal evolution of these ions. A 6th-order Fourier expansion model based empirically on a superposed epoch analysis of geomagnetic storms observed by Polar is presented in this study to provide insight into the evolution of both ionospheric-originating and solar wind-originating plasma throughout geomagnetic storms. At high energies (~200 keV) the flux of O+ and O6+ are seen to become comparable in the outer magnetosphere. Moreover, while the density of O+ is far higher than O6+, the two charge states have comparable pressures in the outer magnetosphere. The temperature of O6+ is generally higher than that of O+, because the O6+ is injected from pre-heated magnetosheath populations before undergoing further heating once in the magnetosphere. A comparison between the model results with O+ observations from the Magnetospheric Multiscale (MMS) mission and the Van Allen Probes provides a validation of the model. In general, this empirical model agrees qualitatively well with the trends seen in both datasets. Quantitatively, the modeled density, pressure, and temperature almost always agree within a factor of at most 10, 5, and 2, respectively.

This paper describes a novel electrostatic analyzer concept to measure suprathermal ions, a Double-Cusp Analyzer for SupraThermals (DCAST) that employs a double-shell cusp structure. Due to the necessity of measuring higher energy levels to cover the suprathermal range, existing ion instruments require greater size and mass. Moreover, observations of potentially low-flux suprathermal ions require a long integration time to fully characterize key ion properties in the plasmas (e.g., anisotropy and energy spectrum) with necessary counting statistics. DCAST covers the suprathermal energy range (2-300 keV/q) spanning heated solar wind and pickup ions; it enables a high cadence, high angular resolution, and wide angle coverage measurement while conserving resources such as mass and size. As a proof-of-concept study, the performance of a prototype DCAST was verified by laboratory measurements (geometric factor, K-factor, and energy resolution), which also involved investigating noise characteristics coming from cross-sector contamination and foreground extreme ultra-violet photons. To understand the specific characteristics of the double-shell type design, the inner and outer sector voltage ratio (RV) effects were examined in terms of the electro-static analyzer performance.This paper describes a novel electrostatic analyzer concept to measure suprathermal ions, a Double-Cusp Analyzer for SupraThermals (DCAST) that employs a double-shell cusp structure. Due to the necessity of measuring higher energy levels to cover the suprathermal range, existing ion instruments require greater size and mass. Moreover, observations of potentially low-flux suprathermal ions require a long integration time to fully characterize key ion properties in the plasmas (e.g., anisotropy and energy spectrum) with necessary counting statistics. DCAST covers the suprathermal energy range (2-300 keV/q) spanning heated solar wind and pickup ions; it enables a high cadence, high angular resolution, and wide angle coverage measurement while conserving resources such as mass and size. As a proof-of-concept study, the performance of a prototype DCAST was verified by laboratory measurements (geometric factor, K-factor, and energy resolution), which also involved investigating noise characteristics coming from...

Understanding the sources and subsequent evolution of plasma in a magnetosphere holds intrinsic importance for magnetospheric dynamics. Previous studies have investigated the balance of ionospheric-originating heavy ions (low charge state) from those of solar wind origin (high charge state) in the magnetosphere of Earth. These studies have suggested a variety of entry mechanisms for solar wind ions to penetrate into the magnetosphere. Following from recently published distributions for oxygen charge states observed by the Polar spacecraft, this paper investigates oxygen charge state flux distributions versus L shell and magnetic latitude (MLAT). By showing these distributions in this frame, and binning by various proxies for magnetospheric dynamics (DST, AE, VSW∗BZ, Pdyn), insight has been gained into the underlying physics at play for oxygen injection. Ionospheric-originating oxygen is observed to depend predominantly on DST, whereas solar wind-originating oxygen is observed to have a strong dependence on solar wind dynamic pressure (Pdyn) at the flanks and on VSW∗BZ at the dayside. This suggests that both Kelvin-Helmholtz instabilities and reconnection play major roles in solar wind ion penetration into a magnetosphere. Additionally, the near-Earth magnetotail reconnection site does not seem to be a major injection site of solar wind-originating plasma in the 1 to 200 keV/e energy range.

A new era of exosphere study is beginning with observations being conducted by orbiting neutral mass spectrometers. We have developed a new analysis framework to improve our ability to invert the detection signal of these instruments into local and near-surface densities. By leveraging Liouville’s theorem we are able to rapidly reach a source solution that best matches exosphere filling processes to an observable signal. Two approaches are developed in this work, the Liouville Algorithm and a traditional Forward Monte Carlo model that acts as validation. The Liouville Algorithm can be applied to the interpretation of photometric observations but is especially powerful for in␣situ study. This type of analysis is motivated by the velocity dependence of mass spectrometers, which demands that the velocity space be fully resolved. Several examples are described to demonstrate the consistency of the two approaches and highlight the strengths of the new algorithm when used in conjunction with or in lieu of Monte Carlo. Our examples focus on Mercury and the BepiColombo instrument SERENA–Strofio, but these methods apply equally to other surface bounded exospheres like those of the Moon or other airless bodies.

We present the two-stage method used to calibrate the electron spectrometer (ELS), part of the plasma spectrometer (CAPS) on board the Cassini spacecraft currently in orbit around Saturn. The CAPS-ELS is a top-hat electrostatic analyser designed to measure electron fluxes between 0.5 eV and 26 keV. The on-ground calibration method described here includes the production of photoelectrons, which are energised and passed into the CAPS-ELS in a purpose designed calibration facility. Knowledge of the intensity of these incident electrons and the subsequent instrument output provides an on-ground calibrated geometric factor. Comparative studies of physical quantities such as plasma density and electron differential flux calculated using on-ground calibration factor with the quantities deduced from the wave experiment and high energy electron detector provide in-flight calibration. The results of this are presented together with a comparison of the experimentally calibrated values with simulated calibration values.

In July 2004 the Cassini spacecraft began its orbital tour in the Saturnian system and performed 74 orbits during the nominal mission (July 2004–July 2008) providing data from nearly all local times at various distances and latitudes relative to the planet. The particles and field instruments onboard the spacecraft were essentially operating continuously offering the possibility to study the global configuration and the dynamics of the second largest magnetosphere in our solar system extensively. One of those instruments aboard Cassini is the Low Energy Magnetospheric Measurement System (LEMMS), one of three particle detectors of the Magnetospheric Imaging Instrument (MIMI). MIMI/LEMMS measures the intensity, energy spectra and pitch angle distributions of energetic ions ( E > 30 keV ) and electrons ( E > 20 keV ) separately. The measured energetic particle distributions together with the measured magnetic field provide a very powerful tool to investigate the Saturnian magnetosphere in those regions covered by the Cassini orbits. This paper will give an overview of the energetic particle measurements of the MIMI/LEMMS sensor in the Saturnian system. In the first part of the paper synoptic maps will be shown where all the data are presented as a function of various trajectory parameters of the spacecraft. Secondly bi-directional electron distributions along the magnetic field direction will be described as a feature in the Saturnian system. Thirdly the particle parameters in the inner magnetosphere with absorption signatures of the various moons are presented. Fourthly it will be shown that the region around about 15 R S seems to be a characteristic region where depletion signatures in energetic particle distributions are very often observed. At the end of this work a 60 min intensity periodicity in the MIMI/LEMMS data is discussed.

The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission to Mercury offers our first opportunity to explore this planet’s miniature magnetosphere since the brief flybys of Mariner 10. Mercury’s magnetosphere is unique in many respects. The magnetosphere of Mercury is among the smallest in the solar system; its magnetic field typically stands off the solar wind only ~1000 to 2000 km above the surface. For this reason there are no closed drift paths for energetic particles and, hence, no radiation belts. Magnetic reconnection at the day side magnetopause may erode the subsolar magnetosphere, allowing solar wind ions to impact directly the regolith. Inductive currents in Mercury’s interior may act to modify the solar wind interaction by resisting changes due to solar wind pressure variations. Indeed, observations of these induction effects may be an important source of information on the state of Mercury’s interior. In addition, Mercury’s magnetosphere is the only one with its defining magnetic flux tubes rooted beneath the solid surface as opposed to an atmosphere with a conductive ionospheric layer. This lack of an ionosphere is probably the underlying reason for the brevity of the very intense, but short-lived, ~l-2 min, substorm-like energetic particle events observed by Mariner 10 during its first traversal of Mercury’s magnetic tail. Because of Mercury’s proximity to the sun, 0.3-0.5 AU, this magnetosphere experiences the most extreme driving forces in the solar system. All of these factors are expected to produce complicated interactions involving the exchange and recycling of neutrals and ions among the solar wind, magnetosphere, and regolith. The electrodynamics of Mercury’s magnetosphere are expected to be equally complex, with strong forcing by the solar wind, magnetic reconnection, and pick-up of planetary ions all playing roles in the generation of field-aligned electric currents. However, these field-aligned currents do not close in an ionosphere, but in some other manner. In addition to the insights into magnetospheric physics offered by study of the solar wind-Mercury system, quantitative specification of the “external” magnetic field generated by magnetospheric currents is necessary for accurate determination of the strength and multipolar decomposition of Mercury’s intrinsic magnetic field. MESSENGER’S highly capable instrumentation and broad orbital coverage will greatly advance our understanding of both the origin of Mercury’s magnetic field and the acceleration of charged particles in small magnetospheres. In this article, we review what is known about Mercury’s magnetosphere and describe the MESSENGER science team’s strategy for obtaining answers to the outstanding science questions surrounding the interaction of the solar wind with Mercury and its small, but dynamic, magnetosphere.

The Neutral Particle Detector (NPD), an Energetic Neutral Atom (ENA) sensor of the Analyzer of Space Plasmas and Energetic Atoms (ASPERA-3) on board Mars Express, detected intense fluxes of ENAs emitted from the subsolar region of Mars. The typical ENA fluxes are (4–7) × 10 5 cm −2 sr −1 s −1 in the energy range 0.3–3 keV. These ENAs are likely to be generated in the subsolar region of the martian exosphere. As the satellite moved away from Mars, the ENA flux decreased while the field of view of the NPD pointed toward the subsolar region. These decreases occurred very quickly with a time scale of a few tens of seconds in two thirds of the orbits. Such a behavior can be explained by the spacecraft crossing a spatially constrained ENA jet, i.e., a highly directional ENA emission from a compact region of the subsolar exosphere. This ENA jet is highly possible to be emitted conically from the subsolar region. Such directional ENAs can result from the anisotropic solar wind flow around the subsolar region, but this can not be explained in the frame of MHD models.

We have analysed ion escape at Mars by comparing ASPERA-3/Mars Express ion measurements and a 3-D quasi-neutral hybrid model. As Mars Express does not have a magnetometer onboard, the analysed IMA data are from an orbit when the IMF clock angle was possible to determine from the magnetic field measurements of Mars Global Surveyor. We found that fast escaping planetary ions were observed at the place which, according to the 3-D model, is anticipated to contain accelerated heavy ions originating from the martian ionosphere. The direction of the interplanetary magnetic field was found to affect noticeably which regions can be magnetically connected to Mars Express and to the overall 3-D Mars–solar wind interaction.

We present measurements with an Energetic Neutral Atom (ENA) imager on board Mars Express when the spacecraft moves into Mars eclipse. Solar wind ions charge exchange with the extended Mars exosphere to produce ENAs that can spread into the eclipse of Mars due to the ions' thermal spread. Our measurements show a lingering signal from the Sun direction for several minutes as the spacecraft moves into the eclipse. However, our ENA imager is also sensitive to UV photons and we compare the measurements to ENA simulations and a simplified model of UV scattering in the exosphere. Simulations and further comparisons with an electron spectrometer sensitive to photoelectrons generated when UV photons interact with the spacecraft suggest that what we are seeing in Mars' eclipse are ENAs from upstream of the bow shock produced in charge exchange with solar wind ions with a non-zero temperature. The measurements are a precursor to a new technique called ENA sounding to measure solar wind and planetary exosphere properties in the future.

The neutral particle detector (NPD) on board Mars Express has observed energetic neutral atoms (ENAs) from a broad region on the dayside of the martian upper atmosphere. We show one such example for which the observation was conducted at an altitude of 570 km, just above the induced magnetosphere boundary (IMB). The time of flight spectra of these ENAs show that they had energies of 0.2–2 keV/amu, with an average energy of ∼1.1 keV/amu. Both the spatial distribution and the energy of these ENAs are consistent with the backscattered ENAs, produced by an ENA albedo process. This is the first observation of backscattered ENAs from the martian upper atmosphere. The origin of these ENAs is considered to be the solar wind ENAs that are scattered back by collision processes in the martian upper atmosphere. The particle flux and energy flux of the backscattered ENAs are 0.9 – 1.3 × 10 7 cm −2 s −1 and ∼ 9.5 × 10 9 eV cm −2 s −1 , respectively.

The Electron Spectrometer (ELS) instrument of the ASPERA-3 package on the Mars Express satellite has recorded photoelectron energy spectra up to apoapsis (∼10,000 km altitude). The characteristic photoelectron shape of the spectrum is sometimes seen well above the ionosphere in the evening sector across a wide range of near-equatorial latitudes. Two numerical models are used to analyze the characteristics of these high-altitude photoelectrons. The first is a global, multi-species MHD code that produces a 3-D representation of the magnetic field and bulk plasma parameters around Mars. It is used here to examine the possibility of magnetic connectivity between the high-altitude flanks of the martian ionosheath and the subsolar ionosphere. It is shown that some field lines in this region are draped interplanetary magnetic lines while others are open field lines (connected to both the IMF and the crustal magnetic field sources). The second model is a kinetic electron transport model that calculates the electron velocity space distribution along a selected, non-uniform, magnetic field line. It is used here to simulate the high-altitude ELS measurements. It is shown that the photoelectrons are essentially confined to the source cone, as governed by magnetic field inhomogeneity along the field line. Reasonable agreement is shown between the data and the model results, and a method is demonstrated for inferring properties of the local and photoelectron source region magnetic field from the ELS measurements. Specifically, the number of sectors in which photoelectrons are measured is a function of the magnetic field intensity ratio and the field's angle with respect to the detector plane. In addition, the sector of the photoelectron flux peak is a function of the magnetic field azimuthal angle in the detector plane.

The ELectron Spectrometer (ELS) from the Analyzer of Space Plasmas and Energetic Atoms (ASPERA-3) flown on the Mars Express spacecraft has an 8% energy resolution, combined with the capability to oversample the martian electron distribution. This makes possible the resolution and identification of electrons generated as a result of the He 304 A ionization of CO 2 at the martian exobase on the dayside of the planet. Ionospheric photoelectrons were observed during almost every pass into the ionosphere and CO 2 photoelectron peaks were identified near the terminator. Atmospherically generated CO 2 photoelectrons are also observed at 10,000 km altitude in the martian tail near the inner magnetospheric boundary. Observations over a wide range of spacecraft orbits showed a consistent presence of photoelectrons at locations along the inner magnetospheric boundary and in the ionosphere, from an altitude of 250 to 10,000 km.

Mars Express (MEX) Analyser of Space Plasmas and Energetic Atoms (ASPERA-3) data is providing insights into atmospheric loss on Mars via the solar wind interaction. This process is influenced by both the interplanetary magnetic field (IMF) in the solar wind and by the magnetic ‘anomaly’ regions of the martian crust. We analyse observations from the ASPERA-3 Electron Spectrometer near to such crustal anomalies. We find that the electrons near remanent magnetic fields either increase in flux to form intensified signatures or significantly reduce in flux to form plasma voids. We suggest that cusps intervening neighbouring magnetic anomalies may provide a location for enhanced escape of planetary plasma. Initial statistical analysis shows that intensified signatures are mainly a dayside phenomenon whereas voids are a feature of the night hemisphere.

The magnetospheric imaging instrument (MIMI) is a neutral and charged particle detection system on the Cassini orbiter spacecraft designed to perform both global imaging and in-situ measurements to study the overall configuration and dynamics of Saturn’s magnetosphere and its interactions with the solar wind, Saturn’s atmosphere, Titan, and the icy satellites. The processes responsible for Saturn’s aurora will be investigated; a search will be performed for substorms at Saturn; and the origins of magnetospheric hot plasmas will be determined. Further, the Jovian magnetosphere and Io torus will be imaged during Jupiter flyby. The investigative approach is twofold. (1) Perform remote sensing of the magnetospheric energetic (E > 7 keV) ion plasmas by detecting and imaging charge-exchange neutrals, created when magnetospheric ions capture electrons from ambient neutral gas. Such escaping neutrals were detected by the Voyager 1 spacecraft outside Saturn’s magnetosphere and can be used like photons to form images of the emitting regions, as has been demonstrated at Earth. (2) Determine through in-situ measurements the 3-D particle distribution functions including ion composition and charge states (E > 3 keV/e). The combination of in-situ measurements with global images, together with analysis and interpretation techniques that include direct “forward modeling” and deconvolution by tomography, is expected to yield a global assessment of magnetospheric structure and dynamics, including (a) magnetospheric ring currents and hot plasma populations, (b) magnetic field distortions, (c) electric field configuration, (d) particle injection boundaries associated with magnetic storms and substorms, and (e) the connection of the magnetosphere to ionospheric altitudes. Titan and its torus will stand out in energetic neutral images throughout the Cassini orbit, and thus serve as a continuous remote probe of ion flux variations near 20R S (e.g., magnetopause crossings and substorm plasma injections). The Titan exosphere and its cometary interaction with magnetospheric plasmas will be imaged in detail on each flyby. The three principal sensors of MIMI consists of an ion and neutral camera (INCA), a charge-energy-mass-spectrometer (CHEMS) essentially identical to our instrument flown on the ISTP/Geotail spacecraft, and the low energy magnetospheric measurements system (LEMMS), an advanced design of one of our sensors flown on the Galileo spacecraft. The INCA head is a large geometry factor (G ~ 2.4 cm2 sr) foil time-of-flight (TOF) camera that separately registers the incident direction of either energetic neutral atoms (ENA) or ion species (≥5° full width half maximum) over the range 7 keV/nuc < E < 3 MeV/nuc. CHEMS uses electrostatic deflection, TOF, and energy measurement to determine ion energy, charge state, mass, and 3-D anisotropy in the range 3 ≤ E ≤ 220 keV/e with good (~0.05 cm2 sr) sensitivity. LEMMS is a two-ended telescope that measures ions in the range 0.03 ≤ E ≤ 18 MeV and electrons 0.015 ≤ E < 0.884 MeV in the forward direction (G ~ 0.02 cm2 sr), while high energy electrons (0.1–5 MeV) and ions (1.6–160 MeV) are measured from the back direction (G ~ 0.4 cm2 sr). The latter are relevant to inner magnetosphere studies of diffusion processes and satellite microsignatures as well as cosmic ray albedo neutron decay (CRAND). Our analyses of Voyager energetic neutral particle and Lyman-a measurements show that INCA will provide statistically significant global magnetospheric images from a distance of ~60 RS every 2–3 h (every ~10 min from ~20 RS). Moreover, during Titan flybys, INCA will provide images of the interaction of the Titan exosphere with the Saturn magnetosphere every 1.5 min. Time resolution for charged particle measurements can be

The potentiality of exploring the plasma dynamical behavior of Mercury via energetic neutral atom (ENA) analysis is briefly discussed. The analysis of the Hermean ENA would give clues to the global magnetospheric configuration, clarifying crucial issues like the existence of radiation belts. In order to demonstrate the feasibility of ENA imaging in the Hermean environment, an empirical model of the equatorial proton distributions, originally developed for the Earth's magnetosphere, is here modified in order to be applicable to the expected conditions, according to the present knowledge of the planet. Simulations of the ENA radiation are then presented and discussed. It is shown that the simulated radiation is significantly high, especially when pointing the field of view towards the Hermean surface. The way of determining the proton flux distributions and the cross-tail electric field characteristics by using the ENA signal is discussed. Finally, a preliminary design of an ENA detector suitable for the ESA mission to Mercury ‘BepiColombo’ is proposed and discussed.

We report on energetic particle measurements from the Low-Energy Magnetospheric Measurement System (LEMMS) aboard the Cassini spacecraft during the Earth swing-by maneuver in August 1999. LEMMS is capable of identifying the energy and incidence direction of energetic ions and electrons with energies of a few tens of keV to several tens of MeV. Cassini flew by Earth with a velocity of 16 kms−1 or 9 Earth radii (RE) per hour, which means that the entire day side magnetosphere was passed within 1 hour. This fast flyby trajectory of the spacecraft provided a snapshot of the Earth's magnetosphere where key regions were passed within a few hours instead of typically tens of hours or days for orbiting spacecraft. The measurements provide a means to show the performance of the instrument's capabilities as well as to test models of energetic particle distributions within the Earth's magnetosphere [e.g. Sibeck et al., 1987]. Particle pitch angle distributions (PADs) on the inbound pass along the 1300 magnetic local time (MLT) meridian gradually changed from normal distributions (maximum at 90° pitch angle) to butterfly pitch angle distributions (maximum at intermediate pitch angles). Close to the plasmapause, LEMMS observed field-aligned bidirectional distributions. During the outbound pass through the inner magnetosphere at 0130 MLT, pitch angle distributions are generally close to isotropic. Shortly after a substorm onset, identified by ground-based observations and aboard the Polar spacecraft [Khan et al., this issue], LEMMS observed an energy-time dispersed enhancement in the differential flux of low-energy electrons. We further address the question of whether signatures of the Earth magnetotail were observed at distances beyond 5000 RE when Cassini passed through the Earth's downstream region. LEMMS measured a series of particle increases during that time. The angular distributions during these enhancements show that most of these particles did indeed arrive from the Earth's direction. Nevertheless, the lack of singly charged heavy ions does not allow for a definite identification of this event as a deep-tail encounter.

Galileo, as the first orbiting spacecraft in an outer planet's magnetosphere, provides the opportunity to study global energetic ion distributions in Jupiter's magnetosphere. We present directional anisotropies of energetic ion distributions measured by the Galileo Energetic Particles Detector (EPD). The EPD measurements of proton (80–1050 keV), oxygen (26–562 keV/nucleon), and sulfur (16–310 keV/nucleon) distributions cover a wide energy range. Spatially, the data set includes measurements from 6 to 142 Jovian radii (RJ) and covers all local times inside the Jovian magnetosphere. For each species a single detector head scans almost the entire sky (≈ 4π sr), producing the three-dimensional angular distributions from which the anisotropies are derived. Consequently, the resulting anisotropy estimates are both global and robust. Such anisotropies, generally produced by convective flow, ion intensity gradients, and field-aligned components, have long been used to estimate flow velocities and to locate spatial boundaries within magnetospheres. They can therefore provide vital information on magnetospheric circulation and dynamics. We find that the EPD measured anisotropies in the Jovian magnetosphere are dominated by a component in the corotational direction punctuated by episodic radial components, both inward and outward. Under the assumption that anisotropies are produced predominantly by convective flow, we derive flow velocities of protons, oxygen ions, and sulfur ions. The validity of that approach is supported by the fact that these three independently derived flow velocities agree, to a large extent, in this approximation. Thus, for the first time, we are able to derive the global flow pattern in a magnetosphere of an outer planet. In a comparison between the first-order EPD flow velocities and those predicted by a magnetohydrodynamic (MHD) simulation of the Jovian magnetosphere, we find that qualitatively the directions appear similar, although no firm evidence of steady outflow of ions has been observed at distances covered by Galileo. A first rough comparison indicates that the measured first-order flow velocities are higher by at least a factor of 1.5 than the MHD simulation results.

Since December 1995 the Galileo spacecraft is in orbit around Jupiter. Up to now the spacecraft performed 25 orbits through the Jovian system within nearly 4 years providing an excellent data coverage in local time specifically for distances r

In addition to earlier found variations of the Martian magnetotail thickness as a function of the solar wind ram pressure, succeeding analysis of the magnetotail boundary crossings by Phobos 2 orbiter revealed modulation of magnetopause positions by planetary crustal magnetization. It was shown that magnetic anomalies mostly influence the magnetotail thickness when they are approaching the terminator region due to planetary rotation. The areographic position of the equatorial regions of Mars, which efficiently increase planetary magnetotail thickness, corresponds to strongly magnetized regions recently mapped by the Mars Global Surveyor. Local enhancement of the magnetotail thickness due to non-uniform crustal magnetization can be as high as 500–1000 km.

The dependence of the energy of ions on their mass could be a useful tool for the identification of possible acceleration processes in the Martian plasma sheet. The study is based on a few cases when heavier ions and protons were registered simultaneously in the plasma sheet by the TAUS energy spectrometer, and on published data of the ASPERA instrument. The analysis of mass dependence of the ion energy in the plasma sheet suggests that frictional preacceleration of ions probably in the pole regions of the Martian magnetosphere can be responsible for not more than 40% of the energy gain of heavy ions in the plasma sheet, the cross-tail current sheet acceleration yields up to 70%, and from 30 to 70% of the heavy ion energy is provided by the acceleration due to magnetic field line stresses (and/or electric field aligned acceleration).

The role of different processes which could be responsible for deceleration of the solar wind upstream of the martian bow shock is discussed. It is shown that the geometry of the foreshock strongly controls variations of the solar wind velocity. Deceleration of the solar wind was observed on three elliptical orbits when the spacecraft approached the planet inside the foreshock. Variations of the solar wind speed are not consistent with the ones expected for ring-like distribution of pickup protons. Effects related with deceleration in the foreshock and transient phenomena (Alfven waves) dominate and mask mass-loading effects if they do exist.

Data on heavy ion (mass/charge ratio m i q > 3 ) fluxes in the Martian plasma sheet collected by the TAUS experiment aboard the Phobos 2 orbiter and data of the MAGMA magnetometer are analyzed statistically. The analysis suggests that acceleration due to magnetic field line stresses and acceleration in the central current sheet could account for the observations, though planetary ions are likely to be preaccelerated to velocities of 40 – 50 km/s. The effectiveness of these acceleration processes of heavy ions in cases of high and low solar wind ram pressure is considered.

During Galileo's orbit G2 in 1996 the Energetic Particles Detector (EPD) onboard the spacecraft detected a number of particle bursts with large radial/antisunward anisotropies in the distant Jovian magnetotail [Krupp et al., 1998]. In this letter we focus on a detailed analysis of one of the bursts. Prior to the onset of the burst, particle intensities at low energies increase over several hours. This phase can be interpreted as a plasma loading phase. It ends after the onset of strong distortions in the magnetic field with a bipolar excursion of the north-south component being the most prominent feature. The subsequent plasma sheet encounters show that the plasma sheet has thinned considerably. Accelerated/heated ion beams first from the Jovian direction and then later from the tail direction are seen at the plasma sheet and lobe interfaces and intense radio and plasma wave emissions are detected. The event is tentatively interpreted as a dynamical process, where the Jovian magnetotail is internally driven unstable by mass loading of magnetic flux tubes.

Following the discovery of the terrestrial ionospheric outflow in the early 1970s, and the confirmation of its significance in the late 1970s and the 1980s, a large number of studies investigated the processes leading to the escape of ionospheric ions to the magnetosphere. In general, ionospheric outflow was found to increase proportionally with geomagnetic activity. In the late 1980s and the 1990s several studies showed that whenever the abundance of ionospheric origin ions in the magnetosphere is extraordinarily high, their effects on geospace activity are substantial. It has been suggested that the large abundance of ionospheric origin ions regulates major dynamic processes like storms and substorms. This appears to be the case during great magnetic storms. A number of factors have been identified, which have a critical influence on the extent and the characteristics of ionospheric outflow. There are certainly several internal geospace factors, but a major influence on ionospheric outflow presumably originates in certain interplanetary structures. Such structures are for example high-speed streams, magnetic clouds, and coronal mass ejections. We will discuss the relationship between interplanetary structures and ionospheric outflow, on the basis of both theoretical expectations and observational facts.

In March 1991, a great magnetic storm occurred, with [ Dst ] reaching peak values of more than 300 nT. We present energetic ion measurements from the Magnetospheric Ion Composition Spectrometer (MICS) on board the Combined Release and Radiation Effects Satellite (CRRES), which monitored the dramatic compositional variations in the inner magnetosphere during the storm. While the inner magnetosphere was dominated by protons before the storm, the MICS observations show that the abundance of ionospheric-origin O' ions progressively increased during the main phase of the storm. Eventually the O + ions dominated, contributing more than 70% of the total particle energy density near storm maximum, which is to be compared to a quiet-time contribution of less than 10%. We suggest that the final deep drop in Dst at storm maximum, is due to a new population that is effectively fed into the inner magnetosphere from the polar ionosphere. The main agent of this ionospheric “intervention” is O + , which becomes the dominant ion species at the maximum epoch of great storms, at least around solar maximum.

Interplanetary magnetic clouds appear to have a wide range of ability to affect the magnetospheric energetic particle populations. This “effectiveness” or lack thereof, shows up in both the electron and ion components of the radiation belts. The ring current ions are intensified and energised by the geomagnetic storm response to the cloud. The ion composition of the ring current may also change due to source variations in the solar wind and ionosphere. Seven of the magnetic cloud events during the first year of the GGS Polar mission are compared as to their effectiveness to change the intensity and composition of the ring current ion populations. Data from the CAMMICE MICS ion composition instrument on Polar are presented to quantify the ring current response to each event. The January 10, 1997 event was observed to have a relatively modest effect on the ring current. In contrast, the cloud that arrived on May 15, 1997 resulted in ring current ion energy density enhancements approximately one order of magnitude larger than the pre-storm values.

The Charge and Mass Magnetospheric Ion Coimposition Experiment (CAMMICE) on the board the POLAR spacecraft observed more than 70 cusp energetic particle (CEP) eventin 1996. All of these events were associated with a decrease in the magnitude of the local magnetic field measured by the Magnetic Field Experiment (MFE) on POLAR. This is an important discovery, which reveals six interesting features for the CEP events: (1) They all were detected in the dayside at high latitude near apogee; (2) their energies were in excess of 2.4 MeV; (3) an individual CEP event could last for hours; (4) the event-averaged intensity of 1–200 keV/e helium was anticorrelated with the magnitude of the local geomagnetic field but correlated with the turbulent magnetic energy density; (5) the events were associated with an enhancement of the low frequency electromagnetic noise; and (6) a possible seasonal variation was found for the occurence of the events with a maximum in September. The measured high charges state of helium and oxygen ions in the CEP events indicates a solar source for these particles. A possible explanation is that the energetic helium ions are energized from lower energy helium by a local acceleration mechanism associated within the high-altitude dayside cusp. These observations represent a discovery of a major acceleration region of the magnetosphere.

The High-Energy Suprathermal Time-of-Flight sensor (HSTOF) of the Charge, Element, and Isotope Analysis System (CELIAS) on the Solar and Heliospheric Observatory (SOHO) near the Lagrangian point L1 is capable of identifying energetic hydrogen atoms (EHAs) between 55 and 80 keV. Between 1996 February 13 and 1997 August 31, near solar minimum, there were 285 "quiet" days when the interplanetary charged-particle flux was low. During these quiet times, HSTOF scanned the apex of the heliosphere once and the antiapex twice. The flux level and time profile, and hence the arrival direction, of the EHAs accumulated during these quiet times are best interpreted as fluxes of EHAs coming from the heliosheath.

The objective of this paper is to explore the structure and properties of the region between the shocked solar wind and the planetary plasma on the dayside of Mars. This study is based on the observations obtained by the plasma instruments carried on board the Phobos 2 spacecraft. These instruments have independently identified several plasma boundaries in the dayside magnetosphere of Mars, the boundaries are located close to each other but exhibit different plasma features. This investigation leads us to conclude that a dayside region exists on Mars with the following characteristics: (1) the bulk of the shocked solar wind protons are deflected, (2) the total magnetic field increases, (3) a plasma depletion region is formed, (4) both shocked solar wind and planetary plasma are present, (5) accelerated electrons and heavy ions are also present, (6) intensive wave activity is seen in the 5 to 150 Hz frequency interval, and (7) a current layer and associated magnetic shears can be identified. We name this region, located between the ionosphere and the shocked solar wind, the dayside boundary layer or dayside mantle. Its key features are similar to those of the Venus mantle; the variety of names given to the Martian plasma boundaries merely reflects its complicated structure. We conclude that the two boundary layers at Venus and Mars are very similar and that the physical processes (i.e., the interaction of the shocked solar wind with the planetary plasma) at work within these boundary layers are probably similar.

From September to October 1996 the Galileo spacecraft crossed through the distant predawn tail region of the Jovian magnetosphere. The Energetic Particles Detector (EPD) onboard Galileo recorded a series of energetic particle flow bursts in the region beyond 80 R j to the apojove at 113 R j . The events are similar in nature to an event observed with the hot plasma instrument (LECP) onboard Voyager 2. The individual events last for several hours and cover the whole energy range from 15 keV to 55 MeV. The majority of them show considerable intensity increases which are most prominent for heavy ions. The events exhibit high radially outward directed anisotropies suggesting strongly collimated radial outflowing ion beams. The Voyager event was observed beyond the corotation boundary within a magnetospheric boundary layer termed the magnetospheric wind region and consequently it was assumed that the underlying process is connected with a boundary layer instability. However, the Galileo observations show the bursts being embedded in a general corotation flow. It is thus suggested that the flow bursts are driven by an internal plasma sheet process possibly associated with a major re-configuration of the Jovian magnetotail. A series of five very prominent flow bursts observed near apojove of the orbit occurred quasi-periodically with a repetition period of about 2.6±0.2 days which is presumably related to an internal time constant of the Jovian magnetotail.

Measurements with the Energetic Particles Detector (EPD) on Galileo orbit C9 in the Jovian magnetotail revealed the existence of distinct quasi-periodic variations of energetic ion intensities which are superimposed on the well-known 10-hour modulations due to the planetary rotation. The intensity variations are associated with changes of the particle energy spectra and the plasma flow pattern. They are clearly of temporal nature and not the consequence of the spacecraft passing through periodically separated spatial structures. The modulation period is about 3 days. The oscillations are most pronounced throughout the middle magnetotail regime (20 to 80 R j ), however, seem to persist even in the deep tail region. The amplitude of the modulation is dependent on the particle energy. The highest energies measured (about I MeV) show the strongest variations. Energetic particle features with similar periodicity are observed on other Galileo orbits as well. The cause of these modulations is unclear; however, it may be speculated that they correspond to a quasi-periodic transition between two basic states of the Jovian magnetotail which occur with a time constant inherent to the Jovian magnetosphere.

We present first results taken from the high-resolution mass time-of-flight spectrometer (MTOF) of the charge, element, and isotope analysis system (CELIAS) experiment on board the Solar and Heliospheric Observatory (SOHO) spacecraft launched in December 1995, concerning the abundance ratios of neon isotopes in the solar wind. We obtain the isotopic ratios 20Ne/22Ne = (13.8 ± 0.7) and 20Ne/21Ne = (440 ± 110), which agree with the values obtained from the Apollo foil solar wind experiments and which have been derived from measurements of solar particles implanted in lunar and meteoritic samples.

On August 27, 1996, a two-hour energetic heavy ion event (∼ 1 MeV) was detected at 8:25 UT at apogee (∼ 9 R e and an invariant latitude of ∼ 80°), by the Charge and Mass Magnetospheric Ion Composition Experiment onboard POLAR. The event, with a maximum spin averaged peak flux of ∼ 150 particles/(cm 2 -sr-s-MeV), showed three local peaks corresponding to three localized regions; the ion pitch angle distributions in the three regions were different from an isotropic distribution and different from each other. No comparable flux was observed by the WIND spacecraft. The appearance of lower energy He ++ and O >+2 during the event period indicates a solar source for these particles. From region 1 to 2 to 3, the helium energy spectra softened. A distorted magnetic field with three local minima corresponding to the three He peak fluxes was also observed by POLAR. A possible explanation is that the energetic He ions were energized from lower energy helium by a local acceleration mechanism that preferred smaller rigidity ions in the high altitude polar cusp region.

We present the first observations by Polar and 1995-034 of the mid-altitude cusp. On May 29, 1996, the Polar and 1995-034 spacecraft crossed into an extended cusp region. The region was characterized by intense fluxes of solar wind like ions in the energy range 1–10 keV that had angular distributions that showed evidence of flows and trapping. The ion composition data are combined with energetic proton observations from Polar and plasma observations from 1995-034 and DMSP satellites to examine the spatial and temporal extent and plasma characteristics of the cusp during a persistent northward IMF condition. The composition data is consistent with expected solar wind composition with a source temperature of ∼1.25 × 106 K. The combined spacecraft observations show an extended cusp-like region, probably produced by northward IMF.

On January 15, 1994, the ion spectrometer high energy particle - low energy particle detector (HEP-LD) on the Japanese spacecraft Geotail observed five quasi-periodic energetic ion bursts in the deep tail (X=−96 RE). These bursts were associated with plasmoid-like structures in the magnetic field components. In. addition, three multiple TCR groups were identified in the interval. The observations in the distant tail occurred during a time interval of substorm activity which also produced multiple injections in the geosynchronous orbit region. The HEP-LD observations show that Bz bipolar plasmoid-like structures are associated with tailward flowing particle bursts. However, earthward flowing particle bursts are predominantly associated with bipolar signatures in By. In addition, an oxygen burst was seen in the back of a plasmoid (postplasmoid) which showed both By and Bz bipolar magnetic field signatures. The oxygen burst lasted for 23 min, and the density ratio (O/H) reached 15% for the HEP-LD energy range (in the same plasmoid, this ratio was approximately 1% before the oxygen burst). The oxygen burst exhibited a strong beam-like structure which occupied only 6 ∼ 7% of the full solid angle (4π). We suggest that energized oxygen ions of ionospheric origin travel downtail in the narrow postplasmoid-plasma sheet which trails the plasmoid. Furthermore, we suggest that the magnetosphere dissipated larger quantities of energy during this very intense substorm event by ejecting multiple relatively small plasmoids rather than through the formation and ejection of a single large plasmoid.

Solar wind plasma and magnetic data obtained near the Martian terminator bow shock by the TAUS energy spectrometer and the MAGMA magnetometer onboard the Phobos 2 spacecraft are analyzed. It is revealed that on average the solar wind stream is slowing down just upstream of the bow shock. Nearly inverse correlation is found between the values of the velocity decrease and the undisturbed solar wind density for the outbound (mostly quasi-parallel) bow shock crossings, while for the inbound crossings (mostly quasi-perpendicular) this correlation is observed only for the velocity decrease upstream of the shock foot. This result permits us to distinguish between the two possible reasons causing solar wind deceleration: (1) mass loading of the solar wind flow by planetary ions originating from the corona of Mars, and (2) solar wind protons reflected from the bow shock. The solar wind deceleration upstream of the bow shock foot turned to be approximately dawn-dusk symmetric. On the basis of the revealed relation between the velocity decrease and upstream solar wind density, a coronal density profile is deduced which is in agreement with earlier results for the subsolar region. According to this profile, the density of the hot oxygen corona of Mars might be ∼ 3 times higher in the period of observations than the estimations of the "extreme" corona model suggests, even taking into account the possible contribution of the hydrogen corona to the solar wind deceleration effect.

In 1989 the Phobos 2 spacecraft provided sufficient plasma and magnetic field data for statistical investigations of the Martian bow shock structure. Significant magnetic field overshoots were observed in more than 70% of the bow shock crossings detected in the terminator region. According to our studies based on the available low-resolution data set, the features of the Martian overshoots are very similar to those observed in the terrestrial and Venusian bow shock: the magnitude increases with increasing Mach number and the thickness scales as the proton gyroradius computed from the upstream solar wind speed and magnetic field strength. Our investigations seem to support the idea of Mellott and Livesey [1987] suggesting that overshoots may develop not only in supercritical shocks but also when the Mach number is below or around the critical value.

Low energy (