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The Quasi-thermal noise (QTN) technique is a reliable tool to yield accurate measurements of the electron parameters in the solar wind. We apply this method on Parker Solar Probe (PSP) observations to derive the total electron temperature ($T_e$) from the linear fit of the high-frequency part of the QTN spectra acquired by the RFS/FIELDS instrument, and present a combination of 12-day period of observations around each perihelion from Encounter One (E01) to Ten (E10) (with E08 not included) with the heliocentric distance varying from about 13 to 60 solar radii ($R_\odot{}$). We find that the total electron temperature decreases with the distance as $\sim$$R^{-0.66}$, which is much slower than adiabatic. The extrapolated $T_e$ based on PSP observations is consistent with the exospheric solar wind model prediction at $\sim$10 $R_\odot{}$, Helios observations at $\sim$0.3 AU and Wind observations at 1 AU. Also, $T_e$, extrapolated back to 10 $R_\odot{}$, is almost the same as the strahl electron temperature $T_s$ (measured by SPAN-E) which is considered to be closely related to or even almost equal to the coronal electron temperature. Furthermore, the radial $T_e$ profiles in the slower solar wind (or flux tube with larger mass flux) are steeper than those in the faster solar wind (or flux tube with smaller mass flux). More pronounced anticorrelated $V_p$-$T_e$ is observed when the solar wind is slower and closer to the Sun., Comment: 10 pages, 7 figures, and Astronomy & Astrophysics Accepted

International audience; Auroral Kilometric Radiation (AKR) is the strongest terrestrial radio emission, and emanates from the same electron acceleration regions from which particles precipitate into the ionosphere, exciting the aurorae and other phenomena. As such, AKR is a barometer for the state of solar wind - magnetosphere - ionosphere coupling. AKR is anisotropically beamed in a hollow cone from a source region generally found at nightside local times, meaning that a single source region cannot be viewed from all local times in the magnetosphere. In radio data such as dynamic spectra, AKR is frequently observed simultaneously to other radio emissions which can have a similar intensity and frequency range, making it difficult to automatically detect. Building on a previously published pipeline to extract AKR emissions from Wind/WAVES data, in this paper a novel automated AKR burst detection technique is presented and applied to Wind/WAVES data. Over a five year interval, about 5000 AKR bursts are detected with median burst length ranging from about 30 to 60 min. During detected burst windows, higher solar wind velocity is observed, and the interplanetary magnetic field clock angle is observed to tend toward BZ < 0, BY < 0, when compared with the entire statistical interval. Additionally, higher geomagnetic activity is observed during burst windows at polar, high and equatorial latitudes.

International audience; Eclipse observations of the W-L corona show linear rays above active regions at times of solar maximum. We show that these linear rays are also observed in the field-of-view of the C2-LASCO coronagraph, in perfect correspondence with the eclipse results. A selected prominent case taken from the 2001 eclipse observation in Angola is analysed with several different methods, including the use of a synoptic map constructed using SoHO/LASCO C2 images. A clear signature of time variations near the eclipse observation is detected, suggesting that at least some parts of the beam are collimated. These observations strongly suggest high speed streams that apparently ignore the potential large scale coronal magnetic field rooted rather low in the corona. A possible origin is the neutral magnetic points located above the active region. Several mechanisms exist to explain how the plasma is accelerated in these regions to large quasi-relativistic velocities, possibly related to the occurrence of type III radio bursts. We point out a curious analogy with phenomena occurring inside coronal holes.

Context. Dust impacts have been observed using radio and wave instruments onboard spacecraft since the 1980s. Voltage waveforms show typical impulsive signals generated by dust grains. Aims. We aim at developing models of how signals are generated to be able to link observed electric signals to the physical properties of the impacting dust. To validate the model, we use the Time Domain Sampler (TDS) subsystem of the STEREO/WAVES instrument which generates high-cadence time series of voltage pulses for each monopole. Methods. We propose a new model that takes impact-ionization-charge collection and electrostatic-influence effects into account. It is an analytical expression for the pulse and allows us to measure the of amount of the total ion charge, Q, the fraction of escaping charge, ϵ, the rise timescale, τi, and the relaxation timescale, τsc. The model is simple and convenient for massive data fitting. To check our model’s accuracy, we collected all the dust events detected by STEREO/WAVES/TDS simultaneously on all three monopoles at 1AU since the beginning of the STEREO mission in 2007. Results. Our study confirms that the rise time largely exceeds the spacecraft’s short timescale of electron collection. Our estimated rise time value allows us to determine the propagation speed of the ion cloud, which is the first time that this information has been derived from space data. Our model also makes it possible to determine properties associated with the electron dynamics, in particular the order of magnitude of the electron escape current. The obtained value gives us an estimate of the cloud’s electron temperature – a result that, as far as we know, has never been obtained before except in laboratory experiments. Furthermore, a strong correlation between the total cloud charge and the escaping charge allows us to estimate the escaping current from the amplitude of the precursor, a result that could be interesting for the study of the pulses recently observed in the magnetic waveforms of Solar Orbiter or Parker Solar Probe, for which the electric waveform is saturated.

A weak continuous line has been recently discovered onboard Voyager 1 in the interstellar medium, whose origin raised two major questions. First, how can this line be produced by plasma quasi-thermal noise on the Voyager short antenna? Second, why does this line emerge at some distance from the heliopause? We provide a simple answer to these questions, which elucidates the origin of this line. First, a minute quantity of supra-thermal electrons, as generally present in plasmas, whence the qualifier quasi-thermal, can produce a small plasma frequency peak on a short antenna, of amplitude independent of the concentration of these electrons; furthermore, the detection required long spectral averages, alleviating the smallness of the peak compared to the background. We therefore attribute the observed line to a minute proportion of fast electrons that contribute negligibly to the pressure. Second, we suggest that, up to some distance from the heliopause, the large compressive fluctuations ubiquitous in this region prevent the line to emerge from the statistical fluctuations of the receiver noise because it is blurred out by the averaging required for detection,especially in the presence of short-wavelength density fluctuations. These results open up novel perspectives for interstellar missions, by showing that a minute proportion of fast electrons may be sufficient to measure the density even with a relatively short antenna, because the quietness of the medium enables a large number of spectra to be averaged., 5 pages, 4 figures, paper accepted for publication in Astronomy & Astrophysics Letters

Aims. We aim to establish the boundaries of Mercury’s magnetosphere through a comparison of the drops and rises of the electron density revealed by PWI/SORBET, during BepiColombo’s first and second swing-bys of Mercury carried out on 1 October 2021 and 23 June 2022, with global 3D magnetohydrodynamic simulations. Methods. SORBET was switched on during both swing-bys and its radio spectra were re-analysed using a new method based on the theory of the quasi-thermal-noise spectroscopy and adapted to measurements registered with a non-deployed antenna (as planned for the entire cruise phase). In parallel, magnetohydrodynamical (MHD) global simulations of Mercury’s magnetosphere were run under different solar wind conditions. Profiles of the electron density obtained from SORBET data were compared with three MHD simulations, using different values for solar wind sonic Mach numbers and plasma β. Results. Three drops and rises of electron density are clearly identified with the boundaries of the magnetosphere (bow shock, magnetopause, and boundary of a region dominated by closed magnetic field lines) on the inbound part of the first Mercury swing-by. Conclusions. On the inbound part of the first swing-by, a good match is found between the SORBET data and the MHD simulations, revealing the quick reorganisation of the Mercury’s magnetosphere in a variable solar wind. This study also highlights the essential role of the electron density in the future detection of Mercury’s magnetosphere boundaries once BepiColombo will orbit the planet from December 2025.

Auroral Kilometric Radiation (AKR) is terrestrial radio emission that originates in particle acceleration regions along magnetic field lines, coinciding with discrete auroral arcs. AKR viewing geometry is complex due to the confinement of the source regions to nightside local times (LTs) and the anisotropy of the beaming pattern, so observations are highly dependent on spacecraft viewing position. We present a novel, empirical technique that selects AKR emission from observations made with the spin-axis aligned antenna of the Wind/WAVES instrument, based on the rapidly varying amplitude of AKR across spacecraft spin timescales. We apply the technique to Wind/WAVES data during 1999 day of year 227–257, when the Cassini spacecraft flew past Earth and provided an opportunity to observe AKR from two remote locations. We examine the AKR flux and power, with observations made from LTs of 1700–0300 hr having an average power up to 10 4 Wsr -1 larger than those on the dayside and an increasing AKR power observed at higher magnetic latitudes. We perform a linear cross-correlation between the Wind AKR power and the spacecraft magnetic latitude, showing positive then negative correlation as Wind travels from the Northern to Southern magnetic hemisphere. Statistically significant diurnal modulations are found in the whole 30-day period and in subsets of the data covering different local time sectors, indicative of a predominantly geometrical effect for remote AKR viewing. The reproduction of well-known features of the AKR verifies the empirical selection and shows the promise of its application to Wind/WAVES observations.

International audience; The Radio and Plasma Waves (RPW) instrument on the ESA Solar Orbiter mission is designed to measure in situ magnetic and electric fields and waves from the continuum up to several hundred kHz. The RPW also observes solar and heliospheric radio emissions up to 16 MHz. It was switched on and its antennae were successfully deployed two days after the launch of Solar Orbiter on February 10, 2020. Since then, the instrument has acquired enough data to make it possible to assess its performance and the electromagnetic disturbances it experiences. In this article, we assess its scientific performance and present the first RPW observations. In particular, we focus on a statistical analysis of the first observations of interplanetary dust by the instrument’s Thermal Noise Receiver. We also review the electro-magnetic disturbances that RPW suffers, especially those which potential users of the instrument data should be aware of before starting their research work.

Impacts of dust grains on spacecraft are known to produce typical impulsive signals in the voltage waveform recorded at the terminals of electric antennas. Such signals are routinely detected by the Time Domain Sampler (TDS) system of the Radio and Plasma Waves (RPW) instrument aboard Solar Orbiter. We investigate the capabilities of RPW in terms of interplanetary dust studies and present the first analysis of dust impacts recorded by this instrument. We discuss previously developed models of voltage pulses generation after a dust impact onto a spacecraft and present the relevant technical parameters for Solar Orbiter RPW as a dust detector. Then we present the statistical analysis of the dust impacts recorded by RPW/TDS from April 20th, 2020 to February 27th, 2021 between 0.5 AU and 1 AU. The study shows that the dust population studied presents a radial velocity component directed outward from the Sun, the order of magnitude of which can be roughly estimated as $v_{r, dust} \simeq 50$ km.$s^{-1}$. This is consistent with the flux of impactors being dominated by $\beta$-meteoroids. We estimate the cumulative flux of these grains at 1 AU to be roughly $F_\beta \simeq 8\times 10^{-5} $ m$^{-2}$s$^{-1}$, for particles of radius $r \gtrsim 100$ nm. The power law index $\delta$ of the cumulative mass flux of the impactors is evaluated by two differents methods (direct observations of voltage pulses and indirect effect on the impact rate dependency on the impact speed). Both methods give a result $\delta \simeq 0.3-0.4$. Solar Orbiter RPW proves to be a suitable instrument for interplanetary dust studies. These first results are promising for the continuation of the mission, in particular for the in-situ study of the dust cloud outside the ecliptic plane, which Solar Orbiter will be the first spacecraft to explore., Comment: 13 pages, 8 figures

Context.On December 27, 2020, Solar Orbiter completed its first gravity assist manoeuvre of Venus (VGAM1). While this flyby was performed to provide the spacecraft with sufficient velocity to get closer to the Sun and observe its poles from progressively higher inclinations, the Radio and Plasma Wave (RPW) consortium, along with other operational in situ instruments, had the opportunity to perform high cadence measurements and study the plasma properties in the induced magnetosphere of Venus.Aims.In this paper, we review the main observations of the RPW instrument during VGAM1. They include the identification of a number of magnetospheric plasma wave modes, measurements of the electron number densities computed using the quasi-thermal noise spectroscopy technique and inferred from the probe-to-spacecraft potential, the observation of dust impact signatures, kinetic solitary structures, and localized structures at the bow shock, in addition to the validation of the wave normal analysis on-board from the Low Frequency Receiver.Methods.We used the data products provided by the different subsystems of RPW to study Venus’ induced magnetosphere.Results.The results include the observations of various electromagnetic and electrostatic wave modes in the induced magnetosphere of Venus: strong emissions of ∼100 Hz whistler waves are observed in addition to electrostatic ion acoustic waves, solitary structures and Langmuir waves in the magnetosheath of Venus. Moreover, based on the different levels of the wave amplitudes and the large-scale variations of the electron number densities, we could identify different regions and boundary layers at Venus.Conclusions.The RPW instrument provided unprecedented AC magnetic and electric field measurements in Venus’ induced magnetosphere for continuous frequency ranges and with high time resolution. These data allow for the conclusive identification of various plasma waves at higher frequencies than previously observed and a detailed investigation regarding the structure of the induced magnetosphere of Venus. Furthermore, noting that prior studies were mainly focused on the magnetosheath region and could only reach 10–12 Venus radii (RV) down the tail, the particular orbit geometry of Solar Orbiter’s VGAM1, allowed the first investigation of the nature of the plasma waves continuously from the bow shock to the magnetosheath, extending to ∼70RVin the far distant tail region.

Aims. We use multi-spacecraft observations of individual type III radio bursts to calculate the directivity of the radio emission. We compare these data to the results of ray-tracing simulations of the radio-wave propagation and probe the plasma properties of the inner heliosphere. Methods. We used ray-tracing simulations of radio-wave propagation with anisotropic scattering on density inhomogeneities to study the directivity of radio emissions. Simultaneous observations of type III radio bursts by four widely separated spacecraft were used to calculate the directivity and position of the radio sources. The shape of the directivity pattern deduced for individual events is compared to the directivity pattern resulting from the ray-tracing simulations. Results. We show that simultaneous observations of type radio III bursts by four different probes provide an opportunity to estimate the radio source positions and the directivity of the radio emission. The shape of the directivity varies from one event to another and it is consistent with anisotropic scattering of the radio waves.

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.

International audience; The Plasma Wave Investigation (PWI) aboard the BepiColombo Mio (Mercury Magnetospheric Orbiter, MMO) will enable the first observations of electric fields, plasma waves, and radio waves in and around the Hermean magnetosphere and exosphere. The PWI has two sets of receivers (EWO with AM2P, SORBET) connected to two electric field sensors (MEFISTO and WPT) and two magnetic field sensors (SCM: LF-SC and DB-SC). After the launch on October 20, 2018, we began initial operations, confirmed that all receivers were functioning properly, and released the launch locks on the sensors. Those sensors are not deployed during the cruising phase, but the PWI is still capable performing magnetic field observations. After full deployment of all sensors following insertion into Mercury orbit, the PWI will start its measurements of the electric field from DC to 10 MHz using two dipole antennae with a 32-m tip-to-tip length in the spin plane and the magnetic field from 0.3 Hz to 20 kHz using a three-axis sensor and from 2.5 kHz to 640 kHz using a single-axis sensor at the tip of a 4.5-m solid boom extended from the spacecraft’s side panel. Those receivers and sensors will provide (1) in-situ measurements of electron density and temperature that can be used to determine the structure and dynamics of the Hermean plasma environment; (2) in-situ measurements of the electron and ion scale waves that characterize the energetic processes governed by wave–particle interactions and non-MHD interactions; (3) information on radio waves, which can be used to remotely probe solar activity in the heliocentric sector facing Mercury, to study electromagnetic-energy transport to and from Mercury, and to obtain crustal information from reflected electromagnetic waves; and (4) information concerning dust impacts on the spacecraft body detected via potential disturbances. This paper summarizes the characteristics of the overall PWI, including its significance, its objectives, its expected performance specifications, and onboard and ground data processing. This paper also presents the detailed design of the receiver components installed in a unified chassis. The PWI in the cruise phase will observe magnetic-field turbulence during multiple flybys of Earth, Venus, and Mercury. After the Mercury-orbit insertion planned at the end of 2025, we will deploy all sensors and commence full operation while coordinating with all payloads onboard the Mio and MPO spacecraft.

International audience; We discuss the solar wind electron temperatures T-e as measured in the nascent solar wind by Parker Solar Probe during its first perihelion pass. The measurements have been obtained by fitting the high-frequency part of quasithermal noise spectra recorded by the Radio Frequency Spectrometer. In addition we compare these measurements with those obtained by the electrostatic analyzer discussed in Halekas et al. These first electron observations show an anticorrelation between T-e and the wind bulk speed V: this anticorrelation is most likely the remnant of the wellknown mapping observed at 1 au and beyond between the fast wind and its coronal hole sources, where electrons are observed to be cooler than in the quiet corona. We also revisit Helios electron temperature measurements and show, for the first time, that an in situ (T-e, V) anticorrelation is well observed at 0.3 au but disappears as the wind expands, evolves, and mixes with different electron temperature gradients for different wind speeds.

The electric and magnetic field instrument suite FIELDS on board the NASA Parker Solar Probe and the radio and plasma waves instrument RPWS on the ESA Solar Orbiter mission that explore the inner heliosphere are sensitive to signals generated by dust impacts. Dust impacts were observed using electric field antennas on spacecraft since the 1980s and the method was recently used with a number of space missions to derive dust fluxes. Here, we consider the details of dust impacts, subsequent development of the impact generated plasma and how it produces the measured signals. We describe empirical approaches to characterise the signals and compare to a qualitative discussion of laboratory simulations to predict signal shapes for spacecraft measurements in the inner solar system. While the amount of charge production from a dust impact will be higher near the sun than observed in the interplanetary medium before, the amplitude of pulses will be lower because of the different recovery behaviour that varies with the plasma environment. The photocurrent, that is expected to be higher near the Sun, is found to have moderate influence on the spacecraft potential.

Accepted: 2006-01-29, 資料番号: SA1000607000

International audience; We present the solar wind plasma parameters obtained from the Wind spacecraft during more than nine years, encompassing almost the whole solar cycle 23. Since its launch in November 1994 Wind has frequently observed the in-ecliptic solar wind upstream of the Earth's bow shock. The WIND/WAVES thermal noise receiver was specially designed to measure the in situ plasma thermal noise spectra, from which the electron density and temperature can be accurately determined. We present and discuss histograms of such measurements performed from 1994 to 2003. Using these large data sets, we study the density and core temperature variations with solar activity cycle and with different regimes of the solar wind. We confirm the anticorrelation of the electron density with the sunspot number, and obtain a positive correlation of the core temperature, with the sunspot number.

International audience; We present the solar wind plasma parameters obtained from the Ulysses spacecraft during its second pole-to-pole fast latitude scan near the 2001 solar maximum. We study the solar wind properties from the electron density and core temperature measurements made by the radio receiver on Ulysses using the method of quasi-thermal noise spectroscopy. We analyze these parameters as functions of heliographic latitude and distance. We present their histograms normalized to 1 AU and find a bimodal distribution for the electron core temperature. The cooler population can be associated with the fast wind flow emanating from coronal holes present at various latitudes. We discuss a slight north/south asymmetry found for the electron density. Finally, we compare the present results to those obtained during the 1996 solar minimum and 1991 solar maximum.

We update two kinds of results obtained with the SWAN instrument on board SOHO. First, we use H cell data recorded in 2001 and derive the H flow direction in the same way we performed the study at solar minimum. We compare with the Helium flow and doing so we correct for the coordinate system change between the Ulysses and SOHO mission. The deflection plane we obtain is compatible with the previous result within error bars, confirming the predominant role of the interstellar magnetic field. Secondly, we extend the derivation of solar wind ionization temporal evolution as a function of heliolatitude. The pattern for the present solar minimum is strikingly different from the previous minimum, with a much wider slow solar wind equatorial belt which persists until at least 2008. Comparing with synoptic LASCO/C2 electron densities we infer from a preliminary study that the acceleration of the high speed solar wind occurs at a higher altitude during this minimum, a change expansion models should be able to explain., 4 pages, Twelfth International Solar Wind Conference, 21-26 June 2009, Saint-Malo, France

International audience; Among several mechanisms issued from simulation and theoretical studies proposed to account for the nonstationarity of quasi-perpendicular supercritical shocks, one process—the so-called self-reformation—driven by the accumulation of reflected ions in the foot has been intensively analyzed with simulations. Present results based on experimental CLUSTER mission clearly evidence signatures of this self-reformation process for the terrestrial bow shock. The study based on magnetic field measurements includes two parts: (i) a detailed analysis of one typical shock crossing for almost perpendicular shock directions where the risk of pollution by other nonstationarity mechanisms is minimal. A special attention is drawn on appropriate treatment of data to avoid any wrong interpretation. One key result is that the ramp width can reach a very narrow value covering a few electron inertial lengths only; (ii) a statistical analysis allows relating the signatures of this nonstationarity with different plasma conditions and shock regimes. Present results are discussed in comparison with previous simulation works.

We present a kinetic collisionless model of the solar wind generalized to take into account the spiral structure of the interplanetary magnetic field. This model, which also includes Kappa velocity distributions, calculates self-consistently the electric potential profile and derives the solar wind speed and the temperatures of the medium. We study how the inclusion of the spiral geometry changes the plasma parameters compared to the case of a radial magnetic field. Whereas the interplanetary electric potential, the wind density and bulk speed are not significantly changed, we show that the electron and proton temperatures are modified; in particular, we find a decrease of the proton temperature and of its anisotropy, and an increase of the electron temperature. We discuss these results and the validity of the model.

International audience; The measurement of the solar wind electron temperature in the unexplored region between 1 and 45 Rs is of prime importance for understanding the solar wind acceleration. Solar Orbiter's location, combined with the fact that the spacecraft will nearly co-rotate with the sun on some portions of its orbit, will furnish observations placing constraints on solar wind models. We discuss the implementation of the plasma thermal noise analysis for the Solar Orbiter, in order to get accurate measurements of the total electron density and electron temperature and to correct the spacecraft charging effects which affect the electron analyzers.

The Mars Global Surveyor (MGS), Mars Express (MEX), and Venus Express (VEX) experienced several superior solar conjunctions. These conjunctions cause severe degradations of radio signals when the line of sight between the Earth and the spacecraft passes near to the solar corona region. The primary objective of this work is to deduce a solar corona model from the spacecraft navigation data acquired at the time of solar conjunctions and to estimate its average electron density. The corrected or improved data are then used to fit the dynamical modeling of the planet motions, called planetary ephemerides. We analyzed the radio science raw data of the MGS spacecraft using the orbit determination software GINS. The range bias, obtained from GINS and provided by ESA for MEX and VEX, are then used to derive the electron density profile. These profiles are obtained for different intervals of solar distances: from 12Rs to 215Rs for MGS, 6Rs to 152Rs for MEX, and form 12Rs to 154Rs for VEX. They are acquired for each spacecraft individually, for ingress and egress phases separately and both phases together, for different types of solar winds (fast, slow), and for solar activity phases (minimum, maximum). We compared our results with the previous estimations that were based on in situ measurements, and on solar type III radio and radio science studies made at different phases of solar activity and at different solar wind states. Our results are consistent with estimations obtained by these different methods. Moreover, fitting the planetary ephemerides including complementary data that were corrected for the solar corona perturbations, noticeably improves the extrapolation capability of the planetary ephemerides and the estimation of the asteroids masses., Comment: Accepted for publication in A&A

We have developed a simulation model of particle acceleration in coronal shock waves. The model is based on a Monte Carlo method, where particles are traced in prescribed large-scale electromagnetic fields utilizing the guiding center approximation. The particles are scattered in the turbulence according to quasilinear theory, with the scattering amplitude directly proportional to the intensity of Alfv\'en waves at gyro-resonant wavenumbers. The Alfv\'en waves are traced simultaneously with the particles, so that the wave field is propagated outwards from the Sun using WKB propagation supplemented with a phenomenological wavenumber diffusion term and a growth rate computed from the net flux of the accelerated particles. We consider initial wave amplitudes small enough to allow rapid escape of particles from the shock to the ambient medium. Thus, in our model the Alfv\'en waves responsible for the diffusive acceleration of particles are generated by the accelerated particles themselves. In this work, we study the effects of non-constant shock velocity and non-monotonic Alfv\'en velocity on particle acceleration scenarios. We report in particular how the deceleration of a shock affects particle intensity and turbulence power evolution in the vicinity of the shock., Comment: 5 pages, 8 figures. Copyright (2010) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Twelfth International Solar Wind Conference, AIP Conference Proceedings 2010

Most in situ measurements of cosmic dust have been carried out with dedicated dust instruments. However, dust particles can also be detected with radio and plasma wave instruments. The high velocity impact of a dust particle generates a small crater on the spacecraft, and the dust particle and the crater material are vaporised and partly ionised. The resulting electric charge can be detected with plasma instruments designed to measure electric waves. Since 2007 the STEREO/WAVES instrument has recorded a large number of events due to dust impacts. Here we will concentrate on the study of those impacts produced by dust grains originating from the local interstellar cloud. We present these fluxes during five years of the STEREO mission. Based on model calculations, we determine the direction of arrival of interstellar dust. We find that the interstellar dust direction of arrival is ∼260∘, in agreement with previous studies.

[1] We analyze dust impacts recorded by the S/WAVES radio instrument onboard the two STEREO spacecraft near 1 A.U. during the period 2007–2010. The impact of a dust particle on a spacecraft produces a plasma cloud whose associated electric field can be detected by on-board electric antennas. For this study we use the electric potential time series recorded by the waveform sampler of the instrument. The high time resolution and long sampling times of this measurement enable us to deduce considerably more information than in previous studies based on the dynamic power spectra provided by the same instrument or by radio instruments onboard other spacecraft. The large detection area compared to conventional dust detectors provides flux data with a better statistics. We show that the dust-generated signals are of two kinds, corresponding to impacts of dust from distinctly different mass ranges. We propose calibration formulas for these signals and show that we are able to use S/WAVES as a dust detector with convincing results both in the nanometer and micrometer size ranges. In the latter, the orbital motion of the spacecraft enables us to distinguish between interstellar and interplanetary dust components. Our measurements cover the mass intervals ∼10−22–10−20 kg and ∼10−17 − 5 × 10−16 kg. The flux of the larger dust agrees with measurements of other instruments on different spacecraft.

The solar-wind energy flux measured near the ecliptic is known to be independent of the solar-wind speed. Using plasma data from Helios, Ulysses, and Wind covering a large range of latitudes and time, we show that the solar-wind energy flux is independent of the solar-wind speed and latitude within 10%, and that this quantity varies weakly over the solar cycle. In other words the energy flux appears as a global solar constant. We also show that the very high speed solar-wind (VSW > 700 km/s) has the same mean energy flux as the slower wind (VSW < 700 km/s), but with a different histogram. We use this result to deduce a relation between the solar-wind speed and density, which formalizes the anti-correlation between these quantities., 12 pages, 5 figures

Chapman's (1957) conductive model of the solar corona is characterized by a temperature varying as r**(-2/7) with heliocentric distance r. The density distribution in this non-isothermal hydrostatic model has a minimum value at 123 RS, and increases with r above that altitude. It is shown that this hydrostatic model becomes convectively unstable above r = 35 RS, where the temperature lapse rate becomes superadiabatic. Beyond this radial distance heat conduction fails to be efficient enough to keep the temperature gradient smaller than the adiabatic lapse rate. We report the results obtained by Lemaire (1968) who showed that an additional mechanism is then required to transport the energy flux away from the Sun into interplanetary space. He pointed out that this additional mechanism is advection: i.e. the stationary hydrodynamic expansion of the corona. In other words the corona is unable to stay in hydrostatic equilibrium. The hydrodynamic solar wind expansion is thus a physical consequence of the too steep (superadiabatic) temperature gradient beyond the peak of coronal temperature that can be determined from white light brightness distributions observed during solar eclipses. The thermodynamic argument for the existence of a continuous solar wind expansion which is presented here, complements Parker's classical argument based on boundary conditions imposed to the solutions of the hydrodynamic equations for the coronal expansion: i.e. the inability of the mechanical forces to hold the corona in hydrostatic equilibrium. The thermodynamic argument presented here is based on the energy transport equation. It relies on the temperature distribution which becomes super-adiabatic above a certain altitude in the inner corona., Comment: 4 pages, 3 figures, presented at SW12 conference (2009); Copyright 2010 American Institute of Physics 978-0-7354-0759-6/10. This article may be downloaded for personal use only. The following article appeared in CP1216 and may be found at http://proceedings.aip.org

International audience; We describe here a solar wind 1-fluid 1D model which is (1) time-dependent (2) includes transition region down to solar surface (3) uses a moderate number of grid points to enable further generalization to 2D/3D. The present model enables to study the response of the transition region and corona to photospheric pressure perturbations, including the solar wind up to ten solar radii. A simple chromosphere model with uniform temperature is used, in two different forms, either taking into account partial ionization or not. We show here how the amplitude of the response and associated viscous heating vary when attempting to decreasing the dynamical viscosity to approach realistic values.

In coronal outflows and solar winds, the presence of the interplanetary magnetic field and heat fluxes combined with jets and shock waves give rise to important thermal anisotropies and energetic counterstreaming motions of plasma shells. Such anisotropic structures of plasma quickly lead to the onset of the kinetic electromagnetic instabilities which are dependent solely on bulk properties of the plasma and not on resonant interaction with charged particles. Here the interplay between the Weibel and firehose instabilities, both driven by an excess of kinetic energy in the direction of the ambient magnetic field, is considered. Their growth rates and thresholds are evaluated and compared for the electron temperature anisotropies in the solar wind. It is shown that the instability of the Weibel-type, which is improperly known as the “oblique firehose” instability, is the most efficient mechanism of isotropisation limiting the increase of particle velocity anisotropy and thus confirming the observations. These instabilities can explain the origin of interplanetary magnetic field fluctuations, which are expected to enhance along the temperature anisotropy thresholds. ispartof: pages:280-283 ispartof: AIP Conference Proceedings vol:1216 issue:1 pages:280-283 ispartof: International Solar Wind Conference location:Saint-Malo, France date:21 Jun - 26 Jun 2009 status: published

Suprathermal electron heat flux dropouts (HFD) serve as a sensitive test of the magnetic topology of the inner heliosphere. Since the heat flux electron strahl always flows away from the Sun, a heat flux dropout should indicate either that the magnetic field line is completely disconnected from the Sun or that the heat flux strahl is scattered into other pitch angles. We present observations of two suprathermal electron heat flux dropout events observed by the Advanced Composition Explorer (ACE) spacecraft which occur simultaneously with impulsive energetic ion events. Since suprathermal electrons encompass the same velocity range as ions with energies of a few MeV/nucleon, the similarities and differences between them as observed at 1 AU probes the sources and transport of these two species. We compare the two events to show the difference between the signatures of a simple disconnection and a more complicated reconnection scenario. Comparing suprathermal electron modulations with energetic ion modulations is a powerful technique for determining the magnetic topology between particle injection at the Sun and observation at 1 AU.

Observations of energetic ions and electrons from STEREO and ACE have been used to investigate the longitudinal extent of particle emissions from 3He ‐rich solar energetic particle (SEP) events. In the event of 3–4 Nov 2008, ions and electrons were detected 20° ahead and behind the nominal connection from the source region to 1 AU, and electrons were also detected 60° ahead. The results are consistent with those of earlier studies that correlated data from near‐Earth spacecraft with Helios data or with observations of source regions on the Sun.

Since early 2007, significant particle enhancements due to corotating interaction regions (CIRs) have regularly appeared at 1 AU without any appreciable contamination from solar energetic particles (SEPs). In 2009 the prevalence of CIRs diminished as the maximum speed of the high speed solar wind streams in the ecliptic decreased along with the tilt of the heliospheric current sheet. Observations of CIR time profiles at different longitudes from STEREO show delays between the Behind and Ahead spacecraft that are often roughly as expected from the corotation time lag, although small differences in the spacecraft latitudes introduce significant scatter in the time delays. In some cases different features seen at Ahead and Behind suggest that transient disturbances in the solar wind may alter connection to or transport from the shock, or that temporal changes occur in the CIR shock itself. H and He data from STEREO/LET at 1.8–6 MeV/nucleon show that 1) the CIR spectral index at these energies is ~−4, independent of intensity but with considerable variability, 2) the He/H ratio is ~0.03 for larger CIRs but varies systematically with energy and event intensity, and 3) although the correlation between the CIR MeV particle increases and solar wind speed is generally good, many times a high-speed stream is not associated with MeV particles, while at other times a recurring series of CIR particle increases appears only at higher energies and may be associated with current sheet crossings and low speed solar wind.

Whistler waves are present in solar wind plasma. These waves possess characteristic turbulent fluctuations that are characterized typically by the frequency and length scales that are respectively bigger than ion gyro frequency and smaller than ion gyro radius. The electron inertial length is an intrinsic length scale in whistler wave turbulence that distinguishably divides the high frequency solar wind turbulent spectra into scales smaller and bigger than the electron inertial length. We present nonlinear three dimensional, time dependent, fluid simulations of whistler wave turbulence to investigate their role in solar wind plasma. Our simulations find that the dispersive whistler modes evolve entirely differently in the two regimes. While the dispersive whistler wave effects are stronger in the large scale regime, they do not influence the spectral cascades which are describable by a Kolmogorov-like $k^{-7/3}$ spectrum. By contrast, the small scale turbulent fluctuations exhibit a Navier-Stokes like evolution where characteristic turbulent eddies exhibit a typical $k^{-5/3}$ hydrodynamic turbulent spectrum. By virtue of equipartition between the wave velocity and magnetic fields, we quantify the role of whistler waves in the solar wind plasma fluctuations., Comment: To appear in the Proceedings of Solar Wind 12

We have surveyed the spectral and compositional properties of suprathermal (∼40–320 keV nucleon−1) heavy ions during quiet times from 1995 January 1 to 2008 December 31 using instruments on board ACE and Wind spacecraft in the interplanetary (IP) medium near 1 AU. We find that the composition of the quiet‐time suprathermal heavy ion population (3He and C‐Fe) depends on the level of solar activity: the ions exhibit SEP‐like composition signatures during solar maximum and CIR or solar‐wind like composition during solar minimum. In addition, we find that during quietest times in the IP medium, all ion species exhibit suprathermal tails with power‐law spectral indices ranging from 1.27 to 2.29, i.e., significantly different from recent observations and theoretical predictions of a unique 1.5 index. We discuss the implications of these new observations for the origin of the suprathermal ion population near 1 AU.

A familiar feature of turbulence in a low collisionality turbulence is an increase in the electric field spectrum, relative to the magnetic field spectrum, at wavenumbers near the reciprocal of the ion inertial scale. This effect is commonly observed in the solar wind. Here we examine this feature numerically, using a variety of simulations, including compressible Hall MHD, incompressible Hall MHD, and one‐, two‐, and three‐dimensional cases. A feature of this type is even found in a statistical Hall MHD model with no dissipation. This leads to the conclusion that the only requirement for obtaining this dispersive effect is the Hall term in the generalized Ohm’s law. Therefore this observation does not distinguish between whistler and kinetic Alfven waves, between waves and turbulence, nor even between fluid and kinetic plasma models.

In addition to the high temperatures, SOHO/UVCS observations have shown that heavy ions in the polar corona are heated more than protons, and that heavy ion heating is more than mass proportional; further, the perpendicular temperatures are much larger than parallel temperatures. Here, we propose that the more than mass proportional heating of heavy ions in coronal holes is due to the ion reflection at supercritical quasi‐perpendicular shocks and to the ion acceleration by the motional electric field in the shock frame. We also discuss the formation of collisionless shock in the polar corona.

The heliospheric current sheet (HCS) is a permanent solar wind feature, with well predicted Earth passages, but it can be structured and its main orientation can be highly distorted. We report new observations from 2 spacecraft in the solar wind (supported by observations from 3 spacecraft in the nightside magnetosheath), showing an evolution across the Sun‐Earth line of large field reversals adjacent to the HCS. Contrary to a previously reported multi‐spacecraft event, this case shows that the field inversion structure cannot be assumed to be well preserved and close to planar on the scale of the magnetospheric cross‐section. However, both cases indicate the presence of field reversals in an away sector that is connected to the southern solar magnetic hemisphere but lies unexpectedly above a toward sector. Following the interpretation of the reversals as transient outflowing loops, associated initial flow deviations can be envisaged to account for the HCS deformations.

The elemental and ionic composition of the slow (∼400 km/s) solar wind (SW) differs from that of the fast (>600 km/s) SW streams. In particular, the first ionization potential (FIP) effect and the O7/O6 ratios are higher in the slow SW than in the fast SW. While this fundamental difference between fast and slow SW has long been appreciated, there has been no reason to expect or effort to search for systematic variations within given fast or slow SW streams. If present, however, such systematic variations could be diagnostic of the solar processes at the sources of the SW. We suggest two reasons to expect compositional differences between leading and trailing edges of slow SW streams. After selecting two long‐lived fast SW streams from 2005 and 2006 and determining their slow‐fast (SF) and fast‐slow (FS) stream interfaces (SIs), ACE SWICS (version 3) compositional data were obtained for the 1‐day periods of the preceding SF and following FS slow SW streams. The statistics were limited, but no compositional...

Spectral analysis of the high‐quality dual‐frequency data obtained with the radio sounding experiment MaRS on the ESA spacecraft Mars Express has revealed a quasi‐periodic component (QPC) at heliocentric distances from 4 to 10 solar radii. The QPC typically appears as a broad maximum in the temporal frequency fluctuation spectrum centered at a frequency in the range 3.7 mHz

The May and November 2007 ICMEs were observed by both STEREO spacecraft as well as the ACE spacecraft. Simultaneous magnetic cloud fits using a multispacecraft model indicate that the axis of the May 2007 flux rope was perpendicular to the plane of the sky, while axis of the November 2007 event was in the ecliptic plane. A new spatial mapping tool has been developed to understand in‐situ composition in terms of the overall magnetic structure of the ICME. The charges states are low for both of these events; though for the May 2007 event there were enhancements beyond the magnetic cloud, but within the ICME. The tools that have been developed in this effort have been proven useful for gaining insight into ICME structures.

The present study concerns a comparison between the slow solar wind plasma parameters obtained in the extended corona by the UV spectroscopic data from the Ultraviolet Coronagraph Spectrometer (UVCS) onboard SOHO during the minimum of solar activity (1996) and the results of a time‐dependent 2.5 D three‐fluid MHD model of coronal streamer belt. The aim of the study is to improve the knowledge of the slow solar wind acceleration mechanism and the origin of its variability.

Inner‐source pickup ions have been investigated by several workers who all assumed an initial velocity distribution function which is dominated by high velocities in the solar wind velocity frame. This non‐thermal velocity distribution is supposed to be due to the Lorentz force which acts on the freshly ionized particles. However, the location where inner‐source pickup ions are ionized lies close to the Sun (probably between 5 and 25 r⊙). The magnetic field and solar wind velocity are near radial there and the Lorentz force acting on freshly created ions is small. Particles sputtered or ejected with small relative velocities (E

We summarize the properties of the ∼320 interplanetary coronal mass ejections (ICMEs) identified in the near‐Earth solar wind during solar cycle 23 up to mid‐2009 and note a recent increase in the ICME rate that may be associated with the new solar cycle. We also discuss how recent revisions in the ACE/SWICS solar wind composition data require the ICME identification methods described in [1] based on these data to be modified.

The angle between the normal to an interplanetary shock front and the upstream magnetic field (θBn), though often considered to be a property “of the shock,” also depends on the direction of the interplanetary magnetic field through which the shock is traveling. We examine the solar wind context of 105 near‐Earth quasi‐perpendicular fast forward shocks in 1996–2005 identified by θBn⩾80° and/or by evidence of shock drift particle acceleration. These shocks (87% driven by interplanetary coronal mass ejections; ICMEs) were propagating through a variety of solar wind structures including unrelated ICMEs, slow solar wind, high‐speed streams, the heliospheric plasma sheet, and shock sheaths. Magnetic field orientations upstream of the shocks were more likely to be highly inclined to the radial direction than in the solar wind as a whole. We also compare the Fe/O ratio in solar wind and energetic particles (∼1 MeV/n) in the vicinity of these shocks. Average values are similar (∼0.14), but occasionally both are e...

Coronal radio sounding observations were carried out with the radio science experiment MaRS on the ESA spacecraft Mars Express during the period from 25 August to 22 October 2004. Differential frequency and log‐amplitude fluctuations of the dual‐frequency signals were recorded during a period of low solar activity. The data are applicable to low heliographic latitudes, i.e. to slow solar wind. The mean frequency fluctuation and power law index of the frequency fluctuation temporal spectra are determined as a function of heliocentric distance. The radial dependence of the frequency fluctuation spectral index α reflects the previously documented flattening of the scintillation power spectra in the solar wind acceleration region. Temporal spectra of S‐band and X‐band normalized log‐amplitude fluctuations were investigated over the range of fluctuation frequencies 0.01 Hz

Cavitons have been recently discovered in the Earth’s foreshock. Foreshock cavitons have first been predicted by global hybrid simulations and later structures with similar properties were discovered in observational data from the Cluster mission. Cavitons are observed as simultaneous depressions of the magnitude of the magnetic field and plasma density (δB/B, δn/n⩾0.2) that commonly last ⩽30 s and are surrounded by a rim of enhanced values of B and n. They are always immersed in a sea of compressive low frequency waves. Numerical simulations results suggest that the cavitons are produced by the interaction of foreshock sinusoidal plasma waves with perpendicularly propagating compressive fast mode waves. Here we present a study of the waves that were observed in the immediate neighborhood or inside two foreshock cavitons detected by the Cluster I spacecraft. We analyze the frequency, polarization, direction of propagation and compressibility of the waves.

Recent studies have compared properties of the magnetic field in simulations of Hall MHD turbulence with spacecraft data, focusing on methods used to identify classical discontinuities and intermittency statistics. Comparison of ACE solar wind data and simulations of 2D and 3D turbulence shows good agreement in waiting‐time analysis of magnetic discontinuities, and in the related distribution of magnetic field increments. This supports the idea that the magnetic structures in the solar wind may emerge fast and locally from nonlinear dynamics that can be understood in the framework of nonlinear MHD theory. The analysis suggests that small scale current sheets form spontaneously and rapidly enough that some of the observed solar wind discontinuities may be locally generated, representing boundaries between interacting flux tubes.