This article summarizes and aims at comparing the main features of the induced magnetospheres of Mars, Venus and Titan. All three objects form a well-defined induced magnetosphere (IM) and ...magnetotail as a consequence of the interaction of an external wind of plasma with the ionosphere and the exosphere of these objects. In all three, photoionization seems to be the most important ionization process. In all three, the IM displays a clear outer boundary characterized by an enhancement of magnetic field draping and massloading, along with a change in the plasma composition, a decrease in the plasma temperature, a deflection of the external flow, and, at least for Mars and Titan, an increase of the total density. Also, their magnetotail geometries follow the orientation of the upstream magnetic field and flow velocity under quasi-steady conditions. Exceptions to this are fossil fields observed at Titan and the near Mars regions where crustal fields dominate the magnetic topology. Magnetotails also concentrate the escaping plasma flux from these three objects and similar acceleration mechanisms are thought to be at work. In the case of Mars and Titan, global reconfiguration of the magnetic field topology (reconnection with the crustal sources and exits into Saturn’s magnetosheath, respectively) may lead to important losses of plasma. Finally, an ionospheric boundary related to local photoelectron signals may be, in the absence of other sources of pressure (crustal fields) a signature of the ultimate boundary to the external flow.
Context. The Rosetta spacecraft is escorting comet 67P/Churyumov-Gerasimenko from a heliocentric distance of >3.6 AU, where the comet activity was low, until perihelion at 1.24 AU. Initially, the ...solar wind permeates the thin comet atmosphere formed from sublimation. Aims. Using the Rosetta Plasma Consortium Ion Composition Analyzer (RPC-ICA), we study the gradual evolution of the comet ion environment, from the first detectable traces of water ions to the stage where cometary water ions accelerated to about 1 keV energy are abundant. We compare ion fluxes of solar wind and cometary origin. Methods. RPC-ICA is an ion mass spectrometer measuring ions of solar wind and cometary origins in the 10 eV-40 keV energy range. Results. We show how the flux of accelerated water ions with energies above 120 eV increases between 3.6 and 2.0 AU. The 24 h average increases by 4 orders of magnitude, mainly because high-flux periods become more common. The water ion energy spectra also become broader with time. This may indicate a larger and more uniform source region. At 2.0 AU the accelerated water ion flux is frequently of the same order as the solar wind proton flux. Water ions of 120 eV-few keV energy may thus constitute a significant part of the ions sputtering the nucleus surface. The ion density and mass in the comet vicinity is dominated by ions of cometary origin. The solar wind is deflected and the energy spectra broadened compared to an undisturbed solar wind. Conclusions. The flux of accelerated water ions moving from the upstream direction back toward the nucleus is a strongly nonlinear function of the heliocentric distance.
ABSTRACT The Grain Impact Analyzer and Dust Accumulator (GIADA) instrument on board ESA's Rosetta mission is constraining the origin of the dust particles detected within the coma of comet 67 ...P/Churyumov-Gerasimenko (67P). The collected particles belong to two families: (i) compact particles (ranging in size from 0.03 to 1 mm), witnessing the presence of materials that underwent processing within the solar nebula and (ii) fluffy aggregates (ranging in size from 0.2 to 2.5 mm) of sub-micron grains that may be a record of a primitive component, probably linked to interstellar dust. The dynamics of the fluffy aggregates constrain their equivalent bulk density to kg m−3. These aggregates are charged, fragmented, and decelerated by the spacecraft negative potential and enter GIADA in showers of fragments at speeds m s−1. The density of such optically thick aggregates is consistent with the low bulk density of the nucleus. The mass contribution of the fluffy aggregates to the refractory component of the nucleus is negligible and their coma brightness contribution is less than 15%.
We use measurements from the Rosetta plasma consortium Langmuir probe and mutual impedance probe to study the spatial distribution of low‐energy plasma in the near‐nucleus coma of comet ...67P/Churyumov‐Gerasimenko. The spatial distribution is highly structured with the highest density in the summer hemisphere and above the region connecting the two main lobes of the comet, i.e., the neck region. There is a clear correlation with the neutral density and the plasma to neutral density ratio is found to be ∼1–2·10−6, at a cometocentric distance of 10 km and at 3.1 AU from the Sun. A clear 6.2 h modulation of the plasma is seen as the neck is exposed twice per rotation. The electron density of the collisionless plasma within 260 km from the nucleus falls off with radial distance as ∼1/r. The spatial structure indicates that local ionization of neutral gas is the dominant source of low‐energy plasma around the comet.
Key Points
The spatial distribution of plasma around comet 67P is highly structured
Local ionization of neutral gas dominates the plasma environment
Plasma falls off with cometocentric distance as 1/r
Nanodust particles are ubiquitous in the solar system; we may expect that comet 67P/Churyumov–Gerasimenko, the target of the upcoming ROSETTA mission, is also a source of nanodust, that is dust ...particles of nanometer size. Due to their small size and mass, the dust detectors cannot observe them directly neither on the orbiter nor on the Philae lander. However, if nanodust grains get charged, the ion and electron sensors on board of the orbiter might detect them.
In this study we investigate whether this is a realistic option. We show that when the comet activity is low between 3.25 and 2.7AU, the cometary surface and a part of the dust particles get charged due to the charging currents of the solar wind and photoionization as a result of solar radiation. The nucleus׳ surface potential depends on the solar wind density, and it is higher than +4V for solar zenith angle between 0° and 50° for solar wind density n(d=1AU)=4×10−6m−3, and nano-size dust gets immediately accelerated when it collected Q=+1e positive charge due to photoelectron emission. The energy of this charged nanodust is higher than 4eV and it could be detected not far from the subsolar region by the ion and electron sensor (IES) of the Rosetta Plasma Package. In this paper we examine this process in detail.
•Possible charged nanodust flux from 67P/CG is calculated.•A method to detect charged nanodust onboard ROSETTA is given.•Energy spectra of charged dust are estimated in onboard ion and electron spectrometer.
One of the scientific objectives of the Rosetta mission is to investigate the diamagnetic cavity of comet 67P/Churyumov–Gerasimenko. We employed combined data of several instruments of the Rosetta ...Plasma Consortium (RPC) to identify and study diamagnetic cavity crossing events. Using electron data from the Ion Electron Sensor (IES) to complement the Magnetometer (MAG) data enabled us to work out a search criterion for the cavity crossing events based on a unique signature we identified in the electron spectrum. Although this search criterion is insufficient to find all the cavity events, we were able to find an abundance of more than one hundred cavity crossings in the data obtained in the summer of 2015. This unexpectedly high number of events allowed us to study their common features, as well as the shape and extent of the diamagnetic cavity in the terminator plane. The results suggest that in the summer of 2015 there was a cavity around comet 67P, which had a highly variable outer boundary. We present the effects of the diamagnetic cavity on the thermal and suprathermal electron and suprathermal ion content of the plasma, and also the probable mechanisms responsible for these charged particle signatures.
Abstract
After the first detection of the diamagnetic cavity of comet 67P/Churyumov–Gerasimenko, it became apparent that the boundary of this plasma region is very dynamic. To date hundreds of short ...cavity crossing events were detected, none lasting longer than an hour. This intermittent set of short events is very different from the classical cavity observation near 1P/Halley, where Giotto remained continuously inside the cavity. The distance of the cavity boundary at 67P is larger than that predicted by recent models, so it was not clear whether these short cavity-like regions are connected to a global diamagnetic cavity, or they are due to some local effects. Here, we revisit the neutral-drag model of Cravens (1986) and we provide a very good phenomenological approximation for the highly variable size of this dynamic region. The model uses the cometary neutral production rate and the solar wind dynamic pressure as inputs. For the production rate, we use averaged and detrended data derived from Rosetta Orbiter Spectrometer for Ion and Neutral Analysis neutral density measurements. We show that instead of the local neutral pressure, the global production rate drives the size of the cavity. The solar wind pressure is derived from space weather models and independently from the magnetic field measurements of Rosetta Magnetometer (MAG). We accurately estimate the highly variable size of the cavity using this data. Our results suggest that at the time of the measurements a global diamagnetic cavity existed around comet 67P, the size of which varied dynamically following the changing cometary gas production and solar wind pressure.
Solar wind propagation from the point of measurement to an arbitrary target in the heliosphere is an important input for heliospheric, planetary and cometary studies. In this paper a new kinematic ...propagation method, the
magnetic lasso
method is presented. Compared to the simple ballistic approach our method is based on reconstructing the ideal Parker spiral connecting the target with the Sun by testing a previously defined range of heliographic longitudes. The model takes into account the eventual evolution of stream–stream interactions and handles these with a simple model based on the dynamic pressure difference between the two streams. Special emphasis is given to input data cleaning by handling interplanetary coronal mass ejection events as data gaps due to their different propagation characteristics. The solar wind bulk velocity is considered radial and constant. Density and radial magnetic field are propagated by correcting with the inverse square of the radial distance. The model has the advantage that it can be coded easily and fitted to the problem; it is flexible in selecting and handling input data and requires little running time.
We present a statistical study on the interaction between interplanetary coronal mass ejections (ICMEs) and the induced magnetosphere of Venus when the peak magnetic field of the magnetic barrier was ...anomalously large (>65 nT). Based on the entire available Venus Express data set from April 2006 to October 2014, we selected 42 events and analyzed the solar wind parameters, the position of the bow shock, the size and plasma properties of the magnetic barrier, and the position of the ion composition boundary (ICB). It was found that the investigated ICMEs can be characterized with interplanetary shocks and unusually large tangential magnetic fields with respect to the Venus‐Sun line. In most of the cases the position of the bow shock was not affected by the ICME. In a few cases the interaction between magnetic clouds and the induced magnetosphere of Venus was observed. During these events the small magnetosonic Mach numbers inside magnetic clouds caused the bow shock to appear at anomalously large distances from the planet. The positions of the upper and lower boundaries of the magnetic barrier were not affected by the ICMEs. The position of the ICB on the nightside was found closer to the planet during ICME passages which is attributed to the increased solar wind dynamic pressure.
Key Points
Statistical study of the ICME‐Venus interaction
Analysis of solar wind and magnetic barrier conditions during ICME passages
Decreased altitude of the nightside ionosphere during ICME passages
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