Claire Vallat

Birth of a comet magnetosphere: A spring of water ions

The Rosetta mission shall accompany comet 67P/Churyumov-Gerasimenko from a heliocentric distance of >3.6 astronomical units through perihelion passage at 1.25 astronomical units, spanning low and maximum activity levels. Initially, the solar wind permeates the thin comet atmosphere formed from sublimation, until the size and plasma pressure of the ionized atmosphere define its boundaries: A magnetosphere is born. Using the Rosetta Plasma Consortium ion composition analyzer, we trace the evolution from the first detection of water ions to when the atmosphere begins repelling the solar wind (~3.3 astronomical units), and we report the spatial structure of this early interaction. The near-comet water population comprises accelerated ions (<800 electron volts), produced upstream of Rosetta, and lower energy locally produced ions; we estimate the fluxes of both ion species and energetic neutral atoms.

Evolution of the ion environment of comet 67P/Churyumov-Gerasimenko - Observations between 3.6 and 2.0 AU

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.

First detection of a diamagnetic cavity at comet 67P/Churyumov-Gerasimenko

Context: The Rosetta magnetometer RPC-MAG has been exploring the plasma environment of comet 67P/Churyumov-Gerasimenko since August 2014. The first months were dominated by low-frequency waves which evolved into more complex features. However, at the end of July 2015, close to perihelion, the magnetometer detected a region that did not contain any magnetic field at all. Aims: These signatures match the appearance of a diamagnetic cavity as was observed at comet 1P/Halley in 1986. The cavity here is more extended than previously predicted by models and features unusual magnetic field configurations, which need to be explained Methods: The onboard magnetometer data were analyzed in detail and used to estimate the outgassing rate. A minimum variance analysis was used to determine boundary normals. Results. Our analysis of the data acquired by the Rosetta Plasma Consortium instrumentation confirms the existence of a diamagnetic cavity. The size is larger than predicted by simulations, however. One possible explanation are instabilities that are propagating along the cavity boundary and possibly a low magnetic pressure in the solar wind. This conclusion is supported by a change in sign of the Sun-pointing component of the magnetic field. Evidence also indicates that the cavity boundary is moving with variable velocities ranging from 230−500 m/s.

Observation of a new type of low-frequency waves at comet 67P/Churyumov-Gerasimenko

Abstract. We report on magnetic field measurements made in the innermost coma of 67P/Churyumov-Gerasimenko in its low-activity state. Quasi-coherent, large-amplitude (δ B/B ~ 1), compressional magnetic field oscillations at ~ 40 mHz dominate the immediate plasma environment of the nucleus. This differs from previously studied cometary interaction regions where waves at the cometary ion gyro-frequencies are the main feature. Thus classical pickup-ion-driven instabilities are unable to explain the observations. We propose a cross-field current instability associated with newborn cometary ion currents as a possible source mechanism.

Cluster encounter with an energetic electron beam during a substorm

During a passage of the midtail plasma sheet, the Cluster 3 spacecraft made an unprecedented high-resolution measurement of a beam of electrons with energies up to 400 keV. The beam was only fully resolved by combining the energy range coverage of the Plasma Electron and Current Experiment and Research with Adaptive Particle Imaging Detector electron detectors and its pitch angle distribution evolved from antiparallel, through counterstreaming to parallel over a period of 20 s. Although energetic electron fluxes (similar to few hundred keV) are frequently observed in the plasma sheet, electron beams of this nature have rarely, if ever, been reported. The global conditions of the magnetosphere at this time were analyzed using multiple spacecraft, ground, and auroral observations, and are shown to be consistent with a near-Earth neutral line substorm scenario. The beam event is clearly associated with a substorm, and we present a discussion on the detailed analysis of the energy and time dependence of the beam in context with several current theories of particle acceleration, finding that the observations do no fit completely with any in a straightforward way.

Science operations planning of the Rosetta encounter with comet 67P/Churyumov-Gerasimenko

Rosetta is a cornerstone mission of the European Space Agency (ESA). It was launched in March 2004 and will rendezvous with comet 67P/Churyumov-Gerasimenko (C-G) in 2014. Rosetta consists of an orbiter and a lander. Rosetta will meet Comet C-G early 2014 at a heliocentric distance of approximately 4 AU after wake up from a 2.5 year phase of deep space hibernation. The lander will be delivered to the surface in Nov. 2014 at around 3 AU from the sun, while the orbiter will continue to follow the comet on its orbit through perihelion until it reaches 2 AU outbound by end of 2015. The Science Operations and Data Handling Concept (SODH concept) deals with the 14 months between lander delivery and end of the nominal mission, the so called escort phase. That mission phase is extraordinarily complex: Approaching the sun the comet becomes increasingly active and its environment is expected to change dramatically and unpredictably. Therefore continuous monitoring of the comet (based on the science data returned) is required to mitigate risks on the spacecraft, mainly due to dust particles emitted from the nucleus. On the other hand, the evolving comet activity poses great scientific opportunities and payload operations are expected to react and adapt in response to these changing activities. In addition, the activity of the comet together with its small size (about 2 km radius) implies that the trajectory of the spacecraft relative to the nucleus may not be predictable for extended periods of time and that active orbit control will be required. The SODH concept foresees a closed loop system between operations planning and data analysis. Scientific operations planning is centralized at the Rosetta Science Operations Centre (RSOC), with an information repository at its core, containing operational inputs provided by the Principal Investigator (PI) teams that are responsible for the payload instruments. At the comet we expect to execute mostly predefined operation blocks. Changes in the comet environment and results of scientific observations feed back into the planning process. The planning process has already started with the baseline planning. It is based on the Rosetta Science Themes, representing the Science Objectives for Rosetta and the associated measurements by the various payload instruments. The instrument teams provide geometrical constraints (e.g. illumination requirements) and resource estimates (power, data volume, number of telecommands) needed for each measurement. The escort phase is divided into several phases. The proposed measurements are ordered based on their contribution to the science objectives to be covered during a given phase. The result will be the baseline plan of typical trajectories and pointing modes for each mission phase and an estimate of required resources, e.g. integration time and data volume. Expected conflicts and prioritization needs will also be identified in this stage. The