Steady-State Magnetohydrodynamic Flow Around an Unmagnetized Conducting Sphere

The non-collisional interaction between conducting obstacles and magnetized plasma winds can be found in different scenarios, from the interaction occurring between regions inside galaxy clusters to the interaction between the solar wind and Mars, Venus, active comets or even the interaction between Titan and the Saturnian's magnetospheric flow. These objects generate, through several current systems, perturbations in the streaming magnetic field leading to its draping around the obstacle's effective conducting surface. Recent observational results suggest that several properties associated with the magnetic field draping, such as the location of the polarity reversal layer of the induced magnetotail, are affected by variations in the conditions of the streaming magnetic field. To improve our understanding of these phenomena, we perform a characterization of several magnetic field draping signatures by analytically solving an ideal problem in which a perfectly conducting magnetized plasma (with frozen-in magnetic field conditions) flows around a spherical body for various orientations of the streaming magnetic field. In particular, we compute the shift of the inverse polarity reversal layer as the orientation of the background magnetic field is changed.Comment: Preprint submitted to Astrophysical Journa

Maximum-variance gradiometer technique for removal of spacecraft-generated disturbances from magnetic field data

In situ measurement of the magnetic field using spaceborne instruments requires a magnetically clean platform and/or a very long boom for accommodating magnetometer sensors at a large distance from the spacecraft body. This significantly drives up the costs and the time required to build a spacecraft. Here we present an alternative sensor configuration and a technique allowing for removal of the spacecraft-generated AC disturbances from the magnetic field measurements, thus lessening the need for a magnetic cleanliness programme and allowing for shorter boom length. The final expression of the corrected data takes the form of a linear combination of the measurements from all sensors, allowing for simple onboard software implementation. The proposed technique is applied to the Service Oriented Spacecraft Magnetometer (SOSMAG) on board the Korean geostationary satellite GeoKompsat-2A (GK2A). In contrast to other missions where multi-sensor measurements were used to clean the data on the ground, the SOSMAG instrument performs the cleaning on board and transmits the corrected data in real time, as needed by space weather applications. The successful elimination of the AC disturbances originating from several sources validates the proposed cleaning technique

A tail like no other

Context. The Rosetta Plasma Consortium (RPC) magnetometer (MAG) data during the tail excursion in March–April 2016 are used to investigate the magnetic structure of and activity in the tail region of the weakly outgassing comet 67P/Churyumov–Gerasimenko (67P). Aims. The goal of this study is to compare the large scale (near) tail structure with that of earlier missions to strong outgassing comets, and the small scale turbulent energy cascade (un)related to the singing comet phenomenon. Methods. The usual methods of space plasma physics are used to analyse the magnetometer data, such as minimum variance analysis, spectral analysis, and power law fitting. Also the cone angle and clock angle of the magnetic field are calculated to interpret the data. Results. It is found that comet 67P does not have a classical draped magnetic field and no bi-lobal tail structure at this late stage of the mission when the comet is already at 2.7 AU distance from the Sun. The main magnetic field direction seems to be more across the tail direction, which may implicate an asymmetric pick-up cloud. During periods of singing comet activity the propagation direction of the waves is at large angles with respect to the magnetic field and to the radial direction towards the comet. Turbulent cascade of magnetic energy from large to small scales is different in the presence of singing as without it

Hydrogen in the extended Venus exosphere

International audienceThe nearly absence of water in the atmosphere of Venus is a major difference to the situation at Earth. The actual content of hydrogen in the exosphere is still an open issue, since no in situ measurements are available yet. A different method uses the presence of proton cyclotron waves as an early tracer of ionized planetary hydrogen picked-up by the solar wind, especially in the region upstream of the bow shock. Here, we report long-term observations over two full Venus-years of upstream proton cyclotron waves by the magnetometer on the Venus Express spacecraft, which indicate permanent ionization and pick-up of hydrogen by the solar wind upstream of the planetary bow shock up to high altitudes. The pick-up protons are shown to be of planetary origin, whereas other sources of neutral hydrogen have only negligible contribution. Therefore, the observation of upstream proton cyclotron waves in the solar wind is a clear indication for the existence of an extended neutral hydrogen corona at Venus, with significant local number densities up to an altitude of eight planetary radii. Recent observations of the exospheric Lyman-α emission also indicate hot neutral hydrogen densities which are higher than expected

Characteristics of quasi-monochromatic ULF waves in the Venusian foreshock

International audienceThe statistical properties of ULF waves observed upstream of Venus foreshock are investigated. The study is restricted to waves which are observed well below the local proton cyclotron frequency. Using the magnetic field observations from Venus Express between May 2006 and February 2012, 115 quasi-monochromatic ULF wave trains have been identified. Statistical results show that the wave periods are mainly from 20 to 30 s in the spacecraft frame, which is about 2-3 times of the local proton cyclotron period. The transverse power dominates the power spectrum, and most of the waves display nearly circular or slightly elliptical polarization in the spacecraft frame. Moreover, these ULF waves mainly have small relative amplitudes with respect to the ambient field magnitude B0 for parallel component (δB||/B0 less than 0.3), while the range of relative amplitudes for perpendicular component δB⊥/B0 is from ~0.1 to ~1.0. Wave propagation angles are mainly less than 30° with respect to the mean magnetic field direction. The obtained results are very similar to the wave properties seen for ULF waves present in the terrestrial foreshock, which suggests that backstreaming ions in the Venusian foreshock form an important energy source for the generation of the waves