R. Gill

Spatial distribution of low-energy plasma around comet 67P/CG from Rosetta measurements

We use measurements from the Rosetta plasma consortium (RPC) Langmuir probe (LAP) and mutual impedance probe (MIP) 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 collisonless plasma within 260 km from the nucleus falls of 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.

Thermal ion imagers and Langmuir probes in the Swarm electric field instruments

The European Space Agencys three Swarm satellites were launched on November 22, 2013 into nearly-polar, circular orbits, eventually reaching altitudes of 460 km (Swarm A and C) and 510 km (Swarm B). Swarms multi-year mission is to make precision, multi-point measurements of low-frequency magnetic and electric fields in Earths ionosphere for the purpose of characterizing magnetic fields generated both inside and external to the Earth, along with the electric fields and other plasma parameters associated with electric current systems in the ionosphere and magnetosphere. Electric fields perpendicular to the magnetic field B→ are determined through ion drift velocity v→i and magnetic field measurements via the relation E→⊥=−v→i×B→. Ion drift is derived from two-dimensional images of low-energy ion distribution functions provided by two Thermal Ion Imager (TII) sensors viewing in the horizontal and vertical planes; v→i is corrected for spacecraft potential as determined by two Langmuir probes (LPs) which also measure plasma density ne and electron temperature Te. The TII sensors use a microchannel-plate-intensified phosphor screen imaged by a charge-coupled device to generate high-resolution distribution images ( 66x40 pixels) at a rate of 16 s−1. Images are partially processed on board and further on the ground to generate calibrated data products at a rate of 2 s−1; these include v→i, E→⊥, and ion temperature Ti in addition to electron temperature Te and plasma density ne from the LPs.

Westward tilt of low‐latitude plasma blobs as observed by the Swarm constellation

In this study we investigate the three-dimensional structure of low-latitude plasma blobs using multi-instrument and multisatellite observations of the Swarm constellation. During the early commissioning phase the Swarm satellites were flying at the same altitude with zonal separation of about 0.5∘ in geographic longitude. Electron density data from the three satellites constrain the blob morphology projected onto the horizontal plane. Magnetic field deflections around blobs, which originate from field-aligned currents near the irregularity boundaries, constrain the blob structure projected onto the plane perpendicular to the ambient magnetic field. As the two constraints are given for two noncoplanar surfaces, we can get information on the three-dimensional structure of blobs. Combined observation results suggest that blobs are contained within tilted shells of geomagnetic flux tubes, which are similar to the shell structure of equatorial plasma bubbles suggested by previous studies.

First results from the Langmuir Probes on the Swarm satellites

Langmuir Probes (LP) are well-proven simple instruments allowing to estimate the electron density (Ne) and temperature (Te) of a plasma. They are also used to estimate the electric potential of satellites to the benefit of other instruments and technical systems. On the Swarm satellites the LPs are part of the Electric Field Instruments (EFI) featuring thermal ion imagers (TII) measuring 3-d ion distributions. The main task of the Langmuir probes is to provide measurements of spacecraft potentials influencing the ions before they enter the TIIs. In addition also electron density (Ne) and temperature (Te) are estimated from EFI LP data. The design of the Swarm LP includes a standard current sampling under sweeps of the bias voltage, and also, for most of the time, a novel ripple technique yielding derivatives of the current-voltage characteristics at three points in a rapid cycle. In normal mode the time resolution of the Ne and Te measurements so becomes only 0.5 s. We show first Ne and Te estimates from the EFI LPs obtained. The data feature very low instrumental noise thanks to the ripple technique. The LP data are compared with observations by incoherent scatter radars, namely EISCAT UHF, VHF, the ESR, and also Arecibo.