K.-H. Glassmeier

Physics of Mass Loaded Plasmas

In space plasmas the phenomenon of mass loading is common. Comets are one of the most evident objects where mass loading controls to a large extent the structure and dynamics of its plasma environment. New charged material is implanted to the fast streaming solar wind by planets, moons, other solar system objects, and even by the interstellar neutral gas flowing through our solar system. In this review we summarize both the current observations and the relevant theoretical approaches. First we survey the MHD methods, starting with a discussion how mass loading affects subsonic and supersonic gasdynamics flows, continuing this with single and multi-fluid MHD approaches to describe the flow when mass, momentum and energy is added, and we finish this section by the description of mass loaded shocks. Next we consider the kinetic approach to the same problem, discussing wave excitations, pitch angle and energy scattering in linear and quasi-linear approximations. The different descriptions differ in assumptions and conclusions; we point out the differences, but it is beyond the scope of the paper to resolve all the conflicts. Applications of these techniques to comets, planets, artificial ion releases, and to the interplanetary neutrals are reviewed in the last section, where observations are also compared with models, including hybrid simulations as well. We conclude the paper with a summary of the most important open, yet unsolved questions.

Little or no solar wind enters Venus' atmosphere at solar minimum.

Venus has no significant internal magnetic field, which allows the solar wind to interact directly with its atmosphere2,3. A field is induced in this interaction, which partially shields the atmosphere, but we have no knowledge of how effective that shield is at solar minimum. (Our current knowledge of the solar wind interaction with Venus is derived from measurements at solar maximum.) The bow shock is close to the planet, meaning that it is possible that some solar wind could be absorbed by the atmosphere and contribute to the evolution of the atmosphere. Here we report magnetic field measurements from the Venus Express spacecraft in the plasma environment surrounding Venus. The bow shock under low solar activity conditions seems to be in the position that would be expected from a complete deflection by a magnetized ionosphere. Therefore little solar wind enters the Venus ionosphere even at solar minimum.

MESSENGER observations of Mercury's dayside magnetosphere under extreme solar wind conditions

CLJ and RMW acknowledge support from the Natural Sciences and Engineering Research Council of Canada, and CLJ acknowledges support from MESSENGER Participating Scientist grant NNX11AB84G. The MESSENGER project is supported by the NASA Discovery Program under contracts NASW- 00002 to the Carnegie Institution of Washington and NAS5-97271 to The Johns Hopkins University Applied Physics Laboratory.

Magnetic Reconnection in the Near Venusian Magnetotail

Magnetic Reconnection Magnetic reconnection (MR) has been observed in the magnetospheres of planets with an intrinsic magnetic field, such as Earth, Mercury, Jupiter, and Saturn. MR is a universal plasma process that occurs in regions of strong magnetic shear and converts magnetic energy into kinetic energy. On Earth, MR is responsible for magnetic storms and auroral events. Using data from the European Space Agency Venus Express spacecraft, Zhang et al. (p. 567, published online 5 April; see the Perspective by Slavin) present surprising evidence for MR in the magnetosphere of Venus, which is a nonmagnetized body. Venus Express observations show that magnetic reconnection occurs in the magnetotail of an unmagnetized planet. Observations with the Venus Express magnetometer and low-energy particle detector revealed magnetic field and plasma behavior in the near-Venus wake that is symptomatic of magnetic reconnection, a process that occurs in Earth’s magnetotail but is not expected in the magnetotail of a nonmagnetized planet such as Venus. On 15 May 2006, the plasma flow in this region was toward the planet, and the magnetic field component transverse to the flow was reversed. Magnetic reconnection is a plasma process that changes the topology of the magnetic field and results in energy exchange between the magnetic field and the plasma. Thus, the energetics of the Venus magnetotail resembles that of the terrestrial tail, where energy is stored and later released from the magnetic field to the plasma.

Toward adapted time‐dependent magnetospheric models: A simple approach based on tuning the standard model

[1] We suggest and test a simple procedure to adapt a magnetic field model by fitting it to observations made simultaneously by several spacecraft. This is done by varying input parameters of a standard model (T96) to find the best fit to the observed field at each time step. As a result we obtain a time-dependent model which can be used for evaluating the quality of the standard model and of the mapping at any particular time, to navigate in the magnetosphere and reproduce its variable configuration during large-scale dynamical events. This procedure was tested using observations made by five Time History of Events and Macroscale Interactions during Substorms (THEMIS) and other complementary (e.g., GOES) spacecraft during the tail season of THEMIS mission (January-March 2008), for which a simplest version of the adapted model was routinely calculated and has been made publicly available. We also use the proton isotropic boundaries observed by low-altitude NOAA spacecraft for independent evaluation of the obtained field models. We found that in quiet conditions deviations of ionospheric footprints between standard and adapted models are generally small (within 1° of latitude), whereas during substorms they may be as large as several degrees, because of stretching and dipolarizations of magnetospheric configuration. We found that the variable tilt of the tail current sheet, partly caused by variations of nonradial component of the solar wind flow, is an additional important factor influencing the modeling result and the mapping quality. By analyzing the adapted models constructed at the time of auroral breakup onset, we conclude that this simple approach is not yet sufficiently accurate to evaluate the source distance in the magnetotail.

Wavelength and direction filtering by magnetic measurements at satellite arrays: Generalized minimum variance analysis

The main goal of the Cluster mission, consisting of four identical spacecraft, is the spatial resolution of plasma structures. For the determination of the wave vectors of a wave field from four positions, classical Fourier analysis is inappropriate. We develop a generalized minimum variance technique which gives a high wave vector resolution though the spatial grid is restricted to only a few sampling positions. This technique uses the amplitude and phase information of the magnetic field from the four satellite positions and determines the optimum wave field corresponding to the measured data. The components of the magnetic field are assumed to be normally distributed. The divergence-free nature of the magnetic field is used as a constraint. Using the magnetic data measured at four positions allows up to seven different wave vectors at one frequency to be uniquely resolved.

Structure of plasmaspheric plumes and their participation in magnetopause reconnection: First results from THEMIS

[1] New observations by the THEMIS spacecraft have revealed dense (>10 cm- 3 ) plasmaspheric plumes extending to the magnetopause. The large scale radial structure of these plumes is revealed by multi-spacecraft measurements. Temporal variations in the radial distribution of plume plasma, caused by azimuthal density gradients coupled with azimuthal flow, are also shown to contribute to plume structure. In addition, flux tubes with cold plume plasma are shown to participate in reconnection, with simultaneous observations of cold ions and reconnection flow jets on open flux tubes as revealed by the loss of hot magnetospheric electrons.

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.

Time History of Events and Macroscale Interactions during Substorms observations of a series of hot flow anomaly events

[1] A series of seven hot flow anomaly (HFA) events has been observed by the Time History of Events and Macroscale Interactions during Substorms (THEMIS) C spacecraft just upstream from the subsolar bow shock from 0100 to 1300 UT on 19 August 2008. Both young (no shocks at edges, two distinct ion populations) and mature (strong shocks at edges, a single hot ion population) HFAs have been observed. Further upstream, THEMIS B observed four proto‐HFAs (density and magnetic field strength depletions, plasma heating but no flow deflections) which later developed into HFAs observed by THEMIS C. We present evidence indicating that electromagnetic right‐hand resonant ion beam instabilities heat ions inside HFAs. Observations of small‐amplitude perturbations (DB/B < 50%) consistent with the resonant ion beam instability in a proto‐HFA, 30 s electromagnetic waves (DB/B ∼ 1) in a young HFA, and magnetic pulsations in a mature HFA (DB/B ∼ 4) indicate that they are at early, middle, and late (nonlinear) stages of the electromagnetic right‐hand resonant ion beam instabilities. Both young and mature HFAs are associated with strong electromagnetic waves near the lower hybrid frequency (0.1–1 Hz). The lower hybrid waves are the likely source of the electron heating inside HFAs. THEMIS B observations of four proto‐HFAs which later developed into HFAs observed by THEMIS C indicate that these four HFAs might extend beyond 14 RE upstream from the bow shock, while the other three HFAs may extend between 5 and 14 RE upstream from the bow shock. We present an example of an HFA that lies displaced toward the side of the tangential discontinuity with a quasi‐parallel bow shock configuration rather than lying centered on the driving interplanetary magnetic field discontinuity.

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.