Both heliophysics and planetary physics seek to understand the complex nature of the solar wind's interaction with solar system obstacles like Earth's magnetosphere, the ionospheres of Venus and Mars, and comets. Studies with this objective are frequently conducted with the help of single or multipoint in situ electromagnetic field and particle observations, guided by the predictions of both local and global numerical simulations, and placed in con- text by observations from far and extreme ultraviolet (FUV, EUV), hard X-ray, and energetic neutral atom imagers (ENA). Each proposed interaction mechanism (e.g., steady or transient magnetic reconnection, local or global magnetic reconnection, ion pick-up, or the KelvinHelmholtz instability) generates diagnostic plasma density structures. The significance of each mechanism to the overall interaction (as measured in terms of atmospheric/ionospheric loss at comets, Venus, and Mars or global magnetospheric/ionospheric convection at Earth) remains to be determined but can be evaluated on the basis of how often the density signatures that it generates are observed as a function of solar wind conditions. This paper reviews efforts to image the diagnostic plasma density structures in the soft (low energy, 0.1-2.0 keV) X-rays produced when high charge state solar wind ions exchange electrons with the exospheric neutrals surrounding solar system obstacles. The introduction notes that theory, local, and global simulations predict the characteristics of plasma boundaries such the bow shock and magnetopause (including location, density gradient, and motion) and regions such as the magnetosheath (including density and width) as a function of location, solar wind conditions, and the particular mechanism operating. In situ measurements confirm the existence of time-and spatial-dependent plasma density structures like the bow shock, magnetosheath, and magnetopause/ionopause at Venus, Mars, comets, and the Earth. However, in situ measurements rarely suffice to determine the global extent of these density structures or their global variation as a function of solar wind conditions, except in the form of empirical studies based on observations from many different times and solar wind conditions. Remote sensing observations provide global information about auroral ovals (FUV and hard X-ray), the terrestrial plasmasphere (EUV), and the terrestrial ring current (ENA). ENA instruments with low energy thresholds (∼ 1 keV) have recently been used to obtain important information concerning the magnetosheaths of Venus, Mars, and the Earth. Recent technological developments make these magnetosheaths valuable potential targets for high-cadence wide-field-of-view soft X-ray imagers.Section 2 describes proposed dayside interaction mechanisms, including reconnection, the Kelvin-Helmholtz instability, and other processes in greater detail with an emphasis on the plasma density structures that they generate. It focuses upon the questions that remain as yet unanswered, such as the significanc...

Results from 2.5-D electromagnetic hybrid simulations show the formation of field-aligned, filamentary plasma structures in the magnetosheath. They begin at the quasi-parallel bow shock and extend far into the magnetosheath. These structures exhibit anticorrelated, spatial oscillations in plasma density and ion temperature. Closer to the bow shock, magnetic field variations associated with density and temperature oscillations may also be present. Magnetosheath filamentary structures (MFS) form primarily in the quasi-parallel sheath; however, they may extend to the quasi-perpendicular magnetosheath. They occur over a wide range of solar wind Alfvénic Mach numbers and interplanetary magnetic field directions. At lower Mach numbers with lower levels of magnetosheath turbulence, MFS remain highly coherent over large distances. At higher Mach numbers, magnetosheath turbulence decreases the level of coherence. Magnetosheath filamentary structures result from localized ion acceleration at the quasi-parallel bow shock and the injection of energetic ions into the magnetosheath. The localized nature of ion acceleration is tied to the generation of fast magnetosonic waves at and upstream of the quasi-parallel shock. The increased pressure in flux tubes containing the shock accelerated ions results in the depletion of the thermal plasma in these flux tubes and the enhancement of density in flux tubes void of energetic ions. This results in the observed anticorrelation between ion temperature and plasma density.

We present the ion distribution functions in the ion foreshock upstream of the terrestrial bow shock obtained with Vlasiator, a new hybrid-Vlasov simulation geared toward large-scale simulations of the Earth's magnetosphere (http://vlasiator.fmi.fi). They are compared with the distribution functions measured by the multispacecraft Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission. The known types of ion distributions in the foreshock are well reproduced by the hybrid-Vlasov model. We show that Vlasiator reproduces the decrease of the backstreaming beam speed with increasing distance from the foreshock edge, as well as the beam speed increase and density decrease with increasing radial distance from the bow shock, which have been reported before and are visible in the THEMIS data presented here. We also discuss the process by which wave-particle interactions cause intermediate foreshock distributions to lose their gyrotropy. This paper demonstrates the strength of the hybrid-Vlasov approach which lies in producing uniformly sampled ion distribution functions with good resolution in velocity space, at every spatial grid point of the simulation and at any instant. The limitations of the hybrid-Vlasov approach are also discussed.

We present simultaneous Time History of Events and Macroscale Interactions during Substormsobservations of plasma parameters upstream in the solar wind and downstream in the magnetosheath (MSH) from 2007 to 2008. We discuss the connection of foreshock (FSH) processes and magnetospheric disturbances to transmission mechanisms in the MSH. In 60% of the analyzed cases, the MSH was strongly influenced by the FSH. We analyze the results as a function of location, time scale, spatial orientation of the observed structures, and the prevailing interplanetary magnetic field (IMF) and solar wind plasma parameters. We find that plasma structures with density enhancement are mostly observed during radial IMF orientations and for small BN , the angle between the upstream magnetic field and the local bow shock normal; the observed structures are pressure balanced with strong anticorrelation between density and temperature; the scale size of the density fluctuations is about 0.4R E . We compare the observations with results from a 2.5-dimensional hybrid simulation to investigate the mechanisms by which the foreshock plasma structures are generated, propagate through the bow shock, and evolve.

[1] In the present statistical study, we discuss a probability of simultaneous observations of the same sign of the magnetic field B Z component in the solar wind and magnetosheath. The analysis is based on 5 min data from four spacecraft (Interball-1, IMP 8, Cluster, and THEMIS) operating in different phases of the solar cycle in the magnetosheath. Their measurements are compared with Wind interplanetary magnetic field (IMF) observations, and other available upstream monitors (ACE, THEMIS B, and OMNI database) are tested for some sets. We can conclude that the probability of observations of the same B Z sign in the solar wind and in the magnetosheath is surprisingly very low from a general point of view. The probability changes through the solar cycle, being larger at the solar minimum. Regardless of the solar cycle phase, this probability is close to 0.5 (random coincidence) for IMF jB Z j < 1 nT, and it is a rising function of the B Z value. Distant solar wind monitors do not guarantee the same sign of the B Z component, even for values of IMF B Z exceeding ±9 nT, but such large values are observed about 3-5% of the time. A better probability profile is reached for a monitor located just upstream (less than 30 R E ), as is demonstrated for the THEMIS project.