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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.

The solar wind-magnetosphere interaction for northward interplanetary magnetic field (IMF) is studied using a newly developed three-dimensional adaptive mesh refinement (AMR) global MHD simulation model. The simulations show that for north-ward IMF the magnetosphere is essentially closed. Reconnection between the IMF and magnetospheric field is limited to finite regions near the cusps. When the reconnection process forms newly closed magnetic field lines on the day side, the solar wind plasma trapped on these reconnected magnetic field lines becomes part of the low-latitude boundary layer (LLBL) plasma and it convects to the nightside along the magnetopause. The last closed magnetic field line marks the topological boundary of the magnetospheric domain. When the last closed magnetic field line disconnects at the cusps and reconnects to the IMF, its plasma content becomes part of the solar wind. Plasma convection in the outer magnetosphere does not directly contribute to the reconnection process. On the dayside the topological boundary between the solar wind and the magnetosphere is located at the inner edge of the magnetopause current layer. At the same time, multiple current layers are observed in the high-altitude cusp region. Our convergence study and diagnostic analysis indicate that the details of the diffusion and the viscous interaction do not play a significant role in controlling the large-scale configuration of the simulated magnetosphere. It is sufficient that these dissipation mechanisms exist in the simulations. In our series of simulations the length of the magnetotail is primarily determined by the balance between the boundary layer driving forces and the drag forces. With a parametric study, we find that the tail length is proportional to the magnetosheath plasma beta near the magnetopause at local noon. A higher solar wind density, weaker IMF, and larger solar wind Mach number results in a longer tail. On the nightside downstream of the last closed magnetic field line the plasma characteristics are similar to that in the magnetotail, posing an observational challenge for identification of the topological status of the corresponding field lines.

We test a new scheme to study the magnetosheath. The scheme uses the solar wind measurements as the input into the gasdynamic convected field model, and the model output is compared with magnetosheath observations. In our four test cases there is a significant overall success in the model prediction. This scheme works better than other methods in magnetosheath studies and is potentially useful for space weather forecasts and nowcasts. The direction of the magnetic field is modeled most accurately. The prediction of the size of the magnetosphere is accurate within a few percent. The predicted thickness of the magnetosheath is accurate up to 90%. With a double-normalization procedure developed in this study, we are able to separate the processes intrinsic in the magnetosheath from those due to large-scale upstream temporal variations. The test cases confirm the existence of a compressional front one third of the distance from the magnetopause to the bow shock near the stagnation streamline. The magnetosheath density profile near the stagnation streamline is consistent with the models that add a compressional front between the two depletion processes described by the plasma depletion model. A major unexpected feature is that the magnetosheath flow pattern is very different from that described by the model and maybe by most other models, including MHD models. The magnetosheath flow near the stagnation streamline does not slow down gradually toward the stagnation point. It moves rapidly until reaching a very small region near the magnetopause.

We propose a new method to study the structure of the magnetosheath and thereby determine the underlying processes that create this structure. This method provides a systematic means of separating perturbations due to the solar wind variations from those generated within the magnetosheath. As a result, we are able to study the magnetosheath processes as well as the dynamic solar wind-magnetopause interaction. We use the solar wind measurements from an upstream monitor as the input to the gasdynamic convected field model and then compare the model output with the in situ magnetosheath observations. We introduce three parameters to scale the model prediction to match the timings of the magnetopause crossing, bow shock crossing, and upstream variations. With this procedure the relationship between the upstream measurements and the magnetosheath observations and the location of the magnetosheath satellite relative to the magnetopause and bow shock boundaries are highly constrained. We then introduce a series of normalization procedures that provide the means to remove the effects of the solar wind variations. The systematic differences between the model prediction and observation indicate physical processes that are not included in the gasdynamic model. An application of this approach is presented in a companion paper.

The interaction of the solar wind with planets with no or only very weak intrinsic magnetic fields, such as Mars and Venus, involves their ionospheres. Single fluid MHD models, which incorporate some of the important mass-loading processes, have been useful in reproducing numerous observed features at these planets, such as the location of the bow shock, but they do have obvious limitations. Our present 3D MHD model is a two-fluid one, which considers protons and the dominant heavy ions in the ionosphere, separately. We have used this model to study the interaction processes at Mars. The model results are in general agreement with the average observed bow shock shape and position and predict reasonable locations for the ionopause. The calculated oxygen escape flux down the tail was estimated to be 2.7 × 10 25 s -1 , which is consistent with Phobos-2 estimates.

The global three-dimensional interaction of Europa with the Jovian magnetosphere is modeled by using a complete set of ideal magnetohydrodynamic (MHD) equations. The model accounts for exospheric mass loading, ion-neutral charge exchange, recombination, and a possible intrinsic dipole magnetic field of Europa. The single-fluid MHD equations are solved by using a modem, finite volume, higher-order, Godunov-type method on an adoptively refined unstructured grid, which allows detailed modeling of the region near Europa while still resolving both the upstream region and the satellite's wake. The magnetic field and plasma density measured during Galileo's EGA flyby of December 19, 1996, are reproduced reasonably well in the simulation. We find the agreement between the data and our model particularly convincing if we assume that the plasma velocity during the EGA flyby deviated from the nominal corotation direction by approximately 20 deg. Evidence from the Galileo energetic particle detector also supports this assumption. In this case, we can fit the data using a dipole with orientation close to that of an induced dipole arising from the interaction of a hypothetical conducting subsurface layer on Europa with the periodically changing magnetic field of Jupiter. However, the magnitude of the dipole in our model is somewhat smaller (70%) than that suggested by Khurana et al. The total mass loading and ion-neutral charge exchange rates are consistent with the estimates of Europa's atmosphere and ionosphere.

This paper presents a computational scheme for compressible magnetohydrodynamics (MHD). The scheme is based on the same elements that make up many modern compressible gas dynamics codes: a high-resolution upwinding based on an approximate Riemann solver for MHD and limited reconstruction; an optimally smoothing multi-stage time-stepping scheme; and solution-adaptive refinement and coarsening. In addition, a method for increasing the accuracy of the scheme by subtracting off an embedded steady magnetic field is presented. Each of the pieces of the scheme is described, and the scheme is validated and its accuracy assessed by comparison with exact solutions. Results are presented for two three-dimensional calculations representative of the interaction of the solar wind with a magenetized planet.

The results of our three-dimensional multiscale magnetohydrodynamic (MHD) model describing the interaction of Saturn's magnetosphere with Titan's upper atmosphere and exosphere are presented. The effects of a conducting ionosphere, exospheric mass loading and ion-neutral charge exchange are taken into consideration. Because the ion gyroradius, in general, is not small compared to the radius of Titan the MHD approximation is not strictly applicable. However, it is still useful for the understanding of the general large-scale features of the magnetospheric interaction. The upstream parameters are set for the typical case when Titan is inside the Saturnian magnetosphere. Although Titan certainly does not have a strong intrinsic magnetic field, some observed structures in Titan's wake might be explained by assuming the existence of a small intrinsic magnetic field. Thus we also considered the possibility that Titan has a small intrinsic dipole moment. The results of the simulations are compared with Voyager measurements and provide estimates of the total mass loading and the rate of ion-neutral charge exchange. The simulation results reproduce the overall behavior of the observations, but it is clear that non-MHD effects also play a role in the interaction between Saturn's magnetosphere and Titan.

Ionization of hot and cold planetary oxygen ions caused by solar radiation, electron impact, and charge exchange with the solar wind protons lead to a heavy plasma population near Venus which is accelerated and incorporated into the solar wind plasma. lonization of hot and cold hydrogen also enhances the solar wind density and reduces its momentum. Charge exchange of the solar wind plasma with hot and cold planetary hydrogen removes a part of this heavy ion population. We include these processes in a three-dimensional magnetohydrodynamic (MHD) model of the solar wind interaction with Venus and analyze their impact on the magnetic and flow field geometry around Venus. We find that photoionization is the most important of these processes if the change in the plasma composition is taken into account. The ionization processes lead to a strong deceleration of the flow around the planet, which in turn results in changes of the magnetic field. Because of the increasing mean mass and density of the plasma near the ionopause velocity gradients develop in the near terminator region, which lead to a local increase of the magnetic field. The inclusion of mass loading in our model moves the bow shock position outward, close to the location observed by the Pioneer Venus Orbiter magnetometer at solar maximum.

The first results for applying a three-dimensional multimedia ideal MHD model for the mass-loaded flow of Jupiter's corotating magnetospheric plasma past Io are presented. The model is able to consider simultaneously physically realistic conditions for ion mass loading, ion-neutral drag, and intrinsic magnetic field in a full global calculation without imposing artificial dissipation. Io is modeled with an extended neutral atmosphere which loads the corotating plasma torus flow with mass, momentum, and energy. The governing equations are solved using adaptive mesh refinement on an unstructured Cartesian grid using an upwind scheme for AHMED. For the work described in this paper we explored a range of models without an intrinsic magnetic field for Io. We compare our results with particle and field measurements made during the December 7, 1995, flyby of to, as published by the Galileo Orbiter experiment teams. For two extreme cases of lower boundary conditions at Io, our model can quantitatively explain the variation of density along the spacecraft trajectory and can reproduce the general appearance of the variations of magnetic field and ion pressure and temperature. The net fresh ion mass-loading rates are in the range of approximately 300-650 kg/s, and equivalent charge exchange mass-loading rates are in the range approximately 540-1150 kg/s in the vicinity of Io.

The results of a three-dimensional adaptive magnetohydrodynamic (MHD) model of the interaction of a magnetized solar wind with a magnetized very local interstellar medium in the presence of neutral interstellar hydrogen are presented. The interplanetary magnetic field is approximated by the Parker spiral, and the direction of the interstellar magnetic field is taken to be arbitrary. It is demonstrated that magnetic field interaction has a very pronounced effect on the structure of the global heliosphere. In particular, it is shown that the interaction of the interstellar wind with the shocked solar wind significantly depends on the direction of the interstellar magnetic field. This effect is mainly manifested in the distances to the heliospheric boundaries and the shape of the heliosphere. Depending on the orientation of the interstellar magnetic field the upstream location of the termination shock is expected to be at 80 ± 10 AU. The termination shock is predicted to be weak in agreement with the available body of observations. It is found that under quiet solar conditions the spiral structure of the interplanetery magnetic field remains imprinted in the solar wind far beyond the termination shock. Numerical simulations indicate that magnetic fields have a stabilizing effect on the heliopause.

The observation of ions created by ionization of cometary gas, either by ground-based observations or by in situ measurements can give us useful information about the gas production and composition of comets. However, due to the interaction of ions with the magnetized solar wind and their high chemical reactivity, it is not possible to relate measured ion densities (or column densities) directly to the parent gas densities. In order to quantitatively analyze measured ion abundances in cometary comae it is necessary to understand their dynamics and chemistry. We have developed a detailed ion–chemical network of cometary atmospheres. We include production of ions by photo- and electron impact-ionization of a background neutral atmosphere, charge exchange of solar wind ions with cometary atoms/molecules, reactions between ions and molecules, and dissociative recombination of molecular ions with thermal electrons. By combining the ion–chemical network with the three-dimensional plasma flow as computed by a new fully three-dimensional MHD model of cometary plasma environments (Gombosi et al. 1996) we are able to compute the density of the major cometary ions everywhere in the coma. The input parameters for our model are the solar wind conditions (density, speed, temperature, magnetic field) and the composition and production rate of the gas. We applied our model to Comet P/Halley in early March 1986, for which the input parameters are reasonably well known. We compare the resulting column density of H 2 O + with ground-based observations of H 2 O + from DiSanti et al. (1990). The results of our model are in good agreement with both the spatial distribution and the absolute abundance of H 2 O + and with their variations with the changing overall water production rate between two days. The results are encouraging that it will be possible to obtain production rates of neutral cometary constituents from observations of their ion products.

Plasma parameters are investigated in the regions upstream and downstream of the bow shock of comet Halley based on the ion spectra measured by the Plasmag-1 instrument aboard the Vega-2 spacecraft. Proton velocities observed by the Solar Direction Analyser and by the Cometary Ram Analyser are compared with the simulated bulk velocity profile provided by a three-dimensional multiscale MHD model (Gombosi et al., 1996). Disregarding the effects of presumably interplanetary disturbances, the simulated and measured values are in good agreement at cometocentric distances larger than about 0.5 million km.

MHD simulation results of the interaction of the expanding atmosphere of comet Hale-Bopp with the magnetized solar wind are presented. At the upstream boundary a supersonic and superalfvenic solar wind enters into the simulation box 25 million km upstream of the nucleus. The solar wind is continuously mass loaded with cometary ions originating from the nucleus. The effects of photoionization, recombination and ion-neutral frictional drag are taken into account in the model. The governing equations are solved on an adaptively refined unstructured Cartesian grid using our MUSCL-type upwind numerical technique, MAUS-MHD (Multiscale Adaptive Upwind Scheme for MHD). The combination of the adaptive refinement with the MUSCL-scheme allows the entire cometary atmosphere to be modeled, while still resolving both the shock and the diamagnetic cavity of the comet. Detailed simulation results for the plasma environment of comet Hale-Bopp for slow and fast solar wind conditions are presented. We also calculate synthetic H2O+, CO+ and soft x-ray images for observing conditions on April 11, 1997.

Since the Cassini spacecraft reached Saturn's orbit in 2004, its instruments have been sending back a wealth of data on the planet's magnetosphere (the region dominated by the magnetic field of the planet). In this Viewpoint, we discuss some of these results, which are reported in a collection of reports in this issue. The magnetosphere is shown to be highly variable and influenced by the planet's rotation, sources of plasma within the planetary system, and the solar wind. New insights are also gained into the chemical composition of the magnetosphere, with surprising results. These early results from Cassini's first orbit around Saturn bode well for the future as the spacecraft continues to orbit the planet.

The large-scale field of the Sun is well represented by its lowest energy (or potential) state. Recent observations, by comparison, reveal that many solar-type stars show large-scale surface magnetic fields that are highly non-potential - that is, they have been stressed above their lowest-energy state. This non-potential component of the surface field is neglected by current stellar wind models. The aim of this paper is to determine its effect on the coronal structure and wind. We use Zeeman-Doppler surface magnetograms of two stars - one with an almost potential, one with a non-potential surface field - to extrapolate a static model of the coronal structure for each star. We find that the stresses are carried almost exclusively in a band of uni-directional azimuthal field that is confined to mid-latitudes. Using this static solution as an initial state for an MHD wind model, we then find that the final state is determined primarily by the potential component of the surface magnetic field. The band of azimuthal field must be confined close to the stellar surface as it is not compatible with a steady-state wind. By artificially increasing the stellar rotation rate we demonstrate that the observed azimuthal fields can not be produced by the action of the wind but must be due to processes at or below the stellar surface. We conclude that the background winds of solar-like stars are largely unaffected by these highly-stressed surface fields. Nonetheless, the increased flare activity and associated coronal mass ejections that may be expected to accompany such highly-stressed fields may have a significant impact on any surrounding planets., Comment: 11 pages, 8 figures, accepted by Monthly Notices of the Royal Astronomical Society

The time-dependent hydrodynamic model of Gombosi et al. (1985) was used to investigate the effects of bulk plasma heating on ion flows in the polar ionosphere. In the analysis, coupled time-dependent continuity, momentum, and energy equations of a two-ion (H/+/ and O/+/) quasi-neutral plasma were solved between 200 and 8000 km for polar wind conditions. In spite of its several limitations, the model can serve to indicate the gross behavior of polar region plasma flows. Two specific cases, ion heating and electron heating, were considered. The calculated temperature and flow profiles suggest that ion heating, rather than electron heating, generates the upwelling ion events, for which large transient O(+) outflows accompanied by elevated ion temperatures were observed.

Evolution of gas and dust distributions following a spatially and temporally localized comet outburst was calculated using a hybrid kinetic - hydrodynamic method. It was found that a comet outburst resulted in significantly increased dust terminal velocities. As these higher terminal velocity values result in larger apex distances, therefore the outburst generates distinct dust envelopes in front of the regular dust coma. Such envelopes were observed at several comets (cf. comet Donati).

A well-developed dusty cometary atmosphere extends to distances over 4 orders of magnitude larger than the size of the nucleus. Similarly, the solar wind-dominated heliosphere extends to about 104 solar radii. The first part of this review explores the similarities and differences between the solar and cometary winds. The paper also presents initial results of a new time-dependent, two-dimensional dusty comet wind calculation.

We perform three-dimensional numerical simulations of stellar winds of early-M dwarf stars. Our simulations incorporate observationally reconstructed large-scale surface magnetic maps, suggesting that the complexity of the magnetic field can play an important role in the angular momentum evolution of the star, possibly explaining the large distribution of periods in field dM stars, as reported in recent works. In spite of the diversity of the magnetic field topologies among the stars in our sample, we find that stellar wind flowing near the (rotational) equatorial plane carries most of the stellar angular momentum, but there is no preferred colatitude contributing to mass loss, as the mass flux is maximum at different colatitudes for different stars. We find that more non-axisymmetric magnetic fields result in more asymmetric mass fluxes and wind total pressures $p_{\rm tot}$ (defined as the sum of thermal, magnetic and ram pressures). Because planetary magnetospheric sizes are set by pressure equilibrium between the planet's magnetic field and $p_{\rm tot}$, variations of up to a factor of $3$ in $p_{\rm tot}$ (as found in the case of a planet orbiting at several stellar radii away from the star) lead to variations in magnetospheric radii of about 20 percent along the planetary orbital path. In analogy to the flux of cosmic rays that impact the Earth, which is inversely modulated with the non-axisymmetric component of the total open solar magnetic flux, we conclude that planets orbiting M dwarf stars like DT~Vir, DS~Leo and GJ~182, which have significant non-axisymmetric field components, should be the more efficiently shielded from galactic cosmic rays, even if the planets lack a protective thick atmosphere/large magnetosphere of their own., Comment: 16 pages, 9 figures, to appear in MNRAS

By analogy with the solar system, it is believed that stellar winds will form bow shocks around exoplanets. For hot Jupiters the bow shock will not form directly between the planet and the star, causing an asymmetric distribution of mass around the exoplanet and hence an asymmetric transit. As the planet orbits thorough varying wind conditions, the strength and geometry of its bow shock will change, thus producing transits of varying shape. We model this process using magnetic maps of HD 189733 taken one year apart, coupled with a 3D stellar wind model, to determine the local stellar wind conditions throughout the orbital path of the planet. We predict the time-varying geometry and density of the bow shock that forms around the magnetosphere of the planet and simulate transit light curves. Depending on the nature of the stellar magnetic field, and hence its wind, we find that both the transit duration and ingress time can vary when compared to optical light curves. We conclude that consecutive near-UV transit light curves may vary significantly and can therefore provide an insight into the structure and evolution of the stellar wind., Comment: 9 Pages, 7 figures. Accepted for publication in Monthly Notices of The Royal Astronomical Society

First results of a three-dimensional multiscale MHD model of the interaction of an expanding cometary atmosphere with the magnetized solar wind are presented. The model starts with a supersonic and super-Alfvenic solar wind far upstream of the comet (25 Gm upstream of the nucleus) with arbitrary interplanetary magnetic field orientation. The solar wind is continuously mass loaded with cometary ions originating from a 10-km size nucleus. The effects of photoionization, electron impact ionization, recombination, and ion-neutral frictional drag are taken into account in the model. The governing equations are solved on an adaptively refined unstructured Cartesian grid using our new multiscale upwind scalar conservation laws-type numerical technique (MUSCL). We have named this the multiscale adaptive upwind scheme for MHD (MAUS-MHD). The combination of the adaptive refinement with the MUSCL-scheme allows the entire cometary atmosphere to be modeled, while still resolving both the shock and the diamagnetic cavity of the comet. The main findings are the following: (1) Mass loading decelerates the solar wind flow upstream of the weak cometary shock wave (M approximately equals 2, M(sub A) approximately equals 2), which forms at a subsolar standoff distance of about 0.35 Gm. (2) A cometary plasma cavity is formed at around 3 x 10(exp 3) km from the nucleus. Inside this cavity the plasma expands outward due to the frictional interaction between ions and neutrals. On the nightside this plasma cavity considerably narrows and a relatively fast and dense cometary plasma beam is ejected into the tail. (3) Inside the plasma cavity a teardrop-shaped inner shock is formed, which is terminated by a Mach disk on the nightside. Only the region inside the inner shock is the 'true' diamagnetic cavity. (4) The model predicts four distinct current systems in the inner coma: the density peak current, the cavity boundary current, the inner shock current, and finally the cross-tail current. (5) The calculated plasma parameters (magnetic field, plasma density, speed, and temperature) are in very good agreement with published Giotto observations.

A new two-dimensional axisymmetric MHD model is used to study the interaction of the solar wind with Venus under conditions where the interplanetary field is approximately aligned with the solar wind velocity. This numerical model solves the MHD transport equations for density, velocity, pressure, and magnetic field on an adaptively refined, unstructured grid system. This use of an adaptive grid allows high spatial resolution in regions of large density/velocity gradients and yet can be run on a workstation. The actual grid sizes vary from about 0.06 R(sub v) near the bowshock to 2 R(sub v) in the unperturbed solar wind. The results of the calculations are compared with observed magnetic field values obtained from the magnetometer on the Pioneer Venus Orbiter, at a time when the angle between the solar wind velocity vector and the interplanetary magnetic field (IMF) was only 7.6 deg. Good qualitative agreement between the observed and calculated field behavior is found. The overall results suggest that the induced magnetotail disappears when the IMF is radial for an extended time period and implies that it weakens when the field rotated through a near-radial orientation.

International audience; Comets considered to be pristine objects contain key information about the early formation of the solar system. Their volatile components can provide clues about the origin and evolution of gases and ices in the comets. Measurements with ROSINA/RTOF at 67P/Churyumov-Gerasimenko have now allowed, for the first time, a direct in situ high-time resolution measurement of the most abundant cometary molecules originating directly from a comet's nucleus over a long time-period, much longer than any previous measurements at a close distance to a comet between 3.1 and 2.3 au. We determine the local densities of H2O, CO2, and CO, and investigate their variabilities.

The heating and acceleration of the solar wind is an active area of research. Alfvén waves, because of their ability to accelerate and heat the plasma, are a likely candidate in both processes. Many models have explored wave dissipation mechanisms which act either in closed or open magnetic field regions. In this work, we emphasize the boundary between these regions, drawing on observations which indicate unique heating is present there. We utilize a new solar corona component of the Space Weather Modeling Framework, in which Alfvén wave energy transport is self-consistently coupled to the magnetohydrodynamic equations. In this solar wind model, the wave pressure gradient accelerates and wave dissipation heats the plasma. Kolmogorov-like wave dissipation as expressed by Hollweg along open magnetic field lines was presented in van der Holst et al. Here, we introduce an additional dissipation mechanism: surface Alfvén wave (SAW) damping, which occurs in regions with transverse (with respect to the magnetic field) gradients in the local Alfvén speed. For solar minimum conditions, we find that SAW dissipation is weak in the polar regions (where Hollweg dissipation is strong), and strong in subpolar latitudes and the boundaries of open and closed magnetic fields (where Hollweg dissipation is weak). We show that SAW damping reproduces regions of enhanced temperature at the boundaries of open and closed magnetic fields seen in tomographic reconstructions in the low corona. Also, we argue that Ulysses data in the heliosphere show enhanced temperatures at the boundaries of fast and slow solar wind, which is reproduced by SAW dissipation. Therefore, the model's temperature distribution shows best agreement with these observations when both dissipation mechanisms are considered. Lastly, we use observational constraints of shock formation in the low corona to assess the Alfvén speed profile in the model. We find that, compared to a polytropic solar wind model, the wave-driven model with physical dissipation mechanisms presented in this work is more aligned with an empirical Alfvén speed profile. Therefore, a wave-driven model which includes the effects of SAW damping is a better background to simulate coronal-mass-ejection-driven shocks. © 2012. The American Astronomical Society. All rights reserved. Fil: Evans, Rebekah M.. Boston University; Estados Unidos Fil: Opher, M.. Boston University; Estados Unidos Fil: Oran, R.. University Of Michigan, Ann Arbor; Estados Unidos Fil: van der Holst, Bartholomeus. University Of Michigan, Ann Arbor; Estados Unidos Fil: Sokolov, I.V.. University Of Michigan, Ann Arbor; Estados Unidos Fil: Frazin, Richard A.. University Of Michigan, Ann Arbor; Estados Unidos Fil: Gombosi, Tamas I.. University Of Michigan, Ann Arbor; Estados Unidos Fil: Vasquez, Alberto Marcos. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentina

[1] In order to better describe the space plasmas where pressure anisotropy has prominent effects, we extend the BATS-R-US magnetohydrodynamics (MHD) model to include anisotropic pressure. We implement the anisotropic MHD equations under the double adiabatic approximation with an additional pressure relaxation term into BATS-R-US and perform global magnetospheric simulations. The results from idealized magnetospheric simulations confirm previous studies: pressure anisotropy widens the magnetosheath, increases the density depletion in the vicinity of the magnetopause, enhances the nightside plasma pressure, and introduces an eastward ring current. In addition, we find that the flow speed in the magnetotail is significantly reduced by including pressure anisotropy in MHD simulations. Our model is validated through comparing the simulations to the THEMIS data on both the dayside and nightside of the magnetosphere during quiet times. The comparison to the results from isotropic MHD simulations implies that although anisotropic MHD is comparable to isotropic MHD in matching the measurement, it improves the simulated plasma velocity in some cases.

Magnetic loops are building blocks of the closed-field corona. While active region loops are readily seen in images taken at EUV and X-ray wavelengths, quiet Sun loops are seldom identifiable and therefore difficult to study on an individual basis. The first analysis of solar minimum (Carrington Rotation 2077) quiet Sun (QS) coronal loops utilizing a novel technique called the Michigan Loop Diagnostic Technique (MLDT) is presented. This technique combines Differential Emission Measure Tomography (DEMT) and a potential field source surface (PFSS) model, and consists of tracing PFSS field lines through the tomographic grid on which the Local Differential Emission Measure (LDEM) is determined. As a result, the electron temperature Te and density Ne at each point along each individual field line can be obtained. Using data from STEREO/EUVI and SOHO/MDI, the MLDT identifies two types of QS loops in the corona: so-called "up" loops in which the temperature increases with height, and so-called "down" loops in which the temperature decreases with height. Up loops are expected, however, down loops are a surprise, and furthermore, they are ubiquitous in the low-latitude corona. Up loops dominate the QS at higher latitudes. The MLDT allows independent determination of the empirical pressure and density scale heights, and the differences between the two remain to be explained. The down loops appear to be a newly discovered property of the solar minimum corona that may shed light on the physics of coronal heating. The results are shown to be robust to the calibration uncertainties of the EUVI instrument., Comment: Accepted for publication in The Astrophysical Journal, waiting for the full biblio info

Received 25 October 2011; revised 30 April 2012; accepted 1 May 2012; published 27 June 2012. [1] We investigate the plasma environment of comet 67P/Churyumov-Gerasimenko, the target of the European Space Agency’s Rosetta mission. Rosetta will rendezvous with the comet in 2014 at almost 3.5 AU and follow it all the way to and past perihelion at 1.3 AU. During its journey towards the inner solar system the comet’s environment will significantly change. The interaction of the solar wind with a well developed neutral coma leads to the formation of an upstream bow shock and, closer to the comet, the inner shock separating the solar wind, with cometary pick-up ions mass-loaded, from the inner cometary ions which are dragged outward through abundant collisions and charge exchange with the expanding neutral gas. As a consequence the interplanetary magnetic field is prevented from penetrating the innermost region of the comet, the so-called magnetic cavity. We use our magnetohydrodynamics model BATSRUS (Block-Adaptive-Tree-Solarwind-Roe-Upwind-Scheme) to simulate the solar wind – comet interaction. The model includes photoionization, ion-electron recombination, and charge exchange. Under certain conditions our model predicts an unstable plasma flow at the inner shock. We show that the plasma shear flow around the magnetic cavity can lead to Kelvin-Helmholtz instabilities. We investigate the onset of this phenomenon with change of heliocentric distance and furthermore show that a previously stable magnetic cavity boundary can become unstable when the neutral gas is predominately released from the dayside of the comet.

[1] We use the Polar Wind Outflow Model (PWOM) to study the geomagnetically quiet conditions in the polar cap during solar maximum. The PWOM solves the gyrotropic transport equations for O+, H+, and He+ along several magnetic field lines in the polar region in order to reconstruct the full 3D solution. We directly compare our simulation results to the data based empirical model of Kitamura et al. (2011) of electron density which is based on 63 months of Akebono satellite observations. The modeled ion and electron temperatures are also compared with a statistical compilation of quiet time data obtained by the EISCAT Svalbard Radar (ESR) and Intercosmos Satellites. The data and model agree reasonably well, albeit with some differences. This study shows that photoelectrons play an important role in explaining the differences between sunlit and dark results of electron density, ion composition, as well as ion and electron temperatures of the quiet time polar wind solution. Moreover, these results provide an initial validation of the PWOM's ability to model the quiet time “background” solution.

The recent solar minimum with very low activity provides us a unique opportunity for validating solar wind models. During CR2077 (2008 November 20 through December 17), the number of sunspots was near the absolute minimum of solar cycle 23. For this solar rotation, we perform a multi-spacecraft validation study for the recently developed three-dimensional, two-temperature, Alfvén-wave-driven global solar wind model (a component within the Space Weather Modeling Framework). By using in situ observations from the Solar Terrestrial Relations Observatory (STEREO) A and B, Advanced Composition Explorer (ACE), and Venus Express, we compare the observed proton state (density, temperature, and velocity) and magnetic field of the heliosphere with that predicted by the model. Near the Sun, we validate the numerical model with the electron density obtained from the solar rotational tomography of Solar and Heliospheric Observatory/Large Angle and Spectrometric Coronagraph C2 data in the range of 2.4 to 6 solar radii. Electron temperature and density are determined from differential emission measure tomography (DEMT) of STEREO A and B Extreme Ultraviolet Imager data in the range of 1.035 to 1.225 solar radii. The electron density and temperature derived from the Hinode/Extreme Ultraviolet Imaging Spectrometer data are also used to compare with the DEMT as well as the model output. Moreover, for the first time, we compare ionic charge states of carbon, oxygen, silicon, and iron observed in situ with the ACE/Solar Wind Ion Composition Spectrometer with those predicted by our model. The validation results suggest that most of the model outputs for CR2077 can fit the observations very well. Based on this encouraging result, we therefore expect great improvement for the future modeling of coronal mass ejections (CMEs) and CME-driven shocks. © 2012. The American Astronomical Society. All rights reserved. Fil: Jin, M.. University Of Michigan, Ann Arbor; Estados Unidos Fil: Manchester, W.B.. University Of Michigan, Ann Arbor; Estados Unidos Fil: Van Der Holst, B.. University Of Michigan, Ann Arbor; Estados Unidos Fil: Gruesbeck, J.R.. University Of Michigan, Ann Arbor; Estados Unidos Fil: Frazin, R.A.. University Of Michigan, Ann Arbor; Estados Unidos Fil: Landi, E.. University Of Michigan, Ann Arbor; Estados Unidos Fil: Vasquez, Alberto Marcos. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentina Fil: Lamy, Philippe. Laboratoire D'astrophysique de Marseille; Francia Fil: Llebaria, Antoine. Laboratoire D'astrophysique de Marseille; Francia Fil: Fedorov, A.. Laboratoire D'astrophysique de Marseille; Francia Fil: Toth, Gabor. University Of Michigan, Ann Arbor; Estados Unidos Fil: Gombosi, Tamas I.. University Of Michigan, Ann Arbor; Estados Unidos

[1] Physics-based global magnetosphere modeling requires large computational resources. It is still impractical to resolve the computational domain to the point where numerical errors become negligible. One possible way of reducing numerical diffusion is the “Boris correction”: the semirelativistic magnetohydrodynamics equations are solved with an artificially reduced speed of light. Here we introduce a new alternative approach, an Implicit Scheme with Limited Numerical Dissipation (ISLND). The fully implicit time stepping provides stability, and the wave speeds are limited in the dissipative numerical fluxes only. This limiting only affects the numerical scheme, and it does not modify the equations being solved. This approach can be employed for most total variation diminishing schemes. The differences between the Boris and ISLND schemes are demonstrated in simple numerical tests. We also perform several simulations for two magnetic storms using the global magnetosphere, the ionosphere electrodynamics, and the inner magnetosphere models of the Space Weather Modeling Framework, and we compare the Boris scheme with the limited numerical dissipation method and also with the unmodified base scheme at various grid resolutions. We find that for these particular simulations the Boris scheme and the ISLND scheme produce comparable results, both being significantly less diffusive than the unmodified scheme.

We carry out a numerical simulation depicting the effects of a Coronal Mass Ejection (CME) event on a close-in giant planet in an extrasolar system. We drive the CME in a similar manner as in simulations of space weather events on Earth. The simulation includes the planetary orbital motion, which leads to the forming of a comet-like planetary magnetotail which is oriented almost perpendicular to the direction of propagation of the CME. The combination of this feature and the fact that the CME does not expand much by the time it reaches the planet leads to a unique CME-magnetosphere interaction, where the CME itself is highly affected by the presence of the planetary magnetosphere. We find that the planet is well-shielded from CME penetration, even for a relatively weak internal magnetic field. The planetary angular momentum loss associated with such an event is negligible compared to the total planetary angular momentum. We also find that the energy which is deposited in the magnetosphere is much higher than in the case of the Earth, and our simulation suggests there is a large-scale change in the orientation of the magnetosphere-ionosphere current system during the CME event., 16 pages, 10 figures, accepted to ApJ

We present a focused parameter study of solar wind - magnetosphere interaction for the young Sun and Earth, $~3.5$ Ga ago, that relies on magnetohydrodynamic (MHD) simulations for both the solar wind and the magnetosphere. By simulating the quiescent young Sun and its wind we are able to propagate the MHD simulations up to Earth's magnetosphere and obtain a physically realistic solar forcing of it. We assess how sensitive the young solar wind is to changes in the coronal base density, sunspot placement and magnetic field strength, dipole magnetic field strength and the Sun's rotation period. From this analysis we obtain a range of plausible solar wind conditions the paleomagnetosphere may have been subject to. Scaling relationships from the literature suggest that a young Sun would have had a mass flux different from the present Sun. We evaluate how the mass flux changes with the aforementioned factors and determine the importance of this and several other key solar and magnetospheric variables with respect to their impact on the paleomagnetosphere. We vary the solar wind speed, density, interplanetary magnetic field strength and orientation as well as Earth's dipole magnetic field strength and tilt in a number of steady-state scenarios that are representative of young Sun-Earth interaction. This study is done as a first step of a more comprehensive effort towards understanding the implications of Sun-Earth interaction for planetary atmospheric evolution.

Numerical solutions to the time-dependent kinetic equation, which describes the transport of superthermal electrons in the plasmasphere between the two conjugate ionospheres, are presented. The model calculates the distribution function as a function of time, field-aligned distance, energy, and pitch-angle. The processes of refilling, depleting, and establishing steady-state conditions of super- thermal electrons in the plasmasphere are discussed. A distinctive feature of the ionospheric plasma is the presence of superthermal electrons. Superthermal electrons, which are created by ionization of the atmosphere by solar radiation or energetic particles of magnetospheric origin, take an active part in a number of physical processes affecting the Earth's ionosphere and plasmasphere. Knowledge of the distribution function of superthermal electrons is required when solving such geophysical problems as heating of the thermal ionospheric and plasmaspheric plasmas, optical emissions and ionization of the upper atmosphere, and wave generation in the ionospheric and plasmaspheric plasmas. One impo.rtant aspect of the superthermal electron problem is their transport through the plasmasphere. The superthermal electrons escaping from the ionosphere experience small-angle scattering when traveling through the plasmasphere as a result of the Coulomb interaction with the thermal plasma. Due to this scattering, some of the superthermal electrons are scattered to the outside of the loss cone and undergo magnetic reflection, i.e. they become trapped. Sanatani and Hanson (1970) and Nagy and Banks (1970) presented qualitative discussions of electron trapping and the resulting increase in plasmaspheric heating, but the first attempts of quantitative calculations were those of Gastman (1973), Takahashi (1973), Swartz et al. (1975), Lejeune and Worsmer (1976) and Khazanov et. al. (1977). Among the numerous papers devoted to the superthermal electron transport between the magnetoconjugate regi. ons of the ionosphere, no results have yet been published which describe the evolution and formation of superthermal electron fluxes under nonstationary conditions. Such models may be needed when the local source of superthermal electrons increases or decreases rather sharply, such as during sunrise and sunset, auroral precipitation events, or plasmaspheric millling. During these short periods, strongly anisotropic distributions of sUperthermal electrons are generated, which may be sources of Plasma instabilities and anomalous heating of the thermal plasma.

A one-dimensional hybrid particle code is used to study the interactions between upflowing thermal ions from conjugate ionospheres. The simulation model allows for multiple species, convection of plasmaspheric flux tubes, and Coulomb self-collisions which conserve momentum and energy locally. The model incorporates a variable-flux boundary condition where the flux, at the boundaries, approaches zero as the plasmasphere fills and equilibrium conditions are reached. The effects of two important processes on plasmaspheric refilling have been considered. The first includes convection of the plasmaspheric flux tube. The second is the interaction of ionospheric thermal plasma and particle injection from an external source. Particle injection seems to play an important role in the evolution of the total particle distribution on the early timescales (t less than 1 hour); however, for late timescales (t larger than 8 days) the thermal plasma from the ionosphere dominates the particle distribution.

A numerical solution has been found for the hyperbolic initial-value problem, which follows from the 16-moment set of equations describing the transport of a plasma in a strong magnetic field. In addition to the time-dependent evolution of mass and momentum, these equations describe the variations in anisotropic temperatures and heat fluxes for both ion and electron species. The numerical solution employs a high-order Godunov method to achieve third-order accuracy in space and second-order accuracy in time. The method includes an approximate Riemann solver which is suitable for a system of equations that cannot entirely be expressed in conservation form. In addition, a new technique has been developed to overcome the stiffness that occurs in regions where plasma flows are strongly reactive. Simple test cases showing correct wave behavior are presented.

The interactions between upflowing thermal ions from conjugate ionospheres are modeled using a 1-dimensional hybrid particle code. The simulation model allows for multiple species, convection of plasmaspheric flux tubes, and Coulomb self collisions which conserve momentum and energy. Plasmaspheric refilling is studied, in which convection of the plasmaspheric flux tube and particle injection from an external source is included. The interaction of ionospheric thermal plasma and the injected plasma seems to play an important role in the evolution of the total particle distribution on the early time scales (t

It is shownthat the presenceof hot, anisotropicprotons of ring current in outer plasmaspherelead to the modificationof the dispersionof A- and FMS-wavesin the low frequencyregion. In this case,a periodic instabilityof A-wavesis observed,and during their propagation,the changeof polarization is possible. This can explainthe presenceof right-handpolarizationwavesin the ring current zone.

The cometary ion pickup process along the sun-comet line at Comet Halley is investigated using a quasi-linear diffusion model including both pitch angle and energy diffusion, adiabatic compression, and convective motion with the solar wind flow. The model results are compared with energetic ion distributions observed by instruments on board the Giotto spacecraft. The observed power spectrum index of magnetic turbulence (gamma) is 2-2.5. The present simulation shows that when gamma was 2, the calculated proton distributions were much more isotropic than the observed ones. The numerical solutions of the quasi-linear diffusion equations show that the isotropization of the pickup ion distribution, particularly at the pickup velocity, is not complete even close to the bow shock. Given the observed turbulence level, quasi-linear theory yields pickup ion energy distributions that agree with the observed ones quite well and easily produces energetic ions with energies up to hundreds of keV.

We have obtained expressions for the moments of the quasi-linear integral of anisotropic plasma collisions. Resonant and adiabatic interactions of particles with an arbitrary wave mode are considered. Expressions for the moments describing the interactions of ion-cyclotron waves with ring current and magnetospheric helium ions have been derived as an example, using the obtained results.

[1] The major momentum‐loading source in Saturn’s magnetosphere, Enceladus, has been studied with seven Cassini flybys between 2005 and 2008. In this paper, we first use parameter tests with our 3‐D magnetohydrodynamic simulation to demonstrate and determine the sensitivity of the interaction to both electron impact rates and charge‐ exchange rates. We also investigate the reasons behind our previous discovery that in the plume, within about two Enceladus radii of the plume’s source, the momentum‐loading rates per unit ion and neutral density are orders of magnitude lower than at greater distances. We find that depletion of hot electrons and variations in charge‐exchange rates are two possible explanations for such a reduction of the momentum‐loading rates. Assisted by the Cassini observations, we use our understanding of the plasma interaction to determine the temporal variation of Enceladus’ neutral plume, which is important in understanding its origin, as well as the geological evolution of this icy moon. We base our study on magnetometer observations during all seven flybys to present the first comparative analysis to all flybys in 2005 and 2008. It is found that the maximum variation in gas production rates is one third the largest rate. The plasma momentum‐ loading rate ranges from 0.8 to 1.9 kg/s, which is consistent with previous studies.

We perform a numerical study of the evolution of a Coronal Mass Ejection (CME) and its interaction with the coronal magnetic field based on the May 12, 1997, CME event using a global MagnetoHydroDynamic (MHD) model for the solar corona. The ambient solar wind steady-state solution is driven by photospheric magnetic field data, while the solar eruption is obtained by superimposing an unstable flux rope onto the steady-state solution. During the initial stage of CME expansion, the core flux rope reconnects with the neighboring field, which facilitates lateral expansion of the CME footprint in the low corona. The flux rope field also reconnects with the oppositely orientated overlying magnetic field in the manner of the breakout model. During this stage of the eruption, the simulated CME rotates counter-clockwise to achieve an orientation that is in agreement with the interplanetary flux rope observed at 1 AU. A significant component of the CME that expands into interplanetary space comprises one of the side lobes created mainly as a result of reconnection with the overlying field. Within 3 hours, reconnection effectively modifies the CME connectivity from the initial condition where both footpoints are rooted in the active region to a situation where one footpoint is displaced into the quiet Sun, at a significant distance ($\approx 1R_\odot$) from the original source region. The expansion and rotation due to interaction with the overlying magnetic field stops when the CME reaches the outer edge of the helmet streamer belt, where the field is organized on a global scale. The simulation thus offers a new view of the role reconnection plays in rotating a CME flux rope and transporting its footpoints while preserving its core structure.

[1] We use a single‐fluid global MHD model to study the solar wind control of large‐ scale mass loading and plasma release phenomena in the Kronian magnetosphere. We show that, at high solar wind dynamic pressure, the loss of plasma in the magnetotail is continuous. At medium dynamic pressure, plasmoids are pinched off periodically along an X‐line in the postmidnight sector through a cascade of helical reconnection. Plasmoids have a magnetic topology of a helical flux rope with its ends anchoring in the polar regions of Saturn. With decreasing dynamic pressure, the repetition period of plasmoids gradually increases. A higher mass‐loading rate or a higher axial tilt of Saturn makes the repetition period longer. At low dynamic pressure, the release of plasmoids becomes quasi‐periodic or chaotic. The pressure control of the repetition period is very similar to the behavior of a dripping faucet. The mass and volume of the closed magnetosphere are smaller at lower dynamic pressures because of a relatively longer X‐line. In our simulations, large‐scale plasmoids are responsible for less than 8% of the total mass loss, and the rest of the plasma is lost via cross‐field diffusion or other small‐scale mechanisms.

The development of global magnetospheric models, such as Space Weather Modeling Framework (SWMF), which can accurately reproduce and track space weather processes has high practical utility. We present an interval on 5 June 1998, where the location of the polar cap boundary, or open-closed field line boundary (OCB), can be determined in the ionosphere using a combination of instruments during a period encompassing a sharp northward to southward interplanetary field turning. We present both point- and time-varying comparisons of the observed and simulated boundaries in the ionosphere and find that when using solely the coupled ideal magnetohydrodynamic magnetosphere-ionosphere model, the rate of change of the OCB to a southward turning of the interplanetary field is significantly faster than that computed from the observational data. However, when the inner magnetospheric module is incorporated, the modeling framework both qualitatively, and often quantitatively, reproduces many elements of the studied interval prior to an observed substorm onset. This result demonstrates that the physics of the inner magnetosphere is critical in shaping the boundary between open and closed field lines during periods of southward interplanetary magnetic field (IMF) and provides significant insight into the 3-D time-dependent behavior of the Earth's magnetosphere in response to a northward-southward IMF turning. We assert that during periods that do not include the tens of minutes surrounding substorm expansion phase onset, the coupled SWMF model may provide a valuable and reliable tool for estimating both the OCB and magnetic field topology over a wide range of latitudes and local times.