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Modelling and simulation of the interaction of the Solar Wind with Mercury and Mars
Abstract
The purpose of this work is to study the interactions between the Solar Wind (SW) and Mercury and/or Mars by means of numerical simulations. In the context of previous observations made Mariner 10 and MESSENGER, and the preparation of the future mission Bepi Colombo, we developed a three-dimensional and parallel hybrid model of the interaction of the SW with Mercury's magnetosphere. We also investigated the reflection of incident particles on the Martian bow shock, using a test-particle program combined with hybrid simulation results. Despite the fact that observations are necessary for studying the interaction of the SW with a planetary obstacle, they are not sufficient and numerical modelling represents an essential tool to complete observations analysis. In the second chapter we introduce the different simulation models used to study the interactions of the SW with Mercury and Mars. We describe in particular the building steps of our Mercury hybrid model. The results concerning the hermean environment, obtained with models which preceded ours, are described in the first part of the Chapter 3. In the following of this chapter, we present our results about the intrinsic magnetic field and the magnetospheric plasma of Mercury. In the Chapter 4, we describe the hybrid model and the test-particle program we used to investigate the reflection of SW and planetary protons on the Martian bow shock. Then we exhibit the results. Although this study has been led on the Martian bow shock, it can be easily adapted on Mercury.
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The influence of the interplanetary magnetic field on solar wind-magnetosphere interactions is described. A finite-conductivity solution for the magnetic field B in the magnetosheath is constructed, and it holds even close to the magnetopause, where the frozen-flux approximation is invalid. The electric field E implied by the solar wind plasma bulk motion is determined, too. Magnetic field diffusion caused by finite conductivity results in only a partial screening of the outer field in a dissipative layer near the magnetopause, with residual Bn and Et penetrating into the magnetosphere. The dependence of the magnetic and electric fields at the magnetopause on the magnetic Reynolds number, Rm, is Bn-Rm-p and Et-Rm-p; Bt-Rmp, and En-Rmp where p=0.25. The Poynting vector flux over the magnetopause is independent of Rm and equal to -2.6×1011W for usual solar wind and IMF parameters. Assuming E11=0 on open magnetic field lines, it is possible to determine the electric field within the region of open magnetic field lines using IMF-dependent boundary conditions at the magnetopause. Field-aligned currents in this region (or in the polar cap) are derived from the equation Div (ΣE)=j11, where the ionospheric conductivity Σ is assumed uniform. Sunward convection for northward IMF is explained. A shift of the cusp projection in the ionosphere due to the IMF effect is presented. For northward IMF a two-vortex convection pattern in the polar cap is obtained. Comparison of model predictions with experimental data shows good agreement in the dayside polar cap. The proposed model provides a self-consistent analytic solution of the problem of IMF penetration into the magnetosphere.
The MErcury, Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission will send the first spacecraft to orbit the planet Mercury. A miniaturized set of seven instruments, along with the spacecraft telecommunications system, provide the means of achieving the scientific objectives that motivate the mission. The payload includes a combined wide- and narrow-angle imaging system; γ-ray, neutron, and X-ray spectrometers for remote geochemical sensing; a vector magnetometer; a laser altimeter; a combined ultraviolet-visible and visible-infrared spectrometer to detect atmospheric species and map mineralogical absorption features; and an energetic particle and plasma spectrometer to characterize ionized species in the magnetosphere.
We examine the
circulation of heavy ions of planetary origin within Mercury’s magnetosphere.
Using single particle trajectory calculations, we focus on the dynamics of
sodium ions, one of the main species that are ejected from the planet’s
surface. The numerical simulations reveal a significant population in the
near-Mercury environment in the nightside sector, with energetic (several keV)
Na + densities that reach several tenths cm-3 at planetary
perihelion. At aphelion, a lesser (by about one order of magnitude) density
contribution is obtained, due to weaker photon flux and solar wind flux. The
numerical simulations also display several features of interest that follow
from the small spatial scales of Mercury’s magnetosphere. First, in contrast
to the situation prevailing at Earth, ions in the magnetospheric lobes are
found to be relatively energetic (a few hundreds of eV), despite the low-energy
character of the exospheric source. This results from enhanced centrifugal
acceleration during E × B transport over the polar cap. Second,
the large Larmor radii in the mid-tail result in the loss of most Na + into the
dusk flank at radial distances greater than a few planetary radii. Because
gyroradii are comparable to, or larger than, the magnetic field variation
length scale, the Na + motion is also found to be non-adiabatic throughout most
of Mercury’s equatorial magnetosphere, leading to chaotic scattering into the
loss cone or meandering (Speiser-type) motion in the near-tail. As a direct
consequence, a localized region of energetic Na + precipitation develops at the
planet’s surface. In this region which extends over a wide range of
longitudes at mid-latitudes ( ~ 30°–40°), one may expect additional
sputtering of planetary material.Key words. Magnetospheric physics
(planetary magnetospheres) – Space plasma physics (charged particle motion
and acceleration; numerical simulation studies)
A new ``Paraboloidal'' model of Mercury's magnetospheric magnetic field based upon the earlier terrestrial model and using similar techniques is developed. The model describes the field of Mercury's dipole, which is considered to be offset from the planet's center; the magnetopause currents driven by the solar wind; and the tail current system including the cross-tail currents and their closure currents at the magnetopause. The effect of the interplanetary magnetic field (IMF) is modeled as a partial penetration of the IMF into the magnetosphere. The goals of the present work are (1) to develop an easily usable, yet robust model of Mercury's magnetospheric magnetic field and (2) to produce an improved ``unified'' determination of Mercury's magnetic dipole moment which fits the measurements taken during both Mariner 10's first and third flybys. This new model of Mercury's magnetosphere is described and used to determine a best Mercury magnetic dipole moment of 192 nT R M 3, from the two Mariner 10 flybys, a value which is intermediate between the various estimates produced by previous models. The best fit to the Mariner 10 measurements gives the dipole offset 0.18 R M above the equatorial plane. The new Paraboloidal model is used to predict the configuration of this miniature magnetosphere under average and extreme solar wind conditions.
The Mars Global Surveyor spacecraft was inserted into an elliptical orbit around Mars on September 12, 1997. It includes the MAG/ER instrument with two magnetometers providing in-situ sensing of the ambient magnetic field and an electron reflectometer measuring the local distribution function of the electrons in the energy range of 10 eV to 20 keV. This statistical study deals with the identification and the position of the Bow Shock (BS) and of another plasma boundary, the Magnetic Pile-up Boundary (MPB), proved as permanent by MAG/ER. During the first year of the MGS mission, a total of 290 orbits have been considered to fit the geometric characteristics of these boundaries. The position and shape of these boundaries are compared with previous studies. Good agreement is found with the Phobos 2 observations, suggesting than the mean bow shock and MPB locations are independent of solar cycle phase. The great number of crossings shows that the Bow Shock position and nightside MPB position are highly variable.
The reflection of solar wind protons on the Martian bow shock (BS) is investigated by means of three-dimensional simulation models. A two steps approach is adopted to allow a detailed analysis of the reflected population. Firstly, the 3-dimensional hybrid model of Modolo et al. (2005) is used to compute a stationary state of the interaction of the solar wind (SW) with Mars. Secondly, the motion of test particles is followed in the electromagnetic field computed by the hybrid simulation meanwhile detection criteria defined to identify reflected protons are applied. This study demonstrates some effects of the large curvature of a planetary BS on the structure of the foreshock. Reflected protons encounter the BS in a region encompassing parts of the quasi-perpendicular and quasi-parallel shocks, and exit the shock mainly from the quasi-parallel region. The energy spectrum of all reflected protons extends from 0 to almost 15keV. A virtual omnidirectional detector (VOD) is used to compute the local omnidirectional flux of reflected protons at various locations upstream of the BS. Spatial variations of this omnidirectional flux indicate the location and spatial extent of the proton foreshock and demonstrate its shift, increasing with the distance downstream, in the direction opposite to the motional electric field of the SW. Local energy spectra computed from the VOD observations demonstrate the existence of an energy gradient along the direction of the convection electric field.
Unattenuated solar photo rate coefficients and excess energies for dissociation, ionization, and dissociative ionization are presented for atomic and molecular species that have been identified or are suspected to exist in the atmospheres of planets, satellites (moons), comets, or as pollutants in the Earth atmosphere. The branching ratios and cross sections with resonances have been tabulated to the greatest detail possible and the rate coefficients and excess energies have been calculated from them on a grid of small wavelength bins for the quiet and the active Sun at 1 AU heliocentric distance.
A unique feature of Mercury's space environment is its strongly coupled surface-exosphere-magnetosphere-solar wind system, which can be remotely monitored by space missions such as Mariner 10, MESSENGER and soon BepiColombo and by ground-based observatories. Mercury's exosphere is a very complex medium with only a few species detected so far, including atomic hydrogen. H has only been detected once by Mariner 10 in 1974-1975 and represents a tracer of the interaction between the solar wind and Mercury. The PHEBUS instrument onboard the BepiColombo ESA/JAXA mission to Mercury is a dual-channel EUV-FUV spectrometer capable of detecting faint emissions including H I Lyman- at 121.6 nm. The first part of this thesis focuses on the radiometric modelling and simulation of the optical return of PHEBUS. To prepare for in-flight and orbit spectral calibrations, a set of reference stars is determined and evaluated to match the resolution and spectral range of the detector. Predictions on the possibility of detection of a wide range of emission lines in space and in Mercury's exosphere are given (science performance). PHEBUS is based on SPICAV, the UV spectrometer onboard Venus Express and can use similar techniques to perform the relevant calibrations. Therefore, a study of the star occultation events of SPICAV is performed in the second part of this thesis. The stars' spectra are extracted, analysed and convoluted with the instrument function for possibility of future use with PHEBUS. The results are stored in the calibration database for the "Cross-calibration of past FUV experiments" workgroup of ISSI. In parallel to the development of new dedicated instruments, such as PHEBUS with high sensitivity and spectral resolution, state-of-the-art simulations of the exosphere of Mercury are also needed. In the third part of this thesis the 3D Monte Carlo hydrogen model SPERO is constructed. SPERO is the first fully consistent 3D exospheric model of hydrogen at Mercury, taking into account source and loss mechanisms such as thermal desorption, photoionisation and solar radiation pressure. Thermal desorption is assumed to be the dominant source of exospheric hydrogen. Surface densities as well as exospheric densities, temperatures and velocities are computed up to 8 Mercury radii. A sensitivity study is carried out highlighting the uncertainties in the source and loss mechanisms, and resulting in a day/night density and temperature asymmetry. Using the computed densities in a radiative transfer model makes it possible to compare with the Mariner 10 Lyman- emission data and later on possibly the hydrogen data return of the MASCS instrument onboard NASA MESSENGER.
When the PHOBOS-2 spacecraft entered the Martian ion foreshock region and was magnetically connected with the bow shock, the mass spectrometer ASPERA recorded enhanced proton fluxes in the perpendicular and antisunward directions. The third elliptical orbit on February 8, 1989 has been analysed in detail in this respect and we have concluded that the observed disturbances might have resulted from gyrating reflected ions and field aligned beams propagating upstream from the bow shock. Comparison with a simple model of specular reflection can partially explain observations. Disturbances recorded inside the foot seem to result from kinetic processes mentioned by Moses et al. [1988]. The density of the backstreaming ions at the Martian bow shock reached 0.4 cm−3 and could account for 60% of the observed solar wind velocity decrease just upstream of the shock. The Martian upstream region is very compact and that results in a shortage of space for different instabilities to develop. So no reflected beam disruptions were observed, only beam-like distributions were recorded.
The measurements carried out by the plasma spectrometer ASPERA, on-board the Phobos 2 spacecraft show that the Martian bow shock is characterized by a sudden increase of ionization of the neutral corona. It acts as a source of new ions that can strongly modify the process of ion heating behind the shock front. The loss of momentum of solar wind protons due to their interaction with exospheric ions may lead to an increase in the effective scale of the obstacle.
This paper presents a new solar Extreme Ultraviolet (EUV) flux model for aeronomic calulations (EUVAC), which is based on the measured F74113 solar EUV reference spectrum. The model provides fluxes in the 37 wavelength bins that are in widespread use. This paper also presents cross sections to be used with the EUVAC flux model to calculate photoionization rates. The flux scaling for solar activity is accomplished using a proxy-based on the F10.7 index and its 81-day average together with the measured solar flux variation from the EUVS instrument on the Atmosphere Explorer E satellite. This new model produces 50-575 A integrated EUV fluxes in good agreement with rocket observations. The solar cycle variation of the chromospheric fluxes agrees well with the measured variation of the Lyman alpha flux between 1982 and 1988. In addition, the theoretical photoelectron fluxes, calculated using the new EUV flux model, are in good agreement with the solar minimum photoelectron fluxes from the Atmosphere Explorer E satellite and also with the solar maximum photoelectron fluxes from the Dynamics Explorer satellite. Its relative simplicity coupled with its ability to reproduce the 50-575 A solar EUV flux as well as the measured photoelectron spectrum makes the model well suited for aeronomic applications. However, EUVAC is not designed to accurately predict the solar flux variability for numerous individual lines.
This short note classifies the interactions between different objects in
the solar system and their environments with the solar wind or
magnetospheric plasma. The classification is performed using a simple
but physically justified M-B diagram, where M is the total mass of the
neutral gas around the object and B is the magnetic field (either
intrinsic or external) at the object. The solar system objects form
groups within the diagram that correspond to the following general
classes: magnetospheres, mini-magnetospheres, induced magnetospheres,
cometary and Moon-like interactions. The Martian magnetic anomalies are
unique objects. We also identify a class termed as combined
magnetospheres, which does not contain any objects that meet the present
conditions.
The magnetic cusp of a planetary magnetosphere allows solar wind plasma to gain access to the planet's magnetosphere and, for Mercury, the surface. From measurements by the MESSENGER Magnetometer we have characterized the magnetic field in the northern cusp region of Mercury. The first six months of orbital measurements indicate a mean latitudinal extent of the cusp of ∼11, and a mean local time extent of 4.5hrs, at spacecraft altitudes. From the average magnetic pressure deficit in the cusp, we estimate that (1.10.6)×10 24 protons s-1 bombard the surface over an area of (5.21.6)×1011m2 near the northern cusp. Plasma pressures in the cusp are 40% higher when the interplanetary magnetic field (IMF) is anti-sunward than when it is sunward. The influence of the IMF direction does not overcome the north-south asymmetry of Mercury's internal field, and particle flux to the surface near the southern cusp is predicted to be a factor of 4 greater than in the north. The higher particle flux impacting the surface in the south should lead to a greater exospheric source from the south and a higher rate of space weathering than in the area of the northern cusp.
During the first and second Mercury flyby the MESSENGER spacecraft detected a dawn side double-current sheet inside the Hermean magnetosphere that was labeled the “double magnetopause” (Slavin, J.A. et al. [2008]. Science 321, 85). This double current sheet confines a region of decreased magnetic field that is referred to as Mercury’s “dayside boundary layer” (Anderson, M., Slavin, J., Horth, H. [2011]. Planet. Space Sci.). Up to the present day the double current sheet, the boundary layer and the key processes leading to their formation are not well understood. In order to advance the understanding of this region we have carried out self-consistent plasma simulations of the Hermean magnetosphere by means of the hybrid simulation code A.I.K.E.F. (Müller, J., Simon, S., Motschmann, U., Schüle, J., Glassmeier, K., Pringle, G.J. [2011]. Comput. Phys. Commun. 182, 946–966). Magnetic field and plasma results are in excellent agreement with the MESSENGER observations. In contrast to former speculations our results prove this double current sheet may exist in a pure solar wind hydrogen plasma, i.e. in the absence of any exospheric ions like sodium. Both currents are similar in orientation but the outer is stronger in intensity. While the outer current sheet can be considered the “classical” magnetopause, the inner current sheet between the magnetopause and Mercury’s surface reveals to be sustained by a diamagnetic current that originates from proton pressure gradients at Mercury’s inner magnetosphere. The pressure gradients in turn exist due to protons that are trapped on closed magnetic field lines and mirrored between north and south pole. Both, the dayside and nightside diamagnetic decreases that have been observed during the MESSENGER mission show to be direct consequences of this diamagnetic current that we label Mercury’s “boundary-layer-current“.
Telescopic measurements using the double-image technique, during transits of Mercury across the solar disk, give 2429 ± 18 km for the mean radius of Mercury along the two meridians centered at longitudes 90° and 270°, respectively (IAU system).
Observations of sodium emission from Mercury can be used to describe the spatial and temporal patterns of sources and sinks in the planet's surface-boundary-exosphere. We report on new data sets that provide the highest spatial resolution of source regions at polar latitudes, as well as the extraordinary length of a tail of escaping Na atoms. The tail's extent of ~1.5 degrees (nearly 1400 Mercury radii) is driven by radiation pressure effects upon Na atoms sputtered from the surface in the previous ~15 hours. Wide-angle filtered-imaging instruments are thus capable of studying the time history of sputtering processes of sodium and other species at Mercury from ground-based observatories in concert with upcoming satellite missions to the planet. Plasma tails produced by photo-ionization of Na and other gases in Mercury's neutral tails may be observable by in-situ instruments.
Gross characteristics of the vector magnetic field perturbations detected by the Triad satellite at 800-km altitude in the auroral oval are analyzed as a function of dipole local time (DLT) under a broad range of magnetic activity. The perturbations are always transverse to the main field and thus result from field-aligned currents. During the period 1400 DLT through dusk to 2300 DLT, the perturbation throughout the oval is directed eastward; the inferred field-aligned currents flow downward (into the ionosphere) in the equatorward part of the disturbance region and upward (out of the ionosphere) in the poleward part. During the epoch 2400 DLT through dawn to 1000 DLT, the magnetic perturbation is westward and the current flow upward in the equatorward part and downward in the poleward part, characteristics reversed from those in the 1400-2300 DLT epoch. Either set can appear in the transition period, 2300-2400 DLT.
In the present paper, direct observations in space are used as a basis to study the status of our knowledge of shock structure from a plasma physics point of view. The history of collisionless shock experience is briefly reviewed. A reasonably documented picture of collisionless shock is derived from a discussion of the earth's bow shock system, planetary shocks, and astrogenic shocks. Methods of observing collisionless shock in space are described, and an example is given of the application of paired satellite measurements to the determination of the properties of the earth's bow shock system.
Despite the fact the Mercury is a small, rocky, burned planet, so close to the Sun that it is hard to be seen from Earth except during twilight, it has played a very important role in the history of astronomy and fundamental physics and plays a crucial role in the understanding of the formation and evolution of our Solar System.
A database, the largest one to date, of magnetosheathencounters by geosynchronous satellites during 1986-1992 and 1999-2000and upstream observations by Wind, IMP 8, or Geotail in the solar wind areused to estimate the forecasting capability of the models of Chao et al. [2001], Shue et al. [1998], and Petrinec and Russell [1996]. For each of the1-min resolution data points obtained by GOES spacecraft, we check the followingtwo things: if the magnetosheath was observed by the spacecraft, and if eachof the three models predicted a magnetosheath encounter by the spacecraft.Three parameters are defined to quantify the models' forecasting capability:probability of prediction (PoP), probability of detection (PoD), and falsealarm rate (FAR). A higher PoP and PoD with a lower FAR imply a better forecastingmodel. In the 1986-1992 period we found that most of the magnetosheathencounters observed at 6.6 REare detected. In particular, the Chaoet al. [2001] model predicts the lowest FAR compared withthose of the other two models. We have also studied the magnetosheath encountersmade by GOES spacecraft for the period from 1999 to 2000 using Wind and Geotailas the solar wind monitor. This independent database of magnetosheath encountersduring 1999-2000 confirms our previous anticipations. The PoD of Petrinec and Russell [1996]model (94%) is much higher than those of Shueet al. [1998] (74%) and Chaoet al. [2001] (84%) models. The Chao et al. [2001] model has a higher PoPthan the other two models. The values of FAR for Chao et al. [2001], Shue et al. [1998], and Petrinec and Russell [1996] models are 25,27, and 40%, respectively.
The latest measurements from the two encounters of the MESSENGER spacecraft in year 2008 have discovered several interesting features of the magnetosphere of Mercury. We have performed high-resolution 3D hybrid model calculations to simulate the solar wind interaction with the Hermean magnetosphere during the first two Mercury encounters of the MESSENGER spacecraft in 2008. It is found that the global structure of the Hermean magnetosphere is significantly controlled by the direction of the interplanetary magnetic field. The bow shock size and shape and the magnetotail configuration have very large differences in these two encounters with northward-pointing and southward-pointing interplanetary magnetic field, respectively. Comparisons are also given with the observed magnetic field profiles and the computational results. In general, good agreement can be found including the interesting feature of the relatively thick magnetopause current layer at outbound measurements. Our work shows that 3D hybrid simulation is a promising method to study in detail the Hermean magnetosphere in parallel with the post-MOI observations of the MESSENGER spacecraft and the Bepi-Colombo mission in future.
We report observations of electrostatic ion acoustic waves and Langmuir waves during a recent lunar encounter by the Wind spacecraft. These waves are observed when Wind is magnetically connected to the nominal wake and at distances greater than 8 lunar radii from the wake. When interpreted in the context of a simple time-of-flight model, these observations imply the existence of a system of currents and disturbed particle distributions that extends far into solar wind.
Mercury is difficult to observe because it is so close to the Sun. However, when the angle of the ecliptic is near maximum in the Northern Hemisphere, and Mercury is near its greatest eastern elongation, it can be seen against the western sky for about a half hour after sunset. During these times, we were able to map sodium D2 emission streaming from the planet, forming a long comet-like tail. On May 26, 2001 (UT) we mapped the tail downstream to a distance of about 40,000 km. Sodium velocities in the tail increased to about 11 km/sec at 40,000 km as the result of radiation pressure acceleration. On June 05, 2000 (UT) we mapped the cross-sectional extent of the tail at a distance of about 17,500 km downstream. At this distance, the half-power full-width of the emission was about 20,000 km. We estimated the transverse velocity of sodium in the tail to range from 2 to 4 km/sec. The velocities we observed imply source velocities from the planet surface of the order of 5 km/sec, or 4 eV. Particle sputtering is a likely candidate for production of sodium atoms at these velocities. The total flux of sodium in the tail was approximately 1 x 1023 atoms/sec, which corresponds to 1 to 10 % of the estimated total production rate of sodium on the planet.
Recent results of 3D hybrid simulations of Mercury's magnetosphere revealed its basic structure with well pronounced cusp regions and also a closed ion ring forms around the planet. In general the plasma within the magnetosphere is more energetic in the high solar wind pressure case and the ion foreshock contains hotter magnetosheath plasma. Particles originating from the planet disperse throughout the magnetosphere, with the greatest congregation occuring in the inner magnetospheric drift-driven rings in both cases. These planetary particles can also leak upstream into the foreshock region. We study changes in properties and structure of the Mercury's magnetosphere with different solar wind beta, in particular the magnetosheath. We also predict values of basic plasma parameters obtained during the flyby of MESSENGER in January 2008.
Paper presents Vela measurements demonstrating that positive ions from solar wind are often accelerated at bow shock and emitted outward into free flowing wind.
We investigate the parallel acceleration of ions inside Mercury's magnetosphere. We focus on centrifugal effects associated with the large scale plasma convection. We demonstrate that, because curvature radii of the E × B drift paths are much smaller at Mercury than at Earth, centrifugal effects are enhanced and lead to prominent parallel energization during transport from high to low latitudes. For moderate convection rates, model trajectory calculations reveal that these E × B related centrifugal effects yield energization of heavy ions sputtered from Mercury's surface (e.g., Na+) of several tens or a few hundreds of eV within minutes. This suggests that, at Mercury, material of planetary origin is rapidly raised up to plasma sheet energies. Such a situation contrasts with that prevailing at Earth where these centrifugal effects are relatively weak, yielding energization of ionosphere originating ions of at most a few tens of eV on the time scale of hours.
A numerical MHD code is used to simulate the dynamical responses of Mercury's magnetosphere to different orientations of the interplanetary magnetic field (IMF) and values of the solar wind dynamical pressure. It is found that the IMF orientation is very effective in changing the size of the polar cap and, hence, the surface area subject to direct solar wind sputtering. Even though the polar cap size is not significantly changed by the solar wind dynamic pressure variation for a fixed IMF direction, the global configuration and size of the Hermean magnetosphere are highly responsive. The observed time variations of the atomic sodium emission at Mercury, hence, might indeed be correlated with the solar wind condition.
Short time variations of Mercury's exosphere cannot be tracked easily from ground based observatories because of the difficulty of distinguishing them from Earth atmospheric effects. On July 13th 2008, using THEMIS solar telescope, we were able to simultaneously measure brightness, Doppler shift and width of the exospheric sodium D2 emission line during half a day with a resolving power of ˜370,000. Mercury's exosphere displayed an emission brightness peak in the Northern hemisphere which vanished in few hours and a more persistent Southern Hemispheric peak. The bulk Doppler shift of the exosphere suggests a period of strong escape from Mercury. The global changes of the Doppler shift and of the Doppler width suggest that a cloud of sodium atoms ejected before or at the beginning of our sequence of observations passed through THEMIS field of view moving anti-sunward. A preferentially southern ejection of sodium atoms leading to the observed persistent southern emission peak is consistent with the orientation of the Interplanetary Magnetic Field during that period.
We present the first combined intensity and temperature maps of sodium in Mercury's exosphere, made possible by the use of the THEMIS solar telescope on Tenerife in the Canary Islands. The intensity maps clearly show high-latitude peaks, and temperatures inferred from spectral line widths suggest that these regions are either slightly hotter than the rest of the exosphere or much smaller than observed. These brighter, warmer regions are also observed, for the first time, to appear within few Earth hours which strongly suggest that they are produced by solar wind sputtering. This highly capable instrument obtained these data during daylight, highlighting the unique potential for THEMIS to undertake continuous multi-hour and multi-day datasets in conjunction with the MESSENGER mission to Mercury.
Ion measurements made by the Phobos 2 spacecraft revealed an extensive, ever-persisting escape of planetary ions in the Martian tail. Observations indicated that in large regions of the Martian tail, most ions were of planetary origin. The measurements suggested that the solar wind "pickup" of planetary ions is an important factor in the global Mars-solar wind interaction. However, how in detail the newly formed planetary ions affect the formation of the observed global plasma and field regions remained unanswered. In this paper the escape of Martian ions is studied with a quasi-neutral hybrid simulation model. The used hybrid model includes two ion species: the solar wind protons and the planetary ions. This approach makes it possible to study the asymmetries associated with the finite ion gyroradius as well as the difference between the motion of the solar wind protons and planetary ions. The source of the escaping ions is studied indirectly by producing planetary ions from two ion sources: the neutral corona (photoionization) and the ionosphere. The simulation is found to reproduce many observed features of the Mars-solar wind interaction in general and those of the planetary ions in particular. Ion escape within the optical shadow forms an antisunward flowing beam of ~1 keV planetary ions, much as observed. A total mass loss rate of ~100-400 g s-1 is found to be large enough to reproduce several plasma and magnetic field features observed by Phobos2 on its circular orbits. Simulations produce a wide region behind the planet where the density and the particle flux of H+ ions are much smaller than their solar wind values, resembling the observed "proton cavity." The work also depicts how the "ionotail," formed mainly by the escaping planetary ions, is embedded in the Martian magnetotail. The runs also result in a notable asymmetry with respect to the direction of the convectice electric field in the solar wind which is caused by the finite ion gyroradius. The magnetopause also demonstrates the asymmetry: the magnetopause is a sharp boundary or a smooth transition layer, depending on the direction of the upstream convective electric field. Overall, the work illustrates the importance of the plasma-neutral interactions in the global Mars-solar wind interaction.
Atmospheric effects of precipitating solar wind protons in the Martian atmosphere are studied. The proton flux to the atmosphere is derived from a newly developed global quasineutral hybrid simulation which includes solar wind H+ ions and planetary O+ ions. The motion of the precipitating particles in the atmosphere is followed, and the effects of collisions to atmospheric neutrals are studied by a collision-to-collision Monte Carlo algorithm. Maximum atmospheric effects are estimated by using a fully absorbing boundary condition in the hybrid model where all solar wind protons are allowed to precipitate into the atmosphere without reflection. The developed mass-loaded hybrid code is found to reproduce many of the observed plasma and field features near Mars. When the vertical profiles of the energy deposition rates, CO2+ ionization rates, and Lyman alpha emission rates are calculated at different solar zenith angles, the maximum atmospheric effects on the dayside under average solar wind conditions are found to be typically a few percent of the effects of EUV radiation. On the nightside the proton precipitation is estimated to be intensive enough to be able to produce the measured ionospheric electron densities. The analysis illustrates that the atmospheric effects are strongly coupled with the global plasma interaction process between Mars and the solar wind.
Vapor clouds generated by oblique impacts were observed spectroscopically. The observations here concentrate on the earliest downrange-moving component, among multiple components of vapor clouds generated by hypervelocity impacts of quartz projectiles into dolomite targets. The spectrum of impact vapor simultaneously exhibits blackbody radiation, molecular band emission, and atomic line emission, but their relative ratios change with time and space. The spectrum of the earliest component is dominated by the line/band emissions. The strong band/line emissions demonstrate the presence of a gas phase. The ratio of line/band emission to blackbody radiation is higher in near-vertical impacts than shallower angle impacts. Ratios of normalized intensities of emission lines indicate that a Boltzmann distribution with an equilibrium temperature ranging from 4000 K to 6000 K approximates well the distribution of calcium atoms in energy levels in the impact vapor. Furthermore, the temperature of impact vapor appears to be controlled by the vertical component of the impact velocity. The degree of self-absorption of a calcium emission line, which comes from vaporized dolomite target, indicates that only a very small mass of impact vapor is involved in the observed atomic radiation process. This observation, as well as the short lifetime and extremely high temperature, suggests that jetting may be the dominant source of the line emissions in the earliest stage vapor component at impact velocities less than 6 km/s. The experimental techniques and analysis methods presented in this study are applicable to other components of impact-generated vapor and will provide new information on vaporization mechanisms in hypervelocity impacts.
The author presents results from efforts to develop a more realistic data based model of the magnetic field of the earth as it extends into the magnetosphere, and is draped into the magnetotail. The author attempts in this work to address what are considered to be several of the major deficiencies of the previous work. These include a model which did not well merge into the magnetopause, a poor treatment of the K{sub p} index, and inaccurate treatments of the B{sub z} component in the equatorial magnetotail. These efforts are compared to spacecraft based measurements.
Surface-bounded exospheres have been detected at the Moon, Mercury, and Europa and almost certainly exist about other objects. Historically, the first of these systems to be observed was the lunar exosphere, where He and Ar were detected by the Apollo spacecraft. The Hermean exosphere is archetypical of these systems in that it is part of a coupled system including the surface and magnetosphere interacting dynamically with the solar wind and fields. Studies of the Hermean exosphere heretofore have neglected or only superficially considered these interactions. We will review the current state of knowledge of the exospheres of Mercury and the Moon and discuss areas in which our knowledge is most incomplete. We will focus on the exosphere as part of a coupled system including the surface at its base and the particle, field, and interplanetary environment as both a source and sink for neutrals. Apollo era instruments made unambiguous detections of 36Ar, 40Ar, and He and placed stringent upper limits on other species [Hodges, 1975; Hodges 1973]. Post-Apollo work began with the discovery of Na and K using ground-based techniques [Potter and Morgan, 1988]. There have also been attempts to use the International Ultraviolet Explorer and the Hubble Space Telescope to detect additional species. The Mariner 10 mission included two encounters with Mercury and one distant flyby in 1974-1975: (1) March 29, 1974 (nightside pass with closest approach at 723-km altitude), (2) September 21, 1974 (distant dayside pass), and (3) March 16, 1975 (nightside pass with closest approach at 327-km altitude). Since then there has been no other spacecraft mission to Mercury. In the 2 decades since the Mariner 10 mission, slow but steady progress has been made in understanding the Hermean system, but our knowledge of this innermost terrestrial planet is extremely limited. It is clear that many theories about the origin and evolution of Mercury that were proposed before Mariner 10 are inconsistent with what we now know about the system. Substantial progress will be made only by returning to the planet. This review aims to present a tool for mission planning as well as an introduction and overview of the exosphere.
We present images of Mercury's thermal emission that were obtained with the Berkeley-Maryland-Illinois Array (BIMA) millimeter interferometer at a wavelength of 0.3 cm and with the VLA (Very Large Array) at wavelengths from 1.3 to 20.5 cm. These images are analyzed with detailed thermophysical and radiative transfer models that are based in part on a lunar analogy. We constrain the thermophysical model with Mariner 10 infrared measurements of Mercury's night-side surface temperature and show that Mercury's regolith, like that of the Moon, consists of a thermally insulating surface layer, with a thickness of a few centimeters, atop a highly compacted region that extends to a depth of several meters. The radiative transfer model is constrained in part by linear polarization images that we obtained with the VLA at wavelengths from 2.0 to 20.5 cm. These images reveal wavelength-dependent scattering at the surface boundary and rms surface slopes that range from 15 deg at lambda 2.0 cm to 10 deg at lambda 6.2 cm. We develop a method for constraining the microwave opacity at each wavelength by modeling diurnal brightness variations over the resolved disk and find that Mercury's regolith is at least two to three times more transparent than the lunar maria and at least 40% more transparent than the lunar highlands. This difference is likely due to lower Fe and Ti abundances in Mercury's regolith, which is consistent with Mercury's high visual albedo and suggests that most of Mercury's surface is an extreme example of the lunar highlands.
A phenomenological magnetic field model for the earth's magnetosphere is constructed from a dipole field and a uniform field directed sunward in the northern hemisphere and antisunward in the southern hemisphere. The properties of this simple model are compared with those of several other quantitative models. The present model is found to be more suitable for calculations than some other simple models in cases where the distant (greater than 13 earth radii) magnetotail configuration is important. Moreover, this model is easily adaptable to changes in the field geometry and to the description of magnetotail asymmetries.
The global structure of collisionless bow shocks is investigated using a 2.5-dimensional electromagnetic hybrid code. This allows us to study the macrostructure of the shock while accounting for microphysical processes at the shock. The study entails the interaction of solar wind with magnetic dipoles of varying strength. For very weak dipoles the interaction does not lead to formation of a shock since the obstacle is not strong enough for the flow to become subsonic. For lager dipole strengths, a bow shock/wave is formed due to the presence of a plasma stagnation region in front of the dipole. It is found that the quasi-perpendicular part of this boundary corresponds to a true shock wave, whereas the quasi-parallel side consists of a magnetosonic wave followed by a rotational discontinuity. The backstreaming ions in the foreshock of this interaction lead to the generation of parallel propagating sinusoidal waves. These waves result in beam scattering, however, do not affect the solar wind. The formation of quasi-parallel shock is tightly connected to the generation of oblique compressional waves. These waves are generated by backstreaming ions having a beam-ring distribution function and are an inherent part of the shock dissipation processes. The results demonstrate that the two classes of 30 s ULF waves observed in the ion foreshock are unrelated. The results also demonstrate that at least in 2.5-dimensional, plasma scales determine the nature of the bow shock to a large extent although system size can influence both particle acceleration and evolution of ULF waves.
Mercury appears to have a tectonic framework and diastrophic history not
found on other terrestrial planets explored to date. On the part of the
planet viewed by Mariner 10, only two localized areas show evidence of
tensional stresses, both of which are apparently associated with the
Caloris basin. Lobate scarps occur in the remainder of the explored
region and appear to be primarily reverse or thrust faults which have
resulted from compressive stresses acting on a global scale. The period
of compression represented by these scarps occurred during the final
phase of heavy bombardment on Mercury and was probably caused by crustal
shortening due to a small decrease in the planet's radius.
Stratigraphic, volumetric, and albedo considerations together with
distribution indicate that the majority of smooth plains on Mercury were
produced by volcanism which occurred at the close of the period of late
heavy bombardment similar to that on the moon and Mars. Several
generations of plains are evident; the oldest may have resulted in part
from an early differentiation of the planet.
In addition to providing the first in situ evidence of a magnetosphere at Mercury, the first flyby by Mariner 10 inspired reports of Earth-like substorms. While the small scales at Mercury should make the substorm timescale there much shorter than it is at the Earth, these early interpretations may have too readily assumed that the substorm requirement of an energy storage and release phase occurs. Instead, its size should make Mercury's magnetosphere especially prone to disturbances that are purely ``driven'' by the changing external boundary conditions on the magnetosphere imposed by the solar wind. These result simply from the magnetosphere's relatively unhindered reconfiguration in response to the varying interplanetary parameters, including the IMF southward component. In this paper we demonstrate that the ``disturbed'' structure observed outbound from closest approach during the first Mariner 10 flyby can alternately be explained as a consequence of a typical period of rotating IMF. We use an appropriately modified IMF-dependent terrestrial magnetosphere model scaled for Mercury, together with an assumed, reasonable IMF time series, to reproduce the magnetic field signature during the disturbed outbound pass segment. This result suggests that rapid restructuring of the small magnetosphere in response to changing local interplanetary conditions may dominate the magnetospheric dynamics at Mercury. Future Mercury magnetosphere missions should be instrumented to distinguish between this driven magnetospheric dynamism and any internal Earth-like substorm processes. These results also remind us that driven reconfigurations must always be considered in studies of transients in the terrestrial magnetosphere.
Following the recent optical discovery of intense sodium D-line emission from Mercury, we explore the scenario of an extended exosphere of sodium and other metallic atoms. It is shown that the strong effect of solar radiation pressure acceleration would permit the escape of Na atoms from Mercury's surface even if they are ejected at velocity less than the surface escape velocity. Fast photoionization of the Na atoms is effective in limiting the tailward extension of the sodium exosphere, however. The subsequent loss of the photoions to the magnetosphere could be a significant source of the magnetospheric plasma. The recirculation of the magnetospheric charged particles to the planetary surface could also play an important role in maintaining an extended sodium exosphere as well as a magnetosphere of sputtered metallic ions.
The surface morphology and optical properties of Mercury resemble those of the moon in remarkable detail and record a very
similar sequence of events. Chemical and mineralogical similarity of the outer layers of Mercury and the moon is implied;
Mercury is probably a differentiated planet with a large iron-rich core. Differentiation is inferred to have occurred very
early. No evidence of atmospheric modification of landforms has been found. Large-scale scarps and ridges unlike lunar or
martian features may reflect a unique period of planetary compression near the end of heavy bombardment by small planetesimals.
Radio observations of Mercury made with the VLA; once in 1986, and on
two dates in February of 1988 are presented. These observations are the
first to spatially map both hot regions associated with the theoretical
hot poles. These 'hot poles' are separated by 180 deg and are a result
of the unusual diurnal heating from Mercury's 3/2 spin-orbit resonance
and eccentric orbit. The highest resolution data maps areas of the
planet as small as 330 km. Maps of total intensity, brightness
temperature, polarized intensity, fractional polarization,
depolarization, and spectral index are included. It is found that the
subsurface thermal emissions from Mercury are characteristic of
blackbody reradiation from the solar insolation over a diurnal cycle.
These observations to produce full-disk thermophysical models are used.
The one-dimensional, time-dependent heat-diffusion equation for all
observed disk elements at each epoch in order to constrain
thermophsyical parameters and properties of the subsurface material are
solved. Using typical lunar values for several of the parameters, it is
possible to reproduce the temperature morphology and most of the
observed temperature values. It is found that the best-fit models
require a substantial contribution of the heat transport in the
subsurface to be radiative in nature. The primary difficulty in the
models is in predicting the observed temperature differences as a
function of frequency.
A fully developed bow shock and magnetosheath were observed near Mercury, providing unambiguous evidence for a strong interaction between Mercury and the solar wind. Inside the sheath there is a distinct region analogous to the magnetosphere or magnetotail of Earth, populated by electrons with lower density and higher temperature than the electrons observed in the solar wind or magnetosheath. At the time of encounter, conditions were such that a perpendicular shock was observed on the inbound leg and a parallel shock was observed on the outbound leg of the trajectory, and energetic plasma electron events were detected upstream from the outbound shock crossing. The interaction is most likely not atmospheric, but the data clearly indicate that the obstacle to solar wind flow is magnetic, either intrinsic or induced. The particle fluxes and energy spectra showed large variations while the spacecraft was inside the magnetosphere, and these variations could be either spatial or temporal.
A simple model of the atomic sodium exosphere is used in conjunction
with a magnetospheric field model to study the ion trajectories of
Na+ ions in the magnetosphere of Mercury. It is found that
low-energy (<3 keV) ions usually encounter the planetary surface at
high latitude as a result of the finite size of the planet and the E
× B drift pattern of the ions. On the other hand, high-energy
(>10 keV) particles tend to hit the planetary surface on the
nightside hemisphere at low latitudes. Such precipitation patterns of
the exospheric ions might have important implication on the recent
observations of enhancement of sodium emissions at high latitude regions
of Mercury.
In 1974 a number of observations of the magnetic field at about 1 AU were already available and the concept of coronal magnetic field expansion in the heliosphere well established. On the other hand, only a few, sporadic, in situ field observations were available at heliocentric distances significantly inside the terrestrial orbit.
We report spectroscopic measurements of sodium in Mercury's atmosphere for the period July 1985-May 1988. Na abundance varies from <1 x 1010 atoms cm-2 in equatorial regions in March 1987 to as much as =~1.5 x 1012 atoms cm-2 in May 1988. All observations were made using a high-resolution echelle spectrograph on the 61-in. telescope on Mt. Bigelow, Arizona. Strong high-latitude enhancements are common, with the added characteristic that there is usually more at one hemisphere than the other. There is an obvious diurnal correlation, with morning abundances significantly higher than afternoon. We see notable Na emission enhancements when our spectrograph slit is placed over the radar-bright spots at 340? mercurian longitude and 55?N and 25?S latitude. We do not see any trend in column abundance with increasing radiation acceleration. We do not see a significant correlation of abundance with either solar F10.7 flux or sunspot number. Other larger variations occur and must be caused by other factors. Copyright 1997 Academic Press.