A 3D hybrid simulation study of the electromagnetic field distributions in the lunar wake

A three-dimensional hybrid code is used to study the electromagnetic disturbances in the solar wind that arise due to the absorption effect of the Moon. Due to the nearly insulating nature of the Moon, interplanetary magnetic fields (IMFs) can move through the interior without hindrance. However, the near-vacuum created in the wake region due to the lunar absorption effect will lead to enhancement of the strength of the magnetic field by a factor of about 1.4 in the middle of the lunar wake and lead to depletions at two sides. The situations arising from different orientations of the interplanetary magnetic fields relative to the radial direction are compared. Asymmetries of the inward diffusions both along and perpendicular to the field lines are also observed. The electric field formed from the plasma convection could reach a magnitude of 0.2–0.8 mV/m at the border of the wake. The role of the electric field on the inward accelerations is important to the geometry of the lunar wake.     

Topology of induced lunar magnetic fields

Using the asymmetric theory of lunar induction derived by Schubertet al. (1973a), we have obtained the total and induced magnetic field line structure within the Moon and the diamagnetic cavity. Total field distributions are shown for orientations of the oscillating interplanetary field parallel, perpendicular and at 45° to the cavity axis. Induced field lines are shown only for the orientations of the interplanetary field parallel and orthogonal to the cavity axis. When compared with the field lines derived using the long wavelength limit of spherically symmetric vacuum induction theory, the configurations obtained using the asymmetric theory exhibit significant distortion. For all orientations of the interplanetary field, the field lines are strongly compressed on the sunlit hemisphere because of the confining solar wind pressure at the lunar surface and the exclusion of the field by the lunar core. Field line compression is also observed in the antisolar region in agreement with the experimental observations of Schubertet al. (1973b). and Smithet al. (1973). For the parallel orientation of the interplanetary field, antisolar compression is caused by cavity confinement of the induced field. For the interplanetary field perpendicular to the cavity axis there is, in addition to compression by the cavity boundary, redistribution of field lines from the sunlit to the night side. In this case field lines entering the Moon just forward of the limb pass through the lunar crust on the night side and then exit forward of the limb. This phenomenon manifests itself as a displacement of the null in the induced magnetic field at the surface sunward of the limb, in striking similarity to the magnetospheric field lines of the Earth.Paper dedicated to Professor Harold C. Urey on the occasion of his 80th birthday on 29 April, 1973.     

Anomalous deformation of the Earth's bow shock in the lunar wake: Joint measurement by Chang’E-1 and SELENE

Because the solar wind (SW) flow is usually super-sonic, a fast-mode bow shock (BS) is formed in front of the Earth's magnetosphere, and the Moon crosses the BS at both dusk and dawn flanks. On the other hand, behind of the Moon along the SW flow forms a tenuous region called lunar wake, where the flow can be sub-Alfvénic (and thus sub-sonic) because of its low-density status. Here we report, with joint measurement by Chang’E-1 and SELENE, that the Earth's BS surface is drastically deformed in the lunar wake. Despite the quasi-perpendicular shock configuration encountered at dusk flank under the Parker-spiral magnetic field, no clear shock surface can be found in the lunar wake, while instead gradual transition of the magnetic field from the upstream to downstream value was observed for a several-minute interval. This finding suggests that the ‘magnetic ramp’ is highly broadened in the wake where a fast-mode shock is no longer maintained due to the highly reduced density. On the other hand, observations at the 100 km altitude on the dayside show that the fast-mode shock is maintained even when the width of the downstream region is smaller than a typical scale length of a perpendicular shock. Our results suggest that the Moon is not so large to eliminate the BS at 100 km altitude on the dayside, while the magnetic field associated with the shock structure is drastically affected in the lunar wake.     

Low energy neutral atoms imaging of the Moon

Imaging of low-energy neutral atoms (LENAs) in the vicinity of the Moon can provide wide knowledge of the Moon from the viewpoint of plasma physics and planetary physics. At the surface of the Moon, neutral atoms are mainly generated by photon-stimulated desorption, micrometeorite vaporization and sputtering by solar wind protons. LENAs, the energetic neutral atoms with energy range of 10-500 eV, are mainly created by sputtering of solar wind particles. We have made quantitative estimates of sputtered LENAs from the Moon surface. The results indicate that LENAs can be detected by a realistic instrument and that the measurement will provide the global element maps of sputtered particles, which substantially reflect the surface composition, and the magnetic anomalies. We have also found that LENAs around dark regions, such as the permanent shadow inside craters in the pole region, can be imaged. This is because the solar wind ions can penetrate shaded regions due to their finite gyro-radius and the pressure gradient between the solar wind and the wake region. LENAs also extend our knowledge about the magnetic anomalies and associated mini-magnetosphere systems, which are the smallest magnetospheres as far as one knows. It is thought that no LENAs are generated from mini-magnetosphere regions because no solar wind may penetrate inside them. Imaging such void areas of LENAs will provide another map of lunar magnetic anomalies.     

Satellite measurements of the moon's magnetic field: A preliminary report

A preliminary analysis of the data from the UCLA magnetometer on board the Apollo 15 subsatellite indicates that remnant magnetization is a characteristic property of the Moon, that its distribution is such as to produce a rather complex pattern or fine structure, and that a detailed mapping of its distribution is feasible with the present experiment. The analysis also shows that lunar induction fields produced by transients in the interplanetary magnetic field are detectable at the satellite orbit so that in principle the magnetometer data can be used to determine the latitudinal and longitudinal as well as radial dependences of the distribution of electrical conductivity within the Moon. Finally, the analysis indicates that the plasma void or diamagnetic cavity which forms behind the Moon when the Moon is in the solar wind, is detectable at the satellite's orbit and that the flow of the solar wind near the limbs is usually rather strongly disturbed.Publication No. 981. Institute of Geophysics and Planetary Physics.     

First lunar wake passage of ARTEMIS: Discrimination of wake effects and solar wind fluctuations by 3D hybrid simulations

The spacecraft P1 of the new ARTEMIS (Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun) mission passed the lunar wake for the first time on February 13, 2010. We present magnetic field and plasma data of this event and results of 3D hybrid simulations. As the solar wind magnetic field was highly dynamic during the passage, a simulation with stationary solar wind input cannot distinguish whether distortions were caused by these solar wind variations or by the lunar wake; therefore, a dynamic real-time simulation of the flyby has been performed. The input values of this simulation are taken from NASA OMNI data and adapted to the P1 data, resulting in a good agreement between simulation and measurements. Combined with the stationary simulation showing non-transient lunar wake structures, a separation of solar wind and wake effects is achieved. An anisotropy in the magnitude of the plasma bulk flow velocity caused by a non-vanishing magnetic field component parallel to the solar wind flow and perturbations created by counterstreaming ions in the lunar wake are observed in data and simulations. The simulations help to interpret the data granting us the opportunity to examine the entire lunar plasma environment and, thus, extending the possibilities of measurements alone: A comparison of a simulation cross section to theoretical predictions of MHD wave propagation shows that all three basic MHD modes are present in the lunar wake and that their expansion governs the lunar wake refilling process.     

Electromagnetic induction in the moon

Electromagnetic induction in a stratified Moon with a trailing cavity is discussed. The influence of the Moon wake is studied by using a two-layer lunar model with a perfectly conductive core. The magnetic field is shown to be independent of the wake length when that quantity is greater than 3 lunar radii. Regions on the sunlit and dark sides where the magnetic field may be described in terms of its first spatial harmonic have been distinguished, together with the corresponding errors admitted. It is in these regions that the electrical conductivity of the Moon can be found with very high accuracy, by simultaneous observations on the lunar surface and in the undisturbed solar wind. Results of these observations can be conveniently related to values of the apparent resistivity. Translated by Miss Eva Vokálová of the Astronomical Institute, Charles University, Prague, Czechoslovakia.     

Orbital mapping of the lunar magnetic field

Magnetometer data obtained during the first four lunations after the deployment of the Apollo 15 subsatellite have been used to construct contour maps of the lunar magnetic field referred to 100 km altitude. These contour maps cover a relatively small band on the lunar surface. Within the region covered there is a marked near side-far side asymmetry. The near-side field is generally weaker and less structured than the far-side field. The strongest intrinsic lunar magnetic field detected is between the craters Van de Graaff and Aitken, centered at 20°S and 172°E. The variation in field strength with altitude for this feature suggests that its scale size is on the order of 80 km. A magnetization contrast between this region and its surroundings of the order of 6 × 10^–5 emu-cm^–3 is obtained assuming a 10-km thick slab. Preliminary Apollo 16 magnetometer data at extremely low altitude (0 to 10 km) show a very structured magnetic field with field strengths up to 56. Large compressions in the magnetic field magnitude, just above the lunar limb regions, are occasionally detected when the Moon is in the solar wind. The occurrence of limb compressions is strongly dependent on the selenographic coordinates of the lunar region on the solar wind terminator beneath the orbit of the sub-satellite. The discovery of remanent magnetization of varying strength over much of the lunar surface and its correlation with limb compression source regions supports the hypothesis that limb compressions are due to the deflection of the solar wind by regions of strong magnetization at the lunar limbs. If this hypothesis is correct, then the map of lunar regions associated with compressions indicates that the northerly equatorial region on the far side is less strongly magnetized than the southerly equatorial region on the far side.Paper dedicated to Professor Harold C. Urey on the occasion of his 80th birthday on 29 April, 1973.     

Photoelectrons and solar wind/lunar limb interaction

It is suggested that boundary conditions for solar wind/lunar limb interactions are active. The whole-Moon limb does not evoke a shock cone because warm (13 eV/electron) solar wind electrons are replaced by cool (2 eV/electron) photoelectrons that are ejected from the generally smooth areas of the lunar terminator illuminated at glazing angles by the Sun. A localized volume of low thermal pressure is created in the solar wind by these cool photoelectrons. The solar wind expands into this turbulence-suppressive volume without shock production. Conversely, directly illuminated highland areas exchange hot photoelectrons (> 20 eV/electron) for warm solar wind electrons. The hot electrons generate a localized pressure increase (p) in the adjacent solar wind flow which evokes a shock streamer in the solar wind. Shock streamers are identifiable by a coincident increase in the magnitude (B p) of the solar wind magnetic field immediately external to the lunar wake. Shock occurrence is controlled by lunar topography, solar activity in the hard ultraviolet (> 20 eV), solar wind electron density and thermal velocity, and the intensity of the solar wind magnetic field.Paper dedicated to Professor Harold C. Urey on the occasion of his 80th birthday on 29 April 1973.The Lunar Science Institute is operated by the Universities Space Research Association under Contract No. NSR 09-051-001 with the National Aeronautics and Space Administration.     

The effects of boundary condition asymmetries on the interplanetary magnetic field-moon interaction

Boundary condition asymmetries inherent in the solar wind flow past the Moon are included in a cylindrical model of the interplanetary magnetic field - Moon interaction. Numerical examinations of the sunward side response of this model are compared in the frequency domain with those of symmetrically excited spherical and cylindrical models and two characteristic differences are observed: the response of the asymmetric model is depressed at low frequencies due to magnetic diffusion around a conducting core, and is flattened at high frequencies because of the finite application time of the incident interplanetary magnetic field. The diffusion of field lines around the core is also evident in the time response of the model in the anti-solar cavity. The above features of the lunar response resulting from boundary condition asymmetries are shown to be evident in observational measurements.     

A theory for the interpretation of lunar surface magnetometer data

The solution to the problem of the motion of the Moon relative to spatial irregularities in the interplanetary magnetic field is found. The lunar electrical conductivity is modeled by a two-layer conductivity profile. For the interaction of the Moon with the corotating sector structure of the interplanetary magnetic field it is found that the magnetic field in the lunar shell is the superposition of an oscillatory uniform field, an oscillatory dipole field and anoscillatory field that is toroidal about the axis of the motional electric field. With various lunar conductivity models and the theory of this paper, lunar surface magnetometer data can be quantitatively interpreted to yield information on the conductivity and consequently the temperature of the lunar core.Presently visiting the Max-Planck-Institut für Physik und Astrophysik, München, Germany.     

Analysis of the complex solar particle event on April 29–30, 1973

Based on the data of the high-apogee satellite Prognoz-3, the April 29–30, 1973 solar particle event is analysed. The event's complex energetic particle, interplanetary magnetic field and solar wind plasma properties are discussed. The unusual behaviour of solar particles up to energies 100 MeV can well be explained in terms of the interaction with an interplanetary shock wave system passing the Earth. Assuming that the structure of the interplanetary shock wave system is similar to that considered first by Parker (1961) and Gold (1959) and reviewed later by Hundhausen (1972) and Dryer (1974, 1975), the main characteristics of the energetic particle fluxes, solar wind and interplanetary magnetic field can be understood.     

A global model of the internal magnetic field of the Moon based on Lunar Prospector magnetometer observations

A preliminary model of the internal magnetic field of the Moon is developed using a novel, correlative technique on the low-altitude Lunar Prospector magnetic field observations. Subsequent to the removal of a simple model of the external field, an internal dipole model is developed for each pole-to-pole half-orbit. This internal dipole model exploits Lunar Prospector's orbit geometry and incorporates radial and theta vector component data from immediately adjacent passes into the model. These adjacent passes are closely separated in space and time and are thus characteristic of a particular lunar regime (wake, solar wind, magnetotail, magnetosheath) or regimes. Each dipole model thus represents the correlative parts of three adjacent passes, and provides an analytic means of continuing the data to a constant surface of 30 km above the mean lunar radius. The altitude-normalized radial field from the wake and tail regimes is used to build a model in which 99.2% of the 360 by 360 bins covering the lunar surface are filled. This global model of the radial magnetic field is used to construct a degree 178 spherical harmonic model of the field via the Driscoll and Healy sampling theorem. Terms below about degree 150 are robust, and polar regions are considered to be the least reliable. The model resolves additional detail in the low magnetic field regions of the Imbrium and Orientale basins, and also in the four anomaly clusters antipodal to the large lunar basins. The model will be of use in understanding the sources of the internal field, and as a first step in modeling the interaction of the internal field with the solar wind.     

Lunar surface potential and electric field

The Moon has no significant atmosphere, thus its surface is exposed to solar ultraviolet radiation and the solar wind. Photoemission and collection of the solar wind electrons and ions may result in lunar surface charging. On the dayside, the surface potential is mainly determined by photoelectrons, modulated by the solar wind;while the nightside surface potential is a function of the plasma distribution in the lunar wake. Taking the plasma observations in the lunar environment as inputs, the global potential distribution is calculated according to the plasma sheath theory, assuming Maxwellian distributions for the surface emitted photoelectrons and the solar wind electrons. Results show that the lunar surface potential and sheath scale length change versus the solar zenith angle, which implies that the electric field has a horizontal component in addition to the vertical one. By differentiating the potential vertically and horizontally, we obtain the global electric field. It is found that the vertical electric field component is strongest at the subsolar point,which has a magnitude of 1 V m-1. The horizontal component is much weaker, and mainly appears near the terminator and on the nightside, with a magnitude of several mV m-1. The horizontal electric field component on the nightside is rotationally symmetric around the wake axis and is strongly determined by the plasma parameters in the lunar wake.     

Lunar Dust: Properties and Investigation Techniques

Physical conditions in the near-surface layer of the Moon are overviewed. This medium is formed in the course of the permanent micrometeoroid bombardment of the lunar regolith and due to the exposure of the regolith to solar radiation and high-energy charged particles of solar and galactic origin. During a considerable part of a lunar day (more than 20%), the Moon is passing through the Earth’s magnetosphere, where the conditions strongly differ from those in the interplanetary space. The external effects on the lunar regolith form the plasma-dusty medium above the lunar surface, the so-called lunar exosphere, whose characteristic altitude may reach several tens of kilometers. Observations of the near-surface dusty exosphere were carried out with the TV cameras onboard the landers Surveyor 5, 6, and 7 (1967–1968) and with the astrophotometer of Lunokhod-2 (1973). Their results showed that the near-surface layer glows above the sunlit surface of the Moon. This was interpreted as the scattering of solar light by dust particles. Direct detection of particles on the lunar surface was made by the Lunar Ejects and Meteorite (LEAM) instrument deployed by the Apollo 17 astronauts. Recently, the investigations of dust particles were performed by the Lunar Atmosphere and Dust Environment Explorer (LADEE) instrument at an altitude of several tens of kilometers. These observations urged forward the development of theoretical models for the lunar exosphere formation, and these models are being continuously improved. However, to date, many issues related to the dynamics of dust and the near-surface electric fields remain unresolved. Further investigations of the lunar exosphere are planned to be performed onboard the Russian landers Luna-Glob and Luna-Resurs.     

The interaction of the solar wind with lunar magnetic anomalies

A simplified model for the interaction of the cold solar wind with lunar magnetic anomalies is considered. Since on the illuminated side of the Moon the dynamic pressure of the solar wind significantly exceeds the magnetic pressure of the anomalies, upward propagation of the lunar field is possible only by means of diffusion. This process does not depend on the velocity but only on the concentration of the solar wind and the characteristic size of anomalies. Theoretical calculations are compared with the data of Apollo 12 and Explorer 35.     

Density cavity observed over a strong lunar crustal magnetic anomaly in the solar wind: A mini-magnetosphere?

We present observations of what may be the inner region of a lunar mini-magnetosphere. If so, these likely represent the first such observations. Previous studies of solar wind interaction with lunar crustal magnetic fields found increased particle fluxes associated with magnetic amplifications, suggesting a shock/sheath region. The central density cavity expected in the inner mini-magnetosphere (if analogous to other planetary magnetospheres) has proven elusive. We now present Lunar Prospector fly-throughs of a density cavity near a strong crustal magnetic source in the solar wind, and compare these unique observations with typical orbits in the solar wind and wake. We observed the density cavity on two consecutive orbits on July 14, 1999 with optimal viewing geometry, downstream from one of the strongest lunar crustal sources (an anomaly centered at 235E, 20S), during very unusual solar wind conditions. We found no other similar features in the solar wind in 7 months of low-altitude orbits, suggesting that fully formed lunar mini-magnetospheres are rare and/or difficult to observe from orbit.     

Cross-correlation of solar wind parameters with sunspots (‘Long-term variations’) at 1 AU during cycles 21 and 22

Long-term variations of solar wind parameters at 1 AU are correlated with sunspots for the time interval 1973 to 1993 (solar cycles 21, 22). Using theNear-Earth Heliosphere Data OMNI the plasma density, the magnitude of the interplanetary magnetic field, the solar wind velocity and the solar wind temperature show consistent long-term variations in each cycle (21 and 22) — pointing to specifictime-lags in the coupling between sunspots (and the underlying convection zone), the solar corona and the solar wind parameters at 1 AU (ecliptic).     

Frozen fields

Magnetic fields due to permanent magnetization of planetary crusts and interiors have been clearly detected only for the Earth and Moon. However, they are likely to be a ubiquitous property of silicate and partially silicate objects in the solar system. An indication that this is true is the recent indirect evidence from the Galileo flybys that the asteroids Gaspra and Ida have intrinsic magnetic fields. Lunar paleomagnetism differs substantially from terrestrial paleomagnetism in part because the dominant ferromagnetic carriers are metallic Fe-Ni grains rather than iron oxides such as magnetite. The distribution of metallic iron remanence carriers on the Moon is influenced strongly by impact processes. In addition, large-scale lunar impacts may have produced transient magnetic fields capable of imparting magnetization with or without a former core dynamo. An unresolved issue of lunar paleomagnetism is the origin of swirl-like albedo markings associated with the strongest magnetic anomalies detected from orbit. The interpretation of solar wind magnetic field perturbations during the Gaspra and Ida flybys as due to intrinsic asteroidal magnetic fields has been supported by detailed magnetohydrodynamic simulations. The inferred magnetization limits for Gaspra are consistent with a wide variety of meteorite types and do not allow firm constraints to be imposed on Gaspra's bulk composition.     

Spatial structure of transverse oscillations in the interplanetary magnetic field

The spatial structure of the transverse oscillations in the interplanetary magnetic field at 1 AU is studied by comparing the simultaneous observations by Explorer 33 and 35 satellites at the maximum separation of about 200R _E. The anisotropy characteristics of these oscillations suggest that the oscillations sampled are Alfvén waves. It is found that the size of the region of the wave coherence is related to the solar wind velocity; the size is 80R _E when the wind velocity is lower than 500 km s^–1 but becomes less than this when the wind velocity is higher. An inference is made that the solar atmospheric turbulence contributing to the faster solar wind is finer in scale than that associated with the slower wind.A postgraduate student at the Tokai University