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 This finding suggests that the ‘magnetic ramp’ is highly broadened in the wake where a fast-mode shock is no longer maintained due to the highly reduced density.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.     

On the origin of energetic particles in the foreshock region of the Earth’s bow shock

We consider the processes related to the formation of the so-called foreshock region upstream of the Earth’s bow shock. We suggest a model based on the surfing of pick-up ions in the bow shock front in terms of which the ion acceleration mechanism in the front can be explained. We ascertain the physical conditions under which the accelerated ions lie upstream of the shock front and determine the direction of motion of the energetic ions. We conclude that it is this population of energetic ions (longitudinal beams) that plays a major role in forming the ion foreshock boundary.     

Hypervelocity Impact Experiments in the Laboratory Relating to Lunar Astrobiology

The results of a set of laboratory impact experiments (speeds in the range 1–5 km s⁻¹) are reviewed. They are discussed in the context of terrestrial impact ejecta impacting the Moon and hence lunar astrobiology through using the Moon to learn about the history of life on Earth. A review of recent results indicates that survival of quite complex organic molecules can be expected in terrestrial meteorites impacting the lunar surface, but they may have undergone selective thermal processing both during ejection from the Earth and during lunar impact. Depending on the conditions of the lunar impact (speed, angle of impact etc.) the shock pressures generated can cause significant but not complete sterilisation of any microbial load on a meteorite (e.g. at a few GPa 1–0.1% of the microbial load can survive, but at 20 GPa this falls to typically 0.01–0.001%). For more sophisticated biological products such as seeds (trapped in rocks) the lunar impact speeds generate shock pressures that disrupt the seeds (experiments show this occurs at approximately 1 GPa or semi-equivalently 1 km s⁻¹). Overall, the delivery of terrestrial material of astrobiological interest to the Moon is supported by these experiments, although its long term survival on the Moon is a separate issue not discussed here.     

Bow shock protons in the lunar environment

Protons from the Earth's bow shock are observed by the Suprathermal Ion detector Experiment (SIDE) in two regions of the lunar orbit. The dawn region begins at the dawn side bow shock crossing and ends ç 5 days later and the dusk region begins at ç 2 days prior to entering the dusk side magnetosheath and ends at the inbound bow shock crossing. Dusk and dawn refer to a terrestrial coordinate system. The dominant contribution to the ion spectra observed by the SIDE in these regions is from particles with energies between ç 750 eV q^–1 and 3500 eV q^–1. 3500 eVq^–1 is the upper limit of the energy range of the detector. Analysis of simultaneous data from the Explorer 35 magnetometer and the SIDE indicates that the observability of bow shock protons at the lunar distance is dependent on the configuration of the interplanetary magnetic field.Paper presented at the Conference on Interactions of the Interplanetary Plasma with the Modern and Ancient Moon, sponsored by the Lunar Science Institute, Houston, Texas and held at the Lake Geneva Campus of George Williams College, Wisconsin, between September 30 and October 4, 1974.     

Excitation of alfven turbulence in the solar wind ahead of the Earth bow shock by beams of high-velocity protons

One of the mechanisms of Alfven turbulence generation in the foreshock region is investigated by the example of the Earth bow shock. The effect of the temperature of high-velocity beams on characteristics of generated disturbances is examined. It is shown that the beam temperature has a significant impact on transverse scales of disturbances. The higher the temperature, the greater the limitations on transverse wavelengths. The development of instability in the propagation of reflected, intermediate, and diffusion proton beams in the foreshock region of the Earth bow shock is considered. Perturbation motion dynamics in foreshock region is analyzed.     

Foreshock cavities and internal foreshock boundaries

We present two case studies of cluster encounters with foreshock cavities. For one event, we are able, for the first time, to accurately relate the observation of a foreshock cavity to the measured position of the bow shock. This allows us to compute the shock angle, a vital parameter in models of foreshock cavity formation, with greater confidence than any previous study. This cavity appears to be elongated along the magnetic field and we use the multispacecraft nature of the Cluster mission to constrain its field-parallel and -perpendicular extent. We show that this event is embedded within a region of field-aligned ion beams. This is the first time a foreshock cavity has been shown to be surrounded by foreshock ion beams. A second foreshock cavity is associated with a small rotation in the interplanetary magnetic field (IMF). We show that this event appears on the boundary between an interval when the spacecraft were inside the ion foreshock, and an excursion upstream. This is the first report of a foreshock cavity observed during the traversal of the global foreshock. This second event has some features expected from the new Sibeck et al. (2008) model of cavities as brief encounters with a spatial boundary in the global foreshock.     

Observations of electron plasma waves upstream of the Jovian bow shock

The Ulysses spacecraft encountered the planet Jupiter in February 1992, on its journey towards high heliospheric latitude. During the approach to the planet, as well as on the outbound pass, while receding from the Jovian bow shock, the Plasma Frequency Receiver that is part of the Unified Radio and Plasma Wave experiment (URAP) recorded bursts of plasma waves in the frequency range of a few kHz. These emissions, first observed by the PWS experiment onboard the Voyager spacecraft, have been identified as upstream electron plasma waves. In this paper, we present the first analysis of the characteristics of these emissions, which are very similar to those found in the Earth's electron foreshock, upstream of the Earth's bow shock. These bursty emissions, with a peak frequency very close to the local electron plasma frequency F_pe, have a typical electric field amplitude in the range 0.01–0.1 mV m⁻¹, with some bursts above 1 mV m⁻¹. The frequency bandwidth over which significant power can be found above the instrument background noise ranges from below 0.2 F_pc to about 2 F_pc. On the basis of our present knowledge of similar emissions observed at Earth, we suggest that the broadband emissions are triggered by suprathermal (a few tens of eV) electrons, streaming back from Jupiter's bow shock.     

Energetic lunar nighttime ion events

The Rice University Suprathermal Ion Detector Experiment regularly observes ion events normally ranging from 250 eV q^?1 to 1000 eV q^?1 all through the lunar night. These ion events occur most often 2 to 3 days prior to the sunrise terminator. There is also a secondary activity peak 3 to 4 days after local sunset on the Moon. The events are normally of 4 hr or less in duration and the integral flux is 10⁶ ions cm^?2 s^?1 ster^?1. This article discusses the character of these events and presents the preliminary findings of a detailed study begun on this subject.     

Primary and secondary magnetizations in lunar rocks - Implications for the ancient magnetic field of the Moon

The nature of the ancient magnetic field of the Moon, in which lunar rocks acquired their remanent magnetism, has emerged as an important potential source of evidence, if somewhat controversial, for a lunar core which at a period in the Moon's history was the source of the magnetic field. Many of the lunar rocks possess a stable, primary remanence (NRM) with characteristics consistent with and indicative of thermo-remanent magnetization, acquired when the rocks cooled in an ambient magnetic field. Also present are secondary components of magnetization, one type of which appears to have been acquired between collection on the Moon and reception in the laboratory and others which were apparently acquired on the Moon.An important question to be answered is whether meteorite impacts play any part in lunar magnetism, either in modifying pre-existing magnetizations or by imparting a shock remanent magnetism (SRM) in a transient magnetic field associated with the impact. With current knowledge, SRM, in either a global lunar magnetic field of a transient field, and TRM cannot be distinguished, and in the paper the secondary magnetization characteristic of lunar rocks are examined to investigate whether their nature favours the presence of a permanent lunar magnetic field or whether they are consistent with an origin as a transient field-generated SRM.Besides terrestrial processes of secondary magnetization, such as viscous, chemical and partial thermoremanent magnetization, possible processes peculiar to the Moon are discussed and their likely importance assessed in relation to lunar sample history. The nature of the secondary magnetizations appear to be best explained on the assumption that they are due to one or more of the processes that require an ambient lunar field, namely viscous, partial thermoremanent and shock magnetization. When associated with other types of evidence obtained from lunar magnetism studies, investigations of lunar sample remanent magnetism now favours the existence of an ancient lunar magnetic field.     

Shock metamorphic history of >4 Ga Apollo 14 and 15 zircons

During impact events, zircons develop a wide range of shock metamorphic features that depend on the pressure and temperature conditions experienced by the zircon. These conditions vary with original distance from impact center and whether the zircon grains are incorporated into ejecta or remain within the target crust. We have employed the range of shock metamorphic features preserved in >4 Ga lunar zircons separated from Apollo 14 and 15 breccias and soils in order to gain insights into the impact shock histories of these areas of the Moon. We report microstructural characteristics of 31 zircons analyzed using electron beam methods including electron backscatter pattern (EBSP) and diffraction (EBSD). The major results of this survey are as follows. (1) The abundance of curviplanar features hosting secondary impact melt inclusions suggests that most of the zircons have experienced shock pressures between 3 and 20 GPa; (2) the scarcity of recrystallization or decomposition textures and the absence of the high‐pressure polymorph, reidite, suggests that few grains have been shocked to over 40 GPa or heated above 1000 °C in ejecta settings; (3) one grain exhibits narrow, arc‐shaped bands of twinned zircon, which map out as spherical shells, and represent a novel shock microstructure. Overall, most of the Apollo 14 and 15 zircons exhibit shock features similar to those of terrestrial zircon grains originating from continental crust below large (~200 km) impact craters (e.g., Vredefort impact basin), suggesting derivation from central uplifts or uplifted rims of large basins or craters on the Moon and not high‐temperature and ‐pressure ejecta deposits.     

Evidence for accelerated electrons upstream from Jupiter's bow shock: Ulysses results

Several transient increases of electrons with energies in the range 40–100 keV have been detected upstream and immediately downstream from the Jovian bow shock (and only in these regions), by instruments on the Ulysses spacecraft during February 1992. The energy spectra of these electrons differ markedly from the energy spectrum of the trapped magnetospheric electrons measured by the same instrument. Two populations of the upstream electrons were identified. Type I electrons appear at times when the direction of the interplanetary magnetic field at the spacecraft could have been tangent to the Jovian bow shock surface thus paralleling, for the first time at another planetary bow shock, the rather well understood situation at Earth's bow shock. Type II electrons have the same energy spectrum as Type I electrons, but are not so clearly associated with the tangent field-line condition. They occur at high southerly latitudes only while the Type I electrons are detected both on the inbound and outbound passages. Type II electrons have never been reported at the Earth's bow shock or any other planetary bow shock. Under the assumption that the field line that goes through Ulysses connects to the bow shock in a straight line, two possible explanations for the Type II electrons may be: (1) very large distortions of the bow shock surface, perhaps caused by deformations of the magnetopause, may permit the tangent condition; and (2) upstream electrons are preferentially, but not necessarily, accelerated when the IMF is tangent to the bow-shock surface.     

Upstream proton cyclotron waves at Venus

Data from the magnetometer MAG aboard the Venus Express S/C are investigated for the occurrence of cyclotron wave phenomena upstream of the Venus bow shock. For an unmagnetized planet such as Venus and Mars the neutral exosphere extends into the on-flowing solar wind and pick-up processes can play an important role in the removal of particles from the atmosphere. At Mars upstream proton cyclotron waves were observed but at Venus they were not yet detected. From the MAG data of the first 4 months in orbit we report the occurrence of proton cyclotron waves well upstream from the planet, both outside and inside of the planetary foreshock region; pick-up protons generate specific cyclotron waves already far from the bow shock. This provides direct evidence that the solar wind is removing hydrogen from the Venus exosphere. Determining the role the solar wind plays in the escape of particles from the total planetary atmosphere is an important step towards understanding the evolution of the environmental conditions on Venus. The continual observations of the Venus Express mission will allow mapping the volume of escape more accurately, and determine better the present rate of hydrogen loss.     

Laboratory studies on seismic and electrical properties of the moon

Laboratory measurements of seismic wave velocities and electrical properties of Apollo lunar samples and similar material of terrestrial origin are discussed in this paper. Measurements of the electrical properties show that in the frequency range above a few hundred Hz the outer region of the Moon may be considered as a low loss dielectric. This observation supports a longstanding speculation that dry, powdered rocks in which the dielectric loss tangent is frequency-independent over a wide range of frequency are present in the uppermost lunar surface layers. The surface layers of the Moon are likely to have an extremely low electrical conductivity. Thus future electromagnetic probing of the Moon to a few hundred kilometer depth is possible in the few kHz frequency range. Based on ultrasonic experiments with pressure as a variable, we next present the elastic constants and equations of state of lunar materials and characteristic dispersion of seismic wave velocities of the Moon. We find thatP andS wave velocities increase sharply within the first 30 km depth and then level off gradually. Combining this observation with lunar seismic and geophone data, we believe that the first 30 km of the Moon may be interpreted as a scattering region. If H₂O exists on the Moon, H₂O may occur at some shallow depth beneath the outermost surface layer in solid ice interlocking cracks and pores and mineral grains. The rocks in this permafrost state have relatively low seismic velocity and highQ. If permafrost does exist, we would expect a wide range of electrical conductivity and dielectric constant. Future electromagnetic probing of the Moon should yield very usefull information on the physical state of the lunar interior; when this electrical information is combined with the seismic information, we should learn much more about the internal constitution and the state of the Moon than is known today.     

X-ray fluorescence observations of the moon by SMART-1/D-CIXS and the first detection of Ti Kα from the lunar surface

The demonstration of a compact imaging X-ray spectrometer (D-CIXS), which flew on ESA's SMART-1 mission to the Moon (Racca et al., 2001; Foing et al., 2006), was designed to test innovative new technologies for orbital X-ray fluorescence spectroscopy. D-CIXS conducted observations of the lunar surface from January 2005 until SMART-1 impacted the Moon in September 2006. Here, we present scientific observations made during two solar flare events and show the first detection of Titanium Kα from the lunar surface. We discuss the geological implications of these results. We also discuss how experience from D-CIXS has aided the design of a similar instrument (Chandrayaan-1 X-ray Spectrometer (C1XS)) that was launched on the 22nd October 2008 on India's Chandrayaan-1 mission to the Moon.     

Observations of water vapor ions at the lunar surface

The Apollo 14 Suprathermal Ion Detector Experiment observed a series of bursts of 48.6 eV water vapor ions at the lunar surface during a 14-h period on March 7, 1971. The maximum flux observed was 10⁸ ions cm^–2 s^–1 sr^–1. These ions were also observed at Apollo 12, 183 km to the west. Evaluation of specific artificial sources including the Apollo missions and the Russian Lunokhod leads to the conclusion that the water vapor did not come from a man-made source. Natural sources exogenous to the Moon such as comets and the solar wind are also found to be inadequate to explain the observed fluxes. Consequently, these water vapor ions appear to be of lunar origin.Paper dedicated to Prof. Harold C. Urey on the occasion of his 80th birthday on 29 April 1973.     

Magnetospheric protons and electrons encountered by the moon in the plasma sheet

During its passage through the geomagnetic tail, the Moon encounters the plasma sheet. Properties of plasma sheet electrons and protons, first detected at lunar distances by Explorer 35, are described. The electrons have a rapidly fluctuating non-Maxwellian energy distribution with a mean energy of several hundred electron volts and density ç 0.2 cm^?3. The protons, of energy ç 1 keV, were usually detected above the instrument background when flowing towards the Earth at ç 200 km s^?1. Implications for migration of grains on the lunar surface are also pointed out and it is suggested that strong terrestrial polar winds in the early history of the Earth-Moon system may have caused some erosion of the Earth-facing side of the Moon, and that gravitational shielding of interplanetary rock flux by the Earth may also be an explanation of the relative smoothness of the front side.     

Electron plasma oscillations associated with type III radio emissions and solar electrons

An extensive study of the IMP-6 and IMP-8 plasma and radio wave data has been performed to try to find electron plasma oscillations associated with type III radio noise bursts and low-energy solar electrons. This study shows that electron plasma oscillations are seldom observed in association with solar electron events and type III radio bursts at 1.0 AU. In nearly four years of observations only one event was found in which electron plasma oscillations are clearly associated with solar electrons. For this event the plasma oscillations appeared coincident with the development of a secondary maximum in the electron velocity distribution functions due to solar electrons streaming outwards from the Sun. Numerous cases were found in which no electron plasma oscillations with field strengths greater than 1 μV m^?1 could be detected even though electrons from the solar flare were clearly detected at the spacecraft. For the one case in which electron plasma oscillations are definitely produced by the electrons ejected by the solar flare the electric field strength is relatively small, only about 100 μV m^?1. This field strength is about a factor of ten smaller than the amplitude of electron plasma oscillations generated by electrons streaming into the solar wind from the bow shock. Electromagnetic radiation, believed to be similar to the type III radio emission, is also observed coming from the region of the more intense electron plasma oscillations upstream of the bow shock. Quantitative calculations of the rate of conversion of the plasma oscillation energy to electromagnetic radiation are presented for plasma oscillations excited by both solar electrons and electrons from the bow shock. These calculations show that neither the type III radio emissions nor the radiation from upstream of the bow shock can be adequately explained by a current theory for the coupling of electron plasma oscillations to electromagnetic radiation. Possible ways of resolving these difficulties are discussed.     

The sodium tail of the Moon

During the few days centered about new Moon, the lunar surface is optically hidden from Earth-based observers. However, the Moon still offers an observable: an extended sodium tail. The lunar sodium tail is the escaping “hot” component of a coma-like exosphere of sodium generated by photon-stimulated desorption, solar wind sputtering and meteoroid impact. Neutral sodium atoms escaping lunar gravity experience solar radiation pressure that drives them into the anti-solar direction forming a comet-like tail. During new Moon time, the geometry of the Sun, Moon and Earth is such that the anti-sunward sodium flux is perturbed by the terrestrial gravitational field resulting in its focusing into a dense core that extends beyond the Earth. An all-sky camera situated at the El Leoncito Observatory (CASLEO) in Argentina has been successfully imaging this tail through a sodium filter at each lunation since April 2006. This paper reports on the results of the brightness of the lunar sodium tail spanning 31 lunations between April 2006 and September 2008. Brightness variability trends are compared with both sporadic and shower meteor activity, solar wind proton energy flux and solar near ultra violet (NUV) patterns for possible correlations. Results suggest minimal variability in the brightness of the observed lunar sodium tail, generally uncorrelated with any single source, yet consistent with a multi-year period of minimal solar activity and non-intense meteoric fluxes.     

The relationship of electron plasma oscillations to type III radio emissions and low-energy solar electrons

An extensive study of the IMP-6 and IMP-8 plasma and radio wave data has been performed to try to find electron plasma oscillations associated with type III radio noise bursts and low energy solar electrons. This study shows that electron plasma oscillations are seldom observed in association with solar electron events and type III radio bursts at 1.0 AU. In nearly four years of observations only one event was found in which electron plasma oscillations are clearly associated with solar electrons. Numerous cases were found in which no electron plasma oscillations with field strengths greater than 1 V/m could be detected even though electrons from the solar flare were clearly detected at the spacecraft.For the one case in which electron plasma oscillations are definitely produced by the electrons ejected by the solar flare, the electric field strength is very small, only about 100 V/m. This field strength is about a factor of ten smaller than the amplitude of electron plasma oscillations generated by electrons streaming into the solar wind from the bow shock. Electromagnetic radiation, believed to be similar to the type III radio emission, is also observed coming from the region of more intense electron plasma oscillations upstream of the bow shock. Quantitative calculations of the rate of conversion of the plasma oscillation energy to electromagnetic radiation are presented for plasma oscillations excited by both solar electrons and electrons from the bow shock. These calculations show that neither the type III radio emissions nor the radiation from upstream of the bow shock can be adequately explained by a current theory for the coupling of electron plasma oscillations to electromagnetic radiation. Possible ways of resolving these difficulties are discussed.     

Electron content near the lunar surface using dual-frequency VLBI tracking data in a single lunar orbiter mission

In VLBI observations of Vstar, a subsatellite of the Japanese lunar mission SELENE, there were opportunities for lunar grazing occultation when Vstar was very close to the limb of the Moon. This kind of chance made it possible to probe the thin plasma layer above the Moon's surface as a meaningful by-product of VLBI,by using the radio occultation method with coherent radio waves from the S/X bands.The dual-frequency measurements were carried out at Earth-based VLBI stations. In the line-of-sight direction between the satellite and the ground-based tracking station where VLBI measurements were made, the effects of the terrestrial ionosphere, interplanetary plasma and the thin lunar ionosphere mixed together in the combined observables of dual-frequency Doppler shift and phase shift. To separate the variation of the ionospheric total electron content(TEC) near the surface of the Moon from the mixed signal, the influences of the terrestrial ionosphere and interplanetary plasma have been removed by using an extrapolation method based on a short-term trend. The lunar TEC is estimated from the dual-frequency observation for Vstar from UT 22:18to UT 22:20 on 2008 June 28 at several tracking stations. The TEC results obtained from VLBI sites are identical, however, they are not as remarkable as the result obtained at the Usuda deep space tracking station.