Energetic neutral atom occultation: New remote sensing technique to study the lunar exosphere

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113,, doi: /2008ja013356, 2008 Energetic neutral atom occultation: New remote sensing technique to study the lunar exosphere Y. Futaana, 1 S. Nakano, 2,3 M. Wieser, 1 and S. Barabash 1 Received 28 April 2008; revised 8 August 2008; accepted 21 August 2008; published 6 November [1] We propose a new technique to investigate the lunar exosphere by measuring energetic neutral atoms (ENAs). The lunar exosphere is a typical exosphere of celestial bodies without a dense atmosphere and without a global magnetosphere, but our knowledge of it is incomplete because of a very limited number of observations. In this paper, we present a feasibility study for detecting the charge exchange solar wind ENAs in the vicinity of the Moon by developing a numerical model that simulates the interaction between the solar wind and the lunar exosphere. The calculated ENA flux reaches 10 7 cm 2 s 1, which corresponds to 3% of the solar wind flux. The flux is high enough to be measured by existing ENA sensors. We also conducted many runs with different exospheric parameters. The charge exchange ENA flux is then found to depend both on the positions of the satellite and on the exosphere parameters. This result indicates that inversion techniques can in principle be applied in the future to reconstruct the exosphere profile from the obtained ENA flux data. Citation: Futaana, Y., S. Nakano, M. Wieser, and S. Barabash (2008), Energetic neutral atom occultation: New remote sensing technique to study the lunar exosphere, J. Geophys. Res., 113,, doi: /2008ja Introduction [2] It is often said that the Moon has no atmosphere. However, this phrase is not altogether true. Even though the density is very small (10 6 cm 3 at the surface) [e.g., Stern, 1999; Wurz et al., 2007], the Moon possesses a tenuous atmosphere. This tenuous atmosphere is called the exosphere or the surface-bound exosphere (SBE) [Stern, 1999] because the condition for an exosphere (the scale height is longer than the mean free path) is satisfied at the surface. SBE bodies are very common in the solar system (for example, Mercury, Io, Europa, and Callisto). However, our understanding of the composition and structure is incomplete even for the Moon. One example of the unknown characteristics of the lunar SBE is that the sum of the number density of identified species does not reach the total number density derived from total pressure measured by Apollo [Stern, 1999]. The main reason for our ignorance is the very limited number of observations (see Stern [1999, Table 1] for details), which stems from the severe lower limit of sensitivities of existing sensors under the tenuous density environment. [3] In this paper, we propose a new technique to investigate the lunar exosphere at the terminator region by using solar wind energetic neutral atom (ENA) measurements. The technique is an analogy of the occultation (sounding) 1 Swedish Institute of Space Physics, Kiruna, Sweden. 2 Institute of Statistical Mathematics, Research Organization of Information and Systems, Tokyo, Japan. 3 Also at Japan Science and Technology Agency, Tokyo, Japan. Copyright 2008 by the American Geophysical Union /08/2008JA technique widely used in planetary missions [e.g., Pätzold et al., 2007; Imamura et al., 2008]. Instead of using radio waves or photons, we use ENAs which have originated from the solar wind to probe the exosphere. The variations of ENA flux carry information on the lunar exospheric densities integrated over the line of sight (LOS) of the ENA sensor, and, inversely, we can in principle reconstruct the lunar exospheric density profile by measuring the LOS dependency of the ENA flux. [4] Measurements of ENAs opened a new window for investigations of solar wind interaction with solar system bodies. A number of spacecraft carried ENA sensors into space in order to investigate plasma phenomena around the Earth [e.g., Barabash, 1995; Mitchell et al., 2000; Pollock et al., 2000; Moore et al., 2000]. With the Cassini spacecraft, an ENA sensor was inserted into orbit around Saturn [Krimigis et al., 2005; Mitchell et al., 2005], and on its way to Saturn, it visited Jupiter [Mauk et al., 2003]. Mars Express and Venus Express are equipped with ENA sensors, and they are carrying out ENA observations in the vicinity of Mars and Venus [Barabash et al., 2006, and references therein]. As a future mission, IBEX [McComas et al., 2004], Chandrayaan-1 [Bhardwaj et al., 2005], and Bepi Colombo [Barabash et al., 2001] will be launched to investigate the heliosphere, the Moon, and Mercury, respectively, using ENAs. [5] The source of the solar wind ENAs is the solar wind protons. When the solar wind protons experience charge exchange reactions with cold neutral atoms, they are neutralized and become solar wind ENAs. The solar wind ENAs have already been observed by several missions. For example, around the Earth, Collier et al. [2001] identified the solar wind ENAs using the IMAGE satellite. 1of7

2 Figure 1. Illustration of the charge exchange mechanism and the basic idea of ENA occultation. Around Mars, Mars Express detected the solar wind ENAs in the wake region [Brinkfeldt et al., 2006] and flank region [Futaana et al., 2006a]. In the Venusian environment, Galli et al. [2008] reported the first observations of tailward solar wind ENA flow. [6] Using these tailward solar wind ENA flows, similar kinds of ENA occultation techniques have been examined at the wake region of Mars [Kallio et al., 2006; Brinkfeldt et al., 2006]. Bepi Colombo also plans to measure the Mercury exospheric profile using ENAs [Barabash et al., 2001]. The advantage of the application of this technique to the Moon lies not only within the field of the lunar sciences. The lunar environment is favorable for establishing the ENA sounding technique under the rather simple conditions of solar wind interaction with the Moon because of the lack of global magnetosphere or dense ionosphere. For example, ENA sounding at Mars is quite sensitive to many external parameters, most likely caused by the existence of dynamic plasma boundaries which reform the solar wind velocity vectors [Kallio et al., 2006; Brinkfeldt et al., 2006]. [7] One of the interesting regions to be explored by this technique is the lunar terminator region. The characteristic phenomena are ongoing around a transition region between sunlit and dark regions. The surface potential, which is mainly dominated by photoionization by UV light and ambient electrons, changes from positive (sunlit) to negative (dark) around the terminator [Vondrak, 1988; Halekas et al., 2005]. Because of the strong transitions of the electric field, charged dust can be blown more easily than over other places. Magnetic anomalies can also interact with the solar wind much more easily compared to other places, creating limb compressions [Russell and Lichtenstein, 1975]. An ENA occultation is one of the favorable ways of investigating this active region. [8] In this paper, we demonstrate the feasibility of lunar exosphere sounding by calculating the parameter dependence of the ENA images. We also discuss the effect of the lunar magnetic anomaly. In section 2, we review ENA generation mechanisms in the vicinity of the Moon. In section 3, we describe model calculations of ENA generation, and we discuss the feasibility of the technique in section 4. Section 5 contains our conclusions. 2. ENA Generation Mechanisms [9] The major mechanisms for the generation of neutral atoms due to solar wind interaction with celestial bodies that have been considered are thermal desorption (TD), solar wind ion sputtering (SWIS), photon-stimulated desorption (PSD) and electron-stimulated desorption (ESD), and micrometeoroid impact vaporization (MIV). The typical energy of neutral atoms which are generated by these mechanisms is less than 1 ev, and only those generated by SWIS can exceed 10 ev [e.g., Barabash et al., 2001; Futaana et al., 2006b]. Here we would like to discuss another ENA generation mechanism around the Moon: the charge exchange reaction between solar wind particles and the exosphere (Figure 1). [10] When the solar wind particles (mainly protons) traverse the tenuous exosphere, some of them experience the charge exchange reaction H þ sw þ M ex! H E sw þ M þ ex ; where H + sw and H E sw are the solar wind protons and solar wind hydrogen ENAs, M ex is the exospheric atoms or molecules, and M + ex is the ionized exospheric ions. During the charge exchange reaction, the energy loss of the solar wind particles is negligibly small (<10 ev) [e.g., Basu et al., 1993] compared to their original energy (1 kev). Therefore, generated neutral atoms keep energies about 1 kev (400 km s 1 ), so they cannot be a source of the lunar exosphere. However, the flux of the ENA provides information about the exospheric density along the LOS, and in sections 3 and 4, we discuss the feasibility of detecting the charge exchange solar wind ENAs using test particle simulations. 3. Model Description [11] In order to conduct feasibility studies of the ENA occultation technique for an exospheric study of the Moon, we developed an ENA generation model on the basis of the charge exchange mechanism between the lunar exosphere and the solar wind. For each run, we launched 2 million imaginary solar wind protons and calculated the charge exchange ENA flux by integrating the contribution of all the proton trajectories. Here we used the velocity-dependent charge exchange cross section [Barnett et al., 1990; Stancil et al., 1999] as discussed below. [12] In our model, the solar wind is assumed to consist solely of protons because they are the most abundant species in the solar wind. In addition, the alpha particles, which are the second major component, will not normally be neutralized by charge exchange reactions because they are in a double-charged state. This assumption is reasonable from an instrumental point of view because some ENA sensors are able to separate H and He ENAs [e.g., Kazama et al., 2006]. The density, velocity, and temperature of the solar wind protons can be specified as an input parameter. ð1þ 2of7

3 Table 1. Lunar Exosphere Models Used in This Work and Its Reference Parameters Name Surface Density (cm 3 ) Scale Height (km) Cross Section (cm 2 ) O proxy atmosphere (1 2) He atmosphere The reference values used here are n SW =9cm 3, V SW = 400 km s 1, and V th 31.5 km s 1 (corresponding temperature is K), respectively, where V th is the thermal velocity. The gaussian distribution function is assumed for the initial velocity distribution for test particles. In general, the solar wind protons can be considered as a natural source proton beam. We also assumed here a uniform interplanetary magnetic field (B IMF = 5 nt) perpendicular to the lunar ecliptic plane. The uniform convectional electric field (E C = V SW B IMF ) also exists. For simplicity, the imaginary solar wind particles behave independently of the background solar wind flow. This means that no feedback processes to background conditions are regarded in the model. The Moon is treated as a sphere which absorbs all the solar wind protons and ENAs when they hit the surface. The lunar wake is not taken into account in the calculations for reasons of simplicity. Note that more realistic magnetic and electric field models, such as wake structures, existence of magnetic anomalies, and surface charging, will be implemented in the near future. [13] On the other hand, the most ambiguous factor here is the composition and height profile of the lunar exosphere. The composition affects the cross section of the charge exchange reactions with solar wind protons and thus affects the production of the ENAs. Stern [1999, Table 1] presents a good summary of which components have ever been found in the lunar exosphere, while the total density of them does not reach the lunar exospheric density of 10 6 cm 3. [14] Given this ambiguity of the exospheric composition and structures, we adopted two models of the exosphere: oxygen proxy atmosphere and helium atmosphere (Table 1). One may say that this is a rough assumption. However, this assumption is acceptable from a cross-section point of view because most of the major lunar exosphere (except He) is thought to have mostly the same ionization potential energy as the O atom (13.6 ev), and therefore the cross-section model is more or less comparable to that of proton-oxygen charge exchange (hereafter we call this modeled atmosphere an O proxy atmosphere). For example, according to the existing observations summarized by Stern [1999], the largest possible concentration of lunar exosphere ever found is OH with an upper limit of <10 6 cm 3 whose potential energy is 13 ev. Another main contribution of Ar (10 5 cm 3 ) has a potential energy of 15.8 ev. [15] Then, we could refer to two velocity-dependent charge exchange cross-section models: we adopted here the model by Stancil et al. [1999] for proton-oxygen reactions and the model by Barnett et al. [1990] for proton-helium reactions. The typical cross sections in the energy range we are interested in (0.4 2 kev amu 1 )are (1 2) cm 2 for proton-oxygen reactions. On the other hand, the He atmosphere should be treated in a different model from the O proxy atmosphere because the cross section is very small ( cm 2 ). [16] We need to model the exosphere profile properly. However, as mentioned in section 1, the structure of the lunar exosphere is quite unknown. Here we assumed the density of O proxy atmosphere at the surface to be 10 6 cm 3. LACE mass spectroscopy [Hoffman et al., 1973] concluded that the He atmosphere density is and cm 3 for dayside and nightside, respectively. Here we assumed 10 4 cm 3 for the helium atmosphere. [17] The scale height of the exosphere is the other unknown parameter. We used 100 and 1280 km for O proxy and He atmosphere as references for our calculation owing to the following discussion. If we could assume that the source of the exosphere was under the thermal equilibrium and the height profile of the density is in the exponential form, the scale height can be calculated to be 1000 km for He and 100 km for O proxy atmosphere (OH or Ar (20 amu)), assuming 400 K at the surface. Note that the H atom density is extremely low (<17 cm 3 ) according to the investigations by UV spectroscopy on Apollo 17 [Feldman and Morrison, 1991]. This assumption would be acceptable if we compare it with the ground observations of Na (23 amu) and K (40 amu): the scale height at very low altitude atmosphere is 120 ± 42 km and 90 ± 20 km, respectively [Potter and Morgan, 1988]. [18] Since the lunar exosphere is not in a state of equilibrium because of its collisionless feature, these approximations are quite rough, especially at the higher altitude. For example, Mendillo et al. [1997] mentioned that Na and K atmosphere extends up to 10 lunar radii according to the ground observations. Wurz et al. [2007] demonstrated in their Monte Carlo simulation that the neutral atoms generated by the solar wind sputtering, which is nonthermal production, form a major component. However, we should emphasize here that the height profile of the neutral density can in principle be reconstructed by the observations of ENAs and inversion methods of the obtained data even though the distribution is not exponential. In addition, understanding the height profile of the exosphere density is one advantage of the observations and the inversion using the ENA occultation technique that we propose in this paper. The dependency in the scale height is discussed in section Discussion [19] Figure 2a shows the expected image of the charge exchange ENAs generated by the O proxy atmosphere. The satellite orbit is in the noon-midnight meridian with a height of 100 km. The center of each image is the Sun direction, and the curve is the limb of the Moon. Grids are shown every 10. The images are taken in the specified solar zenith angles (SZAs) (i.e., Sun-Moon center-satellite angle). In this orbit, the satellite goes to the geometric umbra with SZA 109. [20] As seen in Figure 2a, the ENA flux is beam-like with an angular spread of 10. This is consistent with the assumed thermal spread of the solar wind. Since the angular resolution of existing ENA sensors is typically 5 30, the taken images will be coarse. For example, the Neutral Particle Detector on board Mars Express and Venus Express 3of7

4 Figure 2. (a) Calculated charge exchange solar wind ENA images. The satellite is located at 100 km from the lunar surface with a solar zenith angle of every 10. The center of each image is the Sun direction, and the grids are shown every 10. (b) The solar zenith angle dependence of the total flux and the maximum differential flux of the ENA. The dashed line at 109 is the geometric umbra of the spacecraft. has an angular resolution of 5 30 [Barabash et al., 2006]. The Low-Energy Neutral Atom imager on board IMAGE has an angular resolution of [Moore et al., 2000]. [21] Figure 2b shows the solar zenith angle (satellite position) dependence of the total ENA flux and the maximum differential flux. The solar zenith angle can be converted to the time of the observation by the orbiting satellite for the actual instances. The maximum flux of the charge exchange ENAs is the order of 10 7 particles cm 2 s 1 (3% of the solar wind proton flux), and the maximum differential flux of the charge exchange ENAs reaches 10 8 particles cm 2 sr 1 s 1 at the terminator region. This is well above the typical detection limits for ENA instruments in this energy range. For example, according to Kazama et al. [2006], the typical one-count level of the ENAs is particles cm 2 sr 1 s 1. The ENA flux is relatively constant over the sunlit hemisphere because only the solar wind ENAs generated at the altitude above the satellite can be detected in this region. The maximum count rate is expected when the satellite is entering into shadow (109 ; dashed line) because the solar wind can penetrate down to a very low altitude around the terminator region, where more frequent charge exchange reaction is expected. The count rate will go down very quickly as the satellite goes deep into the umbra. [22] Note that we can distinguish the charge exchange solar wind ENAs from those coming from the lunar surface by energy analysis because the charge exchange ENAs are mostly composed of hydrogen with the specific energy of the solar wind energy (1 kev). The MIV, PSD, ESD, and SWIS generate various species of ENAs, and even though hydrogen ENAs are generated, the particle energy is very low (<10 ev for MIV, PSD, and ESD and <100 ev for SWIS) [see Futaana et al., 2006b]. Scattered ENAs at the lunar surface may have wider energy distribution functions because of the energy loss during the collision with surface materials. [23] Figure 3 shows the solar wind bulk velocity dependence of the total flux and maximum differential flux for the Figure 3. velocity. The ENA flux dependence on the solar wind 4of7

5 Figure 4. exosphere. The calculated ENA images using different scale height models of the oxygen proxy O proxy atmosphere and the He proxy atmosphere. The satellite is assumed to be placed at the solar zenith angle of 110. The ENA flux originated from the O proxy exosphere has a small dependence on the solar wind velocity, while that from the He exosphere is quite sensitive to solar wind velocity. This dependence reflects the velocity-dependent cross section for proton-helium charge exchange. However, the absolute flux of ENAs originated from the He exosphere is more than 3 orders lower than that from the O proxy exosphere. Therefore, we conclude that measurement of the He exosphere is quite difficult, at least for the thermal component assumed in our model. [24] Figure 4 represents the ENA images generated from the O proxy exosphere with three scale height models. The satellite is assumed to be located at a solar zenith angle of 110. Three images show clear differences in fluxes and in angular width of counts. The total flux and the maximum differential flux vary times, which may be large enough differences to invert the scale height from obtained ENA images in the future. [25] At the end of this section, we will mention the application of this technique to the study of magnetic anomalies. In the previous work by Futaana et al. [2006b] global imaging of magnetic anomalies using sputtered ENAs from the lunar surface was proposed. They proposed that the solar wind protons reach the lunar surface directly, sputtering surface materials in most areas of the Moon. These sputtered particles can be measured as ENAs. However, solar wind protons cannot reach the lunar surface if strong magnetic anomalies provide magnetic shielding for the solar wind. Under the conditions, these shielded areas can be measured as a void of sputtered ENA signal. They claim that this sputtered ENA imaging is expected to make another surface map of the lunar magnetic anomalies, or, in other words, a solar wind protection map. [26] If we employ the occultation technique proposed in this paper in addition to the above imaging of the surface mapping, we may conduct 3-D imaging of the lunar minimagnetosphere interaction with the solar wind in the terminator region [e.g., Russell and Lichtenstein, 1975; Lin et al., 1998; Harnett and Winglee, 2002; Futaana et al., 2003]. Considering that magnetic anomalies are strong enough to interact with the solar wind, generating a minimagnetosphere system [Lin et al., 1998; Harnett and Winglee, 2002], the solar wind protons cannot fly close to the surface because the solar wind protons are deflected in front of the minimagnetosphere. Since most of the ENAs are generated in the area close to the surface, the solar wind ENA flux behind the magnetic anomalies will be depleted by the existence of the minimagnetosphere. The height scale of this ENA flux depletion reflects the height scale of the minimagnetosphere, which would be quite important information for a study of the nature of the smallest known magnetosphere. 5. Concluding Remarks [27] We have discussed the feasibility of lunar ENA occultations with two different models of the lunar exosphere and conclude that the ENAs generated by charge exchange mechanisms can be detected by existing ENA sensors. For precise observations of the ENA occultation, we may mention the existing ENA sensors with the following features. Typical angular resolution of ENA sensors is This is very close to the angle of the solar wind thermal spread (sin 1 (V th /V sw ), where V th is the thermal velocity and V sw is the bulk velocity of the solar wind). Therefore, an improvement of angular resolution is preferred for this study. On the other hand, the energy resolution and the mass resolution of existing sensors are sufficient for the exospheric soundings. [28] As a plan for the near future, the inversion algorithm should be developed. This paper only shows the feasibility of the detection of the exospheric ENAs and exhibits the clear dependences on exospheric parameters (forward problem). The envisioned inversion technique is as follows. An orbiter equipped with an ENA sensor provides the time series of the ENA flux. The ENA flux at a specific time corresponds to the charge exchange occurrence, which is proportional to the lunar exospheric neutral density, integrated over the line of sight (LOS). As the orbiter goes around the Moon, the orbiter changes its location and attitude. The ENA flux then changes in time because of the change of the LOS. The problem is, therefore, to choose the best model to explain the time series of the ENA flux observations in a reasonable mathematical way. Actually, this is a typical example of the inverse problem in a mathematical sense. For example, a tomographic inversion can be used to solve this kind of problem [e.g., C:son Brandt et al., 2002; DeMajistre et al., 2004; Nakano et al., 2008]. This kind of tomographic inversion becomes a nonlinear problem; therefore, multiple ENA flux calcula- 5of7

6 tions demonstrated in this paper are needed. We should develop an efficient algorithm for solving the forward problem for the ENA flux calculation discussed here because the current algorithm for calculating the solar wind ENA flux is actually not efficient enough to make iterative calculations from the calculation time point of view (even though it may not be unrealistic). However, from the calculations in section 4, we found that we may ignore the loss of the solar wind proton flux because of its low flux (3% of the solar wind flux) under usual conditions. Under this assumption, it may not be needed to repeat the calculation of the forward problem because the inversion can be treated as a simple linear program, which can be readily solved. [29] On the other hand, we need to mention several open characteristics to be considered for the inversion in practice. One is the possibility of multiple scale heights due to a mixture of exospheric species with different masses or with different temperatures. The nonexponential form of the density profile [e.g., Potter and Morgan, 1988] may be another problem. The day-night asymmetry of the exospheric density profile would become important in practice as well. [30] Finally, we should mention the possibility of observing charge exchange ENAs in the lunar limb region with the Chandrayaan-1 satellite. Chandrayaan-1 provides an ENA sensor (CENA) at the Moon for the first time [Bhardwaj et al., 2005]. The sensor field of view is the nadir direction because the main targets are SWIS ENAs from the lunar surface. However, since the total aperture is 160 and the lunar apparent diameter is 140, the residual 20 point the open space. When the orbital and attitude configurations are satisfied, observations of charge exchange ENAs may be possible. 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