detecting neutral atoms from beyond the heliopause with ... - IOPscience

The Astrophysical Journal, 695:L58–L61, 2009 April 10  C 2009.

doi:10.1088/0004-637X/695/1/L58

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DETECTING NEUTRAL ATOMS FROM BEYOND THE HELIOPAUSE WITH INTERSTELLAR BOUNDARY EXPLORER J. Heerikhuisen and N. V. Pogorelov Department of Physics and Center for Space Physics and Aeronomic Research, University of Alabama, Huntsville, AL 35899, USA; [email protected], [email protected] Received 2008 December 9; accepted 2009 February 25; published 2009 March 23

ABSTRACT Theoretical work concerning neutral atoms to be observed by the recently launched Interstellar Boundary Explorer (IBEX) spacecraft has focused on neutral hydrogen atoms created by charge exchange in the inner heliosheath. In this Letter, we explore the effects of including H atoms born through charge exchange in the outer heliosheath and the nearby interstellar medium, with energies between 10 eV and ∼70 eV. Although such atoms are generally not regarded as energetic neutral atoms (ENAs), they can nevertheless be observed by IBEX due to the spacecraft’s orbital speed around the Sun. We find that the apparent boost in particle energy due to this motion allows IBEX to observe interstellar neutral atoms, and that the flux of atoms at the lowest energies observed by IBEX will be dominated by atoms originating from outside the heliopause. We show, using simulated data from a self-consistent model of the heliosphere, that the first two energy channels on IBEX will be dominated by outer heliosheath ENAs, and that these flux maps can, in principle, be used to deduce the asymmetry of the outer heliosphere caused by the interstellar magnetic field. The work presented in this Letter employs the simplifying assumption that radiation pressure exactly balances gravity for an H atom, and the predictions made here are only appropriate for solar minimum conditions. Key words: ISM: kinematics and dynamics – magnetic fields – solar wind

et al. 2008; Burlaga et al. 2008; Decker et al. 2008) crossed the SW TS and entered the inner heliosheath. The 10 AU difference in the observed TS distance (94 AU for Voyager 1 and 84 AU for Voyager 2) suggests an inherent asymmetry in the heliosphere, most likely caused by the influence of the interstellar magnetic field (Pogorelov et al. 2004; Opher et al. 2006) but mediated by the presence of neutral hydrogen (Pogorelov et al. 2007). However, given the three year difference in crossing times, some of this apparent asymmetry is likely due to time-dependent effects, though an inherent asymmetry is still believed to exist (Washimi et al. 2007). The observed asymmetry of the heliosphere is one of the few clues we have to deduce the interstellar magnetic field strength and orientation. We will show that H atoms from the outer heliosheath can potentially add to the information we have about heliospheric asymmetry. When an interstellar H atom charge exchanges in the inner heliosheath, it effectively interchanges momentum with a proton from the plasma, resulting in a cold slow proton and a hot energetic H atom (i.e., an ENA). Due to the high thermal speed and relatively slow bulk speed of the plasma, ENAs can have trajectories oriented toward the inner heliosphere, and these are the atoms that IBEX will primarily be mapping. IBEX will measure ENA flux between 10 eV and 6 keV, corresponding to a minimum impact speed onto the detector of 44 km s−1 . However, due to the fact that IBEX orbits the Earth, which in turn orbits the Sun (at ∼30 km s−1 ), H atoms as slow as 14 km s−1 may be detected in the lowest energy channel in the ecliptic plane. Since the flow speed of the LISM relative to the Sun is 26 km s−1 , primary interstellar H and somewhat slower hotter H atoms born in the outer heliosheath are both, in principle, detectable by IBEX. The question is what fraction of these slow atoms can reach 1 AU without being ionized due to impact by electrons, protons, or photons, since the survival probability decreases exponentially for low-energy atoms. In anticipation of IBEX data, a number of recent papers have used models of the heliosphere to estimate the flux of ENAs at

1. INTRODUCTION The recently launched Interstellar Boundary Explorer (IBEX), one of NASA’s small explorer (SMEX) missions (McComas et al. 2004, 2006), will construct all-sky maps of energetic neutral atoms (ENAs) originating from the distant heliosphere, by observing these directly from an eccentric Earth orbit. These maps will, for the first time, provide a global picture of the region carved out of the local interstellar cloud by the solar wind (SW), known as the heliosphere. Although IBEX will only observe neutral atoms, each of these has properties associated with the region of plasma where it was born out of a charge-exchange encounter between a plasma proton and an interstellar neutral atom. Neutral atoms created in this way may travel many hundreds of astronomical units unimpeded, and can therefore be used as probes of distant places in the heliosphere (Hsieh & Gruntman 1993; Gruntman et al. 2001; Fahr et al. 2007). The colliding plasma flows of the SW and local interstellar medium (LISM) create a teardrop-shaped heliosphere that is separated from the LISM by a tangential discontinuity known as the heliopause (HP). The supersonic SW crosses the termination shock (TS), whereupon it flows through the “inner heliosheath” and down the heliotail. Measurements of the flow speed and temperature of interstellar helium (M¨obius et al. 2004) indicate that the LISM flow is also supersonic, suggesting a bow shock which separates the LISM and the “outer heliosheath” on the interstellar side of the HP. Meanwhile neutrals from the partially ionized (∼25%) interstellar medium flow through the SW– LISM interaction region and couple to the ionized component through charge exchange. However, the large mean free paths of neutrals (of the order of global heliospheric scales) make this coupling weak, and allow for nonlocal coupling of disparate plasma regions (see, for example, Zank 1999). Recently, Voyager 1 (Stone et al. 2005; Decker et al. 2005; Burlaga et al. 2005) and Voyager 2 (Stone et al. 2008; Richardson L58

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1 AU (Fahr & Lay 2000; Gruntman et al. 2001; Heerikhuisen et al. 2008, 2007; Sternal et al. 2008; Prested et al. 2008). All these studies, however, focused on the hot slow plasma of the inner heliosheath as the creation site of ENAs. In this Letter, we will focus on the lowest three energy channels of IBEX (10–73 eV), and decompose our simulated ENA maps into the three ENA source components based on the H-atom creation site: LISM, outer heliosheath, and inner heliosheath. In Section 2 we present our model, in Section 3 the results, and finish with conclusions in Section 4.

2. THE MODEL The model heliosphere used in the calculations presented here is based on our coupled ion-neutral code (Heerikhuisen et al. 2008), where we use MHD to describe the ionized component and couple this self-consistently to a kinetic Monte Carlo model for H atoms. As in that paper, we employ a Lorentzian distribution function for SW protons, and set κ = 1.63 based on Voyager 1 LECP data (Decker et al. 2005). The latter gives the SW distribution function a narrow core and broad powerlaw tail, corresponding to the core and pickup ion distributions that co-move as the SW, but which do not thermalize into a single distribution. The suprathermal ions also allow for ENAs to be created with energies of several keV, and these in turn increase the flow of energy from the inner heliosheath to the outer heliosheath (Heerikhuisen et al. 2008). The boundary conditions chosen for the calculations in this Letter are the same as Pogorelov et al. (2008a, 2008b), and are described in detail therein. In short, we use a uniform SW and choose the interstellar magnetic field B∞ to lie in the so-called hydrogen deflection plane (Lallement et al. 2005) pointing south at an angle of 30◦ to the ecliptic, and having a magnitude of 3 μG. Magnetic fields of this strength and orientation can introduce asymmetry in the TS shape consistent with the Voyager 1 and 2 observations (Pogorelov et al. 2008a, 2008b, 2009). One difference in the current calculation is that we have adopted the newer charge-exchange cross-section of Lindsay & Stebbings (2005). The procedure for creating all-sky maps from our simulated heliosphere starts with a steady-state configuration obtained from our MHD-plasma/kinetic-neutral code (Pogorelov et al. 2008c). Each subsequent charge-exchange event contributes some small fraction of the computational weight of the particle to the ENA flux. We assume that this contribution is isotropic in the frame of the plasma, then trace a straight line from the charge-exchange location to the Sun. When we consider only ENAs from the inner heliosheath, most of the ionization losses that an ENA receives occur close to the Sun where the concentration of “ionizers” is high. Hydrogen atoms from the outer heliosheath, however, also undergo significant charge-exchange losses while traveling through the relatively dense plasma outside the heliopause, and so we also take these losses into account. Once we have tracked the particle and losses to 100 AU, we institute the analytic formula (1) for subsequent losses (Heerikhuisen et al. 2008). This formula takes into account the fact that IBEX, as a Sun-pointed spinner, detects ENAs in the plane perpendicular to the Sun–Earth line (path “B” in Figure 1). Detection at this location results in more ionization losses than the shortest path to 1 AU (path “A” in Figure 1). In Equation (1), w is the particle weight, β is the total loss rate due to ionization by photons, electrons, and protons, and v0 is the particle’s

Figure 1. Cartoon showing the Sun (yellow), the Earth’s orbit (dashed), and IBEX’s observing geometry. Neutrals are detected by IBEX in a plane that is tangent to the 1 AU sphere, along the trajectory marked “B.” These neutrals experience more ionization losses than those traveling along trajectory “A,” though the latter are not detectable by IBEX.

velocity in the Sun frame given in AU/s:    r 2 πβ E , β(r) = 6×10−7 w1 = w100 exp − . (1) 2v0 r Note that this formula assumes the same photoionization rate for all directions, while studies show that the ionization rate is higher over the poles (Bzowski 2008). Since our investigation focuses on interstellar neutrals arriving in the ecliptic plane, the latitudinal dependence of the ionization rate is not so important. Once we have the depleted particle at the tangent point between the Sun–Earth line and its velocity vector, we take half the computational weight and accumulate this onto a map where we add the Earth’s orbital speed (30 km s−1 ), and the other half onto a map where the orbital speed is subtracted. The above assumption of straight trajectories corresponds to setting the outward force of solar radiation pressure to exactly balance the inward force of gravity. In a recent paper, Bzowski (2008) carefully investigated the dynamics of H atoms in the heliosphere and found that the assumption of straight trajectories is almost valid at solar minimum, but that this assumption breaks down at solar maximum, especially for the relatively low energies considered here. Specifically, Bzowski (2008) found that for solar minimum conditions the deflection in particle orbits is about 1◦ and that the energy boost due to gravity is about 10% at 10 eV. Of course, interstellar neutrals observed in the lowest energy band will only have an energy of about 5 eV in the Sun frame, so the above are lower limits. These considerations suggest that the directional properties discussed in the following sections are not valid during solar maximum, and should be considered as an idealized limit for solar minimum conditions. 3. RESULTS In an effort to mimic the IBEX data products (due for public release in the middle of 2009), we have created all-sky maps of ENA flux using the procedure described in Section 2. Figure 2 shows maps of ENAs in the 10–19 eV energy range, corresponding to the lowest energy channel of IBEX. To aid analysis, we have split the ENA flux data into forward and backward look directions of the moving spacecraft (the left and

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Figure 2. All-sky maps of neutral atom flux (cm2 sr s keV)−1 looking forward (left) and backward (right) along the orbit of the Earth around the Sun for the lowest IBEX energy band (10–19 eV). Here we have used the Mollweide projection, with the ecliptic plane running horizontally through the center. The central “bull’s eye” is aligned with the inflow direction of interstellar helium and the heliotail is on the far left and right. The top plots are for primary interstellar H atoms, middle is for outer heliosheath atoms, and bottom is for H born in the inner heliosheath. The actual map produced by IBEX is the sum of all six of these, and is shown in the left plot of Figure 4. Note that primary interstellar atoms can only be seen when IBEX moves toward the interstellar flow direction, and hence the top right plot shows no signal at all.

right plots of Figure 2, respectively), and the region of creation due to charge exchange (LISM top, outer heliosheath middle, and inner heliosheath lower). Note that the inflow direction of LISM neutrals originates from 5◦ above the ecliptic plane. For the lowest IBEX energy band, considered in Figure 2, outer heliosheath neutrals from the forward direction dominate the neutral atom flux. Primary interstellar neutrals show a comparatively weak signal from the forward direction at this energy, and are not detectable from the backward direction at any energies. Neutrals from the inner heliosheath, however, show a stronger signal from the backward direction, due to the fact that, unlike neutrals from beyond the heliopause, these H atoms tend to have energies much greater than a few tens of eV. The double source structure observed from interstellar neutrals is caused by the projection of the interstellar flow along the look direction reducing the apparent energy of off-center atoms to this energy bin, while H atoms originating from the center of the plot end up in the next energy channel shown in Figure 3. Figure 3 shows similar maps to Figure 2 but this time for the second energy channel of 19–38 eV. In this case outer heliosheath neutrals still dominate the flux, but LISM neutrals have a stronger signal than inner heliosheath neutrals. The relative signal strengths of LISM neutrals can be understood in terms of their flow speeds and thermal properties. The LISM flows at 26 km s−1 and has a thermal speed of 10 km s−1 , which, when combined with the spacecraft motion, gives rise to an observed energy of about 23 eV. The relatively low thermal speed results in most LISM neutrals being observed in the second energy channel. A similar analysis of outer heliosheath

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Figure 3. All-sky maps of neutral atom flux (cm2 sr s keV)−1 looking forward (left) and backward (right) along the orbit of the Earth around the Sun for the lowest IBEX energy band (19–38 eV). The same projection is used here as in Figure 2.

neutrals, with a bulk speed of 8 km s−1 and a thermal speed of 30 km s−1 , gives 17 eV, suggesting a split among the first and second channels. Note that outer heliosheath neutrals are more difficult to analyze in this way since the outer heliosheath plasma temperature varies from less than 10,000 K at the bow shock to about 50,000 K at the heliopause. Figure 4 shows maps of the total neutral atom flux as they would be observed by IBEX, for the first three energy channels 10–19 eV (left), 19–38 eV (middle), and 38–73 eV (right). At the lowest energy, it is clear that the “angel wings” feature of the inner heliosheath neutrals from Figure 2 dominates the full map. The full map of the second energy channel, Figure 4 (middle), is dominated by LISM and outer heliosheath neutrals, while by the third energy channel inner heliosheath neutrals, especially those from the distant heliotail, begin to dominate the flux observed by IBEX. The interstellar flow in the simulation originates from the center of the “bull’s eye” on the all-sky maps. This direction is determined from helium observations. Lallement et al. (2005) suggested that the deflection of hydrogen away from the helium flow direction, from Lyα measurements of the SW, was due to the interaction of interstellar H atoms with the plasma of the outer heliosheath (helium interaction is much weaker), and that the plane of this deflection should coincide with the plane determined by the interstellar magnetic field and velocity vectors. The 19–38 eV all-sky map (the middle plot of Figure 4) shows a clear deflection and elongation in this plane. This suggests that IBEX might be able to provide additional observational data related to the interstellar magnetic field orientation, which is still unknown. Although the deflection seen in this plot is close to the observed 4◦ and aligns with the plane of the interstellar magnetic field chosen for this calculation, actual IBEX observations will be contaminated by the lensing effects of gravity and radiation pressure, and the signal-to-noise ratio of the instrument.

Figure 4. All-sky maps of the total neutral atom flux observed by IBEX in the first three energy ranges (left to right): 10–19, 19–38, and 38–73 eV.

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DETECTING NEUTRAL ATOMS FROM BEYOND THE HELIOPAUSE WITH IBEX 4. CONCLUSIONS

We have presented results from our MHD-plasma/kineticneutral code and provided neutral atom flux estimates relevant to the IBEX mission. The focus of this Letter is on the lowest energy channels of IBEX, where the orbital motion of the spacecraft can boost the energy of relatively slow atoms from beyond the heliopause into the IBEX energy range. We show that the effect of spacecraft motion is important for the lowest three energy channels of IBEX, corresponding to energies below about 70 eV. We find that neutrals from beyond the heliopause, as opposed to those originating from charge exchange in the inner heliosheath, dominate the H-atom flux in the 10–19 eV and 19–38 eV bands. The lowest energy band has a distinct “angel wings” feature caused by a selection effect that puts neutrals from the center of the plot into the next energy channel due to an energy boost associated with the hydrogen bulk flow. This selection effect also means that the second energy channel has interstellar neutrals focused close to the interstellar flow source. We find, however, that the peak H-atom flux in our idealized calculations is offset by about 4◦ in the plane of the interstellar magnetic field—as expected from Lyα studies of the SW (Lallement et al. 2005) as well as numerical investigations (Izmodenov et al. 2005; Pogorelov et al. 2008b). As discussed earlier, our assumption of straight trajectories renders this deflection result invalid for solar maximum conditions, and only approximately valid during solar minimum. We have shown that IBEX will be able to image the outer heliosheath using its lowest energy channels. The geometry of this region is strongly influenced by the LISM magnetic field, while the plasma properties are mediated by neutrals of both interstellar and solar origin. Neutrals born through charge exchange in the SW, for example, can move outward and charge exchange in the outer heliosheath, thereby heating this region. Primary interstellar neutrals, on the other hand, charge exchange in the outer heliosheath forming the “hydrogen wall” and, by depositing their momentum during such collisions, act to compress the heliosphere. On the basis of the model heliosphere used here, it is expected that the Voyager spacecraft will not cross the heliopause until 2022 (assuming that their RTG’s continue generating power until then), so that remote diagnostics of this region by IBEX will be our main source of data over the coming decade. This work was supported by NASA grants NNG06GD48G, NNX07AH18G, NNX08AJ33G, and NNX08AJ21G. Calculations were performed on the Columbia supercomputer at NASA

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