A New Instrument for Measuring the Near-Solar Neutral Atom Population

1. Abstract Neutral-atom physics in the heliosphere is still a relatively unexplored topic since we lack adequate in-situ data with which to form theories.  In particular, the formation of pick-up ions in near-solar regions depends on significant neutral populations that we so far have not detected except through optical means.  Unfortunately, current neutral atom detectors are generally large and heavy.  Microelectromechanical systems (MEMS) technologies allow for the creation of detectors of similar or greater sensitivity for far smaller mass, volume, and energy costs, ideal for spacecraft with tightly constrained design budgets.  We present here a Coke-can sized neutral-atom instrument design for Solar Probe that uses MEMS devices massing only a few grams. We measure neutrals ranging from thermal energies to 5keV/amu. Dust Cloud Science Motivation Interplanetary neutral atoms have two major sources: the interstellar medium and the so-called inner-source dust arising from the asteroids and comets. These populations, which vary significantly over heliolatitude and heliocentric distance, concern us because they are the sources of pickup ions and additionally may contribute significantly to the pressure in the inner heliosphere (Schwadron et al, 2000). While the interstellar-source neutrals can and have been deduced through in-situ measurements of pickup ion populations at 1-5 AU (Gloeckler et al, 2000, 2001, 2004 ApJ), the inner-source neutral population is far more difficult to determine. So far there have been no in-situ measurements near the sun, and thus we only have models to tell us about the neutral density, composition, and velocity. In-situ measurement of the neutral population at 4-20 Rs would provide a valuable test of models currently based only on remote-sensing (spectroscopy) and distant pickup ion measurements. Freestanding Grating • Figure 7: Grating Fabrication Process • Boron-dope support structure into bulk silicon • Deposit 0.5 μm Si3N4 layer • Back-etch bulk to free membrane and support structure • Laser-etch nitride membrane • Conformal deposition of conductor Alternate Detector: Faraday Cup Array • Figure 8: Cup Fabrication Process • Deep-etch bulk silicon • Grow oxide insulators • Conformal conductor deposition • Separate conductors • Rear-connection and circuitry (standard CMOS process) Instrument requirements • Solar wind and UV rejection • Full-range mass spectrum (non-stepped) • 5% mass resolution (H, He,C,N,O, Ne) • 1 minute time-resolution Conclusions Instrument Details The instrument presented here aims to answer a very specific science question: what is the inner source of pickup ions? The composition of the near-solar neutral population will answer this question. We have demonstrated that the interstellar neutrals comprise only a negligible fraction of the populations in the near-solar region, and thus any measured neutrals can be assumed to be of local origin. We have also demonstrated that the periapsis of Solar Probe’s orbit will be an ideal time to make the necessary measurements. We have designed an instrument that occupies only a volume similar to a Coke can and masses under 1 kg. The instrument will have only a short time to perform its measurements, and thus has been designed to have high time resolution. References • Gloeckler et al (2004), A&A, 426, 845 • Gold et al (2001), Plan. And Space Sci., 49, 1467-1479 • Gruntman, M. (1995), Applied Optics, 34, 25, 5732-5737 • Joglekar et al (2004), PNAS, 101, 16, 5856-5861 • Ruciński et al (1996), Space Sci. Rev., 78, 73-84 • Schwadron et al (2000), J. Geophys. Res., 105, 7465-7472 • Wilck and Mann (1996), Planet. Space Sci., 44, 493-499 • Wimmer-Schweingruber and Bochsler (2003), Geophy. Res. Lett., 30, 1077 • Burns et al (1979), Icarus 40, 1-48 • Bzowski and Krolikowska (2005), A&A, 435, 723-732 • Cranmer, S. (1998), Astrophys. J., 508, 925-939 • Cranmer et al (1999), Astrophys. J., 511, 481-501 • Darling et al (2002), Sensors and Actuators A, 95 • Gloeckler et al (2000), J. Geophys. Res., 105, 7459-7463 • Gloeckler and Geiss (2001), Space Sci. Rev., 97, 169-181 • Gloeckler et al (2004), Astrophys. J., 604, L121-L124 The authors would like to thank for their help Dr. John Raymond of Harvard University and Robert Lundgren, Dr. Patrick Koehn, Dr. Susan Lepri, Jason Gilbert, and David Sigler of the University of Michigan. This work is supported by award number NNG04GL44H of the Graduate Student Research Program of the National Aeronautics and Space Administration. A New Instrument for Measuring the Near-Solar Neutral Atom Population P. Mukherjeea, T. Zurbuchena, and F. Herrerob aDepartment of Atmospheric, Oceanic, and Space Science The University of Michigan, Ann Arbor, MI 48109 bDetector Systems Branch Goddard Space Flight Center, Greenbelt, MA 20771 Mission Specifics Enabling Technologies Figure 3: Solar Probe will swing around the sun, approaching as close as 4 solar radii at periapsis. The heat-shield will always face the sun, protecting the instruments. The proposed instrument will be aimed to pick up the maximum flux at periapsis as seen below. The dust cloud surrounding the sun roughly follows 7:1 aspect ratio ellipsoids of constant density, with a profile proportional to 1/r along the ecliptic plane. The grating will have 50-70 nanometer slits, which will block Lyman-alpha photons while allowing particles through. In addition, it’s likely that a significant fraction of those particles will be ionized via tunneling interactions with the slit walls. This behavior will require testing. Figure 4: As Solar Probe approaches the sun, its velocity increases monotonically until periapsis at 4 solar radii. In addition, it is rapidly moving into denser regions of the dust cloud (Fig. 3). flux=density*velocity The very sharp spike in the flux at periapsis should allow for short integration times for neutral population measurements. • Figure 1: The inner-source neutral density is far higher than the interstellar-source density in regions very close to the sun. Assumptions for these profiles are: • Interstellar neutrals follow an e-λ/r falloff from the termination shock, where λ=u/τ; τ=1.08*10-7 s-1 is the ionization rate (Ruciński et al, 1996) • Inner-source helium density taken as 5% of the hydrogen density (Cranmer et al, 1998) as per solar wind ratios as a worst-case. Efforts to further miniaturize the design may involve the use of this microfabricated faraday cup array (based on Darling et al, 2002). These individually-addressable cups are essentially just deep-trench capacitors with very low cup-to-cup and high cup-to-ground capacitances. Figure 2: Dust grains follow Keplerian orbits, and near the sun these orbital velocities can rise to a significant fraction of the solar wind speed. where G is the gravitational constant and β is the ratio of radiation and gravitational forces (Burns et al, 1979). We assumed circular orbits, 1μm dust grains, and solar minimum conditions at the ecliptic. If the neutrals come from the dust grains, and after ionization are picked up by the solar wind, they still have a large azimuthal velocity component. Open Questions: Is the inner source primarily the sungrazing comets, the asteroids, or both? Is the velocity of the dust source important to the neutral and/or pickup ion distributions? Figure 6: 2-D instrument diagram. The time-of-flight region is essentially the same as that of FIPS (Gold et al 2001) with modified voltages and the UV grating is described in Fig 7. Figure 5: 3-D instrument view. The device is clamped to the spacecraft on the bottom, and the aperture is aimed 31 degrees off-axis for ideal periapsis measurements.