sign in sign up
<<
Home , Energetic neutral atom

International Workshop on Instrumentation for Planetary Missions (2012) 1032.pdf

A NEUTRAL INSTRUMENT FOR IO OBSERVATIONS AND OTHER PLANETARY APPLICATIONS. M. R. Collier1, J. W. Keller1, and M. D. Shappirio1, 1NASA’s Goddard Space Flight Center, Greenbelt, Maryland 20771

Introduction: We describe an approach to re- remotely the Jovian Io torus from even motely image the dynamics of the Io plasma torus us- as far as outside of the Jovian . ing low energy neutral . Such a capability will Instrument Description: The Jovian low energy improve understanding of the interrelation between neutral atom instrument concept is shown in Figure 1. volcanic, plasma torus, and Jovian magnetospheric The top section of the instrument consists of a passive mass and energy-exchange processes. conversion surface neutral atom imager with a 360 Charge Exchange and Energetic Neutral Atom degree field-of-view. In this design, energetic neutral (ENA) Imaging: Energetic neutral atoms (ENAs) are atoms enter the instrument and hit a venetian-blind produced when an energetic (greater than a few eV assembly of passive conversion surfaces at a small - for example, a corotating ion in the Io plasma torus) angle. undergoes a charge exchange reaction with a cold neu- The venetian-blind assembly increases the effective tral atom (such as the neutral and that surface area of the passive conversion surfaces. A frac- populate the torus). The cold neutral atom gives up an tion of the incident neutral atoms then become con- to the ion, neutralizing it. As a result, a cold verted to negative ion and an energetic neutral atom are created. The re- and traverse a torroidal sulting ENA is no longer confined by the magnetic analyzer, for energy field and, in the case of charge exchange in the Jovian measurement (shown in Io torus, has sufficient energy to escape ’s grav- purple in Figure 1). The ity. The energetic neutral atom travels ballistically negative ions then in- teract with a thin car- away from the location of the charge exchange reac- bon foil to generate tion which is generally energy and con- secondary serving. Thus, measurement of ENA fluxes allows us that are steered to hit an to image remotely plasma populations that in the past array of six microchan- had to be studied in-situ. nel plates to produce a In particular, ENA imaging allows (i) direct obser- start signal and to de- vation of charge exchange processes that result in the termine the direction of transfer of both mass and energy, for example the in- the incoming neutral troduction of cold ions into the Io torus via charge ex- atom. The ions upon Figure 2 - The MINI-ME instru- change, (ii) remote monitoring of plasma populations exiting the carbon foil ment (upper near corner of space- from a distance, for example outside of the Jovian craft) on the FASTSAT spacecraft fly through the time-of- prior to launch. magnetosphere, (iii) the acquisition of global informa- flight chamber at the tion about plasma populations, for example, their time bottom of the figure and are turned around in a quad- and spatial variability and ratic potential which, like an harmonic oscillator, pro- (iv) the ability to study duces a time-of-flight (i.e. period of oscillation) inde- fundamental plasma pendent of energy but dependent on the mass of the through temporal incident ion. The ions then hit a stop microchanel plate variations in ENA flux, allowing a determination of their time-of-flight and for example the effects of hence mass. wave processes [1]. As described above, the Io ENA employs In the case of the Jo- a quadratic potential high resolution mass analyzer (the vian Io plasma torus, chamber at the bottom in the concept figure). The high ENA imaging will allow resolution mass spectrometer which takes the place of the simple detector plane used on previous ENA in- us to study the torus struments represents a major advance for this type of morphology, composi- instrumentation. Its performance has been electrostati- tion, and time-variability cally modeled using simion code. The simulations remotely. Here we dis- show that for a linear electric field design we will Figure 1 - Concept for a low cuss an instrument con- energy neutral atom imager for achieve M/ΔM~50, remarkably good for an energetic observing Io torus composition cept based on flight- neutral atom imager (as opposed to a thermal quad- and variability. proven technology to International Workshop on Instrumentation for Planetary Missions (2012) 1032.pdf

rupole mass analyzer), and necessary for separating molecular species. Estimate of Iogenic Neutral Atom Flux and In- strument Count Rate: Eviatar and Barbosa [2] based on charge exchange of heavy ions in the Io torus calcu- lated a creation rate Sn for fast (~70 km/s) neutral at- 28 -1 oms from the Io torus of Sn=5.7x10 s . They model this oxygen and sulfur neutral wind as flowing out in the form of a cylinder of total height 2 RJ based on the geometry of the torus. Thus, if the neutral atom camera were observing the Io torus from outside the Jovian magnetosphere at even as far as 100 RJ, the neutral 5 2 flux, ΦN~Sn/2π/(100RJ)/(2RJ)=9x10 /cm /s. This is a very large flux and a very observable flux - for com- parison, it is about six times the oxygen flux [3]. The top end of the Io ENA telescope concept Figure 3 - Data from the March 31, 2011 event including shown in Figure 1 has flight heritage based on the MINI-ME neutral atom data, PISA electrometer data, and Miniature Imager for Neutral Ionospheric atoms and AMPERE current profiles. Magnetospheric Electrons (MINI-ME) instrument the MINI-ME calibration data, will allow us to infer launched on NASA’s FASTSAT-HSV01 (Fast the presence of molecular neutral atoms from the Io Affordable Science and Technology Satellite torus at a high cadence while the time-of-flight unit Huntsville-01 mission) by the DoD Space Test Pro- will provide composition information on the observed gram-S26 (STP-26) as a secondary payload from molecules. Kodiak, Alaska on 19 November 2010 [4]. MINI-ME, shown on the FASTSAT spacecraft in Figure 2, has The top end of the Io ENA camera shown in Figure collected data in full science mode as of the time of 1 is very similar to MINI-ME and provides compel- this writing for over 18 months. It responds to neutral ling, flight-based, evidence for the soundness of the atoms in the energy range from a few eV up to about design. Based on the calibration of the 700 eV, although the same design could go signifi- FASTSAT/MINI-ME instrument, the total instrument efficiency (including conversion, transmission, and cantly higher in energy. Note that energetic neutral -3 2 oxygen from the Io torus will be at about 300 eV and MCPs) is ~10 , and the aperature size is ~1 cm for a energetic neutral S from the Io torus will be at about single angular sector. Thus, the count rate, RN, ob- served by the Io ENA camera in the sector facing the 600 eV, well within the energy range of the MINI-ME 5 2 -3 instrument. Io torus at 100 RJ would be RN~9x10 /cm /s•10 •1 cm2 = 900 counts/s. This rate would, of course, be The first science results from the FASTSAT/MINI- larger at closer distances. ME instrument were reported at the Fall American Geophysical Union 2011 meeting [5]. One of the Assuming the efficiency of the time-of-flight unit is strengths of the MINI-ME design that will prove par- about 10%, this means even at 100 RJ the Io ENA ticularly useful for observing energetic Iogenic mole- camera would be observing about 100 counts per sec- ond in the time-of-flight spectrum, statistics that would cules such as SO2 is its ability to detect not only ele- ments but also molecules. Because some of the inci- make it extremely easy to observe time variability in dent energetic molecules dissociate on the highly pol- the composition of the Io torus and link it to both sub- ished tungsten conversion surfaces and the dissociation sequent Jovian magnetospheric activity as well as geo- results in the products moving at the same velocity as logic activity on Io itself. the original molecule, MINI-ME observes distinct References: [1] Espley J. R. et al. (2005) J. Geo- peaks at energies in the ratios of the individual specie phys. Res., 110, A09S33, doi: 10.1029/2004JA010935 mass to the molecular mass. For example, the top two [2] Eviatar A. and Barbosa D. D. (1984) J. Geophys. plots in Figure 3 show flight spectra from a MINI-ME Res., 89, 7393. [3] Bame S.J. et al. (1975) Solar Phys., pass of the aurora oval, based on FASTSAT/PISA and 43, 463-473. [4] Rowland, D. E. et al. (2011) IEEEAC AMPERE data shown in the lower figures. In particu- paper #1425. [5] Collier, M. R. et al. (2011) AGU Fall lar, the spectrum on the right shows two distinct peaks Meeting Abstract SM31A-2087. with energies of about 100 eV, the dissociated oxygen, Acknowledgments: This work was supported in and 200 eV, the undissociated O2. This characteristic part by the Proposal Design Lab program of the Plane- response to neutral molecules, which was observed in tary Division at Goddard Space Flight Center.