Oxygen ions observed near Saturn's A ring

Science

Volumn 307, Issue 5713, 2005, Pages 1260-1262

  Waite Jr , J H ^a   Cravens, T E ^b   Ip, W H ^c   Kasprzak, W T ^d   Luhmann, J G ^e   McNutt, R L ^f   Niemann, H B ^d   Yelle, R V ^g   Mueller Wodarg, I ^h   Ledvina, S A ^e   Scherer, S ^a  

^a UNIVERSITY OF MICHIGAN   (United States)

^b UNIVERSITY OF KANSAS   (United States)

^c NATIONAL CENTRAL UNIVERSITY   (Taiwan)

^d NASA GODDARD SPACE FLIGHT CENTER   (United States)

^e UNIVERSITY OF CALIFORNIA   (United States)

^f JOHNS HOPKINS UNIVERSITY   (United States)

^g UNIVERSITY OF ARIZONA   (United States)

^h IMPERIAL COLLEGE LONDON   (United Kingdom)

Author keywords

[No Author keywords available]

Indexed keywords

IONOSPHERE; IONS; MASS SPECTROMETERS; MOLECULES; OXYGEN; PHOTOIONIZATION; PROTONS; SIGNAL PROCESSING; SOLAR RADIATION; SPACECRAFT;

BACKGROUND SIGNALS; ION AND NEUTRAL MASS SPECTROMETER (INMS); OXYGEN IONS;

PLANETS;

OXYGEN;

ION; OXYGEN; SATURN;

ARTICLE; ASTRONOMY; ATMOSPHERE; ION NEUTRAL MASS SPECTROMETER; MASS SPECTROMETER; PRIORITY JOURNAL; SIGNAL PROCESSING; SPACE FLIGHT;

ATMOSPHERE; EXTRATERRESTRIAL ENVIRONMENT; ICE; IONS; MASS SPECTROMETRY; OXYGEN; PROTONS; SATURN; SPACECRAFT;

EID: 20044378288     PISSN: 00368075     EISSN: None     Source Type: Journal    
DOI: 10.1126/science.1105734     Document Type: Article

References (25)

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11. + is seen at mass 31 as well as at 32. This is not a problem for lower mass numbers or for lower speeds, such as the 6-km/s ram speed (i.e., head-on gas speed) that the INMS will experience at Titan.
12. Ions created in the ring-plane environment from the ionization of neutral species moving at the local keplerian speed (≈15 km/s at the outer A ring) respond to the saturnian magnetic and corotation electric fields by both gyrating about the magnetic field and moving around the planet at the corotation speed (≈21 km/s at the outer A ring). In addition to this motion in the ring plane (which can be called horizontal), the ions should also have a small (≈1 km/s) velocity component along the magnetic field (approximately perpendicular to the ring plane, or vertical), which they inherit from the motion of the parent neutral atoms and molecules acquired during the surface ejection process and/or during gas interactions with the grains (1, 5, 6). The statistical distribution of these ions form a circle, or ring, in velocity space. This type of pick-up ion distribution is well known in other space environments (22, 23). Near the outer A ring, the gyration speed of a newly formed ion in the co-rotating frame of reference is 5 to 6 km/s (i.e., the ring radius). The ring distribution can evolve into other distributions (such as a shell distribution) resulting from collisions or wave-particle interactions associated with magnetic fluctuations or waves (22).
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14. 2, where v is measured in kilometers per second. For A = 1, 16, and 32 and v = 15 km/s (the INMS compensation speed on this occasion), E = 1.2 eV, 19 eV, and 37 eV, respectively. We then found count rates per unit ion flux for A = 1, 16, and 32 of 0.0045, 0.010, and 0.0240, respectively.
15. During its normal open-source (ions or neutrals) operation in the ionosphere and atmosphere of Titan, where the molecules are cold and the relative velocity is dominated by the spacecraft ram velocity, the entire distribution is within the instrument's compensation region volume (10), defined by the energy/velocity acceptance along the FOV and the velocity width perpendicular to this associated with the angular width of the FOV. However, during operations in the period after orbit insertion (over the A ring), only a small fraction of the ion distribution is captured in the compensation region.
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25. We thank R. Johnson and F. Crary for useful and insightful discussions and comments. We are also very grateful for the hard work and dedication of our engineering, technical, and operational staff: G. Fletcher, E. Walter, R. Miller, J. Xu, J. Parajeko, D. Gell, and R. Thorpe. We thank D. Eddy, I. Robertson, and T. Hunt-Ward for technical help with the figures and text Financial support from the NASA Cassini project is acknowledged.