ions in the Saturnian Magnetosphere

Transcription

1 Main Rings Rhea Titan Enceladus torus Neutral H 2 and H 2 + ions in the Saturnian Magnetosphere Wendy Tseng 1, R. Johnson 1, M. Thomsen 2, T. Cassidy 3 and M. Elrod 1 1 University of Virginia, USA 2 Los Alamos, USA 3 JPL NRAO postdoc symposium, 11 April 2011

2 Outline To compare the morphology of H 2 + torus observed by Cassini Plasma Analyzer (CAPS) Neutral H 2 : H2 source rates H2 Lifetime Spatial H2 density distribution H 2 + ions: H2 + ion source rates H2 + ion density distribution

3 Rhea» Major neutral sources: 1. O 2 and H2 from the main rings 2. Water-group neutrals from Enceladus plumes and the inner icy satellites 3. N 2, CH 4 and H 2 from Titan s exosphere 3

4 Source Mechanisms: 1. Main Rings photolytic decomposition of water ice: H 2 :O 2 =2:1 (Johnson et al., 2006) ~2.0 x10 26 H 2 s -1 at Cassini Saturn Orbital Insertion in 2004 (solar incident angle = 24 at SOI) H 2 O 2 (Tseng et al. 2011) H 2 solar photons 4 O 2

5 Evidences of the Ring atmosphere/ ionosphere O + 2 O + Cassini - Ion and Neutral Mass Spectrometer (Waite et al., 2005) Cassini Plasma Analyzer (Tokar et al., 2005) Existence of the ring O2 atmosphere/ionosphere -- confirmed by Cassini 5

6 Uncertainty of the source rate: Recycling Effect From our simulated O2 + ion density (Tseng et al., 2010), a factor of 10 lower compared to CAPS data at SOI Enceladus-originated material (O, OH, O +, and OH + ) deposited on the rings to form O2 and H2 via grainsurface chemistry (Ip, 1995; Tseng and Ip, 2011) H 2 O 2 H 2 solar photons O 2 O 6 O

7 2. Enceladus water plumes photodissociation of H 2 O dominates in most regions H 2 O + hv -> H 2 + O We used the Enceladus water torus distribution (Cassidy and Johnson, 2010) to estimate the H2 source rate ~3x10 26 H 2 s -1 with an average H 2 O source rate 1.0x10 28 s -1 (Tseng et al.,2011 ) Uncertainty: variable H 2 O ejection rates in different times (4x x10 28 s -1 : Smith et al., 2010) 7

8 3. Rhea Cassini-CAPS Martens et al., (2008) O + 2 in the outer magnetosphere 8

9 Rhea: detection of O2/CO2 atmosphere Day-and-Night Asymmetry Cassini -- INMS Teolis et al.,

10 Rhea s exosphere and its H2 torus Source mechanism: radiolytic decomposition of surface ice by energetic plasma particles (H 2 :O 2 =2:1) Plasma energy deposition: from thermal and suprathermal electrons: Cassini plasma data -- Schippers et al (2008) from thermal ions and energetic ions: Cassini plasma data (Johnson et al., 2008; Dialynas et al., 2009) ~3.5 x10 25 H 2 s -1 (comparable to Teolis et al. 2010) Uncertainty: highly variable hot-electron density in the magnetosphere from time to time 10

11 4. Titan Composition: N2 (~95%), CH4 (~4%) and H2 (~1%) thermal escape ~ 1 x10 28 H 2 s -1 (Cassini-INMS; Cui et al., 2008) Uncertainty: a factor of 2 (different model interpretation, Strobel et al., 2008 ) 11

12 H2 Lifetime -- To constraint the morphology of the H2 cloud Chemistry: Photo-dissociations & ionization Electron-impact ionizations and dissociations Charge-exchange reactions Plasma Environment: The W + and H + ions as well as the cold/hot electrons from Cassini in-situ measurements (Persoon et al.,2008; Wilson et al., 2008; Sittler et al., 2008; Schippers et al., 2008)

13 Neutral H 2 lifetime at equator in unit of Saturn s radius

14 Neutral H 2 lifetime at equator in unit of Saturn s radius

15 Neutral H 2 lifetime at equator Peak density of hot electron in unit of Saturn s radius

16 Neutral H 2 column density (#/cm 2 ) azimuthally averaged Main Rings uncertainty for source rate Enceladus Titan Rhea? in unit of Saturn s radius

17 Trapped in the main rings! 1. Re-emit in thermal velocity H 2 Titan H 2 2. Very long lifetime Rhea

18 Morphology of H 2 + torus observed by CAPS H 2 column density / ionization time = H 2 + ion source rate (no plasma diffusion) Main Rings Enceladus Titan Rhea

19 H 2 + ionization source rate at equator (#/cm 2 /s) Large ionization rate Titan Neutrals have long lifetimes here and trapped by the rings

20 Compare to the Cassini plasma data Plasma Diffusion!! Thomsen et al Tseng et al.,

21 Compare to the CAPS data Add the plasma diffusion the plasma residence time of ~ 2 days (Chen et al., 2010) in inner magnetosphere: the H2 + ion column density is ~1.5x10 10 cm -2 around Rhea Assuming 0.5 Saturn rotation period as the average lifetime at Titan s orbit, we obtain a H2 + ion column density of ~1.2x10 9 cm -2 Both are in agreement with the CAPS data (Thomsen et al., 2010; Tseng et al., 2011) 19

22 Summary Neutral H 2 lifetime: highest in the main rings (photo-processes) lowest around 9.0 R S (peak density of hot electron) Neutral H 2 : Main rings and Enceladus water torus: the primary sources of H 2 in inner region Titan: a significant source in the outer magnetosphere H 2 + ionization source rates 3 peaks located in main rings, Rhea and Titan The estimated H2 + ion densities are in agreement with the CAPS data.