UCL DEPARTMENT OF SPACE AND CLIMATE PHYSICS MULLARD SPACE SCIENCE LABORATORY Modelling Cometary Sodium Tails Kimberley S Birkett ([email protected]), Geraint H Jones, Andrew J Coates Mullard Space Science Laboratory, University College London, UK. The Centre for Planetary Science at University College London/Birkbeck, UK. 1. Introduction 3. Model Outline 5. Comet ISON (C/2012 S1) Preliminary Model • Neutral sodium emission first detected in 1882. • New, fully heliocentric distance dependent cometary sodium Monte-Carlo model including Results • First neutral sodium tail detections: for the first time the crucial parameter of cometary orbital motion. • Results for Earth (Celestial Sphere) § Newall et al, 1910 – Comet 1910a – Extended D emission (no tail reported) ) • Heliocentric velocity dependence of radiation pressure (due to 2 § Nguyen, Huu, Doan, 1960 – Comet Mrkos (C/1957 P1) – 7 degree tail § Collision Radius: Variable (see panel 3) resonance fluorescence in the sodium D lines) acting on reported (no discussion of morphology, only spectra) § Time step: Hours sodium atoms in simulation is characterised by velocity § Number of neutral sodium atoms produced per time step= 1000 (constant) § Cremonese et al, 1997 – Comet Hale-Bopp (C/1995 O1) dependent acceleration, in the same manner as that used § Solar disk is visible near perihelion Combi et al, by Combi et al. 1997 Acceleration (cm/s • Uses collision radius, given by (Whipple et al, 1976): Heliocentric Velocity (km/s) Cremonese et al, Q σ R 1997 coll coll : Collision Radius (cm) v : Outflow velocity (cm/s) Rcoll = -1 2 Q : Production Rate (s ) σcoll : Collision Cross Section (cm ) 4⇡v Orange Filter Image Ion Filter Image § Inside collision radius sodium atoms travel radially outwards (to consider collisional Wide field CoCam images of Comet Hale-Bopp (C/1995 O1) effects within dense part of coma). • Morphology of sodium tails characterised by heliocentric velocity dependent § Outside collision radius sodium atoms feel effects of radiation pressure. radiation pressure. • Collision radius estimated for different heliocentric Collision Radius Estimate for Comet ISON 18.00 27/11/2013 12.00 02/12/2013 • Radiation pressure is dependent on heliocentric velocity due to presence of strong distances, r , using simple scaling arguments sodium D absorption lines in the solar spectrum, i.e. Doppler shift of incident (Combi et al, 2012; Swamy et al, 2010). • Results for SOHO/LASCO C3 Coronagraph 4 1 3.5 radiation directly determines the radiation pressure it exerts on the cometary § Greater than 1AU: Q r − ; v r − 2 ; R coll r− § Collision Radius: Variable (see panel 3) / 2 / 1 / 1.5 sodium atom. § Less than 1AU: Q r − ; v r − 2 ; Rcoll r− § Time step: Hours / / / § Less than 0.8AU: Scaling arguments break § Number of neutral sodium atoms produced per time step= 1000 (constant) § Outer circles (blue) indicate SOHO/LASCO C3 field of view. down at small heliocentric distances and § Inner circles (green) indicate SOHO/LASCO C2 field of view. log (collision radiusin km) therefore approximate collision radius at these § Offset between model and angle due to solar rotation angle. positions as a constant approximately equal to log (heliocentric distance in AU) the width of the sodium tail observed at comet McNaught (C/2006 P1). • Average lifetime of photoionisation: ⌧ =1 . 69 10 5 s (~2 days) at 1AU from theoretical values, based on conclusions from previous sodium⇥ tail models contradicting experiments (Combi et al, 1997; Heubner et al, 1993) • Initially built on model developed by Brown et al (Brown et al, 1998). • Considers weightings due to three different brightness effects important in characterising • Unique evolution of sodium tails can result in interesting features: e.g. Sunward the morphology of the neutral sodium tail: 12.00 28/11/2013 00.00 30/11/2013 Stagnation Region 1. Neutral sodium lifetime, where t is the age of the sodium atom and ⌧ is its average lifetime (Brown et al, 1998) 2. Variation of sodium emission due to velocity/acceleration spectrum § Preliminary analysis of LASCO 3. Solar flux variation with heliocentric distance (dist) data has not revealed a distinct 2. Why Sodium? sodium tail. • Source of sodium in comets is currently unknown. Various authors have suggested: t 1 § Analysis is on-going. § Dust tail (Wilson et al, 1998; Rauer et al., 1998) C = R exp 2 § Combinations of Dust and Nuclear/Near-Nuclear Sources (Brown et al., 1998; (dist) Orange Filter C3 Image Blue Filter C3 Image 1 ⇥ ⌧ 2 3 11.4912.00 28/11/2013 28/11/2013 00.00 11.5530/11/2013 28/11/2013 Cremonese et al., 2002) ✓ ◆ *Curved box around current comet nucleus position (20 degrees in Right Ascension and Declination) indicates edge of simulation § Combination of Plasma and Nuclear/Near-Nuclear Sources (Combi et al, 1997) region • Strong neutral sodium doublet (resonance fluorescence) transition is easy to detect, 4. Comet Hale-Bopp (C/1997 O1) Preliminary Model even when column density is low. Results 6. Conclusions/Further Work • Sodium elsewhere in the solar system has been used as a tracer for chemical and • Modelling is vital to understanding sodium tail observations. physical processes that are hard to detect directly. Sodium has been detected in: Model Result 21.00 16/04/1997 • Successful start in producing fully heliocentric distance dependent model. § Earth’s atmosphere - thought to have been deposited Collision Radius: 4.e4 km (as • Model parameters need to be finely calibrated to comets of different sodium by meteorite impact used by Brown et al, 1998) production rates (including variable production rates) at different heliocentric § Earth’s moon (Matta et al., 2009) Time step: Hours distances. § Mercury (Baumgardner et al., 2008) Number of neutral sodium atoms • Analysis of potential sodium tail observations at comet ISON are on-going. § Jupiter’s Moon: Io – gave evidence for shape of produced per time step= 1000 • In the future this model will be applied to other comets exhibiting sodium tail features plasma torus (Thomas, 1992; Mendillo et al., 2007) (e.g. comet McNaught) § Saturn’s Moon: Enceladus – sodium detected by CoCam image of Hale-Bopp • If interested in collaboration, please get in touch! [email protected] Cassini, but not in spectra taken from Earth, providing taken using the sodium filter. insight into plume formation (Postberg et al., 2009; Sodium tail of planet Mercury 21.08 16/04/1997 Acknowledgements: Birkett is supported by an RAS Travel Grant; Project supported by as viewed from Earth in 2007 STFC; CoCam image courtesy Hale-Bopp international team; LASCO images courtesy Schneider et al., 2009) (Baumgardner et al., 2008) *Curved box around current comet nucleus position (20 degrees in Right Ascension and Declination) indicates edge of simulation region of PI R. Howard (US Naval Research Laboratory) .