Pickup Proton Instabilities and Scattering in the Distant Solar Wind and the Outer Heliosheath: Hybrid Simulations

Transcription

1 Pickup Proton Instabilities and Scattering in the Distant Solar Wind and the Outer Heliosheath: Hybrid Simulations Kaijun Liu 1,2, Eberhard Möbius 2,3, S. P. Gary 2,4, Dan Winske 2 1 Auburn University, Auburn, AL Los Alamos National Laboratory, Los Alamos, NM University of New Hampshire, Durham, NH Space Science Institute, Boulder, CO AGU Fall Meeting, December 3-7,

2 2 The Heliosphere and the Interstellar Medium Plasma flows in the heliosphere and the outer heliosheath + PUIs accumulate in the solar wind flow with increasing distance from the Sun + Termination shock: Solar wind transition from super- to submagnetosonic flow + Solar wind neutrals can pass through heliopause and become PUIs in the outer heliosheath + Bow wave instead of bow shock: Recent results revealed that the interstellar plasma flow is sub-magnetosonic so it goes through a bow wave instead of a shock as it approaches the heliopause [McComas et al., 2012] Artwork by Giacomo Marchesi

3 3 Pickup ions in the solar wind Interstellar medium: + Cold plasma, mostly protons+electrons + Cold neutral gas, mostly hydrogen Interstellar plasma flows around heliosphere, but interstellar gas flows into heliosphere Interstellar neutrals charge exchange with solar wind ions and leads to fast pickup ions in the solar wind Artwork by Giacomo Marchesi

4 4 Pickup ions in the heliosheath Pickup ions in the solar wind and heliosheath are sources of energetic neutral atoms (ENAs), including some directed back toward Earth IBEX in Earth orbit; measures ENAs from charge exchange in + Inner Heliosheath + Interstellar medium Artwork by Giacomo Marchesi

5 5 IBEX Observations The prime result from IBEX is a ribbon of intense ENAs + Correlated with the direction of the interstellar magnetic field + Stable over several years with small-scale variations <1 year (From McComas et al., 2009)

6 6 IBEX Ribbon: Six Possible Sources No well-accepted model for the ribbon yet Challenge: Successful model must explain correlation with interstellar magnetic field (From McComas et al., 2010)

7 7 Secondary ENA mechanism Involves three charge-exchange processes: 1. Solar wind ions become outward-directed solar wind ENAs 2. Solar wind ENAs flow outward across the heliopause, become interstellar pickup ions 3. Interstellar pickup ions become inward directed ENAs, which flow back across the heliopause toward Earth Major problem: 1. Interstellar pickup ions form thin velocity-ring distribution 2. Proton ring cyclotron instability scatters and broadens pickup ion distribution 3. Only a narrow velocity-ring distribution can yield a narrow ribbon in ENAs [Herrikhuisen et al., 2010] 4. Florinski et al. [2010] shows scattering of interstellar pickup ions is fast, making it difficult for the secondary ENA mechanism

8 8 Objectives We use the Los Alamos hybrid simulation code to study pickup proton instabilities and scattering in both + Solar wind + Interstellar medium * Results published in JGR: Liu et al., 2012

9 9 Simulation Plasma Parameters Eberhard Möbius, at LANL on sabbatical from UNH, has developed an analytic model for pickup ion parameters in the solar wind An analytic model of the IBEX ribbon with neutral solar wind based ion pickup beyond the heliopause has also been developed, which provides pickup ion parameters in the present study for the outer heliosheath (See Poster SH11A-2184 by Möbius et al. for details)

10 10 Results: A Representative Simulation Top: Instability development shows 4 phases: 1. Quiescent (insufficient PUI density) 2. Exponential growth 3. Linear temporal growth 4. Quasi-steady Bottom: mean square pitch angle change, < 2 >, of all the PUIs reveals the pitch angle scattering + Scattering is insignificant during the quiescent phase + PUIs are substantially scattered during the exponential growth phase + Scattering onset density is defined as the PUI density when < 2 > reaches 0.2 ( ~25.6 o )

11 11 Results: A Representative Simulation Evolution of PUI velocity distribution + PUIs remain close to a ring distribution during the quiescent phase + PUIs are substantially scattered during the exponential growth phase + PUI distribution continues to broaden toward a fully isotropic shell in the linear temporal growth phase

12 12 Results: A Representative Simulation Pitch-angle scattering of the newly-injected pickup ions is quick and steady during the linear temporal growth phase Comparable scattering rate during the linear temporal growth phase is consistent with the relatively unchanged wave energy spectra at k i >0.1 Scattering time is defined as the time when < 2 > reaches 0.2

13 13 Results: Energy Conversion Factor Linear temporal growth phase Energy conversion factor, pu : How much newly injected PUI kinetic energy is converted to wave energy + Insensitive to injection rate, injection speed + Decreases with background plasma beta: warmer plasma leads to stronger cyclotron damping pu ~0.01 suggests that ( B/B) 2 ~0.3 upstream of the termination shock but ( B/B) 2 <10-4 at 1AU

14 14 Results: Scattering Time and Onset Density Scattering time/rate in the linear temporal growth phase decreases/increases with injection rate Scattering onset density increases with injection rate Scaling relations predict that the scattering time in the outer heliosheath close to the heliopause is 10s of days and the scattering onset density is ~20% of the local PUI density

15 15 Relevance to the Secondary ENA Ribbon Mechanism The simulation follows a plasma parcel moving in the reference frame of the heliosphere As the plasma parcel approaches heliopause, pickup ions accumulate Pickup ions retain their ring velocity distribution in the quiescent phase when the plasma parcel is relatively far away from heliopause and contribute to the ENA ribbon Pickup ions are scattering toward isotropy once the exponential growth phase starts, which relates to the Florinski et al. [2010] calculation * See Poster SH11A-2184 by Möbius et al. for details. The results have been submitted to ApJ: Möbius et al., 2012

16 16 Conclusions Accumulated PUIs in the solar wind and in the outer heliosheath drive enhanced magnetic fluctuations and lead to pitch angle scattering of the PUIs Significant scattering occurs after the accumulated PUI density exceeds a critical scattering onset density, which increases with PUI injection rate Scattering rate during the linear temporal growth phase is quick and steady Significant scattering of PUIs in the outer heliosheath occurs in a relatively limited spatial range close to the heliopause. PUIs beyond this region retain their ring velocity distribution and can contribute to the ENA ribbon

17 17 Future Work Scattering is still fast in the outer heliosheath. But this is a preliminary study and lots of important factors need to be included in the future The inherent background magnetic turbulence neglected can scatter the PUIs, but also might act as a damping for the instabilities [Florinski et al., 2010; Gamayunov et al., 2010] The 1-D nature of the simulations excludes the effects of obliquely-propagating ion cyclotron waves. The narrow velocity ring may also broaden in the direction perpendicular to B due to lower hybrid waves or ion Bernstein waves Injection of PUIs from other sources neglected could provide further damping: a hotter neutral distribution that stem from inner heliosheath subsonic solar wind and from PUIs in the supersonic SW + Charge exchange between interstellar neutrals and interstellar plasma ions Uncertainty due to extrapolation of the simulation results calls for an analytic model like in Lee and Gary, 1991 The new view of bow wave instead of bow shock may lead to changes in the plasma parameters in the outer heliosheath and modifies the present analysis

18 18 References Florinski, V., G. P. Zank, J. Heerikhuisen, Q. Hu, and I. Khazanov (2010), Stability of a pickup ion ring-beam population in the outer heliosheath: Implications for the IBEX ribbon, Astrophys. J., 719, Gamayunov, K., M. Zhang, and H. Rassoul (2010), Pitch angle scattering in the outer heliosheath and formation of the Interstellar Boundary Explorer ribbon, Astrophys. J., 725, Heerikhuisen, J., et al. (2010), Pick-up ions in the outer heliosheath: A possible mechanism for the Interstellar Boundary Explorer ribbon, Astrophys. J., 708, L126, doi: / /708/2/l126. Lee, M. A. and S. P. Gary (1991), Quasi-linear evolution of ULF waves excited by cometary ion pickup, J. Geophys. Res., 96(A12), 21,319 21,327, doi: /91ja Liu, K., E. Möbius, S. P. Gary, and D. Winske (2012), Pickup proton instabilities and scattering in the distant solar wind and the outer heliosheath: Hybrid simulations, J. Geophys. Res., 117, A10102, doi: /2012ja McComas, D. J., et al. (2009), Global observations of the interstellar interaction from the Interstellar Boundary Explorer (IBEX), Science, 326, , doi: /science McComas, D. J., et al. (2010), Evolving outer heliosphere: Large-scale stability and time variations observed by the interstellar boundary explorer, J. Geophys. Res., 115, A09113, doi: /2010ja McComas, D. J., et al. (2012), The heliosphere s interstellar interaction: No bow shock, Science, 336, , doi: /science Möbius, E., K. Liu, H. O. Funsten, S. P. Gary, and D. Winske (2012), Analytical model of the IBEX ribbon due to neutral solar wind based ion pickup outside the heliospheric boundary, Astrophys. J., submitted. Stone, E. C. (2001), News from the edge of interstellar space, Science, 293, 55 56, doi: /science Zank, G. P. (1999), Interaction of the solar wind with the local interstellar medium: A theoretical perspective, Space Sci. Rev., 89, , doi: /a: