Fluxes of energetic neutral helium atoms from the heliosheath … · 2014. 8. 19. · s sr keV LD...
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Fluxes of energetic neutral helium atoms from the heliosheath and the IBEX Ribbon Pawel Swaczyna, Stan Grzedzielski, and Maciej Bzowski Space Research Centre of the Polish Academy of Sciences, Warsaw, Poland EGU2014-223 Introduction Full sky maps of energetic neutral hydrogen atoms (H ENA) obtained with IBEX, revealed a bright, arc-like Ribbon, which dominates over the heliosheath emis- sion on large swaths of the sky. Potentially, the helium ENA emission could give complementary information about the heliosphere and its environment. Helium binary interactions Hydrogen → important charge-exchange with protons (H + +H 0 → H 0 +H + ) → other: almost negligible if the energy is small . 100 keV Helium: I three charge-states He 2+ (α-particles), He + and He 0 I a number of different reactions that change charge state of He ion or atom, including ionization (ion), charge-exchange (cx), double charge-exchange between He 2+ and He 0 (2cx) I contributions depends on assumed conditions in plasma (different in the inner heliosheath, interstellar medium, and hot interstellar bubbles) For energy . 10 keV the mean free path (m.f.p.) against He ENA ionization is up to one order of magnitude longer than the m.f.p. of H ENA. For assumed Local Interstellar Medium (LISM) conditions: n H = 0.194 cm -3 ,n p = 0.056 cm -3 ,n He = 0.0153 cm -3 ,n He + = 0.0096 cm -3 , m.f.p. of He ENA: 7800 AU; H ENA 870 AU at E = 5 keV. He 2+ He + He 0 ion cx H + ion He 0 ion H 0 cx He + cx H 0 cx H + cx He + ion H 0 cx He 0 cx H 0 ion 2cx He 0 0.1 1 10 100 1000 100 1000 10 4 10 5 10 6 Energy keV Mean free path AU ion H cx H cx He ion He He ENA H ENA Figure 1. Left panel – diagram presenting the important binary interactions in the heliosphere and LISM. Right panel – the m.f.p. against H/He ENA ionization in the LISM. For He ENA, contributing reactions are presented by color lines. Heliosheath signal (Grzedzielski et al. 2013 & 2014) To asses the inner heliosheath contribution to He ENA fluxes we use a simple axisymmetrical analytical model by Suess & Nerney (1990): I circular termination shock (TS) at 94 AU (as measured by Voyager 2) I distance to heliopause at the Voyager 1 trajectory: 121 AU I plasma density in the inner heliosheath 0.002 cm -3 I post-termination-shock bulk plasma velocity 150 km/s I He ion spectra consist of solar wind particles and pick-up ions (PUI) → PUI – assumed κ-distribution consistent with high energy Voyager data 0° 30° 60° 90° 120° 150° 180° 0.01 0.1 1 10 100 Angular distance from the nose Flux ENAcm 2 s sr keV 0.5 keV 1 keV 2 keV 5 keV 10 keV 20 keV Figure 2. Simulated He ENA intensities from the inner he- liosheath as functions of an- gular distance from the nose (the symmetry axis). For en- ergy 100 - 200 keV the re- sults are consistent with the HSTOF measurements. IBEX Ribbon – two hypothetical sources (Swaczyna et al. 2014) Secondary ENA Model B 1 2 3 4 5 6 7 Heliosphere Local Interstellar Medium Extraheliospheric Model Local Interstellar Cloud Local Bubble Contact layer Heliosphere Production of ENA in the LISM just outside the he- liopause in the direction where the line-of-sights are perpendicular to the direction of the magnetic field. Production of ENA in the contact layer between the Local Interstellar Cloud (LIC) and a hypothetical bay of the Local Bubble (LB). Mechanism The solar wind ions (1) are neutralized inside the TS (2) and create a flux of primary ENA (3). The primary ENA are ionized (4) and became PUIs (5), which after subsequence neutralization (6) produce a thin disk of secondary ENA emission (7). The neutral hydrogen and helium atoms from the LIC evaporate into the LB. Part of the suprathermal ions in the LB after neutralization on the neutrals are a source of the observed ENA. The Ribbon is a geo- metric effect of different integration path lengths. Basic model Adapted analytical model by M ¨ obius et al. (2012) with heliolatitude dependence of the solar wind parame- ters (Sok ´ ol et al. 2013), the LISM densities as pre- sented in the section “Helium binary interactions”. Adapted analytical model by Grzedzielski et al. (2010), set (4) of parameters in the plane model. En- ergy dependence based on the hydrogen ENA mea- sured in the ribbon direction near the nose. Geometry Assumed small circle: center (λ, β)=(219.2 ◦ , 39.9 ◦ ); radius 74.5 ◦ , width 15 ◦ (Funsten et al. 2013) Governed by the extinction in the LIC, planar inter- face between the LIC and LB with the smallest dis- tance in the center of the Ribbon. Results – maps of expected He ENA flux in the two hypotheses 2 keV 90 60 30 0 30 60 90 120 150 180 150 120 90 60 30 0 30 60 RC V1 V2 Nose Tail 90 60 30 0 30 60 90 120 150 180 150 120 90 60 30 0 30 60 RC V1 V2 Nose Tail Flux ENAcm 2 s sr keV 0.1 1 0.2 0.3 0.5 2 5 keV 90 60 30 0 30 60 90 120 150 180 150 120 90 60 30 0 30 60 RC V1 V2 Nose Tail 90 60 30 0 30 60 90 120 150 180 150 120 90 60 30 0 30 60 RC V1 V2 Nose Tail Flux ENAcm 2 s sr keV 0.1 0.2 0.03 0.3 0.05 0.5 10 keV 90 60 30 0 30 60 90 120 150 180 150 120 90 60 30 0 30 60 RC V1 V2 Nose Tail 90 60 30 0 30 60 90 120 150 180 150 120 90 60 30 0 30 60 RC V1 V2 Nose Tail Flux ENAcm 2 s sr keV 0.01 0.1 0.02 0.2 0.03 0.05 Figure 3. Left column – the heliosheath signal with the secondary ENA Ribbon. Right column – the he- liosheath signal with the extraheliospheric Ribbon. Hydrogen Ribbon – comparison between data and models Extraheliospheric Secondary ENA IBEX Map 1-6 1.11 keV 90 60 30 0 30 60 90 120 150 180 150 120 90 60 30 0 30 60 RC V1 V2 Nose Tail 90 60 30 0 30 60 90 120 150 180 150 120 90 60 30 0 30 60 RC V1 V2 Nose Tail 90 60 30 0 30 60 90 120 150 180 150 120 90 60 30 0 30 60 RC V1 V2 Nose Tail Differential Flux ENAcm 2 s sr keV 0 50 100 150 200 250 2.73 keV 90 60 30 0 30 60 90 120 150 180 150 120 90 60 30 0 30 60 RC V1 V2 Nose Tail 90 60 30 0 30 60 90 120 150 180 150 120 90 60 30 0 30 60 RC V1 V2 Nose Tail 90 60 30 0 30 60 90 120 150 180 150 120 90 60 30 0 30 60 RC V1 V2 Nose Tail Differential Flux ENAcm 2 s sr keV 0 20 40 60 Figure 4. Comparison of the fluxes of hydrogen ENA observed by IBEX during the first 3 years (McComas et al. 2012) with the fluxes in the models of the Ribbon. The heliospheric ENA are not included in the models plots. Conclusions – expected properties of the He ENA signal Ratio (E ∼ 1 keV/nuc) Sec. ENA Extrahel. He-to-H ENA flux from the Ribbon 0.0001 0.01 The Ribbon-to-heliosheath He ENA flux 0.5 50 I The expected heliospheric signal is highly concentrated in the direction of the heliospheric tail: Flux(tail)/Flux(nose) ∼ 10 2 I If observed hydrogen atoms from the Ribbon are the secondary ENA then observation of the He ENA signal from the Ribbon is not likely due to too small amount of helium in the neutral solar wind. I If the Ribbon is produced as in the extraheliospheric model then the He ENA signal from the Ribbon dominates over the heliosheath signal except for the heliospheric tail and should be potentially easily detectable. Outlook I The long mean free path against ionization of keV He ENA in the LISM and low heliosheath signal in large part of the sky make He ENA a good candidate for studies of the LISM structure at distances comparable with the distance to the LIC edge (0.05 pc ≈ 10 000 AU, Redfield & Linsky 2000). I For two-dimensional sources (e.g. interfaces), also secondary ENA, i.e., pro- duced from the ionized and then neutralized primary ENA, should give a non-negligible contribution and be included into future considerations. References Funsten et al. 2013, ApJ, 776, 30 Grzedzielski et al. 2010, ApJL, 715, L84 Grzedzielski et al. 2013, A&A, 549, A76 Grzedzielski et al. 2014, A&A, 563, A134 McComas et al. 2012, ApJS, 203, 1 M¨ obius et al. 2013, ApJ, 766, 129 Redfield & Linsky 2000, ApJ, 534, 825 Sok ´ ol et al. 2013, Solar Phys. 285, 167 Suess & Nerney 1990, JGR, 95, 6403 Swaczyna et al. 2014, ApJ, 782, 106 This work was supported by the Polish National Science Centre grant 2012-06-M-ST9-00455. http://www.cbk.waw.pl/en http://www.cbk.waw.pl/ ∼ pswaczyna [email protected]
Transcript of Fluxes of energetic neutral helium atoms from the heliosheath … · 2014. 8. 19. · s sr keV LD...
Fluxes of energetic neutral helium atoms from the heliosheath and the IBEX RibbonPaweł Swaczyna, Stan Grzedzielski, and Maciej Bzowski
Space Research Centre of the Polish Academy of Sciences, Warsaw, Poland
EGU2014-223
Introduction
Full sky maps of energetic neutral hydrogen atoms (H ENA) obtained with IBEX,revealed a bright, arc-like Ribbon, which dominates over the heliosheath emis-sion on large swaths of the sky. Potentially, the helium ENA emission could givecomplementary information about the heliosphere and its environment.
Helium binary interactions
Hydrogen→ important charge-exchange with protons (H+ + H0→ H0 + H+)→ other: almost negligible if the energy is small . 100 keV
Helium:I three charge-states He2+ (α-particles), He+ and He0
I a number of different reactions that change charge state of He ion or atom,including ionization (ion), charge-exchange (cx), double charge-exchangebetween He2+ and He0 (2cx)
I contributions depends on assumed conditions in plasma (different in theinner heliosheath, interstellar medium, and hot interstellar bubbles)
For energy . 10 keV the mean free path (m.f.p.) against He ENA ionization isup to one order of magnitude longer than the m.f.p. of H ENA.For assumed Local Interstellar Medium (LISM) conditions:nH = 0.194 cm−3, np = 0.056 cm−3, nHe = 0.0153 cm−3, nHe+ = 0.0096 cm−3,m.f.p. of He ENA: 7 800 AU; H ENA 870 AU at E = 5 keV.
He2+
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Figure 1. Left panel – diagram presenting the important binary interactions inthe heliosphere and LISM. Right panel – the m.f.p. against H/He ENA ionizationin the LISM. For He ENA, contributing reactions are presented by color lines.
Heliosheath signal (Grzedzielski et al. 2013 & 2014)
To asses the inner heliosheath contribution to He ENA fluxes we use a simpleaxisymmetrical analytical model by Suess & Nerney (1990):I circular termination shock (TS) at 94 AU (as measured by Voyager 2)I distance to heliopause at the Voyager 1 trajectory: 121 AUI plasma density in the inner heliosheath 0.002 cm−3
I post-termination-shock bulk plasma velocity 150 km/sI He ion spectra consist of solar wind particles and pick-up ions (PUI)→ PUI – assumed κ-distribution consistent with high energy Voyager data
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Figure 2. Simulated He ENAintensities from the inner he-liosheath as functions of an-gular distance from the nose(the symmetry axis). For en-ergy 100 − 200 keV the re-sults are consistent with theHSTOF measurements.
IBEX Ribbon – two hypothetical sources (Swaczyna et al. 2014)
Secondary ENA Model
B
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Heliosphere
Local Interstellar
Medium
Extraheliospheric Model
Local Interstellar Cloud
Local Bubble
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Heliosphere
Production of ENA in the LISM just outside the he-liopause in the direction where the line-of-sights areperpendicular to the direction of the magnetic field.
Production of ENA in the contact layer between theLocal Interstellar Cloud (LIC) and a hypothetical bayof the Local Bubble (LB).
Mechanism
The solar wind ions (1) are neutralized inside the TS(2) and create a flux of primary ENA (3). The primaryENA are ionized (4) and became PUIs (5), whichafter subsequence neutralization (6) produce a thindisk of secondary ENA emission (7).
The neutral hydrogen and helium atoms from the LICevaporate into the LB. Part of the suprathermal ionsin the LB after neutralization on the neutrals are asource of the observed ENA. The Ribbon is a geo-metric effect of different integration path lengths.
Basic model
Adapted analytical model by Mobius et al. (2012) withheliolatitude dependence of the solar wind parame-ters (Sokoł et al. 2013), the LISM densities as pre-sented in the section “Helium binary interactions”.
Adapted analytical model by Grzedzielski et al.(2010), set (4) of parameters in the plane model. En-ergy dependence based on the hydrogen ENA mea-sured in the ribbon direction near the nose.
Geometry
Assumed small circle: center (λ, β)=(219.2◦, 39.9◦);radius 74.5◦, width 15◦ (Funsten et al. 2013)
Governed by the extinction in the LIC, planar inter-face between the LIC and LB with the smallest dis-tance in the center of the Ribbon.
Results – maps of expected He ENA flux in the two hypotheses
2keV
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Figure 3. Left column – the heliosheath signal with the secondary ENA Ribbon. Right column – the he-liosheath signal with the extraheliospheric Ribbon.
Hydrogen Ribbon – comparison between data and models
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Figure 4. Comparison of the fluxes of hydrogen ENA observed by IBEX duringthe first 3 years (McComas et al. 2012) with the fluxes in the models of theRibbon. The heliospheric ENA are not included in the models plots.
Conclusions – expected properties of the He ENA signal
Ratio (E ∼ 1 keV/nuc) Sec. ENA Extrahel.He-to-H ENA flux from the Ribbon 0.0001 0.01
The Ribbon-to-heliosheath He ENA flux 0.5 50
I The expected heliospheric signal is highly concentrated in the direction ofthe heliospheric tail: Flux(tail)/Flux(nose) ∼ 102
I If observed hydrogen atoms from the Ribbon are the secondary ENA thenobservation of the He ENA signal from the Ribbon is not likely due to toosmall amount of helium in the neutral solar wind.
I If the Ribbon is produced as in the extraheliospheric model then the HeENA signal from the Ribbon dominates over the heliosheath signal exceptfor the heliospheric tail and should be potentially easily detectable.
Outlook
I The long mean free path against ionization of keV He ENA in the LISMand low heliosheath signal in large part of the sky make He ENA a goodcandidate for studies of the LISM structure at distances comparable withthe distance to the LIC edge (0.05 pc ≈ 10 000 AU, Redfield & Linsky 2000).
I For two-dimensional sources (e.g. interfaces), also secondary ENA, i.e., pro-duced from the ionized and then neutralized primary ENA, should give anon-negligible contribution and be included into future considerations.
References
Funsten et al. 2013, ApJ, 776, 30Grzedzielski et al. 2010, ApJL, 715, L84Grzedzielski et al. 2013, A&A, 549, A76Grzedzielski et al. 2014, A&A, 563, A134McComas et al. 2012, ApJS, 203, 1
Mobius et al. 2013, ApJ, 766, 129Redfield & Linsky 2000, ApJ, 534, 825Sokoł et al. 2013, Solar Phys. 285, 167Suess & Nerney 1990, JGR, 95, 6403Swaczyna et al. 2014, ApJ, 782, 106
This work was supported by the Polish National Science Centre grant 2012-06-M-ST9-00455.
http://www.cbk.waw.pl/en http://www.cbk.waw.pl/∼pswaczyna [email protected]
