Potential of Energetic Neutral Helium Atoms to Resolve ...swaczyna/conf/... · 2014 AGU Fall Meeting Talk SH21D-02 Helium is the second most abundant species in the Universe!He ENA

Motivation and aimsBackground of heliospheric signal

Extraheliospheric signalSummary and conclusions

Potential of Energetic Neutral Helium Atoms to Resolve Structureof the Local Interstellar Medium within 0.1 parsec

Paweł Swaczyna, Stan Grzedzielski, and Maciej Bzowski

[email protected]

Space Research Centre of the Polish Academy of Sciences

2014 AGU Fall MeetingSan Francisco, CA, December 16, 2014

1 / 12 P. Swaczyna, S. Grzedzielski, and M. Bzowski Potential of He ENA to Resolve Structure of the LISM within 0.1 pc



Motivation and aims

2014 AGU Fall Meeting

Talk SH21D-02

Helium is the second most abundant species in the Universe→ He ENA fluxes should be detectable by well designed instrument.

Expected He ENA fluxes from the heliosphere are low.The mean free path of helium atoms in the LISM is long.→ Chance to observe extraheliospheric sources

Sun is close to the edge of the Local Interstellar Cloud (within 0.1 pc).Observation of sources from such a distance is likely.

Outline:

1 Expected emission from the inner heliosheath and the IBEX Ribbon

2 Range of He ENA in the LISM and exemplary model of emission

3 Prerequisites for IMAP ENA detector




Inner heliosheath signal


Talk SH21D-02

æ

æ

æ

Inner heliosheath

Dis

tan

ce

toth

en

ose:

10°

45°

90°

135°

170°

1 10 100 100010-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

1

10

102

Energy @keV�nD

Diff.

inte

nsity@H

cm

2s

sr

ke

V�nL-

1D

He +PUI at TS

He

EN

A

HSTOFHeliotail

Axisymmetrical model of incompressibleplasma flow (Suess and Nerney 1990),parameters reflect Voyagers results

Spherical symmetry of supersonic solarwind

Helium ENA originate from He+ PUI in theinner heliosheath

Simulated intensities agree with HSTOF(Grzedzielski et al. 2014, A&A 563, A134)

Tail-to-nose intensity ratio: 10 – 50

Sharp drop of intensity above 100 keV/n:j(1 MeV/n) = 10−6j(100 keV/n)

Expected inner heliosheath He ENA flux 100 – 1000 times smaller than observed H ENA flux




Secondary ENA from the IBEX Ribbon


Talk SH21D-02D

iff.

inte

nsity@H

cm

2s

sr

ke

V�nL-

1D

Inner heliosheath Secondary ENA Total

1 1010-3

10-2

10-1

1A: Heliolatitude: 0°

He ENA

1 1010-3

10-2

10-1

1B: Heliolatitude: 45°

He ENA

1 1010-3

10-2

10-1

1C: Heliolatitude: 70°

He ENA

Energy @keV�nD

Adapted analytical model by Möbius et al. 2012

Relevant reactions for helium included(Swaczyna et. al. 2014, ApJ 782:106)

Fast/slow supersonic solar wind (SSW) included(Sokół et al. 2013)

Geometry of the Ribbon as observed from the IBEX(Funsten et al. 2013, Schwadron et al. 2014)

Amount of He ENA from the Secondary ENAmechanism much smaller due to:

smaller probability of He2+ neutralization in the SSWlonger mean free paths for ionization of primary HeENA and second neutralization

Secondary ENA mechanism provide a signal at most ofthe order of the inner heliosheath emission




Maps of expected heliospheric flux (IHS + Sec. ENA)


Talk SH21D-02

-150-120-90-60-30

03060 90120150

180

-60

-30

0

30

60

V1

V2

NoseTailA

B

CSimulated

He ENA

1 keV�n

Diff. intensity @Hcm2 s sr keV�nL-1D

10.5 20.30.2

-150-120-90-60-30

03060 90120150

180

-60

-30

0

30

60

V1

V2

NoseTailA

B

CSimulated

He ENA

2 keV�n


0.1 0.50.30.20.05

-150-120-90-60-30

03060 90120150

180

-60

-30

0

30

60

V1

V2

NoseTailA

B

CSimulated

He ENA

3 keV�n


0.10.050.030.02

-150-120-90-60-30

03060 90120150

180

-60

-30

0

30

60

V1

V2

NoseTailA

B

CSimulated

He ENA

4 keV�n


0.01 0.050.030.02

Dominating part of heliospheric He ENA flux could be potentially observed from the heliotail




Mean free path of He ENA in the LISM


Talk SH21D-02

0.1 1 10 100 1000

1

2

5

10

20

Energy @keV�nD

Mean

free

path@1

000

AUD

ion H

cx&ion H+

cx He+

ion e-

Densities in the closest LISM:H 0.19 cm−3 He 0.015 cm−3

H+ 0.06 cm−3 He+ 0.009 cm−3

(Frisch et al. 2011)

The longest mean free path (m.f.p)(>7000 AU) for E = 0.5− 3 keV/n

Electron impact ionization makes theLISM opaque above a few tens keV/n

Sources of suprathermal ions operating in the LISM within distances comparable with the m.f.p.could produce observable He ENA fluxes




He ENA intensities from extraheliospheric sources


Talk SH21D-02

{

IMAP

d

0seLISM

source region

jENA(E) = e− d

λLISM(E)

∫ se

0e− s

λsource(E){

jHe+ (E)[σ

cxHe+H(E)nH + σ

cxHe+He(E)nHe

]+ jHe2+ (E)σ2cx

He2+He(E)nHe

}ds

The expected flux depends on:extinction in the LISM to the edge of the source region

extintion in the source region

intensities of suprathermal helium ions in the source region

distribution of the neutrals in the source region

integration over line-of-sight in the source region

Distribution of Ions and neutrals in general are not homogeneous in the source region.Thus appropriate quantities should be treated as s dependent.




Exemplary model of the extraheliospheric source (1)


Talk SH21D-02

Adapted model of ENA production at the contact layer between the Local Interstellar Cloud andLocal Bubble originally proposed by Grzedzielski et al. (2010) as the source for the IBEX Ribbon

(Swaczyna et al. 2014, ApJ 782:106)

Heliosphere

Local Interstellar Cloud

Grzedzielski et al. took low H+ density (<0.03 cm−3) toextend mean free path of H ENA in the LIC.Here: H+ density in the LIC: 0.06 cm−3

Large temperature in the LB (TLB = 106 K)→ plasma completely ionized deep inside LB, densities:H+ 0.005 cm−3 and He2+ 0.0005 cm−3

Neutral atoms (H, He) evaporate from the LIC and create aboundary layer in the LB (source region for ENA)

Assumed κ-distribution in the LB with κ = 2→ high energy tail is the source for suprathermal ions

Assumed planar geometry of the interface




Exemplary model of the extraheliospheric source (2)


Talk SH21D-02

1 10 10010-5

10-4

10-3

10-2

10-1

1

10

Energy @keV�nD

Diff.

inte

nsity@H

cm

2s

sr

keV�nL-

1D

2,000 AU

5,000 AU

10,000 AU

20,000 AU50,000 AU

Innerheliosheath

He ENA

Signal in the model up to 50 times larger than theheliospheric signal

Signal is strongly attenuated above a few tens of keVby electron impact ionization

Detectable from distances . 20,000 AU ≈ 0.1 pc

0 20 40 60 800.0

0.5

1.0

1.5

Distance from the center @degD

Rela

tive

inte

nsity 0.5 keV

1 keV 2ke

V

5 keV

10keV20

keV

He ENA

d0 = 2,000 AU

0 20 40 60 800.0

0.2

0.4

0.6

0.8

1.0

Distance from the center @degD

Rela

tive

inte

nsity

0.5keV

1keV

2keV

5keV

10keV20

keVHe ENA

d0 = 20,000 AU




Perspective for He ENA observations


Talk SH21D-02

The He ENA m.f.p. against ionization in the LISM is almost constant between 0.5 – 3 keV.

→ Ratios of intensities at different energies are not affected by attenuation in the LISM.

→ Potential for studies of energy spectra and physics in the source region.

Determination of distance to the source should be also possible:

→ Differential attenuation of signal at energies with different m.f.p. could be determined if ion

spectra in the source region are known (e.g. power-law)

Combination of maps of He ENA intensities at different energies could allow tomography of

shape of the source region.

The shortest distance to the edge of the LIC is probably smaller than 0.1 pc (20,000 AU) over

large part of the sky (Redfield and Linsky 2000, 2014).

It is likely that processes operating at this edge could produce suprathermal ions.




Prerequisites for IMAP ENA detector


Talk SH21D-02

Mass spectrometer should be included in the ENA detector

Energy channels should cover energies smaller than a few tens keV/n

→ Expected signal above 100 keV/n is very small thus could be hard to detect.

Sensitivity (effective cross section times field of view) of detector should be

increased at least by an order of magnitude with respect to IBEX

Better angular and energy resolution is not needed.

→ Better resolution will decrease sensitivity and make He ENA harder to detect.




Take home message


Talk SH21D-02

Expected signal of He ENA emission from the heliosphere is small.

The long He ENA mean free path against ionization in the LISM make theENA sources detectable at distances up to 0.1 pc.

Observation is most likely for energies . 50 kev/n.

High sensitivity ENA detector with mass spectrometer on IMAP couldmake He ENA detection possible.


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Potential of Energetic Neutral Helium Atoms to Resolve ...swaczyna/conf/... · 2014 AGU Fall Meeting Talk SH21D-02 Helium is the second most abundant species in the Universe!He ENA