Space Research Centre of the Polish Academy of Sciences ...swaczyna/conf/swaczyna_egu2015.pdfSpace...
1
Energetic neutral helium atoms as a tool to study the heliosphere and the Local Interstellar Medium Pawel Swaczyna, Stan Grzedzielski, and Maciej Bzowski Space Research Centre of the Polish Academy of Sciences, Warsaw, Poland EGU2015-723 1. Introduction More than 5 years of the IBEX observations shows the unique role of Energetic Neutral Atoms (ENA) in remote sampling of the plasma in the heliosphere and its vicinity (Mc- Comas et al. 2014). However, ionization of the hydrogen ENA in the Local Interstellar Medium (LISM) make the H ENA fluxes extinct at distances ∼1000 AU. One of the most popular models of the LISM suggests coexistence of warm partially ionized clouds embedded in the fully ionized plasma of the bay of the Local Bubble. The two clouds closest to the Sun are probably the Local Interstellar Cloud (LIC) and Galactic (G) Cloud (Redfield & Linsky 2008, Linsky & Redfield 2014). The structure of the clouds could be resolved using atomic absorption lines in high-resolution spectra of the closest stars. This method requires the minimal thickness of the observed cloud of the order of 0.1 pc, and thus gives only an upper limit for the distance to the LIC boundary (Wood et al. 2000). Helium due to the higher ionization energy could extend the ability of the ENA imaging of the LISM to distances unavailable for the telescopic observations of absorption lines. This, however, requires an ENA detector with mass spectrometer in future missions. 2. He ENA from the heliosphere The heliospheric H ENA signal observed by the IBEX could be split into two main sources. One is the inner heliosheath (IHS), where Pick-Up Ions (PUI) created in the supersonic so- lar wind could be neutralized by charge-exchange with interstellar neutrals. The other one is the outer heliosheath (OHS) source due to the Secondary ENA Mechanism (Heerikhuisen et al. 2010), in which the neutral solar wind could produce ENA flux backwards into the Sun by two subsequent charge-exchange in the region of the sky where interstellar magnetic field is perpendicular to the line-of-sight. For simulation of the IHS signal we use axisymmetrical model of incompressible plasma flow (Suess & Nerney 1990), with parameters reflecting Voyagers results (Grzedzielski et al. 2014). The He ENA fluxes originate from the He + PUI for which we evolve spectra assumed at the termination shock. The expected Secondary He ENA fluxes were calculated using generalized analytical model by M¨ obius et al. (2013) with the heliolatitudial structure of the supersonic solar wind obtained from interplanetary scintillations (Sok ´ ol et al. 2013). 3. Extraheliospheric sources Suprathermal helium ions in the LISM after charge-exchange with the neutrals produce fluxes of He ENA. Long mean free path against ionization (Figure 1) makes them observ- able at distances comparable to the distance to the LIC edge. { ENA detector d 0 s e LISM source region j ENA (E )= e - d λ LISM Z s e 0 e - s λ source n j He + σ cx He + H n H + σ cx He + He n He + j He 2+ σ 2cx He 2+ He n He o ds The expected flux depends on the extinction in the LISM to the edge of the source region and the properties of the source region: extinction, intensities of suprathermal helium ions, distribution of neutrals, and dimension over the line-of-sight. 0.1 1 10 100 1000 0.5 1 5 10 Energy [keV/n] Mean free path [ 1000 AU ] He ENA ion H cx&ion H + cx He + ion e - Figure 1. — The He ENA mean free path (m.f.p.) against ionization in the LISM with the densities: H 0.19 cm -3 ,H + 0.06 cm -3 , He 0.015 cm -3 , He + 0.009 cm -3 . The longest m.f.p. (>7000 AU) have He ENA with energies 0.5 - 3 keV/n. Notice that electron impact ionization makes the LISM opaque above a few tens keV/n. 4. Simulated signal from the heliosphere The simulated heliospheric He ENA signal is concentrated in the heliotail direction (Figure 2). In the model He + PUI are distributed almost homogeneously in the IHS, thus the expected signal grows as the IHS thickness grows in the tail direction. This is possible due to lower rate of He + –He charge-exchange in comparison with the rate of H + –H charge-exchange. The Secondary ENA mechanism provides a signal at most of the order of the IHS emission (Figure 3). The peak energy depends on heliolatitude, as observed for H ENA. 150 120 90 60 30 0 30 60 90 120 150 180 60 30 0 30 60 V1 V2 Nose Tail A B C Simulated He ENA 1 keVn Diff. intensity cm 2 s sr keVn 1 1 0.5 2 0.3 0.2 150 120 90 60 30 0 30 60 90 120 150 180 60 30 0 30 60 V1 V2 Nose Tail A B C Simulated He ENA 2 keVn Diff. intensity cm 2 s sr keVn 1 0.1 0.5 0.3 0.2 0.05 150 120 90 60 30 0 30 60 90 120 150 180 60 30 0 30 60 V1 V2 Nose Tail A B C Simulated He ENA 3 keVn Diff. intensity cm 2 s sr keVn 1 0.1 0.05 0.03 0.02 150 120 90 60 30 0 30 60 90 120 150 180 60 30 0 30 60 V1 V2 Nose Tail A B C Simulated He ENA 4 keVn Diff. intensity cm 2 s sr keVn 1 0.01 0.05 0.03 0.02 Figure 2. — Maps of simulated He ENA intensities from the heliosphere. Notice the logarithmic scale in the intensities. Figure 3. — He ENA spectra of the IBEX Ribbon in three directions decomposed into the IHS and Secondary ENA components. 5. Exemplary model of extraheliospheric source Heliosphere Local Interstellar Cloud As an exemplary model we adapt the model of ENA produc- tion at the contact layer between the Local Interstellar Cloud and Local Bubble originally proposed by Grzedziel- ski et al. (2010) as the source for the IBEX Ribbon. In the cases considered by Grzedzielski et al. (2010) the ex- pected He ENA fluxes are larger than the heliospheric emis- sion (Swaczyna et al. 2014). Here we extend the considered distance d 0 to the source region up to ∼0.1 pc. We assume planar interface, thus the closest path define the center of circular symmetry. We assume that the Local Bubble far away from the interface is filled with hot (10 6 K), low density completely ionized plasma (n p = 0.005 cm -3 , n α = 0.0005 cm -3 ). The ion distribution function is assumed to be κ-distribution with κ = 2. The signals obtained from the model are up to ∼50 times larger than the helio- spheric signal for energies .10 keV/n (Figure 4). Above few tens of keV/n the fluxes are strongly attenuated by electron impact ionization. The spatial shape of the signal changes with the distance to the interface (Figure 5). 1 10 100 10 -5 10 -4 10 -3 10 -2 10 -1 1 10 Energy [keV/n] Diff. int. [( cm 2 s sr keV / n ) - 1 ] 2 000 AU 5 000 AU 10 000 AU 20 000 AU 50 000 AU Inner heliosheath He ENA Figure 4. — Simulated He ENA spectra with different distances to the interface in the closest path di- rection. The IHS signal at 90 ◦ from the nose is also plotted for com- parison. Notice that for energies 0.5 - 3 keV/n the spectra are shifted only vertically due to almost con- stant mean free path against ioniza- tion in this energy range (Figure 1). 0 20 40 60 80 0.0 0.5 1.0 1.5 θ [deg] Relative intensity 0.5 keV 1 keV 2 keV 5 keV 10 keV 20 keV He ENA d 0 = 2 000 AU 0 20 40 60 80 0.0 0.2 0.4 0.6 0.8 1.0 θ [deg] Relative intensity 0.5 keV 1 keV 2 keV 5 keV 10 keV 20 keV He ENA d 0 = 20 000 AU Figure 5. — Relative signal strength as a function of angular distance from the center of symmetry for two selected distances to the interface. Notice the maximum at ∼70 ◦ for the lowest energies in the left panel. 6. Conclusions & Perspectives I Expected signal of the He ENA emission from the heliosphere is small except for the heliotail. I The long He ENA m.f.p. against ionization in the LISM makes the hypothetical extraheliospheric ENA sources detectable at distances up to 20000 AU / 0.1 pc. I Successful observations are most likely for energies . 50 kev/n. I High sensitivity ENA detector with mass spectrometer could make He ENA detection possible. 7. References Grzedzielski et al. 2010, ApJL 715, L84 Grzedzielski et al. 2014, A&A 563, A134 Heerikhuisen et al. 2010, ApJL 708, L126 Linsky & Redfield 2014, Ap&SS 354, 29 McComas et al. 2014, ApJS 213, 20 M¨ obius et al. 2013, ApJ 766, 129 Redfield & Linsky 2008, ApJ 673, 283 Sok ´ ol et al. 2013, Solar Phys. 285, 167 Suess & Nerney 1990, JGR 95, 6403 Swaczyna et al. 2014, ApJ 782, 106 Wood et al. 2000, ApJ 542, 411 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 Space Research Centre of the Polish Academy of Sciences ...swaczyna/conf/swaczyna_egu2015.pdfSpace...
Energetic neutral helium atoms as a tool to study the heliosphere and the Local Interstellar MediumPaweł Swaczyna, Stan Grzedzielski, and Maciej Bzowski
Space Research Centre of the Polish Academy of Sciences, Warsaw, Poland
EGU2015-723
1. Introduction
More than 5 years of the IBEX observations shows the unique role of Energetic NeutralAtoms (ENA) in remote sampling of the plasma in the heliosphere and its vicinity (Mc-Comas et al. 2014). However, ionization of the hydrogen ENA in the Local InterstellarMedium (LISM) make the H ENA fluxes extinct at distances ∼1000 AU.
One of the most popular models of the LISM suggests coexistence of warm partially ionizedclouds embedded in the fully ionized plasma of the bay of the Local Bubble. The two cloudsclosest to the Sun are probably the Local Interstellar Cloud (LIC) and Galactic (G) Cloud(Redfield & Linsky 2008, Linsky & Redfield 2014). The structure of the clouds could beresolved using atomic absorption lines in high-resolution spectra of the closest stars. Thismethod requires the minimal thickness of the observed cloud of the order of 0.1 pc, and thusgives only an upper limit for the distance to the LIC boundary (Wood et al. 2000).
Helium due to the higher ionization energy could extend the ability of the ENA imaging ofthe LISM to distances unavailable for the telescopic observations of absorption lines. This,however, requires an ENA detector with mass spectrometer in future missions.
2. He ENA from the heliosphere
The heliospheric H ENA signal observed by the IBEX could be split into two main sources.One is the inner heliosheath (IHS), where Pick-Up Ions (PUI) created in the supersonic so-lar wind could be neutralized by charge-exchange with interstellar neutrals. The other one isthe outer heliosheath (OHS) source due to the Secondary ENA Mechanism (Heerikhuisenet al. 2010), in which the neutral solar wind could produce ENA flux backwards into the Sunby two subsequent charge-exchange in the region of the sky where interstellar magneticfield is perpendicular to the line-of-sight.
For simulation of the IHS signal we use axisymmetrical model of incompressible plasmaflow (Suess & Nerney 1990), with parameters reflecting Voyagers results (Grzedzielski et al.2014). The He ENA fluxes originate from the He+ PUI for which we evolve spectra assumedat the termination shock. The expected Secondary He ENA fluxes were calculated usinggeneralized analytical model by Mobius et al. (2013) with the heliolatitudial structure of thesupersonic solar wind obtained from interplanetary scintillations (Sokoł et al. 2013).
3. Extraheliospheric sources
Suprathermal helium ions in the LISM after charge-exchange with the neutrals producefluxes of He ENA. Long mean free path against ionization (Figure 1) makes them observ-able at distances comparable to the distance to the LIC edge.{
ENA
detector
d
0seLISM
source region
jENA(E) = e− dλLISM
∫ se
0e− s
λsource
{jHe+
[σcx
He+HnH + σcxHe+HenHe
]+ jHe2+σ2cx
He2+HenHe
}ds
The expected flux depends on the extinction in the LISM to the edge of the source regionand the properties of the source region: extinction, intensities of suprathermal helium ions,distribution of neutrals, and dimension over the line-of-sight.
0.1 1 10 100 10000.5
1
5
10
Energy [keV/n]
Mea
nfre
epa
th[1
000
AU]
He ENA
ion H
cx&ion H+
cx He+
ion e-
Figure 1. — The He ENA mean free path(m.f.p.) against ionization in the LISM withthe densities: H 0.19 cm−3, H+ 0.06 cm−3,He 0.015 cm−3, He+ 0.009 cm−3.The longest m.f.p. (>7000 AU) have HeENA with energies 0.5 − 3 keV/n. Noticethat electron impact ionization makes theLISM opaque above a few tens keV/n.
4. Simulated signal from the heliosphere
The simulated heliospheric He ENA signal is concentrated in the heliotail direction(Figure 2). In the model He+ PUI are distributed almost homogeneously in the IHS,thus the expected signal grows as the IHS thickness grows in the tail direction. This ispossible due to lower rate of He+–He charge-exchange in comparison with the rate ofH+–H charge-exchange.
The Secondary ENA mechanism provides a signal at most of the order of the IHSemission (Figure 3). The peak energy depends on heliolatitude, as observed for H ENA.
-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
Diff. intensity @Hcm2 s sr keV�nL-1D
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
Diff. intensity @Hcm2 s sr keV�nL-1D
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
Diff. intensity @Hcm2 s sr keV�nL-1D
0.01 0.050.030.02
Figure 2. — Maps of simulated He ENA intensities from the heliosphere. Notice thelogarithmic scale in the intensities.
Figure 3. — He ENA spectra of the IBEX Ribbon in three directions decomposed intothe IHS and Secondary ENA components.
5. Exemplary model of extraheliospheric source
Heliosphere
Local Interstellar Cloud
As an exemplary model we adapt the model of ENA produc-tion at the contact layer between the Local InterstellarCloud and Local Bubble originally proposed by Grzedziel-ski et al. (2010) as the source for the IBEX Ribbon. Inthe cases considered by Grzedzielski et al. (2010) the ex-pected He ENA fluxes are larger than the heliospheric emis-sion (Swaczyna et al. 2014).Here we extend the considered distance d0 to the sourceregion up to ∼0.1 pc. We assume planar interface, thus theclosest path define the center of circular symmetry.
We assume that the Local Bubble far away from the interface is filled with hot (106 K),low density completely ionized plasma (np = 0.005 cm−3, nα = 0.0005 cm−3). The iondistribution function is assumed to be κ-distribution with κ = 2.
The signals obtained from the model are up to ∼50 times larger than the helio-spheric signal for energies .10 keV/n (Figure 4). Above few tens of keV/n the fluxesare strongly attenuated by electron impact ionization. The spatial shape of the signalchanges with the distance to the interface (Figure 5).
1 10 10010-5
10-4
10-3
10-2
10-1
1
10
Energy [keV/n]
Diff
.int
.[(c
m2s
srke
V/n)-
1 ]
2 000 AU
5 000 AU
10000 AU
20 000 AU50 000 AU
Innerheliosheath
He ENA
Figure 4. — Simulated He ENAspectra with different distances tothe interface in the closest path di-rection. The IHS signal at 90◦ fromthe nose is also plotted for com-parison. Notice that for energies0.5−3 keV/n the spectra are shiftedonly vertically due to almost con-stant mean free path against ioniza-tion in this energy range (Figure 1).
0 20 40 60 800.0
0.5
1.0
1.5
θ [deg]
Rel
ativ
ein
tens
ity
0.5 keV1 keV 2
keV
5 keV
10 keV20 keV
He ENA
d0 = 2 000 AU
0 20 40 60 800.0
0.2
0.4
0.6
0.8
1.0
θ [deg]
Rel
ativ
ein
tens
ity
0.5keV
1 keV
2keV
5keV
10 keV20 keVHe ENA
d0 = 20 000 AU
Figure 5. — Relative signal strength as a function of angular distance from the centerof symmetry for two selected distances to the interface. Notice the maximum at ∼70◦
for the lowest energies in the left panel.
6. Conclusions & Perspectives
I Expected signal of the He ENA emission from the heliosphere is small except for the heliotail.I The long He ENA m.f.p. against ionization in the LISM makes the hypothetical extraheliospheric ENA sources
detectable at distances up to 20 000 AU / 0.1 pc.I Successful observations are most likely for energies . 50 kev/n.I High sensitivity ENA detector with mass spectrometer could make He ENA detection possible.
7. References
Grzedzielski et al. 2010, ApJL 715, L84Grzedzielski et al. 2014, A&A 563, A134Heerikhuisen et al. 2010, ApJL 708, L126Linsky & Redfield 2014, Ap&SS 354, 29McComas et al. 2014, ApJS 213, 20Mobius et al. 2013, ApJ 766, 129Redfield & Linsky 2008, ApJ 673, 283Sokoł et al. 2013, Solar Phys. 285, 167Suess & Nerney 1990, JGR 95, 6403Swaczyna et al. 2014, ApJ 782, 106Wood et al. 2000, ApJ 542, 411
This work was supported by the Polish NationalScience Centre grant 2012-06-M-ST9-00455.
http://www.cbk.waw.pl/en http://www.cbk.waw.pl/∼pswaczyna [email protected]
