Interstellar Magnetic Fields
in the Solar Vicinity
P. C. Frisch et al.
University of Chicago
Midwest Magnetic Field Workshop, May 2011
1
et al…..
B-G Andersson, SOFIA, USRA, CA
Andrei Berdyugin, Tuorla Observatory, University of Turku, Finland
Daiane Breves Seriacopi, U. Sao Paulo, Brazil
Herbert O. Funsten, Los Alamos National Laboratory, Los Alamos
Antonio M. Magalhaes, Inst. de Astro., Geo. & Ciencias Atmos., U. Sao Paulo, Brazil
David J. McComas, Southwest Research Institute, San Antonio, TX
Vilppu Piirola, Tuorla Observatory, University of Turku, Finland
Nathan A. Schwadron, University of New Hampshire, Durham, NH
Jonathan D. Slavin, SAO-Harvard, Cambridge, MA
Sloane J. Wiktorowicz, University of California, Berkeley
2
OUTLINE
1.
2.
3.
4.
5.
6.
7.
8.
Local interstellar magnetic field (ISMF) and our
magnetically shaped heliosphere
Local ISMF from the IBEX Ribbon
Local ISMF strength if in magnetic and gas
pressures are in equilibrium local ISM
Local ISMF direction from linearly polarized starlight
ISMF direction from Loop I radio continuum
ISMF direction from nearby pulsars
Implications: Kinematical and ISMF suggest Sun in
superbubble fragment
Summary
3
The Heliosphere
as a Gauge for Strength and
Direction of ISMF
4
Sun and Local Interstellar Medium (LISM):
The Sun is in an Intermediate Velocity Cloud system
(Frisch, APOD; Frisch, Redfield, Slavin 2011)
5
Voyager 1 & 2 in Inner Heliosheath
V1 crossed solar
Wind termination
Shock at 94 AU
In 2004
Termination shock
Asymmetry caused
By angle between
Interstellar (IS) gas
And ISMF
V2 crossed solar
Wind termination
Shock at 84 AU
In 2007
Asymmetry of heliopause
Shown by 5o offset between
upwind IS Heo and Ho directions
6
Heliosphere as gauge of ISMF
(Pogorelov et al. 2009)
• MHD models of the heliosphere, with ions coupled to neutral H by charge
exchange, predict the heliosphere configuration
•Magnetic field needed to explain observed heliosphere properties
•10 AU difference between Voyager 1 crossing the termination shock 2004
at 94 AU and Voyager 2 crossing the termnation shock in 2007.
• 5 degree offset between velocity vectors of interstellar Ho and Heo flow in
heliosphere
• Direction of subsonic solar wind flow in inner heliosheath from Voyager 2
data
•3 kHz emissions detected by both V1 and V2 from beyond the heliopause
•IBEX Ribbon
7
Magnetically Shaped Heliosphere for three ISMF directions
(Opher et al. 2009)
8
IBEX maps Energetic Neutral Atoms (ENAs)
for energies 0.05-6 keV
• IBEX measures energetic neutral atoms formed by chargeexchange between interstellar neutral Ho and solar wind and
pickup ions (McComas et al. 2009 Science)
•IBEX discovered ‘Ribbon’ of ENAs that is seen where the radial
sightline is orthogonal to the ISMF as it drapes over the
heliopause
•Pickup ion (PUI) forms when 26 km/s interstellar Ho gives e- to solar
wind ion
• Mechanism creates slow PUI and fast neutral (ENA)
• PUIs become suprathermal in inner heliosheath
•Takes three years for 440 km/s solar wind ion or PUI to propagate to
inner heliosheath, charge exchange with interstellar Ho , and return to
be measured by IBEX at 1 AU
9
Flow of neutral interstellar gas through heliosphere
creates pickup ions and energetic neutral atoms (ENAs)
ENAs
(A) PUI Pitch
Angles heavily
Scattered by
turbulence
(B) PUI Pitch
Angles
Ring-beam
ENAs
ENAs
ISMF
10
Interstellar Boundary Explorer (IBEX) Spacecraft
launched late 2008;
Four ENA skymaps collected to date
• Two huge aperture single pixel ENA cameras:
– IBEX-Lo (~10 eV to 2 keV)
– IBEX-Hi (~300 eV to 6 keV)
• Simple sun-pointed spinner (4 rpm)
(McComas et al. 2009, SSR)
11
IBEX skymaps of Energetic Neutral Atoms
Ecliptic coords
Galactic coords
Ribbon only
(Schwadron et al. 2011)
12
MHD heliosphere model of Pogorelov et al. (2009, ApJL) was
Basis for Identifying Ribbon with locus of sightlines that are perpendicular
To ISMF draping over the heliosphere
•MHD model predated Ribbon discovery
•Ribbon match where the ISMF leaves
strongest imprint on the heliopause, for 10 AU
outside of heliopause
• Model ISM:
•|B|=3 G
•n(p+)=0.06 cm-3, n(Ho)=0.15 cm-3
• Model is multifluid MHD model with plasma
and neutrals kinetically coupled
•
(Schwadron et al. 2009)
13
Ribbon seen where sightline is perpendicular to ISMF
Draping over the heliosphere, e.g. where “B-dot-R=0”
(McComas et al. 2009)
(Pogorelov et al. 2009)
14
Modeling the Origin of the Ribbon quantitatively
(Heerikhuisen et al. 2010)
• ENAs carry solar wind momentum
outside of heliopause
• New charge-change creates secondary
pickup ions form outside of heliopause
•Third charge-change creates ENAs
traveling in all directions
• These ENAs are seen by IBEX as a
Ribbon if the PUIs maintain pitch angles of
about 90 deg long-enough to form new
ENA.
• Florinski (2011) argues that magnetic
turbulence isotropizes the PUI angular
distribution quickly (few minutes) compared
to time-scales of charge-exchange (2
years)
• Gamaunov et al. (2011) argue that locally
generated small scale turbulence (ion
cyclotron waves,10-4 AU) marginally
stabilizes pitch angles so that Ribbon
forms.
15
Best heliosphere values for
Strength and direction of ISMF shaping heliosphere
• Method: Model ENA Ribbon (Heerikhuisen
et al. 2010)
• |B| = 3 G
• Direction: l,b = 36o, 53o
(Ribbon arc center)
• Large scale magnetic turbulence can
inhibit the isotropization of pitch-angles
due to small-scale turbulence, and
stabilize pitch angles around pseudoring beam (G., Zhang et al. 2011)
• Method: Center of Ribbon arc (Funsten et
al. 2009)
• Direction: l,b = 33o, 55o
• Method: Model heliosphere asymmetries
and H-He offset angle (Opher et al. 2008)
• |B| = 5 G
•Direction: l,b = 15o±4o, 33o±6o
16
Strength of Local ISMF if
gas and magnetic pressures
are equal, =1
17
Obtain interstellar gas pressure at the heliosphere
•
Temperature of ISM at heliosphere derived from
Ulysses observations of HeI 25 km/s flow inside
of heliosphere (caveat: new IBEX results)
•
Predict the density of ISM at heliosphere from
photoionization models (CLOUDY code, Slavin
and Frisch 2008)
•
Local Interstellar Cloud (LIC) observed in UV
towards  CMa gives sight-line properties
•
Constrain with ISM data from inside and outside
of heliosphere
–
Electron density from CII*/CII, MgI/MgII, HI and HeI
–
Local interstellar cloud in  CMa sightline
–
Interstellar HeI inside heliosphere
–
Pickup ion data on Ne, Ar, He
( Redfield Linsky 2011)
18
In circumheliospheric ISM B=2.7 G
if thermal and magnetic pressures equal
• Results:
– n(HI)~0.19 cm-3, n(e)~0.07 cm-3
– He ~39% ionized, H ~25% ionized
– If gas and magnetic pressures are
equal,
 ~1, then B=2.7 G
•
Weakness:
•
Need new HST data for Sirius,
a much closer star (2.7 pc)
•
HeI temperature (and density)
inside of heliosphere may
change (e.g. new IBEXLO data)
19
(Slavin and Frisch 2008)
Local Interstellar Magnetic
Field Direction from
Polarized Starlight
20
ISMF direction from weak linear polarization of
nearby stars in galactic center hemisphere
(discovered by Tinbergen, 1982)
Polarized starlight of
stars within D<500 pc
Shows Loop I
Configuration
(Mathewson & Ford 1994)
(Tinbergen 1982)
• Local Patch of dust covers of Loop I
region; polarization ~0.02%
•Nearest star in Tinbergen sample is 6 pc
away towards heliosphere nose
•36 Oph has very efficient polarization per
unit column density (compared to distant
stars)
• Most local ISM within 15 pcs
• D < 30--40 pc test the ISM close to the
heliosphere
21
North Polar Spur synchrotron emission indicates polarization plane due to
magnetically aligned interstellar dust is parallel to ISMF direction
•
Polarized
synchrotron
emission
Polarized starlight
•
•
•
•
820 MHz
starlight
(Data from Berkhuijsen (1973), Mathewson & Ford (1974),
And Berdyugin, unpublished)
•
ISMF in North Polar Spur is traced
by both synchrotron emission and
stellar polarization caused by
magnetically aligned interstellar
dust
Synchrotron emission polarized
perpendicular to ISMF
Therefore polarization vector from
dust opacity is parallel to ISMF
Interstellar dust is ~1% gas mass
Polarizations are proportional to
dust mass but depends on viewing
angle, clumpiness of dust, and
number of magnetic field directions
in sightline
If only one magnetic field direction
in sightline, e.g. nearby stars,
polarization more efficient.
22
Method for Fitting a magnetic field to the
polarization position angles
1.
Assemble data on interstellar polarizations of stars within 40 pc
T=5000-10000 K
(Redfield and Linsky 2004)
23
Polarization Data used in Paper I
• These data give interstellar polarizations of stars 5-40 pc away
• Factor of 70 range in sensitivities of data used in first analyses
•
•
•
•
•
•
Frisch et al. (2011) - 1sigma >/= 0.003% (NOT and LNA)
Planet Pol (Bailey et al. 2010) - 1 sigma ~ 0.0004%
Wiktorowicz (in Frisch et al. 2010)- 1-sigma~0.0004%
Santos et al. (2010) - 1 sigma ~ 0.02%
Tinbergen (1982), Piirola (1977) - 1sigma~0.007%
IBEX Ribbon: IBEX-HI 1 keV ENAs, for fluxes > 1.5*mean flux
Ecliptic coords.
Galactic coords.
24
Method for Fitting a magnetic field to the
polarization position angles
1.
Assemble data on interstellar polarizations of stars within 40 pc
2.
Select spatial subsample of data (stars within 90o of heliosphere nose)
3.
Assume that the subsample polarizations trace a single ISMF direction
4.
Systematically rotate polarization position angles into all possible directions for
dipole field, and see which direction does the best job of aligning the
polarization vectors with a meridian of the coordinate system
5.
Minimize Fi to find ISMF direction Bi , eg minimize the mean of all position
angles j for j stars for ISMF direction Bi :
6.
Paper I: Weight all data points equally (because of S/N
differences.
7.
Paper II (in progress): Better spatial coverage of high S/N
data so use weighted fits (factor = G(sigma,P))
25
Find best-fitting ISMF to polarization data by minimizing
F=mean(|sin(PA)|) for all possible ISMF directions on sky
•
Paper I: Best fitting interstellar magnetic
field direction from unweighted optical
polarization data:
– Galactic coordinates: L = 38o B=23o
(+/- 35o)
– Ecliptic coordinates:  = 263o =37o
(+/- 35o)
•
Paper II: Unweighted fit with new data
– Results same, uncertainties smaller
•
Paper II: Weighted fit with new data (in
preparation)
–
–
–
–
ISMF direction changes by 12o
Uncertainties larger
G alactic coordinates: L = 49o, B=28o
Ecliptic coordinates:  = 260o,  = 49o
26
Strength and direction of
Local ISMF from four
pulsars in third galactic
quadrant, 130-300 pc away
27
B-direction from Pulsar measurements: Obtain
ISMF direction and strength from ratio of pulsar
Faraday rotation and dispersion measures
• Four pulsars (130-300 pc) in
third galactic quadrant where see
low density ISM
• |B|=3.3 G
• ISMF field is directed up out of
the ecliptic plane, or
– towards L,B=5o,42o
– towards =232o,18o
• This direction is 22o from center
of IBEX ENA Ribbon arc
• B|| from ratio of pulsar rotation
and dispersion measures:
•Ref: Salvati 2010
28
Direction of Local ISMF obtained
from spherically complete Loop I
superbubble shell (e.g.North Polar
Spur) consistent with ISMF
directions from polarization data
and IBEX Ribbon
29
Very Local ISMF direction determined from the Loop I radio continuum
is consistent with other local ISMF directions
Loop I fit with two
magnetic superbubble
Shells, S1 & S2
Sun in S1 shell
(Frisch Mueller 2011)
Wolleben (2007) modeled 1.3 GHz
Loop I as two “S1” & “S2” subshells
ISMF governing
shell geometry:
L,B=71o±48o, 18o±48o
ISMF direction
of S1 shell agrees
with other indicators of
local ISMF direction
30
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Implication: Both local ISMF direction and cloud kinematics
suggest solar location in expanding Loop S1 (black dots).
LSR velocity vectors
ISMF direction suggests
Sun in compressed
S1 shell of expanding
Loop I
Kinematics of interstellar
Clouds within 15 pc suggest
Association with S1 shell
32
SUMMARY
•
•
•
•
•
Linearly polarized starlight traces interstellar
magnetic field (ISMF)
Optical polarizations of stars within 40 pc gives
ISMF direction near Sun. Direction ~30o from
Ribbon ISMF direction if a dipole field
Some polarizations trace same ISMF as IBEX
Ribbon
Astronomical data on nearest pulsars and radio
continuum polarization give similar ISMF directions
Implications: Kinematical and magnetic field support
hypothesis that Sun is inside of fragment of
expanding superbubble shell
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The general flow past the Sun of interstellar
gas within 15 pc is clumpy and turbulent
•
•
•
•
•
•
•
Most nearby ISM is within 15 pc
LSR velocity of bulk flow past Sun is
-17 ± 5 km/s from L,B=335o, -7o
Clouds fill 6% to 19% of close space
Warm and low density:
– T=2,000--12,000 K
– N=0.1-0.2 /cc
The velocity of interstellar HeI in the
heliosphere (Witte 2004) disagrees
by ~2 km/s with velocity of ISM in
the upwind direction
Sun near edge of of a cloud
The magnetic field in the Tinbergen
polarization patch could from a
separate kinematical component
than ISMF affecting heliosphere.
Ref: Frisch, Redfield, Slavin ARAA 2011
34
Global ISMF near Sun related to solar position in
the Local Bubble
•
•
•
•
Sun in very low density Local Bubble cavity (n<0.005 /cc).
Solar location inside of Local Bubble gives rise to easy expansion of supernova
remnants, with swept-up magnetic field, into low density region around the Sun.
Loop I, formed by supernova in Scorpius-Centaurus, is one of these remnants.
All models of the North Polar Spur (or Loop I) supernova remnant place Sun in
rim, if it is spherical.
(Frisch, APOD)
35