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CDP Workshop on Magnetic Fields and Structures Magnetic fields in the interstellar medium, galactic


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Title: CDP Workshop on Magnetic Fields and Structures Magnetic fields in the interstellar medium, galactic

CDP Workshop on Magnetic Fields and
StructuresMagnetic fields in the interstellar
medium,galactic halos, and intergalactic medium
  • Jeffrey Linsky
  • University of Colorado
  • February 13, 2008
  • File CDP_ism.ppt

Magnetic fields in low density media Topics to
be covered
  • Magnetic field at the edge of the heliosphere
  • Properties of the ISM
  • Methods for measuring magnetic fields in the ISM
  • Magnetic field strucure in galaxies

The solar wind structure is controlled by the
magnetic field
  • Coronal hole regions have open magnetic fields,
    high speed (800 km/s) flows and low densities.
  • Near the equator complex field structures
    dominate with low speed (400 km/s) flows and
    high densities.
  • ?vconstant
  • dM/dt2x10-14 Msun /yr
  • 1x109 kg/s

Heliosphere/ISM Interaction Terminology
  • Termination shock where the supersonic solar
    wind (400-800 km/s) becomes subsonic and heated
    (94 AU for Voyager 1)
  • Heliopause Interface around the Sun between the
    subsonic solar wind and ISM plasma (150 AU)
  • Bow shock where the incoming ISM (26 km/s)
    becomes subsonic (250 AU). May not shock depends
    on magnetic field.
  • Hydrogen wall Pileup of neutral H gas mostly in
    upwind direction with charge exchange (150-250
  • Plasma models include electromagnetic and
    gravity forces on all ionized particles (either
    as one or multifluid models)
  • Kinetic models treat neutral particles (e.g. H)
    with long path lengths by Boltzmann equation or
    Monte Carlo techniques.
  • Pickup ions interstellar neutrals that are
    ionized (photoionization or charge exchange) and
    captured by the solar wind magnetic field and
    accelerated at the termination shock.

Interaction of Stars with their LISM
Heliosphere is the structure caused by the
momentum balance (?v2) between the outward moving
solar wind and the surrounding interstellar
medium. Magnetized solar wind extends out to
heliopause, diverts plasma around Solar System,
and modulates the cosmic ray flux into Solar
System. Most neutrals stream in unperturbed,
except neutral hydrogen, which due to charge
exchange reactions, is heated and decelerated
forming Hydrogen Wall (log NH (cm-2)
14.5). Reviews of heliospheric modeling Wood
(2004), Zank (1999), and Baranov (1990).
Reviews of interaction of LISM with heliosphere
Solar Journey Frisch (2006), and Redfield (2006)
Müller (2004)
What has Voyager 1 told us about the Termination
  • Voyager 1 was launched 5 Sept 1977, passed
    Jupiter and Saturn and crossed the termination
    shock (TS) on 16 Dec 2004 at 94 AU from the Sun.
  • Onboard detectors measured the magnetic field and
    energies and directions of energetic electrons,
    protons, and He nuclei.
  • Weak shock velocity jump (r2.6).
  • TS not spherical due to magnetic field.
  • TS moves in and out with the solar cycle.
  • Solar wind in heliosheath slower, hotter, and
    denser than inside the TS.
  • The TS was expected to be the location where
    anomalous cosmic rays (ACRs) are accelerated, but
    peak in ACR flux not at the TS.

Change in Magnetic Field Strength and Direction
(heliographic) at the TS Burlaga (2005) Science
309, 2027
Change in Magnetic Field at the TS Burlaga et
al. (2005)
  • Magnetic compression ratio across TS 3/-1.
  • Complete change in average field strength perhaps
    due to thermalization of the plasma by the TS.
  • Inward moving TS (gt90 km/s) may have crossed
    Voyager 1 Jokipii (2005) ApJ 631, L163.
  • 1nT10µG

Effect of the Interstellar Magnetic Field on the
Heliosphere (Opher et al. 2006, 2007)
  • 3-D MHD adaptive grid models (BATS-R-US code)
    with interstellar magnetic field (B1.8µG)
    a30-60 degrees from inflow direction. (60-90
    degrees from Galactic plane).
  • VSW450 km/s, vism25.5 km/s Parker spiral B2µG
    at equator
  • Produces a N-S asymmetry in the TS and
    heliosheath and a deflection in the current sheet
  • TS moved in 2.0 AU for Voyager 1 with TSP
    streaming outward from Sun along a spiral field
    line that does not first go through the TS.
  • Moves TS inward more in S (Voyager 2) than N
    (Voyager 1).
  • Where Voyager 2 crosses the TS will be the
    critical test of value of a.

Observations of Neutral Hydrogen in the
Heliosphere from Lyman-a Backscattering
Measurements References
  • Bertaux et al. (1995) Solar Phys. 162, 403
    describes the SWAN (Solar Wind Anisotropies)
    experiment on SOHO (Solar and Heliospheric
    Observatory) satellite.
  • Lallement et al. (2005) Science 307, 1447.
  • Quémerais et al. (2006) AA 455, 1135

SWAN maps Backscattered Lyman-a Radiation from
Neutral Hydrogen Flowing into the Heliosphere
using a Hydrogen Gas Absorption Cell Spectrometer
  • Maximum in the Lyman-a sky glow when inflowing H
    has maximum Doppler shift relative to SOHO.
  • Maps show Lyman-a glow at different times of year
    when SOHO has different velocities relative to
    the H inflow vector.

Deflection of Neutral H vs. He due to charge
exchanged Solar Wind Protons becoming H
  • Incoming He atoms retain the ISM inflow direction
    because no charge exchange.
  • H secondaries will be deviated only if the
    interstellar magnetic field is inclined relative
    to the gas flow direction.
  • Measured deviation is 4/-1 degrees.

Inferring the Direction of the Local Interstellar
Magnetic Field
  • MHD models consistent with the deviation of H
    relative to He predict the magnetic field
    direction in Galactic coordinates (40-60 degrees
    from Galactic plane).
  • Magnetic field is parallel to the edges of the
    LIC and G clouds and likely compressed by the
    relative motion of the two clouds.

Techniques for measuring magnetic field
properties (B and orientation) in the ISM
  • Polarization of starlight measures field
    orientation only
  • Zeeman splitting of the HI 21cm line and other
    lines in the radio spectrum.
  • Faraday rotation of linear polarization.
  • Synchrotron radiation at radio wavelengths.

Polarization of starlight by aligned dust grains
(Davis Greenstein ApJ 114, 206 (1951))
  • Polarization by absorption and scattering of
    background starlight by dust grains partially
    aligned by the magnetic field.
  • Component of light with E vector along the long
    axis of grains (containing some iron) is
    partially attenuated.
  • Spinning grains aligned by torque due to
    paramagnetic relaxation of iron in the grains.
    Makes the short axis of the grains the axis of
  • Maximum light extinction for E perpendicular to
  • Main result magnetic field oriented parallel to
    the plane of the Galaxy out to several hundred pc.

Why should the polarization be aligned with the
magnetic field?
  • Alignment mechanisms (e.g., Alfven waves) predict
    that the rotation axis is perpendicular to the
    long dimension and parallel to B.
  • Extinction coefficient larger along long axis ?
    maximum polarization when B in plane of the sky.
  • So, polarization and B are coaligned.

Jones et al. ApJ 389, 602 (1992))
Orientation of the Galactic magnetic field from
starlight polarization (Heiles ApJ 462, 316
  • Plots show optical polarization from reanalysis
    of Mathewson Ford (1970) compilation.
  • Assume that linear polarization aligned parallel
    to B.
  • Conclusion magnetic field lines are spiral but
    do not follow the spiral arms of the Galaxy in
    detail. Maybe a field reversal just inside the
    solar orbit.

Measuring interstellar magnetic fields using
Zeeman polarization of the HI 21cm line
  • Early observations showing circular polarization
    differences from two lines of sight.
  • Line of sight B 6.71.5µG, 6.91.2µG.
  • Troland Heiles (ApJ 260, L19 (1982)).

Statistics of magnetic field strengths in the
Galaxy from Zeeman 21cm measurements (Heiles
Troland ApJ 624, 773 (2005))
  • Data from Arecibo telescope Stokes V survey in HI
    21cm line.
  • Typical parameters for cold neutral medium (CNM)
    at right.
  • In CNM the magnetic field dominates the thermal
  • Turbulence and magnetism are in approximate

Median value of the total magnetic field in CNM,
Btot6.01.8µG (Heiles and Troland (2003))
Stokes V data measures line of sight value of B
not the total value. Inferring the median value
of Btot from the Blos data involves modelling
(see the paper).
Measuring the interstellar magnetic field
strength and orientation using Faraday rotation
techniques (e.g., Rand Lyne MNRAS 268, 497
  • Faraday rotation of a linearly polarized wave
    propagating along B is due to interaction of E
    with electrons right-hand mode travels faster
    than left-hand mode ? rotation of polarization.
  • Observe pulsars through the ionized component of
    the ISM.
  • RM ?² and measured from Stokes Q and U vectors.

Results ltBgt1.4µG with 5µG variations 2 field
reversals inside solar location B oriented
toward l885
New study of interstellar B using pulsar
rotation measures (Han et al. ApJ 642, 868 (2006))
Inferring the Direction of the Local Interstellar
Magnetic Field
  • MHD models consistent with the deviation of H
    relative to He predict the magnetic field
    direction in Galactic coordinates (30-60 degrees
    from Galactic plane).
  • Magnetic field is parallel to the edges of the
    LIC and G clouds and likely compressed by the
    relative motion of the two clouds.

Measuring interstellar magnetic fields from
synchrotron emission
Power law energy spectrum of relativistic
electrons (assumed).
Synchrotron emissivity for the electron energy
Electrons emit peak power at this frequency.
Since the synchrotron emission depends on B and
f(E), one must measure or assume one to get the
other. Near Earth f(E) is measured, except at low
energies, ? Btot5.0µG. Elsewhere in Galaxy,
assume same f(E) as locally or some other
Radio intensity and polarization map of the
spiral galaxy NGC 4254 (Chyzy et al. AA 474, 415
Radio intensity and polarization map of the
barred spiral NGC 1365 (Moss et al. AA 465, 157
Classical Theoretical Models of the
ISMReferences Assumptions
  • Field, Goldsmith, Habing (1969) ApJ 155, L149
  • McKee, Ostriker (1977) ApJ 218, 148 (CNM,
  • Wolfire et al.(2003) ApJ 587,278 (3-phase)
  • Ferrière (2001) Rev.Mod.Phys. 73(4)1031 (ISM
  • Cox(2005) ARAA 43, 337 (important review)
  • Hydrostatic equilibrium (only thermal pressure
  • Thermal equilibrium
  • Heating by UV photoelectric (on gas and grains)
    (depends on SN rate)
  • Cooling (radiation by forbidden lines, PAHs)
  • Thermally unstable P(n) curve allows coexistence
    of 3 phases at n0.3 to 30/cm³, P/k1700-4400,
  • Nonthermal pressure terms not included

Constituents of the Local Bubble Pressure
Components (K/cm³)
  • Weight of overlying gas Pmid/k 22,000
  • Cosmic rays Pcr/k 9300
  • Magnetic pressure Pmag/k B²/8pk
  • 7200 (B/5µG)²
  • Ram pressure Pram/k ?v²/k 2400(ntot/0.3)(v/8
  • Thermal pressure Pth/k ntotT2300 (but a wide
  • PmagPth when Beq2.8µG
  • PmagPcr estimated from observed synchrotron

Measurements of thermal gas pressures
  • In LIC, P/k 2300 from spectral line widths and
    gas densities.
  • Jenkins Tripp (2001) study of C I fine
    structure excitation (STIS 1.5 km/s spectra).
  • Mean thermal P/k2240.
  • 15 of gas at P/kgt5,000
  • A very small amount of gas at P/kgt100,000.
    Turbulent compression? Very small sizes (0.01 pc)?

C I fine structure line spectra obtained by
Jenkins Tripp (2001) with STIS at 1.5 km/s
Thermal Pressure Disequilibrium
  • Theoretical two-phase equilibrium ISM model
    (Wolfire et al. 2003 Cox 2005).
  • Blue dotted curve is 10X higher heating rate.
  • Black dashed line is total midplane pressure due
    to overlying matter.
  • Orange dashed line is mean magnetic pressure.
  • Nonthermal pressure (dynamic, magnetic, cosmic
    ray) dominates the thermal pressure. Therefore, a
    wide range of pressures (1700-20,000 can be
    compensated by different nonthermal terms.
  • Is this the whole story? Dynamic pressures can be
    measured by high-resolution UV spectroscopy.

Some common themes and conclusions concerning
magnetic fields in astrophysics
  • Magnetic fields are everywhere and generally play
    an important role in creating observed phenomena
    (even when not anticipated by theoreticians).
  • Over 21 orders of magnitude, magnetic field
    strengths are usually close to equipartition,
    ßPother/Pmag 100few.
  • Measuring B involves thoughtful (and usually
    innovative) application of a few basic physical
  • Without magnetic fields the universe would be far
    less interesting.

Magnetic fields are to astrophysics as sex is to
Zank Frisch (1999)
How large of a density increase is needed to
significantly alter the structure of the
NH(LISM) 10 cm-3
Increase the density of the surrounding LISM by
only a factor of 50 (nH from 0.2 to 10 cm-3) and
the termination shock shrinks from 100 AU to 10
Consequences of a compressed heliosphere
(1) Cosmic rays flux modulated by magnetized
solar wind Cloud nucleation, increase in
planetary albedo (Marsh Svensmark 2000, Carslaw
et al. 2002) Lightning production increase
(Gurevich Zybin 2005) CR alters ozone layer
chemistry (Randall et al. 2005) CR source DNA
mutation (Reedy et al. 1983) (2) Direct
deposition of ISM material onto planetary
atmosphere Dust deposition could trigger
snowball Earth episode (Pavlov et al.
2005) Create mesospheric ice clouds, increase in
planetary albedo (McKay Thomas 1978)
NH(LISM) 0.2 cm-3
Müller (2004)
Measuring the Properties of Interstellar Clouds
  • Linsky et al. (2000) ApJ 528, 756
  • Redfield Linsky (2000) ApJ 534, 825
  • Redfield Linsky (2002) ApJS 139, 439
  • Redfield Linsky (2004) ApJ 602, 776
  • Redfield Linsky (2004) ApJ 613, 1004
  • Redfield Linsky (2007) ApJ, in press

Nearby warm clouds in the interstellar medium.
Mic cloud is the warmest (10,000 K)
Heliosphere/ISM interaction Examples of Code
  • Monte Carlo codes beginning with Baranov Malama
    (1993, 1995).
  • Hydrodynamic four-fluid codes beginning with Zank
    et al. (1996) (one fluid for protons, 3 fluids
    for hydrogenprimary, secondary, tertiary atoms)
  • Hybrid codes beginning with Muller et al. (2000).
  • MHD codes (cf. Opher 2004).

Stone et al. (Science 309, 2017 (2005))
  • A Energetic proton intensity streaming from the
  • B Termination Shock Particle (TSP) streaming
    anisotropy decreases when cross the TS due to
    scattering by magnetic turbulence.
  • C Intensities of high-energy protons and
  • D Intensities of Galactic cosmic rays and He
    ions (anomalous cosmic rays).