Download Leaky Stars: Pulsations, Waves, and Turbulence in Stellar Winds

Leaky Stars:
Pulsations, Waves, and Turbulence
in Stellar Winds
across the H-R Diagram
Steven R. Cranmer & many others
Harvard-Smithsonian Center for Astrophysics
Leaky Stars:
Pulsations,
Outline:
Waves, and Turbulence
Stellar
• Background: in
history
& basicWinds
physics
across
the
H-R
Diagram
• The Sun: coronal heating & fast solar wind
• Hot stars (O, B, W-R): pulsations & radiation-driving
• Cool stars (T Tau, Mira): chromospheric flows?
Steven R. Cranmer & many others
Harvard-Smithsonian Center for Astrophysics
Motivations . . .
Solar corona & wind:
• “Space weather” can affect satellites, power
grids, and astronaut safety.
• Sun’s mass-loss history may have impacted
planetary formation / atmospheres.
• The Sun is a “benchmark” for many basic
processes in plasma physics.
Stellar winds:
• Mass loss affects evolutionary tracks
(isochrones, cluster HB/RGB), SN yields.
• Hot-star winds influence ISM abundances &
ionization state of Galaxy.
• Spectroscopy of wind lines
extragalactic
standard candles?
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
First observations of stellar outflows ?
• Coronae & Aurorae seen since antiquity . . .
• “New stars”
1572: Tycho’s supernova
1600: P Cygni outburst
(“Revenante of the Swan”)
1604: Kepler’s supernova
in “Serepentarius”
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
Brief history: solar wind
• 1860–1950: Evidence slowly builds for outflowing magnetized plasma in the
solar system:
• solar flares  aurora, telegraph snafus, geomagnetic “storms”
• comet ion tails point anti-sunward (no matter comet’s motion)
• 1958: Eugene Parker proposed that the hot corona provides enough gas pressure
to counteract gravity and accelerate a “solar wind.”
• 1962: Mariner 2 provided direct confirmation!
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
Brief history: stellar winds
• Milne (1924): rad. pressure
can eject atoms/ions from
stellar atmospheres.
• P Cygni profiles = winds:
» O, B, WR, LBVs: Beals (1929);
Swings & Struve (1940)
» G, K, M giants, supergiants:
Adams & MacCormack (1935);
Deutsch (1956)
• Also: IR excesses, maser
O supergiant (Morton 1967)
M supergiant (Bernat 1976)
emission, “plain” blueshifts.
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
Schematic H-R Diagram
106
I
104
III
102
V
Sun
1
10–2
O
30,000
B
A
10,000
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram

F G
6,000
K
M
3,000
S. R. Cranmer
CfA Colloquium, March 9, 2006
Stellar winds
106
no coronae?
I
"cool"
dense
radiatively
driven winds
104
(slow?)
winds
"warm"
hybrid
winds
III
102
Be stars
"hot"
solar-type
winds
V
Sun
1

flare
stars
10–2
O
30,000
B
A
10,000
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
F G
6,000
K
M
3,000
S. R. Cranmer
CfA Colloquium, March 9, 2006
Convection zones
106
I
deep
core
convection
104
fully
convective
III
102
V
Sun
1

subsurface
convection
10–2
O
30,000
B
A
10,000
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
F G
6,000
K
M
3,000
S. R. Cranmer
CfA Colloquium, March 9, 2006
Temperatures in outer atmospheres
Sun
Hot star (O, B)
Cool star (K, M)
?
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
One-page stellar wind physics
• Momentum conservation:
To sustain a wind, /t = 0 ,
and RHS must be “tuned:”
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
One-page stellar wind physics
• Momentum conservation:
To sustain a wind, /t = 0 ,
and RHS must be “tuned:”
• Energy conservation:
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
One-page stellar wind physics
• Momentum conservation:
To sustain a wind, /t = 0 ,
and RHS must be “tuned:”
• Energy conservation:
• Photosphere (& most of hot-star wind)
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
One-page stellar wind physics
• Momentum conservation:
To sustain a wind, /t = 0 ,
and RHS must be “tuned:”
• Energy conservation:
• Chromosphere: heating  rad. losses
• Photosphere (& most of hot-star wind)
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
One-page stellar wind physics
• Momentum conservation:
To sustain a wind, /t = 0 ,
and RHS must be “tuned:”
• Energy conservation:
• Transition region & low corona
• Chromosphere: heating  rad. losses
• Photosphere (& most of hot-star wind)
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
One-page stellar wind physics
• Momentum conservation:
To sustain a wind, /t = 0 ,
and RHS must be “tuned:”
• Energy conservation:
• Extended corona & cool-star wind
• Transition region & low corona
• Chromosphere: heating  rad. losses
• Photosphere (& most of hot-star wind)
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
Solar convection & surface waves
• Cool stars with sub-photospheric convection undergo “p-mode” oscillations:
• Lighthill (1952) showed how
turbulent motions generate
acoustic power; more recently
generalized to MHD . . .
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
Solar convection & surface waves
• Cool stars with sub-photospheric convection undergo “p-mode” oscillations:
• Lighthill (1952) showed how
turbulent motions generate
acoustic power; more recently
generalized to MHD . . .
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
Coronal heating mechanisms
• A surplus of proposed models! (Mandrini et al. 2000; Aschwanden et al. 2001)
• Where does the mechanical
vs.
energy come from?
• How is this energy coupled
to the coronal plasma?
• How is the energy dissipated
waves
shocks
eddies
(“AC”)
interact with
inhomog./nonlin.
vs.
twisting
braiding
shear
(“DC”)
turbulence
reconnection
and converted to heat?
collisions (visc, cond, resist, friction) or collisionless
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
Coronal heating mechanisms
• A surplus of proposed models! (Mandrini et al. 2000; Aschwanden et al. 2001)
• Where does the mechanical
vs.
energy come from?
• How is this energy coupled
to the coronal plasma?
• How is the energy dissipated
waves
shocks
eddies
(“AC”)
interact with
inhomog./nonlin.
vs.
twisting
braiding
shear
(“DC”)
turbulence
reconnection
and converted to heat?
collisions (visc, cond, resist, friction) or collisionless
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
Inter-granular bright points
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
An Alfvén wave heating model
• Cranmer & van Ballegooijen (2005) built a model of the global properties of
incompressible non-WKB Alfvenic turbulence along an open flux tube.
• Background plasma properties (density, flow speed, B-field strength) were fixed
empirically; wave properties were modeled with virtually no “free” parameters.
• Lower boundary condition: observed horizontal motions of G-band bright points.
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
MHD turbulence
• It is highly likely that somewhere in the outer solar
atmosphere the fluctuations become turbulent and
cascade from large to small scales:
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
MHD turbulence
• It is highly likely that somewhere in the outer solar
atmosphere the fluctuations become turbulent and
cascade from large to small scales:
• With a strong background field, it is
easier to mix field lines (perp. to B)
than it is to bend them (parallel to B).
• Also, the energy transport along the
Z–
Z+
field is far from isotropic:
Z–
(e.g., Dmitruk et al. 2002)
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
Turbulent heating rate
• Solid curve: predicted Qheat
for a polar coronal hole.
• Dashed RGB regions:
empirical estimates of heating
rate of primary plasma
(models tuned to match
conditions at 1 AU).
• What is really needed are
direct measurements of the
plasma (atoms, ions,
electrons) in the acceleration
region of the solar wind!
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
UVCS / SOHO
• SOHO (the Solar and Heliospheric Observatory) was launched in Dec. 1995 with
12 instruments probing solar interior to outer heliosphere.
• The Ultraviolet Coronagraph Spectrometer
(UVCS) measures plasma properties of
coronal protons, ions, and electrons between
1.5 and 10 solar radii.
• Combines occultation with spectroscopy to
do what neither alone could accomplish.
slit field of view:
• Mirror motions select height
• Instrument rolls indep. of spacecraft
• 2 UV channels: LYA & OVI
• 1 white-light polarimetry channel
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
UVCS results: solar minimum (1996-1997 )
• The fastest solar wind flow is expected to come from dim “coronal holes.”
• In June 1996, the first measurements of heavy ion (e.g., O+5) line emission in the
extended corona revealed surprisingly wide line profiles . . .
On-disk profiles: T = 1–3 million K
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
Off-limb profiles: T > 200 million K !
S. R. Cranmer
CfA Colloquium, March 9, 2006
The impact of UVCS
UVCS/SOHO has led to new views of the acceleration regions of the solar wind.
Key results include:
• The fast solar wind becomes supersonic
much closer to the Sun (~2 Rs) than
previously believed.
• In coronal holes, heavy ions (e.g., O+5)
both flow faster and are heated hundreds
of times more strongly than protons and
electrons, and have anisotropic
temperatures. (Kohl et al. 1997, 1998)
• At very large heights in bright
streamers, the heavy ions begin to depart
from thermal equilibrium in a similar way
to coronal holes.
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
Ion cyclotron waves in the corona?
• UVCS observations have rekindled theoretical efforts to understand heating and
acceleration of the plasma in the (collisionless?) acceleration region of the wind.
• Ion cyclotron waves (10–10,000 Hz)
suggested as a “natural” energy source that
can be tapped to preferentially heat &
accelerate heavy ions.
MHD turbulence
something
else?
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
Alfven wave’s
oscillating
E and B fields
ion’s Larmor
motion around
radial B-field
cyclotron resonancelike phenomena
S. R. Cranmer
CfA Colloquium, March 9, 2006
Switch gears . . .
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
Hot star winds: radiative driving
• Bound electron resonances have higher crosssections than free electrons (higher “Q”)
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
Hot star winds: radiative driving
• Bound electron resonances have higher crosssections than free electrons (higher “Q”)
• More acceleration facilitates more forcing!
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
CAK wind theory
• Highly nonlinear momentum equation has a steady-state solution only for a
specific maximal mass-loss rate (“eigenvalue”); Castor, Abbott, & Klein (1975)
• Results agree well
with observations.
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
More about pulsations . . .
• Interior: discrete
spectrum of “standing
waves”
• Exterior: continuous
spectrum of “traveling
waves”
• Nonradial pulsations (NRP) are observable
at the photosphere via Doppler-shifted line
profile variations:
• For hot stars, these are mainly “g-modes”
(excited by deep-core convection & various
opacity/ionization instabilities)
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
Acoustic-gravity waves: evanescence?
• In an isothermal, hydrostatic atmosphere,
acoustic waves conserve energy density by
growing in amplitude: δE ~ ρ (δv2)
• There is an acoustic cutoff frequency, below
which waves are evanescent (non-propagating)
• Most (low-order) oscillations are evanescent in
a stellar photosphere.
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
height
S. R. Cranmer
CfA Colloquium, March 9, 2006
Acoustic-gravity waves: evanescence?
• In an isothermal, hydrostatic atmosphere,
acoustic waves conserve energy density by
growing in amplitude: δE ~ ρ (δv2)
• There is an acoustic cutoff frequency, below
which waves are evanescent (non-propagating)
• Most (low-order) oscillations are evanescent in
a stellar photosphere.
height
v/cs = 0.01
v/cs = 0.1
• When a subsonic wind is considered, ALL frequencies are able to propagate!
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
Wave leakage: 1D simulation results
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
Stronger pulsational amplitude
• Instead of varying base density by 1% (0.99 to 1.01), vary it by a factor of 60
(i.e., 1/60 to 60), for ω/ωac ~ 0.3 :
• In the supersonic wind, acoustic waves are modified by the radiative force into the
so-called “Abbott waves:” CAK critical point = sub
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
super-Abbott flow!
S. R. Cranmer
CfA Colloquium, March 9, 2006
Synthesized P Cygni profiles
• Profiles computed using “SEI”
(Sobolev w/ Exact Integration):
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
Synthesized P Cygni profiles
• Profiles computed using “SEI”
(Sobolev w/ Exact Integration):
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
• BW Vulpeculae (large-amplitude β
Cep pulsator), seen with IUE:
S. R. Cranmer
CfA Colloquium, March 9, 2006
Be stars: “decretion disks”
• Be stars are non-supergiant B-type stars
with emission in hydrogen Balmer lines.
• Be stars are rapid rotators, but are not
rotating at “critical” / “breakup:”
Vrot  (0.5 to 0.9) Vcrit
• How does angular momentum get
added to the circumstellar gas ?
Hints:
• Many (all?) Be stars undergo NRPs.
• Rivinius et al. (1998, 2001) found correlations between emission-line “outbursts”
and constructive interference (“beating”) between NRP periods.
• Ando (1986) & Saio (1994) suggested that nonadiabatic NRPs could transfer
angular momentum outwards: similar to wave “radiation pressure!”
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
Cool stars: younger/older Sun
• How do outflows & inflows co-exist around
young T Tauri stars, and does the disk
accretion power the wind?
• If not, then is the wind similar to the
“mature” solar wind?
• How can there be fast/supersonic
winds in the chromospheres of
both young & evolved solar-mass
stars?
waves/shocks?
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
Cool stars: Mira supergiants
• How are the presumably cool (“corona-free”)
winds of red giants and supergiants
accelerated?
• How do these winds affect the shapes of the
planetary nebulae that are formed at the end
of stellar evolution?
• High-luminosity: radiative driving... of dust
• Shock-heated “calorispheres” (Willson 2000)
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
Conclusions
• Stellar winds & circumstellar waves affect a
broad swath of astrophysical problems.
• Observations: spectroscopy is key!
More plasma diagnostics
Better understanding!
• The surprisingly extreme plasma conditions in
solar coronal holes (T ion >> Tp > Te ) have
guided theorists to discard some candidate
processes, further investigate others, and have
cross-fertilized other areas of plasma physics
& astrophysics.
• Future observational programs are needed:
next-generation UVCS, high-res UV stellar
spectroscopy, Stellar Imager (interferometry).
For more information: http://cfa-www.harvard.edu/~scranmer/
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
Extra slides . . .
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
The need for both
solar-disk & coronagraph observations
• On-disk measurements
help reveal basal coronal
heating & lower boundary
conditions for solar wind.
• Off-limb measurements (in solar wind “acceleration region” ) allow dynamic
non-equilibrium plasma states to be followed as the asymptotic conditions at 1 AU
are gradually established.
Occultation is required because
extended corona is 5 to 10 orders of
magnitude less bright than the disk!
Spectroscopy provides detailed
plasma diagnostics that imaging
alone cannot.
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
Spectroscopic diagnostics
• Off-limb photons formed by both collisional excitation/de-excitation and resonant
scattering of solar-disk photons.
• Profile width depends on line-of-sight component of velocity distribution (i.e.,
perp. temperature and projected component of wind flow speed).
• Total intensity depends
on the radial component
of velocity distribution
(parallel temperature and
main component of wind
flow speed), as well as
density.
• If atoms are flow in the same direction as incoming
disk photons, “Doppler dimming/pumping” occurs.
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
Doppler dimming & pumping
• After H I Lyman alpha, the O VI 1032, 1037 doublet are the next brightest lines in
the extended corona.
• The isolated 1032 line Doppler dims like
Lyman alpha.
• The 1037 line is “Doppler pumped” by
neighboring C II line photons when O5+
outflow speed passes 175 and 370 km/s.
• The ratio R of 1032 to 1037 intensity
depends on both the bulk outflow speed
(of O5+ ions) and their parallel
temperature. . .
• The line widths constrain perpendicular
temperature to be > 100 million K.
• R < 1 implies anisotropy!
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
Coronal holes: over the solar cycle
• Even though large coronal holes have similar outflow speeds at 1 AU (>600 km/s),
their acceleration (in O+5) in the corona is different! (Miralles et al. 2001)
Solar minimum:
Solar maximum:
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
Thin tubes merge into supergranular funnels
Peter (2001)
Tu et al.
(2005)
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
Resulting wave amplitude (with damping)
• Transport equations solved for 300 “monochromatic” periods (3 sec to 3 days), then
renormalized using photospheric power spectrum.
• One free parameter: base “jump amplitude” (0 to 5 km/s allowed; 3 km/s is best)
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
Anisotropic MHD cascade
• Can MHD turbulence generate ion cyclotron waves? Many models say no!
• Simulations & analytic models
predict cascade from small to
large k ,leaving k ~unchanged.
“Kinetic Alfven waves” with
large k do not necessarily
have high frequencies.
• In a low-beta plasma, KAWs
are Landau-damped, heating
electrons preferentially!
• Cranmer & van Ballegooijen
(2003) modeled the anisotropic
cascade with advection &
diffusion in k-space and found
some k “leakage” . . .
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
Streamers: open and/or closed?
Wang et al. (2000)
• High-speed wind: strong connections to the largest coronal holes
• Low-speed wind: still no agreement
on the full range of coronal sources:
hole/streamer boundary (streamer “edge”)
streamer plasma sheet (“cusp/stalk”)
small coronal holes
active regions (some with streamer cusps)
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
Streamers with UVCS
• Streamers viewed “edge-on”
look different in H0 and O+5
• Ion abundance depletion in
“core” due to grav. settling?
• Brightest “legs” show
negligible outflow, but
abundances consistent with
in situ slow wind.
• Higher latitudes and upper
“stalk” show definite flows
(Strachan et al. 2002).
• Stalk also has preferential
ion heating & anisotropy,
like coronal holes! (Frazin
et al. 2003)
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
Heating above & below the critical point
• Why does the critical point matter? Leer & Holzer (1980), Pneuman (1980):
SUBSONIC coronal heating:
“puffs up” scale height,
draws more particles into wind:
M
vs.
u
SUPERSONIC coronal heating:
subsonic region is unaffected.
Energy flux has nowhere else
to go:
M same, u
• Even if heating is the same (hole vs. streamer), moving rcrit changes the above!
• Also, changing f(r) changes where the Alfven wave flux is the strongest:
FA   <v 2>VA  Br
FA (crit)
Br (crit)
Hypothesis:
all flux tubes have same FA
1
---------  --------  ---FA
Br
f
(Kovalenko 1978; Wang & Sheeley 1991)
• But how is an increased “Alfven wave flux” linked to actual heating?
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
Why is the fast/slow wind fast/slow?
• Several ideas exist; one powerful one relates flux tube expansion to wind speed
(Wang & Sheeley 1990). Physically, the geometry determines location of Parker
critical point, which determines how the “available” heating affects the plasma:
SUPERSONIC coronal heating:
subsonic region is unaffected.
Energy flux has nowhere else to
go:
M same, u
vs.
SUBSONIC coronal heating:
“puffs up” scale height, draws
more particles into wind:
M
u
• MHD turbulence heating rates give temperatures consistent with UVCS & in situ.
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
CMEs
• Coronal mass ejections (CMEs) are an efficient way for the Sun to shed twisted
magnetic fields (and net helicity?) and generated solar energetic particles (SEPs).
Coronagraph images contain
much information, but
spectroscopy also provides:
• Heating rates and energy
budget (DEM)
• 3D velocity field & chirality;
twisting/unwtisting rates
• conditions in shocks
(preferential ion accel.)
• conditions in current sheets
(reconnection rates)
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
UVCS CME results: Doppler shifts
Feb. 12, 2000
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
Intensity
Width
Shift
(Lyman alpha) April 18, 2000
S. R. Cranmer
CfA Colloquium, March 9, 2006
UVCS CME results: Reconnection physics
• On several occasions, narrow brightening
in Fe XVIII (Te ~ 6 MK) appears in the
probable location of a current sheet.
• Lin et al. (2005) also saw Lyman alpha
“closing down” in the sheet: one can
measure reconnection rate (Vin / Vout )
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006
The Need for Better Observations
Even though UVCS/SOHO has made significant advances,
• We still do not understand the physical processes that heat and
accelerate the entire plasma (protons, electrons, heavy ions),
(Our understanding of ion cyclotron resonance is
based essentially on just one ion!)
• There is still controversy about whether the fast solar wind occurs
primarily in dense polar plumes or in low-density inter-plume
plasma,
• We still do not know how and where the various components of
the variable slow solar wind are produced (e.g., “blobs”).
UVCS has shown that answering these questions is possible, but
cannot make the required observations.
Leaky Stars: Pulsations, Waves, and Turbulence in
Stellar Winds across the H-R Diagram
S. R. Cranmer
CfA Colloquium, March 9, 2006