Transcript No Slide Title

Plasma Unbound:
New Insights into
Coronal Heating and
Solar / Stellar Winds
Steven R. Cranmer
Harvard-Smithsonian Center for Astrophysics
Plasma Unbound:
Outline:
New Insights
•
•
into
OverviewCoronal
and brief historical
background
Heating
and
Heating the coronal base
(reconnection & turbulence)
Solar/Stellar
Winds
3. Heating and accelerating the extended corona
(waves & turbulence)
4. The young Sun
accretion-powered winds?
Steven R. Cranmer
Harvard-Smithsonian Center for Astrophysics
Motivations
Solar corona & solar wind:
• Space weather can affect satellites, power grids,
and astronaut safety.
• The Sun’s mass-loss & X-ray history impacted
planetary formation and atmospheric erosion.
• The Sun is a “laboratory without walls”
for many basic processes in physics, at
regimes (T, P) inaccessible on Earth!
• plasma physics
• nuclear physics
• non-equilibrium thermodynamics
• electromagnetic theory
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
The extended solar atmosphere
Heating is everywhere . . .
. . . and everything is in motion
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
The extended solar atmosphere
Heating is everywhere . . .
. . . and everything is in motion
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
Too-brief history
• Total eclipses let us see the vibrant outer
solar corona: but what is it?
• 1870s: spectrographs pointed at corona:
• Fraunhofer lines (not moon-related)
• unknown bright lines
• 1930s: Lines identified as highly ionized
ions: Ca+12 , Fe+9 to Fe+13  it’s hot!
• 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.”
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
In situ solar wind: properties
• 1962: Mariner 2 detected two phases of solar wind: slow (mostly) + fast streams
• Uncertainties about which type is “ambient”
persisted because measurements were limited to
the ecliptic plane . . .
• Ulysses left the ecliptic; provided 3D view of
the wind’s source regions.
fast
slow
600–800
300–500
Tp (105 K)
2.4
0.4
Te (105 K)
1.0
1.3
> mion/mp
< mion/mp
low
high
speed (km/s)
Tion / Tp
O7+/O6+, Mg/O
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
By ~1990, it was clear the
fast wind needs something
besides gas pressure to
accelerate so fast!
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
Ulysses’ view over the poles
McComas et al. (2008)
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
Exploring the solar wind (1970s to present)
Space probes have pushed out the boundaries of the “known” solar wind . . .
• Helios 1 & 2: inner solar wind (Earth to Mercury)
• Ulysses: outer solar wind (Earth to Jupiter, also flew over N/S poles)
• Voyager 1 & 2: far out past Pluto: recently passed the boundary between the
solar wind and the interstellar medium
• CLUSTER: multiple spacecraft probe time and space variations simultaneously
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
Particles are not in “thermal equilibrium”
…especially in the
high-speed wind.
mag.
field
WIND at 1 AU
(Steinberg et al. 1996)
WIND at 1 AU
(Collier et al. 1996)
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Helios at 0.3 AU
(e.g., Marsch et al. 1982)
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
Overview of coronal observations
• Plasma at 106 K emits most of its spectrum in the UV and X-ray . . .
Coronal hole (open)
“Quiet”
regions
Active
regions
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
Solar wind: connectivity to the corona
• 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
Wang et al. (2000)
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
1990s: SOHO’s new view of the corona
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
The coronal heating problem
• We still don’t understand the physical processes responsible for heating up the
coronal plasma.
A lot of the heating occurs in a narrow “shell.”
• Most suggested ideas involve 3 general steps:
1. Churning convective motions that tangle
up magnetic fields on the surface.
2. Energy is stored in tiny twisted & braided
magnetic flux tubes.
3. Collisions between ions and electrons
(i.e., friction?) release energy as heat.
Heating
Solar wind acceleration!
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
Coronal heating mechanisms
• So many ideas, taxonomy is needed!
(Mandrini et al. 2000; Aschwanden et al. 2001)
• Where does the mechanical
energy come from?
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
vs.
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
Coronal heating mechanisms
• So many ideas, taxonomy is needed!
(Mandrini et al. 2000; Aschwanden et al. 2001)
• Where does the mechanical
vs.
energy come from?
waves
shocks
eddies
• How rapidly is this energy
coupled to the coronal
plasma?
(“AC”)
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
vs.
twisting
braiding
shear
(“DC”)
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
Coronal heating mechanisms
• So many ideas, taxonomy is needed!
(Mandrini et al. 2000; Aschwanden et al. 2001)
• Where does the mechanical
vs.
energy come from?
waves
shocks
eddies
• How rapidly is this energy
coupled to the coronal
plasma?
(“AC”)
interact with
inhomog./nonlin.
vs.
twisting
braiding
shear
(“DC”)
turbulence
reconnection
• How is the energy dissipated
and converted to heat?
collisions (visc, cond, resist, friction) or collisionless
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
Turbulence
• It is highly likely that somewhere in the solar
atmosphere, the fluctuations become turbulent
and cascade from large to small scales.
• The original Kolmogorov (1941) theory of
incompressible fluid turbulence describes a
constant energy flux from the largest “stirring”
scales to the smallest “dissipation” scales.
• Largest eddies have kinetic energy ~ ρv2 and a
turnover time-scale  =l/v, so the rate of transfer
of energy goes as ρv2/ ~ ρv3/l .
• Dimensional analysis can give the spectrum of
energy vs. eddy-wavenumber k: Ek ~ k–5/3
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
Turbulence in coronal loops?
• Many stochastic processes can be described roughly using a turbulent “language.”
• Coronal loops are always in motion, with
waves & bulk flows propagating back and
forth along the field lines.
• Counter-propagating Alfvén waves interact
over shorter time intervals  thus the
cascade takes longer to develop.
• However, the weaker character of the cascade makes it able to “send” more
energy down the ladder of ever-smaller eddies, and thus lead to more dissipation!
n = 0 (Kolmogorov), 3/2 (Gomez et al. 2000), 5/3
(Kraichnan), 2 (van Ballegooijen; Rappazzo et al.)
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
Reconnection & Turbulence
• Ultimately, the actual dissipation and heating in loops
appears to occur in regions of magnetic reconnection.
• This is still understandable from a turbulence paradigm,
since on its smallest scales, MHD turbulence tends to:
» break up into thin reconnecting sheets.
» accelerate electrons along the field to generate currents.
• Even pre-existing current
sheets are unstable in a
variety of ways to growth of
turbulent motions, which
may dominate the energy
loss & particle acceleration.
Dmitruk et al. (2004)
Rappazzo et al. (2008)
Onofri et al. (2006)
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
The need for extended heating
• The basal coronal heating problem is not yet
solved, but the field seems to be “homing in on”
the interplay between emerging flux, reconnection,
turbulence, and helicity (shear/twist).
X
• Above ~2 Rs , some other kind of energy deposition is needed in order to . . .
» accelerate the fast solar wind
(without
artificially boosting mass loss and peak Te ),
» produce the proton/electron temperatures
seen in situ (also magnetic moment!),
» produce the strong preferential heating and
temperature anisotropy of ions (in the
wind’s acceleration region) seen with UV
spectroscopy.
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
Waves? Start in the photosphere . . .
• The photosphere shows convective motion on a broad range of time/space scales:
β << 1
β~1
β>1
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
Alfvén waves from cradle to grave
• In dark intergranular lanes, strong-field photospheric flux tubes are shaken by an
observed spectrum of horizontal motions.
• In mainly open-field regions, Alfvén waves propagate up along the field, and partly
reflect back down (non-WKB).
• Nonlinear couplings allow a (mainly perpendicular) turbulent cascade, terminated
by damping → gradual heating over several solar radii.
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
MHD turbulence: two kinds of “anisotropy”
• Outside closed loops, we can revert to
“standard” Kolmogorov (1941) scaling,
but with two modifications:
Z–
Z–
• With a strong background field, it is
easier to mix field lines (perp. to B) than
it is to bend them (parallel to B).
Z+
• Also, the energy transport along the
field is far from isotropic.
(e.g., Hossain et al. 1995; Matthaeus et al. 1999;
Dmitruk et al. 2001, 2002; Oughton et al. 2006)
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
“The kitchen sink”
• Cranmer, van Ballegooijen, & Edgar (2007) computed self-consistent solutions of
waves & background one-fluid plasma state along various flux tubes... going from
the photosphere to the heliosphere.
• Ingredients:
• Alfvén waves: non-WKB reflection with full
spectrum, turbulent damping, wave-pressure
acceleration
• Acoustic waves: shock steepening, TdS &
conductive damping, full spectrum, wave-pressure
acceleration
• Radiative losses: transition from optically thick
(LTE) to optically thin (CHIANTI + PANDORA)
• Heat conduction: transition from collisional
(electron & neutral H) to collisionless “streaming”
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
Results: turbulent heating & acceleration
T (K)
Ulysses
SWOOPS
Goldstein et al.
(1996)
reflection
coefficient
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
Multi-fluid collisionless effects?
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
Multi-fluid collisionless effects?
O+5
O+6
protons
electrons
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
Exploring the extended corona
• “Off-limb” measurements (in the 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.
• The Ultraviolet Coronagraph Spectrometer
(UVCS) on SOHO combines these features to
measure plasma properties of coronal protons,
ions, and electrons between 1.5 and 10 solar radii.
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
Preferential ion heating & acceleration
• UVCS observations have rekindled theoretical efforts to understand heating and
acceleration of the plasma in the (collisionless?) acceleration region of the wind.
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
Preferential ion heating & acceleration
• 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?
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Alfven wave’s
oscillating
E and B fields
ion’s Larmor
motion around
radial B-field
cyclotron resonancelike phenomena
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
The Young Sun . . .
• Kelvin-Helmholz contraction:
ISM cloud fragment becomes a
protostar; gravitational energy
converted to heat.
• Hayashi track: protostar reaches
approx. hydrostatic equilibrium, but
slower gravitational contraction
continues. Observed as the T Tauri
phase.
• Henyey track: Tcore reaches ~107 K
and hydrogen burning dominates;
some accretion and gravitational
contraction remain, but both slow to
a halt at ZAMS.
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
T Tauri stars: active accretion & outflows
• T Tauri stars exhibit signatures of disk accretion (outer parts), “magnetospheric
accretion streams” & X-ray corona (inner parts), and various (polar?) outflows.
• Nearly every observational diagnostic varies in time, sometimes with stellar
rotation, but often more irregularly.
(Romanova et
al. 2007)
(Matt & Pudritz 2005, 2008)
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
Accretion-driven T Tauri winds
• Cranmer (2008) extended the solar wave/turbulence models to outer atmospheres
of young, solar-type stars.
• The impact of inhomogeneous “clumps” on the stellar surface generates MHD
waves that propagate horizontally (like solar Moreton & EIT waves!).
• These “extra” waves input orders of magnitude more energy into a turbulent MHD
cascade, and can give rise to stellar winds with dM/dt up to 106 times solar!
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
Accretion-driven T Tauri winds
• Results: wind mass loss rate increases
~similarly to the accretion rate.
• For high enough densities, radiative
cooling “kills” coronal heating!
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
Synergy with other systems
• Pulsating hot (O, B, Wolf-Rayet) stars: Pulsations “leak” outwards as non-WKB
waves and shocks. New insights from solar wave theory are being extended.
• Red giants & supergiants: Strong pulsations & shocks, with dense/slow winds
(radiatively cooled?) and variability from dust, thermal instabilities, etc.
• AGN accretion flows: A similarly collisionless (but pressure-dominated) plasma
undergoing anisotropic MHD cascade and kinetic wave-particle interactions...
Freytag et al. (2002)
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
Conclusions
• The Sun/heliosphere system is a nearby
“laboratory without walls” for studying
plasma physics in regimes of parameter space
inaccessible in Earth-based laboratories.
• Theoretical advances in MHD turbulence
continue to feed back into global models of
coronal heating and the solar wind.
• The extreme plasma conditions in coronal
holes (Tion >> Tp > Te ) have guided us to
discard some candidate processes, further
investigate others, and have cross-fertilized
other areas of plasma physics & astrophysics.
For more information: http://www.cfa.harvard.edu/~scranmer/
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
Extra slides . . .
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
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”
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
The solar activity cycle
Yohkoh/SXT
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
What produces “emission lines” in a spectrum?
• There are 2 general ways of producing extra photons at a specific wavelength.
• Both mechanisms depend
Incoming particle
on the quantum nature of
atoms: “bound” electrons
have discrete energies . . .
• The incoming particle can
be either:
Energy
re-emitted
as light
Electron
absorbs
energy
• A free electron from some other ionized
atom (“collisional excitation”)
• A photon at the right wavelength from the
bright solar disk (“resonant scattering”)
• There is some spread in wavelength
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
The UVCS instrument on SOHO
• 1979–1995: Rocket flights and Shuttle-deployed Spartan 201 laid groundwork.
• 1996–present: 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
reveal the solar wind acceleration region!
slit field of view:
• Mirror motions select height
• UVCS “rolls” independently of spacecraft
• 2 UV channels: LYA (120–135 nm)
OVI (95–120 nm + 2nd ord.)
• 1 white-light polarimetry channel
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
UVCS results: over the poles (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
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Off-limb profiles: T > 100 million K !
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
Emission lines as plasma diagnostics
• Many of the lines seen by UVCS are formed by resonantly scattered disk photons.
• If profiles are Doppler shifted up or down in wavelength (from the known rest
wavelength), this indicates the bulk flow speed along the line-of-sight.
• The widths of the profiles tell us about random motions along the line-of-sight
(i.e., temperature)
• The total intensity (i.e., number of
photons) tells us mainly about the
density of atoms, but for resonant
scattering there’s also another
“hidden” Doppler effect that tells
us about the flow speeds
perpendicular to the line-of-sight.
• If atoms are flow in the same direction as incoming
disk photons, “Doppler dimming/pumping” occurs.
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
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!
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
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:
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
Waves: remote-sensing techniques
The following techniques are direct… (UVCS ion heating was more indirect)
• Intensity modulations . . .
• Motion tracking in images . . .
Tomczyk et al.
(2007)
• Doppler shifts . . .
• Doppler broadening . . .
• Radio sounding . . .
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
Alfvén waves: from Sun to Earth
• Velocity amplitudes of fluctuations measured (mainly) perpendicular to the
background magnetic field.
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
Why is the fast (slow) wind fast (slow)?
• What determines how much energy and
momentum goes into the solar wind?
Waves & turbulence input from below?
vs.
Reconnection & mass input from loops?
• Cranmer et al. (2007) explored
the wave/turbulence paradigm
with self-consistent 1D models
of individual open flux tubes.
• Boundary conditions imposed
only at the photosphere (no
arbitrary “heating functions”).
• Wind acceleration determined by a combination of
magnetic flux-tube geometry, gradual Alfvén-wave
reflection, and outward wave pressure.
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
Do blobs trace out the slow wind?
• The blobs are very lowcontrast and thus may be
passive “leaves in the wind.”
Sheeley et al.
(1997)
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
Polar plumes and jets
• Dense, thin flux tubes permeate polar coronal holes. They
live for about a day, but can recur from the same footpoint
over several solar rotations.
• Short-lived “polar jets” are energetic events that appear to
eject plasma into the solar wind. Hinode/XRT
(DeForest et al. 1997)
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
The solar chromosphere
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
The need for chromospheric heating
Not huge in radial extent,
but contains order of
magnitude more mass
than the layers above . . .
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
“Traditional” chromospheric heating
• Vertically propagating acoustic waves
conserve flux (in a static atmosphere):
Bird (1964)
• Amplitude eventually reaches Vph and
wave-train steepens into a shock-train.
• Shock entropy losses go into heat; only
works for periods <
~ 1–2 minutes…
• New idea: “Spherical” acoustic wave
fronts from discrete “sources” in the
photosphere (e.g., enhanced turbulence
or bright points in inter-granular lanes)
may do the job with longer periods.
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
Time-dependent chromospheres?
• Carlsson & Stein (1992, 1994, 1997, 2002, etc.) produced 1D time-dependent
radiation-hydrodynamics simulations of vertical shock propagation and
transient chromospheric heating. Wedemeyer et al. (2004) continued to 3D...
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
Runaway to the transition region (TR)
• Whatever the mechanisms for heating, they must be balanced by radiative losses to
maintain chromospheric T.
• When shock strengths
“saturate,” heating depends
on density only:
• Why then isn’t the corona 109 K? Downward heat conduction smears out the
“peaks,” and the solar wind also “carries” away some kinetic energy. Conduction
also steepens the TR to be as thin as it is.
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
Strongest fields in supergranular “funnels?”
Peter (2001)
Fisk
(2005)
Tu et al. (2005)
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
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)
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
Coronal mass ejections
• Forbes & Priest (1995) and Lin & Forbes (2000) developed a theory of CMEs as a
loss of magnetostatic equilibrium in a twisted “flux rope.”
• The current sheet energizes both the CME (above)
and a “two-ribbon flare” (below)
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009
Overview of “in situ” fluctuations
• Fourier transform of B(t), v(t), etc., into frequency:
Magnetic Power
f -1 “energy containing range”
• How much of the “power”
is due to spacecraft flying
through flux tubes rooted
on the Sun?
f -5/3
“inertial range”
The inertial range is a
“pipeline” for transporting
magnetic energy from the
large scales to the small
scales, where dissipation
can occur.
f -3
“dissipation
range”
few hours
Plasma Unbound: New Insights Coronal Heating
and Solar/Stellar WInds
0.5 Hz
Steven Cranmer, CfA
HAO Colloquium, March 4, 2009