turbulent heating of the corona and solar wind: the...
TRANSCRIPT
Turbulent heating of the corona and solar wind: the heliospheric dark energy problem
Stuart D. Bale Physics Department and Space Sciences Lab
University of California, Berkeley, USA
IAP 2012 seminar, PSFC/MIT, January 11, 2012
The Sun A boring, middle-aged star
G type, population 1, ‘yellow dwarf’
Photospheric blackbody ~5000-6000K
Sunspots and ‘active regions’
Impulsive Solar Activity - ‘Carrington Event – September 1-2, 1859 - Brilliant, intense aurora borealis (18 hrs later) - Disruption of telegraph services - Once per 500 years (ice cores) - Solar-terrestrial connection
- Interplanetary space is not empty!
Comet tails Comets have two tails - A ‘Dust tail’ is diffuse and follows the comet’s orbit (Keplerian) - A ‘Gas tail’ which points away from the Sun (Biermann)
dust
gas
Comet tails - 2010
The solar corona
1919 eclipse photo, Sobral
1571, Caron
The solar corona Coronal structure often resembles magnetic lines of force
Eclipse observations show the ‘solar corona’ Thomson-scattered white light
The corona is very hot and magnetized Hydro Scale height (H ~ kT/mg) not consistent with simple hydrostatic equilibrium Using 6000 degrees C as a temperature, if the atmosphere is hydrogen then H = 175 km (110 miles) – solar radius is much larger Instead, from the eclipses the scale height is clearly comparable to the radius of the Sun, or H = 695,500 km (430,000 miles) - So the corona is very hot or we have some new, lighter elements ‘ coronium’
Early spectrographic measurements of mysterious emission lines helped the confusion (again ‘coronium’) Edlén (1942) identified line emission with highly ionized Fe implying electron temperatures of T >106 C
The corona is very hot and magnetized
K corona – photospheric light scattered from electrons – spectrum is washed out by Doppler-shift
The corona is a tenous, hot magnetized plasma
(Cranmer et al., 2008)
An important measurement: perpendicular heating
F corona – photospheric light scattered from dust, solar spectrum remains – ‘zodiacal light’
E corona – emission lines from ionized, heavy elements in the corona – UV-soft x-ray
- H and He are fully ionized – no emission - Minor ions are partially (and often highly) ionized - Polarization/splitting of emission lines gives line-of-
sight magnetic field (~1 G)
A summary (to 1950’s) - The solar photosphere is ~6000K and macroscopically
‘homogeneous’ (and β = nkT/(Β2/2µ0) > 1) – a ‘fluid’
- Impulsive events and flares on the Sun produce activity in the Earth’s ionosphere – transit time is hours (slow) - space is not empty!
- Comet ‘gas’ tails point away from the Sun – fast flow!
- The solar corona is tenuous and highly structured – often organized by magnetic ‘lines of force’
- The solar corona is very hot (> 106 K) – this temperature inversion is puzzling (2nd law of thermodynamics)
- The high coronal temperature and emission lines suggest that the gas is highly ionized, i.e. a magnetized collisionless plasma (β << 1)
Parker’s solar wind model
A ‘solar wind’ is accelerated from the corona
- Hydrostatic solution (similar to Bondi accretion)
- Predicts a supersonic atmosphere ‘wind’
- Similar to ‘de Laval nozzle’ or a jet engine
- Requires energy input at the base. kTph is not nearly enough! Requires nonthermal energy
- ‘Alfven point’ in magnetized plasma determines extent of corona - corotation
Mariner 2 measurements
Parker’s solar wind is confirmed The solar wind is highly variable
The solar wind is heated continuously
- Helios spacecraft measurements from 0.3 – 1 AU
- Voyager spacecraft measurements outward
- Tp ~ 1/r
- Free expansion predicts a much more rapid decay
- Requires continuous, distributed energy input
Kinetic Physics in the Corona and Solar Wind
• There are very few collisions in the solar wind
• Not in thermal equilibrium
• Large temperature anisotropies – heating is organized by magnetic field
• Different temperatures
• Relative drifts
B
THe/TH
Re
lativ
e F
req
ue
nc
y
B
(Marsch et al)
(Kasper et al)
electrons
The solar wind is bimodal
Summary #2 - The corona requires a non-thermal source of heat
- A sufficiently heated corona will expand super-sonically and super-Alfvenically to form a ‘solar wind’
- The expanding solar wind requires additional heating
The large coronal magnetic energy density is a sufficient energy source! This is our ‘dark energy’. But problems remain:
1. How are the magnetic fields created and transported
2. How is the magnetic energy converted to thermal energy: magnetic reconnection, shocks, waves and turbulence
3. What is the role of ambipolar electric fields?
source of energy Photospheric motion, granulation, footpoint shuffling
Alfven waves in the corona
NASA Solar Dynamics Observatory (SDO) – Advanced Imaging Assembly (AIA) High cadence, high resolution 193Å coronal imaging (Vourlidas and Stenborg) Steady outflows – reconnection? Alfven wave Poynting flux?
Alfven waves CoMP at NSO FeXIII at 1074.7 nm - ‘Waves’ are faster than sound - Abundant in the solar wind
(Belcher and Davis) - Propagate along magnetic field - Low intensity (in white light)
Hinode (JAXA) CaII measurements
Plasma wave launching - Footpoint ‘shuffling’ generates currents, magnetic fields - Alfven waves propagate upward - Produce a turbulent cascade that terminates in damping - Damping heats the plasma
Turbulent ‘eddies’
evolution
viscous damping
Neutral fluid turbulence
Magnetized turbulent ‘eddies’
evolution
collisionless damping
Ambient magnetic field
Magnetic Plasma Turbulence
Magnetic Plasma Turbulence
Perpendicular cascade
Electric field measurements
Voltage
ELECTRIC FIELD MEASUREMENTS IN THE MAGNETOSPHERE
,= d 2cI,
V2=O
251
f
Fig. 2.
Distance 2 b
Three-electrode probe system. Potential along a line in the plasma through the probes and along a line through the lead ABD.
~\To feVc'~ / kTi v 2 " - - - . e x p - - + iph~ (17)
I1 + I2 = 4~r~ne ~/2Trine ~,kTJ + X2~rni + 16 4he l"
These equations are valid only when V1, V2 and Vc are nonpositive. If VB is chosen e.g. so that V2 = 0, then according to Equation (14) 1/1 = - e E ' d . By eliminating E' from the Equations (13) and inserting 11 and I2 from Equations (15) and (16), and using the fact that V~ given above is small we get
2 ne/--/~Te.(1 exp (\ld/eV~ = J ~ Rr - " t o " - - ( 1 8 ) VR + Vc ~ / m e
where Vo is the floating potential defined in Section 5. Equations (9) and (11) are still valid (with V=0 in Equation (9)). Equation (9)
can also be obtained by combining Equations (13)-(16), and will accordingly read
R r 2 > (Rr2)min __ 1 /mekZ e (19) = ome 2 ~ 2~
which means that
= c~e k \kTe,l]"
Values of (Rr2)mi, for three-electrode systems are given in Table III. By eliminating
(Fahleson)
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- Voltage probes (and spacecraft) are Langmuir probes- Current balance (thermal, photoelectron, secondaries) determine floating voltage
- Bias current minimizes voltage variations due to natural currents- Unbiased probes measure primarily current variations - this is historically the case for SW experiments
Perpendicular cascade
(Bale et al., 2005)
Summary #3 - Alfven waves appear to be generated low in the corona.
- Magnetized turbulence is measured in the outer heliosphere – consistent with a perpendicular cascade
- However, remote-sensing (UVCS) ion temperatures suggest cyclotron heating and hence significant high frequency compressive waves.
- What is the role of reconnection? Ambipolar electric fields? Shocks?
- We need radial profiles, we need to get inside of the Alfven radius (10-15 Rs)
- We need modern, high-quality in situ measurements in the inner heliosphere
NASA Solar Probe Plus
- NASA ‘Living with a Star’ Mission - Recommended by NAS for 30 years
- Most ambitious NASA ‘Heliophysics’ mission
- Launch in 2018 - Mostly in situ instruments - Perihelion at 9.5 Rs –
within the Alfven radius - Lots of orbits…
Solar Probe Plus
The ‘FIELDS’ Experiment for Solar Probe Plus
4 x Voltage (electric field) sensors 3 x Magnetometers
SWEAP (Kasper, SAO+UCB) Solar wind plasma
FIELDS (Bale, UC Berkeley) Electric and magnetic fields
ISIS (McComas, SwRI) Energetic particles
WISPR (Howard, NRL) White light imager
- Excellent magnetic and electric fields - Excellent plasma measurements
Solar Probe Plus
Requires heroic thermal engineering! - TPS ~ 2000C - FIELDS antennas ~ 1300C - FIELDS magnetometers ~ -100C
Requires some interesting ops - Initial warm up of radiators - Dust environment - Cp/Cg problems - Solar panels and power
Solar Probe Plus
2018 launch 35 Rs initial perihelion 7 x Venus Gravity Assist (300 km Venus flyby) 9.5 Rs final perihelion End 2027
Solar Probe Plus
Launch July 2018
Solar Probe Plus
Advanced Technology Solar Telescope (ATST)
- 4.2m, off-axis pupil AO telescope on Haleakala, Maui
- Optimized for dynamic range and low scattering
- Will resolve magnetic (electric?) fields with ~70km resolution at ~1-2 Rs
- Connection with Solar Probe Plus
- Funded by US NSF, first light in ~2017
Perspective - Magnetic fields are likely to provide the missing ‘dark’
energy for coronal heating and solar wind acceleration
- Magnetized turbulence is likely to generate the required solar wind continuous heating
- The plasma physics remains an open question - Alfven waves and turbulence - Magnetic reconnection - Shock waves - Ambipolar fields
- Physics may be similar to collisionless accretion
- The coming decade will be a ‘golden age’ for coronal and solar wind physics: STEREO, SDO, IRIS, Solar Orbiter, Solar Probe Plus, FASR, and ATST