Download Lecture 19: The Solar Magnetic Field

The Solar Magnetic Field and Solar Activity
Sunspot Characteristics:
•  Detailed magnetograms of Sunspots show that they have regions
of N-S and S-N magnetic field.
•  One spot is ALWAYS N-S,
while the other is S-N.
N Rota(on •  At any given time the direction
of the N-S to S-N arrangement
is the same for ALL sunspots on
the Sun (but switches north and
south of solar equator).
S Sunspot Characteristics:
•  Sunspots are transient features in the solar atmosphere. Their
total number changes with time as well.
•  Sunspots are typically paired
on the surface of the Sun.
•  They are often seen
connected to filaments on the
surface.
•  They are clustered near the
middle regions of the Sun and
rotate with it.
•  Sunspots are not ‘dark’, but cool (about 3000K).
Sunspot Characteristics:
Umbra: dark center of the sunspot Penumbra: lighter color region surrounding the umbra •  Constant area frac(on •  Diameter = 20,000-­‐60,000 km Color is due to magne(c field orienta(on •  Umbra – ver(cal field •  Penumbra – inclined field Not So Dark….
•  Sunspots have strong fields that
contain lots of plasma that doesn’t
want to move.
•  This would be ok, except that the
Sun’s convection zone would like to
move neutral material to the surface.
•  The ‘magnetic bubble’ around the sunspot prevents convection
from being as efficient. So less energy is delivered to the surface,
and the gas is cooler there (only 3000K).
Sunspot Characteristics:
• The number of sunspots waxes and wanes on an 11 year cycle • Orienta(on flips every 11 years too (N-­‐S è S-­‐N) to (S-­‐N è N-­‐S) • Takes 22 years for orienta(on to flip and return to previous state Sunspot Cycle – 11 year average Cycle 21, June 1976: 10 years, 3 months Cycle 22, Sept. 1986: 9 years, 8 months Cycle 23, May 1996: 12 years, 6 months Cycle 24, Dec. 2008: ? hZp://www.solen.info/solar/cyclcomp.html Solar Flares:
Flares are massive eruptions on the solar surface.
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Releases EM radia(on, energe(c par(cles, wave mo(ons and shock waves Time scale: minutes to hours More frequent during solar maximum (10s of events/day) HoZer than the corona Light reaches Earth in ~8.3 min; par(cles can arrive soon aber Energy released could power the US for decades, but it’s <0.1 PSun Solar Flares: Radiation
First detected in 1859-Carrington and Hodgeson via visible light.
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Difficult to detect in visible light – they don’t perturb the total amount of white light very much Much more intense in X-­‐rays and radio frequencies (RF) – much higher intensity than normal Small amounts of gamma rays can also be produced from nuclear reac(ons triggered in the chromosphere by high energy protons and ions Need space based telescopes to observe much of this radia(on – blocked by Earth’s atmosphere Solar Flares: Radiation
Bremsstrahlung (breaking radia(on) However, the electrons in the
compressed plasma are propelled
with such velocity that they trigger
intense X-Ray emissions as they
pass hot ions (Bremsstrahlung).
Synchrotron Radia(on In addition, the electrons coil around
field lines, which in turn produces
oscillatory emission in the form of
Radio Waves (Synchrotron).
Sunspots and Rotation:
• The number of sunspots waxes and wanes on an 11 year cycle • Orienta(on flips every 11 years too (N-­‐S è S-­‐N) to (S-­‐N è N-­‐S) • Takes 22 years for orienta(on to flip and return to previous state Differential Rotation:
•  Recall that the Sun is rotating differentially with FAR more
variation than the Earth.
•  Just as in the Earth, the motions of plasma in the deep
convection zone generate a magnetic field.
•  The solar field is produced closer to the ‘surface’ and is
affected by rotation more strongly.
•  At the equator the Sun rotates once every 25 days. At the
poles it rotates every 36 days.
•  This has some impressive consequences for the Sun.
Differential Magnetic Field.
Because the plasma inside the Sun is bound to the rotation of the
neutral convection zone, the magnetic field is going to be stretched
out by the differential rotation of the neutral Sun!
This process takes some time, but eventually the field gets wrapped
up, just like a tether ball. And just like a tether ball, the Sun’s magnetic
field bounces back!
Solar Activity:
•  This magnetic cycle is the reason why the Sun appears active
and it sets the table for ALL space weather.
•  Sunspots are the most
common result of this, but not
the most energetic.
•  The occur where the
magnetic field bursts out from
the twisted lines of the field.
•  Every 11 years the field
‘snaps back’ and the process
starts again.
•  However, the ‘new’ field has
changed sign!
Solar Dynamo Sunspots Revealed:
•  Sunspot characteristics make a lot of sense when we consider
the magnetic Sun….
•  Sunspot number is tied to how
wrapped up the field is by
differential rotation.
•  The region where sunspots form
is where the field gets the most
wrapped up.
•  The orientation of the N-S pairs
is due to the orientation of the
solar field and how it changes
with cycle.
Differential Rotation:
•  Recall that the Sun is rotating differentially with FAR more
variation than the Earth.
•  Just as in the Earth, the
motions of plasma in the
deep convection zone
generate a magnetic field.
•  The solar field is produced
closer to the ‘surface’ and is
affected by rotation more
strongly.
•  This has some impressive
consequences for the Sun.
The Earth’s magne(c field reverses too… just not on an 11 year cycle Glatzmeier and Roberts Solar flares and the Magne(c Field Flares occur in regions of rapid magnetic field re-alignment.
1.  Coronal loop 2.  Field begins to inflate 3.  Field twists as sunspots move at different speeds due to differen(al rota(on 4.  Field begins to pinch inwards (field lines of opposite sign aZract) 5.  Magne(c field breaks due to shear forces 6.  Plasma blob is accelerated upward and addi(onal plasma is accelerated back towards the chromosphere Magne(c Reconnec(on and Solar Flares Sunspot Forma(on and Solar Flare The 11 Year Cycle Magne(c Field Map Corona X-­‐ray Emissions Classification of Solar Flares:
GOES satellite at looks light intensity between 0.1 – 0.8 nm and 0.5 – 4 nm Each category is subdivided 1-­‐9 (ex: X1, X2…X9) Class Peak Intensity (W/m2) between 0.1 and 0.8 nm Effect on Earth B I < 10-­‐6 C 10-­‐6 ≤ I < 10-­‐5 Minor events – few no(ceable consequences M 10-­‐5 ≤ I < 10-­‐4 Medium events –brief radio blackouts near poles X I ≥ 10-­‐4 Major events – planet wide radio blackouts