Lecture19_SolarMagneticField.pptxSunspot Characteristics:
• 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).
• 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
S
Rota(on
• Sunspots are typically paired on the surface of the Sun.
• Sunspots are transient features in the solar atmosphere. Their
total number changes with time as well.
• 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:
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: ?
• 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.
• 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).
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.
• This has some impressive consequences for the Sun.
• At the equator the Sun rotates once every 25 days. At the poles
it rotates every 36 days.
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 number is tied to how wrapped up the field is by
differential rotation.
• Sunspot characteristics make a lot of sense when we consider the
magnetic Sun….
• 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
Sunspot Forma(on and Solar Flare
Magne(c Reconnec(on and Solar Flares
The 11 Year Cycle
Classification of Solar Flares:
GOES satellite at looks light
intensity between 0.1 – 0.8 nm
and 0.5 – 4 nm
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