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 Rotaon S • 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).

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 fracon • Diameter = 20,000-60,000 km

Color is due to magnec field orientaon • Umbra – vercal 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 •Orientaon flips every 11 years too (N-S è S-N) to (S-N è N-S) •Takes 22 years for orientaon 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: ?

hp://www.solen.info/solar/cyclcomp.html Solar Flares:

Flares are massive eruptions on the solar surface. • Releases EM radiaon, energec parcles, wave moons and shock waves • Time scale: minutes to hours • More frequent during solar maximum (10s of events/day) • Hoer than the corona • Light reaches Earth in ~8.3 min; parcles can arrive soon aer • 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 reacons triggered in the chromosphere by high energy protons and ions • Need space based telescopes to observe much of this radiaon – blocked by Earth’s atmosphere Solar Flares: Radiation

Bremsstrahlung (breaking radiaon)

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 Radiaon 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 •Orientaon flips every 11 years too (N-S è S-N) to (S-N è N-S) •Takes 22 years for orientaon 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 magnec field reverses too… just not on an 11 year cycle

Glatzmeier and Roberts Solar flares and the Magnec 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 differenal rotaon 4. Field begins to pinch inwards (field lines of opposite sign aract) 5. Magnec field breaks due to shear forces 6. Plasma blob is accelerated upward and addional plasma is accelerated back towards the chromosphere Sunspot Formaon and Solar Flare Magnec Reconnecon and Solar Flares The 11 Year Cycle

Magnec 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) Effect on Earth between 0.1 and 0.8 nm B I < 10-6 C 10-6 ≤ I < 10-5 Minor events – few noceable consequences M 10-5 ≤ I < 10-4 Medium events –brief radio blackouts near poles X I ≥ 10-4 Major events – planet wide radio blackouts