The Solar Magnetic Field and Solar Activity
24
The Solar Magnetic Field and Solar Activity
Transcript of The Solar Magnetic Field and Solar Activity
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
• 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
