PX282 - G5 - solar activity cycle

image: hathaway NASA/ARC

• the solar magnetic activity varies in an 11 year cycle
• approaching the maximum, the number of sunspots increases
• at the start of each cycle, sun spots are seen at high latitudes
• later, they emerge closer to the equator
• the x-ray emission also increases

images: carroll & ostile, an introduction to modern astrophysics, 2007

• the magnetic field cycles between toroidal and poloidal due to differential rotation
• this causes the magnetic field to be reversed
• this is called the solar dynamo model

images: carroll & ostile, an introduction to modern astrophysics, 2007

• at the beginning, few sunspots with magnetic field close to a dipole
• differential rotation drags the field lines from poloidal to toroidal
• twisted magnetic flux loops emerge, giving sunspots
• meridional flows drag them towards equator
• opposite polarity toroidal components cancel
• solar magnetic field reverts to dipole with opposite polarity

images: carroll & ostile, an introduction to modern astrophysics, 2007

• the large-scale magnetic field (solid lines) is similar to a dipole (dashed lines)

• coronal plasma can escape along 'open field lines' as a solar wind

• this causes the ion tail of comets and can also erode planet atmospheres

• at earth, solar winds with velocities $\sim 500km{s}^{-1}$, and number densities $\sim 7\times {10}^6{m}^{-3}$ can be seen

• this is comparable to the coronal temperatures, but an additional energy source is needed to explain this temperature
• it is believed that the winds are assisted by alfven waves propagating along magnetic field lines excited by convection at the photosphere

image: steven cranmer

mass loss rate

• considering the mass of a thin shell in wind around the sun with a radius $r:$

• this is negligible compared to the solar mass (c.f. AGB stars, the mass loss rate is $\sim {10}^{-4}{M}_{\odot }y{r}^{-1}$)
• this is still significant for the loss of angular momentum and spin is slowed down
• the wind is force to corotate with the sun, ie. the sun exerts a torque on the wind, transferring angular momentum and slowing down solar rotation
• the angular momentum:

• $J/m$ is called the specific angular momentum
• there is a significant magnetic braking

image: NASA, lines represent magnetic field lines and arrows represent solar winds

• solar wind blows a bubble in interstellar medium (ISM), the heliosphere $\sim 120$ AU
• magnetic reconnection in coronal flux loops can drive solar flares, and coronal mass ejections (CMEs)
  • solar flares are sudden released of energy as x-rays, $\gamma$-rays, radio and optical wavelength emissions
  • CME is the ejection of plasma from the solar corona
• these drive space weather in earth (like aurora)
• they also cause erosion of planet atmosphere and affect the habitability