Solar Activity & the Earth Astronomy 12

Solar seismology is the study of the Sun's interior by analyzing waves in the Sun's atmosphere.  Just like the Earth's interior can by analyzed by studying Earthquakes, astronomers can also learn about the Sun's interior by analyzing waves in its gases.

Waves similar to those of earthquakes travel through the Sun and make its surface heave like an ocean or a bubbling pan of hot oatmeal.  The rising and falling surface gas makes a regular pattern, which can be detected as a doppler shift of the moving material.  Astronomers next use computer models of the Sun to predict how the observed surface waves are affected by conditions in the Sun's deep interior.  With this technique, astronomers can measure the density and temperature deep within the Sun from the pattern and speed of the waves in its atmosphere.  The results they find agree well with hose of the current models, indicating that our understanding of the Sun is correct.

The cause of the Sun's magnetic field and the sunspot cycle are not well understood, but it is believed to come from the movement of charged particles in the convection zone of the Sun.  The Sun isn't a giant ball of gas, it is a giant ball of hot ionized gas called a plasma.  An ionized gas has free flowing positive and negative charges.  Moving charges induce a magnetic field.  So as the plasma is moved in the convection zone a magnetic field is created.

The Sun is not a solid ball, but rather like a fluid.  It exhibits differential rotation, meaning the surface moves at different speeds depending on the latitude.  The Sun spins fastest at the equator and slowest at the poles.  This results in the magnetic-field lines getting wound up.  When the winding gets extreme, the magnetic field lines buckle, and twist breaking the surface and creating sunspots.  Sometimes these twists can actually snap, causing solar flares at those locations on the surface. The magnetic field cause activity on the sun’s surface.

At the peak of the cycle, the polarity of the field flips, during a time of maximum sunspot activity.

The sun's magnetic influence extends well past the planets and into interstellar space.  This region, called the heliosphere, acts as a magnetic shield against charged particles from deep space called cosmic rays.  It is the solar wind that spreads the Sun's magnetic field throughout the solar system. 

Granules, supergranules, spicules, and the solar wind occur continuously.  These are features of the quiet Sun.  But the Sun's atmosphere is periodically disrupted by magnetic fields that stir things up, creating the active Sun.  The Sun's most obvious transient features are sunspots, regions of the photosphere that appear dark because they are cooler than the rest of the Sun's lower atmosphere.  Sometimes sunspots occur in isolation, but often they arise in clusters called sunspot groups

Solar minimum and maximum (for sunspot activity) can range from 8 to 14 year cycle (typically 11?) (sunspots - where powerful magnetic fields occur)

The Sunspot Cycle:

Like other transient features of the active Sun, the average number and location of sunspots vary in predictable cycles.  As shown below, the last sunspot maximum was in 2014 followed by a sunspot minimum in 2020.  The average sunspot cycle lasts approximately 11 years.  During a sunspot minimum, the Sun is almost devoid of sunspots, as it was in September 2020.

Galileo observed the first sunspots in 1613, and since then sunspot activity has been monitored.  Although more accurately in later dates.  Below is a 400 year map of the sunspot cycle.  Notice that the peaks are not always the same height.  Some sunspot maximums are much stronger than others.  The last solar maximum in 2014 was much less intense than previous maximums.  As you can see this has happened before with the Dalton Minimum and Maunder Minimum.

The Solar Cycle:

The solar cycle is different but related to the sunspot cycle.  The solar cycle also has to do with the Sun's magnetic field but instead of sunspots forming from magnetic field lines bursting through the photosphere it has to do with the pole reversal of the Sun's magnetic field.  Since the magnetic field of the Sun is formed from moving charged particles it is not stable (nor is the Earth's magnetic field).  So the north and south poles of the Sun actually switch places every 11 years.  The Solar cycle is measured from the start of the Sun's north magnetic pole being close to its north geographic pole, the pole reversal when the north magnetic pole is close to its south geographic pole, and back again. 

Sunspots have shown that our Sun rotates about once every 27 days.  So stars with a slower rotation than our Sun should have a shorter solar cycle, while stars that rotate more quickly should have a longer one.

Aurora:

the northern lights are caused by charged particles (protons and electrons) blasted out from the Sun in all directions that collide with gases in the Earth's atmosphere after being trapped by the Earth's magnetic field.

Those particles have traveled 150 million kilometres from the Sun to reach Earth, which takes about 2 - 4 days.  They are part of the solar wind.  As Earth orbits the Sun, a portion of those particles collide with the Earth's magnetic field.  A lot of particles are deflected by the magnetic field but some become trapped in Earth's magnetic field.  The trapped ones are directed towards the north and south magnetic poles.  They spiral around the magnetic filed lines into Earth's atmosphere.

As the charged particles spiral closer to the Earth they interact with gases in the Earth's atmosphere.  The charged particles have energy which they transfer to the gas.  This moves the gas's electron into an excited state where the electron orbits the nucleus a little further away than usual.  The atom is unstable like this so the electron jumps back down.  When it does, it releases the energy as light. (More on this in the next lesson).  Light from millions of these interactions is what causes the northern lights.

The colours you see come from interactions with Oxygen and Nitrogen in Earth's atmosphere and happen at different altitudes above the ground.  Red occurs highest in the atmosphere while green is the most common colour seen.  The purple colour is lowest in the atmosphere but still well over 50 km above the surface of the Earth.

Solar storms have a range of intensity just like earthquakes and hurricanes but they are broken into 3 different categories.  Radio blackouts, Proton storms and Geomagnetic storms.  Each on a 5 point scale from minor to Extreme.