Physics 320
Astrophysics I:  Lecture #23
Prof. Dale E. Gary
NJIT

Solar Activity

A. The Magnetic Sun

B. Solar Activity

The Sun goes through an 11-year activity cycle, where the number of sunspots becomes almost zero, then grows to a large number, then back to near zero over approximately 11 years.  Here is the changing sunspot number over the last 11 years.  In 2007 we were in sunspot minimum, when there were almost no sunspots, but for some reason the minimum continued for three years, only starting to rise in late 2009.  The dotted and dashed curves are two predictions of what the sunspot number may be next year. 

The plot below shows the previous sunspot cycle (#23) and the start of the current cycle (#24) in a somewhat different way.  The chart on the right is just the sunspot number as before, but the chart on the left shows the latitude dependence of the sunspot locations on the Sun.  Notice that early in the cycle, the sunspots generally start appearing near 30 degrees latitude on the Sun.  Over the cycle they occur at progressively lower latitudes. At the current time of the new solar cycle, they appear again at high latitudes, and late in the cycle they appear near the equator.  This is called a butterfly diagram.


The next plot shows the butterfly diagram for the last 140 years!  Notice that the sunspot cycle is very regular, but seems to have been growing stronger over the more recent cycles. However, there have been predictions that the solar cycle is declining again, and this seems to be borne out by the latest solar cycle.


This butterfly diagram and at least two other effects, Joy's Law, and Hale's Law, can be explained in a rough way by the operation of a solar dynamo.  A 2D movie shows the influence of "meridianal flow." With the addition of Hale's Law, the solar cycle is actually seen to be a 22-year cycle. Here is the "magnetic butterfly diagram."

Magnetic fields can be measured in the photosphere using the Zeeman Effect.  Below is an "image" of magnetic field strength showing not only the strength of the sunspot regions, but also their magnetic polarity.  White in the image represents fields coming out of the Sun (north magnetic polarity) while black represents fields going into the Sun (south polarity).
 


Magnetic Fields: The sunspots in the image at right are revealed to be areas of strong, opposite polarity magnetic fields, which extend to even larger areas than the spots themselves.

Notice the lighter areas, called faculae, which are found near sunspots and are also areas of strong magnetic field.

Sunspots and their surrounding magnetic field areas are called active regions.  Active regions are the site of solar flares, vast explosions of energy that send high-energy particles (electrons and protons mostly, but some helium atoms and a very few heavier elements) into space.  They are also involved in a phenomenon called coronal mass ejections (CMEs), which are spectacular "bubbles" of magnetic field and plasma that lift off from the Sun and expand quickly into interplanetary space.  Sometimes these CMEs are directed toward Earth, and when they run into the Earth's magnetosphere they can cause magnetic storms. Here is a Sun to Earth movie using actual data. See the SDO daily data page. Here is a link to a recent solar flare seen with NJIT's Expanded Owens Valley Solar Array.