Solar Cycle 25 is coming to life. For the second time this month, sunspots belong to Solar Cycle 25 have been identified. A sunspot, numbered AR2744, emerged in the Sun’s southern hemisphere on July 8th, 2019 and has been identified as being part of Solar Cycle 25 according to spaceweather.com.
The solar cycle is a cycle of low to high magnetic activity that the Sun goes through approximately every 11 years. The sun is currently in a period of minimum magnetic activity at the end of Solar Cycle 24. The appearance of sunspot AR2744 heralds the possible start of Solar Cycle 25.
How is AR2744 indentified as being part of Solar Cycle 25?
A key piece of evidence relies on identifying the polarity of the magnetic fields of sunspots. Sunspots generally group together in pairs to form the poles of solar magnets as if a giant horseshoe magnet was placed under the Sun’s surface.
One sunspot of each pair has a positive (“+” or “north”) polarity where the magnetic field is directed outward. The other spot in the pair has the opposite negative polarity (“-” or “south”) where the magnetic field is directed inward. The polarity of sunspots is highlighted in magnetogram images which show the strength of magnetic fields and their polarity. The image below is a magnetogram image of the area around AR2744 where areas with “+” polarity appear as green while areas of “-” polarity appear as red/orange.
Sunspot pairs are usually oriented parallel to the Sun’s equator so a sunspot pair can be classified as either:
- “+/-” as in the image above, or
- the opposite orientation “-/+”, as if the horseshoe magnet was rotated 180 degrees.
Sunspots in the Sun’s southern hemisphere from the old Solar Cycle 24 have a -/+ polarity. AR2744 appeared in the southern hemisphere but its polarity, as seen in the above image is “+/-“, making it a part of Solar Cycle 25. This reasoning is based on G.H. Hale’s observations in the first decade of the 20th century:
- The same polarity pattern is seen in each solar hemisphere. For example, all cycle 24 sunspots in the southern hemisphere have the same “-/+” polarity.
- Opposite polarity pattern are seen in north vs southern hemisphere. So all cycle 24 sunspots in the northern hemisphere have “-/+” polarity.
- Polarity patterns reverse in each subsequent solar cycle. The “-/+” polarity of cycle 24 southern spots flips to “+/-” for cycle 25.
It is common for sunspots belonging to both cycles to appear during the transition from one cycle to the next so we will have to keep an eye out for more spots belonging to cycle 25.