The Sun’s magnetic field goes through an approximately 11-year cycle during which it completely changes its polarity (i.e., the Sun’s South and North Poles trade places.) This phenomenon, called the Solar Cycle (or Schwabe cycle), leads to significant fluctuations in the frequency and intensity of coronal mass ejections, sunspots, solar flares, etc. These disruptions are so massive that their effects are often felt even on Earth through aurora viewings, large geomagnetic storms, increased cosmic radiation, etc. Disturbances to electricity grids, satellite/radio communication, and spacecraft are not uncommon.
Solar cycles fluctuate between periods of minimum sunspot activity (Solar Minima) and periods of maximum sunspot activity (Solar Maxima). Post the completion of two solar cycles (together called a Hale cycle), the Sun’s magnetic field gets restored to its original state. Geological studies indicate that the 11-year solar cycle, and the 22-year Hale cycle may have more or less remained the same for 700-800 million years. Several additional theories have been associated with the solar cycle – such as the Gleissberg Cycle, Gnevyshev–Ohl Rule, Suess Cycle, Waldmeier Effect, etc.
The 1989 power blackout in Canada, and the 2003 power blackout across Eastern US and Canada are seminal examples of the impact of large geomagnetic storms. More recently, 40 Starlink satellites were destroyed by a relatively minor geomagnetic storm in February 2022. Hence, space weather predictions, including forecasting solar cycles, are extremely important. The solar cycle is reported to have been first discovered in 1843, but a lot is still unknown even to this day. An international team of experts (called the Solar Cycle Prediction Panel) makes predictions about the upcoming solar cycles. Observers from around 80 stations worldwide collect and mine data related to sunspots, and other aspects. However, while the cycles are repetitive, it is difficult to predict their intensity or exact duration (these cycles generally range from 8 to 14 years, 11 being close to the average) with high accuracy.
We are currently in solar cycle 25, which began in December 2019 and is expected to continue till 2030-31. In general, when the Sun’s magnetic field flips, the ripple effects are seen throughout the solar system, and beyond. Venus, on account of being without a protective magnetosphere, is estimated to be the hardest hit. In Dec 2006, a small but powerful coronal mass ejection from the Sun ripped away massive amounts of oxygen from Venus, thereby rendering it inhospitable for life. Other planets are impacted as well. For instance, some researchers believe that the appearance and disappearance of white clouds on the surface of Neptune is due to this 11 year-cycle. Neptune’s cloud cover appears to increase significantly during the peak sun activity period, and vice versa. While this observation may be considered more correlation than causation at this point, research on this topic does open up exciting avenues for further exploration and discovery.
Recent research indicates that the solar cycle might be caused by the alignment between Venus, Earth, and Jupiter, which occurs once every 11 years. These three planets have the strongest gravitational effects on the Sun, and displace the Sun’s plasma by about 1 mm during this alignment. While this displacement is minuscule, it is enough to disrupt the overall plasma circulation inside the Sun, thereby affecting the Sun’s magnetic field through a mechanism called the alpha effect. Having said that, there are still a lot of unanswered questions. For instance, it has been recently observed that extremely high-energy gamma rays emanating from the Sun, but none of the current models can explain the reason.
Studies on solar cycles and allied areas have received a massive boost in the past decade due to technological advancements, increasing global interest, and greater investments by public and private institutions. A lot of interesting developments are happening – here’s an example in which the polar field strength is used as the primary feature for solar cycle forecasting. Innovations in the form of advanced simulations, improved observational techniques, more comprehensive datasets, and new solar-dynamo theories are becoming the norm. Hopefully, all these measures will allow humans to predict better (and prepare against) disruptive space weather changes, including solar cycle effects.