9:00 AM | *An update on Solar Cycle 25...a somewhat surprising start to the new cycle in terms of sunspot activity...the connection to the "Great American" total solar eclipse of April 2024*

There was an explosion yesterday, July 31st, on the sun’s northeastern limb which may indicate a more active period in coming days. It has been relatively quiet on the sun in recent days despite an overall quick start to solar cycle 25. This new sunspot group which resulted in yesterday’s solar flare will rotate into a position that soon puts the Earth in a more direct “line of fire”….something to monitor. Courtesy spaceweather.com, NASA's Solar Dynamics Observatory (SDO)

Overview

Solar cycle 25 began during late 2019 and is expected to continue until about 2030 with a peak around 2025.  This current solar cycle is the 25th since 1755 when extensive recording of sunspot activity began. Sunspot activity has ramped up significantly since the beginning of cycle 25 featuring 57% spotless days in 2020, 18% in 2021, and only 1 day so far this year without visible sunspots. In fact, the start to solar cycle 25 has been somewhat surprising with more sunspot activity than most earlier predictions had suggested.

The observed and predicted solar cycle is depicted in “Sunspot Number”, the black line represents the monthly averaged data and the purple line represents a 13-month weighted, smoothed version of the monthly averaged data.  The forecast for the current solar cycle is given by the red line. Plot courtesy NOAA/Space Prediction Center

Solar cycle predictions

Solar cycle predictions are used by various agencies and industry groups and are very difficult as our overall knowledge of the sun is still rather limited. Many solar scientists would suggest, for example, that we really don’t have a good idea about what happens inside the sun.  It is thought that the solar magnetic field is generated by a dynamo inside the star, a rotating, convecting, and electrically conducting fluid that converts kinetic energy into magnetic energy, spinning a magnetic field out into space around the Sun. Sunspots are thought to be related to the sun’s rotation and the solar equator actually rotates faster than the poles. If straight magnetic field lines running longitudinally were to be dragged along with this rotation, they would become stretched and eventually tangled, generating temporary, localized regions of strong magnetic fields, or sunspots.

Daily observations of the number of sunspots since 1 January 1900 according to Solar Influences Data Analysis Center (SIDC). The thin blue line indicates the daily sunspot number, while the dark blue line indicates the running annual average. The recent low sunspot activity is clearly reflected in the recent low values for the total solar irradiance. Data source: WDC-SILSO, Royal Observatory of Belgium, Brussels. Last day shown: 30 June 2022. Last diagram update: 1 July 2022.

Despite our limited knowledge of the sun, predictions are indeed made for solar cycle activity as, for example, they are important for determining the life span of satellites in low-Earth orbit, as the drag on the satellites correlates with the solar cycle. A higher solar maximum decreases satellite life and a lower solar maximum extends satellite life.  Also, the prediction gives a rough idea of the frequency of space weather storms of all types, from radio blackouts to geomagnetic storms to radiation storms, so is used by many industries to gauge the expected impact of space weather in the coming years. The current solar cycle, 25, appears to be more active than the consensus prediction with more sunspot activity at this stage compared to many of the earlier predictions. There certainly is still a chance that solar cycle 25 ends up as another weak cycle with less sunspot activity compared to normal. If so, this would continue a recent trend that has featured weakening solar cycles since cycle 21 peaked around 1980.

Despite a faster than expected start for solar cycle 25 in terms of overall sunspot activity, it is still featuring a somewhat similar amplitude at this stage to other “weak” solar cycles in the past including the just prior #24. Plot courtesy WDC

Cosmic rays

One of the ways to determine the stage of a solar cycle is to evaluate the level of cosmic rays that are reaching the Earth. Galactic cosmic rays from distant supernova explosions pass through the solar system and are high-energy particles that impact the Earth’s atmosphere. Most of the incoming cosmic ray particles are protons and they actually arrive as individual particles – not in the form of a ray as the term “ray” would suggest. Cosmic rays from deep space are repelled by solar activity; when one goes up, the other goes down. The sun makes it more difficult for these cosmic rays to reach Earth. Simply put, when the sun is active, its magnetic field gets stronger and more tangled. Cosmic rays have trouble penetrating the magnetic thicket. Also, individual coronal mass ejections (CMEs) sweep aside cosmic rays, causing sharp reductions called "Forbush Decreases."

Most high-energy radiation doesn't come from the sun; it comes from deep space. Every day galactic cosmic rays from distant supernova explosions pass through the Solar System. When they hit the top of Earth's atmosphere, they create a secondary spray of radiation, which we measure using sensors onboard our balloons. Measurements since the onset of solar cycle 25 show a 15% reduction in the atmosphere high over California. ((Courtesy spaceweather.com).

One of the best ways to determine the influx of cosmic rays on Earth is to measure radiation levels high up in the atmosphere. In fact, measurements using “cosmic ray balloons” in the high atmosphere over California have shown a 15% drop in radiation levels since the early days of solar cycle 25.  Indeed, the most recent measurement on July 23rd was the lowest level recorded during the past 6 years.  These type of measurements are done on a regular basis through the use of balloons and are sponsored by “spaceweather.com” (more information here). This finding of subsiding cosmic radiation is not surprising as the onset of solar cycle 25 – a cycle that is more active than anticipated - has naturally led to the decrease in cosmic radiation reaching the Earth.  Solar maximum for cycle 25 is expected sometime during 2025, so the downward trend in cosmic radiation should continue for a few years to come. 

Impact of solar cycle 25 on the “Great American” total solar eclipse of April 2024

Total solar eclipses provide rare opportunities to gather information for many scientific disciplines including solar dynamics, heliophysics and atmospheric science.  As an example, these events are great for studying the sun’s wispy outer atmosphere known as the corona as the brightness of the sun typically drowns out this outer layer. Indeed, not even a 99 percent eclipse will reveal the sun’s corona.  Temperatures in the corona can reach 1 million °C, making the region much hotter than the solar surface, which is “just” 6,000 °C or so. How the corona gets so hot has puzzled scientists for decades and solar scientists gather useful data during total solar eclipses with the next opportunity here coming in 2024.

The expected solar maximum phase of cycle 25 may actually turn out to be very advantageous for scientists to study the corona during the April 2024 eclipse. Current trends point to the sun having about 125 sunspots when it peaks and this may be not far past the time of the total solar eclipse in April 2024.  A more active sun usually is correlated with a more dynamic corona which is making for a lot of excitement in the solar scientist community regarding the April 2024 total solar eclipse.

Meteorologist Paul Dorian
Arcfield
arcfieldweather.com

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