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Solar Cycle 25
Image of the Sun captured at the Alqueva Lake Observatory through the telescopes used in the Solar Observation sessions
The Solar Cycle
In the 19th century, with the studies carried out by Samuel Heinrich Schwabe, astronomers became aware of the periodicity of the appearance of sunspots on the surface of the Sun, with an average period of approximately 11 years. As new phenomena on the surface of the Sun were discovered, such as solar flares, filaments, prominences, among others, it was discovered that the intensity and frequency of these phenomena varied depending on the number of sunspots. The number of sunspots is today the metric used to measure solar activity (more sunspots, more activity), which in itself has measurable effects on our planet. Geomagnetic storms caused by solar activity affect the functioning of satellites, power outages, variations in the Earth's climate, the aurora borealis, and much more. The impact of solar activity on the Earth and our technology has created the need to look for models with a better ability to predict solar activity.
Sunspot activity over the past 400 years has shown that the amplitude of the sunspot cycle varies from one cycle to another. The average cycle has about 150 sunspots during the period of maximum activity. However, occasionally, as was the case during the period known as the Maunder minimum between 1645 and 1715, solar activity can become so weak that it appears to disappear for several decades.
In addition to the 11-year solar cycle, there is a larger 88-year cycle called the Gleissberg cycle, which is associated with an amplitude modulation of the 11-year cycles. In other words, the solar cycles increase and decrease in intensity over time, and this variation in intensity is mediated by the Gleissberg cycle.
Figure 1: Plots show sunspot counts from 1750 to 2024. The 11-year cycle is easily observed. The longer 88-year cycle is observed by the variation in the maximum number of sunspots from cycle to cycle. Reference: Solar Cycle Science
In December 2019 the Sun entered the twenty-fifth solar cycle, SC25. Since then the number of sunspots has been gradually increasing.
What are sunspots?
Figure 2: Video corresponds to a time lapse of HMIIC images from the NASA – Solar Dynamics Observatory telescope, captured throughout the day 10/5/2024. The dark spots observed on the surface of the Sun are sunspots.
Sunspots are a characteristic phenomenon of the solar photosphere, which is one of several layers of the Sun. Just as in the Earth's atmosphere we have the troposphere, stratosphere, mesosphere and thermosphere, on the Sun, in simplified form, we have the photosphere, chromosphere and corona. The photosphere is a layer about 400km thick and has an average temperature of 5500ºC.
The spots represent regions where the temperature is lower, consequently making this region less luminous; the hotter it is, the brighter it is. As the region of the spots has a temperature of around 3500ºC, the luminosity of the region is lower than the hotter surface around it, giving the appearance of black spots on the solar surface.
Figure 3: Image of a sunspot captured by NASA's National Solar Observatory's Inouye Solar Telescope (DKIST). Two distinct regions of the sunspot can be distinguished.
Sunspots appear in both solar hemispheres. At the beginning of the cycle, they appear in smaller numbers and sizes at higher latitudes. As the cycle progresses, the number of spots increases as they approach the equator.
Although the recording of sunspots began in the 18th century and the discovery of the cyclical nature of the number of sunspots was made shortly thereafter, it took 150 years for George Ellery Hale, in 1908, to discover that sunspots are a manifestation of the activity of the solar magnetic field.
Figure 4: Video corresponds to a time lapse of HMIBC images from the NASA – Solar Dynamics Observatory telescope, captured throughout the day 10/5/2024.
Figure 4 shows the intensity of the magnetic field on the surface of the Sun. The scale from green to blue indicates the intensity of the northern polarity of the magnetic field. The scale from yellow to red indicates the intensity of the southern polarity of the magnetic field. The regions of greatest magnetic field intensity correspond to the sunspot regions. Nowadays it is impossible to understand solar phenomenology without understanding in depth the physics of the solar magnetic field.
The increase in the number of sunspots throughout the cycle brings with it an increase in the number of solar flares.
Very (very, very, very) simply, sunspots are regions where energy accumulates. Sometimes the sun can release this energy in the form of eruptions, as was the case on October 3, 2024, when the largest eruption in the last 17 years was recorded.
Solar eruptions, the Earth and the Aurora Borealis
Figure 5: Video corresponds to a time lapse of AIA 171 images from the NASA – Solar Dynamics Observatory telescope, captured throughout the day 10/3/2024. The X9.05 intensity eruption occurred at 12:17 UT.
On July 28, 2023, we also had a less intense eruption. This occurred closer to the solar perimeter, so it is possible to better observe the size of these phenomena. For scale purposes, along the solar equator we place around 110 Earths.
Figure 6: Video corresponds to a time lapse of AIA 304 images from the NASA – Solar Dynamics Observatory telescope, captured throughout the day 07/28/2023. The M4.11 intensity eruption occurred at 15:58 UT.
Some of these eruptions, composed of highly energetic particles, manage to reach Earth and interact with our planet's magnetic field. This interaction is what causes the aurora borealis. Therefore, the aurora borealis is a direct manifestation on Earth of solar activity 150 million kilometers away.
Throughout 2024, it was possible to observe the Northern Lights in Portugal, as solar activity continues to increase. Throughout 2025 and early 2026, it will be possible to continue observing these events in Portugal, if the intensity of the eruptions allows. However, although we can observe these phenomena in Portugal, the auroras will always be more intense at higher latitudes, an experience certainly not to be missed.
Butterfly Diagram
Figure 7: The top graph is a butterfly diagram and shows the progression of sunspots throughout the cycle. The vertical axis of this graph is the sine of latitude (e.g., sine of 30º is 0.50). Sunspots appear at higher latitudes and move closer to the equator throughout the cycle. The bottom graph shows the increase in percentage coverage of the sun's surface by sunspots. Reference: Solar Cycle Science
Butterfly diagrams illustrate the distribution of sunspots and magnetic flux on the Sun's surface and their variations over time. Cycles typically overlap by 2 to 3 years, meaning that before one cycle is over, the next cycle is already beginning. At the beginning of each cycle, active regions emerge at latitudes of about 30°. As the cycle progresses, active regions emerge closer and closer to the equator, an effect known as Sporer's Law. The cancellation of magnetic polarity at each latitude and across the equator leaves behind an excess of the next polarity that is carried to the poles. The north and south poles have opposite polarities that reverse from cycle to cycle. The time of this polar field reversal is near the maximum of the solar cycle.
Solar maximum in 2025
Peak intensity is expected for July 2025 with a maximum number of 120 sunspots.
Figure 8: Forecast of Solar Cycle 25 compared to previous cycles. Reference: Solar Cycle Science
The probability of observing auroras in Portugal will be greater in 2025, depending on the intensity of solar flares. Geomagnetic storms are associated with interference in communications by or with satellites, cause interference in radio or GPS signals and can cause problems in the national electricity grid.
Solar Cycle 25 at the Lake Alqueva Observatory
Throughout the year 2025 we will have a Sun full of activity to observe in Solar Observations .
With special telescopes we can observe the solar chromosphere and photosphere. We observe filaments, sunspots, flares, coronal ejections and the granular surface of the chromosphere. All these phenomena will have a greater intensity and frequency due to the greater degree of activity of the Sun. At the end of the session, if atmospheric conditions allow it, we will capture an image of the Sun for our visitors.
In Astronomical Observations we may be lucky enough to catch auroras in the sky.
These events will be very difficult to predict in advance but the sky at the Alqueva Lake Observatory is more than dark enough to observe these events.
Only in 2024, we had the opportunity to observe auroras on two different days, one of them with our visitors on October 5th at 11 pm.