Like the number of sunspots, the occurrence of solar flares follows an approximately 11-year solar cycle.
But as current Solar cycle 25 is nearing its peak, what is the number solar flare to match the previous, smaller solar cycle of 24?
Due to the change in flare calibration levels since 2020, you will find two answers to this question online – but only one is correct.
Related: The Sun’s magnetic field is about to flip. Here’s what you can expect.
The Sun follows an 11-year solar cycle of waxing and waning activity. The solar cycle is usually measured by the number of visible sunspots sun, with records dating back over 270 years. Most solar flares come from sunspots, so more sun spots — you get more flares.
Solar flares are categorized into flare classes, classified by the magnitude of the soft X-rays observed in a narrow wavelength range of 0.1-0.8 nm. Flare classes are C, M, and X, each 10 times stronger than the previous one. (Flame levels are then divided by number, e.g. M2, X1, etc.). Flares of these categories (except for the largest X-class events) tend to follow the solar cycle closely.
In terms of sunspot numbers, solar cycle 25 (our current cycle) exceeded the sunspot levels of solar cycle 24 (which peaked in 2014). With a higher number of sunspots, we would also expect a higher number of flares. This is the case but the difference is far from what some would believe.
Recalibrating solar flare levels
How do solar flares compare between solar cycles 24 and 25? It seems like a simple enough question, but it’s clouded by the National Oceanic and Atmospheric Administration’s (NOAA) recalibration of 2020 solar flare levels.
X-ray radiation levels from solar flares have been measured since 1974. X-rays do not penetrate Earth’s atmosphere, and can therefore only be measured by detectors on Earth-orbiting satellites. For 50 years, these solar flare detectors have been located on NOAA’s GOES satellites. As technology improves and older technology declines, newer detectors are launched on the newer GOES satellites to continue the continuous observation of solar flares. GOES-18 (the 18th satellite in the sequence) is the current satellite responsible for primary X-ray observations, which has launched in 2022.
These flare-defining X-rays are measured by X-ray sensors aboard NOAA’s GOES satellites. The GOES satellite has been around for decades, and their solar X-ray detectors operate for several years until a newer replacement is launched. pic.twitter.com/3INrEKPFWFMay 5, 2024
Because flare levels have been measured (and their classes defined) by detectors on multiple satellites/instruments, corrections are sometimes necessary to account for slight differences in calibration from one detector to another.
In 2010-2020, flare levels were defined by measurements from GOES-14 and GOES-15. This period covered solar maximum solar cycle period 24 until the end of that cycle. However, after the launch of these two satellites, a calibration discrepancy was found between GOES-14/15 and all the previous ones GOES X-ray detectors. To correct this, all science data from 1974-2010 (from the GOES-1 to GOES-13 satellites) was readjusted to match the new calibration that was considered correct at the time. The result of this was that the threshold for each flare class increased by 42%, meaning that a single solar flare in 2010 had to be 42% larger than the 2009 flare to be given the same X-class level.
However, and here comes the twist: after switching to the GOES-16 data on the new detector, it was found that the original calibration (from 1974-2010) was correct all along, and the 2010-2020 calibration was incorrect. . This meant that in 2020 all the previous ones data (from 1974-2020) have been recalibrated again to their previous correct levels, lowering the threshold of the various flare classes). With a lower flare threshold, this meant that strong C-class (C7+) flares became M-class events, and strong M-class (M7+) flares became X-class flares. Therefore, an X-class solar flare was much easier to achieve in 2021 than in in 2019. This recalibration in 2020 therefore increased the number of higher class eruptions in solar cycle 24 than originally reported.
Inconsistencies in the number of flares
After recalibrating solar flare levels in 2020, NOAA has republished its historical scientific flare files with the correct levels. However, archived operational data, which report solar flare levels as originally reported at the time, have not been recalibrated. As a result, various flare lists compiled and analyzed by third parties may use either recalibrated scientific data or uncalibrated operational data when comparing solar flare levels between solar cycles. The first comparison gives the correct results, while the second compares current flare levels from cycle 25 to severely underestimated flare levels from previous cycles, resulting in scientifically incorrect comparisons. Let’s compare some data!
Comparison of solar cycle 24 and solar cycle 25
The graphs below show the number of solar flares occurring in each flare class compared between solar cycles 24 and 25. The blue, orange, and red lines show the number of class C, M, and X flares, multiplied by 0.1, 1, and 10 to plot the data on the same axis.
The thicker lines show these data for solar cycle 25 (with years from 2021 on the X-axis). The thinner, less saturated lines show the same data for solar cycle 24 (plotted against years since 2010).
This first chart shows this comparison from historical flare traffic data, i.e. excluding the 2020 recalibration.
This graph shows a huge difference in flare levels from cycles 24 to 25. According to this graph, the total number of flares in class M and X in cycle 25 already overtaken a total of 24 cycle flares in less than half the time. This is a catchy headline and has been shared by several prominent social media accounts. But as impressive as this statistic sounds, it is incorrect. As we discussed, this graph does not account for the 42% recalibration of flare levels.
This second graph shows the same comparison, this time from accurate scientific data.
This chart shows the correct comparison of solar flares between Cycles 24 and 25. As you can see, although the number of flares in Cycle 25 is still ahead of Cycle 24 at each flare level, the difference is much smaller than that shown in the previous chart. The operational data underestimate the number of cycle 24 bursts by almost half, a significant difference. In fact, the number of X-class solar flares in Cycle 24 is only half of the total in Cycle 24, and even had fewer X-class flares until the recent solar activity from the famous active regions AR 13663 and AR 13664. This chart also shows that although in In May 2024, there was a lot of X-class activity in these active regions, this level of activity is not unprecedented – with solar cycle 24, there was a similar spike in flares towards the end of 2015.
So remember, if you see comparisons of solar cycle flare levels online, be sure to check if they are using historical operational data (wrong) or recalibrated science data (correct).