Like the number of sunspots, the occurrence of solar flares follows a solar cycle of approximately 11 years.
But like the current Solar cycle 25 is approaching its peak, how are the numbers? solar flares How does it compare to the previous, smaller Solar Cycle 24?
Due to a change in flare calibration levels starting in 2020, you will find two answers to this question online, but only one is correct.
Related: The Sun’s Magnetic Field Is About to Reverse. Here’s What to Expect.
The Sun follows an 11-year solar cycle of waxing and waning activity. The solar cycle is usually measured by the number of sunspots visible on the sunwith records dating back over 270 years. Most solar flares come from sunspots, so with more sunspots — you will have more flares.
Solar flares are classified into classes, ranked by the magnitude of soft X-rays observed in a narrow wavelength range of 0.1 to 0.8 nm. Flare classes are C-class, M-class, and X-class, each 10 times stronger than the previous one. (Flare levels are further subdivided by a number, e.g. M2, X1, etc.). Flares in these categories (except for the largest X-class events) tend to closely follow the solar cycle.
In terms of sunspot numbers, Solar Cycle 25 (our current cycle) has surpassed the sunspot levels of Solar Cycle 24 (which peaked in 2014). With higher sunspot numbers, we also expect higher numbers of solar flares. It’s the case, but the difference is far from what some would have you believe.
Recalibration of solar flare levels
How do solar flares compare between solar cycles 24 and 25? This question seems simple enough, but it is muddied by a recalibration of solar flare levels in 2020 from the National Oceanic and Atmospheric Administration (NOAA).
X-ray levels from solar flares have been measured since 1974. X-rays do not penetrate the earth’s atmosphereand can therefore only be measured by detectors on Earth-orbiting satellites. For the past 50 years, these solar flare detectors have been placed on NOAA’s GOES satellites. As technology improves and older technologies degrade, new detectors are launched on new GOES satellites to ensure continued observation of solar flares. GOES-18 (the 18th satellite in the sequence) is the current satellite responsible for primary X-ray observations, having launched in 2022.
These flare-defining X-rays are measured by X-ray sensors aboard NOAA’s GOES satellites. The GOES satellites have been around for decades, and their solar X-ray detectors are operational for a few years until a new replacement is launched. pic.twitter.com/3INrEKPFWFMay 5, 2024
Since flare levels were measured (and their classes defined) by detectors on multiple satellites/instruments, corrections are sometimes necessary to account for slight differences in calibration between detectors.
From 2010 to 2020, flare levels were defined by measurements from GOES-14 and GOES-15. This period covered the solar maximum period of solar cycle 24, until the end of this cycle. However, during the launch of these two satellites, a calibration difference was discovered between GOES-14/15 and all previous GOES X-ray detectors. To address this issue, the science data from 1974 to 2010 (from GOES-1 to GOES-13 satellites) were all readjusted to match the new calibration, which was considered correct at the time. The result was that the threshold for each flare class increased by 42%, meaning that an individual solar flare in 2010 would have to be 42% larger than a flare in 2009, to get the same X-class rating.
However, and this is where the problem lies: after switching to GOES-16 data on a new detector, it was discovered that the original calibration (from 1974 to 2010) was correct all along, and that the calibration from 2010 to 2020 was incorrect. This means that in 2020, all previous The data (from 1974 to 2020) was again recalibrated to its previous correct levels, lowering the threshold for the different flare classes. With a lower flare threshold, this meant that strong C-class flares (C7+) became M-class events, and strong M-class flares (M7+) became X-class flares. An X-class solar flare was therefore much easier to obtain in 2021 than in 2019. This 2020 recalibration therefore increased the number of higher-class flares in solar cycle 24 compared to what was originally reported.
A discrepancy in the number of flares
Following the recalibration of solar flare levels in 2020, NOAA republished its historical scientific flare datasets with the correct levels. However, the archived operational data, which lists solar flare levels as they were originally reported at the time, has not been recalibrated. As a result, different solar flare lists compiled and analyzed by third parties may use either the recalibrated scientific data or the unrecalibrated operational data to compare solar flare levels between solar cycles. The former comparison yields correct results, while the latter compares current solar flare levels from cycle 25 with severely underestimated solar flare levels from previous cycles, producing 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, in a comparison between solar cycles 24 and 25. The blue, orange and red lines show the number of C, M and X class flares respectively, multiplied by 0.1, 1 and 10 to plot the data on the same axis.
The thicker lines show this data for solar cycle 25 (with years since 2021 on the X-axis). The thinner, less saturated lines show the same data for solar cycle 24 (plotted relative to years since 2010).
This first graph shows this comparison based on historical torch operating data, i.e. without taking into account the 2020 recalibration.
This graph shows a massive difference in flare levels between cycles 24 and 25. According to this graph, the total number of M and X class flares during cycle 25 was already exceeded The total number of surges in cycle 24, in less than half the time. This is a punchy headline that has been posted by several prominent social media accounts. But as punchy as this statistic may seem, it is incorrect. As we have seen, this graph does not account for the 42% recalibration of surge levels.
This second graph shows the same comparison, this time based on precise scientific data.
This graph 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 always higher than that of cycle 24 at each flare level, the difference is much smaller than that shown in the previous graph. The operational data underestimates the number of flares in cycle 24 by almost half, which is a significant difference. In reality, the number of X-class solar flares in Cycle 24 is only half the total number in Cycle 24, and there were even fewer X-class flares until the recent solar activity from the famous active regions AR 13663 and AR 13664. This chart also shows that while May 2024 saw strong X-class activity in these active regions, this level of activity is not unprecedented – Solar Cycle 24 saw a similar surge in flares towards the end of 2015.
So remember, if you see the solar cycle flare level comparison online, be sure to check whether they are using historical operational data (incorrect) or recalibrated scientific data (correct).