Like the number of sunspots, the occurrence of solar flares follows the approximately 11-year solar cycle.
But as a current Solar cycle 25 approaches the peak, what are the number of solar flares stacking up against the earlier, smaller Solar Cycle 24?
Due to a change in ignition calibration levels from 2020, you will find two answers to this question online – but only one is correct.
Connected: The sun’s magnetic field is about to flip. Here’s what to expect.
The Sun follows an 11-year solar cycle of waxing and waning. The solar cycle is usually measured by the number of sunspots visible in it SUN, with records dating back over 270 years. Most solar flares originate from sunspots, so with more sun spots — you’ll get more fire.
Solar flares are categorized into flare classes, classified by the magnitude of soft X-rays observed in a narrow wavelength range of 0.1–0.8 nm. The ignition classes are C-class, M-class and X-class, each 10 times stronger than the previous one. (The ignition levels are then subdivided by a number, eg M2, X1, etc.). Flares of 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 counts, we would also expect higher flare counts. This is 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 seems like a simple enough question, but it is clouded by a recalibration of solar flare levels in 2020 by the National Oceanic and Atmospheric Administration (NOAA).
Solar flare X-ray levels have been measured since 1974. X-rays do not penetrate the Earth’s atmosphere, and thus can only be measured by detectors on Earth-orbiting satellites. For 50 years, these solar flare detectors have been deployed on NOAA’s GOES satellites. As technology improves and older technology breaks down, newer detectors are launched on newer GOES satellites to continue the ongoing 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 X-rays that define the flare are measured by X-ray sensors aboard the NOAA GOES satellites. The GOES satellite has been around for decades, and their solar X-ray detectors are operational for several years, until a newer replacement is launched. pic.twitter.com/3INrEKPFWFMay 5, 2024
Because flare levels are measured (and their classes determined) by detectors across multiple satellites/instruments, corrections are sometimes needed to account for small differences in calibration from one detector to another.
From 2010-2020, flare levels were determined from measurements from GOES-14 and GOES-15. This period covered solar maximum period of solar cycle 24, until the end of that cycle. However, with the launch of these two satellites, a calibration discrepancy was discovered between GOES-14/15 and all before GO X-ray detectors. To fix this, the science data from 1974-2010 (from the GOES-1 to GOES-13 satellites) were all readjusted to match the new calibration, which was believed to be accurate at that time. The result of this was that the threshold for each flare class was increased by 42%, meaning that an individual solar flare in 2010 had to be 42% larger than a 2009 flare to be given the same class X level.
However, and here comes the twist: after switching the GOES-16 data to a new detector, it was found that the original calibration (from 1974-2010) had been correct all along, and the 2010-2020 calibration was incorrect. This would mean that in 2020, all before data (from 1974-2020) were recalibrated back to their previously accurate levels, lowering the threshold of the various ignition classes). With a lower flare threshold, this meant that strong C-class (C7+) flares became M-class events, and M-class (M7+) strong flares became X-class flares. Therefore, a class solar flare X was much easier to reach in 2021 than in 2019. Therefore, this 2020 recalibration increased the number of higher-class flares in Solar Cycle 24 than originally reported.
A mismatch in ignition numbers
After recalibrating solar flare levels for 2020, NOAA re-released their historical scientific data sets of flares with the correct levels. However, archived operations data, which list the levels of solar flares as originally reported at the time, were not recalibrated. A consequence of this is that the 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 accurate results, while the second compares actual ignition levels from cycle 25 with grossly underestimated ignition levels from previous cycles, producing scientifically inaccurate 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 within each flare class, in a comparison between solar cycles 24 and 25. The blue, orange, and red lines show the number of class C, M, and X 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 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 ignition performance data, ie. not taking into account the 2020 recalibration.
This graph shows a massive change in ignition levels from cycles 24 to 25. According to this graph, the total number of class M and X ignitions in cycle 25 has already overcome total number of flashes from cycle 24, in less than half the time. This is a witty headline and was posted by some popular social media accounts. But as stark as this statistic sounds, it is inaccurate. As we have discussed, this scheme does not take into account the 42% recalibration of the ignition levels.
This second chart shows the same comparison, this time from exact scientific data.
This chart shows the exact comparison of solar flares between cycles 24 and 25. As you can see, although the number of flares of Cycle 25 is still ahead of Cycle 24 at every flare level, the discrepancy is much smaller than that shown in the chart of previous The operations data underestimate the number of Cycle 24 flares by nearly half, a significant difference. In reality, the number of X-class solar flares in cycle 24 is only half of the total amount of cycle 24, and there were even fewer X-class flares until the last solar activity from the famous active regions AR 13663 and AR 13664. This graph also shows that although May 2024 saw a lot of X-class activity from these active regions, this level of activity is not unprecedented – with Solar Cycle 24 experiencing a similar jump in flares towards the end of 2015.
So remember, if you see comparisons of Solar Cycle flare levels online, be sure to check whether they are using historical operational data (inaccurate) or scientific recalibrated data (accurate).