SC25 Tracking


Welcome to the STCE's Solar Cycle 25 tracking page! Here we monitor the ongoing solar cycle 25 (SC25) by providing graphs on the evolution of some important parameters in the space weather domain such as the sunspot number, flaring activity, coronal mass ejections and geomagnetic indices. This page is updated about once every 4 months, and allows for comparisons with previous solar cycles. Table underneath can be used to quickly navigate to your parameter of interest. Enjoy!

Latest update: 26 October 2021 - Next update: February 2022


Sunspot number Hemispheric sunspot activity Butterfly-diagram & Latitude
10,7 cm radio flux Number of M- and X-class flares Number of Coronal Mass Ejections
Number of Prominences Number of proton events Ap-index
> 2 MeV Electron fluence Number of Ground Level Enhancements Dst-index
                                                 Cosmic rays  


Sunspot number


The sunspot number provides the longest record on solar activity, dating back all the way to the early 17th century. The graph above shows the monthly and SILSO smoothed monthly sunspot number (SIDC/SILSO smoothing formula) for the previous 2 solar cycles, SC23 and SC24. The prediction for the ongoing solar cycle 25 which started in December 2019, is displayed in green and derived using the parametrized equations from Hathaway et al. (1994). Amplitude (115 +/-10) and timing of the maximum (July 2025 +/- 8 months) are based on the results from the international SC25 Prediction Panel, but in the above graph the timing has been advanced to August 2024 (SC25 panel update). The expected rise time is now 56 months, still remaining well above the average rise time of the previous 24 solar cycles (about 53 months). More infographics on the sunspot number and SC25 are at a dedicated SILSO webpage. Sunspot number data from SILSO, World Data Center for the Sunspot Index, Royal Observatory of Belgium, Brussels, 1749-present.


Hemispheric sunspot activity


Solar activity is not equally distributed over the two solar hemispheres. Indeed, there can be significant differences over longer time scales (several years) in the hemispheric activity levels. The above graph shows the evolution of the sunspot area of the solar hemispheres based on the Greenwich sunspot areas (1874-present), with "blue" indicating a dominating northern hemisphere, and "red" a dominating southern hemisphere. These data are maintained by and available at the Solar Cycle Science group. From the maximum of SC18 till the maximum of SC20, the northern solar hemisphere was clearly dominating solar activity. The descending branch of the last 4 solar cycles was clearly influenced by the southern hemisphere. Currently (October 2021), the southern hemisphere is slightly dominating.


Similar to the sunspot area, a graph can be created for the evolution of the sunspot number of the solar hemispheres. These data are maintained by SILSO. Originally, these existed only for the last 3 solar cycles, starting in 1992 (See these SILSO datafiles). The newly extended hemispheric sunspot numbers series are based on the sunspot areas listed in the Greenwich photographic catalogue, and are calibrated on the existing SILSO reference hemispheric database (Veronig et al., 2021). In the graph above, "green" is indicating a dominating northern hemisphere, and "red" a dominating southern hemisphere. For obvious reasons, the graph is very similar to the one of the hemispheric sunspot area.


Butterfly-diagram & Latitude


The distribution of the sunspot groups over the solar disk can be seen in the so-called butterfly diagram, displaying for every month the latitude of every individual sunspot group (data from the Solar Cycle Science website ) that has appeared. It clearly shows that during the period of solar cycle minimum, the sunspot groups of the old solar cycle appear ever closer to the solar equator, while the sunspots of the new solar cycle manifest themselves again at high latitudes. As the pattern resembles the wings of a butterfly, it is called a butterfly diagram. The first such butterfly diagram was published by E. Walter Maunder in the Monthly Notices of the Royal Astronomical Society in 1904. The sunspot groups that have emerged over the last few months all belong to the new SC25 and continue to reveal more of the outer tips of the new butterfly's wings. From 16 till 18 July 2021, a sunspot was visible near the southeast solar limb at the very high southern latitude of -43 degrees. NOAA 2844 was a quiet and magnetically simple region, and the first region of the new SC25 of which the latitude exceeded 40 degrees. See also this STCE Newsitem.


The graph above shows the smoothed (Meeus, 1958) average latitude of the sunspot regions from SC12 to 24 (brown; the thin dashed lines show the standard deviations), and for SC25 (green). The SC12-24 and SC25 latitudes are fit such that the months of lowest smoothed latitudes coincide (month "0"). SC25 latitude reached a minimum in February 2019 (6.3 degrees, so the active regions emerged very close to the solar equator), and has peaked in August 2020 (23.2 degrees). This peak seems to have occured slightly earlier and to be slightly higher than the average SC12-24 values, but there's nothing out of the ordinary. More info on the average latitude of sunspot groups is in this STCE Newsitem.


10.7 cm radio flux


Every day, the Dominion Radio Astrophysical Observatory (DRAO) in Penticton, British Columbia, Canada, measures and publishes the values of the 10.7 cm solar radio flux. The wavelengths around 10.7 cm (frequencies around 2800 MHz) are ideal for monitoring the solar activity, as they are very sensitive to conditions in the upper chromosphere and at the base of the corona. As a result, this radio flux is widely used in solar research and space weather applications such as e.g. satellite drag. It is expressed in solar flux units (with 1 sfu = 10-22 W m-2 Hz-1), and measured three times daily: around 17UT (18UT in winter), 20UT (local noon at Penticton), and 23UT (22UT in winter). The 20UT value is usually taken as the radio flux for that particular day (Tapping, 2013).

In the graph above, the adjusted data (corrected for the varying Sun-Earth distance) have been used, with data till December 2017 coming from NGDC/NOAA archive, and from National Resources Canada (NRCan) afterwards. The data were smoothed using Meeus smoothing formula (Meeus, 1958) and cover the averages from SC19 to SC24. The dashed lines reflect values including the standard deviations. The SC25 radioflux remains at the low end of the SC19-24 average. It is within 1 sfu compared to values of the previous solar cycle (SC24) at this stage of the cycle (16 months after smoothed minimum).


Number of M- and X-class flares


Solar flares are known to cause radio black-outs at high frequencies (HF; 3-30 MHz). Some flares may be accompanied by strong radio emission affecting radar and GNSS frequencies (see the ROB/GNSS website), thus causing problems with air traffic operations and with GPS-based communication and navigation services. Solar flares are recorded in the wavelength band from 1 to 8 Angstrom (0.1-0.8 nm) by the GOES (NOAA/SWPC). There are 3 distinct classes: C, M, and X standing for small (or "Common"), Medium, and large (or "eXtreme") flares. The range is logarithmic, each class being 10 times stronger than the previous one, and within each category ranging from 1 to 9 (e.g. a C9 flare, an M3 flare,...).

In the graph above, the yearly number of flares are binned in 3 classes: M1 to M4, M5 to M9, and the X-class flares. The thin green line represents the yearly International Sunspot Number (ISN). The values for the current year are obviously preliminary. The data sources are NOAA/NGDC (1976-July 2017), and the NOAA/SWPC Weekly bulletins (2017-present). Rescaling of the flares' peak intensity (pre-GOES-16 era) has been performed in accordance with guidelines at NGDC/NOAA. So far (mid-October 2021), SC25 produced 14 flares in the M1_M4 range, as well as 1 X-class flare (X1.5 on 3 July 2021 by NOAA 12838 - see this STCE Newsitem). As the maximum sunspot number for SC25 is thought to be similar to that of SC24, the number of M- and X-class flares is also expected to be very similar (rescaled!), with 1075 M- and 76 X-class flares forecasted for the entire duration of SC25 (see Janssens, 2021).


Number of proton events


An increased level of energetic protons may constitute a radiation threat to astronauts, in particular during their extra-vehicular activities (space walks). Proton events can increase the radiation dose of the crew and passengers on transpolar flights, and can cause communication problems over the polar areas (the so-called "Polar Cap Absorption"). These particles also give satellites a hard time. They can create malfunctions in the onboard electronic circuitry, degrade solar panel efficiency, and increase the noise in star-tracking systems. The parameter used to gauge the proton flux is the > 10 MeV proton flux, as measured by the GOES (NOAA/SWPC). The proton flux is expressed in proton flux units or pfu (1 pfu = 1 proton / cm2 s sr), with the alert threshold at 10 pfu.

The graph above shows the yearly number of proton events, binned per peak flux range in accordance with the corresponding NOAA S-scale. Since the GOES measurements started in 1976, there have been about 60 proton events per decade (see NOAA/SESC and NOAA/SWPC for the source data). Note there are large differences between solar cycles. For example, during SC24, there were only 42 proton events with no peaks larger than 10,000 pfu observed. The first proton event of SC25 took place on 29 May 2021, reaching a peak flux of only 15 pfu (STCE Newsitem).


Number of Ground Level Enhancements


Occasionally, during proton events there may be a substantial increase of protons with energies of 500 MeV or more. The secondary particles resulting from the interaction of these highly energetic protons with the Earth's upper atmosphere may shower down all the way to the Earth's surface, where they are measured with neutron monitors (RMI/Dourbes, NMDB). The increase in the number of neutrons is then called a Ground Level Enhancement (GLE). Specifically, a GLE is registered when there are near-time coincident and statistically significant enhancements of the count rates of at least two differently located neutron monitors including at least one neutron monitor near sea level and a corresponding enhancement in the proton flux measured by a space-borne instrument (Poluianov et al., 2017). The list of GLEs is currently maintained by the University of Oulu. GLEs are even rarer than proton events, with a frequency of only about 10 per decade (see also these STCE Newsitems here and here). During the previous SC24, only 2 GLEs have been recorded (17 May 2012 and 10 september 2017). So far during SC25, no GLEs have been observed.


Number of Prominences


Solar filaments are clouds of ionized gas above the solar surface squeezed between magnetic regions of opposite polarity. Being cooler and denser than the plasma underneath and their surroundings, they appear as dark lines when seen on the solar disk and as bright blobs when seen near the solar limb (then they are called "prominences"). Special filters are required to observe these features, and one such a filter is the Hydrogen-alpha (H-alpha) line in the red part of the solar spectrum at 656.3 nm. Prominences have been observed by members of the solar section of the British Astronomical Association (BAA) for decades.

The graph above shows the monthly and smoothed (Meeus, 1958) monthly number of prominences since 1992. Note that, over time, these observations have been performed by different observers with different instruments, so this may affect in part the observed evolution. Though the most recent minimum was deeper than the previous one at the onset of SC24, the current values have been rising since February 2019 -10 months prior to the sunspot number minimum- and have since then exceeded the values recorded around the SC24 minimum.


Number of Coronal Mass Ejections


Solar eruptions are often (but not always) accompanied by coronal mass ejections (CMEs). These clouds of charged particles, when directed to Earth, may eventually disturb the Earth's magnetic field and cause a strong geomagnetic storms. This in turn may result in problems with the power grids and pipelines, issues with satellites, satellite communication and GNSS based services, and radio communication problems amongst many other. The aurorae (or polar lights) are a spectacular but unharmful phenomenon.

CMEs can be observed using coronagraphs on board satellites such as SOHO and STEREO. Tools are used to automatically detect CMEs in these coronagraphic images. At the ROB, such a software tool was developed during the early years of this century (see Robbrecht and Berghmans, 2004 and Robbrecht et al., 2009 ). CACTus is short for Computer Aided CME Tracking, and its data are available at a dedicated ROB website. Note that different instruments and different methodologies may give different values for the number of CMEs (for a discussion see Lamy et al., 2019).

The graph above shows the daily and smoothed (Meeus, 1958) daily CME rate, averaged per month and corrected for the duty cycle (i.e. the time for which images are available). The interesting part in this graph is that though SC24 was a weaker solar cycle compared to SC23, the maximum CME rate was comparable to and even a bit higher than its predecessor. This was explained by Gopalswamy and collaborators in 2014 as being due to a significant reduction in the total pressure (magnetic + plasma) in the heliosphere, leading to an anomalous expansion of CMEs (more halo CMEs) and a relatively higher number of CMEs altogether. The excess CME expansion contributed to the diminished effectiveness of CMEs in producing geomagnetic storms during cycle 24, with no (zero) extremely severe geomagnetic storms observed during SC24. For SC25, at this stage of the solar cycle (15 months since smoothed minimum), the smoothed daily CME-rate is at similar levels than at the same time during SC24.    




The Ap-index is a parameter expressing the level of disturbance of the geomagnetic field. More details on this and other geomagnetic indices can be found at the STCE's SWx Classification Page. Geomagnetic data are maintained by the Kyoto World Data Center for Geomagnetism (1932-present).
The Ap-index reached its smoothed (Meeus, 1958) minimum in April 2020, only 4 months after sunspot minimum. Since SC17, this minimum typically occurs between 4 to 15 months after sunspot minimum. The smoothed monthly Ap is well below the longtime Ap-average of SC17-24, as demonstrated by the graph above.
The table underneath summarizes the highest monthly Ap-value per solar cycle (since 1932). Note how the highest values almost always happen in spring or autumn, an effect that is at least in part explained by Russell and McPherron (1973). Also, the highest Ap values usually occur 2 to 4 years after solar cycle maximum. This is most likely due to a combination of the number of equatorial coronal holes which maximizes during the declining phase of the solar cycle, and the occurence of isolated but very active sunspot regions that still may occur during that time. Of note too is that there was not a single extremely severe geomagnetic storm (Kp = 9, Ap = 400 nT) in SC24. The highest monthly Ap-value so far this solar cycle was reached in March 2021 (10.1 nT).


SC Timing Ap (nT)
17 March 1940 36.5
18 September 1951 39.9
19 September 1957 49.3
20 April 1973 29.8
21 September 1982 35.9
22 June 1991 44.4
23 October 2003 34.7
24 September 2017 18.1
25 March 2021 10.1




The storm-time disturbance index Dst (Sugiura, 1964) is designed to measure the magnetic signature of magnetospheric currents, in particular -but not restricted to- the ring current. Dst is computed using 1-hour values of the horizontal H-component by four low latitude observatories (list at ISGI) sufficiently distant from the auroral and equatorial electrojets to inhibit noise from these two sources. The values are expressed in nT and mostly negative in case of a strong geomagnetic disturbance, as the enhanced ring current tends to counteract (weaken) the Earth's magnetic field. The Dst index is derived and maintained by the World Data Center for Geomagnetism at Kyoto, Japan (WDC Kyoto; 1957-present).

Since its inception in 1957, there have been 660 days with at least one 1-hour interval when Dst reached -100 nT or lower (more negative, i.e. stronger). These numbers have been grouped in 5 bins, as displayed in the graph above. There are only 12 days when a Dst < -350 nT was reached, the last one dating back from 8 November 2004 (Dst = -374 nT). Note the very weak SC24. So far, SC25 has no entries in this graph.


The > 2 MeV Electron fluence


The > 2 MeV electron flux in the outer radiation belt can be significantly influenced by strong interplanetary CMEs and high speed solar wind streams (HSS) impacting the Earth’s magnetosphere (Kavanagh and Denton, 2007 ). It is measured by the GOES satellites (NOAA/SWPC) in their geostationary orbit, which is located in the Earth’s outer radiation belt. High fluxes of energetic electrons are associated with a type of spacecraft charging referred to as deep-dielectric charging, which may eventually result in anomalous behaviour of the spacecraft systems and in their temporary or permanent loss of functionality. See the SWx Classification page for more info and terminology.

Because the > 2 MeV electron flux at geostationary orbit has diurnal variations, a daily value of the electron fluence which is an accumulation over 24h of the electron flux, in units of electrons / (cm2 sr day) is in use. Fluence values greater than 5 . 107 electrons / (cm2 sr day) are indicative of adverse space weather conditions hazardous to geosynchronous satellites. Fluence values in excess of 5 . 109 electrons / (cm2 sr day) are considered to be very high (see NRCan).

The graph above shows the daily electron fluence (blue) and the 365-days smoothed electron flunce (orange - see also Poblet et al, 2020) since mid-1997. The data were obtained from NOAA/SWPC's The Weekly, which is online and archived. The highest electron fluence since 1997 was recorded on 29 July 2004, when it reached a value of 9.3 . 109 electrons / (cm2 sr day). From 2003 to 2008, and again from 2015-2019, elevated fluence levels were recorded because of the declining phase of the solar cycle when (equatorial) coronal holes and the extensions of polar coronal holes are most numerous. The two dips early 2002 and mid 2014 mark solar cycle maximum when the polar magnetic fields were reversing their polarity and coronal holes were pretty much absent and in the process of being recreated. In 2009, the electron fluence was relatively low compared to previous years because of the deep minimum during the SC23-SC24 transition when both the solar wind speed and the sunspot number were at very low levels. During the most recent minimum, there was also a dip in the electron fluence, but values remained an order of magnitude greater than in 2009. Note that the values for the electron fluence have been obtained by different GOES.


Cosmic rays


When galactic cosmic rays (GCR) or high-energetic (> 500 MeV) solar protons interact with particles in the Earth's upper atmosphere, the created secondary particles can reach all the way down to the Earth's surface where they can be measured by e.g. neutron monitors such as in Dourbes, Belgium (see also the NMDB as well as GLEs on this page). Different neutron monitors give different neutron counts, which is due to different instrumentation, different sensitivities and cut-off rigidity (i.e. a quantitative measure of the shielding provided by the Earth's magnetic field against CR particles).

The evolution of the neutron counts anti-correlate with the solar cycle activity. During a solar cycle maximum, the turbulent solar wind and the continued series of CMEs provide sort of a magnetic shield against the GCR, and neutron counts are at their lowest. During solar cycle minimum, this magnetic shield is much less prominent allowing the GCR an easier access to the earth environment and thus resulting in elevated neutron counts. See this STCE Newsitem for more information.

The graph above shows the evolution of the (unsmoothed) monthly neutron count (counts/minute ; corrected for air pressure and efficiency) as monitored by the Cosmic Rays station of the University of Oulu, Finland. This station has one of the longest running databases for this parameter, starting in 1964. Currently (2021), neutron counts are recovering from the solar cycle minimum values. These counts were, at their maximum, similar to those during the previous solar cycle minimum in 2008-2009.




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