Solar cycle 24 is the most recently completed solar cycle, the 24th since 1755, when extensive recording of solar sunspot activity began. It began in December 2008 with a minimum smoothed sunspot number of 2.2, and ended in December 2019. Activity was minimal until early 2010. It reached its maximum in April 2014 with a 23 months smoothed sunspot number of 81.8. This maximum value was substantially lower than other recent solar cycles, down to a level which had not been seen since cycles 12 to 15 (1878-1923).
Predictions
Prior to the minimum between the end of Solar Cycle 23 and the beginning of Solar Cycle 24, two theories predicted how strong Solar Cycle 24 would be. One camp postulated that the Sun retained a long memory (Solar Cycle 24 would be active) while the other asserted that it had a short memory (quiet). Prior to 2006, the difference was substantial with a minority of researchers predicting "the smallest solar cycle in 100 years." Another group of researchers, including one at NASA, predicted that it "looks like its going to be one of the most intense cycles since record-keeping began almost 400 years ago."
The delayed onset of high latitude spots indicating the start of Solar Cycle 24 led the "active cycle" researchers to revise their predictions downward and the consensus by 2007 was split 5-4 in favor of a smaller cycle. By 2012, consensus was a small cycle, as solar cycles are much more predictable 3 years after minima.
In May 2009 the NOAA Space Weather Prediction Center's Solar Cycle 24 Prediction Panel predicted the cycle to peak at 90 sunspots in May 2013. In May 2012 NASA's expert David Hathaway predicted a peak in Spring of 2013 with about 60 sunspots.
NASA funded and used Ken Schatten's physics-based models, which utilized a solar Dynamo model, to accurately predict the low. This method used the correlation between solar magnetic field strength at solar minimum to sunspot number at solar maximum to accurately predict the peak solar flux of each of the last three solar cycles. Schatten's predictions become accurate as early as solar minima, 5–6 years before solar max.
Results
In early 2013, after several months of calm, it was obvious that the active 2011 was not a prelude to a widely predicted late 2012-early 2013 peak in solar flares, sunspots and other activity. This unexpected stage prompted some scientists to propose a "double-peaked" solar maximum, which then occurred. The first peak reached 99 in 2011 and the second peak came in early 2014 at 101.
Speculation
According to NASA, the intensity of geomagnetic storms during Solar Cycle 24 may be elevated in some areas where the Earth's magnetic field is weaker than expected. This fact was discovered by the THEMIS spacecraft in 2008. A 20-fold increase in particle counts that penetrate the Earth's magnetic field may be expected. Solar Cycle 24 has been the subject of various hypotheses and commentary pertaining to its potential effects on Earth.
While acknowledging that the next solar maximum will not necessarily produce unusual geomagnetic activity, astrophysicist Michio Kaku took advantage of the media focus on the 2012 phenomenon to draw attention to the need to develop strategies for coping with the terrestrial damage that such an event could inflict. He asserted that governments should ensure the integrity of electrical infrastructure, so as to prevent a recurrence of disruption akin to that caused by the solar storm of 1859.
The current solar cycle is currently the subject of research, as it is not generating sunspots in the expected manner. Sunspots did not begin to appear immediately after the last minimum (in 2008) and although they started to reappear in late 2009, they were at significantly lower rates than anticipated.
On April 19, 2012, the National Astronomical Observatory of Japan predicted that the Sun's magnetic field would assume a quadrupole configuration.
Throughout 2012, NASA posted news releases discrediting the 2012 phenomenon and the so-called Mayan prophecy, delinking them from solar activity and space weather.
Events
2008
On 4 January 2008, an active region appeared with magnetic polarity reversed compared to that expected by Hale's law for solar cycle 23. This presaged the start of solar cycle 24—though did not mark its official beginning. The region was located at the relatively high latitude 30° N which, according to Spörer's law, provided further evidence for the arrival of cycle 24. The NOAA assigned it the active region number AR10981.
Only a few sunspots were observed on the surface of the Sun throughout 2008. The smoothed monthly sunspot number reached a minimum of 2.2 in December 2008, therefore an international panel of scientists declared that month as solar minimum and the beginning of Solar Cycle 24.
2009
Solar activity remained extremely low throughout 2009. The observed monthly sunspots exceeded 10 only in December.
2010
On 19 January 2010, active region AR11041 produced an M2.3-class flare, the first flare of cycle 24 above M-class. It was followed by an M1.7-class flare seven hours later and four consecutive M-class flares the next day. Among the four flares, the strongest reached a strength of M3.4.
On 12 February 2010, active region AR11046 produced an M8.3-class flare. Later in the month, active regions AR11045 and AR11046 unleashed a total of nine M-class flares.
On 5 April 2010, the first coronal mass ejection (CME) of cycle 24 erupted at an active region causing a G3 (strong) geomagnetic storm on Earth. The Kp index, which quantifies disturbances in the horizontal component of Earth's magnetic field, reached a value of 7.
On 1 and 2 August 2010, a series of four large CMEs were observed erupting from the Sun's Earth-facing side. These CMEs were likely connected to a C3.2-class flare from active region AR11092 despite the CMEs taking place about 400,000 km apart from the region. On 4 August 2010, a G2 (moderate) geomagnetic storm caused aurorae to be visible in the northern hemisphere at latitudes as far south as 45° N near Michigan and Wisconsin in the United States, and Ontario, Canada. European observers reported sightings as far south as Denmark near latitude 56° N. The aurorae were reportedly green in color due to the interaction of the solar particles with oxygen atoms in the relatively denser atmosphere of southern latitudes.
On 14 August 2010, a C4.4-class flare produced the first solar radiation storm of cycle 24. The proton storm event was minor, rating at S1, and was easily absorbed by the Earth's ionosphere.
On 6 November 2010, active region AR11121 emitted an M5.4 flare.
2011
February
Peaking at 01:56 UT on February 15, 2011, sunspot group 1158 produced an X2.2-class solar flare. Dubbed the Valentine's Day solar event by the scientific community, it was the first Solar Cycle 24 flare reaching X class level. In fact, it was the first of its class since December 2006. NOAA issued an R3 (strong) radio blackout alert pertaining this prominent x-ray flux event. In addition to flashing Earth with X and UV radiation, the explosion also hurled a CME in Earth's direction. The magnetosphere was impacted on February 18. The CME struck a minor G1-level geomagnetic storm.
Shortly before, on February 13, sunspot 1158 had unleashed an M6.6-class solar flare. On February 18, the same active region produced another x-ray burst with the same strength. 13 M-class bursts were registered in February 2011.
March
A CME exploded from the vicinity of sunspot 1164 during the late hours of March 7, 2011. It leapt away from the Sun traveling ~2200 km/s, making it the fastest CME since September 2005.
On March 9, active region 1166 erupted in an X1.5 flare. An R3-level radio blackout was reported. The related CME caused a G2 geomagnetic storm two days later. 21 M-class flares were registered this month.
July
Sunspot 1260 produced an M9.3-class solar flare on July 30, 2011. Because of its brevity, the eruption did not hurl a substantial cloud of ionized material or CME toward Earth, so it was not geoeffective.
August
On August 5, 2011, the combined cloud of three consecutive CMEs produced brilliant aurorae, reported as far south as Oklahoma and Alabama. The geomagnetic storm reached a G4 (severe) level, enough to create power outages. It was one of the strongest geomagnetic storms in years. In the southern hemisphere, aurorae could be seen as far north as South Africa, Southern Chile and Southern Australia. The CMEs were hurled by three M-class flares erupting in active sunspot 1261: M1.4 on August 2, M6.0 on August 3 and M9.3 on August 4.
On August 9 at 08:05 UT, sunspot 1263 produced a massive X6.9-class solar flare, the third X-flare of Solar Cycle 24 and the most powerful so far (as of May 2013). There was also a CME associated with this burst. Although the flare was not Earth-directed, radiation created waves of ionization in Earth's upper atmosphere, briefly disrupting communications at some VLF and HF radio frequencies. An R3-level (strong) radio blackout alert was issued. A proton event greater than 10 MeV (million electron volts) and exceeding 10 pfu (proton flux units) was also reported, so an S1-level solar radiation storm was also issued.
September
Sunspot 1283 erupted with an M5.3-class solar flare on September 6 at 01:50 UT. An R2 (moderate) blackout radio alert was issued. The burst was Earth-directed. Just 21 hours later, an X2.1-class flare – some four times stronger than the earlier flare – erupted from the same sunspot region. NOAA detected an R3 (strong) radio blackout and an S1 (minor) solar radiation storm. The combined CMEs of these bursts arrived at Earth on September 9, provoking a G3 (strong) geomagnetic storm.
The next day, September 7, an X1.8-class solar flare erupted from sunspot 1283, producing an S1 solar radiation storm. A fourth flare, an M6-class, was ejected by the same sunspot on September 8.
This sequence of flares produced waves of ionization in Earth's upper atmosphere, briefly altering the propagation of low-frequency radio signals around Earth. Moreover, the eruptions hurled clouds of plasma in its direction. CME impacts, strong geomagnetic storms and aurorae were registered from September 9 onwards.
Then, on September 22, an X1.4-class solar flare erupted out of sunspot 1302. An R3-level radio blackout was registered. The blast produced a significant CME, but was not Earth directed. Two days later, an X1.9-class flare, followed in the next 31 hours by a spectacular string of 14 M-class flares, the biggest being two M7 flares, was mostly unleashed out of the same sunspot. The first two explosions, X1.9 and M7.1, propelled a pair of closely spaced CMEs. A G4 (severe) geomagnetic storm was reported on September 26.
In total, the Sun produced four X flares and 31 M flares in September 2011, one of the most active months of Solar Cycle 24 so far.
October
The Sun unleashed eight M-class flares this month, being the strongest the M3.9 event, followed by an Earth-directed CME, produced by sunspot 1305 on October 2. Just in the eve, sunspots 1302 and 1305 had emitted flares almost at the same time; the first event was a C-class and the second one reached a M1.2 category. This double eruption, which hurled a double CME as well, were particularly interesting as coincided with the arrival of a comet, discovered by amateur astronomers the previous day, that disintegrated in spectacular fashion when it plunged into the Sun. A very similar scenario happened on May 10–11, 2011.
November
On November 3, 2011, active region 1339, one of the largest sunspots in years - 40,000 km wide and at least twice that in length - unleashed an X1.9-class solar flare. Waves of ionization in the upper atmosphere created an R3 (strong) radio blackout. The related CME was not headed for Earth. 13 M-class flares were registered this month. November 2011 may be considered the most active month of the current Cycle 24 so far, as monthly sunspot count was nearly 100 (96.7) and the same went for the F10.7 Solar Flux (the radio emission from the Sun at a wavelength of 10.7 cm) that racked up a value of 153.1. However, these numbers are well below those of Cycle 23 at its peak. Cycle 23 peak sunspot count was 170 and its F10.7 was about 235.
December
Solar activity increased again in late December, with the Sun unleashing eight M-flares. The most intense flare, produced by sunspot 1385, was an M4.0 event on December 25. The year 2011 ended up with 111 M-class and 8 X-class solar flares.