Solar cycle 24
Solar cycle 24 was the 24th solar cycle since 1755, when reliable sunspot observations began, spanning from December 2008 to December 2019 and characterized by its unusually weak activity compared to recent cycles.[1][2] It began following a prolonged solar minimum with a smoothed sunspot number of 2.2, the lowest in over a century, and exhibited a double-peaked maximum, with the first peak in 2012 and the primary maximum in April 2014 reaching a smoothed sunspot number of 116.4—about 35% weaker than cycle 23.[1][3] The cycle's duration of approximately 11 years aligned with the typical 11-year periodicity of solar activity, driven by the Sun's dynamo processes that reverse its magnetic polarity roughly every decade.[4] Despite its subdued intensity, solar cycle 24 produced several notable space weather events, including the strongest flare of the cycle—an X9.3-class eruption on September 6, 2017, from active region AR12673, which triggered radio blackouts and auroral displays. Earlier highlights included an X6.9 flare on August 9, 2011, from active region AR11263, the most powerful Earth-directed event early in the cycle, and multiple coronal mass ejections (CMEs) that caused geomagnetic storms, such as the July 2012 event from AR1520, which rivaled the 1859 Carrington Event in potential impact had it been Earth-facing.[5][6] Overall, the cycle featured fewer large sunspots and reduced high-energy particle events than predecessors, with only one solar proton event exceeding 10,000 particles/cm²/sr/s compared to four in each of cycles 21–23, leading to milder geoeffectiveness for satellite operations and power grids.[7][8] The cycle's asymmetry, with the northern solar hemisphere peaking earlier and more prominently than the southern, contributed to its irregular profile and highlighted ongoing challenges in solar dynamo modeling.[9] Observations from missions like NASA's Solar Dynamics Observatory provided unprecedented data on these dynamics, revealing weaker heliospheric magnetic fields and lower solar wind speeds that influenced Earth's magnetosphere. As the weakest cycle since the early 20th century, solar cycle 24 offered insights into long-term solar variability, including potential links to grand minima like the Maunder Minimum, though it did not descend to such extremes.[10]Predictions and Expectations
Early Forecasts
Predictions for solar cycles have historically relied on precursor methods, which analyze indicators from preceding cycles to forecast the amplitude and timing of the next cycle. These include geomagnetic indices such as the aa or Ap indices, which measure Earth's magnetic field disturbances caused by solar activity, and trends in sunspot areas or polar magnetic field strengths observed during the declining phase of the previous cycle. Such methods assume that solar dynamo processes, which generate the Sun's magnetic field, exhibit patterns that can be extrapolated, often using statistical models or physics-based simulations calibrated against historical data from cycles 1 through 23.[11] Early forecasts for Solar Cycle 24, issued between 2000 and 2006, showed significant divergence, reflecting the limitations of these precursor techniques amid varying interpretations of Cycle 23 data. NASA's David Hathaway and Robert Wilson, using solar meridional circulation speeds as a precursor, predicted a peak smoothed sunspot number of 145 ± 30 in late 2011 or early 2012. Updating this with geomagnetic aa index analysis, they later forecasted an even stronger cycle at 160 ± 25, peaking around 2012. In contrast, Leif Svalgaard and colleagues, basing their model on polar magnetic field strengths—a key precursor for cycle amplitude—anticipated a weak cycle with a maximum of only 70 ± 2 sunspots. Mausumi Dikpati's team at the National Center for Atmospheric Research employed a flux-transport dynamo model incorporating sunspot area trends from Cycle 23, yielding a high prediction of 155 to 180 sunspots at maximum in 2012–2013. These estimates, compiled by organizations like NASA and NOAA, highlighted a range from below-average to above-average activity, with no consensus emerging before 2007.[11][12][13] Uncertainties in these early predictions were amplified by the unusually prolonged minimum at the end of Cycle 23, which lasted longer than typical 11-year cycles and featured exceptionally low activity. Observations indicated over 800 spotless days—days without visible sunspots—during the transition from Cycle 23 to Cycle 24, surpassing records from previous minima and complicating precursor signals like geomagnetic indices. This deep minimum, with 266 spotless days alone in 2008, suggested potential disruptions in the solar dynamo, leading models to produce wide-ranging outcomes depending on assumptions about rise time and memory from prior cycles.[14][15]| Year | Authors/Team | Method | Predicted Peak Sunspot Number | Expected Maximum Year |
|---|---|---|---|---|
| 2004 | Hathaway & Wilson (NASA) | Meridional circulation precursor | 145 ± 30 | 2011–2012 |
| 2005 | Svalgaard et al. | Polar magnetic fields | 70 ± 2 | ~2013 |
| 2006 | Dikpati et al. (NCAR) | Flux-transport dynamo with sunspot areas | 155–180 | 2012–2013 |
| 2006 | Hathaway & Wilson (NASA) | Geomagnetic aa index | 160 ± 25 | 2012 |