Even when a solar cycle is close to its maximum, it may happen from time to time there are periods with low solar activity. This is especially true when the maximum is predicted to be already low to medium, as is the case for ongoing solar cycle 24.
Space weather forecasters can issue an alert when solar activity is extremely low. This "all quiet alert" is a message sent when quiet space weather conditions are expected for the next 48 hours or until further notice. This implies that:
- the solar x-ray output is expected to remain below C-class level (so B-class flares at most);
- no geomagnetic storming is in the offing (so Kp remains below 5);
- the high-energy proton fluxes are expected to remain below the event threshold.
This kind of alerts are used by satellite operators. Indeed, when they are maneuvering the satellites, these become particularly vulnerable, so it's very useful to be sure really nothing will happen then.
Usually, the number and complexity of the sunspot groups determine whether or not the aforementioned criteria are met. But also other solar features can affect radiation and geomagnetic conditions. Indeed, a well-aimed coronal hole can cause geomagnetic storming, and it is well known that some erupting filaments are accompanied by C-class (or higher) flares and proton events.
Nonetheless, one would expect these all quiet conditions are only satisfied during solar cycle minimum. In 2008, the doldrums year of the last solar cycle transit, such conditions were predicted for one day out of every three, with less than 3% misses.
As it turns out, "all quiet periods" can also occur during the years of maximum solar cycle activity. The most recent all quiet alert was issued on August 20 by space weather forecasters of the SIDC ). It was halted on August 22, when active regions were appearing at the solar east limb, thus increasing the potential on small C-class flares. As shown in the USET and PROBA2 images for those days, the sun was populated by few and simple sunspots, with no large filaments or coronal holes on the disk. The 22 August PROBA2-image heralds the arrival of possibly flare-active regions at the south-east limb (bright).
As NOAA's satellite and ground-based recordings for the 20-22 August period show, there were no C-class flares, no proton events, and geomagnetic conditions were unsettled at most. All quiet on the solar front!
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