The term “space weather” is used to designate processes occurring on the Sun or in the Earth’s magnetosphere, ionosphere, and thermosphere that could have multi-faceted impacts with the potential to affect a wide range of space-based and terrestrial assets and services. Space weather phenomena such as solar flares, radiation storms, and geomagnetic storms are some potential concerns for aviation.

This chapter provides an overall introduction to space weather general theory.

This chapter also outlines the potential effects of space weather on the aircraft, including communications, navigation [e.g., Global Positioning System (GPS)], radiation exposure, and radiation effects on avionics.

The Sun is the dominant source of the conditions commonly described as space weather. Emissions from the Sun are both continuous (e.g., solar luminescence and solar wind) and eruptive (e.g., coronal mass ejections (CME) and flares). These solar eruptions may cause radio blackouts, magnetic storms, ionospheric storms, and radiation storms at Earth.

Similar to the charged particles that come from the Sun, Galactic Cosmic Rays (GCR) are charged particles that originate in more distant supernovae and contribute to the space weather conditions near Earth. Essentially, these charged particles comprise a steady drizzle of radiation at Earth.

The sum of the solar and non-solar components equals the full extent of the potential radiation dose received. The size of the GCR flux varies inversely with the sunspot cycle (sunspots are described in Section 23.4); that is, during sunspot minimums when the interplanetary environment near Earth is laminar and steady, the GCR component is large due to its easier access to the near-Earth environment. At sunspot maximum, the turbulence and energetics associated with solar eruptions reduce GCR access to the vicinity of the Earth.

The Sun is a variable star. That means the balance between the continuous emissions and the eruptive emissions changes with time. One metric that is commonly used to track this variability is the occurrence of sunspots. Astronomers have made sunspot observations continuously for hundreds, maybe even thousands, of years. Though the underlying physics is complex it is currently the subject of ongoing research and exploration. On average, sunspots come and go in an 11-year period. The magnitude and duration of individual cycles varies, but typically more eruptive events occur near the solar maximum, while few are observed near solar minimum. All solar electromagnetic emissions, from radio to gamma rays, are also stronger during solar maximum and less intense near solar minimum.

Because space weather activity varies with sunspot activity, they are often used as a proxy index for changing space weather conditions. This is because sunspots, by their very nature, exist due to strong local magnetic fields. Severe space weather can occur with rapid changes in these magnetic fields. While sunspots are easily seen, other events such as GCR, CMEs, and increased solar wind are more difficult to observe from the ground and may not be related to long historical records of sunspots.

The solar wind is the continuous flow away from the Sun of charged particles and magnetic field, called plasma. Solar wind is a consequence of the very high temperature of the solar corona (uppermost region of the Sun’s atmosphere) and the resultant expansion of the plasma into space.

The solar wind carries the energy from most solar eruptions that affect the near-Earth environment. The sole exception, solar flare photons consisting of light and x rays carry the energy released in solar flares. Even in the absence of an eruption, the constant flow of plasma fuels Earth’s geomagnetic field. The solar wind may be fast and energetic if an eruption occurs, or it can gradually increase due to a coronal-hole structure that allows unimpeded high-speed solar wind to escape from the corona. As seen from the Earth, the Sun rotates on approximately a 27-day period, so well-established coronal-hole structures that persist for several months will swing by Earth on schedule, roughly every 27 days.

Most solar eruptions originate in areas that have strong magnetic fields. Usually marked with sunspots, these areas are commonly called active regions. Active regions are numerous and common during solar maximum and scarce during solar minimum.

Flares and CMEs are the two major types of solar eruptions. They may occur independently or at the same time. Solar flares have been recognized for more than 100 years, as they can be seen from the ground. In the past 50 years, Hydrogen-Alpha (656.3 nanometer wavelength) filter-equipped ground-based telescopes have been used to observe flares.

Flares are characterized by a very bright flash phase that may last for a few minutes to a few hours during the largest flares. Flares can emit at all frequencies across the electromagnetic emission spectrum, from gamma rays to radio.

CMEs, in contrast to solar flares, are difficult to detect; they are not particularly bright and may take hours to fully erupt from the Sun. CMEs literally are an eruption of a large volume of the solar outer atmosphere, the corona. Prior to the satellite era, they were very difficult to observe. The energy released in a large solar flare is on par with that released in a CME; however, CMEs are far more effective in perturbing Earth’s magnetic field and are known to cause the strongest magnetic storms. A typical travel time for a CME from the Sun to Earth may range from less than one day to more than four days. The travel time of the electromagnetic emission produced during flares, by comparison, is at the speed of light. They instantaneously affect the day side of Earth upon observation.