Space Weather

23.1 Introduction

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.

23.2 The Sun—Prime Source of Space Weather

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.

23.3 The Sun’s Energy Output and Variability

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.

23.4 Sunspots and the Solar Cycle

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.

23.5 Solar Wind

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.

23.6 Solar Eruptive Activity

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.

The frequency of solar flares and CMEs tracks with the solar cycle. As many as 25 solar flares may occur per day during the maximum phase of the solar cycle. At solar minimum, it may take six months or more for 25 flares to occur. CME frequency varies from about five per day near solar maximum to one per week or longer at solar minimum.

Many CMEs observed lifting off the Sun miss Earth due to the CME’s direction of travel.

23.7 Geospace

Geospace is the volume of space that surrounds Earth, influenced by the Earth’s magnetic field in the solar wind. If Earth did not have a magnetic field, the solar wind would blow past unimpeded, affected only by the mass of Earth and its atmosphere. Earth’s magnetic field extends outward in all directions. This forms a cocoon for the planet, protecting it from the flow of the solar wind. The cocoon is called the magnetosphere. The magnetosphere typically extends towards the Sun about 10 Earth radii on the day side and stretches away from the Sun many times more on the night side. The shape is similar to a comet tail, with it being extended during strong solar wind conditions and less during quieter times. On its flanks, the magnetosphere extends outward roughly 20 Earth radii in the dawn and dusk sectors. The magnetosphere deflects most of the energy carried by the solar wind, while making a fraction of it available to be absorbed by the near-Earth system. When the Sun is active and CMEs interact with Earth, the additional energy disrupts the magnetosphere, resulting in a magnetic storm. Then, over time, the magnetosphere adjusts through various processes and once more returns to normal. The most visible manifestation of the energy being absorbed from the solar wind into the magnetosphere is the aurora, both in the Northern and Southern Hemispheres. The aurora occurs when accelerated electrons, and sometimes protons, from the Sun follow the magnetic field of Earth down to the polar regions, where they collide with oxygen and nitrogen atoms and molecules in Earth’s upper atmosphere. In these collisions, the electrons transfer their energy to the atmosphere, thus exciting the atoms and molecules to higher energy states. When they relax to lower energy states, they release their energy in the form of light. Simply put, the more energy in the solar wind, the brighter and more widespread the aurora glow becomes.

Nearer to Earth is another region called the ionosphere. It is a shell of weak plasma, where electrons and ions exist embedded in the neutral atmosphere. The ionosphere begins at roughly 80 km in altitude and extends out many Earth radii, at the topside.

Extreme Ultraviolet (EUV) solar emissions create the ionosphere by ionizing the neutral atmosphere. The electrons and ions created by this process then engage in chemical reactions that progress faster in the lower ionosphere. The ionosphere changes significantly from day to night. When the Sun sets, chemical processes, together with other dynamic processes, allow some of the ionization to remain until the new day brings the solar EUV once again. An important point is that the energy that comes from the Sun in the solar wind makes its way to the ionosphere, where it alters the ambient conditions during space weather storms.

23.8 Galactic Cosmic Rays (GCR)

Galactic Cosmic Rays, more commonly known as GCR, is a consequence of distant supernovae raining charged particles, heavy ions, protons, and electrons onto the inner heliosphere. The abundance of GCR is inversely rated to the solar cycle. At solar maximum, when the solar wind flow is turbulent and strong, the GCR flux is inhibited and therefore low. At solar minimum, the GCR flux increases by about 25 percent in the near-Earth environment. When high-energy GCR enter Earth’s atmosphere, it creates a cascade of interactions resulting in a range of secondary particles, including neutrons that make their way to Earth’s surface.

23.9 Geomagnetic Storms

Geomagnetic storms are strong disturbances to Earth’s magnetic field in the solar wind. These storms pose problems for many activities, technological systems, and critical infrastructure. The topology of Earth’s magnetic field changes in the course of a storm, as the near-Earth system attempts to adjust to the jolt of energy from the Sun. CMEs and the shocks they drive are often the causative agent and can send the geomagnetic field into a disturbed state.

The most obvious and probably the only pleasing attribute of an energized geomagnetic field is the auroras. Geomagnetic storms tend to brighten auroras and allow them to move equatorward. The duration of geomagnetic storms is usually on the order of days. The strongest storms may persist for almost one week. A string of CMEs may cause prolonged disturbed periods related to the additional energy being pumped toward the Earth.

Although the frequency of geomagnetic storms reflects the solar cycle, a closer look shows a bimodal distribution. Large numbers of storms cluster at solar maximum resulting from frequent CMEs, and again in the declining phase due to high-speed solar wind streams. Typically, the most intense storms occur near solar maximum, with weaker storms occurring during the declining phase.

23.10 Solar Radiation Storms

Solar radiation storms occur when large quantities of charged particles, primarily protons, are accelerated by processes at or near the Sun and then bathe the near-Earth environment with these charged particles. These particles cause an increase in the radiation dose to humans and increase the possibility of single-event upsets in electronics. Earth’s magnetic field and atmosphere offer some protection from this radiation, but protection decreases with altitude, latitude, magnetic field strength, and direction. The polar regions on Earth are the most open to these charged particles. The magnetic field lines at the poles extend vertically downwards, intersecting Earth’s surface. This allows the particles to spiral down the field lines and penetrate into the atmosphere and increase the ionization.

A significant factor related to the criticality of the radiation increase at Earth is the energy distribution of the solar protons. Protons of varying energies will bathe Earth as a function of the site of the eruption at the Sun and the magnetic connection between the Sun and Earth. High-energy protons cause radiation dose increases that are of concern to human beings. Lower energy protons have little effect on humans but have a severe impact on the polar ionosphere.

The duration of solar radiation storms is a function of the magnitude of the solar eruption as well as the energy level of protons. For events that are of a large magnitude but low energy, the duration may last for one week. Events that are of high energy may last for only a few hours. Diversity in the duration of solar radiation storms exists, just as there are many factors that contribute to the acceleration and propagation of the charged particles near Earth.

Solar radiation storms can occur at any point in the solar cycle but tend to be most common during the years around solar maximum.

23.11 Ionospheric Storms

Ionospheric storms arise from large influxes of solar particle and electromagnetic radiation. There is a strong coupling between the ionosphere and the magnetosphere, which means both regimes can be disturbed concurrently.

The symptoms of an ionospheric storm include enhanced currents, turbulence and wave activity, and a nonhomogeneous distribution of free electrons. This clustering of electrons that leads to scintillation of signals passing through the cluster is particularly problematic for the Global Navigation Satellite System (GNSS), including the United States’ GPS.

The duration of the ionospheric storm impact may range from a few minutes to days-long prolonged events. As a general rule, these ionospheric storms mimic the duration of geomagnetic storms. The intensity of ionospheric storms varies significantly as a function of local time, season, and time within the solar cycle.

The frequency of occurrence of ionospheric storms is also similar to geomagnetic storms with one important caveat. The near-equatorial ionosphere, a band extending approximately ±10° in latitude on either side of the magnetic equator, can be very disturbed in the post-sunset to near-midnight hours, even in the absence of a geomagnetic storm. This behavior is related to the internal electrodynamics of the ionosphere rather than external stimulation from the Sun.

23.12 Solar Flare Radio Blackouts

Radio blackouts primarily affect high frequency (HF) (3–30 megahertz (MHz)), although detrimental effects may spill over to VHF (30–300 MHz) and beyond, resulting in fading and diminished ability for reception. The blackouts are a consequence of enhanced electron densities caused by the emissions from solar flares that ionize the sunlit side of Earth.

The process consists of x ray and EUV bursts from a solar flare, increasing the number of free electrons in the atmosphere below 90 km; this in turn increases their interaction with the neutral atmosphere that increases the amount of radio energy lost as radio waves pass through this region. During a large flare event, the amount of radio energy lost is sufficient to make the return signal from the ionosphere too small to be useful with normal radio receivers. The net effect of this process is a blackout for HF transmissions. The duration of dayside solar flare radio blackouts closely follows the duration of the solar flares that cause the blackouts, beginning with the arrival of the x ray and EUV photons and abating with their diminution. Usually, the radio blackouts last for several minutes, but they can last for hours.

23.13 Effects of Space Weather on Aircraft Operations

23.13.1 Communications

High frequency communications (HF COM) at low- to mid-latitudes are used by aircraft during transoceanic flights and routes where line-of-sight VHF communication is not an option. HF enables a skip mode to send a signal around the curvature of Earth. HF COM on the Earth’s day side can be adversely affected when a solar flare occurs and its photons rapidly alter the electron density of the lower altitudes of the ionosphere, causing fading, noise, or a total blackout. Usually these disruptions are short-lived (tens of minutes to a few hours), so the outage ends fairly quickly.

HF COM at high latitudes and polar regions are adversely affected for longer periods, sometimes days, due to some space weather events. The high latitude and polar ionosphere are a sink for charged particles that alter the local ionization and provide steep local ionization gradients to deflect HF radio waves, as well as increase local absorption.

Satellite communication (SATCOM) signals pass through the bulk of the ionosphere and are a popular means of communicating over a wide area. The frequencies normally used for SATCOM are high enough for the ionosphere to appear transparent. However, when the ionosphere is turbulent and nonhomogeneous, an effect called scintillation (a twinkling in both amplitude and phase) is imposed upon the transmitted signal. Scintillations can result in loss-of-lock and the inability for the receiver to track a Doppler-shifted radio wave.

23.13.2 Navigation and GPS

Space weather adversely affects GPS in three ways: it increases the error of the computed position, it causes a loss-of-lock for receivers, and it overwhelms the transmitted signal with solar radio noise.

23.13.3 Radiation Exposure to Flightcrews and Passengers

Solar radiation storms occurring under particular circumstances cause an increase in radiation dose to flightcrews and passengers. As high polar latitudes and high altitudes have the least shielding from the particles, the threat is the greatest for higher altitude polar flights. The increased dose is much less of an issue for low and midlatitude flights.

23.13.4 Radiation Effects on Avionics

The electronic components of aircraft avionic systems are susceptible to damage from the highly ionizing interactions of cosmic rays, solar particles, and the secondary particles generated in the atmosphere. As these components become increasingly smaller, and therefore more susceptible, the risk of damage also increases.