Solar Storm (Solar Radiation Storm) (S Scale)

Unique identifier / Notation

ET0007

Synonyms

Solar energetic particle events,

Solar proton events.

Solar radiation storms occur when large quantities of charged particles, primarily protons, accelerated by eruptive processes at or near the Sun reach the near-Earth environment (NOAA, 2019).

Primary reference(s)

NOAA, 2019. Solar Radiation Storm. Space Weather Prediction Center, National Oceanic and Atmospheric Administration (NOAA). Accessed 16 October 2020.

Additional scientific description

The Earth’s magnetic field and atmosphere generally protect life on Earth from this particle radiation, but that shielding depends on latitude, magnetic field strength and direction. In the polar regions, the magnetic field lines intersecting the Earth’s surface allow lower energy particles to penetrate into the atmosphere. Solar radiation storms thus often result in polar cap events (PCA), which occur in limited areas around geomagnetic poles; they may last more than a week (NOAA, 2019).

Higher energy particles can penetrate the magnetic field and reach spacecraft orbiting at lower latitudes, in particular the International Space Station. High altitude orbits such as the geosynchronous orbit used by spacecraft delivering many commercial and governmental services are not protected by the magnetic field.

A factor of criticality in a radiation storm is the energy spectrum of the solar protons. High-energy protons cause single event upsets in spacecraft electronics and increase the harmful radiation dose of exposed human beings, such as in manned spaceflights. Lower energy protons have a severe impact on the polar ionosphere and affect High Frequency propagation at high latitude. The severity of radiation storms can thus be characterised by the flux of charged particles (typically as a 5-minute average) above a given energy threshold such as 10 or 100 MeV. For example, the National Oceanic and Atmospheric Administration (NOAA) scale characterises a radiation storm as extreme when the 5-minute flux above 10 MeV exceeds 105 particles•s- 1•sr-1•cm-2. In large magnitude solar eruptions, high-energy events may last only a few hours while low-energy events may last up to a week (NOAA, 2019).

Solar radiation storms occur when a large-scale magnetic reconfiguration, causing a coronal mass ejection and often an associated solar flare, accelerates charged particles in the solar atmosphere to very high velocities. The most common particles are protons, but ions of other low mass (helium to iron) elements are often present and contribute significantly to radiation damage. All these ions can be accelerated to large fractions of the speed of light. At these velocities, the particles can traverse the 150 million km from the Sun to Earth in just tens of minutes or less. When they reach Earth, the fast-moving ions penetrate the magnetosphere that shields Earth from lower energy charged particles. Once inside the magnetosphere, low energy (<1 GeV) particles are guided down the magnetic field lines and penetrate into the atmosphere near the north and south poles (NOAA, 2019).

Metrics and numeric limits

NOAA categorises solar radiation storms using the NOAA Space Weather Scale on a scale from S1 to S5 (NOAA, 2011; see graphic below). The scale is based on measurements of energetic protons taken by the GOES satellite in geosynchronous orbit. The start of a solar radiation storm is defined as the time when the flux of protons at energies ≥10 MeV equals or exceeds 10 proton flux units (1 pfu = 1 particle•cm-2•s-1•ster-1). The end of a solar radiation storm is defined as the last time the flux of ≥10 MeV protons is measured at or above 10 pfu. This definition allows multiple injections from flares and interplanetary shocks to be encompassed by a single solar radiation storm. A solar radiation storm can persist for periods ranging from hours to days.

Scale	Description	Effect	Physical measure (Flux level of >= 10 MeV particles)	Average Frequency (1 cycle = 11 years)
S 5	Extreme	Biological: Unavoidable high radiation hazard to astronauts on EVA (extra-vehicular activity); passengers and crew in highflying aircraft at high latitudes may be exposed to radiation risk. Satellite operations: Satellites may be rendered useless, memory impacts can cause loss of control, may cause serious noise in image data, star-trackers may be unable to locate sources; permanent damage to solar panels possible. Other systems: Complete blackout of HF (high frequency) communications possible through the polar regions, and position errors make navigation operations extremely difficult.	105	Fewer than 1 per cycle
S 4	Severe	Biological: Unavoidable radiation hazard to astronauts on EVA; passengers and crew in high-flying aircraft at high latitudes may be exposed to radiation risk. Satellite operations: May experience memory device problems and noise on imaging systems; star-tracker problems may cause orientation problems, and solar panel efficiency can be degraded. Other systems: Blackout of HF radio communications through the polar regions and increased navigation errors over several days are likely.	104	3 per cycle
S 3	Strong	Biological: Radiation hazard avoidance recommended for astronauts on EVA; passengers and crew in high-flying aircraft at high latitudes may be exposed to radiation risk. Satellite operations: Single-event upsets, noise in imaging systems, and slight reduction of efficiency in solar panel are likely. Other systems: Degraded HF radio propagation through the polar regions and navigation position errors likely	103	10 per cycle
S 2	Moderate	Biological: Passengers and crew in high-flying aircraft at high latitudes may be exposed to elevated radiation risk. Satellite operations: Infrequent single-event upsets possible. Other systems: Small effects on HF propagation through the polar regions and navigation at polar cap locations possibly affected.	102	25 per cycle
S 1	Minor	Biological: None. Satellite operations: None. Other systems: Minor impacts on HF radio in the polar regions.	10	50 per cycle

Key relevant UN convention / multilateral treaty

Not identified.

Examples of drivers, outcomes and risk management

Solar radiation storms cause several impacts near Earth. When energetic protons collide with satellites or humans in space, they can penetrate deep into the object that they collide with and cause damage to electronic circuits or biological DNA. Also, when the energetic protons collide with the atmosphere, they ionise the atoms and molecules thus creating free electrons. These electrons create a layer near the bottom of the ionosphere that can absorb HF radio waves making radio communication difficult or impossible.

NOAA’s Space Weather Prediction Center currently forecasts the probability of S1 (minor radiation storm) occurrence as part of their 3-day forecast and forecast discussion products and issues a warning for an expected S1 or higher event; as well as a warning for when the 100 MeV proton level is expected to reach 1 pfu. It also issues alerts for when each NOAA Space Weather Scale Radiation Storm level is reached (S1–S5) and/or when the 100 MeV protons reach 1 pfu (NOAA, 2019).

Some impacts from solar radiation storms can impair the health and operation of satellites and International Space Station operations and crew, and impact HF communication in the polar regions, affecting transpolar commercial airline operations.

In 2019, the International Civil Aviation Organization established a global network to feed space weather advisories into the existing global aviation system (ICAO, 2019). It currently comprises three centres: one operated by a consortium of Australia, Canada, France and Japan, another by a consortium comprising Austria, Belgium, Cyprus, Finland, Germany, Italy, Netherlands, Poland and the United Kingdom; and a third operated by the United States. Further centres are planned to be in operation by 2022.