Frequently Used Space Weather Terms and Events

Coronal Hole (CH)

A coronal hole is an opening in the Sun's atmosphere associated with open magnetic field lines. High-speed solar winds originate from within these holes which appear as dark zones when viewed at different optical wavelengths. Coronal holes can occur at any time during a solar cycle, but are typically larger and more frequent towards solar maximum.

Coronal Mass Ejection (CME)

A coronal mass ejection (CME) is an eruption of material from the solar corona, observed with a white-light coronagraph.

The material consists of plasma made up primarily of electrons and protons (in addition to small quantities of heavier elements such as helium, oxygen, and iron), plus the entrained coronal magnetic field. When the plasma cloud sweeps past the Earth as an ICME (Interplanetary CME), it may disrupt the Earth's magnetosphere, compressing it on the dayside and extending the nightside tail. When the magnetosphere reconnects on the nightside, it creates trillions of watts of power which is directed back towards the Earth's upper atmosphere. This process is responsible for a particularly strong light show known as the aurora borealis or northern lights in the northern hemisphere and the aurora australis or southern lights in the Southern Hemisphere. CME events, along with solar flares, can disrupt radio transmissions, trigger geomagnetic storms that can in rare instances, cause damage to satellites and electrical transmission lines.

CME captured by LASCO C2

Geomagnetic Storm

A geomagnetic storm is a major disruption of Earth's magnetosphere. When unusually strong surges of solar wind (charged particles from the Sun) sweep past Earth, this effect causes variations in the magnetic field which surrounds the planet. Geomagnetic storms can cause disruptions to high frequency (HF) radio communications through the polar zones, GPS positioning errors, and is also responsible for the northern and southern lights. In extreme cases, voltage control related damage can be inflicted upon power grid transmission lines and lead to power blackouts.

NOAA / SWPC classifies geomagnetic storms on a level ranging between G1 (minor), G2 (Moderate), G3 (Strong), G4 (Severe) and G5 (Extreme).

For full impacts, click on the SWPC grid below.

The Aurora

Kp Index Graph

Interplanetary Magnetic Field (IMF)

This is another name for the Sun's magnetic field. The IMF is huge and goes beyond any of the planets in our solar system, hence the name interplanetary. The magnetic field of the Sun is carried into space via the solar wind and is twisted into a spiral by the Sun's rotation.

One important aspect of the IMF that can affect geomagnetic activity around Earth is known as the Bz. Earth also has a magnetic field which forms a bubble around our planet called the Magnetosphere. This bubble deflects the solar wind. Earth's magnetic field comes into contact with the Sun's magnetic field in a place called the magnetopause. Earth's magnetic field points north. When the Sun's magnetic field points south, also known as southward Bz, it may cancel Earth's magnetic field at point of contact. When the Bz is south the 2 fields link up. This basically opens up a door that may allow energy from the solar wind to reach Earth's atmosphere.

Prominence (Filament)

A prominence is a large, bright plasma feature extending outward from the Sun's surface when viewed against the darkness of space around the edges (limbs) of the Sun. Prominences are anchored to the Sun's surface in the photosphere, and extend outwards into the Sun's hot outer atmosphere, called the corona. This same feature is called a filament when viewed above the solar surface and appear as dark, dense lines. They are one of the most common solar features, especially towards solar maximum. Most times they remain magnetically anchored in place or fall back toward the Sun, although every now and then they can collapse and lead to large eruptions known as coronal mass ejections.



Radiation Storm

Energetic solar events such as solar flares can sometimes propel energetic particles such as protons into space and towards Earth at a very high rate of speed. So fast in fact, they can travel the 150 million kilometers from the Sun to Earth in less than half hour.

When they reach Earth, the fast moving protons can penetrate the magnetosphere that shields our planet from lower energy charged particles. Once inside the magnetosphere, the particles are guided down the magnetic field lines and penetrate into the atmosphere near the north and south poles.

During a radiation storm, energetic protons will collide with the atmosphere, ionizing atoms and molecules thus creating free electrons. These electrons create a layer near the bottom of the ionosphere that can absorb radio waves making high frequency (HF) radio communication greatly degraded.

NOAA / SWPC classifies radiation storms on a level ranging between S1 (minor), S2 (Moderate), S3 (Strong), S4 (Severe) and S5 (Extreme). Besides interference or blackouts to HF radio through the polar regions, other impacts during stronger events can also include exposure to radiation for passengers and crew in aircraft flying at high latitudes and possible issues or damage to satellites.

For full impacts, click on the SWPC grid below.

Radio Blackout

A radio blackout occurs during a solar flare in near real time and affects the sunlit side of Earth. This can cause disturbances in the ionosphere due to increased levels of X-ray and extreme ultraviolet (EUV) radiation. When moderate to strong solar flares take place, certain frequency ranges in the HF spectrum (3-30 MHz typical) may be degraded or blacked out while a flare is in progress on the sunlit side of the earth.

NOAA / SWPC classifies radio blackouts on a level and flare threshold ranging between R1 (M1) (minor), R2 (M5) (Moderate), R3 (X1) (Strong), R4 (X10) (Severe) and R5 (X20) (Extreme). While minor and brief radio blackouts are common during solar maximum, the stronger of these events R3 and above can lead to widespread or complete loss of HF communications on the sunlit side of Earth for several hours.

For full impacts, click on the SWPC grid below.

Solar Flare (SF)

A solar flare is a violent explosion in the Sun's atmosphere with an energy equivalent to tens of millions of hydrogen bombs. Solar flares take place in the solar corona and chromosphere, heating plasma to tens of millions of kelvins and accelerating the resulting electrons, protons and heavier ions to near the speed of light. They produce electromagnetic radiation across the electromagnetic spectrum at all wavelengths from long-wave radio to the shortest wavelength gamma rays. Most flares occur around sunspots, where intense magnetic fields emerge from the Sun's surface into the corona. The energy efficiency associated with solar flares may take several hours or even days to build up, but most flares take only a matter of minutes to release their energy.

Solar flares are classified as A, B, C (Minor), M (moderate to strong) or X (Major) according to the peak flux (in watts per square meter, W/m2) of 100 to 800 picometer X-rays near Earth, as measured by the GOES spacecraft. Each class has a peak flux ten times greater than the preceding one, with X class flares having a peak flux of order 10-4 W/m2. Within a class there is a linear scale from 1 to 9, so an X2 flare is twice as powerful as an X1 flare, and is four times more powerful than an M5 flare. The more powerful M and X class flares are often associated with a variety of effects on the near-Earth space environment. Although the GOES classification is commonly used to indicate the size of a flare, it is only one measure.

Flare observed by SDO / AIA.

Solar Flux (SFI)

The 10.7 cm (2800 MHz) radio flux, commonly called the solar flux or solar flux index (SFI), is the amount of solar noise (light) that is emitted by the sun at 10.7 cm radio wavelengths. The solar flux is measured and reported at approximately 1700 UT daily by the Penticton Radio Observatory in British Columbia, Canada. Values are not corrected for variations resulting from the eccentric orbit of the Earth around the Sun.

The solar flux is used as a basic indicator of solar activity. It can vary from values below 50 to values in excess of 300 (representing very low solar activity and high to very high solar activity respectively). Values in excess of 200 occur typical during the peak of the solar cycles.

The solar flux is closely related to the amount of ionization taking place at F2 layer heights (heights sensitive to long-distance radio communication). High solar flux values generally provide good ionization for long-distance communications at higher than normal frequencies. Low solar flux values can restrict the band of frequencies which are usable for long distance communications. The solar flux is measured in "solar flux units" (s.f.u.). One s.f.u. is equivalent to 10^-22 Wm^-2 Hz^-1.

Solar Wind

The solar wind is a stream of energetic particles (plasma) flowing outwards from the Sun that consists mostly of electrons and protons. The Sun's magnetic field is also carried into our solar system via the solar wind and extends beyond each of our solar systems planets. This is known as the interplanetary magnetic field (IMF). The solar wind moves past Earth at a rate of below 300 km/s during quiet periods to upwards of 900 km/s during periods of extreme solar activity. Unlike the wind we can feel within the atmosphere on Earth, you would not feel the solar wind due to the airless vacuum of space.

Sudden Impulse (SI)

A sudden magnetic impulse represents a rapid momentary fluctuation of the geomagnetic field over a period of only a few minutes. It is generally associated with coronal mass ejections (CMEs) produced by energetic solar events and can (but need not always) be followed by increased geomagnetic activity. This event is typically used to define the exact moment a CME, also known as an interplantery shock passes Earth.

Sunspots (Active Regions)

A sunspot is a region on the Sun's surface (photosphere) that is marked by a lower temperature than its surroundings, and intense magnetic activity. Although they are blindingly bright, at temperatures of roughly 5000 Kelvin, the contrast with the surrounding material at some 6000 K leaves them clearly visible as dark spots. Interestingly, if they were isolated from the surrounding photosphere they would be brighter than an electric arc.

The darkest core of a sunspot is called the Umbra, whereas the lighter surrounding is called the Penumbra.

Sunspots typically begin to appear during the rise of a new 11 year (average) sunspot cycle with peak activity during what it known as solar maximum. It is also quite common to see no sunspots for months on end during the quiet period in between solar cycles known as solar minimum.

A single sunspot or cluster of spots within a sunspot grouping will often be referred to as an active region (AR).

Sunspot Magnetic Class

The magnetic class of sunspots is important in determining how potentially volatile particular active regions may be. Sunspots are regularly observed using instruments capable of determining the magnetic polarity of sunspots and active regions. By also applying laws which have been formulated over the years, visual observations can also be used to establish the magnetic polarity and complexity of spot groups. There are basically 7 magnetic types of sunspots that are classified. They are described as follows:


A - Alpha (single polarity spot).
B - Beta (bipolar spot configuration).
G - Gamma (atypical mixture of polarities).
BG - Beta-Gamma (mixture of polarities in a dominantly bipolar configuration).
D - Delta (opposite polarity umbrae within single penumbra).
BD - Beta with a Delta configuration.
BGD - Beta-Gamma with a Delta configuration.

Example: A region labelled as having a magnetic classification of BG indicates that the sunspot region contains a mixture of magnetic polarities, but the dominant polarity of the group is bipolar. Potentially very powerful and potent regions are those which have classifications of BG, BD and BGD. As magnetic complexity increases, the ability of an active region to generate major energetic events likewise increases.

Sunspot Number (SSN)

This represents the number of observed sunspots and sunspot groups on the Earth facing solar surface (Visible Disk). This number varies in phase with the solar flux index and typically at its highest level during the peak of a solar cycle, although highs and lows can be observed at any point during a cycle.

The spot count for an active region with only 1 spot visible starts with a minimum number of 11 (1 spot group = (10) + 1 visible spot = (1) , making a grand total of 11). Each visible spotted region that qualifies for active region designation will have its own individual spot count for the day. Typically to qualify, a sunspot region must be large enough to be seen via telescopic observations and be visible for at least 24 hours.

The daily total sunspot number reported is calculated using the following formula. R= Ns + 10 * Ng, with Ns the number of spots and Ng the number of groups counted over the entire solar disk.

Sweep Frequency Events (Type II, III, IV and V events)

Energetic solar events often produce characteristic radio "bursts". These bursts are generated by solar material plunging through the solar corona. Type III and type V events are caused by particles being ejected from the solar environment at near relativistic speeds. Type II and IV events are caused by slower-moving solar material propagating outward at speeds varying between approximately 800 and 1600 kilometers per second. Type II and IV radio bursts are of particular importance.

These sweep frequency radio events are signatures of potentially dense solar material which has been ejected from the solar surface (CME). If the region responsible for these events is well positioned, the expelled solar material may succeed in impacting with the Earth. Such an impact often causes an SSC followed by Minor to Major geomagnetic storm conditions and significantly degraded radio propagation conditions. It is therefore interesting to pay attention to events which cause Type II and/or IV radio sweep events, since they may indicate the potential for increased magnetic activity (and decreased propagation quality) within 36-72 hours typical. It should be noted, however, that predicting degraded terrestrial conditions is significantly more complex than simply observing whether the energetic event had an associated Type II or IV radio sweep. Flare position, proton spectra, flare size, event duration, event intensity and a host of other variables must be analyzed before a qualitative judgement can be made.

It should also be noted that sweep frequency radio events are capable of producing Short Wave Fades (SWFs) and Sudden Ionospheric Disturbances (SIDs). Depending on the severity of the event, the duration of SWFs and SIDs may last in excess of several hours with typical values being approximately 30 minutes. SWFs and SIDs cause absorption of radio signals (due to intense ionization) at frequencies up to and well in excess of 500 MHz. Microwave continuum bursts can affect frequencies up to 30 GHz. Frequencies in the HF region can be completely blacked out for a period of time during intense energetic events.

Tenflare (10cm Radio Burst)

A tenflare, sometimes called a 10cm radio burst, is associated with optical and x-ray flares. Solar flares emit radiation over a very wide range of frequencies. One of the more significant frequencies observed is the 10.7 cm wavelength band (2695 MHz). When a solar flare erupts, "noise" from the flare is received over this very wide range of frequencies.

When the noise received on the 10.7 cm wavelength band surpasses 100% of the background noise level during a solar flare, a Tenflare is said to be in progress. The more intense solar flares are associated with tenflares. Almost all major flares are associated with tenflares.

Generally, the greater the intensity of the burst of noise observed at the 10.7 cm wavelength band, the more significant the flare is said to be. The duration of the tenflare can also be used to determine the severity of the flare.