Summary forecast explanation
The summary forecast on the IPS website home page distills in simple terms the details of the full forecast that is issued each day.
The summary usually includes the following information for the indicated day.
- the expected level of solar flare activity
- whether coronal mass ejection effects are expected
- whether coronal hole effects are expected
- the expected range of the solar wind speed
- the expected level of geomagnetic activity
- whether short-wave radio fadeouts are expected
- the expected conditions for high-frequency radio communications
The summary forecast often also mentions:
- recent or upcoming events and conditions
- the likelihood of auroras
- anything else of interest
The various parts of the forecast are explained in more detail below.
Solar flare activity
Solar flare activity summarises the number and strength of expected solar X-ray flares. Activity is reported in the following categories:
|flares weaker than C-class
|flares of C-class strength
|1 – 4 flares of M-class strength
|5 or more flares of M-class strength, or 1 – 4 flares of M5 or greater strength
|5 or more flares of M5 or greater strength
The X-ray flare classes in order of increasing strength are A, B, C, M and X, corresponding to levels of the solar X-ray flux, measured over the 0.1 – 0.8 nm wavelength range in Watts per square metre by the GOES-15 satellite:
|greater than 10-8 W/m²
|greater than 10-7 W/m²
|greater than 10-6 W/m²
|greater than 10-5 W/m²
|greater than 10-4 W/m²
Within each class, flares are further categorised by a number suffix (M1.2, M5, etc), with larger numbers indicating stronger flares within that category.
Coronal mass ejections
A coronal mass ejection (CME) is an ejection of a large amount of solar plasma (mostly protons and electrons) and magnetic fields from the Sun. Most CMEs are ejected into space nowhere near the Earth. Those that do impact Earth can disturb the Earth's magnetic field and lead to a subsequent disruption of the ionosphere (possibly affecting high-frequency radio communication).
The amount of material in a CME varies widely, but the average mass has been estimated as being around 1.6 x 1012 kg (less than a millionth of the mass of Earth's atmosphere). The speed at which a CME travels also varies a lot, being on average around 500 km/s. At this speed, a CME takes 3-4 days to reach Earth. Some CMEs get here in half the time.
The mass and speed of a CME can be estimated by observing its departure from the sun and from spectrographs of associated radio bursts (which reach Earth only minutes after a CME occurs). Images and movies of CMEs erupting from the sun can be obtained from the LASCO instrument on the SOHO spacecraft (head-on view) and from the SECCHI COR2 instruments on the two STEREO spacecraft (rearwards side-on views).
In addition to mass and speed, the orientation of a CME's magnetic field also affects the impact it will have if it reaches Earth. This factor can't currently be measured until the CME is almost upon us (at the ACE spacecraft) where the CME's effect on the solar wind can be observed. Large sustained negative values of the Bz component of the solar wind's magnetic field are a good indicator that a CME will have a strong effect on the Earth's magnetic field.
Coronal hole effects
A coronal hole is a low density region of the sun's corona with relatively low temperature (somewhat less than the usual 1 million °C of the solar corona). Coronal holes are a source of high speed solar wind streams, which can induce moderate disturbances in the Earth's magnetic field and ionosphere.
A coronal hole can persist for several rotations of the sun (each rotation taking around 27 days). Thus, its effect at Earth can repeat every 27 days or so as the solar wind stream from the coronal hole sweeps past Earth like a lighthouse beam (even more like a rotary lawn sprinkler).
Coronal holes can be observed using images (usually at X-ray wavelengths) obtained from various satellites, the highest quality ones being lately from SDO (see AIA 193, AIA 211, AIA 335).
Solar wind speed
Solar wind is the outflow of solar material from the hot, unstable corona. The solar wind expands into interplanetary space at a speed of about 400 km/s (this can vary dramatically), carrying with it the magnetic fields that originate in the Sun.
Expected solar wind speed is reported in the following categories:
|up to 400 km/s
|400 – 500 km/s
|500 – 600 km/s
|over 600 km/s
High solar wind speeds can be the result of coronal holes or CMEs and can cause disturbances in Earth's magnetic field and subsequent ionospheric disruption, especially if they occur in conjunction with large sustained negative values of the Bz component of the solar wind's magnetic field.
Activity in the Earth's magnetic field is reported in categories that refer to values of the A-index, which indicates the daily average level of the field's disturbance.
|less than 8
|8 – 15
|16 – 29
|30 – 49
|50 – 99
|100 or more
|any level of storm
An A-index can be calculated for any location where there is a measuring station, and can be averaged over several stations to give an A-index for a region, or for the entire planet (the planetary A-index is referred to as Ap).
The related K-index also measures geomagnetic field disturbance, but over a 3-hour period. It has values from 0-9.
When strong solar flares occur, the lower level of the ionosphere (the D region) can become more highly ionised than usual. This can cause radio communication through that region to become impractical due to increased absorption of the radio signal by the flare-enhanced D region. Sometimes, the whole HF radio spectrum is affected, but normally the lower (short-wave) frequencies are affected most, hence the term short-wave fadeout (SWF).
High-frequency communication conditions
The quality of radio propagation on a high-frequency (HF) communication circuit depends on many factors including the electron content and distribution in the ionosphere, the presence of ionospheric disturbances and irregularities, and the occurrence of short-wave fadeouts. HF propagation conditions can also vary with the latitude of the circuit, being generally poorer for higher latitude circuits.
Propagation conditions in the IPS forecasts are classified in general terms as poor, fair, normal or good for each of three geomagnetic latitude bands. For Australia, these correspond to:
|0 – 20° (equatorial, Darwin)
|20 – 60° (Townsville, Brisbane, Sydney, Perth, Melbourne, Hobart)
|60 – 90° (Macquarie Island, Antarctica)
Auroras are visible as a steady glow or as moving curtains of light in the night sky. They result from the collision of charged particles from the Earth's magnetosphere with atoms in the upper atmosphere. These charged particles are accelerated down magnetic field lines towards the Earth's atmosphere in regions around the poles known as auroral ovals.
The different colours visible in auroras depend on the atoms that participate in the collisions and the interactions that occur with the charged particles.
The positions of the auroral ovals vary seasonally and their width increases along with increased geomagnetic activity. Normally, auroras are seen only near the poles. Visibility of auroras at lower latitudes (eg, Southern Australia) usually corresponds with the occurrence of major geomagnetic storms.