Solar Disturbances that Affect Radio Propagation: Flares, Coronal Holes & CMEs

The Sun has a major influence on ionospheric radio propagation, and solar disturbances like Solar Flares, Coronal Holes and Coronal Mass Ejections have a major impact.

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Solar effects on propagation includes:
The Sun & its structure Sunspots Solar disturbances SID sudden ionospheric disturbance Auroras & propagation

Ionospheric propagation: Ionospheric propagation

The condition of the Sun has a major impact on ionospheric radio propagation - it is primarily radiation from the Sun that gives rise to the ionosphere.

As a result it is hardly surprising that the state of the Sun has a huge impact on the state of the ionosphere and in turn the conditions for radio propagation.

The Sun exhibits some major disturbances and these can significantly impact the condition of the ionosphere: disturbances like solar flares, coronal holes and coronal mass ejections or CMEs all have a major impact.

These affect a variety of forms of HF radio communications including two way radio communications, maritime mobile radio communications, general mobile radio communications using the HF bands, point to point radio communications, radio broadcasting and amateur radio communications.

Solar Wind

Under steady conditions the Sun emits a constant stream of material: plasma. This is understandably a very hot and ionised stream of material, that the Sun emits in all directions.

This is called the Solar Wind, and it travels at speeds between about 300 and 800 km/s - very fast.

The actual source of the Solar Wind is the Corona where the temperature is so high and material so excited by this that the gravitational forces of the Sun are overcome and material escapes.

The material escapes at high velocities, typically between about 300 and 800 km/s. The exact mechanism for this and the speeds reached is not fully understood.

The amount of material escaping varies, and is considerably affected by solar disturbances.

Some of the solar wind naturally reaches the Earth, and under steady solar conditions this is mainly deflected by the Earth's magnetic field, but when the solar wind is very high, this can lead to visible auroras and geomagnetic and ionospheric storms.

Solar disturbances

The Sun is basically a huge energy ball, emitting colossal amounts of energy, and as a result it is hardly surprising that there are disturbances.

These disturbances come in a variety of forms and although we know a lot about them, there is still a huge amount we do not know. For obvious reasons it is not possible to get too close to study them in detail, and we cannot explore below the surface.

The main disturbances that are of interest include solar flares, coronal holes and coronal mass ejections.

Solar flares

Solar flares are enormous explosions that occur on the surface of the Sun. They result in the emission of colossal mounts of energy. In addition to this, the larger solar flares also eject large amounts of material mainly in the form of protons.

Flares erupt in just a few minutes with apparently no warning. When they occur the material is heated to millions of degrees Celsius and it leaves the surface of the Sun in a huge arch, returning some time later. The flares normally occur near sunspots, often along the dividing line between them where there are oppositely directed magnetic forces.

It is the magnetic fields appear to be responsible for the solar flares. When the magnetic field between the sunspots becomes twisted and sheared the magnetic field lines may cross and reconnect with enormous explosive energy.

When this occurs an eruption of gases takes place through the solar surface, and it extends several tens of thousands of miles out from the surface of the Sun and follow the magnetic lines of force to form a solar flare. The gases from within the sun start to rise and the area becomes heated even more and this causes the level of visible radiation and other forms of radiation to increase.

Solar Flare - which can affect radio communications on the Earth — Solar flare
*Image courtesy NASA*

During the first stages of the solar flare, high velocity protons are ejected. These travel at around a third the speed of light. Then, about five minutes into the solar flare, lower energy particles follow. This material follows the arc of the magnetic lines of force and returns to the Sun, although some material is ejected into outer space especially during the larger flares.

• Effect of solar flares: For most solar flares, the main effect felt on Earth is an increase in the level of solar radiation, although some material can be ejected into space.

The radiation from the flare covers the whole electromagnetic spectrum and elements such as the ultra-violet, X-rays and the like will affect the levels of ionisation in the ionosphere and hence it has an effect on radio communications via the ionosphere. Often an enhancement in ionospheric HF propagation is noticed as the higher layers of the ionosphere have increased levels of iononisation.

However if the levels of ionisation in the lower levels start to rise then this can result in higher levels of attenuation of the radio communications signals and poor conditions may be experienced. Additionally an increase in the level of background noise at VHF can also be detected easily.

Another effect that may be noticed is that the level of received background noise increases and this can be detected mainly at VHF and above where other forms of noise are much lower.

Flares generally only last for about a maximum of an hour, after which the surface of the Sun returns to normal although some Post Flare Loops remain for some time afterwards. The flares affect radio propagation and radio communications on Earth and the effects may be noticed for some time afterwards.

• Solar Flare Classifications: Flares are classified by their intensity at X-ray wavelengths, i.e. wavelengths between 1 - 8 Angstroms. The X-Ray intensity from the Sun is continually monitored by the National Oceanic and Atmospheric Administration (NOAA) using detectors on some of its satellites. Using this data it is possible to classify the flares.

Solar Flare Classifications
Flare Classification	Details
X Class Flares	X class is the classification for the largest flares; these are major events that can trigger Ionospheric radio propagation blackouts around the whole world and long-lasting radiation storms in the upper atmosphere.
M Class Flares	M class flares are a tenth the size of X class ones and are described as being medium-sized. They generally cause brief radio blackouts that affect Earth's polar regions and sometimes minor radiation storms follow.
C Class Flares	C class flares are small and they give rise to very few noticeable consequences on Earth. At its peak, a C-class flare is a tenth of the size of an M class flare.
B Class Flares	These flares are a tenth the size of the C class ones.
A Class Flares	A class flares are a tenth of the size and intensity of B class ones and they have no noticeable impacts on Earth and radio communications experience no impact.

Within each class the intensity is graded between 1 and 9. Accordingly an M class flare may be graded as M9 if it is the the most intense in that band, whereas one just falling into the M class would be graded as M1. Similar gradings follow for other classes.

The only exception is the X class flares that can go beyond X9 to accommodate very large flares. one example was a flare that occurred in 2003. It was so powerful that it overloaded the measuring equipment which only went as far as X17. It was later estimated that it could have been as large as X45.

It is found that the occurrence of these flares correlate well with the sunspot cycle, increasing in number towards the peak of the sunspot cycle.

Coronal Holes

Coronal holes are another important feature of solar activity and can have an impact on HF ionospheric radio communications.

Coronal holes appear as dark regions in the corona of the Sun when viewed by extreme ultraviolet, EUV or soft x-ray imaging.

They were first discovered after X-ray telescopes were first launched into space and being above the Earth's atmosphere they were able to study the structure of the corona across the solar disc.

Coronal holes are associated with "open" magnetic field lines Their open magnetic structure allows plasma to escape from the Sun and can be seen as an increase in the level of the solar wind. The solar wind streams from coronal holes travel very fast and as a result it is often referred to as a high speed stream in the context of analysis of structures in interplanetary space.

Although coronal holes can develop at any time and in location on the Sun, they are more common and persistent during the years around solar minimum. Some of the more long lasting coronal holes can last through several of the 27 day solar rotations.

Although coronal holes can appear at any point on the Sun, they are most common and also long lasting when they occur in the solar polar regions.

It is also possible for coronal holes to develop in isolation from the polar holes and it is also found that one can extend and this part split off to become a new hole in its own right.

It is found that the long lasting coronal holes are sources for high speed solar wind streams. When one of these high speed stream interacts with the relatively slower steady solar wind, a compression region will be created, and this is known as a co-rotating interaction region, or CIR.

In view of the speed and amount of the solar wind from some coronal holes, it can be sufficient to create some geomagnetic storms, which in turn create disturbances to the ionosphere and ionospheric radio communications.

CMEs

Coronal mass ejections, CMEs, are yet another form of solar disturbance these can have a major effect on the Earth's magnetic field and on radio communications. They can cause brilliant auroral displays as well as disrupting radio communications and affecting satellites as well.

Although much greater in their impact than flares in many respects, CMEs were not discovered until spacecraft could observe the Sun from space. The reason for this is that Coronal Mass Ejections, CMEs can only be viewed by looking at the corona of the Sun, and until the space age this could only be achieved during an eclipse. As eclipses occur very infrequently and only last for a few minutes. Using a space craft the corona could be seen when viewing through a coronagraph, a specialised telescope with what is termed an occulting disk enabling it to cut out the main area of the Sun and only view the corona. This enabled the corona to be viewed.

Although ground based coronagraphs are available, they are only able to view the very bright innermost area of the corona. Space based ones are able to gain a very much better view of the corona extending out to very large distances from the Sun and in this way see far more of the activity in this region, and hence view CMEs.

For many years it was thought that solar flares were responsible for ejecting the masses of particles that gave rise to the auroral disturbances that are experienced on earth. Now it is understood that CMEs are the primary cause.

CMEs are huge occurrences where vast amounts of plasma are out from the Sun, and along with this there are large magnetic fields that expand outwards.

In terms of their size a CME might easily eject many billions of tons of plasma or coronal material. Accompanying the material as it expands outwards from the Sun, there is an embedded magnetic field which is far greater than the background field - the interplanetary magnetic field.

The speeds at which the material from a coronal mass ejection travels can range from anywhere between 250 km/s to 3000km/s - a truly enormous speed. This means that the fastest CMEs can reach the Earth in as little as 15 hours or so, whereas the slower ones may take a few days, if they are in the direction of the Earth.

The larger CMEs often start as a result of stressed and twisted magnetic field realigning themselves to less stressful configurations. Often this process start with a solar flare releasing electromagnetic energy, but then the explosive release of plasma sends out the huge quantities of material.

Coronal mass ejections often occur around sunspot groups where there are strong localised regions of stressed magnetic flux. As a result many of the larger CMEs tend to occur around periods of the solar maximum.

However CMEs can also occur in other areas, for example where relatively cool and denser plasma is trapped around the inner corona region of the Sun by magnetic flux. These are often associated with various filaments and prominences. When the flux "ropes" reconfigure the denser filament or prominence can collapse back to the solar surface and either it may be quietly reabsorbed, or alternatively a CME may result.

For the most explosive CMEs, where the plasma is emitted at very fast speeds - faster than the background solar wind, then a shockwave can result. This can cause an increased level of radiation giving rise to a more intense geomagnetic storm.

When the increased level of plasma from a CME reaches the Earth, the first part to be experienced is the shock wave. This is followed by the plasma itself, and this may take some hours or days to pass.

Dependent upon the intensity of the CME, a geomagnetic storm is likely to result. Much of the material will be deflected by the Earth's magnetic field, but some material will enter byt he poles where the earth's magnetic field enters.

The result is likely to be increased auroral activity and an ionospheric storm which will result in disruption to HF ionospheric radio communications.

Solar disturbances are responsible for many of the major changes in the ionosphere. The effects of both CMEs and solar flares can cause major changes to ionospheric radio propagation, often disrupting them for hours or sometimes days. As a result a knowledge of when they are happening, and their size can help in predicting what ionospheric radio conditions may be like.