Solar Flares and Disturbances for Radio Propagation
- an overview of the basics of solar flares and other disturbances including
CMEs and how they affect ionospheric HF radio propagation conditions for two way
radio communications, maritime mobile radio communications, point to point radio
communications and radio broadcasting.
The condition of the Sun has a major impact on ionospheric
radio propagation. Accordingly it affects 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.
As the Sun provides the radiation that governs the state of
the ionosphere and hence HF radio propagation, any flares or other disturbances
are of great importance. Under some circumstances these can enhance radio
communications and the HF radio propagation conditions. Under other
circumstances they can disrupt radio communications on the HF bands, while at
the same time providing some radio propagation conditions that can be used at
VHF by radio amateurs.
there are a number of types of disturbance that are of
particular interest for radio communications. Flares are one of the most
obvious. However, apart from solar flares there are other disturbances that
occur. One is the coronal mass ejection, and there are also coronal holes.
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
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. This
radiation 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. Additionally an increase in the level of background noise at VHF can
also be detected easily.
Flares generally only last for about 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. The
largest flares are termed X-Class flares. M-Class flares are smaller, having a
tenth the X-Ray intensity of the X-Class ones. C-Class flares then have a tenth
the intensity of the M-Class ones.
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.
CMEs
Coronal mass ejections, CMEs, are another form of disturbance
that can affect radio communications. Although much greater 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.
Coronal Mass Ejections, CMEs are huge bubbles of gas that are
threaded with magnetic field lines, and the bubbles are ejected over the space
of several hours. 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.
It is now understood that CMEs disrupt the steady flow of the
solar wind producing a large increase in the flow. This may result in large
disturbances that might strike the Earth if they leave the Sun in the direction
of the Earth.
Coronal Mass Ejections, CMEs are often associated with solar
flares eruptions but they can also occur on their own. Like solar flares their
frequency varies according to the position in the sunspot cycle, peaking around
the sunspot maximum, and falling around the minimum. At solar minimum there may
be about one each week whilst at the peak two or three may be observed each day.
Fortunately they do not all affect the Earth. Material is thrown out from the
Sun in one general direction and only if this is on an intersecting trajectory
will it affect the Earth.
Coronal Holes
Coronal holes are another important feature of solar
activity. They are regions where the corona appears dark. 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 and are often although not exclusively found at the Sun's poles. The
high-speed solar wind is known to originate from them and this has an impact on
ionospheric radio propagation conditions and hence on all HF radio
communications.
Summary
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.
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