Space Weather and Radio Communications

In the darkness of space between the Sun and Earth, electromagneticradiation from radio waves to gamma-rays as well as high energy chargedparticles stream constantly toward the Earth. Meanwhile, in the rarefied upperatmosphere of the Earth far above the highest clouds, magnetic storms rage andhowling winds drive atmospheric currents around the globe. Conditions in theEarth's upper atmosphere and further out into space, driven by powerfulemissions from the Sun, are referred to as "space weather".

Underlying space weather is the Sun, the output of which varies over an 11year cycle. Conditions in the upper atmosphere vary greatly at different pointson the globe with the change in the zenith angle of the Sun, just as with the"regular" weather on the surface of the Earth. Conditions also vary throughoutthe day and night and with the seasons. For the radio frequency communicator,managing the effects of space weather is essential to achieving reliablecommunications 24 hours a day.

Frequencies in the High Frequency (HF: 3 - 30 MHz) band are particularlysensitive to space weather. HF, which is suited to radio communications oververy long distances, is important for defence forces, emergency services,remote broadcasters, aviation and marine operators. Communications from VLF tosatellite are also affected by space weather making its prediction and theunderstanding of its effects invaluable.

Solar Activity and the Ionosphere

HF radio, which can be effective over extremely long distances, utilises aportion of the upper atmosphere known as the ionosphere. The ionosphere, whichreflects HF radio waves, is created by solar radiation and is a part of thespace weather environment. The ionosphere extends from around 50 km to 500 kmin altitude and is characterised by the presence of free electrons which canrefract (bend) and sometimes reflect radio waves back to Earth. The greaterthe density of free electrons, the greater the frequency of radio waves thatcan be reflected.

The free electrons in the ionosphere result from ionisation of atoms andmolecules by solar radiation. Variations in chemical composition andatmospheric dynamics lead to the formation of a number of distinct bands orlayers. These regions of particularly high electron density are labelled inorder of increasing height as the D, E, F1 and F2 regions.

The F2 region (sometimes just called the F region), stretching from 200 kmto 500 km in altitude is the most important part of the ionosphere for radiocommunicators. It is highest in altitude and thus provides the greatestcommunication range. It also reflects the highest frequencies, which is vitalsince absorption (attenuation) of HF decreases with increasing frequency. Itis also the only layer that is ionised sufficiently to reflect HF both day andnight. The lowest part of the ionosphere, the D region, is also very importantas it attenuates rather than reflects radio waves.

With the coming of night and the absence of solar ionising radiation, theelectron density in the D, E and F1 regions becomes very low. The electrondensity of the F2 region is also reduced at night but persists in a weakenedstate due to winds in the upper atmosphere which carry electrons from day-sideto night-side. Thus, reflections from the night-side ionosphere occur only fromthe F2 region (called the F region at night).

Both the D and F regions of the ionosphere are highly sensitive tovariations in space weather and solar activity. The interaction of HF radiowaves with the D and F regions varies greatly with the seasons, throughout theday and night and throughout the solar cycle.

The Solar Cycle and Sunspots

Space weather is driven by the Sun and follows the "solar cycle" closely.This cycle is typically about 11 years in duration and is manifest in many ofthe radiative and magnetic properties of the Sun. The solar cycle is definedin terms of "sunspots" on the solar disc. Sunspots are regions of extremelyintense localised magnetic fields which appear darker than the surroundingsurface. A sunspot region on the solar surface is akin to an extreme lowpressure system or cyclone on Earth with intense magnetic fields rather thanextreme winds.

At times, sunspots are rare and the solar disc appears almost withoutblemish. This occurs at solar minimum, the start of a solar cycle. Later,sunspots become common and it is normal to see numerous large sunspots, oftenassembled in complex groups, spread across the solar disc. The peak of thesolar cycle, when sunspots are most numerous, is known as solar maximum.

There is a standardised way of counting sunspots present on the solar disc,to give the sunspot number or SSN, which is the traditional indicator of solaractivity and the progress of the solar cycle.

Sunspots and the Ionosphere

The presence of sunspots is of particular importance to the HF communicator.Overlying and surrounding sunspots are particularly hot, bright areas calledplage (after the French for "beach" and for the colour of sand). Plage regionsproduce Extreme Ultra-Violet radiation (EUV) in especially large quantities andit is EUV which causes ionisation of the all important F region.

At solar minimum, when there are no plage regions, there is less EUV andless ionisation of the F region. Consequently, the frequencies reflected bythe F region are lower. At the peak of the solar cycle, solar maximum,ionisation is greatest and the frequencies reflected by the ionosphere arehighest. Higher frequencies mean reduced attenuation by the D region andincreased range.

Solar Flares and HF Fadeouts

Sunspots are also the site of solar flares which are huge explosivedischarges on the surface of the Sun. Massive amounts of energy in the form ofradiation and matter are bound by the intense magnetic fields of a sunspotregion. During a flare, the magnetic field structure collapses, releasing theenergy and matter into space.

X-rays released by flares bombard the Earth causing sudden and intenseionisation of the D region. This leads to increased D region attenuation of HFwaves and in some cases total absorption of all HF frequencies for severalhours. An "HF fadeout" or "Sudden Ionospheric Disturbance (SID) only affectsHF circuits that have ionospheric reflection points in the sunlit hemisphereof the Earth (that pass through the D region). Night-side ionosphericreflection points are unaffected, being in the Earth's shadow and shieldedfrom solar radiation.

Solar flares large enough to cause a total HF blackout, occur on about 300days per 11 year cycle and are most common around solar mximum.

Further Ionospheric Disturbances and HF Communications

Often associated with large solar flares is the release of huge quantitiesof solar matter, referred to as a Coronal Mass Ejection (CME). A CME moves awayfrom the Sun between about 400 to 1000 km/s, expanding as it does, so that itimpacts the Earth usually within two to five days. The Earth's magnetic fieldis buffeted violently by the CME, initially as a shock and then as a period oflarge fluctuations to the geomagnetic field.

Coronal holes are another solar phenomena that also leads to geomagneticdisturbances on Earth. A coronal hole is like a window in the magnetic fieldstructure of the Sun's corona that allows solar particles to flow more freelyoutward from the Sun towards the Earth.

CMEs are most likely to occur at solar maximum. Coronal holes are mostcommon during the declining part of the solar cycle, from maximum to minimum,when the Sun's magnetic field is decreasing in strength.

Of concern to the HF radio communicator is that a significant geomagneticdisturbance or geomagnetic storm in the upper atmosphere initiates anionospheric storm. When this occurs, usable HF frequencies are greatly reducedand irregularities in the ionosphere result in signals travelling by multiplepaths (leading to signal fading).

Major geomagnetic and ionospheric storms can last for several days, causingsevere disruption to HF communications. Associated with such storm periods areaurorae, one of the better known ionospheric phenomena.

Ionospheric Prediction and HF frequency management

In the previous section of this article, HF radio communications via theionosphere were discussed and the connection between the Sun and the ionosphereestablished. It was seen how the ionosphere is created by solar radiation andalso how solar events can regularly disrupt radio communications which rely onthe ionosphere.

One could be forgiven for thinking that with so many cataclysmic solarevents going on and their effects on the ionosphere that HF is impractical.Fortunately, solar events that disrupt HF communications are not every dayoccurrences, happening on perhaps one hundred days per 11 year solar cycle.Events causing severe disruptions lasting for several hours that affect theentire HF spectrum are rarer still, occurring just a dozen or so times percycle.

With constant observations of the Sun-Earth environment, disruptive solarevents can be largely predicted and warnings and alerts issued in advance. TheAustralian Government agency responsible for monitoring space weather andissuing regular forecasts and warnings of imminent solar activity is theBureau of Meterology's space weather branch, Bureau of Meteorology - Space Weather Services.

Predicting Solar Flares

The prediction of large solar flares has traditionally relied onobservations of sunspot regions using ground based telescopes such as at theSWS Solar Observatory located at Culgoora, NSW. At present, ground basedflare prediction is undergoing something of a revolution with new highresolution images of the magnetic field structures of sunspots becomingavailable. These new images, provided by the GONG project of which the SWS/USAFLearmonth Solar Observatory is a participant, provide new insights into thedetailed magnetic behaviour of sunspots. Space weather agencies such as SWSalong with solar researchers expect that by analysing the solar flares of thecoming cycle with these new insights, we will be able to even better predictthe size and timing of major flare events.

Predicting Ionospheric Disturbances

The response of the ionosphere to a geomagnetic disturbance resulting froma CME or Coronal Hole (see Part One) is called an ionospheric storm. The effecton the ionosphere is complicated and depends on the time of day, the season andthe latitude. However, severe geomagnetic storms invariably lead to severeionospheric storms and depressed HF conditions.

During an ionospheric storm HF communication is likely to experience thegreatest problems at higher latitudes (away from the equator). This is alsowhere the well known ionospheric phenomena, the aurora, is to be seen duringan ionospheric storm. The greater the disturbance, the more equator-ward HFcommunication problems (and aurora) are experienced.

SWS Solar observatories located in northwest NSW and WA monitor solar radioemissions to provide critical information such as the velocity of CMEs and thedistribution of solar RF interference. SWS uses this information along withobservations from ground based magnetic observing stations, NASA satellitebased solar wind data and many years of experience to predict the location,intensity and duration of ionospheric storms. With continual advances insatellite based observations of the Sun-Earth environment, such as the NASASTEREO program and the growing body of ionospheric data and models, ionosphericpredictions for radio communicators have been refined over the last 20 years.

Ionospheric monitoring and HF frequency management

Perhaps of greatest value, however, to the regular HF communicator isday-to-day frequency selection in response to the ever changing conditions inthe ionosphere. While the solar cycle and solar events are very important indetermining the frequencies supported by the ionosphere, equally largevariations are observed with latitude, with the seasons and throughout the dayand night.

SWS operates a large network and collects data from ionosondes locatedthroughout the Australasian, Antarctica, and the Pacific which constantly"sound" the ionosphere. Ionosondes produce an "ionogram" showing the range offrequencies reflected vertically by the ionosphere and the altitude they arereflected from.

Using real-time and historical ionospheric data in conjunction withsophisticated HF propagation models SWS is able to provide detailed HFfrequency guides for any communication circuit at any time of the day or night.Predictions of optimum usable frequencies are offered in real-time or predictedin advance and are freely available online.

One of the most popular SWS frequency guides is the HAP chart (Hourly AreaPrediction), which gives the best HF frequency to use from any location intoany area of the globe specified. HAP charts are available in real-time based oncurrent observed conditions, or in advance with conditions predicted days,months or (with declining confidence) even years in advance, for any hour ofthe day.

Other SWS on-line products can provide a more detailed frequency guide oncea circuit is specified. URSL predictions are simple to use, giving upper (U),recommended (R), secondary (S) and lowest (L) frequencies for a given circuit.GRAFEX predictions provide much more detail. Frequencies for differentpropagation modes via E and F layers and lower limits due to D regionabsorption are given, for every hour of the day; sufficient information for aknowledgeable HF operator to communicate reliably and confidently at any hourof the day.

The products mentioned above are available online through the SWS website.SWS has also developed two commercial software packages for HF communicationsengineers; ASAPS and GWPS. ASAPS (Advanced Stand Alone Prediction System)incorporates all the features of the HF predictions described above whileallowing inclusion of specific transmitting and receiving antennaspecifications for field strength and SNR calculations. GWPS (Ground WavePrediction System) is for predictions of HF propagation via ground-wave only.It includes transmitting and receiving antenna specification and transmitterpower and makes detailed predictions of range and receiver field strength fordifferent levels of man-made noise.

Space weather effects on GPS and Satellite Communications

While GHz signals such as those used in GPS systems pass straight throughthe ionosphere, they suffer a time delay as a result of the presence of so manyfree electrons. This typically results in positional errors of 5 to 10 metres,which can increase to many tens of metres under extreme ionospheric conditions.

SWS produces detailed world maps of Total Electron Count (TEC) which areindicative of the expected time delay and are able to be used by satelliteoperators and GPS receivers to make positional corrections for the ionosphere.

The Future of HF and Space-Weather Monitoring

With the development of digital HF mobiles able to provide email andinternet connections, HF appears to be undergoing something of a resurgence.HF communication via the ionosphere is free and with proper frequencymanagement, HF can provide reliable long distance communications 24 hours a day.

With the increasing dependence on satellite based navigation systems suchas GPS, which are vulnerable to large solar and ionospheric disturbances, spaceweather alert systems will be increasingly important in the future.

Bureau of Meteorology - Space Weather Services continues to develop space weather andionospheric forecasting and is committed to providing new services that meetthe demands of the changing communications environment. SWS welcomes anyenquiries about the Sun and its effects on the Earth and about the services weprovide in support of HF communications and space weather.

Material prepared by Dr. Andrew McDonald