HF Comms.: D-Region Absorption

Short Wave Fade and Polar Cap Absorption events are types of Sudden Ionospheric Disturbances (SID) where emissions from solar activity cause enhance ionization of the D-layer thereby elevating the LUF. At times the enhanced absorption may be strong enough to close the HF propagation window completely - this is called a Short Wave Blackout.

At lower/mid latitudes, intense X-rays from a solar flare can cause a Short Wave Fade (SWF) event. SWF events can only occur on the sunlit hemisphere of the Earth and are most intense at the sub-solar point. They persist for the duration of the flare - tens of minutes to hours.

In the auroral/polar zone, energetic protons (>10 MeV) produced by processes at the Sun and in interplanetary space arrive at Earth and enter the atmosphere over the polar regions and can result in a Polar Cap Absorption (PCA) event. PCA events caused by protons from intense flares start within 15 minutes to 2 hours of the flare and last anywhere from about an hour to several days (with an average of around 24 to 36 hours). CME??

Ability to forecast events that cause D-Region Absorption

Flares normally occur in solar active regions. Observations of such regions sometimes suggest that a flare could occur, but not when it will occur. It is therefore possible to provide a probability that a region will produce a flare of a particular size in the next 12 to 24 hours, especially for regions that have already exhibited enhanced activity. There are rules of thumb for the duration of a flare, given its magnitude, but these fail if the flaring structure is very large - these produce long duration events that can last for many hours.

Solar particle events that result in radiation storms are even more difficult to forecast - only a few flares produce this type of event. Observations do suggest that once a regions has produced a particle an event, it could produce another; also, once an event has started researchers are having some success in calculating the peak intensity and duration.

Some radiation storm are caused by the shock fronts moving through the heliosphere, for example resulting from a coronal mass ejection (CME). In these cases, it may be possible to determine that a storm is likely before it really starts because the shock front takes time to propagate.

HF Comms.: Geomagnetic Storm??

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Ability to forecast Geomagnetic Storms??

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Disruption of trans-ionospheric propagation

• The quality of communications can be disrupted if the Total Electron Content (TEC) is too high, or if there is scintillation.
• Position resolution provided by Satellite Navigation systems such as GPS (and Galileo) can be degraded by high TEC (the signal is refracted and the extra path length causing timing errors) and by scintillation (the receiver can loose lock).
• There are additional problems related to satellite based augmentation systems (SBAS) such as WAAS, EGNOS, etc. At mid-latitudes, gradients in TEC can be quite large during geomagnetic storms and the SBAS cannot accommodate them either in accurately measuring the gradients (the spatial density of reference receivers is too small) or in updating the ionospheric correction model (the data rate is inadequate). The problem is aggrevated if reference stations have difficulties contacting sufficent spacecraft when gathering the data needed to calculate the correction matrix.

Ability to forecast Ionospheric Disturbances

Scintillation at mid and lower latitudes is caused by other mechanisms and is most likely to occur between local sunset and midnight.

Geomagnetic storms manifest themselves as deviations in the Earth's magnetic field from the quiet conditions that extend over wide geographic areas (from high-latitude to mid-latitude and equatorial regions). The disturbances in the geomagnetic fields are caused by fluctuations in the solar wind impinging on the Earth. They may be limited to the polar regions, unless the interplanetary magnetic field (IMF) carried by the solar wind has long periods (several hours or more) of southward component (Bz<0) with large magnitudes (greater than 10-15 nT) - the occurrence of such a period stresses the magnetosphere continuously, causing the magnetic field disturbances to reach the equatorial region. The manifestations of storms are strong deviations in the Earth's magnetic field from the quiet conditions that extend over wide geographic areas: from high-latitude to mid-latitude and equatorial regions.

Radiation

The bottom panel of the plot shows neutron flux measured at ground level and is a measure of the cosmic ray flux. Decreases can be caused by the passage of CMEs (so called Forbush Decreases); rises can be caused by solar flares.

If an intense solar flare occurs, the level of radiation can increase. The top two panels show the X-ray flux and proton flux measured by the GOES spacecraft - if these show significant increases there may be enhanced radiation levels.

Legislation

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