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The following Y2K material has been kept available by MITRE for historical purposes only and has not been updated unless noted.
| GEOMAGNETIC AND IONOSPHERE STORMS AND THEIR POSSIBLE IMPACTS |
Sudden ionospheric disturbances (SIDs) increase the ion density of the lower ionosphere, expand its vertical extent, and lower the level of the lower ionosphere. The most notable SID is the Short Wave Fade.
Refraction of HF signals mostly occur in the highest portion of the ionosphere (or F-layer), where the number of free electrons is greatest. Frequencies below a "Maximum Usable Frequency (MUF)" are bent back toward the Earth, while those above the MUF do not encounter sufficient bending and pass through the ionosphere into space.
Below a "Lowest Usable Frequency" (LUF), below which signals don't get through the lowest portion of the ionosphere (or D-layer). This occurs because each time a radiowave passes through the D-layer, it will cause ionized atmospheric particles to oscillate. Many ionized particles will collide with the still relatively dense neutral air molecules in the D-layer, causing radiowave energy to be damped (i.e., converted to heat). The lower the frequency, the greater the degree of signal absorption.
The HF propagation window is the range of frequencies between the LUF and MUF. HF operators choose propagation frequencies within this window, so their signals will pass through the absorbing D-layer and subsequently refract from the F-layer.
The LUF and MUF curves show a normal, daily variation. During early afternoon, incoming photoionizing solar radiation is at a maximum, so the D- and F-layers are strong and the LUF and MUF are elevated. During the night, the removal of ionizing sunlight causes all ionospheric layers to weaken (some layers disappear altogether), and the LUF and MUF become depressed.
When the X-ray radiation emitted during a solar flare significantly enhances D-layer ionization and absorption (thereby elevating the LUF) it can cause a Short Wave Fade (SWF) Event. A SWF can occur over the entire sunlit hemisphere of the Earth facing the sun and it can last anywhere from tens of minutes to an hour or two. This enhanced absorption can become strong enough to close the HF propagation window completely, which is referred to as a Short Wave Blackout.
Scintillation of radiowave signals is the rapid, random variation in signal amplitude, phase, and/or polarization caused by small scale irregularities in the electron density along a signal's path. The result is signal fading and data drop-outs on satellite command uplinks, data downlinks, or on communication signals. Ionospheric radiowave scintillation is very similar to the visual twinkling of starlight or heat shimmer over a hot road caused by atmospheric turbulence.
Scintillation tends to be a highly localized effect. Only if the signal path penetrates an ionospheric region where these small scale electron density irregularities are occurring will an impact be felt. Low latitude, nighttime links with geosynchronous communications satellites are particularly vulnerable to intermittent signal loss due to scintillation.
Scintillation tends to be most severe within 20 degrees of the geomagnetic equator. It is also strongest from local sunset until just after midnight, and during periods of high solar activity. In the auroral and polar regions, scintillation is strong, especially at night, and its influence increases with higher levels of geomagnetic activity. Knowledge of those time periods and portions of the ionosphere where conditions are conducive to scintillation permits organizations to reschedule activities and/or to switch to less susceptible radio frequencies or satellite links.
GPS satellites, which are located at semi-synchronous altitude, are also vulnerable to scintillation. Signal strength enhancements and fades, as well as phase changes, due to scintillation can cause a GPS receiver to lose signal lock with a particular satellite. The reduction in the number of simultaneously useable GPS satellites may result in a potentially less accurate position fix.
The Total Electron Content (TEC) along the path of a GPS signal can introduce a positioning error. Just as the presence of free electrons in the ionosphere caused HF radiowaves to be bent (or refracted), the higher frequencies used by GPS satellites will suffer some bending (although to a much lesser extent than with HF radiowaves). This signal bending increases the signal path length. In addition, passage through an ionized medium causes radiowaves to be retarded somewhat from the speed of light. Both the longer path length and slower speed can introduce an error into a GPS location fix.
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This page is provided and maintained by our Website Administrators Last modified: Thursday, 14-Feb-2008 09:21:04 EST |