IONOSPHERE

The Ionosphere

Ionosphere: A collection of ionized particles and electrons in the uppermost portion of the earth's atmosphere which is formed by the interaction of the solar wind with the very thin air particles that have escaped the earth's gravity. These ions are responsible for the reflection or bending of radio waves occurring between certain critical frequencies with these critical frequencies varying with the degree of ionization. As a result, radio waves having frequencies higher the lowest usable frequency (LUF) but lower than the maximum usable frequency (MUF) are propagated over large distances. Finally, predictions for the LUF and MUF at different times and regions around the world can be found by searching the world wide web for propagation forecasts.

D-Layer: The lowest part of the ionosphere, the D-layer appears at an altitude of 50-95km. This layer has a negative effect on radio waves because it only absorbs radio-energy, particularly those frequencies below 7MHz. It develops shortly after sunrise and disappears shortly after sunset. This layer reaches maximum ionization when the sun is at its highest point in the sky and this layer is also responsible the the complete absorption of sky waves from the 80m and 160m amateur bands as well as the AM broadcast band during the daytime hours.

E-layer: This part of the ionosphere is located just above the D-layer at an altitude of 90-150km. This layer can only reflect radio waves having frequencies less than 5MHz. It has a negative effect on frequencies above 5MHz due to the partial absorption of these higher frequency radio waves. The E-layer develops shortly after sunrise and it disappears a few hours after sunset. The maximum ionization of this layer is reached around midday and the ions in this layer are mainly O₂⁺.

E_s-layer: Also called the sporadic E-layer. This layer is characteristically very different from the normal E-layer. Its altitude may vary anywhere between 80km and 120km. This extraordinary part of the ionosphere is capable of reflecting radio waves well into the VHF-band (30-300 MHz) and even into the lower parts of the UHF-band (300-3000 MHz). It is still a mystery as to how this layer actually develops, but, it is clear that this layer appears mostly during the summer months and briefly at mid-winter, with the peak occurring in the early summer. Furthermore, it can appear at any time of the day, with a preference for the late morning and early evening. The sporadic E-layer may produce skip distances ranging from 400km to 2000km, with unusually high signal strengths. Even with a fraction of a Watt and a small ground plane antenna, long range contacts are very common.

F-layer: Highest part of the ionosphere. The F-layer appears a few hours after sunset, when the F1- and F2-layers merge. The F-layer is located between 250km and 500km in altitude. Even well into the night, this layer may reflect radio waves up to 20 MHZ, and occasionally even up to 25 MHZ. Ions in the lower part of the F-layer are mainly NO⁺ and are predominantly O⁺ in the upper part.

F1-layer: The F1-layer is located between 150km and 200km in altitude and it occurs during daylight hours. Just before sunrise, the sun begins to shine on the upper part of the atmosphere containing the F-layer. Due to an unclear physical mechanism, the sunlight causes this F-layer to split into two distinct layers called the F1- and F2-layers. The maximum ionization of the F1-layer is reached at midday; this layer merges with the F2-layer a few hours after sunset to reform the F-layer. Finally, this layer reflects radio waves only up to about 10MHz.

F2-layer: This important layer of the ionosphere is the upper most part of the earth's atmosphere and it is located between 250km and 450km in altitude with occasional altitudes extending beyond 600km. At the higher latitudes north or south of the equator, this layer is located at lower altitudes. Near the equator, this layer can be located at twice the altitude as compared to the higher latitudes. About an hour before sunrise, this layer starts to develop as the F-layer begins to split (see F1-layer above). The maximum ionization of the F2-layer is usually reached one hour after sunrise and it typically remains at this level until shortly after sunset. However, this layer shows great variability with peaks in the maximum ionization occurring at any time during the day, displaying its sensitivity to rapidly changing solar activity and major solar events. In contrast to all other layers of the ionosphere, the maximum ionization of the F2-layer usually peaks during the winter months. Most importantly, this layer can reflect radio waves up to 50MHz during a sunspot maximum and maximum usable frequencies (MUF) can extend beyond 70MHz on rare occasions.

Geomagnetic field (GMF): The magnetic field which originates from the rotation of the molten iron core of our planet. This magnetic field produces the well known magnetic flux lines which run between the two magnetic poles allowing us to navigate by use of a compass. The shape of the geomagnetic field, GMF, is very similar to a water drop, with the tail pointing away from the sun. This shape is formed by a constant stream of charged particles originating from the sun (i.e. solar wind) and exerting a constant "pressure" on the side facing the sun. The GMF plays a major role in the dynamics of the earth's atmosphere and without the protection of our GMF, which traps charged particles before they reach the earth's surface, our planet's surface would be undergoing a constant bombardment of these charged particles. Furthermore, without this charged particle trap, the ionosphere would cease to exist and without an ionosphere, sky wave propagation wound not exist and neither would DX contacts! Finally, the GMF is weakest near the polar regions and strongest near equatorial regions and on the night side of the earth opposite the sun, the GMF can extend millions of kilometers into space. Because of the importance of the GMF in trapping charged particles necessary for sky wave propagation, the short term variability of the GMF influences propagation; therefore, these short term variations are included in propagation forecasts. These forecasts categorize the GMF into the following categories: quiet, unsettled, active, minor storm, major storm, severe storm, very severe storm (very rare).