Radio Communication and the IonosphereThe ionosphere controls HF propagation, based on factors like solar activity, time of day, and frequency.
The ionosphere is a region in the upper-atmosphere that meets space (exosphere).
Figure 1: Ionosphere (Thermosphere) is part Earth's Atmosphere
When solar energy ionizes gases 48 to 800 kilometers above the earth's surface, it produces a mixture of free electrons and ions (known as "plasma").
The high densities of free electrons significantly affect radio communication.
Scientists divide the ionosphere into regions, D, E and F:
- D Region
- E Region
- F Region
The largest part of the HF absorption takes place in the D-region, in particular frequencies lower than 10 MHz.
The typical (noon time) free electron density ~1010 electrons/m3 at 60-90 km, consists NO+ Nitric Oxide ionized by solar light at 1216Å UVC.
What are the differences among ionospheric region D, E, and F? a summary
The ionosphere ↗ is divided into regions, D, E, and F from about 48 up to about 800 km above the earth`s surface, characterized by different free electron densities, and ion compositions.
Figure 2: Illustration of Ionospheric regions
During the day the F-region splits into two region called F1 and F2, while the D-region vanishes completely at night. These regions do not have sharp boundaries, and the altitudes at which they occur vary during the course of a day and from season to season.
What makes the ionospheric regions distinct? See graph of plasma density vs altitude.
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Figure 3: Typical Ionospheric electron density profiles
The above graph is based on a review from U.C.Berkeley by Bob Brown Ph.D, NM7M (SK)↗
- E-region ~1012 electrons/m3 at 150-800 km, consists H+, He+ Hydrogen and Helium ionized ↗ by 100-1000Å EUV
- E-region ~1011 electrons/m3 at 90-150 km, consists O2+ Oxygen ionized ↗ by 10-100Å EUV
- D-region ~1010 electrons/m3 at 50-90 km, consists NO+ Nitric Oxide ionized ↗ by 1216Å UVC Moreover, X-ray Solar Flares, 1-10 Å (hard X-ray), enhance D-region, causing Blackout events.
As the ionosphere changes, numerous natural and man made systems are affected:
Read more about the impact of the ionosphere on radio communication.
- The density of free electrons in the ionophere has an effect on radio waves. Thus it plays an important role in modern communication and navigation systems. As a result, we can obtain signals from radio and GPS systems. In both cases, the composition and density of the ionosphere can significantly affect the signals.
- The ionosphere can be influenced by the weather. Different weathers like thunderstorm systems, hurricanes can create pressure waves that ripple up into the Ionosphere. NASA has done research on this matter and within the past 15 years, NASA satellites revealed the connections between the weather condition changes in Earth’s Ionosphere. The ionosphere is impacted also by space weather.
- The ionosphere is home to several spacecraft, including the international space station. This suggests that these satellites may be affected by the ionosphere's constantly shifting properties. This also covers the sudden surges of charged particles, which can shorten satellites' orbital lifetimes and increase drag.
The project "Understanding HF Propagation" focuses on near-real-time indicators and explains what they mean.