The Ionosphere and Skywave Propagation
Ionization of D, E and F regions Free-electron density Skywaves

This page is part of the project "Understanding HF Propagation."
by Doron Tal, 4X4XM

The ionosphere enables skywave

Figure 1: Skywave, LOS and ground wave

The ionosphere is a layer of the upper atmosphere detailed in Figure 2. Let's focus on the mesosphere and ionosphere, which are located between 50 to 700 kilometers above Earth's surface:

Figure 2: Ionosphere (Thermosphere) is part Earth's Atmosphere

EUV sunlight ionizes atoms and molecules, releasing free electrons and ions that combine to form plasma. Hence the name Ionosphere.

Figure 3: Plasma State courtesy of P.R.A. Co., Ltd. (Plasma Related Alliance), Japan

The ionosphere is divided into regions at altitudes from 48 to 600 kilometers above the earth's surface.

Figure 4: Ionospheric regions illustration

This figure shows ionospheric regions, labeled D, E, F1, and F2, with an additional label F on the night side. It highlights the variation in ionosphere regions between day and night, with the day side on the left and the night side on the right. This visual representation helps understand how the ionosphere's structure changes with the time of day.

These regions are distinguished by their free-electron densities.

Figure 5: Typical Distributions of Free Electrons in the Ionosphere

The above graph is based on a review from U.C.Berkeley by Bob Brown Ph.D, NM7M (SK) ↗

HF radio waves aimed at the inosphere can be bounced, allowing for global communication channels.

Figure 6: Complex skywave modes:
F Skip / ¹F¹E, E-F Ducted, F Chordal, E-F ocasional and sporadic E^↗.
Diagram courtesy of ASWFC↗ ibid Section 2.4, Fig.2.4.

Figure 6 shows different modes of radio wave propagation in the ionosphere, including ionospheric tilt, chordal mode, ducted mode, sporadic E, F skip, 1F1E, and 1F1Es1F. It highlights how radio waves interact with the E and F regions, illustrating their travel paths over long distances.

Summary:
This page explains how the ionosphere enables HF radio communication by forming plasma layers (Figures 1–2) and organizing into distinct regions—D, E, and F (Figure 4). Changes in electron density with altitude and time (Figure 5) allow various skywave propagation modes, including skip, ducted, and chordal paths (Figure 6).

The project "Understanding HF Propagation" focuses on near-real-time indicators and explains what they mean.

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