--- title: "Ionosphere layers and characteristics" subject: "Propagation and Antenna" chapter: 4 key: "semester_6/propagation_and_antenna:ionosphere-layers-characteristics" source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/ionosphere-layers-characteristics site: BCT Engineering --- # Ionosphere layers and characteristics Propagation and Antenna · Chapter 4: Propagation and Radio Frequency Spectrum - Asked in: 2071 Magh, 5+3 marks - Video: [Sky Wave Propagation (Basics, Ionosphere Layers, Multi Hope Propagation, Issues & Parameters)](https://www.youtube.com/watch?v=AhgFZBQLLxA) — Engineering Funda - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/ionosphere-layers-characteristics - Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-4 ## Question Explain the different layers of ionosphere. What are the major characteristics of ionosphere? ## Short answer The ionosphere comprises D (50–90 km, day-only, absorbs), E (90–140 km, reflects MF), F1 (150–250 km, day) and F2 (250–400 km, highest density, main HF reflector). Its refractive index n=√(1−81N/f²) bends waves; critical frequency fc=9√Nmax, MUF=fc secθi. It varies with sun and can cause fading and blackouts. ## Answer #### Layers of the ionosphere The **ionosphere** is the region of the upper atmosphere, about 50–400 km high, ionised by solar UV and X-ray radiation. It is divided into layers by height and electron density: ```text height (km) 400 |~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ F2 (250-400) 300 | /\ day & night 220 |~~~~~~~/~~\~~~~~~~~~~~~~~~~~~ F1 (150-250) | / \ day only 110 |~~~~~/~~~~~~\~~~~~~~~~~~~~~~~ E (90-140) 70 |::::/::::::::\::::::::::::::: D (50-90) | / sky \ day only 0 |__/____________\_____________ earth Tx skip Rx ``` | Layer | Height | Electron density (per m³) | Present | Main effect | |---|---|---|---|---| | D | 50–90 km | 10⁸–10¹⁰ | Day only | Absorbs LF/MF sky wave; reflects VLF | | E | 90–140 km (≈110) | ≈10¹¹ | Mostly day (weak at night) | Reflects MF and lower HF; fc ≈ 3–4 MHz | | Sporadic Eₛ | ≈110 km | Patchy, high | Irregular | Occasional VHF long-distance | | F1 | 150–250 km (≈200) | ≈2–5 × 10¹¹ | Day only | Some HF reflection, absorption; fc ≈ 4–5 MHz | | F2 | 250–400 km | ≈10¹² (highest) | Day and night | Main HF reflector; fc ≈ 5–12 MHz; longest hops | - **D layer:** lowest; daytime only; high collision rate, so it absorbs MF and low-HF waves; reflects VLF/LF. - **E layer:** reflects MF and low HF; weak at night; sporadic-E patches can reflect VHF. - **F1 layer:** daytime only; merges into F2 at night; partly reflects and partly absorbs HF. - **F2 layer:** highest and densest; present day and night; main layer for long-distance HF; single hop up to ≈ 4000 km. #### Major characteristics of the ionosphere 1. **Variable ionisation:** depends on the sun, so it changes with time of day (diurnal), season, latitude and the 11-year sunspot cycle. 2. **Refractive index less than 1:** n = √(1 − 81N/f²); decreases as electron density rises, so waves bend back to earth. 3. **Frequency dependent:** higher frequencies penetrate further; above the critical frequency (fc = 9√Nmax) at vertical incidence, and above the MUF for oblique incidence, waves escape. 4. **Absorption:** proportional to collision frequency and to 1/f², greatest in the D layer. 5. **Irregularities and disturbances:** sporadic E, sudden ionospheric disturbances (solar flares) and ionospheric storms cause fading and blackouts. 6. **Virtual height** is always more than actual height. 7. Acts as a dispersive, time-varying, slightly anisotropic medium (because of earth's magnetic field, causing Faraday rotation). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which ionospheric layer is present both day and night and is the main HF reflector? A. D B. E C. F1 D. F2 Answer: D. F2 (250-400 km) has the highest density and persists at night; F1 and D are daytime only. (Key point: Layer present day and night) 2. What is the main effect of the D layer? A. Reflects HF over long distances B. Absorbs LF/MF sky waves due to high collision rate C. Provides highest electron density D. Exists only at night Answer: B. D is lowest, daytime only, and absorbs MF and low HF waves. (Key point: D layer role) 3. Why do waves bend back toward earth in the ionosphere? A. n is greater than 1 and rises with N B. n is less than 1 and decreases as electron density increases C. n is constant D. n depends only on temperature Answer: B. n = sqrt(1 - 81N/f^2) is below 1 and falls as N increases with height. (Key point: Refractive index) 4. How does ionospheric absorption depend on frequency? A. Proportional to f B. Proportional to 1/f^2 and to collision frequency C. Independent of f D. Proportional to f^2 Answer: B. Absorption is greatest at low frequency and in the D layer where collisions are frequent. 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