Assertion (A) : The ionosphere is that region of the earth’s atmosphere in which the constituent gases are ionised by radiations from outer space. Reason (R) : Throughout the ionosphere, there are several layers in which ionization density either reaches maximum or remains almost constant. It depend on the intensity of sun. It also depends upon the atmospheric pressure.
Both (A) and (R) are true, but (R) is not the correct explanation of (A).
Assertion — TRUE. The ionosphere is the region roughly 50 to 400 km up in which ultraviolet and X-radiation from the sun, together with cosmic rays, strip electrons from the gas molecules. The resulting free-electron density is what allows the layer to refract radio waves back to earth — the basis of long-distance HF (sky-wave) communication.
Reason — TRUE. The ionisation is indeed layered, and for a sound physical reason: ionising radiation is strongest at the top but the gas is thinnest there, while lower down the gas is dense but the radiation has already been absorbed. The product of the two peaks at particular heights, producing the D, E, F1 and F2 layers:
| Layer | Height | Behaviour |
|---|---|---|
| D | 50–90 km | absorbs HF by day, vanishes at night |
| E | ~110 km | reflects MF/HF, weakens at night |
| F1, F2 | 150–400 km | merge at night; the main reflecting layer |
and the statement's dependence on solar intensity is right too — ionisation varies with time of day, season and the eleven-year sunspot cycle, which is why HF propagation changes so dramatically between day and night.
Does R explain A? No. The assertion states what the ionosphere is and what creates it — ionisation by radiation from space. The reason describes the internal structure that results: how many layers there are and how their density varies. Structure is a consequence of the ionisation, not an explanation of why the region is ionised. A genuine explanation of A would have to invoke the ionising power of solar UV and X-rays acting on the upper atmosphere.
Both statements are therefore true, but the second elaborates rather than explains.
Hence, both (A) and (R) are true, but (R) is not the correct explanation of (A).
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