Which of the following is indicated by a sudden rise in radio background noise across a large portion of the HF spectrum?
Why Solar flares and CME impacts blast the Earth with broadband radio energy (solar radio bursts) along with X-rays and particles, and the sudden flood of noise shows up across a wide swath of the HF spectrum at once. A rise that affects many bands simultaneously points to a source outside the station and outside the ionosphere, and the Sun is the only thing big enough to do it everywhere at once. The same event often produces a sudden ionospheric disturbance, so the noise rise is frequently followed by HF signals fading out on the sunlit side of the Earth.
Watch out A temperature inversion is a tropospheric effect that creates ducting at VHF and UHF; it bends signals but does not raise HF noise levels. Transequatorial and long-path openings are propagation paths, which change signal strength on particular routes rather than lifting the noise floor across the whole HF range.
Sun sneezes, the whole HF band hisses: broadband noise jump means flare or CME.