Question E3C12:
Which of the following is indicated by a sudden rise in radio background noise across a large portion of the HF spectrum?  

Choices:  
A: A temperature inversion has occurred  
B: A coronal mass ejection impact or a solar flare has occurred  
C: Transequatorial propagation on 6 meters is likely  
D: Long-path propagation on the higher HF bands is likely    

Correct Answer is  B: A coronal mass ejection impact or a solar flare has occurred  

To calculate what a sudden, broadband rise in HF noise means, think “solar event.”

Why the correct answer is correct
- Solar flares often produce strong solar radio bursts that raise the noise floor simultaneously across many HF bands. This appears as a sudden, wideband “hash” or hiss on the S‑meter.
- Flares also increase X‑ray/EUV flux, causing a Sudden Ionospheric Disturbance (SID) on the sunlit side, often followed by shortwave fadeouts. A CME impact can also elevate noise and disturb HF, but the immediate, sudden broadband noise is most characteristic of a flare’s radio burst.

Why the other choices are incorrect
- Temperature inversion (A): This is a tropospheric phenomenon that enhances VHF/UHF ducting, not HF. It does not cause a sudden HF-wide noise rise.
- Transequatorial propagation on 6 meters (C): TEP affects 50 MHz (VHF), typically near equinoxes and late afternoon/evening. It does not manifest as broadband HF noise.
- Long-path propagation on higher HF bands (D): Long-path is about geometry and timing (often near grayline), not a sudden, wideband rise in background noise.

Useful rules of thumb
- Sudden, broadband HF noise increase across many bands → think “solar flare/solar radio burst.”
- Flare timing vs. CME:
  - Flares: effects are immediate on the dayside (minutes). Can cause SIDs and radio bursts.
  - CMEs: effects arrive later (≈ 1–3 days), leading to geomagnetic storms and auroral absorption.
- Flare classes by X‑ray flux (GOES):
  - C: $10^{-6}$ to $10^{-5}\ \text{W/m}^2$
  - M: $10^{-5}$ to $10^{-4}\ \text{W/m}^2$
  - X: $\ge 10^{-4}\ \text{W/m}^2$
  M and X flares are most associated with strong radio bursts and HF disruption.
- Geomagnetic activity indicators:
  - $K_p \ge 5$ indicates a geomagnetic storm (often from a CME), which can degrade HF but is not typically a sudden, instantaneous HF-wide noise spike.