Question E3C01:
What is the cause of short-term radio blackouts?  

Choices:  
A: Coronal mass ejections  
B: Sunspots on the solar equator  
C: North-oriented interplanetary magnetic field  
D: Solar flares    

Correct Answer is  D: Solar flares  

Short-term radio blackouts are caused by solar flares (correct).

Why this is correct:
- Solar flares emit intense X-ray and extreme-UV radiation that reaches Earth in about 8 minutes, immediately boosting D-layer ionization on the sunlit side.
- This sudden ionization greatly increases HF absorption (a Sudden Ionospheric Disturbance/Dellinger fadeout), wiping out HF communications for minutes to an hour or so.

Why the other choices are incorrect:
- Coronal mass ejections (A): CMEs arrive much later (tens of hours to days) and mainly cause geomagnetic storms, aurora, and longer-term disturbances, not the immediate, short-lived blackouts.
- Sunspots on the solar equator (B): Sunspots correlate with flare probability but do not directly cause blackouts; it’s the flare’s X-ray burst that does.
- North-oriented interplanetary magnetic field (C): A northward IMF is typically less geoeffective; southward IMF favors geomagnetic coupling. IMF orientation doesn’t cause immediate HF blackouts.

Useful rules of thumb:
- Onset and location: Daylit path only; starts within minutes of a flare.
- Duration: Typically minutes to an hour.
- Frequency dependence: Lower HF is absorbed more strongly; if conditions allow, try higher HF bands.
- Absorption scaling (conceptual):
  $$\text{Absorption} \propto \frac{N_e \,\nu}{f^2}$$
  where $N_e$ is electron density, $\nu$ is collision frequency, and $f$ is RF frequency. Thus, more D-layer ionization and lower $f$ mean more loss.
- NOAA “R-scale”: M- and X-class flares produce R1–R5 radio blackouts; stronger (X-class) flares cause deeper, broader HF outages.