Question E3C08:
Which of the following is the space-weather term for an extreme geomagnetic storm?  

Choices:  
A: B9  
B: X5  
C: M9  
D: G5    

Correct Answer is  D: G5  

To identify an extreme geomagnetic storm, use NOAA’s Space Weather Scales:
- Geomagnetic storms use the G-scale: G1 (Minor) to G5 (Extreme).
- Therefore, G5 is the term for an extreme geomagnetic storm.

Why D (G5) is correct:
- G5 is the NOAA designation for an extreme geomagnetic storm, typically associated with a planetary K-index of $K_p = 9$.

Why the others are incorrect:
- A (B9): This is a very weak solar flare classification (B-class), not a geomagnetic storm term.
- B (X5): This is a strong X-class solar flare magnitude, used for X-ray flare strength and radio blackout scale (R-scale), not the geomagnetic storm scale.
- C (M9): This is a high M-class flare level, again a flare/X-ray term, not a geomagnetic storm term.

Useful rules of thumb:
- Geomagnetic storm scale: G1–G5 roughly maps to $K_p = 5$ to $K_p = 9$; G5 corresponds to $K_p = 9$.
- Flare (X-ray) classes by peak flux (1–8 Å band):
  - $B_n: n \times 10^{-7}\ \text{W/m}^2$
  - $C_n: n \times 10^{-6}\ \text{W/m}^2$
  - $M_n: n \times 10^{-5}\ \text{W/m}^2$
  - $X_n: n \times 10^{-4}\ \text{W/m}^2$
- Scale letters:
  - G = Geomagnetic storm (Kp-based)
  - R = Radio blackout (X-ray flares, M/X classes)
  - S = Solar radiation storm (proton flux)