Question E3C04:
What does the value of Bz (B sub z) represent?  

Choices:  
A: Geomagnetic field stability  
B: Critical frequency for vertical transmissions  
C: North-south strength of the interplanetary magnetic field  
D: Duration of long-delayed echoes    

Correct Answer is  C: North-south strength of the interplanetary magnetic field  

Correct answer: C — North-south strength of the interplanetary magnetic field

Why C is correct:
- $B_z$ is the z-component of the interplanetary magnetic field (IMF), reported in nanotesla (nT), in geocentric solar magnetospheric (GSM) coordinates.
- It measures the north-south orientation of the IMF:
  - $B_z < 0$ (southward) strongly couples with Earth’s northward magnetic field, promoting magnetic reconnection, geomagnetic storms, aurora, absorption, and HF disruption.
  - $B_z > 0$ (northward) tends to be quieter for geomagnetic conditions.

Useful relationships and rules of thumb:
- Vector form: $$\mathbf{B}_{IMF} = (B_x, B_y, B_z)$$ where $B_z$ is the north–south component.
- Quick rule: 
  - If $B_z < -5$ nT for 30+ minutes, expect rising $K_p$, auroral activity, disturbed HF (especially on polar paths).
  - If $B_z > 0$, conditions are generally more stable.

Why the other choices are incorrect:
- A: Geomagnetic field stability is indicated by indices like $K$, $K_p$, and $A_p$, not $B_z$. $B_z$ is an upstream solar wind parameter, not a stability index.
- B: Critical frequency for vertical incidence is $f_oF2$ (and related MUF values), not $B_z$.
- D: Long-delayed echoes (LDE) durations are propagation anomalies not described by $B_z$; $B_z$ is a magnetic field component, not a timing measure.