Question E3C05:
What orientation of Bz (B sub z) increases the likelihood that charged particles from the Sun will cause disturbed conditions?  

Choices:  
A: Southward  
B: Northward  
C: Eastward  
D: Westward    

Correct Answer is  A: Southward  

To understand Bz:
- Bz is the north–south component of the interplanetary magnetic field (IMF) in geocentric solar magnetospheric (GSM) coordinates.
- Earth’s dayside magnetic field is northward (+Z). Solar wind coupling is strongest when the IMF is oppositely directed.

Why A (Southward) is correct:
- A southward (negative) Bz is opposite Earth’s northward field, which promotes magnetic reconnection at the dayside magnetopause.
- This opens field lines, allowing solar wind energy and charged particles to enter the magnetosphere/ionosphere, increasing geomagnetic activity (Kp, AE) and causing disturbed HF conditions (auroral absorption, disturbances).

Why the other choices are incorrect:
- B (Northward): Aligns with Earth’s field, suppressing reconnection; reduces coupling and geomagnetic disturbance.
- C (Eastward) and D (Westward): These describe the By component (east–west). By influences the location/shape of auroral features but is not the primary driver of storm-time energy input; storm likelihood mainly tracks Bz.

Useful rules of thumb and formulas:
- Rule: Disturbances increase when $B_z < 0$ (southward), especially if sustained.
- Practical threshold: Sustained $B_z \lesssim -5$ to $-10$ nT for a few hours, with higher solar wind speed, often yields Kp ≥ 5.
- IMF clock-angle coupling:
  $$\epsilon \propto v\,B^2\,\sin^4\!\left(\frac{\theta}{2}\right)$$
  where $v$ is solar wind speed, $B$ is IMF magnitude, and the clock angle
  $$\theta = \tan^{-1}\!\left(\frac{|B_y|}{B_z}\right)$$
  Coupling maximizes when $\theta \approx 180^\circ$ (strong southward $B_z$).