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

*Correct Answer is  A: Southward*  

### AI Response: (may contain errors)  

To understand the orientation of $B_z$ (the vertical component of the interplanetary magnetic field) and its effect on charged particles from the Sun, we need to consider how the Earth's magnetic field interacts with solar wind.

1. **Correct Answer: Southward ($B_z$ is negative)**  
   - When $B_z$ is oriented southward, it aligns opposite to the Earth's magnetic field. This alignment allows for a more efficient coupling of the solar wind's magnetic field with the Earth's magnetic field. As a result, charged particles from the Sun can more easily enter the Earth's magnetosphere, leading to increased geomagnetic activity and disturbed conditions, such as auroras.

2. **Why the Other Choices are Incorrect:**
   - **Northward ($B_z$ is positive)**: A northward orientation of $B_z$ tends to repel the solar wind's magnetic field, reducing the likelihood of charged particles entering the magnetosphere. This results in calmer geomagnetic conditions.
   - **Eastward ($B_z$ is horizontal)**: An eastward orientation does not significantly enhance the interaction between the solar wind and the Earth's magnetic field. It does not create the same coupling effect as a southward orientation.
   - **Westward ($B_z$ is horizontal)**: Similar to the eastward orientation, a westward $B_z$ does not facilitate the entry of charged particles into the magnetosphere and does not lead to disturbed conditions.

3. **Useful Rules of Thumb:**
   - A southward $B_z$ increases the likelihood of geomagnetic storms.
   - A northward $B_z$ generally indicates stable conditions.
   - The interaction of solar wind with the Earth's magnetic field is crucial for understanding space weather phenomena.

In summary, a southward orientation of $B_z$ enhances the entry of solar wind particles into the Earth's magnetosphere, leading to disturbed conditions, while northward and horizontal orientations do not facilitate this interaction.