A: The earth's field departs from its dipole shape substantially at large distances (greater than about 30,000 km).
R: At large distances, the field gets modified due to the field of ions in motion in the ionosphere which is sensitive to extra-terrestrial distubances such as, the solar wind.

Options

A. If both Assertion & Reason are the true and the reason is the correct explanation of the assertion, then mark (1)
B. If both Assertion & Reason are true but the reson is not the correct explanation of the assertion, then mark (2)
C. If Assertion is true statement but Reason is false, then mark (3)
D. If both Assertion and Reason are false statements, then mark (4)

Answer

To solve the question regarding the Earth's magnetic field and its behavior at large distances, we will break down the assertion (A) and reason (R) step by step.

### Step-by-Step Solution:

1. **Understanding the Assertion (A)**:
- The assertion states that the Earth's magnetic field departs from its dipole shape at distances greater than about 30,000 km.
- This means that while the Earth's magnetic field can be approximated as a dipole (like a bar magnet) close to the Earth, at larger distances, this approximation no longer holds true.

2. **Analyzing the Reason (R)**:
- The reason provided states that at large distances, the Earth's magnetic field is modified due to the motion of ions in the ionosphere.
- The ionosphere is a layer of the Earth's atmosphere that contains a high concentration of ions and electrons, which can affect the magnetic field.

3. **Connecting A and R**:
- The motion of ions in the ionosphere can create additional magnetic fields that interact with the Earth's magnetic field.
- These interactions can lead to distortions in the magnetic field, deviating from the ideal dipole shape.

4. **Considering External Influences**:
- The reason also mentions that the ionosphere is sensitive to extraterrestrial disturbances such as solar wind.
- Solar wind consists of charged particles emitted by the sun, which can further influence the behavior of the ionosphere and, consequently, the Earth's magnetic field.

5. **Conclusion**:
- Both the assertion and the reason are correct. The Earth's magnetic field does indeed depart from a dipole shape at large distances due to the influence of the ionosphere and external factors like solar wind.
- Therefore, the answer to the question is that both the assertion (A) and the reason (R) are correct, and the reason correctly explains the assertion.

### Final Answer:
Both the assertion and the reason are correct, and the reason explains the assertion.

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