Which of the following is independent quantities is not used to specify the earth's magnetic field?

Options

A. Magnetic declination `(theta)`
B. Magnetic field `(delta)`
C. Horizontal component of earth's field `(B_(H))`
D. Vertical component of earth's field `(B_(V))`

Explanation

To determine which independent quantity is not used to specify the Earth's magnetic field, we will analyze each option provided in the question step by step.

### Step 1: Understand Magnetic Declination
- **Definition**: Magnetic declination (denoted as θ) is the angle between the magnetic north (the direction a compass points) and true north (the direction of the geographic North Pole).
- **Usage**: It is used to correct compass readings and is an important parameter in defining the Earth's magnetic field.

### Step 2: Understand Magnetic Dip
- **Definition**: Magnetic dip (or magnetic inclination) is the angle that the Earth's magnetic field makes with the horizontal plane at a given location.
- **Usage**: This quantity is significant in characterizing the magnetic field because it indicates how much the magnetic field lines are inclined.

### Step 3: Understand Horizontal Component of Earth's Magnetic Field
- **Definition**: The horizontal component of the Earth's magnetic field (denoted as B_H) is the component of the magnetic field that lies in the horizontal plane.
- **Usage**: This component is crucial for navigation and is used to describe the magnetic field in a specific area.

### Step 4: Understand Vertical Component of Earth's Magnetic Field
- **Definition**: The vertical component of the Earth's magnetic field is the component that acts perpendicular to the horizontal plane.
- **Usage**: While it is a part of the magnetic field, it is not typically used as an independent quantity to specify the Earth's magnetic field.

### Conclusion
After analyzing all four options, we find that the vertical component of the Earth's magnetic field is the quantity that is not used as an independent specification of the Earth's magnetic field. Therefore, the correct answer is:

**D. Vertical component of Earth's magnetic field**