The direction of magnetic field at any location on the earth’s surface is commonly specified in terms of ________.
both field declination and field inclination
Understanding Earth's Magnetic Field Direction
The Earth acts like a giant magnet, creating a magnetic field around it. This magnetic field is very important for us, protecting us from harmful solar radiation and also being used for navigation with compasses. At any specific location on the Earth's surface, the magnetic field has both a strength (magnitude) and a direction.
To completely specify the direction of the Earth's magnetic field at a given point, we need two angles. These angles describe the orientation of the magnetic field vector relative to the geographic north and the horizontal plane. These two angles are known as magnetic declination and magnetic inclination.
What is Magnetic Declination?
Magnetic declination is the angle between the geographic north (true north) and the direction indicated by a magnetic compass (magnetic north). Geographic north is the direction towards the geographic North Pole. Magnetic north is the direction towards the Earth's magnetic North Pole, which is not exactly at the same location as the geographic North Pole. Magnetic declination is typically measured in degrees east or west of geographic north.
Think of it this way: If you stand somewhere and point towards true north (like on a map), and then see where your compass needle points, the angle between those two directions is the magnetic declination at your location.
What is Magnetic Inclination?
Magnetic inclination, also known as the angle of dip, is the angle between the Earth's magnetic field lines and the horizontal plane at a specific location. At the magnetic equator, the field lines are nearly horizontal, so the inclination is close to 0 degrees. As you move towards the magnetic poles, the field lines become steeper. At the magnetic North Pole, the field lines point almost vertically downwards (inclination is close to +90 degrees), and at the magnetic South Pole, they point almost vertically upwards (inclination is close to -90 degrees).
Imagine a dip needle (a compass needle free to pivot vertically). The angle it makes with the horizontal is the magnetic inclination.
Why Both are Needed to Specify Direction
The Earth's magnetic field at any point in space can be represented by a vector. A vector in three dimensions requires two angles to specify its direction relative to a reference system. In the case of the Earth's magnetic field, these two angles are:
- Magnetic Declination: Specifies the horizontal direction of the field vector relative to geographic north.
- Magnetic Inclination: Specifies the vertical direction (dip) of the field vector relative to the horizontal plane.
Together, magnetic declination and magnetic inclination uniquely define the direction of the Earth's magnetic field at any location on the surface.
Analyzing the Options
Let's look at the given options:
- Option 1: field declination
Magnetic declination only gives the horizontal angle, not the complete direction of the field in 3D space. - Option 2: field
This term is too general and does not specify how the direction is measured or described. - Option 3: both field declination and field inclination
As explained above, these two angles together define the direction of the magnetic field vector at a point. Declination gives the angle in the horizontal plane, and inclination gives the angle relative to the horizontal plane. This provides the complete 3D direction. - Option 4: horizontal component of the field
The horizontal component is part of the magnetic field vector, and specifying its direction gives only the horizontal orientation (which is related to declination), but not the vertical orientation (inclination) or the strength of the field. It doesn't fully specify the direction of the *total* magnetic field.
Therefore, to completely specify the direction of the magnetic field at a location, both magnetic declination and magnetic inclination are needed.
Revision Table: Geomagnetic Elements
The direction and intensity of the Earth's magnetic field at a point are described by quantities called geomagnetic elements. The main elements are:
| Element | Description |
|---|---|
| Magnetic Declination (D) | Angle between geographic north and magnetic north (horizontal direction). |
| Magnetic Inclination (I) | Angle between the magnetic field vector and the horizontal plane (vertical direction/dip). |
| Horizontal Component (H) | The magnitude of the magnetic field vector projected onto the horizontal plane. |
| Vertical Component (Z) | The magnitude of the magnetic field vector projected onto the vertical direction. |
| Total Magnetic Field (F or B) | The magnitude of the total magnetic field vector at the point. |
These elements are related by mathematical equations:
\( H = F \cos(I) \)
\( Z = F \sin(I) \)
\( F^2 = H^2 + Z^2 \)
\( \tan(I) = Z/H \)
The declination \(D\) relates the horizontal component \(H\) to the geographic north direction.
Additional Information on Earth's Magnetic Field
The Earth's magnetic field is not static; it changes over time. This change is called secular variation. The magnetic poles also drift over time. The Earth's magnetic field is generated primarily by the motion of molten iron in the outer core, a process called the geodynamo.
Navigational charts and systems often provide local magnetic declination information to help users correct their compass readings to find true north. Magnetic inclination is important for scientific study of the Earth's interior and for some applications like directional drilling.
Understanding magnetic declination and inclination is fundamental to fields like navigation, geophysics, and surveying.
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