What is the main characteristic of the Van Allen belts?

The main characteristic of the Van Allen belts is that they are filled with charged particles. These belts are regions of space around the Earth, created by the Earth's magnetic field, where high-energy particles, mainly electrons and protons, are trapped. The charged particles originate from solar wind and cosmic rays, and they are held in these belts due to the influence of the Earth's magnetic field.

This phenomenon is critical for understanding various aspects of space weather and its impact on satellite operations, as well as providing insight into the Earth's magnetic environment. The presence of these charged particles can lead to phenomena such as auroras when the particles interact with the Earth's atmosphere.

The other options describing the Van Allen belts do not accurately reflect their nature or composition. For instance, the belts do not primarily consist of gas particles from the Earth's atmosphere, nor are they located within the Earth's crust, which is solid and separate from these regions in space. Dark matter, on the other hand, refers to a hypothetical form of matter that accounts for gravitational effects in the universe but does not pertain to the characteristics of the Van Allen belts.