Roman numerals in an outline signify
details
main ideas
subdivisions
secondary headings

When writing papers, various types of numerals are used for various purposes. Roman numerals are commonly used to signify main ideas. They would be commonly combined with arabic numerals and uppercase and lower-case letters in perfectly segmenting the paper which is important if you're writing important academic research papers.

The answer is atmosphere. there are four spheres of the earth: 1. geosphere (from greek word 'geo' = ground), 2. hydrosphere (from greek word 'hydro' = water), 3. biosphere (from greek word 'bio' = life), and 4. atmosphere (from greek word 'atmo' = air). a magnetosphere is a region of space around astronomical objects controlled by the magnetic field of that object. through the process of elimination, since lightning is present in the air, it can be found in the atmosphere.