I understand the electromagnetic spectrum travels below speed c in Earth’s atmosphere. Is there any part of the spectrum which remains at speed c, or indeed anything which travels at speed c in the atmosphere?
The atmospheric effect on the speed of electromagnetic radiation becomes more and more negligible the higher the energy of the incoming photon. gamma rays will travel very close to c through the atmosphere.
Most portions of the EM spectrum have a phase velocity in excess of c.
Some portions I suspect have a group velocity in excess of c, but are highly absorbing at these wavelengths - so you ultimately cannot trasmit information faster than c.