The ionosphere is the part of the atmosphere that is ionized by solar radiation, and too tenuous to be cooled by contact with other air. It forms the inner edge of the magnetosphere and has practical importance because it reflects radio waves to distant places on Earth.
The ionosphere is generally recognized to have three, sometimes four, layers. The D layer is the innermost layer (approximately 50 to 95 km above the surface of the Earth), and mostly absorbs radio waves. The E layer is the middle layer and influences the propagation of radio waves. The F layer (or F region; approximately 160 and 400 km above the surface of the Earth) consists of layers of increased free-electron density caused by the ionizing effect of solar radiation. The F layer combines into one layer at night in sunlight divides into two layers, the F1 and F2. The F layers are responsible for most skywave propagation of radio, and are thickest and most reflective of radio on the side of the Earth facing the sun.
The ionosphere is a region that contains ions: directly above the mesosphere and directly below the exosphere. Within the thermosphere layer, ultraviolet radiation causes ionization creating the ionosphere (up to 550 km. in the Earth’s atmosphere). The Earth’s ionosphere, though protected from direct solar wind scouring by the magnetosphere (and the Earth’s magnetic field), is a shield of layers that absorbs most energetic wavelengths in the atmosphere. The ionosphere state can be predicted by monitoring sunspots which increase the solar winds. The solar wind’s stream of particles (mostly high-energy protons ~ 500 keV) are ejected from the Sun’s upper atmosphere. The interactions between the solar wind and the ionosphere induces energy into the Earth’s magnetic field (and effects the telluric currents). Scientists believe that the Schumann resonance is due to the space between the surface of the Earth and the ionosphere acting as aresonant cavity that is then excited by energy from lightning strikes.
The physics of the ionosphere and space plasmas where recombination and collisional excitation (i.e. radiative processes) are of interest currently because of it’s not approximately fulfilled: in particular, for the electrons. The assumption of the Maxwell-Boltzmann distribution yield quantitatively wrong results and even prevent a correct qualitative understanding of the physics involved. The open system space tether, which uses the ionosphere, is being researched. The space tether uses plasma contactors and the ionosphere as parts of a circuit to extract energy from the Earth’s magnetic field by electromagnetic induction.
Scientists also are exploring the structure of the ionosphere by bouncing radio waves of different frequencies from it, and using special receivers to detect how the reflected waves have changed from the transmitted waves. Project HAARP (High Frequency Active Auroral Research Program) investigation are focused to “understand, simulate, and control ionospheric processes that might alter the performance of communication and surveillance systems” and started in 1993 for a proposed twenty year experiment. CUTLASS (Co-operative UK Twin Located Auroral Sounding System) researches the ionosphere using radar.
Scientists are also examining the ionosphere by the changes to radio waves from satellites and stars transmitted through. The Arecibo radio telescope located in Puerto Rico, was originally intended to the study of Earth’s ionosphere.