Incoherent Scatter

The incoherent scatter mechanism

Incoherent Scatter (IS) radar is the most powerful ground based technique for the study of the Earth's ionosphere and its interactions with the upper atmosphere, the magnetosphere and the interplanetary medium (solar wind).

The 'incoherent' scatter echo is the result of the scattering of electromagnetic energy, radiated from the radar, by electrons in the ionospheric plasma, which are themselves controlled by the much slower, massive positive ions. Most of the power is due to scattering from electron density fluctuations caused by the presence of these ions and the frequency spectrum of the received signal provides information about their temperature, composition and velocity. In addition, the typically double-peaked ion-line spectrum of the incoherent scatter echo also contains information about the temperature of the electrons themselves. Besides the "ion-line", the incoherent scatter spectrum also contains other, normally weaker, components including two "plasma-" or "electron-lines" representing scattering processes where the electrons act as if the ions were absent.

The total returned power depends on the number of electrons and gives an estimate of the ionospheric electron density; the width of the spectrum depends on the ratio of the ion temperature to ion mass and the overall shift of the spectrum corresponds to the bulk motion of the ions. The shape of the ion line spectrum is a sensitive function of the ratio of the electron and ion temperatures.

With suitable assumptions about the concentrations of different ions in the ionosphere, the basic parameters of electron density, electron temperature, ion temperature and ion velocity are routinely derived by all Incoherent Scatter radars. Atitudes below about 120 km, collisions between the ions and the neutral atmosphere affect the incoherent scattering process and result in a single-humped spectrum from which the frequency of ion collisions with the neutral molecules of the upper atmosphere can also be deduced.

From these basic results, many further ionospheric and upper atmospheric parameters can be deduced, though not all together nor in all altitude regimes. These include: ion composition, electric field strength, conductivity and current, Joule and particle heating rates, neutral air temperature, composition and wind speed, fluxes of heat and plasma along the Earth's magnetic field lines and, with additional information from the electron lines, electric current density in the direction of the magnetic field and part of the spectrum of supra-thermal electrons.

Additionally, powerful radars such as EISCAT can observe scattering from ionization irregularities set up by plasma and neutral atmosphere instabilities, allowing a range of E-region and mesospheric phenomena to be observed directly. These observations, and planned expansions to study tropospheric and stratospheric phenomena, have recently added a substantial new component to the observing programmes of several radars, particularly in the case of the EISCAT VHF system where the capabilities of the radar support important investigations into the coupling between the lower atmosphere and the ionosphere.

Incoherent scatter radars

Typical Incoherent Scatter radars radiate effective powers measured in gigawatts, but the returned signals normally represent only picowatts.

Powerful multi-mega-watt transmitters, large high-gain antennas (typically at least 1000 m2 in area), sensitive receivers and sophisticated radar control and data acquisition systems are all necessary for the sucful detection and evaluation of the weak incoherent scatter echoes received from the ionosphere.

The EISCAT radars can currently make measurements covering all altitudes from about 50 km to more than 2500 km. Structures smaller than 200 m can be resolved and studied on time scales ranging from tens of minutes to well under a second, depending on the geophysical phenomena. The measurements describe many of the fundamental characteristics of the ionosphere and upper atmosphere and support a wide range of scientific investigations aimed at understanding the complex effects of the Sun on our environment.

Incoherent Scatter radar systems provide a wealth of observational data and are complemented by detailed observations from balloons, rockets and satellites as well as a wide range of ground-based instruments including magnetometers, all-sky cameras, ionosondes and coherent (auroral) backscatter radars. Incoherent Scatter radars have attracted many such instruments to their vicinity and will continue to provide the focus of substantial research efforts for the foreseeable future.

Ionospheric Heating

In addition to studies of the natural ionosphere and neutral atmosphere, a range of ionospheric modification experiments, performed to improve our understanding of fundamental plasma processes, are also conducted using powerful high- frequency transmitters located close to the EISCAT radar site in Tromsų. This 'Heating' facility is an integral part of the EISCAT Scientific Association.