There are multiple techniques that can be employed to study the Sun and solar wind. Remote-sensing observations include radio and visible/white-light observations of the Sun from the ground, and observations in the ultraviolet (UV) and X-ray bands from space along with any other parts of the electromagnetic (EM) spectrum. The density irregularities in the solar wind can also be observed using ground-based or space-based coronagraphs for the inner solar wind as it emerges from the Sun's corona, by space-based white-light heliospheric imagers, and by radio measurements of distant compact radio sources to indirectly observe the solar wind through scintillation in the radio signal received from distant astronomical or artificial radio sources.

In-situ measurements of solar-wind velocity, density, magnetic field, temperature, and other plasma and field parameters are also of high importance. An advantage of in-situ measurements is that the physical parameters recorded are measurements of the primary solar wind parameters at that point in space and time. However, the obvious disadvantage to this is that measurements can be made only at that particular position of the spacecraft at certain times. Some regions are only accessed rarely, if ever; for example, in-situ measurements inside of 0.3AU (the perihelion of the orbit of the planet Mercury) or outside of the ecliptic plane of the solar system are not available at present. Only the Ulysses spacecraft data set is available for past years outside of the ecliptic (1990 launch, mission data from 1994 to 2009) and some measurements are available from the twin Helios spacecraft both outside of the ecliptic and just inside the orbit of Mercury (1974 and 1976 launches, mission data from 1975 to 1985). Remote-sensing observations have the advantage of covering the solar wind over a wide range of heliocentric distances and all heliographic latitudes (including out of the ecliptic plane) and at almost any time; this is especially true of radio observations. The disadvantage to remote-sensing observations is that the primary parameters are not measured but are inferred from the observations.

Observing radio waves

The radio spectrum itself is broken down into various segments as defined through radio astronomy (see Table 13.1). It is important to remember that frequencies below about 10MHz (sometimes as high as below 30 MHz, …