Of course there are imagers looking at pictures of aurora and airglow, there are lidar radars that send up very precise and short light pulses that then reflect back from different layers, there are passive measurements, there are the Esrange rocket launches and balloons. But with the EISCAT radars were mostly looking at radio waves. We’re measuring what is called geospace so near space and we’re looking in a specific location of that, but it's really a global system. Energy gets thrown into the atmosphere and causes these waves that propagate out and sometimes the waves interact with the plasma and cause these irregularities that make gps measurements hard because the gps signals have to go through this ionised plasma. Most of what we’re studying is the ionosphere, 70km and up, where you have auroras. Auroras follow the solar cycles. They are also there in the summer but you don’t see it. That's one of the nice things with radar you can see the effects of the aurora the whole year round, not just when it's dark in winter. We’re looking at the ionised portion of the earth’s atmosphere. We can say something about the neutral atmosphere as well but what we directly study is ionised part. There are a lot of different kinds of physics that can be studied here.

— Craig Heinselman, EISCAT Director in discussion with HH and AM, 2018, IRF, Kiruna

A radar is an active sensor. Using acoustic waves, electronic beams, electromagnetic waves or other flows of energy or particle flow a radar ‘pokes’ the object of study in order to then reconstruct its properties from the modification of the flow. Active sensing was developed to study the characteristics of the ionosphere, an electrically active region in the upper atmosphere, situated 70–80 kilometers above the surface of the earth. Guglielmo Marconi’s transatlantic test in 1901 had shown that there was an atmospheric buffer zone that propagated radio waves, making it possible to bounce radio waves from one hemisphere to another
(see Colours, Communication, Commons).  The discovery of the ionosphere spurred a broad interest in radio techniques and opened a new field in earth sciences through which the upper atmosphere became an object of experimentation, and not only observation undercutting traditional distinctions between field and laboratory sciences.

Building upon propagation studies, atmospheric radio sounding was developed for sending radio waves to the ionosphere and detecting their return. While sounding-echo experiments could not modify the ionosphere, it could design and manipulate the transmitted radio waveforms so that their return scattering or deflection could be measured more clearly. Active sensing brought new meanings to experimentation, and introduced a distinct approach to probing the atmosphere that involved instrument design as well as novel modes of experimentation, that instead of manipulating the object of enquiry changed features, such as frequency and waveform of the wave or particle being emitted by the active sensor. By bombarding its object with waves or particle beams and by imaging the returning signals, active sensing emerged as a way of seeing the world through electromagnetic waves (see Posthuman perspective). This shift profoundly affected scientific research, but importantly also warfare, and the development of military and commercial communication technologies.

︎P ...︎Q - S︎... C︎