If you ‘google’ ACTIVE EXPERIMENTS IN THE IONOSPHERE, you will get over 350K results, including the following:
by Michael Mendillo, Jeffrey Baumgardner and Peter J. Sultan, Center for Space Physics, Boston University, Boston MA.

I.1  Introduction to ACTIVE EXPERIMENTS: “…barium plasma injections in the ionosphere or magnetosphere result in observable effects… Ionospheric heating effects generally last as long as the heating is applied, often in the 10’s of minutes domain. The end result is that active experiments require a comprehensive set of multi-diagnostic instruments that can handle the small time constants associated with the induced effects. …[the Chapter 9, page 250 pdf concentrates] on the chemical release experiments used in ionospheric investigations…

I.2  Types of Chemical Release Experiments
Chemically induced ionospheric modification experiments involve either the enhancement of ambient plasma densities or their depletion. …the most widely used technique involves the release of barium …an element that is ionized by solar ultraviolet… Strontium (Sr) vapor has been used in conjunction with Ba [barium] release experiments …releases of other species such as trimethyl aluminum (TMA), cesium, calcium, etc.
The second type of chemical release experiment involves the release of molecules that react very rapidly with ambient ionospheric plasma…These…modify the ionosphere via recombination chemistry (plasma neutralization) that leaves the ionospheric F-region plasma in a depleted (“ionospheric hole”) state.

II.  Incoherent Scatter Radar Techniques Applied to Active Experiments
Incoherent scatter radar…is based on the fact that an upwardly propagating radio wave that is higher in frequency than the plasma frequency anywhere along its path will be weakly re-radiated by the free electrons of the ionosphere … Incoherent scatter radar is a unique and ideal diagnostic instrument…a typical chemical release cloud from a sounding rocket payload reaches an approximate maximum radius of …50 km…

…in 1985…the Space Shuttle’s OMS engines were used to generate an ionospheric hole above New England… [one wonders how many such “holes” have been generated in the ionosphere since 1985!]
Arecibo in Puerto Rico is an Incoherent Scatter Radar Facility.

A problem unique to ACTIVE EXPERIMENTS is that the composition of the observed ionospheric plasma is changed by the released species, and the radar is often faced with ions not usually present in the natural ionosphere. A spectral analysis program that is not expected to handle unusual ions will misinterpret the presence of those ions as changes in other parameters (i.e., temperature). [In my words they are creating false ‘compositions’ with these injections and analyzing mixtures of ions that don’t exists in the natural ionosphere!]

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