The medium you refer to is the media referred to in the third article in the OP.There is an acceptable term for medium, that is the intergalactic medium, or IGM for short.
'NASA's Voyager Hits New Region at Solar System Edge'
"Voyager is showing that what is outside is pushing back. ... Like cars piling up at a clogged freeway off-ramp, the increased intensity of the magnetic field shows that inward pressure from interstellar space is compacting it."
It is not the particles of matter which exist in quantities less than in any vacuum artificially created on Earth which are pushing back and exerting inward pressure toward the solar system.
If the three articles I linked to are corect then it is the dark matter, which the particles of matter exist in, which is the interstellar medium. It is the dark matter which is displaced by the matter the solar system consists of which is pushing back and exerting inward pressure toward the solar system.
"a "field" in physics may be envisioned as if space were filled with interconnected vibrating balls and springs, and the strength of the field can be visualized as the displacement of a ball from its rest position"
A "field" in physics is space filled with dark matter, and the strength of the field is the displacement of the dark matter from its relativisitc rest position. This is what is referred to in the third article as the deformation of continuous media. The media is the intergalactic medium.