Virtual instrument is the collection of data obtained from one instrument or resultat of processing of some data which can be written as a sequence of data records associated with time sequence.
For example : magnetic field vector with time of measurements or electron energy spectra (array) with time of measurements.
Each real space instrument can be divided in several virtual instruments. Auxillary data (orbits, attitudes, calibration coefficients (if they depend on time) are virtual instruments as well.