  For each photon wave packet of extragalactic light, the dispersion by
line-of-sight intergalactic plasma causes an increase in the envelope width and
a chirp (drift) in the carrier frequency. It is shown that for continuous
emission of many temporally overlapping wave packets with random epoch phases,
such as quasars in the radio band, this in turn leads to quasi-periodic
variations in the intensity of the arriving light on timescales between the
coherence time (defined as the reciprocal of the bandwidth of frequency
selection, taken here as of order 0.01 GHz for radio observations) and the
stretched envelope, with most of the fluctuation power on the latter scale
which is typically in the millisecond range for intergalactic dispersion. Thus,
by monitoring quasar light curves on such short scales, it should be possible
to determine the line-of-sight plasma column along the many directions and
distances to the various quasars, affording one a 3-dimensional picture of the
ionized baryons in the near universe.
