  TianQin is a proposed space-borne gravitational wave (GW) observatory
composed of three identical satellites orbiting around the geocenter with a
radius of $10^5$ km. It aims at detecting GWs in the frequency range of 0.1 mHz
-- 1 Hz. The detection of GW relies on the high precision measurement of
optical path length at $10^{-12}$~m level. The dispersion of space plasma can
lead to the optical path difference (OPD, $\Delta l$) along the propagation of
laser beams between any pair of satellites. Here, we study the OPD noises for
TianQin. The Space Weather Modeling Framework is used to simulate the
interaction between the Earth magnetosphere and solar wind. From the
simulations, we extract the magnetic field and plasma parameters on the orbits
of TianQin at four relative positions of the satellite constellation in the
Earth magnetosphere. We calculate the OPD noise for single link, Michelson
combination, and Time-Delay Interferometry (TDI) combinations ($\alpha$ and
$X$). For single link and Michelson interferometer, the maxima of $|\Delta l|$
are on the order of 1 pm. For the TDI combinations, these can be suppressed to
about 0.004 and 0.008 pm for $\alpha$ and $X$. The OPD noise of the Michelson
combination is colored in the concerned frequency range; while the ones for the
TDI combinations are approximately white. Furthermore, we calculate the ratio
of the equivalent strain of the OPD noise to that of TQ, and find that the OPD
noises for the TDI combinations can be neglected in the most sensitive
frequency range of TQ.
