  The TianQin space Gravitational Waves (GW) observatory will contain 3
geocentric and circularly orbiting spacecraft with an orbital radius of 10^5
km, to detect the GW in the milli-hertz frequency band. Each spacecraft pair
will establish a 1.7*10^5 km-long laser interferometer immersed in the solar
wind and the magnetospheric plasmas to measure the phase deviations induced by
the GW. GW detection requires a high-precision measurement of the laser phase.
The cumulative effects of the long distance and the periodic oscillations of
the plasma density may induce an additional phase noise. This paper aims to
model the plasma induced phase deviation of the inter-spacecraft laser signals,
using a realistic orbit simulator and the Space Weather Modeling Framework
(SWMF) model. Preliminary results show that the plasma density oscillation can
induce the phase deviations close to 2*10^-6 rad/Hz^1/2 or 0.3pm/Hz^1/2 in the
milli-hertz frequency band and it is within the error budget assigned to the
displacement noise of the interferometry. The amplitude spectrum density of
phases along three arms become more separated when the orbital plane is
parallel to the Sun-Earth line or during a magnetic storm. Finally, the
dependence of the phase deviations on the orbital radius is examined.
