  TianQin is a proposed space-based gravitational wave detector designed to
operate in circular high Earth orbits. As a sequel to [Zhang et al. Phys. Rev.
D 103, 062001 (2021)], this work provides an analytical model to account for
the perturbing effect of the Earth's gravity field on the range acceleration
noise between two TianQin satellites. For such an ``orbital noise,'' the
Earth's contribution dominates above $5\times 10^{-5}$ Hz in the frequency
spectrum, and the noise calibration and mitigation, if needed, can benefit from
in-depth noise modeling. Our model derivation is based on Kaula's theory of
satellite gravimetry with Fourier-style decomposition, and uses circular
reference orbits as an approximation. To validate the model, we compare the
analytical and numerical results in two main scenarios. First, in the case of
the Earth's static gravity field, both noise spectra are shown to agree well
with each other at various orbital inclinations and radii, confirming our
previous numerical work while providing more insight. Second, the model is
extended to incorporate the Earth's time-variable gravity. Particularly
relevant to TianQin, we augment the formulas to capture the disturbance from
the Earth's free oscillations triggered by earthquakes, of which the mode
frequencies enter TianQin's measurement band above 0.1 mHz. The analytical
model may find applications in gravity environment monitoring and
noise-reduction pipelines for TianQin.
