  Observations of transmission spectra reveal that hot Jupiters and Neptunes
are likely to possess escaping atmospheres driven by stellar radiation.
Numerous models predict that magnetic fields may exert significant influences
on the atmospheres of hot planets. Generally, the escaping atmospheres are not
entirely ionized, and magnetic fields only directly affect the escape of
ionized components within them. Considering the chemical reactions between
ionized components and neutral atoms, as well as collision processes, magnetic
fields indirectly impact the escape of neutral atoms, thereby influencing the
detection signals of planetary atmospheres in transmission spectra. In order to
simulate this process, we developed a magneto-hydrodynamic multi-fluid model
based on MHD code PLUTO. As an initial exploration, we investigated the impact
of magnetic fields on the decoupling of H$^+$ and H in the escaping atmosphere
of the hot Neptune GJ436 b. Due to the strong resonant interactions between H
and H$^+$, the coupling between them is tight even if the magnetic field is
strong. Of course, alternatively, our work also suggests that merging H and
H$^+$ into a single flow can be a reasonable assumption in MHD simulations of
escaping atmospheres. However, our simulation results indicate that under the
influence of magnetic fields, there are noticeable regional differences in the
decoupling of H$^+$ and H. With the increase of magnetic field strength, the
degree of decoupling also increases. For heavier particles such as O, the
decoupling between O and H$^+$ is more pronounced. Our findings provide
important insights for future studies on the decoupling processes of heavy
atoms in the escaping atmospheres of hot Jupiters and hot Neptunes under the
influence of magnetic fields.
