Kinetic nature of Jovian magnetodisk

Jovian magnetosphere has a huge equatorial plasma disk, which is also known as the current sheet or magnetodisk. This current sheet enlarges the subsolar magnetosphere size more than twice compared to a purely planetary dipole magnetosphere. Past and current space missions, such as Galileo, JUNO, etc. have shown showed the presence of different sorts of particles in the plasma disk that emphasizes the multi-kinetic nature of Jovian magnetodisk.

In our work we develop a multi-spices self-consistent model of the Jovian plasmadisk based on the kinetic approach. As the initial magnetic field model we use a model of infinitely thin magnetodisk (Alexeev & Belenkaya, 2005). Calculations revealed that different spices contribute to the current at different scales, also it was possible to show that different particles with different energies and pitch angles can significantly increase the electric current locally. The kinetic scales of the self-consistent plane current sheets have been studied by Sasunov et al. (2015, 2021) and we, following these trajectories methods, generalized the results to the geometry of the radial plasma outflow.

Shortly a plan of the study is:

1. We start from test particles calculation, in which magnetic field is determined as a sum of the Jupiter's dipole and equatorial disk current's field from the Alexeev and Belenkaya (2005). The described region is located from the planet, R_J, till spherical surface with radius about 100 R_J. The main attention is focused on the middle magnetosphere between 20 R_J and 60 R_J. As demonstrated by our analysis, near to the equatorial plane the magnetospheric field can be described by the simple model with opposite direction of the B_ρ and B_φ in the northern and southern hemispheres. Both components of the field vector, as well as normal component B_θ, decrease with distance ρ as ρ^-1. As a result, the same behavior will have an azimuthal and radial currents in the disk. The field lines inclination angle to equatorial plane is small and approximately constant (about 20°).

2. Outside of the plasma disk (z> D, where D~2.5 R_J is a disk thickness) the adiabatic approach is valid, and we can calculate the moments of the distribution function f(r,v) in a drift approximation. Out of the disk the electric current created by particles is small, but near the disk (z< D) the particles with small pitch angles form the current. If particle density is high enough, then 2B_ρ=μ₀ J_φ, where J_φ=∫ev_φf(r,v)dГ, where dГ is a volume element of the phase space. So, the structure is self-consistent. The plasma sources include both iogenic and ionospheric (upper thermospheric) cold ions, which are accelerated by electric field at the disk region. Finally, the plasma velocity is about Alfvenic velocity.

3. In numerical calculations we control a phase position at z=0 (in the equatorial plane).

4. Comparison of the results with trajectories which can be found from condition of the particle magnetic moment conversation.

5. Azimuthal symmetry leads to conversation of the general moment of particle and this integral of motion limits the radial distance interval, in which the equatorial distances of the guiding center field lines are confined.

Figure 1. 3D trajectories of particles with different phase angles, crossing the infinitely thin current sheet at z=0 (coloured red and brown). Magnetic field line is shown (purple dotted line). Here, six particles with small pitch-angles, which crossed equatorial plane 4 times before reflection from equatorial plane (3 particles) or going to bottom hemisphere (also 3 particles) are shown. All particles have the same initial pitch-angle but different Larmor phases. The destiny of particles depends on the phase value, but a final (after moving away from the disk) pitch angle is the same and is determined by magnetic moment conservation.

References

1) Alexeev, I. I. and Belenkaya, E. S.: Modeling of the Jovian Magnetosphere, Ann. Geophys., 23, 809–826, https://doi.org/10.5194/angeo-23-809-2005, 2005.

2) Sasunov, Y. L., Khodachenko, M. L., Alexeev, I. I., Belenkaya, E. S., Semenov, V. S., Kubyshkin, I. V., and Mingalev, O. V. (2015), Investigation of scaling properties of a thin current sheet by means of particle trajectories study. J. Geophys. Res. Space Physics, 120, 1633– 1645. doi: 10.1002/2014JA020486.

3) Yu. L. Sasunov, M. L. Khodachenko, I. V. Kubyshkin, N. Dwivedi, I. I. Alexeev, E. S. Belenkaya, H. V. Malova, and N. Kulminskaya, "Transient particle acceleration by a dawn–dusk electric field in a current sheet", Physics of Plasmas 28, 042902 (2021) https://doi.org/10.1063/5.0037060

Acknowledgements

This research work is partly supported by the joint RFBR and DFG grant No 21-52-12025\21. Authors thanks the Europlanet RI 2024 grant and virtual observatory VESPA for cooperations.