During magnetospheric substorms, plasma from magnetic reconnection in the magnetotail is thought to reach the inner magnetosphere and form a partial ring current. We simulate this process using a fully kinetic 3D particle-in-cell (PIC) numerical code along with a global magnetohydrodynamics (MHD) model. The PIC simulation extends from the solar wind outside the bow shock to beyond the reconnection region in the tail, while the MHD code extends much further and is run for nominal solar wind parameters and a southward interplanetary magnetic field. By the end of the PIC calculation, ions and electrons from the tail reconnection reach the inner magnetosphere and form a partial ring current and diamagnetic current. The primary source of particles to the inner magnetosphere is bursty bulk flows (BBFs) that originate from a complex pattern of reconnection in the near-Earth magnetotail at xGSM=-18 RE to -30 RE. Most ion acceleration occurs in this region, gaining from 10 to 50 keV as they traverse the sites of active reconnection. Electrons jet away from the reconnection region much faster than the ions, setting up an ambipolar electric field allowing the ions to catch up after approximately 10 ion inertial lengths. The initial energy flux in the BBFs is mainly kinetic energy flux from the ions, but as they move earthward, the energy flux changes to enthalpy flux at the ring current. The power delivered from the tail reconnection in the simulation to the inner magnetosphere is >2x1011 W, which is consistent with observations.

Using the magnetic field observed by Galileo during two flybys of Callisto, Khurana et al. (1998) demonstrated that Callisto generates a strong induction response to the time-varying primary field, indicative of the presence of a subsurface ocean. In contrast, Hartkorn and Saur (2017) modeled the atmosphere and ionosphere of Callisto and suggested that the ionosphere could be responsible for a significant part of the observed magnetic fields. Thus, they concluded that the water ocean might be located much deeper than previously thought or might not exist at all. While Khurana et al. (1998) did not account for the induction within a conductive ionosphere, Hartkorn and Saur (2017) overestimated the conductivity of the ionosphere by using Cowling conductivity which is not applicable for the situation at Callisto. In this paper, we re-analyzed the S-band open-loop one-way Doppler data of the Galileo spacecraft with the aim to derive the electron density (ED) and neutral density (ND) profiles of Callisto and address its implication in terms of moon’s conductivities and interiors. Using modern orbit determination software, MONTE, and the most up-to-date information on the Jovian system, we reconstructed the Galileo orbit with a full dynamical approach. The estimated rms values of the Doppler residuals for baseline measurement vary from 0.01-0.08 Hz, well within the expected noises of the radio signals. We used these residuals to derive the ED profiles using the technique discussed in Verma et al., (2019). We found an appreciable ionosphere for C22 and C23 Ingress occultations with peak densities of 15600±900 cm-3 and 17700±600 cm-3, respectively. For other cases, the detections do not exceed the 3-σ level. While the general features of the EDs are consistent with Kliore et al. (2002), our estimated 1-σ formal uncertainties are 2-3 times better presumably because of the constrained Galileo’s orbit. Assuming O2 as the major component of the Callisto’s atmosphere, the estimated ND (weighted mean) at the surface is 2.0±0.33 x10-10 cm-3 which corresponds to a column density of 3.9±0.35 x10-16 cm-2 (see Figure). Finally, we will use these density profiles to constrain the ionospheric conductivities and address their implications in terms of the presence of a subsurface ocean.