Surface charging is one of the most common causes of spacecraft anomalies. When and to what potential the spacecraft is charged are two important questions in space weather. Here, for a Chinese geosynchronous navigation satellite, we infer the extreme negative surface charging potentials from the ion differential fluxes measured by a low-energy ion spectrometer. Without the solar eclipse effect away from the midnight, the charging potentials are found to have a negative limit which is determined by the maximum SuperMAG electrojet index in the preceding 2 hr. Such an empirical relation can be reasonably explained by the dependence of 1--50 keV electron fluxes on substorm strength. Similar relations may also exist for other inner magnetospheric spacecraft in the non-eclipse region, which would be useful for spacecraft engineering and space weather alerts.

In preparation for future human habitats on Mars, it is important to understand the Martian radiation environment. Mars does not have an intrinsic magnetic field and Galactic cosmic ray (GCR) particles may directly propagate through and interact with its atmosphere before reaching the surface and subsurface of Mars. However, Mars has many high mountains and low-altitude craters where the atmospheric thickness can be more than 10 times different from one another. We thus consider the influence of the atmospheric depths on the Martian radiation levels including the absorbed dose, dose equivalent and body effective dose rates induced by GCRs at varying heights above and below the Martian surface. The state-of-the-art Atmospheric Radiation Interaction Simulator (AtRIS) based on GEometry And Tracking (GEANT4) Monte Carlo method has been employed for simulating particle interactions with the Martian atmosphere and terrain. We find that higher surface pressures can effectively reduce the heavy ion contribution to the radiation, especially the biologically weighted radiation quantity. However, enhanced shielding (both by the atmosphere and the subsurface material) can considerably enhance the production of secondary neutrons which contribute significantly to the effective dose. In fact, both neutron flux and effective dose peak at around 30 cm below the surface. This is a critical concern when using the Martian surface material to mitigate radiation risks. Based on the calculated effective dose, we finally estimate some optimized shielding depths, under different surface pressures (corresponding to different altitudes) and various heliospheric modulation conditions. This may serve for designing future Martian habitats.

In the past, few studies have attempted to determine some of the internal properties of Coronal Mass Ejections (CMEs), which were limited to a certain position or a certain time. For understanding the evolution of internal thermodynamic state of CMEs during their heliospheric propagation, we improve the self-similar flux rope internal state (FRIS) model, which is constrained by measured propagation and expansion speed profiles of a CME. We implement the FRIS model to a CME erupted on 2008 December 12 and probe the internal state of the CME. We find that the polytropic index of the CME plasma decreased continuously from 1.8 to 1.35 as the CME propagated out from 6 Rs to 14 Rs, implying that the CME released heat before it reached adiabatic state and then absorbed heat. We also estimate the entropy changing and heating rate of the CME. Our results show that the thermal force inside the CME is the internal driver of CME expansion while Lorentz force prevented the CME from expanding. It is noted that centrifugal force originated due to poloidal motion of the plasma inside the flux rope decreased with fastest rate and Lorentz force decreased slightly faster than thermal pressure force as the CME moved away from the Sun. We further extrapolate the internal thermodynamic properties of the CME up to 1 AU and compare with in situ observations. The limitations of the model and approximations made in the study would also be discussed.

Magnetic flux ropes (MFRs) are believed to be one kind of fundamental structures in solar eruptions, of which the formation is being intensely studied. Here we report a rapid buildup process of an MFR during a confined X2.2 class flare that occurred on 2017 September 6 in NOAA AR 12673, three hours after which the MFR erupted as a major coronal mass ejection (CME) accompanied by an X9.3 class flare. For the X2.2 class flare, we do not find separation of the two flare ribbons and clear CME signature in the coronagraph images, suggesting it to be confined. For the X9.3 class flare, apparent separation of the two ribbons and a CME show it to be an eruptive one. We perform a time sequence of nonlinear force-free fields (NLFFFs) extrapolations covering the two flares and find that: although the flux-weighted mean twist number of the MFR was almost unchanged prior to the eruptive flare, the axial flux and magnetic helicity of the MFR were dramatically enhanced after the confined flare, as much as 86% for the former and 260% for the latter. The above three parameters were all significantly reduced after the eruptive flare. It clearly evidences the buildup and release of the MFR during the confined and eruptive flare, respectively. The buildup of the MFR may be achieved by reconnection during the confined flare. We also calculate the pre-flare distributions of the decay index above the main polarity inversion line (PIL) and do not find any significant difference. It indicates that the enhancement of the MFR flux may play a role in facilitating the subsequent successful eruption.