تعاملات پلاسمایی خارجی با اجسام غیر مغناطیسی در سیستم خورشیدی
Abstract: The absence of a protecting magnetic field, such as the dipole magnetic field around Earth, makes the interaction of solar wind with unmagnetized objects particularly interesting. Long-term evolution of the object’s surface and atmosphere is closely tied to its interaction with the outer space environment. The ionospheric plasma layer around unmagnetized objects acts as an electrically conducting transition layer between lower atmospheric layers and outer space. This study considers two distinct types of unmagnetized objects: Titan and comet 67P/Churyumov-Gerasimenko (67P/CG). For many years, Titan has been a key target of the National Aeronautics and Space Administration (NASA) Cassini mission investigations; and the European Space Agency (ESA) Rosetta spacecraft explored comet 67P/CG for more than two years. Ionospheric composition and primary ion production rate profiles for Titan are modeled for various solar activity conditions. Photoionization is the main source of ion production on the dayside; on the nightside, electron-impact ionization is the main ionization source. This dissertation uses model results and in-situ measurements by the Ion and Neutral Mass Spectrometer (INMS) and the Langmuir Probe (LP) onboard the Cassini spacecraft to show that while the solar activity cycle impacts the primary ion species significantly, there is little effect on heavy ion species. Solar cycle modulates the Titan’s ionospheric chemistry. The solar cycle effects of on each ion species are quantified n this work. In some cases the solar zenith angle significantly overshadows the solar cycle effects. How each individual ion reacts to changes in solar activity and solar zenith angle is discussed in details. A method to disentangle these effects in ion densities is introduced. At comet 67P/CG, the fast-moving solar wind impacts the neutral coma. Two populations of electrons are recognizable in the cometary plasma. These are the hot suprathermal electrons, created by photoionization or electron-impact ionization, and the cold/thermal electrons. Even though photoionization is the dominant source of ion production, electron-impact ionization can be as high as the photoionization for certain solar events. At 3 AU, electron energy spectra from in-situ measurements of the Ion and Electron Sensor (IES) instrument exhibit enhancement of electron fluxes at particular energies. Model-data comparisons show that the flux of electrons is higher than the typical solar wind and pure photoionization fluxes. The probable cause of this enhancement is the ambipolar electric field and/or plasma compression. This research also discusses formation of a new boundary layer around the comet near perihelion, similar to the diamagnetic cavity at comet 1P/Halley. At each crossing event to the diamagnetic cavity region, flux of suprathermal electrons with energies between 40 to 250 eV drops. The lower flux of solar wind suprathermal electrons in that energy range can cause this flux drop.