External Plasma Interactions with Nonmagnetized Objects in the Solar System

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

Solar wind magnetic holes can cross the bow shock and enter the magnetosheath

Solar wind magnetic holes are localized depressions of the magnetic field strength, on timescales of seconds to minutes. We use Cluster multipoint measurements to identify 26 magnetic holes which are observed just upstream of the bow shock and, a short time later, downstream in the magnetosheath, thus showing that they can penetrate the bow shock and enter the magnetosheath. For two magnetic holes, we show that the relation between upstream and downstream properties of the magnetic holes are well described by the MHD (magnetohydrodynamic) Rankine&ndash;Hugoniot (RH) jump conditions. We also present a small statistical investigation of the correlation between upstream and downstream observations of some properties of the magnetic holes. The temporal scale size and magnetic field rotation across the magnetic holes are very similar for the upstream and downstream observations, while the depth of the magnetic holes varies more. The results are consistent with the interpretation that magnetic holes in Earth's and Mercury's magnetosheath are of solar wind origin, as has previously been suggested. Since the solar wind magnetic holes can enter the magnetosheath, they may also interact with the magnetopause, representing a new type of localized solar wind&ndash;magnetosphere interaction. &nbsp;</p

Estimating the solar wind pressure at comet 67P from Rosetta magnetic field measurements

Aims: The solar wind pressure is an important parameter of space weather, which plays a crucial role in the interaction of the solar wind with the planetary plasma environment. Here we investigate the possibility of determining a solar wind pressure proxy from Rosetta magnetic field data, measured deep inside the induced magnetosphere of comet 67P/Churyumov-Gerasimenko. This pressure proxy would be useful not only for other Rosetta related studies but could also serve as a new, independent input database for space weather propagation to other locations in the Solar System. Method: For the induced magnetospheres of comets the magnetic pressure in the innermost part of the pile-up region is balanced by the solar wind dynamic pressure. Recent investigations of Rosetta data have revealed that the maximum magnetic field in the pile-up region can be approximated by magnetic field measurements performed in the inner regions of the cometary magnetosphere, close to the boundary of the diamagnetic cavity, from which the external solar wind pressure can be estimated. Results: We were able to determine a solar wind pressure proxy for the time interval when the Rosetta spacecraft was located near the diamagnetic cavity boundary, between late April 2015 and January 2016. We then compared our Rosetta pressure proxy to solar wind pressure extrapolated to comet 67P from near-Earth. After the exclusion of disturbances caused by transient events, we found a strong correlation between the two datasets

A Single Deformed Bow Shock for Titan-Saturn System

During periods of high solar wind pressure, Saturn's bow shock is pushed inside Titan's orbit exposing the moon and its ionosphere to the solar wind. The Cassini spacecraft's T96 encounter with Titan occurred during such a period and showed evidence for shocks associated with Saturn and Titan. It also revealed the presence of two foreshocks: one prior to the closest approach (foreshock 1) and one after (foreshock 2). Using electromagnetic hybrid (kinetic ions and fluid electrons) simulations and Cassini observations, we show that the origin of foreshock 1 is tied to the formation of a single deformed bow shock for the Titan‐Saturn system. We also report the observations of a structure in foreshock 1 with properties consistent with those of spontaneous hot flow anomalies formed in the simulations and previously observed at Earth, Venus, and Mars. The results of hybrid simulations also show the generation of oblique fast magnetosonic waves upstream of the outbound Titan bow shock in agreement with the observations of large‐amplitude magnetosonic pulsations in foreshock 2. We also discuss the implications of a single deformed bow shock for new particle acceleration mechanisms and also Saturn's magnetopause and magnetosphere

Drivers of magnetic field amplification at oblique shocks: in-situ observations

Collisionless shocks are ubiquitous structures throughout the universe. Shock waves in space and astrophysical plasmas convert the energy of a fast-flowing plasma to other forms of energy, including thermal and magnetic energies. Plasma turbulence and high-amplitude electric and magnetic fluctuations are necessary for effective energy conversion and particle acceleration. We survey and characterize in-situ observations of reflected ions and magnetic field amplification rates at quasi-perpendicular shocks under a wide range of upstream conditions. We report magnetic amplification factors as high as 25 times the upstream magnetic field in our current dataset. Reflected ions interacting with the incoming plasma create magnetic perturbations which cause magnetic amplification in upstream and downstream regions of quasi-perpendicular shocks. Our observations show that in general magnetic amplification increases with the fraction of reflected ions, which itself increases with Mach number. Both parameters plateau once full reflection is reached. Magnetic amplification continuously increases with the inverse of the magnetization parameter of the upstream plasma. We find that the extended foot region upstream of shocks and nonlinear processes within that region are key factors for intense magnetic amplification. Our observations at non-relativistic shocks provide the first experimental evidence that below a certain magnetization threshold, the magnetic amplification efficiency at quasi-perpendicular shocks becomes comparable to that at the quasi-parallel shocks

Estimating the solar wind pressure at comet 67P from Rosetta magnetic field measurements

Aims: The solar wind pressure is an important parameter of space weather, which plays a crucial role in the interaction of the solar wind with the planetary plasma environment. Here we investigate the possibility of determining a solar wind pressure proxy from Rosetta magnetic field data, measured deep inside the induced magnetosphere of comet 67P/Churyumov-Gerasimenko. This pressure proxy would be useful not only for other Rosetta related studies but could also serve as a new, independent input database for space weather propagation to other locations in the Solar System. Method: For the induced magnetospheres of comets the magnetic pressure in the innermost part of the pile-up region is balanced by the solar wind dynamic pressure. Recent investigations of Rosetta data have revealed that the maximum magnetic field in the pile-up region can be approximated by magnetic field measurements performed in the inner regions of the cometary magnetosphere, close to the boundary of the diamagnetic cavity, from which the external solar wind pressure can be estimated. Results: We were able to determine a solar wind pressure proxy for the time interval when the Rosetta spacecraft was located near the diamagnetic cavity boundary, between late April 2015 and January 2016. We then compared our Rosetta pressure proxy to solar wind pressure extrapolated to comet 67P from near-Earth. After the exclusion of disturbances caused by transient events, we found a strong correlation between the two datasets

Planar Coherent Waves observed in Magnetospheric Multi Scale Fast Plasma Imager Phase Space Measurements: Part 2

Various wave vector estimation methods like Bellan’s method, multi-phase component analysis, and k-filtering with DivB=0 rely on the assumption that one dominant wave vector exists per frequency bin. They can’t resolve multiple wave fronts in a bin. Wave distribution function analysis could be used to tackle this. However, in this study we explore a different route where the response of phase space density (PSD) of the ions and electrons due the superposition of two wave vectors are visualized in phase space. A plasma wave is characterized by the cyclic exchange of energy between particles and fields that compose the wave mode, along with possible resonant interactions. The response of phase space to both of these interactions is a function of wave vector direction. Burst mode MMS-FPI measurements with a Nyquist frequency of 3 Hz for the ions and 17 Hz for the electrons will be used in this study, allowing the investigation of waves below these frequencies. The focus will be on waves in the ion cyclotron frequency range. First, we will use linear theory to explore how FPI PSD should respond to one and two wave vectors as function of wave amplitude and PSD one count level, exploring various projections of PSD. Then we will re-analyze a kinetic Alfven wave (KAW) event with oblique propagation in the magnetopause boundary layer for which the FPI PSD is highly unstable to field aligned ion cyclotron waves (ICW) to see if this event could actually be explained as a superposition of an ICW and its reflection

Evaluating the deHoffmann-Teller cross-shock potential at real collisionless shocks

Shock waves are common in the heliosphere and beyond. The collisionless nature of most astrophysical plasmas allows for the energy processed by shocks to be partitioned amongst particle sub-populations and electromagnetic fields via physical mechanisms that are not well understood. The electrostatic potential across such shocks is frame dependent. In a frame where the incident bulk velocity is parallel to the magnetic field, the deHoffmann-Teller frame, the potential is linked directly to the ambipolar electric field established by the electron pressure gradient. Thus measuring and understanding this potential solves the electron partition problem, and gives insight into other competing shock processes. Integrating measured electric fields in space is problematic since the measurements can have offsets that change with plasma conditions. The offsets, once integrated, can be as large or larger than the shock potential. Here we exploit the high-quality field and plasma measurements from NASA's Magnetospheric Multiscale mission to attempt this calculation. We investigate recent adaptations of the deHoffmann-Teller frame transformation to include time variability, and conclude that in practice these face difficulties inherent in the 3D time-dependent nature of real shocks by comparison to 1D simulations. Potential estimates based on electron fluid and kinetic analyses provide the most robust measures of the deHoffmann-Teller potential, but with some care direct integration of the electric fields can be made to agree. These results suggest that it will be difficult to independently assess the role of other processes, such as scattering by shock turbulence, in accounting for the electron heating