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The nature of the solar wind-planetary interaction depends on the properties of the solar wind, such as the interplanetary magnetic field (IMF), as well as the intrinsic magnetism and/or ionospheric properties of the planet. The volume of space that bounds this interaction region defines a planet's magnetosphere. The plasma environment around Mars behaves as a hybrid magnetosphere that exhibits characteristics of both induced and intrinsic magnetospheric processes. This hybrid nature is the result of strong crustal magnetic anomalies at Mars that are unequally distributed across the surface of the planet. These anomalies form "mini-magnetospheres" that interact with the solar wind. Magnetic reconnection is a ubiquitous space plasma physics phenomenon that reassigns magnetic field lines to new arrangements and mixes and energizes previously separated plasma populations. In this work, we analyze the local and global effects of magnetic reconnection via in situ orbital observations made by the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. Our results demonstrate that: (1) magnetic reconnection between the crustal anomalies and the IMF is the main driver for the formation of bundles of twisted magnetic flux known as magnetic flux ropes. We compiled a database of magnetic flux rope observations made by MAVEN and concluded that 91% were likely formed via magnetic reconnection. Flux rope formation illustrates examples of mini-magnetospheric processes occurring at Mars. (2) Magnetic reconnection also plays a role in discrete aurora formation within two mini-magnetospheres. We demonstrate that 87% of discrete aurora observations detected by the Imaging Ultraviolet Spectrograph onboard MAVEN occurred when the local IMF and underlying mini-magnetospheres are oriented in a way that favors magnetic reconnection. This work explains previously reported trends in discrete aurora detections and lays a framework for understanding Martian discrete aurora from a mini-magnetospheric point of view. (3) Our global analysis of the distribution of crustal anomalies at Mars measured by MAVEN has revealed that magnetic reconnection between the crustal fields and the IMF is favored when the IMF points southward. This result holds a variety of interesting implications regarding global measurements of magnetic connectivity at Mars. This work demonstrates that magnetic reconnection at Mars produces byproducts we also observe at planets with global intrinsic magnetic fields such as flux ropes, and aurora, as well as organizes large-scale plasma phenomena at Mars. The importance of magnetic reconnection within the Martian magnetosphere highlights its hybrid nature and underlines the limitless variety of solar wind-magnetosphere interactions that are possible both within our solar system and beyond.

We have developed global magnetohydrodynamic (MHD) models of the magnetospheres of Jupiter and Saturn motivated by the need to better understand the global structure and dynamics of the magnetospheres, their interaction with the solar wind and the plasma sources internal to them. The models are also used both as planning tools for the Cassini mission to Saturn and to give a global perspective to the measurements. Our model of Jupiter's magnetosphere is the first to include the Io mass loading region and to solve for the plasma flow in the inner magnetosphere. With the model we study the bow shock and magnetopause crossings made by Cassini and Galileo. In addition, we examine the field-aligned currents in Jupiter's inner magnetosphere with a height integrated ionospheric model coupled to the magnetosphere at the inner boundary. We find that the model describes the state of the magnetosphere at the time of Cassini quite well. The models of Saturn's magnetosphere that we present represent the first and only global models of the Kronian magnetosphere to date. With the models we study the effects of different source terms and different solar wind conditions on the configuration of the magnetosphere. Although simpler models are useful for understanding the relative roles of the icy satellite and Titan plasma sources and the configuration for different solar wind conditions, these models cannot fully account for the plasma in the inner magnetosphere of Saturn. The higher source rate gives good agreement with the mass densities measured by Voyager. For this case, we study the general structure of the magnetosphere as well as some applications of the model to the Cassini mission. We have examined the plasma environment at the satellites of Saturn, provided information about the plasma ram direction and extracted data from the model along the Cassini tour. We have carried out two simulations of the two-body, coupled Saturn-Titan system with Titan in super-fast magnetosonic flow. In each case, a fast-magnetosonic bow shock as well as a tail shock form at Titan. When Titan is located just outside Saturn's bow shock in the solar wind, the interaction between the bow shocks of the two bodies is quite complicated. A dominating feature is the sunward flow in the magnetosheath which results in a tangential discontinuity forming at Titan.