Plasma, one of the four fundamental states of matter, prevails the universe and accounts for 90\% of the known masses. Interaction between the solar wind, a space plasma with solar origin, and the terrestrial magnetic field shapes the space climate that is crucial for our modern society that heavily dependent on electricity, electronics and satellites. Waves ubiquitously grow, propagate, interact with other waves and plasmas, and eventually damps away in plasmas, significantly altering plasma dynamics and energy transport. Measurements of both plasma particles and electromagnetic fields allow probing wave-plasma interactions of interest.

Part one of the thesis presents a few new results relating to the electron heat flux in the solar wind. Electron heat flux is a poorly understood quantity in weakly collisional or collisionless astrophysical and space plasmas, but it is crucial to modeling large scale systems such as galaxy clusters and stellar winds. We present a statistical study of the electron heat flux in the solar wind and confirm that it is bounded from above by power laws of electron beta. We consider various collisionless processes that potentially reduce the heat flux. In particular, the whistler heat flux instability (WHFI) has long been considered to constrain heat flux in the high beta regime. We show for the first time local generation of whistler waves in the solar wind using high-cadence simultaneous particles and wave measurements onboard ARTEMIS spacecraft. We present the statistical properties of the whistler waves in the solar wind at 1 AU, with evidence supporting WHFIs generating the observed whistler waves. However, we argue that the wave amplitude is too small to effectively reduce the electron heat flux. Accompanied by the electron heat flux is measurable electron bulk drifts with respect to the solar wind protons, which significantly modify Landau resonance of kinetic Alfv en waves (KAWs). We consider the effects of potential KAW instabilities in the context of electron heat flux inhibition.

Part two of the thesis considers a type of electrostatic structure known as the electron phase space hole (EPSH). It is formed in the non-linear stage of plasma streaming instabilities and remains highly stable for long duration. We address for the first time 3D configuration of EPSHs by analyzing multi-spacecraft (using NASA MMS) passing of the same EPSH. The length scale perpendicular to the background magnetic field is directly measured with significant implications to electron motions inside EPSHs. In addition, statistical study of such multi-spacecraft observation reveals strong correlation among parameters of fast EPSHs.

Spacecraft observations from the interplanetary medium of our solar system reveal the presence of a magnetized super-sonic flow emanating from the sun, commonly known as the solar wind. Empirically, in-situ measurements from spacecraft suggest that the solar wind is in a turbulent state frequently occurring fluid-like systems. Though theories of non-magnetized hydrodynamic turbulence have been successfully adapted to account for plasma dynamics relevant to the solar wind (e.g. strong magnetization, multi-particle composition, non-viscous dissipation, and weak collisionality), there is lacking consensus regarding the physical processes responsible for empirically observed phenomena: e.g. compressible fluctuations, intermittent coherent features, injection of energy at large scales, and particle heating. Interpreting in-situ spacecraft measurements is often complicated by limitations associated with single point me which most often consist of a single point (or at best a few points) located near Earth. At the largest physical scales, processes associated with solar wind generation and evolution consist of temporal variation over the 11 year solar cycle, with spatial gradients extending over the large scale heliosphere, ~200 AU. At the smallest scales, heating and dissipation process can occur on electron kinetic scales corresponding to ~ kHz frequencies and centimeter length scales in the inner heliosphere. Even in observing fluid-like magnetohydrodynamic (MHD) fluctuations of the solar wind, ``easily'' measurable by spacecraft at 1 AU, significant ambiguity exists in distinguishing effects associated with plasma transport from the processes related to the generation (heating and acceleration) of the solar wind in the inner-heliosphere.

The source of the solar wind is the corona, a hot magnetized upper-atmosphere of our sun with ambient temperatures ranging from 10^5-10^6 Kelvin: orders of magnitude larger than the solar photospheric surface at 5800 Kelvin. Even the roughest estimation of the coronal energy budgets suggest that the magnetic field must be responsible for heating the corona to these temperatures. However, the specific processes which drive coronal heating, and subsequently accelerate the solar wind, are yet unknown; though many models of coronal heating exist, little empirical evidence is currently available to distinguish between theories.

The NASA Parker Solar Probe (PSP) mission, launched in August 2018, recently became the closest human-made object to orbit the sun. During its closest perihelion approach, PSP will reach an altitude of 9.8 solar radii (0.045 AU), well within the expected boundary between the solar wind corona, known as the Alfven point. By measuring the local plasma environment, PSP will provide an empirical understanding of the processes responsible for coronal heating and solar wind acceleration which cannot be observed using remote sensing techniques. In addition, through studying the turbulent environment present in the inner heliosphere, PSP will inevitably make significant contribution to our understanding of magnetized turbulence and the role it plays in shaping astrophysical systems.

This dissertation highlights the development of observational techniques and instrumentation used in studying nonlinear dynamic processes, e.g. turbulence and plasma instabilities, in astrophysical plasmas. Part 1 consists of a discussion of incompressible magnetohydrodynamic turbulence in the solar wind and the observed coupling with compressible fluctuations. Chapter 1 contains an overview of the historical and mathematical development of MHD turbulence based on both empirical observations from spacecraft and theory of hydrodynamic turbulence. Chapter 2 contains original research on the effect of intermittency on the observational signatures of MHD turbulence. Chapter 3 discusses the the nature of compressible fluctuations in the solar wind based on the mathematical and observational techniques developed in Chapter 2. Chapter 4 describes an observational study which examines the existence of parametric mode coupling in the solar wind which could drive compressible fluctuations as well as initiate non-linear turbulent interactions in the heliosphere.

Part 2 surveys the calibration and operation of the PSP/FIELDS magnetometer suite. Chapter 5 highlights the operation and calibration of the PSP/FIELDS DC fluxgate magnetometer (MAG). Chapter 6 consists of an overview of the PSP/FIELDS search coil magnetometer (SCM) and an in depth discussion of instrument calibration through the framework of linear time invariant filter design. Chapter 7 describes a merged fluxgate and search coil data product for PSP created using optimal filter design techniques.

Solar flares are the most vigorous explosive phenomena in our solar system. They release up to $\sim10^{33}$ erg of magnetic energy in the Sun's corona in times that range from minutes to hours. Some 10 to 50\% of the flare energy goes into electron acceleration. Among other processes, when these electrons interact with the ambient plasma, they produce bremsstrahlung radiation in hard X-rays (HXRs). Analyses of flare HXRs are critical for understanding energy release dynamics, acceleration mechanisms, and their connection with other phenomena in the corona. One of these phenomena is coronal heating, an open problem in heliophysics. This problem seeks to clarify why the Sun's coronal temperature is up to three orders of magnitude higher than that at the Sun's surface.

Coronal temperatures demand a mean energy input between $\sim10^5$ and $2\times10^7$ erg cm$^{-2}$ s$^{-1}$. Multiple observations have proven that medium and large-size flares together do not contribute enough energy to account for these input power requirements. Instead, a popular idea proposes that the solar atmosphere is filled with small impulsive heating events releasing magnetic energy in the corona, called nanoflares. If nanoflares follow the same physics as their larger counterparts, they should emit hard X-rays (HXRs) but with substantially fainter intensity. A copious and continuous presence of nanoflares would result in sustained HXR emission. These nanoflares could deliver sufficient energy into the Sun's corona, to account for its high temperatures. To date, there has not been any direct detection of such persistent HXRs emitted from the quiescent Sun. However, $\sim12$ days of solar off-pointing observations of the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) during periods of quiescent activity led to HXR upper limits. In the 6-12 keV energy range, e.g., this upper limit is $9.5\times 10^{-4}$ photons s$^{-1}$ cm$^{-2}$ keV$^{-1}$.

Observing faint HXR emission is challenging because it demands instruments with high sensitivity and dynamic range. RHESSI has insufficient sensitivity to detect such faint sources, especially in the form of a broad, diffuse signal rather than the bright, compact signals for which RHESSI was designed. The Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket experiment excels in these two attributes. FOXSI has a sensitivity of $\sim$ 0.0032 photons cm$^{-2}$ s$^{-1}$ keV$^{-1}$ ($\sim$50 times that of RHESSI) at 8 keV and a dynamic range of $\sim$100 for sources $>30$ arcsec apart. FOXSI achieves such a superior performance by pairing nested grazing-incidence Wolter-I mirrors with low-noise semiconductor detectors optimized for high energies. FOXSI's direct focusing capabilities allow quiet regions of the corona to be isolated to look for the presence of HXR sources.

This thesis constrains the quiet Sun emission in the 5-10 keV energy range using FOXSI observations from the second and third rocket flights (FOXSI-2 and -3). To fully characterize FOXSI's sensitivity, this thesis presents a thorough optics calibration and a ray-tracing simulation to assess ghost ray backgrounds generated by sources outside of the telescope field of view. This work demonstrates a Bayesian approach to provide upper thresholds of quiet Sun HXR emissions and probability distributions for the expected flux of a quiet-Sun HXR source when it is assumed to exist. For FOXSI-2 and -3, such upper limits are $4.5\times 10^{-2}$ photons s$^{-1}$ cm$^{-2}$ keV$^{-1}$ and $6.0\times10^{-4}$ photons s$^{-1}$ cm$^{-2}$ keV$^{-1}$, respectively (both in the 5-10 keV energy range). These two limits are similar to that of RHESSI in the 6-12 keV energy band ($9.5\times 10^{-4}$ photons s$^{-1}$ cm$^{-2}$ keV$^{-1}$) but with an important difference: it took $\sim$1/2600 less integration time for FOXSI to get enough statistics to yield these equivalent limits. The FOXSI-2 limit presented in this doctoral work is the first-ever quiet Sun upper threshold in HXR estimated from observations performed during a period of high solar activity. This dissertation's quiet Sun HXR analyses during a solar cycle minimum are the first scientific results that use the $\sim6.5$ minutes of the FOXSI-3 rocket observations. A possible future spacecraft using FOXSI's concept would allow enough observation time to constrain the current HXR quiet Sun limits further or perhaps even make direct detections. This last objective would demand observations of a few hours at the very least and (ideally simultaneous) onboard measurements of the backgrounds. Any upper quiet Sun HXR limit constrains the parameter space (e.g., the index and cutoff energy for thick target power-law models) that nanoflare electron energy distribution can have. The limits found in this doctoral work suggest very steep spectra, i.e., $>5$ power-law indexes when we assume nanoflare accelerated electrons follow a thick target model (in agreement with earlier RHESSI-based results).

Aluminum-26 (Al-26) is a radioactive isotope produced in massive star processes. High-resolution spectroscopy of its 1.809 MeV gamma-ray (γ-ray) decay signature constrains the dynamics of its emission as it is ejected from its progenitor sites and incorporated into the interstellar medium of the Milky Way Galaxy. Imaging reveals dominant emission in the Inner Galaxy and emission in localized regions of massive star activity. Taken together, spectroscopy and imaging of Al-26 shed light on the chemical evolution of the Galaxy over millions of years.

The Compton Spectrometer and Imager (COSI) is a compact Compton telescope designed to measure astrophysical γ-rays of energy 0.2–5 MeV. Its high-purity germanium detectors track incident photons as they Compton scatter throughout the detector volume. In 2016, COSI flew on a NASA ultra-long duration balloon for 46 days. This dissertation details the first analysis of Al-26 in the flight data and reports a measurement of 3.7σ significance above background and an Inner Galaxy flux of (8.6 ± 2.5) × 10−4 ph cm−2 s−1. All scientific achievements from the flight are predicated on calibrations performed before launch. These calibration procedures were repeated in advance of an intended 2020 balloon flight. Analyzing calibration data validates instrument performance and informs studies of detector effects, including charge sharing and charge trapping, that complicate the measurement process.

The next generation of COSI as a NASA Small Explorer satellite is slated for launch in 2027. It is anticipated to strengthen the spectroscopic measurement of Al-26 in the balloon flight and yield the most detailed images of Al-26 to date. Extensive testing of the imaging algorithm in COSI’s new analysis toolkit is presented as a first step towards producing these images. The desired culmination of these efforts is an enhanced understanding of Galactic Al-26 by way of thorough calibrations, novel insight into undesirable detector effects, and advanced analyses of high-resolution data from the COSI balloon and satellite instruments.

High-energy transient events can be used to probe the extreme physics that power them, as well as the properties of matter in violent astrophysical environments such as in the vicinity of compact objects or the birthplaces of black holes. X-ray binary outbursts provide important insights into the physics of accretion and the nature of black holes and neutron stars. Meanwhile, gamma-ray bursts (GRBs) carry signatures of the powerful progenitors that produce them, yet the origins of their prompt emission and their jet structure remain unclear. Spectral and timing analyses have been effective investigative tools for these extreme settings, especially as telescopes advance and models increase in predictive power. Pairing them with linear polarization analyses of high-energy emissions can add even more information about the emission mechanisms and source geometries of accreting black holes and GRBs.

The Compton Spectrometer and Imager (COSI) is a soft gamma-ray (0.2-5 MeV) telescope designed to study astrophysical sources including accreting black holes and GRBs. It has significant heritage as a balloon-borne telescope, and was selected as a NASA Small Explorer (SMEX) slated to launch on a satellite in 2027. COSI employs a compact Compton telescope designed to conduct high-resolution spectroscopy, imaging over a wide field-of-view, polarization studies, and effective suppression of background events. Compton telescopes detect multiple interactions from individual incoming photons, allowing for polarization information to be captured through measurements of the distribution of azimuthal angles. While the standard method relies on binning the photons to produce and fit an azimuthal scattering angle distribution, improved polarization sensitivity is obtained by using additional information to more accurately weigh each event’s contribution to the likelihood statistics. In this work, we report validations of COSI’s capabilities as a polarimeter. Furthermore, we develop tools that enable future spectral and polarimetric analyses of COSI GRB observations.

We also present the first observed outburst from the transient X-ray binary source MAXI J0637-430, based on observations from the Nuclear Spectroscopic Telescope Array (NuSTAR) and the Neil Gehrels Swift Observatory X-Ray Telescope (Swift/XRT). We study the source's transition from a soft state dominated by disk-blackbody emission to a hard state dominated by a power-law or thermal Comptonization component, with NuSTAR providing the first coverage of MAXI J0637-430 above 10 keV. These broadband spectra show that a two-component model does not provide an adequate description of the soft state spectrum. As such, we test alternative excess emission models such as blackbody emission from the plunging region, a reflection component with a Comptonization continuum, and a reflection component of a blackbody illuminating the disk. The study demonstrates the importance of broadband spectral analyses of accreting compact objects. We include a discussion on how joint spectral and polarization analyses could be conducted for long-duration transient sources with COSI in the future.

The Sun’s atmosphere is a complex and dynamic magnetized plasma and extends all theway from its visible surface out into interplanetary space, carving out a bubble in the inter- stellar medium which is called the heliosphere. All interactions between the Sun and life on Earth are channelled through this medium. Of particular importance to making Sun-Earth connections are the regions called the corona and the inner heliosphere. These two regimes are strongly coupled together but their mutual boundary may be regarded as the location where the dynamic pressure of the outflowing solar wind overcomes the magnetic pressure of the Sun’s intrinsic field. By inner heliosphere, we focus on the portion of the Sun’s sphere of influence which extends out to 1 au and therefore is most relevant to the Earth and humanity.

Our most complete understanding of the corona and heliosphere comes from large scalephysical models which can fill in information about a plasma on a 3D grid. In 2018, Parker Solar Probe (PSP) was launched into an orbit taking it closer to the Sun than any human- made object in history. This has presented an opportunity to directly probe regions of the heliosphere which had hitherto could only be accessed with global modelling. In this body of work we use new data from PSP to improve our knowledge and understanding of this global structure and further derive novel constraints on plasma models of the corona and heliosphere.

Specifically, we first introduce a framework for evaluating models of the coronal magneticfield, which sets how the solar wind emerges and shapes the inner heliosphere. In addition to new PSP data which provides direct boundary conditions on the magnetic skeleton of the corona, we show how it is important to make use of pre-existing observational capabilities to constrain the sizes of coronal holes and the locations of high plasma density indicating the topology of the coronal streamer belt. We illustrate how models must be constrained at multiple boundaries to give an accurate representation and that focusing on individual specific metrics can lead to different conclusions about optimum model parameters.

Next, we use the full data set of the heliospheric magnetic field taken by Parker Solar Probein its first four years on orbit to directly measure the heliospheric magnetic field down to 0.13 au and compare directly to the large scale expectations of the Parker magnetic field. We present evidence that at 0.13 au the heliospheric magnetic field remains latitudinally isotropic, indicating the coronal field has already relaxed to this state within this radius. We measure the open magnetic flux and confirm it is conserved between 1 au and PSP’s closest approach to date. This conservation implies a deficit in open magnetic flux according to coronal models with typically accepted model parameters. We also compare the mean direction of the heliospheric magnetic field to the expectation of the Parker spiral model, finding very good agreement which is tending to improve with closing distance from the sun as the ratio of average field strength to random fluctuations increases.

Third, we present a study in which we determine Parker Solar Probe’s magnetic connectivityback to specific coronal sources for its first solar encounter. This exercise allows determi- nation of specific locations on the Sun which emit solar wind plasma later measured by PSP, and therefore contextualises its measurements. This application of combining coronal modelling and PSP data shows how making these connections is a vital building block for understanding other peculiar plasma physics observed as PSP as it has explored new re- gions of the inner heliosphere. Further, it allows disambiguation of spatial and temporal phenomena.

Finally, we present recent work using observations by Parker Solar Probe and other 1 auspacecraft to localise type III radio bursts, an impulsive solar ejection of electron beams, from emission at the solar surface out into the inner heliosphere. These events have the potential to act as passive tracers of coronal and heliospheric structure. We comment on the future prospects of using this localisation to constrain magnetic connectivity and density structure.

We close with a summary of these results and the outlook for further improvement of ourunderstanding of the coupled corona and inner heliosphere ans PSP continues to approach the Sun and as other advances in space based instrumentation are made, such as the gradual escape of the Solar Orbiter to higher latitudes.

The individual investigations, which are briefly introduced above, are united in highlightingseveral specific advances in our understanding of the Sun’s atmosphere facilitated by the ad- dition of Parker Solar Probe to humanity’s suite of heliospheric instrumentation. Specifically, we exemplify how multi-point, multi-spacecraft and multi-messenger observations at differ- ent heliographic locations are vital in making progress in constraining our physical models; using just one vantage point or one physical observable can lead to false conclusions about model optimisation. We also observe an underlying thread of the surprising utility of the very simplest model representations of the corona and heliosphere, for example a current- free corona and essentially hydrodynamic heliosphere can accurately predict the magnetic polarity structure, and even the velocity stream structure measured in situ by PSP. Lastly, we verify that as one would expect from sending an instrument to never-before explored regions of interplanetary space, new gaps in our understanding are identified. For example, confirming that coronal models do not open enough magnetic flux to the inner heliosphere, or showing at several points that while we make substantial progress exploring closer to the Sun, a lack of far-side and high latitude remote sensing (most critically of the photospheric magnetic field), remains a big limitation to accurately reproducing the physical structure of the heliosphere.

γ-rays are an indispensable probe of neutron star and black hole systems, some of the most extreme environments in our universe. The penetrating power of γ-rays allows us to probe deep within these often obscured systems. In particular, polarization measurements of neutron stars and black holes can give essential clues about γ-ray emission mechanisms, source geometry, and magnetic field structure where spectral, temporal, and imaging analysis fall short.

The Compton Spectrometer and Imager (COSI) is one of the few γ-ray telescopes designed as a polarimeter. COSI is a wide-field balloon-borne soft γ-ray (0.2-5 MeV) telescope, and one of its main goals is to measure the polarization of γ-rays emitted by compact objects. As a Compton telescope, COSI is inherently sensitive to polarization: polarized photons preferentially Compton scatter orthogonally to their polarization direction. In May of 2016, COSI was launched from Wanaka, New Zealand, on NASA’s new super pressure balloon and flew for 46 days before the flight was terminated in Peru. The Crab nebula, Cygnus X-1, and Centaurus A are among the compact objects detected during the 2016 flight.

A key step to performing imaging, spectral, and polarization analysis of the sources detected during the 2016 flight is to accurately simulate the detector response. To do so, I developed a detailed detector effects engine which applies the intrinsic detector performance to Monte Carlo simulations. With accurate simulations of the instrument in place, I developed a spectral analysis pipeline for sources detected by COSI. As needed for spectral analysis and polarimetry, I developed a background subtraction technique for broadband, persistent sources that utilizes the COMPTEL data space. I verified these new analysis methods using the COSI observation of GRB 160530A: after subtracting the background using the COMPTEL data space method and fitting the spectrum, the spectral parameters are consistent with those measured by another instrument.

I used the COMPTEL data space background subtraction algorithm to fit the Crab spectrum, but concluded that the algorithm is limited in estimating the background accurately enough in cases where the source is background-dominated. I used simulations to assess the prospects for polarimetry of COSI’s observation of the Crab and inferred that we require a better mechanism of albedo radiation background rejection to perform polarimetry of this particular observation. I note that similar observations from the COSI instrument on a satellite platform would lead to more success in measuring the spectra and polarization properties of compact objects, and that the analysis methods developed in this work could be easily applied.

In this work, we introduce background relevant to and our own analyses of collisionless shock observations in the heliosphere. The observations of interest are in situ measurements collected by spacecraft immersed in the plasma. We introduce basic concepts in the physics of shocks and space plasmas and describe some specific techniques appropriate for the analysis of shock observations. Then we discuss the spacecraft mission that provided the data for our studies: a four-probe constellation known as Magnetospheric Multiscale (MMS). Instruments providing the most necessary measurements to our work (electric fields, magnetic fields, and particle distributions and moments) are discussed individually. The subsequent three chapters, detailing the analyses we performed on this data, represent the work we have published on the subject.

In the first paper, we compare shock normals, planarities, and Normal Incidence Frame (NIF) cross-shock potentials determined from electric field measurements and proxies, for a subcritical (Fast Magnetosonic Mach number M_F=1.1±0.1) interplanetary (IP) shock and a supercritical bow shock (M_F=2.13±0.04). The low-Mach shock’s cross-shock potential was 26±6V. The shock scale was 33km, too short to allow comparison with proxies from ion moments. Proxies from electron moments provided potential estimates of 40±5V. Shock normals from magnetic field minimum variance analysis were nearly identical, indicating a planar front. The high-Mach shock’s cross-shock potential was estimated to be from 290 to 440V from the different spacecraft measurements, with shock scale 120km. Reflected ions contaminated the ion-based proxies upstream, whereas electron-based proxies yielded reasonable estimates of 250±50V. Shock normals from electric field maximum variance analysis differed, indicating a rippled front.

For the second paper, we investigate the dependence of shock parameters (speed v_sh, normal n ̂, and angle θ_Bn) on the choice of upstream and downstream regions for 51 bow shock crossings in MMS Fast Survey data. We summarize guidelines for selecting stream regions based on the magnetic field and particle moments. Preferred upstream and downstream combinations were identified by minimizing RH conservation errors. Comparing parameters from different up/downstream combinations provided a measure of how stream region choices affect the parameters. Shifting from the preferred stream region combination to another would cause <5° change in n ̂ for 90% of shocks, <15km/s change in v_sh of 70% of shocks, and <5° change in θ_Bn for 84% of shocks. All parameters would shift by more than their standard deviations σ. The most robust is n ̂, which would change by <1σ for 22% and <3σ for 86% of shocks, while v_sh is the least robust, changing by <3σ for only 12% of shocks. Summary plots and detailed lists of parameters are provided in a separate Supplement, freely available at https://doi.org/10.5281/zenodo.3583341.

In the third paper, we return to the IP shock that was recorded crossing the MMS constellation on 2018 January 8. Plasma measurements upstream of the shock indicate efficient proton acceleration in the IP shock ramp: 2-7 keV protons are observed upstream for about three minutes (~8000 km) ahead of the IP shock ramp, outrunning the upstream waves. The differential energy flux (DEF) of 2-7 keV protons decays slowly with distance towards the upstream region (dropping by about half within 8 Earth radii from the ramp) and is lessened by a factor of about four downstream from the ramp (within a distance comparable to the gyroradius of ~keV protons). Comparison with test-particle simulations has confirmed that the mechanism accelerating the solar wind protons and injecting them upstream is classical shock drift acceleration. This example of observed proton acceleration by a low-Mach, quasi-perpendicular shock may be applicable to astrophysical contexts, such as supernova remnants or the acceleration of cosmic rays.

Radio waves at the local plasma frequency and its harmonic are generated upstream of collisionless shocks in foreshock regions which are magnetically connected to the shock. The radio waves are created in a multi-step process which involves the acceleration of electrons at the shock front, growth of electrostatic Langmuir waves driven by the accelerated electron beam, and conversion of the Langmuir waves into radio waves.

These radio waves can be used to remotely determine properties of the shock. For example, Type II solar radio burst observations yield information about the radial speed and angular extent of the coronal mass ejection-driven shock associated with the burst. However, in order to completely understand the generation of the radio waves and interpret the remote observations, in situ spacecraft measurements of the shock-accelerated electrons and Langmuir waves are necessary.

In this thesis, a brief introduction to the heliospheric environment is followed by a survey of the basic principles of collisionless shocks, a detailed discussion of the process of generating shock-accelerated electrons and the resulting plasma waves, and a description of the relevant instrumentation on board the Wind and STEREO spacecraft. Following this review material, several results based on in situ observations are presented:

(1) High cadence electron measurements made by Wind in the foreshock region of several IP shocks allow for a determination of the spatial scales of the source regions of Type II radio bursts. The sizes of the observed foreshock source regions are comparable to the size of the terrestrial bow shock.

(2) Langmuir waves upstream of IP shocks can be used as a diagnostic signature of foreshock electrons. Using a large database of shocks observed by Wind, different shock parameters are statistically tested for their effectiveness at accelerating electrons.

(3) Using a new type of electron detector on STEREO, the limits of the Fast Fermi theory for electron acceleration at suprathermal energies are examined. Preliminary results suggest that the mechanism may hold beyond the regime where the Larmor radius of electrons is much smaller than the scale sizes of the shock.

The prospects for future work in this area are discussed in the conclusion, and a description of an experimental antenna calibration procedure known as rheometry is included as an appendix.

Parker Solar Probe (PSP), launched in late 2018, is a mission designed to sample the near-Sun environment and solar corona, and answer broad open questions concerning coronal energy flow and solar wind dynamics. The SPAN-Ion instrument is an on-board electrostatic analyzer responsible for measuring 3D ion velocity distribution functions (VDFs). In the first part of this thesis, we give an overview of SPAN-Ion, its intrinsic uncertainties, and discuss the effect of its finite field-of-view on moment measurements. We then move on to study magnetic switchbacks, rapid radial reversals of the magnetic field. While their role in young solar wind dynamics and precise generation mechanisms are still unclear, their ubiquity marks them out as an important early PSP observation. Using MHD invariants to probe their macroscale structure, we show that they are localised S-shaped folds in the magnetic field with internally backward propagating Alfvénic fluctuations, which has important implications for studies of small-scale turbulence using such invariants. Using fits to SPAN-Ion data, we then investigate alpha particle density, abundance, and velocity fluctuations inside and outside individual switchbacks, showing that there are no consistent compositional changes inside vs outside, but argue that these findings cannot yet be used to definitively rule in favour of one particular switchback generation mechanism (although they may be able to in the future). We also show that alpha particle speeds may be enhanced, decreased, or remain constant during a switchback, depending on the relative values of the alpha proton drift and the local wave phase speed, in contrast to the always positive proton velocity spikes. In the final part we study the alpha VDFs in more detail, focussing on characterising secondary alpha populations or alpha ``beams”. These have been essentially unstudied relative to their proton beam counterparts. We find they are generally more dense and slower moving than proton beams, and occur less frequently. We report time localised correlations between proton and alpha normalised heat flux, suggesting the existence of a common mechanism for producing beams in each species. We then perform a case study of an ion scale wave event, showing for the first time an active role being played by the alphas, specifically the alpha beam population, in driving solar wind plasma unstable and locally generating right-handed fast magnetosonic waves. The predicted wave frequencies, polarisations, and times of occurrence agree remarkably well with the observations. Such wave events are important for understanding the mechanisms of energy exchange between waves and particles that may be responsible for in-situ heating of the solar wind.