Galactic Cosmic Rays (GCR) are the predominant source of highly energetic particles in the inner heliosphere during solar quiet times. These particles are fully ionized atoms that are accelerated to ...near-relativistic speeds during events of extreme energy release throughout the Milky Way Galaxy and beyond. Some GCR particles eventually find their way to the outer edges of the heliosphere and a portion of those are able to propagate to 1 AU. GCR have sufficient energy to ionize atoms and molecules in the matter that they impact, causing radiation damage to both robotic and biologic materials. Understanding the flux and spectrum of GCR is of great importance to future robotic and human explorers venturing beyond low-Earth orbit. In this dissertation, we use the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) instrument along with modeling efforts to study a variety of phenomena that can influence the energetic particle flux in the near-Moon environment, including Interplanetary Coronal Mass Ejections (ICMEs), Corotating Interaction Regions (CIRs) and the Earth's magnetotail. As part of this study, the CRaTER instrument and its calibration are discussed in detail. A new model is developed to better predict the transit times of Interplanetary Coronal Mass Ejections and the associated drops in GCR flux called Forbush decreases. This model could provide a more accurate estimate of an ICME's arrival time within hours of ejection from the Sun. An important model discrepancy is resolved by using the CRaTER instrument to measure GCR while the Moon is in the Earth's magnetotail. Previous studies that predicted shielding of GCR by the magnetotail are disproven; we find no evidence for a drop in GCR, intensity as a result of passage through the magnetotail. We use the CRaTER instrument to investigate step-like durable decreases in GCR flux with time. We find that these decreases occurred when CIRs convected past the observing spacecraft shortly after solar minimum, presumably caused by the more effective shielding provided by the outward propagating magnetic structures. A change in the proton linear energy transfer spectrum is observed in conjunction with the GCR flux decrease.
Data from the first two orbits of the Sun by Parker Solar Probe reveal that the solar wind sunward of 50 solar radii is replete with plasma waves and instabilities. One of the most prominent plasma ...wave power enhancements in this region appears near the electron cyclotron frequency (f_ce). Most of this wave power is concentrated in electric field fluctuations near 0.7 f_ce and f_ce, with strong harmonics of both frequencies extending above f_ce. At least two distinct, often concurrent, wave modes are observed, preliminarily identified as electrostatic whistler-mode waves and electron Bernstein waves. Wave intervals range in duration from a few seconds to hours. Both the amplitudes and number of detections of these near-f_ce waves increase significantly with decreasing distance to the Sun, suggesting that they play an important role in the evolution of electron populations in the near-Sun solar wind. Correlations are found between the detection of these waves and properties of solar wind electron populations, including electron core drift, implying that these waves play a role in regulating the heat flux carried by solar wind electrons. Observation of these near-f_ce waves is found to be strongly correlated with near-radial solar wind magnetic field configurations with low levels of magnetic turbulence. A scenario for the growth of these waves is presented which implies that regions of low-turbulence near-radial magnetic field are a prominent feature of solar wind structure near the Sun.
Context: The analysis of the thermal part of velocity distribution functions (VDF) is fundamentally important for understanding the kinetic physics that governs the evolution and dynamics of space ...plasmas. However, calculating the proton core, beam and alpha-particle parameters for large data sets of VDFs is a time consuming and computationally demanding process that always requires supervision by a human expert. Aims: We developed a machine learning tool that can extract proton core, beam and alpha-particle parameters using images (2-D grid consisting pixel values) of VDFs. Methods: A database of synthetic VDFs is generated, which is used to train a convolutional neural network that infers bulk speed, thermal speed and density for all three particle populations. We generate a separate test data set of synthetic VDFs that we use to compare and quantify the predictive power of the neural network and a fitting algorithm. Results: The neural network achieves significantly smaller root-mean-square errors to infer proton core, beam and alpha-particle parameters than a traditional fitting algorithm. Conclusion: The developed machine learning tool has the potential to revolutionize the processing of particle measurements since it allows the computation of more accurate particle parameters than previously used fitting procedures.
Parker Solar Probe (PSP) observes unexpectedly prevalent switchbacks, which are rapid magnetic field reversals that last from seconds to hours, in the inner heliosphere, posing new challenges to ...understanding their nature, origin, and evolution. In this work, we investigate the thermal states, electron pitch angle distributions, and pressure signatures of both inside and outside switchbacks, separating a switchback into spike, transition region (TR), and quiet period (QP). Based on our analysis, we find that the proton temperature anisotropies in TRs seem to show an intermediate state between spike and QP plasmas. The proton temperatures are more enhanced in spike than in TR and QP, but the alpha temperatures and alpha-to-proton temperature ratios show the opposite trends, implying that the preferential heating mechanisms of protons and alphas are competing in different regions of switchbacks. Moreover, our results suggest that the electron integrated intensities are almost the same across the switchbacks but the electron pitch angle distributions are more isotropic inside than outside switchbacks, implying switchbacks are intact structures but strong scattering of electrons happens inside switchbacks. In addition, the examination of pressures reveals that the total pressures are comparable through an individual switchback, confirming switchbacks are pressure-balanced structures. These characteristics could further our understanding of ion heating, electron scattering, and the structure of switchbacks.
Parker Solar Probe (PSP) data recorded within a heliocentric radial distance of 0.3 AU have revealed a magnetic field dominated by Alfvénic structures that undergo large local variations or even ...reversals of the radial magnetic field. They are called magnetic switchbacks, they are consistent with folds in magnetic field lines within a same magnetic sector, and are associated with velocity spikes during an otherwise calmer background. They are thought to originate either in the low solar atmosphere through magnetic reconnection processes, or result from the evolution of turbulence or velocity shears in the expanding solar wind. In this work, we investigate the temporal and spatial characteristic scales of magnetic switchback patches. We define switchbacks as a deviation from the nominal Parker spiral direction and detect them automatically for PSP encounters 1, 2, 4 and 5. We focus in particular on a 5.1-day interval dominated by switchbacks during E5. We perform a wavelet transform of the solid angle between the magnetic field and the Parker spiral and find periodic spatial modulations with two distinct wavelengths, respectively consistent with solar granulation and supergranulation scales. In addition we find that switchback occurrence and spectral properties seem to depend on the source region of the solar wind rather than on the radial distance of PSP. These results suggest that switchbacks are formed in the low corona and modulated by the solar surface convection pattern.
Parker Solar Probe (PSP) routinely observes magnetic field deflections in the solar wind at distances less than 0.3 au from the Sun. These deflections are related to structures commonly called ...'switchbacks' (SBs), whose origins and characteristic properties are currently debated. Here, we use a database of visually selected SB intervals - and regions of solar wind plasma measured just before and after each SB - to examine plasma parameters, turbulent spectra from inertial to dissipation scales, and intermittency effects in these intervals. We find that many features, such as perpendicular stochastic heating rates and turbulence spectral slopes are fairly similar inside and outside of SBs. However, important kinetic properties, such as the characteristic break scale between the inertial to dissipation ranges differ inside and outside these intervals, as does the level of intermittency, which is notably enhanced inside SBs and in their close proximity, most likely due to magnetic field and velocity shears observed at the edges. We conclude that the plasma inside and outside of a SB, in most of the observed cases, belongs to the same stream, and that the evolution of these structures is most likely regulated by kinetic processes, which dominate small scale structures at the SB edges.
During Parker Solar Probe's first two orbits there are widespread observations of rapid magnetic field reversals known as switchbacks. These switchbacks are extensively found in the near-Sun solar ...wind, appear to occur in patches, and have possible links to various phenomena such as magnetic reconnection near the solar surface. As switchbacks are associated with faster plasma flows, we questioned whether they are hotter than the background plasma and whether the microphysics inside a switchback is different to its surroundings. We have studied the reduced distribution functions from the Solar Probe Cup instrument and considered time periods with markedly large angular deflections, to compare parallel temperatures inside and outside switchbacks. We have shown that the reduced distribution functions inside switchbacks are consistent with a rigid phase space rotation of the background plasma. As such, we conclude that the proton core parallel temperature is the same inside and outside of switchbacks, implying that a T-V relationship does not hold for the proton core parallel temperature inside magnetic field switchbacks. We further conclude that switchbacks are consistent with Alfvénic pulses travelling along open magnetic field lines. The origin of these pulses, however, remains unknown. We also found that there is no obvious link between radial Poynting flux and kinetic energy enhancements suggesting that the radial Poynting flux is not important for the dynamics of switchbacks.
Parker Solar Probe (PSP) achieved its first orbit perihelion on November 6, 2018, reaching a heliocentric distance of about 0.165 au (35.55 R\(_\odot\)). Here, we study the evolution of fully ...developed turbulence associated with the slow solar wind along the PSP trajectory between 35.55 R\(_\odot\) and 131.64 R\(_\odot\) in the outbound direction, comparing observations to a theoretical turbulence transport model. Several turbulent quantities, such as the fluctuating kinetic energy and the corresponding correlation length, the variance of density fluctuations, and the solar wind proton temperature are determined from the PSP SWEAP plasma data along its trajectory between 35.55 R\(_\odot\) and 131.64 R\(_\odot\). The evolution of the PSP derived turbulent quantities are compared to the numerical solutions of the nearly incompressible magnetohydrodynamic (NI MHD) turbulence transport model recently developed by Zank et al. (2017). We find reasonable agreement between the theoretical and observed results. On the basis of these comparisons, we derive other theoretical turbulent quantities, such as the energy in forward and backward propagating modes, the total turbulent energy, the normalized residual energy and cross-helicity, the fluctuating magnetic energy, and the correlation lengths corresponding to forward and backward propagating modes, the residual energy, and the fluctuating magnetic energy.
One of the main discoveries from the first two orbits of Parker Solar Probe (PSP) was the presence of magnetic switchbacks, whose deflections dominated the magnetic field measurements. Determining ...their shape and size could provide evidence of their origin, which is still unclear. Previous work with a single solar wind stream has indicated that these are long, thin structures although the direction of their major axis could not be determined. We investigate if this long, thin nature extends to other solar wind streams, while determining the direction along which the switchbacks within a stream were aligned. We try to understand how the size and orientation of the switchbacks, along with the flow velocity and spacecraft trajectory, combine to produce the observed structure durations for past and future orbits. We searched for the alignment direction that produced a combination of a spacecraft cutting direction and switchback duration that was most consistent with long, thin structures. The expected form of a long, thin structure was fitted to the results of the best alignment direction, which determined the width and aspect ratio of the switchbacks for that stream. The switchbacks had a mean width of \(50,000 \, \rm{km}\), with an aspect ratio of the order of \(10\). We find that switchbacks are not aligned along the background flow direction, but instead aligned along the local Parker spiral, perhaps suggesting that they propagate along the magnetic field. Since the observed switchback duration depends on how the spacecraft cuts through the structure, the duration alone cannot be used to determine the size or influence of an individual event. For future PSP orbits, a larger spacecraft transverse component combined with more radially aligned switchbacks will lead to long duration switchbacks becoming less common.
The solar wind escapes from the solar corona and is accelerated, over a short distance, to its terminal velocity. The energy balance associated with this acceleration remains poorly understood. To ...quantify the global electrostatic contribution to the solar wind dynamics, we empirically estimate the ambipolar electric field (\(\mathrm{E}_\parallel\)) and potential (\(\Phi_\mathrm{r,\infty}\)). We analyse electron velocity distribution functions (VDFs) measured in the near-Sun solar wind, between 20.3\,\(R_S\) and 85.3\,\(R_S\), by the Parker Solar Probe. We test the predictions of two different solar wind models. Close to the Sun, the VDFs exhibit a suprathermal electron deficit in the sunward, magnetic field aligned part of phase space. We argue that the sunward deficit is a remnant of the electron cutoff predicted by collisionless exospheric models (Lemaire & Sherer 1970, 1971, Jockers 1970). This cutoff energy is directly linked to \(\Phi_\mathrm{r,\infty}\). Competing effects of \(\mathrm{E}_\parallel\) and Coulomb collisions in the solar wind are addressed by the Steady Electron Runaway Model (SERM) (Scudder 2019). In this model, electron phase space is separated into collisionally overdamped and underdamped regions. We assume that this boundary velocity at small pitch angles coincides with the strahl break-point energy, which allows us to calculate \(\mathrm{E}_\parallel\). The obtained \(\Phi_\mathrm{r,\infty}\) and \(\mathrm{E}_\parallel\) agree well with theoretical expectations. They decrease with radial distance as power law functions with indices \(\alpha_\Phi = -0.66\) and \(\alpha_\mathrm{E} = -1.69\). We finally estimate the velocity gained by protons from electrostatic acceleration, which equals to 77\% calculated from the exospheric models, and to 44\% from the SERM model.
