1,721,256 research outputs found
On the Sun's faintest coronal hard X-rays
Get PDFSolar flares are the most vigorous explosive phenomena in our solar system. They release up to 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 and erg cm s. 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, 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 photons s cm keV.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 0.0032 photons cm s keV (50 times that of RHESSI) at 8 keV and a dynamic range of 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 photons s cm keV and photons s cm keV, 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 ( photons s cm keV) but with an important difference: it took 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 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)
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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Ion Distribution Functions in the Near-Sun Solar Wind
Get PDFParker 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
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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Multi-Spacecraft Observations of Collisionless Shocks in the Heliosphere
Get PDFIn 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
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Studies of Ionosphere/Thermosphere Plasma-Neutral Coupling in Response to Various Energy Drivers
Get PDFExtending from approximately 85 to 1,000 km above Earth’s surface, the terrestrial ionosphere-thermosphere (I-T) system holds growing significance as we increasingly rely on space-based infrastructure. The I-T system comprises a mixture of ionized and neutral gases whose dynamics are tightly coupled, generating a variety of complex phenomena, some of whichremain poorly characterized and understood. This dissertation presents studies investigating three of the most enigmatic I-T phenomena, each driven by a distinct energy source. Specifically, we examine:
1. rapid electrodynamic changes and plasma redistribution following explosive events in the lower atmosphere, with a particular focus on the 2022 eruption of the Hunga Tonga-Hunga Ha’apai (hereafter ‘Tonga’) volcano,
2. thermospheric wind perturbations linked to the abrupt changes in solar inputs triggered daily by the setting sun, and
3. aurora-like glows in the subauroral ionosphere associated with rapid ion flows in the upper ionosphere during geomagnetically active periods.
Each study employs both observational data and theoretical modeling to examine the coupled plasma-neutral response to the energy driver and to characterize and explain the ensuing phenomenon.
The first study delves into the ionospheric effects of the 2022 Tonga volcanic eruption. This eruption drove global-scale atmospheric waves that propagated into space and propelledionospheric disturbances. This dissertation investigates the ionospheric consequences of the eruption within about 5,000 km of the volcano. The study demonstrates the immediate
large-scale electrodynamic effects of the eruption using observations from NASA’s Ionospheric Connection Explorer (ICON) satellite. Extreme (>100 m/s) east-west and vertical ion drifts are observed thousands of kilometers away from the volcano within an hour of the eruption, before the arrival of any known neutral atmospheric wave. The measured ion drifts are magnetically conjugate to the ionospheric E region about 400 km from Tonga. A theoretical calculation shows that the observed ion drifts are consistent with the ionospheric E region dynamo effects of an expanding neutral atmospheric wavefront with a large (>200 m/s) neutral wind amplitude. The analysis suggests that the thermospheric neutral winds initiated by the eruption interacted with the E Region ionospheric plasma and created strong electric potentials which propagated along Earth’s magnetic field via Alfv ́en waves and caused the observed plasma drifts in the opposite hemisphere. These observations are the first direct detection in space of the rapid and extreme electrodynamic consequences of
a volcanic eruption and contributes to our understanding of the coupling between the lower atmosphere and I-T system following explosive events such as this eruption.
The second study considers the daily effect of the setting sun on the I-T system. The moving solar terminator (ST) generates atmospheric disturbances, broadly termed solar terminator waves (STWs). Despite theoretically recurring daily, STWs remain poorly understood, partially due to measurement challenges near the ST. By presenting analysis of neutral wind data from the Michelson Interferometer for Global High-resolution Thermospheric Imaging (MIGHTI) onboard the ICON satellite, this dissertation reveals observations of STW signatures in thermospheric neutral winds, including the first observed meridional wind signatures. Seasonal analysis demonstrates that STWs are most prominent during solstices, when they intersect the ST about ∼ 20◦ latitude from the equator in the winter hemisphere and have phase fronts inclined at a ∼ 40◦ angle to the ST. This work provides the first observed STW altitude profiles, revealing large (>200 km) vertical wavelengths above 200 km. Comparing these observations to four different models suggests the STWs likely originate directly or indirectly with waves from below 97 km. These results indicate that STWs may play an under-recognized role in the daily variability of the I-T system, warranting further study.
Finally, this dissertation considers aurora-like emissions which arise equatorward of the auroral oval in conjunction with extremely fast ionospheric ion flows. The ‘picket fence’ is a captivating visual phenomenon featuring vibrant green streaks. It is often observed concurrently with and at lower altitudes than the rare purpleish-white arc called STEVE (Strong Thermal Emission Velocity Enhancement). Despite its aurora-like appearance, recent studies suggest that the picket fence may not be driven by magnetospheric particle precipitation but instead by local electric fields parallel to Earth’s magnetic field. This dissertation evaluates the parallel electric fields hypothesis by quantitatively comparing picket fence spectra with the emissions generated in a kinetic model driven by local parallel electric fields energizing ambient electrons in a realistic neutral atmosphere. The results demonstrate that, at a typical picket fence altitude of 110 km, parallel electric fields between 40 and 70 Td (∼80 to 150 mV/m at 110 km) energize ambient electrons sufficiently so that, when they collide with neutrals, they reproduce the observed ratio of N2 first positive to atomic oxygen green line emissions, without producing N2+ first negative emissions, consistent with the features observed in picket fence spectra. These findings establish a quantitative connection between ionospheric electrodynamics and observable picket fence emissions, offering verifiable targets for future models and experiments.
The work presented in this dissertation has contributed to ongoing I-T research as well as spawned new research directions, including providing benchmarks for more detailed modeling studies of the Tonga volcanic eruption, demonstrating the need for in-depth modeling follow-up studies to examine the origin of STWs and their effects on the ionosphere, and leading to a proposal for a rocket campaign to measure the parallel electric fields that may drive picket fence emissions for the first time
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Measuring the polarization of compact objects with the Compton Spectrometer and Imager
Get PDFγ-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
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Observational Signatures of Nonlinear Interactions in the Solar Wind
Get PDFSpacecraft 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
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