R-3 Repository :: Browsing by Author "Alexander, David"

Browsing by Author "Alexander, David"

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Dynamics and Evolution of Solar Eruptive Prominences
(2014-04-24) Zhu, Chunming; Alexander, David; Toffoletto, Frank R.; Niu, Fenglin
Solar eruptive phenomena, including eruptive solar prominences/filaments, solar flares and Coronal Mass Ejections (CMEs), have severe impact on the Earth’s space environment and human activities: so-called Space Weather. The dynamics and evolution of the prominences/filaments are important for our understanding of the initiation processes that drive CMEs and lead to drastic energy release in the solar flares. This thesis focuses on recent progresses on the destabilization and subsequent eruption of the prominences/filaments, via three primary case studies that elucidate the most important activities occurring in the eruptive prominence: eruption of a bifurcated solar filament, interchange reconnection facilitating a filament eruption, and the interaction of two distinct filaments with subsequent production of solar flares. In Chapter 2, we study a partial eruption of a bifurcated filament which exhibited clear and strong kinking motion of the filament axis (∼ 120◦ rotation). Seven mass transfer events are identified and are thought to also transfer magnetic flux from the lower to upper branch, leading to the generation of ideal instabilities, that subsequently triggered the eruption of the upper branch. In Chapter 3, we present evidence of interchange reconnection driven by the interaction of an erupting filament with a nearby coronal hole that leads to the eruption of this filament. Kinking motions in this filament serves to bring the magnetic field of its eastern leg in close contact with the unipolar magnetic field of the coronal hole where it drives the reconnection that governs the subsequent evolution of the fila- ment and coronal hole boundary. The observed EUV brightenings and bi-directional flows in the contact layer formed by this interaction, along with the occurrence of type III radio bursts that are strongly related to escaping electrons along open fields, provide corroborative evidence for the occurrence of reconnection at this location. A consequence of this interaction was the development of a complex CME, that displayed both open and closed features: we believe this is the first time such a CME configuration has been observed directly in association with a filament eruption. In Chapter 4, an interaction between two filaments, rarely reported before, is identified and studied. This complex interaction is responsible for the production of a hard x-ray coronal source as part of a C3.0 class solar flare. The observed hard x-ray coronal source occurring between the two filaments, driven by a convergence of the filaments, and a newly formed hot plasma layer, indicate that magnetic reconnection occurred between the magnetic fields associated with both filaments. The eruption of the filaments later led to the onset of a much larger solar flare, class M2.9, as expected from the standard flare model. It is interesting to note that both loop shrinkage and supra-arcade downflows (SADs) are present during this M2.9 flare.
Dynamics of solar eruptive filaments
(2008) Liu, Rui; Alexander, David
The solar filament is one of the most important structures that lead to the destabilization of the solar corona, thereby driving the space weather in the Earth space. The dynamics of solar eruptive filaments is crucial for us to understand the physics governing the initiation of coronal mass ejections (CMEs). In this thesis we concentrate on kinking filaments and asymmetric eruptive filaments, which feature unique dynamic evolutions with implication of distinct initiation mechanisms. Kinking filaments with their warped axes are generally regarded as the 'fingerprint' of the MHD helical kink instability. Theoretical/numerical modelings of the kink instability in the solar context have raised a number of interesting issues which can only be fully addressed with detailed observational inputs. Our study on the kink evolution in a number of filament eruptions with a wide range of different natures provide a complete picture of how the kink instability works in the interactions of the filament with its magnetic environment. Our work has shown evidence supporting the writhing motion of the filament spine as a precursor of eruptive phenomena in the solar corona, and as a key component in regulating the nature of the eruption, in terms of full, partial or failed eruptions. The dynamic evolution of both kinking and asymmetric eruptive filaments has significant impacts on the production of hard X-ray emission. We have identified two types of asymmetric eruptive filaments: whipping-like and zipping-like, which are associated with the shifting of hard X-ray sources in different ways. Both can be understood in terms of how the highly sheared filament channel field responds to an external asymmetric magnetic confinement. In kinking filaments, our study suggests that two distinct processes take place during the kink evolution, leading to two types of HXR emission with different morphological connections to the overall magnetic configuration. Self-consistent, qualitative models are constructed in both studies. These results improve our understanding of the physical processes leading to the destabilization and eruption of solar filaments, and have significant impact on the modeling of the CME initiation and evolution.
Evidence for Impulsive Heating of Active Region Coronal Loops
(2013-07-24) Reep, Jeffrey; Bradshaw, Stephen; Alexander, David; Ecklund, Karl
We present observational and numerical evidence supporting the theory of impulsive heating of the solar corona. We have run numerical simulations solving the hydrodynamic equations for plasma confined to a magnetic flux tube, for the two distinct cases of steady and impulsive heating. We find that steady heating cannot explain the observed amount of low-temperature plasma in active regions on the sun. The results for impulsive heating closely match those of the observations. The ratio of heating time to cooling time predominantly determines the observed temperature distribution of the plasma. We have also identified an observational bias in calculating intensities of spectral lines in previous studies, which causes an under-estimation of low-temperature plasma. We predict Doppler shifts in the observed line emission that are in agreement with observations, and which may serve as a diagnostic of the strength of heating. We conclude that impulsive heating of active region coronal loops is more likely than steady heating.
Experimental Characterization of Plasma Detachment from Magnetic Nozzles
(2013-09-16) Olsen, Christopher; Cloutier, Paul A.; Chang-Diaz, Franklin R.; Alexander, David; Tittel, Frank K.
Magnetic nozzles, like Laval nozzles, are observed in several natural systems and have application in areas such as electric propulsion and plasma processing. Plasma flowing through these nozzles is inherently tied to the field lines and must separate for momentum redirection or particle transport to occur. Plasma detachment and associated mechanisms from a magnetic nozzle are investigated. Experimental results are presented from the plume of the VASIMR® VX-200 device flowing along an axisymmetric magnetic nozzle and operated at two ion energies to explore momentum dependent detachment. The argon plume expanded into a 150m3 vacuum chamber where the background pressure was low enough that charge-exchange mean-free-paths were longer than experiment scale lengths. This magnetic nozzle system is demonstrated to hydrodynamically scale up to astrophysical plasmas, particularly the solar chromosphere, implying general relevance to all systems. Plasma parameters were mapped over a large spatial range using measurements from multiple plasma diagnostics. The data show that the plume does not follow the magnetic field lines. A mapped integration of the ion flux shows the plume may be divided into three regions where 1) the plume briefly follows the magnetic flux, 2) diverges quadratically before 3) expanding with linear trajectories. Transitioning from region 1→2, the ion flux departs from the magnetic flux suggesting ion detachment. An instability forms in region 2 driving an oscillating electric field that causes ions to expand before enhancing electron cross-field transport through anomalous resistivity. Transitioning from region 2→3 the electric field dissipates, the trajectories linearize, and the plume effectively detaches. A delineation of sub-to-super Alfvénic flow aligns well with the inflection points of the linearization without a change in magnetic topology. The detachment process is best described as a two part process: First, ions detach by a breakdown of the magnetic moment when the quantity |v/fcLB| becomes of order unity. Second, the turbulent electric field enhances electron transport up to a factor of 4±1 above collisional diffusion; electron cross-field velocities approximate that of the ions and depart on more centralized field lines. Electrons are believed to detach by breakdown of magnetic moment further downstream in the weaker magnetic field.
Exploring the Effects of Stellar Magnetism on the Potential Habitability of Exoplanets
(IOP Publishing, 2024) Atkinson, Anthony S.; Alexander, David; Farrish, Alison O.
Considerable interest has centered on Earth-like planets orbiting in the circumstellar habitable zone (CHZ) of its star. However, the potential habitability of an exoplanet depends upon a number of additional factors, including the presence and strength of any planetary magnetic field and the interaction of this field with that of the host star. Not only must the exoplanet have a strong enough magnetic field to shield against stellar activity, but it must also orbit far enough from the star to avoid direct magnetic connectivity. We characterize stellar activity by the star’s Rossby number, Ro, the ratio of stellar rotation rate to convective turnover time. We employ a scaled model of the solar magnetic field to determine the star’s Alfvén radius, the distance at which the stellar wind becomes super-Alfvénic. Planets residing within the Alfvén surface may have a direct magnetic connection to the star and therefore not be the most viable candidates for habitability. Here, we determine the Rossby number of a sample of 1053 exoplanet-hosting stars for which the rotation rates have been observed and for which a convective turnover time can be calculated. We find that 84 exoplanets in our sample have orbits which lie inside the CHZ and that also lie outside the star’s Alfvén surface: 34 of these have been classified as terran (11) or superterran (23) planets. Applying the Alfvén surface habitability criterion yields a subset of the confirmed exoplanets that may be optimal targets for future observations in the search for signatures of life.
High Temperature Plasma Dynamics in Solar Flares
(2021-08-13) Mandage, Revati Sudam; Bradshaw, Stephen; Alexander, David; Gonnermann, Helge
Spatially resolved spectroscopic observations show wing enhancements and broadening in extreme ultraviolet emission from particular hot iron lines. Several physical processes ranging from plasma turbulence, magnetic perturbations to non-Gaussian ion populations, and non-thermal physics have been proposed to play a role in their formation. In this thesis I investigate in detail the role of plasma dynamics in spectral line shapes by studying the wing enhancements of Fe XXIII and XXIV observed during solar flares, using a field-aligned hydrodynamic model. First I examine how plasma dynamics in a single, monolithic flaring loop contributes to the formation of line asymmetry. This is done by running 35 simulations that use the observed values and their uncertainties for the driving electron beam parameters. Next I study the effect of flaring loop length on spectral line shape and broadening by running simulations with different loop lengths and the same beam parameters. The presence of sub-resolution structures, confirmed by increasingly high-resolution observations, and observational difficulties in isolating a monolithic loop from nearby loops, necessitate an investigation into the effect of superposed dynamics on some line asymmetries. Hence, I design multiloop models that are representative of three possible configurations of loops. Here I study how the resultant Fe XXIII spectral line profiles differ in each case and examine the differences between these multiloop models and the single loop model. I also briefly explore the role of a constant time delay in heating successive sub-loops of a multiloop configuration on the spectral line shape. The results show that the single loop model can successfully reproduces line asymmetries, and the loop length plays an important role in explaining some of the key observations such as the positive correlation between Doppler shifts and line widths, and broad but symmetric hot Fe lines. For a multiloop model with sub-loops of the same length, an imposed heating time delay is an important factor that significantly alters the line profile shape from the single loop case. Whereas, multiloop models with sub-loops of varied lengths predict significantly different line profiles, such as asymmetric lines for longer durations and with large blue-shifts, without the necessity of introducing time delays.
Hydrodynamic Modeling of Heating Processes in Solar Flares
(2014-10-09) Reep, Jeffrey; Bradshaw, Stephen J; Alexander, David; Warburton, Tim
This thesis examines the heating of the solar atmosphere due to energy release in solar flares. A one-dimensional hydrodynamic model, which solves the equations of conservation of mass, momentum, and energy along a magnetic flux tube, is described in detail and employed to study the dynamic response of the solar atmosphere to large amounts of energy release from the magnetic field. A brief introduction to the solar atmosphere and solar flares, from both observational and theoretical perspectives, is given. Then, the hydrodynamic model is described, along with derivations of energy deposition due to a beam of highly energetic electrons colliding with the ambient atmosphere (and their implementation in the model is explained). Using this model of heating along with RHESSI-derived parameters of observed flares, the sensitivity of the GOES flare classification to the parameters of the electron beam (the non-thermal energy, the power-law index of the electron distribution, and the low-energy cut-off) are examined, and clear correlations are determined. Next, the response of the atmosphere to heating due to isoenergetic beams of electrons are studied, to elucidate the importance of electrons at different energy. It is found that at high total energy fluxes, the energy of individual electrons are unimportant, but that at lower fluxes, lower energy electrons are significantly more efficient at heating the atmosphere and driving chromospheric evaporation than high energy electrons. It is also found that the threshold for explosive evaporation is strongly dependent on the cut-off energy, as well as the beam flux. A case study of a well-observed flare is performed. The flare, a C-class flare that occurred on 28 November 2002, was modeled for various cases of heating due to a beam of electrons, in situ coronal heating, and a hybrid model that combines both forms of heating. It is found that the observation of X-ray source heights seen with RHESSI are most consistent with a hybrid model. The results indicate that the energy must be partitioned between thermal and kinetic energy, and the implications are discussed. This work is then summarized, and future avenues of research are discussed. Improvements that can be made to the model, the forward modeling of emission, and comparisons to observations are discussed.
Light-guide snapshot imaging spectrometer for remote sensing applications
(The Optical Society, 2019) Wang, Ye; Pawlowski, Michal E.; Cheng, Shuna; Dwight, Jason G.; Stoian, Razvan I.; Lu, Jiawei; Alexander, David; Tkaczyk, Tomasz S.
A fiber-based snapshot imaging spectrometer was developed with a maximum of 31853 (~188 x 170) spatial sampling and 61 spectral channels in the 450nm-750nm range. A compact, custom-fabricated fiber bundle was used to sample the object image at the input and create void spaces between rows at the output for dispersion. The bundle was built using multicore 6x6 fiber block ribbons. To avoid overlap between the cores in the direction of dispersion, we selected a subset of cores using two alternative approaches; a lenslet array and a photomask. To calibrate the >30000 spatial samples of the system, a rapid spatial calibration method was developed based on phase-shifting interferometry (PSI). System crosstalk and spectral resolution were also characterized. Preliminary hyperspectral imaging results of the Rice University campus landscape, obtained with the spectrometer, are presented to demonstrate the system’s spectral imaging capability for distant scenes. The spectrum of different plant species with different health conditions, obtained with the spectrometer, was in accordance with reference instrument measurements. We also imaged Houston traffic to demonstrate the system’s snapshot hyperspectral imaging capability. Potential applications of the system include terrestrial monitoring, land use, air pollution, water resources, and lightning spectroscopy. The fiber-based system design potentially allows tuning between spatial and spectral sampling to meet specific imaging requirements.
Magnetic helicity injection and velocity characteristics of rotating sunspots
(2011) Zhu, Chunming; Alexander, David
This thesis presents calculations of the magnetic helicity injection due to rotating sunspots and a determination of the characteristics of the rotating sunspots in the active regions with simple magnetic configurations. Four active regions are investigated to study the relationship between rotating sunspots and magnetic helicity. The observations indicate that significantly more helicity is injected during the period of rotation in polarities with strong magnetic field. This may be a result of the emergence of a magnetic flux rope from below the solar surface. Moreover, our preliminary study on a large sample of 90 active regions shows that the level of flaring activity increases with the rate of helicity injection. Finally, a statistical study is carried out to determine the relation between rotating sunspots and the emergence of magnetic flux tubes. Among 82 active regions which exhibit flux emergence, 93% are associated with rotating sunspots. Among 50 active regions without well-defined flux emergence, 60% of sunspots are observed to be rotating, though relatively slowly. In addition, we find that sense of the rotation (i.e., clockwise or counter-clockwise) of the sunspots shows a weak hemispherical tendency.
Magnetic Modeling of the Solar Corona
(2011) Sandman, Anne; Alexander, David
The magnetic field is the dominant force in the Sun's outer atmosphere, the corona. It determines the large-scale structure of the corona, governs the small-scale activity that heats the plasma, and powers energetic transient phenomena. The study of the coronal magnetic field is an important component of efforts to understand the corona as a whole, but the high plasma temperature and relative weakness of the field in the corona preclude direct measurement of the field in this region. Theoretical modeling, therefore, provides critical support to our investigation of the coronal magnetic field. This thesis contains an exploration of magnetic modeling in the context of the physics of the solar atmosphere. Using the novel stereoscopic capability of the Solar Terrestrial Relations Observatory, we compare the 3D coronal field structure with various models to study the distribution of the magnetic field and large-scale currents in several active regions. In addition to using existing models, we developed an original modeling approach: using several magnetic dipoles under the solar surface, configured based on comparison with the observed field structure, we overcome certain weaknesses inherent in existing models. The misalignment angles between the vectors of the reconstructed and model fields measure the departure of the model field from the observed field structure, indicating the non-potentiality of the coronal field. The dipole modeling method achieves a significant reduction in misalignment compared with previously established models, implying a closer agreement between our model field and observed field structures. We use the misalignment measurements to examine the free energy contained in active region magnetic fields and find that following major flares, the field relaxes substantially from a high-shear configuration to a low-shear configuration, indicating a significant loss of free energy in the field. The results in this thesis yield insight into the physics of the solar atmosphere and provide a means to better understand the complex region between the photosphere and corona. Improved understanding of this region will elucidate how the field connects the two regions, and the way in which energy is transported from the convective solar interior into the corona and heliosphere.
Mass composition and dynamics in quiet sun prominences
(2009) Kilper, Gary K.; Alexander, David
Solar prominences are transient phenomena in the solar atmosphere that display highly dynamic activity and can result in dramatic eruptions, ejecting a large amount of material into the heliosphere. The dynamics of the prominence plasma reveal information about its interaction with the magnetic field of the prominence, while the eruptions are associated with coronal mass ejections, which greatly affect space weather near Earth and throughout the solar system. My research on these topics was conducted via observational analyses of the partially-ionized prominence material, its composition, and the dynamics over time in prominences that range in activity from quiescent to highly active. The main results are evidence that (1) in quiescent prominences, neutral He is located more in the lower part of the structure, (2) a higher level of activity in prominences is related to a mixing of the material, and (3) an extended period of high activity and mixing occurs prior to eruptions, possibly due to mass loading. In addition, innovative modifications to analytical techniques led to measurements of the material's mass, composition, and small-scale dynamics.
Modeling of Solar-Stellar Connections and the Heliophysics of Exoplanet Systems
(2021-04-28) Farrish, Alison O.; Alexander, David
In the past two and a half decades, advances in the field of exoplanet detection have confirmed more than 4,000 known planets outside of our Solar System \cite{exoplanets}. With this wealth of data, the field is now poised to transition from a phase of detection to one of more in-depth characterization of planetary processes and evolution. Exoplanet systems are of interest not only for the potential for habitability, but also in the opportunity they provide for the study of comparative heliophysics - the similarities and differences in physical interactions between the central host star and any associated planets. In applying solar- and heliophysics-based knowledge and tools to the study of exoplanet systems, we can expand our understanding of the breadth of possible star-planet interactions and the influence of stellar behavior on planetary environments and processes such as atmospheric loss, planetary magnetosphere dynamics, ionospheric emission, and more. We present here a series of studies of solar-stellar connections and the heliophysics of exoplanet systems, employing a surface flux transport treatment of photospheric flux emergence, migration, and dispersal, and the application of this solar-based modeling framework to exoplanet host stars. In Chapter 1, we describe the state of the field of exoplanet characterization, relevant solar physics concepts, and the context for making solar-stellar comparisons. Chapter 2 comprises the methodology employed in modeling stellar photospheres, coronae, and asterospheres with relevance to exoplanet space weather environments. Chapter 3 expands upon the solar-stellar connection, demonstrating the application of magnetic flux transport modeling to the simulation of stellar activity across a broad population of cool stars. Chapter 4 details our modeling of magnetic and energetic environments driving star-planet interaction. In Chapter 5, we present examples of applications of this work to planetary response modeling, detailed investigations of stellar extreme ultraviolet (EUV) emission, and young Sun analogs. Future applications of our integrated modeling approach, particularly in comparisons with solar dynamo models and upcoming observing campaigns from Parker Solar Probe and James Webb Space Telescope, are discussed in Chapter 6.
Modeling Stellar Activity-rotation Relations in Unsaturated Cool Stars
(IOP Publishing, 2021) Farrish, Alison O.; Alexander, David; Johns-Krull, Christopher M.; Li, Minjing
We apply a surface flux transport model developed for the Sun to reconstruct the stellar activity-rotation relationship, LX/Lbol versus Ro, observed for unsaturated cool stars (Rossby numbers Ro ≳ 0.1). This empirical flux transport model incorporates modulations of magnetic flux strength consistent with observed solar activity cycles, as well as surface flux dynamics consistent with observed and modeled stellar relationships. We find that for stellar flux models corresponding to a range of 0.1 ≲ (Ro/RoSun) ≲ 1.2, the LX/Lbol versus Ro relation matches well the power-law behavior observed in the unsaturated regime of cool stars. Additionally, the magnetic activity cycles captured by the stellar simulations produce a spread about the power-law relation consistent with that observed in cool star populations, indicating that the observed spread may be caused by intrinsic variations resulting from cyclic stellar behavior. The success of our flux transport modeling in reproducing the observed activity relationship across a wide range of late-F, G, K, and M stars suggests that the photospheric magnetic fields of all unsaturated cool stars exhibit similar flux emergence and surface dynamic behavior, and may hint at possible similarities in stellar dynamo action across a broad range of stellar types.
Modeling the Space Weather Environment of Terrestrial Exoplanets
(2021-04-28) Sciola, Anthony; Toffoletto, Frank; Alexander, David
The majority of currently known terrestrial exoplanets orbit close to their host stars, on the order of 0.05 AU. Such planets orbiting M Dwarf stars, assuming an Earth-like atmosphere, have the potential to possess liquid water on their surface, a key requirement for habitability of life as we are familiar with. However such close proximity to the star is also likely to result in the planet experiencing stellar wind pressures orders of magnitude greater than that at Earth. An understanding of whether the planet possesses an intrinsic magnetic field, or magnetosphere, and how this interacts with the extreme stellar wind, is necessary in order to constrain other parameters such as atmospheric loss rate. There is a unique radio emission which is expected to be commonly produced by magnetized planets, and is produced by every magnetized Solar System planet. Despite estimates predicting that such emission from Jupiter-sized set of exoplanets should be observable, there have been no confirmed detections thus far. This thesis utilizes magnetohydrodynamic (MHD) models coupled to the Rice Convection Model (RCM), originally developed to simulate Earth’s magnetosphere, to better understand the magnetic environments of terrestrial exoplanets. The first project adapts a coupled MHD+RCM model to simulate the environment of Proxima Centauri b, and estimates the rate of atmospheric loss via charge exchange. The second addresses the calculation of expected radio emission from a given exoplanetary environment, both analytically and numerically, which includes the effects of both ionospheric saturation, and secondary inner magnetosphere currents, on the radio signal’s location of emission and total signal power. This work may be used to better determine which star-planet systems may be more likely to produce observable radio emission, and are therefore better targets for future observation.
Multi-wavelength analysis of solar transient phenomena
(2009) Coyner, Aaron J.; Alexander, David
Solar transient phenomena such as solar flares and coronal mass ejection are some of the most energetic and explosive phenomena affecting the solar environment. Emission signatures within solar flares provide direct insight into the physical mechanisms involved in the flaring process as well as the role the magnetic field topology plays in the energy release and particle transport within flares. Specifically, the work here addresses the temporal and spatial relationships between ultraviolet and hard X-ray flare emissions while also addressing the relationship between hard X-ray emission evolution in flares and the development of quasi-separatrix layers (QSLs) within the magnetic structure of the flaring region. As a final component, we address the implications of pre-event solar conditions such as magnetic configuration and flare productivity on the particle composition of solar energetic particle (SEP) events seen at 1AU. Specifically, we find that co-spatial and co-temporal UV and hard X-ray emission expected in 1-D loop flare models only account for a portion of the observed flare emission, and a complete explanation of the flaring process must take into account more complex and time-varying magnetic topologies along with contributions from multiple physical processes. Finally, we find, for particle events, that closed magnetic configurations at higher energies result in higher average Fe/O enhancements while the amount of open field and the active region appear to have no direct relationship to the observed SEP compositions.
Nonthermal hard X-ray flux saturation in solar flares
(2005) Daou, Antoun Georges; Alexander, David
We use the unprecedented spectral and spatial resolution of RHESSI to explore the behavior of electrons and their associated currents in solar flares. Spectral images are used to determine an estimate of the effective surface area for the different independent substructures within each event. The incident electron spectra at those flaring footpoints are derived from the RHESSI photon spectra. We find that, over a wide range of flare X-ray magnitudes, the integrated photon flux above 20 keV asymptotically approaches a limiting value, suggesting a saturation of the photon production in flares. The inferred particle fluxes in the beam, together with this saturation limit, are used to determine the energy loss mechanism dominating the energetic particle transport in solar flares.
Observational analysis of the compositional variation in solar filaments
(2007) Kilper, Gary K.; Alexander, David
Solar filaments have long been associated with coronal mass ejections and Space Weather phenomena. Filaments are dynamcal, and changes in their overall mass or its distribution could trigger an eruption. However, a comprehensive study of the elemental composition, abundance, and dynamics in a variety of filaments had not yet been conducted. The research work presented here is a detailed study of twenty filaments with various properties, which was conducted by analyzing the absorption due to the filaments in cotemporal He I (10830 A) and Haalpha (6563 A) images. The results show bands of relative helium deficits and surpluses that imply a stratification of filament material, possibly due to neutral atom cross-field diffusion. For more highly-varying material; there is a weak direct relation between a change in absorption and a change in the relative abundance of helium, and erupting filaments exhibit increasing absorption and a spatial homogenization of material in the pre-erupting filament section.
Observational and theoretical interpretation of energetic particle transport in solar flares
(2008) Daou, Antoun Georges; Alexander, David
The combination of excellent space-based remote sensing, and image reconstruction techniques, as well as improvements in numerical modeling, help enhance our understanding of particle transport in solar flares. We conduct a rigorous analysis of flare hard X-ray emission using the unprecedented spectral and spatial resolution of the RHESSI telescope data in order to better understand the spectral properties of the emitting electron population in solar flares. We complete our study with a forward-fit to the data using a Fokker-Planck kinetic code, to numerically model the particle transport in phase-space in realistic magnetic geometries and for different particle injection profiles.
Radiometric and design model for the tunable light-guide image processing snapshot spectrometer (TuLIPSS)
(Optical Society of America, 2021) Zheng, Desheng; Flynn, Christopher; Stoian, Razvan I.; Lu, Jiawei; Cao, Haimu; Alexander, David; Tkaczyk, Tomasz S.; Tkaczyk, Tomasz S.
The tunable light-guide image processing snapshot spectrometer (TuLIPSS) is a novel remote sensing instrument that can capture a spectral image cube in a single snapshot. The optical modelling application for the absolute signal intensity on a single pixel of the sensor in TuLIPSS has been developed through a numerical simulation of the integral performance of each optical element in the The tunable light-guide image processing snapshot spectrometer (TuLIPSS) is a novel remote sensing instrument that can capture a spectral image cube in a single snapshot. The optical modelling application for the absolute signal intensity on a single pixel of the sensor in TuLIPSS has been developed through a numerical simulation of the integral performance of each optical element in the TuLIPSS system. The absolute spectral intensity of TuLIPSS can be determined either from the absolute irradiance of the observed surface or from the tabulated spectral reflectance of various land covers and by the application of a global irradiance approach. The model is validated through direct comparison of the simulated results with observations. Based on tabulated spectral reflectance, the deviation between the simulated results and the measured observations is less than 5% of the spectral light flux across most of the detection bandwidth for a Lambertian-like surface such as concrete. Additionally, the deviation between the simulated results and the measured observations using global irradiance information is less than 10% of the spectral light flux across most of the detection bandwidth for all surfaces tested. This optical modelling application of TuLIPSS can be used to assist the optimal design of the instrument and explore potential applications. The influence of the optical components on the light throughput is discussed with the optimal design being a compromise among the light throughput, spectral resolution, and cube size required by the specific application under consideration. The TuLIPSS modelling predicts that, for the current optimal low-cost configuration, the signal to noise ratio can exceed 10 at 10 ms exposure time, even for land covers with weak reflectance such as asphalt and water. Overall, this paper describes the process by which the optimal design is achieved for particular applications and directly connects the parameters of the optical components to the TuLIPSS performance.
Report on RSI-CNES Workshop: Sustainable Space Exploration
(James A. Baker III Institute for Public Policy of Rice University, 2023) Alexander, David; Jernigan, Mark; Mbezal, Hugues; Baker Institute Science and Technology Policy Program
This report documents a workshop between the Rice Space Institute (RSI) and Centre national d'études spatiales (CNES) held at the Rice Global Paris Center on December 4 - 6, 2023. The workshop centered on addressing the technical and policy challenges facing a sustainable human presence in space and identifying the various government and commercial demand signals that drive the technology needs and timeline, as well as considering the policy, ethical and regulatory changes/approaches required to tackle these needs.

Browsing by Author "Alexander, David"

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