Assessing the validity of location-based social media in the study of spatial processes

The advent of big spatial data has created new opportunities for studying geographic phenomena. Open mapping projects, citizen science initiatives, and location-based social media all fall under the umbrella of volunteered geographic information and are now frequently used spatial data sources. The fact that these sources are user-contributed as opposed to gathered by experts has raised significant concerns over data quality. While data accuracy, particularly in open mapping projects (e.g., OpenStreetMap), has been given considerable attention, far less has been paid to data validity, specifically on location-based social media. In this three article dissertation, I explore the validity of location-based social media in the study of spatial processes. In the first article, I implement a survey on the Oklahoma State campus to explore college students' behaviors and perceptions of location-based social media and note differences in terms of gender, race, and academic standing. The second and third articles are empirical studies utilizing geolocated data from Twitter, a popular social media platform. The second article makes use of precise location data (e.g., latitude - longitude) and uses geographically weighted regression to explore the patterns of non-English Twitter usage in Houston, Texas. The third article uses general location data (e.g., city) to explore the patterns of #BlackLivesMatter and counter-protest content across the states of Louisiana and Texas. The results of these studies collectively provide an optimistic, though cautionary, outlook on the use of location-based social media data in geography

Measuring Social Interaction Potential in Oklahoma City and Tulsa

Social degradation is now a serious problem in cities in the United States. People are interacting with their neighbors, families, and those within their community less frequently. Some major causes of this are central city decline, urban sprawl, and suburbanization. People within cities are now farther away from each other and from activity locations, making social interaction more difficult. Recently, time geography has researchers rethinking traditional approaches to these topics along with studies in transportation, accessibility, and mobility. This subdiscipline of geography is based on an individualized approach that incorporates the constraints that all humans face on a day-to-day basis. In my thesis I evaluate the social interaction potential (SIP) of working populations in the urbanized areas of Oklahoma City and Tulsa, focusing on coupling constraints that limit the opportunities for individuals to meet face-to-face at certain activity locations and interact with each other. Using restaurants as activity locations, I calculate the amount of time that can potentially be spent interacting with others at activity locations based on different combinations of work locations and home locations within the two cities. Several variables strongly affect the calculation of SIP such as the population within each zone, the driving time to activity locations, and the distance from all other zones. While Oklahoma City and Tulsa are similar in terms land area, population, restaurant location distribution, and recent capital improvement projects, the two have different urban forms which affect the patterns of SIP. While there is generally more SIP near the center of both cities, Oklahoma City shows a more dispersed and sectoral pattern with "fingers" of SIP extending from the downtown area, while Tulsa shows a more central city phenomenon with high concentration of SIP in the center and a steep drop off in SIP away from the center.Geograph

Ionized Gas in the Smith Cloud

We present WHAM observations of Halpha, [N II], and [S II] in the Smith Cloud. A map of Halpha emission from the cloud shows ionized gas coincident with the brightest H I emission, but nearly-as-bright Halpha in some regions with faint H I. The ionized mass of the cloud is at least as large as the neutral mass, > 10^6 M_sun. Ionized gas in the core of the Smith Cloud has an electron temperature 6000 K < T < 16000 K. The observed ratio [N II] / Halpha = 0.39 \pm 0.09 shows that the cloud has a non-primordial nitrogen abundance, 0.1 - 1 times solar.Comment: 4 pages, 2 figures. To appear in the proceedings of "The Role of Disk-Halo Interaction in Galaxy Evolution: Outflow vs Infall?", EAS Publication Serie

Confirmation of a Faraday Rotation Measure Anomaly in Cygnus

We confirm the reality of a reversal of the sign of the Faraday Rotation Measure in the Galactic plane in Cygnus (Lazio et al, 1990), possibly associated with the Cygnus OB1 association. The rotation measure changes by several hundred rad/m$^2$ over an angular scale of $2-5^{\circ}$. We show that a simple model of an expanding plasma shell with an enhanced density and magnetic field, consistent with observations of H$\alpha$ emission in this part of sky, and physically associated with a superbubble of the Cygnus OB1 association, can account for the magnitude and angular scale of this feature.Comment: Submitted to the Astrophysical Journa

Photoionization of High Altitude Gas in a Supernova-Driven Turbulent Interstellar Medium

We investigate models for the photoionization of the widespread diffuse ionized gas in galaxies. In particular we address the long standing question of the penetration of Lyman continuum photons from sources close to the galactic midplane to large heights in the galactic halo. We find that recent hydrodynamical simulations of a supernova-driven interstellar medium have low density paths and voids that allow for ionizing photons from midplane OB stars to reach and ionize gas many kiloparsecs above the midplane. We find ionizing fluxes throughout our simulation grids are larger than predicted by one dimensional slab models, thus allowing for photoionization by O stars of low altitude neutral clouds in the Galaxy that are also detected in Halpha. In previous studies of such clouds the photoionization scenario had been rejected and the Halpha had been attributed to enhanced cosmic ray ionization or scattered light from midplane H II regions. We do find that the emission measure distributions in our simulations are wider than those derived from Halpha observations in the Milky Way. In addition, the horizontally averaged height dependence of the gas density in the hydrodynamical models is lower than inferred in the Galaxy. These discrepancies are likely due to the absence of magnetic fields in the hydrodynamic simulations and we discuss how magnetohydrodynamic effects may reconcile models and observations. Nevertheless, we anticipate that the inclusion of magnetic fields in the dynamical simulations will not alter our primary finding that midplane OB stars are capable of producing high altitude diffuse ionized gas in a realistic three-dimensional interstellar medium.Comment: ApJ accepted. 17 pages, 7 figure

The Turbulent Warm Ionized Medium: Emission Measure Distribution and MHD Simulations

We present an analysis of the distribution of H-alpha emission measures for the warm ionized medium (WIM) of the Galaxy using data from the Wisconsin H-Alpha Mapper (WHAM) Northern Sky Survey. Our sample is restricted to Galactic latitudes |b| > 10. We removed sightlines intersecting nineteen high-latititude classical H II regions, leaving only sightlines that sample the diffuse WIM. The distribution of EM sin |b| for the full sample is poorly characterized by a single normal distribution, but is extraordinarily well fit by a lognormal distribution, with = 0.146 +/- 0.001 and standard deviation 0.190 +/- 0.001. drops from 0.260 +/- 0.002 at Galactic latitude 10<|b|<30 to 0.038 +/- 0.002 at Galactic latitude 60<|b|<90. The distribution may widen slightly at low Galactic latitude. We compare the observed EM distribution function to the predictions of three-dimensional magnetohydrodynamic simulations of isothermal turbulence within a non-stratified interstellar medium. We find that the distribution of EM sin |b| is well described by models of mildy supersonic turbulence with a sonic Mach number of ~1.4-2.4. The distribution is weakly sensitive to the magnetic field strength. The model also successfully predicts the distribution of dispersion measures of pulsars and H-alpha line profiles. In the best fitting model, the turbulent WIM occupies a vertical path length of 400-500 pc within the 1.0-1.8 kpc scale height of the layer. The WIM gas has a lognormal distribution of densities with a most probable electron density n_{pk} = 0.03 cm^{-3}. We also discuss the implications of these results for interpreting the filling factor, the power requirement, and the magnetic field of the WIM.Comment: 16 pages, 13 figures, ApJ in press. Replacement reflects version accepted for publicatio