Browsing by Author "Xu, Z."

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    The Atmospheric-Ionospheric-Magnetospheric Responses to the 2015 St. Patrick's Day Geomagnetic Storm at High Latitudes
    Xu, Z.; Clauer, C. Robert; Chu, Xinzhao; Hartinger, Michael D.; Zhao, Jian (2016-12-15)
    The atmospheric-ionospheric-magnetospheric (AIM) system response to extreme solar wind conditions depends on the solar wind driving conditions, ionospheric configurations, and neutral atmospheric transportation. The 17 March 2015 geomagnetic storms driven by coronal mass ejections (CME) provide an opportunity to investigate how the global AIM response depends on the solar wind inputs. In this study, multiple instruments, including lidars, magnetometers, HF radars, satellites, and others, are combined to provide global, coordinated coverage in the AIM system. First, we examined the ionospheric responses at high latitude regions in both the northern and southern hemispheres, by using the conjugate West Greenland and Antarctic magnetometer chains to remotely sense several current systems. There were dramatic differences between the intensity, duration, and spatial structure of the current systems between hemispheres. Then, we examined the neutral atmospheric response and its connection with the MI systems in the high latitude regions with the Fe Boltzmann Lidar observations at the McMurdo station in Antarctica. The neutral Fe layer observed by Lidar from abnormally high altitudes (nearly 160km) is enhanced during the storm. It should be associated with not only the neutral atmospheric factors but also MI factors such as Joule heating and ionospheric electromagnetic drifting. These multiple instrument observations present an overall picture and help understand the AIM coupling mechanisms better.
  • Factors influencing the profitability of fertilizer use on maize in Zambia
    Xu, Z.; Guan, Z.; Jayne, T. S.; Black, R. (International Association of Agricultural Economists, 2009)
    Despite government subsidies and the potential benefits to yield and income, fertilizer use in most of Africa remains low. Determining the barriers to its use and the factors influencing its profitability is critical in increasing its popularity. This study uses longitudinal farm data in a generalized asymmetrical production model in order to approximate maize yields in response to household features, traditional production inputs, and government programs in Zambia. The analysis determined that both input and non-input factors, such as market conditions, application rate, fertilizer availability, animal use, and even death of a family member, played roles in determining yield response and overall profitability of fertilizer use on maize. This research illustrates the importance of considering household heterogeneity in regards to fertilizer use and yield response in both productions models and, perhaps more importantly, in government programming and planning.
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    First observations of core-transiting seismic phases on Mars
    Irving, J. C. E.; Lekic, V.; Durán, C.; Drilleau, M.; Kim, D.; Rivoldini, A.; Khan, A.; Samuel, H.; Antonangeli, D.; Bruce Banerdt, W.; Beghein, C.; Bozdagk, E.; Ceylan, S.; Charalambous, C.; Clinton, J.; Davis, P.; Garcia, R.; Giardini, D.; Catherine Horleston, A.; Huang, Q.; Hurst, K. J.; Kawamura, T.; King, Scott D.; Knapmeyer, M.; Li, J.; Lognonné, P.; Maguire, Ross; Panning, M. P.; Plesa, A. C.; Schimmel, M.; Schmerr, N. C.; Stählerc, S. C.; Stutzmann, E.; Xu, Z. (Proceedings of the National Academy of Sciences, 2023-05-02)
    We present the first observations of seismic waves propagating through the core of Mars. These observations, made using seismic data collected by the InSight geophysical mission, have allowed us to construct the first seismically constrained models for the elastic properties of Mars core. We observe core-Transiting seismic phase SKS from two farside seismic events detected on Mars and measure the travel times of SKS relative to mantle traversing body waves. SKS travels through the core as a compressional wave, providing information about bulk modulus and density. We perform probabilistic inversions using the core-sensitive relative travel times together with gross geophysical data and travel times from other, more proximal, seismic events to seek the equation of state parameters that best describe the liquid iron-Alloy core. Our inversions provide constraints on the velocities in Mars core and are used to develop the first seismically based estimates of its composition. We show that models informed by our SKS data favor a somewhat smaller (median core radius = 1,780 to 1,810 km) and denser (core density = 6.2 to 6.3 g/cm3) core compared to previous estimates, with a P-wave velocity of 4.9 to 5.0 km/s at the core mantle boundary, with the composition and structure of the mantle as a dominant source of uncertainty. We infer from our models that Mars core contains a median of 20 to 22 wt% light alloying elements when we consider sulfur, oxygen, carbon, and hydrogen. These data can be used to inform models of planetary accretion, composition, and evolution.
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    Interhemispheric Asymmetries in the Ground Magnetic Response to Interplanetary Shocks: The Role of Shock Impact Angle
    Xu, Z.; Hartinger, Michael D.; Oliveira, Denny M.; Coyle, Shane; Clauer, C. Robert; Weimer, Daniel R.; Edwards, T. R. (2020-03)
    Interplanetary (IP) shocks drive magnetosphere-ionosphere (MI) current systems that in turn are associated with ground magnetic perturbations. Recent work has shown that IP shock impact angle plays a significant role in controlling the subsequent geomagnetic activity and magnetic perturbations; for example, highly inclined shocks drive asymmetric MI responses due to interhemispherical asymmetric magnetospheric compressions, while almost head-on shocks drive more symmetric MI responses. However, there are few observations confirming that inclined shocks drive such asymmetries in the high-latitude ground magnetic response. We use data from a chain of Antarctic magnetometers, combined with magnetically conjugate stations on the west coast of Greenland, to test these model predictions (Oliveira & Raeder, 2015, https://doi.org/10.1002/2015JA021147; Oliveira, 2017, https://doi.org/10.1007/s13538-016-0472-x). We calculate the time derivative of the magnetic field (partial derivative B/partial derivative t) in each hemisphere separately. Next, we examine the ratio of Northern to Southern Hemisphere partial derivative B/partial derivative t intensities and the time differences between the maximum. partial derivative B/partial derivative t immediately following the impact of IP shocks. We order these results according to shock impact angles obtained from a recently published database with over 500 events and discuss how shock impact angles affect north-south hemisphere asymmetries in the ground magnetic response. We find that the hemisphere the shock strikes first usually has (1) the first response in partial derivative B/partial derivative t and (2) the most intense response in partial derivative B/partial derivative t. Additionally, we show that highly inclined shocks can generate high-latitude ground magnetic responses that differ significantly from predictions based on models that assume symmetric driving conditions.
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    Surface waves and crustal structure on Mars
    Kim, D.; Banerdt, W. B.; Ceylan, S.; Giardini, D.; Lekic, V.; Lognonne, P.; Beghein, C.; Beucler, E.; Carrasco, S.; Charalambous, C.; Clinton, J.; Drilleau, M.; Duran, C.; Golombek, M.; Joshi, R.; Khan, A.; Knapmeyer-Endrun, B.; Li, J.; Maguire, Ross; Pike, W. T.; Samuel, H.; Schimmel, M.; Schmerr, N. C.; Stahler, S. C.; Stutzmann, E.; Wieczorek, M.; Xu, Z.; Batov, A.; Bozdag, E.; Dahmen, N.; Davis, P.; Gudkova, T.; Horleston, A.; Huang, Q.; Kawamura, T.; King, Scott D.; McLennan, S. M.; Nimmo, F.; Plasman, M.; Plesa, A. C.; Stepanova, I. E.; Weidner, E.; Zenhausern, G.; Daubar, I. J.; Fernando, B.; Garcia, R. F.; Posiolova, L.; Panning, M. P. (AAAS, 2022-10-28)
    We detected surface waves from two meteorite impacts on Mars. By measuring group velocity dispersion along the impact-lander path, we obtained a direct constraint on crustal structure away from the InSight lander. The crust north of the equatorial dichotomy had a shear wave velocity of approximately 3.2 kilometers per second in the 5- to 30-kilometer depth range, with little depth variation. This implies a higher crustal density than inferred beneath the lander, suggesting either compositional differences or reduced porosity in the volcanic areas traversed by the surface waves. The lower velocities and the crustal layering observed beneath the landing site down to a 10-kilometer depth are not a global feature. Structural variations revealed by surface waves hold implications for models of the formation and thickness of the martian crust.