Browsing by Author "Burchill, Johnathan K."

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    Correction: Swarm Thermal Ion Imager measurement performance
    (2024-03-11) Burchill, Johnathan K.; Knudsen, David J.
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    Examining Surface Effects on the Swarm Satellite Langmuir Probes
    (2019-08-23) Quinn, Candice; Burchill, Johnathan K.; Knudsen, David J.; Brown, Jo C.; Cully, Christopher M.
    The Swarm satellites are equipped with two Langmuir probes to study the plasma environment around Earth. One probe is coated in Titanium nitride and the other in gold, providing a unique opportunity to study surface effects for different coatings. At their altitudes there are neutral particles that may contaminate the probes which adds an uncertainty in their measurements in the form of a time delay. The time delay is most evident by a shift in the current-voltage curve when the upsweep is compared to the downsweep. Using a simple model to fit a simulated current-voltage curve to the sampled curve, it is found that two of the three satellites have a time delay that ranges between 0.627 ms and 1.34 ms. In addition to contamination, there may be additional sources of measurement error that stem from the local environment that are not studied as they are not linked to contamination.
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    How variable are Birkeland currents?
    (2023-07-21) Burchill, Johnathan K.
    Abstract I address the problem of estimating the time-rate-of-change of high-latitude Birkeland currents by using a string-of-pearls formation of satellites. Space series are calculated by linear interpolation of measurements made at the revisit times of the satellites. A lower bound on the total time derivative can be estimated as a function of distance along the orbit. Space series of the vertical component of electric current density, used as a proxy for field-aligned (Birkeland) current density at high latitude, are estimated from the along-track spatial derivative of Swarm magnetic field measurements residual to the CHAOS-7 internal field model. The results reveal non-negligible total time derivatives over periods shorter than 2 mins. Auroral Birkeland current densities derived from single-satellite traversals of magnetic field gradients can change dramatically in the time it takes a single satellite to cross a large-scale current system. In one example, during an overflight by the Swarm satellites of the THEMIS Fort Yukon all-sky imager on 1 December 2013, the vertical current density poleward of a visually quiescent auroral arc changes from $$\sim 0.3\ \mu \,\hbox {A}/\hbox {m}^{2}$$ ∼ 0.3 μ A / m 2 upward to $$\sim 1.0\ \mu \,\hbox {A}/\hbox {m}^{2}$$ ∼ 1.0 μ A / m 2 downward in 13.7 s (corresponding to an along-track separation of Swarm A and B of 104 km). The variability of Auroral Birkeland currents, between 25 November 2013 and 31 December 2013, as estimated by the median of $$|dj_z/dt|$$ | d j z / d t | , reaches $$15\ \hbox {nA}/\hbox {m}^{2}/\textrm{s}$$ 15 nA / m 2 / s in the northern dayside auroral zone and exceeds $$30\ \hbox {nA}/\hbox {m}^{2}/\textrm{s}$$ 30 nA / m 2 / s in the pre-noon sector of the southern hemisphere. Graphic Abstract
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    Swarm Thermal Ion Imager measurement performance
    (2022-12-09) Burchill, Johnathan K.; Knudsen, David J.
    Abstract We assess the performance of the thermal ion imaging (TII) technique as conceived for the Swarm Earth Explorer satellites. Analysis, simulation, and laboratory testing performed prior to flight provided estimates of systematic and random error sources of the electric field instrument’s vector ion drift, electric field, and ion kinetic temperature measurements. An end-to-end instrument simulator, consisting of models of the TIIs on a prototypical Swarm satellite orbiting Earth with ionospheric plasma, electric field, and magnetic field inputs, was used to generate TII sensor data (level 0 instrument data). These data were processed with a prototype processor (the level 1b processor) to characterize theoretical measurement performance. We describe the methodology used to assess TII measurement uncertainty and present the main findings of the end-to-end measurement performance study. In addition, we assess the measurement performance achieved during approximately eight years of orbital operations. Example measurements demonstrate the quality of ion drift velocity. Unprocessed TII imagery reveals spurious signals which can affect measurement performance. Availability of such imagery has proven vital for diagnosing measurement anomalies associated with sensor operation and spacecraft–plasma interactions. Graphical Abstract