One of the major changes we’ve made since last year’s expedition is the extension of our imaging diagnostic from a single all-sky camera + magnetometer to an improved suite of multiple, individually-deployable observation units. The benefits of this new instrument array are manyfold. First, production of several units in parallel has refined the design and assembly of the units and has made them more robust. Second, having several units provides redundancy to protect against downtimes during repairs or from lack of data owing to inauspicious placement of a unit. Further, imaging the aurora and measuring local changes in magnetic field in several distinct locations opens up an entirely new direction of potential scientific investigation: tomographic reconstruction of the distribution of certain constituent species in the ionosphere.
The aurora’s airglow has 3-dimensional extent and evolves in time, tracing out the dynamics of the atmospheric emitters as illuminated by the precipitating solar wind primaries. Quantitative determination of the shape and evolution of the airglow is a major stepping stone in uncovering much interesting physics associated with the aurora, including the magnetohydrodynamics governing atmospheric plasmas and the spectrum of auroral primaries. It’s desirable, then, to extend the all-sky time lapses we made last year to be full 3-dimensional movies of the aurora. This can be achieved through the use of multiple cameras. The basic principle is familiar – in the same way our eyes digest an object’s parallax to infer depth, the systematic comparison of images taken of the airglow at multiple known locations can help us infer the total shape.
A well-known and ideally-implemented example of tomographic reconstruction is in magnetic resonance imaging (MRI). A sample is imaged from all directions with fine angular resolution, allowing 3D density profiles to be extracted algorithmically. Several groups have successfully applied this principle to auroral airglow imaging despite the limited viewing angles and we’re attempting to recreate these efforts with simple, inexpensive hardware. Our goal is to deploy several observation units spaced by dozens of kilometers in order to image the airglow along multiple lines of sight. With knowledge of the latitude, longitude, and altitude of each unit, time-synchronized pictures can be used to correct an initial guess of what the airglow’s 3D density profile may be. Done at each exposure, this procedure will yield a 3D movie of the aiglow’s evolution.
The major contribution to the aurora’s brightness is the 557.7nm OI emission line, primarily from the oxygen-dense lower ionosphere. We’re not using any optical filters, so this means we’ll be using our field-deployable observation units to perform optical tomographic reconstruction of the density of atomic oxygen plasma. This data can be benchmarked against our simultaneously-recorded vector magnetic field data, as one should expect a large current sheet to produce a magnetic field. This observational scheme serves to investigate intermediate-scale spatiotemporal dynamics that can be hard to access by large satellite arrays and localized sounding rocket measurements.
Check back in for updates on this effort as we build out our data analysis pipeline!