Expansion and Contraction of the Auroral Oval Area

Abstract

<p>The aurora is a visible manifestation of Earth’s coupling to near-Earth space. The emitted light is produced by charged particles that precipitate into the upper atmosphere. These particles are usually located on closed magnetic field lines that connect directly between the northern and southern hemisphere. As a result, Earth’s aurora often appears in an oval shape surrounding the magnetic pole. Inside the oval, at high magnetic latitudes, is a region with open magnetic field lines that extend into the solar wind. This region of open magnetic flux is the polar cap and is a consequence of the Dungey cycle: Reconnection between the solar wind magnetic field and closed terrestrial field lines at the dayside magnetopause produces open field lines which are transported to the nightside where they are again closed by reconnection. A fundamental property of the magnetosphere-ionosphere system is that changes in the amount of open magnetic flux is equal to the net difference between the dayside and nightside reconnection rates. That is, the polar cap expands when dayside reconnection dominates and contracts when nightside reconnection dominates. This is known as the expanding/contracting polar cap paradigm, and has been studied extensively in the last few decades. The expansion and contraction of the aurora itself has received less attention. In this work, we use global auroral images to study the spatiotemporal evolution of the auroral oval. We investigate how the solar wind, open flux and auroral flux covary. Furthermore, we attempt to determine how well a pure fluid description of the auroral zone can explain the observed evolution.</p>

Robust Estimates of Spatiotemporal Variations in the Auroral Boundaries Derived From Global UV Imaging

“…We have used our own method to detect initial boundaries from auroral images to construct the models, but any boundary data set from any source can be used (the data can be regular or irregular in both space and time). Both the processed data set and the code is publicly available (Ohma et al., 2023a, 2023b, 2023c). Since the model approach presented here provides the boundary normal velocity directly, it is ideally suited to be used in conjunction with ionospheric data assimilation techniques such as the Assimilative Mapping of Ionospheric Electrodynamics procedure (Lu, 2017; Richmond & Kamide, 1988) and the Local mapping of polar ionospheric electrodynamics (Lompe) technique (Hovland et al., 2022; Laundal et al., 2022) to estimate regional or global reconnection rates.…”

“…Auroral boundaries identified and used in this study (Ohma et al., 2023b), background‐corrected images (Ohma et al., 2023c), and the code to make these data sets (Ohma et al., 2023a) are available. The most recent version of the code is also available at https://github.com/aohma/fuvpy/.…”

Background removal from global auroral images: Data-driven dayglow modeling

“…We describe the approach in Section 3 and discuss the benefits and limitations of the proposed method in Section 4. The Python code to use this method and the data set used in this publication are publicly available (Ohma et al, 2022a(Ohma et al, , 2022b.…”