Assessment of the Origin of a Plasma Depletion Band Over the United States During the 8 September 2017 Geomagnetic Storm

The development of an intense total electron content (TEC) depletion band over the United States during the 8 September 2017 geomagnetic storm was understood as the extension of an equatorial plasma bubble (EPB) to midlatitudes in previous studies. However, this study reports non‐EPB aspects within this phenomenon. First, the simultaneous emergence of the TEC depletion band at midlatitudes and EPBs in the equatorial region indicates that the midlatitude TEC depletion band is not initiated by an EPB. Second, the intensification of TEC depletion at midlatitudes during the decay of TEC depletion at intermediate latitudes is anomalous. Third, the location of the TEC depletion band at midlatitudes is inconsistent with the EPB location estimated from zonal plasma motion. Given ionospheric perturbations in North America from the beginning of the storm, it is plausible that the TEC depletion band was locally generated in association with these perturbations. Plain Language Summary Intense plasma depletions occasionally occur at midlatitudes during geomagnetic storms. Due to their morphological similarity to plasma bubbles that develop in the equatorial region, midlatitude depletions are often considered extensions of equatorial plasma bubbles (EPBs) to midlatitudes. However, midlatitude depletions are also recognized as locally generated phenomena. During the 8 September 2017 geomagnetic storm, an anomalously large total electron content (TEC) depletion band emerged in TEC maps over the American sector. This feature appears as a single structure, extending from the equatorial region to midlatitudes in both hemispheres. While this phenomenon is commonly understood as the extension of an EPB to midlatitudes, this study reports non‐EPB aspects that were not discussed in previous studies. Key Points A new perspective on the interpretation of a plasma depletion band over the United States during the 8 September 2017 storm is presented Equatorial plasma bubble (EPB) was proposed as its source in previous studies, but we report non‐EPB characteristics in this phenomenon Considering its emergence time, location relative to EPB, and intensification with time, this event can be a local midlatitude phenomenon