Unusual Phase Alignment of Nighttime Electrified Medium‐Scale Traveling Ionospheric Disturbance After Moderate Geomagnetic Storms and Response to Substorms

Patgiri, Dipjyoti and Otsuka, Yuichi and Shreedevi, P. R. and Kakoti, Geetashree and Ranjan, Alok Kumar and Rathi, Rahul and Shinbori, Atsuki and Nishioka, Michi and Perwitasari, Septi (2025) Unusual Phase Alignment of Nighttime Electrified Medium‐Scale Traveling Ionospheric Disturbance After Moderate Geomagnetic Storms and Response to Substorms. Geophysical Research Letters, 52 (20). ISSN 0094-8276

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Abstract

Plain Language Summary: Plasma instabilities, driven by different forcings, give rise to density irregularities in the Earth's ionosphere that significantly disrupt trans‐ionospheric radio wave propagation, thereby affecting space‐based communication and navigation systems. Among these irregularities, medium‐scale traveling ionospheric disturbances (MSTIDs) are quasi‐periodic perturbations in F‐region plasma density, typically characterized by horizontal wavelengths of a few hundred kilometers. Nighttime MSTIDs in the northern hemisphere typically exhibit a northwest–southeast alignment and propagate southwestward, often generated through Perkins instability seeded by E–F region coupling, and are commonly known as electrified MSTIDs (EMSTIDs). Northeast‐southwest aligned EMSTID was observed in detrended total electron content maps over Japan on 17 March 2003, following two moderate geomagnetic storms. A comparative analysis of thermospheric wind and vertical ion drift measurements between the storm day and quiet days revealed significant storm‐time modifications in both wind and electric fields. These disturbed conditions likely created a favorable environment for the growth of Perkins instability, leading to the generation of the atypically aligned EMSTID. Interestingly, ionosonde observation revealed that Perkins instability is sufficiently enhanced to generate EMSTIDs, even with weak E‐F region coupling within or across hemispheres. Additionally, EMSTID amplitudes showed instantaneous modulation with substorm's expansion/recovery phases, suggesting the penetration of electric fields.