7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

GNSS observations of traveling ionospheric disturbances in the ionosphere
Y. Otsuka1, A. Shinbori1, T. Sori1, M. Nishioka2, M. Nishioka2, S. Perwitasari2
1
Institute for Space-Earth Environmental Research (ISEE), Nagoya University
2
National Institute of Information and Communications Technology (NICT)
e-mail (speaker): otsuka@isee.nagoya-u.ac.jp

Traveling Ionospheric Disturbance (TID) is aphenomenon characterized by wave-like structures in theelectron density of the ionosphere. Since the 1960s,numerous observational and theoretical studies on TIDs
have been conducted[1]. Based on their temporal andspatial scales, TIDs with horizontal wavelengths ofseveral hundred kilometers and periods ranging from 15minutes to 1 hour are classified as Medium-Scale TIDs
(MSTIDs). MSTIDs have been believed to be caused byatmospheric gravity waves (GWs) propagating from thelower atmosphere to the ionosphere.
In recent years, Global Navigation Satellite System(GNSS) data have been utilized to measure Total
Electron Content (TEC) along the path of radio waves
propagating from satellites to receivers. By utilizing TEC
data from multiple satellites and a dense network ofGNSS receivers, researchers have been able to reveal
two-dimensional horizontal structures of TIDs.
Consequently, it has been discovered that MSTIDs
exhibit different characteristics between daytime andnighttime (Fig. a). During nighttime, MSTIDs display awavefront elongating from northwest (NW) to southeast(SE) in the northern hemisphere. The polarizationelectric field is considered a contributing factor tonighttime MSTIDs. Generation mechanism for thenighttime MSTIDs can be explained as follows[2]:MSTIDs, which are plasma density perturbations,produce spatial inhomogeneity of the Pedersen
conductivity (𝛿𝛿𝛴𝛴𝑃𝑃 ). In the mid-latitude F region, theelectric current 𝑱𝑱 flows northeastward during nighttime.
When 𝛿𝛿𝛴𝛴𝑃𝑃 has a structure elongated from NW to SE, 𝑱𝑱
traverses 𝛿𝛿𝛴𝛴𝑃𝑃 . For this case, the polarization electricfield (𝛿𝛿𝑬𝑬), which is northeastward (southwestward) inthe regions of low (high) 𝛿𝛿𝛴𝛴𝑃𝑃 , should be generated tomaintain a divergence-free 𝑱𝑱. This electric field moves
the plasma upward (downward) by 𝑬𝑬 × 𝑩𝑩 drift, causing
the electron density perturbations, that is MSTID. The
sporadic E (Es) layer also plays an important role for theFigure 1. TEC variations over Japan[2]. MSTIDs
during (left) daytime and (right) nighttime are seen.

MSTID generation through the polarization electricfields generated by Es layer instability.
Most of the MSTIDs that appear during the daytime
propagate equatorward and occur most frequently in
winter. This feature is consistent at different longitudinal
sectors in both the northern and southern hemispheres.
The preference of equatorward propagation is one of thereasons why daytime MSTIDs are considered to becaused by atmospheric gravity waves[2]. Through thecollision of neutral gases with ions in the F-region, theions move along the geomagnetic field lines ( 𝑩𝑩 ),whereas ion motion across 𝑩𝑩 is restricted because theion–neutral collision frequency is smaller than thegyrofrequency. This directivity of ion motion causes
directivity in the response of the plasma density
variations to the gravity waves. The largest-amplitude
TEC perturbations could be caused by gravity waves
propagating equatorward because the neutral windoscillation parallel to 𝑩𝑩 is larger for gravity waves
propagating equatorward than for those propagating inother directions. Such a directivity in the response of theplasma to the gravity waves could be responsible for theequatorward propagation of the daytime MSTIDs.
Following major earthquakes such as the 2004
Sumatra-Andaman earthquake (M9.1) and the 2011
Tohoku earthquake (M9.1), TEC variations propagatingradially from the epicenter at various propagation
velocities were observed. TEC variations propagating ata few km/s could be induced by acoustic waves launchedfrom the Rayleigh-wave propagating on the surface[3, 4].
TEC variations propagating at a sound velocity in thethermosphere are also observed. After the 15 January2022 Hunga Tonga-Hunga Ha’apai volcanic eruption,
TEC variations associate with the air pressure waves
propagating radially from the volcano as a Lamb modein the troposphere were observed[5]. Effects of theelectric field transmission along the geomagnetic fieldfrom the other hemisphere on MSTID generation is alsoseen.
In this presentation, various types of MTIDs observed byGNSS-TEC will be shown.
References[1] R.D. Hunsucker, Reviews of Geophysics, 20, 293–315, doi:10.1029/RG020i002p00293 (1982)[2] Y. Otsuka, In Ionosphere Dynamics and Applications,doi:10.1002/9781119815617.ch18 (2021)[3] Y. Otsuka et al., Earth Planets Space, 58, 159-165,
doi:10.1186/BF03353373 (2006)[4] T. Tsugawa et al., Earth Planets Space, 63, 66,
doi:10.5047/eps.2011.06.035 (2011)[5] A. Shinbori et al., Earth Planets Space, 74, 106, doi:
10.1186/s40623-022-01665-8