Thawing of ice-rich permafrost and subsequent ground subsidence can form characteristic landforms, and the resulting topography they create are collectively called “thermokarst”. The impact of wild...

Post-wildfire surface deformation near Batagay, Eastern Siberia, detected by L-band and C-band InSAR

Sporadic-E (Es) is a layer of ionization that irregularly appears within the E region of the ionosphere and is known to generate an unusual propagation of very high frequency waves over long distances. The detailed spatial structure of Es remains unclear due to the limited spatial resolution in the conventional ionosonde observations. We detect midlatitude Es by interferometric synthetic aperture radar (InSAR), which can clarify the spatial structure of Es with unprecedented resolution. Moreover, we use the range split-spectrum method (SSM) to separate dispersive and nondispersive components in the InSAR image. While InSAR SSM largely succeeds in decomposing into dispersive and nondispersive signals, our results indicate that small-scale dispersive signals due to the total electron content anomalies are accompanied by nondispersive signals with similar spatial scale at the same locations. We also examine the effects of higher-order terms in the refractive index for dispersive media. Both of these detected Es episodes indicate that smaller-scale dispersive effects originate from higher-order effects. We interpret that the smaller-scale nondispersive signals could indicate the emergence of nitric oxide (NO) generated by the reactions of metals, Mg and Fe, with nitric oxide ion (NO+) during the Es.

/pdf/midlatitude-sporadic-e-episodes-viewed-by-l-band-split-1nu7vcntgz.pdf

Midlatitude sporadic-E episodes viewed by L-band split-spectrum InSAR

Large earthquakes disturb the ionosphere in several different ways, and we can observe them with GNSS networks as changes in TEC. First, I discuss coseismic disturbances that appear ~10 minutes after earthquakes as sudden TEC oscillations propagating towardmagnetic equator with the acoustic wave speed (~1 km/s). Coseismic uplift/subsidence excite acoustic waves, and components with periods of 4–5 minutes propagate to the ionospheric F region and cause TEC oscillations. An empirical law is proposed to relate their amplitudes to moment magnitudes (Mw) of earthquakes using ~30 events Mw6.6–9.2. A very large earthquake is often followed by resonant atmospheric oscillation lasting for hours. Rayleigh surface waves also make acoustic waves and cause TEC oscillations thousands of kilometers away from epicenters. Secondly, I review current knowledge of TEC changes occurring immediately before large earthquakes. They start 10–80 minutes before earthquakes and make positive electron density anomalies above faults about to rupture. Comparison of 18 earthquakes Mw7.3–9.2 suggests positive correlations with Mw in their leading times, changes in TEC rates, and cumulative anomalies. The three-dimensional structure of electron density anomalies suggest they are formed by E× B drift caused by the penetration of electric fields, possibly made by surface charges, through the ionosphere.

Ionospheric Disturbances Related to Earthquakes

We studied ionospheric irregularities caused by midlatitude sporadic-E (Es) in Japan using ionospheric total electron content (TEC) data from a dense GNSS array, GEONET, with a 3D (three-dimensional) tomography technique. Es is a thin layer of unusually high ionization that appears at altitudes of ~ 100 km. Here, we studied five cases of Es irregularities in 2010 and 2012, also reported in previous studies, over the Kanto and Kyushu Districts. We used slant TEC residuals as the input and estimated the number of electron density anomalies of more than 2000 small blocks with dimensions of 20–30 km covering a horizontal region of 300 × 500 km. We applied a continuity constraint to stabilize the solution and performed several different resolution tests with synthetic data to assess the accuracy of the results. The tomography results showed that positive electron density anomalies occurred at the E region height, and the morphology and dynamics were consistent with those reported by earlier studies.

/pdf/3d-tomography-of-midlatitude-sporadic-e-in-japan-from-gnss-12163zez7y.pdf

3D tomography of midlatitude sporadic-E in Japan from GNSS-TEC data

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2017JA025133

Strong Sporadic E Occurrence Detected by Ground-Based GNSS

Kilometer-scale fine structures of midlatitude sporadic E (Es) plasma patches have been directly imaged for the first time by an interferogram derived from L band Advanced Land Observation Satellite/Phased Array-type L band Synthetic Aperture Radar data obtained over southwestern Japan. The synthetic aperture radar interferogram captured the eastern part of a large-scale frontal structure of daytime midlatitude Es which spans over 250 km in the east-northeast to west-southwest direction. Fine structures are characterized by frontal and disc-shaped patches which are elongated in the same direction as the large-scale frontal structure. Length and width of the disc-shaped patches are 10–20 km and 5–10 km, respectively, and they are quasi-periodically located with a typical separation of 10–15 km. The Kelvin-Helmholtz instability with the vertical shear of zonal winds is considered to be the most likely candidate for the generation mechanism of the frontal patch and disc-shaped patches aligned in the zonal direction.

/pdf/imaging-the-midlatitude-sporadic-e-plasma-patches-with-a-shs8hrz6nf.pdf

Imaging the midlatitude sporadic E plasma patches with a coordinated observation of spaceborne InSAR and GPS total electron content

/pdf/a-step-by-step-recipe-of-band-spli3ng-technique-for-isola8on-4bpy200lt8.pdf

A step-by-step recipe of band-spli3ng technique for isola8on of ionospheric signal in L-band InSAR data

Small-scale horizontal structures of daytime midlatitude sporadic E are studied by interferometric synthetic aperture radar (InSAR) and GPS total electron content (TEC) observations over Japan. With GPS-TEC observations, sporadic E with foEs higher than 16 MHz can be detected [Maeda and Heki, 2014]. A dense array of GPS receiving stations in Japan (GEONET) enables us to image horizontal shapes of sporadic E by plotting vertical TEC anomalies on a map. Such TEC maps revealed that sporadic E over Japan has a common shape which is elongated in the east-west (E-W) direction with typical length and width of ~200 km and ~20 km, respectively, regardless of occurrence latitudes [Maeda and Heki, 2014; 2015]. In this study, we observed smaller-scale structures by InSAR as well as GPS-TEC observations. The spatial resolution is ~300 m for InSAR and ~2 km for GPS-TEC (by analyzing raw slant TEC time series). The results show that small-scale plasma patches are embedded in large scale frontal structures and such small patches are quasi-periodically located both in zonal and meridional directions. There are two major candidates for the generation mechanism of daytime sporadic E structure, i.e, namely, atmospheric gravity waves, and the Kelvin-Helmholtz (K-H) instability. We speculate that the K-H instability in the neutral atmosphere under the presence of vertical shear of zonal winds would create billow structures elongated in the zonal direction. On the other hand, secondary instability, such as secondary turbulance in the neutral atmosphere or gradient-drift instability are the two major possible drivers for the patch structuring in the meridional direction.

/pdf/possible-shear-instability-in-the-daytime-midlatitude-1i4x5gq2sr.pdf

Takato Suzuki

Papers

Imaging the midlatitude sporadic E plasma patches with a coordinated observation of spaceborne InSAR and GPS total electron content

Midlatitude sporadic-E episodes viewed by L-band split-spectrum InSAR

A step-by-step recipe of band-spli3ng technique for isola8on of ionospheric signal in L-band InSAR data

Possible shear instability in the daytime midlatitude sporadic-E observed with InSAR and GPS-TEC