Morphological characteristics of daytime mid-latitude sporadic-E (Es) patches are studied by two-dimensional total electron content (TEC) maps drawn using the Japanese dense network of Global Positioning System (GPS) receivers. By analyzing over 70 cases, we found that their horizontal shapes are characterized by frontal structure typically elongated in east-west by~100 km. They are observed to migrate mainly northward in the morning and southward in the afternoon with speeds of 30-100 m/s. This may reflect the velocities of neutral winds controlled by the atmospheric tides. Such frontal structures are often found to include smaller scale structures.

[1] We observed two-dimensional structure and time evolution of ionospheric irregularities caused by midlatitude sporadic E (E s ) over Japan as positive anomalies of total electron content (TEC) by analyzing the data from the nationwide Global Positioning System (GPS) array. In this paper we report a case study of strong E s observed in the local evening of 21 May 2010, over Tokyo, Japan. In the slant TEC time series, E s showed a characteristic pulse-like enhancement of~1.5 TEC units lasting for~10 min. We plotted these positive TEC anomalies on the subionospheric points of station-satellite pairs to study the horizontal structure of the E s irregularity. We confirmed that the irregularity existed at the height of 106 km by comparing the data of multiple GPS satellites, which is consistent with the local ionosonde observations. The horizontal shapes of the E s irregularity showed frontal structures elongated in E-W, spanning~150 km in length and~30 km in width, composed of small patches. The frontal structure appears to consist of at least two parts propagating in different directions: one moved eastward by~60 m s À1 , and the other moved southwestward by~80 m s À1 . Similar TEC signatures of E s were detected by other GPS satellites, except one satellite that had line of sight in the N-S direction which dips by 40-50°toward north, which indicates the direction of plasma transportation responsible for the E s formation. We also present a few additional observation results of strong E s irregularities.Citation: Maeda, J., and K. Heki (2014), Two-dimensional observations of midlatitude sporadic E irregularities with a dense GPS array in Japan, Radio Sci., 49, 28-35,

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.

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.

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.