Sporadic-E Experiment Over Kyushu Research Articles

Electron temperature in nighttime sporadic E layer at mid-latitude

Abstract. Electron temperature in the sporadic E layer was measured with a glass-sealed Langmuir probe at a mid-latitude station in Japan in the framework of the SEEK (Sporadic E Experiment over Kyushu)-2 campaign which was conducted in August 2002. Important findings are two fold: (1) electron temperature and electron density vary in the opposite sense in the height range of 100–108 km, and electron temperature in the Es layer is lower than that of ambient plasma, (2) electron temperature in these height ranges is higher than the possible range of neutral temperature. These findings strongly suggest that the heat source that elevates electron temperature much higher than possible neutral temperature exists at around 100 km, and/or that the physical parameter values, which are used in the present theory to calculate electron temperature, are not proper.

A thermal mechanism for generation of small‐scale irregularities in the ionospheric E region

We present a mechanism for small‐scale irregularity generation in the E region due to a thermal instability driven by the neutral wind. The polarization electric field induced by the neutral wind causes factional heating due to ion neutral collisions. In plasma‐depleted regions this leads to enhanced plasma pressure and plasma is forced out, enhancing the plasma depletion. When heat advection is included, the threshold for instability is lower than neutral winds commonly observed in the midlatitude E region. Another factor contributing to the instability growth is the dependence of the ion neutral collisional frequency on ion temperature. This also results in a smaller threshold neutral wind due to both a reduced ion cooling rate during ion‐neutral energy exchange and heat advection. In a standard two‐fluid treatment the thermal theory predicts two preferable altitude ranges for small‐scale irregularity generation: between 93 and 102 km and near 110 km altitude. We find that the thermal processes may be responsible for the continuous and “thick” continuous echoes observed by the middle and upper atmosphere radar and by the frequency agile radar during the Sporadic E Experiment over Kyushu (SEEK) campaign for 3.2‐m and 6.1‐m field‐aligned irregularities, respectively. The theory may explain the SEEK campaign observations of 6.1‐m echoes at as high as 150 km altitude if the irregularity elongation is stronger than that due to ambipolar diffusion. The predicted phase velocities are much smaller than the ion acoustic velocity, which allows us to associate the instability with the type 2 radar echoes.

Semidiurnal behavior of quasi‐periodic echoes in the mid‐latitude Es region observed with the Chung‐Li VHF Radar

The range rate of the so‐called quasi‐periodic (QP) echoes from nighttime mid‐latitude sporadic‐E regions observed with the Chung‐Li VHF radar during the SEEK (Sporadic‐E Experiment over Kyushu) campaign displayed semidiurnal characteristics. The time periods before ∼2200 LT and after ∼0400 LT were dominated by the negative range rate QP (NQP) echoes. In contrast, a preponderance of positive range rate QP (PQP) echoes occurred between 2200 and 0400 LT. The slopes of NQP echoes found during the evening period steepened with time and became positive around 2200 LT. Similar behavior was also found prior to ∼0400 LT when PQP echoes were replaced by NQP echoes during the SEEK campaign. The sign of the range rate changed around 2200 and 0400 LT on the six consecutive nights. Semidiurnal neutral‐wind variations are proposed to explain the slope sign reversals in the observations. These results, together with previous findings, suggest that semidiurnal tides and long‐period waves (with 30 minute periodicity) may be playing a role, in addition to atmospheric gravity waves, in the formation of QP echoes.

The SEEK (Sporadic‐E Experiment over Kyushu) Campaign

The SEEK (Sporadic‐E Experiment over Kyushu) campaign was conducted in late August 1996 from the southern region of Kyushu, Japan, to investigate the mechanism for the generation of quasi‐periodic (QP) radar backscatter from field‐aligned irregularities imbedded in nighttime sporadic‐E (Es) layers. SEEK was designed to determine in‐situ small‐scale electrodynamical properties using two sounding rockets and large‐scale dynamics and electrodynamics using ground‐based sensors, which included a transportable radar and other radio and optical instruments deployed in the vicinity of the rocket range. It was observed by this campaign that Es layers existed in a convergent wind shear region, where large electric fields were induced and when active atmospheric gravity waves existed in the mesosphere. However, there was little evidence which positively supported the hypothesis that Es layers were deeply modulated in altitude.

E region observations over Chung‐Li during the SEEK Campaign

During the Sporadic E Experiment over Kyushu (SEEK) campaign E‐region field aligned irregularities (FAI) were observed with the Chung‐Li VHF Radar (operated on 52 MHz) in Taiwan between August 15–22, 1996. The characteristics of the quasi‐periodical echoes from the E‐region about 80 km north of the Chung‐Li radar are studied. During the same period ionogram records were taken with the Digisonde located at the Chung‐Li radar site. Strong sporadic‐E layers were detected with the Digisonde when the FAI, observed with the VHF radar, were also very intense. The variations of sporadic E‐layer critical frequency and of the field‐aligned irregularities were studied. The latter were estimated to be located at altitudes 95–115 km occurring in layers of 5–15 km thickness, moving downwards at a rate of about 2.5 km/h. There seemed to be a change over of the FAI echoes from lower to upper E‐region every night approximately at 23 hour LT. The distinction between post‐sunrise and post‐sunset FAI echoes observed earlier by MU radar was not clearly seen in Chung‐Li. A preliminary examination of the morphology of the fine structures within the layers indicates quasi‐periodic features with 5–10 minutes period. Many of these periodic fine structures were observed at the Chung‐Li VHF radar to have opposite slopes than those detected earlier with the MU radar in Japan, which is located 1500 km north of the Chung‐Li VHF radar.

The SEEK Chemical Release Experiment: Observed neutral wind profile in a region of sporadic E

Wind measurements made with the chemical release technique during a sporadic E layer event are presented. The data were obtained as part of the Sporadic E Experiment over Kyushu (SEEK) sounding rocket campaign. The winds show a strong maximum in the lower E region approaching 150 m s−1 near 105‐km altitude and a large shear below the maximum. The large shear was within a few kilometers altitude of the peak in the electron densities measured on the downleg of the rocket trajectory. Calculations of the Richardson numbers for the wind profile show that the altitude range near the layer was highly unstable.

Possible evidence of gravity wave coupling into the mid‐latitude F region ionosphere during the SEEK Campaign

On five of eight observation nights during the 1996 SEEK (Sporadic E Experiment over Kyushu) campaign, Japan, unusual “wave‐like” structures were imaged in the 630 nm thermospheric nightglow emission. Four of these events were observed to travel towards the southwest, providing new evidence in support of recent theories describing the coupling of medium‐scale gravity waves into the mid‐latitude F region ionosphere. Available ionosonde data and the visual characteristics of the wave structures indicate no association with the occurrence of mid‐latitude spread F or F region upwellings. Instead, the data support the novel concept of feedback from the ionosphere into the gravity wave, via the Perkins instability, to enhance its visibility.

DC electric field measurement in the SEEK Campaign

DC electric fields were measured by the Electric Field Detector ‐ Beam experiment (EFD‐B) onboard the sounding rocket S‐310‐26 above the E‐region ionosphere during the Sporadic‐E Experiment over Kyushu (SEEK) campaign. The sounding rocket was launched into the ionosphere when a strong quasi‐periodic (QP) echo structure appeared. The EFD‐B measured both the electric field and the magnetic field from X+169 s until X+232 s (altitude of 167 km to 173 km). Accuracy of the measurements were 0.19 mV/m for the electric field and 1.9 nT for the magnetic field. The magnetic field measurements were in very good agreement with a model field (IGRF 95). The electric fields, which were projections of the ionospheric electric fields to the altitude of the observation point along the magnetic field line, reached to 11.5 mV/m at maximum. The observed electric field is much larger than expected, and is associated with the 4 km periodic structure of the electric field which may be related to the atmospheric gravity wave activity in the E region.

Quasi‐periodic echoes observed with the Chung‐Li VHF Radar during the SEEK Campaign

The 52‐MHz Chung‐Li VHF radar, located in Taiwan, was operated during the SEEK (Sporadic‐E Experiment over Kyushu) campaign to investigate the so‐called quasi‐periodic (QP) echoes that occur from nighttime sporadic E (Es) layers. Highlights of this study are presented in this paper, using measurements made on August 17, 1996, to illustrate the echo characteristics. QP echoes displaying 5 to 10 minute periods were observed throughout this night. A new finding is the occurrence of QP echoes whose striations were almost vertically oriented when plotted using a height‐time‐intensity (HTI) format. This observation provides the first unambiguous evidence for the existence of steep spatial tilts in Es layers that are responsible for QP echoes. Other characteristics from three periods during this night are described including striations with positively sloped striations. These characteristics are discussed in light of existing models for QP echoes.

Structures in sporadic‐E observed with an impedance probe during the SEEK Campaign: Comparisons with neutral‐wind and radar‐echo observations

In order to clarify the origin of the so‐called quasi periodic echoes (QPE) that have been often detected by radar observations in the presence of sporadic‐E (Es) layers in the nighttime midlatitude ionosphere, two sounding rockets were launched during the SEEK (Sporadic‐E Experiment over Kyushu) campaign. Each rocket carried a swept‐frequency impedance probe to measure the E‐region electron‐density (Ne) profile. Using the four Ne profiles obtained during the two rocket flights together with a neutral‐wind profile obtained from a trimethyl aluminum (TMA) chemical release experiment on one of the rockets and QPE obtained with a ground‐based radar, we consider the role of wind shear in the formation of the observed Es layers, and the question of whether QPE are associated with Es layers that are modulated in altitude. The Ne profiles of Es structures that were obtained in the presence of QPE were characterized by the highly concentrated thin layers. The formation of such a thin layer by a neutral‐wind shear process was confirmed in comparison with the TMA measurements. The peak Ne values of the Es layers ranged from 2.2 to 9.3 × 104 el/cm³ near 100‐km altitude. These primary Es layers were accompanied by significant secondary structures that were located about 12 to 20 km above the main Es layers and had peak Ne that ranged from 5.2 × 10³ to 1.3×104 el/cm³. The average altitude profiles of QPE approximately covered the range where the Es‐layer peaks appeared. Our principal finding is that the observed Es structures tended to resemble horizontally stratified layers rather than structures with deep altitude modulation like previous QPE model, although the rocket measurements were separated from those by radar by 90 to 145 km.

Comparison of E‐region electric fields observed with a sounding rocket and a Doppler radar in the Seek Campaign

This paper describes results from simultaneous measurements of mid‐latitude irregularities by a sounding rocket and a Doppler radar. The experiment was conducted on August 26, 1996 as part of the SEEK (Sporadic‐E Experiment over Kyushu) campaign in Japan. Electric field observed with a double‐probe sensor on the rocket showed very large fluctuations of ±10 mV/m where irregularity echoes were quite intense. Doppler velocities observed with the radar agreed well with the electric fields. We estimated growth rates of the gradient‐drift instability, which showed positive values around the major sporadic‐E layer. It was found that, within intense irregularity region with quasi‐periodic structures, there were large polarization electric fields that was likely to be associated with spatial structure of the sporadic‐E layer.

Long base‐line measurements of short‐period mesospheric gravity waves during the SEEK Campaign

Simultaneous observations of short‐period (<1 hour) mesospheric gravity waves were made over an extended base‐line (∼660 km) from two sites in Japan (at Shigaraki and Yamagawa) during the SEEK (Sporadic‐E Experiment over Kyushu) campaign, 9–23 August, 1996. Coincident image data obtained on four nights provided a rare opportunity to investigate the horizontal spatial extent and propagation characteristics of the waves in detail. Surprisingly, out of a total of 12 events observed from Shigaraki and 10 from Yamagawa at most only three exhibited similar propagation characteristics at each site. The majority of the wave displays (∼70–75%) had quite dissimilar characteristics suggesting a preponderance for localized wave motions of limited lifetimes and geographical extent. A marked preference for wave progression towards the N‐NE indicates that the gravity waves imaged in the near infrared OH and OI (557.7 nm) nightglow emissions were probably not directly associated with the observed E‐region irregularities.

Interdisciplinary scientists gather for plasma structure workshop

Two of the most exciting papers presented at the Third Peaceful Valley Workshop were on the nature of plasma structure found in the nighttime midlatitude E and F regions. The first was from a coordinated rocket campaign called Sporadic E Experiment over Kyushu (SEEK) dedicated to the understanding of puzzling quasi‐periodic radar echoes that have been detected in association with sporadic E layers. In‐situ probes on two rockets measured localized electric fields as large as 20 mV/m, confirming theoretical predictions of strong polarization processes that may result from wavelike distortions imposed on normally stratified sporadic E layers. An unexpected result was the discovery of a large localized wind of 150 m/s at E region altitudes and the associated velocity shear, which was likely to lead to Kelvin‐Helmholtz instability.