Roland T. Tsunoda

Onset conditions for equatorial spread F

The problem of day-to-day variability in the occurrence of equatorial spread F (ESF) is addressed using multidiagnostic observations and semiempirical modeling. The observational results are derived from a two-night case study of ESF onset conditions observed at Kwajalein Atoll (Marshall Islands) using the ALTAIR incoherent scatter radar and all-sky optical imaging techniques. The major difference between nights when ESF instabilities did not occur (August 14, 1988) and did occur (August 15, 1988) in the Kwajalein sector was that the northern meridional gradient of 6300-A airglow was reduced on the night of limited ESF activity. Modeling results suggest that this unusual airglow pattern is due to equatorward neutral winds. Previous researchers have shown that transequatorial thermospheric winds can exert a control over ESF seasonal and longitudinal occurrence patterns by inhibiting Rayleigh-Taylor instability growth rates. We present evidence to suggest that this picture can be extended to far shorter time scales, namely, that “surges” in transequatorial winds acting over characteristic times of a few hours to a day can result in a stabilizing influence upon irregularity growth rates. The seemingly capricious nature of ESF onset may thus be controlled, in part, by the inherent variability of low-latitude thermospheric winds.

On the origin of quasi-periodic radar backscatter from midlatitude sporadic E

We describe a newly discovered polarization process that appears to be induced by an atmospheric gravity wave (AGW) when it altitude-modulates a sporadic E (ES) layer in the nighttime, midlatitude ionosphere. This large-scale polarization process appears capable of accounting for three as yet unexplained features found in radar backscatter from field-aligned irregularities in ES layers: (1) kilometer-scale, wavelike variations in the mean Doppler velocity, (2) mean Doppler velocities much larger than background ionospheric motion, and (3) quasi-periodic patterns in backscatter power when plotted as a function of range and time. We show that the polarization electric field develops as a result of the altitude modulation and that its properties are similar to those of the AGW. The novel feature in this process is the spatial modulation that is produced in the field line-integrated Pedersen conductivity by the variation in the ion-neutral collision frequency associated with the varying altitude of the ES layer. The resultant electric field together with the altitude-modulated ES layer then drive the production of secondary plasma waves via the gradient-drift instability. The three puzzling features are shown to be associated with the characteristics of these secondary waves.

Magnetic-field-aligned characteristics of plasma bubbles in the nighttime equatorial ionosphere

Abstract Measurements of both incoherent-scatter (IS) and backscatter from field-aligned irregularities (FAI) were made in 1978 with ALTAIR, a fully-steerable high-power radar, to investigate the magnetic-field-aligned characteristics of equatorial plasma bubbles. By operating the radar in a latitude-scan IS mode, we were able to map the location and percentage depletion of plasma bubbles as a function of altitude and latitude. By showing that backscatter from FAI is spatially collocated with the upper wall of plasma bubbles, we were able to use the spatial displacement of a field aligned backscatter region to estimate the upward bubble velocity. Besides showing that plasma bubbles are indeed aligned along magnetic field lines, we use this data set to show that a plasma bubble with a percentage depletion of as much as 90% does not have as large an upward velocity as predicted by two-dimensional models. Instead, the inferred bubble velocity is shown to be in better agreement with the bubble velocity predicted by theoretical models using flux-tube-integrated values of electron density and Pedersen conductivity. The need to use flux-tube-integrated values when comparing theory and observation is further stressed by the presence of a non-uniform latitudinal distribution of electron density (i.e. the equatorial anomaly) that was found in the latitude-scan data.

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.

Coupled electrodynamics in the nighttime midlatitude ionosphere

We describe a new, self-consistent scenario in which sporadic-E doublets, height bands of upward-displaced F-layer profiles, F-region plasma depletions, and radar backscatter plumes, are all manifestations of a coupled electrodynamical response by the nighttime midlatitude ionosphere to the presence of a traveling ionospheric disturbance (TID). We show that the response consists of (1) formation of image plasma structure in the E region, (2) initiation of a Hall-current-driven polarization process by the E-region plasma structure, and (3) mapping of the polarization electric field to the F region, where it strengthens the electrical properties of the TID that initiated the E-region processes. This scenario provides ready answers for several, hitherto puzzling, questions and a basis for new directions on this research topic.

Spatial structure of the E region field‐aligned irregularities revealed by the MU radar

The MU radar (34.9°N, 136.1°E, geomagnetic latitude 25.0°N) is utilized to observe the spatial structures of the field-aligned irregularities in the midlatitude E region. The multibeam observation in 12 directions has found that the “quasi-periodic” echoes have a monochromatic wave structure propagating toward SSW. Much smaller-scale structure of the irregularities has been observed by the interferometry techniques with three receivers. The results are consistent with a model in which a southward propagating gravity wave modulates the sporadic E layer to generate the quasi-periodic echoes of the irregularities.

First 24.5‐MHz radar measurements of quasi‐periodic backscatter from field‐aligned irregularities in midlatitude sporadic E

We report three new findings from the first observations of quasi-periodic (QP) radar echoes at 24.5-MHz. Most interesting is that QP echoes produced by 6.1-m field-aligned irregularities (FAI) can occur at altitudes as high as 150 km. Because Hall currents are negligible there, the existence of these FAI require interpretation at least in terms of a generalized gradient-drift instability that allows for (1) a transition from a Hall current at low altitudes to a Pedersen current at high altitudes, and (2) ion magnetization effects at altitudes above the lower E region. The second finding is the occurrence of extreme spectral broadening that often accompanies the strongest echoes. The third finding, from data accumulated over a two-month period, is evidence that the longitudinal gradient in conductivity associated with the solar terminator plays a role in the first appearance of QP echoes in the even-ing.

Quasi-periodic radar echoes from midlatitude sporadic E and role of the 5-day planetary wave

Using measurements of magnetic-aspect-sensitive radar echoes from midlatitude sporadic E collected over a two-month period from Tanegashima, Japan, we show that while their occurrence duration from night to night did not exhibit any systematic variation, that of the so-called quasi-periodic (QP) echoes varied sinusoidally with a period of 5 days. We have interpreted this behavior in terms of effects produced by a planetary wave and identified its presence through neutral-wind measurements made with a partial-reflection drift radar located nearby at Yamagawa. We propose that the occurrence of QP echoes is affected both by a contribution of the wind to the dynamo electric field and by the direction of the neutral wind. We argue that because the wind vector of the planetary wave is elliptically polarized at midlatitudes, a preferred wind direction conducive to the generation of QP echoes occurs once every 5 days. On the other hand, this wave is linearly polarized and directed zonally over the geographic equator. The fact that QP echoes are most fully developed at midlatitudes and less so at lower latitudes suggests that zonal flow is not particularly favorable for QP echo production.

Coupling of the Perkins instability and the sporadic E layer instability derived from physical arguments

[i] Tsunoda and Cosgrove [2001] recently pointed out that the F layer and sporadic E (E s ) layers in the nighttime midlatitude ionosphere must be considered electrodynamically as a coupled system in light of the presence of a Hall polarization process in E s layers [Haldoupis et al., 1996; Tsunoda, 1998; Cosgrove and Tsunoda, 2001, 2002a] and the fact that kilometer-scale electric fields map efficiently between the E and F regions. They further noted the apparent presence of positive feedback between processes in those regions. Cosgrove and Tsunoda [2002b, 2003] have since shown that E s layers are unstable with properties not unlike those of the Perkins instability in the F region [Perkins, 1973], motivating the idea that the two instabilities may couple. Finally, Cosgrove and Tsunoda [2004] derived the linear growth rate for the coupled system of a E s layer and the F layer, thus realizing a unified formalism for the Perkins and E s layer (E s L) instabilities. They found that the growth rate was significantly enhanced by the coupling. However, the growth rate computed in Cosgrove and Tsunoda [2004] was expressed only as the largest eigenvalue of a very complex 3 × 3 matrix. In this paper we present a physical interpretation of the E-F coupled-layer (EFCL) instability, and derive the condition for maximal coupling. We obtain a circuit model for the coupled-layer system that provides a physical interpretation for the wavelength dependence of electric field mapping between layers, and allows quantitative predictions. Using the circuit model we derive a rule of thumb for computing the two growth rates of the coupled system from the isolated Perkins and E s L instability growth rates. We compare the result with the exact computation of Cosgrove and Tsunoda 120041.

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.