Nighttime Midlatitude Ionosphere Research Articles

Investigation of Satellite Trace (ST) and Multi-reflected Echo (MRE) ionogram signatures and its possible correlation to nighttime spread F development from Cyprus over the solar mini-max (2009–2016)

Multi-reflected echoes (MREs) and satellite traces (STs) are referred in literature as ionogram signatures of Travelling Ionospheric Disturbances (TIDs) which is a phenomenon that apparently drives spread F development mainly at nighttime mid-latitude ionosphere. A long-term statistical study has been undertaken to investigate the morphological aspect of these signatures over the lower midlatitude European station of Nicosia, Cyprus (35.19°N, 33.38°E geographic; magnetic dip. 29.38°N) by inspecting all ionograms recorded by the DPS-4D digisonde in the interval 2009–2016. The results underline the systematic manifestation of these TID signatures over Cyprus with a possible (although not quite clear) solar activity dependence and a distinctive seasonal and diurnal occurrence rate with a seasonal peak of STs during summer and of MREs during January to April. Based on the experimental results of the present study, the seasonal occurrence rate of MREs and STs is found to increase by 75% and 56% during high solar activity periods. Satellite traces are well known ionogram signatures of TIDs and mostly correlated to the nighttime spread F formation. The occurrence of mid-latitude spread Fs over European longitude sector normally increases during summer. The occurrences of TIDs are also prominent at this interval of the year over nighttime mid-latitude ionosphere. The presence of MREs as an ionogram signature of TIDs over mid-latitude ionosphere is unique in nature.

Effects of sporadic E-layer characteristics on spread-F generation in the nighttime midlatitude ionosphere: A climatological study

The aim of this study is to examine the effects of Es-layer characteristics on spread-F generation in the nighttime midlatitude ionosphere. The Es-layer parameters and spread-F appearance of the 23rd solar cycle (1996–2008) are recorded by the Kokubunji ionosonde. The Es-layer parameters are foEs (critical frequency of Es-layer), fbEs (blanketing frequency of Es-layer), and Δf (≡foEs-fbEs). In order to completely explore the effects, the pre-midnight and post-midnight data are classified by seasons, solar activities, and geomagnetic conditions. Results show that the spread-F occurs more frequently in post-midnight and in summer. And, the occurrence probabilities of spread-F are greater, when the solar activity is lower. For the occurrence probabilities of spread-F versus foEs and Δf under geomagnetic quiet-conditions, the trend is increasing, when the associated probabilities are significant. These indicate that the spread-F occurrence increases with increasing foEs and/or Δf. Further, the increasing trends demonstrate that polarization electric fields generated in Es-layer would be helpful to generate spread-F, through the electrodynamical coupling of Es-layer and F-region. Moreover, this electrodynamical coupling is efficient not only under quiet-conditions but under disturbed-conditions, since the significant increasing trend can also be found under disturbed-conditions. Regarding the occurrence probabilities of spread-F versus fbEs, the evident trends are not in the majority. This implies that fbEs might not be a major factor for the spread-F formation.

Plasma drifts and polarization electric fields associated with TID‐like disturbances in the low‐latitude ionosphere: C/NOFS observations

AbstractMedium‐scale traveling ionospheric disturbances are often observed at the magnetically conjugate points in the nighttime midlatitude ionosphere. It has been suggested that gravity waves disturb the ionosphere and induce electric fields in one hemisphere and that the electric fields are amplified by the Perkins instability and transmitted along the geomagnetic field lines to the conjugate ionosphere, creating similar disturbances there. However, direct observations of electric fields associated with traveling ionospheric disturbances (TIDs) are very few. In this study, we present low‐latitude TID‐like disturbances observed by the Communication/Navigation Outage Forecasting System (C/NOFS) satellite. It is found that ion velocity perturbations are generated in the directions parallel and perpendicular to the geomagnetic field within TIDs. Both the parallel and perpendicular ion velocity perturbations show an in‐phase correlation with the ion density perturbations. For nighttime TIDs, the amplitude of both the parallel and meridional ion velocity perturbations increases almost linearly with the amplitude of the ion density perturbations, and the meridional ion drift is proportional to the parallel ion velocity. For daytime TIDs, the parallel ion velocity perturbation increases with the ion density perturbation, but the meridional ion velocity perturbation does not change much. The observations provide evidence that polarization electric field is generated within TIDs at low latitudes and maps along the geomagnetic field lines over a large distance.

Three‐dimensional simulation of the coupled Perkins and Es‐layer instabilities in the nighttime midlatitude ionosphere

Plasma density structures and associated irregularities in the nighttime midlatitude ionosphere are frequently observed as frontal structures elongated from northwest to southeast (NW‐SE) in the Northern Hemisphere. The frontal structures and the coupling process between the E and F regions are studied with a three‐dimensional numerical model, which can simulate two instability mechanisms: Perkins instability in the F‐region and sporadic‐E (Es)‐layer instability in the E region. The fastest growth of the coupled instability occurs when the unstable conditions on NW‐SE perturbation are satisfied in both regions. The perturbation of F‐region integrated conductivity grows much faster than the isolated Perkins instability. The meridional component of a rotational wind shear blows an existing Es layer southward, and the F‐region structure follows the E‐region drift velocity. The NW‐SE structure in the E region can be formed from random perturbation regardless of the F‐region condition. When the F region is unstable on the NW‐SE perturbation, however, the NW‐SE structure is formed in both regions with a common scale length. We conclude that (1) the Es‐layer instability plays a major role in seeding NW‐SE structure in the F region, and the Perkins instability is required to amplify its perturbation; (2) the rotational wind shear in the E region produces southwestward phase propagation of the NW‐SE structure in both the E and F regions; and (3) the coupling process has a significant effect on the scale of the Es‐layer perturbation rather than the growth rate of the Es‐layer instability.

Wavelength dependence of the <I>E<sub>s</sub></I> layer instability, and of coupling to the F layer, in the nonlinear regime

Abstract. The Es layer instability has been suggested as a participant in the creation of frontal structures observed in both the Es and F layers of the nighttime midlatitude ionosphere, in spite of the fact that the spatial scales of the frontal structures are very different in the two layers. The linear growth rate of the instability has a maxima in the vicinity of the wavelength observed for the Es layer structures (short wavelengths). However, the maxima is non-distinct, and simulations have shown that the instability is extremely nonlinear. Therefore, to understand the wavelength dependence of the Es layer instability it is necessary to factor in nonlinear behavior. Simulations have shown that the instability is active at the wavelengths observed in the F layer, and revealed that the Es layer behavior at these long wavelengths is so nonlinear that the common, highly localized Es layer observation techniques would likely miss the signature, which is highly visible in the F layer. However, there is currently no explanation for why long wavelengths so clearly dominate short (or intermediate) wavelengths in the F layer observations, and this is a weakness in arguments that the Es layer instability participates in the creation of F-region frontal structures. Herein we remove this weakness by showing that longer wavelengths grow to larger amplitudes before eventual nonlinear saturation, and couple more effectively to the F-region.

Response of the nighttime midlatitude ionosphere to the passage of an atmospheric gravity wave

Using the data of the ionospheric vertical sounding in Almaty, the response of various parameters of the nighttime F layer to the passage of an atmospheric gravity wave, generated during the large magnetic storm on July 24–25, 2004, is studied. The analysis of the phase relations between the variations in the electron density at the F layer maximum (NmF), the layer maximum height (hmF), and the layer half-thickness showed that they are determined by the slope of the wave phase front. It is shown that the half-thickness of the layer changes in anti-phase with the variations in NmF2. The known fact that the amplitudes of variations in the critical frequencies of the F2 layer are smaller than the amplitudes of electron density variations at fixed heights is explained.

First three‐dimensional simulation of the Perkins instability in the nighttime midlatitude ionosphere

A three‐dimensional numerical simulation in the nighttime midlatitude ionosphere is developed for the first time and applied to the Perkins instability in the midlatitude F region. Growth of the Perkins instability is successfully reproduced under nighttime condition, and the numerical results basically agree with a linear theory and previous two‐dimensional numerical studies. Northwest‐southeast (NW‐SE) alignment of density perturbations is generated from random seeding by applying a southeastward neutral wind. The perturbations are dominant at altitudes of 200–300 km where a steep density gradient exists, which is consistent with the altitude of 630‐nm airglow emission that often shows NW‐SE alignment. Further simulation in terms of the E‐F coupling can be done in the near future.

Generation of mesoscale <I>F</I> layer structure and electric fields by the combined Perkins and <I>E<sub>s</sub></I> layer instabilities, in simulations

Abstract. The generic equilibrium configuration of the nighttime midlatitude ionosphere consists of an F layer held up against gravity by winds and/or electric fields, and a sporadic E (Es) layer located by a sheared wind field, which experiences the same electric fields as the F layer. This configuration is subject to two large-scale (e.g. >10 km) "layer instabilities": one of the F layer known as the Perkins instability, and another of the Es layer which has been called the Es layer instability. Electric fields on scales larger than (about) 10 km map very efficiently between the Es and F layers, and the two instabilities have a similar geometry, allowing them to interact with one another. As shown through a linear growth rate analysis, the two most important parameters governing the interaction are the relative horizontal velocity between the Es and F layers, and the integrated conductivity ratio ΣH/ΣPF, where ΣH and ΣPF are the field line integrated Hall conductivity of the Es layer, and the field line integrated Pedersen conductivity of the F layer, respectively. For both large and small relative velocities the growth rate was found to be more than double that of the Perkins instability alone, when ΣHΣPF=1.8. However, the characteristic eigenmode varies considerably with relative velocity, and different nonlinear behavior is expected in these two cases. As a follow up to the linear growth rate analysis, we explore in this article the nonlinear evolution of the unstable coupled system subject to a 200 km wavelength initial perturbation of the F layer, using a two-dimensional numerical solution of the two-fluid equations, as a function of relative horizontal velocity and ΣHΣPF. We find that when ΣHΣPF⪝0.5 the Perkins instability is able to control the dynamics and modulate the F layer altitude in 2 to 3 h time. However, the electric fields remain small until the altitude modulation is extremely large, and even then they are not large enough to account for the observations of large midlatitude electric fields. When ΣHΣPF⪞1 the Es layer becomes a major contributor to the F layer dynamics. The Es layer response involves the breaking of a wave, with associated polarization electric fields, which modulate the F layer. Larger electric fields form when the relative velocity between the Es and F layers is large, whereas larger modulations of the F layer altitude occur when the relative velocity is small. In the latter case the F layer modulation grows almost twice as fast (for ΣHΣPF=1) as when no Es layer is present. In the former case the electric fields associated with the Es layer are large enough to explain the observations (~10 mV/m) , but occur over relatively short temporal and spatial scales. In the former case also there is evidence that the F layer structure may present with a southwestward trace velocity induced by Es layer motion.

Azimuth‐dependent Es layer instability: A missing link found

A number of features observed in association with nighttime midlatitude sporadic E (Es) have defied satisfactory explanation, until now. They include Es properties such as semitransparency, tilts, altitude modulation, plasma structure, and frontal structures with preferred alignment in azimuth. Other puzzling manifestations of Es include gigahertz scintillation and anomalously large electric fields (). All can now be interpreted self‐consistently in terms of a recently discovered, azimuth‐dependent Es layer instability [Cosgrove and Tsunoda, 2002b, 2003]. Zonal wind shear turns out to be responsible for both the initial formation of Es (wind shear theory) and its instability to vertical transport, the latter arising through an from Hall polarization. The meridional wind drives an important secondary polarization process that acts through conductivity perturbations to distort and produce a characteristic asymmetry in Es plasma structure, above and below the zonal wind shear node. Stability is restored through reduction in from the overlap of Es structures above and below the wind‐shear node; overlap is produced by antiparallel horizontal transport. Temporal evolution therefore is a cyclical process of Es layer distortion and structure development, followed by horizontal overlap and vertical reconvergence. In this paper we elucidate the underlying physics and discuss how hitherto puzzling observations now provide supporting evidence for this instability. We further endorse the findings that Hall polarization and the Es layer instability must be playing major roles in the coupled electrodynamics and instability of the nighttime midlatitude ionosphere [Tsunoda and Cosgrove, 2001; Cosgrove and Tsunoda, 2004; Cosgrove et al., 2004].

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

Tsunoda and Cosgrove [2001] recently pointed out that the F layer and sporadic E (Es) 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 Es 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 Es 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 Es layer and the F layer, thus realizing a unified formalism for the Perkins and Es layer (EsL) 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 EsL instability growth rates. We compare the result with the exact computation of Cosgrove and Tsunoda [2004].

Instability of the E‐F coupled nighttime midlatitude ionosphere

Tsunoda and Cosgrove [2001] recently pointed out that the F layer and sporadic E (Es) 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 Es 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 Es layers are unstable with properties not unlike those of the Perkins instability in the F region [Perkins, 1973], thus providing yet another reason to consider the behavior of the coupled system. In this paper we show mathematically that the coupled system is indeed unstable with properties that are not only related to but distinct from those of the individual instabilities. A key finding is that this coupled response significantly enhances the growth rate of F region structures, beyond that of the Perkins instability.

A modeling study of the F-region response to SAPS

Sub-auroral polarization stream (SAPS) is the name given to an enhanced westward plasma flow in the night-time mid-latitude ionosphere during geomagnetically disturbed conditions. A description of the SAPS electric field, based on observations from the Millstone Hill incoherent scatter radar, is used here to create a driver for the Utah State University time-dependent ionospheric model in order to assess the effect of the SAPS on the mid-latitude night-time F-region. Tests are conducted for various effective durations of the SAPS, up to 7 h , as well as for different background convection patterns. A longitudinal, or UT, dependence of the ionospheric response is examined. It is found that within the SAPS region, much density structuring occurs, including both enhancements and depletions; in some cases the difference in N m F 2 is as large as a factor of 10.

Simulation of the nonlinear evolution of the sporadic‐E layer instability in the nighttime midlatitude ionosphere

Recently, it has been shown that the configuration of a midlatitude sporadic‐E (Es) layer at a zonal wind‐shear node is unstable at night. The instability is the result of a wind‐shear driven polarization process that occurs when a plane‐wave perturbation in altitude is imposed on the Es layer. The growth rate of the instability depends on the azimuthal alignment of the plane wave distortion, a feature that is reminiscent of the Perkins instability. The plane wave nature of the growing modes, combined with the azimuthal dependance of the growth rate, suggests that the instability may provide an explanation for frontal structures observed in Es layers, which are found to consistently adopt the same azimuthal alignment preferred by the instability. In this paper the nonlinear evolution of the instability is simulated using a flux‐corrected transport finite difference method for the orientation of maximum linear growth rate. It is found that the instability generates significant polarization electric fields that structure the layer; yet the structuring saturates without destroying the layer completely. Decreasing the wind shear decreases the polarization electric field and increases the evolution time scale but otherwise does not profoundly affect the structures that form in the layer. Increasing the F layer conductivity, which is assumed to map perfectly to the Es layer, damps the instability. The results suggest the instability as a possible source of the so‐called quasi‐periodic (QP) echoes, which are coherent radar echoes found in the nighttime midlatitude ionosphere.

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.

On polarized frontal structures, type‐1 and quasi‐periodic echoes in midlatitude sporadic E

Sporadic‐E (ES) patches in the nighttime midlatitude ionosphere often appear to consist of quasi‐periodic (QP) frontal structures, i.e., strips of enhanced conductivity whose cross‐sectional dimensions are small compared to their length. Assuming that such ES patches are usually found at an ion‐convergent node in a wind‐shear profile, we show that the meridional neutral wind at that node would polarize these strips and produce an intense, electrojet‐like current accompanied by an electric field strength in excess of 10 mV/m. The validity of this model seems to be supported by its ability to account for several hitherto unexplained phenomena including type‐1 and QP radar echoes from nighttime ES patches, and the coupled appearance of altitude‐modulated isodensity contours in the F region when frontal structures are present in the E region. While much remains to be done to evaluate the importance of this process, this finding is exciting because it makes plausible the commonplace occurrence of what was previously referred to as anomalous energetics in the midlatitude ionosphere.

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.

Evolutionary process of 10 kilometer scale irregularities in the nighttime midlatitude ionosphere

The nighttime midlatitude ionosphere is numerically simulated to study the evolutionary process of the irregularities that generate the electric field fluctuations observed by satellites. Ionospheres in both hemispheres are included in the simulation, and the evolution of irregularities is studied with given neutral winds in these regions. In each ionosphere, an equation of the field line integrated Pedersen conductivity is derived by integration of the ion momentum equation along the geomagnetic field line. Given initial perturbations are reformed to a sheet‐like structure at the first stage of the simulation. After this reformation, the irregularities start to grow through plasma instability. The sheet‐like structures are also formed in the conjugate ionosphere, where no initial irregularities are given, when the irregularities are sufficiently developed. In a case of an intense neutral wind applied only in the source hemisphere, the growth rate is comparable to that predicted by the linear analysis which includes the electrical coupling between the conjugate ionospheres. When the intense neutral wind is applied in both the conjugate ionospheres, the growth of irregularities is affected by the electric currents in both hemispheres even when the conjugate ionosphere is in winter. The electric current of the conjugate ionosphere can grow or damp the irregularities depending on its direction.

Type-1 echoes from the mid-latitude E-Region ionosphere

Abstract. This paper presents more data on the properties of type-1 irregularities in the nighttime mid-latitude E-region ionosphere. The measurements were made with a 50-MHz Doppler radar system operating in Crete, Greece. The type-1 echoes last from several seconds to a few minutes and are characterized by narrow Doppler spectra with peaks corresponding to wave phase velocities of 250–350 m/s. The average velocity of 285 m/s is about 20% lower than nominal E-region ion-acoustic speeds, probably because of the presence of heavy metallic ions in the sporadic-E-layers that appear to be associated with the mid-latitude plasma instabilities. Sometimes the type-1 echoes are combined with a broad spectrum of type-2 echoes; at other times they dominate the spectrum or may appear in the absence of any type-2 spectral component. We believe these echoes are due to the modified two-stream plasma instability driven by a polarization electric field that must be larger than 10 mV/m. This field is similar in nature to the equatorial electrojet polarization field and can arise when patchy nighttime sporadic-E-layers have the right geometry.

Conjugate occurrence of the electric field fluctuations in the nighttime midlatitude ionosphere

The DE 2 satellite observed electric field fluctuations on the topside of the nighttime midlatitude ionosphere. They extended several hundred kilometers in the latitudinal direction with wavelengths of several tens of kilometers, and their amplitudes were a few millivolts per meter. Such fluctuations were often observed at magnetically conjugate points in the northern and southern hemispheres. These electric field fluctuations are perpendicular to the geomagnetic field. They are not accompanied by any significant plasma depletion or electron temperature variations. Magnetic field fluctuations are sometimes observed simultaneously with electric field fluctuations. We interpret that these fluctuations are caused by field‐aligned currents which flow from the ionosphere in one hemisphere to the conjugate point in the other hemisphere. The power spectrum of these midlatitude electric field fluctuations follows a power law of the form Power α ƒ −n, with the spectral index n of 3.5 to 4.5, which is steeper than that of the electric field fluctuations in the high‐latitude ionosphere or in the equatorial ionosphere. This phenomenon may be related to other ionospheric phenomena, for example, the F region field‐aligned irregularities or spread‐F, observed by ground‐based methods such as the MU radar, but the relationship is not clear.

Characteristics of the mid-latitude ionosphere suitable for modelling ionospheric filtration of ULF waves

An approximate method of one-dimensional modelling of the plasma of the Earth's ionosphere is demonstrated for purposes of studying the ionospheric filtration of ULF waves (micropulsations). Apart from the basic local parameters, characterizing the plasma, also derived local characteristics have been defined, i.e. the mass of the so-called effective ion\(\bar m_i \) and its effective collision frequency\(\bar v_i \). Drawing on existing empirical models of the mid-latitude ionosphere, vertical profiles (50 km ≦ h ≦ 1000 km) were determined of the characteristics Ne ≈ Ni, ve,\(\bar m_i \) and\(\bar v_i \) for the daytime and nighttime mid-latitude ionosphere under low and enhanced solar activity, which can be used to study the ionospheric ULF filter.