Tidal Wind System Research Articles

On the pre-midnight ascent of F-layer in the June solstice during the deep solar minimum in 2008 over the Indian sector

Investigations on the variations of the virtual height (h'F) of the base of the F-layer over Thumba (8.5°N, 77°E, dip lat 0.5°N) in 2002 (high solar activity) and 2008 (very low solar activity) under quiet geomagnetic conditions show characteristic pre-midnight rise of h'F in the June solstice of 2008. Comparison of the h'F variations in 2008 over Thumba and Fortaleza (3.9°S, 38.4°W, dip lat 1.8°S), Brazil, reveals that the pre-midnight rise of h'F is significantly more over Thumba during the June solstice. Drift measurements on-board the Communication/Navigation Outage Forecasting System (C/NOFS) satellite elicit that the midnight upward drift over the Indian sector during the northern summer months of 2009 is the largest, a feature that significantly weakens in 2010. C/NOFS measurements also confirm the electro-dynamical nature of the pre-midnight h'F rise over the Indian sector in the June solstice during the low solar activity. As the equatorial F-region vertical drifts during nighttime are controlled by E-region dynamo driven by tidal wind system, systematic wind measurements at upper mesospheric heights by an MF radar (1.98MHz) from Tirunelveli (8.7°N, 77.8°E, dip lat 0.5°S), India, during 2000–2011 are used to derive the tidal components. This reveals that the phases of both the meridional and zonal components of the diurnal tide regress while the phase of the meridional component of the semidiurnal tide significantly advances with decreasing solar activity with concomitant increases in amplitudes during the summer months. These observations suggest the possible semidiurnal tidal influence on the pre-midnight h'F rise over the Indian sector in the June solstice during low solar activity.

Early Japanese contributions to space weather research (1945–1960)

Abstract. Major contributions by Japanese scientists in the period of 1945 to 1960 are reviewed. This was the period when the foundation of the space weather research was laid by ground-based observations and theoretical research. Important contributions were made on such subjects as equatorial ionosphere in quiet times, tidal wind system in the ionosphere, formation of the F2 layer, VLF propagation above the ionosphere, and precursory phenomena (type IV radio outburst and polar cap absorption) to storms. At the IGY (1957, 1958), research efforts were intensified and new programs in space and Antarctica were initiated. Japanese scientists in this discipline held a tight network for communication and collaboration that has been kept to this day.

Sporadic E: current views and recent progress

Abstract Sporadic E (Es) and related processes are reviewed as functions of viewing system, latitude and altitude. We find that the “windshear theory”—perhaps with a small added external electric field—appears sufficient along with the tidal wind system to explain the mid-latitude layers which we refer to as tidal ion layers (TILs)—layers that have often been identified as sporadic E or as sequential sporadic E. Additionally, it has become clear that the high-latitude, altitude-narrow layers, also often identified as sporadic E, are consistently explained as being formed in appropriate large-scale convective electric field structures with the wind system playing a lesser role. Finally, we find that “true” sporadic E—an altitude-thin E region layer at an unpredictable altitude and/or an unexpected intensity—is found in a rich context of related phenomena that includes the tidal ion layers, electric-field-induced layers, HF/VHF radar quasi-periodic echoing (QPE) regions, and an apparently newly observed phenomenon descriptively termed “ion rain.” We conclude that the QPEs and “ion rain” indicate small horizontal scales and find considerable other evidence of order 10–100 km scale horizontal-structuring of layers which at least hints at an E/F region coupled-electrodynamic process, or processes. These processes apparently include instability-generated 10–100 km horizontal-scale E-fields that—we hypothesize—generate true sporadic E and the related complex layer structures (CLS) via horizontal redistribution of ions.

A proposed temperature dependent mechanism for the formation of sporadic sodium layers

We examine the influence of temperature fluctuations on the formation of sporadic sodium layers (SSLs) with particular emphasis on AIDA (Arecibo Initiative in the Dynamics of the Atmosphere) results. We present evidence suggesting that sodium abundance is very sensitive to the temperature. A 10 K increase in mesopause temperature may double the sodium concentration. Thus the sodium profile may change significantly if appropriate thermal fluctuations due to tides and/or acoustic-gravity waves (AGWs) occur. Gravity wave theory predicts that the ion convergence node, without other influences, coincides with a temperature maximum for a westward propagating wave. In this case, the ion layer coincides with the temperature maximum which results in a higher sodium concentration at or near the ion layer height. This proposed temperature dependency can, for the tidal wind field, account for the observed correlation between sodium and ion column abundances and is supported by the average O 2(0–1) rotational temperature determinations made at Arecibo. Specifically, we propose that the formation of SSLs is due to the temperature fluctuations induced by AGWs, or other wave processes, in conjunction with a background tidal wind system. Additionally, we argue that when an AGW propagates westward, the SSL coincides with an existing tidal ion layer or with a true sporadic- E layer which forms in the net wave field convergence zone. We also note that roughly the same processes may apply to the production of intense sporadic- E layers.

Observations of ion layer motions during the AIDA campaign

The AIDA-89 campaign has yielded the most comprehensive set of low-latitude incoherent scatter radar power profiles and derived electron concentration results ever made. These results have been used to study the time-height trajectories of 80–150 km ion layers and serve to gauge both the periodicity and variability of ion layer structure throughout the campaign. Features of the AIDA ion layer trajectories point to a dynamics ‘zoo’ of processes ranging from multiday-period waves, tides and acoustic-gravity waves (AGWs) to geomagnetic storm effects and evidence of coupled neutral sodium and ion layer/plasma processes. The semidiurnal and diurnal tides are evidenced in the almost always present layers, the Tidal Ion Layers (TILs), which are identified by their regular and periodic trajectories that also display regions of variable mixing or confluence of the various tides. The TILs are contrasted with the truly sporadic layers that include sporadic E and sporadic intermediate layers. The sporadic layers may be formed due to interaction of the tidal wind system with AGWs. The formation process may involve horizontal as well as vertical ion convergence mechanisms and/or various non-linear effects. Limits to the study derive from volume undersampling due to use of the single radar beam.

An upper E region quarterdiurnal tide at Arecibo?

Studies of the time‐height trajectories of ionosphere E region layers at Arecibo have revealed not only the usual diurnal sporadic‐E and semidiurnal intermediate layer structures but also that on occasion, extra layers form and descend through the E region. In particular, during January 1981, four layers per day were observed to descend through the upper E region. This four‐per‐day layer system was remarkably coherent over the 3 days of observation and suggests the existence of a 6‐hour period or quarterdiurnal tidal wind system in the upper E region. Numerical simulations of ion trajectories in a realistic model atmosphere with diurnal, semidiurnal, and quarterdiurnal tidal wind systems of appropriate amplitude and phase structures confirm that the presence of a quarterdiurnal tide is sufficient to explain the observations.

Low‐frequency drift measurements in the upper mesopause region: A useful approach to regular high‐atmosphere neutral wind observations

Results obtained during autumn and winter of 1978–1979 are used to demonstrate that low‐frequency drift measurements provide a useful means of permanently monitoring the circulation and tidal wind systems in the upper mesopause region (90–100 km) and represent a valuable supplement to the ground‐based instrumental pool, particularly with regard to the forthcoming middle atmosphere program (MAP).

Upper atmosphere tides and the vertical motion of ionospheric sporadic layers at Arecibo

The vertical motion of ionospheric sporadic layers in the 80‐ to 140‐km altitude region is examined by using a numerical solution for ion trajectories in a wind system composed of diurnal and semidiurnal tides similar to those observed at Arecibo. The confluence of the two tides in the 100‐ to 110‐km altitude region is shown to produce a variety of layer motion possibilities including ‘quasi‐stagnation’ of the layer as well as rapid upward or downward motions. Experimental layer trajectories are presented which confirm the basic theoretical results and which indicate the continuous opportunity for layers to form in the tidal wind system. The availability of metallic ions for layer formation is discussed as is the observed presence of layers below 90‐km altitude.

The influence of geomagnetic storms on the measured wind in the upper mesopause region deduced from ionospheric drift measurements

Ionospheric drift measurements in the LF range which normally permit reliable monitoring of circulation and tidal wind systems in the upper mesopause region may be misinterpreted during geomagnetic storms. These cause primary disturbances of the wind fields in the upper atmosphere by plasma heating and electric fields. A recording effect arises from an increase in the reflection level, so that the measurements relate to a different altitude with different wind conditions.

Influence of the Tidal Wind System on the Frequency of Sporadic‐E Occurrence

Sporadic‐E (ES) data from Japan were presented to show the temporal variations in sporadic‐E blanketing frequency (ƒbES), virtual height (h′ES), and frequency of occurrence. Tidal winds were substituted into the wind‐shear theory of ES to calculate electron density profiles and determine the influence of the tidal winds in the asymmetry of the ES occurrence variation. Results showed that the tidal wind model used was not strong enough to significantly influence the formation of molecular ion ES layers, but that the winds did have the proper orientation to accumulate and layer metallic ions in the late afternoon. The role of metallic ions in afternoon ES will have to be established.

Longitude and Seasonal Variations of the Sq Current System

The ‘equivalent circuit method’ of deriving electric currents in the ionosphere from assumed values of conductivities and emf's has been described earlier. The ionosphere is represented as a large circuit network, with the Hall effect included, and this network is solved by an iterative procedure on a computer. A single tidal wind system, the ‘1—2’ mode, is used as the driving force of the dynamo. The winds are converted from the geographic to the geomagnetic coordinate system. The international geomagnetic reference field (IGRF) is used to provide magnetic field values for calculating the dynamo emf's and the conductivities. The agreement between the calculated and observed positions of the focuses of the current system is remarkable when we consider that no seasonal variation of the wind was included. Seasonal variations of the reversal times of the equatorial electrojet are obtained in rough agreement with observation, whereas the calculated variations of the intensity of the electrojet do not agree so well.

Effects of dynamo-region electric fields on the magnetosphere

Based on reasonable models of anisotropic electrical conductivities and solar and lunar tidal wind systems in the dynamo region (90–150 km altitude), distributions of electrostatic fields for Sq and lunar variations on quiet days in equinoctial months during moderate sunspot periods are estimated. These electrostatic fields produced in the dynamo region may be communicated to the magnetosphere along the earth's magnetic lines of force without appreciable attenuation. The communicated electrostatic field distributions in the magnetosphere are deduced, and drift motions of magnetospheric plasma particles caused by the coupling of the earth's magnetic and electrostatic fields are computed. An outward drift flow in the daytime and an inward flow at night with a speed of a few meters per second to several hundred occur for Sq. The drift speed for lunar variations is about 5% of that for Sq.

The ionospheric wind dynamo—I: Lunar tide

The differential equation describing the response of the ionosphere to a horizontal wind-induced e.m.f. is derived and the appropriate boundary conditions are discussed. The equation is solved numerically to obtain the electric current distribution produced by several different models of the lunar semidiurnal tide. Tidal wind systems with large vertical wavelength are found to be much more efficient producers of current than wind systems of short wavelength. The current patterns produced by the lunar (2,2) mode with a maximum amplitude of 7.5 m/sec and a vertical wavelength of 85 km are very similar to the L current patterns deduced from geomagnetic observations.

On the significance of ionospheric drift measurements in the LF range

By comparisons with radar meteor wind results from Jodrell Bank and Sheffield it can be shown that closely spaced receiver drift measurements in the low frequency range, as carried out over as many as ten years at two stations in Central Europe, are a useful technique for continuous observation of night-time winds in the lower ionosphere. The results obtained from these measurements, concerning e.g. the seasonal variation of the prevailing wind, the drastic anomalies of the phase of the semi-diurnal tidal wind component during spring and autumn and during some major mid-winter stratospheric warming events, the solar cycle dependence of both the prevailing and tidal wind systems, and the indications for a probable existence of the 26-month zonal wind oscillation also in the lower ionosphere, may be important contributions to further study of the dynamics of the upper atmosphere.

Solar tidal variations of sporadicE

Sporadic E backscatter data have revealed two peaks of Es activity occurring asymmetrically with respect to local noon at about 0900 and 1800 hours. These peaks were shown to move from east to west at approximately the speed of the subsolar point. Analysis of vertical incidence data at or near the backscatter sites failed to reveal definite peaks of ƒoEs, corresponding to the peaks seen in the backscatter data. This leads to the conclusion that the two Es activity peaks in backscatter data represent an increase in the amount of sky area covered by a thin or diffuse type of Es. Fourier analysis of the Es backscatter data reveals 24-hr, 12-hr, and 8-hr components. Magnitudes and phase angles between the Es components and the corresponding components of the ionospheric tidal wind system are very similar and suggest that the tidal winds are the cause of the Es variation. This appears to be the first experimental confirmation of a solar tidal variation that is exclusive of the diurnal zenith angle variation of the normal E layer.

Atmospheric tides and associated magnetic effects

The dynamical theory of atmospheric tides and its relation to observations on the ground and in the upper atmosphere are reviewed briefly. The lunar tidal oscillations of atmospheric pressure observed on the ground are thought to be almost entirely due to gravitational forces, whereas the solar tidal oscillations are believed to be mainly caused by solar heating. Previous theories of resonant, gravitational excitation of the solar tidal oscillations are not consistent with models of the upper atmosphere obtained from rocket measurements. Tidal oscillations at ionospheric heights are inadequately known experimentally and are inadequately explained by present theories, which neglect viscous losses and the ionized nature of the upper atmosphere and whose linear approximations break down at ionospheric height. The dynamo theory attributes the quiet‐day variations of the geomagnetic field to tidal oscillations at ionospheric height. The theory has been used either to derive the magnetic variations from an assumed tidal wind system or to derive the tidal wind system from the observed magnetic variations. The results of both types of calculations lend broad support to the dynamo theory, although neither approach is free of arbitrary assumptions. The additional daily magnetic variations observed during magnetically disturbed days are particularly intense in the auroral zone. These variations are believed to be caused by some form of interaction of the solar wind with the geomagnetic field, but the nature of the interaction is not yet known with certainty. Various proposed theories of these daily disturbance variations are described, and one is discussed in detail. This theory predicts that the generating mechanism responsible for the daily disturbance variations is present, in reduced strength, even on magnetically quiet days. Observations tend to confirm this conclusion.

Theory of the geomagnetic daily disturbance variations

The worldwide ionospheric current system predicted by a previously described theory of the auroral electrojets is computed. Tidal air motions are neglected at first, and the high-latitude current system is assumed to be due entirely to the interaction of magnetospheric rotation with the belt of energetic protons trapped in the distorted geomagnetic field. For a given geomagnetic distortion and trapped proton distribution, the computed current system strongly resembles Chapman's idealized Ds, current system, if sufficiently high E-region electron concentrations independent of geographical position are assumed. As the assumed electron concentrations are decreased, however, the intensity of the computed current system is reduced, first in the auroral zone and then at all latitudes; at the same time the phase of the current system is advanced in the polar cap and is retarded at low latitudes. In view of this limitation of the current system by the available electron concentrations, the large disturbance variations observed during an intense geomagnetic storm can be explained by the present theory only if, in addition to an enhancement in the trapped proton density, greatly enhanced E-region electron concentrations are assumed, particularly in the auroral zone. Such enhanced electron concentrations are known to occur when blanketing sporadic E is observed. The observed correlation between blanketing sporadic E and the strength of the electrojet current is thus predicted by the present theory. The interaction of the tidal wind system with the belt of trapped protons is also considered. The resulting auroral electrojet currents oppose the Ds a current system if the usually accepted pattern of upper atmospheric tidal winds is assumed; the existence of a different wind pattern at high latitudes, at least during geomagnetic storms, is a possibility that cannot altogether be excluded, although it appears unlikely. If the majority of energetic particles in the radiation belt were electrons, their interaction with the usually accepted tidal wind pattern would result in the correct phase for the Ds current system; satellite observations indicate, however, that the majority of the energetic particles are protons. The present theory can also account for the generation of meridional static electric fields of the type that, according to a theory of Megill and Carleton, cause midlatitude red arcs.