This paper deals with how atmospheric gravity waves produce the traveling ionospheric disturbances (TIDs) that are observed by ionosondes. It is shown that, rather than directly producing variations of ionospheric height, a likely mechanism involves changes in ionization density by gradients in the horizontal atmospheric gravity wave air motion. These density changes can be observed as variations of the height of an ionospheric isodensity surface (the usual way of measuring TIDs). This mechanism involving enhancement/depletion of ionospheric density requires quite moderate atmospheric gravity wave air motion speeds, and works well at almost all latitudes.

Propagation mechanisms of lateral (non-great-circle) signals on a high-latitude HF radio path during magnetospheric substorms that occurred in the day-time have been considered. The path is equipped with oblique ionospheric sounding (OIS) from Murmansk to St. Petersburg. The OIS method gives the possibility to determine propagation modes, MOF (maximum observed frequency) values, signal delays, etc. Data of the CUTLASS radar, the IMAGE magnetometer system, the Finnish riometer chain, and the Tromso ionosonde were also used for the analysis. The main results are the following: (1) the lateral signal propagation takes place, as a rule, if the path midpoint is located near the irregularity region that moves sharply from high to low latitudes. The lateral signal propagation appearing during day-time is a new effect. (2) Formation of dense field-aligned irregularities during a substorm leads to decreasing F2MOF values on radio paths. These results can be useful for problems of radiolocation, HF communications and navigation.

Digital ionosonde and magnetometer observations from a polar cap station are used to estimate the fraction of Region 1 current that flows across the polar cap. For a winter case study using data for 2001 Feb 18 the cross-cap current was 3.9×104A Pedersen current and 6×104A Hall current. This total current is only a small percentage, ∼5%, of the Region 1 current.

The midlatitude sporadic E layers form when metallic ions of meteoric origin in the lower thermosphere are converged vertically in a wind shear. The occurrence and strength of sporadic E follow a pronounced seasonal dependence marked by a conspicuous summer maximum. Although this is known since the early years of ionosonde studies, its cause has remained a mystery as it cannot be accounted for by the windshear theory ofEs formation. We show here that the marked seasonal dependence of sporadic E correlates well with the annual variation of sporadic meteor deposition in the upper atmosphere. The later has been established recently from long‐term measurements using meteor radar interferometers in the Northern and Southern Hemispheres. Knowing that the occurrence and strength of sporadic E layers depends directly on the metal ion content, which apparently is determined primarily by the meteoric deposition, the present study offers a cause‐and‐effect explanation for the long‐going mystery of sporadic E layer seasonal dependence.

This paper studies the Arctic ionospheric environment using ionosonde data and assesses the potential of high-frequency surface-wave radar (HFSWR) surveillance for surface vessels and low-altitute air targets. The evaluation is based on sporadic-E (Es) interference in the HFSWR signal. For surveillance problems up to 150 km, such as in the Northwest Passage through Canada's Arctic islands, the ionospheric Es clutter is the dominant clutter that may degrade the performance of the future HFSWR systems. The results show that Es occurs least often at Eureka in winter and at Resolute Bay in summer. Generally, Es occurs less often in the Arctic than at midlatitudes. Observations also suggest that the best time to perform HFSWR surveillance in the Arctic is between approximately 07:00-15:00 UT and 21:00-24:00 UT. During these hours, the number of days that Es interference occurs in a month and the range of frequencies (in the 4-7 MHz range) reflected are minimized compared to other times of the day. These observations suggest that the future HFSWR systems in the Northwest Passage should be designed to operate at higher radio frequencies. For example, a radio frequency higher than 7 MHz can be operated in order to minimize the Es interference during the hours of 07:00-15:00 UT and 21:00-24:00 UT. It is concluded that the Es interference in the HFSWR signal in the Arctic can be equal to or less than that at midlatitudes (e.g., off the east coast of Newfoundland). Furthermore, the overall performance of HFSWR will be better in the Arctic than at midlatitudes due to lower levels of interference of sea clutter and man-made-related noises combined with a much better level of HFSWR propagation.

Scintillation measurements of the 150MHz NNSS beacon satellites at two midlatitude locations spaced approximately 400km east–west revealed that as the east–west zenith angle increased the scintillation index decreased. The explanation of this observation is that the F region irregularities are in the form of (approximately) north–south magnetically field aligned sheets. The likely formation mechanism of the irregularities probably involves the Perkins instability.

This study uses digital ionosonde data from a cusp latitude station (Cambridge Bay, 77° CGM lat.) to study the convection into the polar cap. Days when the IMF magnetic field was relatively steady were used. On many days it was possible to distinguish an interval near noon MLT when the ionosonde data had a different character from that at earlier and later times. Based on our data, and other published measurements, we used the interval 10:00-13:00 MLT as the cusp interval and calculated the convection into the polar cap in this interval. The integrated convection accounted for only ~1/3 of the open polar cap flux. If the convection through the prenoon/postnoon regions on either side of the cusp was calculated the remaining 2/3 of the flux could be accounted for. The characteristics of the prenoon/postnoon regions were different from the cusp region, and we attribute this to transient flank merging versus more steady frontside merging for the cusp. Keywords. Ionosphere (Plasma convection) Magnetospheric physics (Polar cap phenomenon)

This paper studies ionospheric clutter conditions and compares ionosonde measurements in the mid-latitude and arctic regions to determine the most favourable conditions for HFSWR surveillance for surface vessels and low-altitude air targets. The best time to perform HFSWR surveillance is between approximately 06:00-15:00 UT and 20:00-00:00 UT. During these hours, the number of days that sporadic-E interference occurs in a month and the range of frequencies reflected is minimized compared to other times of the day. Of the sites considered, Resolute Bay is the most favourable site for HFSWR surveillance in the summer since sporadic-E interference occurs least often, resulting in reduced signal interference. Similarly, Eureka is the preferred site during the winter months. In addition, the ionosphere at Eureka generally reflects the lowest range of maximum frequencies (similar to 4 - 8 MHz), again resulting in less clutter interference. In all the observations, polar cap sites Eureka and Resolute Bay yield results that are less prone to sporadic-E interference than the mid-latitude site Cambridge Bay.

The mesosphere/lower thermosphere (MLT) wind data from the 46 ground-based (GB) MF and meteor radar (MR) stations, located at the different latitudes over the globe, and the space-based (SB) HRDI data were used for constructing of the empirical global climatic 2-D prevailing wind model at 80–100 km heights for all months of the year. The main data set is obtained during 1990–2001 period. It is shown that the three datasets (MF, MR, HRDI) are mainly well correlated. However, a certain systematic bias between the GB and SB data at 96 km exists, as well as that between the MF and MR data higher 88 km. Simple correction factors are proposed to minimize these biases. The 2-D distant-weighted least-square interpolation procedure for some arbitrary collection of points was used for drawing model contour plots. The model is available in the computer readable form and may be used for construction of the new CIRA model.

In an earlier study based upon medium frequency radar (MFR) data from Saskatoon (52degreesN) the variability of time-sequences of gravity wave (GW) variances was linked to tidal (12,24 h) and planetary wave (2 d) oscillations of the wind. Fifty days of data were chosen from each of winter, spring and autumn seasons for this most comprehensive assessment of wave interactions. While modulations of the GW variances were observed, the results indicated considerable intermittency in the strength and direction of waves from the GW sources.Here we extend the study to other sites in the MLT-MFR (mesosphere, lower-thennosphere) network, and focus upon the planetary waves (PWs) and their modulating influences upon the GW variances. The PW events include the 2 d waves as seen at Saskatoon, London, Hawaii and Christmas Island during the summer of 1994; and 16 d wave activity evidenced at Saskatoon and London throughout 1994, and also during 12 and 7 years of observations, respectively. The modulations of the sequences of GW variances (10-100/150 min, 2-6 h periods) are significant, and the phase-differences between the PW (2 d, 16 d) oscillations in the time-sequences of the winds and of the GW variances allow the propagation directions for the GW fluxes to be inferred. These are eastward in summer months and westward in winter at MLT (60-90 km) altitudes. (C) 2002 Elsevier Science Ltd. All rights reserved.

Equatorial ionospheric plasma bubble irregularity development and dynamics during the major magnetospheric storm of 26 August 1998 are investigated using the data collected by a multistation and multi‐instrument diagnostic network operated at equatorial and low latitude sites in Brazil, and auroral electrojet activity (AU/AL), IMF, and Dst indices. A magnetospheric disturbance onset in the morning of 26 August 1998 was initiated by a solar wind shock and associated IMF Bz polarity reversals and ssc that were soon followed by a succession of substorm‐like auroral electrojet (AE) intensifications and Dst development. An IMF Bz southward turning and associated AE intensifications in the Brazilian dusk sector produced intense prompt penetration eastward electric field that caused large F region vertical drift and consequently the developments of intense postsunset equatorial anomaly and a series of intense plasma bubbles, the latter event lasting the entire night, as observed by digital ionosondes at São Luís (2.33°S, 315.8°E, dip angle: −.5°) and Fortaleza (3.9°S, 321.55°W, dip angle: −9°) and an all‐sky imager, two scanning photometers, and a Digisonde at the low‐latitude site Cachoeira Paulista (22.6°S, 315°E; dip angle: −28°). A notable aspect of the dynamics of the bubbles was their initially very low eastward drift velocity which turned into steadily increasing westward velocity that lasted till early morning hours. The results show for the first time a relationship between the zonal drift velocities of optically observed large‐scale bubbles (tens to hundreds of kilometers) and that of the smaller scale (kilometer sizes) structures as observed by a digital ionosonde. The results point to the dominant role of a disturbance dynamo associated westward thermospheric wind to maintain the plasma irregularity drift increasingly westward going into postmidnight hours. As an important finding, the results further show that significant contribution to the westward plasma bubble irregularity drift, normally attributed to disturbance dynamo effect, could arise from prompt penetration disturbance zonal electric field, in the course of a disturbance sequence lasting several hours. Such effect is attributed to Hall electric field arising from the primary disturbance zonal electric field, under enhanced nighttime ionospheric conductivities produced possibly by storm associated particle precipitation, in the Brazilian longitude sector in agreement with recent evidences [Abdu et al., 1998b].

A network of 15 northern hemisphere radars has been used to measure horizontal winds in the mesosphere and lower thermosphere during the PSMOS campaign of Summer 1999. The radars are sited at latitudes ranging from 21°N to 75°N and longitudes from 142°E to 157°W. The data were examined to investigate the Northern Hemisphere structure of the quasi-2-day planetary wave during the interval June–August. The amplitude of the 2-day wave was found to exhibit great day-to-day variability. In particular, significant periodic fluctuations in amplitude occurred with periods of 8–10 and 14–17 days. These modulations were strongest in July and largely absent in June and August. In July, the wave activity can be resolved into three westward-propagating waves with zonal wave numbers of 2, 3 and 4. The periods associated with these wave numbers were 53–56, 48–50 and 42–43h, respectively. The simultaneous presence of at least two spectral components with periods close to each other may serve to explain the observed amplitude modulations as a result of a beating between different spectral components. An earlier analysis of the planetary-wave field during this interval has revealed a westward propagating ∼16-day wave with zonal wave number 1 (Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002b) 1865–1896). A non-linear interaction between this ∼16-day planetary wave and the (3,0) Rossby-gravity mode (the 2-day-wave) provides a possible mechanism to generate the above ∼42h/wavenumber 4 wave and the ∼55h/wavenumber 2 waves as sum and difference secondary waves. A bispectral analysis was used to further investigate non-linear interactions between members of the planetary-wave field and suggested a number of interactions occur within the planetary-wave field, but that some of the interactions also involve the non-migrating diurnal tide with zonal wavenumber 6.

The mesospheric and lower thermospheric (MLT) winds (60–100 km) obtained by multiple MF radars, located from the arctic to equator at Tromsø (70° N, 19° E), Saskatoon (52° N, 107° W), London (43° N, 81° W), Hawaii (21° N, 157° W) and Christmas Island (2° N, 157° W), respectively, are used to study the planetary-scale 16-day waves. Based on the simultaneous observations (1993/1994), the variabilities of the wave amplitudes, periods and phases are derived. At mid- and high-latitude locations the 16-day waves are usually pervasive in the winter-centred seasons (October through March), with the amplitude gradually decreasing with height. From the subtropical location to the equator, the summer wave activities become strong at some particular altitude where the inter-hemisphere wave ducts possibly allow for the leakage of the wave from the other hemispheric winter. The observational results are in good agreement with the theoretical conclusion that, for slowly westward-traveling waves, such as the 16-day wave, vertical propagation is permitted only in an eastward background flow of moderate speed which is present in the winter hemisphere. The wave period also varies with height and time in a range of about 12–24 days. The wave latitudinal differences and the vertical structures are compared with the Global Scale Wave Model (GSWM) for the winter situation. Although their amplitude variations and profiles have a similar tendency, the discrepancies are considerable. For example, the maximum zonal amplitude occurs around 40° N for radar but 30° N for the model. The phase differences between sites due to the latitudinal effect are basically consistent with the model prediction of equatorward phase-propagation. The global 16-day waves at 95 km from the HRDI wind measurements during 1992 through 1995 are also displayed. Again, the wave is a winter dominant phenomenon with strong amplitude around the 40–60° latitude-band on both hemispheres.Key words. Meteorology and atmospheric dynamics – waves and tides – middle atmosphere dynamics – thermospheric dynamics

During the PSMOS Global-scale tidal variability experiment campaign of June 1–August 31, 1999, a network of radars made measurements of winds, waves and tides in the mesosphere/lower-thermosphere region over a wide range of latitudes. Clear evidence was found that fluctuations in tidal amplitudes occur on a global scale in both hemispheres, and that at least some of these fluctuations are periodic in nature. Modulation of the amplitude of the 12h tide was particularly evident at periods of 10 and 16 days, suggesting a non-linear interaction with planetary waves of those periods to be responsible. In selected cases, the secondary waves predicted from non-linear theory could be identified and their zonal wave numbers determined. In some, but not all, cases the longitudinal structure of the secondary waves supports the theory of planetary-wave/tidal interaction being responsible for the observed tidal modulation. It was noted also that beating between a 12.4-lunar and the solar tide could produce a near 16-day modulation of the 12h tide amplitude that is frequently observed in late summer.

Winds from MF radar observations (60–110km) at Saskatoon (52°N, 107°W) and London (43°N, 81°W), Canada, are processed to investigate the quasi 16-day (12–20 days) waves. The waves at both sites become prominent from late autumn to early spring (October–April) when the zonal background winds are eastward. Throughout the mesosphere they have amplitudes of 5–15m/s, with the zonal component being stronger than the meridional on average. And they exist over a large range of altitudes, generally attenuating with height up to 105 and 95km for Saskatoon and London, respectively. The summer wave activity, however, is weak and limited to a thin layer around the zonal zero-wind line (around 85km); at Saskatoon it is centred near this line but at London it can penetrate 5–10km into the westward flow. The local periods at lower altitudes are longer than that at higher ones, and periods at the two locations at the same time are usually not equal. Although in some situations the phases at London lead those at Saskatoon by tens of degrees, the general phase differences do not appear to have a simple pattern. The lack of coherence or correlation between the two locations for the wave amplitudes, heights of occurrence, and even periods are probably due to the longitudinal and latitudinal differences or localized wave activity. The vertical propagation characteristics for the 16-day wave show a standing wave structure from which the vertical wavelength of ∼40–50km can be derived. This could be caused by interference of the incident wave from the lower atmosphere with the downward reflected wave from near the mesopause. When combining the zonal and meridional components, they demonstrate a quasi-polarization mode of either linear or elliptical characteristic. The possible reasons for these wave characteristics, such as the effect of the background wind, interaction with the gravity waves, and including dissipation and modulation processes, are discussed.

It has become increasingly clear that Gravity Waves (GW) have an essential and often dominant role in the dynamics of the Middle Atmosphere. This leads to them having strong impacts upon the thermal structure and the distribution of atmospheric constituents. However, the radar observations of GW have been limited in their latitudinal extent during the past decade, and although satellite observations are now significantly contributing, global-seasonal climatologies of important characteristics are still inadequate. With regard to models, the inclusion of GW-drag effects has been problematic. Usually no seasonal or latitudinal variation in the subgrid-scale GW-drag parameterization scheme is included, and varieties of parameterization schemes have been used. Although these often make conflicting assumptions, they generally produce similarly acceptable end-products, e.g. zonal-mean zonal wind fields. In this paper, we report upon the beginnings of a substantial program, using observations from a network of MF radars (North America, Pacific and Europe), and data from the Canadian Middle Atmosphere Model (CMAM). This model allows the tidal and planetary wave fields to be assessed, characteristics and climatologies of which are well known from the MF Radars. Here we focus upon the tides. There are useful similarities in the observed and modeled background wind and wave fields, and strong indications that the two non-orographic GW-drag parameterization schemes (Hines; Medvedev–Klaassen) have significant and differing effects upon the dynamics of the modeled atmosphere. It is shown that this comparison process is valuable in the evaluation, and potentially the optimization, of parameterization schemes.

Observations of mean winds and semidiurnal and diurnal tides in the mesosphere/lower-thermosphere (MLT) region were made during the 3-month Planetary-Scale Mesopause Observing System Summer 1999 campaign. Data from 22 ground-based radars (and from two other instruments with measurements for the same period but in 1998) allow us to investigate the ability of the GSWM-00 to simulate the solar tides in the mesopause region (90–95km). Here we have found that the GSWM-00 provides an increasingly reasonable estimate of most of the tidal characteristics in the MLT region. However, the representation of the 24h tide appears superior to that of the 12h tide. Some of these discrepancies are studied in detail. In particular, the observations reveal significant 12h tidal amplitudes at high latitudes in the Northern Hemisphere summer. There is evidence for relation between the longitudinal variability of the mean zonal wind and the tidal characteristics seen from the radar wind measurements in the summer middle latitudes and a quasi-stationary planetary wave with zonal wave number one.

Recent studies using model calculation and ionospheric observations have revealed the existence of an additional layer in the topside equatorial ionosphere, the F3 layer. The observations using bottomside ionograms from locations close to the magnetic equator in Brazilian region have shown that the occurrence of the layer is very high from December to February (local summer) and from June to August (local winter). In fact, for the year 1995 the occurrence of the F3 layer is >75% during the months of January, February and December, and it is >65% for the period of June, July and August (Geofisica Int. 39 (2000) 57). In this work, we use 25 years of data for the months of January and August to investigate how the layer occurrence varies with the magnetic dip angle and solar activity.

Magnetospheric disturbances are known to modify the equatorial thermospheric dynamics and chemistry. The disturbance winds and electric fields in turn could modify equatorial spread F /plasma bubble irregularity development conditions in ways to enhance or inhibit its occurrences. Digital ionosonde/digisonde and optical data from Brazilian equatorial/low latitudes are analyzed during some recent magnetospheric disturbances to examine the control of disturbance electric fields and winds (especially the disturbance meridional winds) on the generation and/or inhibition of the spread F. The SUPIM (Sheffield University Plasmasphere- Ionosphere Model) has been used (based on a new methodology, Souza et al., 2000 JGR in press) to model the maridional wind and equatorial zonal electric field for quiet conditions to be used as reference for determining the disturbance components of these parameters and to identify their possible effects on the spread F developments. Preliminary analysis of some storm event that occurred in 1998 already show disturbance associated post sunset spread F development and associated perturbations (in zonal and meridional) winds as observed over Cachorira Paulista, Fortalez and Sao Luiz. The paper will present results based on analysis for a few quiet days and storm conditions.