武汉上空中层和低热层大气潮汐的流星雷达观测

武汉流星雷达是2002年元月建成的我国第一部全天空流星雷达，本文对2002年2月19日到7月31日流星雷达观测的潮汐的讨论表明，武汉中层顶以周日潮汐为潮汐运动的主要分量，它的强度远大于半日潮汐，周日潮汐和半日潮汐的波源都在80km以下．周日潮汐分量在3、4月份最强，并且经向分量略强于纬向分量．两个分量的峰值在约95km处出现，分别达到44m／s和60m／s．半日潮的最大值24m／s出现在4月初约93km处．周日潮汐和半日潮汐的振幅和相位随时间呈现出拟周期变化的特征，这可能是潮汐与行星波非线形相互作用的结果．观测结果与GSWM模型的比较表明，GSWM模型在相位随高度变化趋势上与观测结果一致，但模型的周日潮相位比观测约超前1—2h，半日潮相位约滞后1—4h．在周日潮汐较强的月份，模型与观测有较大的差异，观测的幅度通常在95km附近有极大值，而模型并没有极大值．GSWM模型对半日潮的幅度的估计通常过小，观测的半日潮汐幅度有时甚至超过模型值的一倍以上．     

Longitudinal MLT wind structure at higher mid-latitudes as seen by meteor radars at central and Eastern Europe (13°E/49°E)

The mid-latitude mesosphere and lower thermosphere (MLT) wind speeds measured by two SKiYMET meteor radars (MRs) at Collm (51°N, 13°E) and Kazan (56°N, 49°E) during 2016–2017 were analyzed to study longitudinal wind structures. The differences between monthly mean prevailing wind speeds and tidal amplitudes were compared with the corresponding differences obtained from TIMED/TIDI satellite winds and gradient wind speeds from the AURA/MLS instrument. It is shown that the MR wind difference between the two sites is statistically significant. The difference of the horizontal prevailing winds can be explained by a superposition of the background zonal flow, which is different at the two latitudes, with stationary planetary waves of different origin. Non-migrating tides contribute significantly to the difference of the semidiurnal tidal winds between the two sites.     

Global structure,seasonal and interannual variability of the eastward propagating tides seen in the SABER/TIMED temperatures (2002–2007)

The present paper is focused on the global spatial (altitude and latitude) structure, seasonal and interannual variability of the most stable in amplitude and phase eastward propagating diurnal and semidiurnal tides with zonal wavenumbers 2 and 3 derived from the SABER/TIMED temperatures for full 6 years (January 2002–December 2007). The tidal results are obtained by an analysis method where the tides (migrating and nonmigrating) and the planetary waves (zonally travelling, zonally symmetric and stationary) are simultaneously extracted from the satellite data. It has been found that the structures of the eastward propagating diurnal tides with zonal wavenumbers 3 and 2 change from antisymmetric with respect to the equator below ∼85 km height, to more symmetric above ∼95 km. The seasonal behavior of the DE3 is dominated by annual variation with maximum in August–September reaching average (2002–2007) amplitude of ∼15 K, while that of the DE2 by semiannual variation with solstice maxima and with average amplitude of ∼8 K. These tides revealed some interannual variability with a period of quasi-2 years. The seasonal behavior of the eastward propagating semidiurnal tide with zonal wavenumber 2 in the southern hemisphere (SH) is dominated by annual variation with maximum in the austral summer (November–January) while that in the northern hemisphere (NH) by semiannual variation with equinoctial maxima. The SE2 maximizes near 115 km height and at latitude of ∼30° reaching an average amplitude of ∼6 K. The seasonal behavior of the eastward propagating semidiurnal tide with zonal wavenumber 3 in both hemispheres indicates a main maximum during June solstice and a secondary one during December solstice. The tide maximizes near 110–115 km height and at a latitude of ∼30° reaching an average amplitude of ∼4.8 K in the SH and ∼4 K in the NH. The tidal structures of the two eastward propagating semidiurnal tides are predominantly antisymmetric about the equator.     

Investigation on the MLT tidal variability during September 2019 minor sudden stratospheric warming

Tidal variability in the mesosphere and lower thermosphere (MLT) during September 2019 Southern hemisphere minor sudden stratospheric warming (SSW) is investigated utilizing ground-based meteor radar wind observations from the equatorial, extratropical, middle, and high latitude stations and global reanalysis dataset. The polar warming is found to move from the mesosphere to the stratosphere until the peak warming day (PWD) of the SSW. The diurnal and semidiurnal tides at individual observational sites do not exhibit any consistent response during the observational interval, but a notable and consistent variability in some specific zonal wavenumber components, i. e., DW1 (migrating diurnal tide), DE3 (nonmigrating eastward wavenumber 3 diurnal tide), and SW2 (migrating semidiurnal tide) is found in the global reanalysis dataset. Incidentally, the warming event occurs during Spring equinox when a dominant seasonal change in the tidal activities generally takes place and hence seasonal variability is also looked into while identifying the SSW impact during the observational interval. It is found that the seasonal broad changes in the DW1, DE3, and SW2 amplitudes can be explained by the variability in the tidal sources, i.e., water vapor, convective activity, ozone, etc during the observational period. However, the extracted short-term variability in the global tidal modes on removing seasonal trend reveals noticeable response in connection with the warming event. The deseasoned amplitude of the DW1 significantly enhances around the PWD at most of the present latitudes. The deseasoned DE3 amplitude responds significantly in the middle atmosphere at low latitudes during the warming phase. The deseasoned SW2 exhibit clear enhancement around the PWD at all the latitudes. However, the deseasoned tidal features do not seem to correlate well with that of the source species unlike the seasonal ones that imply involvement of complex processes during the warming event, seeking further future investigations in this regard.     

Atmospheric tides below 80 km

Measurements and theory of diurnal and semidiurnal tidal oscillations between about 25 and 80 km are reviewed. At latitudes greater than about 30°, meridional (N-S) wind components are consistently in quadrature with and similar to the zonal (E-W) components. The tidal structures are interpreted as a superposition of quasi-steady higher-order modes excited in the troposphere by sources of limited extent (1,000–10,000 km). At latitudes less than about 30°, steady or quasi-steady diurnal and semidiurnal components are not necessarily the dominant components of the daily variation. At high latitudes diurnal phases generally show little change with height in comparison with observations at lower latitudes, in accord with the latitudinal properties of diurnal modes with positive and negative equivalent depths.     

Horizontal winds in the mesosphere at high latitudes

A total of 146 meteorological rocket flights applying the ‘falling sphere’ technique are used to obtain horizontal winds in the mesosphere at polar latitudes, namely at the Andøya Rocket Range (69°N, 125 flights), at Spitsbergen (78°N, 10 flights), and at Rothera (68°S, 11 January flights only). Nearly all flights took place around noon or midnight, i.e., in the same phase of the semidiurnal tide. Meridional winds at 69°N show a clear diurnal tidal variation which is not observed in the zonal winds. The zonal wind climatology shows a transition from summer to winter conditions with the zero wind line propagating upward from 40 km (end of August) to 80 km (end of September). Zonal winds are smaller at Spitsbergen compared to Andøya which is in line with a common angular velocity at both stations. Meridional winds at noon are of similar magnitude at all three stations and are directed towards the north and south pole, respectively. Horizontal and meridional winds generally agree with empirical models, except for the zonal winds at Antarctica which are similar to the NH, whereas there is a significant SH/NH difference in CIRA-1986.     

Early winter meteor wind over Bologna (45°N, 11°E)

The Bologna meteor radar was operational during two winter campaigns, from 6 January 1982 to 1 February 1982 and from 10 December 1982 to 2 February 1983. As occurrence of minor stratospheric warmings has been reported for these intervals, possible effects on meteor wind over Bologna related to this kind of warming are pointed out. Zonal and meridional prevailing winds are found to exhibit the maximum peak to peak value in their oscillations when a minor stratospheric warming reaches such an intensity that ΔT_{(90°N–60°N)} at 10 mbar is reversed. Diurnal and semidiurnal tides exhibit usual amplitude variations, but the semidiurnal tide has a noticeable phase shift at the time of a minor warming occurrence, while a similar shift is less evident in the diurnal tide phase.     

On the specification of TGCM tidal lower boundary conditions from radar wind measurements during the June 1984 GTMS period

Atmospheric tidal theory is applied to radar wind measurements of semidiurnal eastward and northward winds during June 1984 to specify the semidiurnal perturbation geopotential at lower thermosphere heights. Derived amplitudes for the longest-wavelength (2, 2) and (2, 3) tidal modes and their estimated vertical extensions are sufficiently large to modify the latitudinal structure of the thermosphere at levels of 150 km and above. These results can be used as the lower tidal boundary forcing in Thermospheric General Circulation Models (TGCM's) to incorporate the effects of upward propagating tides in simulations of the 26–28 June 1984 Global Thermosphere Mapping Study. (GTMS).     

On the role of tidal winds in the descending of the high type of sporadic layer (Esh)

As the prevailing tidal winds in the E region are generated by heating mechanisms, the dynamics of Es layers impacted by solar tides is a relevant theme in the space weather studies. This paper aims to identify the tidal wind component involved in the mechanism of formation and descending of the high type of sporadic layer (Es_h). The Es_h layers observed at altitudes between around 120 and 150 km in the Brazilian low latitude stations of Jataí and São José dos Campos during the months of April, June, September and December of 2016 are used in this analysis. The height variability and altitude descent of the Es_h layers are analyzed from the h′Es parameter obtained by ionosonde data. In this study, the observational data are compared with the simulations generated by an extended version of the Ionospheric E-Region Model (MIRE). At higher altitudes in the E region, the results show that the prevailing tidal pattern and wind direction controlling the Es_h layer formation and descent are different depending on month: (a) in April and June the zonal wind component and the associated semidiurnal tidal oscillations prevail, with some differences in terms of time of occurrence and descending speeds, and (b) in September and December the diurnal tidal periodicities become dominant, and both the meridional and zonal wind components seem to control the descending of the Es_h layers. Since the role of the tidal periodicities and wind directions changed depending on the month, the results suggest a possible seasonal tidal wind pattern, which is not well understood from the present study but requires further investigation. Other relevant aspects of the observations and the modeling are highlighted and discussed.     

Analysis of coastal altimetry in the Mexican Caribbean

A reprocessing of sea-level anomalies (SLA) resulting from X-TRACK coastal altimetry was carried out for the ENVISAT (2002–2010) and TOPEX/POSEIDON-Jason (1992–2019) satellite missions in the coastal area of the Mexican Caribbean. This consisted of applying a tidal correction to coastal altimetry sea level observations. Harmonic analysis of five coastal tide gauge records was performed to estimate the most important tidal components of the area, resulting on M2, N2, O1, S2, K1, MF, and MM. The tidal signal was reconstructed with the seven tidal components using the TPXO9 model. The SLA signals corrected with the seven tidal components were validated with in situ data from coastal tide gauges. The validation showed that the TPXO9 tidal barotropic model (1/30° grid) used to reconstruct the tidal signal with the seven representative tidal components performed better than the FES2012 global model (1/16° grid) that uses 33 tidal components. The reprocessed SLAs showed clear seasonality with significant signals at 4, 6, and 12 months, with the annual signal being the dominant one. In the Mexican Caribbean coastal zone, oceanographic processes with different scales (from coastal to mesoscale) converge, showing their complexity in the different SLA signals observed. The aim of this work is to contribute to the analysis of coastal altimetry data and understanding the sea level variations in the Mexican Caribbean. This work is the first step in the implementation of methodologies that take advantage of coastal satellite altimetry in the Caribbean Sea.     

Atmospheric tides between 80 km and 120 km

The structure and variability of tides in the 80 – 120 km height region are reviewed. Particularly emphasised are seasonal-latitudinal variations in the vertical structure of diurnal and semidiurnal winds between 70 – 100 km as measured by meteor and partial reflection drift radars, and tidal temperatures determined by incoherent scatter radars between 100 and 140 km. Variations in tidal structures with longitude, from day to day, and during equinoctial transition periods are also addressed.     

Observations of OH(3,1) airglow emission using a Michelson interferometer at 62° S

A Michelson interferometer was used to observe the hydroxyl (OH) emission in the upper mesosphere at the King Sejong Station (62.22° S, 301.25° E), Antarctica. The instrument was installed in February 1999 and has been in routine operation since then. An intensive operational effort has resulted in a substantial data set between April and June, 1999. A spectral analysis was performed on individual data to examine the information of dominant waves. A harmonic analysis was also carried out on the monthly average data to investigate the characteristics of the major low frequency oscillations. The 12-hr temperature oscillations exhibit a striking agreement with a theoretical tidal model, supporting the tidal (migrating) origin. The 8-hr wave is found to be persistent and dominant, reflecting its major role in the upper mesospheric dynamics at the given latitude. The 6-hr oscillation is observed only in May with its value close to the prediction for zonally symmetric tides.     

Rare observation of daytime whistlers at very low latitude (L = 1.08)

The source region and propagation mechanism of low latitude whistlers (Geomag. lat. <30°) have puzzled scientific community for last many decades. In view of recent reports, there is consensus on the source region of low latitude whistlers in the vicinity of the conjugate point. But the plausible conditions of ionospheric medium through which they travel are still uncertain. In addition to that, the whistlers in daytime are never observed at geomagnetic latitudes less than 20°. Here, for the first time, we present a rare observations of whistlers during sunlit hours from a very low-latitude station Allahabad (Geomag. Lat: 16.79°N, L = 1.08) in India on 04 February 2011. More than 90 whistlers are recorded during 1200–1300 UT during which the whole propagation path from lightning source region to whistler observation site is under sunlit. The favorable factors that facilitated the whistlers prior to the sunset are investigated in terms of source lightning characteristics, geomagnetic and background ionospheric medium conditions. The whistler activity period was found to be geomagnetically quiet. However, a significant suppression in ionospheric total electron content (TEC) compared to its quiet day average is found. This shows that background ionospheric conditions may play a key role in low latitude whistler propagation. This study reveals that whistlers can occur under sunlit hours at latitudes as low as L = 1.08 when the source lightning and ionospheric medium characteristics are optimally favorable.     

Neutral temperatures from Thomson scatter measurements: Comparisons with the CIRA(1972)

Neutral exospheric and lower thermospheric (100–130 km) temperatures from Thomson scatter measurements at Millstone Hill (42°N) are compared with CIRA temperatures with a view towards identifying deficiencies in the CIRA and recommending revisions. CIRA models the observed diurnal mean temperatures (T₀) to within 10% over a wide range of solar conditions (75? F_10.7 ? 250), but consistently underestimates the diurnal temperatures with maximum deviations approaching 50% of observed amplitudes (180–240 K) at solar maximum (1200 K ? T₀ ? 1400 K). The observed semidiurnal amplitudes, which lie in the range of 20K–80K, are always underestimated and frequently by more than 50%. In the lower thermosphere, tidal oscillations of temperature of order 20K–40K occur which are not modelled by CIRA. In addition, an analysis of exospheric temperatures at Millstone Hill during a magnetic disturbance indicates a response within 1–2 hours from storm onset, whereas CIRA assumes a 6.7 hour delay. Although some of these deficiences are addressed by the more recent MSIS model, there exists a sufficient data base to recommend several additional revisions to the CIRA temperatures at this time.     

A global view of tidal temperature perturbations above the mesopause: Preliminary model/observation intercomparison

Migrating and nonmigrating diurnal tides in the temperature data from the satellite borne, Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) experiment are compared to model predictions from the Thermosphere Ionosphere Mesosphere Electrodynamics General Circulation Model (TIME-GCM) and from the Global Scale Wave Model (GSWM). The comparative model/observation analysis is performed between 75–120 km altitude and at 7.5°N. The results suggest that interactions between quasi-stationary planetary waves and radiative and latent heat forced tidal components may play an important role in generating the thermal tidal structure above the mesopause though more extended model studies are necessary.     

A comparison of ionosonde measured foF2 and IRI-2016 predictions over China

A comparison of the ionospheric F-region critical frequency (foF2) between ionosonde measurements and IRI-2016 predictions is studied over China during the period from January 2008 to October 2016. Four stations are selected, and the latitude coverage starts at 49.4°N and ends at 23.2°N with a sequential latitude interval of about 10°, the corresponding geomagnetic latitudes are from 39.5°N to 13.2°N. The results show that the variability of the observed foF2 versus latitudes, seasons, local time and levels of solar activity could be well reproduced by IRI-2016. However, the daily lowest value of foF2 from the IRI-2016 prediction occurs earlier than that from the ionosonde. Around the sunrise, the IRI-2016 prediction shows a very sharp rise and grows much faster than the observed foF2 in every month. The foF2 difference between the two options (URSI and CCIR) in IRI-2016 increases as the F10.7 index decreases. During 2008–2009, the annual average deviations of URSI and CCIR range from ?5% to ?10% and from 5% to ?5%, respectively. Generally, the CCIR performs better than URSI during postsunset under low solar activity or in Equatorial Ionization Anomaly (EIA) region over China, while it shows no large difference in performance with URSI in other locations or for other time.     

Observation of the north-south asymmetry in the equatorial spread F intensity at the Brazilian longitude

This study presents an analysis of the observed north-south asymmetry of the range spread F (RSF) intensity at the low latitude region during an equinoctial month of different solar epochs (2002, 2015 and 2017). The ionospheric parameters were obtained during geomagnetic quiet days from four digisonde stations located along the Brazilian longitude, which include a dip equator station (Sao Luiz (SL: 2.33 S, 44.2 W)), conjugate stations (Campo Grande (CG: 20.5°S, 55°W) and Boa Vista (BV: 2.8°N, 60.7°W)) and another low latitude station (Cachoeira Paulista (CP: 22.7°S, 45°W)). The results highlight the competing effect of the post-sunset electric field strength and the trans-equatorial wind on the latitudinal distribution of the irregularity intensity at both hemispheres under varying background ionospheric condition. The RSF intensity was seen to reduce as the solar flux index decreased and the latitudinal peak shifted closer to the dip equator. This was dependent on the variation of the field line mapped irregularity spectrum and the density gradient. Likewise, the north-south asymmetry in the irregularity occurrence was seen to become more significant as a denser ionosphere was observed at the hemisphere with the equatorward meridional wind. This has further proven that the non-linear cascading of the plasma irregularity across the low latitude region is strongly influenced by the local electric field.     

Statistical average estimates of high latitude field-aligned currents from the STARE and SABRE coherent VHF radar systems

Two bistatic VHF radar systems, STARE and SABRE, have been employed to estimate ionospheric electric fields in the geomagnetic latitude range 61.1 – 69.3° (geographic latitude range 63.8 – 72.6°) over northern Scandinavia. 173 days of good backscatter from all four radars have been analysed during the period 1982 to 1986, from which the average ionospheric divergence electric field versus latitude and time is calculated. The average magnetic field-aligned currents are computed using an AE-dependent empirical model of the ionospheric conductance. Statistical Birkeland current estimates are presented for high and low values of the Kp and AE indices as well as positive and negative orientations of the IMF B_z component. The results compare very favourably to other ground-based and satellite measurements.     

Seismogenic ionospheric anomalies associated with the strong Indonesian earthquake occurred on 11 April 2012 (M = 8.5)

Ionospheric perturbations in possible association with a major earthquake (EQ) (M?=?8.5) which occurred in India-Oceania region are investigated by monitoring subionospheric propagation of VLF signals transmitted from the NWC transmitter (F?=?19.8?kHz), Australia to a receiving station at Varanasi (geographic lat. 25.3°N, long 82.99°E), India. The EQ occurred on 11 April 2012 at 08:38:35?h UT (magnitude?≈?8.5, depth?=?10?km, and lat.?=?2.3°N, long.?=?93.0°E). A significant increase of few days before the EQ has been observed by using the VLF nighttime amplitude fluctuation method (fixed frequency transmitter signal). The analysis of total electron contents (TEC) derived from the global positioning system (GPS) at three different stations namely, Hyderabad (latitude 17.38°N, longitude 78.48°E), Singapore (latitude 1.37°N, longitude 103.84°E) and Port Blair (latitude 11.62°N, longitude 92.72°E) due to this EQ has also been presented. Significant perturbation in TEC data (enhancements and depletion) is noted before and after the main shock of the EQ. The possible mechanisms behind these perturbations due to EQ have also been discussed.     

The longitudinal and hemispherical variation of the IRI’s foF2 estimates at ±40° dip angle around 95°E and 130°E sector under fluctuating solar flux conditions

The deviation of the IRI estimates of the monthly mean foF2 in the low mid latitude of 95°E–130°E longitude sector is investigated using simultaneous ground measurements at four stations during 2010–2014. The stations form two conjugate pairs of the same geo-magnetic latitude at two fixed longitudes enabling direct longitudinal and hemispheric comparison. The temporal, spatial, seasonal and solar activity variations of the deviations are discussed with reference to the longitudinal density variation in the transition region between low and midlatitudes. Cases of underestimation/overestimation as well as good estimate are noted. Underestimation (overestimation) in the daytime and overestimation (underestimation) in the nighttime of 95°E (130°E) are common. The longitudinal difference in the measurements suggests negative (positive) foF2 gradient from west to east in daytime (nighttime). In contrast, the IRI predicts flatter or increasing longitudinal profiles from 95°E to 130°E. The local time and longitudinal variation of the IRI deviations can be attributed to the combined role of the longitudinal EIA structure as well as midlatitude zonal wind-magnetic declination effect. The station/season independent deviations relate the role of solar activity representation in the IRI. These deviations may be attributed to the weak IRI response to rapid solar flux fluctuations.