Some results of night-time scintillations at low latitude

Ionospheric scintillation observations of VHF radio signals from FLEETSAT satellite (73°E longitude) at Bhopal from January 1990 to December 1990 have been used to study the characteristic variations of scintillation activity. It is found that scintillation occurrence is essentially a night-time phenomenon and day-time scintillations are very rare. Annual average nocturnal variation of percentage occurrence of scintillations shows maximum at around 2100–2200 hours LT. Seasonally, scintillations are most prominent during equinoxes and least during summer. Geomagnetic disturbances tend to decrease the occurrence of scintillations in the pre-midnight period.     

GPS based TEC measurements for a period August 2008– December 2009 near the northern crest of Indian equatorial ionospheric anomaly region

In recent years, measurements of total electron content (TEC) have gained importance with increasing demand for the GPS-based navigation applications in trans-ionospheric communications. To study the variation in ionospheric TEC, we used the data obtained from GPS Ionospheric Scintillation and TEC monitoring (GISTM) system which is in operation at SVNIT, Surat, India (21.16°N, 72.78°E) located at the northern crest of equatorial anomaly region. The data collected (for the low sunspot activity period from August 2008–December 2009) were used to study the diurnal, monthly, seasonal semi-annual and annual variations of TEC at Surat. It was observed that the diurnal variation at the region reaches its maximum value between 13:00 and 16:00 IST. The monthly average diurnal variations showed that the TEC maximizes during the equinox months followed by the winter months, and are lowest during the summer months. The ionospheric range delay to TEC for the primary GPS signal is 0.162 m per TECU. The diurnal variation in TEC shows a minimum to maximum variation of about 5 to 50 TECU (in current low sunspot activity periods). These TEC values correspond to range delay variations of about 1 to 9 m at Surat. These variations in the range delay will certainly increase in high sunspot activity periods. Detected TEC variations are also closely related to space weather characterizing quantities such as solar wind and geomagnetic activity indices.     

Total electron content at low latitudes

Radio beacon from ATS-6 at 140 MHz was used to measure the changes in the polarization angle (Faraday rotation) at Bombay, Rajkot, Ahmedabad, Udaipur and Patiala during October 1975 to July 1976. In this paper, results of diurnal, seasonal and latitudinal variations in total electron content (TEC) derived from these measurements are reported. The amplitude of diurnal peak is found to be higher at Rajkot, Ahmedabad and Udaipur as compared to that at Patiala or Bombay, indicating that the peak of Appleton anomaly in the latitudinal variation of TEC was close to the latitude of Ahmedabad. The diurnal maximum of TEC occurs around the same time during summer and winter months. The peak electron content shows a semiannual variation at all the stations with large values in equinoxes as compared to winter and summer. The TEC at Bombay shows a seasonal anamoly with high values in winter as compared to summer. The paper describes the development of latitudinal anomaly with the time of the day for different seasons. This anomaly is maximum during 1000 to 1800 LT and is located between 12° and 14° N (dip latitude) in summer and equinoxes and at about 10°N in winter.     

Corotating structures in the solar wind from 111-MHz observations of interplanetary scintillations at large elongations

Results of continuous 111 MHz observations of interplanetary scintillations of the strong radio source 3C 48 at elongations larger than 80? out on the Large Phased Array (LPA) of the Lebedev Physical Institute are reported. The data were taken during a four-year interval, from 2012 to 2015, near the maximum of the 24th solar-activity cycle. The averaged elongation dependence of the scintillation index and similar dependences for individual years during the approach and recession phases suggest the presence of a periodic modulation with a 26-day period, which is masked by day-to-day variations. This periodic modulation can be explained by the existence of a long-lived region of enhanced plasma density adjacent to the solar equator during the solar-activity maximum. It is shown that the scintillation timescale increases in the transition to elongations exceeding 90?.     

Meteotsunamis in the northeastern Gulf of Mexico and their possible link to El Niño Southern Oscillation

Analysis of 20-year time series of water levels in the northeastern Gulf of Mexico has revealed that meteotsunamis are ubiquitous in this region. On average, 1–3 meteotsunamis with wave heights >0.5 m occur each year in this area. The probability of meteotsunami occurrence is highest during March–April and June–August. Meteotsunamis in the northeastern Gulf of Mexico can be triggered by winter and summer extra-tropical storms and by tropical cyclones. In northwestern Florida most of the events are triggered by winter storms, while in west and southwest Florida they appear both in winter and summer. Atmospheric pressure and wind anomalies (periods <6 h) associated with the passage of squalls originated the majority of the observed meteotsunami events. The most intense meteotsunamigenic periods took place during El Niño periods (1997–1998, 2009–2010 and 2015–2016). Meteotsunamis were also active in 2005, a year characterized by exceptionally intense tropical cyclone activity. Meteotsunami incidence varied yearly and at periods between 2 and 5 years. Results from cross-wavelet analysis suggested that El Niño and meteotsunami activity are correlated at annual and longer-period bands.     

ENSO-7赤道西太平洋异常纬向风所驱动的热带太平洋次表层海温距平的循环

近几年的一系列分析研究表明，ENSO与异常东亚冬季风之间有相互影响，持续的强（弱）东亚冬季风通过引起赤道西太平洋地区的西（东）风异常对El Niño/La Niña的发生起着重要作用；赤道太平洋次表层海温异常（SOTA）的年际变化（循环）与ENSO发生有密切关系；ENSO的真正源在西太平洋暖池，暖池正（负）SOTA沿赤道温跃层东传到东太平洋，便导致El Niño/La Niña的爆发；在暖池正（负）SOTA沿赤道东传的同时，有负（正）SOTA沿10°N和10°S纬度带向西传播，从而构成SOTA的循环；热带太平洋SOTA循环的驱动者是赤道西太平洋的异常纬向风。进而可以认为：ENSO实质上是主要由异常东亚季风引起的赤道西太平洋异常纬向风所驱动的热带太平洋次表层海温距平的年际循环。     

Daily variation of geomagnetic field at low latitudes associated with stable solar wind flow

Mean diurnal variation ofH at low and equatorial latitudes is computed for days in the vicinity of passage of ‘quiet’ solar wind. It is shown that the prepassage magnitude of the diurnal variation is appreciably larger when compared to post-passage intervals at low latitudes but the difference vanishes in the electrojet region. It is suggested that the Sq current system moves towards dip equator immediately following quiet wind conditions relative to earlier periods. It is also shown that during conditions of stable solar wind, the solar wind proton density is inversely related to the electrojet strength, while at low latitudes outside the jet influence, there is no clear association.     

Observations of meteorological tsunamis along the south-west Australian coast

Tide gauge data were used to identify the occurrence, characteristics, and cause of tsunamis of meteorological origin (termed ‘meteotsunamis’) along the Western Australian coast. This is the first study to identify meteotsunamis in this region, and the results indicated that they occur frequently. Although meteotsunamis are not catastrophic to the extent of major seismically induced basin-scale events, the wave heights of meteotsunamis examined at some local stations in this study were higher than those recorded through seismic tsunamis. In June 2012, a meteotsunami contributed to an extreme water-level event at Fremantle, which recorded the highest water level in over 115 years. Meteotsunamis (wave heights >0.4 m, when the mean tidal range in the region is ~0.5 m) were found to coincide with thunderstorms in summer and the passage of low-pressure systems during winter. Spectral analysis of tide gauge time series records showed that existing continental seiche oscillations (periods between 30 min and 5 h) were enhanced during the meteotsunamis, with a high proportion of energy transferred to the continental shelf oscillation period. Three recent meteotsunami events (22 March 2010, 10 June 2012, and 7 January 2013) two due to summer thunderstorms and one due to a winter frontal system were chosen for detailed analysis. The meteotsunami amplitudes were up to a factor 2 larger than the local tidal range and sometimes contributed up to 85 % of the non-tidal water signal. A single meteorological event was found to generate several meteotsunamis along the coast, up to 500 km apart, as the air pressure disturbance propagated over the continental shelf; however, the topography and local bathymetry of the continental shelf defined the local sea-level resonance characteristics at each location. With the available data (sea level and meteorological), the exact mechanisms for the generation of the meteotsunamis could not be isolated.     

西北区冰雹日气候分析及预测方法研究

利用1970 —2001 年1 ～12 月中国西北区冰雹日数的气象实测资料，将年冰雹日数发生的变化趋势看作每年的一个气候现象，分析其发生的气候变化趋势、年代际变化以及与大气环流的相关关系，得出多雹年北半球500 h Pa 极涡强、位置偏东，副热带高压面积小、强度弱、位置偏南，乌拉尔山至巴尔喀什湖高度场为正距平，雅库次克至贝加尔湖高度场为负距平。少雹年北半球500 h Pa 极涡弱、位置偏西，副热带高压面积大、强度强、位置偏北，乌拉尔山至巴尔喀什湖高度场为负距平，雅库次克至贝加尔湖高度场为正距平区。从而找出冰雹气候趋势预测的着眼点，建立冰雹短期气候预测概念模型；此模型对冰雹发生变化趋势的预测具有一定的实用价值。     

近300a来古里雅与长江下游温度变化所受太阳活动、火山活动的影响分析

分析了近300a来古里雅冰芯δ¹⁸O值和长江下游温度变化与太阳活动、火山活动的关系.结果表明,无论在年际尺度,还是在10a尺度上,西昆仑山的古里雅冰芯和中国东部的长江下游地区的温度与以太阳辐照和太阳黑子周期长度为代用指标的太阳活动有良好的相关关系.火山活动对古里雅、合肥冬温和杭州春温有明显的降温作用.1810’s—1830’s时段不仅合肥冬温和杭州春温发生了剧烈的变化,而且处于高亚洲中西部的古里雅冰芯δ¹⁸O值也急剧下降,同时在北半球温度重建曲线上这一时段也是一个冷谷,表明该时段的气候变化可能具有全球性,而强烈的火山活动和微弱的太阳活动可能对温度的快速降低起触发作用。     

Corotating and Propagating Disturbances in the Solar Wind Based on Monitoring of Interplanetary Scintillations at the LPA Radio Telescope of the Lebedev Physical Institute in 2017

Monitoring of interplanetary scintillations in 2017 is used as a basis for analyzing the dynamics of scintillation levels in periods preceding the arrival at the Earth of eight large-scale disturbances in the solar wind giving rise to strong geomagnetic storms. In six of the eight events, the dynamics of the scintillation level were mainly determined by the motion of corotating disturbances. In two events, coronal-mass ejections excited in the corona near the western limb of the Sun were observed against the background of corotating disturbances. In one of these cases, a magnetic storm was associated with a corotating flux, and in the other with a powerful propagating disturbance. Comparison with similar data obtained in 2016, also during the descending phase in solar activity, testifies to the existence of corotating disturbances with lifetimes of at least 20 solar rotations. These new results support the earlier conclusion that a weakening of scintillations is observed in the evening sector three to four days before the arrival of the compressed part of a disturbance to the Earth, which could be due to an appreciable lowering of the level of small-scale turbulence in the plasma in an extended region ahead of the frontal part of the disturbance. The interplanetary-scintillation monitoring data for 2017 show that, simultaneously with the associated magnetic storm, there is an enhancement of second-time-scale scintillations, which are most clearly manifest when the storm occurs during the evening or night-time hours. For the events considered, the increase in scintillations accompanying the magnetic storm is associated with an enhancement in the level of small-scale fluctuations in regions of the solar wind adjacent to the Earth when the storm is excited by a corotating disturbance, and with the perturbed ionosphere when the storm is excited by a flare-related disturbance.     

天山山区近40a夏季降水变化及与南北疆的比较

利用新疆1959-1998年的降水资料,分析了天山山区近40a来夏季降水变化特征,并与南疆、北疆进行了比较.结果表明:天山山区近40a来的夏季降水在干湿阶段、最干最湿年份、降水变化的周期方面均与南北疆有别;天山山区夏季降水空间上的同步性变化比南疆及北疆弱一些;天山山区近40a来夏季降水年代际变化与北疆较为相近.新疆近40a来夏季降水最多的年代是90年代,天山山区偏多12%,南疆偏多25%,北疆偏多21%.     

Late Quaternary changes in surface productivity and oxygen minimum zone (OMZ) in the northwestern Arabian Sea: Micropaleontologic and sedimentary record at ODP site 728A

Changes in the abundance of selected planktic foraminiferal species and some sedimentological parameters at ODP site 728A were examined to understand the fluctuations in the surface productivity and deep sea oxygenation in the NW Arabian Sea during last ∼540 kyr. The increased relative abundances of high fertility taxa, i.e., Globigerinita glutinata and Globigerina bulloides mainly during interglacial intervals indicate intense upwelling. Strong SW summer monsoon probably increased the upwelling in the western Arabian Sea during interglacial intervals and caused high surface productivities due to the lateral transport of eutrophic waters. Most of the glacial periods (i.e., MIS 2, 4, 6, 8 and 12) are characterized by higher relative abundances of Neogloboquadrina pachyderma and Neogloboquadrina dutertrei associated with Globigerinoides ruber. The more stratified condition and deep mixed layer due to increased NE winter monsoon are mainly responsible for the higher relative abundances of N. pachyderma during glacial periods. Some of the glacial intervals (i.e., MIS 6 and 8) are also characterized by pteropod spikes reflecting deepening of aragonite compensation depth (ACD) and relatively less intense oxygen minimum zone (OMZ) in this region due to deep sea mixing and thermocline ventilation, and relatively less intense surface productivity during winter monsoon. The interglacial periods are largely devoid of pteropod shells indicating more aragonite dissolution due to increased intensity of OMZ in the northwestern Arabian Sea.     

Wave processes of temperature changes at the surface of the Black Sea

Based on the conception of the cyclic (wave) nature of the changes in the circulation of the atmosphere of the Atlantic-European sector (index of the North Atlantic oscillations) and surface temperature of the Black Sea on scales smaller or equal to the century scale, anomalies of the winter surface temperature are analyzed. It is shown that in the phase of the maximum of the presumably 60-year cycle of changes in the NAO index a negative anomaly of surface temperature forms and in the phase of minimum of this scale of changes of NAO, there is a positive anomaly. Estimates of the possible thermal state of the surface of the Black Sea during the phase of the expected minimum in 2011–2020 of the 60-year cycle (60-year wave) of the NAO index are made. The possibility of a phase shift between changes in solar activity, the NAO index, and anomalies of temperature of the Black Sea on the scales of cyclic (wave) processes larger than 100 years is considered.     

Reorganization of the Asian Monsoon System at About 2. 6 Ma Ago and Its Implications for the Rising of the Tibetan Plateau

REORGANIZATION OF THE ASIAN MONSOON SYSTEM AT ABOUT 2.6 Ma AGO AND ITS IMPLICATIONS FOR THE RISING OF THE TIBETAN PLATEAUheChineseResearchFoundation(KZ 951 A1 2 0 4 )     

Variations in solar and geomagnetic activity with periods near to 1.3 year

It is known that solar wind velocity fluctuates regularly with a period of about 1.3 years. This periodicity (and other signals with periods near to 1.1 and 0.9 years) has also been observed in biological data. The variation is a temporary feature, mostly being observed in the early 1990s. Here, the occurrence of these periodic signals in solar and geomagnetic activity between 1932 and 2005 has been investigated. The signal with 1.3 year period is present in geomagnetic activity only in a short interval after 1990 and to a lesser extent around 1942. At other times the signal is very weak or not present at all. Other periods are much lower amplitude and appear only sporadically throughout the time investigated. A connection between these periods and solar cycles (e.g. different even or odd cycles) has not been proven. It is possible that there is a long-term periodicity in the occurrence of the 1.3 year period but the time series data available is insufficient to confirm this. There are no such periodicities in solar activity. In order to gain a greater understanding of these periodic signals, we should search for their origin in interplanetary space.     

Seasonal cycling of estuarine sediment and contaminant transport

The seasonal cycling of fine sediment in the upper reaches of a hypothetical macrotidal estuary and its possible consequences for the behaviour of a contaminant which partitions between dissolved and particulate forms are investigated theoretically. The simplest one-dimensional models are used as a starting point for future studies: (a) a within-tide hydrodynamic (tidal) model, (b) an associated sediment transport model and (c) a tidally-averaged contaminant dispersal model. The calculations are made for a four-year period and show that a cyclic migration of mobile sediment occurs in the upper reaches of the estuary. Sediment accumulates during spring to late summer, and is redistributed in the lower estuary during high runoff periods (autumn and winter). For a fluvial input of contaminant, the dissolved contaminant levels during summer are greatly depressed below conservative mixing values in the upper (turbidity maximum) region, whereas they are slightly enhanced in the lower reaches. During winter, the levels are substantially greater than conservative values except for a slight depression at very low salinities. Thus, sediment here acts as a source of contaminant for most of the salinity range. For a marine input of contaminant, levels are enhanced above the conservative mixing line at low salinities throughout the year, the effect being much larger during summer.     

Active and break spells of the Indian summer monsoon

In this paper, we suggest criteria for the identification of active and break events of the Indian summer monsoon on the basis of recently derived high resolution daily gridded rainfall dataset over India (1951–2007). Active and break events are defined as periods during the peak monsoon months of July and August, in which the normalized anomaly of the rainfall over a critical area, called the monsoon core zone exceeds 1 or is less than −1.0 respectively, provided the criterion is satisfied for at least three consecutive days. We elucidate the major features of these events. We consider very briefly the relationship of the intraseasonal fluctuations between these events and the interannual variation of the summer monsoon rainfall.     

East Asian monsoon variations during Oxygen Isotope Stage 5: evidence from the northwestern margin of the Chinese loess plateau

We present the results of high-resolution multi-proxy climate studies of the S1 palaeosol, corresponding to oxygen isotope stage (OIS) 5, from the northwestern margin of the Chinese Loess Plateau area. Here, S1 is much thicker (ca. 6–8 m) than in the central Loess Plateau areas (ca. 2 m), where most previous studies have been conducted. Hence, much higher-resolution stratigraphic studies are possible, yielding more insight into the temporal variations of the East Asian monsoon during MIS 5. The frequency-dependent magnetic susceptibility, as well as the concentration of secondary carbonate, is used as an indicator of the summer monsoon intensity, and the median particle size as an indicator of the winter monsoon intensity. The results suggest that the northwestern margin of the Chinese Loess Plateau experienced the strongest summer monsoon intensity in sub-stage (OISS) 5e and the weakest in OISS 5a, among the three warmer periods during stage 5. The summer monsoon was weaker in OISS 5b than in OISS 5d. A dusty interval interrupted the second warmer period (5c) and a soil-forming event interrupted the first colder period (5d). The results also suggest that the directions of changes in the intensities of summer and winter monsoons may not always have been proportionately opposite. For example, the weakest summer monsoon occurred in OISS 5a during which the winter monsoon was not the strongest. We further conclude that the winter monsoon during the last interglacial was probably driven by global ice volume fluctuations, while the summer monsoon was primarily controlled by the northern hemisphere solar insolation and was probably modified by a feedback mechanism. That is, the climatic buffering effect of low-latitudinal oceans may have distorted the response of the summer monsoon to insolation variations. Finally, our results do not show the degree of climatic instability comparable to that recorded in the GRIP ice core for the last interglacial (OISS 5e), even though the study area is situated in a region which has been sensitive to climatic changes.     

Monitoring the three-dimensional ionospheric electron density distribution using GPS observations over China

In this paper, an IRI model assisted GPS-based Computerized Ionospheric Tomography (CIT) technique is developed to inverse the ionospheric electron density (IED) distribution over China. Essentially, an improved algebraic reconstruction technique (IART) is first proposed to reconstruct the ionospheric images with high resolution and high efficiency. A numerical experiment is used to validate the reliability of the method and its advantages to the classical algebraic reconstruction technique (ART). This is then used to reconstruct the IED images using the GPS data in China. The variations of the IED during magnetically quiet and disturbed days are reported and analyzed here. Reconstructed results during magnetically quiet days show some prominent ionospheric features such as the development of equatorial anomaly and the tilt of ionization crest. Meanwhile, ionospheric storm phase effects and disturbed features can also be revealed from the reconstructed IED image under storm conditions. Research shows that the positive storm phase effects usually happen in southern China, and the negative storm phase effects mainly occur in northern China. The equatorial anomaly crest moved to the north in the main phase of the storm. Ionosonde data recorded at Wuhan station provides the verification for the reliability of GPS-based CIT technique.