The features and a possible mechanism of semiannual variation in the peak electron density of the low latitude F2 layer

Ionospheric data observed in 30 stations located in 3 longitude sectors (East Asia/Australia Sector, Europe/Africa Sector and America/East Pacific Ocean Sector) during 1974–1986 are used to analyse the characteristics of semiannual variation in the peak electron density of F2 layer (NmF2). The results indicate that the semiannual variation of NmF2 mainly presents in daytime. In nighttime, except in the region of geomagnetic equator between the two crests of ionospheric equatorial anomaly, NmF2 has no obvious semiannual variation. In the high latitude region, only in solar maxima years and in daytime, there are obvious semiannual variations of NmF2. The amplitude distribution of the semiannual variation of daytime NmF2 with latitude has a “double-humped structure”, which is very similar to the ionospheric equatorial anomaly. There is asymmetry between the Southern and the Northern Hemispheres of the profile of the amplitude of semiannual variation of NmF2 and longitudinal difference. A new possible mechanism of semiannual variation of NmF2 is put forward in this paper. The semiannual variation of the diurnal tide in the lower thermosphere induces the semiannual variation of the amplitude of the equatorial electrojet. This causes the semiannual variation of the amplitude of ionospheric equatorial anomaly through fountain effect. This process induces the semiannual variation of the low latitude NmF2.     

Variability of the 557-nm atmospheric emission

The variability of diurnal, day-to-day, and monthly average rates of the 557.7-nm atmospheric emission (I _557.7) is considered. We use 1997–2010 airglow observation data obtained for the upper atmosphere over Eastern Siberia (52°N, 103°E). The variation coefficient K_V of corresponding quantities is taken as the variability index. For the 23rd solar cycle, we examine the resulting seasonal variation of K_V of monthly averaged I _557.7, the dependence of monthly averaged I _557.7 on solar activity for each month, the variation coefficient of diurnal values of I _557.7 for different seasons of the year, the variability of I _557.7 during some geophysical events, and the correlation of I _557.7 variations with global climatic indices.     

Some aspects of the secular variation of the geomagnetic elements H,Z and F between 1958–1974 in Romania

The paper presents some results concerning the secular variation of H, Z and F in Romania between 1958–1974. Curves of the variation of , Z and F at 31 repeat stations are given. The effect of solar activity on the horizontal component is separated and its characteristics discussed. For the interval 1969.5–1974.5 maps of the smoothed secular variation of H, Z and F are given, with isopores described by curves of second degree with respect to the geographical coordinates.     

Characteristics of the secular variation of the geomagnetic field between 1964–1981 in Romania

The variation of the H, Z, and T components of the geomagnetic field at repeat stations on Romanian territory between 1964 and 1981 is discussed in terms of internal secular and solar cycle related variations. Their geographical distribution is accounted for by the magnetic and electric structure of the interior of the Earth. The effects of magnetic and electromagnetic induction caused by the solar cycle related variation were evaluated.     

Sea-floor dynamic processes as the possible cause of correlations between paleoclimatic and paleomagnetic indices in deep-sea sedimentary cores

Recent publications showing correlations between the variation of intensity of magnetization(J) and paleomagnetic inclinationI on the one hand, and diverse paleoclimatic indices on the other, have been interpreted to represent direct cause and effect relationships between climate and geomagnetism.It is proposed that several sea-floor dynamic processes, which are climatically variable, are at least equally feasible causes of the observed correlations. Thus, for variation inJ, it is a simple matter to invoke variations in such factors as the magnetic particle size fraction below 50 microns, carbonate content, or laminar flow layer thickness, all of which can change with climate. For variation inI, climatically related variation of the velocity gradient in the laminar flow layer can be employed to explain the correlation.Prior to accepting the suggested geomagnetic control of climate as real, it is recommended that simple measurements of the temporal and geographic variation in fabric and composition of the deep-sea sediments involved should be carried out, as a check for the effect of bottom dynamic processes.     

Modeling the Dst variation during magnetic storms

A unified method for calculating the Dst index and its components using models of the magnetospheric magnetic field is proposed. The method is consistent with the procedure for calculating Dst from the ground-based magnetometer data. When calculating Dst, the quiet-day magnetic variation is subtracted from the model variation of the magnetic field of magnetospheric sources. The effect of induced currents flowing in the surface layer of the Earth’s crust is taken into account. The dynamics of the magnetospheric current systems during a storm is studied based on an analysis of the Dst components. The magnetic field components for a “quiet” day in June 1998 are studied. The calculations of the Dst components in the parabolid and T01 models demonstrate that the maximum contributions of the ring current and magnetotail current system to the Dst variation are comparable for the magnetic storm of June 25–26, 1998.     

Obtaining of information about the IMF evolution from an analysis of the geomagnetic field data

A comparison of the time variations in the geomagnetic field characteristics (the u and aa indices of geomagnetic activity) with the variation in the solar magnetic dipole inclination shows close agreement between these variations. The linear correlation coefficients between the u and aa indices, the u index and solar magnetic dipole inclination, and the aa index and solar magnetic dipole inclination are 0.93, 0.45, and 0.49, respectively. This makes it possible to extend studying the IMF evolution in the 11-year cycle of solar activity to the 170-year period beginning from 1835. It has been indicated that the time variation in the heliospheric current sheet (HCS) surface deviation from the solar magnetic equator plane, calculated based on the actual HCS configuration, is in good agreement with the time variation in the amplitude of the Fourier series second harmonics in a harmonic analysis of the series of daily data on the IMF sign in the vicinity of the Earth. The linear correlation coefficient is 0.9 in this case.     

TheQ value variations in the preparing process of rock rupture

In this paper, the "spectral amplitude ratio method" (SAR), "energy method" (EN) and "coda wave method" (CW) are used to calculate theQ value variations of gneiss in the preparing rupture process. The obtained results show that the variation state ofQ values by SAR features the shape of relative stability—gradual increment to the maximum—then decrement and final rupture. The variation state ofQ values by EN is just contrary to that by SAR, i. e. with the shape of stability—decrement—increment—and final rupture. The varation state ofQ values by CW is similar to that by EN, its main frequency features the shape of relatively high value—decrement to the minimum—increment—and final rupture. But to the high frequency (higher than the main frequency), the variation state ofQ values features the shape of the stable value-increment to the maximum-decrement-and final rupture. At the same time, the results by coda wave amplitude spectrum show that, when stress reaches 70% of rupture stress, the high frequency component of S wave rapidly reduces (Q _c increasing); at the time of impending the main rupture, the main frequency component reduces with a large scale (Q _c increasing again), this may be the reason which causes the different variation states of two codaQ values. The result of amplitude spectra of P, S (initial wave) waves also show that with the appearance of microcracks the frequency band of S wave turn to be narrow, the high frequency component is reduced quickly, i. e. the S wave spectra have different variation states with different frequency components. That is why theQ _s obtained by different methods have different variation characteristics.     

The effect of solar activity on the secular variation of the geomagnetic field in Romania

The 11-year solar cycle effect in the geomagnetic components H and Z is made clear for Surlari Observatory and 19 repeat stations for the interval 1952–1974. The correlation with Wolf number and its time derivative is discussed in terms of the effects of the external and induced current systems.The $\text{H}\text{?}$ data available for solar cycle 20 (1964–1976) were processed to give the geographical distribution of the secular variation impulse for epoch 1969.5 in Romania. It is suggested that this distribution might reflect the deep internal structure of the area considered.A qualitative correlation is noted between long-period solar activity and variation of the horizontal component of the geomagnetic field at some repeat stations.     

On the relation of the imminent sudden change in earth resistivity to the active fault and earthquake generating stress field

In this paper, the relations of the changes of earth resistivity (ρ) _s recorded at 100 geoelectrical stations in 31 earthquakes occurred in the continent of China, to the active faults (active abyssal faults or badly active faults near the focal zone) and the causative stress fields are discussed and the following conclusions are obtained:

1. On the condition that a station is near the active fault and in the direction of the causative stress (DCS) of an earthquake (EQ), the immediate variation ofρ _s to the earthquake (called “immediate variation” for short) could be recorded generally at the station.
2. The active fault which lies between a station and the epicenter of an earthquake seems to play a role in “obstructing” the recording of the imminent variation when the strike of the fault is close to the DCS of the earthquake. When that is parrallel with the DCS the “obstructing” function of the fault is strongest; when normal with the DCS, weakest.

The regularity seems to have the universality for moderate earthquakes and strong ones occurred in the continent of China.     

Types of variations in the geomagnetic field spectrum

The purpose of archaeomagnetic research is to investigate the structure of the geomagnetic field and its evolution. This paper is a study of this type. In our preceding studies, we divided the geomagnetic field into dipole and nondipole components. It was then shown that the dipole component consists of the predominant 1200-yr variation and the remainder δ. The δ variation is the subject of this paper. Detailed study and comparison of the characteristic features of two 1200-yr variations (the dipole and δ variations) in both the declination and inclination indicates that, according to most of their features, they can be assigned to different types of waves, namely, traveling waves (the dipole variation) and standing waves (the δ variation). Successive averaging in time and space yielded averaged data on δI and δD not only for various parts of the world (Europe, Asia, and America) but also for the western and eastern hemispheres.     

Seasonal and solar cycle variations of the auroral electrojet indices

Using hourly mean auroral electrojet indices for the past 20 years, we examine the seasonal and solar cycle variations of the AU and AL indices as well as the smaller time-scale fluctuations in these indices. The AU and AL indices maximize during summer and equinoctial months, respectively. By removing the effects of the solar conductance from the AU index, it is found that the electric field contribution to the AU index exhibits the same semiannual variation pattern as the AL index, indicating that the semiannual magnetic variations are controlled by the electric field. Since the auroral electrojets are mostly Hall currents flowing in the east–west direction, the fluctuations of the auroral electrojet indices can be interpreted in terms of fluctuations in the north–south component of the electric field and the Hall conductance. The AU fluctuation is largely due to that of the electric field, while the AL fluctuation is attributed to both the electric field and Hall conductance with their contributions being comparable. The high fluctuation of AL compared to that of AU is attributed to particle precipitation associated with substorm activity. However, the fluctuations of the electric field and conductance do not show any noticeable seasonal dependence. The variation pattern of the yearly mean AL index follows the mirror image of the AU index during the past 20 years, indicating that the absolute values of the two indices are proportional to each other. This suggests again that the electric field is the main modulator of magnetic disturbance. On the other hand, they show a tendency to become higher during the declining phase of the solar cycle. This is the same variation pattern confirmed from the aa index. However, the fluctuations of the electric field and the Hall conductance do not show any apparent dependence on the solar cycle.     

Time Variation of Parameters Related to the Accelerating Preshock Crustal Deformation in the Aegean Area

— The time variation of two parameters related to accelerating seismic deformation before strong earthquakes in the Aegean area is examined. The first is the b parameter of the Gutenberg-Richter relation and the second is the curvature parameter C, which is a measure of deviation of the accelerating preshock deformation from a linear time variation of this deformation. Following two different procedures, it was found that the b value exhibits a decreasing trend prior to the oncoming earthquake, in agreement with the results of laboratory experiments and other independent observations. C values also show a decreasing trend before main shocks. These results indicate that such time variations of these parameters can be considered as precursory phenomena of ensuing strong earthquakes.     

Variation of colour patterns in larval Hydropsyche (Trichoptera): Implications for species identifications and the phylogeny of the genus

Larvae of the lotic caddisfly genus Hydropsyche may occur in densities of thousands to several tens of thousands of individuals/m² across continents and running water types. Therefore, stream ecologists frequently face the task of identifying larvae of this genus. Early keys to larval Hydropsyche of Asia, Europe and North America often used the relatively complicated frontoclypeal colour pattern for species discrimination, whereas equivocal views about the variation of this pattern limited its use in recent keys. Has each species of a given species group a different (although intraspecifically variable) basic pattern of lighter spots on the darker frontoclypeus, or have all species of a group the same basic colour pattern that varies more or less intraspecifically? An answer to this question has obvious implications for the identification of the species as well as for the phylogeny of the genus, so we examined the variation of this colour pattern across ∼11,000 specimens of 10 Hydropsyche species from the Loire River (France).All 10 species had the same basic colour pattern on the frontoclypeus (six contrasting light patches on a dark background) that varied intraspecifically across a certain range of colour contrasts. Comparing other sources illustrating the frontoclypeus of Hydropsyche larvae provided more support for the idea that six contrasting light patches on the frontoclypeus is the typical basic colour pattern that varies within almost all European species; males of these species have a simple phallus form. Two European and many North American species of the genus (from the Ceratopsyche group) have a different basic colour pattern of seven light patches on the frontoclypeus that varies also intraspecifically; males of these species have a complicated phallus form.If the variation of such colour patterns in co-existing populations of several species is known, the relative contrast differences among the individual patches on the frontoclypeus and the form of the individual patches can provide valuable information for rapid species identification. Analysis of this variation may also contribute to phylogenetic studies of the genus Hydropsyche and other hydropsychids.     

The equatorial electrojet and the profile parameters B0 and B1 around midday

This study presents the results of the comparison of B0, B1 and hmF2 with ΔH. B0 and B1 are parameters used in the international reference ionosphere model for the calculation of the F region bottom side profiles. The parameter ΔH obtained from the magnetic data recorded during the International Equatorial Electrojet Year (IEEY) in West Africa is used to describe the strength of the equatorial electrojet. This work covers the years 1993 and 1994, two years of low and moderate solar activity. The result shows that the electric field (E), which drives the equatorial electrojet, plays a major role in the variation of the thickness and the height of the F2 layer. However, the variation of the shape of the bottomside F2 is not sensitive to the electric field.     

Latitudinal dependence of the seasonal variation of particulate extinction in the UTLS over the Indian longitude sector during volcanically quiescent period based on lidar and SAGE-II observations

The altitude profiles of particulate extinction in the upper troposphere and lower stratosphere (UTLS) obtained from SAGE-II in the latitude region 0–30°N over the Indian longitude sector (70–90°E) are used to study the latitudinal variation of its annual pattern in this region during the volcanically quiescent period of 1998–2003. The SAGE-II data is compared with the lidar measurements from Gadanki (13.5°N, 79.2°E) when the satellite had an overhead occultation pass over a small geographical grid centered at this location. The particulate optical depth (τ_p) in the UT region shows a general decrease with increase in latitude and a pronounced summer–winter contrast with relatively low values during winter and high values during summer. In general, these variations are in accordance with the latitudinal variation of convective available potential energy (CAPE) and thunderstorm activity, which are good representative indices of tropospheric convection. While the particulate extinction (and τ_p) in the 18–21 km (LS₁) region is relatively low in the equatorial region up to 15°N, it shows an increase in the off-equatorial region, beyond 15°N. While the annual variation of τ_p in the LS₁ region is almost insignificant near the equator, it is rather well pronounced in latitude region between 10 and 15°N with relatively high values during winter and low values during summer. Beyond 20°N, this shows a prominent peak during summer. At a higher altitude, the 21–30 km (LS₂) region, the latitude variation of τ_p shows a different pattern with high values near the equator and low values in the off-equatorial region confirming the existence of a stratospheric aerosol reservoir. Low values of τ_p at lower regime (LS₁) near the equator could be due to rapid transport of particulates from the near equatorial region to higher latitudes, while the equatorial high at upper regime (LS₂) could be due to lofting and subsequent accumulation.     

A computational study of the role of particle size standard deviation on the collision frequency in differential settling

The results of a simple computational model for differential settling are presented illustrating the significant role that particle size distribution plays in collision frequency and sedimentation rate of particles in a quiescent environment. The model tracks a large number of particles(order 10~5) with log-normally distributed diameters,as they settle at their Stokes settling velocities.Particle collisions are detected and result in larger particles that fall more rapidly.A number of simplifying assumptions are made in the model in order to avoid empirical correlations for phenomena such as collision efficiency and particle shape.These simplifying assumptions were needed to isolate and quantify the role of the particle size distribution.Simulated concentration profiles indicate that,even in the absence of collisions, the standard deviation(σ_D) of the particle size strongly influences the bulk mass settling rate as,for largerσ_D, more mass is concentrated in larger,faster falling particles.The collision frequency is also a strong function ofσ_D. For a given mass concentration the collision frequency first increases linearly with increasingσ_D as greater variation in particle size leads to greater variation in particle velocity,and shorter times for particles to catch each other.For largerσ_D more mass is concentrated in larger particles,so,for a given mass concentration,there are fewer particles per unit volume,increasing the mean distance between the particles and reducing the collision frequency.The implications of these results for sedimentation measurement using optical attenuation techniques are discussed.     

A baseline study of metal contamination along the Namibian coastline for Perna perna and Choromytilus meridionalis

The use of bivalves such as the brown mussel (Perna perna) and the black mussel (Choromytilus meridionalis) is common in the study of marine pollution and the effect of these pollutants on ecosystems and are important in both economic and ecological roles. Namibian marine ecosystems are threatened by pollution from mining, commercial fishing and population growth. The aims of this study were to determine baseline metal concentrations, spatial variation and variation between species. Metal levels in C. meridionalis from Guano Platform (GP) are the lowest of all the sites. The most polluted sites are Rocky Point (RP), Halifax Island (HIL) and between Walvis Bay and Swakopmund (WS). The bioaccumulation of metals between P. perna and C. meridionalis were not uniform for all metals. Overall the study indicates the condition of the coastline to be mostly normal, with Cd and Pb levels being of concern.     

Coda Q Estimates in the Andaman Islands Using Local Earthquakes

The attenuation property of Andaman Island has been investigated analyzing coda waves from 57 local earthquakes in the magnitude range of 2.0–4.9, using the single backscattering model. These earthquakes waveforms, recorded on five broadband seismographs sited over the island from north to south during Nov. 2003 to March 2004, have been used to calculate the frequency dependent Coda Q (Q _c ) applying the time domain coda-decay method. The Coda Q, computed at central frequencies from (0.5–12) Hz and five-lapse time windows from 40 to 80 s, progressively increases from 105 f ^0.88 in the north Andaman to 135 f ^0.79 in the south Andaman with an average of 119 f ^0.80. The average Q _c values vary from 75 ± 42 at 0.5 Hz to 697 ± 54 at 12 Hz central frequency for 40 s lapse time window, while for 80 s lapse time window its variation is from 117 ± 38 at 0.5 Hz to 1256 ± 115 at 12 Hz. The Q _c estimated at different lapse times manifests a significant variation from 122f ^0.75 to 174f ^0.73, corresponding to lapse time window lengths of 40 and 80 s, respectively. The variation of Q _c with frequency, lapse time and also with the location of seismograph reflects the marked structural and compositional inhomogeneity with depth along the Andaman Islands. These observations are well correlated with the seismicity pattern and distinct high angle subduction beneath the island.     

Reflection of the solar wind parameters in the CR geomagnetic cutoff rigidity before a strong magnetic storm in November 2003

The time variations in the CR geomagnetic cutoff rigidity and their relation to the interplanetary parameters and the Dst index during a strong magnetic storm of November 18–24, 2003, have been analyzed. The Tsyganenko (Ts03) model of a strongly disturbed magnetosphere [Tsyganenko, 2002a, 2002b; Tsyganenko et al., 2003] have been used to calculate effective geomagnetic thresholds with the help of the method for tracing CR particle trajectories in the magnetospheric magnetic field. The geomagnetic thresholds have been calculated using the method of global spectrographic survey (GSS), based on the data from the global network of CR stations, and the results have been compared with the effective geomagnetic cutoff rigidities. The daily anisotropy of effective geomagnetic thresholds during the Dst variation minimum have been estimated. The relation of the theoretical and experimental geomagnetic thresholds, obtained using the GSS method, to the interplanetary parameters and Dst variation is analyzed. The Dst variations, IMF B _z , and solar wind density are most clearly defined in the geomagnetic thresholds during this storm. The correlation between B _y and experimental geomagnetic thresholds is higher than such a correlation between this parameter and theoretical thresholds by a factor 2–3, which suggests that a real dawn-dusk asymmetry during this storm was stronger than such an asymmetry represented by the Ts03 model.