Electric fields and large-scale undulations in the evening sector of the diffuse auroral zone

The synchronous observations of strong electric fields and large-scale undulations observed on December 12, 2004, in the evening sector of the diffuse auroral zone 0900-1000 UT (~1700-1800 MLT) have been analyzed. The appearance of strong northward electric field at ~0900 UT was almost simultaneously registered at Tixie Bay ionospheric station (71.6° N, 128.9° E, L =, 5.6) and on the DMSP F15 satellite. At 0910-1000 UT, the all-sky TV camera at Tixie Bay and the DMSP satellites (F13, F14, and F15) registered eight undulations propagating westward at a velocity of 0.7—0.8 km/s. The undulation parameters registered during the TV observations agree with the satellite measurements. The distinctive feature of the analyzed event consists in that an intense electric field and undulations were localized within the diffuse zone in the region of increased precipitation of keV electrons. A comparison of the ground-based and satellite measurements made it possible to draw the conclusion on the necessary conditions for formation of diffuse undulations.     

Climatic characteristics of the ionosphere over Irkutsk. Observations and comparison with the IRI-2001 model

The morphological peculiarities of the behavior of the electron density over Irkutsk (52.3° N, 104.3° E) have been investigated by the current-median method. The observations are compared with the IRI model data at the decay phase of solar activity (2003–2006). Systematic discrepancies between the prediction and observations are discussed.     

Morphology and causes of the Weddell Sea anomaly

The zone of anomalous diurnal variations in foF2, which is characterized by an excess of nighttime foF2 values over daytime ones, has been distinguished in the Southern Hemisphere based on the Intercosmos-19 satellite data. In English literature, this zone is usually defined as the Weddell Sea anomaly (WSA). The anomaly occupies the longitudes of 180°–360° E in the Western Hemisphere and the latitudes of 40°–80° S, and the effect is maximal (up to ∼5 MHz) at longitudes of 255°–315° E and latitudes of 60°–70° S (50°–55° ILAT). The anomaly is observed at all levels of solar activity. The anomaly formation causes have been considered based on calculations and qualitative analysis. For this purpose, the longitudinal variations in the ionospheric and thermospheric parameters in the Southern Hemisphere have been analyzed in detail for near-noon and near-midnight conditions. The analysis shows that the daytime foF2 values are much smaller in the Western Hemisphere than in the Eastern one, and, on the contrary, the nighttime values are much larger, as a result of which the foF2 diurnal variations are anomalous. Such a character of the longitudinal effect mainly depends on the vertical plasma drift under the action of the neutral wind and ionization by solar radiation. Other causes have also been considered: the composition and temperature of the atmosphere, plasma flows from the plasmasphere, electric fields, particle precipitation, and the relationship to the equatorial anomaly and the main ionospheric trough.     

Solar energetic events in the years 1996–2004. The analysis of their geoeffectiveness

Data concerning solar energetic events, published in 1996–2004 by the USAF/NOAA in the form of daily reports, have been collected. The analysis of the particular event types indicates that the degree of their geoeffectiveness depends on their size and on their solar disc location. The mere information that a solar X-flare (XRA) event or a Long Duration XRA Event (LDE) has occurred on the solar disc is insufficient to produce a relevant forecast of geomagnetic disturbances. The probability increases if the XRA is of class X which has occurred on the solar disk in central region (30 °E, 30 °W; 30 °S, 30 °N). XRAs associated with metric type II and IV radio bursts (RSP II and RSP IV), which occurred on the solar disc in this region will very probably cause a geomagnetic disturbance not only if X class are involved, but also M class and B–C class. The Disappearance of Solar Filament (DSF) data cannot be used in forecasting geomagnetic disturbances. The geoeffective and nongeoeffective DSFs are too disproportional. jboch@ig.cas.cz fridrich@geomag.sk     

Morphological analysis of the winter nighttime ionosphere at mid-latitudes of the Asian region

We present a study of peculiarities of the winter nighttime maximum in the critical frequencies f ₀ F2 at mid-latitudes of the Asian region. The data of stations located at different longitudes and close latitudes have been used in the analysis: Novosibirsk (54.8°N, 83.2°E), Irkutsk (52.5°N, 104.0°E), and Khabarovsk (48.5°N, 135.1°E). It has been found that the nighttime maximum in f ₀ F2 is observed after midnight (∼0200–0400 LT) and is a stable feature of the quiet ionosphere from the middle of October to the middle of March at low solar activity (SA) at all analyzed stations. This interval decreases with increasing SA. The difference between the maximal and minimal f ₀ F2 values in nighttime hours is the largest in December–January, and its amplitude is almost independent of SA. Variations in the critical frequency of the h _m F2 layer are inversely related to those in the height of the maximum. We have studied periods when the difference between the daytime and nighttime values of f ₀ F2 is less than 2 MHz. The intervals of observations of such events at different longitudes do not coincide. No dependence of the winter nighttime maximum amplitude on magnetic activity has been found.     

Influence of a tropical cyclone on the upper ionosphere according to tomography sounding data over Sakhalin Island in November 2007

Tomography sounding data for the first half of November 2007 are presented. The sounding was conducted over three points located at the same meridian—Yuzhno-Sakhalinsk (47° N, 143° E), Poronaisk (49° N, 143° E), and Nogliki (51° N, 143° E)—in order to find the possible influence of a tropical cyclone on the upper ionosphere. A change in the foF2 parameter by on average no more than 10–20% is a possible response of the upper ionosphere localized over the tropical cyclone (TC) zone (in the given case, 25°–30° northward and 5°–20° eastward) at a distance of approximately 3800–5500 km from it. A decrease or, vice versa, an increase in foF2 is related to the delay of the measurement moment relative to the beginning of the TC action. The complexity of a morphological analysis of the given event is that a tropical cyclone is a “wideband” (in the longitudinal and, to a lesser degree, in the latitudinal directions) and lasting disturbance source.     

Structure of the crust and uppermost mantle from broadband seismic array in the western Dabie Mountains,east central China

A broadband seismic array of 7 stations was set up in the western Dabie Mountains (31°20′-31°50′N, 114°30′-115°E). Teleseismic events from May 2001 to November 2001 were collected and analyzed by radial receiver function to determine the S-wave velocity structure of the crust and uppermost mantle. The crustal thickness is 32-38 km beneath the array. The crust-mantle boundary appears as a gently north-dipping velocity discontinuity, but turns to be a velocity gradient beneath a station near the Qiliping shea...     

Detection of the wave-like structures in the F-region electron density: Two station measurements

Comparative studies of short-term ionospheric variability in the F region ionosphere during rapid sequence sounding campaign “HIRAC/SolarMax” (23–29 April 2001) are presented. The ionospheric short-term fluctuations have been studied in detail using measurements from vertical sounding at Ebro (40.8 °N, 0.5 ° E) and Průhonice (49.9 °N, 14.5 °E) in the period range from 15 minutes to 2 hours. The electron density measurements contain variations that indicate the possible presence of propagating gravity waves. Regular wave-like bursts were found during quiet days at both stations in electron concentration in F region, with an increase of the oscillation activity after sunrise and then during late afternoon, and at sunset and after sunset. Solar Terminator is assumed to be one of the sources of the regular wave bursts detected in the ionosphere during campaign HIRAC. As expected, substantial intensification in longer period gravity waves was found to occur during the disturbed period on April 28. Particular enhancement of the wave-like activity during disturbed day is discussed, being significant evidences of a change of the wave-like activity pattern at a height around 200 km.     

Influence of meteorological and wave processes on the lower ionosphere during solar minimum conditions according to the data on midlatitude VLF-LF propagation

The statistical characteristics of the intensity of VLF-LF radio signals transmitted from the midlatitude radio stations and recorded by the receiver at the Mikhnevo geophysical observatory (54.94°N, 37.73°E; Institute of Geosphere Dynamics, Russian Academy of Sciences) in 2007–2010 are analyzed. The experiments revealed strong variations in the intensity of radio signals during the deep solar minimum conditions, when the medium does not experience impacts from above associated with solar and geomagnetic activity. We relate the observed variations to the disturbances from below, which are caused by the meteorological and wave processes occurring in the lower atmosphere.     

A neurocomputing approach to predict monsoon rainfall in monthly scale using SST anomaly as a predictor

A relationship between summer monsoon rainfall and sea surface temperature anomalies was investigated with the aim of predicting the monthly scale rainfall during the summer monsoon period over a section (80°–90°E, 14°–24°N) of eastern India that depends heavily upon the rainfall during the summer monsoon months for its agricultural practices. The association between area-averaged rainfall of June over the study zone and global sea surface temperature (SST) anomalies for the period 1982–2008 was examined and the variability of rainfall in monthly scale was calculated. With a view to significant variability in the rainfall in the monthly scale, it was decided to implement the artificial neural network (ANN) for forecasting the monthly scale rainfall using the SST anomalies as a predictor. Finally, the potential of ANN in this prediction has been assessed.     

Possible precursory accelerated Benioff strain in the region of Sistan Suture Zone of Eastern Iran

We investigated whether accelerated seismic strain release precedes large earthquakes occurring in and around the Sistan Suture Zone, Eastern Iran. Online catalogs of teleseismic events occurring post-1960 within the region 27.0°–37.0°N, 55.0°–65.0°E, report five M _w > 7.0 earthquakes, namely, 1968 Dasht-e-Bayaz, 1978 Tabas, 1979 Khuli-Buniabad, 1981 Sirch and 1997 Zirkuh-e-Q’aenat events. We defined four earthquake test episodes, 1968–1978, 1978–1981, 1979–1981, and 1981–1997, with all catalogued intermediate events having magnitudes within 2.0 units that of the final large event. Using the 1968 event as the starting point, we investigated possible increased moderate earthquake activity patterns prior to the large events of 1978, 1981 and 1997 by examining if the cumulative Benioff strain released from such preceding events followed a power law time-to-failure. Our investigation seem to suggest that the 1978, 1981 and 1997 events (i) followed a period of accelerated moderate earthquake activity and (ii) the radius of their optimal critical region, R, scaled with their magnitude, M, according to the scaling law log R ∝ 0.36 M. Our suggestions conform to those proposed by similar investigations in varied seismotectonic regimes.     

SAR arc occurrence frequency during cycle 23 of solar activity

The occurrence frequency of SAR arcs during 1997–2006 has been analyzed based on the photometric observations at the Yakutsk meridian (Maimaga station, corrected geomagnetic coordinates: 57° N, 200° E). SAR arcs appeared in 114 cases (～500 h) during ～370 nights of observations (～3170 h). The occurrence frequency of SAR arcs increases to 27% during the growth phase of solar activity and has a clearly defined maximum ～36% at a decline of cycle 23. The SAR arc registration frequency corresponds to the variations in geomagnetic activity in this solar cycle. The dates, UT, and geomagnetic latitudes of photometric observations are presented for 1997–2006.     

Estimate of seasonal changes in the intensity of the infrared atmospheric system of molecular oxygen

On the basis of measurements of the intensity of 1.58-μm emissions of the Infrared Atmospheric System of molecular oxygen (IRAO₂) conducted at the Zvenigorod scientific station of the Institute of Atmospheric Physics of the Russian Academy of Sciences (φ = 55.7°N, λ = 36.8°E), seasonal variations are estimated for various solar zenith angles. Their amplitude has the maximum value at the solar zenith angles χ _S ∼ 105–110°. It decreases at χ _S ∼ 125–130° and tends to zero at χ _S ∼ 80–85°. The comparison of currently measured values of the 1.58-μm emission intensity of the Infrared Atmospheric System of molecular oxygen with published data on the intensity of this emission obtained in 1961–1966 reveals their decrease over approximately 50 years. This fact is in good agreement with similar behavior of the emission intensity of atomic oxygen (557.7 nm) over the period considered.     

The response of TEC at quasi-conjugate point GPS stations during solar flares

Global Positioning System (GPS) derived total electron content (TEC) measurements were analyzed to investigate the ionospheric response during the X-class solar flare event that occurred on 5-6 December 2006 at geomagnetic conjugate stations: Syowa, Antarctica (SYOG) (GC: 69.00°S, 39.58°E; CGM: 66.08°S, 71.65°E) and árholt, Iceland (ARHO) (GC: 66.19°N, 342.89°E; CGM: 66.37°N, 71.48°E). Bernese GPS software was used to derive the TEC maps for both stations. The focus of this study is to determine the symmetry or asymmetry of TEC values which is an important parameter in the ionosphere at conjugate stations during these solar flare events. The results showed that during the first flares on 5 December, effects were more pronounced at SYOG than at ARHO. However, on 6 December, the TEC at ARHO showed a sudden spike during the flare with a different TEC variation at SYOG.     

Seasonal and UT variations of the position of the auroral precipitation and polar cap boundaries

The data of the DMSP F7 spacecraft are used for studying the influence of the geomagnetic dipole tilt angle on the latitudinal position of auroral precipitation boundaries in the nighttime (2100–2400 MLT) and daytime (0900–1200 MLT) sectors. It is shown that, in the nighttime sector, the high-latitude zone of soft diffuse precipitation (SDP) and the boundary of the polar cap (PC) at all levels of geomagnetic activity are located at higher and lower latitudes relative to the equinox period in winter and summer, respectively. The position of boundaries of the diffuse auroral precipitation zone (DAZ) located equatorward from the auroral oval does not depend on the season. In the daytime sector, the inverse picture is observed: the SDP precipitation zone takes the most low-latitude and high-latitude positions in the winter and summer periods, respectively. The total value of the displacements from winter to summer of both the nighttime and daytime boundaries of the PC is ∼2.5°. A diurnal wave in the latitudinal position of the nighttime precipitation boundaries is detected. The wave is most pronounced in the periods of the winter and fall seasons, is much weaker in the spring period, and is almost absent in summer. The diurnal variations of the position of the boundaries are quasi-sinusoidal oscillations with the latitude maximum and minimum at 0300–0500 and 1700–2100 UT, respectively. The total value of the diurnal displacement of the boundaries is ∼2.5° of latitude. The results obtained show that, undergoing seasonal and diurnal variations, the polar cap is shifted as a whole in the direction opposite to the changes in the tilt angle of the geomagnetic dipole. The seasonal displacements of the polar cap and its diurnal variations in the winter period occur without any substantial changes in its area.     

我国低纬电离层闪烁和周跳的统计特征比较

本文利用位于我国中南部电离层闪烁监测台网2012年至2015年的观测数据,比较分析了GPS（Global Positioning System）信号闪烁与周跳的统计特征以及太阳活动和地磁扰动对闪烁与周跳的影响.结果表明,闪烁活动与周跳出现随地方时、月份、太阳活动和地磁扰动变化的统计特征类似,且周跳出现的可能性随S4指数增高显著增大,说明闪烁与周跳存在密切的关联,是引起周跳的一种重要因素.一天之中,闪烁和周跳主要出现在日落后至黎明前,午夜前出现最频繁,白天仅偶尔出现.在赤道异常峰及其邻近区域,一年之中,闪烁和周跳主要出现在春秋季,春季闪烁活动和周跳出现明显比秋季频繁,呈现春秋不对称性,冬夏季节闪烁和周跳都很少出现.闪烁活动与周跳出现的逐年变化显著依赖太阳活动水平,随太阳活动水平升高而增强,而地磁扰动与闪烁活动与周跳出现呈负相关,地磁扰动对闪烁活动与周跳出现整体上起抑制作用.平均而言,越靠近磁赤道的台站闪烁活动越频繁,随纬度升高,闪烁活动频次逐渐降低,且闪烁活动的开始时间随纬度升高而滞后,暗示引起GPS信号闪烁的电离层不规则结构主要起源于磁赤道区.此外,分析还发现,闪烁活动与周跳出现的空域有相当好的一致性,主要分布在观测点上空仰角55°以下、方位角150°~240°的空域内.     

VLF electromagnetic background in the seismoactive area of Northern Vietnam

The preliminary results of observations of VLF electromagnetic signals (atmospherics) in the mountains of North Vietnam (Dien Bien Phu, 21°23′50″ N, 103°0′28″ E) are presented. Primarily, signals of typical atmospherics with a maximum in the frequency range of 4–9 kHz were observed; sometimes they were accompanied by so-called “tails” at frequencies less than 1 kHz, and also tweeks of usually short duration (about 10 ms) were observed. Several parameters of the ionosphere, as well as the distance to sources of atmospherics (in accordance to the data of the World Wide Lightning Location Network (TOGA)), were estimated by spectral-temporal characteristics of tweeks.     

Frequency analysis of the 2004 Sumatra-Andaman earthquake using spectral seismograms

Frequency analysis of the Sumatra-Andaman earthquake of 2004, one of the most significant and best-recorded earthquakes, is based on spectral seismograms obtained from their broadband seismograms. The Sumatra-Andaman earthquake is found to have a wide-range frequency content of P-wave radiation during the rupturing process. On the basis of stacking spectral seismograms we distinguished four time events of the rupturing process of a total length of about 540 s. The frequency, f _max, is the highest for the first event (0.163 Hz in time interval 0–88 s), lowest for the second — which is the strongest (0.075 Hz in time interval 88–204 s). For third and fourth events frequencies are similar (0.089 and 0.082 Hz in time intervals 204–452 and 452–537 s, respectively). The frequency also shows an azimuthal dependence (±0.02 Hz). Azimuths for which the frequency, f _max, has maximum and minimum values are 203–222° and 23–42°, respectively. These observations are discussed in relation to previously published papers on this topic.     

Seismic tomography of the crust and upper mantle in the Bohai Bay Basin and its adjacent regions

In the Bohai Bay Basin and its adjacent regions(112°―124°E,34°―42°N),there exists abundant gas-petroleum while modern inter-plate seismic activity is robust.Although the tectonic structure of this region is very complicated,plenty of geological,geophysical and geochemical data and results are obtained through previous researches.On the basis of absorbing previous results,especially various kinds of geological and geophysical results,we collect and process the arrival time of P-wave phases of local events and tele-seismic events recorded by the station within this region from 1978 to 2004,build the responding initial model,and image the velocity structure of the crust and upper mantle of this region via tomography.The perturbation images of various depths and velocity profiles imply that the velocity structure of the crust and upper mantle in the Bohai Bay Basin and its adjacent regions is mainly influenced by the surface tectonic units,and is characterized by "Stripped along east-west,and zoned along south-north";some large-scaled faults penetrate Moho and lithosphere,and provide the channels for the basic lava or hot mass upwelling from the mantle.