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1.
Complex ionograms from the Intercosmos-19 satellite with strongly delayed and sometimes multiple reflections from the Earth are considered. An analysis shows that these reflections are usually associated with sharp horizontal gradients of the ionospheric plasma. Such gradients are formed on the walls of the main ionospheric trough, at peaks of electron density, and on the inner and, especially frequently, on the outer slope of the crest of the equatorial anomaly. In one case, distant reflections from the Earth (DREs) formed near the equator, when the satellite in perigee was lower than the F2-layer maximum height. A quantitative interpretation of the most typical cases of DREs is given based on ray tracing. For this purpose, the model of the ionosphere under the satellite is developed, ray paths are calculated, and model ionograms are formed. The good agreement between experimental and model ionograms allows us to conclude that the task of interpreting complicated ionograms obtained by Intercosmos-19 with DRE has been solved successfully. 相似文献
2.
Unusual complex ionograms obtained by the Intercosmos-19 satellite are considered, in which four diffuse clouds with a characteristic shape are strung like pearls on the main path of the reflected signal. Ray tracing has been used to show that they are associated with 26 layers of irregularities located at altitudes from hmFs2 up to ~900 km. The sizes of the irregularities range from a few kilometers to 100 kilometers, and the intensity of δNe reaches 100%. The heights of irregular layers increase towards the equator, together with a rise of the F2 layer, and are not associated with magnetic field lines. Complex ionograms have been observed on the outer slope and at the top of the crest of the equatorial anomaly. They are probably caused by the processes occurring in the equatorial ionosphere. 相似文献
3.
Longitudinal and local time variations in the structure of the equatorial anomaly under high solar activity in the equinox are considered according to the Intercosmos-19 topside sounding data. It is shown that the anomaly begins to form at 0800 LT, when the southern crest is formed. The development of the equatorial anomaly is associated with well-known variations in the equatorial ionosphere: a change in the direction of the electric field from the west to the east, which causes vertical plasma drift W (directed upward) and the fountain effect. At 1000 LT, both anomaly crests appear, but they become completely symmetrical only by 1400 LT. The average position of the crests increases from I = 20° at 1000 LT to I = 28° at 1400 LT. The position of the crests is quite strong, sometimes up to 15°, varies with longitude. The foF2 value above the equator and the equatorial anomaly intensity (EAI) at 1200–1400 LT vary with the longitude according to changes in the vertical plasma drift velocity W. At this time, four harmonics are observed in the longitudinal variations of W, foF2, and EAI. The equatorial anomaly intensity increases to the maximum 1.5–2 h after the evening burst in the vertical plasma drift velocity. Longitudinal variations of foF2 for 2000–2200 LT are also associated with corresponding variations in the vertical plasma drift velocity. The equatorial anomaly intensity decreases after the maximum at 2000 LT and the crests decrease in size and shift towards the equator, but the anomaly is well developed at midnight. On the contrary, after midnight, foF2 maxima in the region of the anomaly crests are farther from the equator, but this is obviously associated with the action of the neutral wind. At 0200 LT, in contrast to the morning hours, only the northern crest of the anomaly is clearly pronounced. Thus, in the case of high solar activity during the equinoxes, a well-defined equatorial anomaly is observed from 1000 to 2400 LT. It reaches the maximum at 2000 LT. 相似文献
4.
G. L. Gdalevich A. Kh. Depueva N. I. Izhovkina V. D. Ozerov 《Geomagnetism and Aeronomy》2010,50(1):69-78
The results of the Cosmos-900 satellite observ ations of plasma density inhomogeneities in the geomagnetic equator region
and the longitudinal distributions of the equatorial spread-F, according to the Intercosmos-19 satellite data are presented. It is show n that the dependence of radiosignal propagation
in the ionosphere on geophysical parameters is related to development of the electrostatic instability of the inhomo-geneous
ionospheric plasma. The longitudinal dependence of the spread-F, can reflect the influence of the energetic sources, located outside the ionospheric layer that scatters a radio pulse, on
the ionosphere. The manifestation of the longitudinal effect in the equatorial spread-F, in the Atlantic region can be explained by the influence of the cone instability on the plasma electrodynamics in the South
Atlantic geomagnetic anomaly. 相似文献
5.
Additional strongly remote (up to 2000 km) radio-signal reflection traces on Intercosmos-19 ionograms obtained in the equatorial ionosphere have been considered. These traces, as a rule, begin at frequencies slightly lower than the main trace cutoff frequencies, which indicates that an irregularity with a decreased plasma density exists here. The waveguide stretched along the magnetic-field line is such an inhomogeneity in the equatorial ionosphere. The ray tracing confirm that radio waves propagate in a waveguide and make it possible to determine the typical waveguide parameters: ?δN e ≥ 10%, with a diameter of 15–20 km. Since the waveguide walls are smooth, an additional trace is always recorded distinctly even in the case in which main traces were completely eroded by strong diffusivity. Only one additional trace (of the radio signal X mode) is usually observed one more multiple trace is rarely recorded. Waveguides can be observed at all altitudes of the equatorial ionosphere at geomagnetic latitudes of ±40°. The formation of waveguides is usually related to the formation of different-scale irregularities in the nighttime equatorial ionosphere, which result in the appearance of other additional traces and spread F. 相似文献
6.
G. F. Deminova 《Geomagnetism and Aeronomy》2010,50(2):181-186
Using the data of the topside ionosphere sounding from the Intercosmos-19 satellite, longitudinal variations in foF2 at low latitudes at the daytime hours are considered. It is obtained that these variations in particular days in the majority
of cases have a regular wave-like character with periods of about 75°–100° in longitude and amplitudes on the average of 2–4
MHz. In other words, along the valley and crests of the equatorial anomaly, a structure with four maximums and four minimums
which have a tendency to be located near certain longitudes (the same in all seasons) is observed. The variations in foF2 along the crests of the equatorial anomaly are usually in anti-phase to variations along its valley. Comparing the characteristics
of this wavelike structure at the daytime and nighttime hours, we obtained that the average positions of its extremes at the
nighttime hours are shifted eastwards by 10°–50° relative to the daytime extremes. As a cause of formation of such a structure,
high harmonics of atmospheric tides are assumed which, uplifting from below to heights of the E region, via the electric currents in this region influence the longitudinal structure of the electrodynamic plasma drift
over the equator and by that impact the structure of the entire daytime low-latitude ionosphere. 相似文献
7.
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. 相似文献
8.
The characteristics of ionospheric scintillations at Rajkot in the equatorial anomaly crest region in India are described for the years 1987–1991 by monitoring the 244-MHz transmission from the satellite FLEETSAT. This period covers the ascending phase of solar cycle 22. Scintillations occur predominantly in the pre-midnight period during equinoxes and winter seasons and in the post-midnight period during summer season. During equinoxes and winter, scintillation occurrence increases with solar activity, whilst in summer it is found to decrease with solar activity. Statistically, scintillation occurrence is suppressed by magnetic activity. The characteristics observed during winter and equinoxes are similar to those seen at the equatorial station, Trivandrum. This, coupled with the nature of the post-sunset equatorial F-region drift and h‘F variations, supports the view that at the anomaly crest station, scintillations are of equatorial origin during equinox and winter, whilst in summer they may be of mid-latitude type. The variations in scintillation intensity (in dB) with season and solar activity are also reported. 相似文献
9.
P. T. Jayachandran P. Sri Ram V. V. Somayajulu P. V. S. Rama Rao 《Annales Geophysicae》1997,15(2):255-262
The unique geometry of the geomagnetic field lines over the equatorial ionosphere coupled with the E–W electric field causes the equatorial ionization anomaly (EIA) and equatorial spread-F (ESF). lonosonde data obtained at a chain of four stations covering equator to anomaly crest region (0.3 to 33 °N dip) in the Indian sector are used to study the role of EIA and the associated processes on the occurrence of ESF. The study period pertains to the equinoctial months (March, April, September and October) of 1991. The ratios of critical frequency of F-layer (f0F2) and electron densities at an altitude of 270 km between Ahmedabad (33 °N dip) and Waltair (20 °N dip) are found to shoot up in the afternoon hours on spread-F days showing strengthening of the EIA in the afternoon hours. The study confirms the earlier conclusions made by Raghava Rao et al. and Alex et al. that a well-developed EIA is one of the conditions conducive for the generation of ESF. This study also shows that the location of the crest is also important in addition to the strength of the anomaly. 相似文献
10.
The main factors controlling NmF2 longitudinal variations at mid- and subauroral latitudes have been studied. The data of the Intercosmos-19 topside sounding,
obtained at high solar activity for summer nighttime conditions, have been used in the analysis. The contributions of the
solar ionization, neutral wind, and temperature and composition of the thermosphere to NmF2 longitudinal variations have been estimated based on ionospheric models. It has been indicated that NmF2 variations in the unsunlit midlatitude ionosphere mainly depends on the residual electron density and its decay under the
action of recombination. At subauroral latitudes under summer nighttime conditions, the ionosphere is partially sunlit, and
ionization by solar radiation mainly contributes to NmF2 longitudinal variations, whereas the effect of the neutral wind is slightly less significant. These results also indicate
how the contribution of different factors to NmF2 longitudinal variations changes at different latitudes. 相似文献
11.
Variations of the total electron content according to the index IONEX IGS in the period of preparation of the earthquake in
Haiti (M7.9) on January 12, 2010, are considered. The situation is exceptional owing to the unique position of the island
of Haiti relative to the structure of the ionosphere over the Caribbean Sea: the ionospheric region over Haiti is in the trough
formed by the northern slope of the equatorial anomaly and additional maximum formed at latitudes of approximately 30° N within
this longitudinal interval. Distortion of the shape of the equatorial anomaly, total decrease in the electron content in the
equatorial anomaly a few days prior to the earthquake, increase in the electron concentration directly over the earthquake
epicenter a few days prior to the earthquake, increase in the additional maximum at latitudes of ∼30° N, and formation of
an additional maximum in the Southern Hemisphere in the region conjugated to the additional maximum in the Northern Hemisphere
in the periods of its intensification are observed. The configuration of the equatorial anomaly is restored after the earthquake. 相似文献
12.
Ian J. Walker 《地球表面变化过程与地形》1999,24(5):437-448
Lee-side windspeed and sediment transport were measured over a small (1·2 m) transverse ridge in the Silver Peak dunefield, west-central Nevada, USA, using an intensive array of 25 cup anemometers and seven total flux traps. During crest-transverse and transporting flow conditions (u0·3crest ≈ 8·4 m s−1), windspeed near the surface of the lee slope averaged half (48 per cent) that of crest speeds. Dimensionless speeds in the separation zone ranged from 0·2 to 0·8 that of the outer flow (u12). Along the boundary of the separation cell, windspeed increased by 10 per cent of the crest speed before separation. Equilibrium of upper and lower wake regions was not observed by the documented eight dune heights, suggesting that wake recovery may not occur over closely spaced dunes. Sediment transport measured directly on both the lee slope and interdune surfaces averaged approximately 15 per cent of crest inputs. This suggests that a significant amount (c. 70–95 per cent) of sediment transported over the crest moved as fallout. For this data set, flux was approximately proportional to the cube of the near-surface windspeed (u0·3) and in general there was an order of magnitude difference between flux measured at the crest and that measured within the separation zone. Transport direction in the separation zone was acutely oblique to the incident direction owing to secondary flow deflection. Beyond the interdune, transport direction progressed from oblique to crest-transverse. This indicates that an appreciable amount of sediment may move laterally along the lee slope and interdune corridor under crest-transverse flows. Regarding the grain size and sorting properties of transported sediment, there was no significant difference in mean grain size over the dune, although in general particles were finer and more poorly sorted in the lee. Copyright © 1999 John Wiley & Sons, Ltd. 相似文献
13.
14.
With the use of data from topside sounding on board the Interkosmos-19 (IK-19) satellite, the region of permanent generation of large-scale irregularities in the daytime winter ionosphere of the Southern Hemisphere is differentiated. This region is characterized by low values of foF2 and hmF2 and occupies a rather large latitudinal band, from the equatorial anomaly ridge to ~70° S within the longitudinal range from 180° to 360°. Irregularities with a dimension of hundreds kilometers are regularly observed in the period from 0700–0800 to 1800–1900 LT, i.e., mainly in the daytime. In the IK-19 ionograms, they normally appear in the form of an extra trace with a critical frequency higher than that of the main trace reflected from the ionosphere with lower density. The electron density in the irregularity maximum sometimes exceeds the density of the background ionosphere by nearly a factor of 3. A model of the ionosphere with allowance for its irregular structure was created, and it was shown on the basis of trajectory calculations how the IK-19 ionograms related to these irregularities are formed. A possible mechanism of the generation of large-scale irregularities of the ionospheric plasma is discussed. 相似文献
15.
This work is devoted to a numerical simulation of the equatorial ionosphere, performed using the GSM TIP model completed with
a new block for calculating the electric field. It has been indicated that the usage of the wind system calculated according
to the MSIS-90 model makes it possible to reproduce the electromagnetic drift velocities at the equator, the effect of the
F2-layer stratification, and the appearance of the F3 layer in the equatorial ionosphere. The calculations performed using the modified GSM TIP model made it possible to detect
a maximum in the electron density vertical profile at an altitude of ∼1000 km, formed by H+ ions, which we called the G layer. If this layer actually exists, it can be observed during sounding the low-latitude ionosphere from satellites during
dark time of day. 相似文献
16.
当喷泉效应较弱而双峰结构发展不充分的时候,可能在赤道异常区仅能够观测到一个电子密度的峰值,称之为单峰现象.本文利用CHAllenging Minisatellite Payload卫星在2001-2010年的电子密度数据给出了单峰的发生规律,单峰在地方时早上08：00-10：00和下午16：00-19：00发生率高,发生位置在经度上呈现多波数分布,尤其在10：00-18：00明显：在分季时多呈现四波,而在冬至季时以三波为主.单峰发生多的经度,正好对应着双峰的结构特征较弱之处.究其原因,是非迁移潮的DE2和DE3分量调制了背景风场和大气发电机电场,在电场和喷泉效应减弱的经度,双峰结构难以形成时,就会表现为单峰结构.本文扩展了对单峰现象的地方时、季节和经度分布等规律的了解,明确了非迁移潮在其中施加的影响,由此,单峰同双峰现象一样可以用于研究非迁移潮对热层-电离层的作用. 相似文献
17.
Summary The evolution of the opinions as to the problem of the triaxiality of the Earth in the period prior to satellite geodesy can be seen, e.g., in[1–18]. Recently the opinion has been voiced that triaxiality is a result of the mathematical treatment of data rather than reality[19–21], especially since this is a comparatively small parameter. This opinion is not in contradiction with the results of satellite observations[22–28], but the non-zero values of the harmonic coefficients of the second degree and second order are a reality, they yield a value of the equatorial flattening of about1/90 000, and the representation of the equatorial section by an ellipse is justified even if the harmonics n=3, k=1 and n=3, k=3 have amplitudes only about half as small, and some other parameters might occur with just as much justification besides triaxiality. 相似文献
18.
A. V. Pavlov N. M. Pavlova S. F. Makarenko V. N. Shubin 《Geomagnetism and Aeronomy》2009,49(2):219-226
The occurrence probabilities of the first and second anomalous nighttime local maximums in the diurnal variations in the electron density at a maximum of the ionospheric F 2 layer (NmF2) in the region where the crest (hump) of the equatorial anomaly originates in the northern geographic hemisphere have been studied using the data of the stations for vertical sounding of the ionosphere (Paramaribo, Dakar, Quagadougou, Ahmedabad, Delhi, Calcutta, Chongoing, Guangzhou, Taipei, Chung-Li, Okinawa, Yamagawa, Panama, and Bogota) from 1957 to 2004. It has been demonstrated that the anomalous nighttime NmF2 maximums are least frequently formed at ～53° geomagnetic longitude. The calculations have indicated that the studied probabilities are independent of solar activity. Geomagnetic activity weakly affects the rate of occurrence of the first nighttime NmF2 maximum at geomagnetic longitudes of approximately 140° to 358°. At geomagnetic longitudes of approximately 16° to 70° (i.e., in the longitudinal zone of a decreased occurrence frequency of anomalous nighttime maximums), the occurrence probability of the first anomalous nighttime NmF2 maximum under geomagnetically quiet conditions is pronouncedly lower than under geomagnetically disturbed conditions. The dependence of the occurrence probabilities of the first and second anomalous nighttime NmF2 maximums on the month number in a year has been studied. 相似文献
19.
Xinmiao Zhang Zhengyu Zhao Yuannong Zhang Chen Zhou 《Journal of Atmospheric and Solar》2010,72(11-12):869-875
An exceptionally long total solar eclipse occurred over the Yangtze River Basin in the mid-latitudes of China on 22 July 2009. The moon’s umbral shadow crossed through the ionospheric equatorial anomaly region. During the solar eclipse, new ionospheric behaviors were observed using a multi-station sounding approach. These new phenomena include: (1) visible Doppler spreading of F layer echoes at multiple group distances during the solar eclipse period, (2) strong ionospheric response near the peak of the northern equatorial anomaly crest and (3) synchronous oscillations in the Es and F layer during the recovery phase of the solar eclipse. 相似文献
20.
Dario Dello Iacono Aldo Zollo Maurizio Vassallo Tiziana Vanorio Sebastien Judenherc 《Bulletin of Volcanology》2009,71(3):275-284
In September 2001, an extensive active-seismic investigation (Serapis experiment) was carried out in the Gulfs of Naples and
Pozzuoli, with the aim of investigating and reconstructing the shallow crustal structure of the Campi Flegrei caldera, and
possibly identifying its feeding system at depth. The present study provides a joint analysis of the very shallow seismic
reflection data and tomographic images based on the Serapis dataset. This is achieved by reflection seismic sections obtained
by the 3D data gathering and through refined P-velocity images of the shallowest layer of Pozzuoli Gulf (z < 1,000 m). From the refined Vp model, the overall picture of the velocity distribution confirms the presence of a complex
arc-shaped anomaly that borders the bay offshore. The deeper part of the anomaly (beneath 700 m, with Vp > 3,500 m/s) correlates
with units made up of agglomerate tuffs and interbedded lava, which form the southern edge of the caldera, which was probably
formed following the two large ignimbritic eruptions that marked the evolutionary history of the area under study. The upper
part of the anomaly that tends to split into two parallel arcs is correlated with dikes, volcanic mounds and hydrothermal
alteration zones noted in previous shallow reflection seismic analyses. The depth of the transition between the upper and
lower parts of the anomaly is characterized by an abrupt Vp increase on the one-dimensional (1D) profiles extracted from the
3D tomographic model and by the presence of a strong reflector located at about 0.6/0.7 s Two Way Time (TWT) on Common Mid
Point gathers. The move-out velocity analysis and stack of the P–P and P–S reflections at the layer bottom allowed to estimate
relatively high Vp/Vs values (3.7 ± 0.9). This hypothesis has been tested by a theoretical rock physical modeling of the Vp/Vs
ratio as a function of porosity suggesting that the shallow layer is likely formed by incoherent, water saturated, volcanic
and marine sediments that filled Pozzuoli Bay during the post-caldera activity. 相似文献