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1.
During the 6th August 1995, the CUTLASS Finland HF radar ran in a high time resolution mode, allowing measurements of line-of-sight convection velocities along a single beam with a temporal resolution of 14 s. Data from such scans, during the substorm expansion phase, revealed pulses of equatorward flow exceeding 600 m s–1 with a duration of 5 min and a repetition period of 8 min. Each pulse of enhanced equatorward flow was preceded by an interval of suppressed flow and enhanced ionospheric Hall conductance. These transient features, which propagate eastwards away from local midnight, have been interpreted as ionospheric current vortices associated with fieldaligned current pairs. The present study reveals that these ionospheric convection features appear to have an accompanying signature in the magnetosphere, comprising a dawnward perturbation and dipolarisation of the magnetic field and dawnward plasma flow, measured in the geomagnetic tail by the Geotail spacecraft, located at L = 10 and some four hours to the east, in the postmidnight sector. These signatures are suggested to be the consequence of the observation of the same field aligned currents in the magnetosphere. Their possible relationship with bursty Earthward plasma flow and magnetotail reconnection is discussed. 相似文献
2.
The CUTLASS Finland HF radar has been operational since February 1995. The radar frequently observes backscatter during the midnight sector from a latitude range 70–75° geographic, latitudes often associated with the polar cap. These intervals of backscatter occur during intervals of substorm activity, predominantly in periods of relatively quiet magnetospheric activity, with Kp during the interval under study being 2-and KP for the day being only 8-. During August 1995 the radar ran in a high time resolution mode, allowing measurements of line-of-sight convection velocities along a single beam with a temporal resolution of 14s, and measurement of a full spatial scan of line-of-sight convection velocities every four minutes. Data from such scans reveal the radar to be measuring return flow convection during the interval of substorm activity. For three intervals during the period under study, a reduction in the spatial extent of radar backscatter occurred. This is a consequence of D region HF absorption and its limited extent in the present study is probably a consequence of the high latitude of the substorm activity, with the electrojet centre lying between 67° and 71° geomagnetic latitude. The high time resolution beam of the radar additionally demonstrates that the convection is highly time dependent. Pulses of equatorward flow exceeding 600 m s–1 are observed with a duration of 5 min and a repetition period of 8 min. Their spatial extent in the CUTLASS field of view was 400–500 km in longitude, and 300–400 km in latitude. Each pulse of enhanced equatorward flow was preceded by an interval of suppressed flow and enhanced ionospheric Hall conductance. The transient features are interpreted as being due to ionospheric current vortices associated with field aligned current pairs. The relationship between these observations and substorm phenomena in the magnetotail is discussed. 相似文献
3.
High-time resolution CUTLASS observations and ground-based magnetometers have been employed to study the occurrence of vortical flow structures propagating through the high-latitude ionosphere during magnetospheric substorms. Fast-moving flow vortices (800 m s–1) associated with Hall currents flowing around upward directed field-aligned currents are frequently observed propagating at high speed (1 km s–1) azimuthally away from the region of the ionosphere associated with the location of the substorm expansion phase onset. Furthermore, a statistical analysis drawn from over 1000 h of high-time resolution, nightside radar data has enabled the characterisation of the bulk properties of these vortical flow systems. Their occurrence with respect to substorm phase has been investigated and a possible generation mechanism has been suggested. 相似文献
4.
Mid-latitude ionospheric perturbation associated with the Spacelab-2 plasma depletion experiment at Millstone Hill 总被引：1，自引：0，他引：1
Elevation scans across geomagnetic mid latitudes by the incoherent scatter radar at Millstone Hill captured the ionospheric response to the firing of the Space Shuttle Challenger OMS thrusters near the peak of the F layer on July 30, 1985. Details of the excitation of airglow and the formation of an ionospheric hole during this event have been reported in an earlier paper by Mendillo et al.. The depletion (factor 2) near the 320 km Shuttle orbital altitude persisted for 35 min and then recovered to near normal levels, while at 265 km the density was reduced by a factor of 6; this significant reduction in the bottomside F-region density persisted for more than 3 hours. Total electron content in the vicinity of the hole was reduced by more than a factor of 2, and an oscillation of the F-region densities with 40-min period ensued and persisted for several hours. Plasma vertical Doppler velocity varied quasi-periodically with a 80-min period, while magnetic field variations observed on the field line through the Shuttle-burn position exhibited a similar 80-min periodicity. An interval of magnetic field variations at hydromagnetic frequencies (95 s period) accompanied the ionospheric perturbations on this field line. Radar observations revealed a downward phase progression of the 40-min period density enhancements of -1.12° km–1, corresponding to a 320-km vertical wavelength. An auroral-latitude geomagnetic disturbance began near the time of the Spacelab-2 experiment and was associated with the imposition of a strong southward IMF Bz across the magnetosphere. This created an additional complication in the interpretation of the active ionospheric experiment. It cannot be determined uniquely whether the ionospheric oscillations, which followed the Spacelab-2 experiment, were related to the active experiment or were the result of a propagating ionospheric disturbance (TID) launched by the enhanced auroral activity. The most reasonable conclusion is that the ionospheric oscillations were a result of the coincident geomagnetic disturbance. The pronounced depletion of the bottomside ionosphere, however, accentuated the oscillatory behavior during the interval following the Shuttle OMS burn. 相似文献
5.
We utilise high-time resolution measurements from the PACE HF radar at Halley, Antarctica to explore the evolution of the ionospheric response during the first few minutes after enhanced reconnection occurs at the magnetopause. We show that the plasma velocity increases associated with flux transfer events (FTEs) occur first 100–200 km equatorward of the region to which magnetosheath (cusp) precipitation maps to the ionosphere. We suggest that these velocity variations start near the ionospheric footprint of the boundary between open and closed magnetic field lines. We show that these velocity variations have rise times 100 s and fall times of 10 s. When these velocity transients reach the latitude of the cusp precipitation, sometimes the equatorward boundary of the precipitation begins to move equatorward, the expected and previously reported ionospheric signature of enhanced reconnection. A hypothesis is proposed to explain the velocity variations. It involves the rapid outflow of magnetospheric electrons into the magnetosheath along the most recently reconnected field lines. Several predictions are made arising from the proposed explanation which could be tested with ground-based and space-based observations. 相似文献
6.
The 1964 Prince William Sound earthquake (March 28, 1964;M
w
=9.2) caused crustal deformation over an area of approximately 140,000 km2 in south central Alaska. In this study geodetic and geologic measurements of this surface deformation were inverted for the slip distribution on the 1964 rupture surface. Previous seismologic, geologic, and geodetic studies of this region were used to constrain the geometry of the fault surface. In the Kodiak Island region, 28 rectangular planes (50 by 50 km each) oriented 218°N, with a dip varying from 8o nearest the Aleutian trench to 9o below Kodiak Island, define the rupture surface. In the Prince William Sound region 39 planes with variable dimensions (40 by 50 km near the trench, 64 by 50 km inland) and orientation (218°N in the west and 270°N in the east) were used to approximate the complex faulting. Prior information was introduced to constrain offshore dip-slip values, the strike-slip component, and slip variation between adjacent planes. Our results suggest a variable dip-slip component with local slip maximums occurring near Montague Island (up to 30 m), further to the east near Kayak Island (up to 14 m), and trenchward of the northeast segment of Kodiak Island (up to 17m). A single fault plane dipping 30°NW, corresponding to the Patton Bay fault, with a slip value of 8 m modeled the localized but large uplift on Montague Island. The moment calculated on the basis of our geodetically derived slip model of 5.0×1029 dyne cm is 30% less than the seismic moment of 7.5×1029 dyne cm calculated from long-period surface waves (Kanamori, 1970) but is close to the seismic moment of 5.9×1029 dyne cm obtained byKikuchi andFukao (1987). 相似文献
7.
We present for the first time a statistical study of 50 keV ion events of a magnetospheric origin upstream from Earths bow shock. The statistical analysis of the 50–220 keV ion events observed by the IMP-8 spacecraft shows: (1) a dawn-dusk asymmetry in ion distributions, with most events and lower intensities upstream from the quasi-parallel pre-dawn side (4 LT-6 LT) of the bow shock, (2) highest ion fluxes upstream from the nose/dusk side of the bow shock under an almost radial interplanetary magnetic field (IMF) configuration, and (3) a positive correlation of the ion intensities with the solar wind speed and the index of geomagnetic index Kp, with an average solar wind speed as high as 620 km s–1 and values of the index Kp 2. The statistical results are consistent with (1) preferential leakage of 50 keV magnetospheric ions from the dusk magnetopause, (2) nearly scatter free motion of 50 keV ions within the magnetosheath, and (3) final escape of magnetospheric ions from the quasi-parallel dawn side of the bow shock. An additional statistical analysis of higher energy (290–500 keV) upstream ion events also shows a dawn-dusk asymmetry in the occurrence frequency of these events, with the occurrence frequency ranging between 16%-34% in the upstream region. 相似文献
8.
Lunar and solar atmospheric tidal oscillations have been determined with satisfactory accuracy from 17 years 11 months of mean sea-level barometric pressure observations taken at Nandi, Fiji. In many respects, the results are consistent with previous tidal determinations in the south-west Pacific region, although these are few and widely scattered. However, the mean annual amplitude of the lunar tide at Nandi, as determined in this study, 88 b, is much greater than might have been expected from currently available global amplitude maps. Nevertheless, the probable correctness of this result has been confirmed by the analysis of nearly 6 years of similar data from Nausori (130 km E.S.E. of Nandi), which yielded a mean annual lunar amplitude of 88 b, compared with a Nandi amplitude of 83 b for a closely corresponding period. 相似文献
9.
Approximately one thousand microearthquakes with body-wave magnitude mb have been located in northern Venezuela and the southern Caribbean region (9–12° N; 64–70° W) since the installation in 1980 of the Venezuelan Seismological Array, together with forty events of mb 4, one of them with surface-wave magnitude Ms 6. Focal depths are in the range of 0 to <15 km. This geologically complex region is part of the boundary between the Caribbean and the South American Plates. Epicentral locations indicate that this E–W oriented portion of the boundary is formed by two 400 km long subparallel fault zones: San Sebastián fault zone (SSF), 20 km north of Caracas along the coast; and La Victoria fault zone (LVF), 25 km south of the city. They are clearly delineated by the microseismicity. New composite focal mechanism solutions (CFMS) along these faults show right-lateral strike-slip (RLSS) motion on nearly E–W oriented fault planes. NW-striking subsidiary active faults occur in the region and intercept the two main E–W fault zones. These interceptions show high levels of microearthquake activity and seismic moment release when compared to other portions of both, the main and subsidiary faults. New CFMS at those fault crossing sites show NW-striking RLSS motion and normal faulting, in an en-echelon-like structural behavior. Geological data and quantitative comparisons with other transcurrent plate boundaries in the world suggest that the rate of plate motion in this area is on the order of 20 mm/y. Several moderate and large shocks have occurred along the SSF and LVF since 1640, including an Ms 7.6 event in 1900 on SSF. Although the region may be relatively far from a repeat of this earthquake, seismicity data indicate that strong shocks could take place along segments of the seismically active faults identified in this study. 相似文献
10.
J. R. Kayal 《Pure and Applied Geophysics》1982,120(4):809-819
The regional variation of the seismic velocity-ratio () over a 200 km long traverse has been studied by means of microearthquake surveys. The Wadati-plot method is used with a minimum of four P and S arrivals for each of 49 earthquakes. The area as a whole is found to be characterized by a value of 1.74–1.76 for earthquakes of depth 12–40 km, except in a 50 km long section near Wellington, where is low at 1.60. This low has been attributed to the fault zones in the region. A small change of is observed between the upper crust (5 km) and lower crust (12 km), but there is no change of between the lower crust and uppermost mantle. 相似文献
11.
CUTLASS Finland radar observations of the ionospheric signatures of flux transfer events and the resulting plasma flows 总被引：1，自引：0，他引：1
The CUTLASS Finland radar has been run in a two-beam special scan mode, which offered excellent temporal and spatial information on the flows in the high-latitude ionosphere. A detailed study of one day of this data revealed a convection reversal boundary (CRB) in the CUTLASS field of view (f.o.v) on the dayside, the direction of plasma flow either side of the boundary being typical of a dawn-cell convection pattern. Poleward of the CRB a number of pulsed transients are observed, seemingly moving away from the radar. These transients are identified here as the ionospheric signature of flux transfer events (FTEs). Equatorward of the CRB continuous backscatter was observed, believed to be due to the return flow on closed field lines. The two-beam scan offered a new and innovative opportunity to determine the size and velocity of the ionospheric signatures associated with flux transfer events and the related plasma flow pattern. The transient signature was found to have an azimuthal extent of 1900 ± 900 km and an poleward extent of 250 km. The motion of the transient features was in a predominantly westward azimuthal direction, at a velocity of 7.5 ± 3 km. 相似文献
12.
Phase velocities of Rayleigh waves for the Adriatic Sea area are obtained in the period range 25–190 sec along the path (l'Aquila-Trieste) AQU-TRI and 20–167 sec along the path (Trieste-Bari) TRI-BAI.The phase velocities are systematically higher than the known values for the surrounding regions. The data inversion indicates the presence of a lithosphere typical of stable continental areas with clear high-velocity lid (V
s
4.6 km/sec) overlying a well developed low velocity zone (V
s
4.2 km/sec).P. F. Geodinamica C.N.R., Roma Pubbl. N. 189. 相似文献
13.
HF radar measurements were performed near the magnetic equator in Africa (Korhogo 9°2463N–5°3738W) during the International Equatorial Electrojet Year (1993–1994). The HF radar is a high–resolution zenithal radar. It gives ionograms, Doppler spectra and echo parameters at several frequencies simultaneously. This paper presents a comparative study of the daytime ionospheric structures observed during 3 days selected as representative of different magnetic conditions, given by magnetometer measurements. Broad Doppler spectra, large echo width, and amplitude fluctuations revealed small-scale instability processes up to the F-region peak. The height variations measured at different altitudes showed gravity waves and larger-scale disturbances related to solar daytime influence and equatorial electric fields. The possibility of retrieving the ionospheric electric fields from these Doppler or height variation measurements in the presence of the other possible equatorial ionospheric disturbances is discussed. 相似文献
14.
SuperDARN studies of the ionospheric convection response to a northward turning of the interplanetary magnetic field 总被引：1，自引：0，他引：1
J. R. Taylor S. W. H. Cowley T. K. Yeoman M. Lester T. B. Jones R. A. Greenwald G. Sofko J.-P. Villain R. P. Lepping M. R. Hairston 《Annales Geophysicae》1998,16(5):549-565
The response of the dayside ionospheric flow to a sharp change in the direction of the interplanetary magnetic field (IMF) measured by the WIND spacecraft from negative Bz and positive By, to positive Bz and small By, has been studied using SuperDARN radar, DMSP satellite, and ground magnetometer data. In response to the IMF change, the flow underwent a transition from a distorted twin-cell flow involving antisunward flow over the polar cap, to a multi-cell flow involving a region of sunward flow at high latitudes near noon. The radar data have been studied at the highest time resolution available (2 min) to determine how this transition took place. It is found that the dayside flow responded promptly to the change in the IMF, with changes in radar and magnetic data starting within a few minutes of the estimated time at which the effects could first have reached the dayside ionosphere. The data also indicate that sunward flows appeared promptly at the start of the flow change (within 2 min), localised initially in a small region near noon at the equatorward edge of the radar backscatter band. Subsequently the region occupied by these flows expanded rapidly east-west and poleward, over intervals of 7 and 14 min respectively, to cover a region at least 2 h wide in local time and 5° in latitude, before rapid evolution ceased in the noon sector. In the lower latitude dusk sector the evolution extended for a further 6 min before quasi-steady conditions again prevailed within the field-of-view. Overall, these observations are shown to be in close conformity with expectations based on prior theoretical discussion, except for the very prompt appearance of sunward flows after the onset of the flow change. 相似文献
15.
A wide set of dynamics phenomena (i.e., Geodynamics, Post Glacial Rebound, seismicity and volcanic activity) can produce time gravity changes, which spectrum varies from short to long (more than 1 year) periods. The amplitude of the gravity variations is generally in the order of consequently their detection requires instruments with high sensitivity and stability: then, high quality experimental data. Spring and superconducting gravimeters are intensively used with this target and they are frequently jointed with tiltmeters recording stations in order to measure the elasto-gravitational perturbation of the Earth. The far-field effects produced by large earthquakes on records collected by spring gravimeters and tiltmeters are investigated here. Gravity and tilt records were analyzed on time windows spanning the occurrence of large worldwide earthquakes; the gravity records have been collected on two stations approximately 600 km distant. The background noise level at the stations was characterized, in each season, in order to detect a possible seasonal dependence and the presence of spectral components which could hide or mask other geophysical signals, such as, for instance, the highest mode of the Seismic Free Oscillation (SFO) of the Earth. Some spectral components (6.5; 8; 9; 14, 20, 51) have been detected in gravity and tilt records on the occasion of large earthquakes and the effect of the SFO has been hypothesized. A quite different spectral content of the EW and NS tiltmeter components has been detected and interpreted as a consequence of the radiation pattern of the disturbances due to the earthquakes. Through the analysis of the instrumental sensitivity, instrumental effects have been detected for gravity meters at very low frequency.F.S.E. (Fondo Sociale Europeo -European Community -) 相似文献
16.
Ionosphere-magnetosphere disturbances at high latitudes, e.g. magnetic substorms, are accompanied by energetic particle precipitation and strong variations of the ionospheric electric fields and currents. These might reasonably be expected to modify the local atmospheric electric circuit. We have analysed air-earth vertical currents (AECs) measured by a long wire antenna at Esrange, northern Sweden during 35 geomagnetic substorms. Using superposed epoch analysis we compare the air-earth current variations during the 3 h before and after the time of the magnetic X-component minimum with those for corresponding local times on 35 days without substorms. After elimination of the average daily variation we can conclude that the effect of substorms on AEC is small but distinguishable. It is speculated that the AEC increases observed during about 2 h prior to the geomagnetic X-component minimum, are due to enhancement of the ionospheric electric field. During the subsequent 2 h of the substorm recovery phase, the difference between substorm and quiet atmospheric currents decreases. The amplitude of this substorm variation of AEC is estimated to be less than 50% of the amplitude of the diurnal variation in AEC during the same time interval. The statistical significance of this result was confirmed using the Van der Waerden X-test. This method was further used to show that the average air-earth current and its fluctuations increase during late expansion and early recovery phases of substorms. 相似文献
17.
Summary The effect of the IMF sector boundary crossing (IMF SBC) in the vorticity area index (VAI) — the well-known dip in the VAI after IMF SBC — is found to be independent of the IMF SBC effect in the cosmic ray flux. This finding refutes a recent suggestion by Lundstedt [1] that the IMF SBC effect in VAI is caused by a decrease in cosmic ray flux, but supports the concept of the IMF SBC effects in the ionosphere and atmosphere developed by Latovika [2–4]. Cosmic rays seem to affect the troposphere in another way.
¶rt;mu nu mau nam aum n ( ) a u¶rt; na¶rt;u aumu () — um uu n — a¶rt; auu m ma nm uu . mm mam nam ¶rt;a n¶rt;u ¶rt;m¶rt;a [1], m m a nuu nma uu , n¶rt;¶rt;uam nu m u u am, aum amu [2–4]. am m uu u m um a mn ¶rt;u a.相似文献
18.
aamuam mam uu mn nma ma-au a. a¶rt; uua¶rt;uma mnam, uuma mnn¶rt;mu u mn nma m ¶rt;uamau au uu uma, n¶rt; m n¶rt; mn nm nu¶rt; a. 2. a u au Q=1,99 u 2,06×10–6 /2 am ¶rt;au mu n ua amu, ma mu (2,5×10–6 a/2 ) aum nam mm ¶rt;uana. um mm am na m¶rt;, ¶rt;¶rt; nu¶rt;m am mam. 相似文献
19.
Martin Šamaj Reviewer J. Horák Reviewer Z. Jaňour 《Studia Geophysica et Geodaetica》1982,26(2):187-192
ama mam maum n¶rt; n¶rt;um ma u ¶rt;, ¶rt;u ¶rt;unu nu nu au muu u. ¶rt;aa ¶rt; aa a uu au mm ¶rt;uuu n m¶rt;a am. u ¶rt;a n¶rt;aam m ¶rt; muu m nau am u u a um anu u m umu au, m ¶rt;aa ¶rt; a nm n¶rt;um nmam an¶rt;u mauu nu . 相似文献
20.
Karel Pěč Jana Pěčová Oldřich Praus Reviewer M. Hvoždara 《Studia Geophysica et Geodaetica》1977,21(3-4):383-393
n ¶rt;a, n¶rt;mau 531 au ¶rt; u aum m u¶rt;uu n mumu ana¶rt;, ¶rt; u -m n, aauum ¶rt;um u u amuaa n¶rt;naa ma ua aama. uu nmam an¶rt;u ¶rt; u aum aam ¶rt;au cuu uP
n
m
, n¶rt;am mn n=1, 2, 3 u 5 (m n). u uua ¶rt;a¶rt;amu uu n¶rt;mauu uum au (a. 1) u u n aumam uu nmu, m n¶rt;mam u¶rt; am uuu ¶rt; u aum (u. 1–4). annuau 2
u 5
mn nm ma am mmmu m (u. 5, 6). ama uuu u m aam amu uu uma. 相似文献