bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetosheath electrons in anomalously low density solar wind observed by Geotail
Authors: Kasaba Y. , Terasawa T. , Tsubouchi K. , Mukai T. , Saito Y. , Matsumoto H. , Kojima H. , Steinberg J. , McComas D. , Skoug R. , Matsui H. , Hoshino M.
Published: February 2004

We present an overview of the magnetosheath electron observations above 60 eV in the anomalously low density solar wind on 11 May 1999 observed by Geotail. We observed anisotropic ‘strahl’ component not only in the solar wind but also in the magnetosheath. Strahl is observed during the interval of 6–21h UT and most enhanced in the interval surrounding the solar wind density minimum in 18–19h UT. Its temperature seems positively correlated to its flux. We also evaluate the density and temperature of low energy isotropic component. The variation of density is associated with proton density. Its temperature is continuous, and suddenly decreased after the density minimum in ∼18–19h UT. We also compared our downstream data with upstream one observed by the ACE spacecraft. We conclude that the electron acceleration and heating at the bow shock is executed in the ordinal mechanisms even in the low density solar wind.
Coronal mass ejections detected in solar mean magnetic field
Authors: Boberg Fredrik , Lundstedt Henrik
Published: February 2004

One minute resolution solar mean magnetic field sequences during 74 coronal mass ejection events are analyzed using wavelet transforms. A significant fraction of the resulting wavelet power spectra shows a characteristic peak at the time of the coronal mass ejection. With an average period of 13 minutes, the signature in the solar mean field is most likely the result of global waves trigged by the departing CME.
Geomagnetic disturbances at high latitudes during very low solar wind density event
Authors: Papitashvili V. O. , Clauer C. R. , Christiansen F. , Pilipenko V. A. , Popov V. A. , Rasmussen O. , Suchdeo V. P. , Watermann J. F.
Published: February 2004

In this letter, we report geomagnetic observations over Greenland and Antarctica during May 11, 1999, when the solar wind almost disappeared. Greenland magnetometers show no magnetic activity in the nominal auroral zone but disturbances reached ∼200 nT at higher latitudes; however, conjugate observations in Antarctica show much weaker activity ∼40 nT. Analyzed data provide evidence that the cusp current system could be extended far to the dusk (dawn) in the northern (southern) polar region. This is consistent with the Ørsted satellite observations, where field‐aligned currents were detected near cusp, over the northern geomagnetic pole, and in the nightside polar cap; however, almost no were detected over the southern polar cap. We conclude that the low‐density solar wind quieted activity at auroral latitudes (”loading‐unloading” processes); but the “directly‐driven” convection observed over both polar caps was mainly unaffected by the significant inflation of the magnetosphere.
Photometric measurement of cold helium ions in the magnetotail by an EUV Scanner onboard planet‐B: Evidence of the existence of cold plasmas in the near‐Earth plasma sheet
Authors: Yoshikawa I. , Yamazaki A. , Shiomi K. , Yamashita K. , Takizawa Y. , Nakamura M.
Published: February 2004

The first optical measurement of He+ ions in the plasma sheet was successful by using the XUV scanner onboard Planet‐B. The intensity of He II (304 Å) was ∼0.02 Rayleigh. This result encourages future imaging missions for the whole magnetosphere by detecting He II (304 Å) emission. This is also a proof of the existence of cold plasmas in the plasma sheet. The existence of cold ions brings a new question: How can they remain cold in the plasma sheet?
Three spacecraft observations of solar wind discontinuities
Authors: Horbury Timothy S. , Burgess David , Fränz Markus , Owen Christopher J.
Published: February 2004

Observations of solar wind magnetic field discontinuities using 3 spacecraft allow their orientations to be estimated. During 5 days when Geotail, Wind and IMP 8 were between 6 × 104 and 4 × 105 km apart, 35 events identified using the Tsurutani‐Smith method were detected in all 3 magnetic field data sets. Normals estimated from inter‐spacecraft timings showed that very few were unambiguous rotational discontinuities, with 77% likely to be tangential, with < 20% of the magnetic field at the discontinuity threading the normal plane. However, previous single spacecraft studies using minimum variance suggest that most discontinuities are rotational. Minimum variance analysis resulted in many normal estimates lying far from the timing‐derived normals. While some of this discrepancy is likely to be due to random errors in minimum variance vectors, there appears to be a class of events with small field magnitude changes where the minimum variance directions and discontinuity normals are approximately perpendicular, probably due to surface waves on the discontinuities.
Enhancement of optical aurora triggered by the solar wind negative pressure impulse (SI−)
Authors: Sato Natso , Murata Yozo , Yamagishi Hisao , Yukimatu Akira S. , Kikuchi Masayuki , Watanabe Masakazu , Makita Kazuo , Yang Huigen , Liu Ruiyuan , Rich Frederick J.
Published: February 2004

A geomagnetic negative sudden impulse (SI−) occurred on 3 August 1997 in association with a sudden decrease of solar wind dynamic pressure. The discrete auroras observed by an all‐sky TV imager at Zhongshan in Antarctica (magnetic latitude is ∼74.5°S) suddenly enhanced their luminosity and moved poleward at the time of the SI−. Following the primary enhancement, three cycles of quasi‐periodic luminosity pulsations were observed with periods of about 10–14 min and their average position drifted equatorward. The luminosity pulsations showed one‐to‐one correspondence with the magnetic pulsations, with the characteristic features of a field‐line resonance. The low‐altitude DMSP‐F13 satellite was almost directly over Zhongshan and observed accelerated electron precipitation with an inverted ‘V' structure and an upward field‐aligned current. These signatures suggest that the solar wind negative pressure impulse triggered a field‐line resonance of the geomagnetic field. The resonance caused the upward and downward field‐aligned current sheets, and the field‐aligned electron acceleration.
High‐latitude HF‐induced airglow displaced equatorwards of the pump beam
Authors: Kosch M. J. , Rietveld M. T. , Hagfors T. , Leyser T. B.
Published: February 2004

HF‐induced airglow at 630 nm was observed by the Digital All‐sky Imager, located near Skibotn in Norway, at F‐region altitudes above the EISCAT HF facility near Tromsø on 21 February 1999. The transmitter was operated in a 4‐min on, 4‐min off sequence at 4.04 MHz O‐mode with the beam pointing vertically. The airglow reached a peak intensity of about 100 R above background and appeared equatorward of the HF beam's projection on the reflection altitude, which was obtained from ionograms. Generally, the region of maximum airglow was displaced towards the magnetic field line (zenith angle = 12.8°S) passing through the HF facility. This is a unique feature of these observations. From mid‐latitude studies, such airglow is thought to be excited either by electrons energised to several eV by plasma turbulence, or by thermal electron temperature enhancement. Such localisation towards the magnetic field is unexpected for both mechanisms of airglow generation and suggests this feature may be important at high latitudes.
Polar cap index (PC) as a proxy for ionospheric electric field in the near‐pole region
Authors: Troshichev O. A. , Lukianova R. Yu. , Papitashvili V. O. , Rich F. J. , Rasmussen O.
Published: February 2004

The ion drift measurements made by a number of DMSP satellites during some intervals in 1991, 1997, and 1998 are utilized for estimation of the ionospheric electric fields over the near‐pole region; these estimates are then compared with the Polar Cap (PC) magnetic activity index obtained from ground geomagnetic observations at Qaanaaq (former Thule, Greenland) and Vostok (Antarctica). The analysis shows that the polar cap electric field is primarily controlled by variations in the near‐Earth's interplanetary electric field. The relationship between the polar cap ionospheric electric field and the PC‐index can be approximated by a quadratic polynomial. The polar cap ionospheric electric field tends to saturate at the asymptote of ∼45–50 mV/m when the PC index reaches large positive values (PC>10); the residual electric field (for near‐zero interplanetary electric field applied to the Earth's magnetosphere) is ∼12 mV/m. It is concluded that the PC‐index can serve as a proxy of the ionospheric electric fields in the near‐pole region.
The role of interplanetary magnetic field and solar wind in modulating both galactic cosmic rays and geomagnetic activity
Authors: Sabbah I.
Published: February 2004

We show that the enhancements in both the upper limiting rigidity of the cosmic ray solar diurnal variation and the geomagnetic activity during the decline of solar activity are related to high values of the product of the solar wind speed and the interplanetary magnetic field strength (VB). The upper limiting rigidity correlates very well with the magnitude of VB. This reflects the effect of both interplanetary magnetic field (IMF) and solar wind speed in modulating cosmic rays. The upper limiting rigidity is correlated with the geomagnetic activity as well. However, it is the value of the product VB that directly modulates the geomagnetic activity. The product VB is more important for both cosmic ray and geomagnetic activity modulations rather than the IMF alone.
Lion roar trapping in mirror modes
Authors: Treumann R. A. , Georgescu E. , Baumjohann W.
Published: February 2004

We show that whistler mode noise generated inside mirror modes (Lion Roars) by a remaining electron temperature anisotropy may become trapped in the mirror bottle. The trapping selects a narrow range of wave numbers and restricts the width of the lion roar frequency band in agreement with recent observations.
Mesospheric and lower thermospheric manifestations of a stratospheric warming event over Eureka, Canada (80°N)
Authors: Walterscheid R. L. , Sivjee G. G. , Roble R. G.
Published: February 2004

We have examined lidar and airglow data and National Weather Service analysis fields for a stratospheric sudden warming event in February 1993. The lidar and airglow measurements recorded temperature changes in the mesosphere and lower thermosphere over Eureka, Canada (80°N). In addition, the event was simulated by the National Center for Atmospheric Research TIME‐GCM. The observations, analysis fields, and the simulation results taken together indicate a connection between the stratospheric warming of February 1993 and alternating regions of cooling and warming above the main warming in the lower stratosphere (however, movement of the polar vortex complicates the interpretation for the second of two warming episodes in the upper stratosphere during the event). The sudden warming was associated with cooling observed in the OH airglow and predicted by the model. This cooling preceded the warming in the lower stratosphere.
Radar measurements of electric fields in the topside of the equatorial electrojet: First results
Authors: Tsunoda Roland T. , Ecklund Warner L. , Johnston Paul E.
Published: February 2004

We show that the vertical polarization electric field (Ep) in the topside of the equatorial electrojet can be determined from the Doppler spectra of type‐2 echoes obtained with a radar that is appropriately displaced in latitude from the magnetic dip equator. Using a 49.92 MHz radar on Christmas Island (2.9° magnetic dip latitude), we found unexpectedly large Ep at altitudes at least as high as 120 km over the dip equator. Another new and related finding is the transient appearance of type‐1 echoes at 99 km over Christmas Island, which likely was produced by an Ep of at least 11 mV/m that must have appeared at 114 km altitude over the dip equator. Elevated electron temperatures inferred from the type‐1 Doppler spectra are consistent with the presence of anomalous energetics.
Magnetospheric sash dependence on IMF direction
Authors: Siscoe G. L. , Erickson G. M. , Ö Sonnerup B. U. , Maynard N. C. , Siebert K. D. , Weimer D. R. , White W. W.
Published: February 2004

The magnetospheric sash is a ribbon of weak field shaped like a horseshoe with its open ends adjacent to the north and south dayside, magnetopause cusps and its closed end forming the cross‐tail current sheet. The clock angle of the sash in the dawn‐dusk meridian plane (as seen from the sun) rotates from 0° to 90° as the clock angle of the interplanetary magnetic field (IMF) rotates from 0° to 180°. We use a global MHD simulation to obtain the sash clock angles for IMF clock angles of 45°, 90°, and 135°. Remarkably, the results are very close to the clock angle of the magnetic null points obtained by superposing a uniform field representing the IMF on a dipole field representing the earth. Contours of magnetic field strength on cross sections perpendicular to the solar wind flow direction show how the sash evolves tailward from the dayside cusps.
Density extremes in the solar wind
Authors: Crooker N. U. , Shodhan S. , Gosling J. T. , Simmerer J. , Lepping R. P. , Steinberg J. T. , Kahler S. W.
Published: February 2004

The largest density variations in the solar wind occur near the streamer belt, where frequently they are associated with interplanetary coronal mass ejections (ICMEs). They tend to be embedded in larger‐scale regions of high pressure: high densities in ICME sheaths and corotating interaction regions and low densities in structures with distinctive, high‐magnetic‐pressure profiles, sometimes within ICMEs. On average, however, ICME densities are similar to ambient‐wind densities. For a set of 34 ICMEs identified in Wind data as magnetic clouds, the average density was 11 cm−3 in both the clouds and all slow wind during the same period. A set of low‐density structures observed earlier by ISEE 3 recurred for three solar rotations, possibly owing to recurrence of the streamer belt itself, with its frequent transient outflows. Density averages less than 1 cm−3 show a possible solar cycle variation which peaks 1–2 years prior to the peak of ICME signatures.
Ground magnetic signatures of ULF and substorm activity during an interval of abnormally weak solar wind on May 11, 1999
Authors: Rostoker Gordon
Published: February 2004

On May 11, 1999 the solar wind density became extremely low, dropping slowly over several hours to below 1 particle/cc at a time when the wind speed was relatively low (∼360–380 km/s) and the north‐south component of the interplanetary magnetic field (IMF) was fluctuating within ±2 nT of zero. During this period, we tried to establish whether or not ground based magnetometers detected any unusual signatures over and above a weakened version of the normal signatures of activity. In this paper we report that, although the oval was contracted, the activity along the oval demonstrated no unusual characteristics. However, at high latitudes there was significant ULF activity in a latitudinally localized strip during the nighttime hours. We contend that this activity is associated with resonant oscillations in the magnetotail which couple to field lines around the high latitude edge of the plasma sheet.
Observations of an anomalously low frequency Alfvén continuum in an abnormally expanded magnetosphere
Authors: Mathie R. A. , Mann I. R.
Published: February 2004

We analyze a Pc5 ULF pulsation event that occurred on 12 May, 1999, following an extended interval of extremely low solar wind dynamic pressure (ram pressure dropped to ∼0.2nPa; bow shock standoff position estimated at ∼60RE). Using the IMAGE magnetometer array, we show that the extended magnetosphere at this time gave rise to an Alfvén continuum of field line oscillations in the outer magnetosphere of anomalously low frequency, consistent with field lines mapping out to large distances within the enlarged magnetospheric cavity. We demonstrate that the pulsation event was characterized by discrete, low frequency signals consistent with cavity/waveguide modes, and that these discrete modes couple to high latitude field line resonances during the event. Our observations suggest that cavity/waveguide modes continue as the dominant source of energy input to geomagnetic pulsations even during intervals when the magnetosphere is abnormally extended.
On the modulation of galactic cosmic ray intensity during solar activity cycles 19, 20, 21, 22 and early 23
Authors: Van Allen James A.
Published: February 2004

Cross‐plots of annual mean sunspot number against the annual mean intensity of galactic cosmic rays for the period 1953–1999 show a striking difference between such modulation loops for solar activity cycles 19 and 21 and those for cycles 20 and 22. The loops for cycles 19 and 21 are broad ovals whereas those for cycles 20 and 22 are nearly flat. A complementary feature of the data is that the cosmic ray intensity decreases more rapidly as the sunspot number increases following solar activity minima when the solar polar magnetic parameter A is negative than when A is positive. It is argued that these facts give some support to including gradient and curvature drifts in theories of the transport of cosmic rays in the heliosphere. Further development of the modulation loop for cycle 23 is awaited with special interest.
Response of the equatorial and polar magnetosphere to the very tenuous solar wind on May 11, 1999
Authors: Farrugia C. J. , Singer H. J. , Evans D. , Berdichevsky D. , Scudder J. D. , Ogilvie K. W. , Fitzenreiter R. J. , Russell C. T.
Published: February 2004

We examine effects in the equatorial and polar magnetosphere during 9–13 May, 1999. Earth's field at geostationary orbit became closely dipolar for ∼16 hours when solar wind densities nsw were <1 cm−3. Electron precipitation in the northern polar cap intensified as nsw decreased, with significant fluxes up to ∼15 keV energy on May 11. The simultaneous precipitation void in the southern polar cap implies a very pronounced north‐south asymmetry, also reflected in the hemispherical power deposition. With an intense and collimated strahl, these observations support the ideas of Fairfield and Scudder [1985] on the preferential entry of the strahl into the northern hemisphere under the observed IMF away sector as a source of the north‐south precipitation asymmetry. The polar rain north‐south asymmetry argues against an ejecta source for the solar wind drop‐out. The temporal profiles of solar wind parameters were very asymmetric with respect to the time of minimum nsw, and strong compressions and substorm activity prevailed as nsw recovered.
Hemispheric asymmetries in ionospheric electrodynamics during the solar wind void of 11 May 1999
Authors: Knipp D. J. , Lin C.‐H. , Emery B. A. , Ruohoniemi J. M. , Rich F. J. , Evans D. S.
Published: February 2004

We use the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) procedure to produce composite maps of high‐latitude ionospheric parameters for 11 May 1999. On that day there was a near void in the solar wind momentum flux. Yet, focused high‐latitude interactions occurred at Earth. Our maps of convection, Joule heating, particle precipitating power and field‐aligned current show a striking asymmetry between hemispheres. In the latter three parameters there is at least a five‐fold difference in values between the hemispheres. The northern hemisphere polar cap particle precipitation is the strongest we have ever mapped. Strong electrodynamic interactions were focused above 70° north magnetic latitude. In the southern hemisphere only weak, irregular patterns are mapped. We attribute these differences to the strong interplanetary magnetic field spiral configuration, the sunward tilt of the northern hemisphere and the direct beam or “strahl” of solar electrons impinging on the northern polar cap during the momentum void.
Ion sound waves in the topside low latitude ionosphere
Authors: Huba J. D. , Joyce G. , Fedder J. A.
Published: February 2004

In this letter we report new modeling results that show the generation of ion sound waves in the topside low latitude ionosphere. The waves are excited at sunrise and sunset; they persist ∼1–3 hrs at altitudes above the O+/H+ transition altitude (∼1000 km) with periods on the order of 10's min. The waves are the result of the rapid heating and cooling of the lower ionosphere that occurs at sunrise and sunset. At sunrise, photoelectron heating produces strong upward plasma flows along the geomagnetic field. These flows lead to a local compression and heating of the plasma at the apex of the field line which in turn generates ion sound waves. At sunset, the waves are produced by a rapid cooling of the plasma. We suggest that this phenomenon may be observable with the Arecibo radar facility.
Implications of depleted flux tubes in the Jovian magnetosphere
Authors: Russell C. T. , Kivelson M. G. , Kurth W. S. , Gurnett D. A.
Published: February 2004

A rare but persistent phenomenon in the jovian magnetosphere is the occurrence of apparently depleted flux tubes, whose magnetic pressures are significantly above ambient levels. These flux tubes occur about 0.25% of the observing time in the region of the Io torus in the Galileo high resolution data. The importance of these tubes is that they can return to the inner magnetosphere the magnetic flux that has been convected radially outward with the iogenic plasma to the tail. The paucity of these tubes is consistent with the expected flux return rates if the tubes are moving inward at an average rate of about 5–10 km/s in the torus. Depleted flux tubes have yet to be observed inside of the Io orbit where the plasma beta is lower than in the hot torus. Estimates of the plasma density outside the tube from plasma wave measurements enable the average perpendicular temperature to be obtained from the magnetic field change. Extrapolating this temperature back to Io, we obtain an average ion temperature of approximately 60 eV. These values are generally consistent with earlier Voyager observations but on the low side of their range of uncertainty, and agree quite well with contemporaneous Galileo measurements where these are available.
Initial observations of polar mesospheric summer echoes using the EISCAT Svalbard Radar
Authors: Hall C. M. , Röttger J.
Published: February 2004

Polar Mesospheric Summer Echoes (PMSE) have been observed at various frequencies from MF to UHF and at various latitudes. Here, we report the observation of PMSE over Svalbard (79°N) at 500 MHz. Although PMSE have been reported earlier at such high latitude at VHF (notably around 50 MHz), they had hitherto not been reported at such high frequency as 500 MHz. Furthermore, this observation represents the first one using a frequency between 224MHz and 933MHz. The PMSE scattering mechanism is still open to debate and is certainly frequency dependent: the new observations at 500 MHz will help to identify the transition frequency between different mechanisms.
A correlation between extended intervals of Ulf wave power and storm‐time geosynchronous relativistic electron flux enhancements
Authors: Mathie R. A. , Mann I. R.
Published: February 2004

Recent research has highlighted the possibility that large‐amplitude ULF pulsations may act as an acceleration mechanism for generating relativistic electron populations in the outer zone magnetosphere during storm‐time. We examine solar wind characteristics, Pc5 ULF wave power and outer magnetospheric measurements of high energy electron flux during the recurrent fast solar wind speed streams which occurred during the first half of 1995. We find a close correlation between extended intervals of significant pulsation power and observations of enhanced relativistic (>2 MeV) electron flux in the outer zone magnetosphere, suggesting that these two features may be causally related. We demonstrate that significant electron flux increases at geosynchronous orbit are only observed in response to ULF wave power which is sustained at high levels over a number of days following storm onset. Our observations suggest that ULF pulsations may be the likely acceleration mechanism for generating storm‐time MeV “killer” electrons in the magnetosphere.
Modeling ionospheric absorption modified by anomalous heating during substorms
Authors: Milikh G. M. , Dimant Y. S. , Shao X. , Guzdar P. N. , Sharma A. S. , Papadopoulos K. , Burns E. M. , Goodrich C. C. , Rosenberg T. J. , Weatherwax A. T. , Wiltberger M. J. , Lyon J. G. , Fedder J. A.
Published: February 2004

Riometers monitor the changes in ionospheric conductivity by measuring the absorption of very high frequency radio noise of galactic origin passing through the ionosphere. In this Letter the absorption of radio signals by a thin layer of ionospheric plasma, produced by ionization due to energetic precipitating electrons, is modeled by taking into account strong turbulent heating caused by instabilities. The precipitating electron population is obtained from a global MHD simulation of the magnetosphere, along with the electric fields which excite the Farley‐Buneman instability and lead to turbulent electron heating. A comparison, the first of its kind, of the data from polar and sub‐auroral riometers for the magnetic cloud event of January 10, 1997 shows good agreement. The ionospheric conductance modified by turbulent electron heating can be used to improve the magnetosphere — ionosphere coupling in the current global MHD models.
The formation of meso‐ and macroporous gas hydrates
Authors: Kuhs Werner F. , Klapproth Alice , Gotthardt Frank , Techmer Kirsten , Heinrichs Till
Published: February 2004

We present results of experimental studies on the formation of gas hydrates (clathrates) at conditions of geophysical interest. Clathrate hydrates formed by a reaction of gas at ice Ih surfaces are always found to be mesoporous to macroporous with pores sizes between 100 to 400 nm and pore volumes of approximately 25–40% for CH4, Ar and N2 hydrate, and smaller pores of 20 to 100 nm with a porosity of approximately 10–20% for CO2 hydrate. The three‐dimensional sponge‐like microstructure occurs in single crystalline grains of typically a few µm size and was observed by field‐emission scanning electron microscopy. It forms over a wide range of p‐T conditions below the ice Ih melting. The porous microstructure is stable for at least several months, even close to the clathrate decomposition, and is proposed to be formed by local differences in the energy balance between hydrate formation and ice decompositon. The results presented are considered of potential major importance for the understanding of the behaviour of natural gas hydrates found e.g. in polar ice sheets and permafrost regions, and also in some celestial bodies.
Cooling of the South China Sea by the Toba Eruption and correlation with other climate proxies ∼71,000 years ago
Authors: Huang Chi‐Yue , Zhao Meixun , Wang Chia‐Chun , Wei Ganjian
Published: February 2004

The Toba tephra layer has been identified in core MD972151 in the southern South China Sea (SCS), northeast of the Indonesia Toba caldera. This affords us an opportunity to directly determine a 1°C cooling for ca.l kyr on the SCS following the Toba eruption (71 Ka) during the marine isotope stage 5a–4 transition, using century‐scale sea surface temperature records. This cooling event in the SCS is well correlated with several coeval proxies such as increased East Asia winter monsoon intensity, increased ice‐rafted detritus in the North Pacific Ocean sediments, and decreased δ18O in Greenland ice core. Such correlation suggests a climate change where cold climate signals originated in the Northern Hemisphere ice sheets, transferred southward by the winter monsoon, and cooled the SCS.
Thermal satellite images and boundary layer structures in desert marginal areas
Authors: Nichol Janet E. , Grove A. T.
Published: February 2004

A series of parallel striations has been observed on thermal satellite images of a desert marginal area in West Africa. The available climatic data suggest their identification as helical roll vortices of the type associated with the formation of cloud streets. This is because the wavelength of the striations conforms to the expected depth of the Atmospheric Boundary Layer (ABL) expected for such roll structures at the time of the image, and varies spatially according to the influence of ground surface roughness on ABL depth. Knowledge of helical roll vortices is usually derived from vertical temperature and wind profiles and radar reflectivity fields over time, whereas the image data presented here permit visualisation of the horizontal component, and thus examination of their interaction with surface phenomena. Association of the observed striations with systems of stabilised dunes in the study area, due to their similarity in length, direction and spacing, would support hypotheses of a causal relationship between helical roller vortices and linear dune systems.
Getting more for less: The unusual efficiency of fluid flow in fractures
Authors: Bai Taixu , Pollard David D.
Published: February 2004

Although it is recognized that the opening of fractures changes with changing fracture spacing, it is a common practice to consider only the effects of loading and rock properties, and to assume that opening of all fractures is independent of fracture spacing. Thus, one would expect that the more fractures contained in a rock mass, the greater the fluid flow rate. Using the cubic law and the Finite Element Method, we have investigated the volumetric flow rate through a number of equally‐spaced fractures in a fractured rock layer as a function of the ratio of fracture spacing to layer thickness, under a remote extension. Results show that there is an optimum value for the ratio of fracture spacing to layer thickness that yields the maximum flow rate. This value is independent of the fluid properties, the head gradient, the applied extension, the internal fluid pressure and the overburden stress.
Reconstructing the temporal dynamics of snow cover from observations
Authors: Perona Paolo , D'Odorico Paolo , Porporato Amilcare , Ridolfi Luca
Published: February 2004

A nonlinear differential equation for the description of the mean annual behavior of snow depth at a point is reconstructed from measured time series of temperature, snow fall, and snow cover, using the so‐called Trajectory Method proposed by Eisenhammer et al. [1991]. The temperature and snow fall are used as independent variables of a polynomial equation with nonlinearities up to the third degree. The reconstructed model not only reproduces the original data very well, but also captures the main behavior of the system when forced by different climatic conditions.
Insolation‐weighted assessment of northern hemisphere snow‐cover and sea‐ice variability
Authors: Pielke R. A. , Liston G. E. , Robock A.
Published: February 2004

The effects of sea‐ice and snow‐cover trends on the surface energy budget are assessed by scaling with the direct solar insolation. We have found that, consistent with other studies, an early spring melt has occurred in recent years in the unweighted data, but this trend is muted when the data are weighted by solar insolation. The onset of sea‐ice growth and snow in the Fall, however, has no significant trend during the period of record. The effect of sea ice on the reflection of sunlight is largest in May and June when significant sea‐ice coverage remains and the sun angle is high. Snow cover, in contrast, has its largest effect on the reflection of sunlight in April.
Paleoclimatic significance of the temperature‐dependent susceptibility of Holocene Loess along a NW‐SE transect in the Chinese Loess Plateau
Authors: Deng Chenglong , Zhu Rixiang , Verosub K. L. , Singer M. J. , Yuan Baoyin
Published: February 2004

We have conducted mineral magnetic investigations of samples of modern dune sand, pristine loess and present‐day loess along a NW‐SE transect from the northern to the central loess plateau in China. The methods used include the temperature‐dependence of susceptibility (TDS) before and after citrate‐bicarbonate‐dithionite (CBD) treatment as well as X‐ray diffraction (XRD) analysis. The XRD analyses demonstrate that magnetite and hematite both exist in the Chinese loess‐paleosol sequence and its modem source area, but the TDS measurements show that magnetite is the predominant contributor to magnetic susceptibility. Maghemite is present in the pristine loess and the present‐day loess due to pedogenesis. The pedogenic processes that produce the maghemite are closely linked to paleoclimate, and for this transect, precipitation appears to be the most important climatic variable. We suggest that TDS can be used as a useful method for the analysis of pedogenesis and climatic change.
Urban subsidence inside the city of Napoli (Italy) Observed by satellite radar interferometry
Authors: Tesauro M. , Berardino P. , Lanari R. , Sansosti E. , Fornaro G. , Franceschetti G.
Published: February 2004

Satellite radar interferometry reveals subsidence occurring in an area of about 2 km by 2 km inside the city of Napoli, Italy. The observations show a maximum (vertical) displacement of about 5 cm between 1992/93 and 1996, while the deformation signal decreases between 1996 and 1998. The investigated area is densely populated and has been influenced, between 1992 and 1995, by intensive construction of a new underground railway. These interferometric results demonstrate the high correlation between the observed deformation and the excavation of the tunnels for the development of the underground.
Heat flow scaling for mantle convection below a conducting lid: Resolving seemingly inconsistent modeling results regarding continental heat flow
Authors: Lenardic A. , Moresi L.
Published: February 2004

Numerical models of mantle convection below a conducting lid, meant to mimic a continent, have been used 1) to argue that heat flow variations in stable continental regions result principally from lithospheric thickness variations, and 2) to argue that the relationship between lithospheric thickness and continental heat flow is too weak to allow for this. We reconcile these results using a theoretical heat flow scaling which shows that the relationship between heat flow and lid thickness can take on two end‐member forms. As the ratio of lid thickness to convecting layer depth increases, the effects of lid thickness variations on heat flow move from weak to strong. Modeling studies that concluded that lithospheric thickness variations could lead to significant continental heat flow variations assumed a convecting layer depth appropriate to upper mantle convection and, thus, maximized the relative thickness of the continental lithosphere, i.e., the conducting lid. Studies that concluded that the relationship is weak assumed whole mantle convection which placed them on the flatter end of the heat flow versus relative lid thickness curve.
Natural fluorinated organics in fluorite and rocks
Authors: Harnisch Jochen , Frische Matthias , Borchers Reinhard , Eisenhauer Anton , Jordan Armin
Published: February 2004

Results of measurements of fluorinated compounds in gasses extracted from igneous and metamorphic rocks are reported. A new extraction method analogous to a pepper mill for geological samples is described. It permits extraction at low temperatures and ensures a rapid transfer of extracted gases from active surfaces to cryogenic pre‐concentration loop. Values for CF4, CF3Cl, CF2Cl2, CFCl3, CHF3, SF6 and NF3 in fluorites, granites, basalts and other igneous and metamorphic rocks are reported. It is proposed that trifluoroacetic acid (TFA) that was recently discovered in various environmental archives could also origin from similar geogenic sources.
Magnetic polarity structure of the lower oceanic crust
Authors: Allerton Simon , Tivey Maurice A.
Published: February 2004

A newly developed rock drill was used to take the first fully oriented samples from in‐situ gabbros exposed on Atlantis Bank, which formed 11 m.y. ago on the Southwest Indian Ridge (SWIR). These paleomagnetic data locate the seabed outcrop of a normal polarity magnetic lineation within an outcrop of lower crust that correlates with overlying deep‐towed and sea‐surface magnetic field measurements. A systematic lateral offset between the seabed boundary and measured anomalies implies that the reversal boundary dips ∼25° to the south, away from the ridge axis. Paleomagnetic results suggest that following magnetization acquisition, the platform tilted south by ∼20° (range: 33 south° to 7° north), implying an original reversal boundary dip of only ∼5°.
High pressure phase transformation of jadeite and stability of NaAlSiO4 with calcium‐ferrite type structure in the lower mantle conditions
Authors: Tutti Faramarz , Dubrovinsky Leonid S. , Saxena Surendra K.
Published: February 2004

Phase transformation of Jadeite clinopyroxene was studied at temperatures of 800 and 2200 °C at pressures between 30–75 GPa in externally electrically heated and internally laser‐heated diamond anvil cells. It has been found that Jadeite transforms to the assemblage of NaAlSiO4 (calcium‐ferrite type structure) plus stishovite at high pressure and temperatures. Therefore NaAlSiO4 calcium‐ferrite is stable at the deep lower mantle pressure and temperature conditions.
Ground deformation near Gada ‘Ale Volcano, Afar, observed by radar interferometry
Authors: Amelung Falk , Oppenheimer Clive , Segall P. , Zebker H.
Published: February 2004

Radar interferometric measurements of ground‐surface displacement using ERS data show a change in radar range, corresponding to up to 12 cm of subsidence near Gada ‘Ale volcano in northern Afar, Ethiopia, that occurred between June 1993 and May 1996. This is the area of lowest topography within the Danakil Depression (−126 m). Geodetic inverse modeling and geological evidence suggest a volcanic origin of the observed deformation; it was probably caused by a combined process of magma withdrawal from a larger reservoir and normal faulting. There is no evidence of subaerial eruption. This is the only identifiable deformation event during June 1993–October 1997 in the 80 km long Erta ‘Ale volcanic range, indicating surprising inactivity elsewhere in the range.
Fuzzy patches on the Earth's core‐mantle boundary?
Authors: Garnero Edward J. , Jeanloz Raymond
Published: February 2004

Recent seismological investigations reveal the presence of highly anomalous structures at the base of the mantle, modeled as patches ≤5–50 km thick having ultralow‐velocities (−δVP∼10–20%, −δVS∼10–50%). Waveform modeling shows seismological data are compatible with the patches exhibiting a wide range of density increases, up to δρ∼60%, which can be ascribed to chemical contamination of the deep mantle by the core. Not all anomalies require lowermost mantle partial melting, and may be located just below or right at the core‐mantle boundary (CMB): a ∼1–3 km thick zone of finite rigidity (crystallization?) at the top of the outer core or, more generally, of gradational properties across the CMB can also explain observations. Fuzzy patches at the boundary may be zones of intense chemical and physical interactions between the mantle and core.
Detailed plate boundary structure off northeast Japan coast
Authors: Takahashi Narumi , Kodaira Shuichi , Tsuru Tetsuro , Park JinOh , Kaneda Yoshiyuki , Kinoshita Hajimu , Abe Shintaro , Nishino Minoru , Hino Ryota
Published: February 2004

In 1997, a seismic experiment using an airgun array and ocean bottom seismographs (OBSs) was performed in the forearc region of the northeast Japan (NEJ) arc. The objectives of this experiment were to clarify whole of the velocity structure around the forearc region of NEJ arc including a detailed plate boundary structure and the heterogeneous structure. In this paper, we estimated the heterogeneous velocity structure around the forearc region of northeast Japan by applying 2‐D ray tracing and travel time inversion to the airgun‐OBS data. The depth where the island arc Moho comes into contact with the subducting oceanic crust is about 20 km. We suggest the existence of an oceanic layerl overlying the oceanic crust subducting down to at least near 18 km by comparing observed waveforms with these calculated using the reflectivity method.
Displacement field and fault model for the September 7, 1999 Athens Earthquake inferred from ERS2 Satellite radar interferometry
Authors: Kontoes C. , Elias P. , Sykioti O. , Briole P. , Remy D. , Sachpazi M. , Veis G. , Kotsis I.
Published: February 2004

On September 7, 1999, a moderate (Mw=5.9) normal faulting earthquake occurred in the northwest of Athens (Hellas) causing heavy damages and casualties. Using interferometric combinations of ERS2 SAR images, we analyzed the coseismic deformation field. Two fringes are observed south of the Fili mountain, up to the coastline of the Elefsis gulf. They correspond to 56 mm increase in slant range. Modeling the earthquake as a dislocation in an elastic half‐space, we inverted the interferometric data to assess the fault location and geometry and the amplitude of the coseismic slip. The model suggests ∼300 mm slip on an 18 km long blind fault composed of two pieces. The intersection of the fault plane with the Earth surface is located in the Fili mountain with a ∼N120° orientation.
Pitfalls in GPR data interpretation: Differentiating stratigraphy and buried objects from periodic antenna and target effects
Authors: Radzevicius Stanley J. , Guy Erich D. , Daniels Jeffrey J.
Published: February 2004

Periodic events in ground penetrating radar (GPR) data may result from antenna and target effects rather than reflections from geologic features. One of the most common pitfalls in GPR data interpretation is to identify each event on a radar cross‐section as scattering from a discrete horizon, without considering other possible sources of these events. Soil electrical properties and surface roughness affect ground penetrating radar antenna radiation and waveform characteristics. An impedance mismatch occurs over soils with electrical properties different than those for which the antennas were designed to perform optimally over, and results in periodic ring‐down, which can be misinterpreted as stratigraphy or multiple reflections. In addition, target resonance can introduce additional periodic features that can lead to misinterpretation in regards to the number of targets present. Co‐pole and cross‐pole antenna configurations can be combined with polarization dependent scattering characteristics of subsurface objects to recognize and reduce antenna ring‐down for improved imaging and interpretation.
The characterization of landslide size distributions
Authors: Stark Colin P. , Hovius Niels
Published: February 2004

Landslide size distributions generally exhibit power‐law scaling over a limited scale range. The range is set by the mapping resolution, by the number of observed events, and by the slope failure process itself. This property of self‐similarity is an important insight into the physics of hillslope failure. Typically, however, a large proportion of the landslide data does not fit a simple power law. These data are always ignored in order to characterize the scaling. We show that landslide data sets from New Zealand and Taiwan exhibit two scaling regimes, separated by a crossover scale that is purely an artefact of mapping resolution. Below this scale the landslide data are undersampled. We propose a general model for the size distribution of observed landslides which can account for the whole population of mapped slope failures. The model quantifies the undersampling of smaller landslides and provides an improved estimation of the power‐law scaling of larger landslides. Estimates of this scaling suggest that the area disturbed by landsliding, and perhaps the landslide sediment yield, are essentially dependent on the frequency of smaller landslides. Higher resolution landslide maps will be required in order to quantify these fluxes. Our results also indicate that the probability of extreme landslide events is less than previous studies would predict.
Estimates of geothermal gradients and heat flow from BSRs along the Western Continental Margin of India
Authors: Rao Y. Hanumantha , Subrahmanyam C. , Sharma S. R. , Rastogi A. A. , Deka B.
Published: February 2004

Geothermal gradients and heat flow were estimated from the position of Bottom Simulating Reflectors (BSRs) identified on seismic reflection sections from the Western Continental Margin of India (WCMI). The estimated geothermal gradients along the WCMI range between 35 to 65 °C/km and heat flow varies from 50 to 130 mW/m². The geothermal gradient structure of WCMI inferred from the BSRs, shows a landward decrease in heat flow coupled with a high heat flow zone in the north and relatively low heat flow zone at the southern end. A decrease of sediment thickness proximal to the ocean/continent boundary could explain these high heat flow distribution trends west of the Laccadive ridge. The Bombay‐Saurashtra offshore region emerges as a zone of high heat flow and its proximity to the Deccan volcanic activity at 65 Ma needs further study.
Paleohydrogeology at 5‐ to 50‐kilometer depths of accretionary prisms: The Franciscan Complex, California
Authors: Sadofsky Seth J. , Bebout Gray E.
Published: February 2004

High‐P/T metamorphism of the Franciscan Complex represents evolving subduction‐zone processes at depth. Varied peak P‐T conditions (100–350°C and 3–10 kbar) promote differences in porosity, permeability, and rheology, potentially impacting fluid mobility and entrainment of seawater. Local‐scale isotopic buffering of CaCO3 veins by exchange with adjacent host‐rocks obscures assessment of fluid sources (most vein δ13CVPDB=−11.0 to −3.0‰, δ18OVSMOW=+12.0 to +18.5‰). Some veins with elevated δ18O (relative to rock‐buffered values) may reflect up‐dip flow of H2O released at greater depths. Differences in vein δ13C in adjacent coherent greywacke and shaley mélange zones of lower‐grade rocks may be due to preferential infiltration of mélange zones by deeply derived CH4‐bearing fluids or to vein formation over a range of T. Calculated fluid‐δ18O for lower‐T veins spans the range of fluids venting in active accretionary prisms and producing forearc serpentinite seamounts. Calcite cement in the Coastal Belt is absent in higher‐grade rocks, suggesting that cement is lost to decarbonation reactions like those invoked to elevate fluid alkalinity in Marianas seamounts.
On the age and morphology of the reunion event
Authors: Baksi Ajoy K. , Hoffman Kenneth A.
Published: February 2004

Data pertinent to the age of the normal polarity Reunion “Event” (or subchron), and whether it is a split feature, are critically examined. New 40Ar/39Ar age determinations on lavas from two sequences on Reunion Island, along with available potassium‐argon (K‐Ar), 40Ar/39Ar and astrochronological analyses, indicate an age of 2.15–2.13 Ma. Although the possibility cannot be completely ruled out, at present there is no convincing evidence supporting the contention of a short duration of normal polarity some 70–90 kyr later.
Soil diffuse degassing and thermal energy fluxes from the Southern Lakki Plain, Nisyros (Greece)
Authors: Brombach Tatjana , Hunziker Johannes C. , Chiodini Giovanni , Cardellini Carlo , Marini Luigi
Published: February 2004

Two diffuse soil CO2 flux surveys from the southern Lakki plain show that CO2 is mainly released from the hydrothermal explosion craters. The correspondence between high CO2 fluxes and elevated soil temperatures suggests that a flux of hot hydrothermal fluids ascends towards the surface. Steam mostly condenses near the surface and the heat given off is conductively transferred to the atmosphere through the soil, accompanied by a large CO2 flux. It was calculated, that 68 t d−1 of hydrothermal CO2 are released through the total surveyed area of ∼1.3 km². Admitting that a steam flux of 2200 t d−1 accompanies this CO2 flux, the thermal energy released through steam condensation amounts to 58 MW.