bluuebunny/crossref_metadata_2025 huggingface dataset:
Stationary magnetospheric convection on November 24, 1981. 1. A case study of "pressure gradient/minimum-B" auroral arc generation
Authors: Galperin Y. I. , Bosqued J. M.
Published: June 2005

Abstract. We present two case studies in the night and evening sides of the auroral oval, based on plasma and field measurements made at low altitudes by the AUREOL-3 satellite, during a long period of stationary magnetospheric convection (SMC) on November 24, 1981. The basic feature of both oval crossings was an evident double oval pattern, including (1) a weak arc-type structure at the equatorial edge of the oval/polar edge of the diffuse auroral band, collocated with an upward field-aligned current (FAC) sheet of \\sim1.0 µA m-2, (2) an intermediate region of weaker precipitation within the oval, (3) a more intense auroral band at the polar oval boundary, and (4) polar diffuse auroral zone near the polar cap boundary. These measurements are compared with the published magnetospheric data during this SMC period, accumulated by Yahnin et al. and Sergeev et al., including a semi-empirical radial magnetic field profile BZ in the near-Earth neutral sheet, with a minimum at about 10-14 RE. Such a radial BZ profile appears to be very similar to that assumed in the "minimum- B/cross-tail line current" model by Galperin et al. (GVZ92) as the "root of the arc", or the arc generic region. This model considers a FAC generator mechanism by Grad-Vasyliunas-Boström-Tverskoy operating in the region of a narrow magnetic field minimum in the near-Earth neutral sheet, together with the concept of ion non-adiabatic scattering in the "wall region". The generated upward FAC branch of the double sheet current structure feeds the steady auroral arc/inverted-V at the equatorial border of the oval. When the semi-empirical BZ profile is introduced in the GVZ92 model, a good agreement is found between the modelled current and the measured characteristics of the FACs associated with the equatorial arc. Thus the main predictions of the GVZ92 model concerning the "minimum-B" region are consistent with these data, while some small-scale features are not reproduced. Implications of the GVZ92 model are discussed, particularly concerning the necessary conditions for a substorm onset that were not fulfilled during the SMC period.Key words. Magnetospheric physics (auroral phenomena; magnetospheric configuration and dynamics; plasma sheet).
Pc3 pulsations during variable IMF conditions
Authors: Villante U. , Lepidi S. , Francia P. , Vellante M. , Meloni A. , Lepping R. P. , Mariani F.
Published: June 2005

Abstract. Pc3 geomagnetic field fluctuations detected at low latitude (L'Aquila, Italy) during the passage of a high velocity solar wind stream, characterized by variable interplanetary magnetic field conditions, are analyzed. Higher frequency resonant fluctuations and lower frequency phenomena are simultaneously observed; the intermittent appearance and the variable frequency of the longer period modes can be well interpreted in terms of the variable IMF elements; moreover their polarization characteristics are consistent with an origin related to external waves propagating in antisunward direction. A comparison with simultaneous observations performed at Terra Nova Bay (Antarctica) provides additional evidence for a clear relationship between the IMF and Pc3 pulsations also at very high latitudes.Key words. Magnetospheric physics (MHD waves and instabilities; solar wind · magnetosphere interactions)
Auroral electrojet dynamics during magnetic storms, connection with plasma precipitation and large-scale structure of the magnetospheric magnetic field
Authors: Feldstein Y. I. , Gromova L. I. , Grafe A. , Meng C.-I. , Kalegaev V. V. , Alexeev I. I. , Sumaruk Y. P.
Published: June 2005

Abstract. Effect of the equatorward shift of the eastward and westward electrojets during magnetic storms main phase is analyzed based on the meridional chains of magnetic observatories EISCAT and IMAGE and several Russian observatories (geomagnetic longitude ~110°, corrected geomagnetic latitudes 74°F 51°.) Magnetic storms of various Dst index intensity where the main phase falls on 1000 UT - 2400 UT interval were selected so that one of the observatory chains was located in the afternoon - near midnight sector of MLT. The eastward electrojet center shifts equatorward with Dst intensity increase: when Dst ~ - 50 nT the electrojet center is located at F ~ 62°, when Dst ~ -300 nT it is placed at F ~54°. The westward electrojet center during magnetic storms main phase for intervals between substorms shifts equatorward with Dst increase: at F~ 62° when Dst ~ -100 nT and at F ~ 55° when Dst ~ -300 nT. During substorms within the magnetic storms intervals the westward electrojet widens poleward covering latitudes F~ 64°- 65°. DMSP (F08, F10 and F11) satellite observations of auroral energy plasma precipitations at upper atmosphere altitudes were used to determine precipitation region structure and location of boundaries of various plasma domains during magnetic storms on May 10-11, 1992, February 5-7 and February 21-22, 1994. Interrelationships between center location, poleward and equatorward boundaries of electrojets and characteristic plasma regions are discussed. The electrojet center, poleward and equatorward boundaries along the magnetic observatories meridional chain were mapped to the magnetosphere using the geomagnetic field paraboloid model. The location of auroral energy oxygen ion regions in the night and evening magnetosphere is determined. Considerations are presented on the mechanism causing the appearance in the inner magnetosphere during active intervals of magnetic storms of ions with energy of tens KeV. In the framework of the magnetospheric magnetic field paraboloid model the influence of the ring current and magnetospheric tail plasma sheet currents on large-scale magnetosphere structure is considered.Key words. Ionosphere (particle precipitation) · Magnetospheric physics (current systems; magnetospheric configuration and dynamics).
Anisotropy of ionospheric irregularities determined from the amplitude of satellite signals at a single receiver
Authors: Tereshchenko E. D. , Khudukon B. Z. , Kozlova M. O. , Nygrén T.
Published: June 2005

Abstract. A new method of determining the anisotropy parameters of small-scale irregularities in the ionospheric F region is presented and experimental results are shown. The method is based on observations of amplitude fluctuations of radio waves transmitted by satellites flying above the F region. In practice, Russian navigational satellites are used and both the amplitude and the phase of the received signal is measured on the ground level. The method determines both the field-aligned anisotropy and the field-perpendicular anisotropy and orientation of the spatial spectrum of the irregularities, assuming that the contours of constant power have an elliptic shape. A possibility of applying the method to amplitude tomography is also discussed. Using a chain of receivers on the ground level, one could locate the regions of small-scale irregularities as well as determine their relative intensities. Then the large-scale background structures could be mapped simultaneously by means of ordinary ray tomography using the phase observations, and therefore the relations of small-scale and large-scale structures could be investigated.Key words. Ionosphere (auroral ionosphere; ionospheric irregularities; instruments and techniques)
Nongyrotropic particle distributions in space plasmas
Authors: Motschmann U. , Glassmeier K. H. , Brinca A. L.
Published: June 2005

Abstract. In nonstationary, strong inhomogeneous or open plasmas particle orbits are rather complicated. If the nonstationary time scale is smaller than the gyration period, if the inhomogeneity scale is smaller than the gyration radius, i.e. at magnetic plasma boundaries, or if the plasma has sources and sinks in phase space, then nongyrotropic distribution functions occur. The stability of such plasma configurations is studied in the framework of linear dispersion theory. In an open plasma nongyrotropy drives unstable waves parallel and perpendicular to the background magnetic field, whereas in the gyrotropic limit the plasma is stable. In nonstationary plasmas nongyrotropy drives perpendicular unstable waves only. Temporal modulation couples a seed mode with its side lobes and thus it renders unstable wave growth more difficult. As an example of an inhomogeneous plasma a magnetic halfspace is discussed. In a layer with thickness of the thermal proton gyroradius a nongyrotropic distribution is formed which may excite unstable parallel and perpendicular propagating waves.Key words. Interplanetary physics (plasma waves and turbulence) · Ionosphere (plasma waves and instabilities) · Magnetospheric physics (plasma waves and instabilities)
A flux transfer event observed at the magnetopause by the Equator-S spacecraft and in the ionosphere by the CUTLASS HF radar
Authors: Neudegg D. A. , Yeoman T. K. , Cowley S. W. H. , Provan G. , Haerendel G. , Baumjohann W. , Auster U. , Fornacon K.-H. , Georgescu E. , Owen C. J.
Published: June 2005

Abstract. Observations of a flux transfer event (FTE) have been made simultaneously by the Equator-S spacecraft near the dayside magnetopause whilst corresponding transient plasma flows were seen in the near-conjugate polar ionosphere by the CUTLASS Finland HF radar. Prior to the occurrence of the FTE, the magnetometer on the WIND spacecraft ~226 RE upstream of the Earth in the solar wind detected a southward turning of the interplanetary magnetic field (IMF) which is estimated to have reached the subsolar magnetopause ~77 min later. Shortly afterwards the Equator-S magnetometer observed a typical bipolar FTE signature in the magnetic field component normal to the magnetopause, just inside the magnetosphere. Almost simultaneously the CUTLASS Finland radar observed a strong transient flow in the F region plasma between 78° and 83° magnetic latitude, near the ionospheric region predicted to map along geomagnetic field lines to the spacecraft. The flow signature (and the data set as a whole) is found to be fully consistent with the view that the FTE was formed by a burst of magnetopause reconnection.Key words. Interplanetary physics (ionosphere-magnetosphere interaction) · Magnetospheric physics (magnetopause · cusp · and boundary layers; solar wind-magnetosphere interactions)
A new method to reconstruct the ionospheric convection patterns in the polar cap
Authors: Israelevich P. L. , Ershkovich A. I.
Published: June 2005

Abstract. A new method to reconstruct the instantaneous convection pattern in the Earth's polar ionosphere is suggested. Plasma convection in the polar cap ionosphere is described as a hydrodynamic incompressible flow. This description is valid in the region where the electric currents are field aligned (and hence, the Lorentz body force vanishes). The problem becomes two-dimensional, and may be described by means of stream function. The flow pattern may be found as a solution of the boundary value problem for stream function. Boundary conditions should be provided by measurements of the electric field or plasma velocity vectors along the satellite orbits. It is shown that the convection pattern may be reconstructed with a reasonable accuracy by means of this method, by using only the minimum number of satellite crossings of the polar cap. The method enables us to obtain a reasonable estimate of the convection pattern without knowledge of the ionospheric conductivity.Key words. Ionosphere (modelling and forecasting; plasma convection; polar ionosphere)
ELF wave generation in the ionosphere using pulse modulated HF heating: initial tests of a technique for increasing ELF wave generation efficiency
Authors: Barr R. , Stubbe P. , Rietveld M. T.
Published: June 2005

Abstract. This paper describes the results of a preliminary study to determine the effective heating and cooling time constants of ionospheric currents in a simulated modulated HF heating, `beam painting' configuration. It has been found that even and odd harmonics of the fundamental ELF wave used to amplitude modulate the HF heater are sourced from different regions of the ionosphere which support significantly different heating and cooling time constants. The fundamental frequency and its odd harmonics are sourced in a region of the ionosphere where the heating and cooling time constants are about equal. The even harmonics on the other hand are sourced from regions of the ionosphere characterised by ratios of cooling to heating time constant greater than ten. It is thought that the even harmonics are sourced in the lower ionosphere (around 65 km) where the currents are much smaller than at the higher altitudes around 78 km where the currents at the fundamental frequency and odd harmonics maximise.Key words. Electromagnetics (antennae) · Ionosphere (active experiments) · Radio science (non linear phenomena)
Correlations between SAR arc intensity and solar and geomagnetic activity
Authors: Lobzin V. V. , Pavlov A. V.
Published: June 2005

Abstract. We present a study of statistical relationships between SAR arc intensities acquired by the Pacific Northwest Laboratory Photometer Network during 1978-1988 and solar and geomagnetic activity indices Dst, F10.7, and Kp by use of the method of multiple regression analysis. We found significant correlations between intensity and all of the indices involved. In the present work we show for the first time that the partial correlation coefficients depend on the time offset, t, between the time of SAR arc intensity observations and the onset of the geomagnetic storm recovery phase, with the largest correlations being observed when  8 h < t < 16 h. It is also shown that there are significant differences between partial correlation coefficients calculated for SAR arcs associated with strong  (Dstmin< -100 nT) and weak (Dstmin>-100 nT) geomagnetic storms. We observe also that the multiple correlation coefficients for strong storms are much larger than for weak ones. We found that the variations in the electron temperature, Te, in the SAR arc region are not mainly produced by variations in the electron density of the ionosphere but are strongly driven by the additional heating of the electron gas due to an interaction of the ring current ions and the plasmaspheric electrons. As a result, variations of Te in the SAR arc region with characteristic time scales from several minutes to several hours are stipulated by time variations of ring current parameters.Key words. Atmospheric composition and structure (airglow and aurora) · Ionosphere (ionosphere · atmosphere interactions; ionosphere · magnetosphere interactions)
The influence of global warming in Earth rotation speed
Authors: Abarca del Rio R.
Published: June 2005

Abstract. The tendency of the atmospheric angular momentum (AAM) is investigated using a 49-year set of monthly AAM data for the period January 1949-December 1997. This data set is constructed with zonal wind values from the reanalyses of NCEP/NCAR, used in conjunction with a variety of operationally produced AAM time series with different independent sources and lengths over 1976-1997. In all the analyzed AAM series the linear trend is found to be positive. Since the angular momentum of the atmosphere-earth system is conserved this corresponds to a net loss of angular momentum by the solid earth, therefore decreasing the Earth rotation speed and increasing the length of day (LOD). The AAM rise is significant to the budget of angular momentum of the global atmosphere-earth system; its value in milliseconds/century (ms/cy) is +0.56 ms/cy, corresponding to one-third of the estimated increase in LOD (+1.7 ms/cy). The major contribution to this secular trend in AAM comes from the equatorial Tropopause. This is consistent with results from a previous study using a simplified aqua-planet model to investigate the AAM variations due to near equatorial warming conditions. During the same time interval, 1949-1997, the global marine + land-surface temperature increases by about 0.79 °C/cy, showing a linear correspondence between surface temperature increase and global AAM of about 0.07 ms per 0.1 °C. These results imply that atmospheric angular momentum may be used as an independent index of the global atmosphere's dynamical response to the greenhouse forcing, and as such, the length of day may be used as an indirect indicator of global warming.Key words. Meteorology and atmospheric dynamics (general circulation) · Geodesy
Post-noon two-minute period pulsating aurora and their relationship to the dayside convection pattern
Authors: Milan S. E. , Yeoman T. K. , Lester M. , Moen J. , Sandholt P. E.
Published: June 2005

Abstract. Poleward-moving auroral forms, as observed by meridian-scanning photometers, in the vicinity of the cusp region are generally assumed to be the optical signature of flux transfer events. Another class of quasi-continuous, short period (1-2 min) wave-like auroral emission has been identified, closely co-located with the convection reversal boundary in the post-noon sector, which is similar in appearance to such cusp aurora. It is suggested that these short period wave-like auroral emissions, the optical signature of boundary plasma sheet precipitation in the region 1 field-aligned current system, are associated with ULF magnetohydrodynamic wave activity, which is observed simultaneously by ground magnetometer stations. This association with ULF wave activity is strengthened by the observation of several harmonic frequencies in the pulsation spectrum, each an overtone of the fundamental standing wave resonance frequency.Key words. Magnetospheric physics (auroral phenomena; magnetopause · cusp · and boundary layers; MHD waves and instabilities)
A comparison of velocity measurements from the CUTLASS Finland radar and the EISCAT UHF system
Authors: Davies J. A. , Lester M. , Milan S. E. , Yeoman T. K.
Published: June 2005

Abstract. The CUTLASS Finland radar, which comprises an integral part of the SuperDARN system of HF coherent radars, provides near continuous observations of high-latitude plasma irregularities within a field-of-view which extends over some four million square kilometres. Within the Finland radar field-of-view lie both the EISCAT mainland and EISCAT Svalbard incoherent scatter radar facilities. Since the CUTLASS Finland radar commenced operation, in February 1995, the mainland EISCAT UHF radar has been run in common programme 1 and 2 modes for a total duration exceeding 1000 h. Simultaneous and spatially coincident returns from these two radars over this period provide the basis for a comparison of irregularity drift velocity and F-region ion velocity. Initial comparison is limited to velocities from four intervals of simultaneous radar returns; intervals are selected such that they exhibit a variety of velocity signatures including that characteristic of the convection reversal and a rapidly fluctuating velocity feature. Subsequent comparison is on a statistical basis. The velocities measured by the two systems demonstrate reasonable correspondence over the velocity regime encountered during the simultaneous occurrence of coherent and incoherent scatter; differences between the EISCAT UHF measurements of F-region ion drift and the irregularity drift velocities from the Finland radar are explained in terms of a number of contributing factors including contamination of the latter by E-region echoes, a factor which is investigated further, and the potentially deleterious effect of discrepant volume and time sampling intervals.Key words. Ionosphere (ionospheric irregularities; plasma convection)
Effect of suprathermal electrons on the intensity and Doppler frequency of electron plasma lines
Authors: Guio P. , Lilensten J.
Published: June 2005

Abstract. In an incoherent scattering radar experiment, the spectral measurement of the so-called up- and downshifted electron plasma lines provides information about their intensity and their Doppler frequency. These two spectral lines correspond, in the backscatter geometry, to two Langmuir waves travelling towards and away from the radar. In the daytime ionosphere, the presence of a small percentage of photoelectrons produced by the solar EUV of the total electron population can excite or damp these Langmuir waves above the thermal equilibrium, resulting in an enhancement of the intensity of the lines above the thermal level. The presence of photo-electrons also modifies the dielectric response function of the plasma from the Maxwellian and thus influences the Doppler frequency of the plasma lines. In this paper, we present a high time-resolution plasma-line data set collected on the Eiscat VHF radar. The analysed data are compared with a model that includes the effect of a suprathermal electron population calculated by a transport code. By comparing the intensity of the analysed plasma lines data to our model, we show that two sharp peaks in the electron suprathermal distribution in the energy range 20-30 eV causes an increased Landau damping around 24.25 eV and 26.25 eV. We have identified these two sharp peaks as the effect of the photoionisation of N2 and O by the intense flux of monochromatic HeII radiation of wavelength 30.378 nm (40.812 eV) created in the chromospheric network and coronal holes. Furthermore, we see that what would have been interpreted as a mean Doppler drift velocity for a Maxwellian plasma is actually a shift of the Doppler frequency of the plasma lines due to suprathermal electrons.Key words. Ionosphere (electric fields and currents; solar radiation and cosmic ray effects)
On the variability of I(7620 Å)/I(5577 Å) in low altitude aurora
Authors: Llewellyn E. J. , Gattinger R. L. , Vallance Jones A.
Published: June 2005

Abstract. An auroral electron excitation model, combined with simple equilibrium neutral and ion chemistry models, is used to investigate the optical emission processes and height profiles of I(5577 Å) and I(7620 Å) in the 90 to 100 km altitude region. It is shown that the apparent discrepancies between ground-based and rocket-borne auroral observations of the I(7620 Å)/I(5577 Å) ratio are due to the extreme height variation of this intensity ratio in the 90 to 100 km region.Key words. Atmospheric composition and structure (airglow and aurora)
Cooling rate of thermal electrons by electron impact excitation of fine structure levels of atomic oxygen
Authors: Pavlov A. V. , Berrington K. A.
Published: June 2005

Abstract. The atomic oxygen fine structure cooling rate of thermal electrons based on new effective collision strengths for electron impact excitation of the ground-state 3P fine-structure levels in atomic oxygen have been fitted to an analytical expression which is available to the researcher for quick reference and accurate computer modeling with a minimum of calculations. We found that at the F region altitudes of the ionosphere the new cooling rate is much less than the currently used fine structure cooling rates (up to a factor of 2-4), and this cooling rate is not the dominant electron cooling process in the F region of the ionosphere at middle latitudes.Key words. Atmospheric composition and structure (thermosphere · composition and density) · Ionosphere (mid-latitude ionosphere; modelling and forecasting).
The role of bromine and chlorine chemistry for arctic ozone depletion events in Ny-Ålesund and comparison with model calculations
Authors: Martinez M. , Arnold T. , Perner D.
Published: June 2005

Abstract. During the Arctic Tropospheric Ozone Chemistry (ARCTOC) campaigns at Ny-Ålesund, Spitsbergen, the role of halogens in the depletion of boundary layer ozone was investigated. In spring 1995 and 1996 up to 30 ppt bromine monoxide were found whenever ozone decreased from normal levels of about 40 ppb. Those main trace gases and others were specifically followed in the UV-VIS spectral region by differential optical absorption spectroscopy (DOAS) along light paths running between 20 and 475 m a.s.l.. The daily variation of peroxy radicals closely followed the ozone photolysis rate J(O3(O1D)) in the absence of ozone depletion most of the time. However, during low ozone events this close correlation was no longer found because the measurement of radicals by chemical amplification (CA) turned out to be sensitive to peroxy radicals and ClOx. Large CA signals at night can sometimes definitely be assigned to ClOx and reached up to 2 ppt. Total bromine and iodine were both stripped quantitatively from air by active charcoal traps and measured after neutron activation of the samples. Total bromine increased from background levels of about 15 ppt to a maximum of 90 ppt during an event of complete ozone depletion. For the spring season a strong source of bromine is identified in the pack ice region according to back trajectories. Though biogenic emission sources cannot be completely ruled out, a primary activation of halogenides by various oxidants seems to initiate an efficient autocatalytic process, mainly driven by ozone and light, on ice and perhaps on aerosols. Halogenides residing on pack ice surfaces are continuously oxidised by hypohalogenous acids releasing bromine and chlorine into the air. During transport and especially above open water this air mixes with upper layer pristine air. As large quantities of bromine, often in the form of BrO, have been observed at polar sunrise also around Antarctica, its release seems to be a natural phenomenon. The source strength of bromine from halogen activation on the pack ice, as based on the measured inorganic bromine levels, averages about 1012 Br-atoms m-2 s-1 during sunlit periods in Arctic spring. The total source strength of inorganic bromine from sunlit polar regions may therefore amount to 30 kt y-1.Key words. Atmospheric composition and structure (troposphere · composition and chemistry; instruments and techniques)
Mountain wave motions determined by the Esrange MST radar
Authors: Réchou A. , Barabash V. , Chilson P. , Kirkwood S. , Savitskaya T. , Stebel K.
Published: June 2005

Abstract. A European campaign of ground-based radar, lidar and optical measurements was carried out during the winter of 1996/1997 (28 December-2 February) to study lee waves in the northern part of Scandinavia. The participants operated ozone lidars, backscatter lidars and MST radars at ALOMAR/Andoya and Esrange/Kiruna, and an ALIS imaging system in Kiruna. The Andoya site was generally windward of the Scandinavian mountains, the Kiruna site on the leeward side. The goal of the experiment was to examine the influence of lee waves on the formation of Polar Stratospheric Clouds (PSCs). This paper studies the radar data from MST-radar ESRAD located at Esrange [68.°N, 21.°E], i.e. in the lee of the mountains. We present three cases where strong lee waves were observed: in one case they propagated upwards to the lower stratosphere and in the other two cases they were trapped or absorbed in the troposphere. We examine the local waves and the direction and strength of the local wind using the radar, the synoptic meteorological situation using weather maps (European Meteorological Bulletin) and the synoptic stratospheric temperatures using ECMWF data. We observed that waves propagate up to the stratosphere during frontal passages. When anticyclonic ridges are present, the propagation to the stratosphere is very weak. This is due to trapping of the waves at or below the tropopause. We also show that the radar data alone can be used to characterise the different weather conditions for the three cases studied (through the variation of the height of the tropopause). The synoptic stratospheric temperatures in the three cases were similar, and were above the expected threshold for PSC formation. Lidar and visual observation of PSCs and nacreous clouds, respectively, showed that these were present only in the case when the lee waves propagated up to the lower stratosphere.Key words. Atmospheric composition and structure (aerosols and particles) · Electromagnetic (wave propa- gation) · Meteorology and atmospheric dynamics (mesoscale meteorology)
VHF radar observations of gravity waves at a low latitude
Authors: Dutta G. , Bapiraju B. , Balasubrahmanyam P. , Aleem Basha H.
Published: June 2005

Abstract. Wind observations made at Gadanki (13.5°N) by using Indian MST Radar for few days in September, October, December 1995 and January, 1996 have been analyzed to study gravity wave activity in the troposphere and lower stratosphere. Horizontal wind variances have been computed for gravity waves of period (2-6) h from the power spectral density (PSD) spectrum. Exponential curves of the form eZ/H have been fitted by least squares technique to these variance values to obtain height variations of the irregular winds upto the height of about 15 km, where Z is the height in kilometers. The value of H, the scale height, as determined from curve fitting is found to be less than the theoretical value of scale height of neutral atmosphere in this region, implying that the waves are gaining energy during their passage in the troposphere. In other words, it indicates that the sources of gravity waves are present in the troposphere. The energy densities of gravity wave fluctuations have been computed. Polynomial fits to the observed values show that wave energy density increases in the troposphere, its source region, and then decreases in the lower stratosphere.Key words. Meteorology and atmospheric dynamics (middle atmosphere dynamics; turbulence; waves and tides)
Meso-scale effects of tropical deforestation in Amazonia: preparatory LBA modelling studies
Authors: Dolman A. J. , Silva Dias M. A. , Calvet J.-C. , Ashby M. , Tahara A. S. , Delire C. , Kabat P. , Fisch G. A. , Nobre C. A.
Published: June 2005

Abstract. As part of the preparation for the Large-Scale Biosphere Atmosphere Experiment in Amazonia, a meso-scale modelling study was executed to highlight deficiencies in the current understanding of land surface atmosphere interaction at local to sub-continental scales in the dry season. Meso-scale models were run in 1-D and 3-D mode for the area of Rondonia State, Brazil. The important conclusions are that without calibration it is difficult to model the energy partitioning of pasture; modelling that of forest is easier due to the absence of a strong moisture deficit signal. The simulation of the boundary layer above forest is good, above deforested areas (pasture) poor. The models' underestimate of the temperature of the boundary layer is likely to be caused by the neglect of the radiative effects of aerosols caused by biomass burning, but other factors such as lack of sufficient entrainment in the model at the mixed layer top may also contribute. The Andes generate patterns of subsidence and gravity waves, the effects of which are felt far into the Rondonian area The results show that the picture presented by GCM modelling studies may need to be balanced by an increased understanding of what happens at the meso-scale. The results are used to identify key measurements for the LBA atmospheric meso-scale campaign needed to improve the model simulations. Similar modelling studies are proposed for the wet season in Rondonia, when convection plays a major role.Key words. Atmospheric composition and structure (aerosols and particles; biosphere-atmosphere interactions) · Meterology and atmospheric dynamics (mesoscale meterology)
Compressional wave events in the dawn plasma sheet observed by Interball-1
Authors: Verkhoglyadova O. , Agapitov A. , Andrushchenko A. , Ivchenko V. , Romanov S. , Yermolaev Yu.
Published: June 2005

Abstract. Compressional waves with periods greater than 2 min (about 10-30 min) at low geomagnetic latitudes, namely compressional Pc5 waves, are studied. The data set obtained with magnetometer MIF-M and plasma analyzer instrument CORALL on board the Interball-1 are analyzed. Measurements performed in October 1995 and October 1996 in the dawn plasma sheet at -30 RE ≤ XGSM and |ZGSM| ≤ 10 RE are considered. Anti-phase variations of magnetic field and ion plasma pressures are analyzed by searching for morphological similarities in the two time series. It is found that longitudinal and transverse magnetic field variations with respect to the background magnetic field are of the same order of magnitude. Plasma velocities are processed for each time period of the local dissimilarity in the pressure time series. Velocity disturbances occur mainly transversely to the local field line. The data reveal the rotation of the velocity vector. Because of the field line curvature, there is no fixed position of the rotational plane in the space. These vortices are localized in the regions of anti-phase variations of the magnetic field and plasma pressures, and the vortical flows are associated with the compressional Pc5 wave process. A theoretical model is proposed to explain the main features of the nonlinear wave processes. Our main goal is to study coupling of drift Alfven wave and magnetosonic wave in a warm inhomogeneous plasma. A vortex is the partial solution of the set of the equations when the compression is neglected. A compression effect gives rise to a nonlinear soliton-like solution.Key words. Magnetosphere physics (magnetotail) · Space plasma physics (kinetic and MHD theory; non-linear phenomena)
Late spring ultraviolet levels over the United Kingdom and the link to ozone
Authors: Austin J. , Driscoll C. M. H. , Farmer S. F. G. , Molyneux M. J.
Published: June 2005

Abstract. Erythemally-weighted ultraviolet (UVery) levels measured over southern England, during anticyclonic weather between 30 April and 2 May, 1997, were almost 50 higher than normally expected for clear skies and were similar to mid-summer values for the first time since measurements began in 1990. Investigation of this episode suggests that a combination of both meteorological and chemical effects were responsible for generating record low ozone amounts for the time of year. Further, comparisons between the A band ultraviolet (315 to 400 nm wavelength) amounts, and radiative calculations confirm that the high UVery was primarily due to the reduction in total ozone. These results are contrasted with a similar period for 1998, in which near climatological ozone amounts were measured. The prospects for enhanced UVery levels in future years are briefly reviewed in the light of expected increases in stratospheric halogen levels and greenhouse gases.Key words. Atmospheric composition and structure (middle atmosphere · composition and chemistry) · Meterology and atmospheric dynamics (middle atmosphere dynamics; radiative processes)
A distribution law for relative humidity in the upper troposphere and lower stratosphere derived from three years of MOZAIC measurements
Authors: Gierens K. , Schumann U. , Helten M. , Smit H. , Marenco A.
Published: June 2005

Abstract. Data from three years of MOZAIC measurements made it possible to determine a distribution law for the relative humidity in the upper troposphere and lower stratosphere. Data amounting to 13.5% of the total were obtained in regions with ice supersaturation. Troposphere and stratosphere are distinguished by an ozone concentration of 130 ppbv as threshold. The probability of measuring a certain amount of ice supersaturation in the troposphere decreases exponentially with the degree of ice supersaturation. The probability of measuring a certain relative humidity in the stratosphere (both with respect to water and ice) decreases exponentially with the relative humidity. A stochastic model that naturally leads to the exponential distribution is provided. Mean supersaturation in the troposphere is about 15%, whereas ice nucleation requires 30% supersaturation on the average. This explains the frequency of regions in which aircraft induce persistent contrails but which are otherwise free of clouds. Ice supersaturated regions are 3-4 K colder and contain more than 50% more vapour than other regions in the upper troposphere. The stratospheric air masses sampled are dry, as expected, having mean relative humidity over water of 12% and over ice of 23%, respectively. However, 2% of the stratospheric data indicate ice supersaturation. As the MOZAIC measurements have been obtained on commercial flights mainly between Europe and North America, the data do not provide a complete global picture, but the exponential character of the distribution laws found is probably valid globally. Since water vapour is the most important greenhouse gas and since it might enhance the anthropogenic greenhouse effects via positive feedback mechanisms, it is important to represent its distribution correctly in climate models. The discovery of the distribution law of the relative humidity makes possible simple tests to show whether the hydrological cycle in climate models is represented in an adequate way or not.Key words. Atmospheric composition and structure (troposphere · composition and chemistry)
Letter to the Editor: First direct observations of the reduced striations at pump frequencies close to the electron gyroharmonics
Authors: Honary F. , Robinson T. R. , Wright D. M. , Stocker A. J. , Rietveld M. T. , McCrea I.
Published: June 2005

Abstract. It is well known that the ionospheric plasma response to high-power HF radio waves changes drastically as the heater frequency approaches harmonics of the electron gyrofrequency. These include changes in the spectrum of the stimulated electromagnetic emission, reduction in the anomalous absorption of low-power diagnostic waves propagating through the heated volume, and reduction in the large scale F-region heating. Theoretical models as well as previous experimental evidence point towards the absence of small-scale field-aligned plasma density irregularities at pump frequencies close to electron gyroharmonics as the main cause of these changes. Results presented in this paper are the first direct observations of the reduced striations at the 3rd gyroharmonic made by the CUTLASS radar. In addition, simultaneous EISCAT observations have revealed that the "enhanced ion-line" usually present in the EISCAT ion-line spectrum during the first few seconds after heater switch on, persisted at varying strengths while the heater was transmitting at frequencies close to the 3rd electron gyroharmonics.Key words. Ionosphere (active experiments; ionospheric irregularities) · Radio science (ionospheric physics)
Letter to the Editor: Spatial and seasonal variations of the foF2 long-term trends
Authors: Danilov A. D. , Mikhailov A. V.
Published: June 2005

Abstract. Using a method suggested by the authors earlier, the long-term trends of the F2-layer critical frequency, foF2 are derived for a set of ionospheric stations with a wide latitudinal and longitudinal coverage. All the trends are found to be negative. A pronounced dependence on geomagnetic latitude is found, the trend magnitude increasing with the latter. No globe scale longitudinal effect in trends is detected. For the majority of the stations there is also a pronounced seasonal effect, the trend magnitude being higher in summer than in winter.Key words. Ionosphere (ionospheric disturbances; mid-latitude ionosphere)
Collisionless electrons in a thin high Mach number shock: dependence on angle and b
Authors: Gedalin M. , Griv E.
Published: June 2005

Abstract. It is widely believed that electron dynamics in the shock front is essentially collisionless and determined by the quasistationary magnetic and electric fields in the shock. In thick shocks the electron motion is adiabatic: the magnetic moment is conserved throughout the shock and v2^ ∝ B. In very thin shocks with large cross-shock potential (the last feature is typical for shocks with strong electron heating), electrons may become demagnetized (the magnetic moment is no longer conserved) and their motion may become nonadiabatic. We consider the case of substantial demagnetization in the shock profile with the small-scale internal structure. The dependence of electron dynamics and downstream distributions on the angle between the shock normal and upstream magnetic field and on the upstream electron temperature is analyzed. We show that demagnetization becomes significantly stronger with the increase of obliquity (decrease of the angle) which is related to the more substantial influence of the inhomogeneous parallel electric field. We also show that the demagnetization is stronger for lower upstream electron temperatures and becomes less noticeable for higher temperatures, in agreement with observations. We also show that demagnetization results, in general, in non-gyrotropic down-stream distributions.Key words. Interplanetary physics (interplanetary shocks; planetary bow shocks)
Two types of ELF hiss observed at Varanasi, India
Authors: Singh D. K. , Singh Ashok K. , Patel R. P. , Singh R. P. , Singh A. K.
Published: June 2005

Abstract. The morphology of ELF hiss events observed at low-latitude ground station Varanasi (L = 1.07, geomagnetic latitude 14°55'N) are reported, which consist of two types: (1) events which propagated in ducted mode along the geomagnetic field line corresponding to observing station Varanasi and (2) events which propagated in ducted mode along higher L-values (L = 4–6), after reaching the lower edge of ionosphere excite the Earth-ionosphere wave guide and propagate towards equator to be received at Varanasi. To understand the generation mechanism of ELF hiss, incoherent Cerenkov radiated power from the low latitude and middle latitude plasmasphere are evaluated. Considering this estimated power as an input for wave amplification through wave-particle interaction, the growth rate and amplification factor is evaluated which is too small to explain the observed wave intensity. It is suggested that some non-linear mechanism is responsible for the generation of ELF hiss.Key words. Ionosphere (equatorial ionosphere; ionosphere · magnetosphere interactions; wave · particle interactions)
Ring current influence on auroral electrojet predictions
Authors: Gleisner H. , Lundstedt H.
Published: June 2005

Abstract. Geomagnetic storms and substorms develop under strong control of the solar wind. This is demonstrated by the fact that the geomagnetic activity indices Dst and AE can be predicted from the solar wind alone. A consequence of the strong control by a common source is that substorm and storm indices tend to be highly correlated. However, a part of this correlation is likely to be an effect of internal magnetospheric processes, such as a ring-current modulation of the solar wind-AE relation. The present work extends previous studies of nonlinear AE predictions from the solar wind. It is examined whether the AE predictions are modulated by the Dst index.This is accomplished by comparing neural network predictions from Dst and the solar wind, with predictions from the solar wind alone. Two conclusions are reached: (1) with an optimal set of solar-wind data available, the AE predictions are not markedly improved by the Dst input, but (2) the AE predictions are improved by Dst if less than, or other than, the optimum solar-wind data are available to the net. It appears that the solar wind-AE relation described by an optimized neural net is not significantly modified by the magnetosphere's Dst state. When the solar wind alone is used to predict AE, the correlation between predicted and observed AE is 0.86, while the prediction residual is nearly uncorrelated to Dst. Further, the finding that Dst can partly compensate for missing information on the solar wind, is of potential importance in operational forecasting where gaps in the stream of real time solar-wind data are a common occurrence.Key words. Magnetospheric physics (solar wind · magnetosphere interactions; storms and substorms)
Non-magnetic aspect sensitive auroral echoes from the lower E region observed at 50 MHz
Authors: Rüster R. , Schlegel K.
Published: June 2005

Abstract. Backscatter from E-region irregularities was observed at aspect angles close to 90° (almost parallel to the direction of the magnetic field) using the ALOMAR SOUSY radar at Andoya/Norway. Strong electric fields and increased E-region electron temperatures simultaneously measured with the incoherent scatter facility EISCAT proved that the Farley-Buneman plasma instability was excited. In addition, strong particle precipitation was present as inferred from EISCAT electron densities indicating that the gradient drift instability may have been active, too. Backscatter at such large aspect angles was not expected and has not been observed before. The characteristics of the observed echoes, however, are in many aspects completely different from usual auroral radar results: the Doppler velocities are only of the order of 10 m/s, the half-width of the spectra is around 5 m/s, the echoes originate at altitudes well below 100 km, and they seem to be not aspect-sensitive with respect to the magnetic field direction. We, therefore, conclude that the corresponding irregularities are not caused by the mentioned instabilities and that other mechanism have to be invoked.Key words. Ionosphere (plasma waves and instabilities; ionosphere irregularities; particle precipitaion) · Meteorology and atmospheric dynamics (middle atmosphere dynamics)
On the possible role of cusp/cleft precipitation in the formation of polar-cap patches
Authors: Walker I. K. , Moen J. , Kersley L. , Lorentzen D. A.
Published: June 2005

Abstract. The work describes experimental observations of enhancements in the electron density of the ionospheric F-region created by cusp/cleft particle precipitation at the dayside entry to the polar-cap convection flow. Measurements by meridian scanning photometer and all-sky camera of optical red-line emissions from aurora are used to identify latitudinally narrow bands of soft-particle precipitation responsible for structured enhancements in electron density determined from images obtained by radio tomography. Two examples are discussed in which the electron density features with size scales and magnitudes commensurate with those of patches are shown to be formed by precipitation at the entry region to the anti-sunward flow. In one case the spectrum of the incoming particles results in ionisation being created, for the most part below 250 km, so that the patch will persist only for minutes after convecting away from the auroral source region. However in a second example, at a time when the plasma density of the solar wind was particularly high, a substantial part of the particle-induced enhancement formed above 250 km. It is suggested that, with the reduced recombination loss in the upper F-region, this structure will retain form as a patch during passage in the anti-sunward flow across the polar cap.Key words. Ionosphere (ionospheric irregularities; particle precipitation; polar ionosphere)
Trends in the characteristics of the annual and semiannual variations observed in the radio wave absorption in the lower ionosphere
Authors: Laštovička J. , Pancheva D.
Published: June 2005

Abstract. The continuous increase in concentration of greenhouse gases in the atmosphere is expected to cool higher levels of the atmosphere. There is some direct and indirect experimental evidence of long-term trends in temperature and other parameters in the mesosphere and lower thermosphere (MLT). Here we look for long-term trends in the annual and semiannual variations of the radio wave absorption in the lower ionosphere, which corresponds to the MLT region heights. Data from central and southeastern Europe are used. A consistent tendency to a positive trend in the amplitude of the semiannual wave appears to be observed. The reality of a similar tendency in the amplitude of the annual wave is questionable in the sense that the trend in the amplitude of the annual wave is probably induced by the trend in the yearly average values of absorption. The phases of both the annual and semiannual waves display a forward tendency, i.e. shift to an earlier time in the year. A tentative interpretation of these results in terms of changes of the seasonal variation of temperature and wind at MLT heights does not contradict the trends observed in those parameters.Key words. Ionosphere (ionosphere – atmosphere interactions; mid-latitude ionosphere) · Meteorology and atmospheric dynamics (middle atmosphere dynamics)