bluuebunny/crossref_metadata_2025 huggingface dataset:
Low frequency waves in plasmas with spatially varying electron temperature
Authors: Guio P. , Børve S. , Pécseli H. L. , Trulsen J.
Published: April 2005

Abstract. Low frequency electrostatic waves are studied in magnetized plasmas with an electron temperature which varies with position in a direction perpendicular to the magnetic field. For wave frequencies below the ion cyclotron frequency, the waves need not follow any definite dispersion relation. Instead a band of phase velocities is allowed, with a range of variation depending on the maximum and minimum values of the electron temperature. Simple model equations are obtained for the general case which can be solved to give the spatial variation of a harmonically time varying potential. A simple analytical model for the phenomenon is presented and the results are supported by numerical simulations carried out in a 2½-dimensional particle-in-cell numerical simulation. We find that when the electron temperature is striated along B0 and low frequency waves (ω ≪ Ωci) are excited in this environment, then the intensity of these low frequency waves will be striated in a manner following the electron temperature striations. High frequency ion acoustic waves (ω ≫ Ωci) will on the other hand have a spatially more uniform intensity distribution.Key words: Ionosphere (plasma temperature and density) · Radio science (waves in plasma) · Space plasma physics (numerical simulation studies)
The effect of magnetic substorms on near-ground atmospheric current
Authors: Belova E. , Kirkwood S. , Tammet H.
Published: April 2005

Abstract. Ionosphere-magnetosphere disturbances at high latitudes, e.g. magnetic substorms, are accompanied by energetic particle precipitation and strong variations of the ionospheric electric fields and currents. These might reasonably be expected to modify the local atmospheric electric circuit. We have analysed air-earth vertical currents (AECs) measured by a long wire antenna at Esrange, northern Sweden during 35 geomagnetic substorms. Using superposed epoch analysis we compare the air-earth current variations during the 3 h before and after the time of the magnetic X-component minimum with those for corresponding local times on 35 days without substorms. After elimination of the average daily variation we can conclude that the effect of substorms on AEC is small but distinguishable. It is speculated that the AEC increases observed during about 2 h prior to the geomagnetic X-component minimum, are due to enhancement of the ionospheric electric field. During the subsequent 2 h of the substorm recovery phase, the difference between "substorm" and "quiet" atmospheric currents decreases. The amplitude of this "substorm" variation of AEC is estimated to be less than 50% of the amplitude of the diurnal variation in AEC during the same time interval. The statistical significance of this result was confirmed using the Van der Waerden X-test. This method was further used to show that the average air-earth current and its fluctuations increase during late expansion and early recovery phases of substorms.Key words: Ionosphere (electric fields and currents) · Magnetospheric physics (storms and substorms) · Meteorology and atmospheric dynamics (atmospheric electricity)
On the fast movements of the thermosphere: an interpretation
Authors: Lilensten J. , Amblard P. O.
Published: April 2005

Abstract. We examine the oscillations of the meridional neutral wind in the F region as seen by the EISCAT radar. We propose an interpretation in term of eddies (tourbillons) of typical size of a few tens to a few hundreds of kilometers. The observed rotation velocity is a few hundreds of meters per second. We suggest that the tourbillons are a common feature of thermospheric movements. We propose an optical experiment to check the validity of this assumption.Key words: Atmospheric composition and structure (thermosphere · composition and chemistry) · Ionosphere (ionosphere · atmosphere interactions)
Enhancement of stratospheric aerosols after solar proton event
Authors: Shumilov O. I. , Kasatkina E. A. , Henriksen K. , Vashenyuk E. V.
Published: April 2005

Abstract. The lidar measurements at Verhnetulomski observatory (68.6°N, 31.8°E) at Kola peninsula detected a considerable increase of stratospheric aerosol concentration after the solar proton event of GLE (ground level event) type on the 16/02/84. This increase was located at precisely the same altitude range where the energetic solar protons lost their energy in the atmosphere. The aerosol layer formed precipitated quickly (1–2 km per day) during 18, 19, and 20 February 1984, and the increase of R(H) (backscattering ratio) at 17 km altitude reached 40% on 20/02/84. We present the model calculation of CN (condensation nuclei) altitude distribution on the basis of an ion-nucleation mechanism, taking into account the experimental energy distribution of incident solar protons. The meteorological situation during the event was also investigated.
Application of stochastic inversion in auroral tomography
Authors: Nygrén T. , Markkanen M. , Lehtinen M. , Kaila K.
Published: April 2005

Abstract. A software package originally developed for satellite radio tomography is briefly introduced and its use in two-dimensional auroral tomography is described. The method is based on stochastic inversion, i.e. finding the most probable values of the unknown volume emission rates once the optical measurements are made using either a scanning photometer or an auroral camera. A set of simulation results is shown for a different number and separations of optical instruments at ground level. It is observed that arcs with a thickness of a few kilometers and separated by a few tens of kilometers are easily reconstructed. The maximum values of the inversion results, however, are often weaker than in the model. The most obvious reason for this is the grid size, which cannot be much smaller than the arc thickness. The grid necessarily generates a spatial averaging effect broadening the arc cross-sections and reducing the peak values. Finally, results from TV-camera observations at Tromsø and Esrange are shown. Although these sites are separated by more than 200 km, arcs close to Tromsø have been successfully reconstructed.
Pyramidal ice crystal scattering phase functions and concentric halos
Authors: Liu C. , Jonas P. R. , Saunders C. P. R.
Published: April 2005

Abstract. Phase functions have been calculated using the Monte Carlo/geometric ray tracing method for single hexagonal pyramidal ice crystals (such as solid and hollow bullets) randomly oriented in space and horizontal plane, in order to study the concentric halo formations. Results from three dimensional model calculations show that 9° halo can be as bright as the common 22° halo for pyramidal angle of 28°, and the 18°, 20°, 24° and 35° halos cannot be seen due to the strong 22° halo domination in the scattering phase function between 18° and 35°. For solid pyramidal ice crystals randomly oriented horizontally, the 35° arc can be produced and its intensity depends on the incident ray solar angle and the particle aspect ratio.
On the seasonal effect of orbital variations on the climates of the next 4000 years
Authors: Adem Julián
Published: April 2005

Abstract. A northern hemisphere thermodynamic climate model is used to compute the effect of the insolation anomalies due to orbital variations in the climates for the next 4000 years. Berger's mean monthly anomalies of insolation are used in the computations, which are carried out for 1000, 2000, 3000 and 4000 years after present (kyears AP). The numerical simulations show that in the climates of the next 4000 years, due to the effect of the orbital variations, springs and summers will be increasingly dryer and falls and winters wetter than the present climate.
Predicted signatures of pulsed reconnection in ESR data
Authors: Davis C. J. , Lockwood M.
Published: April 2005

Abstract. Early in 1996, the latest of the European incoherent-scatter (EISCAT) radars came into operation on the Svalbard islands. The EISCAT Svalbard Radar (ESR) has been built in order to study the ionosphere in the northern polar cap and in particular, the dayside cusp. Conditions in the upper atmosphere in the cusp region are complex, with magnetosheath plasma cascading freely into the atmosphere along open magnetic field lines as a result of magnetic reconnection at the dayside magnetopause. A model has been developed to predict the effects of pulsed reconnection and the subsequent cusp precipitation in the ionosphere. Using this model we have successfully recreated some of the major features seen in photometer and satellite data within the cusp. In this paper, the work is extended to predict the signatures of pulsed reconnection in ESR data when the radar is pointed along the magnetic field. It is expected that enhancements in both electron concentration and electron temperature will be observed. Whether these enhancements are continuous in time or occur as a series of separate events is shown to depend critically on where the open/closed field-line boundary is with respect to the radar. This is shown to be particularly true when reconnection pulses are superposed on a steady background rate.
Auroral-arc splitting by intrusion of a new convection channel
Authors: Frey H. U. , Haerendel G. , Buchert S. , Lanchester B. S.
Published: April 2005

Abstract. During a run of the Common Programme Three of the EISCAT radar the splitting of an auroral arc was observed by high time-resolution, ground-based cameras when the UHF radar beam was close to the arc. The evening eastward electrojet situation with a large-scale northward ionospheric electric field was disturbed by the intrusion of a convection channel with southward electric field from the east. The interaction of the new convection channel with the auroral arc caused changes in arc brightness and arc splitting, i.e. the creation of a new arc parallel to the pre-existing auroral arc. The event is described as one possibility for the creation of parallel arcs during slightly disturbed magnetic conditions far from the Harang discontinuity.
On the current-voltage relationship in auroral breakups and westwards-travelling surges
Authors: Olsson A. , Eriksson A. I. , Janhunen P.
Published: April 2005

Abstract. Auroral precipitating electrons pass through an acceleration region before entering the atmosphere. Regardless of what produces it, a parallel electric field is assumed to cause the acceleration. It is well known that from kinetic theory an expression for the corresponding upward field-aligned current can be calculated, which under certain assumptions can be linearized to j∥=KV. The K constant, referred to as the Lyons-Evans-Lundin constant, depends on the source density and thermal energy of the magnetospheric electrons; it is an important parameter in magnetosphere-ionosphere coupling models. However, the K parameter is still rather unknown, and values are found in a wide range of 10–8–10–10 S m–2. In this study, we investigated how the type of auroral structure affects the K values. We look at onset and westwards-travelling surge (WTS) events and make comparisons with earlier results from observations of more stable auroral arcs. A new analysis technique for studying those magnetospheric parameters using ground-based measurements is introduced. Electron density measurements are taken with the EISCAT radar, and through an inversion technique the flux-energy spectra are calculated. Source densities, thermal energies and potential drops are estimated from fittings of accelerated Maxwellian distributions. With this radar technique we have the possibility to study the changes of the mentioned parameters during the development of onsets and the passage of surges over EISCAT. The study indicates that the linearization of the full Knight formulation holds even for the very high potential drops and thermal temperatures found in the dynamic onset and WTS events. The values of K are found to be very low, around 10–11 S m–2 in onset cases as well as WTS events. The results may establish a new technique where ionospheric measurements are used for studying the ionosphere-magnetosphere coupling processes.
Auroral ionospheric conductivities: a comparison between experiment and modeling, and theoretical f10.7-dependent model for EISCAT and ESR
Authors: Lilensten J. , Blelly P. L. , Kofman W. , Alcaydé D.
Published: April 2005

Abstract. Several authors have published models of auroral conductances in the past. Some of them are based on theoretical approaches; others are the result of observations. The data bases for the latter models range from 8 h to 3 years of experiments. In this paper, we show the results of our own modeling, based on a coupled ki- netic/fluid approach. We compare these results to the sta- tistical model based on 3 years of experiment. We then model the auroral ionospheric conductances above EISCAT and the EISCAT-Svalbard Radar for different solar-activity indices. These modeled conductances are fitted with a simple law depending on the f10.7 index and on the solar zenith angle.
PMSE observations at three different frequencies in northern Europe during summer 1994
Authors: Bremer J. , Hoffmann P. , Manson A. H. , Meek C. E. , Rüster R. , Singer W.
Published: April 2005

Abstract. Simultaneous observations of polar mesosphere summer echoes (PMSE) have been carried out during summer 1994 in northern Norway using three radars on different frequencies: the ALOMAR SOUSY radar at Andenes on 53.5 MHz, the EISCAT VHF radar at Tromsø on 224 MHz and the MF radar at Tromsø on 2.78 MHz. During the common measuring period in July/August 1994, PMSE could be detected at 224 and 53.5 MHz, and there are strong hints that PMSE also occur at 2.78 MHz. Reliable correlations between hourly backscattered power values indicate that the PMSE structures have zonal extensions of more than 130 km and can be detected at very different scales (half wavelength) between 0.67 (EISCAT VHF radar) and 54 m (MF radar). Using the wind values derived by the MF radar it can be shown that the mesospheric wind field influences the structure of PMSE. The diurnal variation of PMSE is strongly connected with tidal-wind components, whereas spatial differences of PMSE can partly be explained by the mean wind field.
Calibration of a numerical ionospheric model with EISCAT observations
Authors: Blelly P.-L. , Lilensten J. , Robineau A. , Fontanari J. , Alcaydé D.
Published: April 2005

Abstract. A set of EISCAT UHF and VHF observations is used for calibrating a coupled fluid-kinetic model of the ionosphere. The data gathered in the period 1200- 2400 UT on 24 March 1995 had various intervals of interest for such a calibration. The magnetospheric activity was very low during the afternoon, allowing for a proper examination of a case of quiet ionospheric conditions. The radars entered the auroral oval just after 1900 UT: a series of dynamic events probably associated with rapidly moving auroral arcs was observed until after 2200 UT. No attempts were made to model the dynamical behaviour during the 1900–2200 UT period. In contrast, the period 2200–2400 UT was characterised by quite steady precipitation: this latter period was then chosen for calibrating the model during precipitation events. The adjustment of the model on the four primary parameters observed by the radars (namely the electron concentration and temperature and the ion temperature and velocity) needed external inputs (solar fluxes and magnetic activity index) and the adjustments of a neutral atmospheric model in order to reach a good agreement. It is shown that for the quiet ionosphere, only slight adjustments of the neutral atmosphere models are needed. In contrast, adjusting the observations during the precipitation event requires strong departures from the model, both for the atomic oxygen and hydrogen. However, it is argued that this could well be the result of inadequately representing the vibrational states of N2 during precipitation events, and that these factors have to be considered only as ad hoc corrections.
Errors due to random noise in velocity measurement using incoherent-scatter radar
Authors: Williams P. J. S. , Etemadi A. , McCrea I. W. , Todd H.
Published: April 2005

Abstract. The random-noise errors involved in measuring the Doppler shift of an 'incoherent-scatter' spectrum are predicted theoretically for all values of Te/Ti from 1.0 to 3.0. After correction has been made for the effects of convolution during transmission and reception and the additional errors introduced by subtracting the average of the background gates, the rms errors can be expressed by a simple semi-empirical formula. The observed errors are determined from a comparison of simultaneous EISCAT measurements using an identical pulse code on several adjacent frequencies. The plot of observed versus predicted error has a slope of 0.991 and a correlation coefficient of 99.3%. The prediction also agrees well with the mean of the error distribution reported by the standard EISCAT analysis programme.
Ionospheric composition measurement by EISCAT using a global fit procedure
Authors: Cabrit B. , Kofman W.
Published: April 2005

Abstract. The aim of this paper is to show the gain of the global fit procedure compared to the classical gate-by-gate analysis in the simultaneous determination of the temperatures and composition profiles in the lower F region. A method for the evaluation of the transition altitude and the width of the transition region between molecular and atomic ions is described in the case of the EISCAT CP1 experiment; it is validated using a simulated experiment. It is shown that alternating-code gates have to be processed together with the long-pulse gates. Our new method is applied on six days of actual data. The results obtained during quiet periods are compared with independent estimations.
Current state-of-the-art for the measurement of non-Maxwellian plasma parameters with the EISCAT UHF Facility
Authors: Hubert D. , Leblanc F. , Gaimard P.
Published: April 2005

Abstract. New results on the information that can be extracted from simulated non-Maxwellian incoherent radar spectra are presented. The cases of a pure ionosphere and of a composite ionosphere typical of a given altitude of the auroral F region are considered. In the case of a pure ionosphere of NO+ or O+ ions it has been shown that the electron temperature and the electron density can be derived from a Maxwellian analysis of radar spectra measured at aspect angles of 0° or 21° respectively; the ion temperature and ion temperature anisotropy can be derived from a non- constraining model such as the 1D Raman fitting of a complementary measurement made at an aspect angle larger than 0° for the NO+ ions, or at an aspect angle larger than 21° for the O+ ions. Moreover with such measurements at large aspect angles, the shape of the velocity ion distribution functions can simultaneously be inferred. The case of a composite ionosphere of atomic O+ and molecular NO+ ions is a difficult challenge which requires simultaneously a complementary measurement of the electron temperature to provide the ion composition and the electron density from the incoherent radar spectra at a specific aspect angle of 21°; hence, a model dependent routine is necessary to derive the ion temperatures and ion temperature anisotropies. In the case where the electron temperature is not given, a routine which depends on ion distribution models is required first: the better the ion distribution models are, the more accurately derived the plasma parameters will be. In both cases of a composite ionosphere, the 1D Raman fitting can be used to keep a check on the validity of the results provided by the ion distribution model dependent routine.
Upstream waves and field line resonances: simultaneous presence and alternation in Pc3 pulsation events
Authors: Verõ J. , Lühr H. , Vellante M. , Best I. , Střeštik J. , Miletits J. C. , Holló L. , Szendrõi J. , Zieger B.
Published: April 2005

Abstract. Based on a detailed study of Pc3 events at an array between L = 1.5 and 3 in Central Europe, the authors found quick changes between upstream waves (UW, i.e. pulsation directly driven by UW) and field line resonance (FLR, i.e. azimuthal oscillations of geomagnetic field lines). The alternation of the two types is especially characteristic (and the UW part stronger) if the interplanetary magnetic field (IMF) is highly variable. Events due to field line resonance may have a structure consisting of multiple lines with frequencies differing by about 10%, corresponding to neighbouring shells of field lines separated by about 100 km at the surface. This coincides with previous findings (about 10% at a meridional distance of 80 km). The frequency of the UW type is well correlated with the frequency of waves in the interplanetary medium. Additionally, there are signals of unidentified origin which also seem to be influenced by IMF.>Key words. Magnetosphere Physics · MHD waves and instabilities · Plasmasphere · Solar wind/magnetosphere interactions
The spatial relationship between active regions and coronal holes and the occurrence of intense geomagnetic storms throughout the solar activity cycle
Authors: Bravo S. , Cruz-Abeyro J. A. L. , Rojas D.
Published: April 2005

Abstract. We study the annual frequency of occurrence of intense geomagnetic storms (Dst < –100 nT) throughout the solar activity cycle for the last three cycles and find that it shows different structures. In cycles 20 and 22 it peaks during the ascending phase, near sunspot maximum. During cycle 21, however, there is one peak in the ascending phase and a second, higher, peak in the descending phase separated by a minimum of storm occurrence during 1980, the sunspot maximum. We compare the solar cycle distribution of storms with the corresponding evolution of coronal mass ejections and flares. We find that, as the frequency of occurrence of coronal mass ejections seems to follow very closely the evolution of the sunspot number, it does not reproduce the storm profiles. The temporal distribution of flares varies from that of sunspots and is more in agreement with the distribution of intense geomagnetic storms, but flares show a maximum at every sunspot maximum and cannot then explain the small number of intense storms in 1980. In a previous study we demonstrated that, in most cases, the occurrence of intense geomagnetic storms is associated with a flaring event in an active region located near a coronal hole. In this work we study the spatial relationship between active regions and coronal holes for solar cycles 21 and 22 and find that it also shows different temporal evolution in each cycle in accordance with the occurrence of strong geomagnetic storms; although there were many active regions during 1980, most of the time they were far from coronal holes. We analyse in detail the situation for the intense geomagnetic storms in 1980 and show that, in every case, they were associated with a flare in one of the few active regions adjacent to a coronal hole.
Characteristics of very large aspect angle E-region coherent echoes at 933 MHz
Authors: Jackel B. J. , Moorcroft D. R. , Schlegel K.
Published: April 2005

Abstract. The EISCAT UHF radar system was used to study the characteristics of E-region coherent backscatter at very large magnetic aspect angles (5–11°). Data taken using 60 μs pulses during elevation scans through horizontally uniform backscatter permitted the use of inversion techniques to determine height profiles of the scattering layer. The layer was always singly peaked, with a mean height of 104 km, and mean thickness (full width at half maximum) of 10 km, both independent of aspect angle. Aspect sensitivities were also estimated, with the Sodankylä-Tromsø link observing 5 dB/degree at aspect angles near 5°, decreasing to 3 dB/degree at 10° aspect angle. Observed coherent phase velocities from all three stations were found to be roughly consistent with LOS measurements of a common E-region phase velocity vector. The E-region phase velocity had the same orientation as the F-region ion drift velocity, but was approximately 50% smaller in magnitude. Spectra were narrow with skewness of about +1 (for negative velocities), increasing slightly with aspect angle.
Uncertainties in field-line tracing in the magnetosphere. Part II: the complete internal geomagnetic field
Authors: Willis D. M. , Robin Singh J. , Freeman K. S. C.
Published: April 2005

Abstract. The discussion in the preceding paper is restricted to the uncertainties in magnetic-field-line tracing in the magnetosphere resulting from published standard errors in the spherical harmonic coefficients that define the axisymmetric part of the internal geomagnetic field (i.e. gn0 ± δgn0). Numerical estimates of these uncertainties based on an analytic equation for axisymmetric field lines are in excellent agreement with independent computational estimates based on stepwise numerical integration along magnetic field lines. This comparison confirms the accuracy of the computer program used in the present paper to estimate the uncertainties in magnetic-field-line tracing that arise from published standard errors in the full set of spherical harmonic coefficients, which define the complete (non-axisymmetric) internal geomagnetic field (i.e. gnm ± δgnm and hnm ± δhnm). An algorithm is formulated that greatly reduces the computing time required to estimate these uncertainties in magnetic-field-line tracing. The validity of this algorithm is checked numerically for both the axisymmetric part of the internal geomagnetic field in the general case (1 ≤ n ≤ 10) and the complete internal geomagnetic field in a restrictive case (0 ≤ m ≤ n, 1 ≤ n ≤ 3). On this basis it is assumed that the algorithm can be used with confidence in those cases for which the computing time would otherwise be prohibitively long. For the complete internal geomagnetic field, the maximum characteristic uncertainty in the geocentric distance of a field line that crosses the geomagnetic equator at a nominal dipolar distance of 2 RE is typically 100 km. The corresponding characteristic uncertainty for a field line that crosses the geomagnetic equator at a nominal dipolar distance of 6 RE is typically 500 km. Histograms and scatter plots showing the characteristic uncertainties associated with magnetic-field-line tracing in the magnetosphere are presented for a range of illustrative examples. Finally, estimates are given for the maximum uncertainties in the locations of the conjugate points of selected geophysical observatories. Numerical estimates of the uncertainties in magnetic-field-line tracing in the magnetosphere, including the associated uncertainties in the locations of the conjugate points of geophysical observatories, should be regarded as "first approximations'' in the sense that these estimates are only as accurate as the published standard errors in the full set of spherical harmonic coefficients. As in the preceding paper, however, all computational techniques developed in this paper can be used to derive more realistic estimates of the uncertainties in magnetic-field-line tracing in the magnetosphere, following further progress in the determination of more accurate standard errors in the spherical harmonic coefficients.
On storm weakening during substorm expansion phase
Authors: Siscoe G. L. , Petschek H. E.
Published: April 2005

Abstract. Iyemori and Rao recently presented evidence that the strength of a magnetic storm, as measured by -Dst, weakens, or its rate of growth slows, during the substorm expansion phase. Yet the expansion phase is known to inject energetic particles into the ring current, which should strengthen the storm. We propose to reconcile these apparently contradictory results by combining the virial theorem and a principle of energy partitioning between energy storage elements in a system with dissipation. As applied to the unloading description of the substorm expansion phase, the virial theorem states that -Dst is proportional to the sum of the total magnetic energy and twice the total kinetic energy in the magnetosphere including the tail. Thus if expansion phase involves converting magnetic energy stored in the tail into kinetic energy stored in the ring current, a drop in -Dst during expansion phase requires that less than half the drop in magnetic energy goes into the ring current, the rest going into the ionosphere. Indeed Weiss et al., have estimated that the energy dissipated in the ionosphere during expansion phase is twice that injected into the ring current. This conclusion is also consistent with the mentioned energy partitioning principle, which requires that more energy be dissipated than transferred between storage elements. While Iyemori and Rao's observations seem to contradict the hypothesis that storms consist at least in part of a sum of substorms, this mode of description might nonetheless be preserved by including the substorm's growth-phase contribution. Then the change in storm strength measured from before the growth phase to after the expansion phase is positive, even though the expansion phase alone makes a negative contribution.
Guiding MF waves from the Earth's surface into space
Authors: Blagoveshchensky D. V. , Dobroselsky K. A. , Maltseva O. A.
Published: April 2005

Abstract. The insufficiently known phenomenon of MF- wave propagation from the Earth's surface through the magnetosphere (guiding) to the conjugate hemisphere and back to the transmitter has been experimentally studied. Computer modelling fulfilled on the basis of ray tracing showed that guiding was possible only from area of the main ionospheric trough. The effect of MF guiding is most useful for the diagnostics of the plasmapause, poleward edge of the trough, the diffuse precipitation boundary and so on.
Effect of equatorial ionization anomaly on the occurrence of spread-F
Authors: Jayachandran P. T. , Sri Ram P. , Somayajulu V. V. , Rama Rao P. V. S.
Published: April 2005

Abstract. The unique geometry of the geomagnetic field lines over the equatorial ionosphere coupled with the E-W electric field causes the equatorial ionization anomaly (EIA) and equatorial spread-F (ESF). Ionosonde data obtained at a chain of four stations covering equator to anomaly crest region (0.3 to 33 °N dip) in the Indian sector are used to study the role of EIA and the associated processes on the occurrence of ESF. The study period pertains to the equinoctial months (March, April, September and October) of 1991. The ratios of critical frequency of F-layer (ƒ0F2) and electron densities at an altitude of 270 km between Ahmedabad (33 °N dip) and Waltair (20 °N dip) are found to shoot up in the afternoon hours on spread-F days showing strengthening of the EIA in the afternoon hours. The study confirms the earlier conclusions made by Raghava Rao et al. and Alex et al. that a well-developed EIA is one of the conditions conducive for the generation of ESF. This study also shows that the location of the crest is also important in addition to the strength of the anomaly.
Ocean-tide parameters from the simultaneous long-period analysis of the orbits of Starlette and Stella
Authors: Harwood N. M. , Swinerd G. G.
Published: April 2005

Abstract. The orbits of two geodetic satellites, Starlette and Stella, have been analysed in order to determine ocean-tide parameters. The orbit of Starlette has been determined over a three-year period and Stella over a one-year period. Long-period analysis techniques have been used to determine the evolutions of the orbital inclination, eccentricity and right ascension of the ascending node for each satellite due to ocean tides. The ocean-tide parameters have been determined in a simultaneous fitting of the theoretical orbital variations to the observed variations. The results are compared with ocean-tide models.
Ionospheric storm effects in the nighttime E region caused by neutralized ring current particles
Authors: Bauske R. , Noël S. , Prölss G. W.
Published: April 2005

Abstract. During magnetic storms an anomalous increase in the ionization density of the nighttime E region is observed at low and middle latitudes. It has been suggested that this effect is caused by the precipitation of neutralized ring current particles. Here a coupled ring current decay-ionosphere model is used to confirm the validity of this explanation.
Self-consistent modelling of the daytime electron density profile in the ionospheric F region
Authors: Mikhailov A. , Schlegel K.
Published: April 2005

Abstract. A theoretical self-consistent method for the description of daytime Ne(h) profiles in the ionospheric F region measured by EISCAT is proposed. It is based on the use of a theoretical F-region model and measured electron density, Ne(h), electron, Te(h), and ion temperature, Ti(h), and field-aligned plasma drift Vl(h) profiles. The method describes the observed Ne(h) profile with high accuracy for quiet and disturbed conditions. Two versions of the method are considered: in the first the exospheric temperature Tex is derived from a procedure minimizing [log(Ne(h)obs / Ne(h)cal]2, in the second Tex is deduced from the ion energy conservation in the F region. The method allows us to infer from the incoherent-scatter observations: concentrations of atomic oxygen, [O], molecular oxygen, [O2], molecular nitrogen, [N2] the vertical plasma drift, W, the exospheric temperature. Tex, and the shape parameter S in the neutral temperature profile. The ratio ([O+]/Ne) calculated by the theoretical model is used to correct Te(h), Ti(h) and Ne(h) profiles routinely measured with EISCAT which are known to depend strongly on the actual applied ion-composition model. Such a correction is especially important for geomagnetically disturbed periods when the F region is strongly enriched with molecular ions. We conclude that four of the six thermospheric parameters, namely [O], [N2], W and Tex can be confidently inferred from the EISCAT observations, while the other two derived parameters, [O2] ans S are less reliable. The method can be used for the analysis of long-term (seasonal, solar cycle) as well as for day-to-day variations of the thermospheric parameters and the F-region ion composition using daytime incoherent-scatter observations.
Parametrization of spectra of plasma bubble induced VHF satellite scintillations and its geophysical significance
Authors: Vijayakumar P. N. , Pasricha P. K.
Published: April 2005

Abstract. An important component of ionospheric plasma irregularity studies in the Indian low latitudes involves the study of the plasma bubbles which produce intense scintillations of the transionospheric satellite signals. Many such plasma bubble induced (PBI) scintillation events were identified while recording 244 MHz signal from the geostationary satellite Fleetsat (73°E) at Delhi (28.6°N, 77.2°E) during March-April 1991. This type of scintillations represents changes in plasma processes. These scintillations are spectrally analyzed using an autoregressive (AR) scheme, which is equivalent to maximum entropy method of spectrum analysis, amenable to extracting optimum spectral content from short data lengths (20 – 40 s). Each spectrum is assigned a level of detectability using the final prediction error (FPE) derived from the optimum filter order required to resolve the spectrum. Lower detectability together with a higher order filter indicate a higher level of coherence for the plasma irregularities (discrete structures). Consistent patterns for these scintillations emerge from the present analysis as follows: (1) the initial and final phases of a scintillation patch display quasiperiodic oscillations. Their corresponding spectra show dominant (Gaussian shaped) spectral features with detectability levels of –6 dB to –12 dB and requiring a higher order (>6) AR filter for their spectral resolution. These are most likely associated with discrete "filament-like" or "sheet-like" plasma structures that exist near the bubble walls. (2) Two main features of the scintillation spectra could be positively associated with the well-developed plasma bubble stage: (a) spectra displaying a power-law process with a single component spectral slope between 1.6 to 3.0. Generally such spectra are resolved with a 2nd order filter and have a 1 dB to 6 dB of detectability. (b) Spectra displaying a double slope, indicating an inner and an outer scale regime for the power-law irregularities. These spectra are resolved with higher order filters (>3 but <7) and possess detectability levels of –1 dB to 3 dB. These spectra display finer spectral changes, perhaps indicative of the nature of continuously evolving plasma irregularities. As an example, an analysis of a single scintillation patch is presented to highlight the geophysical significance of the present approach. Some important parameters used in the AR scheme of spectral analysis are given in the Appendix.
The effects of nitric oxide cooling and the photodissociation of molecular oxygen on the thermosphere/ionosphere system over the Argentine Islands
Authors: Wells G. D. , Rodger A. S. , Moffett R. J. , Bailey G. J. , Fuller-Rowell T. J.
Published: April 2005

Abstract. In the past the global, fully coupled, time-dependent mathematical model of the Earth's thermosphere/ionosphere/plasmasphere (CTIP) has been unable to reproduce accurately observed values of the maximum plasma frequency, foF2, at extreme geophysical locations such as the Argentine Islands during the summer solstice where the ionosphere remains in sunlight throughout the day. This is probably because the seasonal dependence of thermospheric cooling by 5.3 µm nitric oxide has been neglected and the photodissociation of O2 and heating rate calculations have been over-simplified. Now we have included an up-to-date calculation of the solar EUV and UV thermospheric heating rate, coupled with a new calculation of a diurnally varying O2 photodissociation rate, in the model. Seasonally dependent 5.3 µm nitric oxide cooling is also included. With these important improvements, it is found that model values of foF2 are in substantially better agreement with observation. The height of the F2-peak is reduced throughout the day, but remains within acceptable limits of values derived from observation, except at around 0600 h LT. We also carry out two studies of the sensitivity of the upper atmosphere to changes in the magnitude of nitric oxide cooling and photodissociation rates. We find that hmF2 increases with increased heating, whilst foF2 falls. The converse is true for an increase in the cooling rate. Similarly increasing the photodissociation rate increases both hmF2 and foF2. These changes are explained in terms of changes in the neutral temperature, composition and neutral wind.
The World Neutron Monitor Network as a tool for the study of solar neutrons
Authors: Usoskin I. G. , Kovaltsov G. A. , Kananen H. , Tanskanen P.
Published: April 2005

Abstract. The use of the World Neutron Monitor Network to detect high-energy solar neutrons is discussed in detail. It is shown that the existing network can be used for the routine detection of intense sporadic solar-neutron events whenever they occur. A technique is suggested involving the weighted summation of responses of separate monitors to solar neutrons. It is demonstrated that the use of this method improves the significance of solar-neutron event detection. Different results of the simulation of the neutron-monitor sensitivity to solar neutrons have been tested with respect to their application for practical use. It is shown that the total number of neutrons with energy above 300 MeV injected from the Sun during a solar flare can be estimated directly from the time-integrated neutron-monitor response to solar neutrons without any model assumptions. The estimation technique has been developed.
Density fluctuations measured by ISEE 1-2 in the Earth's magnetosheath and the resultant scattering of radio waves
Authors: Lacombe C. , Steinberg J.-L. , Harvey C. C. , Hubert D. , Mangeney A. , Moncuquet M.
Published: April 2005

Abstract. Radio waves undergo angular scattering when they propagate through a plasma with fluctuating density. We show how the angular scattering coefficient can be calculated as a function of the frequency spectrum of the local density fluctuations. In the Earth's magnetosheath, the ISEE 1-2 propagation experiment measured the spectral power of the density fluctuations for periods in the range 300 to 1 s, which produce most of the scattering. The resultant local angular scattering coefficient can then be calculated for the first time with realistic density fluctuation spectra, which are neither Gaussian nor power laws. We present results on the variation of the local angular scattering coefficient during two crossings of the dayside magnetosheath, from the quasi-perpendicular bow shock to the magnetopause. For a radio wave at twice the local electron plasma frequency, the scattering coefficient in the major part of the magnetosheath is b(2fp) ≃ 0.5 – 4 × 10–9 rad2/m. The scattering coefficient is about ten times stronger in a thin sheet (0.1 to1RE) just downstream of the shock ramp, and close to the magnetopause.
Geomagnetic activity and wind velocity during the Maunder minimum
Authors: Mendoza B.
Published: April 2005

Abstract. Following a given classification of geomagnetic activity, we obtained aa index values for the Maunder minimum (1645–1715). It is found that the recurrent and fluctuating activities were not appreciable and that the shock activity levels were very low. The aa index level was due almost entirely to the quiet days. Calculated average solar-wind velocities were 194.3 km s–1 from 1657 to 1700 and 218.7 km s–1 from 1700 onwards. Also, the coronal magnetic field magnitude and southward interplanetary magnetic field component Bz were lower. It is concluded that the nearly absent levels of geomagnetic activity during this period were due to lower coronal and Bz magnetic field magnitudes as well as to the continuous impinging on the Earth of a slow wind.
Range finding of Alfvén oscillations and direction finding of ion-cyclotron waves by using the ground-based ULF finder
Authors: Guglielmi A. , Kangas J. , Milling D. , Orr D. , Pokhotelov O.
Published: April 2005

Abstract. A new approach to the problem of direction and distance finding of magnetospheric ULF oscillations is described. It is based on additional information about the structure of geoelectromagnetic field at the Earth's surface which is contained in the known relations of the theory of magnetovariation and magnetotelluric sounding. This allows us to widen the range of diagnostic tools by using observations of Alfvén oscillations in the Pc 3–5 frequency band and the ion-cyclotron waves in the Pc 1 frequency band. Preliminary results of the remote sensing of the magnetosphere at low-latitudes using the MHD ranger technique are presented. The prospects for remote sensing of the plasmapause position are discussed.
Observations of atmospheric gravity waves by radio interferometry: are results biased by the observational technique?
Authors: Mercier C. , Jacobson A. R.
Published: April 2005

Abstract. In this paper we present a quantitative comparison between a large data base of medium-scale atmospheric gravity waves (AGWs) observed by radio interferometry of transionospheric radio sources and the results of a numerical simulation of the observed effects. The simulation includes: (i) the propagation and dissipation of AGWs up to ionospheric heights and (ii) the calculation of the subsequent slant TEC perturbations integrated along the path to the radio sources. We show that the observed azimuthal distribution of AGWs can be deeply biased. Predicted results are found to be consistent with previous extensive observations using radio beacons aboard geostationary satellites. These observations are rediscussed in view of the present predictions.
A new airborne Polar Nephelometer for the measurement of optical and microphysical cloud properties. Part II: Preliminary tests
Authors: Crépel O. , Gayet J.-F. , Fournol J.-F. , Oshchepkov S.
Published: April 2005

Abstract. A new optical sensor, the airborne Polar Nephelometer, has been tested in an open wind tunnel. The wind tunnel was operated in cloudy conditions including either cloud water droplets or ice crystals, or a mixture of these particles. The sensor is designed to measure the optical and microphysical parameters of cloud particles sized from a few micrometers to about 500 µm diameter. Basically, the probe measures the scattering phase function of an ensemble of cloud particles which intersect a collimated laser beam near the focal point of a paraboloidal mirror. From the measured scattering phase function the retrieval of the droplet-size spectra and subsequent derived quantities such as liquid water content and size parameters can be calculated using an inversion method. The particle phase discrimination (water droplets/ice particles) can be derived from the shape of the scattering phase function and the sensitivity of the probe allows the detection of small ice crystals (typically of 5 µm diameter). The paper describes the preliminary results obtained by the prototype version of the Polar Nephelometer in various cloudy conditions. These results are compared with direct microphysical measurements obtained by usual PMS probes also mounted in the wind tunnel. Complementary results obtained in a cold chamber are presented in order to illustrate the reliability of the Polar Nephelometer in the presence of small ice crystals.
Impact of warm ENSO events on atmospheric circulation and convection over the tropical Atlantic and West Africa
Authors: Janicot S.
Published: April 2005

Abstract. Empirical studies have shown that warm El Nino/Southern Oscillation (ENSO) episodes are associated during northern summer with, first, a southward location of the intertropical convergence zone (ITCZ) over the tropical Atlantic, and, second, a weakened convection over West Africa where the ITCZ is near its mean latitude. A modelling experiment presented here is used to help explain this apparent contradiction. In simulated ENSO conditions, the ITCZ is located southwards over the tropical Atlantic. Over West Africa the intertropical front is also displaced southwards, but more slightly; the ITCZ is located at its climatological latitude and the vertical development of convective clouds over West and Central Africa is reduced due to dynamical subsidence in the upper levels.
Evidence for the role of the diabatic heating in synoptic scale processes: a case study example
Authors: Martin M. L. , Luna M. Y. , Valero F.
Published: April 2005

Abstract. The quasigeostrophic theory is used to address the role of diabatic forcing in synoptic scale processes over Iberia. A parametrization of diabatic heating is obtained in terms of a thermodynamic variable called the ice-liquid water potential temperature which is conservative under all phase changes of water. A case study objectively selected by means of a rotated principal component analysis over the diabatic field is analyzed to test the proposed parametrization. This study highlights the fact that the magnitudes of diabatic forcing and dynamic forcing are very nearly the same throughout the troposphere. The results also show that the composite diabatic heating is a better representation for both cloudiness and precipitation fields than the dynamic forcing.
An overview of a heavy rain event in southeastern Iberia: the role of large-scale meteorological conditions
Authors: Valero F. , Luna M. Y. , Martín M. L.
Published: April 2005

Abstract. The heavy rain event of 30 January to 4 February 1993 in the Segura Basin in SE Iberia is investigated. The study emphasizes the atmospheric processes that led to thunderstorm development. The meteorological scenario has been separated into preconvective and convective periods and into large-scale and subsynoptic settings. The warm, moist air flow from the Mediterranean Sea, channelled by a high-over-low blocking pattern with a cold front over southeastern Iberia appears to be the main reason that helped trigger a severe weather occurrence. The study of the mesoscale environment parameters showed an enhanced conditional unstability through a deep troposphere layer with a moderate to strong vertical wind shear that promoted conditions to allow organization of long-lived convective structures.