bluuebunny/crossref_metadata_2025 huggingface dataset:
The F-region trough: seasonal morphology and relation to interplanetary magnetic field
Authors: Voiculescu M. , Virtanen I. , Nygrén T.
Published: June 2010

Abstract. We present here the results of a statistical study of the ionospheric trough observed in 2003 by means of satellite tomography. We focus on the seasonal morphology of the trough occurrence and investigate the trough latitude, width and the horizontal gradients at the edges, at different magnetic local times, as well as their relations to geomagnetic activity and the interplanetary magnetic field. A seasonal effect is noticed in the diurnal variation of the trough latitude, indicating that summer clearly differs from the other seasons. In winter the troughs seem to follow the solar terminator. The width of the trough has a diurnal variation and it depends on the season, as well. The broadest troughs are observed in winter and the narrowest ones in summer. A discontinuity in the diurnal variation of the trough latitude is observed before noon. It is suggested that this is an indication of a difference between the generation mechanisms of morningside and eveningside troughs. The density gradients at the edges have a complex dependence on the latitude of the trough and on geomagnetic activity. The photoionization and the auroral precipitation are competing in the formation of the trough walls at different magnetic local times. An important finding is that the interplanetary magnetic field plays a role in the occurrence of the trough at different levels of geomagnetic activity. This is probably associated with the topology of the polar cap convection pattern, which depends on the directions of the IMF components By and Bz.
Spectral analysis of 10-m resolution temperature profiles from balloon soundings over Beijing
Authors: Wu Y. , Xu J. , Yuan W. , Chen H. , Bian J.
Published: June 2010

Abstract. Vertical temperature profiles with a height resolution of 10 m have been measured in the troposphere and lower stratosphere during March and April 2003 over the Beijing Meteorological Observatory. This resolution allows us to study temperature spectra up to higher wave numbers than many published papers. Our purposes in this study are to examine the spectral character of normalized temperature fluctuations in the 2.90–8.01 km (troposphere) and 14.65–19.76 km (lower stratosphere) altitude ranges and to compare them with model spectra. Vertical wave number spectra of six temperature profiles are presented. Results indicate that mean spectral slopes are about −1.9 in the troposphere and −2.2 in the lower stratosphere, which is believed to be the shallowest slopes ever measured by balloon-borne radiosonde soundings. Mean spectral amplitudes at m=1/(100 m) are about 17 times larger in the troposphere and 4 times larger in the lower stratosphere than the predicted saturated spectral amplitudes. These results show that the observed temperature spectra do not obey current gravity wave saturation models, the "universal" atmospheric spectrum model, or the wind-shifting model, in both slope and amplitude.
Sunset transition of negative charge in the D-region ionosphere during high-ionization conditions
Authors: Verronen P. T. , Ulich Th. , Turunen E. , Rodger C. J.
Published: June 2010

Abstract. The solar proton event of October 1989 and especially the sunset of 23 October is examined in this study of negative ion chemistry, which combines measurements of nitric oxide, electron density, and cosmic radio noise absorption with ion and neutral chemistry modelling. Model results show that the negative charge transition from electrons to negative ions during sunset occurs at altitudes below 80 km and is dependent on both ultraviolet and visible solar radiation. The ultraviolet effect is mostly due to rapid changes in atomic oxygen and O2(1Δg), while the decrease in NO3- photodetachment plays a minor role. The effect driven by visible wavelengths is due to changes in photodissociation of CO3- and the subsequent electron photodetachment from O-, and at higher altitudes is also due to a decrease in the photodetachment of O2-. The relative sizes of the ultraviolet and visible effects vary with altitude, with the visible effects increasing in importance at higher altitudes, and they are also controlled by the nitric oxide concentration. These modelling results are in good agreement with EISCAT incoherent scatter radar and Kilpisjärvi riometer measurements.
Probing of medium-scale traveling ionospheric disturbances using HF-induced scatter targets
Authors: Blagoveshchenskaya N. F. , Borisova T. D. , Kornienko V. A. , Moskvin I. V. , Rietveld M. T. , Frolov V. L. , Uryadov V. P. , Kagan L. M. , Yampolski Yu. M. , Galushko V. L. , Koloskov A. V. , Kasheev S. B. , Zalizovski A. V. , Vertogradov G. G. , Vertogradov V. G. , Kelley M. C.
Published: June 2010

Abstract. Experimental results from the Tromsø and Sura heating experiments at high and mid-latitudes are examined. It is shown that the combination of HF-induced target and bi-static HF Doppler radio scatter observations is a profitable method for probing medium-scale traveling ionospheric disturbances (TIDs) at high and mid-latitudes. HF ionospheric modification experiments provide a way of producing the HF-induced scatter target in a controlled manner at altitudes where the sensitivity to TIDs is highest. Bi-static HF Doppler radio scatter observations were carried out on the London-Tromsø-St. Petersburg path in the course of a Tromsø heating experiment on 16 November 2004 when the pump wave was reflected from an auroral Es-layer. During Sura heating experiments on 19 and 20 August 2004, when the HF pump wave was reflected from the F2 ionospheric layer, multi-position bi-static HF Doppler radio scatter observations were simultaneously performed at three reception points including St. Petersburg, Kharkov, and Rostov-on-Don. Ray tracing and Doppler shift simulations were made for all experiments. A computational technique has been developed allowing the reconstruction of the TID phase velocities from multi-position bi-static HF Doppler scatters. Parameters of medium-scale TIDs were found. In all experiments they were observed in the evening and pre-midnight hours. TIDs in the auroral E-region with periods of about 23 min were traveling southward at speeds of 210 m/s. TIDs in the mid-latitudinal F-region with periods from 20 to 45 min travelled at speeds between 40 and 150 m/s. During quiet magnetic conditions the waves were traveling in the north-east direction. In disturbed conditions the waves were moving in the south-west direction with higher speeds as compared with quiet conditions. Possible sources for the atmospheric gravity waves at middle and high latitudes are discussed.
A first comparison of irregularity and ion drift velocity measurements in the E-region
Authors: Makarevich R. A. , Honary F. , Howells V. S. C. , Koustov A. V. , Milan S. E. , Davies J. A. , Senior A. , McCrea I. W. , Dyson P. L.
Published: June 2010

Abstract. E-region irregularity velocity measurements at large flow angles with the STARE Finland coherent VHF radar are considered in context of the ion and electron velocity data provided by the EISCAT tristatic radar system, CUTLASS Finland coherent HF radar, and IMAGE fluxgate magnetometers. The data have been collected during a special experiment on 27 March 2004 during which EISCAT was scanning between several E- and one F-region altitudes along the magnetic field line. Within the E-region, the EISCAT measurements at two altitudes of 110 and 115 km are considered while the electron velocity is inferred from the EISCAT ion velocity measurements at 278 km. The line-of-sight (l-o-s) VHF velocity measured by STARE VHF los is compared to the ion and electron velocity components (Vi0 comp and Ve0 comp) along the STARE l-o-s direction. The comparison with Ve0 comp for the entire event shows that the measurements exhibit large scatter and small positive correlation. The correlation with Ve0 comp was substantial in the first half of the interval under study when Ve0 comp was larger in magnitude. The comparison with Vi0 comp at 110 and 115 km shows a considerable positive correlation, with VHF velocity being typically larger (smaller) in magnitude than Vi0 comp at 110 km (115 km) so that VVHF los appears to be bounded by the ion velocity components at two altitudes. It is also demonstrated that the difference between VVHF los and Vi0 comp at 110 km can be treated, in the first approximation, as a linear function of the effective backscatter height heff also counted from 110 km; heff varies in the range 108–114 km due to the altitude integration effects in the scattering cross-section. Our results are consistent with the notion that VHF velocity at large flow angles is directly related to the ion drift velocity component at an altitude heff.
3-D GCM modelling of thermospheric nitric oxide during the 2003 Halloween storm
Authors: Dobbin A. L. , Griffin E. M. , Aylward A. D. , Millward G. H.
Published: June 2010

Abstract. Numerical modelling of thermospheric temperature changes associated with periods of high geomagnetic activity are often inaccurate due to unrealistic representation of nitric oxide (NO) densities and associated 5.3-μm radiative cooling. In previous modelling studies, simplistic parameterisations of NO density and variability have often been implemented in order to constrain thermospheric temperature predictions and post storm recovery timescales during and following periods of high auroral activity. In this paper we use the University College London (UCL) 3-D Coupled Thermosphere and Middle Atmosphere (CMAT) General Circulation Model to simulate the 11-day period from 23 October to 3 November 2003, during which the Earth experienced some of the largest geomagnetic activity ever recorded; the so called "Halloween storm". This model has recently been updated to include a detailed self consistent calculation of NO production and transport. Temperatures predicted by the model compare well with those observed by the UCL Fabry Perot Interferometer at Kiruna, northern Sweden, when changes in solar and auroral activity are taken into account in the calculation of NO densities. The spatial distribution of predicted temperatures at approximately 250-km altitude is also discussed. Simulated NO densities at approximately 110 km are presented. Large quantities of NO are found to be present at to the equator, one to two days after the most intense period of geomagnetic activity. This is the first 3-D GCM simulation of NO production and transport over the 2003 Halloween storm period.
Applications of Kalman filters based on non-linear functions to numerical weather predictions
Authors: Galanis G. , Louka P. , Katsafados P. , Pytharoulis I. , Kallos G.
Published: June 2010

Abstract. This paper investigates the use of non-linear functions in classical Kalman filter algorithms on the improvement of regional weather forecasts. The main aim is the implementation of non linear polynomial mappings in a usual linear Kalman filter in order to simulate better non linear problems in numerical weather prediction. In addition, the optimal order of the polynomials applied for such a filter is identified. This work is based on observations and corresponding numerical weather predictions of two meteorological parameters characterized by essential differences in their evolution in time, namely, air temperature and wind speed. It is shown that in both cases, a polynomial of low order is adequate for eliminating any systematic error, while higher order functions lead to instabilities in the filtered results having, at the same time, trivial contribution to the sensitivity of the filter. It is further demonstrated that the filter is independent of the time period and the geographic location of application.
Statistical study of the location and size of the electron edge of the Low-Latitude Boundary Layer as observed by Cluster at mid-altitudes
Authors: Bogdanova Y. V. , Owen C. J. , Fazakerley A. N. , Klecker B. , Rème H.
Published: June 2010

Abstract. The nature of particle precipitations at dayside mid-altitudes can be interpreted in terms of the evolution of reconnected field lines. Due to the difference between electron and ion parallel velocities, two distinct boundary layers should be observed at mid-altitudes between the boundary between open and closed field lines and the injections in the cusp proper. At lowest latitudes, the electron-dominated boundary layer, named the "electron edge" of the Low-Latitude Boundary Layer (LLBL), contains soft-magnetosheath electrons but only high-energy ions of plasma sheet origin. A second layer, the LLBL proper, is a mixture of both ions and electrons with characteristic magnetosheath energies. The Cluster spacecraft frequently observe these two boundary layers. We present an illustrative example of a Cluster mid-altitude cusp crossing with an extended electron edge of the LLBL. This electron edge contains 10–200 eV, low-density, isotropic electrons, presumably originating from the solar wind halo population. These are occasionally observed with bursts of parallel and/or anti-parallel-directed electron beams with higher fluxes, which are possibly accelerated near the magnetopause X-line. We then use 3 years of data from mid-altitude cusp crossings (327 events) to carry out a statistical study of the location and size of the electron edge of the LLBL. We find that the equatorward boundary of the LLBL electron edge is observed at 10:00–17:00 magnetic local time (MLT) and is located typically between 68° and 80° invariant latitude (ILAT). The location of the electron edge shows a weak, but significant, dependence on some of the external parameters (solar wind pressure, and IMF BZ- component), in agreement with expectations from previous studies of the cusp location. The latitudinal extent of the electron edge has been estimated using new multi-spacecraft techniques. The Cluster tetrahedron crosses the electron and ion boundaries of the LLBL/cusp with time delays of 1–40 min between spacecraft. We reconstruct the motion of the electron boundary between observations by different spacecraft to improve the accuracy of the estimation of the boundary layer size. In our study, the LLBL electron edge is distinctly observed in 87% of mid-altitude LLBL/cusp crossings with clear electron and ion equatorward boundaries equivalent to 35% of all LLBL/cusp crossings by Cluster. The size of this region varied between 0°–2° ILAT with a median value of 0.2° ILAT. Generally, the size of the LLBL electron edge depends on the combination of many parameters. However, we find an anti-correlation between the size of this region and the strength of the IMF, the absolute values of the IMF BY- and BZ-components and the solar wind dynamic pressure, as is expected from a simple reconnection model for the origin of this region.
Longitudinal drift of substorm electrons as the reason of impulsive precipitation events and VLF emissions
Authors: Lubchich A. A. , Yahnin A. G. , Titova E. E. , Demekhov A. G. , Trakhtengerts V. Yu. , Manninen J. , Turunen T.
Published: June 2010

Abstract. Using the data from satellite CRRES and three geostationary LANL spacecraft, the propagation of an electron cloud from midnight to the evening sector is investigated. An electron cloud was injected during a weak isolated substorm that developed on a quiet geomagnetic background. It is found that within the local time sector from 03:00 until at least 08:00 MLT, the propagation of electrons at perpendicular pitch-angles is well described by a simple model of drift in the dipole magnetic field. The flux levels in the field-aligned electrons increase simultaneously with the flux at perpendicular pitch angles, which is attributed to the pitch angle diffusion by the whistler mode. This pitch-angle diffusion leads to precipitation of electrons from a drifting cloud and an increase in the ionospheric electron density, simultaneously observed above Tromsø, Norway, by the EISCAT UHF radar in the morning sector (04:40–05:25 MLT). The precipitation develops as quasi-periodic pulses with a period of about 100 s. We discuss the models of pulsating precipitation due to the whistler cyclotron instability and show that our observations can be explained by such a model.
On the scaling of waiting-time distributions of the negative IMF Bz component
Authors: D'Amicis R. , Bruno R. , Bavassano B. , Carbone V. , Sorriso-Valvo L.
Published: June 2010

Abstract. Statistics associated with the fluctuations in solar wind parameters show a remarkable dependence on the solar activity phase. In particular, we focus our attention on the waiting-time statistics governing the MHD fluctuations of the z-component of the interplanetary magnetic field, which are important within the framework of the Sun-Earth connections, and briefly discuss the preliminary results. Data from several spacecrafts, covering different phases of the solar cycle and different radial distances, are used. We found that propagating Alfvénic fluctuations and convected structures strongly influence the statistics which vary from quasi-Poissonian to power law.
The relationship between small-scale and large-scale ionospheric electron density irregularities generated by powerful HF electromagnetic waves at high latitudes
Authors: Tereshchenko E. D. , Khudukon B. Z. , Rietveld M. T. , Isham B. , Hagfors T. , Brekke A.
Published: June 2010

Abstract. Satellite radio beacons were used in June 2001 to probe the ionosphere modified by a radio beam produced by the EISCAT high-power, high-frequency (HF) transmitter located near Tromsø (Norway). Amplitude scintillations and variations of the phase of 150- and 400-MHz signals from Russian navigational satellites passing over the modified region were observed at three receiver sites. In several papers it has been stressed that in the polar ionosphere the thermal self-focusing on striations during ionospheric modification is the main mechanism resulting in the formation of large-scale (hundreds of meters to kilometers) nonlinear structures aligned along the geomagnetic field (magnetic zenith effect). It has also been claimed that the maximum effects caused by small-scale (tens of meters) irregularities detected in satellite signals are also observed in the direction parallel to the magnetic field. Contrary to those studies, the present paper shows that the maximum in amplitude scintillations does not correspond strictly to the magnetic zenith direction because high latitude drifts typically cause a considerable anisotropy of small-scale irregularities in a plane perpendicular to the geomagnetic field resulting in a deviation of the amplitude-scintillation peak relative to the minimum angle between the line-of-sight to the satellite and direction of the geomagnetic field lines. The variance of the logarithmic relative amplitude fluctuations is considered here, which is a useful quantity in such studies. The experimental values of the variance are compared with model calculations and good agreement has been found. It is also shown from the experimental data that in most of the satellite passes a variance maximum occurs at a minimum in the phase fluctuations indicating that the artificial excitation of large-scale irregularities is minimum when the excitation of small-scale irregularities is maximum.
Assigning the causative lightning to the whistlers observed on satellites
Authors: Chum J. , Jiricek F. , Santolik O. , Parrot M. , Diendorfer G. , Fiser J.
Published: June 2010

Abstract. We study the penetration of lightning induced whistler waves through the ionosphere by investigating the correspondence between the whistlers observed on the DEMETER and MAGION-5 satellites and the lightning discharges detected by the European lightning detection network EUCLID. We compute all the possible differences between the times when the whistlers were observed on the satellite and times when the lightning discharges were detected. We show that the occurrence histogram for these time differences exhibits a distinct peak for a particular characteristic time, corresponding to the sum of the propagation time and a possible small time shift between the absolute time assigned to the wave record and the clock of the lightning detection network. Knowing this characteristic time, we can search in the EUCLID database for locations, currents, and polarities of causative lightning discharges corresponding to the individual whistlers. We demonstrate that the area in the ionosphere through which the electromagnetic energy induced by a lightning discharge enters into the magnetosphere as whistler mode waves is up to several thousands of kilometres wide.
Solar cycle dependent characteristics of the equatorial blanketing Es layers and associated irregularities
Authors: Devasia C. V. , Sreeja V. , Ravindran S.
Published: June 2010

Abstract. The occurrence of blanketing type Es (Esb) layers and associated E-region irregularities over the magnetic equatorial location of Trivandrum (8.5° N; 77° E; dip ~0.5°) during the summer solstitial months of May, June, July and August has been investigated in detail for the period 1986–2000 to bring out the variabilities in their characteristics with the solar cycle changes. The study has been made using the ionosonde and magnetometer data of Trivandrum from 1986–2000 along with the available data from the 54.95 MHz VHF backscatter radar at Trivandrum for the period 1995–2000. The appearance of blanketing Es layers during these months is observed to be mostly in association with the occurrence of afternoon Counter Electrojet (CEJ) events. The physical process leading to the occurrence of a CEJ event is mainly controlled by the nature of the prevailing electro dynamical/neutral dynamical conditions before the event. Hence it is natural that the Esb layer characteristics like the frequency of occurrence, onset time, intensity, nature of gradients in its top and bottom sides etc are also affected by the nature of the background electro dynamical /neutral dynamical processes which in turn are strongly controlled by the solar activity changes. The occurrence of Esb layers during the solstitial months is found to show very strong solar activity dependence with the occurrence frequency being very large during the solar minimum years and very low during solar maximum years. The intensity of the VHF radar backscattered signals from the Esb irregularities is observed to be controlled by the relative roles of the direction and magnitude of the prevailing vertical polarization electric field and the vertical electron density gradient of the prevailing Esb layer depending on the phase of the solar cycle. The gradient of the Esb layer shows a more dominant role in the generation of gradient instabilities during solar minimum periods while it is the electric field that has a more dominant role during solar maximum periods.
Comparison of the magnetic equivalent convection direction and ionospheric convection observed by the SuperDARN radars
Authors: Benkevitch L. V. , Koustov A. V. , Liang J. , Watermann J. F.
Published: June 2010

Abstract. SuperDARN radar and high-latitude magnetometer observations are used to statistically investigate quality of the convection direction estimates from magnetometer data if assumption is made that the magnetic equivalent convection vector (MEC) corresponds to the convection direction. The statistics includes five full days, ~75 000 of joint individual measurements for different seasons. It is demonstrated that the best (worst) agreement between the MEC and ionospheric convection occurs for the sunlit, summer (dark, winter) ionosphere. Overall, the MEC direction is reasonable (deviates less than 45° from the SuperDARN direction) in at least ~55% of points and it is better for the latitudes of the auroral oval. In terms of the magnetic local time, the agreement is the best (worst) in the dusk (early morning) sector. Possible reasons for differences between the MEC and ionospheric convection directions are discussed.
Linear prediction studies for the solar wind and Saturn kilometric radiation
Authors: Taubenschuss U. , Rucker H. O. , Kurth W. S. , Cecconi B. , Zarka P. , Dougherty M. K. , Steinberg J. T.
Published: June 2010

Abstract. The external control of Saturn kilometric radiation (SKR) by the solar wind has been investigated in the frame of the Linear Prediction Theory (LPT). The LPT establishes a linear filter function on the basis of correlations between input signals, i.e. time profiles for solar wind parameters, and output signals, i.e. time profiles for SKR intensity. Three different experiments onboard the Cassini spacecraft (RPWS, MAG and CAPS) yield appropriate data sets for compiling the various input and output signals. The time period investigated ranges from DOY 202 to 326, 2004 and is only limited due to limited availability of CAPS plasma data for the solar wind. During this time Cassini was positioned mainly on the morning side on its orbit around Saturn at low southern latitudes. Four basic solar wind quantities have been found to exert a clear influence on the SKR intensity profile. These quantities are: the solar wind bulk velocity, the solar wind ram pressure, the magnetic field strength of the interplanetary magnetic field (IMF) and the y-component of the IMF. All four inputs exhibit nearly the same level of efficiency for the linear prediction indicating that all four inputs are possible drivers for triggering SKR. Furthermore, they act at completely different lag times ranging from ~13 h for the ram pressure to ~52 h for the bulk velocity. The lag time for the magnetic field strength is usually beyond ~40 h and the lag time for the y-component of the magnetic field is located around 30 h. Considering that all four solar wind quantities are interrelated in a corotating interaction region, only the influence of the ram pressure seems to be of reasonable relevance. An increase in ram pressure causes a substantial compression of Saturn's magnetosphere leading to tail collapse, injection of hot plasma from the tail into the outer magnetosphere and finally to an intensification of auroral dynamics and SKR emission. So, after the onset of magnetospheric compression at least ~1.2 rotations of the planet elapse until intensified SKR emission is visible in a Cassini-RPWS dynamic spectrum.
Multiple triangulation analysis: application to determine the velocity of 2-D structures
Authors: Zhou X.-Z. , Zong Q.-G. , Wang J. , Pu Z. Y. , Zhang X. G. , Shi Q. Q. , Cao J. B.
Published: June 2010

Abstract. In order to avoid the ambiguity of the application of the Triangulation Method (multi-spacecraft timing method) to two-dimensional structures, another version of this method, the Multiple Triangulation Analysis (MTA) is used, to calculate the velocities of these structures based on 4-point measurements. We describe the principle of MTA and apply this approach to a real event observed by the Cluster constellation on 2 October 2003. The resulting velocity of the 2-D structure agrees with the ones obtained by some other methods fairly well. So we believe that MTA is a reliable version of the Triangulation Method for 2-D structures, and thus provides us a new way to describe their motion.
Simultaneous lidar observations of a polar stratospheric cloud on the east and west sides of the Scandinavian mountains and microphysical box model simulations
Authors: Blum U. , Khosrawi F. , Baumgarten G. , Stebel K. , Müller R. , Fricke K. H.
Published: June 2010

Abstract. The importance of polar stratospheric clouds (PSC) for polar ozone depletion is well established. Lidar experiments are well suited to observe and classify polar stratospheric clouds. On 5 January 2005 a PSC was observed simultaneously on the east and west sides of the Scandinavian mountains by ground-based lidars. This cloud was composed of liquid particles with a mixture of solid particles in the upper part of the cloud. Multi-colour measurements revealed that the liquid particles had a mode radius of r≈300 nm, a distribution width of σ≈1.04 and an altitude dependent number density of N≈2–20 cm−3. Simulations with a microphysical box model show that the cloud had formed about 20 h before observation. High HNO3 concentrations in the PSC of 40–50 weight percent were simulated in the altitude regions where the liquid particles were observed, while this concentration was reduced to about 10 weight percent in that part of the cloud where a mixture between solid and liquid particles was observed by the lidar. The model simulations also revealed a very narrow particle size distribution with values similar to the lidar observations. Below and above the cloud almost no HNO3 uptake was simulated. Although the PSC shows distinct wave signatures, no gravity wave activity was observed in the temperature profiles measured by the lidars and meteorological analyses support this observation. The observed cloud must have formed in a wave field above Iceland about 20 h prior to the measurements and the cloud wave pattern was advected by the background wind to Scandinavia. In this wave field above Iceland temperatures potentially dropped below the ice formation temperature, so that ice clouds may have formed which can act as condensation nuclei for the nitric acid trihydrate (NAT) particles observed at the cloud top above Esrange.
Rotation of the magnetic field in Earth's magnetosheath by bulk magnetosheath plasma flow
Authors: Longmore M. , Schwartz S. J. , Lucek E. A.
Published: June 2010

Abstract. Orientations of the observed magnetic field in Earth's dayside magnetosheath are compared with the predicted field line-draping pattern from the Kobel and Flückiger static magnetic field model. A rotation of the overall magnetosheath draping pattern with respect to the model prediction is observed. For an earthward Parker spiral, the sense of the rotation is typically clockwise for northward IMF and anticlockwise for southward IMF. The rotation is consistent with an interpretation which considers the twisting of the magnetic field lines by the bulk plasma flow in the magnetosheath. Histogram distributions describing the differences between the observed and model magnetic field clock angles in the magnetosheath confirm the existence and sense of the rotation. A statistically significant mean value of the IMF rotation in the range 5°-30° is observed in all regions of the magnetosheath, for all IMF directions, although the associated standard deviation implies large uncertainty in the determination of an accurate value for the rotation. We discuss the role of field-flow coupling effects and dayside merging on field line draping in the magnetosheath in view of the evidence presented here and that which has previously been reported by Kaymaz et al. (1992).
Correcting the geomagnetic IHV index of the Eskdalemuir observatory
Authors: Martini D. , Mursula K.
Published: June 2010

Abstract. We study here the recently proposed measure of local geomagnetic activity called the IHV (Inter-Hour Variability) index calculated for the Eskdalemuir (ESK) station. It was found earlier that the ESK IHV index depicts an artificial, step-like increase from 1931 to 1932. We show here that this increase is due to the fact that the values of the magnetic field components of the ESK observatory stored at the World Data Center are two-hour running averages of hourly data stored in ESK yearbooks. Two-hour averaging greatly reduces the variability of the data which leads to artificially small values of the IHV index in 1911–1931. We also study the effect of two-hour averaging upon hourly mean and spot values using 1-minute data available for recent years, and calculate the correction factors for the early years, taking into account the weak dependence of correction factors on solar activity. Using these correction factors, we correct the ESK IHV indices in 1912–1931, and revise the estimate of the centennial change based on them. The effect of correction is very significant: the centennial increase in the ESK IHV-raw (IHV-cor) index in 1912–2000 changes from 73.9% (134.4%) before correction to 10.3% (25.3%) thereafter, making the centennial increase at ESK quite similar to other mid-latitude stations. Obviously, earlier long-term studies based on ESK IHV values are affected by the correction and need to be revised. These results also strongly suggest that the ESK yearbook data should be digitized and the hourly ESK data at WDC should be replaced by them.
Sub-keV ring current ions as the tracer of substorm injection
Authors: Yamauchi M. , Lundin R.
Published: June 2010

Abstract. The dynamics of the energy-latitude dispersed sub-keV trapped ions inside the ring current region, the so-called wedge-like dispersions structure, were statistically studied using Viking satellite data. Probabilities with/without these signatures at various local times in the dayside are obtained in terms of different time-lags from the substorm activity monitored by the AE index. The structure appears in the early morning sector within a few hours after the substorm, and it slowly propagates eastward while decaying with a time scale of several hours. The result qualitatively confirmed the previous model that the wedge-like dispersions are originated from past substorm-related plasma injections into the nightside ring current region, and that the dispersion is formed when these injected plasma slowly moves eastward to the dayside by the drift motion (E×B (eastward), grad-
Change in ozone depletion rates beginning in the mid 1990s: trend analyses of the TOMS/ SBUV merged total ozone data, 1978-2003
Authors: Krzyscin J. W.
Published: June 2010

Abstract. Statistical analyses have been applied to the gridded monthly means of total ozone from combined TOMS and SBUV measurements (version 8 of the data) for the period 1978-2003. We focus on the detection of a change in the trend pattern by searching for a turnaround in the previous downward trend. The ozone time series have been examined separately for each grid point and season, taking into account the various descriptions of the trend term: double-linear, proportional to the index of the overall chlorine content in the stratosphere, and a smooth curve without an a priori defined shape (the output of the regression model). Standard explanatory variables representing physical and chemical processes known to influence the ozone distribution have been considered: Mg II index, QBO wind at 10 and 30 hPa, zonal wind anomalies at 50 hPa along the 60° north or 60° south circle, the index of the stratospheric aerosols loading in the NH or SH, and the tropopause pressure. The multivariate adaptive regression splines methodology is used to find an optimal set of the explanatory variables and shape of the trend curve. The statistical errors of the models' estimates have been calculated using block bootstrapping of the models' residuals. The results appear to be consistent among models using different formulations of the trend pattern. The 2003 level of total ozone after the removal of the variations due to the parameterized dynamical/chemical forcing on the ozone is still below the long-term (1978-2003) mean level over the extratropical regions. The deficit is ~2-5% in the NH and much larger in the SH and exhibits clear seasonal variability, ~15% in autumn, ~10% in winter, and ~-5% in spring and summer. The present total ozone level is higher beyond the tropics than that in the mid 1990s but it is too early to announce a beginning of the ozone recovery there because of the trend uncertainties, due to errors of the regression estimates for individual grid points and longitudinal variability of the trend pattern. A rigorous statistical test has shown the statistically significant turnaround for some grid points over the extratropical region and a deepening of the ozone negative trend has not been found for any grid point.
Seasonal and diurnal variation of lightning activity over southern Africa and correlation with European whistler observations
Authors: Collier A. B. , Hughes A. R. W. , Lichtenberger J. , Steinbach P.
Published: June 2010

Abstract. Lightning Imaging Sensor (LIS) data have been analysed to ascertain the statistical pattern of lightning occurrence over southern Africa. The diurnal and seasonal variations are mapped in detail. The highest flash rates (107.2 km-2 y-1) occur close to the equator but maxima are also found over Madagascar (32.1 km-2 y-1) and South Africa (26.4 km-2 y-1). A feature of the statistics is a relatively steady contribution from over the ocean off the east coast of South Africa that appears to be associated with the Agulhas current. Lightning statistics are of intrinsic meteorological interest but they also relate to the occurrence of whistlers in the conjugate region. Whistler observations are made at Tihany, Hungary. Statistics reveal that the period of most frequent whistler occurrence does not correspond to the maximum in lightning activity in the conjugate region but is strongly influenced by ionospheric illumination and other factors. The whistler/flash ratio, R, shows remarkable variations during the year and has a peak that is narrowly confined to February and March.
Observations of concentrated generator regions in the nightside magnetosphere by Cluster/FAST conjunctions
Authors: Hamrin M. , Marghitu O. , Rönnmark K. , Klecker B. , André M. , Buchert S. , Kistler L. M. , McFadden J. , Rème H. , Vaivads A.
Published: June 2010

Abstract. Here and in the companion paper, Marghitu et al. (2006), we investigate plausible auroral generator regions in the nightside auroral magnetosphere. In this article we use magnetically conjugate data from the Cluster and the FAST satellites during a 3.5-h long event from 19-20 September 2001. Cluster is in the Southern Hemisphere close to apogee, where it probes the plasma sheet and lobe at an altitude of about 18 RE. FAST is below the acceleration region at approximately 0.6 RE. Searching for clear signatures of negative power densities, E·J<0, in the Cluster data we can identify three concentrated generator regions (CGRs) during our event. From the magnetically conjugate FAST data we see that the observed generator regions in the Cluster data correlate with auroral precipitation. The downward Poynting flux observed by Cluster, as well as the scale size of the CGRs, are consistent with the electron energy flux and the size of the inverted-V regions observed by FAST. To our knowledge, these are the first in-situ observations of the crossing of an auroral generator region. The main contribution to E·J<0 comes from the GSE EyJy. The electric field Ey is weakly negative during most of our entire event and we conclude that the CGRs occur when the duskward current Jy grows large and positive. We find that our observations are consistent with a local southward expansion of the plasma sheet and/or rather complicated, 3-D wavy structures propagating over the Cluster satellites. We find that the plasma is working against the magnetic field, and that kinetic energy is being converted into electromagnetic energy. Some of the energy is transported away as Poynting flux.
Indirect mapping of the source of the oppositely directed fast plasma flows in the plasma sheet onto the auroral display
Authors: Yahnin A. G. , Despirak I. V. , Lubchich A. A. , Kozelov B. V. , Dmitrieva N. P. , Shukhtina M. A. , Biernat H. K.
Published: June 2010

Abstract. Data from Polar and Geotail spacecraft are combined to investigate the relationship between locations of active auroras and the magnetotail plasma sheet region where reversed fast plasma flows are generated during substorms. Using the magnetospheric magnetic field model, it is shown that at the beginning of the tailward fast flow the ionospheric footprint of the spacecraft measuring the flow tends to be located poleward of the auroral bulge. The spacecraft within the earthward flow is mapped equatorward of the poleward edge of the auroral bulge. We conclude that a source of the fast plasma flows is conjugated with the poleward edge of the auroral bulge. Analysis of the behavior of the plasma and the magnetic field in the vicinity of the source of the diverging flows allows us to conclude that the source region, interpreted as the magnetic reconnection site, coincides with the region of the cross-tail current reduction, and the tailward propagation of the region is associated with the tailward propagation of the current disruption front.
Long-term modulation of the cosmic ray fluctuation spectrum
Authors: Starodubtsev S. A. , Usoskin I. G. , Grigoryev A. V. , Mursula K.
Published: June 2010

Abstract. Here we study the power level of rapid cosmic ray fluctuations in the frequency range of 10-4-1.67·10-3 Hz (periods from 10 min to about 3 h), using measurements by space-borne instruments for the period since 1974. We find that the power level of these fluctuations varies over the solar cycle, but the phase of this variation depends on the energy of cosmic ray particles. While the power level of these fluctuations in the higher energy channels (corresponding to galactic cosmic rays) changes in phase with the solar cycle, the fluctuation level for lower energy channels (predominantly of solar/interplanetary origin) is roughly in an opposite phase with the solar cycle. The results prove conclusively that these fluctuations originate in the near-Earth space, excluding their atmospheric or magnetospheric origin. We present these new results and discuss a possible scenario explaining the observed energy-dependence.
IMF effect on sporadic-E layers at two northern polar cap sites: Part II – Electric field
Authors: Nygrén T. , Aikio A. T. , Voiculescu M. , Ruohoniemi J. M.
Published: June 2010

Abstract. This paper is the second in a series on a study of the link between IMF and sporadic-E layers within the polar cap. In Paper I (Voiculescu et al., 2006), an analysis of the sporadic-E data from Thule and Longyearbyen was presented. Here we concentrate on the electric field mechanism of sporadic-E generation. By means of model calculations we show that the mechanism is effective even at Thule, where the direction of the geomagnetic field departs from vertical only by 4. The model calculations also lead to a revision of the electric field theory. Previously, a thin layer was assumed to grow at a convergent null in the vertical ion velocity, which is formed when the electric field points in the NW sector. Our calculations indicate that in the dynamic process of vertical plasma compression, a layer is generated at altitudes of high vertical convergence rather than at a null. Consequently, the layer generation is less sensitive than previously assumed to fluctuations of the electric field direction within the NW sector. The observed diurnal variations of sporadic-E occurrence at Longyearbyen and Thule are compared with the diurnal variations of the electric field, calculated using a representative range of IMF values by means of the statistical APL model. The results indicate that the main features of Es occurrence can be explained by the convection pattern controlled by the IMF. Electric fields calculated from the IMF observations are also used for producing distributions of sporadic-E occurrence as a function of electric field direction at the two sites. A marked difference between the distributions at Thule and Longyearbyen is found. A model estimate of the occurrence probability as a function of electric field direction is developed and a reasonable agreement between the model and the experimental occurrence is found. The calculation explains the differences between the distributions at the two sites in terms of the polar cap convection pattern. The conclusion is that the electric field is the major cause for sporadic-E generation and, consequently, IMF has a clear control on the occurrence of sporadic E within the polar cap.
Comparison of large-scale Birkeland currents determined from Iridium and SuperDARN data
Authors: Green D. L. , Waters C. L. , Anderson B. J. , Korth H. , Barnes R. J.
Published: June 2010

Abstract. The Birkeland currents, J||, electrically couple the high latitude ionosphere with the near Earth space environment. Approximating the spatial distribution of the Birkeland currents may be achieved using the divergence of the ionospheric electric field, , assuming zero conductance gradients such that . In this paper, electric field data derived from the Super Dual Auroral Radar Network (SuperDARN) are used to calculate , which is compared with the Birkeland current distribution derived globally from the constellation of Iridium satellites poleward of 60° magnetic latitude. We find that the assumption of zero conductance gradients is often a poor approximation. On the dayside, in regions where the SuperDARN electric field is constrained by radar returns, the agreement in the locations of regions of upward and downward current between and J|| obtained from Iridium data is reasonable with differences of less than 3° in the latitudinal location of major current features. It is also shown that away from noon, currents arising from conductance gradients can be larger than the component. By combining the estimate in regions of radar coverage with in-situ estimates of conductance gradients from DMSP satellite particle data, the agreement with the Iridium derived J|| is considerably improved. However, using an empirical model of ionospheric conductance did not account for the conductance gradient current terms. In regions where radar data are sparse or non-existent and therefore constrained by the statistical potential model the approximation does not agree with J|| calculated from Iridium data.
Space weather impact on the equatorial and low latitude F-region ionosphere over India
Authors: Dabas R. S. , Das R. M. , Vohra V. K. , Devasia C. V.
Published: June 2010

Abstract. For a detailed study of the space weather impact on the equatorial and low latitude F-region, the ionospheric response features are analysed during the periods of three recent and most severe magnetic storm events of the present solar cycle which occurred in October and November 2003, and November 2004. The F-layer base height (h'F), peak height (hmF2) and critical frequency (foF2) data, from Trivandrum, an equatorial station and Delhi, a low latitude location, are examined during the three magnetic storm periods. The results of the analysis clearly shows that the height of the F-region (both h'F and hmF2), at the equator and low latitude, simultaneously increases by 200 to 300 km, in association with maximum negative excursion of Dst values around the midnight hours with a large depletion of ionization over the equator, which is followed by an ionization enhancement at low latitude during the recovery phase of the storm. At Delhi, fast variations up to 200 m/s are also observed in the F-layer vertical upward/downward velocity, calculated using Doppler shifts, associated with the maximum negative excursion of Dst. This shows that during magnetic disturbances, the equatorial ionization anomaly (EIA) expands to a much wider latitude than the normal fountain driven by the E/F-layer dynamo electric fields. It is also observed that during the main phase of the storm, at low latitude there is generally an enhancement of F-region ionization with an increase in h'F/hmF2 but in the equatorial region, the ionization collapses with a decrease in h'F/hmF2, especially after sunset hours. In addition, at the equator the normal pre-sunset hours' enhancement in h'F is considerably suppressed during storm periods. This might be due to changes in magnitude and direction of the zonal electric field affecting the upward E×B drift and hence the plasma distribution in the form of a decrease in electron density in the equatorial region and an increase in the low latitude region. In association with disturbance electric fields, the enhanced storm-induced equatorward meridional winds in the thermosphere can also further amplify the F-layer height rise at low latitudes during the post-midnight hours, as observed in two of the storm periods.