bluuebunny/crossref_metadata_2025 huggingface dataset:
The role of external triggers in flow shear arcs in the dayside aurora
Authors: Sandholt P. E. , Farrugia C. J.
Published: June 2010

Abstract. In case studies we relate dayside auroral transients to IMF By-distorted plasma convection cells based on high-resolution observations from the ground. We selected three days representing positive and negative IMF By conditions when SuperDARN returned reliable dayside convection patterns in the sector of our optical observations from Ny Ålesund, Svalbard (76° MLAT). We combine two perspectives on the dayside aurora, the local and the global. In the first we derive the fine-structure of dayside precipitation/convection as a function of magnetic latitude (MLAT) and magnetic local time (MLT), which is necessary to understand the local M-I coupling processes (Birkeland current structure). The larger perspective (quasi-global dayside aurora) may be used to shed light on the solar wind-magnetosphere interconnection topology. The auroral morphology consists of brightening events and poleward moving auroral forms (PMAFs) in the pre- and postnoon sectors longitudinally separated by a band of strongly attenuated aurora near noon. We find that the MLT-dependent spatial structure in the dayside aurora (PMAFs/prenoon – "midday gap aurora" – PMAFs/postnoon) which is present during stable IMF conditions is altered by temporal structure during intervals of IMF/solar wind plasma transients. The focus is on the PMAF substructure (so-called "rebrightening forms") which we identify as dynamical plasma flow shear arcs (FSAs) in By-distorted dawn- and dusk-centered convection cells in the close vicinity of the cusp.
Daily variation at three Antarctic geomagnetic observatories within the polar cap
Authors: Pietrolungo M. , Lepidi S. , Cafarella L. , Santarelli L. , Di Mauro D.
Published: June 2010

Abstract. In this work we present a statistical analysis of the diurnal variation as observed at three Antarctic observatories located at different positions within the polar cap during the year 2006. Data used for the analysis are from the Italian geomagnetic observatory at Mario Zucchelli Station (formerly Terra Nova Bay, geographic latitude 74.7° S, corrected geomagnetic latitude 80.0° S), from the French-Italian observatory at Concordia Station (75.1° S, 88.9° S) and from the French observatory at Dumont D'Urville (66.7° S, 80.4° S), which are located in pairs at the same geographic and corrected geomagnetic latitude; such a position allows to distinguish whether the geographic or the geomagnetic reference system is better suitable to describe the observed phenomena at so high latitudes. The peculiarities of the daily variation as observed during this year and its relation with the observatory location and magnetospheric and interplanetary conditions were analysed. Data were also studied taking into account different Lloyd seasons. The results indicate that the 24-h variation is quite persistent, but its amplitude strongly depends on season and global geomagnetic activity: indeed, it almost vanishes during local winter for quiet geomagnetic conditions; this reduction is more evident at the stations closer to the geographic pole, where the solar radiation reduction during winter is more dramatic. The Interplanetary Magnetic Field orientation has been found to be important in that the north-south and the east-west components control the amplitude and the diurnal pattern of the variation, respectively.
Polar observations of ion/electron bursts at the pre-dawn polar cap boundary: evidence for internal reconnection of overdraped lobe flux
Authors: Sandholt P. E. , Farrugia C. J.
Published: June 2010

Abstract. Observations made by Polar of ion-electron bursts on the dawn side of the polar cap are presented. They occurred when conditions external to the magnetosphere corresponded to that of the sheath region of a magnetic cloud, which was characterized by very high densities/dynamic pressure and a magnetic field which was strong in all components and which was tilted antisunward (Bx<0) and northward (Bz>0) with its clock angle lying between 20 and 90° (By: 8–15 nT). A clear temporal development in the energy range spanned by the individual ion bursts (from 0.2–2 keV to 1–10 keV) was present. We relate this to a corresponding temporal evolution in the cloud sheath field and plasma. We analyze the solar wind-magnetosphere aspects of the observations using the concepts of (i) (i) overdraped lobe flux, (ii) Bx- and By-regulated sequential reconnections in opposite hemispheres (magnetopause and internal modes), and (iii) newly-closed magnetic flux. In particular, we find that the most energetic ion bursts (accompanied by bi-directionally streaming electrons at 1–10 keV and intense magnetosheath-origin fluxes) are located on newly closed field lines generated by internal reconnection occurring between overdraped lobe field lines and the closed geomagnetic field. This result corroborates a topology of lobe reconnection under conditions of dipole tilt and/or nonzero IMF Bx component advanced by Watanabe et al. (2006), which in our case is adapted to nonzero IMF By conditions.
Multi-instrument observations of nightside plasma patches under conditions of IMF Bz positive
Authors: Wood A. G. , Pryse S. E. , Middleton H. R. , Howells V. S. C.
Published: June 2010

Abstract. Results are presented from two multi-instrument case studies showing patches of cold, long-lived plasma in the winter nightside ionosphere during times when the z-component of the Interplanetary Magnetic Field (IMF Bz) was positive. These enhancements were coincident with the antisunward convective plasma drift, flowing from polar to nightside auroral latitudes. In the first case, on 5 December 2005 with IMF By negative, two regions of enhanced electron density were observed extended in MLT in the magnetic midnight sector separated by lower densities near midnight. It is likely that the earlier enhancement originated on the dayside near magnetic noon and was transported to the nightside sector in the convective flow, whilst the later feature originated in the morning magnetic sector. The lower densities separating the two enhancements were a consequence of a pair of lobe cells essentially blocking the direct antisunward cross polar flow from the dayside. A second case study on 4 February 2006 with IMF By positive revealed a single nightside enhancement likely to have originated in the morning magnetic sector. These multi-instrument investigations, incorporating observations by the EISCAT radar facility, the SuperDARN network and radio tomography, reveal that plasma flowing from the dayside can play a significant role in the nightside ionosphere under conditions of IMF Bz positive. The observations are reinforced by simulations of flux-tube transport and plasma decay.
Polyphase alternating codes
Authors: Markkanen M. , Vierinen J. , Markkanen J.
Published: June 2010

Abstract. We present a new class of alternating codes. Instead of the customary binary phase codes, the new codes utilize either p or p–1 phases, where p is a prime number. The first class of codes has code length pm, where m is a positive integer, the second class has code length p–1. We give an actual construction algorithm, and explain the principles behind it. We handle a few specific examples in detail. The new codes offer an enlarged collection of code lengths for radar experiments.
Bistatic observations of large and small scale ULF waves in SPEAR-induced HF coherent backscatter
Authors: Yeoman T. K. , Baddeley L. J. , Dhillon R. S. , Robinson T. R. , Wright D. M.
Published: June 2010

Abstract. HF radar backscatter which has been artificially-induced by a high power RF facility has been demonstrated to provide ionospheric electric field data of unprecedented temporal resolution and accuracy. Here such data, induced by the SPEAR high power radar on Svalbard, are used to investigate ULF wave processes observed by the CUTLASS HF radars. Observations are presented of both waves with a large-scale nature, driven externally to the magnetosphere and those with small azimuthal scale lengths, driven by wave-particle interactions. For ULF wave events with large azimuthal scale lengths an excellent agreement in the observed wave polarisation ellipse is found between the radar observations and ground-based magnetometer data. In contrast, for the small scale events, no ground-based magnetic counterpart is observed. Indeed the data from the two CUTLASS radars seem inconsistent, and each radar must be interpreted separately, as the spatial resolution of the radars is sufficient to resolve the wave characteristics along the radar beams, but insufficient to resolve the wave characteristics across the beams. A high azimuthal wave number (m) wave with a period of 300 s and m~−60 is observed to occur over Svalbard at ~14:00 magnetic local time. This confirms the existence of waves driven by wave-particle interactions with trapped particle populations in the outer magnetosphere. A comparison of the observed wave characteristics with previous, lower latitude, observations suggests that these high latitude waves have a similar azimuthal scale size to those generated in the inner magnetosphere; the azimuthal wave number of −60 observed in the present study is comparable to previous values of −20– −50, but suggests an increase of m with latitude. A similar energy source in drifting proton populations is also suggested, but with lower characteristic proton energies of 10 keV implicated at high latitude, compared to the 20–60 keV energies invoked for previous lower latitude observations.
The EISCAT meteor code
Authors: Wannberg G. , Westman A. , Kero J. , Szasz C. , Pellinen-Wannberg A.
Published: June 2010

Abstract. The EISCAT UHF system has the unique capability to determine meteor vector velocities from the head echo Doppler shifts measured at the three sites. Since even meteors spending a very short time in the common volume produce analysable events, the technique lends itself ideally to mapping the orbits of meteors arriving from arbitrary directions over most of the upper hemisphere. A radar mode optimised for this application was developed in 2001/2002. A specially selected low-sidelobe 32-bit pseudo-random binary sequence is used to binary phase shift key (BPSK) the transmitted carrier. The baud-length is 2.4 μs and the receiver bandwidth is 1.6 MHz to accommodate both the resulting modulation bandwidth and the target Doppler shift. Sampling is at 0.6 μs, corresponding to 90-m range resolution. Target range and Doppler velocity are extracted from the raw data in a multi-step matched-filter procedure. For strong (SNR>5) events the Doppler velocity standard deviation is 100–150 m/s. The effective range resolution is about 30 m, allowing very accurate time-of-flight velocity estimates. On average, Doppler and time-of-flight (TOF) velocities agree to within about one part in 103. Two or more targets simultaneously present in the beam can be resolved down to a range separation <300 m as long as their Doppler shifts differ by more than a few km/s.
Plasma convection in the magnetotail lobes: statistical results from Cluster EDI measurements
Authors: Haaland S. , Paschmann G. , Förster M. , Quinn J. , Torbert R. , Vaith H. , Puhl-Quinn P. , Kletzing C.
Published: June 2010

Abstract. A major part of the plasma in the Earth's magnetotail is populated through transport of plasma from the solar wind via the magnetotail lobes. In this paper, we present a statistical study of plasma convection in the lobes for different directions of the interplanetary magnetic field and for different geomagnetic disturbance levels. The data set used in this study consists of roughly 340 000 one-minute vector measurements of the plasma convection from the Cluster Electron Drift Instrument (EDI) obtained during the period February 2001 to June 2007. The results show that both convection magnitude and direction are largely controlled by the interplanetary magnetic field (IMF). For a southward IMF, there is a strong convection towards the central plasma sheet with convection velocities around 10 km s−1. During periods of northward IMF, the lobe convection is almost stagnant. A By dominated IMF causes a rotation of the convection patterns in the tail with an oppositely directed dawn-dusk component of the convection for the northern and southern lobe. Our results also show that there is an overall persistent duskward component, which is most likely a result of conductivity gradients in the footpoints of the magnetic field lines in the ionosphere.
Effects on magnetic reconnection of a density asymmetry across the current sheet
Authors: Tanaka K. G. , Retinò A. , Asano Y. , Fujimoto M. , Shinohara I. , Vaivads A. , Khotyaintsev Y. , André M. , Bavassano-Cattaneo M. B. , Buchert S. C. , Owen C. J.
Published: June 2010

Abstract. The magnetopause (MP) reconnection is characterized by a density asymmetry across the current sheet. The asymmetry is expected to produce characteristic features in the reconnection layer. Here we present a comparison between the Cluster MP crossing reported by Retinò et al. (2006) and virtual observations in two-dimensional particle-in-cell simulation results. The simulation, which includes the density asymmetry but has zero guide field in the initial condition, has reproduced well the observed features as follows: (1) The prominent density dip region is detected at the separatrix region (SR) on the magnetospheric (MSP) side of the MP. (2) The intense electric field normal to the MP is pointing to the center of the MP at the location where the density dip is detected. (3) The ion bulk outflow due to the magnetic reconnection is seen to be biased towards the MSP side. (4) The out-of-plane magnetic field (the Hall magnetic field) has bipolar rather than quadrupolar structure, the latter of which is seen for a density symmetric case. The simulation also showed rich electron dynamics (formation of field-aligned beams) in the proximity of the separatrices, which was not fully resolved in the observations. Stepping beyond the simulation-observation comparison, we have also analyzed the electron acceleration and the field line structure in the simulation results. It is found that the bipolar Hall magnetic field structure is produced by the substantial drift of the reconnected field lines at the MSP SR due to the enhanced normal electric field. The field-aligned electrons at the same MSP SR are identified as the gun smokes of the electron acceleration in the close proximity of the X-line. We have also analyzed the X-line structure obtained in the simulation to find that the density asymmetry leads to a steep density gradient in the in-flow region, which may lead to a non-stationary behavior of the X-line when three-dimensional freedom is taken into account.
Development of radio acoustic sounding system (RASS) with Gadanki MST radar – first results
Authors: Chandrasekhar Sarma T. V. , Narayana Rao D. , Furumoto J. , Tsuda T.
Published: June 2010

Abstract. A high-power acoustic exciter was designed and developed for the Gadanki MST Radar to facilitate observations in the Radio Acoustic Sounding System (RASS) mode. Sweep range of acoustic signal frequencies was set to 94–125 Hz so as to satisfy Bragg matching condition for temperature range of −90°–40°C between surface and the tropopause (about 17 km). Raytracing of acoustic wave propagation was used to predict the antenna beam directions along which optimum RASS echoes could be obtained. During the RASS observation period of about 18 h on 23–24 July 2006 height profiles of atmospheric virtual temperature were obtained between 1.5 km and 10 km and occasionally up to 14 km. In comparison with the three simultaneous radiosonde launches, RASS derived temperature profiles had the r.m.s. discrepancy of about 1 K, although deviation of the RASS results sometimes appeared when the radial wind velocity was not fully available for the correction of apparent sound speed. This study has successfully demonstrated capability of the RASS application with the Gadanki MST radar, which will be used for continuous monitoring of the temperature profiles in the troposphere and lower stratosphere region in the tropics.
Ring current decay time model during geomagnetic storms: a simple analytical approach
Authors: Monreal MacMahon R. , Llop-Romero C.
Published: June 2010

Abstract. The ring current growth and decay, characterized by the Dst index, has been studied for thirty years using the Burton et al. (1975) equation. The original formula is based on the restriction of the DPS (Dessler, Parker, and Schoppke) theorem and assuming a constant decay time of particles. The decay time scale is important because the energy injection rate cannot be determined it without the knowledge of this parameter. In a previous work, instead of using a constant value, we introduced the decay time of particles in the energy rate balance equation as a continuous function of the absolute value of the pressure corrected Dst index to avoid the reported discontinuities determining it. Here, based on the DPS restriction, we extend our previous empirical work to obtain analytically the proposed continuous function considering losses due to a global resistive force as a product of viscous-like, and other related dissipation processes. We test our model predicting Dst for a couple of specific storm events and also comparing our results with forecasts of a good reference model appeared in the literature.
High time resolution boundary layer description using combined remote sensing instruments
Authors: Gaffard C. , Nash J. , Walker E. , Hewison T. J. , Jones J. , Norton E. G.
Published: June 2010

Abstract. Ground based remote sensing systems for future observation operations will allow continuous monitoring of the lower troposphere at temporal resolutions much better than every 30 min. Observations which may be considered spurious from an individual instrument can be validated or eliminated when considered in conjunction with measurements from other instruments observing at the same location. Thus, improved quality control of atmospheric profiles from microwave radiometers and wind profilers should be sought by considering the measurements from different systems together rather than individually. In future test bed deployments for future operational observing systems, this should be aided by observations from laser ceilometers and cloud radars. Observations of changes in atmospheric profiles at high temporal resolution in the lower troposphere are presented from a 12 channel microwave radiometer and 1290 MHz UHF wind profiler deployed in southern England during the CSIP field experiment in July/August 2005. The observations chosen were from days when thunderstorms occurred in southern England. Rapid changes near the surface in dry layers are considered, both when rain/hail may be falling from above and where the dry air is associated with cold pools behind organised thunderstorms. Also, short term variations in atmospheric profiles and vertical stability are presented on a day with occasional low cloud, when thunderstorms triggered 50 km down wind of the observing site Improved quality control of the individual remote sensing systems need to be implemented, examining the basic quality of the underlying observations as well as the final outputs, and so for instance eliminating ground clutter as far as possible from the basic Doppler spectra measurements of the wind profiler. In this study, this was performed manually. The potential of incorporating these types of instruments in future upper air observational networks leads to the challenge to improve the observing systems and also to exploit measurements at high temporal resolution in numerical weather prediction. These examples are intended to inform potential operational users of the changes in atmospheric structure that can be observed with the new types of observing system.
Derivation of a planetary ionospheric storm index
Authors: Gulyaeva T. L. , Stanislawska I.
Published: June 2010

Abstract. The planetary ionospheric storm index, Wp, is deduced from the numerical global ionospheric GPS-IONEX maps of the vertical total electron content, TEC, for more than half a solar cycle, 1999–2008. The TEC values are extracted from the 600 grid points of the map at latitudes 60° N to 60° S with a step of 5° and longitudes 0° to 345° E with a step of 15° providing the data for 00:00 to 23:00 h of local time. The local effects of the solar radiant energy are filtered out by normalizing of the TEC in terms of the solar zenith angle χ at a particular time and the local noon value χ0. The degree of perturbation, DTEC, is computed as log of TEC relative to quiet reference median for 27 days prior to the day of observation. The W-index map is generated by segmentation of DTEC with the relevant thresholds specified earlier for foF2 so that 1 or −1 stands for the quiet state, 2 or −2 for the moderate disturbance, 3 or −3 for the moderate ionospheric storm, and 4 or −4 for intense ionospheric storm at each grid point of the map. The planetary ionospheric storm Wp index is obtained from the W-index map as a latitudinal average of the distance between maximum positive and minimum negative W-index weighted by the latitude/longitude extent of the extreme values on the map. The threshold Wp exceeding 4.0 index units and the peak value Wpmax≥6.0 specify the duration and the power of the planetary ionosphere-plasmasphere storm. It is shown that the occurrence of the Wp storms is growing with the phase of the solar cycle being twice as much as the number of the magnetospheric storms with Dst≤−100 nT and Ap≥100 nT.
A multi-diagnostic approach to understanding high-latitude plasma transport during the Halloween 2003 storm
Authors: Yin P. , Mitchell C. N. , Spencer P. , McCrea I. , Pedersen T.
Published: June 2010

Abstract. During the Halloween 2003 storm event, significant electron density enhancements at elevated F-layer altitudes were recorded by the EISCAT and ESR radars in northern Europe between 20:00 and 24:00 UT on 30 October. At the same time, a sequence of optical images from Qaanaaq in northern Greenland captured a series of eastward-propagating polar cap patches. In this paper, an advanced 4-D tomographic method based on the assimilation of global GPS data, coupled to a predictive Kalman filtering technique, has been used to reveal the linkage between these ionospheric structures. The combination of the various data sources has clearly established the time history of this extreme event, in which high-density plasma was uplifted in the dayside ionosphere and convected anti-sunward across the polar cap to European high latitudes at an elevated F-layer. Using this multi instrument approach, we can differentiate between those density structures observed at the ESR which occurred as a result of cross-polar transport and those more likely to have been produced by in-situ soft particle precipitation, a distinction which is supported by the ESR and EISCAT data. The multi-diagnostic approach reported here has the potential significantly to extend our current understanding of high latitude plasma transport and the origin of electron density enhancements.
The possible statistical relation of Pc1 pulsations to Earthquake occurrence at low latitudes
Authors: Bortnik J. , Cutler J. W. , Dunson C. , Bleier T. E.
Published: June 2010

Abstract. We examine the association between earthquakes and Pc1 pulsations observed at a low-latitude station in Parkfield, California. The period under examination is ~7.5 years in total, from February 1999 to July 2006, and we use an automatic identification algorithm to extract information on Pc1 pulsations from the magnetometer data. These pulsations are then statistically correlated to earthquakes from the USGS NEIC catalog within a radius of 200 km around the magnetometer, and M>3.0. Results indicate that there is an enhanced occurrence probability of Pc1 pulsations ~5–15 days in advance of the earthquakes, during the daytime. We quantify the statistical significance and show that such an enhancement is unlikely to have occurred due to chance alone. We then examine the effect of declustering our earthquake catalog, and show that even though significance decreases, there is still a statistically significant daytime enhancement prior to the earthquakes. Finally, we select only daytime Pc1 pulsations as the fiducial time of our analysis, and show that earthquakes are ~3–5 times more likely to occur in the week following these pulsations, than normal. Comparing these results to other events, it is preliminarily shown that the normal earthquake probability is unaffected by geomagnetic activity, or a random event sequence.
The Pedersen current carried by electrons: a non-linear response of the ionosphere to magnetospheric forcing
Authors: Buchert S. C. , Tsuda T. , Fujii R. , Nozawa S.
Published: June 2010

Abstract. Observations by the EISCAT Svalbard radar show that electron temperatures Te in the cusp electrojet reach up to about 4000 K. The heat is tapped and converted from plasma convection in the near Earth space by a Pedersen current that is carried by electrons due to the presence of irregularities and their demagnetising effect. The heat is transfered to the neutral gas by collisions. In order to enhance Te to such high temperatures the maximally possible dissipation at 50% demagnetisation must nearly be reached. The effective Pedersen conductances are found to be enhanced by up to 60% compared to classical values. Conductivities and conductances respond significantly to variations of the electric field strength E, and "Ohm's law" for the ionosphere becomes non-linear for large E.
Statistical analysis of thermospheric gravity waves from Fabry-Perot Interferometer measurements of atomic oxygen
Authors: Ford E. A. K. , Aruliah A. L. , Griffin E. M. , McWhirter I.
Published: June 2010

Abstract. Data from the Fabry-Perot Interferometers at KEOPS (Sweden), Sodankylä (Finland), and Svalbard (Norway), have been analysed for gravity wave activity on all the clear nights from 2000 to 2006. A total of 249 nights were available from KEOPS, 133 from Sodankylä and 185 from the Svalbard FPI. A Lomb-Scargle analysis was performed on each of these nights to identify the periods of any wave activity during the night. Comparisons between many nights of data allow the general characteristics of the waves that are present in the high latitude upper thermosphere to be determined. Comparisons were made between the different parameters: the atomic oxygen intensities, the thermospheric winds and temperatures, and for each parameter the distribution of frequencies of the waves was determined. No dependence on the number of waves on geomagnetic activity levels, or position in the solar cycle, was found. All the FPIs have had different detectors at various times, producing different time resolutions of the data, so comparisons between the different years, and between data from different sites, showed how the time resolution determines which waves are observed. In addition to the cutoff due to the Nyquist frequency, poor resolution observations significantly reduce the number of short-period waves (<1 h period) that may be detected with confidence. The length of the dataset, which is usually determined by the length of the night, was the main factor influencing the number of long period waves (>5 h) detected. Comparisons between the number of gravity waves detected at KEOPS and Sodankylä over all the seasons showed a similar proportion of waves to the number of nights used for both sites, as expected since the two sites are at similar latitudes and therefore locations with respect to the auroral oval, confirming this as a likely source region. Svalbard showed fewer waves with short periods than KEOPS data for a season when both had the same time resolution data. This gives a clear indication of the direction of flow of the gravity waves, and corroborates that the source is the auroral oval. This is because the energy is dissipated through heating in each cycle of a wave, therefore, over a given distance, short period waves lose more energy than long and dissipate before they reach their target.
Solar and geomagnetic activity effects on mid-latitude F-region electric fields
Authors: Kumar V. V. , Parkinson M. L. , Dyson P. L. , Polglase R.
Published: June 2010

Abstract. Diurnal patterns of average F-region ionospheric drift (electric field) and their dependence on solar and geomagnetic activity have been defined using digital ionosonde Doppler measurements recorded at a southern mid-latitude station (Bundoora 145.1° E, 37.7° S geographic, 49° S magnetic). A unique database consisting of 300 907 drift velocities was compiled, mostly using one specific mode of operation throughout 1632 days of a 5-year interval (1999–2003). The velocity magnitudes were generally larger during the night than day, except during the winter months (June–August), when daytime velocities were enhanced. Of all years, the largest drifts tended to occur during the high speed solar wind streams of 2003. Diurnal patterns in the average quiet time (AE<75 nT) meridional drifts (zonal electric field) peaked at up to ~6 m s−1 poleward (0.3 mV m−1 eastward) at 03:30 LST, reversing in direction at ~08:30 LST, and gradually reaching ~10 m s−1 equatorward at ~13:30 LST. The quiet time zonal drifts (meridional electric fields) displayed a clear diurnal pattern with peak eastward flows of ~10 m s−1 (0.52 mV m−1 equatorward) at 09:30 LST and peak westward flows around midnight of ~18 m s−1 (0.95 mV m−1 poleward). As the AE index increased, the westward drifts increased in amplitude and they extended over a greater fraction of the day. The perturbation drifts changed in a similar way with decreasing Dst except the daytime equatorward flows strengthened with increasing AE index, whereas they became weak for Dst<−60 nT. The responses in all velocity components to changing solar flux values were small, but net poleward perturbations during the day were associated with large solar flux values (>192×10−22 W m−2 Hz−1). These results help to more fully quantify the response of the mid-latitude ionosphere to changing solar and geomagnetic conditions, as required to refine empirical and theoretical models of mid-latitude electric fields.
Transmission code optimization method for incoherent scatter radar
Authors: Vierinen J. , Lehtinen M. S. , Orispää M. , Virtanen I. I.
Published: June 2010

Abstract. When statistical inversion of a lag profile is used to determine an incoherent scatter target, the posterior variance of the estimated target can be used to determine how well a set of transmission codes perform. In this work we present an incoherent scatter radar transmission code optimization search method suitable for different modulation types, including binary phase, polyphase and amplitude modulation. We found that the combination of amplitude and phase modulation provides better performance than traditional binary phase coding, in some cases giving better accuracy than alternating codes.
Diagnostics of active and eruptive prominences through hydrogen and helium lines modelling
Authors: Labrosse N. , Vial J.-C. , Gouttebroze P.
Published: June 2010

Abstract. In this study we show how hydrogen and helium lines modelling can be used to make a diagnostic of active and eruptive prominences. One motivation for this work is to identify the physical conditions during prominence activation and eruption. Hydrogen and helium lines are key in probing different parts of the prominence structure and inferring the plasma parameters. However, the interpretation of observations, being either spectroscopic or obtained with imaging, is not straightforward. Their resonance lines are optically thick, and the prominence plasma is out of local thermodynamic equilibrium due to the strong incident radiation coming from the solar disk. In view of the shift of the incident radiation occuring when the prominence plasma flows radially, it is essential to take into account velocity fields in the prominence diagnostic. Therefore we need to investigate the effects of the radial motion of the prominence plasma on hydrogen and helium lines. The method that we use is the resolution of the radiative transfer problem in the hydrogen and helium lines out of local thermodynamic equilibrium. We study the variation of the computed integrated intensities in H and He lines with the radial velocity of the prominence plasma. We can confirm that there exist suitable lines which can be used to make a diagnostic of the plasma in active and eruptive prominences in the presence of velocity fields.
Causal relationships between eruptive prominences and coronal mass ejections
Authors: Filippov B. , Koutchmy S.
Published: June 2010

Abstract. A close association between eruptive prominences and CMEs, both slow and fast CMEs, was reported in many studies. Sometimes it was possible to follow the material motion starting from the prominence (filament) activation to the CME in the high corona. Remnants of the prominence were found in the bright core of the CME. However, detailed comparisons of the two phenomena reveal problems in explaining CMEs as a continuation of filament eruptions in the upper corona. For example, the heliolatitudes of the disappeared filaments and subsequent coronal ejections sometimes differ by tens of degrees. In order to clear up the problems appearing when considering this association EP-CME, we tentatively analyse the more general question of the dynamics of the generic magnetic flux rope. Prominences and filaments are the best tracers of the flux ropes in the corona long before the beginning of the eruption. A twisted flux rope is held by the tension of field lines of photospheric sources until parameters of the system reach critical values and a catastrophe happens. We suggest that the associated flux rope height above the photosphere is one of these parameters and that it is revealed by the measured height of the filament. 80 filaments were analysed and we found that eruptive prominences were near the so-called limit of stability a few days before their eruptions. We suggest that a comparison of actual heights of prominences with the calculated critical heights from magnetograms could be systematically used to predict filament eruptions and the corresponding CMEs.
A statistical approach for identifying the ionospheric footprint of magnetospheric boundaries from SuperDARN observations
Authors: Lointier G. , Dudok de Wit T. , Hanuise C. , Vallières X. , Villain J.-P.
Published: June 2010

Abstract. Identifying and tracking the projection of magnetospheric regions on the high-latitude ionosphere is of primary importance for studying the Solar Wind-Magnetosphere-Ionosphere system and for space weather applications. By its unique spatial coverage and temporal resolution, the Super Dual Auroral Radar Network (SuperDARN) provides key parameters, such as the Doppler spectral width, which allows the monitoring of the ionospheric footprint of some magnetospheric boundaries in near real-time. In this study, we present the first results of a statistical approach for monitoring these magnetospheric boundaries. The singular value decomposition is used as a data reduction tool to describe the backscattered echoes with a small set of parameters. One of these is strongly correlated with the Doppler spectral width, and can thus be used as a proxy for it. Based on this, we propose a Bayesian classifier for identifying the spectral width boundary, which is classically associated with the Polar Cap boundary. The results are in good agreement with previous studies. Two advantages of the method are: the possibility to apply it in near real-time, and its capacity to select the appropriate threshold level for the boundary detection.
Reconnection in a slow Coronal Mass Ejection
Authors: Poletto G. , Bemporad A. , Landini F. , Romoli M.
Published: June 2010

Abstract. This paper aims at studying reconnection occurring in the aftermath of the 28 May 2004, CME, first imaged by the LASCO (Large Angle and Spectrometric Coronagraph) C2 at 11:06 UT. The CME was observed in White Light and UV radiation: images acquired by the LASCO C2 and C3 coronagraphs and spectra acquired by UVCS (Ultraviolet Coronagraph Spectrometer) allowed us to identify the level at which field lines, stretched outwards by the CME ejection, reconnect below the CME bubble. As the CME propagates outwards, reconnection occurs at increasingly higher levels. The process goes on at a low pace for several hours: here we give the profile of the reconnection rate vs. heliocentric distance over a time interval of ≈14 h after the CME onset, extending estimates of the reconnection rate to larger distances than previously inferred by other authors. The reconnection rate appears to decrease with time/altitude. We also calculate upper and lower limits to the density in the diffusion region between 4 and 7 R⊙ and conclude by comparing estimates of the classical and anomalous resistivity in the diffusion region with the value inferred from the data. The latter turns out to be ≥5 order of magnitudes larger than predicted by classical or anomalous theories, pointing to the need of identifying the process responsible for the observed value.
Why are CMEs large-scale coronal events: nature or nurture?
Authors: van Driel-Gesztelyi L. , Attrill G. D. R. , Démoulin P. , Mandrini C. H. , Harra L. K.
Published: June 2010

Abstract. The apparent contradiction between small-scale source regions of, and large-scale coronal response to, coronal mass ejections (CMEs) has been a long-standing puzzle. For some, CMEs are considered to be inherently large-scale events – eruptions in which a number of flux systems participate in an unspecified manner, while others consider magnetic reconnection in special global topologies to be responsible for the large-scale response of the lower corona to CME events. Some of these ideas may indeed be correct in specific cases. However, what is the key element which makes CMEs large-scale? Observations show that the extent of the coronal disturbance matches the angular width of the CME – an important clue, which does not feature strongly in any of the above suggestions. We review observational evidence for the large-scale nature of CME source regions and find them lacking. Then we compare different ideas regarding how CMEs evolve to become large-scale. The large-scale magnetic topology plays an important role in this process. There is amounting evidence, however, that the key process is magnetic reconnection between the CME and other magnetic structures. We outline a CME evolution model, which is able to account for all the key observational signatures of large-scale CMEs and presents a clear picture how large portions of the Sun become constituents of the CME. In this model reconnection is driven by the expansion of the CME core resulting from an over-pressure relative to the pressure in the CME's surroundings. This implies that the extent of the lower coronal signatures match the final angular width of the CME.
Processes and mechanisms governing the initiation and propagation of CMEs
Authors: Vršnak B.
Published: June 2010

Abstract. The most important observational characteristics of coronal mass ejections (CMEs) are summarized, emphasizing those aspects which are relevant for testing physical concepts employed to explain the CME take-off and propagation. In particular, the kinematics, scalings, and the CME-flare relationship are stressed. Special attention is paid to 3-dimensional (3-D) topology of the magnetic field structures, particularly to aspects related to the concept of semi-toroidal flux-rope anchored at both ends in the dense photosphere and embedded in the coronal magnetic arcade. Observations are compared with physical principles and concepts employed in explaining the CME phenomenon, and implications are discussed. A simple flux-rope model is used to explain various stages of the eruption. The model is able to reproduce all basic observational requirements: stable equilibrium and possible oscillations around equilibrium, metastable state and possible destabilization by an external disturbance, pre-eruptive gradual-rise until loss of equilibrium, possibility of fallback events and failed eruptions, relationship between impulsiveness of the CME acceleration and the source-region size, etc. However, it is shown that the purely ideal MHD process cannot account for highest observed accelerations which can attain values up to 10 km s−2. Such accelerations can be achieved if the process of reconnection beneath the erupting flux-rope is included into the model. Essentially, the role of reconnection is in changing the magnetic flux associated with the flux-rope current and supplying "fresh" poloidal magnetic flux to the rope. These effects help sustain the electric current flowing along the flux-rope, and consequently, reinforce and prolong the CME acceleration. The model straightforwardly explains the observed synchronization of the flare impulsive phase and the CME main-acceleration stage, as well as the correlations between various CME and flare parameters.
A comparison of coronal mass ejections identified by manual and automatic methods
Authors: Yashiro S. , Michalek G. , Gopalswamy N.
Published: June 2010

Abstract. Coronal mass ejections (CMEs) are related to many phenomena (e.g. flares, solar energetic particles, geomagnetic storms), thus compiling of event catalogs is important for a global understanding these phenomena. CMEs have been identified manually for a long time, but in the SOHO era, automatic identification methods are being developed. In order to clarify the advantage and disadvantage of the manual and automatic CME catalogs, we examined the distributions of CME properties listed in the CDAW (manual) and CACTus (automatic) catalogs. Both catalogs have a good agreement on the wide CMEs (width>120°) in their properties, while there is a significant discrepancy on the narrow CMEs (width≤30°): CACTus has a larger number of narrow CMEs than CDAW. We carried out an event-by-event examination of a sample of events and found that the CDAW catalog have missed many narrow CMEs during the solar maximum. Another significant discrepancy was found on the fast CMEs (speed>1000 km/s): the majority of the fast CDAW CMEs are wide and originate from low latitudes, while the fast CACTus CMEs are narrow and originate from all latitudes. Event-by-event examination of a sample of events suggests that CACTus has a problem on the detection of the fast CMEs.
Two-spacecraft reconstruction of a magnetic cloud and comparison to its solar source
Authors: Möstl C. , Miklenic C. , Farrugia C. J. , Temmer M. , Veronig A. , Galvin A. B. , Vršnak B. , Biernat H. K.
Published: June 2010

Abstract. This paper compares properties of the source region with those inferred from satellite observations near Earth of the magnetic cloud which reached 1 AU on 20 November 2003. We use observations from space missions SOHO and TRACE together with ground-based data to study the magnetic structure of the active region NOAA 10501 containing a highly curved filament, and determine the reconnection rates and fluxes in an M4 flare on 18 November 2003 which is associated with a fast halo CME. This event has been linked before to the magnetic cloud on 20 November 2003. We model the near-Earth observations with the Grad-Shafranov reconstruction technique using a novel approach in which we optimize the results with two-spacecraft measurements of the solar wind plasma and magnetic field made by ACE and WIND. The two probes were separated by hundreds of Earth radii. They pass through the axis of the cloud which is inclined −50 degree to the ecliptic. The magnetic cloud orientation at 1 AU is consistent with an encounter with the heliospheric current sheet. We estimate that 50% of its poloidal flux has been lost through reconnection in interplanetary space. By comparing the flare ribbon flux with the original cloud fluxes we infer a flux rope formation during the eruption, though uncertainties are still significant. The multi-spacecraft Grad-Shafranov method opens new vistas in probing of the spatial structure of magnetic clouds in STEREO-WIND/ACE coordinated studies.
A study of fine structure of diffuse aurora with ALIS-FAST measurements
Authors: Sergienko T. , Sandahl I. , Gustavsson B. , Andersson L. , Brändström U. , Steen Å.
Published: June 2010

Abstract. We present results of an investigation of the fine structure of the night sector diffuse auroral zone, observed simultaneously with optical instruments (ALIS) from the ground and the FAST electron spectrometer from space 16 February 1997. Both the optical and particle data show that the diffuse auroral zone consisted of two regions. The equatorward part of the diffuse aurora was occupied by a pattern of regular, parallel auroral stripes. The auroral stripes were significantly brighter than the background luminosity, had widths of approximately 5 km and moved southward with a velocity of about 100 m/s. The second region, located between the region with auroral stripes and the discrete auroral arcs to the north, was filled with weak and almost homogeneous luminosity, against which short-lived auroral rays and small patches appeared chaotically. From analysis of the electron differential fluxes corresponding to the different regions of the diffuse aurora and based on existing theories of the scattering process we conclude the following: Strong pitch angle diffusion by electron cyclotron harmonic waves (ECH) of plasma sheet electrons in the energy range from a few hundred eV to 3–4 keV was responsible for the electron precipitation, that produced the background luminosity within the whole diffuse zone. The fine structure, represented by the auroral stripes, was created by precipitation of electrons above 3–4 keV as a result of pitch angle diffusion into the loss cone by whistler mode waves. A so called "internal gravity wave" (Safargaleev and Maltsev, 1986) may explain the formation of the regular spatial pattern formed by the auroral stripes in the equatorward part of the diffuse auroral zone.
Survey of ELF-VLF plasma waves in outer radiation belt observed by Cluster STAFF-SA experiment
Authors: Pokhotelov D. , Lefeuvre F. , Horne R. B. , Cornilleau-Wehrlin N.
Published: June 2010

Abstract. Various types of plasma waves have profound effects on acceleration and scattering of radiation belt particles. For the purposes of radiation belt modeling it is necessary to know statistical distributions of plasma wave parameters. This paper analyzes four years of plasma wave observations in the Earth's outer radiation belt obtained by the STAFF-SA experiment on board Cluster spacecraft. Statistical distributions of spectral density of different plasma waves observed in ELF-VLF range (chorus, plasmaspheric hiss, magnetosonic waves) are presented as a function of magnetospheric coordinates and geomagnetic activity indices. Comparison with other spacecraft studies supports some earlier conclusions about the distribution of chorus and hiss waves and helps to remove the long-term controversy regarding the distribution of equatorial magnetosonic waves. This study represents a step towards the development of multi-spacecraft database of plasma wave activity in radiation belts.