bluuebunny/crossref_metadata_2025 huggingface dataset:
On the relationship between aspect sensitivity, wave activity, andmultiple scattering centers of mesosphere summer echoes: a case study using coherent radar imaging
Authors: Chen J.-S. , Hoffmann P. , Zecha M. , Röttger J.
Published: June 2010

Abstract. A mesosphere-summer-echo layer, observed by the OSWIN VHF radar (54.1°N, 11.8°E) with vertical and 7° oblique radar beams, was examined using the method of coherent radar imaging (CRI). We disclosed the echo events having multiple scattering centers (MSC) in the radar volume by means of the high angular resolution of the CRI technique and found that the MSC events occurred more frequently in the upper portion of the echo layer. More examinations showed that the characteristics were different between the upper and lower portions of the layer. For example, the differences in echo power between vertical and oblique beams changed mostly from positive to negative along the increase of altitude, and strong turbulent echoes were seen in the upper portion of the layer. These observations indicate that the aspect sensitivity of the echoes became less and less with the increase of altitude. Moreover, the scattering centers of the echoes were close to zenith for the lower portion of the layer but were usually several degrees from the zenith for the upper portion of the layer. Observable wave-like variation in the scattering center was also seen in the upper part of the layer. Based on these features, we drew some conclusions for this case study: (a) the MSC events might result from the slanted layer/anisotropic structure tilted by short-wave activities, (b) the tilt angle of the layer structure could be 6°–10°, causing the echo power received by the 7° oblique beam was larger than or comparable to that received by the vertical beam, and (c) short-wave activities not only tilted the layer structure, but also induced isotropic irregularities. Key words. Meteorology and atmospheric dynamics (middle atmosphere dynamics). Radio science (interferometry; instruments and techniques)
Seasonal variations of O2 atmospheric and OH(6−2) airglowand temperature at mid-latitudes from SATI observations
Authors: López-González M. J. , Rodríguez E. , Wiens R. H. , Shepherd G. G. , Sargoytchev S. , Brown S. , Shepherd M. G. , Aushev V. M. , López-Moreno J. J. , Rodrigo R. , Cho Y.-M.
Published: June 2010

Abstract. More than 3 years of airglow observations with a Spectral Airglow Temperature Imager (SATI) installed at the Sierra Nevada Observatory (37.06°N, 3.38°W) at 2900m height have been analyzed. Values of the column emission rate and vertically averaged temperature of the O2 atmospheric (0–1) band and of the OH Meinel (6–2) band from 1998 to 2002 have been presented. From these observations a clear seasonal variation of both emission rates and rotational temperatures is inferred at this latitude. It is found that the annual variation of the temperatures is larger than the semi-annual variation, while for the emission rates the amplitudes are comparable. Key words. Atmospheric composition and structure (airglow and aurora; pressure density and temperature; instruments and techniques)
The statistical dependence of auroral absorption on geomagnetic and solar wind parameters
Authors: Kavanagh A. J. , Kosch M. J. , Honary F. , Senior A. , Marple S. R. , Woodfield E. E. , McCrea I. W.
Published: June 2010

Abstract. Data from the Imaging Riometer for Ionospheric Studies (IRIS) at Kilpisjärvi, Finland, have been compiled to form statistics of auroral absorption based on seven years of observations. In a previous study a linear relationship between the logarithm of the absorption and the Kp index provided a link between the observations of precipitation with the level of geomagnetic activity. A better fit to the absorption data is found in the form of a quadratic in Kp for eight magnetic local time sectors. Past statistical investigations of absorption have hinted at the possibility of using the solar wind velocity as a proxy for the auroral absorption, although the lack of available satellite data made such an investigation difficult. Here we employ data from the solar wind monitors, WIND and ACE, and derive a linear relationship between the solar wind velocity and the cosmic noise absorption at IRIS for the same eight magnetic local time sectors. As far as the authors are aware this is the first time that in situ measurements of the solar wind velocity have been used to create a direct link with absorption on a statistical basis. The results are promising although, it is clear that some other factor is necessary in providing reliable absorption predictions. Due to the substorm related nature of auroral absorption, this is likely formed by the recent time history of the geomagnetic activity, or by some other indicator of the energy stored within the magnetotail. For example, a dependence on the southward IMF (interplanetary magnetic field) is demonstrated with absorption increasing with successive decreases in Bz; a northward IMF appears to have little effect and neither does the eastward component, By. Key words. Magnetospheric physics (energetic particles, precipitating; solar wind-magnetosphere interactions) – Ionosphere (modeling and forecasting)
Investigation of subauroral ion drifts and related field-aligned currents and ionospheric Pedersen conductivity distribution
Authors: Figueiredo S. , Karlsson T. , Marklund G. T.
Published: June 2010

Abstract. Based on Astrid-2 satellite data, results are presented from a statistical study on subauroral ion drift (SAID) occurrence. SAID is a subauroral phenomenon characterized by a westward ionospheric ion drift with velocity greater than 1000m/s, or equivalently, by a poleward-directed electric field with intensity greater than 30mV/m. SAID events occur predominantly in the premidnight sector, with a maximum probability located within the 20:00 to 23:00 MLT sector, where the most rapid SAID events are also found. They are substorm related, and show first an increase in intensity and a decrease in latitudinal width during the expansion phase, followed by a weakening and widening of the SAID structures during the recovery phase. The potential drop across a SAID structure is seen to remain roughly constant during the recovery phase. The field-aligned current density and the height-integrated Pedersen conductivity distribution associated with the SAID events were calculated. The results reveal that the strongest SAID electric field peaks are associated with the lowest Pedersen conductivity minimum values. Clear modifications are seen in the ionospheric Pedersen conductivity distribution associated with the SAID structure as time evolves: the SAID peak is located on the poleward side of the corresponding region of reduced Pedersen conductivity; the shape of the regions of reduced conductivity is asymmetric, with a steeper poleward edge and a more rounded equatorward edge; the SAID structure becomes less intense and widens with evolution of the substorm recovery phase. From the analysis of the SAID occurrence relative to the mid-latitude trough position, SAID peaks are seen to occur relatively close to the corresponding mid-latitude trough minimum. Both these features show a similar response to magnetospheric disturbances, but on different time scales - with increasing magnetic activity, the SAID structure shows a faster movement towards lower latitudes than that of the mid-latitude trough. From the combined analysis of these results, we conclude that the SAID generation mechanism cannot be regarded either as a pure voltage generator or as a pure current generator, applied to the ionosphere. While the anti-correlation between the width and the peak intensity of the SAID structures with substorm evolution indicates a magnetospheric source acting as a constant voltage generator, the ionospheric modifications and, in particular the reduction in the conductivity for intense SAID structures, are indicative of a constant current system closing through the ionosphere. The ionospheric feedback mechanisms are seen to be of major importance for sustaining and regulating the SAID structures. Key words. Ionosphere (mid-latitude ionosphere; electric fields and currents; ionosphere-magnetosphere interactions)
Relation of polar auroral arcs to magnetotail twisting and IMF rotation: a systematic MHD simulation study
Authors: Kullen A. , Janhunen P.
Published: June 2010

Abstract. We investigate with the help of a magnetohydrodynamic (MHD) model how the large-scale topology of the magnetosphere develops for a constant interplanetary magnetic field (IMF) with different IMF clock angles and for an IMF By sign change during northward IMF. A detailed examination of the topological changes in the tail and the ionosphere for different IMF conditions shows a good agreement with observational results. The MHD simulations for different constant IMF clock angle cases show the expected field-line bending and tail twisting for nonzero IMF By. The tail becomes longer and at its tailward end stronger twisted for IMF Bz>∣By∣ than for IMF Bz
Slow shock interactions in the heliosphere using an adaptive grid MHD model
Authors: Wu C.-C. , Dryer M. , Wu S. T.
Published: June 2010

Abstract. A one-dimensional (1-D), time-dependent, adaptive-grid MHD model with solar wind structure has been used in the past to study the interaction of shocks. In the present study, we wish to study some fundamental processes that may be associated with slow shock genesis and their possible interactions with other discontinuities. This adaptive-grid model, suitable for appropriate spatial and temporal numerical simulations, is used for this purpose because its finer grid sizes in the vicinity of the steep gradients at shocks make it possible to delineate the physical parameters on both sides of the shocks. We found that a perturbation with deceleration of solar wind will generate an ensemble consisting of a forward slow shock, a fast forward wave and a reverse slow shock. On the other hand, a perturbation with an increase in acceleration of solar wind will generate both a slow shock and a fast shock. These two perturbations, although not unique, may be representative of momentum and pressure changes at the solar surface. During the transition of a fast shock overtaking a slow shock from behind, the slow shock might disappear temporarily. Also, during the process of the merging of two slow shocks, a slow shock-like structure is formed first; later, the slow shock-like structure evolves into an intermediate shock-like structure. This intermediate shock-like structure then evolves into an intermediate wave and a slow shock-like structure. Finally, the slow shock-like structure evolves into a slow shock, but the intermediate wave disappears by interacting with the non-uniform solar wind. This complex behavior demonstrates the non-unique nature of the formation of slow shocks, intermediate shocks and their derivative structures. We emphasize the main aim of this work to be both: (a) non-unique input physical parameters to explain the paucity of observed slow shocks, as well as (b) the impossibility of backward tracing to the history of input boundary conditions in view of the present inability to describe unambiguous inputs at the Sun.
Large-scale velocity fluctuations in polar solar wind
Authors: Bavassano B. , Bruno R. , D'Amicis R.
Published: June 2010

Abstract. The 3-D structure of the solar wind varies dramatically along the Sun's activity cycle. In the present paper we focus on some properties of the polar solar wind. This is a fast, teneous, and steady flow (as compared to low-latitude conditions) that fills the high-latitude heliosphere at low solar activity. The polar wind has been extensively investigated by Ulysses, the first spacecraft to perform in-situ measurements in the high-latitude heliosphere. Though the polar wind is quite a uniform flow, fluctuations in its velocity do not appear negligible. A simple way to characterize the solar wind structure is that of performing a multi-scale statistical analysis of the wind velocity differences. The occurrence frequency distributions of velocity differences at time lags from 1 to 1024h and the corresponding values of mean, standard deviation, skewness, and kurtosis have been obtained. A comparison with previous results in ecliptic wind at both low and high solar activity has been performed. It comes out that the kind of trend observed in the distributions for changing scale is the same for the different solar wind regimes. Differences between different flows just have an effect on the values of the distribution moments and the scales at which the transition from non-Gaussian to Gaussian-like behaviours occurs. This is typical of systems in which random fluctuations are mixed to coherent structures of some characteristic size, in other words, systems where long-range correlations cannot be neglected.
Letter to the Editor Why do ultra-low-frequency MHD oscillations with a discrete spectrum exist in the magnetosphere?
Authors: Leonovich A. S. , Mazur V. A.
Published: June 2010

Abstract. A new concept of the global magnetospheric resonator is suggested for fast magnetosonic waves in which the role of the resonator is played by the near-Earth part of the plasma sheet. It is shown that the magnetosonic wave is confined in this region of the magnetosphere within its boundaries. The representative value of the resonator's eigenfrequency estimated at f~1MHz is in good agreement with observational data of ultra-low-frequency MHD oscillations of the magnetosphere with a discrete spectrum (f~0.8, 1.3, 1.9, 2.6...MHz). The theory explains the ground-based localization of the oscillations observed in the midnight-morning sector of the high-latitude magnetosphere.
Planetary wave coupling in the middle atmosphere (20-90km): A CUJO study involving TOMS, MetO and MF radar data
Authors: Chshyolkova T. , Manson A. H. , Meek C. E. , Avery S. K. , Thorsen D. , MacDougall J. W. , Hocking W. , Murayama Y. , Igarashi K.
Published: June 2010

Abstract. The atmospheric coupling due to Planetary Waves (PW) in the middle atmosphere (20-90km) has been studied using TOMS, MetO and MFR data. The wavelet and wave number analyses have been applied to all parameters at five CUJO (Canada US Japan Opportunity) locations. The CUJO network covers latitudes of 31-52° N and longitudes from 81° W to 142° E, and allows for the assessment of longitudinal variability. The results of temporal and spectral comparisons show that the total ozone (TOMS) and MetO temperatures at low stratospheric heights (typically 100mbar) have high values of correlation as well as similar spectral content. The eastward motions dominate at low stratospheric heights (100mbar), while westward motions became comparable or even stronger in the upper stratosphere (0.46mbar). During the summer months a reduction of PW activity has been observed in the stratosphere, especially at its upper heights, and in the upper middle atmosphere. The MetO (0.32mbar, 55km) and MFR winds (circa 60km) are in good general agreement, especially for the zonal component. Several examples of planetary wave activity at different atmospheric levels throughout the middle atmosphere have been presented. These examples include an eastward propagating 15-day disturbance with wave number 6, that has been observed only at low stratospheric heights; long-period (20-30 days) oscillations with wave number ~1 that have been detected in a wide height range (20-90km); and an oscillation with period near 16 days that was found only at mesospheric heights.
Mean diurnal variations of noctilucent clouds during 7 years of lidar observations at ALOMAR
Authors: Fiedler J. , Baumgarten G. , von Cossart G.
Published: June 2010

Abstract. From 1997 to 2003, noctilucent clouds (NLC) were observed by lidar above the ALOMAR observatory in Northern Norway (69° N) during a total of 1880 measurement hours. This data set contains NLC signatures for 640h, covering all local times, even during the highest solar background conditions. After data limitation imposing a threshold value of 4x10-10m-1sr-1 for the volume backscatter coefficient of the NLC particles, a measure for the cloud brightness, local time dependencies of the NLC occurrence frequency, altitude, and brightness were determined. On average, over the 7 years NLC occurred during the whole day and preferably in the early morning hours, with a maximum occurrence frequency of ~40% between 4 and 7 LT. Splitting the data into weak and strong clouds yields almost identical amplitudes of diurnal and semidiurnal variations for the occurrence of weak clouds, whereas the strong clouds are dominated by the diurnal variation. NLC occurrence, altitude, as well as brightness, show a remarkable persistence concerning diurnal and semidiurnal variations from 1997 to 2003, suggesting that NLC above ALOMAR are significantly controlled by atmospheric tides. The observed mean anti-phase behavior between cloud altitude and brightness is attributed to a phase shift between the semidiurnal components by ~6h. Investigation of data for each individual year regarding the prevailing oscillation periods of the NLC parameters showed different phase relationships, leading to a complex variability in the cloud parameters.
Estimates of magnetotail reconnection rate based on IMAGE FUV and EISCAT measurements
Authors: Østgaard N. , Moen J. , Mende S. B. , Frey H. U. , Immel T. J. , Gallop P. , Oksavik K. , Fujimoto M.
Published: June 2010

Abstract. Dayside merging between the interplanetary and terrestrial magnetic fields couples the solar wind electric field to the Earth's magnetosphere, increases the magnetospheric convection and results in efficient transport of solar wind energy into the magnetosphere. Subsequent reconnection of the lobe magnetic field in the magnetotail transports energy into the closed magnetic field region. Combining global imaging and ground-based radar measurements, we estimate the reconnection rate in the magnetotail during two days of an EISCAT campaign in November-December 2000. Global images from the IMAGE FUV system guide us to identify ionospheric signatures of the open-closed field line boundary observed by the two EISCAT radars in Tromsø (VHF) and on Svalbard (ESR). Continuous radar and optical monitoring of the open-closed field line boundary is used to determine the location, orientation and velocity of the open-closed boundary and the ion flow velocity perpendicular to this boundary. The magnetotail reconnection electric field is found to be a bursty process that oscillates between 0mV/m and 1mV/m with ~10-15min periods. These ULF oscillations are mainly due to the motion of the open-closed boundary. In situ measurements earthward of the reconnection site in the magnetotail by Geotail show similar oscillations in the duskward electric field. We also find that bursts of increased magnetotail reconnection do not necessarily have any associated auroral signatures. Finally, we find that the reconnection rate correlates poorly with the solar wind electric field. This indicates that the magnetotail reconnection is not directly driven, but is an internal magnetospheric process. Estimates of a coupling efficiency between the solar wind electric field and magnetotail reconnection only seem to be relevant as averages over long time intervals. The oscillation mode at 1mHz corresponds to the internal cavity mode with additional lower frequencies, 0.5 and 0.8mHz, that might be modulated by solar wind pressure variations.
Excitation of plasma waves by unstable photoelectron and thermal electron populations on closed magnetic field lines in the Martian ionosphere
Authors: Borisov N. , Nielsen E.
Published: June 2010

Abstract. It is argued that anisotropic electron pitch angle distributions in the closed magnetic field regions of the Martian ionosphere gives rise to excitation of plasma instabilities. We discuss two types of instabilities that are excited by two different populations of electrons. First, the generation of Langmuir waves by photoelectrons with energies of the order of 10eV is investigated. It is predicted that the measured anisotropy of their pitch angle distribution at the heights z≈400km causes excitation of waves with frequencies f~30kHz and wavelengths λ~30m. Near the terminators the instability of the electrostatic waves with frequencies of the order of or less than the electron gyrofrequency exited by thermal electrons is predicted. The typical frequencies of these waves depend on the local magnitude of the magnetic field and can achieve values f~3-5kHz above strong crustal magnetic fields.
Pc3-4 ULF waves observed by the SuperDARN TIGER radar
Authors: Ponomarenko P. V. , F. W. Menk F. W. , Waters C. L. , Sciffer M. D.
Published: June 2010

Abstract. Despite extensive research, the mechanisms for propagation of Pc3-4 energy from the generation region at the bow shock to the high-latitude ionosphere remain unresolved. We used high temporal (6-12s) and spatial (45km) resolution data from the SuperDARN TIGER radar (Tasmania) to examine Pc3-4 wave signatures at the F-region heights. We focus on a case study on 28 September 2000, when large-amplitude band-limited Pc3-4 oscillations were observed across 10-20 range gates in beam #4 (which points towards the CGM pole) for about four hours preceding MLT noon. These waves were detected in sea-scatter echoes reflected from the ionospheric footprint of the plasmatrough. Nearby ground magnetometer data from Macquarie Island showed very similar variations in both the north-south and east-west components. The radar data revealed the occasional presence of quasi-FLR (field-line resonance) spatial structures with frequencies much higher than those of the local fundamental FLR modes. Detailed spectral analysis of the ionospheric and ground data shows that these structures most probably correspond to a 3rd-harmonic, poloidal-mode FLR. Such observations suggest that compressional Pc3-4 waves produced in the upstream solar wind travel earthward from the magnetopause in the magnetic equatorial plane depositing energy into the Alfvenic modes, as either forced or 3rd-harmonic FLR that reach ionospheric heights along magnetic field lines.
Origin of type-2 thermal-ion upflows in the auroral ionosphere
Authors: Kagan L. M. , St.-Maurice J.-P.
Published: June 2010

Abstract. The origin of thermal ion outflows exceeding 1km/s in the high-latitude F-region has been a subject of considerable debate. For cases with strong convection electric fields, the "evaporation" of the ions due to frictional heating below 400-500km has been shown to provide some satisfactory answers. By contrast, in the more frequent subclass of outflow events observed over auroral arcs, called type-2, there is no observational evidence for ion frictional heating. Instead, an electron temperature increase of up to 6000° K is observed over the outflow region. In this case, field-aligned electric fields have long been suspected to be involved, but this explanation did not seem to agree with expectations from the ion momentum balance. In the present work we provide a consistent scenario for the type-2 ion upflows based on our case study of an event that occurred on 20 February 1990. We introduce, for the first time, the electron energy balance in the analysis. We couple this equation with the ion momentum balance to study the salient features of the observations and conclude that type-2 ion outflows and the accompanying electron heating events are indeed consistent with the existence of a field-aligned electric field. However, for our explanation to work, we have to require that an allowance be made for electron scattering by high frequency turbulence. This turbulence could be generated at first by the very fast response of the electrons themselves to a newly imposed electric field that would be partly aligned with the geomagnetic field. The high frequencies of the waves would make it impossible for the ions to react to the waves. We have found the electron collision frequency associated with scattering from the waves to be rather modest, i.e. comparable to the ambient electron-ion collision frequency. The field-aligned electric field inferred from the observations is likewise of the same order of magnitude as the normal ambipolar field, at least for the case that we have studied in detail. We propose that the field-aligned electric field is maintained by the north-south motion of an east-west arc. The magnetic perturbation associated with the arc itself converts a small fraction of the perpendicular electric field into a field parallel to the total magnetic field, while the north-south motion ensures that the conversion never stops.
Statistical study of the proton isotropy boundary
Authors: Lvova E. A. , Sergeev V. A. , Bagautdinova G. R.
Published: June 2010

Abstract. Based on a large data set of polar NOAA-type satellite observations we studied the latitude-MLT shape of the 80keV proton isotropy boundary (IB) as a function of the solar wind parameters and magnetic activity. Using "snapshots" of isotropy boundaries near-simultaneously crossed at four points we found that its equatorward expansion, as well as its dawn-dusk shift, depends mostly on the AE-index and on the corrected Dst*, whereas the amplitude of the IB daily variation is mostly controlled by the solar wind dynamic pressure. Applying a nonlinear, multi-parametric, least-square regression procedure, the empirical relationship describing the IB latitude as a function of MLT and AE, Pd, Dst* parameters was obtained. Comparing it with the predictions from the Tsyganenko-2001 model we found a good agreement during the quiet time but some important differences during the disturbed periods. Interpretation of these results in terms of the properties of the magnetospheric configuration is briefly discussed.
Magnetosheath waves under very low solar wind dynamic pressure: Wind/Geotail observations
Authors: Farrugia C. J. , Gratton F. T. , Gnavi G. , Matsui H. , Torbert R. B. , Fairfield D. H. , Ogilvie K. W. , Lepping R. P. , Terasawa T. , Mukai T. , Saito Y.
Published: June 2010

Abstract. The expanded bow shock on and around "the day the solar wind almost disappeared" (11 May 1999) allowed the Geotail spacecraft to make a practically uninterrupted 54-h-long magnetosheath pass near dusk (16:30-21:11 magnetic local time) at a radial distance of 24 to 30 RE (Earth radii). During most of this period, interplanetary parameters varied gradually and in such a way as to give rise to two extreme magnetosheath structures, one dominated by magnetohydrodynamic (MHD) effects and the other by gas dynamic effects. We focus attention on unusual features of electromagnetic ion wave activity in the former magnetosheath state, and compare these features with those in the latter. Magnetic fluctuations in the gas dynamic magnetosheath were dominated by compressional mirror mode waves, and left- and right-hand polarized electromagnetic ion cyclotron (EIC) waves transverse to the background field. In contrast, the MHD magnetosheath, lasting for over one day, was devoid of mirror oscillations and permeated instead by EIC waves of weak intensity. The weak wave intensity is related to the prevailing low solar wind dynamic pressures. Left-hand polarized EIC waves were replaced by bursts of right-hand polarized waves, which remained for many hours the only ion wave activity present. This activity occurred when the magnetosheath proton temperature anisotropy (= ) became negative. This was because the weakened bow shock exposed the magnetosheath directly to the (negative) temperature anisotropy of the solar wind. Unlike the normal case studied in the literature, these right-hand waves were not by-products of left-hand polarized waves but derived their energy source directly from the magnetosheath temperature anisotropy. Brief entries into the low latitude boundary layer (LLBL) and duskside magnetosphere occurred under such inflated conditions that the magnetospheric magnetic pressure was insufficient to maintain pressure balance. In these crossings, the inner edge of the LLBL was flowing sunward. The study extends our knowledge of magnetosheath ion wave properties to the very low solar wind dynamic pressure regime.
Time evolution of electric fields and currents and the generalized Ohm's law
Authors: Vasyliūnas V. M.
Published: June 2010

Abstract. Fundamentally, the time derivative of the electric field is given by the displacement-current term in Maxwell's generalization of Ampère's law, and the time derivative of the electric current density is given by the generalized Ohm's law. The latter is derived by summing the accelerations of all the plasma particles and can be written exactly, with no approximations, in a (relatively simple) primitive form containing no other time derivatives. When one is dealing with time scales long compared to the inverse of the electron plasma frequency and spatial scales large compared to the electron inertial length, however, the time derivative of the current density becomes negligible in comparison to the other terms in the generalized Ohm's law, which then becomes the equation that determines the electric field itself. Thus, on all scales larger than those of electron plasma oscillations, neither the time evolution of J nor that of E can be calculated directly. Instead, J is determined by B through Ampère's law and E by plasma dynamics through the generalized Ohm's law. The displacement current may still be non-negligible if the Alfvén speed is comparable to or larger than the speed of light, but it no longer determines the time evolution of E, acting instead to modify J. For theories of substorms, this implies that, on time scales appropriate to substorm expansion, there is no equation from which the time evolution of the current could be calculated, independently of ∇xB. Statements about change (disruption, diversion, wedge formation, etc.) of the electric current are merely descriptions of change in the magnetic field and are not explanations.
Interhemispheric asymmetries in the occurrence of magnetically conjugate sub-auroral polarisation streams
Authors: Parkinson M. L. , Pinnock M. , Wild J. A. , Lester M. , Yeoman T. K. , Milan S. E. , Ye H. , Devlin J. C. , Frey H. U. , Kikuchi T.
Published: June 2010

Abstract. Earthward injections of energetic ions and electrons mark the onset of magnetospheric substorms. In the inner magnetosphere (L4), the energetic ions drift westward and the electrons eastward, thereby enhancing the equatorial ring current. Wave-particle interactions can accelerate these particles to radiation belt energies. The ions are injected slightly closer to Earth in the pre-midnight sector, leading to the formation of a radial polarisation field in the inner magnetosphere. This maps to a poleward electric field just equatorward of the auroral oval in the ionosphere. The poleward electric field is subsequently amplified by ionospheric feedback, thereby producing auroral westward flow channels (AWFCs). In terms of electric field strength, AWFCs are the strongest manifestation of substorms in the ionosphere. Because geomagnetic flux tubes are essentially equi-potentials, similar AWFC signatures should be observed simultaneously in the Northern and Southern Hemispheres. Here we present magnetically conjugate SuperDARN radar observations of AWFC activity observed in the pre-midnight sector during two substorm intervals including multiple onsets during the evening of 30 November 2002. The Northern Hemisphere observations were made with the Japanese radar located at King Salmon, Alaska (57, and the Southern Hemisphere observations with the Tasman International Geospace Environment Radar (TIGER) located at Bruny Island, Tasmania (55. LANL geosynchronous satellite observations of energetic ion and electron fluxes monitored the effects of substorms in the inner magnetosphere (L6). The radar-observed AWFC activity was coincident with activity observed at geosynchronous orbit, as well as westward current surges in the ionosphere observed using ground-based magnetometers. The location of AWFCs with respect to the auroral oval was inferred from FUV auroral images recorded on board the IMAGE spacecraft. DMSP SSIES ion drift measurements confirmed the presence of AWFCs equatorward of the auroral oval. Systematic asymmetries in the interhemispheric signatures of the AWFCs probably arose because the magnetic flux tubes were distorted at L shells passing close to the substorm dipolarisation region. Transient asymmetries were attributed to the development of nearby field-aligned potential drops and currents.
Electric current and magnetic field geometry in flapping magnetotail current sheets
Authors: Runov A. , Sergeev V. A. , Baumjohann W. , Nakamura R. , Apatenkov S. , Asano Y. , Volwerk M. , Vörös Z. , Zhang T. L. , Petrukovich A. , Balogh A. , Sauvaud J.-A. , Klecker B. , Rème H.
Published: June 2010

Abstract. Using four-point magnetic field measurements by the Cluster spacecraft, we statistically analyze the magnetic field and electric current configurations during rapid crossings of the current sheet observed in July-October 2001 at geocentric distances of 19 RE. The database includes 78 crossings, specially selected to apply multi-point data analysis techniques to calculate vector derivatives. Observed bipolar variations of jz, often with | jz |>jy, indicate that the electric currents follow kinks of the current sheet. The current density varies between 5-25nA/m2. The half-thickness of the current sheet during flapping varies over a wide range, from 1 to 20 ion thermal gyroradii (Lcp), calculated from average temperature and lobe magnetic field for each crossing). We found no relationship between the tilt angle of the current sheet normal and the half-thickness. In 68 cases the magnetic field curvature vector has a positive (earthward) X-component. Ten cases with a negative (tailward) curvature, associated with reconnection, were detected within 0<YGSM<7 RE. The minimum curvature radii vary mainly between 1 and 10 Lcp, and the adiabaticity parameter κ≤1 for 73% of the events. The electric current density during flapping is often off-central, i.e. the main current density is shifted from the neutral sheet (| Bx |<5nT) to the Northern or Southern Hemisphere. This is most likely a temporal effect related to the flapping. The analysis shows that the flapping motion of the current sheet is associated with kink-like waves on the sheet surface. The kink fronts, tilted in the Y-Z plane, moved toward dawn in the morning half and toward dusk in the evening half of the magnetotail.
Interplanetary magnetic field control of Saturn's polar cusp aurora
Authors: Bunce E. J. , Cowley S. W. H. , Milan S. E.
Published: June 2010

Abstract. Dayside UV emissions in Saturn's polar ionosphere have been suggested to be the first observational evidence of the kronian "cusp" (Gérard et al., 2004). The emission has two distinct states. The first is a bright arc-like feature located in the pre-noon sector, and the second is a more diffuse "spot" of aurora which lies poleward of the general location of the main auroral oval, which may be related to different upstream interplanetary magnetic field (IMF) orientations. Here we take up the suggestion that these emissions correspond to the cusp. However, direct precipitation of electrons in the cusp regions is not capable of producing significant UV aurora. We have therefore investigated the possibility that the observed UV emissions are associated with reconnection occurring at the dayside magnetopause, possibly pulsed, akin to flux transfer events seen at the Earth. We devise a conceptual model of pulsed reconnection at the low-latitude dayside magnetopause for the case of northwards IMF which will give rise to pulsed twin-vortical flows in the magnetosphere and ionosphere in the vicinity of the open-closed field-line boundary, and hence to bi-polar field-aligned currents centred in the vortical flows. During intervals of high-latitude lobe reconnection for southward IMF, we also expect to have pulsed twin-vortical flows and corresponding bi-polar field-aligned currents. The vortical flows in this case, however, are displaced poleward of the open-closed field line boundary, and are reversed in sense, such that the field-aligned currents are also reversed. For both cases of northward and southward IMF we have also for the first time included the effects associated with the IMF By effect. We also include the modulation introduced by the structured nature of the solar wind and IMF at Saturn's orbit by developing "slow" and "fast" flow models corresponding to intermediate and high strength IMF respectively. We then consider the conditions under which the plasma populations appropriate to either sub-solar reconnection or high-latitude lobe reconnection can carry the currents indicated. We have estimated the field-aligned voltages required, the resulting precipitating particle energy fluxes, and the consequent auroral output. Overall our model of pulsed reconnection under conditions of northwards and southwards IMF, and for varying orientations of IMF By, is found to produce a range of UV emission intensities and geometries which is in good agreement with the data presented by Gérard et al. (2004). The recent HST-Cassini solar wind campaign provides a unique opportunity to test the theoretical ideas presented here.
Cosmic ray cutoff prediction using magnetic field from global magnetosphere MHD simulations
Authors: Weygand J. M. , Raeder J.
Published: June 2010

Abstract. Relativistic particles entering the Earth's magnetosphere, i.e. cosmic rays and solar energetic particles, are of prime space weather interest because they can affect satellite operations, communications, and the safety of astronauts and airline crews and passengers. In order to mitigate the hazards that originate from such particles one needs to predict the cutoff latitudes of such particles as a function of their energies and the state of the magnetosphere. We present results from a new particle tracing code that is used to determine the cutoff latitudes of 8-15Men-1 alpha particles during the 23/24 April, 1998 geomagnetic storm and the preceding quiet time. The calculations are based on four different geomagnetic field models and compared with SAMPEX observations of alpha particles in the same energy range. The geomagnetic field models under consideration are: (i) the International Geomagnetic Reference Field (IGRF) model, (ii) the Tsyganenko "89" model (T89c), (iii) the Tsyganenko "96" model (T96), and (iv) a global magnetohydrodynamic (MHD) model of Earth's magnetosphere. Examining 11 SAMPEX cutoff latitude observations we find that the differences between the observed and the predicted cutoff latitudes are 2.3° ± 2.0° (mean) and 7.9° (maximum difference) for the IGRF model; 3.9° ± 2.4° (mean) and 6.9° (maximum difference) for the T89c model; 4.0° ± 1.4° (mean) and 5.5° (maximum difference) for the T96 model; and 2.5° ± 1.7° (mean) and 7.0° (maximum difference) for the MHD model. All models generally predict cutoff latitudes equatorward of the SAMPEX observations. The MHD model results also show steeper cutoff energy gradients with latitude compared to the empirical models and more structure in the cutoff energy versus latitude function, presumably due to the presence of boundary layers in the MHD model.
Radial diffusion modeling with empirical lifetimes: comparison with CRRES observations
Authors: Shprits Y. Y. , Thorne R. M. , Reeves G. D. , Friedel R.
Published: June 2010

Abstract. A time dependent radial diffusion model is used to quantify the competing effects of inward radial diffusion and losses on the distribution of the outer zone relativistic electrons. The rate of radial diffusion is parameterized by Kp with the loss time as an adjustable parameter. Comparison with HEEF data taken over 500 Combined Release and Radiation Effects Satellite (CRRES) orbits indicates that 1-MeV electron lifetimes near the peak of the outer zone are less than a day during the storm main phase and few days under less disturbed conditions. These values are comparable to independent estimates of the storm time loss rate due to scattering by EMIC waves and chorus emission, and also provide an acceptable representation of electron decay rates following the storm time injection. Although our radial diffusion model, with data derived lifetimes, is able to simulate many features of the variability of outer zone fluxes and predicts fluxes within one order of magnitude accuracy for most of the storms and L values, it fails to reproduce the magnitude of flux changes and the gradual build up of fluxes observed during the recovery phase of many storms. To address these differences future modeling should include an additional local acceleration source and also attempt to simulate the pronounced loss of electrons during the main phase of certain storms.
Characteristics of the near-tail dawn magnetopause and boundary layer
Authors: Paschmann G. , Haaland S. , Sonnerup B. U. Ö. , Hasegawa H. , Georgescu E. , Klecker B. , Phan T. D. , Rème H. , Vaivads A.
Published: June 2010

Abstract. The paper discusses properties of the near-tail dawnside and boundary layer, as obtained from Cluster plasma and magnetic field measurements during a single skimming orbit on 4 and 5 July 2001 that included 24 well-defined crossings by all four spacecraft. As a result of variations of the interplanetary magnetic field, the magnetic shear across the local varied between ~0° and ~180°. Using an improved method, which takes into account acceleration and thickness variation, we have determined the orientation, speed, thickness and current for the 96 individual crossings. The orientations show clear evidence of surface waves. Magnetopause thicknesses range from ~100 to ~2500km, with an average of 753km. The speeds range from less than 10up to more than 300, with an average of 48. Both results are consistent with earlier ISEE and AMPTE results obtained for the dayside magnetopause. Importantly, scaling the thicknesses to the ion gyro radius or the ion inertial length did not reduce the large dynamic range. There is also no significant dependence of thickness on magnetic shear. Current densities range from ~0.01 up to ~0.3uA, with an average value of 0.05 . By including some extra crossings that did not involve all four spacecraft, we were able to apply the Walén test to a total of 60 by Cluster 1 and 3, and have classified 19 cases as rotational discontinuities (RDs), of which 12 and 7 were sunward and tailward of an X-line, respectively. Of these 60 crossings, 26 show no trace of a boundary layer. The only with substantial boundary layers are into the plasma mantle. Of the 26 without a boundary layer, 8 were identified as RDs. Since reconnection produces wedge-shaped boundary layers emanating from the X-line, RDs without may be considered close to the X-line, in which case the observed magnetic shear and Alfvén Mach number should be representative of the conditions at the X-line itself. It is therefore important that four of the eight cases had shear angles ≤100, i.e. the reconnecting fields were far from being anti-parallel, and that all eight cases had Alfvén Mach numbers MA>1 in the adjoining magnetosheath. Another important conclusion can be drawn from the without a that were tangential discontinuities (TDs). To observe TDs with no at such large distances from the subsolar point appears to rule out diffusion over large portions of the as an effective means for plasma transport across the magnetopause.
Solar activity explored with new wavelet methods
Authors: Lundstedt H. , Liszka L. , Lundin R.
Published: June 2010

Abstract. In order to improve the forecasts of the impact of solar activity on the terrestrial environment on time scales longer than days, improved understanding and forecasts of the solar activity are needed. The first results of a new approach of modelling and forecasting solar activity are presented. Time series of solar activity indicators, such as sunspot number, group sunspot number, F10.7, E10.7, solar magnetic mean field, Mount Wilson plage and sunspot index, have been studied with new wavelet methods; ampligrams and time-scale spectra. Wavelet power spectra of the sunspot number for the period 1610 up to the present show not only that a dramatic increase in the solar activity took place after 1940 but also that an interesting change occurred in 1990. The main 11-year solar cycle was further studied with ampligrams for the period after 1850. time-scale spectra were used to examine the processes behind the variability of the solar activity. Several interesting deterministic and more stochastic features were detected in the time series of the solar activity indicators. The solar nature of these features will be further studied. Keywords. Solar physics, astrophysics and astronomy (Magnetic fields; Stellar interiors and dynamo theory) – Space plasma physics (nonlinear phenomena)
Weak interaction in the African Easterly Jet
Authors: Lenouo A. , Nkankam Kamga F. , Yepdjuo E.
Published: June 2010

Abstract. Low-frequency, African Easterlies Waves (AEW) are examined as disturbances embedded in the mid-tropospheric easterly jet of the African low troposphere. The solution to the nonlinear vorticity equation relevant to the description of waves is sought in the form of triplet waves. The latest suggest a unified method to determine their kinetics characteristic and to explain the mechanism of energy exchange between their different modes. The period of energy interaction between different modes of the global wave is equal to 3.5 days when the wave packet is moving with a group velocity dependent on the mean basic flow. The effects of nonlinearity are also identified, and it is noted that the horizontal shears of mean flow, as well as the temporal variation of the amplitude wave functions, are the controlling factors. Keywords. Meteorology and atmospheric dynamics (Synoptic-scale meteorology; Tropical meteorology; Waves and tides)
Thermospheric density structures over the polar regions observed with CHAMP
Authors: Schlegel K. , Lühr H. , St.-Maurice J.-P. , Crowley G. , Hackert C.
Published: June 2010

Abstract. We report on the unexpected detection of considerable structure in high latitude thermospheric densities, as derived from an accelerometer onboard the CHAMP satellite. The width of the structures, which can either be maxima or minima, varies between a few hundred km and 2000 km. The amplitudes of these density extrema can reach 50% of ambient. Maxima cluster around 75° (N and S), while minima are found closer to the poles. In a magnetic latitude-magnetic local time frame the maxima are found mainly around the cusp region. Overall, the observed structures somewhat resemble so-called density cells previously found in model calculations. However the models generate their cells around 140–300 km altitude and show little, if any remnant at 400 km or above. This has to be contrasted with the fact that the CHAMP observations were obtained near 430 km altitude. We have explored Joule heating as a possible mechanism for the generation of the structures, at least in density enhancement regions, using Hall currents measured on CHAMP and simultaneous incoherent scatter measurements with EISCAT. However, the electric fields were usually quite small during the period of observation, making the quest for an explanation for the structures all the more challenging. Keywords. Meteorology and atmospheric dynamics (Thermospheric dynamics) – Magnetospheric physics (Polar cap phenomena) – Atmospheric composition and structure (Pressure, density, and temperature)