bluuebunny/crossref_metadata_2025 huggingface dataset:
Axi-symmetric models of auroral current systems in Jupiter's magnetosphere with predictions for the Juno mission
Authors: Cowley S. W. H. , Deason A. J. , Bunce E. J.
Published: June 2010

Abstract. We develop two related models of magnetosphere-ionosphere coupling in the jovian system by combining previous models defined at ionospheric heights with magnetospheric magnetic models that allow system parameters to be extended appropriately into the magnetosphere. The key feature of the combined models is thus that they allow direct connection to be made between observations in the magnetosphere, particularly of the azimuthal field produced by the magnetosphere-ionosphere coupling currents and the plasma angular velocity, and the auroral response in the ionosphere. The two models are intended to reflect typical steady-state sub-corotation conditions in the jovian magnetosphere, and transient super-corotation produced by sudden major solar wind-induced compressions, respectively. The key simplification of the models is that of axi-symmetry of the field, flow, and currents about the magnetic axis, limiting their validity to radial distances within ~30 RJ of the planet, though the magnetic axis is appropriately tilted relative to the planetary spin axis and rotates with the planet. The first exploration of the jovian polar magnetosphere is planned to be undertaken in 2016–2017 during the NASA New Frontiers Juno mission, with observations of the polar field, plasma, and UV emissions as a major goal. Evaluation of the models along Juno planning orbits thus produces predictive results that may aid in science mission planning. It is shown in particular that the low-altitude near-periapsis polar passes will generally occur underneath the corresponding auroral acceleration regions, thus allowing brief examination of the auroral primaries over intervals of ~1–3 min for the main oval and ~10 s for narrower polar arc structures, while the "lagging" field deflections produced by the auroral current systems on these passes will be ~0.1°, associated with azimuthal fields above the ionosphere of a few hundred nT.
New results on equatorial thermospheric winds and the midnight temperature maximum
Authors: Meriwether J. , Faivre M. , Fesen C. , Sherwood P. , Veliz O.
Published: June 2010

Abstract. Optical observations of thermospheric winds and temperatures determined with high resolution measurements of Doppler shifts and Doppler widths of the OI 630-nm equatorial nightglow emission have been made with improved accuracy at Arequipa, Peru (16.4° S, 71.4° W) with an imaging Fabry-Perot interferometer. An observing procedure previously used at Arecibo Observatory was applied to achieve increased spatial and temporal sampling of the thermospheric wind and temperature with the selection of eight azimuthal directions, equally spaced from 0 to 360°, at a zenith angle of 60°. By assuming the equivalence of longitude and local time, the data obtained using this technique is analyzed to determine the mean neutral wind speeds and mean horizontal gradients of the wind field in the zonal and meridional directions. The new temperature measurements obtained with the improved instrumental accuracy clearly show the midnight temperature maximum (MTM) peak with amplitudes of 25 to 200 K in all directions observed for most nights. The horizontal wind field maps calculated from the mean winds and gradients show the MTM peak is always preceded by an equatorward wind surge lasting 1–2 h. The results also show for winter events a meridional wind abatement seen after the MTM peak. On one occasion, near the September equinox, a reversal was observed during the poleward transit of the MTM over Arequipa. Analysis inferring vertical winds from the observed convergence yielded inconsistent results, calling into question the validity of this calculation for the MTM structure at equatorial latitudes during solar minimum. Comparison of the observations with the predictions of the NCAR general circulation model indicates that the model fails to reproduce the observed amplitude by a factor of 5 or more. This is attributed in part to the lack of adequate spatial resolution in the model as the MTM phenomenon takes place within a scale of 300–500 km and ~45 min in local time. The model shortcoming is also attributed in part to the need for the model to include a hydrodynamical mechanism to describe the merging of the zonal wind with the meridional tidal winds that converge onto the geographical equator. Finally, a conclusion of this work is that the MTM compressional heating takes place along the perimeter of the pressure bulge rather than within the bulge, an issue previously not appreciated.
Observations of equatorial mesospheric winds over Cariri (7.4° S) by a meteor radar and comparison with existing models
Authors: Buriti R. A. , Hocking W. K. , Batista P. P. , Medeiros A. F. , Clemesha B. R.
Published: June 2010

Abstract. Mesospheric winds observed with a meteor radar at Cariri (7.4° S, 36.5° W), Brazil, during the period of July 2004 to June 2005, show a clear semiannual oscillation known as the Mesospheric Semiannual Oscillation (MSAO), which maximizes in the zonal mean wind mainly at 82 km, with amplitude decreasing with height. Maximum westward winds for the MSAO occurred in March and September. The meridional wind, on the other hand, presented a clear annual variation maximizing in December. On average, the amplitude of the meridional MSAO was smaller than the zonal MSAO component. Comparison with models shows on occasions that there are significant differences between the observed winds and the CIRA (Cospar International Reference Atmosphere) and HWM93 (Horizontal Wind Model) models. In addition, diurnal and semidiurnal parameters were calculated and compared to the GSWM model. Other results observed during one year of data are presented in this work.
Comparison of Birkeland current observations during two magnetic cloud events with MHD simulations
Authors: Korth H. , Anderson B. J. , Lyon J. G. , Wiltberger M.
Published: June 2010

Abstract. Low altitude field-aligned current densities obtained from global magnetospheric simulations are compared with two-dimensional distributions of Birkeland currents at the topside ionosphere derived from magnetic field observations by the constellation of Iridium satellites. We present the analysis of two magnetic cloud events, 17–19 August 2003 and 19–21 March 2001, where the interplanetary magnetic field (IMF) rotates slowly (~10°/h) to avoid time-aliasing in the magnetic perturbations used to calculate the Birkeland currents. In the August 2003 event the IMF rotates from southward to northward while maintaining a negative IMF By during much of the interval. During the March 2001 event the IMF direction varies from dawnward to southward to duskward. We find that the distributions of the Birkeland current densities in the simulations agree qualitatively with the observations for northward IMF. For southward IMF, the dayside Region-1 currents are reproduced in the simulation, but appear on average 5° further poleward of their observed location, while the nightside Region-1 currents and the Region-2 currents are largely under-represented. Comparison of the observed and simulated Birkeland current distributions, which are intimately related to the plasma drifts at the ionosphere, shows that the ionospheric convection pattern in the MHD model and its dependence on the IMF orientation is essentially correct. The Birkeland total currents in the simulations are about a factor of 2 larger than observed during southward IMF. For Bz>0 the disparity in the total current is reduced and the simulations for purely northward IMF agree with the observations to within 10%. The disparities in the magnitudes of the Birkeland currents between the observations and the simulation results are a combined effect of the simulation overestimating the ionospheric electric field and of the Iridium fits underestimating the magnetic perturbations. The marginal difference in the latitude resolution of the ionospheric grids in the simulations and the observations is shown to have only secondary effect on the magnitudes of the Birkeland currents. The electric potentials in the simulation for southward IMF periods are twice as large as those obtained from measurements of the plasma drift velocities by DMSP, implying that the reconnection rates in the simulation are too large.
Lower hybrid waves at the shock front: a reassessment
Authors: Walker S. N. , Balikhin M. A. , Alleyne H. St. C. K. , Hobara Y. , André M. , Dunlop M. W.
Published: June 2010

Abstract. The primary process occurring at a collisionless shock is the redistribution of the bulk upstream energy into other degrees of freedom. One part of this process results in the acceleration of electrons at the shock front. Accelerated electrons are observed at the terrestrial and other planetary shocks, comets, and their effects are observed in astrophysical phenomena such as supernova remnants and jets in the form of X-ray bremsstrahlung radiation. One of the physical models for electron acceleration at supercritical shocks is based on low-hybrid turbulence due to the presence of reflected ions in the foot region. Since lower hybrid waves propagate almost perpendicular to the magnetic field they can be simultaneously in resonance with both the unmagnetised ions (ω=Vik⊥) and magnetised electrons (ω=Vek||). In this paper, Cluster observations of the electric field are used to study the occurrence of lower hybrid waves in the front of the terrestrial bow shock. It is shown that the lower hybrid waves exist as isolated wave packets. However, the very low level of the observed lower hybrid turbulence is too small to impart significant energisation to the electron population.
Long-term wave growth and its linear and nonlinear interactions with wind fluctuations
Authors: Ge Z. , Liu P. C.
Published: June 2010

Abstract. Following Ge and Liu (2007), the simultaneously recorded time series of wave elevation and wind velocity are examined for long-term (on Lavrenov's τ4-scale or 3 to 6 h) linear and nonlinear interactions between the wind fluctuations and the wave field. Over such long times the detected interaction patterns should reveal general characteristics for the wave growth process. The time series are divided into three episodes, each approximately 1.33 h long, to represent three sequential stages of wave growth. The classic Fourier-domain spectral and bispectral analyses are used to identify the linear and quadratic interactions between the waves and the wind fluctuations as well as between different components of the wave field. The results show clearly that as the wave field grows the linear interaction becomes enhanced and covers wider range of frequencies. Two different wave-induced components of the wind fluctuations are identified. These components, one at around 0.4 Hz and the other at around 0.15 to 0.2 Hz, are generated and supported by both linear and quadratic wind-wave interactions probably through the distortions of the waves to the wind field. The fact that the higher-frequency wave-induced component always stays with the equilibrium range of the wave spectrum around 0.4 Hz and the lower-frequency one tends to move with the downshifting of the primary peak of the wave spectrum defines the partition of the primary peak and the equilibrium range of the wave spectrum, a characteristic that could not be revealed by short-time wavelet-based analyses in Ge and Liu (2007). Furthermore, these two wave-induced peaks of the wind spectrum appear to have different patterns of feedback to the wave field. The quadratic wave-wave interactions also are assessed using the auto-bispectrum and are found to be especially active during the first and the third episodes. Such directly detected wind-wave interactions, both linear and nonlinear, may complement the existing theoretical and numerical models, and can be used for future model development and validation.
Parameterization of ionization rate by auroral electron precipitation in Jupiter
Authors: Hiraki Y. , Tao C.
Published: June 2010

Abstract. We simulate auroral electron precipitation into the Jovian atmosphere in which electron multi-directional scattering and energy degradation processes are treated exactly with a Monte Carlo technique. We make a parameterization of the calculated ionization rate of the neutral gas by electron impact in a similar way as used for the Earth's aurora. Our method allows the altitude distribution of the ionization rate to be obtained as a function of an arbitrary initial energy spectrum in the range of 1–200 keV. It also includes incident angle dependence and an arbitrary density distribution of molecular hydrogen. We show that there is little dependence of the estimated ionospheric conductance on atomic species such as H and He. We compare our results with those of recent studies with different electron transport schemes by adapting our parameterization to their atmospheric conditions. We discuss the intrinsic problem of their simplified assumption. The ionospheric conductance, which is important for Jupiter's magnetosphere-ionosphere coupling system, is estimated to vary by a factor depending on the electron energy spectrum based on recent observation and modeling. We discuss this difference through the relation with field-aligned current and electron spectrum.
Rotational temperature of N2+ (0,2) ions from spectrographic measurements used to infer the energy of precipitation in different auroral forms and compared with radar measurements
Authors: Jokiaho O. , Lanchester B. S. , Ivchenko N. , Daniell G. J. , Miller L. C. H. , Lummerzheim D.
Published: June 2010

Abstract. High resolution spectral data are used to estimate neutral temperatures at auroral heights. The data are from the High Throughput Imaging Echelle Spectrograph (HiTIES) which forms part of the Spectrographic Imaging Facility (SIF), located at Longyearbyen, Svalbard in Norway. The platform also contains photometers and a narrow angle auroral imager. Quantum molecular spectroscopy is used for modelling N2+ 1NG (0,2), which serves as a diagnostic tool for neutral temperature and emission height variations. The theoretical spectra are convolved with the instrument function and fitted to measured rotational transition lines as a function of temperature. Measurements were made in the magnetic zenith, and along a meridian slit centred on the magnetic zenith. In the results described, the high spectral resolution of the data (0.08 nm) allows an error analysis to be performed more thoroughly than previous findings, with particular attention paid to the correct subtraction of background, and to precise wavelength calibration. Supporting measurements were made with the Svalbard Eiscat Radar (ESR). Estimates were made from both optical and radar observations of the average energy of precipitating electrons in different types of aurora. These provide confirmation that the spectral results are in agreement with the variations observed in radar profiles. In rayed aurora the neutral temperature was highest (800 K) and the energy lowest (1 keV). In a bright curling arc, the temperature at the lower border was about 550 K, corresponding to energies of 2 keV. The radar and modelling results confirm that these average values are a lower limit for an estimation of the characteristic energy. In each event the energy distribution is clearly made up of more than one spectral shape. This work emphasises the need for high time resolution as well as high spectral resolution. The present work is the first to provide rotational temperatures using a method which pays particular attention to errors in measurement and fitting, and background subtraction.
Complexity in the high latitude HF radar spectral width boundary region
Authors: Parkinson M. L. , Hannah K. M. , Dyson P. L.
Published: June 2010

Abstract. SuperDARN radars are sensitive to the collective Doppler characteristics of decametre-scale irregularities in the high latitude ionosphere. The radars routinely observe a distinct transition from large spectral width (>100 m s−1) located at higher latitudes to low spectral width (<50 m s−1) located at lower latitudes. Because of its equatorward location, the TIGER Tasmanian radar is very sensitive to the detection of the spectral width boundary (SWB) in the nightside auroral ionosphere. An analysis of the line-of-sight velocities and 2-D beam-swinging vectors suggests the meso-scale (~100 km) convection is more erratic in the high spectral width region, but slower and more homogeneous in the low spectral width region. The radar autocorrelation functions are better modelled using Lorentzian Doppler spectra in the high spectral width region, and Gaussian Doppler spectra in the low spectral width region. However, paradoxically, Gaussian Doppler spectra are associated with the largest spectral widths. Application of the Burg maximum entropy method suggests the occurrence of double-peaked Doppler spectra is greater in the high spectral width region, implying the small-scale (~10 km) velocity fluctuations are more intense above the SWB. These observations combined with collective wave scattering theory imply there is a transition from a fast flowing, turbulent plasma with a correlation length of velocity fluctuations less than the scattering wavelength, to a slower moving plasma with a correlation length greater than the scattering wavelength. Peak scaling and structure function analysis of fluctuations in the SWB itself reveals approximately scale-free behaviour across temporal scales of ~10 s to ~34 min. Preliminary scaling exponents for these fluctuations, αGSF=0.18±0.02 and αGSF=0.09±0.01, are even smaller than that expected for MHD turbulence.
COSTEP/SOHO observations of energetic electrons far upstream of the Earth's bow-shock
Authors: Klassen A. , Gómez-Herrero R. , Böhm E. , Müller-Mellin R. , Heber B. , Wimmer-Schweingruber R.
Published: June 2010

Abstract. We have analyzed 124 electron bursts at energies above 0.25 MeV observed with the EPHIN/COSTEP instrument onboard the SOHO spacecraft far upstream of the Earth's bow-shock at the libration point L1 from 1996 through 2005. Most of the bursts were observed during low solar activity (in 1996–1997 and in 2005) and all 124 bursts were not associated with solar particle events. It is shown that some upstream events are detected at energies above 0.7 MeV. We find that the event occurrence number shows a distinct seasonal variation with maxima around equinoxes and minima near solstices. This together with a close correspondence between the event occurrence number with maxima in solar wind speed (Vsw), geomagnetic activity index (Ap) and in the southward interplanetary magnetic field (IMF) component (Bz) indicates that the observed events can be explained in terms of leakage of magnetospheric particles during enhanced geoactivity rather than by acceleration at the Earth's bow-shock.
Ionospheric feedback effects on the quasi-stationary coupling between LLBL and postnoon/evening discrete auroral arcs
Authors: Echim M. M. , Roth M. , De Keyser J.
Published: June 2010

Abstract. We discuss a model for the quasi-stationary coupling between magnetospheric sheared flows in the dusk sector and discrete auroral arcs, previously analyzed for the case of a uniform height-integrated Pedersen conductivity (ΣP). Here we introduce an ionospheric feedback as the variation of ΣP with the energy flux of precipitating magnetospheric electrons (εem). One key-component of the model is the kinetic description of the interface between the duskward LLBL and the plasma sheet that gives the profile of Φm, the magnetospheric electrostatic potential. The velocity shear in the dusk LLBL plays the role of a generator for the auroral circuit closing through Pedersen currents in the auroral ionosphere. The field-aligned current density, j||, and the energy flux of precipitating electrons are given by analytic functions of the field-aligned potential drop, ΔΦ, derived from standard kinetic models of the adiabatic motion of particles. The ionospheric electrostatic potential, Φi (and implicitely ΔΦ) is determined from the current continuity equation in the ionosphere. We obtain values of ΔΦ of the order of kilovolt and of j|| of the order of tens of μA/m2 in thin regions of the order of several kilometers at 200 km altitude. The spatial scale is significantly smaller and the peak values of ΔΦ, j|| and εem are higher than in the case of a uniform ΣP. Effects on the postnoon/evening auroral arc electrodynamics due to variations of dusk LLBL and solar wind dynamic and kinetic pressure are discussed. In thin regions (of the order of kilometer) embedding the maximum of ΔΦ we evidence a non-linear regime of the current-voltage relationship. The model predicts also that visible arcs form when the velocity shear in LLBL is above a threshold value depending on the generator and ionospheric plasma properties. Brighter arcs are obtained for increased velocity shear in the LLBL; their spatial scale remains virtually unmodified. The field-aligned potential drop tends to decrease with increasing LLBL density. For higher values of the LLBL electron temperature the model gives negative field-aligned potential drops in regions adjacent to upward field-aligned currents.
Cluster observations on the thin current sheet in the magnetotail
Authors: Cai C. L. , Dandouras I. , Rème H. , Cao J. B. , Zhou G. C. , Parks G. K.
Published: June 2010

Abstract. A magnetotail event on 15 September 2001 is analyzed, during which the Cluster spacecraft observed both embedding and bifurcation of the Thin Current Sheet (TCS). It is indicated that the ion anisotropy and nongyrotropy are responsible for those new equilibrium features that represent deviations from the conventional Harris model. Measurements show that an embedded proton TCS manifests a pressure anisotropy with p||>p⊥, simultaneously associated with a density embedding; while a bifurcated oxygen ion TCS exhibits a pressure anisotropy mainly with p||
p⊥ is the necessary marginal firehose stability condition. The local flapping motion of the TCS was observed, and some particular features such as the solitary wave-like behavior and the kink motion in the plane perpendicular to the Sun-Earth direction were revealed. The present investigation intimates the complexity of equilibria of the magnetotail current sheet and declares the crucial role played by the ion kinetics in the dynamics of TCSs.
The burst of solar and geomagnetic activity in August–September 2005
Authors: Papaioannou A. , Mavromichalaki H. , Eroshenko E. , Belov A. , Oleneva V.
Published: June 2010

Abstract. During the August–September 2005 burst of solar activity, close to the current solar cycle minimum, a significant number of powerful X-ray flares were recorded, among which was the outstanding X17.0 flare of 7 September 2005. Within a relatively short period (from 22 August to 17 September) two severe magnetic storms were also recorded as well as several Forbush effects. These events are studied in this work, using hourly mean variations of cosmic ray density and anisotropy, derived from data of the neutron monitor network. During these Forbush effects the behavior of high energy cosmic ray characteristics (density and anisotropy) is analyzed together with interplanetary disturbances and their solar sources, and is compared to the variations observed in geomagnetic activity. A big and long lasting (~6 h) cosmic ray pre-decrease (~2%) is defined before the shock arrival on 15 September 2005. The calculated cosmic ray gradients for September 2005 are also discussed.
Intensive radiosonde observations of the diurnal tide and planetary waves in the lower atmosphere over Yichang (111°18' E, 30°42' N), China
Authors: Huang C. M. , Zhang S. D. , Yi F.
Published: June 2010

Abstract. The characteristics of diurnal tide and planetary waves (PWs) in the troposphere and lower stratosphere (TLS) over Yichang (111°18' E, 30°42' N) were studied by using the data from intensive radiosonde observations in August 2006 (summer month) and January 2007 (winter month) on an eight-times-daily basis. The radiosonde observations of the diurnal tide and PWs in the TLS in the mid-latitudes have seldom been reported. We find that there exists dominant diurnal oscillations in the TLS over Yichang. The observed diurnal tide consists of significant nonmigrating components, which may be owning to the local latent heat release. Since the nonmigrating tides are usually composed of high order modes with smaller vertical wavelengths, which are prone to dissipation in comparison with the low order modes, the observational tidal amplitudes decrease sharply at several heights. Some evident discrepancies between the observations and the GSWM-02 are found, which may result mainly from the inaccurate prediction of the nonmigrating tidal components by the GSWM-02. And, due to the evident seasonal differences of the water vapor mixing ratio disturbance and the tropospheric jet induced turbulence in winter, the diurnal tides in the summer and winter months have some different characteristics. Besides the diurnal tide, obvious quasi 7-day PW (QSDPW) and quasi 10-day PW (QTDPW) are also recognized from our observations in both the summer and winter months. The QSDPWs in the troposphere in both the summer and winter months show a standing wave structure, while the QTDPWs generally exhibit traveling wave characteristics. Spectral analyses reveal that some waves with periods around that of the diurnal tide are generated due to the interactions of the diurnal tide and PWs and the tidal amplitudes are modulated by the PWs, indicating the extensive coupling between the diurnal tide and PWs. Moreover, our observations manifest that the PWs can exert great impacts on the tropospheric jet in winter and the tropopause in both the summer and winter months.
Dendroclimatic transfer functions revisited: Little Ice Age and Medieval Warm Period summer temperatures reconstructed using artificial neural networks and linear algorithms
Authors: Helama S. , Makarenko N. G. , Karimova L. M. , Kruglun O. A. , Timonen M. , Holopainen J. , Meriläinen J. , Eronen M.
Published: June 2010

Abstract. Tree-rings tell of past climates. To do so, tree-ring chronologies comprising numerous climate-sensitive living-tree and subfossil time-series need to be "transferred" into palaeoclimate estimates using transfer functions. The purpose of this study is to compare different types of transfer functions, especially linear and nonlinear algorithms. Accordingly, multiple linear regression (MLR), linear scaling (LSC) and artificial neural networks (ANN, nonlinear algorithm) were compared. Transfer functions were built using a regional tree-ring chronology and instrumental temperature observations from Lapland (northern Finland and Sweden). In addition, conventional MLR was compared with a hybrid model whereby climate was reconstructed separately for short- and long-period timescales prior to combining the bands of timescales into a single hybrid model. The fidelity of the different reconstructions was validated against instrumental climate data. The reconstructions by MLR and ANN showed reliable reconstruction capabilities over the instrumental period (AD 1802–1998). LCS failed to reach reasonable verification statistics and did not qualify as a reliable reconstruction: this was due mainly to exaggeration of the low-frequency climatic variance. Over this instrumental period, the reconstructed low-frequency amplitudes of climate variability were rather similar by MLR and ANN. Notably greater differences between the models were found over the actual reconstruction period (AD 802–1801). A marked temperature decline, as reconstructed by MLR, from the Medieval Warm Period (AD 931–1180) to the Little Ice Age (AD 1601–1850), was evident in all the models. This decline was approx. 0.5°C as reconstructed by MLR. Different ANN based palaeotemperatures showed simultaneous cooling of 0.2 to 0.5°C, depending on algorithm. The hybrid MLR did not seem to provide further benefit above conventional MLR in our sample. The robustness of the conventional MLR over the calibration, verification and reconstruction periods qualified it as a reasonable transfer function for our forest-limit (i.e., timberline) dataset. ANN appears a potential tool for other environments and/or proxies having more complex and noisier climatic relationships.
Mesospheric dust and its secondary effects as observed by the ESPRIT payload
Authors: Havnes O. , Surdal L. H. , Philbrick C. R.
Published: June 2010

Abstract. The dust detector on the ESPRIT rocket detected two extended dust/aerosol layers during the launch on 1 July 2006. The lower layer at height ~81.5–83 km coincided with a strong NLC and PMSE layer. The maximum dust charge density was ~−3.5×109 e m−3 and the dust layer was characterized by a few strong dust layers where the dust charge density at the upper edges changed by factors 2–3 over a distance of ≲10 m, while the same change at their lower edges were much more gradual. The upper edge of this layer is also sharp, with a change in the probe current from zero to IDC=−10−11 A over ~10 m, while the same change at the low edge occurs over ~500 m. The second dust layer at ~85–92 km was in the height range of a comparatively weak PMSE layer and the maximum dust charge density was ~−108 e m−3. This demonstrates that PMSE can be formed even if the ratio of the dust charge density to the electron density P=NdZd /n_e≲0.01. In spite of the dust detector being constructed to reduce possible secondary charging effects from dust impacts, it was found that they were clearly present during the passage through both layers. The measured secondary charging effects confirm recent results that dust in the NLC and PMSE layers can be very effective in producing secondary charges with up to ~50 to 100 electron charges being rubbed off by one impacting large dust particle, if the impact angle is θi≳20–35°. This again lends support to the suggested model for NLC and PMSE dust particles (Havnes and Næsheim, 2007) as a loosely bound water-ice clump interspersed with a considerable number of sub-nanometer-sized meteoric smoke particles, possibly also contaminated with meteoric atomic species.
Initiation of ballooning instability in the near-Earth plasma sheet prior to the 23 March 2007 THEMIS substorm expansion onset
Authors: Zhu P. , Raeder J. , Germaschewski K. , Hegna C. C.
Published: June 2010

Abstract. In this work, we analyze the ballooning stability of the near-Earth plasma sheet prior to the initial expansion onset of the 23 March 2007 THEMIS substorm event. Using solar wind data from WIND satellite observation for the substorm event as an input at dayside, we reconstructed a sequence of global magnetospheric configurations around the expansion onset by means of OpenGGCM simulation. These simulations have reproduced most of the salient features, including the onset timing, observed in the THEMIS substorm event (Raeder et al., 2008). The ballooning instability criterion is evaluated using eigenvalue analyses for the near-Earth plasma sheet region when the configuration attains a quasi-static equilibrium condition prior to the onset. Our analysis of the evolution of the near-Earth magnetotail region during the substorm growth phase (10:10 UT to 10:43 UT) reveals a correlation between the breaching of the ballooning stability condition and the initial expansion onset in the temporal domain. The analysis indicates that the ballooning instability is at first initiated by the stretching of closed field lines near the region with a local nonzero minimum in normal component of magnetic field (Bz>0) associated with the formation of a bipolar-flow pattern in the near-Earth plasma sheet around 10 RE. The subsequent initiation of ballooning instability with enhanced growth rate occurs (about 28 min later) on those most stretched closed field lines Earthward of the neutral line following the formation and the tailward recession of a plasmoid.
Effects of solar wind density on auroral electrojets and brightness under influence of substorms
Authors: Shue J.-H. , Kamide Y. , Gjerloev J. W.
Published: June 2010

Abstract. Using the auroral electrojet indices and Polar Ultraviolet Imager auroral images, we examined two fortuitous events during which the solar wind density had clear enhancements while the other solar wind parameters were relatively constant. Two electrojet enhancements were found in each event. The first electrojet enhancement was likely to be related to a substorm in which an auroral bulge appeared at premidnight. The second electrojet enhancement was driven by the density enhancement in the solar wind. The auroral oval became wider in latitude and the auroral distribution became dispersed after the density enhancement arrived at the Earth. The total auroral power integrated over the entire nightside region from 50 to 80° MLAT, however, did not increase significantly in response to the density enhancement. Our interpretation is that the substorm that occurred prior to the solar wind density enhancement had drained out a significant portion of the stored energy in the magnetotail; therefore, less precipitation energy was deposited into the auroral ionosphere by the density enhancement.
Mass analysis of charged aerosol particles in NLC and PMSE during the ECOMA/MASS campaign
Authors: Robertson S. , Horányi M. , Knappmiller S. , Sternovsky Z. , Holzworth R. , Shimogawa M. , Friedrich M. , Torkar K. , Gumbel J. , Megner L. , Baumgarten G. , Latteck R. , Rapp M. , Hoppe U.-P. , Hervig M. E.
Published: June 2010

Abstract. MASS (Mesospheric Aerosol Sampling Spectrometer) is a multichannel mass spectrometer for charged aerosol particles, which was flown from the Andøya Rocket Range, Norway, through NLC and PMSE on 3 August 2007 and through PMSE on 6 August 2007. The eight-channel analyzers provided for the first time simultaneous measurements of the charge density residing on aerosol particles in four mass ranges, corresponding to ice particles with radii <0.5 nm (including ions), 0.5–1 nm, 1–2 nm, and >3 nm (approximately). Positive and negative particles were recorded on separate channels. Faraday rotation measurements provided electron density and a means of checking charge density measurements made by the spectrometer. Additional complementary measurements were made by rocket-borne dust impact detectors, electric field booms, a photometer and ground-based radar and lidar. The MASS data from the first flight showed negative charge number densities of 1500–3000 cm−3 for particles with radii >3 nm from 83–88 km approximately coincident with PMSE observed by the ALWIN radar and NLC observed by the ALOMAR lidar. For particles in the 1–2 nm range, number densities of positive and negative charge were similar in magnitude (~2000 cm−3) and for smaller particles, 0.5–1 nm in radius, positive charge was dominant. The occurrence of positive charge on the aerosol particles of the smallest size and predominately negative charge on the particles of largest size suggests that nucleation occurs on positive condensation nuclei and is followed by collection of negative charge during subsequent growth to larger size. Faraday rotation measurements show a bite-out in electron density that increases the time for positive aerosol particles to be neutralized and charged negatively. The larger particles (>3 nm) are observed throughout the NLC region, 83–88 km, and the smaller particles are observed primarily at the high end of the range, 86–88 km. The second flight into PMSE alone at 84–88 km, found only small number densities (~500 cm−3) of particles >3 nm in a narrow altitude range, 86.5–87.5 km. Both positive (~2000 cm−3) and negative (~4500 cm−3) particles with radii 1–2 nm were detected from 85–87.5 km.
Asymmetries in the equatorial electrojet around N-E Brazil sector
Authors: Rastogi R. G. , Trivedi N. B.
Published: June 2010

Abstract. The paper examines the data of geographic northward (X), eastward (Y) and vertical (Z) components of the magnetic field from a dense array of 26 vector magnetometers operated in N-NE Brazil from November 1990 to March 1991. As expected, the daily variation of X showed a minor maximum around 03:00–04:00 LT and a major maximum around 12:00 LT. The daily range of ΔY showed a strong minimum around noon at all stations. The combined ΔY and ΔX indicated the direction of the equatorial electrojet currents to be flowing along 25° north of east at the centre and 20° north of east at the edges of the equatorial electrojet (EEJ) belt. The centre of the EEJ as defined by the zero intercept of the Z versus latitude was found to be near 1.0° S dip latitude. The electrojet current was stronger in the northern half than in the southern half of the electrojet belt. These anomalies are suggested to be due to the abnormal distribution of the mean magnetic field in this region.
Shape, size, velocity and field-aligned currents of dayside plasma injections: a multi-altitude study
Authors: Marchaudon A. , Cerisier J.-C. , Dunlop M. W. , Pitout F. , Bosqued J.-M. , Fazakerley A. N.
Published: June 2010

Abstract. On 20 February 2005, Cluster in the outer magnetosphere and Double Star-2 (TC-2) at mid-altitude are situated in the vicinity of the northern cusp/mantle, with Cluster moving sunward and TC-2 anti-sunward. Their magnetic footprints come very close together at about 15:28 UT, over the common field-of-view of SuperDARN radars. Thanks to this conjunction, we determine the velocity, the transverse sizes, perpendicular and parallel to this velocity, and the shape of three magnetic flux tubes of magnetosheath plasma injection. The velocity of the structures determined from the Cluster four-spacecraft timing analysis is almost purely antisunward, in contrast with the antisunward and duskward convection velocity inside the flux tubes. The transverse sizes are defined from the Cluster-TC-2 separation perpendicular to the magnetic field, and from the time spent by a Cluster spacecraft in one structure; they are comprised between 0.6 and 2 RE in agreement with previous studies. Finally, using a comparison between the eigenvectors deduced from a variance analysis of the magnetic perturbation at the four Cluster and at TC-2, we show that the upstream side of the injection flux tubes is magnetically well defined, with even a concave front for the third one giving a bean-like shape, whereas the downstream side is far more turbulent. We also realise the first quantitative comparison between field-aligned currents at Cluster calculated with the curlometer technique and with the single-spacecraft method, assuming infinite parallel current sheets and taking into account the velocity of the injection flux tubes. The results agree nicely, confirming the validity of both methods. Finally, we compare the field-aligned current distribution of the three injection flux tubes at the altitudes of Cluster and TC-2. Both profiles are fairly similar, with mainly a pair of opposite field-aligned currents, upward at low-latitude and downward at high-latitude. In terms of intensity, the field-aligned currents at Cluster are two to three times less intense than at TC-2 for the first two flux tubes, in agreement with magnetic field line convergence. For the third flux tube, the intensity is equal, which is explained by the fact that TC-2 crosses the tube on its edge. Finally, the analysis of the ion and electron moments at Cluster shows that the field-aligned currents result from a small difference between upward ion and electron fluxes.
Occurrence of polar mesosphere summer echoes at very high latitudes
Authors: Zecha M. , Röttger J.
Published: June 2010

Abstract. Observations of polar mesosphere summer echoes (PMSE) have been carried out during the summer periodes 1999–2001 and 2003–2004 at the very high latitude of 78° N using the SOUSY Svalbard Radar (53.5 MHz) at Longyearbyen. Although the measurements could not be done continuously in these seasons, PMSE have been detected over more than 6600 h of 9300 h of observation time overall. Using this data base, particular PMSE occurrence characteristics have been determined. PMSE at Svalbard appear from the middle of May to the end of August with an almost permanent total occurrence in June and July. Diurnal variations are observable in the height-depend occurrence rates and in PMSE thickness, they show a maximum around 09:00–10:00 UTC and a minimum around 21:00–22:00 UTC. PMSE occur nearly exclusively between a height of 80 km and 92 km with a maximum near 85 km. However, PMSE appear not simultaneously over the entire height range, the mean vertical PMSE extension is around 4–6 km in June and July. Furthermore, typically PMSE are separated into several layers, and only 30% of all PMSE are single layers. The probability of multiple layers is greater in June and July than at the beginning and the end of the PMSE season and shows a marked 5-day-variation. The same variation is noticeable in the seasonal dependence of the PMSE occurrence and the PMSE thickness. We finally discuss potential geophysical processes to explain our observational results.
Small-scale structures in neutrals and charged aerosol particles as observed during the ECOMA/MASS rocket campaign
Authors: Strelnikov B. , Rapp M. , Strelnikova I. , Engler N. , Latteck R.
Published: June 2010

Abstract. We present results of in situ measurements of neutral temperature during the ECOMA/MASS rocket campaign. We present and compare results of turbulence measurements conducted simultaneously by both in situ and doppler radar techniques. We show that the derived values of the turbulence energy dissipation rates are similar on average. We also find a region with a near adiabatic lapse rate with turbulence detected at the upper and lower edge. We note that it is consistent with expectation for a Kelvin-Helmholtz instability. We also present an estimate of the Schmidt numbers, Sc, for the charged aerosols that utilizes in situ measured small-scale density fluctuations of charged aerosols and both in situ and radar turbulence measurements. The derived Schmidt numbers fall within the range between 100 and 4500. This result agrees with previous estimates based on multi-frequency observations of PMSE (Rapp et al., 2008) and also with estimates of microphysical parameters presented in the companion paper by Rapp et al. (2009).
High time resolution PFISR and optical observations of naturally enhanced ion acoustic lines
Authors: Michell R. G. , Lynch K. A. , Heinselman C. J. , Stenbaek-Nielsen H. C.
Published: June 2010

Abstract. Observations of naturally enhanced ion acoustic lines (NEIALs) taken with the Poker Flat Incoherent Scatter Radar (PFISR) using a mode with very high time resolution are presented. The auroral event took place over Poker Flat, Alaska on 8 February 2007 at 09:35 UT (~22:00 MLT), and the radar data are complemented by common-volume high-resolution auroral imaging. The NEIALs occurred during only one of the standard 15-s integration periods. The raw data of this time show very intermittent NEIALs which occur only during a few very short time intervals (≤1 s) within the 15-s period. The time sampling of the raw data, ~19 ms on average, allows study of the time development of the NEIALs, though there are indications that even finer time resolution would be of interest. The analysis is based on the assumption that the NEIAL returns are the result of Bragg scattering from ion-acoustic waves that have been enhanced significantly above thermal levels. The spectra of the raw data indicate that although the up- and down-shifted shoulders can both become enhanced at the same time, (within 19 ms), they are most often enhanced individually. The overall power in the up-and down-shifted shoulders is approximately equal throughout the event, with the exception of one time, when very large up-shifted power was observed with no corresponding down-shifted power. This indicates that during the 480 μs pulse, the strongly enhanced ion-acoustic waves were only traveling downward and not upward. The exact time that the NEIALs occurred was when the radar beam was on the boundary of a fast-moving (~10 km/s), bright auroral structure, as seen in the high resolution auroral imaging of the magnetic zenith. When viewed with high time resolution, the occurrence of NEIALs is associated with rapid changes in auroral luminosity within the radar field of view due to fast-moving auroral fine structures.
Simultaneous observation of ionospheric plasma bubbles and mesospheric gravity waves during the SpreadFEx Campaign
Authors: Takahashi H. , Taylor M. J. , Pautet P.-D. , Medeiros A. F. , Gobbi D. , Wrasse C. M. , Fechine J. , Abdu M. A. , Batista I. S. , Paula E. , Sobral J. H. A. , Arruda D. , Vadas S. L. , Sabbas F. S. , Fritts D. C.
Published: June 2010

Abstract. During the Spread F Experiment campaign, under NASA Living with a Star (LWS) program, carried out in the South American Magnetic Equator region from 22 September to 8 November 2005, two airglow CCD imagers, located at Cariri (7.4° S, 36.5° W, geomag. 11° S) and near Brasilia (14.8° S, 47.6° W, geomag. 10° S) were operated simultaneously and measured the equatorial ionospheric bubbles and their time evolution by monitoring the airglow OI 6300 intensity depletions. Simultaneous observation of the mesospheric OH wave structures made it possible to investigate the relationship between the bubble formation in the ionosphere and the gravity wave activity at around 90 km. On the evening of 30 September 2005, comb-like OI 6300 depletions with a distance of ~130 km between the adjacent ones were observed. During the same period, a mesospheric gravity wave with a horizontal wavelength of ~130 km was observed. From the 17 nights of observation during the campaign period, there was a good correlation between the OI 6300 depletion distances and the gravity wave horizontal wavelengths in the mesosphere with a statistically significant level, suggesting a direct contribution of the mesospheric gravity wave to plasma bubble seeding in the equatorial ionosphere.
Observation of an inner magnetosphere electric field associated with a BBF-like flow and PBIs
Authors: Johansson T. , Bonnell J. W. , Cully C. , Donovan E. , Raeder J. , Eriksson S. , Andersson L. , Ergun R. E. , Angelopoulos V. , McFadden J. , Glassmeier K.-H. , Mann I.
Published: June 2010

Abstract. Themis E observed a perpendicular (to the magnetic field) electric field associated with an Earthward plasma flow at XGSM=−9.6 RE on 11 January 2008. The electric field observation resembles Cluster observations closer to Earth in the auroral region. The fast plasma flow shared some characteristics with bursty bulk flows (BBFs) but did not meet the usual criteria in maximum velocity and duration to qualify as one. Themis C observed the same flow further downtail but Themis D, separated by only 1 RE in azimuthal direction from Themis E, did not. At the time of the electric field and ion flow event, the all-sky imager and ground-based magnetometer at Rankin Inlet observed Poleward Boundary Intensifications (PBIs) and a negative bay signature. None of the other Themis ground-based observatories recorded any significant auroral or magnetic field activity, indicating that this was a localized activity. The joint Themis in situ and ground-based observations suggest that the two observations are related. This indicates that auroral electric fields can extend to regions much farther out than previously seen in Cluster observations.