bluuebunny/crossref_metadata_2025 huggingface dataset:
On the response of the equatorial and low latitude ionospheric regions in the Indian sector to the large magnetic disturbance of 29 October 2003
Authors: Manju G. , Kumar Pant T. , Ravindran S. , Sridharan R.
Published: June 2010

Abstract. The present paper investigates the response of the equatorial and low latitude ionosphere over the Indian longitudes to the events on 29 October 2003 using ionosonde data at Trivandrum (8.5° N (0.5° N geomagnetic), 77° E) and SHAR (13.7° N (5.7° N geomagnetic), 80.2° E), ground-based magnetometer data from Trivandrum and Total Electron Content (TEC) derived from GPS data at the locations of Ahmedabad (23° N (15° N geomagnetic), 72° E), Jodhpur (26.3° N (18.3° N geomagnetic), 73° E) and Delhi (28° N (20° N geomagnetic), 77° E). Following the storm sudden commencement, the TEC at all the three stations showed an overall enhancement in association with episodes of inter-planetary electric field penetration. Interestingly, real ionospheric height profiles derived using the ionosonde data at both Trivandrum and SHAR showed significant short-term excursions and recoveries. In the post noon sector, these features are more pronounced over SHAR, an off equatorial station, than those over Trivandrum indicating the increased effects of neutral winds.
Semidiurnal tidal signature in sporadic E occurrence rates derived from GPS radio occultation measurements at higher midlatitudes
Authors: Arras C. , Jacobi C. , Wickert J.
Published: June 2010

Abstract. GPS (Global Positioning System) Radio occultation (RO) measurements from CHAMP, GRACE and FORMOSAT-3/COSMIC satellites at Northern Hemisphere midlatitides (50°–55° N) are analysed to obtain the diurnal variation of sporadic E layer occurrence frequency in 2006 and 2007. Interconnections with zonal wind shears measured by meteor radar at Collm (51.3° N, 13° E), Germany, are investigated. According to theory, maximum Es occurrence is expected when the zonal wind shear, which is mainly produced by the semidiurnal tide in midlatitudes, is negative. This is confirmed by the present measurements and analysis.
A summary of observational records on periodicities above the rotational period in the Jovian magnetosphere
Authors: Kronberg E. A. , Woch J. , Krupp N. , Lagg A.
Published: June 2010

Abstract. The Jovian magnetosphere is a very dynamic system. The plasma mass-loading from the moon Io and the fast planetary rotation lead to regular release of mass from the Jovian magnetosphere and to a change of the magnetic topology. These regular variations, most commonly on several (2.5–4) days scale, were derived from various data sets obtained by different spacecraft missions and instruments ranging from auroral images to in situ measurements of magnetospheric particles. Specifically, ion measurements from the Galileo spacecraft represent the periodicities, very distinctively, namely the periodic thinning of the plasma sheet and subsequent dipolarization, and explosive mass release occurring mainly during the transition between these two phases. We present a review of these periodicities, particularly concentrating on those observed in energetic particle data. The most distinct periodicities are observed for ions of sulfur and oxygen. The periodic topological change of the Jovian magnetosphere, the associated mass-release process and auroral signatures can be interpreted as a global magnetospheric instability with analogies to the two step concept of terrestrial substorms. Different views on the triggering mechanism of this magnetospheric instability are discussed.
Observation of mesospheric gravity waves at Comandante Ferraz Antarctica Station (62° S)
Authors: Bageston J. V. , Wrasse C. M. , Gobbi D. , Takahashi H. , Souza P. B.
Published: June 2010

Abstract. An airglow all-sky imager was operated at Comandante Ferraz Antarctica Station (62.1° S, 58.4° W), between April and October of 2007. Mesospheric gravity waves were observed using the OH airglow layer during 43 nights with good weather conditions. The waves presented horizontal wavelengths between 10 and 60 km and observed periods mainly distributed between 5 and 20 min. The observed phase speeds range between 5 m/s and 115 m/s; the majority of the wave velocities were between 10 and 60 m/s. The waves showed a preferential propagation direction towards the southwest in winter (May to July), while during spring (August to October) there was an anisotropy with a preferential propagation direction towards the northwest. Unusual mesospheric fronts were also observed. The most probable wave source could be associated to orographic forcing, cold fronts or strong cyclonic activity in the Antarctica Peninsula.
Statistical properties of Joule heating rate, electric field and conductances at high latitudes
Authors: Aikio A. T. , Selkälä A.
Published: June 2010

Abstract. Statistical properties of Joule heating rate, electric field and conductances in the high latitude ionosphere are studied by a unique one-month measurement made by the EISCAT incoherent scatter radar in Tromsø (66.6 cgmlat) from 6 March to 6 April 2006. The data are from the same season (close to vernal equinox) and from similar sunspot conditions (about 1.5 years before the sunspot minimum) providing an excellent set of data to study the MLT and Kp dependence of parameters with high temporal and spatial resolution. All the parameters show a clear MLT variation, which is different for low and high Kp conditions. Our results indicate that the response of morning sector conductances and conductance ratios to increased magnetic activity is stronger than that of the evening sector. The co-location of Pedersen conductance maximum and electric field maximum in the morning sector produces the largest Joule heating rates 03–05 MLT for Kp≥3. In the evening sector, a smaller maximum occurs at 18 MLT. Minimum Joule heating rates in the nightside are statistically observed at 23 MLT, which is the location of the electric Harang discontinuity. An important outcome of the paper are the fitted functions for the Joule heating rate as a function of electric field magnitude, separately for four MLT sectors and two activity levels (Kp<3 and Kp≥3). In addition to the squared electric field, the fit includes a linear term to study the possible anticorrelation or correlation between electric field and conductance. In the midday sector, positive correlation is found as well as in the morning sector for the high activity case. In the midnight and evening sectors, anticorrelation between electric field and conductance is obtained, i.e. high electric fields are associated with low conductances. This is expected to occur in the return current regions adjacent to auroral arcs as a result of ionosphere-magnetosphere coupling, as discussed by Aikio et al. (2004) In addition, a part of the anticorrelation may come from polarization effects inside high-conductance regions, e.g. auroral arcs. These observations confirm the speculated effect of small scale electrodynamics, which is not included in most of the global modeling efforts of Joule heating rate.
The characteristics of field-aligned currents associated with equatorial plasma bubbles as observed by the CHAMP satellite
Authors: Park J. , Lühr H. , Stolle C. , Rother M. , Min K. W. , Michaelis I.
Published: June 2010

Abstract. Field-aligned currents (FACs) generate magnetic deflections perpendicular to the ambient Earth magnetic field. We investigate the characteristics of FACs associated with equatorial plasma bubbles (EPBs) as deduced from magnetic field measurements by the CHAMP satellite. Meridional magnetic deflections inside EPBs show a clear hemispheric anti-symmetry for events observed before 21:00 LT: inward in the Northern Hemisphere and outward in the Southern Hemisphere. When an eastward electric field is assumed the magnetic signature signifies a Poynting flux directed downward along the magnetic field lines. This means that FACs are driven by a high-altitude equatorial source. Such a scheme cannot be drawn as strictly from our observations after 22:00 LT, possibly because of a westward turning of the electric field inside EPBs and/or a decay of EPBs later at night. The perpendicular magnetic deflection is tilted by 40° from the magnetic meridional plane in westward direction, which implies that the depleted volume of EPBs, as well as the FAC structure, is tilted westward by 40° above the magnetic equator. The peak-to-peak amplitude of the FAC density is found to range typically between 0.1–0.5 μA/m2. The field-aligned sheet current density and the diamagnetic current strength show no correlation.
Radar observations of artificial E-region field-aligned irregularities
Authors: Nossa E. , Hysell D. L. , Fallen C. T. , Watkins B. J.
Published: June 2010

Abstract. Artificial E region field aligned plasma density irregularities (FAIs) were generated using HAARP in four different experimental modes and observed with a coherent scatter radar imager located 450 km to the southwest where it could detect field-aligned backscatter. The experiments were conducted in July of 2008, during the Polar Aeronomy and Radio Science Summer School (PARS), during quiet conditions in the daytime when the E layer was dense and absorption was modest. The echoes observed during zenith and magnetic zenith heating experiments were deflected from their nominally anticipated horizontal positions toward the midpoint position. The occurrence of hysteresis when heating with amplitude modulated pulses implied the development of the resonance instability, although the threshold for the onset of instability appeared to be higher than what has been predicted theoretically. Heating experiments involving pump frequencies slightly above and below the second electron gyroharmonic frequency produced no significant differences in the observed echoes. Finally, heating with a pump frequency slightly above the E region critical frequency appears to have produced FAIs at two distinct altitudes where the upper-hybrid resonance condition could be satisfied.
Uncertainties in satellite remote sensing of aerosols and impact on monitoring its long-term trend: a review and perspective
Authors: Li Z. , Zhao X. , Kahn R. , Mishchenko M. , Remer L. , Lee K.-H. , Wang M. , Laszlo I. , Nakajima T. , Maring H.
Published: June 2010

Abstract. As a result of increasing attention paid to aerosols in climate studies, numerous global satellite aerosol products have been generated. Aerosol parameters and underlining physical processes are now incorporated in many general circulation models (GCMs) in order to account for their direct and indirect effects on the earth's climate, through their interactions with the energy and water cycles. There exists, however, an outstanding problem that these satellite products have substantial discrepancies, that must be lowered substantially for narrowing the range of the estimates of aerosol's climate effects. In this paper, numerous key uncertain factors in the retrieval of aerosol optical depth (AOD) are articulated for some widely used and relatively long satellite aerosol products including the AVHRR, TOMS, MODIS, MISR, and SeaWiFS. We systematically review the algorithms developed for these sensors in terms of four key elements that influence the quality of passive satellite aerosol retrieval: calibration, cloud screening, classification of aerosol types, and surface effects. To gain further insights into these uncertain factors, the NOAA AVHRR data are employed to conduct various tests, which help estimate the ranges of uncertainties incurred by each of the factors. At the end, recommendations are made to cope with these issues and to produce a consistent and unified aerosol database of high quality for both environment monitoring and climate studies.
Coronal magnetic fields inferred from IR wavelength and comparison with EUV observations
Authors: Liu Y.
Published: June 2010

Abstract. Spectropolarimetry using IR wavelength of 1075 nm has been proved to be a powerful tool for directly mapping solar coronal magnetic fields including transverse component directions and line-of-sight component intensities. Solar tomography, or stereoscopy based on EUV observations, can supply 3-D information for some magnetic field lines in bright EUV loops. In a previous paper \\citep{liu08} the locations of the IR emission sources in the 3-D coordinate system were inferred from the comparison between the polarization data and the potential-field-source-surface (PFSS) model, for one of five west limb regions in the corona (Lin et al., 2004). The paper shows that the region with the loop system in the active region over the photospheric area with strong magnetic field intensity is the region with a dominant contribution to the observed Stokes signals. So, the inversion of the measured Stokes parameters could be done assuming that most of the signals come from a relatively thin layer over the area with a large photospheric magnetic field strength. Here, the five limb coronal regions are studied together in order to study the spatial correlation between the bright EUV loop features and the inferred IR emission sources. It is found that, for the coronal regions above the stronger photospheric magnetic fields, the locations of the IR emission sources are closer to or more consistent with the bright EUV loop locations than those above weaker photospheric fields. This result suggests that the structures of the coronal magnetic fields observed at IR and EUV wavelengths may be different when weak magnetic fields present there.
Kelvin-Helmholtz billows and their effects on mean state during gravity wave propagation
Authors: Liu X. , Xu J. , Gao H. , Chen G.
Published: June 2010

Abstract. The Kelvin-Helmholtz (KH) billows which appear in the process of gravity wave (GW) propagation are simulated directly by using a compressible nonlinear two-dimensional gravity wave model. The differences between our model and others include: the background field has no special initial configuration and there is no initial triggering mechanism needed in the mesosphere and lower thermosphere (MLT) region to excite the KH billows. However, the initial triggering mechanism is performed in the lower atmosphere through GW, which then propagate into the MLT region and form billows. The braid structures and overturning of KH billows, caused by nonlinear interactions between GWs and mean flow, can be resolved precisely by the model. These results support the findings in airglow studies that GWs propagating from below into the MLT region are important sources of KH billows. The onset of small scale waves and the wave energy transfer induce the shallower vertical wave number power spectral densities (PSD). However, most of the slopes are steeper than the expected kz−3 power law, which indicates that GWs with 10 km vertical wavelength are still a dominant mode. The results also show that the evolution of mean wind vary substantially between the different processes of GWs propagation. Before the KH billows evolve, the mean wind is accelerated greatly by GWs. By contrast, as the KH billows evolve and mix with mean flow, the mean wind and its peak value decrease.
Phase-coded pulse aperiodic transmitter coding
Authors: Virtanen I. I. , Vierinen J. , Lehtinen M. S.
Published: June 2010

Abstract. Both ionospheric and weather radar communities have already adopted the method of transmitting radar pulses in an aperiodic manner when measuring moderately overspread targets. Among the users of the ionospheric radars, this method is called Aperiodic Transmitter Coding (ATC), whereas the weather radar users have adopted the term Simultaneous Multiple Pulse-Repetition Frequency (SMPRF). When probing the ionosphere at the carrier frequencies of the EISCAT Incoherent Scatter Radar facilities, the range extent of the detectable target is typically of the order of one thousand kilometers – about seven milliseconds – whereas the characteristic correlation time of the scattered signal varies from a few milliseconds in the D-region to only tens of microseconds in the F-region. If one is interested in estimating the scattering autocorrelation function (ACF) at time lags shorter than the F-region correlation time, the D-region must be considered as a moderately overspread target, whereas the F-region is a severely overspread one. Given the technical restrictions of the radar hardware, a combination of ATC and phase-coded long pulses is advantageous for this kind of target. We evaluate such an experiment under infinitely low signal-to-noise ratio (SNR) conditions using lag profile inversion. In addition, a qualitative evaluation under high-SNR conditions is performed by analysing simulated data. The results show that an acceptable estimation accuracy and a very good lag resolution in the D-region can be achieved with a pulse length long enough for simultaneous E- and F-region measurements with a reasonable lag extent. The new experiment design is tested with the EISCAT Tromsø VHF (224 MHz) radar. An example of a full D/E/F-region ACF from the test run is shown at the end of the paper.
The sporadic radiant and distribution of meteors in the atmosphere as observed by VHF radar at Arctic, Antarctic and equatorial latitudes
Authors: Younger P. T. , Astin I. , Sandford D. J. , Mitchell N. J.
Published: June 2010

Abstract. Results are presented of a study of the temporal and spatial variability in meteor count rate observations from three VHF meteor radars. These radar are located in the Arctic (at Esrange, 68° N), in the Antarctic (at Rothera, 68° S) and near the Equator (on Ascension Island, 8° S). It is found that for all three locations there is a strong diurnal cycle in observed hourly meteor counts and the time of maxima and minima in these counts depends on the month of the year. In addition, at high latitude there is a strong annual cycle in observed monthly-mean meteor counts, whereas for the radar at low latitude there is a semi-annual cycle. At high latitude there is also an annual cycle in the mean height at which meteors are observed. However, no such annual cycle is found in observed meteor count rates from the low latitude radar. The meteor count data from all the radars are combined to investigate the sporadic radiant distribution (i.e. the distribution of direction of arrival on the celestial sphere of sporadic meteors). This combined radiant distribution shows that there are six main source regions for meteors. The latitudinal and temporal dependence in observed meteor count rates appears to result from a combination of the sporadic radiant distribution, annual fluctuations in atmospheric density, the sensitivity of the radar to meteors from different source directions and the temporal and spatial variability in meteor fluxes.
Dispersion analysis of spaced antenna scintillation measurement
Authors: Grzesiak M. , Wernik A. W.
Published: June 2010

Abstract. We present a dispersion analysis of the phase of GPS signals received at high latitude. Basic theoretical aspects for spectral analysis of two-point measurement are given. To account for nonstationarity and statistical robustness a power distribution of the windowed Fourier transform cross-spectra as a function of frequency and phase is analysed using the Radon transform.
Dust specific extinction cross-sections over the Eastern Mediterranean using the BSC-DREAM model and sun photometer data: the case of urban environments
Authors: Gerasopoulos E. , Kokkalis P. , Amiridis V. , Liakakou E. , Perez C. , Haustein K. , Eleftheratos K. , Andreae M. O. , Andreae T. W. , Zerefos C. S.
Published: June 2010

Abstract. In this study, aerosol optical depth (AOD) measurements, from a MFR sun photometer operating in Athens, were compared with columnar dust loading estimations, from the BSC-DREAM model, during identified dust events, in order to extract the typical specific extinction cross-section for dust over the area. The selected urban environment of Athens provided us with the opportunity to investigate the mixing of dust and urban pollution and to estimate the contribution of the latter. The specific extinction cross-section for dust at 500 nm was found to be equal to σ500*=0.64±0.04 m2 g, typical for medium to large distances from dust sources, with weak wavelength dependence in the visible and near infrared band (0.4–0.9 μm). The model showed a tendency to underpredict AOD levels for increasing values of the Ångström exponent, indicative of fine particles of anthropogenic origin inside the boundary layer. On average we found an AOD under-prediction of 10–15% for Ångström exponents in the range of 0 to 1 and 30–40% in the range of 1 to 2. Additionally, modelled surface concentrations were evaluated against surface PM10 measurements. Model values were lower than measured surface concentrations by 30% which, in conjunction with large scatter, indicated that the effect of the boundary layer anthropogenic contribution to columnar dust loadings is amplified near the ground.
Influences on the radius of the auroral oval
Authors: Milan S. E. , Hutchinson J. , Boakes P. D. , Hubert B.
Published: June 2010

Abstract. We examine the variation in the radius of the auroral oval, as measured from auroral images gathered by the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) spacecraft, in response to solar wind inputs measured by the Advanced Composition Explorer (ACE) spacecraft for the two year interval June 2000 to May 2002. Our main finding is that the oval radius increases when the ring current, as measured by the Sym-H index, is intensified during geomagnetic storms. We discuss our findings within the context of the expanding/contracting polar cap paradigm, in terms of a modification of substorm onset conditions by the magnetic perturbation associated with the ring current.
Solar stereoscopy – where are we and what developments do we require to progress?
Authors: Wiegelmann T. , Inhester B. , Feng L.
Published: June 2010

Abstract. Observations from the two STEREO-spacecraft give us for the first time the possibility to use stereoscopic methods to reconstruct the 3-D solar corona. Classical stereoscopy works best for solid objects with clear edges. Consequently an application of classical stereoscopic methods to the faint structures visible in the optically thin coronal plasma is by no means straight forward and several problems have to be treated adequately: 1) First there is the problem of identifying one-dimensional structures – e.g. active region coronal loops or polar plumes- from the two individual EUV-images observed with STEREO/EUVI. 2) As a next step one has the association problem to find corresponding structures in both images. This becomes more difficult as the angle between STEREO-A and B increases. 3) Within the reconstruction problem stereoscopic methods are used to compute the 3-D-geometry of the identified structures. Without any prior assumptions, e.g., regarding the footpoints of coronal loops, the reconstruction problem has not one unique solution. 4) One has to estimate the reconstruction error or accuracy of the reconstructed 3-D-structure, which depends on the accuracy of the identified structures in 2-D, the separation angle between the spacecraft, but also on the location, e.g., for east-west directed coronal loops the reconstruction error is highest close to the loop top. 5) Eventually we are not only interested in the 3-D-geometry of loops or plumes, but also in physical parameters like density, temperature, plasma flow, magnetic field strength etc. Helpful for treating some of these problems are coronal magnetic field models extrapolated from photospheric measurements, because observed EUV-loops outline the magnetic field. This feature has been used for a new method dubbed "magnetic stereoscopy". As examples we show recent application to active region loops.
Overturning instability in the mesosphere and lower thermosphere: analysis of instability conditions in lidar data
Authors: Hurd L. , Larsen M. F. , Liu A. Z.
Published: June 2010

Abstract. Resonant sodium lidar measurements from the transition region between the mesosphere and lower thermosphere have revealed frequently-occurring overturning events characterized by vertical scales of ~3–6 km and timescales of several hours. Larsen et al. (2004) proposed that a convective roll instability, similar to that found in the planetary boundary layer, is the likely mechanism responsible for the events. This type of instability requires an inflection point in the background winds near the center of the vortex roll with a low static stability region capped by an inversion. The earlier paper argued that the conditions required to support the instability are common in the altitude range where the features are found. In this paper, we use data from the University of Illinois sodium lidar that was located at the Starfire Optical Range near Albuquerque, New Mexico, and from the Maui/MALT Lidar Facility in Hawaii and present several cases that are used to examine the behavior of the inflection point in detail as a function of time during the evolution of the overturning event. In addition, we examine the background static stability conditions using the temperature data from the lidar.
Magnetospheric solitary structure maintained by 3000 km/s ions as a cause of westward moving auroral bulge at 19 MLT
Authors: Yamauchi M. , Dandouras I. , Daly P. W. , Stenberg G. , Frey H. U. , Lindqvist P.-A. , Ebihara Y. , Nilsson H. , Lundin R. , Rème H. , André M. , Kronberg E. A. , Balogh A. , Henderson M.
Published: June 2010

Abstract. In the evening equatorial magnetosphere at about 4 RE geocentric distance and 19 MLT, the four Cluster spacecraft observed a solitary structure with a width of about 1000~2000 km in the propagation direction. The solitary structure propagates sunward with about 5~10 km/s carrying sunward electric field (in the propagation direction) of up to about 10 mV/m (total potential drop of about 5~10 kV), depletion of magnetic field of about 25%, and a duskward E×B convection up to 50 km/s of He+ rich cold plasma without O+. At the same time, auroral images from the IMAGE satellite together with ground based geomagnetic field data showed a westward (sunward at this location) propagating auroral bulge at the magnetically conjugate ionosphere with the solitary structure. The solitary structure is maintained by flux enhancement of selectively 3000 km/s ions (about 50 keV for H+, 200 keV for He+, and 750 keV for O+). These ions are the main carrier of the diamagnetic current causing the magnetic depletion, whereas the polarization is maintained by different behavior of energetic ions and electrons. Corresponding to aurora, field-aligned accelerated ionospheric plasma of several keV appeared at Cluster from both hemispheres simultaneously. Together with good correspondence in location and propagation velocity between the auroral bulge and the solitary structure, this indicates that the sunward moving auroral bulge is caused by the sunward propagation of the solitary structure which is maintained by energetic ions. The solitary structure might also be the cause of Pi2-like magnetic variation that started simultaneously at Cluster location.
A study of evolution/suppression parameters of equatorial postsunset plasma instability
Authors: Oyekola O. S.
Published: June 2010

Abstract. Evening equatorial pre-reversal vertical ion E×B
Magnetosheath excursion and the relevant transport process at the magnetopause
Authors: Cai C. L. , Dandouras I. , Rème H. , Cao J. B. , Zhou G. C. , Shen C. , Parks G. K. , Fontaine D.
Published: June 2010

Abstract. A large-amplitude excursion of the magnetosheath (MS) in quiet solar wind conditions on 17 March 2004 was recorded simultaneously by the Cluster and TC-1 spacecraft. During this period, the IMF Bz was entirely northward. The coherence between the bow shock motion and magnetopause (MP) motion is revealed and the excursion velocities of the bow shock motion are analyzed. In addition, the relevant plasma transport phenomenon in the form of flux fluctuations below the ion gyrofrequency at the MP is exposed and is interpreted as manifestation of the drift instability. Correlated observations on charge accumulation and electrostatic potential perturbation are recorded by electron measurements in high energy regime, and also the eventual cross-field vortex motion in the nonlinear stage and the consequential mass exchange are exhibited. The present investigation gives some new insight into the MS plasma transport mechanism across the subsolar MP region in quiet solar wind conditions during a period of northward IMF.
Observations of plasma vortices in the vicinity of flow-braking: a case study
Authors: Keika K. , Nakamura R. , Volwerk M. , Angelopoulos V. , Baumjohann W. , Retinò A. , Fujimoto M. , Bonnell J. W. , Singer H. J. , Auster H. U. , McFadden J. P. , Larson D. , Mann I.
Published: June 2010

Abstract. We examine fast plasma flows and magnetic field fluctuations observed by THEMIS at 03:00–03:30 UT on 12 December 2007. All THEMIS probes are situated in the near-Earth plasma sheet (XSM>−10 RE) with 1–2 RE spacecraft separations in azimuthal and radial directions. We focus on the observations of plasma convective flows made simultaneously by more than one THEMIS probe. At about 03:10 UT and 03:14 UT, the THEMIS P2 probe observed earthward flows of >100 km/s. The THEMIS P1 probe, located duskward and earthward of P2, observed tailward flows under a positive Bz. The inner most probe THEMIS P4, located at almost the same MLT as THEMIS P1 and P2, did not see any clear flow. We examine the convective flow patterns for the THEMIS observations. We conclude that plasma vortices are formed near the region where the earthward flows slow down and turn in azimuthal directions.
Spatial aliasing and distortion of energy distribution in the wave vector domain under multi-spacecraft measurements
Authors: Narita Y. , Glassmeier K.-H.
Published: June 2010

Abstract. Aliasing is a general problem in the analysis of any measurements that make sampling at discrete points. Sampling in the spatial domain results in a periodic pattern of spectra in the wave vector domain. This effect is called spatial aliasing, and it is of particular importance for multi-spacecraft measurements in space. We first present the theoretical background of aliasing problems in the frequency domain and generalize it to the wave vector domain, and then present model calculations of spatial aliasing. The model calculations are performed for various configurations of the reciprocal vectors and energy spectra or distribution that are placed at different positions in the wave vector domain, and exhibit two effects on aliasing. One is weak aliasing, in which the true spectrum is distorted because of non-uniform aliasing contributions in the Brillouin zone. It is demonstrated that the energy distribution becomes elongated in the shortest reciprocal lattice vector direction in the wave vector domain. The other effect is strong aliasing, in which aliases have a significant contribution in the Brillouin zone and the energy distribution shows a false peak. These results give a caveat in multi-spacecraft data analysis in that spectral anisotropy obtained by a measurement has in general two origins: (1) natural and physical origins like anisotropy imposed by a mean magnetic field or a flow direction; and (2) aliasing effects that are imposed by the configuration of the measurement array (or the set of reciprocal vectors). This manuscript also discusses a possible method to estimate aliasing contributions in the Brillouin zone based on the measured spectrum and to correct the spectra for aliasing.
The Goddard multi-scale modeling system with unified physics
Authors: Tao W.-K. , Anderson D. , Chern J. , Entin J. , Hou A. , Houser P. , Kakar R. , Lang S. , Lau W. , Peters-Lidard C. , Li X. , Matsui T. , Rienecker M. , Schoeberl M. R. , Shen B.-W. , Shi J. J. , Zeng X.
Published: June 2010

Abstract. Recently, a multi-scale modeling system with unified physics was developed at NASA Goddard. It consists of (1) a cloud-resolving model (CRM), (2) a regional-scale model, the NASA unified Weather Research and Forecasting Model (WRF), and (3) a coupled CRM-GCM (general circulation model, known as the Goddard Multi-scale Modeling Framework or MMF). The same cloud-microphysical processes, long- and short-wave radiative transfer and land-surface processes are applied in all of the models to study explicit cloud-radiation and cloud-surface interactive processes in this multi-scale modeling system. This modeling system has been coupled with a multi-satellite simulator for comparison and validation with NASA high-resolution satellite data. This paper reviews the development and presents some applications of the multi-scale modeling system, including results from using the multi-scale modeling system to study the interactions between clouds, precipitation, and aerosols. In addition, use of the multi-satellite simulator to identify the strengths and weaknesses of the model-simulated precipitation processes will be discussed as well as future model developments and applications.
The influence of solar wind on extratropical cyclones – Part 2: A link mediated by auroral atmospheric gravity waves?
Authors: Prikryl P. , Muldrew D. B. , Sofko G. J.
Published: June 2010

Abstract. Cases of mesoscale cloud bands in extratropical cyclones are observed a few hours after atmospheric gravity waves (AGWs) are launched from the auroral ionosphere. It is suggested that the solar-wind-generated auroral AGWs contribute to processes that release instabilities and initiate slantwise convection thus leading to cloud bands and growth of extratropical cyclones. Also, if the AGWs are ducted to low latitudes, they could influence the development of tropical cyclones. The gravity-wave-induced vertical lift may modulate the slantwise convection by releasing the moist symmetric instability at near-threshold conditions in the warm frontal zone of extratropical cyclones. Latent heat release associated with the mesoscale slantwise convection has been linked to explosive cyclogenesis and severe weather. The circumstantial and statistical evidence of the solar wind influence on extratropical cyclones is further supported by a statistical analysis of high-level clouds (<440 mb) extracted from the International Satellite Cloud Climatology Project (ISCCP) D1 dataset. A statistically significant response of the high-level cloud area index (HCAI) to fast solar wind from coronal holes is found in mid-to-high latitudes during autumn-winter and in low latitudes during spring-summer. In the extratropics, this response of the HCAI to solar wind forcing is consistent with the effect on tropospheric vorticity found by Wilcox et al. (1974) and verified by Prikryl et al. (2009). In the tropics, the observed HCAI response, namely a decrease in HCAI at the arrival of solar wind stream followed by an increase a few days later, is similar to that in the northern and southern mid-to-high latitudes. The amplitude of the response nearly doubles for stream interfaces associated with the interplanetary magnetic field BZ component shifting southward. When the IMF BZ after the stream interface shifts northward, the autumn-winter effect weakens or shifts to lower (mid) latitudes and no statistically significant response is found at low latitudes in spring-summer. The observed effect persists through years of low and high volcanic aerosol loading. The similarity of the response in mid-to-high and low latitudes, the lack of dependence upon aerosol loading, and the enhanced amplitude of the effect when IMF BZ component shifts southward, favor the proposed AGW link over the atmospheric electric circuit (AEC) mechanism (Tinsley et al., 1994). The latter requires the presence of stratospheric aerosols for a significant effect and should produce negative and positive cloud anomalies in mid-to-high and low latitudes, respectively. However, if the requirement of aerosols for the AEC mechanism can be relaxed, the AGW and AEC mechanisms should work in synergy at least in mid-to-high latitudes.
Role of gravity wavelike seed perturbations on the triggering of ESF – a case study from unique dayglow observations
Authors: Sreeja V. , Vineeth C. , Pant Tarun Kumar , Ravindran Sudha , Sridharan R.
Published: June 2010

Abstract. First observational evidence, from the Indian longitudes, for the presence of gravity wavelike perturbations with periods of 20–30 min, acting as probable seeds for Equatorial Spread F (ESF) irregularities is described. The study is based on the daytime optical measurements of the mesopause temperature and the intensity of the thermospheric O(1D) 630.0 nm dayglow emissions using the unique MultiWavelength Dayglow PhotoMeter from Trivandrum (8.5° N; 77° E; dip lat ~0.5° N), a dip equatorial station. Measurements during the equinoctial months of a solar maximum (2001) and a solar minimum year (2006) have been used in this study. It is shown that under identical background ionospheric conditions within a solar epoch, the power of the gravity waves have a deterministic role in the generation of ESF. The mesopause temperature simultaneously observed, indicate that possible source regions for these perturbations lie in the lower atmosphere.
Search for an onset mechanism that operates for both CMEs and substorms
Authors: Siscoe G. L. , Kuznetsova M. M. , Raeder J.
Published: June 2010

Abstract. Substorms and coronal mass ejections have been cited as the most accessible examples of the explosive energy conversion phenomenon that seems to characterize one of the behavior modes of cosmic plasmas. This paper addresses the question of whether these two examples – substorms and CMEs – support or otherwise the idea that explosive energy conversion is the result of a single process operating in different places and under different conditions. As a candidate mechanism that might be common to both substorms and CMEs we use the Forbes catastrophe model for CMEs because before testing it appears to have the potential, suitably modified, to operate also for substorms. The essence of the FCM is a sudden onset of an imbalance of the forces acting on an incipient CME. The imbalance of forces causes the CME to start to rise. Beneath the rising CME conditions develop that favor the onset of magnetic reconnection which then releases the CME and assists its expulsion. Thus the signature of the FCM is a temporally ordered sequence in which there is first the appearance of force imbalance which leads to upward (or outward) motion of the CME which leads to magnetic reconnection under it which expedites rapid expulsion. We look for the FCM signature in the output of two global magnetospheric MHD simulations that produce substorm-like events. We find the ordered sequence of events as stated but with a significant difference: there is no plasmoid prior to the onset of rapid reconnection, that is, there is no counterpart to the incipient CME on which an imbalance of forces acts to initiate the action in the FCM. If this result – that rapid tailward motion precedes the rapid reconnection of substorm expansion – is ultimately verified by other studies, it suggests that a description of the cause of substorm expansion should identify the cause of the preceding rapid tailward motion, since this leads necessarily to rapid reconnection, whatever the reconnection mechanism turns out to be. Clearly then, it is important to identify the cause of the preceding tailward motion.
The potential of the synergistic use of passive and active remote sensing measurements for the validation of a regional dust model
Authors: Amiridis V. , Kafatos M. , Perez C. , Kazadzis S. , Gerasopoulos E. , Mamouri R. E. , Papayannis A. , Kokkalis P. , Giannakaki E. , Basart S. , Daglis I. , Zerefos C.
Published: June 2010

Abstract. A long-lasting Saharan dust event affected Europe on 18–23 May 2008. Dust was present in the free troposphere over Greece, in height ranges between the surface and approximately 4–5 km above sea level. The event was monitored by ground-based CIMEL sunphotometric and multi-wavelength combined backscatter/Raman lidar measurements over Athens, Greece. The dust event had the maximum of its intensity on 20 May. Three-dimensional dust spatial distribution over Greece on that day is presented through satellite synergy of passive and active remote sensing using MODIS and CALIPSO data, respectively. For the period under study, the ground-based measurements are used to characterize the dust event and evaluate the latest version of the BSC Dust Regional Atmospheric Modeling (BSC-DREAM) system. Comparisons of modeled and measured aerosol optical depths over Athens show that the Saharan dust outbreak is fairly well captured by BSC-DREAM simulations. Evaluation of BSC-DREAM using Raman lidar measurements on 20 May shows that the model consistently reproduces the dust vertical distribution over Athens.
Changes in the response of the AL Index with solar cycle and epoch within a corotating interaction region
Authors: McPherron R. L. , Kepko L. , Pulkkinen T. I. , Hsu T. S. , Weygand J. W. , Bargatze L. F.
Published: June 2010

Abstract. We use observations in the solar wind and on the ground to study the interaction of the solar wind and interplanetary magnetic field with Earth's magnetosphere. We find that the type of response depends on the state of the solar wind. Coupling functions change as the properties of the solar wind change. We examine this behavior quantitatively with time dependent linear prediction filters. These filters are determined from ensemble arrays of representative events organized by some characteristic time in the event time series. In our study we have chosen the stream interface at the center of a corotating interaction region as the reference time. To carry out our analysis we have identified 394 stream interfaces in the years 1995–2007. For each interface we have selected ten-day intervals centered on the interface and placed data for the interval in rows of an ensemble array. In this study we use Es the rectified dawn-dusk electric field in gsm coordinates as input and the AL index as output. A selection window of width one day is stepped across the ensemble and for each of the nine available windows all events in a given year (~30) are used to calculate a system impulse response function. A change in the properties of the system as a consequence of changes in the solar wind relative to the reference time will appear as a change in the shape and/or the area of the response function. The analysis shows that typically only 45% of the AL variance is predictable in this manner when filters are constructed from a full year of data. We find that the weakest coupling occurs around the stream interface and the strongest well away from the interface. The interface is the time of peak dynamic pressure and strength of the electric field. We also find that coupling appears to be stronger during recurrent high-speed streams in the declining phase of the solar cycle than it is around solar maximum. These results are consistent with the previous report that both strong driving (Es) and high dynamic pressure (Pdyn) reduce the coupling efficiency. Although the changes appear to be statistically significant their physical cause cannot be uniquely identified because various properties of the solar wind vary systematically through a corotating interaction region. It is also possible that the quality of the propagated solar wind data depends on the state of the solar wind. Finally it is likely that the quality of the AL index during the last solar cycle may affect the results. Despite these limitations our results indicate that the Es-AL coupling function is 50% stronger outside a corotating interaction region than inside.
A global model of the ionospheric F2 peak height based on EOF analysis
Authors: Zhang M.-L. , Liu C. , Wan W. , Liu L. , Ning B.
Published: June 2010

Abstract. The ionospheric F2 peak height hmF2 is an important parameter that is much needed in ionospheric research and practical applications. In this paper, an attempt is made to develop a global model of hmF2. The hmF2 data, used to construct the global model, are converted from the monthly median hourly values of the ionospheric propagation factor M(3000)F2 observed by ionosondes/digisondes distributed globally, based on the strong anti-correlation existed between hmF2 and M(3000)F2. The empirical orthogonal function (EOF) analysis method, combined with harmonic function and regression analysis, is used to construct the model. The technique used in the global modelling involves two layers of EOF analysis of the dataset. The first layer EOF analysis is applied to the hmF2 dataset which decomposed the dataset into a series of orthogonal functions (EOF base functions) Ek and their associated EOF coefficients Pk. The base functions Ek represent the intrinsic characteristic variations of the dataset with the modified dip latitude and local time, the coefficients Pk represents the variations of the dataset with the universal time, season as well as solar cycle activity levels. The second layer EOF analysis is applied to the EOF coefficients Pk obtained in the first layer EOF analysis. The coefficients Ak, obtained in the second layer EOF analysis, are then modelled with the harmonic functions representing the seasonal (annual and semi-annual) and solar cycle variations, with their amplitudes changing with the F10.7 index, a proxy of the solar activity level. Thus, the constructed global model incorporates the geographical location, diurnal, seasonal as well as solar cycle variations of hmF2 through the combination of EOF analysis and the harmonic function expressions of the associated EOF coefficients. Comparisons between the model results and observational data were consistent, indicating that the modelling technique used is very promising when used to construct the global model of hmF2 and it has the potential of being used for the global modelling/mapping of other ionospheric parameters. Statistical analysis on model-data comparison showed that our constructed model of hmF2, based on the EOF expansion method, compares better with the observational data than the model currently used in the International Reference Ionosphere (IRI) model.
Analysis and modeling of ducted and evanescent gravity waves observed in the Hawaiian airglow
Authors: Simkhada D. B. , Snively J. B. , Taylor M. J. , Franke S. J.
Published: June 2010

Abstract. Short-period gravity waves of especially-small horizontal scale have been observed in the Maui, Hawaii airglow. Typical small-scale gravity wave events have been investigated, and intrinsic wave propagation characteristics have been calculated from simultaneous meteor radar wind measurements. Here we report specific cases where wave structure is significantly determined by the local wind structure, and where wave characteristics are consistent with ducted or evanescent waves throughout the mesopause region. Two of the documented events, exhibiting similar airglow signatures but dramatically different propagation conditions, are selected for simple numerical modeling case studies. First, a Doppler-ducted wave trapped within relatively weak wind flow is examined. Model results confirm that the wave is propagating in the 85–95 km region, trapped weakly by evanescence above and below. Second, an evanescent wave in strong wind flow is examined. Model results suggest an opposite case from the first case study, where the wave is instead trapped above or below the mesopause region, with strong evanescence arising in the 85–95 km airglow region. Distinct differences between the characteristics of these visibly-similar wave events demonstrate the need for simultaneous observations of mesopause winds to properly assess local propagation conditions.