Showing papers in "Annales Geophysicae in 2012"
TL;DR: In this paper, multiple dipolarization fronts (DFs) were observed by Cluster spacecraft in the magnetotail during a substorm, which were kinetic structures, embedded in the bursty plasma flow, and moved earthward (mainly) and dawnward.
Abstract: Multiple dipolarization fronts (DFs) were observed by Cluster spacecraft in the magnetotail during a substorm. These DFs were kinetic structures, embedded in the bursty plasma flow, and moved earthward (mainly) and dawnward. Intense electric field, parallel and perpendicular currents were detected in the DF layer. These front layers were energy dissipation region (load region) where the energy of electromagnetic fields were transferred to the plasma thermal and kinetic energy. This dissipation was dominated by electrons. There were enhancements of plasma waves around the DF region: wavelet results show that wave activities around the ion cyclotron frequency in the front layer were generated by Alfven ion cyclotron instability; whistler waves were also detected before, during and after the DFs, which are triggered by electron temperature anisotropy and coincident with enhancement of energetic electron fluxes. The observation of these waves could be important for the understanding of evolution of DF and electron energization during the substorm. We discuss the generation mechanism of the DFs and suggest that these DFs were generated in the process of transient reconnection, and then traveled toward the Earth.
140 citations
TL;DR: In this article, the authors studied the effect of the bow shock on the magnetosphere and the ionosphere, and found that the jets could have a scale size of up to a few RE but less than 6 RE transverse to the XGSE axis.
Abstract: . It has recently been proposed that ripples inherent to the bow shock during radial interplanetary magnetic field (IMF) may produce local high speed flows in the magnetosheath. These jets can have a dynamic pressure much larger than the dynamic pressure of the solar wind. On 17 March 2007, several jets of this type were observed by the Cluster spacecraft. We study in detail these jets and their effects on the magnetopause, the magnetosphere, and the ionospheric convection. We find that (1) the jets could have a scale size of up to a few RE but less than ~6 RE transverse to the XGSE axis; (2) the jets caused significant local magnetopause perturbations due to their high dynamic pressure; (3) during the period when the jets were observed, irregular pulsations at the geostationary orbit and localised flow enhancements in the ionosphere were detected. We suggest that these inner magnetospheric phenomena were caused by the magnetosheath jets.
107 citations
TL;DR: In this paper, a nonlinear model approach with 13 model coefficients and a few empirically fixed parameters was proposed to model and predict the spatial and temporal variations of the peak electron density height.
Abstract: . The F2-layer peak density height hmF2 is one of the most important ionospheric parameters characterizing HF propagation conditions. Therefore, the ability to model and predict the spatial and temporal variations of the peak electron density height is of great use for both ionospheric research and radio frequency planning and operation. For global hmF2 modelling we present a nonlinear model approach with 13 model coefficients and a few empirically fixed parameters. The model approach describes the temporal and spatial dependencies of hmF2 on global scale. For determining the 13 model coefficients, we apply this model approach to a large quantity of global hmF2 observational data obtained from GNSS radio occultation measurements onboard CHAMP, GRACE and COSMIC satellites and data from 69 worldwide ionosonde stations. We have found that the model fits to these input data with the same root mean squared (RMS) and standard deviations of 10%. In comparison with the electron density NeQuick model, the proposed Neustrelitz global hmF2 model (Neustrelitz Peak Height Model – NPHM) shows percentage RMS deviations of about 13% and 12% from the observational data during high and low solar activity conditions, respectively, whereas the corresponding deviations for the NeQuick model are found 18% and 16%, respectively.
93 citations
TL;DR: In this article, the authors used reaction rates on cold binary aerosol in a model of stratospheric chemistry to derive a threshold temperature, TACL, for the onset of chlorine activation.
Abstract: . Low stratospheric temperatures are known to be responsible for heterogeneous chlorine activation that leads to polar ozone depletion. Here, we discuss the temperature threshold below which substantial chlorine activation occurs. We suggest that the onset of chlorine activation is dominated by reactions on cold binary aerosol particles, without the formation of polar stratospheric clouds (PSCs), i.e. without any significant uptake of HNO3 from the gas phase. Using reaction rates on cold binary aerosol in a model of stratospheric chemistry, a chlorine activation threshold temperature, TACL, is derived. At typical stratospheric conditions, TACL is similar in value to TNAT (within 1–2 K), the highest temperature at which nitric acid trihydrate (NAT) can exist. TNAT is still in use to parameterise the threshold temperature for the onset of chlorine activation. However, perturbations can cause TACL to differ from TNAT: TACL is dependent upon H2O and potential temperature, but unlike TNAT is not dependent upon HNO3. Furthermore, in contrast to TNAT, TACL is dependent upon the stratospheric sulfate aerosol loading and thus provides a means to estimate the impact on polar ozone of strong volcanic eruptions and some geo-engineering options, which are discussed. A parameterisation of TACL is provided here, allowing it to be calculated for low solar elevation (or high solar zenith angle) over a comprehensive range of stratospheric conditions. Considering TACL as a proxy for chlorine activation cannot replace a detailed model calculation, and polar ozone loss is influenced by other factors apart from the initial chlorine activation. However, TACL provides a more accurate description of the temperature conditions necessary for chlorine activation and ozone loss in the polar stratosphere than TNAT.
89 citations
TL;DR: In this paper, the authors present multi spacecraft measurements in the magnetosheath (MSH) and in the solar wind (SW) by Interball, Cluster and Polar, demonstrating that coherent structures with magnetosonic Mach number up to 3 generate transient and anomalous boundary dynamics, which may cause substantial displacements of the magnetospheric boundaries and the riddling of peripheral boundary layers.
Abstract: . We present multi spacecraft measurements in the magnetosheath (MSH) and in the solar wind (SW) by Interball, Cluster and Polar, demonstrating that coherent structures with magnetosonic Mach number up to 3 – Supermagnetosonic Plasma Streams (SPS) – generate transient and anomalous boundary dynamics, which may cause substantial displacements of the magnetospheric boundaries and the riddling of peripheral boundary layers. In this regard, for the first time, we describe a direct plasma penetration into the flank boundary layers, which is a candidate for being the dominant transport mechanism for disturbed MSH periods. Typically SPS's have a ram pressure exceeding by several times that of the SW and lead to long-range correlations between processes at the bow shock (BS) and at the magnetopause (MP) on one side and between MSH and MP boundary layers on the other side. We demonstrate that SPS's can be observed both near the BS and near the MP and argue that they are often triggered by hot flow anomalies (HFA), which represent local obstacles to the SW flow and can induce the SPS generation as a means for achieving a local flow balance. Finally, we also discuss other causes of SPS's, both SW-induced and intrinsic to the MSH. SPS's appear to be universal means for establishing a new equilibrium between flowing plasmas and may also prove to be important for astrophysical and fusion applications.
87 citations
TL;DR: In this paper, the authors reported on the longitudinal, local time and seasonal occurrence of equatorial plasma bubbles (EPBs) and L band (GPS) scintillations over equatorial Africa.
Abstract: . We report on the longitudinal, local time and seasonal occurrence of equatorial plasma bubbles (EPBs) and L band (GPS) scintillations over equatorial Africa. The measurements were made in 2010, as a first step toward establishing the climatology of ionospheric irregularities over Africa. The scintillation intensity is obtained by measuring the standard deviation of normalized GPS signal power. The EPBs are detected using an automated technique, where spectral analysis is used to extract and identify EPB events from the GPS TEC measurements. Overall, the observed seasonal climatology of the EPBs as well as GPS scintillations in equatorial Africa is adequately explained by geometric arguments, i.e., by the alignment of the solar terminator and local geomagnetic field, or STBA hypothesis (Tsunoda, 1985, 2010a). While plasma bubbles and scintillations are primarily observed during equinoctial periods, there are longitudinal differences in their seasonal occurrence statistics. The Atlantic sector has the most intense, longest lasting, and highest scintillation occurrence rate in-season. There is also a pronounced increase in the EPB occurrence rate during the June solstice moving west to east. In Africa, the seasonal occurrence shifts towards boreal summer solstice, with fewer occurrences and shorter durations in equinox seasons. Our results also suggest that the occurrence of plasma bubbles and GPS scintillations over Africa are well correlated, with scintillation intensity depending on depletion depth. A question remains about the possible physical mechanisms responsible for the difference in the occurrence phenomenology of EPBs and GPS scintillations between different regions in equatorial Africa.
80 citations
TL;DR: In this article, magnetic holes with relatively small scale sizes, detected by Cluster and TC-1 in the magnetotail plasma sheet, are studied and it is found that these magnetic holes are spatial structures and they are not magnetic depressions generated by the flapping movement of the magnetic current sheet.
Abstract: . Magnetic holes with relatively small scale sizes, detected by Cluster and TC-1 in the magnetotail plasma sheet, are studied in this paper. It is found that these magnetic holes are spatial structures and they are not magnetic depressions generated by the flapping movement of the magnetotail current sheet. Most of the magnetic holes (93%) were observed during intervals with Bz larger than Bx, i.e. they are more likely to occur in a dipolarized magnetic field topology. Our results also suggest that the occurrence of these magnetic holes might have a close relationship with the dipolarization process. The magnetic holes typically have a scale size comparable to the local proton Larmor radius and are accompanied by an electron energy flux enhancement at a 90° pitch angle, which is quite different from the previously observed isotropic electron distributions inside magnetic holes in the plasma sheet. It is also shown that most of the magnetic holes occur in marginally mirror-stable environments. Whether the plasma sheet magnetic holes are generated by the mirror instability related to ions or not, however, is unknown. Comparison of ratios, scale sizes and propagation direction of magnetic holes detected by Cluster and TC-1, suggests that magnetic holes observed in the vicinity of the TC-1 orbit (~7–12 RE) are likely to be further developed than those observed by Cluster (~7–18 RE).
72 citations
TL;DR: In this paper, the authors consider the mechanism of ion acceleration by dipolarization fronts in the Earth's magnetotail and develop the analytical model of the ion resonance interaction with dipolarisation fronts to describe the observed acceleration.
Abstract: . In this paper, we consider the mechanism of ion acceleration by dipolarization fronts in the Earth's magnetotail. The statistics of dipolarization front observations by Interball-tail have been collected from 1995 to 1998 (51 events). We demonstrate that near dipolarization fronts bursts of energetic ions are often observed with an average energy of about 100–200 keV. We develop the analytical model of the ion resonance interaction with dipolarization fronts to describe the observed acceleration. We compare the model and the observations to estimate the width of fronts along the dawn-dusk direction, Ry. The mean value is ⟨ Ry ⟩ ~6 RE.
70 citations
TL;DR: In this article, the authors carried out a survey of all Double Star 1 magnetopause crossings, using a combination of ion and magnetic field measurements, and identified 17 candidate events from the entire TC-1 mission.
Abstract: The Kelvin-Helmholtz Instability (KHI) can drive waves at the magnetopause. These waves can grow to form rolled-up vortices and facilitate transfer of plasma into the magnetosphere. To investigate the persistence and frequency of such waves at the magnetopause we have carried out a survey of all Double Star 1 magnetopause crossings, using a combination of ion and magnetic field measurements. Using criteria originally used in a Geotail study made by Hasegawa et al. (2006) (forthwith referred to as H2006), 17 candidate events were identified from the entire TC-1 mission (cover- ing 623 orbits where the magnetopause was sampled), a majority of which were on the dayside of the terminator. The relationship between density and shear velocity was then in- vestigated, to identify the predicted signature of a rolled up vortex from H2006 and all 17 events exhibited some level of rolled up behavior. The location of the events had a clear dawn-dusk asymmetry, with 12 (71 %) on the post noon, dusk flank suggesting preferential growth in this region.
68 citations
TL;DR: In this paper, the authors reconstruct the realistic distribution of chorus wave-normals in radiation belts for all magnetic latitudes using three-dimensional ray tracing technique, which employs K. Ronnmark's WHAMP to solve hot plasma dispersion relation along the wave packet trajectory.
Abstract: . Discrete ELF/VLF (Extremely Low Frequency/Very Low Frequency) chorus emissions are one of the most intense electromagnetic plasma waves observed in radiation belts and in the outer terrestrial magnetosphere. These waves play a crucial role in the dynamics of radiation belts, and are responsible for the loss and the acceleration of energetic electrons. The objective of our study is to reconstruct the realistic distribution of chorus wave-normals in radiation belts for all magnetic latitudes. To achieve this aim, the data from the electric and magnetic field measurements onboard Cluster satellite are used to determine the wave-vector distribution of the chorus signal around the equator region. Then the propagation of such a wave packet is modeled using three-dimensional ray tracing technique, which employs K. Ronnmark's WHAMP to solve hot plasma dispersion relation along the wave packet trajectory. The observed chorus wave distributions close to waves source are first fitted to form the initial conditions which then propagate numerically through the inner magnetosphere in the frame of the WKB approximation. Ray tracing technique allows one to reconstruct wave packet properties (electric and magnetic fields, width of the wave packet in k-space, etc.) along the propagation path. The calculations show the spatial spreading of the signal energy due to propagation in the inhomogeneous and anisotropic magnetized plasma. Comparison of wave-normal distribution obtained from ray tracing technique with Cluster observations up to 40° latitude demonstrates the reliability of our approach and applied numerical schemes.
58 citations
TL;DR: In this article, the authors present an investigation of magnetic flux ropes observed by the four Cluster spacecraft during periods of magnetic reconnection in the Earth's magnetotail, using a list of 21 Cluster encounters with the reconnection process in the period 2001-2006 identified in Borg et al.
Abstract: . We present an investigation of magnetic flux ropes observed by the four Cluster spacecraft during periods of magnetic reconnection in the Earth's magnetotail. Using a list of 21 Cluster encounters with the reconnection process in the period 2001–2006 identified in Borg et al. (2012), we present the distribution and characteristics of the flux ropes. We find 27 flux ropes embedded in the reconnection outflows of only 11 of the 21 reconnection encounters. Reconnection processes associated with no flux rope observations were not distinguishable from those where flux ropes were observed. Only 7 of the 27 flux ropes show evidence of enhanced energetic electron flux above 50 keV, and there was no clear signature of the flux rope in the thermal particle measurements. We found no clear correlation between the flux rope core field and the prevailing IMF By direction.
TL;DR: In this article, the authors present a near real-time (NRT) high resolution water vapour distribution models based on GNSS observations delivered from Ground Base Augmentation Systems (GBAS) and ground meteorological data.
Abstract: . The near real-time (NRT) high resolution water vapour distribution models can be constructed based on GNSS observations delivered from Ground Base Augmentation Systems (GBAS) and ground meteorological data. Since 2008 in the territory of Poland, a GBAS system called ASG-EUPOS (Active Geodetic Network) has been operating. This paper addresses the problems concerning construction of the NRT model of water vapour distribution in the troposphere near Poland. The first section presents all available GNSS and ground meteorological stations in the area of Poland and neighbouring countries. In this section, data feeding scheme is discussed, together with timeline and time resolution. The high consistency between measured and interpolated temperature value is shown, whereas some discrepancy in the pressure is observed. In the second section, the NRT GNSS data processing strategy of ASG-EUPOS network is discussed. Preliminary results show fine alignment of the obtained Zenith Troposphere Delays (ZTDs) with reference data from European Permanent Network (EPN) processing center. The validation of NRT troposphere products against daily solution shows 15 mm standard deviation of obtained ZTD differences. The last section presents the first results of 2-D water vapour distribution above the GNSS network and application of the tomographic model to 3-D distribution of water vapour in the atmosphere. The GNSS tomography model, working on the simulated data from numerical forecast model, shows high consistency with the reference data (by means of standard deviation 4 mm km−1 or 4 ppm), however, noise analysis shows high solution sensitivity to errors in observations. The discrepancy for real data preliminary solution (measured as a mean standard deviation) between reference NWP data and tomography data was on the level of 9 mm km−1 (or 9 ppm) in terms of wet refractivity.
TL;DR: In this article, the authors present in situ observations of meteoric smoke particles (MSP) obtained during three sounding rocket flights in December 2010 in the frame of the final campaign of the Norwegian-German ECOMA project (ECOMA = Existence and Charge state Of meteoric Smoke particles in the Middle Atmosphere).
Abstract: . We present in situ observations of meteoric smoke particles (MSP) obtained during three sounding rocket flights in December 2010 in the frame of the final campaign of the Norwegian-German ECOMA project (ECOMA = Existence and Charge state Of meteoric smoke particles in the Middle Atmosphere). The flights were conducted before, at the maximum activity, and after the decline of the Geminids which is one of the major meteor showers over the year. Measurements with the ECOMA particle detector yield both profiles of naturally charged particles (Faraday cup measurement) as well as profiles of photoelectrons emitted by the MSPs due to their irradiation by photons of a xenon-flash lamp. The column density of negatively charged MSPs decreased steadily from flight to flight which is in agreement with a corresponding decrease of the sporadic meteor flux recorded during the same period. This implies that the sporadic meteors are a major source of MSPs while the additional influx due to the shower meteors apparently did not play any significant role. Surprisingly, the profiles of photoelectrons are only partly compatible with this observation: while the photoelectron current profiles obtained during the first and third flight of the campaign showed a qualitatively similar behaviour as the MSP charge density data, the profile from the second flight (i.e., at the peak of the Geminids) shows much smaller photoelectron currents. This may tentatively be interpreted as a different MSP composition (and, hence, different photoelectric properties) during this second flight, but at this stage we are not in a position to conclude that there is a cause and effect relation between the Geminids and this observation. Finally, the ECOMA particle detector used during the first and third flight employed three instead of only one xenon flash lamp where each of the three lamps used for one flight had a different window material resulting in different cut off wavelengths for these three lamp types. Taking into account these data along with simple model estimates as well as rigorous quantum chemical calculations, it is argued that constraints on MSP sizes, work function and composition can be inferred. Comparing the measured data to a simple model of the photoelectron currents, we tentatively conclude that we observed MSPs in the 0.5–3 nm size range with generally increasing particle size with decreasing altitude. Notably, this size information can be obtained because different MSP particle sizes are expected to result in different work functions which is both supported by simple classical arguments as well as quantum chemical calculations. Based on this, the MSP work function can be estimated to lie in the range from ~4–4.6 eV. Finally, electronic structure calculations indicate that the low work function of the MSP measured by ECOMA indicates that Fe and Mg hydroxide clusters, rather than metal silicates, are the major constituents of the smoke particles.
TL;DR: In this article, the seasonal changes in aerosol characteristics using a classification scheme proposed by Gobbi et al. (2007) were analyzed using Spectral aerosol measurements using the Microtops-II sun photometer (MT-II) during April 2009-March 2010 and analyzed to study the seasonal effects.
Abstract: . The present study focuses on analyzing the seasonal changes in aerosol characteristics using a classification scheme proposed by Gobbi et al. (2007). This scheme is based on the correlation between the Angstrom exponent (α) in the 440–870 nm range and the difference in α values [dα = α (440–675 − α(675–870)] including the size of fine-mode particles (Rf) and the fine-mode fraction (η). The classification scheme can therefore provide information on the aerosol characteristics and their modification in transit. Spectral aerosol measurements using the Microtops-II sun photometer (MT-II) have systematically been conducted in Hyderabad, India during April 2009–March 2010 and analysed to study the seasonal effects. The results reveal a seasonal dependence, i.e. the presence of fine-mode aerosols under turbid atmospheres in winter and post-monsoon, a mixture of fine and coarse aerosol types in pre-monsoon and a significant influence of marine mixed with dust air masses during the monsoon season. The identification of the aerosol source type and the modification processes are discussed along with clustered air-mass trajectory analysis. Furthermore, we have also checked the consistency of this scheme with the findings arrived from the columnar size distributions (CSDs) computed by numerical inversion of spectral AOD using King's inversion algorithm and the scatter plot between AOD and spectral α. The comparison clearly demonstrates the usefulness of the classification scheme and highlights its advantages for the monitoring and study of seasonal variation of the aerosol types and the modification processes in the atmosphere.
TL;DR: In this paper, a comparison of Collection 5 (C005), aerosol optical depth (AOD) retrieved by MODIS, with AERONET-observed AOD over Kanpur (an urban site), Gandhi College (a rural site) and Nainital (a relatively clean site) in the Indo Gangetic Plain (IGP).
Abstract: . Moderate resolution imaging spectroradiometer (MODIS) sensors, onboard Terra and Aqua, have been observing the Earth since start of 2000 and mid 2002, respectively. The present study provides a comparison of Collection 5 (C005), aerosol optical depth (AOD) retrieved by MODIS, with AERONET-observed AOD over Kanpur (an urban site), Gandhi College (a rural site) and Nainital (a relatively clean site) in the Indo Gangetic Plain (IGP). The results show that at Kanpur, MODIS retrievals are well within the prelaunch uncertainty ± 0.05 ±0.15 τ, and a good correlation (R2 > 0.7 for both Terra and Aqua). Nainital also shows good retrieval (R2 > 0.8 for Terra and R2 > 0.68 for Aqua), as more than 66% of total collocations are within the prelaunch uncertainty. However, it is seen that there is significant overestimation in this case, especially in the months of winter. Gandhi College poses a challenge to MODIS retrieval, as here
TL;DR: In this paper, the complete plasma and Maxwell's equations are derived rigorously from the complete ionospheric electrodynamics, provided that appropriate limits and approximations are assumed, which leads to a picture of the underlying physical processes that can be rather different from the commonly presented intuitive or ad hoc explanations.
Abstract: . The conventional equations of ionospheric electrodynamics, highly succesful in modeling observed phenomena on sufficiently long time scales, can be derived rigorously from the complete plasma and Maxwell's equations, provided that appropriate limits and approximations are assumed. Under the assumption that a quasi-steady-state equilibrium (neglecting local dynamical terms and considering only slow time variations of external or aeronomic-process origin) exists, the conventional equations specify how the various quantities must be related numerically. Questions about how the quantities are related causally or how the stress equilibrium is established and on what time scales are not anwered by the conventional equations but require the complete plasma and Maxwell's equations, and these lead to a picture of the underlying physical processes that can be rather different from the commonly presented intuitive or ad hoc explanations. Particular instances include the nature of the ionospheric electric current, the relation between electric field and plasma bulk flow, and the interrelationships among various quantities of neutral-wind dynamo.
TL;DR: In this article, the authors used a combination of single-pulse-Doppler, time-of-flight and pulse-topulse phase correlation measurements to determine the radial velocity to within a few tens of metres per second with 3.12 ms time resolution.
Abstract: . We have developed an automated analysis scheme for meteor head echo observations by the 46.5 MHz Middle and Upper atmosphere (MU) radar near Shigaraki, Japan (34.85° N, 136.10° E). The analysis procedure computes meteoroid range, velocity and deceleration as functions of time with unprecedented accuracy and precision. This is crucial for estimations of meteoroid mass and orbital parameters as well as investigations of the meteoroid-atmosphere interaction processes. In this paper we present this analysis procedure in detail. The algorithms use a combination of single-pulse-Doppler, time-of-flight and pulse-to-pulse phase correlation measurements to determine the radial velocity to within a few tens of metres per second with 3.12 ms time resolution. Equivalently, the precision improvement is at least a factor of 20 compared to previous single-pulse measurements. Such a precision reveals that the deceleration increases significantly during the intense part of a meteoroid's ablation process in the atmosphere. From each received pulse, the target range is determined to within a few tens of meters, or the order of a few hundredths of the 900 m long range gates. This is achieved by transmitting a 13-bit Barker code oversampled by a factor of two at reception and using a novel range interpolation technique. The meteoroid velocity vector is determined from the estimated radial velocity by carefully taking the location of the meteor target and the angle from its trajectory to the radar beam into account. The latter is determined from target range and bore axis offset. We have identified and solved the signal processing issue giving rise to the peculiar signature in signal to noise ratio plots reported by Galindo et al. (2011), and show how to use the range interpolation technique to differentiate the effect of signal processing from physical processes.
TL;DR: The formation of the halo may not be a continuous process, but one that occurs, in part, in bursts in regions where the conditions responsible for the scattering are optimum as discussed by the authors.
Abstract: . Observations of a continual erosion of the strahl and build up of the halo with distance from the sun suggests that, at least in part, the halo may be formed as a result of scattering of the strahl. This hypothesis is supported in this paper by observation of intense scattering of strahl electrons, which gives rise to a proto-halo electron population. This population eventually merges into, or becomes the halo. The fact that observations of intense scattering of the strahl are not common implies that the formation of the halo may not be a continuous process, but one that occurs, in part, in bursts in regions where the conditions responsible for the scattering are optimum.
TL;DR: In this article, the authors used data from six sounding rockets carried different dust detectors, as well as probes for electrons and ions, to assess whether the existence of meteoric dust removes more electrons than would be expected from gas phase ion chemistry alone.
Abstract: . The ionosphere is always assumed to contain equal numbers of positive and negative charges in a given volume (quasineutrality). Hence fewer electrons than positive charges are an indication of negative charges other than electrons. Theories predict and in-situ mass spectrometer measurements confirmed that these negative charges are negative ions, but recent experimental results suggest that other scavengers of free electrons can also be active in the mesosphere. Outside the polar summer mesosphere this additional removal of electrons is today believed to be due to meteoric dust, which maximises in the mesosphere. Data predominantly from the recent ECOMA flights are used to test this presumption. Six sounding rockets carried different dust detectors, as well as probes for electrons and ions. With such an instrumental ensemble one can assess whether indeed the existence of meteoric dust removes more electrons than would be expected from gas phase ion chemistry alone. Other factors potentially impacting on electron removal are also discussed in the paper.
TL;DR: In this paper, the authors examined data from a topside ionosphere and two magnetospheric missions (CHAMP, Cluster and Geotail) for signatures of ultra low frequency (ULF) waves during the exceptional 2003 Halloween geospace magnetic storm, when Dst reached −380 nT.
Abstract: . We examine data from a topside ionosphere and two magnetospheric missions (CHAMP, Cluster and Geotail) for signatures of ultra low frequency (ULF) waves during the exceptional 2003 Halloween geospace magnetic storm, when Dst reached ~−380 nT. We use a suite of wavelet-based algorithms, which are a subset of a tool that is being developed for the analysis of multi-instrument multi-satellite and ground-based observations to identify ULF waves and investigate their properties. Starting from the region of topside ionosphere, we first present three clear and strong signatures of Pc3 ULF wave activity (frequency 15–100 mHz) in CHAMP tracks. We then expand these three time intervals for purposes of comparison between CHAMP, Cluster and Geotail Pc3 observations but also to be able to search for Pc4–5 wave signatures (frequency 1–10 mHz) into Cluster and Geotail measurements in order to have a more complete picture of the ULF wave occurrence during the storm. Due to the fast motion through field lines in a low Earth orbit (LEO) we are able to reliably detect Pc3 (but not Pc4–5) waves from CHAMP. This is the first time, to our knowledge, that ULF wave observations from a topside ionosphere mission are compared to ULF wave observations from magnetospheric missions. Our study provides evidence for the occurrence of a number of prominent ULF wave events in the Pc3 and Pc4–5 bands during the storm and offers a platform to study the wave evolution from high altitudes to LEO. The ULF wave analysis methods presented here can be applied to observations from the upcoming Swarm multi-satellite mission of ESA, which is anticipated to enable joint studies with the Cluster mission.
TL;DR: A data assimilation tool is implemented using a 3-D radiation belt model and an ensemble Kalman filter approach and major improvements compared to pure physics based model are presented.
Abstract: . In this study we implement a data assimilation tool using a 3-D radiation belt model and an ensemble Kalman filter approach. High time and space reanalysis of the electron radiation belt fluxes is obtained over the time period 5 October to 25 October 1990 by combining sparse observations with the Salammbo 3-D model in an optimal way. The convergence of the ensemble Kalman filter is analyzed carefully. The risk of using a biased physical model is discussed and relative consequences are highlighted. Finally, a validation against CRRES data and major improvements compared to pure physics based model are presented.
TL;DR: In this paper, a review of recent theories of electrodynamics processes associated with transient luminous events (TLEs) in a strong thunderstorm system has been presented, where the authors examine the simplest versions of these theories in order to make their physics as transparent as possible.
Abstract: . Transient luminous events (TLEs) occasionally observed above a strong thunderstorm system have been the subject of a great deal of research during recent years. The main goal of this review is to introduce readers to recent theories of electrodynamics processes associated with TLEs. We examine the simplest versions of these theories in order to make their physics as transparent as possible. The study is begun with the conventional mechanism for air breakdown at stratospheric and mesospheric altitudes. An electron impact ionization and dissociative attachment to neutrals are discussed. A streamer size and mobility of electrons as a function of altitude in the atmosphere are estimated on the basis of similarity law. An alternative mechanism of air breakdown, runaway electron mechanism, is discussed. In this section we focus on a runaway breakdown field, characteristic length to increase avalanche of runaway electrons and on the role played by fast seed electrons in generation of the runaway breakdown. An effect of thunderclouds charge distribution on initiation of blue jets and gigantic jets is examined. A model in which the blue jet is treated as upward-propagating positive leader with a streamer zone/corona on the top is discussed. Sprite models based on streamer-like mechanism of air breakdown in the presence of atmospheric conductivity are reviewed. To analyze conditions for sprite generation, thunderstorm electric field arising just after positive cloud-to-ground stroke is compared with the thresholds for propagation of positively/negatively charged streamers and with runway breakdown. Our own estimate of tendril's length at the bottom of sprite is obtained to demonstrate that the runaway breakdown can trigger the streamer formation. In conclusion we discuss physical mechanisms of VLF (very low frequency) and ELF (extremely low frequency) phenomena associated with sprites.
TL;DR: In this article, the authors investigated geoeffectivity of near-Earth magnetic clouds during two periods concentrated around the last two solar minima, and found that during low and moderate solar activity, the magnetic properties of opposite polarity bipolar clouds may depend significantly on the surrounding solar wind structure.
Abstract: . In this paper, we have investigated geoeffectivity of near-Earth magnetic clouds during two periods concentrated around the last two solar minima. The studied magnetic clouds were categorised according to the behaviour of the Z-component of the interplanetary magnetic field (BZ) into bipolar (BZ changes sign) and unipolar (BZ maintains its sign) clouds. The magnetic structure of bipolar clouds followed the solar cycle rule deduced from observations over three previous solar cycles, except during the early rising phase of cycle 24 when both BZ polarities were identified almost with the same frequency. We found a clear difference in the number of unipolar clouds whose axial field points south (S-type) between our two study periods. In particular, it seems that the lack of S-type unipolar clouds contributed to relatively low geomagnetic activity in the early rising phase of cycle 24. We estimated the level of magnetospheric activity using a Dst prediction formula with the measured BZ and by reversing the sign of BZ. We found that bipolar clouds with fields rotating south-to-north (SN) and north-to-south (NS) were equally geoeffective, but their geoeffectiveness was clearly modified by the ambient solar wind structure. Geoeffectivity of NS-polarity clouds was enhanced when they were followed by a higher-speed solar wind, while the majority of geoeffective SN-polarity clouds lacked the trailing faster wind. A leading shock increased the geoeffectiveness of both NS- and SN-polarity clouds, in particular, in the case of an intense storm. We found that in 1995–1998, SN-polarity clouds were more geoeffective, while in 2006–2011 NS-polarity clouds produced more storms. A considerably larger fraction of events were trailed by a higher-speed solar wind during our latter study period, which presumably increased geoeffectivity of NS-polarity. Thus, our study demonstrates that during low and moderate solar activity, geoeffectivity of opposite polarity bipolar clouds may depend significantly on the surrounding solar wind structure. In addition, different polarities also give different temporal storm evolutions: a storm from an SN-polarity cloud is expected to occur, on average, half-a-day earlier than a storm from an NS-polarity cloud.
TL;DR: In this paper, the ionosonde NmF2 data and the JPL TEC maps were collected to investigate the equinoctial asymmetries in ionospheric electron density and its variation with solar activity.
Abstract: . The ionosonde NmF2 data (covering several solar cycles) and the JPL TEC maps (from 1998 through 2009) were collected to investigate the equinoctial asymmetries in ionospheric electron density and its variation with solar activity. With solar activity increasing, the equinoctial asymmetry of noontime NmF2 increases at middle latitudes but decreases or changes little at low latitudes, while the equinoctial asymmetry of TEC increases at all latitudes. The latitudinal feature of the equinoctial asymmetry at high solar activity is different from that at low solar activity. The increases of NmF2 and TEC with the solar proxy P = (F10.7+F10.7A)/2 also show equinoctial asymmetries that depend on latitudes. The increase rate of NmF2 with P at March equinox (ME) is higher than that at September equinox (SE) at middle latitudes, but the latter is higher than the former at the EIA crest latitudes, and the difference between them is small at the EIA trough latitudes. The phenomenon of higher increase rate at SE than at ME does not appear in TEC. The increase rate of noontime TEC with P at ME is higher than that at SE at all latitudes, and the difference between them peaks at both sides of dip equator. It is mentionable that the equinoctial asymmetries of NmF2 and TEC increase rates present some longitudinal dependence at low latitude. The influences of equinoctial differences in the thermosphere and ionospheric dynamics processes on the equinoctial asymmetry of the electron density were briefly discussed.
TL;DR: In this article, a complex water Atmospheric correction algorithm scheme (CAAS) was proposed to estimate the aerosol and sun glint contributions in the whole spectrum of ocean colour from a knowledge of the total and Rayleigh corrected radiances.
Abstract: . The current SeaDAS atmospheric correction algorithm relies on the computation of optical properties of aerosols based on radiative transfer combined with a near-infrared (NIR) correction scheme (originally with assumptions of zero water-leaving radiance for the NIR bands) and several ancillary parameters to remove atmospheric effects in remote sensing of ocean colour. The failure of this algorithm over complex waters has been reported by many recent investigations, and can be attributed to the inadequate NIR correction and constraints for deriving aerosol optical properties whose characteristics are the most difficult to evaluate because they vary rapidly with time and space. The possibility that the aerosol and sun glint contributions can be derived in the whole spectrum of ocean colour solely from a knowledge of the total and Rayleigh-corrected radiances is developed in detail within the framework of a Complex water Atmospheric correction Algorithm Scheme (CAAS) that makes no use of ancillary parameters. The performance of the CAAS algorithm is demonstrated for MODIS/Aqua imageries of optically complex waters and yields physically realistic water-leaving radiance spectra that are not possible with the SeaDAS algorithm. A preliminary comparison with in-situ data for several regional waters (moderately complex to clear waters) shows encouraging results, with absolute errors of the CAAS algorithm closer to those of the SeaDAS algorithm. The impact of the atmospheric correction was also examined on chlorophyll retrievals with a Case 2 water bio-optical algorithm, and it was found that the CAAS algorithm outperformed the SeaDAS algorithm in terms of producing accurate pigment estimates and recovering areas previously flagged out by the later algorithm. These findings suggest that the CAAS algorithm can be used for applications focussing in quantitative assessments of the biological and biogeochemical properties in complex waters, and can easily be extended to other sensors such as OCM-2, MERIS and GOCI.
TL;DR: In this paper, the authors investigate how much the modeled ring current depends on the representations of magnetic and electric fields and boundary conditions used in simulations, and they find that different combinations of the magnetic and electrical field configurations and boundary condition result in very different modeled ring currents, and therefore the physical conclusions based on simulation results can differ significantly.
Abstract: . The main point of the paper is to investigate how much the modeled ring current depends on the representations of magnetic and electric fields and boundary conditions used in simulations. Two storm events, one moderate (SymH minimum of −120 nT) on 6–7 November 1997 and one intense (SymH minimum of −230 nT) on 21–22 October 1999, are modeled. A rather simple ring current model is employed, namely, the Inner Magnetosphere Particle Transport and Acceleration model (IMPTAM), in order to make the results most evident. Four different magnetic field and two electric field representations and four boundary conditions are used. We find that different combinations of the magnetic and electric field configurations and boundary conditions result in very different modeled ring current, and, therefore, the physical conclusions based on simulation results can differ significantly. A time-dependent boundary outside of 6.6 RE gives a possibility to take into account the particles in the transition region (between dipole and stretched field lines) forming partial ring current and near-Earth tail current in that region. Calculating the model SymH* by Biot-Savart's law instead of the widely used Dessler-Parker-Sckopke (DPS) relation gives larger and more realistic values, since the currents are calculated in the regions with nondipolar magnetic field. Therefore, the boundary location and the method of SymH* calculation are of key importance for ring current data-model comparisons to be correctly interpreted.
TL;DR: In this article, a combination of ground-based Global Positioning System (GPS) total electron content (TEC), and the meridian chain of ionosondes distributed along the Latin America longitude of ~280° E was used to investigate positive ionospheric storm effects.
Abstract: . Positive ionospheric storm effects that occurred during the superstorm on 20 November 2003 are investigated using a combination of ground-based Global Positioning System (GPS) total electron content (TEC), and the meridian chain of ionosondes distributed along the Latin America longitude of ~280° E. Both the ground-based GPS TEC and ionosonde electron density profile data reveal significant enhancements at mid-low latitudes over the 280° E region during the main phase of the November 2003 superstorm. The maximum enhancement of the topside ionospheric electron content is 3.2–7.7 times of the bottomside ionosphere at the locations of the ionosondes distributed around the mid- and low latitudes. Moreover, the height of maximum electron density exceeds 400 km and increases by 100 km compared with the quiet day over the South American area from middle to low latitudes, which might have resulted from a continuous eastward penetration electric field and storm-generated equatorward winds. Our results do not support the conclusions of Yizengaw et al. (2006), who suggested that the observed positive storm over the South American sector was mainly the consequence of the changes of the bottomside ionosphere. The so-called "unusual" responses of the topside ionosphere for the November 2003 storm in Yizengaw et al. (2006) are likely associated with the erroneous usage of magnetometer and incomplete data.
TL;DR: In this paper, the authors presented the thermal forcing of the semidiurnal, terdiurnal, and 6-h components of the migrating tide induced by ozone heating in stratosphere and lower mesosphere.
Abstract: . This paper presents the thermal forcing of the semidiurnal, terdiurnal, and 6-h components of the migrating tide induced by ozone heating in stratosphere and lower mesosphere. The heating as a function of local time is determined from the global ozone observed by the Microwave Limb Sounder on the Aura satellite. The harmonic components of the heating rates of the semidiurnal, terdiurnal and the 6-h periodicities are calculated using the Strobel/Zhu parameterized model and then decomposed into Hough modes. Seasonal variations of each harmonic component and its Hough modes are presented. For all three tidal components, the majority of the annual mean O3 heating projects onto symmetric modes. The semiannual variation is a prominent signal in almost all of the symmetric Hough modes near the stratopause. The strongest annual variation takes place in the asymmetric modes. The results also show that, during the solstice season, the maximum forcing of the diurnal and terdiurnal component occurs in the summer hemisphere while the maximum forcing of the semidiurnal and 6-h components occurs in the winter hemisphere. The global mean ozone density and the tidal components of the ozone heating rate are different between December–January and June–July. The asymmetry in the heating is primarily due to the 6.6% annual variation in the solar energy input into the Earth's atmosphere due to the annual variation of the Sun-Earth distance.
TL;DR: In this article, the authors used the total electron content (TEC) data observed from global GPS network in the regions of North America, Europe, and East Asia during the magnetic storm of 7~10 November 2004.
Abstract: . Larger-scale traveling ionospheric disturbances (LSTIDs) were studied using the total electron content (TEC) data observed from global GPS network in the regions of North America, Europe, and East Asia during the magnetic storm of 7~10 November 2004. 4 LSTID events were detected in North America, 4 in Europe, and 3 in East Asia. The parameters of the 11 LSTID events, such as the propagation azimuth (the angle with respect to north, taking clockwise as positive), horizontal phase velocity and damping rate were determined. Our results showed two new propagation features of the LSTIDs. One was the latitudinal dependence of the LSTIDs' propagation azimuths. The LSTIDs tended to deflect more to west from south as they propagated to lower latitudes, which indicated that the Coriolis force was one of the main causes of the LSTIDs' southwestward deviation. The other was the different mean horizontal phase velocities of LSTIDs among different regions. The mean horizontal phase velocity of LSTIDs was 422 ± 36 m s−1 in North America, 381 ± 69 m s−1 in Europe, and 527 ± 21 m s−1 in East Asia, respectively. The results also indicated that the amplitudes of LSTIDs decreased during their propagation for every event, and the daytime damping rates were more than 1 time larger than the nighttime ones due to different ion drag between daytime and nighttime. The source regions of the LSTIDs were likely to be located between geomagnetic latitudes of 68° N and 62° N in North America, and between 65° N and 57° N in Europe, according to the variation of magnetic H component observed in these two regions.
TL;DR: In this paper, a neural network-based model for estimating the probability occurrence of geomagnetic storms following halo coronal mass ejection (CME) and related interplanetary (IP) events is presented.
Abstract: . Estimating the geoeffectiveness of solar events is of significant importance for space weather modelling and prediction. This paper describes the development of a neural network-based model for estimating the probability occurrence of geomagnetic storms following halo coronal mass ejection (CME) and related interplanetary (IP) events. This model incorporates both solar and IP variable inputs that characterize geoeffective halo CMEs. Solar inputs include numeric values of the halo CME angular width (AW), the CME speed (Vcme), and the comprehensive flare index (cfi), which represents the flaring activity associated with halo CMEs. IP parameters used as inputs are the numeric peak values of the solar wind speed (Vsw) and the southward Z-component of the interplanetary magnetic field (IMF) or Bs. IP inputs were considered within a 5-day time window after a halo CME eruption. The neural network (NN) model training and testing data sets were constructed based on 1202 halo CMEs (both full and partial halo and their properties) observed between 1997 and 2006. The performance of the developed NN model was tested using a validation data set (not part of the training data set) covering the years 2000 and 2005. Under the condition of halo CME occurrence, this model could capture 100% of the subsequent intense geomagnetic storms (Dst ≤ −100 nT). For moderate storms (−100