bluuebunny/crossref_metadata_2025 huggingface dataset:
A new three‐dimensional magnetopause model with a support vector regression machine and a large database of multiple spacecraft observations
Authors: Wang Y. , Sibeck D. G. , Merka J. , Boardsen S. A. , Karimabadi H. , Sipes T. B. , Šafránková J. , Jelínek K. , Lin R.
Published: April 2013

We present results from a new three‐dimensional empirical magnetopause model based on 15,089 magnetopause crossings from 23 spacecraft. To construct the model, we introduce a Support Vector Regression Machine (SVRM) technique with a systematic approach that balances model smoothness with fitting accuracy to produce a model that reveals the manner in which the size and shape of the magnetopause depend upon various control parameters without any assumptions concerning the analytical shape of the magnetopause. The new model fits the data used in the modeling very accurately, and can guarantee a similar accuracy when predicting unseen observations within the applicable range of control parameters. We introduce a new error analysis technique based upon the SVRM that enables us to obtain model errors appropriate to different locations and control parameters. We find significant east‐west elongations in the magnetopause shape for many combinations of control parameters. Variations in the Earth's dipole tilt can cause significant magnetopause north/south asymmetries and deviation of the magnetopause nose from the Sun‐Earth line nonlinearly by as much as 5 Re. Subsolar magnetopause erosion effect under southward IMF is seen which is strongly affected by solar wind dynamic pressure. Further, we find significant shrinking of high‐latitude magnetopause with decreased magnetopause flaring angle during northward IMF.
Analysis of experimentally validated trans‐ionospheric attenuation estimates of VLF signals
Authors: Graf K. L. , Lehtinen N. G. , Spasojevic M. , Cohen M. B. , Marshall R. A. , Inan U. S.
Published: April 2013

Accurate models of trans‐ionospheric propagation are needed to assess the role of Earth‐originating very low frequency (VLF) electromagnetic waves in radiation belt dynamics. Recent studies have called the relatively crude early trans‐ionospheric models into question, finding that they underestimate the attenuation by 20–100 dB. A full wave model that includes all of the relevant physics has recently become available and experimentally verified to within a few decibels via comparison to more extensive satellite data. Using this model, we discuss the importance of wave polarization, incidence angle, bearing, ground conductivity, horizontal distance from the source, and the ionospheric profile, all of which are demonstrated to play a significant role in the trans‐ionospheric propagation. trans‐ionospheric attenuation estimates are provided both for the case of vertical incidence of a whistler‐mode wave and for the case of magnetospheric injection from a dipolar terrestrial VLF source. These estimates agree with observation to within ±6 dB. The remaining discrepancy may be attributable to ionospheric variation and/or factors not captured by our horizontally stratified model. On the basis of the full wave treatment presented herein, we find that the earlier work showing a >20 dB overestimation by traditional models results from the unrealistic simplifying assumption that the wave is vertically incident onto the ionosphere, exacerbated by the fact that most of the satellite data used for comparison came at large horizontal distances (hundreds of kilometers) from the source.
Constructing the frequency and wave normal distribution of whistler‐mode wave power
Authors: Watt C. E. J. , Degeling A. W. , Rankin R.
Published: April 2013

We introduce a new methodology that allows the construction of wave frequency distributions due to growing incoherent whistler‐mode waves in the magnetosphere. The technique combines the equations of geometric optics (i.e., raytracing) with the equation of transfer of radiation in an anisotropic lossy medium to obtain spectral energy density as a function of frequency and wavenormal angle. We describe the method in detail and then demonstrate how it could be used in an idealized magnetosphere during quiet geomagnetic conditions. For a specific set of plasma conditions, we predict that the wave power peaks off the equator at ∼15° magnetic latitude. The new calculations predict that wave power as a function of frequency can be adequately described using a Gaussian function, but as a function of wavenormal angle, it more closely resembles a skew normal distribution. The technique described in this paper is the first known estimate of the parallel and oblique incoherent wave spectrum as a result of growing whistler‐mode waves and provides a means to incorporate self‐consistent wave‐particle interactions in a kinetic model of the magnetosphere over a large volume.
Modeling terrestrial gamma ray flashes produced by relativistic feedback discharges
Authors: Liu Ningyu , Dwyer Joseph R.
Published: April 2013

This paper reports a modeling study of terrestrial gamma ray flashes (TGFs) produced by relativistic feedback discharges. Terrestrial gamma ray flashes are intense energetic radiation originating from the Earth's atmosphere that has been observed by spacecraft. They are produced by bremsstrahlung interactions of energetic electrons, known as runaway electrons, with air atoms. An efficient physical mechanism for producing large fluxes of the runaway electrons to make the TGFs is the relativistic feedback discharge, where seed runaway electrons are generated by positrons and X‐rays, products of the discharge itself. Once the relativistic feedback discharge becomes self‐sustaining, an exponentially increasing number of relativistic electron avalanches propagate through the same high‐field region inside the thundercloud until the electric field is partially discharged by the ionization created by the discharge. The modeling results indicate that the durations of the TGF pulses produced by the relativistic feedback discharge vary from tens of microseconds to several milliseconds, encompassing all durations of the TGFs observed so far. In addition, when a sufficiently large potential difference is available in thunderclouds, a self‐propagating discharge known as the relativistic feedback streamer can be formed, which propagates like a conventional positive streamer. For the relativistic feedback streamer, the positive feedback mechanism of runaway electron production by the positrons and X‐rays plays a similar role as the photoionization for the conventional positive streamer. The simulation results of the relativistic feedback streamer show that a sequence of TGF pulses with varying durations can be produced by the streamer. The relativistic streamer may initially propagate with a pulsed manner and turn into a continuous propagation mode at a later stage. Milliseconds long TGF pulses can be produced by the feedback streamer during its continuous propagation. However, the continuous propagation of the streamer tends to be unstable, because it does not expand like the conventional positive streamer. Its head electric field continues to increase and can reach half of the conventional breakdown threshold field, which results in an ion density of 3– 10×1014m−3 in the channel immediately following the head. The spatial width of the high field region in the streamer head is about 100 m and the streamer speed is about 5×105 m/s. As a result, conventional positive streamers can be initiated from thundercloud hydrometeors or inhomogeneities of enhanced conductivities of millimeter sizes in the relativistic feedback streamer head and the positive streamers may further result in the formation of leaders. In addition, a relativistic feedback streamer can result in a charge moment change of several tens of coulomb‐kilometers in a few tens of milliseconds, indicating that the relativistic feedback discharge process could be an important component for thundercloud charge dynamics.
Spatial variation of energy conversion at the Earth's magnetopause: Statistics from Cluster observations
Authors: Anekallu Chandrasekhar R. , Palmroth M. , J. Koskinen Hannu E. , Lucek E. , Dandouras I.
Published: April 2013

We investigate magnetopause energy conversion in a large statistical data set utilizing Cluster spacecraft observations. We have compiled a database of about 4000 magnetopause crossings from Cluster spacecraft 1 measurements during years 2001–2008. We have estimated the local energy conversion across the magnetopause for these crossings using Generic Residue Analysis and analyzed the spatial distribution of load and generator regions during dayside and lobe reconnection as a function of the interplanetary magnetic field (IMF) magnitude and solar wind dynamic pressure. We found scatter in the load and the generator regions on the magnetopause surface. Categorizing the crossings into equatorward or tailward of the cusp improves the organization of the load and generator regions on the surface. During dayside reconnection, equatorward (tailward) of the cusp indicates more load (generator) than generator (load) and is in agreement with theory. During lobe reconnection, we find that a faint load region dominates both equatorward and tailward of the cusp. We compare these statistics with Grand Unified Magnetosphere Ionosphere Coupling Simulation version 4 (GUMICS 4) global magnetohydrodynamic simulation results and find that there is a reasonable agreement, although disagreements are also found especially during lobe reconnection. We also investigate the influence of IMF magnitude on the load and generator locations and suggest that the spatial mixing of load and generators is due to rapid movement of the magnetopause surface which in turn moves the locations where load and generator processes appear. The solar wind dynamic pressure controls the magnitude of energy conversion across the magnetopause such that higher dynamic pressures lead to more energy conversion. A similar dependence is observed for IMF magnitude as well.
Current structure and nonideal behavior at magnetic null points in the turbulent magnetosheath
Authors: Wendel D. E. , Adrian M. L.
Published: April 2013

The Poincaré index indicates that the Cluster spacecraft tetrahedron entraps a number of 3‐D magnetic nulls during an encounter with the turbulent magnetosheath. Previous researchers have found evidence for reconnection at one of the many filamentary current layers observed by Cluster in this region. We find that many of the entrained nulls are also associated with strong currents. We dissect the current structure of a pair of spiral nulls that may be topologically connected. At both nulls, we find a strong current along the spine, accompanied by a somewhat more modest current perpendicular to the spine that tilts the fan toward the axis of the spine. The current along the fan is comparable to the that along the spine. At least one of the nulls manifests a rotational flow pattern in the fan plane that is consistent with torsional spine reconnection as predicted by theory. These results emphasize the importance of examining the magnetic topology in interpreting the nature of currents and reconnection in 3‐D turbulence.
An observation‐based GCR model of heavy nuclei: Measurements from CRIS onboard ACE spacecraft
Authors: Zhao L.‐L. , Qin G.
Published: April 2013

We present an observation‐based elemental Galactic Cosmic Ray (GCR) heavy nuclei spectra model, based on ACE/CRIS measurements. Then we extrapolate the spectra model to the lower energy range of ACE/SIS instrument. In addition, we compare the modeling results with both the CRIS and SIS measurements. The flux of lower energetic particles measured by SIS are despiked since there are Solar Energetic Particle (SEP) events. The good agreement between the modeling and the observation results, especially for the solar minimum, indicates the validity of our model in the energy range 30–500 MeV/nuc. Compared with two GCR radiation environment models, CRÈME96/Nymmik model and Badhwar & O'Neill model, our model can provide an improved fit to the GCRs spectra measured by ACE. Furthermore, our model is a phenomenological one, without consideration of the physical process during GCRs' propagating through the heliosphere. Therefore, it is more straightforward and applicable in practice. Finally, the model can reproduce and predict the GCR spectra in the past and future, respectively, based on the strong correlation between the model parameters and solar activity indicated by sunspot number.
Analysis of the experiment on registration of X‐rays from the stepped leader of a cloud‐to‐ground lightning discharge
Authors: Babich Leonid P. , Kutsyk Igor M. , Donsko Evgeni N. , Dwyer Joseph R.
Published: April 2013

Using a Monte Carlo technique to simulate the transport of runaway electrons (REs) and X‐rays in the atmosphere and through attenuators covering detectors, we have modeled the results of experiments to detect X‐rays from triggered lightning and stepped leaders of a natural cloud‐to‐ground lightning. In the model, bremsstrahlung of high‐energy runaway electrons (REs) generated at the leader front is assumed to be the origin of X‐rays. Specific fluxes (per one RE) of photons and bremsstrahlung energy at the detectors were calculated. The analysis was executed with monoenergetic and exponential initial energy distributions of REs with different angular distributions. To reproduce the detected radiation energy of ~1–2 MeV, a generation of ~1010–1011 REs per flash is required in the case with the beam angular distribution of monoenergetic REs with the energy in the range 1–10 MeV. The same result was obtained with the exponential energy distribution of REs with the average energy 7 MeV, i.e., with the average energy in the RE avalanche. The electric field amplifies the flux of the radiation energy, and the amplification becoming stronger as the RE source approaches the ground. In the case with an isotropic angular distribution of REs in the bottom hemisphere, with no electric field, (4–5) × 109 REs are required for reproducing ~1–2 MeV of detected X‐ray energy. In addition, fluxes of photons and fluxes of their energy at the detectors, energy distributions of photons and their average energy were calculated.
Comparison of zonal neutral winds with equatorial plasma bubble and plasma drift velocities
Authors: Chapagain Narayan P. , Fisher Daniel J. , Meriwether John W. , Chau Jorge L. , Makela Jonathan J.
Published: April 2013

A one‐year dataset spanning March 2011 to March 2012 of coincident observations of nighttime thermospheric zonal neutral winds, equatorial plasma bubble (EPB) velocities, and zonal plasma drifts is used to examine the relationship between the thermosphere and the ionosphere near the geomagnetic equator over Peru. Thermospheric neutral winds are determined by using a bistatic Fabry‐Perot interferometer (FPI) experiment located at Merihill and Nazca in Peru. The ambient plasma drift velocities were obtained using the incoherent scatter radar at the Jicamarca Radio Observatory in Peru. The EPB zonal velocities were estimated utilizing images of the OI 630.0 nm emission recorded by a narrow‐field optical imaging system at the Cerro Tololo Inter‐American Observatory in Chile. The joint analysis of these datasets illustrates that the nighttime and night‐to‐night variations in the zonal neutral winds, EPB velocities, and plasma drifts are well correlated. This consistent result of the local time variations of the neutral winds with that of EPB and plasma drifts illustrates that the F‐region dynamo is, in general, fully activated. However, at times, the magnitude of the EPB velocities and the plasma drifts are different from the neutral winds. It is plausible that such a difference is due either to the effect of polarization electric fields developed inside the EPB or due to the latitudinal gradient of the neutral winds and EPB velocity measurements since the EPB velocities are estimated at a higher latitude, corresponding to an apex altitude of ~400 km, than the wind estimates, which derive from an apex altitude of ~250 km.
Links between the plasmapause and the radiation belt boundaries as observed by the instruments CIS, RAPID, and WHISPER onboard Cluster
Authors: Darrouzet F. , Pierrard V. , Benck S. , Lointier G. , Cabrera J. , Borremans K. , Ganushkina N. Yu , Keyser J. De
Published: April 2013

In the present work, we study the relations between the position of the plasmapause and the position of the radiation belt boundaries. The Cluster mission offers the exceptional opportunity to analyze those different regions of the inner magnetosphere with identical sensors on multiple spacecraft. We compare the positions of the radiation belt edges deduced from CIS (Cluster Ion Spectrometry) observations (electrons with energy >2 MeV) with the positions of the plasmapause derived from WHISPER (Waves of HIgh frequency and Sounder for Probing of the Electron density by Relaxation) data (electron plasma frequency). In addition, we compare those results with the edges positions determined from RAPID (Research with Adaptive Particle Imaging Detectors) observations (electrons with energy between 244.1 and 406.5keV). The period of 1 April 2007 to 31 March 2009 has been chosen for the analysis because at that time Cluster's perigee was located at lower radial distances than during the earlier part of the mission. The perigee was then as close as 2 RE, deep inside the plasmasphere and the radiation belts. This time period corresponds to a long solar activity minimum. Differences are observed between the radiation belt boundary positions obtained from the two different instruments: The radiation belt positions are related to the energy bands. The results show that the plasmapause position is more variable than the radiation belt boundary positions, especially during small geomagnetic activity enhancements. A correspondence is observed between the plasmapause position determined by WHISPER and the outer edge of the outer radiation belt of energetic electrons (>2 MeV) observed by CIS. This result is unexpected since previous studies based on other spacecraft observations indicated a correlation between the inner edge of the outer belt and the plasmapause. However, during higher geomagnetic activity time periods, the plasmapause is located closer to the inner boundary of the outer radiation belt. Also, the thickness of the slot region is found to follow the global evolution of the geomagnetic activity.
The day‐to‐day longitudinal variability of the global ionospheric density distribution at low latitudes during low solar activity
Authors: Pacheco E. E. , Yizengaw E.
Published: April 2013

One important characteristic of longitudinal variability of the ionosphere is the global wavenumber‐4 signature. Recent investigations have focused mainly on the climatological pattern during daytime and evening sectors. We investigate the day‐to‐day variability of the wavenumber‐4 structure of the longitudinal ionospheric density distribution using the global total electron content (TEC) measurements from Global Positioning Systems receivers on the ground. The quiet time (Kp ≤ 3) day‐to‐day occurrence of the wavenumber‐4 is obtained during periods of low solar flux conditions for the years 2008 and 2009. We find that the wavenumber‐4 structure occurs at all local time sectors; however, the daytime TEC wavenumber‐4 structures are clearer and can persist until the midnight hours. The most significant occurrence is observed during the 1000–2400 LT sector while the minimum number of wavenumber‐4 structure is observed between the 0400 and 0600 LT sector. Around the nighttime sector, more wavenumber‐4 occurrence is observed during the premidnight sector than the postmidnight hours. The seasonal occurrence probability of the wavenumber‐4 pattern is at a maximum during the March–April equinox and June–July solstice. December–January is the period when the wavenumber‐4 occurrence is less dominant than the rest of the year.
The hemispheric conjugate observation of postnoon “bright spots”/auroral spirals
Authors: Hu Ze‐Jun , Yang Hui‐Gen , Hu Hong‐Qiao , Zhang Bei‐Chen , Huang De‐Hong , Chen Zhuo‐Tian , Wang Q.
Published: April 2013

Using the Polar ultraviolet imager (UVI) above the Arctic and the all‐sky camera (ASC) at Chinese Zhongshan Station (ZHS) in Antarctica, an outstanding hemispheric conjugate observation of postnoon “bright spots” in the Arctic and auroral spirals in Antarctica is presented. Multiple bright auroral spirals, which are counterclockwise rotation viewed along the direction of the magnetic field, in postnoon auroral arcs are seen on the field of view (FOV) of ASC at ZHS in the Southern Hemisphere, while multiple bright spots are seen at the conjugate FOV of ASC on postnoon auroral oval in the Northern Hemisphere by Polar UVI. We consider that the auroral spirals in postnoon arcs are the visible characteristic of postnoon UV bright spots on ground‐based observation and suggest that the current sheet instability above the parallel electric field region, which could produce the arcs, is the origin for bright spots occurring at the ionosphere, and the hemispheric symmetry/asymmetry of postnoon upward field‐aligned currents, which is affected by the IMF BY and season, can control the conjugacy/nonconjugacy of postnoon bright spots/auroral spirals between the two hemispheres.
Evidence from radial velocity measurements of a global electric field in Saturn's inner magnetosphere
Authors: Wilson R. J. , Bagenal F. , Delamere P. A. , Desroche M. , Fleshman B. L. , Dols V.
Published: April 2013

The presence of a convection pattern in the Saturnian inner magnetosphere was first observed in moon signatures that were displaced from their expected locations. A noon to midnight directed electric field can account for these positional offsets, and also can explain asymmetries found in plasma temperatures between the day and night sides. By assuming magnetic field models and drift times, estimates of the electric field strength have been made. However the origin of the electric field remains a mystery. This paper, which presents new evidence for the convection pattern, uses measured in situ velocities with measured magnetic field values, at a range of radial distances without the restriction of requiring the presence of a moon. Advances in extracting plasma parameters with reduced uncertainties clearly show the convection pattern that was previously hidden when uncertainties were large. We confirm the earlier findings that there is a distinct drift velocity superposed on the corotating plasma that pushes the plasma toward ∼dawn that dominates over the general plasma outflow. We find that an electric field directed toward 01–02 h Saturnian local time best describes this drift, and that the value of the electric field strength is not constant with radial distance.
Annual asymmetry in thermospheric density: Observations and simulations
Authors: Lei Jiuhou , Dou Xiankang , Burns Alan , Wang Wenbin , Luan Xiaoli , Zeng Zhen , Xu Jiyao
Published: April 2013

In this paper, the Challenging Minisatellite Payload (CHAMP) and Gravity Recovery and Climate Experiment (GRACE) observations during 2002–2010 are utilized to study the variation of the annual asymmetry in thermospheric density at 400 km under low solar activity condition (F10.7 = 80) based on the method of empirical orthogonal functions (EOFs). The derived asymmetry index (AI) in thermospheric density from the EOF analysis shows a strong latitudinal variation at night but varies a little with latitudes in daytime. Moreover, it exhibits a terdiurnal tidal signature at low to middle latitudes. The global mean value of the AI is 0.191, indicating that a 47% difference in thermosphere between the December and June solstices in the global average. In addition, the NCAR Thermosphere‐Ionosphere Electrodynamics Global Circulation Model (TIEGCM) is used to explore the possible mechanisms responsible for the observed annual asymmetry in thermospheric density. It is found that the standard simulations give a lower AI and also a weaker day‐to‐night difference. The simulated AI shows a semidiurnal pattern in the equatorial and low‐latitude regions in contrast with the terdiurnal tide signature seen in the observed AI. The daily mean AI obtained from the simulation is 0.125, corresponding to a 29% December‐to‐June difference in thermospheric density at 400 km. Further sensitivity simulations demonstrated that the effect of the varying Sun‐Earth distance between the December and June solstices is the main process responsible for the annual asymmetry in thermospheric density, while the magnetic field configuration and tides from the lower atmosphere contribute to the temporal and spatial variations of the AI. Specifically, the simulations show that the Sun‐Earth distance effect explains 93% of the difference in thermospheric density between December and June, which is mainly associated with the corresponding changes in neutral temperature. However, our calculation from the density observations reveals that the varying Sun‐Earth distance effect only accounts for ~67% of the December‐to‐June difference in thermosphere density, indicating that the TIEGCM might significantly underestimate the forcing originating from the lower atmosphere.
General solution for calculating polarization electric fields in the auroral ionosphere and application examples
Authors: Amm O. , Fujii R. , Vanhamäki H. , Yoshikawa A. , Ieda A.
Published: April 2013

We devise an approach to calculate the polarization electric field in the ionosphere, when the ionospheric conductances, the primary (modeled) or the total (measured) electric field, and the Cowling efficiency are given. In contrast to previous studies, our approach is a general solution which is not limited to specific geometrical setups, and all parameters may have any kind of spatial dependence. The solution technique is based on spherical elementary current (vector) systems (SECS). This way, we avoid the need to specify explicit boundary conditions for the searched polarization electric field of its potential which would be required if the problem was solved in a differential equation approach. Instead, we solve an algebraic matrix equation, and the implicit boundary condition that the divergence of the polarization electric field vanishes outside our analysis area is sufficient. In order to illustrate our theory, we then apply it to two simple models of auroral electrodynamic situations, the first being a mesoscale strong conductance enhancement in the early morning sector within a relatively weak southward primary electric field, and a morning sector auroral arc with only a weak conductance enhancement, but a large southward primary electric field at the poleward flank of the arc. While the significance of the polarization electric field for maximum Cowling efficiency is large for the first case, it is rather minor for the second one. Both models show that the polarization electric field effect may not only change the magnitude of the current systems but also their overall geometry. Furthermore, the polarization electric field may extend into regions where the primary electric field is small, thus even dominating the total electric field in these regions. For the first model case, the total Joule heating integrated over the analysis area decreases by a factor of about 4 for maximum Cowling efficiency as compared to the case of vanishing Cowling efficiency. Furthermore, for this case the resulting total electric field structurally shows a strong resemblance to that frequently observed during auroral omega band events.
Estimating the open magnetic flux from the interplanetary and ionospheric conditions
Authors: Wang C. , Xia Z. Y. , Peng Z. , Lu Q. M.
Published: April 2013

The open magnetic flux (FPC) is a key parameter to study magnetospheric dynamical process, which is closely related to magnetic reconnections in the dayside magnetopause and magnetotail. Using global MHD simulations, we find that the open magnetic flux FPC can be estimated through a combined parameter f by FPC=0.89f/(f+0.20)+0.52, where the parameter
is a function of the solar wind velocity (vSW), the solar wind number density (nSW), the southern interplanetary magnetic field (IMF) strength (BS), and the ionospheric Pederson conductance (ΣP). The comparison with the limited observational FPCdata available in the literature shows its promise in estimating the open magnetic flux from the interplanetary and ionospheric conditions. The open magnetic flux (FPC) may be served as a key space weather forecast element in the future.
Solar origin of heliospheric magnetic field inversions: Evidence for coronal loop opening within pseudostreamers
Authors: Owens M. J. , Crooker N. U. , Lockwood M.
Published: April 2013

The orientation of the heliospheric magnetic field (HMF) in near‐Earth space is generally a good indicator of the polarity of HMF foot points at the photosphere. There are times, however, when the HMF folds back on itself (is inverted), as indicated by suprathermal electrons locally moving sunward, even though they must ultimately be carrying the heat flux away from the Sun. Analysis of the near‐Earth solar wind during the period 1998–2011 reveals that inverted HMF is present approximately 5.5% of the time and is generally associated with slow, dense solar wind and relatively weak HMF intensity. Inverted HMF is mapped to the coronal source surface, where a new method is used to estimate coronal structure from the potential‐field source‐surface model. We find a strong association with bipolar streamers containing the heliospheric current sheet, as expected, but also with unipolar or pseudostreamers, which contain no current sheet. Because large‐scale inverted HMF is a widely accepted signature of interchange reconnection at the Sun, this finding provides strong evidence for models of the slow solar wind which involve coronal loop opening by reconnection within pseudostreamer belts as well as the bipolar streamer belt. Occurrence rates of bipolar‐ and pseudostreamers suggest that they are equally likely to result in inverted HMF and, therefore, presumably undergo interchange reconnection at approximately the same rate. Given the different magnetic topologies involved, this suggests the rate of reconnection is set externally, possibly by the differential rotation rate which governs the circulation of open solar flux.
Are trends in total electron content (TEC) really positive?
Authors: Laštovička Jan
Published: April 2013

The total electron content (TEC) is a primary ionospheric parameter as it can be studied globally using signals of GNSS satellites. Lean et al. (2011, JGR) in the first study of long‐term trends in TEC found positive trends for the period 1995–2010 based on TEC derived from global GPS observations. However, the positive trends seem to be inconsistent with both observed and modeled trends in foF2. Here we try to shed light on this puzzle using historical Faraday rotation TEC measurements and foF2 data over the period 1995–2010. We use data from European area where Lean et al. (2011) report remarkably stronger than average positive trend of TEC. These data suggest no trend of TEC in this region in the past but also almost no trend of foF2 over 1995–2010 compared with a negative trend in solar cycles 21 and 22. This means that the trends for 1995–2010 are more positive than trends for the period 1976–1996 and that the trends for 1995–2010 are not representative for the previous period. The magnitude of trends for 1995–2010 is to some extent uncertain due to the uncertain level of the solar EUV flux in the deep solar minimum in 2008 and 2009.
Preliminary investigation of real‐time mapping of foF2 in northern China based on oblique ionosonde data
Authors: Zhou Chen , Wang Ruopeng , Lou Wenyu , Liu Jing , Ni Binbin , Deng Zhongxin , Zhao Zhengyu
Published: April 2013

A real‐time mapping model of foF2 in northern China was established using neural networks (NNs). To avoid the local minimum problem associated with traditional NNs, a newly improved genetic algorithm‐based NN (GA‐NN) was developed using the input parameters of solar activities, geomagnetic activities, neutral winds, seasonal information, and geographical coordinates. The foF2 data were extracted by inversing the oblique ionograms obtained from the oblique ionosondes of the China Ground‐based Seismo‐ionospheric Monitoring Network every 30 min for the period from August 2009 to December 2011. The data associated with five transmitter stations (Beijing, Changchun, Qingdao, Xinxiang, and Suzhou) and one receiver station in Binzhou were considered the input parameters for the real‐time foF2 mapping model, and the data from the Dalian and Jinyang transmitter stations were used to verify the results. The Jining transmitter station data were used to test the capability of the model. The root‐mean‐square error and percent deviation were calculated to estimate the performance of the model. The correlation coefficient was used to evaluate the correlation of observed and predicted values. In addition, observations of foF2 from the vertical ionosondes at Beijing, Changchun, Qingdao, and Suzhou stations are compared with the model prediction of foF2. The results indicate that the developed real‐time foF2 mapping model based upon genetic algorithm‐based NN is very promising for ionospheric studies.
Seasonal and local time variation of ionospheric migrating tides in 2007–2011 FORMOSAT‐3/COSMIC and TIE‐GCM total electron content
Authors: Chang Loren C. , Lin Chien‐Hung , Liu Jann‐Yenq , Balan Nanan , Yue Jia , Lin Jia‐Ting
Published: April 2013

This study examines the seasonal and interannual variation of the major migrating tidal components in midlatitude to low‐latitude total electron content (TEC) observations from the FORMOSAT‐3/COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate) satellite constellation from 2007 to 2011. Although the absolute amplitudes of the TEC zonal mean and migrating tidal components show a strong positive relation to the increasing and decreasing phases of the solar cycle, the relative tidal amplitudes following normalization by maximum background values show a more varied response to solar activity levels. Features of ionospheric local time variation produced by individual migrating tidal components are consistent from year to year, with DW1 forming the equatorial daytime peak in TEC, SW2 corresponding to the generation of the equatorial ionization anomaly (EIA) crests, and TW3 contributing to the TEC trough between the EIA crests. Numerical experiments using Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (TIE‐GCM) are also performed to determine the sensitivity of the ionospheric migrating tides to upward propagating migrating tidal components from the neutral mesosphere and lower thermosphere (MLT). Zonal mean TECs decrease when MLT tidal forcing is applied and are particularly sensitive to the MLT DW1. Most of the ionospheric SW2 response is attributable to MLT SW2 forcing, enhancing the EIA crests by amplifying the equatorial fountain. TW3 in the model is generated through both in situ photoionization and nonlinear interaction between DW1 and SW2.
Simulated ring current response during periods of dawn‐dusk oriented interplanetary magnetic field (By)
Authors: Mitchell Elizabeth J. , Fok Mei‐Ching H. , Lopez Ramon E. , Lyon John G.
Published: April 2013

Ring current formation is mainly attributed to enhanced global magnetospheric convection and particle injection. One of the indicators of enhanced global magnetospheric convection is the transpolar potential. The transpolar potential has been shown to respond to dawn‐dusk oriented interplanetary magnetic field (IMF), enhancing as the IMF magnitude grows. This suggests that the ring current should respond to dawn‐dusk oriented IMF. This work examines the ring current response during periods of dawn‐dusk oriented IMF using the Lyon‐Fedder‐Mobarry and Comprehensive Ring Current Model simulations. Exploring three hypotheses, this work shows through simulation results that the ring current does respond during periods of dawn‐dusk oriented IMF, that the inner magnetospheric response is different for periods of dawn‐dusk oriented IMF than for periods of southward IMF, and that these differences are attributed to both lower magnetospheric convection on closed field lines and the location of the reconnection region on the nightside. Specifically, the simulation results show that as the magnitude of the dawn‐dusk oriented IMF increases, producing a corresponding increase in the transpolar potential, the ring current response increases. The response is always much less than a comparable southward IMF would produce. This lower response is due to both magnetospheric convection on closed field lines, which builds the ring current but at a slower rate, and flank reconnection, which allows energy to flow through the inner magnetosphere without building the ring current plasma population (as a conduit).
WINDII observations and TIME‐GCM simulations of O(1S) polar spirals during geomagnetic disturbances
Authors: Shepherd Gordon G. , Roble Raymond G. , Cho Young‐Min
Published: April 2013

The daytime O(1S) emission at 557.7 nm observed at 250 km with the Wind Imaging Interferometer (WINDII) is dominantly excited by photoelectron impact on atomic oxygen, so the volume emission rate is normally a measure of the atomic oxygen concentration. Daily polar maps of the 250 km volume emission rate during geomagnetic disturbances display polar spirals extending out of the auroral region and down to the equator. Since the local time is fixed for a given latitude for a single day, the spiral maps form a spatio‐temporal pattern in which the longitudinal variations cannot be distinguished from those in universal time. Simulations for 2 January 1993 implemented with the National Center for Atmospheric Research Thermosphere‐Ionosphere‐Mesosphere‐Electrodynamics General Circulation Model (TIME‐GCM) using the equivalent satellite perspective show a remarkably similar spiral pattern. Hourly universal time simulations reveal a rapid equatorward expansion of the spirals during this modest geomagnetic event of Kp = 4.7, which are mirrored in the meridional and vertical winds. Simulations of the electron density show that the emission within the spirals is caused by the recombination of O2+ ions with electrons, and not the atomic oxygen enhancement itself. All of this strongly suggests that the spirals are in fact large‐scale Traveling Ionospheric Disturbances (TIDs), and comparisons of the WINDII data made with TID observations reported in the literature, including those made on the same day, support this conclusion. The TIME‐GCM simulations suggest that a component of the spirals originates in the lower atmosphere and appears at thermospheric heights.
Sounding of the plasmasphere by Mid‐continent MAgnetoseismic Chain (McMAC) magnetometers
Authors: Chi P. J. , Engebretson M. J. , Moldwin M. B. , Russell C. T. , Mann I. R. , Hairston M. R. , Reno M. , Goldstein J. , Winkler L. I. , Cruz‐Abeyro J. L. , Lee D.‐H. , Yumoto K. , Dalrymple R. , Chen B. , Gibson J. P.
Published: April 2013

We present a statistical analysis on the plasmaspheric mass density derived from the field line resonance (FLR) observations by the Mid‐continent MAgnetoseismic Chain (McMAC). McMAC consists of nine stations in the United States and Mexico along the 330° magnetic longitude, spanning L‐values between 1.5 and 3.4. Using the gradient method and an automated procedure for FLR detection, we studied a full year of McMAC observations between July 2006 and June 2007. We find that the rate of FLR detection can reach as high as 56% around local noon at L = 2.7, and the detection rates at higher and lower L‐values decline due to the occasional presence of the plasmapause and weaker FLR signals, respectively. At L‐values between 1.8 and 3.1, the inferred equatorial plasma mass density follows the L‐dependence of L−4. By comparing the mass density with the electron density, we found that the ion mass gradually decreased from 1.7 amu at L = 1.8 to 1 amu at L = 3.1. The plasma mass density exhibits an annual variation that maximizes in January, and at L = 2.4 the ratio between January and July densities is 1.6. Our observations also show a local time dependence of plasmaspheric mass density that stays steady in the morning and rises postnoon, a phenomenon that may be attributed to the equatorial ionization anomaly as a part of the plasma neutral coupling at low latitude.
On the numerical simulation of particle dynamics in the radiation belt: 2. Procedure based on the diagonalization of the diffusion tensor
Authors: Camporeale E. , Delzanno G. L. , Zaharia S. , Koller J.
Published: April 2013

In this paper, we conclude the survey and comparison of different numerical methods used to solve the diffusion equation for particle dynamics in the Earth's radiation belt, initiated in Camporeale et al. (2013). Here we focus on the diagonalization procedure introduced by Albert and Young (2005) that, by performing a change of coordinates, solves the diffusion equation in a space where the mixed diffusion terms are null. We describe the diagonalization procedure and its numerical implementation, which is not as straightforward as the implementation of a traditional solver in a rectangular domain. We compare the computing times with and without the diagonalization procedure, and we conclude that this procedure is generally not advantageous from the point of view of computational efficiency.
Substorm onset and expansion phase intensification precursors seen in polar cap patches and arcs
Authors: Nishimura Y. , Lyons L. R. , Shiokawa K. , Angelopoulos V. , Donovan E. F. , Mende S. B.
Published: April 2013

Localized, transient polar cap flows impinging on the poleward boundary of the nightside auroral oval have been suggested to precede poleward boundary intensifications (PBIs), which are often followed by auroral streamers, some of which propagate equatorward and appear to lead to substorm auroral onset. While echo coverage often limits radar flow measurements, imaging of polar cap patches and arcs has the potential to monitor localized polar cap flows by tracing emission structures, previously associated with enhanced flows, over long distances. We use such imaging to examine if polar cap patches and arcs moving over large distances can be seen as possible precursors to presubstorm‐onset PBIs. We find evidence that such features, which are longitudinally narrow, propagate from the dayside polar region toward the nightside polar cap boundary prior to pre‐onset PBIs. This sequence indicates that transient activity in the dayside polar region may initiate polar cap patches and arcs that propagate across the polar cap and are followed by pre‐onset PBIs. Furthermore, we find evidence that expansion‐phase active aurora expanded poleward soon after additional polar cap patches and arcs reached close to the nightside polar cap boundary. The expansion phase auroral activity significantly weakened when polar cap patches/arcs disappeared. Our findings suggest that plasma transport originating from the dayside and reaching the nightside open‐closed boundary may trigger in plasma sheet flow bursts and play a crucial role in pre‐ and post‐onset auroral activity. Polar cap imaging offers the possibility for monitoring such localized, transient plasma transport over large distances.
Topside electron density profiles observed at low latitudes by COSMIC and compared with in situ ion densities measured by C/NOFS
Authors: Lai Pei‐Chen , Burke William J. , Gentile L. C.
Published: April 2013

Electron density height profiles (EDPs) play critical roles in accurately tracing the propagation of radio frequency signals through the topside ionosphere. The Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) satellites were designed to provide this information during GPS radio occultation events. However, due to the reliance of COSMIC data reduction on inverse Abel transformations, questions persist about the accuracy of its EDP products. The Communication/Navigation Outage Forecasting System (C/NOFS) satellite in a low‐inclination orbit offers new opportunities for empirically testing the accuracy of topside EDPs from COSMIC over the altitude range 400 to 800 km. We report here on the results of a feasibility study covering the interval 24–25 October 2011 in which a large magnetic storm occurred. We identified universal times when C/NOFS passed within 6° × 60° rectangles centered at the latitudes and longitudes of the tangent line for COSMIC EDPs. We also compared COSMIC EDPs with those predicted by two widely used models. Our analysis shows that during a very large fraction of the available events, ion densities measured by C/NOFS were in closer agreement with COSMIC EDPs than were model predictions at the same altitudes. We also use the COSMIC and C/NOFS data sets to assess stormtime effects on the low‐latitude ionosphere. During the main phase of the storm both local densities and vertical total electron content rose on the dayside, but decreased on the nightside. During recovery both quantities quickly reverted toward their quiet‐time levels.
Inner magnetospheric heavy ion composition during high‐speed stream‐driven storms
Authors: Forster D. R. , Denton M. H. , Grande M. , Perry C. H.
Published: April 2013

Ion composition data, taken by the Combined Release and Radiation Effects Satellite Magnetospheric Ion Composition Spectrometer instrument, are investigated across eight high‐speed solar wind‐stream‐driven storms (HSSs) during 1991. The HSSs are identified using solar wind data from OMNI alongside geomagnetic indices, and the behavior of ions in the energy range 31.2–426.0 keV is investigated. A case study of the single HSS event that occurred on 30 July 1991 is performed, and superposed epoch analyses of five events are conducted. The data show evidence of a local minimum (dropout) in the flux and partial number density of ionic species H+, He+, He++, and O+ close to the onset of magnetospheric convection. The flux and number density rapidly fall and then recover over a period of hours. The initial rapid recovery in number density is observed to consist primarily of lower‐energy ions. As the number density reaches its maximum, the ions show evidence of energization. Heavy ion‐to‐proton ratios are observed to decrease substantially during these HSS events.
Evaluation of 1‐D and 3‐D seismic models of the Pacific lower mantle with S, SKS, and SKKS traveltimes and amplitudes
Authors: Thorne Michael S. , Zhang Yang , Ritsema Jeroen
Published: April 2013

In this study, we analyzed the seismic phases S, SKS, and SKKS from 31 deep‐focus earthquakes in the Tonga‐Fiji region recorded in North America between epicentral distances of 85° and 120°. The differential traveltimes and amplitude ratios for these phases reveal clear epicentral distance trends not predicted by standard one‐dimensional (1‐D) reference Earth models. The increase of the S/SKS amplitude ratio up to a factor of 10 is accompanied by an increase of the S‐SKS differential traveltime of up to 10 s. SKKS‐SKS differential traveltimes of 2–3 s and SKKS/SKS amplitude ratios of a factor of 2–4 across the epicentral range have maxima near 107°. We examined these observations using full (1‐D and 3‐D) waveforms for three 1‐D seismic velocity profiles for the central Pacific region and for the tomographic model S40RTS including modifications: different regularization parameters, great‐circle path azimuthal variation, strength of S wave velocity perturbations, S wave velocity gradients in the lower mantle, and ultra–low velocity zones. To explain these data, we constructed a hybrid model that combines both features of S40RTS and short‐wavelength features from the 1‐D profiles. The large‐scale seismic structure is represented by S40RTS. Embedded within S40RTS are a 20 km thick ultra–low velocity zone at the core‐mantle boundary near the source side and a 200 km thick negative velocity gradient zone near the receiver side of the paths. Our analysis demonstrates that the S wave velocity structure of the Pacific large low shear‐velocity province cannot be interpreted solely by global tomographic or regional modeling approaches in exclusion of each other.