bluuebunny/crossref_metadata_2025_split huggingface dataset:
Comparative analysis of low‐altitude ENA emissions in two substorms
Authors: Buzulukova N. , Fok M.‐C. , Roelof E. , Redfern J. , Goldstein J. , Valek P. , McComas D. 
Published: April 2013

AbstractWe report on the dynamics of low‐altitude energetic neutral atom (ENA) emissions during two substorms that occurred during the main phases of two storms: (1) a CIR‐driven storm on 11 October 2008 and (2) a coronal mass ejection (CME)‐driven storm on 5 April 2010. For both of these storms, we have complementary spacecraft and ground‐based observations. The dual‐spacecraft Two Wide‐angle Imaging Neutral‐atom Spectrometers (TWINS) mission obtained ENA images containing low‐altitude emissions (LAEs). Substorm dynamics is inferred from THEMIS all‐sky imagers. TWINS‐observed LAEs are compared with trapped/loss cone proton fluxes from the low‐altitude NOAA/MetOp spacecraft constellation. We find that the timing and intensity profiles of LAEs are different for the two selected events. For the 11 October 2008 event, the LAEs rise during substorm recovery phase and storm main phase. On 5 April 2010, the LAEs tend to peak near substorm onset. We argue that the different LAE behavior results from different pitch‐angle distributions (PADs) of the ion source population. Ion PADs are isotropic during substorm recovery phase for the 11 October 2008 event and have empty loss cone for the 5 April 2010 event. For both cases, LAE intensification marks the onset of activity in the magnetotail and precedes the large substorm onset. We conclude that the LAE production starts in the transition region between the magnetotail and ring current and may expand/move into the inner magnetosphere together with ring current formation.
The composition of Mars' topside ionosphere: Effects of hydrogen
Authors: Matta Majd , Withers Paul , Mendillo Michael 
Published: April 2013

AbstractA one‐dimensional model of the Martian ionosphere is used to explore the importance of atomic and molecular hydrogen chemistry in the upper atmosphere and ionosphere. Neutral and ionized H and H2 undergo chemical reactions that lead to the production of the hydrogenated ions: H+, H2+, H3+, OH+, HCO+, ArH+, N2H+, HCO2+, and HOC+. Simulations are conducted for the cases of photochemistry only and photochemistry coupled with transport in order to asses the separate effects of plasma diffusion in the topside ionosphere. For both of these cases, the sensitivity of the ionosphere is tested for (1) molecular hydrogen abundance and (2) reaction rate, k1, for the charge exchange between H+ and H2. Results are reported for midday solar minimum conditions. We find that the ionospheric composition of Mars is sensitive to H2 abundance, but relatively insensitive to the reaction rate, k1. Depending on the conditions simulated, the topside ionosphere can contain appreciable amounts of hydrogenated species such as H3+, OH+, and HCO+. Comparisons are made with Viking ion density measurements as well as with results of other published Mars ionospheric models. Future comparisons with more extensive ion composition will be available when the Mars Atmosphere and Volatile Evolution mission arrives at Mars.
Electron fluxes and pitch‐angle distributions at dipolarization fronts: THEMIS multipoint observations
Authors: Runov A. , Angelopoulos V. , Gabrielse C. , Zhou X.‐Z. , Turner D. , Plaschke F. 
Published: April 2013

AbstractTaking advantage of multipoint observations from a Cluster‐like Time History of Events and Macroscale Interactions during Substorms (THEMIS) probe configuration repeated in three events, we study pitch‐angle distributions (PAD) of lower energy (0.2–keV) electrons and omnidirectional energy‐time spectrograms of higher energy (30–500 keV) electrons observed at and near dipolarization fronts in the plasma sheet. Recent observations have shown that dipolarization fronts in the plasma sheet provide an impulsive electric field suggested to cause electron energization and dispersionless injections. Increase and decrease in energetic electron flux are equally probable at the fronts, however. Our case studies demonstrate increased energetic electron flux in the front's central region but decreased flux on its dusk side, where diverted plasma flow forms a vortex. An electric field associated with this vortex causes the electron flux decrease. We also find that shorter‐term energetic flux decreases, often observed before injections, coincide with a dip in the northward magnetic field ahead of the front. We attribute these decreases to particle energy loss via the inverse betatron effect. Our case studies reveal that pancake‐type (maximum at 90° pitch angle) and cigar‐type (maxima at 0 and 180°) PADs coexist at the same front. Our data analysis suggests that energetic electron PADs are mainly pancake type near the neutral sheet (|Bx| < 5 nt) and mainly cigar type at |Bx| > 10 nt. These results, to be confirmed in statistical studies, provide important constraints for further modeling of electron energization and transport toward the inner magnetosphere.
Terrestrial myriametric radio burst observed by IMAGE and Geotail satellites
Authors: Fung Shing F. , Hashimoto Kozo , Kojima Hirotsugu , Boardsen Scott A. , Garcia Leonard N. , Matsumoto Hiroshi , Green James L. , Reinisch Bodo W. 
Published: April 2013

AbstractWe report the simultaneous detection of a terrestrial myriametric radio burst (TMRB) by IMAGE and Geotail on 19 August 2001. The TMRB was confined in time (0830–1006 UT) and frequency (12–50 kHz). Comparisons with all known nonthermal myriametric radiation components reveal that the TMRB might be a distinct radiation with a source that is unrelated to the previously known radiation. Considerations of beaming from spin‐modulation analysis and observing satellite and source locations suggest that the TMRB may have a fan beamlike radiation pattern emitted by a discrete, dayside source located along the poleward edge of magnetospheric cusp field lines. TMRB responsiveness to IMF Bz and By orientations suggests that a possible source of the TMRB could be due to dayside magnetic reconnection instigated by northward interplanetary field condition.
Ordering of injection events within Saturnian SLS longitude and local time
Authors: Kennelly T. J. , Leisner J. S. , Hospodarsky G. B. , Gurnett D. A. 
Published: April 2013

AbstractPeriodicities similar to the Saturn kilometric radio (SKR) emissions have been observed throughout the magnetosphere in both the magnetic field and the plasma. An outstanding question is what mechanism links these periodicities between the inner and outer magnetospheres. It had been postulated that the interchange instability, where narrow injections return the magnetic flux carried by the bulk plasma outflow, could play a role in determining the periodicities, but early analysis found no ordering of the injection events in the SKR‐derived Saturnian longitude system (SLS). In this study, we reexamine this possibility by limiting our data set to the young injection events observed by the Cassini radio and plasma wave science instrument. We find that the young injection events observed near midnight local time are strongly ordered by SLS. Further, this ordering varies with the Saturnian season. Pre‐equinox, the northern hemisphere's longitude system controls the event occurrence. Post‐equinox, the events are ordered by the southern hemisphere‐derived longitude system. We suggest that this may be an effect in the variations in ionospheric conductivity or due to change in the magnetosphere's orientation relative to the solar wind.
He2+ transport in the Martian upper atmosphere with an induced magnetic field
Authors: Shematovich V. I. , Bisikalo D. V. , Stenberg G. , Barabash S. , Diéval C. , Gérard J.‐C. 
Published: April 2013

AbstractSolar wind helium may be a significant source of neutral helium in the Martian atmosphere. The precipitating particles also transfer mass, energy, and momentum. To investigate the transport of He2+ in the upper atmosphere of Mars, we have applied the direct simulation Monte Carlo method to solve the kinetic equation. We calculate the upward He, He+, and He2+ fluxes, resulting from energy spectra of the downgoing He2+ observed below 500 km altitude by the Analyzer of Space Plasmas and Energetic Atoms 3 instrument onboard Mars Express. The particle flux of the downward moving He2+ ions was 1–2 × 106 cm–2 s–1, and the energy flux is equal to 9–10 × 10–3 erg cm–2 s–1. The calculations of the upward flux have been made for the Martian atmosphere during solar minimum. It was found, that if the induced magnetic field is not introduced in the simulations the precipitating He2+ ions are not backscattered at all by the Martian upper atmosphere. If we include a 20 nT horizontal magnetic field, a typical field measured by Mars Global Surveyor in the altitude range of 85–500 km, we find that up to 30%–40% of the energy flux of the precipitating He2+ ions is backscattered depending on the velocity distribution of the precipitating particles. We thus conclude that the induced magnetic field plays a crucial role in the transport of charged particles in the upper atmosphere of Mars and, therefore, that it determines the energy deposition of the solar wind.
Observation and theory of whistler wave generation by high‐power HF waves
Authors: Kuo Spencer , Cheng Wei‐Te , Pradipta R. , Lee M. C. , Snyder Arnold 
Published: April 2013

AbstractSummer midnight and afternoon VLF wave generation comparison experiments were conducted on 25 July and 27 July 2011, respectively, using two CW HF X‐mode waves with 11 VLF frequency differences from 2 to 21.5 kHz. The background magnetic variations were at comparable levels. During the afternoon experiment, the D region absorption was significant and increasing. The number of the ionogram echoes decreased during the afternoon experiment. VLF signals were detected from 2 to 7.6 kHz in both experiments, showing an inverse frequency dependence of intensity, although signal intensity (except at 5.5 kHz) detected during the midnight experiment was stronger than the corresponding afternoon intensity before observing a decrease of the number of ionogram echoes. However, VLF signals from 11.5 to 21.5 (except at 19.6 kHz) were also generated in the afternoon experiment concurrent with a decrease of the O‐mode ionosonde echoes from 2 to 4 MHz. The concurrence of a decrease of the afternoon ionogram echoes, the unexpected generation of VLF waves at higher frequencies, and the increasing D region absorption throughout the experiment may be explained by the generation of large‐scale density irregularities, which scatter the ionosonde signals as well as couple with the modulated electrojet to generate whistler waves. A theoretical formulation of the coupling mechanism for the whistler wave generation is presented.
Dependence of the oblique propagation of ULF foreshock waves on solar wind parameters
Authors: Hsieh W.‐C. , Shue J.‐H. 
Published: April 2013

AbstractAccording to theory, foreshock ultra‐low frequency (ULF) waves are allowed to propagate mainly along the ambient magnetic field; however, waves obliquely propagating at a noticeably large angle can be easily observed. Although the mechanism of wave refraction was proposed to resolve the disagreement, doubts on the application of wave refraction to oblique propagation still remain. With the Time History of Events and Macroscale Interactions during Substorms (THEMIS) data, we examine the dependence of the oblique propagation of the ULF waves on solar wind parameters. The distributions of the oblique propagation angle θkB2 are spread out over a wide range of the solar wind parameters. From the mechanism of wave refraction, we derive the θkB2 as a function of the upstream propagation angle θkB1, the ratio of solar wind speed to upstream and downstream wave phase speed , and the angle between the interplanetary magnetic field (IMF) and solar wind velocity θBV. Both of the prediction and the observation show the irrelevance of the wave frequency, the sense of polarization, the strength of the IMF, and solar wind speed to the oblique propagation. We find that the prediction from wave refraction is in good agreement with the observation, especially along the track of prediction lines with the same θkB1 and .
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

AbstractIn 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

AbstractWe 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.
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

AbstractA 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.
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

AbstractRing 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

AbstractThe 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.