bluuebunny/crossref_metadata_2025 huggingface dataset:
Production of small‐scale Alfvén waves by ionospheric depletion, nonlinear magnetosphere‐ionosphere coupling and phase mixing
Authors: Russell A. J. B. , Wright A. N. , Streltsov A. V.
Published: April 2013

Rockets and satellites have previously observed small‐scale Alfvén waves inside large‐scale downward field‐aligned currents, and numerical simulations have associated their formation with self‐consistent magnetosphere‐ionosphere coupling. The origin of these waves was previously attributed to ionospheric feedback instability; however, we show that they arise in numerical experiments in which the instability is excluded. A new interpretation is proposed in which strong ionospheric depletion and associated current broadening (a nonlinear steepening/wave‐breaking process) form magnetosphereionosphere waves inside a downward current region and these oscillations drive upgoing inertial Alfvén waves in the overlying plasma. The resulting waves are governed by characteristic periods, which are a good match to previously observed periods for reasonable assumed conditions. Meanwhile, wavelengths perpendicular to the magnetic field initially map to an ionospheric scale comparable to the electron inertial length for the low‐altitude magnetosphere, but become shorter with time due to frequency‐based phase mixing of boundary waves (a new manifestation of phase mixing). Under suitable conditions, these could act as seeds for the ionospheric feedback instability.
Thermospheric atomic oxygen density estimates using the EISCAT Svalbard Radar
Authors: Vickers H. , Kosch M. J. , Sutton E. , Ogawa Y. , La Hoz C.
Published: April 2013

Coupling between the ionized and neutral atmosphere through particle collisions allows an indirect study of the neutral atmosphere through measurements of ionospheric plasma parameters. We estimate the neutral density of the upper thermosphere above ~250 km with the European Incoherent Scatter Svalbard Radar (ESR) using the year‐long operations of the International Polar Year from March 2007 to February 2008. The simplified momentum equation for atomic oxygen ions is used for field‐aligned motion in the steady state, taking into account the opposing forces of plasma pressure gradients and gravity only. This restricts the technique to quiet geomagnetic periods, which applies to most of the International Polar Year during the recent very quiet solar minimum. The method works best in the height range ~300–400 km where our assumptions are satisfied. Differences between Mass Spectrometer and Incoherent Scatter and ESR estimates are found to vary with altitude, season, and magnetic disturbance, with the largest discrepancies during the winter months. A total of 9 out of 10 in situ passes by the CHAMP satellite above Svalbard at 350 km altitude agree with the ESR neutral density estimates to within the error bars of the measurements during quiet geomagnetic periods.
Universal time control of AKR: Earth is a spin‐modulated variable radio source
Authors: Morioka Akira , Miyoshi Yoshizumi , Kurita Satoshi , Kasaba Yasumasa , Angelopoulos Vassilis , Misawa Hiroaki , Kojima Hirotsugu , McFadden James P.
Published: April 2013

Auroral kilometric radiation (AKR) is known to be transient emissions generated by rapidly accelerated electrons together with sudden auroral activation in the polar magnetosphere. In contrast, the characteristics and relationship with the auroral acceleration of rather continuous AKR emissions are not well understood. We examine the emission using long‐term data and report that the continuous AKR emission frequency changes with universal time (UT) as the Earth rotates, indicating that the Earth is a spin‐modulated variable radio source. The observed UT variation of AKR frequency means that the acceleration altitude changes periodically with planetary rotation. The observations indicate that the diurnal wobble of the tilted geomagnetic field in the solar wind flow alters the magnetosphere‐ionosphere (M‐I) coupling state in the polar magnetosphere, giving rise to periodic variation of auroral particle acceleration altitude. These observations of planetary radio wave properties provide insight into the physics of planetary particle acceleration.
Planetary period magnetic field oscillations in Saturn's magnetosphere: Postequinox abrupt nonmonotonic transitions to northern system dominance
Authors: Provan G. , Cowley S. W. H. , Sandhu J. , Andrews D. J. , Dougherty M. K.
Published: April 2013

We examine the “planetary period” magnetic field oscillations observed in the “core” region of Saturn's magnetosphere (dipole L ≤ 12), on 56 near‐equatorial Cassini periapsis passes that took place between vernal equinox in August 2009 and November 2012. Previous studies have shown that these consist of the sum of two oscillations related to the northern and southern polar regions having differing amplitudes and periods that had reached near‐equal amplitudes and near‐converged periods ~10.68 h in the interval to ~1 year after equinox. The present analysis shows that an interval of strongly differing behavior then began ~1.5 years after equinox, in which abrupt changes in properties took place at ~6‐ to 8‐month intervals, with three clear transitions occurring in February 2011, August 2011, and April 2012, respectively. These are characterized by large simultaneous changes in the amplitudes of the two systems, together with small changes in period about otherwise near‐constant values of ~10.63 h for the northern system and ~10.69 h for the southern (thus, not reversed postequinox) and on occasion jumps in phase. The first transition produced a resumption of strong southern system dominance unexpected under northern spring conditions, while the second introduced comparably strong northern system dominance for the first time in these data. The third resulted in suppression of all core oscillations followed by re‐emergence of both systems on a time scale of ~85 days, with the northern system remaining dominant but not as strongly as before. This behavior poses interesting questions for presently proposed theoretical scenarios.
Meridional electric fields in layer‐type and clump‐type plasma structures in midlatitude sporadic E region: Observations and plausible mechanisms
Authors: Chu Y.H. , Yang K.F. , Wang C.Y. , Su C.L.
Published: April 2013

The use of multibased line interferometry technique operated by a VHF coherent scatter radar with relatively broad radar beam pattern makes it possible to reveal zonal drift velocity of 3 m field‐aligned irregularities (FAIs) inside moving layer‐type or clump‐type plasma structures at kilometer scale. We present compelling evidences in this article showing that, no matter what types of the plasma structures, the apparent zonal drift velocity of the 3 m FAIs is always linearly dependent on the zonal trace velocity (or bulk velocity) of the kilometer‐scale plasma structure. This feature seems to imply that the bulk motion of the large‐scale plasma structure may significantly affect the observed 3 m FAI Doppler velocity, which should be removed to obtain true FAI drift velocity. In addition, there is an evident tendency for the 3 m FAIs in the layer‐type plasma structure to drift in the same zonal direction as the moving plasma structure. However, the correlation between the drift direction of the 3 m FAIs and the moving direction of the large‐scale plasma structure is indistinct for the clump‐type plasma structure. The meridional electric fields estimated from the true zonal FAI drift velocities for the layer‐type plasma structures moving in the west (east) direction point in the northward/upward (southward/downward) direction. Statistical results show that the mean value of the meridional electric field inside the layer‐type plasma structure is approximately 2.7–2.8 mV/m, about one order of magnitude larger than that for the clump‐type plasma structure. Mechanisms that are, respectively, responsible for the generations of the layer‐type and clump‐type plasma structures are discussed in this article. It is believed that neutral wind shear is very likely the main cause of the layer‐type plasma structure formation, while the clump‐type plasma structure may be associated with propagating gravity wave.
Geomagnetic and auroral activity driven by corotating interaction regions during the declining phase of Solar Cycle 23
Authors: Luan Xiaoli , Wang Wenbin , Lei Jiuhou , Burns Alan , Dou Xiankang , Xu Jiyao
Published: April 2013

A superposed epoch analysis is performed to investigate the relative impact of the solar wind/interplanetary magnetic field (IMF) on geomagnetic activity, auroral hemispheric power, and auroral morphology during corotating interaction regions (CIRs) events between 2002 and 2007, when auroral images from Thermosphere Ionosphere Mesosphere Energetics and Dynamics/Global Ultraviolet Imager were available. Four categories of CIRs have been compared. These were classified by the averaged IMF Bz and the time of maximum solar wind dynamic pressure around the CIR stream interface or onset time. It is found that during CIR events: (1) The peaks of auroral power and Kp were largely associated with dominant southward Bz, whereas auroral activity also became stronger with increases of solar wind speed, density, and dynamic pressure. (2) The percentage and absolute increases of auroral hemispheric power with solar wind speed were much greater under dominantly northward Bz conditions than under dominantly southward Bz conditions. (3) The enhancement of the auroral power and Kp with increasing solar wind speed followed the same pattern, for both dominantly southward and northward Bz conditions, regardless of the behavior of solar wind density and dynamic pressure. These results suggest that, during CIR events, southward Bz played the most critical role in determining geomagnetic and auroral activity, whereas solar wind speed was the next most important contributor. The solar wind dynamic pressure was the less important factor, as compared with Bz and solar wind speed. Relatively strong auroral precipitation energy flux (> ~3 mW/m2) occurred in a wider auroral oval region after the stream interface than before it for both dominantly northward and southward Bz conditions. These conditions enhanced the auroral hemispheric power after the stream interface. Intense auroral precipitation (> ~4 mW/m2) generally occurred widely at night under dominantly southward Bz conditions, but the location of this precipitation in the auroral oval was different when it was associated with different solar wind density and speed conditions.
Kinetic simulations of 3‐D Farley‐Buneman turbulence and anomalous electron heating
Authors: Oppenheim M. M. , Dimant Y. S.
Published: April 2013

Electric fields map from the magnetosphere to the E region ionosphere where they drive the intense currents of the auroral electrojet. Particularly during geomagnetic storms and substorms, these currents become sufficiently intense to develop Farley‐Buneman (FB) streaming instabilities and become turbulent. This leads to anomalous electron heating which can raise the electron temperature from 300 K to as much as 4000 K and, also, modifies auroral conductivities. This paper describes the first fully kinetic 3‐D simulations of electric field‐driven turbulence in the electrojet and compares the results with 2‐D simulations and observations. These simulations show that 3‐D turbulence can dramatically elevate electron temperatures, enough to explain the observed heating. They also show the saturated amplitude of the waves; coupling between linearly growing modes and damped modes; the propagation of the dominant modes at phase velocities near the acoustic velocity, slower than in 2‐D simulations; and anomalous cross‐field electron transport, leading to a greatly increased E region Pedersen conductivity. These simulations provide information useful in accurately modeling FB turbulence and represent significant progress in understanding the electrojet.
Lower thermospheric‐enhanced sodium layers observed at low latitude and possible formation: Case studies
Authors: Xue X. H. , Dou X. K. , Lei J. , Chen J. S. , Ding Z. H. , Li T. , Gao Q. , Tang W. W. , Cheng X. W. , Wei K.
Published: April 2013

We report two lower thermospheric‐enhanced sodium layer (TeSL) cases observed at a low‐latitude station, Lijiang, China (26.7°N, 100.0°E), on 10 March and 10 April 2012, respectively. The TeSLs in the two cases were located at altitudes near 122 and 112 km, respectively. In addition, strong sporadic sodium layers (SSLs) near 100 km accompanied the TeSL observed on 10 March 2012. Both the TeSLs and SSLs exhibited tidal‐induced downward motion. The adjacent ground‐based and space‐borne ionospheric radio observations showed strong Es layers before the appearance of the TeSLs, suggesting an “ Es– TeSLs (SSLs)” chain formed through the tidal wind shear mechanism. Assuming that the vertical tidal wavelengths remain unchanged, it is found that in different regions caused by the tidal wind shear, different TeSLs evolution processes are expected: (1) in a tidal‐convergence region, a TeSL/SSL with a downward propagation phase is enhanced due to a rapid decrease in the Na+lifetime at the lower altitude; (2) in an ion convergence‐divergence interface region, a TeSL/SSL will still follow the tidal downward phase progression, but sodium density does not exhibit evident enhancement; and (3) when a TeSL/SSL enters into a tidal wind‐divergence zone, the layer density tends to decrease.
Joint responses of geosynchronous magnetic field and relativistic electrons to external changes in solar wind dynamic pressure and interplanetary magnetic field
Authors: Li L. Y. , Cao J. B. , Yang J. Y. , Dong Y. X.
Published: April 2013

This paper studied statistically the joint responses of magnetic field and relativistic (>0.5 MeV) electrons at geosynchronous orbit to 201 interplanetary perturbations during 6 years from 2003 (solar maximum) to 2008 (solar minimum). The statistical results indicate that during geomagnetically quiet times (HSYM > −30 nT, and AE < 200 nT), ~47.3% changes in the geosynchronous magnetic field and relativistic electron fluxes are caused by the combined actions of the enhancement of solar wind dynamic pressure (Pd) and the southward turning of interplanetary magnetic field (IMF) (ΔPd > 0.4 nPa and IMF Bz < 0 nT), and only ~18.4% changes are due to single dynamic pressure increase (ΔPd > 0.4 nPa, but IMF Bz > 0 nT), and ~34.3% changes are due to single southward turning of IMF (IMF Bz < 0 nT, but |ΔPd| < 0.4 nPa). Although the responses of magnetic field and relativistic electrons to the southward turning of IMF are weaker than their responses to the dynamic pressure increase, the southward turning of IMF can cause significant dawn‐dusk asymmetric perturbations that the magnetic field and relativistic electron fluxes increase on the dawnside (LT ~ 00:00–12:00) but decrease on the duskside (LT ~ 13:00–23:00) during the quiet times. Furthermore, the variation of relativistic electron fluxes is adiabatically controlled by the magnitude and elevation angle changes of magnetic field during the single IMF southward turnings. However, the variation of relativistic electron fluxes is independent of the change in magnetic field in some magnetospheric compression regions during the solar wind dynamic pressure enhancements (including the single pressure increases and the combined external perturbations), indicating that nonadiabatic dynamic processes of relativistic electrons occur there.
Oxygen‐hydrogen differentiated observations from TWINS: The 22 July 2009 storm
Authors: Valek P. W. , Goldstein J. , McComas D. J. , Ilie R. , Buzulukova N. , Fok M.‐C. , Perez J. D.
Published: April 2013

The 22 July 2009 magnetic storm is the first significant storm during the emergence of the recent prolonged solar cycle minimum. This moderate storm (minimum Dst approximately ≤ −78 nT) has received a good deal of attention in the community. We present here global observations of the H and O populations in the inner magnetosphere using Energetic Neutral Atom (ENA) observations from the TWINS mission. We develop and provide the methodology for separating H and O ENAs, based on mass dependent differences in the pulse height distributions of the microchannel plate (MCP) based detectors. We present the first composition separated H and O ENA images at central energies of 16 and 32 keV. We also show that TWINS has sufficient angular resolution to separate the High Altitude Emissions (HAEs) from the Low Altitude Emissions (LAEs). We observe that all ENA emissions in this energy range quickly rise, but the O ENAs have a larger relative increase, and stay at elevated levels much longer, well into the recovery phase.
Statistical study of geomagnetic field line resonances observed by CHAMP and on the ground
Authors: Heilig B. , Sutcliffe P. R. , Ndiitwani D. C. , Collier A. B.
Published: April 2013

Studies of single events showing the signatures of Pc3 pulsation field line resonances (FLRs) observed in magnetic field data by CHAMP above the E‐region ionosphere and at conjugate ground stations at L < 2 have previously been reported. Notable features of FLRs observed in these investigations were a Doppler shift in the satellite azimuthal component and a 90° rotation of the polarization ellipse between CHAMP and the ground. In this report, we extend the previous work by means of the first statistical investigation of the same features for a large number of events using CHAMP and ground magnetic field data. Our statistical study confirms the Doppler shifts to higher or lower frequencies relative to the ground FLR frequencies for equatorward or poleward passes of CHAMP over the ground station. Our statistical investigation shows the amount of rotation of the FLR polarization ellipses between the ionosphere and the ground to be ~100° on average, rather than the theoretically predicted 90°. This is explained by the polarization ellipses on the ground being oriented close to N‐S irrespective of the orientation above the ionosphere, which is −13° on average. We find no evidence of ionospheric conductivity gradients associated with sunrise and sunset affecting the amount of rotation of the polarization ellipses as predicted by more recent theoretical advances. We are not able to provide conclusive evidence on the relationship between the polarization ellipse rotation and azimuthal wave number as predicted in recent numerical modeling.
VLF wave intensity in the plasmasphere due to tropospheric lightning
Authors: Colman J. J. , Starks M. J.
Published: April 2013

A climatology of VLF (very low frequency) wave intensity from lightning in the plasmasphere is constructed. Starting from Optical Transient Detector/Lightning Imaging Sensor (OTD/LIS) lightning data representing 1995–2005, a climatology of strikes is assembled with 1° × 1° latitude‐longitude spatial resolution, averaged into 2 h bins for each month of the year. Assuming a linear relationship between optical flash rate and VLF power flux, and that the VLF amplitude drops off as one over distance, a proxy for VLF power is developed. A typical lightning spectrum is applied and the values are scaled by appropriate transionospheric absorptions for each time and place. These values are mapped along geomagnetic field lines in order to compare them to E‐field spectral densities measured by the DEMETER satellite between 2005 and 2009. An overview of the DEMETER survey mode data is presented which leads to the best scaling of the lightning VLF climatology in LEO (low earth orbit). The resulting data set represents a monthly, 2‐hour, solar minimum climatology of VLF wave intensity from lightning in LEO. Finally, the E‐field spectral densities are converted to Poynting flux, mapped to the plasmasphere, and converted to B‐field spectral densities. Good overall agreement is found with previous observations and estimates. This new climatology is expected to have a significant impact on calculations of pitch‐angle diffusion for relativistic electrons in the inner radiation belt.
Magnetospheric convection and magnetopause shadowing effects in ULF wave‐driven energetic electron transport
Authors: Degeling A. W. , Rankin R. , Murphy K. , Rae I. J.
Published: April 2013

Magnetospheric radiation belt electron transport in the presence of ULF waves and a convection electric field is investigated using a model that includes a nightside plasma sheet source and electron losses by magnetopause shadowing. Narrow‐band ULF waves launched from a prescribed dayside magnetopause source are shown to interact with trapped and untrapped equatorially mirroring electrons within the magnetosphere. For magnetic moments less than 8 keV/nT and a strong convection electric field (in the order of 5 mV/m), we find that a limb of untrapped plasma sheet electrons extending across the dayside magnetosphere into the afternoon sector provides a phase space density (PSD) source for injection to lower L‐shells by ULF waves, causing a rapid enhancement in PSD. However, the same ULF wave activity gives rise to a rapid dropout in PSD for electrons with a higher magnetic moment or in the presence of a weaker convection electric field, since in this case the plasma sheet electrons escape through the magnetopause before they reach the afternoon sector. In their place, a lack of PSD, or PSD “holes” can be periodically injected from the magnetopause to lower L‐shells by ULF waves, leading to the rapid depletion in average PSD. In each case, the magnitude and extent in L‐shell of the PSD enhancement or depletion is strongly dependent on the amplitude of ULF waves in the afternoon sector, and is significantly augmented by the overlap of drift‐resonant islands and an associated stochastic transport layer.
Modulation of auroral electrojet currents using dual modulated HF beams with ELF phase offset, a potential D‐region ionospheric diagnostic
Authors: Gołkowski M. , Cohen M. B. , Moore R. C.
Published: April 2013

Experiments at the ionospheric heating facility of the High Frequency Active Auroral Research Program (HAARP) are performed employing dual HF beams amplitude modulated at ELF/VLF with a phase offset between the two modulation waveforms. The amplitude of the observed ELF/VLF waves is strongly dependent on the imposed ELF/VLF phase offset, the modulation waveform, and the orientation of the HF beams. Data from two ground stations are interpreted using simulations of modulated heating power envelopes as well as a comprehensive model of ionospheric ELF/VLF generation. It is found that two colocated vertical beams HF beams excite a single ionospheric ELF/VLF source, but independent ELF/VLF sources can be induced in the ionospheric region above the heater if the HF beams are offset from zenith to intersect at their 3 dB points. Furthermore, the use of two vertical HF beams with ELF phase offset is found to be a potential diagnostic method for the ionospheric D region.
Characteristic dimension of electromagnetic ion cyclotron wave activity in the magnetosphere
Authors: Lee Jeongwoo , Min Kyungguk , Kim Kap‒Sung
Published: April 2013

In this paper, we estimate the size of coherent activity of electromagnetic ion cyclotron (EMIC) waves using the multi‒spacecraft observations made during the Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission. We calculate the cross‒correlations between EMIC wave powers measured by different THEMIS spacecraft, plot them over the separation distances between pairs of observing spacecraft, and determine the 1/e folding distance of the correlations as the characteristic dimension of the coherent wave activity. The characteristic radius in the direction transverse to the local magnetic field is found to lie in rather a wide range of 1500–8600 km varying from the AM to PM sectors and also from hydrogen to helium bands. However, the characteristic dimensions normalized by either gyroradius or wavelength fall into narrower ranges almost independent of the emission band and event location. Specifically, the coherent dimension is found to be 10–16 times gyroradius of 100 keV protons and 2–3 times local wavelength. The former may give a useful scale for the source dimension, and the latter suggests that the EMIC wave activity maintains coherency only up to a couple of wavelengths.
Three‐dimensional structure of magnetic reconnection in the magnetotail from Geotail observations
Authors: Nagai T. , Shinohara I. , Zenitani S. , Nakamura R. , Nakamura T. K. M. , Fujimoto M. , Saito Y. , Mukai T.
Published: April 2013

Three‐dimensional structure of magnetic reconnection in the near‐Earth magnetotail is explored using Geotail observations from 1996 to 2012. Magnetic reconnection is identified as a simultaneous plasma flow and magnetic field reversal near the neutral sheet with intense out‐of‐plane electron currents. There are 30 events in the region of −32 < X < −18 RE (in the aberrated GSM coordinates) in the magnetotail. The dawn‐dusk width (in the y direction) of the magnetic reconnection site is probably less than 8 RE with its center in the premidnight region. The magnetic field structure in the x‐z plane does not change significantly in the full width of the magnetic reconnection site. The ion inflow‐outflow structure shows a marked edge effect in the duskside. As inflow, ions move toward the equatorial plane with an additional dawnward motion. This dawnward motion is more evident in the duskside. As outflows, ions escape tailward and earthward with a duskward motion, as expected from the Speiser motion, except at the duskside edge. At the duskside edge, almost all ions move dawnward, and only part of high‐energy ions can escape earthward and tailward. There is a possibility that the dawnward flow consists of adjacent plasma sheet plasmas transported by the pressure gradient force at the duskside edge. The present results would be the first step to construct a three‐dimensional structure of magnetic reconnection in various cosmic plasmas.
A novel technique for rapid L∗ calculation: Algorithm and implementation
Authors: Min Kyungguk , Bortnik Jacob , Lee Jeongwoo
Published: April 2013

Computing the magnetic drift invariant, L∗, rapidly and accurately has always been a challenge to magnetospheric modelers, especially given the importance of this quantity in the radiation belt community. Min et al. (2013) proposed a new method of calculating L∗ using the principle of energy conservation. Continuing with the approach outlined therein, the present paper focuses on the technical details of the algorithm to outline the implementation, systematic analysis of accuracy, and verification of the speed of the new method. We also show new improvements which enable near real‐time computation of L∗. The relative error is on the order of 10−3 when ∼0.1 RE grid resolution is used and the calculation speed is about 2 s per particle in the popular Tsyganenko and Sitnov 05 model (TS05). Based on the application examples, we suggest that this method could be an added resource for the radiation belt community.
Modeling ionospheric super‐fountain effect based on the coupled TIMEGCM‐SAMI3
Authors: Lu G. , Huba J. D. , Valladares Cesar
Published: April 2013

Recently, efforts have been undertaken to develop a coupled thermosphere‐ionosphere‐plasmasphere model based on two well‐established models, namely, the Thermosphere‐Ionosphere‐Mesosphere General Circulation Model (TIMEGCM) developed at the National Center for Atmospheric Research and the SAMI3 ionosphere model developed at the Naval Research Laboratory. This paper presents the first results from the coupled model on the investigation of a prompt penetration electric field (PPEF) event that took place on 9 November 2004. The coupled model eliminates two major upper boundary limitations of the stand‐alone TIMEGCM, e.g., the upper boundary height and the prescribed O+ fluxes at the upper boundary. It is found that the F‐layer peak height is raised above 800 km in response to the large PPEF. The O+ fluxes in the top ionosphere vary drastically during the course of the PPEF, with strong upward and downward fluxes with a magnitude greater than 109 cm−2 s−1 in localized regions. For the first time, the coupled model allows us to simulate and visualize the super‐fountain effect on a global scale. Future model development is also envisaged, including the implementation of a more realistic magnetic field model and a fully two‐way coupling between neutrals and ions.
Near‐lunar proton velocity distribution explained by electrostatic acceleration
Authors: Hutchinson I. H.
Published: April 2013

The observation of parallel ion velocity in the near‐lunar wake approximately equal to external solar wind velocity can be explained within uncertainties by an analytic electrostatic expansion model. The one‐dimensional model frequently used is inadequate because it does not account for the moon's spherical shape. However, application of a more recent generalization to three dimensions of the solution along characteristics predicts higher velocities and is probably sufficient to account for the SARA observations on the Chandrayaan‐1 spacecraft.
Three‐dimensional magnetic flux rope structure formed by multiple sequential X‐line reconnection at the magnetopause
Authors: Zhong J. , Pu Z. Y. , Dunlop M. W. , Bogdanova Y. V. , Wang X. G. , Xiao C. J. , Guo R. L. , Hasegawa H. , Raeder J. , Zhou X. Z. , Angelopoulos V. , Zong Q. G. , Fu S. Y. , Xie L. , Taylor M. G. G. T. , Shen C. , Berchem J. , Zhang Q. H. , Volwerk M. , Eastwood J. P.
Published: April 2013

On 14 June 2007, four Time History of Events and Macroscale Interactions during Substorms spacecraft observed a flux transfer event (FTE) on the dayside magnetopause, which has been previously proved to be generated by multiple, sequential X‐line reconnection (MSXR) in a 2‐D context. This paper reports a further study of the MSXR event to show the 3‐D viewpoint based on additional measurements. The 3‐D structure of the FTE flux rope across the magnetospheric boundary is obtained on the basis of multipoint measurements taken on both sides of the magnetopause. The flux rope's azimuthally extended section is found to lie approximately on the magnetopause surface and parallel to the X‐line direction; while the axis of the magnetospheric branch is essentially along the local unperturbed magnetospheric field lines. In the central region of the flux rope, as distinct from the traditional viewpoint, we find from the electron distributions that two types of magnetic field topology coexist: opened magnetic field lines connecting the magnetosphere and the magnetosheath and closed field lines connecting the Southern and Northern hemispheres. We confirm, therefore, for the first time, the characteristic feature of the 3‐D reconnected magnetic flux rope, formed through MSXR, through a determination of the field topology and the plasma distributions within the flux rope. Knowledge of the complex geometry of FTE flux ropes will improve our understanding of solar wind‐magnetosphere interaction.
Solar periodicity in energetic ions at Saturn
Authors: Carbary J. F. , Roelof E. C. , Mitchell D. G. , Hamilton D. C.
Published: April 2013

Energetic protons (2.8–78 keV) and water group ions (8.8–78 keV) observed in Saturn's magnetosphere using the Magnetospheric Imaging Instrument/Charge‐Energy‐Mass Spectrometer instrument from 2005 through 2012 were subjected to a Lomb periodogram analysis with the period window extending from 0.5 to 50 days, and the data constrained to the spatial region between 10 RS (1 RS = 60,268 km) and the magnetopause. Both the protons and water group ions exhibited solar periodicity at ~26 days and harmonics thereof. For all ions, the 26 day periodicities were strong on the dayside and duskside, weak at dusk, and virtually nonexistent on the nightside. The solar periodicity was evident during both the first and second halves of the 7 year time span, but strongest in the first half from January 2004 to July 2008. Some of the ion spectral peaks, especially for the water group ions, can be associated with orbital motion of the spacecraft. The 26 day periodicities are likely caused by corotating interaction regions in the solar wind that periodically sweep past Saturn, regularly compressing its dayside magnetosphere, but not its nightside.
Flux tube analysis of L‐band ionospheric scintillation
Authors: Shume E. B. , Mannucci A. J. , Butala M. D. , Pi X. , Valladares C. E.
Published: April 2013

This manuscript presents magnetic flux tube analysis of L‐band signal scintillation in the nighttime equatorial and low‐latitude ionosphere. Residues of the scintillation index estimated from the L‐band signals received from Geostationary Earth Orbit (GEO) satellites are employed in the analysis. The estimates have been shown to be associated with simultaneous GPS VTEC variations derived from JPL's GIPSY‐GIM package. We have applied the wavelet decomposition technique simultaneously on the time series in a flux tube over the equatorial and low‐latitude regions. The technique decomposes the signal to identify the dominant mode of variabilities and the temporal variations of scintillation‐producing irregularities in the context of a flux tube. Statistically significant regions of the wavelet power spectra considered in our study have mainly shown that (a) dominant plasma irregularities associated with variabilities in a flux tube have periods of about 4 to 15 minutes (horizontal irregularity scales of about 24 to 90 km). These periods match short period gravity waves, (b) scintillation‐producing irregularities are anisotropic along the flux tube and in the east‐west direction, and (c) the occurrences of scintillation‐producing irregularities along the flux tube indicate that the entire flux tube became unstable. However, plasma instability occurrences were not simultaneous in most cases along the flux tube, there were time delays of various orders. Understanding the attributes of L‐band scintillation‐producing irregularities could be important for developing measures to mitigate L‐band signal degradation.
Influence of the El Niño Southern Oscillation on the middle and upper atmosphere
Authors: Pedatella N. M. , Liu H.‐L.
Published: April 2013

Based on Whole Atmosphere Community Climate Model (WACCM) simulations, Pedatella and Liu (2012) recently demonstrated that significant interannual variability occurs in migrating and nonmigrating tides in the mesosphere and lower thermosphere (MLT) due to the El Niño Southern Oscillation (ENSO). The role of changes in tropospheric forcing, changes in the zonal mean atmosphere, and planetary wave‐tide interactions on generating the tidal variability in the MLT are investigated in the present study. The ENSO‐driven variability in the migrating diurnal tide (DW1) is found to be primarily due to changes in the tropospheric forcing of the DW1. Changes in tropospheric forcing are also the source of the changes in the eastward propagating nonmigrating diurnal tide with zonal wave number 3 (DE3). However, changes in the zonal mean atmosphere also contribute to interannual variability of the DE3 due to the ENSO. Variability in the eastward propagating nonmigrating diurnal tide with zonal wave number 2 (DE2) is largely due to changes in the background atmosphere, with a smaller additional contribution due to changes in tropospheric forcing. Variability in the westward propagating semidiurnal tide with zonal wave number 4 (SW4) is believed to be due to changes in planetary waves during the ENSO which will enhance generation of the SW4 through the nonlinear interaction of the migrating semidiurnal tide and stationary planetary waves with zonal wave number 2. The influence of the interannual tidal variability on the longitude structure of the low‐latitude ionosphere is also investigated in the present study. Comparison of El Niño and La Niña time periods reveals that the ENSO introduces changes of ~2–4 ms−1 in the daytime vertical drift velocity at certain longitudes. Simulation results further illustrate that the variability in the vertical drift velocity drives interannual variability in the low‐latitude daytime F region maximum electron density (NmF2). The results demonstrate that the ENSO introduces variability of ~10–30% in the MLT and ~10–15% in the ionosphere. The ENSO should therefore be considered as a potentially significant source of variability in the Earth's upper atmosphere.
Anisotropic structures of oceanic slab and mantle wedge in a deep low‐frequency tremor zone beneath the Kii Peninsula, SW Japan
Authors: Saiga Atsushi , Kato Aitaro , Kurashimo Eiji , Iidaka Takashi , Okubo Makoto , Tsumura Noriko , Iwasaki Takaya , Sakai Shin'ichi , Hirata Naoshi
Published: April 2013

Anisotropy is an important feature of elastic wave propagation in the Earth and can arise from a variety of ordered architectures such as fractures with preferential alignments or preferred crystal orientations. We studied the regional variations in shear wave anisotropy around a deep Low‐Frequency Earthquake (LFE) zone beneath the Kii Peninsula, SW Japan, using waveforms of local earthquakes observed by a dense linear array along the LFE zone. The fast directions of polarization are subparallel to the strike of the margin for both crustal and intraslab earthquakes. The delay time of the split shear waves in intraslab earthquakes is larger than that in crustal earthquakes and shows a down‐dip variation across the LFE zone. This indicates that anisotropy exists in the mantle wedge and in the lower crust and/or oceanic slab. We explain the observed delay time of 0.015–0.045 s by suggesting that the mantle wedge consists of a deformed, 1–15 km thick serpentine layer if the mantle wedge is completely serpentinized. In addition to high‐fluid pressures within the oceanic crust, the sheared serpentine layer may be a key factor driving LFEs in subduction zones.
Incorporating fault zone head wave and direct wave secondary arrival times into seismic tomography: Application at Parkfield, California
Authors: Bennington Ninfa L. , Thurber Clifford , Peng Zhigang , Zhang Haijiang , Zhao Peng
Published: April 2013

We present a three‐dimensional (3D) P wave velocity (Vp) model of the Parkfield region that utilizes existing P wave arrival time data, including fault zone head waves (FZHWs), and data from direct wave secondary arrivals (DWSAs). The first‐arrival and DWSA travel times are obtained as the global‐ and local‐minimum travel time paths, respectively. The inclusion of FZHWs and DWSAs results in as much as a 5% and a 10% increase in the across‐fault velocity contrast, respectively, for the Vp model at Parkfield relative to that of Thurber et al. [2006]. Viewed along strike, three pronounced velocity contrast regions are observed: a pair of strong positive velocity contrasts (SW fast), one NW of the 1966 Parkfield earthquake hypocenter and the other SE of the 2004 Parkfield earthquake hypocenter, and a strong negative velocity contrast (NE fast) between the two hypocenters. The negative velocity contrast partially to entirely encompasses peak coseismic slip estimated in several slip models for the 2004 earthquake, suggesting that the negative velocity contrast played a part in defining the rupture patch of the 2004 Parkfield earthquake. Following Ampuero and Ben‐Zion (2008), the pattern of velocity contrasts is consistent with the observed bilateral rupture propagation for the 2004 Parkfield earthquake. Although the velocity contrasts also suggest bilateral rupture propagation for the 1966 Parkfield earthquake, the fault is creeping to the NW here, i.e., exhibiting velocity‐strengthening behavior. Thus, it is not surprising that rupture propagated only SE during this event.
Tidal modulation and back‐propagating fronts in slow slip events simulated with a velocity‐weakening to velocity‐strengthening friction law
Authors: Hawthorne J. C. , Rubin A. M.
Published: April 2013

We examine tidal modulation and back‐propagating fronts in simulated slow slip events using a rate and state friction law that is steady state velocity weakening at low slip rates and velocity strengthening at high slip rates. Tidal forcing causes a quasi‐sinusoidal modulation of the slip rate during the events, with the maximum moment rate occurring close to or slightly after the maximum applied stress. The amplitude of modulation scales linearly with the tidal load and increases as the tidal period increases relative to the timescale for state evolution. If we choose parameters so that the model matches the observed tidal modulation of slip in Cascadia, it can reproduce only a subset of the stress drops inferred from observations and only in a limited portion of parameter space. The tidal forcing also causes back‐propagating fronts to form and move back through the region that has already ruptured. The stress drop that drives these back‐propagating fronts sometimes comes from the tidal load and sometimes from a stress recovery that occurs behind the front in tidal and non‐tidal simulations. We investigate the slip and propagation rates in the back‐propagating fronts and compare them with observations. The modeled fronts propagate too slowly to be good representations of the fronts inferred from tremor observations. For the simulated fronts to propagate at the observed speeds, the stress drops driving them would have to be more than 70 % of the stress drop driving the forward‐propagating front.
Evolution of dike opening during the March 2011 Kamoamoa fissure eruption, Kīlauea Volcano, Hawai`i
Authors: Lundgren Paul , Poland Michael , Miklius Asta , Orr Tim , Yun Sang‐Ho , Fielding Eric , Liu Zhen , Tanaka Akiko , Szeliga Walter , Hensley Scott , Owen Susan
Published: April 2013

The 5–9 March 2011 Kamoamoa fissure eruption along the east rift zone of Kīlauea Volcano, Hawai`i, followed months of pronounced inflation at Kīlauea summit. We examine dike opening during and after the eruption using a comprehensive interferometric synthetic aperture radar (InSAR) data set in combination with continuous GPS data. We solve for distributed dike displacements using a whole Kīlauea model with dilating rift zones and possibly a deep décollement. Modeled surface dike opening increased from nearly 1.5 m to over 2.8 m from the first day to the end of the eruption, in agreement with field observations of surface fracturing. Surface dike opening ceased following the eruption, but subsurface opening in the dike continued into May 2011. Dike volumes increased from 15, to 16, to 21 million cubic meters (MCM) after the first day, eruption end, and 2 months following, respectively. Dike shape is distinctive, with a main limb plunging from the surface to 2–3 km depth in the up‐rift direction toward Kīlauea's summit, and a lesser projection extending in the down‐rift direction toward Pu`u `Ō`ō at 2 km depth. Volume losses beneath Kīlauea summit (1.7 MCM) and Pu`u `Ō`ō (5.6 MCM) crater, relative to dike plus erupted volume (18.3 MCM), yield a dike to source volume ratio of 2.5 that is in the range expected for compressible magma without requiring additional sources. Inflation of Kīlauea's summit in the months before the March 2011 eruption suggests that the Kamoamoa eruption resulted from overpressure of the volcano's magmatic system.
The 2011 Mw 7.1 Van (Eastern Turkey) earthquake
Authors: Elliott J.R. , Copley A.C. , Holley R. , Scharer K. , Parsons B.
Published: April 2013

We use interferometric synthetic aperture radar (InSAR), body wave seismology, satellite imagery, and field observations to constrain the fault parameters of the Mw 7.1 2011 Van (Eastern Turkey) reverse‐slip earthquake, in the Turkish‐Iranian plateau. Distributed slip models from elastic dislocation modeling of the InSAR surface displacements from ENVISAT and COSMO‐SkyMed interferograms indicate up to 9 m of reverse and oblique slip on a pair of en echelon NW 40 °–54 ° dipping fault planes which have surface extensions projecting to just 10 km north of the city of Van. The slip remained buried and is relatively deep, with a centroid depth of 14 km, and the rupture reaching only within 8–9 km of the surface, consistent with the lack of significant ground rupture. The up‐dip extension of this modeled WSW striking fault plane coincides with field observations of weak ground deformation seen on the western of the two fault segments and has a dip consistent with that seen at the surface in fault gouge exposed in Quaternary sediments. No significant coseismic slip is found in the upper 8 km of the crust above the main slip patches, except for a small region on the eastern segment potentially resulting from the Mw 5.9 aftershock on the same day. We perform extensive resolution tests on the data to confirm the robustness of the observed slip deficit in the shallow crust. We resolve a steep gradient in displacement at the point where the planes of the two fault segments ends are inferred to abut at depth, possibly exerting some structural control on rupture extent.
A model of fracture nucleation, growth and arrest, and consequences for fracture density and scaling
Authors: Davy Philippe , Le Goc Romain , Darcel Caroline
Published: April 2013

In order to improve discrete fracture network (DFN) models, which are increasingly required into groundwater and rock mechanics applications, we propose a new DFN modeling based on the evolution of fracture network formation—nucleation, growth, and arrest—with simplified mechanical rules. The central idea of the model relies on the mechanical role played by large fractures in stopping the growth of smaller ones. The modeling framework combines, in a time‐wise approach, fracture nucleation, growth, and arrest. It yields two main regimes. Below a certain critical scale, the density distribution of fracture sizes is a power law with a scaling exponent directly derived from the growth law and nuclei properties; above the critical scale, a quasi‐universal self‐similar regime establishes with a self‐similar scaling. The density term of the dense regime is related to the details of arrest rule and to the orientation distribution of the fractures. The DFN model, so defined, is fully consistent with field cases former studied. Unlike more usual stochastic DFN models, ours is based on a simplified description of fracture interactions, which eventually reproduces the multiscale self‐similar fracture size distribution often observed and reported in the literature. The model is a potential significant step forward for further applications to groundwater flow and rock mechanical issues.
Fracture patterns in nonplane strain boudinage—insights from 3‐D discrete element models
Authors: Abe Steffen , Urai Janos L. , Kettermann Michael
Published: April 2013

We use 3‐D Discrete Element Method simulations to model the evolution of boudin structures in a layered material under nonplane strain conditions. As the models are shortened perpendicular to the layer orientation, they are extended at different rates in the two layer‐parallel directions. The particular emphasis of the study is on the orientation of fractures between the boudin blocks. The results show that the fracture orientation distribution is closely connected to the ratio of the two layer‐parallel extension rates. The anisotropy of the fracture orientation distribution increases systematically from no anisotropy at isotropic layer‐parallel extension to a highly anisotropic distribution in case of uniaxial extension. We also observe an evolution of the anisotropy of fracture orientation distribution with increasing deformation in each individual model from a high‐initial anisotropy towards a value characteristic for the ratio of the layer‐parallel extension rates. The observations about the relation between the strain ratios and the fracture patterns do have the potential to serve as the basis for a new method to analyze strains in naturally boudinaged rocks.