bluuebunny/crossref_metadata_2025 huggingface dataset:
How simulated fluence of photons from terrestrial gamma ray flashes at aircraft and balloon altitudes depends on initial parameters
Authors: Hansen R. S. , Østgaard N. , Gjesteland T. , Carlson B.
Published: April 2013

Up to a few years ago, terrestrial gamma ray flashes (TGFs) were only observed by spaceborne instruments. The aircraft campaign ADELE was able to observe one TGF, and more attempts on aircraft observations are planned. There is also a planned campaign with stratospheric balloons, COBRAT. In this context an important question that arises is what count rates we can expect and how these estimates are affected by the initial properties of the TGFs. Based on simulations of photon propagation in air we find the photon fluence at different observation points at aircraft and balloon altitudes. The observed fluence is highly affected by the initial parameters of the simulated TGFs. One of the most important parameters is the number of initial photons in a TGF. In this paper, we give a semi‐analytical approach to find the initial number of photons with an order of magnitude accuracy. The resulting number varies over several orders of magnitude, depending mostly on the production altitude of the TGF. The initial production altitude is also one of the main parameters in the simulations. Given the same number of initial photons, the fluence at aircraft and balloon altitude from a TGF produced at 10 km altitude is 2–3 orders of magnitude smaller then a TGF originating from 20 km altitude. Other important parameters are altitude distribution, angular distribution and amount of feedback. The differences in altitude, altitude distribution and amount of feedback are especially important for the fluence of photons observed at altitudes less than 20 km, and for instruments with a low‐energy threshold larger than 100 keV. We find that the maximum radius of observation in 14 km for a TGF with the intensity of an average RHESSI TGF is smaller than the results reported by Smith et al. (2011), and our results support the conclusion in Gjesteland et al. (2012) and Østgaard et al. (2012) that TGFs probably are a more common phenomenon than previously reported.
Magnetopause surface waves: THEMIS observations compared to MHD theory
Authors: Plaschke F. , Angelopoulos V. , Glassmeier K.‐H.
Published: April 2013

On 5 and 6 October 2011, the three inner THEMIS spacecraft observed oscillatory magnetopause (MP) motion in the early afternoon sector over extended periods of time, caused by the passage of surface waves. The close configuration of the spacecraft tangential to the MP allowed for determination of wave propagation velocities via cross‐correlation analysis. This enabled us to compare observations with solutions of the surface wave dispersion relation derived in the framework of ideal MHD theory. We find that observations and theoretical predictions are in quantitative agreement if a high plasma flow velocity (of around 340 km∕s) can be assumed in the plasma depletion layer on the magnetosheath side of the MP. Although the surface waves propagated more slowly than the plasma on the magnetosheath side, they were not subject to the Kelvin‐Helmholtz instability. We suggest that waves generated in the quasi‐parallel bow shock or upstream foreshock regions may have impinged on the MP generating the surface waves. Additionally, the spacecraft separation in the boundary normal direction enabled us to infer directly the waveform and MP boundary layer structure. Interestingly, the trailing edges of the waves were more inclined than the leading edges, contrary to theory and prior observations of Kelvin‐Helmholtz‐driven surface waves.
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

We 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

Periodicities 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.
The magnetospheric banana current
Authors: Liemohn Michael W. , Ganushkina Natalia Yu. , Katus Roxanne M. , De Zeeuw Darren L. , Welling Daniel T.
Published: April 2013

It is shown that the banana current, a current system in the inner magnetosphere closing entirely within the magnetosphere (i.e., not through the ionosphere or on the magnetopause) but not circumflowing around the Earth, is a regular feature of near‐Earth space. Closure options for the eastward asymmetric current on the inside of a localized pressure peak were explored, with the conclusion that the current must close via westward current around the outside of the high pressure region. It is a current that encircles a pressure peak and, therefore, whenever there is a pressure peak in the inner magnetosphere, a banana current exists. If multiple pressure peaks exist in the inner magnetosphere, then multiple banana currents will also coexist. Its occurrence rate is equal to that of the partial ring current, defined here as westward magnetospheric current that closes through field‐aligned currents into and out of the ionosphere. Its magnitude can reach a few mega‐amps during the main phase of storms, but drops to <0.1 MA during extended quiet intervals. The magnetic perturbation related to this current is strong within the region of high plasma pressure that it encircles, but is otherwise very weak outside of the banana current loop because the oppositely‐directed current flow on either side of the loop largely cancels each other. In general, its related magnetic field is a few nanotesla of northward perturbation for both ground‐based and geosynchronous magnetometers, making it difficult to magnetically detect. The banana current is placed in the context of the other near‐Earth nightside current systems.
On the application of ensemble modeling techniques to improve ambient solar wind models
Authors: Riley Pete , Linker Jon A. , Mikić Zoran
Published: April 2013

Ensemble modeling is a method of prediction based on the use of a representative sample of possible future states. Global models of the solar corona and inner heliosphere are now maturing to the point of becoming predictive tools; thus, it is both meaningful and necessary to quantitatively assess their uncertainty and limitations. In this study, we apply simple ensemble modeling techniques as a first step towards these goals. We focus on one relatively quiescent time period, Carrington rotation 2062, which occurred during the late declining phase of solar cycle 23. To illustrate and assess the sensitivity of the model results to variations in boundary conditions, we compute solutions using synoptic magnetograms from seven solar observatories. Model sensitivity is explored using (1) different combinations of models, (2) perturbations in the base coronal temperature (a free parameter in one of the model approximations), and (3) the spatial resolution of the numerical grid. We present variance maps, “whisker” plots, and “Taylor” diagrams to summarize the accuracy of the solutions and compute skill scores, which demonstrate that the ensemble mean solution outperforms any of the individual realizations. Our results provide a baseline against which future model improvements can be compared.
Auroral particle precipitation characterized by the substorm cycle
Authors: Wing Simon , Gkioulidou Matina , Johnson Jay R. , Newell Patrick T. , Wang Chih‐Ping
Published: April 2013

Substorms release a large amount of energy, some of which is used to energize the precipitating particles in the polar region. Superposed epoch analysis was performed with 11 years of DMSP SSJ/4/5 data to characterize the substorm cycle of the diffuse, monoenergetic, and broadband/wave precipitating electrons and precipitating ions. Although substorms only increase the ion pressure by 30%, they increase the power of the diffuse, monoenergetic, and wave electron aurora by 310%, 71%, and 170%, respectively. Substorms energize the ion aurora mainly in the 21:00–05:00 magnetic local time (MLT) sector. The dynamics of the diffuse electron aurora are different from those of the other two electron aurorae. The expansion phase duration is approximately 15 min for the monoenergetic and wave electron aurorae, whereas it is 1 h for the diffuse electron aurora. The monoenergetic and wave electron aurorae appear to complete the substorm cycle within a 5 h interval, whereas the diffuse electron aurora takes more than 5 h. The diffuse electron aurora power and energy flux start increasing at 15 min before the substorm onset, whereas those for the monoenergetic and wave electron aurorae start increasing at 1 h and 15 min before the onset. The increase in the monoenergetic electron aurora power and energy flux may result from the increase in the magnetotail stretching and region‐1 field‐aligned current during the growth phase. The monoenergetic electrons may also be associated with fast flows, which have been previously observed more frequently in the dusk‐midnight sector.
Particle and magnetic field properties of the Saturnian magnetosheath: Presence and upstream escape of hot magnetospheric plasma
Authors: Sergis N. , Jackman C. M. , Masters A. , Krimigis S. M. , Thomsen M. F. , Hamilton D. C. , Mitchell D. G. , Dougherty M. K. , Coates A. J.
Published: April 2013

We analyze plasma, energetic particle, and magnetic field data from all available Cassini passes through the Saturnian magnetosheath between July 2004 and July 2011 and provide a statistical overview of particle and field properties. The results show that magnetosheath plasma has an average number density of ~0.1 cm−3 and a temperature of ~300 eV. The measured magnetic field strength is ~1 nT, and the plasma beta is in the range of 10 to 100. The prevailing flow and magnetic field configuration is close to that theoretically expected, with flow speed values of ~200 km/s. Compositional data reveal that although at low energies (few keV) there is a strong presence of water group ions (W+) forming localized structures we refer to as W+ “islands” that travel downstream convected in the plasma flow. Under average magnetic field conditions in the Saturnian magnetosheath, the kinetic properties of these hot W+ ions can enable escape upstream from the bow shock. Based on the measured particle and field distributions and the modeled bow shock and magnetopause positions, we describe the energetic ion escape as a function of energy and pitch angle and successfully predict the energy distribution of the escaping W+ ions. Comparison with the ion spectra measured in the nearby solar wind confirms that the suggested escape mechanism due to large ion gyroradii is sufficient to explain the observed leakage of heavy energetic ions upstream from the Saturnian bow shock.
The role pickup ions play in the termination shock
Authors: Ariad D. , Gedalin M.
Published: April 2013

The termination shock (TS) is a quasi‐perpendicular shock with broad regions on both sides that are populated by high energy ions. These energetic particles play an important role in the formation of the TS structure, which distinguishes it from planetary bow shocks. The pressure of the pickup ions is calculated by solving numerically the Vlasov equation in a time‐stationary one‐dimensional model shock with the parameters taken from the TS observations. The solution is obtained by backward tracing of the ion trajectories in the shock front. It is found that, in order to maintain the shock stationarity, the upstream density of the pickup ions should be comparable to the density of the solar wind protons.
Nightside electron precipitation at Mars: Geographic variability and dependence on solar wind conditions
Authors: Lillis Robert J. , Brain David A.
Published: April 2013

Electron precipitation is usually the dominant source of energy input to the nightside Martian atmosphere, with consequences for ionospheric densities, chemistry, electrodynamics, communications, and navigation. We examine downward‐traveling superthermal electron flux on the Martian nightside from May 1999 to November 2006 at 400 km altitude and 2 A.M. local time. Electron precipitation is geographically organized by crustal magnetic field strength and elevation angle, with higher fluxes occurring in regions of weak and/or primarily vertical crustal fields, while stronger and more horizontal fields retard electron access to the atmosphere. We investigate how these crustal field‐organized precipitation patterns vary with proxies for solar wind (SW) pressure and interplanetary magnetic field (IMF) direction. Generally, higher precipitating fluxes accompany higher SW pressures. Specifically, we identify four characteristic spectral behaviors: (1) “stable” regions where fluxes increase mildly with SW pressure, (2) “high‐flux” regions where accelerated (peaked) spectra are more common and where fluxes below ~500 eV are largely independent of SW pressure, (3) permanent plasma voids, and (4) intermittent plasma voids where fluxes depend strongly on SW pressure. The locations, sizes, shapes, and absence/existence of these plasma voids vary significantly with solar wind pressure proxy and moderately with IMF proxy direction; average precipitating fluxes are 40% lower in strong crustal field regions and 15% lower globally for approximately southwest proxy directions compared with approximately northeast directions. This variation of the strength and geographic pattern of the shielding effect of Mars' crustal fields exemplifies the complex interaction between those fields and the solar wind.
Coincident particle and optical observations of nightside subauroral proton precipitation
Authors: Søraas Finn , Laundal Karl M. , Usanova Maria
Published: April 2013

We report two instances of nightside subauroral proton precipitation observed during a geomagnetic storm: (1) An arc at 蝶 50° magnetic latitude (MLAT), which extended roughly from midnight to 6 magnetic local time (MLT), and (2) A corotating spot at 51.5° MLAT, postmidnight. The proton precipitation was observed by both the IMAGE SI‐12 proton aurora (Doppler‐shifted Lyman‐ α) imager, and low‐altitude Polar‐orbiting Operational Environmental Satellites (POESs) that serendipitously traversed both the arc and the spot, providing in situ particle measurements. The Lyman‐alpha emission and the particle observations match closely. The particle measurements showed that the energies of the precipitating protons extended to several hundred keV, but no enhancements in the protons below 20 keV were observed. Cluster observations showed that the ring current contained protons with energies extending from tens of eV to several hundred keV; thus, the particles are scattered into the atmosphere from an existing reservoir. Outside the luminous regions, but at the same magnetic latitude, the POES satellites observed localized regions with enhanced proton fluxes outside, but close to, the loss cone. This shows that the protons along a large region in MLT are subjected to pitch angle scattering. Model calculations of the plasmasphere show that the arc and the proton precipitation are located just inside the plasmapause. The intensity and longitudinal extension of the arc are modulated by the dynamical pressure in the solar wind. In the dusk sector, ground stations recorded electromagnetic ion cyclotron waves associated with localized proton precipitation just inside the plasmapause. This suggests that the proton precipitation is caused by wave/particle interaction.
Can a nightside geomagnetic Delta H observed at the equator manifest a penetration electric field?
Authors: Wei Y. , Fraenz M. , Dubinin E. , He M. , Ren Z. , Zhao B. , Liu J. , Wan W. , Yumoto K. , Watari S. , Alex S.
Published: April 2013

A prompt penetration electric field (PPEF) usually manifests itself in the form of an equatorial ionospheric electric field being in correlation with a solar wind electric field. Due to the strong Cowling conductivity, a PPEF on the dayside can be inferred from Delta H (ΔH), which is the difference in the magnitudes of the horizontal (H) component between a magnetometer at the magnetic equator and one off the equator. This paper aims to investigate the performance of ΔH in response to a PPEF on the nightside, where the Cowling conductivity is not significant. We first examine the strongest geomagnetically active time during the 20 November 2003 superstorm when the Dst drops to −473 nT and show that the nightside ΔH can indeed manifest a PPEF but with local time dependence and longitude dependence. We then examine a moderately active time by taking advantage of the multiple‐penetration event during 11–16 November 2003 when the Dst remains greater than −60 nT. During this event, a series of PPEF pulses recorded in Peru, Japan, and India form a database, allowing us to examine PPEF effects at different local times and longitudes. The results show that (1) the nightside ΔH was caused by attenuation of the effects of the polar electric field with decreasing latitude; (2) the nightside ΔH can manifest a PPEF at least in the midnight‐dawn sector (0000–0500 LT), but not always; and (3) the magnitude of the nightside ΔH in the midnight‐dawn sector in Peru is on average only 1/18 of that of the dayside ΔH in response to a given PPEF.
Interplanetary shock–induced current sheet disturbances leading to auroral activations: THEMIS observations
Authors: Zhou Xiaoyan , Zhou Xu‐Zhi , Angelopoulos Vassilis , Shi Quanqi , Wang Chih‐Ping , Frey Harald
Published: April 2013

We present a unique case of shock‐generated tail current sheet (CS) disturbances measured by five THEMIS spacecraft in a tail‐aligned configuration from −11 to −17 Re on 3 March 2009 when an interplanetary shock passed by the Earth. The spacecraft were ideally near the tail CS center. The IMF Bz was weakly southward with a 2 h average of −0.4 nT in the shock upstream. At 0602 UT when the shock impinged at the subsolar magnetopause, compressional waves were launched and propagated at a speed of ~2900 km s−1 into the tail. Their arrival presented a discontinuity in the magnetic field and plasma density around a THEMIS outer probe. Then, the CS thickness started decreasing and the tail reconnection rate started increasing. About 3–4 min later when the shock in the solar wind compressed the tail magnetopause at the down‐tail location of the THEMIS spacecraft, the CS thinning became more significant and abrupt. Consequently, the CS thickness reduced by ~80% from ~16 × 103 to ~3 × 103 km in ~7 min X = −11 Re. At the same time, earthward fast flows and multiple dipolarization fronts were detected in the tail CS and auroral activity similar to a small substorm on the ground. We found that the tail CS and plasma sheet experienced a two‐step evolution, which is attributed to the much faster propagation of compressional waves inside the magnetosphere than the shock in the solar wind. During the first step, the local magnetic field and plasma varied with low fluctuations. In the second step, the CS and plasma sheet became very disturbed with fast flows and significant waves presented. Observations also indicated that the magnetotail is not in pressure equilibrium during the shock compression.
Characteristics of the Poynting flux and wave normal vectors of whistler‐mode waves observed on THEMIS
Authors: Li Wen , Bortnik J. , Thorne R. M. , Cully C. M. , Chen L. , Angelopoulos V. , Nishimura Y. , Tao J. B. , Bonnell J. W. , LeContel O.
Published: April 2013

The characteristics of the Poynting flux and wave normal vectors of whistler‐mode waves outside the plasmapause are investigated for the lower (0.1–0.5 fce) and upper bands (0.5–0.8 fce), where fce is the equatorial electron cyclotron frequency. To analyze the wave properties, we utilized high‐resolution waveform data from multiple THEMIS spacecraft in the near‐equatorial magnetosphere from June 2008 to November 2012. Full measurements of the wave electric and magnetic fields are used to calculate the Poynting fluxes and construct the wave normal vectors, which are then used to calculate the polar and azimuthal angles with respect to the background magnetic field. Statistical results show that the majority of whistler‐mode waves propagate away from the magnetic equator, suggesting that the major source region is very close to the equator. The lower band wave normal angle distribution shows a major peak close to the field line direction and a secondary peak near the resonance cone. In contrast, the wave normal distribution of upper band waves exhibits a broad distribution between 0° and 60° with the largest probability at ~0°. The azimuthal component of the wave normal vector predominantly points radially outward for both lower and upper band waves, but a tendency for azimuthal propagation is observed for lower band waves in the day and dusk sectors probably due to pronounced azimuthal density gradients in the afternoon sector. Our statistical results provide crucial information on the Poynting fluxes and wave normal vectors of whistler‐mode waves, which play a significant role in radiation belt electron dynamics.
Effects of the surface conductivity and the IMF strength on the dynamics of planetary ions in Mercury's magnetosphere
Authors: Seki K. , Terada N. , Yagi M. , Delcourt D.C. , Leblanc F. , Ogino T.
Published: April 2013

To examine the effects of planetary surface conductivity and the southward interplanetary magnetic field (IMF) strength on ion dynamics, systematic trajectory tracings of Na+ ions were performed in the electric and magnetic field configurations obtained from magnetohydrodynamics (MHD) simulations of the solar wind‐Mercury interaction. Comparison with a previous study, which used an analytical model that rescaled the Earth's magnetosphere and assumed the existence of the distant neutral line (DNL) in Mercury's magnetotail, shows a drastic change in the Na+ precipitation pattern due to the formation of the near‐Mercury neutral line (NMNL) in MHD simulations. The Na+ precipitation band at approximately 30° of latitude (LAT), which was obtained in the previous study, disappeared in the equivalent low‐conductivity MHD case due to the NMNL formation, while the NMNL formation causes high‐energy Na+ precipitation in the equatorial region. The change in strength of the southward IMF (sBz) alters the location of the NMNL and the Na+ precipitation pattern. In the low‐conductivity sBz = 5 case, both the equatorial precipitation and the Na+ band at approximately LAT = 30 are formed. In the high‐conductivity sBz = 5 case, magnetospheric convection through the polar regions is suppressed, which results in a region of dense Na+ near the planet. These results suggest that the precipitation pattern of planetary ions onto Mercury's surface changes significantly with the activity level of Mercury's magnetosphere. It is also suggested that observations of the magnetospheric convection, the distribution of Na+ ions around the planet, or the precipitation pattern of Na+ ions onto the planetary surface can provide us information about the surface conductivity.
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

Solar 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.
Radio emissions from terrestrial gamma‐ray flashes
Authors: Dwyer Joseph R. , Cummer Steven A.
Published: April 2013

The theory of radio frequency (RF) emissions by terrestrial gamma‐ray flashes (TGFs) is developed. These radio emissions, which are separate from the emission caused by lightning, are produced by the electric currents generated by runaway electrons and resulting low‐energy electrons and ions. It is found from theory that the radio frequency pulses produced by TGFs are large enough to measure. Features of these signals depend strongly on certain aspects of the runaway acceleration process and so should provide additional information about the source mechanism(s) of this interesting atmospheric phenomenon. The RF emissions from several TGF models are calculated and compared with measurements of TGF‐associated radio pulses, and it is found that the measured energy densities at higher frequencies support a source mechanism that involves a very large (>104) number of distinct seed particle injections. In particular, the results are consistent with the relativistic feedback discharge model of TGFs.
The analysis of electron fluxes at geosynchronous orbit employing a NARMAX approach
Authors: Boynton R. J. , Balikhin M. A. , Billings S. A. , Reeves G. D. , Ganushkina N. , Gedalin M. , Amariutei O. A. , Borovsky J. E. , Walker S. N.
Published: April 2013

The methodology based on the Error Reduction Ratio (ERR) determines the causal relationship between the input and output for a wide class of nonlinear systems. In the present study, ERR is used to identify the most important solar wind parameters, which control the fluxes of energetic electrons at geosynchronous orbit. The results show that for lower energies, the fluxes are indeed controlled by the solar wind velocity, as was assumed before. For the lowest energy range studied here (24.1 keV), the solar wind velocity of the current day is the most important control parameter for the current day's electron flux. As the energy increases, the solar wind velocity of the previous day becomes the most important factor. For the higher energy electrons (around 1 MeV), the solar wind velocity registered 2 days in the past is the most important controlling parameter. Such a dependence can, perhaps, be explained by either local acceleration processes due to the interaction with plasma waves or by radial diffusion if lower energy electrons possess higher mobility. However, in the case of even higher energies (2.0 MeV), the solar wind density replaces the velocity as the key control parameter. Such a dependence could be a result of solar wind density influence on the dynamics of various waves and pulsations that affect acceleration and loss of relativistic electrons. The study also shows that statistically the variations of daily high energy electron fluxes show little dependence on the daily averaged Bz, daily time duration of the southward IMF, and daily integral (where Bs is the southward component of IMF).
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

Summer 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.
Theoretical study: Influence of different energy sources on the cusp neutral density enhancement
Authors: Deng Yue , Fuller‐Rowell Timothy J. , Ridley Aaron J. , Knipp Delores , Lopez Ramon E.
Published: April 2013

Simulations with the Global Ionosphere‐Thermosphere Model (GITM) show that both Poynting flux and soft electron precipitation are important in producing neutral density enhancements near 400 km altitude in the cusp that have been observed by the Challenging Minisatellite Payload (CHAMP) satellite. Imposing a Poynting flux of 75 mW/m2 in the cusp within the model increases the neutral density by 34%. The direct heating from 100 eV, 2 mW/m2 soft electron precipitation produces only a 5% neutral density enhancement at 400 km. However, the associated enhanced ionization in the F‐region from the electron precipitation leads to a neutral density enhancement of 24% through increased Joule heating. Thus, the net effect of the soft electron is close to 29%, and the combined influence of Poynting flux and soft particle precipitation causes a more than 50% increase in neutral density at 400 km, which is consistent with CHAMP observations in extreme cases. The effect of electron precipitation on the neutral density at 400 km decreases sharply with increasing characteristic energy such that 900 eV electrons have little effect on neutral density. Finally, the impact of 2 keV, 0.3 mW/m2 proton precipitation on the neutral density is negligible due to a lowering of the altitude of Joule heating.
Elastic collisions of interstellar helium atoms with solar wind protons
Authors: Gruntman Mike
Published: April 2013

Relative motion of the Sun with respect to the surrounding local interstellar medium results in entering of interstellar helium atoms into the heliosphere. Current models of helium flowing into the solar system do not include elastic collisions of atoms with solar wind ions. It was predicted in 1986, without quantitative elaboration, that such collisions would enhance wings of directional distributions of helium atom fluxes. This paper focuses on a theoretical treatment of elastic collisions of interstellar He atoms with the solar wind protons resulting in increased wing intensities, called the helium flux halo in contradistinction to the flux core. We concentrate on directional distributions of He atom intensities at 1 AU from the Sun for observers at rest and moving with the Earth and confirm the formation of the flux halo. We show that the collision‐produced halo would often dominate He atom intensities at angles larger than 30°–35° from the maximum intensity direction in the flux core. A comparison with direct measurements of interstellar helium atom fluxes is beyond the scope of this paper.
MHD modeling of the double‐gradient (kink) magnetic instability
Authors: Korovinskiy D. B. , Divin A. , Erkaev N. V. , Ivanova V. V. , Ivanov I. B. , Semenov V. S. , Lapenta G. , Markidis S. , Biernat H. K. , Zellinger M.
Published: April 2013

The paper presents the detailed numerical investigation of the “double‐gradient mode,” which is believed to be responsible for the magnetotail flapping oscillations—the fast vertical (normal to the layer) oscillations of the Earth's magnetotail plasma sheet with a quasiperiod ∼100–200 s. The instability is studied using the magnetotail near‐equilibrium configuration. For the first time, linear three‐dimensional numerical analysis is complemented with full 3‐D MHD simulations. It is known that the “double‐gradient mode” has unstable solutions in the region of the tailward growth of the magnetic field component, normal to the current sheet. The unstable kink branch of the mode is the focus of our study. Linear MHD code results agree with the theory, and the growth rate is found to be close to the peak value, provided by the analytical estimates. Full 3‐D simulations are initialized with the numerically relaxed magnetotail equilibrium, similar to the linear code initial condition. The calculations show that current layer with tailward gradient of the normal component of the magnetic field is unstable to wavelengths longer than the curvature radius of the field line. The segment of the current sheet with the earthward gradient of the normal component makes some stabilizing effect (the same effect is registered in the linearized MHD simulations) due to the minimum of the total pressure localized in the center of the sheet. The overall growth rate is close to the theoretical double‐gradient estimate averaged over the computational domain.
Climatology of the migrating terdiurnal tide (TW3) in SABER/TIMED temperatures
Authors: Pancheva D. , Mukhtarov P. , Smith A. K.
Published: April 2013

The present paper is focused on the global spatial (altitude and latitude) structure and seasonal and interannual temporal variability of the migrating terdiurnal tide (TW3) seen in 8 years (2002–2009) of observations of the kinetic temperature measured by the SABER instrument on the TIMED satellite. The tides (migrating and nonmigrating) and the planetary waves (zonally traveling and stationary) are simultaneously extracted from the satellite data. It is found that the SABER TW3 tide is dominated by different Hough modes below and above ~80 to 90 km height; in the mesosphere, it reflects mainly the evanescent feature of the first symmetric mode, and in the lower thermosphere, propagating modes with shorter vertical wavelengths dominate its structure. The seasonal behavior of the terdiurnal tide at low latitudes is dominated by an annual variation with local summer amplification, while that at midlatitudes is a combination of semiannual variation with maxima in equinoxes and an annual variability with winter enhancement. The Hough modes of the solar heating from WACCM heating to a large extent determine the predominantly antisymmetric features of the SABER TW3 tide during the solstice but are not able to explain well the predominant symmetric features in the equinoxes. The TW3 tide reveals some interannual variability with a period of quasi‐2 years which indicates an enhancement during the eastward phase of the stratospheric QBO.
Statistics of the tripolar electrostatic solitary waves within magnetic reconnection diffusion region in the near‐Earth magnetotail
Authors: Li S. Y. , Zhang S. F. , Cai H. , Chen X. Q. , Deng X. H.
Published: April 2013

In this paper, we report the observations and statistical characteristics of tripolar electrostatic solitary waves (ESWs) along the plasma sheet boundary layer near the magnetic reconnection X line in the near‐Earth magnetotail. Within reconnection diffusion region, the tripolar ESWs are ample and are continuously observed during one burst interval (8.75 s) of the Geotail/WaveForm Capture in the neutral plasma sheet where β > 1 on 10:20 UT, 2 February 1996. The tripolar ESW is suggested to be one kind of steady‐going solitary structure. More than 200 waveforms with clear tripolar characteristics are differentiated for statistical analysis, and result shows that (1) their amplitude is within 100–>500 μV/m, with an average amplitude of about 254 μV/m; (2) the pulse width of the tripolar ESWs is 0.5–1.0 ms, with an average value of about 0.75 ms; (3) it is asymmetrical in both the amplitude and pulse width of the tripolar ESWs: most part of the tripolar ESWs (about 76.5%) are asymmetrical in the amplitude of one hump and the other one, and more than 75% (about 177 amount the 236 waveforms) of the tripolar ESWs are asymmetrical in the time duration of the two humps in the waveform; (4) most of the tripolar ESWs are with the potential humps of 10–60 mV, small ratio of them with potential humps larger than 100 mV. The tripolar ESWs with net potential drop of about 10–50 mV can be interpreted as “weak” double layers. The possible generation mechanism of tripolar ESWs and their role in reconnection are discussed by studying the particle distribution during which the tripolar ESWs are continuously observed. The observation of tripolar ESWs presents evidence of complex structure of electron holes within the reconnection diffusion region and is helpful to the understanding of the energy release process of reconnection.
Axis and velocity determination for quasi two‐dimensional plasma/field structures from Faraday's law: A second look
Authors: Sonnerup Bengt U. Ö. , Denton Richard E. , Hasegawa Hiroshi , Swisdak M.
Published: April 2013

We re‐examine the basic premises of a single‐spacecraft data analysis method, developed by Sonnerup and Hasegawa (2005), for determining the axis orientation and proper frame velocity of quasi two‐dimensional, quasi‐steady structures of magnetic field and plasma. The method, which is based on Faraday's law, makes use of magnetic and electric field data measured by a single spacecraft traversing the structure, although in many circumstances the convection electric field, − v × B, can serve as a proxy for E. It has been used with success for flux ropes observed at the magnetopause but has usually failed to provide acceptable results when applied to real space data from reconnection events as well as to virtual data from numerical MHD simulations of such events. In the present paper, the reasons for these shortcomings are identified, analyzed, and discussed in detail. Certain basic properties of the method are presented in the form of five theorems, the last of which makes use of singular value decomposition to treat the special case where the magnetic variance matrix is non‐invertible. These theorems are illustrated using data from analytical models of flux ropes and also from MHD simulations as well as a 2‐D kinetic simulation of reconnection. The results make clear that the method requires the presence of a significant, non‐removable electric field distribution in the plane transverse to the invariant direction and that it is sensitive to deviations from strict two‐dimensionality and strict time stationarity.
Low latitude thermospheric responses to magnetic storms
Authors: Earle G. D. , Davidson R. L. , Heelis R. A. , Coley W. R. , Weimer D. R. , Makela J. J. , Fisher D. J. , Gerrard A. J. , Meriwether J.
Published: April 2013

Thermospheric density and neutral velocity perturbations associated with three magnetic storms in the autumn season of 2011 are examined using data from the neutral wind meter (NWM) on the Communication/Navigation Outage Forecast System (C/NOFS) satellite. The data from perigee passes near 400 km altitude show marked increases in neutral density during the storms and associated increases in horizontal neutral flow speeds. These thermospheric responses are characterized by enhanced meridional neutral flows with peak perturbation amplitudes near 100 m/s and relative neutral density enhancements ranging from 50–100%. The increases in the neutral density and meridional flow velocity at equatorial latitudes occur about 5–7 h after the initial perturbations are observed in the z component of the interplanetary magnetic field (IMF), and they persist for 20–30 h. The perturbations in the neutral density are in good agreement with temperature increases predicted by an empirical model that has been validated using data from the CHAMP and Gravity Recovery and Climate Experiment missions, with a maximum lag time of ~1–1.5 h between the model temperature increases and the observed density perturbations. The model temperatures are in excellent agreement with ground‐based low‐latitude temperature measurements during the storms. Ground‐based wind measurements during one of the storms provide additional data for comparison with the perturbation wind amplitudes measured aboard the satellite.
Three‐dimensional modeling of the electromagnetic characteristics of equatorial plasma depletions
Authors: Dao E. , Seyler C. E. , Kelley M. C.
Published: April 2013

We derive and employ an algorithm for the three‐dimensional fluid treatment of ionospheric plasma, including the complete set of electromagnetic fields. Traditionally, simulations of ionospheric plasma instabilities assume electrostatic physics and/or simplify the problem in two dimensions while the dynamics along the Earth's magnetic field lines are neglected. Explored are the 3‐D electric and magnetic fields resulting from current divergence, current flows, and Alfvén waves associated with low‐latitude plasma irregularity dynamics. We investigate both dynamic and static electromagnetic properties. For the first time, we present simulations of the Alfvénic dynamics that map electric fields along the Earth's magnetic field lines. We also find that 1‐D theory can significantly overestimate 3‐D ambipolar electric fields.
Electron source at the outer boundary of the radiation belts: Storm time case
Authors: Lui A. T. Y.
Published: April 2013

We examine the strength of the electron source at the outer boundary of the radiation belts using multisatellite observations from Time History of Events and Macroscale Interactions during Substorms during one moderate geomagnetic storm. The electron phase space density (PSD) as a function of the first adiabatic invariant for an equatorially mirroring population over a broad energy range is used to determine the source strength. It is found that the source strength from dipolarization at the outer boundary of the radiation belts during this storm period is well above the observed phase space densities of relativistic electrons inside the outer radiation belt, indicating that the inward radial transport can be a potential source of the relativistic electrons within the radiation belts, even in the strong pitch angle diffusion limit.
Local‐time‐dependent low‐altitude ion spectra deduced from TWINS ENA images
Authors: Goldstein J. , McComas D. J. , Valek P. , Redfern J. , Søraas F. , Bazell D.
Published: April 2013

In this paper, we analyze Two Wide‐angle Imaging Neutral‐atom Spectrometers (TWINS) stereo observations of energetic neutral atoms (ENAs) produced from the low altitude emission (LAE) region during the interval 1130–1146 UT on 6 April 2010. Geometrical calculations determine the geophysical locations of pixels at or near the LAE limb and the associated uncertainties. For our event, the two TWINS imagers observed a broad (8.4–9.2 h wide in magnetic local time (MLT)) region of LAEs on the opposing limb, possibly containing an ion boundary near dusk. The most intense LAEs were detected in a narrow range of magnetic latitude (67°–74°) and pitch angle (112°–116°). We implement a simplified thick‐target approximation (TTA) to obtain ion spectra from TWINS LAEs and perform a validation study using a conjunction of the TWINS‐observed LAE crescents with a simultaneous NOAA 17 polar‐orbit pass slightly west of the TWINS LAEs. Since TTA is limited to the brightest portion of LAEs, we apply our analysis for pixels with at least 30% of the peak value. TWINS ion spectra are calculated for individual pixels spanning several hours of MLT. The spectra exhibit a pronounced local time dependence. For more westward MLT (and more equatorward latitude), there is a shift toward spectra that are more energetic and peaked. This spatial dependence is consistent with ion drift theory and previous observations. The peaked LAE‐derived ion spectra of 6 April 2010 are notably different than those observed during much weaker disturbances, but are consistent with LAE observations from similar activity levels. These results demonstrate that with proper caution in interpreting the results, TWINS ENA imaging resolves MLT‐dependent (and to a limited extent, latitude‐dependent) low‐altitude ion spectral shape information, simultaneously across a broad range of MLT. This study advances previous results that considered much coarser MLT structure in LAEs and augments previous statistical spectral analysis of in situ data.
Dependence of the oblique propagation of ULF foreshock waves on solar wind parameters
Authors: Hsieh W.‐C. , Shue J.‐H.
Published: April 2013

According 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 .