bluuebunny/crossref_metadata_2025 huggingface dataset:
Simultaneous observation of traveling ionospheric disturbances in the Northern and Southern Hemispheres
Authors: Valladares C. E. , Villalobos J. , Hei M. A. , Sheehan R. , Basu Su. , MacKenzie E. , Doherty P. H. , Rios V. H.
Published: June 2010

Abstract. Measurements of total electron content (TEC) using 263 GPS receivers located in the North and South America continents are presented to demonstrate the simultaneous existence of traveling ionospheric disturbances (TID) at high, mid, and low latitudes, and in both Northern and Southern Hemispheres. The TID observations pertain to the magnetically disturbed period of 29–30 October 2003 also known as the Halloween storm. The excellent quality of the TEC measurements makes it possible to calculate and remove the diurnal variability of TEC and then estimate the amplitude, wavelength, spectral characteristics of the perturbations, and the approximate velocity of the AGW. On 29 October 2003 between 17:00 and 19:00 UT, there existed a sequence of TEC perturbations (TECP), which were associated with the transit of atmospheric gravity waves (AGW) propagating from both auroral regions toward the geographic equator. A marked difference was found between the northern and southern perturbations. In the Northern Hemisphere, the preferred horizontal wavelength varies between 1500 and 1800 km; the propagation velocity is near 700 m/s and the perturbation amplitude about 1 TEC unit (TECu). South of the geographic equator the wavelength of the TECP is as large as 2700 km, the velocity is about 550 m/s, and the TECP amplitude is 3 TECu. Concurrently with our observations, the Jicamarca digisonde observed virtual height traces that exhibited typical features that are associated with TIDs. Here, it is suggested that differences in the local conductivity between northern and southern auroral ovals create a different Joule heating energy term. The quality of these observations illustrates the merits of GPS receivers to probe the ionosphere and thermosphere.
The equatorial E-region and its plasma instabilities: a tutorial
Authors: Farley D. T.
Published: June 2010

Abstract. In this short tutorial we first briefly review the basic physics of the E-region of the equatorial ionosphere, with emphasis on the strong electrojet current system that drives plasma instabilities and generates strong plasma waves that are easily detected by radars and rocket probes. We then discuss the instabilities themselves, both the theory and some examples of the observational data. These instabilities have now been studied for about half a century (!), beginning with the IGY, particularly at the Jicamarca Radio Observatory in Peru. The linear fluid theory of the important processes is now well understood, but there are still questions about some kinetic effects, not to mention the considerable amount of work to be done before we have a full quantitative understanding of the limiting nonlinear processes that determine the details of what we actually observe. As our observational techniques, especially the radar techniques, improve, we find some answers, but also more and more questions. One difficulty with studying natural phenomena, such as these instabilities, is that we cannot perform active cause-and-effect experiments; we are limited to the inputs and responses that nature provides. The one hope here is the steadily growing capability of numerical plasma simulations. If we can accurately simulate the relevant plasma physics, we can control the inputs and measure the responses in great detail. Unfortunately, the problem is inherently three-dimensional, and we still need somewhat more computer power than is currently available, although we have come a long way.
Mapping of coma anisotropies to plasma structures of weak comets: a 3-D hybrid simulation study
Authors: Gortsas N. , Motschmann U. , Kührt E. , Knollenberg J. , Simon S. , Boesswetter A.
Published: June 2010

Abstract. The effects of coma anisotropies on the plasma environment of comets have been studied by means of a 3-D hybrid model which treats electrons as a massless, charge-neutralizing fluid, whereas ion dynamics are covered by a kinetic approach. From Earth-based observations as well as from in-situ spacecraft measurements the shape of the coma of many comets is ascertained to be anisotropic. However, most plasma simulation studies deploy a spherically symmetric activity pattern. In this paper anisotropy is studied by considering three different coma shape models. The first model is derived from the Haser model and is characterised by spherically symmetry. This reference model is then compared with two different neutral gas shape models: the dayside restricted model with no nightside activity and a cone shaped model with opening angle of π/2. In all models the integrated surface activity is kept constant. The simulations have been done for the Rosetta target comet 67P/Churyumov-Gerasimenko for two heliocentric distances, 1.30 AU and 3.25 AU. It is found that shock formation processes are modified as a result of increasing spatial confinement. Characteristic plasma structures of comets such as the bow shock, magnetic barrier region and the ion composition boundary exhibit a shift towards the sun. In addition, the cone shaped model leads to a strong increase of the mass-loaded region which in turn leads to a smooth deceleration of the solar wind flow and an increasing degree of mixture between the solar wind and cometary ion species. This creates an additional transport channel of the magnetic field from the magnetic barrier region away which in turn leads to a broadening of this region. In addition, it leads to an ion composition boundary which is only gradually developed.
Storm induced large scale TIDs observed in GPS derived TEC
Authors: Borries C. , Jakowski N. , Wilken V.
Published: June 2010

Abstract. This work is a first statistical analysis of large scale traveling ionospheric disturbances (LSTID) in Europe using total electron content (TEC) data derived from GNSS measurements. The GNSS receiver network in Europe is dense enough to map the ionospheric perturbation TEC with high horizontal resolution. The derived perturbation TEC maps are analysed studying the effect of space weather events on the ionosphere over Europe. Equatorward propagating storm induced wave packets have been identified during several geomagnetic storms. Characteristic parameters such as velocity, wavelength and direction were estimated from the perturbation TEC maps. Showing a mean wavelength of 2000 km, a mean period of 59 min and a phase speed of 684 ms−1 in average, the perturbations are allocated to LSTID. The comparison to LSTID observed over Japan shows an equal wavelength but a considerably faster phase speed. This might be attributed to the differences in the distance to the auroral region or inclination/declination of the geomagnetic field lines. The observed correlation between the LSTID amplitudes and the Auroral Electrojet (AE) indicates that most of the wave like perturbations are exited by Joule heating. Particle precipitation effects could not be separated.
The influence of geomagnetic storms on the estimation of GPS instrumental biases
Authors: Zhang W. , Zhang D. H. , Xiao Z.
Published: June 2010

Abstract. An algorithm has been developed to derive the ionospheric total electron content (TEC) and to estimate the resulting instrumental biases in Global Positioning System (GPS) data from measurements made with a single receiver. The algorithm assumes that the TEC is identical at any point within a mesh and that the GPS instrumental biases do not vary within a day. We present some results obtained using the algorithm and a study of the characteristics of the instrumental biases during active geomagnetic periods. The deviations of the TEC during an ionospheric storm (induced by a geomagnetic storm), compared to the quiet ionosphere, typically result in severe fluctuations in the derived GPS instrumental biases. Based on the analysis of three ionospheric storm events, we conclude that different kinds of ionospheric storms have differing influences on the measured biases of GPS satellites and receivers. We find that the duration of severe ionospheric storms is the critical factor that adversely impacts the estimation of GPS instrumental biases. Large deviations in the TEC can produce inaccuracies in the estimation of GPS instrumental biases for the satellites that pass over the receiver during that period. We also present a semi quantitative analysis of the duration of the influence of the storm.
Negative correlation between terminal velocity and VHF radar reflectivity: observation and plausible explanation
Authors: Su C.-L. , Chu Y.-H. , Lo I.-Y.
Published: June 2010

Abstract. The correlation between precipitation backscatter P (or radar reflectivity Z) and reflectivity-weighted terminal velocity Vt has long been thought to be positive. Namely, the larger the magnitude of the terminal velocity is, the stronger the radar reflectivity will be, and vice versa. However, we will show in this article the observational evidences of negative Vt–P correlation made with the Chung-Li VHF radar. It is found that the negative Vt–P correlation can occur in the regions from close to ground to well above the melting layer. In addition, there is a strong tendency for the negative
Ionospheric storms at geophysically-equivalent sites – Part 1: Storm-time patterns for sub-auroral ionospheres
Authors: Mendillo M. , Narvaez C.
Published: June 2010

Abstract. The systematic study of ionospheric storms has been conducted primarily with groundbased data from the Northern Hemisphere. Significant progress has been made in defining typical morphology patterns at all latitudes; mechanisms have been identified and tested via modeling. At higher mid-latitudes (sites that are typically sub-auroral during non-storm conditions), the processes that change significantly during storms can be of comparable magnitudes, but with different time constants. These include ionospheric plasma dynamics from the penetration of magnetospheric electric fields, enhancements to thermospheric winds due to auroral and Joule heating inputs, disturbance dynamo electrodynamics driven by such winds, and thermospheric composition changes due to the changed circulation patterns. The ~12° tilt of the geomagnetic field axis causes significant longitude effects in all of these processes in the Northern Hemisphere. A complementary series of longitude effects would be expected to occur in the Southern Hemisphere. In this paper we begin a series of studies to investigate the longitudinal-hemispheric similarities and differences in the response of the ionosphere's peak electron density to geomagnetic storms. The ionosonde stations at Wallops Island (VA) and Hobart (Tasmania) have comparable geographic and geomagnetic latitudes for sub-auroral locations, are situated at longitudes close to that of the dipole tilt, and thus serve as our candidate station-pair choice for studies of ionospheric storms at geophysically-comparable locations. They have an excellent record of observations of the ionospheric penetration frequency (foF2) spanning several solar cycles, and thus are suitable for long-term studies. During solar cycle #20 (1964–1976), 206 geomagnetic storms occurred that had Ap≥30 or Kp≥5 for at least one day of the storm. Our analysis of average storm-time perturbations (percent deviations from the monthly means) showed a remarkable agreement at both sites under a variety of conditions. Yet, small differences do appear, and in systematic ways. We attempt to relate these to stresses imposed over a few days of a storm that mimic longer term morphology patterns occurring over seasonal and solar cycle time spans. Storm effects versus season point to possible mechanisms having hemispheric differences (as opposed to simply seasonal differences) in how solar wind energy is transmitted through the magnetosphere into the thermosphere-ionosphere system. Storm effects versus the strength of a geomagnetic storm may, similarly, be related to patterns seen during years of maximum versus minimum solar activity.
Evolution of dipolarization in the near-Earth current sheet induced by Earthward rapid flux transport
Authors: Nakamura R. , Retinò A. , Baumjohann W. , Volwerk M. , Erkaev N. , Klecker B. , Lucek E. A. , Dandouras I. , André M. , Khotyaintsev Y.
Published: June 2010

Abstract. We report on the evolution of dipolarization and associated disturbances of the near-Earth current sheet during a substorm on 27 October 2007, based upon Cluster multi-point, multi-scale observations of the night-side plasma sheet at X~−10 RE. Three dipolarization events were observed accompanied by activations on ground magnetograms at 09:07, 09:14, and 09:22 UT. We found that all these events consist of two types of dipolarization signatures: (1) Earthward moving dipolarization pulse, which is accompanied by enhanced rapid Earthward flux transport and is followed by current sheet disturbances with decrease in BZ and enhanced local current density, and subsequent (2) increase in BZ toward a stable level, which is more prominent at Earthward side and evolving tailward. During the 09:07 event, when Cluster was located in a thin current sheet, the dipolarization and fast Earthward flows were also accompanied by further thinning of the current sheet down to a half-thickness of about 1000 km and oscillation in a kink-like mode with a period of ~15 s and propagating duskward. Probable cause of this "flapping current sheet" is shown to be the Earthward high-speed flow. The oscillation ceased as the flow decreased and the field configuration became more dipolar. The later rapid flux transport events at 09:14 and 09:22 UT took place when the field configuration was initially more dipolar and were also associated with BZ disturbance and local current density enhancement, but to a lesser degree. Hence, current sheet disturbances induced by initial dipolarization pulses could differ, depending on the configuration of the current sheet.
The linear dependence of GHz scintillation on electron density observed in the equatorial anomaly
Authors: Whalen J. A.
Published: June 2010

Abstract. The dependence of scintillation caused by equatorial bubbles on the electron density in the post sunset anomaly cannot generally be determined because the irregularities that produce the scintillation obscure the measurement of the electron density. This work addresses this problem with the introduction of a new technique based on the finding that anomaly electron density is so regular that its LT distribution can be fit by a polynomial over its post-sunset extent. By means of this polynomial, gaps in the data caused by the irregularities can be inferred where otherwise not measurable. Observations are of amplitude scintillation at 1.5 GHz from the Marisat satellite and of NmF2 measured by ionospheric soundings at Ascension I. in Mar 2001 during solar maximum. The result is that the well defined maximum of the S4 scintillation index is a linear function of coinciding NmF2. Evidence that supports this result has been found in 3 differing cases: The first finds that both S4 and NmF2 decrease linearly with LT; the second, that both occurrence of scintillation and NmF2 increase linearly with F10.7 cm solar flux; the third finds a linear relation between occurrence of scintillation at 137 MHz and NmF2. The importance of these results is that scintillation is caused by the bubble, but its magnitude is determined by the magnitude of the electron density that the bubble intersects. A further implication is that, given the unpredictability of bubbles, if NmF2 could be predicted, the maximum level of S4 possible could be predicted if a bubble were to occur.
Summer planetary-scale oscillations: aura MLS temperature compared with ground-based radar wind
Authors: Meek C. E. , Manson A. H.
Published: June 2010

Abstract. The advent of satellite based sampling brings with it the opportunity to examine virtually any part of the globe. Aura MLS mesospheric temperature data are analysed in a wavelet format for easy identification of possible planetary waves (PW) and aliases masquerading as PW. A calendar year, 2005, of eastward, stationary, and westward waves at a selected latitude is shown in separate panels for wave number range −3 to +3 for period range 8 h to 30 days (d). Such a wavelet analysis is made possible by Aura's continuous sampling at all latitudes 82° S–82° N. The data presentation is suitable for examination of years of data. However this paper focuses on the striking feature of a "dish-shaped" upper limit to periods near 2 d in mid-summer, with longer periods appearing towards spring and fall, a feature also commonly seen in radar winds. The most probable cause is suggested to be filtering by the summer jet at 70–80 km, the latter being available from ground based medium frequency radar (MFR). Classically, the phase velocity of a wave must be greater than that of the jet in order to propagate through it. As an attempt to directly relate satellite and ground based sampling, a PW event of period 8d and wave number 2, which appears to be the original rather than an alias, is compared with ground based radar wind data. An appendix discusses characteristics of satellite data aliases with regard to their periods and amplitudes.
Cluster and ACE observations of phase synchronization in intermittent magnetic field turbulence: a comparative study of shocked and unshocked solar wind
Authors: Chian A. C.-L. , Miranda R. A.
Published: June 2010

Abstract. We apply two distinct nonlinear techniques, kurtosis and phase coherence index, to analyze the modulus of interplanetary magnetic field data |B| measured by Cluster and ACE spacecraft from 1 to 3 February 2002. High degree of phase synchronization is found across a wide range of time scales, from 1 s to 104 s, in the magnetic field fluctuations, both in the shocked solar wind upstream of Earth's bow shock and in the unshocked ambient solar wind at the L1 Lagrangian point. This is the first direct measurement of phase coherence in the ambient solar wind turbulence. We show that phase synchronization related to nonlinear multiscale interactions is the origin of the departure from Gaussianity in the intermittent magnetic field turbulence. In particular, we demonstrate that at small scales near the spectral break the intermittency level of Cluster is lower than ACE, which may be a signature of the reflected ions from the shock.
Large enhancements in low latitude total electron content during 15 May 2005 geomagnetic storm in Indian zone
Authors: Dashora N. , Sharma S. , Dabas R. S. , Alex S. , Pandey R.
Published: June 2010

Abstract. Results pertaining to the response of the equatorial and low latitude ionosphere to a major geomagnetic storm that occurred on 15 May 2005 are presented. These results are also the first from the Indian zone in terms of (i) GPS derived total electron content (TEC) variations following the storm (ii) Local low latitude electrodynamics response to penetration of high latitude convection electric field (iii) effect of storm induced traveling atmospheric disturbances (TAD's) on GPS-TEC in equatorial ionization anomaly (EIA) zone. Data set comprising of ionospheric TEC obtained from GPS measurements, ionograms from an EIA zone station, New Delhi (Geog. Lat. 28.42° N, Geog. Long. 77.21° E), ground based magnetometers in equatorial and low latitude stations and solar wind data obtained from Advanced Composition Explorer (ACE) has been used in the present study. GPS receivers located at Udaipur (Geog. Lat. 24.73° N, Geog. Long. 73.73° E) and Hyderabad (Geog. Lat. 17.33° N, Geog. Long. 78.47° E) have been used for wider spatial coverage in the Indian zone. Storm induced features in vertical TEC (VTEC) have been obtained comparing them with the mean VTEC of quiet days. Variations in solar wind parameters, as obtained from ACE and in the SYM-H index, indicate that the storm commenced on 15 May 2005 at 02:39 UT. The main phase of the storm commenced at 06:00 UT on 15 May with a sudden southward turning of the Z-component of interplanetary magnetic field (IMF-Bz) and subsequent decrease in SYM-H index. The dawn-to-dusk convection electric field of high latitude origin penetrated to low and equatorial latitudes simultaneously as corroborated by the magnetometer data from the Indian zone. Subsequent northward turning of the IMF-Bz, and the penetration of the dusk-to-dawn electric field over the dip equator is also discernible. Response of the low latitude ionosphere to this storm may be characterized in terms of (i) enhanced background level of VTEC as compared to the mean VTEC, (ii) peaks in VTEC and foF2 within two hours of prompt penetration of electric field and (iii) wave-like modulations in VTEC and sudden enhancement in hmF2 within 4–5 h in to the storm. These features have been explained in terms of the modified fountain effect, local low latitude electrodynamic response to penetration electric field and the TIDs, respectively. The study reveals a strong positive ionospheric storm in the Indian zone on 15 May 2005. Consequences of such major ionospheric storms on the systems that use satellite based navigation solutions in low latitude, are also discussed.
THEMIS observation of a substorm event on 04:35, 22 February 2008
Authors: Liu J. , Angelopoulos V. , Frey H. , McFadden J. , Larson D. , Glassmeier K. , Mende S. , Russell C. T. , Rae I. J. , Murphy K. R. , Apatenkov S.
Published: June 2010

Abstract. We report on THEMIS in-situ and ground-based observations during a substorm between 04:30~04:50 UT on 22 February 2008. The spacecraft (probes) were aligned along the tail between XGSM=−5 RE to −25 RE. The most distant probe P1 (X=−24.5 RE) detected two successive tailward moving bipolar magnetic structures. P2 (X=−18 RE), P3 (X=−11 RE), P4 (X=−10.5 RE) all captured signatures related to the Earthward movement of a magnetic structure. THEMIS ground stations and all-sky imagers also recorded Pi2 pulsations and a sudden brightening in a white-light auroral imager followed by poleward expansion. We perform a detailed timing analysis of probe and ground-based data and reconstruct the time sequence of phenomena during this substorm. The earliest sign of substorm onset was the bipolar perturbation in the northward component of the magnetic field (interpreted as the result of reconnection onset) at P1 at 04:35:16 UT and corresponding magnetic perturbation at P2 at 04:35:14 UT. Auroral onset was seen at or before 04:36:55 UT, consistent with the visual onset of high-latitude magnetic pulsations at around that time. Earthward flows at P3 and P4 seen at ~04:36:03 UT, and dipolarization onset at ~04:36:50 UT, were observed at almost the same time as the ground onset signature, implying that near-Earth dipolarization happened in the aftermath of tail reconnection but not significantly ahead of the auroral intensification. Reconnection in the tail preceded ground onset and near-Earth dipolarization (current disruption) by ~2 min. Two reconnection pulses (the first one weaker than the second one) accompanied by correlative increases of cumulative magnetic flux transfer into the reconnection region were observed. A direct association of the reconnection pulses with two auroral intensifications can be made, suggesting that tail reconnection, like the auroral expansion, advances in steps rather than continuously.
Profile of the horizontal wind variance near the ground in near neutral flow – K-theory and the transport of the turbulent kinetic energy
Authors: Yahaya S. , Frangi J. P.
Published: June 2010

Abstract. This paper deals with the characteristics of the atmospheric turbulent flow in the vicinity of the ground, and particularly with the profile of the horizontal wind variance. The study is based on experimental measurements performed with fast cup anemometers located near the ground at 5 different levels (from 0.25 to 4 m) and sampled at 1 Hz. The experiment was carried over two agricultural plots with various tillage treatments in a fallow semiarid area (Central Aragon, Spain). The results of this study reveal that near the ground surface and under moderate wind, the horizontal wind variance logarithmically increases with height, in direct relationship with the friction velocity and the roughness length scale. A theoretical development has allowed us to link this behaviour to the modeling of the turbulent kinetic energy (TKE) transport through the eddy diffusivity. Thus, the study proposes a formulation of the similarity universal function of the horizontal wind variance. Besides, the formulation offers a new method for the determination of the friction velocity and the roughness length scale and can be used for the evaluation of the TKE transport rate.
The presence of large sunspots near the central solar meridian at the times of major geomagnetic storms
Authors: Willis D. M. , Henwood R. , Stephenson F. R.
Published: June 2010

Abstract. A further study is made of the validity of a technique developed by the authors to identify historical occurrences of intense geomagnetic storms, which is based on finding approximately coincident observations of sunspots and aurorae recorded in East Asian histories. Previously, the validity of this technique was corroborated using scientific observations of aurorae in Japan during the interval 1957–2004 and contemporaneous white-light images of the Sun obtained by the Royal Greenwich Observatory, the Big Bear Solar Observatory, the Debrecen Heliophysical Observatory, and the Solar and Heliospheric Observatory spacecraft. The present investigation utilises a list of major geomagnetic storms in the interval 1868–2008, which is based on the magnitude of the AA* magnetic index, and reconstructed solar images based on the sunspot observations acquired by the Royal Greenwich Observatory during the shorter interval 1874–1976. It is found that a sunspot large enough to be seen with the unaided eye by an "experienced" observer was located reasonably close to the central solar meridian for almost 90% of these major geomagnetic storms. Even an "average" observer would easily achieve a corresponding success rate of 70% and this success rate increases to about 80% if a minority of ambiguous situations are interpreted favourably. The use of information on major geomagnetic storms, rather than modern auroral observations from Japan, provides a less direct corroboration of the technique for identifying historical occurrences of intense geomagnetic storms, if only because major geomagnetic storms do not necessarily produce auroral displays over East Asia. Nevertheless, the present study provides further corroboration of the validity of the original technique for identifying intense geomagnetic storms. This additional corroboration of the original technique is important because early unaided-eye observations of sunspots and aurorae provide the only possible means of identifying individual geomagnetic storms during the greater part of the past two millennia.
Circulation of the thermocline salinity maximum waters off the Northern Brazil as inferred from in situ measurements and numerical results
Authors: Silva A. C. , Bourles B. , Araujo M.
Published: June 2010

Abstract. High resolution hydrographic observations of temperature and salinity are used to analyse the subsurface circulation along the coast of North Brazil, off the Amazon mouth, between 2° S and 6° N. Observations are presented from four cruises carried out in different periods of the year (March–May 1995, May–June 1999, July–August 2001 and October–November 1997). Numerical model outputs complement the results of the shipboard measurements, and are used to complete the descriptions of mesoscale circulation. The Salinity Maximum Waters are here analyzed, principally in order to describe the penetration of waters originating in the Southern Hemisphere toward the Northern Hemisphere through the North Brazil Current (NBC)/North Brazil Undercurrent (NBUC). Our results show that, if the Equatorial Undercurrent (EUC) is fed by Northern Atlantic Waters, this contribution may only occur in the ocean interior, east of the western boundary around 100 m depth. Modeling results indicate a southward penetration of the Western Boundary Undercurrent (WBUC) below the thermocline, along the North Brazilian coast into the EUC or the North Equatorial Undercurrent (NEUC) (around 48° W–3° N). The WBUC in the region does not flow more south than 3° N. The northern waters are diverted eastward either by the NBC retroflection or by the northern edge of the associated clockwise rings. The existence of subsurface mesoscale rings associated to the NBC retroflection is evidenced, without any signature in the surface layer, so confirming earlier numerical model outputs. These subsurface anticyclones, linked to the NBC/NBUC retroflection into the North Equatorial Undercurrent and the EUC, contribute to the transport of South Atlantic high salinity water into the Northern Hemisphere.
Physical meaning of the equinoctial effect for semi-annual variation in geomagnetic activity
Authors: Yoshida A.
Published: June 2010

Abstract. Physical meaning of the equinoctial effect for semi-annual variation in geomagnetic activity is investigated based on the three-hourly am index and solar wind parameters. When the z component of the interplanetary magnetic field (IMF) in geocentric solar magnetospheric (GSM) coordinates is southward, am indices are well correlated with BsVx2, where Bs is the southward component of the IMF and Vx is the solar wind velocity in the sun-earth direction. The am-BsVx2 relationship, however, depends on the range of Vx2: the am in higher ranges of Vx2 tends to be larger than am in lower ranges of Vx2 for the same value of BsVx2 for both equinoctial and solstitial epochs. Using the data sets of the same Vx2 range, it is shown that distribution of points in the am-BsVx2 diagram at the solstitial epochs overlaps with that at the equinoctial epochs and the average am values in each BsVx2 bin in solstitial epochs are closely consistent with those in equinoctial epochs, if Vx2 for each point at solstices are reduced to Vx2sin2 (Ψ) where Ψ is the geomagnetic colatitude of the sub-solar point. Further, it is shown that monthly averages of the am index in the long period is well correlated with the values of sin2(ψ) for the middle day of each month. These findings indicate that the factor that contributes to the generation of geomagnetic disturbance is not the velocity of the solar wind, but the component of the solar wind velocity perpendicular to the dipole axis of the geomagnetic field. The magnitude of the perpendicular velocity component varies semi-annually even if the solar wind velocity remains constant, which is considered to be the long-missed key factor causing the equinoctial effect.
Longitudinal development of a substorm brightening arc
Authors: Shiokawa K. , Ieda A. , Nakajima A. , Sakaguchi K. , Nomura R. , Aslaksen T. , Greffen M. , Spanswick E. , Donovan E. , Mende S. B. , McFadden J. , Glassmeier K.-H. , Angelopoulos V. , Miyashita Y.
Published: June 2010

Abstract. We present simultaneous THEMIS-ground observations of longitudinal (eastward) extension of a substorm initial-brightening arc at Gillam (magnetic latitude: 65.6°) at 08:13 UT on 10 January 2008. The speed of the eastward arc extension was ~2.7 km/s. The extension took place very close to the footprints of the longitudinally separated THEMIS E and D satellites at ~12 RE. The THEMIS satellites observed field dipolarization, weak earthward flow, and pressure increase, which propagated eastward from E to D at a speed of ~50 km/s. The THEMIS A satellite, located at 1.6 RE earthward of THEMIS E, observed fluctuating magnetic field during and after the dipolarization. The THEMIS E/D observations suggest that the longitudinal extension of the brightening arc at substorm onset is caused by earthward flow braking processes which produce field dipolarization and pressure increase propagating in longitude in the near-earth plasma sheet.
Disruption of magnetospheric current sheet by quasi-electrostatic field
Authors: Liu W. W. , Liang J.
Published: June 2010

Abstract. Recent observational evidence has indicated that local current sheet disruptions are excited by an external perturbation likely associated with the kinetic ballooning (KB) instability initiating at the transition region separating the dipole- and tail-like geometries. Specifically a quasi-electrostatic field pointing to the neutral sheet was identified in the interval between the arrival of KB perturbation and local current disruption. How can such a field drive the local current sheet unstable? This question is considered through a fluid treatment of thin current sheet (TCS) where the generalized Ohm's law replaces the frozen-in-flux condition. A perturbation with the wavevector along the current is applied, and eigenmodes with frequency much below the ion gyrofrequency are sought. We show that the second-order derivative of ion drift velocity along the thickness of the current sheet is a critical stability parameter. In an E-field-free Harris sheet in which the drift velocity is constant, the current sheet is stable against this particular mode. As the electrostatic field grows, however, potential for instability arises. The threshold of instability is identified through an approximate analysis of the theory. For a nominal current sheet half-thickness of 1000 km, the estimated instability threshold is E~4 mV/m. Numerical solutions indicate that the two-fluid theory gives growth rate and wave period consistent with observations.
Development of substorm bulges during different solar wind structures
Authors: Despirak I. V. , Lubchich A. A. , Yahnin A. G. , Kozelov B. V. , Biernat H. K.
Published: June 2010

Abstract. Using data from WIND spacecraft, we investigated the difference in substorm bulge development during different types of solar wind flow: solar wind recurrent streams (RS), corotating interaction regions (CIR), magnetic clouds (MC), and the region of interaction of magnetic clouds with undisturbed solar wind (Sheath). The RS/CIR and MC/Sheath structures were examined for the periods December 1996–July 1997; January 2000–December 2000; October 2001. All available auroral substorms observed by the Ultra Violet Imager onboard the Polar spacecraft during these periods were studied. It is shown that the largest latitudinal and longitudinal sizes of the auroral bulge expansions are during CIR and Sheath intervals. We found a difference in auroral bulge parameters for MC- and RS-associated substorms. In contrast to substorms associated with RS, the latitudinal size of the auroral bulge during MC is smaller, but longitudinal size is larger. As consequence, the ratio between longitudinal and latitudinal sizes for MC-associated substorms is also larger. We suggest that the latter feature is explained by different configuration of the near-Earth magnetotail during RS and MC.
Intermittency of storms and substorms: is it related to the critical behaviour?
Authors: Dobias P. , Wanliss J. A.
Published: June 2010

Abstract. Intermittency is one of the possible means of quantifying dynamics of fractal processes. In this paper, the analysis of the intermittency of magnetospheric storms and substorms is presented. The analysis allows for a classification of the processes in terms of the power-law scaling of the magnitude of deviations of the index values from the values at quiet times (normal state), and the relative timings of occurrences of such deviations. These are expressed in terms of the co-dimension and the Fano factor. The relationship between the two is related to the nature of the processes behind the observed storm and substorm dynamics. The results suggest that there is a similarity between the two, and therefore it is possible that there are common dynamical processes behind the storms and substorms. In particular, it appears that both of them behave consistently with what would be expected for critical systems, which is consistent with the conclusions of several previous works.
Equinoctial asymmetry of a low-latitude ionosphere-thermosphere system and equatorial irregularities: evidence for meridional wind control
Authors: Maruyama T. , Saito S. , Kawamura M. , Nozaki K. , Krall J. , Huba J. D.
Published: June 2010

Abstract. Nocturnal ionospheric height variations were analyzed along the meridian of 100° E by using ionosonde data. Two ionosondes were installed near the magnetic conjugate points at low latitudes, and the third station was situated near the magnetic equator. Ionospheric virtual heights were scaled every 15 min and vertical E×B drift velocities were inferred from the equatorial station. By incorporating the inferred equatorial vertical drift velocity, ionospheric bottom heights with the absence of wind were modeled for the two low-latitude conjugate stations, and the deviation in heights from the model outputs was used to infer the transequatorial meridional thermospheric winds. The results obtained for the September and March equinoxes of years 2004 and 2005, respectively, were compared, and a significant difference in the meridional wind was found. An oscillation with a period of approximately 7 h of the meridional wind existed in both the equinoxes, but its amplitude was larger in September as compared to that in March. When the equatorial height reached the maximum level due to the evening enhancement of the zonal electric field, the transequatorial meridional wind velocity reached approximately 10 and 40 m/s for the March and September equinoxes, respectively. This asymmetry of the ionosphere-thermosphere system was found to be associated with the previously reported equinoctial asymmetry of equatorial ionospheric irregularities; the probability for equatorial irregularities to occur is higher in March as compared to that in September at the Indian to Western Pacific longitudes. Numerical simulations of plasma bubble developments were conducted by incorporating the transequatorial neutral wind effect, and the results showed that the growth time (e-folding time) of the bubble was halved when the wind velocity changed from 10 to 40 m/s.
Stormtime dynamics of the global thermosphere and equatorial ionosphere
Authors: Burke W. J. , Huang C. Y. , Sharma R. D.
Published: June 2010

Abstract. During magnetic storms the development of equatorial plasma bubbles (EPBs) and distributions of thermospheric densities are strongly influenced by the histories of imposed magnetospheric electric (εM) fields. Periods of intense EPB activity driven by penetration εM fields in the main phase are followed by their worldwide absence during recovery. A new method is applied to estimate global thermospheric energy (Eth) budgets from orbit-averaged densities measured by accelerometers on polar-orbiting satellites. During the main phase of storms Eth increases as long as the stormtime εM operates, then exponentially decays toward quiet-time values during early recovery. Some fraction of the energy deposited at high magnetic latitudes during the main phase propagates into the subauroral ionosphere-thermosphere where it affects chemical and azimuthal-wind dynamics well into recovery. We suggest a scenario wherein fossils of main phase activity inhibit full restoration of quiet-time dayside dynamos and pre-reversal enhancements of upward plasma drifts near dusk denying bottomside irregularities sufficient time to grow into EPBs.
Solar and magnetospheric forcing of the low latitude thermospheric mass density as observed by CHAMP
Authors: Müller S. , Lühr H. , Rentz S.
Published: June 2010

Abstract. We have studied the dependence of the thermospheric mass density at equatorial latitudes on the influence of various drivers. This statistical study is based on CHAMP accelerometer measurements. Our aim is to delineate the influences of the different contributions. For the isolation of the effects we make use of a dedicated data selection procedure and/or removal of disturbing effects. In a first step all readings are normalised to an altitude of 400 km. For the investigation of the solar influences only magnetically quiet days (Ap≤15) are considered. The dependence on solar flux can well be described by a linear relation within the flux range F10.7=80–240. The slope is twice as steep on the day side as on the night side. The air density exhibits clear annual and semi-annual variations with maxima at the equinoxes and a pronounced minimum around June solstice. The thermosphere maintains during quiet days a day to night mass density ratio very close to 2, which is independent of solar flux level or season. The magnetospheric input causing thermospheric density enhancement can well be parameterised by the am activity index. The low latitude density responds with a delay to changes of the index by about 3 h on the dayside and 4–5 h on the night side. The magnetospheric forcing causes an additive contribution to the quiet-time density, which is linearly correlated with the am index. The slopes of density increases are the same on the day and night sides. We present quantitative expressions for all the dependences. Our results suggest that all the studied forcing terms can be treated as linear combinations of the respective contribution.
Geomagnetic storm effects on GPS based navigation
Authors: Rama Rao P. V. S. , Gopi Krishna S. , Vara Prasad J. , Prasad S. N. V. S. , Prasad D. S. V. V. D. , Niranjan K.
Published: June 2010

Abstract. The energetic events on the sun, solar wind and subsequent effects on the Earth's geomagnetic field and upper atmosphere (ionosphere) comprise space weather. Modern navigation systems that use radio-wave signals, reflecting from or propagating through the ionosphere as a means of determining range or distance, are vulnerable to a variety of effects that can degrade the performance of the navigational systems. In particular, the Global Positioning System (GPS) that uses a constellation of earth orbiting satellites are affected due to the space weather phenomena. Studies made during two successive geomagnetic storms that occurred during the period from 8 to 12 November 2004, have clearly revealed the adverse affects on the GPS range delay as inferred from the Total Electron Content (TEC) measurements made from a chain of seven dual frequency GPS receivers installed in the Indian sector. Significant increases in TEC at the Equatorial Ionization anomaly crest region are observed, resulting in increased range delay during the periods of the storm activity. Further, the storm time rapid changes occurring in TEC resulted in a number of phase slips in the GPS signal compared to those on quiet days. These phase slips often result in the loss of lock of the GPS receivers, similar to those that occur during strong(>10 dB) L-band scintillation events, adversely affecting the GPS based navigation.
Impacts of boundary layer turbulence and land surface process parameterizations on simulated sea breeze characteristics
Authors: Miao J.-F. , Wyser K. , Chen D. , Ritchie H.
Published: June 2010

Abstract. This paper investigates the sensitivity of sea breeze (SB) simulations to combinations of boundary-layer turbulence and land-surface process parameterizations implemented in the MM5 mesoscale meteorological mode for an observed SB case over the Swedish west coast. Various combinations from four different planetary boundary layer (PBL) schemes [Blackadar, Gayno-Seaman (GS), Eta, MRF], and two land surface model (LSM) schemes (SLAB, Noah) with different complexity are designed to simulate a typical SB case over the Swedish west coast. The simulations are conducted using two-way interactively nested grids. Simulated 10-m winds are compared against observed near-surface wind data from the GÖTE2001 campaign to examine the diurnal cycle of wind direction and speed for SB timing. The SB (vertical) circulation is also compared in the different experiments. The results show that the different combinations of PBL and LSM parameterization schemes result in different SB timing and vertical circulation characteristics. All experiments predict a delayed SB. The vertical component of the SB circulation varies in the experiments, among which the GS PBL scheme produces the strongest SB circulation. Evident differences between the SLAB and Noah LSMs are also found, especially in maximum of updraft and downdraft velocities of the SB vertical circulation. The results have significant implications for convective initiation, air quality studies and other environmental problems in coastal areas.
Substorm activity in Venus's magnetotail
Authors: Volwerk M. , Delva M. , Futaana Y. , Retinò A. , Vörös Z. , Zhang T. L. , Baumjohann W. , Barabash S.
Published: June 2010

Abstract. The magnetotail of the induced magnetosphere of Venus is investigated through the magnetic field and plasma data of Venus Express. A comparison is made between two neutral sheet crossings. One crossing shows the magnetic field is rather quiet and the plasma instrument indicates a change from energetic (few 100 eV) to low energy (few 10 eV) ions. The other crossing shows more dynamics in the magnetic field, including signatures that are interpreted as characteristic of a reconnection site, and the plasma instrument indicates ions that are energized to 1500 to 2000 eV, in the same magnetospheric region where in the first crossing only low energy ions showed up.
On a magnetosphere disturbed by solar wind; observations of macroelectrons
Authors: Wodnicka E. B.
Published: June 2010

Abstract. Three-dimensional electromagnetic full kinetic particle code (a version of TRISTAN) is used to study the interaction of a weakly-magnetized object with a solar wind of low density. The details of two magnetospheric processes – wave activity and energetic electrons appearing at the flanks of the magnetosphere – are presented. The results of the simulation are compared with known magnetospheric data.
High resolution observations of turbulence in the troposphere and lower stratosphere over Gadanki
Authors: Dutta G. , Ajay Kumar M. C. , Vinay Kumar P. , Rao P. V. , Bapiraju B. , Aleem Basha H.
Published: June 2010

Abstract. High resolution (150 m) wind measurements from 13–17 July 2004 by Mesosphere-Stratosphere-Troposphere (MST) radar and 15–16 July 2004 by Lower Atmospheric Wind Profiler (LAWP) have been used to study the time variation of turbulence intensity. Layers of higher turbulence are observed in the lower stratosphere on 15–16 July which give rise to mixing in the region. Enhancement in short-period gravity wave activity and turbulent layers are observed after 22:00 LT which could be due to a dry convection event that occurred at that time. The breakdown of the convectively generated high frequency waves seems to have given rise to the turbulence layers. Wind shear is found to be high above the easterly jet, but very poor correlation is observed between square of wind shear and turbulence parameters in the region. The heights of the turbulent layers in the lower stratosphere do not correlate with levels of minimum Richardson number. A monochromatic inertia gravity wave could be identified during 13–17 July 2004. A non-linear interaction between the waves of different scales as proposed by Hines (1992) might also be responsible for the breakdown and generation of turbulence layers.
E- and F-region coupling between an intense sporadic E layer and a mesoscale traveling ionospheric disturbance
Authors: Swartz W. E. , Kelley M. C. , Aponte N.
Published: June 2010

Abstract. Considerable controversy exists concerning the relative roles of the E- and F-regions in controlling irregularity formation in the mid-latitude ionosphere, largely because electric fields due to a plasma instability in one region may map to the other if the perpendicular scales are sufficiently large. In this paper we look at a particularly fortuitous data set in which both E- and F-region observations were made using incoherent scatter radar, GPS, lidar, and coherent scatter radar. In this event, a dense, patchy sporadic E layer was detected that exhibited type I (two-stream) coherent echoes while, at the same time, the F-layer plasma was highly structured with plasma drifts of hundreds of m/s. We examine this event in the context of a mesoscale traveling ionospheric disturbance. The data presentation will be followed by comparison with current theories for coupling between these two regions.