bluuebunny/crossref_metadata_2025 huggingface dataset:
Localization of wave fields in lower hybrid cavities
Authors: Tjulin A. , Eriksson A. I. , André M.
Published: June 2010

Abstract. We investigate lower hybrid wave trapping in cylindrically symmetric density depletions in the electrostatic approximation. Our investigation is inspired by previous observations of such trapping by spacecraft in the auroral region at altitudes up to about 2000km, and the recent discovery of this phenomenon at altitudes above 20000km in the inner magnetosphere. No particular shape is assumed for the density depletion, which need not be strictly zero outside some value of the radial coordinate r. Important previously known properties concerning parabolic density depletions extending to finite r are shown to hold also for arbitrary shapes and infinite extent: for a given parallel wave number kz, modes below the ambient lower hybrid frequency fLH are trapped in the density depletion (in the sense that they are evanescent outside the cavity), have a discrete spectrum and rotate in a left-handed sense, while there is a continuous spectrum of freely propagating right-handed rotating modes above fLH. New results are such that even though the density depletion may go to zero slowly with increasing r, and thus be essentially infinite in extent, there is a maximum distance within which a trapped mode with given kz and azimuthal mode number m may propagate. Furthermore, we find that for any monotonic density cavity and given kz, there is a local relation between plasma density gradient and the lowest possible frequency that can be trapped. We combine our theoretical results with spacecraft observations to find an upper bound on kz. Our examples indicate that the length of the cavities is larger than the width by a factor of at least 100.
Decoding of Barker-coded incoherent scatter measurements by means ofmathematical inversion
Authors: Damtie B. , Lehtinen M. S. , Nygrén T.
Published: June 2010

Abstract. The standard analysis of Barker-coded incoherent scatter experiments is based on a matched filter with an impulse response which is a mirror image of the code itself. The method produces small sidelobes which cause contamination from regions outside the nominal range gate. A corresponding effect is also encountered in the lag direction, where individual lag estimates are biased by the variation of the plasma autocorrelation function around the nominal lag value. The present paper introduces a new method of analysing Barker-coded experiments by means of stochastic inversion. Since it does not apply a decoding filter, it does not suffer from drawbacks caused by the sidelobes of the range ambiguity function. The method combines the profile of each full lag and a number of surrounding fractional lags into a single inversion problem. Error analysis also indicates that the statistical accuracy given by inversion is better than that obtained by means of standard decoding. Furthermore, the inversion method gives a possibility to reduce the bias due to the variation of the autocorrelation around the nominal lag. In this paper the method is described and applied to data obtained by means of the EISCAT Svalbard radar. In addition, it is shown that mathematical inversion can be used instead of the the conventional height integration. Key words. Radio science (ionospheric physics; signal processing; instruments and techniques)
Cluster survey of the high-altitude cusp properties: a three-year statistical study
Authors: Lavraud B. , Fedorov A. , Budnik E. , Grigoriev A. , Cargill P. J. , Dunlop M. W. , Rème H. , Dandouras I. , Balogh A.
Published: June 2010

Abstract. The global characteristics of the high-altitude cusp and its surrounding regions are investigated using a three-year statistical survey based on data obtained by the Cluster spacecraft. The analysis involves an elaborate orbit-sampling methodology that uses a model field and takes into account the actual solar wind conditions and level of geomagnetic activity. The spatial distribution of the magnetic field and various plasma parameters in the vicinity of the low magnetic field exterior cusp are determined and it is found that: 1) The magnetic field distribution shows the presence of an intermediate region between the magnetosheath and the magnetosphere: the exterior cusp, 2) This region is characterized by the presence of dense plasma of magnetosheath origin; a comparison with the Tsyganenko (1996) magnetic field model shows that it is diamagnetic in nature, 3) The spatial distributions show that three distinct boundaries with the lobes, the dayside plasma sheet and the magnetosheath surround the exterior cusp, 4) The external boundary with the magnetosheath has a sharp bulk velocity gradient, as well as a density decrease and temperature increase as one goes from the magnetosheath to the exterior cusp, 5) While the two inner boundaries form a funnel, the external boundary shows no clear indentation, 6) The plasma and magnetic pressure distributions suggest that the exterior cusp is in equilibrium with its surroundings in a statistical sense, and 7) A preliminary analysis of the bulk flow distributions suggests that the exterior cusp is stagnant under northward IMF conditions but convective under southward IMF conditions.
A comparison of wave mode identification techniques
Authors: Walker S. N. , Sahraoui F. , Balikhin M. A. , Belmont G. , Pinçon J. L. , Rezeau L. , Alleyne H. , Cornilleau-Wehrlin N. , André M.
Published: June 2010

Abstract. The four point measurements available from the Cluster mission enable spatiotemporal effects in data sets to be resolved. One application of these multipoint measurements is the determination of the wave vectors and hence the identification of wave modes that exist within the plasma. Prior to multi-satellite missions, wave identification techniques were based upon the interpretation of observational data using theoretically defined relations. However, such techniques are limited by the quality of the data and the type of plasma model employed. With multipoint measurements, wave modes can be identified and their wave directions determined purely from the available observations. This paper takes two such methods, a phase differencing technique and k-filtering and compares their results. It is shown that both methods can resolve the k vector for the dominant mirror mode present in the data. The phase differencing method shows that the nature of the wave environment is constantly changing and as such both methods result in an average picture of the wave environment in the period analysed. The k-filtering method is able to identify other modes that are present.
Retrieval of water vapor using SSM/I and its relation with the onset of monsoon
Authors: Singh R. P. , Dey S. , Sahoo A. K. , Kafatos M.
Published: June 2010

Abstract. The seasonal variations and interannual variability of total precipitable water (TPW) deduced from the Special Sensor Microwave Imager (SSM/I) satellite over oceanic regions of the Indian sub-continent during the years between 1988 to 1998 show characteristic behavior. The weekly patterns of TPW are found to be closely related to the dynamics of the climatic conditions and the onset date of monsoon. The present results show that the satellite monitoring of TPW may prove as a good and reliable indicator in forecasting Indian monsoon.
Characterization of VHF radar observations associated with equatorial Spread F by narrow-band optical measurements
Authors: Sekar R. , Chakrabarty D. , Narayanan R. , Sripathi S. , Patra A. K. , Subbarao K. S. V.
Published: June 2010

Abstract. The VHF radars have been extensively used to investigate the structures and dynamics of equatorial Spread F (ESF) irregularities. However, unambiguous identification of the nature of the structures in terms of plasma depletion or enhancement requires another technique, as the return echo measured by VHF radar is proportional to the square of the electron density fluctuations. In order to address this issue, co-ordinated radar backscatter and thermospheric airglow intensity measurements were carried out during March 2003 from the MST radar site at Gadanki. Temporal variations of 630.0-nm and 777.4-nm emission intensities reveal small-scale ("micro") and large-scale ("macro") variations during the period of observation. The micro variations are absent on non-ESF nights while the macro variations are present on both ESF and non-ESF nights. In addition to the well-known anti-correlation between the base height of the F-region and the nocturnal variation of thermospheric airglow intensities, the variation of the base height of the F-layer, on occasion, is found to manifest as a bottomside wave-like structure, as seen by VHF radar on an ESF night. The micro variations in the airglow intensities are associated with large-scale irregular plasma structures and found to be in correspondence with the "plume" structures obtained by VHF radar. In addition to the commonly observed depletions with upward movement, the observation unequivocally reveals the presence of plasma enhancements which move downwards. The observation of enhancement in 777.4-nm airglow intensity, which is characterized as plasma enhancement, provides an experimental verification of the earlier prediction based on numerical modeling studies.
Equatorial ionospheric zonal drift model and vertical drift statistics from UHF scintillation measurements in South America
Authors: Sheehan R. E. , Valladares C. E.
Published: June 2010

Abstract. UHF scintillation measurements of zonal ionospheric drifts have been conducted at Ancon, Peru since 1994 using antennas spaced in the magnetic east-west direction to cross-correlate geo-synchronous satellite signals. An empirical model of average drift over a wide range of Kp and solar flux conditions was constructed from successive two-dimensional fits of drift vs. the parameters and day of year. The model exhibits the typical local time trend of maximum eastward velocity in the early evening with a gradual decrease and reversal in the early morning hours. As expected, velocities at all hours increase with the solar flux and decrease with Kp activity. It was also found that vertical drifts could contribute to the variability of drift measurements to the east of Ancon at a low elevation angle. The vertical drift at the ionospheric intersection to the east can be estimated when combined with nearly overhead observations at Ancon or a similar spaced-antenna site at Antofagasta, Chile. Comparisons on five days with nearly simultaneous measurements of vertical drift by the Julia radar at Jicamarca, Peru show varying agreement with the spaced-antenna estimates. Statistical results from 1997 to 2001 generally agree with radar and satellite studies.
Longitudinal differences observed in the ionospheric F-region during the major geomagnetic storm of 31 March 2001
Authors: Sahai Y. , Fagundes P. R. , Becker-Guedes F. , Abalde J. R. , Crowley G. , Pi X. , Igarashi K. , Amarante G. M. , Pimenta A. A. , Bittencourt J. A.
Published: June 2010

Abstract. A new ionospheric sounding station using a Canadian Advanced Digital Ionosonde (CADI) was established for routine measurements by the "Universidade do Vale do Paraiba (UNIVAP)" at São José dos Campos (23.2° S, 45.9° W), Brazil, in August 2000. A major geomagnetic storm with gradual commencement at about 01:00 UT was observed on 31 March 2001. In this paper, we present and discuss salient features from the ionospheric sounding measurements carried out at S. J. Campos on the three consecutive UT days 30 March (quiet), 31 March (disturbed) and 1 April (recovery) 2001. During most of the storm period, the foF2 values showed negative phase, whereas during the two storm-time peaks, large F-region height variations were observed. In order to study the longitudinal differences observed in the F-region during the storm, the simultaneous ionospheric sounding measurements carried out at S. J. Campos, El Arenosillo (37.1° N, 6.7° W), Spain, Okinawa (26.3° N, 127.8° E), Japan and Wakkanai (45.5° N, 141.7° E), Japan, during the period 30 March-1 April 2001, have been analyzed. A comparison of the observed ionospheric parameters (h'F and foF2) in the two longitudinal zones (1. Japanese and 2. Brazilian-Spanish) shows both similarities and differences associated with the geomagnetic disturbances. Some latitudinal differences are also observed in the two longitudinal zones. In addition, global ionospheric TEC maps from the worldwide network of GPS receivers are presented, showing widespread TEC changes during both the main and recovery phases of the storm. The ionospheric sounding measurements are compared with the ASPEN-TIMEGCM model runs appropriate for the storm conditions. The model results produce better agreement during the quiet period. During the disturbed period, some of the observed F-region height variations are well reproduced by the model results. The model foF2 and TEC results differ considerably during the recovery period and indicate much stronger negative phase at all the stations, particularly at the low-latitude ones.
Geomagnetic storm and equatorial spread-F
Authors: Becker-Guedes F. , Sahai Y. , Fagundes P. R. , Lima W. L. C. , Pillat V. G. , Abalde J. R. , Bittencourt J. A.
Published: June 2010

Abstract. In August 2000, a new ionospheric sounding station was established at Sao Jose dos Campos (23.2° S, 45.9° W; dip latitude 17.6° S), Brazil, by the University of Vale do Paraiba (UNIVAP). Another ionospheric sounding station was established at Palmas (10.2° S, 48.2° W; dip latitude 5.5° S), Brazil, in April 2002, by UNIVAP in collaboration with the Lutheran University Center of Palmas (CEULP), Lutheran University of Brazil (ULBRA). Both the stations are equipped with digital ionosonde of the type known as Canadian Advanced Digital Ionosonde (CADI). In order to study the effects of geomagnetic storms on equatorial spread-F, we present and discuss three case studies, two from the ionospheric sounding observations at Sao Jose dos Campos (September and November 2000) and one from the simultaneous ionospheric sounding observations at Sao Jose dos Campos and Palmas (July 2003). Salient features from these ionospheric observations are presented and discussed in this paper. It has been observed that sometimes (e.g. 4-5 November 2000) the geomagnetic storm acts as an inhibitor (high strong spread-F season), whereas at other times (e.g. 11-12 July 2003) they act as an initiator (low strong spread-F season), possibly due to corresponding changes in the quiet and disturbed drift patterns during different seasons.
Evidence for direct solar control of the mesopause dynamics through dayglow and radar measurements
Authors: Pant T. K. , Tiwari D. , Sridharan S. , Sridharan R. , Gurubaran S. , Subbarao K. S. V. , Sekar R.
Published: June 2010

Abstract. The day-to-day measurements of the daytime intensities of hydroxyl (OH) Meinel (8-3) band airglow emissions at 731.6 and 740.2nm carried out from the equatorial station Thiruvananthapuram (8.5° N, 76.5° E, 0.5° dip) during the period of January-March 2001 have been investigated. This investigation provides evidence for the presence of a long period (≈16 days) wave modulating these intensities at the mesopause altitudes. Simultaneous radar measurements of zonal wind at ~87km, i.e. mesopause from Tirunelveli (8.7° N, 77.8° E, 0.33° dip), a location nearby, also reveal the presence of these long period oscillations. The daytime airglow and zonal wind undergo changes simultaneously. Similar modulations are seen in the solar 10.7cm flux also preceding dayglow and wind variabilities by 4-5 days. It is inferred in the present case that the changes in the solar flux are the cause of the generation of this long period wave in the atmosphere below the mesosphere. The oscillations in the measured dayglow intensities in the mesopause region and the winds at ~87km are resulting from the modulation caused by this wave in this region after a delay of 4-5 days.
Cup anemometer response to the wind turbulence-measurement of the horizontal wind variance
Authors: Yahaya S. , Frangi J. P.
Published: June 2010

Abstract. This paper presents some dynamic characteristics of an opto-electronic cup anemometer model in relation to its response to the wind turbulence. It is based on experimental data of the natural wind turbulence measured both by an ultrasonic anemometer and two samples of the mentioned cup anemometer. The distance constants of the latter devices measured in a wind tunnel are in good agreement with those determined by the spectral analysis method proposed in this study. In addition, the study shows that the linear compensation of the cup anemometer response, beyond the cutoff frequency, is limited to a given frequency, characteristic of the device. Beyond this frequency, the compensation effectiveness relies mainly on the wind characteristics, particularly the direction variability and the horizontal turbulence intensity. Finally, this study demonstrates the potential of fast cup anemometers to measure some turbulence parameters (like wind variance) with errors of the magnitude as those deriving from the mean speed measurements. This result proves that fast cup anemometers can be used to assess some turbulence parameters, especially for long-term measurements in severe climate conditions (icing, snowing or sandy storm weathers).
Longitudinal and latitudinal variations in dynamic characteristics of the MLT (70−95km): a study involving the CUJO network
Authors: Manson A. H. , Meek C. E. , Chshyolkova T. , Avery S. K. , Thorsen D. , MacDougall J. W. , Hocking W. , Murayama Y. , Igarashi K. , Namboothiri S. P. , Kishore P.
Published: June 2010

Abstract. The newly-installed MFR (medium frequency radar) at Platteville (40°N, 105°W) has provided the opportunity and impetus to create an operational network of middle- latitude MFRs stretching from 81°W–142°E. CUJO (Canada U.S. Japan Opportunity) comprises systems at London (43°N, 81°W), Platteville (40°N, 105°W), Saskatoon (52°N, 107°W), Wakkanai (45°N, 142°E) and Yamagawa (31°N, 131°E). It offers a significant 7000km longitudinal sector in the North American-Pacific region, and a useful range of latitudes (12–14°) at two longitudes. Annual climatologies involving both height and frequency versus time contour plots for periods from 8h to 30 days, show that the changes with longitude are very significant and distinctive, often exceeding the local latitudinal variations. Comparisons with models and the recent UARS-HRDI global analysis of tides are discussed. The fits of the horizontal wave numbers of the longer period oscillations are provided in unique frequency versus time contour plots and shown to be consistent with the expected dominant modes. Annual climatologies of planetary waves (16 day, 2 day) and gravity waves reveal strong seasonal and longitudinal variations. Key words. Meteorology and atmospheric dynamics (middle atmosphere dynamics; waves and tides; climatology)
F-region ionospheric perturbations in the low-latitude ionosphere during the geomagnetic storm of 25-27 August 1987
Authors: Pavlov A. V. , Fukao S. , Kawamura S.
Published: June 2010

Abstract. We have presented a comparison between the modeled NmF2 and hmF2, and NmF2 and hmF2 which were observed at the equatorial anomaly crest and close to the geomagnetic equator simultaneously by the Akita, Kokubunji, Yamagawa, Okinawa, Manila, Vanimo, and Darwin ionospheric sounders and by the middle and upper atmosphere (MU) radar (34.85° N, 136.10° E) during the 25-27 August 1987 geomagnetically storm-time period at low solar activity near 201°, geomagnetic longitude. A comparison between the electron and ion temperatures measured by the MU radar and those produced by the model of the ionosphere and plasmasphere is presented. The corrections of the storm-time zonal electric field, EΛ, from 16:30 UT to 21:00 UT on 25 August bring the modeled and measured hmF2 into reasonable agreement. In both hemispheres, the meridional neutral wind, W, taken from the HWW90 wind model and the NRLMSISE-00 neutral temperature, Tn, and densities are corrected so that the model results agree with the ionospheric sounders and MU radar observations. The geomagnetic latitude variations in NmF2 on 26 August differ significantly from those on 25 and 27 August. The equatorial plasma fountain undergoes significant inhibition on 26 August. This suppression of the equatorial anomaly on 26 August is not due to a reduction in the meridional component of the plasma drift perpendicular to the geomagnetic field direction, but is due to the action of storm-time changes in neutral winds and densities on the plasma fountain process. The asymmetry in W determines most of the north-south asymmetry in hmF2 and NmF2 on 25 and 27 August between about 01:00-01:30 UT and about 14:00 UT when the equatorial anomaly exists in the ionosphere, while asymmetries in W, Tn, and neutral densities relative to the geomagnetic equator are responsible for the north-south asymmetry in NmF2 and hmF2 on 26 August. A theory of the primary mechanisms causing the morning and evening peaks in the electron temperature, Te, is developed. An appearance, magnitude variations, latitude variations, and a disappearance of the morning Te peaks during 25-27 August are caused by variations in EΛ, thermospheric composition, Tn, and W. The magnitude of the evening Te peak and its time location are decreased with the lowering of the geomagnetic latitude due to the weakening of the effect of the plasma drift caused by W on the electron density. The difference between 25 August and 26-27 August in an appearance, magnitude and latitude variations, and a disappearance of the evening Te peak is caused by variations in W, the thermospheric composition, Tn, and EΛ.
Estimating uncertainties in incoherent scatter radar parameters from random variations in time series data
Authors: Davis C. J. , McCrea I. W.
Published: June 2010

Abstract. The variation of parameters in time series data from the EISCAT UHF incoherent scatter radar has been used to estimate the uncertainties in measurements of electron concentration, electron and ion temperature and line-of-sight ion velocity. Good agreement was found with the results of previous analyses that estimated uncertainties in velocity and electron temperature from similar data using a complementary method. By contrast, uncertainties in ion temperature and electron concentration estimated from the same time series data were found to differ from the expected theoretical values by factors of 1.8 and 2.7, respectively. It was found that the relative sizes of these uncertainties more closely matched those predicted by a previous Monte Carlo simulation than values predicted by currently accepted theoretical formulae. Although the specific measured and theoretical uncertainties reported here relate to a very simple kind of ISR experiment, the method itself is general and can be applied to data from any incoherent scatter radar.
Observational study of generation conditions of substorm-associated low-frequency AKR emissions
Authors: Olsson A. , Janhunen P. , Hanasz J. , Mogilevsky M. , Perraut S. , Menietti J. D.
Published: June 2010

Abstract. It has lately been shown that low-frequency bursts of auroral kilometric radiation (AKR) are nearly exclusively associated with substorm expansion phases. Here we study low-frequency AKR using Polar PWI and Interball POLRAD instruments to constrain its possible generation mechanisms. We find that there are more low-frequency AKR emission events during wintertime and equinoxes than during summertime. The dot-AKR emission radial distance range coincides well with the region where the deepest density cavities are seen statistically during Kp>2. We suggest that the dot-AKR emissions originate in the deepest density cavities during substorm onsets. The mechanism for generating dot-AKR is possibly strong Alfvén waves entering the cavity from the magnetosphere and changing their character to more inertial, which causes the Alfvén wave associated parallel electric field to increase. This field may locally accelerate electrons inside the cavity enough to produce low-frequency AKR emission. We use Interball IESP low-frequency wave data to verify that in about half of the cases the dot-AKR is accompanied by low-frequency wave activity containing a magnetic component, i.e. probably inertial Alfvén waves. Because of the observational geometry, this result is consistent with the idea that inertial Alfvén waves might always be present in the source region when dot-AKR is generated. The paper illustrates once more the importance of radio emissions as a powerful remote diagnostic tool of auroral processes, which is not only relevant for the Earth's magnetosphere but may be relevant in the future in studying extrasolar planets.
Characteristic properties of Nu whistlers as inferred from observations and numerical modelling
Authors: Shklyar D. R. , Chum J. , Jiříček F.
Published: June 2010

Abstract. The properties of Nu whistlers are discussed in the light of observations by the MAGION 5 satellite, and of numerically simulated spectrograms of lightning-induced VLF emissions. The method of simulation is described in full. With the information from this numerical modelling, we distinguish the characteristics of the spectrograms that depend on the site of the lightning strokes from those that are determined mainly by the position of the satellite. Also, we identify the region in the magnetosphere where Nu whistlers are observed most often, and the geomagnetic conditions favouring their appearance. The relation between magnetospherically reflected (MR) whistlers and Nu whistlers is demonstrated by the gradual transformation of MR whistlers into Nu whistlers as the satellite moves from the high-altitude equatorial region to lower altitudes and higher latitudes. The magnetospheric reflection of nonducted whistler-mode waves, which is of decisive importance in the formation of Nu whistlers, is discussed in detail.
Mesopause dynamics from the scandinavian triangle of radars within the PSMOS-DATAR Project
Authors: Manson A. H. , Meek C. E. , Hall C. M. , Nozawa S. , Mitchell N. J. , Pancheva D. , Singer W. , Hoffmann P.
Published: June 2010

Abstract. The "Scandinavian Triangle" is a unique trio of radars within the DATAR Project (Dynamics and Temperatures from the Arctic MLT (60–97km) region): Andenes MF radar (69°N, 16°E); Tromsø MF radar (70°N, 19°E) and Esrange "Meteor" radar (68°N, 21°E). The radar-spacings range from 125-270km, making it unique for studies of wind variability associated with small-scale waves, comparisons of large-scale waves measured over small spacings, and for comparisons of winds from different radar systems. As such it complements results from arrays having spacings of 25km and 500km that have been located near Saskatoon. Correlation analysis is used to demonstrate a speed bias (MF smaller than the Meteor) between the radar types, which varies with season and altitude. Annual climatologies for the year 2000 of mean winds, solar tides, planetary and gravity waves are presented, and show indications of significant spatial variability across the Triangle and of differences in wave characteristics from middle latitudes. Key words: Meteorology and atmospheric dynamics (middle atmosphere dynamics; waves and tides: instrument and techniques)
The storm-time ring current: a statistical analysis at two widely separated low-latitude stations
Authors: Francia P. , Villante U. , Adorante N. , Gonzalez W. D.
Published: June 2010

Abstract. We conducted a statistical analysis of the geomagnetic field variations during the storm main phase at two low-latitude stations, separated by several hours in magnetic local time, in order to investigate the asymmetry and longitudinal extent of the storm-time ring current. The results show evidence for an asymmetric current which typically extends from evening to noon and, during moderate solar wind electric field conditions, up to the early morning, confirming the important role of the magnetospheric convection in the ring current energization. We also analyzed a possible relationship between the local current intensity during the storm main phase and the substorm activity observed at different time delays τ with respect to the storm onset. The results show a significant anticorrelation for τ =-1h, indicating that if the substorm activity is high just before the storm, a weaker ring current develops.
The heliospheric modulation of cosmic ray boron and carbon
Authors: Potgieter M. S. , Langner U. W.
Published: June 2010

Abstract. The observed boron to carbon ratio (B/C) at Earth provides a good measure of the overall secondary to primary ratio of galactic cosmic rays. This makes B/C an important constraint and test for the validity and general applicability of theoretical and numerical models of galactic propagation and heliospheric modulation. For this purpose, the modulation of boron and carbon in the heliosphere must be understood in greater detail. The latest approach to heliospheric modulation, using a numerical model containing a termination shock, a heliosheath and particle drifts, is used to the study the modulation of the two species. This model also includes a more comprehensive set of diffusion coefficients. From this and previous work follows that the model is compatible with a variety of observations, for seven species, i.e. protons, anti-protons, electrons, positrons, helium, boron, and carbon, with the same set of parameters for both solar magnetic polarity cycles. Despite the rather flat interstellar spectrum for carbon below 100MeV/nuc, the modulated spectra at 1AU look very similar for boron and carbon, caused by adiabatic energy losses, implying that the carbon modulation should have a much larger radial gradient in the outer heliosphere below ~200-500MeV/nuc than boron. Significant modulation can be caused by the heliosheath but it is strongly dependent on energy and on the field polarity, with almost no effect at high energies to the largest effect at low energies. The solar wind termination shock has an important effect on the B to C ratio in the heliosphere, although small at Earth, during the A<0 cycle, with E<~600MeV/nuc, but it seems less significant for the A>0 cycle and with increasing tilt angles. Drift models produce different spectra for consecutive solar minimum conditions which may account for the modulation level differences between observations around 100MeV/nuc compared to around 500MeV/nuc. All factors taken into account, heliospheric modeling indicates that the interstellar spectra for B and C need further refinement around 1GeV/nuc, in order to fit observations over a wide energy range at Earth and that this refinement probably has to take into account the proposed contribution of a local interstellar carbon component. These results confirm that this numerical model with a TS can reasonably reproduce the B and C modulation between the outer boundary and Earth, making it a reasonable approximation for both polarity cycles from solar minimum to moderate solar activity.
Dayside and nightside contributions to the cross polar cap potential: placing an upper limit on a viscous-like interaction
Authors: Milan S. E.
Published: June 2010

Abstract. Observations of changes in size of the ionospheric polar cap allow the dayside and nightside reconnection rates to be quantified. From these it is straightforward to estimate the rate of antisunward transport of magnetic flux across the polar regions, quantified by the cross polar cap potential ΦPC. When correlated with upstream measurements of the north-south component of the IMF, ΦPC is found to increase for more negative Bz, as expected. However, we also find that ΦPC does not, on average, decrease to zero, even for strongly northward IMF. In the past this has been interpreted as evidence for a viscous interaction between the magnetosheath flow and the outer boundaries of the magnetosphere. In contrast, we show that this is the consequence of flows excited by tail reconnection, which is inherently uncorrelated with IMF Bz.
Meteor radar observations of atmospheric waves in the equatorial mesosphere/lower thermosphere over Ascension Island
Authors: Pancheva D. , Mitchell N. J. , Younger P. T.
Published: June 2010

Abstract. Some preliminary results about the planetary wave characteristics observed during the first seven months (October 2001-April 2002) of observations over Ascension Island (7.9°S, 14.4°W) are reported in this study. The zonal wind is dominated by the 3–7-day waves, while the meridional component – by the quasi-2-day wave. Two wave events in the zonal wind are studied in detail: a 3–4-day wave observed in the end of October/November and the 3–6-day wave in January/February. The moderate 3- and 3.2-day waves are interpreted as an ultra-fast Kelvin wave, while for the strong 4-day wave we are not able to make a firm decision. The 6-day wave is interpreted as a Doppler-shifted 5-day normal mode, due to its very large vertical wavelength (79km). The quasi-2-day wave seems to be present almost continuously in the meridional wind, but the strongest bursts are observed mainly in December and January. The observed period range is large, from 34 to 68h, with some clustering around 43–44 and 50h. The estimated vertical wavelengths indicate shorter lengths during the equinoxes, in the range of 25-30km, and longer ones, ∼40–50km, in January/February, when the 48-h wave is strongest. Key words. Meteorology and atmospheric dynamics middle atmosphere dynamics, waves and tides)
Stochastic Maximum Likelihood (SML) parametric estimation of overlapped Doppler echoes
Authors: Boyer E. , Petitdidier M. , Larzabal P.
Published: June 2010

Abstract. This paper investigates the area of overlapped echo data processing. In such cases, classical methods, such as Fourier-like techniques or pulse pair methods, fail to estimate the first three spectral moments of the echoes because of their lack of resolution. A promising method, based on a modelization of the covariance matrix of the time series and on a Stochastic Maximum Likelihood (SML) estimation of the parameters of interest, has been recently introduced in literature. This method has been tested on simulations and on few spectra from actual data but no exhaustive investigation of the SML algorithm has been conducted on actual data: this paper fills this gap. The radar data came from the thunderstorm campaign that took place at the National Astronomy and Ionospheric Center (NAIC) in Arecibo, Puerto Rico, in 1998.
Aspect sensitivity in the VHF radar backscatters studied using simultaneous observations of Gadanki MST radar and GPS sonde
Authors: Ghosh A. K. , Das S. S. , Patra A. K. , Rao D. N. , Jain A. R.
Published: June 2010

Abstract. Simultaneous observations made on four days using the MST radar and GPS-sonde at Gadanki (13.5° N, 79.2° E), a tropical station in India, are presented to address the aspect sensitivity of radar backscatters observed at different heights. The observations show that wherever stability parameter N2 is high, vertical shear of horizontal wind is low and Richardson number (Ri) is high, the aspect sensitivity is high indicating that the aspect sensitive radar backscatters are due to thermal structures in the atmosphere. Such a case can be seen very clearly in the upper troposphere and lower stratosphere. At some heights, where N2 is high, Ri is high, but shears are relatively weak, the aspect sensitivity is found to almost disappear, indicating that some amount of shear provides favorable conditions for causing aspect sensitivity. Aspect sensitivity does not occur at all where N2 is low or negative and Ri is low in spite of wind shear being either high or low, indicating that the regions are well mixed and hence turbulent. The study also shows a power difference in the symmetric beams. A case study on this aspect suggests that this asymmetry is due to the tilting of layers by the action of atmospheric waves. There is indication that these waves are generated through Kelvin-Helmholtz-instability (KHI).
Temporal evolution of two auroral arcs as measured by the Cluster satellite and coordinated ground-based instruments
Authors: Aikio A. T. , Mursula K. , Buchert S. , Forme F. , Amm O. , Marklund G. , Dunlop M. , Fontaine D. , Vaivads A. , Fazakerley A.
Published: June 2010

Abstract. The four Cluster s/c passed over Northern Scandinavia on 6 February 2001 from south-east to north-west at a radial distance of about 4.4 RE in the post-midnight sector. When mapped along geomagnetic field lines, the separation of the spacecraft in the ionosphere was confined to within 110km in latitude and 50km in longitude. This constellation allowed us to study the temporal evolution of plasma with a time scale of a few minutes. Ground-based instrumentation used involved two all-sky cameras, magnetometers and the EISCAT radar. The main findings were as follows. Two auroral arcs were located close to the equatorward and poleward edge of a large-scale density cavity, respectively. These arcs showed a different kind of a temporal evolution. (1) As a response to a pseudo-breakup onset, both the up- and downward field-aligned current (FAC) sheets associated with the equatorward arc widened and the total amount of FAC doubled in a time scale of 1–2min. (2) In the poleward arc, a density cavity formed in the ionosphere in the return (downward) current region. As a result of ionospheric feedback, a strongly enhanced ionospheric southward electric field developed in the region of decreased Pedersen conductance. Furthermore, the acceleration potential of ionospheric electrons, carrying the return current, increased from 200 to 1000eV in 70s, and the return current region widened in order to supply a constant amount of return current to the arc current circuit. Evidence of local acceleration of the electron population by dispersive Alfvén waves was obtained in the upward FAC region of the poleward arc. However, the downward accelerated suprathermal electrons must be further energised below Cluster in order to be able to produce the observed visible aurora. Both of the auroral arcs were associated with broad-band ULF/ELF (BBELF) waves, but they were highly localised in space and time. The most intense BBELF waves were confined typically to the return current regions adjacent to the visual arc, but in one case also to a weak upward FAC region. BBELF waves could appear/disappear between s/c crossings of the same arc separated by about 1min. Key words. Ionosphere (electric fields and currents) – Magnetospheric physics (auroral phenomena; magnetosphereionosphere interactions)
Magnetic local time, substorm, and particle precipitation-related variations in the behaviour of SuperDARN Doppler spectral widths
Authors: Parkinson M. L. , Chisham G. , Pinnock M. , Dyson P. L. , Devlin J. C.
Published: June 2010

Abstract. Super Dual Auroral Radar Network (DARN) radars often detect a distinct transition in line-of-sight Doppler velocity spread, or spectral width, from <50ms–1 at lower latitude to >200ms–1 at higher latitude. They also detect a similar boundary, namely the range at which ionospheric scatter with large spectral width suddenly commences (i.e. without preceding scatter with low spectral width). The location and behaviour of the spectral width boundary (SWB) (and scatter boundary) and the open-closed magnetic field line boundary (OCB) are thought to be closely related. The location of the nightside OCB can be inferred from the poleward edge of the auroral oval determined using energy spectra of precipitating particles measured on board Defence Meteorology Satellite Program (DMSP) satellites. Observations made with the Halley SuperDARN radar (75.5° S, 26.6° W, geographic; –62.0°Λ) and the Tasman International Geospace Environment Radar (TIGER) (43.4° S, 147.2° E; –54.5°Λ) are used to compare the location of the SWB with the DMSP-inferred OCB during 08:00 to 22:00 UT on 1 April 2000. This study interval was chosen because it includes several moderate substorms, whilst the Halley radar provided almost continuous high-time resolution measurements of the dayside SWB location and shape, and TIGER provided the same in the nightside ionosphere. The behaviour of the day- and nightside SWB can be understood in terms of the expanding/contracting polar cap model of high-latitude convection change, and the behaviour of the nightside SWB can also be organised according to substorm phase. Previous comparisons with DMSP OCBs have proven that the radar SWB is often a reasonable proxy for the OCB from dusk to just past midnight (Chisham et al., 2004). However, the present case study actually suggests that the nightside SWB is often a better proxy for the poleward edge of Pedersen conductance enhanced by hot particle precipitation in the auroral zone. Simple modeling implies that the large spectral widths must be caused by ~10-km scale velocity fluctuations. Key words. Ionosphere (auroral ionosphere; ionospheremagnetosphere interactions) – Magnetospheric physics (storms and substorms)
Repetitive X-line Hall current structures over the dawnside ionosphere induced by successive exo-magnetosphere pressure pulses
Authors: Sarafopoulos D. V.
Published: June 2010

Abstract. This work is a synthesis of observational and magnetosphere model produced results. In the first place, we observe geographic latitude-dependent delays in signature arrival times at dawnside ground magnetograms. We use the IMAGE chain ground station magnetograms associated with in-situ observations obtained from the Wind, ACE, IMP-8, LANL 97A and Cluster satellites. We demonstrate that the structures under study of ground signatures are directly dictated by successive exo-magnetosphere pressure pulses applied along the magnetopause. In one case, using the Cluster configuration, we determine the magnetopause surface wave velocity ~210km·s–1, the wavelength λ≈16 RE and the azimuthal wave number m≅6. For this case, via Tsyganenko's T96 model, the positions from the ground stations are traced out along the magnetic field lines, and their conjugate points over the XYGSM plane are determined. In this way, we have found that especially the conjugate points corresponding to the highest latitude stations are systematically dispersed along the X-axis (ranging up to ~8 RE and consequently, each of these points is associated with a different amount of magnetopause displacement dictated by the pressure wave. The local magnetopause compressions produce increments of the cross tail electric field, which is directly mapped over the ionosphere plane, where successive X-line Hall current structures are developed. Key words. Ionosphere (electric fields and currents) – Magnetospheric physics (Magnetosphere-ionosphere interactions, Electric fields)
Local wavelet correlation: applicationto timing analysis of multi-satellite CLUSTER data
Authors: Soucek J. , Dudok de Wit T. , Dunlop M. , Décréau P.
Published: June 2010

Abstract. Multi-spacecraft space observations, such as those of CLUSTER, can be used to infer information about local plasma structures by exploiting the timing differences between subsequent encounters of these structures by individual satellites. We introduce a novel wavelet-based technique, the Local Wavelet Correlation (LWC), which allows one to match the corresponding signatures of large-scale structures in the data from multiple spacecraft and determine the relative time shifts between the crossings. The LWC is especially suitable for analysis of strongly non-stationary time series, where it enables one to estimate the time lags in a more robust and systematic way than ordinary cross-correlation techniques. The technique, together with its properties and some examples of its application to timing analysis of bow shock and magnetopause crossing observed by CLUSTER, are presented. We also compare the performance and reliability of the technique with classical discontinuity analysis methods. Key words. Radio science (signal processing) – Space plasma physics (discontinuities; instruments and techniques)
A joint Cluster and ground-based instruments study of two magnetospheric substorm events on 1 September 2002
Authors: Draper N. C. , Lester M. , Wild J. A. , Milan S. E. , Provan G. , Grocott A. , Cowley S. W. H. , Bogdanova Y. , Dewhurst J. P. , Fazakerley A. N. , Davies J. A. , Bosqued J.-M.
Published: June 2010

Abstract. We present a coordinated ground- and space-based multi-instrument study of two magnetospheric substorm events that occurred on 1 September 2002, during the interval from 18:00 UT to 24:00 UT. Data from the Cluster and Polar spacecraft are considered in combination with ground-based magnetometer and HF radar data. During the first substorm event the Cluster spacecraft, which were in the Northern Hemisphere lobe, are to the west of the main region affected by the expansion phase. Nevertheless, substorm signatures are seen by Cluster at 18:25 UT (just after the expansion phase onset as seen on the ground at 18:23 UT), despite the ~5 RE} distance of the spacecraft from the plasma sheet. The Cluster spacecraft then encounter an earthward-moving diamagnetic cavity at 19:10 UT, having just entered the plasma sheet boundary layer. The second substorm expansion phase is preceded by pseudobreakups at 22:40 and 22:56 UT, at which time thinning of the near-Earth, L=6.6, plasma sheet occurs. The expansion phase onset at 23:05 UT is seen simultaneously in the ground magnetic field, in the magnetotail and at Polar's near-Earth position. The response in the ionospheric flows occurs one minute later. The second substorm better fits the near-Earth neutral line model for substorm onset than the cross-field current instability model. Key words. Magnetospheric physics (Magnetosphereionosphere interactions; Magnetic reconnection; Auroral phenomenon)
A statistical study of unstable particle populations in the global ringcurrent and their relation to the generation of high m ULF waves
Authors: Baddeley L. J. , Yeoman T. K. , Wright D. M. , Trattner K. J. , Kellet B. J.
Published: June 2010

Abstract. The first statistical study of the unstable proton populations which contain "free energy" required to drive small-scale poloidal mode ULF waves in the magnetosphere between L-shell locations of 6 and 9 is presented. The data examined are all in the form of Ion Distribution Functions (IDFs) covering a particle energy range of 0.025keV to 328keV, amassed over 2.5 years from the TIMAS and CAMMICE (MICS) instruments on-board the Polar spacecraft. Any free energy which is available to drive a resonant wave mode manifests itself as a positive gradient region in the IDF. A new analysis technique applied to the data, allows for the first time, the amount of free energy contained in each IDF to be quantified. The results show that IDFs are a common occurrence in the magnetosphere at these L-shells, although they are most common in the dawn/pre-noon sector. Lower energy (10–45keV) protons are the most commonly observed unstable populations and also contain the largest amounts of free energy (>1010J). Positive gradient regions at higher energies (>100keV) are rarely observed and also contain greatly reduced free energies (<109J). Key words. Magnetospheric physics (Energetic particles, trapped; MHD waves and instabilities) – Space plasma physics (wave-particle interactions)