bluuebunny/crossref_metadata_2025 huggingface dataset:
Meteor head echo altitude distributions and the height cutoff effect studied with the EISCAT HPLA UHF and VHF radars
Authors: Westman A. , Wannberg G. , Pellinen-Wannberg A.
Published: June 2010

Abstract. Meteor head echo altitude distributions have been derived from data collected with the EISCAT VHF (224MHz) and UHF (930MHz) high-power, large-aperture (HPLA) radars. At the high-altitude end, the distributions cut off abruptly in a manner reminiscent of the trail echo height ceiling effect observed with classical meteor radars. The target dimensions are shown to be much smaller than both the VHF and the UHF probing wavelengths, but the cutoff heights for the two systems are still clearly different, the VHF cutoff being located several km above the UHF one. A single-collision meteor-atmosphere interaction model is used to demonstrate that meteors in the (1.3–7.2)µg mass range will ionise such that critical electron density at 224MHz is first reached at or around the VHF cutoff altitude and critical density at 930MHz will be reached at the UHF cutoff altitude. The observed seasonal variation in the cutoff altitudes is shown to be a function of the seasonal variation of atmospheric density with altitude. Assuming that the electron density required for detection is in the order of the critical density, the abrupt altitude cutoffs can be explained as a consequence of the micrometeoroid joint size-speed distribution dropping off so fast at the large-mass, high-velocity end that above a certain altitude the number of detectable events becomes vanishingly small. Conversely, meteors at the low-mass end of the distribution will be gradually retarded such that the ionisation they generate never reaches critical density. These particles will remain unobservable.Key words. Radio science (instruments and techniques) – Interplatery physics (interplanetary dust) – General or miscellaneous (new fields)
Freak waves - more frequent than rare!
Authors: Liu P. C. , Pinho U. F.
Published: June 2010

Abstract. Contrary to the widely held notion that considers the occurrence of freak waves in the ocean as being rare, from an examination of five years of wave measurements made in the South Atlantic Ocean, we found the occurrence of freak waves is actually more frequent than rare. Key words. Meteorology and atmospheric dynamics (waves and tides) – Oceanography: physical (surface waves and tides; air-sea interaction)
Average characteristics of the midtail plasma sheet in different dynamic regimes of the magnetosphere
Authors: Dmitrieva N. P. , Sergeev V. A. , Shukhtina M. A.
Published: June 2010

Abstract. We study average characteristics of plasma sheet convection in the middle tail during different magnetospheric states (Steady Magnetospheric Convection, SMC, and substorms) using simultaneous magnetotail (Geotail, 15-35 RE downtail) and solar wind (Wind spacecraft) observations during 3.5 years. (1) A large data set allowed us to obtain the average values of the plasma sheet magnetic flux transfer rate (Ey and directly compare it with the dayside transfer rate (Emod for different magnetospheric states. The results confirm the magnetic flux imbalance model suggested by Russell and McPherron (1973), namely: during SMC periods the day-to-night flux transport rate equals the global Earthward plasma sheet convection; during the substorm growth phase the plasma sheet convection is suppressed on the average by 40%, whereas during the substorm expansion phase it twice exceeds the day-to-night global flux transfer rate. (2) Different types of substorms were revealed. About 1/3 of all substorms considered displayed very weak growth in the tail lobe magnetic field before the onset. For these events the plasma sheet transport was found to be in a balance with the day-to-night flux transfer, as in the SMC events. However, the lobe magnetic field value in these cases was as large as that in the substorms with a classic growth phase just before the onset (both values exceed the average level of the lobe field during the SMC). Also, in both groups similar configurational changes (magnetic field stretching and plasma sheet thinning) were observed before the substorm onset. (3) Superimposed epoch analysis showed that the plasma sheet during the late substorm recovery phase has the characteristics similar to those found during SMC events, the SMC could be a natural magnetospheric state following the substorm.
Magnetic turbulent spectra in the magnetosheath: new insights
Authors: Sahraoui F. , Belmont G. , Pinçon J. L. , Rezeau L. , Balogh A. , Robert P. , Cornilleau-Wehrlin N.
Published: June 2010

Abstract. The spectrum of the magnetic fluctuations measured by the Cluster satellites in the inner magnetosheath is investigated using the k-filtering technique. On a case study, it is shown first that the wave vectors calculated from the Flux Gate Magnetometer (FGM) data fit well with those determined from the Spatio-Temporal Analysis of Field Fluctuations (STAFF) data for their common range of frequency, which allows one to confirm that the high pass filter applied to STAFF data does not alter the spatial characteristics of its spectra. Both analyses confirm the dominance of the mirror mode for frequencies up to 1.4Hz. Furthermore, by comparing the experimental charateristics of the identified mirror mode to the prediction of the linear theory, it is shown that the predicted maximum growth rate is observed in the frequency range 0-0.15Hz, i.e. the FGM range. All the rest of the mirror mode, identified for higher frequencies is more likely to be a non linear extension of the most instable one. This cascade on the spatial scales is, in turn, observed in the satellite frame as a temporal spread due to Doppler shift. Further implications on the real nature of the observed spectrum are discussed.
Bow shock specularly reflected ions in the presence of low-frequency electromagnetic waves: a case study
Authors: Meziane K. , Mazelle C. , Wilber M. , LeQuéau D. , Eastwood J. P. , Rème H. , Dandouras I. , Sauvaud J. A. , Bosqued J. M. , Parks G. K. , Kistler L. M. , McCarthy M. , Klecker B. , Korth A. , Bavassano-Cattaneo M.-B. , Lundin R. , Balogh A.
Published: June 2010

Abstract. An energetic ion (E≤40) event observed by the CLUSTER/CIS experiment upstream of the Earth's bow shock is studied in detail. The ion event is observed in association with quasi-monochromatic ULF MHD-like waves, which we show modulate the ion fluxes. According to three statistical bow shock position models, the Cluster spacecrafts are located at ~0.5 Re from the shock and the averaged bow shock θBn0 is about ~30°. The analysis of the three-dimensional angular distribution indicates that ions propagating roughly along the magnetic field direction are observed at the onset of the event. Later on, the angular distribution is gyrophase-bunched and the pitch-angle distribution is peaked at α0~θBn0, consistent with the specular reflection production mechanism. The analysis of the waves shows that they are left-handed in the spacecraft frame of reference (right-handed in the solar wind frame) and propagate roughly along the ambient magnetic field; we have found that they are in cyclotron-resonance with the field-aligned beam observed just upstream. Using properties of the waves and particles, we explain the observed particle flux-modulation in the context of θBn changes at the shock caused by the convected ULF waves. We have found that the high count rates coincide with particles leaving the shock when θBn angles are less than ~40°, consistent with the specular reflection hypothesis as the production mechanism of ions.
Cluster observes formation of high-beta plasma blobs
Authors: Haerendel G. , Georgescu E. , Glassmeier K. H. , Klecker B. , Bogdanova Y. V. , Rème H. , Frey H.
Published: June 2010

Abstract. Late in a sequence of four moderate substorms on 26 July 2001, Cluster observed periods of a few minutes durations of high-beta plasma events (B<10nT, β=2-30), connected with dipolarizations of the magnetic field. Cluster was located near 02:45 MLT, at R=19RE and at about 5°N GSM. These events began late in the recovery phase of the second and about 5min before onset of the third substorm and lasted for three hours, way beyond the recovery phase of the fourth substorm. The most remarkable observation is that the onset coincided with the arrival of energetic (E~7keV) O+ ions and energetic electrons obviously from the ionosphere, which tended to dominate the plasma composition throughout the remaining time. The magnetic flux and plasma transport is continuously directed equatorward and earthward, with oscillatory east-west movements superposed. Periods of the order of 5-10min and strong correlations between the magnetic elevation angle and log β (correlation coefficient 0.78) are highly reminiscent of the high-beta plasma blobs discovered with Equator-S and Geotail between 9 and 11RE in the late night/early morning sector (Haerendel et al., 1999). We conclude that Cluster observed the plasma blob formation in the tail plasma sheet, which seems to occur predominantly in the recovery and post-recovery phases of substorms. This is consistent with the finding of Equator-S and Geotail. The origin is a pulsed earthward plasma transport with velocity amplitudes of only several tens of km/s.
Multipoint investigation of the source region of storm-time chorus
Authors: Santolík O. , Gurnett D. A. , Pickett J. S.
Published: June 2010

Abstract. In this case study we investigate the source region of whistler-mode chorus located close to the geomagnetic equator at a radial distance of 4.4 Earth radii. We use measurements from the four Cluster spacecraft at separations of less than a few hundreds of km, recorded during the geomagnetic storm of 18 April 2002. The waveforms of the electric field fluctuations were obtained by the WBD instruments in the frequency range 50Hz-9.5kHz. Using these data, we calculate linear and rank correlation coefficients of the frequency averaged power-spectral density measured by the different spacecraft. Those coefficients have been recently shown to decrease with spacecraft separation distance perpendicular to the static magnetic field cchor03 with a characteristic scale length of 100km. We find this characteristic scale varying between 60 and 200km for different data intervals inside the source region. We examine possible explanations for the observed large scatter of the correlation coefficients, and we suggest a simultaneously acting effect of random positions of locations at which the individual chorus wave packets are generated. The statistical properties of the observations are approximately reproduced by a simple 2-D model of the source region, assuming a perpendicular half-width of 35km (approximately one wavelength of the whistler-mode waves) for the distribution of power radiated from individual active areas.
Observation of continuum radiations from the Cluster fleet: first results from direction finding
Authors: Décréau P. M. E. , Ducoin C. , Le Rouzic G. , Randriamboarison O. , Rauch J.-L. , Trotignon J.-G. , Vallières X. , Canu P. , Darrouzet F. , Gough M. P. , Buckley A. M. , Carozzi T. D.
Published: June 2010

Abstract. The Cluster fleet offers the first possibility of comparing non-thermal terrestrial continuum radiation from similarly equipped nearby observation points. A very rich data set has already been acquired on the Cluster polar orbit, between 4 and 19 Earth radii geocentric distances, and preliminary analysis has been carried out on these emissions. We focus in this paper on direction finding performed from all four spacecraft as a means to locate the position of the sources of this continuum radiation. Directions are derived from spin modulation properties, under the usual analysis assumptions of the wave vector of the radiation lying in the plane containing the spin axis and the antenna position at electric field minimum. All the spin axes of the four Cluster spacecraft are aligned perpendicular to the ecliptic, thus the aligned spacecraft antenna spin planes provide redundant 2-D views of the propagation path of the radiation and source location. Convincing 2-D triangulation results have been obtained in the vicinity of the source region. In addition, the out of spin plane component of the wave vector reveals itself to a certain extent through directivity characteristics compared at different distances of the spin plane to the ecliptic. The four case events studied (two of them taken near apogee, the other two near perigee) have confirmed general properties derived from previous observations: trapping in the lower frequency range, radiation escaping into the magnetosheath region in the higher frequency range. All propagation directions are compatible with source positions in the plasmapause region, however, at a significant distance from the equator in one case. Our observations have also revealed new properties, like the importance of small-scale density irregularities in the local amplification of continuum radiation. We conclude that more detailed generation and propagation models are needed to fit the observations.
Comparison of the measured and modeled electron densities, and electron and ion temperatures in the low-latitude ionosphere during 19-21 March 1988
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, Chung-Li, Manila, Vanimo, and Darwin ionospheric sounders and by the middle and upper atmosphere (MU) radar at Shigaraki (34.85°N, 136.10°E, Japan) during the 19-21 March 1988 geomagnetically quiet time period at moderate solar activity near approximately the same geomagnetic meridian of 201°. A comparison between the electron, Te, and ion, Ti, temperatures measured by the MU radar and those produced by the model of the ionosphere and plasmasphere is presented for 19-21 March 1988. It is shown that there is a large disagreement between the measured and modeled hmF2 from about 07:00 UT to about 11:00 UT if the equatorial ExB drift given by Scherliess and Fejer (1999) is used. The required equatorial upward ExB drift is weaker from 03:14 UT to 11:14 UT than that given by Scherliess and Fejer (1999) for the studied time period. The required modification of the ExB drift weakens the effect of the fountain in NmF2 bringing the modeled and measured hmF2 and NmF2, into reasonable agreement. The depth of the equatorial NmF2 trough in the calculated NmF2 is approximately consistent with the measured depth if the modified equatorial ExB drift is used. It has been found that the north-south asymmetries in the observed NmF2 and hmF2 about the geomagnetic equator are mainly caused by the asymmetry in the neutral wind about the geomagnetic equator. In the Northern Hemisphere, the meridional neutral wind taken from the HWW90 wind model and the NRLMSISE-00 atomic oxygen density are corrected so that the model results agree with the ionospheric sounders and MU radar observations. A theory of the primary mechanisms causing the latitude dependence of the morning and evening peaks in Te is developed. The latitude dependence of the magnitudes of these peaks in Te is interpreted in terms of the corresponding dependence of the electron density. The relative role of the ExB drift and the plasma drift caused by the neutral wind in the formation and the dependence of the magnitudes of the morning and evening electron temperature peaks on the geomagnetic latitude is studied.
Brightness variations of the northern 630nm intertropical arc and the midnight pressure bulge over Eritrea
Authors: Wiens R. H. , Habtemichael S. , Andemariam F. , Welday K. , Criswick J. , Brown S. , Sargoytchev S.
Published: June 2010

Abstract. The nightglow brightness at 630nm from the thermospheric O(1D) layer was monitored nightly at Asmara, Eritrea (15.4° N, 39.9° E, 7° N dip) with an all-sky imager. Averages of north-south strips of the images enabled contour plots of brightness on a latitude vs. local time grid. The contours show the movement of the intertropical arc southward before midnight, staying just north of Asmara after midnight, and gradually brightening to a maximum at 02:00h local civil time, 02:00 LT, after which it disappears before dawn. It is argued that all features of the plots can be explained by known mechanisms capable of driving ions along magnetic field lines, including the fountain effect, summer to winter transequatorial winds, and the midnight pressure bulge. The 02:00 LT brightness maximum is the most striking and the most persistent feature in the data. The persistence of the location of the 02:00 LT brightening is attributed to a pressure bulge centered on the geographic equator at midnight and extending to higher latitudes with increasing local time in both the winter and the summer hemispheres. The bulge is shown to be stronger near solstice than near equinox, confirming earlier work.
Common volume coherent and incoherent scatter radar observations of mid-latitude sporadic E-layers and QP echoes
Authors: Hysell D. L. , Larsen M. F. , Zhou Q. H.
Published: June 2010

Abstract. Common-volume observations of sporadic E-layers made on 14-15 June 2002 with the Arecibo incoherent scatter radar and a 30MHz coherent scatter radar imager located on St. Croix are described. Operating in dual-beam mode, the Arecibo radar detected a slowly descending sporadic E-layer accompanied by a series of dense E-region plasma clouds at a time when the coherent scatter radar was detecting quasi-periodic (QP) echoes. Using coherent radar imaging, we collocate the sources of the coherent scatter with the plasma clouds observed by Arecibo. In addition to patchy, polarized scattering regions drifting through the radar illuminated volume, which have been observed in previous imaging experiments, the 30MHz radar also detected large-scale electrostatic waves in the E-region over Puerto Rico, with a wavelength of about 30km and a period of about 10min, propagating to the southwest. Both the intensity and the Doppler shifts of the coherent echoes were modulated by the wave.
Effect of planetary waves on cooling the upper mesosphere and lower thermosphere by the CO2 15-µm emission
Authors: Shved G. M. , Ogibalov V. P. , Pogoreltsev A. I.
Published: June 2010

Abstract. The steady-state 2-D linearized model of global-scale waves, calibrated according to available observations, is used to evaluate planetary-wave perturbations of temperature from the surface up to the height of about 165km. The maximum order of perturbation amplitudes in the upper mesosphere and lower thermosphere is found to be 15K for the ultra-fast Kelvin wave (UFKW) of the 3.5-day period, 8K for the 10-day wave, 5K for the 2- and 16-day waves, and 2K for the 5-day wave. The wave-caused variation in heat influx in the CO2 15-µm band, averaged over the wave period, depends on both the amplitude of temperature and temperature profile in the atmosphere unperturbed by the wave. An additional increase in radiative cooling is the prevailing effect of planetary waves in the upper mesosphere and lower thermosphere. The UFKW results in an increasing cooling rate up to ~0.1 of the cooling rate in the unperturbed atmosphere. The tangible contributions of the 2-, 10-, and 16-day waves are questionable. The contribution of the 5-day wave is negligible.
Simulation studies on the tomographic reconstruction of the equatorial and low-latitude ionosphere in the context of the Indian tomography experiment: CRABEX
Authors: Thampi S. V. , Pant T. K. , Ravindran S. , Devasia C. V. , Sridharan R.
Published: June 2010

Abstract. Equatorial ionosphere poses a challenge to any algorithm that is used for tomographic reconstruction because of the phenomena like the Equatorial Ionization Anomaly (EIA) and Equatorial Spread F (ESF). Any tomographic reconstruction of ionospheric density distributions in the equatorial region is not acceptable if it does not image these phenomena, which exhibit large spatial and temporal variability, to a reasonable accuracy. The accuracy of the reconstructed image generally depends on many factors, such as the satellite-receiver configuration, the ray path modelling, grid intersections and finally, the reconstruction algorithm. The present simulation study is performed to examine these in the context of the operational Coherent Radio Beacon Experiment (CRABEX) network just commenced in India. The feasibility of using this network for the studies of the equatorial and low-latitude ionosphere over Indian longitudes has been investigated through simulations. The electron density distributions that are characteristic of EIA and ESF are fed into various simulations and the reconstructed tomograms are investigated in terms of their reproducing capabilities. It is seen that, with the present receiver chain existing from 8.5° N to 34° N, it would be possible to obtain accurate images of EIA and the plasma bubbles. The Singular Value Decomposition (SVD) algorithm has been used for the inversion procedure in this study. As is known, by the very nature of ionospheric tomography experiments, the received data contain various kinds of errors, like the measurement and discretization errors. The sensitivity of the inversion algorithm, SVD in the present case, to these errors has also been investigated and quantified.
Adaptive sidelobe control for clutter rejection of atmospheric radars
Authors: Kamio K. , Nishimura K. , Sato T.
Published: June 2010

Abstract. Clutter rejection is among the most important issues in radar signal processing, for which the adaptive antenna technique can be a powerful means. Compared to other applications of the adaptive antenna, however, atmospheric radars require strict conditions, which have prevented application of this technique; the main antenna beam pattern should not be altered since the target region is defined by its shape. In particular, the loss of the antenna gain should be kept to no more than about 0.5dB, in order to maintain the high sensitivity of the system. Also, clutter from surrounding mountains is often stronger than the desired weak scattering from atmospheric turbulence. We introduce a new algorithm which satisfies the above conditions, and confirms its capability by applying it to actual data taken by the MU radar. This paper presents the first report that demonstrates the effectiveness of the adaptive antenna technique in atmospheric radar applications. Despite the fact that no information is given on the spectral features of the desired and undesired signals, only the clutter echoes from surrounding mountains were effectively cancelled without affecting the desired echoes from atmospheric turbulence.
An attempt to calibrate the UHF strato-tropospheric radar at Arecibo using NexRad radar and disdrometer data
Authors: Kafando P. , Petitdidier M.
Published: June 2010

Abstract. The goal of this paper is to present a methodology to calibrate the reflectivity of the UHF Strato-Tropospheric (ST) radar located at NAIC in Puerto Rico. The UHF lower relevant altitude is at 5.9km, the melting layer being at around 4.8km. The data used for the calibration came from the observations of clouds, carried out with Strato-Tropospheric dual-wavelength (UHF and VHF) radars and a disdrometer; those instruments being located on the NAIC site in Arecibo, Puerto Rico. The National Weather Service operates other instruments like the radiosondes and the NexRad Radar in other sites. The proposed method proceeds in two steps. The first consists of the comparison between the NexRad reflectivity and the reflectivity computed from the drop size distributions measured by the disdrometer for one day with a noticeable rainfall rate. In spite of the distance of both instruments, the agreement between the reflectivities of both instruments is enough good to be used as a reference for the UHF ST radar. The errors relative at each data set is found to be 2.75dB for the disdrometer and 4dB for the NexRad radar, following the approach of Hocking et al. (2001). The inadequacy between the two sampled volume is an important contribution in the errors. The second step consists of the comparison between the NexRad radar reflectivity and the UHF non-calibrated reflectivity at the 4 altitudes of common observations during one event on 15 October 1998. Similar features are observed and a coefficient is deduced. An offset around 4.7dB is observed and the correlation factor lies between 0.628 and 0.730. According to the errors of the data sets, the precision on the calibration is of the order of 2dB. This method works only when there are precipitation hydrometeors above the NAIC site. However, the result of the calibration could be applied to other data obtained during the campaign, the only constraint being the same value of the transmitter power. Key words. Meteorology and atmospheric dynamics (tropical meteorology; remote sensing; instruments and techniques)
Ozone and water vapour in the austral polar stratospheric vortex and sub-vortex
Authors: Peet E. , Rudakov V. , Yushkov V. , Redaelli G. , MacKenzie A. R.
Published: June 2010

Abstract. In-situ measurements of ozone and water vapour, in the Antarctic lower stratosphere, were made as part of the APE-GAIA mission in September and October 1999. The measurements show a distinct difference above and below the 415K isentrope. Above 415K, the chemically perturbed region of low ozone and water vapour is clearly evident. Below 415K, but still above the tropopause, no sharp meridional gradients in ozone and water vapour were observed. The observations are consistent with analyses of potential vorticity from the European Centre for Medium Range Weather Forecasting, which show smaller radial gradients at 380K than at 450K potential temperature. Ozone loss in the chemically perturbed region above 415K averages 5ppbv per day for mid-September to mid-October. Apparent ozone loss rates in the sub-vortex region are greater, at 7ppbv per day. The data support, therefore, the existence of a sub-vortex region in which meridional transport is more efficient than in the vortex above. The low ozone mixing ratios in the sub-vortex region may be due to in-situ chemical destruction of ozone or transport of ozone-poor air out of the bottom of the vortex. The aircraft data we use cannot distinguish between these two processes. Key words. Meteorology and atmospheric dynamics polar meteorology) – Atmospheric composition and structure (middle atmosphere–composition and chemistry)
Arctic and Antarctic polar mesosphere summer echoes observed with oblique incidence HF radars: analysis using simultaneous MF and VHF radar data
Authors: Ogawa T. , Nozawa S. , Tsutsumi M. , Arnold N. F. , Nishitani N. , Sato N. , Yukimatu A. S.
Published: June 2010

Abstract. Polar mesosphere summer echoes (PMSEs) have been well studied using vertical incidence VHF radars at northern high-latitudes. In this paper, two PMSE events detected with the oblique incidence SuperDARN HF radars at Hankasalmi, Finland (62.3° N) and Syowa Station, Antarctica (69.0° S), are analyzed, together with simultaneous VHF and medium-frequency (MF) radar data. Altitude resolutions of the HF radars in the mesosphere and the lower thermosphere are too poor to know exact PMSE altitudes. However, a comparison of Doppler velocity from the HF radar and neutral wind velocity from the MF radar shows that PMSEs at the HF band appeared at altitudes within 80-90km, which are consistent with those from previous vertical incidence HF-VHF radar results. The HF-VHF PMSE occurrences exhibit a semidiurnal behavior, as observed by other researchers. It is found that in one event, PMSEs occurred when westward semidiurnal winds with large amplitude at 85-88km altitudes attained a maximum. When the HF-VHF PMSEs were observed at distances beyond 180km from MF radar sites, the MF radars detected no appreciable signatures of echo enhancement. Key words. Meteorology and atmospheric dynamics (middle atmosphere dynamics; thermospheric dynamics; waves and tides)
The six-month line in geomagnetic long series
Authors: Le Mouël J. L. , Blanter E. , Shnirman M.
Published: June 2010

Abstract. Daily means of the horizontal components X (north) and Y (east) of the geomagnetic field are available in the form of long series (several tens of years). Nine observatories are used in the present study, whose series are among the longest. The amplitudes of the 6-month and 1-year periodic variations are estimated using a simple but original technique. A remarkably clear result emerges from the complexity of the geomagnetic data: the amplitude of the 6-month line presents, in all observatories, the same large variation (by a factor of 1.7) over the 1920–1990 time span, regular and quasi-sinusoidal. Nothing comparable comes out for the annual line. The 6-month line results from the modulation by an astronomical mechanism of a magnetospheric system of currents. As this latter mechanism is time invariant, the intensity of the system of currents itself must present the large variation observed on the 6-months variation amplitude. This variation presents some similarities with the one displayed by recent curves of reconstructed solar irradiance or the "Earth's temperature". Finally, the same analysis is applied to the aa magnetic index.Key words. Geomagnetism and paleomagnetism (time variations, diurnal to secular). Magnetospheric physics (current systems; polar cap phenomena)
Interchange instability of a curved current layerconvecting in the magnetosheath from the bow shock towards themagnetopause
Authors: Arshukova I. L. , Erkaev N. V. , Biernat H. K.
Published: June 2010

Abstract. This paper deals with nonsteady perturbations of the magnetosheath parameters which are related to variations of the interplanetary magnetic field from north to south under a constant solar wind dynamic pressure. The magnetic field changes its direction within a thin layer which is convected with the plasma from the bow shock to the ionopause. In the course of time, this current layer is amplified during its motion towards the magnetopause. The intensity of the current is increasing, the layer thickness is decreasing, and the gradients of parameters are becoming much sharper while the layer is approaching the magnetopause. The curvature radius of this layer is decreasing while it is draping around the magnetopause. This curved layer structure with reversed magnetic field in the magnetosheath is found to be unstable with respect to the interchange instability. The growth rate of the instability is obtained for different positions of the layer. Key words. Magnetospheric physics (magnetosheath)
Letter to the EditiorTesting the hypothesis of the influence of neutral turbulence on the deduction of ambipolar diffusivities from meteor trail expansion
Authors: Hall C. M. , Aso T. , Tsutsumi M. , Nozawa S. , Manson A. H. , Meek C. E.
Published: June 2010

Abstract. Fading times of radar echoes from underdense meteor trails in the upper mesosphere/lower thermosphere are commonly used to determine ambipolar diffusivities and hence ambient temperature. Diffusivities are generally expected to increase exponentially with height through the region from which the meteor trail echoes are obtained, viz., typically 70-110km altitude for a ~30-MHz radar. In practice, however, this is more the exception: unexpectedly large diffusivities are obtained in the lower part of the regime, and unexpectedly low values are obtained in the upper part; only in the few kilometres on either side of the maximum in echo occurrence (viz., 90km for a 30-MHz radar) does the diffusivity profile behave as expected. Hall (2002) hypothesised that neutral turbulence might be enhancing expansion of the meteor trail in the lower part of the regime. In this communication, due to results only available since the publication of Hall's suggestion, we are able to refute the hypothesis.
Spatial and temporal variations of small-scale plasma turbulence parameters in the equatorial electrojet: HF and VHF radar observational results
Authors: Manju G. , Viswanathan K. S. , Ravindran S.
Published: June 2010

Abstract. The spatial and temporal variations of various parameters associated with plasma wave turbulence in the equatorial electrojet (EEJ) at the magnetic equatorial location of Trivandrum (8.5° N, 77° E; dip 0.5° N) are studied for the first time, using co-located HF (18MHz) and VHF (54.95MHz) coherent backscatter radar observations (daytime) in the altitude region of 95-110km, mostly on magnetically quiet days. The derived turbulence parameters are the mean electron density irregularity strength (δn/n), anomalous electron collision frequency (νe*) and the corrected east-west electron drift velocity (Vey). The validity of the derived parameters is confirmed using radar data at two different frequencies and comparing with in-situ measurements. The behaviour of δn/n in relation to the backscattered power during weak and strong EEJ conditions is also examined to understand the growth and evolution of turbulence in the electrojet.
High-latitude electromagnetic and particle energy flux during an event with sustained strongly northward IMF
Authors: Korth H. , Anderson B. J. , Frey H. U. , Waters C. L.
Published: June 2010

Abstract. We present a case study of a prolonged interval of strongly northward orientation of the interplanetary magnetic field on 16 July 2000, 16:00-19:00 UT to characterize the energy exchange between the magnetosphere and ionosphere for conditions associated with minimum solar wind-magnetosphere coupling. With reconnection occurring tailward of the cusp under northward IMF conditions, the reconnection dynamo should be separated from the viscous dynamo, presumably driven by the Kelvin-Helmholtz (KH) instability. Thus, these conditions are also ideal for evaluating the contribution of a viscous interaction to the coupling process. We derive the two-dimensional distribution of the Poynting vector radial component in the northern sunlit polar ionosphere from magnetic field observations by the constellation of Iridium satellites together with drift meter and magnetometer observations from the Defense Meteorological Satellite Program (DMSP) F13 and F15 satellites. The electromagnetic energy flux is then compared with the particle energy flux obtained from auroral images taken by the far-ultraviolet (FUV) instrument on the Imager for Magnetopause to Aurora Global Exploration (IMAGE) spacecraft. The electromagnetic energy input to the ionosphere of 51 GW calculated from the Iridium/DMSP observations is eight times larger than the 6 GW due to particle precipitation all poleward of 78° MLAT. This result indicates that the energy transport is significant, particularly as it is concentrated in a small region near the magnetic pole, even under conditions traditionally considered to be quiet and is dominated by the electromagnetic flux. We estimate the contributions of the high and mid-latitude dynamos to both the Birkeland currents and electric potentials finding that high-latitude reconnection accounts for 0.8 MA and 45kV while we attribute <0.2MA and ~5kV to an interaction at lower latitudes having the sense of a viscous interaction. Given that these conditions are ideal for the occurrence of the KH instability at the magnetopause and hence the viscous interaction, this result suggests that the viscous interaction is a small contributor to coupling solar wind energy to the magnetosphere-ionosphere system.
Excitation of Alfvén impulse by the anomalous resistance onset on the auroral field lines
Authors: Pilipenko V. , Mazur N. , Fedorov E. , Uozumi T. , Yumoto K.
Published: June 2010

Abstract. The onset of anomalous resistance in a layer on auroral field lines is shown to be accompanied by the excitation of an Alfvénic impulse (AI). The generated AI marks the transition of the global magnetosphere-ionosphere instability into an explosive phase with positive feedback. The spatial structure of this impulse both in space and on the ground has been described analytically and numerically under the thin layer approximation. The field-aligned currents transported by the Alfvénic impulse are concentrated near the edges of the layer with anomalous resistivity, whereas the reverse currents are spread throughout the layer. For some parameters of the layer the ionospheric attenuation of even small-scale structures is not dramatically large, so the magnetic response to the generated AI may be observed on the ground. The considered event on 3 January 1997 with magnetic-auroral intensification observed by ground magnetometers and Polar UV imager agrees with the model proposed.
Artificial optical emissions at HAARP for pump frequencies near the third and second electron gyro-harmonic
Authors: Kosch M. J. , Pedersen T. , Hughes J. , Marshall R. , Gerken E. , Senior A. , Sentman D. , McCarrick M. , Djuth F. T.
Published: June 2010

Abstract. High-power high-frequency radio waves beamed into the ionosphere cause plasma turbulence, which can accelerate electrons. These electrons collide with the F-layer neutral oxygen causing artificial optical emissions identical to natural aurora. Pumping at electron gyro-harmonic frequencies has special significance as many phenomena change their character. In particular, artificial optical emissions become strongly reduced for the third and higher gyro-harmonics. The High frequency Active Auroral Research Program (HAARP) facility is unique in that it can select a frequency near the second gyro-harmonic. On 25 February 2004, HAARP was operated near the third and passed through the second gyro-harmonic for the first time in a weakening ionosphere. Two novel observations are: firstly, a strong enhancement of the artificial optical emission intensity near the second gyro-harmonic, which is opposite to higher gyro-harmonics; secondly, the optical enhancement maximum occurs for frequencies just above the second gyro-harmonic. We provide the first experimental evidence for these effects, which have been predicted theoretically. In addition, irregular optical structures were created when the pump frequency was above the ionospheric critical frequency.Keywords. Active experiments – Auroral ionosphere – Wave-particle interactions
Results of the Rio de Janeiro magnetic observations 1781−1788
Authors: Vaquero J. M. , Trigo R. M.
Published: June 2010

Abstract. We have compiled a relatively extensive catalogue of geomagnetic declination measurements performed by the Portuguese Bento Sanches Dorta in Rio de Janeiro (Brazil) between 1781 and 1788. All the information was retrieved from printed documents compiled in the first three volumes of the Memories of the Portuguese Royal Academy of Sciences. During this period Sanches Dorta performed roughly 20000 individual observations, however, only a fraction of this data is presently available. In this work, all available information about these measurements is analysed. The annual change (variability and trend) and the daily cycle of geomagnetic declination at monthly scale are discussed. Keywords. Geomagnetism and paleomagnetism, History of geophysics, Geomagnetic field declination
Magnetohydrodynamic waves within the medium separated by the plane shock wave or rotational discontinuity
Authors: Lubchich A. A. , Despirak I. V.
Published: June 2010

Abstract. Characteristics of small amplitude plane waves within the medium separated by the plane discontinuity into two half spaces are analysed. The approximation of the ideal one-fluid magnetohydrodynamics (MHD) is used. The discontinuities with the nonzero mass flux across them are mainly examined. These are fast or slow shock waves and rotational discontinuities. The dispersion equation for MHD waves within each of half space is obtained in the reference frame connected with the discontinuity surface. The solution of this equation permits one to determine the wave vectors versus the parameter cp, which is the phase velocity of surface discontinuity oscillations. This value of cp is common for all MHD waves and determined by an incident wave or by spontaneous oscillations of the discontinuity surface. The main purpose of the study is a detailed analysis of the dispersion equation solution. This analysis let us draw the following conclusions. (I) For a given cp, ahead or behind a discontinuity at most, one diverging wave can transform to a surface wave damping when moving away from the discontinuity. The surface wave can be a fast one or, in rare cases, a slow, magnetoacoustic one. The entropy and Alfvén waves always remain in a usual homogeneous mode. (II) For certain values of cp and parameters of the discontinuity behind the front of the fast shock wave, there can be four slow magnetoacoustic waves, satisfying the dispersion equation, and none of the fast magnetoacoustic waves. In this case, one of the four slow magnetoacoustic waves is incident on the fast shock wave from the side of a compressed medium. It is shown that its existence does not contradict the conditions of the evolutionarity of MHD shock waves. The four slow magnetoacoustic waves, satisfying the dispersion equation, can also exist from either side of a slow shock wave or rotational discontinuity. (III) The expressions determining the polarisation of the MHD waves are derived in the reference frame connected with the discontinuity surface. This form of presentation is much more convenient in investigating the interaction of small perturbations with MHD discontinuities. It is shown that the perturbations of the velocity and magnetic field associated with the surface magnetoacoustic wave have the elliptic polarisation. Usually the planes of polarisation for the perturbations of the velocity and magnetic field are not coincident with each other. Keywords. Space plasma physics (Discontinuities; Shock waves) – Interplanetary physics (Discontinuities; Interplanetary shocks) – Magnetospheric physics (Solar windmagnetosphere interactions)
Transforming in-situ observations of CME-driven shock accelerated protons into the shock's reference frame.
Authors: Robinson I. M. , Simnett G. M.
Published: June 2010

Abstract. We examine the solar energetic particle event following solar activity from 14, 15 April 2001 which includes a "bump-on-the-tail" in the proton energy spectra at 0.99 AU from the Sun. We find this population was generated by a CME-driven shock which arrived at 0.99 AU around midnight 18 April. As such this population represents an excellent opportunity to study in isolation, the effects of proton acceleration by the shock. The peak energy of the bump-on-the-tail evolves to progressively lower energies as the shock approaches the observing spacecraft at the inner Lagrange point. Focusing on the evolution of this peak energy we demonstrate a technique which transforms these in-situ spectral observations into a frame of reference co-moving with the shock whilst making allowance for the effects of pitch angle scattering and focusing. The results of this transform suggest the bump-on-the-tail population was not driven by the 15 April activity but was generated or at least modulated by a CME-driven shock which left the Sun on 14 April. The existence of a bump-on-the-tail population is predicted by models in Rice et al. (2003) and Li et al. (2003) which we compare with observations and the results of our analysis in the context of both the 14 April and 15 April CMEs. We find an origin of the bump-on-the-tail at the 14 April CME-driven shock provides better agreement with these modelled predictions although some discrepancy exists as to the shock's ability to accelerate 100 MeV protons. Keywords. Solar physics, astrophysics and astronomy (Energetic particles; Flares and mass ejections) – Space plasma physics (Transport processes)
Vertical structure of currents in the vicinity of the Iceland-Scotland Ridge
Authors: Carollo C. , Astin I. , Graff J.
Published: June 2010

Abstract. An analysis of current profiles carried out on moored Acoustic Doppler Current Profiler data sets recorded during the Variability of Exchanges In the Northern Seas project is presented here for several locations in three sections in the vicinity of the Iceland-Scotland Ridge: (1) north of the Faroes, (2) in the Faroe-Bank Channel and (3) in the Faroe-Shetland Channel. Total currents have been decomposed into three components, namely the mean residual flow, tide and surge. The mean residual flow is found to be dominant. Results for the major tidal constituents (M2, S2, N2, O1 and K1) are shown and discussed. It is found that the predominant tidal harmonic M2 becomes steered through depth to align with the bottom topography. The mean residual flow is found to be generally larger than the surge, particularly in the Faroe-Bank Channel below 500m depth where it is the dominant component. Here tidal rectification, i.e. the topographic rectification of tidal currents originating in nonlinearities that rectify the oscillatory tidal motion, is identified as the process enhancing the large mean residual currents found. From the current structure, two water masses are identified in the channel: the upper slowing moving inflow water and the colder outflow water characterised by a 3–6-day periodicity. In the Faroe-Shetland Channel the flow is characterised by large tidal currents, particularly in shallow waters. Instead, north of the Faroes none of the component was identified as dominant. The results show that the variability of the current components is strongly dependent upon topography and water depth. Keywords. Oceanography: Physical (Currents; General circulation; General or miscellaneous)
Comparison of DMSP cross-track ion drifts and SuperDARN line-of-sight velocities
Authors: Drayton R. A. , Koustov A. V. , Hairston M. R. , Villain J.-P.
Published: June 2010

Abstract. Cross-track ion drifts measured by the DMSP satellites are compared with line-of-sight SuperDARN HF velocities in approximately the same directions. Good overall agreement is found for a data set comprising of 209 satellite passes over the field of view of nine SuperDARN radars in both the Northern and Southern Hemispheres. The slope of the best linear fit line relating the SuperDARN and DMSP velocities is of the order of 0.7 with a tendency for SuperDARN velocities to be smaller. The agreement implies that the satellite and radar data can be merged into a common set provided that spatial and temporal variations of the velocity as measured by both instruments are smooth. Keywords. Ionosphere (Ionospheric irregularities; Plasma convection; Auroral ionosphere)
Cluster observations of bounday layer structure and a flux transfer event near the cusp
Authors: Fear R. C. , Fazakerley A. N. , Owen C. J. , Lahiff A. D. , Lucek E. A. , Balogh A. , Kistler L. M. , Mouikis C. , Rème H.
Published: June 2010

Abstract. On the 25th January 2002 between 10:00 and 12:00 UT, the four Cluster spacecraft passed through the northern high-latitude cusp, the dayside magnetosphere and into the magnetosheath in a linear formation. In the magnetosphere the PEACE electron spectrometers on the four spacecraft all observed a series of transient bursts of magnetosheath-like plasma, but without bipolar magnetic signatures in the magnetopause normal component as might be expected if the plasma had been injected by transient reconnection (flux transfer events – FTEs). Reordering the data using the magnetopause transition parameter reveals that these plasma observations, the related variations in the magnetic field and the balance of magnetic and thermal gas pressures are consistent with transient entries into a stable high-latitude boundary layer structure. However, once some of the spacecraft entered the magnetosheath, FTE signatures were observed outside the magnetopause at the same time as some of the boundary layer entries occurred at the other spacecraft inside. Thus, (a) the lack of a bipolar BN signature is inconsistent with the traditional picture of a magnetospheric FTE, and (b) the cause of the observed entry of the spacecraft into the boundary layer (pressure pulse or passing magnetosheath FTE) can only be determined by spacecraft observations in the magnetosheath. Keywords. Magnetospheric physics (Magnetopause, cusp and bondary layers; Solar wind- magnetosphere interactions; Magnetosheath)