bluuebunny/crossref_metadata_2025 huggingface dataset:
Low-latitude ionospheric turbulence observed by Aureol-3 satellite
Authors: Hobara Y. , Lefeuvre F. , Parrot M. , Molchanov O. A.
Published: June 2010

Abstract. Using PSD (Power Spectral Density) data on electron density and electric field variations observed on board Aureol-3 satellite at low-to-mid-latitude ionosphere we analyze a scale distribution of the ionospheric turbulence in a form k-α, where k is the wave number and α is the spectral index. At first, high-resolution data in the near-equator region for several orbits have been processed. In this case the frequency range is from 6Hz to 100Hz (corresponding spatial scales from 80m to 1.3km), each power spectrum obeys a single power law fairly well, and the mean spectral indices are rather stable with αN=2.2±0.3 and αE=1.8±0.2, for the density and electric field, respectively. Then we produce a statistical study of 96 electric field bursts in the frequency range 10-100Hz from low-time resolution data (filter bank envelope). These bursts concentrate on the side of the Equatorial Anomaly crest (geomagnetic latitude 30-40°). Spectral indices of the bursts vary in the interval αE=2.0-2.5 but are fairly stable in seasons and local times. The electric field power of the burst has rather a large variability but has a relative increase in mean values for the summer and winter, as well as the daytime. The effect of major seismic activities toward the ionospheric turbulence is not conclusive either for the refractive index or for the electric field power. However, the mean value for the electric field power of bursts during seismic periods is larger than that for non seismic periods, and the statistical difference of the mean values is rather significant.
A Monte Carlo model of auroral hydrogen emission line profiles
Authors: Gérard J.-C. , Shematovich V. I. , Bisikalo D. V. , Lummerzheim D.
Published: June 2010

Abstract. Hydrogen line profiles measured from space-borne or ground-based instruments provide useful information to study the physical processes occurring in the proton aurora and to estimate the proton flux characteristics. The line shape of the hydrogen lines is determined by the velocity distribution of H atoms along the line-of-sight of the instrument. Calculations of line profiles of auroral hydrogen emissions were obtained using a Monte Carlo kinetic model of proton precipitation into the auroral atmosphere. In this model both processes of energy degradation and scattering angle redistribution in momentum and charge transfer collisions of the high-energy proton/hydrogen flux with the ambient atmospheric gas are considered at the microphysical level. The model is based on measured cross sections and scattering angle distributions and on a stochastic interpretation of such collisions. Calculations show that collisional angular redistribution of the precipitating proton/hydrogen beam is the dominant process leading to the formation of extended wings and peak shifts in the hydrogen line profiles. All simulations produce a peak shift from the rest line wavelength decreasing with increasing proton energy. These model predictions are confirmed by analysis of ground-based H-β line observations from Poker Flat, showing an anti-correlation between the magnitude of the peak shift and the extent of the blue wing of the line. Our results also strongly suggest that the relative extension of the blue and red wings provides a much better indicator of the auroral proton characteristic energy than the position of the peak wavelength.
Alfvénic fluctuations in "newborn"' polar solar wind
Authors: Bavassano B. , Pietropaolo E. , Bruno R.
Published: June 2010

Abstract. The 3-D structure of the solar wind is strongly dependent upon the Sun's activity cycle. At low solar activity a bimodal structure is dominant, with a fast and uniform flow at the high latitudes, and slow and variable flows at low latitudes. Around solar maximum, in sharp contrast, variable flows are observed at all latitudes. This last kind of pattern, however, is a relatively short-lived feature, and quite soon after solar maximum the polar wind tends to regain its role. The plasma parameter distributions for these newborn polar flows appear very similar to those typically observed in polar wind at low solar activity. The point addressed here is about polar wind fluctuations. As is well known, the low-solar-activity polar wind is characterized by a strong flow of Alfvénic fluctuations. Does this hold for the new polar flows too? An answer to this question is given here through a comparative statistical analysis on parameters such as total energy, cross helicity, and residual energy, that are of general use to describe the Alfvénic character of fluctuations. Our results indicate that the main features of the Alfvénic fluctuations observed in low-solar-activity polar wind have been quickly recovered in the new polar flows developed shortly after solar maximum. Keywords. Interplanetary physics (MHD waves and turbulence; Sources of the solar wind) – Space plasma physics (Turbulence)
Correlation between cosmic noise absorption and VHF coherent echo intensity
Authors: Makarevitch R. A. , Honary F.
Published: June 2010

Abstract. We present examples and statistical analysis of the events with statistically significant correlation between the cosmic noise absorption (CNA) and the signal-to-noise ratio (SNR) of the VHF coherent echo intensity in the area monitored simultaneously by an imaging riometer and two oblique-sounding coherent VHF radars in Northern Scandinavia. By only considering the observations from the narrow riometer beams comparable (in terms of the intersection with the ionosphere) with the VHF radar cells, we identify ~200 one-hour high correlation periods (HCPs) for 2 years near the solar cycle maximum, 2000–2001. The HCP occurrence is maximized in the afternoon (12:00–17:00 UT, MLT≅UT+3), with the secondary peak near the midnight (21:00–02:00 UT). Relative to the VHF echo occurrence, HCPs occur more frequently from 11:00 to 20:00 UT. The diurnal variation of HCP occurrence is similar to that of the 1-h intervals with the lowest mean absorption A<0.25dB. The HCPs are observed more frequently during the winter months, which, combined with the fact that VHF echoes observed during HCPs exhibit features typical for field-aligned E-region irregularities, makes their association with the polar mesospheric echoes (for which some positive CNA/SNR correlation has been reported in the past) very unlikely. Instead, we attribute the high positive CNA/SNR correlation to the synchronous, to a first approximation, variation of the particle fluxes for two different but close sets of energies. By considering the dependence of the CNA/SNR correlation coefficients for both VHF radars (CA1 and CA2) upon the correlation between SNRs for two radars (C12), we show that both coefficients, CA1 and CA2, and the agreement between them decrease drastically with a C12 decrease, which we interpreted through the progressively increasing role of the spatial inhomogeneity of the processes leading to the enhanced CNA and SNR. In this situation, a similarity between the radio signal collection areas should become important, and we demonstrate that the HCP occurrence and mean correlation coefficient decrease as the riometer beams and radar cells become less comparable in terms of mutual orientation and closeness between the points of maximum sensitivity. Keywords. Ionosphere (Auroral ionosphere; Particle precipitation; Instruments and techniques)
Comparison between CNA and energetic electron precipitation: simultaneous observation by Poker Flat Imaging Riometer and NOAA satellite
Authors: Tanaka Y.-M. , Ishii M. , Murayama Y. , Kubota M. , Mori H. , Yamamoto M.-Y. , Kadokura A. , Lummerzheim D. , Desrochers J. , Evans D. S.
Published: June 2010

Abstract. The cosmic noise absorption (CNA) is compared with the precipitating electron flux for 19 events observed in the morning sector, using the high-resolution data obtained during the conjugate observations with the imaging riometer at Poker Flat Research Range (PFRR; 65.11° N, 147.42° W), Alaska, and the low-altitude satellite, NOAA 12. We estimate the CNA, using the precipitating electron flux measured by NOAA 12, based on a theoretical model assuming an isotropic pitch angle distribution, and quantitatively compare them with the observed CNA. Focusing on the eight events with a range of variation larger than 0.4dB, three events show high correlation between the observed and estimated CNA (correlation coefficient (r0)>0.7) and five events show low correlation (r0<0.5). The estimated CNA is often smaller than the observed CNA (72% of all data for 19 events), which appears to be the main reason for the low-correlation events. We examine the assumption of isotropic pitch angle distribution by using the trapped electron flux measured at 80° zenith angle. It is shown that the CNA estimated from the trapped electron flux, assuming an isotropic pitch angle distribution, is highly correlated with the observed CNA and is often overestimated (87% of all data). The underestimate (overestimate) of CNA derived from the precipitating (trapped) electron flux can be interpreted in terms of the anisotropic pitch angle distribution similar to the loss cone distribution. These results indicate that the CNA observed with the riometer may be quantitatively explained with a model based on energetic electron precipitation, provided that the pitch angle distribution and the loss cone angle of the electrons are taken into account. Keywords. Energetic particles, precipitating – Energetic particles, trapped – Ionosphere-magnetosphere interactions
Zonal wave numbers 1-5 in planetary waves from the TOMS total ozone at 65° S
Authors: Grytsai A. , Grytsai Z. , Evtushevsky A. , Milinevsky G. , Leonov N.
Published: June 2010

Abstract. Planetary waves in the total ozone at the southern latitude of 65° S are studied to obtain the main characteristics of the zonal wave numbers 1–5. The TOMS total ozone data were used to analyze the amplitude and periodicity variations of the five spectral components during August-December of 1979–2003. A presence of the shorter period of waves 1–3 in 1996 (7 days) in comparison with 2002 (8–12 days) is revealed which can be attributed to the distinction in conditions of typical and anomalously weak stratospheric polar vortex, probably, a strong and weak mean zonal wind. The interannual variations of the monthly and 5-month mean amplitudes of the zonal wave numbers 1–5 are described. Wave 1 has the largest amplitude in October (up to 139 DU in 2000) and increasing amplitude trend (15 DU/decade for October 1979–2003). The 5-month mean amplitudes averaged over 1979–2003 are 53.6, 29.9, 15.5, 10.5, and 7.8 DU for the wave number sequence 1, 2, 3, 4 and 5, respectively. For the stationary components the amplitudes are 38.3, 4.8, 1.8, 1.2, 0.7 DU, respectively. Thus, the stationary component of wave 1 and the traveling one of waves 2–5 are predominant. The tendencies in a long-term change in the wave number amplitude can be explained by taking into account the degree of wave deformation of the stratospheric polar vortex edge, net meridional displacements of the lower stratosphere air, and the difference between the total ozone loss and negative trends in the polar and mid-latitude regions. Keywords. General circulation – Middle atmosphere dynamics – Waves and tides
Effects of D-region RF heating studied with the Sodankylä Ion Chemistry model
Authors: Enell C.-F. , Kero A. , Turunen E. , Ulich Th. , Verronen P. T. , Seppälä A. , Marple S. , Honary F. , Senior A.
Published: June 2010

Abstract. The upper mesosphere and lower thermosphere, or ionospheric D region, is an atmospheric layer which is difficult to access experimentally. A useful method that also has a large potential for further studies is artificial heating of electrons by means of powerful radio transmitters. Here we estimate the effect of D-region heating for a few typical cases of high electron density – daylight, typical auroral electron precipitation, and a solar proton event – by coupling a model of RF electron heating to the Sodankylä Ion Chemistry (SIC) model. The predicted effects are among others an increase in the ratio of the concentration of negative ions to that of free electrons, and an increase in the absorption of cosmic noise as measured by riometers. For the model runs presented in this paper we have calculated the absorption for the frequency (38.2MHz) of the IRIS imaging riometer in Kilpisjärvi, Finland, as observing the ionosphere above the EISCAT Heater in Tromsø, Norway. The predicted enhancements of the absorption are 0.2–0.8dB, an effect which is clearly detectable. Keywords. Ionosphere (Active experiments; Ion chemistry and composition; Wave propagation)
The Bonn University lidar at the Esrange: technical description and capabilities for atmospheric research
Authors: Blum U. , Fricke K. H.
Published: June 2010

Abstract. The Bonn University operates a Rayleigh/Mie/Raman backscatter lidar at the Esrange near the Swedish city of Kiruna, north of the Arctic circle. The lidar system covers the atmosphere from about 4 km to 100 km altitude and it is capable of measuring aerosols in the tropo-, strato-, and mesosphere, as well as of determining temperature profiles in the aerosol-free part of the atmosphere (i.e. above 30-km altitude). Density tuned fixed-spacer etalons provide daylight capability and thus increased sensitivity to noctilucent clouds during polar summer. Polarisation measurements allow liquid and solid phase discrimination for aerosol and cloud particles in the tropo- and stratosphere. The derived temperature profiles can be used for the detection and analysis of atmospheric gravity waves. Although several lidar experiments are situated in polar latitudes, a comprehensive instrument which covers the troposphere, stratosphere, and mesosphere, is daylight capable, and observes temperature profiles, as well as aerosols, is exceptional. In this article a technical description, in particular, of the optical configuration of this experiment is given, as well as an overview of achievable geophysical parameters. The potential for geophysical analyses is shown. Keywords. Atmospheric composition and structure (Instruments and techniques; Aerosols and particles; Pressure, density and temperature) – Meteorology and atmospheric dynamics (Middle atmosphere dynamics)
A quantitative analysis of the diurnal evolution of Ionospheric Alfvén resonator magnetic resonance features and calculation of changing IAR parameters
Authors: Hebden S. R. , Robinson T. R. , Wright D. M. , Yeoman T. , Raita T. , Bösinger T.
Published: June 2010

Abstract. Resonance features of the Ionospheric Alfvén Resonator (IAR) can be observed in pulsation magnetometer data from Sodankylä, Finland using dynamic spectra visualizations. IAR resonance features were identified on 13 of 30 days in October 1998, with resonance structures lasting for 3 or more hours over 10 intervals. The diurnal evolution of the harmonic features was quantified for these 10 intervals using a manual cursor-clicking technique. The resonance features displayed strong linear relationships between harmonic frequency and harmonic number for all of the time intervals studied, enabling a homogeneous cavity model for the IAR to be adopted to interpret the data. This enabled the diurnal variation of the effective size of the IAR to be obtained for each of the 10 time intervals. The average effective size was found to be 530 km, and to have an average variation of 32% over each time interval: small compared to the average variation in Alfvén velocity of 61%. Thus the diurnal variation of the harmonics is chiefly caused by the changing plasma density within the IAR due to changing insolation. This study confirms Odzimek (2004) that the dominating factor affecting the IAR eigenfrequencies is the variation in the Alfvén velocity at the F-layer ion-density peak, with the changing IAR size affecting the IAR eigenfrequencies to a smaller extent. Another IAR parameter was derived from the analysis of the IAR resonance features associated with the phase matching structure of the standing waves in the IAR. This parameter varied over the time intervals studied by 20% on average, possibly due to changing ionospheric conductivity. Keywords. Ionosphere (Auroral ionosphere;Wave propagation) – Radio science (Electromagnetic noise and interference)
Induction effects on ionospheric electric and magnetic fields
Authors: Vanhamäki H. , Viljanen A. , Amm O.
Published: June 2010

Abstract. Rapid changes in the ionospheric current system give rise to induction currents in the conducting ground that can significantly contribute to magnetic and especially electric fields at the Earth's surface. Previous studies have concentrated on the surface fields, as they are important in, for example, interpreting magnetometer measurements or in the studies of the Earth's conductivity structure. In this paper we investigate the effects of induction fields at the ionospheric altitudes for several realistic ionospheric current models (Westward Travelling Surge, Ω-band, Giant Pulsation). Our main conclusions are: 1) The secondary electric field caused by the Earth's induction is relatively small at the ionospheric altitude, at most 0.4 mV/m or a few percent of the total electric field; 2) The primary induced field due to ionospheric self-induction is locally important, ~ a few mV/m, in some "hot spots", where the ionospheric conductivity is high and the total electric field is low. However, our approximate calculation only gives an upper estimate for the primary induced electric field; 3) The secondary magnetic field caused by the Earth's induction may significantly affect the magnetic measurements of low orbiting satellites. The secondary contribution from the Earth's currents is largest in the vertical component of the magnetic field, where it may be around 50% of the field caused by ionospheric currents. Keywords. Geomagnetism and paleomagnetism (geomagnetic induction) – Ionosphere (electric fields and currents)
Polar cap influx
Authors: MacDougall J. , Jayachandran P. T.
Published: June 2010

Abstract. This study uses digital ionosonde data from a cusp latitude station (Cambridge Bay, 77° CGM lat.) to study the convection into the polar cap. Days when the IMF magnetic field was relatively steady were used. On many days it was possible to distinguish an interval near noon MLT when the ionosonde data had a different character from that at earlier and later times. Based on our data, and other published measurements, we used the interval 10:00-13:00 MLT as the cusp interval and calculated the convection into the polar cap in this interval. The integrated convection accounted for only ~1/3 of the open polar cap flux. If the convection through the prenoon/postnoon regions on either side of the cusp was calculated the remaining 2/3 of the flux could be accounted for. The characteristics of the prenoon/postnoon regions were different from the cusp region, and we attribute this to transient flank merging versus more steady frontside merging for the cusp. Keywords. Ionosphere (Plasma convection) Magnetospheric physics (Polar cap phenomenon)
Interhemispheric observations of the ionospheric signature of tail reconnection during IMF-northward non-substorm intervals
Authors: Grocott A. , Yeoman T. K. , Milan S. E. , Cowley S. W. H.
Published: June 2010

Abstract. This paper presents the first interhemispheric radar observations interpreted as the ionospheric response to tail reconnection during IMF-northward non-substorm intervals. SuperDARN measurements of plasma convection in the nightside ionospheres of both hemispheres, taken on 21–22 February and 26–27 April 2000, show bursts of flow in the midnight sector which are understood to be characteristic of such phenomena. Upstream interplanetary magnetic field data confirm that the field orientation at the dayside magnetopause was northwards, but with a significant IMF By component (negative during the first interval, positive during the second), for many hours prior to the bursts being observed. During the By-negative interval the bursts were directed westwards in the Northern Hemisphere and eastwards in the Southern Hemisphere; during the By-positive interval their directions were reversed. These two asymmetries between the different orientations of IMF By and between the two hemispheres are key to our understanding of the magnetospheric phenomenon responsible for generating the bursts. They provide further evidence in support of the idea that the bursts are a result of reconnection in an asymmetric tail under the prolonged influence of IMF By. Concurrent data from ground magnetometers and geosynchronous satellites confirm that the bursts have no associated substorm characteristics, consistent with previous studies. Keywords. Ionosphere (Plasma convection; Ionospheremagnetosphere interactions) – Magnetospheric Physics (Magnetotail)
Statistics of a parallel Poynting vector in the auroral zone as a function of altitude using Polar EFI and MFE data and Astrid-2 EMMA data
Authors: Janhunen P. , Olsson A. , Tsyganenko N. A. , Russell C. T. , Laakso H. , Blomberg L. G.
Published: June 2010

Abstract. We study the wave-related (AC) and static (DC) parallel Poynting vector (Poynting energy flux) as a function of altitude in auroral field lines using Polar EFI and MFE data. The study is statistical and contains 5 years of data in the altitude range 5000–30000 km. We verify the low altitude part of the results by comparison with earlier Astrid-2 EMMA Poynting vector statistics at 1000 km altitude. The EMMA data are also used to statistically compensate the Polar results for the missing zonal electric field component. We compare the Poynting vector with previous statistical DMSP satellite data concerning the electron precipitation power. We find that the AC Poynting vector (Alfvén-wave related Poynting vector) is statistically not sufficient to power auroral electron precipitation, although it may, for Kp>2, power 25–50% of it. The statistical AC Poynting vector also has a stepwise transition at R=4 RE, so that its amplitude increases with increasing altitude. We suggest that this corresponds to Alfvén waves being in Landau resonance with electrons, so that wave-induced electron acceleration takes place at this altitude range, which was earlier named the Alfvén Resonosphere (ARS). The DC Poynting vector is ~3 times larger than electron precipitation and corresponds mainly to ionospheric Joule heating. In the morning sector (02:00–06:00 MLT) we find that the DC Poynting vector has a nontrivial altitude profile such that it decreases by a factor of ~2 when moving upward from 3 to 4 RE radial distance. In other nightside MLT sectors the altitude profile is more uniform. The morning sector nontrivial altitude profile may be due to divergence of the perpendicular Poynting vector field at R=3–4 RE. Keywords. Magnetospheric physics (Auroral phenomena; Magnetosphere-ionosphere interactions) – Space plasma physics (Wave-particle interactions)
A case study testing the cavity mode model of the magnetosphere
Authors: Sarafopoulos D. V.
Published: June 2010

Abstract. Based on a case study we test the cavity mode model of the magnetosphere, looking for eigenfrequencies via multi-satellite and multi-instrument measurements. Geotail and ACE provide information on the interplanetary medium that dictates the input parameters of the system; the four Cluster satellites monitor the magnetopause surface waves; the POLAR (L=9.4) and LANL 97A (L=6.6) satellites reveal two in-situ monochromatic field line resonances (FLRs) with T=6 and 2.5 min, respectively; and the IMAGE ground magnetometers demonstrate latitude dependent delays in signature arrival times, as inferred by Sarafopoulos (2004b). Similar dispersive structures showing systematic delays are also extensively scrutinized by Sarafopoulos (2005) and interpreted as tightly associated with the so-called pseudo-FLRs, which show almost the same observational characteristics with an authentic FLR. In particular for this episode, successive solar wind pressure pulses produce recurring ionosphere twin vortex Hall currents which are identified on the ground as pseudo-FLRs. The BJN ground magnetometer records the pseudo-FLR (alike with the other IMAGE station responses) associated with an intense power spectral density ranging from 8 to 12 min and, in addition, two discrete resonant lines with T=3.5 and 7 min. In this case study, even though the magnetosphere is evidently affected by a broad-band compressional wave originated upstream of the bow shock, nevertheless, we do not identify any cavity mode oscillation within the magnetosphere. We fail, also, to identify any of the cavity mode frequencies proposed by Samson (1992). Keywords. Magnetospheric physics (Magnetosphereionosphere interactions; Solar wind-magnetosphere interactions; MHD waves and instabilities)
Polar F-layer model-observation comparisons: a neutral wind surprise
Authors: Sojka J. J. , David M. , Schunk R. W. , van Eyken A. P.
Published: June 2010

Abstract. The existence of a month-long continuous database of incoherent scatter radar observations of the ionosphere from the EISCAT Savlbard Radar (ESR) at Longyearbyen, Norway, provides an unprecedented opportunity for model/data comparisons. Physics-based ionospheric models, such as the Utah State University Time Dependent Ionospheric Model (TDIM), are usually only compared with observations over restricted one or two day events or against climatological averages. In this study, using the ESR observations, the daily weather, day-to-day variability, and month-long climatology can be simultaneously addressed to identify modeling shortcomings and successes. Since for this study the TDIM is driven by climatological representations of the magnetospheric convection, auroral oval, neutral atmosphere, and neutral winds, whose inputs are solar and geomagnetic indices, it is not surprising that the daily weather cannot be reproduced. What is unexpected is that the horizontal neutral wind has come to the forefront as a decisive model input parameter in matching the diurnal morphology of density structuring seen in the observations.
Transpolar aurora: time evolution, associated convection patterns, and a possible cause
Authors: Blomberg L. G. , Cumnock J. A. , Alexeev I. I. , Belenkaya E. S. , Bobrovnikov S. Yu. , Kalegaev V. V.
Published: June 2010

Abstract. We present two event studies illustrating the detailed relationships between plasma convection, field-aligned currents, and polar auroral emissions, as well as illustrating the influence of the Interplanetary Magnetic Field's y-component on theta aurora development. The transpolar arc of the theta aurorae moves across the entire polar region and becomes part of the opposite side of the auroral oval. Electric and magnetic field and precipitating particle data are provided by DMSP, while the POLAR UVI instrument provides measurements of auroral emissions. Ionospheric electrostatic potential patterns are calculated at different times during the evolution of the theta aurora using the KTH model. These model patterns are compared to the convection predicted by mapping the magnetopause electric field to the ionosphere using the Paraboloid Model of the magnetosphere. The model predicts that parallel electric fields are set up along the magnetic field lines projecting to the transpolar aurora. Their possible role in the acceleration of the auroral electrons is discussed. Keywords. Ionosphere (Plasma convection; Polar ionosphere) – Magnetospheric physics (Magnetosphereionosphere interactions)
Study of the solar wind coupling to the time difference horizontal geomagnetic field
Authors: Wintoft P.
Published: June 2010

Abstract. The local ground geomagnetic field fluctuations (Δ B) are dominated by high frequencies and 83% of the power is located at periods of 32 min or less. By forming 10-min root-mean-square (RMS) of Δ B a major part of this variation is captured. Using measured geomagnetic induced currents (GIC), from a power grid transformer in Southern Sweden, it is shown that the 10-min standard deviation GIC may be computed from a linear model using the RMS Δ X and Δ Y at Brorfelde (BFE: 11.67° E, 55.63° N), Denmark, and Uppsala (UPS: 17.35° E, 59.90° N), Sweden, with a correlation of 0.926±0.015. From recurrent neural network models, that are driven by solar wind data, it is shown that the log RMS Δ X and Δ Y at the two locations may be predicted up to 30 min in advance with a correlation close to 0.8: 0.78±0.02 for both directions at BFE; 0.81±0.02 and 0.80±0.02 in the X- and Y-directions, respectively, at UPS. The most important inputs to the models are the 10-min averages of the solar wind magnetic field component Bz and velocity V, and the 10-min standard deviation of the proton number density σn. The average proton number density n has no influence. Keywords. Magnetospheric physics (Solar wind - magnetosphere interactions) – Geomagnetism and paleomagnetism (Rapid time variations)
Assessment of ionospheric Joule heating by GUMICS-4 MHD simulation, AMIE, and satellite-based statistics: towards a synthesis
Authors: Palmroth M. , Janhunen P. , Pulkkinen T. I. , Aksnes A. , Lu G. , Østgaard N. , Watermann J. , Reeves G. D. , Germany G. A.
Published: June 2010

Abstract. We investigate the Northern Hemisphere Joule heating from several observational and computational sources with the purpose of calibrating a previously identified functional dependence between solar wind parameters and ionospheric total energy consumption computed from a global magnetohydrodynamic (MHD) simulation (Grand Unified Magnetosphere Ionosphere Coupling Simulation, GUMICS-4). In this paper, the calibration focuses on determining the amount and temporal characteristics of Northern Hemisphere Joule heating. Joule heating during a substorm is estimated from global observations, including electric fields provided by Super Dual Auroral Network (SuperDARN) and Pedersen conductances given by the ultraviolet (UV) and X-ray imagers on board the Polar satellite. Furthermore, Joule heating is assessed from several activity index proxies, large statistical surveys, assimilative data methods (AMIE), and the global MHD simulation GUMICS-4. We show that the temporal and spatial variation of the Joule heating computed from the GUMICS-4 simulation is consistent with observational and statistical methods. However, the different observational methods do not give a consistent estimate for the magnitude of the global Joule heating. We suggest that multiplying the GUMICS-4 total Joule heating by a factor of 10 approximates the observed Joule heating reasonably well. The lesser amount of Joule heating in GUMICS-4 is essentially caused by weaker Region 2 currents and polar cap potentials. We also show by theoretical arguments that multiplying independent measurements of averaged electric fields and Pedersen conductances yields an overestimation of Joule heating. Keywords. Ionosphere (Auroral ionosphere; Modeling and forecasting; Electric fields and currents)
Range imaging observations of PMSE using the EISCAT VHF radar: Phase calibration and first results
Authors: Fernandez J. R. , Palmer R. D. , Chilson P. B. , Häggström I. , Rietveld M. T.
Published: June 2010

Abstract. A novel phase calibration technique for use with the multiple-frequency Range IMaging (RIM) technique is introduced based on genetic algorithms. The method is used on data collected with the European Incoherent SCATter (EISCAT) VHF radar during a 2002 experiment with the goal of characterizing the vertical structure of Polar Mesosphere Summer Echoes (PMSE) over northern Norway. For typical Doppler measurements, the initial phases of the transmitter and receiver are not required to be the same. The EISCAT receiver systems exploit this fact, allowing a multi-static configuration. However, the RIM method relies on the small phase differences between closely spaced frequencies. As a result, the high-resolution images produced by the RIM method can be significantly degraded if not properly calibrated. Using an enhanced numerical radar simulator, in which data from multiple sampling volumes are simultaneously generated, the proposed calibration method is validated. Subsequently, the method is applied to preliminary data from the EISCAT radar, providing first results of RIM images of PMSE. Data using conventional analysis techniques, and confirmed by RIM, reveal an often-observed double-layer structure with higher stability in the lower layer. Moreover, vertical velocity oscillations exhibit a clear correlation with the apparent motion of the layers shown in the echo power plots.
The effect of E-region wave heating on electrodynamical structures
Authors: Noël J.-M. A. , St.-Maurice J.-P. , Blelly P.-L.
Published: June 2010

Abstract. We show that heating by large amplitude E-region plasma waves at high latitudes can at times substantially enhance the electro-dynamical response of the ionosphere. This is made manifest through an increase in parallel current densities and parallel electric fields generated at the edge of arcs in the E and lower F-region of the ionosphere, in response to sharp cutoffs in precipitation with an otherwise uniform differential energy flux. The enhancement is rooted in a reduction in electron recombination that occurs in response to higher electron temperatures triggered by the generation of strong electric fields near the edge of the arc. The reduced recombination rate, in turn, leads to enhanced conductivity gradients near the edge of the arc, which, in turn, drives more intense parallel currents and stronger local electric fields. Keywords. Ionosphere (Electric fields and curents; Plasma temperature and density) – Space plasma physics (Numerical simulation studies)
The relationship between auroral hiss at high altitudes over the polar caps and the substorm dynamics of aurora
Authors: Titova E. E. , Yahnin A. G. , Santolík O. , Gurnett D. A. , Jirícek F. , Rauch J.-L. , Lefeuvre F. , Frank L. A. , Sigwarth J. B. , Mogilevsky M. M.
Published: June 2010

Abstract. Strong variations of intensity and cutoff frequency of the auroral hiss were observed by INTERBALL-2 and POLAR satellites at high altitudes, poleward from the auroral oval. The hiss intensifications are correlated with the auroral activations during substorms and/or pseudo-breakups. The low cutoff frequency of auroral hiss increases with the distance between the aurora and the satellite footprint. Multicomponent wave measurements of the hiss emissions on board the POLAR spacecraft show that the horizontal component of the Poynting flux of auroral hiss changes its direction in good accordance with longitudinal displacements of the bright auroras. The vertical component of the Poynting flux is directed upward from the aurora region, indicating that hiss could be generated by upgoing electron beams. This relationship between hiss and the aurora dynamics means that the upgoing electron beams are closely related to downgoing electron beams which produce the aurora. During the auroral activations the upgoing and downgoing beams move and change their intensities simultaneously. Keywords. Magnetospheric physics (Auroral phenomena; Plasma waves and instabilities; Storms and substorms)
Coordinated studies of the geospace environment using Cluster, satellite and ground-based data: an interim review
Authors: Amm O. , Donovan E. F. , Frey H. , Lester M. , Nakamura R. , Wild J. A. , Aikio A. , Dunlop M. , Kauristie K. , Marchaudon A. , McCrea I. W. , Opgenoorth H.-J. , Strømme A.
Published: June 2010

Abstract. A little more than four years after its launch, the first magnetospheric, multi-satellite mission Cluster has already tremendously contributed to our understanding about the coupled solar wind - magnetosphere - ionosphere system. This is mostly due to its ability, for the first time, to provide instantaneous spatial views of structures in the system, to separate temporal and spatial variations, and to derive velocities and directions of moving structures. Ground-based data have an important complementary impact on Cluster-related research, as they provide a larger-scale context to put the spacecraft data in, allow to virtually enlarge the spacecrafts' field of view, and make it possible to study in detail the coupling between the magnetosphere and the ionosphere in a spatially extended domain. With this paper we present an interim review of cooperative research done with Cluster and ground-based instruments, including the support of other space-based data. We first give a short overview of the instrumentation used, and present some specific data analysis and modeling techniques that have been devised for the combined analysis of Cluster and ground-based data. Then we review highlighted results of the research using Cluster and ground-based data, ordered into dayside and nightside processes. Such highlights include, for example, the identification of the spatio-temporal signatures of the different modes of reconnection on the dayside, and the detailed analysis of the electrodynamic magnetosphere-ionosphere coupling of bursty bulk flows in the tail plasma sheet on the nightside. The aim of this paper is to provide a "sourcebook" for the Cluster and ground-based community that summarises the work that has been done in this field of research, and to identify open questions and possible directions for future studies. Keywords. Ionosphere (Auroral ionosphere) – Magnetospheric physics (Magnetosphere-ionosphere interactions; General or miscellanous)
Creation of the substorm current wedge through the perturbation of the directly driven current system: a new model for substorm expansion
Authors: Rostoker G. , Friedrich E.
Published: June 2010

Abstract. The past four decades have seen a considerable amount of research on the study of magnetospheric substorms, and over most of these years the expansive phase of the substorm has been associated with the development of a three dimensional current system that has been termed the substorm current wedge. This current system has been thought to be a consequence of the short-circuiting of crosstail current through the ionosphere, and is viewed as a distinctive current system operating independently from the directly driven current with which it co-exists. The purpose of this paper is to show that the substorm current wedge should be viewed as an equivalent current system rather than a real current system. It will be shown that the magnetic perturbation pattern associated with the current wedge can be modeled as purely a perturbation of the directly driven current system in the midnight sector. Keywords. Magnetospheric physics (Auroral phenomena; Current systems; Magnetotail; Storms and substorms
Cluster observation of continuous reconnection at dayside magnetopause in the vicinity of cusp
Authors: Zheng Y. , Le G. , Slavin J. A. , Goldstein M. L. , Cattell C. , Balogh A. , Lucek E. A. , Rème H. , Eastwood J. P. , Wilber M. , Parks G. , Retinò A. , Fazakerley A.
Published: June 2010

Abstract. In this paper, we present a case study of continuous reconnection at the dayside magnetopause observed by the Cluster spacecraft. On 1 April 2003, the four Cluster spacecraft experienced multiple encounters with the Earth's dayside magnetopause under a fairly stable southwestward interplanetary magnetic field (IMF). Accelerated plasma flows, whose magnitude and direction are consistent with the predictions of the reconnection theory (the Walén relation), were observed at and around the magnetopause current layer for a prolonged interval of ~3 h at two types of magnetopause crossings, one with small magnetic shears and the other one with large magnetic shears. Reversals in the Y component of ion bulk flow between the magnetosheath and magnetopause current layer and acceleration of magnetosheath electrons were also observed. Kinetic signatures using electron and ion velocity distributions corroborate the interpretation of continuous magnetic reconnection. This event provides strong in-situ evidence that magnetic reconnection at the dayside magnetopause can be continuous for many hours. However, the reconnection process appeared to be very dynamic rather than steady, despite the steady nature of the IMF. Detailed analysis using multi-spacecraft magnetic field and plasma measurements shows that the dynamics and structure of the magnetopause current layer/boundary can be very complex. For example, highly variable magnetic and electric fields were observed in the magnetopause current layer. Minimum variance analysis shows that the magnetopause normal deviates from the model normal. Surface waves resulting from the reconnection process may be involved in the oscillation of the magnetopause. Keywords. Magnetospheric physics (Magnetopause, cusp and boundary layers; Solar wind-magnetosphere interactions) – Space plasma physics (magnetic reconnection)
An imaging interferometry capability for the EISCAT Svalbard Radar
Authors: Grydeland T. , Chau J. L. , La Hoz C. , Brekke A.
Published: June 2010

Abstract. Interferometric imaging (aperture synthesis imaging) is a technique used by radio astronomers to achieve angular resolution that far surpasses what is possible with a single large aperture. A similar technique has been used for radar imaging studies of equatorial ionospheric phenomena at the Jicamarca Radio Observatory. We present plans for adding an interferometric imaging capability to the EISCAT Svalbard Radar (ESR), a capability which will contribute significantly to several areas of active research, including naturally and artificially enhanced ion-acoustic echoes and their detailed relation in space and time to optical phenomena, polar mesospheric summer echoes (PMSE), and meteor studies. Interferometry using the two antennas of the ESR has demonstrated the existence of extremely narrow, field-aligned scattering structures, but having only a single baseline is a severe limitation for such studies. Building additional IS-class antennas at the ESR is not a trivial task. However, the very high scattering levels in enhanced ion-acoustic echoes and PMSE means that a passive receiver antenna of more modest gain should still be capable of detecting these echoes. In this paper we present simulations of what an imaging interferometer will be capable of observing for different antenna configurations and brightness distributions, under ideal conditions, using two different image inversion algorithms. We also discuss different antenna and receiver technologies.
Flux pile-up and plasma depletion at the high latitude dayside magnetopause during southward interplanetary magnetic field: a cluster event study
Authors: Moretto T. , Sibeck D. G. , Lavraud B. , Trattner K. J. , Rème H. , Balogh A.
Published: June 2010

Abstract. An event of strong flux pile-up and plasma depletion at the high latitude magnetopause tailward of the cusp has been analyzed based on observations by the suite of Cluster spacecraft. The multi-satellite analysis facilitates the separation of temporal and spatial features and provides a direct estimate for the strength of the plasma depletion layer for this event. A doubling of the magnetic field strength and a forty percent reduction of the density are found. Our analysis shows that roughly half of the total magnetic field increase occurs within 0.6 RE of the magnetopause and another quarter within a distance of 1.2 RE. In addition, the plasma depletion signatures exhibit temporal variations which we relate to magnetopause dynamics. Keywords. Magnetospheric physics (Magnetopause, Cusp and boundary layers; Magnetosheath; Solar windmagnetosphere interactions)
Fine-time energetic electron behavior observed by Cluster/RAPID in the magnetotail associated with X-line formation and subsequent current disruption
Authors: Vogiatzis I. I. , Fritz T. A. , Zong Q.-G. , Baker D. N. , Sarris E. T. , Daly P. W.
Published: June 2010

Abstract. Energetic electrons with 90deg pitch angle have been observed in the magnetotail at ~19 RE near local midnight during the recovery phase of a substorm event on 27 August 2001 (Baker et al., 2002). Based on auroral images Baker et al. (2002) placed the substorm expansion phase between ~04:06:16 and ~04:08:19 UT. The electron enhancements perpendicular to the ambient magnetic field occurred while the Cluster spacecraft were on closed field lines in the central plasma sheet approaching the neutral sheet. Magnetic field and energetic particle measurements have been employed from a number of satellites, in order to determine the source and the subsequent appearance of these electrons at the Cluster location. It is found that ~7.5 min after an X-line formation observed by Cluster (Baker et al., 2002) a current disruption event took place inside geosynchronous orbit and subsequently expanded both in local time and tailward, giving rise to field-aligned currents and the formation of a current wedge. A synthesis of tail reconnection and the cross-tail current disruption scenario is proposed for the substorm global initiation process: When a fast flow with northward magnetic field, produced by magnetic reconnection in the midtail, abruptly decelerates at the inner edge of the plasma sheet, it compresses the plasma populations earthward of the front, altering dynamically the Bz magnetic component in the current sheet. This provides the necessary and sufficient conditions for the kinetic cross-field streaming/current (KCSI/CFCI) instability (Lui et al., 1990, 1991) to initiate. As soon as the ionospheric conductance increases over a threshold level, the auroral electrojet is greatly intensified (see Fig. 2 in Baker et al., 2002), which leads to the formation of the substorm current wedge and dipolarization of the magnetic field. This substorm scenario combines the near-Earth neutral line and the current disruption for the initiation of substorms, at least during steady southward IMF. One can conclude the following: The observations suggest that the anisotropic electron increases observed by Cluster are not related to an acceleration mechanism associated with the X-line formation in the midtail, but rather these particles are generated in the dusk magnetospheric sector due to the longitudinal and tailward expansion of a current disruption region and subsequently observed at the Cluster location with no apparent energy dispersion. Keywords. Magnetospheric physics (Magnetotail; Plasma convection; Storms and substorms)