Showing papers in "Annales Geophysicae in 2011"

Pre-monsoon aerosol characteristics over the Indo-Gangetic Basin: implications to climatic impact

Abstract: . Sun/sky radiometer observations over the Indo-Gangetic Basin (IGB) region during pre-monsoon (from April–June 2009) have been processed to analyze various aerosol characteristics in the central and eastern IGB region, represented by Kanpur and Gandhi College, respectively, and their impacts on climate in terms of radiative forcing. Monthly mean aerosol optical depth (AOD at 500 nm) and corresponding Angstrom Exponent (AE at 440–870 nm, given within the brackets) was observed to be about 0.50 (0.49) and 0.51 (0.65) in April, 0.65 (0.74) and 0.67 (0.91) in May and 0.69 (0.45) and 0.77 (0.71) in June at Kanpur and Gandhi College, respectively. Results show a positive gradient in AOD and AE from central to eastern IGB region with the advancement of the pre-monsoon, which may be caused due to diverse geographical location of the stations having different meteorological conditions and emission sources. Relatively lower SSA was observed at the eastern IGB (0.89) than the central IGB (0.92) region during the period, which suggests relative dominance of absorbing aerosols at the eastern IGB as compared to central IGB region. The absorbing aerosol optical properties over the station suggest that the atmospheric absorption over central IGB region is mainly due to dominance of coarse-mode dust particles; however, absorption over eastern IGB region is mainly due to dominance of fine-particle pollution. The derived properties from sun/sky radiometer during pre-monsoon period are used in a radiative-transfer model to estimate aerosol radiative forcing at the top-of-the atmosphere (TOA) and at the surface over the IGB region. Relatively large TOA and surface cooling was observed at the eastern IGB as compared to the central IGB region. This translates into large heating of the atmosphere ranging from 0.45 to 0.55 K day−1 at Kanpur and from 0.45 to 0.59 K day−1 at Gandhi College.

A statistical and event study of magnetotail dipolarization fronts

Abstract: A study of dipolarization fronts of the Earth's magnetotail has been performed using seven years (2001–2007) of Cluster data Events both with and without high-speed earthward flows are included A superposed epoch analysis of the data shows that the dipolarization is preceeded by a decrease of Bz before the increase The duration of the dipolarization tends to be decreasing with increasing velocity of the plasma flows The thickness of the dipolarization front is on average 18 plasma inertial lengths, independent of the plasma velocity We find that the events fall into two categories: Earthward and tailward moving dipolarizations The dipolarization fronts can be assumed to be tangential discontinuities and the currents on the front have mainly a perpendicular component

Climatology of GPS phase scintillation and HF radar backscatter for the high-latitude ionosphere under solar minimum conditions

Abstract: . Maps of GPS phase scintillation at high latitudes have been constructed after the first two years of operation of the Canadian High Arctic Ionospheric Network (CHAIN) during the 2008–2009 solar minimum. CHAIN consists of ten dual-frequency receivers, configured to measure amplitude and phase scintillation from L1 GPS signals and ionospheric total electron content (TEC) from L1 and L2 GPS signals. Those ionospheric data have been mapped as a function of magnetic local time and geomagnetic latitude assuming ionospheric pierce points (IPPs) at 350 km. The mean TEC depletions are identified with the statistical high-latitude and mid-latitude troughs. Phase scintillation occurs predominantly in the nightside auroral oval and the ionospheric footprint of the cusp. The strongest phase scintillation is associated with auroral arc brightening and substorms or with perturbed cusp ionosphere. Auroral phase scintillation tends to be intermittent, localized and of short duration, while the dayside scintillation observed for individual satellites can stay continuously above a given threshold for several minutes and such scintillation patches persist over a large area of the cusp/cleft region sampled by different satellites for several hours. The seasonal variation of the phase scintillation occurrence also differs between the nightside auroral oval and the cusp. The auroral phase scintillation shows an expected semiannual oscillation with equinoctial maxima known to be associated with aurorae, while the cusp scintillation is dominated by an annual cycle maximizing in autumn-winter. These differences point to different irregularity production mechanisms: energetic electron precipitation into dynamic auroral arcs versus cusp ionospheric convection dynamics. Observations suggest anisotropy of scintillation-causing irregularities with stronger L-shell alignment of irregularities in the cusp while a significant component of field-aligned irregularities is found in the nightside auroral oval. Scintillation-causing irregularities can coexist with small-scale field-aligned irregularities resulting in HF radar backscatter. The statistical cusp and auroral oval are characterized by the occurrence of HF radar ionospheric backscatter and mean ground magnetic perturbations due to ionospheric currents.

MHD waves generated by high-frequency photospheric vortex motions

Abstract: . In this paper, we discuss simulations of MHD wave generation and propagation through a three-dimensional open magnetic flux tube in the lower solar atmosphere. By using self-similar analytical solutions for modelling the magnetic field in Cartesian coordinate system, we have constructed a 3-D magnetohydrostatic configuration which is used as the initial condition for non-linear MHD wave simulations. For a driver we have implemented a high-frequency vortex-type motion at the footpoint region of the open magnetic flux tube. It is found that the implemented swirly source is able to excite different types of wave modes, i.e. sausage, kink and torsional Alfven modes. Analysing these waves by magneto-seismology tools could provide insight into the magnetic structure of the lower solar atmosphere.

The solar and interplanetary causes of the recent minimum in geomagnetic activity (MGA23): a combination of midlatitude small coronal holes, low IMF B Z variances, low solar wind speeds and low solar magnetic fields

Abstract: . Minima in geomagnetic activity (MGA) at Earth at the ends of SC23 and SC22 have been identified. The two MGAs (called MGA23 and MGA22, respectively) were present in 2009 and 1997, delayed from the sunspot number minima in 2008 and 1996 by ~1/2–1 years. Part of the solar and interplanetary causes of the MGAs were exceptionally low solar (and thus low interplanetary) magnetic fields. Another important factor in MGA23 was the disappearance of equatorial and low latitude coronal holes and the appearance of midlatitude coronal holes. The location of the holes relative to the ecliptic plane led to low solar wind speeds and low IMF (Bz) variances (σBz2) and normalized variances (σBz2/B02) at Earth, with concomitant reduced solar wind-magnetospheric energy coupling. One result was the lowest ap indices in the history of ap recording. The results presented here are used to comment on the possible solar and interplanetary causes of the low geomagnetic activity that occurred during the Maunder Minimum.

Comprehensive calculation of the energy per ion pair or W values for five major planetary upper atmospheres

Abstract: . The mean energy W expended in a collision of electrons with atmospheric gases is a useful parameter for fast aeronomy computations. Computing this parameter in transport kinetic models with experimental values can tell us more about the number of processes that have to be taken into account and the uncertainties of the models. We present here computations for several atmospheric gases of planetological interest (CO2, CO, N2, O2, O, CH4, H, He) using a family of multi-stream kinetic transport codes. Results for complete atmospheres for Venus, Earth, Mars, Jupiter and Titan are also shown for the first time. A simple method is derived to calculate W of gas mixtures from single-component gases and is conclusively checked against the W values of these planetary atmospheres. Discrepancies between experimental and theoretical values show where improvements can be made in the measurement of excitation and dissociation cross-sections of specific neutral species, such as CO2 and CO.

Characteristics of the terrestrial field-aligned current system

Abstract: . We present the first ever comprehensive statistical study of the spatiotemporal characteristics of field-aligned currents in the terrestrial magnetosphere-ionosphere system using multi point measurements. We determine how the FAC density, variability and scale size are coupled. The three ST 5 satellites were in a pearls-on-a-string formation making measurements of the magnetic field with variable inter-spacecraft separations ranging from a few seconds to about 10 min. More than 4700 sets of satellite passes are analyzed using a robust correlation analysis aimed at determining the variability of the FAC system as a function of scale size and satellite spacing. We find significant differences between the FAC characteristics on the dayside and on the nightside in terms of dynamics of the current systems. On the dayside the FAC characteristics are found to be independent of IMF Bz and geomagnetic activity while the nightside indicates increased variability during disturbed conditions. The boundary separating highly and poorly correlated FACs can be fitted by a linear line for satellite separations shorter than 60 s (dayside) and 160 s (nightside). We interpret this as the dayside and nightside magnetospheric reconfiguration times respectively. For times exceeding this the FAC characteristics are suggested to be controlled by the solar wind (dayside) and plasma sheet (nightside) dynamics. Finally, the characteristics of FAC system with scale sizes larger than ~200 km (at ionospheric altitude) appear to be stable and repeatable on time scales of the order of a minute (i.e. comparable to the low-altitude orbiting satellite's traverse time across the auroral belt). In this sense, our results effectively validate the Iijima and Potemra (1978) assumption that on average the large-scale currents with scale sizes of the Region1 and Region2 are quasi-persistently significant in the transport of energy and momentum between the magnetosphere and the ionosphere.

On the origin of mesoscale TIDs at midlatitudes

Abstract: . A recent breakthrough experiment by Ogawa et al. (2009) showed that Mesoscale Traveling Ionospheric Disturbances (MSTIDs), a common phenomenon at midlatitudes, originate in the auroral zone as gravity waves. Curiously, however, the latter do not seem to be related to magnetic activity. These atmospheric waves are common at high latitudes (Bristow and Greenwald, 1996; Bristow et al., 1996), and we argue here that, as they propagate to lower latitudes, Joule damping reduces the gravity wave spectrum to waves suffering the weakest damping. The direction of weakest damping corresponds to the direction predicted by the Perkins instability (Perkins, 1973) for nighttime MSTIDs. The daytime features reported by Ogawa et al. (2009) are very likely due to classical gravity wave interaction with the F-region ionosphere.

Magnetopause reconnection across wide local time

Abstract: During April to July 2007 a combination of 10 spacecraft provided simultaneous monitoring of the dayside magnetopause across a wide range of local times. The array of four Cluster spacecraft, separated at large distances (10 000 km), were traversing the dawn-side magnetopause at high and low latitudes; the five THEMIS spacecraft were often in a 4 + 1 grouped configuration, traversing the low latitude, dusk-side magnetosphere, and the Double star, TC-1 spacecraft was in an equatorial orbit between the local times of the THEMIS and Cluster orbits. We show here a number of near simultaneous conjunctions of all 10 spacecraft at the magnetopause. One conjunction identifies an extended magnetic reconnection X-line, tilted in the low latitude, sub-solar region, which exists together with active anti-parallel reconnection sites extending to locations on the dawn-side flank. Oppositely moving FTE's are observed on all spacecraft, consistent with the initially strong IMF By conditions and the comparative locations of the spacecraft both dusk-ward and dawn-ward of noon. Comparison with other conjunctions of magnetopause crossings, which are also distributed over wide local times, supports the result that reconnection activity may occur at many sites simultaneously across the sub-solar and flank magnetopause, but linked to the large scale (extended) configuration of the merging line; broadly depending on IMF orientation. The occurrence of MR therefore inherently follows a "component" driven scenario irrespective of the guide field conditions. Some conjunctions allow the global magnetopause response to IMF changes to be observed and the distribution of spacecraft can directly confirm its shape, motion and deformation at local noon, dawn and dusk-side, simultaneously.

On the nature of particle energization via resonant wave-particle interaction in the inhomogeneous magnetospheric plasma

Abstract: . When a quasi-monochromatic wave propagating in an inhomogeneous magnetoplasma has sufficiently large amplitude, there exist phase-trapped resonant particles whose energy increases or decreases depending on the "sign" of inhomogeneity. The variation of energy density of such particles can greatly exceed the wave energy density which contradicts energy conservation under the prevalent assumption that the wave serves as the energy source or sink. We show that, in fact, the energy increase (or decrease) of phase-trapped particles is related to energy transfer from (to) phase untrapped particles, while the wave basically mediates the energization process. Virtual importance of this comprehension consists in setting proper quantitative constraints on attainable particle energy. The results have immediate applications to at least two fundamental problems in the magnetospheric physics, i.e. particle dynamics in the radiation belts and whistler-triggered emissions.

Data derived NARMAX Dst model

Abstract: . The NARMAX OLS-ERR methodology is applied to identify a mathematical model for the dynamics of the Dst index. The NARMAX OLS-ERR algorithm, which is widely used in the field of system identification, is able to identify a mathematical model for a wide class of nonlinear systems using input and output data. Solar wind-magnetosphere coupling functions, derived from analytical or data based methods, are employed as the inputs to such models and the outputs are geomagnetic indices. The newly deduced coupling function, p1/2V4/3BTsin6(θ/2), has been implemented as an input to model the Dst dynamics. It was shown that the identified model has a very good forecasting ability, especially with the geomagnetic storms.

Current sheets from Ulysses observation

Abstract: . Current sheet is a significant source of solar wind MHD turbulence intermittency. It has long been recognized that these structures can arise from non-linear interactions of MHD turbulence. Alternatively, they may also be relic structures in the solar wind that have a solar origin, e.g., magnetic walls of flux tubes that separate solar wind plasma into distinct parcels. Identifying these structures in the solar wind is crucial to understanding the properties of the solar wind MHD turbulence. Using Ulysses observations we examine 3-year worth of solar wind magnetic field data when the Ulysses is at low latitude during solar minimum. Extending the previous work of Li (2007, 2008), we develop an automatic data analysis method of current sheet identification. Using this method, we identify more than 28000 current sheets. Various properties of the current sheet are obtained. These include the distributions of the deflection angle across the current sheet, the thickness of the current sheet and the waiting time statistics between current sheets.

Interhemispheric comparison of GPS phase scintillation at high latitudes during the magnetic-cloud-induced geomagnetic storm of 5–7 April 2010

Abstract: . Arrays of GPS Ionospheric Scintillation and TEC Monitors (GISTMs) are used in a comparative scintillation study focusing on quasi-conjugate pairs of GPS receivers in the Arctic and Antarctic. Intense GPS phase scintillation and rapid variations in ionospheric total electron content (TEC) that can result in cycle slips were observed at high latitudes with dual-frequency GPS receivers during the first significant geomagnetic storm of solar cycle 24 on 5–7 April 2010. The impact of a bipolar magnetic cloud of north-south (NS) type embedded in high speed solar wind from a coronal hole caused a geomagnetic storm with maximum 3-hourly Kp = 8- and hourly ring current Dst = −73 nT. The interhemispheric comparison of phase scintillation reveals similarities but also asymmetries of the ionospheric response in the northern and southern auroral zones, cusps and polar caps. In the nightside auroral oval and in the cusp/cleft sectors the phase scintillation was observed in both hemispheres at about the same times and was correlated with geomagnetic activity. The scintillation level was very similar in approximately conjugate locations in Qiqiktarjuaq (75.4° N; 23.4° E CGM lat. and lon.) and South Pole (74.1° S; 18.9° E), in Longyearbyen (75.3° N; 111.2° E) and Zhongshan (74.7° S; 96.7° E), while it was significantly higher in Cambridge Bay (77.0° N; 310.1° E) than at Mario Zucchelli (80.0° S; 307.7° E). In the polar cap, when the interplanetary magnetic field (IMF) was strongly northward, the ionization due to energetic particle precipitation was a likely cause of scintillation that was stronger at Concordia (88.8° S; 54.4° E) in the dark ionosphere than in the sunlit ionosphere over Eureka (88.1° N; 333.4° E), due to a difference in ionospheric conductivity. When the IMF tilted southward, weak or no significant scintillation was detected in the northern polar cap, while in the southern polar cap rapidly varying TEC and strong phase scintillation persisted for many hours. This interhemispheric asymmetry is explained by the difference in the location of solar terminator relative to the cusps in the Northern and Southern Hemisphere. Solar terminator was in the immediate proximity of the cusp in the Southern Hemisphere where sunlit ionospheric plasma was readily convected into the central polar cap and a long series of patches was observed. In contrast, solar terminator was far poleward of the northern cusp thus reducing the entry of sunlit plasma and formation of dense patches. This is consistent with the observed and modeled seasonal variation in occurrence of polar cap patches. The GPS scintillation and TEC data analysis is supported by data from ground-based networks of magnetometers, riometers, ionosondes, HF radars and all-sky imagers, as well as particle flux measurements by DMSP satellites.

Apparent temperature anisotropies due to wave activity in the solar wind

Abstract: . The fast solar wind is a collisionless plasma permeated by plasma waves on many different scales. A plasma wave represents the natural interplay between the periodic changes of the electromagnetic field and the associated coherent motions of the plasma particles. In this paper, a model velocity distribution function is derived for a plasma in a single, coherent, large-amplitude wave. This model allows one to study the kinetic effects of wave motions on particle distributions. They are by in-situ spacecraft measured by counting, over a certain sampling time, the particles coming from various directions and having different energies. We compare our results with the measurements by the Helios spacecraft, and thus find that by assuming high wave activity we are able to explain key observed features of the measured distributions within the framework of our model. We also address the recent discussions on nonresonant wave–particle interactions and apparent heating. The applied time-averaging procedure leads to an apparent ion temperature anisotropy which is connected but not identical to the intrinsic temperature of the underlying distribution function.

Suprathermal electron acceleration during reconnection onset in the magnetotail

Abstract: . We study one event of reconnection onset associated to a small substorm on 27 September 2006 by using Cluster observations at inter-spacecraft separation of about 10 000 km. We focus on the acceleration of suprathermal electrons during different stages of reconnection. We show that several distinct stages of acceleration occur: (1) moderate acceleration during reconnection of pre-existing plasma sheet flux tubes, (2) stronger acceleration during reconnection of lobe flux tubes, (3) production of the most energetic electrons within dipolarization fronts (magnetic pile-up regions). The strongest acceleration is reached at the location of Bz maxima inside the magnetic pile-up region where the reconnection jet stops. Very strong localized dawn-dusk electric field are observed within the magnetic pile-up regions and are associated to most of the magnetic flux transport.

Proton/electron temperature ratio in the magnetotail

Abstract: We study the temperature ratio of protons and electrons based on statistics of Cluster observations in the Earth's magnetotail from 2001 to 2004 During these years, from June to November, the Cluster spacecraft visited the plasma sheet at the distance r~−19 RE We use proton and electron moments collected in the central region of the plasma sheet (|Z|

Properties of standing Kruskal-Schwarzschild-modes at the magnetopause

Abstract: . The radial, oscillatory motion of the Earth's magnetopause has been found to occur predominantly with some distinct, sometimes called "magic" frequencies, which have been attributed to magnetospheric wave guide modes, typical solar wind variations or, more recently, surface waves on the magnetopause standing between the northern and southern ionospheres. In this paper we present for the first time a derivation of these surface waves, denominated as Kruskal-Schwarzschild-modes (KS-modes), in the approximation of the ideal, single-fluid magnetohydrodynamic theory for incompressible plasmas. The calculations are performed in the simplified geometry of the box magnetosphere with the magnetopause being a plane between two plasma regimes of homogeneous conditions. The reflection of the KS-modes at the ionospheres is being discussed. Under the given assumptions and realistic conditions the validity of the calculations is shown to be limited to cases of parallel or anti-parallel background magnetic fields on both sides of the magnetopause, respectively. For these cases a detailed discussion of the mode structure is presented. The magnetopause when affected by a KS-mode is found to resemble a membrane under tension with respect to its motion; the ionospheres act as supporting points of the membrane and the KS-modes correspond in this picture to their eigenmodes of oscillation. Localized pressure enhancements in the magnetosheath are discussed as possible excitation mechanism for the KS-modes.

Climatology of the inter-hemispheric field-aligned current system in the equatorial ionosphere as observed by CHAMP

Abstract: . From geomagnetic field observations of CHAMP during 2001–2009 we extracted characteristic signatures of inter-hemispheric field-aligned currents (IHFACs) in the equatorial ionosphere. The results are in general agreement with previous observations. Nighttime IHFACs are negligibly small. Solstitial IHFACs flow from the summer to winter (from winter to summer) hemisphere at dawn (around noon). Duskside IHFACs flow southbound irrespective of season. We have also found some new IHFAC properties, which may have been predicted by theories, but are not yet given observational support. IHFACs clearly exhibit a longitude dependence, which is modulated by the South Atlantic Anomaly, the offset between geographic and magnetic equators, and tidal waves. IHFACs show little dependence on the solar cycle. We provide a comprehensive assessment of the IHFAC modulation by non-migrating tides.

Photospheric magnetic vortex structures

Abstract: . Using direct numerical magneto-hydrodynamic (MHD) simulations, we demonstrate the evidence of two physically different types of vortex motions in the solar photosphere. Baroclinic motions of plasma in non-magnetic granules are the primary source of vorticity in granular regions of the solar photosphere, however, there is a significantly more efficient mechanism of vorticity production in strongly magnetised intergranular lanes. These swirly motions of plasma in intergranular magnetic field concentrations could be responsible for the generation of different types of MHD wave modes, for example, kink, sausage and torsional Alfven waves. These waves could transport a relevant amount of energy from the lower solar atmosphere and contribute to coronal plasma heating.

Vertical electron density and topside effective scale height ( H T ) variations over the Indian equatorial and low latitude stations

Abstract: . Understanding the vertical electron density profile, which is the altitudinal variation of ionospheric electron density distribution is an important aspect for the ionospheric investigations. In this paper, the bottom-side electron density profiles derived from ground based ionosonde data and the ROCSAT-1 in-situ electron density data were used to determine the estimates of the topside electron density profiles using α-Chapman function over an equatorial station Trivandrum (8.47° N, 76.91° E) and a low latitude station Waltair (17.7° N, 83.3° E) in the Indian region. The reconstructed electron density profiles are compared with IRI (2007) model derived vertical electron density profiles which resulted in significant deviations between the two different profiles. Both the reconstructed electron density profiles and the IRI model derived profiles are integrated independently to derive the Total Electron Content (TEC) values which are compared with GPS derived TEC values. TEC values derived from the reconstructed electron density profiles give better estimates with the GPS-TEC compared to those of IRI model derived TEC values. Compared to the GPS-TEC, the IRI model is underestimating the TEC values during day-time and is overestimating during night-time at both the stations. The percentage deviations of IRI derived TEC from GPS-TEC are larger compared to those between reconstructed profile derived TEC and GPS-TEC. F2-layer peak electron density, peak height and electron density at ROCSAT altitudes (≈600 km) are used to derive the effective scale heights (HT) of the topside ionosphere during the period from July 2003 to June 2004. The diurnal and seasonal variations of HT and E×B drift velocities are presented in this paper. The diurnal variation of the effective scale height (HT) shows peak values around noon hours with higher values during day-time and lower values during night-time both at Trivandrum and Waltair. The E×B drift velocities at both the places also have shown a clear diurnal variation with a negative peak around 04:00 LT and maximum during day-time hours. The higher and lower values of HT seem to be associated with positive and negative phases of the E×B drift velocities, respectively.

Modeling ionospheric foF2 by using empirical orthogonal function analysis

Abstract: . A similar-parameters interpolation method and an empirical orthogonal function analysis are used to construct empirical models for the ionospheric foF2 by using the observational data from three ground-based ionosonde stations in Japan which are Wakkanai (Geographic 45.4° N, 141.7° E), Kokubunji (Geographic 35.7° N, 140.1° E) and Yamagawa (Geographic 31.2° N, 130.6° E) during the years of 1971–1987. The impact of different drivers towards ionospheric foF2 can be well indicated by choosing appropriate proxies. It is shown that the missing data of original foF2 can be optimal refilled using similar-parameters method. The characteristics of base functions and associated coefficients of EOF model are analyzed. The diurnal variation of base functions can reflect the essential nature of ionospheric foF2 while the coefficients represent the long-term alteration tendency. The 1st order EOF coefficient A1 can reflect the feature of the components with solar cycle variation. A1 also contains an evident semi-annual variation component as well as a relatively weak annual fluctuation component. Both of which are not so obvious as the solar cycle variation. The 2nd order coefficient A2 contains mainly annual variation components. The 3rd order coefficient A3 and 4th order coefficient A4 contain both annual and semi-annual variation components. The seasonal variation, solar rotation oscillation and the small-scale irregularities are also included in the 4th order coefficient A4. The amplitude range and developing tendency of all these coefficients depend on the level of solar activity and geomagnetic activity. The reliability and validity of EOF model are verified by comparison with observational data and with International Reference Ionosphere (IRI). The agreement between observations and EOF model is quite well, indicating that the EOF model can reflect the major changes and the temporal distribution characteristics of the mid-latitude ionosphere of the Sea of Japan region. The error analysis processes imply that there are seasonal anomaly and semi-annual asymmetry phenomena which are consistent with pre-existing ionosphere theory.

Numerical simulations of local circulations and pollution transport over Reunion Island

Abstract: . A series of high-resolution (1 km) numerical simulations with a limited-area numerical model has been performed over Reunion Island. In the dynamical context of a regular maritime flow perturbed by a major topographic obstacle such as Reunion Island, the objectives are to identify the main atmospheric circulations at local-scale over the island and to improve the understanding of local-scale transport and dispersion of pollutants emitted from local sources. To investigate the effects of topography and land surface heating on low-level flows over the island, simulations representative of austral winter were performed in idealized conditions keeping the radiative forcing plus a background east-south-easterly synoptic flux of varying strengths, typical of the prevailing trade-wind conditions. The numerical experiments show mainly that flow splitting of the trade-wind occurs around the island, with enhanced winds blowing along the coast lines parallel to the synoptic flux, due to the lateral constriction of the flow by the island and resulting Venturi effect. Blocking occurs on the island side facing the trade-wind. The north-western area on the leeside is screened from the trade-wind by high mountains, and this enables the development of diurnal thermally-induced circulations, combining downslope and land-breeze at night, and upslope and sea breeze at daytime. Flow splitting is modulated by radiative convergence toward the island at daytime, and divergence from the island at night. Stronger winds than the large-scale trade-wind occur along the coast at daytime (Venturi effect), whereas at night, the trade-wind flow is pushed few kilometres offshore by divergence of cooled air from the land. At night, the trade-wind flow is pushed few kilometres offshore by divergence of cooled air from the land. Consequently, a number of processes of pollution transport and dispersion have been identified. Vortices in the wake of the island were found to cause counterflow circulation and trapping of polluted air masses near the north-western coast. These air masses may in turn be sucked by anabatic wind systems during daytime (upslope and sea breezes) in the cirques and up to the summits of the island, and especially to Piton Maido (2200 m) where a new observatory of the Indian Ocean background atmosphere is being built. A "cap effect" above the mountains downstream from the volcano (to the south-east of the island), and especially above Piton Maido, might occur in case of development of inland and upslope breezes on the west coast. In this case, air pumped from lower layers may protect the observatory from the volcanic plume forced to pass over a "cap" of low-level air clean of volcanic emissions.

High-resolution wave number spectrum using multi-point measurements in space – the Multi-point Signal Resonator (MSR) technique

Abstract: . A new analysis method is presented that provides a high-resolution power spectrum in a broad wave number domain based on multi-point measurements. The analysis technique is referred to as the Multi-point Signal Resonator (MSR) and it benefits from Capon's minimum variance method for obtaining the proper power spectral density of the signal as well as the MUSIC algorithm (Multiple Signal Classification) for considerably reducing the noise part in the spectrum. The mathematical foundation of the analysis method is presented and it is applied to synthetic data as well as Cluster observations of the interplanetary magnetic field. Using the MSR technique for Cluster data we find a wave in the solar wind propagating parallel to the mean magnetic field with relatively small amplitude, which is not identified by the Capon spectrum. The Cluster data analysis shows the potential of the MSR technique for studying waves and turbulence using multi-point measurements.

EISCAT-Cluster observations of quiet-time near-Earth magnetotail fast flows and their signatures in the ionosphere

Abstract: . We report observations of a sequence of quiet-time Earthward bursty bulk flows (BBFs) measured by the Cluster spacecraft in the near-tail plasma sheet (XGSM ~ −12 to −14 RE) in the evening sector, and by simultaneous high-resolution measurements in the northern conjugate ionosphere by the EISCAT radars, a MIRACLE all-sky camera and magnetometers, as well as a meridian-scanning photometer (MSP) in the Scandinavian sector on 17 October 2005. The BBFs at Cluster show signatures that are consistent with the plasma "bubble" model (Chen and Wolf, 1993, 1999), e.g. deflection and compression of the ambient plasma in front of the Earthward moving bubble, magnetic signatures of a flow shear region, and the proper flows inside the bubble. In addition, clear signatures of tailward return flows around the edges of the bubble can be identified. The duskside return flows are associated with significant decrease in plasma density, giving support to the recent suggestion by Walsh et al. (2009) of formation of a depleted wake. However, the same feature is not seen for the dawnside return flows, but rather an increase in density. In the ionosphere, EISCAT and optical measurements show that each of the studied BBFs is associated with an auroral streamer that starts from the vicinity of the polar cap boundary, intrudes equatorward, brakes at 68–70° aacgm MLAT and drifts westward along the proton oval. Within the streamer itself and poleward of it, the ionospheric plasma flow has an equatorward component, which is the ionospheric manifestation of the Earthward BBF channel. A sharp velocity shear appears at the equatorward edge of a streamer. We suggest that each BBF creates a local velocity shear in the ionosphere, in which the plasma flow poleward of and inside the streamer is in the direction of the streamer and southeastward. A northwestward return flow is located on the equatorward side. The return flow is associated with decreased plasma densities both in the ionosphere and in the magnetosphere as measured by EISCAT and Cluster, respectively. In summary, we present the first simultaneous high-resolution observations of BBF return flows both in the plasma sheet and in the ionosphere, and those are in accordance with the bubble model. The results apply for the duskside return flows, but the manifestation of dawnside return flows in the ionosphere requires further studies. Finally, EISCAT measurements indicate increased nightside reconnection rate during the ~35-min period of BBFs. We suggest that the observed temporal event of IMF rotation to a more southward direction produces enhanced open flux transport to the nightside magnetotail, and consequently, the nightside reconnection rate is increased.

Statistical characteristics of gravity wave activities observed by an OH airglow imager at Xinglong, in northern China

Abstract: . An all-sky airglow imager (ASAI) was installed at Xinglong, in northern China (40.2° N, 117.4° E) in November 2009 to study the morphology of atmospheric gravity waves (AGWs) in the mesosphere and lower thermosphere (MLT) region. Using one year of OH airglow imager data from December 2009 to November 2010, the characteristics of short-period AGWs are investigated and a yearlong AGW climatology in northern China is first ever reported. AGW occurrence frequency in summer and winter is higher than that in equinoctial months. Observed bands mainly have horizontal wavelengths from 10 to 35 km, observed periods from 4 to 14 min and observed horizontal phase speeds in the range of 30 to 60 m s−1. Most of the bands propagate in the meridional direction. The propagation directions of the bands show a strong southwestward preference in winter, while almost all bands propagate northeastward in summer. Although the wind filtering in the middle atmosphere may control AGW propagations in the zonal direction, the non-uniform distribution of wave sources in the lower atmosphere may contribute to the anisotropy in the meridional direction in different seasons. Additionally, as an indication of local instability, the characteristics of ripples are also analyzed. It also shows seasonal variations, occurring more often in summer and winter and mainly moving westward in summer and eastward in winter.

Interplanetary coronal mass ejections in the near-Earth solar wind during the minimum periods following solar cycles 22 and 23

Abstract: . In this paper we examine the occurrence rates and properties of interplanetary coronal mass ejections (ICMEs) and solar activity levels during the minima following solar cycle 22 (January 1995–December 1997) and 23 (January 2007–April 2010) minima using observations from the OMNI data base. Throughout the minimum following cycle 22 the CME and ICME rates roughly tracked each other, while for the minimum following cycle 23 they diverged. During the minimum after solar cycle 23, there were large variations in the streamer belt structure. During the lowest activity period of cycle 23 (based on sunspot numbers), the ICME rate was about four times higher than during a similar activity period of cycle 22. We propose that this relatively high ICME rate may be due to CME source regions occurring at lower heliolatitudes and due to equatoward deflection of slow and weak CMEs originating from the mid- and high-heliolatitudes. The maximum magnetic fields of the ICMEs identified during the minimum following cycle 23 were ~30 % lower and their radial widths were ~15 % lower compared to the ICMEs observed during the minimum following solar cycle 22. The weak and small ICMEs may result from intrinsically weak CMEs and/or they may represent stronger CMEs that are encountered far away from the center.

Mid-latitude Summer Nighttime Anomaly (MSNA) - Observations and model simulations

Abstract: . In this paper, we present model simulations of the Mid-latitude Summer Nighttime Anomaly (MSNA) in the Northern Hemisphere, which is characterized by noon-time dip and evening maximum in the diurnal variation of the ionospheric density. The simulations are carried out using SUPIM (Sheffield University Plasmasphere Ionosphere Model) for solar minimum at 135° E longitude where MSNA is most pronounced in the Northern Hemisphere. The simulations are used to understand the relative importance of electric fields, and zonal and meridional winds in the formation of MSNA. The wind velocities measured by the Middle and Upper atmosphere radar (MU radar) and those obtained from the horizontal wind model (HWM93) are used. The results show that the formation of MSNA is closely related to the diurnal variation of the neutral winds with little contribution from the changes in the electric fields. The observed features of MSNA are better reproduced when MU radar winds are used as model input rather than HWM winds.

Exploring planetary magnetic environments using magnetically unclean spacecraft: a systems approach to VEX MAG data analysis

Abstract: . In situ measurements of the magnetic field are vital to the study of many fundamental problems in planetary research. Therefore the magnetometer experiment is a key element of the payload of Venus Express. In addition to the interaction of the solar wind with Venus, these measurements are crucial for the study of atmospheric escape and detection of lightning. However, the methodology for the magnetic field measurements had to be different to the traditional approach, because Venus Express is not a magnetically clean spacecraft. A technique based on two-point simultaneous measurements of the magnetic field and systems identification software is used to separate the natural magnetic field from the spacecraft generated interference. In this paper an overview of the techniques developed to separate these two field types and the results achieved for 1 Hz Venus Express data are presented. Previous publications suggest that the resulting Venus Express cleaned data is of comparable quality to measurements made from onboard magnetically clean spacecraft (Zhang et al., 2008a, b; Slavin et al., 2009).

The TWINS exospheric neutral H-density distribution under solar minimum conditions

Abstract: . Terrestrial exospheric atomic hydrogen (H) resonantly scatters solar Lyman-α (121.567 nm) radiation, observed as the glow of the H-geocorona. The Two Wide-angle Imaging Neutral-atom Spectrometers (TWINS) satellites are equiped with two Lyman-α line-of-sight Detectors (LADs) each. Since during the past solar minimum conditions the relevant solar control parameters practically did not vary, we are using LAD data between June and September 2008 to create a time averaged hydrogen geocorona model representative for these solar minimum conditions. In this averaged model we assume that the H-geocorona is longitudinally symmetric with respect to the earth-sun line. We find a 3-dimensional H-density distribution in the range from 3 to 8 earth radii which with some caution can also be extrapolated to larger distances. For lower geocentric distances than 3 earth radii a best fitting r-dependent Chamberlain (1963)-like model is adapted. Main findings are larger than conventionally expected H-densities at heights above 5 RE and a pronounced day-to-night side H-density asymmetry. The H-geocorona presented here should serve as a reference H-atmosphere for the earth during solar minimum conditions.

Statistical evidence for O + energization and outflow caused by wave-particle interaction in the high altitude cusp and mantle

Abstract: We present a statistical study of the low (<1 Hz) frequency electric and magnetic field spectral densities observed by Cluster spacecraft in the high altitude cusp and mantle region At the O+ gyro