Auroral E-region electron density gradients measured with EISCAT

In the theory of E-region plasma instabilities, the ambient electric field and electron density gradient are both included in the same dispersion relation as the key parameters that provide the energy for the generation and growth of electrostatic plasma waves. While there exist numerous measurements of ionospheric electric fields, there are very few measurements and limited knowledge about the ambient electron density gradients, N_e, in the E-region plasma. In this work, we took advantage of the EISCAT CP1 data base and studied statistically the vertical electron density gradient length, L_z = N_e/(dN_e/d_z), at auroral E-region heights during both eastward and westward electrojet conditions and different ambient electric field levels. Overall, the prevailing electron density gradients, with L_z ranging from 4 to 7 km, are found to be located below 100 km, but to move steadily up in altitude as the electric field level increases. The steepest density gradients, with L_z possibly less than 3 km, occur near 110 km mostly in the eastward electrojet during times of strong electric fields. The results and their implications are examined and discussed in the frame of the linear gradient drift instability theory. Finally, it would be interesting to test the implications of the present results with a vertical radar interferometer.     

TheT e-T n difference in the lowerE region and radio-wave absorption

Summary The difference observed between the electron temperature, T _e , and the neutral atmosphere temperature, T _n , is shown to be able to contribute significantly to the observed variability of ionospheric radio-wave absorption (A1 andHF-A3 measurements).     

Electron temperatures during rapid subauroral ion drift events

Examples of data from DE-2 satellite instruments are presented. These illustrate the behaviour of plasma parameters in the F-region and adjacent topside ionosphere during rapid sub-auroral ion drift (SAID) events. In particular, a variety of behaviours of the electron temperature (T_e) is demonstrated, both within and equatorward of the SAID region. The Sheffield University plasmasphere-ionosphere model (SUPIM) is used to perform calculations in which a model SAID is applied to a plasma flux tube. The model results indicate that strongly elevated ion temperature (a recognised signature of SAID events) is on occasion sufficient to raise T_e to observed values by ion-electron heat transfer. On other occasions, an additional heat source is required. It is suggested that such a source for the electron gas may be due to interaction between the ring current and the plasmasphere at high altitudes. The magnitude of the downward heat flux is consistent with that necessary to produce sub-auroral red arcs. The resulting strongly heated electron gas causes vibrational excitation of molecular nitrogen in the thermosphere.     

First EISCAT measurement of electron-gas temperature in the artificially heated D-region ionosphere

The ionospheric electron gas can be heated artificially by a powerful radio wave. According to our modeling, the maximum effect of this heating occurs in the D-region where the electron temperature can increase by a factor of ten. Ionospheric plasma parameters such as N_e,T_e and T_i are measured by EISCAT incoherent scatter radar on a routine basis. However, in the D-region the incoherent scatter echo is very weak because of the low electron density. Moreover, the incoherent scatter spectrum from the D-region is of Lorentzian shape which gives less information than the spectrum from the E- and F-regions. These make EISCAT measurements in the D-region difficult. A combined EISCAT VHF-radar and heating experiment was carried out in November 1998 with the aim to measure the electron temperature increase due to heating. In the experiment the heater was switched on/off at 5 minute intervals and the integration time of the radar was chosen synchronously with the heating cycle. A systematic difference in the measured autocorrelation functions was found between heated and unheated periods.     

Some F2-layer effects during the January06-11,1997 cedar storm period as observed with theMillstoneHill incoherent scatter facility

An observed daytime negative storm effect on 10 January and strongpost-midnight N_m F₂ increases on 7 and 8 January followed by a steep T_e decrease down to T_n are analyzed. Daytime neutral composition and T_n(h) were inferred from incoherent scatter (IS) radar observations using aself-consistent approach to F₂ -layer modelling. Observed daytime T_e , T_i , N_e profiles were corrected for the disturbed day using the calculated ioncomposition. The main reason for the observed N_m F₂ negative storm effectis the O/(N₂ O₂ ) ratio decrease in the thermosphere. The calculated O/N₂ ratio at hm F₂ is less by a factor of 3.7 than MSIS prediction for 10January and is close to MSIS for quiet days. Strong Nm F₂ night-time increase by afactor of 3 is due to a plasma compression mechanism under the action of observed EB drift(westward E-field) moving plasma from higher L shells to lower ones and squeezing it into the F₂ -region. Moderate (1-2)10 ⁸ cm ^-2 s ^-1 O fluxes in agreement with IS observations were shown to be sufficient to account for the night-timeN_m F₂ increase. This elevation in the F₂ -region electron densityis shown to be the main reason for the observed steep Te post-midnight decrease. 1999 ElsevierScience Ltd. All rights reserved.     

Upward continuation of Dome-C airborne gravity and comparison with GOCE gradients at orbit altitude in east Antarctica

An airborne gravity campaign was carried out at the Dome-C survey area in East Antarctica between the 17th and 22nd of January 2013, in order to provide data for an experiment to validate GOCE satellite gravity gradients. After typical filtering for airborne gravity data, the cross-over error statistics for the few crossing points are 11.3 mGal root mean square (rms) error, corresponding to an rms line error of 8.0 mGal. This number is relatively large due to the rough flight conditions, short lines and field handling procedures used. Comparison of the airborne gravity data with GOCE RL4 spherical harmonic models confirmed the quality of the airborne data and that they contain more high-frequency signal than the global models. First, the airborne gravity data were upward continued to GOCE altitude to predict gravity gradients in the local North-East-Up reference frame. In this step, the least squares collocation using the ITGGRACE2010S field to degree and order 90 as reference field, which is subtracted from both the airborne gravity and GOCE gravity gradients, was applied. Then, the predicted gradients were rotated to the gradiometer reference frame using level 1 attitude quaternion data. The validation with the airborne gravity data was limited to the accurate gradient anomalies (T_XX, T_YY, T_ZZ and T_XZ) where the long-wavelength information of the GOCE gradients has been replaced with GOCO03s signal to avoid contamination with GOCE gradient errors at these wavelengths. The comparison shows standard deviations between the predicted and GOCE gradient anomalies T_XX, T_YY, T_ZZ and T_XZ of 9.9, 11.5, 11.6 and 10.4 mE, respectively. A more precise airborne gravity survey of the southern polar gap which is not observed by GOCE would thus provide gradient predictions at a better accuracy, complementing the GOCE coverage in this region.     

Reference evapotranspiration trends and their sensitivity to climatic change on the Tibetan Plateau (1970–2009)

Evapotranspiration is an important component of the water and energy balance. It is dependent on climate. Precipitation, solar radiation, temperature, humidity, and wind all contribute to the rate of evapotranspiration. In this study, the temporal trends of reference evapotranspiration (ET_ref) and four main ET_ref drivers, namely, mean air temperature (T_a), wind speed (u₂), net radiation (R_n) and actual vapour pressure (e_a) from 1970 to 2009, were calculated based on 75 meteorological stations on the Tibetan Plateau. The results showed that the ET_ref on the Tibetan Plateau decreased on average by 0.6909 mm a^‐1a^‐1 from 1970 to 2009. T_a and e_a showed an increasing trend, whereas u₂ and R_n exhibited a decreasing trend. To explore the underlying causes of the ET_ref variation, an attribution analysis was performed to quantify the contribution of T_a, u₂, R_n and e_a, which showed that the changes in u₂, R_n and e_a produced the negative effect, whereas T_a produced the positive effect on ET_ref rates. The changes in u₂ were found to produce the largest decrease (?0.7 mm) in ET_ref, followed by e_a (?0.4 mm) and R_n (?0.1 mm). Although the significant increase in T_a had a large positive effect (0.51 mm) on ET_ref rates, changes in the other three variables each reduced ET_ref rates, resulting in an overall negative trend in ET_ref. Copyright © 2012 John Wiley & Sons, Ltd.     

Convection in a rotating cylinder with its sidewall having a finite thermal conductance

Abstract

An investigation is made of steady thermal convection of a Boussinesq fluid confined in a vertically-mounted rotating cylinder. The top and bottom endwall disks are thermal conductors at temperatures T_t and T_b with δT = T_t ? T_b >0. The vertical sidewall has a finite thermal conductance. A Newtonian heat flux condition is adopted at the sidewall. The Rayleigh number of the fluid system is large to render a boundary layer-type flow. Finite-difference numerical solutions to the full Navier-Stokes equations are obtained. The vertical motions within the buoyancy layer along the sidewall induce weak meridional flows in the interior. Because of the Coriolis acceleration, the meridional flows give rise to azimuthal flows relative to the rotating container. Strong vertical gradients of azimuthal flows exist in the regions near the endwalls. As the stratification effect increases, concentration of flow gradients in thin endwall boundary layers becomes more pronounced. The azimuthal flow field exhibits considerable horizontal gradients. The temperature field develops horizontal variations superposed on the dominant vertical distribution. As either the sidewall thermal conductance or the stratification effect decreases, the temperature distribution tends to the profile varying linearly with height. Comparisons of the sizes of the dynamic effects demonstrate that, in the bulk of flow field, the vertical shear of azimuthal velocity is supported by the horizontal temperature gradient, resulting in a thermal-wind relation.     

Effective radius of heating of the lower ionosphere by intense shortwave radiation

The results of a theoretical analysis of the radial distribution of electron temperature T _e in the area of heating of the lower ionosphere by intense shortwave radiation are presented. It was established that effective radius r _eff of heating at a certain height may differ significantly from the characteristic radius of illumination of the ionosphere (a) by radiation at this height. At the boundary of the heating area (r = r _eff ), the characteristic radial scale of T _e changes is less than the corresponding scale of changes in the squared amplitude of the radiation electric field, and it is almost independent of the amplitude value; i.e., the formation of a relatively strong T _e gradient at such a boundary is a common feature of heating of the lower ionosphere by intense shortwave radiation.     

Variations of daytime and nighttime electron temperature and heat flux in the upper ionosphere,topside ionosphere and lower plasmasphere for low and high solar activity

A database of the electron temperature (T_e) comprising of most of the available LEO satellite measurements is used for studying the solar activity variations of T_e. The T_e data are grouped for two levels of solar activity (low LSA and high HSA), five altitude ranges between 350 and 2000 km, and day and night. By fitting a theoretical expression to the T_e values we obtain variation of T_e along magnetic field lines and heat flux for LSA and HSA. We have found that T_e increases with increase in solar activity at low and mid-latitudes during nighttime at all altitudes studied. During daytime the T_e response to solar activity depends on latitude, altitude, and season. This analysis shows existence of anti-correlation between T_e and solar activity at mid-latitudes below 700 km during the equinox and winter day hours. Heat fluxes show small latitudinal dependence for daytime but substantial for nighttime.     

The behaviour of electron temperature under non-stationary conditions in dependence on geomagnetic latitude

Summary The electron temperature was monitored using a radiofrequency probe situated on the board of satellite INTERCOSMOS-8 from 30th Nov. to 25th Dec., 1972. All data were obtained under quiet geomagnetic conditions K_p<2⁺. The processing of data enabled us to compare the behaviour of the electron temperature T_e under non-stationary conditions of local sunrise and sunset for various geomagnetic latitudes. The results showed a different behaviour of the electron temperature in mid-latitudes and in the region of the magnetic equator in dependence on angle x.     

Global modelling study (GSM TIP) of the ionospheric effects of excited N2, convection and heat fluxes by comparison with EISCAT and satellite data for 31 July 1990

Near-earth plasma parameters were calculated using a global numerical self-consistent and time-dependent model of the thermosphere, ionosphere and protonosphere (GSM TIP). The model results are compared with experimental data of different origin, mainly EISCAT measurements and simultaneous satellite data (N_e and ion composition). Model runs with varying inputs of auroral FAC distributions, temperature of vibrationally excited nitrogen and photoelectron energy escape fluxes are used to make adjustments to the observations. The satellite data are obtained onboard Active and its subsatellite Magion –2 when they passed nearby the EISCAT station around 0325 and 1540 UT on 31 July 1990 at a height of about 2000 and 2200 km, respectively. A strong geomagnetic disturbance was observed two days before the period under study. Numerical calculations were performed with consideration of vibrationally excited nitrogen molecules for high solar-activity conditions. The results show good agreement between the incoherent-scatter radar measurements (N_e, T_e, T_i) and model calculations, taking into account the excited molecular nitrogen reaction rates. The comparison of model results of the thermospheric neutral wind shows finally a good agreement with the HWM93 empirical wind model.     

The elastic thickness of the lithosphere in the Pacific Ocean

In this study, we present determinations of the effective elastic thicknessT_e of the oceanic lithosphere along Pacific chains or archipelagoes.T_e is determined by computing the deflection of a continuous elastic plate under the load of volcanoes, and constrained by geoid heights provided by SEASAT. In the South Central Pacific, estimates of 14 km for the Marquesas and 6 km or less for the Pitcairn-Mururoa-Gloucester chain are in good agreement with a previous work in this region (Cook-Austral and Society chains). Around the Line Islands chain, SEASAT data reveal that the bathymetry is poorly known, preventing fine analysis. Meanwhile,T_e looks globally very low ( 6 km), except for three volcanoes but these results may be unreliable. The Easter chain features lowT_e values ( 6 km), with no noticeable variation along the chain. Higher values are found for a Samoan island, Manuae (24 km), and along the Hawaiian-Emperor seamounts chain (from 32 km at the eastern end of the chain to 21.5 km for the Hawaiian volcanoes, and from 25.5 to 15 km for the Emperor seamounts). The large number ofT_e estimates obtained in this study points out a noticeable difference between North and South Pacific results. Those from the North Pacific agree with the general trend (increase with the square root of age plate at loading time), while those from the South Central Pacific are much lower, according to their plate age. These lowT_e results from the South Pacific are only partly explained by taking account of thermal perturbations using the rejuvenation model. Therefore, these results then point out a regional difference in oceanic lithosphere.     

Using equilibrium temperature to assess thermal disturbances in rivers

Flow regulation is widely known to modify the thermal regime of rivers. Here, we examine the sensitivity of an empirical approach, the Equilibrium Temperature Concept (ETC), to detect both the effects of hydraulic infrastructures on the annual thermal cycle and the recovery of the thermal equilibrium with the atmosphere. Analysis was undertaken in a Pyrenean river (the Noguera Pallaresa, Ebro basin) affected by a series of reservoirs and hydropower plants. Equilibrium temperature (T_e) is defined as the water temperature (T_w) at which the sum of all heat fluxes is zero. Based on the assumption of a linear relationship between T_e and T_w, we identified changes in the T_e–T_w regression slope, used as an indicator of a thermal alteration in river flow. We also assessed the magnitude of the alteration by examining the regression slope and its statistical significance. Variations in the regression parameters were used as indicators of the influence of factors other than atmospheric conditions on water temperature. Observed T_w showed a linear relationship with T_e at all river stations. However, the slopes of the T_e–T_w relationship appeared to be lower in the reaches downstream from hydraulic infrastructures, particularly below large dams. A seasonal analysis indicated that T_e–T_w relationships had higher slopes and lower p‐values during autumn, while no significant differences were found at other seasons. Although thermal characteristics did not strongly depend on atmospheric conditions downstream of hydraulic infrastructures, the river recovered to pre‐alteration conditions with distance downstream, indicating the natural tendency of water to attain thermal equilibrium with the atmosphere. Accepting associated uncertainties, mostly because of the quality of the data and the lack of consideration of other factors influencing the thermal regime (e.g. discharge), ETC appears to be a simple and effective method to identify thermal alterations in regulated rivers. Copyright © 2015 John Wiley & Sons, Ltd.     

VLF quarter-gyrofrequency plasmaspheric emissions as observed by intercosmos 24 and magion 2 satellites

Summary Quarter-gyrofrequency plasmaspheric emissions with spectral properties differing from those of discrete plasmaspheric emissions, usual in active intervals, have been observed by low-altitude Intercosmos 24 and Magion 2 satellites during periods in which geomagnetic activity decreases. Their occurrence in satellite records shows very good correlation with simultaneously observed subauroral electron temperature enhancements and increase of electron temperature anisotropyT _e being larger than T _e . An analysis of the observed wave characteristics is given. Propagation of the emissions within the plasmasphere is discussed. It is shown that the region where they are observed at low altitudes can be closely connected along geomagnetic field lines with the equatorial region of their origin.     

Correlations between SAR arc intensity and solar and geomagnetic activity

We present a study of statistical relationships between SAR arc intensities acquired by the Pacific Northwest Laboratory Photometer Network during 1978–1988 and solar and geomagnetic activity indices Dst, F10.7, and Kp by use of the method of multiple regression analysis. We found significant correlations between intensity and all of the indices involved. In the present work we show for the first time that the partial correlation coefficients depend on the time offset, t, between the time of SAR arc intensity observations and the onset of the geomagnetic storm recovery phase, with the largest correlations being observed when 8 d t d 16 h. It is also shown that there are significant differences between partial correlation coefficients calculated for SAR arcs associated with strong (Dst_min > −100 nT) and weak (Dst_min > −100 nT) geomagnetic storms. We observe also that the multiple correlation coefficients for strong storms are much larger than for weak ones. We found that the variations in the electron temperature, T_e, in the SAR arc region are not mainly produced by variations in the electron density of the ionosphere but are strongly driven by the additional heating of the electron gas due to an interaction of the ring current ions and the plasmaspheric electrons. As a result, variations of T_e in the SAR arc region with characteristic time scales from several minutes to several hours are stipulated by time variations of ring current parameters.     

A study of regional gravity field recovery from GOCE vertical gravity gradient data in the Auvergne test area using collocation

Gravity field and steady-state Ocean Circulation Explorer (GOCE) is the first satellite mission that observes gravity gradients from the space, to be primarily used for the determination of high precision global gravity field models. However, the GOCE gradients, having a dense data distribution, may potentially provide better predictions of the regional gravity field than those obtained using a spherical harmonic Earth Geopotential Model (EGM). This is investigated in Auvergne test area using Least Squares Collocation (LSC) with GOCE vertical gravity gradient anomalies (T_zz), removing the long wavelength part from EGM2008 and the short wavelength part by residual terrain modelling (RTM). The results show that terrain effects on the vertical gravity gradient are significant at satellite altitude, reaching a level of 0.11 E?tv?s unit (E.U.) in the mountainous areas. Removing the RTM effects from GOCE T_zz leads to significant improvements on the LSC predictions of surface gravity anomalies and quasigeoid heights. Comparison with ground truth data shows that using LSC surface free air gravity anomalies and quasi-geoid heights are recovered from GOCE T_zz with standard deviations of 11 mGal and 18 cm, which is better than those obtained by using GOCE EGMs, demonstrating that information beyond the maximal degree of the GOCE EGMs is present. Investigation of using covariance functions created separately from GOCE T_zz and terrestrial free air gravity anomalies, suggests that both covariance functions give almost identical predictions. However, using covariance function obtained from GOCE T_zz has the effect that the predicted formal average error estimates are considerably larger than the standard deviations of predicted minus observed gravity anomalies. Therefore, GOCE T_zz should be used with caution to determine the covariance functions in areas where surface gravity anomalies are not available, if error estimates are needed.     

Isostasy of the Eastern Anatolia (Turkey) and Discontinuities of its Crust

This paper presents a probable isostatic model of the East Anatolian Region, which lies in a belt of significant plate movements. Probable locations of the horizontal and vertical discontinuities in the crust structure were determined using the normalized full gradient (NFG) method. For the purpose of explaining the mechanism that supports topography corresponding to the crust thickness in the region, calculations of effective elastic thickness (T _e) were carried out initially by utilizing admittance and misfit functions. According to these results, the effective elastic thickness value obtained was less than the crust thickness, even though the isostatic model does not conform with the Airy model. Consequently, it was assumed that there could be problems beneath the crust. Hence, the NFG method was applied on the Bouguer gravity data of the region in order to investigate probable discontinuities in the crust structure. According to the NFG results, vertical structural transitions were observed at a depth ranging between 10 and 30 km, which begin immediately north of the Bitlis Zagros Suture Zone (BZSZ) and continue in a northerly direction. The relationship between the effective elastic thickness (T _e; 13 km in average as determined in the last stage), and the seismogenic zone in the region was investigated. If the T _e value happens to be less then the crustal thickness, then one can say that there are problems in the crustal structure of the region similar to Eastern Anatolia. Indeed, when NFG results of the study area are examined, numerous vertical and horizontal discontinuities in the crust can be observed. These discontinuities, which correspond to low Bouguer gravity anomalies and shallow focal depth-earthquakes, are probably the source of the factors which rule the tectonic mechanism of the region.     

The effective elastic lithosphere under the Cook-Austral and Society islands

We have determined the elastic thicknessT_e of the oceanic lithosphere along two volcanic chains of the South Central Pacific: Cook-Austral and Society islands. We used a three-dimensional spatial method to model the lithospheric flexure assuming a continuous elastic plate. The model was constrained by geoid height data from the SEASAT satellite.Along the Cook-Austral chain the elastic thickness increases westward, from 2–4 km at McDonald hot spot to 14 km at Rarotonga. At McDonald seamount, however, the data are better explained by a local compensation model. The observed trend shows an increase ofT_e with age of plate at loading time. However, the elastic layer under the Cook-Austral appears systematically thinner by several kilometers than expected for “normal” seafloor, suggesting that substantial thermal thinning has taken place in this region. Considering the apparent thermal age of the plate instead of crustal age improves noticeably the results. Along the Society chainT_e varies from 20 km under Tahiti to 13 km under Maupiti which is located 500 km westward. When plotting together the Society and Cook-AustralT_e results versus age of load, we notice that within the first five million years after loading,T_e decreases significantly while tending rapidly to an equilibrium value. This may be interpreted as the effect of initial stress relaxation which occurs just after loading inside the lower lithosphere and suggests that the presently measured elastic thickness under the very young Tahiti load ( 0.8 Ma) is not yet the equilibrium thickness.