Author name code: duhau ADS astronomy entries on 2022-09-14 author:Duhau, Silvia ------------------------------------------------------------------------ Title: Solar magnetic variability and climate Authors: de Jager, Cornelis; Duhau, Silvia; Nieuwenhuizen, Adrianus Bibcode: 2020smvc.book.....D Altcode: When Kees de Jager returned to his birth ground Texel, he was asked by the director of Royal Institute for Sea Research, Prof. Dr. J. de Leeuw, to investigate the relationship between solar activity and climate. This research, done over a period of 15 years, generated about 10 papers with remarkable new insights. In 2019 Kees decided to compile the papers into a scientific book. This book covers:

The equatorial and poloidal magnetic fields and their relation to the Earth's NH temperature.

The notions phase diagram of the solar dynamo, Transition Point and Grand Episode.

The granular scale magnetic fields or 'bright spots'.

Telescopes for large-field high-resolution imaging of the sun.

The expectations for future solar activity derived from the decomposition of the solar dynamo into 8 'modes'. This leads to prediction of the sunspot maxima till 2130.

The statistical start of the modern heating and solar variability as part of the modern heating curve.

The non-linear time difference between the solar magnetic variations and terrestrial warming. Title: The relation between the average northern hemisphere ground temperature and solar equatorial and polar magnetic activity Authors: De Jager, Cornelis; Nieuwenhuizen, A. C. T.; Nieuwenhuizen, H.; Duhau, S. Bibcode: 2018PAIJ....2..175D Altcode: 2018PAIJ....2..175J No abstract at ADS Title: A Remarkable Recent Transition in the Solar Dynamo Authors: de Jager, C.; Akasofu, S. -I.; Duhau, S.; Livingston, W. C.; Nieuwenhuijzen, H.; Potgieter, M. S. Bibcode: 2016SSRv..201..109D Altcode: 2016SSRv..tmp...64D We summarize the major aspects of the remarkable, fairly long lasting period (∼ 2005 to ∼ 2010) of low solar activity, that we will call the Transition. It is the transitional stage between the Grand Maximum of the 20th century and a forthcoming (most probably Regular) episode of solar activity. The various kinds of activity in the functioning of the equatorial components of the solar dynamo before and during the Transition are summarized. While the behavior of unipolar magnetic regions and their rest-latitudes already gave very early indications - mid 20th century - of the forthcoming Transition, more such indications became available around 1995 and the main part of it occurred between 2005 and 2010. Some of the inferences are discussed. We submit the hypothesis that the solar tachocline undergoes pulsations and we present some helioseismic evidences. In that scenario we find that its equatorial part has moved downward over a fairly small semi-amplitude (∼ 0.03 solar radii) during the time of the Transition. There are several indications, apart from this `pulsation', that the tachocline may even be pulsating with still smaller amplitudes in more modes. We speculate about the physical mechanism(s). Title: Reply to “The influence of planetary attractions on the solar tachocline” by N. Scafetta, O. Humlum, J.E. Solheim, K. Stordahl Authors: Callebaut, D.; de Jager, C.; Duhau, S. Bibcode: 2013JASTP.102..372C Altcode: There are some evidences that sunspot cycle and solar inertial force variability are related.It is known that differential rotation drives sunspot cycle.Solar inertial force is too weak to drive differential rotation.A mechanism able to amplify solar inertial force to its required strength is still missing. Title: Sudden transitions and grand variations in the solar dynamo, past and future</xref> Authors: De Jager, Cornelis; Duhau, Silvia Bibcode: 2012JSWSC...2A..07D Altcode: The solar dynamo is the exotic dance of the sun's two major magnetic field components, the poloidal and the toroidal, interacting in anti-phase. On the basis of new data on the geomagnetic aa index, we improve our previous forecast of the properties of the current Schwabe cycle #24. Its maximum will occur in 2013.5 and the maximum sunspot number Rmax will then be 62 ± 12, which is within the bounds of our earlier forecasts. The subsequent analysis, based on a phase diagram, which is a diagram showing the relation between maximum sunspot numbers and minimum geomagnetic aa index values leads to the conclusion that a new Grand Episode in solar activity has started in 2008. From the study of the natural oscillations in the sunspot number time series, as found by an analysis based on suitable wavelet base functions, we predict that this Grand Episode will be of the Regular Oscillations type, which is the kind of oscillations that also occurred between 1724 and 1924. Previous expectations of a Grand (Maunder-type) Minimum of solar activity cannot be supported. We stress the significance of the Hallstatt periodicity for determining the character of the forthcoming Grand Episodes. No Grand Minimum is expected to occur during the millennium that has just started.

The full text of all quoted papers by the two authors of this paper can be consulted at www.cdejager.com/sun-earth-publications/. Title: The influence of planetary attractions on the solar tachocline Authors: Callebaut, Dirk K.; de Jager, Cornelis; Duhau, Silvia Bibcode: 2012JASTP..80...73C Altcode: We present a physical analysis of the occasionally forwarded hypothesis that solar variability, as shown in the various photospheric and outer solar layer activities, might be due to the Newtonian attraction by the planets. We calculate the planetary forces exerted on the tachocline and thereby not only include the immediate forces but we also take into account that these planetary or dynamo actions occur during some time, which demands integration. As an improvement to earlier research on this topic we reconsider the internal convective velocities and we examine several other effects, in particular those due to magnetic buoyancy and to the Coriolis force. The main conclusion is that in its essence: planetary influences are too small to be more than a small modulation of the solar cycle. We do not exclude the possibility that the long term combined action of the planets may induce small internal motions in the sun, which may have indirectly an effect on the solar dynamo after a long time. Title: Quantifying and specifying the solar influence on terrestrial surface temperature Authors: de Jager, C.; Duhau, S.; van Geel, B. Bibcode: 2010JASTP..72..926D Altcode: This investigation is a follow-up of a paper in which we showed that both major magnetic components of the solar dynamo, viz. the toroidal and the poloidal ones, are correlated with average terrestrial surface temperatures. Here, we quantify, improve and specify that result and search for their causes. We studied seven recent temperature files. They were smoothed in order to eliminate the Schwabe-type (11 years) variations. While the total temperature gradient over the period of investigation (1610-1970) is 0.087 °C/century; a gradient of 0.077 °C/century is correlated with the equatorial (toroidal) magnetic field component. Half of it is explained by the increase of the Total Solar Irradiance over the period of investigation, while the other half is due to feedback by evaporated water vapour. A yet unexplained gradient of -0.040 °C/century is correlated with the polar (poloidal) magnetic field. The residual temperature increase over that period, not correlated with solar variability, is 0.051 °C/century. It is ascribed to climatologic forcings and internal modes of variation. We used these results to study present terrestrial surface warming. By subtracting the above-mentioned components from the observed temperatures we found a residual excess of 0.31° in 1999, this being the triangularly weighted residual over the period 1990-2008. We show that solar forcing of the ground temperature associated with significant feedback is a regularly occurring feature, by describing some well observed events during the Holocene. Title: Solar Gleissberg periodicities in relation to grand episodes Authors: Duhau, S.; de Jager, C. Bibcode: 2009EGUGA..11.6131D Altcode: The solar Gleissberg cycle is not restricted to one single periodicity, as was assumed by its discoverer. Later, it was shown that there are at least two components, each of which dominating in another period of time. We connect this observation to an earlier finding, viz. that the occurrence of the Grand Episodes is related to the behaviour of the solar dynamo in connection to its so-called Transition Point: another Grand Episode starts when the curve defined by the proxies of the poloidal and toroidal magnetic field components has crossed or passed along the Transition Point's coordinates. Here, we present evidence that each Grand Episode is related to one or more specific components of the Gleissberg cycle; each of them with its own period length. This finding must have implications for our understanding of the dynamo mechanism Title: Forecasting the parameters of sunspot cycle 24 and beyond Authors: de Jager, C.; Duhau, S. Bibcode: 2009JASTP..71..239D Altcode: Solar variability is controlled by the internal dynamo which is a non-linear system. We develop a physical-statistical method for forecasting solar activity that takes into account the non-linear character of the solar dynamo. The method is based on the generally accepted mechanisms of the dynamo and on recently found systematic properties of the long-term solar variability. The amplitude modulation of the Schwabe cycle in dynamo's magnetic field components can be decomposed in an invariant transition level and three types of oscillations around it. The regularities that we observe in the behaviour of these oscillations during the last millennium enable us to forecast solar activity. We find that the system is presently undergoing a transition from the recent Grand Maximum to another regime. This transition started in 2000 and it is expected to end around the maximum of cycle 24, foreseen for 2014, with a maximum sunspot number Rmax=68±17. At that time a period of lower solar activity will start. That period will be one of regular oscillations, as occurred between 1730 and 1923. The first of these oscillations may even turn out to be as strongly negative as around 1810, in which case a short Grand Minimum similar to the Dalton one might develop. This moderate-to-low-activity episode is expected to last for at least one Gleissberg cycle (60-100 years). Title: Episodes of relative global warming Authors: de Jager, C.; Duhau, S. Bibcode: 2009JASTP..71..194D Altcode: Solar activity is regulated by the solar dynamo. The dynamo is a non-linear interplay between the equatorial and polar magnetic field components. So far, in Sun-climate studies, only the equatorial component has been considered as a possible driver of tropospheric temperature variations. We show that, next to this, there is a significant contribution of the polar component. Based on direct observations of proxy data for the two main solar magnetic fields components since 1844, we derive an empirical relation between tropospheric temperature variation and those of the solar equatorial and polar activities. When applying that relation to the period 1610-1995, we find some quasi-regular episodes of residual temperature increases and decreases, with semi-amplitudes up to ~0.3 °C. The present period of global warming is one of them. Title: The Solar Dynamo and Its Phase Transitions during the Last Millennium Authors: Duhau, S.; de Jager, C. Bibcode: 2008SoPh..250....1D Altcode: 2008SoPh..tmp..112D We analyze the variation of the solar-dynamo magnetic-field components during the last millennium through a study of their proxy data. We introduce a phase diagram with as abscissa and ordinate the proxies of the values of the toroidal and poloidal magnetic field components. In this diagram the dynamo system appears to regularly cross a well-defined point, which we call the Transition Point. Such crossings occurred five times during the past millennium. Each of these crossings preceded a Grand Episode, either a Minimum or a Maximum one. In addition to these two types of quasiperiodic behavior, a third type consisting of weaker quasiregular oscillations (R) around the Transition Point's coordinates is identified. These periods appear to last one or two times the Gleissberg cycle length. Between the various types of episodes there are brief phase transitions. We identify two types of such phase transitions. Title: A long-term relationship between solar magnetic field, geomagnetic field and Earth's rotation rate. Authors: Duhau, S.; de Jager, C. Bibcode: 2008AGUSMGP31C..06D Altcode: All solar and solar terrestrial variables observables at ground level undergo time changes that have their origins - some of them only partially - in solar magnetic field variations. We have found that solar magnetic field amplitude cycle may be well described as the superposition of a attractor constant level, and oscillations around it that are well represented by the Gleissberg cycle - this last related to Grand Maxima and Grand Minima occurrence - and a decadal oscillation that contains the odd-even rule. In longer term scales a millennial oscillation must be also considered. Here we compare that signals in sunspot number, geomagnetic Mayaud index, sudden storm commencement (SI) index, length of day variation and the velocity of the westward drift and dipolar intensity of the geomagnetic field. Very well defined relationships that depend on the variable are found. In particular we need to divide the decadal variation in a Hale cycle and a semi-secular oscillation since the Hale oscillation in the Earth's rotation rate and in Geomagnetic field are not related and the semi secular oscillation is strongly amplified in these two variables. The origin of the so found relationship, that may help to reconstruct shorter time series of the related variables from other known since longer, is discussed. Title: Solar Activity Earth Rotation Rate and Global Surface Temperature Long-term Variations. Authors: Duhau, S. Bibcode: 2007AGUSMGP54A..03D Altcode: Empirical evidences of the relationship that does exist between solar activity, Earth rotation rate and global surface temperature long-term modulations and the mechanisms able to explain such a relationship are reviewed. In particular we discuss the possibility that these modulations are excited either externally by planetary spin-orbit interaction, or internally to the sun , so controlling the intensity and frequency of geomagnetic storm time variations that in turn excite length of day and secular geomagnetic variations by electromagnetic induction. Title: On the Physical Origin of the Cycles in Long-term Modulation of Solar Activity Authors: Duhau, S.; de Jager, C. Bibcode: 2007AGUSMSH23B..03D Altcode: By a Morlet wavelet analysis in sub harmonics of the 11 year fundamental frequency of sunspot number as a proxy for the toroidal component of solar dynamo magnetic field the evolution for the last 400 years of four well defined cycles- a decadal, a semi-secular, the Gleissberg and the Suess ones - in the modulation of this component of the solar dynamo field is found. The properties of these cycles as seen in geomagnetic index aa and Si as proxy data for polar dynamo field and CME's frequency and intensity, respectively, are described . From this procedure and by analyzing longer proxy time series in the light of the known non-linear properties of solar dynamo system, the meaning of the four cycles and its variability are discussed. Title: Long Term Variations in Solar Magnetic Field, Geomagnetic Field and Climate Authors: Duhau, S. Bibcode: 2006apri.meet...18D Altcode: No abstract at ADS Title: Global Earth surface temperature changes induced by mean Sun dynamo magnetic field variations Authors: Duhau, Silvia Bibcode: 2003ESASP.535..317D Altcode: 2003iscs.symp..317D There are some evidences that support global climatic change by cosmic rays and ionosphere-ground electrical circuit variations; in turn these variations are strongly linked to storms sudden commencement. Here we define the SSC index as the product of the amplitude by the duration time of each storm sudden commencement. A wavelet multi-resolution analysis of yearly means of global temperature, radiative input and SSC index time series since 1868 performed. It is found that for time scales larger than 60 years a 70% of the long-term NH change in Earth surface temperature might be attributed to SSC index and the remainder 30% to solar irradiance. This finding is discussed in the light of long-term evolution of solar magnetic dynamo field and related parameters. In particular it is shown that the long-term surface temperature variation has its maximum value around 1995 and monotonically decreases afterward. This tendency could be maintained for the forthcoming years following the descending chaotic transition that solar dynamo system started around 1993. Title: Total ozone content changes associated to solar dynamo magnetic field variations. Authors: Duhau, Silvia; Martínez, Ernesto A. Bibcode: 2003ESASP.535..819D Altcode: 2003iscs.symp..819D There are some evidences that support the occurrence of total ozone content changes by high energetic particle flux intensity variations. In turn, these last are strongly linked to sudden commencement storms (SSC's). Therefore, an analysis of total ozone content in selected ground stations, radiative input and SSC index yearly means time series, the last defined as the product of the amplitude by the duration time of each storm, is performed. It is found that long-term ozone variations are mostly related to SSC index long-term variations in all the stations, with different behaviours that depend on geographical location. In particular, the strong decrease of total ozone content during cycle 22 is related to the very high SSC values that occurred during that sunspot cycle and is remarkably strong in Faraday and Halley Bay stations. In all the stations the minimum in total ozone content long term variation is reached around 1993 and is followed by an steady increase, that would continue along the forthcoming years following the descending chaotic transition that solar global magnetic field dynamo system started around 1993. Title: On the nature of the steep changes in solar and geomagnetic activity after 1705, 1923 and 1993 Authors: Duhau, Silvia Bibcode: 2003ESASP.535...91D Altcode: 2003iscs.symp...91D It was shown that the 11-year cycle amplitude modulation has suffered an ascending chaotic transition after 1923, leading the solar mean magnetic field almost to duplicate in less than 30 years, and that a descending transition started at year 1993. To investigate the nature of these steep changes we perform a multi-resolution wavelet analysis of the relationship between long term evolution in Wolf sunspot number, geomagnetic index aa time series and 158-day periodicity in sunspots (1844-2002). The same methodology is applied to the study of the sudden commencement index (SSC) (1968-1993), defined for each storm as the product of its amplitude and rise time. We find that the chaotic ascending transition is synchronic with the strengthening of the 158-day periodicity, while the SSC index is inversely related to the 158 days periodicity and had reached during sunspot cycle 22 its largest value since 1868. In the light of dynamo theory we conclude that new magnetic flux has been injected to the dynamo layer from the convective region during the ascending transition wile during cycle 22 strong toroidal fields were ejected by the solar dynamo system leading to intense sudden commencement geomagnetic storms. Title: An Early Prediction of Maximum Sunspot Number in Solar Cycle 24 Authors: Duhau, S. Bibcode: 2003SoPh..213..203D Altcode: A non-linear coupling function between sunspot maxima and aa minima modulations has been found as a result of a wavelet analysis of geomagnetic index aa and Wolf sunspot number yearly means since 1844. It has been demonstrated that the increase of these modulations for the past 158 years has not been steady, instead, it has occurred in less than 30 years starting around 1923. Otherwise sunspot maxima have oscillated about a constant level of 90 and 141, prior to 1923 and after 1949, respectively. The relevance of these findings regarding the forecasting of solar activity is analyzed here. It is found that if sunspot cycle maxima were still oscillating around the 141 constant value, then the Gnevyshev-Ohl rule would be violated for two consecutive even-odd sunspot pairs (22-23 and 24-25) for the first time in 1700 years. Instead, we present evidence that solar activity is in a declining episode that started about 1993. A value for maximum sunspot number in solar cycle 24 (87.5±23.5) is estimated from our results. Title: The sudden increase of solar and geomagnetic activity after 1923 as a manifestation of a non-linear solar dynamo Authors: Duhau, S.; Chen, Ch. Y. Bibcode: 2002GeoRL..29.1628D Altcode: 2002GeoRL..29m...6D There are some indications that the coupling function between the polar and the toroidal solar magnetic field strengths are time dependent. Therefore, as proxy data of these strengths, the relationship between long-term modulation of solar and geomagnetic activity is analyzed here by means of a wavelet multi-resolution analysis of yearly means of sunspot number and geomagnetic index aa since 1844. It is found that aa index and relative sunspot number long-term modulations oscillate around constant levels that have increased to be 1.9 and 1.6, respectively of its values prior 1923. The oscillations of the aa and Rz long-term modulation appear to be well represented by the superposition of a decadal cycle and the Gleissberg cycle. The amplitude ratio and the phase shift between the Rz and aa decadal cycles changed sharply at year 1923, and the Gleissberg cycle was interrupted to be restarted at year 1949. The synchronicity of these changes with temporal changes in several solar and solar-terrestrial variables indicates that the non-linear nature of the solar dynamo has lead to a major change of its background state after 1923. Title: A method for the interpretation of three-dimensional equatorial GDV fields Authors: Duhau, S.; Martínez, E. A. Bibcode: 1998EP&S...50..141D Altcode: The equatorial GDV fields have been widely measured in North-South magnetometer chains and the data so obtained interpreted by assuming that these variations are bi-dimensional. This hypothesis limits its application to the inference of the noon amplitude of the ionospheric current and to the case on which the suspected local Earth's structure does not have discontinuities running along the direction normal to that of the electrojet current vector. In this work we develop a method to interpret three-dimensional GDV fields, based on the Riesz and in the Fourier integral transforms. This method consists in a numerical code that allows to separate, in real time, any three-dimensional low frequency field, and a system equations to infer the ionospheric current system at daylight times and to predict the field induced from the external GDV field in a layered Earth's model. We discuss the application of the method to data obtained in North-South magnetometer chains, and, by analyzing a particular case--data from the Indian geomagnetic observatories--we illustrate how the method increases the amount of information that may be obtained from these data. Title: An explanation of the kilometer scale wave in the equatorial electrojet Authors: Duhau, Silvia; de Mendoza, Diego Hurtado Bibcode: 1996GeoRL..23.2185D Altcode: Density irregularities in the equatorial electrojet are dominated by kilometer scale waves. In this report we combine a few simple ideas about strong turbulence and non-local effects due to the vertical structure of the equatorial E-region. This leads to a preference for kilometer scale waves. We include the vertical current conservation effect, the eddy turnover, and the growth velocity. We determine the saturation of density perturbation driven by the gradient drift instability. This heuristic theory predicts phenomena consistent with sounding rocket and coherent radar observations. It also gives same general clues about the interaction between the ambient and the turbulent scale flows. Title: On the origin of the fluctuations in the length of day and in the geomagnetic field on a decadal time scale Authors: Duhau, Silvia; Martínez, Ernesto A. Bibcode: 1995GeoRL..22.3283D Altcode: There is, at present, strong evidence that on a decadal time scale the excess length of day variations are caused by the exchange of angular momentum between the liquid core and the solid mantle. Nevertheless the mechanism which facilitates the momentum exchange is still not understood. In the present work a mechanism by which the magnetospheric equatorial ring current system controls the momentum exchange is introduced, some empirical insights about that mechanism are obtained from relevant data and the magnitudes of the involved magnetic fields are evaluated from these data. Title: Depth of the conductosphere under the Indian shield Authors: Favetto, A.; Osella, A. M.; Duhau, S. Bibcode: 1992PEPI...71..180F Altcode: The depth at which the highly conducting layer of the upper mantle, the conductosphere, begins under the Indian shield is estimated. To do this, the geomagnetic daily variations measured north of the magnetic equator are separated into contributions of internal and external origin, and from the latter an analytical representation for the ionospheric current system is inferred. The external field produced by this current and the field induced by it in a two-layer conductivity model of the Earth consisting of an insulator overlying a perfect conductor are obtained. The total field thus calculated is compared with the data to obtain the conductosphere depth. The conductosphere below the Indian shield is found to be located at approximately 1000 km, well below the average global value, in agreement with previous findings below the African shield. This result supports the hypothesis that there is a correlation between the deep conducting structure of the Earth and some tectonic features. Title: Solar wind stream interaction: Electron temperature and heat flux rise in the low-speed stream Authors: Alexander, P.; Duhau, S. Bibcode: 1990JGR....9519149A Altcode: The strong compression produced in two-stream interaction regions in the solar wind is a local source of heating. The study of the distribution of that energy between heat and internal energy provides valuable information about transport processes. In the present work, the electron heat flux and temperature rise in the compression produced within the low-speed portion of the interaction region is predicted using a new heat conduction law valid for collisionless plasmas with isotropic electron temperature, introduced recently by one of the authors. Equations are found for the electron heat flux and temperature rise as functions of two parameters, one related to the strength of the compression and the other one to the heat flux at the boundaries of the region under study. These equations lead to agreement between theory and experiment. Title: A comparison between the experimentally and theoretically determined equatorial electrojet electric field Authors: de La Vega, M.; Duhau, S. Bibcode: 1989JGR....9412061D Altcode: Laminar theories of electrojet dynamics predict that the primary eastward electric field is almost constant with height. Nevertheless, according to previous findings of the authors, the electrojet intensity predicted by these theories agrees with the measured one only if that component of the electric field is assumed to be almost half its measured F region value at E region heights. In the present paper this apparent contradiction is investigated further by contrasting the physical hypotheses on which theoretical models rely with empirical data and taking into account the turbulent contribution to the Pedersen conductivity as proposed by Rogister (1971). It is found that turbulence due to either gradient drift or two-stream instabilities, or both, reduces the secondary electric field by increasing the Pedersen conductivity from 30% to 100% of its laminar value within almost all the E region, and that the geomagnetic field lines seem not to be equipotential, a result that cannot be explained within the framework of present theories. Title: Absolute stability in a collisionless electron-heat-conducting plasma in strong magnetic fields Authors: de la Torre, A.; Duhau, S. Bibcode: 1989JPlPh..41..107D Altcode: The dispersion relation obtained from a linear analysis of the hydrodynamic system of equations of Duhau is used to study the behaviour of the fast and slow magnetosonic and entropy modes in an electron-heat-flux-conducting plasma. The evolution of the hydrodynamic modes different from the Alfvén mode are studied as the electron heat flux is increased from zero as well as around the borders of overstable regions, for any anisotropy condition of the ions. The development of the domains of mirror and electron-heat-flux overstabilities are established and the regions of absolute stability are shown Title: Meridional electric currents in the equatorial F-region Authors: Louro, A. A.; Duhau, S. Bibcode: 1988JATP...50..977L Altcode: A study of the boundary conditions for the equatorial thermospheric transport equations has led to the theoretical prediction of the vertical electric field at the base of the F-region. This model is applied to the calculation of the F-region electric current field in the meridional plane as a function of time and the east-west magnetic field generated by these currents. In particular, the field at sunset is compared with the observations made by Magsat. Title: The zonal neutral wind in the equatorial thermosphere Authors: Louro, A. A.; Duhau, S. Bibcode: 1988JGR....93.9987L Altcode: A theoretical prediction of zonal neutral winds at the equator at F region heights is presented, using values for the ion velocity calculated with a model also due to the authors. The results, and the dynamic properties of the thermosphere that may be inferred from them, are viewed in light of information obtained from recent measurements performed with the satellite Dynamics Explorer 2. The calculated values of the zonal neutral wind agree in general with the measured values in phase; the predicted amplitude is smaller during the early nighttime, and to a lesser extent during the daytime period. The model predicts practically constant values of the zonal neutral wind with height, and a superrotation less than 5%, in agreement with the experimental findings. Title: Boundary conditions for the neutral and ionospheric transport equations at the base of the equatorial F region Authors: Duhau, S.; Louro, A. A. Bibcode: 1987JGR....9213629D Altcode: In previous works, the authors used a simple model of the equatorial E and F regions to predict simultaneously zonal neutral and ion velocities and vertical currents at the base of the F region. In this work, the model is extended to include effects not considered previously: ion-neutral drag at the F region base, the E region dynamo electric field, and the F region zonal electric field. The model is used to predict the variables mentioned above as functions of time and latitude. The results of the calculations show first that the ion velocity is quite sensitive to the combination of the three effects, and a substantial improvement over previous results is found upon comparison with meaured values. The calculated zonal neutral wind also reveals the influence of ion drag through its latitudinal variations. Finally, the current density perpendicular to the geomagnetic field exhibits two pronounced peaks of the order of 10-7 A m-2 sunrise and sunset at the dip equator. Title: Effect of the electron temperature in the electron number density and dynamics of the equatorial E-region Authors: Duhau, S.; de La Vega, M.; Azpiazu, M. C. Bibcode: 1987P&SS...35....1D Altcode: The electron rate of recombination and collision frequency of electrons with neutrals depend on the electron temperature, which is, at E-region heights, greater than the neutral temperature even for geomagnetically quiet conditions; therefore in order to perform theoretical computations of electron number and electric current densities, the usual hypothesis that both temperatures are equal at those heights is removed in the present paper. Instead, an empirical model of E-region electron temperature is used and all the data were carefully selected so that they correspond to equivalent ionospheric and solar conditions. It is found that: the electron number density, depends strongly on the electron temperature, being well predicted by the present model; the shape of the vertical profile of the electrical conductivity is mainly determined by the electron number density profile, and the shape of the vertical current density thus calculated is in good agreement with the measured one. At the dip equator, a discrepancy between the intensity of the measured and computed current densities around the height of the electrojet peak still remains, which could be due to an anomalous reduction of the electric field there, as is suggested by the agreement found a few degrees beyond the dip. Title: Hydromagnetic waves for a collisionless plasma in strong magnetic fields Authors: Duhau, S.; de La Torre, A. Bibcode: 1985JPlPh..34...67D Altcode: A hydrodynamic system of equations, valid in the limit in which the Larmor radius and the electron to ion mass ratio are both zero, and including the thermo-dynamic variables and the energy equation of the electrons, is used to investigate the propagation of small-amplitude waves in a collisionless heat-conducting plasma. The result is compared with that derived from the Chew, Goldberger & Low equations. It is found that for zero heat flux, the inclusion of the electron pressure does not change the number and characteristic of the modes but modifies the mirror stability criterion. In the general case, the phase speed is symmetric with respect to two axes: one parallel to the heat flux vector and the other normal to it. The heat flux generates a new mode and couples strongly the slow and fast magnetosonic modes whose wavenumber vectors have projections in the positive flux vector direction, giving rise to a new overstability whose existence does not depend on the ion anisotropy. Title: Empirical model of the E-region electron temperature around noon and at low magnetic activity Authors: Duhau, S.; Azpiazu, M. C. Bibcode: 1985P&SS...33..909D Altcode: Electron temperature measurements made with Langmuir probes at E-region heights together with deviative absorption data show that the electrons are not in thermal equilibrium with the neutrals. Moreover, for very quiet days ( Ap ⩽ 7, Kp ⩽ 1 + throughout the whole day) and hours close to noon the quotients between the electron and neutral gas temperature profiles have a similar behaviour. In this paper Te profiles measured in situ with Langmuir probes and Tn, profiles given by neutral atmosphere models, both in the specified ionospheric conditions, are used to compute T e/T n profiles. Each of the profiles thus obtained is fitted by a Lorentzian curve and the variation with F10.7 of its parameters is also fitted by simple mathematical expressions. Title: Evidence of mutual induction between the ionosphere and the earth at equatorial latitudes Authors: Duhau, S.; Osella, A. M. Bibcode: 1985JGR....90.4434D Altcode: The mutual induction between the ionosphere and the earth has been suggested as one of the possible causes of the longitudinal inhomogeneities in the phase velocity of type II irregularities in the equatorial electrojet. From the analysis of the geomagnetic daily variations at equatorial latitudes in Peru, Nigeria, and central Africa, latitudinal profiles of the integrated current density and clear evidence of latitudinal discontinuities in the distribution of the earth conductivity were found in previous work. In the present paper a very high correlation is found between tectonic features and inhomogeneities both in the phase velocity and in the integrated current density. This shows that ionospheric inhomogeneities are related to specific lateral discontinuities in the conductivity of the upper layers of the earth. This result gives enough insight into that effect to allow its inclusion in future theoretical models. Title: Hydromagnetic equations for collisionless plasmas in strong magnetic fields Authors: Duhau, S. Bibcode: 1984JPlPh..32...23D Altcode: The Chew, Goldberger & Low equations are a one-fluid system for the thermodynamic variables of the ions that are coupled to the electrons only through the electromagnetic variables. The magnitude of these variables in a collisionless plasma is re-examined in the present paper and it is found that, in the limit in which the Larmor radius and the electron-to-ion mass ratio are both zero, the current, in the plane normal to the magnetic field, is entirely transported by the electrons in the reference frame that moves with the bulk velocity. The first-order electric field contributes to the ion equation of motion with a zero-order term that couples the thermodynamic variables of both species. So the energy equation of the electrons must now be included in the equation set; to close this equation, a simple mathematical representation of the measured quasi-stationary velocity distribution function of this species is used. A two-fluid equation system in the limit in which the Larmor radius and the electron-to-ion mass ratio are both zero is then found. Title: Description of the coastal effect at equatorial latitudes with applications to the Peruvian and Nigerian zones Authors: Duhau, S.; Osella, A. M. Bibcode: 1984P&SS...32..845D Altcode: The problem of the electromagnetic induction produced by a localized and an extended ionospheric current near an ocean coast, over a mantle of infinite conductivity, has been reduced to the solution of an integral equation where the induced current density appears in an implicit form. This formalism is applied to calculate the field induced by the geomagnetic daily variation due to the presence of the ocean at the Peruvian and Nigerian equatorial zones. Title: The current flow between the equatorial E and F region Authors: Duhau, S.; Louro, A. A. Bibcode: 1983JGR....88.9205D Altcode: In dynamical calculations inthe equatorial F region it is usually assumed that the current flowing through the base of the F region does so exclusively along the geomagnetic field lines. This hypothesis is revised in the present paper, and it is found that the major contirbution to the total flow at that height is given by the current normal to the field lines. Title: A theoretical model of equatorial F region dynamics Authors: Duhau, S.; Louro, A. A. Bibcode: 1983JGR....88.9213D Altcode: A simple dynamical model of the equatorial F region is presented in which this section of the ionosphere is coupled to the E region, which is represented by a thin horizontal current layer, and the vertical current density at the base of the F region is taken to be composed entirely of current normal to the geomagnetic field lines, according to a previous findings by the authors. The new boundary condition for the current density at the interface between the E and F layer simplies the system of equations. The model is applied to calculate the zonal ion velocity and the vertical current at the boundary between the E and F regions. Title: Depth of the nonconducting layer at the Nigerian Dip Equator Authors: Duhau, S.; Osella, A. M. Bibcode: 1983JGR....88.5523D Altcode: The geomagnetic daily variations at the Niegerian dip equator are analyzed to find the thickness of the nonconducting layer in this zone. To do so, the external current system was determined from the external part of the geomagnetic daily variations by using a method discussed in a previous work. The field induced by this system was calculated assuming a two-layer model for the earth conductivity. Fitting this induced field to the internal part of of the geomagnetic daily variations yielded a thickness of 500 km for the nonconducting layer in the internal of 100 km south to 500 km north of the dip equator. However, there is evidences showing a latitudinal gradient in the thickness of this interval. Title: Daytime E-region ion and nitric oxide densities Authors: Azpiazu, M. C.; Duhau, S. Bibcode: 1982JPhD...15..933A Altcode: Rocket observations of ion number densities in the E-region show that the relative concentration of its major constituents, n(NO+)/n(O2+), varies drastically with solar activity and depends on the electron and the nitric oxide concentrations. In this paper, a simple steady-state model is presented in which, by means of the electric quasi-neutrality condition, the continuity equations of both ions have been uncoupled and a one-to-one relationship between the electron and nitric oxide concentrations has been obtained. This model is applied to the calculation of the relative ion number densities and the nitric oxide concentration at middle latitudes, near noon under quiet geomagnetic and low solar activity conditions and a good agreement with the observation is found. Title: A correlation between measured E-region current and geomagnetic daily variation at equatorial latitude. Authors: Duhau, S.; Osella, A. M. Bibcode: 1982JGG....34..213D Altcode: The usual methods of separation of the geomagnetic daily variations into parts of external and internal origin at equatorial latitudes have been revised to remove any previous assumption about the internal current, so that the separation may be performed in a zone of anomalous earth conductivity. The resulting procedure has been applied to obtain the distribution of the ionospheric current from the external field, at the South American dip equator and the result has been compared with previous measurements of the E-region current. Title: Indication of anomalous conductivity at the nigerian dip equator Authors: Duhau, S.; Romanelli, L.; Hirsch, F. A. Bibcode: 1982P&SS...30...97D Altcode: The geomagnetic daily variations at the Nigerian dip equator have been analyzed with the methodology introduced in a previous paper. It has been found that the height integrated current presents a notoriously higher amplification in Nigeria than in Peru. It has also been found that there exists a strong and inhomogeneous anomaly in the Earth's conductivity in Nigeria. And contrary to what is usually accepted, it is shown that its latitudinal distribution can not be precisely determined until the distribution and magnitude of the ionospheric currents at F-region heights is more accurately known. Title: Non-thermal equilibrium between electrons and neutrals at ionospheric E-region heights Authors: Duhau, S.; Azpiazu, M. C. Bibcode: 1981GeoRL...8..819D Altcode: The results of measurements of electron temperature in the E-region by incoherent scatter radar and Langmuir probes are contradictory. To provide an additional empirical evidence a correlation is found between the collision frequency data obtained by the method of deviative absorption and F10.7 (for a height of 110 km) at noon. A remarkable agreement is found between these results and those obtained with the collision frequencies calculated from temperatures measured with probes on quiet days. We conclude that, contrary to what is usually assumed, electrons and neutrals are not in thermal equilibrium in the E-region, the quotient between the electron and neutral gas temperatures (Te/Tn) depends strongly on F10.7, and that there is a mechanism whereby electrons are heated, totally localized in the E-region. Title: The ionospheric current at the Nigerian equator as determined from the geomagnetic daily variations Authors: Duhau, S.; Romanelli, L. Bibcode: 1981P&SS...29..703D Altcode: A critical analysis of the theoretical methods for obtaining the ionospheric and induced currents from the geomagnetic variations is performed, and the ionospheric currents in the electrojet are analyzed, through the geomagnetic variations in the proximity of the dip equator in Nigeria. For this purpose, a method, previously introduced by the authors, is applied, which makes it possible to discuss the contribution of the currents induced in the Earth by those variations. The result is compared with that obtained from the variations in the South American area. It is found that the amplifications could be very different in the two areas. Title: Electromagnetic induction at the south american geomagnetic equator as determined from measured ionospheric currents Authors: Duhau, S.; Romanelli, L. Bibcode: 1979JGR....84.1849D Altcode: The latitudinal distribution of the equatorial height-integrated current density has been obtained from available experimental data and correlated with the latitudinal distribution of the horizontal component of the daily geomagnetic variations to determine the importance of the internal part of the electrojet field. A simple method is introduced that makes it possible to analyze the importance of the internal electroject field and simultaneously to separate the planetary field. Contradicting what has been assumed in recent works and confirming earlier results, it has been found that the current induced in the earth by the electrojet contributes significantly to daily geomagnetic variations at South American longitudes. Title: Kelvin-Helmholtz instability of anisotropic plasma in a magnetic field Authors: Duhau, S.; Gratton, J. Bibcode: 1975JPlPh..13..451D Altcode: The Kelvin-Helmholtz problem is analyzed by a set of general hydromagnetic equations, which includes ideal magnetohydrodynamic and Chew-Goldberger-Low models as particular cases. A formalism is given that facilitates comparison between results from different models. A sheared flow is one in which the velocity has no component in the y direction, and such that the x and z components of the velocity depend on the y co-ordinate. A sheared field is defined similarly. The differential equations for linear modes of oscillation of a sheared flow in a sheared magnetic field is obtained; and the energy of these modes is studied. As a particular case of oscillations of a sheared flow, the properties of the modes excited by arbitrary modulation of a tangential discontinuity are studied. The relationship between radiation of waves from such a discontinuity and instability of the system is brought out by considering the system energy. Domains of absolute stability are given; and the different hydromagnetic models are compared by examining the predicted domains. It is found that anisotropy plays an important role in the conditions of stability. Title: La descripción hidrodinámica del plasma y su aplicación al problema de Kelvin-Helmholtz Title: La descripción hidrodinámica del plasma y su aplicación al problema de Kelvin-Helmholtz Title: The hydrodynamic description of plasma and its application to the Kelvin-Helmholtz problem; Authors: Duhau, Silvia Bibcode: 1974PhDT.......123D Altcode: No abstract at ADS Title: Hydromagnetic oscillations of a tangential discontinuity in the Chew, Goldberger, and Low approximation. Authors: Duhau, S.; Gratton, F.; Gratton, J. Bibcode: 1970PhFl...13.1503D Altcode: The differential equation for linear modes of oscillation of plane parallel flows of plasmas along an external magnetic field in the Chew, Goldberger, and Low approximation is obtained. Properties of modes for a tangential discontinuity are studied for the case when the surface is modulated along the magnetic field. Overstable modes found by other authors are shown to be spurious. Regions of existence of modes, proper frequencies, and spatial dependence of the perturbation are given. It is found that, broadly speaking, low β plasmas should be free of surface instabilities for all values of the flow velocity, whereas high β plasmas can be unstable if the flow velocity is nearly sonic. Changes in the anisotropy do not substantially affect the general picture of the problem.