duhau

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Author name code: duhau
ADS astronomy entries on 2022-09-14
author:Duhau, Silvia

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Title: Solar magnetic variability and climate
Authors: de Jager, Cornelis; Duhau, Silvia; Nieuwenhuizen, Adrianus
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.

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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.
2018PAIJ....2..175D Altcode: 2018PAIJ....2..175J
No abstract at ADS

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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.
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).

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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.
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.

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Title: Sudden transitions and grand variations in the solar dynamo,
past and future&lt;/xref&gt;
Authors: De Jager, Cornelis; Duhau, Silvia
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 <A
href="http://www.cdejager.com/sun-earth-publications/">www.cdejager.com/sun-earth-publications/</A>.

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Title: The influence of planetary attractions on the solar tachocline
Authors: Callebaut, Dirk K.; de Jager, Cornelis; Duhau, Silvia
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.

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Title: Quantifying and specifying the solar influence on terrestrial
surface temperature
Authors: de Jager, C.; Duhau, S.; van Geel, B.
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.

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Title: Solar Gleissberg periodicities in relation to grand episodes
Authors: Duhau, S.; de Jager, C.
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

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Title: Forecasting the parameters of sunspot cycle 24 and beyond
Authors: de Jager, C.; Duhau, S.
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).

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Title: Episodes of relative global warming
Authors: de Jager, C.; Duhau, S.
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.

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Title: The Solar Dynamo and Its Phase Transitions during the Last
Millennium
Authors: Duhau, S.; de Jager, C.
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.

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Title: A long-term relationship between solar magnetic field,
geomagnetic field and Earth's rotation rate.
Authors: Duhau, S.; de Jager, C.
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.

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Title: Solar Activity Earth Rotation Rate and Global Surface
Temperature Long-term Variations.
Authors: Duhau, S.
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.

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Title: On the Physical Origin of the Cycles in Long-term Modulation
of Solar Activity
Authors: Duhau, S.; de Jager, C.
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.

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Title: Long Term Variations in Solar Magnetic Field, Geomagnetic
Field and Climate
Authors: Duhau, S.
2006apri.meet...18D Altcode:
No abstract at ADS

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Title: Global Earth surface temperature changes induced by mean Sun
dynamo magnetic field variations
Authors: Duhau, Silvia
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.

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Title: Total ozone content changes associated to solar dynamo magnetic
field variations.
Authors: Duhau, Silvia; Martínez, Ernesto A.
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.

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Title: On the nature of the steep changes in solar and geomagnetic
activity after 1705, 1923 and 1993
Authors: Duhau, Silvia
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.

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Title: An Early Prediction of Maximum Sunspot Number in Solar Cycle 24
Authors: Duhau, S.
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.

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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.
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.

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Title: A method for the interpretation of three-dimensional equatorial
GDV fields
Authors: Duhau, S.; Martínez, E. A.
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.

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Title: An explanation of the kilometer scale wave in the equatorial
electrojet
Authors: Duhau, Silvia; de Mendoza, Diego Hurtado
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.

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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.
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.

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Title: Depth of the conductosphere under the Indian shield
Authors: Favetto, A.; Osella, A. M.; Duhau, S.
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.

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Title: Solar wind stream interaction: Electron temperature and heat
flux rise in the low-speed stream
Authors: Alexander, P.; Duhau, S.
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.

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Title: A comparison between the experimentally and theoretically
determined equatorial electrojet electric field
Authors: de La Vega, M.; Duhau, S.
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.

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Title: Absolute stability in a collisionless electron-heat-conducting
plasma in strong magnetic fields
Authors: de la Torre, A.; Duhau, S.
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

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Title: Meridional electric currents in the equatorial F-region
Authors: Louro, A. A.; Duhau, S.
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.

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Title: The zonal neutral wind in the equatorial thermosphere
Authors: Louro, A. A.; Duhau, S.
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.

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Title: Boundary conditions for the neutral and ionospheric transport
equations at the base of the equatorial F region
Authors: Duhau, S.; Louro, A. A.
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.

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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.
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.
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 &amp; 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.
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.
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.
1984JPlPh..32...23D Altcode:
The Chew, Goldberger &amp; 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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
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.
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.