hoyng

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Author name code: hoyng
ADS astronomy entries on 2022-09-14
author:"Hoyng, Peter"

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Title: Mode analysis of numerical geodynamo models
Authors: Schrinner, M.; Schmitt, D.; Hoyng, P.
2011PEPI..188..185S Altcode: 2011arXiv1101.3022S
It has been suggested in Hoyng (2009) that dynamo action can be analysed
by expansion of the magnetic field into dynamo modes and statistical
evaluation of the mode coefficients. We here validate this method by
analysing a numerical geodynamo model and comparing the numerically
derived mean mode coefficients with the theoretical predictions. The
model belongs to the class of kinematically stable dynamos with a
dominating axisymmetric, dipolar and non-periodic fundamental dynamo
mode. Our present study supports that contributions from higher
order modes to the magnetic field result from the deformation of the
fundamental mode by the turbulent flow. The analysis requires a number
of steps: the computation of the so-called dynamo coefficients, the
derivation of the temporally and azimuthally averaged dynamo eigenmodes
and the decomposition of the magnetic field of the numerical geodynamo
model into the eigenmodes. For the determination of the theoretical mode
excitation levels the turbulent velocity field needs to be projected
on the dynamo eigenmodes. We compare the theoretically and numerically
derived mean mode coefficients and find reasonably good agreement
for most of the modes. Some deviation might be attributable to the
approximation involved in the theory. Since the dynamo eigenmodes
are not self-adjoint, a spectral interpretation of the eigenmodes is
not possible.

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Title: An efficient method for computing the eigenfunctions of the
dynamo equation
Authors: Schrinner, M.; Schmitt, D.; Jiang, J.; Hoyng, P.
2010A&A...519A..80S Altcode: 2009arXiv0911.3279S
 Aims: We present an elegant method of determining the
eigensolutions of the induction and dynamo equations in a fluid
embedded in a vacuum. Methods: The magnetic field is expanded
in a complete set of functions. The new method is based on the
biorthogonality of the adjoint electric current and the vector
potential with an inner product defined by a volume integral over
the fluid domain. The advantage of this method is that the velocity
and the dynamo coefficients of the induction and the dynamo equation
do not have to be differentiated and thus even numerically determined
tabulated values of the coefficients produce reasonable results. 
Results: We provide test calculations and compare with published results
obtained by the classical treatment based on the biorthogonality of the
magnetic field and its adjoint. We especially consider dynamos with
mean-field coefficients determined from direct numerical simulations
of the geodynamo and compare with initial value calculations and the
full MHD simulations.

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Title: Saturation and time dependence of geodynamo models
Authors: Schrinner, M.; Schmitt, D.; Cameron, R.; Hoyng, P.
2010GeoJI.182..675S Altcode: 2009arXiv0909.2181S
In this study we address the question under which conditions a
saturated velocity field stemming from geodynamo simulations leads
to an exponential growth of the magnetic field in a corresponding
kinematic calculation. We perform global self-consistent geodynamo
simulations and calculate the evolution of a kinematically advanced
tracer field. The self-consistent velocity field enters the induction
equation in each time step, but the tracer field does not contribute to
the Lorentz force. This experiment has been performed by Cattaneo and
Tobias and is closely related to the test field method by Schrinner
et al. We find two dynamo regimes in which the tracer field either
grows exponentially or approaches a state aligned with the actual
self-consistent magnetic field after an initial transition period. Both
regimes can be distinguished by the Rossby number and coincide with
the dipolar and multipolar dynamo regimes identified by Christensen and
Aubert. Dipolar dynamos with low Rossby number are kinematically stable
whereas the tracer field grows exponentially in the multipolar dynamo
regime. This difference in the saturation process for dynamos in both
regimes comes along with differences in their time variability. Within
our sample of 20 models, solely kinematically unstable dynamos show
dipole reversals and large excursions. The complicated time behaviour
of these dynamos presumably relates to the alternating growth of
several competing dynamo modes. On the other hand, dynamos in the low
Rossby number regime exhibit a rather simple time dependence and their
saturation merely results in a fluctuation of the fundamental dynamo
mode about its critical state.

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Title: Statistical dynamo theory: Mode excitation
Authors: Hoyng, P.
2009PhRvE..79d6320H Altcode: 2009arXiv0903.3844H
We compute statistical properties of the lowest-order multipole
coefficients of the magnetic field generated by a dynamo of arbitrary
shape. To this end we expand the field in a complete biorthogonal set of
base functions, viz. B=∑kak(t)bk(r)
. The properties of these biorthogonal function sets are treated
in detail. We consider a linear problem and the statistical
properties of the fluid flow are supposed to be given. The turbulent
convection may have an arbitrary distribution of spatial scales. The
time evolution of the expansion coefficients ak is
governed by a stochastic differential equation from which we infer
their averages ⟨ak⟩ , autocorrelation functions
⟨ak(t)ak∗(t+τ)⟩ , and an equation for
the cross correlations ⟨akaℓ∗⟩ . The
eigenfunctions of the dynamo equation (with eigenvalues λk
) turn out to be a preferred set in terms of which our results assume
their simplest form. The magnetic field of the dynamo is shown
to consist of transiently excited eigenmodes whose frequency and
coherence time is given by Iλk and -1/Rλk ,
respectively. The relative rms excitation level of the eigenmodes,
and hence the distribution of magnetic energy over spatial scales,
is determined by linear theory. An expression is derived for
⟨|ak|2⟩/⟨|a0|2⟩ in
case the fundamental mode b0 has a dominant amplitude, and
we outline how this expression may be evaluated. It is estimated that
⟨|ak|2⟩/⟨|a0|2⟩∼1/N
, where N is the number of convective cells in the dynamo. We show that
the old problem of a short correlation time (or first-order smoothing
approximation) has been partially eliminated. Finally we prove that
for a simple statistically steady dynamo with finite resistivity all
eigenvalues obey Rλk&lt;0 .

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Title: Analysis of the variability of the axial dipole moment of a
numerical geodynamo model
Authors: Kuipers, J.; Hoyng, P.; Wicht, J.; Barkema, G. T.
2009PEPI..173..228K Altcode:
We have analysed the time evolution of the axial dipole moments
(ADMs) from three numerical geodynamo models by relating it to
the Fokker-Planck equation governing the systematic and random ADM
motion. We have determined the effective growth rate of the ADM and
the diffusion coefficient D characterising its random fluctuations. We
find that the numerical ADM data exhibit a nonlinear quenching that
is not significantly different from that of the Sint-2000 data. The
quenching is only partly due to a reduction of the r.m.s. convective
flow speed with increasing ADM. Our results suggest that in these
numerical models similar mechanisms may be at work as in the earth's
core, and that the results of Brendel et al. [Brendel, K., Kuipers,
J., Barkema, G.T., Hoyng, P., 2007. An analysis of the fluctuations
of the geomagnetic dipole. Phys. Earth Planet. Inter. 162, 249-255]
are unlikely to be an artifact caused by the restricted length of
the dataset. They also suggest that the dynamics of the ADM is that
of a Brownian particle (i.e. driven by additive noise) in a bistable
potential, and we illustrate some consequences of this idea.

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Title: An analysis of the fluctuations of the geomagnetic dipole
Authors: Brendel, K.; Kuipers, J.; Barkema, G. T.; Hoyng, P.
2007PEPI..162..249B Altcode: 2007arXiv0707.0623B
The time evolution of the strength of the Earth's virtual axial
dipole moment (VADM) is analyzed by relating it to the Fokker-Planck
equation, which describes a random walk with VADM-dependent drift
and diffusion coefficients. We demonstrate first that our method
is able to retrieve the correct shape of the drift and diffusion
coefficients from a time series generated by a test model. Analysis
of the Sint-2000 data shows that the geomagnetic dipole mode has a
linear growth time of 20-7+13 kyear, and that the nonlinear quenching
of the growth rate follows a quadratic function of the type [1-(]. On
theoretical grounds, the diffusive motion of the VADM is expected to
be driven by multiplicative noise, and the corresponding diffusion
coefficient to scale quadratically with dipole strength. However,
analysis of the Sint-2000 VADM data reveals a diffusion which depends
only very weakly on the dipole strength. This may indicate that the
magnetic field quenches the amplitude of the turbulent velocity in
the Earth's outer core.

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Title: Relativistic astrophysics and cosmology : a primer
Authors: Hoyng, Peter
2006racp.book.....H Altcode:
No abstract at ADS

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Title: Relativistic Astrophysics and Cosmology
Authors: Hoyng, Peter
2006rac..book.....H Altcode:
No abstract at ADS

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Title: End-to-end simulations for the LISA Technology Package
Authors: Hannen, V. M.; Smit, M.; Hoyng, P.; Selig, A.; Schleicher, A.
2003CQGra..20S.261H Altcode:
We present an end-to-end simulation facility which has been developed in
the framework of the LISA Technology Package (LTP) architect study for
SMART-2, the technology demonstration mission that precedes LISA. The
simulator evolves positions and orientations of the spacecraft and two
test masses contained in the inertial sensors of LTP under the influence
of external and internal forces and torques and under the influence of
control loops for satellite drag-free control (DFC) and electrostatic
test mass suspension. Altogether, a coupled system with 18 degrees of
freedom is solved numerically. A series of test runs has been performed
to verify the correct functioning of the various models contained in
the end-to-end simulator and to provide a preliminary assessment of the
performance of DFC algorithms and control laws for test mass suspension,
which are currently foreseen for use in the basic operation mode of LTP.

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Title: The field, the mean and the meaning
Authors: Hoyng, Peter
2003and..book....1H Altcode: 2003eclm.book....1H
No abstract at ADS

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Title: Solar Cycle
Authors: Ossendrijver, M.; Hoyng, P.
2002eaa..bookE1980O Altcode:
The term solar cycle refers to a quasi-periodic variation with a period
of about 11 years, visible in many of the Sun's observables. The
solar cycle is most easily observed in indices directly related to
the Sun's magnetic field, such as SUNSPOTS. During the last decades,
solar-cycle variations have also been found in many other aspects of
the Sun (irradiance, surface flows, coronal shape, oscil...

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Title: A theoretical analysis of the observed variability of the
geomagnetic dipole field
Authors: Hoyng, P.; Schmitt, D.; Ossendrijver, M. A. J. H.
2002PEPI..130..143H Altcode:
We present a detailed analysis of the Sint-800 virtual axial dipole
moment (VADM) data in terms of an αΩ mean field model of the geodynamo
that features a non-steady generation of poloidal from toroidal magnetic
field. The result is a variable excitation of the dipole mode and the
overtones, and there are occasional dipole reversals. The model permits
a theoretical evaluation of the statistical properties of the dipole
mode. We show that the model correctly predicts the distribution of
the VADM and the autocorrelation function inferred from the Sint-800
data. The autocorrelation technique allows us to determine the turbulent
diffusion time τd= R2/ β of the geodynamo. We
find that τd is about 10-15 kyr. The model is able to
reproduce the observed secular variation of the dipole mode, and the
mean time between successive dipole reversals. On the other hand, the
duration of a reversal is a factor ∼2 too long. This could be due to
imperfections in the model or to unknown systematics in the Sint-800
data. The use of mean field theory is shown to be selfconsistent.

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Title: Magnetic field reversals and secular variation in a bistable
geodynamo model
Authors: Schmitt, D.; Ossendrijver, M. A. J. H.; Hoyng, P.
2001PEPI..125..119S Altcode:
A method is described which enables the calculation of statistical
properties of geodynamo models. The result is a Fokker-Planck equation
for the probability distribution of the dipole moment. The method
only requires a non-oscillatory, predominantly dipolar magnetic field
and helicity fluctuations. The fluctuations perturb the fundamental
dynamo mode and lead to the excitation of higher modes. This results in
stochastic oscillations of the dipole field amplitude in a bistable
potential with minima representing normal and reversed polarity,
and occasional jumps between them. The shape of the potential is
determined by supercritical dynamo excitation and nonlinear limitation
of field growth. Application of the method to a mean-field dynamo model
with random fluctuations of the helicity parameter α reproduces the
observed relation between the secular variation and the reversal rate
of the geomagnetic field, as well as the amplitude distribution of
the dipole field inferred from the Sint-800 record.

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Title: The geodynamo as a bistable oscillator
Authors: Hoyng, P.; Ossendrijver, M. A. J. H.; Schmitt, D.
2001GApFD..94..263H Altcode:
Our intent is to provide a simple and quantitative understanding of the
variability of the axial dipole component of the geomagnetic field on
both short and long time scales. To this end we study the statistical
properties of a prototype nonlinear mean field model. An azimuthal
average is employed, so that (1) we address only the axisymmetric
component of the field, and (2) the dynamo parameters have a
random component that fluctuates on the (fast) eddy turnover time
scale. Numerical solutions with a rapidly fluctuating alpha reproduce
several features of the geomagnetic field: (1) a variable, dominantly
dipolar field with additional fine structure due to excited overtones,
and sudden reversals during which the field becomes almost quadrupolar,
(2) aborted reversals and excursions, (3) intervals between reversals
having a Poisson distribution. These properties are robust, and appear
regardless of the type of nonlinearity and the model parameters. A
technique is presented for analysing the statistical properties of
dynamo models of this type. The Fokker-Planck equation for the amplitude
a of the fundamental dipole mode shows that a behaves as the position
of a heavily damped particle in a bistable potential ~(1-a^2)^2,
subject to random forcing. The dipole amplitude oscillates near the
bottom of one well and makes occasional jumps to the other. These
reversals are induced solely by the overtones. Theoretical expressions
are derived for the statistical distribution of the dipole amplitude,
the variance of the dipole amplitude between reversals, and the mean
reversal rate. The model explains why the reversal rate increases
with increasing secular variation, as observed. Moreover, the present
reversal rate of the geodynamo, once per (2-3)x10^5years, is shown
to imply a secular variation of the dipole moment of ~15% (about the
current value). The theoretical dipole amplitude distribution agrees
well with the Sint-800 data.

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Title: Cyclic Evolution of Sunspots: Gleaning New Results from
Old Data
Authors: Solanki, S. K.; Fligge, M.; Pulkkinen, P.; Hoyng, P.
2000JApA...21..163S Altcode:
No abstract at ADS

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Title: Book Review: The little book of the big bang : a cosmic primer
/ Copernicus, 1998/Springer, 1998
Authors: Hoyng, P.
1999SSRv...88..608H Altcode:
No abstract at ADS

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Title: The geodynamo as a bistable oscillator
Authors: Ossendrijver, M. A. J. H.; Hoyng, P.; Schmitt, D.
1999AGAb...15...73O Altcode: 1999AGM....15..J13O
Paleomagnetism has convincingly shown that the Earth has reversed
the polarity of its almost dipolar magnetic field many times in the
past. The origin of the field is generally ascribed to dynamo action
in the liquid outer core. Numerical simulation of a mean-field model
demonstrates that random fluctuations of the dynamo alpha-effect
cause occasional rapid magnetic reversals, like those of the
Earth's field. The fluctuations perturb the fundamental stationary
dipolar mode and lead to the excitation of higher oscillatory dynamo
modes. This results in stochastic oscillations of the dipole field
amplitude in a bistable potential with minima representing normal and
reversed polarity, and occasional jumps between them. The shape of
the potential is determined by supercritical dynamo excitation and
nonlinear limitation of field growth. A statistical analysis shows
that the model reproduces the essential features of the geomagnetic
secular variation and reversal record.

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Title: Op zoek naar de grens van het heelal.
Authors: Hoyng, P.
1998Zenit..25..340H Altcode:
No abstract at ADS

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Title: Book Review: SCORe '96 / Kluwer, 1997 &amp; 1998
Authors: Hoyng, P.
1998SSRv...85..552H Altcode:
No abstract at ADS

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Title: Chromospheric and Photospheric Activity of the Sun as a Star
Using SOLSTICE Data
Authors: Catalano, S.; Lanza, A. F.; Brekke, P.; Rottman, Gary J.;
Hoyng, P.
1998ASPC..154..584C Altcode: 1998csss...10..584C
The distribution of photospheric and chromospheric active regions
(spots and plages) on the surface of stars is mainly recovered from
the deconvolution of light or line flux curves. Here we check the
reliability of the maximum entropy technique to derive surface maps from
the rotational modulation of the continuum and line fluxes observed
in active stars. We compare the solar chromospheric map obtained from
the deconvolution of daily average of the C 2 line flux at 133.5 nm
measured by SOLSTICE, and solar Hα images contemporaneously obtained
at Catania observatory. Hα plages may not exactly overlay C 2 plages,
however the agreement between the reconstructed and the direct images
is strikingly good. Only small (&lt;20circ-30circ
in diameter) or low contrast (Delta I/I &lt; 0.3) structures are not
recovered. We use also the Wolf sunspot number to study the longitude
distribution of the active regions in photosphere and chromosphere,
contrasting the solar behaviour with that of other active stars.

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Title: A Model for a Thin Magnetised Disc in LMC X-3
Authors: Hoyng, P.
1998IAUS..188..379H Altcode:
Dynamo action in turbulent accretion discs will generate randomly
oriented magnetic fields. The mean field &lt;! !B ! !&gt; can be
assumed to be zero. We have determined the radial distribution of the
r.m.s. magnetic field (i.e. the six quantities &lt; ! !B
theta 2 ! !&gt;, &lt; ! !B theta Br
! !&gt;, etc.). The disc structure is determined with the help of
the standard thin disc equations by allowing for the influence of the
magnetic field on temperature and angular momentum. This work is an
extension of that of Schramkowski et al. (1996) who considered only
vertical turbulent transport of magnetic energy. We include also radial
transport, energetically unimportant, but essential in that it fixes
the radial distribution of the r.m.s. field. Our approach is still
kinematic. It is found that a magnetic disc is thinner and hotter than
its non-magnetised counterpart. If the escaping magnetic energy heats a
disc corona, the radial distribution of the r.m.s. magnetic field may
be inferred from the `ratio' of the thermal emission of the disc and
the nonthermal emission of the corona. From the X-ray spectrum of black
hole candidate LMC X-3 we confirm values in the range of 106
- 107G found by Schramkowski et al. (1996). Schramkowski
G.P. et al.: 1996, A&amp;A 315, 638.

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Title: Mean magnetic field and energy balance of Parker's surface-wave
dynamo.
Authors: Ossendrijver, A. J. H.; Hoyng, P.
1997A&A...324..329O Altcode:
We study the surface-wave dynamo proposed by Parker (1993) as a model
for the solar dynamo, by solving equations for the mean magnetic
field {vec}(B)_0_, as well as for the mean 'magnetic energy tensor'
((T))=&lt;{vec}(BB)&gt;/8π. This tensor provides information about the
energy balance, rms field strengths and correlation coefficients between
field components. The main goal of this paper is to check whether the
equations for {vec}(B)_0_ and ((T)) are compatible, i.e. whether both
have "reasonable" solutions for a set of "reasonable" parameters. We
apply the following constraints: {vec}(B)_0_ has a period of 22 years
and, taking into account the effect of period variations, a decay time
of 10 dynamo periods, and ((T)) is marginally stable. We find that
under these constraints, the equations for {vec}(B)_0_ and ((T)) are
compatible only if, apart from turbulent transport out of the dynamo
region, an additional energy sink is introduced. If this extra term is
omitted, then marginal stability of ((T)) requires a turbulent diffusion
in the convection zone of the order β_2_&gt;3x10^14^cm^2^/s, whereas
the conditions on {vec}(B)_0_ require β_2_=~10^12^cm^2^/s. Furthermore,
the rms surface field strength, the maximum rms field strength and the
magnetic energy flux through the upper surface of the convection zone
cannot simultaneously assume solar values. We explore the possibility
that the extra energy sink is provided by resistive dissipation,
hitherto not accounted for in the equation for ((T)), by considering
various cases. We demonstrate that with a heuristically modified
equation for ((T)), the inconsistencies can be removed. Our results
suggest that resistive dissipation is the dominant sink of magnetic
energy, and that resistive heating may amount to several percent of
the solar luminosity.

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Title: GRB 960720 and GRB 970111
Authors: in 't Zand, J.; Heise, J.; Hoyng, P.; Jager, R.; Piro, L.;
Costa, E.; Feroci, M.; Frontera, F.; dal Fiume, D.; Orlandini, M.;
Pizzichini, G.; Nicastro, L.; Muller, J.
1997IAUC.6569....1I Altcode: 1997IAUC.6569Q...1I
J. in 't Zand, J. Heise, P. Hoyng, and R. Jager, Space Research
Organization of the Netherlands (SRON), Utrecht; L. Piro, E. Costa, and
M. Feroci, Istituto Astrofisica Spaziale, CNR, Frascati; F. Frontera,
D. Dal Fiume, M. Orlandini, G. Pizzichini, and L. Nicastro, Istituto
Tecnologie e Studio delle Radiazioni Extraterrestri, CNR, Bologna; and
J. Muller, BeppoSAX, Rome, and SRON, Utrecht, write: "The positions of
the x-ray counterparts to GRB 960720 and GRB 970111 as determined with
the BeppoSAX Wide Field Cameras (cf. IAUC 6467 and 6533, respectively)
have been refined through an improvement of the spatial calibration of
the instruments. The calibration concerns small misalignments between
both cameras and between the cameras and the satellite coordinate
system, the image scales of both WFCs, and the modeling of the point
spread function. The calibration was tested with 123 detections of
catalogued x-ray sources with accurately known positions, as found in 15
SAX WFC observations with a variety of orientation angles. These sources
have a representative range of intensities and off-axis angles. None of
the detections has a difference between catalogue and measured source
positions &gt; 3'.9, and 97 percent have a difference of &lt; 3'. In a
subset of fourteen sources having significance levels that are within
a factor of 2 of the levels of GRB 960720 and GRB 970111, one source
shows a difference of &gt; 3' from the catalogue position. For GRB
960720, we find a 3'-radius error circle centered on R.A. = 17h30m37s,
Decl. = +49o05'.8 (equinox 2000.0). This position is 10'.5 from the
previously reported position, for which a 10' error circle radius was
reported. Neither the previously reported SAX NFI source (IAUC 6480)
nor the optical source (IAUC 6526) lies within the error circle. The
new error circle does include the radio-loud quasar 4C 49.29 (z =
1.038). The error circle for GRB 970111 is centered on R.A. = 15h28m15s,
Decl. = +19o36'.3. This center is 4'.2 from the previously reported
position, and the circle overlaps the IPN annulus reported by Hurley
et al. (IAUC 6545). None of the previously reported SAX NFI sources
(Butler et al., IAUC 6539), RASS sources (Voges et al., IAUC 6539),
radio source (Frail et al., IAUC 6545), and ROSAT HRI source (IAUC 6567)
lies within the 3'- radius error circle."

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Title: Is the Solar Cycle Timed by a Clock?
Authors: Hoyng, P.
1996SoPh..169..253H Altcode:
Dicke (1978) has argued that the phase of the solar cycle appears to
be coupled to an internal clock: shorter cycles are usually followed
by longer ones, as if the Sun remembers the correct phase. The data
set is really too short to demonstrate the presence of a phase memory,
but phase and amplitude of the cycle are strongly correlated for 300
yr or more. It is shown that this memory effect can be explained
by mean field theory in terms of fluctuations in α, which induce
coherent changes in the frequency and amplitude of a dynamo wave. It
is concluded that there is neither a strong observational indication
nor a theoretical need for an extra timing device, in addition to the
one provided by dynamo wave physics.

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Title: Does the sun have a memory?
Authors: Ossendrijver, A. J. H.; Hoyng, P.; Schmitt, D.
1996NAWG.1996..247O Altcode:
The authors study the effect of stochastic fluctuations in α using
a simple axisymmetric mean field dynamo model. There are two main
features: Firstly, the excitation of higher eigenmodes of the dynamo
equation, providing a possible explanation of asymmetries between the
two hemispheres and, close to the equator, the formation of small
regions of the "wrong" polarity. Secondly, a correlation between
fluctuations in phase and amplitude of the dipole field. Phase and
amplitude are not locked onto one another, i.e. the dynamo has
no "memory". The authors use the phase-amplitude correlation to
compare the model with sunspot data, where a similar correlation is
observed. This provides a rough estimate of the required strength
of the α-fluctuations. It is found that α, defined as an azimuthal
average, has fluctuations of the order δα/α0 ≍ 3 at
mid latitudes.

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Title: Flow and distribution of magnetic energy in accretion disks:
a kinematic approach.
Authors: Schramkowski, G. P.; van Niekerk, E. C. M.; Hoyng, P.;
Achterberg, A.
1996A&A...315..638S Altcode:
By taking account of the statistical nature of kinematic mean field
dynamo theory, it is pointed out that the mean magnetic field &lt;B&gt;
should always vanish. A more fruitful approach therefore seems to be to
study the dynamics of the second moment &lt;BB&gt; of magnetic field
which contains information on mean magnetic energy and statistical
correlations between the various components of the magnetic field. The
general dynamics of this tensor are discussed. As an application
a stationary solution for zero helicity is presented in a local
approximation. It is found that such a dynamo can only be maintained by
compressible turbulence. The implications of this non-helical solution
for the structure and emission of stationary thin accretion disks
are discussed. The limits of applicability of the kinematic approach
are briefly discussed. We apply our model to the inner region of the
accretion disk around the black hole candidate LMC X-3. We find typical
rms magnetic field strength values in the range 4x10^6^-4x10^7^Gauss.

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Title: Book reviews
Authors: Shore, Steven N.; van der Kruit, P. C.; Achterberg, A.;
Hoyng, P.; de Jager, Cornelis
1996SSRv...78..557S Altcode:
No abstract at ADS

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Title: Stochastic and nonlinear fluctuations in a mean field dynamo.
Authors: Ossendrijver, A. J. H.; Hoyng, P.
1996A&A...313..959O Altcode:
We study the effect of rapid stochastic fluctuations in the kinetic
helicity in a plane parallel mean field dynamo model for the
Sun. The α-parameter has a fluctuating component δα=α-α_0_,
which is modelled as a random forcing term. The fluctuations give
rise to variations in the amplitude and phase of the dynamo wave,
such that shorter cycles have higher amplitudes, as is observed in
the solar cycle. By making a second order expansion close to the
unperturbed marginally stable dynamo wave we are able to go beyond
the weak forcing limit studied by Hoyng. We show that with increasing
strength of the forcing the effective dynamo frequency decreases. We
introduce a simple non-linearity to model α-quenching and derive a
set of linear equations for the mean field, valid in the weak forcing
case. With α-quenching, phase and amplitude fluctuations are bounded,
but still correlated. The strength of the α-quenching is measured by a
parameter q=-(T_e_/α_0_)(dα/dT)|_T_e__, where T_e_ is the equilibrium
value of the toroidal field. We make a comparison with sunspot data, and
conclude that these are well explained by the model if δα/α_0_=~2.2
and q=~0.7. Finally we briefly consider the alternative possibility of
fluctuations caused by nonlinear dynamics, without external forcing
(δα=0). We show that the resulting phase-amplitude diagram does
not agree with observations. Although this is no proof that the
phase-amplitude correlation cannot be reproduced by nonlinear chaos,
we conclude that stochastic noise provides a more natural explanation.

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Title: Stochastic excitation and memory of the solar dynamo.
Authors: Ossendrijver, A. J. H.; Hoyng, P.; Schmitt, D.
1996A&A...313..938O Altcode:
We consider a simple axisymmetric mean field dynamo model for the
Sun in the α{OMEGA}-limit and study the effect of rapid, latitude
dependent stochastic fluctuations in α. The fluctuations excite
overtones of the fundamental mode of the mean magnetic field. We
decompose the mean field into eigenmodes and derive an equation for
the mode coefficients. Transient mode excitation gives rise to a
mean field with spatial and temporal variability, and may provide an
explanation for grand minima, the observed phase-amplitude correlation,
North-South asymmetries and, close to the equator, reverse polarity
regions in the solar butterfly diagram. We find that the North-South
asymmetry often peaks near the activity minimum, in agreement with
the observations. The most likely candidate for the origin of the
fluctuations are giant cells. Sunspot data are well reproduced if α,
defined as an azimuthal average, has fluctuations δα of the order
δα/α_0_=~3 at colatitude θ=60deg, assuming that there are 20 giant
cells from pole to pole, with a coherence time of 1 month. The model
predicts that the resulting phase and amplitude fluctuations of the
fundamental mode are correlated for about 90 dynamo periods.

---------------------------------------------------------
Title: Book reviews
Authors: Humphreys, R. M.; Kemp, S.; Savonije, G.; van der Hucht,
K. A.; van der Kruit, P. C.; Miley, G.; Bumba, V.; van Nieuwkoop,
J.; van Hoolst, T.; Cox, A.; Rutten, R. J.; Kleczek, J.; de Jager,
Cornelis; Jerzykiewicz, M.; Zwaan, C.; Poedts, S.; Sakai, Jun-Ichi;
Pecker, J. -C.; Heikkila, W.; de Jong, T.; Wilson, P. R.; Müller,
E. A.; Hoyng, P.; Icke, V.; Shore, S. N.; Achterberg, A.; Lucchin, F.;
Butcher, H.; Ne'Eman, Y.; Heidmann, J.; Belton, M. J. S.; de Graauw,
Th.; Waters, L. B. F. M.; Pacini, F.; Hultqvist, B.; Akasofu, S. -I.;
Vial, J. -C.; Schatzman, E.; van der Laan, H.; Cole, K. D.; Vanbeveren,
D.; Southwood, D.; van der Klis, M.; Katgert, Peter
1996SSRv...76..339H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: STARS - Seismic Telescope for Astrophysical Research from
Space. Report on the phase A study.
Authors: Badiali, M.; Catala, C.; Favata, F.; Fridlund, M.; Frandsen,
S.; Gough, D. O.; Hoyng, P.; Pace, O.; Roca-Cortés, T.; Roxburgh,
I. W.; Sterken, C.; Volonté, S.
1996star.book.....B Altcode:
STARS is an asteroseismology mission, which, through the acquisition of
very accurate light curves, will detect oscillations in a wide variety
of stars. This will allow, for the first time, the internal structure
of stars of different age, composition and spectral type, to be studied
directly, bringing the spectacular successes of helioseismology within
reach for a large number of other stars spreading over most of the
Hertzsprung-Russell diagram, including solar-type stars in open
clusters. The results of the phase A study are presented in this report.

---------------------------------------------------------
Title: Solar and stellar dynamos
Authors: Hoyng, P.
1996ASPC..109...59H Altcode: 1996csss....9...59H
No abstract at ADS

---------------------------------------------------------
Title: STARS: Seismic Telescope for Astrophysical Research from Space
Authors: Badiali, M.; Catala, C.; Favata, F.; Fridlund, M.; Frandsen,
S.; Gough, D. O.; Hoyng, P.; Pace, O.; Roca-Cortes, T.; Roxburgh,
I. W.; Sterken, C.; Volonte, A.
1996ESADS...4....1B Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Does the Sun Have a Memory?
Authors: Ossendrijver, A. J. H.; Hoyng, P.; Schmitt, D.
1996sgmf.proc..247O Altcode:
We study the effect of stochastic fluctuations in alpha using
a simple axisymmetric mean feld dynamo model. There are two main
features. Firstly, the excitation of higher eigenmodes of the dynamo
equation, providing a possible explanation of asymmetries between the
two hemispheres and, close to the equator, the formation of small
regions of the 'wrong' polarity. Secondly, a correlation between
fluctuations in phase and amplitude of the dipole field. Phase and
amplitude are not locked onto one another, i.e. the dynamo has no
'memory'. We use the phase-amplitude correlation to compare the
findings of our model with sunspot data, where a similar correlation
is observed. This provides a rough estimate of the required strength
of the alpha-fluctuations. It is found that alpha, defined as an
azimuthal average, has fluctuations of the order delta alpha / alpha ~
3 at mid latitudes.

---------------------------------------------------------
Title: Book Review: Lectures on solar and planetary dynamos [invited
papers] / Cambridge U Press, 1995
Authors: Hoyng, P.; Somov, B. V.; Hoyng, P.
1995SoPh..160...61H Altcode: 1995SoPh..160...61P
No abstract at ADS

---------------------------------------------------------
Title: Grail: a proposal for a gravitational Radiation Antenna in
the Netherlands
Authors: Frossati, G.; Steel, D.; de Zeeuw, T.; Gaemers, K.; Linde,
F.; van den Heuvel, E.; van den Klis, M.; Sloot, P.; Hoekstra, A.;
de Ronde, J.; van den Graaf, H.; Hartjes, F.; van Holten, J. W.;
Nooren, G. J.; Oberski, J.; de Waele, A.; van den Heijden, R. W.;
Rogalla, H.; Flokstra, J.; Ter Brake, H.; Veldhuis, D.; Fluitman,
J. H.; Bereschot, J. W.; Eldersboek, M.; Lodder, J. C.; de Haab, P.;
Hoyng, P.; Spallicci, A.
1995grai.rept....1F Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Dynamo spectroscopy.
Authors: Hoyng, P.; Schutgens, N. A. J.
1995A&A...293..777H Altcode:
We study the properties of the magnetic field {vec}(B)
of a dynamo by expanding the field in a special set of
base functions, namely the eigenfunctions of its dynamo equation
[{vec}(B)={SIGMA}sigma_csigma^(t){vec}(b)sigma^({vec}(r))].
We prove that the time evolution of the mean &lt;csigma^&gt;
and the frequency spectrum of csigma^(t) are
completely determined by the eigenvalue λsigma_ of
the dynamo equation. The spectrum is a Lorentzian with central
frequency ωsigma_=Imλsigma_ and width
γsigma_=-Reλsigma_. This property had been
conjectured by Hoyng (1988), and is now shown to hold for arbitrary
dynamos. The proof is relatively straightforward, and we point out the
error in the analysis of Hoyng (1988). Finally, we illustrate how this
property sheds light on the physical meaning of the dynamo equation,
and we compare the predicted spectra qualitatively with recent studies
of the global properties of the solar magnetic field.

---------------------------------------------------------
Title: The effect of random alpha-fluctuations and the global
properties of the solar magnetic field.
Authors: Hoyng, P.; Schmitt, D.; Teuben, L. J. W.
1994A&A...289..265H Altcode:
We study the effect of rapid random fluctuations in the dynamo parameter
α in a simple axisymmetric mean-field dynamo. The model is 1D; it is
a shell with latitude-dependent fields. Radial turbulent diffusion is
modeled by a prescribed factor exp(imagkr)/r in the field potentials. We
consider mainly linear models. The fluctuations excite overtones of
the fundamental mode which are otherwise damped. Butterfly diagrams
and frequency spectra S_l_(ν) of the Legendre expansion coefficients
c_l_(t) of the toroidal mean field [B(θ,t)={SIGMA}_l_c_l_(t)P_l_(cos
θ)] are compared with observations of the solar magnetic field. The
results are: (1). The model accounts for the observed relative phases
of the coefficients c_l_(t) for odd l at the frequency 1/(22yr)
of the fundamental mode, and potentially also for their relative
amplitudes. (2). The spectra S_l_(ν) are broad and featureless for
even l, while for odd l the frequency 1/(22yr) of the fundamental
mode dominates. They agree qualitatively with the results of Gokhale
and coworkers. It is unlikely that the modal structure in the surface
field for even l found by Stenflo and colleagues can be produced by mean
field models with random α-fluctuations. (3). Butterfly diagrams have
a solar-type structure for 1&lt;~kR&lt;~5 (R = position of the bottom
of the convection zone). The fluctuations generate many phenomena also
seen in the solar cycle, such as slow activity modulations due to mode
beating, accidental disappearances of the activity, transequatorial
activity, and sometimes quite persistent North-South asymmetries in
the butterfly diagram. The character of the latter depends sensitively
on the value of kR. (4). The amplitudes of the eigenmodes are shown
to behave as randomly excited coupled oscillators, which allows us
to explain the excitation levels of the overtones, their frequency
spectra and the structure of the North-South asymmetries. (5). In the
latitude region where the dynamo operates the local fluctuations in
({vec}(u).∇ x {vec}(u))τ_c_ are ~60 to 70 times larger than the
mean value of ({vec}(u).∇ x {vec}(u))τ_c_, and the fluctuations in
α are 6 to 7 times larger than the mean value of α.

---------------------------------------------------------
Title: Distribution and flow of magnetic energy in an accretion disk
Authors: Schramkowski, G. P.; van Niekerk, E. C. M.; Hoyng, P.;
Achterberg, A.
1994SSRv...68..329S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The solar dynamo
Authors: Hoyng, P.
1994ASIC..433..387H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Random forcing and the mode structure of the solar magnetic
field
Authors: Schmitt, D.; Hoyng, P.; Teuben, L. J. W.
1994smf..conf..110S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Helicity fluctuations in mean field theory: an explanation
for the variability of the solar cycle?
Authors: Hoyng, P.
1993A&A...272..321H Altcode:
We consider the effect of fluctuations δα(t) in the
mean helicity α0 (both assumed independent of
position) on a plane dynamo wave. The time scale τc
of the fluctuations is much shorter than the diffusion time
1/β0κ2(β0κ2τc
≪ 1; β0 = turbulent diffusion
coefficient; κ = wave number). We distinguish
weak and strong random forcing, according to whether
(δαr.m.s./α0)√β0κ2τc
is small or large with respect to 1, and we present a detailed analysis
of the weak forcing case. Simple equations are derived for the
phases and the logarithmic amplitudes of the poloidal and toroidal
mean field, in which the forcing terms appear as additive noise. Phase
difference and amplitude ratio of the poloidal and toroidal (mean) field
are subject to small fluctuations of constant r.m.s. magnitude. Simple
expressions are derived for the r.m.s. phase shift, amplitude
drift and quality factor of the toroidal (mean) field. These
depend on the fluctuations only through the quantity D = ¼(
δαr.m.s./α0)2β0κ2τc
which plays the role of a diffusion coefficient. The results are:
(1). Phase shift Δ and logarithmic amplitude Λ each perform a
random walk; (2). In the α2-limit these random walks are
uncorrelated; the phase is very stable but the amplitude is completely
irregular; (3). In the αω-limit there exists a correlation: Λ +
Δ ≃ 0, which persists for many dynamo periods. The quality factor
is then given by Q = 1/D. The model is then applied to the solar
dynamo. The predicted correlation Λ + Δ ≃ 0 implies that weaker
(stronger) cycles last longer (shorter) than average, which is a
well-known observed feature of the solar cycle. We define Λ and Δ
using the epochs of solar maxima and the sunspot numbers, and show that
Λ + Δ ≃ 0 is obeyed rather well. This indicates that fluctuations
in the mean helicity are an important mechanism causing the observed
phase and amplitude variations of the solar cycle. Simulations show
many features also seen in the solar cycle, such as quasi-periodicity,
intermittency and long periods of low activity. Further inferences are:
(1). The sunspot numbers appear to be proportional to the strength of
the toroidal field; (2). The quality factor Q is about 10, which makes
the solar dynamo a border-line case between weak and strong forcing;
(3). The solar data indicate that it is necessary to allow for nonlinear
effects; (4). The mean helicity fluctuations δα(t) are caused by very
large spatial fluctuations in the local helicity. This could explain
the discrepancy between theoretical estimates for α0
and values derived from mean field models.

---------------------------------------------------------
Title: Prisma - the First Space Mission to See Inside the Stars
Authors: Appourchaux, T.; Gough, D. O.; Catala, C.; Frandsen, S.;
Frohlich, C.; Hoyng, P.; Jones, A.; Lemaire, P.; Tondello, G.; Weiss,
W. W.
1993ASPC...42..411A Altcode: 1993gong.conf..411A
No abstract at ADS

---------------------------------------------------------
Title: Can Mean Helicity Fluctuations Explain the Variability of
the Solar Cycle?
Authors: Hoyng, P.
1993IAUS..157...71H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Distribution of magnetic energy in -dynamos, I: The method
Authors: van Geffen, J. H. G. M.; Hoyng, P.
1993GApFD..71..187V Altcode:
In this paper a method for solving the equation for the mean magnetic
energy &lt;BB&gt; of a solar type dynamo with an axisymmetric convection
zone geometry is developed and the main features of the method are
described. This method is referred to as the finite magnetic energy
method since it is based on the idea that the real magnetic field B of
the dynamo remains finite only if &lt;BB&gt; remains finite. Ensemble
averaging is used, which implies that fields of all spatial scales are
included, small-scale as well as large-scale fields. The method yields
an energy balance for the mean energy density B2/8 of the dynamo, from
which the relative energy production rates by the different dynamo
processes can be inferred. An estimate for the r.m.s. field strength
at the surface and at the base of the convection zone can be found by
comparing the magnetic energy density and the outgoing flux at the
surface with the observed values. We neglect resistive effects and
present arguments indicating that this is a fair assumption for the
solar convection zone. The model considerations and examples presented
indicate that (1) the energy loss at the solar surface is almost
instantaneous; (2) the convection in the convection zone takes place
in the form of giant cells; (3) the r.m.s. field strength at the base
of the solar convection zone is no more than a few hundred gauss; (4)
the turbulent diffusion coefficient within the bulk of the convection
zone is about 1014cm2s-1, which is an order of magnitude larger than
usually adopted in solar mean field models.

---------------------------------------------------------
Title: PRISMA: Probing Stars from Core to Corona
Authors: Appourchaux, T.; Catala, C.; Catalano, S.; Fransden, S.;
Fröhlich, C.; Gough, D. O.; Hoyng, P.; Jones, A.; Lemaire, P.;
Tondello, G.; Weiss, W.
1993ASSL..183..505A Altcode: 1993pssc.symp..505A
No abstract at ADS

---------------------------------------------------------
Title: Stochastic excitation of global magnetic fields by fluctuations
in the mean helicity
Authors: Hoyng, P.; van Geffen, J. H. G. M.
1993GApFD..68..203H Altcode:
The mean field B in the dynamo equation can be interpreted as a
longitude-averaged field, and this implies that there are fluctuations
in the mean parameters characterizing the turbulent flow. In this paper
we study the effect of fluctuations in the mean helicity numerically
and analytically in a very simple spherical 2-dynamo: there is no
differential rotation and the non-fluctuating parts of and do not depend
on position (we call this the 2-sphere dynamo). The dynamo equation then
contains a term × (t)B, which describes the effect of the fluctuations
in the mean helicity. We show that this type of random forcing implies
that the dynamo has to operate (slightly) subcritically, and that in
addition many eigenmodes are excited, rather than only the fundamental
mode. The advantage of this simple α2-dynamo model is that we can
support the numerical results with analytical estimates, for instance,
for the value of the dynamo number at which the dynamo operates,
the relative excitation levels of the modes, and their spectra. This
is achieved with the help of an expansion technique: B is expanded
in terms of a complete, orthogonal set of eigenfunctions. We have
taken the mean helicity fluctuations to be position-independent for
simplicity. This, however, renders the dynamo model so simple that only
dipole fields are excited and magnetic field reversals are absent. We
also briefly study the effect of non-linearities, in particular of
α-effect quenching. Non-linearities provide a reference level to the
fundamental mode, but do not affect the relative excitation levels of
the modes.

---------------------------------------------------------
Title: Book review
Authors: Hoyng, P.
1992SoPh..137..203H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Book-Review - Helioseismology from Space
Authors: Foing, B. H.; Hoyng, P.
1992SoPh..137..203F Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Mean Field Dynamo Theory
Authors: Hoyng, P.
1992ASIC..373...99H Altcode: 1992sla..conf...99H
No abstract at ADS

---------------------------------------------------------
Title: The velocity sensitivity of resonant scattering spectrometers
employing a piezoelastic modulator
Authors: Hoyng, P.
1991SoPh..133...43H Altcode:
The resonant scattering spectrometers of the IRIS ground-based
network for measuring whole-disc solar velocity oscillations make
use of a piezoelastic modulator. The velocity noise generated by
this optical component is analysed with particular emphasis on the
required stability of the amplitude of oscillation, a. The product
of the absolute stability ¦ a − am¦/amand
the relative stability δar.m.s./am may not be
larger than 10 −4 to 10 −5 (depending on
specific wishes), where amis the optimum amplitude. The
velocity noise due to photon statistics is slightly enhanced, but
other instrumental sources of velocity noise remain unaffected.

---------------------------------------------------------
Title: Non-Steady Global Magnetic Fields in Kinematic Theory
Authors: van Geffen, J. H. G. M.; Hoyng, P.; Zwaan, C.
1991LNP...380..129V Altcode: 1991IAUCo.130..129V; 1991sacs.coll..129V
The dynamo equation for the mean field (B) contains a random forcing
term of unknown magnitude, which is therefore always omitted. The
influence of this term is potentially large. To evaluate its effect,
we employ ensemble averaging. If an ensemble average is used, there
is no random forcing term in the dynamo equation. The effect of
fluctuations is that the ensemble members get out of phase, so that
(B) 0. The damping time of (B) can be found by requiring that the mean
energy (BB) remains finite. The eigenvalues of the dynamo equation then
all have negative real parts. Im determines the period, and -Re/Im the
relative period stability of the dynamo. We have developed a code to
solve the equation for (BB) in a spherical shell (the convection zone),
assuming axisymmetry. We report our first results, which do not yet
include differential rotation. Using spherically symmetric boundary
conditions, we reproduce the well known 2-dynamo, whose behaviour is
known analytically. For instance, for an 2-dynamo located in a shell
with inner boundary at R/2, we find that (BB) remains finite for
R2/ = 1.48, where 3 represents turbulent diffusion and .y turbulent
vorticity. Taking = 1/4(βy)1/2 — a factor of four below maximum
helicity — implies that we have a dynamo number Ca ≡ Rα/β =
0.30. Using this value we find a damping time of 6 X 10-2 R 2/β
for (B), which is a measure for the coherence time of B in a single
ensemble member. This result implies that the large-scale field of
this particular α2-dynamo reorganizes its structure completely on
a time scale of only about one year (for solar values of R and β),
and it shows the enormous influence of random forcing in general.

---------------------------------------------------------
Title: Book reviews
Authors: Haubold, Hans Joachim; Snijders, M. A. J.; van Rensbergen,
W.; de Jager, Cornelis; van de Weygaert, R.; Achterberg, A.; Linssen,
P. F. J.; Hoyng, P.; Kleczek, J.; Bauer, Siegfried J.; Hayakawa, S.
1990SSRv...54..187H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Mechanisms for Dynamo Mode Excitation
Authors: Hoyng, P.
1990IAUS..142...45H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Book Review: Instabilities in space and laboratory plasmas. /
CUP, 1986.
Authors: Hoyng, P.
1990SSRv...54..192H Altcode: 1990SSRv...54..192M
No abstract at ADS

---------------------------------------------------------
Title: Excitation of Dynamo Modes
Authors: Hoyng, P.
1990IAUS..138..359H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Turbulent transport of magnetic fields. V. Distribution of
magnetic energy in a simple 2-dynamo
Authors: van Geffen, J. H. G. M.; Hoyng, P.
1990GApFD..53..109V Altcode:
In this paper we analyse the stationary mean energy density tensor
Tij = BiBj for the x2-sphere. This model is one of the simplest
possible turbulent dynamos, originally due to Krause and Steenbeck
(1967): a conducting sphere of radius R with homogeneous, isotropic and
stationary turbulent convection, no differential rotation and negligible
resistivity. The stationary solution of the (linear) equation for Tij
is found analytically. Only Trr, Tθθ and Tφ φ are unequal to zero,
and we present their dependence on the radial distance r. The stationary solution depends on two coefficients describing the
turbulent state: the diffusion coefficient β≈ &lt; u2 &gt; τ c/3
and the vorticity coefficient γ≈ &lt;|dtri ×u|2&gt;τc/3 where
u(r, t) is the turbulent velocity and c its correlation time. But
the solution is independent of the dynamo coefficient α≈-&lt;
u· dtri ×u&gt; τc/3 although α does occur in the equation for
Tij. This result confirms earlier conclusions that helicity is not
required for magnetic field generation. In the stationary state,
magnetic energy is generated by the vorticity and transported to the
boundary, where it escapes at the same rate. The solution presented
contains one free parameter that is connected with the distribution
of B over spatial scales at the boundary, about which Tij gives no
information. We regard this investigation as a first step towards the
analysis of more complicated, solar-type dynamos.

---------------------------------------------------------
Title: On the Sensitivity of Resonant Scattering Spectrometers for
Whole-Disk Solar Velocity Oscillation Measurements
Authors: Hoyng, P.
1989ApJ...345.1088H Altcode:
The sensitivity of resonant scattering spectrometers for measuring
solar velocity oscillations in integrated light is analyzed. It is
shown that, with the help of simple analytical tools and qualitative
arguments, most of the contributions to the instrumental sensitivity
can be estimated. Although the frequency dependence of the individual
sources of instrument noise cannot always be predicted reliably in
this way, it is possible to derive useful estimates of instrumental
parameters via comparison with a desired noise level.

---------------------------------------------------------
Title: Turbulent transport of magnetic fields. IV - Damping of the
mean field (B) in alpha-squared dynamos with alpha proportional to
COS Theta
Authors: van Geffen, J. H. G. M.; Hoyng, P.
1989A&A...213..429V Altcode:
It is proven that the dynamo equation for the mean field (B) of the
spherical alpha-squared dynamo with alpha proportional to cos Theta
and constant beta have only exponentially damped solutions, if the
mean energy density (B-squared) is nonzero and finite. The origin and
interpretation of this phenomenon is reviewed.

---------------------------------------------------------
Title: Book Review: The internal solar angular velocity. / Reidel,
1987
Authors: Hoyng, P.
1989SSRv...49..438H Altcode: 1989SSRv...49..438D
No abstract at ADS

---------------------------------------------------------
Title: Turbulent Transport of Magnetic Fields. III. Stochastic
Excitation of Global Magnetic Modes
Authors: Hoyng, P.
1988ApJ...332..857H Altcode:
Studies outlining how the role of fluctuation in the turbulent
convection can be analyzed with ensemble averages are reviewed and the
relation between the observed magnetic field and the ensemble average
magnetic field is solved by expanding the observed field in terms of
eigenfunctions of the dynamo equation. The effect of the fluctuations
is shown to take the form of stochastic excitation of all eigenmodes
of the dynamo equation, rather than steady excitation of only the
fundamental eigenmode, as in traditional dynamo models. The solar
dynamo is considered and the occurrence of coronal holes and of field
reversals in the earth's dynamo are discussed. A comparison is made
between this problem and the problem of scalar transport.

---------------------------------------------------------
Title: Book review
Authors: Hoyng, P.
1988SoPh..117..203H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Book-Review - Advances in - and Asteroseismology
Authors: Christensen-Dalsgaard, J.; Frandsen, S.; Hoyng, P.
1988SoPh..117..203C Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Turbulent transport of magnetic fields. I - A simple mechanical
model. II - The role of fluctuations in kinematic theory
Authors: Hoyng, P.
1987A&A...171..348H Altcode:
In the present paper, a simple mechanical model is analysed that
reproduces a few basic features of the average magnetic field in the Sun
and the Earth. It provides an intuitive picture of how the α-effect
operates in combination with differential rotation. The model serves
three purposes: (1) it enhances an intuitive understanding of the
behaviour of a magnetic field in turbulent fluids; (2) it shows that
kinematic models (and ultimately, non-linear models also) must allow for
fluctuations as one is essentially dealing with a stochastic process;
(3) it serves as a starting point for a re-analysis of the kinematic
dynamo problem.

---------------------------------------------------------
Title: Turbulent Transport of Magnetic Fields - Part Two - the Role
of Fluctuations in Kinematic Theory
Authors: Hoyng, P.
1987A&A...171..357H Altcode:
It is argued that due to fluctuations in the turbulent convection,
the ensemble averaged field &lt; B &gt; of a dynamo must damp
out to zero if &lt;BB&gt;, in particular &lt; B2 &gt;,
is to remain finite. This idea is supported by two case studies:
a simple α2 dynamo and the αΩ dynamo in the local
approximation. Apart from the well-known constants α and β, the
equation for &lt; BB &gt; contains the parameter γ ≡ &lt; ∣
V × u1∣2 &gt; τc/3, that also
occurred in the model analysed the preceding paper. These results lead
to a new statistical formulation of kinematic dynamo theory, which
must eventually be extended to nonlinear theory. New aspects are:
1) Turbulent transport of B is a stochastic process and fluctuations
have a large influence; 2) B in the dynamo fluctuates on all spatial
scales and bears only a statistical relation to &lt; B &gt;; 3)
The frequency stability δω/ω of αΩ dynamos and the average time
between spontaneous reversals in α2 dynamos are determined
by kinematic theory; for the solar dynamo δω/ω is estimated to
be larger than observed, but nonlinear effects may cure this; 4) The
average magnetic stress tensor (i. e. energy density, Lorentz force and
field cross-correlation coefficients) as well as γ are determinable
quantities (for the Sun γ ≍ 10-10 s-1); 5) In
principle, helical turbulence (α ≠ 0) is not necessary for dynamo
action; 6) The global resonances found by Stenflo and Vogel, and the
appearance of coronal holes may correspond to stochastic excitations of
overtones of the dynamo equation. The meaning of the ensemble average,
the energy balance of the dynamo and the physics of turbulent mixing
are discussed in detail. The limitations are: zero resistivity, and
homogeneous, isotropic turbulence with a short correlation time.

---------------------------------------------------------
Title: Generalized Compton-Getting transformation for nonrelativistic
particles.
Authors: Stevens, G. A.; Hoyng, P.
1986A&A...168..354S Altcode:
The problem of how to construct the particle distribution function in
a frame of reference that moves with respect to the frame in which
the observations were made is considered. The starting point of the
analysis is to expand the velocity distribution functions in both
frames of reference in spherical harmonics. A solution of this so-called
transformation problem is given in integral form, together with a method
that allows practical application. The (optimum) data acquisition and
data reduction method is discussed in detail. Six examples are given
that illustrate various aspects of the transformation problem. Finally,
the transformation method is tested and conditions for a reliable
construction of the three-dimensional distribution function in the
moving frame are investigated. The statistical aspects of the problem
are briefly discussed.

---------------------------------------------------------
Title: Book-Review - Localisation and Orientation in Biology and
Engineering
Authors: Ubbels, G. A.; Hovenier, J. W.; Russell, C. T.;
Nieuwenhuijzen, H.; Heintze, J. R. W.; Linssen, P. F. J.; van der
Kruit, P. C.; Thé, P. S.; Somogyi, Antal J.; Cuperman, S.; Rephaeli,
Y.; Stirpe, G. M.; Kleczek, J.; Kresák, L.; de Kool, M.; Wapstra,
A. H.; Swanenburg, B. N.; Hoyng, P.; Reijnen, G. C. M.; Somov,
B. V.; van der Hucht, K. A.; Sehnal, L.; Namba, O.; Schwartz, A. W.;
v. D. Stadt, H.; McNally, D.; de Hoop, D.
1986SSRv...44..393U Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Book-Review - Seismology of the Sun and the Distant Stars
Authors: Gough, D. O.; Hoyng, P.
1986SSRv...44..401G Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Observations of Steady Anomalous Magnetic Heating in Thin
Current Sheets
Authors: Martens, P. C. H.; van den Oord, G. H. J.; Hoyng, P.
1985SoPh...96..253M Altcode:
A faint steadily emitting loop-like structure has been observed
by HXIS in its low energy channels (3.5-8.0 keV) on November 5/6,
1980. These HXIS observations have permitted us to follow the thermal
evolution of this loop for a period of about 15 hr and from this study
we conclude that only a fraction of 0.1% of the volume of the loop is
steadily heated at the rather large rate of 0.6 erg cm-3
s-1. We interpret this heating as the dissipation of magnetic
fields in thin current sheets and we find that the dissipation with
classical resistivity is very unlikely, while ion-kinetic tearing, as
proposed by Galeev et al. (1981), suits the observations very well. The
enhancement of the resistivity over the classical resistivity then
turns out to be a factor 4 × 104. Dissipation in extremely
thin sheets via the ion-acoustic instability (Duijveman et al., 1981)
cannot be completely excluded when the cross-field heat conductivity
is anomalously enhanced by a factor 400.

---------------------------------------------------------
Title: Microwave and X-ray observations of delayed brightenings at
sites remote from the primary flare locations
Authors: Nakajima, H.; Dennis, B. R.; Hoyng, P.; Nelson, G.; Kosugi,
T.; Kai, K.
1985ApJ...288..806N Altcode:
Five examples of solar flares observed with the 17-GHz interferometer
at Nobeyama in which a secondary microwave burst occurred at a
distance of 100,000 km to 1,000,000 km from the primary flare site
are presented. The secondary microwave burst in all five cases had
a similar time profile to the primary burst with a delay of 2 to
25 s. The velocity of a triggering agent inferred from this delay
and spatial separation is 10,000 km to 100,000 km/s. The intensity
of the secondary burst was a factor of 3 to 25 smaller than that of
the primary burst in all events except for one case in which it was
a factor of 2 larger. The polarization degree of the secondary burst
at 17 GHz was 35 percent, significantly higher than the average value
for typical impulsive bursts. Two of the events were accompanied by
meterwave type III/V bursts located high in the corona between the
primary and secondary sites. For two of the other events, X-ray images
of the secondary source were obtained with the hard-X-ray imaging
spectrometer on the Solar Maximum Mission. These observations strongly
suggest that the distant microwave bursts were produced by electrons
with energies of 10 keV to 100 keV which were channeled along a huge
loop from the main flare site to the remote location.

---------------------------------------------------------
Title: Kinematic dynamo theory for an arbitrary mean flow.
Authors: Hoyng, P.
1984ESASP.220...97H Altcode: 1984ESPM....4...97H
The problem of an arbitrary, incompressible mean flow v0 in
kinematic dynamo theory is amenable to a systematic treatment leading
to an intuitively simple result. The author's approach is based on
the theory of stochastic differential equations.

---------------------------------------------------------
Title: Microwave and X-ray observations of delayed brightenings at
sites remote from the primary flare locations
Authors: Nakajima, H.; Dennis, B. R.; Hoyng, P.; Nelson, G.; Kosugi,
T.; Kai, K.
1984STIN...8433326N Altcode:
Five examples of solar flares observed with the 17-GHz interferometer
at Nobeyama in which a secondary microwave burst occurred at a
distance of 100,000 km to 1,000,000 km from the primary flare site
are presented. The secondary microwave burst in all five cases had
a similar time profile to the primary burst with a delay of 2 to 25
s. The velocity of a triggering agent inferred from this delay and
spatial separation is 10,000 km to 100,000 km/s. The intensity of the
secondary burst was a factor of 3 to 25 smaller than that of the primary
burst in all events except for one case in which it was a factor of 2
larger. The polarization degree of the secondary burst at 17 GHz was
35%, significantly higher than the average value for typical impulsive
bursts. Two of the events were accompanied by meterwave type III/V
bursts located high in the corona between the primary and secondary
sites. For two of the other events, X-ray images of he secondary source
were obtained with the hard-X-ray imaging spectrometer on the Solar
Maximum Mission. These observations strongly suggest that the distant
microwave bursts were produced by electrons with energies of 10 keV
to 100 keV which were channeled along a huge loop from the main flare
site to the remote location.

---------------------------------------------------------
Title: Microwave and X-ray Observations of Delayed Brightenings at
Sites Remote from the Primary Flare Locations
Authors: Nakajima, H.; Dennis, B. R.; Hoyng, P.; Nelson, G.; Kosugi,
T.; Kai, K.
1984BAAS...16..524N Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Report of ESA's topical team on solar and heliospheric physics.
Authors: Christensen-Dalsgaard, J.; Delache, P.; Hoyng, P.; Priest,
E. R.; Schwenn, R.; Stenflo, J. O.
1984ESASP1070...26C Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Hard X-Ray Studies of the Large Coronal Feature on June
29, 1980
Authors: Harrison, R. A.; Simnett, G. M.; Hoyng, P.; Lafleur, H.;
van Beek, H. F.
1984sii..conf..287H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Evidence for Extensive Magnetic Structures Between Two Active
Regions from Studies of Flares on June 24, 1980
Authors: Simnett, G. M.; Harrison, R. A.; Hoyng, P.; van Beek, H. F.
1984sii..conf..273S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Analysis of the 1980 November 18 limb flare observed by the
hard X-ray imaging spectrometer (HXIS)
Authors: Haug, E.; Elwert, G.; Hoyng, P.
1984AdSpR...4g.211H Altcode: 1984AdSpR...4..211H
X-ray images of the 18 November 1980 limb flare taken by the HXIS
instrument aboard SMM were analysed. The hard X-rays originated from
three spots on the SW limb of the solar disk with different altitudes
and time evolution. The locations of the brightest spots in hard and
soft X-rays are compared with the predictions of flare models. The
X-ray spctra from the pixels with highest count rates can be fitted by
power laws. The spatial variation of the spectral index is in agreement
with the existence of a non-thermal electron component.

---------------------------------------------------------
Title: Imaging of Impulsive Solar Flare Phenomena
Authors: Duijveman, A.; Hoyng, P.
1983SoPh...86..279D Altcode:
We review some recent advances in our understanding of impulsive solar
flare phenomena obtained through new hard X-ray and radio imaging
instruments (the Solar Maximum Mission and Hinotori satellites, the
VLA and VLBI).

---------------------------------------------------------
Title: Microwave and hard X-ray imaging of a solar flare on 1980
November 5
Authors: Hoyng, P.; Marsh, K. A.; Zirin, H.; Dennis, B. R.
1983ApJ...268..865H Altcode:
VLA and SMM hard X ray data on the solar flares of November 5, 1980 are
analyzed and compared with data from other sources. The VLA provided
measurements at 15 GHz at 10 sec intervals, using left and right
circular polarizations with a 0.6 arcsec resolution. The hard X ray
imaging spectrometer on the SMM obtained data in six bands from 3.5-30
keV, with 8 x 8 arcsec resolution and 1.5 sec separation. The data were
examined for a possible nonthermal source for the microwave component
of the emissions detected, the origin of 16-30 keV excess fluxes,
the relation between the X ray and microwave sources, the magnetic
connection between observed loops, and the physical characteristics
of the radiating loop. The data were consistent with a model that
assumes fast electrons are accelerated to a single power-law energy
distribution and freely stream along the magnetic field. The data also
agreed with a thick-target model for solar flare X ray emission.

---------------------------------------------------------
Title: The structure and evolution of a solar flare as observed in
3.5 30 keV X-rays
Authors: Harrison, R. A.; Simnett, G. M.; Hoyng, P.; Lafleur, H.;
van Beek, H. F.
1983SoPh...84..237H Altcode:
On July 5, 1980 the Hard X-Ray Imaging Spectrometer on board the Solar
Maximum Mission observed a complex flare event starting at 22 : 32 UT
from AR 2559 (Hale 16955), then at N 28 W 29, which developed finally
into a 2-ribbon flare. In this paper we compare the X-ray images with
Hα photographs taken at the Big Bear Solar Observatory and identify
the site of the most energetic flare phenomena. During the early
phases of the event the hard X-rays (&gt;16 keV) came from a compact
source located near one of the two bright Hα kernels; we believe the
latter are at the footpoints of a compact magnetic loop. The kernel
identified with the X-ray source is immediately adjacent to one of the
principal sunspots and in fact appears to `rotate' around the sunspot
over 90° in the early phase of the flare. Two intense X-ray bursts
occur at the site of the rotating kernel, and following each burst
the loop fills with hot, X-ray emitting plasma. If the first burst is
interpreted as bremsstrahlung from a beam of electrons impinging on a
collisionally dominated medium, the energy in such electrons, &gt;16
keV, is ∼ 5 × 1030 erg. The altitude of the looptop is
7-10 × 103 km. The temperature structure of the flare is
extremely non-homogeneous, and the highest temperatures are found in
the top of the loop.

---------------------------------------------------------
Title: X-Ray Imaging of Three Flares during the Impulsive Phase
Authors: Duijveman, A.; Hoyng, P.; Machado, M. E.
1982SoPh...81..137D Altcode:
The impulsive phases of three flares that occurred on April 10, May 21,
and November 5, 1980 are discussed. Observations were obtained with the
Hard X-ray Imaging Spectrometer (HXIS) and other instruments aboard
SMM, and have been supplemented with Hα data and magnetograms. The
flares show hard X-ray brightenings (16-30 keV) at widely separated
locations that spatially coincide with bright Hα patches. The bulk of
the soft X-ray emission (3.5-5.5 keV) originates from in between the
hard X-ray brightenings. The latter are located at different sides
of the neutral line and start to brighten simultaneously to within
the time resolution of HXIS. Concluded is that: The bright hard X-ray
patches coincide with the footpoints of loops.

---------------------------------------------------------
Title: Impulsive Acceleration and Heating in Flares
Authors: Hoyng, P.
1982Obs...102..119H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Observations of a POST Flare Radio Burst in X-Rays
Authors: Svestka, Z.; Hoyng, P.; van Tend, W.; Boelee, A.; de Jager,
C.; Stewart, R. T.; Acton, L. W.; Bruner, E. C.; Gabriel, A. H.;
Rapley, C. G.; de Jager, C.; LaFleur, H.; Nelson, G.; Simnett, G. M.;
van Beek, H. F.; Wagner, W. J.
1982SoPh...75..305S Altcode:
More than six hours after the two-ribbon flare of 21 May 1980, the
hard X-ray spectrometer aboard the SMM imaged an extensive arch above
the flare region which proved to be the lowest part of a stationary
post-flare noise storm recorded at the same time at Culgoora. The X-ray
arch extended over 3 or more arc minutes to a projected distance of
95 000 km, and its real altitude was most probably between 110 000
and 180 000 km. The mean electron density in the cloud was close to
109 cm−3 and its temperature stayed for many
hours at a fairly constant value of about 6.5 × 106 K. The
bent crystal spectrometer aboard the SMM confirms that the arch emission
was basically thermal. Variations in brightness and energy spectrum at
one of the supposed footpoints of the arch seem to correlate in time
with radio brightness suggesting that suprathermal particles from
the radio noise regions dumped in variable quantities into the low
corona and transition layer; these particles may have contributed to
the population of the arch, after being trapped and thermalized. The
arch extended along the H∥ = 0 line thus apparently
hindering any upward movement of the upper loops reconnected in the
flare process. There is evidence from Culgoora that this obstacle may
have been present above the flare since 15-30 min after its onset.

---------------------------------------------------------
Title: Origin and Location of the Hard X-Ray Emission in a Two-Ribbon
Flare
Authors: Hoyng, P.; Duijveman, A.; Machado, M. E.; Rust, D. M.;
Svestka, Z.; Boelee, A.; de Jager, C.; Frost, K. T.; Lafleur, H.;
Simnett, G. M.; van Beek, H. F.; Woodgate, B. E.
1981ApJ...246L.155H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Fast plasma heating by anomalous and inertial resistivity
effects in the solar atmosphere
Authors: Duijveman, A.; Hoyng, P.; Ionson, J. A.
1981ApJ...245..721D Altcode:
A simple model is presented to describe fast plasma heating by anomalous
and inertial resistivity effects. It is noted that a small fraction of
the plasma contains strong currents that run parallel to the magnetic
field and are driven by an exponentiating electric field. The anomalous
character of the current dissipation derives from the excitation of
electrostatic ion-cyclotron and/or ion-acoustic waves. The possible
role of resistivity deriving from geometrical effects ('inertial
resistivity') is also considered. Using a marginal stability analysis,
equations for the average electron and ion temperatures are derived
and numerically solved. No loss mechanisms are taken into account. The
evolution of the plasma is described as a path in the drift velocity
diagram, where the drift velocity is plotted as a function of the
electron to ion temperature ratio.

---------------------------------------------------------
Title: The Relationship Between the Microwave and Hard X-Ray Sources
in a Solar Flare
Authors: Marsh, K. A.; Zirin, H.; Hoyng, P.; Dennis, B. R.
1981BAAS...13..889M Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The limb flare of 1980 April 30 as seen by the hard X-ray
imaging spectrometer
Authors: van Beek, H. F.; de Jager, C.; Schadee, A.; Svestka, Z.;
Boelee, A.; Duijveman, A.; Galama, M.; Hoekstra, R.; Hoyng, P.; Fryer,
R.; Simnett, G. M.; Imhof, J. P.; LaFleur, H.; Maseland, H. V. A. M.;
Mels, W. M.; Schrijver, J.; van der Laan, J. J. M.; van Rens, P.; van
Tend, W.; Werkhoven, F.; Willmore, A. P.; Wilson, J. W. G.; Machado,
M. E.; Zandee, W.
1981ApJ...244L.157V Altcode:
X-ray imaging of the limb event of 1980 April 30 shows that the flaring
involved two distinct components: a pointlike component, which was the
source of the initial hard X-ray burst and an extensive tongue reaching
some 30,000 km above the limb. The tongue had a higher temperature
than the other parts of the structure and seemed to be enhanced by
energetic electrons that derived their energy from the initial source.

---------------------------------------------------------
Title: Hard X-ray imaging of two flares in active region 2372
Authors: Hoyng, P.; Duijveman, A.; Boelee, A.; de Jager, C.; Galama,
M.; Hoekstra, R.; Imhof, J.; Lafleur, H.; Machado, M. E.; Fryer, R.
1981ApJ...244L.153H Altcode:
Hard X-ray images of two flares observed by the Hard X-ray Imaging
Spectrometer (HXIS) aboard SMM on 1980 April 7 and 10 are discussed. A
comparison with H-alpha images and the photospheric magnetic field
maps shows that the emission originates in (arcades of) loops which
differ greatly in the hardness of the X-ray spectra. On April 7 the
hardest X-ray emission coincided with the brightest H-alpha patch. On
April 10 the most intense X-ray emission appeared to be concentrated
in a looplike structure with a softer spectrum at the top and a
harder spectrum in the legs. Temperature estimates from flux ratios
in different energy bands tend to confirm that small, hot components
are embedded in more extensive, cooler flaring regions.

---------------------------------------------------------
Title: Structural development of the X-ray limb flare of 30 April 1980
Authors: de Jager, C.; Hoyng, P.; Lafleur, H.; Schadee, A.; Svestka,
Z.; van Beek, H. F.; van Tend, W.; Fryer, R.; Simnett, G. M.
1981AdSpR...1m.251D Altcode: 1981AdSpR...1..251D
We describe the development of the limb flare of 30 April 1980, 20:20
UT, as observed by the Hard X-ray Imaging Spectrometer (HXIS) aboard
the Solar Maximum Mission (SMM). It consisted of a short-lived bright
nucleus (FWHM &lt; 10,000 km), just inside the Sun's limb; a longer
lasting tongue, extending to a height of ~ 30,000 km, and a more
complicated feature, approximately situated at the Sun's limb. The
tongue was a pre-existing magnetic structure that started emitting
X-rays only a few seconds after the bright nucleus, and which had a
slightly higher temperature than the nucleus; its X-ray emission may
be caused by electrons escaped from the nucleus.

---------------------------------------------------------
Title: Solar maximum mission experiment: Early results of the hard
X-ray imaging experiment
Authors: Boelee, A.; de Jager, C.; Duijveman, A.; Galama, M.; Hoekstra,
R.; Hoyng, P.; Imhof, J. P.; Lafleur, H.; Maseland, H. V. A. M.; Mels,
W. A.; Schadee, A.; Schrijver, J.; Svestka, Z.; van Beek, H. F.;
van Rens, P.; van der Laan, J. J. M.; van Tend, W.; Werkhoven, F.;
Wiersma, G.; Zandee, W.; Simnett, G. M.; Charlton, C. P.; Fryer, R.;
Willmore, A. P.; Wilson, J. W. G.; Machado, M. E.
1981AdSpR...1m.255B Altcode: 1981AdSpR...1R.255B
We have selected four widely different flares from the early
period of operations of the Hard X-Ray Imaging Spectrometer (HXIS)
on SMM to illustrate the characteristic imaging properties of this
experiment. For the small flare of April 4, 1980, we demonstrate the
instrument's capability for locating a compact source. In the weak,
but extensive, flare of April 6 we show how well the instrument can
display spatial structure, and also the low level of the instrument
background. In the 1B flare of April 7 we are able to locate positions
of the X-ray emission in the soft and hard channels, and estimate
the positional variations of the emission patches. Finally, in the IN
flare of April 10, which produced the strongest hard X-ray burst we
have seen so far, we repeat some of the studies made for the April 7
event, and also demonstrate the capability of the HXIS instrument to
study the development, with high time resolution, of individual 8”
× 8” elements of the flare.

---------------------------------------------------------
Title: Solar Maximum Mission experiment - Early results of the hard
X-ray imaging experiment
Authors: Simnett, G. M.; Charlton, C. P.; Fryer, R.; Boelee, A.;
de Jager, C.; Duijveman, A.; Galama, M.; Hoekstra, R.; Hoyng, P.;
Imhof, J. P.
1981AdSpR...1m.255S Altcode: 1981AdSpR...1..255S
Four widely different flares from the early period of operations
of the Hard X-ray Imaging Spectrometer (HXIS) on SMM have been
selected to illustrate the characteristic imaging properties of this
experiment. For the small flare of April 4, 1980, the instrument's
capability for locating a compact source is demonstrated. In the weak,
but extensive, flare of April 6, the ability of the instrument to
display spatial structure, and also the low level of the instrument
background, are shown. In the 1B flare of April 7, positions of the
X-ray emission in the soft and hard channels are capable of being
located, and the positional variations of the emission patches can
be estimated. Finally, in the 1N flare of April 10, which produced
the strongest hard X-ray burst seen so far, some of the studies made
for the April 7 event are repeated, and the capability of the HXIS
instrument to study the development, with high time resolution, of
individual 8 x 8 arcsec elements of the flare is also demonstrated.

---------------------------------------------------------
Title: Structural development of the X-ray limb flare of 30 April
1980.
Authors: de Jager, C.; Fryer, R.; Hoyng, P.; Lafleur, H.; Schadee,
A.; Simnett, G. M.; Svestka, Z.; van Beek, H. F.; van Tend, W.
1981hea..conf..251D Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Solar Maximum Mission experiment: early results of the hard
X-ray imaging experiment.
Authors: Simnett, G. M.; Boelee, A.; Charlton, C. P.; de Jager, C.;
Duijveman, A.; Fryer, R.; Galama, M.; Hoekstra, R.; Hoyng, P.; Imhof,
J. P.; Lafleur, H.; Machado, M. E.; Maseland, H. V. A. M.; Mels,
W. A.; Schadee, A.; Schrijver, J.; Svestka, Z.; van Beek, H. F.;
van Rens, P.; van der Laan, J. J. M.; van Tend, W.; Werkhoven, F.;
Wiersma, G.; Willmore, A. P.; Wilson, J. W. G.; Zandee, W.
1981hea..conf..255S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Strong Langmuir wave turbulence - Some results with
selfconsistent Landau damping
Authors: van Grunsven, T. F. J.; Hoyng, P.; Nicholson, D. R.
1980A&A....91....7V Altcode:
An initial value problem of the high-frequency Langmuir wave electric
field in two spatial dimensions expressed by the split-timestep Fourier
method is solved for the case when two equally strong wave vector pumps
are initialized in the k-space. One case corresponds to weak Langmuir
turbulence indicating that time evolution agrees with weak turbulence
concepts of Landau damping and three-wave decay interactions. In the
case of strong Langmuir turbulence, the soliton collapse to the smallest
length scales is prevented by Landau damping. It is concluded that the
shape of electron velocity distribution is approximately constant up to
a specific maximum velocity, but rapidly decreases at larger velocities.

---------------------------------------------------------
Title: Impulsive electron acceleration to energies of tens of kT/e/
by Langmuir wave turbulence
Authors: Hoyng, P.; Duijveman, A.; van Grunsven, T. F. J.; Nicholson,
D. R.
1980A&A....91...17H Altcode:
An expression which describes the evolution of the tail of electron
distribution determined by the spatial inhomogeneity of the Langmuir
wave and escape of fast electrons from the acceleration region is
applied to specific situations. One case analyzed a many-current-layer
model with properties for accelerating electron streams which generate
type III solar radio bursts; the model shows that it can emit an
electron flux of 10 to the 32nd/s, with a switch-on time of 0.05 s,
and a maximum acceleration energy of 50 keV. Another problem involves
a single current layer (shock wave) accelerating electrons to energies
of 10 kT(e); the electrons can be additionally accelerated by a second
stage process. The first (fast stage) electron acceleration to 100
to 1000 kT(e) can only result from high frequency electrostatic wave
turbulence if the shock has a complex structure.

---------------------------------------------------------
Title: Recent Results from the Solar Maximum Mission: Observational,
Interpretational and Theoretical Aspects
Authors: Hoyng, P.
1980BAAS...12..903H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Multifarious Spatial Structure in a Compact Hard X-Ray Flare
Authors: Harrison, R. A.; Vanbeek, H. F.; Hoyng, P.; Lafleur, H.;
Simnett, G. M.
1980BAAS...12..911H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: A Study of the Impulsive Flares and Homologous Flares From
AR2372 From April 6-13, Using Hard X-Ray Images
Authors: Simnett, G. M.; Machado, M.; Harrison, R. A.; Hoyng, P.;
Lafleur, H.; Svestka, Z.; Vanbeek, H. F.
1980BAAS...12S.899S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Hard X-Ray Images of the Continuously Active Region 2478 from
June 3-6, 1980
Authors: Fryer, R. J.; Hoyng, P.; Lafleur, H.; Simnett, G. M.; van
Beek, H. V.
1980BAAS...12..892F Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Brief Report of Meeting - IAU / Solar Maximum Year
Authors: Švestka, Zdenk; Van Hoven, Gerard; Hoyng, Peter; Kuperus, Max
1980SoPh...67..379S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The hard X-ray imaging spectrometer (HXIS).
Authors: van Beek, H. F.; Hoyng, P.; Lafleur, B.; Simnett, G. M.
1980SoPh...65...39V Altcode:
The HXIS, a joint instrument of the Space Research Laboratory at
Utrecht, The Netherlands, and the Department of Space Research of
the University of Birmingham, U.K., images the Sun in hard X-rays: Six
energy bands in energy range 3.5-30 keV, spatial resolution 8″ over Ø
2'40″ and 32″ over Ø 6'24″ field of view, and time resolution of
0.5-7 s depending on the mode of operation. By means of a `flare flag'
it alerts all the other SMM instruments when a flare sets in and informs
them about the location of the X-ray emission. The experiment should
yield information about the position, extension and spectrum of the hard
X-ray bursts in flares, their relation to the magnetic field structure
and to the quasi-thermal soft X-rays, and about the characteristics
and development of `type IV' electron clouds above flare regions.

---------------------------------------------------------
Title: A Model for Impulsive Electron Acceleration to Energies of
Tens of Kt/e
Authors: Hoyng, P.; Duijveman, A.; van Grunsven, T. F. J.; Nicholson,
D. R.
1980IAUS...91..299H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Energetic particles in solar flares
Authors: Ramaty, R.; Paizis, C.; Colgate, S. A.; Dulk, G. A.; Hoyng,
P.; Knight, J. W.; Lin, R. P.; Melrose, D. B.; Orrall, F.; Shapiro,
P. R.
1980sfsl.work..117R Altcode: 1980sofl.symp..117R
The various manifestations of energetic particles in solar
flares are examined, and possible mechanisms for the acceleration
of these particles are considered. Hard X-ray observations and
possible mechanisms for the production of the dominant form of solar
energetic particles, electrons with energies between 10 and 100 keV,
are discussed, with consideration of thin-target models, thick-target
models and thick-target models with reverse currents, and first-phase
acceleration mechanisms for energetic electrons emitting impulsive
microwave and fast-drift Type III radio bursts as well as impulsive hard
X rays, which are detected themselves 20 min after the flare at 1 AU
are considered. Radio evidence on the number, energy and pitch-angle
distributions of energetic particles produced during solar flares
is summarized, and observations at 1 AU of proton and electron energy
spectra, the proton/electron ratio and energetic particle events rich in
He-3 from solar flares are discussed. Finally, consideration is given to
gamma-ray evidence of nuclear reactions in flares and white-light flares

---------------------------------------------------------
Title: Relaxation and bremsstrahlung of thick-target electron streams:
a simple application of the Legendre expansion method.
Authors: Hoyng, P.; Melrose, D. B.; Adams, J. C.
1979ApJ...230..950H Altcode:
The Legendre series expansion of the three-dimensional equations for
electron velocity and Langmuir wave distributions provides a numerical
treatment for the relaxation of a stationary, axisymmetric electron
stream in a homogeneous and fully ionized hydrogen plasma. Both
quasilinear and Coulomb interactions figure in the treatment. The
electrons are represented by a Fokker-Planck system of equations,
which allows elimination of the distinction between background and
stream electrons. A computation of the bremsstrahlung associated with
the isotropic component of the electron distribution yields a simple
model for radiation from thick-target electron streams, for example
in impulsive hard X-ray emission from solar flares. Results of the
study suggest power law-like bremsstrahlung spectra as they are often
observed can be produced quite readily.

---------------------------------------------------------
Title: Practical use of the Legendre-expanded quasi-linear equations
for anisotropic particles and Langmuir waves.
Authors: Hoyng, P.; Stevens, G. A.; Melrose, D. B.
1979ApJ...230..961H Altcode:
The Legendre expansion for the three-dimensional quasilinear
equations for axisymmetric streams of particles and Langmuir waves
is discussed. In particular, it is shown that a well-posed numerical
problem based on the Legendre expansion can be defined for conditions
equivalent to a decomposition into an arbitrary number of mutually
coinciding one-dimensional streams of particles and waves. The
difficulty of negative distribution functions occurring in the Legendre
expansion has also proven tractable. The Legendre expansion method
is applicable to the study of the suprathermal streams of electrons
which occur frequently in astrophysical plasmas.

---------------------------------------------------------
Title: Energetic particles in solar flares. Chapter 4 in the
proceedings of the 2nd SKYLAB Workshop on Solar Flares
Authors: Ramaty, R.; Colgate, S. A.; Dulk, G. A.; Hoyng, P.; Knight,
J. W., III; Lin, R. P.; Melrose, D. B.; Paizis, C.; Orrall, F.;
Shapiro, P. R.
1978epsf.proc.....R Altcode:
The recent direct observational evidence for the acceleration of
particles in solar flares, i.e. radio emission, bremsstrahlung
X-ray emission, gamma-ray line and continuum emission, as well as
direct observations of energetic electrons and ions, are discussed
and intercorrelated. At least two distinct phases of acceleration of
solar particles exist that can be distinguished in terms of temporal
behavior, type and energy of particles accelerated and the acceleration
mechanism. Bulk energization seems the likely acceleration mechanism
for the first phase while Fermi mechanism is a viable candidate for
the second one.

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Title: Nonlinear Langmuir waves during type III solar radio bursts.
Authors: Nicholson, D. R.; Goldman, M. V.; Hoyng, P.; Weatherall, J. C.
1978ApJ...223..605N Altcode:
Type III solar radio bursts are thought to be associated with
intense levels of electron-beam-excited Langmuir waves. The nonlinear
evolution of these waves, in time and in two spatial dimensions, due to
their coupling to other waves is studied numerically. For parameters
appropriate to one-half the earth-sun distance, nonlinear effects are
found to be important, as in previous one-dimensional work. However, a
new and important phenomenon, two-dimensional soliton collapse, is found
to occur. This collapse, induced directly by the wave-packet nature of
the beam-excited waves, produces two-dimensional wave spectra extending
over a much broader range of wave numbers than has been predicted by
inhomogeneous quasi-linear theory. The results compare favorably with
certain aspects of recent observations. The background magnetic field
is neglected; while substantially justified for the present parameters,
this neglect may require reexamination at locations closer to the sun.

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Title: Diagnostics of solar flare hard X-ray sources.
Authors: Hoyng, P.; Knight, J. W.; Spicer, D. S.
1978SoPh...58..139H Altcode:
The dynamics of hard X-ray producing electron beams in solar flares
can be strongly affected by the occurrence of a reverse current. The
parameter diagram for a beam can be divided into three regimes, one
of which is the usual thick target case, the two others being due to
two different possible consequences of the reverse current. The use
of this parameter diagram as a possible diagnostic tool for solar
flare hard X-ray sources is discussed, together with the necessary
observations and their interpretation.

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Title: Legendre expansion of the quasi-linear equations for
anisotropic particles and Langmuir waves.
Authors: Hoyng, P.; Melrose, D. B.
1977ApJ...218..866H Altcode:
The quasi-linear diffusion and friction coefficients for axisymmetric
electron distributions interacting with Langmuir waves are evaluated
explicitly by expanding the distribution of waves in Legendre
polynomials. The quasi-linear equations are then reduced to a
form in which both the distributions of waves and of particles are
simultaneously expanded in Legendre polynomials, and all coefficients
are evaluated explicitly. It is argued that such expansions are likely
to be justified in practice and that the results obtained should prove
useful in discussing quasi-linear relaxation under various conditions in
three dimensions rather than one dimension. New results are anticipated
for the problem of the propagation of electron streams causing type
III solar radio bursts. The influence of the magnetic field on the
Langmuir waves is neglected.

---------------------------------------------------------
Title: Book reviews
Authors: Reijnen, G. C. M.; Ness, Norman F.; Kliore, Arvydas J.;
Sonnenschein, F. J.; Hoogenboom, A. M.; Hack, Margherita; Johnson,
F. S.; Reijnen, G. C. M.; Kleczek, J.; van Bueren, H. G.; de Graaff,
W.; Hoyng, P.; Swider, W.; Grevesse, N.
1977SSRv...20..677R Altcode:
No abstract at ADS

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Title: A two-component model of impulsive microwave burst emission
consistent with soft and hard X rays.
Authors: Boehme, A.; Fuerstenberg, F.; Hildebrandt, J.; Saal, O.;
Krueger, A.; Hoyng, P.; Stevens, G. A.
1977SoPh...53..139B Altcode:
A two-component (core-halo) emission model has been applied
reconciling hard and soft X-ray burst emissions with the microwave
burst radiation. The core region is represented by a nonthermal
energy distribution (Maxwellian+power law tail) and assumed to be
surrounded by a thermal halo. Parameters characterizing the energy
distribution and emission measures have been derived numerically from
soft and hard X-ray measurements. Using an artificial magnetic field
model the microwave flux spectrum has been calculated on the basis of
gyro-synchrotron emission and absorption by solving the equation of
radiation transfer along the ray trajectories. Open parameters were
used to adapt the spectrum to the radio measurements.

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Title: Radiation from a source in a cold magnetoactive plasma,
re-examined. Application to cyclotron and multipole radiation
Authors: Hoyng, P.; Stevens, G. A.
1977PhFl...20..520H Altcode:
An analysis is presented of the radiation emitted by an arbitrary
source embedded in a cold, magnetoactive plasma and physically
distinct from the latter. The plasma is supposed to be infinite and
homogeneous; its dielectric properties are described by a dielectric
tensor ɛ. Expressions for the radiation fields are derived using
the technique of Fourier decomposition. An expression for the vector
potential is constructed and elaborated as far as possible for an
arbitrary current source. The approach differs from that in previous
work on technical points, the main one being the sequence in which the
various integrations are carried out. The radiation flux is defined on
the basis of Poynting's vector S; a distinction is made between current
sources behaving as a given function of time and randomly fluctuating
sources. In the latter case an ensemble average is preferred over
a time average. A comparison is made with existing treatments in the
literature, and a variety of defects is pointed out. The general result
for the radiation flux is then specified for cyclotron radiation from a
stationary ensemble of electrons and for multipole radiation. Throughout
the paper a compact notation is used based on the work of Bremmer.

---------------------------------------------------------
Title: Book reviews
Authors: Reijnen, G. C. M.; Kleczek, J.; Millman, Peter M.; Vesseur,
H. J. A.; Bar-Nun, Akiva; de Jager, C.; van Albada, T. S.; Rawer,
K.; Hinze, J. O.; Trümper, J.; de Jager, Cornelis; Müller, O.;
Kovalevsky, J.; Hammerschlag, R. H.; Hoyng, Peter
1977SSRv...20..235R Altcode:
No abstract at ADS

---------------------------------------------------------
Title: On the Nature of Impulsive Electron Acceleration in Solar
Hard X-ray Flares. II. A Theory
Authors: Hoyng, P.
1977A&A....55...31H Altcode:
Summary. The suggestion is elaborated that shock wave generated
Langmuir waves accelerate electrons in the adjoining plasma. Langmuir
wave generation can be achieved in ion- acoustic unstable shocks by
induced bremsstrahlung from electrons. A crude model analysis shows the
Langmuir waves to have short wavelengths, k kD/4, while propagating
almost parallel to the shock plane. It is possible that sufficient
power in Langmuir waves is generated to explain the observed scale of
electron acceleration. The evolution of the Langmuir wave distribution
emerging from the shock is analyzed including the effects of spatial
gradients. The initial stage is dominated by nonlinear Landau damping,
which isotropizes the waves and increases their wavelengths, leading
eventually to the onset of the modulational instability. Virtually
all wave energy is now converted into fast electrons by Landau
damping. Depending on parameters, these processes are completed within
a distance of the order of 100 m from the shock. Next follows an
extended nonthermal region in which fast electrons (+ selfconsistent
level of Langmuir waves) slowly relax to thermal equilibrium. The
length of this region is t0 -t0 km and depends among other factors,
on the amount of fast electron scattering. The resulting fast electron
velocity distribution is not discussed. However, there is hope for
a universal distribution because strong wave-wave interactions take
place before acceleration, while redistribution of energy between fast
electrons occurs afterwards. Key words: solar flare hard X-rays shock
wave stochastic acceleration parametric instability

---------------------------------------------------------
Title: On the nature of impulsive electron acceleration in solar
hard X-ray flares. I. Inferences from observations.
Authors: Hoyng, P.
1977A&A....55...23H Altcode:
The nature of the impulsive electron acceleration regularly observed
in solar flares is investigated on the basis of relevant observations,
primarily in hard X-rays. Essential observational data are reviewed
for flares that most clearly exhibit electron acceleration, the
bremsstrahlung interpretation is adopted, and acceleration by a dc
field or electron runaway is ruled out. Acceleration by collective
wave-particle interactions is considered, with attention focused on
Langmuir waves. It is proposed that Langmuir waves are most likely
generated in the slow shock waves of Petschek's (1964) model of
magnetic-field-line merging, after which they propagate outward and
accelerate electrons in a relatively large volume. The generation
mechanism of such waves in ion-acoustic turbulent shocks is then
analyzed, and the question of electron acceleration is examined in
some detail. The evolution of the Langmuir-wave distribution resulting
in electron acceleration is computed along with the relaxation of the
fast electrons back to thermal equilibrium.

---------------------------------------------------------
Title: High time resolution analysis of solar hard X-ray flares
observed on board the ESRO TD-1A satellite.
Authors: Hoyng, Peter; Brown, John C.; van Beek, H. Frank
1976SoPh...48..197H Altcode:
The Utrecht solar hard X-ray spectrometer S-100 on board the ESRO TD-1A
satellite covers the energy range above 25 keV with 12 logarithmically
spaced channels. Continuous sun-pointing is combined with high time
resolution: 1.2 s for the four low energy channels (25-90 keV) and
4.8 s for the others. It is emphasized that the instrument design and
calibration yield data virtually free of pile-up and other instrumental
defects.

---------------------------------------------------------
Title: An Error Analysis of Power Spectra
Authors: Hoyng, P.
1976A&A....47..449H Altcode:
Summary. An expression is derived for the uncertainty in the Fourier
power spectrum due to noise present in the data. We consider both
series with Poisson-distributed noise as well as with normal-distributed
noise. The expressions for the errors involve only the power spectrum
itself and they can be readily generalized to arbitrarily distributed
noise. A working example is discussed. Key words: error analysis -
power spectra - Fourier transform

---------------------------------------------------------
Title: Betatron acceleration in a large solar hard X-ray burst.
Authors: Brown, J. C.; Hoyng, P.
1975ApJ...200..734B Altcode:
The problem of diagnosing flare particle acceleration mechanisms from
hard X-ray bursts is discussed, and it is argued that the electron
trap model of bursts is more amenable to observational investigation
at present than models of thick-target type. It is then shown that
data for the large X-ray burst of 1972 August 4 are consistent with
the source electrons being trapped in a very large vibrating coronal
magnetic bottle. Furthermore, the observations show that the burst time
profile is not dominated by collisional losses. It is proposed instead
that the entire profile is essentially determined by betatron action
of the varying trap field on the electrons. This betatron model is
then analyzed in detail and shown to predict very well the observed
correlation of electron flux and spectral index in this event when
it is supposed that the electrons are initially produced by runaway
in a direct electric field. Comparison of the model with observations
permits inference of the approximate form of magnetic field evolution
in the trap. Finally the physics behind this field evolution is briefly
considered. Subject headings: flares, solar - X-rays, solar

---------------------------------------------------------
Title: Hard X-rays from the sun.
Authors: van Beek, H. F.; Hoyng, P.; de Jager, C.; Stevens, G. A.
1975NTNA...41..101V Altcode:
No abstract at ADS

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Title: High Time Resolution Analysis of Solar Flares Observed on
the ESRO Td-Ia Satellite
Authors: Hoyng, P.; Brown, J. C.; Stevens, G.; van Beek, H. F.
1975IAUS...68..233H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Studies on hard X-ray emission from solar flares and on
cyclotron radiation from a cold magnetoplasma
Authors: Hoyng, P.
1975PhDT.........1H Altcode:
This thesis proposes an interpretation of hard X-ray emission from solar
flares and presents a theoretical study on the generation of cyclotron
radiation by a source in a cold magnetically active plasma. Hard
X-ray observations by the ESRO TD-1A satellite are analyzed, the
solar flares of May 18 and August 4, 1972, are examined in detail,
and a set of consistent parameters are derived for the flare of
May 18 from observations of hard-X rays and simultaneously emitted
centimeter radio waves. A model for the flare of August 4 is suggested
in which a coronal magnetic trap is filled with fast electrons by
some acceleration process, trap eigenmode oscillations are excited,
and expansion occurs. It is proposed that electrons in ordinary solar
flares are accelerated by resonant interactions with Langmuir waves
generated in thin current sheets and propagating laterally from the
sheets. In the cyclotron study, the radiation flux is defined on the
basis of Poynting's vector, and an expression for the harmonic frequency
is found which differs nontrivially from the one commonly used.

---------------------------------------------------------
Title: High time resolution analysis of solar flares observed on
the ESRO TD-1A satellite.
Authors: Hoyng, P.; Brown, J. C.; Stevens, G.; van Beek, H. F.
1975IAUS...68Q.233H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Studies on hard X-ray emission from solar flares and on
cyclotron radiation from a cold magnetoplasma.
Authors: Hoyng, P.
1975UtrOv.311.....H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: On the Determination of the Photospheric Velocity Distribution
from Profiles of Weak Fraunhofer Lines
Authors: Rutten, R. J.; Hoyng, P.; de Jager, C.
1974SoPh...38..321R Altcode:
We derive the conditions under which the profile of a weak Fraunhofer
line can be described as the convolution of the separate profiles
of damping, thermal and non-thermal motions at the average depth of
formation of the line. The average velocity distribution along the line
of sight, rather than its customary chosen macro- and micro-turbulent
components, is then found from the deconvolution of the observed profile
with the known other contributions. Reversely, the observed profiles
can be compared to predicted profiles on the basis of De Jager's (1974)
theoretical turbulence broadening curves.

---------------------------------------------------------
Title: On the Formation and Unfolding of Pulse Height Distributions
Authors: Hoyng, P.; Stevens, G. A.
1974Ap&SS..27..307H Altcode:
Spacecraft measurements of X-ray or particle pulse height distributions
have become increasingly accurate during the last fifteen years,
and they will continue to do so. The present paper deals with the
question how one can reconstruct original photon or particle spectra
from measured pulse height distributions. The statistical aspects
of the formation of pulse height distributions are investigated. A
method is presented that allows for a reliable reconstruction of the
original spectrum. Its essentials are the formulation and subsequent
solution of a matrix equation connecting pulse height distribution
with photon/particle spectrum; an error analysis of the reconstructed
spectrum is given. The present method has two advantages over the
usualχ 2-minimum method: It is able to recover more
spectral detail and it requires less computing time. Finally, a
numerical example is given.

---------------------------------------------------------
Title: Interpretation of a Hard Solar X-Ray Burst
Authors: Hoyng, P.; Stevens, G. A.
1973sari.conf...97H Altcode:
No abstract at ADS