3,705 research outputs found
Cosmic ray modulation with a Fisk-type heliospheric magnetic field and a latitude-dependent solar wind speed
No full textThe effect of a latitude-dependent solar wind speed on a Fisk heliospheric magnetic field [Fisk, L. A. Motion of the footpoints of heliospheric magnetic field lines at the Sun: implications for recurrent energetic particle events at high heliographic latitudes. J. Geophys. Res. 101, 15547–15553, 1996] was first discussed by Schwadron and Schwadron and McComas [Schwadron, N.A. An explanation for strongly underwound magnetic field in co-rotating rarefaction regions and its relationship to footpoint motion on the the sun. Geophys. Res. Lett. 29, 1–8, 2002. and Schwadron, N.A., McComas, D.J. Heliospheric “FALTS”: favored acceleration locations at the termination shock. Geophys. Res. Lett. 30, 41–1, 2003]. Burger and Sello [Burger, R.A., Sello, P.C. The effect on cosmic ray modulation of a Parker field modified by a latitudinal-dependent solar wind speed. Adv. Space Res. 35, 643–646, 2005] found a significant effect for a simplified 2D version of a latitude-dependent Fisk-type field while Miyake and Yanagita [Miyake, S., Yanagita, S. The effect of a modified Parker field on the modulation of the galactic cosmic rays. In: Proceedings of 30th International Cosmic Ray Conference. Merida, Mexico, vol. 1, 445–448, 2007] found a smaller effect. The current report improves on a previous attempt Hitge and Burger [Hitge, M., Burger, R.A. The effect of a latitude-dependent solar wind speed on cosmic-ray modulation in a Fisk-type heliospheric magnetic field. In: Proceedings of 30th International Cosmic Ray Conference. Merida, Mexico, vol. 1, pp. 449–450, 2007] where the global change in the solar wind speed and not the local speed gradient was emphasized. The sheared Fisk field of Schwadron and McComas [Schwadron, N.A., McComas, D.J. Heliospheric “FALTS”: Favored acceleration locations at the termination shock. Geophys. Res. Lett. 30, 41–1, 2003.) is similar to the current Schwadron–Parker hybrid field. Little difference is found between the effects of a Parker field and a Schwadron–Parker hybrid field on cosmic-ray modulation, in contrast to the results of Burger and Sello and Miyake and Yanagita [Burger, R.A., Sello, P.C. The effect on cosmic ray modulation of a Parker field modified by a latitudinal-dependent solar wind speed. Adv. Space Res. 35, 643–646, 2005 and Miyake, S., Yanagita, S. The effect of a modified Parker field on the modulation of the galactic cosmic rays. In: Proceedings of 30th International Cosmic Ray Conference. Merida, Mexico, vol. 1, pp. 445–448, 2007]. The two-dimensional approximation used by these authors is therefore inadequate to model the complexities of the actual three-dimensional field. We also show that a Fisk-type field with a latitude-dependent solar wind speed (Schwadron–Parker hybrid field) decreases both the relative amplitude of recurrent cosmic ray intensity variations and latitude gradients and yields similar constants of proportionality for these quantities as for the constant solar wind speed case
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No full textHere is a curious book. Its title-page declares "The Artist's Book of Fables" but its pre-title-page has "Fables, Original and Selected, with a Memoir of the Author." After that title-page, it is identical with "Fables, Original and Selected" as in our copy printed by John Murray in 1833. There is again an AI at the front and an index of engravings and engravers at the back. I found that copy twenty years ago. I had found an inferior copy twenty-two years before that. At that time, I noted Aesopic fables here including "Stone Broth" and "The Mouse and the Oyster."This is a hardbound book (hard cover)James Northcote, R.A
Modeling drift along the heliospheric wavy neutral sheet
No full textDrift along the wavy heliospheric neutral sheet is believed to play an important role in cosmic-ray modulation and can explain the peaked versus flat intensity profiles during consecutive solar magnetic epochs. Modulation models are becoming more and more realistic and in order to determine the role of the wavy neutral sheet more accurately, we revisit a previous calculation for drift along it. While mathematically correct, we argue that the previous expression for neutral sheet drift, which follows naturally from the standard expression for gradient and curvature drift, must be adapted in order for the drift speed to be less than particle speed. We compare the effect of both the previous and the current more accurate version of neutral sheet drift on cosmic-ray modulation with results obtained by other methods
An investigation into a generalized Fisk-type heliospheric magnetic field
No full textMSc (Space Physics), North-West University, Potchefstroom Campus, 2017The existence of a Fisk-type heliospheric magnetic field (HMF) has been contested since the
Fisk field [Fisk, 1996] challenged the traditional view of the HMF first proposed by Parker
[1958]. Several modifications of the original Fisk field model have been published in the past
[Schwadron, 2002], of which the Fisk-Parker hybrid HMF model of Burger and Hitge [2004] is
one. A detailed derivation and the uniqueness of the Fisk-Parker hybrid magnetic field expressions
are shown in this study for the first time. This study also presents a divergence-free
generalised Fisk HMF model to further test the existence of a Fisk-type field. The generalised
Fisk field model implements a spatially dependent differential rotation rate of the photosphere
by using newly developed transformations to map a magnetic field line from the solar wind
source surface to the photosphere. The footpoint trajectories on the source surface as well the
magnetic field line configurations are shown. The data analyses methods of Forsyth et al. [2002]
are used to search for a signature of a generalised Fisk field model during solar minimum conditions
using the magnetic field data from the first solar orbit of Ulysses. The generalised Fisk
field agrees better with the observed magnetic field during all intervals scanned by Ulysses except
during one interval. The 26-day recurrent cosmic-ray variations from data collected from
the global neutron monitor network are investigated and agree well with the results of Richardson
et al. [1999] and Paizis et al. [1999]. The results of this study provides further support for the
existence of a Fisk-type HMF.
Die bestaan van ’n Fisk-tipe heliosferiese magneetveld (HMV) is betwis sedert the Fisk veld
[Fisk, 1996] die tradisionele uitgangspunt van die eerste HMV uitgedaag het wat voorgestel
is deur Parker [1958]. Verskeie veranderinge van die oorspronklike Fisk veld is gepubliseer
in die verlede [Schwadron, 2002], waarvan die Fisk-Parker hibried HMV van Burger and Hitge
[2004] een is. ’n Gedetailleerde afleiding en uniekheid van die Fisk-Parker hibried magneetveld
vergelykings word vir die eerste keer voorgestel in hierdie studie. Hierdie studie stel ook
die divergensie-vrye veralgemeende Fisk HMV model bekend om die bestaan van ’n Fisk-tipe
veld verder te toets. Die veralgemeende Fisk model implementeer ’n ruimtelike afhanklike
differensi¨ele rotasie tempo van die fotosfeer deur gebruik te maak van nuwe transformasies
om magneetveldlyne van die sonwindbronoppervlak na die fotosfeer te spoor. Die voetpuntbane
op die bronoppervlak sowel as die magneetveldlynopset word gewys. Die data-ontleding
metodes van Forsyth et al. [2002] word gebruik om te soek vir ’n handtekening van die veralgemeende Fisk veld gedurende son-minimum toestande deur gebruik te maak van die magneetveld data van die eerste sonomwenteling van Ulysses. Die veralgemeende Fisk veld stem beter ooreen met die geobserweerde magneetveld gedurende al die intervalle wat geskandeer is deur Ulysses behalwe vir een interval. Die 26-dag herhalende kosmiesestraalvariasies van die data versamel deur die globale neutron monitor netwerk is ondersoek en stem baie goed ooreen met die resultate van Richardson et al. [1999] en Paizis et al. [1999] Die resultate van hierdie studie gee verdere ondersteuning vir die bestaan van ’n Fisk-tipe HMV.Master
A Fisk‐Parker Hybrid Heliospheric Magnetic Field with a Solar‐Cycle Dependence
No full textWe present a refinement of the Fisk-Parker hybrid field of Burger and Hitge which now includes a region bordering the solar rotational equator where magnetic field footpoint motion occurs only through diffusive reconnection. The hybrid field, therefore, only occurs above a certain latitude in a given hemisphere, and in the equatorial region the field is a pure Parker field. We also propose a simple qualitative model for the solar cycle dependence of the hybrid field, taking into account changes in the tilt angle of the heliospheric current sheet and the latitudinal extend of the polar coronal hole on the photosphere and on the source surface over the course of a solar activity cycle. We find that the amplitude of magnetic field fluctuations for assumed solar minimum parameters would not be observable above the background noise (see Roberts and coworkers). We also show that for these parameters, periodicities associated with differential footpoint motion would be barely distinguishable from rigid rotation at the solar equatorial rate. We point out that the question of periodicities in magnetic field data is perhaps more complicated than previously thought. We confirm the result of Burger and Hitge that a Fisk-type heliospheric magnetic field provides a natural explanation for the observed linear relationship between the amplitude of the recurrent cosmic-ray variations and the global latitude gradient (see Zhang). We show that this relationship holds for helium, protons, and electrons. Moreover, we show that the constant of proportionality is larger when qA > 0 than when qA < 0, as inferred from observations by Richardson and coworker
Sensitivity of cosmic-ray proton spectra to the low-wavenumber behavior of the 2D turbulence power spectrum
No full textIn this study, a novel ab initio cosmic ray (CR) modulation code that solves a set of stochastic transport equations
equivalent to the Parker transport equation, and that uses output from a turbulence transport code as input for the
diffusion tensor, is introduced. This code is benchmarked with a previous approach to ab initio modulation. The
sensitivity of computed galactic CR proton spectra at Earth to assumptions made as to the low-wavenumber
behavior of the two-dimensional (2D) turbulence power spectrum is investigated using perpendicular mean free
path expressions derived from two different scattering theories. Constraints on the low-wavenumber behavior of
the 2D power spectrum are inferred from the qualitative comparison of computed CR spectra with spacecraft
observations at Earth. Another key difference from previous studies is that observed and inferred CR intensity
spectra at 73 AU are used as boundary spectra instead of the usual local interstellar spectrum. Furthermore, the
results presented here provide a tentative explanation as to the reason behind the unusually high galactic proton
intensity spectra observed in 2009 during the recent unusual solar minimu
Effects of various dissipation range onset models on the 26-day variations of low-energy galactic cosmic-ray electrons
No full textThe effect of various models presented by Leamon et al. (2000) for the dissipation range cutoff wavenumber on the 26-day variations of galactic cosmic-ray electrons in a Fisk-Parker hybrid field is investigated, by means of a three-dimensional steady-state numerical modulation code. Analytical expressions for the mean free paths parallel and perpendicular to the heliospheric magnetic field are adapted from the works of Teufel and Schlickeiser (2003) and Shalchi et al. (2004), respectively. Note that only solar minimum conditions are considered, and that only qualitative agreement with data is sought. Effective diffusion for galactic electrons pertaining to 26-day variations is found to be dominated by the ratio of the perpendicular to parallel mean free paths at low energies, and the relationship between changes in cosmic-ray intensities and the modulation parameter postulated by Zhang (1997) is found to no longer hold when this ratio drops below a critical value. Use of ion inertial scale dependent models for the dissipation range cutoff leads to possible second linearities in the relative amplitudes as functions of latitude gradient
A comparison of turbulence-reduced drift coefficients of importance for the modulation of galactic cosmic-ray protons in the supersonic solar wind
No full textThe study of the modulation of cosmic rays in the heliosphere relies heavily on a thorough understanding of the transport of these charged particles in the turbulent solar wind. Drift effects due to gradients and the curvature of the background magnetic field have long been known to be reduced in the presence of turbulence, and as such, several forms for the drift coefficient that include the effect of turbulence have been proposed. The present study aims to investigate the qualitative effects of various turbulence-reduced drift coefficients on cosmic ray intensities computed using an ab initio 3D steady-state cosmic-ray modulation code. Results from a two-component turbulence transport models are used as inputs for the basic turbulence quantities. Furthermore, an expression for the perpendicular mean free path is derived here from a modification of the non-linear guiding center theory of Matthaeus et al. (2003) assuming a 2D turbulence power spectrum with a k-1k-1 energy range wavenumber dependence, and is used in conjunction with the various proposed turbulence-reduced drift coefficients. Cosmic-ray intensities computed using different drift coefficients but assuming the same turbulence conditions are found to differ widely. This study emphasises the need to gain a better understanding of the effect of turbulence on drifts in the heliosphereNational Research Foundation (NRF), South Afric
Cosmic-ray modulation: an ab initio approach
No full textA better understanding of cosmic-ray modulation
in the heliosphere can only be gained through a proper understanding
of the effects of turbulence on the diffusion and drift
of cosmic rays. We present an ab initio model for cosmic-ray
modulation, incorporating for the first time the results yielded
by a two-component turbulence transport model. This model
is solved for periods of minimum solar activity, utilizing
boundary values chosen so that model results are in fair to
good agreement with spacecraft observations of turbulence
quantities, not only in the solar ecliptic plane but also along
the out-of-ecliptic trajectory of the Ulysses spacecraft.
These results are employed as inputs for modelled slab
and 2D turbulence energy spectra. The latter spectrum is
chosen based on physical considerations, with a drop-off
at the very lowest wavenumbers commencing at the 2D
outerscale. There currently exist no models or observations
for this quantity, and it is the only free parameter
in this study. The modelled turbulence spectra are used
as inputs for parallel mean free path expressions based
on those derived from quasi-linear theory and perpendicular
mean free paths from extended nonlinear guiding
center theory. Furthermore, the effects of turbulence on
cosmic-ray drifts are modelled in a self-consistent way,
employing a recently developed model for drift along
the wavy current sheet. The resulting diffusion coefficients
and drift expressions are applied to the study of
galactic cosmic-ray protons and antiprotons using a threedimensional,
steady-state cosmic-ray modulation code, and
sample solutions in fair agreement with multiple spacecraft
observations are presented.National Research
Foundation (NRF
A generalized Fisk-type HMF: implications of spatially dependent photospheric differential rotation
No full textThe existence of a Fisk-type heliospheric magnetic field (HMF) has been debated ever since Fisk proposed an alternative to the traditional view of the HMF first proposed by Parker. Several modifications of the original Fisk field model have been published in the past, for example, the Schwadron field and the Fisk–Parker hybrid HMF model. This study presents a new generalized Fisk HMF model that implements for the first time a spatially dependent differential rotation rate of the photosphere, by mapping magnetic field lines from the solar wind source surface to the photosphere. Data analysis methods of Forsyth et al. are used to search for a signature of a Fisk-type field during solar minimum conditions, using the magnetic field data from the first solar orbit of the Ulysses spacecraft. The new generalized Fisk field agrees better with the observed magnetic field winding angle than a standard Parker field during the majority of intervals scanned by Ulysse
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