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Pisanko, Yuri V.

doi:10.1023/a:1004930904888

Cited by 6 publications

(2 citation statements)

References 8 publications

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“…Smith 1979, Sakurai 1989), the coronal magnetic field (e.g. Altschuler et al 1977, Nakagawa 1997), the solar helmet streamers (Cuperman et al 1990), the solar wind polar streams (Pisanko 1997) and magnetic clouds (e.g. Burlaga 1988, p 89, Yang 1989, Kumar and Rust 1996, Vandas et al 1996.…”

Section: Introductionmentioning

confidence: 99%

Magnetized vortex tubes in the solar wind plasma

Polygiannakis

Moussas

2000

Plasma Phys. Control. Fusion

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We make new applications of our previously proposed method for estimating the strainrate tensor and vorticity vector in plasmas (concerning the local deformation and self rotation of the plasma fluid elements, respectively) solely from magnetic field time series. Here we use solar wind measurements of Ulysses spacecraft made in the outer heliosphere, on and off the ecliptic plane, during the period 1990-1998. The application results imply that the solar wind plasma is, to a good approximation, weakly incompressible, being nearly incompressible on the local magnetic field-normal plane, while expanding at maximum strain rate in the magnetic field direction. This property is theoretically expected, at least for low and intermediate beta plasmas, and supports previous arguments for two-dimensional MHD turbulence in the solar wind. In the magnetic induction equation the vorticity term is favoured, being at least an order of magnitude larger than the strain-rate term, thus explaining the magnetic field alignment of the minimum magnetic field variance and the random wandering of the magnetic field's vector tip on a sphere, both being well known, general features of the heliospheric magnetic field fluctuations. Further, the solar wind is found dominated by magnetized vortex sheet structures (MVS), on the tangential plane of which lie the, (not generally aligned) average vectors of magnetic field, vorticity and plasma velocity in the solar-corotating frame of reference. These coplanarity properties are shown to be consistent with a theoretically predicted force-free state, minimizing the total energy while conserving a generalized helicity function. The theory additionally implies that the (not directly measured by Ulysses) electric current density also lies on the MVS tangential plane, hence the MVS also constitute current sheets. The MVS spatial orientation implies that the MVS are wrapped in the form of magnetized vortex tubes with axes aligned to the average magnetic field. The vector couplings, characterizing the MVS, are found to weaken with increasing heliodistance and near the heliospheric current sheet.

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Section: Introductionmentioning

confidence: 99%

Magnetized vortex tubes in the solar wind plasma

Polygiannakis

Moussas

2000

Plasma Phys. Control. Fusion

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“…Field-aligned electric current density (force-free fields), being the energy-minimizing state with the constraint of constant magnetic helicity (Woltjer 1958, Taylor 1974, was used in the solar wind for modelling magnetic cloud configurations (see e.g. Burlaga (1988) and p 89) and the fast polar streams (Pisanko, 1997). 'Alfvénic', i.e.…”

Section: Introductionmentioning

confidence: 99%

On the role of strain rate and vorticity in plasmas

Polygiannakis

Moussas

1999

Plasma Phys. Control. Fusion

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It is shown that the magnetic field's local evolution in plasmas is directly affected by an interplay between the deformation (strain rate) and the self-rotation (vorticity) of the elementary plasma fluid volumes. At regions of strong strain rate, the fast local convergence or divergence of the flow causes exponential increase or decrease of the field's components, respectively. At regions of strong vorticity, faster directional than magnitude variations of the field occur, explaining the field alignment of the field minimum-variance direction and the random wandering of the field's tip on a sphere, both observed in the solar wind plasma, even in the absence of Alfvén waves. We further investigate the coupling between the maximum strain-rate direction and the local magnetic field, that was previously deduced from magnetic field measurements in the outer heliosphere. Cases of long-lasting, non-Parkerian, radial heliospheric magnetic field are also shown to be periods of field-aligned strain rate. A statistical proof for this alignment is given, assuming that the small-scale field fluctuations are weakly stationary and time reversible. We further propose a generalization of the Stokesian fluid stress-strain relation to the case of one-fluid, collisionless MHD plasmas, including the effects of turbulent viscosity and magnetically induced shearing motion. For a negligible or isotropic or 'field-aligned' thermal pressure tensor, the proposed 'Stokesian plasma' relation implies the field alignment of the solar wind plasma strain-rate direction and leads to anisotropic stress-strain balance equations, related to those of 'firehose' plasma instability. The field alignment of a principal strain-rate direction leads to simplifications in the magnetic-induction equation, especially in the case of force-free fields. For the simplified case of homogeneous, isotropic and incompressible plasma turbulence, the proposed stress-strain-rate relation implies that the velocity and magnetic field inertial range spectra should be identical, further reducing to the Kolmogorov-like k −5/3 law for scale-invariant eddy cascade at constant energy-dissipation rate.

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Automated Coronal Hole Detection Using Local Intensity Thresholding Techniques

2009

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We identify coronal holes using a histogram-based intensity thresholding technique and compare their properties to fast solar wind streams at three different points in the heliosphere. The thresholding technique was tested on EUV and X-ray images obtained using instruments onboard STEREO, SOHO and Hinode. The full-disk images were transformed into Lambert equal-area projection maps and partitioned into a series of overlapping sub-images from which local histograms were extracted. The histograms were used to determine the threshold for the low intensity regions, which were then classified as coronal holes or filaments using magnetograms from the SOHO/MDI. For all three instruments, the local thresholding algorithm was found to successfully determine coronal hole boundaries in a consistent manner. Coronal hole properties extracted using the segmentation algorithm were then compared with in situ measurements of the solar wind at ∼ 1 AU from ACE and STEREO. Our results indicate that flux tubes rooted in coronal holes expand super-radially within 1 AU and that larger (smaller) coronal holes result in longer (shorter) duration highspeed solar wind streams.

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Cited by 6 publications

References 8 publications

Magnetized vortex tubes in the solar wind plasma

Magnetized vortex tubes in the solar wind plasma

On the role of strain rate and vorticity in plasmas

Automated Coronal Hole Detection Using Local Intensity Thresholding Techniques

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