MAGNETIC HELICITY SPECTRUM OF SOLAR WIND FLUCTUATIONS AS A FUNCTION OF THE ANGLE WITH RESPECT TO THE LOCAL MEAN MAGNETIC FIELD

Journal Article · Fri Jun 10 00:00:00 EDT 2011 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/734/1/15· OSTI ID:21576687

Podesta, J J

Magnetic field data acquired by the Ulysses spacecraft in high-speed streams over the poles of the Sun are used to investigate the normalized magnetic helicity spectrum {sigma}{sub m} as a function of the angle {theta} between the local mean magnetic field and the flow direction of the solar wind. This spectrum provides important information about the constituent modes at the transition to kinetic scales that occurs near the spectral break separating the inertial range from the dissipation range. The energetically dominant signal at scales near the thermal proton gyroradius k{sub perpendicular{rho}i} {approx} 1 often covers a wide band of propagation angles centered about the perpendicular direction, {theta} {approx_equal} 90{sup 0} {+-} 30{sup 0}. This signal is consistent with a spectrum of obliquely propagating kinetic Alfven waves with k{sub perpendicular} >> k{sub ||} in which there is more energy in waves propagating away from the Sun and along the direction of the local mean magnetic field than toward the Sun. Moreover, this signal is principally responsible for the reduced magnetic helicity spectrum measured using Fourier transform techniques. The observations also reveal a subdominant population of nearly parallel propagating electromagnetic waves near the proton inertial scale k{sub ||} c/{omega}{sub pi} {approx} 1 that often exhibit high magnetic helicity |{sigma}{sub m}| {approx_equal} 1. These waves are believed to be caused by proton pressure anisotropy instabilities that regulate distribution functions in the collisionless solar wind. Because of the existence of a drift of alpha particles with respect to the protons, the proton temperature anisotropy instability that operates when T{sub pperpendicular}/T{sub p||} > 1 preferentially generates outward propagating ion-cyclotron waves and the fire-hose instability that operates when T{sub pperpendicular}/T{sub p||} < 1 preferentially generates inward propagating whistler waves. These kinetic processes provide a natural explanation for the magnetic field observations.

• OSTI ID: 21576687
• Journal Information: Astrophysical Journal, Vol. 734, Issue 1; Other Information: DOI: 10.1088/0004-637X/734/1/15; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTROPHYSICS
COSMOLOGY AND ASTRONOMY
ALFVEN WAVES
ALPHA PARTICLES
ANISOTROPY
DISTRIBUTION FUNCTIONS
FLUCTUATIONS
FOURIER TRANSFORMATION
HELICITY
HOSE INSTABILITY
ION PLASMA WAVES
MAGNETIC FIELDS
MAGNETOHYDRODYNAMICS
PROTON TEMPERATURE
PROTONS
SOLAR WIND
SPECTRA
SUN
TURBULENCE
WAVE PROPAGATION
WHISTLERS
BARYONS
CHARGED PARTICLES
ELECTROMAGNETIC RADIATION
ELEMENTARY PARTICLES
FERMIONS
FLUID MECHANICS
FUNCTIONS
HADRONS
HYDRODYNAMICS
HYDROMAGNETIC WAVES
INSTABILITY
INTEGRAL TRANSFORMATIONS
ION WAVES
IONIZING RADIATIONS
MAIN SEQUENCE STARS
MECHANICS
NOISE
NUCLEONS
PARTICLE PROPERTIES
PLASMA INSTABILITY
PLASMA MICROINSTABILITIES
PLASMA WAVES
RADIATIONS
RADIO NOISE
RADIOWAVE RADIATION
SOLAR ACTIVITY
STARS
STELLAR ACTIVITY
STELLAR WINDS
TRANSFORMATIONS
VARIATIONS