Anisotropy of magnetic field fluctuations in an average interplanetary magnetic cloud at 1 AU

摘要

A subset of magnetic clouds (MCs) identified over 8.6 years of the WIND mission are examined for a pattern of anisotropic magnetic field fluctuations within their extent at 1 AU. This subset (N = 42) consists of MCs taken from a comprehensive survey of WIND Magnetic Field Investigation data. Root mean square deviations of the axial field (RMSA) and the perpendicular field (RMS ; i.e., that lying in the cross-sectional plane of the MC), rendered in a magnetic cloud coordinate system, are computed on three timescales, 8, 30, and 60 min, and a fluctuation anisotropy quantity [A(t) = RMSA /RMS ? 1] is calculated for each timescale. The 42 MCs are separated by relative closest approach (Y 0/R 0) into two distinct regions [|Y 0/R 0| < 0.33 (inner) and 0.33 < |Y 0/R 0| < 0.66 (middle)], where R 0 is a model-estimated MC radius and Y 0 is a model-estimated magnitude of the closest-approach vector, and superimposed within each region. We show a clear and distinct magnetic field fluctuation anisotropy profile within each region. Additionally, this profile closely resembles that of the pitch angle of the field implying that the field fluctuations tend to be predominately perpendicular to the field itself. Thus transverse fluctuations are most common in MCs in the range (1.4–10.4) × 10~(-4) Hz.