Study Of Magnetic Fluctuations Research Articles

The variation in the magnetic fluctuation anisotropy across the front boundaries of magnetic clouds and its geomagnetic response

A statistical study of magnetic fluctuations near the front boundaries of magnetic clouds is approached with the method of Minimum Variance Analysis, based on the data of Imp8 and Wind spacecraft. New discoveries are that (1) fluctuation anisotropy tends to increase across the front boundaries of magnetic clouds; (2) there is a good correlation between the fluctuation anisotropy and the geomagnetic activity indices; and (3) in some cases, although there is southward field component immediately after the front boundary, Kp index descends (or Dst index ascends) with a corresponding decrease of the fluctuation anisotropy; in other cases with no distinct southward field component, Kp index ascends (or Dst index descends) with a corresponding increase of the fluctuation anisotropy. Thus we suggest that the fluctuation anisotropy might be a useful indicator in diagnosing the magnetic activities of magnetic clouds.

A study of magnetic fluctuations and their anomalous scaling in the solar wind: the Ulysses fast-latitude scan

Abstract. The solar wind is a highly turbulent and intermittent medium at frequencies between 10-4 and 10-1 Hz. Power spectra are used to look at fluctuations in the components of the magnetic field at high frequencies over a wide range of latitudes. Results show steady turbulence in the polar regions of the Sun and a more varied environment in the equatorial region. The magnetic field fluctuations exhibit anomalous scaling at high frequencies. Various models have been proposed in an attempt to better understand the scaling nature of such fluctuations in neutral fluid turbulence. We have used the Ulysses fast latitude scan data to perform a wide ranging comparison of three such models on the solar wind magnetic field data: the well-known P model, in both its Kolmogorov and Kraichnan forms, the lognormal cascade model and a model adapted from atmospheric physics, the G infinity model. They were tested by using fits to graphs of the structure function exponents g(q), by making a comparison with a non-linear measure of the deviation of g(q) from the non-intermittent straight line, and by using extended self similarity technique, over a large range of helio-latitudes. Tests of all three models indicated a high level of intermittency in the fast solar wind, and showed a varied structure in the slow wind, with regions of apparently little intermittency next to regions of high intermittency, implying that the slow wind has no uniform origin. All but one of the models performed well, with the lognormal and Kolmogorov P model performing the best over all the tests, indicating that inhomogeneous energy transfer in the cascade is a good description. The Kraichnan model performed relatively poorly, and the overall results show that the Kraichnan model of turbulence is not well supported over the frequency and distance ranges of our data set. The G infinity model fitted the results surprisingly well and showed that there may very well be important universal geometrical aspects of intermittency over many physical systems.

Magnetic Fluctuations for Small Magnetic Prandtl Numbers

Numerous studies of magnetic fluctuations with a zero mean-field for small magnetic Prandtl numbers (Prm ≪ 1) show that magnetic fluctuations cannot be generated by turbulent fluid flow with the Kolmogorov energy spectrum. In addition, the generation of magnetic fluctuations with a zero mean-field for Prm ≪ 1 were not observed in numerical simulations. However, in astrophysical plasmas the magnetic Prandtl numbers are small and magnetic fluctuations are observed. Thus a mechanism of generation of magnetic fluctuations for Prm ≪ 1 still remains poorly understood. On the other hand, in astrophysical applications (e.g., solar and stellar convection zones, galaxies, accretion disks) the turbulent velocity field cannot be considered as a divergence-free.

Properties of whistler mode waves between 0.3 and 1.0 AU from Helios observations

A study of magnetic fluctuations in the solar wind in the distance range from 0.3 to 1 AU and the frequency range 4.7 Hz‐2.2 kHz by the Technical University of Braunschweig search‐coil magnetometer experiment onboard Helios 1 and 2 during the time intervals from December 1974 to April 1975 for Helios 1 and January 1976 to May 1976 for Helios 2 yields the following results: (1) Near 1 AU electromagnetic wave modes are restricted to frequencies less than 220 Hz for 99% of the time. Inside 0.5 AU the waves are restricted to less than 470 Hz in 94% of all cases. (2) For a spectral density [P(f)]1/2; ∝ fα with the quantity √P measured in nT/√Hz the spectral index α varies between −1.4 and −2.0 with a most probable value of −1.7 and a slight tendency toward steeper spectra towards the sun. (3) No clear correlation between simple plasma parameters and magnetic field magnitude F on one side and spectral densities on the other side is found. The proportionality between spectral density and F is simply attributed to the radial variation. (4) High wave intensities occur in the vicinity of stream interfaces with a decrease in wave intensity with increasing distance from the stream interaction region. (5) Magnetic field energy density above 4.7 Hz varies between 5 × 10−18 erg/cm³ and 10−14 erg/cm³ with a maximum of occurrence of 5 × 10−17 erg/cm³ near 1 AU and 1.6 × 10−15 erg/cm³ near 0.3 AU. The values are therefore orders of magnitude below the total magnetic field energy density. (6) The fluctuations are tentatively interpreted as locally generated whistler waves.