Eugene H. Levy

Effects of particle drifts on the solar modulation of galactic cosmic rays

Gradient and curvature drifts in an Archimedian-spiral magnetic field are shown to produce a significant effect on the modulation of galactic cosmic rays by the solar wind. The net modulations, heliocentric radial gradient, and average energy change of particles which reach the inner solar system are significantly reduced. The effects of drifts are due to the fact that cosmic rays for which the drift velocity is comparable to the wind velocity or larger, have more rapid access to the inner solar system than in the absence of drifts.

Kinematic reversal schemes for the geomagnetic dipole.

Fluctuations in the distribution of cyclonic convective cells, in the earths core, can reverse the sign of the geomagnetic field. Two kinematic reversal schemes are discussed. In the first scheme, a field maintained by cyclones concentrated at low latitude is reversed by a burst of cyclones at high latitude. Conversely, in the second scheme, a field maintained predominantly by cyclones in high latitudes is reversed by a fluctuation consisting of a burst of cyclonic convection at low latitude. The precise fluid motions which produce the geomagnetic field are not known. However, it appears that, whatever the details are, a fluctuation in the distribution of cyclonic cells over latitude can cause a geomagnetic reversal.

Magnetic reconnection flares in the protoplanetary nebula and the possible origin of meteorite chondrules.

Abstract Many primitive meteorites are composed largely of chondrules, small once-molten beads of glassy rock. The existence of chondrules poses a basic problem for our understanding of the photoplanetary nebula inasmuch as the chondrules seem to have been melted by very short-lived, transient heating events in otherwise cool nebular sorroundings. In this paper, the possibility is investigated that meteorite chondrules formed as a result of melting of protosolar nebula dust balls by the energy released from magnetic flares in the nebulas corona. Analysis of the energy that could be released by magnetic reconnection events in nebular coronal flares shows that previously existing dust balls could be heated transiently to temperatures sufficiently high (above 1700°K) to cause the short-lived melting events that are needed to account for the existence of chondrules. The release of flare energy at rates sufficient to account for chondrule melting requires that the flares occur in the presence of magnetic fields somewhat in excess of 5 G in a low-density coronal region of the disk. Nebular magnetic fields of this strength are in accord with magnetizing fields that have been inferred from the measured remanent magnetization of primitive meteorites.

The interplanetary magnetic field structure

THE interplanetary magnetic field is the extension, by the solar wind, of the solar magnetic field1. The detailed connection between the solar and interplanetary fields depends, however, on the precise manner in which the wind originates in the solar atmosphere and involves a number of complicated dynamical questions which are not yet fully understood2. Thus knowledge of the three-dimensional structure of the interplanetary magnetic field is important to our understanding of the solar atmosphere and solar wind flow. Up to now, direct measurements have been confined to the close vicinity of the solar equatorial plane; as a result our ideas about the overall structure of the interplanetary field are speculative. Here we suggest that the cosmic-ray flux provides some indirect information about the morphology of the interplanetary field.

Generation of dynamo magnetic fields in protoplanetary and other astrophysical accretion disks

A computational method for treating the generation of dynamo magnetic fields in astrophysical disks is presented. The numerical difficulty of handling the boundary condition at infinity in the cylindrical disk geometry is overcome by embedding the disk in a spherical computational space and matching the solutions to analytically tractable spherical functions in the surrounding space. The lowest lying dynamo normal modes for a thick astrophysical disk are calculated. The generated modes found are all oscillatory and spatially localized. Tha potential implications of the results for the properties of dynamo magnetic fields in real astrophysical disks are discussed. 30 references.

Oscillating dynamo magnetic field in the presence of an external nondynamo field - The influence of a solar primordial field

Dynamo magnetic fields are self-excited and, once started, can perpetrate themselves with no outside source of magnetic flux, as long as the necessary fluid motions persist. Such dynamo fields behave completely independently of the fields overall polarity. In the presence of an external field of separate origin this polarity symmetry of the dynamo states is broken; the dynamo states become asymmetric with respect to polarity. In this paper a calculation is performed of the characteristics of a spherical shell dynamo in the presence of a fossil magnetic field penetrating into the dynamo from below. The asymmetric periodic states are found as a function of the strength of underlying fossil field. Applying these results to the sun, there appears to be no evidence of any intense large-scale primordial magnetic flux, having either dipole-like or quadrupole-like symmetry about the suns equator, penetrating into the convection zone from the suns radiative core. Indeed, the calculations indicate, even on the basis of the presently crude observations, that any such primordial field must have an intensity smaller than a few gauss.

Magnetic dynamo in the moon: a comparison with the Earth.

The assumption that the moon had an internal magnetic field produced in the same way as the geomagnetic field requires that the moon rotated faster than the angular velocity at which it would break up. This suggests that a lunar dynamo is not a tenable explanation for the magnetic remanence observed on the moon.

Oscillating dynamo in the presence of a fossil magnetic field - The solar cycle

Hydromagnetic dynamo generation of oscillating magnetic fields in the presence of an external, ambient magnetic field introduces a marked polarity asymmetry between the two halves of the magnetic cycle. The principle of oscillating dynamo interaction with external fields is developed, and a tentative application to the sun is described. In the sun a dipole moment associated with the stable fluid beneath the convection zone would produce an asymmetrical solar cycle.

Dynamo magnetic field modes in thin astrophysical disks : an adiabatic computational approximation

An adiabatic approximation is applied to the calculation of turbulent MHD dynamo magnetic fields in thin disks. The adiabatic method is employed to investigate conditions under which magnetic fields generated by disk dynamos permeate the entire disk or are localized to restricted regions of a disk. Two specific cases of Keplerian disks are considered. In the first, magnetic field diffusion is assumed to be dominated by turbulent mixing leading to a dynamo number independent of distance from the center of the disk. In the second, the dynamo number is allowed to vary with distance from the disks center. Localization of dynamo magnetic field structures is found to be a general feature of disk dynamos, except in the special case of stationary modes in dynamos with constant dynamo number. The implications for the dynamical behavior of dynamo magnetized accretion disks are discussed and the results of these exploratory calculations are examined in the context of the protosolar nebula and accretion disks around compact objects.

Generation of dynamo magnetic fields in thin Keplerian disks

The combined action of nonuniform rotation and helical convection in protoplanetary disks, in the Galaxy, or in accretion disks surrounding black holes and other compact objects, enables an alpha-omega dynamo to generate a large-scale magnetic field. In this paper, the properties of such magnetic fields are investigated using a two-dimensional, partially numerical method. The structures of the lowest-order steady state and oscillatory modes are calculated for two kinds of external boundary conditions. A quadruple, steady state, highly localized mode is the most easily excited for low values of the dynamo number. The results indicate that, except under special conditions, disk dynamo modes tend to consist of relatively localized rings structures. For large values of the dynamo number, the magnetic field consists of a number of quasi-independent, spatially localized modes generated in various concentric rings filling the disk inward of a dynamo generation front. 36 refs.