Dipole model of the Earth's magnetic field

About: Dipole model of the Earth's magnetic field is a research topic. Over the lifetime, 2756 publications have been published within this topic receiving 83021 citations.

Propagation of solar cosmic rays through interplanetary magnetic field

Abstract: In recent years evidence has been accumulating that low-energy solar cosmic-ray particles arrive at the earth after intense solar disturbances. The time-delay of polar-cap absorptions and of geomagnetic storms, which follow after an intense solar radio outburst of type IV, indicates that solar cosmic-ray particles originating to the west of the solar central meridian reach the earth earlier and more easily than those from the east. The model of the interplanetary magnetic field, which may be appropriate for the explanation of this evidence, is as follows: The interplanetary magnetic field is formed by the outward-streaming solar winds which carry the imbedded solar magnetic fields and the lines of force also linked with the sun itself. The rotation of the sun produces a curvature of streams and consequently of the lines of force, which are convex toward the west. Since the injected solar particles will tend to travel along the existing magnetic lines of force, this model explains the western excess of the arrival of solar cosmic rays as well as the inequality of their travel time with respect to the heliographic position. The estimated intensity of the magnetic field is of the order of 10−3 to 10−5 gauss near the earth's orbit.

Field line resonant pulsations associated with a strong dayside ionospheric shear convection flow reversal

Abstract: We discuss a set of coordinated observations from the arrays of Greenland ground magnetometers, Sondrestrom incoherent scatter radar, and the DMSP, GOES 7, and IMP 8 satellites during 1000–1400 UT on August 4, 1991. The work presented here follows work presented by Clauer and Ridley [1995]. In the previous work we show that this particular interval is characterized by a large positive interplanetary magnetic field (IMF) By and near-zero IMF Bz components. Associated with these conditions is a very strong ionospheric convection shear reversal boundary in the dayside noon and prenoon sector in the northern hemisphere. The convection reversal boundary is observed to be very dynamic, showing wave-like displacements of several degrees in invariant latitude with a tailward phase propagation. These variations are associated with magnetic pulsations with a period of about 34 min. We have suggested that these waves are produced by a Kelvin-Helmholtz instability at the shear convection reversal boundary. We present here a more detailed analysis of the magnetic variations from the Greenland arrays of magnetometers. Our new findings show that equatorward of the convection reversal boundary, there is power in the pulsation spectra in bands with periods of about 34, 17, 12, and 8 min. Examination of the northward component of the pulsations along a meridional chain of stations ranging from 76.23° to 66.86° invariant latitude shows high coherence between stations and a change in relative phase of about 160° over the observed latitude range for the 8-min. pulsations. These observations are consistent with the expected signature of a field line resonance. We suggest that the source of the resonance is the disturbance generated at the shear convection reversal boundary which then produces resonances on nearby, equatorward closed field lines. In this case we suggest that the source is the Kelvin-Helmholtz wave established at the east-west convection shear which develops near local noon and prenoon regions during large IMF By positive conditions. This is possibly the first observation of the field line resonance associated with such a source.

Magnetar activity via the density–shear instability in Hall-MHD

Abstract: We investigate the density-shear instability in Hall-MHD via numerical simulation of the full non-linear problem, in the context of magnetar activity. We confirm the development of the instability of a plane-parallel magnetic field with an appropriate intensity and electron density profile, in accordance with analytic theory. We find that the instability also appears for a monotonically decreasing electron number density and magnetic field, a plane-parallel analogue of an azimuthal or meridional magnetic field in the crust of a magnetar. The growth rate of the instability depends on the Hall properties of the field (magnetic field intensity, electron number density and the corresponding scale-heights), while being insensitive to weak resistivity. Since the Hall effect is the driving process for the evolution of the crustal magnetic field of magnetars, we argue that this instability is critical for systems containing strong meridional or azimuthal fields. We find that this process mediates the formation of localised structures with much stronger magnetic field than the average, which can lead to magnetar activity and accelerate the dissipation of the field and consequently the production of Ohmic heating. Assuming a 5 × 1014G magnetic field at the base of crust, we anticipate that magnetic field as strong as 1015G will easily develop in regions of typical size of a few 102 meters, containing magnetic energy of 1043erg, sufficient to power magnetar bursts. These active regions are more likely to appear in the magnetic equator where the tangential magnetic field is stronger.

The Formation of Large-Scale Current Sheets within Magnetic Clouds

Abstract: Magnetic clouds are a class of interplanetary coronal mass ejections (CME) predominantly characterised by a smooth rotation in the magnetic field direction, indicative of a magnetic flux rope structure. Many magnetic clouds, however, also contain sharp discontinuities within the smoothly varying magnetic field, suggestive of narrow current sheets. In this study we present observations and modelling of magnetic clouds with strong current sheet signatures close to the centre of the apparent flux rope structure. Using an analytical magnetic flux rope model, we demonstrate how such current sheets can form as a result of a cloud’s kinematic propagation from the Sun to the Earth, without any external forces or influences. This model is shown to match observations of four particular magnetic clouds remarkably well. The model predicts that current sheet intensity increases for increasing CME angular extent and decreasing CME radial expansion speed. Assuming such current sheets facilitate magnetic reconnection, the process of current sheet formation could ultimately lead a single flux rope becoming fragmented into multiple flux ropes. This change in topology has consequences for magnetic clouds as barriers to energetic particle propagation.