Earth's magnetic field

About: Earth's magnetic field is a research topic. Over the lifetime, 20360 publications have been published within this topic receiving 446747 citations. The topic is also known as: magnetic field of Earth & geomagnetic field.

Ørsted Initial Field Model

Abstract: Magnetic measurements taken by the Orsted satellite during geomagnetic quiet conditions around Jan-uary 1, 2000 have been used to derive a spherical harmonic model of the Earth's magnetic field for epoch 2000.0. The maximum degree and order of the model is 19 for internal, and 2 for external, source fields; however, coefficients above degree 14 may not be robust. Such a detailed model exists for only one previous epoch, 1980. Achieved rms misfit is < 2 nT for the scalar intensity and < 3 nT for one of the vector components perpendicular to the magnetic field. For scientific purposes related to the Orsted mission, this model supercedes IGRF 2000.

Global Configuration of a Magnetic Cloud

Abstract: A magnetic cloud associated with a 2N flare on January 1, 1978 was observed by IMP-8, Helios A, Helios B, and Voyager 2 The variation of the magnetic field observed at each spacecraft is represented to good approximation by Lundquist's solution for a cylindrically symmetric force-free magnetic field with constant alpha A least-squares fit of Lundquist's solution to the data from each spacecraft gives the local orientation of the axis of the magnetic cloud The times of the estimated boundaries of the magnetic cloud at each spacecraft, together with the speeds of the boundaries and the spacecraft position, give the positions of the boundaries at a given time From these results the magnetic cloud is determined to resemble a flux rope whose minor radius is approximately 015 AU at 1 AU, and whose radius of curvature at 1 AU is approximately 1/3 AU

The polar wind

Abstract: The polar wind is an ambipolar outflow of thermal plasma from the terrestrial ionosphere at high latitudes to the magnetosphere along geomagnetic field lines. The polar wind plasma consists mainly of H+, He+, and O+ ions and electrons. Although it was initially believed that O+ ions play a major role only at low altitudes, it is now clear from observations that relatively large amounts of suprathermal and energetic O+ ions are present in the polar magnetosphere. Recently, thermal O+ outflow has been observed at altitudes of 5000–10,000 km together with H+ and He+ ions. The polar wind undergoes four major transitions as it flows from the ionosphere to the magnetosphere: (1) from chemical to diffusion dominance, (2) from subsonic to supersonic flow, (3) from collision-dominated to collisionless regimes, and (4) from heavy to light ion composition. The collisions are important up to about 2500 km, after which the ions and electrons exhibit temperature anisotropies. The direction of the anisotropy varies with geophysical conditions. The polar wind outflow varies with season, solar cycle, and geomagnetic activity. The O+ flux exhibits a summer maximum, while the H+ flux reaches a maximum in the spring. The He+ flux increases by a factor of 10 from summer to winter. At both magnetically quiet and active times the integrated H+ ion flux is largest in the noon sector and smallest in the midnight sector. The integrated upward H+ ion flux exhibits a positive correlation with the interplanetary magnetic field. In the sunlit polar cap the photoelectrons can increase the ambipolar electric field, which in turn increases the polar wind ion outflow velocities. The outflowing polar wind plasma flux tubes also convect across the polar cap. When the flux tubes cross the cusp and nocturnal auroral regions, the plasma can be heated and become unstable. Similar mixing of hot magnetospheric plasma with cold polar wind may result in instabilities. A number of free energy sources in the polar wind, including temperature anisotropy, relative drift between species, and spatial inhomogeneities, feed various fluid and kinetic instabilites. The instabilities can produce plasma energization and cross-field transport, which modify the large-scale polar wind outflow.

Common origin of positive ionospheric storms at middle latitudes and the geomagnetic activity effect at low latitudes

Abstract: Anomalous increases of the ionization density at middle latitudes (positive ionospheric storms) and anomalous increases of the neutral gas density at low latitudes (the geomagnetic activity effect) are prominent features of upper atmospheric storms. The present study investigates the idea that both phenomena are caused by traveling atmospheric disturbances (TADs). According to theory, such TADs are generated during magnetic substorm activity and propagate with high velocity from polar to equatorial latitudes. To examine the above hypothesis, magnetic, ionospheric, and neutral atmospheric data are compared for five different disturbance events. These case studies demonstrate that (1) there is a good temporal correlation between magnetic substorm activity at high latitudes, daytime positive ionospheric storms at middle latitudes, and the geomagnetic activity effect at low latitudes; (2) the initial phase of positive ionospheric storms propagates with high velocity toward lower latitudes; (3) this velocity is roughly consistent with the time lag of the geomagnetic activity effect at low latitudes; (4) the ionospheric disturbance is a conjugate phenomenon of global extent; and (5) it cannot be explained as an electric field effect. In summary, our data are fully consistent with the idea that TADs are responsible for both positive ionospheric storms at middle latitudes and the geomagnetic activity effect at low latitudes.