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.

Seasonal variations of high‐latitude field‐aligned currents inferred from Ørsted and Magsat observations

Abstract: [1] In this paper we report on field-aligned currents inferred from high-precision three-component geomagnetic field observations made on board the Danish satellite Orsted over polar regions. Because of a slow drift in local time of the satellite orbit through the “noon-midnight” sector, we were able to study the seasonal dependence of the dynamic properties of the dayside and nightside field-aligned current systems over the Northern and Southern Hemispheres. We find an average over-the-pole distance between dayside and nightside currents of 32° during summer but 37° during winter and 36° during equinox. The decrease in the size of the summer polar cap is caused by a shift of both daytime and nighttime current systems to higher magnetic latitudes. For comparison, the dawn-dusk cross-polar distance of the Region 1/Region 2 field-aligned currents has been determined from high-precision data observed by Magsat, a satellite flown in 1979–1980 in a “dawn-dusk” orbit. The latter results show that the dawn-dusk distance between R1/R2 currents exhibits little seasonal dependence and amounts to ∼34° for all seasons in both polar caps. The seasonal dependence is confirmed for the high-latitude field-aligned intensities; they are larger by a factor of 1.5–1.8 in the sunlit (summer) polar cap in comparison with the winter hemisphere. Our results suggest that the R1/R2 and dayside field-aligned currents are well balanced between the pairs of downward/upward currents for all seasons as well as between hemispheres during equinox. We were not able to confirm results reported in earlier studies that the net currents tend to increase with an enhancement of ionospheric conductivity caused by the solar illumination or substorm activity.

A qualitative study of the reconnection between the Earth's magnetic field and an interplanetary field of arbitrary orientation

Abstract: To the present date only the reconnection process for exactly antiparallel fields has been discussed in detail. In magnetospheric terms this restricts us to the consideration only of southward interplanetary fields. The qualitative study presented here shows how reconnection takes place between arbitrarily oriented fields in infinite and finite geometries. The process may best be thought of as a continuous exchange of field-line partners during the time a field line maps into the diffusion region. In a finite geometry the diffusion regions lie on field lines which connect neutral points of the field configuration, and along which a potential drop is imposed. Although the discussion here centers principally on the magnetosphere, the described reconnection process and field topology should also be applicable to other astrophysical problems.

Near-bottom magnetic anomalies, asymmetric spreading, oblique spreading, and tectonics of the Mid-Atlantic Ridge near lat 37°N

Abstract: A detailed study of the magnetic anomalies of the Mid-Atlantic Ridge crest near lat 37 °N (FAMOUS) was conducted using a deeply towed instrument package. The most recent expression of the accreting plate boundary in rift valley 2 is an alternating series of linear central volcanoes and depressions that are marked by a sharp maximum in crustal magnetization only 2 to 3 km wide. Spreading in the FAMOUS area is highly asymmetric, with rates of 13.4 mm/yr to the east and 7.0 mm/yr to the west. At 1.7 m.y. B.P., the sense of asymmetry reverses in direction, with spreading faster to the west; this results in a gross symmetry when averaged through time. The change in spreading asymmetry occurred in less than 0.15 m.y. Spreading in the FAMOUS area is 17° oblique. Even on a fine scale there is no indication of readjustment to an orthogonal plate-boundary system. Spreading has been stably oblique for at least 8 m.y., even through a change in spreading direction. The presence of negative polarity crust within the Brunhes normal epoch in the inner floor has been observed and may be due to old crust left behind or to recording of a geomagnetic field event. Crustal magnetization decays to 1/e its initial value in less than 0.6 m.y. The rapid decay may be facilitated by very intense crustal fracturing observed in the inner floor. Crustal magnetic sources may be approximated (mathematically) by a uniformly magnetized layer 700 m thick. Magnetic studies indicate that over 90 percent of the extrusive volcanism occurs within the rift inner floor and is extremely rare in the rift mountains. Magnetic anomaly transition widths vary from 1 to 8 km with time and appear to reflect a time-varying median-valley structure. The valley has either a wide inner floor and narrow terraces, in which case the volcanic zone is wide and magnetic anomalies are poorly recorded (wide transition widths), or it has a narrow inner floor and wide terraces — the volcanic zone is then narrow and anomalies are clearly recorded (narrow transition widths). The median valley of any ridge segment varies between these two structures with time. At present rift valley 2 has a narrow inner floor and volcanic zone (1 to 3 km), whereas rift valley 3 is at the opposite end of the cycle with a wide inner floor and volcanic zone (9 to 11 km).