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.

Resonant Magnetic Field Sensor With Frequency Output

Abstract: This paper presents a novel type of resonant magnetic field sensor exploiting the Lorentz force and providing a frequency output. The mechanical resonator, a cantilever structure, is embedded as the frequency-determining element in an electrical oscillator. By generating an electrical current proportional to the position of the cantilever, a Lorentz force acting like an additional equivalent spring is exerted on the cantilever in the presence of a magnetic field. Thus, the oscillation frequency of the system, which is a function of the resonator's equivalent spring constant, is modulated by the magnetic field to be measured. The resonant magnetic field sensor is fabricated using an industrial CMOS process, followed by a two-mask micromachining sequence to release the cantilever structure. The characterized devices show a sensitivity of 60 kHz/Tesla at their resonance frequency f0 =175 kHz and a short-term frequency stability of 0.025 Hz, which corresponds to a resolution below 1 muT. The devices can thus be used for Earth magnetic field applications, such as an electronic compass. The novel resonant magnetic field sensor benefits from an efficient continuous offset cancellation technique, which consist in evaluating the frequency difference measured with and without excitation current as output signal

The Lake Mungo geomagnetic excursion

Abstract: Archaeomagnetic studies have been made of prehistoric aboriginal fireplaces occurring along the ancient shore of Lake Mungo, a dried out lake in southeastern Australia. Directions of magnetization preserved in ovenstones and baked hearths show that wide departures of up to 120° from the axial dipole field direction occurred about 30000 years ago. The determination of the variation in geomagnetic field strength from the baked material is complicated by non-ideal behaviour during Thellier’s double heating method. The problem appears to arise from the subsequent (postfiring) formation of iron oxyhydroxides during a period in which the water level in the lake rose. During laboratory heatings these oxyhydroxides dehydrate causing the non-ideal behaviour observed. The ancient field strengths deduced are therefore probably minimum values. The geomagnetic excursion recorded between at least 30780 ±520 and 28140 ±370 years b.p. on the conventional radiocarbon time scale is associated with very high field strengths between 1 and 2 Oe (1 Oe » 79.6 A m_ 1 ). The field strength subsequently decreased to between 0.2 and 0.3 Oe after the excursion. This main excursion is referred to as the Lake Mungo geomagnetic excursion. There is evidence that a second excursion associated with low field strengths of 0.1-0.2 Oe occurred around 26000 years b.p. A review of geomagnetic excursions less than 40 000 years in age shows that it may be premature to assume that these are world-wide synchronous features. The range of ages and their groupings in different parts of the world may indicate they are temporary non-dipole features of continental extent. However, the duration of most excursions (order of 103 years) is very similar to that of polarity transitions and this could indicate they are aborted reversals.

Statistical properties and possible supply mechanisms of tailward cold O + beams in the lobe/mantle regions

Abstract: We investigate statistical properties of cold O+ beams (COBs) streaming tailward at the velocity nearly equal to the major H+ component, which were observed by Geotail/low-energy particle (LEP) instrument in the tail lobe/mantle regions at geocentric distance between 8 and 210 RE (Earth radii) during the solar-minimum period (October 1993 to March 1995). The average O+ density is ∼1.3×10−3 cm−3, which corresponds to ∼1.2 % of the proton component. Properties of the flow velocity show that it is not the weakening of the magnetospheric convection but the large parallel velocity which enables the O+ ions to remain still in the distant lobe/mantle regions. The occurrence frequency of COBs suggests that O+ ions tend to exist in the mantle-like region rather than tenuous “pure lobe” and that their existence has a clear correlation with the geomagnetic activity. On the basis of the EMF By dependence of the double-peaked COB distribution along dawn-dusk direction, it is shown that COBs exist mostly on loaded quadrants in the north–south and dawn-dusk asymmetry of sheath plasma entry caused by the IMF By effect on the dayside reconnection process. The concentration on the loaded quadrants can be seen even in geomagnetic storms. It suggests that frequent COB occurrence at active times is mainly due to the southward orientation of IMF rather than the increase of dynamic pressure itself during the geomagnetic storms. That is, the statistics show that COBs are abundant at geomagnetically active times on loaded quadrants resulting from the dayside reconnection process, where the ions of solar wind origin bear the major component, and their field-aligned velocity is larger than usual. These COBs should originate in the dayside magnetosphere and/or the polar cap regions. From the COB energy of several keV, which is rather higher than that of cusp/cleft ion outflows, the necessity of extra energization(s) to elevate parallel velocity is suggested. Clear IMF By dependence, on one hand, provides other possibilities of the COB's source such as the energetic UFI beams and the equatorially trapped ions. The requirements for each candidate so as to be a main contributor to COBs are also discussed.

Variability in the maximum height of the ionospheric F2-layer over Millstone Hill (September 1998–March 2000); influence from below and above

Abstract: . The basic aim of this ‘case study’ is to investigate the variability in the maximum height of the ionospheric F2-layer, hmF2, with periods of planetary waves (2–30 days), and to make an attempt to determine their origin. The hourly data of hmF2 above Millstone Hill (42.6° N, 71.5° W) during 01 September 1998 - 31 March 2000 were used for analysis. Three types of disturbances are studied in detail: (i) the 27- day oscillations observed in the hmF2 above Millstone Hill are generated by the geomagnetic activity and by the global-scale 27-day wave present in the zonal mesosphere/lower thermosphere (MLT) neutral wind. The time delay between the 27-day oscillation in the zonal wind and that in the hmF2 is found to be 5–6 days, while between the 27-day oscillation in the geomagnetic activity and that in the hmF2 is found to be 0.8–1 day; (ii) the 16-day oscillation in the hmF2 observed during summer 1999 is probably generated by the global scale 16-day modulation of the semidiurnal tide observed in the MLT region during PSMOS campaign in June–August. We found that if the modulated semidiurnal tide mediates the planetary wave signature in the ionosphere, this planetary wave oscillation has to be best expressed in the amplitude and in the phase of the 12-h periodicity of the ionosphere; and (iii) the third type of disturbances studied is the quasi-2- day activity in the hmF2 that increases during geomagnetic disturbances. The strong pseudo diurnal periodicities generated during the geomagnetic storms can interact between each other and produce the quasi-2-day oscillations in the ionosphere. Key words. Ionosphere (ionosphere-atmosphere interactions; ionosphere-magnetoshpere interactions; wave propagation)