South Atlantic Geomagnetic Anomaly Research Articles

On the Recurrence of the South Atlantic Geomagnetic Anomaly: Paleomagnetic Evidence from Late Pleistocene and Holocene Chilean Volcanic Rocks

On the Recurrence of the South Atlantic Geomagnetic Anomaly: Paleomagnetic Evidence from Late Pleistocene and Holocene Chilean Volcanic Rocks

Palaeomagnetic evidence for the persistence or recurrence of geomagnetic main field anomalies in the South Atlantic

We present a dataset of a full-vector palaeomagnetic study of Late Pleistocene lavas from the island Tristan da Cunha in the South Atlantic Ocean. The current day geomagnetic field intensity in this region is approximately 25 mu T, compared to an expected value of similar to 43 mu T;this phenomenon is known as the South Atlantic geomagnetic Anomaly (SAA). Geomagnetic field models extending back to the last 10 ka find no evidence for this being a persistent feature of the geomagnetic field, albeit, all models are constructed from data which is particularly sparse in the southern hemisphere. New Ar-40/Ar-39 incremental heating dating indicates the studied lavas from Tristan da Cunha extruded between 90 and 46 ka. Palaeointensity estimations of eight lava flows made using the Thellier method yield an average palaeointensity of 18 +/- 6 mu T and virtual axial dipole moment (VADM) of 3.1 +/- 1.2 x 10(22) Am-2. The lava flows demonstrate four time intervals comparable to the present day SAA, where the average VADM of the Tristan da Cunha lavas is weaker than the global VADM average. This suggests a persistent or recurring low intensity anomaly to the main geomagnetic field similar to the SAA existed in the South Atlantic between 46 and 90 ka. (C) 2016 Elsevier B.V. All rights reserved.

Geomagnetic forecasts driven by thermal wind dynamics in the Earth's core

There exists a fundamental as well as practical interest in being able to accurately forecast the future evolution of Earth's magnetic field at decadal to secular ranges. This work enables such forecasts by combining geomagnetic data with an Earth-like numerical model of a convection-driven fluid dynamo. The underlying data assimilation framework builds on recent progress in inverse geodynamo modelling, a method which estimates an internal dynamic structure for Earth's core from a snapshot of the magnetic field and its instantaneous rate of change at the surface, and takes advantage of linear relationships and long-range correlations between observed and hidden state variables. Here the method is further evolved into a single-epoch ensemble Kalman filter, in order to initialise at a given epoch an ensemble of states compatible with the observations and representative of the uncertainties in the estimation of hidden quantities. The ensemble dynamics, obtained by subsequent numerical integration of the prognostic model equations, are found to be governed by a thermal wind balance or equilibrium between buoyancy forces, the Coriolis force and the pressure gradient. The resulting core fluid flow pattern is a quasi-steady eccentric gyre organised in a column parallel to Earth's rotation axis, in equilibrium with a longitudinal hemispheric convective density anomaly pattern. The flow provides induction for the magnetic field, which also undergoes a realistic amount of diffusion. Predictions of the present magnetic field from data taken within the past century show that the ensemble has an average retaining good consistency with the true geomagnetic evolution and an acceptable spread well representative of prediction errors, up to at least a secular range. The predictability of the geodynamo thus appears to significantly exceed previous theoretical expectations based on the chaotic divergence of ensemble members. The assimilation generally outperforms the linear mathematical extrapolations from a 30-yr prediction range onwards, with a 40 per cent improvement in Earth-surface error at a secular range. The geomagnetic axial dipole decay observed over the past two centuries is predicted to continue at a similar pace in the next century, with a further loss of 1.1 ± 0.3 µT by year 2115. The focal (or minimum intensity) point of the South Atlantic geomagnetic anomaly is predicted to enter the South Pacific region in the next century, with the anomaly itself further deepening and widening. By year 2065, the minimum intensity is predicted to decrease by 1.46 ± 0.4 µT at the Earth surface and the focal point to move 12.8 ± 1.4 deg westwards with a slight northward component. This corresponds to a drift rate of 0.26 deg yr −1 , similar to the typical geomagnetic westward drift observed over the past four centuries. The same drift rate is also predicted until 2115 with a further (but more uncertain) intensity decrease

New geomagnetic field observations in the South Atlantic Anomaly region

Three new geomagnetic observatories have been established recently around the South Atlantic geomagnetic Anomaly by GeoForschungsZentrum Potsdam(GFZ), Germany, in collaboration with other institutions. In Bolivia, the collaboration is with Universidad Mayor de San Andres, LaPaz, while Hermanus Magnetic Observatory (HMO) in South Africa has assisted with a new observatory in Namibia. The third observatory was set up on the island of St. Helena with logistical support from the IDA seismological network, University of California at San Diego, USA. All these observatories are operated remotely with a minimum amount of building infrastructure and without permanent staff. People living nearby have been trained to carry out the required absolute measurements for a few hours per week. In this paper we report on our experiences, challenges and solutions in setting up nearly automated observatories in remote locations in order to obtain high quality geomagnetic data. These new data, complemented by annual repeat station surveys in southern Africa, will provide valuable geomagnetic field information on the South Atlantic Anomaly changes in this area of extremely rapid decrease of field intensity.

Unstable plasma irregular structures in the topside ionosphere and spread-F

The results of the Cosmos-900 satellite observ ations of plasma density inhomogeneities in the geomagnetic equator region and the longitudinal distributions of the equatorial spread-F, according to the Intercosmos-19 satellite data are presented. It is show n that the dependence of radiosignal propagation in the ionosphere on geophysical parameters is related to development of the electrostatic instability of the inhomo-geneous ionospheric plasma. The longitudinal dependence of the spread-F, can reflect the influence of the energetic sources, located outside the ionospheric layer that scatters a radio pulse, on the ionosphere. The manifestation of the longitudinal effect in the equatorial spread-F, in the Atlantic region can be explained by the influence of the cone instability on the plasma electrodynamics in the South Atlantic geomagnetic anomaly.

Analysis of the observation of particle detector inside ‘CBERS-1’ satellite under solar quiet conditions

Based on the knowledge and related theory of earth’s radiation belt, the data of energetic particles observed by detectors onboard ‘CBERS-1’ satellite at a solar synchronous orbit were analyzed. It is proved that the observational results are in agreement with the theoretical description of the radiation belt structures. Analysis of more than 3 years’ data showed clearly that under quiet solar conditions, at a height of about 800 km the energetic particles were mainly located in three regions: northern auroral belt (40°–80°), southern auroral belt (−40°–−80°) and South Atlantic Geomagnetic Anomaly Region (SAA). Actually, this is for the global distribution, at each longitude the latitudinal coverage is much narrower and particles are along the geomagnetic latitude of about ±60°. The species of particles in different regions and their counting rates are different. In SAA, usually both electrons and protons are observed, which should come from inner radiation belt; in polar regions only energetic electrons are observed under the quiet condition, which belongs to the outer radiation belt. The distribution of outer radiation electrons is asymmetrical for longitudes as well as northern and southern polar regions. These asymmetries can be explained with the reflecting altitudes of the mirror points of charged particles at the same L shell.

Whistler observations of the plasmasphere/plasmapause from stations of the British Antarctic Survey

VLF whistler observations from Eights Station, Antarctica were crucial to the discovery and exploration of the plasmapause (“Carpenter's knee”) by Don Carpenter in the mid-1960s. The Weddell Sea sector of Antarctica is particularly well suited to such work because of the high whistler rate (conjugate to thunderstorm regions), proximity to the ground footprint of the average plasmapause, low electromagnetic noise levels (far from power lines, etc.), low ionospheric absorption (in winter), and wave amplification due to the South Atlantic Geomagnetic Anomaly. VLF recordings have been made at Halley, Antarctica, (76°S,27°W, L∼4.3) since 1967; the station is located on a similar L-shell to Eights and its successor Siple, but eastward in longitude by about 2 h in magnetic local time. In this paper, we review some of the research on the structure and dynamics of the plasmasphere/plasmapause which has been based on whistler data from Halley. In particular, the use of Halley and Siple as a station pair corotating with the Earth through the Sun–Earth frame has enabled the complex dynamics of the duskside bulge region to be better understood. For example, features consistent with narrow dense sunward-pointing plasma tails, have been delineated. Whistler data from Halley have also provided much information on fine structure within the plasmasphere. The paper discusses some important results of inner plasmaspheric probing using fixed-frequency (∼20 kHz) whistler data from the lower latitude Faraday station (65° S, 64° W,L∼2.3) , including annual density variations and magnetic storm effects, and concludes by indicating some directions for the future.

Electrostatic Instability and Plasma Irregularities in the Topside Ionosphere above the South-Atlantic Geomagnetic Anomaly

The data of measuring the plasma density in the topside ionosphere for the South-Atlantic geomagnetic anomaly region are presented. It is shown that irregular plasma structures with a wide spectrum of irregularity scale (including large-scale structures with a dimension of order of some hundred kilometers) can be generated in the fields of electrostatic turbulence in inhomogeneous plasma.

Thermospheric and mesospheric temperatures during geomagnetic storms at 23°S

Night-time thermospheric temperatures, T63o, and mesospheric rotational temperatures, T(OH) and T(O 2), have been measured at Cachoeira Paulista (23°S, 45°W, 16°S dip latitude), located in both the equatorial ionospheric anomaly and the South Atlantic Geomagnetic Anomaly, with a Fabry-Perot interferometer and a multi-channel tilting filter-type photometer, respectively. The thermospheric temperatures are obtained from the Doppler line broadening of the OI 630.0 nm emission and the mesospheric rotational temperatures from the OH(9,4) and O 2A(0,1) band emissions. Measurements made during three geomagnetic storms showed that the nocturnal mean values of T 630 during the recovery phase of the storms were lower than those observed during quiet time and from model predictions. Also, the nocturnal mean value of the T 630 soon after the SSC event on 27 June 1992 was higher than the quiet time and model predictions. The observed mesospheric nocturnal mean rotational temperatures, T(O 2) and T(O 2), were unaffected by the storms. A comparison of the night-time observed temperatures T 630, T(OH) and T(O 2) with those calculated using the MSIS-86 model is also presented.

Riometer absorption events at SANAE, L=4.0

The absorption of cosmic radio noise passing through the ionosphere may be described as a function of radio wave frequencyA(f e ) αf e -n , with n ≈ 2.0 for spatially uniform precipitation of electrons and n < 2.0 for spatially nonuniform precipitation. Using multifrequency riometer recordings at SANAE, the following observations are reviewed: (1) The frequency distribution of the power index, n, obtained from 4 min averaged absorptions during 1983, shows a most probable value around n ≈ 1.5, indicating that mostly energetic electrons are precipitated spatially structured onto the upper atmosphere, as in optical aurora. (2) Multifrequency riometer recordings suggest that field-aligned ionospheric irregularities have scattered additional cosmic radio waves from the central region of the Galaxy into the fields of views of the riometer antennae during an auroral absorption event in the early morning hours of 27 July, 1982. With the power reflectivity by ionospheric irregularities inversely proportional to the fourth power of radio wave frequency, as required by the Bragg condition, an estimated 70% increase in the 20 MHz radio flux at 01:22 UT, at the strong absorption peak, can explain the strongly reduced absorption observed in 20 MHz relative to 30 and 51.4 MHz. (3) Gradual increases in absorptions observed at all three riometer frequencies from onset at 11:50 UT of the largest solar proton ground level enhancement on 29 September, 1989, until 18:00 UT, suggest diffusion of the much more intense low energy protons from the polar cap to the L=4.0 geomagnetic field shell and subsequent precipitation at SANAE due to the South Atlantic Geomagnetic Anomaly. (4) The flux of electron energy deposited per second at SANAE is closely related to geomagnetic activity, but has a lower maximum during the years 1971 and 1980 of solar polar magnetic reversals than in the years 1976 and 1986/87 of minimum solar activity. (5) A significant correlation has been found between the arrival of single-hop whistlers and 30 MHz riometer absorption events, using point statistics. The maximum absorption at 30 MHz was ∼0.04 dB with a delay of 3 ± 2 s relative to the whistler.

Night-time d-region electron density variations observed in the South Atlantic geomagnetic anomaly in association with solar-proton events

Very low frequency signals (VLF) received in Atibaia (Brazil) during the occurrence of the two major solar-proton events of August 1972 and July 1974 were analysed. Phase and amplitude changes observed at night were introduced in the model given by Wait and Spies (1964) properly adapted to the ionospheric conditions of the South Atlantic Geomagnetic Anomaly (SAGA) region. Electron density disturbed profiles for “ L” shell intervals 1.15< L<2.0 and 1.08< L<1.15 were deduced. The results highlight the great amount of ionization produced at low “ L” values due to the precipitation of energetic particles in the SAGA, mainly at night. Some discrepancies among the electron density profiles proposed by different authors are discussed.

Trimpi events on low latitude paths: An investigation of gyroresonance interactions at low L-values

We report on Trimpi events observed at Durban ( L = 1.69, 29°53′ S, 31°00′ E) and investigate the efficacy of gyroresonance scattering in precipitating electrons into the atmosphere at low L (<2). The rate of occurrence of Trimpis at Durban is less than one per day. Our observations include a number of daytime events on OMEGA signals from La Reunion. Using the full relativistic equations of motion, a test particle simulation is employed to find the region in parameter space where large pitch angle scattering occurs. We find that at low L the conditions for pitch angle scattering are less favourable than at higher L ( L ∼ 4). Resonant electrons have high (relativistic) energies, interaction times are of the order of milliseconds ( T i ∼ 5 ms) and large wave amplitudes ( B w ∼ 200 pT) are required at whistler frequencies to produce pitch angle changes of greater than 1°. Large pitch angle scattering is needed near Durban since particles near the loss cone will have been lost in the South Atlantic Geomagnetic Anomaly. We note that the radio frequencies transmitted into the magnetosphere from lightning are too low to give effective electron scattering at low L. We suggest an explanation for the low rate of occurrence of Trimpis at Durban.

D-region electron density diurnal changes observed in association with the PCA event of July 1974

Well defined diurnal phase and amplitude deviations of very low frequency radio signals propagation were detected during the solar proton event that occurred from 3 to 8 July 1974. Phase advances observed on signals transmitted from GBR (U.K.), received at Atibaia (Brazil) and at Inubo Radio Observatory (Japan), were compared with simultaneous NWC (Australia) transmissions received at Atibaia, and at Syowa Station (Antarctica). Emphasis was given to the propagation paths NWC-Atibaia and NWC-Syowa, because both propagate completely in the southern hemisphere, crossing regions where the Earth's magnetic field behaves anomalously. The comparison allowed the determination of parameters typical of the D-region at a given height in the lower ionosphere, for geomagnetic regions defined through the McLlwain parameter( L). An exponential model was adopted to fit the vertical distribution of the diurnal electron density. The experimental results showed a delayed contribution to the total ionization observed which was attributed to a slow precipitation of energetic particles in the South Atlantic Geomagnetic Anomaly region.

Diurnal lower ionosphere electron density variations observed in the South Atlantic Geomagnetic Anomaly and sub-Antarctic region during the occurrence of the major PCA event of 4 August 1972

VLF phase and amplitude signals simultaneously received in Atibaia and Curitiba, both in Brazil, were analysed during the occurrence of the major proton flare event of 4 August 1972. Phase advances recorded in Stockholm (Sweden) and Inubo (Japan) were also included for comparison purposes. Particular attention was devoted to the NWC (Australia)-Atibaia (Brazil) propagation-path which passes through disturbed regions (geomagnetically), namely the sub-Antarctic region and the South Atlantic Geomagnetic Anomaly (SAGA), and which propagates completely in the southern hemisphere. Following the current mechanisms that allow charged particles to precipitate at low altitudes in the ionosphere ( Dregion), four regions were defined based on the most significant ‘ L’ value involved. By means of successive comparisons with low and mid latitude propagation paths, some parameters typical of the lower ionosphere were deduced for the southern hemisphere in association with PCA events detected in the polar caps. Fitting exponential models to the data analysed, daytime electron density profiles were obtained for different ‘ L’ values throughout the disturbed period.

The longitudinal dependence of whistler and chorus characteristics observed on the ground near L = 4

Whistler activity at L ≃ 4 is known to be a function of longitude, peaking in the Weddell Sea sector of Antarctica; a combination of source and propagation factors, the latter possibly partly associated with the South Atlantic geomagnetic anomaly, is believed to be responsible. There is evidence, for example from satellite surveys, that chorus and hiss activity may also be longitude dependent. To investigate this further, we have compared VLF data from four L ≃ 4 Antarctic stations from a 2‐day period in June 1982. Siple, Halley, and Sanae form a closely spaced (∼20°–0° geomagnetic longitude) triplet, while Kerguelen is ∼120° (geomagnetic) to the east, on the opposite side of the anomaly. To a large extent there was a repeatable diurnal variation in activity at all stations on the two days. Events observed at Siple tended to be similar to those observed ∼9 hours earlier (the same MLT) at Kerguelen on the same day. There was a very marked drop‐off in both whistler and VLF emission activity between Siple and Halley on the one hand and Sanae on the other. The reason for this is not clear; it may be either a source effect such as the lower occurrence of lightning over eastern North America compared to the adjacent Atlantic Ocean, or else a wave‐particle interaction effect whereby the conditions for wave growth or amplification are more favorable, or substorm particle injections penetrate the magnetosphere more deeply, at the longitude of Siple than further east. Comparison of the spectral forms of whistler mode activity at neighboring stations suggests that wave generation occurs simultaneously over relatively wide longitude (or local time) sectors (≳30° or 2 hours). Individual interaction regions are smaller than this, ≲5° in longitude, comparable with the previously inferred sizes of whistler ducts. Data sets with longer time coverage and better spatial resolution are required to answer some of the problems raised by this limited study.

Comment on “Highly relativistic magnetospheric electrons: A role in coupling to the middle atmosphere?”

It is argued that Baker et al. (1987) may have underestimated the electron precipitation rate in the southern hemisphere in the region extending west to about 90 deg in longitude from the southern extension of the center of the South Atlantic geomagnetic anomaly (SAGA). In a reply, it is noted that Baker et al. restricted their study to the case of uniform precipitation in latitude and longitude throughout the outer-zone area, thus entailing a great simplification of the real-world situation. Comments are made on the effects of the SAGA on electron precipitation.

Temperature structure of plasma bubbles in the low latitude ionosphere around 600 km altitude

The electron temperature inside plasma bubbles at a height of 600 km was first measured by means of Japan's seventh scientific satellite Hinotori which is an equator orbiting satellite with an inclination of 31°. During the period between June 1981 and February 1982, 724 plasma bubbles were detected and studied. The electron temperature inside the plasma bubbles is either higher or lower than that outside and can also be equal to the electron temperature outside, depending on the local time and on the place where the data were taken. Heating of electrons inside plasma bubbles sometimes occurs over the South Atlantic geomagnetic anomaly and over the Hawaiian anomaly where particle precipitation can frequently be observed.

Night-time electron density of the lower ionosphere in the South Atlantic Geomagnetic Anomaly region obtained with a LF/VLF oblique ionosonde

Studies of the reflectivity of the night-time lower ionosphere in the region of the South Atlantic Geomagnetic Anomaly have been carried out for the period August 1980 to July 1981, using data collected by an oblique ionosonde at low/very low frequency, in the southern part of Brazil, near the centre of the Anomaly. From these studies monthly average behaviour of heights and reflection coefficients of the lower ionosphere between 15 and 60 kHz have been deduced. Assuming an exponential model of the electron density distribution in the lower ionosphere the appropriate numerical parameters were calculated using a trial-and-error approach with ‘full wave’ calculations and iterative computational techniques. For the period considered three sets of parameters could satisfactorily represent the lower ionosphere in the anomalous region and in the period analysed: one valid from August to November 1980, one valid from December 1980 to March 1981 and the other valid from May to July 1981. April 1981 seemed to present anomalous behaviour.

Electron precipitation near L=4: Longitudinal variation

Observations of penetrating electron precipitation at two L ∼ 4 stations in the southern hemisphere, Kerguelen and Siple Station, Antarctica, are summarized. The nearly complete absence of precipitation at Kerguelen and its frequent occurrence at Siple Station is explained in terms of their location with respect to the center of the South Atlantic geomagnetic anomaly at L ∼ 4. Conclusions which result from this explanation are discussed along with questions which remain concerning penetrating electron precipitation.

Observations in the South Atlantic geomagnetic anomaly with intercosmos-Bulgaria-1300 during a geomagnetic storm

The region of South Atlantic Geomagnetic Anomaly (SAGA) was investigated by the Intercosmos-Bulgaria-1300 satellite, launched on August 7, 1981. On the basis of data obtained from 15 orbits during increased geomagnetic activity in August 1981, a map of the Anomaly was elaborated. Two centres of activity were identified. By means of the EMO-5 electrophotometer on board the Intercosmos-Bulgaria-1300 satellite, the atmosphere glow in lines λ 5577 Å, λ 6300 Å and λ 4278 Å was studied.