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This paper presents the regular variations of terrestrial magnetic field recorded by a new magnetic Observatory Medea, Algeria (geographic latitude: 36.85°N, geographic longitude: 2.93°E, geomagnetic latitude: 27.98°N, geomagnetic longitude: 77.7°E) during 2008–2011. The diurnal and seasonal variations of the solar quiet (Sq) variations are analyzed. The results show differences in the diurnal pattern of the northward-component Sq variation (SqX) at different seasons. The seasonal variation of SqX is similar in different years. The diurnal pattern of SqX from July through September cannot be explained by an equivalent current system that is symmetric about the noon time sector. The observations indicate that the major axis of the elliptic current system is tilted towards the equator in the morning hours during those months. The diurnal pattern of SqY indicates southward currents in the morning and northward currents in the afternoon, except during February–March 2009 when there is apparently no southward current during the morning. For the other months, the observations indicate that the maximum northward current intensity in the afternoon tends to be greater than the maximum southward current intensity in the morning. This is because of the UT variation of the Sq current system. That is, the pattern and strength of the Sq current system are different when SqY is measured in the morning around 8 UT and in the afternoon around 14 UT. The amplitude of these extreme varies linearly with the solar cycle. For the SqY component, the changes in the morning maximum have an annual variation while that of the afternoon minimum has a semi-annual variation. These variations are attributed to seasonal variations in the ionospheric E-region conductivity and atmospheric tidal winds. The field-aligned currents can also contribute to the seasonal variation of SqY. However, the two-dimensional approach used in this article does not allow us to quantitatively determine their influences. [ABSTRACT FROM AUTHOR]

The Maâdna structure is located approximately 400 km south of Algiers (33°19′ N, 4°19′ E) and emplaced in Upper-Cretaceous to Eocene limestones. Although accepted as an impact crater on the basis of alleged observations of shock-diagnostic features such as planar deformation features ( PDFs) in quartz grains, previous works were limited and further studies are desirable to ascertain the structure formation process and its age. For this purpose, the crater was investigated using a multidisciplinary approach including field observations, detailed cartography of the different geological and structural units, geophysical surveys, anisotropy of magnetic susceptibility, paleomagnetism, and petrography of the collected samples. We found that the magnetic and gravimetric profiles highlight a succession of positive and negative anomalies, ones that might indicate the occurrence of a causative material which is at least in part identical. Geophysical analysis and modeling suggest the presence of this material within the crater at a depth of about 100 m below the surface. Using soil magnetic susceptibility measurements, the shallowest magnetized zone in the central part of the crater is identified as a recently deposited material. Paleomagnetic and rock magnetic experiments combined with petrographic observations show that detrital hematite is the main magnetic carrier although often associated with magnetite. A primary magnetization is inferred from a stable remanence with both normal and reverse directions, carried by these two minerals. Although this is supposed to be a chemical remagnetization, its normal polarity nature is considered to be a Pliocene component, subsequent to the crater formation. The pole falls onto the Miocene-Pliocene part of the African Apparent Polar Wander Path ( APWP). Consequently, we estimate the formation of the Maâdna crater to have occurred during the time period extending from the Late Miocene to the Early Pliocene. Unfortunately, our field and laboratory investigations do not allow us to confirm an impact origin for the crater as neither shatter cones, nor shocked minerals, were found. A dissolved diapir with inverted relief is suggested as an alternative to the impact hypothesis, which can still be considered as plausible. Only a drilling may provide a definite answer. [ABSTRACT FROM AUTHOR]

We investigated how the noise in satellite magnetic data affects magnetic lithospheric field models derived from these data in the special case where this noise is correlated along satellite orbit tracks. For this we describe the satellite data noise as a perturbation magnetic field scaled independently for each orbit, where the scaling factor is a random variable, normally distributed with zero mean. Under this assumption, we have been able to derive a model for errors in lithospheric models generated by the correlated satellite data noise. Unless the perturbation field is known, estimating the noise in the lithospheric field model is a non-linear inverse problem. We therefore proposed an iterative post-processing technique to estimate both the lithospheric field model and its associated noise model. The technique has been successfully applied to derive a lithospheric field model from CHAMP satellite data up to spherical harmonic degree 120. The model is in agreement with other existing models. The technique can, in principle, be extended to all sorts of potential field data with "along-track" correlated errors. [ABSTRACT FROM AUTHOR]