The present paper contains the archeomagnetic results for the inclination of ancient magnetic field in Bulgaria. An amplitude of nearly 14° and a period between 700-900 years for the secular variation of the inclination are obtained.
The methods and apparatus used are described. Two examples of application of Thellier's method are given.

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The use of partial alternating field demagnetization and, to a lesser extent, partial thermal demagnetization has been found useful as a means of removing secondary components of magnetization from rocks used in certain methods of determining paleointensities. During the course of this investigation, which was carried out on lavas erupted during historic times, it was also learned that a partial remanence characterized by relatively low coercivity possessed blocking temperatures throughout the blocking temperature spectrum. On the other hand, a partial remanence characterized by low blocking temperatures possessed only low coercivity.

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Continental or oceanic basaltic rocks which contain optically homogeneous titanomagnetites are defined as magnetic class I rocks and exhibit thermochemical remanent magnetization (TCRM) behavior when heated. Heating of magnetic class I rocks results in an increase in:
(a) Curie point (T_c)
(b) susceptibility (X)
(c) coercive force (H_c)
(d) remanent magnetization (J_r)
(e) demagnetization stability
(f) saturation magnetization (J_s).
Though the titanomagnetites are optically homogeneous, they are chemically heterogeneous, and may contain textures which cannot be optically resolved. The extent of optically unresolved textures and degree of chemical heterogeneity will depend on the previous history of the sample. The development of iron-oxide textures and correlative magnetization behavior can be understood through experimental studies of magnetic class I rocks. The magnetic characteristics of rocks which exhibit the TCRM behavior are presented and discussed. Three processes can be operative in the development of TCRM in basaltic rocks:
(a) thermal process
(b) thermochemical process
(c) oxidation process.

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Consideration is given to changes in the extent of shell splitting due to the presence of superimposed electric fields. In displacing the spiralling-mirroring domain of trapped particles, such superimposed fields constitute a pitch angle dispersion process. This produces a corresponding change in particle radial displacement; and so is accompanied by a modification in the overall shell splitting.

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Various observed characteristics of the six types of stress-affected remanent magnetization of igneous rocks are theoretically derived for a model rock in which a large number of ferrimagnetic minerals having an isotropic magnetostriction are distributed at random.
For small magnetic fields (H) and small uniaxial compressions (P) irreversible displacements of 90° domain walls alone are subjected to a uniaxial compression in the acquisition process of remanent magnetization. Hence Rayleigh's law of the magnetic hysteresis loop is adopted in examining the effect of H and P on the displacement of 90° walls.
Results of calculations based on these assumptions give theoretical curves of J_R(P₊P₀H₊H₀) vs P/H, J_R(H₊H₀P₊P₀) vs P/H, J_R(H₊P₊P₀H₀) vs H/P, J_R(P₊H₊P₀H₀) vs H/P, J_R(H₊P₊P₀H₀) vs H/P and J_R(P₊H₊H₀P₀) vs H/P. These theoretical curves reasonably well fit the observed curves, having all observed characteristics of respective types of stress-affected remanent magnetization. The effect of P on the remanent magnetization is a little smaller in the case of P⊥H than in the case of P//H in both the experiment and the theory. However, numerical disagreement between experimental and theoretical results are still noticed in regard to details of the phenomena.

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Analogue model measurements of electromagnetic variations over an anisotropic conducting cube embedded in a conducting flat model earth in the near field of an oscillating sheet current, line current, vertical magnetic dipole, and a horizontal magnetic dipole are presented. The E_x/H_y and H_z/H_y ratios are given as a function of frequency for several depths and orientations of the anisotropic cube and horizontal distances of the cube from the overhead field sources.

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Thermomagnetic analysis along with conventional reflection microscopy and magnetic powder analysis can be used to evaluate the alteration state of the magnetic fraction in a rock. The alteration state is positive or negative depending on the sense of change in a magnetic parameter (±ΔM) observed during thermomagnetic analysis. Grain size, sample preparation, experimental atmosphere, and heating rate are factors which must be considered in interpreting the results of thermomagnetic analysis. The effects of the experimental factors are more pronounced in basic than in acidic rocks because of the greater amount of titanium and other cations in the iron oxides. The effects of experimental factors are likewise more pronounced in extrusive than in intrusive basic rocks because of the more variable cooling conditions and the smaller more chemically heterogeneous iron oxides.

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A brief discussion is given of the thermal and alternating field demagnetization methods in estimating paleomagnetic field intensities. It appears that reliable paleomagnetic intensity estimates may be obtained by alternating field demagnetization if (1) the results can be successfully extrapolated towards the highest microcoercivities of the NRM and of the TRM, and (2) the saturation magnetization has not changed appreciably after heating the sample to the Curie temperature and subsequent cooling. This would imply that the application of the “complete alternating field method” is not necessarily less laborious than that of the thermal method for which thermomagnetic analyses probably are not essential.

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The apparent resonance frequencies of the earth-ionosphere cavity as determined from the location of the maxima of the electric or magnetic field components in the amplitude-frequency plane depend on the source-observer separation. Here the variations of these apparent resonance frequencies with source range are presented for a realistic propagation constant (or ionospheric) model and for a single dipole source. The data presented have an application to the interpretation of experimentally determined diurnal variations of the apparent resonator mode frequencies.

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A new method of spherical harmonic analysis which utilizes both scalar and vector observations is presented and applied to two distinct samples of data obtained between 1900 and 1965. The oblateness of the earth is taken into account and time derivatives up to 2nd order are determined along with the main field. One of the resulting models has been employed in the officially adopted International Geomagnetic Reference Field.

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