Polar cap and auroral oval areas are presented for the period 1900–2015 using a model of the magnetosphere and the International Geomagnetic Reference Field Version 12 (IGRF‐12) model for the geomagnetic field. The effect of the main field long‐term changes is addressed considering steady interplanetary conditions. Until ~1940 both northern and southern polar cap areas increased with time, consistent with the geomagnetic dipole moment decrease during this period. Thereafter, while the southern polar cap area continued to increase steadily, the northern polar cap area has been decreasing rapidly. This hemispherical dichotomy is related to asymmetry in the core field and its secular variation. In the southern polar region the surface field intensity has been decreasing throughout the entire period, whereas in the northern polar region an intensity decrease turned to an increase from 1940 to present day. This recent surface intensity increase stems from intensification of the flux patch below Siberia and cessation of weakening of the flux patch below North America, both on the core‐mantle boundary. The surprising temporal decrease in the area of the northern polar cap has important implications for space weather and communication systems. While the decreasing dipole intensity allows more particles to penetrate the atmosphere, a shrinking northern auroral oval implies an increase in particle precipitation density, resulting in more severe damage to airplanes and ships positioning systems, spacecraft electronic systems and airline passengers when passing above this region. Further study is needed to observationally establish that the northern auroral oval, as defined by optical occurrences of aurora, is indeed shrinking.

The nonlinear behavior of acoustic waves and their dissipation in the upper atmosphere is studied on the example of infrasound waves generated by vertical motion of the ground surface during the Mw 8.3 earthquake that occurred about 46 km from Illapel, Chile on 16 September 2015. To conserve energy, the amplitude of infrasound waves initially increased as the waves propagated upward to the rarefied air. When the velocities of air particles became comparable with the local sound speed, the nonlinear effects started to play an important role. Consequently, the shape of waveform changed significantly with increasing height, and the original wave packet transformed to the “N‐shaped” pulse resembling a shock wave. A unique observation by the continuous Doppler sounder at the altitude of about 195 km is in good agreement with full wave numerical simulation that uses as boundary condition the measured vertical motion of the ground surface.

Changes in the Earth's magnetic field can deeply modify the polar caps and auroral zones, which are the regions of most frequent precipitation of energetic particles. The present field is characterized by a dominant dipole plus weaker multipolar components. The field varies greatly in time, with the most drastic changes being polarity reversals that take place on average every ∼200 000 yr. During a polarity transition the field magnitude may diminish to about 10 per cent of its value prior to the reversal due to a decreasing dipolar component and by becoming mostly multipolar in nature. Polar caps depend on the geomagnetic field configuration so changes in their morphology are expected as a consequence of the variation and reversal of this field. We model polar caps' location by considering a superposition of the internal geomagnetic field and a uniform external field and then following the open field lines to the Earth's surface. Polar caps' location and shape for different magnetic field reversal scenarios are analysed in this work. Two polar caps near the present dipole axis intersection with the Earth's surface prevail for a dipole decrease to a certain extent, below which the southern hemisphere polar cap moves to mid-latitudes. An axial dipole collapse gives a pair of polar caps both at mid-latitudes of the southern hemisphere, while in a dipole rotation scenario the polar caps reside at the equator. If reversals occur due to an energy cascade from the dipole to higher degrees, more than two polar caps may appear. In our energy cascade scenario, four polar caps at various latitudes of both hemispheres prevail. These results indicate that during reversals auroral zones may reach mid-and low-latitude regions, and the atmosphere may become more vulnerable to the direct effect of energetic particle precipitation. This vulnerability is particularly striking at the southern hemisphere where reversed flux patches appear on the core-mantle boundary and weak intensity characterizes the present field at the Earth's surface.