Induction arrow spatial and temporal variations

Abstract

The induction arrow C is a characteristic of the variable geomagnetic field at one observation point (x, y) for a certain period T of geomagnetic variations B. It visually describes the magnitude and direction of the deviation of the geomagnetic field vector from the horizontal plane. The induction arrow is nonzero only when the vertical component Bz appears. Bz appears in three cases: 1) electrical conductivity’s horizontal gradients under the surface of the globe; 2) incomplete compensation of the vertical component of the primary field of the ionosphere-magnetosphere source by the secondary field induced in the horizontally layered Earth. The degree of compensation systematically changes during the day, year..., leading to temporal variations in the induction arrow (source effect); 3) the appearance of not-removed noise or lithosphere emission containing Bz. This article presents the spatial and temporal variations of the induction arrows. Real (in-phase) and imaginary (out-of-phase) induction arrows (vertical response functions or tippers) were obtained for every day with proper observations from 1991 to 2014 years for five intervals of periods: 150—300 s, 300—600 s, 600—1200 s, 1200—2400 s, 2400—3600 s, calculated from three components of geomagnetic field recorded at 137 observatories of the global network INTERMAGNET. To reduce the scatter, the daily values were recalculated to monthly values. Such global material from +87° to –88° of geomagnetic latitude was obtained for the first time and its analysis yielded new scientific results. The annual variations (with a period one year) are visible at about 2/3 of the observatories (at the other observatories, they are below the background level of shorter period variations and/or noise). Its amplitude strongly depends on the geomagnetic latitude and sometimes reaches such a high value as 0.4—0.5 (peak-to-peak) at high (>65°) latitudes and varies within 0.01—0.15 at middle and low latitudes. Previous studies at middle latitudes discovered that the annual variations in the northern component Au is positive everywhere (maximum in June, minimum in December) and proposed, as a global source model for the annual variations explanation, the ring current at the height 3―6 Earth’s radii. We discovered that at high latitudes, Au is usually negative. We propose to explain all the observed features of the induction arrow’s annual variations by variations of the ionosphere currents in the aurora zone. 11-year variations are found at ≈30 % of observatories located at all latitudes but more frequently in aurora zones. At a few observatories, trends (monotonous changes in arrows) were found. The largest trend of magnitude 0.2 for all periods was found in southern Greenland, where glaciers have been melting rapidly over the past 30 years, which makes it possible to associate both trends with global warming. In a few aurora observatories of North America trends of magnitude ≈0.1 were noticed. In these same years, the North Magnetic Pole unusually rapidly migrated for 1100 km in 23 years. It can be assumed that these trends are related to changes in the aurora oval position. At geomagnetic latitudes from –78° to +78°, the main harmonic (24 hours=86400 s) of daily variations of the geomagnetic field is always accompanied by at least one higher order harmonic 24/n (n=1÷7). At higher latitudes >78° of both hemispheres only the fundamental 24-hour harmonic is visible. Induction arrow temporal variations create difficulties in determining its constant component used to study the electrical conductivity of the Earth’s crust and upper mantle but can be very useful for geodynamic processes study