[1] Geomagnetic field data with high time resolution (typically 1 s) have recently become more commonly acquired by ground stations. Such high time resolution data enable identifying Pi2 pulsations which have periods of 40-150 s and irregular (damped) waveforms. It is well-known that pulsations of this type are clearly observed at mid-and low-latitude ground stations on the nightside at substorm onset. Therefore, with 1-s data from multiple stations distributed in longitude around the Earth's circumference, substorm onset can be regularly monitored. In the present study we propose a new substorm index, the Wp index (Wave and planetary), which reflects Pi2 wave power at low-latitude, using geomagnetic field data from 11 ground stations. We compare the Wp index with the AE and ASY indices as well as the electron flux and magnetic field data at geosynchronous altitudes for 11 March 2010. We find that significant enhancements of the Wp index mostly coincide with those of the other data. Thus the Wp index can be considered a good indicator of substorm onset. The Wp index, other geomagnetic indices, and geosynchronous satellite data are plotted in a stack for quick and easy search of substorm onset. The stack plots and digital data of the Wp index are available at the Web site (http://s-cubed.info) for public use. These products would be useful to investigate and understand space weather events, because substorms cause injection of intense fluxes of energetic electrons into the inner magnetosphere and potentially have deleterious impacts on satellites by inducing surface charging.

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A magnetic amphidrome is defined as a place where changes of the magnetic field over time, as measured by a total‐field magnetometer, are reduced to the point of being negligible. The reduction is caused by destructive interference between the vertical and horizontal components of the time‐varying field. At an ideal amphidrome, variations with time are suppressed completely and the total‐magnetic‐field magnitude is steady. Such a phenomenon may be expected to depend on the frequency content of the time variations in the vertical and horizontal components. The subject is treated first in terms of the quiet daily variation, [Formula: see text], which is studied on a global basis. It is seen that there are magnetic latitude bands, north and south of the equator, where the quiet daily variation is minimal. These zones are called the “diurnal doldrums.” In addition to this global pattern, the magnetic daily variation is modified by Earth’s conductivity structure locally, and [Formula: see text] amphidrome behavior may be aided or obstructed locally. The second part of the paper treats the magnetic “rapid fluctuations.” A simple condition for an amphidrome is that the direction of Earth’s main magnetic field be parallel to the normal of the “preferred plane” in which the small vector changes of rapid magnetic fluctuations tend to lie. Examples are given of observed data for Australia, and a numerical model of Australian electrical‐conductivity structure is used to predict amphidromes regionally. Formal treatment of the preferred‐plane concept involves taking the out‐of‐phase (or quadrature) part of the induction phenomenon into account as well, and a parameter is proposed which may be contoured to show an amphidrome minimum. The phenomena of amphidromes are fundamental for magnetic mapping procedures. Near amphidromes, the fluctuating magnetic fields of Earth are suppressed, and their capacity for introducing error into magnetic survey data is reduced correspondingly. The case of a “complete” or “ultimate” amphidrome, applying to both diurnal and rapid fluctuations, may be expected to be rare. None is known at present. The reason is that the diurnal doldrums, favoring [Formula: see text] amphidromes, occur at low latitudes. Rapid‐fluctuation amphidromes, however, are more likely to occur at mid‐ to high latitudes.

S U M M A R YOcean swells have a magnetic signal, caused by the motional induction of sea water moving in the steady main magnetic field of Earth. To check the character of such signals at the sea surface, a magnetometer has been set free from a ship to float unrestricted on the surface of the ocean for periods of several days. The path of the floating magnetometer was tracked by satellite; this procedure enabled also the eventual recovery of the magnetometer by the ship.Superimposed upon a background of slow change of magnetic field, as the magnetometer drifted across different patterns of crustal magnetization, are high-frequency signals generated by the strong ocean swell present at the time. These wave-generated signals are typically up to 5 nT trough-to-peak, consistent with theory for their generation by ocean swells several metres trough-to-peak in height.The power spectra of the magnetic signals show a consistent peak at period 13 s, appropriate for the known characteristics of ocean swell in the area. The power spectra then exhibit a strong (−7 power) fall-off as period decreases below 13 s. This strong fall-off is consistent with oceanographic observations of the spectra of surface swell, combined with motional induction theory.

Abstract. Most magnetic mapping exercises involve measuring the amplitude, or total-field component, of Earth's magnetic field. Removing the time-varying part of the field is a task of data reduction, and the most common time variation is the quiet daily variation, $q. It is thus valuable to have for reference type curves of the quiet daily variation in the total field. To meet this need, global data obtained during the 1965 International Year of the Quiet Sun have been used to derive type curves describing the $q variation of the total magnetic field, in addition to the traditional field components. As for the traditional components, the total-field curves show significant seasonal and latitudinal variability in amplitude and phase. The effect of the equatorial electrojet is clearly evident. In both hemispheres of the globe there are bands of reduced amplitude in total-field signal between the equator and the path of the $q focus. These bands, here termed the total-field "doldrums", persist throughout the year.