Earth's magnetic field

About: Earth's magnetic field is a research topic. Over the lifetime, 20360 publications have been published within this topic receiving 446747 citations. The topic is also known as: magnetic field of Earth & geomagnetic field.

The 13.5‐day periodicity in the Sun, solar wind, and geomagnetic activity: The last three solar cycles

Abstract: We make a detailed analysis of the 13.5-day periodicity of the solar chromosphere, the near-Earth solar wind, interplanetary magnetic field and geomagnetic activity during the last three solar cycles. The 13.5-day periodicity is a real quasi-periodicity whose amplitude varies sizably with time, attaining occasionally values larger than, for example, the amplitude of the 27-day periodicity. In case of heliospheric and geomagnetic variables, intervals of large 13.5-day periodicity are due to the occurrence at 1 AU of two high-speed streams per solar rotation. According to the tilted solar dipole model, such two-stream structures appear if the heliospheric current sheet is sufficiently narrow and tilted. We show that, during the main two-stream structures, the interplanetary magnetic field (IMF) indeed had a persistent two-sector structure, and the heliosheet was sizably tilted. Multiple IMF sector structure is thus excluded as the main cause for 13.5-day periodicity in solar wind and geomagnetic activity. We determine the exact time and phase (solar longitude) of all intervals of significant 13.5-day periodicity during the last three solar cycles. We find that even the longest intervals of two-stream structure (up to 2 years) consist of separate activations. Each of the main activations of the 13.5-day (as well as 27-day) periodicity has a nearly equal length of a few (about 4) solar rotations only. This gives new, interesting information about the solar dynamics related to the development of the dipole tilt. Using the phase of the main 13.5-day activations, we could determine the longitudinal position of the solar dipole tilt for all major activations. We note that this position can abruptly change by even 90 deg between two successive 13.5-day activations. For each of the three solar cycles studied, the largest two-stream structures were found in the late declining phase of the cycle. On the other hand, the main activations of the 13.5-day periodicity of solar variables, which are due to two active solar longitudes approximately 180° apart, tend to occur around solar maxima.

Major geomagnetic storms (Dst ≤ −100 nT) generated by corotating interaction regions

Abstract: [1] Seventy-nine major geomagnetic storms (minimum Dst ≤ −100 nT) observed in 1996 to 2004 were the focus of a “Living with a Star” Coordinated Data Analysis Workshop (CDAW) in March 2005. In nine cases, the storm driver appears to have been purely a corotating interaction region (CIR) without any contribution from coronal mass ejection-related material (interplanetary coronal mass ejections (ICMEs)). These storms were generated by structures within CIRs located both before and/or after the stream interface that included persistently southward magnetic fields for intervals of several hours. We compare their geomagnetic effects with those of 159 CIRs observed during 1996–2005. The major storms form the extreme tail of a continuous distribution of CIR geoeffectiveness which peaks at Dst ∼ −40 nT but is subject to a prominent seasonal variation of ∼40 nT which is ordered by the spring and fall equinoxes and the solar wind magnetic field direction toward or away from the Sun. The O'Brien and McPherron (2000) equations, which estimate Dst by integrating the incident solar wind electric field and incorporating a ring current loss term, largely account for the variation in storm size. They tend to underestimate the size of the larger CIR-associated storms by Dst ∼ 20 nT. This suggests that injection into the ring current may be more efficient than expected in such storms. Four of the nine major storms in 1996–2004 occurred during a period of less than three solar rotations in September to November 2002, also the time of maximum mean IMF and solar magnetic field intensity during the current solar cycle. The maximum CIR-storm strength found in our sample of events, plus additional 23 probable CIR-associated Dst ≤ −100 nT storms in 1972–1995, is (Dst = −161 nT). This is consistent with the maximum storm strength (Dst ∼ −180 nT) expected from the O'Brien and McPherron equations for the typical range of solar wind electric fields associated with CIRs. This suggests that CIRs alone are unlikely to generate geomagnetic storms that exceed these levels.

The Wind magnetic cloud and events of October 18–20, 1995: Interplanetary properties and as triggers for geomagnetic activity

Abstract: Late on October 18, 1995, a magnetic cloud arrived at the Wind spacecraft ≈ 175 RE upstream of the Earth. The cloud had an intense interplanetary magnetic field that varied slowly in direction, from being strongly southward to strongly northward during its ≈ 30 hours duration, and a low proton temperature throughout. From a linear force free field model the cloud was shown to have a flux rope magnetic field line geometry, an estimated diameter of about 0.27 AU, and an axis that was aligned with the Y axis(GSE) within about 25°. A corotating stream, in which large amplitude Alfven waves of about 0.5 hour period were observed, was overtaking the cloud and intensifying the fields in the rear of the cloud. The prolonged southward magnetic field observed in the early part of the cloud produced a geomagnetic storm of Kp = 7 and considerable auroral activity late on October 18. About 8 hours in front of the cloud an interplanetary shock occurred. About three-fourths the way into the cloud another apparent interplanetary shock was observed. It had an unusual propagation direction, differing by only 21° from alignment with the cloud axis. It may have been the result of the interaction with the postcloud stream, compressing the cloud, or was possibly due to an independent solar event. It is shown that the front and rear boundaries of the cloud and the upstream driven shock had surface normals in good agreement with the cloud axis in the ecliptic plane. The integrated Poynting flux into the magnetosphere, which correlated well with geomagnetic indices, jumped abruptly to a high value upon entry into the magnetic cloud, slowly decreased to zero near its middle, and again reached substantial but sporadic values in the cloud-stream interface region. This report aims to support a variety of ISTP studies ranging from the solar origins of these events to resulting magnetospheric responses.

Simulation studies of ionospheric electric fields and currents in relation to field‐aligned currents, 2. Substorms

Abstract: Computer simulation studies of the electric fields and currents in the global ionosphere produced by field-aligned electric currents for quiet periods are conducted. The steady state equations for current conservation are solved numerically by assuming (1) several divided regions of the global earth (such as the polar cap, auroral zone, and middle-low latitudes), (2) exponentially distributed anisotropic electric conductivities for each zone with a continuous change at the boundaries of the regions, and (3) exponentially distributed downward and upward field-aligned current intensities in the auroral region, assumptions based on our current knowledge of auroral phenomena and geomagnetic variations as well as rocket and satellite measurements of field-aligned currents. Resultant computer-plotted diagrams include equipotential contours of the electric fields, vector distributions of the electric fields and currents, and electric current patterns equivalent to the magnetic field effect produced by the field-aligned and real ionospheric currents. One of the merits of this simulation method is that the three-dimensional current system can roughly be estimated from the equivalent current system obtained from ground-based geomagnetic data alone. This paper also provides a foundation for a similar study of substorms. The following main results are obtained: (1) Conductivity inhomogeneity alters considerably the electric field pattern that has previously been obtained by assuming the uniform conductivity distribution. (2) Even a slight conductivity enhancement along the nightside auroral belt results in a large modification of the electric field. (3) The existence of the strong conductivity gradients and the field-aligned currents in the equatorward half of the auroral oval reduces the electric field in the middle and low latitudes. This corresponds to the ‘shielding’ effect of the electric field inside the Alfven layer in the magnetotail. (4) Seasonal changes in the polar cap conductivities cause surprisingly large effects on the electric fields and currents. (5) The equivalent ionospheric currents differ significantly from real ionospheric currents in both intensity and direction.