Magnetic field reversals and galactic dynamos

We argue that global magnetic field reversals similar to those observed in the Milky Way occur quite frequently in mean-field galactic dynamo models that have relatively strong, random, seed magnetic fields that are localized in discrete regions. The number of reversals decreases to zero with reduction of the seed strength, efficiency of the galactic dynamo and size of the spots of the seed field. A systematic observational search for magnetic field reversals in a representative sample of spiral galaxies promises to give valuable information concerning seed magnetic fields and, in this way, to clarify the initial stages of galactic magnetic field evolution.     

The palaeogeomagnetic field strength,variations in reversal frequency,and geomagnetic dynamo models

Abstract

The geomagnetic field and its frequent polarity reversals are generally attributed to magnetohydrodynamic (MHD) processes in the Earth's metallic and fluid core. But it is difficult to identify convincingly any MHD timescales with that over which the reversals occur. Moreover, the geological record indicates that the intervals between the consecutive reversals have varied widely. In addition, there have been superchrons when the reversals have been frequent, and at least two, and perhaps three, 35-70 Myr long superchrons when they were almost totally absent. The evaluation of these long-term variations in the palaeogeophysical record can provide crucial constraints on theories of geomagnetism, but it has generally been limited to only the directional or polarity data. It is shown here that the correlation of the palaeogeomagnetic field strength with the field's protracted stability during a fixed polarity superchron provides such a constraint. In terms of a strong field dynamo model it leads to the speculation that the magnetic Reynolds number, and the toroidal field, increase substantially during a superchron of frequent reversals.     

First-order reversal curve (FORC) diagrams for pseudo-single-domain magnetites at high temperature

The recently developed first-order reversal curve (FORC) technique for rapidly examining magnetic domain state has great potential for paleomagnetic and environmental magnetic investigations. However, there are still some gaps in the basic understanding of FORC diagrams, in particular the behavior of pseudo-single-domain (PSD) grains and the contribution of magnetostatic interactions. In this paper we address some of these problems. We report the first FORC diagrams measurements on narrowly sized and well-characterized synthetic PSD through multidomain (MD) magnetite samples. The FORC diagrams evolve with grain size from single-domain (SD)-like to MD-like through the PSD grain size range. Since each sample contains grains of essentially a single size, individual PSD grains evidently contain contributions from both SD-like and MD-like magnetic moments, in proportions that vary with grain size; the evolving FORC diagrams cannot be due to physical mixtures of SD and MD grains of widely different sizes. The FORC diagrams were all asymmetric. Small PSD samples have FORC diagrams with a distinctive closed-contour structure. The distributions of the larger MD grains display no peak, and lie closer to the interaction-field axis. To assess the effect of magnetostatic interactions, we measured FORC diagrams between room temperature and the Curie temperature. On heating the FORC distributions contract without changing shape until ∼500°C. Above this temperature the diagrams become more MD-like, and in addition become more symmetric. The temperature dependence of the interaction-field parameter is proportional to that of the saturation magnetization, in accordance with Néel’s interpretation of the Preisach diagram. The decrease in asymmetry with heating suggests that the origin of the asymmetry lies in magnetostatic interactions. The magnetic hysteresis parameters as a function of temperature were determined from the FORC curves. As the grain size decreased the normalized coercive force was found to decrease more rapidly with temperature.     

Trends in Cretaceous and Tertiary geomagnetic reversals

The statistical theory of modulated renewal processes is used to analyze the polarity reversal scales of Larson & Hilde (1975) and LaBreque, Kent & Cande (1977). The results suggest that the trending effect in these data may be modelled by a rate parameter with an exponential quadratic trend. Short times in one polarity state tend to be followed by short times in the other state. The graphical analysis points to the possibility of an undulating pattern in reversal rates. The empirical distributions of the normal and reversed polarities show slight differences in comparison with each other in most of the statistical tests, but a moving-window analysis indicates possible serial effects for the normal times. As a rough approximation, a statistical two-state model for reversals might be realistic, for example, an alternating renewal process under relaxed assumptions. There has been a gradual stepping-up of the minimum reversal rate from the Oligocene to the present, but little change in the observed range of the reversal rates. With the long quiet (Mercanton) interval removed from the data, the average time spent in the reversed polarity state is slightly greater than for the normal state. A change in statistical properties for the entire set of data considered as a single sample occurs around the Eocene-Oligocene transition (Middle Eocene on earlier time scales). The analyses of the statistical second-order properties of the entire sequence of 271 observations, and the subsets of normal and reversed between-times, reject a renewal hypothesis if theoretical statistical considerations are strictly applied to the results (although this hypothesis is not rejected for the Oligocene to Recent observations); this result is at variance with some geophysical models. A short appendix on the theory of point processes is provided to aid the general reader in following the arguments used in this paper.     

白垩纪中期地球磁场与全球地质现象(英文)

白垩纪超静磁带(CNS)是指白垩纪中期约40 Ma内(Aptian Santonian, 121~83 Ma)地球磁场没有出现倒转的异常现象。这一现象引起地球学界的极大关注,原因在于它与白垩纪中期(124~90 Ma)其他一系列事件在时间上非常吻合,包括洋底扩张速率的快速增加和洋底高原、海山链、大型火成岩省等生产速率的快速增加。过去20多年许多研究强调所有这些现象与下地幔对流的联系。近期研究的一个重要的进展在于把地球磁场的变化(反转频率和古强度)与许多重要的地质和地球物理过程结合起来,如周期性地幔对流、地幔柱活动、全球热流、真极移、大型火成岩省的产生和生物群集绝灭。文中回顾了白垩纪地球磁场研究以及与白垩纪中期所发生的全球性的地质事件之间的可能存在的关联,并讨论在这一重要研究领域内未来研究的发展方向。     

Geomagnetic events and relative palaeointensity variations during the past 300 ka as recorded in Kolbeinsey Ridge sediments, Iceland Sea: indication for a strongly variable geomagnetic field

 Magnetostratigraphic analyses of five sediment cores recovered from the Kolbeinsey Ridge area revealed consistent records of several geomagnetic events linked with low relative palaeointensities within the past 300 ka. Interpretation of various rock magnetic parameters clearly rule out the possibility that the recorded non-normal polarity directions are linked to a deviating magnetomineralogical fraction or a distorted magnetic fabric. Therefore, these directions are interpreted as true recordings of geomagnetic field variations. Hysteresis parameters and thermomagnetic measurements revealed pure pseudo single-domain (PSD) magnetite with Curie temperatures of 580  °C as the dominant remanence carrier mineral. Due to the homogeneity of the rock magnetic parameters, the sediments are also suitable for relative palaeointensity determinations. Oxygen-isotope stratigraphies for two of the cores provided a time frame to estimate the age ranges for the recorded geomagnetic field variations. The obtained record of ten events of up to 12 ka duration during the past 300 ka and the related record of relative palaeointensity indicate a fairly dynamic character of the Earth's magnetic field, which is in clear contrast to published polarity time scales. Received: 2 July 1998 / Accepted: 5 February 1999     

Impact of beaver dam analogues on hydrology in a semi-arid floodplain

Natural beaver ponds help connect the stream to the floodplain, maintain late summer low flows and reduce peak flow during high flow events by offering temporary surface water (SW) storage. When beavers are extirpated from the landscape, stream degradation often ensues. This study assesses the impact of beaver dam analogues (BDA) as a stream restoration technique to help maintain low flow water levels and enhance stream-floodplain interactions on a seasonal basis in Red Canyon Creek, Lander, WY. BDAs increased SW and groundwater (GW) levels, favoured the occurrence of flow reversals (i.e., stream-to-floodplain GW flow) during high flow events associated with mid-winter and early-spring thaw events, and reduced the groundwater-to-stream hydraulic gradient on an annual basis. Although GW temperatures varied seasonally, relatively cooler GW temperatures were observed in the BDA impacted reach compared to the control reach away from BDA influence. BDAs however did not significantly impact stream temperatures. Overall, results suggest that when installed in sequence, BDA complexes can successfully reconnect the stream to its floodplain, and ultimately increase SW-GW exchange at the floodplain scale by allowing flow reversals to occur and by reducing the GW to stream hydraulic gradient. Although BDAs built with fence posts, willow branches, sediments and small boulders are naturally porous and require regular maintenance, this study also highlights the viability of small BDAs as a restoration practice to enhance landscape resilience to drought and high flow events in deeply incised channels where beavers would not come back naturally.     

Enigmatic solar wind disappearance events – Do we understand them?

At the Sun-Earth distance of one astronomical unit (1 AU), the solar wind is known to be strongly supersonic and super Alfvenic with Mach and Alfven numbers being on average 12 and 9 respectively. Also, solar wind densities (average ∼10cm^-3) and velocities (average ∼450kms^-1) at 1AU, are known to be inversely correlated with low velocities having higher than average densities andvice versa. However, on May 11 and 12 1999 the Earth was engulfed by an unusually low density (< 0.1cm^-3) and low velocity (< 350km s^-1) solar wind with an Alfven Mach number significantly less than 1. This was a unique low-velocity, low-density, sub-Alfvénic solar wind flow which spacecraft observations have shown lasted more than 24 hours. One consequence of this extremely tenuous solar wind was a spectacular expansion of the Earth’s magnetosphere and bow shock. The expanding bow shock was observed by several spacecraft and reached record upstream distances of nearly 60 Earth radii, the lunar orbit. The event was so dramatic that it has come to be known asthe solar wind disappearance event. Though extensive studies of this event were made by many authors in the past, it has only been recently shown that the unusual solar wind flows characterizing this event originated from a small coronal hole in the vicinity of a large active region on the Sun. These recent results have put to rest speculation that such events are associated with global phenomenon like the periodic solar polar field reversal that occurs at the maximum of each solar cycle. In this paper we revisit the 11 May 1999 event, look at other disappearance events that have ocurred in the past, examine the reasons why speculations about the association of such events with global phenomena like solar polar field reversals were made and also examine the role of transient coronal holes as a possible solar source for such events.