Local time features of geomagnetic jerks

Nagao, Hiromichi; Iwamoto, Toshihiko; Higuchi, Tomoyuki; Nakano, S.; Araki, Tohru

doi:10.1186/bf03351712

Cited by 10 publications

(12 citation statements)

References 28 publications

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“…Other recent works have confirmed the internal origin of jerks, for example, Le Huy et al (1998) who performed spherical harmonic models of the 1969, 1978 and 1991 events ( X , Y and Z components). In addition, Nagao et al (2002) investigated geomagnetic monthly means, obtained from hourly mean values at each local time, of the three magnetic components of the same jerk. They concluded that the distribution of the jerk is independent of local time and that it cannot be explained by external currents.…”

Section: Introductionmentioning

confidence: 99%

Can a 1-D mantle electrical conductivity model generate magnetic jerk differential time delays?

Pinheiro¹,

Jackson²

2008

Geophysical Journal International

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S U M M A R Y Worldwide jerks occurring in 1969Worldwide jerks occurring in , 1978Worldwide jerks occurring in and 1991 are not simultaneous at the earth's surface, showing differential delays of about 2 yr. One way to explain this intriguing temporal pattern is to consider the earth's mantle as a conductor. Consequently the geomagnetic field observed at the earth's surface will correspond to a filtered version of the original field generated at the core-mantle boundary (CMB). We developed the forward approach to this problem assuming the temporal part of the jerk as a simultaneous impulse in the third time-derivative of the magnetic field at the CMB. Two synthetic spherical harmonic models of the jerk amplitudes are built by using two different power spectra. The effect of the mantle is illustrated by a 1-D radial electrical conductivity model that acts as linear, causal and time-invariant filter. The key point is that the mantle filter is different for each harmonic degree. Therefore, because the mixing of harmonics varies with location, distinct time delays will exist at the earth's surface. By using Backus' mantle filter theory, we demonstrate that a simple 1-D mantle electrical conductivity model is able to generate differential jerk time delays that depend strongly on the jerk morphology input model. We also illustrate that the time delays will vary for each component of the magnetic field.

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Section: Introductionmentioning

confidence: 99%

Can a 1-D mantle electrical conductivity model generate magnetic jerk differential time delays?

Pinheiro¹,

Jackson²

2008

Geophysical Journal International

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“…With observatory data, jerk occurrence times for all sv components at the Earth's surface have already been detected, for example, by De Michelis et al (2000), Nagao et al (2002) and Nagao et al (2003). Concerning the CMB region, an investigation by Dormy & Mandea (2005) for a non‐conducting mantle showed that it was not possible to find a sharp jerk signal in the component at the CMB.…”

Section: Resultsmentioning

confidence: 99%

“…It can also be equivalently described as a jump in the time derivative of the geomagnetic sv, that is, in the geomagnetic secular acceleration. In former studies, the surface distribution of these jumps was addressed by the term ‘jerk morphology’ (De Michelis et al 1998; 2000; Le Huy et al 2000; Nagao et al 2002; Pinheiro & Jackson 2008). While this distribution was referred to a fixed mean jerk occurrence time, we can extend this concept to a ‘dynamical jerk morphology’, which is composed of jumps moving at the Earth's surface and at the CMB: Related to each secular acceleration component, a temporally variable surface function can be defined.…”

Section: Geomagnetic Jerksmentioning

confidence: 99%

“…Similarly to the early work of Ha Duyen Chau et al (1981) and others (e.g. De Michelis et al 1998, 2000; Nagao et al 2002; Chambodut & Mandea 2005), we have constructed an algorithm which fits temporally straight lines to the data function of the sv component considered at a (ϑ, ϕ) position. The algorithm is implemented for a 1°× 1° grid of a global surface (Earth's surface or CMB) in the following steps:…”

Section: Geomagnetic Jerksmentioning

confidence: 99%

“…1969, 1978, 1991, 2003). Most of the jerks were observed globally, but some have been detected in a few observatories only, such as the event around 1985, which seemingly occurred only in the southern hemisphere (Courtillot et al 1984; Stewart 1991; Alexandrescu et al 1995, 1996; Macmillan 1996; De Michelis et al 1998, 2000; Mandea et al 2000; Nagao et al 2002; De Michelis & Tozzi 2005; Olsen & Mandea 2007; Tozzi et al 2009).…”

Section: Introductionmentioning

confidence: 99%

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The 1991 geomagnetic jerk as seen at the Earth's surface and the core-mantle boundary

Ballani

Hagedoorn

Wardinski

et al. 2010

Geophysical Journal International

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S U M M A R YWe determine the occurrence times of geomagnetic impulses (jerks) around the year 1991 in the three geomagnetic secular variation components for the Earth's surface by a simple optimization algorithm. The geomagnetic field models we use are the low-degree parts of the models CM4 and C 3 FM. We find that the temporal jerk pattern can be detected in fields (n ≤ 4), from which the spherical harmonic degrees n = 2 or n = 3 (tangential) and n = 4 (radial) are representative.To calculate the secular variation components at the core-mantle boundary (CMB) we apply the non-harmonic downward continuation (NHDC) method. For the mantle conductivity, three estimates, dependent on the radius, are assumed with conductances between 10 7 S and 2 × 10 8 S. The knowledge of the secular variation components at the CMB allows us to track the global distributions of jerk occurrence times in dependence on the mantle conductivity estimate.We find for each component a typically shaped, global topology for the location of the jerk occurrence times at the Earth's surface and the CMB. For the tangential (ϑ, ϕ)-components, these global topologies show always the well-known temporal bimodality on each surface. Another characteristic feature is found for the jerk of the r-component. It displays a double jerk centred around 1991 consisting of a v-shaped and a reversely v-shaped part, which are significantly correlated.Between the CMB and the Earth's surface, we find time delays in the range of 1-2 yr for the tangential and less than 1 yr for the radial jerk components. To understand these time delays, comparisons at fixed locations are carried out to check the influence of the respective conductivity function. For studying the time delay effects, we apply the inversion set-up of the NHDC to calculate simulated temporal oscillations and derive analytical expressions approximating the phase shifts. We find that jerk occurrence time delays and simulated phase shifts of temporal oscillations have a similar behaviour with respect to the influence of the conductivity and for the radial and the tangential components, respectively.In addition, a new concept for determining a jerk amplitude is presented briefly. This so-called dynamical jerk morphology, which forms a portion of the geomagnetic secular acceleration, is defined for each component by a time function on the considered surface. Its temporal motion patterns at the CMB are likely related with jerk originating processes in the fluid outer core.

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