scholarly journals A homogeneous aa index: 1. Secular variation

2018 ◽  
Vol 8 ◽  
pp. A53 ◽  
Author(s):  
Mike Lockwood ◽  
Aude Chambodut ◽  
Luke A. Barnard ◽  
Mathew J. Owens ◽  
Ellen Clarke ◽  
...  

Originally complied for 1868–1967 and subsequently continued so that it now covers 150 years, the aa index has become a vital resource for studying space climate change. However, there have been debates about the inter-calibration of data from the different stations. In addition, the effects of secular change in the geomagnetic field have not previously been allowed for. As a result, the components of the “classical” aa index for the southern and northern hemispheres (aa S and aa N) have drifted apart. We here separately correct both aa S and aa N for both these effects using the same method as used to generate the classic aa values but allowing δ, the minimum angular separation of each station from a nominal auroral oval, to vary as calculated using the IGRF-12 and gufm1 models of the intrinsic geomagnetic field. Our approach is to correct the quantized a K -values for each station, originally scaled on the assumption that δ values are constant, with time-dependent scale factors that allow for the drift in δ. This requires revisiting the intercalibration of successive stations used in making the aa S and aa N composites. These intercalibrations are defined using independent data and daily averages from 11 years before and after each station change and it is shown that they depend on the time of year. This procedure produces new homogenized hemispheric aa indices, aa HS and aa HN, which show centennial-scale changes that are in very close agreement. Calibration problems with the classic aa index are shown to have arisen from drifts in δ combined with simpler corrections which gave an incorrect temporal variation and underestimate the rise in aa during the 20th century by about 15%.

2018 ◽  
Vol 8 ◽  
pp. A58 ◽  
Author(s):  
Mike Lockwood ◽  
Ivan D. Finch ◽  
Aude Chambodut ◽  
Luke A. Barnard ◽  
Mathew J. Owens ◽  
...  

Paper 1 (Lockwood et al., 2018) generated annual means of a new version of the aa geomagnetic activity index which includes corrections for secular drift in the geographic coordinates of the auroral oval, thereby resolving the difference between the centennial-scale change in the northern and southern hemisphere indices, aaN and aaS. However, other hemispheric asymmetries in the aa index remain: in particular, the distributions of 3-hourly aaN and aaS values are different and the correlation between them is not high on this timescale (r = 0.66). In the present paper, a location-dependant station sensitivity model is developed using the am index (derived from a much more extensive network of stations in both hemispheres) and used to reduce the difference between the hemispheric aa indices and improve their correlation (to r = 0.79) by generating corrected 3-hourly hemispheric indices, aaHN and aaHS, which also include the secular drift corrections detailed in Paper 1. These are combined into a new, “homogeneous” aa index, aaH. It is shown that aaH, unlike aa, reveals the “equinoctial”-like time-of-day/time-of-year pattern that is found for the am index.


2012 ◽  
Vol 1 (2) ◽  
pp. 85-101 ◽  
Author(s):  
E. Kozlovskaya ◽  
A. Kozlovsky

Abstract. Seismic broadband sensors with electromagnetic feedback are sensitive to variations of surrounding magnetic field, including variations of geomagnetic field. Usually, the influence of the geomagnetic field on recordings of such seismometers is ignored. It might be justified for seismic observations at middle and low latitudes. The problem is of high importance, however, for observations in Polar Regions (above 60° geomagnetic latitude), where magnitudes of natural magnetic disturbances may be two or even three orders larger. In our study we investigate the effect of ultra-low frequency (ULF) magnetic disturbances, known as geomagnetic pulsations, on the STS-2 seismic broadband sensors. The pulsations have their sources and, respectively, maximal amplitudes in the region of the auroral ovals, which surround the magnetic poles in both hemispheres at geomagnetic latitude (GMLAT) between 60° and 80°. To investigate sensitivity of the STS-2 seismometer to geomagnetic pulsations, we compared the recordings of permanent seismic stations in northern Finland to the data of the magnetometers of the IMAGE network located in the same area. Our results show that temporary variations of magnetic field with periods of 40–150 s corresponding to regular Pc4 and irregular Pi2 pulsations are seen very well in recordings of the STS-2 seismometers. Therefore, these pulsations may create a serious problem for interpretation of seismic observations in the vicinity of the auroral oval. Moreover, the shape of Pi2 magnetic disturbances and their periods resemble the waveforms of glacial seismic events reported originally by Ekström (2003). The problem may be treated, however, if combined analysis of recordings of co-located seismic and magnetic instruments is used.


Author(s):  
Bruno Zossi ◽  
Mariano Fagre ◽  
Hagay Amit ◽  
Ana G. Elias

Palaeomagnetic methods can extend the documentary record of changes in the Earth’s magnetic field far into the past. Tolerable agreement is found between various methods, demonstrating the geophysical value of palaeomagnetic experiments. Combining results from the different approaches of investigating secular change can lead to a better perspective and to superior models of geomagnetic field behaviour. Lake sediments have recently been found to hold remarkably detailed signatures of past field changes. A mathematical approach to formulating an empirical description of global geomagnetic field behaviour is proposed and applied to palaeomagnetic data spanning the last 10 ka.


2002 ◽  
Vol 347 (1-3) ◽  
pp. 179-187 ◽  
Author(s):  
J.M Torta ◽  
A De Santis ◽  
M Chiappini ◽  
R.R.B von Frese

1993 ◽  
Vol 23 (3) ◽  
pp. 337-342 ◽  
Author(s):  
H.M. McKay

Fine-root electrolyte leakage from 2-year-old planting stock of three conifer species was measured on 13 dates between October 1989 and April 1990. The three species (Piceasitchensis (Bong.) Carr., Pseudotsugamenziesii (Mirb.) Franco, and Larixleptolepis (Sieb. & Zucc.) Gord.) were raised using either transplanting or undercutting and wrenching regimes. On each lifting date, excised fine roots were stored at 1 °C for 30 and 90 days. The deterioration of the fine roots was assessed by comparing electrolyte leakage values before and after storage. As the lifting date progressed from October to mid-December all three species became less adversely affected by storage, indicating that roots undergo physiological changes that increase their tolerance to prolonged cold storage. The interaction between lifting date and storage duration on fine-root quality differed in the three species. For example, Piceasitchensis could tolerate 90 days' storage commencing at any time from mid-December till early April with negligible root damage. Pseudotsugamenziesii could not tolerate 90 days' storage at any time of year, and tolerance to 30 days' storage was limited and could be substantially reversed. Fine roots of Larixleptolepis were only slightly damaged after 90 days' storage beginning from mid-November to the end of December.


2005 ◽  
Vol 153 (1) ◽  
pp. 73-79 ◽  
Author(s):  
Olga Giménez-Palop ◽  
Gabriel Giménez-Pérez ◽  
Dídac Mauricio ◽  
Eugenio Berlanga ◽  
Neus Potau ◽  
...  

Objective: Ghrelin is a gastric peptide that plays a role in appetite stimulation, energy balance and possibly in insulin resistance. Hyperthyroidism is a situation where negative energy balance and insulin resistance coexist, while in hypothyroidism a positive energy balance and normal insulin sensitivity predominate. We investigated ghrelin levels and their relationship with hunger, food intake and both anthropometric and insulin resistance parameters in patients with thyroid dysfunction. Design and methods:We studied 24 hyperthyroid and 17 hypothyroid patients before and after normalisation of thyroid hormone levels and their respective body mass index (BMI)-matched control group. We measured plasma ghrelin levels, homeostasis model assessment of insulin resistance (HOMA-IR) index, a hunger score, mean three-day calorie intake and anthropometric parameters. Results: In hyperthyroidism, HOMA-IR index was higher (3.21 ± 0.60 vs 1.67 ± 0.15mMmU/l; P = 0.014, t test for independent data) and ghrelin levels were lower (463.6 ± 36.4 vs 561.1 ± 32.1 pg/ml; P = 0.041, Mann–Whitney U-test) than in its control group and both normalised after treatment (HOMA-IR: 2.28 ± 0.38mMmU/l; P = 0.106, t test for independent data, and ghrelin: 539.7 ± 45.4 pg/ml; P = 0.549, Mann–Whitney U-test). Glucose, as a component of HOMA-IR index was the only predictor for ghrelin levels (β = −0.415, P = 0.044, stepwise multiple regression analysis). In hypothyroidism, HOMA-IR index and ghrelin levels were similar to those in its control group both before and after treatment. In both thyroid dysfunction states, no correlations were observed between changes in ghrelin levels and in free T4, free T3, anthropometric parameters, total calorie intake and hunger score. Conclusions: In thyroid dysfunction states, ghrelin levels seemed to be in relation to insulin resistance and not to energy balance and food intake regulation, as seen in other physiological and pathological states.


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