On the Influence of the Data Sampling Interval on Computer-Derived K-Indices

The K index was devised by Bartels et al. (1939) to provide an objective monitoring of irregular geomagnetic activity. The K index was then routinely used to monitor the magnetic activity at permanent magnetic observatories as well as at temporary stations. The increasing number of digital and sometimes unmanned observatories and the creation of INTERMAGNET put the question of computer production of K at the centre of the debate. Four algorithms were selected during the Vienna meeting (1991) and endorsed by IAGA for the computer production of K indices. We used one of them (FMI algorithm) to investigate the impact of the geomagnetic data sampling interval on computer produced K values through the comparison of the computer derived K values for the period 2009, January 1st to 2010, May 31st at the Port-aux-Francais magnetic observatory using magnetic data series with different sampling rates (the smaller: 1 second; the larger: 1 minute). The impact is investigated on both 3-hour range values and K indices data series, as a function of the activity level for low and moderate geomagnetic activity

A homogeneous aa index: 1. secular variation

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 (aaS and aaN) have drifted apart. We here separately correct both aaS and aaN for both these effects using the same method as used to generate the classic aa values but allowing {\delta}, 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 aK-values for each station, originally scaled on the assumption that {\delta} values are constant, with time-dependent scale factors that allow for the drift in {\delta}. This requires revisiting the intercalibration of successive stations used in making the aaS and aaN 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, aaHS and aaHN, 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 {\delta} combined with simpler corrections which gave an incorrect temporal variation and underestimate the rise in aa during the 20th century by about 15%

A homogeneous aa index: 2. hemispheric asymmetries and the equinoctial variation

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

Time-of-day/time-of-year response functions of planetary geomagnetic indices

Aims: To elucidate differences between commonly-used mid-latitude geomagnetic indices and study quantitatively the differences in their responses to solar forcing as a function of Universal Time (UT), time-of-year (F), and solar-terrestrial activity level. To identify the strengths, weaknesses and applicability of each index and investigate ways to correct for any weaknesses without damaging their strengths. Methods: We model how the location of a geomagnetic observatory influences its sensitivity to solar forcing. This modelling for a single station can then be applied to indices that employ analytic algorithms to combine data from different stations and thereby we derive the patterns of response of the indices as a function of UT, F and activity level. The model allows for effects of solar zenith angle on ionospheric conductivity and of the station’s proximity to the midnight-sector auroral oval: it employs coefficients that are derived iteratively by comparing data from the current aa index stations (Hartland and Canberra) to simultaneous values of the am index, constructed from chains of stations in both hemispheres. This is done separately for 8 overlapping bands of activity level, as quantified by the am index. Initial estimates were obtained by assuming the am response is independent of both F and UT and the coefficients so derived were then used to compute a corrected F-UT response pattern for am. This cycle was repeated until it resulted in changes in predicted values that were below the adopted uncertainty level (0.001%). The ideal response pattern of an index would be uniform and linear (i.e., independent of both UT and F and the same at all activity levels). We quantify the response uniformity using the percentage variation at any activity level, V = 100(S/), where S is the index’s sensitivity at that activity level and S is the standard deviation of S: both S and S were computed using the 8 UT ranges of the 3-hourly indices and 20 equal-width ranges of F. As an overall metric of index performance, we take an occurrence-weighted mean of V, Vav, over the 8 activity-level bins. This metric would ideally be zero and a large value shows that the index compilation is introducing large spurious UT and/or F variations into the data. We also study index performance by comparisons with the SME and SML indices, compiled from a very large number of stations, and with an optimum solar wind “coupling function”, derived from simultaneous interplanetary observations. Results: It is shown that a station’s response patterns depend strongly on the level of geomagnetic activity because at low activity levels the effect of solar zenith angle on ionospheric conductivity dominates over the effect of station proximity to the midnight-sector auroral oval, whereas the converse applies at high activity levels. The metric Vav for the two-station aa index is modelled to be 8.95%, whereas for the multi-station am index it is 0.65%. The ap (and hence Kp) index cannot be analyzed directly this way because its construction employs tabular conversions, but the very low Vav for am allows us to use / to evaluate the UT-F response patterns for ap. This yields Vav = 11.20% for ap. The same empirical test applied to the classical aa index and the new “homogenous” aa index, aaH (derived from aa using the station sensitivity model), yields Vav of, respectively, 10.62% (i.e., slightly higher than the modelled value) and 5.54%. The ap index value of Vav is shown to be high because it exaggerates the average semi-annual variation and has an annual variation giving a lower average response in northern hemisphere winter. It also contains a strong artefact UT variation. We derive an algorithm for correcting for this uneven response which gives a corrected ap value, apC, for which Vav is reduced to 1.78%. The unevenness of the ap response arises from the dominance of European stations in the network used and the fact that all data are referred to a European station (Niemegk). However, in other contexts, this is a strength of ap, because averaging similar data gives increased sensitivity and more accurate values on annual timescales, for which the UT-F response pattern is averaged out

Three years continuous record of the Earth’s magnetic field at Concordia Station (DomeC, Antarctica)

The magnetic observatory deployed at DomeC, Antarctica, in the French-Italian base known as Concordia hasnow been permanently running for more than three years. This paper focuses on these long-term results whichare more relevant for an observatory intended to provide absolute values of the field. The problems whichemerged in this fairly long record are discussed and solutions suggested to upgrade the observatory to the standardsof an absolute one (i.e. Intermagnet standards).Mailing address: Dr. Aude Chambodut, Ecole et Observatoiredes Sciences de la Terre 5, rue Descartes 67084, StrasbourgCedex, France; e-mail: [email protected]

Semi-annual, annual and Universal Time variations in the magnetosphere and in geomagnetic activity: 4. Polar Cap motions and origins of the Universal Time effect

We use the am, an, as and the a-sigma geomagnetic indices to the explore a previously overlooked factor in magnetospheric electrodynamics, namely the inductive effect of diurnal motions of the Earth’s magnetic poles toward and away from the Sun caused by Earth’s rotation. Because the offset of the (eccentric dipole) geomagnetic pole from the rotational axis is roughly twice as large in the southern hemisphere compared to the northern, the effects there are predicted to be roughly twice the amplitude of those in the northern hemisphere. Hemispheric differences have previously been discussed in terms of polar ionospheric conductivities generated by solar photoionization, effects which we allow for by looking at the dipole tilt effect on the time-of-year variations of the indices. The electric field induced in a geocentric frame is shown to also be a significant factor and gives a modulation of the voltage applied by the solar wind flow in the southern hemisphere that is typically a 30% diurnal modulation for disturbed intervals rising to about 76% in quiet times. For the northern hemisphere these are 15% and 38% modulations. Motion towards/away from the Sun reduces/enhances the directly-driven ionospheric voltages and reduces/enhances the magnetic energy stored in the tail and we estimate that approximately 10% of the effect appears in directly driven ionospheric voltages and 90% in changes of the rate of energy storage or release in the near-Earth tail. The hemispheric asymmetry in the geomagnetic pole offsets from the rotational axis is shown to be the dominant factor in driving Universal Time (UT) variations and hemispheric differences in geomagnetic activity. Combined with the effect of solar wind dynamic pressure and dipole tilt on the pressure balance in the near-Earth tail, the effect provides an excellent explanation of how the observed Russell-McPherron pattern with time-of-year F and UT in the driving power input into the magnetosphere is converted into the equinoctial F - UT pattern in average geomagnetic activity (after correction is made for dipole tilt effects on ionospheric conductivity), added to a pronounced UT variation with minimum at 02-10UT. In addition, we show that the predicted and observed UT variations in average geomagnetic activity has implications for the occurrence of the largest events that also show the nett UT variation

International Geomagnetic Reference Field: the 12th generation

The 12th generation of the International Geomagnetic Reference Field (IGRF) was adopted in December 2014 by the Working Group V-MOD appointed by the International Association of Geomagnetism and Aeronomy (IAGA). It updates the previous IGRF generation with a definitive main field model for epoch 2010.0, a main field model for epoch 2015.0, and a linear annual predictive secular variation model for 2015.0-2020.0. Here, we present the equations defining the IGRF model, provide the spherical harmonic coefficients, and provide maps of the magnetic declination, inclination, and total intensity for epoch 2015.0 and their predicted rates of change for 2015.0-2020.0. We also update the magnetic pole positions and discuss briefly the latest changes and possible future trends of the Earth’s magnetic fiel

Geomagnetic Field, IGRF

International audienc

Observatoire magnétique de Concordia/Dôme C (DMC)

The shelters of the Concordia/DomeC Magnetic Observatory, area of the base called "the magnetic village".Les abris de l'Observatoire Magnétique de Concordia/Dome C, zone de la base nommée "le village magnétique"

Cave de l'abri dit 'Variomètre' à l'Observatoire Magnétique de Concordia/Dôme C (DMC)

Vault of the 'Variometer' shelter at the Concordia/Dome C Magnetic Observatory. The depth of 2 meters in the ice leads to a rapid formation of frost. The staircase to go down and the scalar magnetometer to measure the field intensity are visible.Cave de l'abri dit 'Variomètre' à l'Observatoire Magnétique de Concordia/Dôme C. La profondeur de 2 mètres dans la glace entraîne une formation rapide de givre. L'escalier permettant de descendre ainsi que le magnétomètre scalaire permettant de mesurer l'intensité du champ sont visible