Magnetic field power spectra and magnetic radial diffusion coefficients using CRRES magnetometer data (original) (raw)
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The geomagnetic power spectrum
Geophysical Journal International, 2008
Combining CHAMP satellite magnetic measurements with aeromagnetic and marine magnetic data, the global geomagnetic field has now been modelled to spherical harmonic degree 720. An important tool in field modelling is the geomagnetic power spectrum. It allows the comparison of field models estimated from different data sets and can be used to identify noise levels and systematic errors. A correctly defined geomagnetic power spectrum is flat (white) for an uncorrelated field, such as the Earth's crustal magnetic field at long wavelengths. It can be inferred from global spherical harmonic models as well as from regional grids. Marine and aeromagnetic grids usually represent the anomaly of the total intensity of the magnetic field. Appropriate corrections have to be applied in estimating the geomagnetic power spectrum from such data. The comparison of global and regional spectra using a consistently defined azimuthally averaged geomagnetic power spectrum facilitates quality control in field modelling and should provide new insights in magnetic anomaly interpretation.
Power-spectrum analyses have been carried out on two data sets of the geomagnetic K-index from the Athens and Sofia magnetic Observatories. For the period between 19561984, periodicities of about 2.8 and 6 months have been obtained. Similar results were found by the auto-correlation technique. Both periods are significant to 0.05 and 0.01 level, respectively. In our attempt to explain transient geomagnetic disturbances caused by other parameters, the K-index was correlated to cosmic-ray and aurora intensity. The best correlation coefficient between K-variations and cosmic-ray data from Athens Neutron-Monitor Station was 0.58 and between K-index and Auroral,activity index was 0.47.
Annales Geophysicae, 2010
We analyze spectral parameters of the geomagnetic disturbances within the 1-4 mHz (Pc5/Pi3) frequency range for 29 observatories from polar to auroral latitudes. The main object of this study is the broadband (noise) background under quiet and moderately disturbed conditions. To obtain a quantitative description of background high-latitude long period ULF activity the log-log dependence of the spectral power on frequency is expanded over Legendre polynomials, and the coefficients of this expansion (spectral moments) are used to describe the most common features of these spectra. Not only the spectral power, but also the spectral slope and higher spectral moments, averaged over relatively long time intervals, demonstrate a systematic dependence on corrected geomagnetic (CGM) latitude, , and magnetic local time, MLT. The 2-D distributions of the spectral moments in-MLT coordinates are characterized by existence of structures, narrow in latitude and extended in MLT, which can be attributed to the projections of different magnetospheric domains. Spatio-temporal distributions of spectral power of elliptically (P-component) and randomly (Ncomponent) polarized signal are similar, but not identical. The N-component contribution to the total signal becomes non-negligible in regions with a high local activity, such as the auroral oval and dayside polar cusp. The spectral slope indicates a larger relative contribution of higher frequencies upon the latitude decrease, probably, as a result of the resonant effects in the ULF noise. The higher spectral moments are also controlled mostly by CGM latitude and MLT and are fundamentally different for the polarized and non-polarized components. This study is a step towards the construction of an empirical model of the ULF wave power in Earth's magnetosphere.
2015
In this thesis we investigate and analyse the nature and behaviour of the external field variations of the geomagnetic field for quiet and moderately disturbed days using geomagnetic observatory measurements and field models. We use spherical harmonic modelling, led by available geomagnetic observatory measurements and past models (particularly the Comprehensive Model of Sabaka et al. 2004). As an initial step, we extended the lifespan of the Comprehensive Model (CM4) beyond its 2002.5 lifespan to allow for use of current data. We produced profile plots of the diurnal field and generated global maps of the field and compare these with the CM4 model, to see how well the CM4 model could reasonably predict ground variation of the diurnal field outside its lifespan and for days away from quiet time. The comparison shows that away from quiet time period, the CM4 model is producing more reasonable predictions than expected, despite the lack of active data in the original model dataset. Th...
Regular daily variations in satellite magnetic total intensity data
Annales Geophysicae, 2007
Regular magnetic daily quiet time (Sq) variations in total intensity of about 30 nT amplitude are determined in Universal Time (UT) from satellite magnetic field measurements. The CHAMP satellite traverses all hours of local time in 132 days and the Sq variations in total intensity are therefore calculated as an average over the 132 days for each hour of UT. Results are compared with the Sq daily variations in total intensity for the region above the ionosphere calculated from Malin's (1973) spherical harmonic analysis of the Sq Fourier coefficients for hourly mean value magnetic data from a global distribution of ground-based magnetic observatories. From the reasonable agreement between the two calculations, we conclude that low-Earth orbit satellites that traverse all hours of local time can determine Sq variations in total intensity above the ionosphere.
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Space Weather, 2015
In an attempt to develop better forecasts of geomagnetic activity, hourly ranges of geomagnetic data are analyzed with a focus on how the data are distributed. A lognormal distribution is found to be able to characterize the magnetic data for all observatories up to moderate disturbances with each distribution controlled by the mean of the logarithm of the hourly range. In the subauroral zone, the distribution deviates from the lognormal, which is interpreted as motion of the auroral electrojet toward the equator. For most observatories, a substantial deviation from the lognormal distribution was noted at the higher values and is best modeled with a power law extrapolation, which gives estimates of the extreme values that may occur at observatories which contribute to the disturbance storm time (Dst) index and in Canada. This paper applies the principles of Riley [2012] to magnetic measurements from the geomagnetic observatories of the International Real-time Magnetic Observatory Network (INTERMAGNET) [Kerridge, 2001], see http://www.intermagnet.org/ to investigate the applicability of the technique to hourly ranges of the magnetic field. A detailed statistical investigation of both the general population and extreme events is carried out.
Geomagnetic response to sudden expansions of the magnetosphere
Journal of Geophysical Research, 1988
The geomagnetic response to five successive sudden expansions of the magnetosphere was examined by the use of magnetic data observed on the ground and by satellites. At the geosynchronous orbit between 0800 and 1100 LT the magnetic field component parallel to Earth's rotation axis decreased successively. The amplitude and the fall time of each decre. me were 20-30 nT and 2.5-3.5 min, respectively. The decre. me was propagated about 10 rnin later to the distance of about 31 R E from Earth in the antisunward direction, indicating propagation speed of about 300 km/s. The H component of ground magnetograms from low-latitude stations showed decreazes with waveform similar to that at the geosynchronous orbit, but each decre. me at the dayside equator was greatly enhanced and preceded by a short small positive impulse. Each of the corresponding geomagnetic variations at high latitude stations consisted of two successive sharp pulses of opposite sense with 2-3 min duration. The dominant component and the sense of these high-latitude pulses were highly dependent upon local time and latitude. The distribution of equivalent ionospheric current arrows for each highlatitude pulse showed clear twin vortices centered at 70-76 ø geomagnetic latitude in the dayside and was approximately symmetric with respect to the noon meridian. The current direction of the vortices was reversed from the first pulse to the second. It suggests successive appearance of a dawn-todusk and then a dusk-to-dawn electric field, both of which were transmitted from the magnetosphere to the polar ionosphere. The effect of ionospheric currents due to these polar electric fields was superposed on the simple magnetic decrease produced by an expansion of the whole magnetosphere and produced the complex waveform distribution on the ground. and modified by Araki et al. [1984]. The magnetic observa
Analysis of 11 Myr of geomagnetic intensity variation
We have conducted a detailed exploratory analysis of an 11 million year long almost continuous record of relative geomagnetic palcointensity from a sediment core acquired on Deep Sea Drilling Project Leg 73, at Site 522 in the South Atlantic. We assess the quality of the palcointensity record using spectral methods and conclude that the relative intensity record is minimally influenced by climate variations. Isothermal remanence is shown to be the most effective normalizer for these data, although both susceptibility and anhysteretic remanence are also adequate. Statistical analysis shows that the palcointensity variations follow a gamma distribution, and are compatible with predictions from modified palcosecular variation models and global absolute palcointensity data. When subdivided by polarity interval, the variability in palcointensity is proportional to the average, and further, the average is weakly correlated with interval length. Spectral estimates for times from 28.77 until 22.74 Ma, when the reversal rate is about 4 Myr -1, are compatible with a Poisson model in which the spectrum of intensity variations is dominated by the reversal process in the frequency range 1-50 Myr -1. In contrast, between 34.7 and 29.4 Ma, when the reversal rate is about 1.6 Myr -1, the spectra indicate a different secular variation regime. The magnetic field is stronger, and more variable, and a strong peak in the spectrum occurs at about 8 Myr -1. This peak may be a reflection of the same signal as recorded by the small variations known as tiny wiggles seen in marine magnetic anomaly profiles. 17,735 17,736 CONSTABLE ET AL.: 11 MYR OF GEOMAGNETIC INTENSITY
Terrestrial, Atmospheric and Oceanic Sciences, 2008
On 24 March 1991, global ultra-low-frequency (ULF) pulsations (1.1-3.3 mHz) observed in the magnetosphere as well as on the ground were studied via analyzing magnetic field data obtained from a global network, comprising ground-based observatories and geosynchronous satellites. In the magnetosphere, the compressional and transverse components of the magnetic fields recorded at two satellites, GOES 6 and GOES 7, showed dominant fluctuations when they were in the vicinity of the noon sector, whereas the transverse fluctuations became dominant when they were at the dawn side. Similarly, on the ground, the H and D components had major fluctuations along with an increase in amplitude from low to high geomagnetic latitudes. In addition, the amplitude of the ULF pulsation was enhanced at the dawn and dusk sides. The geomagnetic pulsations propagated anti-sunward and were of counterclockwise and clockwise elliptical polarizations at the dawn and dusk sides respectively. The counterclockwise elliptical polarization reversed to a clockwise elliptical polarization at geomagnetic local noon and linear polarization was observed during the reversal. It appears that the analysis of the global network data not only provided us with a study of the characteristics of the waves in the magnetosphere and on the ground but also provided us with correlations between the geosynchronous and ground observations, which should be essential to the determination of possible mechanisms of this storm-related wave event.