The analysis of daily dynamics of geomagnetic field variations spectrum under quiet and disturbed conditions

doi:10.21203/rs.3.rs-3339206/v1

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The analysis of daily dynamics of geomagnetic field variations spectrum under quiet and disturbed conditions

https://doi.org/10.21203/rs.3.rs-3339206/v1

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The results of static analysis of measurement records of three components of the Earth's magnetic field obtained at the Alma-Ata Observatory [43.25°N; 76.95°E] are presented. The daily dynamics of the spectrum of measurements of the parameters of the geomagnetic field obtained by adding the spectra of magnetically quiet days with a maximum daily Ap index of less than 5 for the period from 2010 to 2018 was considered. Regular fluctuations in the range of acoustic-gravity waves are revealed. The power spectrum of measurement records of three magnetic field components during a geomagnetic storm (Kp = 8) and under quiet geomagnetic conditions (Kp = 2) was also studied.

geomagnetic field

ultra-low frequency fluctuation

acoustic-gravity wave

Daily changes in the magnetic field are determined by the dynamics of ionospheric and magnetospheric current systems which in turn depend on the interaction of the solar radiation flux with atmospheric gas. Atmospheric energy and the exchange of entropy between the atmosphere and the space has been understood in a number of works from the perspective of nonequilibrium thermodynamics. To describe dynamic processes in the atmosphere, the atmosphere has been modelled as a two-component medium consisting of gas and radiation – that comes from the Sun and from the Earth’s surface – that interacts with it (Essex, 1984; Somsikov, 2014; Kshevetskii et al. 2021). Andreev et al. (2018) present the results of calculations of the dispersion relation obtained for linearized system of gas-radiation equations. They show that the disequilibrium of the atmosphere caused by the interaction of radiation with atmospheric gas makes a significant contribution to the spectrum of wave disturbances for heights up to 100 km. The results of theoretical calculations of atmospheric disturbance spectra for different at different levels of light exposure to the Sun are consistent with the observed spectra of atmospheric pressure disturbances, disturbances of electron concentration in the ionosphere, and variations in the intensity of cosmic rays (Antonova et al. 2006). With an increase in the magnitude of the solar radiation flux, one can observe an increase in the natural oscillation frequencies of the nonequilibrium atmosphere, while the magnitude of the shift of the spectrum of the natural oscillations of the atmosphere into the high-frequency region increases with altitude. Somsikov (2011), based on statistical analysis of variations of three components of the Earth's magnetic field, found that during the daytime and nighttime periods, there is a clear asymmetry of the spectra of variations with periods corresponding to the spectrum of acoustic-gravity waves. An explanation of the observed asymmetry is proposed. It is based on the mechanism in which the spectrum of acoustic-gravitational waves changes because of the interaction of atmospheric gas with solar radiation.

The presence of regular fluctuations in the daily pattern of changes in geomagnetic parameters reflects the state of the geomagnetic field at a given time in a given region. The fact that the ionospheric and geomagnetic wave disturbances are linked requires us to take into account their regular variations in the study of shock-acoustic waves from seismic and other sources of disturbances. When developing methods for processing and analyzing variations in the Earth's magnetic field, it is necessary to take into account both regular daily variations and their distinctive features during periods of helio-geophysical events.

The purpose of this article is to analyze the daily dynamics of the spectrum of variations of the geomagnetic field in calm and disturbed conditions.

This paper presents the results of a study of regular fluctuations in the diurnal features of the parameters of the geomagnetic field based on long-term statistical data. The distinctive features of ultra-low-frequency fluctuations on a magnetically disturbed day (Kp = 8) and on a magnetically calm day (Kp = 2) are shown.

The Burlaga-Klein approach (Burlaga and Klein, 1986) and the Higuchi method (Higuchi, 1988, 1990) were used for fractal analysis to study the dynamics of their scale characteristics (spectral indices, fractal sizes) depending on geomagnetic conditions. The revealed regular fluctuations of the geomagnetic field should be taken into account when analyzing data, for example, during strong earthquakes in the Central Asian region.

Measurements of three components of the geomagnetic field obtained at the Alma-Ata Observatory for the period from 2010 to 2018 were used to identify the features of daily changes in the spectrum of geomagnetic field disturbances. Alma-Ata Observatory (geomag.ionos.kz), the only geomagnetic observatory in the Central Asian region that is part of the international INTERMAGNET network. The international code of the observatory is AAA; geographical coordinates [43.25°N; 76.95°E]; geomagnetic coordinates [34.3°N, 152.7°E]. The real-time registration of X-, Y-, Z-components of geomagnetic field variations is carried out on the basis of Lemi-008 and Lemi-018M ferrosonde magnetometers in 1 second resolution. To analyze the heliogeophysical situation, the planetary Kp-index and Ap-index were used based on the data from the Kyoto International Data Center, Japan, (http://swdcwww.kugi.kyoto-u.ac.jp).

To study the regular fluctuations of the geomagnetic field, monthly periodograms were constructed based on minute data from the Alma-Ata magnetic Observatory for the period from 2010 to 2018 by adding up the spectra of magnetically quiet days with a maximum daily Ap index of less than 5. Examples of periodograms for the X, Y and Z components of variations in the Earth's magnetic field are shown in Fig. 1 and Fig. 2. On the horizontal axis, the time UT in hours is shown (for Almaty, the local time UT + 6), on the vertical axis – the period of disturbances in minutes.

The constructed periodograms show that during the winter (January) and equinoctial (March, September) months at night, regular disturbances with periods from 70 to 110 minutes, the duration of which is about 2 hours, are present in the data on the X- and Y-components of the magnetic field. In July, during the daytime, regular disturbances with periods from 80 to 130 minutes are present in the data on the X-component of the magnetic field; there are no regular disturbances in the data on the Y-component. In the data on the Z-component of the magnetic field, regardless of the season, during the local pre-noon time of day (10–12 LT), there are regular disturbances with periods from 110 to 180 minutes, the duration of which is about 2–3 hours. The revealed periods are characteristic of acoustic-gravity waves generated in the auroral zones of the Northern and Southern hemispheres, regardless of the level of magnetic activity, and propagating from high latitudes to medium and low latitudes (Hocke and Schlegel, 1996; Vadas and Liu, 2009). The passage of the solar terminator is also a regular source of acoustic-gravity waves in the atmosphere (Somsikov et al., 2011).

3.2 Spectral analysis of the ULF geomagnetic data

To study ultra-low frequency fluctuations (ULF) of the geomagnetic field, 24 sets of 1-hour time series were formed for each day, each includes N = 3600 second values of the X-, Y-, Z-components of the geomagnetic field, and for them, we used the traditional method of Fast Fourier Transform FFT (Fast Fourier Transform, FFT) to calculate the values of the slope β of the spectrum. Technique to calculate the power spectral density of the ULF signals S(f ) given in the work of (Smirnova et al. 2001).

Examples of data processing by the FFT method for a local noon (12:00–13:00 LT) on a magnetically disturbed day on September 8, 2017, Kp = 8 are shown in Fig. 2 and when the geomagnetic conditions become quiet on October 4, 2017, Kp = 2, are shown in Fig. 3.

The variations of the X-, Y-, Z-components of the geomagnetic field with removed trend are shown, the straight line represents the slope of the β spectrum. The magnetically disturbed day (Kp = 8) was selected from the period of a very large magnetic storm that occurred on September 6–9, 2017. According to NASA's Solar Dynamics Observational Laboratory (https://sdo.gsfc.nasa.gov/) the magnetic storm was caused by a series of solar flares, including the X9.3 flare, the most intense flare recorded during the current solar cycle.

On a perturbed day (the geomagnetic storm) in local time interval of 12–13 hours, the amplitude of the perturbations is largest for X-component, it reach 5–15 nT with periods from 2 to 5 min. The slope of the spectrum, β is 3.84, 3.95 and 3.13, calculated from the X-, Y-, Z-components, respectively. On a geomagnetic quiet day, in local time interval of 12–13 hours, the amplitude of disturbances is largest in Z-component, it reach 0.5 nT with periods of less than 1 min. The slope of the β spectrum is 2.55, 2.99 and 2.08, calculated from the X-, Y-, Z-components, respectively, which corresponds to the fractional Brownian motion in geomagnetic quiet day (Smirnova et al. 2001).

ULF disturbances with amplitudes from tenths to tens of Nt and are divided into regular geomagnetic pulsations, Pc, and irregular, pulsed, Pi. Depending on the periods, geomagnetic pulsations are classified as follows: Pc1 (T = 0.2-5 c), Pi1 (T = 1–40 c); Pc2 (T = 5–10 c), Pi2 (T = 40–150 c); Pc3 (T = 10–45 c), Pi3 (T > 150 c); Pc4 (T = 45–150 c); Pc5 (T = 150–600 c); Pc6 (T > 600 c).

The question on the nature of generating regular pulsations of the geomagnetic field remains open today. Waves in the transition region between the solar wind and the magnetosphere can serve as a powerful source of magnetic pulsations (Schott et al. 2004). The resonant excitation of Pc3-Pc5 pulsations in the Earth's magnetosphere has been repeatedly discussed in the literature. Some authors explain the often observed harmonic structure of Pc3-Pc4 spectra after sudden onset magnetic storms (SC) by resonance effects (Kleimenova and Kozyreva, 2014). A number of papers discuss the possibility of direct transfer of Pc5 pulsations from the solar wind to the magnetosphere, while the spectrum of pulsations may differ from the resonant frequencies of the magnetosphere (Villante et al. 2003; Kessel et al. 2004). Pi pulsations are typical for night hours and for periods of geomagnetic disturbances.

In this study, we examined regular fluctuations of the geomagnetic field based on minute data of the Alma-Ata magnetic Observatory for the period from 2010 to 2018 by adding up the spectra of magnetic quiet days with a maximum daily Ap index of less than 10. We showed presence of disturbances in the frequency range of acoustic-gravitational waves.

The distinctive features of ultra-low-frequency fluctuations on a geomagnetic disturbed day (Kp = 8) and on a magnetically quiet day (Kp = 2) are shown. On a disturbed day, the amplitude of the oscillations is 5–15 nT with periods from 2 to 5 minutes, whereas on a magnetically calm day, the amplitude of the oscillations is the largest in the Z-component, they are 0.5±0.15 nT with periods of less than 1 min.

Studying the geophysical layers of the Earth as a joint dynamic system is a matter of scientific interest. Geophysicists are investigating the variability and interrelation of the ionosphere, magnetosphere and atmosphere, the source of which is the corpuscular flows of magnetized plasma that accompanies the solar wind. This leads to variations of different spectra and with different periods occur in measurements of geomagnetic parameters. The relevance of the study of regular fluctuations of the variable geomagnetic field and the identification of its multi-scale local features is associated with the modern needs of geophysical fundamental and applied science, focused on the creation of adequate theories and mathematical models.

Availability of data and materials

The Kp index data is available at the World Geomagnetism Data Center, Kyoto, wdc.kugi.kyoto-u.ac.jp.

Geomagnetic measurements are available in the International Network of Real-time Magnetic Observatories (INTERMAGNET), which can be downloaded from https://www.intermagnet.org .

Geomagnetic measurements are available on the website of the geomagnetic observatory Alma-Ata geomag.ionos.kz.

Monthly periodograms for the X, Y and Z components of the magnetic field based on the data of the Alma-Ata Observatory [43.25°N; 76.95°E] for the period from 2010 to 2018 are available at https://drive.google.com/file/d/1ylxybkB30bUgYCH1iHIFoGJ4l5rjOLxr/view?usp=sharing.

Competing interests

The authors declare that they have no competing interests.

Funding

This research has been funded by the Science Committee of the Ministry of Science and Higher Education of the Republic of Kazakhstan (Grant No. AP09259554).

Authors' contributions

All authors have contributed to the development of the concept and design of the study. Materials preparation, data collection and analysis were carried out by [Saule Mukasheva], [Alexey Andreyev], [Olga Sokolova], [Vitaliy Kapytin]. The first draft of the manuscript was written by [Saule Mukasheva and Alexey Andreyev], and all the authors commented on previous versions of the manuscript. All authors have read and approved the final manuscript.

Acknowledgements

We thank the Institute of Geophysics of the CAS, for supporting its operation and INTERMAGNET for promoting high standards of magnetic observatory practice (www.intermagnet.org).

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The analysis of daily dynamics of geomagnetic field variations spectrum under quiet and disturbed conditions

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Abstract

Figures

1 Introduction

2 Data and methodology

3 Observational results

3.2 Spectral analysis of the ULF geomagnetic data

Conclusions

Declarations

References

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