A regional geomagnetic field model over Southern Africa derived with harmonic splines from Swarm satellite and ground-based data recorded between 2014 and 2019

A regional harmonic spline geomagnetic main field model, Southern Africa Core Field Model (SACFM-3), is derived from Swarm satellite and ground-based data for the southern African region, in the eastern part of the South Atlantic Anomaly (SAA) where the field intensity continues to decrease. Using SACFM-3 and the global CHAOS-6-×9 model, a detailed study was conducted to shed light on the high spatial and temporal geomagnetic field variations over Southern Africa between 2014 and 2019. The results show a steady decrease of the radial component Z in almost the entire region. In 2019, its rate of decrease in the western part of the region has reached high values, 76 nT/year and 78 nT/year at Tsumeb and Keetmanshoop magnetic observatories, respectively. For some areas in the western part of the region the radial component Z and field intensity F have decreased in strength, from 1.0 to 1.3% and from 0.9 to 1.2%, respectively, between the epochs 2014.5 and 2019.5. There is a noticeable decrease of the field intensity from the south-western coast of South Africa expanding towards the north and eastern regions. The results show that the SAA area is continuing to grow in the region. Abrupt changes in the linear secular variation in 2016 and 2017 are confirmed in the region using ground-based data, and the X component shows an abrupt change in the secular variation in 2018 at four magnetic observatories (Hermanus, Hartebeesthoek, Tsumeb and Keetmanshoop) that needs further investigation. The regional model SACFM-3 reflects to some extent these fast core field variations in the Z component at Hermanus, Hartebeesthoek and Keetmanshoop observatories. Graphical Abstract

Diurnal variation effect in marine magnetometric surveys: clues from surveys in southeast Brazil

The diurnal variation of the magnetic field cannot be predicted or modeled and for that reason, it is monitored during the magnetic surveys, usually by a stationary magnetometer. However, marine surveys have a practical issue with diurnal monitoring, owing to the distance between the survey, stationary magnetometers, and magnetic observatories. This work aims to verify the use of nearby magnetic observatories to estimate the diurnal variation correction in different marine surveys and evaluate its effectiveness. In this study, we selected surveys at the continental shelf near Santos city (Survey 1), continental slope next to the first survey location (Survey 2), continental shelf near Ubatuba city (Survey 3), and Mamanguá ria in the Paraty city (Survey 4), all southeast to the Brazilian coast. The crossing points were implemented to compare the magnetic field values at different times and days at the same measurement point, before and after the correction. Afterwards, we measure the Pearson’s Correlation of the raw data and the diurnal corrected data in all crossing points of each survey which showed an improvement after correction by the value approximating to 1, which indicates a very well correlation. The Ubatuba and Mamanguá surveys allowed comparing the observatory correction results with the base magnetometer results that were rather similar. Our analyses indicate a satisfactory diurnal correction using the observatory data and the crossing points approach, which can be used for every marine magnetometric survey worldwide placed near the coast (< 280 km) that do not have a stationary magnetometer available.

The relevance of local magnetic records when using extreme space weather events as benchmarks

Space weather indices introduced for scientific purposes are commonly used to quantify operational nowcast of the geospace state during extreme space weather events. Some indices, such as the Disturbance storm time (Dst) index, have been applied to situations for which they are not originally intended. This raises a question about suitability as a space weather benchmark. In analysing historical records for different magnetometers at low- and mid-latitude, we find periods with longitudinal asymmetry in magnetic response that suggest important signals from individual magnetometers are being averaged out of the Dst record. This asymmetry develops as a double spike in the H-component: one negative in the observatories in the day sector and one positive in the observatories in the night sector. These spikes develop in short-time (about 2 hours) and pose a potential hazardous effect for users affected by space weather. The results from historical events have been reinforced with the systematic study of magnetic records during extreme events (Dst ≤ -200 nT and AL ≤ -2000 nT) in the period 1998-2017 from six magnetic observatories at about 40° magnetic latitude. Moreover, we show that the largest asymmetries take place during the early main phase and are recorded in narrow local time sectors. An important outcome of these results is that space weather benchmarks should be based on local records instead of the commonly used global indices. This action improves two important aspects of space weather: the assessment of historical extreme events and that of the needs of users.

Historical <i>K</i> index data collection of Soviet magnetic observatories, 1957–1992

The K index is one of the oldest universal indices of geomagnetic activity, introduced in 1938 by Julius Bartels, that is still being widely used. Up to the present day, long-term time series of homogeneous K index records have been accumulated at data repositories all over the world. The multidecadal practice of its application makes it an indispensable source of information for the retrospective analysis of solar–terrestrial interaction for nearly eight solar cycles. Most significantly, while studying the historical geomagnetic data, K index data sheets are in certain cases far easier for automated analysis than the conventional analogue magnetograms. The presented collection includes the results of the K index determination at 41 geomagnetic observatories of the former USSR for the period from July 1957 to the early 1990s. This unique collection was formed at the World Data Center for Solar-Terrestrial Physics in Moscow. The historical data, which are offered to the international scientific community, cover the second half of the 20th century and can be used for the retrospective analysis and study of geomagnetic events in the past, as well as for data validation or forecasting (Sergeyeva et al., 2020). The dataset is available at: https://doi.org/10.1594/PANGAEA.922233, last access: 16 September 2020.

Development and implementation of a low-cost long-period telluric recorder for deep Earth electrical investigations

&lt;div&gt; &lt;div role=&quot;gridcell&quot; data-scope=&quot;messages_table&quot; data-recover=&quot;true&quot;&gt; &lt;div data-testid=&quot;messenger_incoming_text_row&quot;&gt; &lt;div&gt; &lt;div&gt;Magnetotelluric loggers are key instruments for deep geophysical studies of crust and mantle. However, conducting a large-scale survey requires the implementation of a series of magnetotelluric instruments to complete the measurements in an efficient time. The main efforts and costs of a magnetotelluric survey are devoted to magnetic recordings. Therefore, using a compination of magnetotelluric stations along with parallel tellurics recorders can significantly reduce the time and costs needed to complete a regional survey. Based on this motivation, we present the construction, implementation and case studies of a long period telluric recorder (LPTR). The telluric recorder is based on a 24 bit ADC with a multiplexer that enables 2 differential channels devoted to the Ex and Ey telluric components. The multiplexer is adjusted to provide 1sample per second from each channel that corresponds to 2Hz sampling rate at the ADC. The multiplexing at this rate reduces the ADC efficient resolution to 20 bit. As the full measuring range is +/- 1.25V the least significant bit LSB is about 2.4 micro V. The output of the ADC is transferred via USB to a mini PC for time stamping and saving. The time of each record is provided from a GPS with accuracy of 1 ms. The LPTR is connected to the ground using a Cu-CuSo&lt;sub&gt;4&lt;/sub&gt; nonpolarizable electrodes. The electrodes are specially constructed to provide good and longterm connection to the ground in arid environments. The LPTR has been tested throughout several field implementations in Egypt. The setup for contiuous telluric acquisition is realized in Moghra, Dakhla, Farafra and in Fayoum. These locations covers a variety of northern and southern Egypt as well as western desert and Nile valy. During the test implementations the recorder is put to run parallel to an ADU07-e magnetotelluric system for 1-3 days then for 2-4 months to be compared and integrated with the magnetic observatories at Fayoum and Abo Simble. Both observatories are running MAGSON fluxgate magnetometers at a sampling rate of 1 Hz. The resultant data showed that the LPTR synchronizes with the ADU07-e at periods from 5s and with the magnetic observatory data at periods 25s. This indicates an efficient low-cost system that can be used for deep Earth resistivity investigations. A case study of 2-4 months of continuous telluric recordings that have been processed with magnetic observatories data provided impedances for periods up to 42000 seconds. The results are 1D modeled for depths of more than 800KM. A comparison between the obtained 1D MT model and global Earth-models (LITHO1) based on seismological data shows a quite good matching at the deep interfaces like upper crust, middle crust and lower crust. The delineation of seismic discontinuities at 410 KM and 680 KM shows corresponding clear change in resistivity at 410 KM and then at 700 KM as well.&lt;/div&gt; &lt;/div&gt; &lt;/div&gt; &lt;/div&gt; &lt;/div&gt;

K indices and K-derived magnetic activity indices: context’s reminder

&lt;p&gt;The K index was devised by Bartels et al. (1939) to provide an objective monitoring of irregular geomagnetic activity at subauroral latitudes. K indices are based upon geomagnetic disturbances, measured in horizontal geomagnetic components at magnetic observatories, after &amp;#171; eliminating &amp;#187; the regular daily variation. An individual K index is an integer in the range 0 to 9 corresponding to a class that contains the largest range of geomagnetic disturbances (in either of the two horizontal components) during a 3-hour UT interval. Limits of range vary from one observatory to another since they depend on the corrected geomagnetic latitude of the observatory.&lt;/p&gt;&lt;p&gt;A great number of Space Weather applications rely on K-derived magnetic activity indices at subauroral latitudes. These historical indices; endorsed by IAGA such as Kp, aa and am; represent unprecedented homogeneous time series, up to more than 150 years, highly valuable for all studies related to long-term geomagnetic activity.&lt;/p&gt;&lt;p&gt;However, one has to keep in mind that local K indices and subauroral related ones (K-derived) were developed during other time, under specific societal and technological conditions.&lt;/p&gt;&lt;p&gt;We recall the local K indices derivation processes and characteristics to enlight possible nowadays drawbacks and their simple mitigations.&lt;/p&gt;

Local diurnal variations of the geomagnetic field during magnetically quiet conditions

&lt;p&gt;The Earth&amp;#8217;s magnetic field as measured from ground-based magnetometers is composed of a variety of fields generated by diverse sources, spanning a broad amplitude and frequency spectrum. Long-term variable sources induce smooth changes, whereas short-term variable sources are able to induce rapid spikes in the geomagnetic field. An important aspect of Space Weather research is to understand the contribution and impact of each of these sources. In particular, knowing the amplitude and frequency of steady-like sources, like diurnal variations, enables us to determine the impact of sudden and hazardous events such as solar storms. The basic approach to this challenge is to identify the quiet magnetic field information within the recorded time-varying signal.&lt;br&gt;In this work, we examine the variance of the magnetically quiet diurnal and semi-diurnal components of the geomagnetic field, as recorded by ground-based magnetic observatories of the INTERMAGNET network. These variations are extracted by applying appropriately designed digital filters on the geomagnetic field time series. The residual signal is analysed in terms of local time and seasonal variations for selected locations under quiet magnetic conditions. This approach allows us to evaluate the applicability of the introduced filtering method. The obtained results improve our understanding of the driving sources of quiet currents such as the Sq current and the variations of their distributions with respect to regular solar irradiance variations. They will also contribute to a better extraction and description of the remaining/residual signal related to solar wind stimuli (e.g. ICMEs, CIRs) causing magnetic storms.&lt;/p&gt;

Historical <i>K</i> index data collection of Soviet magnetic observatories, 1957–1992

K index is one of the oldest universal indices of geomagnetic activity, introduced in 1938 by Julius Bartels, that is still being widely used. Up to the present day, long-term timeseries of homogeneous K index records have been accumulated at data repositories all over the world. The multidecadal practice of its application makes it an indispensable source of information for retrospective analysis of solar-terrestrial interaction for nearly eight Solar cycles. Most significantly, while studying the historical geomagnetic data, K index datasheets are in certain cases far easier for automated analysis than the conventional analogue magnetograms. The presented collection includes the results of the K index determination at 41 geomagnetic observatories of the former USSR for the period from July 1957 to early 1990s. This unique collection was formed at the World Data Center for Solar-Terrestrial Physics in Moscow. The historical data, which are offered to the international scientific community, cover the second half of the 20th century and can be used for retrospective analysis and study of geomagnetic events in the past as well as for data validation or forecasting (Sergeyeva et al., 2020). The dataset is available at: https://doi.org/10.1594/PANGAEA.922233, last access: 16 September 2020.

Geoinformation system for analyzing the dynamics of extreme geomagnetic disturbances from observations of ground stations

The paper is concerned with an approach to developing a specialized web-GIS based on a microservice architecture that provides analytical control of the disturbed component of geomagnetic field variations, according to observation data from magnetic observatories and variational stations published on the SuperMAG portal (http://supermag.jhuapl.edu/). A method of spatial interpolation of geomagnetic data implemented in the proposed web-GIS, together with the proposed scheme for ranking and interpreting them, as well as a visualization method in the form of isolines, allows a user to track the structure, observe the dynamics, identify probable regions, duration and time intervals of the occurrence of extreme geomagnetic disturbances.