The Sheshan Geomagnetic Observatory (International
Association of Geomagnetism and Aeronomy (IAGA) code SSH), China was built in Xujiahui, Shanghai in 1874 and moved to Sheshan, Shanghai at the end of 1932. So far, the SSH has a history of nearly 150 years. It is one
of the earliest geomagnetic observatories in China and one of the
geomagnetic observatories with the longest history in the world. In this
paper, we present the rescue and quality control (QC) of the historical data at the
SSH from 1933 to 2019. The rescued data are the absolute hourly mean values
(AHMVs) of declination (D), horizontal (H), and vertical (Z) components. Some of these data are paper-based
records and some are stored in a floppy disk in BAS, DBF, MDB, and other
file storage formats. After digitization and format transformation, we
imported the data into the Toad database to achieve the unified data
management. We performed statistics of completeness, visual analysis,
outliers detects, and data correction on the stored data. We then conducted
the consistency test of daily variation and secular variation (SV) by
comparing the corrected data with the data of the reference observatory, and the computational data of the COV-OBS model, respectively. The consistency test reveals good agreement. However, the individual data should be used with caution because these data are suspicious values, but there is not any explanation or change registered in the available metadata and logbooks. Finally, we present examples of the datasets in discriminating geomagnetic jerks and study of storms. The digitized and quality-controlled AHMVs data are available at: 10.5281/zenodo.7005471 (Zhang et al.,
2022).
Introduction
Geomagnetic observation data contains abundant solar–terrestrial spatial
information, which is widely used in geoscience and space science research.
The observation data with time resolutions of 1 s to 1 h are usually used to study various short-period magnetic events such as pulsation, geomagnetic crochet, geomagnetic bay, and magnetic storm (Zhao et al., 2019),
and to monitor and predict the electromagnetic environment in
solar–terrestrial space. At the same time, it also has important
applications in detecting underground electrical structures and evaluating
the impact of geomagnetic induced current (GIC) on underground metal pipe
network, transmission network, communication cables, high-speed railway
lines, and other major projects (Kappenman, 1996; Bolduc et al., 1998, 2002;
Boteler et al., 1998; Liu et al., 2008, 2016; Liu et al., 2009; Guo et al.,
2015). Observation data with time resolution of 1 h to hundreds of years are
usually used for the study of the geomagnetic field and its secular variation (SV),
such as geomagnetic jerk (Courtillot and Mouël, 1984; Xu, 2009),
magnetic pole movement, dipole magnetic moment change, westward drift, etc.,
which are of great significance for understanding the material flow inside
the core and at the core mantle boundary.
The development and application of geomagnetism depends on long-term data
accumulation. The long-term operation of the geomagnetic observatory is very
important for the study of the geomagnetic field (Linthe et al., 2013). It
is especially valuable to study the variation characteristics of the
geomagnetic field from decades to hundreds of years (Clarke et al., 2009; Zhang et
al., 2008b). Using the latest scientific and technological means to analyze
the continuous geomagnetic observation data as long as possible, to obtain
the variation information of the geomagnetic field, has always been a method
often used by scientific researchers. However, not all data can be directly
provided to researchers, because some data still exist only in the form of
hard copy, and even some data face the risk of serious damage and loss due
to improper storage conditions. Therefore, it is very important to rescue
and digitize these data as soon as possible. High-quality data are the basis
of scientific research and the prerequisite for obtaining valuable results
(Linthe et al., 2013). Scientists around the world have paid more and more
attention to the accumulation of observation data, the rescue of historical
data and the sharing of scientific data resources (Curto and Marsal., 2007;
Peng et al., 2007; Chulliat et al., 2009; Korte et al., 2009; Dawson et al., 2009; Reay et al., 2013; Morozova et al.,
2014, 2021; Sergeyeva et al., 2021; Dong et al., 2009; Zhao et al., 2017;
Thomson, 2020).
The rescue, recovery, digitization, and the quality control (QC) of historical
geomagnetic data are of extraordinary importance for the geomagnetic
community (Rasson et al., 2011). This paper presents the collection,
collation, digitization, quality control, and correction of the
historical data of the Sheshan Geomagnetic Observatory (International
Association of Geomagnetism and Aeronomy code SSH) from 1933 to 2019. The SSH
Geomagnetic Observatory is the geomagnetic observatory with the longest
history in China. Although many efforts have been made (Gao and Hu, 1993), the existing data are still insufficient. Our work aims at filling the lack of observation data at the SSH observatory since 1933, presenting the data of absolute hourly mean values (AHMVs) collected from 1933 to 2019.
This paper is organized as follows. Section 2 describes the data acquisition
method, providing information about SSH observatory history, data sources,
and the digitization method. Section 3 introduces the quality control of the
digitized data. Section 4 describes the correction of the selected problem
data. Section 5 describes the validation of the corrected series by
comparing it with reference series and Sect. 6 presents application examples
of the datasets. Concluding remarks are given in Sect. 7.
Data production methodsThe Sheshan geomagnetic observatory
The first step of the data-rescue process was to collect resources scattered
in different locations, which exist in various forms, including data and
metadata that may have an influence on data rescue (observatory relocation,
instrument replacement, replacement of observers, environmental change,
etc.). We have carefully examined the documentation stored in the SSH,
Geomagnetic Network of China (GNC), and reference room of the Institute of
Geophysics, China Earthquake Administration (IGPCEA). The reference room is a resource center of the IGPCEA, used to collect books, journals, papers, monographs, unpublished reports, internal textbooks, research reports, reference documents, and scientific research achievements related to the discipline. It took us nearly 2 months to collect resources. The
documentation consulted includes the Observatory Communication Journal, Geomagnetic Observation Report, Chronicles of China Geomagnetic Observatory, and postal letters. The metadata are mainly stored in the Chronicles of China Geomagnetic Observatory and the Geomagnetic Observation Report. An example of the cover of the bibliographic documents is shown in Fig. 1. The data of 1933–1954 were recorded in the Geomagnetic Observation Report. The observation was interrupted from April 1945 to December 1946 due to World War II. The data of 1955–1994 were stored in the DBF format. The data of 1995–2001 were stored in the BAS format. The data from 2002 to 2006 were stored in the Access database in the MDB format. The DBF, BAS, and MDB are all data file storage formats. The DBF is a tabular data file stored in binary and is the database format used by dBase and FoxPro databases in DOS systems. The BAS file format is written in the BASIC language, a plain-text data storage format. The MDB format is a storage format used by Microsoft Access software that can generally be opened directly with ACCESS. The data of 2007, 2008, and 2010 were lost for unknown reasons. The data from August to December 2011, and July to October 2019 were missed due to the failure of absolute observation instruments. Data for other years from 2009 to 2019 are stored in the Oracle database.
Cover of the (a)Chronicles of China Geomagnetic Observatory (Department of Science, Technology and
Monitoring, CEA, 1984) and (b)Geomagnetic Observation Report (Institute of Geophysics, Chinese Academy of Sciences, 1965).
The Sheshan Geomagnetic Observatory (SSH) is presently run by IGPCEA and has been in operation for almost 150 years. Its predecessors are the observatory in Xujiahui, Shanghai, and that in Lujiabang, Jiangsu Province. It was established in Xujiahui in 1874 and began continuous geomagnetic observation since then. It was moved from Xujiahui to Lujiabang in 1908 and then from Lujiabang to Sheshan in December 1932. The Sheshan Geomagnetic
Observatory is located in Sheshan (latitude: 31.1∘ N,
longitude: 121.2∘ E), 20 km to the southwest of Shanghai city. The
geology of the vicinity of the observatory is Upper Jurassic to Down
Cretaceous Andesite. The gradient of the field is about 2–3 nT m-1. The
earliest absolute house and recording room were built in 1932, they are made
of non-magnetic material. The regular observation began in 1933.
Table 1 shows the absolute and relative instruments in the SSH observatory from 1933 to 2019 and the measured geomagnetic elements at different periods. The information was retrieved from the bibliographic documents mentioned above. The first instrument set included an Elliott (D measurement), a Smith (H measurement), and a Schulze (I measurement) as absolute instruments since 1933. The continuous recordings of magnetic variations of declination (D), horizontal (H), and vertical (Z) were obtained respectively with a horizontal variometer (Toepfer) and a vertical intensity variometer (Godhavn) since 1933. Later, a few replacements of instruments took place in the SSH observatory (Table 1). During this period, many jumps were seen in the relative recorded data due to the adjustment of the variometer, the lightning stroke, earthquakes, and other reasons. These jumps have been corrected by the baseline, so that the absolute value is not affected. By 2000, the SSH observatory was equipped with digital instruments. On 1 January 2003, the Schmidt Standard Theodolite was replaced by a DIM-100/353766 Fluxgate Theodolite and an Overhauser Effect Proton Precession Magnetometer GSM-19F replaced the Proton Precession Magnetometer CZM-2.
Summary of instruments in the period from 1933 to 2019 at SSH.
Because some records are handwritten or manually mimeographed, it is
impossible to automate the digitization process. To facilitate the
digitization and further application of these records, all the documents
were photographed. It is also useful for checking the consistency of
digitized data and source data in the future. It is not good for old paper copies to be carried around too much and as soon as we have a digital picture, which is fast to make, we can store the respective papers in their normal archive place again with the usual temperature, humidity, etc. Using the character-recognition program to recognize the photos and compare the consistency with the paper data, it was found that the recognition effect of the character was not ideal. It may be due to the light color of the handwriting, or some of the handwriting is fuzzy and unclear. Therefore, the digitization was mainly performed by key input. We digitized the AHMVs of the three components of declination (D), horizontal (H), and vertical (Z). We designed a set of Excel templates to unify the data-entry format. The digital templates are very similar to the original data source to keep track of our work. The input templates include three workbooks, which are used to store the AHMVs of 1 year, including the AHMVs of the D, H, and Z components. Every AHMVs workbook consists of 14 worksheets, including
text description, data worksheets from January to December of every year, and
automatic summary worksheets. The monthly data worksheet header includes the
station code, measuring point ID, date, large value, and 24 hourly mean values. The large value is a fixed value every month. The purpose of entering a large value is to facilitate the rapid entry of each value. The 24 AHMVs can be calculated by adding the large values to the 24 hourly mean values
respectively. For example, the large value in January 1985 was 33 300. For
each hourly mean value of this month, we only need to input the digits after
thousands' digit. If the input value of 0 h on 1 January was 146.1, we
can get the AHMV at this moment by 33 300 plus 146.1, and so on. Missing
values were marked as “99 999”. An example of the Excel tables with digitized data is presented in Fig. 2. The “key input” approach is slower but has the lower error rate (Capozzi et al., 2020). After each month of data entry, we cross checked the digitized data with the original source values in order to identify and remove transcription errors. Using this approach, it took us half a year to digitize the 1933–1954 data from paper records.
An example of the Excel tables with digitized data.
Import data stored in various formats into Oracle database
We developed a data convert software (Fig. 3) to import data stored in
various formats (XLS, DBF, BAS, and MDB) into a unified Oracle database. In
this way, all AHMVs were stored in the same database in a unified format. We
call these stored data without any correction the original AHMVs data; it
is convenient for the subsequent analysis and application. This allows us to
examine the data using Geomagnetic Data Processing Software (GDPST)
developed by the Geomagnetic Network of China (Zhang et al., 2016). The GDPST was developed based on the Oracle database. It provides a convenient way of data processing and comparative analysis. The software also has the functions of the data query, data backup, and data download, etc.
Data import software.
Quality control of digitized data of SSH observatory
The purpose of quality control (QC) is to check the completeness and
reliability of geomagnetic observation data. The quality of geomagnetic data
is often affected by the changes in the instrument or environmental
conditions of the measurements, like the repair or re-calibration of the
instrument, instrument replacement, observatory relocation, gradual changes
of the observation environment, changes in observing process, etc. (Morozova
et al., 2014; Zhang et al., 2016). Most such changes can lead to what we call data problems and can be defined as sudden breaks and jumps in the series of geomagnetic data, or gradual biases, or noise and change of transfer function, etc. Correction of problem data before any subsequent analyses is highly desirable (Mestre et al., 2013).
In this study, the QC was performed in order to check the quality of the
rescued data. The inspection contents include evaluating the completeness of
data, the accuracy of daily variation, the stability of secular variation,
and analyzing the influence factors of data quality.
The completeness of data
Based on the original AHMVs, the annual completeness is calculated using
the following Eq. (1):
C=(We-Wm)/We,
where C is the completeness of the AHMVs, We is the number of expected data in the chosen period, Wm is the number of missing data, see Fig. 4
for the completeness of data. The series in this study consists of data
measured from 1933 to 2019. It has 5 larger gaps with a total of 66 months
of data missing; the number of missing data accounts for 6.5 % of the
total. All gaps were not replaced by interpolation in this study. War and instrument failure are the main reasons for the gaps of observation.
The completeness of the AHMVs from 1933 to 2019.
The accuracy of the rescued data
We have designed a strict quality-control procedure to ensure the accuracy
of the rescued data. It consists of the following three steps:
Preliminary analysis of the series, detection of outliers.
In order to avoid the adverse impact of extreme data on the overall trend,
we filtered out clearly obvious outliers by the appropriate filtering
function of Excel, such as the missing values which were marked as “99 999”, obvious input error, and so on.
Visual analysis of the series and their first-time derivative (FTD) at different timescales.
After removing the obvious outliers, we plotted AHMVs of the geomagnetic field components D, H, and Z for all time from 1933 to 2019 (see Fig. 5). It can be seen from the figure that the D, H, and Z components have obvious trends. It is the secular variation (SV) in geomagnetic field with time. The additional signal in the plots mainly comes from the activity of external field. The most significant influence is on both the horizontal components D and H; its influence on the vertical component is minor. In the plots, we do not see obvious step and peak interference.
We checked the AHMV plots of the SSH month by month and found that sometimes the geomagnetic changes were quiet and regular, sometimes violent and irregular, and most of the days the geomagnetic changes were superimposed on the regular quiet day changes with some disturbances of different shapes and amplitude. As shown in Fig. 6, taking the AHMV record of January 1955 as an example, it can be seen from the figure that the record includes both regular periodic quiet day changes and complex perturbation, and the geomagnetic storms of 18–19 January are violent disturbances.
In order to eliminate the impact of trend and detect the data problems more
effectively, we plotted the first-time derivative (FTD) of AHMVs for D, H, and Z components (Fig. 7). We calculated the FTD using the consecutive values of hourly series (Morozova et al., 2014). For all geomagnetic components, the FTD is calculated asdX/dt(hour)=(X(hour)-X(hour-1))/1,where X denotes the geomagnetic field components D, H, and Z.
The FTD plots are also particularly useful in evaluating artificial noise,
especially interference in the shape of steps or spikes (Linthe et al.,
2013; Pang et al., 2013; Chen et al., 2014). It can be seen from the figure
that the data after the FTD eliminates the trend change, and the data are
steady, going up and down within a certain range; ΔD varies between
-13.4 and 12.6 min, ΔH varies between -302 and 203 nT, and
ΔZ varies between -70 and 62 nT.
The tolerance test detects the outliers and compare with geomagnetic indices.
For a large set of data with a normal or approximately normal distribution,
99.7 % of the values are distributed in the (μ-3σ, μ+3σ) interval, where σ and μ are the standard
deviation and mean for all time, respectively. The values beyond this interval are generally considered as outliers. We presented the histograms of the FTD of D, H, and Z components between 1933 and 2019 in Fig. 8, which aimed at detecting the outliers further. The distribution can be well modeled by the Gaussian probability density function (solid red curve). The vertical dashed red lines indicate the lower and upper limits obtained by applying the criteria (μ-3σ) and (μ+3σ). We found that more than 98.6 % of the FTD data points fall within the range of 3 times the standard deviations.
The AHMV plots of D, H, and Z components for all
time from 1933 to 2019.
The AHMV record of January 1955 at SSH.
The FTD plots of D, H, and Z components.
The histogram of the FTD of between 1933 and 2019.
For the FTD data exceeding 3 times the standard deviation, we defined
them as FTD outliers. We need to confirm whether the outlier is related to
geomagnetic activity. Two conventional indices to describe geomagnetic activity are Kp and Dst; Kp has no linear relationship with the geomagnetic
activity, so the ap index was introduced. The ap is expressed in “ap units”: 1 ap unit equals 2 nT (Menvielle et al., 2011). Currently, Kp and ap indices are produced by the GeoForschung Zentrum (GFZ) Potsdam, Germany (Kp and ap values since 1932 are available online at https://www.gfz-potsdam.de/, last access: 18 November 2022, GeoForschung Zentrum Potsdam Website, 2022); Dst indices are currently produced by the
World Data Center for Geomagnetism, Kyoto (Dst values since 1957 are
available online at http://wdc.kugi.kyoto-u.ac.jp/, last access: 18 November 2022, World Data Center for Geomagnetism, Kyoto, 2022). The comparative
analysis (Fig. 9) also shows that the geomagnetic components have a good
correlation with Dst and ap indices, especially H and Dst. It should be
noted that the H component in Fig. 9 has eliminated periodic changes such
as secular variation and seasonal variation.
Comparative analysis of the H components with Dst and ap
indices.
We compared the FTD outliers with the ap and Dst indices trying to establish the cause of the FTD out of tolerance, and took corresponding measures:
(a) When the ap is greater than or equal to 24 nT, or Dst is less -30 nT, the outlier was considered to be caused by geomagnetic activity. The AHMVs at
the corresponding time were not corrected. (b) When the ap is less than 24 nT and Dst is greater than -30 nT, we carefully looked for the cause of each FTD outlier by comparing the daily variation curves of multiple
observatories (Sheshan, Chongming, Wuhan, Guangzhou, or Nanjing), and
further consult the available documentation (Observatory Communication Journal, Geomagnetic Observation Report, Chronicles of China Geomagnetic Observatory, and postal letters).
A preliminary analysis found that for the D component, 65.6 % of the outliers were related to geomagnetic disturbance; the 33.5 % showed that no abnormality was found in the daily change curve; the remaining 82 values were questionable. For the H component, 80.5 % of the outliers were related to geomagnetic disturbance; the 17.6 % showed that no abnormality was found in the daily change curve; the remaining 199 values were questionable. For the Z component, 99.6 % of the outliers were related to geomagnetic disturbance; the 0.4 % showed that no abnormality was found in the daily change curve; the remaining 112 values were questionable. A total of 393 FTD outliers were questionable and no relevant and useful information was recorded in the available documentation. The AHMVs at the corresponding time were not corrected but were marked as questionable data in the datasets, the quality flag was QC =Q. As shown in Fig. 10, taking the FTD outliers of 19 October 2013 as an example, a clear deviation was found in the data between 08:00 to 13:00 from the real geomagnetic characteristics. Due to the lack of complete documentation, the questionable data were not corrected, just made the marks in the datasets, QC =Q. (c) When the ap index is less than 24 nT and Dst is greater than -30 nT, a change is registered in the available documentation. These data can be accepted to be corrected. In Table 2, we listed the date of the data to be corrected and the reasons recorded in the daily log and annual report. It took place only on 1 January 2003, a modern absolute instrument named Fluxgate Theodolite DIM-100 replaced the geomagnetic induction instrument Schulze, and an Overhauser Effect Proton Precession
Magnetometer GSM-19F replaced the Proton Precession Magnetometer CZM-2. These
changes led to sudden steps in size of about 1.9′, 8, 40, and 35 nT in
D, H, Z, and F components, respectively. The log recorded the exact jumps in the measurements during the change of instruments at that time (SSH observatory, 2004).
The contrast curve of SSH, COM, WHN, and
GZH from 15 to 24 October 2013.
Date and the reasons recorded of the data to be corrected.
DateMetadataTime intervalCorrection values (new values - old values) 1 January 2003Instrument replacements1 January 2003–31 December 2019D: 1.9′H: 8 nTZ: 40 nTF: 35 nTCorrection of the data problems
As was mentioned above, we only corrected the data of D, H, Z, and F
components that occurred after 1 January 2003. The break arose due to the
installation of new instruments in 1 January 2003. The corrected value is 1.9′ for the D component, 8 nT for the H component, 40 nT for the Z component, and 35 nT for F
component (see Table 2). As shown in Figs. 11 and 12, we gave the hourly
and daily mean values curve of the D, H, and Z components before and after correction from 1 January 2001 to 31 December 2004. It can be seen that
the quality of data has been greatly improved after correction.
The curve of the AHMVs shows obvious annual and seasonal variations. The seasonal variation shows that the variation range is large in summer and small in winter.
The absolute daily mean value (ADMV) curve of the D component shows an obvious long-term trend of slow
decline from 2001 to 2004. No obvious change characteristics can be seen in the Z curve.
Plot of hourly mean values before and after correction.
Plot of daily mean values before and after correction.
Mutual comparison is an important method to check data quality. In general,
the difference of the same component between two stations close to each
other is small and stable. We also compared the data of the SSH before and after correction with those data from the Chongming Geomagnetic Observatory (IAGA code COM), which is the nearest observatory from the SSH (Fig. 13). The differences of the three components before correction
are as follows: ΔD varies between -1.0 and 2.4 min, ΔH varies between -2 and 14 nT, and ΔZ varies between -46 and 19 nT. The differences of the three components after correction are as follows: ΔD varies between -0.3 and 1.3 min, ΔH varies between -7 and 24 nT, and ΔZ varies between -20 and 4 nT. The standard deviations of the differences of the three components before correction are as follows: 1.1 min, 3 nT, and 20 nT. The standard deviations of the differences of the three components after correction are as follows: 0.3 min, 3 nT and 3.3 nT. Again, it shows that the quality of observation data is improved after correction.
Plot of difference between the SSH and COM before and after
correction.
Validation of the corrected series by compare with reference series
Intercomparison of time series for geomagnetic elements from adjacent observatories or geomagnetic models is also an important method to test
accuracy and stability of data (Curto and Marsal., 2007). Firstly, we
compared the rescued data with those data from the reference observatory. In
China, the regular observation of most geomagnetic observatories began in
the 1980s. Only eight geomagnetic observatories were established during the
international geophysical year (Rasson et al., 2011). Among the eight
observatories, the Guangzhou observatory (GZH) is closer to the Sheshan observatory.
It started observation early, and the quality of observation data was good.
So, we selected the GZH as the reference observatory. The GZH is located in
Guangzhou City, Guangdong Province, about 1240 km northeast of the SSH.
It began geomagnetic observation in 1957. Due to the interference of
Guangzhou metro operation, the new site, Gaoyao Liantang Town, was selected in 1996. The construction of the new observatory was completed at the end of
2001. The geomagnetic observation records officially began in the new place on 1 January 2002.
We used GDPST which offers very useful diagnostic procedures of the data
quality to plot intercomparison of the value curve and their difference curve
from the SSH and GZH observatories on hourly and daily timescales to detect data with potential quality issues year by year. As an example, we presented
AHMVs, ADMVs and their difference curves of the SSH and COM observatories in
Fig. 14. At Fig. 14 in the upper panel (a), the AHMV and their difference
curves are depicted, while in the lower panel (b), the ADMVs and their
difference curves are plotted. On hourly scales, the single components D, H, and Z of the SSH and GZH behave roughly identical, and their difference series slowly fluctuates (due to geomagnetic activity) around a certain range. Spikes are caused in most cases by external disturbances. On daily timescales, the components D, H, and Z are roughly identical, but their differences clearly coincide with the variation of the geomagnetic field. It is because the distance between the two observatories are too far to completely offset the influence of internal and external source fields in different regions.
AHMVs, ADMVs and their difference curves of the SSH and GZH
observatories. (a) AHMVs and their difference curves. (b) ADMVs and their difference curves.
Comparing the measured values with the calculated values of the model for a
long-time scale is not only an important means to check the stability of
secular variation, but also an important means to evaluate the accuracy of
the model (Zhang et al., 2008b; Chen et al., 2012). One of the aims of
geomagnetic observatories is the monitoring of SV (Reda et al., 2011).
Secondly, to monitor the SV of SSH, we compared the series curve of the annual mean values (AMVs) for the X, Y, and Z components calculated from the rescued records with these data calculated from the COV-OBS model (Gillet et al., 2013; Huder et
al., 2020). The COV-OBS.x2 model covers the period from 1840 to 2020. The
data source of the model is from observatory data, satellite data, and the
older surveys. The model can give the field contributions from the internal
and external sources.
As can be seen from the Fig. 15, the change trends of X, Y, and Z components from the SSH and COV-OBS model are very consistent. The X component increased year by year before 1962 and generally decreased after 1962; from 1933 to 2019, the Y component shows a general downward trend and the Z component shows a general upward trend. There are differences between AMVs of the SSH and these data calculated from the COV-OBS model, the X component varies from -210 to -276 nT, the Y component varies from 17 to 94 nT, and the Z component varies from 198 to 289 nT. According to the preliminary analysis, the main reasons for the large difference between the SSH and COV-OBS model may be the local magnetic anomaly in the Sheshan area, the uneven distribution of global stations, the lack of modeling data, and data-quality problems in the SSH. This fully illustrates the importance of continuous and high-quality data in magnetic field modeling.
The AMV and their difference curves of X, Y, Z
components calculated from the rescued records and the COV-OBS model.
The SV of the annual Y series from the SSH and
COV-OBS model.
Application examples of SSH datasets
We calculated the first-time derivative (FTD) using a difference between 2
consecutive years as SV and plotted the SV from the rescued data and COV-OBS
model to detect possible geomagnetic jerks over the past 90 years. For all
geomagnetic components, the SV is calculated as
dX/dt(year)=(X(year)-X(year-1))/1,
where X denotes the geomagnetic field components D, H, and Z.
List of geomagnetic storms that have occurred at the SSH since
1933.
SNStart timeEnd timeVariation amplitude SNStart timeEnd timeVariation amplitude HDZHDZ122 August 1937 03:0023 August 1937 15:00329157822227 August 1978 02:0031 August 1978 20:0031210455222 January 1938 02:0023 January 1938 24:00366113402313 July 1982 16:00C53815656325 January 1938 11:0027 January 1938 10:0033316039245 September 1982 22:008 September 1982 02:0038514868424 March 1940 13:0026 March 1940 08:00534109762521 September 1982 03:0023 September 1982 21:003061006152 March 1947 08:004 March 1947 22:003387468264 February 1983 16:006 February 1983 20:003038833624 January 1949 18:00C33312753276 February 1986 13:0010 February 1986 03:0032816236712 May 1949 06:0015 May 1949 18:00431113872813 March 1989 01:0015 March 1989 22:00629185114819 March 1950 05:0019 March 1950 23:00383135842920 October 1989 09:0023 October 1989 10:003189663913 September 1957 00:0014 September 1957 16:00557152112309 April 1990 08:0011 April 1990 24:0038894731029 September 1957 00:002 October 1957 10:0038411886318 November 1991 06:0010 November 1991 03:00326163541111 February 1958 01:0013 February 1958 06:00649210133329 May 1992 19:0012 May 1992 07:0043411680128 July 1958 07:0010 July 1958 11:0038612696336 April 2000 16:007 April 2000 20:0032113053133 September 1958 08:006 September 1958 13:00306107653415 July 2000 14:0016 July 2000 18:00317130681415 July 1959 08:0017 July 1959 02:004881501153531 March 2001 00:001 April 2001 15:00447175971531 March 1960 09:002 April 1960 23:003321261063624 November 2001 05:0025 November 2001 24:0031787781630 April 1960 12:001 May 1960 20:0036497823729 October 2003 06:00C330160501712 November 1960 13:0014 November 1960 23:00387140493830 October 2003 16:002 November 2003 21:00323140421830 September 1961 21:001 October 1961 20:0034047333920 November 2003 08:0021 November 2003 24:00461100391925 May 1967 12:0029 May 1967 20:0045911680407 November 2004 18:00C453120342023 March 1969 18:0025 March 1969 24:003039564419 November 2004 18:0012 November 2004 24:00343123392125 March 1976 13:0027 March 1976 24:0033393534215 May 2005 02:0016 May 2005 18:003789830
Note C in “End time”: followed by another storm.
The geomagnetic storm occurred on 11 February 1958.
Geomagnetic jerks are defined as V-like or Λ-like changes in the SV
and occur in a time period of a few months (Courtillot and Le Mouël, 1984;
Morozova et al., 2014; Kang et al., 2020). “The geomagnetic jerks are due
to interactions of the core field and the rapid time-varying core flow”
(Kuang and Tangborn, 2011). Since Malin and Hodder (1982), Courtillot and Le Mouël (1984) discovered the geomagnetic jerk in 1969, 10 jerks have been detected in observatories from 1933 to 2020, of which 1969, 1978 (Alexandrescu et
al., 1996), 1991 (De Michelis et al., 1998), 1999 (Mandea et al., 2000;
Zhang et al., 2008a), 2003 (Mandea and Olsen, 2007; Feng et al., 2018; He et
al., 2019), 2007 (Kotzé, 2010; Chulliat et al., 2010), and 2014
(Brown et al., 2016; Kloss and Finlay, 2019; Finlay et al., 2016; Kang et
al., 2020) were global events. In addition, there were two local events
which occurred in 1949 (Mandea et al., 2000) and 2011 (Chulliat and Maus,
2014; Kotzé and Korte, 2016). In 2017, there were similar characteristics
of geomagnetic jerks, which may be a new geomagnetic jerk (He et al., 2019;
Pavón-Carrasco et al., 2021). The jerks are more easily seen in the
eastward component (Y) of the geomagnetic secular variation. Not all jerks can
be detected around the world, some seem to be seen only in limited regions
(Morozova et al., 2014), and its occurrence time is not exactly the same at each
observatory.
Figure 16 gives the SV of the annual Y series from the SSH and COV-OBS model,
and the dotted blue line is the third-order moving average curve of the Y
component SV at the SSH. Nine jerks are clearly seen in the plot. They occurred
in 1950, 1971, 1978, 1993, 1999, 2004, 2008, 2013, and 2018. Except for the
jerks that occurred in 1978 and 1999, other events were a little later as seen at the SSH. Between 2008 and 2017, only one jerk (in 2013) is seen at the SSH, which is inconsistent with the study of other scholars mentioned above. They observed jerk events in 2011 and 2014, respectively. We cannot see a potential jerk in 2011, maybe because of the data gap until 2011.
A geomagnetic storm is a global phenomenon of magnetic disturbance. At low
and mid-latitudes, it mainly manifests itself as a decrease in the
horizontal geomagnetic field (H) during a geomagnetic storm. According to
the Kakioka Magnetic Observatory Website (2022), a total of 67 very large
geomagnetic storms (variation range of the H component >300 nT) have
occurred since 1933. Referring to the start and end time of the geomagnetic
storm announced by the website, we studied the geomagnetic storms that were
recorded in the data series of the SSH since 1933. We found a total of 42 very large geomagnetic storms (variation range of the H component >300 nT) in these data series (Table 3). Figure 17 shows an example of a very large geomagnetic storm that occurred on 11 February 1958. A sudden storm commencement (SSC) occurred at 01:00 on 11 February 1958. It is the sign of the start
of the geomagnetic storm. This storm lasted about 53 h and ended at 06:00
on the 13 February 1958. The maximum variation amplitude of D, H and Z during the geomagnetic storm is 210, 649 and 133 nT respectively.
Data availability
The digitized and quality-controlled absolute hourly
mean value (AHMV) data are available at:
10.5281/zenodo.7005471 (Zhang et al., 2022). The data are
provided in Microsoft Excel format, including the observed AHMV files of the three components (D, H and Z) at the SSH for 1933–2019, and also the metadata files about the datasets.
Conclusion
This paper presents the acquisition process, quality control, data
correction, quality examination, and application examples of the datasets
of the SSH from 1933 to 2019. The quality examination results show that the
corrected data have a good agreement with the reference observatory data and
the model data. This fully indicates that the rescued data are of good
quality. The datasets are valuable for studying the geomagnetic daily
variation, geomagnetic field model construction, and secular variation. It
should be noted that the data marked with Q (QC =Q) are used with caution for the reasons mentioned above. A few problems were found in the
acquisition of geomagnetic historical datasets: (1) the rescue of paper data is a time-consuming and laborious work. For example, the font color of some reports is light, which makes automatic recognition difficult. We can only recognize and input by key. (2) Some metadata of the SSH has been missing, which brings difficulties to the identification and correction of data. Therefore, we believe that we should do our best to rescue historical data to avoid the irreparable losses over time. Our plan for the next phase is to rescue the historical data from 1874 to 1932 at SSH, as well as from other observatories, to provide more high-quality data for the geomagnetic science community.
Author contributions
SZ performed data correction, quality assessment, and the analysis of application example. SZ prepared the manuscript. CF calculated the X, Y, and Z components from the COV-OBS model, drew pictures, and performed revision of the manuscript. GZ was responsible for the digitization of paper data from 1933 to 1945 of the SSH and provided digital series of the geomagnetic components of 2009 and 2011. CC designed a set of Excel templates. JW developed the data import software. SH and PG collected and organized metadata. GC sorted out the geomagnetic storm list and geomagnetic indices.
Competing interests
The contact author has declared that none of the authors has any competing interests.
Disclaimer
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Acknowledgements
We thank SSH, GNC, and the reference room of the Institute of Geophysics, China Earthquake Administration for providing the valuable data resources. We are grateful to editors and anonymous reviewers for their helpful reviews. We
sincerely thank all the staff who have ever worked or are currently working
at the SSH. We also thank GeoForschung Zentrum Potsdam, Germany, for providing online ap indices, World Data Center for Geomagnetism, Kyoto, for providing
online Dst indices, and the Kakioka Magnetic Observatory of the Japan
Meteorological Agency for providing the online magnetic storm catalog.
Financial support
This work was funded by the National Key R&D Program of China (grant no. 2017YFC1500205), the National Natural Science of Foundation of China (grant no. 41974073), the Macau Foundation, and the pre-research project of Civil Aerospace Technologies of China (grant no. D020308).
Review statement
This paper was edited by Kirsten Elger and reviewed by two anonymous referees.
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