On Geomagnetic Data

Data available from the DACGSM (Data Analysis Center for Geomagnetism and Space Magnetism) WWW page are classified to
Geomagnetic indices, Geomagnetic Field Data at the Observatories, Geomagnetic Field Models and others.

Geomagnetic indices describe how the geomagnetic field is disturbed. The following geomagnetic indices are available from the DACGSM (Data Analysis Center for Geomagnetism and Space Magnetism) WWW page.

1. AE index (explanation):
   index that reflects electric currents flowing in the high-latitude ionosphere where the auroras appear.
2. Dst index (explanation):
   index that expresses disturbance of mid-latitude geomagnetic field parallel to geomagnetic dipole axis.
3. ASY index (explanation in PDF):
   index that expresses longitude-dependent disturbance of mid-latitude geomagnetic field, but SYM-H component is simlar to the Dst index.

4. Kp and ap indices:
   indices that indicate global geomagnetic field disturbance, derived by GeoForschungsZentrum (GFZ), Potsdam, Germany.
5. International Q- and D-days:
   List of 5 and 10 quietest days and 5 most disturbed days in a month, derived from the values of Kp index.

Geomagnetic field has been observed routinely at geomagnetic observatories and recorded as "geomagnetic field data". Geomagnetic field data are essential for research in geomagnetism: for instance, they are indispensable to derivation of geomagnetic indices and geomagnetic field models. Geomagnetic field is a vector and has three components like north-south, east-west, and up-down elements. Geomagnetic field data that are available on the DACGSM WWW page are classified into three types, that is, "final", "provisional", and "quick look", depending on the accuracy of measurement or delay time.

Among these three types, "quick look" means that data are collected via Internet, as promptness is a top priority. Quick look data are not calibrated (not checked to remove noises and not corrected with absolute measurement), thus they can be used just to monitor present status of geomagnetic field. The DACGSM WWW page provides not only plots of quick look data but also plots of quick look data observed at Shigaraki and Mineyama by DACGSM.

Provisional data are possible to be replaced by more accurate data, though they have already checked once or more times. Final data are basically definitive. DACGSM serves provisional and final data of some geomagnetic observatories from the WWW page. A list of these geomagnetic observatories can be found in "Online Data Catalog", which is updated automatically every week, and Page of "Catalogue search and plot" is also available. You may also see the "Newly Arrived Online-available Data" page.

There is another classification of geomagnetic data, that is, "digital" and "analog". Digital data are recorded electrically in recording devices such as hard disk drives, while analog data are recorded in printing papers or films, which are traditional recording devices since 19th century. Digital data can be subclassified by adding a statement of their time resolution (1-sec, 1-min, 1-hour, and so on). 1-sec digital data are suitable to investigate rapid variations of geomagnetic field, but they are available only for limited number of geomagnetic observatories and limited time periods. 1-min digital data make the primary constituents of digital data, thus you can find 1-min digital data of numerous observatories for recent periods. 1-hour digital data are adequate for analysis of long-term variations of geomagnetic field, because they can be generated by digitization of analog data and are available for periods of about 100 years. However, it should be noted that 1-hour digital data cannot be used to analyze geomagnetic phenomena with time scale less than hours. If you are interested in lower time resolution data (e.g., 1-year data), please visit World Data Center for Geomagnetism, Edinburgh, UK or Data Center in Boulder, Colorado, USA. The latter also provides Paleomagnetic Data.

Analog data are also referred to as "Magnetogram", and they are subclassified into "Normal run magnetogram" and "Rapid run magnetogram", depending on their recording speed. Rapid run magnetograms show geomagnetic field variations with shorter time scale than normal run magnetogtrams. Some observatories also provide "storm magnetogram" which is of low sensitivity for recording severe geomagnetic disturbance. DACGSM is generating digital images of magnetograms, which are thought to be potentially useful, and provides them from the WWW page. DAGGGM also provides digital images of tellurigram, which are analogue records of "Earth Current".

You may also visit WWW page of geomagnetic observatories, some of which provide geomagnetic field data.

Geomagnetic field models are useful to estimate roughly the field. IGRF, which is derived by IAGA at every five years, are available from the DACGSM WWW pages which give magnetic charts, coefficients, model value at a point and others. Transformation of coordinate which is from Geomagnetic (Dipole) to Geographic or from Geographic to Geomagnetic (Dipole) can be also possible.

The following contents are also available from the DACGSM WWW page:

1. Online Catalogue for Archieved Data [Status of Data Collection].
   (Newly arrived Data, Catalogue search, Catalogue search and plot and Data Catalogue in PDF)
2. Publication List
3. Height profile of Ionospheric conductivity model and Height integrated Ionospheric conductivity model [from IRI model]