Daily Magnetograms – NAO Rozhen

Geomagnetic variations and K-index

The graphs show the real-time variations in the local components of the Earth's magnetic field and estimated K-indices at National Astronomical Observatory (NAO) Rozhen, Bulgaria.

The three components of the magnetic intensity vector X, Y, Z and the total intensity (absolute value) F are in nT (nanotesla). The observations are performed continuously using a geophysical vector magnetometer MG-01, located at the National Astronomical Observatory – Rozhen in the south region of Bulgaria (Geodetic coordinates: 41°41′45′′N; 24°44′21′′E; Elevation: 1731 m). The sampling period of the raw data is 1 second. These raw measuremens are centered around zero-second of every minute and filtered using an IAGA recomended Gaussian low-pass filter. Thus, 1-minute averaged data are produced witch are shown on the graphs. The page is updated every 10 minutes with new values. Time is UTC and is measured in hours.

To estimate the amplitude of the local variations in the intensity of the geomagnetic field, the calculation of the so-called K-index has been internationally adopted. It shows the correspondence, expressed in quasi-logarithmic scale, between the amplitude of the disturbances in the horizontal component of Earth's magnetic field and an integer number, in the range from 0 to 9. As the K-index value increases by one unit, the amplitude increases approximately twice. The index is calculated every three hours. The amplitude is determined using the difference between the maximal and minimal intensity values measured for the last three hours.

An important part of the K-index calculating procedure is the determination of the so-called Solar quiet (Sq) curve, consisting of the undisturbed solar variations of the geomagnetic field. It shows the regular diurnal changes in the Earth's magnetosphere as a consequence of the planet's rotation around its axis. During this process, on the side illuminated by the Sun, the ionization caused by solar UV radiation leads to the formation of huge currents (10⁷ A) flowing in the upper layers of the atmosphere (the so-called ionosphere) at 90 - 150 km altitude above the surface. This current system forms the diurnal "normal" variations in the magnetosphere, which are not related to the solar wind and the disturbances known as magnetic storms. Therefore, for the correct determination of the local K-indexes, it is necessary to subtract the Sq curve from the observational data. Unfortunately, this curve is "buried" under the magnetic perturbations caused by the solar wind, which in turn leads to difficulties in its determination in pure form.

Different approaches to solving this problem exist. One of them, probably the most accurate one, involves statistically averaging the data from "quiet" days for a given period of the year, when no noticeable fluxes of charged solar particles are observed to be attacking the Earth's magnetosphere. The obtained average Sq curve possesses a high degree of certainty. Unfortunately, this methodology requires a solid base of observational data covering a period of at least several years, which is still lacking in our observations. That is why we have adopted a different approach to the estimation of K-indices. For each three-hour observation period, as a first approximation of the real Sq curve, a regression line is calculated using least squares fit, and then subtracted from the values ​​of the corresponding magnetic data. This results in the pure magnetospheric perturbations, from which the maximum magnitude of the disturbance during the three-hour period is determined. This procedure is performed independently for the X and Y components of the geomagnetic field, and finally, the maximum of the two values ​​is taken. The local K-index is calculated using the table below.

It is important to note that the described procedure so far provides only a preliminary estimate of the local K-indices, and the values ​​shown on the page although correct, should not be taken as a final result. As more of our observational data are accumulated in the future, we expect that the procedure will be improved and refined.

A higher value of the K-index corresponds to a greater disturbance in the magnetic field. Values of 0, 1, 2, 3 indicate a quiet field or low geomagnetic activity and are usually shown in green color. A value of 4 means a moderate activity and is displayed in yellow. Values of 5 and higher mean a magnetic storm and are displayed in red color.

The Earth's magnetic field has different intensities in the different latitude regions. The same process (or perturbation) in the magnetosphere can be detected having a different amplitude in two magnetic observatories located on the surface. As we approach the magnetic poles, the amplitude increases. An individual table is calculated for each observational location, showing the relationship between the amplitudes of the geomagnetic disturbances and a given index number. Thus, a correlation between the values of the K-index for the various magnetic observatories around the world is ensured. The following table is used in the region of Bulgaria:

In geophysical science, the so-called planetary index or Kp-index is used. It is estimated as a weighted mean of the local K-indexes calculated in 13 geomagnetic observatories located in the mid-latitude region. The accuracy of the Kp-index is 1/3. The index values are published daily by international geomagnetic centers.