The storm began quite unexpectedly.
The strongest geomagnetic storm for the last seven months began on Earth almost precisely at the same time as the solar eclipse reached its maximal stage, the culmination of which occurred at GMT 02:35 (06:35 Moscow time). The ten-point Kp-index of geomagnetic activity, which only exceeded level 3 once in July, registered a rapid jump to Kp=6 within an hour, corresponding to a level 2 magnetic storm. Oscillations of such amplitude in the magnetic field have not been observed since October, 11th of last year, which is more than seven months ago.
The unified scale of force for magnetic storms was introduced by the US National Oceanic and Atmospheric Administration (NOAA) in November, 1999, by analogy with the Botsford scale, used for estimating wind velocity. As with the Botsford scale, each level of which is characterised not only by figures, but also by vivid pictures of the observed phenomena (from the light swaying of trees to the destruction of capital structures), the scale of space weather categorises storms not only using figures, but also by means of processes occurring in the Earth's magnetosphere.
According to this scale, level 2 storms can influence power systems located at high latitudes and affect space vehicles - leading to differences in their orbital drift compared with drift predictions. Such storms can also cause deterioration of high-frequency radio wave distribution at high latitudes and can be accompanied by polar auroras at latitudes up to 50 degrees.
If we reject mystical causes for the storm, its sudden appearance is genuinely amazing. The Sun is in a state of total calm, there are no sunspots or even visible active areas on its surface. Furthermore, no flashes or mass ejections have been registered for the last several days.
In such cases, scientists often explain unexpected oscillations in the Earth's magnetic field in terms of the passage of our planet through the interface of the interplanetary magnetic field. The Earth's magnetosphere is closely interconnected with that of the Sun, being in certain equilibrium with it in accordance with the law of action and reaction.
If the Sun's magnetic field is, for some reason or other, "torn" from the lines of the Earth, the magnetic field of our planet "loses support" and starts to oscillate, searching for a new equilibrium point. However, these oscillations are never very significant; otherwise powerful magnetic storms would occur every 14 days (the period of half-turn of the Sun and its magnetic fields).
However, even this explanation does not fit the given case. The Earth crossed the sector boundary of the magnetic field without incident two days ago, on July 20th, and is now moving within the sector with negative magnetic field polarity (negative sign). Passage to the following sector with positive magnetic field polarity (positive sign) will occur no sooner than August 2nd-3rd, 2009.
The only thing we can do is return to the mystical explanations: for example, the lines of magnetic connection between the Earth and the Sun were broken by the passage of the Moon.
However, this conclusion is not scientifically viable as the Moon does not have its own magnetic field. A better explanation, in term of physics, would be that the electrical charge, which can be discharged on to the Earth (but only under certain conditions), can be accumulated on the surface of the Moon under the influence of the Sun's ionizing radiation. An example of these special conditions might be: when a planet and satellite pass close to one another and there is a certain configuration of magnetic fields, at which point they can act as electricity conductors. However, this scenario has not yet been theoretically substantiated.
We can only add that the magnetic storm lasted approximately 9 hours. The magnetosphere returned to a state of calm at around GMT 12.00 (16:00 Moscow time).