The Geomagnetic Storm of May 1921

The Geomagnetic Storm of May 15-16, 1921


A geomagnetic storm is a temporary disturbance of the earth’s magnetosphere caused by a solar wind gusts or due to solar coronal mass ejection. The increase in solar wind pressure compresses the magnetosphere as the solar wind’s magnetic field interacts with the earth’s magnetic field as energy is transferred to the magnetosphere. This causes an increase in movement of plasma through magnetosphere and an increase in electric current in the magnetosphere and the ionosphere.


The geomagnetic storm of 1921 known as the great geomagnetic storm of 1921 was an extreme but rare event. The financial loss from such a storm could reach $2 trillion in the first year alone (1). Because this event occurred before extensive interconnectivity of electrical systems and the general electrical dependency across infrastructures, so the financial losses and effects were not that devastating despite the ground currents being higher in magnitude than the 1989 geomagnetic storm. The precursor to this storm was a major sunspot big enough to be sighted with the naked eye through smoked glass (2).The scientist gave the size of the sunspot at that time to be 94,000 by 21,000 miles in size (3). Northern Lights appeared in eastern United States.

In the great geomagnetic storm of 1921, disturbance levels as high as 5000 nT/min may have occurred with a Dst range as much as -900 nT. This Dst is 50% larger than 1989 Dst, which is the largest on record (4).


The geomagnetic storm of 1921 caused a worldwide disruption of telegraph service the entire signal and switching system of the New York Central Railroad was put out of operation. The currents induced in some telegraph wires were strong enough to cause fire; as a result operators got injured by the exploding consoles. In New Zealand there were erratic signals at all radio stations, violet fluctuation of current affected telephone lines and unusual interferences occurred at some telephone exchanges (5).

The cause of the New York Central railroad outage was the ground current that had invaded the electrical system. However, the radio waves were strengthened during the storm which resulted in strong intercontinental reception but electric lights were not affected (6).

If this storm were to occur today, it would melt and burn the copper windings of more than 350 high voltage transformers in US with the possibility of leaving 130million without power (7). Because of the interconnectivity of electrical components in todays developed world, the effects of power loss would ripple across the social infrastructure causing cascading failures. Most transformers from space weather impacts cannot be repaired at all, and 500 kV and 765kV transformers are no more manufactured in US. Plus these transformers weigh 100 tons and the backlog is of 3 years. Thus full recovery from such an event will take 4-10 years.

Figure 1: The interconnectivity of US electrical grid lines (

The figure 1 shows the network of extra high voltage long-distance transmission lines across the US and how they are interconnected in today’s world. The circles show geomagnetically induced currents flowing in each transformer and the red circled area is the region that is susceptible to system collapse if an event like 1921 were to occur today.



The geomagnetic storm can leave extensive footprints with power grids being most vulnerable to disturbances from the space environment.  The rate of change of the horizontal component of the geomagnetic field (dBh/dt) primarily creates geo-electric fields that cause the flow of GIC in power grids. The impulsive geomagnetic environment can develop and propagate rapidly exposing large geographic areas to its influence.

Large GICs (as in the case of May 1921 storm) are most closely related to geomagnetic field disturbances that have high rate of change, hence high cadence and dBh/dt analysis which can be used to quantify GIC. The GIC threats are traditionally been associated with auroral electroject intensifications at mid and high latitude locations (4).

However for analysis there is limited data available. Long-term detailed climatology on impulsive geomagnetic field disturbances is not available. Silverman and Cliver’s recent paper does provide a descriptive on the solar and aurora observations for the 1921 storm. There have been a number of plots from this area that have been preserved and provide some perspective into the event. However, there are no other observation details from Northern Europe that would confirm such a large impulsive disturbance (8).The most detailed information on the climatology of impulsive disturbances can only be derived for the last 20-25 years of storms as there is availability of digitally recorded data.


Filed Under: Space Physics

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Hello, My name is Talha and I am a recent graduate of University College London in Electronic and Electrical Engineering and Masters in Space Science and Engineering. I have extensive experience in teaching Maths/Further Maths and Physics in O and A levels. I can help with course work, .... Read More