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31 July, 2021.

Can Solar Wind Velocity Predict Storm Strength?

The solar wind emanating from the Sun shapes the geomagnetic field of the Earth - our protective shield. But can it reliably predict how stressed that shield becomes during space weather storms?

28 February 2005 | This question has been a point of debate amongst researchers for a long time. Many have claimed that solar wind velocity is related to the strength of geomagnetic storms. Many others have suggested that the link is weak. What is known for certain is that the link appears strongest between the north-south component of the interplanetary magnetic field (IMF) and geomagnetic storms; when the IMF is directed southward, storm strength is strongest. But does a correlation exist between the solar wind velocity and geomagnetic storm strength? The proponderance of models in existence that make use of solar wind velocity suggests that such a relationship should exist. Some of the most sophisticated models in the world use the velocity of the solar wind to predict such things as the intensity of geomagnetic storms based upon the Dst index, which describes the intensity of the ring of electrical current that flows around the equatorial regions of the Earth. During intense geomagnetic storms, the strength of the ring current increases and this causes a correponding decrease in the strength of the magnetic field on the ground near the equatorial regions. That measured decrease is known as the Dst index, and has been used to quantize geomagnetic storm strength.

Predicting the strength of the ring current has become an important point of study because many different types of anomalies can be correlated to ring-current intensity. For example, damaging effects of space weather are most frequently observed when the Dst index drops below -300 nanoTeslas (nT). A value of zero nT is expected during background quiet conditions. The most intense Dst event that was unambiguously recorded by ground magnetometers was on 13 March 1989 when the Dst index fell to -589 nT. There are many models that have been created by scientists that attempt to predict Dst intensities through the use of various solar wind quantities, such as the velocity of the solar wind and the strength and orientation of the IMF. How well these models perform, or how reliable they may be, is determined in-part by how well each of the individual solar wind quantities correlate to storm strength.

A new study by Dr. R. P. Kane[1] of the Instituto Nacional de Pesquisas Espaciais, Sao Jose dos Campos, Brazil, sheds new light on this problem. Dr. Kane examined 30 years of data from 1973 to 2003 to determine whether solar wind velocity could be used to establish a relationship with geomagnetic storm strength (Dst). He found that solar wind velocity has virtually no relationship with geomagnetic storm strength, citing several notable cases where strong geomagnetic storms were associated with weak solar wind velocities, and several other cases where high solar wind velocities were associated with weak geomagnetic storms. For example, on 26 September 1989, a moderately strong geomagnetic storm with a respectable Dst index of -151 nT was associated with a weak solar wind flow measuring only 374 kilometers per second (km/sec). Conversely, the disturbance of 4 August 1972 was associated with a very high solar wind velocity of over 2,100 km/sec yet produced a similar strength geomagnetic storm having a Dst value of -152 nT. By comparison, the intense geomagnetic storm of 13 March 1989 (having a Dst value of -589 nT) was associated with a solar wind velocity of 550-800 km/sec. The scatter in solar wind velocity to Dst values is large. So large, in fact, that there is notable concern that models currently in use to predict Dst intensities may be performing no better by considering solar wind velocity than they would be if they disregarded it altogether. He suggests that no reasonable method yet exists to accurately predict Dst values prior to the arrival of the disturbance at the Earth. He notes with some embarrasment, that over the course of the last 60 years, our ability to predict geomagnetic storm intensity has not improved very much beyond what the pioneers in this field (Chapman and Bartels) could do in 1940.

Dr. Kane does point out, however, that solar wind velocity does have legitimate applications in determining the potential arrival times of coronal mass ejections (CMEs). Yet he also emphasizes that even the inclusion of solar wind velocities in time-of-arrival calculations is only accurate to within about 24 hours, if the CMEs impact the Earth at all (15% of all halo CME's don't reach the Earth). This rather poor prediction efficiency is a symptom of the need for additional strong research to attempt to find a better proxy for storm strength from solar observations.

This research was published in the Journal of Geophysical Research, volume 110 on 25 February 2005 (A02213, doi:10.1029/2004JA010799, 2005) by the American Geophysical Union.

[1]R.P. Kane (2005), How good is the relationship of solar and interplanetary plasma parameters with geomagnetic storms?, Journ. Geophys. Res., 110, A02213, doi:10.1029/2004JA010799, 2005