Geology topics

Kalevi Mursula

Publications and source records attributed to Kalevi Mursula.

5 recordsLinked to original sources

USGS2026

Near-Earth solar wind speed of fast coronal mass ejections

Using solar wind and ground magnetometer data for solar cycles 23–25, extreme-value power-law models are developed that quantify relationships between near-Earth solar wind speed and Sun-to-Earth transit times of interplanetary coronal mass ejections (ICMEs). The models of near-Earth solar wind speed, conditional on ICME transit time, are (a) approximately stable for transit times as short as 15 hr, (b) sublinear, consistent with hydrodynamic drag acting on ICMEs during transit, with linear (ballistic) models not supported by the data, (c) representative of most of the information content of the data, and (d) insensitive to the solar cycle and interplanetary preconditioning. Applying the models to the September 1859 Carrington event ( 𝑇 =17.6 hours), we estimate an ICME-average speed of 𝑉 𝑎 =102⁢1 1164 896 km/s and a 1-hr ICME-maximum speed of 𝑉 𝑚 =157⁢5 1795 1382 km/s. These estimates indicate that Carrington-class magnetic storms do not require exceptionally extreme solar wind speeds, with values lower than some previous estimates.

JGR Space Physics↗

USGS2026

Interplanetary electric fields for extreme magnetic storms

Using a list of sudden-commencement storms, the ring-current index, and 1-h near-Earth solar-wind measurements from solar cycles 20–25, we develop extreme-value statistical models relating storm intensity 𝐷 =max⁡{−𝐷⁢𝑠⁢𝑡} to the storm main-phase maximum duskward interplanetary electric field 𝐸 . The conditional relationship 𝐷|𝐸 is demonstrably sublinear—linear models are confidently rejected—indicating saturation of magnetospheric response under extreme solar-wind forcing. An event like that of July 2012 ( 𝐸 =69.6 mV/m), if Earth-directed, would be associated with a median storm intensity of 𝐷 =49⁢5 648 378 nT. Storms comparable to March 1989 ( 𝐷 =594 nT) correspond to electric fields of 𝐸 =5⁢4 76 39 mV/m, while Carrington-class storms ( 𝐷 =964 nT) correspond to 𝐸 =9⁢5 133 68 mV/m—substantially lower than several previous estimates. These results indicate that solar-wind conditions capable of driving extremely intense magnetic storms are less exceptional, and potentially more frequent, than previously thought.

Geophysical Research Letters↗

USGS2025

What is the lowest latitude of discrete aurorae during superstorms?

From a survey of published accounts of visual sightings of aurorae, a compilation is presented of the lowest identified geomagnetic latitude at which discrete aurorae were seen at local zenith during magnetic storms having intensities with maximum − Dst > 200 nT. The compilation includes data for the superstorms of 2 September 1859, 4 February 1872, and 15 May 1921. A statistical model is developed representing the equatorward boundary of discrete aurorae versus storm intensity. The model indicates that a once-per-century storm would likely induce discrete aurorae at zenith down to a geomagnetic latitude of 34 ° . Insofar as aurorae can be taken as a proxy for electrojet currents, such a storm would expose many nighttime electric-power systems, in the contiguous United States or Europe, to high levels of geomagnetic disturbance. A Carrington-class storm would induce discrete aurorae down to 24 ° . These exposures are much greater than those indicated in recent numerical simulations of extreme magnetic storms. Using the model to infer storm intensity from reports of low-latitude aurorae, a storm on 28 August 1859, likely had maximum − Dst = 673 nT. That this storm occurred just a few days before the Carrington storm of 2 September (maximum − Dst = 964 nT) deserves attention. A storm that occurred on 17 September 1770 is estimated to have had maximum − Dst = 928 nT. The vision of Ezekiel could have been inspired by aurorae from a storm with maximum − Dst = 550 nT.

Space Weather↗

USGS2024

Challenging ring-current models of the Carrington storm

A detailed analysis is made of horizontal-component geomagnetic-disturbance data acquired at the Colaba observatory in India recording the Carrington magnetic storm of September 1859. Prior to attaining its maximum absolute value, disturbance at Colaba increased with an e -folding timescale of 0.46 hr (28 min). Following its maximum, absolute disturbance at Colaba decreased as a trend having an e -folding timescale of 0.31 hr (19 min). Both of these timescales are much shorter than those characterizing the drift period of ring-current ions. Furthermore, over one 28-min interval when absolute disturbance was increasing, the data indicate an absolute rate of change of ≥2,436 nT/hr. If this is representative of disturbance generated by a symmetric magnetospheric ring current, then, assuming a standard and widely used parameterization, an interplanetary electric field of ≥451 mV/m is indicated. An idealized and extreme solar-wind dynamic pressure could, conceivably, reduce this bound on the interplanetary electric field to ≥202 mV/m. If the parameterization for electric-field extrapolation is accurate, but the field strengths obtained are deemed implausible, then it can be concluded that the Colaba disturbance data were significantly affected by partial-ring, field-aligned, or ionospheric currents. The same conclusion is supported by the shortness of the e -folding timescales characterizing the Colaba data. Several prominent studies of the Carrington event need to be reconsidered.

Journal of Geophysical Research Space Physics↗

USGS2024

On the uncertain intensity estimate of the 1859 Carrington storm

A study is made of the intensity of the Carrington magnetic storm of September 1859 as inferred from visual measurements of horizontal-component geomagnetic disturbance made at the Colaba observatory in India. Using data from modern observatories, a lognormal statistical model of storm intensity is developed, to characterize the maximum-negative value of the storm-time disturbance index (maximum – Dst) versus geomagnetic disturbance recorded at low-latitude observatories during magnetic storms. With this model and a recently published presentation of the Colaba data, the most likely maximum – Dst of the Carrington storm and its credibility interval are estimated. A related model is used to examine individual Colaba disturbance values reported for the Carrington storm. Results indicate that only about one in a million storms with maximum – Dst like the Carrington storm would result in local disturbance greater than that reported from Colaba. This indicates that either the Colaba data were affected by magnetospheric-ionospheric current systems in addition to the ring current, or there might be something wrong with the Colaba data. If the most extreme Colaba disturbance value is included in the analysis, then, of all hypothetical storms generating the hourly average disturbance recorded at Colaba during the Carrington storm, the median maximum – Dst = 964 nT, with a 68% credibility interval of [855,1087] nT. If the most extreme Colaba disturbance value is excluded from the analysis, then the median maximum – Dst = 866 nT, with a 68% credibility interval of [768,977] nT. The widths of these intervals indicate that estimates of the occurrence frequency of Carrington-class storms are very uncertain, as are related estimates of risk for modern technological systems.

Journal of Space Weather and Space Climate↗