Geoeffectiveness and efficiency of CIR, Sheath and ICME in generation of magnetic storms

We investigate relative role of various types of solar wind streams in generation of magnetic storms. On the basis of the OMNI data of interplanetary measurements for the period of 1976-2000 we analyze 798 geomagnetic storms with Dst < -50 nT and their interplanetary sources: corotating interaction regions (CIR), interplanetary CME (ICME) including magnetic clouds (MC) and Ejecta and compression regions Sheath before both types of ICME. For various types of solar wind we study following relative characteristics: occurrence rate; mass, momentum, energy and magnetic fluxes; probability of generation of magnetic storm (geoeffectiveness) and efficiency of process of this generation. Obtained results show that despite magnetic clouds have lower occurrence rate and lower efficiency than CIR and Sheath they play an essential role in generation of magnetic storms due to higher geoeffectiveness of storm generation (i.e higher probability to contain large and long-term southward IMF Bz component).

💡 Research Summary

The authors performed a comprehensive statistical study of the relative importance of three major solar‑wind structures—corotating interaction regions (CIR), sheath regions preceding interplanetary coronal mass ejections (ICME), and the ICME bodies themselves (magnetic clouds, MC, and ejecta)—in generating geomagnetic storms. Using the OMNI database for the interval 1976‑2000, they identified 798 storms with Dst ≤ ‑50 nT and classified the interplanetary driver for each event based on a set of plasma and magnetic‑field criteria (velocity, density, temperature, β‑parameter, temperature‑to‑expected‑temperature ratio, thermal pressure, magnetic field). Gaps in the data were compensated by normalising occurrence rates to the total observation time of each year.

The study first quantifies the occurrence rates of the different solar‑wind types. Steady streams (fast, slow, heliospheric current sheet) account for roughly 60 % of the total observation time, while disturbed streams (CIR, sheath, MC, ejecta) occupy 25‑50 % depending on the phase of the solar cycle. Mass, momentum, and energy fluxes peak in CIR and sheath intervals, whereas magnetic flux is highest in magnetic clouds. However, the integrated (time‑averaged) losses of these quantities are larger for the steady fast and slow streams because of their longer duration.

Next, the authors evaluate “geoeffectiveness”, defined as the probability that a given solar‑wind type will produce a storm. Of the 798 storms, 464 (58 %) could be linked to a specific solar‑wind driver; the remaining 42 % lack sufficient plasma data for classification. Magnetic clouds, despite being the least frequent, exhibit the highest geoeffectiveness (≈ 45 % of MC intervals contain a storm) because they more often contain prolonged, intense southward IMF Bz. CIR and sheath intervals are more common but have lower geoeffectiveness (≈ 30‑32 %) due to shorter or more intermittent southward Bz episodes. Ejecta show the lowest probability (≈ 28 %).

The paper then introduces “efficiency”, the ratio of geomagnetic response to the integrated solar‑wind driver. By correlating Dst (and the pressure‑corrected Dst*) with the time‑integrated Bz (∫Bz dt) and the convective electric field Ey (∫Ey dt) for each driver type, linear regression yields distinct slopes: CIR ≈ 0.42 nT /(h·nT), sheath ≈ 0.38, ejecta ≈ 0.35, and MC ≈ 0.24. Thus, MCs are the least efficient at converting southward Bz (or Ey) into a Dst depression, while CIR and sheath are the most efficient. Similar analyses for the planetary‑range indices Kp and AE reveal non‑linear, saturated relationships, with CIR and sheath again showing higher efficiency than MC and ejecta.

A novel methodological element is the double‑superposed epoch analysis (DSEA). The authors align all storms at two reference times—storm onset (t = 0) and Dst minimum (t = 6)—and stretch/compress the intervals so that the main phase duration is identical for every event. This permits a direct comparison of the temporal evolution of Bz, Ey, and geomagnetic indices across different driver types. The DSEA confirms that the pre‑history of Bz/Ey (approximately the two‑hour lag between Bz minimum and Dst peak) influences the storm development, emphasizing that instantaneous values alone are insufficient for accurate forecasting.

Key insights from the study are:

1. Magnetic clouds, although rare and less efficient per unit of southward Bz, dominate storm generation because they more reliably provide long‑lasting southward IMF.
2. CIR and sheath regions are frequent and energetically potent, delivering high instantaneous fluxes, but their lower geoeffectiveness limits their overall contribution.
3. The linear relationship between Dst and integrated Bz/Ey provides a useful quantitative metric for driver efficiency and can be incorporated into real‑time storm prediction models.
4. The DSEA technique offers a robust framework for assessing driver‑storm coupling that accounts for both driver intensity and temporal structure.

Overall, the paper delivers a thorough comparative assessment of solar‑wind drivers, integrating occurrence statistics, physical fluxes, probabilistic storm generation, and energy‑conversion efficiency. These results refine our understanding of how different interplanetary structures contribute to geomagnetic activity and provide practical parameters for improving space‑weather forecasting.