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The largest geomagnetic storms of solar cycle 24 so far occurred on 2015March 17 and June 22 with $D_{\rm st}$ minima of $-223$ and $-195$ nT,respectively. Both of the geomagnetic storms show a multi-step development. Weexamine the plasma and magnetic field characteristics of the driving coronalmass ejections (CMEs) in connection with the development of the geomagneticstorms. A particular effort is to reconstruct the in situ structure using aGrad-Shafranov technique and compare the reconstruction results with solarobservations, which gives a larger spatial perspective of the source conditionsthan one-dimensional in situ measurements. Key results are obtained concerninghow the plasma and magnetic field characteristics of CMEs control thegeomagnetic storm intensity and variability: (1) a sheath-ejecta-ejectamechanism and a sheath-sheath-ejecta scenario are proposed for the multi-stepdevelopment of the 2015 March 17 and June 22 geomagnetic storms, respectively;(2) two contrasting cases of how the CME flux-rope characteristics generateintense geomagnetic storms are found, which indicates that a southwardflux-rope orientation is not a necessity for a strong geomagnetic storm; and(3) the unexpected 2015 March 17 intense geomagnetic storm resulted from theinteraction between two successive CMEs plus the compression by a high-speedstream from behind, which is essentially the "perfect storm" scenario proposedby \citet[][i.e., a combination of circumstances results in an event of unusualmagnitude]{liu14a}, so the "perfect storm" scenario may not be as rare as thephrase implies.

Plasma and Magnetic Field Characteristics of Solar Coronal Mass Ejections in Relation to Geomagnetic Storm Intensity and Variability