Owens et al. (2021) showed with the 150-year homogenized geomagnetic index aaHaa_H that extreme geomagnetic storms are not only more frequent in the active phase of the solar cycle and in larger cycles, but also cluster at different times within the active phase depending on whether the solar cycle is odd- or even-numbered: extreme events concentrate in the early active phase of even cycles and the late active phase of odd cycles. The physical mechanism behind this parity dependence is unresolved. The open problem is to determine whether it arises from the solar-cycle-dependent polarity of coronal mass ejection (CME) flux-rope axial fields (Hale's law), from the ambient overlying coronal magnetic field swept up by fast CMEs — and if so whether the added geoeffectiveness manifests as enhanced flux-rope field or enhanced sheath field — or from parity-dependent modulation of the heliospheric magnetic-field strength, using in situ observations and/or physics-based modelling.

Background

The roughly 11-year solar cycle modulates space weather: geomagnetic storms, which are global disturbances of Earth's magnetic field, occur more often near solar maximum than minimum. The dominant drivers of large storms are coronal mass ejections (CMEs) — eruptions of magnetized plasma from the Sun — which near Earth appear as interplanetary CMEs (ICMEs) typically containing a magnetic flux rope (a helical, rope-like magnetic structure) preceded by a compressed, turbulent sheath region formed by the CME ploughing through the ambient solar wind. Storm intensity is controlled mainly by the southward interplanetary magnetic field, which enables reconnection with Earth's day-side field and injects energy into the magnetosphere; both the flux-rope field and the shocked sheath field can supply it. Because extreme storms are by definition rare, their occurrence statistics must be extracted from very long records. The aaaa index is a 150-year measure of global geomagnetic activity built from pairs of magnetometer stations; aaHaa_H is a homogenized version that corrects for station changes, secular variation of Earth's main field, and other inhomogeneities (Lockwood et al., 2018).

Owens, Lockwood, Barnard, Scott, Haines and Macneil (Solar Physics 296, 82, 2021) fitted probabilistic occurrence models to the aaHaa_H record and found: (1) storms of all magnitudes occur more often in an 'active phase' spanning solar-cycle phases 0.18–0.79; (2) the available data are consistent with larger cycles producing proportionally more extreme events; and (3) extreme events cluster early in the active phase of even-numbered cycles but late in odd-numbered cycles — for the six largest events (those exceeding the 99.99th percentile of one-day aaHaa_H values) the split is 3 early/even and 3 late/odd, with probability p=0.016p = 0.016 of arising by chance. Crucially, they found no difference in storm occurrence averaged over odd versus even cycles, only in the timing within the cycle.

The Sun's magnetic cycle is 22 years long (the Hale cycle): sunspot polarities reverse every 11-year cycle (Hale's law), and the polar coronal fields reverse near each maximum. The heliospheric polarity state is conventionally labelled qA>0qA>0 when the northern solar polar cap has positive (outward) polarity. Statistically, CME flux-rope axial-field polarities follow Hale's law and so are opposed in odd and even cycles (Bothmer and Rust, 1997; Lynch et al., 2005). However, magnetic clouds (ICME flux ropes) of opposite polarity show no obvious difference in geoeffectiveness (Fenrich and Luhmann, 1998; Kilpua et al., 2012), and sheath fields are geoeffective in their own right (Tsurutani et al., 1988). A 22-year cycle in overall geomagnetic and solar-wind activity has long been known (Cliver et al., 1996), and cosmic-ray and heliospheric data indicate that the heliospheric magnetic field — the Sun's magnetic field carried through interplanetary space by the solar wind — is directly modulated by the Hale cycle, with different field strengths in the two polarity states (Thomas, Owens and Lockwood, 2014). Statistical studies of magnetic clouds and their sheaths confirm that the largest storms often arise from combinations of sheath southward field and southward-leading flux ropes (Li, Luhmann and Lynch, 2018; Huttunen et al., 2005).

Owens et al. (2021) therefore proposed — as a speculative interpretation requiring direct coronal and heliospheric observations and modelling — that during qA>0qA>0 intervals the large-scale coronal field near the heliographic equator is on average southward, so that fast CMEs drag out and amplify this overlying field, tipping already-severe events into the extreme category; whether this extra field would appear as additional flux-rope field or as enhanced sheath field was explicitly stated to be unclear, and parity-dependent heliospheric field strength was offered as a further candidate. Testing this picture is hampered by the small number of extreme events in even the longest homogeneous records (14 complete cycles, six 99.99th-percentile events in 150 years).

Problem Statement

Determine the physical origin of the odd/even (Hale-cycle parity) timing modulation of extreme geomagnetic storms reported by Owens et al. (2021): extreme storms cluster in the early active phase of even-numbered solar cycles and the late active phase of odd-numbered cycles, with no difference in occurrence averaged over odd and even cycles. Specifically, establish — from in situ observations and/or physics-based modelling — whether this parity dependence arises from (i) the solar-cycle-dependent polarity of CME flux-rope axial fields under Hale's law, (ii) the polarity of the ambient overlying coronal magnetic field swept up by fast CMEs, and if so whether the added geoeffectiveness manifests as additional flux-rope field or as enhanced sheath field, or (iii) parity-dependent modulation of the heliospheric magnetic-field strength. A complete answer must discriminate among these mechanisms with a stated statistical significance, be consistent with the absence of any average odd/even difference in storm occurrence and with prior null results on magnetic-cloud polarity geoeffectiveness, and hold despite the finite length of homogeneous geomagnetic records (14 complete cycles; six 99.99th-percentile events in 150 years of aaHaa_H data).

References

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