[Paper Review] GCR intensity during the sunspot maximum phase and the inversion of the heliospheric magnetic field

М. Б. Крайнев, G. A. Bazilevskaya+3 more|arXiv (Cornell University)|Sep 2, 2015

Solar and Space Plasma Dynamics4 references3 citations

TL;DR

This study analyzes galactic cosmic ray (GCR) intensity, heliospheric magnetic field (HMF) polarity, and sunspot activity during the maximum phase of solar cycle 24, revealing an extended HMF inversion period and anomalous GCR behavior. Using a classification of HMF polarity distributions, it finds that SC 24's HMF inversion phase was the longest on record, with GCR intensity showing delayed and unusual double-gap structure and strong energy hysteresis, suggesting prolonged modulation effects in the current solar cycle's maximum phase.

ABSTRACT

The maximum phase of the solar cycle is characterized by several interesting features in the solar activity, heliospheric characteristics and the galactic cosmic ray (GCR) intensity. Recently the maximum phase of the current solar cycle (SC) 24, in many relations anomalous when compared with solar cycles of the second half of the 20-th century, came to the end. The corresponding phase in the GCR intensity cycle is also in progress. In this paper we study different aspects of the sunspot, heliospheric and GCR behavior around this phase. Our main conclusions are as follows: 1) The maximum phase of the sunspot SC 24 ended in 06.2014, the development of the sunspot cycle being similar to those of SC 14, 15 (the Glaisberg minimum). The maximum phase of SC 24 in the GCR intensity is still in progress. 2) The inversion of the heliospheric magnetic field consists of three stages, characterized by the appearance of the global heliospheric current sheet (HCS), connecting all longitudes. In two transition dipole stages beside the global HCS there are additional local HCSs, while the inversion stage lies between two transition dipole ones and there is no global HCS in this stage. The inversion stage of the current SC 24 is the longest when compared with those for SC 21-23. The second transition dipole stage and hence the whole inversion period of the heliospheric magnetic field in SC 24 provisionally ended in 08.2014. 3) The behavior of the GCR intensity in the period of the sunspot maximum phase and the inversion of the heliospheric magnetic fields for SC 21-23 demonstrates all the characteristic features for this period: the two-gap structure corresponding to two-peak structure in the sunspot activity, and the energy hysteresis. In the current SC 24 the GCR intensity shows rather unusual features and we should wait for one or even two years to see the whole picture.

Motivation & Objective

• To investigate the anomalous behavior of the maximum phase of solar cycle 24 in comparison to previous cycles (SC 20–23), particularly in sunspot area and GCR intensity.
• To clarify the physical meaning of heliospheric magnetic field (HMF) polarity inversion using a proposed classification of HMF polarity distributions.
• To examine the correlation between HMF polarity evolution, sunspot activity, and GCR intensity modulation during high solar activity phases.
• To assess whether the observed GCR intensity patterns in SC 24—especially delayed double-gap structure and energy hysteresis—reflect a deviation from the established behavior in earlier cycles.

Proposed method

• Utilized Carrington rotation-averaged sunspot area data (Sss(t)) from Greenwich-USAF and pre-1870 reconstructions for solar activity proxy.
• Employed heliospheric magnetic field (HMF) parameters from the Wilcox Solar Observatory, including quasi-tilt angle (αqt), high-latitude line-of-sight component (Bls^pol), and spherical harmonic coefficients.
• Applied HMF strength (Bhmf) data from the OMNI database to assess modulation effects on GCRs.
• Used stratospheric balloon monitoring (RBM) data from Murmansk and Moscow as proxies for intermediate (Jint) and low-energy (Jlow) GCR intensities (Teff ≈ 3 GeV and few hundred MeV).
• Incorporated neutron monitor data (Moscow) as a proxy for high-energy GCR intensity (Jhigh, Teff ≈ 15 GeV).
• Classified HMF polarity distributions into three stages: 'transition dipole' (A>0 and A<0) and 'inversion' (no global HCS), with αqt used to define transitions between stages.

Experimental results

Research questions

• RQ1 How does the maximum phase of solar cycle 24 differ from previous cycles in terms of sunspot area and GCR intensity behavior?
• RQ2 What is the duration and structure of the HMF polarity inversion in SC 24 compared to SC 21–23, and how does it relate to the heliospheric current sheet (HCS) configuration?
• RQ3 To what extent do the double-gap structure in GCR intensity and energy hysteresis in SC 24 deviate from the patterns observed in earlier even and odd-numbered cycles?
• RQ4 How do the timing and morphology of HMF polarity transitions correlate with the Gnevyshev gap and sunspot activity peaks in the maximum phase?
• RQ5 What does the delayed onset of the first GCR intensity gap in SC 24 imply about the evolution of HMF strength and cosmic ray modulation?

Key findings

• The maximum phase of solar cycle 24 in sunspot area ended in June 2014, with the cycle's development resembling that of SC 14 and SC 15 (the Glaisberg minimum).
• The HMF inversion in SC 24 was the longest on record, with the 'inversion' stage (no global HCS) lasting significantly longer than in SC 21–23.
• The second 'transition dipole' stage of HMF polarity inversion in SC 24 ended in August 2014, though additional Carrington rotations with this configuration were expected into 2015 due to low N-hemisphere high-latitude SMF strength.
• The GCR intensity in SC 24 exhibited a delayed first gap (February 2012) and no visible second gap for high-energy particles, indicating anomalous modulation behavior.
• The energy hysteresis loop for SC 24 was wide, consistent with expectations for an even-numbered cycle, but the full structure remains unresolved due to ongoing maximum phase.
• The HMF 'inversion' period in SC 24 was centered slightly before the Gnevyshev gap in sunspot area and HMF strength, aligning with the established pattern observed in earlier cycles.