[Paper Review] Long-term trends of magnetic bright points: I. Number of MBPs at disc centre

D. Utz, R. Müller+6 more|arXiv (Cornell University)|Nov 24, 2015

Solar and Space Plasma Dynamics56 references3 citations

TL;DR

This study analyzes long-term trends in magnetic bright points (MBPs) using eight years of Hinode/SOT G-band data (2006–2014), revealing that MBP numbers at the solar equator follow the solar cycle with a 2.5-year lag and are strongly modulated by hemisphere-specific activity. The results indicate that MBPs near the equator are linked to sunspot activity belts, with up to 60% of maximum MBP counts persisting during minima, suggesting a connection to both global and local dynamo processes.

ABSTRACT

Context. The Sun shows an activity cycle that is caused by its varying global magnetic field. During a solar cycle, sunspots, i.e. extended regions of strong magnetic fields, occur in activity belts that are slowly migrating from middle to lower latitudes, finally arriving close to the equator during the cycle maximum phase. While this have been well known for centuries, much less is known about the solar cycle evolution of small-scale magnetic fields. Aims. To address this question, we study magnetic bright points (MBPs) as proxies for such small-scale, kG solar magnetic fields. This study is based on a homogeneous data set that covers a period of eight years. Methods. An automated MBP identification algorithm was applied to the synoptic Hinode/SOT G-band data over the period November 2006 to August 2014, i.e. covering the decreasing phase of Cycle 23 and the rise, maximum, and early decrease of Cycle 24. This data set includes, at the moment of investigation, a total of 4 162 images, with about 2.9 million single MBP detections. Results. After a careful preselection and monthly median filtering of the data, the investigation revealed that the number of MBPs close to the equator is coupled to the global solar cycle but shifted in time by about 2.5 years. Furthermore, the instantaneous number of detected MBPs depends on the hemisphere, with one hemisphere being more prominent, i.e. showing a higher number of MBPs. After the end of Cycle 23 and at the starting point of Cycle 24, the more active hemisphere changed from south to north. Conclusions. These findings suggest that there is indeed a coupling between the activity of MBPs close to the equator with the global magnetic field. The results also indicate that a significant fraction of the magnetic flux that is visible as MBPs close to the equator originates from the sunspot activity belts.

Motivation & Objective

• To investigate whether small-scale magnetic fields, as traced by magnetic bright points (MBPs), exhibit long-term activity cycle behavior similar to sunspots.
• To determine the temporal relationship between MBP numbers and the global solar magnetic cycle, particularly during the transition from solar cycle 23 to 24.
• To assess the influence of hemispheric asymmetry on MBP distribution and activity.
• To evaluate whether persistent MBP activity during solar minima indicates a local dynamo contribution beyond the global cycle.

Proposed method

• An automated MBP detection algorithm was applied to 4,162 synoptic Hinode/SOT G-band images collected from November 2006 to August 2014.
• Monthly median filtering and data preselection were used to reduce noise and isolate long-term trends from short-term variability.
• MBP counts were analyzed as a function of time and latitude, focusing on the equatorial region and hemispheric differences.
• The temporal evolution of MBP numbers was compared with the sunspot number and solar cycle phase to assess phase lag and correlation.
• Instrumental aging effects were evaluated to ensure that observed trends were physical rather than artifacts of data degradation.
• Hemispheric asymmetry was quantified by comparing MBP counts in northern and southern solar hemispheres over time.

Experimental results

Research questions

• RQ1 Is there a long-term cycle in the number of magnetic bright points (MBPs) at solar disc center, and if so, how does it relate to the sunspot cycle?
• RQ2 Does the MBP activity cycle exhibit a time lag relative to the sunspot cycle, and what might explain such a delay?
• RQ3 How does the distribution and number of MBPs vary between the northern and southern solar hemispheres over the solar cycle?
• RQ4 What fraction of maximum MBP activity persists during solar minimum, and what does this imply about the origin of small-scale magnetic flux?

Key findings

• The number of MBPs near the solar equator shows a strong correlation with the global solar cycle, but with a time lag of approximately 2.5 years.
• The more active hemisphere shifted from south to north around the minimum between solar cycles 23 and 24, indicating a reversal in hemispheric dominance.
• Clear peaks in MBP numbers were observed at latitudes of ±7°, coinciding with the positions of sunspot activity belts at the end of the solar cycle.
• Even during solar minimum, up to 60% of the maximum MBP detection rate was observed, suggesting a persistent source of small-scale magnetic flux.
• The observed trends and hemispheric asymmetry are robust against instrumental aging effects, indicating they are physically real.
• The results imply that a significant fraction of equatorial MBPs originate from decaying sunspot activity belts, while residual activity may stem from a local surface dynamo.