Bimodal Distribution of Magnetic Fields and Areas of Sunspots

TL;DR

This study analyzes sunspot magnetic fields and areas, revealing a bimodal distribution linked to sunspot development stages and magnetic inclination differences, using data from SDO/HMI.

Contribution

It uncovers a bimodal distribution of magnetic flux in sunspots and relates it to penumbra formation stages and magnetic field inclination.

Findings

Bimodal distribution of average magnetic flux density in sunspots.

Relation between sunspot size and magnetic flux density.

Connection between magnetic field inclination and sunspot development stages.

Abstract

We applied automatic identification of sunspot umbrae and penumbrae to daily observations from the Helioseismic Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) to study their magnetic flux (B) and area (A). The results confirm a previously known logarithmic relationship between the area of sunspots and their maximum flux density. In addition, we find that the relation between average magnetic flux (Bavg) and sunspot area shows a bimodal distribution: for small sunspots and pores (A < 20 millionth of solar hemisphere, MSH), Bavg = 800 G (gauss), and for large sunspots (A > 100 MSH), Bavg is about 600 G. For intermediate sunspots, average flux density linearly decreases from about 800 G to 600 G. A similar bimodal distribution was found in several other integral parameters of sunspots. We show that this bimodality can be related to different stages of sunspot penumbra formation and can be explained by the difference in average inclination of magnetic fields at the periphery of small and large sunspots.