Tilt Angle and Footpoint Separation of Small and Large Bipolar Sunspot Regions Observed with HMI

TL;DR

This study analyzes how tilt angle and footpoint separation evolve during the emergence and decay of bipolar sunspot regions, revealing size-dependent behaviors and confirming the existence of two distinct sunspot populations.

Contribution

It provides a detailed, high-cadence analysis of tilt and separation dynamics across different sunspot sizes using HMI data, highlighting size-dependent evolution patterns.

Findings

Large regions increase tilt angle during decay

Small regions often show negative tilt during emergence

Footpoint separation grows over 72 hours for most regions

Abstract

We investigate bipolar sunspot regions and how tilt angle and footpoint separation vary during emergence and decay. The Helioseismic and Magnetic Imager on board the Solar Dynamic Observatory collects data at a higher cadence than historical records and allows for a detailed analysis of regions over their lifetimes. We sample the umbral tilt angle, footpoint separation, and umbral area of 235 bipolar sunspot regions in Helioseismic and Magnetic Imager - Debrecen Data (HMIDD) with an hourly cadence. We use the time when the umbral area peaks as time zero to distinguish between the emergence and decay periods of each region and we limit our analysis of tilt and separation behavior over time to within +/-96 hours of time zero. Tilt angle evolution is distinctly different for regions with small (~30 MSH), midsize (~50 MSH), and large (~110 MSH) maximum umbral areas, with 45 and 90 MSH being useful divisions in separating the groups. At the peak umbral area, we determine median tilt angles for small (7.6 degrees), midsize (5.9 degrees) and large (9.3 degrees) regions. Within +/-48 hours of the time of peak umbral area, large regions steadily increase in tilt angle, midsize regions are nearly constant, and small regions show evidence of negative tilt during emergence. A period of growth in footpoint separation occurs over a 72-hr period for all of the regions from roughly 40 to 70 Mm. The smallest bipoles (< 9 MSH) are outliers in that they do not obey Joy's law and have a much smaller footpoint separation. We confirm Munoz-Jaramillo et al. (2015) results that the sunspots appear to be two distinct populations.