Invisible sunspots and rate of solar magnetic flux emergence

TL;DR

This study analyzes how sunspot visibility affects observed parameters, revealing significant center-to-limb variation and underestimation of growth phase durations, using a large dataset of sunspot regions.

Contribution

It introduces a new visibility function for sunspots and quantifies the impact of visibility biases on solar magnetic flux emergence measurements.

Findings

44% of new sunspot regions in the West go undetected

Visibility of small sunspots varies strongly with position on the solar disk

Growth phase durations of active regions are underestimated in previous studies

Abstract

We study the visibility of sunspots and its influence on observed values of sunspot region parameters. We use Virtual Observatory tools provided by AstroGrid to analyse a sample of 6862 sunspot regions. By studying the distributions of locations where sunspots were first and last observed on the solar disk, we derive the visibility function of sunspots, the rate of magnetic flux emergence and the ratio between the durations of growth and decay phases of solar active regions. We demonstrate that the visibility of small sunspots has a strong center-to-limb variation, far larger than would be expected from geometrical (projection) effects. This results in a large number of young spots being invisible: 44% of new regions emerging in the West of the Sun go undetected. For sunspot regions that are detected, large differences exist between actual locations and times of flux emergence, and the apparent ones derived from sunspot data. The duration of the growth phase of solar regions has been up to now underestimated.