Tilt angle and lifetime of sunspot groups

Author:

Nagovitsyn Yury A12ORCID,Osipova Aleksandra A1ORCID,Pevtsov Alexei A3ORCID

Affiliation:

1. Central Astronomical Observatory of the Russian Academy of Sciences at Pulkovo, Pulkovskoe sh. 65/1, St Petersburg 196140, Russia

2. St Petersburg State University of Aerospace Instrumentation, Bol’shaya Morskaya ul. 67, St Petersburg 190000, Russia

3. National Solar Observatory, 3665 Discovery Drive, 3rd Floor, Boulder, CO 80303, USA

Abstract

ABSTRACT We use the Catalogue of Solar Activity (CSA) to study the latitudinal variations of tilt of solar active regions. The tilt angles β are computed taking into account changes of the heliographic grid with latitude φ. We show that when sunspot groups of different sizes and lifetimes are included, a classical representation of the Joy’s law as a linear function of latitude (β ∝ φ) is only the first approximation valid within a limited range of latitudes (−25° ≤ φ ≤ +25°). Outside this range, the functional dependence β = f(φ) becomes non-linear. Separating the data set on large long-living groups (LLG) and small short-living groups (SSG) reveals two quite different dependencies in β = f(φ): non-linearity in tilt is only present in LLGs and the steepness of linear section of β = f(φ) fit is higher for LLGs. This suggests a difference in the physical properties of two populations of solar groups, which could be hypothesized as an indication of different localization of subsurface zones of their formation in the framework of a distributed dynamo. However, since CSA contains the coordinates of sunspots averaged over the lifetime (or disc passage) of each group, one cannot rule out that the difference in tilts of SSG and LLG groups may be affected by the evolution of tilt angles during the lifetime/disc passage of the groups.

Funder

Russian Foundation for Basic Research

Ministry of Science and Higher Education of the Russian Federation

Publisher

Oxford University Press (OUP)

Subject

Space and Planetary Science,Astronomy and Astrophysics

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