Determine the Sun's Rotational Period Using SOHO Images

Stellar & Galactic Astronomy Lab 01 — Solar Rotation Copyright 2021, James T. Hooten, All Rights Reserved 01-1 01 The S un’s Period of Rotation Purpose The purpose of this laboratory activity is to empirically determine the rotational period of the Sun using solar images from the SOHO spacecraft. This lab is written for both face-to-face and online labs. In some cases, special instructions are included for online students only. For this lab you will utilize the following item(s): Name of Item Item Description Lab 01 Template-Solar Rotation.pdf The file you are currently reading Lab 01 Data File-Solar Rotation.xls Excel file for online submissions Lab 01 Questions-Solar Rotation Online quiz for Follow-Up questions Excel Spreadsheet software Background Information The Sun is the star nearest to Earth. Because of this we know more about the Sun than any other star. Like all rotating bodies, the Sun has a north pole and a south pole, and the Sun rotates about its polar axis. The time required for the Sun to rotate once on its axis is considerably longer than that for Earth. Unlike the Earth, the Sun is not solid and rotates differently at different latitudes. This means the Sun rotates at a different rate near the equator compared to near the poles. This is known as differential rotation (see Figure 01.01).

Stellar & Galactic Astronomy Lab 01 — Solar Rotation 01-2 Copyright 2021, James T. Hooten, All Rights Reserved Figure 01.01: Differential Rotation on the Sun The Sun displays differential rotation because it is not solid. It is a gigantic ball of mostly hydrogen plasma. Rotation is faster near the equator and slower near the poles. Occasionally, dark spotted regions appear on the “surface” of the Sun. These sunspots are cooler areas of strong magnetic activity in the photospheric layer of the Sun. Sunspots are usually short-lived and are seen to move across the visible face of the Su n due to the Sun’s rotation. Visually, over the course of several days, an observer can determine the rotational period of the Sun by measuring the changing positions of sunspots. The rotational period is the amount of time required for the Sun to rotate once on its axis. This time will vary by latitude due to differential rotation.

Stellar & Galactic Astronomy Lab 01 — Solar Rotation Copyright 2021, James T. Hooten, All Rights Reserved 01-3 Figure 01.02: Sunspots Visible on the Sun’s Photosphere Sunspots appear as dark areas on the Sun’s visible surface (photosphere). The dark center of the sunspot is called the umbra. The average temperature of the photosphere is about 10,000°F, while that of the umbra is about 6500°F. The lighter area surrounding the sunspot’s umbra is the penumbra. The Solar and Heliospheric Observatory, known as SOHO, is a satellite that continuously observes the Sun. It was launched on December 2, 1995 and is still an ongoing mission that is a joint project of both the European Space Agency (ESA) and the National Aeronautics and Space Administration (NASA). What’s to Come? For this lab, you will use SOHO satellite observational images of sunspots to determine the rotational period of the Sun at different latitudes. The images are at the end of this lab.

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