Solar Active Region Spotter

Intro

Solar physicists try to understand the Sun and how it works. An important part of this is studying the transient features like active regions -- huge knots of magnetic field that push outward through the solar surface and are associated with things like prominences and flares. Eruptions that come from prominences and flares can launch magnetic fields and plasma (charged gas) into space and towards the Earth, which can affect satellites, astronauts, and even our power grid!

Because of these impacts, the National Oceanic and Atmospheric Administration (NOAA) operates a Space Weather Prediction Center. This center carefully identifies and tracks active regions due to their apparent role in producing space weather, numbering every active region that appears or rotates into view as the Sun turns (1 solar rotation is about 27 days). NOAA does not, however, track which active regions persist for multiple solar rotations. This becomes a challenge for researchers looking to study the life cycle of active regions, as older active regions will be given new numbers. This project will help researchers know which active regions persist for many months, allowing us to build a database of linked NOAA active region numbers and lowering the barrier for long-term active region study. This information will in turn enable research into the contribution of active regions to the strangely high temperatures of the Sun’s corona, a mystery that's puzzled solar scientists for decades!

Data

The data that you'll be viewing is from the Solar Dynamics Observatory (SDO), specifically the instruments called the Atmospheric Imaging Assembly (AIA) and the Helioseismic and Magnetic Imager (HMI). AIA views the Sun in 7 extreme ultraviolet (EUV) channels, which allow us to see the Sun at temperatures ranging from roughly 50,000 K to 10,000,000 K. We use three of them for this project: 171, 193, and 211 Angstroms. These focus on the 500,000 -- 2,000,000 K range, in which active region details are most apparent.

HMI, on the other hand, makes maps of the magnetic field at the solar surface. These grayscale images - called magnetograms - show active regions as strong, irregular shapes in black and white to denote positive and negative magnetic fields. Time lapse movies of the magnetograms over several days allow us to track how an active region is changing through time.

Magnetograms are very precise measurements of light coming from the Sun to determine the magnetic field direction along the line of sight to the observer. This is measured using a fascinating phenomenon of light, called the Zeeman effect. Energetic atoms emit light at very specific wavelengths. If that light passes through a magnetic field, the light will get split into two very slightly different groups; the "distance" between the two groups (how strongly they're split) tells you how strong the field was that did the splitting! Since active regions are really big knots of strong magnetic field, it's important to look at their magnetic "footprint" in order to study them.

This project makes primary use of the magnetic field data from HMI. Plasma, which is what the AIA data images, moves much faster than the magnetic fields at the Solar surface do. Because the magnetic signature of an active region changes more slowly than its associated plasma activity, HMI data is more useful to us in recognizing a single active region through time.

All of SDO's data is available in near-real time and shared online via open archives. There are many great external resources for viewing this data online. Many of these are linked on the front page of this project; Helioviewer and its desktop application JHelioviewer are our personal favorites, but the Sun In Time and Solar Monitor are also excellent resources. The Space Weather Data Portal is particularly great for those looking to compare data between various instruments.

Definitions

The corona is the Sun’s atmosphere: while the whole Sun is made of very hot plasma, the corona is the part that is transparent in visible light. It also has some very intriguing features, such as its temperature – the corona is almost a thousand times hotter than the surface below it!

Active regions are bright spots on the solar disk that form due to the Sun’s naturally churning plasma tangling its magnetic field. Knots of magnetic field flux rise to the visible solar surface and poke out through the solar atmosphere, creating the bipolar regions we view in magnetograms (the magnetic field leaves and re-enters the surface, which appears as paired positive and negative polarity regions). Active regions are associated with several other solar phenomena, such as prominences/filaments, solar flares, and sunspots.

Prominences or filaments are mostly-horizontal magnetic fields full of cool plasma that appear dark to the eye. Areas of strong magnetic field, referred to as sunspots, can make up these large dark regions on the solar disk. Coronal loops, which are thin arcs of magnetic field that glow with plasma, also commonly extend out from active regions. The only difference between these terms are where they were traditionally observed – prominences were seen on the limb (edge) of the Sun extending above the surface, while filaments were seen on-disk and thus appeared as dark streaks. They are, however, the same structure.

Coronal loops are structures which appear as thin loops of magnetic field glowing with hot plasma, which fan out in large numbers from the base of active regions. They appear like bundles or yarn or thread looped together in AIA views of the solar surface.

Coronal mass ejections (CMEs) are large eruptions from the Sun of both plasma and magnetic field. These enormous bubbles, which grow to many times the size of the Sun as they speed through space, can interact with the magnetic field of Earth. This can either trigger the beautiful aurora borealis (or australis) or cause disruptions to GPS and the electric grid, depending on the strength of the interaction.

The solar cycle describes the periodicity of solar activity and magnetic fields in the Sun. The main bipolar field of the Sun reverses every 11 years, which directly correlates with a periodic ebb and flow of solar activity. Active regions are most common near the peak of the solar cycle - midway between reversals of the Sun’s bipolar magnetic field. This is also when the Sun’s magnetic field is at its most complex.