Sunspots, solar flares, and aurorae

15 July 2012 by Phil Bull

A recent solar flare originating from a large sunspot

Oxford astronomer and exoplanet researcher Amy McQuillan was in Tel Aviv this week for a research visit. Taking photos early one evening, she noticed something strange on the disc of the Sun. A little while later, she posted what she'd seen on FaceBook:

I took some pretty photos of the sunset this evening, but was annoyed that dust on my lens was causing a black spot on the Sun. I've now noticed it appeared on the same place on the Sun, even when the camera and the cloud positions had changed... Have I photographed a sunspot tonight?

Sunspots are dark regions that we observe on the disc of the Sun, caused by intense magnetic fields that sometimes form on its surface. These strong fields result in a slight cooling of the surrounding solar gas, which makes it glow less brightly. The regions therefore appear darker when compared to gas nearby that's outside the strong field, and appear as spots on the Sun.

So was it a sunspot? Of course, simply leaving that sort of question unanswered isn't really an option for us scientist types, so Amy set to work on trying to identify the mysterious black smudge:

I compared it to the current SDO Intensitygram, and get a pretty good match if I rotate about 40 degrees clockwise from a vertical axis of rotation. I'm not sure what it should be... What do you think? It would be amazing if I've actually photographed a sunspot from a beach in Tel Aviv, considering I'm here to study stellar rotation using spot modulation in light curves!

Amy's picture is shown below. SDO is the NASA Solar Dynamics Observatory, a satellite that continuously monitors the Sun using an array of specialised instruments. You can see the most recent images here; the "Intensitygram" that Amy mentioned (near the bottom of that page) is a good way of picking out sunspots, and is shown on the left of her picture. On the right is the shot she took herself, using a standard digital SLR camera.

Sunspots?: NASA SDO image of the Sun (left) and Amy's digital SLR photo (right).

There are a few things to notice in that picture. The first is the dark blob, just left of centre - the suspected sunspot. It looks pretty similar to the (much higher resolution) SDO image, but the shape is still quite hard to make out. More on that in a bit. The second is the shape and colour of the Sun - in Amy's photo, there is a strong colour gradient, from yellow at the top to a deep orange at the bottom, and an odd stretching of the disc near the bottom. Both of these effects are caused by the Earth's atmosphere; around sunset, the Sun is close to the horizon, and so light must travel through more of the atmosphere on its way to our eyes. Because of this, it gets refracted (bent) and scattered more strongly than when the Sun is high in the sky. The amount of refraction changes rapidly close to the horizon, and so you get a distortion of the image. The amount of scattering changes rapidly too, and so you also get different colours.

On seeing Amy's photos, another Oxford astronomer, Calum Brown, applied some image enhancement techniques to them to see whether he could pick out any more detail:

Enhanced image: Enhanced, false-colour versions of the SDO image (left) and Amy's photo (right). Note how the smaller sunspots are now visible in the photo.

Again, the SDO image is on the left, and the photo is on the right. Calum used false colour to highlight features in the enhanced images. The large blue/green blob to the left of the photo seems to match the SDO image quite well - that looks like our sunspot! But the big spot is actually part of a cluster, the smaller members of which you should now be able to pick out near the bottom of Amy's picture as small yellow blobs.

This cluster of sunspots has been dubbed AR1520 by NASA (see here for a high-res image), and was responsible for generating a gigantic X-ray flare a few days ago (don't miss the amazing photo on that page, and an animation of the magnetic field here). The flare sent a stream of hot, magnetised gas (called a Coronal Mass Ejection) barrelling through the Solar System, which slammed into Earth's atmosphere yesterday evening at about 7pm Oxford time. It's expected to cause a temporary increase in geomagnetic activity and aurorae, amongst other things. There's nothing much to worry about - this sort of thing happens all the time - although some particularly violent events in the past have managed to cause damage to power grids and disrupt communications for a while.

So, there you have it - a chance blip in a photo and some clever research lead us to an active region on the Sun that is having a real effect on the Earth as we speak!

(N.B. Thinking of taking pictures of the Sun with your camera? Be very careful! Never look directly at the Sun, especially through a camera or other types of lens - it can very easily blind you permanently. See here for more advice on how to take photos safely.)

Image credit: NASA, Amy McQuillan, Calum Brown, and NASA SDO.

Categories: astrophotography | sun | Amy McQuillan | Calum Brown