Sen—The Sun features regularly in news stories at the moment. Explosions (called solar flares) and eruptions (called coronal mass ejections) that are produced in its atmosphere capture our attention because they are both beautiful and dangerous. They cause stormy weather in the space around the Earth, that we call space weather, which can severely disrupt the technology that modern society relies on. But something else has been happening on the Sun in the last few years that is much more subtle and hard to see. It’s something I’ve been following with the Hinode satellite and it’s related to the Sun’s global magnetic field.
Hinode is a Japanese satellite that has three telescopes onboard, including a spectrometer called EIS that was built by a consortium led by my group at UCL. It’s the mission that I am most heavily involved with at the moment and it gives us a great view of the Sun in the visible, ultraviolet and X-ray parts of the electromagnetic spectrum. It’s a satellite that can also measure the Sun’s magnetic field and this is what I have had my eye on.
The Sun’s magnetic field pulses in strength and complexity over an 11-year cycle—it’s the very slow heartbeat of the Sun. In recent years the Sun has been in the rise phase of its cycle and I have been eagerly waiting for the maximum of the cycle to be reached. This is a time when flares and coronal mass ejections are at their most frequent. It’s also a time when the whole magnetic field of the Sun reverses its orientation—signaling that the peak is here.
The Solar Optical Telescope (SOT) on Hinode has been monitoring the Sun’s magnetic field with a particular focus on the poles. Globally the Sun’s magnetic field is like that of a bar magnet with a north and south magnetic pole which are seen at the heliographic poles. This is where the reversal is seen—what was the north magnetic pole becomes a south pole and vice versa. This reversal is driven by a slow migration of tiny patches of magnetic field from decaying sunspots that get carried to the poles by fluid flows near the solar surface. Once there, they overwhelm and replace the previous magnetic polarity. It’s hard to see these magnetic patches, but SOT is a superb telescope and it has given a great view of the small patches of magnetic field accumulating at the Sun’s poles.
It’s been interesting to watch the poles changing with Hinode. The observations have shown that this process isn’t a mirror image in both hemispheres—both poles don’t reverse at the same time. Also, the reversal didn’t happened at the time when solar maximum was predicted to occur (mid-2013) by a range of forecasters. Hinode observations showed that the north heliographic pole started to reverse its magnetic polarity back in 2012, in advance of the predicted solar max, whilst the magnetic field at the south heliographic pole is lagging behind and is only just flipping now. The Sun may be taking its time, but the polar reversal shows us that solar max is here.
The Sun at solar minimum imaged by the EIS telescope on the Hinode mission. Image credit: JAXA/UKSA/NASA/Ignacio Ugarte Urra
The Sun at solar maximum imaged by the EIS telescope on the Hinode mission. Image credit: JAXA/UKSA/NASA/Ignacio Ugarte Urra