Exploring our star from space: UK-led space instruments spearheading studies of the Sun and heliosphere

Tuesday 15th July, 2014

Our guest speaker was Prof Richard Harrison who is Head of Space Physics and Chief Scientist at Rutherford Appleton Laboratory who came to talk to us about "Exploring our star from space: UK-led space instruments spearheading studies of the Sun and heliosphere". He is the UK Principal Investigator for NASA's STEREO spacecraft that are currently in orbit about the Sun. He has been a solar physicist for about 35 years and so, naturally, he studies the solar atmosphere and space weather.

He explained that as our civilisation becomes more dependent on technology in our everyday activities it is important that we learn to monitor and forecast the Sun's effect on all this equipment. For solar storms can wreak havoc on power distribution systems as shown by the coronal mass ejection that knocked out the power transmission system in Quebec, Canada in March 1989.

A coronal mass ejection (CME) is an eruption on the Sun that sends huge quantities of solar material along with its inherent magnetic fields into space. These eruptions are unpredictable and the only warning we get is from appropriate instruments on spacecraft between Earth and the Sun. If one of these CMEs is headed our way then we can take appropriate action and power down satellites and power transmission systems, change aircraft flightpaths and warn any astronauts on the International Space Station to take cover.

Currently, there are a number of spacecraft observing the Sun and the weather it sends our way. The oldest spacecraft is the Solar and Heliospheric Orbiter or SOHO which has been operating since 1996. This joint ESA-NASA mission orbits around a point between the Earth and Sun known as L1 where the gravitational forces of the two bodies allow it to stay roughly in the same position as the Earth moves around the Sun. It was only meant to last a couple of years but it has recently had its mission extended until December 2016.

The mission that Prof Harrison is involved with is NASA's STEREO spacecraft. These are actually a pair of spacecraft that are orbiting the Sun with the Earth but one is slightly inside Earth's orbital path and the other slightly outside. This positioning means that one craft is slowly getting ahead of the Earth (STEREO-A) and the other lagging more and more behind (STEREO-B). This intentional placement allows each craft to gradually map more of the Sun and when both images are combined to give scientists a stereoscopic image of solar phenomena.

There is a slight drawback to these orbits, though, in that at some point the spacecraft will effectively disappear behind the Sun; otherwise known as superior conjunction. When this happens the radio receivers on Earth will not be able to distinguish STEREO's signal from the Sun's radiation. Communication with the spacecraft will cease and the satellites will each go into safe mode without collecting data for a time. This will happen for STEREO-A from March 24th to July 7th, 2015 and for STEREO-B from January 22nd to March 23rd, 2015. However, this means that at least one spacecraft, therefore, will always be collecting data.