Ulysses encounters massive coronal ejection from the Sun
19 June 2001
On 10 May, most of the instruments on board Ulysses recorded their highest readings during the ten and a half years that the spacecraft has been in orbit. The cause was a spectacular coronal mass ejection (CME) which had left the Sun three days previously, heading towards the position in space that Ulysses was occupying at the time."Ulysses saw the highest magnetic field and plasma densities it has ever witnessed. It was a very fortuitous encounter," says Richard Marsden, Ulysses project scientist.
CMEs are ejections of vast amounts of matter from the Sun's atmosphere or corona, which occur most frequently around solar maximum when the Sun's 11-year activity cycle is at a peak. Activity on the Sun is still high after last autumn's solar maximum.
When a CME is ejected in the direction of Earth, the magnetised cloud of charged particles hitting the Earth's magnetosphere causes spectacular auroras and can disrupt telecommunications and electrical systems on the ground. So far, CMEs have been studied almost entirely from the perspective of Earth, either from ground-based solar observatories or Earth-orbiting spacecraft. Now, however, the Ulysses data are adding another view.
Ulysses' close encounter with the CME occurred just as the spacecraft was approaching the ecliptic plane (the plane in which the planets orbit the Sun) on its journey from the Sun's south to north pole. The spacecraft is now on the short leg of its 6.2 year orbit, known as the fast latitude scan, which takes it from pole to pole in just 320 days.
Ulysses completed its south polar pass in January, crossed the ecliptic on 25 May and will begin its north polar pass in September. Viewed from Earth, the spacecraft was off the west limb of the Sun when the CME hit.
A new perspective on CMEs should come from comparisons between the Ulysses data and observations made by SOHO, the Earth-orbiting ESA-NASA solar observatory, which first recorded the CME as it left the Sun on 7 May.
"With this particular geometry, we thought something interesting would happen - and it did," says Marsden. "The optical instruments on SOHO get their best views of CMEs when these are ejected off the limbs of the Sun as seen from Earth, and Ulysses was ideally placed to intercept the heart of this CME as it swept out into space."
The radio instrument on board Ulysses was the first to receive warning of the CME's departure from the Sun. "We detected slowly-drifting (type II) radio emission, beginning at 1:30pm on 7 May, which told us that a CME-driven interplanetary shock was coming our way. It was the most dramatic type II event we've seen on Ulysses," says Robert MacDowall from the Goddard Space Flight Center and principal investigator for URAP (Unified Radio and Plasma Wave Investigation).
The shock, travelling at an average speed of 900 km/s, arrived at the spacecraft at 1:35am on 10 May, followed a few hours later by a very dense region of charged particles.
"This event is very unusual because we intercepted an extremely dense plug of material in the ejecta which looks like the manifestation of a filament that lifted off the Sun," adds MacDowall. A filament, or solar prominence, is a dense tube of material held in a magnetic loop above the Sun's surface. CMEs can occur after a change in the magnetic field causes the filament to collapse onto the surface and precipitate a massive explosion.
"To our knowledge, such high particle densities (100 particles per cubic centimetre) associated with ejecta have not been seen since the event of 11 January 1997 at the NASA Wind spacecraft," adds Mike Reiner, from the Catholic University, Washington D.C. and a URAP team member. That CME was the cause of great disturbances in the Earth's neighbourhood and damaged several satellites.
The magnitude of the CME was also evident in the magnetic field data. "By 9am on 10 May, the magnetic field was so compressed that its strength was higher than at any time in the past ten years. At 34nT (nano-Tesla), it was seven times the normal value of the magnetic field seen at Ulysses," says Andre Balogh, from Imperial College, London and principal investigator for the magnetometer.
"We now want to follow the way the different magnetic loops raced out from the Sun so that we can understand the complexity of this CME. Looking at the SOHO images and our magnetic field data, we can only admire its complexity, which was beyond anything you will read about in a text book," says Balogh.
By unravelling the complicated structure of CMEs, like the one that engulfed Ulysses, it should be possible to predict the effects these CMEs can have on our planet and its magnetosphere.
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