Solar Physics: LASCO

The Large Angle Spectroscopic Coronagraph

The Large Angle Spectroscopic Coronagraph consists of three coronagraphs which image the solar corona from 1.1 to 32 solar radii. It is one of 12 instruments aboard the Solar and Heliospheric Observatory (SOHO).
C1 was a spectral imager of the low corona, with a field of view from from 1.1 to 3 solar radii. It is no longer operating, ceasing to deliver data around June, 1998. C2 and C3 are white light imagers, with fields of view from 1.5 to 6 solar radii (C2) and 3.7 to 30 solar radii (C3).
Reference: Brueckner et al., Solar Physics 162:357-402, 1995.

Click on each thumbnail to see the full image sequence, or click here for all three.

These three sequences show a Coronal Mass Ejection as it travels through the field of the three coronagraphs that comprise LASCO.
Towards the top-left of the top panel, from the C1 telescope, a magnetic loop structure near to the solar surface is seen to erupt outwards. This image shows the emission from highly ionized iron atoms (Fe XIV) and indicates temperatures of up to 2 million Kelvin.
The C2 telescope (middle panel) allows us to track the morphology of this ejection in detail as it expands out to the left of the solar disk to 6 solar radii. Here the scattered sunlight from the excess mass which has been ejected into the corona is observed.
The material can be tracked further still from the Sun as it moves into the field of the the C3 telescope (bottom panel) at speeds of around 900 km/s. Panel three shows the ejection as it passes through the field of view of the outermost coronagraph.
LASCO collaborates well with other instruments on SOHO and other spacecraft.

LASCO, in conjunction with the other instruments aboard SOHO, is able to assist in addressing some of the fundamental questions about the solar corona and this is reflected in the research being done.

Why is the corona so hot?

The coronal plasma is sandwiched between cold space and the relatively cool solar surface, yet is maintained at a temperature of several million Kelvin without any obvious energy source.

How is the solar wind accelerated?

There is a constant outflow of plasma from the Sun. Particles travelling at speeds of up to 700 km/s (depending on the magnetic structures at their source) are being driven to the very edge of the solar-system. The mechanism behind this acceleration and the location at which it occurs is not fully understood.

What causes coronal mass ejections?

Bubbles of plasma are ejected into the corona reasonably frequently, something like once per day, and become more frequent at solar maximum. They are often associated with surface phenomenon such as erupting filaments but the exact relationship isn't fully understood. Large amounts of kinetic energy are involved with up to 10^13 Kg of material being accelerated to around 400 km/s.

How does the global corona evolve?

LASCO is able to study the effects of emerging magnetic flux at the solar surface on the large scale corona at heights well beyond that previous available. It is already clear that the fairly stable structures seen at solar minimum are becoming more complex in only the first year or so of the increase towards solar maximum.

Space Weather

It has long been known that CMEs are responsible for large disturbances in the Earth's magnetosphere known as geomagnetic storms. As such, they are the main contributer to space weather.

The above picture shows a C3 image of what is called a halo CME. These are believed to be directed along the Earth-Sun line and around half are directed toward the Earth. When they arrive they may cause large disturbances in the Earth's magnetosphere and have been known to cause blackouts, satellite failures and increase radiation exposure to astronauts and spaceborn equipment.