Cluster finds giant gas vortices at the edge of Earth's magneticbubble (Forwarded)

ESA News
http://www.esa.int

12 August 2004

Cluster finds giant gas vortices at the edge of Earth's magnetic bubble

ESA's quartet of space-weather watchers, Cluster, has discovered vortices of
ejected solar material high above the Earth. The superheated gases trapped in
these structures are probably tunnelling their way into the Earth's magnetic
'bubble', the magnetosphere. This discovery possibly solves a 17-year-mystery of
how the magnetosphere is constantly topped up with electrified gases when it
should be acting as a barrier.

The Earth's magnetic field is our planet's first line of defence against the
bombardment of the solar wind. The solar wind itself is launched from the Sun
and carries the Sun's magnetic field throughout the Solar System. Sometimes this
magnetic field is aligned with Earth's and sometimes it points in the opposite
direction.

When the two fields point in opposite directions, scientists understand how
'doors' in Earth's field can open. This phenomenon, called 'magnetic
reconnection', allows the solar wind to flow in and collect in the reservoir
known as the boundary layer. On the contrary, when the fields are aligned they
should present an impenetrable barrier to the flow. However, spacecraft
measurements of the boundary layer, dating back to 1987, present a puzzle
because they clearly show that the boundary layer is fuller when the fields are
aligned than when they are not. So how is the solar wind getting in?

Thanks to the data from the four formation-flying spacecraft of ESA's Cluster
mission, scientists have made a breakthrough. On 20 November 2001, the Cluster
flotilla was heading around from behind Earth and had just arrived at the dusk
side of the planet, where the solar wind slides past Earth's magnetosphere.
There it began to encounter gigantic vortices of gas at the magnetopause, the
outer 'edge' of the magnetosphere.

"These vortices were really huge structures, about six Earth radii across," says
Hiroshi Hasegawa, Dartmouth College, New Hampshire who has been analysing the
data with help from an international team of colleagues. Their results place the
size of the vortices at almost 40 000 kilometres each, and this is the first
time such structures have been detected.

These vortices are known as products of Kelvin-Helmholtz instabilities (KHI).
They can occur when two adjacent flows are travelling with different speeds, so
one is slipping past the other. Good examples of such instabilities are the
waves whipped up by the wind slipping across the surface of the ocean. Although
KHI-waves had been observed before, this is the first time that vortices are
actually detected.

When a KHI-wave rolls up into a vortex, it becomes known as a 'Kelvin Cat's
eye'. The data collected by Cluster have shown density variations of the
electrified gas, right at the magnetopause, precisely like those expected when
travelling through a 'Kelvin Cat's eye'.

Scientists had postulated that, if these structures were to form at the
magnetopause, they might be able to pull large quantities of the solar wind
inside the boundary layer as they collapse. Once the solar wind particles are
carried into the inner part of the magnetosphere, they can be excited strongly,
allowing them to smash into Earth's atmosphere and give rise to the aurorae.

Cluster's discovery strengthens this scenario but does not show the precise
mechanism by which the gas is transported into Earth's magnetic bubble. Thus,
scientists still do not know whether this is the only process to fill up the
boundary layer when the magnetic fields are aligned. For those measurements,
Hasegawa says, scientists will have to wait for a future generation of
magnetospheric satellites.

More about ...

* Cluster factsheet
http://www.esa.int/esaSC/SEMYN5T1VED_index_0.html

Related articles

* Space weather
http://www.esa.int/esaSC/SEMLC2T1VED_index_0.html
* Cluster's new view of near-Earth space
http://www.esa.int/esaCP/ESA6VTTM5JC_index_0.html
* How the Sun affects us on Earth
http://www.esa.int/esaSC/SEML7BS1VED_foryou_0.html
* What are solar flares?
http://www.esa.int/esaSC/SEMHKP7O0MD_index_0.html

IMAGE CAPTIONS:

[Image 1:
http://www.esa.int/export/esaCP/SEMT..._index_1.html]
This figure shows a three-dimensional cut-away view of Earth’s magnetosphere.
The curly features sketched on the boundary layer are the Kelvin-Helmholtz
vortices discovered by Cluster. They originate where two adjacent flows travel
with different speed. In this case, one of the flows is the heated gas inside
the boundary layer of the magnetosphere, the other the solar wind just outside
it. The arrows show the direction of the magnetic field, in red that associated
with the solar wind and in green the one inside Earth’s magnetosphere. The white
dashed arrow shows the trajectory followed by Cluster.

Credits: H. Hasegawa (Dartmouth College)

[Image 2:
http://www.esa.int/export/esaCP/SEMT...html#subhead1]
This computer simulation shows how the density of the electrified gas is
expected to vary across the vortices along Cluster’s trajectory (white dashed
line). The density is lower inside the boundary layer (blue region) and higher
outside, in the region dominated by the solar wind (shown in red). The density
variations measured by the instruments on board Cluster match those predicted by
this model.

Credits: H. Hasegawa (Dartmouth College)