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Space weather and the ionosphere

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Space weather is the dynamics of the region of space surrounding the Earth. It involves complex interactions between solar radiation, solar wind, the Earth's magnetic field, and the upper extremes of the atmosphere. Understanding it is of great importance, as space weather events can interfere with communications, damage satellites and overload electrical grids. It can also affect navigation and timing signals (see the positioning navigation and timing page).

The uppermost part of the atmosphere includes what is known as the ionosphere. It starts at an altitude of about 60km, and extends for hundreds of kilometres upwards. Whilst the ionosphere is so thin as to be almost vacuum (and indeed the International Space Station and some satellites orbit within its upper reaches), it is not entirely empty. The gas within it is exposed to hard ultraviolet light from the sun, which can knock electrons away from their atoms, leaving positively charged ions. (This ionisation can also be caused by energetic particles originating from the Sun.) The low density of the gas makes it difficult for an electron to find and recombine with an ion, and so a significant fraction of the gas remains ionised. The free electrons within the ionosphere have a profound effect on radio communication, as they can reflect and delay radio signals.

Tomography of the ionosphere

(Key people: Alex Chartier, Paul Spencer, Cathryn Mitchell, Nathan Smith, Christopher Benton, Jenna Tong)

When radio waves travel from a satellite to a ground-based receiver, they are delayed by the electrons in the ionosphere. (This delay is roughly proportional to the number of electrons present in a column lying along the raypath.) By using techniques similar to those used in X-ray CT scanners, it is possible to combine a set of measured delays into a 3D map of the ionosphere.

However, whilst CT scanners can take large numbers of measurements at precisely controlled positions, ionospheric tomography must make do with relatively few measurements along paths which depend on the (ever changing) position of satellites in the sky. A particular problem is the paucity of horizontal measurements (which can only be obtained during the short intervals when a satellite is very close to the horizon, or from satellite-to-satellite observations). Fortunately, the missing information can be partially compensated for, by making realistic assumptions about the distribution of the electrons.

Software

Invert has created a software package (known as Multi-Instrument Data Analysis System or MIDAS) to perform 3D tomography of the ionosphere. Measurements obtained from a network of ground-based and space-based GPS receivers can be assimilated into the algorithm, together with point estimates of the local electron density. The resulting reconstructions provide a highly accurate three-dimensional image of the Earth's ionosphere and its evolution with time.


Example MIDAS output, showing a solar storm.

Please contact Cathryn Mitchell if you are interested in using this software.

Realtime tomography

MIDAS is used to create realtime maps of the ionosphere over Europe:

Data assimilation

(Key people: Nathan Smith, Cathryn Mitchell, Alex Chartier, Siân Jenkins)

Many geophysical processes are dynamic in nature and mathematical models can be used to describe the evolution of those processes. However the models may have many degrees of freedom and must be constrained in some manner so as to replicate what is actually seen in practice. Real-world data can be used to learn or optimise model parameters and thereby constrain model behaviour. This is data assimilation. The Invert group is primarily interested in using this methodology for nowcasting/imaging and forecasting the ionosphere.

Polar regions

(Key people: Cathryn Mitchell, Joe Kinrade, Zama Katamzi, Ping Yin)

The Earth's poles are of particular interest to Invert, due to the rich physical phenomena which result from the Earth's magnetic field directing charged particles from the Sun to high latitudes. Understanding the polar ionosphere is also of major practical importance, due to the increasing amount of airline traffic taking polar routes. Ionospheric storms can affect disrupt navigation and block radio communication, and so an increased understanding of these storms is needed.

One consequence of ionospheric storms is scintillation, whereby the strength and apparent direction of a radio signal passing through the ionosphere is observed to change rapidly. (The effect is analogous to stars twinkling in the night sky.) In extreme cases, scintillation can make GPS completely unusable, and so being able to predict and mitigate it is of great importance.


Cathryn sets up a GPS scintillation monitoring station in Antarctica.

Invert has recently installed a set of scintillation receivers in Antarctica. (See the Antarctica 2010 and Antarctica 2011 pages for respective details of the primary mission to install the receivers, and the secondary mission to service, and collect data from the receivers.) One of these is at the south pole itself, whilst more are located over a range of far-southern latitudes. These devices take 50 GPS measurements per second, and by comparing the subtle differences between nearby measurements are able to detect and accurately quantify scintillation.

We also have a scintillation receiver running in Tromso (in the far north of Norway), and are planning to place another in Kiruna (in the far north of Sweden).

The equatorial anomaly

(Key people: Christopher Benton, Jenna Tong, Julian Rose)

The equatorial anomaly is the region of increased ionisation present around the magnetic equator. It is home to a variety of interesting features, such as prevailing electrical currents, and persistent regions or above-average or below-average ionisation. As with polar regions, ionospheric scintillation is particularly common in the vicinity of the magnetic equator. To study this, we have recently installed a scintillation receiver in Cape Verde, on the island of Sâo Vicente. Data from this receiver can be seen here.


The Cape Verde Atmospheric Observatory, at which our new scintillation receiver is located.

There is still much to be understood about the equatorial anomaly. An example of an unexplained phenomenon can be seen in this set of GPS scintillation measurements taken during an ionospheric storm over South East Asia. The pink crosses show measurements taken from two scintillation receivers located in Hue and Hoc Mon. The positions correspond to points where a path from a GPS satellite to a receiver intersects the ionosphere. The magnitude of the scintillation is represented by the superimposed pink circles (with a plain cross representing negligible scintillation). The coloured background gives the total electron content in the ionosphere.

1 April 2006 was a quiet day, with typical levels of scintillation. 5 April 2006 was during an ionospheric storm, with elevated TEC values, but similar levels of scintillation. On 6 April 2006, the ionospheric storm was still in progress, but scintillation was suppressed. We are interested in deducing the cause of this phenomenon.

The plasmasphere

(Key people: Talini Pinto Jayawardena, Robert Watson, Cathryn Mitchell, Julian Rose)

The plasmasphere is a torus-shaped region of plasma (ionised gas) extending from the top of the equatorial ionosphere, up to a distance of about 30,000km. It is much less dense than the ionosphere, but also much larger.


Region around Earth, with the plasmasphere shown in orange. (Image courtesy NASA.)

The group has recently won a competition to fly their TOPCAT payload aboard the UKube-1 satellite. This will study both the plasmasphere and the upper reaches of the ionosphere.