Imaging ionospheric plasma flow


Introduction

A brief tutorial


Introduction

The ionosphere is the region of the Earth's atmosphere between 60 and 1000 km. In this region free electrons and ions can exist for a considerable period of time, resulting in the formation of a plasma gas containing charged particles. This plasma is not stationary, but is constantly circulating around. Flow patterns develop which can, for example, carry plasma from the dayside to the nightside or vice-versa. The plasma flow pattern is related to the space weather, and thus to the strength and direction of the interplanetary magnetic field and the solar wind.

Radars tramsmit radio waves into the upper atmosphere and receive the returned signal. The returned signal will contain information on the speed and direction of the plasma flow. In the Northern hemisphere there is a chain of 8 high-latitude radars which are used to study high-latitude flow of plasma. This chain is called SuperDARN. The figure below is a diagram showing the Northern hemisphere SuperDARN radars and the area viewed by each radar.



Individually each radar will be able to give us information on how the plasma flows in the area in front of the radar. We can combine plasma information from all the 8 radars to give us a global picture of how the plasma flows over the entire high-latitude Northern hemisphere region. These global patterns are very useful from a space weather perspective, allowing us to view changes in the Earth's magnetosphere and ionosphere.

The technique that allows us to combine information on the flow of ionospheric plasma from all the eight SuperDARN radars, is called the map-potential technique. The map-potential technique will look at the returned radar signal from all the 8 radars to determine the overall plasma flow. In areas where there is no signal (either because the region is not being viewed by any radar, or because there is no returned signal) the map-potential technique uses plama flow patterns from a statistical model which is based on the conditions of the interplanetary magnetic field.

Below you will be shown how to run the map-potential technique for one day of data.


A brief tutorial

1) Go into your user area. There you will find a number of files (under the 'desktop' menu click on 'open unix shell', then type 'ls -l' in the window that appears. This will list the files in your directory.) . There are two files (.fit and .inx) each from 7 of the SuperDARN radars for 11 January 2001, 18-20 UT. The letter after the date will tell you which radar the data comes from. The .fit and .inx data tells us which type of files they are. The .fit files contain the data from the radars in a specific format, the .inx file is a index to the .fit file. The .fit files you have here contain 2 hours of data from each radar (all 1200 observation points for each 2 minute scan with the radio waves). These files contain information on the plasma flow observed by each individual radar.

Running the map-potential technique will allow you to combine the radar measurements from all the different radar and to visualise the result.

1) In your user area there is a file called map_setup. If you type

. map_setup (i.e. '.' 'map_setup')

the map_potential technique will be initiated.

2)The map_potential technique works by gridding all the available radar data onto a global files. The gridded data from each radar are called grid files. The inidividual grid files from all the radars are then combined into one large file called a radvec file.

In order to make the grid and radvec files for a file starting at any particular time, type :

make_radvec yyyymmddhh

If you now type 'ls -l' again you should find more files have appeared.

3)> When the radvec file has been created, you will be able to make maps showing the plasma flow. The map-potential technique is run using a widget. Typing

map_start

will make this widget appear.


The map_potential widget

This is the map_potential widget which is used when plotting the plasma flow pattern.



In order to run the map-potential technqiue, all you have to do is to enter the file name of the radvec file (remebering to type enter afterwards). You then need to choose the ACE spacecraft and to enter a delay time of 50 minutes. This is the travel time between when the ACE spacecraft observes the Inteplantary Magnetic Field (IMF) and when starts to affect the plasma flow in the upper atmosphere. If you now press execute, you will see a series of maps showing the plasma flow patterns over the northern hemisphere. The maps are updated every 2 minutes, the text on the screen will tell you how many vectors there are every 2 mins.

The maps show magnetic local noon at the top, i.e. the 'nose' of the magnetosphere. (MLT stands for Magnetic Local Time)

The dashed concentric circles show the magnetic latitudes.

The capital letters show the current location of the radar stations.

The maps show you velocity vectors of the plasma - the coloured dots show the data points and the coloured lines the direction and size of the velocity.

The contours show the flow patterns found using a combination of the map-potential model and the data from the radars. The contour lines show the direction of the flow. The number on the contours gives the electric potential - which is related to the velocity. This is much like weather maps where there are contours showing the air pressure over the earth, and the closeness of the contours determines the local wind speed.

In the bottom right hand corner there is an arrow showing you the direction of the IMF as measured by the ACE spacecraft (called the clock-angle). The spacecraft measures the x,y and z components of the IMF. If the arrow points upwards the IMF z component is positive, if it points downwards the IMF z component is negative. If the arrow points to the right then the IMF By component is positive and if it points to the left than the IMF By component is negative. The direction of y and z components of the IMF has a great effect on the plasma flow patterns.

It is easier to create a postscript(a graphics file) file of the plots and then look at the plots using an program called 'Ghostview'. If you press the "PS/COLOUR" on the "Output device" option, then you will create a file called idl.ps. Typing:

ghostview idl.ps

will allow you to view the output file and to view one plot after the other in your own time. Click on the list of page numbers and press return to view different pages.

Describe the plasma flow patterns you observe. In which direction does the plasma flow ? Look at the plots for 18:40 and 18:50. How are they different, is there any obvious change in the IMF between these two times ? (Hint, look at the clock-angle arrows on the plot and at the figure below showing the x,y and z component of the IMF between 1700 and 2000. Don't forget the delay time when studying the figure.)