Using ULF Waves to Remote Sense Magnetospheric Plasma Mass Density


Ultra-low frequency (ULF; 1-100 mHz) field line resonances may be used to remote sense the plasma mass density in the magnetosphere as a function of both time and radial distance. A particular type of plasma wave in the magnetosphere has its energy guided along the magnetic field. When it encounters the ionosphere, the energy reflects back into space. At the appropriate frequency, a field line resonance (FLR) is formed. The basic idea for remote sensing is to detect the resonance signature of these resonant low frequency plasma waves. This information may then be used to estimate the plasma mass density in space.

The latitude of the FLR detection determines how far out into space the plasma mass density is estimated. The technique used is slightly different for high compared with low latitudes. Ground based magnetometer arrays are used for the FLR detection process. At the moment we can monitor the plasma mass density in the ranges:

The ULF magnetometer data used to estimate the plasma mass densities comes from
Space Physics Group, University of Newcastle, Newcastle, NSW, Australia
Space Physics University of Alberta , Edmonton, Alberta, Canada
SAMNET at Lancaster University, UK.

Briefly, the procedure is as follows:
  1. Obtain the ULF magnetometer data
  2. Calculate the ULF resonant frequencies. For an example, see Waters et al., 1991
  3. Solve the ULF wave equation for the plasma mass densities,
  4. There are two different techniques depending on the latitude:
Solving the wave equation requires the magnetic field topology be known. At low latitudes we use the IGRF and for high latitudes we use the IGRF with the Tsyganenko models. The integration of the Tsyganenko magnetic field models was done in collaboration with Dr Eric Donovan, now at University of Calgary. This work was reported in J. Geophys. Res., 101, p24737, 1996 Abstract.

Other relevant references can be found here. A number of examples are shown below:

High Latitudes - Type 1 Plots

(a) Magnetometer data from CANOPUS, Canadian Space Agency magnetometer array [Churchill line data provided by Prof. J.C. Samson] may be used to find the FLRs.
The plasma density plots are calculated assuming the Tsyganenko, 1989 magnetic field topology.
There are 2 types of plots available. The first show the density [H+ cm-3], colour coded, on an xy plane with time on the x-axis and distance in Earth radii on the y-axis. Some examples are:

7 June, 1990 1 Feb, 1995 2 Feb, 1995 3 Feb, 1995 8 Feb, 1995

(b) Magnetometer data from the IMAGE and SAMNET arrays may also be used.
These intruments are uniquely placed to remote sense the plasmapause.

31 Aug,1990 20 Aug, 1996 11 Mar, 1998

High Latitudes - Type 2 Plots

The second type of plots are in polar coordinates.
Radial distance from Earth and magnetic local time (for CANOPUS) are arranged on a circular map. Local noon at the top, dawn on the right:

14-19 Oct, 1990 20-25 Oct, 1990 26-31 Oct, 1990

Low Latitudes

A technique recently developed in the Space Physics Group at Newcastle is the Harmonic Derived Density (HARDD) method. This uses the harmonics of ULF field line resonances to estimate the plasma mass density at discrete locations along a magnetic flux tube. This makes the determination of plasma mass density independent of the way the palsma is distributed along the field topology. An example is:

8 Sept, 1989

Data Requests

Researchers working in space physics may request estimates of plasma mass densities. If you wish to obtain these, there are a number of points you should be aware of: To request data send email to C.L. Waters

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