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:
- TIME : Dawn until dusk and sometimes for a few hours predawn
- Low latitudes; 1.8Re to 2.8Re
- High latitudes; 4.5Re to 12Re
The ULF magnetometer data used to estimate the plasma mass densities comes from
Briefly, the procedure is as follows:
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.
- Obtain the ULF magnetometer data
- Calculate the ULF resonant frequencies. For an example, see
Waters et al., 1991
- Solve the ULF wave equation for the plasma mass densities,
- There are two different techniques depending on the latitude:
- High latitudes: estimates the plasma mass density in the equatorial plane (where Zgsm=0)
- Low latitudes: estimates the plasma mass density at discrete locations along the magnetic flux tube (altitude) without assuming any apriori plasma mass distribution function
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.
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
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
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
- The ULF data come from different research groups who need to apply for funding in order to keep their instruments working. Please visit and read any rules these groups have regarding the use of their data.
- The high latitude data is easier to calculate compared with the low latitude data. The low latitude estimates are calculated by solving a non-linear scheme which requires a certain amount of trial and error to get it to work.
- We can only provide plasma mass density estimates if we can detect ULF resonances for your particular interval.
- This technique is usually limited to the local daytime although occasionally the ULF resonances can be detected in the predawn hours.
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