DMSP SSJ4 - Precipitating Ion/Electron Data

COLOR SPECTROGRAMS

Color spectrograms present ion or electron differential number flux (number of particles falling on a square centimeter per second from each steradian of solid angle per eV of energy range) vs. both energy of the particles and the time of the particle observation. The differential flux is color coded and presented as colored dots on a vertical line. One vertical line represents the observed flux in one or more seconds. In the example below, each vertical line is the average of 3 seconds of data.

The SSJ4 data are sometimes presented in a format devised by Johns Hopkins University / Applied Physics Lab in which the data are plotted as differential energy flux and the range of ion energies (the y-axis) is turned upside down. This is just another way to present the same data.

This is a typical color spectrogram of the SSJ4 data obtained in one polar pass of a DMSP F13 spacecraft during the crossing of the northern polar region on 97 May 15 during a geomagnetic storm. Data from the SSIES ion drift meter are plotted below the SSJ4 to show the regions of sunward and anti-sunward flow of the ionospheric plasma. This plot was created using an IDL program which is also available upon request.

SSJ4 color spectrogram[Click on image to obtain a full size copy of the image.]

AVERAGE ENERGY, TOTAL ENERGY FLUX AND TOTAL NUMBER FLUX

It is often useful to sum (or integrate) the data across the enegy range of the instrument. The three parameters that are typically obtained in this manner are the averge energy (EAVE), the total energy flux (JETOT) and the total number flux (JTOT). JTOT and JETOT are calculated by using the following equations:

Equations: JTOT=Sum(j(Ei)*Energy/step) and JETOT=Sum(Ei*j(Ei)*Enegy/step)

where J(Ei) is the differential number flux for the channel with central energy Ei. JTOT is measured in units of particles/(cm^2 ster s) and JETOT is measured in units of keV/(cm^2 ster s). The average energy is then:

EAVE=JETOT / JTOT

Note that the equation integrates across only 19 channels. Channels 10 and 11 of the instrument are set at the same energy and the data at this energy should be used only once. EAVE is measured in units of keV.

Some Limitations of the SSJ4 Data:

  1. The SSJ4 instrument has a limited field of view. It is often, but not always, correct to assume that the flux of particles flowing down the field line is omnidirectional. Also, the SSJ4 makes no measurement of the particles flowing up the field line.
  2. The SSJ4 instrument has a limited energy range. At times, a significant amount of energy may be flowing into the ionosphere from the magnetosphere from ions and/or electrons with energies > 30 keV.
  3. Because of the DMSP spacecraft potential in sunlight is typically -20 V to -25V, photoelectrons ( 0 eV - 10 eV ) often are deflected back to the spacecraft and flow into the lowest energy channel ( 30 eV ) of the electron detector. Also, ambient ions ( 0.25 eV ) often are accelerated into the lowest energy channel ( 30 eV ) of the ion detector. This inflates the value for the integrated number flux and integrated energy flux.
  4. The SSJ4 instrument detects a spectrum over a period of 1 second. In an active auroral arc, the fluxes may change more quickly than this and create an un-physical spectrum.
  5. WARNING: The F13 low energy ion data (30 eV to 1 keV) from early 1996 to current and the F15 low energy ion data from mid-January 2000 to current should not be used.

Go to DMSP Home Page