Protons-Saturn

Description of the Cassini CAPS IMS time-of-flight (TOF) raw data files and the higher level MATLAB data files and curve fitting programs, respectively.

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Using an IDL program that reads the raw Cassini CAPS singles, A-cycle data, and TOF composition, B-cycles data, we generated ST TOF spectra as shown in Figure 4 of the paper. See directory NASA\caps2004183. We then took the peak channel centered on the proton location in TOF to construct an energy spectrum of proton counts versus energy (see Figure 11 in the paper). This data was then loaded into another IDL program to convert this spectrum into a weighting function, see Eqs. 3 and 4 in the paper, that was then applied to the singles data as described in the paper. This IDL program was also used to generate the various data sets used by the MATLAB curve fitting tool to make 2D fits as described in the paper. We also computed, using Spice Kernals, the angle between the IMS look direction for all eight angular sectors relative to the co-rotation direction, respectively. This is shown in Figure 2 of the paper. These angles were inserted into Equation 5 of the paper that was also used in the MATLAB 2D fits. Each spectrum is one B-cycle apart in time. 

After the MATLAB fits were done, the fit parameter, proton density nP in #/cm3, proton temperature TP in eV, proton rotational speed VP in km/s, and spacecraft potential SC in Volts, were loaded into the same IDL program, different branch, to make the plots shown in Figure 13 of the paper. For the fits we used a convected Maxwellian velocity distribution function (VDF).

A different IDL program is used to make plots of ion cts vs TOF channel number (512 channels). There are 2048 chan that are collapsed to 512. Channels by selecting every fourth channel.

In summary: 

1. The TOF data we manually computed the energy spectrum for the protons.

2. Then using IDL program we computed the parameters for the weighting function (derived from the TOF data) that was used to weight the singles data and make the data files for MATLAB curve fitting app.

3. MATLAB using the convected Maxwellian function with GF described in the paper to perform energy-angle fits.

4. The fit results (proton density, spacecraft potential, proton temperature, proton rotation speed) were then plotted as shown in the paper (data pts vs energy and fits super-imposed) using same IDL program but using a different portion of the code. 

5. The data files used by the MATLAB curve fitting routine were eout (eV), aout (angle of angular sector used in degrees), yout (weighted singles counts plus other constants given in Eq. 5 of the paper, and sout (standard deviation of yout for the 2D fits. Note, the data is 1D but repeated three times with only the angle aout changing (15 points per angular sector). Some of the other parameters were used to make eout, aout, yout and sout.