Manifests as isotropic background count rate that is independent of energy.
DIS Penetrating Radiation Correction Methods
1. Correct Moments using background estimates provided in the DIS moments CDF files:
We average the lowest 5 values (not energies) of differential energy flux in the 'energyspectr_omni' data product to obtain an estimate of the background. This differential energy flux can be used to form a ‘background skymap’ of phase space densities that can be numerically integrated in parallel with the normal processing, providing effective plasma parameters for the background signal.
The validity of this background estimate is broadly applicable, but breaks down in hot magnetosheath intervals where there is at least some valid signal at all energies.
Because the background cannot be reliably estimated for all cases, these estimates are NOT used to automatically correct the L2 moments. Instead, we provide users with the means to correct L2 moments themselves if the background estimate appears to be appropriate for the purpose.
Three variables are provided in the DIS moments CDF:
| Variable | Definition / Notes |
|---|---|
| energyspectr_bg | background differential energy flux measurement |
numberdensity_bg | effective number density of the background signal (referred to as nbg below) |
| pres_bg | effective scalar pressure of the background signal |
Note: By definition, the bulk velocities and off-diagonal pressure tensor components of the background signal are zero
FPI DIS Moments can be corrected as follows:
| Quantity | Background Correction | |
|---|---|---|
Density | ncorr = nL2 - nbg | |
Velocity | Vi,corr = nL2 Vi,L2 / ncorr | |
| Pressure Tensor | Definition of pressure tensor component: | Pij = int (vi vj f d3v) - m n Vi Vj |
For an isotropic background: | int (vi vj f d3v)= Pbg if i=j, and is 0 otherwise | |
| Therefore: | Pij,corr = Pij,L2 - int (vi vj fbg d3v) + m nL2 Vi,L2 Vj,L2 – m ncorr Vi,corr Vj,corr | |
| Temperature Tensor | Tij,corr = Pij,corr / (ncorr kB) | |