1. Auroral Oval Position

The location of the auroral oval is a fundamental parameter of both theoretical and practical importance (for example, to space weather forecasting). Nonetheless no accepted standard exists for determining the oval position, even on a historical basis. No single data technique or instrument has the global coverage and continuity for making such a determination, yet combining disparate data sets introduces many difficulties. The need for a calibration standard should be clear.

The DMSP particle data set was chosen as a standard for compelling reasons. DMSP data is not affected by sunlight or darkness, is available in both hemispheres, and has the requisite longevity (about two decades and continuing). The sensitivity of the particle data (a few time 10-2 ergs/cm2 s ) is many times greater than is possible from either ground-based or space-borne imagers. The detailed spectral information and sensitivity make it possible to determine boundaries of maximum geophysical significance, and indeed, the JHU/APL automated DMSP boundaries have been well-documented in the refereed literature to correlate well with other geophysical phenomena.

The equatorward boundary used in OVATION is the b1e boundary defined by Newell et al. [1996]. This equatorward edge of the soft electron precipitation was chosen because it corresponds fairly well with the existing equatorward boundary introduced by Gussenhoven et al. [1981] and Hardy et al. [1981], and which has long been used by the Air Force Space Weather Command, and distributed over the internet. The usefulness of this boundary has been adequately established both in the literature and on a practical basis.

The poleward boundary used in OVATION is intended to be the open/closed boundary (for example, the first encounter with polar rain, cusp, or mantle, or open LLBL on the dayside). More details about the poleward boundary selection are given by Sotirelis et al. [1998] and Sotirelis and Newell [2000]. The latter paper describes the shapes assumed for the auroral oval boundaries in order to extend them to local times with no data.

Other data sets do not necessarily have the capability to measure these boundaries directly. However the cross-calibration procedure establishes the typical offset between the boundaries these other instruments DO measure, and the ones chosen above, as a function of MLT. Moreover, a standard deviation (also a function of MLT) is determined, so that the correct weight can be given to each instrument. A more formal explication of the cross-calibration is given below.

1. Determine the average difference (as a function of MLT)
   <b_eq - b_eqs>         <b_p - b_ps>
   
   
2. Determine the standard deviations:
   s²_eqs = (1/n) S_i (b_eq - b_eqs)²
   s²_ps  = (1/n) S_i (b_ep - b_eps)²
   each of which are functions of MLT.
   
   
3. The Auroral Boundaries Project incorporates all data sources, weighted by 1/s²_eqs and 1/s²_eqs
   

2. Polar Cap Flux and b2i

The polar cal flux, F_PC, given by the OVATION plotting routine represents the amount of open geomagnetic flux, in megawebers. F_PC, as calculated by OVATION, correlates well with the total auroral power as estimated by POLAR UVI [Newell et al., 2001]

b2i is the DMSP magnetotail stretching index. It is the latitude at which magnetic field lines are no longer curved enough to pitch angle scatter high energy ions into the loss cone. As implemented on the DMSP particle data set, the main reference is Newell et al. [1998].

3. Precipitation Intensity

The precipitaion intensity option is based on the Boundary-Oriented Precipitation Model [Sotirelis and Newell, 2000]. The boundaries are chosen according to the actual measurements of the auroral oval position (as described in Section 1 above). The intensity is calibrated by b2i. The current version does not yet incorporate actual measured precipitation intensities to update the statistical model.

References