We have developed a method to forecast the short-term space weather risk to satellites based upon the current value of Dst. However, Dst measurements are not instantly available, because preparing it is a complex process. Several versions of Dst are released, the earliest being quick-look Dst which is released between 12 and 36 hours after the measurements are made.
To predict the current value of Dst we use the most recent quick-look Dst value and real-time solar wind data measured by the ACE satellite, using a method based upon that of O'Brien and McPherron.
The practical application of the method involves several stages, augmented by test criteria for accuracy of the Dst prediction, which in turn determine the timing of the process.
The timing of the process is determined automatically with the aim of performing the nowcast as soon as is realistically possible. Attempts at a nowcast, subject to the following tests, will be made half an hour after the last successful nowcast. Nowcasting is defined as predicting the next hour on from the previous nowcast irrespective of the actual time. Any break down in the program will result in the program being reset and the nowcast will be set to the present hour.
The process described above is augmented by several approximations, restrictions and tests must be made due to the nature of the data available.
The data set of solar wind properties from the ACE satellite contains data gaps ranging from several hours of missing data to the occasional minute of bad data. The solar wind parameters necessary are the solar wind speed v, the density of the solar wind n and the z component of the interplanetary magnetic field. Solar wind speed and density are recorded by the SWEPAM instrument, Bz by the MAG instrument, both instruments give minute values but data gaps and the final minute given by SEC are not always concurrent.
The satellite risk assessment is based on the nowcast of Dst. The aim being to make this nowcast as soon as possible, while maintaining an acceptable level of accuracy. Thus several criteria are applied to the solar wind parameters from SEC before hourly averages are calculated.
The first and third conditions are regarded as non-critical in that a data gap in the ACE data will be recorded for the nowcast time and a nowcast of Dst will be made by using a longer time step in the Burton equation.
The second criterion is regarded as the critical condition, such that delaying the nowcast could possibly make a better more accurate nowcast. As such from this criterion a delay time is calculated after which an attempt at calculating the nowcast will be re-attempted. The delay time is given by the end of the UT hour wishing to be nowcasted minus the time of the final value of solar wind data propagated to the earth.
The point within the vertical dashed lines is the prediction of Dst for the latest hour we can make a prediction for - the "nowcast." It is this value which is stored as our prediction for any future analysis. Note the predictions up to this nowcast from the final quick-look Dst value are not fixed but may change as quick-look Dst is updated or as more solar wind data is obtained allowing interpolation across data gaps. However the nowcast made for each hour is stored as is, and will not be updated even if subsequently an improved prediction for that hour (now lagging real time) could be made.
The plot highlights the 4 data conditions
How we've done in the last week
How we've done in the last month