IVM Data Description

1. Length of a file

Ultimately we will redo these files so that they will all be a single orbit in length starting at the apogee of the orbit. For the present time we are delivering the files to the public in the same form as we get them from the Air Force. That means each file can range from only a fraction of an orbit (~25 minutes) to nearly two complete orbits. Even though it has this odd cadence, all the data are present and any gaps here indicate either bad data that has been filtered out or else the complete loss of data for that time period.


2. Parameters on the graph

Six geophysical parameters are presented on the graph: the ion temperature (Ti), the ion flow in the ram (i.e.--spacecraft velocity) direction (+Vx in the direction of spacecraft velocity), the ion flows in the horizontal and vertical directions (Vy and Vz) at right angles to the ram direction (+Vz is upward, +Vy is roughly northward), the total ion density (Ni) (th e sum of O+ and H+ and He+ concentrations), and the light ion density (the sum of H+ and He+ concentrations).

As can be seen here there are data gaps where we have filtered out bad data. In general Ti and the densities can be trusted to be valid through out the entire orbit and at all altitudes. However the flow data can only be trusted at lower altitudes. The analysis of the ram velocity becomes more uncertain as the percentage of the light ions becomes predominate. Under those conditions, the analysis routine sets Vx to zero and solves for the other parameters. (Note that the Vy or Vz data are also removed here since the drift meter does not produce usable data under these conditions.) Likewise the crosstrack ion drift meter only works reliably when the percentage of O+ is very high and since the percentage of O+ decreases with increasing altitude, therefore the best crosstrack data come in the region near perigee and on the dayside. (However, data from perigee on the nightside or near apogee on the dayside can also be acceptable, depending on the conditions.) We are currently working on determining the criteria for quality flags on the crosstrack data. Until we have those in place please view the crosstrack data with caution and do NOT use these data in a presentation or publication without first checking with one of the CINDI researchers.

This second plot shows a better orbit where the perigee occurs at about 0030 LT. In this case there is no period where Vx is set to zero, however there are still periods where the crosstrack data are filtered out as bad.

So why is this happening? C/NOFS was launched in spring 2008 during the bottom of an unusually quiet minimum in the solar cycle. The early results from CINDI have emphasized how quiet the ionosphere is and how low the transition height (altitude where the density of O+ is equal to the density of H+) is. This is much lower than we anticipated when the instruments were designed and the orbit was selected. The good news is that the percentage of good data per orbit will increase as time goes by for two reasons: 1) solar activity will increase, heating the atmosphere and ionosphere thus pushing the transition height up to higher altitudes and 2) the satellite's orbit will decay placing more of the pass at low altitudes and regions of higher O+ density.


3. Data in the ascii file

At present we are not presenting the data in numerical form. We are only presenting the plots of the data here. Once we have developed reliable quality flags to go with the data, then we will begin serving the numerical data as well as the plots. Please check back here regularly.


4. How these data are processed

The initial data are binned in half second segments. Each RPA sweep takes one-half second. The IDM data are sampled at 48 Hz for each component and averaged into half-second segments. These half-second data (Ti, Vx, Vy, Vz, Ni, and Ni-light) are then passed through a 15-point median filter to remove fliers. (Note that fill data are excluded, so if there are fill data in the 15 point window, the next good data past the fill data are included in the envelope for the filtering.) The resulting filtered data are then averaged using a 9-point running average. (Again, fill data are excluded as described above.) The plots show the data in the full half second resolution. The ascii data files (when they go on-line) will give the data at a one-second resolution.