The atmospheric density model used in ODTK, Jacchia, and MSIS variants, takes as input Kp or Ap, F10, and F10bar.

NOTE: CIRA 1972, an empirical model of atmospheric temperature and densities as recommended by the Committee on Space Research (COSPAR), is also known as Jacchia 1971.

NOTE: Jacchia-Bowman models require additional indices which are input as described below.

Specify the method of input for solar and geomagnetic activity for both general and Jacchia-Bowman specific inputs. Select between Read from File and Static Values:

Applies to general solar and geomagnetic activity inputs, Jacchia-Bowman specific files are discussed below. The following file formats are supported:

- Space Weather flux file (.txt), available at www.celestrak.com/SpaceData and sponsored by CSSI and AGI (default). Use the FluxData.InputFluxFile attribute to select this file.
- Flux (*.fxm) file.
- Schatten predict (*.dat) file.

If you decide to use an .fxm file, the following types are available:

- stkFluxGeoMag.fxm
- stkFluxGeoMag.5yr.fxm

The two version of each type of file, which are updated daily on AGI's FTP site, contain identical data, except that the first contains more historical data and therefore takes longer to read. These files contain sections of measured and predicted values. The measured data contains daily values for F10/F10bar and Ap and Kp values every 3 hours. The predicted data sections contain daily predictions for F10/F10bar and for Ap (Ap is converted to Kp internally). The Space Weather (.txt) files also contain monthly predicts of F10/F10bar. The F10StartTime and F10StopTime fields (read-only) display the F10/F10bar data time span (measured over observed data) based on the selected file. Similarly the ApStartTime and ApEndTime display the Ap/Kp data time span (measured over observed data). ODTK processes which use the F10 or Ap data will use all data from the file (observed plus predicted).

Any satellite propagated in ODTK for which drag is calculated will use the same interface to get F10, F10bar and Kp. If the file is specified, all satellites will use values from that file.

Applies to general solar and geomagnetic activity inputs, Jacchia-Bowman specific files are discussed below. Instead of reading data from a file, you can input specific values for F10, F10bar and either Ap or Kp. If these static values are used, they will be the same for all satellites. If you enter a Kp value, the Ap value is recalculated, and vice versa.

If the boolean attribute ValidateFileSpan is set to true (the default), then, when the user runs the filter with LastMeasurement as the Stop Mode, the span from filter start to filter start + threshold is checked to see if the observed EOP and flux data cover the span. When running the filter with a Stop mode that provides a definitive stop time, the actual filter stop time is checked, and the threshold is ignored. If the observed EOP or flux data does not cover the span, a warning message is issued. The user can set a WarnThreshold attribute (the default is 1 day, and days are the default unit).

Select one of the following options for the KpUpdate attribute:

Kp Subsampling Options | |
---|---|

Option | Description |

3-Hourly | When this option is selected, the subfield KPSubSamplingRatio
does not show. This means that when a K_{p}/a_{p}
value is required for ODTK in a particular 3-hour window, it will
be derived from a stored 3-hour K_{p}/a_{p}
constant, where each pair of adjacent 3-hour constants has a
discontinuity at the interior 3-hour boundary. |

3-Hourly Cubic Spline | Updates by interpolating the 3-hourly data using natural cubic splines. |

3-Hourly Interpolated | When this option is selected, the subfield KPSubSamplingRatio
displays. The integer 1 (recommended) will select one third-order
interpolating polynomial spline per 3-hour
K_{p}/a_{p} window. The integer 2 will select two
third-order interpolating polynomial splines per 3-hour
K_{p}/a_{p} window. The integer n will select n
third-order interpolating polynomial splines per 3-hour
K_{p}/a_{p}P window. |

Daily | When this option is selected, a K_{p}/a_{p}
value is derived from a stored daily K_{p}/a_{p}
constant, where each pair of adjacent daily constants has a
discontinuity at the interior daily boundary. |

Jacchia-Bowman atmospheric density models (the 2008 model is currently supported in ODTK) require additional indicators of solar and geomagnetic activity. The required indices can be obtained at sol.spacenvironment.net/~JB2008/indices.html.

Solar and geomagnetic data are provided in separate files at sol.spacenvironment.net/~JB2008/indices.html. These files are read in their respective provided formats by ODTK.

NOTE: The solar flux indices F10 and F10bar are included in both the Jacchia-Bowman specific files and the general solar and magnetic indices file described above, though the reference time of day for the daily provided value may be different. When the Jacchia-Bowman 2008 atmospheric density model is used, the F10 and F10bar values from the Jacchia-Bowman specific files are used.

Instead of reading data from a file, you can input specific values for F10, F10bar, M10, M10bar, S10, S10bar, Y10, Y10bar and DstDTc. If these static values are used, they will be the same for all satellites.

Here are some useful descriptions of solar flux (F10):

NorthWest Research Associates Space Weather Services: The F10.7 index is a measure of the noise level generated by the sun at a wavelength of 10.7 cm at the earth's orbit. The global daily value of this index is measured at local noon at the Penticton Radio Observatory in Canada.

Australian Space Weather Agency (link to Sunspots -> 'The Ten Centimetre Solar Radio Flux): The radio emission from the sun at a wavelength of 10.7 centimetres (often called "the 10 cm flux") has been found to correlate well with the sunspot number. Sunspot number is defined from counts of the number of individual sunspots as well as the number of sunspot groups and must be reduced to a standard scale taking into account the differences in equipment and techniques between observatories. On the other hand, the radio flux at 10.7 centimetres can be measured relatively easily and quickly and has replaced the sunspot number as an index of solar activity for many purposes.

As used in ODTK, F10bar, or average F10, is the statistical mean of the F10 values over 81 days, which is approximately 3 solar rotations, or 3 * ~27 days. The Sun rotates once every ~27 days. According to NASA's Marshall Space Flight Center:

Since the Sun is a ball of gas it does not have to rotate rigidly like the solid planets and moons do. In fact, the Sun's equatorial regions rotate faster (taking only about 24 days) than the polar regions (which rotate once in more than 30 days).

The mean value of F10 for a given day is centered on that day, with the span of the average starting 40 days prior and ending 40 days following the given day.

The following are technical descriptions of Kp and Ap. For further information see the National Geodetic Data Center site.

**NOTE:** The 'p' in 'Kp' and 'Ap' stands for
'planetary'.

K indices isolate solar particle effects on the Earth's magnetic field; over a 3-hour period, they classify into disturbance levels the range of variation of the more unsettled horizontal field component. Each activity level relates almost logarithmically to its corresponding disturbance amplitude. Three-hour indices discriminate conservatively between true magnetic field perturbations and the quiet-day variations produced by ionospheric currents. K indices range in 28 steps from 0 (quiet) to 9 (greatly disturbed) with fractional parts expressed in thirds of a unit. A K-value equal to 27, for example, means 2 and 2/3 or 3-; a K-value equal to 30 means 3 and 0/3 or 3 exactly; and a K-value equal to 33 means 3 and 1/3 or 3+. The arithmetic mean of the K values scaled at 13 selected observatories gives Kp.

The a-index ranges from 0 to 400 and represents a K-value converted to a linear scale in gammas (nanoTeslas)--a scale that measures equivalent disturbance amplitude of a station at which K=9 has a lower limit of 400 gammas.

In summary, Kp and ap indices are used to quantify solar particle effects on the Earth's magnetic field; the Kp index is one of the input parameters to the atmospheric density model for drag calculations.

ODTK 6.5