Frequently Asked Questions about the DMSP SSIES data

  1. What are the DMSP spacecraft?
  2. What is the SSIES thermal instrument complement?
  3. Where are the SSIES instruments located on DMSP and what coordinate system is being used?
  4. What specific geophysical parameters are available here that are derived from these instruments?
  5. What portion of the Earth do these spacecraft cover?
  6. What part of the polar regions do these spacecraft cover?
  7. What time period does this database cover?
  8. What is the resolution of the data provided here?
  9. Are there any higher resolution data available?
  10. How are the data presented on this website?
  11. How are these parameters derived from the RPA?
  12. How are these parameters derived from the IDM?
  13. How are these parameters derived from the scintillation meter?
  14. How are these parameters derived from the Langmuir probe?
  15. Are there any other parameters derived from the SSIES instruments that are not available on this website?
  16. How do I tell whether these parameters are good and reliable or not?
  17. What do these quality flags mean and how are they derived?
  18. What could be causing the degradation of the data?
  19. What about the F14 data on-line here?
  20. Has the corotation component been taken into account in the flow data?
  21. Are the flow data rezeroed based on the flow data in the middle latitude regions outside of the auroral regions?
  22. Why don't the flows always go to zero in the middle latitude regions outside of the auroral regions?  Shouldn't they?
  23. Is there any way to correct for this?
  24. Are there any special periods of time where data are missing or not available?
  25. Where are the data about the electrostatic potentials you folks have derived from the flow data (the NADIA program)?
  26. I want a large amount of data (like an entire month of continuous data or more) and I don't want to have to go through the tedious process of downloading the data orbit by orbit.  Is there another way to obtain the data?
  27. How do I credit you folks if I use these data in a paper or talk?
  28. I need some data from an event that occurred this morning / yesterday / last week / three weeks ago and it's not on-line here. Where is it?

Answers:

  1. What are the DMSP spacecraft?

   The Defense Meteorological Satellite Program (DMSP) is a series of polar orbiting weather satellites at an altitude of about 840 km operated by the United States Department of Defense.  The spacecraft are in 96 degree inclination orbits which means the orbit precesses through 360 degrees per year, which means the orbit stays locked to a local time throughout the year.  The first DMSP spacecraft was launched January 19, 1965 and the DOD keeps at least two in operation at any one time.  (Sometimes as many as four have been operating simultaneously.)  More information on the DMSP spacecraft can be found here. There is a parallel civilian weather spacecraft called TIROS-POES (Polar Orbiting Earth Satellite), but those do not carry any thermal plasma instruments.

2. What is the SSIES thermal instrument complement?

   Starting with DMSP-F8 (launched summer 1987) the DMSP program flew the Special Sensors-Ions, Electrons, and Scintillation (SSIES) thermal plasma analysis package on the spacecraft.  The SSIES instruments were built by the Center for Space Sciences at the University of Texas at Dallas.  Each package include a Retarding Potential Analyzer (RPA), an Ion Drift Meter (IDM), a scintillation meter, and a Langmuir probe.

3. Where are the SSIES instruments located on DMSP and what coordinate system is being used?

   The figure below shows a schematic drawing of the DMSP spacecraft in flight. DMSP is a three-axis stabilized spacecraft and the view is of the front side of the spacecraft as it moves around the Earth. Thus the velocity vector is out of the page towards the viewer and the RPA, IDM, and scintillation meter are hanging off the bottom side of the spacecraft and facing into the ram direction. For the SSIES data here we use a coordinate system where +x is out of the page (towards the viewer), +y is horizontal to the right, and +z is vertical upward away from the center of the Earth.

 

 

4. What specific geophysical parameters are available here that are derived from these instruments?

   The RPA, IDM, and scintillation meter are all variants of Faraday cups facing into the direction of travel of the spacecraft and mounted on a metal plate which is held at the same potential as the ambient plasma. The Langmuir probe sits on a boom which extends 76.2 centimeters (30 inches) from the spacecraft.

   The RPA measures the ion flux entering the instrument as a function of the retarding potential (positive) applied to the incoming ions.From this data the RPA can measure the thermal ion flow speed in the direction of the spacecraft (VX), the ion temperature (Ti) and the H+, He+ and O+ fractional composition of the plasma (fH+, fHe+, and fO+). A more detailed explanation of the RPA and how we analyze the data is available here.

   The IDM has a detector divided into four sections, each of which measures the ion current falling on it. By comparing the differences in the currents and knowing the geometry of the instrument we deduce the arrival angles of the ions and hence the cross-track velocities of the plasma. Thus the IDM provides measurements of the cross-track horizontal ion flow (VY ) and the vertical flow (VZ). A more detailed explanation of the IDM and how we analyze the data is available here.

   The scintillation meter is a simple Faraday cup which measures the total ion current entering the cup. From that we calculate the ion density of the plasma (Ni) and this measurement serves as a check on the ion density measurements made by the RPA. Further, the scintillation meter measures the ion density with a resolution of 24 Hz compared to the once every 4 seconds (0.25 Hz) resolution of the RPA. A more detailed explanation of the scintillation meter and how we analyze the data is available here.

    The Langmuir probe is a sphere on boom 76.2 centimeters (30 inches) away from the spacecraft which collects electrons from the ambient plasma. Calculations based on the collected current as a function of the bias voltage on the probe allow for the determination of the electron density (NE) and temperature (TE). A more detailed explanation of the Langmuir probe and how we analyze the data is available here. It should be noted that because of the relatively straightforward analysis of this data, in the block 2 SSIES (F11 through F15) the calculations were done by the onboard microprocessor and only the values of the electron density and temperature were transmitted to the ground. However during selected periods (such as the Leonid meteor shower in 2001) the raw current telemetry from the Langmuir were transmitted to the ground instead of the RPA currents, thus causing the loss of all RPA data during these periods (see question 24 below)

   Thus combining the output from the four SSIES instruments gives us the complete three-dimensional flow vector of the thermal plasma, the plasma density and composition, along with the temperature of the ions and the electrons.

 

5. What portion of the Earth do these spacecraft cover?

      Basically the DMSP fly in one of two local time configurations: roughly dawn-dusk and roughly 0930-2130 LT. The dawn-dusk DMSPs are F8, F11, and F13 while the rest are in the 0930-2130 orientation. Since the orbital period is about 105 minutes, this means that the track moves about 25 degrees westward in longitude between successive passes. Over a period of about 3 days a given satellite will pass at least once within 1 degree of almost every point on Earth. (For more details on the geographic coverage, go to the spacecraft background page.)

6. What part of the polar regions do these spacecraft cover?

   Because of the inclination of the orbit, no DMSP ever goes above 84 degrees north or south latitude. However most polar studies using DMSP data are interested in the data in magnetic local time and magnetic latitude. Because the Earth's dipole is tilted relative to the Earth's spin axis, this coordinate system "rocks" back and forth under the satellite's track over the course of a day. Thus in magnetic latitude-MLT coordinates the satellite's track wanders about depending on the time of day and thus forming a band of tracks. Click here for diagrams showing the extent of these bands for each satellite.

7. What time period does this database cover?

   Initially this database covers only the operational spacecraft (F12 through F15) during the 1 January 2000 through 31 December 2002 period. We hope to expand the coverage beyond this (both forward with the ongoing operations and back into the historical data), but that is dependent on further funding. Meanwhile there are some special event periods outside of this three-year period that we have processed and a complete listing can be found here. If you have a special event or campaign period you would like processed, please send us the request and we will consider it.

8. What is the resolution of the data provided here?

   All data provided here are at a 4-second resolution to match the resolution of the RPA data.

9. Are there any higher resolution data available?

   The IDM samples the cross track ion velocities 6 times per second for both the horizontal and vertical flows. Thus 24 samples for each component are averaged to match the 4-second resolution of the RPA. The scintillation meter samples the ion density 24 times per second, so 96 samples (minus any range change flags) are averaged to match the 4-second resolution of the RPA. These higher resolution data are available as a special request by emailing us for more information.

10. How are the data presented on this website?

   When you go to our data search page, you are given two ways of searching for the data. The first is by asking for a start date and then requesting a list of files (for up to five days at a time). The second is by choosing a specific date and then asking for a specific time period (of up to 100 minutes).

   If you take the first option, say you want to see data from 22 June 2001 (day of year = 173), you would enter that information into the blanks. You have the option of searching for all the available DMSP spacecraft (the default setting) or clicking on one of the radio buttons to search for data from only one spacecraft. After hitting the submit button, a new page appears with a list of the names of the datafiles from that day. Each file consists of a single orbit of data starting at the equatorial crossing of the spacecraft as it is moving northward (the duskside for F12 through F15). The filename contains the information about the time and which spacecraft these data are from. For example, the file named rl011731033.f13 contains the data from F13, year 2001 (01), doy 173, starting at 1033. To further make this clear, the webpage gives the date and starting time (June 22 2001 10:33 UT) on the same line. Clicking on that filename brings up a new page with a line plot of the data for that orbit and two more links. One link will plot the VY data in both polar regions on polar dials. (Both the line plot and the polar plots are jpeg files you can save through your browser.) The other link brings up an ascii file of the data for this orbit which can be saved to you computer through your browser. The format for this data file is found here. The idea for this is that you can easily browse through the line plots to find a period or event you are looking for, and once you have found it, then you can download only the data you are interested in using.

   The second option allows you to specify a particular period (less than 100 minutes) of data from a particular spacecraft to examine in the same manner as described above. This comes in particularly handy with examining a period which spans two consecutive orbits. The routine on our site will splice together the data from the two files to make a continuous plot and ascii data file for the user.

11. How are these parameters derived from the RPA?

   The retarding potential analyzer (RPA) is a Faraday cup looking into the direction of the spacecraft's travel. A repeller grid charged slightly negative on the front prevents any electrons from entering the cup. The ions must pass through a second set of repeller grids before reaching the collector. Over the course of 4 seconds the voltage on this set of grids is swept from -3 volts (all the ions reach the collector) to +12 volts (no ions reach the collector). The current on the collector plotted as a function of the retarding voltage forms a curve which is analyzed to obtain the velocity of the ions along the spacecraft's path (VX), the temperature of the ions (Ti), the total ion density (Ni), and the fractional composition of the plasma in H+, He+ and O+ (fH+, fHe+, and fO+). For a more complete description of this analysis, click here.

12. How are these parameters derived from the IDM?

   The drift meter (IDM) is a Faraday cup looking into the direction of the spacecraft's travel. A repeller grid charged slightly negative on the front prevents any electrons from entering the cup. The aperture on the IDM is square and the collector in back consists of four separate plates. As the ions enter the cup, if there is any cross-track velocity, then there will be an imbalance on the number of ions (current measured) hitting one plate versus the other. By measuring the differences in the current and knowing the geometry of the IDM, we can calculate the cross track velocities. The spacecraft velocity (generally about 7.5 km/s) is always larger than the cross track ion velocities. The limit on the measurable cross track velocities is ±3.0 km/s. For a more complete description of this analysis, click here.

13. How are these parameters derived from the scintillation meter?

   The scintillation meter is a simple Faraday cup looking into the ram direction of the spacecraft's travel. A repeller grid in front is charged slightly negative to prevent the entry of electron, so the collector in back measures the current from all ions entering the cup. Knowing the speed of the spacecraft and the size of the aperture it is a straightforward calculation to convert the measured current to the ion density along the column sampled. The current is measured 24 times per second (once about every 300 m). In addition, there are six bandpass filters connected to the electrometer which given information on structures (density variations) on scales from 12-24 Hz to 956-2263 Hz. A more complete description of the instrument and analysis is available here.

14. How are these parameters derived from the Langmuir probe?

   The Langmuir probe is a gold-plated, sphere (4.45 cm diameter) surrounded by a gold-plated spherical (5.72 cm diameter) grid screen. The sphere is always kept at a constant +20 V with respect to the grid while both are swept through a range of potentials from -4 V to +4 V (relative to the ambient plasma potential). At negative potential extreme of -4V all the electrons are repelled by the outer grid (and all ions are repelled by the positive potential on the sphere), so no current is collected by the sphere. As the potential goes positive electrons get past the grid and are collected by the sphere, thus generating the measured current. Above a certain positive potential all the electrons within the Debye shielding region around the probe are collected, so the collected current remains constant with increasing potential. The shape and size of the current vs. potential curve are determined by the electron density (NE) and temperature (TE) in the plasma, thus an analysis of the current as a function of potential can determine these two parameters. A more detailed description of this analysis can be found here.

15. Are there any other parameters derived from the SSIES instruments that are not available on this website?

   There are filter data from the scintillation meter and instrument housekeeping data we process but do not place on the website. As noted in question 9 above, we also have higher resolution cross-track horizontal ion flow (VY) and the vertical flow (VZ) from the IDM and higher resolution ion density (Ni) data from the duct meter. Contact us if you are interested in these data.

16. How do I tell whether these parameters are good and reliable or not?

   This is the key question of any scientific dataset. Over the 15 years we have worked with the DMSP SSIES data we have discovered a surprisingly large number of ways in which the data can behave oddly. In an ideal system we would have each orbit checked both by computer algorithms and double-checked by a person before we allowed it to be placed in a public database. However, given the size of this database (12 satellite years at 4-second resolution with more to come) and the limited staff resources and time, we simple cannot examine all the data by hand to guarantee that every single orbit on-line here is perfect and error-free. It is entirely possible that when you select an orbit, it may be the first time a human has actually examined that particular data. The best we were able to do is develop an algorithm which assigns a quality flag to each 4-second set of RPA-derived data and each 4-second set of IDM-derived data. Further information can be found on the Quality Flags page.

17. What do these quality flags mean and how are they derived?

   The two quality flags (one for the RPA data and the other for the IDM data) are given in the ascii listing of the orbit's parameters. The code is 1 = good, high confidence in the data; 2 = caution, concern about this data; 3 = poor quality, bad data, do not use; 4 = undetermined. The procedure for determining the flags is described in detail here along with examples of various degrees of quality data. The good news is that we try to err on the side of caution, so any data with a quality flag of 1 can be used with a high degree of confidence. Note: While the data have been preprocessed, the quality flags are determined 'on the fly' when you request the data.  This is done to allow us to refine and improve the algorithm. Thus it is possible that quality flags will not match if you request the data once, then rerequest the same data several months later. In general if you are looking at a period of several minutes of continuous data which are consistently rated good (1) or bad (3) it is highly unlikely that any change in our algorithm will change those flags. However if your period of interest has a mixture of good (1), caution (2) and/or bad (3), then it is possible that some of the flags may change as we upgrade the algorithm. Further information can be found on the Quality Flags page.

18. What could be causing the degradation of the data?

   Many things can affect the quality of the SSIES data. However the most common problem is a relatively high (more than 15%) concentration of light ions (H+ and He+) in the plasma. The RPA and IDM were designed to operate in a predominately O+ environment, so as the percentage of light ions increases, the quality of the data is degraded. Another common problem occurs with the RPA. Since it takes 4 seconds (~28-30 km) for the RPA to complete a single sweep, the analysis assumes that the thermal plasma is uniform during that period. However, if the plasma has a significant variation in either density or velocity on a scale smaller than this, then the current varies erratically during the sweep and no analysis is possible. This frequently is seen in the auroral region. In these cases, fill data are placed in the parameters derived from the RPA and the RPA flag is set to 3 (bad). Since the quality flag for the IDM are based in part on the RPA data, the associated IDM quality flag is set to 4 (undetermined). A more complete discussion of these problems and their effect on the data is available here.

19. What about the F14 data on-line here?

   In order for the RPA, IDM, and scintillation meter to function properly, they must operate at the same potential as the ambient plasma. (Obviously if they were charged to say +30 volt, no ions would be reaching them at all, while if they were charged to -30 volts, the ions would be accelerating as they entered the instruments, thus throwing off the analysis.) To ensure that this condition is met, the instruments are electrically separated from the rest of the spacecraft and a device called SENPOT continually adjusts the ground plane of these instruments relative to the spacecraft's potential to hold them all at the plasma potential. Unfortunately on 2 September 1999 the SENPOT on F14 failed. While the spacecraft is in darkness essentially all of the spacecraft is at the ambient plasma potential, but once it goes into daylight, the photoionization of the metal illuminated by the sunlight drives the spacecraft negative. Thus there is an unknown negative potential applied to the RPA, IDM and scintillation meter and this corrupts the data. We are working to see if there is a way to recover some or all of these data, but for the time being we are flagging all the daylight F14 data in this database as bad (quality flag 3). A more detailed description of the problem is available here.

20. Has the corotation component been taken into account in the flow data?

   Because the Earth rotates beneath the spacecraft's track, there is an eastward flow component in the raw data (in the nonrotating frame). However, almost all other data observations and analysis of the ionosphere (e.g. Earth-based radars) are done in the corotating frame (the frame of reference rotating with the Earth). So for each four-second set of data we calculate the eastward flow at the spacecraft's altitude, then remove that vector component from the horizontal ion flow data (VX and VY). A more detailed description of this procedure is available here.

21. Are the flow data rezeroed based on the flow data in the middle latitude regions outside of the auroral regions?

   No. In the data from the NADIA program that we have provided in the past, the two crosstrack flows (VY and VZ) were rezeroed relative to the flows outside of the auroral region (generally about 45 to 50 degrees magnetic latitude).However, for this dataset we are presenting the all three flow velocities as they are observed by the SSIES instruments (other than correcting for the corotation component described above).

22. Why don't the flows always go to zero in the middle latitude regions outside of the auroral regions? Shouldn't they?

   In principle, yes, all these flows should be zero. However there are some real non-zero middle latitude flows called SAPS (also known as SAIDs). Aside from that we do see non-zero flows in the middle latitudes (say between 30 and 50 degrees magnetic latitude) frequently. Most of these can be explained as erroneous measurements caused by the effect of the light ions (see question 18) on the drift meter.

23. Is there any way to correct for this?

   Yes, there are several ways, but for right now we are not doing any corrections here. (We may do some later as part of providing the electrostatic potential data, see question 25 below.) For the time being we leave any rezeroing or correcting of the data to the discretion of the user.

24. Are there any special periods of time where data are missing or not available?

   A more complete list of periods can be found here. The main period of incomplete data is from 6 November 2001 (starting at about 14:40 UT) to 20 November 2001 (ending at about 18:03 UT). This period covers one of the Leonid meteor showers. During this time, the Langmuir probe raw data were transmitted rather than the RPA data to determine if it was possible to detect micrometeorite hits on the spacecraft. Because of the lack of RPA data during these periods, there are no ram drift, ion temperature, or composition data during this period. In addition, since the quality flags for the IDM are based on RPA parameters, all the IDM data quality flags are set to undetermined (flag = 4) during this period.

25. Where are the data about the electrostatic potentials you folks have derived from the flow data (the NADIA program)?

   In the past Dr. Hairston has provided calculations of the potential along the spacecraft's track derived from the NADIA program. We are in the process of adapting and upgrading the NADIA program to be run using the data from this database rather. Eventually we hope to distribute these data as well through this website. Until then, please direct all requests for potential data directly to Dr. Hairston.

26. I want a large amount of data (like an entire month of continuous data or more) and I don't want to have to go through the tedious process of downloading the data orbit by orbit. Is there another way to obtain the data?

   If you wish to acquire a large amount of data, please contact us directly and we can make alternative arrangements to deliver the data to you.

27. How do I credit you folks if I use these data in a paper or talk?

   If you simply use the data, we would appreciate being credited in the acknowledgments of your paper (or presentation) and please drop us an email letting us know. This gives us statistics we can use in getting further funding to expand this resource. An example would be something like "we gratefully acknowledge the Center for Space Sciences at the University of Texas at Dallas and the US Air Force for providing the DMSP thermal plasma data." If you have questions about the data and contact us about checking it or explaining it before you use it, then we would like the opportunity to be included as a coauthor.

28. I need some data from an event that occurred this morning / yesterday / last week / three weeks ago and it's not on-line here. Where is it?

   We do not receive the DMSP data in a realtime manner. We get the telemetry on CDs mailed to us once a month. Generally we get the telemetry from the previous month on about the second week of the current month and we process it as soon as time permits. If you need data from a recent event such as this, please keep checking back regularly.


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