Fundamental Technologies

Ulysses HISCALE Pages

Guide to Scientific Use of Ulysses HISCALE Data Available Through VHO

Authors:
Thomas P. Armstrong and Errol J. Summerlin
Armstrong@ftecs.com   Errol.Summerlin@nasa.gov

Date: November 17, 2009, revised 8/16/2010

Question 1: What is the information available in the HISCALE data sets?

Answer 1: The HISCALE instrument and investigation operated from day 318, 1990 (November 17) until spacecraft shutdown on June 30, 2009. It measured directional fluxes of electrons, protons, and ions with energies from several 10’s of keV up to about 10 MeV at time resolutions ranging from 12 seconds up to 17 minutes. Summaries of passbands, orientations, and geometrical factors of the instrument relevant to the data as it exists in the VHO/VEPO path are given in the appendices included here and much more exhaustively in the HISCALE Data Analysis Handbook at url http://hiscale.ftecs.com/uly-handbk-oview.html. More helpful information about the data set and instrument specifics is provided at url http://data.ftecs.com/VHO/VEPO/ulysses_hiscale/metadata/. The Data Analysis Handbook is intended to be the permanent reference files of technical information about the instrument and analysis software; the metadata files will be revised and extended from time to time. HISCALE data users should become familiar with this material and pose specific questions about it to armstrong@ftecs.com.

Question 2: What is the directional resolution and orientation of the observations?

Answer 2: 4 single parameter solid state detector assemblies (LEMS30, LEFS60, LEMS120, and LEFS150) and one three element solid state detector telescope (CA60, aka WART) on HISCALE provide angular coverage. The number at the end of the name indicates the detector’s orientation with respect to the spin-axis of the spacecraft which points towards earth at all times. As the spacecraft spins, the data is collected in 4 or 8 sectors (sectors are essentially fractions of a spin; 4 for LEMS30 and LEFS150 and 8 for LEMS120, LEFS60 and CA60). The opening angles for the telescopes are 51 degrees for the two LEMS assemblies, 53 degrees for the two LEFS assemblies, and 45 degrees for the CA60 (composition aperture) telescope. The sectors combined with the detectors oriented in different directions with respect to the spin axis gives coverage in almost all directions, but the angular resolution is somewhat limited. Sectored data contains look directions of each sector in the spacecraft frame in spacecraft coordinates. Spin averaged data is also available (see the print group descriptions on VHO).

Question 3: To which data sets does this FAQ apply?

Answer 3: In the 4 telescopes that provide angular coverage, the electrical impulses generated by an event are sent to two places for analysis: the MFSA and the Rate Channels. Data from both these analyses is available through VHO and this FAQ applies to both. The MFSA data linked through VHO has had a maximum of post-processing done on it and is called Level 3 MFSA data (pg80 through pg92 are the last 4 characters of these data sets; see the appendix). The Rate Channel data has had a minimum of post-processing done and is called Level 1 Rate Channel data (DG65 through DG72 are the last 4 characters of these data sets; see the appendix). Additionally, the CA60 telescope determines ion composition and all the PHA (pulse height analysis) events detected by this telescope are linked through VHO. This FAQ will provide a brief description of how these data are used to determine ion composition, but the details of this analysis are beyond the scope of this FAQ as they require specialized knowledge of the HISCALE instrument. Higher level data products use the CA60 PHA data to discriminate between species and provide composition, eliminating the need for typical users to perform this analysis themselves. Finally, the ephemeris and spacecraft orientation data file (EPH at the end of the filename) is linked through VHO.

Question 4: How are the measurements made?

Answer 4: At the base of each of the 4 telescopes providing angular coverage is a silicon surface barrier detector called the M, M’, F, and F’ detectors. When a charged particle strikes these detectors, it creates an electrical impulse that is sent one of two places. For MFSA data, it is converted from an analog to a digital signal and binned according to the amplitude of the pulse into one of 32 energy bins (only 25 of which contain useable data. See chapter 4 of the HISCALE Data Analysis Handbook for more information). For the Rate Channel data with fewer energy bins and much higher time resolution, the signal is input to a set of voltage discriminators that place it in an energy bin that is logically vetoed by a following detector, thus assuring that the particle did not penetrate the first detector. These bins correspond to different incident particle energies. An exhaustive compilation of incident energies and channel efficiencies for all particle species is provided in excel workbooks at the url: http://data.ftecs.com/VHO/VEPO/ulysses_hiscale/metadata/range-energy%20summaries/. The passbands associated with each of the rate channels are tabulated in the appendices of this document for the species that are expected to dominate the response of the channel in question. Separate identification of species fluxes of electrons, protons, helium, and heavier ion species requires consideration all of the HISCALE data, especially that from the CA60 telescope. Events are binned according to sector and time. Thus, for each accumulation time (1024 seconds) there are 768 bins of data (32 energy channels for each of 24 sectors on the spacecraft). While these 4 telescopes can separate ions from electrons (see question 9 for more info), they cannot distinguish between varieties of ions by themselves. The last telescope, the CA60, uses 3 detectors to analyze incoming ions. The first detector is very thin and ions are expected to penetrate it. The second detector is intended to stop the particle and the third detector is used to reject overly energetic particles based on co-incidence. The energies deposited in the two detectors, if plotted, follow a unique track for each species. The proximity of a particular event pair to these tracks determines its species.

Question 5: What are the energy and time resolutions of the observations?

Answer 5: MFSA data has a 17 minute (1024 second) accumulation time and 25 usable energy bins. For convenience, data are averaged to longer time scales as well (1 hour and 1 day). Rate Channel data has 12 second time resolution and 7-9 energy bins and is also averaged to longer time scales (5 minute, 1 hour, and 1 day). The bin boundaries are described in the parameter definitions on VHO but can also be found in the appendix.

Question 6: What post-processing has been done on the data?

Answer 6: The data sets available on VHO vary in the amount of post-processing done on the data. Level 1 Rate Channel are simply the counting rates in the various energy bins. They have not been background corrected nor converted to physical units, and ions have not been separated from protons. The Level 3 MFSA data has had several post-processing steps performed. First, backgrounds have been subtracted (see question 7). Then using composition measurements from the CA60 telescope, the ions have been separated into protons and Z>1 (mostly alpha particles; see questions 8 and 9). Then, the geometric factors of the instrument are applied to produce physical units of flux. This provides a flux for each energy channel in the frame of the spacecraft for each sector. Finally, a Compton-Getting transformation is performed to provide fluxes in the solar wind frame. The details of this transformation can be found here.

Questions and answers continued

Return to Ulysses HISCALE Data User's Guide table of contents.
Return to Fundamental Technologies Home Page.

Last modified Sept. 21, 2010
T. Hunt-Ward
tizby@ftecs.com