; FARAD ;--------------------------------------------------------------- ;! add ionospheric Faraday rotation to CL table ;# Task Calibration Polarization VLA ;----------------------------------------------------------------------- ;; Copyright (C) 1995 ;; Associated Universities, Inc. Washington DC, USA. ;; ;; This program is free software; you can redistribute it and/or ;; modify it under the terms of the GNU General Public License as ;; published by the Free Software Foundation; either version 2 of ;; the License, or (at your option) any later version. ;; ;; This program is distributed in the hope that it will be useful, ;; but WITHOUT ANY WARRANTY; without even the implied warranty of ;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the ;; GNU General Public License for more details. ;; ;; You should have received a copy of the GNU General Public ;; License along with this program; if not, write to the Free ;; Software Foundation, Inc., 675 Massachusetts Ave, Cambridge, ;; MA 02139, USA. ;; ;; Correspondence concerning AIPS should be addressed as follows: ;; Internet email: aipsmail@nrao.edu. ;; Postal address: AIPS Project Office ;; National Radio Astronomy Observatory ;; 520 Edgemont Road ;; Charlottesville, VA 22903-2475 USA ;----------------------------------------------------------------------- ;--------------------------------------------------------------- FARAD LLLLLLLLLLLLUUUUUUUUUUUU CCCCCCCCCCCCCCCCCCCCCCCCCCCCC FARAD Task to add ionospheric Faraday rotation to CL table INNAME Input UV file name (name) INCLASS Input UV file name (class) INSEQ 0.0 9999.0 Input UV file name (seq. #) INDISK 0.0 9.0 Input UV file disk unit # INFILE Ionosonde data file. SOURCES Source list ' '=>all. TIMERANG Time range to use. ANTENNAS Antennas to correct. SUBARRAY 0.0 9999.0 Subarray; 0 => 1. GAINVER 0.0 9999.0 CL table version to update OPCODE Operation code. 'TEC ' => use TEC data 'F0F2' => use f0F2 data 'MOD ' => use Chiu model ' ' => use model BPARM Model parameters: 1 = altitude of F2 layer 2 = R1 sunspot number ---------------------------------------------------------------- FARAD Task: This task estimates the ionospheric Faraday rotation measure and adds it to a CL table. Adverbs: INNAME.....Input UV file name (name). Standard defaults. INCLASS....Input UV file name (class). Standard defaults. INSEQ......Input UV file name (seq. #). 0 => highest. INDISK.....Disk drive # of input UV file. 0 => any. INFILE.....Name of file containing ionospheric data. SOURCES....list of sources to process. '*' = all; a "-" before a source name means all except ANY source named. TIMERANG...Time range of the data to be used. In order: Start day, hour, min. sec, end day, hour, min. sec. Days relative to ref. date. ANTENNAS...A list of the antennas to be modified. If any number is negative then all antennas listed are NOT to be modified. All 0 => use all. SUBARRAY...The subarray to modify. Do only one at a time. GAINVER....The CL table version number which is to be updated. NOTE: There is NO default value. OPCODE.....Operation code (see also EXPLAIN FARAD): 'TEC ' => use TEC data. INFILE should be the name of a file containing TEC data in the format distributed by the Boulder station. Uses BPARM(1) or BPARM(2). 'F0F2' => use critical frequency data (f0F2). Note that the second character is a zero. Not yet implemented. 'MOD ' => use Chiu model. Uses BPARM(2) and may use BPARM(1). The default is to use the Chiu model. BPARM......Model parameters: BPARM(1) Altitude of F2 layer (km). 0 => calculate using Chiu model. BPARM(2) Mean monthly Zurich sunspot number (R1). ---------------------------------------------------------------- FARAD: Task to calculate line-of-sight, free-electron density and Faraday rotation measure of the ionosphere and to enter these in a calibration (CL) table. Documentor: C. Flatters Related Programs: CALIB, PCAL, SPLIT, TABCOP, TABED This task estimates the Faraday rotation measure of the ion- osphere and enters it in a CL table. If polarization calibra- tion is requested, later AIPS calibration tasks will use this estimate to remove the phase difference between right and left circular polarization introduced by ionospheric Faraday rotation. Note that although phase calibration eliminates differences in ionospheric Faraday rotation between antennae from the calibrated data it introduces these differences into the polarization leakage terms (which are calculated by PCAL); it is therefore necessary to know the ionospheric Faraday rotation for every antenna in the array if polarization leakage terms are to be properly calibrated. FARAD makes a number of simplifying assumptions about the form and behaviour of the ionosphere. These are described in the following sections. If you have not used FARAD before you should read these sections to familiarize yourself with its limitations. APPROXIMATIONS IONOS calculates the free-electron column density from the zenith column density of free electrons, usually referred to as the total electron content (or TEC) of the ionosphere, assuming that the ionosphere is approximated by a thin sheet; the adverb OPTYPE selects the means used to derive the TEC. Since the F2 layer of the ionosphere dominates the total electron content and since the distribution of free electrons in the F2 layer is normally strongly peaked the thin sheet approximation is usually valid. It may, however, break down for the daytime ionosphere when solar activity is high (near solar maximum). Given the free-electron column density the rotation measure is calculated assuming an offset dipole model for the Earth's magnetic field. The F2 layer is assumed to be at a constant height for the duration of the observations. The assumed height may be entered directly as BPARM(1) or you may force FARAD to calculate it using the Chiu model (see below) by setting BPARM(1) to zero. The height of the F2 layer is normally between 300 and 400 km, depending on solar activity. The diurnal variation in altitude is negligable (of the order of 1% or less) but there is some variation with magnetic longitude and if observing a source away from the zenith you may be looking through the ionosphere at a significantly different magnetic longitude. However, at most latitudes the following approximation will become invalid long before the assumption of constant height. FARAD calculates (or reads) the TEC for a single reference position and applies it to the selected antennae assuming that the TEC is a function of local mean time and does not depend on geographic latitude. This approximation is only valid over small distances and only antennae within a few degrees of the reference point in latitude and longitude should be selected. For OPTYPE = 'TEC' and OPTYPE = 'F0F2' the reference point is, of course, the position at which the measurements were taken; for OPTYPE = 'MOD' the reference point is the latitude and longitude of the first selected antenna. OPTYPE = 'TEC' As TEC measurements (usually derived from measuring the Faraday rotation of a signal from a geostationary satellite) gives FARAD exactly what it needs, this is the preferred method of running FARAD. INFILE should be a file containing hourly TEC measurements in the format used by the Boulder monitoring station (described below). Other monitoring stations may also use this format. TEC data from Boulder, which is used to calibrate VLA polarization data, is distributed with AIPS. The input files may be found in the directory AIPSIONS and follow the naming convention TECB.yy, where yy is the last two digits of the year. TEC indicates that the files contain TEC data, B indicates Boulder. To access the TEC data for 1989, for example, use INFILE = 'AIPSIONS:TECB.89'. OPTYPE = 'F0F2' (NOT YET AVAILABLE) The most commonly available information on the electron content of the ionosphere is the critical frequency of the F2 layer (f0F2). Many ionospheric monitoring stations distribute hourly values of f0F2 in a standard format. The f0F2 values give the peak density of the F2 layer, which can be converted to a TEC provided that the profile of the F2 layer is known. FARAD calculates an equivalent thickness assuming a Chapman profile for the F2 layer; since Chapman profiles depend only on the altitude of the peak (which is assumed constant) the equivalent thickness is only calculated once. Note that the F2 layer may not be described by a simple Chapman profile in the daytime close to solar maximum. OPTYPE = 'MOD' If there is no ionospheric data available OPTYPE = 'MOD' calculates the TEC using an empirical model due to Y. T. Chiu (J. At. Terr. Phys. 37:1563, 1975). The model is simplified somewhat by assuming a constant altitude and slab thickness for the F2 layer (as for OPCODE = 'F0F2 above). The only input to this model is the mean monthly Zurich sunspot number (R1), which is used as an indicator of solar activity. The Chiu model does not therefore reproduce day-to-day variations in the ionosphere (note that use of daily values of R1 appears to exaggerate day-to-day variations). The Chiu model is claimed to be accurate to within 20%, but appears to fall apart badly for the daytime ionosphere near solar maximum. This model is only the default since it requires no external data. If ionospheric measurements exist for your data, you should use them; the Chiu model should only be used as a last resort! THE BOULDER TEC FORMAT TEC data from Boulder, Colorado is made available in the form of a text file. All records in the file are 79 or 80 characters in length (the 80th character is stripped from downloadable files). The file begins with a single header record followed by an unlimited number of data records. The header record contains the following fields (optional items are marked with an asterisk) Column Description ====== =========== 01 identifier for the type of data (7 for TEC) 02 blank 03-20 station name 21-30 * satellite name 31-34 * satellite East longitude 35-40 * beacon frequency 41-45 * latitude of SIP (420 km altitude) 46-50 * East longitude of SIP (420 km altitude) 51-60 * East longitude of time zone used for data 61-70 * Conversion factor (units of ambiguity) in units of electrons per meter squared per unitpf ambiguity (eg. pi radians) times 10 to the -15th 71-80 * Method of data reduction FR: Faraday rotation GD: group delay DCP: differential carrier phase Each data record contains the following fields: Column Description ====== =========== 01-02 range and type of data 11: hourly values for 00-11 hours 12: hourly values for 12-23 hours 21: median data for 00-11 hours 22: median data for 12-23 hours 03-05 station computer code (eg. 840 for Boulder) 06-07 year - 1900 08-09 month number 10-11 day of month for hourly data; otherwise 40 for medians or 50 for median count 12-13 characteristic (70 for TEC data) 14-73 data (twelve fields of five characters each) 74-76 latitude of SIP (degrees) 76-79 East longitude of SIP (degrees) 80 time U: UT L: local time If column 80 is not present FARAD will assume that times are in UT. Each five-character data field has the following structure Character Description 1-3 TEC in units of 10 to the 15th (no decimal point) 4 blank 5 data qualifier C: no data M: add 1000 to data N: add 2000 to data