Data Set Overview : This data set tabulates local electron densities and local magnetic field strengths obtained from Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) Active Ionospheric Sounding (AIS) mode ionograms. The electron density is obtained by measuring the increment in frequency between the plasma frequency harmonics, visible as bright vertical lines at low frequency and delay time in many MARSIS ionograms. Similarly, the local magnetic field strength is found by measuring the difference in delay time between electron cyclotron echoes, visible as bright horizontal lines at low frequency on many ionograms. Both the measured quantity and the derived result are included in the archive product. We also include a data quality flag giving the impression of the archivist as to how reliable each result is. This archive product includes only data from the nominal mission, which starts about orbit 1844 and ends on orbit 2539. Parameters : This data set consists of electron plasma density and magnetic field magnitudes along with spacecraft position information in a Mars - Sun coordinate system as well as data quality flags. Processing : The local electron densities are obtained from individual ionograms by measuring the frequency spacing between plasma frequency harmonics, primarily visible at sounding frequencies below 1 MHz. These harmonics appear as bright vertical lines at approximately constant frequency intervals. The local plasma frequency is equal to the frequency interval between the vertical lines. An adjustable scale with digital readout coordinated with the computer screen cursor is used on the graphical representation of the ionogram to manually measure the interval between harmonic lines. A quality flag is assigned by the operator to each measurement. The criteria for assigning the various values of this flag are given elsewhere in this document. The measured plasma frequency and its corresponding quality flag are written to the output file. During further processing the plasma frequency is converted to electron density using the well-known formula: n_e [cm^-3] : (f_pe [MHz] /8.980e-3) ^2 where n_e is the electron density in particles per cubic centimeter and f_pe is the electron plasma frequency in megahertz. For more complete explanation of this process, and for discussion of how the plasma frequency harmonics are generated, please refer to Gurnett et al. (2005, 2008), Morgan et al. (2008, 2013a, 2013b), and Duru et al. (2008). A fully automated method of obtaining the local plasma frequency is given by Andrews et al. (2013). The local magnetic field strengths are obtained from individual ionograms by measuring the time-delay spacing between electron cyclotron echoes, visible as equally spaced bright horizontal at low frequency on an ionogram. The constant time-delay interval between the echoes is equal to the electron cyclotron period, i. e., the inverse of the electron cyclotron frequency. An adjustable scale with digital readout coordinated with the computer screen cursor is used on the graphical representation of the ionogram to manually measure the time interval between electron cyclotron echoes. A quality flag is assigned by the operator to each measurement. The criteria for assigning the various values of this flag are given elsewhere in this document. The measured electron cyclotron period and its corresponding quality flag are written to the output file. The electron cyclotron period is converted to magnetic field strength by using the well-known relation between magnetic field strength and electron cyclotron frequency, written here in terms of the electron cyclotron period: |B| [nT] : 1000/(28 * T [ms]) where T is the measured electron cyclotron period in milliseconds and |B| is magnitude of the magnetic field in nanotesla. For a more complete explanation of this process along with discussion of how the electron cyclotron echoes are generated, please refer to Gurnett et al. (2005, 2008), Akalin et al. (2010), and Morgan et al. (2011). Data : The electron plasma density and magnetic field magnitude data set is provided as simple ASCII SPREADSHEET objects with fields delimited via commas. Each row in a spreadsheet contains a fixed number of fields, however some fields may have a null entry, i.e. two adjacent commas with no value. This occurs when one of the measurments (either the plasma density or B-field magnitude) was unclear. In these case only the measurement which could be made with a mimimum of confidence is included and the other is left empty. One row in each spreadsheet corresponds to a single frame in the upstream MEX-M-MARSIS-3-RDR-AIS-V1.0 dataset. Ancillary Data : Ancillary spacecraft position data are provided adjacent to the measurements to which they pertain. In addition to the primary measurements, each row a product contains up to 2 data quality flags and a handful of spacecraft ephemerides considered relevent by the dataset producers. Coordinate System : The primary data are scalar measurements that reflect conditions local to the spacecraft. Because these quantities are scalars, no coordinate system is required for their intrinsic description; however the corresponding spacecraft position within the Mars-Sun system may be important in analyzing these products. As a convenience to the archive user, the following position data fields are provided inline within the data product files: ALTITUDE ABOVE THE GEODE PLANETOGRAPHIC LATITUDE PLANETOCENTRIC LATITUDE LOCAL TRUE SOLAR TIME WEST LONGITUDE MARS-SUN DIRECTION DISTANCE (X) MARS-SUN PERPENDICULAR DIRECTION DISTANCE (RHO) SOLAR ZENITH ANGLE A description of each field may be found by reading any data label on this volume. Further position information may be found in the GEO_INDEX.TAB file in the INDEX directory. Also, the spacecraft event time field provided as input to the NAIF Spice toolkit may be used to generate coordinate values in almost any imaginable system. Software : As the data are formatted as simple ASCII spreadsheets, no programs are provided on this volume. Confidence Level Overview : For electron cyclotron echoes the data quality flags are given as follows: 0 - NO echoes 1 - 1 or 2 echoes, look faint 2 - 1 clearer echo, with more echoes 3 - Best (this usually is multiple, 5 or greater, that gives the best confidence aligning the trace For plasma frequency harmonics the data quality flags are given as follows: 0 - NO plasma frequency harmonics 1 - Harmonics are visible but there is significant doubt as to the correct value of the interval between harmonics. This doubt occurs because the harmonics are blurred and difficult to locate precisely or because the frequency interval between harmonics is not unique. 2 - The harmonics are well determined with a unique frequency interval between them. Review : The MARSIS Electron Plasma Density and Magnetic Field Magnitude data will be reviewed internally by the Mars Express MARSIS team prior to release to the PSA. The data set will also be peer reviewed by the PSA and PDS. Data Coverage and Quality : Data coverage is effected by MARSIS being a dual-mode instrument. In general, MARSIS coverage of the nightside of Mars is dedicated to Subsurface mode, since that is when the ionosphere interferes least with subsurface measurements. This means that when Mars Express periapsis is primarily on the nightside, MARSIS AIS measurements are limited to altitudes above about 850 km, about 5 minutes on either side of periapsis. Early in the mission, there are also some partial orbits with a ten minute gap poleward of periapsis. These partial orbital passes are not guaranteed to be included in this archive set. A full orbital pass in the MARSIS nominal mission consists of approximately 35 min 50 seconds of continuously acquired ionograms. For both plasma frequency harmonic's electron density measurements and electron cyclotron echo-magnetic field strength measurements, all full orbital passes between orbits 1844 and 2539, 2005-06-22T22:49:03 through 2006-01-03T18:38:17, are here archived. These orbits and dates approximately bracket the beginning of regular data collection for the nominal mission of MARSIS and the end of the nominal mission. For the nominal mission volume the archived electron cyclotron echo magnetic field strength data include a number of orbits prior to orbit 1994 which were taken during the commissioning phase of MARSIS. Both archived electron cyclotron echo-magnetic field strength and plasma frequency harmonic-electron density data include a number of partial orbits that were collected as targets of opportunity. We include such data with no guarantee of completeness of coverage. The uncertainty in frequency measurements for MARSIS AIS is discussed by Morgan et al. (2013). MARSIS AIS samples 160 frequencies over 1.26 seconds in collecting data for an ionogram. These frequencies are spaced at somewhat irregular intervals in order to avoid interference features and discontinuities in the receiver sensitivity; however, the general trend in spacing between sampled frequencies is quasilogarithmic. The implication of this sampling regime is that over most of the sampled frequencies, between 0.5 MHz and 5.5 MHz, the fractional uncertainty in frequency is approximately 2%. At frequencies below 0.5 MHz, down to the minimum sampling frequency of 0.1 MHz, the uncertainty increases sharply to around 12%. The vertical lines that yield the local plasma frequency virtually always occur below frequencies of 1 MHz and above 0.3 MHz. Figure 3 of Morgan et al. (2013) indicates that measurement error will be below 4% in this range and will center about 2.5%. Because the local electron density is proportional to the square of the plasma frequency, the fractional error on the electron density will be approximately twice that on the plasma frequency or about 5%. The time width of an electron cyclotron echo on an ionogram is usually about twice the time width of the sounding pulse, which is 91.4 microseconds. Therefore, we estimate the error in any single measurement of the electron cyclotron period as +/- 0.1828 milliseconds. Because the fractional error in magnetic field strength is equal to the fractional error in time, and the absolute error in time is constant, the fractional error in magnetic field increases as the electron cyclotron period decreases and the magnetic field increases. We can take into account the effect of having more electron cyclotron echoes to measure when the magnetic field is high by dividing by the square root of the number of echoes. The resulting estimated percent errors are as follows: T_echo ms |B| nT Error on |B| % --------- ------ -------------- 0.25 40.7 13% 1.17 30.6 6.0% 2.08 17.2 4.5% 3.0 11.9 3.8% 3.9 9.1 3.2% 4.8 7.4 3.0% 5.7 6.2 2.8% 6.6 5.3 2.5% 7.6 4.7 2.3% Limitations : Because the local electron density and magnetic field strength are collected at most one time from an ionogram, the frequency of collection is 1/7.543 Hz or 0.133 Hz. Practical limits on the plasma frequency harmonics are 0.03 to .75 MHz or local electron density of 10 to 7000 cm^-3. Practical limits on the electron cyclotron period are 7.5 ms to 200 microseconds or 5 nT to 180 nT. The measurements described in this document are not observed when the Mars Express spacecraft is outside of the magnetosheath of Mars because (1) the values are typically too low to be measured and (2) the solar wind flow around the spacecraft convects the plasma away from the spacecraft before it can interact with the antenna and receiver of the MARSIS sounder. This effect is explained in detail in the reference DURUETAL2008.

Standard Quality Flags: Confidence in the measurements provided by this dataset is indicated in a set of two quality flags. For plasma frequency harmonics the data quality flag values are: 1 - Harmonics are visible but there is significant doubt as to the correct value of the interval between harmonics. This doubt occurs because the harmonics are blurred and difficult to locate precisely or because the frequency interval between harmonics is not unique. 2 - Best quality. Harmonics are clear with well-defined interval between them. For electron cyclotron echoes the data quality flag value are: 1 - 1 or 2 echoes, look faint 2 - 1 clearer echo, with more echoes 3 - Best quality, with 5 or more clear echoes, giving the best confidence aligning the trace. Verification flags: For a limited set of data, the electron densities have been verified by a second independent measurement. For the points where a verification has been attempted, the verification flag values are: empty field - No verification attempted 1 - Verification attempted; difference between archived value and independently measured value is greater than or equal to one frequency pixel, which is 0.01 MHz in the frequency range where this method of measurement is applicable. 2 - Verification attempted; difference between archived value and independently measured value is less than one frequency pixel, 0.01 MHz.