Preliminary Level 1b plasma dataset
|Sat||TEXT||N/A||Spacecraft identifier: A,B, or C|
|Date||TEXT (23 characters)||N/A||Date in format: yyyy-mo-dyThr:mi:sc.msc|
|Day2000||INTEGER||MD2000||Day of observation, days since 2000-01-01, UTC|
|Msec||INTEGER||Ms||Millseconds of day of observation, UTC|
|Latitude||REAL||Deg||Position in ITRF - Latitude|
|Longitude||REAL||Deg||Position in ITRF - Longitude|
|Height||REAL||Km||Height above reference ellipsoid|
|Radius||REAL||Km||Position in ITRF - Radius|
|Sza||REAL||Deg||Solar zenith angle|
|Ne||REAL||cm-3||Plasma density (electrons)|
|Te||REAL||K||Plasma electron temperature|
|Flag||INTEGER||N/A||Quality flag (see sect 1.4.7 and Table 2 for more details)|
Table 1: Langmuir probe dataset fields
Latitude, longitude, height, radius
The spacecraft location (latitude, longitude, height, radius) is calculated from TLEs, provided by ESOC FOS, and the usual sgdp4 software. The accuracy is completely sufficient when working with LP data alone. But it is not a precise orbit determination and users shall be careful when using the LP data in conjunction with other Swarm data products, particularly with magnetic data.
Solar zenith and azimuth angles
The reference for calculating the solar zenith angle and azimuth (in geodetic coordinates, not relative to the satellite) is:
- SUN_ALG1 - sun position according algorithm 1, short version
- in R. Grena (2012), Five new algorithms for the computation of sun position from 2010 to 2110, Solar Energy, vol. 86(5), 1323-1337, doi:10.1016/j.solener.2012.01.024.
High and low gain probes
Each Swarm satellite has two Langmuir probes. Almost always one probe is set to high gain, and the other to low gain. This is necessary to cover the full range of plasma densities in the ionosphere at Swarm altitudes.
Only one set of plasma parameters, namely density, electron temperature and spacecraft potential, is derived from the raw L0 data, which have the measurements from both probes. The users of the L1b products need to worry about low or high gain only in order to understand some of the limitations of the data and the flags field, as it will be detailed in the following sections.
The electron (plasma) density is derived from the high gain ion current (when the probe potential is a few Volts negative). Instrumentally this always works, flags indicating overflow or a bad 0-tracking (explanation below) should not be considered for the density.
The electron density has not been calibrated yet (an absolute validation will be possible using Incoherent Scatter radars). According to a first quick comparison with other independent measurements and models, the current values are up to a few 10% too high at low density, but a more systematic work on this is ongoing.
The natural temperature unit for a Langmuir probe is eV. Nevertheless, in the present dataset the temperature is provided in K, which is more suitable for most scientific uses.
As for the density, Te has not been calibrated yet; a first quick comparison with other independent measurements and models suggests that it might be up to several hundred K too high.
As described above, a sweep is performed every 128 s. The sweep measurements are not directly used for producing this data set. But the sweeps have an effect on Te: right after each sweep the temperature is higher by up to about 100 K (0.01 eV) and decreases slowly, over several seconds, to values just before the sweep. The time series of Te data shows upward jumps every 128 s. This effect is most pronounced at high densities, and becomes almost invisible at low density. The cause is not fully understood. Efforts to understand the effect and possibly mitigating it are going on. Very likely it is not a true temperature variation.
We report about short duration events (few seconds) where Te varies strongly and erratically and reaches extremely high values. An example can be seen in Figure 1. Such events occur at all latitudes, but more often at high ones. The problem is under investigation and in the next release of the preliminary data these events could be flagged relatively reliably. Until then it is recommended to use Te only in event studies where these bursts are easily visible in plots of the data, and the user can manually mask them.
Under certain (rare) instrumental conditions the temperature estimates should not be used, but it is still provided to complete the data row with a valid density. This is described as follows: to estimate the electron temperature Te, the current at a suitable positive bias with respect to the spacecraft potential needs to be measured. To set this bias in an optimal way, a so-called zero current tracking is performed, which occasionally fails. When this happens, then often it fails only on one probe, the high gain one, sometimes also only on the low gain probe. If 0-tracking is successful with only one probe, this probe is used for estimating Te. Very rarely, <0.1 % of all times, 0-tracking fails on both probes, and it is recommend to discard then Te. This is flagged in data by setting Flag ≥ 192.
Figure 1: Example of Te unexpected features
The spacecraft potential Vs is mainly needed for computing the Thermal Ion Imagers data. For this data set always the measurements from the low gain probe were used, except when the 0-tracking on this probe failed.
The spacecraft potential shows inverse tops similar to Te right after sweeps.
The quality flags are bit-coded numbers (from bit 0 to bit 8) converted in decimal values in the dataset.
The meaning of some of the flag bits have already been described above. Bit 8 indicates duplicated normal mode data at sweeps, which can optionally be discarded or replaced by interpolated data. When bit 6 and 7 are both set (<0.1 % of all data), both probes had a failed 0-tracking, and temperature and spacecraft potential, but not density, should be discarded/interpolated.
For completeness here is an explanation of the remaining flag bits in use, which, however, the user could normally ignore:
- Bits 0 and 1 indicate, whether the probes are set to high or low gain. The nominal situation is: Bit 0=1 and bit 1=0, i.e. probe 1 is the high gain one, and probe 2 the low gain one. Exceptions are certain periods in the commissioning phase (until Jan 2, 2014), and later some short periods, ~1 min, probably related to instrument switch ons.
- Bits 2-5 indicate overflows on the electron current side (positive bias). They are expected for the high gain probe when the density is sufficiently high. The data processor uses automatically only the non-overflowing probes to estimate Te. There are a few rare special conditions when the overflow flags are relevant.
In Table 2 an overview of the Langmuir probes flags is given.
|0 (bit0=0, bit1=0)||Probe 1 & Probe 2 in low gain (never happened to date)|
|1 (bit0=1, bit1=0)||Probe 1 = high gain, Probe 2 = low gain (nominal situation)|
|2 (bit0=0, bit1=1)||Probe 1 = low gain, Probe 2 = high gain (short periods during Commissioning Phase)|
|3 (bit0=1, bit1=1)||Probe 1 & Probe 2 in high gain (never happened to date)|
|4 - 7 (bit 2 activated)||overflows on the linear electron current region for Probe 1|
|8 - 11 (bit 3 activated)||overflows on the linear electron current region for Probe 2|
|12 - 15 (bits 2-3 activated)||overflows on the linear electron current region for both Probes 1 & 2|
|16 - 19 (bit 4 activated)||overflows on the retarded electron current region for Probe 1|
|20 - 31 (bits 2-3-4 activated)||Combinations of the above situations|
|32 - 35 (bit 5 activated)||overflows on the retarded electron current region for Probe 2|
|36 - 63 (bits 2-3-4-5 activated)||Combinations of the above situations|
|64 - 127 (bit 6 activated)||Zero tracking failure on Probe 1 (+ overflow combinations)|
|128 - 191 (bit 7 activated, bit 6 non activated)||Zero tracking failure on Probe 2 (+ overflow combinations)|
|192 - 255 (bit 6-7 both activated)||Zero tracking failure on both probes (+ overflow combinations): electron temperature and spacecraft potential not usable.|
|≥ 256 (bit 8 activated)||Timestamps corresponding to sweep mode intervals: to be discarded|
Table 2: Langmuir Probes quality flags description
The Thermal Ion Imagers Dataset
Data are provided in ZIP-archived files with the following naming convention:
- PREL indicates the data are preliminary;
- X is the satellite letter, one of A, B or C;
- TII1B indicates this file contain a subset of TII-related L1b plasma products;
- YYYYMMDDTHHMMSS marks the beginning of the interval;
- yyyymmddThhmmss marks the end of the interval;
- VVVV is the dataset version
Each file un-compresses to a NASA/CDF file containing the following variables (Table 3):
|Timestamp||double||s||1||Times are seconds from 1 Jan 2000 00:00:00 UT. To convert to modified Julian day: tMJD2000=Timestamp/86400.|
|latitude||double||deg.||1||ITRF spherical latitude, derived from L1b Medium Or-bit Determination (MOD).|
|longitude||double||deg.||1||ITRF spherical longitude, derived from L1b MOD.|
|radius||double||m||1||ITRF spherical radius, derived from L1b MOD.|
|v_SC||double||m/s||3||Satellite velocity vector in North, East, Centre (NEC) frame.|
|v_ion||double||m/s||3||Ion velocity vector in NEC frame.|
|E||double||mV/m||3||Electric field vector in NEC frame.|
|v_ion_H||double||m/s||2||Horizontal sensor ion velocity in TII coordinates (x and y).|
|v_ion_V||double||m/s||2||Vertical sensor ion velocity in TII coordinates (x and y).|
|rms_fit_H||double||DN||1||RMS error from TII x-profile 2-Gaussian fits, H sensor.|
|rms_fit_V||double||DN||1||RMS error from TII x-profile 2-Gaussian fits, V sensor.|
|var_x_H||double||Pixel2||1||Variance of 16 Hz X moment, H sensor.|
|var_y_H||double||pixel2||1||Variance of 16 Hz Y moment, H sensor.|
|var_x_V||double||pixel2||1||Variance of 16 Hz X moment, V sensor.|
|var_y_V||double||pixel2||1||Variance of 16 Hz Y moment, V sensor.|
|SAA||int32||N/A||1||South Atlantic Anomaly proximity indicator.|
|Flags_TII||int32||N/A||1||TII quality flag.|
|Flags_Platform||int32||N/A||1||Satellite platform flag.|
|Maneuver_Id||int32||N/A||1||Satellite manoeuvre identification code.|
Table 3: TII CDF file content
The CDF also includes annotations for measurement units, labels, descriptions, and processor version.
The users shall be aware of the following information:
- For the time being only data from Swarm A and B are distributed. The TII instrument on Swarm C is still undergoing intense calibration activities and the data distribution is therefore delayed.
- The "Flags_TII", "Flags_Platform", and "Maneuver_Id" variables are described on the Level-1b product definition page.
- Satellite positions are derived from the 1 Hz MOD L1b measurements, and are cubically-interpolated at the TII measurement times.
- Spacecraft potential is provided by IRF as part of the Langmuir Probe (LP) dataset, and is based on estimates from LP Probe 2 only. Accordingly, the TII electromotive force (emf) corrections are made relative to the position of Probe 2 only.
- Electron density, which is used in the TII quality flag calculation, is provided by IRF as well.
- Magnetotorquer-induced velocity deflections are not calculated.
- TII profile fits are weighted evenly.
- Measurement errors are not calculated.
- TII quality flag greater than or equal to 20, indicates that data are affected by serious quality issues and should not be used.
Known problems or limitations
All data should be treated with caution as to its interpretation. The following issues are known, but not yet always properly flagged:
- Large offsets in ion drift, electric field, and ion temperature associated with imperfect calibrations, or with artifacts in the TII detector. Currently, variations in measurements relative to the back-ground signal are more scientifically meaningful than absolute values.
- Noisy ion velocities, ion temperatures, and electric fields. This occurs at low plasma density due to low signal-to-noise ratio.
- Measurement jitter occurred routinely prior to May 2014 due to sub-optimal configuration of on-board image processing algorithms in the EFI TII flight software.
Where to find data?
The interested user shall first visit the Swarm Data Access web page and follow the instructions in order to register in the "MyEarthnet" system and understand the structure of the Swarm FTP server. Then, he/she can access the ESA dissemination server and download Swarm data.
In particular, once logged into the FTP server, the preliminary plasma dataset can be found here:
Sub-folders inside are rather self-explanatory: Langmuir Probe and Thermal Ion Imagers datasets are given in separate folders and further divided by Spacecraft.