Swarm is an Earth Observation mission of the European Space Agency (ESA) with the primary objective to measure Earth's magnetic field and its temporal variations, which enables investigations of, e.g. the core dynamics, geodynamo processes, and core–mantle interactions . Further, the Swarm mission is devoted to characterize the ionospheric electric fields, currents, and other ionospheric plasma processes. The space segment consists of three identical satellites, which carry a diverse set of instruments to achieve the ambitious mission objectives: a vector field magnetometer (VFM) and an absolute scalar magnetometer (ASM) for collecting high-resolution magnetic field measurements, three star trackers for accurate attitude determination, a dual-frequency GPS receiver for precise orbit determination, an accelerometer to retrieve measurements of the satellite's non-gravitational acceleration, and an electric field instrument (EFI) composed of two Langmuir probes (LPs) and two thermal ion imagers (TIIs) for the plasma and electric field-related measurements. The three satellites were launched in 2013 into the same near-polar orbits. Shortly after launch, the satellites were manoeuvred into a constellation in which two satellites, Swarm A and Swarm C, fly side by side with 1.4∘ separation in longitude at the Equator at an altitude of 462 km (initial altitude), and the third satellite, Swarm B, flies at a higher altitude of 511 km (initial altitude). Due to the difference in altitude, the orbital planes precess at different rates such that the angle between Swarm B's orbital plane and those of the other two satellites slowly change over time. In 2018, Swarm B's orbital plane was perpendicular to those of Swarm A and Swarm C, while by the end of 2021, Swarm B will be counter-rotating with respect to the lower-flying pair, which will result in a conjunction every 47 min.

With its plasma instrumentation (LP and TII, ), Swarm is an excellent mission to investigate and survey the ionosphere, its structure and dynamics. Recently, Swarm measurements brought more understanding of the space weather.

As reported by , Swarm EFI measurements helped to advance the understanding of the auroral phenomenon known as “Steve”, which is visible as subauroral purple emission. By using Swarm LP electron density and temperature measurements this study demonstrates that Steve events are not only associated with intense subauroral ion drifts but also with peaks of plasma temperatures and extremely low densities . The results presented in provided additional understanding of the Steve phenomena, expanding the knowledge needed for proper numerical simulations. More recently, discussed the possibility to use electron density measurements collected by LPs aboard Swarm, to derive a proxy for the ionospheric turbulence. This work suggests that, looking at the scaling features of the density fluctuations for different locations and geomagnetic activity levels, it is possible to distinguish two families of density fluctuations, one of which is most probably related to turbulent processes . Furthermore, the long time coverage of the Swarm mission offers the possibility to perform extended statistical analysis on the climatology of plasma irregularities via plasma-related measurements, as the in situ electron density data from the Swarm LP. The first global statistics obtained by in situ measurements of plasma variations observed by Swarm mission, confirmed the presence of three main regions of strong ionospheric irregularities: the magnetic Equator extending from post-sunset to early morning, the auroral ovals, and the polar caps . The long-term behaviour of density gradients and fluctuations has been studied by using Swarm data , where LP density measurements are used to catch ionospheric structures and irregularities. This new statistical study describes phenomena already explored by past missions, but also reveals a new anomaly that is the persistence of strong density fluctuations in the southern polar cap during local summer (December solstice) . The morning overshoot consists in a rapid increase of electron temperature in the early morning hours at low latitudes. Its dependence on geographic regions, local time, seasons and geomagnetic activity has been presented by , by using electron temperature and density LP data at two different altitudes of the Swarm satellites, and ISS/FPMU (International Space Station/Floating Potential Measurement Unit) measurements . Plasma density and temperature hemispherical asymmetries have been extensively investigated in ionospheric physics, and recently discussed by . In their study plasma density from both Swarm and CHAMP measurements is used, demonstrating the importance of multi mission synergies and long mission life-time to statistically investigate ionospheric phenomena. For a comprehensive list of scientific results obtained with the support of the Swarm data, we refer to the Swarm webpage . It is worth to emphasize the role of LP plasma measurements in the recent ionospheric research field. As discussed above, LP data contributed to many scientific results advantaged by Swarm long-time mission coverage, multi-point in situ measurements as per Swarm spacecraft constellation, and continuously improved LP data quality. These studies demonstrated the scientific valence of LP measurements, and thus, the importance of an accurate monitoring of LP data with the scope to maintain high instrument performance and data processing accuracy.

The LPs are relatively simple instruments which are immersed into a plasma to measure electron density, Ne, and electron temperature, Te. Owing to their simplicity, relatively small weight and low power consumption, LPs have been used on many satellite missions . Examples are Demeter , Rosetta , and Swarm . The science data derived from LPs aboard Swarm are part of the Level 1B (L1B) products and are obtained from the PLASMA operational processor. The LP data are available at both 2 and 1 Hz cadence. The algorithm of the PLASMA processor is described in the L1b Plasma Algorithm document . To support the scientific research, the Swarm data products are continuously monitored for quality control (QC) and improved by the ESA/ESRIN Swarm Data Innovation and Science Cluster (DISC) data quality team. Also, users community feedback are essentials to improve the Swarm data product quality. Most of the feedback actually results in recommendations which are the drivers to elaborate and introduce improvements in the data processing algorithms . It is worth to specify that data quality is here intended as the goodness of the data product as output of a processing process. The data quality can be qualified by comparison with other dataset (in situ or ground measurement), validation with numerical or empirical models, or derived by statistical data analysis of the product itself. In our definition of data quality, if the data product is subject to a low level of errors as derived from statistical analysis or known issues, or/and has a high agreement with other dataset (model or spacecraft observations), then the quality of the data is considered good. In this paper the EFI-LP L1B data quality evolution introduced in the current baseline is described and the data quality status is statistically investigated. Known issues and future perspectives are discussed as well.

The Swarm LPs have been described by including its “harmonic mode” with the sinusoidally modulated probe bias, in which the instrument is operated most of the times. Also the model equations that are assumed to determine the plasma density, the electron temperature and the spacecraft potential from the currents and admittances (which are the reciprocal of impedance) computed on board for given biases, are therein included. Complementing the description in , we add here a detailed description of the LP instruments' functionalities and operational settings.

The two probes are mounted on the earthward edge of the ram panel as illustrated in Fig. . They are separated by 30 cm and located relatively close to the faceplate of the TII, which is also mounted on the ram panel. The LPs are expected to provide accurate and independent estimates of the spacecraft potential, which is in principle needed to process the TII data.

The probes are also expected to provide plasma densities and electron temperatures over the entire range of signal magnitudes encountered along the orbit. Experimental results demonstrated that when the probes are immersed in the satellite plasma sheath, the estimation of spacecraft potential me by effected , but so far the accuracy of current and density estimation from Swarm LP is not effected by this issue. The Swarm orbits will cover a limited range of altitudes from about 520 km shortly after launch to 250 km close to re-entry. But they sample practically all latitudes and local times, which results in a relatively large and dynamic signal range. Densities ranging from few hundreds cm-3 to several millions cm-3 can occur, representing more than 4 orders of magnitude. Avoiding an automatic gain control, which would potentially interfere with reliable and accurate current measurements, both probes are typically operated with fixed but different gains, called low and high gain . By electronically coupling a second shunt resistor in parallel, the mode is low gain which allows higher probe currents to be measured without ADC (analogue-to-digital converter) overflows. The term “gain” should be understood here rather as a sensitivity of the current measurement than an amplification. The ratio between high and low gain is about 50. High or low gain can be set by a telecommand from ground and usually one of the probes is in high and the other in low gain. Table  summarizes the differences between the probes and their gain operations.

The surface material of one of the probes is titanium nitride (TiN), which had previously been used in several space missions, for example, Rosetta and Demeter . Out of concern of the aggressive chemical reactivity of ionospheric oxygen (O), the other probe surface is made of gold-plated (Au) titanium (Ti), which is a novelty in space. It is known that Ti is very difficult to electroplate (e.g. ); however, a small company with experience in gold-plating jewellery made of titanium was given the contract. Testing before launch did not reveal any problems with the Au probes even after baking with temperatures of up to 300 ∘C and after exposure to ultrasound. Both the nitration to TiN and the gold-plating are supposed to prevent strong oxidation of the Ti surface. Probably this would negatively affect the performance of the probe, because TiO is a relatively poor conductor. Presently, after more than 7 years in an O-dominated atmosphere, there are no conclusive indications in the in-orbit data that degradation in the form of serious oxidation of any of the probes has occurred, or that either of the two methods is preferable compared to the other one.

The L1B PLASMA processor, which is used to generate the LP data products, is organized according to the simple flowchart reported in Fig. . It uses as inputs the L1B products, containing position and velocity of the satellite, auxiliary data, and EFI-LP Level 0 (L0) data, to obtain three L1B and one Level 1A (L1A) data products. The auxiliary data contain information that support the Swarm data processing, such as physical constants, or instrumental calibration parameters obtained during ground tests. The L0 data contain raw measurements from each Swarm instrument and are essential to generate the L1 products. The EFI-LP L1A product (EFIX_LP_1A) contains information about the LP configuration, ion and electron currents in different regimes, and bias voltages. The L1B product LP_X_CA_1B delivers the LP calibration parameters derived for each probe by the L1B PLASMA processor. Finally, the EFIX_LP_1B and EFIXLPI_1B products provide the plasma parameters as density, electron temperature, plasma potential, together with the spacecraft position and the flags indicating a possible source of error for each data point. The EFIX_LP_1B are available at a 2 Hz sample rate. By simple linear interpolation of these products at full UTC second, the EFIXLPI_1B data product is obtained at 1 Hz sample rate. Also, the LPs operate in different modes. The “harmonic mode” (HM) consists of sinusoidal varying biases applied to the LPs. Each HM cycle lasts for 0.5 s, and during this time the HM currents and admittances are measured. To our knowledge, this method to obtain the current–voltage (I–V) characteristic of the space plasma is being used in orbit for the first time. The HM operates most of the time, while the classical “sweep mode” occurs every 128 s and lasts for 1 s. In sweep mode, the I–V curve is measured traditionally by scanning the probe bias over a range that stretches from a dominant ion current (at negative bias) to a saturated electron current (positive bias). Sweep mode data are not used in the L1B PLASMA processor but are separately analysed and provided as an additional “advanced” product. Furthermore, every 6 h, a calibration mode is activated and a calibration data packet is generated on board. These data are used for calibration purposes, and during the calibration mode, short data gaps are registered in L1B PLASMA products. The EFI-LP data products, and the other Swarm L1B products, are provided in daily files with a latency of 4 d. Detailed information on Swarm L1B processors and data products is provided by . The Swarm products are freely accessible through the ESA dissemination server . The next two sections describe the recent data products evolution and data quality characterization.

The Swarm LP measurements have a high value for scientific investigations. However, few anomalies affect the LP measurements which are continuously monitored and investigated by the ESA data quality team and the scientific community. The source of these anomalies is only partially understood, which leaves open questions in both physical and instrumental domains. This section is dedicated to the description of the occurrence of one of these anomalies, namely the occurrence of extremely high values in the electron temperature measurements, which has a large impact on the data quality and scientific investigations. In addition, we describe the calibration of LP measurements, which will be introduced in the next baseline (06).

The quality control and validation activities performed by the data quality team in the frame of the ESA Swarm DISC reveal the good quality and instrument performance of the Langmuir probes aboard the Swarm satellites. The analysis demonstrated that the current baseline 05 plasma data products are substantially improved with respect to the previous baseline (04). In particular, the electron temperature measurements are more stable and, on average, smaller with respect to the older baseline. The changes introduced in the current baseline lead to the recovery of past data gaps, increasing the data coverage and reducing the possibility of future failures for generating the data. The LP measurements have captured the ionospheric plasma variability over more than half of a solar cycle, which revealed that the data quality depends on the solar activity, as shown in Fig. . In particular, plasma density measurements are more accurate during higher solar activity. On the contrary, electron temperature measurements are more stable during low solar activity. These results are highly related to the LP instrumental settings and are well tracked by the monitoring of the data quality. Plasma density LP data have good agreement with TII faceplate measurements, particularly on the day side. However, the comparison between the two datasets demonstrates a weaker correlation on the night side. The disagreement in nocturnal regions is partially related to the fact that the LP processing algorithm assumes that the plasma is composed of singly ionized oxygen only. Investigations are ongoing in order to include molecular ions in the plasma algorithm and to improve the quality of the plasma density computation in future baselines. The next release of the L1B LP data products will include the calibration parameters for plasma density and electron temperature, which are statistically derived from a comparison with ground measurements. Furthermore, a dedicated flag will be introduced to identify the extreme values of electron temperature. These changes will further improve the quality of the Swarm LP L1B data products and will further promote their application to a broad range of ionospheric studies.