Spaceweather Products
GFZ Potsdam

Total electron content derived from GPS observations on board the satellites is a major parameter in space weather that describes the variability of the ionosphere and plasmasphere. Regional and global TEC maps are in use as well as results from 3D global assimilation models that are intensively used to re-analyse, nowcast and forecast the ionospheric electron density evolution, see e.g. [1].

The Rate Of change of TEC (ROT) has been proposed to monitor small-scale variability [2]. It is known that large fluctuations in electron content through which the GPS ray is passing can seriously affect GNSS and create radio wave scintillations that degrade significantly solutions for positioning and navigation [1]. With the demand of enhanced accuracy of GNSS alerts on its reliability increase more and more in importance.

The new Swarm product, ROT, combined with the existing Swarm products, Total Electron Content (TEC), electron density (Ne), and Ionospheric Bubble Index (IBI) has a high-level importance for the end-user community. These product combinations are highly relevant for users in navigation and communications, while strong plasma gradients cause GPS signal degradation or even loss of GPS signal [3]. ROT is a relevant space weather asset irrespective of geomagnetic activity, e.g., they do occur not only during geomagnetic storms.

ROT can be derived from the operational product (time series of the ionospheric TEC) from the Swarm mission: TECxTMS_2F, where x = A, B, or C for Swarm A, Swarm B, or Swarm C, respectively. The Swarm TEC data is available as a 1-s (1 Hz) time series since 2014-Jun-15 06:43:00 and as a 10-s (0.1 Hz) time series from 2013-Nov-22 (mission start) to 2014-Jun-15 06:42:59. Therefore, ROT data can be derived as a 1-s (1 Hz) time series, which is based on the first time derivative of the TEC observations:

$${\mathrm{ROT}}_n=\left|\frac{{\mathrm{TEC}}_n-{\mathrm{TEC}}_{n-1}}{t_n-t_{n-1}}\right|$$

Here, n is the TEC’s nth measurement and t_n is the time stamp of this measurement. ROT is measured in the unit TECU/s, where 1 TECU corresponds to 10¹⁶ electrons/m². The gaps in TEC data with duration of 6 seconds or more (t_n-t_n-1>5 s) are excluded from the ROT calculation avoiding the severe artificial changes in ROT. In order to identify ROT values that are enhanced compared to climatological expectations, the knowledge of the background ROT is crucial. Climatological maps derived for each satellite separately is used to identify the three threshold values for normal, enhanced, and severe TEC changes. The green, yellow, and red colour convention provides thresholds for normal (0≤ROT≤0.1), enhanced (0.1<ROT≤0.4), and severe (0.4<ROT) TEC changes in units of TECU/s, respectively.

Note: For the sake of user friendly vizualisation in the ascending and descending orbit plots, all ROT>1 TECU/s values are reduced to ROT=1 TECU/s. In contrast, the archive data files provide the true, unmodified ROT values.

References

[1] "Ionospheric space weather effects monitored by simultaneous ground and space based GNSS signals"
Jakowski, N., V. Wilken, S. Schlueter, S. M. Stankov, and S. Heise (2005), J. Atmos. Sol. Terr. Phys., 67, 1074–1084, doi:10.1016/j.jastp.2005.02.023

[2] "Challenges of Real-Time Monitoring of Ionospheric Perturbations and TEC Fluctuations with GPS Single Station"
Cokrlic M. (2014), Mitigation of Ionospheric Threats to GNSS: an Appraisal of the Scientific and Technological Outputs of the TRANSMIT Project, Dr. Riccardo Notarpietro (Ed.), ISBN: 978-953-51-1642-4, InTech, doi:10.5772/58780

[3] "The Swarm satellite loss of GPS signal and its relation to ionospheric plasma irregularities"
Xiong, C., Stolle, C., and Lühr, H. (2016), Space Weather, 14, 563–577, doi:10.1002/2016SW001439

[4] "Long-term analysis of ionospheric polar patches based on CHAMP TEC data"
Noja, M., C. Stolle, J. Park, and H. Lühr (2013), Radio Sci., 48, 289–301, doi:10.1002/rds.20033