Retrieval and Validation of Ionospheric Indices for Ship-based Monitoring over the Arctic

Abstract

Global Navigation Satellite Systems (GNSS) offer a world-wide signal coverage and have gained importance for remote sensing applications on a global scale over the last three decades. A GNSS data set for ionospheric scintillation studies is considered, here, that has been recorded on the German research icebreaker Polarstern. The ship-based setup contributed to the expedition of a Multidisciplinary Observatory in the Arctic (MOSAiC). The expedition comprised an eight-month drift period with the pack ice through the central Arctic Ocean (October 2019 to May 2020) and a four-month period with cruising and drifting parts in the Atlantic sector of the Arctic including Svalbard and Fram Strait (June 2020 to September 2020). The primary goal of the MOSAiC scintillation study is to prepare for ionospheric monitoring using GNSS in the Central Arctic far north from existing GNSS stations. The drift of Polarstern gives an excellent opportunity with a one-year data set. The GNSS receiver products consist of 60-sintegrated phase and amplitude indices as well as a 15-s-resolved slant total electron content (TEC) product. The Rate-Of-TEC index (ROTI) is computed with 60-s-resolution using the slant TEC product. An adapted azimuth masking is applied for the ship-based products to avoid ship structure shadowing of the GNSS links. Ancillary data of the ship's vertical displacement (heave) and surrounding sea-ice concentration are used to analyze the impact of sea-state-induced heave and of changing multipath conditions (water or sea-ice reflections). In general, the index values (phase, amplitude and ROTI) are low during the one year of MOSAiC expedition. The phase index does not exceed the weak scintillation level (0.3 rad). Occasionally, the amplitude index increases to a moderate level (0.5). The daily mean values of ROTI are below 0.5 TEC units per minute. The low index level can be explained by calm space weather conditions in the 2019/2020 period that lies close to the solar minimum. Severe geomagnetic storms (activity index Ap > 0) are not found and the solar radiation index is also low (F10.7 at about 70 sfu). The ship-based results are validated against station data from Ny-Alesund, Svalbard (GNSS index data provided by University of Oslo and magnetic field amplitudes by University of Tromsø). Ship and station show good agreement in ROTI results for the colocation period (June 2020) when the ship was close to Ny-Alesund. In both cases, ROTI values are correlated with magnetic field fluctuations (correlation coefficients of 0.7 and 0.6 for station and ship, respectively). The ship's phase and amplitude index data do not show relation to magnetic field fluctuations. Significant impact on the amplitude index occurs due to multipath conditions (correlation coefficient of 0.7 with respect to sea-ice concentration). In conclusion, ship-based ionospheric monitoring in the Arctic is feasible using GNSS data. Under the here-given calm space weather conditions best sensitivity is found for ROTI retrievals.