Swarm Satellites Research Articles

The ESA Swarm mission to help ionospheric modeling: a new NeQuick topside formulation for mid-latitude regions

The ionospheric topside representation made by the NeQuick model is improved by correcting the H0 parameter used by the model to calculate the topside scale height. The task is accomplished by fitting the NeQuick topside analytical function through two anchor points: the F2-layer absolute electron density maximum; the electron density value as measured by Swarm satellites from December 2013 to September 2018. Specifically, two-dimensional grids of H0 as a function of foF2 and hmF2 are obtained, one by employing data from Swarm A and Swarm C, both flying at about 460 km of altitude, and the other one by employing data from Swarm B, flying at about 520 km of altitude. These two grids are used to calculate corrected values of H0, which allows a more reliable description of the topside region. The new NeQuick formulation is statistically validated by comparing corresponding modeled vertical total electron content values to those derived from COSMIC/FORMOSAT-3 measured Radio Occultation profiles, and those measured by Swarm satellites. The results show that the proposed formulation significantly improves the topside description made by the NeQuick model at mid latitudes, for both high and low solar activities. The International Reference Ionosphere model might benefit of its inclusion.

Magnetic Field Oscillations Observed by Swarm Satellites in the Nightside Upper Ionosphere During Low‐Latitude Pi2 Pulsations

AbstractPi2 oscillations (40–150 s) in the nightside upper ionosphere are studied using magnetic field data acquired by multiple Swarm spacecraft in low‐Earth orbit and at the low‐latitude Bohyun (BOH, L = 1.3) station on 22 October 2014. Four Pi2 events were identified from the BOH data near midnight (magnetic local time = 1.5 hr), while Swarm‐A, Swarm‐B, and Swarm‐C spacecraft were orbiting in the premidnight (magnetic local time = 21–22 hr) meridian from 70° to −60° in magnetic latitude at ∼450‐ to 500‐km altitudes. Unlike previous low‐Earth orbit studies, which used a single point observation, the latitudinal structure of the amplitude and phase of ionospheric magnetic field perturbations can be determined by simultaneous multipoint observations along the latitude at a constant radial distance. We observed that the horizontal H component of BOH data is well correlated with the compressional (Bz) component of ionospheric magnetic fields when Swarm spacecraft were at |magnetic latitude| < 30° with or without an accompanying ionospheric field perturbation in the radial (Bx) component, depending on the latitude of the spacecraft. It is found that the phase and amplitude relationship between Bx and Bz along the latitude is consistent with the model ionospheric field perturbations at 500‐km altitude, which are associated with a plasmaspheric resonance excited in a dipole numerical simulation. This indicates that the latitudinal variation of the ionospheric Pi2 pulsations in both Bx and Bz components is the consequence of the spatial mode structure in the north‐south direction of trapped fast mode waves inside the plasmasphere.

Determination of Swarm Front Plate’s Effective Cross Section From Kinetic Simulations

The front plates and embedded particle sensor shells that are part of the electric field instrument (EFI) on the Swarm satellites have recently been used as planar Langmuir probes, as an additional diagnostic tool to infer environment parameters. The interpretation of measured currents in terms of the plasma density or incoming flow speed, however, requires a knowledge of the front plate effective cross section $A_{\textrm {eff}}$ . Measurements made under various space plasma conditions have led to the conclusion that this cross section is generally larger than the known geometrical cross section $A_{\textrm {geo}}$ . Interpretations of measurements have thus been made using fixed relative enhancements of $A_{\textrm {geo}}$ ranging from 8% to 17%. In this paper, results from kinetic simulations are presented, from which the effective cross section can be determined over a range of plasma parameters. These are used to shed light on the physical mechanisms responsible for this enhancement and construct an empirical fit to the relative enhancement $\delta $ , where $A_{\textrm {eff}} = A_{\textrm {geo}}(1 + \delta)$ , and, in turn, enable improvements in the accuracy of inferred plasma parameters.

Field‐Aligned and Horizontal Currents in the Northern and Southern Hemispheres From the Swarm Satellite

AbstractWe present statistical investigation of the high‐latitude ionospheric current systems in the Northern Hemisphere (NH) and Southern Hemisphere (SH) during low (Kp < 2) and high (Kp ≥ 2) geomagnetic activity levels. Nearly 4 years of vector magnetic field measurements are analyzed from the two parallel flying Swarm A and C satellites using the spherical elementary current system method. The ionospheric horizontal and field‐aligned currents (FACs) for each auroral oval crossing are calculated. The distributions of the mean values of FACs as well as the horizontal curl‐free and divergence‐free currents in magnetic latitude and magnetic local time for each hemisphere and activity level are presented. To estimate the NH/SH current ratios for the two activity levels, we remove seasonal bias in the number of samples and in the Kp distribution by bootstrap resampling. This is done in such a manner that there are equal number of samples from each season in each Kp bin. We find that for the low activity level, the currents in the NH are stronger than in the SH by 12±4% for FAC, 9±2% for the horizontal curl‐free current, and 8±2% for the horizontal divergence‐free current. During the high activity level, the hemispheric differences are not statistically significant. This suggests that the local ionospheric conditions, such as magnetic field strength or daily variations in insolation, may be important and play a larger role during quiet than disturbed periods. This issue must be studied further.

Multipoint Field‐Aligned Current Estimates With Swarm

AbstractField‐aligned currents (FACs) are a key component of the magnetosphere‐ionosphere system, providing the transfer of energy and momentum between the distant collision‐free magnetospheric plasma and the collisional ionosphere. The European Space Agency's Swarm mission offers a unique opportunity to explore the FACs low‐altitude end by a broad range of single‐spacecraft and multispacecraft techniques. The present technical report demonstrates the application of dual‐ and three‐satellite FAC estimation methods based on the least squares approach, with the goal to enhance the use of Swarm data in exploring the high‐latitude current structures, beyond the official FAC product presently available to the end user. The dual‐satellite method presents some clear advantages since it provides stabler solutions, can be applied on a more general spacecraft configuration, and offers a robust error estimation scheme. Consequently, the method provides significantly more data near the singularity where the satellites' orbits intersect, allows FAC estimation with configurations that involves the upper Swarm satellite, or to fine tune the constellation geometry to the problem at hand. Similarly, the three‐satellite method, meant to be applied when Swarm forms a close configuration, brings additional valuable information, associated to a different (larger) scale. Moreover, it can estimate the FAC density with high time resolution when instantaneous measurements are used. The performance of the methods are thoroughly analyzed both on Swarm events and on simulated data, and the results are compared with other available methods. Particular emphasis is put on how different FAC estimation methods complement each other and provide consistent results.

Geomagnetic secular variation changes in Southern Africa during the SWARM period 2013 - 2018

Geomagnetic field observations from 4 magnetic observatories located in Southern Africa located at Hermanus (HER), Hartebeesthoek (HBK), Keetmanshoop (KMH) and Tsumeb (TSU) have been analysed with the main purpose to identify abrupt secular variation changes on time scales of less than 1 year. Removal of an annual variation resulting from large-scale magnetospheric and ionospheric currents by means of 12-month differences of the respective observatory monthly mean of northward component X, eastward component Y and vertical component Z, revealed clear evidence of several geomagnetic secular variation changes that took place in this region during the period between 2013 and 2018. The geomagnetic field model CHAOS6-x7, based exclusively on SWARM satellite and magnetic observatory data during this period, has been used to determine secular acceleration patterns across Southern Africa. The results obtained revealed that the observed secular variation changes took place over a range of strengths in the respective X, Y and Z components at every magnetic observatory. In addition, the respective observatories in the region also exhibited strong individual characteristics. The findings in this investigation show once again that the southern African region is characterised by unpredictable abrupt geomagnetic secular variation changes that have the potential to render linear prediction models less accurate.

Mid and low-latitudinal ionospheric field-aligned currents derived from the Swarm satellite constellation and their variations with local time, longitude, and season

We use the Field Aligned Currents (FAC) data of the 10 quietest days of each month of 2014–2018, the first four years of FAC observational data, derived from Swarm A and C satellites observations to study their characteristics within 15–55° latitude in the northern and −15° to −55° latitude in the southern hemispheres. Results show that in summer months, the noon time FAC have the largest variations. Over the same sector region and the same local time, the FAC have the same trend irrespective of season.Examinations of seasonal patterns indicate that, both dawn and dusk times FACs in both hemispheres always have negative (southbound) variations during summer months. In winter months the dusk time FAC becomes positive only in the southern hemisphere, while the dawn time FAC is ambiguous as it becomes negative only at low latitudes. The noon time FAC is strongly positive (northbound), in both hemispheres in summer months, but during winter months it becomes southbound at mid-latitudes and remains northbound at low latitudes. Over the African, Asian and American sectors, the summer months FAC have identical structure, while in winter months the direction of the dawn and noon times FACs are ambiguous.At the foci points of the solar quiet (SQ) current, the northern hemispheric FAC has the same variation as those observed by Park et al. (2011) and Luhr et al. (2015) during summer months, while during winter months the noon time FAC is still weakly positive. The latitudinal variation of the FAC with respect to longitudes shows that the southern hemispheric FAC at mid-latitudes is always negative at eastern longitudes irrespective of season. It suggests that the FAC is longitudinally dependent.Unlike previous ground based studies, the satellite data not only shows a single prominent negative (southbound) FAC over the African sector region around equinoxes, but it shows that during the March equinox the dusk time FAC is always negative in both hemispheres except at western longitudes 45°, where it has positive patterns. In the middle of summer and winter (months 7 and 1), the noon time FAC varies from positive in summer to negative in winter and the dawn time FAC varies from negative in summer to positive in winter which agrees with the theoretical prediction. The dusk time FAC in the northern hemisphere remains negative which agrees the observations of Park et al. (2011) and Luhr et al. (2015), while in the southern hemisphere it changes from negative in summer to positive in winter.

A new parameterized approach for ionospheric tomography

GNSS ionospheric tomography technique is capable to reconstruct the high-quality 3D ionospheric electron density (IED) images with a relatively low cost. We present a new parameterized approach for refining the voxel-based ionospheric tomography modeling. This approach is different to most other voxel-based techniques as they assume a homogeneous IED distribution in each voxel that is unreasonable for the tomography modeling. In this method, IED of any point within a voxel is determined via vertically exponential interpolation and horizontally inverse distance weighted interpolation from the IED values at the eight corners of that voxel. The parameterized tomography is tested with real data collected over the period of June 1–30, 2015 from 45 GPS stations in south China. The superiority of the new parameterized method is verified by comparison with the traditional nonparametric method. The new parameterized method outperforms the traditional method by 12%, 10%, 5% and 2% for vertical resolutions of 25 km, 50 km, 75 km and 100 km, respectively, in the self-consistency validation by GPS data. Such improvements are 20%, 24%, 22% and 16%, respectively, when assessed by the Swarm in situ IEDs. In terms of the vertical layer discretization, configurations using the resolution of 25 km generally performs better than the other three vertical resolutions. Overall, the parameterized method using a vertical resolution of 25 km achieves the best performance from the comprehensive comparisons with ionospheric data derived by GPS, ionosonde and Swarm satellites.

Magnetic Field and Electron Density Data Analysis from Swarm Satellites Searching for Ionospheric Effects by Great Earthquakes: 12 Case Studies from 2014 to 2016

We analyse Swarm satellite magnetic field and electron density data one month before and one month after 12 strong earthquakes that have occurred in the first 2.5 years of Swarm satellite mission lifetime in the Mediterranean region (magnitude M6.1+) or in the rest of the world (M6.7+). The search for anomalies was limited to the area centred at each earthquake epicentre and bounded by a circle that scales with magnitude according to the Dobrovolsky’s radius. We define the magnetic and electron density anomalies statistically in terms of specific thresholds with respect to the same statistical quantity along the whole residual satellite track (|geomagnetic latitude| ≤ 50°, quiet geomagnetic conditions). Once normalized by the analysed satellite tracks, the anomalies associated to all earthquakes resemble a linear dependence with earthquake magnitude, so supporting the statistical correlation with earthquakes and excluding a relationship by chance.

Two Typical Ionospheric Irregularities Associated With the Tropical Cyclones Tembin (2012) and Hagibis (2014)

AbstractTwo remarkable ionospheric irregularities named equatorial plasma bubble (EPB) and medium‐scale traveling ionospheric disturbance (MSTID) are verified by using multi‐instrumental observations, for example, the ground‐based GPS networks, ionosonde stations, and Swarm satellites, when the tropical cyclones Tembin and Hagibis approached Hong Kong on 26 August 2012 and 15 June 2014, respectively. The low‐latitude plasma bubble over an area of 105–120 °E in longitude was observed during 12:30–17:00 universal time (UT) on 26 August 2012. GPS observations from magnetically conjugate locations indicate that the nighttime bubble during 20:30–01:00 local time (LT = UT + 8h) on 26 August should be formed from the magnetically equatorial region rather than drifted from the west (eastward drift) or generated locally. Different from the EPB, during another cyclone on 15 June 2014, the northwest‐southeast aligned nighttime MSTID was verified in midlatitude regions at a mean horizontal velocity of 156 m/s southwestward propagation during 12:30–17:30 UT when Hagibis was near the mainland coast. By comparing the ionospheric observations during the two cyclones, differences are identified: small‐scale irregularities associated with plasma bubble cause obvious perturbations of the rate of total electron content index with the value of ~3.0 TECU/min; while the MSTID in midlatitude only cause relatively slight rate of total electron content index disturbances with the value of ~1.5 TECU/min. In addition, the magnetic conjugacy of EPB and MSTID in two hemispheres during the passage of tropical cyclones has been also discussed in this study.

An Opposite Response of the Low‐Latitude Ionosphere at Asian and American Sectors During Storm Recovery Phases: Drivers From Below or Above

AbstractIn this study, we focus on the recovery phase of a geomagnetic storm that happened on 6–11 September 2017. The ground‐based total electron content data, as well as the F region in situ electron density, measured by the Swarm satellites show an interesting feature, revealing at low and equatorial latitudes on the dayside ionosphere prominent positive and negative responses at the Asian and American longitudinal sectors, respectively. The global distribution of thermospheric O/N2 ratio measured by global ultraviolet imager on board the Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics satellite cannot well explain such longitudinally opposite response of the ionosphere. Comparison between the equatorial electrojet variations from stations at Huancayo in Peru and Davao in the Philippines suggests that the longitudinally opposite ionospheric response should be closely associated with the interplay of E region electrodynamics. By further applying nonmigrating tidal analysis to the ground‐based total electron content data, we find that the diurnal tidal components, D0 and DW2, as well as the semidiurnal component SW1, are clearly enhanced over prestorm days and persist into the early recovery phase, indicating the possibility of lower atmospheric forcing contributing to the longitudinally opposite response of the ionosphere on 9–11 September 2017.

Impact of typhoon Vongfong 2014 on the ionosphere and geomagnetic field according to Swarm satellite data: 1. Wave disturbances of ionospheric plasma

The article considers the influence of large atmospheric processes on the ionosphere by the example of tropical typhoon Vongfong 2014. We use data obtained from three SWARM satellite missions (450–500 km altitude). We discuss two possible mechanisms of transfer of atmospheric disturbances to ionospheric heights. The first mechanism is the generation of acoustic-gravity waves (AGWs); the second mechanism considers the excitation of electric fields in the atmosphere. We propose new techniques for detecting the ionospheric response to AGW, which rely on low-orbit satellite data. The first technique is based on determination of relative electron density variations in the frequency band from 15 to 150–180 s, corresponding to certain scales of AGW. The second technique estimates space-time derivatives of the electron density, measured by two nearby SWARM satellites. We present and estimate the characteristic magnitudes of ionospheric response effects, their localization and spatial-temporal characteristics for the large tropical cyclone under study.

Влияние тайфуна Vongfong 2014 на ионосферу и геомагнитное поле по данным спутников Swarm: 1. Волновые возмущения ионосферной плазмы

The article considers the influence of large atmospheric processes on the ionosphere by the example of tropical typhoon Vongfong 2014. We use data obtained from three SWARM satellite missions (450–500 km altitude). We discuss two possible mechanisms of transfer of atmospheric disturbances to ionospheric heights. The first mechanism is the generation of acoustic-gravity waves (AGWs); the second mechanism considers the excitation of electric fields in the atmosphere. We propose new techniques for detecting the ionospheric response to AGW, which rely on low-orbit satellite data. The first technique is based on determination of relative electron density variations in the frequency band from 15 to 150–180 s, corresponding to certain scales of AGW. The second technique estimates space-time derivatives of the electron density, measured by two nearby SWARM satellites. We present and estimate the characteristic magnitudes of ionospheric response effects, their localization and spatial-temporal characteristics for the large tropical cyclone under study.

Steve: The Optical Signature of Intense Subauroral Ion Drifts

AbstractLittle is currently known about the optical phenomenon known as Steve. The first scientific publication on the subject suggests that Steve is associated with an intense subauroral ion drift (SAID). However, additional inquiry is warranted as this suggested relationship as it is based on a single case study. Here we present eight occurrences of Steve with coincident or near‐coincident measurements from the European Space Agency's Swarm satellites and show that Steve is consistently associated with SAID. When satellite observations coincident with Steve are compared to that of typical SAID, we find the SAID associated with Steve to have above average peak ion velocities and electron temperatures, as well as extremely low plasma densities.

A Comparison of Cross‐Track Ion Drift Measured by the Swarm Satellites and Plasma Convection Velocity Measured by SuperDARN

AbstractCross‐track ion drifts measured by the Swarm A satellite are compared with colocated line‐of‐sight Super Dual Auroral Radar Network (SuperDARN) velocities in approximately the same directions. More than 200 Swarm A passes over four polar cap SuperDARN radars in the Northern and Southern Hemispheres are considered. Overall, the Swarm‐based velocities are larger than the SuperDARN velocities; the slope of the best fit line to the data is ~0.67. Somewhat stronger differences are found when Swarm A measurements for the entire year 2016 are compared with SuperDARN vector data from global‐scale convection maps. Swarm ion drift data demonstrate known features of the high‐latitude convection patterns, for example, reverse convection cells at interplanetary magnetic field Βz > 0. The latitudes of the convection reversal boundary inferred from SuperDARN are found to be in reasonable agreement with those determined from Swarm A and Swarm B, with Swarm‐based latitudes occurring roughly 1° more equatorward, typically.

Flow Velocity and Field‐Aligned Current Associated With Field Line Resonance: SuperDARN Measurements

AbstractThis study presents observations of the two‐dimensional velocities and field‐aligned currents of magnetospheric field line resonances. Knowledge of the two‐dimensional plasma flows is essential for determining the field‐aligned current structure of the field line resonance and for understanding the role that field line resonances play in the generation of some auroral arc forms. In this work the full velocity vectors are determined from a high‐resolution fit to the Super Dual Auroral Radar Network (SuperDARN) line‐of‐sight data. Two discrete field line resonance events are presented, one at 4.0 mHz (4‐ to 5‐min period) and the other at 0.6 mHz (30‐min period). Band‐pass filtering the north‐south and east‐west components of the total convection velocity vectors reveals the two‐dimensional velocity field specific to an individual field line resonance. The field line resonance velocities exhibit periodic vorticity, which correspond to periodic poleward moving bands of enhanced field‐aligned current. Assuming a constant ionospheric Pedersen conductance of 10 S, peak field‐aligned current values are estimated to be 2–4 μA/m2. The field‐aligned currents alternate between upward and downward directed and peak in magnitude ~0.5–1° poleward and/or equatorward of the peak in field line resonance flow velocity. A Swarm satellite overpass occurs directly over the 0.6‐mHz field line resonance. The SuperDARN field‐aligned current profile along the Swarm trajectory is found to be similar in magnitude and structure to that determined from Swarm‐A and Swarm‐C in situ measurements.

Modelling of electromagnetic signatures of global ocean circulation: physical approximations and numerical issues

The interactions of flowing electrically conductive seawater with Earth’s magnetic field generate electric currents within the oceans, as well as secondary electric currents induced in the resistive solid Earth. The ocean-induced magnetic field (OIMF) is an observable signature of these currents. Ignoring tidally forced ocean flows, the global ocean circulation system is driven by wind forcing on the ocean surface and by the temperature- and salinity-dependent buoyancy force. Ocean circulation’s magnetic signals contribute to the total magnetic field observed at the Earth’s surface or by low-orbit satellite missions. In this paper, we concentrate on accurate numerical modelling of the OIMF employing various approaches. Using a series of numerical test cases in different scenarios of increasing complexity, we evaluate the applicability of the unimodal thin-sheet approximation, the importance of galvanic coupling between the oceans and the underlying mantle (i.e. the bimodal solution), the effects of vertical stratification of ocean flow as well as the effects of vertical stratification of both oceanic and underlying electrical conductivity, and the influence of electromagnetic self-induction. We find that the inclusion of galvanic ocean-mantle coupling has the largest effect on the predicted OIMF. Self-induction is important only on the largest spatial scales, influencing the lowest spherical harmonic coefficients of the OIMF spectrum. We find this conclusion important in light of the recent Swarm satellite mission which has the potential to observe the large-scale OIMF and its seasonal variations. The implementation of fully three-dimensional ocean flow and conductivity heterogeneity due to bathymetry, which substantially increases the computational demands of the calculations, can play some role for regional studies, or when a more accurate OIMF prediction is needed within the oceans, e.g. for comparison with seafloor observations. However, the large-scale signals at the sea surface or at satellite altitude are less affected.

Preliminary validation of in situ electron density measurements onboard CSES using observations from Swarm Satellites

A preliminary validation of the in situ electron density data obtained by CSES, a newly launched electromagnetic satellite in China on Feb. 2, 2018, using the Swarm satellites LP measurements, is carried out in this study to assess the consistency and reliability of the LAP observations onboard CSES. Three ways are implemented to achieve the goal: (1) comparison of the global distribution patterns for both the daytime and nighttime data, (2) comparison of the data between CSES and Swarm at intersecting ground tracks with similar local time, and (3) comparison of the relative relationship between nighttime and daytime data using the nighttime enhancement space climatology phenomenon. The results show that the in situ electron densities estimated by LAP onboard CSES are in good agreement with data from the Swarm satellites in terms of global distributions, local details and relationship between nighttime and daytime data. A systematic bias between them is also found: LAP density estimates are much lower than that of Swarm satellites. However, the highly consistent relative variations between the two datasets proves the electron density dataset of LAP onboard CSES for now is applicable for the relative data variation applications on ionosphere-related studies. Further calibration of the LAP dataset is recommended using electron density data obtained by different measurements.

Geomagnetic conjugate observations of ionospheric disturbances in response to a North Korean underground nuclear explosion on 3 September 2017

Abstract. We report observations of ionospheric disturbances in response to a North Korean underground nuclear explosion (UNE) on 3 September 2017. By using data from IGS (International GNSS Service) stations and Swarm satellites, geomagnetic conjugate ionospheric disturbances were observed. The observational evidence showed that UNE-generated ionospheric disturbances propagated radially from the UNE epicenter with a velocity of ∼280 m s−1. We propose that the ionospheric disturbances are results of electrodynamic process caused by LAIC (lithosphere–atmosphere–ionosphere coupling) electric field penetration. The LAIC electric field can also be mapped to the conjugate hemispheres along the magnetic field line and consequently cause ionospheric disturbances in conjugate regions. The UNE-generated LAIC electric field penetration plays an important role in the ionospheric disturbances in the region of the nuclear test site nearby and the corresponding geomagnetic conjugate points.

The Swarm Satellite Trio Studies Earth and Its Environment

8th Swarm Data Quality Workshop; Frascati, Italy, 8–12 October 2018