Swarm Science Results after Two Years in Space

Editorial Board (December 2016)
Editor-in-Chief
Yasuo Ogawa, Tokyo Institute of Technology, Japan

Vice Editors-in-Chief
Masato Furuya, Hokkaido University, Japan
Solid Earth Science

Nozomu Nishitani, Nagoya University, Japan
Space Science

Editors
Yosuke Aoki
Nanan Balan
Stephen Bannister
Shanaka de Silva
Koji Fukuma
Lin-Ni Hau
Hauke Hussmann
Attila Komjathy
Alexey Kuvshinov

Huixin Liu
Takuto Maeda
Koji Matsumoto
Mitsuhiro Nakagawa
Junichi Nakajima
Yasuhiro Nariyuki
Azusa Nishizawa
Keiji Ohtsuki
Taku Ozawa

Takeshi Sakanoi
Martha K. Savage
Benoit Taisne
Hiroshi Takenaka
Hiroaki Toh
Yih-Min Wu
Akimasa Yoshikawa

Hiroo Kanamori
Jun'ichiro Kawaguchi
Takafumi Matsui
Hitoshi Mizutani

Toshitaka Tsuda
Zhongliang Wu
Kiyoshi Yomogida

Advisory Board
Benjamin Fong Chao
Bernard Chouet
Yoshimori Honkura
Toshihiko Iyemori

Journal Scope
Earth, Planets and Space (EPS) is the official journal of the Society of Geomagnetism and Earth, Planetary
and Space Sciences, The Seismological Society of Japan, The Volcanological Society of Japan, The
Geodetic Society of Japan, and The Japanese Society for Planetary Sciences.
EPS is a peer-reviewed, open-access journal published under SpringerOpen. It is an international journal
covering scientific articles in the entire field of earth and planetary sciences, particularly geomagnetism,aeronomy, space science, seismology, volcanology, geodesy, and planetary science. EPS also welcomesarticles in new and interdisciplinary subjects, and technical reports on instrumentation and software.
The journal was launched in 1998 and has since published over 2700 articles. All of them are available forfree on SpringerLink:
http://link.springer.com/journal/40623
More information about the journal, its article collections, and the submission process is available on thejournal homepage:
http://earth-planets-space.springeropen.com
Submit your next research article to Earth, Planets and Space via:
https://www.editorialmanager.com/epsp
Yours sincerely,
Prof. Yasuo Ogawa
Editor-in-Chief, Earth, Planets and Space
editor-in-chief@earth-planets-space.org

Earth, Planets and Space
Olsen et al. Earth, Planets and Space (2016) 68:172
DOI 10.1186/s40623-016-0546-6

Open Access

PREFACESpecial issue “Swarm science results
after 2 years in space”
Nils Olsen1*, Claudia Stolle2, Rune Floberghagen3, Gauthier Hulot4 and Alexey Kuvshinov5

field contributions, in particular during geomagnetic
quiet conditions, is discussed by Stolle et al. (2016).
Five contributions discuss the magnetic field producedby ionospheric and magnetospheric currents: Chulliat
et al. (2016) present a climatological model of the ionospheric currents responsible for geomagnetic daily variations at non-polar latitudes, while the work of Laundal
et al. (2016) concentrates on a consistent description ofhorizontal ionospheric and field-aligned currents in thepolar ionosphere, in particular regarding their dependence on solar irradiation that controls ionospheric conductivity. A scheme for estimating the polar ionosphericcurrents that form the Polar Electrojets on an orbit-byorbit basis is presented by Aakjær et al. (2016), while Tozzi
et al. (2016) discuss unmodelled magnetic field contributions in satellite-based magnetic field models. Michelis
et al. (2016) present a study of high-latitude magnetic field
variations during the St. Patrick’s Day Storm.
The same event is investigated by Pignalberi et al.
(2016) using Swarm plasma density measurements, andby Cherniak and Zakharenkova (2016) using GPS datafrom ground and Swarm.
Calibration of the electric field instrument of Swarm ispresented by Fiori et al. (2016). A combination of electric,
magnetic, and TEC observations has been used by Astafyeva et al. (2016) to investigate the magnetic storm of
22–23 June 2015. Aoyama et al. (2016) combine groundmagnetic data and Swarm TEC observations to study
possible ionospheric effects of the 2015 eruption of a volcano in Chile, Zakharenkova et al. (2016) used GPS and
Swarm plasma observations to study equatorial plasma
density irregularities in the topside ionosphere, andXiong et al. (2016) performed a scale analysis of equatorial plasma irregularities.
van den IJssel et al. (2016) describe improvements of
Swarm GPS antenna settings to enhance high-precision
positioning of the spacecraft, and da Encarnação et al.
(2016) discuss various attempts to determine monthly

Swarm is a three-satellite constellation mission launchedby the European Space Agency (ESA) on 22 November 2013. It consists of three identical spacecraft, twoof which (Swarm Alpha and Swarm Charlie) are flying almost side-by-side in polar orbits at lower altitude(about 470 km in September 2016) with an East-Westseparation of 1.4◦ in longitude corresponding to 155 km
at the equator. The third satellite (Swarm Bravo) is in aslightly higher orbit (about 520 km altitude in September2016). Each of the three satellites carry a magnetometry
package (consisting of absolute scalar magnetometer,
fluxgate vector magnetometer, and star imager) for measuring the direction and strength of the magnetic field,
and instruments to measure plasma and electric fieldparameters as well as gravitational acceleration. Timeand position are provided by on-board GPS. The configuration of the various instruments on each of the three
Swarm spacecraft is shown in Fig. 1. More information
about the mission can be found at http://earth.esa.int/
swarm.
The 21 articles collected in this special issue were stimulated by the Joint Inter-Association Symposium “JA4Results from Swarm, Ground Based Data and Earlier
Satellite Missions” at the 26th General Assembly of theInternational Union of Geodesy and Geophysics (IUGG)
held in Prague in July 2015.
Tøffner-Clausen et al. (2016) report on the advanced
calibration of the magnetometry package of the Swarmsatellites. Finlay et al. (2016) and Olsen et al. (2016) present models of Earth’s core magnetic field, while Thébault
et al. (2016) and Kotsiaros (2016) determine models of
the lithospheric field. The importance of high-resolution
magnetic field models for studying external magnetic*Correspondence: nio@space.dtu.dk
1
Division of Geomagnetism, DTU Space, Technical University of Denmark,
Diplomvej 371, 2800 Kongens Lyngby, Denmark
Full list of author information is available at the end of the article© The Author(s) 2016. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License(http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium,provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license,
and indicate if changes were made.

1

Swarm Science Results after Two Years in Space
Olsen et al. Earth, Planets and Space (2016) 68:172

Page 2 of 3

Fig. 1 Various scientific instruments on board each of the three Swarm satellites (ESA/AOES Medialab)energy and enthusiastic spirit, from beginning to end ofeven the most intense meetings, was extremely appreciated. He had a remarkable ability to help solve problems
and convince anyone to do the right thing in the interestof the project and science, sometimes even very late in theevening, and always in his unique and particularly cheerful way, whatever the circumstances.
Gernot passed away in March 2016 after a lengthy,
hard, and unfair fight against cancer. He, and his legendary laugh, will be sorrowfully missed by his friends andcolleagues.

snapshot models of the large-scale part of Earth’s gravity
field from Swarm GPS observations.
Finally, the processing of the Swarm accelerometer data
is described by Siemes et al. (2016).
The work reported in this issue could not have beenachieved without the unfailing support of our deeply
regretted friend and colleague Gernot Plank (Fig. 2), towhich we wish to dedicate this special issue of Earth Planets Space. Gernot has been one of the major contributors
to the success of the Swarm mission. With great enthusiasm he played a considerable role in the establishment of
Swarm SCARF, the Swarm Level-2 data processing facility. Swarm SCARF was key to the project and all scientificresults presented in the present special issue (as well an
in a previous issue, see Earth Planets Space, Vol. 65, Issue11) heavily rely on the success of this facility. Gernot’s

Authors’ contributions
All authors of this article served as guest editors for this special issue. All
authors read and approved the final manuscript.
Author details
1
Division of Geomagnetism, DTU Space, Technical University of Denmark,
Diplomvej 371, 2800 Kongens Lyngby, Denmark. 2 Helmholtz‑Zentrum
Potsdam, Deutsches GeoForschungsZentrum GFZ, 14473 Potsdam, Germany.
3
Directorate of Earth Observation Programmes, ESRIN, Via Galileo Galilei 2,00044 Frascati, Italy. 4 Equipe de Géomagnétisme, Institut de Physique duGlobe de Paris, Sorbonne Paris Cité, Université Paris Diderot, CNRS, 1 rue Jus‑sieu, 75005 Paris, France. 5 Institute of Geophysics, ETH Zürich, Sonneggstrasse
5, 8092 Zürich, Switzerland.
Acknowledgements
We thank the authors of the papers in this special issue, and the referees
who served to evaluate the contributions and gave helpful comments andsuggestions.
Received: 10 October 2016 Accepted: 10 October 2016

ReferencesAakjær CD, Olsen N, Finlay CC (2016) Determining polar ionospheric electrojet
currents from Swarm satellite constellation magnetic data. Earth Planets
Space 68:140. doi:10.1186/s40623‑016‑0509‑y
Aoyama T, Iyemori T, Nakanishi K, Nishioka M, Rosales D, Veliz O, Safor EV (2016)
Localized field‑aligned currents and 4‑min TEC and ground magnetic
oscillations during the 2015 eruption of Chile’s Calbuco volcano. Earth
Planets Space 68:148. doi:10.1186/s40623‑016‑0523‑0
Astafyeva E, Zakharenkova I, Alken P (2016) Prompt penetration electric fields
and the extreme topside ionospheric response to the June 22–23, 2015
geomagnetic storm as seen by the Swarm constellation. Earth Planets
Space 68:152. doi:10.1186/s40623‑016‑0526‑x

Fig. 2 Gernot Plank (1967–2016)2

Earth, Planets and Space
Olsen et al. Earth, Planets and Space (2016) 68:172

Page 3 of 3

Cherniak I, Zakharenkova I (2016) High‑latitude ionospheric irregularities: dif‑ferences between ground‑ and space‑based GPS measurements during
the 2015 St. Patrick’s Day Storm. Earth Planets Space 68:136. doi:10.1186/
s40623‑016‑0506‑1
Chulliat A, Vigneron P, Hulot G (2016) First results from the Swarm dedicatedionospheric field inversion chain. Earth Planets Space 68:104. doi:10.1186/
s40623‑016‑0481‑6
da Encarnação JT et al (2016) Gravity field models derived from Swarm GPS
data. Earth Planets Space 68:127. doi:10.1186/40623‑016‑0499‑9
Finlay CC, Olsen N, Kotsiaros S, Gillet N, Tøffner‑Clausen L (2016) Recent
geomagnetic secular variation from Swarm and ground observatories asestimated in the CHAOS‑6 geomagnetic field model. Earth Planets Space68:112. doi:10.1186/s40623‑016‑0486‑1
Fiori RAD, Koustov AV, Boteler DH, Knudsen DJ, Burchill JK (2016) Calibration
and assessment of Swarm ion drift measurements using a compari‑son with a statistical convection model. Earth Planets Space 68:100.
doi:10.1186/s40623‑016‑0472‑7
Kotsiaros S (2016) Toward more complete magnetic gradiometry with
the Swarm mission. Earth Planets Space 68:130. doi:10.1186/
s40623‑016‑0498‑x
Laundal KM, Finlay CC, Olsen N (2016) Sunlight effects on the 3D polar current
system determined from low earth orbit measurements. Earth Planets
Space 68:142. doi:10.1186/s40623‑016‑0518‑x
Michelis PD, Consolini G, Tozzi R, Marcucci MF (2016) Observations of high‑latitude geomagnetic field fluctuations during St. Patrick’s Day Storm:
Swarm and SuperDARN measurements. Earth Planets Space 68:105.
doi:10.1186/s40623‑016‑0476‑3
Olsen N, Finlay CC, Kotsiaros S, Tøffner‑Clausen L (2016) A model of earth’s
magnetic field derived from 2 years of Swarm satellite constellation data.
Earth Planets Space 68:124. doi:10.1186/s40623‑016‑0488‑z

Pignalberi A, Pezzopane M, Tozzi R, Michelis PD, Coco I (2016) Comparison
between IRI and preliminary Swarm langmuir probe measurements dur‑ing the St. patrick storm period. Earth Planets Space 68:93. doi:10.1186/
s40623‑016‑0466‑5
Siemes C et al (2016) Swarm accelerometer data processing from raw accel‑
erations to thermospheric neutral densities. Earth Planets Space 68:92.
doi:10.1186/s40623‑016‑0474‑5
Stolle C, Michaelis I, Rauberg J (2016) The role of high‑resolution geomagnetic
field models for investigating ionospheric currents at low earth orbit
satellites. Earth Planets Space 68:110. doi:10.1186/s40623‑016‑0494‑1
Thébault E, Vigneron P, Langlais B, Hulot G (2016) A Swarm lithosphericmagnetic field model to SH degree 80. Earth Planets Space 68:126.
doi:10.1186/s40623‑016‑0510‑5
Tøffner‑Clausen L, Lesur V, Olsen N, Finlay CC (2016) In‑flight scalar calibrationand characterisation of the Swarm magnetometry package. Earth Planets
Space 68:129. doi:10.1186/s40623‑016‑0501‑6
Tozzi R, Mandea M, Michelis PD (2016) Unmodelled magnetic contributions
in satellite‑based models. Earth Planets Space 68:108. doi:10.1186/
s40623‑016‑0484‑3
van den Ijssel J, Forte B, Montenbruck O (2016) Impact of Swarm GPS receiver
updates on POD performance. Earth Planets Space 68:85. doi:10.1186/
s40623‑016‑0459‑4
Xiong C, Stolle C, Lühr H, Park J, Fejer BG, Kervalishvili GN (2016) Scale analysis
of equatorial plasma irregularities derived from Swarm constellation.
Earth Planets Space 68:121. doi:10.1186/s40623‑016‑0502‑5
Zakharenkova I, Astafyeva E, Cherniak I (2016) GPS and in situ Swarm observa‑
tions of the equatorial plasma density irregularities in the topside iono‑
sphere. Earth Planets Space 68:120. doi:10.1186/s40623‑016‑0490‑5

3Swarm Science Results after Two Years in SpaceFRONTIER LETTER Open Access

In-flight scalar calibration and characterisation of the Swarm
magnetometry package
Lars Tøffner-Clausen*, Vincent Lesur, Nils Olsen and Christopher C. Finlay
Earth, Planets and Space 2016, 68:129 DOI: 10.1186/s40623-016-0501-6
Received: 29 January 2016, Accepted: 29 June 2016, Published: 22 July 2016

Abstract
We present the in-flight scalar calibration and characterisation of the Swarm magnetometry package consisting of theabsolute scalar magnetometer, the vector magnetometer, and the spacecraft structure supporting the instruments. A
significant improvement in the scalar residuals between the pairs of magnetometers is demonstrated, confirming the highperformance of these instruments. The results presented here, including the characterisation of a Sun-driven disturbancefield, form the basis of the correction of the magnetic vector
measurements from Swarm which is applied to the Swarm Level 1bmagnetic data.

Keywords: Geomagnetism; Magnetometer; Instrument calibration;
Satellite; Swarm

Figure 1*Corresponding author: Lars Tøffner-Clausen, lastec@space.dtu.dk

FULL PAPER

Open Access

Impact of Swarm GPS receiver updates on POD performance
Jose van den IJssel*, Biagio Forte and Oliver Montenbruck
Earth, Planets and Space 2016, 68:85 DOI: 10.1186/s40623-016-0459-4
Received: 4 January 2016, Accepted: 3 May 2016, Published: 23 May 2016

Abstract
The Swarm satellites are equipped with state-of-the-art Global Positioning System (GPS) receivers, which are used for the precise geolocationof the magnetic and electric field instruments, as well as for the determination of the Earth’s gravity field, the total electron content andlow-frequency thermospheric neutral densities. The onboard GPS receivers deliver high-quality data with an almost continuous data rate.
However, the receivers show a slightly degraded performance when flying over the geomagnetic poles and the geomagnetic equator, due toionospheric scintillation. Furthermore, with only eight channels available for dual-frequency tracking, the amount of collected GPS tracking
data is relatively low compared with various other missions. Therefore, several modifications have been implemented to the Swarm GPS
receivers. To optimise the amount of collected GPS data, the GPS antenna elevation mask has slowly been reduced from 10° to 2°. To improve
the robustness against ionospheric scintillation, the bandwidths of the GPS receiver tracking loops have been widened. Because thesemodifications were first implemented on Swarm-C, their impact can be assessed by a comparison with the close flying Swarm-A satellite. Thisshows that both modifications have a positive impact on the GPS receiver performance. The reduced elevation mask increases the amount of
GPS tracking data by more than 3 %, while the updated tracking loops lead to around 1.3 % more observations and a significant reduction intracking losses due to severe equatorial scintillation. The additional observations at low elevation angles increase the average noise of thecarrier phase observations, but nonetheless slightly improve the resulting reduced-dynamic and kinematic orbit accuracy as shown by
independent satellite laser ranging (SLR) validation. The more robust tracking loops significantly reduce the large carrier phase observation
errors at the geomagnetic poles and along the geomagnetic equator and do not degrade the observations at midlatitudes. SLR validation
indicates that the updated tracking loops also improve the reduced-dynamic andkinematic orbit accuracy. It is expected that the Swarm gravity field recovery will8
7.9
Swarm-A
benefit from the improved kinematic orbit quality and potentially also from the
Swarm-B
7.8
Swarm-C
7.7
expected improvement of the kinematic baseline determination and the
7.6
anticipated reduction in the systematic gravity field errors along the geomagnetic
7.5
7.4
equator. Finally, other satellites that carry GPS receivers that encounter similar
7.3
7.2
disturbances might also benefit from this analysis.
83

86 C 88 C

88 A Bnr tracked GPS

0.1 Hz 1 Hz FoV 80

Keywords: Swarm; GPS; Precise orbit determination; Ionospheric scintillation;
Elevation mask; Tracking loop

7.1
7

TL C

Jan
Mar
2014

May

Jul

Sep

Nov

Jan
Mar
2015

Figure 1*Corresponding author: Jose van den IJssel, j.a.a.vandenijssel@tudelft.nl
4

May

TL A TL B

Jul

Sep

Nov

Earth, Planets and Space

FULL PAPER

Open Access

Swarm accelerometer data processing from raw accelerations tothermospheric neutral densities
Christian Siemes*, João de Teixeira da Encarnação, Eelco Doornbos, Jose van den IJssel, Jiří Kraus,
Radek Pereštý, Ludwig Grunwaldt, Guy Apelbaum, Jakob Flury and Poul Erik Holmdahl Olsen
Earth, Planets and Space 2016, 68:92 DOI: 10.1186/s40623-016-0474-5
Received: 31 December 2015, Accepted: 18 May 2016, Published: 28 May 2016

Abstract
The Swarm satellites were launched on November 22, 2013, and carry accelerometers and GPS receivers as part of their scientific payload. The
GPS receivers do not only provide the position and time for the magnetic field measurements, but are also used for determining nongravitational forces like drag and radiation pressure acting on the spacecraft. The accelerometers measure these forces directly, at much finer
resolution than the GPS receivers, from which thermospheric neutral densities can be derived. Unfortunately, the acceleration measurements
suffer from a variety of disturbances, the most prominent being slow temperature-induced bias variations and sudden bias changes. In thispaper, we describe the new, improved four-stage processing that is applied for transforming the disturbed acceleration measurements intoscientifically valuable thermospheric neutral densities. In the first stage, the sudden bias changes in the acceleration measurements aremanually removed using a dedicated software tool. The second stage is the calibration of the accelerometer measurements against the
non-gravitational accelerations derived from the GPS receiver, which includes the correction for the slow temperature-induced bias variations.
The identification of validity periods for calibration and correction parameters is part of the second
stage. In the third stage, the calibrated and corrected accelerations are merged with the nongravitational accelerations derived from the observations of the GPS receiver by a weighted averagein the spectral domain, where the weights depend on the frequency. The fourth stage consists oftransforming the corrected and calibrated accelerations into thermospheric neutral densities. We
present the first results of the processing of Swarm C acceleration measurements from June 2014 to
May 2015. We started with Swarm C because its acceleration measurements contain much lessdisturbances than those of Swarm A and have a higher signal-to-noise ratio than those of Swarm B.
The latter is caused by the higher altitude of Swarm B as well as larger noise in the acceleration
measurements of Swarm B. We show the results of each processing stage, highlight the difficulties
encountered, and comment on the quality of the thermospheric neutral density data set.

Keywords: Swarm mission; Accelerometry; Thermospheric neutral density; Non-gravitational
accelerations

Figure 1*Corresponding author: Christian Siemes, Christian.Siemes@esa.int

FULL PAPER

Open AccessComparison between IRI and preliminary Swarm Langmuirprobe measurements during the St. Patrick storm period
Alessio Pignalberi, Michael Pezzopane*, Roberta Tozzi, Paola De Michelis and Igino Coco
Earth, Planets and Space 2016, 68:93 DOI: 10.1186/s40623-016-0466-5
Received: 30 December 2015, Accepted: 6 May 2016, Published: 28 May 2016

Abstract
Preliminary Swarm Langmuir probe measurements recorded during March 2015, a period of time including the St. Patrick
storm, are considered. Specifically, six time periods are identified: two quiet periods before the onset of the storm, two periods
including the main phase of the storm, and two periods during the recovery phase of the storm. Swarm electron density
values are then compared with the corresponding output given by the International Reference Ionosphere (IRI) model,
according to its three different options for modelling the topside ionosphere. Since the Swarm electron density
measurements are still undergoing a thorough validation, a comparison with IRI in terms of absolute values would have notbeen appropriate. Hence, the similarity of trends embedded in the Swarm and IRI time series is investigated in terms ofPearson correlation coefficient. The analysis shows that the electron density
representations made by Swarm and IRI are different for both quiet and disturbed
periods, independently of the chosen topside model option. Main differences
between trends modelled by IRI and those observed by Swarm emerge, especially atequatorial latitudes, and at northern high latitudes, during the main and recovery
phases of the storm. Moreover, very low values of electron density, even lower than
2 × 104 cm−3, were simultaneously recorded in the evening sector by Swarm satellites
at equatorial latitudes during quiet periods, and at magnetic latitudes of about ±60°
during disturbed periods. The obtained results are an example of the capability of
Swarm data to generate an additional valuable dataset to properly model thetopside ionosphere.

Keywords: IRI model; Swarm data; Topside electron density; St. Patrick storm
Figure 1*Corresponding author: Michael Pezzopane, michael.pezzopane@ingv.it
5

Swarm Science Results after Two Years in Space

FULL PAPER

Open Access

Calibration and assessment of Swarm ion drift measurements
using a comparison with a statistical convection model
R. A. D. Fiori*, A. V. Koustov, D. H. Boteler, D. J. Knudsen and J. K. Burchill
Earth, Planets and Space 2016, 68:100 DOI: 10.1186/s40623-016-0472-7
Received: 22 January 2016, Accepted: 12 May 2016, Published: 7 June 2016

Abstract
The electric field instruments onboard the Swarm satellites make high-resolution
measurements of the F-region ion drift. This paper presents an initial investigation of
preliminary ion drift data made available by the European Space Agency. Based on datataken during polar cap crossings, we identify large offsets in both the along-track andcross-track components of the measured ion drift. These offsets are removed by zeroing
drift values at the low-latitude boundary of the high-latitude convection pattern. This
correction is shown to significantly improve agreement between the Swarm ion drift
measurements and velocity inferred from a radar-based statistical convection model for
periods of quasi-stability in the solar wind and interplanetary magnetic field. Agreement ismost pronounced in the cross-track direction (R = 0.60); it improves slightly (R = 0.63) if data
are limited to periods with IMF Bz < 0. The corrected Swarm data were shown to properly
identify the convection reversal boundary for periods of IMF Bz < 0, in full agreement withprevious radar and satellite measurements, making Swarm ion drift measurements avaluable input for ionospheric modeling.

ab

Keywords: Ion drift; Swarm electric field instrument; Ionospheric plasma flow

Figure 1*Corresponding author: R. A. D. Fiori, robyn.fiori@canada.ca

FULL PAPER

Open AccessFirst results from the Swarm Dedicated Ionospheric Field
Inversion chain
A. Chulliat*, P. Vigneron and G. Hulot
Earth, Planets and Space 2016, 68:104 DOI: 10.1186/s40623-016-0481-6
Received: 24 February 2016, Accepted: 30 May 2016, Published: 13 June 2016

Abstract
Data-based modeling of the magnetic field originating in the Earth’s ionosphere is challenging due to the multiple timescales
involved and the small spatial scales of some of the current systems, especially the equatorial electrojet (EEJ) that flows along
the magnetic dip equator. The Dedicated Ionospheric Field Inversion (DIFI) algorithm inverts a combination of Swarm satellite
and ground observatory data at mid- to low latitudes and provides models of the solar-quiet (Sq) and EEJ magnetic fields onthe ground and at satellite altitude. The basis functions of these models are spherical harmonics in quasi-dipole coordinatesand Fourier series describing the 24-, 12-, 8- and 6-h periodicities, as well as
Local time at the equator (northward tracks)
the annual and semiannual variations. A 1-D conductivity model of the Earthand a 2-D conductivity model of the oceans and continents are used toseparate the primary ionospheric field from its induced counterpart. First
results from the DIFI algorithm confirm several well-known features of theseasonal variability and westward drift speed of the Sq current systems. They
also reveal a peculiar seasonal variability of the Sq field in the Southernhemisphere and a longitudinal variability reminiscent of the EEJ wave-4
Local time differences at the equator
structure in the same hemisphere. These observations suggest that the Sq
and EEJ currents might be electrically coupled, but only for some seasonsand longitudes and more so in the Southern hemisphere than in the
Northern hemisphere.
24

LT (hrs)

18

12

6

0
2014.0A
B

2014.25

2014.52014.75

2015.02015.252015.52014.75

2015.02015.252015.50.50LT (hrs)

-0.5

-1

-1.5

-2

Keywords: Swarm; Geomagnetism; Ionosphere; Sq; Equatorial electrojet*Corresponding author: A. Chulliat, arnaud.chulliat@noaa.gov
6-2.5
2014.0A-B

2014.25

2014.5Figure 1

Earth, Planets and Space

FULL PAPER

Open AccessObservations of high-latitude geomagnetic field fluctuations during
St. Patrick’s Day storm: Swarm and SuperDARN measurements
Paola De Michelis*, Giuseppe Consolini, Roberta Tozzi and Maria Federica Marcucci
Earth, Planets and Space 2016, 68:105 DOI: 10.1186/s40623-016-0476-3
Received: 29 January 2016, Accepted: 18 May 2016, Published: 21 June 2016

Abstract

Keywords: High-latitude phenomena; Solar wind–magnetosphere

YExternal [nT]

YTotal [nT]

XExternal [nT]

XTotal [nT]

The aim of this work is to study the properties of the magnetic field’s fluctuations produced by ionospheric and magnetosphericelectric currents during the St. Patrick’s Day geomagnetic storm (17 March 2015). We analyse the scaling features of the external
contribution to the horizontal geomagnetic field recorded simultaneously by the three satellites of the Swarm constellation during aperiod of 13 days (13–25 March 2015). We examine the different latitudinal structure of the geomagnetic field fluctuations andanalyse the dynamical changes in the magnetic field scaling features during the development of the geomagnetic storm. Analysis
reveals consistent patterns in the scaling properties of magnetic fluctuations and striking changes between the situation before thestorm, during the main phase and recovery phase. We discuss these dynamicalchanges in relation to those of the overall ionospheric polar convection andpotential structures as reconstructed using SuperDARN data. Our findings
suggest that distinct turbulent regimes characterised the mesoscale magnetic
field’s fluctuations and that some factors, which are known to influencelarge-scale fluctuations, have also an influence on mesoscale fluctuations. The
obtained results are an example of the capability of geomagnetic field
fluctuations data to provide new insights about ionospheric dynamics andionosphere–magnetosphere coupling. At the same time, these results could
open doors for development of new applications where the dynamicalchanges in the scaling features of the magnetic fluctuations are used as local
indicators of magnetospheric conditions.

interactions; Ionospheric turbulence; Swarm magnetic
measurements

Figure 1*Corresponding author: Paola De Michelis, paola.demichelis@ingv.it

FULL PAPER

Open Access

Unmodelled magnetic contributions in satellite-basedmodelsRoberta Tozzi*, Mioara Mandea and Paola De Michelis
Earth, Planets and Space 2016, 68:108 DOI: 10.1186/s40623-016-0484-3
Received: 30 January 2016, Accepted: 7 June 2016, Published: 28 June 2016

Abstract
A complex system of electric currents flowing in the ionosphere and magnetosphere originates from the interaction of thesolar wind and the Interplanetary Magnetic Field (IMF) with the Earth’s magnetic field. These electric currents generatemagnetic fields contributing themselves to those measured by both ground observatories and satellites. Here, low-resolution(1 Hz) magnetic vector data recorded between 1 March 2014 and 31 May 2015 by the recently launched Swarm constellation
are considered. The core and crustal magnetic fields and part of that originating in the magnetosphere are removed from
Swarm measurements using CHAOS-5 model. Low- and mid-latitude residuals of the geomagnetic field representing theionospheric and the unmodelled magnetospheric contributions are investigated, in the Solar Magnetic frame, according to
the polarity of IMF By (azimuthal) and Bz (north–south) components and to different geomagnetic activity levels. The proposed
approach makes it possible to investigate the features of unmodelled contributions due to the external sources of thegeomagnetic field. Results show, on one side, the existence of a relation between the analysed residuals and IMF components
By and Bz, possibly due to the long distance effect of high-latitude field-aligned currents. On the other side, they suggest thepresence of a contribution due to the partial ring current
that is activated during the main phase of geomagnetic
storms. The perturbation observed on residuals is also
compatible with the effect of the net field-aligned
currents. Moreover, we have quantitatively estimated theeffect of these current systems on computed residuals.

Keywords: Swarm magnetic vector data; Interplanetary
Magnetic Field; Magnetic models;
Field-aligned currents; Partial ring current

Figure 1*Corresponding author: Roberta Tozzi, roberta.tozzi@ingv.it
7

Swarm Science Results after Two Years in Space

FULL PAPER

Open Access

Recent geomagnetic secular variation from Swarm and ground observatories
as estimated in the CHAOS-6 geomagnetic field model
Christopher C. Finlay*, Nils Olsen, Stavros Kotsiaros, Nicolas Gillet and Lars Tøffner-Clausen
Earth, Planets and Space 2016, 68:112 DOI: 10.1186/s40623-016-0486-1
Received: 31 December 2015, Accepted: 9 June 2016, Published: 11 July 2016

Abstract
We use more than 2 years of magnetic data from the Swarm mission, and monthly means from 160 ground observatories as available in March
2016, to update the CHAOS time-dependent geomagnetic field model. The new model, CHAOS-6, provides information on time variations of
the core-generated part of the Earth’s magnetic field between 1999.0 and 2016.5. We present details of the secular variation (SV) and secular
acceleration (SA) from CHAOS-6 at Earth’s surface and downward continued to the core surface. At Earth’s surface, we find evidence forpositive acceleration of the field intensity in 2015 over a broad area around longitude 90°E that is also seen at ground observatories such as
Novosibirsk. At the core surface, we are able to map the SV up to at least degree 16. The radial field SA at the core surface in 2015 is found to
be largest at low latitudes under the India–South-East Asia region, under the region of northern South America, and at high northern latitudes
under Alaska and Siberia. Surprisingly, there is also evidence for significant SA in the central Pacific region, for example near Hawaii whereradial field SA is observed on either side of a jerk in 2014. On the other hand, little SV or SA has occurred over the past 17 years in the southern
polar region. Inverting for a quasi-geostrophic core flow that accounts for this SV, we obtain a prominent planetary-scale, anti-cyclonic, gyre
centred on the Atlantic hemisphere. We also find oscillations of non-axisymmetric, azimuthal, jets at low latitudes, for example close to 40°W,
that may be responsible for localized SA oscillations. In addition to scalar data fromØrsted, CHAMP, SAC-C and Swarm, and vector data from Ørsted, CHAMP and Swarm,
CHAOS-6 benefits from the inclusion of along-track differences of scalar and vector fielddata from both CHAMP and the three Swarm satellites, as well as east–west differences
between the lower pair of Swarm satellites, Alpha and Charlie. Moreover, groundobservatory SV estimates are fit to a Huber-weighted rms level of 3.1 nT/year for the
eastward components and 3.8 and 3.7 nT/year for the vertical and southward
components. We also present an update of the CHAOS high-degree lithospheric field,
making use of along-track differences of CHAMP scalar and vector field data to produce
a new static field model that agrees well with the MF7 field model out to degree 110.

Keywords: Geomagnetism; Secular variation; Field modelling; Core dynamics; Swarm

Figure 1*Corresponding author: Christopher C. Finlay, cfinlay@space.dtu.dk

FULL PAPER

Open Access

Scale analysis of equatorial plasma irregularities derived
from Swarm constellation
Chao Xiong*, Claudia Stolle, Hermann Lühr, Jaeheung Park, Bela G. Fejer and Guram N. Kervalishvili
Earth, Planets and Space 2016, 68:121 DOI: 10.1186/s40623-016-0502-5
Received: 11 December 2015, Accepted: 28 June 2016, Published: 15 July 2016

Abstract
In this study, we investigated the scale sizes of equatorial plasma irregularities (EPIs) using measurements from the Swarmsatellites during its early mission and final constellation phases. We found that with longitudinal separation between Swarmsatellites larger than 0.4°, no significant correlation was found any more. This result suggests that EPI structures include
plasma density scale sizes less than 44 km in the zonal direction. During the Swarm earlier mission phase, clearly better EPI
correlations are obtained in the northern hemisphere, implying more fragmented irregularities in the southern hemisphere
where the ambient magnetic field is low. The previously reported inverted-C shell structure of EPIs is generally confirmed by
the Swarm observations in the northern hemisphere, but with various tilt angles. From the Swarm spacecrafts with zonal
separations of about 150 km, we conclude that larger zonal scale sizes of irregularities exist during the early evening hours(around 1900 LT).

Keywords: Equatorial plasma irregularities;ab

Ionospheric scale lengths; Swarm
constellation

Figure 1*Corresponding author: Chao Xiong, bear@gfz-potsdam.de8Earth, Planets and Space

FULL PAPER

Open Access

Gravity field models derived from Swarm GPS dataJoão Teixeira da Encarnação*, Daniel Arnold, Aleš Bezděk, Christoph Dahle, Eelco Doornbos,
Jose van den IJssel, Adrian Jäggi, Torsten Mayer-Gürr, Josef Sebera, Pieter Visser
and Norbert Zehentner
Earth, Planets and Space 2016, 68:127 DOI: 10.1186/s40623-016-0499-9
Received: 31 December 2015, Accepted: 29 June 2016, Published: 20 July 2016

Abstract
It is of great interest to numerous geophysical studies that the time series of global gravity field models derived from Gravity
Recovery and Climate Experiment (GRACE) data remains uninterrupted after the end of this mission. With this in mind, someinstitutes have been spending efforts to estimate gravity field models from alternative sources of gravimetric data. This study
focuses on the gravity field solutions estimated from Swarm global positioning system (GPS) data, produced by the
Astronomical Institute of the University of Bern, the Astronomical Institute (ASU, Czech Academy of Sciences) and Institute ofGeodesy (IfG, Graz University of Technology). The three sets of solutions are based on different approaches, namely thecelestial mechanics approach, the acceleration approach and the short-arc approach, respectively. We derive the maximum
spatial resolution of the time-varying gravity signal in the Swarm gravity field models to be degree 12, in comparison with themore accurate models obtained from K-band ranging data of
GRACE. We demonstrate that the combination of the GPS-driven
models produced with the three different approaches improves
the accuracy in all analysed monthly solutions, with respect to anyof them. In other words, the combined gravity field model
consistently benefits from the individual strengths of eachseparate solution. The improved accuracy of the combined modelis expected to bring benefits to the geophysical studies during
the period when no dedicated gravimetric mission is operational.

Keywords: Gravity field; Swarm; High–low satellite-to-satellite
tracking; GRACE

Figure 1*Corresponding author: João Teixeira da Encarnação, J.G.deTeixeiradaEncarnacao@tudelft.nl

FULL PAPER

Open AccessToward more complete magnetic gradiometry with the
Swarm mission
Stavros Kotsiaros
Earth, Planets and Space 2016, 68:130 DOI: 10.1186/s40623-016-0498-x
Received: 31 December 2015, Accepted: 27 June 2016, Published: 22 July 2016

Abstract
An analytical and numerical analysis of the spectral properties of the gradient tensor, initially performed by Rummel and vanGelderen (Geophys J Int 111(1):159–169, 1992) for the gravity potential, shows that when the tensor elements are grouped
into sets of semi-tangential and pure-tangential parts, they produce almost identical signal content as the normal element.
Moreover, simple eigenvalue relations can be derived between these sets and the spherical harmonic expansion of thepotential. This theoretical development generally applies to any potential field. First, the analysis of Rummel and van Gelderen(1992) is adapted to the magnetic field case and then the elements of the magnetic gradient tensor are estimated by 2 yearsof Swarm data and grouped into Γ(1)={[∇B]rθ, [∇B]rφ} resp. Γ(2)={[∇B]θθ−[∇B]φφ, 2[∇B]θφ}. It is shown that the estimatedcombinations Γ(1) and Γ(2) produce similar signal content as the theoretical radial gradient Γ(0)={[∇B]rr}. These results
demonstrate the ability of multi-satellite missions such as Swarm, which cannot
directly measure the radial gradient, to retrieve similar signal content by means of10
R (r ) @ 460 km
the horizontal gradients. Finally, lithospheric field models are derived using the
R (r ) @ 460 km(1)(2)
gradient combinations Γ and Γ and compared with models derived from10
R (r ) @ 460 km
traditional vector and gradient data. The model resulting from Γ(1) leads to a very
R (r ) @ 0 km
10similar, and in particular cases improved, model compared to models retrieved by
R (r ) @ 0 km
10
R (r ) @ 0 km
using approximately three times more data, i.e., a full set of vector, North–Southand East–West gradients. This demonstrates the high information content of Γ(1).
102Power Rn [nT2/km ]

−3

(0)
n h
(1)
n h

−4

(2)
n h

−5

(0)
n h
(1)
n h

−6

(2)
n h

−7

−8

10

Keywords: Crustal field; Swarm gradients; Field modeling

20

30

40

50

60

degree n

Figure 1

Corresponding author: Stavros Kotsiaros, skotsiaros@space.dtu.dk
9

70

80

90

Swarm Science Results after Two Years in Space

FULL PAPER

Open Access

Determining polar ionospheric electrojet currents from Swarmsatellite constellation magnetic data
Cecilie Drost Aakjær*, Nils Olsen and Christopher C. Finlay
Earth, Planets and Space 2016, 68:140 DOI: 10.1186/s40623-016-0509-y
Received: 8 February 2016, Accepted: 6 July 2016, Published: 5 August 2016

Abstract
We determine the strength and location of the ionospheric currents responsible for the polar electrojets from magnetic datacollected by the Swarm satellite constellation on an orbit-by-orbit basis. The ionospheric currents are modelled using a simple, yetrobust, method by a series of line currents at 110 km altitude (corresponding to the ionospheric E-layer) perpendicular to the satellite
orbit, separated by 1° (about 113 km). We assess the reliability of our method, with the aim of a possible near-real-time application. A
study of the effect of different regularization methods is therefore carried out. An L1 model regularization of the second-order spatialdifferences, and robust treatment of the data (to account for non-Gaussian error distributions), yields the most encouraging results.
We apply our approach to two three-weekly data periods in March 2014
(geomagnetic quiet conditions) and March 2015 (more disturbed
conditions), respectively. Our orbit-by-orbit approach also allows the
temporal evolution of the polar electrojets to be investigated. We findremarkable agreement of the ionospheric activity in Northern andSouthern polar regions, with correlation exceeding 0.9 for periods longer
than two days. Reliability of the approach is shown by three key results:(1) a common regularization parameter for all orbits with enough data
coverage, (2) 0.95 squared coherence with the Auroral Electrojet index,
and (3) 0.97 squared coherence is found between the side-by-side flying
satellites, Alpha and Charlie, indicating a method invariant to smallchanges in data input. All these results indicate a possible automatednear-real-time application.

Keywords: Geomagnetism; Field modelling; Ionosphere; Swarm

Figure 1*Corresponding author: Cecilie Drost Aakjær, cda@space.dtu.dk

FULL PAPER

Open Access

Sunlight effects on the 3D polar current system determinedfrom low Earth orbit measurements
Karl M. Laundal*, Christopher C. Finlay and Nils Olsen
Earth, Planets and Space 2016, 68:142 DOI: 10.1186/s40623-016-0518-x
Received: 29 January 2016, Accepted: 29 July 2016, Published: 11 August 2016

Abstract
Interaction between the solar wind and the Earth’s magnetosphere is associated with large-scalecurrents in the ionosphere at polar latitudes that flow along magnetic field lines (Birkeland
currents) and horizontally. These current systems are tightly linked, but their global behaviors are
rarely analyzed together. In this paper, we present estimates of the average global Birkeland
currents and horizontal ionospheric currents from the same set of magnetic field measurements.
The magnetic field measurements, from the low Earth orbiting Swarm and CHAMP satellites, areused to co-estimate poloidal and toroidal parts of the magnetic disturbance field, represented inmagnetic apex coordinates. The use of apex coordinates reduces effects of longitudinal andhemispheric variations in the Earth’s main field. We present global currents from bothhemispheres during different sunlight conditions. The results show that the Birkeland currents
vary with the conductivity, which depends most strongly on solar EUV emissions on the daysideand on particle precipitation at pre-midnight magnetic local times. In sunlight, the horizontal
equivalent current flows in two cells, resembling an opposite ionospheric convection pattern,which implies that it is dominated by Hall currents. By combining the Birkeland current maps andthe equivalent current, we are able to calculate the total horizontal current, without any
assumptions about the conductivity. We show that the total horizontal current is close to zero in
the polar cap when it is dark. That implies that the equivalent current, which is sensed by groundmagnetometers, is largely canceled by the horizontal closure of the Birkeland currents.

Keywords: Polar ionospheric currents; Birkeland currents; Equivalent currents; Apex coordinates;
LEO magnetic field

12

North

06

18

South

18

06

10

-50◦

00

−0.4

downward

−0.2

0.0

µA/m2

Figure 1*Corresponding author: Karl M. Laundal, karl.laundal@ift.uib.no

50◦

00
12

0.2

0.4

upwardEarth, Planets and SpaceLETTER 

Open Access

The role of high-resolution geomagnetic field models for investigating
ionospheric currents at low Earth orbit satellites
Claudia Stolle*, Ingo Michaelis and Jan Rauberg
Earth, Planets and Space 2016, 68:110 DOI: 10.1186/s40623-016-0494-1
Received: 27 January 2016, Accepted: 15 June 2016, Published: 8 July 2016

Abstract
Low Earth orbiting geomagnetic satellite missions, such as the Swarm satellite mission, are the only means to monitor and investigate
ionospheric currents on a global scale and to make in situ measurements of F region currents. High-precision geomagnetic satellite
missions are also able to detect ionospheric currents during quiet-time geomagnetic conditions that only have few nanotesla
amplitudes in the magnetic field. An efficient method to isolate the ionospheric signals from satellite magnetic field measurements
has been the use of residuals between the observations and predictions from empirical geomagnetic models for other geomagnetic
sources, such as the core and lithospheric field or signals from the quiet-timemagnetospheric currents. This study aims at highlighting the importance of highresolution magnetic field models that are able to predict the lithospheric field and thatconsider the quiet-time magnetosphere for reliably isolating signatures from ionosphericcurrents during geomagnetically quiet times. The effects on the detection of ionosphericcurrents arising from neglecting the lithospheric and magnetospheric sources are
discussed on the example of four Swarm orbits during very quiet times. The respective
orbits show a broad range of typical scenarios, such as strong and weak ionosphericsignal (during day- and nighttime, respectively) superimposed over strong and weaklithospheric signals. If predictions from the lithosphere or magnetosphere are notproperly considered, the amplitude of the ionospheric currents, such as the midlatitude
Sq currents or the equatorial electrojet (EEJ), is modulated by 10–15 % in the examples
shown. An analysis from several orbits above the African sector, where the lithosphericfield is significant, showed that the peak value of the signatures of the EEJ is in error by 5
% in average when lithospheric contributions are not considered, which is in the range of
uncertainties of present empirical models of the EEJ.

Keywords: Geomagnetic field; Ionospheric current; Geomagnetic models

Figure 1*Corresponding author: Claudia Stolle, claudia.stolle@gfz-potsdam.de

LETTER 

Open AccessGPS and in situ Swarm observations of the equatorial
plasma density irregularities in the topside ionosphere
Irina Zakharenkova*, Elvira Astafyeva and Iurii Cherniak
Earth, Planets and Space 2016, 68:120 DOI: 10.1186/s40623-016-0490-5
Received: 31 December 2015, Accepted: 14 June 2016, Published: 15 July 2016

Abstract
Here we study the global distribution of the plasma density irregularities in the topside ionosphere by using the concurrent
GPS and Langmuir probe measurements onboard the Swarm satellites. We analyze 18 months (from August 2014 till January2016) of data from Swarm A and B satellites that flew at 460 and 510 km altitude, respectively. To identify the occurrence of
the ionospheric irregularities, we have analyzed behavior of two indices ROTI and RODI based on the change rate of total
electron content and electron density, respectively. The obtained results demonstrate a high degree of similarities in theoccurrence pattern of the seasonal and longitudinal distribution of the topside ionospheric irregularities derived from bothtypes of the satellite observations. Among the seasons with good data
coverage, the maximal occurrence rates for the post-sunset equatorial a
irregularities reached 35–50 % for the September 2014 and March
2015 equinoxes and only 10–15 % for the June 2015 solstice. For theequinox seasons the intense plasma density irregularities were more
frequently observed in the Atlantic sector, for the December solstice in
the South American–Atlantic sector. The highest occurrence rates forthe post-midnight irregularities were observed in African longitudinalbsector during the September 2014 equinox and June 2015 solstice.
The observed differences in SWA and SWB results could be explainedby the longitude/LT separation between satellites, as SWB crossed thesame post-sunset sector increasingly later than the SWA did.
24

SWA
SWB

LT, hours

20
16
12

8
4
0

A

S

O

N

D

J

F

M

A

M

J

J

A

S

O

N

D

J

3

LT, hours

2.99

2.59

2

2.20

1.81

1.44

1

1.05

0.67

0

Keywords: Swarm; Plasma irregularities; Topside ionosphere; GPS;
Langmuir probe; ROTI

A

201 4

S

O

N

D

J

2 0 15

F

M

A

M

J

MonthsFigure 1*Corresponding author: Irina Zakharenkova, zakharen@ipgp.fr
11

J

A

S

O

N

D

J

2016

Swarm Science Results after Two Years in SpaceLETTER 

Open Access

A model of Earth’s magnetic field derived from 2 years of Swarmsatellite constellation data
Nils Olsen*, Christopher C. Finlay, Stavros Kotsiaros and Lars Tøffner-Clausen
Earth, Planets and Space 2016, 68:124 DOI: 10.1186/s40623-016-0488-z
Received: 31 December 2015, Accepted: 13 June 2016, Published: 20 July 2016

Abstract
More than 2 years of magnetic field data taken by the three-satellite constellation mission Swarm are used to derive a modelof Earth’s magnetic field and its time variation. This model is called SIFMplus. In addition to the magnetic field observations
provided by each of the three Swarm satellites, explicit advantage is taken of the constellation aspect of Swarm by including
East–West magnetic intensity and vector field gradient information from the lower satellite pair. Along-track differences of themagnetic intensity as well as of the vector components provide further information concerning the North–South gradient.
The SIFMplus model provides a description of the static lithospheric field that is very similar to models determined from
CHAMP data, up to at least spherical harmonic degree n=75. Also the core field part of SIFMplus, with a quadratic timedependence for n≤6 and a linear time dependence for n=7–15, demonstrates
the possibility to determine high-quality field models from only 2 years ofa
Swarm data, thanks to the unique constellation aspect of Swarm. To account forthe magnetic signature caused by ionospheric electric currents at polar
latitudes we co-estimate, together with the model of the core, lithospheric andlarge-scale magnetospheric fields, a magnetic potential that depends on quasidipole latitude and magnetic local time.
Rn [nT2 ]101

Keywords: Geomagnetism; Field modeling; Swarm satellites

MF7
CHAOS-6
SIFMplus
CHAOS-6 - MF7
SIFM - MF7
SIFMx - MF7

100

SIFMplus noMLT - MF7
SIFMplus - MF7

20

30

40

50

60

70

80

degree n

Figure 1*Corresponding author: Nils Olsen, nio@space.dtu.dk

LETTER 

Open Access

A Swarm lithospheric magnetic field model to SH
degree 80
Erwan Thébault*, Pierre Vigneron, Benoit Langlais and Gauthier Hulot
Earth, Planets and Space 2016, 68:126 DOI: 10.1186/s40623-016-0510-5
Received: 9 February 2016, Accepted: 8 July 2016, Published: 20 July 2016

Abstract
The Swarm constellation of satellites was launched in November 2013 and since then has delivered high-quality scalar andvector magnetic field measurements. A consortium of several research institutions was selected by the European Space
Agency to provide a number of scientific products to be made available to thescientific community on a regular basis. In this study, we present the dedicatedlithospheric field inversion model. It uses carefully selected magnetic field scalarand vector measurements from the three Swarm satellites between March 2014 andDecember 2015 and directly benefits from the explicit expression of the magnetic
field gradients by the lower pair of Swarm satellites. The modeling scheme is a
two-step one and relies first on a regional modeling approach that is very sensitive
to small spatial scales and weak signals which we seek to describe. The final model
is built from adjacent regional solutions and consists in a global spherical harmonics
model expressed between degrees 16 and 80. The quality of the derived model isassessed through a comparison with independent models based on Swarm and theCHAMP satellites. This comparison emphasizes the high level of accuracy of thecurrent model after only 2 years of measurements but also highlights the possible
improvements which will be possible once the lowest two satellites reach lower
altitudes.

Keywords: Geomagnetism; Swarm; Lithosphere

Figure 2*Corresponding author: Erwan Thébault, erwan.thebault@univ-nantes.fr
12Earth, Planets and SpaceLETTER 

Open Access

High-latitude ionospheric irregularities: differences between
ground- and space-based GPS measurements during the 2015
St. Patrick’s Day storm
Iurii Cherniak* and Irina Zakharenkova
Earth, Planets and Space 2016, 68:136 DOI: 10.1186/s40623-016-0506-1
Received: 31 December 2015, Accepted: 6 July 2016, ublished: 30 July 2016

Abstract
We present an analysis of ionospheric irregularities at high latitudes during the 2015 St. Patrick’s Day storm. Our
study used measurements from ~2700 ground-based GPS stations and GPS receivers onboard five low earth orbit(LEO) satellites—Swarm A, B and C, GRACE and TerraSAR-X—that had close orbit altitudes of ~500 km, and the
Swarm in situ plasma densities. An analysis of the rate of TEC index (ROTI) derived from LEO–GPS data, together
with Swarm in situ plasma probe data, allowed us to examine the topside ionospheric irregularities and tocompare them to the main ionospheric storm effects observed in ground-based GPS data. We observed strong
ionospheric irregularities in the topside ionosphere during the storm’s main phase that were associated withstorm-enhanced density (SED) formation at mid-latitudes and further evolution of the SED plume to the polar
tongue of ionization (TOI). Daily ROTI maps derived from ground-based and LEO–GPS measurements show thepattern of irregularities oriented in the local noon–midnight direction, which is a signature of SED/TOI
development across the polar cap region. Analysis of the Swarm in situ plasma measurements revealed that,during the storm’s main phase, all events with extremely enhanced plasma densities (>106 el/cm3) in the polar cap
were observed in the Southern Hemisphere. When Swarm satellites crossed these enhancements, degradation of
GPS performance was observed, with a sudden decrease in the number of GPS satellites tracked. Our findings
indicate that polar patches and TOI structures in the topside ionosphere were predominantly observed in theSouthern Hemisphere, which had much higher plasma densities than the Northern Hemisphere, where SED/TOI
structures have already been reported earlier. LEO–GPS data (ROTI and topside TEC) were consistent with theseresults.

Keywords: Topside ionosphere irregularities; Geomagnetic storm; GPS; ROTI; SED; TOI; Polar patch
Figure 1*Corresponding author: Iurii Cherniak, tcherniak@ukr.net

LETTER 

Open Access

Localized field-aligned currents and 4-min TEC and groundmagnetic oscillations during the 2015 eruption of Chile’s
Calbuco volcano
Tadashi Aoyama*, Toshihiko Iyemori, Kunihito Nakanishi, Michi Nishioka, Domingo Rosales,
Oscar Veliz and Erick Vidal Safor
Earth, Planets and Space 2016, 68:148 DOI: 10.1186/s40623-016-0523-0
Received: 17 February 2016, Accepted: 11 August 2016, Published: 31 August 2016

Abstract
The Calbuco volcano in southern Chile erupted on April 22, 2015. About 2 h after the first eruption, a Swarm satellite passed
above the volcano and observed enhancement of small-amplitude (~0.5 nT) magnetic fluctuations with wave-packet
structure which extends 15° in latitude. Similar wave packet is seen at the geomagnetic
SW-B
conjugate point of the volcano. Just after the eruption, geomagnetic fluctuations with the°Nspectral peaks around the vertical acoustic resonance periods, 215 and 260 s, were also
20
observed at Huancayo Geomagnetic Observatory located on the magnetic equator. Besides
these observations, around 4-min, i.e., 175, 205 and 260 s, oscillations of total electron°°
60 W
40 ° W°
80 W
0content (TEC) were observed at global positioning system stations near the volcano. The
horizontal propagation velocity and the spatial scale of the TEC oscillation are estimated to be
720 m/s and 1600 km, respectively. These observations strongly suggest that the atmospheric°
20 S
waves induced by explosive volcanic eruption generate TEC variation and electric currents.
The Swarm observation may be explained as a manifestation of their magnetic effects
GPS stations°
observed in the topside ionosphere.
40 S

Keywords: Magnetic field; Magnetic fluctuation; Field-aligned current; Acoustic gravity
wave; Middle latitudes; Atmospheric wave; Ionospheric dynamo; Volcanic
eruption; Total electron content; Ionospheric disturbance*Corresponding author: Tadashi Aoyama, aoyama@kugi.kyoto-u.ac.jp
13

°
60 S

Figure 1

Swarm Science Results after Two Years in SpaceLETTER 

Open Access

Prompt penetration electric fields and the extreme topside ionospheric
response to the June 22–23, 2015 geomagnetic storm as seenby the Swarm constellation
Elvira Astafyeva*, Irina Zakharenkova and Patrick Alken
Earth, Planets and Space 2016, 68:152 DOI: 10.1186/s40623-016-0526-x
Received: 5 February 2016, Accepted: 20 August 2016, Published: 6 September 2016

Abstract
Using data from the three Swarm satellites, we study the ionospheric response to the intense geomagnetic storm of June 22–23, 2015. With theminimum SYM-H excursion of −207 nT, this storm is so far the second strongest geomagnetic storm in the current 24th solar cycle. A specificconfiguration of the Swarm satellites allowed investigation of the evolution of the storm-time ionospheric alterations on the day- and thenightside quasi-simultaneously. With the development of the main phase of the storm, a significant dayside increase of the vertical total electron
content (VTEC) and electron density Ne was first observed at low latitudes on the dayside. From ~22 UT of 22 June to ~1 UT of 23 June, thedayside experienced a strong negative ionospheric storm, while on the nightside an extreme enhancement of the topside VTEC occurred at
mid-latitudes of the northern hemisphere. Our analysis of the equatorial electrojet variations obtained from the magnetic Swarm data indicatesthat the storm-time penetration electric fields were, most likely, the main driver of the observed
ionospheric effects at the initial phase of the storm and at the beginning of the main phase. The
dayside ionosphere first responded to the occurrence of the strong eastward equatorial electricfields. Further, penetration of westward electric fields led to gradual but strong decrease of theplasma density on the dayside in the topside ionosphere. At this stage, the disturbance dynamo
could have contributed as well. On the nightside, the observed extreme enhancement of the Ne
and VTEC in the northern hemisphere (i.e., the summer hemisphere) in the topside ionosphere wasmost likely due to the combination of the prompt penetration electric fields, disturbance dynamo
and the storm-time thermospheric circulation. From ~2.8 UT, the ionospheric measurements fromthe three Swarm satellites detected the beginning of the second positive storm on the dayside,which was not clearly associated with electrojet variations. We find that this second storm might
be provoked by other drivers, such as an increase in the thermospheric composition.

Keywords: Swarm mission; Geomagnetic storm; Ionosphere; TEC; Electron density; Prompt
penetration electric field; EEJ/EEF; Counter electrojet; Topside ionosphere*Corresponding author: Elvira Astafyeva, astafyeva@ipgp.fr

14

Figure 1

Information for Contributors
General1.

Manuscripts should be submitted through the journal’s Editorial Manager system(https://www.editorialmanager.com/epsp/default.aspx).

2.

Only papers not previously published will be accepted, and the emphasis is on theoriginality of concept or observation.

3.

Four varieties of article types are available.
3.1“Full papers” without word limit.

3.2“Letters” have a limit of 11,000 words including legends for figures and tables. One
figure or table not exceeding one A4 page is counted as 500 words.

4.

3.3

“Frontier Letters” can be submitted upon invitation by the Editor-in-Chief.

3.4“Technical reports” describe the technical development of instrument or software.

Authors retain the copyright, and distribution of articles is free under cc-by license.

Technical
1.

Manuscripts should be written in English with double spacing.

2.

Each manuscript should be organized in the following order: title, authors’ names,
affiliations, abstract, key words, main text, acknowledgement, appendix, references, andsupplementary materials.

3.

The corresponding author should be clearly indicated.

4.

The abstract is limited to 300 words and should be informative and include principal
findings and conclusions. Please avoid abbreviations.

5.

The main text can have multiple free headings.

6.

High-resolution figures should be provided in the following format; PDF (preferred
format for diagrams), PNG (preferred format for photos or images), EPS ,TIFF, JPEG, BMP.

7.

References must follow the Springer Basic style.

Correspondence
If you have any questions, please contact editorial@earth-planets-space.com.

© EPS Steering Committee 2016 All rights reserved.
A Grant-in-Aid for Publication of Scientific Research Results (251001) from Japan
Society for the Promotion of Science is used for printing.

Contents
Special issue “Swarm science results after 2 years in space”. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nils Olsen, Claudia Stolle, Rune Floberghagen, Gauthier Hulot and Alexey Kuvshinov
In-flight scalar calibration and characterisation of the Swarm magnetometry package. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Lars Tøffner-Clausen, Vincent Lesur, Nils Olsen and Christopher C. Finlay
Impact of Swarm GPS receiver updates on POD performance . . . . . . . . . . Jose van den IJssel, Biagio Forte and Oliver Montenbruck
Swarm accelerometer data processing from raw accelerations to thermospheric neutral densities. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Christian Siemes, João de Teixeira da Encarnação, Eelco Doornbos, Jose van den IJssel, Jiří Kraus,
Radek Pereštý, Ludwig Grunwaldt, Guy Apelbaum, Jakob Flury and Poul Erik Holmdahl Olsen

Comparison between IRI and preliminary Swarm Langmuir probe measurements during the St. Patrick storm period. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Alessio Pignalberi, Michael Pezzopane, Roberta Tozzi, Paola De Michelis and Igino CocoCalibration and assessment of Swarm ion drift measurements using a comparison with a statistical convection model. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . R. A. D. Fiori, A. V. Koustov, D. H. Boteler, D. J. Knudsen and J. K. Burchill
First results from the Swarm Dedicated Ionospheric Field Inversion chain . . . . . . . . . . . . . . . . . . A. Chulliat, P. Vigneron and G. Hulot
Observations of high-latitude geomagnetic field fluctuations during St. Patrick’s Day storm: Swarm and SuperDARN
measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Paola De Michelis, Giuseppe Consolini, Roberta Tozzi and Maria Federica Marcucci
Unmodelled magnetic contributions in satellite-based models . . . . . . . . . . . Roberta Tozzi, Mioara Mandea and Paola De Michelis
Recent geomagnetic secular variation from Swarm and ground observatories as estimated in the CHAOS-6 geomagnetic fieldmodel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Christopher C. Finlay, Nils Olsen, Stavros Kotsiaros, Nicolas Gillet and Lars Tøffner-Clausen
Scale analysis of equatorial plasma irregularities derived from Swarm constellation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chao Xiong, Claudia Stolle, Hermann Lühr, Jaeheung Park, Bela G. Fejer and Guram N. Kervalishvili
Gravity field models derived from Swarm GPS data
. . . . . . . . . . . . . . . . . . . . . João Teixeira da Encarnação, Daniel Arnold, Aleš Bezděk, Christoph Dahle, Eelco Doornbos, Jose van den IJssel,
Adrian Jäggi, Torsten Mayer-Gürr, Josef Sebera, Pieter Visser and Norbert Zehentner

Toward more complete magnetic gradiometry with the Swarm mission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Stavros Kotsiaros
Determining polar ionospheric electrojet currents from Swarm satellite constellation magnetic data
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cecilie Drost Aakjær, Nils Olsen and Christopher C. Finlay
Sunlight effects on the 3D polar current system determined from low Earth orbit measurements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Karl M. Laundal, Christopher C. Finlay and Nils Olsen
The role of high-resolution geomagnetic field models for investigating ionospheric currents at low Earth orbit satellites. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Claudia Stolle, Ingo Michaelis and Jan Rauberg
GPS and in situ Swarm observations of the equatorial plasma density irregularities in the topside ionosphere. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Irina Zakharenkova, Elvira Astafyeva and Iurii Cherniak
A model of Earth’s magnetic field derived from 2 years of Swarm satellite constellation data
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nils Olsen, Christopher C. Finlay, Stavros Kotsiaros and Lars Tøffner-Clausen
A Swarm lithospheric magnetic field model to SH degree 80. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Erwan Thébault, Pierre Vigneron, Benoit Langlais and Gauthier Hulot
High-latitude ionospheric irregularities: differences between ground- and space-based GPS measurements during the2015 St. Patrick’s Day storm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Iurii Cherniak and Irina Zakharenkova
Localized field-aligned currents and 4-min TEC and ground magnetic oscillations during the 2015 eruption of Chile’s
Calbuco volcano. . . . . . . . Tadashi Aoyama, Toshihiko Iyemori, Kunihito Nakanishi, Michi Nishioka, Domingo Rosales, Oscar Veliz and Erick Vidal Safor
Prompt penetration electric fields and the extreme topside ionospheric response to the June 22–23, 2015 geomagnetic
storm as seen by the Swarm constellation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Elvira Astafyeva, Irina Zakharenkova and Patrick Alken

1
4
4

5
5
6
6
7
7
8
8

9
9
10
10
11
11
12
12
13

13
14

Cover image: Olsen, N. et al., Earth, Planets and Space 2016, 68:172 DOI: 10.1186/s40623-016-0546-6