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Downloaded from Ftp://Spdf.Gsfc Laundal et al. Earth, Planets and Space (2016) 68:142 DOI 10.1186/s40623-016-0518-x FULL PAPER Open Access Sunlight efects on the 3D polar current system determined from low Earth orbit measurements Karl M. Laundal1,2* , Christopher C. Finlay3 and Nils Olsen3 Abstract Interaction between the solar wind and the Earth’s magnetosphere is associated with large-scale currents 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, are used to co-estimate poloidal and toroidal parts of the magnetic disturbance field, represented in magnetic apex coordi- nates. The use of apex coordinates reduces effects of longitudinal and hemispheric variations in the Earth’s main field. We present global currents from both hemispheres during different sunlight conditions. The results show that the Bir- keland currents vary with the conductivity, which depends most strongly on solar EUV emissions on the dayside and 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 and the 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 ground magnetometers, is largely canceled by the horizontal closure of the Birkeland currents. Keywords: Polar ionospheric currents, Birkeland currents, Equivalent currents, Apex coordinates, LEO magnetic field Introduction the field lines are almost radial, this current can be inter- Te ionospheric current system can be decomposed into preted as the divergence-free component of the horizontal horizontal and field-aligned (Birkeland) currents. Te current. Te magnetic fields associated with curl-free hor- former is often further decomposed into Hall and Ped- izontal currents and the Birkeland currents cancel (Fuku- ersen currents, which are defined relative to the convec- shima 1994; Vasyliunas 2007, and references therein). tion electric field in the ionosphere. It is not possible to Te global average high-latitude equivalent current separate these current components using only ground during active geomagnetic conditions flows in two cells, magnetometers. Instead, one can estimate an equivalent which meet in the polar cap (Vestine et al. 1947; Friis- current, which is the two-dimensional current, flowing Christensen and Wilhjelm 1975; Weimer et al. 2010; at some fixed height, which corresponds to the observed Laundal et al. 2016). Te pattern resembles reverse iono- magnetic field disturbances (e.g., Chapman and Bartels spheric convection during similar conditions, with some 1940; Amm 1997). On ground at high latitudes, where clear differences: Te current in the polar cap is on aver- age much more tilted, toward dawn on the dayside, with respect to the noon–midnight meridian than the convec- *Correspondence: [email protected] tion streamlines. Te current cell at dawn is also stronger 1 Birkeland Centre for Space Science, University of Bergen, Allegt. 55, Bergen, Norway compared to the dusk cell than what is observed in the Full list of author information is available at the end of the article convection. Vasyliunas (1970) explained this difference as © 2016 The Author(s). 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. Laundal et al. Earth, Planets and Space (2016) 68:142 Page 2 of 19 a mismatch between the Hall current and the observed field on ground is perpendicular to that associated with divergence-free currents. Friis-Christensen and Wilhjelm Birkeland currents in space. (1975) showed that the equivalent current is more similar In this study, we present global statistical patterns of to the convection pattern during summer conditions, and both the horizontal equivalent current and the Birke- less so during winter. land currents, estimated using magnetic field measure- From satellites at sufficiently low altitudes, the associ- ments from the CHAMP and Swarm satellites, without ated magnetic field is readily detectable. It was magnetic any assumptions about conductivity. Te calculations of field measurements from ∼800 km altitude, using the the equivalent currents and Birkeland currents are based TRIAD satellite, that enabled Iijima and Potemra (1978) on a decomposition of the field into poloidal and toroidal to reveal the large-scale Region-1 (R1) and Region-2 components (Backus 1986; Olsen 1997; Weimer 2001). (R2) Birkeland current systems, which is prevalent dur- We use the magnetic apex coordinates systems defined ing geomagnetic active conditions. Te R1 currents are by Richmond (1995), which are non-orthogonal since located close to the polar cap boundary, the regions at they take the non-dipole terms in the terrestrial mag- polar latitudes which are threaded by field lines con- netic field into account. Te currents are well organized nected to the interplanetary magnetic field (IMF). in these coordinates, and therefore, fewer parameters Tey flow upward at the dusk side of the polar cap and are required to describe them. Te results also become downward at dawn. Te R2 currents are located equa- more invariant with respect to spatial (longitudinal and torward of the R1 currents and have opposite polari- hemispheric) and temporal variations in the Earth’s field, ties. Te large-scale Birkeland current system has later which means that observed hemispheric and longitudinal been measured and characterized by several authors. variations can be interpreted independently of local field Friis-Christensen et al. (1984) used ground magnetic structures (e.g., Laundal and Gjerloev 2014). field measurements in combination with assumptions Te technique is presented in detail in the next section. on ionospheric conductivity (Kamide et al. 1981) to cal- “Field and current variations with sunlight conditions” culate maps of the high-latitude electric field and Bir- section contains current patterns at high latitudes, sorted keland currents. Papitashvili et al. (2002) and Weimer according to sunlight conditions during relatively strong (2001) used satellite data, from the Ørsted and Magsat solar wind driving. Te results are discussed in “Discus- satellites and from Dynamics Explorer 2, respectively, sion” section, and “Conclusions” section concludes the to develop spherical harmonic models of the Birkeland paper. currents, resolving variations with seasons and the ori- entation of the IMF. He et al. (2012) used CHAMP data Technique to develop a global model of Birkeland currents using We use vector magnetic field data collected by low Earth empirical orthogonal functions. Juusola et al. (2014) orbit satellites from the Swarm constellation (Novem- also used CHAMP measurements to study global Bir- ber 2013–September 2015) and from CHAMP (August keland currents, with the spherical elementary current 2000–September 2010), sampling one vector field datum technique (Amm 1997). With the use of relatively crude every 30 s. Te latest update of the CHAOS geomagnetic but numerous measurements of the magnetic field from field model (Finlay et al. 2015), CHAOS-5, is used to the constellation of ≈70 commercial Iridium satellites, obtain estimates of the time-dependent field originating global maps of the Birkeland currents are now available in Earth’s core, the static lithospheric field and the large- at high cadence (10 min), through the Active Magneto- scale magnetospheric field, which are subtracted from sphere and Planetary Electrodynamics Response Exper- the observations leaving a residual that includes the sig- iment (AMPERE; Anderson et al. 2000; Waters et al. nature of the polar current systems of interest. Te vector 2001). field magnetic data were rotated from the magnetometer Laundal et al. (2015) used the magnetic field and cur- frame to the geographic frame using Euler angles co- rents from AMPERE to compare observations in space estimated during the construction of the CHAOS-5 field with equivalent currents measured with ground mag- model. netometers. Tey found that sunlight conditions strongly Te magnetic field perturbation vectors used in this r 6615 affect the equivalent current and how it relates to the Bir- study were measured at radii between = km and r 6902 keland current. In darkness in the polar cap, the equiva- = km (geodetic heights between 248 and 546 km). lent current measured from ground tends to align with At these altitudes, the satellites fly above most of the hor- the negative ionospheric closure of the Birkeland cur- izontal ionospheric currents, below the magnetospheric rents, consistent with the actual horizontal current there currents, and through the currents connecting
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