Magnetospheric Dynamo Driving Large‐scale Birkeland Currents

Abstract. The spatial distributions of large-scale field-aligned Birkeland currents have been derived using magnetic field data obtained from the Iridium constellation of satellites from February 1999 to December 2007. From this database, we selected intervals that had at least 45% overlap in the large-scale currents between successive hours. The consistency in the current distributions is taken to indicate stability of the large-scale magnetosphere–ionosphere system to within the spatial and temporal resolution of the Iridium observations. The resulting data set of about 1500 two-hour intervals (4% of the data) was sorted first by the interplanetary magnetic field (IMF) GSM clock angle (arctan(By/Bz)) since this governs the spatial morphology of the currents. The Birkeland current densities were then corrected for variations in EUV-produced ionospheric conductance by normalizing the current densities to those occurring for 0° dipole tilt. To determine the dependence of the currents on other solar wind variables for a given IMF clock angle, the data were then sorted sequentially by the following parameters: the solar wind electric field in the plane normal to the Earth–Sun line, Eyz; the solar wind ram pressure; and the solar wind Alfvén Mach number. The solar wind electric field is the dominant factor determining the Birkeland current intensities. The currents shift toward noon and expand equatorward with increasing solar wind electric field. The total current increases by 0.8 MA per mV m−1 increase in Eyz for southward IMF, while for northward IMF it is nearly independent of the electric field, increasing by only 0.1 MA per mV m−1 increase in Eyz. The dependence on solar wind pressure is comparatively modest. After correcting for the solar dynamo dependencies in intensity and distribution, the total current intensity increases with solar wind dynamic pressure by 0.4 MA/nPa for southward IMF. Normalizing the Birkeland current densities to both the median solar wind electric field and dynamic pressure effects, we find no significant dependence of the Birkeland currents on solar wind Alfvén Mach number.

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Comparison of the observed dependence of large-scale Birkeland currents on solar wind parameters with that obtained from global simulations

Annales Geophysicae ◽

10.5194/angeo-29-1809-2011 ◽

2011 ◽

Vol 29 (10) ◽

pp. 1809-1826 ◽

Cited By ~ 17

Author(s):

H. Korth ◽

L. Rastätter ◽

B. J. Anderson ◽

A. J. Ridley

Keyword(s):

Magnetic Field ◽

Solar Wind ◽

Large Scale ◽

Coupled Model ◽

Total Current ◽

Modeling Framework ◽

Rate Of Increase ◽

Solar Wind Parameters ◽

The University ◽

Birkeland Currents

Abstract. Spatial distributions of the large-scale Birkeland currents derived from magnetic field data acquired by the constellation of Iridium Communications satellites have been compared with global-magnetosphere magneto-hydrodynamic (MHD) simulations. The Iridium data, spanning the interval from February 1999 to December 2007, were first sorted into 45°-wide bins of the interplanetary magnetic field (IMF) clock angle, and the dependencies of the Birkeland currents on solar wind electric field magnitude, Eyz, ram pressure, psw, and Alfvén Mach number, MA, were then examined within each bin. The simulations have been conducted at the publicly-accessible Community Coordinated Modeling Center using the University of Michigan Space Weather modeling Framework, which features a global magnetosphere model coupled to the Rice Convection Model. In excess of 120 simulations with steady-state conditions were executed to yield the dependencies of the Birkeland currents on the solar wind and IMF parameters of the coupled model. Averaged over all IMF orientations, the simulation reproduces the Iridium statistical Birkeland current distributions with a two-dimensional correlation coefficient of about 0.8, and the total current agrees with the climatology averages to within 10%. The total current for individual events regularly exceeds those computed from statistical distributions by factors of ≥2, resulting in larger disparities between observations and simulations. The simulation results also qualitatively reflect the observed increases in total current with increasing Eyz and psw, but the model underestimates the rate of increase by up to 50%. The equatorward expansion and shift of the large-scale currents toward noon observed for increasing Eyz are also evident in the simulation current patterns. Consistent with the observations, the simulation does not show a significant dependence of the total current on MA.

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Comparison of large-scale Birkeland currents determined from Iridium and SuperDARN data

Annales Geophysicae ◽

10.5194/angeo-24-941-2006 ◽

2006 ◽

Vol 24 (3) ◽

pp. 941-959 ◽

Cited By ~ 20

Author(s):

D. L. Green ◽

C. L. Waters ◽

B. J. Anderson ◽

H. Korth ◽

R. J. Barnes

Keyword(s):

Electric Field ◽

Large Scale ◽

Radar Data ◽

Space Environment ◽

Near Earth Space ◽

Radar Network ◽

High Latitude Ionosphere ◽

Ionospheric Electric Field ◽

Birkeland Currents

Abstract. The Birkeland currents, J||, electrically couple the high latitude ionosphere with the near Earth space environment. Approximating the spatial distribution of the Birkeland currents may be achieved using the divergence of the ionospheric electric field, , assuming zero conductance gradients such that . In this paper, electric field data derived from the Super Dual Auroral Radar Network (SuperDARN) are used to calculate , which is compared with the Birkeland current distribution derived globally from the constellation of Iridium satellites poleward of 60° magnetic latitude. We find that the assumption of zero conductance gradients is often a poor approximation. On the dayside, in regions where the SuperDARN electric field is constrained by radar returns, the agreement in the locations of regions of upward and downward current between and J|| obtained from Iridium data is reasonable with differences of less than 3° in the latitudinal location of major current features. It is also shown that away from noon, currents arising from conductance gradients can be larger than the component. By combining the estimate in regions of radar coverage with in-situ estimates of conductance gradients from DMSP satellite particle data, the agreement with the Iridium derived J|| is considerably improved. However, using an empirical model of ionospheric conductance did not account for the conductance gradient current terms. In regions where radar data are sparse or non-existent and therefore constrained by the statistical potential model the approximation does not agree with J|| calculated from Iridium data.

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Large-scale Birkeland currents in the dayside polar region during strongly northward IMF: A new Birkeland current system

Journal of Geophysical Research Atmospheres ◽

10.1029/ja089ia09p07441 ◽

1984 ◽

Vol 89 (A9) ◽

pp. 7441-7452 ◽

Cited By ~ 214

Author(s):

T. Iijima ◽

T. A. Potemra ◽

L. J. Zanetti ◽

P. F. Bythrow

Keyword(s):

Large Scale ◽

Polar Region ◽

Current System ◽

Birkeland Currents

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Extended period of polar cap auroral display: auroral dynamics and relation to the IMF and the ionospheric convection

Annales Geophysicae ◽

10.1007/s00585-995-0854-9 ◽

1995 ◽

Vol 13 (8) ◽

pp. 854-862 ◽

Cited By ~ 5

Author(s):

V. G. Vorobjev ◽

S. V. Leontyev ◽

Ya. I. Feldstein

Keyword(s):

Large Scale ◽

Interplanetary Space ◽

Extended Period ◽

Auroral Oval ◽

Polar Cap ◽

Substorm Activity ◽

Convection Patterns ◽

Northern And Southern Hemispheres ◽

Birkeland Currents ◽

The Imf

Abstract. An unusually extended period (5 h) of polar cap auroral display on 3 August 1986 is examined. Auroras have been investigated using ground-based data as well as measurements from the IMP-8 spacecraft in interplanetary space and simultaneous observations from the polar-orbiting satellites Viking and DE-1 in the northern and southern hemispheres, respectively. It is found that visible Sun-aligned arcs are located inside the transpolar band of the θ-aurora observed from the satellite in ultraviolet wavelengths. The transpolar band can contain several Sun-aligned arcs that move inside the band toward the morning or evening side of the auroral oval independent of the direction of the band movement. Intensifications of polar cap auroras with durations of up to about 30 min are observed. No change has been found in either IMF parameters or substorm activity that can be related to these intensifications. The θ-aurora occurred during a 2-h period when the B z-component of the IMF was negative. A tendency is noted for dawnward (duskward) displacement of the transpolar band when By>0 (By<0) in the southern hemisphere. Simultaneous observations of auroral ovals during interplanetary Bz<0, By<0 and Bx>0 in both hemispheres and convection patterns for Bz<0 and By<0 have been displayed using satellite and ground-based measurements. It was found that the transpolar band of the <theta>-aurora in the sunlit hemisphere was situated in the region of large-scale downward Birkeland currents.

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Statistical Birkeland current distributions from magnetic field observations by the Iridium constellation

Annales Geophysicae ◽

10.5194/angeo-26-671-2008 ◽

2008 ◽

Vol 26 (3) ◽

pp. 671-687 ◽

Cited By ~ 99

Author(s):

B. J. Anderson ◽

H. Korth ◽

C. L. Waters ◽

D. L. Green ◽

P. Stauning

Keyword(s):

Magnetic Field ◽

Statistical Analysis ◽

Large Scale ◽

Field Observations ◽

Marked Contrast ◽

Data Set ◽

Current Distributions ◽

Magnetometer Data ◽

Pure States ◽

Birkeland Currents

Abstract. We present a statistical analysis of Birkeland currents derived from Iridium magnetometer data acquired in the Northern Hemisphere to determine the dependence of large-scale currents on the interplanetary magnetic field (IMF) direction. Because the Iridium data span nearly seven years, we can restrict analysis to only those intervals with stable currents. We used image comparison to quantify the consistency between successive one-hour current distributions and selected 1550 two-hour intervals, 5% of the data, for analysis. Results include: no statistically significant average currents are present poleward of 80° during southward IMF; Region-2 currents are weak and confined to latitudes >65° during northward IMF; there is marked contrast between currents for northward and southward IMF but the evolution of the patterns is continuous with IMF rotation. The directions of flows inferred from the most poleward currents are more consistent with theoretical expectations of transport away from magnetopause reconnection than previous results. We attribute the differences to the restriction in this analysis to intervals having relatively stable distributions of current so that the data set corresponds more nearly to pure states of the system.

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