Want a more indepth look at how the solar wind magnetic field’s By component can impact the magnetosphere? Then you have come to the right place. Here is a brief update from one of our research projects

 

The Interplanetary Magnetic Field (IMF) can have an east-west, or By, component. This magnetic field component can produce a range of interhemispheric asymmetries, including interhemispheric asymmetries in the field-aligned currents (or Birkeland currents) at the polar regions. Several steps lead to this asymmetry. First, the location of magnetic reconnection occurs at high latitude instead of in the equatorial region. If By<0 when the IMF By component dominates, reconnection occurs pre-noon for the northern hemisphere and post-noon for the southern hemisphere. If instead By>0, reconnection occurs post noon in the northern hemisphere and pre-noon in the south. Second, the asymmetry in the reconnection pattern leads to a distortion of the two-cell plasma convection pattern in the polar ionosphere. This distortion results in overlapping Birkeland currents in the cusp region such that for By>0 nT, the cusp’s throat opens towards the dusk in the northern hemisphere and towards the dawn in the southern hemisphere. This pattern reverses for IMF By<0 nT. The shearing of the dayside two cell convection is shown in Figure 1 where the top row shows the pattern for IMF By>0 and the bottom row display the configuration for IMF By<0. The left column displays the pattern for the northern hemisphere and the right gives the southern hemisphere pattern.

Figure 1 Expected shear in Region-1 Birkeland currents. Adapted from Strangeway et al. (2000), following Cowley (1981) and Burch and Reiff (1985). The top shows the shear for IMF B_y > 0, while the bottom row corresponds to IMF B_y < 0. The current pattern is shown using a “glass Earth” projection, with dawn on the right-hand side of each plot for both northern and southern hemispheres. Noon is towards the top of each plot. Open circles indicate currents flowing out of the ionosphere, mainly on the dusk-side, with Xs indicating currents flowing into the ionosphere, mainly on the dawn-side, as expected for Region-1 currents. The contours show the associated flow pattern, and the dashed line the nominal open-closed field line boundary.

Observations of this shear in the Birkeland currents for positive and negative orientation of IMF By have been made by the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) mission. Anderson et al. [2008] provide examples of both singular events as well as statistical results. Furthermore, most magnetospheric models take into account the direction of IMF By, but it has not yet been shown that these models can reproduce the shear in the Birkeland currents in the cusp simultaneously in both hemispheres for By<0 nT and By>0 nT.

 

On 8 October 2012, a magnetic storm occurred. During this storm, the IMF Bz component was strongly negative for most of the storm and the IMF By component switched from strongly positive after the first approximately 6 hours to strongly negative for about 8 hours. Thus, this storm presents an excellent opportunity to examine the asymmetry in the Birkeland current observations and the results from the Space Weather Modeling Framework (SWMF) MHD model.

 

Figures 1 and  2 show the IMF, solar wind plasma, geomagnetic indices, northern and southern hemisphere AMPERE observations of the Birkeland-like currents (i.e., these currents are perpendicular to the ionosphere and not true field-aligned currents), and the SWMF model results of the Birkeland currents. The top panel in the left column displays the IMF components, including the Bz component in green and the By component in blue. The two panels below the IMF show the solar wind velocity vector and the solar wind dynamic pressure. The start of the storm occurs with the sharp jump in the dynamic pressure. The second panel from the bottom in the left column presents the SuperMAG auroral upper electrojet (SMU) (black curve) and SuperMAG auroral lower electrojet (SML) (red curve) indices, and in the bottom panel is the SuperMAG ring current index. The later index demonstrates that this event is a moderate storm with an SMR value of about – 100 nT. The four panels on the right side of the figure display the Birkeland currents looking down upon the northern magnetic pole. The color bar indicates the current density. The left columns are the observations from the AMPERE mission, and the right columns are the results from the SWMF MHD model. The top row shows the current pattern in the northern hemisphere, and the bottom row shows the currents from the southern hemisphere. Note that the southern hemisphere currents are shown as a glass earth projection where the observer is looking through a glass earth at the current pattern. In each panel, the magnetic noon (towards the Sun) is located at the top; magnetic midnight is at the bottom, dawn is on the right, and dusk is on the left. The black dashed circles indicate 10° magnetic latitudes from 80° MLat down to 40° MLat.

The panels on the right side of Figure 1 show the Birkeland currents 23:50 UT on 8 October 2012 when the IMF Bz component is strongly negative (-14 nT) and the IMF By component is positive (~7 nT). In the northern hemisphere, the AMPERE observations show the blue downward currents (into the ionosphere) overlapping the red upward currents at about 75° MLat in the noon sector such that the throat of the cups opens towards dusk. Although the shear in the currents is not as pronounced, the opposite occurs in the southern hemisphere AMPERE observations at -80° MLat in the noon sector, but there appears to be an arc of downward current at higher and lower latitudes with respect to the red upward currents. The same shear in the Birkeland currents is also present in the northern hemisphere SWMF results at 75° MLat in the noon sector. However, the shear in the Birkeland currents is more apparent in the south at -75° MLat in the noon sector than the AMPERE observations demonstrating that the throat of the cusp opens towards the dawn. Furthermore, in the southern hemisphere, the SWMF Birkeland current appears to be significantly larger in magnitude.

 

 

Figure 2 Ionospheric Birkeland current at 2350 UT on 8 October 2012. Left Column: IMF (top panel), solar wind velocity (second panel), dynamic pressure (third panel), SMU and SML indices (fourth panel), and SMR index (bottom panel). Middle Column: Northern (top panel) and southern (bottom panel) hemisphere AMPERE observations of the Birkeland-like currents. Right column: the SWMF model results of the Birkeland currents with the northern hemisphere in the top panel and the southern hemisphere in the bottom panel.

 

Figure 3 Ionospheric Birkeland current at 2350 UT on 8 October 2012. This figure has the same format as Figure 1.

Figure 2 has the same format as Figure 1 and displays the reverse Birkeland current pattern in the noon sector after the IMF By component has rotated from approximately 7 nT to -8 nT. In the Northern hemisphere, the AMPERE observations show the red upward currents overlapping the blue downward currents at about 78° MLat in the noon sector such that the throat of the cups opens towards the dawn. The opposite pattern occurs in the southern hemisphere AMPERE observations at -80° MLat in the noon sector, where the throat of the cups opens towards dusk. A similar shear in the Birkeland currents is also present in the northern hemisphere SWMF results at 75° MLat in the noon sector. However, the upward red Birkeland currents are not as clear. The shear in the Birkeland currents is less pronounced in the southern at -75° MLat in the noon sector than the AMPERE observations. Similar to Figure 1, in both the northern and southern hemispheres, the SWMF Birkeland currents are significantly larger in magnitude than the AMPERE currents.

 

In summary, the AMPERE Birkeland-like currents demonstrate that the throat of the cusp opens in the opposite direction in the northern and southern hemisphere during strong IMF By conditions and this pattern reverse when the IMF By component flips. Furthermore, the SWMF MHD model can roughly reproduce this shear in the Birkeland currents and the reversal in the Birkeland currents when the By changes direction. However, the SWMF does not precisely reproduce the cusp Birkeland current pattern, and in most cases, the magnitude of the Birkeland currents is more prominent in the SWMF model than in the observations. Finally, comparing the AMPERE and SWMF of the Birkeland current pattern in the dusk, midnight, and dawn sectors shows that the SWMF model’s results poorly reproduce the ionospheric current pattern. Why this is the case is not clear at this time.

 

 

References

Anderson, B. J., Korth, H., Waters, C. L., Green, D. L., and Stauning, P. (2008). Statistical Birkeland current distributions from magnetic field observations by the Iridium constellation. In Annales Geophysicae, Vol. 26, No. 3, pp. 671-687. No. 3, pp. 671-687.

 

Burch, J. L.,. and P. H. Reiff (1985), Field-aligned currents and ion convection at high altitudes, Adv. Space Res., 5, 23-40. Doi: 10.1016/0273-1177(85)90113-9.

 

Cowley, S. W. H. (1981). Magnetospheric asymmetries associated with the y-component of the IMF, Planet. Space Sci., 29, 79-96. Doi: 10.1016/0032-0633(81)90141-0.

 

Strangeway, C. T. Russell, C. W. Carlson, J. P. McFadden, R. E. Ergun, M. Temerin, D. M. Klumpar, W. K. Peterson, and T. E. Moore (2000), Cusp field-aligned currents and ion outflows, J. Geophys. Res., 105, 21,129-21,142. Doi: 10.1029/2000JA900032.