Introduction

The IMF clock angle (θ_CA) has been known to lead to the relative displacement of the auroral locations in the northern and southern hemispheres, as well as to a twist of the entire magnetotail configuration. However, no direct comparison has been made between the IMF-induced twisting configuration in the magnetotail and the resulting conjugate auroral displacement in both hemispheres. Here we present a case study providing strong evidence for a twisted near-Earth tail configuration that is responsible for the time sequence of conjugate auroral features associated with relative interhemispheric displacement during a weak substorm, using the IMF measurements obtained from the THEMIS satellites, the near-Earth tail magnetic field measurements from the Cluster satellites, and the ground-based optical auroral measurements from all-sky cameras (ASC) at the Iceland-Syowa geomagnetic conjugate points.

Results

Recently, Motoba et al. [2010] have presented the interhemispheric conjugate auroral features during a weak substorm event on 21 September 2009, using the ground-based ASC observations at the northern and southern geomagnetic conjugate points: Tjörnes (TJO, 66.2°N, 342.9°E) in Iceland and Syowa Station (SYO, 69.0°S, 39.6°E) in Antarctica. In our previous study, we revealed that the time-dependent motion of the relative MLT displacement of the conjugate points deduced from a detailed comparison of both ASC images was strongly controlled by the varying IMF B_y polarity. During this event, four Cluster spacecraft were located in the postmidnight sector 11-14 R_E downtail, in magnetic conjunction with the ground-based optical observations, as shown in Figure 1. Such an ideal location of Cluster allows us to confirm how the in situ tail field configuration varied during the relative MLT displacement of the conjugate aurora in the ionosphere.

Figure 1. Locations of four Cluster spacecraft in the (a) XZ GSM plane and (b) XY GSM plane, for the interval from 0024 UT (open circles) to 0100 UT (solid circles) on 21 September 2009. For reference, T96 magnetic field lines traced from the four Cluster positions at 0024 UT to both ionospheres are indicated by colored solid lines. (c) Cluster footprints mapped onto the Northern Hemisphere.

Click each image to enlarge.

A manifestation of the IMF-induced magnetotail twisting is believed to appear mainly in the y component of the in situ tail field. In order to identify the magnetotail twisting signature at Cluster, Figure 2a displays time series of the y component of the residual magnetic field (ΔB_y, black lines) at each Cluster satellite obtained by subtracting the modeled field (T96 and IGRF) from the measured field, together with the time-shifted variation of the IMF θ_CA data (gray lines) measured by THEMIS B. We found that both ΔB_y field and time-shifted IMF θ_CA variations were correlated moderately well. Especially, for the time interval of about 30 min from the auroral breakup onset (0024 UT, marked by vertical dotted line), the maximum correlation coefficients (0.56 to 0.61) between the ΔB_y field at Cluster and IMF θ_CA were obtained by delaying the IMF data by 52±1 min from the time at which the IMF reached the subsolar magnetopause. The longer time delay could correspond to the time scale required for the IMF-induced magnetotail reconfiguration at the Cluster locations.

Figure 2. y components (ΔBy, black) of residual magnetic field at four Cluster spacecraft, together with time-shifted IMF θCA (gray). Each maximum correlation coefficient (Max CC) between ΔBy and IMF θCA and its corresponding time shift (Δt) are shown at the upper right. (b) Latitudinal and (c) MLT displacements of the northern footprint of SYO relative to TJO deduced from conjugate auroral images. Red and blue lines represent relative displacements of TJO-SYO conjugate points inferred from Østgaard et al.'s [2005] function (Østgaard05) and those from the T96 model using the time-shifted (51 min) IMF values, respectively.

Click each image to enlarge.

Figures 2b and 2c show the temporal variations of the latitudinal and MLT displacements (gray solid circles) of the northern footprint of SYO relative to TJO during substorm, respectively, which are deduced from the ground-based optical images [for details see Motoba et al., 2010]. We found that the relative MLT displacement of the conjugate points varied roughly coincident with the ΔB_y variations at Cluster. In addition, the correlation coefficient of the observed relative MLT displacement with the results obtained via the Østgaard et al.'s [2005] empirical function (red line) and the T96 model (blue line) peaked when the inputted IMF data were delayed by 51 min from the subsolar magnetopause. It is also emphasized that the time shift of 51 min applied to the IMF data was consistent with that obtained from the correlation with the in situ ΔB_y field at Cluster.

Conclusion

We have presented that the in situ ΔB_y field variations at the Cluster locations conjugate to the ground-based ASC measurements were correlated moderately well with the time-shifted IMF θ_CA variation. The maximum correlation coefficients between the ΔB_y field at Cluster and IMF θ_CA were obtained by delaying the IMF data by 52±1 min from the time at which the IMF reached the subsolar magnetopause. Moreover, the IMF θ_CA related ΔB_y variation at Cluster coincided well with the temporal variation of relative MLT displacement of the conjugate aurora observed at the TJO-SYO pair. In our case, the time lag at the maximum correlation with the IMF variation was common in both the in situ ΔB_y variation at Cluster and the relative MLT displacement of the conjugate aurora in the ionosphere. Therefore, these results suggest that the relative MLT displacement of the conjugate auroral locations in the nightside ionosphere during a weak substorm is directly controlled by a twist of the central near-Earth tail field that has relatively longer reconfiguration time to IMF changes.

Source

Motoba, T., K. Hosokawa, Y. Ogawa, N. Sato, A. Kadokura, S. C. Buchert, and H. Rème (2011), In situ evidence for interplanetary magnetic field induced tail twisting associated with relative displacement of conjugate auroral features, J. Geophys. Res., 116, A04209, doi:10.1029/2010JA016206.

Biographical Note

Tetsuo Motoba is a postdoctoral fellow at the National Institute of Polar Research. His current research interest focuses on the relationship between aurora and magnetospheric dynamics.

Please send comments/suggestions to

Emmanuel Masongsong / emasongsong@igpp.ucla.edu