2018 THEMIS SCIENCE NUGGETS

Magnetospheric convection in a twisted magnetotail

by T. Pitkänen
Shandong University, Weihai, China
Umeå University, Sweden

Introduction

The north-south component of the interplanetary magnetic field (IMF) carried by the solar wind plays an important role in generation of disturbances such as geomagnetic storms and substorms in the geospace. Much less is known of the effects caused by the IMF dawn-dusk component. Theoretical considerations, modeling and observations however suggest that under the influence of a nonzero dawn-dusk IMF, specifically the magnetotail may experience large-scale deformations. The magnetotail may get twisted from the symmetrical configuration including a rotation of the plasma sheet around the Sun-Earth axis and bending of closed magnetic field lines in the dawn-dusk direction. How this tail twisting affects plasma and magnetic flux transport, termed as magnetospheric convection, is still an open question. Previous studies addressing this question have mainly been based on ionospheric observations, but they suggest an existence of a hemispheric asymmetry in tail magnetospheric convection.

Here, we investigate the influence of a nonzero dawn-dusk IMF on tail magnetospheric convection by analyzing THEMIS observations during a time interval which exhibits clear signatures of a twisted magnetotail. During the time interval, the THEMIS satellites formed a tail-aligned constellation, which provides us a unique opportunity to study the twist effects using simultaneous measurements from different tail distances.

Figure 1. Solar wind and magnetotail observations on January 1-2, 2009 during the time interval of the tail twisting signatures. (a) IMF, (b-d) magnetic field (FGM), electric field (EFI) and two components of the ion velocity perpendicular to the magnetic field (ESA) for THEMIS-B, and (e-g) the same parameters, but for THEMIS-C. The green, yellow and red shading indicate intervals of stable tail B_y > 0 (positive twist), fluctuating tail B_y during transition, and stable tail B_y < 0 (negative twist), respectively.

Observations

Figure 1 shows IMF and THEMIS data from the time interval of interest on January 1-2, 2009. The panels from top to bottom display (a) the IMF, (b-d) magnetic field, electric field and two components of the ion velocity perpendicular to the magnetic field for THEMIS-B, and (e-g) the same parameters for THEMIS-C. The satellites were moving tail- and dawnward in the postmidnight sector: THEMIS-B farther away, XGSM ~ -10 to -21 R_E (R_E = Earth radius) and THEMIS-C closer to Earth, XGSM ~ -8 to -14 R_E during the color shaded intervals, respectively. In the top panel, one can see a gradual change in the direction of the dawn-dusk IMF component (B_y, green curve) from positive to negative and back to positive between 19:20 and 02:48 UT. The tail magnetic field B_y component measured by THEMIS-B appears to respond to this by a similar behavior with a time delay (green curve in Fig. 1b). Also, tail B_y at THEMIS-C mimics the IMF B_y behavior with a time delay after ~ 23 UT when the satellite exited the inner magnetosphere distances (green curve in Fig. 1e). The observed change in tail B_y with reversals is interpreted as a signature of a change in the tail twist mode (from a positive mode to negative and back to positive) as a response to the changing IMF. Thus, the time interval enables us to study the magnetotail under both twist modes, although the positive twist is interpreted to occur weaker than the negative twist.

Figure 2. The tail convection electric field components versus tail B_x for THEMIS-B earthward flow samples during (a-f) the interval of the negative twist (red subinterval) and (g-l) the positive twist (latter green subinterval in Figure 1). The electric field obtained directly from the EFI instrument and by using ion velocity (-Vi x B) are indicated, and their comparison confirms that the tail electric field appears mainly owing to the ExB drift. The black vertical lines indicate the median value of the electric field component for each hemisphere. Plus and minus mark the median electric field direction, and brackets around a sign indicate that there is significant variation in the data around zero.

The findings indicate that the plasma convection driven by the electric and magnetic fields is twist-mode dependent, as demonstrated by Figure 2 in the case of earthward ion flow samples measured by THEMIS-B: Whereas the Y component of the convection electric field (E_y) is generally positive (dawn-to-dusk) for both of the twist modes, E_z has clearly an opposite direction and there is an indication of a similar situation for E_x, although the component is subject to a major variation around zero during the negative twist. The measurements from the both magnetic hemispheres suggest that at least the E_y and E_z components preserve their orientation across the neutral sheet. The measurements by THEMIS-C support these results and suggest that a quasi-collinear electric field is measured throughout the tail where the tail twist influences irrespective of the tail distance from Earth. Figure 3 summarizes the results for the convection electric field in the GSM YZ plane, including tailward flow. For the positive twist and tailward flow samples there is some uncertainty in the results and thus two possible scenarios are shown.

Figure 3. Schematic illustration of the convection electric field in the plasma sheet in the GSM YZ plane when seen from the tail toward Earth as inferred from the results. For simplicity, cases of pure earthward or tailward flow are assumed. (a) and (b) are for the negative twist and (c) and (d) for the positive twist. (d’) The electric field proposed by the THEMIS-C yellow subinterval for the positive twist and tailward flow.

Figure 4 displays how the plasma flow is affected. Shown is the dawn-dusk ion velocity component perpendicular to the magnetic field for (a) earthward and (b) tailward flow samples measured by THEMIS-B during the negative twist. The distributions of the data points form clear patterns in which the velocity component tends to be in the opposite directions in the two magnetic hemispheres.

Figure 4. The dawn-dusk ion velocity component perpendicular to the magnetic field versus tail B_x measured by THEMIS-B for (a) earthward and (b) tailward flow samples during the interval of the negative twist. The perpendicular speed in the GSM XY plane for a flow sample is between 50 and 200 km/s. The color scale from blue to red measures the density of the data points in arbitrary units. The percentage in the right upper corner indicates the percentage of the data points lying (a) in the gray-shaded and (b) in the white quadrants, respectively.

Conclusion

These THEMIS observations indicate that a prevailing nonzero dawn-dusk IMF component can have an influence not only on the magnetotail configuration but also on magnetospheric convection. Whereas the response of the tail plasma sheet magnetic field to nonzero IMF B_y appears as a change of tail B_y to a collinear direction with IMF B_y (equivalent of bending or twisting of closed magnetic field lines), the response of the convection consists of a hemispheric asymmetry in the dawn-dusk flow component. Under the influence of IMF B_y in a twisted tail, the convection and the corresponding convection electric field can have such a configuration that the convection has an opposite dawn-dusk direction in the northern and southern plasma sheet. This yields for both earthward and tailward flows. The findings for the earthward flows match with the previous observations in the nightside ionosphere, which often show an interhemispheric asymmetry in plasma convection under nonzero IMF B_y conditions. The role of the tailward flows deserves further investigation.

The results of this study thus imply that a nonzero IMF B_y component can have a major influence on the plasma and magnetic flux transport in the Earth's magnetotail. In future work, we aim to study the flow asymmetry statistically to find out what is its significance on magnetotail dynamics in general. We plan to examine satellite data from several space missions including THEMIS, ESA's Cluster and ISAS’s/NASA's Geotail as well as the NASA’s new MMS mission.

Reference

Pitkänen, T., A. Kullen, Q. Q. Shi, M. Hamrin, A. De Spiegeleer, and Y. Nishimura, Convection electric field and plasma convection in a twisted magnetotail: A THEMIS case study 1-2 January 2009, J. Geophys. Res. Space Physics, 123, 7486-7497, doi:10.1029/2018JA025688, 2018.

Biographical Note

Timo Pitkänen is a Visiting Research Scientist at the Institute of Space Sciences, Shandong University, and a Researcher in the Space Physics Group at Umeå University. His current research is focused on the solar wind and interplanetary magnetic field influence on the nightside geospace.

Please send comments/suggestions to

Emmanuel Masongsong / emasongsong @ igpp.ucla.edu