Introduction

Substorm events are periods lasting around 2-3 hours during which energy originating from the Sun enters inside the Earth's magnetosphere and is explosively dissipated. Substorms are known to be initiated in the tail of the magnetosphere, close to the Earth, at around 10 Earth's radii in the nightside and propagate tailward. In February 2007, the STEREO-B spacecraft passed through the distant tail of the magnetosphere which allowed searching for substorm effects at 1,500,000 km from the Earth. Furthermore, during this period, ground stations were operating in the frame of the THEMIS multi-satellite missions and provided a precise timing of the onset of substorms as they are linked to the break-up of auroral forms in the Earth's atmosphere/ionosphere.

Figure 1. From top to bottom, the variation of the AE index, of the Bz component of the Interplanetary Magnetic Field (IMF) and of the solar wind velocity. The fourth and fifth panels show the energy spectrograms of protons measured in solar wind onboard STEREO-A and similar measurements performed onboard STEREO-B on its travel away from the Moon.

Click each image to enlarge.

Observations

In Figure 1, we compare the strength of the electric current flowing in the Earth's ionosphere, the auroral electrojets (AE), with the changes of the interplanetary magnetic field (Bz) and with the velocity of the solar wind (SW bulk velocity). The figure also displays the color coded energy of the solar wind measured onboard the two STEREO spacecraft. STEREO-A is located inside the solar wind, ahead of the Earth, while the STEREO-B spacecraft skims behind the Earth, along the tail of the magnetosphere at distances ranging from 200 to 350 RE. As shown in Figure 1 the activity of the electrojet is enhanced by high-speed solar wind and varying interplanetary magnetic field, Bz. At the end of the period, the electrojet is reduced and the solar wind velocity is small. It is during that period the response of the far tail to magnetic activity was studied.

Figure 2. From 0100 to 1900 UT on 21 February 2007: the variations of the AE index, the Bx component of the magnetic field recorded at XGSM = -255 RE onboard STEREO-B, and the electron density measured by the Solar Wind Electron Analyzer (SWEA) on STEREO-B. The three vertical dashed lines indicate the onset of AE enhancements.

Click each image to enlarge.

Figure 2 provides a comparison between the variation of the electric current flowing in the Earth's ionosphere (AE) with the changes of the magnetic field and the plasma density recorded at 255 RE from the Earth's in the distant tail of the magnetosphere. Three main AE increases are detected, around 03:40, 08:30 and 14:10 UT. During major disturbances, the ionospheric current can be 10 times higher, so these changes are particularly weak. The modifications of the far tail are slightly delayed in time with respect to the ionospheric events. The duration of the far tail perturbations is almost the same as that of the electrojet enhancements. During high-density episodes the satellite is outside the magnetosphere (inside the magnetosheath), in flowing solar plasma, while during low-density periods, the satellite is located inside the magnetosphere.

Figure 3. Keogram (magnetic latitude-time) giving the latitudinal variation of the auroral luminosity recorded at INUVIK. These "white light" data are from THEMIS ground based observatories. A narrow band of auroral luminosity is drifting equatorward from 0800 to 0828 UT on 21 February 2007 during the growth phase of the substorm. At 0828 UT the aurora breaks up.

Click each image to enlarge.

Use of the ground-based THEMIS network of ground stations allowed us to catch the exact time of the auroral break-up, occurring in coincidence with the second event. The corresponding data are displayed in Figure 3. The time of the auroral onset, measured in the high latitude station INUVIK, was 08:28 UT when the aurora broke up and auroral forms quickly propagated northward with visible intensifications at 0828 UT, 0833 UT and 0841 UT. This represents a clear onset of a substorm associated with the minor AE enhancement seen at 0828 UT in Figure 2. This substorm onset preceded the exit of the STEREO-B satellite into the magnetosheath at 0841:50 UT by less than 14 min. Assuming that the two events are related (Figure 2), we find a propagation velocity of about 1800 km/s for the information to travel from about -20 RE, where the substorm is expected to be initiated, to the STEREO-B location at X = -255 RE.

Conclusion

The large-scale dynamic evolution of the magnetotail in relation to substorms is most commonly understood and described in terms of magnetic reconnection, neutral line formation, and plasmoid ejection into the distant tail, as well as current disruption and diversion in the inner tail. We have presented a case study of the dynamic changes seen in the far tail (255 RE) related to enhancement of weak to very weak geomagnetic activity (AE < 100 nT). STEREO-B was located close to the boundary separating the lobe from the boundary layer/magnetosheath. During a 24 h period, three very weak auroral electrojet perturbations were surprisingly well-correlated with motions of the far tail. The spacecraft passed from the lobe to the boundary layer or to the magnetosheath, very shortly after each AE perturbation. These boundary motions can hardly be related to a plasmoid, as a widening of the tail is expected from such a high-pressure structure. Indeed plasmoids lead a satellite located in the distant magnetosheath to pass inside the lobe, i.e., the reverse motion of what is observed for the cases presented here. For one of the AE enhancements, ground measurement of auroral luminosity has provided a precise timing of the substorm onset related to the AE enhancement, so that an estimation of the propagation speed of the tail disturbance can be computed. This speed, 1800 km/s, greatly exceeds the typical plasmoid propagation velocity (less than 800 km/s). This fast propagation speed implies that the tail disturbance is due to a large-scale wave propagating inside the lobe, which tends to reduce the magnetic pressure.

Source

Sauvaud, J.-A., A. Opitz, L. Palin, B. Lavraud, C. Jacquey, L. Kistler, H. U. Frey, J. Luhmann, D. Larson, and C. T. Russell (2011), Far tail (255 RE) fast response to very weak magnetic activity, J. Geophys. Res., 116, A03215, doi:10.1029/2010JA016077.

Biographical Note

For more than 25 years, Jean-André Sauvaud, formerly Director of the Centre d'Étude Spatiale des Rayonnements (CESR) in Toulouse, has made outstanding contributions to our understanding of Earth's magnetosphere, covering a wide range of topics, from radiation belt dynamics, time-dispersed auroral particle injections, the contribution of ionospheric particles, magnetopause and plasmasheet boundary layer properties, substorm processes, to energetic particle precipitation caused by ground-based transmitters.

Please send comments/suggestions to

Emmanuel Masongsong / emasongsong@igpp.ucla.edu