Ultra-low-frequency Waves In The Magnetotail

Ultra-low-frequency (ULF) waves with periods between 1 and 1000 s play an important role in the Earth’s magnetosphere. They act as an energy sink, transfer or dissipation. The magnetotail is an extremely complex system, in which there are many dynamic processes. And ULF waves are essential to magnetotail dynamics. The magnetotail has three main regions, i.e. the plasma sheet, plasma sheet boundary layer and lobe. Our works mainly focus on ULF waves or oscillations in the plasma sheet and lobe. We used Cluster data with years from 2001 to 2009 to study the ULF waves/oscillations in these two regions. We summary our results as follows:1. ULF waves/oscillations in the plasma sheetPi2 pulsations (periods:40-150 s) are an important phenomenon during substorm. There are plenty of Pi2-band waves/oscillations in the plasma sheet. We investigated the generation mechanisms of Pi2-band waves in this region. We examined the relation between these waves and dynamic processes in the plasma sheet (fast flows and substorm activity) and the change of the solar wind velocity direction. Our results suggested that fast flows could be the main driver of Pi2-band waves/oscillations in the plasma sheet, especially considering that most of these waves are compressional. The relatively small number of other events indicated that other mechanisms also play a role in creating Pi2-band waves/oscillations in the plasma sheet but are relatively rare. In all wave events of this study, the ion thermal pressure and magnetic pressure vary in anti-phase, suggesting that these waves have the slow-mode feature.Pi2-band waves/oscillations in the near-Earth plasma sheet could be associated with Pi2 pulsations on the ground. We studied a wavy current sheet event between 1235 and 1300 UT on 15 October 2004. Three Pi2 pulsations at～1236,～1251 and～1255 UT are observed at the Tixie (TIK) station located near the foot-points of Cluster. The mechanism creating the Pi2 (period-40 s) onset at～1236 UT is unclear. The second Pi2 (period～90 s, onset at～1251 UT) is associated with a strong field-aligned current, which has a strong transverse component of the magnetic field, observed by Cluster with a time delay ～60 s. We suggested that it is caused by Alfven waves bouncing between the northern and southern ionosphere. For the third Pi2 (period～60 s) there is almost no damping at the first three periods. They occur in conjunction with periodic field-aligned currents one-to-one with 72 s delay, and we suggested that it is generated by these periodic field-aligned currents.A kink-like neutral sheet oscillation event has also been found to be associated with Pi2 pulsations on the ground. The oscillations with periods between 40 and 60 s are dominant in BX and BY, and accompanied by strong field-aligned currents, which show one-to-one correlated with the Pi2 pulsations recorded by KTN and TIK stations. The westward longitudinal propagating speed of the Pi2 is ～6 km/s, comparable to the velocity (-4 km/s) of the oscillations projecting onto the same latitudinal ground. The above findings suggested that the kink-like neutral sheet oscillations are viable candidate for the generation of high-latitude Pi2 pulsations via field-aligned currents. The magnetic double-gradient instability can explain these neutral sheet oscillations.Recently, waves near dipolarization fronts (DFs) have been reported. We analysed magnetic compressional structures (or oscillations) ahead of a DF. We found that the structures, observed near the neutral sheet, are mainly compressional and dominant in Bz; they are almost non-propagating relative to the local ion bulk flow and their lengths are several local proton gyroradius; the ion density increases when BT decreases; ions are partially trapped by the structures with parallel and perpendicular velocities varying in anti-phase; and local conditions are favorable for excitation of the mirror instability. Therefore, we suggested that these structures are mirror mode-like, and local conditions ahead of the DF are viable for exciting the mirror instability to generate mirror mode waves or structures.2. ULF waves/oscillations in the magnetotail lobeThough the magnetotail lobe is quiet, there are plenty of ULF waves. We investigated 263 Pi2-band and 161 Pc5-band (periods:150-600 s) waves in the magnetotail lobe. Our findings were as follows:(1) 90% of the mean wave amplitudes within Pi2-band (Pc5-band) are below-0.25 nT (0.36 nT) for the transverse components, and ～0.16 nT (0.39 nT) for the compressional component; (2) Pi2-band waves are more likely to occur in the lobe region close to the plasma sheet, while Pc5-band waves can occur throughout the lobe; (3) The amplitudes of the lobe waves and the AE index are weakly correlated; However, the amplitudes tend to be larger when the AE index is larger; (4) The amplitudes also tend to be larger when the solar wind velocity, the solar wind dynamic pressure (Psw) or its variations (△Psw) are larger; The correlation coefficient between the amplitudes of the Pc5-band waves and APsw is up to～0.58. We suggested that both dynamic processes in the plasma sheet boundary layer or plasma sheet (inner sources) and solar wind conditions (outer sources) can both contribute to the generation of the lobe ULF waves; Pi2-band waves are effected more by inner sources, while strong APsw can drive compressional Pc5-band waves in the magnetotail lobe.Next, we investigated 3 magnetic oscillation events with periods 2 to 20 min in the lobe and their relationship with solar wind parameters. Our findings were as follows: (1) the event on 4 October 2009, oscillating mainly in Bx and Bz, is observed in the southern lobe with a velocity of (-204,-68,-541) km/s; its polarization is counterclockwise; the oscillations correspond to the variations of Psw one to one cycle, and the tendency of the total pressure variations in the lobe is similar to Psw; (2) the event on 11 August 2004 is dominant in BX and BY with significant changes in the IMF BY and the Y component of the solar wind velocity (Vy, sw); its velocity is (-548,-316, 109) km/s; the wavy profiles between BY and the IMF BY are similar and out of phase; (3) the BY oscillations of the event on 20 August 2003 are different from the Bx and Bz oscillations; BY varies in anti-phase with the IMF BY. Our results supported the explanation that strong quasi-period variations of Psw can squeeze the magnetotail to generate the Bx and BY oscillations in the lobe, and we suggested that strong quasi-period variations in the IMF BY and VY, sw contribute to the generation of the Bx and BY oscillations.