Multipoint observations of Pi2 pulsations and correlation with dynamic processes in the near-Earth magnetotail on March 18,2009

Simultaneous measurements from THEMIS spacecraft,GOES-11 and ground stations(Canadian Array for Realtime Investigations of Magnetic Activity or CARISMA,and 210°magnetic meridian or MM)on March 18,2009 allow the study of dynamic processes in the near-Earth magnetotail and corresponding Pi2 pulsations on the ground in great detail.Fast earthward flows along with traveling Alfvén waves and fast mode waves in the Pi2 band were observed by three Time History of Events and Macroscale Interactions during Substorms(THEMIS)probes(P3,P4 and P5)in the near-Earth plasmasheet.At the mid-to high-latitude nightside,the CARISMA stations located near the foot points of the three probes recorded Pi2s with two periods,about 80 s after the earthward fast flows observed by the P4 probe.The long-period Pi2(140–150 s)belongs to the transient response Pi2(TR Pi2),since the travel time of the Alfvén waves between the plasma sheet and CARISMA stations is very close to half the period of the long-period Pi2.The short-period Pi2(60–80 s)has the same period band as the perpendicular velocity of the fast flows,which indicates that it may relate to the inertial current caused by periodic braking of the earthward fast flows.The 210°MM stations located at the low-latitude duskside also observed Pi2s with the same start time,waveform and frequency,about^120 s after the earthward fast flows.Strong poloidal oscillations are shown by GOES-11(~23 MLT)and the compressional component(Bb)is highly correlated with H components of the 210°MM stations,whereas the other two components(Br and Be)are not.These results confirm that the low-latitude Pi2s are generated by cavity mode resonance,which is driven by an impulsive broadband source in the near-Earth magnetotail.

Nightside field-aligned current during the April 6,2000 superstorm

Using the east-west geomagnetic disturbance fields observed from stations at mid-latitudes,we investigate the characteristics of field-aligned currents on the nightside during the April 6,2000 superstorm.The results indicate that there is an eastward disturbance on the nightside of Northern Hemisphere during the main phase of magnetic storm,while the interplanetary magnetic field(IMF)is continued southward.The positive disturbances in the postmidnight are larger than that in the premidnight.This suggests that upward field-aligned currents develop on the nightside,when the IMF is directed southward.The peak of upward fieldaligned currents is located in postmidnight,and it will be more obvious at substorm expansion.We get the conclusion that both partial ring current and region-2 field-aligned current shift to dusk sector.The upward region-2 current is decreased at substorm onset,and intensified after it.We also investigate the relationship between the upward field-aligned currents on the nightside and the auroral electrojets at high latitudes.It shows a good correspondence between region-2 current and the westward electrojet in the postmidnight.We suppose that they are both associated with the convection electric field.

Ground magnetic characteristics of the storm-time ring current Asymmetry

Using the minute data of the H component of geomagnetic field from the 20°E magnetic meridian chain and the 30°N magnetic latitudinal chain, the temporal evolution characteristics of the equatorial ring current during the storm on November 7-10, 2004 are studied. It is indicated that the UT-MLT and UT-MLAT graphics extremely exhibit the local time distribution, latitudinal variation and temporal evo- lution of the H component. The results show: (1) The UT-MLT contour clearly shows the increasing of the H component mostly around noon during the initial phase, representing the geomagnetic effect from the magnetopause current system. During the main phase, most negative values of the H com- ponent appear around the dusk-side, indicating the dawn-dusk asymmetric distribution of the ring cur- rent. (2) The contour of UT-MLAT suggests the latitudinal variation of the H component decreasing with the enhancement of the latitudes during geomagnetic storm, which is in good agreement with the Dst index. The latitudinal variations provide a new sight for describing the temporal characteristics of the intensity of the storm-time ring current. (3) Both the contours of UT-MLT and UT-MLAT are useful to monitor the space-time distribution of the equatorial ring current.

Latitudinal shift and tilt of the ring current during magnetic storms

The equatorial ring current (ERC) theory suggested that the distribution of global disturbed horizontal geomagnetic field only depends on the cosine of station’s latitude. However,we always observe a lar-ger ΔH at higher latitude stations than lower ones,implying that the ERC could tilt or/and shift with respect to the equatorial plane during intense storms. In this paper,we analyze 11 intense magnetic storms from 2000 to 2004,and introduce two configurational factors to characterize the topology of storm time ring current. The results show that ERC has occasionally deviated off equatorial plane with both tilt angle δt≈13°―25° and latitude shift δs≈0°―21.8°. The ground disturbed field distribution should be improved as ΔHk = ΔHmaxcos(k-δ ),which agree well with the geomagnetic observations.