[1] This paper reports on several substorms observed under northward interplanetary magnetic field (IMF) conditions, the intensity of which was at least as significant as that of typical substorms under moderately southward IMF conditions. Such northward IMF periods were identified during the recovery phase of three strong storms. In the case of each storm, two or more substorms occurred successively, being separated by $1.8-5 h, while the IMF condition continued persistently northward. The substorms are clearly evidenced by auroral and other complementary observations. For the most intense substorms, the auroral breakup occurred at the magnetic latitude of $58°, and for the others it was between 60°and 65°. The polar cap size prior to each onset was substantial despite the northward IMF conditions. The auroral expansion following each onset lasted from a few up to several magnetic local time hours and exhibited a clear poleward expansion feature. For most of the events studied, geosynchronous magnetic dipolarizations preceded by field stretching and/or energetic particle injections occurred. The occurrence of such (intense) substorms implies that a certain (large) amount of energy remains in the tail even under northward IMF conditions. The occurrence of two or more successive substorms further implies that even after the release of a certain amount of energy triggered by the substorm, the tail can still have a substantial amount of energy left, which can be released by a subsequent substorm(s). We conjecture that an intense substorm during a northward IMF period can be expected when such a period belongs to the recovery phase of an intense storm mainly because of large energy loading done by preceding southward IMF B z during the storm's main (and some early recovery) phase. In addition we argue that substorm energy can also be supplied by other mechanisms of the solar wind-magnetosphere coupling under northward IMF conditions such as dayside reconnection in the presence of a substantial IMF B y component.

On April 13 (day 103), 2001, 0700–1400 UT, the Polar satellite experienced different plasma regimes (i.e., magnetosphere, magnetosheath, and solar wind) because of the solar wind dynamic pressure variations and its high orbital inclination near the subsolar magnetopause meridian. When Polar was in the magnetosheath, quasiperiodic spacecraft potential (SP) variations, corresponding to density variations, with a recurrence time of ∼3–10 min were observed. Using simultaneous solar wind observations, it was confirmed that the magnetosheath SP variations were inherent in the solar wind. We observed an almost one‐to‐one correspondence between the SP variations and the geomagnetic field perturbations at lower latitudes (L = 1.1–2.8) on the nightside. At higher latitudes (L = 2.9–6.1) on the dayside, however, the field perturbations are more complicated than the magnetosheath SP variations. This suggests that if the magnetospheric perturbations produced by the external source (solar wind/magnetosheath pressure variations) deeply penetrate into the magnetosphere, the lower‐latitude data on the nightside are important to monitor the external source variations. In addition, we observed the radial electric field oscillations excited nearly simultaneously with the magnetic field enhancement, associated with a sudden increase in the solar wind dynamic pressure, when Polar was in the magnetosphere. These oscillations may be considered as transient standing Alfvén waves excited by externally applied pressure changes as reported by previous studies.

[1] A clear bipolar (negative/positive) signature in the E y component was observed by three spacecraft on Cluster in the magnetotail during the passage of a solar wind discontinuity on October 11, 2001 (day 284), which caused a sudden commencement (sc) on the ground. The positive E y perturbation was accompanied by the northward/ dawnward plasma flow and the B x enhancement, which is the dominant magnetic field component. The estimated E y from the plasma flow and magnetic field was in good agreement with the observed E y . Thus the positive E y perturbation can be interpreted to be the signature of inward plasma motions, corresponding to the compression of the magnetopause. During the interval of the negative E y perturbation, the magnetic field increase was mainly due to the increase in B y and B z , not in B x . Unlike the positive E y perturbation, the negative E y perturbation was not consistent with the estimated E y . Therefore the negative E y signature does not seems to have a direct connection with plasma motions. The observed field and flow variations may be associated with a deformation of the magnetotail due to the solar wind discontinuity moving tailward. The direction and shape of the sc front propagating tailward can be confirmed by three spacecraft of Cluster. We also observed quasiperiodic geomagnetic perturbations at the low-latitude ground station Kakioka (L = 1.25) following the sc event. They were highly correlated with the magnetic field perturbations at Cluster in the magnetotail (X gse = $12 R E ). We show that the source of these perturbations is the quasiperiodic solar wind variations superposed on the increased solar wind pressure behind the interplanetary discontinuity.

[1] Kokubun (1983) reported the local time variation of normalized amplitude of sudden commencement (SC) with a strong day-night asymmetry (i.e., maximum amplitude near noon and minimum amplitude near midnight) at geosynchronous orbit with 81 SC events. Further careful inspection of Kokubun's local time distribution reveals that the normalized SC amplitudes in the prenoon sector (MLT = 9-12) are larger than those in the postnoon sector (MLT = 12-15). That is, there is a morning-afternoon asymmetry in the normalized SC amplitudes. Until now, however, there are no studies on this SC-associated morning-afternoon asymmetry at geosynchronous orbit. Motivated by this previous observation, we investigate a large data set (422 SC events in total) of geosynchronous SC observations and confirm that the geosynchronous SC amplitudes normalized to SYM-H are larger in the morning sector than in the afternoon sector. This morning-asymmetry is probably caused by the enhancement of partial ring current, which is located in the premidnight sector, due to solar wind dynamic pressure increase. We also examine seasonal variations of the normalized SC amplitude and find that the SC-associated geosynchronous magnetic field perturbations are dependent on seasons of the year. This may be due to the location of the magnetopause current and cross-tail current enhanced during the SC interval with respect to geosynchronous spacecraft position.

[1] Pi2 pulsations observed in the inner magnetosphere have been explained as radially trapped fast mode waves in the plasmasphere (i.e., plasmaspheric resonance). This model suggests that these waves can be globally detected at all local times in the inner magnetosphere when azimuthal propagation is allowed. There are no reports of Pi2-associated fast mode waves on the dayside in the inner magnetosphere, however. In this case study we focus on a Pi2 pulsation that was observed by the low-latitude Bohyun station (L = 1.35) in the postmidnight sector (MLT = 3.1) at 1853 UT on 27 February 2008. During the Pi2 event, Time History of Events and Macroscale Interactions during Substorms (THEMIS)-E was near the dawnside inner magnetosphere (MLT = 5.1 and L = 2.6), and THEMIS-D was near the duskside inner magnetosphere (MLT = 17.8 and L = 3.1), which are transition regions between nightside and dayside. On the dawnside, THEMIS-E observed poloidal oscillations characterized by the azimuthal component of the electric field (dE y ) and the radial (dB x ) and compressional (dB z ) components of the magnetic field. These components had high coherence (>0.8) with a low-latitude Pi2 pulsation at Bohyun. We confirmed that the poloidal oscillations are radially standing fast mode waves excited by plasmaspheric resonance. On the duskside, however, no poloidal oscillations in the Pi2 frequency band were detected at THEMIS-D, indicating that the plasmaspheric resonance may not establish itself globally. We suggest that there is strong longitudinal attenuation of fast mode waves near the duskside, which may be due to complicated duskside plasmapause structures. In addition, a well-defined, trapped fast mode oscillation would be expected as a two-dimensional mode structure near the meridian plane of a source region.