No abstract

The magnetosphere‐ionosphere coupling is achieved, essentially, by a superposition of quasi‐stationary and time‐dependent field‐aligned currents (FACs), over a broad range of spatial and temporal scales. The planarity of the FAC structures observed by satellite data and the orientation of the planar FAC sheets can be investigated by the well‐established minimum variance analysis (MVA) of the magnetic perturbation. However, such investigations are often constrained to a predefined time window, i.e., to a specific scale of the FAC. The multiscale field‐aligned current analyzer, introduced here, relies on performing MVA continuously and over a range of scales by varying the width of the analyzing window, appropriate for the complexity of the magnetic field signatures above the auroral oval. The proposed technique provides multiscale information on the planarity and orientation of the observed FACs. A new approach, based on the derivative of the largest eigenvalue of the magnetic variance matrix with respect to the length of the analysis window, makes possible the inference of the current structures' location (center) and scale (thickness). The capabilities of the FAC analyzer are explored analytically for the magnetic field profile of the Harris sheet and tested on synthetic FAC structures with uniform current density and infinite or finite geometry in the cross‐section plane of the FAC. The method is illustrated with data observed by the Cluster spacecraft on crossing the nightside auroral region, and the results are cross checked with the optical observations from the Time History of Events and Macroscale Interactions during Substorms ground network.

Global Pi2 pulsations have mainly been associated with either low/middle latitudes or middle/ high latitudes and, as a result, have been treated as two different types of Pi2 pulsations, either the plasmaspheric cavity resonance or the transient response of the substorm current wedge, respectively. However, in some reports, global Pi2 pulsations have a single period spanning low/middle/high latitudes. This "super" global type has not yet been satisfactorily explained. In particular, it has been a major challenge to identify the coupling between the source region and the ground. Here we report two consecutive super global Pi2 events which were observed over a wide latitudinal and longitudinal range. Using four spacecraft that were azimuthally spread out in the nightside and one spacecraft in the tail lobe, it was possible to follow the Pi2 signal along various paths with time delays from the magnetotail to the ground. Furthermore, it was found that the global pulsations were a combination of various modes including the transient Alfvén and fast modes, field line resonance, and possibly a forced cavity-type resonance. As for the source of the Pi2 periodicity, oscillatory plasma flow inside the plasma sheet during flow braking (e.g., interchange oscillations) is a likely candidate. Such flow modulations, resembling the ground Pi2 pulsations, were recorded for both events.

A substorm recovery event in the early morning sector is explored by means of ground and spacecraft data. The ground data are provided by stations of the MIRACLE network, in northern Scandinavia and Svalbard, while spacecraft data are observed by the Cluster satellites, toward the end of the recovery phase. Additional information is provided by the Fast Auroral SnapshoT (FAST) satellite, conjugate to Cluster 3 (C3). A prominent signature in the Cluster data is the low‐frequency oscillations of the perturbation magnetic field, in the Pc5 range, interpreted in terms of a motion of quasi‐stationary mesoscale field‐aligned currents (FACs). Ground magnetic pulsations in the Ps6 range suggest that the Cluster observations are the high‐altitude counterpart of the drifting auroral undulations, whose features thus can be explored closely. While multiscale minimum variance analysis provides information on the planarity, orientation, and scale of the FAC structures, the conjugate data from FAST and from the ground stations can be used to resolve also the azimuthal motion. A noteworthy feature of this event, revealed by the Cluster observations, is the apparent relaxation of the twisted magnetic flux tubes, from a sequence of 2‐D current filaments to an undulated current sheet, on a timescale of about 10 min. This timescale appears to be consistent with the drift mirror instability in the inner magnetosphere, mapping to the equatorward side of the oval, or the Kelvin‐Helmholtz instability related to bursty bulk flows farther downtail, mapping to the poleward side of the oval. However, more work is needed and a better event statistics, to confirm these tentative mechanisms as sources of Ω‐like auroral undulations during late substorm recovery.