A. Keiling

Alfvén waves and Poynting flux observed simultaneously by Polar and FAST in the plasma sheet boundary layer

We present the first simultaneous observations of Alfven waves at Polar and FAST altitudes, ∼7 R E geocentric and ∼3500 km, respectively, at ∼23 MLT in the main phase of a major geomagnetic storm on 22 October 1999. We compare the Poynting flux for these waves and the electron energy flux at the two spacecraft. We also present a new method of Alfven wave analysis, examining Poynting flux magnitude and directionality along with the perturbation electric to magnetic field ratio of these waves as a function of wave temporal scale (frequency). The results of this analysis are compared with those expected from kinetic Alfven wave models. There is a mean net loss of ∼2.1 ergs cm -2 s -1 (mW m -2 ) in earthward Poynting flux over the altitude region between Polar and FAST, a mean net increase in earthward electron energy flux of up to ∼ 1.2 ergs cm -2 s -1 over the same region, frequency characteristics consistent with a mixture of Alfven waves obeying the kinetic Alfven wave dispersion relation mixed with some coupling to the ionosphere, and high-frequency kinetic Alfven wave generation between Polar and FAST. Current models are found to be generally consistent with the study results but are not yet sufficiently well formulated to account for the details, including evidence for temporal and/or spatial modulation of reflectivity.

Pi2 pulsations observed with the Polar satellite and ground stations: Coupling of trapped and propagating fast mode waves to a midlatitude field line resonance

Simultaneous measurements from the Polar satellite and several ground stations of two substorm-related Pi2 pulsation events (separated by ∼6 min) provide evidence for radially trapped and propagating fast mode waves and a coupled field line resonance (FLR). The Pi2 pulsations were observed at five ground stations located between 2130 and 2330 magnetic local time (MLT) ranging from L=1.83 to 3.75, which showed nearly identical waveforms in the H component with a frequency of ∼20 mHz. Five additional ground stations located between L=4.48 and ∼15 (on similar meridians) recorded weaker less-correlated signals. The pulsations were also detected both simultaneously and with a time delay of ∼38 s at two low-latitude stations (L=1.17 and 1.23) on the dayside at ∼1030 and ∼0612 MLT, respectively, indicating the global extend of the pulsations. The nightside ground data showed an amplitude maximum and a phase reversal in the H component between L=3.4 and 3.75. During the oscillations the Polar satellite moved (on the same meridian as the ground stations in the nightside) from 14° to 10° magnetic latitude and from L=4.1 to 3.7. Electric and magnetic field measurements also showed two Pi2 pulsation events (∼20 mHz) in both the compressional (Bz and Ey) and transverse (By and Ex) mode components with waveforms almost identical to the ground signals. Whereas the first Pi2 had a standing wave structure in the compressional mode, the second Pi2 was a propagating wave. Both Pi2s had standing wave signatures in the transverse mode. The amplitude of the compressional magnetic field component (Bz) was ∼40% of that of the azimuthal component (By). Although the two Pi2 events showed equal amplitudes in the H component of ground data, Polar recorded much larger oscillations in the azimuthal magnetic field component (By) of the first Pi2 event; the fast mode amplitude (Bz) was nearly unchanged for both Pi2s. This suggests that Polar was at or near a localized FLR excited by the oscillations of the fast mode waves. During the in situ FLR observation, Polars footpoint was closest to the ground stations which recorded the amplitude maximum and the phase reversal in the H component.

Plasma sheet dynamics observed by the Polar spacecraft in association with substorm onsets

We present observations of the Polar spacecraft of magnetospheric substorm signatures in the plasma sheet midway along auroral field lines between the ionosphere and the equatorial plasma sheet. On October 17, 1997, Polar was located in the onset meridian in conjunction with the Scandinavian magnetometer chain (International Monitor for Auroral Geomagnetic Effects; IMAGE). In addition, a geostationary spacecraft, LANL-97A, was located near the onset meridian. On August 29, 1997, Polar was magnetically conjugate to the Canadian magnetometer chain (Canadian Auroral Network for the OPEN Program Unified Study; CANOPUS) ∼ 5 hours east of the onset meridian. In both cases, substorm activity was manifested as strong magnetic (20 nT) and electric (40 mV m−1) field variations with bursts of parallel Poynting flux (∼ 1 erg cm−2 s−1), predominantly directed toward the ionosphere. In the first event Polar was located in the plasma sheet near the plasma sheet boundary, and the field variations were initiated at the ground onset. In the second event, Polar crossed the plasma sheet boundary to the tail lobes a few minutes prior to a local plasma sheet expansion. As Polar was engulfed by the plasma sheet, the field variations occurred in the previously quiet plasma sheet boundary. This coincided with the auroral bulge reaching the CANOPUS stations. We compare these two events and argue that the field variations were most probably signatures of the reconnection of open field lines and the subsequent enhanced earthward flows. Furthermore, weak flow bursts were observed at Polar in both events ∼ 9 min before the onset. In the first event, a gradual development toward a negative bay and a burst of Pi2 pulsations were associated with the flow bursts. We anticipate that these signatures, often described in terms of pseudobreakups, were a precursor of the substorm onset, the initiation of the reconnection of closed field lines.

Magnetosphere‐ionosphere coupling of global Pi2 pulsations

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 ...

Low‐altitude electron acceleration due to multiple flow bursts in the magnetotail

At 10:00 UT on 25 February 2008, Cluster 1 spacecraft crossed the near-midnight auroral zone, at about 2R(E) altitude, while two of the Time History of Events and Macroscale Interactions During Sub ...