Geotail observations of low‐frequency waves and high‐speed earthward flows during substorm onsets in the near magnetotail from 10 to 13 R_E

Abstract

1] Geotail electric and magnetic field data from five substorms were used to examine the relationship between low-frequency waves and high-speed earthward flows at radial distances between 10 and 13 R E . Strong compressional fluctuations of the magnetic field in the Pi2 frequency range (0.007-0.03 Hz) were observed during the 26 April 1995 substorm and the other four substorms studied, in association with high-speed earthward flows and magnetic field dipolarizations. However, the maximum earthward flow was generally observed 30 s to a few minutes after the start of the dipolarizations and magnetic field fluctuations in the Pi2 frequency range. Waves near the ion gyrofrequency ($0.1-1.0 Hz) and the lower hybrid frequency ($5-16 Hz) were also observed during all five substorms. The maximum amplitudes at these frequencies were not observed until after the start of the magnetic field dipolarization and earthward flow, which does not appear to be consistent with local substorm initiation by a cross-field current driven instability. Recent work has shown the importance of high-speed earthward flow bursts as drivers of substorm activity. However, we found that the earthward kinetic energy flux was much smaller than the Poynting flux or the thermal energy. This is consistent with the idea that currents driven by thermal pressure gradients and magnetic field changes are responsible for a major part of the substorm current wedge.

Statistical and superposed epoch study of dipolarization events using data from Wind perigee passes

“…At 03:50:52 UT the magnetic field inclination to the equatorial plane at Wind reached a maximum value close to 30 • . Large amplitude fluctuations of the magnetic field were also observed after the start of the magnetic field dipolarization, similar to fluctuations of the magnetic field observed during dipolarizations by Geotail (Sigsbee et al, 2002;Nagai et al, 2000). Although Wind was located close to the equatorial plane during this event, the 3-DP plasma data indicate that the spacecraft was situated outside of a thinned plasma sheet before the dipolarization.…”

“…The durations of the earthward flow bursts and dipolarization events observed by Geotail at radial distances Lopez et al, 1988) Maximum inclination angle >20 • (From the results of Ratio of maximum inclination to initial >1.5 inclination (From the result of Rate of change of the magnetic field >1.5 • /min inclination (this paper) of 10 to 13 R E were on the order of 5-10 min (e.g. Sigsbee et al, 2002;Fairfield et al, 1998;Nagai et al, 2000). A minimum rate of change in the magnetic field inclination of 1.5 • /min was initially chosen to aid in selecting Wind dipolarization events, since it would produce the required minimum 15 • change in inclination over a 10-min time period.…”

Separating spatial and temporal variations in auroral electric and magnetic fields by Cluster multipoint measurements

“…The periods of the observed waves are interesting, since they are greater than those associated with standing Alfvén waves in the ionospheric Alfvén resonator (T ∼1 s), but shorter than periods associated with the Pi2 pulsations associated with substorm onset (T ∼40-150 s) or field line resonances (T ∼100-1000 s), although they could be harmonics of these. Recently fluctuations with shorter periods than those of Pi2 have been observed in the magnetotail in association with substorm onset, but these are not likely to be Alfvénic and furthermore have periods shorter than 10 s (Sigsbee et al, 2002). Another possibility is that they are Alfvén waves excited by the large field-aligned currents surrounding the "wave guide" region and propagated into it.…”

In-situ observation of ULF wave activities associated with substorm expansion phase onset and current disruption

“…Liang (jliang@phys.ucalgary.ca) waves are typically representative of the plasma instability modes responsible for transient magnetospheric reconfigurations, among which the substorm expansion phase (EP) onset and the accompanying current disruption (CD) process are outstanding examples. Magnetic field fluctuations with wide frequency range during substorm intervals have been investigated by many researchers based upon different spacecraft measurements and analysis approaches (Holter et al, 1995;Lui and Najmi, 1997;Sigsbee et al, 2002;Shiokawa et al, 2005;Saito et al, 2008a, b;Lui et al, 2008). As a methodological preparations as well as noteworthy predecessors to this study we refer the readers to Holter et al (1995) and Saito et al (2008a, b) for an example of the use of wavelet analysis on a time-frequency decomposition of ULF wave components, and the efforts to discern observed wave components in terms of known plasma modes.…”