Using a common methodology to analyze data from the Active Magnetospheric Particle Tracer Explorer/Ion Release Module (AMPTE/IRM) and International Sun-Earth Explorer 2 (ISEE 2) satellites we report on the statistical properties of bursty bulk flow events (BBFs) in the inner plasma sheet (IPS). A positive correlation between BBFs and the AE index suggests that BBFs are predominantly geomagnetically active time phenomena. Earthward BBFs are more frequent close to midnight and away from Earth, up to a distance of approximately 19 R(sub E). Tailward BBFs are very infrequent in the IRM data set and somewhat less infrequent in the ISEE 2 data set in the region of the satellites' spatial overlap, possibly due to the more active conditions prevailing during the ISEE 2 mission in that region. However, in both data sets the ratio of tailward to earthward BBFs increases with distance from Earth; more than 20% of all BBFs are anti-sunward tailward of X = -19 R(sub E) in the ISEE 2 data set. BBFs are responsible for 60-100% of the measured earthward transport of mass, energy and magnetic flux past the satellite in the regions of maximum occurrence rate, even though they last approximately 10-15% of the IPS observation time there. Thus BBFs represent the primary transport mechanism at those regions. The one-to-one correspondence between BBFs and substorm phase, as well as the relative contribution of BBFs to the total transport observed during substorms are questions that await further investigation based on multi instrument studies of individual events.

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Statistical characteristics of bursty bulk flow events

The authors present an analysis of plasma wave measurements in the plasma sheet in the frequency range from 0.1 mHz to 8 Hz. They compute power spectra in several different bands, and look at this data against location in the plasma sheet, magnetic field, and magnetospheric activity. In general the power spectra increase with decreasing frequency over the full range. Wave power is enhanced during periods of enhanced activity, or substorms. Fluctuations are in the range of nanotesla, and tend to be stronger along X{sub GSM} as opposed to the two orthogonal directions.

Low‐frequency waves in the near‐Earth plasma sheet

[1] From July to October 2001, the Cluster satellites were located within the magnetotail and crossings of the neutral sheet were frequently observed by the FGM instrument [Balogh et al., 2001]. The neutral sheet often appears to be moving relative to the spacecraft. Multiple crossings of the neutral sheet on August 3, 2001 are analysed. These neutral sheet crossings were rather unusual. Spacecraft 2, which is in the northwestern part of the Cluster tetrahedron, leads the crossing from the north to the south lobe which implies a strongly twisted magnetotail on a local scale. Examining the solar wind interplanetary magnetic field conditions, we postulate that a wavy motion in the dawn-dusk direction is superimposed on an extremely twisted and warped neutral sheet.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2002GL015544

A wavy twisted neutral sheet observed by CLUSTER

Four years of IMP 8 magnetic field measurements, 1978-1982, when ISEE 3 took upstream interplanetary magnetic field (IMF) measurements, have been analyzed to produce full cross-section magnetic maps of the magnetotail at about 33RE downwind from Earth. This paper describes how the field geometry in the cross-sectional plane responds to different IMF orientations: dominant By, dominant +Bz, and dominant −Bz. The dominant By case exhibits marked departures from bilateral symmetry that have the sense of superimposing a fraction of the IMF on the symmetrical tail field. However, the “superimposed” perturbation field, measured as a fraction of the IMF, is highly nonuniform: It is maximum in the flanks of the plasma sheet and minimum in the lobes. There is also a rotation of the current sheet which varies with IMF strength and with distance. The two dominant Bz cases show no systematic departures from bilateral symmetry. However, the shapes and the relative sizes of the dipolar and flaring field regions of their cross sections are markedly different. The difference field, obtained by subtracting the positive Bz case from the negative Bz case, shows that the strongest perturbations run the north-south extent of the flanks, instead of residing in the lobes, as might be expected from the dayside reconnection model.

Interplanetary magnetic field control of magnetotail magnetic field geometry: IMP 8 observations

[1] Sudden impulses (SIs) are an important source of ultra low frequency (ULF) wave activity throughout the Earth's magnetosphere. Most SI-induced ULF wave events have been reported in the dayside magnetosphere; it is not clear when and how SIs drive ULF wave activity in the nightside plasma sheet. We examined the ULF response of the nightside plasma sheet to SIs using an ensemble of 13 SI events observed by THEMIS (Timed History of Events and Macroscale Interactions during Substorms) satellites (probes). Only three of these events resulted in ULF wave activity. The periods of the waves are found to be 3.3, 6.0, and 7.6 min. East-west magnetic and radial electric field perturbations, which typically indicate the toroidal mode, are found to be stronger and can have phase relationships consistent with standing waves. Our results suggest that the two largest-amplitude ULF responses to SIs in the nightside plasma sheet are tailward-moving vortices, which have previously been reported, and the dynamic response of cross-tail currents in the magnetotail to maintain force balance with the solar wind, which has not previously been reported as a ULF wave driver. Both mechanisms could potentially drive standing Alfven waves (toroidal modes) observed via the field-line resonance mechanism. Furthermore, both involve frequency selection and a preference for certain driving conditions that can explain the small number of ULF wave events associated with SIs in the nightside plasma sheet.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2012JA017984

THEMIS observations of ULF wave excitation in the nightside plasma sheet during sudden impulse events

Eight years of Interplanetary Monitoring Platform (IMP) 8, four years of International Sun Earth Explorer (ISEE) 2, and one year of IMP 7 magnetometer data have been combined to produce an 'image' of the average magnetic field for a YZ cross section (aberrated GSM) of the magnetotail at a downtail distance of 25 R(sub E). The shape of the neutral sheet and magnetopause boundaries can be observed directly from the images. A fitting function that qualitatively matches the observed boundary shape can then be chosen. This approach improves on previous fits to possible unsuitable functional forms specified independently of the data. In addition, as a refinement of previous studies, we have corrected for varying solar wind dynamic pressure and the effects of tail flaring. We find the magnetopause is displaced above the XY plane with increasing dipole tilt. The neutral sheet is found to curve slightly more than the model of Fairlfield (1980) during times of large dipole tilt and near the flanks appears to differ substantially from the neutral sheet shape given by the analytic model of Voigt (1984), the more recent neutral sheet model of Dandouras (1988) based on the Voigt model, and the semi-empirical model of Tsyganenko (1989).

/pdf/imaging-the-effect-of-dipole-tilt-on-magnetotail-boundaries-3k85hr3dzb.pdf

Imaging the effect of dipole tilt on magnetotail boundaries

Ultralow frequency waves in the magnetotails of the earth and the outer planets

An MHD simulation of the terrestrial magnetosphere, rescaled to represent the Uranian magnetotail, is carried out. The 3p immersion can be explained in terms of possible extreme departures from average plasma sheet shapes in the Uranian magnetosphere. The orientation of the Uranian dipole and rotation axes produce a dynamically curved plasma sheet which is an unusual feature of the Uranian magnetosphere.

/pdf/a-pincer-shaped-plasma-sheet-at-uranus-4wva199byt.pdf

C. Max Hammond

Papers

Imaging the effect of dipole tilt on magnetotail boundaries

Ultralow frequency waves in the magnetotails of the earth and the outer planets

A pincer-shaped plasma sheet at Uranus