Tailward propagation of Pi2 waves in the Earth's magnetotail lobe

Journal of Geophysical Research, 2004

Annales Geophysicae, 2011

We have analyzed an event on 14 February 2003 in which Cluster satellites and the CPMN ground magnetometer chain made simultaneous observations of a Pi 2 pulsation along the same meridian. Three of the four Cluster satellites were located outside the plasmasphere, while the other one was located within the plasmasphere. By combining the multipoint observations in space and the multipoint observations on the ground, we have obtained a detailed L-profile of the Pi 2 signatures, which has not been done in the past. In addition, we have used a method called Independent Component Analysis (ICA) to separate out other superposed waves with similar spectral components. The result shows that the wave phase of the Pi 2 was the same up to L ∼ 3.9 (corresponding to the plasmasphere), became earlier up to L ∼ 4.1 (corresponding to the plasmapause boundary layer), and showed a delaying tendency up to L ∼ 5.9 (corresponding to the plasmatrough). This systematic phase pattern, obtained for the first time by a combination of a ground magnetometer chain and multisatellites along a magnetic meridian with the aid of ICA, supports the interpretation that a Pi 2 signal propagated from a farther source and reached the plasmasphere.

Journal of Geophysical Research: Space Physics, 2009

The objective of this study is to understand better the propagation of Pi 2 waves in the nighttime region. We examined Pi 2 oscillations that showed high correlation between high-and low-latitude Magnetic Data Acquisition System/Circum Pan-Pacific Magnetometer Network stations (correlation coefficient: jgj ! 0.75). For each horizontal component (H and D) we examined the magnetic local time (MLT) dependence of the delay time of high-latitude Pi 2 oscillations that corresponds to the highest correlation with the low-latitude Pi 2 oscillation. We found the delay time of the high-latitude H showed remarkable MLT dependence, especially in the premidnight sector: we found that in the premidnight sector the high-latitude H oscillation tends to delay from the low-latitude oscillation (<100 s). On the other hand, the delay time of the high-latitude D oscillation was not significant ($±10 s) in the entire nighttime sector. We propose a Pi 2 propagation model to explain the observed delay time of high-correlation highlatitude H. The model quantitatively explains the trend of the event distribution. We also examined the spatial distribution of high-correlation Pi 2 events relative to the center of auroral breakups. It was found that the high-correlation Pi 2 events tend to occur away from the center of auroral breakups by more than 1.5 MLT. The present result suggests that the high-correlation H component Pi 2 oscillations at high latitude are a manifestation of forced Alfvén waves excited by fast magnetosonic waves.

Journal of Geophysical Research: Space Physics, 2004

Using data from the four Cluster spacecraft, estimates of the motion of isolated magnetic field dips and peaks in the magnetosheath are presented. Their motion and orientation are deduced using four‐spacecraft timing, with typical spacecraft separations of ∼100 km. A set of 39 clear isolated dips and 12 peaks, spread over 4 days, is considered. The calculated velocities in the plasma frame are scattered around zero, with a width of ∼30 km/s and an average absolute value of 21 km/s. This places an upper limit on the speed of these structures, and within timing errors these values are consistent with the structures being stationary in the plasma frame. Two events had estimated speeds of over half the local Alfvén speed; it is shown that these two estimates were unreliable as a result of uncertainties in timings. The inferred propagation speeds were typically slower than local wave speeds (95% less than half the local Alfvén speed), even allowing for their orientations (85% less than t...

Journal of geomagnetism and geoelectricity, 1988

2015

Author(s): Ream, Jodie Barker | Advisor(s): Walker, Raymond J; Ashour-Abdalla, Maha | Abstract: Pi2 pulsations are magnetic field fluctuations with periods between 40 an 150 seconds observed on the ground in conjunction with the onset of magnetospheric substorms. Pi2 period perturbations are also observed in magnetic field and plasma observations in space in conjunction with fast earthward flows leading to several theories concerning how and where the pulsations are generated. We investigate the source and propagation of Pi2 period pulsations through the magnetosphere, tracing the disturbances from their origin in the magnetotail through the inner edge of the plasma sheet and into the inner magnetosphere. Several models for the generation of Pi2 pulsations have been constructed by using satellite and ground-based observations. Our approach is to use global magnetohydrodynamic (MHD) computer codes to simulate the Earth&#39;s magnetosphere during substorms to determine where the Pi2 p...

Annales Geophysicae, 1996

The terrestrial magnetosheath contains a rich variety of low-frequency (: proton gyrofrequency) fluctuations. Kinetic and fluid-like processes at the bow shock, within the magnetosheath plasma, and at the magnetopause all provide sources of wave energy. The dominance of kinetic features such as temperature anisotropies, coupled with the high-conditions, complicates the wave dispersion and variety of instabilities to the point where mode identification is difficult. We review here the observed fluctuations and attempts to identify the dominant modes, along with the identification tools. Alfve´n/ioncyclotron and mirror modes are generated by ¹ , /¹ # '1 temperature anisotropies and dominate when the plasma is low or high, respectively. Slow modes may also be present within a transition layer close to the subsolar magnetopause, although they are expected to suffer strong damping. All mode identifications are based on linearized theory in a homogeneous plasma and there are clear indications, in both the data and in numerical simulations, that nonlinearity and/or inhomogeneity modify even the most basic aspects of some modes. Additionally, the determination of the wave vector remains an outstanding observational issue which, perhaps, the Cluster mission will overcome.

Pi2 pulsations are a category of ULF waves with periods between 40-150 seconds frequently observed by ground-based magnetometers predominantly during substorm onset. The origin of these pulsations has been attributed to the coupling of Alfvénic oscillations associated with the generation of the substorm current wedge, and fast-mode compressional waves moving radially inward from the tail, including plasmaspheric cavity modes at low-latitudes. It has recently been suggested that the frequencies of observed night-side auroral zone and low-latitude Pi2 pulsations, or Pi2 waveforms on the flanks, may be due to periodic variations in the sunward plasma flow from the tail such as during multiple bursty bulk flows (BBFs). Using a favourable conjunction of the Geotail satellite with the CARISMA groundbased magnetometers on 23rd December 2000, the relationship between the frequency of Pi2 pulsations observed on the ground and periodicity in Earthward plasma flows has been investigated. Enhanced Earthward flows were seen during periods of substorm activity; however, using time-series analysis a direct link was not observed between the periodicity in the flow-bursts and the periodicity of pulsations within the Pi2 waveband.

Advances in Space Research, 2005

The large scale structure of the current sheet in the terrestrial magnetotail is often represented as the superposition of a constant northward-oriented magnetic field component (B z) and a component along the Earth-Sun direction (B x) that varies with distance from the center of the sheet (z 0 in GSM) as in a Hams neutral sheet. The latter implies that B x = B Lx tanh((z À z 0)/h) where B Lx is the magnitude of the B x component in the northern lobe. Correspondingly, the cross-tail current should be approximated by J y = (B Lx /h) sech 2 ((z À z 0)/h). Using data from the fluxgate magnetometer (FGM) on the Cluster II spacecraft tetrad, we have used measured fields and currents to ask if this model represents the large-scale properties of the system. During very quiet crossings of the plasmasheet, we find that the model gives a reasonable estimate of the trend of the average current and field distributions, but during disturbed intervals, the best fit fails to represent the data. If, however, the parameters z 0 and h of the model are taken as variable functions of time, the fits can be reasonably good. The temporal variation of the fit parameter h that characterizes the thickness of the current sheet can be interpreted in terms of thinning during the growth phase of a substorm and thickening following the expansion phase. Ground signatures that give insight into the local time of substorm onset can be used to interpret the response of the plasmasheet to substorm related changes of the global system. During a substorm, the field magnitude in the central plasmasheet fluctuates at the period of Pi2 pulsations.

Journal of Geophysical Research: Space Physics, 2013

Using a global magnetohydrodynamic (MHD) simulation of the magnetosphere during a disturbed interval on 14 September 2004, we have investigated fluctuations in plasma properties of the magnetotail in the Pi2 range and their relationship to dipolarization fronts (DFs). Results from the MHD simulation indicate that this event is a very active interval with variable convection and disorder in the tail on a range of scales as small as 1 R E. DFs are observed in the simulation at the leading edge of fast earthward flows that originate from reconnection regions that form between-15 and-30 R E in the tail. Pi2 period fluctuations are identified in pressure, magnetic field, and velocity components inside-13 R E following each burst of DFs in the midnight sector. The fluctuations observed in the pressure appear to be generated by the successive DFs as they approach the interface between stretched tail field lines and dipolar field lines. Fluctuations in the velocity may be the result of interactions between successive DFs and are amplified directly following the passage of the DFs as they propagate earthward. Although the limited azimuthal extent of the pulsations near the plasma sheet, just inside of the braking region, makes it difficult to draw a direct comparison between the ground-based measurements and the pulsations at-6 R E , the temporal evolution of the simulated DFs and Pi2 pulsations approximately reproduces the timing of the variations observed by satellites and ground-based instruments. Therefore, we have been able to use the global simulation to track the bursty flows, dipolarization fronts, and associated Pi2 period fluctuations throughout the entire magnetosphere in order to understand the sources of the changes measured in the near-Earth region.