Generation of Pi2 pulsations by intermittent earthward propagating dipolarization fronts: An MHD case study

Abstract

[1] 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.

Propagation of Pi2 pulsations through the braking region in global MHD simulations

“…Using a selection of 48 Pi2 events that occurred when Time History of Events and Macroscale Interactions during Substorms (THEMIS) was in the inner magnetosphere, Kwon et al [2012] has shown that space and ground observations of Pi2 are coherent if the satellite is within about 3 h of MLT of the ground station. Ream et al [2013] showed that the pulsations observed in the UCLA simulation are coherent for ∼2.5 h MLT at a radial distance of 6 R E , in good agreement with the observational results. The upper limit of the coherence in the LFM results is 2 h MLT.…”

“…The substorm selected for this case study occurred on 14 September 2004. Observations of this event have previously been presented by Cao et al [2008Cao et al [ , 2012 and Ream et al [2013] and are shown for the interval from 1700 to 1900 UT in Figure 1. The panels from top to bottom show the AL index [Davis and Sugiura, 1966] (Figure 1a), B H from the Urumqi magnetometer (northwest China; 43.80 ∘ latitude, 87.70 ∘ longitude) ( Figure 1b), B H filtered to Pi2 frequencies ( Figure 1c), and magnetic field observations from the Double Star (TC1) satellite located in the plasma sheet at (−10.2, −1.6, 1.2) R E GSM (Figure 1d).…”

“…Perturbations in B z develop with the flow channel and the DF that forms at the earthward edge of the flow channel. Ream et al [] showed that once the flow channel reaches ∼−12 R E , the frequency of the perturbations is within the Pi2 spectrum. The fact that the frequency of the perturbations increases as the flow channel propagates earthward supports the transient response (TR) model for Pi2 generation.…”

“…They also concluded that if the Alfvén waves were properly reflected by the ionosphere, the TR Pi2 would be established. A recent event study of a substorm on 14 September 2004, simulated by using the UCLA 3‐D global magnetohydrodynamic (MHD) model [ Ream et al , ], examined whether flow bursts generate Pi2 period fluctuations in the tail as they travel earthward from the reconnection region. The authors showed that inside ∼−12 R E the earthward velocity, magnetic field, and pressure all fluctuate at Pi2 frequencies when a flow channel is present.…”

“…They also concluded that if the Alfvén waves were properly reflected by the ionosphere, the TR Pi2 would be established. A recent event study of a substorm on 14 September 2004, simulated by using the UCLA 3-D global magnetohydrodynamic (MHD) model [Ream et al, 2013], examined whether flow bursts generate Pi2 period fluctuations in the tail as they travel earthward from the REAM ET AL.…”