THEMIS observations of consecutive bursts of Pi2 pulsations during weak geomagnetic times

Ching-Chang Cheng (鄭慶章 )

Faculty of Physics, Department of Electronic Engineering, National Formosa University, Hu-Wei, Taiwan

Presented at PSSC, NCKU, Taiwan on March 17, 2009

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• 1. Introduction • 2. Data presentation • 2.1. THEMIS and geostationary observations• 2.2. Hodogram analysis• 2.3. Waveform comparison• 3. Summary• 4. Prospects for ERG & SCOPE missions• 5. Acknowledgments

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1. Introduction Pi2 pulsations in a period of 40 to 150 seconds (6-25 mHz):impulsive and damped oscillations of geomagnetic fields

A group of Pi2s generally occurred successively during substorm onsets [see review by Saito, 1969].

Some early evidence of multiple onsets accompanied by Pi2s in a magnetospheric substorm. [e.g. Clauer and McPherron, 1974]

Recent reports [e.g. Sutcliffe, 1998; Rostoker, 2000 ; Sutcliffe and Lyons, 2002] show the occurrence of successive Pi2 pulsations during extremely quiet times.

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Sutcliffe and Lyons [2002] pointed out that they are associated with poleward boundary intensifications (PBIs) and enhanced energetic particle flux in the plasma sheet.

Rostoker [2000] demonstrated that both Pi2s and weak substorm activity occurred along the nightside auroral oval. Since the magnetic disturbance at the equatorward edge of the oval was not the same as that at the poleward edge that fit the description of PBIs, Rostoker [2000] also suggested that they could be independent and might have different physical origins.

What causes the recurrent onsets of quiet-time Pi2s is still uncertain. Namely, to determine if the source mechanism of quiet-time Pi2s is different from the one during substorm times is an important issue in magnetospheric physics.

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2. Data presentation2.1. THEMIS and geostationary observations

6Red marks THEMIS-E footprints by T89 model

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THEMIS-E and ground observations of consecutive Pi2s

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Stations in the meridian along THEMIS-E footprints

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GOES 12 and LANL-94 observations

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Sketch of the substorm current wedge from McPherron et al. [1973]

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Reproduced from Pashin et al. [1982]

Current line

Current sheet

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Fig. 1 in Lester et al. [1984]

CCW polarization

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2.2. Hodogram analysis CW: Clockwise polarization; CCW: Counterclockwise polarization

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2.3. Waveform comparison

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3. Summary

• On 20 April 2007, four Pi2 pulsation bursts occurred successively while the AE index was less than 100 nT.

• Their wave polarization and magnetic variation patterns look like the ones affected by the substorm current wedge.

• They have the characteristics of a fast magnetospheric cavity mode.

• They can be explained by the coupling of a fast magnetospheric cavity mode driven by fast compressional waves to field line resonances owing to the impulsive source at the magnetotail.

• This event thus suggests that the source of Pi2s at times of weak geomagnetic activity can be the same as during substorms.

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4. Prospects for ERG & SCOPE missions

• How does the substorm-like current wedge form during weak geomagnetic times?

By braking BBFs? or with current disruption due to plasma instability?

• Where does it initiate ?

• How wide is it in the local time sector? 21-03 LT?

• Is its location and width tied to the plasmapause shape?

• Are they internally affected by the near-Earth magnetotail conditions? or externally by incident solar wind conditions?

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5. Acknowledgments

The THEMIS data were obtained via CDAWeb and AIDA at NCU, Taiwan. The GOES 12 magnetic field data were provided by H. Singer at NOAA via CDAWeb. The electron flux data at LANL 1994-084 were provided by LANL via CDAWeb. This work was supported by National Science Council of R. O. C. on Taiwan under the grant NSC 97-2111-M-150-001, and by National Space Organization under the grant 97- NSPO(B)-SP-FA07-01(A). The work at NCU was supported in part by Ministry of Education under the Aim for Top University Progr

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