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Impulsively Excited Nightside Ultralow Frequency Waves Simultaneously Observed on and off the Magnetic Equator
  • Published Date:
    2018
  • Source:
    Geophysical Research Letters, 45(16), 7918-7926.
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  • Description:
    The Arase spacecraft is capable of observing ultralow frequency waves in the inner magnetosphere at intermediate magnetic latitudes, a region sparsely covered by previous spacecraft missions. We report a series of impulsively excited fundamental toroidal mode standing Alfven waves in the midnight sector observed by Arase outside the plasmasphere at magnetic latitudes 13-24 degrees. The wave onsets are concurrent with Pi2 onsets detected by the Van Allen Probe B spacecraft at the magnetic equator in the duskside plasmasphere and by ground magnetometers at low latitudes. The duration of each toroidal wave packet is similar to 20 min, which is much longer than that of the corresponding Pi2 wave packet. The toroidal waves cannot be the source of high-latitude Pi2 waves because they were not detected on the ground near the magnetic field footprint of Arase. Overall, the toroidal wave event lasted more than 2 hr and allowed us to use the wave frequency to estimate the plasma mass density at L = 6.1-8.3. The mass density (in amu/cm(3)) is higher than the electron density (cm(-3)) by a factor of similar to 6, which implies that 17-33% of the ions were O+. Plain Language Summary Magnetospheric substorms begin with a sudden change of the configuration of the magnetotail, which excites magnetohydrodynamic waves with periods of the order of 100 s. The Arase satellite detects these waves at distances less than similar to 6 Earth radii and magnetic latitudes as high as similar to 40 degrees, a region sparsely covered by previous spacecraft missions. On 24 May 2017, Arase, located off the equator, observed substorm-associated resonant oscillation of magnetic field lines (standing waves) continuously for 2 hr, whereas a Van Allen Probes spacecraft, located on the equator, observed short-lived oscillations with different frequencies. This multipoint observation indicates that different wave modes are excited from a common impulsive source located in the near-Earth magnetotail. The waves at Arase allow us for the first time to continuously monitor the plasma mass density in the nightside magnetosphere during substorms.
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