Magnetospheric compression due to impact of enhanced solar wind dynamic pressure Pdyn has long been considered as one of the generation mechanisms of electromagnetic ion cyclotron (EMIC) waves. With the Van Allen Probe-A observations, we identify three EMIC wave events that are triggered by Pdyn enhancements under prolonged northward interplanetary magnetic field (IMF) quiet time preconditions. They are in contrast to one another in a few aspects. Event 1 occurs in the middle of continuously increasing Pdyn while Van Allen Probe-A is located outside the plasmapause at postmidnight and near the equator (magnetic latitude (MLAT) ~ −3°). Event 2 occurs by a sharp Pdyn pulse impact while Van Allen Probe-A is located inside the plasmapause in the dawn sector and rather away from the equator (MLAT ~ 12°). Event 3 is characterized by amplification of a preexisting EMIC wave by a sharp Pdyn pulse impact while Van Allen Probe-A is located outside the plasmapause at noon and rather away from the equator (MLAT ~ −15°). These three events represent various situations where EMIC waves can be triggered by Pdyn increases. Several common features are also found among the three events. (i) The strongest wave is found just above the He+ gyrofrequency. (ii) The waves are nearly linearly polarized with a rather oblique propagation direction (~28° to ~39° on average). (iii) The proton fluxes increase in immediate response to the Pdyn impact, most significantly in tens of keV energy, corresponding to the proton resonant energy. (iv) The temperature anisotropy with T⊥ > T|| is seen in the resonant energy for all the events, although its increase by the Pdyn impact is not necessarily always significant. The last two points (iii) and (iv) may imply that in addition to the temperature anisotropy, the increase of the resonant protons must have played a critical role in triggering the EMIC waves by the enhanced Pdyn impact.
Bibliographical noteFunding Information:
This research was supported by the Korea Astronomy and Space Science Institute under the R&D program supervised by the Ministry of Science, ICT and Future Planning. Work at Los Alamos was performed under the auspices of the U.S. Department of Energy, with support from the NASA Van Allen Probes mission. The Van Allen Probes data used for this paper can be obtained from the NASA's CDAWeb FTP server, http://cdaweb.gsfc.nasa.gov, and at the EMFISIS website for the magnetic field and plasma density data, http://emfisis.physics.uiowa.edu/, and at the Los Alamos National Laboratory ftp site, ftp://stevens.lanl.gov/pub/projects/rbsp/rbspa/hope, for the HOPE particle data. The solar wind data used for this work were obtained at the NASA's CDAWeb, OMNI FTP server, http://omniweb.gsfc.nasa.gov. The GOES magnetic field data can be obtained at ftp://satdat.ngdc.noaa.gov/sem/goes/data/new_avg. The ground geomagnetic data used in this paper can be obtained at http://www.bcmt.fr/data_download.php for the station AAE, at the DTU site (http://www.space.dtu.dk) for the station TDC, and at the USGS site (http://geomag.usgs.gov) for the station GUA. The THEMIS data analysis software to calculate the WNA and ellipticity is freely available at the THEMIS website, http://themis.ssl.berkeley.edu/software.shtml.
©2016. American Geophysical Union. All Rights Reserved.
- EMIC waves
- Van Allen Probes
- dynamic pressure