2008 THEMIS SCIENCE NUGGETS

Multipoint Observations of Magnetospheric Compression-related EMIC Pc1 Waves

by Maria Usanova

Introduction

Electromagnetic ion cyclotrons (EMIC) are believed to play an important role in near-Earth space by interacting with populations of energetic ions and electrons trapped in the Earth's magnetic field. Of particular interest is the possibility that EMIC waves remove so-called satellite “killer” electrons from space. EMIC waves are generated in the equatorial magnetosphere via the interaction with a population of “hot” protons (with temperatures of ~10 - 100 keV or 100-1000 MK) at frequencies ranging between 0.2 – 5 Hz, also classified by space physicists as Pc 1. These waves travel great distances from the source region and can be detected by magnetometers both in space and on the ground. In this study, we investigate conditions favorable for EMIC wave excitation as well as radial localization of the waves.

Observations

At around 14:00 UT on 29th June 2007 a slow and dense solar wind hit the Earth’s magnetosphere and triggered a strong magnetospheric compression that lasted ~4.5 hours. Coincidently with the compression, and lasting throughout its duration, the Canadian array of ground magnetometers (CARISMA; www.carisma.ca) saw clear and long-lasting EMIC emissions; the most intense waves appear to coincide with the peak solar wind pressure.

CARISMA and GOES Observations
Figure 1. (a) Solar wind pressure (red line) and the magnitude
of the magnetic field observed on the dayside by the GOES
12 satellite (blue line), from 12:00-20:00 UT; (b) – (g)
Fourier spectrograms of magnetic field from selected CARISMA stations.
Click here to enlarge the image.

In space, the same EMIC waves were observed consistently by three THEMIS spacecraft for a period of 35 minutes as they consecutively crossed the same region of space in a “string-of-pearls” configuration.

THEMIS Observations of EMIC Waves
Figure 2. (Orbit plot of THEMIS C, D, E (top two panels) between
14:00 - 16:00 UT; map showing the locations of CARISMA sites
and magnetic footprints of THEMIS satellites during the EMIC
wave event, starting at 14:35 UT, 14:40 UT and 15:00 UT,
respectively (third panel); and outbound orbit plot of THEMIS
C, D, E superposed over dipole field lines between 14:00 – 16:00 UT.
Thick lines show the locations where each spacecraft observed
EMIC waves (THEMIS C and D were so close that their
trajectories practically coincide in the figure).
Click here to enlarge the image.

Figure 3 (a) shows the spectrogram and time-series of magnetic field registered by one of the CARISMA magnetometers. Fourier spectrograms of the magnetic field from THEMIS D, C and E are shown in Figure 3 (b – d). The EMIC spectrograms show significant similarity from spacecraft to spacecraft, even though these three satellites traverse the same region at progressively later times. The red lines in Figure 3 (b - d) show the THEMIS spacecraft surface voltages, which represent a proxy for the ambient plasma density. Low negative values of surface voltages correspond to the higher density of surrounding plasma. The EMIC waves appear to be confined within the high-density side of this voltage decrease and localized in a region of approximately one Earth radius wide. Analysis of magnetic field data obtained from the ground-based magnetometers allowed us to conclude that the density drop observed on THEMIS in fact corresponds to the crossing of the plasmasphere boundary (plasmasphere is an inner dense region of the magnetosphere that rotates with the Earth and mainly formed by ions of atmospheric origin).

Spectrograms from CARISMA and THEMIS instruments
Figure 3. (a) Fourier spectrogram (top panel) and time-series
(bottom panel) of magnetic field at Gillam; (b) - (d) Fourier spectral
density (top panel) and waveforms (bottom panel) of magnetic field at
THEMIS D, C, and E; over-plotted are the spacecraft voltage (red
line), the local (black line) and the equatorial (white line) helium
gyrofrequency. (e) geocentric radial distance of THEMIS C, D and E
as a function of Universal Time. Thick lines show the intervals where each
spacecraft sees EMIC waves.
Click here to enlarge the image.

Conclusion

Our observations suggest solar wind density enhancements and magnetospheric compressions may be an important source for radially localized EMIC emissions close to the plasmasphere boundary. This study may have wider importance for inner magnetosphere energetic particle dynamics.

Biographical Note Maria Usanova is a PhD student at the University of Alberta, Canada. Her research interests focus on the electromagnetic ion cyclotron (EMIC) waves and their role in the dynamics of the Earth’s radiation belts.



Please send comments/suggestions to
Emmanuel Masongsong / emasongsong@igpp.ucla.edu