Introduction
The Earth's magnetosphere is an inhomogeneous medium where the fast and shear Alfvèn magnetohydrodynamic (MHD) wave modes can couple. In a case of resonant mode coupling known as field line resonance (FLR), the shear Alfvèn mode can rapidly build to large amplitude at the frequency that matches the fast mode driver. The energy transfer associated with FLR has previously been shown to be significant; for example, energy deposition in the ionosphere has been compared to energy deposition during a substorm and can significantly alter ionsopheric conductivities. We present observations from multiple THEMIS probes and IMAGE ground magnetometers, tracing the energy transfer from magnetopause boundary undulations to an FLR and finally to the ionosphere.
Observations
The field line resonance (FLR) event occurred on 31 October 2008 during a period of low geomagnetic activity and high solar wind speed (>600 km/s, Figure 1a). This suggests that magnetopause may be Kelvin-Helmholtz unstable. At ~0330 UT, THEMIS-B is located at the magnetopause and THEMIS-C is located in the magnetosphere at about 10 Earth radii from the Earth; both probes are near the magnetic equator and the dawn meridian (Figure 1c). THEMIS-B observes the signature of magnetopause surface waves and THEMIS-C observes ULF waves of the same periodicity (Figure 1c). We conclude that the surface waves are the driver of the ULF waves.
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| Figure 1. a) From top to bottom, solar wind velocity, dynamic pressure, IMF Bx (GSE/GSM), IMF By (GSM), IMF Bz (GSM), and the AE index from OMNIweb. b) From top to bottom, a dynamic energy flux spectrogram from TH-B (electrons), the plasma velocity component normal to the magnetopause inferred from TH-B ESA ion measurements, and detrended total magnetic field from TH-C. c) The orbits of TH-B and TH-C in the GSM equatorial plane during the interval from 0230 to 0400 UT. |
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THEMIS-C observes wave polarization features that are consistent with a standing shear Alfvèn wave known as the toroidal mode; specifically, there is a localized peak in amplitude in both the radial electric and east-west magnetic field and harmonics are observed in dynamic spectrograms (Figure 2, panels 1-4, 6), the electric and magnetic field are 90 degrees out of phase (Figure 2, panel 5), and the frequency of 5 mHz is consistent with what is expected for a toroidal mode in this region. THEMIS-C also observes a rotation in phase of 180 degrees in the radial electric field across the amplitude peak, suggestive of a field line resonance (Figure 2, panels 6 and 7).
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| Figure 2. From top to bottom, radial electric field, radial electric field dynamic power spectra, east-west magnetic field, east-west magnetic field spectra, band pass filtered (3-6 mHz) radial electric (black) and east-west magnetic field (blue) data, instantaneous amplitude of radial electric field, instantaneous phase of radial electric field, radial Poynting vector (black) and time averaged radial Poynting vector (red), east-west Poynting vector, field-aligned Poynting vector. The time average is computed with a running 10 minute boxcar window. |
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In the bottom three panels of Figure 2, we show observations of the Poynting vector from THEMIS-C. There is strong net energy transfer towards the magnetotail when the FLR is observed, as expected for a surface wave driver. The net energy transfer towards the ionosphere is comparable to the energy transfer directed towards the Earth. The net energy transfer, when mapped to the ionosphere, is ~0.70 mW/m2.
We use ground magnetometers from the IMAGE array to remove time-space ambiguity from the THEMIS-C observations. Using a latitudinal chain, we observe the expected features of FLR that THEMIS-C observes; the 5 mHz amplitude is peaked at the locations close to the ground track of THEMIS-C, and there is an 180 degree rotation for the two stations closest to THEMIS-C. These observations show that the wave activity evolved in time and was closely tied to the boundary undulations that THEMIC-B observes, but that THEMIS-C was also fortuitously located near the center of the FLR during the time the FLR was being driven. Using these ground observations and a statistical study of ionospheric conductivities, we find that the Joule dissipation rate in the ionosphere is comparable to the energy input to the ionosphere from the FLR (0.70 mW/m2).
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| Figure 3. a) The positions of IMAGE magnetometer stations are plotted on a geographic grid (dotted lines) and solid black lines of constant McIlwain L parameter are overplotted in steps of 2 Re. The red line is the ground track of TH-C, mapped from TH-C's position using field line tracing in the Mead and Fairfield magnetic field model. b) In each panel, the H component of the magnetic field with hourly means subtracted is plotted for pairs of stations. From top to bottom, the H component of the magnetic field observed at HOR and BJN, BJN and SOR, and SOR and MUO is shown. A black line indicates a time when a 180 degree phase difference between BJN and SOR is particularly clear, whereas the other stations are in phase. |
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Conclusions
We observed a field line resonance in the dawn sector being driven by magnetopause surface waves. THEMIS-C is ideally placed near the magnetic equatorial plane such that it observes comparable radial and field-aligned (towards ionosphere) energy flux; in other words, it directly observes the coupling between the fast mode driven by the surface waves and the shear Alfven wave mode (toroidal mode). Finally, ground observations confirm the location of the FLR and suggest that Joule dissipation is an important damping mechanism for the toroidal mode excited by the FLR. These observations demonstrate the importance of FLR as a mechanism for energy transfer in the Earth's magnetosphere.
Source
Hartinger, M., V. Angelopoulos, M. B. Moldwin, K. Glassmeier, and Y. Nishimura (2011), Global energy transfer during a magnetospheric field line resonance, Geophys. Res. Lett., 38, L12101, doi:10.1029/2011GL047846.
Biographical Note
Michael Hartinger is a graduate student in the Earth and Space Sciences Department at UCLA.
Please send comments/suggestions to
Emmanuel Masongsong / emasongsong@igpp.ucla.edu
