Drift-Kinetic Alfvén Waves Observed near a Reconnection X Line in the Earth’s Magnetopause

We identify drift-kinetic Alfvén waves in the vicinity of a reconnection X line on the Earth’s magnetopause. The dispersive properties of these waves have been determined using wavelet interferometric techniques applied to multipoint observations from the Cluster spacecraft. Comparison of the observed wave dispersion with that expected for drift-kinetic Alfvén waves shows close agreement. The waves propagate outwards from the X line suggesting that reconnection is a kinetic Alfvén wave source. Energetic ${\mathrm{O}}^{+}$ ions observed in these waves indicate that reconnection is a driver of auroral ion outflow.

Figure 1

(a) Schematic of X line crossing; (b) Plasma density from ion spectrometer (red) and spacecraft potential (black) also shows the local Alfvén speed in ${\mathrm{ms}}^{-1}$ divided by ${10}^6$ and plasma beta multiplied by 10 on the same scale; (c) Electric field in GSM coordinates under the assumption that $\mathbf{E}·{\mathbf{B}}_0=0$ ($y$ and $z$ components have been shifted positive by 100 and $200\mathrm{mV}/\mathrm{m}$, respectively); (d) Magnetic field in GSM coordinates; (e) Field-aligned wave Poynting flux; (f) and (g) Electron energy flux and pitch angle spectra; (h) and (i) Oxygen ion pitch angle and energy spectra from Cluster 4. All data are from Cluster 3 unless otherwise stated.

Figure 2

(a) $E_x$ wavelet spectra; (b) $B_y$ wavelet spectra; (c) $E_x/B_y$ wavelet spectra; (d) two-point wavelet coherence between transverse $B_y$ measurements from Cluster 1 and Cluster 4; (e) Phase difference between $B_y$ measurements from Cluster 1 and Cluster 4. Coordinates are in the field-aligned system defined in the text.

Figure 3

Averaged results from 14:55–14:56:30 UT; (a) Wave vector from interferometry in field-aligned coordinates as defined in the text; (b) Wave frequency for the drift-kinetic wave from the solution of Eq. (6); (c) Observed $E_{\perp }/B_{\perp }$ ratio (black), predicted result from the homogeneous theory (blue) and the predicted result from Eq. (7) (red).

Figure 4

Average results from 14:58–15:04 UT; (a) Perpendicular wave number from interferometry; (b) Observed $E_{\perp }/B_{\perp }$ normalized by the local Alfvén speed (black) and the predicted result as described in the text (blue).