Observations of Slow Electron Holes at a Magnetic Reconnection Site

We report in situ observations of high-frequency electrostatic waves in the vicinity of a reconnection site in the Earth’s magnetotail. Two different types of waves are observed inside an ion-scale magnetic flux rope embedded in a reconnecting current sheet. Electron holes (weak double layers) produced by the Buneman instability are observed in the density minimum in the center of the flux rope. Higher frequency broadband electrostatic waves with frequencies extending up to $f_{pe}$ are driven by the electron beam and are observed in the denser part of the rope. Our observations demonstrate multiscale coupling during the reconnection: Electron-scale physics is induced by the dynamics of an ion-scale flux rope embedded in a yet larger-scale magnetic reconnection process.

Figure 1

Crossing of a magnetic flux rope close to a reconnection site by Cluster. Panels on the top show schematic of the crossing (adopted after Ref. 12) and location of the SC in the plane normal to the current sheet. Below are Cluster observations: magnetic field components in the Geocentric Solar Magnetospheric system (GSM) measured by fluxgate magnetometer [19] (c)–(e), current computed by the curlometer technique [20], and plasma density derived from the SC potential measured by EFW [15]. For this event the GSM system is close to the current sheet aligned system, with $Y$ GSM being the current direction and $Z$ GSM normal to the current. The spatial scale on the top of (c) shows the SC position in the $X$ direction (with respect to the center of the flux rope). The ion inertial length in the lobe is $c/{\omega }_{pi}=1300\mathrm{km}$, $n=0.03{\mathrm{cm}}^{-3}$.

Figure 2

Detailed observation of the electric fields inside the flux rope at C2 (SC coordinates). (a) Magnetic field components at C2 in the SC coordinate system (close to GSM), with $X$ and $Y$ being in the SC spin plane. (b) Plasma density and (c) $Y$ component of the electric field, which is closest to being parallel in the center of the rope. Perpendicular (d) and parallel (e) to $B$ components of the electric field as well as the electrostatic potential (f) in the center of the rope. The horizontal bars in (b) mark the times at which the electron measurements are available at respective SC. The horizontal bar in (c) marks the interval for which the spectra shown in Fig. 3c are computed.

Figure 3

Electron distributions (a),(b) and wave spectrum (c) observed in the dense part of the rope. (a) Electron differential energy flux at C1 measured by the plasma electron and current experiment [21]. (b) Cuts of the distribution function at 0 and 90° pitch angles (thin lines) which are fitted by Maxwellian distributions (thick lines). (c) Spectra of the parallel and perpendicular components of the electric field during the time interval 18:43:07.5–08.6 UT [marked by the black bar in Fig. 2c]. The vertical line marks the electron plasma frequency $f_{pe}\sim 3\mathrm{kHz}$.