A mechanism for heating electrons in the magnetopause current layer and adjacent regions

Abstract

Abstract. Taking advantage of the string-of-pearls configuration of the five THEMIS spacecraft during the early phase of their mission, we analyze observations taken simultaneously in the magnetosheath, the magnetopause current layer and the magnetosphere. We find that electron heating coincides with ultra low frequency waves. It seems unlikely that electrons are heated by these waves because the electron thermal velocity is much larger than the Alfvén velocity (V a ). In the short transverse scale (k ⊥ ρ i >> 1) regime, however, short scale Alfvén waves (SSAWs) have parallel phase velocities much larger than V a and are shown to interact, via Landau damping, with electrons thereby heating them. The origin of these waves is also addressed. THEMIS data give evidence for sharp spatial gradients in the magnetopause current layer where the highest amplitude waves have a large component δB perpendicular to the magnetopause and k azimuthal. We suggest that SSAWs are drift waves generated by temperature gradients in a high beta, large T i /T e magnetopause current layer. Therefore these waves are called SSDAWs, where D stands for drift. SSDAWs have large k ⊥ and therefore a large Doppler shift that can exceed their frequencies in the plasma frame. Because they have a small but finite parallel electric field and a magnetic component perpendicular to the magnetopause, they could play a key role at reconnecting magnetic field lines. The growth rate depends strongly on the scale of the gradients; it becomes very large when the scale of the electron temperature gradient gets below 400 km. Therefore SSDAW's are expected to limit the sharpness of Correspondence to: P. Robert (patrick.robert@lpp.polytechnique.fr) the gradients, which might explain why Berchem and Russell (1982) found that the average magnetopause current sheet thickness to be ∼400-1000 km (∼500 km in the near equatorial region).

MMS Observations of Reconnection at Dayside Magnetopause Crossings During Transitions of the Solar Wind to Sub‐Alfvénic Flow

“…What remains is the kinetic Alfvén wave mode. This wave mode has significant perpendicular V and B fluctuations at low frequencies and a weak total pressure fluctuation (Chaston et al, ; Hollweg, ; Roux et al, ). This mode is presumed to be driven on field lines that resonate with global modes generated in the magnetosphere due to disturbances.…”

Drift waves, intense parallel electric fields, and turbulence associated with asymmetric magnetic reconnection at the magnetopause

“…These data suggest that ion response alone does not maintain quasineutrality and that the corrugated drift wave has variation along B . Strong V e z motion is associated with intense E || at high frequencies [ Newman et al, ; Roux et al, ; Stawarz et al, ]. Since the bursts of E || have short durations (tens of milliseconds), it is possible that the observations in Figure i are not representative since they are averaged over 30 ms.…”

“…Even with such small statistics, these observations suggest that asymmetric magnetic reconnection may be inherently turbulent. Furthermore, the low‐frequency B fluctuations and the strong E || fluctuations seem to be connected through strong field‐aligned currents [ Roux et al, ; Stawarz et al, ]. The low‐frequency B fluctuations and the strong E || fluctuations may influence magnetic reconnection.…”

Quasi-parallel electron beams and their possible application in inferring the auroral arc's root in the magnetosphere

“…The most common manifestation of the electromagnetic whistler-mode wave in the inner magnetosphere is the chorus wave within the frequency range 0.1-0.9 f ce . Readers are referred to Thorne et al (2010) (Roux et al, 2011). Using the KAW dispersion relation in Roux et al (2011) with observed local plasma parameters, we estimate that, to be able to interact with ∼ 100 eV electrons via Landau resonance, k ⊥ must be on the order of ∼ 0.1 km −1 .…”

“…Readers are referred to Thorne et al (2010) (Roux et al, 2011). Using the KAW dispersion relation in Roux et al (2011) with observed local plasma parameters, we estimate that, to be able to interact with ∼ 100 eV electrons via Landau resonance, k ⊥ must be on the order of ∼ 0.1 km −1 . With the observed flow magnitudes of several tens of km s −1 , had such short-scale KAW existed, it would have been Doppler-shifted up to ∼ 1 Hz range.…”