tokens: conclusions of this paper are correct and valuable. The data presented here suggest another mechanism for the ECD instability. The LHD electric fields of 20 mV m −1 in B ∼ 15 nT produce ExB drift of 1300 km s −1 , comparable to the electron thermal speed. Since this field exists in narrow channels with a width of a few electron gyroradii, only electrons can participate, making ion–electron drift V d ∼ te . Incidentally, this instability attracted attention of researchers working with Hall ion thrusters, because it affects their performance (Ducrocq et al. 2006 ; Boeuf Garrigues 2018 ). In these devices, the instability is also caused by the ExB drift of electrons only, because ions have gyroradius larger than the drift channels. The theory of ECD instability that links electron–cyclotron harmonics with ion-acoustic waves and wavelengths at scales below the electron gyroradius provides convincing explanation for waves in Fig. 2(a) , which exhibit both large E ∥ and E ⊥ , and contain f ce harmonics and Doppler shifted structures smaller than electron gyroradius. 3 WHAT HEATS IONS AND ELECTRONS? We focus now on the fundamental question: What kinds of processes can increase the ion energy by 400 eV (from 50 to 450 eV), and the electron energy by 50 eV (from 20 to 70 eV), within a few seconds, or during one proton gyroperiod, as seen in Fig. 1 . Additional constraint is that heating of electrons is isotropic, while that of ions is mostly perpendicular. The mechanism of stochastic ion heating on LH waves related to condition (1 ) has been explained by Karney ( 1979 ), and on other wave structures by other authors: Cole ( 1976 ), McChesney et al. ( 1987 ), Balikhin et al. ( 1993 ), Mishin Banaszkiewicz ( 1998 ), Stasiewicz et al. ( 2000 , 2013 ), Stasiewicz ( 2007 ), Vranjes Poedts ( 2010 ), and Yoon Bellan ( 2019 ). The essence is that particles stressed by strong gradients lose adiabaticity and can be scattered along the electric field for direct acceleration by both DC and wave fields. The computed χ (Fig. 4b ) shows values greatly exceeding 1, needed to stochastically perturb proton orbits; however, significant ion heating is observed when χ > 10. Formally, the error of χ is equivalent to the error of the electric field measurements, i.e. ∼1 mV m −1 , or 10 per cent of large-amplitude fields, because of gain uncertainty in amplifiers (Lindqvist et al. 2016 ). However, in the case of ECD waves with wavelengths of the electron gyroradius, the gradient computations underestimate real values because the spacecraft separations are larger than wavelengths. In Fig. 4(c) , we show the difference in E y components measured by satellites 1 and 2. Similar differences are seen on any pair of the satellites. A difference Δ E y ≈100 mV m −1 on scales of the electron gyroradius r e ≈ 1 km in the field B = 15 nT would produce χ ≈ 4600, which could effectively demagnetize electrons and subject them to stochastic heating. Applying 10 per cent error to this value
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