tokens: at 3 s resolution. Magnetic field data is provided by the MFI instrument (Lepping et al. 1995 ) and density and velocity data by the 3DP instrument (Lin et al. 1995 ) measured during a stationary slow stream. The start time of the interval is 12:00 2000 April 4 and stop time is 16:00 of 2000 April 6. During this interval the solar wind speed remained about 300 km s −1 . As it is seen from Fig. 1 , λ 0.03 except for very small τ. We suggest that this enhancement at the small scales is probably due to measurement uncertainties (which are known strongly affecting plasma parameters at high frequencies Kasper et al. 2006 ; Gogoberidze et al. 2012 ) rather than indicating a real enhancement of compressible perturbations. To support this suggestion, we present another important characteristic of compressible/non-Alfvénic perturbations (Tu Marsch 1995 ; Bruno Carbone 2013 ) by dashed line in Fig. 1 . As can be seen λ m is approximately constant and does not increase with decreasing time-scale. Figure 1. The ratio of dimensionless rms amplitudes (solid line) and (dashed line, see the text for details) as a function of time-scale for a stationary stream of the slow solar wind. The above consideration suggests that the thermal-speed spectra reported by Safrankova et al. ( 2016 ) are in fact the pseudo-thermal spectra dominated by the magnetic fluctuations rather than the temperature fluctuations. This conclusion is supported by the fact that the level of spectra reported by Safrankova et al. ( 2016 ) cannot be caused lonely by the temperature fluctuations. Indeed, the spectrum of the (pseudo-)thermal speed in their fig. 1(b) is only about three times lower than the spectrum of the bulk speed driven by Alfvén waves (their fig. 1a), whereas the authentic temperature fluctuations should be at least one order less energetic (Goldsein Siscoe 1972 ; Tu et al. 1991 ; Tu Marsch 1995 ). To be more specific, let us utilize the straightforward expression relating the bulk velocity fluctuations δ V Bn measured by Safrankova et al. ( 2016 ) and magnetic fluctuations due to Alfvén waves. By means of ( 34 ), we can eliminate from ( 31 ) thus arriving to the following estimation for the velocity spectra ratio: With reasonable values T 0∥ / T 0⊥ ∼ 0.5 or T 0∥ / T 0⊥ ∼ 1.8 and , ( 35 ) easily explains the modest ratio ≳ 1/3 between 〈|δ V Tn | 2 〉 B and deduced from figs 1(a) and (b) by Safrankova et al. ( 2016 ). With autentic temperature perturbations this ratio would be at least one order less (Goldsein Siscoe 1972 ; Tu et al. 1991 ; Tu Marsch 1995 ). 4 DISCUSSION AND CONCLUSIONS We have shown that the nature of 'temperature fluctuations' derived from the currents of Faraday cup(s) is strongly affected by the temperature anisotropy. If the temperature anisotropy is close to zero, then the derived 'temperature fluctuations' are dominated by the perturbations of real temperature (thermal speed). However, when the temperature anisotropy is finite, as is typical
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