The work is devoted to development and testing of new fluxgate magnetometer installed on-board of Acess-II sounding rocket. Both pre-flight and space testing of the instrument are described.

Temperature stability is an important factor of magnetometer operation, especially in the case of spacecraft on-board installation. Declared sensitivity temperature dependence of 13-17 ppm/deg is exactly a thermal expansion coefficient of feedback coils. Is it wholly satisfactory for this mission? There is no description of how this figure was measured. Moreover, temperature behavior of polynomial coefficients for non-linearity correction (which seem to be not dependent on feedback coils), has not been addressed at all. Your consideration on the subject would be relevant and instructive.

Line 35: “20 pT/Hz” should be 20 pT/sqrtHz

Line 211: “inside a single-axis four-layer mumetal magnetic shield (Figure 5a)” According to Fig.5 and its legend, it is “a three-layer mumetal magnetic shield”.

Line 223: “Robust linear regression was used to fit a linear trend to the noise floor from 0.05 to 1.0 Hz, and this trend was evaluated at 1 Hz” Consider “Robust linear regression was used to fit a linear trend from 0.05 to 1.0 Hz, and the noise floor was evaluated at 1 Hz”.

Line 245: Sensitivity figure for Z direction seems to be erroneous.

Line 302: “computationally highpass filtered below 1 Hz”. Should it be “over 1 Hz”?

The authors present the design of a prototype fluxgate magnetometer based in its pre-flight characteristics and an evaluation of its performance during a short flight aboard sounding rocket ACES-II. The paper is well written, understandable and an appropriate number of citations is included. The ability of the new magnetometer to perform geophysical magnetic field measurements in the space environment is clearly demonstrated.

General comments:

The primary (science) magnetometer, which is the main reference for the in-flight comparison, should be briefly discussed and relevant literature should be cited.

The pre-flight calibration of the three Euler angles of the rotation matrix and its accuracy should be discussed, as it is assumed to be the main cause of the difference between the prototype and the primary (science) magnetometer.

The section on in-flight performance would benefit from some improvements and/or clarifications:

• It is not clear for how long the magnetometer was actively measuring.
• It is said that a quiet period between 17:24:00 and 17:24:30 was used for the in-flight calibration, but not the entire 30 seconds are shown in Figure 6. What do the data look like before the “quiet period”?
• The measured and the modelled field obviously agree to within 25 nT RMS outside of the scientifically interesting period. What is the standard deviation of the difference within the mentioned crossing of the active auroral arc? The plots in Figure 6 do not indicate a big difference between the two phases.
• The performance discussion would benefit from plotting the difference between the prototype and science magnetometer to deepen the demonstration of the good match. The mentioned alignment mismatch between the two sensors could be calibrated based on the flight data which would further reduce the reported RMS deviation.
• A comparison of the high pass filtered data from both magnetometers would show that also the actual science event was measured correctly by the prototype sensor.

Specific comments:

Line 35: The instrumental noise of the MMS sensors in low range is less than 8 pT/sqrt(Hz).

Line 146: … to measure thermal electrons.

Line 231: … detailed science analysis of it will …