## 5 Discussion and Conclusions  
Our instrument concept MPA aims at answering science questions on small-scale processes that are generally believed to be important for the solar-wind heating and acceleration. We study the ability of a magnetostatic plasma instrument to resolve proton and \(\alpha\)-particle VDFs with a high time resolution. We find that MPA can achieve this goal. MPA is based on the dependence of the gyro-radius of a charged particle travelling across a constant magnetic field on the particle velocity. The gyro-radius being proportional to the speed, by measuring the gyro-radius, we indirectly measure the particle speed. To obtain the best velocity resolution, we place our position-sensitive sensor such that it measures the diameter of the particle's circular motion. The velocity resolution of the instrument is then determined by the width of the pixels in the sensor plane, the aperture length and the value of the magnetic field strength. Whilst reducing the pixel size improves the velocity resolution, it deteriorates the counting statistics as fewer particles hit each pixel. The key feature of MPA is that the Lorentz force naturally separates all incoming particles according to their speed. Thus, in one acquisition step, a full 1D cut of the VDF across all energies is obtained. This operating principle enables a high time resolution (5 ms). However, even in this instrument design, the time resolution is limited by  
Figure 14: Estimated (red and green) and model-input (blue and black) distribution functions. The estimation of the distribution function is based on the counting results shown in Figure 13.  
Figure 13: Counts as a function of hit distance for \(\alpha\)-particles and protons. counting statistics. To overcome inaccuracies due to counting statistics, we can enlarge the aperture, by increasing either the field of view or the aperture area, to the detriment of velocity resolution. Our assumptions made for the sake of simplicity must be discussed and assessed in further detailed engineering work. A magnetic circuit simulation must be conducted to model the exact magnetic field between the two magnets. Shielding considerations are always important in the context of scientific space instrumentation, where magnetic cleanliness is crucial. A trade-off study between resolution and count rate will be necessary to decide on the best sensor, magnetic field strength and aperture geometry for our measurements. Moreover, the dependency of the count measurements on particle energy, mass, charge and hit angle must be addressed during calibration phases. Figure 1 of [PERSON] and [PERSON] [20] and Figure 3 of [PERSON] and [PERSON] [29] highlight the strong dependence of the detector efficiency on mass and charge of the analyzed particles. Consequently, it will be necessary to characterize the detector for ions such as \(He^{2+}\), \(C^{6+}\), \(C^{5+}\), \(O^{6+}\), \(O^{7+}\), etc. found in the solar wind [30; 31]. However, since MPA separates ion species with identical inflow speeds by a distance proportional to \(m/q\), a direct characterization of ion populations is possible. Finally, MPA has a narrow field of view (\(5^{\circ}\times 5^{\circ}\)), making 3D measurements impossible. Three-dimensional measurements of the solar wind were achieved, for example, by [PERSON] et al. [24] using Helios data between 0.3 and 1 au. These measurements have revealed that often temperature anisotropies occur with either \(T_{\parallel}/T_{\perp}>1\) or \(T_{\parallel}/T_{\perp}<1\)[24], where \(T_{\perp}(T_{\parallel})\) is the temperature perpendicular (parallel) to the background magnetic field. These anisotropies are most pronounced at small heliocentric distances (around 0.3 au). Thus, 3D in-situ measurements are of interest to base and confirm analytical models of heating processes [32; 33] and when the VDF differs strongly from a Maxwellian.