Development of a Double Hemispherical Probe for Improved Space Plasma Measurements

Abstract

Langmuir probes have been widely used for space plasma measurements for decades. However, there are still challenges in the interpretation of their measurements. Due to the interaction of the ambient plasma with a spacecraft and an onboard probe itself, the local plasma conditions around the probe could be very different from the true ambient plasma of interest. These local plasma conditions are often anisotropic and/or inhomogeneous. Most of the Langmuir probes that are made of a single electrode have difficulties to remove these local plasma effects, introducing errors in the derived plasma characteristics. Directional probes are able to characterize anisotropic and inhomogeneous plasmas. The split Langmuir probe and the Segmented Langmuir Probe have been developed to characterize the plasma flow in the Earth's ionosphere. Here we introduce a new type of a directional Langmuir probe, the Double Hemispherical Probe (DHP), to improve the space plasma measurements in a broad range of scenarios: (a) low‐density plasmas, (b) high surface‐emission (photo and/or secondary electron emission) environments, (c) flowing plasmas, and (d) dust‐rich plasma environments. The DHP consists of two identical hemispheres that are electrically insulated and swept with the same voltages simultaneously. The difference currents between the two hemispheres are used to characterize the anisotropic/inhomogeneous plasma conditions created around the probe, which will be then removed or minimized on the interpretation of their current‐voltage curves. This paper describes the basic concept and design of the DHP sensor, as well as its initial results tested in the laboratory plasma environments.

A Double Hemispherical Probe for Characterizing and Minimizing the Self‐Wake Effects on Probe Measurements

“…10.1029/2020JA028508 plasma around the probe, which can be removed or minimized to derive the true ambient plasma characteristics (Wang et al, 2018). In low-density plasmas (Case i), probes can be engulfed in the Debye sheath of SC, causing measurement errors.…”

“…To test the DHP and understand the current collection by probes in plasma flows, we used a laboratory model 4 mm in diameter with a 0.5 mm insulating rod (Figure 2a; Wang et al, 2018) placed in the Colorado Solar Wind Experiment (CSWE) device (Figure 2b; Ulibarri et al, 2017). The DHP is made of two halves (HS1 and HS2) of ball bearings that are electrically isolated from each other with a Kapton tape.…”

“…The DHP is a spherical Langmuir probe that consists of two hemispheres that are electrically isolated from each other (Wang et al, 2018). The two hemispheres are swept together with the same bias voltage to obtain two independent I‐V curves.…”

“…These probes have been mainly used to measure ion flows in Earth's ionosphere. We have pursued the development of the Double Hemispherical Langmuir Probe (DHP) that is intended to improve space plasma measurements in the following nonideal situations (Wang et al, 2018): (i) low‐density plasmas, (ii) flowing plasmas, (iii) environments of high surface emission of photoelectrons and/or secondary electrons, and (iv) dust‐rich plasmas.…”

“…The two hemispheres are swept together with the same bias voltage to obtain two independent I‐V curves. The differences between the two I‐V curves determine the anisotropic local plasma around the probe, which can be removed or minimized to derive the true ambient plasma characteristics (Wang et al, 2018). In low‐density plasmas (Case i), probes can be engulfed in the Debye sheath of SC, causing measurement errors.…”