DC Langmuir Probe for Measurement of Space Plasma: A Brief Review

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  • ABSTRACT

    Herein, we discuss the in situ measurement of the electron temperature in the ionosphere/plasmasphere by means of DC Langmuir probes. Major instruments which have been reported are a conventional DC Langmuir probe, whose probe voltage is swept; a pulsed probe, which uses pulsed bias voltage; a rectification probe, which uses sinusoidal signal; and a resonance cone probe, which uses radio wave propagation. The content reviews past observations made with the instruments above. We also discuss technical factors that should be taken into account for reliable measurement, such as problems related to the contamination of electrodes and the satellite surface. Finally, we discuss research topics to be studied in the near future.


  • KEYWORD

    instrumentation , electrode contamination , electron temperature , ionosphere

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  • [Fig. 1.] Electric circuit for probe measurement in space: An electrode of any shape (sphere, cylinder, or plane) is injected into the ionosphere/plasmasphere. The current (electron and ion current) flows into the electrode, satellite frame (rocket body), and ambient plasma, and it returns again to the electrode. A DC Langmuir probe assumes an infinite conductive surface area of the counter electrode (satellite surface, or conductive part of the rocket). For tiny satellites, the surface area becomes insufficient for DC probe measurement.
    Electric circuit for probe measurement in space: An electrode of any shape (sphere, cylinder, or plane) is injected into the ionosphere/plasmasphere. The current (electron and ion current) flows into the electrode, satellite frame (rocket body), and ambient plasma, and it returns again to the electrode. A DC Langmuir probe assumes an infinite conductive surface area of the counter electrode (satellite surface, or conductive part of the rocket). For tiny satellites, the surface area becomes insufficient for DC probe measurement.
  • [Fig. 2.] I-V curve (Langmuir probe characteristic) when the electrode is clean and unclean: The horizontal axis is the probe voltage, and the vertical axis is the current. Note that the I-V curve provided by a contaminated electrode is not the same when the probe voltage is increased from low voltage to higher voltage. An I-V curve provided by a clean (ion bombarded in plasma) electrode shows no serious effects due to surface contamination.
    I-V curve (Langmuir probe characteristic) when the electrode is clean and unclean: The horizontal axis is the probe voltage, and the vertical axis is the current. Note that the I-V curve provided by a contaminated electrode is not the same when the probe voltage is increased from low voltage to higher voltage. An I-V curve provided by a clean (ion bombarded in plasma) electrode shows no serious effects due to surface contamination.
  • [Fig. 3.] Anomalous heating of thermal electrons around the Sq focus (Sq focus anomaly): (a) Height profiles of electron temperature measured by an electron temperature probe (ETP) show a sharp maximum increase in the midst of the Sq current system at an altitude of 110 km. It is noted that the high Te measured by ETP reflects the existence of a high-energy tail in the energy distribution of thermal electrons. (b) Possible mechanism of the Sq focus anomaly: The potential in the winter hemisphere is higher than that in the summer hemisphere. Electrons that exist below ~150 km are accelerated by an electric field which might exist along a magnetic field line, originally produced by the potential difference. It may be noted that electrons in the summer hemisphere can travel along the magnetic line of force to the winter hemisphere.
    Anomalous heating of thermal electrons around the Sq focus (Sq focus anomaly): (a) Height profiles of electron temperature measured by an electron temperature probe (ETP) show a sharp maximum increase in the midst of the Sq current system at an altitude of 110 km. It is noted that the high Te measured by ETP reflects the existence of a high-energy tail in the energy distribution of thermal electrons. (b) Possible mechanism of the Sq focus anomaly: The potential in the winter hemisphere is higher than that in the summer hemisphere. Electrons that exist below ~150 km are accelerated by an electric field which might exist along a magnetic field line, originally produced by the potential difference. It may be noted that electrons in the summer hemisphere can travel along the magnetic line of force to the winter hemisphere.
  • [Fig. 4.] The second-harmonic components of probe current. The secondharmonic component is in semi-logarithmic scale. (a) In the height range between 88.6 km and 124.6 km, the curve shows the existence of a high energy tail. The curve at the height of 98.4 km shows especially high Te as well as a high energy tail. (b) The linear part of the curve is reduced as the height increases from 128.8 km to 250.5 km, which suggests that the electron energy of the thermal range becomes Maxwellian as the height increases.
    The second-harmonic components of probe current. The secondharmonic component is in semi-logarithmic scale. (a) In the height range between 88.6 km and 124.6 km, the curve shows the existence of a high energy tail. The curve at the height of 98.4 km shows especially high Te as well as a high energy tail. (b) The linear part of the curve is reduced as the height increases from 128.8 km to 250.5 km, which suggests that the electron energy of the thermal range becomes Maxwellian as the height increases.
  • [Fig. 5.] Example global map of electron temperature at 600 km for three different seasons in the longitude bands of 210°?285° and 285°?360°. The longitude band of 210°?285° is a region where the magnetic meridional plane tilts eastward about 10°, while in the longitude band of 285°?360°, the magnetic meridional plane tilts tremendously. Due to the tilting of the magnetic meridional plane in the longitude band of 285°?360°, zonal wind, as well as meridional wind, influences the plasma behavior.
    Example global map of electron temperature at 600 km for three different seasons in the longitude bands of 210°?285° and 285°?360°. The longitude band of 210°?285° is a region where the magnetic meridional plane tilts eastward about 10°, while in the longitude band of 285°?360°, the magnetic meridional plane tilts tremendously. Due to the tilting of the magnetic meridional plane in the longitude band of 285°?360°, zonal wind, as well as meridional wind, influences the plasma behavior.
  • [Fig. 6.] (a) Local time variation of Te in the plasma bubble. The letters A, B, C, D, E , and F denote the data obtained in the longitude bands of 0˚- 60˚, 60˚ - 120 ˚, 120˚ - 180˚, 180˚ - 240˚, 240˚- 300˚, and 300˚ - 360˚, respectively. In (b), the upper panel shows the total number of plasma bubble (grey shade), and that of the bubble inside which Te is higher than that of ambient plasma (black shade). The lower panel shows the locations where Te inside the plasma bubble is higher (triangular), and lower (circle). The size of the triangle and circle shows the degree of the Te difference from that of ambient plasma (after Oyama et al. 1988).
    (a) Local time variation of Te in the plasma bubble. The letters A, B, C, D, E , and F denote the data obtained in the longitude bands of 0˚- 60˚, 60˚ - 120 ˚, 120˚ - 180˚, 180˚ - 240˚, 240˚- 300˚, and 300˚ - 360˚, respectively. In (b), the upper panel shows the total number of plasma bubble (grey shade), and that of the bubble inside which Te is higher than that of ambient plasma (black shade). The lower panel shows the locations where Te inside the plasma bubble is higher (triangular), and lower (circle). The size of the triangle and circle shows the degree of the Te difference from that of ambient plasma (after Oyama et al. 1988).
  • [Fig. 7.] Height profile of the electron temperature up to 8,000 km, which was calculated from the second-harmonic components measured by the AKEBONO satellite (Balan et al. 1996a). The second harmonic component, which is picked up from the probe current when a small sinusoidal signal is superimposed on an electrode, provides the second derivative of the I-V curve. The second derivative of the I-V curve still indicates the exponential function from which Te is calculated, if the thermal electrons of the ambient plasma are Maxwellian (after Balan et al. 1996a).
    Height profile of the electron temperature up to 8,000 km, which was calculated from the second-harmonic components measured by the AKEBONO satellite (Balan et al. 1996a). The second harmonic component, which is picked up from the probe current when a small sinusoidal signal is superimposed on an electrode, provides the second derivative of the I-V curve. The second derivative of the I-V curve still indicates the exponential function from which Te is calculated, if the thermal electrons of the ambient plasma are Maxwellian (after Balan et al. 1996a).
  • [Fig. 8.] Plot of altitude/local time of Te at two geomagnetic latitude zones (GLAT 0°?15°, and GLAT 15°?30°), observed by the Akebono satellite. Altitude is indicated by circles every 2,000 km. Note that Te is about 8,000K at 22 LT at 4,000 km for GLAT 15°?30°, while for GLAT 0°?15° at 22 LT, Te at 4,000 km is < 5,000K. This suggests that the magnetic tube at GLAT 15°?30° is larger than that at GLAT 0°?15°, causing slow Te increase in the early morning, and slow decrease of Te at night at GLAT 15°?30˚, where heat capacity is larger than at GLAT 0°-15˚.
    Plot of altitude/local time of Te at two geomagnetic latitude zones (GLAT 0°?15°, and GLAT 15°?30°), observed by the Akebono satellite. Altitude is indicated by circles every 2,000 km. Note that Te is about 8,000K at 22 LT at 4,000 km for GLAT 15°?30°, while for GLAT 0°?15° at 22 LT, Te at 4,000 km is < 5,000K. This suggests that the magnetic tube at GLAT 15°?30° is larger than that at GLAT 0°?15°, causing slow Te increase in the early morning, and slow decrease of Te at night at GLAT 15°?30˚, where heat capacity is larger than at GLAT 0°-15˚.