10:45 〜 12:15
[PEM10-P09] Detection of the lower hybrid resonance (LHR) in the ionosphere by a new wideband impedance probe installed on SS-520-3
キーワード：観測ロケットSS-520-3、インピーダンスプローブ、低域混成共鳴 (LHR)、電子衝突周波数、電離圏イオン組成、IRI-2016モデル
The lower hybrid resonance (LHR) has been detected in the ionosphere by NEI/PWM (Number density of Electron measurement by Impedance probe/Plasma Wave Monitor installed on the Sounding Rocket SS-520-3. SS-520-3 was launched from Svalbard on Nov. 4, 2021 to perform observations of the ion acceleration and heating processes in the cusp region. In addition to the determination of electron number density from probe capacitance Cp minimum at upper hybrid resonance (UHR) in a frequency range from 0.1 to 20 MHz, NEI/PWM was designed to measure the Cp minimum at LHR in a frequency range from 5 to 11 kHz in order to establish a new method for estimation of the ionospheric ion composition from the detected LHR frequency.
As shown by Balmain [1964], the impedance of short dipole antenna in a plasma around UHR frequency can be derived using the dielectric tensor with terms of electron. The probe impedance in a plasma around LHR frequency also can be derived using the dielectric tensor with terms of electron and ion. It is expected from the theory that a pair of minimum and maximum is found around LHR frequency in each frequency sweep of the probe capacitance measurement. In the observation, such frequency profile was found in a frequency range from 7 to 9 kHz only when the probe is in the wake in an altitude range from 550 to 750 km. In addition, the frequency profile was found multiple times in one frequency sweep.
The results can be explained as follows: In the observation in the altitude range, electron number density was about 5 x 1010 m-3 in the ram side of the rocket and 3 x 1010 m-3 in the wake, electron temperature was ~2400 K, and rocket velocity was ~3 km/s. From the IRI-2016 model, it is expected that the ratio of ion temperature to electron temperature is 75%, and abundances of heavy ions as O+ and N+ is 95%. From the above parameters, the electron collision frequency is estimated to be 25 Hz [Itikawa, 1971]. On the basis of the theory, the frequency profile of the probe capacitance around LHR becomes unclear when the electron collision frequency is higher than 100 Hz. From Bernoulli's principle, the ratio of ion density in the ram side of the rocket to the background can be estimated to be 6. So, the electron collision frequency in the ram side could increase to 150 Hz. Since the ion thermal velocity (~1 km/s) is less than the rocket velocity (~3 km/s) while the electron thermal velocity (~200 km/s) is lager than the rocket velocity, the lengths of ion and electron wakes are expected to be 0.8 m and 4 mm, respectively. Since the probe (1.2 m) is as long as the ion wake but much larger than the electron wake, the ion number density is expected to be much less than the electron number density around the probe in the wake side of the rocket. The electron collision frequency around the probe in the ion wake is therefore estimated to be <15 Hz. It is also expected that ion wake is fluttered. It could be the reason why the probe capacitance minimum is found multiple times in one frequency sweep. When the abundance of heavy ion as O+ and N+ is 95 and 90%, the LHR frequency is expected to be around 7 kHz, and 8-9 kHz, respectively. The observed frequency range (7-9 kHz) suggest that the abundances of heavy ion is from 90 to 95%, which is a little less than that from IRI-2016 model.
As shown by Balmain [1964], the impedance of short dipole antenna in a plasma around UHR frequency can be derived using the dielectric tensor with terms of electron. The probe impedance in a plasma around LHR frequency also can be derived using the dielectric tensor with terms of electron and ion. It is expected from the theory that a pair of minimum and maximum is found around LHR frequency in each frequency sweep of the probe capacitance measurement. In the observation, such frequency profile was found in a frequency range from 7 to 9 kHz only when the probe is in the wake in an altitude range from 550 to 750 km. In addition, the frequency profile was found multiple times in one frequency sweep.
The results can be explained as follows: In the observation in the altitude range, electron number density was about 5 x 1010 m-3 in the ram side of the rocket and 3 x 1010 m-3 in the wake, electron temperature was ~2400 K, and rocket velocity was ~3 km/s. From the IRI-2016 model, it is expected that the ratio of ion temperature to electron temperature is 75%, and abundances of heavy ions as O+ and N+ is 95%. From the above parameters, the electron collision frequency is estimated to be 25 Hz [Itikawa, 1971]. On the basis of the theory, the frequency profile of the probe capacitance around LHR becomes unclear when the electron collision frequency is higher than 100 Hz. From Bernoulli's principle, the ratio of ion density in the ram side of the rocket to the background can be estimated to be 6. So, the electron collision frequency in the ram side could increase to 150 Hz. Since the ion thermal velocity (~1 km/s) is less than the rocket velocity (~3 km/s) while the electron thermal velocity (~200 km/s) is lager than the rocket velocity, the lengths of ion and electron wakes are expected to be 0.8 m and 4 mm, respectively. Since the probe (1.2 m) is as long as the ion wake but much larger than the electron wake, the ion number density is expected to be much less than the electron number density around the probe in the wake side of the rocket. The electron collision frequency around the probe in the ion wake is therefore estimated to be <15 Hz. It is also expected that ion wake is fluttered. It could be the reason why the probe capacitance minimum is found multiple times in one frequency sweep. When the abundance of heavy ion as O+ and N+ is 95 and 90%, the LHR frequency is expected to be around 7 kHz, and 8-9 kHz, respectively. The observed frequency range (7-9 kHz) suggest that the abundances of heavy ion is from 90 to 95%, which is a little less than that from IRI-2016 model.