5:15 PM - 6:45 PM
[PEM17-P16] Examination of electron density in the Earth's magnetosphere and ionosphere using the satellite potential observed by the Arase
Keywords:Arase spacecraft, BepiColombo spacecraft, magnetosphere and ionosphere, electron density, spacecraft potential
The Arase spacecraft has surveyed geospace since March 2017 and provided a variety of data that contributes to the determination of electron density. Density is the fundamental information that determines the structure of magnetosphere, plasmasphere, and ionosphere. It also determines the dispersion relation of waves, which affects their growth, decay, and propagation.
For Arase, electron density in the data archive is based on the UHR (upper hybrid resonance) frequency (time resolution: 1 min), which is identified by a combination of automatic and visual determination in the electric spectrum (10 kHz to 10 MHz) of the PWE/HFA in conjunction with the observed magnetic field strength. This method can derive the electron density with high accuracy, including in regions with a low-temperature component below 10 eV. However, it is difficult in the regions where stronger waves are observed near the UHR frequency or in low-density regions where the UHR wave intensity is weak. Therefore, there are several regions where density is not determined well. Other spacecraft have used low-energy particle data for plasma density. However, electrostatic plasma analyzers onboard the Arase, LEPe and LEPi, do not cover below ~20 eV, so density derivation from particle measurements require assumptions on the distribution function below 20 eV.
Spacecraft potential, which is determined by the balance between photoelectron outflow and ambient electron inflow, can also derive the electron density. The accuracy is less, because the spacecraft potential depends on the photoelectron and secondary electron yields, shape and attitude of the spacecraft, and the ambient electron temperature. However, the observed quantities can be easily obtained with some accuracies and 1-spin (about 8 sec) resolution without visual determination or specific assumptions. Therefore, it is potentially possible to derive density data with some degree of confidence. Compared with the relationship between plasma density (determined by ion measurements) and spacecraft potential in Geotail and Cluster, it is known that electron density derived from Arase's spacecraft potential has large errors below 1/cc. There may be caused by inaccurate electron density derived from the UHR frequency, the insufficient antenna length of 15 m (Geotail and Cluster: 50 m) which means that Debye length is easily larger than the antenna length in low-energy region, etc.
In order to solve those issues, we started to evaluate the accuracy of plasma density derivations throughout the entire Arase orbit. First, we analyze the correlation between spacecraft potential and UHR frequency from April 2017 to April 2022 (with electron density information derived by UHR frequency) and evaluate its location and time dependences. And we investigate the cases in which electron density derived from UHR is far from the one in proportion to 'exp (spacecraft potential)' or the potential-density experimental relationships in Geotail and Cluster. Next, we will also make a comparison with the low-energy electron and ion density and temperature data derived from LEPe and LEPi.
The spacecraft potential is relatively measured on the potential of the wire-antenna probe with a bias current which compensate for the imbalance between outgoing photoelectrons and incoming ambient electrons. The accuracy and stability of probe potential also affects to the accuracy of the electric field and low-frequency waves. Fluctuations in spacecraft potential seen in low-density region may indicate a problem with the stability of the probe potential. In this case, the spacecraft potential problem is directly related to the accuracy of electric field measurements of Arase, which has been the subject of much controversy. This work will also contribute to the accuracy of density and electric field measurements of BepiColombo/Mio spacecraft using the same type of probes, which will start observations on orbit in 2026.
For Arase, electron density in the data archive is based on the UHR (upper hybrid resonance) frequency (time resolution: 1 min), which is identified by a combination of automatic and visual determination in the electric spectrum (10 kHz to 10 MHz) of the PWE/HFA in conjunction with the observed magnetic field strength. This method can derive the electron density with high accuracy, including in regions with a low-temperature component below 10 eV. However, it is difficult in the regions where stronger waves are observed near the UHR frequency or in low-density regions where the UHR wave intensity is weak. Therefore, there are several regions where density is not determined well. Other spacecraft have used low-energy particle data for plasma density. However, electrostatic plasma analyzers onboard the Arase, LEPe and LEPi, do not cover below ~20 eV, so density derivation from particle measurements require assumptions on the distribution function below 20 eV.
Spacecraft potential, which is determined by the balance between photoelectron outflow and ambient electron inflow, can also derive the electron density. The accuracy is less, because the spacecraft potential depends on the photoelectron and secondary electron yields, shape and attitude of the spacecraft, and the ambient electron temperature. However, the observed quantities can be easily obtained with some accuracies and 1-spin (about 8 sec) resolution without visual determination or specific assumptions. Therefore, it is potentially possible to derive density data with some degree of confidence. Compared with the relationship between plasma density (determined by ion measurements) and spacecraft potential in Geotail and Cluster, it is known that electron density derived from Arase's spacecraft potential has large errors below 1/cc. There may be caused by inaccurate electron density derived from the UHR frequency, the insufficient antenna length of 15 m (Geotail and Cluster: 50 m) which means that Debye length is easily larger than the antenna length in low-energy region, etc.
In order to solve those issues, we started to evaluate the accuracy of plasma density derivations throughout the entire Arase orbit. First, we analyze the correlation between spacecraft potential and UHR frequency from April 2017 to April 2022 (with electron density information derived by UHR frequency) and evaluate its location and time dependences. And we investigate the cases in which electron density derived from UHR is far from the one in proportion to 'exp (spacecraft potential)' or the potential-density experimental relationships in Geotail and Cluster. Next, we will also make a comparison with the low-energy electron and ion density and temperature data derived from LEPe and LEPi.
The spacecraft potential is relatively measured on the potential of the wire-antenna probe with a bias current which compensate for the imbalance between outgoing photoelectrons and incoming ambient electrons. The accuracy and stability of probe potential also affects to the accuracy of the electric field and low-frequency waves. Fluctuations in spacecraft potential seen in low-density region may indicate a problem with the stability of the probe potential. In this case, the spacecraft potential problem is directly related to the accuracy of electric field measurements of Arase, which has been the subject of much controversy. This work will also contribute to the accuracy of density and electric field measurements of BepiColombo/Mio spacecraft using the same type of probes, which will start observations on orbit in 2026.