Cold ion streams consisting of double proton populations and singly charged oxygen observed at the distant magnetopause by Geotail: A case study

Abstract

The Geotail plasma and magnetic field observations reveal that multiple ion streams with different energies coexist at the magnetopause separating the magnetosheath and the lobe/mantle in the distant magnetotail at (−187, −23, −2) RE in the GSM coordinates. The ion streams sometimes consist of three components flowing tailward. On the basis of E × B drift analysis, we have identified that two of the three streams are protons, while the residual one of the highest energy (>7 keV/q) consists of O+. The direction of the perpendicular velocities of these ion components is toward the magnetosheath from the lobe/mantle. The higher‐energy (∼2 keV/q) proton component is detected only at the magnetopause, while the other two components (the lower‐energy (<1 keV/q) protons and O+) can also be observed in the lobe/mantle. In the lobe/mantle, the bulk velocity of the O+ flow is always nearly equal to that of the lower‐energy proton stream. In the magnetosheath, the two proton streams completely merge with each other, and the O+ component cannot be recognized clearly. In the magnetopause current layer, the lower‐energy proton component is significantly accelerated in the direction perpendicular to the magnetic field due to electric field enhancement, whereas the parallel speeds of the higher‐energy protons change according to the energization rate in the current layer. The temperature of the higher‐energy proton stream is generally 50–100 eV, approximately twice as large as that of the lower‐energy proton stream in the lobe/mantle and slightly higher than that of protons in the magnetosheath. These signatures of multiple ion streams are discussed in terms of magnetic field reconnection at the lobe/mantle magnetopause.