Plasmoid‐associated energetic ion bursts in the deep geomagnetic tail: Properties of plasmoids and the postplasmoid plasma sheet

Abstract

The properties of energetic ions (>35 keV) observed in plasmoids and in the postplasmoid plasma sheet in the distant geomagnetic tail by the energetic particle anisotropy spectrometer instrument on ISEE 3 are investigated and compared with simultaneous thermal electron and magnetic field data. Two representative plasmoid‐associated bursts are discussed in detail, followed by the results of a survey of 37 such events. It is found that the bulk speeds deduced from the angular anisotropies of the energetic ions, assumed to be protons, agree with those deduced from the thermal electrons to within ∼200 km s−1 in the hot plasma regions, showing that the ions are convected with the plasma. Average speeds are 660 km s−1 in plasmoids and 840 km s−1 in the postplasmoid plasma sheet. In the energetic particle boundary layers surrounding the hot plasma regions, however, the ion anisotropies are consistent with energetic proton bulk speeds (∼1000 km s−1) which considerably exceed local plasma speeds (∼200 km s−1) such that the ions form a beam, originating in the plasma sheet, which flows along newly reconnected magnetic field lines through plasma which has tail lobe characteristics. In the plasma sheet regions a correlation is found between the energetic proton intensity and the thermal electron temperature Te, such that the intensity increases by 2 orders of magnitude as kTe increases from ∼50 to ∼200 eV. To investigate this further, the thermal ion temperature Ti, not measured on ISEE 3, is estimated from pressure balance arguments. It is found that Ti and Te are correlated, with Ti ≈ 8Te in the plasmoids and with Ti ≈ 11Te in the postplasmoid plasma sheet, such that the energetic ion fluxes are also correlated with (and presumably controlled by) Ti. Characterizing the energetic protons as a Maxwellian distribution with density np* and temperature Tp* then yields typical energies kTp* ≃ 10 keV, compared with kTi ≃ 1 keV and kTe ≃ 100 eV, and densities np* ≃ 6 × 10−3 cm−3, compared with ni ≃ ne ≃ 0.16 cm−3. Thus the energetic protons form a tail to the thermal population which is “non‐Maxwellian” in the sense that their characteristic temperature is much larger than thermal values. The energetic proton bulk speeds are also integrated to investigate plasmoid lengths. It is found that one‐loop plasmoids (as inferred from field data) have an average length ∼60 RE, while multiple‐loop plasmoids have an average length ∼110 RE. For single‐loop plasmoids the observed time delay in their appearance in the distant tail following substorm onset is consistent with pinching off at a near‐Earth neutral line (given some acceleration as the plasmoid moves downtail), while for multiple‐loop plasmoids, agreement is found only if the first loop alone is considered. This result suggests that subsequent loops are formed after substorm onset, either through tailward retreat of the substorm neutral line and further pinching off or through loop formation within the postplasmoid plasma sheet.