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An open magnetopause model of the Earth's distant tail based on ISEE 3 evidence

1988; American Geophysical Union; Volume: 93; Issue: A3 Linguagem: Inglês

10.1029/ja093ia03p01795

2156-2202

S. Grzȩdzielski, Wiesław M. Macek,

Geomagnetism and Paleomagnetism Studies

It is shown that an “open” hydromagnetic equilibrium model of the Earth's distant magnetotail can describe the gross features of the cross‐tail dimensions, the magnetic field intensity, and the average plasma density in the tail lobes, as measured by ISEE 3. The tail flaring rate changes (decreases) downstream at around X ∼ −(100–120) R E . Beyond that distance our model predicts a noncircular flattened tail whose average diameter is 2 R T = 2|yz| 1/2 = 60–70 R E at down‐tail distances of X ∼ −200 R E and 2R T ∼ 70–110 R E at X ∼ −1000 R E . These values agree with measurements by ISEE 3 and Pioneer 7, respectively. The degree of flattening y/z (east‐west/north‐south dimension ratio) is related to the degree of anisotropy of the plasma pressure (p ∥ /p ⊥ ratio) and to the degree of anisotropy of the field pressure in the solar wind. The most probable flattened tail, elongated in the east‐west direction, is obtained in our model (for p ∥ /p ⊥ ∼ 1) with an y/z ratio of 1.4–1.7 at X ∼ −200 R E and 7–12 at X ∼ −;1000 R E . A tail cross section elongated in the north‐south direction (y/z < 1) would require a large p ∥ /p ⊥ ratio, exceeding the threshold for the excitation of the fire hose instability in the solar wind. The magnetopause in our model is assumed to be partially open as implied by the observed electron distribution functions, by magnetic field data, and by the presence of mantle‐type plasma in the distant tail lobes. The degree of openness α, defined as the fraction of the ambient interplanetary magnetic field lines that become connected to the lobe field lines, is evaluated to be α ∼ 0.1. If the initial magnetic flux in each lobe is taken to be 0.65×10 9 Wb and if α = 0.1, we obtain a decrease of the average (X aberrated) component of the tail lobe field from |B T | = 7.5–8.5 nT at X ∼ −200 R E to |B T | ≲ 5 nT at X ∼ −1000 R E . The related lobe plasma density increase between these distances is from n T = 0.3–0.4 to 0.8–1.5 cm −3 . The field and plasma values obtained are in reasonable agreement with the observations. The total length of the tail depends on α, and the value α = 0.1 allows the tail to reach X ∼ −(2–3) × 10³ R E as supported by the evidence of magnetospheric tail‐associated phenomena seen in Pioneer 7 data. Higher values of up to α = 0.2 cannot be ruled out on the basis of plasma data, but in that case the magnetic flux loss from the lobes becomes too fast to permit the tail to extend beyond X ∼ −(700–1000) R E .