An ISEE 3 study of average and substorm conditions in the distant magnetotail

Abstract

Average and substorm conditions in the lobe and plasma sheet regions of the earth's magnetotail are studied as a function of downstream distance and east‐west location using ISEE 3 magnetometer and plasma analyzer measurements. On the basis of 756 magnetopause crossings a low‐latitude magnetotail diameter of 60±5 RE at |X| = 130 ‐ 225 RE is determined. The strength of the lobe magnetic field from |X| = 20 to 130 RE is shown to fall off as X−0.53±0.05. Flaring ceases on average at |X| = 120 ± 10 RE with a relatively constant BL = 9.2 nT beyond that distance. The ratios |By/Bx| and |Bz/Bx| in the translunar tail lobes are small and relatively constant with mean values of 0.10 and 0.06, respectively. These results are shown to be in good agreement with the Coroniti‐Kennel flaring tail models of lobe magnetic field configuration with slightly enhanced By due to Maxwell stresses exerted at the magnetopause by the solar wind. The plasma parameters Vx, ne, βe, and MA in the lobes all increase with distance down the tail while Te decreases. The mean values of these lobe quantities at |X| = 200 ‐ 220 RE are Vx = −200 ‐ 250 km/s, n = 0.1 ‐ 0.2 cm−3, βe = 0.02 ‐ 0.05, MA = 0.3 ‐ 0.4, and Te = 5 ‐ 8×105 °K. Strong density and weak velocity and temperature gradients are observed as ISEE 3 moves from the center of the lobes out toward the magnetopause. In particular, factor of 3–6 increases in plasma density are observed as the spacecraft moves from the center of the tail at |Y′| < 10 RE (Y′ refers to the aberrated GSM system) toward the dawn and dusk portions of lobes at |Y′| > 20 RE. Good agreement is found between the leaky magnetopause model of Pilipp and Morfill (1978) and the strong density/weak velocity gradients observed in the lobes. Substorm activity, as measured by AE(9), is only weakly correlated with magnetic field strength, electron beta, or Alfvénic Mach number in the lobes at |X| > 200 RE. The plasma sheet magnetic field intensity and electron temperature decrease with increasing downstream distance, while flow speed, density, and Alfvénic Mach number all increase. Average plasma sheet parameters at |X| = 200 ‐ 220 RE are B = 4.0 nT, Vx = −500 km/s, ne = 0.3 cm−3, Te = 1.2×106 °K, and MA = 2.7. Electron beta is independent of downstream distance with a mean value of approximately 0.7. On the basis of pressure balance arguments the estimated total plasma beta in the |X| > 60 RE plasma sheet is 4.5, and the Ti/Te ratio is 5.5. With respect to reconnection, the most significant results are the correlations between Bz, Vx, and AE(9) in the plasma sheet, the variation in these parameters with X and ± Y, and their implications for the location of the distant neutral line. The highest tailward flow speeds are found to be proportional to the magnitude of the embedded southward Bz. Furthermore, both tailward Vx and southward Bz are shown to be well correlated with AE(9). Earthward of |X| = 100 RE the average Bz is northward and the flow is on average sub‐Alfvénic. Between |X| = 100 and 180 RE the flow becomes predominantly tailward and super‐Alfvénic, MA = 1 ‐ 2, across the entire width of the tail. However, the average magnetic field is found to be southward only in a 10 RE wide region near the aberrated noon‐midnight meridian. At |X| = 180 ‐ 225 RE the flow speed is somewhat higher, MA = 2 ‐ 3, and the width of the region of southward Bz grows to 30 RE. Tailward flow speed and electron temperature exhibit maxima in the central portion of the plasma sheet where Bz < 0. However, at |Y′| > 15 RE, where Bz > 0, the flow direction remains tailward, albeit at a reduced speed. The magnetic field results are interpreted in terms of a curved distant neutral line which is located at |X| = 100 ‐ 140 RE near local midnight. Along the flanks of the magnetotail closed field lines are apparently still being swept tailward at |X| = 200 RE. In summary, there is general agreement between the ISEE 3 magnetic field and plasma measurements in the distant magnetotail and many of the predictions of reconnection theory, but the existing theoretical models will have to be extended to three dimensions before these observations can be fully understood.