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 predominan...

Observations of slow flows in the distant plasma sheet

“…In reality, the region of acceleration may be some distance from the observation site. Slavin et al [1985] have described the change in lobe density and magnetic field with respect to downtail distance. They find that in the region X > -100 RE, the lobe density increases with downtail distance, as the plasma mantle fills in the lobes, while the magnetic field strength decreases.…”

A simple model of the flux content of the distant magnetotail

“…[33] The model has not explicitly addressed the issue of closed field line regions sometimes observed in the flanks of the magnetotail at distances of a few hundred R E [e.g., Slavin et al, 1985]; it has been suggested that these are associated with a curved distant X-line [Slavin et al, 1985] or are nearEarth flank field lines which have been carried downtail by diffusive momentum transfer from the magnetosheath [e.g., Mist and Owen, 2002]. We offer an alternative interpretation, noting that draped c+ open field lines produced by lobe reconnection will coat the flanks of the tail with regions of B z -positive magnetic field, which could be misinterpreted as closed field lines if viewed in the equatorial plane.…”

“…The transport of open flux from the dayside to the nightside owing to the antisunward flow of the solar wind is now known as the Dungey convection cycle [Dungey, 1961[Dungey, , 1963 and results in the formation of a magnetotail of length $1000 R E [Dungey, 1965]. There is also a ''disconnected'' tail, which comprises the highly kinked solar wind field lines newly reconnected at C. It is now thought that reconnection can take place at two locations in the magnetotail: at a ''distant'' X-line $150 R E downtail during quiet times [e.g., Slavin et al, 1985], and a ''near-Earth'' X-line during substorms near 20-30 R E [e.g., Siscoe and Cummings, 1969;McPherron et al, 1973;Russell and McPherron, 1973]. The only tail X-line that is of importance to this study is the one that is actively reconnecting open lobe field lines, and in the rest of this paper tail reconnection is considered as a single process.…”

Boundary layer formation in the magnetotail: Geotail observations and comparisons with a global MHD simulation

“…It is also generally assumed, that reconnection of lobe field lines occurs at almost any time. During geomagnetically quiet times, reconnection is believed to occur in the distant tail at an X-line that is located around 100 RE from earth [Slavin et al, 1985], while during geomagnetically active times, one or more X-lines develop closer to earth, leading to the formation of plasmoids or flux ropes that are ejected through the distant tail [Hones, 1979]. The models predict that convection associated with the reconnection sites is generally earthward on the earthward side of an X-line and tailward on the tailward side [Dungey, 1961;Cowley, 1980].…”