Tailward Streaming Research Articles

Association between energetic particle bursts and Birkeland currents in the geomagnetic tail

IMP 8 observations in the geomagnetic tail of energetic particles at hundreds of keV and of the magnetic field are examined for a substorm period from 1700 to 1800 UT on March 3, 1976. For this interval, it is found that bursts of energetic protons detected at a downstream distance of ∼37 RE in the midnight tail region are accompanied by east‐west magnetic field perturbations indicative of Birkeland current sheets. The magnitude of the magnetic field perturbations ranges from ∼1 to 12 nT, corresponding to current densities of 1 to 9 mA/m when integrated over the current sheet thickness. These values are comparable to previous observations of Birkeland currents in the near‐earth tail (X > −20 RE) and with the auroral‐arc‐associated Birkeland current densities at low altitudes scaled out to the tail region. The favorable comparison suggests that the closure of small‐scale Birkeland current systems, at least in these instances, lies tailward of ∼40 RE. The observed Birkeland current system and the magnitude of current densities are found to be consistent with ion tearing instability in the tail. The inferred wavelength of the tearing mode along the tail axis is ∼2 to 17 RE. In the context of the tearing model, the presence of Birkeland currents and the occasional isotropic distribution of energetic electrons (Ee > 0.22 MeV) suggest that the energetic particle events in this study occur on closed magnetic field lines, even though strong tailward streaming dominates the particle anisotropy in these events.

Simultaneous observation of the plasma sheet in the near Earth and distant magnetotail: ISEE‐1 and ISEE‐3

We present particle data from the spacecraft 1981‐025 and 1982‐019 at geosynchronous orbit (6.6 RE), from ISEE‐1 in the near‐earth geomagnetic tail and from ISEE‐3 in the distant geomagnetic tail at ∼ 130 RE obtained during the first half of March 22, 1983. The spacecraft observations are supplemented by ground‐based magnetograms from near local‐midnight stations. Two substorm particle injections were detected at ∼ 0715 and 0806 UT at geosynchronous orbit. At the same time suprathermal plasma sheet protons are observed at ISEE‐1. Multiple entries and exits into and out of the plasma sheet are recorded at ISEE‐3 in the distant tail. A substorm recovery is indicated by a poleward leap of the substorm electrojet at 0900 UT. After 0900 UT ISEE‐1 and ISEE‐3 entered the plasma sheet simultaneously within a few minutes. At the boundary of the recovering plasma sheet strong earthward streaming (ISEE‐1) and tailward streaming (ISEE‐3) suprathermal protons are observed, indicating that the two spacecraft were on opposite sides of the particle source. This represents the first measurement of ion beams at the plasma sheet boundary occurring simultaneously in the near‐earth region and in the deep tail flowing in opposite directions.

Streaming reversal of energetic particles in the magnetotail during a substorm

Reversal from tailward streaming to earthward streaming of energetic ions at 0.29–0.50 MeV during a substorm on February 3, 1978, is studied with measurements of energetic particles, plasma, and magnetic field from the IMP 8 spacecraft near the dusk flank of the magnetotail. Four new features emerge when high time resolution data are examined in detail. The times of reversal from tailward to earthward streaming of energetic ions and from tailward to earthward plasma flow do not coincide. Second, the velocity distribution in the tailward flowing plasma has a crescent shape, whereas the velocity distribution in the earthward flowing plasma resembles a convecting Maxwellian. Third, tailward streaming of energetic ions is sometimes detected in northward magnetic field regions and conversely, earthward streaming in southward field environments. Fourth, energetic ions streaming earthward are occasionally present in conjunction with a strong tailward streaming population in the same energy range. These new features suggest that the streaming reversal of energetic ions and the plasma flow reversal in this event are due to the spacecraft traversing different plasma regions during the substorm‐associated configurational change of the plasma sheet and the magnetotail and is unrelated to the motion of an acceleration region such as an X type neutral line moving past the spacecraft.

Several features of the earthward and tailward streaming of energetic protons (0.29–0.5 MeV) in the Earth's plasma sheet

The characteristics of earthward and tailward streaming of energetic protons (0.29–0.50 MeV) in the magnetotail at downstream distances of 20 to 40 RE are examined with ∼5.5‐min averaged data from the APL/JHU Charged Particle Measurements Experiment on board the IMP 7 and IMP 8 spacecraft. On the basis of observations from September 1972 to May 1978 it is found that the occurrence frequency of energetic magnetospheric protons streaming either tailward or earthward with a front‐to‐back flux ratio of >2 is at least 23%. Tailward streaming is found to be prevalent in the postmidnight plasma sheet, while earthward streaming is more frequent in the premidnight sector. The particle spectrum is progressively harder from the dawn flank to the dusk flank of the plasma sheet and is generally harder for tailward streaming than for earthward streaming. It is suggested that the dawn‐dusk reversal in the dominant streaming direction results from an underlying circulation pattern of energetic protons in the magnetotail, tailward in the postmidnight region and earthward in the premidnight region.

Multiple crossings of a very thin plasma sheet in the Earth's magnetotail

High‐resolution magnetic field, plasma, and energetic particle data from the IMP 8 spacecraft have been used to study multiple crossings of the earth's magnetotail plasma sheet when it becomes thin during magnetospheric substorms. IMP 8 is found to traverse the entire thickness of the β > 1 plasma sheet in times that are as short as 10 s. Such traversals recur on a time scale of several minutes, and they are associated with high velocity plasma flows that are usually directed tailward but are occasionally directed earthward for brief intervals. We argue that these observations can be explained by rapid oscillations of a plasma sheet that is only a few thousand kilometers thick, a dimension that is comparable to the gyroradius of energetic protons. When ions of 50 keV and 290 keV are detected at a location on tail lobe field lines near the plasma sheet, they are often seen to be moving in the dawn to dusk direction and may be undergoing serpentine cross tail motion. Differences in the angular distributions of the two energies indicate that the higher energy ions are preferentially located on field lines deeper in the tail lobe. This result is consistent with ion acceleration at a neutral line with subsequent drift in a cross‐tail electric field that moves the lower energy particles closer to the equatorial plane. A neutral line acceleration model is also supported by our observation that tailward streaming energetic electrons are occasionally present at the lobe‐plasma sheet interface.

Earthward transport of energetic protons in the Earth's plasma sheet

The transport of magnetospheric energetic protons (Ep ∼ 0.29‐0.50 MeV) in the earth’s plasma sheet at downstream distances of ∼ 30‐45 Re is studied on a statistical basis with 5.5 min average data from the APL/JHU Charged Particle Measurement Experiment on the IMP 7 and IMP 8 spacecraft. The analysis shows that there is a net earthward transport of these particles, although tailward streaming is observed as frequently as earthward streaming. The net transport is earthward for all Kp conditions and for low as well as high flux thresholds. The averaged net particle flux is ∼ 2 × 10²cm−2sec−1, which translates into averaged particle and energy fluxes over the plasma sheet region of ∼ 4 × 1022sec−1 and ∼ 2 × 1016ergs sec−1, respectively. It is suggested that this particle influx could be a possible source of protons for the earth’s outer radiation belt.

Relevance of southward magnetic fields in the neutral sheet to anisotropic distribution of energetic electrons and substorm activity

The implications of southward magnetic fields at the magnetotail neutral sheet to the development of streaming anisotropy of energetic electrons and magnetospheric substorm activity are examined. Magnetic field and energetic particle measurements from the Imp 6 spacecraft, the AE index, and global auroral images from DMSP spacecraft are utilized in this study. Criteria are developed to identify events of southward magnetic fields at the neutral sheet which imply the presence of X‐type magnetic neutral lines. Several features of the observations suggest that the southward magnetic fields and the implied X‐type neutral lines are associated with magnetic bubbles in the neutral sheet region. It is found that the signatures of magnetic bubbles are sometimes detected in association with tailward streaming and flux enhancement of energetic electrons (47 keV < E < 350 keV). A cigar‐shaped anisotropy in the energetic electron distribution is frequently but not always observed before the onset of tailward streaming of energetic electrons. The tailward streaming is magnetic field‐aligned and occurs in the form of bursts, suggesting that the generating process is activated somewhat quasi‐periodically and is not in a steady state. Signatures of magnetic bubbles are also detected without any substantial enhancement or detectable tailward streaming of energetic electrons. By comparing Imp 6 observations with the AE index and global auroral images from DMSP spacecraft, it is found that signatures of magnetic bubbles in the neutral sheet are observed during substorms as well as during quiet geomagnetic conditions, indicating that magnetic bubbles are intrinsic features of the neutral sheet in the magnetotail regardless of substorm activity.

Observations of counterstreaming between plasma and energetic particles in the magnetotail

We present fine time resolution (10–20 s) measurements of the energy and angular distributions of three bursts of energetic particles in the magnetotail at distances of approximately 32 RE. These measurements were made near the onset of a magnetic substorm on October 16, 1973, by the NOAA/APL (50 ≤ Ep ≤ 220 keV) and JHU/APL (Ep ≥ 290 keV, Ee ≥ 220 kev) experiments on Imp 7 and the JHU/APL (210 ≤ Ep ≤ 560 keV) experiment on Imp 6. These energetic particle measurements are also correlated with plasma observations made by the University of Iowa Lepedea on Imp 7. These data reveal the presence of two distinct particle populations: (1) an earthward flowing hot plasma usually dominating the particle intensities up to an energy of ∼50 keV and (2) tailward streaming, nonthermal ions and electrons dominating the energy range 200 ≤ Ep ≤ 1 MeV. Nonthermal in this context signifies an energetic particle population bearing no direct relationship with the observed earthward flowing ambient hot magnetotail plasma. A careful examination of the angular distribution of the ions in the transition region (50 ≤ Ep ≤ 220 keV) has allowed us for the first time to separate the above two particle regimes. Although these initial results apply to a limited set of these events, inspection of the NOAA/APL energetic particle experiment data shows counterstreaming to be a common feature of magnetotail energetic particle events. We have been able to obtain, again for the first time, the energy spectrum of the nonthermal magnetospheric burst particles for energies ≥50 keV. These observations place a number of requirements on various theories which attempt to explain the dynamical processes in the magnetotail. No published theory to date seems to be in satisfactory agreement with all aspects of our observations.