29. Particle dynamics in the heliospheric current sheet

V. V. Zharkova1 and O.Khabarova2
1School of Computing, Informatics and Media, University of Bradford
2Heliophysical Laboratory, Institute of Terrestrial Magnetism, Ionosphere and Radiowave Propagation (IZMIRAN), Troitsk, Russia

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Introduction

The interplanetary magnetic field in the heliosphere is divided into two sectors of open magnetic field lines, with opposite magnetic field polarities separated by the equatorial plane, This magnetic field reversal is known as the sector boundary. The sector boundary is considered to be the heliospheric current sheet, which governs the magnetic field reversals from and toward the Sun along the Parker spiral. During the declining phase of solar cycle 22, datasets from  instruments on the Wind spacecraft showed quasi-recurrent patterns of the sector boundaries, and at those boundaries high-energy electrons are found to reverse direction [1]. But the electrons at these boundaries do not always behave in the way that they are expected to, and there are strange distributions of turbulence, ion density and ion velocity around the heliospheric current sheet, which have been difficult to explain. In this nugget, we show results from particle-in-cell simulations of magnetic reconnection in the heliospheric current sheet [2] which reproduce well both the electron and ion distributions, and explain the wave turbulence, providing solutions to 3 long-standing puzzles.

Puzzle number 1: the mismatch between magnetic and electron definitions of the sector boundary

On many occasions, direct measurements of the 3D magnetic field allows us to define reasonably well the time and location of the magnetic field reversal and thus the sector boundary [3-5]. But observations of energetic solar wind electrons reveal that the locations of their motion reversals do not always coincide with those of these magnetic field reversals [6]. The mismatch between the location of the sector boundary deduced from the electrons’ pitch angle, and measured from the change of sign of the interplanetary magnetic field has been proposed to be a consequence of large-scale magnetic field entangling – but this is rather difficult to imagine. It has been proposed that another definition of the sector boundaries be introduced, based on the pitch-angle spectrograms of 320 eV electrons, at the point where intense beams of electrons reverse their direction of motion. However, the electron behaviour with respect to the magnetic structure can be well explained in our model.

Figure 1: Upper: Snapshot of simulated particle trajectories/densities on the x-Vz phase plane in the vicinity of a current sheet. Current sheet parameters are shown under the subfigure. The densities are shown in blue for protons and in black for transit electrons (with higher velocities) and bounced electrons, as a function of distance from the SB. In the PIC simulations the ion/electron mass ratio is 100.Lower: The spectrogram of the electron flux at the energy of 27 eV as a function of pitch angle (WIND SWE data, the electron pitch angle change by 180 degrees is seen in the bottom plot, on the right hand side).

We have used particle-in-cell simulations to examine the behaviour of electrons and ions in a reversing magnetic field, and find that the unexpected observations of electron behaviour can be explained by rather dynamic physical conditions in the current sheet, caused by ongoing magnetic reconnection induced by passing disturbances, such as coronal mass ejections, or shocks. Electrons are accelerated in the reconnection regions, and high energy ‘transit electrons’ leave the current sheet with energies of a few hundreds of eV, where as electrons with energies of a few tens of eV traveling towards the sector boundary cannot reach it, and instead ‘bounce’, turning by 180 degrees before they reach the midplane of the current. The population of bounced electrons forms either a horseshoe shape (for strong interplanetary magnetic fields, of 10-8nT) or medallion shapes (for weaker IMF magnitudes of 10-9nT) which is often observed (Fig.1, bottom plot). The distance at which electrons change their pitch angles is the larger for bigger guide field, again in agreement with observations (see Table 1 in [6]).

Puzzle no. 2 – plasma density variations during sector-boundary crossings.

Across the heliospheric current sheet, the ion density varies rapidly, with a sharp maximum at the sector boundary and smaller maxima on either side (Fig. 2), and substantial large-scale plasma turbulence is often observed in the vicinity [7, 8]. The plasma density is likely to reflect the distribution of trajectories of the transit and bounced protons inside the current sheet, and our simulations show that proton trajectories have a main peak created by the ‘transit’ proton gyration about the sector boundary during acceleration (Fig 2b) while the ‘bounce’ proton gyration is reflected in the two secondary peaks located from the both sides of SB. The transit and bounced electron populations lead to rapid growth of Langmuir turbulence, caused by the Buneman instability.

Figure 2: Upper: The observed charge density in the heliospheric current sheet at the passing the Sector Boundary (blue vertical line. Observations of the double or triple density peaks about the sector boundary for the event shown in Fig. 1. Lower: simulated trajectories/densities for bounced protons (the wider peaks in the top panel), and transit protons (bottom panel and the sharp peak in the centre on the top panel)

Puzzle no. 3 – Ion velocity profiles across the sector boundary.

Figure 3: (a) the solar wind velocity profile around the day of the sector boundary crossing; this shows a superposed epoch analysis of 1322 events; (b) the polarisation electric field in the 3D heliospheric current sheet: the dot-dashed component corresponds to the IMF components typically observed around the sheet.)

Measurements from the Interball-1 and Wind spacecraft, made between 1995 and 2005, revealed rather peculiar profiles of solar wind velocity across the heliospheric current sheet (Fig. 3a). There is an unexplained decrease of the solar wind velocity 1-3 days before the spacecraft cross the sector boundary, and a sharp increase 1-3 days afterwards. Although in about 50% of cases the post-crossing increase can be explained by the presence of a co-rotating interaction region approaching the heliospheric current sheet, the velocity decrease before the SBC has been a mystery.

Our simulations offer an alternative explanation of the observed asymmetric profiles of the solar wind velocity during passage of the sector boundary by the effect of the polarisation (Hall) electric field induced by the separation of electrons and protons with respect to current sheet midplane, which governs the motion of protons and ions about the sector boundary.

Conclusions

In this nugget we illustrate how a view of the heliospheric current sheet which is dynamically reconnecting, thus providing an electric field for the acceleration of electrons and protons, can explain three different aspects of the particle and wave distributions observed during sector boundary crossings. More details can be found in the paper by Zharkova and Khabarova (2012).

References

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