2008 THEMIS SCIENCE NUGGETS

Multi-point In-situ and Ground-based Observations During Auroral Intensifications

by Andrei Runov

Introduction

The Earth’s magnetosphere is formed by a stretching of the Earth’s intrinsic magnetic field by the solar wind, a stream of protons continuously ejected by the Sun. As a result, the magnetotail (an anti-sunward elongated cavity in the solar wind populated by the stretched Earth’s magnetic field and tenuous hot plasmas, partly of solar wind and partly of ionspheric origin) appears on the night-side of the planet. It is very dynamic and sensitive to changes in the solar wind and the interplanetary magnetic field (IMF). Particularly, when the IMF is directed southward, i.e. opposite the Earth's magnetic field, both fields may reconnect which leads to a transfer of the magnetic flux from the day-side to the night-side of the magnetosphere, and an increase of the potential energy, accumulated in the Earth’s magnetotail. An instability in the magnetotail current sheet, separating the oppositely directed magnetic fields, may lead to the release of the stored energy, resulting in energization and acceleration of the magnetotail plasma and excitation of electromagnetic waves. Accelerated by interaction with the waves, particles moving along the magnetic field line may reach the Earth's ionosphere and produce visible aurora. This complex of phenomena is known as a magnetospheric substorm. Although the substorm concept has been known for more than 40 years, the physics of substorms is not well understood.

The plasma population in the magnetotail (the so-called plasma sheet) is not uniform: plasma properties and magnetic fields in the near-Earth plasma sheet differ from those in the mid and/or distant tail. Therefore, energy release processes in the different regions are different. To understand substrom physics, it is essential to monitor mid-tail and the near-Earth regions of the plasma sheet simultaneously. For this, a multi-spacecraft mission called THEMIS was designed. The THEMIS mission includes five identical spacecraft and a dense network of ground-based observatories, monitoring auroral activity and corresponding perturbations in the Earth’s magnetic field.

Observations

Figure 1 presents a schematic of THEMIS spacecraft positions, the measurements in the mid-tail and near-Earth plasma sheet, provided by probe 2 (P2) and probe 4 (P4), respectively, and ground-based measurements from the Gusbay observatory (Eastern Canada). The vertical dashed lines in P2 and P4 stack plots indicate time instances of auroral activation seen by the Gusbay observatory (marked by red arrows in the GBAY data plots).

The ground-based data shows two successive increases of auroral luminosity (upper panel) and corresponding wave-like variations of the Earth’s magnetic field, referred to as Pi2 pulsations. The spacecraft data are shown in the Geocentric Solar Magnetospheric (GSM) coordinate system with X directed sunward, Y directed duskward and Z directed northward along the magnetic dipole. P2, located in the mid-tail, was near the centre of the plasma sheet. The detected variations of the magnetic field (upper panel) indicate that P2 crossed the current sheet (the boundary between Bx>0 and Bx<0). Panels 2 and 3 show energy-time spectrograms of ion population in the plasma sheet at P2's location. The distinctive features are two bursts of high-energy ions (up to 0.5 MeV) near the two auroral onsets. The 4th and 5th panels show the ion concentration and velocity (in the spacecraft frames of reference): two bursts of the tailward (Vx<0) flow, associated with some density reduction, are clearly visible at the time of the two auroral onsets. The two bottom panels show electron energy-time spectrograms. The spectra are rather complex, showing first electron energization, then a dropout of energetic electrons. The spacecraft P1, located further down-tail of P2, generally saw the same signatures about 40 seconds after P2. At around the first onset, P4, located closer to the Earth, saw only small-amplitude variations of the magnetic field. Near the second onset, however, P4 detected dramatic changes: the magnetic field collapsed, ion and electron energy jumped up to 0.5 MeV or more and the ion concentration grew almost in an order of magnitude. The observations aboard P3, located near P4, reveal the same signatures.

Schematic of the THEMIS major conjunction

Figure 1. A schematic of the THEMIS major conjunction; in-situ
observations by THEMIS Probe 2 and 4; ground-based optical
and magnetic observations at GBAY station during 0140 – 0210
UT on March 1, 2008.

Click here to enlarge the image.

A Scenario

A suitable model explaining the observed signatures should include i) a mechanism of plasma acceleration in the mid-tail, ii) a mechanism of particle energization in the near-Earth plasma sheet, and iii) a link between these two processes. The most plausible and commonly accepted mechanism of plasma acceleration due to conversion of the magnetic energy into plasma kinetic and thermal energy is known as magnetic reconnection. According to the reconnection model, tailward flows, which appear tailward of the reconnection region, are associated with southward (negative) Bz. Indeed, the red curve in the P2 magnetic field plot (upper panel) shows clear negative deflection during the first tailward flow bursts and less clearly during the second. Earthward of the reconnection region, the Earthward fast flow with northward (positive) Bz is expected. In our case, however, P2 and P1 are situated tailward of the reconnection site, and P3/ P4 pair are located too far away from the current sheet (Bx at P4 is about 40 nT around the first onset) to detect the Earthward fast flow, since reconnection jets are very narrow in both vertical and azimuthal directions. A fast plasma flow interacting with ambient plasma and magnetic fields can create electric currents flowing along magnetic field lines (field-aligned currents, or FAC) in the ionsphere. These FACs may be detected far from the current sheet as small-amplitude variations of the magnetic field, mainly in the Y and Z components. Indeed, as mentioned above, P3 and P4 detected the magnetic field variations at around the first onset. They may be interpreted as signatures of FAC, associated with the missed Earthward flow.

What happened in the near-Earth plasma sheet then? The Earthward flow brings magnetic flux to the region of the more dipolar magnetic field which leads to a pile-up of the flux. At some stage the sum of the magnetic pressure gradient and the force due to curvature of the magnetic field lines exceeds the plasma pressure gradient, which leads to the instability of the inner edge of the plasma sheet. As a result, magnetic energy is rapidly converted into plasma (particle) energy. This burst-like process is characterized by a fast decrease of Bx and By, a growth of Bz (so called dipolarization), and a sudden increase of plasma energy. All these were observed by P3/P4 during the second onset.

It is worth noting that the second tailward flow was detected by P2 and P1 in the mid-tail almost simultaneously with the bursty energy conversion, observed by P3/P4. Thus, the processes in the mid-tail and near-Earth plasma sheet do not exclude each other and may act at the same time.

Magnetic reconnection scenario

Figure 2. A scenario: Magnetic reconnection (RX) in the
mid-tail --> Fast flow --> Magnetic flux pile-up --> instability
in the near-Earth plasma sheet.

Click here to enlarge the image.

Remaining Questions

Although this scenario may explain the observations, many details of this complex process are still unknown and need to be investigated. Particularly, further studies are needed to understand

  • An initial instability, which makes magnetic reconnection possible;
  • Details of particle (ions and electrons) energization up to 0.5 MeV or even more;
  • The physics of the instability in the near-Earth plasma sheet (commonly referred to as Current Disruption).
  • These questions are to be addressed by further experimental studies as well as by computer simulations and theoretical modeling.

    Biographical Note

    Andrei Runov is a research space physicist with the Institute of Geophysics and Planetary Physics at the University of California, Los Angeles. His current research interests are the physics of magnetospheric substorms, dynamics of space current sheets, and particle energization in space plasmas.



    Please send comments/suggestions to
    Amanda Prentice/aprentice@igpp.ucla.edu