2010 THEMIS SCIENCE NUGGETS

Magnetic Flux Rope Generation by Magnetopause Reconnection at Multiple Sites

by Hiroshi Hasegawa

Introduction

Magnetic flux ropes, consisting of helical magnetic field lines, are often observed at and around the outer boundary of Earth's magnetosphere known as the magnetopause. They are called flux transfer events (FTEs) and are believed to arise from a time-dependent form of magnetic reconnection at the magnetopause. FTEs could be a key element in the solar wind-magnetosphere interaction, because they can be a channel through which plasmas of solar origin are injected into the near-Earth space, and/or an agent regulating the transport of solar wind energy into the magnetotail.

FTEs have often been attributed to reconnection triggered at a single point, or along a line, on the magnetopause. However, there are also suggestions that they are a consequence of reconnection at multiple sites. Thanks to THEMIS multipoint observations of the magnetopause and its surrounding regions, we for the first time identified an FTE that emerged from multiple reconnection sites.

Observations

Figure 1a displays data from four of the THEMIS probes during an FTE on June 14, 2007 (near solstice), observed at the dayside low-latitude magnetopause. The data at ~0400 UT exhibit the typical signatures of FTE: an increase in the magnetic field intensity (|B|) and bipolar variation in the field component (Bx) along the average magnetopause normal. The observed FTE shows a peculiar feature (Vz reversal) as well: it was preceded by a northward plasma jet and was followed by a southward jet. We confirmed that these two jets originated from magnetopause reconnection and, importantly, were directed toward the FTE.

Figure 1. (a) THEMIS observations of a flux transfer event (FTE), the signature of a magnetic flux rope generated at the magnetopause (MP), sandwiched by oppositely directed plasma jets (positive and then negative Vz). (b) Magnetic field structure of the FTE reconstructed from the THEMIS data, with the in-plane field lines represented by black lines and the out-of-plane field Bm shown in color.

Click each image to enlarge.

Figure 1b shows a two-dimensional map of the magnetic field structure of the FTE, reconstructed from the THEMIS data. If the two reconnection jets (along +L and -L directions) were colliding, the flux rope constituting the FTE would be squeezed by the jets. The map indeed shows a consistent feature: the FTE core was elongated in the magnetopause normal (N) direction. Moreover, the THB probe, located on the magnetosheath (shocked solar wind) side of the magnetopause, detected counter-streaming, magnetic-field-aligned beams of heated electrons (Figure 2), probably generated by reconnection; the field lines just outside (magnetosheath side) of the magnetopause were interconnected with geomagnetic field lines on both sides of the FTE. These observations demonstrate that this FTE was generated between two reconnection sites.

Figure 2. Schematic drawing of our FTE, summarizing various features confirmed by THEMIS.

Click each image to enlarge.

Implications

Our analysis not only shows that an FTE can emerge from multiple reconnection sites, but also gives support for a specific model of FTE generation. Some global magnetohydrodynamic simulations of the solar wind-magnetosphere interaction suggest that FTEs are generated more often when the geomagnetic dipole axis is tilted toward or away from the Sun, and that the resulting FTEs tend to move into the winter hemisphere (Figure 3). The THEMIS observation is compatible with this scenario in that our FTE occurred near summer solstice (in the northern hemisphere) and was traveling into the southern, namely winter hemisphere.

Figure 3. Schematic drawing of Earth's magnetosphere (view from dusk), illustrating possible seasonal variations in the generation region and motion of FTEs. The occurrence of magnetopause reconnection at multiple sites, leading to the formation of FTEs and likely preferred for summer/winter (b), could regulate the transport of solar wind energy (magnetic flux) into the magnetotail.

Click each image to enlarge.

The simulations imply that magnetopause reconnection at multiple sites as a whole does not lead to as efficient energy transport as that at a single site. This is because in the presence of multiple reconnection sites, reconnection may not necessarily occur between the IMF and closed geomagnetic field lines; a reconnection site may not create new open field lines, with one end anchored to the Earth and the other end connected to the solar wind (Figure 3b). As a result, the transport of open magnetic flux (solar wind energy) into the tail, which forms the basis of active magnetospheric phenomena including substorms, may become less efficient. If this is right and if more reconnection sites are formed for larger dipole tilt (Figure 3), it is likely that FTEs are indirectly linked with the widely known seasonal variations of geomagnetic activity (weaker activity in summer/winter).

Source

H. Hasegawa, et al. (2010), Evidence for a flux transfer event generated by multiple X-line reconnection at the magnetopause, Geophys. Res. Lett., 37, L16101, doi:10.1029/2010GL044219.

Biographical Note

Hiroshi Hasegawa is an assistant professor at the Institute of Space and Astronautical Science of Japan Aerospace Exploration Agency (JAXA). His research focuses on transport of solar wind mass and energy across the magnetopause and into the magnetosphere.


Please send comments/suggestions to
Emmanuel Masongsong / emasongsong@igpp.ucla.edu