The Earth’s magnetopause is the last barrier between our planet and the charged particles that the Sun expels into interplanetary space, known as the solar wind. It was generally believed that we were especially well-protected when the Sun’s interplanetary magnetic field (IMF) was aligned with the Earth’s equatorial field. However, recent THEMIS observations show that a large volume of solar wind particles may actually enter across the dayside magnetopause even during such “field-aligned” (northward IMF) conditions.

Our report suggests that cracks can develop on the flanks of the magnetopause as well for such nearly “field-aligned” magnetic field conditions and during times of significant wave action along this surface. In support of this interpretation, we present highly illustrative maps which for the first time are based on real data in combination with theoretical considerations. We do not yet know how common or how wide-spread these cracks are on the flanks, but there are some indications from THEMIS that they can occur farther down the flanks.

Observations

Figure 1 illustrates the predicted outward expansion of the flank magnetopause over four of the five THEMIS probes following a sudden drop in the solar wind dynamic pressure at 0645 UT on 8 June 2007. The trailing TH-A probe (magenta color) ended up close to the predicted magnetopause location after the expansion of this surface.

Figure 1. Equatorial THEMIS spacecraft locations (five colored symbols) relative to the Earth and the magnetopause (grid) at 0645 UT on 8 June 2007. The Sun is toward the right. The predicted magnetopause location is shown before (left) and after (right) a sudden drop in solar wind dynamic pressure from 3.4 to 0.9 nPa. Axes tick marks are shown 1 Earth radius (RE) apart.

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TH-A actually crossed the magnetopause frequently during ~0645-0900 UT which suggests a continuous wave action of this surface. Figure 2 displays some TH-A data in support of this interpretation. Rather unexpectedly, we also discovered a common, outward-then-inward, bipolar fluctuation of the magnetic field as the boundary moved Earthward across the TH-A probe. The red vertical lines in Figure 2 show two such examples. Bipolar magnetic field fluctuations are an indication of tightly wound bundles of magnetic field lines which are referred to as flux ropes or magnetic islands. Islands are an expected result if the field is cut and then reconnected by the so-called “magnetic reconnection” process. Since none of the four inner probes detected the same islands that the TH-A probe observed, we estimate that the proposed islands were <0.56 R_E wide.

Figure 2. TH-A ion energy-time spectrogram, magnetic field and total pressure during a shorter time period when magnetic field fluctuations indicate the presence of magnetic islands. Two examples are shown centered at the red vertical lines. The LLBL is a magnetospheric region just inside the magnetopause. The magnetosheath and PDL are two characteristic regions of processed solar wind particles and magnetic fields. The presence of the PDL and its compressed magnetic field strength adjacent to the magnetopause may have facilitated the onset of reconnection.

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Using a novel analysis technique, we were able to use the TH-A data in Figure 2 to reconstruct a ~1000 km-wide region centered about the spacecraft path at the time it observed the first presumed island in Figure 2. The resulting maps in Figure 3 strongly support the magnetic island hypothesis. The reconstructed island was ~1000 km (0.16 R_E) wide, consistent with the maximum 0.56 R_E estimate from the multi-probe separations. Our reconstruction also shows that the island was nearly collocated with a small flow vortex. The deviation from a perfect fit suggests that the magnetic field experienced some evolution as the flow moved it around. There is even some indication of the presence of an old “magnetic reconnection X-line” at (x,y)~(1200,0) km. For more details, see http://www.agu.org/journals/pip/ja/2008JA013505-pip.pdf.

Figure 3. Reconstruction of one magnetic island observed by the TH-A probe at 0653:40 UT. The Sun is toward the right and the magnetosphere is toward the bottom of each panel. The top panel shows stream-lines as black solid lines with color showing the ion temperature. The transformed TH-A velocity observations are shown as white arrows at y=0. The middle panel shows the in-plane magnetic field in black solid lines with the color showing the third Bz component. White arrows along y=0 are the observed TH-A magnetic field. The bottom panel shows the combined stream-line (red) and field-line (black) patterns.

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Figure 4 shows a schematic illustration of the post-noon location of the observed islands (solid dot) as the magnetopause moved Earthward over the TH-A probe. The more common observation of magnetic islands at the sunward-facing side of the magnetopause surface waves suggests that they formed in the vicinity of TH-A following a local compression of the magnetopause. The compression, which may be required to initiate reconnection, could be caused by converging flow toward the magnetopause resulting from neighboring large-scale flow vortices. We propose that the same island formation process might occur farther down the magnetopause flanks as indicated by question marks in Figure 4.

Figure 4. Schematic view of the location of the magnetic islands (solid dots) at the equatorial flank magnetopause. While observed by TH-A at only one location (“FTE”), we speculate that they may be generated farther downtail (question marks) as well. The observed prevalence on the sunward-facing side of the wavy magnetopause may be due to a local compression there. The apparent steepness of the anti-sunward edges of the wave is an artifact of the illustration.

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Implications

The small-scale cracks of the magnetopause as indicated by the presence of the islands may individually be a rather insignificant path for solar particles to enter into the region dominated by the Earth’s protective magnetic field. However, if they are a common occurrence and widely distributed down-stream along the wavy magnetopause, they could as an ensemble provide a larger volume of space for solar wind particles to enter into our near-Earth region of space. More important, perhaps, is what these islands can tell us about the magnetic reconnection process and how it initiates and ceases in the vicinity of large-scale flow vortices along the wavy flank magnetopause.

Biographical Note

Stefan Eriksson is a THEMIS researcher at the Laboratory for Atmospheric and Space Physics, University of Colorado at Boulder. Hiroshi Hasegawa is a researcher at the Japanese Space Agency (JAXA) and an expert in magnetopause surface waves. Wai-Leong Teh is a post-doctoral researcher at Dartmouth College, New Hampshire, who helped develop the novel technique used in this study together with Professor Bengt Sonnerup, Dartmouth College, New Hampshire.