Probing the magnetic structure of a pair of transpolar arcs with a solar wind pressure step

13 We present observations of the northern hemisphere auroras taken with the Far UV cam14 eras onboard the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) space15 craft during a compression of the magnetosphere by a solar wind pressure step on 30 De16 cember 2001. The compression occurs during a period of northward IMF which has given 17 rise to the presence of a pair of transpolar arcs (TPAs) near the dawnside oval. The com18 pression causes a brightening of the oval, from dayside to nightside over the course of 19 10 mins, followed by a brightening of the midnight sector oval and TPAs from nightside 20 to dayside, again over 10 mins. We suggest that the brightening is caused by pitch an21 gle scattering of particles trapped on closed magnetic field lines, and that the sequence 22 of the brightening tracks the solar wind pressure step as it progresses along the length 23 of the magnetotail. Travelling at 600 km s−1, the step reaches up to 90 RE down-tail 24 over the period of brightening, suggesting that the magnetic field lines which map to the 25 TPAs are closed and stretch almost this length down-tail. 26 Plain Language Summary 27 The auroras usually take the form of ovals surrounding the geomagnetic poles, but 28 occasionally an auroral feature bisects the dim region within the ovals: a transpolar arc. 29 Although the geomagnetic conditions that give rise to TPAs are well-understood, there 30 is continued controversy regarding how TPAs are formed and the structure of the mag31 netosphere during their presence: are the magnetic field lines associated with the TPA 32 connected into the interplanetary medium outside the magnetosphere (are open), or do 33 they link from one hemisphere to the other (are closed). In this study we use observa34 tions of the brightening of the auroral oval and a pair of TPAs in response to a sharp 35 increase in the pressure of the solar wind. The oval first brightens from the dayside to 36 the nightside, and then the TPAs brighten from nightside to dayside, allowing us to track 37 the progression of the solar wind step along the length of the magnetotail. This confirms 38 that the TPA field lines are closed and stretch for up to 90 Earth radii down-tail. This 39 allows for the first time the magnetic structure of a TPA to be deduced, probing a re40 gion of the distant magnetotail that is rarely accessed by spacecraft. 41

of the brightening tracks the solar wind pressure step as it progresses along the length 23 of the magnetotail. Travelling at 600 km s −1 , the step reaches up to 90 R E down-tail 24 over the period of brightening, suggesting that the magnetic field lines which map to the 25 TPAs are closed and stretch almost this length down-tail. 26 Plain Language Summary 27 The auroras usually take the form of ovals surrounding the geomagnetic poles, but 28 occasionally an auroral feature bisects the dim region within the ovals: a transpolar arc. 29 Although the geomagnetic conditions that give rise to TPAs are well-understood, there  The main controversy is whether the magnetic field lines associated with the TPA 55 are open (interconnected with the IMF) or closed (connected to the opposite hemisphere).

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The surrounding polar cap is open, magnetically conjugate with the magnetotail lobes.

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An early assumption was that TPAs were associated with flow shears in the polar cap 58 convection pattern driven by lobe reconnection, leading to field-aligned currents carried 59 by precipitating electrons (see, e.g., Carlson and Cowley (2005)). This does not straight-60 forwardly explain the source of these electrons, as the magnetotail lobes are known to 61 be generally devoid of plasma. Also, with a change in the IMF a shear flow can weaken 62 -2-©2020 American Geophysical Union. All rights reserved.    The present study goes some way to answering this question, by considering au-107 roral observations from a period when the magnetosphere is struck by a solar wind pres-108 sure step, which causes a brightening of the auroral oval and a pair of pre-existing TPAs.

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The observed sequence of brightening suggests that the TPAs are indeed closed, and that 110 these closed field lines stretch as far as 90 R E behind the Earth.

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-3-©2020 American Geophysical Union. All rights reserved.      TPAs respond. We note that examination of Fig. 1(b) suggests that although the au-199 roral oval brightens promptly, the brightening of the TPAs is delayed by 10 or more min-200 utes.

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To examine this delay in more detail, Figure 3 presents the sequence of WIC and 202 SI12 images (first and third rows, respectively) from just after the arrival of the step to 203 24 mins after. To aid the eye, the second and fourth rows present difference images, that 204 is, each image subtracted from the image immediately before, with red and blue indi-205 cating that the image has brightened or dimmed, respectively.     its whole length, which corresponds to the brightening of TPA2 seen in Fig. 2(b). 220 We examine this combined brightening of the midnight sector oval and TPA1 in     that they mapped to similar distances down-tail. TPA2, which formed more recently than 302 TPA1, brightened last, suggesting that its field lines mapped even further down-tail. As 303 the two TPAs appeared adjacent to each other in the polar cap, the magnetic mapping 304 from the ionosphere to the distant magnetotail was complex. The inferred mapping of 305 TPA1 suggests that the edges of the TPA may be associated with field-aligned currents 306 which could arise due to a reduction of the cross-tail current in the local time sector of 307 the TPAs. 308 The region to which the TPAs mapped are rarely accessed by spacecraft. This unique 309 set of observations has allowed us to probe the complex magnetic structure of the dis-