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tokens: outlines the global expansion of the FR. Later on, this prominence eruption was accompanied by a CME observed by the Large Angle and Spectrometric Coronagraph (LASCO, Brueckner et al. 1995 ) onboard the SOHO satellite and it is given in the SOHO/LASCO CME catalog 1 ( Yashiro et al. 2004 ). The CME first appears in the LASCO C2 field-of-view at ≈15:45 UT. The speed of the CME leading edge is 452 km s −1 so it is less than twice its velocity in the low corona (Fig. 3 b). At the distances observed by C2, the CME was already decelerating with a mean value of deceleration of 12 m s −2 . 2.2. Photospheric magnetic field We analyze the AR magnetic configuration with the HMI magnetic field data. Since the studied eruption was at the limb on 02 March 2015, the photospheric magnetic field is not available. Therefore, we follow its associated AR evolution when it was on the solar disk. AR NOAA 12290 was on the eastern limb on 18 February 2015. One day later, it started to show on the solar disk as a small bipolar AR with a negative leading and a positive following polarity. During its disk crossing, it progressively grew in size as its magnetic field got dispersed by convective motions. Then, AR 12290 was well in its decay phase when it reached the western limb. Figure 4 shows four magnetograms of the longitudinal field component with a time cadence of one day. The AR was located at ≈15° of longitude after the central meridian on 25 February 2015 in panel a, and at ≈55° of longitude on 28 February 2015 in panel d. The magnetograms are spatially de-rotated to the central meridian so that the spatial extension of the polarities could be compared between the different panels. However, we kept the longitudinal component so projection effects appeared as the AR approached the limb (mostly on the limb side of the leading polarity where fake positive polarities appear). The magnetic field mostly disperses with time in each polarity so that the polarities become more extended. Then, the global bipolar magnetic configuration is progressively larger in size, which is expected to induce a global expansion of the coronal loops. This dispersion also implies that the polarities of opposite signs become into contact at the PIL. This induces cancellations of the magnetic flux, for example, at the locations surrounded by a green ellipse in Figs. 4 a and c. These cancellations of flux are best seen in the movie associated with Fig. 4 . Such cancellations imply a progressive buildup of an FR above the PIL ( van Ballegooijen Martens 1989 ; Amari et al. 2003 ; Green et al. 2018 ). Finally, the FR becomes unstable leading to an eruption (Sect. 2.1 ). Fig. 4. Evolution of the AR magnetic field from 25 to 28 February 2015. The locations of the flux cancellation are shown by green ellipses. The location of the H α filament on 28 February 2015 is displayed in panel d with a red contour. A movie of magnetic field during 25−28 is available online . The above analysis shows that