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¹ European Space Agency, ESTEC, Noordwijk, The Netherlands e-mail: miho.janvier@esa.int

² Université Paris-Saclay, CNRS, Institut d'Astrophysique Spatiale, 91405 Orsay, France

³ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA

⁴ Northumbria University, Newcastle upon Tyne NE1 8ST, UK

⁵ Mullard Space Science Laboratory, UCL, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK

⁶ Leibniz-Institut für Astrophysik Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany

⁷ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Univ. Paris Diderot, Sorbonne Paris Cité, 5 place Jules Janssen, 92195 Meudon, France

⁸ Laboratoire Cogitamus, rue Descartes, 75005 Paris, France

⁹ Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany

¹⁰ Instituto de Astrofísica de Andalucía (IAA-CSIC), Apartado de Correos 3004, 18080 Granada, Spain

¹¹ Physikalisch Meteorologisches Observatorium Davos, World Radiation Center, 7260 Davos, Switzerland

¹² ETH Zürich, Institute for Particle Physics and Astrophysics, Wolfgang-Pauli-Strasse 27, 8093 Zürich, Switzerland

¹³ Astrophysics Research Centre, School of Mathematics and Physics, Queen's University Belfast, University Road, Belfast BT7 1NN, Northern Ireland, UK

¹⁴ Department of Meteorology, University of Reading, Reading, UK

¹⁵ Donostia International Physics Center (DIPC), Paseo Manuel de Lardizabal 4, 20018 San Sebastián, Spain

¹⁶ College of Science, George Mason University, Fairfax, VA 22030, USA

¹⁷ RAL Space, UKRI STFC Rutherford Appleton Laboratory, Harwell, Didcot OX11 0QX, UK

¹⁸ Solar-Terrestrial Centre of Excellence – SIDC, Royal Observatory of Belgium, Ringlaan -3- Av. Circulaire, 1180 Brussels, Belgium

¹⁹ Leibniz-Institut für Sonnenphysik, Schöneckstr. 6, 79104 Freiburg, Germany

²⁰ Adnet Systems, Inc., NASA Goddard Space Flight Center, Code 671, Greenbelt, MD 20771, USA

²¹ Institute of Theoretical Astrophysics, University of Oslo, Oslo, Norway

²² Southwest Research Institute, Boulder, CO, USA

²³ European Space Agency, ESAC, Villanueva de la Cañada, Spain
# Appendix A: Complementary EIS and AIA data analysis explanation

## A.1. EIS and AIA alignment

To compare the EIS data with AIA 304 Å, the two datasets were spatially aligned by extracting slits of emission from the AIA image sequence in order to reproduce the time-y image from the EIS He II 256.32 Å line. The IDL routine *aia_make_eis_raster* was used for the alignment and is available from the GitHub repository pryoung/sdo-idl. Further details are available in EIS Software Note No. 26 (Young 2023a). The best alignment was found by adding an offset of (+8",+15") to the EIS data in addition to the standard EIS-AIA offset obtained from the IDL routine *eis_aia_offset*. All EIS pointings discussed in Sect. 4.2.2 include this correction.

## A.2. LOS to radial speeds conversion

Doppler shifts of the EIS emission lines yield line-of-sight (LOS) plasma speeds, while the tracking of intensity features in the AIA images yields plane-of-sky (POS) speeds. Combined, the two speeds can then give the true plasma velocity, but given the complexity of the filament evolution it can be difficult to associate an imaged feature with a spectral feature. An alternative is to assume the filament erupts radially and then the POS or LOS speed can be converted to the plasma velocity. The position of the filament projected back onto the solar disk can be made based on the flare ribbons that are assumed to be underneath the filament and near the photospheric level. Comparing an AIA 171 Å image from 13:35 UT with the SPICE images at this time (Fig. 11), we estimate the position angle to be 65° (from the Earth-Sun line). Thus the LOS and POS speeds should be multiplied by 2.4 and 1.1, respectively, to estimate the radial speeds.

# Appendix B: SPICE astigmatism and its effect on the Doppler analysis

The astigmatism of the SPICE optics produces false velocity signals that are correlated with the intensity gradients in the image. The primary source of these false velocity signals is due to intensity gradients along the slit, which is negatively correlated with apparent velocity. There is also a smaller (positive) correlation with the intensity perpendicular to the slit.

A simple IDL routine was developed to explore how the intensity gradients could be affecting Fig. 13. The intensity gradient at a pixel location *j* along the slit was estimated by subtracting the intensity at pixel *j* − 1 from that at pixel *j* + 1, and compared with the fitted velocity at pixel *j*. Similarly, the

<figure><img src="image_20.png" /></figure>

**Fig. B.1.** Sample correlations between the intensity gradients along (top) or across (bottom) the SPICE slit with the velocity derived from line fitting. The verticle dashed lines represent the ±1σ range that was excluded from the linear fit (solid line). This sample was taken from the C III raster taken between 13:15:36 and 13:20:57 UT. The strong blueshift from the erupting filament is clearly visible between the dashed lines.

intensity gradient at raster position *i* was estimated by subtracting the intensity at position *i* − 1 from that at *i* + 1. A sample comparison is shown in Fig. B.1. A linear fit was made along each dimension, excluding the region within 1σ of zero gradient, where the real solar signal is assumed to lie. We used the SSW routine *poly_fit.most.pro* to filter out extreme values, but the results do not differ much from what a simple linear fit would give for most images. The slopes of these fits were then multiplied into the gradient along each dimension, and combined to form a proxy for the instrumentally generated velocity. These proxy maps are shown in Fig. B.2.

Examining Fig. B.2, it is clear that the strong blue-shift of the erupting filament in Fig. 13 has no counterpart in the instrumental proxy, and therefore can be safely taken as solar in origin. The proxy images do show velocity signals at the locations of the flare ribbons that are similar to those in Fig. B.2, and thus most are assumed to be instrumental in origin. A possible exception is the strong redshift feature associated with the eastern primary ribbon in the 13:15:36–13:20:57 data; the simulation shows both red and blue shifts, but the data show only redshift, and much stronger than the simulation. Similarly, the strong redshift features seen in the 15:29:46–15:35:06 data do not seem to have an obvious counterpart in the simulated data. However, no attempt is made to interpret these features in the present work.
# Appendix A: Complementary EIS and AIA data analysis explanation

## A.1. EIS and AIA alignment

To compare the EIS data with AIA 304 Å, the two datasets were spatially aligned by extracting slits of emission from the AIA image sequence in order to reproduce the time-y image from the EIS He II 256.32 Å line. The IDL routine *aia_make_eis_raster* was used for the alignment and is available from the GitHub repository pryoung/sdo-idl. Further details are available in EIS Software Note No. 26 (Young 2023a). The best alignment was found by adding an offset of (+8",+15") to the EIS data in addition to the standard EIS-AIA offset obtained from the IDL routine *eis_aia_offset*. All EIS pointings discussed in Sect. 4.2.2 include this correction.

## A.2. LOS to radial speeds conversion

Doppler shifts of the EIS emission lines yield line-of-sight (LOS) plasma speeds, while the tracking of intensity features in the AIA images yields plane-of-sky (POS) speeds. Combined, the two speeds can then give the true plasma velocity, but given the complexity of the filament evolution it can be difficult to associate an imaged feature with a spectral feature. An alternative is to assume the filament erupts radially and then the POS or LOS speed can be converted to the plasma velocity. The position of the filament projected back onto the solar disk can be made based on the flare ribbons that are assumed to be underneath the filament and near the photospheric level. Comparing an AIA 171 Å image from 13:35 UT with the SPICE images at this time (Fig. 11), we estimate the position angle to be 65° (from the Earth-Sun line). Thus the LOS and POS speeds should be multiplied by 2.4 and 1.1, respectively, to estimate the radial speeds.

# Appendix B: SPICE astigmatism and its effect on the Doppler analysis

The astigmatism of the SPICE optics produces false velocity signals that are correlated with the intensity gradients in the image. The primary source of these false velocity signals is due to intensity gradients along the slit, which is negatively correlated with apparent velocity. There is also a smaller (positive) correlation with the intensity perpendicular to the slit.

A simple IDL routine was developed to explore how the intensity gradients could be affecting Fig. 13. The intensity gradient at a pixel location *j* along the slit was estimated by subtracting the intensity at pixel *j* − 1 from that at pixel *j* + 1, and compared with the fitted velocity at pixel *j*. Similarly, the

<figure><img src="image_21.png" /></figure>

**Fig. B.1.** Sample correlations between the intensity gradients along (top) or across (bottom) the SPICE slit with the velocity derived from line fitting. The verticle dashed lines represent the ±1σ range that was excluded from the linear fit (solid line). This sample was taken from the C III raster taken between 13:15:36 and 13:20:57 UT. The strong blueshift from the erupting filament is clearly visible between the dashed lines.

intensity gradient at raster position *i* was estimated by subtracting the intensity at position *i* − 1 from that at *i* + 1. A sample comparison is shown in Fig. B.1. A linear fit was made along each dimension, excluding the region within 1σ of zero gradient, where the real solar signal is assumed to lie. We used the SSW routine *poly_fit.most.pro* to filter out extreme values, but the results do not differ much from what a simple linear fit would give for most images. The slopes of these fits were then multiplied into the gradient along each dimension, and combined to form a proxy for the instrumentally generated velocity. These proxy maps are shown in Fig. B.2.

Examining Fig. B.2, it is clear that the strong blue-shift of the erupting filament in Fig. 13 has no counterpart in the instrumental proxy, and therefore can be safely taken as solar in origin. The proxy images do show velocity signals at the locations of the flare ribbons that are similar to those in Fig. B.2, and thus most are assumed to be instrumental in origin. A possible exception is the strong redshift feature associated with the eastern primary ribbon in the 13:15:36–13:20:57 data; the simulation shows both red and blue shifts, but the data show only redshift, and much stronger than the simulation. Similarly, the strong redshift features seen in the 15:29:46–15:35:06 data do not seem to have an obvious counterpart in the simulated data. However, no attempt is made to interpret these features in the present work.
<figure><img src="image_21.png" /><figcaption>**Fig. B.2.** Simulated Doppler velocity maps representing a proxy for the effect of the astigmatism in the SPICE optics on the velocities derived from line profiles in Figure 13. The times correspond to the observation time.</figcaption></figure>
<figure><img src="image_22.png" /><figcaption>**Fig. B.2.** Simulated Doppler velocity maps representing a proxy for the effect of the astigmatism in the SPICE optics on the velocities derived from line profiles in Figure 13. The times correspond to the observation time.</figcaption></figure>