ToBeOrNotToBe

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Observing the same feature with two spacecraft (SC1, SC2) located at different positions in the heliosphere, is crucial for our understanding of how LOS integration affects remote-sensing image data. Each pixel intensity in an image represents the sum over the intensities that lie along the LOS. We may simply check LOS effects by selecting regions of interest from SC1’s point of view and reconstructing their corresponding LOS for data of SC2. Selected regions of interest (ROI) are EUV wave signatures and coronal dimming regions that occurred on 6 September 2011 in association with a X-class flare/halo CME event. This event was observed on-disk by SDO/AIA and in quadrature by STEREO-A/EUVI, where the eruption site was located close to the Western limb.
Observing the same feature with two spacecraft (SC1, SC2) located at different positions in the heliosphere, is crucial for our understanding of how LOS integration affects remote-sensing image data. Each pixel intensity in an image represents the sum over the intensities that lie along the LOS. We may simply check LOS effects by selecting regions of interest from SC1’s point of view and reconstructing their corresponding LOS for data of SC2. Selected regions of interest (ROI) are EUV wave signatures and coronal dimming regions that occurred on 6 September 2011 in association with a X-class flare/halo CME event. This event was observed on-disk by SDO/AIA and in quadrature by STEREO-A/EUVI, where the eruption site was located close to the Western limb.
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[[File:Nugget_fig1.jpg|700px|thumb|center| 
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Figure 1: Investigating the EUV wave location and kinematics using SDO and STEREO-A data observing the event from two different vantage points.
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EUV wave: as can be seen in Figure 1 [[File:Nugget_fig1.jpg]] the identified EUV wave signatures from SDO/AIA (green and orange dots indicate the the peak and leading front of the wave) do not match those identified from STEREO-A/EUVI (blue and red crosses mark the wave front at the solar limb and at a height of about 90 Mm). Following the reconstructed LOS for the EUV wave positions of SDO (indicated by the lines in the corresponding colors in the upper right panel of Fig.1) discovers why. The signatures observed on-disk in SDO/AIA contain contributions from the expanding CME front higher up in the corona. This significiantly influences the speed measurements of the wave. From STEREO-A’s view the resulting speed of the wave is much lower (''v'' ~ 525km/s) compared to the LOS result obtained from SDO (''v'' ~ 1000km/s).
EUV wave: as can be seen in Figure 1 [[File:Nugget_fig1.jpg]] the identified EUV wave signatures from SDO/AIA (green and orange dots indicate the the peak and leading front of the wave) do not match those identified from STEREO-A/EUVI (blue and red crosses mark the wave front at the solar limb and at a height of about 90 Mm). Following the reconstructed LOS for the EUV wave positions of SDO (indicated by the lines in the corresponding colors in the upper right panel of Fig.1) discovers why. The signatures observed on-disk in SDO/AIA contain contributions from the expanding CME front higher up in the corona. This significiantly influences the speed measurements of the wave. From STEREO-A’s view the resulting speed of the wave is much lower (''v'' ~ 525km/s) compared to the LOS result obtained from SDO (''v'' ~ 1000km/s).
Dimming areas: as given in Figure 2 [[File:Nugget_fig2.jpg]] we reconstruct the three-dimensional position and extension of the dimming areas by selecting different ROIs within the dimming identified on-disk from SDO/AIA. We mark the green region as potential core dimming, the red one as a secondary dimming region, while the cyan oval includes regions of enhanced as well as reduced emission. The orange region marks the cross-check with quiet Sun conditions. Again, the position of the ROIs are transformed to the observation of STEREO-A/EUVI (cf. middle panels of Fig. 3) from which we reconstruct the projected LOS for selected points inside the ROIs (indicated by the colored lines). One can see nicely that the LOS rays cross different regions of the expanding CME body resulting in different intensity contributions when summing up along the LOS. In this way, the CME front adds to the LOS intensity whereas the low density CME body reduces the LOS intensity. For example, the purple location within the cyan oval covers only contributions from the enhanced CME front, while the LOS intensity of the cyan cross contains contributions of both, the expanding CME bubble (here in decreased emission) as well as parts of the front. The reconstructed LOS of the core dimming region (green cross) crosses fully the center of the CME bubble, while the red location crosses first an expanding loop and later on the center of the bubble. Summing up the pixel values along the different LOS’ in STEREO-A/EUVI allow us to compare the cumulated intensity with the observed intensity of these positions in SDO. This is qualitatively true for all the locations selected and shown in the right panel of Fig. 3. One should find: purple is brighter (shows a higher intensity) than cyan, cyan is brighter than red and red is brighter than green. Besides that the core dimming region (green cross) should correspond to a maximum evacuation of plasma, since it is assumed that dense plasma, previously confined by the flux rope, is evacuated there. Indeed this region shows the highest decrease in emission.
Dimming areas: as given in Figure 2 [[File:Nugget_fig2.jpg]] we reconstruct the three-dimensional position and extension of the dimming areas by selecting different ROIs within the dimming identified on-disk from SDO/AIA. We mark the green region as potential core dimming, the red one as a secondary dimming region, while the cyan oval includes regions of enhanced as well as reduced emission. The orange region marks the cross-check with quiet Sun conditions. Again, the position of the ROIs are transformed to the observation of STEREO-A/EUVI (cf. middle panels of Fig. 3) from which we reconstruct the projected LOS for selected points inside the ROIs (indicated by the colored lines). One can see nicely that the LOS rays cross different regions of the expanding CME body resulting in different intensity contributions when summing up along the LOS. In this way, the CME front adds to the LOS intensity whereas the low density CME body reduces the LOS intensity. For example, the purple location within the cyan oval covers only contributions from the enhanced CME front, while the LOS intensity of the cyan cross contains contributions of both, the expanding CME bubble (here in decreased emission) as well as parts of the front. The reconstructed LOS of the core dimming region (green cross) crosses fully the center of the CME bubble, while the red location crosses first an expanding loop and later on the center of the bubble. Summing up the pixel values along the different LOS’ in STEREO-A/EUVI allow us to compare the cumulated intensity with the observed intensity of these positions in SDO. This is qualitatively true for all the locations selected and shown in the right panel of Fig. 3. One should find: purple is brighter (shows a higher intensity) than cyan, cyan is brighter than red and red is brighter than green. Besides that the core dimming region (green cross) should correspond to a maximum evacuation of plasma, since it is assumed that dense plasma, previously confined by the flux rope, is evacuated there. Indeed this region shows the highest decrease in emission.

Revision as of 15:57, 9 August 2016

To be or not to be - the role of projection effects in EUV imaging

Introduction

Coronal mass ejections (CMEs) are often accompanied with activities low in the solar atmosphere including, e.g., extreme-ultraviolet (EUV) waves and coronal dimmings. The bright EUV waves (also known as EIT waves; see nugget 79) are large-scale disturbances propagating through the solar atmosphere most probably driven by the laterally expanding CME flanks. The dark dimming regions represent decreased emission in EUV and soft X-rays, most probably caused by the CME expansion, and are therefore interpreted as low-coronal footprints of CMEs (see e.g. nuggets 114 & 179). Two different types of dimming regions are observed: core or twin dimmings -- stationary, long-lived regions of strongly reduced EUV emission closely located to the eruption site covering opposite magnetic polarities. Secondary or remote dimmings -- more wide spread and less dark regions extending to significant distances away from the eruption site. As the corona is optically thin, bright as well as dark features represent intensities which are integrated along the line-of-sight (LOS). When viewed from single vantage points, EUV waves as well as dimming areas might severely suffer from projection effects, which makes their interpretation tricky.


Two eyes (satellites) are better than one

Observing the same feature with two spacecraft (SC1, SC2) located at different positions in the heliosphere, is crucial for our understanding of how LOS integration affects remote-sensing image data. Each pixel intensity in an image represents the sum over the intensities that lie along the LOS. We may simply check LOS effects by selecting regions of interest from SC1’s point of view and reconstructing their corresponding LOS for data of SC2. Selected regions of interest (ROI) are EUV wave signatures and coronal dimming regions that occurred on 6 September 2011 in association with a X-class flare/halo CME event. This event was observed on-disk by SDO/AIA and in quadrature by STEREO-A/EUVI, where the eruption site was located close to the Western limb.

Figure 1: Investigating the EUV wave location and kinematics using SDO and STEREO-A data observing the event from two different vantage points.

EUV wave: as can be seen in Figure 1 Nugget fig1.jpg the identified EUV wave signatures from SDO/AIA (green and orange dots indicate the the peak and leading front of the wave) do not match those identified from STEREO-A/EUVI (blue and red crosses mark the wave front at the solar limb and at a height of about 90 Mm). Following the reconstructed LOS for the EUV wave positions of SDO (indicated by the lines in the corresponding colors in the upper right panel of Fig.1) discovers why. The signatures observed on-disk in SDO/AIA contain contributions from the expanding CME front higher up in the corona. This significiantly influences the speed measurements of the wave. From STEREO-A’s view the resulting speed of the wave is much lower (v ~ 525km/s) compared to the LOS result obtained from SDO (v ~ 1000km/s).

Dimming areas: as given in Figure 2 Nugget fig2.jpg we reconstruct the three-dimensional position and extension of the dimming areas by selecting different ROIs within the dimming identified on-disk from SDO/AIA. We mark the green region as potential core dimming, the red one as a secondary dimming region, while the cyan oval includes regions of enhanced as well as reduced emission. The orange region marks the cross-check with quiet Sun conditions. Again, the position of the ROIs are transformed to the observation of STEREO-A/EUVI (cf. middle panels of Fig. 3) from which we reconstruct the projected LOS for selected points inside the ROIs (indicated by the colored lines). One can see nicely that the LOS rays cross different regions of the expanding CME body resulting in different intensity contributions when summing up along the LOS. In this way, the CME front adds to the LOS intensity whereas the low density CME body reduces the LOS intensity. For example, the purple location within the cyan oval covers only contributions from the enhanced CME front, while the LOS intensity of the cyan cross contains contributions of both, the expanding CME bubble (here in decreased emission) as well as parts of the front. The reconstructed LOS of the core dimming region (green cross) crosses fully the center of the CME bubble, while the red location crosses first an expanding loop and later on the center of the bubble. Summing up the pixel values along the different LOS’ in STEREO-A/EUVI allow us to compare the cumulated intensity with the observed intensity of these positions in SDO. This is qualitatively true for all the locations selected and shown in the right panel of Fig. 3. One should find: purple is brighter (shows a higher intensity) than cyan, cyan is brighter than red and red is brighter than green. Besides that the core dimming region (green cross) should correspond to a maximum evacuation of plasma, since it is assumed that dense plasma, previously confined by the flux rope, is evacuated there. Indeed this region shows the highest decrease in emission.

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