Eruption of volcano Aoba (Ambae) in Vanuatu archipelago

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Located on the small island Aoba, or Ambae, in the South Pacific nation of Vanuatu, the volcano of the same name erupted in the days from 22 September 2017 onward.

Date & Time
22 September 2017 17:00–25 September 06:30 UTC and 1 October 19:00–21:00 UTC
Satellites
Himawari-8, Suomi-NPP, Sentinel-3
Instruments
AHI, VIIRS, OLCI
Channels/Products
Volcanic Ash & SO2 RGB, Visible, True Colour

By HansPeter Rosesli (Switzerland)

The imaging radiometer AHI on Himawari-8 covered this period with imagery in 16 radiometric bands at 10-minute intervals.

The eruptive plumes did not reach great heights, so passing cloud decks hid or mixed with the volcanic effluents. Nevertheless, the Volcanic Ash & SO2 RGB identified a couple of cyan-coloured SO2 swaths between 22 and 23 September, as shown in the image sequence from 22 Sept 17:00 UTC to 23 Sept 23:00 UTC (MP4, 5 MB).

However, the ash signal did not show up very clearly in these RGBs. While the display of various shades from a dirty red to dark blue (instead of the typical pink) hinted at the presence of ash or other effluents, it manifested rather by its dynamical behaviour, i.e. by plumes originating from the crater area.

The single frames in Figure 1, 23 Sept 00:50 UTC (left slider) and 23 Sept 04:30 UTC (right slider) show examples, where the latter frame also offers a glimpse of the hot spot.

Image comparison
Image from 23 Sept 00:50 UTC Image from 23 Sept 04:30 UTC
Figure 1: Comparison of Himawari-8 images showing the volcanic plume.

Reflected sunlight in the high resolution band VIS0.64 (spatial resolution of 0.5 km v 2 km in the infrared bands used to construct the Ash & SO2 RGB) gave some more details of the various plumes and streamers. The image sequence from 22 Sept 19:00 UTC–25 Sept 06:30 UTC (MP4, 7 MB) is three consecutive days The sequence shows many differently shaped outbursts during the daylight hours.

A couple of individual frames and their Ash & SO2 RGB counterparts may help to identify some of those in the animation.

Figure 2 is the Visible imagery showing a short initial outburst early on 22 September (locally the 23 September), that in the Ash & SO2 RGB appeared as a faint SO2 colour.

Figure 2
 
Figure 2: Himawari-8 Visible 0.64, 22 Sept 21:30 UTC
 

A little over six hours later Figure 3 shows a much stronger outburst. The VIS0.64 indicates a pulsating plume that originated from a tiny dark-grey spot, the active crater of Aoba. Due to the reduced resolution, the Ash & SO2 RGB did not show the knotty pattern, but illustrated the crater as a hot spot in dark blue.

Figure 3
 
Figure 3: Himawari-8 Visible 0.64, 23 Sept 04:40 UTC
 

On 23 September at 21:30 UTC (24 Sept locally) (Figure 4) the Visible imagery showed a southward-pointing plume that mixed with the cloud deck moving through the scene. In the Ash & SO2 RGB that plume was hardly visible.

Figure 4
 
Figure 4: Himawari-8 Visible 0.64, 23 Sept 21:30 UTC
 

One hour later, the plume was still there, and the True Colour RGB from Sentinel-3’s OLCI imaging radiometer showed the situation in more detail.

Figure 5: Sentinel-3, 23 Sept, 22:34 UTC
OLCI True Colour
Figure 6: Suomi-NPP & Himawari-8, 24 Sept, 02:30 UTC
VIIRS Natural Color RGB and Volcanic Ash
AHI Visible 0.64 and Volcanic Ash

Three hours later the scene took on a more chaotic look. On Figure 6, an image couple from Himawari-8 is compared to a Natural Color RGB and an Ash & SO2 RGB from the VIIRS imager on Suomi NPP at 375 m and 750 m spatial resolution, respectively. Note the active crater on the VIIRS images (upper row), a tiny red spot in the Natural Colour RGB and a blue blotch on the Ash&SO2 RGB. On the Natural Colour RGB the crater was surrounded by dark colours, hinting at deposited ash.

Figure 7
 
Figure 7: Himawari-8 Visible animated gif, 1 Oct, 19:00–21:00 UTC

Aoba maintained some activity after 26 September — the animated visible imagery from 1 Oct, 19:00–21:00 UTC (Figure 7) showed a weak plume moving westward in the two hours after sunrise.

 
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