**1. Introduction**

In volcanology, the volcanic cloud-top height (VCTH) is one of the most critical parameters to retrieve. It affects the quantitative estimation of volcanic cloud ash and gases parameters [1–3], the mass eruption rate needed for the transport and deposition models [4–6] and the definition of the most dangerous zone for air traffic. Exploiting their global coverage (in time and space), satellite sensors offer the unique possibility for an effective monitoring of VCTH. In recent years, many techniques have been developed exploiting the dark pixel brightness temperature [2], the CO2 [7,8] and O2 [9,10]

absorption bands, the radio occultation [11] and backward trajectory modelling [12]. Among the different techniques (for a complete review of advantages and drawbacks please refer to [13]) and satellite active systems as CALIPSO [14], several algorithms have been developed exploiting the parallax between remote sensing measurements collected by different views of the same object by using one or more instruments. The use of the parallax was introduced by Prata and Turner [15] using the dual view of the along track scanning radiometer (ATSR) and developed further by Mims et al. [16], Nelson et al. [17], and Flower and Kahn [18] using dedicated multiangle imager spectro radiometer (MISR) measurements. Zakšek et al. [19], Corradini et al. [20], and Merucci et al., [13] developed algorithms based on the combined use of polar-geostationary, ground based-geostationary and geostationary-geostationary measurements respectively.

De Michele et al. [21] generalized the method based on small parallax for virtually all push broom sensor data. The extraction of the VCTH in the form of a plume elevation model from the high-resolution push broom operational land imager (OLI) sensor on board a Landsat-8 satellite, has been demonstrated starting from raw data. The main idea expressed in [21] is that the physical distance between the panchromatic sensor (PAN) and the multi-spectral sensors (MS), both on Landsat-like satellites, yields a baseline and a time lag between the PAN and MS image acquisitions during a single passage of the satellite. This information can be used to extract a spatially detailed map of VCTH from virtually any multi spectral push broom system, called a plume elevation model (PEM).

The main difficulty of the data processing comes from the fact that one Landsat image is composed of 14 focal plane modules (FPMs) arranged in the so called 'staggered' geometry, which makes the joint retrieval of plume velocities and heights challenging. De Michele et al. [21] addressed this problem by reconstructing a new OLI image starting from the raw OLI data stripes (courtesy of National Aeronautics and Space Administration (NASA), pers. comm.). However, the raw Landsat-8 data are not the standard Landsat-8 products provided by NASA/ United States Geological Survey (USGS). The raw data are available only on demand and consist of 14 data stripes (one data stripe for each FPM) multiplied by the number of multispectral bands. The standard product for Landsat 8 is the ortho image, available at no cost for the end-user. For PEM extractions, the advantages of using the standard Landsat products instead of the raw data mainly include the fast -ready to use- availability of the data, free to registered users, which is of major importance during volcanic crises. In this study, we adapt the methodology described in [21] to standard Landsat-8 products, with the aim of simplifying the procedure for routine use, thus widening the usability of this method for producing PEM maps. In this study, the procedure will be applied to the standard Landsat 8 data collected during the 26 October 2013 Mt. Etna eruptive episode and the results compared with those obtained using different satellites systems.

The paper is organized as follows: Section 1 outlines the 26 October 2013 Etna eruption and Section 2 describes the PEM procedure applied to the standard Landsat-8 products. In Section 3 the results obtained are compared with the VCTH retrieved from the PEM procedure applied to the estimations realized using the spinning enhanced visible and infrared imager (SEVIRI) and the moderate resolution imaging spectroradiometer (MODIS). In Sections 4 and 5 the discussion and the conclusions are presented.

#### *The 26 October 2013 Mt. Etna Eruption*

Mt. Etna activity in 2013 was characterized by a sequence of 16 episodes of intense eruptive activity at the summit of the volcano, fed by the New Southeast Crater [22,23]. The 26 October 2013 episodes stand as the 14th of the year and the 39th paroxysm episode of the sequence started earlier in 2011 [24–26]. The eruption occurred after a few months of quiescence and started in the early morning on 25 October, displaying mild intra-crater Strombolian activity. On 26 October, the eruptive activity gradually increased in magnitude and frequency of explosions, and lava started pouring from the crater slowly expanding towards the Valle del Bove. In the early morning of 26 October, the explosion intensity increased markedly, and between 2:00 and 10:00 UTC the activity climaxed into a lava

fountain. Over the paroxysm, the height of the lava fountain steadily reached ~200 m above the crater rim. A significant emission of gas, ash, and lapilli formed an eruptive column that rose convectively several kilometres above the summit of the volcano. The eruption ceased progressively in the late evening, and marked ash fall was reported to be dispersed by the wind southwest of the volcano proximally and distally down to the Ionian Mediterranean Sea [27]. Figure 1 shows the Orthorectified Landsat 8 image collected the 26 October 2013 at 09:37 UTC. The volcanic cloud is clearly visible.

**Figure 1.** Orthorectified Landsat 8 data acquired the 26 October 2013 at 09:37 UTC on Mt. Etna volcano (image courtesy of National Aeronautics and Space Administration (NASA)/United States Geological Survey (USGS)).
