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10 Years of Observations


Ten years ago, on November 2, 2009, the PROBA2 ESA micro-satellite was launched on a Rockot launcher from the Russian launch base Plesetsk, into a sun-synchronous orbit (dawn/dusk polar orbit). More details of the launch and orbit can be found here.

PROBA2 was launched as the second of the European Space Agency's (ESA) fleet of PROBA satellites, which is part of ESA's in-orbit Technology Demonstration programme. PROBA2 hosts 17 new technological developments including two main solar instruments (SWAP and LYRA) designed for studying all events on the Sun that might have implications on the solar-terrestrial connection, both through imaging (SWAP) as well as through irradiance measurements (LYRA). More details on the technology developments can be found here.

Satellite ejection

Figure 1. An Artists impression of the PROBA2 separation from it's launch vehicle, image courtesy of ESA. 

SWAP provides images of the solar corona filtered at a wavelength around 17.4 nm, a bandpass (filter) that corresponds to a temperature of roughly 1 million degrees, allowing us to see the hot solar atmosphere while filtering out the relatively cooler solar surface. SWAP has a cadence (rate at which images are taken) of 1 image every 2 minutes, and observes an exceptionally wide field-of-view (FOV) of 54 arcmin, allowing it to see more structures around the edge of the Sun. SWAP was designed as a compact instrument, with a new off-axis optical design, radiation resistant, and uses a new CMOS-APS detector. SWAP's large FOV was unique in the field of EUV imagers providing the first continuous observations of the extended solar atmosphere.

Figure 2 shows SWAP images from several days spread throughout the mission, the top row shows images from 2010-Feb-20, 2011-Feb-01, 2012-Jan-20, 2013-Feb-05 and, 2014-Jan-28 respectively. The bottom row shows images from, 2015-Jan-19, 2016-Feb-05, 2017-Jan-22, 2018-Feb-02 and, 2019-Feb-01 respectively. The images are processed to show the extended solar atmosphere (the bit around the edge). Observing the structured nature of the extended solar atmosphere is one of the most important observations made by SWAP.

It has long been known that the Sun undergoes an 11-year activity cycle, where solar activity, such as the magnitude and number of flares and coronal mass ejections, fluctuates. The images in Figure 2 reflect this variability through the changing the number of coronal holes (the dark regions) and the number of active regions (the bright structures), which are often the source of the more dramatic solar activity. In 2010 the Sun was reaching the end of its solar minimum phase, its activity increased, peaking at solar maximum around 2014, before once again declining towards solar minimum, which we're currently experiencing (at time of writing). More information about how SWAP observes solar rotation, the solar activity cycle and its different phases can be found here.

SWAP Images

Figure 2. Ten years of SWAP observations, from 2010 to 2014 on the top row and from 2015 to 2019 on the bottom row, showing the changing face of the solar atmosphere.
LYRA is a solar UV-EUV radiometer, monitoring the Sun through 4 channels relevant to Solar Physics, Space Weather and Aeronomy, and hosts one of the first Lyman-Alpha irradiance monitors. Unlike the SWAP imager, LYRA observes the Sun as a star, and measures the UV-EUV output from the whole sun as a single data point. LYRA was the first instrument to incorporate a new type of diamond sensor, designed to withstand the harsh radiation environment of space.
The high signal-to-noise ratio of the LYRA detectors allows them to acquire full-sun measurements at an extremely high-cadence (up to 100 measurements per second). This cadence is required for the detailed study of transient solar events such as solar flares. In particular, LYRA was able to characterise the strongest flare of the current solar cycle, an X9.3 class flare (classifications are catagorised here) on the 6th of September 2017, and make rare observations in its Lyman-Alpha and Herzberg channels. LYRA detected small-amplitude oscillations called quasi-periodic pulsations in its raising phase. Such oscillations, can only be made with high cadence observations, like those of LYRA. Such observations are shedding new light on the flaring mechanism itself.

Figure 3. LYRA light curves of the 6th of September 2017 X9.3 flare. 

Space Weather. SWAP and LYRA were originally designed for studying the Sun and energetic events, such as solar flares, coronal holes and CMEs, that might have implications on the solar-terrestrial connection. Naturally, the observations have become an integral part of several solar-monitoring space weather forecasting centers, and as a consequence PROBA2 has been supported by ESA's Space Situational Awareness (SSA) program since 2013. Several tools have also been developed to utilize the data and track the location of events including the PROBA2 SSA services page, used to track flares and active regions on the Sun, and SWAP/LYRA data products served through the Solar Weather Expert Service Centre (S-ESC) page. Figure 2 above shows several images of the Sun. Active regions (the bright structures) and coronal holes (the dark regions) can clearly be seen in these static images, both of which can have implications for space weather at the Earth. SWAP and LYRA are also monitoring more transient, energetic and eruptive, phenomena such as flares and CMEs, Figure 4 below shows SWAPs ability to track eruptions into the extended solar atmosphere. SWAP and LYRA will continue to monitor such structures providing a warning for potential space-weather events. PROBA2 mission support is foreseen to be continued by ESA’s Space Safety Programme starting in 2020.

Offlimb PROBA2 eruption

Figure 4. A flare (top left panel) and eruption observed by SWAP on 2017-Apr-1. The eruption was traced to over 1-Solar Radii, highlighted by the dashed line. 

Vital Statistics. PROBA2 was launched under the ESA Technology, Engineering and Quality Directorate (D/TEC). After launch, the ESA Science Directorate (D/SCI) supported the scientific exploitation of the PROBA2 science instruments and the Belgian PI-teams were supported by PRODEX. Due to PROBA2's solar monitoring capabilities, it has been supported by ESA's SSE-SSA program in recent years.

Since its launch, PROBA2 has:

  • been in orbit 3653 days.
  • Orbited the Earth ~53,000 times.
  • Produced ~30,000 LYRA data files.
  • Produced ~2,090,000 SWAP images.
  • Passed our ground stations in Redu, Belgium and Svalbard, Norway (Arctic) 32,453 times.
  • Helped produce over 100 peer-reviewed papers, the full list can be seen here.

The PROBA2 Science Centre at the Royal Observatory of Belgium has had the honour of running a guest investigator program. The program has been hugely successful welcoming scientists from across the planet, giving them the opportunity to help operate and understand the instruments on PROBA2, with a view to running dedicated scientific campaigns.  The program has had 8 calls from 2010 to 2018, and welcomed 64 teams, 81 team members from 16 countries.


Figure 5. The ESA Redu station, which tracks, communicates with, and supports the PROBA-series of satellites. 

Heritage. The technology on board PROBA2 has directly fed into several avenues of scientific research and instrumentation. LYRA has helped shape the field of research of the aforementioned quasi-periodic pulsations observed in flares, whereas SWAP has revolutionised how we observe the extended solar atmosphere, and in particular the middle corona, leading to the development of several large field-of-view imagers, especially for space weather monitoring purposes, allowing scientists to link observations of the lower solar atmopsphere to the heliosphere.

The next generation of CMOS detectors, successors to SWAP detector, fed directly into the development of the EUI EUV detectors on Solar Orbiter, ESA's new flagship solar mission, to be launched in 2020. The ESA is also planning a dedicated space weather monitor, to be launched to the L5 Lagrange point (to the side of the Sun from the perspective of the Earth). It's anticipated that the Lagrange mission will host an EUV imager using the same detectors as EUI, producing an exceptionally large field-of-view EUV images. Thiw will allow for the tracking of solar eruptions close to the Sun towards the Earth, providing an early warning to potentially damaging solar events.

Since PROBA2's launch, the ESA's PROBA program has already developed and launched the successful PROBA-V (V for vegetation) mission in May 2013, which provides multispectral images of vegetation cover on a daily and global basis, to study the evolution of vegetation cover. The Vegetation instrument can distinguish between different land cover types and plant species, including crops, to reveal their health, as well as detect water bodies and vegetation burn scars. For the solar community, ESA is currently working with a consortium to develop the PROBA-3 mission. A novel and unique mission to develop the world’s first precision formation flying coronagraph mission. Coronagraphs observe the extended faint solar atmosphere by creating artificial eclipses in space, an occulter is placed in front of the imager blocking the bright solar surface. PROBA-3 will fly a pair of satellites in formation with the occulter on an external spacecraft. The mission will not only help develop formation flying technologies in space, but also provide unique observations of the solar atmosphere.
Figure 6. An Artists impression of the PROBA3 formation flying coronagraph, with the imager on the left platform, and the external occulter on the right spacecraft, blocking the Sun, image courtesy of ESA. 
What next for PROBA2? As discussed above, the Sun undergoes an 11-year activity cycle, and next year will mark the 11th anniversary of the PROBA2 mission and therefore the monitoring of the Sun for a full solar cycle. This landmark period will allow PROBA2 to probe the Sun's evolution over the long term, comparing two solar minimum periods. The instruments themselves are proving robust in the harsh radiation environment. The detectors, which had never been used in space before, still produce a clean signal, with very low levels of noise. SWAP on the other hand has seen less than 10% degradation in the number of active pixels, a remarkable number after 10 years of operations.
To celebrate 10 years of observations, the PROBA2 team are putting together a 10-year anniversary topical collection of articles, a follow-up of the successful 2013 Topical Issue that highlighted the scientific and operational achievements after the first two years of the mission. The 10-year anniversary edition will report on the health and status of the misison as well as various studies ranging from the middle corona to solar flares.
The aforementioned PROBA2 guest investigator program has been extended by BELSPO/PRODEX funding, and the PROBA2 PI-team is currently welcoming research proposals for the ninth round of its Guest Investigator program for research based on SWAP and LYRA data analysis by scientists outside the SWAP and LYRA PI-teams. More information can be found here.
The Sun Then and Now. Figure 7 below shows an image of the Sun from the start of the mission (on the left), and a current image (on the right). A large active region (the bright structure) can clearly be seen in the left image and coronal holes (the dark regions) to the north. SWAP and LYRA will continue to monitor such structures providing a warning for potential space-weather events.

Now and Then

Figure 7. Then & Now, the above figure shows two segments of the Sun, the left segment is from one of the first SWAP images, the right segment is from one of the latest images.