Article - Issue 44, September 2010

Studying the Sun – Enabling extreme ultra violet imaging of the star

Dr Nick Waltham

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A solar eruptive prominence as seen in EUV light through the atmospheric imaging assembly onboard the Solar Dynamics Observatory © NASA

A solar eruptive prominence as seen in EUV light through the atmospheric imaging assembly onboard the Solar Dynamics Observatory © NASA

The Solar Dynamics Observatory project is a new NASA initiative responding to the need to monitor the disruptive influence the Sun has on technological systems. The ambitious project will advance understanding of the physics of the Sun’s atmosphere and how it drives space weather. Dr Nick Waltham, Head of the Imaging Systems Division within the Rutherford Appleton Laboratory, explains more about the engineering behind the UK’s contribution to the high-definition cameras on the observatory.

NASA’s Solar Dynamics Observatory was launched on an Atlas V rocket from Cape Canaveral, Florida, on 11February 2010. This latest space-based observatory is enabling scientists to study the evolution of our Sun’s volatile magnetic fields and the periodic eruption of solar storms in greater detail than ever before.

The Sun is essential in supporting life on Earth and plays a role in our planet’s climate and weather systems. However, it is also an extremely active star with disruptive effects on mankind’s technological systems.

Many will be familiar with the 11-year sunspot solar cycle, a periodic variation in the average number of sunspots seen on the solar disk first discovered in 1843. Further research early in the 20th century found that these sunspots are strongly magnetic and that the solar cycle is actually a magnetic cycle in which the Sun’s magnetic poles reverse approximately every 11 years.

It is widely thought that the Sun’s magnetic field results from a dynamo inside the Sun. The most prominent manifestations of the dynamo are the sunspots and active regions that appear on the surface. Arching above them are coronal loops, magnetic field lines revealed by the hot plasma flowing along them.

Active regions are responsible for the production of violent energy bursts called solar flares and coronal mass ejections in which huge quantities of hot plasma are released from the Sun’s atmosphere and shot out into space. A coronal mass ejection may contain a billion tonnes of coronal plasma and travel at speeds of 1,000kilometres per second.

Those that head towards the Earth can have a potentially disruptive influence on land-based communications and power grid systems as well as on space-based satellites. In 1989 one such solar storm induced DC currents that surged through the Canadian national grid, resulting in equipment failure that left the province of Quebec without power for nine hours.

Solar storms also heat the Earth’s atmosphere, the expansion of which increases the frictional drag on satellites in low Earth orbit and results in orbital decay and re-enter sooner than expected. They also disrupt communications, cause changes in the apparent position of GPS satellites and reduce the power available to satellites by damaging their solar cells.

We need new leading-edge technology to monitor the Sun and understand all of these adverse influences on our vital technological systems, and this motivated the launch of the Solar Dynamics Observatory (SDO) project.

SDO’s mission

SDO spacecraft: Overall length along the sun-pointing axis is 4.5 m, and each side is 2.2 m. The total mass at launch was 3,000 kg. The solar panels of 6.6 m2 provide 1,500 W of power. The four atmospheric imaging assembly telescopes can be seen at the top of the structure © NASA

SDO spacecraft: Overall length along the sun-pointing axis is 4.5 m, and each side is 2.2 m. The total mass at launch was 3,000 kg. The solar panels of 6.6 m2 provide 1,500 W of power. The four atmospheric imaging assembly telescopes can be seen at the top of the structure © NASA

During its nominal five year mission lifetime, extendable to 10 years, the observatory will image the Sun in many wavelengths near simultaneously and with a resolution 10 times higher than the average high-definition television. The first step in NASA’s Living with a Star initiative, this mission is enabling scientists to investigate the causes of solar variability and its effects on Earth.

SDO will look at how the Sun’s magnetic field is generated and structured, how the stored magnetic energy is propagated through the Sun’s atmosphere, the heating of the Sun’s corona to several million degrees Celsius and the space weather generated by the solar wind, solar flares and coronal mass ejections. Advanced on-board imaging instrumentation will deliver the data that scientists hope will provide a more complete understanding of the Sun’s dynamics.

SDO has been manoeuvred into a geosynchronous sun-pointing orbit that allows near-continuous observations of the Sun and the return of science data to a single dedicated ground station located at the White Sands Missile Range, New Mexico, constructed specifically for this mission. The spacecraft transmits data at 150 megabits per second enabling the return of an unprecedented 1.5terabytes of science data every day – almost 50 times more science data than any other mission in NASA history.

Solar imaging

Engineers at the UK’s Rutherford Appleton Laboratory based in Oxfordshire and e2v technologies Ltd, Chelmsford, have provided the camera electronics and charge-coupled device sensors for two of the observatory’s three remote sensing instruments, the Atmospheric Imaging Assembly (AIA) led by the Lockheed Martin Solar and Astrophysics Laboratory in Palo Alto, California and the Helioseismic and Magnetic Imager (HMI) led by Stanford University.

The AIA is studying the evolution of the solar coronal magnetic field, taking images that span 1.3 solar diameters and with a resolution of 1arcsecond. It consists of four telescopes, each with switchable filters to enable imaging in eight discrete extreme ultraviolet (EUV) wavebands, each indicative of a known plasma temperature. The AIA is providing an unprecedented combination of spatial, temporal and area coverage. Previous cameras of this resolution could only view a small region of the Sun’s disk.

The four telescopes can be read out simultaneously and an entire data set can be acquired with a repeat rate of less than 10 seconds. The primary goal of the imager’s science team is to use this information, together with data from other observatory instruments, to advance our understanding of the physics of the Sun’s atmosphere and how it drives space weather.

The HMI is observing the motions of the Sun’s solar surface, or photosphere and studying solar oscillations. It measures the polarisation of the full solar disk with a resolution of 1 arcsecond at a discrete visible-light wavelength of 617 nm. The resulting data is helping scientists to determine the internal sources and mechanisms of solar variability and how the physical processes inside the Sun relate to the surface magnetic field and how the coronal magnetic field affects variability in the extended solar atmosphere.

Imaging requirements

At the heart of these imagers is the charge coupled device (CCD). This was developed primarily as a compact solid-state image sensor for consumer and industrial markets. Today though, it has become the pre-eminent visible and ultraviolet wavelength detector in many fields of scientific research including space-science and both Earth and planetary remote sensing.

A scientific application may typically have requirements for a sensor with many millions of pixels and for the highest possible detection efficiency over the broadest possible spectral range. Wide signal dynamic range is usually another key requirement. Dynamic range is determined by the physical size of the pixels and their capacity to store electrons generated from incident photons and the sensor’s electronic readout noise. Larger pixels can store more electrons, which is why scientific CCDs are routinely very much physically larger than those used in consumer products. But this also causes them to be very much more expensive because both the number of sensors obtainable from a silicon wafer and the yield of fully functional devices decreases very rapidly as the size of the sensor increases.

Front-illuminated sensors were specified for the visible light imaging HMI instrument and back-illuminated extreme ultra-violet (EUV) sensitive sensors for the four AIA telescopes to ensure optimal sensitivity at the EUV wavelengths required for imaging of the Sun’s corona.

Weight and power considerations

Space missions invariably call for the design of extremely light and compact electronic systems of minimal power dissipation, yet they must also survive the vibrations experienced during launch and then the harsh environment of space itself including thermal control and the damaging effects of space radiation. The designer of space electronics therefore faces severe challenges that are compounded by having to work from a relatively small catalogue of high-reliability, space-qualified, radiation-tolerant components that are acceptable and approved for use by international space agencies like NASA and the European Space Agency.

Unlike today’s modern low-voltage digital electronics, CCDs are essentially analogue components that require relatively high drive voltages. Their operation relies on a sequenced clocking of multi-phase electrodes to shift the signal charge collected within the pixels through the structure and to an output charge detection amplifier.

A prime requirement of the CCD design was that it should operate with significantly lower drive voltages than previous e2v CCDs. Our motivation was to greatly simplify the design of the camera’s readout electronics. Here, the challenge lies in the design of circuitry to drive the CCD’s highly capacitive electrodes and to digitise its analogue video output signal with low noise and to high precision. If successful, it would allow us to greatly simplify the design.

Our aims were to ease the selection of appropriately space-qualified components, enable the design of simpler and more compact circuitry, and reduce power dissipation in both the CCD and the electronics. All of these benefits would also increase the reliability of the design. An added bonus would be the CCD’s greater immunity to the accumulative effects of ionising radiation over the long mission lifetime.

We achieved the lower voltage capability by manufacturing the CCD with a thinner insulating dielectric layer between the drive electrodes and the underlying silicon substrate.

We were also able to address the problem of how to convey the very low-level analogue video output signal from the CCD to the camera electronics without any extraneous noise pickup. This usually requires very careful design of the interconnecting harness and rigorous attention to the overall electrical grounding of the CCD-electronics system. To alleviate this issue, we gave each of the CCD’s output amplifiers an additional ‘dummy’ amplifier. The ‘dummy’ receives no signal charge but in all other respects presents an output of near identical electrical characteristics to the real output amplifier. Passing both signals to the camera electronics and subtracting them provides a high degree of common-mode noise rejection to noise induced in the harness.

Two CCDs service the HMI and four are spread across the AIA’s telescopes. Each device is operated and read out through its own dedicated camera electronics box. The walls of this 2.9 kg box are made of 5 mm-thick aluminium to ensure sufficient attenuation of space radiation over the five to 10 year mission life. During exposures the box consumes 12 W of power, rising to 17 W during readout of the CCD. By consumer product standards this is neither small, low-mass nor low-power. On the other hand, a typical high-street digital camera could never provide anything approaching the science-grade photometric and dynamic range performance of our cameras, nor survive the harsh thermal environment of space or the damaging effects of space radiation.

SDO launched

The atmospheric imaging assembly’s many wavebands allow regions on the Sun to be viewed in terms of a range of temperatures. This early image from the Solar Dynamics Observatory is a composite made from three of the wavelength bands, corresponding to temperatures from 0.7 to 2 million degrees (hotter = red, cooler = blue) © NASA

The atmospheric imaging assembly’s many wavebands allow regions on the Sun to be viewed in terms of a range of temperatures. This early image from the Solar Dynamics Observatory is a composite made from three of the wavelength bands, corresponding to temperatures from 0.7 to 2 million degrees (hotter = red, cooler = blue) © NASA

In total we built and qualified six flight camera electronic boxes for the AIA and HMI instruments in the Solar Dynamics Observatory, along with two flight-spare units. All these had to endure rigorous vibration and thermal cycling environmental qualification to ensure they would survive launch and then the harsh space environmental conditions and that they contained no hidden build-quality deficiencies.

The Solar Dynamics Observatory is returning imagery of phenomenal resolution, providing new insight into the evolution of our Sun’s volatile magnetic fields and solar storms. It also promises to open up many new avenues of scientific research as the mission develops.

Today our team’s efforts are undergoing the toughest test of all: 24-hours-a-day service in orbit, returning data that over the coming months and years could, we hope, not only throw light on secrets from within the sun but also spark new approaches to safeguarding from the savage power of the Sun – the technological systems on which the modern world so totally depends.

Ultimately, SDO scientists want to be able to forecast space weather events. By finding answers to the puzzles of the Sun’s magnetic activity, we hope to predict the onset of solar storms so that we on Earth can be pre-warned of potential disruption. Understanding the Sun’s variability may also tell us how the Sun will play a role in our ever changing climate.

See latest images from the Solar Dynamics Observatory at: http://sdo.gsfc.nasa.gov and http://sdowww.lmsal.com/suntoday.html

There are also free apps for iPhone or iPad. See: app 3DSun from iTunes.

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