Staring at the Sun

      On March 13, 1989, the entire province of Quebec was left without power for nine hours when substation transformers burned out after a magnetic storm. The storm was caused by a sudden increase in brightness on the sun’s surface known as a solar flare, which is associated with the release of excessively energetic particles and radiation from every part of the electromagnetic spectrum. Although the energy of a flare is usually only a small fraction of the total energy the sun releases every minute, this can still be more heat and light than would be released by a billion megatons of TNT.

      During a solar flare, a burst of plasma can be accelerated out of the solar corona, where temperatures reach upwards of one million degrees Celsius. These events are called Coronal Mass Ejections (CMEs). When the bulk of these particles encounter the Earth’s magnetic field several days later, the resulting electromagnetic disturbance can strip relay operations and short power lines in our electrical grids. A certain fraction of these particles that is more energized can arrive at the Earth within minutes of a flare, posing a deadly danger to astronauts and satellite hardware in space.

      The 1989 magnetic storm that blacked out Quebec was caused by one of the biggest flares ever to affect Earth, causing 6.5 million dollars in equipment damage. It was, however, only a minor example of the potential disruption solar flares can cause. Imagine an output more than 1.2 million times greater than your household’s electrical system suddenly being injected into your city’s power grid. Few, if any, electrical systems in the world are designed to handle such a load.

      Some satellites already in orbit, such as SOHO (the Solar and Heliospheric Observatory), can sometimes give as much as a few days of warning for the arrival of particles from a large flare or CME. However, bearing in mind the potentially disastrous effects of CMEs, a more reliable and longerrange prediction method would be ideal.

      To predict flares, we must first understand why they occur at all. The process of energy transfer that causes particles to be ejected so rapidly is not well understood. By watching particle interactions in the Sun’s magnetosphere and photosphere during flares (the Sun’s most turbulent times) scientists hope to resolve exactly how particle acceleration is related to the heating of plasma, and to improve their understanding of the sudden particle releases exhibited by the corona.

      UC Berkeley has just built an excellent eye for viewing just these sorts of events: the High Energy Solar Spectroscopic Imager (HESSI). HESSI is capable of seeing a wide spectrum of radiation, from 3 kiloelectron volt (keV) soft xrays to 20 mega-electron volt (MeV) gamma rays, with an energy resolution of between about 1 and 5 keV. It is in these ranges that most of the radiation energy from solar flares is emitted, when x-rays and gamma rays emitted by flare particles interact with each other and matter in the Sun’s photosphere. Rising to a challenge from
      NASA’s Space Science division, a team at UC Berkeley’s Space Sciences Laboratory has modified their original design for HESSI in order to bring the project down to Small Explorer (SMEX) specifications. SMEX spacecraft are built for low cost, “highly focused” missions. A SMEX spacecraft may weigh up to around 250 kg, consume about 200 watts of power on average (about as much as two or three light bulbs), and cost less than $35 million to design and develop.

      In order to fit the bill, the HESSI team has simplified the spacecraft down to one ingenious instrument: an imaging spectrometer utilizing an array of nine germanium crystals cooled to -200 degrees Celsius. The crystals detect the x-rays and gamma rays. A pair of grids above each germanium detector casts shadows on the detector as the spacecraft spins, and the resulting modulation allows scientists to reconstruct a picture of the flare. In its final form, complete with four solar panels, the bright blue and gold HESSI weighs 290 kg and uses only about 220 watts of power. Due to its high angle of orbital inclination (38 degrees in low earth orbit), it will pass over Berkeley several times a day for data downloads and commands. It can view the entire sun while imaging areas with an angular size as small as 2 arcseconds (1 arcsecond being 1/3600 of a degree, or about 1/1800th of the Sun’s disk). HESSI can resolve a simple image in the 100 keV to 1 MeV range in tens of milliseconds. More complex images require an exposure that lasts half the rotation period of the craft: approximately two seconds.

      The most advanced hard x-ray imaging mission ever to be launched, HESSI holds the promise of returning some extraordinary data. A successfully tested HESSI is slated to launch this summer. If all goes well, it will send back more than just important information about the temperamental solar cycle. An x-ray survey of the Crab nebula, a high-resolution study of cosmic gamma ray bursts, and a study of terrestrial sources of gamma rays (e.g. lightning) have also been integrated into the HESSI mission. With its rapidly spinning eye, HESSI may look beyond the placid yellow disk of the Sun to study its violently variable x-ray face. But until then, the HESSI engineers, research scientists, and mission controllers on the ground here in Berkeley are undoubtedly holding their breath.

      To learn more about the HESSI satellite, visit:
      hessi.ssl.berkeley.edu

      Read up on solar flares and other spectacular solar events:
      hesperia.gsfc.nasa.gov/sftheory

      Solar and Stellar Activity Cycles by Peter R. Wilson

      The Heliosphere During the Declining Solar Cycle edited by M.A. Shea

      Leave a Reply