Solar surprises

This August 19, 2004, image from the NASA Solar and Heliospheric Observatory (SOHO) shows a solar flare erupting from a giant sunspot. - NASA/AFP

The first tomographic reconstruction of a coronal mass ejection from the sun is a breakthrough towards a better understanding of the generation and propagation of such ejections, which can produce violent `space weather' events that can disable satellites and endanger astronauts.

MAGNIFICENT aurora displays, power blackouts, disrupted communication, disoriented compasses, disabled satellites and other essential services - what do all these have in common? They are all the results of gigantic storms on the sun that bombard the earth with streams of electromagnetic particles and radiation.

Thousands of millions of tonnes of material are sporadically ejected from the sun and they travel at high speed into interplanetary space. Such transient ejections of material are known as coronal mass ejections (CMEs). They are among the most powerful eruptions in the solar system, but could not be clearly identified until special telescopes, known as coronagraphs, were flown in space - on board the Seventh Orbiting Solar Observatory (OSO-7), Skylab and so on in the early 1970s. There is a compelling body of observational evidence to believe that CMEs are launched when solar magnetic fields become strained and suddenly `snap' to a new configuration, much like a rubber band that has been twisted to the breaking point. These events propel magnetic clouds with a mass of up to 1017 gm to speeds up to 2,600 km/sec into the heliosphere. The nature and cause of CMEs is a fundamental yet unsolved problem. They are often associated with prominence eruptions and/or solar flares.

The sun and the earth are a connected system. Electromagnetic radiation and electrically charged particles stream outward from the sun (the solar wind), envelop the earth, and interact with the earth's magnetic field and terrestrial atmosphere creating an adverse environment. The main goal is to understand the changing flow of energy and matter throughout the sun, heliosphere and planetary environments, thereby exploring the fundamental physical processes of space plasma systems. This ultimately defines the origins and societal impacts of variability in the sun-earth connection.

There are two different types of events in the solar atmosphere that trigger disturbances in the earth's environment. They are solar flares and CMEs. Energy released in a major solar flare is of the order of 1025 Joule, which is comparable in strength to 20 million nuclear bombs, each blowing up with an energy of 100 megatons of TNT (one megaton of TNT is 4.2x1022 ergs). A substantial fraction of this energy goes into accelerating electrons and ions to high (relativistic) speeds. These high energy particles go down towards the sun or out in space and result in enhanced radio, soft X-ray, hard X-ray and gamma ray radiation. A comparable amount of energy is released in expelling matter during a CME.

The most dramatic space weather effects, however, are associated with CMEs. These are sometimes associated with solar flares, and sometimes not, and they now appear to be a primary cause of geomagnetic activity. The primary signature of CMEs in ultraviolet and X-ray images is the creation of post-flare loop systems following the eruption of CMEs. Observations from Yohkoh (a solar observatory satellite) in X-rays and from Solar and Heliospheric Observatory (SOHO) and Transition Region and Coronal Explorer (TRACE) in ultraviolet light reveal extensive arcades associated with CMEs. An opening of magnetic field at the base of CME involves an extensive region on the sun. When an opening of magnetic field avoids any active region, we do not see any flare. However, if it extends into an active region, the subsequently closing field lines are seen as post-flare loops of an eruptive flare.

The coronal mass ejections have been studied primarily from observations using ground-based and space-borne coronagraphs. Observation and interpretation of these events have continuously been carried out with the Large Angle and Spectrometric Coronagraph (LASCO) instrument on SOHO since its launch on December 2, 1995. SOHO is located at the L1 Langrangian point (the point 1.5 million km away from the earth at which gravitational pull of the earth balances that of the sun). LASCO provides CME images of excellent quality, and reveals a large complexity and diversity of forms. The major limitation of CME observations to date has been a lack of three-dimensional structure and trajectory measurements. Speed and direction measurements would allow more accurate prediction of CME impacts that would allow better planning of protective measures such as attitude adjustment and astronaut shielding. If the basic desire is to see the front of a CME heading towards the earth, we just need observations out of the sun-earth line, that is, STEREO (Solar Terrestrial Relations Observatory), and we do not actually need 3D reconstruction. However, 3D measurements would yield better insights in CME generation and propagation and also permit additional tests of CME dynamical models.

In order to understand fully the origin of these powerful blasts and the process that launches them from the sun, we need to see the structure of CMEs in three dimensions. A 3D view is critical for a complete understanding of CMEs to predict better their arrival times and impact angles on the earth. T.G. Moran of the Catholic University, Washington, and J.M. Davila of the National Aeronautics and Space Administration's Goddard Space Flight Centre (NASA-GSFC), Greenbelt, Maryland (Science; 305; 2004; pages 66-70) have analysed two-dimensional images obtained from the LASCO instrument on the SOHO spacecraft in a new way to yield 3D images. Their technique is able to reveal the complex and distorted magnetic fields that travel with the CME cloud and sometimes interact with the earth's own magnetic field, pouring tremendous amounts of energy into the space near the earth. These magnetic fields are invisible. Since the CME gas is electrified, it spirals around the magnetic fields and traces out its shapes. Therefore, a 3D view of the CME plasma gives valuable information on the structure and behaviour of the magnetic fields that power the CME. The new analysis technique for SOHO data determines the 3D structure of a CME by taking a sequence of three images from the LASCO through various polarisers at different angles.

The sun emits unpolarised light. When it is scattered off electrons in the CME plasma, it takes up some polarisation. This means that the electric fields of some of the scattered light are forced to oscillate in certain directions, whereas the electric field in the light emitted by the sun is free to oscillate in all directions. The light from CME structures closer to the plane of the sun (as seen on the LASCO images) had to be more polarised than light from structures farther from that plane. Thus, by computing the ratio of polarised to unpolarised light for each CME structure, one can measure its distance from the plane. This provides the missing 3D view to the LASCO images.

With this technique it has been confirmed that the structure of CMEs directed towards the earth is an expanding arcade of loops, rather than a bubble or rope-like structure. This technique is not new, and has been in use to study relatively static solar structures during eclipses. However, it is applied to fast-moving CMEs for the first time. This method will complement data from the upcoming NASA's STERO mission, scheduled for launch in February 2006. STEREO will use two widely separated spacecraft to construct 3D views of CMEs by combining images from the different vantage points of the twin spacecraft. It is worthwhile to mention here that "halo" CMEs (so called because they appear as halos around the occulting disks) are so weak that we do not see all earth-directed CMEs because they are so far out of the plane of the sky. That means our halo observations and thus our tomographic techniques are probably limited to bigger events. The polarisation technique, STEREO imaging and spectroscopic technique provide different ways of viewing the CME structure and greatly enhance the capability of forecasting `space weather', and unravelling the physical processes that drive CMEs.

Dr. Bhola N. Dwivedi does research in solar physics and teaches Physics in Banaras Hindu University, Allahabad. He is involved in almost all the major solar space experiments, including Skylab, Yohkoh, SOHO and TRACE. His article in Scientific American special issue 2003 bagged him the 2004 SPD/AAS Popular Science Writing Award.