Читать книгу Exploring the Solar System - Peter Bond - Страница 62

Japan has been playing a leading role in solar studies for several decades. The first of its pioneering orbital observatories was Yohkoh, which was launched August 30, 1991. It carried four main instruments: a Hard X‐ray Telescope, the first instrument ever to image high‐energy X‐ray flares; a U.S.–Japanese Soft X‐ray Telescope with a field of view that covered the full solar disk but could also obtain a series of small‐scale, high‐resolution images of flares; a Wide‐Band Spectrometer to observe solar radiation in soft X‐rays, hard X‐rays, and gamma rays, with a fourth detector monitoring Earth's radiation belts; and a UK‐US‐Japanese spectrometer to study specific spectral regions in soft X‐rays.

Yohkoh was the first spacecraft to continuously observe the Sun in X‐rays during an entire sunspot cycle. When it was launched, the Sun was near the peak of its 11‐year cycle, so many active regions and flares were imaged. It then observed the subsequent decline and the start of sunspot cycle 23 in the late 1990s. The spacecraft sent back over six million X‐ray images before it failed in late 2000.

Yohkoh provided important new data about the corona, including information about how and where this multimillion‐degree layer is heated to temperatures hundreds of times greater than the solar surface. By tracking the evolution of the corona, it improved understanding of how the Sun's magnetic fields are deformed, twisted, broken, and reconnected during flares; and how the coronal plasma is heated to millions of degrees by flares. Various structures, known as sigmoids (S‐shaped regions in the corona) and trans‐equatorial interconnecting loops (TILs), were shown to be more likely to be the sites of solar eruptions.

Its successor, Hinode, was also a US‐UK‐Japan collaboration. It was launched on September 23, 2006, into a Sun‐synchronous, near‐polar orbit around Earth that allows it to remain in continuous sunlight for 9 months each year. It carries an optical telescope that includes a high‐resolution imager, a magnetograph that makes rapid observations of the Sun's magnetic and velocity fields, and a spectropolarimeter that makes extremely precise observations of the solar magnetic field. Together these instruments are able, for the first time from space, to measure small changes in the strength and direction of the magnetic field, as well as how these changes coincide with events in the corona. One important product is vector magnetograms that illustrate variations in the strength of the Sun's magnetic field.

Figure 2.41 Hinode is equipped with the highest resolution solar X‐ray telescope ever flown. This full disk image, taken early in the mission, shows features of the X‐ray Sun with a spatial resolution of nearly 1 arc second. These include coronal activity within dark holes near the poles and coronal loops associated with active regions.

(JAXA‐NASA)

Also on board is an advanced version of the Soft X‐ray Telescope flown on Yohkoh. The highest resolution solar X‐ray telescope ever flown, it shows the structure and dynamics of the corona over a wide range of temperatures and a broad field of view. By combining optical and X‐ray observations, it is possible to study how changes in the magnetic field trigger explosive solar events.

An Extreme Ultraviolet Imaging Spectrometer (EIS) provides a key link in the data by observing the chromosphere and transition region that separate the photosphere from the corona. The EIS measures the velocity of solar particles, and the temperature and density of solar plasma.

Its high‐resolution images revealed gigantic arcing magnetic structures that dwarf the underlying sunspots. It found evidence for several mechanisms that may be contributing to the extraordinary heating of the corona, including twisted and tangled magnetic fields that snap and reconnect, extreme turbulence and acoustic waves in the lower atmosphere, magnetic Alfvén waves that propagate upwards at high speed, and X‐ray jets and nanoflares that are continually exploding. Hinode also discovered extended structures at the edges of active regions where material is flowing rapidly outward, possibly contributing as much as a quarter of the total solar wind.

NASA has launched several spacecraft in recent years to investigate different aspects of solar activity. The twin Solar Terrestrial Relations Observatory (STEREO) spacecraft, launched in October 2006, were designed to provide the first simultaneous views of the Sun's Earth‐facing hemisphere and the opposite hemisphere. With one flying ahead of Earth and the other behind it, scientists could produce 3‐D images of Sun–Earth space. These made it possible to pinpoint the location and speed of a CME, and study how it interacted with its surroundings. In this way, a CME could be tracked all the way to Earth and its arrival predicted at least a day in advance. Communications with the STEREO‐B craft were lost in October 2014.

The Solar Dynamics Observatory, which was launched in February 2010, was placed in a geosynchronous orbit around Earth, which enables it to observe the Sun without interruption. The observatory carries three instruments to determine how the Sun's magnetic field is generated, structured and converted into the solar wind, flares, and coronal mass ejections.

The most recent mission is the Parker Solar Probe, which was launched on August 12, 2018. The spacecraft is designed to fly through the Sun's outer atmosphere on numerous occasions to gather data on the processes that heat the corona and accelerate the solar wind. By the end of its seven‐year mission, it will fly within 6 million km of the Sun's visible surface, deep inside the corona.

The Solar Probe carries four instruments. One measures the electric field around the spacecraft and uses three small magnetometers measure magnetic fields. The other instrument suites study energetic particles, and image the corona and solar wind. To withstand the intense temperatures, which will reach almost 1,400ºC, the spacecraft and instruments are protected by a carbon‐composite heat shield.

The European Space Agency is also developing the Solar Orbiter, which will provide complementary observations alongside those of other space observatories. Solar Orbiter was launched on February 9, 2020, the mission will provide close‐up, high‐latitude observations of the Sun from a highly elliptical orbit that will take it well inside the orbit of Mercury. By flying close to the Sun in an inclined orbit, the spacecraft will be able to observe the dynamic solar surface and its connection to the heliosphere for much longer periods than from near‐Earth vantage points.

A 15–60‐minute advance warning of an incoming CME is provided by spacecraft which are parked at the L1 Lagrange Point between the Sun and Earth, such as the Deep Space Climate Observatory (DSCOVR). They detect sudden increases in particle density, interplanetary magnetic field (IMF) strength and solar wind speed when the CME‐associated interplanetary shock arrives ahead of the magnetic cloud.

Much of our knowledge of CMEs has come from the Large Angle and Spectrometric Coronagraph (LASCO) on SOHO. Advance warning of a CME that could be heading toward Earth was provided when LASCO imaged a “halo event,” when the entire Sun appeared to be surrounded by the CME. However, it was still not possible to definitively say if a CME was coming Earthward. Another viewpoint was needed to provide the third dimension.

Between 2006 and 2014, two extra viewpoints were provided by NASA's twin Solar Terrestrial Relations Observatory (STEREO) spacecraft. With one flying ahead of Earth and the other behind it, scientists could produce 3D images of Sun–Earth space. These made it possible to pinpoint the location and speed of a CME, and study how it interacted with its surroundings. In this way, a CME could be tracked all the way to Earth and its arrival predicted at least a day in advance.

The STEREO data showed that almost all CMEs have a common shape, similar to a croissant. This shape is explained by the twisted magnetic flux tubes being wider in the middle and thinner at one end.

CMEs can create major disturbances in the interplanetary medium and in Earth's magnetic field. If they reach Earth, they result in beautiful polar auroras but may also cause large‐scale power cuts, e.g. Quebec in 1989, and problems with spacecraft systems.

The most powerful CME to reach Earth in the last 160 years occurred on September 1, 1859, when solar science was in its infancy. British astronomer Richard Carrington saw a brilliant white flash on the Sun, the first ever observation of a solar flare. Only 17.6 hours later, a massive CME slammed into Earth's magnetic field – arriving much faster than most CMEs.

Campers in the Rocky Mountains woke up in the middle of the night, mistaking the glow of brilliant auroras for sunrise. Even as far south as Cuba, the red illumination of the Northern Lights was bright enough to enable people to read their morning paper.

Figure 2.42 A SOHO coronagraph image showing a spiral‐shaped CME (lower right) erupting from the Sun on June 2, 1998. This CME was rather unusual since the width of the blast was fairly narrow and the strands of plasma were twisting. The LASCO instrument on SOHO blocks the Sun in order to observe coronal structures in visible light. The white circle represents the Sun.

(ESA/NASA)

Figure 2.43 Coronal mass ejections occur when solar magnetic field lines snake around each other, forming the letter “S.” Usually, they go past each other, but if they connect, the mid‐section breaks free and creates a mass ejection. In this artwork, a coronal mass ejection erupts as magnetic field lines reconnect. The yellow arch represents a magnetic flux rope filled with flare plasma. The red, blue, and green lines represent higher magnetic field lines connecting opposite polarities on the solar surface. The dotted line shows the sheared, low‐lying magnetic field. The bright spot is the site of magnetic field reconnection.

(NASA‐MSFC)

Figure 2.44 An S‐shaped structure (sigmoid), in a solar active region is often a precursor to a CME. In this Hinode X‐ray image, a bright sigmoid is observed (right) at the beginning of its eruption on February 12, 2007. The fine structure reveals that the sigmoid is really two opposing “J” shapes wrapping around each other. Sigmoid structures, defined by twisting magnetic fields, can often be observed for several days before a CME occurs.

(Hinode, JAXA, NAOJ, David McKenzie‐University of Montana)

Meanwhile, one of the largest recorded geomagnetic storms electrified telegraph lines, shocking technicians and setting their telegraph papers on fire. Magnetometers around the world recorded strong disturbances in the planetary magnetic field for more than a week.

A Carrington‐class solar superstorm blasted off the Sun on July 23, 2012, but fortunately, it missed the Earth. However, the consequences for our modern society, which is largely dependent on electrical systems and satellites, could have been severe. If a solar storm of that magnitude did strike our planet, the cleanup might cost $2 trillion, according to a study by the National Academy of Sciences.