What is a Solar Flare?:
Solar flares and the often associated coronal mass ejections are the biggest explosions in
today's solar system. Sometimes they have as much energy as a billion megatons of TNT,
several orders of magnitude more than the energy of the recent crash of Comet
Shoemaker-Levy into Jupiter. This tremendous amount of energy is typically released in
only a few minutes. Plasma is heated to tens of millions of degrees and electrons,
protons, and heavy nuclei are accelerated to near the speed of light. Some of these
particles stop in the solar atmosphere and produce the observed X-ray and gamma-ray
emissions. Other particles travel outward from the Sun and are detected in the vicinity of
the Earth and beyond.
Coronal mass ejections were once thought to
be initiated by solar flares. Although some are accompanied by flares, it is now known
that most CMEs are not associated with flares. This has important implications for
understanding and predicting the effects of solar activity on the Earth and in space. If a
CME collides with the Earth, it can excite a geomagnetic storm. Large geomagnetic storms
have, among other things, caused electrical power outages and damaged communications
satellites. In space CMEs typically drive shock waves that produce energetic particles
that can be damaging to both electronic equipment and astronauts that venture outside the
protection of the Earth's magnetic field. Solar flares, on the other hand, directly affect
the ionosphere and radio communications at the Earth, and also release energetic particles
into space. Therefore, to understand and predict "space weather" and the effect
of solar activity on the Earth, an understanding of both CMEs and flares is required.
Where Does the Energy Come From?: It is believed that the energy of a solar flare
comes originally from the violent motions below the solar surface. Prior to a flare, it is
thought that energy builds up from below and is stored in the magnetic field that pervades
the solar atmosphere. This usually occurs near a sunspot, but exactly how it happens and
what triggers it to be suddenly and explosively released is not known.
Why Study Solar Flares?: Because they are such powerful explosions that are little
understood and cannot be predicted. Because they can have dangerous and disruptive
consequences in space and on the Earth. Because the Sun is so much closer and more easily
observed than the other sites in the universe -- such as pulsars, quasars, and black holes
-- where the same high-energy processes are believed to be occurring.
Why Study Solar Flares Now?: The occurrence rate of solar flares varies on an
11-year cycle as illustrated below, where the number of flares recorded per month is
plotted versus time for the last two cycles. The dotted line shows the predicted rate for
the next cycle based on a simple average of the previous two cycles. Most flares,
particularly the rare, large bursts with the most dangerous effects, occur within two to
three years of the peak in the 11-year cycle. HESSI will take full advantage of the next
peak in the solar activity cycle with up to three years of flight operations beginning in
the year 2000.
What Questions will HESSI Address?: How is such a large amount of energy released
so rapidly during a flare? How are so many electrons and protons accelerated so quickly to
such high energies? Where are the electrons and protons accelerated in the solar
atmosphere and where do they deposit their energy?
How will HESSI Address these Questions?: HESSI, shown below, will concentrate on
electrons and protons accelerated in solar flares through observations of the X-rays and
gamma rays that they produce: HESSI will obtain the first ever X-ray and gamma-ray images
of flares from 100 keV to 10 MeV. HESSI will do the first ever nuclear gamma-ray line
spectroscopy of solar flares. HESSI will obtain pictures of flares in X-rays with an
angular resolution of 2 arcseconds, a factor of three better than previously possible.
HESSI will measure X-ray and gamma-ray spectra with less than 1 keV energy resolution, a
factor of 20-40 better than previously possible with scintillation counters.
What Effect Does Solar Activity have on Earth?: Episodic solar activity, including
flares and coronal mass ejections, has a number of terrestrial effects: Energetic
particles are hazards to astronauts and satellites. Geomagnetic disturbances may damage
power systems and disrupt communications on the Earth.
Last Updated by Gilbert Prevost on Monday, 23 March, 1998