by Hazel Bain

What is RHESSI?
Hazel Bain

Hazel Bain

The Sun has recently been getting very active again, and there have been several X-class solar flares.  The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) is one of NASA’s Small Explorer missions (SMEX) and was launched in February 2002.  It is still working after a decade and has now observed over 40,000 solar flares. RHESSI was designed to look at X-rays and Gamma rays which are emitted during solar flares. 

The astronomy group at Glasgow University is part of the RHESSI team. After studying at Glasgow University, I had the opportunity to work at the University of California, Berkeley. My friend Claire Raftery, who studied at Trinity College, Dublin, also works here with me. We enjoy working at Berkeley very much.

How much energy is released during a solar flare?

UK standard Mars bar

During a solar flare a HUGE amount of energy is released. The energy in one UK standard Mars bar (58g) is 260 kcal, that is just over 1000 kJ (unfortunately in 2008 it shrank in size from 62.5g). Note that the USA equivalent is called a Milky Way bar. This sounds like a lot of energy. Indeed, it is a lot of energy, about 1000 times the amount of energy from solar radiation falling on one square metre of the Earth’s atmosphere every second (1.36 kJ per second).  

The energy released in a solar flare can be as high as 1025 Joules, that is the equivalent of roughly 10 billion billion Mars bars!


How can RHESSI help us understand solar flares better?

Solar flare

YOHKOH – Solar flare

Scientists think that the massive explosion (solar flare) is caused by ‘magnetic reconnection’ high in the Sun’s corona. The magnetic energy transfers into kinetic and thermal energy.  Lots and lots of particles (mainly electrons, but also protons and heavy nuclei) are accelerated to very high speeds, a fraction of the speed of light. Many of these electrons zoom downwards into lower parts of the Sun’s atmosphere. At first they have no problem racing down through the solar atmosphere but as they get deeper, the atmosphere gets denser. Eventually they hit the chromosphere, which is too dense for them to pass through and they get stopped. This is a little bit like a game of rugby. If the person with the ball is running for the try line and there are only a few defenders it’s not easy to stop them. However, if there are loads of defenders then the player can’t get past them.

When the electrons get stopped, X-rays are emitted. The place where this happens in the chromosphere is called a footpoint, you’ll see why in a minute. Another thing that happens when the electrons get stopped is that the gas (plasma) in the surrounding area gets heated up. The hot plasma (which can be 10 to 20 Million degrees Celsius) then rises, just as hot air rises on Earth, and forms a hot loop connecting the footpoints. The hot plasma, being ionised, traces out the magnetic field, hence the loop shapes. Now you can see where footpoints get their name, they are the ‘feet’ of the hot loops.

RHESSI solar flare

RHESSI – solar flare

RHESSI helps us to see the X-rays emitted from the footpoints and from the loop. By studying these parts of the flare we can learn a lot about what happens to the electrons during the flare. That is how the magnetic energy is released and transported in a solar flare. Observations from RHESSI can be combined with those from other solar observatories, such as Hinode and SDO, to obtain the maximum amount of information.

Here is a YouTube video of a solar flare seen with TRACE in UV emission on April 21st, 2002. Superimposed are the contours of X-ray emission as seen by RHESSI. The blue contours show more energetic (hard) X-ray emission and the red contours show (soft) X-rays from plasma at 10-20 million degrees.  

More information about solar flares can be found on the Sun|trek website (

See also the NASA RHESSI webpages (