On February 5, 2002, NASA launched what was then called the High Energy
Solar Spectroscopic Imager (HESSI) into orbit. Renamed within months as
the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) after Reuven
Ramaty, a deceased NASA scientist who had long championed the mission,
the spacecraft's job was to observe giant explosions on the sun called
solar flares.
During a solar flare, the gas soars to over 20 million degrees
Fahrenheit, and emits X-rays that scientists can use as fingerprints to
study these events on the sun. X-rays cannot penetrate Earth's
atmosphere, however, so RHESSI observes them from space. Its goal is
simple: to understand how the sun so efficiently shoots out such huge
amounts of energy and particles.
Ten years since its launch, RHESSI has observed more than 40,000
X-ray flares, helped craft and refine a model of how solar eruptions
form, and fueled additional serendipitous science papers on such things
as the shape of the sun and thunder-storm-produced gamma ray flashes.
RHESSI is in a class of NASA spacecraft called Small Explorers --
missions that cost under $120 million with highly focused research
goals. Launching just after the sun reached its period of maximum
activity, known as solar maximum, in 2001, RHESSI was poised to see many
explosive bursts of energy from the sun, including both solar flares
and another eruption of solar material called coronal mass ejections.
"Except, thanks in part to RHESSI, we don't even separate the biggest
explosions into categories like that anymore," says Brian Dennis, the
mission scientist for RHESSI at NASA's Goddard Space Flight Center in
Greenbelt, Md. "RHESSI has taught us that 'Thou shalt instead say Solar
Eruptive Events.' Now we know that one burst of energy in the sun's
atmosphere creates both kinds of eruptions. Part of the energy shoots
into the sky and becomes a CME (coronal mass ejection). Part of the
material is driven down to the sun's surface and appears as the flare.
RHESSI's 10 years of observations have helped fill in the holes in this
picture."
To accomplish this, RHESSI made use of its one, and only one,
instrument, which records both X-rays and even shorter wavelengths of
light, gamma rays. The instrument combines high-resolution solar images
with "spectroscopic" images that delineate the spectrum of energy coming
from any particular point in the image. Together, this helps physically
map out energy levels during an explosive event on the sun, as well as
track the energy's movement.
For example, to understand how a solar flare forms, one has to
understand where the energy to power it comes from. Early RHESSI
observations of a flare on April 15, 2002, showed two X-ray sources --
one higher in the sun's atmosphere and one lower. RHESSI's unique
spectroscopic imaging capability allowed scientists to interpret this to
mean that a huge energy release had originally happened between the two
spots: that crucial initiation in the current picture of solar
eruptions where a single energy burst in the sun's atmosphere, creates
both a CME and a flare.
Since RHESSI observes gamma rays as well as X-rays, it has been able
to compare how events emit these two forms of radiation. X-rays
generally represent electron activity and gamma rays represent activity
from protons and other heavier charged particles called ions, so
comparing both helps show how different particle populations move
around. Scientists have spotted several flares, including their first
one using RHESSI in October 2003, where the gamma rays and X-ray sources
do not line up. Such a spatial difference was entirely unexpected and
suggests that different circumstances guide the movements of different
particle populations.
"This opens up new questions for our model," says Dennis. "The
electrons and ions have different masses, but we'd still expect them to
appear at the same locations in the flare. Perhaps the ions are
accelerated in a different way and end up traveling on different
magnetic field lines from the electrons."
In addition to observations of giant solar flares, RHESSI has
observed more than 25,000 of a smaller version, known as microflares.
These much smaller energy releases are also believed to play a role in
how the sun transports energy from its surface up into its atmosphere.
RHESSI has found that such flares all occur in active regions over the
same range of latitudes on the sun as for the bigger events, so perhaps
they are just smaller versions of the same phenomenon.
Another interesting result from RHESSI has nothing to do with flares
at all, but with the very shape of the sun. Due to its spin, one expects
the sun to be slightly flattened and not a perfect sphere, much the
same as the slight flattening of Earth. Pre-RHESSI measurements of the
sun, however, showed it to be much flatter than physics would dictate,
raising questions of whether scientists had overlooked some fundamental
piece of information about the sun's rotation.
RHESSI helped by providing information simply gathered as part of its
constant assessment of where its instrument points. To keep perfectly
oriented, RHESSI precisely records the position of the sun's horizon, or
"limb," some 16 times per second. With such an extensive collection of
data, scientists could determine the best ever measurement of the sun's
shape. This data suggests the true shape more closely matches what
physics predicts.
RHESSI's monitoring of gamma rays throughout the sky also made it a
prime tool to measure what are called terrestrial gamma-ray flashes
(TGFs), bursts of gamma rays emitted from high in Earth's atmosphere
over lightning storms. The first of these had been spotted before, but
RHESSI showed that they are more common and more luminous than
previously thought. With RHESSI's help, scientists soon realized they
occurred upwards of 50 times per day. Indeed, current numbers suggest
there may be as many as 400 TGFs daily from thunderstorms at different
locations around the world.
"RHESSI made great strides by taking the first high-resolution movies
of flares using their high energy radiation," says Dennis. "The
original mission was only for two years and we quickly achieved our
initial science goals -- but RHESSI didn't stop there. The mission has
been extended several times, and this small mission just keeps going and
going, collecting great data."
In 2009, NASA extended the mission yet again. Now scientists are
working to integrate RHESSI flare observations with data from other
solar telescopes such as the Solar TErrestrial RElations Observatory
(STEREO), Solar Dynamics Observatory (SDO), SOlar and Heliophysics
Observatory (SOHO), and Hinode as they watch the sun's activity rise
toward yet another solar maximum, currently predicted for 2013.
The Explorers Program Office at Goddard provides management and
technical oversight for the RHESSI mission under the direction of NASA's
Science Mission Directorate in Washington, D.C.
For more information about the mission, go to:
http://science.nasa.gov/missions/rhessi/
