RHESSI's Tenth Anniversary

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Number: 169
1st Author: Brian Dennis
2nd Author: Bob Lin
Published: 15 February 2012
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RHESSI's Scientific Legacy

February 5, 2012, marks the 10th anniversary of RHESSI's launch. At this time, it seems appropriate to review the scientific achievements to date, and we present here a top-ten list of the iconic results that we believe will constitute RHESSI's scientific legacy. A much more comprehensive review of RHESSI's scientific achievements can be found in the book recently published in Space Science Reviews (Vol. 159, Issues 1-4) - High-Energy Aspects of Solar Flares.

It is important to remember that RHESSI combines both X-ray and γ-ray imaging spectroscopy in a single instrument, with the stated goal of investigating particle acceleration and energy release in solar flares. The top-ten list consists of items, in our priority order, that show RHESSI's unique contributions in this area of flare research.

Any list of RHESSI's scientific accomplishments would be incomplete without acknowledging the importance of RHESSI's serendipitous contributions to other aspects of solar physics (oblateness), to astrophysics (magnetars), and to Earth sciences (terrestrial gamma-ray flashes). Thus, three additional items are included at the end in those areas of research.

Number 1: Discovery of Gamma-Ray Footpoint Structures

Figure: Overlay of gamma-ray and X-ray contours on TRACE image of the 2003 October 28 flare.

Prior to RHESSI, very little was known about where and how energetic ions are accelerated in solar flares. This is important since these ions may contain as much energy as energetic electrons, both carrying of order ~10-50% of the total energy released in a flare. RHESSI has provided the first-ever imaging of energetic flare ions through their nuclear gamma-ray line emission (Hurford et al. 2003, 2006). In the largest flare, shown in the accompanying figure, double ion footpoints are detected straddling the loop arcade, closely paralleling the electron footpoints. These observations show that the energetic ions are located in small footpoint sources in the vicinity of the flare. This is inconsistent with acceleration over a broad region by a shock front propagating far from the flare as suggested for solar energetic particle (SEP) acceleration by shocks driven by fast CMEs. This strongly suggests that flare ion acceleration is similar to flare electron acceleration, with both possibly related to the process of magnetic reconnection. As shown in the figure, however, the ion footpoints are displaced from the electron footpoints by ~10-20 Mm, for reasons unknown.

RHESSI has also provided the first high resolution spectroscopy of flare γ-ray lines (Smith et al. 2003). Detailed analysis revealed mass-dependent Doppler red shifts of order ~1%, indicating that the emitting ions were traveling downward at an angle of ~30 to 40 degrees to the vertical - likely along tilted magnetic fields. This result, combined with the detection of gamma-ray footpoint emission, shows clearly that the ions must have been accelerated over a relatively small volume in the corona, on closed field lines in the primary flare energy-release region.

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Number 2: Energy Content & Spectrum of Flare Energetic Electrons

Figure: Flare photon flux spectrum.
Figure: Source electron flux spectra.

A crucial question for flares is, how much of the energy released goes into particle acceleration? For the first time, RHESSI was able to show unambiguously that the power-law spectrum of energetic electrons extends down to ~20 keV, and therefore these electrons must contain a large fraction, ~10-50%, of the total energy released in many flares. RHESSI’s uniquely fine energy resolution (~1 keV FWHM) is sufficient to resolve the steep high-energy fall-off of the hot flare thermal continuum, allowing for the accurate determination of the energy above which the hard X-ray emission must be non-thermal (e.g., Holman et al. 2003). For bright solar flares, RHESSI‘s measurements of the hard X-ray continuum are the best ever obtained for an astrophysical source. They are precise enough for model-independent deconvolution using powerful mathematical techniques developed for RHESSI observations (e.g., Kontar et al. 2011) to obtain the spectrum of the bremsstrahlung-emitting source electrons. For flare hard X-ray spectra with a flattening at low energies, the derived electron source spectra appear to show a roll-off around 20 - 40 keV (Kasparova et al. 2005), but after correcting for albedo (the Compton scattering of the source hard X-rays by the solar photosphere), using a newly-developed Green’s function method (Kontar et al. 2006), all the derived source electron power-law spectra extend with no roll-off down to <~20 keV and sometimes as low as ~12 keV (see also Sui et al. 2005, 2007), where the hot flare thermal emission dominates. Power-law spectra extending that low imply that the source energetic electrons must contain a large fraction of the energy released in many flares.

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Number 3: Ubiquitous Nonthermal Emissions from the Corona, & Bulk Energization

Figure: X-ray images of flare on 20 Jan. 2005 showing coronal hard X-ray source.

RHESSI's excellent energy resolution, in combination with its imaging capability, made it possible for the first time to cleanly measure nonthermal emissions from the corona (e.g., Battaglia & Benz 2006). These observations reveal that a non-thermal component is present in essentially all flares (Krucker & Lin 2008). RHESSI discovered that coronal nonthermal emissions are found in a large variety of associations such as in the pre-flare phase (Lin et al. 2003), in the absence of footpoint emission (Veronig & Brown 2004), associated with jets (Bain et al. 2009), with Coronal Mass Ejections (Krucker et al. 2007), and also in the gamma-ray range (Krucker et al. 2008). These observations indicate that a significant fraction of the total accelerated electrons are in the corona at one time, clearly favoring an acceleration site in the corona. Some rare, extremely bright sources provide crucial tests of the maximal efficiency of different acceleration models. The extreme brightness suggests that all electrons within the source are accelerated in a bulk energization process (Krucker et al. 2010; Ishikawa et al. 2011). Simultaneous microwave observations indicate that the energy in accelerated electrons at the peak of the event is of the same order of magnitude as the magnetic energy (i.e., plasma beta near unity). This indicates an extremely efficient conversion of magnetic energy into kinetic energy.

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Number 4: Double Coronal X-ray Sources

Figure: Twin coronal X-ray sources with footpoints and presumptive location of energy release site indicated.

The detection of twin coronal X-ray sources, one at the top and the other above the top of the flare loops. For both sources, higher energy and, therefore, higher temperature emission is shifted toward the region between the sources. This provided the strongest evidence yet for energy release in the corona between the two sources as indicated in the figure. Several other similar events have been reported, e.g., Liu and Petrosian, but it is not known how common this phenomenon might be.

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Number 5: Microflares and the Quiet Sun

Figure: Location of ~25,000 microflares on the solar disc.

RHESSI could readily observe both hard and soft X-rays from microflares, determining both spectrum and location. The figure shows the solar coordinates of some 25,000 of them. Their distribution in space shows that they all come from active regions within the active North and South latitude bands. This establishes conclusively that flares, and these microflares, do not heat the quiet corona.

Related to this, RHESSI has established the lowest upper limits on quiet-Sun hard X-ray emission in the absence of active regions, placing strong constraints on any non-thermal energy release by nanoflares or other phenomena, and even on axions produced in the core of the Sun.

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Number 6: Initial Downward Motion of X-ray Sources

Figure: Altitude variations of the coronal X-ray source observed during the 2002 April 14–15 flare.

Sui and Holman (2003) also reported the initial downward motion of the coronal X-ray source prior to the previously reported continuous upward motion. This was further detailed by Sui et al. (2004), where it was shown that the rate of altitude increase correlated with the hard X-ray flux, suggesting that it was related to the energy release rate. This initial downward motion has been seen now in other flares (e.g., Ji et al. 2008) and at other wavelengths, and it appears to be associated with the propagation of reconnection along flare ribbons, but its interpretation is still unclear.

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Number 7: HXR Flare Ribbons

Figure: Hard X-ray and white light imaging of a flare ribbon.

With RHESSI’s unprecedented (~2 arcsec) spatial resolution, detailed hard X-ray imaging of flare ribbons finally became possible. This showed elongated structures along the ribbon that closely match the white light sources, but only partially match the EUV sources (Liu et al. 2007; Dennis & Pernak 2009; Krucker et al. 2011). The width of the hard X-ray sources, however, was often unresolved even by RHESSI, yielding an upper limit of ~1 arcsec for the dimension. This indicates that the number density of precipitating electrons is even larger than previously thought. These observations challenge our current understanding of the standard thick-target model and contribute new insights to our understanding of electron acceleration and transport.

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Number 8: Location of Superhot X-ray Source

Figure: Spectrum and image during during the 2002 July 23 X4.8 flare.

Superhot (defined as >30 MK) flare plasmas were discovered about three decades ago. (Previous instrumentation could only unambiguously measure flare plasma temperatures up to 10-20 MK). RHESSI’s high spectral and spatial resolution has allowed the soft X-ray emission during the 2002 July 23 X4.8 flare to be separated into two spatially distinct isothermal coronal X-ray sources, a superhot (>30 MK) source high in the corona, and a normal ~20 MK source located at a lower altitude (Caspi & Lin 2010). The super-hot source was present at a high altitude even during the flare pre-impulsive phase when no HXR footpoint emission was detected. This suggests a coronal origin for the superhot component, while the normal ~20 MK plasma originates primarily from chromospheric evaporation. The superhot and hot plasmas thus arise from fundamentally different physical processes.

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Number 9: Photosphere as a Compton or "Dentist's" Mirror

Figure: Spectra of upwards and downwards directed electrons.

With RHESSI's excellent spectral resolution, we can use the albedo contribution to the measured hard X-ray flux to our advantage. Kontar and Brown (2006) showed that, by considering the Sun’s surface to act as a “Compton mirror,” we can look at the emitting electrons both directly and from behind the source, providing vital information on the directionality of the propagating particles. Using this technique, they determined simultaneously the electron spectra of downward- and upward-directed electrons for two solar flares observed with RHESSI. The results reveal surprisingly near-isotropic electron distributions, which contrast strongly with the expectations from the standard model that invokes strong downward beaming, including a collisional thick-target model. Some would say that this result provides strong evidence against electron beam models in that simple particle beams aren't present in the energetically important particle distributions.

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Number 10: Broadened 511-keV Positron Annihilation Line

Figure: RHESSI count spectra of the solar 511 keV annihilation line during the 2003 October 28 flare.

RHESSI’s high resolution spectroscopy of the flare 511-keV positron annihilation line emission showed a line width of typically >~5 keV, indicating that the temperature of the accelerated-ion interaction region was above 105 K. Later, in some flares, the width of the line narrows to ~1 keV, consistent with annihilation in ionized H at <104 K and ≥1015 cm-3. The full implications of these observations are still unclear but they bring into question the energy source of the heating. We also do not know whether or how the ions alone can produce such a highly dynamic flaring atmosphere, at chromospheric densities that can reach transition-region temperatures with large column depths, and then cool to less than 104 K in minutes while remaining highly ionized.

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Non-flare-related Items

Solar Oblateness

Figure: Space measurements of solar oblateness.

RHESSI's Solar Aspect System has provided the most precise measurements of the shape of the Sun, showing an unexpectedly large flattening compared to what is predicted from solar rotation. Fivian et al. (2008) were able to show that this effect was likely due to magnetic elements in the enhanced network producing emission preferentially at lower latitudes. Once this contribution was removed, the corrected oblateness of the nonmagnetic Sun was determined to be 8.01 +/- 0.14 milli–arc seconds, which is near the value expected from rotation of 7.8 milli–arc seconds, or ~0.001%. This result may explain the variation that had been reported with solar cycle, and it is the most accurate measure of the true oblateness ever made.

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Magnetar Timing and Spectroscopy

Figure: RHESSI 20 - 100 keV light curves of the giant magnetar flare.

RHESSI serendipitously detected a huge flare from the soft-gamma-ray repeater (SGR) 1806–20 that was just 5.25° from the Sun at the time of the observations. SGRs are thought to be magnetars - isolated, strongly magnetized neutron stars with teraGauss exterior magnetic fields and even stronger fields within, making them the most strongly-magnetized objects in the Universe. In the first 0.2 s, the flare released as much energy as the Sun radiates in a quarter of a million years. This observation suggested that a significant fraction of the mysterious short-duration gamma-ray bursts may come from similar extragalactic magnetars.

Later work by Watts and Strohmayer (2006) revealed quasi-periodic oscillations (QPOs) in the RHESSI timing data directly related to the "ringing" modes of the neutron star which, when interpreted as arising from vibrations in the neutron star crust, offer a novel means of testing the neutron star equation of state, crustal breaking strain, and magnetic field configuration.

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Terrestrial Gamma-ray Flashes (TGFs)

Figure: Accumulated spectra of 85 RHESSI TGFs.

RHESSI has detected over 1000 TGFs, which were first discovered with BATSE on the Compton Gamma Ray Observatory and associated with terrestrial lightning. RHESSI showed that TGFs are much more common and luminous than previously thought, and that they extend up to gamma-ray energies beyond 20 MeV, with just the spectral shape predicted by the relativistic runaway model. RHESSI's observations also show that the TGFs come from altitudes of around 15 km, and that they are associated with intra-cloud lightning and not cloud-to-ground lightning or sprites.

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