SDO EVE Flare Observation

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Revision as of 15:06, 12 October 2010

Introduction

The Solar Dynamics Observatory launched in February 2010, and now is providing a 24/7 view of the Sun. These continuous and high temporal observations are very important to studies of solar flares, and are very complimentary to other solar observing instruments, including RHESSI. The full-disk Atmospheric and Imaging Assemble (AIA) instrument provides a suite of images at a range of temperatures every 12 seconds. The Helioseismic and Magnetic Imager (HMI) will provide the photospheric line-of-sight and vector magnetic field instruments to provide context to the overlying magnetic field in active regions and flares. As seen in Fig. 1, the EUV solar spectral irradiance is provided by the EVE instrument from 6-106 nm at 0.1 nm spectral resolution, providing many temperature, density, and energetic diagnostics of solar flares at a wide temperature range of 4.5<log(T)<7.3 (chromospheric to hot coronal emission). The EVE observations of two M-class flares, a compact flare and a two-ribbon flare, are the focus of this nugget, demonstrating the observation capabilities of SDO EVE that are very complimentary to RHESSI.

Figure 1: An example of a single solar EUV spectrum from the SDO EVE instrument. The Red, Blue, and Purple lines are from the three different EVE optical channels, while the Green is the theoretical model from Warren (2001).

M2.0 Compact flare on 12-June-2010

The M2.0 flare that occurred on 12-June-2010 was a compact flare that had a very quick impulsive phase rise due to reconnection of a large single loop bundle proving the energy release, quick heating, and subsequent cooling. The time series of many temperatures of bound-bound emissions are shown in Figure 2 on a linear (2A) and log (2B) scale (see footnote for explanation of how these plots are formed). The cooler emissions that are formed in the chromosphere and transition region footpoints of the flaring loops (log(T)<5.0) show impulsive phase enhancements that are due to collision excitation of the electron beam with the dense, cooler emission plasma of this region followed by spontaneous radiative decay. These impulsive phase enhancements peak before the thermal phase emission of the hot coronal plasma, agreeing with the standard flare theories of energy deposition and heating occurring in the footpoints. These impulsive phase emissions should be well correlated with the thick-target emissions seen in RHESSI hard X-rays. The thermal phase then peaks with the hot coronal log(T)=7.2 plasma, followed by cooling shown as subsequent peaks of the emissions at characteristic temperatures back to pre-flare conditions within ~15 minutes. The strongly peaked impulsive phase and the peak and subsequent cooling of the thermal plasma support the theory that this compact flare had a single reconnection and energy release event for this flare.

Figure 2: Flare plasma temperature evolution and absolute radiated emissions for the M2.0 compact flare on 12-June-2010. The diamonds show the peak emission time for the emission specified by the given color, and therefore the peak plasma temperature.

M1.0 Two-Ribbon flare on 7-Aug-2010

The M1.0 gradual flare on 7-Aug-2010 is an example of a two-ribbon event showing multiple energy releases as reconnection occurs along the flaring arcade. Continuous particle deposition into the footpoints can be seen by the continuous emissions from the chromospheric and transition region plasma. This impulsive phase is then followed by the thermal phase, with thermal emissions with a range of formation temperatures from 6.7<log(T)<7.2 increasing. Interestingly, those at the hotter temperatures (6.8<log(T)<7.2) increase at the same rate and at the same radiative energy output. The hottest temperatures emissions (log(T)=7.2) start to fall off first, at the same time as the impulsive phase, cooler emission peak. This shows the energy deposition rate, and therefore the heating of the plasma in the footpoints starts to fall off, and there is no more energy input to heat the thermal plasma so its emissions start to decrease. Emissions from the cooler plasmas continue to rise then peak in sequence as the hotter plasma cools to these temperatures. All emissions cooler that log(T)=6.8 have almost simultaneous emission peaks at ~18:45 UT, with a curiously constant decrease in emission rates after this time for the rest of the cooling flare (as in all temperature emission have the same slope in the cooling phase).

Also observed in Fig 3 are the emissions in the temperature range from ~5.7<log(T)<6.1 showing an anti-correlated decrease in emissions during the flare. This is the coronal dimming due to the plasma at these temperature either being ejected out of the coronal or, more likely, due to heating and ionization of the characteristic ions at these temperatures to higher ionization states, such as the FE IX emission at 171 A and peak contribution function at log(T)=5.9.

Figure 3: Flare plasma temperature evolution and absolute radiated emissions for the M1.0 two-ribbon flare on 7-Aug-2010. The diamonds show the peak emission time for the emission specified by the given color, and therefore the peak plasma temperature.

Conclusion

Twenty four flares of GOES C2.0 and above have been observed so far since the launch of SDO, 14 of which were observed at various points in the flare evolution by RHESSI. Although SDO EVE has its own limitations when it comes to flares, it does have the capability to derive aspects of solar flare energetics and plasma diagnostics that will help advance and complete the concept of a solar flare developed by previous and current instruments.

Appendix

Notes on 'waterfall' plot formation for those unfamiliar with these type of plots. SDO EVE measures the complete spectrum from 6-106 nm seen in Fig. 1 every 10 seconds at 1 Angstrom spectral resolution. Within this EUV range each bound-bound emission line is assigned a characteristic formation temperature that is the peak of the contribution function from the CHIANTI database. The time series of the spectrally isolated, unblended lines are then placed into deltalog(T)=0.1 bins in the range from 6.8<log(T)<7.3 based on this peak temperature. The cooler emission bins are in various lot(T) ranges, with the given temperature label that corresponds to the various ranges:

• Log(T) Label/Log(T) Range
• 4.9/4.9-5.0
• 5.3/5.1-5.6
• 5.9/5.7-6.1
• 6.3/6.2-6.5
• 6.6/6.6-6.7

Approximately 90 different ions are used to form this data set. Time series are then plotted of each of these log(T) bins on an absolute scale, with the 'pre-flare' value subtracted off; therefore, these plots show the timing and the absolute energy release at the various temperatures throughout the flare. The peak time of each emission temperature bin is also plotted as a diamond symbol, which more easily shows the peak flaring plasma temperature at a given time.

Biographical Note: Phillip Chamberlin is a Research Astrophysicist at NASA's Goddard Space Flight Center. He is the Deputy Project Scientist for SDO as well as a CO-I on the SDO EVE instrument. Tom Woods is the Associate Director of the Technical Division a the University of Colorado's Laboratory for Atmospheric and Space Physics (LASP), and is also the PI of the SDO EVE instrument.