SDO EVE Late Phase Flares

The NASA Solar Dynamics Observatory, or SDO, was launched last year, and the three instruments aboard SDO are providing a wealth of data about the highly variable sun. The Helioseismic and Magnetic Imager, or HMI instrument, is measuring the magnetic fields on the sun and the oscillations at the surface that enable viewing deeper into the sun. The Atmospheric Imaging Assembly, or AIA instrument, obtains high-resolution coronal images at multiple wavelengths in the ultraviolet. The Extreme ultraviolet Variability Experiment, or EVE instrument, observes the solar spectra over the full extreme ultraviolet range with high spectral resolution.

Credit: NASA/Goddard Space Flight Center/CI Lab

X-ray Classification of Solar Flares

A common classification of flare magnitude is based on the peak intensity of the X-ray as measured for more than 30 years by the NOAA GOES satellites. The X-ray flare classification includes a letter, either A, B, C, M, or X, and a number from 1 to 9. The letter represents a factor of 10 change in the X-ray intensity as indicated on the right side of the figure. The number is the intensity within the flare class. For example, the X-ray time series is shown for the C9 flare on May 5, 2010. So far, there have been over 500 C, M, and X class flares during the SDO mission.

Credit: NASA/University of Colorado/Tom Woods

Example of an EUV Late Phase Flare

The SDO observations have revealed a set of flares that have a large second peak for some of the extreme ultraviolet (EUV) emissions. It had previously been known that the EUV emissions have a peak near the time of the flare's X-ray peak, but this second EUV peak is one to five hours later and without a corresponding X-ray peak. We refer to this delayed, second peak as the EUV Late Phase. The time series for the C9 flare on May 5, 2010 show the flare's X-ray peak near 12 UT, followed by the EUV first peak five minutes later, and then the EUV Late Phase peaks more than an hour later. So far, 15% of the flares analyzed during the SDO mission have the EUV Late Phase.
The EUV Late Phase contributes even more flare energy than we originally thought from studying only the X-ray flares. Thus, additional studies are important to understand how much extra energy that the EUV Late Phase provides towards heating and ionizing Earth's atmosphere.

Credit: NASA/University of Colorado/Tom Woods

NOAA Space Weather Prediction Center

The Space Weather Forecast Office is the heart of the NOAA Space Weather Prediction Center (SWPC). The Forecast Office is staffed 24/7 and is the Nations official source of space weather alerts, watches, and warnings.

The Space Weather Prediction Center is one of the nine centers of the National Centers for Environmental Prediction (NCEP) and is the only Center dedicated to space weather forecasting. The NCEP is part of the Nation Weather Service.

The Space Weather Prediction Center also has Research and Development departments who's primary function includes the development and exploration of new models, tools, and data products that can improve the accuracy and lead times of the space weather forecasts.

The NASA SDO research satellite provides valuable data for understanding and forecasting space weather. Data from all three SDO instruments (HMI, AIA, and EVE) are used in the NOAA Forecast Office. The Forecast Office now uses SDO EVE data to provide backup for the GOES X-ray sensor during the regular daily eclipses of the sun that occur in the Fall and Spring. In addition to the operational utility of SDO data, the new knowledge and scientific understanding that is generated by the SDO mission will improve the accuracy of space weather models and therefore, improve the accuracy of future space weather products and forecasts.

Credit: SWPC/NOAA

Solar EUV Radiation in the Upper Atmosphere

This figure shows where the solar EUV radiation is absorbed as a function of wavelength and height in the atmosphere. It also shows the vertical temperature profile of the atmosphere.

Credit: R. Viereck/SWPC/NOAA

Solar Flare Impacts on Earth

This is a movie of a NOAA space weather product which shows how solar flares affect HF communication. Most of the effect comes from the solar x-ray flare; however, understanding the full X-ray and EUV spectral variability will help space weather modelers refine the physical understanding of this complex phenomenon and improve the accuracy of the models. This realtime product , designed to help HF communication customers, can be found at the web site www.swpc.noaa.gov/drap

Credit: SWPC/NOAA

This movie shows a view of the solar corona, glowing brightly in EUV
light, hovering above the solar surface which is dark in that light.
The familiar white-yellow solar surface lies underneath this hot
atmosphere where temperatures exceed a million degrees. The corona is
sometimes briefly visible from Earth during solar eclipses, but can be
viewed at all times with special space-based instruments, such as
SDO's Atmospheric Imaging Assembly (AIA), which observes Extreme
Ultra-Violet (EUV) light using four 16-megapixel cameras
simultaneously.

The AIA images have been given false colors because the human eye
cannot see this light. In this movie, blue shows the coolest regions
(one million degrees), green somewhat warmer regions, and red warm
regions (two million degrees); other AIA images can image even hotter
gases, but these are not shown here. The corona is brightest where
the magnetic field is strongest, most strongly so over solar regions
where dark sunspots are present on its surface.

The Sun's magnetic field, unlike Earth's, is ever changing within
minutes to hours. The corona responds to this evolution by changing in
shape as the field deforms, or in temperature (color) as more or less
heat is deposited by the changing magnetic field. This clip shows 6
hours in the life of the Sun's corona: many small changes can be seen, and
even the Sun's rotation is revealed.

In the upper right of the Sun, a moderate solar flare is observed:
here, the magnetic field becomes unstable and explodes, heating the
coronal gas by electrical currents, and throwing part of the coronal
gas out into interplanetary space in a coronal mass ejection.

We see the flare and ejection occur around noon. Then, about an hour
and a half later, high coronal arches brighten (in what the EVE
instrument observes as the 'late phase'). These arches are the result
of the reformed coronal magnetic field that was breached during the
ejection. This reforming process leaves the local gas at several
million degrees, cooling as it glows. Eventually it reaches
temperatures of one to two million degrees, and then the glow becomes
visible to the narrow AIA filters tuned to these
lower temperatures.

Credit: NASA/SDO/AIA/K. Schrijver/LMATC

This visual shows the same view the previous clip but without the time information.

Credit: NASA/SDO/AIA/K. Schrijver/LMATC

This visual shows the flare and eruption in detail, zoomed in on the
upper-right corner of the Sun's hot atmosphere.

Credit: NASA/SDO/AIA/K. Schrijver/LMATC

This visual shows the flare and eruption in detail, zoomed in on the
upper-right corner of the Sun's hot atmosphere. No Labels.

Credit: NASA/SDO/AIA/K. Schrijver/LMATC

This movie shows the flare and eruption from the previous clips, zoomed in even
more on the region of the explosion. The top row shows three AIA
channels next to a map of the Sun's surface magnetic field (observed
by SDO's Helioseismic and Magnetic Imager). Underneath these images
are two traces of the Sun's brightness as observed by the EVE
instrument, corresponding to the two left-most AIA images.

As the flare goes off, all channels brighten, so much so that
star-shaped diffraction patterns show up caused by AIA's optical
properties; these patterns cross at the locations of maximum
brightness. Then the emission from the flare site itself fades
away. An hour later, a faint high glow is seen in the 94A AIA channel
(green), revealing hot gases well above the flare site. Then the 335A
channel (blue) shows a similar set of bright structures, and finally
the 171A channel (yellow) shows these structures (most clearly as
strands shaped by the Sun's magnetic field). This afterglow, the 'EUV
late phase' of the eruptive flare, reveals that the coronal gas in the
high magnetic arches is cooling, successively showing up in AIA
filters designed to image the glow from gases at temperatures within
limited ranges.

Credit: NASA/SDO/EVE/AIA/HMI/R. Hock/LASP

Diagram of the EUV late phase flare

Using observations from AIA, we can develop a diagram of the C8.8 flare on May 5, 2010 and begin to understand where the EUV late phase originates. The evolution of this flare can be described in five stages.
The first stage or the preflare configuration shows that prior to the flare, the active region contains two sets of nested loops. The inner loops (red) are responsible for the main phase of the flare while the outer loops (blue) are responsible for the EUV late phase. The side lobes (grey) do not change during the flare.
The second stage of this flare's evolution is the main phase of the flare. During the main phase of the flare, the inner loops (red) start to rise and undergo reconnection to create the C8.8 flare. This is what is traditionally called a flare and in many cases, the flare evolution would continue to the last stage: postflare configuration.
In this flare, however, there are two additional stages. During the transitional period, the material ejected during the main phase of the flare continues to push upward. Eventually, it causes the overlying loops to break open. The material can then escape the solar atmosphere and forms a coronal mass ejection (CME).
The outer loops (blue) reform to return the region to equilibrium. In creating the new outer loops, energy is released and emission from the loops forms the EUV late phase.
In the final stage or the postflare configuration, the region returns to a state that looks similar to the preflare configuration with two sets of nested loops.

Credit: NASA/SDO/AIA/R. Hock/University of Colorado

The same graphic as above, without additional labeling.

Credit: NASA/SDO/AIA/R. Hock/University of Colorado

Results of the EBTEL Model

From this diagram and other observations, this flare has three key features:

1. There are two stages of heating with the main phase having more heating than the EUV late phase.
2. Main phase loops are shorter than EUV late phase loops.
3. Main phase is observed in both hot and cool coronal emissions while EUV late phase is observed only in cool coronal emissions.
   We can test these key features of the flare using the EBTEL code and compare to the lightcurves obtained by EVE.
   EBTEL is a radiative transfer code developed by Jim Klimchuk, Spiros Patsourakos and Peter Cargill (The Astrophysical Journal, 682, Issue 2, pp. 1351-1362, 2008) as a way to model the emission from a single loop in the sun's corona. The EBTEL code requires three input parameters: when do you heat the loop, how strongly do you heat it, and what is the length of the loop. The outputs of the EBTEL code are the lightcurves from that single loop for both a hot and cooler EUV emission line that EVE measures.
   From the AIA observations, it is clear that this flare contains many coronal loop. So, we developed a model of this flare that uses 22 EBTEL loops and found the parameters that best match to the EVE lightcurves.
   The output of the model is shown in this figure. The pluses are the EVE observations with the preflare background irradiance subtracted off. The solid black line is the output from the model and agrees with the EVE lightcurves. The individual colored lines are the contribution from each individual coronal loop strand. This model helps to define the physical processes that produce the EUV late phase.

   Credit: NASA/SDO/AIA /EVE/R. Hock/University of Colorado

The same graphic as above, without additional labeling.

Credit: NASA/SDO/AIA /EVE/R. Hock/University of Colorado

Output Parameters of the EBTEL Model

The EBTEL model parameters provide confirmation for the key features of our flare diagram and SDO observations. This figure shows the loop length, heating rate, and peak temperature for each of the EBTEL loops. The colored diamonds are the parameter for each individual coronal loop strand and correspond to the lightcurves in the previous image. Each parameter is plotted as a function of when that loop was heated in the model. In the top panel, the shaded regions are loop length estimates from AIA 17.1 nm images and represent the plane-of-sky or minimum loop lengths.
Loop Lengths: The main phase of the flare is best modeled with short loops while the EUV late phase is modeled with longer loops. The length of both sets of loops is consistent with the loop lengths measured from AIA images.
Heating Rates: Looking at the heating rate parameters, it is clear that there are two stages of heating. The first heating event occurs at the beginning of the flare and involves the heating of small loops. The second heating event occurs through the EUV late phase of the flare and involves longer coronal loops. Individually, these loops experience roughly 1% of the heating that occurs in the main phase of the flare. The heating is also spread out over an hour generating the long secondary flare emission profile that we call the EUV late phase.
Coronal Loop Temperatures: As a result of the lower heating rate during the EUV late phase, the coronal loops involved in that phase do not reach as high a temperature as the loops heated during the main phase. Because the loops are not as hot, they do not emit in the x-ray or hot EUV lines during the EUV late phase.
Using images from AIA and the EBTEL model, we have developed and confirmed the concept of how this particular flare evolved and that explains the origin of the EUV late phase. This EUV late phase flare occurs because (1) there is a set of nested loops, (2) the flare, which occurs in the inner loops, erupts through the overlying loops, and (3) the upper region reconfigures after that eruption and produces the late phase enhancements of the cool corona emissions. These conditions occur fairly frequently on the sun; this flare is not unique. The combination of instruments on SDO has allowed us to explore a previously unnoticed aspect of the evolution of solar flares.

Credit: NASA SDO/AIA/R. Hock/University of Colorado

The same graphic as above, without additional labeling.

Credit: NASA SDO/AIA/R. Hock/University of Colorado