117 research outputs found
EUV Irradiance Observations from SDO/EVE as a Diagnostic of Solar Flares
For the past six years, the EUV Variability Experiment (EVE) onboard the
Solar Dynamics Observatory has been monitoring changes in the Sun's extreme
ultraviolet output over a range of timescales. Its primary function is to
provide measurements of the solar spectral irradiance that is responsible for
driving fluctuations in Earth's ionosphere and thermosphere. However, despite
its modest spectral resolution and lack of spatial information, the EVE
spectral range contains many lines and continua that have become invaluable for
diagnosing the response of the lower solar atmosphere itself to an injection of
energy, particularly during a flare's impulsive phase. In addition, high
temperature emission lines can also be used to track changes in temperature and
density of flaring plasma in the corona. The high precision of EVE observations
are therefore crucial in helping us understand particle acceleration and energy
transport mechanisms during solar flares, as well as the origins of the Sun's
most geoeffective emission.Comment: 10 pages, 7 figures. Submitted to conference proceedings for the
symposium on "Solar and Stellar Flares and their Effects on the Planets" at
the IAU General Assembly in Honolulu, HI, August 201
On the Performance of Multi-Instrument Solar Flare Observations During Solar Cycle 24
The current fleet of space-based solar observatories offers us a wealth of
opportunities to study solar flares over a range of wavelengths. Significant
advances in our understanding of flare physics often come from coordinated
observations between multiple instruments. Consequently, considerable efforts
have been, and continue to be made to coordinate observations among instruments
(e.g. through the Max Millennium Program of Solar Flare Research). However,
there has been no study to date that quantifies how many flares have been
observed by combinations of various instruments. Here we describe a technique
that retrospectively searches archival databases for flares jointly observed by
RHESSI, SDO/EVE (MEGS-A and -B), Hinode/(EIS, SOT, and XRT), and IRIS. Out of
the 6953 flares of GOES magnitude C1 or greater that we consider over the 6.5
years after the launch of SDO, 40 have been observed by six or more instruments
simultaneously. Using each instrument's individual rate of success in observing
flares, we show that the numbers of flares co-observed by three or more
instruments are higher than the number expected under the assumption that the
instruments operated independently of one another. In particular, the number of
flares observed by larger numbers of instruments is much higher than expected.
Our study illustrates that these missions often acted in cooperation, or at
least had aligned goals. We also provide details on an interactive widget now
available in SSWIDL that allows a user to search for flaring events that have
been observed by a chosen set of instruments. This provides access to a broader
range of events in order to answer specific science questions. The difficulty
in scheduling coordinated observations for solar-flare research is discussed
with respect to instruments projected to begin operations during Solar Cycle
25, such as DKIST, Solar Orbiter, and Parker Solar Probe.Comment: 26 pages, 7 figures, 3 tables. Accepted for publication in Solar
Physic
A Hot Microflare Observed With RHESSI and Hinode
RHESSI and Hinode observations of a GOES B-class flare are combined to
investigate the origin of 15 MK plasma. The absence of any detectable hard
X-ray emission coupled with weak blueshifted emission lines (indicating upward
velocities averaging only 14 km/s) suggests that this was a result of direct
heating in the corona, as opposed to nonthermal electron precipitation causing
chromospheric evaporation. These findings are in agreement with a recent
hydrodynamical simulation of microflare plasmas which found that higher
temperatures can be attained when less energy is used to accelerate electrons
out of the thermal distribution. In addition, unusual redshifts in the 2 MK Fe
XV line (indicating downward velocities of 14 km/s) were observed cospatial
with one of the flare ribbons during the event. Downflows of such high
temperature plasma are not predicted by any common flare model.Comment: 6 pages, 4 figures, ApJL (Accepted
Continuum Contributions to the SDO/AIA Passbands During Solar Flares
Data from the Multiple EUV Grating Spectrograph (MEGS-A) component of the
Extreme Ultraviolet Experiment (EVE) onboard the Solar Dynamics Observatory
(SDO) were used to quantify the contribution of continuum emission to each of
the EUV channels of the Atmospheric Imaging Assembly (AIA), also on SDO, during
an X-class solar flare that occurred on 2011 February 15. Both the pre-flare
subtracted EVE spectra and fits to the associated free-free continuum were
convolved with the AIA response functions of the seven EUV passbands at 10 s
cadence throughout the course of the flare. It was found that 10-25% of the
total emission in the 94A, 131A, 193A, and 335A passbands throughout the main
phase of the flare was due to free-free emission. Reliable measurements could
not be made for the 171A channel, while the continuum contribution to the 304A
channel was negligible due to the presence of the strong He II emission line.
Up to 50% of the emission in the 211A channel was found to be due to free-free
emission around the peak of the flare, while an additional 20% was due to the
recombination continuum of He II. The analysis was extended to a number of M-
and X-class flares and it was found that the level of free-free emission
contributing to the 171A and 211A passbands increased with increasing GOES
class. These results suggest that the amount of continuum emission that
contributes to AIA observations during flares is more significant than that
stated in previous studies which used synthetic, rather than observed, spectra.
These findings highlight the importance of spectroscopic observations carried
out in conjunction with those from imaging instruments so that the data are
interpreted correctly.Comment: 9 pages, 6 figures, 1 tabl
The Anomalous Temporal Behaviour of Broadband Ly Emission During Solar Flares From SDO/EVE
Despite being the most prominent emission line in the solar spectrum, there
has been a notable lack of studies devoted to variations in Ly emission
during solar flares in recent years. However, the few examples that do exist
have shown Ly emission to be a substantial radiator of the total energy
budget of solar flares (on the order of 10%). It is also a known driver of
fluctuations in earth's ionosphere. The EUV Variability Experiment (EVE)
onboard the Solar Dynamics Observatory now provides broadband, photometric
Ly data at 10 s cadence with its Multiple EUV Grating
Spectrograph-Photometer (MEGS-P) component, and has observed scores of solar
flares in the 5 years since it was launched. However, the MEGS-P time profiles
appear to display a rise time of tens of minutes around the time of the flare
onset. This is in stark contrast to the rapid, impulsive increase observed in
other intrinsically chromospheric features (H, Ly, LyC, C III,
etc.). Furthermore, the emission detected by MEGS-P peaks around the time of
the peak of thermal soft X-ray emission, rather than during the impulsive phase
when energy deposition in the chromosphere - often assumed to be in the form of
nonthermal electrons - is greatest. Given that spectrally-resolved Ly
observations during flares from SORCE/SOLSTICE peak during the impulsive phase
as expected, this suggests that the atypical behaviour of MEGS-P data is a
manifestation of the broadband nature of the observations. This could imply
that other lines and/or continuum emission that becomes enhanced during flares
could be contributing to the passband. Users are hereby urged to exercise
caution when interpreting broadband Ly observations of solar flares.
Comparisons have also been made with other broadband Ly photometers
such as PROBA2/LYRA and GOES/EUVS-E.Comment: Submitted to A&A Research Notes, 5 pages 4 figure
Decay Phase Cooling and Inferred Heating of M- and X-class Solar Flares
In this paper, the cooling of 72 M- and X-class flares is examined using
GOES/XRS and SDO/EVE. The observed cooling rates are quantified and the
observed total cooling times are compared to the predictions of an analytical
0-D hydrodynamic model. It is found that the model does not fit the
observations well, but does provide a well defined lower limit on a flare's
total cooling time. The discrepancy between observations and the model is then
assumed to be primarily due to heating during the decay phase. The decay phase
heating necessary to account for the discrepancy is quantified and found be
~50% of the total thermally radiated energy as calculated with GOES. This decay
phase heating is found to scale with the observed peak thermal energy. It is
predicted that approximating the total thermal energy from the peak is
minimally affected by the decay phase heating in small flares. However, in the
most energetic flares the decay phase heating inferred from the model can be
several times greater than the peak thermal energy.Comment: Published in the Astrophysical Journal, 201
Solar Flare Impulsive Phase Emission Observed with SDO/EVE
Differential emission measures (DEMs) during the impulsive phase of solar
flares were constructed using observations from the EUV Variability Experiment
(EVE) and the Markov-Chain Monte Carlo method. Emission lines from ions formed
over the temperature range log T = 5.8 - 7.2 allow the evolution of the DEM to
be studied over a wide temperature range at 10s cadence. The technique was
applied to several M- and X-class flares, where impulsive phase EUV emission is
observable in the disk-integrated EVE spectra from emission lines formed up to
3 - 4 MK, and we use spatially-unresolved EVE observations to infer the thermal
structure of the emitting region. For the nine events studied the DEMs
exhibited a two component distribution during the impulsive phase, a low
temperature component with peak temperature of 1 - 2 MK, and a broad high
temperature one from 7 - 30 MK. A bimodal high temperature component is also
found for several events, with peaks at 8 and 25 MK during the impulsive phase.
The origin of the emission was verified using AIA images to be the flare
ribbons and footpoints, indicating that the constructed DEMs represent the
spatially-average thermal structure of the chromospheric flare emission during
the impulsive phase.Comment: 18 pages, 6 figures, accepted for publication in Ap
Velocity Characteristics of Evaporated Plasma Using Hinode/EIS
This paper presents a detailed study of chromospheric evaporation using the
EUV Imaging Spectrometer (EIS) onboard Hinode in conjunction with HXR
observations from RHESSI. The advanced capabilities of EIS were used to measure
Doppler shifts in 15 emission lines covering the temperature range T=0.05-16 MK
during the impulsive phase of a C-class flare on 2007 December 14. Blueshifts
indicative of the evaporated material were observed in six emission lines from
Fe XIV-XXIV (2-16 MK). Upflow velocity (v_up) was found to scale with
temperature as v_up (km s^-1)~8-18 T (MK). Although the hottest emission lines,
Fe XXIII and Fe XXIV, exhibited upflows of >200 km s^-1, their line profiles
were found to be dominated by a stationary component in contrast to the
predictions of the standard flare model. Emission from O VI-Fe XIII lines
(0.5-1.5 MK) was found to be redshifted by v_down (km s^-1)~60-17 T (MK) and
was interpreted as the downward-moving `plug' characteristic of explosive
evaporation. These downflows occur at temperatures significantly higher than
previously expected. Both upflows and downflows were spatially and temporally
correlated with HXR emission observed by RHESSI that provided the properties of
the electron beam deemed to be the driver of the evaporation. The energy flux
of the electron beam was found to be >5x10^10 ergs cm^-2 s^-1 consistent with
the value required to drive explosive chromospheric evaporation from
hydrodynamic simulations.Comment: 9 pages, 7 figures, 1 table, ApJ (Accepted
Detection of 3-Minute Oscillations in Full-Disk Ly Emission During A Solar Flare
In this Letter we report the detection of chromospheric 3-minute oscillations
in disk-integrated EUV irradiance observations during a solar flare. A wavelet
analysis of detrended Lyman-alpha (from GOES/EUVS) and Lyman continuum (from
SDO/EVE) emission from the 2011 February 15 X-class flare (SOL2011-02-15T01:56)
revealed a 3-minute period present during the flare's main phase. The
formation temperature of this emission locates this radiation to the flare's
chromospheric footpoints, and similar behaviour is found in the SDO/AIA
1600\AA\ and 1700\AA\ channels, which are dominated by chromospheric continuum.
The implication is that the chromosphere responds dynamically at its acoustic
cutoff frequency to an impulsive injection of energy. Since the 3-minute period
was not found at hard X-ray energies (50-100 keV) in RHESSI data we can state
that this 3-minute oscillation does not depend on the rate of energization of
non-thermal electrons. However, a second period of 120 s found in both hard
X-ray and chromospheric emission is consistent with episodic electron
energization on 2-minute timescales. Our finding on the 3-minute oscillation
suggests that chromospheric mechanical energy should be included in the flare
energy budget, and the fluctuations in the Lyman-alpha line may influence the
composition and dynamics of planetary atmospheres during periods of high
activity.Comment: 6 pages, 5 figures. Accepted for publication in Astrophysics Journal
Letter
The spectral content of SDO/AIA 1600 and 1700 \AA\ filters from flare and plage observations
The strong enhancement of the ultraviolet emission during solar flares is
usually taken as an indication of plasma heating in the lower solar atmosphere
caused by the deposition of the energy released during these events. Images
taken with broadband ultraviolet filters by the {\em Transition Region and
Coronal Explorer} (TRACE) and {\em Atmospheric Imaging Assembly} (AIA 1600 and
1700~\AA) have revealed the morphology and evolution of flare ribbons in great
detail. However, the spectral content of these images is still largely unknown.
Without the knowledge of the spectral contribution to these UV filters, the use
of these rich imaging datasets is severely limited. Aiming to solve this issue,
we estimate the spectral contributions of the AIA UV flare and plage images
using high-resolution spectra in the range 1300 to 1900~\AA\ from the Skylab
NRL SO82B spectrograph. We find that the flare excess emission in AIA 1600~\AA\
is { dominated by} the \ion{C}{4} 1550~\AA\ doublet (26\%), \ion{Si}{1}
continua (20\%), with smaller contributions from many other chromospheric lines
such as \ion{C}{1} 1561 and 1656~\AA\ multiplets, \ion{He}{2} 1640~\AA,
\ion{Si}{2} 1526 and 1533~\AA. For the AIA 1700~\AA\ band, \ion{C}{1} 1656~\AA\
multiplet is the main contributor (38\%), followed by \ion{He}{2} 1640 (17\%),
and accompanied by a multitude of other, { weaker} chromospheric lines, with
minimal contribution from the continuum. Our results can be generalized to
state that the AIA UV flare excess emission is of chromospheric origin, while
plage emission is dominated by photospheric continuum emission in both
channels.Comment: Accepted for publication in ApJ Skylab NRL SO82B data used in this
work available at http://dx.doi.org/10.5525/gla.researchdata.68
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