15 research outputs found
Association of 3He-Rich Solar Energetic Particles with Large-Scale Coronal Waves
Small 3He-rich solar energetic particle (SEP) events have been commonly
associated with extreme-ultraviolet (EUV) jets and narrow coronal mass
ejections (CMEs) which are believed to be the signatures of magnetic
reconnection involving field lines open to interplanetary space. The elemental
and isotopic fractionation in these events are thought to be caused by
processes confined to the flare sites. In this study we identify 32 3He-rich
SEP events observed by the Advanced Composition Explorer near the Earth during
the solar minimum period 2007-2010 and examine their solar sources with the
high resolution Solar Terrestrial Relations Observatory (STEREO) EUV images.
Leading the Earth, STEREO-A provided for the first time a direct view on
3He-rich flares, which are generally located on the Sun's western hemisphere.
Surprisingly, we find that about half of the 3He-rich SEP events in this survey
are associated with large-scale EUV coronal waves. An examination of the wave
front propagation, the source-flare distribution and the coronal magnetic field
connections suggests that the EUV waves may affect the injection of 3He-rich
SEPs into interplanetary space.Comment: accepted for publication in The Astrophysical Journa
Case studies of multi-day 3He-rich solar energetic particle periods
Context. Impulsive solar energetic particle events in the inner heliosphere
show the long-lasting enrichment of 3He. Aims. We study the source regions of
long-lasting 3He-rich solar energetic particle (SEP) events Methods. We located
the responsible open magnetic field regions, we combined potential field source
surface extrapolations (PFSS) with the Parker spiral, and compared the magnetic
field of the identified source regions with in situ magnetic fields. The
candidate open field regions are active region plages. The activity was
examined by using extreme ultraviolet (EUV) images from the Solar Dynamics
Observatory (SDO) and STEREO together with radio observations from STEREO and
WIND. Results. Multi-day periods of 3He-rich SEP events are associated with ion
production in single active region. Small flares or coronal jets are their
responsible solar sources. We also find that the 3He enrichment may depend on
the occurrence rate of coronal jets.Comment: 7page, 4 figure
Secondary Rayleigh-Taylor type Instabilities in the Reconnection Exhaust Jet as a Mechanism for Supra-Arcade Downflows
Supra-arcade downflows (hereafter referred to as SADs) are low-emission,
elongated, finger-like features usually observed in active-region coronae above
post-eruption flare arcades. Observations exhibit downward moving SADs
intertwined with bright upward moving spikes. Whereas SADs are dark voids,
spikes are brighter, denser structures. Although SADs have been observed for
decades, the mechanism of formation of SADs remains an open issue. In our
three-dimensional resistive magnetohydrodynamic simulations, we demonstrate
that secondary Rayleigh-Taylor type instabilities develop in the downstream
region of a reconnecting current sheet. The instability results in the
formation of low-density coherent structures that resemble SADs, and
high-density structures that appear to be spike-like. Comparison between the
simulation results and observations suggests that secondary Rayleigh-Taylor
type instabilities in the exhaust of reconnecting current sheets provide a
plausible mechanism for observed SADs and spikes
Determination of the Coronal Magnetic Field by Hot Loop Oscillations
We apply a new method to determine the magnetic field in coronal loops using
observations of coronal loop oscillations. We analyze seven Doppler shift
oscillation events detected by SUMER in the hot flare line Fe XIX to obtain
oscillation periods of these events. The geometry, temperature, and electron
density of the oscillating loops are measured from coordinated multi-channel
soft X-ray imaging observations from SXT. All the oscillations are consistent
with standing slow waves in their fundamental mode. The parameters are used to
calculate the magnetic field of coronal loops based on MHD wave theory. For the
seven events, the plasma is in the range 0.15-0.91 with a mean of
0.330.26, and the estimated magnetic field varies between 21-61 G with a
mean of 3414 G. With background emission subtracted, the estimated
magnetic field is reduced by 9%-35%. The maximum backgroud subtraction gives a
mean of 2213 G in the range 12-51 G. We discuss measurement uncertainties
and the prospect of determining coronal loop magnetic fields from future
observations of coronal loops and Doppler shift oscillations.Comment: 26 pages, 7 figures and 5 tables, Accepted for publication in the
Astrophysical Journal (2007, 656, in press
Destabilization of a Solar Prominence/Filament Field System by a Series of Eight Homologous Eruptive Flares
Homologous flares are flares that occur repetitively in the same active
region, with similar structure and morphology. A series of at least eight
homologous flares occurred in active region NOAA 11237 over 16 - 17 June 2011.
A nearby prominence/filament was rooted in the active region, and situated near
the bottom of a coronal cavity. The active region was on the southeast solar
limb as seen from SDO/AIA, and on the disk as viewed from STEREO/EUVI-B. The
dual perspective allows us to study in detail behavior of the
prominence/filament material entrained in the magnetic field of the
repeatedly-erupting system. Each of the eruptions was mainly confined, but
expelled hot material into the prominence/filament cavity system (PFCS). The
field carrying and containing the ejected hot material interacted with the PFCS
and caused it to inflate, resulting in a step-wise rise of the PFCS
approximately in step with the homologous eruptions. The eighth eruption
triggered the PFCS to move outward slowly, accompanied by a weak coronal
dimming. As this slow PFCS eruption was underway, a final ejective flare
occurred in the core of the active region, resulting in strong dimming in the
EUVI-B images and expulsion of a coronal mass ejection (CME). A plausible
scenario is that the repeated homologous flares could have gradually
destabilized the PFCS, and its subsequent eruption removed field above the
acitive region and in turn led to the ejective flare, strong dimming, and CME.Comment: 11 pages, 12 figures, Accepted for publication in Ap
3He-Rich Solar Energetic Particles in Helical Jets on the Sun
Particle acceleration in stellar flares is ubiquitous in the Universe,
however, our Sun is the only astrophysical object where energetic particles and
their source flares can both be observed. The acceleration mechanism in solar
flares, tremendously enhancing (up to a factor of ten thousand) rare elements
like 3He and ultra-heavy nuclei, has been puzzling for almost 50 years. Here we
present some of the most intense 3He- and Fe-rich solar energetic particle
events ever reported. The events were accompanied by non-relativistic electron
events and type III radio bursts. The corresponding high-resolution,
extreme-ultraviolet imaging observations have revealed for the first time a
helical structure in the source flare with a jet-like shape. The helical jets
originated in relatively small, compact active regions, located at the coronal
hole boundary. A mini-filament at the base of the jet appears to trigger these
events. The events were observed with the two Solar Terrestrial Relations
Observatories STEREO on the backside of the Sun, during the period of increased
solar activity in 2014. The helical jets may be a distinct feature of these
intense events that is related to the production of high 3He and Fe
enrichments.Comment: accepted for publication in The Astrophysical Journa
Persistent Doppler shift oscillations observed with HINODE/EIS in the solar corona: spectroscopic signatures of Alfvenic waves and recurring upflows
Using data obtained by the EUV Imaging Spectrometer (EIS) onboard Hinode, we
have per- formed a survey of obvious and persistent (without significant
damping) Doppler shift oscillations in the corona. We have found mainly two
types of oscillations from February to April in 2007. One type is found at loop
footpoint regions, with a dominant period around 10 minutes. They are
characterized by coherent behavior of all line parameters (line intensity,
Doppler shift, line width and profile asymmetry), apparent blue shift and
blueward asymmetry throughout almost the en- tire duration. Such oscillations
are likely to be signatures of quasi-periodic upflows (small-scale jets, or
coronal counterpart of type-II spicules), which may play an important role in
the supply of mass and energy to the hot corona. The other type of oscillation
is usually associated with the upper part of loops. They are most clearly seen
in the Doppler shift of coronal lines with forma- tion temperatures between one
and two million degrees. The global wavelets of these oscillations usually peak
sharply around a period in the range of 3-6 minutes. No obvious profile
asymmetry is found and the variation of the line width is typically very small.
The intensity variation is often less than 2%. These oscillations are more
likely to be signatures of kink/Alfven waves rather than flows. In a few cases
there seems to be a pi/2 phase shift between the intensity and Doppler shift
oscillations, which may suggest the presence of slow mode standing waves
according to wave theories. However, we demonstrate that such a phase shift
could also be produced by loops moving into and out of a spatial pixel as a
result of Alfvenic oscillations. In this scenario, the intensity oscillations
associated with Alfvenic waves are caused by loop displacement rather than
density change.Comment: 9 figures, accepted by Ap
SDO and STEREO Observations of Prominence Dynamics During a Series of Eight Homologous Flares
Homologous flares are eruptive events that occur repetitively in the same active region, with similar structure and morphology. A series of at least eight homologous flares occurred in active region NOAA 11237 over 16 - 17 June 2011. A filament is rooted in the active region with an overlying coronal cavity. The active region appears on the southeast solar limb as seen from SDO/AIA, and on the disk as viewed from STEREO-B/EUVI; the dual perspective allows us to study in detail behavior of prominence/filament material entrained in the magnetic field of the repeatedly-erupting system. Each of the eruptions was mainly confined, with active-region prominence material being ejected from the core of the erupting region onto outer-lobe loops of the active region. The eruption series repeatedly disrupted material of a quiet-Sun extension of the prominence, and that material became suspended at progressively higher heights above the surface. Two final eruptions from the core region destabilized the field holding that material, instigating a coronal mass ejection (CME)