Coronagraph Field Of View Research Articles

Above the Noise: The Search for Periodicities in the Inner Heliosphere

Remote sensing of coronal and heliospheric periodicities can provide vital insight into the local conditions and dynamics of the solar atmosphere. We seek to trace long (one hour or longer) periodic oscillatory signatures (previously identified above the limb in the corona by, e.g., Telloni et al. in Astrophys. J.767, 138, 2013) from their origin at the solar surface out into the heliosphere. To do this, we combined on-disk measurements taken by the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) and concurrent extreme ultra-violet (EUV) and coronagraph data from one of the Solar Terrestrial Relations Observatory (STEREO) spacecraft to study the evolution of two active regions in the vicinity of an equatorial coronal hole over several days in early 2011. Fourier and wavelet analysis of signals were performed. Applying white-noise-based confidence levels to the power spectra associated with detrended intensity time series yields detections of oscillatory signatures with periods from 6 – 13 hours in both AIA and STEREO data. As was found by Telloni et al. (2013), these signatures are aligned with local magnetic structures. However, typical spectral power densities all vary substantially as a function of period, indicating spectra dominated by red (rather than white) noise. Contrary to the white-noise-based results, applying global confidence levels based on a generic background-noise model (allowing a combination of white noise, red noise, and transients following Auchère et al. in Astrophys. J.825, 110, 2016) without detrending the time series uncovers only sporadic, spatially uncorrelated evidence of periodic signatures in either instrument. Automating this method to individual pixels in the STEREO/COR coronagraph field of view is non-trivial. Efforts to identify and implement a more robust automatic background noise model fitting procedure are needed.

Lessons Learned from the Three-view Determination of CME Mass

Abstract With only single-viewpoint, total-brightness images, it is not possible to calculate the true mass of a coronal mass ejection (CME) because total-brightness images do not contain any depth information. If two viewpoints are available, it has been suggested that total-brightness images might be used to simultaneously derive the CME deprojected mass and propagation direction. Exploiting all available data provided by SOHO, STEREO-A, and STEREO-B supplies three combinations of spacecraft pairs for double-viewpoint analysis. We show that, if only aleatory variation is reckoned, then the CME deprojected mass can vary by as much as 100% between different pairs of spacecraft. Going beyond multiple double-viewpoint analyses, we also utilize these spacecraft to simultaneously estimate the CME deprojected mass and propagation direction with triple-viewpoint analysis. However, once again, if only aleatory variation is reckoned, then, for many events, there is no common mass value estimated by the three viewpoints. This indicates that epistemic uncertainty is significant and must be reckoned also. We consider in detail the mass in the coronagraph field of view, from one, two, and three viewpoints, for the CME observed on 2007 December 31, focusing especially on the approximations used and uncertainties in this process. Altogether, we use triple-viewpoint analysis to estimate the mass of eight CMEs. When simultaneously estimating the CME deprojected mass and propagation direction using total-brightness images, the single degree of freedom provided by a third, independent viewpoint clarifies the calculation of, and reduces ambiguity in, the deprojected mass; however, the deprojected mass should not be confused with the true mass.

LARGE SOLAR ENERGETIC PARTICLE EVENTS ASSOCIATED WITH FILAMENT ERUPTIONS OUTSIDE ACTIVE REGIONS

We report on four large filament eruptions (FEs) from solar cycles 23 and 24 that were associated with large solar energetic particle (SEP) events and interplanetary type II radio bursts. The post-eruption arcades corresponded to mostly C-class soft X-ray enhancements, but an M1.0 flare was associated with one event. However, the associated coronal mass ejections (CMEs) were fast (speeds about 1000 km/s) and appeared as halo CMEs in the coronagraph field of view. The interplanetary type II radio bursts occurred over a wide wavelength range indicating the existence of strong shocks throughout the inner heliosphere. No metric type II bursts were present in three events, indicating that the shocks formed beyond 2 to 3 Rs. In one case, there was a metric type II burst with low starting frequency indicating a shock formation height of about 2 Rs. The FE-associated SEP events did have softer spectra (spectral index greater than 4) in the 10 to 100 MeV range, but there were other low-intensity SEP events with spectral indices >/=4. Some of these events are likely FE-SEP events, but were not classified so in the literature because they occurred close to active regions. Some were definitely associated with large active region flares, but the shock formation height was large. We definitely find a diminished role for flares and complex type III burst durations in these large SEP events. Fast CMEs and shock formation at larger distances from the Sun seem to be the primary characteristics of the FE-associated SEP events.

Observing the reconnection region in a transequatorial loop system

A vertical current sheet is a crucial element in many flare/coronal mass ejection (CME) models. For the first time, Liu et al. reported a vertical current sheet directly imaged during the flare rising phase with the EUV Imaging Telescope (EIT) onboard the Solar and Heliospheric Observatory (SOHO). As a follow-up study, here we present the comprehensive analysis and detailed physical interpretation of the observation. The current sheet formed due to the gradual rise of a transequatorial loop system. As the loop legs approached each other, plasma flew at ∼ 6 km s−1 into a local area where a cusp-shaped flare loop subsequently formed and the current sheet was seen as a bright, collimated structure of global length (≥ 0.25 R⊙) and macroscopic width ((5–10)×103km), extending from 50 Mm above the flaring loop to the border of the EIT field of view (FOV). The reconnection rate in terms of the Alfvén Mach number is estimated to be only 0.005–0.009, albeit a halo CME was accelerated from ∼ 400 km s−1 to ∼ 1300 km s−1 within the coronagraph FOV. Drifting pulsating structures at metric frequencies were recorded during the impulsive phase, implying tearing of the current sheet in the high corona. A radio Type III burst occurred when the current sheet was clearly seen in EUV, indicative of accelerated electrons beaming upward from the upper tip of the current sheet. A cusp-shaped dimming region was observed to be located above the post-flare arcade during the decay phase in EIT; both the arcade and the dimming expanded with time. With the Coronal Diagnostic Spectrometer (CDS) aboard SOHO, a clear signature of chromospheric evaporation was seen during the decay phase, i.e., the cusp-shaped dimming region was associated with plasma upflows detected with EUV hot emission lines, while the post-flare loop was associated with downflows detected with cold lines. This event provides a comprehensive view of the reconnection geometry and dynamics in the solar corona.

INTERPLANETARY SHOCKS LACKING TYPE II RADIO BURSTS

We report on the radio-emission characteristics of 222 interplanetary (IP) shocks detected by spacecraft at Sun–Earth L1 during solar cycle 23 (1996 to 2006, inclusive). A surprisingly large fraction of the IP shocks (∼34%) was radio quiet (RQ; i.e., the shocks lacked type II radio bursts). We examined the properties of coronal mass ejections (CMEs) and soft X-ray flares associated with such RQ shocks and compared them with those of the radio-loud (RL) shocks. The CMEs associated with the RQ shocks were generally slow (average speed ∼535 km s−1) and only ∼40% of the CMEs were halos. The corresponding numbers for CMEs associated with RL shocks were 1237 km s−1 and 72%, respectively. Thus, the CME kinetic energy seems to be the deciding factor in the radio-emission properties of shocks. The lower kinetic energy of CMEs associated with RQ shocks is also suggested by the lower peak soft X-ray flux of the associated flares (C3.4 versus M4.7 for RL shocks). CMEs associated with RQ CMEs were generally accelerating within the coronagraph field of view (average acceleration ∼+6.8 m s−2), while those associated with RL shocks were decelerating (average acceleration ∼–3.5 m s−2). This suggests that many of the RQ shocks formed at large distances from the Sun, typically beyond 10 Rs, consistent with the absence of metric and decameter—hectometric (DH) type II radio bursts. A small fraction of RL shocks had type II radio emission solely in the kilometric (km) wavelength domain. Interestingly, the kinematics of the CMEs associated with the km type II bursts is similar to those of RQ shocks, except that the former are slightly more energetic. Comparison of the shock Mach numbers at 1 AU shows that the RQ shocks are mostly subcritical, suggesting that they were not efficient in accelerating electrons. The Mach number values also indicate that most of these are quasi-perpendicular shocks. The radio-quietness is predominant in the rise phase and decreases through the maximum and declining phases of solar cycle 23. About 18% of the IP shocks do not have discernible ejecta behind them. These shocks are due to CMEs moving at large angles from the Sun–Earth line and hence are not blast waves. The solar sources of the shock-driving CMEs follow the sunspot butterfly diagram, consistent with the higher-energy requirement for driving shocks.

On the 3-D reconstruction of Coronal Mass Ejections using coronagraph data

Abstract. Coronal Mass ejections (CMEs) are enormous eruptions of magnetized plasma expelled from the Sun into the interplanetary space, over the course of hours to days. They can create major disturbances in the interplanetary medium and trigger severe magnetic storms when they collide with the Earth's magnetosphere. It is important to know their real speed, propagation direction and 3-D configuration in order to accurately predict their arrival time at the Earth. Using data from the SECCHI coronagraphs onboard the STEREO mission, which was launched in October 2006, we can infer the propagation direction and the 3-D structure of such events. In this review, we first describe different techniques that were used to model the 3-D configuration of CMEs in the coronagraph field of view (up to 15 R⊙). Then, we apply these techniques to different CMEs observed by various coronagraphs. A comparison of results obtained from the application of different reconstruction algorithms is presented and discussed.

Coronal Mass Ejection of 15 May 2001: I. Evolution of Morphological Features of the Eruption

We study the initiation and development of the limb coronal mass ejection (CME) of 15 May 2001, utilizing observations from Mauna Loa Solar Observatory (MLSO), the Solar and Heliospheric Observatory (SOHO), and Yohkoh. The pre-eruption images in various spectral channels show a quiescent prominence imbedded in the coronal void, being overlaid by the coronal arch. After the onset of rapid acceleration, this three-element structure preserved its integrity and appeared in the MLSO MK-IV coronagraph field of view as the three-part CME structure (the frontal rim, the cavity, and the prominence) and continued its motion through the field of view of the SOHO/LASCO coronagraphs up to 30 solar radii. Such observational coverage allows us to measure the relative kinematics of the three-part structure from the very beginning up to the late phases of the eruption. The leading edge and the prominence accelerated simultaneously: the rapid acceleration of the frontal rim and the prominence started at approximately the same time, the prominence perhaps being slightly delayed (4 – 6 min). The leading edge achieved the maximum acceleration amax≈ 600 ± 150 m s−2 at a heliocentric distance 2.4 –2.5 solar radii, whereas the prominence reached amax≈ 380± 50 m s−2, almost simultaneously with the leading edge. Such a distinct synchronization of different parts of the CME provides clear evidence that the entire magnetic arcade, including the prominence, erupts as an entity, showing a kind of self-similar expansion. The CME attained a maximum velocity of vmax≈ 1200 km s−1 at approximately the same time as the peak of the associated soft X-ray flare. Beyond about 10 solar radii, the leading edge of the CME started to decelerate at a≈−20 m s−2, most likely due to the aerodynamic drag. The deceleration of the prominence was delayed for 10 –30 min, which is attributed to its larger inertia.

Geometric localization of STEREO CMEs

We describe a straightforward methodology for determining the location and other gross properties of CMEs within the coronagraph field of view in the upcoming STEREO mission observations. We use geometric triangulation upon a series of lines‐of‐sight taken from two spacecraft views that are locally tangent to the apparent edges of a CME. From the intersections of these lines‐of‐sight, we construct a set of stacked quadrilaterals that fully bound the structure and convey something of its location, shape, and size; a time sequence of such determinations can be used to determine the velocity. The technique is relatively robust and promises a substantial improvement in our capability to locate and characterize CMEs for research as well as forecasting purposes.

Observations of Core Fallback during Coronal Mass Ejections

White-light observations made with the Large Angle and Spectrometric Coronagraph (LASCO) during the present solar maximum have revealed a multitude of faint, inward-moving features at heliocentric distances of r ~ 2-6 R☉. Most of these structures appear to originate above r ~ 3 R☉ and may be signatures of the closing-down of magnetic flux at the boundaries of coronal holes or in the aftermath of coronal mass ejections (CMEs). Here, we present observations of a different type of inflow, in which material within the bright core of a CME collapses back toward the Sun after rising to heights of r ~ 2.5-6 R☉. We have identified roughly 20 such fallback events during 1998-2001. The core structures, which have the form of loops or concave-outward flux ropes, ascend into the coronagraph field of view beyond 2 R☉ with speeds of ~100-400 km s-1 but return with speeds of only ~50-200 km s-1. The initial deceleration rates of ~20-100 m s-2 are comparable to the local gravitational deceleration GM☉/r2 but continually decrease with time. The associated CMEs tend to be impulsive but relatively slow, with the leading front moving outward at ~250-450 km s-1 and often showing some deceleration. It is thus not surprising that some fraction of the core material fails to reach escape speeds, remaining bound to the Sun by gravitational and magnetic tension forces. We suggest that the dynamical behavior of the core may be determined in part by momentum exchanges with the background medium, which consists of ongoing outflows of CME material, ambient solar wind, and inflow streams. In particular, we attribute the asymmetry of the up-down trajectories to the action of such drag forces, whose direction changes from inward to outward as the core decelerates.

Characteristics of coronal mass ejections associated with long‐wavelength type II radio bursts

We investigated the characteristics of coronal mass ejections (CMEs) associated with long‐wavelength type II radio bursts in the near‐Sun interplanetary medium. Type II radio bursts in the decameter‐hectometric (DH) wavelengths indicate powerful MHD shocks leaving the inner solar corona and entering the interplanetary medium. Almost all of these bursts are associated with wider and faster than average CMEs. A large fraction of these radio‐rich CMEs were found to decelerate in the coronagraph field of view, in contrast to the prevailing view that most CMEs display either constant acceleration or constant speed. We found a similar deceleration for the fast CMEs (speed > 900 km s−1) in general. We suggest that the coronal drag could be responsible for the deceleration, based on the result that the deceleration has a quadratic dependence on the CME speed. About 60% of the fast CMEs were not associated with DH type II bursts, suggesting that some additional condition needs to be satisfied to be radio‐rich. The average width (66°) of the radio‐poor, fast CMEs is much smaller than that (102°) of the radio‐rich CMEs, suggesting that the CME width plays an important role. The special characteristics of the radio‐rich CMEs suggest that the detection of DH radio bursts may provide a useful tool in identifying the population of geoeffective CMEs.

X-ray Ejecta, White-Light CMEs and a Coronal Shock Wave

We report on a coronal shock wave inferred from the metric type II burst of 13 January 1996. To identify the shock driver, we examined mass motions in the form of X-ray ejecta and white-light coronal mass ejections (CMEs). None of the ejections could be considered fast (> 400 km s−1) events. In white light, two CMEs occurred in quick succession, with the first one associated with X-ray ejecta near the solar surface. The second CME started at an unusually large height in the corona and carried a dark void in it. The first CME decelerated and stalled while the second one accelerated, both in the coronagraph field of view. We identify the X-ray ejecta to be the driver of the coronal shock inferred from metric type II burst. The shock speed reported in the Solar Geophysical Data (1000–2000 km s−1) seems to be extremely large compared to the speeds inferred from X-ray and white-light observations. We suggest that the MHD fast-mode speed in the inner corona could be low enough that the X-ray ejecta is supermagnetosonic and hence can drive a shock to produce the type II burst.

CMEs observed continuously from the lower corona to the far corona

A campaign of observations with EIT on board SOHO called “CME watch”, is designed to observe the whole sun surface in 195 Å i.e. in a Fe XII emission line, every 17 minutes. In order to analyze the events, we also used He II and coronagraph observations of 97/11/03. By this way, the very beginning of some CMEs was observed. The ejections were seen in Fe XII as a dark bubble propagating in the corona above a prominence, and dimmings produced near an active region and propagating on the solar surface. Two of the ejections coming from the active region were produced with a very short time of delay (1 hour). All the ejections were associated to a CME. However, one CME was not related to a Fe XII low corona event, but maybe this CME had its origin behind the limb. The CME associated with the prominence was quite slow (50 km s −1) and the ones associated with the active region were quite fast (114–490 km s −1). This analysis permitted a better identification of the different CMEs appearing in the coronagraph field of view.

Disruption of a coronal streamer by an eruptive prominence and coronal mass ejection

We describe and analyze in detail the coronal mass ejection of 18 August 1980, using images from the coronagraph on the Solar Maximum Mission (SMM) satellite. The event occurred at the site of a large coronal helmet streamer and evolved into the three‐part structure of a bright frontal shell, followed by a relatively dark space surrounding a bright filamentary core as seen in many mass ejections of the SMM epoch. The bright core can be identified as material from a prominence whose eruption was observed from the ground; this identification is based on (1) the looplike and filamented appearance of the core, (2) its motion along a trajectory that is a good extrapolation of the motion deduced from ground‐based observations of the prominence eruption, and (3) direct observations of Hα emission when the core is in the coronagraph field of view. The mass of the frontal shell is equal to that of the coronal helmet streamer (to the ∼30% accuracy with which mass estimates can be made), indicating that the shell is the coronal material previously in the helmet streamer, displaced and set into motion by the erupting prominence and surrounding cavity. The mass ejected in the bright core (or prominence) is estimated to be ∼50% larger than the “coronal” material in the frontal loop. The total mass of 2.5×1015 g and energy of 5×1031 ergs estimated for this mass ejection are both greater than in typical ejections of the Skylab era but are comparable to the average mass and energy in an interplanetary shock wave.

Observations of a Comet on Collision Course with the Sun

A brilliant new comet (1979 XI: Howard-Koomen-Michels) was discovered in data from the Naval Research Laboratory's orbiting SOLWIND coronagraph. An extensive sequence of pictures, telemetered from the P78-1 satellite, shows the coma, accompanied by a bright and well-developed tail, passing through the coronagraph's field of view at afew million kilometers from the sun. Preliminary orbital calculations based on the observed motion of the comet's head and morphology of the tail indicate that this previously unreported object is a sungrazing comet and may be one of the group of Kreutz sungrazers. It appears from the data that the perihelion distance was less than 1 solar radius, so that the cometary nucleus encountered dense regions of the sun's atmosphere, was completely vaporized, and did not reappear after the time of closest approach to the sun. After this time, however, cometary debris, scattered into the ambient solar wind, caused a brightening of the corona over one solar hemisphere and to heliocentric distances of 5 to 10 solar radii.

Mass flow in loop type coronal transients

The white light coronagraph on Skylab observed many loop type coronal transients. These loops travel through the coronagraph's field of view (2–6R⊙) over a period of a few hours, after which the legs of the loops usually remain visible for a few days. In this paper we investigate the temporal changes in density and mass per unit length measured along the legs of such loops during the several days after the initial eruption. Examination of 8 transients shows that the mass and density in the legs decrease during the few hours after the top of the loop has travelled beyond the coronagraph's field of view. The mass and density then increase slowly, during the next one half to one day, then decrease again over approximately the same period. These changes are generally shown to be too rapid to be explained by solar rotation, indicating that the transient legs have a lifetime of only a few days.