A. De Groof

High-speed coronal rain

At high spatial and temporal resolution, coronal loops are observed to have a highly dynamic nature. Recent observations with SOHO and TRACE frequently show localized brightenings “raining” down towards the solar surface. What is the origin of these features? Here we present for the first time a comparison of observed intensity enhancements from an EIT shutterless campaign with non-equilibrium ionization simulations of coronal loops in order to reveal the physical processes governing fast flows and localized brightenings. We show that catastrophic cooling around the loop apex as a consequence of footpoint-concentrated heating offers a simple explanation for these observations. An advantage of this model is that no external driving mechanism is necessary as the dynamics result entirely from the non-linear character of the problem.

Intensity variations in EIT shutterless mode: Waves or flows?

On 11 July 2001 an EIT shutterless campaign was conducted which provided 120 high-cadence (68 s) 304 Angstrom images of the north eastern quarter of the Sun. The most interesting feature seen in the data is an off-limb half loop structure along which systematic intensity variations are seen which appear to propagate from the top of the loop towards its footpoint. We investigate the underlying cause of these propagating disturbances, i.e. whether they are caused by waves or by plasma flows. First we identify 7 blobs with the highest intensities and follow them along the loop. By means of a location-time plot, bulk velocities can be measured at several locations along the loop. The velocity curve found this way is then compared with characteristic wave speeds and with the free-fall speed in order to deduce the nature of the intensity variations. Additional information on density and temperature is derived by measuring the relative intensity enhancements and comparing the EIT 304 A sequence with Big Bear data and 171 Angstrom data (TRACE/EIT). The combination of all these constraints gives us an insight on the nature and origin of these intensity variations. The idea of slow magneto-acoustic waves is rejected, and we find several arguments supporting that these intensity variations are due to flowing/failing plasma blobs.

LYRA OBSERVATIONS OF TWO OSCILLATION MODES IN A SINGLE FLARE

We analyze light curves from the LYRA irradiance experiment on board PROBA2 during the flare of 2010 February 8. We see both long- and short-period oscillations during the flare. The long-period oscillation is interpreted in terms of standing slow sausage modes; the short-period oscillation is thought to be a standing fast sausage mode. The simultaneous presence of two oscillation modes in the same flaring structure allows for new coronal seismological applications. The periods are used to find seismological estimates of the plasma-β and the density contrast of the flaring loop. Also the wave mode number is estimated from the observed periods.

Detailed comparison of downflows seen both in EIT 30.4 nm and Big Bear H alpha movies

An EIT shutterless campaign was conducted on 11 July 2001 and provided 120 high-cadence (68 s) 30.4 nm images of the north-eastern quarter of the Sun. Systematic intensity variations are seen which appear to propagate along an off-disk loop-like structure. In this paper we study the nature of these intensity variations by confronting the EIT observations studied in De Groof et al. (2004, A&A, 415, 1141) with simultaneous Hα images from Big Bear Solar Observatory. With the goal to carefully co-register the two image sets, we introduce a technique designed to compare data of two different instruments. The image series are first co-aligned and later overplotted in order to visualize and compare the behaviour of the propagating disturbances in both data sets. Since the same intensity variations are seen in the EIT 30.4 nm and in the Hα images, we confirm the interpretation of De Groof et al. (2004, A&A, 415, 1141) that we are observing downflows of relatively cool plasma. The origin of the downflows is explained by numerical simulations of “catastrophic cooling” in a coronal loop which is heated predominantly at its footpoints.

The SWAP EUV Imaging Telescope. Part II: In-flight Performance and Calibration

The Sun Watcher with Active Pixel System detector and Image Processing (SWAP) telescope was launched on 2 November 2009 onboard the ESA PROBA2 technological mission and has acquired images of the solar corona every one to two minutes for more than two years. The most important technological developments included in SWAP are a radiation-resistant CMOS-APS detector and a novel onboard data-prioritization scheme. Although such detectors have been used previously in space, they have never been used for long-term scientific observations on orbit. Thus SWAP requires a careful calibration to guarantee the science return of the instrument. Since launch we have regularly monitored the evolution of SWAP’s detector response in-flight to characterize both its performance and degradation over the course of the mission. These measurements are also used to reduce detector noise in calibrated images (by subtracting dark-current). Because accurate measurements of detector dark-current require large telescope off-points, we also monitored straylight levels in the instrument to ensure that these calibration measurements are not contaminated by residual signal from the Sun. Here we present the results of these tests and examine the variation of instrumental response and noise as a function of both time and temperature throughout the mission.

Fast and Alfvén waves driven by azimuthal footpoint motions - I. Periodic driver

The excitation of Alfv en and fast magneto-acoustic waves in coronal loops driven by footpoint motions is studied in linear, ideal MHD. The analysis is restricted to azimuthally polarized footpoint motions so that only Alfv en waves are directly excited which couple to fast magneto-acoustic waves at later times. In the present study a periodic driver is applied at one end of the loop. The eects of a more realistic random driver are studied in the companion paper De Groof & Goossens (2002) (hereafter referred to as Paper II). The rst part of the paper is devoted to the study of resonant absorption and phase-mixing in the absence of coupling (azimuthal wavenumber ky = 0). Since the density varies across the loop, resonances occur at the magnetic surfaces where the driving frequency equals the local Alfv en frequency. In a second part where Alfv en waves with ky 6 0 coupling to fast waves are taken into account, we nd that the behaviour of the MHD waves is strongly dependent on the driving frequency !d. Especially driving frequencies equal to a quasi-mode frequency seem to make the dierence. The fast waves excited in these cases are global oscillations of the system and form quasi-modes as they are damped through the resonant coupling with Alfv en modes. Since these resonances occur at the same location where the original Alfv en wave peaks, the resonant peak is further amplied. While in most cases coupling has a negative eect on the growth of the directly excited Alfv en waves, driving with a quasi-mode frequency leads to a faster growth of the resonant peaks and a more ecient decrease in length scales than in the uncoupled case.

Study of a Prominence Eruption using PROBA2/SWAP and STEREO/EUVI Data

Observations of the early rise and propagation phases of solar eruptive prominences can provide clues about the forces acting on them through the behavior of their acceleration with height. We have analyzed such an event, observed on 13 April 2010 by SWAP on PROBA2 and EUVI on STEREO. A feature at the top of the erupting prominence was identified and tracked in images from the three spacecraft. The triangulation technique was used to derive the true direction of propagation of this feature. The reconstructed points were fitted with two mathematical models: i) a power-law polynomial function and ii) a cubic smoothing spline, in order to derive the accelerations. The first model is characterized by five degrees of freedom while the second one is characterized by ten degrees of freedom. The results show that the acceleration increases smoothly, and it is continuously increasing with height. We conclude that the prominence is not accelerated immediately by local reconnection, but rather is swept away as part of a large-scale relaxation of the coronal magnetic field.

A virtual appliance as proxy pipeline for the Solar Orbiter/Metis coronagraph

Metis is the coronagraph on board Solar Orbiter, the ESA mission devoted to the study of the Sun that will be launched in October 2018. Metis is designed to perform imaging of the solar corona in the UV at 121.6 nm and in the visible range where it will accomplish polarimetry studies thanks to a variable retarder plate. Due to mission constraints, the telemetry downlink on the spacecraft will be limited and data will be downloaded with delays that could reach, in the worst case, several months. In order to have a quick overview on the ongoing operations and to check the safety of the 10 instruments on board, a high-priority downlink channel has been foreseen to download a restricted amount of data. These so-called Low Latency Data will be downloaded daily and, since they could trigger possible actions, they have to be quickly processed on ground as soon as they are delivered. To do so, a proper processing pipeline has to be developed by each instrument. This tool will then be integrated in a single system at the ESA Science Operation Center that will receive the downloaded data by the Mission Operation Center. This paper will provide a brief overview of the on board processing and data produced by Metis and it will describe the proxy-pipeline currently under development to deal with the Metis low-latency data.

Simulations of the Onset and the Evolution of Coronal Mass Ejections

Abstract Simulations of Coronal Mass Ejections (CMEs) evolving in the interplanetary (IP) space from the Sun up to 1 AU are performed in the framework of ideal magnetohydrodynamics (MHD). The aim is to quantify the effect of the background solar wind and of the CME initiation parameters on the evolution and on the geo-effectiveness of CMEs. The shocks and magnetic clouds related to fast CMEs in the solar corona and interplanetary space play a crucial role in the study of space weather. Better predictions of space weather events require a deeper insight in the physics behind them. Different solar wind models are considered in combination with different CME initiation models: magnetic foot point shearing and magnetic flux emergence. The simulations show that the initial magnetic polarity substantially affects the IP evolution of the CMEs influencing the propagation velocity, the shape, the trajectory (and, thus, the geo-effectiveness).