ABSTRACT
Slow waves are commonly observed on the entire solar atmosphere. Assuming a thin flux tube approximation, the cut-off periods of slow-mode magneto-acoustic-gravity waves that travel from the ...photosphere to the corona were obtained in Costa et al. In that paper, however, a typo in the specific heat coefficient at constant pressure cp value led to an inconsistency in the cut-off calculation, which is only significant at the transition region. Due to the abrupt temperature change in the region, a change of the mean atomic weight (by a factor of approximately 2) also occurs, but is often overlooked in analytical models for simplicity purposes. In this paper, we revisit the calculation of the cut-off periods of magneto-acoustic-gravity waves in Costa et al. by considering an atmosphere in hydrostatic equilibrium with a temperature profile, with the inclusion of the variation of the mean atomic weight and the correction of the inconsistency aforementioned. In addition, we measure the dominant periods near a particular active region (AR 1243) as observed by the Atmospheric Imaging Assembly (AIA) on-board the Solar Dynamic Observatory (SDO) on 2011 July 3 and compare them to our analytical results. The cut-off periods obtained analytically are consistent with the corresponding periods measured in observations.
Generalising the Elsässer variables, we introduce the $Q$-variables. These are more flexible than the Elsässer variables, because they also allow us to track waves with phase speeds different than ...the Alfvén speed. We rewrite the magnetohydrodynamics (MHD) equations with these $Q$-variables. We consider also the linearised version of the resulting MHD equations in a uniform plasma, and recover the classical Alfvén waves, but also separate the fast and slow magnetosonic waves into upward- and downward-propagating waves. Moreover, we show that the $Q$-variables may also track the upward- and downward-propagating surface Alfvén waves in a non-uniform plasma, displaying the power of our generalisation. In the end, we lay the mathematical framework for driving solar wind models with a multitude of wave drivers.
Context.
Recurrent, arc-shaped intensity disturbances were detected by extreme-ultraviolet channels in an active region. The fronts were observed to propagate along a coronal loop bundle rooted in a ...small area within a sunspot umbra. Previous works have linked these intensity disturbances to slow magnetoacoustic waves that propagate from the lower atmosphere to the corona along the magnetic field.
Aims.
The slow magnetoacoustic waves propagate at the local cusp speed, which is equivalent to the sound speed in a low-
β
-regime plasma. However, the measured propagation speeds from the intensity images are usually smaller as they are subject to projection effects due to the inclination of the magnetic field with respect to the line of sight. We aim to understand the effect of projection by comparing observed speeds with those from a numerical model.
Methods.
Using multi-wavelength data, we determined the periods present in the observations at different heights of the solar atmosphere through Fourier analysis. We calculated the plane-of-sky speeds along one of the loops from the cross-correlation time-lags obtained as a function of distance along the loop. We performed a 2D ideal magnetohydrodynamic simulation of an active region embedded in a stratified atmosphere. We drove slow waves from the photosphere with a three-minute periodicity. Synthetic time–distance maps were generated from the forward-modelled intensities in coronal wavelengths and the projected propagation speeds were calculated.
Results.
The intensity disturbances show a dominant period between 2 and 3 min at different heights of the atmosphere. The apparent propagation speeds calculated for coronal channels exhibit an accelerated pattern with values increasing from 40 to 120 km s
−1
as the distance along the loop rises. The propagation speeds obtained from the synthetic time–distance maps also exhibit accelerated profiles within a similar range of speeds.
Conclusions.
We conclude that the accelerated propagation in our observations is due to the projection effect.
Context.
A highly important aspect of solar activity is the coupling between eruptions and the surrounding coronal magnetic field topology, which determines the trajectory and morphology of the ...ejected plasma. Pseudostreamers (PSs) are coronal magnetic structures formed by arcs of twin loops capped by magnetic field lines from coronal holes of the same polarity that meet at a central spine. PSs contain a single magnetic null point in the spine, immediately above the closed field lines, which potentially influences the evolution of nearby flux ropes (FRs).
Aims.
Because of the impact of magnetic FR eruptions on space weather, we aim to improve current understanding of the deflection of coronal mass ejections (CMEs). To understand the net effect of PSs on FR eruptions, it is first necessary to study diverse and isolated FR–PS scenarios that are not influenced by other magnetic structures.
Methods.
We performed numerical simulations in which a FR structure is in the vicinity of a PS magnetic configuration. The combined magnetic field of the PS and the FR results in the formation of two magnetic null points. We evolve this scenario by numerically solving the magnetohydrodynamic equations in 2.5D. The simulations consider a fully ionised compressible ideal plasma in the presence of a gravitational field and a stratified atmosphere.
Results.
We find that the dynamic behaviour of the FR can be categorised into three different classes based on the FR trajectories and whether it is eruptive or confined. Our analysis indicates that the magnetic null points are decisive in the direction and intensity of the FR deflection and their hierarchy depends on the topological arrangement of the scenario. Moreover, the PS lobe acts as a magnetic cage enclosing the FR. We report that the total unsigned magnetic flux of the cage is a key parameter defining whether or not the FR is ejected.
Understanding the deflection of coronal mass ejections (CMEs) is of great interest to the space weather community because of their implications for improving the prediction of CME. This paper aims to ...shed light into the effects of the coronal magnetic field environment on CME trajectories. We analyze the influence of the magnetic environment on the early development of a particular CME event. On 2011 January 24 an eruptive filament was ejected in association with a CME that suffered a large deflection from its source region and expected trajectory. We characterize the 3D evolution of the prominence material using the tie-pointing/triangulation reconstruction technique on EUV and white-light images. To estimate the coordinates in 3D space of the apex of the CME we use a forward-modeling technique that reproduces the large-scale structure of the flux rope-like CME, the Graduated Cylindrical Shell model. We found that the deflection amounts to 42° in latitude and 20° in longitude and that most of it occurs at altitudes below 4R⊙. Moreover, we found a non-negligible deflection at higher altitudes. Combining images of different wavelengths with the extrapolated magnetic field obtained from a potential field source surface model we confirm the presence of two magnetic structures near the erupting event. The magnetic field environment suggests that field lines from the southern coronal hole act as a magnetic wall that produces the large latitudinal deflection; while a nearby pseudostreamer and a northward extension of the southern coronal hole may be responsible for the eastward deflection of the CME.
A critical aspect of solar activity is the coupling between eruptions and the surrounding coronal magnetic field, which determines the trajectory and morphology of the eruptive event. Pseudostreamers ...(PSs) are coronal magnetic structures formed by arcs of twin loops capped by magnetic field lines from coronal holes of the same polarity that meet at a central spine. They contain a single magnetic null point in the spine, potentially influencing the evolution of nearby flux ropes (FRs). To understand the net effect of the PS on FR eruptions is first necessary to study diverse and isolated FR-PS scenarios, which are not influenced by other magnetic structures. We performed numerical simulations in which a FR structure is in the vicinity of a PS magnetic configuration. The combined magnetic field of the PS and the FR results in the formation of two magnetic null points. We evolve this scenario by numerically solving the magnetohydrodynamic equations in 2.5D. The simulations consider a fully ionised compressible ideal plasma in the presence of a gravitational field and a stratified atmosphere. We find that the dynamic behaviour of the FR can be categorised into three different classes based on the FR trajectories and whether it is eruptive or confined. Our analysis indicates that the magnetic null points are decisive in the direction and intensity of the FR deflection and their hierarchy depends on the topological arrangement of the scenario. Moreover, the PS lobe acts as a magnetic cage enclosing the FR. We report that the total unsigned magnetic flux of the cage is a key parameter defining whether the FR is ejected or not.
Slow waves are commonly observed on the entire solar atmosphere. Assuming a thin flux tube approximation, the cut-off periods of slow-mode magneto-acoustic-gravity waves that travel from the ...photosphere to the corona were obtained in Costa et al. (2018). In that paper, however, a typo in the specific heat coefficient at constant pressure \(c_{\mathrm{p}}\) value led to an inconsistency in the cut-off calculation, which is only significant at the transition region. Due to the abrupt temperature change in the region, a change of the mean atomic weight (by a factor of approximately two) also occurs, but is often overlooked in analytical models for simplicity purposes. In this paper, we revisit the calculation of the cut-off periods of magneto-acoustic-gravity waves in Costa et al. (2018) by considering an atmosphere in hydrostatic equilibrium with a temperature profile, with the inclusion of the variation of the mean atomic weight and the correction of the inconsistency aforementioned. In addition, we show that the cut-off periods obtained analytically are consistent with the corresponding periods measured in observations of a particular active region.
Recurrent, arc-shaped intensity disturbances were detected by EUV channels in an active region. The fronts were observed to propagate along a coronal loop bundle rooted in a small area within a ...sunspot umbra. Previous works have linked these intensity disturbances to slow magnetoacoustic waves that propagate from the lower atmosphere to the corona along the magnetic field. The slow magnetoacoustic waves propagate at the local cusp speed. However, the measured propagation speeds from the intensity images are usually smaller as they are subject to projection effects due to the inclination of the magnetic field with respect to the line-of-sight. Here, we aim to understand the effect of projection by comparing observed speeds with those from a numerical model. Using multi-wavelength data we determine the periods present in the observations at different heights of the solar atmosphere through Fourier analysis. We calculate the plane-of-sky speeds along one of the loops from the cross-correlation time lags obtained as a function of distance along the loop. We perform a 2D ideal MHD simulation of an active region embedded in a stratified atmosphere. We drive slow waves from the photosphere with a 3 minutes periodicity. Synthetic time-distance maps are generated from the forward-modelled intensities in coronal wavelengths and the projected propagation speeds are calculated. The intensity disturbances show a dominant period between 2-3 minutes at different heights of the atmosphere. The apparent propagation speeds calculated for coronal channels exhibit an accelerated pattern with values increasing from 40 to 120 km/s as the distance along the loop rises. The propagation speeds obtained from the synthetic time-distance maps also exhibit accelerated profiles within a similar range of speeds. We conclude that the accelerated propagation in our observations is due to the projection effect.
Motivated by the need to improve the ability to forecast whether a certain coronal mass ejection (CME) is to impact Earth, and by the insufficiency of statistical studies that analyze the whole ...erupting system with the focus on the governing conditions under CME deflections, we performed a careful analysis of 13 events along a one-year time interval showing large deflections from their source region. We used telescopes imaging the solar corona at different heights and wavelengths on board the Project for Onboard Autonomy 2 (PROBA2), Solar Dynamics Observatory (SDO), Solar TErrestrial RElations Observatory (STEREO), Solar and Heliospheric Observatory (SOHO) spacecraft and from National Solar Observatory (NSO). By taking advantage of the quadrature position of these spacecraft from October 2010 until September 2011, we inspected the 3D trajectory of CMEs and their associated prominences with respect to their solar sources by means of a tie-pointing tool and a forward model. Considering the coronal magnetic fields as computed from a potential field source surface model, we investigate the roles of magnetic energy distribution and kinematic features in the non-radial propagation of both structures. The magnetic environment present during the eruption is found to be crucial in determining the trajectory of CMEs, in agreement with previous reports.
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