A Decade of Coronagraphic and Spectroscopic Studies of CME-Driven Shocks

Shocks driven by Coronal Mass Ejections (CMEs) are primary agents of space weather. They can accelerate particles to high energies and can compress the magnetosphere thus setting in motion geomagnetic storms. For many years, these shocks were studied only in-situ when they crossed over spacecraft or remotely through their radio emission spectra. Neither of these two methods provides information on the spatial structure of the shock nor on its relationship to its driver, the CME. In the last decade, we have been able to not only image shocks with coronagraphs but also measure their properties remotely through the use of spectroscopic and image analysis methods. Thanks to instrumentation on STEREO and SOHO we can now image shocks (and waves) from the low corona, through the inner heliosphere, to Earth. Here, we review the progress made in imaging and analyzing CME-driven shocks and show that joint coronagraphic and spectrscopic observations are our best means to understand shock physics close to the Sun.Comment: 6 pages, 3 figure

Uncertainties in polarimetric 3D reconstructions of coronal mass ejections

This work is aimed at quantifying the uncertainties in the 3D reconstruction of the location of coronal mass ejections (CMEs) obtained with the polarization ratio technique. The method takes advantage of the different distributions along the line of sight (LOS) of total (tB) and polarized (pB) brightnesses to estimate the average location of the emitting plasma. To this end, we assumed two simple electron density distributions along the LOS (a constant density and Gaussian density profiles) for a plasma blob and synthesized the expected tB and pB for different distances $z$ of the blob from the plane of the sky (POS) and different projected altitudes $\rho$. Reconstructed locations of the blob along the LOS were thus compared with the real ones, allowing a precise determination of uncertainties in the method. Independently of the analytical density profile, when the blob is centered at a small distance from the POS (i.e. for limb CMEs) the distance from the POS starts to be significantly overestimated. Polarization ratio technique provides the LOS position of the center of mass of what we call folded density distribution, given by reflecting and summing in front of the POS the fraction of density profile located behind that plane. On the other hand, when the blob is far from the POS, but with very small projected altitudes (i.e. for halo CMEs, $\rho < 1.4$ R$_\odot$), the inferred distance from that plane is significantly underestimated. Better determination of the real blob position along the LOS is given for intermediate locations, and in particular when the blob is centered at an angle of $20^\circ$ from the POS. These result have important consequences not only for future 3D reconstruction of CMEs with polarization ratio technique, but also for the design of future coronagraphs aimed at providing a continuous monitoring of halo-CMEs for space weather prediction purposes

Super- and Sub-critical Regions in Shocks driven by Radio-Loud and Radio-Quiet CMEs

White-light coronagraphic images of Coronal Mass Ejections (CMEs) observed by SOHO/LASCO C2 have been used to estimate the density jump along the whole front of two CME-driven shocks. The two events are different in that the first one was a "radio-loud" fast CME, while the second one was a "radio quiet" slow CME. From the compression ratios inferred along the shock fronts, we estimated the Alfv\'en Mach numbers for the general case of an oblique shock. It turns out that the "radio-loud" CME shock is initially super-critical around the shock center, while later on the whole shock becomes sub-critical. On the contrary, the shock associated with the "radio-quiet" CME is sub-critical at all times. This suggests that CME-driven shocks could be efficient particle accelerators at the shock nose only at the initiation phases of the event, if and when the shock is super-critical, while at later times they lose their energy and the capability to accelerate high energetic particles.Comment: 7 pages, 5 figures. In press on the "Journal of Advanced Research", Cairo University Pres

Future capabilities of CME polarimetric 3D reconstructions with the METIS instrument: A numerical test

Understanding the 3D structure of coronal mass ejections (CMEs) is crucial for understanding the nature and origin of solar eruptions. To derive information on the 3D structure of CMEs from white-light (total and polarized brightness) images, the polarization ratio technique is widely used. The soon-to-be-launched METIS coronagraph on board Solar Orbiter will use this technique to produce new polarimetric images. We determine the accuracy at which the position of the centre of mass, direction and speed of propagation, and the column density of the CME can be determined along the line of sight. We perform a 3D MHD simulation of a flux rope ejection where a CME is produced. From the simulation we (i) synthesize the corresponding METIS white-light (total and polarized brightness) images and (ii) apply the polarization ratio technique to these synthesized images and compare the results with the known density distribution from the MHD simulation. We find that the polarization ratio technique reproduces with high accuracy the position of the centre of mass along the line of sight. However, some errors are inherently associated with this determination. The polarization ratio technique also allows information to be derived on the real 3D direction of propagation of the CME. In addition, we find that the column density derived from white-light images is accurate and we propose an improved technique where the combined use of the polarization ratio technique and white-light images minimizes the error in the estimation of column densities. Our method allows us to thoroughly test the performance of the polarization ratio technique and allows a determination of the errors associated with it, which means that it can be used to quantify the results from the analysis of the forthcoming METIS observations in white light (total and polarized brightness)

Spectroscopic Detection of Turbulence in Post-CME Current Sheets

Plasma inpost-CMEcurrentsheets(CSs)isexpectedtobehighly turbulent because of the tearing andcoalescence instability and/or local microscopic instabilities. For this reason, in the last decade the inconsistency between the observed (� 10 4 Y10 5 km) and the expected (� 1Y10 m) CS thickness has been tentatively explained in many MHD modelsasaconsequenceof plasmaturbulencethatshouldbeabletosignificantlybroadentheCS.However,fromthe observational point of view, little is known about this subject. A few post-CME CSs have been observed in UVCS spectra as a strong emission in the high-temperature [Fe xviii] line, usually unobservable in the solar corona. In this work, published data on post-CME CSs observed by UVCS are reanalyzed, concentrating for the first time on the evolutionof turbulencederivedfromthenonthermalbroadeningof the[Fe xviii]lineprofiles.Derivedturbulentspeeds are on the order of � 60 km s � 1 a few hours after the CME and slowly decay down to � 30 km s � 1 in the following 2days.Fromthisevolutiontheanomalousdiffusivityduetomicroinstabilitieshasbeenestimated,andthescenarioof multiple small-scale reconnections is tested. Results show that the existence of many (� 10 � 11 to 10 � 17 � CS m � 3 ) microscopic CSs (� CSs) of small sizes (� 10Y10 4 m) could explain not only the high CS temperatures but also the much larger observed thickness of macroscopic CSs, thanks to turbulent broadening. Subject headingg Sun: corona — Sun: coronal mass ejections (CMEs) — Sun: UV radiation — turbulenc

Coronal Magnetic Fields derived with Images acquired during the 21 August 2017 Total Solar Eclipse

The coronal magnetic field, despite its overwhelming importance to the physics and dynamics of the corona, has only rarely been measured. Here, the electron density maps derived from images acquired during the total solar eclipse of August 21st, 2017 are employed to demonstrate a new technique to measure the coronal magnetic fields. The strength of the coronal magnetic fields is derived with a semiempirical formula relating the plasma magnetic energy density to the gravitational potential energy. The resulting values are compared with those provided by more advanced coronal field reconstruction methods based on MHD simulations of the whole corona starting from photospheric field measurements, finding a very good agreement. Other parameters such as the plasma-$\beta$ and Alfv\'en velocity are also derived and compared with those of MHD simulations. Moreover, the plane-of-sky (POS) orientation of the coronal magnetic fields is derived from the observed inclination of the coronal features in the filtered images, also finding a close agreement with magnetic field reconstructions. Hence, this work demonstrates for the first time that the 2D distribution of coronal electron densities measured during total solar eclipses is sufficient to provide the coronal magnetic field strengths and inclinations projected on the POS. These are among the main missing pieces of information that limited so far our understanding of physical phenomena going on in the solar corona.Comment: 20 pages, 11 figure

Low-Frequency Lyα Power Spectra Observed by UVCS in a Polar Coronal Hole

The occurrence of f−1 noise in interplanetary magnetic fields (in the 1 × 10−5 to 1 × 10−4 Hz band) and other plasma parameters has now been known for about 20 years and has been recently identified also in the photospheric magnetic fields. However, the relationship between interplanetary and solar fluctuation spectra and the identification of their sources at the Sun are problems that still need to be addressed. Moreover, interplanetary density and magnetic field power spectra show a f−2 interval at frequencies smaller that ~6 × 10−4 Hz whose source on the Sun is at present not fully understood. In this work we report on the first study of low-frequency density fluctuations in the solar corona at 2.1 R☉. In 2006 June the Ultraviolet Coronagraph Spectrometer (SOHO UVCS) observed over a period of about 9.2 days H Lyα intensity fluctuations at 2.1 R☉ over a polar coronal hole. The Lyα intensity power spectra S(f) (related mainly to density fluctuations) showed a S(f) ∝ f−2 frequency interval between 2.6 × 10−6 and 3.0 × 10−5 Hz and a S(f) ∝ f−1 frequency interval between 3.0 × 10−5 and 1.3 × 10−4 Hz. The detection of a f−2 interval, in agreement with interplanetary density and magnetic field power spectra, has been also predicted in solar wind models as a consequence of phase-mixing mechanisms of waves propagating in coronal holes. High-latitude power spectra show a f−1 band approximately in the same frequency interval where f−1 noise has been detected in interplanetary densities, and interplanetary and photospheric magnetic fields, providing a connection between photospheric, coronal, and interplanetary f−1 noises