Are the Faint Structures Ahead of Solar Coronal Mass Ejections Real Signatures of Driven Shocks?

Moon

et al. 2017

ApJ

Self Cite

It is generally believed that fast coronal mass ejections (CMEs) can generate their associated shocks, which are characterized by faint structures ahead of CMEs in white-light coronagraph images. In this study, we examine whether the observational stand-off distance ratio, defined as the CME stand-off distance divided by its radius, can be explained by bow shock theories. Of 535 SOHO/LASCO CMEs (from 1996 to 2015) with speeds greater than 1000 km s −1 and angular widths wider than 60°, we select 18 limb CMEs with the following conditions: (1) their Alfvénic Mach numbers are greater than one under Mann's magnetic field and Saito's density distributions; and (2) the shock structures ahead of the CMEs are well identified. We determine observational CME stand-off distance ratios by using brightness profiles from LASCO-C2 observations. We compare our estimates with theoretical stand-off distance ratios from gasdynamic (GD) and magnetohydrodynamic (MHD) theories. The main results are as follows. Under the GD theory, 39% (7/18) of the CMEs are explained in the acceptable ranges of adiabatic gamma (γ) and CME geometry. Under the MHD theory, all the events are well explained when we consider quasiparallel MHD shocks with γ=5/3. When we use polarized brightness (pB) measurements for coronal density distributions, we also find similar results: 8% (1/12) under GD theory and 100% (12/12) under MHD theory. Our results demonstrate that the bow shock relationships based on MHD theory are more suitable than those based on GD theory for analyzing CME-driven shock signatures.

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confidence: 78%

Section: Introductionmentioning

confidence: 99%

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confidence: 99%

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Which Bow Shock Theory, Gasdynamic or Magnetohydrodynamic, Better Explains CME Stand-off Distance Ratios from LASCO-C2 Observations ?

Moon

et al. 2017

ApJ

Self Cite

“…Over the last decade, many works focused on the signatures of CME-driven shocks in white-light (WL) observations, namely from the Solar and Heliospheric Observatory (SOHO)/LASCO C2 and C3 coronagraphs and the STEREO/ COR1 and COR2 telescopes (Sheeley et al 2000;Vourlidas et al 2003Vourlidas et al , 2013Gopalswamy et al 2009;Ontiveros & Vourlidas 2009;Gopalswamy & Yashiro 2011;Kim et al 2012;Lee et al 2014). WL data have proven to contain much more information than previously thought, and were used with various techniques to derive shock speeds, shock compression ratios X d u r r = (i.e., the ratio between the upstream u r and the downstream d r plasma densities), geometrical properties of the shock fronts, as well as the strength of the coronal magnetic field encountered by the shocks, and also allowed studies on the correlation between the SEP fluxes and associated CME speeds (e.g., Kahler 2001).…”

Section: Introductionmentioning

confidence: 99%

Plasma Physical Parameters Along Cme-Driven Shocks. Ii. Observation–simulation Comparison

Bacchini

Susino

Бемпорад

et al. 2015

ApJ

In this work, we compare the spatial distribution of the plasma parameters along the 1999 June 11 coronal mass ejection (CME)-driven shock front with the results obtained from a CME-like event simulated with the FLIPMHD3D code, based on the FLIP-MHD particle-in-cell method. The observational data are retrieved from the combination of white-light coronagraphic data (for the upstream values) and the application of the RankineHugoniot equations (for the downstream values). The comparison shows a higher compression ratio X and Alfvénic Mach number M A at the shock nose, and a stronger magnetic field deflection d toward the flanks, in agreement with observations. Then, we compare the spatial distribution of M A with the profiles obtained from the solutions of the shock adiabatic equation relating M A , X, and Bn q (the angle between the upstream magnetic field and the shock front normal) for the special cases of parallel and perpendicular shock, and with a semi-empirical expression for a generically oblique shock. The semi-empirical curve approximates the actual values of M A very well, if the effects of a non-negligible shock thickness sh d and plasma-to magnetic pressure ratio u b are taken into account throughout the computation. Moreover, the simulated shock turns out to be supercritical at the nose and sub-critical at the flanks. Finally, we develop a new one-dimensional Lagrangian ideal MHD method based on the GrAALE code, to simulate the ion-electron temperature decoupling due to the shock transit. Two models are used, a simple solar wind model and a variable-γ model. Both produce results in agreement with observations, the second one being capable of introducing the physics responsible for the additional electron heating due to secondary effects (collisions, Alfvén waves, etc.).

“…We have calculated the shock heights from LASCO-C2 and LASCO-C3 running images. The faint structure surrounding the CME is considered a shock (Lee et al, 2014) and the heights were calculated within the DH-Type II starting and ending durations. The shock signature corresponding to a particular event is identified from the LASCO images at the time of DH starting and ending.…”

Section: Shock Formation Heightsmentioning

confidence: 99%

Halo Coronal Mass Ejections and Their Relation to DH Type-II Radio Bursts

Shanmugaraju

Lawrance²

2015

Sol Phys

A set of 88 halo CMEs observed by the Solar and Heliospheric Observatory/Large Angle Solar Coronagraph (SOHO/LASCO) during the period 2005 to 2010 is considered to study the relationship of these halo CMEs with Type-II radio bursts in the deca-hectametric (DH) wavelength range observed by Wind/(Plasma and Radio Waves: WAVES). Among the 88 events, 39 halo CMEs are found to be associated with DH Type-II radio bursts and their characteristics are analyzed with the following results: i) The heights of the CME leading edge at the time of the starting frequencies of many of the selected DH Type-II events (74 %) are in the range (2 -10 R ) where the shocks are formed. ii) The mean speed of DH-associated halo CMEs (1610 km s −1 ) is nearly twice the mean speed (853 km s −1 ) of halo CMEs without DH Type-II radio bursts, implying that the peak of the Alfvén speed profile in the outer corona where DH Type-II radio bursts start might be around 800 km s −1 . iii) The shock speed of DH Type-II radio bursts calculated using the heights of shock signatures of the corresponding CME events is found to be slightly higher than the CME speed. iv) The CME speed plays a major role in the determination of the ending frequency of DH Type-II radio bursts but not the starting frequency. v) The relationship between the characteristics of DH Type-II radio bursts and CMEs is explained in the context of the universal drift-rate spectrum.