Strength of C oronal M ass E jection-driven S hocks N ear the Sun and I ts I mportance in P redicting S olar E nergetic P article E vents Chenglong Shen 1 , Yuming Wang 1 , 2 ,∗ , Pinzhong Ye 1 , X. P. Zhao 3 , Bin Gui 1 , and S. Wang 1
Strength of Coronal Mass Ejection-driven Shocks Near the Sun and Its Importance in Predicting Solar Energetic Particle Events
Chenglong Shen1, Yuming Wang1,2,∗, Pinzhong Ye1, X. P. Zhao3, Bin Gui1, and S. Wang1
2Department of Computational and Data Sciences, George Mason University, Fairfax, VA22030, USA
3W. W. Hansen Experimental Physics Laboratory, Stanford University, Stanford, CA94305, USA
(∗ Author for correspondence)
To be published in ApJ: doi:10.1086/’521716’
Please email the author for preprints
Coronal shocks are an important structure without direct observationsin solar and space physics. The strength of shocks plays a key role in shockcausingphenomena, such as radio bursts, SEP generation and so on. Radio emissions are one of the first signals of solar eruptions and interplanetary disturbances that could be recorded near the Earth, and SEP events are an important effect of space weather. Therefore, how to accurately evaluate the strength of coronal shocks is extremely important and interesting.
This work will present an improved method of calculating Alfven speed and shock strength (i.e., fast-mode magnetosonic Mach number) near the Sun (above 2 Rs). In the method, observations as many as possible rather than 1Dglobal models are used. Two events, a relatively slow CME on 2001 September15 and a very fast CME on 2000 June 15, are selected to illustrate the calculationprocess. The calculation results suggest that the slow CME drove a strong shockwith Mach number of 3.43~4.18 while the fast CME drove a weak shock withMach number of 1.90~3.21. This is consistent with the radio observationsthat a stronger and longer DH type II radio burst is found during the first event anda shorter type II during the second event. Particularly, the calculation resultsexplain the observational fact that the slow CME produced a major SEP eventwhile the fast CME did not. Through the comparison between the two events,the importance of shock strength in predicting SEP events is addressed.
Calculation process: Step 1
Calculation of the local plasma density at the height of type II radio burst
emitted by using the data of type II radio burst.
The fundamental component of type IIs is used to calculate the local plasma density as
where mpis proton mass with the mean molecular weight (Priest 1982).
Radio observations from Wind/Waves are adopted because the frequency range is 1 – 14 MHz corresponding to the heliocentric distance from 2 – 10 Rs within which shocks are most
efficient for energetic particle generation (Kahler 1994; Cliver et al. 2004) that is we interested in. The type IIs in this range could be called decameter-hectometric (DH) type IIs.
Calculation process: step 2
Calculation of the heliocentric distance where type II radio burst emitted and the speed of shocks at that time by using SOHO/LASCO.
The LASCO height-time profile of the CME responsible for the investigated type II radio burst is used to deduce the emission height. For the CMEs not originating from limb, the following formula is applied to correct the projection effect:
where θ and φ are the latitude and longitude of the source region of a CME, which is determined by examining SOHO/EIT movies, Hmeasureis the measured height in LASCO images and Hheliocentric distance is corrected height.
This step applies two assumptions: (1) the type II radiobursts are generated at the noses of CME driven shocks, (2) the shock standoff distance isrelatively smallnear the Sun, and can be therefore ignored. For DH type IIs, both assumptions are reasonable (Gopalswamy et al. 2005; Cho et al. 2005; Vourlidas et al. 2003; Ciaravella et al. 2006).
Calculation process: step 3
Calculation of the background magnetic field strength at the shock by the CSSS model (Zhao & Hoeksema 1995; Zhao et al. 2002).
The model is used to extrapolate the coronal field with the bottom boundary adopted from the WSO (Wilcox Solar Observatory) synoptic charts.
An error of 20 percent of magnetic filed strength is taken into account to make our calculation results more reliable.
Calculation process: step 4
Calculation of the Alfven speed and fast-mode magnetosonic Mach number.
Fast-mode magnetosonic speed
Fast-mode magnetosonic Mach number
are plasma density, shock speed, magnetic field strength, respectively, and have obtained through step 1 to 3.
is ignored since it is generally about 150 km/s at 5 Rs, the solar radius, and even smaller below that height (Sheeley et al. 1997) that is much less than shock speeds typically of several hundreds km/s.
is sound speed that related to temperature. In our work, an isothermal atmosphere is assumed because the coronal temperature is typically at the order of one million Kelvin throughout the IP space.
CME h-t curve
CME onsets derived from EIT data and the h-t curve
A comparison between a relatively slow CME and a fast CME
Large variation in magnetic field strength at the same height
Radio emission and coronal field for the slow and fast CMEs
Slow CME drove a strong shock, while the fast CME drove a weak shock.
The results are consistent with radio observations and SEP observations.
Large variation in plasma density
Compared with ideal models (one fold Newkirk density model and )
The results from ideal models are contrary to the observed facts.
A comparison between the Mach number and the intensity of radio emissions as a function of height.
The figure shows that the intensity of radio emissions proportionally varies as the Mach number changes.
Another case: An extremely fast CME without SEP on 1999 June 28
Source region location
Type II-like emission
Loc: N27W43, VCME=1586 km/s. The calculation suggests a Mach Number<1.56.