top menu
bottom menu

CMEs and Shocks in Solar Wind

Problem description:

Solar Coronal Mass Ejections (CMEs) are large-scale magnetized plasma structures carrying billions of tons of material that erupt from the Sun and propagate in the heliosphere, interacting in multiple ways with the solar wind (Fig.1). They appear to be a critical element in the solar dynamo which removes the dynamo-generated magnetic flux from the Sun and connects the internal dynamo processes to the external solar environment. The observational data on solar CMEs are related to two spatial domains: the near-Sun region (up to 30Rsun ≈ 0,14 AU) remote-sensed by coronagraphs; and the outer region, including the geo-space and beyond, where in-situ observations are made by spacecraft. The generic term used for CMEs in the solar wind is ejecta. Due to the existence of two spatial domains of observations, the term CME is usually applied for ejecta as they are observed near the Sun (≤ 0,14 AU), whereas at the larger distances they are traditionally called as Interplanetary CMEs (ICMEs).

Traveling outward from the Sun at high speeds (up to thousands kilometers per second), CMEs create major disturbances in the interplanetary medium. Due to the high speed, intrinsic magnetic field and the increased density (as compared to the solar wind background), CMEs produce strong effects on the planetary environments and magnetospheres. Besides of the density and velocity disturbances created in the solar wind plasma, among the main planetary impact factors of CMEs, appear also the associated interplanetary shocks, energetic particles accelerated in the shock regions, and the magnetic field disturbances which create magnetic storms.

 

Fig.1: Typical solar CMEs observed by SoHO/LASCO

 

Solar CMEs and ICMEs are well described in the literature (Gopalswamy et al. 2003, Gopalswamy et al. 2000; St. Cyr et al. 2000) and here we just briefly summarize their main features.

1. CME origin and relation to flares:

CMEs are associated with flares and prominence eruptions and their sources are usually located in active regions and prominence sites. Closed magnetic structure is the basic characteristic of CME Producing regions. Recent studies on temporal correspondence between CMEs and flares provide arguments in favor of the so-called common-cause scenario, according to which flares and CMEs are different manifestations of the same large-scale magnetic processes on the Sun (Zhang et al. 2001). Despite the details of this interconnection still remain to be unclear, it can be definitely stated that an intensive flaring activity on the Sun is accompanied by an increased rate of CME production. The probability of CME-flare association increases with a duration of a flare (Sheeley et al. 1983): ~ 26% for flare duration < 1 h; and 100% for flare duration > 6 h.

2. CME global structure:

Multi-thermal structure of CMEs includes 1) coronal material in the front region (~ 2 MK); 2) a core formed by prominence material (~8000 K), or hot flare plasma (~10 MK). CME/ICME structure evolves during its propagation in the interplanetary space. The following sequence of structures can be related to the ICMEs: a) Interplanetary shock, b) sheath, and c) the ICME itself. The ejecta can contain ordered magnetic field, in which case it is termed as a magnetic cloud (MC) (Burlaga et al.1981). MCs constitute a subset of all ejecta (about 33%).

3. CME speed and acceleration:

The speed of CMEs, VCME , is defined by their tracking in the sky plane on coronagraph images. Thus, the derived VCME is a subject to projection effect errors. Due to the large statistics of the considered SoHO/LASCO CMEs (> 8000), the average speed (» 489 km/s) of CMEs may be considered as a representative quantity (see Fig.2 from Gopalswamy et al. 2003).

 

Fig.2: Speed and angular size distribution of SoHO/LASCO CMEs, observed in 1996-2003

Three basic types of CMEs with respect to their acceleration can be distinguished: 1) Constant-speed CMEs; 2) Accelerated CMEs; 3) Decelerated CMEs. This indicates the presence of three basic forces acting on CMEs during their propagation a) Propelling force (not well known); b) Gravity force; and c) Braking force (interaction with the background medium).

4. CME size:

The CME angular width DCME is measured as the position angle extent of the ejecta in the sky plane. It can be also a subject to projection effect errors. DCME increases on the initial stages of propagation (< 5RSun) and then usually remains constant. Annual average DCME of non-halo CMEs range from 45° (solar minimum) to 61° (close before activity maximum) (Yashiro et al. 2004).

5. CME latitude distribution:

The latitude distribution of CMEs depends on the distribution of the closed field magnetic regions (active regions) on the solar surface. During the rising phase of solar activity cycle, CME latitudes spread gradually from those close to the equator (0°) up to all latitudes (± 90°). At the same time, the majority of eruptions are located within the average latitude interval  ± Q near the equatorial plane, with Q = 60° (Gopalswamy et al. 2003).

6. CME occurrence rate:

CME occurrence rate fCME shows good correlation with the Sunspot Number (SSN). fCME peaks with a delay (about 2 years) after the peak in the SSN. This is connected to the fact that the sunspot activity is confined to the active region belt, whereas CMEs appear at all latitudes during the maximum (Gopalswamy et al. 2003, St. Cyr et al. 2000).

7. CME mass, density, duration: 

CME density nCME at distances ≤ 30RSun ≈ 0.14 AU is estimated from the analysis of associated with CMEs brightness enhancements in the white-light coronagraph images. At large distances (> 0.4 AU) nCME is determined from the in-situ measurements of density in MCs.

CME density dependence on the distance d from the Sun can be presented as a power-law: nCME(d) = n0(d /d0)-3.6, which with n0 = 5x105 ? 5x106 cm-3 and d0 = 3RSun gives a good approximation for the values estimated from the SoHO/LASCO coronograph images.

On the other hand, in-situ measurements of density in magnetic clouds at distances > 0.4 AU give nMC(d) = n 0MC (d / d0) (-2.4 ± 0.3) , with n 0MC = 6.47 ± 0.85 cm-3; d0 = 1 AU. Based on these data Khodachenko et al. (2007 a,b) provided combined CME density approximations (Fig.3): nminejecta(d) = 4.88 (d / d0) -2.3 and nmaxejecta(d) = 7.10 (d / d0) -3.0, where d0 = 1 AU and d is taken in AU.

Average mass of CMEs, MCME, is ≈ 1015g. Average duration of CMEs, tCME at (6 ?10) RSun is ≈ 8 hours.

Fig.3: Observed (dotted lines) and approximated by power-low formulas (dashed lines) minimal and maximal CME and ICME/MC densities as function of distance from the Sun (Khodachenko et al., 2007 a,b)

References:

  • Burlaga, L., Sittler, E., Mariani, F., Schwenn, R., Magnetic loop behind an interplanetary shock - Voyager, Helios, and IMP 8 observations, J. Geophys. Res., 1981, 86, 6673-6684.
  • Gopalswamy, N., Coronal mass ejections: Initiation and detection, Adv. Space Res., 2003, 31, 869-881.
  • Gopalswamy, N., Kaiser, M. L., Thompson, B. J., Burlaga, L. F., Szabo, A., Vourlidas, A., Lara, A., Yashiro, S., Bougeret, J. L., Radio-rich solar eruptive events, Geophys. Res. Lett., 2000, 27, 1427-1430.
  • Khodachenko, M.L., Ribas, I., Lammer, H., Grießmeier, J.-M., Leitner, M., Selsis, F., Eiroa, C., Hanslmeier, A., Biernat, H., Farrugia, C. J., Rucker, H., Coronal Mass Ejection (CME) activity of low mass M stars as an important factor for the habitability of terrestrial exoplanets, Part I: CME impact on expected magnetospheres of Earth-like exoplanets in close-in habitable zones, Astrobiology, 2007a, 7, No.1, 167-184.
  • Khodachenko, M.L., Lammer, H., Lichtenegger, H.I.M., Langmayr, D., Erkaev, N.V., Griessmeier, J.-M., Leitner, M., Penz, T., Biernat, H. K., Motschmann, U., Rucker, H.O., Mass loss of ?Hot Jupiters? ? Implications for CoRoT discoveries. Part I: The importance of magnetospheric protection of a planet against ion loss caused by coronal mass ejections, Planetary and Space Science, 2007b, 55, 631-642.
  • Sheeley, N., Howard, R.A., Koomen, M. J., Michels, D. J., Associations between coronal mass ejections and soft X-ray events, Astrophys. J., 1983, 272, 349-354.
  • St. Cyr, O. C., Howard, R. A., Sheeley, N. R. Jr., Plunkett, S.P., Michels, D. J., Paswaters, S. E., Koomen, M. J., Simnett, G. M., Thompson, B. J., Gurman, J. B., Schwenn, R., Webb, D. F., Hildner, E., Lamy, P. L., Properties of coronal mass ejections: SOHO LASCO observations from January 1996 to June 1998, J. Geophys. Res., 2000, 105, 18169-18185.
  • Yashiro, S., Gopalswamy, N., Michalek, G., St. Cyr, O. C., Plunkett, S. P., Rich, N. B., Howard, R. A., A catalog of white light coronal mass ejections observed by the SOHO spacecraft,  J. Geophys. Res., 2004, 109, A7, A07105, DOI:10.1029/2003JA010282J.
  • Zhang, J., Dere, K. P., Howard, R. A., Kundu, M. R., White, S. M., On the temporal relationship between coronal mass ejections and flares,  Astrophys. J., 2001, 559, 452-642.