Forty-one solar type II radio bursts located close to the solar limb (projected radial distance r 0.8 R ) were observed at 109 MHz by the radioheliograph at the Gauribidanur observatory near Bangalore during the period 1997-2007. The positions of the bursts were compared with the estimated location of the leading edge (LE) of the associated coronal mass ejections (CMEs) close to the Sun. 38/41 of the type II bursts studied were located either at or above the LE of the associated CME. In the remaining 3/41 cases, the burst was located behind the LE of the associated CME at a distance of <0.5 R . Our results suggest that nearly all the metric type II bursts are driven by the CMEs.
We present a statistical study of the characteristics of type-II radio bursts observed in the metric (m) and deca-hectometer (DH) wavelength range during 1997-2008. The collected events are divided into two groups: Group I contains the events of m-type-II bursts with starting frequency ≥ 100 MHz, and group II contains the events with starting frequency of m-type-II radio bursts < 100 MHz. We have analyzed both samples considering three different aspects: i) statistical properties of type-II bursts, ii) statistical properties of flares and CMEs associated with type-II bursts, and iii) time delays between type-II bursts, flares, and CMEs. We find significant differences in the properties of m-type-II bursts in duration, bandwidth, drift rate, shock speed and delay between m-and DH-type-II bursts. From the timing analysis we found that the majority of m-type-II bursts in both groups occur during the flare impulsive phase. On the other hand, the DH-type-II bursts in both groups occur during the decaying phase of the associated flares. Almost all m-DH-type-II bursts are found to be associated with CMEs. Our results indicate that there are two kinds of shock in which group I (high frequency) m-type-II bursts seem to be ignited by flares whereas group II (low frequency) m-type-II bursts are CME-driven.
We studied the characteristics of Coronal Mass Ejections (CMEs) associated with solar flares and DecaHectometric (DH) type II radio bursts, based on source position during 23rd solar cycle (1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007). We classified these CME events into three groups using solar flare locations as, (i) disk events (0-30 • ); (ii) intermediate events (31-60 • ) and (iii) limb events (61-90 • ). Main results from this studies are, (i) the number of CMEs associated with solar flares and DH-type IIs decreases as the source position approaches from disk to limb, (ii) most of the DH CMEs are halo (72%) in disk events and the number of occurrence of halo CMEs decreases from disk to limb, (iii) the average width and speed of limb events (164 • and 1447 km s −1 ) are higher than those of disk events (134 • and 1035 km s −1 ) and intermediate events (146 • and 1170 km s −1 ) and (iv) the average accelerations for disk, intermediate and limb events are −8.2 m s −2 , −10.3 m s −2 and −4.5 m s −2 respectively. These analysis of CMEs properties show more dependency on longitude and it gives strong evidence for projection effect.
We investigated the properties of narrow CMEs (width ≤20°), intermediate CMEs (20°< width < 200°) and wide CMEs (width ≥ 200°) observed during 23rd (1996-1997) and 24th (2008-2009) solar cycle minima and compared their characteristics. We found 1099 narrow CMEs during 24th solar cycle and this is 15 times greater than 23rd solar cycle (75). On the other hand, the number of wide CMEs during 23rd cycle (26) is 6.5 times greater than 24th solar cycle (4) minimum. There is no remarkable difference between the number of intermediate CMEs of 23rd (489) and 24th (506) solar cycle minima. The interesting result is that the CMEs width distributions of the two minima are very different. The median and mean speeds of intermediate CMEs during 24th cycle (195 and 215 km s −1 ) are smaller than those (256 and 290 km s −1 ) during 23rd cycle. There is no significant difference in median and mean speeds of narrow and wide CMEs. During 23rd and 24th solar cycle minima we noted (i) 71% and 83% of narrow CMEs, (ii) 81% and 94% of intermediate CMEs and (iii) 46% and 50% of wide CMEs have speed < 400 km s −1 , respectively. The maximum CME speeds observed during 23rd and 24th cycle are 1556 km s −1 (wide CME) and 1103 km s −1 (intermediate CME), respectively. We noticed that the speed of 24th solar cycle CMEs is smaller than 23rd solar cycle CMEs. The narrow CMEs are found to be distributed at low and high latitudes in 24th cycle, whereas in 23rd cycle, most of the narrow CMEs occurred around the equator.
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