Radio-loud CMEs from the disk center lacking shocks at 1 AU

TL;DR

This study investigates why some radio-loud CMEs originating near the solar disk center do not produce shocks at Earth, revealing they are lower energy events that dissipate before reaching 1 AU and are influenced by other solar processes.

Contribution

It identifies the characteristics and possible reasons for the absence of shocks at 1 AU in certain disk-center radio-loud CMEs, highlighting factors like CME energy, speed, and solar environment effects.

Findings

Approximately 28% of disk-center radio-loud CMEs lack shocks at Earth.

Lower CME energies and specific solar conditions cause shock dissipation before 1 AU.

Shock observation probability increases with CME speed above 1000 km/s and lower frequencies.

Abstract

A coronal mass ejection (CME) associated with a type II burst and originating close to the center of the solar disk typically results in a shock at Earth in 2-3 days and hence can be used to predict shock arrival at Earth. However, a significant fraction (about 28%) of such CMEs producing type II bursts were not associated with shocks at Earth. We examined a set of 21 type II bursts observed by the Wind/WAVES experiment at decameter-hectometric (DH) wavelengths that had CME sources very close to the disk center (within a central meridian distance of 30 degrees), but did not have a shock at Earth. We find that the near-Sun speeds of these CMEs average to ~644 km/s, only slightly higher than the average speed of CMEs associated with radio-quiet shocks. However, the fraction of halo CMEs is only ~30%, compared to 54% for the radio-quiet shocks and 91% for all radio-loud shocks. We conclude that the disk-center radio-loud CMEs with no shocks at 1 AU are generally of lower energy and they drive shocks only close to the Sun and dissipate before arriving at Earth. There is also evidence for other possible processes that lead to the lack of shock at 1 AU: (i) overtaking CME shocks merge and one observes a single shock at Earth, and (ii) deflection by nearby coronal holes can push the shocks away from the Sun-Earth line, such that Earth misses these shocks. The probability of observing a shock at 1 AU increases rapidly above 60% when the CME speed exceeds 1000 km/s and when the type II bursts propagate to frequencies below 1 MHz.