A comparison of CME expansion speeds between solar cycles 23 and 24

TL;DR

This study compares the expansion speeds of limb CMEs between solar cycles 23 and 24, revealing higher speeds and different speed relations in cycle 24, linked to heliospheric pressure differences.

Contribution

It provides the first detailed comparison of CME expansion speeds between cycles 23 and 24, highlighting cycle 24's anomalous CME expansion behavior.

Findings

Cycle 24 CMEs have higher expansion speeds than cycle 23.

The relation between radial and expansion speeds differs between the cycles.

Expansion speed increases more with radial speed in cycle 24.

Abstract

We report on a comparison of the expansion speeds of limb coronal mass ejections (CMEs) between solar cycles 23 and 24. We selected a large number of limb CME events associated with soft X-ray flare size greater than or equal to M1.0 from both cycles. We used data and measurement tools available at the online CME catalog (https://cdaw.gsfc.nasa.gov) that consists of the properties of all CMEs detected by the Solar and Heliospheric Observatory's (SOHO) Large Angle and Spectrometric Coronagraph (LASCO). We found that the expansion speeds in cycle 24 are higher than those in cycle 23. We also found that the relation between radial and expansion speeds has different slopes in cycles 23 and 24. The cycle 24 slope is 45% higher than that in cycle 23. The expansion speed is also higher for a given radial speed. The difference increases with speed. For a 2000 km/s radial speed, the expansion speed in cycle 24 is ~48% higher. These results present additional evidence for the anomalous expansion of cycle 24-CMEs, which is due to the reduced total pressure in the heliosphere.