Evolution of a Long-Duration Coronal Mass Ejection and its Sheath Region Between Mercury and Earth on 2013 July 9-14 (1912.05446v1)

Abstract: Using in situ measurements and remote-sensing observations, we study a coronal mass ejection (CME) that left the Sun on 9 July 2013 and impacted both Mercury and Earth while the planets were in radial alignment (within $3^\circ$). The CME had an initial speed as measured by coronagraphs of 580 $\pm$ 20 km s$^{-1}$, an inferred speed at Mercury of 580 $\pm$ 30 km s$^{-1}$ and a measured maximum speed at Earth of 530 km s$^{-1}$, indicating that it did not decelerate substantially in the inner heliosphere. The magnetic field measurements made by MESSENGER and {\it Wind} reveal a very similar magnetic ejecta at both planets. We consider the CME expansion as measured by the ejecta duration and the decrease of the magnetic field strength between Mercury and Earth and the velocity profile measured {\it in situ} by {\it Wind}. The long-duration magnetic ejecta (20 and 42 hours at Mercury and Earth, respectively) is found to be associated with a relatively slowly expanding ejecta at 1 AU, revealing that the large size of the ejecta is due to the CME itself or its expansion in the corona or innermost heliosphere, and not due to a rapid expansion between Mercury at 0.45 AU and Earth at 1 AU. We also find evidence that the CME sheath is composed of compressed material accumulated before the shock formed, as well as more recently shocked material.

• The paper quantifies the CME's nearly constant velocity (580 km/s near Mercury and 530 km/s near Earth), indicating minimal deceleration.
• The paper measures the CME’s expansion parameter (ζ), uncovering atypically low expansion rates that may refine space weather models.
• The paper examines sheath region evolution, showing a transition from a compact structure at Mercury to an expanded, complex formation at Earth.

Coronal Mass Ejection Dynamics between Mercury and Earth: Analysis and Insights

The paper titled "Evolution of a Long-Duration Coronal Mass Ejection and its Sheath Region Between Mercury and Earth on 2013 July 9-14" provides an in-depth analysis of a coronal mass ejection (CME) captured in situ and through remote sensing as it progressed from the Sun to Earth, impacting Mercury along the way. This analysis allows for comprehensive insights into the expansion and magnetic dynamics of such solar phenomena, which are pivotal drivers of space weather effects on planetary environments.

CME Kinematics and Expansion

Through data from MESSENGER and Wind satellites, combined with coronagraph observations, the researchers documented the CME's consistent velocity profile. The CME maintained a near-constant high speed of approximately 580 km/s from the Sun to Mercury and registered a slightly reduced velocity of 530 km/s at Earth, indicating minimal deceleration in the heliosphere. This consistency highlights the CME's initial expulsion power and the lack of significant interaction decelerations in the intervening space.

A crucial aspect of this research is the CME's expansion dynamics. The paper measured the CME’s spatial scale and duration at both Mercury and Earth, employing a dimensionless expansion parameter, ζ, calculated from local velocity profiles. The findings revealed ζ values at the lower end of typical CME expansion ranges, suggesting an atypical growth constraint, potentially of significance for space weather modeling.

Sheath Region Development

The investigation delved into the complex structure of the CME sheath region between Mercury and Earth. At Mercury, the sheath was succinct, encompassing pre-shocked solar wind material accumulated due to the CME propagation. Conversely, this sheath expanded significantly in size and structure by the time it reached Earth, encompassing a mix of shocked and accumulated solar wind material—a testament to sheath dynamics playing substantial roles in CME impacts on planetary space weather.

Implications for Space Weather Forecasting

The implications of such an in-depth CME paper are extensive. Understanding CME trajectories and structural evolution is vital for predicting space weather events that can disrupt satellite functionality and terrestrial power grids. The research delivers crucial insights into CME dynamics, which can enhance models for anticipating CME impacts on Earth and other planets with magnetospheres.

Future Directions

Further statistical studies, including more events tracked by multiple spacecraft, are necessary to establish broader patterns in CME behavior and to refine the reliability of forecasting models. Upcoming missions, such as the Solar Orbiter and Parker Solar Probe, will provide additional in situ data at varying solar distances, potentially validating or challenging these initial findings regarding CME expansion and sheath formation.

By enhancing the understanding of CME characteristics with such detailed studies, scientists can not only fortify predictive models but also tailor them to consider varying planetary environments across our solar system, thereby improving the safety and reliability of satellite operations and space exploration missions.