Funded by: NextGenerationEU

Project Leader: Davor Sudar


Description:
The Sun is a dynamic and active object composed of hot, ionized plasma. The solar photosphere is the layer from which most visible radiation originates and marks the transition between the dense solar interior and its more tenuous atmosphere. Inside the Sun, plasma exists in a regime where gas pressure dominates over the magnetic field, whereas in the atmosphere the opposite is true, and plasma dynamics are governed primarily by magnetic fields. As a result, numerous dynamic processes occur in the atmosphere, among which eruptive phenomena such as solar flares and coronal mass ejections (CMEs) are particularly significant.

The overall objective of this project is to investigate and improve our understanding of eruptive processes and the general causes leading to solar eruptions. Therefore, the project is divided into three main research areas: 1) Solar Rotation, 2) Eruptive Processes in the Lower Atmosphere and 3) Eruptive Processes in the Middle and Upper Corona and the Heliosphere.

The Sun exhibits differential rotation, with its equator rotating faster than its poles. This phenomenon is a key component of solar dynamo theory and strongly influences solar magnetic activity. Modern research employs various tracers (motion indicators), including sunspots, coronal bright points (CBPs), and helioseismic data to study solar rotation.

Horizontal Reynolds stress is considered the primary driver of the observed differential rotation profile of the Sun. Reynolds stress arises from the correlation between two turbulent velocity components: rotational velocity residuals and meridional motions. For horizontal Reynolds stress to differ from zero, rotational velocity residuals and meridional velocities must be correlated. If the correlation is positive, i.e., rotational velocity residuals that are faster than average are associated with negative meridional velocities (toward the equator), negative Reynolds stress is obtained. The same applies when negative rotational velocity residuals are associated with positive meridional motions. Due to the nature of Reynolds stress, it can only be observed using tracer methods.

Observations reveal negative horizontal Reynolds stress, indicating the transport of angular momentum from higher heliographic latitudes toward lower latitudes (from the poles toward the equator). Although the general role of Reynolds stress appears to be understood, several open questions remain:

- Temporal variations of the rotation profile, both within a single solar cycle and between different cycles

- North–south asymmetry in solar rotation, particularly at active latitudes and in coronal structures

- Asymmetry in the distribution of tracers between the two hemispheres

- Differences in the characteristics of meridional motions among different tracers

- Variations in the magnitude of Reynolds stress between solar cycles

2) Eruptive Processes in the Lower Atmosphere
Solar flares are sudden and intense releases of energy in the solar atmosphere caused by magnetic reconnection. They are observed across the entire electromagnetic spectrum, from gamma rays to radio waves, with the strongest signatures appearing in X-ray and extreme ultraviolet (EUV) wavelengths. In the millimeter (mm) and submillimeter (sub-mm) frequency range (approximately 100 GHz–10 THz), flares remain relatively poorly understood, although modern observations indicate a complex relationship with hard X-ray emission and chromospheric spectral lines.
It is believed that flare mm emission may arise from a combination of different physical mechanisms, primarily thermal bremsstrahlung and gyrosynchrotron radiation, with their relative contributions depending on the flare phase. There are also indications of multiple populations of accelerated electrons contributing to emission in different parts of the spectrum. Spatial analyses show that mm emission is associated with various solar atmospheric structures, including magnetic loop footpoints and tops, flare ribbons, and chromospheric and coronal features. Thanks to its high spatial and temporal resolution, observations with the ALMA radio telescope enable more detailed investigations of these processes.
Although the basic characteristics of flare mm emission have been identified, several important questions remain:
- Dominant emission mechanisms during different flare phases
- Relationships between mm emission and X-ray, EUV, and Hα radiation
- The existence and role of multiple populations of accelerated electrons
- Spatial relationships between emission and magnetic structures
- Contributions of chromospheric and coronal components to total emission

3) Eruptive Processes in the Middle and Upper Corona and the Heliosphere
Coronal mass ejections (CMEs) are the largest eruptive phenomena in the Solar System and are key carriers of solar activity throughout the heliosphere. They involve the ejection of magnetized plasma from the solar corona, most commonly associated with eruptions of twisted magnetic flux-rope structures. Their initiation is believed to result from magnetic instabilities and magnetic reconnection processes, during which energy accumulated through photospheric motions and magnetic flux emergence is released. Because of their impact on planetary environments and technological systems, understanding and predicting CMEs is of critical importance for space weather forecasting.
CMEs are observed in white light using coronagraphs, but such observations are limited by projection effects and a lack of clear source-region information. Therefore, additional indicators from the lower corona and stereoscopic techniques are employed to reconstruct their three-dimensional structure and kinematics. The resulting parameters are used as inputs for heliospheric propagation models. During propagation, CMEs expand, develop complex structures, and may generate shock waves and compression regions.
Their heliospheric counterparts, interplanetary coronal mass ejections (ICMEs), are studied through in situ measurements of plasma and magnetic fields, where they are identified as disturbances in the solar wind. During their evolution through the heliosphere, ICMEs undergo expansion, interact with the surrounding environment, and experience processes such as magnetic reconnection. They also interact with cosmic rays and planetary magnetospheres, potentially leading to geomagnetic storms.
Although the fundamental processes associated with CMEs are known, several important questions remain:
- CME initiation mechanisms and early evolution
- Determination of three-dimensional structure and orientation
- Evolution during propagation and interaction with the surrounding environment
- Connections between coronal observations and in situ measurements
- Conditions leading to the formation of shock waves and compression regions
- Effects on cosmic rays and planetary environments