Solar physics forms a central pillar of Heliophysics, providing the origin point for the magnetic and plasma structures that shape the heliosphere and drive variability throughout the Solar System. From the solar dynamo and the emergence of magnetic fields to the heating of the corona and the eruption of flares and Coronal Mass Ejections (CMEs), solar physics addresses fundamental plasma phenomena with far-reaching heliospheric consequences.

A distinctive feature of solar physics within the context of Heliophysics is its dependence on remote sensing e.g.. Multi-wavelength observations, from EUV and X-ray to white light and radio, provide continuous imaging and spectroscopy of the solar atmosphere. These measurements are essential for diagnosing plasma temperatures, densities, and velocities, and for tracking the evolution of magnetic structures. Remote sensing enables the reconstruction of coronal magnetic fields, the detection of emerging flux, and the monitoring of flares and CMEs in real time.

Europe has established itself as a leader in solar physics through a combination of space missions and ground-based observatories. ESA-led missions such as SoHO and Solar Orbiter have provided critical insights into the solar atmosphere and solar wind e.g.. Ground-based observatories including the Swedish 1 m Solar Telescope, GREGOR telescope, Télescope Héliographique pour l'Étude du Magnétisme et des Instabilités Solaires (THEMIS), and radio facilities such as the Nancay Radioheliograph, and LOw Frequency ARray (LOFAR) contribute high-resolution imaging and radio diagnostics e.g.. The European Solar Telescope (EST), currently under development, will further enhance Europe's capability to probe the structure and evolution of the solar magnetic field at small scales. ESA and European research teams have contributed to and benefited from international missions such as Hinode and IRIS, and are also involved in the upcoming Solar-C (EUVST) mission. These collaborations enhance scientific return and promote coordination across agencies and disciplines.

As such, solar physics is not an isolated discipline, but an integral part of Heliophysics. Its remote sensing capabilities, combined with theoretical modeling and data-driven approaches, provide the starting point for understanding the coupled Sun–heliosphere–planet system. In the next subsection, we explore how solar outputs, including the solar wind and eruptive events, structure the heliosphere and connect to space environments throughout the solar system.

The Sun drives a supersonic solar wind flow, the origin of which is closely linked to one of the major open questions in contemporary physics: the nature of solar coronal heating . This solar wind inflates the heliosphere, a giant plasma bubble that surrounds the Solar System and protects it from interstellar space. The topology and geometry of Earth's and other planets' magnetic fields are heavily influenced by the plasma stream, which mainly consists of protons and electrons emitted from the Sun's surface. Substantial variations in the solar wind's plasma and magnetic field characteristics stem from transient events, such as CMEs or co-rotating interaction regions (CIRs), as well as abrupt polarity changes due to the heliospheric current sheet. This plasma and the associated magnetic field constitute the external input to any magnetospheric processes. Hence, the different sources of the solar wind on the Sun structure the heliosphere see e.g.,.

Our understanding of the heliosphere was shaped by early space missions such as the Pioneers e.g. and Voyagers , by the twin Helios spacecraft and Ulysses , and today by a fleet of spacecraft including assets at the Sun-Earth Lagrange point L1: SoHO , Wind , and ACE . In addition, there are STEREO and Rosetta , while Parker Solar Probe , BepiColombo , and Solar Orbiter are today probing the inner heliosphere. After passing the heliospheric termination shock in 2004 and the heliopause in 2013, Voyager 1 continues to return data from the local interstellar medium . This fleet of missions allows us to track the solar wind from its origins to interplanetary space. Several of these missions are illustrated in Fig. .

Solar Orbiter is an excellent example of an interdisciplinary laboratory, addressing Heliophysics both in situ and from a remote sensing perspective, its value augmented by collaboration with other missions. For instance, exploited simultaneous remote and local observations of the same coronal plasma volume, with Solar Orbiter/Metis and instruments on Parker Solar Probe to determine the coronal heating rate in the slow solar wind, and tracked the radial evolution of turbulence between Parker Solar Probe and Solar Orbiter . tracked the evolution of an interplanetary shock from 0.07 to 0.7 AU. conducted a comprehensive study connecting solar flare–CME initiation to their Earth impact by combining remote sensing, in situ measurements, and modeling, including 3D reconstruction and magnetic flux rope analysis. tracked a CME from 1 AU past Mars, comet 67P/Churyumov-Gerasimenko, Saturn, and New Horizons en route to Pluto. The high-resolution images from Parker Solar Probe and Solar Orbiter also serve as benchmark studies connecting small-scale structures related to solar wind interactions with evolving CMEs . Although not exhaustive, these examples illustrate the rich opportunities provided by the fleet of spacecraft in the Solar System and how they contribute to heliospheric science. Several European missions (Helios, Ulysses, SoHO, Rosetta, Bepi Colombo, and Solar Orbiter) have played and are playing a key role in our understanding of the physics of the heliosphere. They provide the critical link between observations at the smallest distances from the Sun (i.e. Parker Solar Probe) to the farthest reaches that space probes have ever reached (Voyager 1 and 2) and probe the interplanetary medium with unprecedented resolution e.g.. Recent studies show plenty of small-scale solar wind structures, such as mini flux ropes and embedded magnetic fluctuations in the mesoscale regime, related to CMEs that might play an important role in the interaction with the Earth's magnetic field e.g.. Meso-scale solar wind structures may also transfer energy from the Sun to geospace, see for example the review by .

In the context of solar physics, studying solar flares, CMEs, and particle acceleration has profound implications for a wide range of areas within physics. It connects plasma physics, high-energy astrophysics and fundamental particle physics, establishing it as a vital foundation for interdisciplinary research. Recent results linking solar and magnetospheric physics emphasise the various types of solar wind originating from different regions of the Sun and their impact on the magnetosheath . This study revealed that classifications into quasi-parallel and quasi-perpendicular shocks are affected by the different types of solar wind types, suggesting importance of merging research on solar wind sources and dynamics with studies on near Earth plasma environments such as shocks and the magnetosheath.

Among such solar wind sources interplanetary Coronal Mass Ejections (ICMEs) are large-scale solar eruptions that evolve significantly as they travel through the heliosphere. Understanding their structure, expansion, and impact on space weather requires multi-point observations. Fig.  shows an example of multiple spacecraft in situ observations of a CME at different heliocentric distances by Solar Orbiter, Wind, and BepiColombo that were closely aligned with a longitudinal separation of less than 5 °enabling to study radial evolution the ICME. Meanwhile STEREO-A remotely observed the same event from a 75° west of Earth, offering an optimal perspective to image the global structure of the CME. By combining these remote-sensing data with in situ measurements, were able to track the large-scale shape of the CME and its evolution through the inner heliosphere. The observations revealed a flattening of the flux rope cross-section, suggesting that the ICME expansion was neither self-similar nor cylindrically symmetric. Additionally, a comparative analysis of the magnetic field strength between spacecraft indicated a deviation from the expected power-law dependence with distance suggesting a complex evolution of the ICME as previously thought. Other examples of multi-point ICMEs studies can be found in

This section would be incomplete without mentioning the imaging of the far side of the Sun, which is directly relevant to space weather forecasting . The techniques developed for this purpose draw on a variety of disciplines. The first observations were based on the illumination of hydrogen atoms in interplanetary space by intense Lyman α emissions from active regions . Helioseismology is nowadays routinely used for forecasting purposes .

Just as planetary geology is built on earlier scientific understanding of surface processes on Earth, planetary magnetospheric science has been strongly influenced by the earliest observations of the Earth's magnetosphere and the effect of the solar wind on it . Advancement from terrestrial to planetary magnetospheric studies has occurred much faster than in geology.

Decades of theoretical advances started to be confirmed when the first spacecraft, Explorer 1 exited the Earth's atmosphere and discovered the Van Allen Radiation Belts . Theoretical breakthroughs and experiments by luminaries such as Kristian Birkeland, who proposed that beams of electrons could create polar aurora and “polar magnetospheric storms” , now termed “substorms” , and led to the idea that the magnetosphere-ionosphere could form a coupled system. Discoveries of fundamental plasma physical processes, such as magnetic reconnection led to the idea that planetary magnetospheres could really be coupled to the solar wind , where sheared magnetic field lines could be reconfigured to thread a thin current sheet that separates two very disparate plasma regimes.

Fundamental scientific questions that arose from these revolutionary ideas, leading to missions to target the three-dimensional nature of near-Earth space (Cluster), what processes trigger polar magnetospheric substorms (Time History of Events and Macroscale Interactions during Substorms; THEMIS), and what is involved in the process of magnetic reconnection (Magnetospheric Multiscale: MMS). Over the last two decades, the study of the terrestrial magnetosphere has made major leaps forward thanks to multipoint measurements of these three missions, started first by the recently terminated Cluster mission. Cluster has pioneered our three-dimensional view of plasma physics at ion and fluid scales , including the 3-D nature of magnetic reconnection, of Kelvin-Helmholtz instability, and of fast flows and instability in stretched field lines of the magnetotail. The THEMIS mission specifically targeted the scientific question of the causes and consequences of the polar magnetospheric substorms. THEMIS was designed to uniquely determine whether magnetic reconnection or plasma instability detonates the explosive energy release during a substorm . Both of these missions focused on ion and fluid scale physics to determine the global context and consequences of the physical processes. In contrast, the MMS mission focused entirely on microscopic kinetic/electron scales, respectively to understand in unprecedented detail how the fundamental plasma process such as magnetic reconnection occurs.

Taken in isolation, these are three missions that targeted specific scientific questions and gained unrivaled insight into each of these scientific questions. However, thanks to coordinated planning, and a large number of conjunction events between Cluster, THEMIS and MMS, their mutual benefit was far greater than their individual contributions, enabling different spatial and temporal scales to be probed simultaneously for the first time. These missions have gradually untangled the multiscale complexity of space plasmas, leading us to recognise that we are dealing with a system of systems, and one that should in future be planned for at the very start of the mission planning stage. Clearly, further international collaboration via “clusters of Clusters” is required to take the next big steps not only in magnetospheric science, but also in plasma physics across the universe .

The first significant planetary magnetic field observations beyond Earth were made at Jupiter, when Pioneer 10 passed through its magnetosphere in late 1973. This was closely followed by Mariner 10's first flyby of Mercury in March 1974. These events occurred only around 15 years after Luna 1 first left Earth's magnetosphere. In the early years of the space age, terrestrial magnetospheric science taught us a great deal about how our planet's magnetosphere responds to changes in the heliospheric magnetic field and the dynamic and magnetic pressures of the highly variable solar wind.

The exploration of other planets that have a magnetosphere complements our knowledge and understanding of the terrestrial magnetosphere. The heliospheric environment changes with distance from the Sun. For example, Mercury's magnetosphere is much smaller than it would be if it were located at a greater heliocentric distance, where the heliospheric magnetic field is weaker and solar wind number density lower. In addition to the changing heliospheric environment at increasing distances from the Sun, the magnetised planets themselves differ from one another in planetary field strength, planetary rotation rate, alignment of their magnetic and spin axes and internal plasma sources. Thus the study of multiple planetary magnetospheres is worth much more than the sum of the individual parts as it allows us to compare and contrast space plasma physics in a broad range of parameter spaces .

Magnetospheres also provide a unique laboratory in the form of radiation belts, regions where highly energised electrons and various ion species are trapped. Several planets in the Solar System host these regions , with the Earth's radiation belts being among the earliest discoveries of the fledgling field of space science in the late 1950s . Given their dynamic nature, these belts have undergone significant scrutiny . The causes of radiation belt changes are not well understood scientifically; operationally, these same particles can pose serious risks to satellites and astronauts and have driven the growth of a key component of the applied discipline of Space Weather. Interest in the nature of other Solar System radiation belts is growing, particularly in the case of Jupiter, where field strengths are 50 times greater than on Earth, leading to a variety of complex physical processes. Understanding planetary radiation belts could provide a better understanding of exoplanetary systems and emissions .

The Cassini-Huygens mission to Saturn is a prime example of an interdisciplinary project involving various instruments and scientific objectives. The mission's objectives included studying the planet's interior, atmosphere, magnetosphere, rings and moons. One of the Cassini mission's most famous discoveries was the plumes emanating from the icy moon Enceladus. This was first observed through disturbances in magnetometer data . Subsequently, the plumes' appearance , composition and location near the south pole were confirmed using the other instruments on Cassini. Following the detection of magnetic field signatures during the first close flyby of Enceladus, the spacecraft's trajectory was altered to enable closer study of this intriguing moon. What began as a somewhat serendipitous event arguably became one of the mission's most significant discoveries, providing opportunities for instrument teams to collaborate, and widening participation of science communities, e.g., scientists addressing habitability and origin of life in the Solar System. Builds on the legacy of Cassini-Huygens, the L4 mission to Enceladus, the first flagship mission of ESA's Voyage 2050 programme, is planned to further study Enceladus' surface and subsurface, but also external environment, requiring an interdisciplinary approach for assessing external impacts on habitability. Potential habitability is also one of the major target of Juice in the Jovian system exploring the gas giant Jupiter, its space environment, and its moons, Ganymede, Europa and Callisto. In particular Ganymede, with its intrinsic magnetic field, generating a mini magnetosphere, is an especially exciting target for studying complex and unique relationship with the space environment around Jupiter.

The BepiColombo mission is a collaboration between ESA and the Japan Aerospace Exploration Agency (JAXA). It is another excellent example of an interdisciplinary mission that effectively integrates planetary, magnetospheric, and solar wind science. Designed to study Mercury, the planet closest to the Sun, the BepiColombo mission comprises two orbiters: the Mercury Planetary Orbiter (MPO) and the Mercury Magnetospheric Orbiter (MMO, also known as Mio). Each orbiter is designed to investigate different yet interconnected aspects of the planet and its environment. By combining geological, geophysical, and chemical analyses of Mercury's surface with in-depth studies of its magnetosphere and its interactions with the solar wind, BepiColombo bridges multiple scientific disciplines. This approach allows scientists to investigate how the planet's thin exosphere is influenced by internal planetary processes and external solar activity, thereby advancing our understanding of magnetospheric dynamics in an extreme solar environment. Thus, the mission exemplifies the synergy between different fields of space science, offering profound insights into Mercury's evolution and the broader workings of planetary systems. In addition to its primary objectives at Mercury, BepiColombo has already made substantial contributions to heliospheric science during its extended cruise phase. As discussed by , the spacecraft has provided valuable observations of the solar wind, transient events, and planetary environments encountered during its flybys of Earth, Venus, and Mercury. These measurements not only enhance our understanding of space weather in the inner heliosphere but also demonstrate the scientific value of planetary missions beyond their nominal operational phases.

The transport of mass, momentum and energy across boundaries or interaction between the particles and the electromagnetic fields are of interest not only in heliophysical plasmas, but also in plasmas all-over in the universe. Fundamental plasma processes such as magnetic reconnection, waves and turbulence in the boundary region have been studied throughout the Solar System based on in situ and remote observations, as well as simulation studies. The Solar System is the unique place in the universe where the fundamental plasma processes can be studied with in situ measurements for different plasma conditions from the diverse planetary and interplanetary environments. These unique observations allow validation of the models and compare with the remote observations of universal plasma processes.

An example of such plasma processes (shown in Fig. ) is the different studies of the Kelvin–Helmholtz instability in various contexts: (a) near the Sun ; (b) in the solar wind at the boundary of a CME ; (c) at the flank of the Earth's magnetopause ; (d) at the Mars dawnside ionopause ; (e) at the Saturn dawnside magnetopause (f) near the heliopause , and (g) at the edge of galactic jets . By comparing and contrasting these plasma processes, we can study how plasma flows interact with different obstacles throughout the Solar System and in an astrophysical context.

The coupled Ionosphere–Thermosphere–Magnetosphere (ITM) system is a highly dynamic, interconnected region of space close to magnetized planets, such as Earth, where solar and geomagnetic energy inputs drive complex physical processes, see Fig. . The magnetosphere, which is dominated by the planet's magnetic field, interacts with the solar wind, channelling energy and particles towards the high latitude ionospheres. This energy input alters the ionospheric conductivity, driving currents that influence thermospheric winds and temperatures through ion-neutral coupling. The ionisation of the primarily neutral thermosphere can also lead to chemical changes. All thermospheric effects can propagate throughout the ITM system and penetrate deep into the atmosphere. Additionally, the ITM is characterised by numerous interfaces, primarily the gradual transition from the neutral atmosphere (in the mesosphere and below) to the ionised atmosphere, and the shift from a collisional to a collisionless regime. For these reasons, ITM science is inherently interdisciplinary. Therefore, it is crucial to understand the ITM as a coupled system.

The atmosphere and the exosphere mark the end of the process chain for all Solar System objects within the heliosphere. Here, the Sun's energy is deposited through processes such as Joule heating. Understanding the fundamental interactions in the ITM system is therefore essential, particularly on Earth, as these interactions can disrupt human infrastructures during intense space weather events.

The global electric circuit is another example of a system that links the space environment to the different layers of the atmosphere. This is exemplified by events such as sprites and elves during thunderstorms , and more generally by the role of atmospheric electricity .

An additional degree of complexity arises from the coupling with the solar wind and magnetosphere at high latitudes. This coupling is responsible for the main deposition of energy in the system through ionospheric currents and particle precipitation, which also cause the aurora. Excitation by solar illumination and thermospheric winds at the equator generates the equatorial electrojet and fountain. Such electrodynamic couplings are also observed on other planets, but they can differ greatly due to factors such as rapid planetary rotation, the presence or absence of an intrinsic magnetic field, and the density of the atmosphere. This is where past, present and future space exploration missions, combined with detailed physical models, can be crucial in enabling in-depth comparative planetology.

Luminous emissions, of which aurorae are the perfect example, illustrate the beauty of heliophysical processes. New discoveries are still being made in this field, such as Strong Thermal Emission Velocity Enhancement (STEVE) , fragments and dynamic continuum emissions . Aurorae have been observed on various planets in the Solar System across the electromagnetic spectrum, from radio waves to infrared, visible light and ultraviolet radiation. They can be regularly observed with space telescopes such as Hubble and JWST all the way to Uranus, as well as with terrestrial radio telescopes, which also detect radio emissions from Jupiter's and Saturn's moons e.g..

The long-term development of empirical, physical and chemical models e.g. together with the existence of an extensive ground-based infrastructure (see Sect. ) has established Europe as a leading provider of accurate descriptions of the electrodynamics, thermodynamics, and chemistry of the near-Earth space environment. This enables us to unveil its complex physical and chemical processes. The coupling with the lower layers of the atmosphere has also been explored in depth in recent years.

Last but not least, Europe has played a major role in missions such as Swarm , which has combined core magnetic field investigations with ionospheric connectivity and beyond, and SMILE , which investigates the coupling of the magnetosphere. By comparing inputs and impacts at the same time, and associating ground based observations SMILE allow us to access physical parameters that cannot be measured directly, such as conductivities and Joule heating.

As an example, the physical ionosphere model called TRANSport au CARré (TRANSCAR) has been developed, maintained and updated over the last 30 years. Initially developed for high latitudes and open field lines , TRANSCAR was then extended by the IRAP Plasmasphere-Ionosphere Model (IPIM) to encompass closed field lines and the description of the plasmaphere .

Inputs to TRANSCAR/IPIM can be derived from empirical models, directly from data, or adjusted from a combination of data assimilated into these models. For example, the convection electric field can be derived from SuperDARN radar data assimilated into dedicated empirical models e.g.. Conversely, field-aligned currents can be recovered from magnetometers on satellite missions such as CHAMP, IRIDIUM/AMPERE, and Swarm e.g.. It is challenging to model the thermosphere in dynamic situations, but it can be fitted with density data derived from accelerometers on satellites such as CHAMP, Swarm, GRACE and GOCE. Once optimised, TRANSCAR/IPIM simulations can be compared with other datasets for validation and interpretation, such as hmF2 and NmF2 data from ionosondes, electron density, electron and ion temperatures, and ion velocity from incoherent scatter radars, as well as total electron content (TEC) maps from GNSS satellites. simulated the impact of field aligned currents observed with Swarm on the electrodynamics of auroral structures. modelled the effect of a solar high-speed stream (HSS) on the sharp depletion of ionospheric electron density in the F region. The model results were successfully compared with observations from the EISCAT radar and/or Scandinavian ionosondes.

A special category of body in the solar system are those that do not possess a global magnetic field, for example Venus, Mars, Comets and most Moons. This makes their interaction with the solar wind fundamentally different from that of Earth, Mercury, and the giant planets and presents a whole new laboratory of plasma physics to explore. However, missions to these objects are always inter-disciplinary, and thus Heliophysics is often not a priority in the mission objectives. This also offers opportunity to overlap and learn from other disciplines, e.g. about remanent magnetic fields in the Martian crust or solar wind sputtering on surfaces. Even though the communities involved in these objects are often small, they should not be overlooked when proposing new missions to funders.

For example, ESA's Rosetta mission observed the solar wind-comet interaction at comet 67P/Churyumov-Gerasimenko for over two years as well as provided data from Earth, Mars and asteroid flybys. Rosetta has delivered data critical to our understanding of energy transfer in a two-component plasma and shown that aurora-like features can exist at comets . The main drawback of the Rosetta data is its lack of context for the solar wind and the lack of multi-point measurements. Comet Interceptor, ESA's new mission to a comet, has been designed to incorporate three spacecraft, which will provide, for the first time ever, three point measurements of the magnetic field in the plasma environment of a comet .

ESA's Venus Express has been instrumental in understanding the interaction of a completely unmagnetized planet with a strong solar wind . Venus Express was able to show that Venus' atmosphere is continuously eroded by the solar wind and found indications of magnetic reconnection in the tail as well as a plethora of common plasma waves in the entire magnetosphere .

Mars is particularly interesting, as it does not possess a global magnetic field, but only local crustal magnetic fields. These induce asymmetries within the plasma environment that cannot be reproduced elsewhere . ESA's Mars Express has been measuring the plasma in the Martian magnetosphere and ionosphere since 2003. While making many fascinating discoveries on its own on atmospheric and magnetospheric dynamics , it has also shown the strength of multi-point measurements through combination with NASA's MAVEN mission. For example, measurements by the Radiation Assessment Detector (RAD) on NASA's Mars Science Laboratory rover Curiosity and more recently with Fine Resolution Epithermal Neutron Detector (FREND) on Exo-Mars Trace Gas Orbiter (TGO) are also contributing to our understanding of the propagation of CMEs and energetic particles through the Solar System see, e.g.,. These multi-point and multi-instrument measurements will be crucial for further understanding of plasma processes within the complex Martian magnetosphere .

Dusty plasmas have gradually become a discipline on their own, combining aspects of plasma physics, planetary science, astrophysics, materials science, and chemistry. Dusty plasmas are characterised by the presence of micro- to nanometer-sized dust grains that become electrically charged. Charged dust occurs in various regions: in the interplanetary medium , in planetary rings, in cometary tails where solar UV radiation and the solar wind ionize gases and interact with dust , in lunar and asteroid surfaces whose surfaces are eroded by the solar wind , and also in the Earth's mesosphere where noctilucent clouds are linked to dusty plasma produced by meteor ablation. In this context, the Moon stands out as an interesting laboratory not only for dusty plasmas but also for investigating kinetic processes in plasmas and the complex interactions between the solar wind and non-magnetised surface . Some of these are illustrated in Fig. .

As already mentioned, space plasmas offer the possibility to perform in situ investigations of fundamental plasma processes without significantly affecting the surrounding environment. Consequently, they frequently serve as natural laboratories for the observational study of plasma turbulence and kinetic processes associated with shocks or reconnection .

Similarly, laboratory experiments on Earth aim to improve our understanding of related fundamental processes. Heliophysics thus provides many synergies with laboratory plasma experiments. Ground-based plasma devices such as the Large Plasma Device (LAPD), the Facility for Laboratory Reconnection Experiments (FLARE) and PHAse Space MApping (PHASMA) attempt to reproduce phenomena such as plasma waves, magnetic reconnection and particle acceleration . Unlike space plasmas, these devices provide reproducible and, to a certain extent, controllable conditions, thus complementing space observations of these processes when they occur sporadically. Comparisons between space and laboratory plasmas provide new insights into the fundamental physics of plasmas .

Historically, and particularly since the International Geophysical Year (1957–1958), Europe has been a driving force in the installation and operation of ground-based experiments in Heliophysics. This is one of the main reasons for the extensive coverage of instruments and the related scientific and technical expertise of European teams.

Thus, Europe plays a key role in the main instrument networks that provide information on the Ionosphere-Thermosphere-Magnetosphere (ITM) system. These include ground-based magnetometers (INTERMAGNET) for measuring ionospheric currents, ionosondes (GIRO) and GNSS receivers (IGS) for determining ionospheric density and structure, and low-frequency telescopes (LOFAR) and VLF receivers (AWDANet) for studying the dynamics of the mesosphere and thermosphere. Optical airglow observations and meteor measurements by optics and radars also support the studies of mesosphere and lower thermosphere. In the subarctic and arctic regions, specific instruments dedicated to monitoring the polar cap and auroral regions and their complex electrodynamics have long been in place. These include incoherent scatter radars (EISCAT), coherent HF radars (SuperDARN), all-sky cameras (e.g., MIRACLE and ALIS), riometers, Fabry–Pérot interferometers and scanning Doppler images. Some examples of the synergy between ground-based and space-based observations are given by .

The above mentioned ground-based observation networks offer complementary strengths to space missions and significantly enhance Earth system scientific understanding. While satellite instruments provide broad spatial coverage, ground-based networks offer high-resolution, localised measurements. Coordinated campaigns that align satellite overpasses with ground-based measurements with, for instance EISCAT and SuperDARN radars or optical networks with carefully designed special observation mode, allow for targeted investigations of dynamical ITM processes, such as substorms.

Europe also plays a key role in installing ground-based instruments in hard-to-reach regions (e.g. the sub-Antarctic and Antarctic regions, sub-Saharan Africa, South America and East Asia) to monitor the equatorial region and southern auroral and polar zones. To this end, European teams are assisting local teams in the equatorial region with installing ground magnetometers to study the equatorial electrojet and GNSS receivers to investigate equatorial scintillations. They also train the regional teams to maintain and operate these instruments. Around the South Pole, Europe draws on its numerous national sub-Antarctic and Antarctic bases (in the UK, Italy, France, Norway, Sweden, Finland and Germany) to install instruments similar to those used in the Northern Hemisphere (e.g. SuperDARN radars, ground-based magnetometers, cosmic ray detectors and all-sky cameras) and to lead interhemispheric studies.

Several key European instruments, including large (> 1 m) solar telescopes based in the Canary Islands, are also used for solar physics research. France, Germany and Sweden each own a telescope of this size, which they use to study the complex processes occurring on the Sun's surface and in filaments. The 4 m European Solar Telescope (EST), led by the Instituto de Astrofísica de Canarias (IAC, Spain), will also be deployed here.

Long-term solar radio observations have been performed by several countries, including France (Nançay radio observatory), Finland (Metsähovi radio observatory), Germany (Tremsdorf solar radio astronomy observatory) and Italy (Bologna and Cagliari sites), to track types II, III and IV, which are related to solar flares and CMEs . In addition, many smaller solar telescopes and coronagraphs are used across Europe, allowing for synoptic observations of the Sun's surface, corona and filaments. One of the main international networks is SAMNeT, which the UK leads. provides a comprehensive overview of national and international instrumentation and networks that continuously observe the solar surface, Earth's magnetic field, and the ionosphere using both ground- and space-based instruments.

Long-term reconstructions of solar activity underpin a wide range of interdisciplinary science. Centennial- and millennial-scale solar activity reconstructions are used to estimate historical solar forcing, a key input to climate models and understanding the terrestrial system in the past . Secular changes in solar activity, such as the Maunder minimum (1650–1715), also shed light on astrophysical observations of non-magnetically cycling sun-like stars . And more directly, understanding the range of solar activity that has occurred in the past provides the best estimate of what we can expect in the future, which is vital to ensuring societal and technological resilience to space weather .

The long-term solar activity reconstructions, such as those shown in Fig. , are themselves the product of interdisciplinary science. To extend reconstructions further into the past, it is necessary to use increasingly indirect solar activity proxies, often by calibrating against other (shorter duration, but more direct) measures or proxies. Direct, high temporal resolution spacecraft measurements of the near-Earth solar wind properties have been made near-continuously since the mid-1960s, and have been collated in the ongoing OMNI dataset . Over a similar time scale, ground-based neutron monitors can reveal the rate at which galactic cosmic rays (GCRs) produce nuclear reactions with atmospheric molecules . This, in turn, provides information about the solar magnetic field strength, which partially shields Earth from GCRs. Stretching back around 170 years, ground-based magnetometers measure the level of disturbance of the Earth's magnetic field resulting from the solar wind . These long time series of geomagnetic activity can then be coupled to atmosphere and climate models to investigate the connections between solar effects on our past, current and future climate .

Sunspot number counts, which measure visible solar activity, stretch back over four centuries. However, the construction of such a long-term dataset requires the interpretation of historical texts . However, conversion to a more physical measure – such as open solar flux or total solar irradiance – requires considerable modelling and assumptions . On similar timescales, records of auroral occurrence provide about geomagnetic storms, but these data are complicated to use quantitatively, owing to changing patterns of human migration and hence sampling of geomagnetic latitude. But the longest reconstructions come from radionuclides produced in Earth's atmosphere by GCRs. These become locked up in tree rings and ice sheets, potentially enabling solar activity to be reconstructed back millennia . This, however, requires extremely interdisciplinary science, spanning heliospheric, magnetospheric, high-energy physics, climate, glaciology, dendrochronology, etc.

Our list of examples of heliophysics, which highlights the interdisciplinary nature of research, would be incomplete without mentioning the outer edges of the heliosphere and their connection with the interstellar medium.

The heliosphere moves through the Very Local Interstellar Medium (VLISM) at a speed of approximately 26 km s⁻¹ . The interstellar plasma thus encounters an obstacle, the heliosphere, and must flow around it. Similarly, the solar wind must at some point encounter the interstellar plasma. Because both the interstellar plasma and the solar wind are magnetized flows, they do not mix. The interstellar plasma must therefore flow around the heliosphere, and the solar wind must “wrap itself around” the heliosphere. The interface between the solar wind and the interstellar medium is called the heliopause. Information about the obstacle posed by the heliopause is transmitted back into the solar wind, where the termination shock forms, which deflects the solar wind and slows it down . Voyager 1 passed the termination shock in December 2004 at 94 au (Voyager 2 passed it in August 2007 at 84 au). The region between the termination shock and the heliopause is called the heliosheath. Voyager 1 passed the heliopause and entered interstellar space in 2012 , Voyager 2 did so in 2018 . Recent observations by the Interstellar Boundary Explorer IBEX, have shown that there is very likely no bow shock in the interstellar medium as had long been expected , however, this result is still being debated .

The neutral gas of interstellar matter enters the heliosphere uninhibited by the heliospheric boundary layers discussed in the previous paragraph where it can be ionized and picked up by the solar wind, forming a new population of suprathermal ions in the heliosphere, so-called pick-up ions see, e.g., for a review. This mass loading of the solar wind slows it down by about 20 % and heats it beyond about 20–30 au . The pick-up process results in a highly non-thermal velocity distribution function . This distribution is convected outward by the solar wind all the way to the termination shock, where it was widely believed to be further accelerated to form the anomalous component of cosmic rays ACR, see, e.g., . While the flux of low-energy particles increased as Voyager 1 approached the termination shock , it continued to increase beyond the termination shock, which indicates that ACRs are probably accelerated along the flanks of the heliosphere . An alternative explanation involving magnetic reconnection at the compressed heliospheric current sheets around the termination shock was given by .

Since its formation some 4.5 billion years ago, the Solar System and heliosphere have revolved around the galaxy nearly 20 times and on its path must have experienced a wide range of interstellar environments. In high-density environments the heliosphere was compressed to within 25 au . Stronger compressions to within Saturn's orbit were probably achieved by close-by supernova explosions in the galactic neighborhood . Similarly to the records of short-term (on an astrophysical scale) variations in the heliosphere, there are archives of this “galactic voyage” that our heliosphere has undertaken during its history .

The heliosphere is the only “astrosphere” that we can investigate in situ. This system therefore serves as a model for understanding astrospheres surrounding stars that drive stellar winds . These astrospheres strongly influence the space environments of planet-hosting stars and have implications for astrobiology . As such, the heliosphere provides a good example of how research in heliophysics can benefit astrophysics and vice versa, by drawing on the rich and diverse observations of remote astrospheres.

To foster collaboration and innovation within the wider Heliophysics community, it is essential to establish communication platforms and provide a forum for exchanging ideas among scientists involved in Heliophysics research. Low threshold communication channels, such as a dedicated mailing list, a newsletter and a website, which are accessible and open to all community members, regardless of their career stage, are the backbone of this networking. The website is now active at https://www.heliophysics.eu/ (last access: November 2025) and subscription to a regular newsletter is possible at https://spaceweather.gfz.de/helio-europe-mailing-list (last access: November 2025). The latter includes a link for submitting announcements. Most recently the EHC has set up a LinkedIn account, for further communication and interaction (https://www.linkedin.com/company/heliophysics (last access: November 2025)).

The planning, development, and operation of space missions is a cross-generational effort. Therefore, the continuous training and carrying over of expertise is essential for the success of Heliophysics as a field. The future of Heliophysics relies on well-trained Early Career Researchers (ECRs) who are motivated to pursue a scientific career in this field and to advance the future space-mission development. In this regard, the support of ECRs must lie at the heart of all activities by the ECRs. As one component of this effort, we aim to establish a monthly EHC seminar series titled “HelioMeet”. This initiative is designed to provide a platform for ECRs to present their latest research, engage in meaningful discussions, explore interdisciplinary connections across various Heliophysics subfields, and encourage putting problems in a broader perspective. HelioMeet will amplify the voices of early-career researchers within the broader scientific community, helping them establish professional networks, gain visibility, and contribute actively to shaping the future of Heliophysics.

Our community already routinely practices openly sharing data from different sources and instruments. However, finding information about accessing data products can sometimes be difficult. Maintaining public data archives, communicating about free access to data and making space-based and ground-based data linkable are thus key to the success of Heliophysics data analysis. Community efforts and a communication platform for effective information exchange are needed to leverage synergies between data providers, scientific facilities, and the providers of data analysis tools. Heliophysics is connected to many other fields of science, such as exoplanet research, laboratory plasma physics and astrophysical plasma research. Therefore, the Heliophysics community is interested in and invested in exchanging with related fields outside its scope for mutual benefit.

Modern Heliophysics research increasingly depends on integrated data access, analysis, and modeling across traditionally separate domains, from solar observations to planetary magnetospheres, and combining multi-messenger data from both space- and ground-based platforms. However, the lack of shared infrastructure, standards, and sustained support for tools and data systems remains a major bottleneck to interdisciplinary science. This is being addressed with activities such as the International Heliophysics Data Environment Alliance IHDEA, . To address Europe's scientific and strategic goals in Heliophysics, coordinated development investment in open-source software, data infrastructure, and community-led initiatives is essential.

A robust and interoperable ecosystem, where researchers can seamlessly work across missions, instruments, and domains, requires common data models, open documentation, reusable software libraries, and standardised metadata standards. Some domains have developed powerful tools, particularly in Python, for example the community-led SunPy project , which provides core functionality for solar data analysis and is now foundational in the Heliophysics ecosystem. Yet many of these efforts across Heliophysics still operate in isolation, are inconsistently maintained, or lack interoperability, making it difficult to integrate data or replicate results across subfields. The Python in Heliophysics Community (PyHC) builds on this foundation by promoting best practices, code compatibility, and sustainable development across a suite of open-source tools, including SunPy, and mission-specific packages. While PyHC originated in the US, there is a clear opportunity for European leadership, through ESA, national agencies, Horizon Europe programmes, and community networks, to shape its direction, and coordinate development across European tools and archives.

Europe already leads in open data access through resources like ESA's Heliophysics Archives. But infrastructure alone is not enough, long-term support for software maintenance, documentation, and developer coordination is essential, yet often underfunded or overlooked.

To enable interdisciplinary Heliophysics, data and software must be recognised as critical scientific outputs. Embracing open science and FAIR principles will support transparency, reuse, and broader participation. With targeted investment and coordination, Europe is well positioned to lead the development of a sustainable, interoperable ecosystem for Heliophysics research.

The EHC encourages the exploitation of public events such as solar eclipses and Long Nights of Science to raise the profile of the community. In this respect, we aim to coordinate public events across Europe to promote public engagement. It is imperative to involve schools to ensure the next generation's interest in science. Citizen science involves inviting the general public to participate in active research by analysing data, finding events or attending public talks at local, regional, national or international levels. These activities can be coordinated by sharing presentation materials and data among EHC scientists through EHC repositories.

The EHC encourages the systematic use of the keywords “Heliophysics” and “EHC” (for example, as ORCID keywords) to increase the discipline's visibility and foster community cohesion. The term “Heliophysics” has now officially been added to the Merriam-Webster dictionary.

We envisage the EHC as a community-driven initiative where Heliophysics researchers are actively engaged at all career stages. Regardless of the organisational structure that the EHC adopts, all roles must be determined through transparent, open and inclusive practices. Based on our experience of national organisations such as MIST in the UK, ATST in France and AEF in Germany, we recognise that a low-overhead approach can lead to higher productivity, efficiency and inclusivity levels than a more formal organisational or management structure. Membership in the EHC must be open to heliophysicists from all career stages, and we envisage that membership can be simply acquired by registering for the EHC mailing list. In this way, the threshold for joining the EHC is very low, enabling participation for a wide range of community members.

In an initial attempt to maintain momentum, the team of the ISSI Forum, joined by several volunteers, decided to set up four working groups that will focus on the following topics:

The EHC interim steering committee

As the authors of this paper do not represent the diversity of the EHC community, we would like to emphasise that the structuring phase is still ongoing. Depending on community interest and engagement, the working groups and their members will be revised and completed. The society is committed to equality, diversity, and inclusion, as will be reflected in the dedicated section on its website and will be addressed by a separate working group.

There are national and international organisations that coordinate scientific communities engaged in Heliophysics. We believe that effective coordination and collaboration with these organisations is essential for our multidisciplinary research, which is based on internationally coordinated space and ground-based observations of the solar system. The Heliophysics research pursued by EHC is directly relevant to space weather science, so close communication with E-SWAN is essential. However, while Heliophysics contains the science of space weather, it does not address more applied questions such as impacts on infrastructure see also the introductory note in.

The EHC can serve as a hub for different national organisations (MIST, ATST, AEF, etc.) to plan next-generation missions or observations at a European level, involving countries with and without such national organisations. We anticipate that this coordination will enable EHC to join the worldwide ground observation network more effectively. EHC is anticipated to play a key role in the planning of the internationally coordinated space-based observations, such as the ISTP-NEXT proposal , for solar and terrestrial missions, as well as other planetary missions throughout the solar system. These communications among the different space agencies and the international scientific communities are expected to keep track of what missions are planned globally, and how to prepare to take advantage of emerging opportunities for collaboration. Coordination with international programmes such as SCOSTEP (Scientific Committee on Solar-Terrestrial Physics) or ILWS (International Living with the Stars) that are promoting interdisciplinary research and collaboration will strengthen Heliophysics discipline worldwide.