This paper presents the laboratory characterization of a compact CubeSat optical transceiver developed under the UK Space Agency's Autonomous Laser Intersatellite Gigabit Network (ALIGN) programme. A Position Sensing Detector (PSD) and Avalanche Photodiode (APD) are integrated within the receiver to simultaneously close a PAT feedback loop and carry high-speed data. Tests at 1550 nm using OOK modulation achieved a bit error rate (BER) below $3.8 \times 10^{-3}$ at $1.25 \text{Gb} \mathrm{s}^{-1}$ with a 65 V APD bias. The $200 \mu \mathrm{m}$ IAG200H5 device reached a measured −3 dB bandwidth of 715 MHz and the PSD subsystem drew 5.2 W, both within CubeSat power and performance budgets. These results validate the receiver architecture and provide a measurement-backed basis for an in-orbit PAT demonstration.

Free-space optical (FSO) inter-satellite links (OISLs) offer dramatically higher data throughput than traditional radio-frequency (RF) systems for satellite constellations. This paper presents the Free-space Optical Communications Unit for Satellites (FOCUS), a compact 2U payload for 6U CubeSats—the 6U form factor referring to a ∼10×10×30cm nanosatellite platform—demonstrating autonomous >1Gbps optical links at ranges up to 1000km in Low Earth Orbit (LEO). The terminal employs non-return-to-zero on-off keying (NRZ-OOK) modulation at 1.25Gbps. Component-level characterisation using an arbitrary waveform generator (AWG) test setup demonstrates receiver sensitivity below −37dBm, measured bit error rate (BER) as low as 10−8 under laboratory conditions, closed-loop pointing accuracy below 50μrad using a fast-steering mirror (FSM), and AWG-based end-to-end throughput characterisation at 1.25Gbps. Compared with prior CubeSat-class OISL demonstrations, FOCUS achieves the highest reported ISL data rate at equivalent or better receiver sensitivity within a 2U payload volume. Two 6U CubeSats are planned to launch, subject to funding confirmation, as part of the Autonomous Laser InterSatellite Gigabit Network (ALIGN) mission, funded by the UK Space Agency, to validate optical ISL performance at orbital ranges from 100km to 1000km.

Solar flare ribbons are extensive brightenings in the chromosphere during flares, often showing fine scale structuring that reflects the underlying energy release. Using high cadence imaging from the Swedish 1-m Solar Telescope/CRISP during an X1.5-class limb flare on 10 June 2014, we identify and track 232 coherent, thread-like substructures, which we term for the first time “riblets”. From a statistical analysis, riblets have well defined lifetimes and plane-of-sky speeds (typically 5-15 s and 50-150 km/s respectively), establishing them as distinct ribbon substructures. From analysis of their temporal distributions, their distance-time (X-T) evolution uniquely reveal approximately linear and non-linear (accelerating/decelerating) classes, a discrepancy that may be influenced by projection geometry. From analysis of their spatial distributions, we find no clear correspondence between the properties of adjacent riblets, suggesting that local atmospheric conditions (fine-scale thermodynamic and/or magnetic structuring) govern their kinematics more than spatial variations in electron-beam energy flux. From analysis of their spectral distributions, clusters of riblets do show temporal and spatial coincidence with hard X-ray emission signatures, consistent with episodic electron-beam injection into the chromosphere. Using Fermi/GBM spectroscopy, we derive thick-target parameters suitable for flare simulations, with representative values δ_ low≈ 5.93, E_ c≈ 24.7 keV, and an implied beam energy flux ℱ_ beam≈ 1.5× 10^10 erg cm^-2 s^-1 (based on RHESSI footpoint area). Together, these results identify riblets as the fundamental building block of flare ribbons and provide quantitative constraints for forward tests of riblet formation mechanisms.

Solar vortices are fundamental components of solar atmospheric dynamics, serving as natural laboratories for magnetic field twisting, energy concentration and transport, wave guidance, and plasma coupling across atmospheric layers. Numerical and observational studies show that solar vortices are intimately connected to key physical processes including magnetic reconnection, atmospheric heating, turbulence, and wave generation. This white paper, prepared for the UK Space Frontiers 2035 call, outline five high-priority scientific questions addressing vortex generation mechanisms, cross-layer coupling, magnetic restructuring, collective wave-guidance structures, and their role in triggering explosive events and modulating the solar wind. Key observations and capabilities required to make significant advancements over the coming decade are identified. The UK solar physics community has established world-leading expertise in vortex dynamics, combining strengths in high-resolution observations, MHD turbulence theory, numerical modelling, and space instrumentation. UK researchers have made foundational contributions to Solar Orbiter, delivered critical systems for DKIST, and maintain active involvement in MUSE and SOLAR-C EUVST missions. Our technical approach centres on developing next-generation instrumentation: a multi-band, space-qualified system employing four tunable Fabry-Pérot Interferometers providing diffraction-limited, high-cadence spectropolarimetric coverage from the deep photosphere to the low corona. This capability will be validated through a staged mission architecture beginning with balloon-borne demonstrators. Continuing this effort over the coming decade is vital to maintain UK leadership in this field and achieve the goals of roadmap for solar system research.

Magnetoconvective simulations of the solar photosphere show ubiquitous formation of vortices in intergranular lanes, which are thought to contribute to the energy transfer between atmospheric layers. Observationally, these vortices are often reported as photospheric intensity vortices, with their flow field assumed parallel to the plane of sky, making their identification a challenge. The most common method to detect these vortices is to infer the photospheric velocity field using Fourier local correlation tracking (FLCT) and identifying vortical motions within. To validate FLCT as a tool for inferring photospheric intensity vortex flows, FLCT was performed on TiO 705.8 nm observations from the Visible Broadband Imager at the Daniel K. Inouye Solar Telescope, as well as on synthetic intensity images derived from the MURaM code. Using Γ-functions on velocity vectors, vortices were identified while characterizing their statistical properties. Vortices in MURaM FLCT velocity fields were then compared with those in the MURaM-simulated velocity field. It was found that the FLCT kernel size has a significant impact on recovered vortex properties and quantity. However, statistical distributions of the properties of vortex lifetimes and areas remain similar. Furthermore, vortices in the MURaM-simulated velocity field were found to be much smaller than those in MURaM FLCT-derived velocity fields, with their overlap in space being 0.4%–19.7%, depending on the FLCT kernel size. The vast majority of those that do match are coincidental and arise from particularly large choices of kernel size. Therefore, FLCT was determined to be unreliable for inferring photospheric intensity vortices in intergranular lanes.

The solar chromosphere is permeated by complex magnetic fields that guide plasma flows and energy into the corona. This work presents a detailed analysis of a unique, high-resolution observation of a giant chromospheric spiral structure that emerges due to a large magnetic pore, captured by the Swedish 1-m Solar Telescope (SST). A comprehensive data analysis pipeline is developed to automatically detect the edges of 2255 plasma flows (loops) that constitute the spiral, and these are used to extract the kinematics of flows propagating along the magnetic field. The analysis reveals three primary insights into the spiral's physics. First, magnetic curvature is correlated with oscillatory flow dynamics, i.e. regions of high loop curvature exhibit a statistically significant excess of higher-order oscillation modes compared to straighter loops; it is also correlated with higher intensity and longer periods. Second, spatial distribution of oscillation period shows an inverse trend, decreasing from ∼3.5 minutes in the pore to ∼3 minutes in the outer spiral arms. This is interpreted as a signature of the overlying trans-equatorial quadrupolar coronal loop system compressing the pore's field lines into a near-horizontal orientation, producing a period gradient that challenges the standard expanding canopy model. Finally, the emission signature confirms that oscillating threads represent localised channels of brightness that lie within cooler, absorbing loop material. This study provides the first statistical analysis of oscillatory flows in a large-scale spiral, probing energy flow through the chromosphere through curved magnetic structure.

Free-space optical communication offers extremely high-rate data links for the inter-satellite CubeSat (nanosatellite) missions that have limited power and mass (size) constraints. This paper presents a comprehensive experimental characterisation of a sub-200 g erbium-doped fibre amplifier (EDFA) subsystem, evaluating optical power scaling and thermal stability in seeded and unseeded operation as well as beam profiling across drive conditions. Although EDFA is widely used in more than 40% of optical low Earth orbit missions, published subsystem-level characterisation under realistic spacecraft constraints remains limited. The amplifier used in this study produces a steady 425 mW optical output at 55 degrees C with a 2.1% beam-divergence variation under seeded operation at 5 A pump current under all drive conditions. Unseeded operation, on the other hand, shows a thermal roll-off from 470 mW to 400 mW and a temperature that is around 25 degrees C higher (75-80 degrees C). This is the first thermal comparison of seeded and unseeded modes in a CubeSat-scale amplifier. DC-DC conversion efficiency of 85-92% maintains total power consumption below 23 W, well within typical 60 W CubeSat electrical power system limits. These validated performance parameters provide essential design inputs for power-thermal modelling, link-budget analysis, and optical-terminal integration planning for nanosatellite free-space communication systems.

Beacon and data lasers are the critical components of CubeSat free-space optical inter-satellite links (OISL), yet component-level characterisation data for both is largely absent from open literature. This paper reports bench measurements of the dual-wavelength laser system in the ALIGN (Autonomous Laser Inter-Satellite Gigabit Network) FOCUS (Free-space Optical Communications Unit for Satellites) terminal - a 6U CubeSat payload targeting 1 Gbps inter-satellite links in low Earth orbit (mission target) - comprising a 976 nm beacon and a 1550 nm data laser. We characterised light-current behaviour, beam divergence across drive current ranges, and spectral properties of both lasers using flight-representative drive electronics. The 976 nm beacon produced 0.32 W at $540 ~\text{mA}(60 \%$ above the 0.2 W requirement) and offered current-controlled divergence from 4.25° (wide acquisition) to 2.73° (fine tracking) with no moving parts. The 1550 nm data laser seed confirmed output above the 1 mW threshold at the laser front end, while the full amplified system produced stable Gaussian beam profiles across all drive conditions with divergence of $5.33^{\circ}-5.44^{\circ}$. After collimation through the FOCUS optical head, transmitted divergence measured $82 \mu \text{rad}$ and $117 \mu \text{rad}$ full-angle at 976 nm and 1550 nm respectively, with $19.7 \mu \text{rad}$ inter-beam co-alignment. Spectral measurements confirmed 976.99 nm centre wavelength with 5.929 nm FWHM for the beacon, and 1549.7 nm centre wavelength with $\text{FWHM} \leq 0.5 ~\text{nm}$ for the data laser. These results represent one of the few open, peer-reviewed componentlevel datasets for this class of dual-wavelength CubeSat optical terminal.

In this work, a novel pointing, acquisition, and tracking (PAT) model is proposed and developed based on a new custom-designed 1.5U Free-space Optical Communications Unit for Satellites (FOCUS) payload. FOCUS is expected to be demonstrated as part of the Autonomous Laser Inter-Satellite Gigabit Network (ALIGN) UK space mission, which aims to achieve a full-duplex optical inter-satellite link between two 6U CubeSats in the same LEO plane at data rates > 1 Gbps with the FOCUS performance tests conducted across link distances ranging up to 1000 km. The key original contributions of this paper are three-fold; (i) a review of the state-of-the-art PAT models implemented in optical satellite communication missions to date; (ii) the ALIGN and FOCUS concept-of-operations in context of PAT; and (iii) algorithm development and numerical analysis of the FOCUS PAT sequence. The proposed PAT model, leading from coarse to fine line-of-sight pointing, utilizes a combination of open and closed loop schemes based on CubeSat’s attitude determination and control system and mirror-based steering mechanisms, where the optical transmitter and receiver follow the scan-stare strategy to precisely locate each other within seconds in a field-of-uncertainty defined by both orbital and optical misalignment errors. ALIGN is funded by the UK Space Agency (UKSA) via the National Space Innovation Programme (NSIP) and it is anticipated to be launched in late 2027 or later.

This paper examines the growing adoption of laser communication (lasercom) in space missions and payloads for identifying emerging trends and key technology drivers of future optical communications satellite systems. It also presents a comprehensive overview of commercially available and custom-designed lasercom terminals, outlining their characteristics and specifications to meet the evolving demands of global satellite networks. The analysis explores the technical considerations and challenges associated with integrating lasercom terminals into LEO constellations and the Inter-satellite communications service provision in LEO due to their power, size, and weight constraints. By analyzing advancements in CubeSat lasercom technology designed to cater for the emergence of future mega constellations of interacting small satellites, the paper underscores its promising role in establishing high-performance satellite communication networks for future space exploration and data transmission. In addition, a brief overview of our ALIGN planned mission is provided, which highlights the main key operational features in terms of PAT and link budget analysis.

In this analysis, we use spectroscopic observations of the quiet Sun made by the IRIS instrument and investigate wave propagation. We analyze various spectral lines formed in different atmospheric layers, such as the photosphere, chromosphere, and transition region. We examine the Doppler velocity time series at various locations in the quiet Sun to determine the dominant oscillation periods. Our results executing statistical analysis resemble those of the classical physical scenario, indicating that the photosphere is mainly characterized by the dominant 5 minute period, while the chromosphere is primarily associated with the 3 minute oscillation period. In the transition region, we observe a variety of oscillation periods, with dominant periods of 3, 8, and 12 minutes. We estimate the cutoff frequency by deducing the phase difference between two Doppler velocity time series obtained from spectral line pairs in different atmospheric layers formed at different temperatures. This reveals a significant correlation between 3 minute periods in the transition region and photospheric oscillations, suggesting that these oscillations in the transition region might propagate from the photosphere. Additionally, we analyze the phase difference between chromospheric oscillations and photospheric oscillations, demonstrating that only the 3 minute oscillations propagate upward. Based on the statistical analyses, we suggest the presence of magnetoacoustic waves in the solar atmosphere, some of which are propagating from the lower solar atmosphere upward, while some others are propagating downward. The transition region carries both long-period oscillations generated in situ and some photospheric oscillations that are also able to reach there from below.

We present the formation of quasi-periodic cool spicule-like jets in the solar atmosphere using 2.5-D numerical simulation in two-fluid regime (ions+neutrals) under the presence of thermal conduction and ion-neutral collision. The nonlinear, impulsive Alfvénic perturbations at the top of the photosphere trigger field aligned magnetoacoustic perturbations due to ponderomotive force. The transport of energy from Alfvén pulse to such vertical velocity perturbations due to ponderomotive force is considered as an initial trigger mechanism. Thereafter, these velocity perturbations steepen into the shocks followed by quasi-periodic rise and fall of the cool jets transporting mass in the overlying corona. This article is part of the theme issue 'Partially ionized plasma of the solar atmosphere: recent advances and future pathways'.

Precision beam pointing plays a critical role in free-space optical communications terminals in uplink, downlink and inter-satellite link scenarios. Among the various methods of beam steering, the use of fast steering mirrors (FSM) is widely adopted, with many commercial solutions employing diverse technologies, particularly focusing on small, high-bandwidth mirrors. This paper introduces a method using lightweight, commercial off-the-shelf components to construct a custom closed-loop steering mirror platform, suitable for mirror apertures exceeding 100 mm. The approach involves integrating optical encoders into two off-the-shelf open-loop actuators. These encoders read the signal reflected on purposefully diamond-machined knurled screw knobs, providing maximum contrast between light and dark lines. The resulting steering mirror has the potential to complement or replace FSM in applications requiring a larger stroke, at the expense of motion speed. In the presented setup, the mirror tilt resolution achieved based on the encoder closed-loop signal feedback is 45 μrad, with a mean slew rate of 1.5 mrad/s. Importantly, the steering assembly is self-locking, requiring no power to maintain a steady pointing angle. Using the mirror to actively correct for a constantly moving incoming beam, a 5-fold increase in concentration of the beam spot on the center of the detector was obtained compared to a fixed position mirror, demonstrating the mirrors ability to correct for satellite platform jitter and drift.

Using Swedish 1 m Solar Telescope Crisp Imaging Spectro-Polarimeter 6563 & Aring; (H alpha) observations and Mancha3D simulations, we analyze the formation and evolution of falling knots beneath a hedgerow prominence. By comparing the observed knot widths and kinematics to those of a parametric survey of simulations, we estimate the range of magnetic field values and characteristic wavelengths to test if the magnetic Rayleigh-Taylor instability (MRTI) can provide a physically meaningful explanation. We recover observational parameters using a novel semiautomated method and find knot velocities with a mean of -9.68 km s-1 and a mean width of 614 km. Our simulations survey a range of critical wavelengths, lambda c , of 100 to 500 km, and magnetic field strengths, B 0, of 1 to 20 G, finding the closest match to observations around lambda c = 300 km, and B 0 = 2 to 6 G. As both the observational and simulated values match expected values, we conclude that the MRTI can provide a physically meaningful explanation of this observation. Additionally, we also predict that the Daniel K. Inouye Solar Telescope will be able to observationally recover secondary instabilities on the leading edge of the falling mass through applying a point-spread function to an example from the simulated results.

Measuring magnetic fields in the inner corona, the interface between the solar chromosphere and outer corona, is of paramount importance if we aim to understand the energetic transformations taking place there, and because it is at the origin of processes that lead to coronal heating, solar wind acceleration, and of most of the phenomena relevant to space weather. However, these measurements are more difficult than mere imaging because polarimetry requires differential photometry. The coronal magnetograph mission (CMAG) has been designed to map the vector magnetic field, line-of-sight velocities, and plane-of-the-sky velocities of the inner corona with unprecedented spatial and temporal resolutions from space. This will be achieved through full vector spectropolarimetric observations using a coronal magnetograph as the sole instrument on board a spacecraft, combined with an external occulter installed on another spacecraft. The two spacecraft will maintain a formation flight distance of 430 m for coronagraphic observations, which requires a 2.5 m occulter disk radius. The mission will be preferentially located at the Lagrangian L5 point, offering a significant advantage for solar physics and space weather research. Existing ground-based instruments face limitations such as atmospheric turbulence, solar scattered light, and long integration times when performing coronal magnetic field measurements. CMAG overcomes these limitations by performing spectropolarimetric measurements from space with an external occulter and high-image stability maintained over time. It achieves the necessary sensitivity and offers a spatial resolution of 2.5″ and a temporal resolution of approximately one minute, in its nominal mode, covering the range from 1.02 solar radii to 2.5 radii. CMAG relies on proven European technologies and can be adapted to enhance any other solar mission, offering potential significant advancements in coronal physics and space weather modeling and monitoring.

Studies on small-scale jets’ formation, propagation, evolution, and role, such as type I and II spicules, mottles, and fibrils in the lower solar atmosphere’s energetic balance, have progressed tremendously thanks to the combination of detailed observations and sophisticated mathematical modelling. This review provides a survey of the current understanding of jets, their formation in the solar lower atmosphere, and their evolution from observational, numerical, and theoretical perspectives. First, we review some results to describe the jet properties, acquired numerically, analytically and through high-spatial and temporal resolution observations. Further on, we discuss the role of hydrodynamic and magnetohydrodynamic instabilities, namely Rayleigh–Taylor and Kelvin–Helmholtz instabilities, in jet evolution and their role in the energy transport through the solar atmosphere in fully and partially ionised plasmas. Finally, we discuss several mechanisms of magnetohydrodynamic wave generation, propagation, and energy transport in the context of small-scale solar jets in detail. This review identifies several gaps in the understanding of small-scale solar jets and some misalignments between the observational studies and knowledge acquired through theoretical studies and numerical modelling. It is to be expected that these gaps will be closed with the advent of high-resolution observational instruments, such as Daniel K. Inouye Solar Telescope, Solar Orbiter, Parker Solar Probe, and Solar CubeSats for Linked Imaging Spectropolarimetry, combined with further theoretical and computational developments.

Vortex flows, related to solar convective turbulent dynamics at granular scales and their interplay with magnetic fields within intergranular lanes, occur abundantly on the solar surface and in the atmosphere above. Their presence is revealed in high-resolution and high-cadence solar observations from the ground and from space and with state-of-the-art magnetoconvection simulations. Vortical flows exhibit complex characteristics and dynamics, excite a wide range of different waves, and couple different layers of the solar atmosphere, which facilitates the channeling and transfer of mass, momentum and energy from the solar surface up to the low corona. Here we provide a comprehensive review of documented research and new developments in theory, observations, and modelling of vortices over the past couple of decades after their observational discovery, including recent observations in Hα , innovative detection techniques, diverse hydrostatic modelling of waves and forefront magnetohydrodynamic simulations incorporating effects of a non-ideal plasma. It is the first systematic overview of solar vortex flows at granular scales, a field with a plethora of names for phenomena that exhibit similarities and differences and often interconnect and rely on the same physics. With the advent of the 4-m Daniel K. Inouye Solar Telescope and the forthcoming European Solar Telescope, the ongoing Solar Orbiter mission, and the development of cutting-edge simulations, this review timely addresses the state-of-the-art on vortex flows and outlines both theoretical and observational future research directions.

Magnetism dominates the structure and dynamics of the solar corona. To understand the true nature of the solar corona and the long-standing coronal heating problem requires measuring the vector magnetic field of the corona at a sufficiently high resolution (spatially and temporally) across a large Field-of-View (FOV). Despite the importance of the magnetic field in the physics of the corona and despite the tremendous progress made recently in the remote sensing of solar magnetic fields, reliable measurements of the coronal magnetic field strength and orientation do not exist. This is largely due to the weakness of coronal magnetic fields, previously estimated to be on the order of 1-10 G, and the difficulty associated with observing the extremely faint solar corona emission. With the Solar cUbesats for Linked Imaging Spectro-polarimetry (SULIS) mission, we plan to finally observe, in detail and over the long-term, uninterrupted measurements of the coronal magnetic vector field using a new and very affordable instrument design concept. This will be profoundly important in the study of local atmospheric coronal heating processes, as well as in measuring the nature of magnetic clouds, in particular, within geoeffective Earth-bound Coronal Mass Ejections (CMEs) for more accurate forecasting of severe space weather activity.

ABSTRACT We report on the properties of coronal loop foot-point heating with observations at the highest resolution, from the CRisp Imaging Spectro-Polarimeter located at the Swedish 1-m Solar Telescope and co-aligned NASA Solar Dynamics Observatory observations, of Type II spicules in the chromosphere and their signatures in the extreme ultraviolet (EUV) corona. Here, we address one important issue, as to why there is not always a one-to-one correspondence, between Type II spicules and hot coronal plasma signatures, i.e. beyond TR temperatures. We do not detect any difference in their spectral properties in a quiet Sun region compared to a region dominated by coronal loops. On the other hand, the number density close to the foot-points in the active region is found to be an order of magnitude higher than in the quiet Sun case. A differential emission measure analysis reveals a peak at ∼5 × 105 K of the order of 1022 cm−5 K−1. Using this result as a constraint, we conduct numerical simulations and show that with an energy input of 1.25 × 1024 erg (corresponding to ∼10 RBEs contributing to the burst) we manage to reproduce the observation very closely. However, simulation runs with lower thermal energy input do not reproduce the synthetic AIA 171 Å signatures, indicating that there is a critical number of spicules required in order to account for the AIA 171 Å signatures in the simulation. Furthermore, the higher energy (1.25 × 1024 erg) simulations reproduce catastrophic cooling with a cycle duration of ∼5 h, matching a periodicity we observe in the EUV observations.

The National Science Foundation’s Daniel K. Inouye Solar Telescope (DKIST) will revolutionize our ability to measure, understand, and model the basic physical processes that control the structure and dynamics of the Sun and its atmosphere. The first-light DKIST images, released publicly on 29 January 2020, only hint at the extraordinary capabilities that will accompany full commissioning of the five facility instruments. With this Critical Science Plan (CSP) we attempt to anticipate some of what those capabilities will enable, providing a snapshot of some of the scientific pursuits that the DKIST hopes to engage as start-of-operations nears. The work builds on the combined contributions of the DKIST Science Working Group (SWG) and CSP Community members, who generously shared their experiences, plans, knowledge, and dreams. Discussion is primarily focused on those issues to which DKIST will uniquely contribute.