An Overview of Solar Orbiter Observations of
Interplanetary Shocks in Solar Cycle 25
ABSTRACT
Interplanetary shocks are fundamental constituents of the heliosphere, where they form as a result of solar activity. We use previously unavailable measurements of interplanetary shocks in the inner heliosphere provided by Solar Orbiter, and present a survey of the first 100 shocks observed in situ at different heliocentric distances during the rising phase of solar cycle 25. The fundamental shock parameters (shock normals, shock normal angles, shock speeds, compression ratios, Mach numbers) have been estimated and studied as a function of heliocentric distance, revealing a rich scenario of configurations. Comparison with large surveys of shocks at 1 au show that shocks in the quasi-parallel regime and with high speed are more commonly observed in the inner heliosphere. The wave environment of the shocks has also been addressed, with about 50% of the events exhibiting clear shock-induced upstream fluctuations. We characterize energetic particle responses to the passage of IP shocks at different energies, often revealing complex features arising from the interaction between IP shocks and pre-existing fluctuations, including solar wind structures being processed upon shock crossing. Finally, we give details and guidance on the access use of the present survey, available on the EU-project “solar energetic particle analysis platform for the inner heliosphere” (SERPENTINE) website. The algorithm used to identify shocks in large datasets, now publicly available, is also described.
行星际激波是日球层（heliosphere）的基本组成部分，它们是太阳活动的结果。我们利用 Solar Orbiter (SO) 提供的以前无法获得的内日球层行星际激波的测量数据，并对第25太阳周期上升阶段在不同日心距离上原位观测到的前100次激波进行了调查。基本激波参数（激波法线、激波法线角、激波速度、压缩比、马赫数）作为日心距离的函数进行了估计和研究，揭示了丰富的配置场景。与对1~AU的激波的大规模调查比较表明，在内日球层中更常观察到准平行状态和高速的激波。激波的波动环境也得到了解决，大约50%的事件表现出明显的激波引起的上游波动。我们描述了高能粒子对不同能量的激振通过的反应，通常揭示了激振与先前存在的波动之间相互作用产生的复杂特征，包括在激波穿越时处理的太阳风结构。最后，我们给出了目前调查的详细信息和使用指南，该调查可在欧盟项目“内日球层太阳高能粒子分析平台”（SERPENTINE）网站上获得。本文还描述了用于识别大型数据集中冲击的算法，这些数据集现已公开。
1 Introduction
Shocks are ubiquitously observed in astrophysical environments, where they are believed to play a crucial role in energy conversion and particle acceleration (e.g. Bykov et al., 2019). Despite decades of research, the mechanisms by which shocks mediate such processes of energy conversion and particle acceleration are still a matter of debate (Lee et al., 2012). In general, shocks are abrupt transitions between supersonic and subsonic flows, converting directed bulk flow energy (upstream) into heat and magnetic energy (downstream) (Marcowith et al., 2016). In the collisionless case, a fraction of the available energy can be channelled in the production of energetic particles (e.g., Drury, 1983).
Heliospheric shocks are unique as accessible by direct spacecraft observations, and thus represent the missing link to astrophysical systems only observable remotely, like in the case of spectacular radiation emission due to shock–accelerated particles in supernova remnants (e.g. Giuffrida et al., 2022). Most of our knowledge is built around direct observations of the Earth’s bow shock, resulting from the interaction between the supersonic solar wind and the Earth’s magnetosphere, which represents an obstacle to its propagation (Eastwood et al., 2015). Since the early predictions and evidence due to the IMP8 mission (Dungey, 1979) to the modern NASA Magnetospheric MultiScale mission (MMS; Burch et al., 2016) elucidating the details of how energy is partitioned across the shock transition (Schwartz et al., 2022), the Earth’s bow shock has been an invaluable resource to understand shock behaviour down to the smallest, kinetic scales. In the past decades, particles reflected by the Earth’s shock and the fluctuation they induce in the upstream plasma (namely particle and wave foreshocks) have been extensively documented using the large spacecraft fleet now orbiting Earth (e.g., Wilkinson, 2003; Wilson III, 2016), often combined with numerical efforts (e.g., Kartavykh et al., 2013; Turc et al., 2023).
Interplanetary (IP) shocks travel in the heliosphere driven by eruptive phenomena like Coronal Mass Ejections (CMEs) and solar wind Stream Interaction Regions (SIRs) (Burlaga, 1971; Richardson, 2018; Webb & Howard, 2012). IP shocks are much less investigated than Earth’s bow shock due several to observational challenges, due to their higher speed with respect to the Earth’s bow shock posing a stronger constraint on needed time resolution to resolve the shock transition, and due to the lower number of multi-spacecraft observations Cohen et al. (2019). Therefore, IP shocks allow us to access a poorly explored regime of shock dynamics, including shock evolution from their origin at the Sun and into the interplanetary medium (Richardson, 2011). IP shocks are typically weaker and with larger radii of curvature compared to the Earth’s bow shock (e.g., Reames, 1999), and their waves and particle foreshocks are much less well-characterised than their terrestrial counterparts, with several studies highlighting fundamental differences between them. For example, using the Solar Terrestrial Relations Observatory (STEREO) mission (Kaiser et al., 2008), Kajdič et al. (2012) has shown that upstream waves are somewhat irregular at IP shocks, and are sometimes observed without corresponding shock-reflected particle populations as would be expected. Blanco-Cano et al. (2016) surveyed IP shocks observed by STEREO from 2007 to 2010 and showed that significant suprathermal particle populations are more likely to be found at CME-driven shocks at 1 AU with respect to SIR-driven ones due to different evolutionary features of such structures. Transient structures, routinely observed at Earth’s bow shock and known to play a fundamental role in energy conversion and particle acceleration (Plaschke et al., 2018), are very rarely observed at IP shocks, with little evidence of upstream shocklets (Lucek & Balogh, 1997; Wilson et al., 2009; Trotta et al., 2023a) and downstream jets (Hietala et al., 2024).
Figure 1: Solar Orbiter trajectory (black line) throughout our statistical campaign in a fixed Earth–Sun frame. The red dots represent IP shock crossings, and the blue and yellow dot represent the Earth and the Sun, respectively. (Solar Orbiter model: esa.com).
Additionally, IP shocks provide insights about how the shock system evolves in time and through its spatial propagation, an aspect that cannot be investigated for Earth’s bow shock. From this point of view, multi-spacecraft observations leveraging different heliospheric vantage points are crucial to reconstruct fundamental properties of the shock’s system (Lugaz et al., 2024). Crucially, novel missions like NASA’s Parker Solar Probe (PSP; Fox et al., 2016) and ESA’s Solar Orbiter (Müller et al., 2020) are probing the inner heliosphere with state-of-the-art instrumentation, thereby yielding previously unavailable datasets and therefore provide an unprecedented opportunity for discovery in IP shocks. The importance of such inner heliospheric observers has been highlighted by several recent works exploiting useful line-ups among them and the existing near-Earth spacecraft fleet (e.g., Trotta et al., 2024, 2024a; Davies et al., 2024). Furthermore, the exploitation of this tantalizing observational window is particularly timely given the peak of activity of solar cycle 25, modulating IP shocks occurrence (Oh et al., 2007) and motivating shock surveying efforts with both long- and short-term impacts (see, for example, Oliveira, 2023).
Table 1: Summary of the shock properties and parameters provided in the Solar Orbiter shock list. An interactive and downloadable list can be found on the SERPENTINE data center (see Appendix B ).
Parameter Content Units Shock ID Shock unique identifier - Shock Date Date of shock observation UT Shock Time Time of the shock crossing UT Heliocentric distance Distance from the Sun AU Solar-MACH config Link to orbit configuration plot - Associated SEP event ID of associated SEP event - Mean upstream B/B0
- Upstream density Mean upstream ion density cm−3 Upstream Mean upstream plasma beta - Upstream 𝐁 Mean upstream 𝐁 vector nT n^_Shock Mean shock normal vector - _Bn
Mean shock normal angle ∘ V_Shock Mean shock speed km/s M_A Alfvénic Mach number - M_fms Fast Magnetosonic Mach number - r_B Mean magnetic compression ratio - r_Gas Mean gas compression ratio - Structures - two hours Structuring across shock - Structures - 8 minutes Structuring across shock - Notes E.g., data availability - Identified with E.g., TRUFLS, Visual inspection - Possibility of multi-SC observation Other SC within 0.2 AU - Wave foreshock Present/Absent - Foreshock extent Duration of wave foreshock minutes Frequency range Frequencies of enhanced wave activity Hz Low energy particle reflection Present/Absent - Proton response at selected energies No response/Spike/Plateau/Irregular - Proton peak delay t_peak−t_shock For selected energies minutes Electron response at selected energies - - Notes on particle response - -
激波在天体物理环境中普遍被观测到，人们认为它们在能量转换和粒子加速中起着关键作用（例如，Bykov等人，2019）。尽管已有数十年的研究，激波介导能量转换和粒子加速的机制仍是一个有争议的问题（Lee等人，2012）。一般来说，激波是超音速和亚音速流动之间的突然转变，将定向的整体流动能量（上游）转换为热能和磁能（下游）（Marcowith等人，2016）。在无碰撞的情况下，部分可用能量可以引导用于产生高能粒子（例如，Drury，1983）。
日球层激波的独特之处在于可以直接通过航天器观测，因此代表了与只能通过远程观测的天体物理系统缺失的联系，例如超新星遗迹中由于激波加速粒子而产生的壮观辐射发射（例如，Giuffrida等人，2022）。我们的大部分知识都是围绕对地球弓形激波的直接观测构建的，这是由超音速太阳风与地球磁层相互作用产生的，地球磁层对其传播构成障碍（Eastwood等人，2015）。从早期IMP8任务的预测和证据（Dungey，1979）到现代NASA磁层多尺度任务（MMS；Burch等人，2016）阐明了能量如何在激波过渡中分配（Schwartz等人，2022），地球的弓形激波一直是理解激波行为直至最小动力学尺度的宝贵资源。在过去的几十年中，地球激波反射的粒子及其在上游等离子体中引起的波动（即粒子和波前激波）已使用现在环绕地球的大型航天器舰队进行了广泛记录（例如，Wilkinson，2003；Wilson III，2016），通常与数值研究相结合（例如，Kartavykh等人，2013；Turc等人，2023）。
行星际（IP）激波在日球层中传播，由日冕物质抛射（CMEs）和太阳风相互作用区域（SIRs）等爆发性现象驱动（Burlaga，1971；Richardson，2018；Webb & Howard，2012）。由于观测挑战、相对于地球弓形激波更高的速度对所需时间分辨率提出更严格的要求，以及多航天器观测数量较少（Cohen等人，2019），行星际激波的研究远不如地球弓形激波深入。因此，行星际激波使我们能够探索激波动力学中研究较少的领域，包括激波从太阳起源到行星际介质的演化（Richardson，2011）。与地球弓形激波相比，行星际激波通常较弱，曲率半径较大（例如，Reames，1999），其波动和粒子前激波的特征远不如地球对应物明确，多项研究强调了它们之间的根本差异。例如，使用太阳-地球关系观测台（STEREO）任务（Kaiser等人，2008），Kajdič等人（2012）表明行星际激波上游的波动有些不规则，有时观察到没有相应的激波反射粒子群体，这与预期不符。Blanco-Cano等人（2016）调查了2007至2010年间STEREO观测到的行星际激波，并表明在1天文单位处，由于不同结构的演化特征，日冕物质抛射驱动的激波更有可能发现显著的超热粒子群体，而与太阳风相互作用区域驱动的激波相比则不然。地球弓形激波中经常观察到的瞬态结构已知在能量转换和粒子加速中起基本作用（Plaschke等人，2018），但在行星际激波中很少观察到，几乎没有上游激波子波（Lucek & Balogh，1997；Wilson等人，2009；Trotta等人，2023a）和下游射流的证据（Hietala等人，2024）。
此外，行星际激波（IP shocks）提供了关于激波系统如何随时间和空间传播而演化的见解，这是地球弓形激波无法研究的一个方面。从这个角度来看，利用不同的日球层视角进行多航天器观测对于重建激波系统的基本特性至关重要（Lugaz等人，2024）。至关重要的是，像NASA的Parker Solar Probe（PSP；Fox等人，2016）和ESA的Solar Orbiter（Müller等人，2020）这样的新型任务正在用最先进的仪器探测内日球层，从而产生以前无法获得的数据集，并为行星际激波的发现提供了前所未有的机会。这种内日球层观测者的重要性已经被最近几项利用它们与现有近地航天器队列之间有用排列的研究所强调（例如，Trotta等人，2024，2024a；Davies等人，2024）。此外，考虑到太阳周期25的活动高峰正在调制行星际激波的发生（Oh等人，2007），并激发具有长期和短期影响的激波调查工作（例如，参见Oliveira，2023），因此对这种诱人的观测窗口的利用特别及时。
在本工作中，我们展示了使用Solar Orbiter在不同日心距离处观察到的行星际激波（IP shocks）的广泛调查。这项激波调查是在欧洲项目“内日球层太阳高能粒子分析平台”（SERPENTINE）的框架内进行的，并通过项目数据中心公开提供。此处讨论的版本可通过Zenodo（Trotta等人，2024b）引用。
在第2节中，我们介绍了所使用的数据集。
第3节描述了整个调查中使用的激波识别和特征化方法，讨论了为每个事件计算的关键参数，并总结了为每个观察到的行星际激波提供的信息。
在第4节中，我们描述了这项统计工作的初步结果，展示了激波参数与日心距离的一般趋势，以及与1天文单位处先前观察结果的关系（第4.1节）。我们还讨论了波前激波的一般特性，以及新型Solar Orbiter载荷观察到的行星际激波通过后高能粒子的响应（分别见第4.2节和第4.3节）。
在第5节中，我们给出了结论。
最后，在附录A和B中，我们提供了此处开发和使用的激波搜索软件的详细信息，以及如何使用此目录及其未来通过SERPENTINE数据中心实施的信息。