Jonathan Rae

“The Grand European Heliospheric Observatory” An integrated ESA approach to challenges in Solar and Solar-Terrestrial physics. Ian McCrea RAL Space, STFC Rutherford Appleton Laboratory, UK Jonathan Rae Mullard Space Science Laboratory, UCL, UK 1 “The Grand European Heliospheric Observatory” An integrated ESA approach to challenges in solar and solar-terrestrial physics. Abstract This short document is being submitted to the ESA White Paper exercise for Voyage 2050 missions, though it is perhaps not a white paper in the traditional sense. Rather than focusing in detail on a particular science area, our document is a suggestion that the future ESA science programme should take a holistic and balanced view of the scientific disciplines which underpin space weather, these being solar, heliospheric, magnetospheric, ionospheric and thermospheric physics. Each of these science areas individually justifies its own place in a future ESA Science Programme, though at the terrestrial end of the process chain there is scope for strategic overlap with the ESA Earth Explorer programme, as well as for creative arrangements to facilitate the combined use of ground-based instruments. We believe that, when future missions in the areas of solar and solar-terrestrial physics are considered, it would be appropriate to ensure that a reasonable balance is maintained across the sub-disciplines listed above. This could, of course, be done by the selection of more than one such mission at various points during the Voyage 2050 programme. An interesting way of achieving the same outcome, however, might be the creation of an ESA L-class opportunity that combined multiple missions from across the fields of solar, heliospheric, magnetospheric and ionospheric physics. Such a concept would not only address major challenges in the Solar-Terrestrial physics, but also underpin the European space weather applications which derive from it. Although these are supported elsewhere in the ESA programme (e.g. in the Space Situational Awareness programme), this is not necessarily done in a way which allows fundamental underpinning research. An L-class opportunity, combining multiple science missions in the way that we are suggesting, could provide the framework for an integrated and collaborative approach to constitute a “Grand European Heliospheric Observatory”. Introduction Space weather should really be understood, not as a science area in itself, but as an application of science, bringing together knowledge from across multiple scientific disciplines (the obvious ones being solar physics, heliospheric physics, magnetospheric physics, ionospheric physics and atmospheric science; but also linking with fields such as materials science and magnetotellurics). This knowledge is used to address the problems of predicting and mitigating the effects of energetic processes, initially arising from solar activity, on human society and its technology, either on the Earth’s surface, in its atmosphere or in geospace. While the implementation of space weather observations, models and mitigation strategies tends to be operationally-focused and impact-led, and therefore rightly belongs outside the ESA science programme, the selection of these tools and techniques is underpinned by a huge body of scientific knowledge, which cuts across multiple disciplines. Thus, while space weather is increasingly regarded as an important societal hazard, as reflected by its inclusion in the civil contingency strategies of multiple countries, there remains a compelling need to advance the underlying science, as this will ultimately enable monitoring, modelling and mitigation strategies to be improved. This underpinning science is not space weather, but consists of basic research into the fundamental physical processes connecting the solar interior with the Earth’s environment. As such, it most certainly belongs in the ESA science programme domain, of which it should be an important part, given the already advanced development of the relevant observing 2 techniques and the relative accessibility of the key regions (when compared to interplanetary missions, for example). This document is, therefore, an attempt to make the case for a cross-disciplinary ESA approach to advancing the science which underpins space weather. It calls for the adoption of a coordinated approach, to avoid the fragmentation which tends to result when each community individually proposes its own favourite missions, without necessarily considering how these might fit together into a broader picture. We are making the suggestion that ESA, while perhaps stopping short of a formal roadmap, should exercise a strategic approach to mission selection in this area, taking into account the most important areas in which knowledge is still lacking and recognising that addressing such knowledge gaps would not just benefit science in itself, but also the space weather applications areas which depend on such scientific understanding. It is worth mentioning that this strategy might even extend further than the selection of space missions for the ESA Science Programme. We note that the chain of physical processes underpinning space weather extends far down into the terrestrial upper atmosphere, even down to the Earth’s surface. Within the last few years, ESA has brought forward some missions within its Earth Explorer programme with the potential to address scientific questions important to space weather (examples include SWARM and the recent selection of the Daedalus mission). Hence coordination between these two ESA programmes seems very desirable, in order to forward progress in this science area. We should also mention the fact that, in principle, ESA has the possibility to contribute funding to ground-based instruments where the use of these would enhance the effectiveness of its selected space missions. While this has not often happened in practice, multiple aspects of the science covered here would benefit from ground-based support, ranging from high-resolution observations by solar telescopes, through radio techniques to measure solar energy release and solar wind propagation, to ground-based radar measurements of the processes connecting the ionosphere and magnetosphere and the eventual dissipation of this energy in the terrestrial atmosphere. Overview of key science areas Below we set out, at very high level, ten science areas, which we believe could achieve both significant scientific impact and potential synergistic operational benefits. These benefits to operations would arise, among other things, from enabling scientific progress which drives the development of better modelling and prediction, giving the operational missions e.g. in the ESA Space Situational Awareness programme a better context in which to work. The following text is certainly not intended to be a detailed science case, as each of these areas could justify a “white paper” in its own right. Although we present some very high-level ideas, it is also not the intention of this document to propose new mission concepts, since multiple concepts are presented in the other White papers connected to this exercise. Rather the following is meant as a summary of areas that a balanced programme could potentially address. Such a programme could, of course, be realised not only through opportunities in Voyage 2050, but by strategic collaborations with other agencies. It is worth noting that, in some of these areas, ESA is or has recently been involved in missions which address parts of this science, though improvement and some level of continuity of observations is highly desirable, not least given the inherent long-term variability of the solar-terrestrial system. 1: Long-term modulation of solar activity The present epoch of solar activity appears to be particularly interesting, as recent solar cycles seem to be displaying a marked decrease in sunspot activity, with possible evidence that the Sun may be entering a new “grand minimum” of activity, on a scale not seen for more than three centuries. While the causes of such “grand minima” remain disputed, historical proxy data for solar magnetic field 3 suggest that these phenomena might recur every 350-400 years, so that the coming grand minimum (if confirmed as such) might be one of a series, the last of which was the Maunder minimum of 1645- 1715. Although the mechanisms behind such variability are unclear, an idea attracting interest is that long-period perturbations to the dynamics of the solar interior might change the effectiveness of the magnetic field dynamo, hence inhibiting the emergence of solar active regions to the photospheric surface (see the section on flux emergence below). While this idea remains very controversial, it might potentially mean that observations of magnetic transport and flux emergence made in the interval between now and 2055 (the predicted end of the grand minimum) would provide a very interesting data set for comparison with observations made earlier and later. Goal What is the long-term variability of the solar dynamo, what are its causes and how does this affect solar activity? Concepts Long-term measurements of solar magnetic field at the photosphere and below. Comparison of “grand minimum” conditions with the behaviour of the “normal Sun”. Mission Scenarios SDO-type mission(s) in solar orbit, ensuring sustained high-resolution magnetic field measurements of the Sun over multiple solar cycles, especially during the potential “grand minimum”. Perhaps consider a mission in solar polar orbit,

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