The science of large scale heliosphere and missions that made it possible (K. Dialynas, 16/5/2023)
Channel: Academy of Athens
Date: 2023-05-22

K. Dialynas
Center of Space Research & Technology, Academy of Athens

Abstract:
The exploration of interplanetary space and our sun’s astrosphere, the heliosphere, has made a big leap over the past two decades, due to the path-breaking observations of the two Voyager spacecraft, launched more than 45 years ago.
Launched in 1977, the Voyager 1 and Voyager 2 missions (V1 & V2) surveyed the interplanetary space for 27-30 years. Their measurements (see review article from Dialynas et al.[1]) showed a general decrease in ion intensities as the inverse square of the radial distance from the Sun with a minimum at about the year 2000, followed by a gradual increase thereafter. These measurements revealed also for the first time the evolution of the outward propagating solar wind (SW) through the interplanetary space, together with the effects of Solar Particle Events and Corotating Interaction Regions, from 1 AU up to the Termination Shock (TS).
The crossings of V1 & V2 from the TS in 2004 and 2007, at distances of ∼94 and ∼84 AU, respectively, led to the discovery of the previously unknown reservoir of ions and electrons that constitute the heliosheath (HS). The V1 & V2 crossings of the heliopause (HP), in 2012 and 2018 at ∼122 AU and ∼119 AU, respectively, pinpointed the extent of the upwind heliosphere’s expansion into the Verly Local Interstellar Medium (VLISM) and its rough symmetry[2]. The Voyagers also showed for the first time that the helioshpere filters substantially the very high energy Galactic Cosmic Rays (GCR), providing an immense shield that protects our solar system from the harsh galactic radiation (overall, ∼14% of GCRs reach 1 AU). Both crossings of the HP were associated with a virtual depletion of particles of solar origin at all levels, an abrupt increase of GCR, magnetic field and plasma density upstream at the HP, whereas the temperature was found higher than expected. All parameters were previously significantly underestimated in most models of the heliosphere.
Apart from the similarities between the V1 and V2 crossings, some substantial and puzzling differences were also identified. For example, the V1 crossing of the HP was associated with the discovery of a flow stagnation region that was observed before the boundary, possibly due to flux tube (FT) interchange instability at the boundary. The use of LECP measurements in V1, revealed the existence of a radial inflow of ∼40-139 keV ions within the HS, for ∼9-10 AU before the HP crossing and a radial outflow over a spatial scale of ∼28 AU past the HP, that corresponds to an ion population leaking from the HS into IS space. Unlike V1, which found two interstellar flux tubes that had invaded the HS with strong anticorrelations in GCRs, V2 found no similar precursors to the HP. Clearly, the interstellar (IS) space upstream at the HP is not a “calm sea” of rarefied plasma and fields.
The Voyager in-situ observations were complemented by global images using remotely sensed ENAs (5.2-55 keV) from Cassini/Ion and Neutral Camera (INCA), showing that the heliosphere forms a roughly symmetric time-dependent obstacle to the inward interstellar flow, following the outward propagating changes of the solar wind pressure through the solar cycle[3]. Further, the Webinar: TUESDAY 16 May 2023, 12:00 local time (UTC+3) The science of the large scale heliosphere and the missions that made it possible K. Dialynas Center of Space Research & Technology, Academy of Athens heliosphere responds promptly to those changes, within ∼2-3 yrs, in both the nose and antinose directions. Since the V1&2 became interstellar (IS) missions, it was identified that the primary driver of the interaction of the heliosphere with the VLISM is the pressure of the IS magnetic field. The solar wind plasma is confined between the TS and the HP globally, forming a heliosheath that exhibits diamagnetic behavior and interacts directly with the strong interstellar magnetic field. 
In this talk I will provide a brief overview of the findings from the Cassini and Voyager missions since the Voyagers crossed the TS, following their survey through the heliosheath and their respective crossings from the HP, out to the VLISM. The combined in-situ ions and remotely sensed ENAs are being used within the SHIELD DRIVE Science Center (BU) to develop a “digital twin” of the Heliosphere and also guide the requirements for the particle and fields measurements from the future Interstellar Probe mission[4,5].
1. Dialynas K. et al. Space Science Reviews, 218:21 (2022).
2. Krimigis, S. M. et al., Nature Astronomy, 3:997-1006 (2019).
3. Dialynas K. et al., Nature Astronomy, 1:0115 (2017).
4. Brandt, P. C. et al., Space Science Reviews, 219:18 (2023).
5. Dialynas, K. et al., Front. Astron. Space Sci., 10 (2023).