Determining the direction of the local interstellar magnetic field (LISMF) is important for understanding the heliosphere’s global structure, the properties of the interstellar medium, and the propagation of cosmic rays in the local galactic medium. Measurements of interstellar neutral atoms by Ulysses for He and by SOHO/SWAN for H provided some of the first observational insights into the LISMF direction. Because secondary neutral H is partially deflected by the interstellar flow in the outer heliosheath and this deflection is influenced by the LISMF, the relative deflection of H versus He provides a plane - the so-called - B-V plane in which the LISMF direction should lie. IBEX subsequently discovered a ribbon, the center of which is conjectured to be the LISMF direction. The most recent He velocity measurements from IBEX and those from Ulysses yield a B-V plane (Figure 6) with uncertainty limits that contain the centers of the IBEX ribbon at 0.7-2.7 keV.

IBEX Discoveries of the Global Heliosphere from Energetic Neutral Atoms and Preparations for IMAPIBEX Discoveries of the Global Heliosphere from Energetic Neutral Atoms and Preparations for IMAP

Nathan Schwadron1, Dave McComas2, Eberhard Moebius1, Len Burlaga3, John Richardson4 and the IBEX Team

1: The University of New Hampshire, Durham NH 03824; 2: Southwest Research Institute, San Antonio TX, 78228; 3: Goddard Space Flight Institute, Greenbelt MD 20771; 4: Massachusetts Institute of Technology, Cambridge MA 02139

Our piece of cosmic real-estate, the heliosphere, is the domain of all human existence --an astrophysical case-history of the successful evolution of life in a habitable system. By exploring our global heliosphere and its myriad interactions, we develop key physical knowledge of the interstellar interactions that influence exoplanetary habitability as well the history and destiny of our solar system. IBEX was the first mission to explore the global heliosphere and in concert with Voyager 1 and Voyager 2 is discovering a fundamentally new and uncharted physical domain of the outer heliosphere. In parallel, Cassini/INCA maps the global heliosphere at energies (~5-55 KeV) above those measured by IBEX. The enigmatic IBEX ribbon and the INCA belt were unanticipated discoveries demonstrating that much of what we know or think we understand about the outer heliosphere needs to be revised. Remarkably, the combination of observations of the ribbon, the belt and the globally distributed flux have provided a picture not only of the global heliosphere, but also the interstellar magnetic field, which has a strength and direction that can be directly compared to Voyager 1 observations. Currently, unraveling the interstellar magnetic field and its influences on the flows and structure of the heliosheath is an area of remarkably rapid discovery. The next quantum leap enabled by IMAP will open new windows on the frontier of Heliophysics at a time when the space environment is rapidly evolving. IMAP, like ACE before it, will be a keystone of the Heliophysics System Observatory. IMAP with 100 times the combined resolution and sensitivity of IBEX and INCA will discover the substructure of the IBEX ribbon and will reveal in unprecedented resolution global maps of our heliosphere. The remarkable synergy between IMAP, Voyager 1 and Voyager 2 will remain for at least the next decade as Voyager1 pushes further into the interstellar domain and Voyager 2 moves through the heliosheath .

 

Our piece of cosmic real-estate, the heliosphere, is the domain of all human existence --an astrophysical case-history of the successful evolution of life in a habitable system. By exploring our global heliosphere and its myriad interactions, we develop key physical knowledge of the interstellar interactions that influence exoplanetary habitability as well the history and destiny of our solar system. IBEX was the first mission to explore the global heliosphere and in concert with Voyager 1 and Voyager 2 is discovering a fundamentally new and uncharted physical domain of the outer heliosphere. In parallel, Cassini/INCA maps the global heliosphere at energies (~5-55 KeV) above those measured by IBEX. The enigmatic IBEX ribbon and the INCA belt were unanticipated discoveries demonstrating that much of what we know or think we understand about the outer heliosphere needs to be revised. Remarkably, the combination of observations of the ribbon, the belt and the globally distributed flux have provided a picture not only of the global heliosphere, but also the interstellar magnetic field, which has a strength and direction that can be directly compared to Voyager 1 observations. Currently, unraveling the interstellar magnetic field and its influences on the flows and structure of the heliosheath is an area of remarkably rapid discovery. The next quantum leap enabled by IMAP will open new windows on the frontier of Heliophysics at a time when the space environment is rapidly evolving. IMAP, like ACE before it, will be a keystone of the Heliophysics System Observatory. IMAP with 100 times the combined resolution and sensitivity of IBEX and INCA will discover the substructure of the IBEX ribbon and will reveal in unprecedented resolution global maps of our heliosphere. The remarkable synergy between IMAP, Voyager 1 and Voyager 2 will remain for at least the next decade as Voyager1 pushes further into the interstellar domain and Voyager 2 moves through the heliosheath .

 

AbstractAbstractIBEX images energetic neutral atoms (ENAs) produced in the complex boundar regions that mediate the interaction between the solar wind and the Local Interstellar Medium(LISM). Further, IBEX determines the properties of the LISM directly by measuring interstellar neutrals that pass through the boundaries and propagate inward to 1 AU. The history and future of our heliosphere in the galaxy is key to understanding the condition on our evolving planet. As we ask about the habitability of other planets surrounding other stars, we grapple with understanding the complex environments and interactions in the local parts of the galaxy where these stars exist. Our own heliosphere is an astrophysical case-history of the successful evolution of life in a habitable system.

IBEX images energetic neutral atoms (ENAs) produced in the complex boundar regions that mediate the interaction between the solar wind and the Local Interstellar Medium(LISM). Further, IBEX determines the properties of the LISM directly by measuring interstellar neutrals that pass through the boundaries and propagate inward to 1 AU. The history and future of our heliosphere in the galaxy is key to understanding the condition on our evolving planet. As we ask about the habitability of other planets surrounding other stars, we grapple with understanding the complex environments and interactions in the local parts of the galaxy where these stars exist. Our own heliosphere is an astrophysical case-history of the successful evolution of life in a habitable system.

MotivationsMotivations

Heliotail structure, asymmetric lobes and regions of fast and slow wind. IBEX measurements of ENAs have for the first time mapped out the structure of our heliosphere’s tail [7], which is structured like a four-leaf clover (Figure 2). The four-leaf clover shape of the observed heliotail is attributed to the fact that the Sun has been sending out fast solar wind near its poles and slower wind near its equator for a number of years (interpretation and analysis of ENAs emitted from the heliotail must properly account for significant time delays). This is a common pattern in the most recent phase of the suns 11-year activity cycle. These results also showed that the clover shape does not align symmetrically about the ecliptic plane, but instead, is rotated slightly, indicating that as it moves further away from the sun and its magnetic influence, the charged particles begin to be nudged into a new orientation, aligning with the magnetic field orientation of the LISM.

Maximum pressure region in the inner heliosheath, which deflects heliosheath flows. Plasma flows measured by Voyager 2 [37] show a clear rotation away from radially outward with increasing penetration into the inner heliosheath while the overall flow speed remains roughly constant. However, the direction of rotation is far more into the transverse, and less into the polar direction, than predicted. No current model reproduces the key observational results of 1) the direction of flow rotation or 2) constancy of the flow speed. In recent work [11], IBEX observations of the plasma pressure in the inner heliosheath [10] were used to show that the flow direction is consistent with flow away from the region of maximum pressure in the inner heliosheath, 20∼ ◦ south of the upwind direction. It was shown further [11] that the dominance of the suprathermal ion pressure in the inner heliosheath measured by IBEX can explain both the observed flow rotation and constancy of the flow speed. These results highlight the importance of IBEX observations of suprathermal ions in the inner heliosheath for understanding the local insitu observations from the Voyager spacecraft, and this key region of the heliospheres interstellar interaction more generally.

REFERENCESREFERENCES

The IBEX RibbonThe IBEX Ribbon

(c)

(d)

(e)

(f)

The IBEX mission has already made numerous fundamental and sweeping discoveries (Figure 1), many unanticipated, about the local interstellar magnetic field (LISMF) and its influence, the boundaries of our solar system, their spatial structure, and the properties of the LISM. In the first extended mission, IBEX continued to discover the properties and fundamental physical processes (Figure 1) that regulate the global heliosphere, the LISM, and the IBEX ribbon while also revealing critical new information about global magnetospheric and lunar interactions with the solarwind. Outside the IBEX ribbon, IBEX observes the globally distributed flux (GDF) thought to be generated predominantly from the inner heliosheath. Observing the emissions of the ribbon, the GDF, and neutrals from the interstellar medium, IBEX has made sweeping discoveries allowing us to understand the global structure of the heliosphere, how it evolves and is influenced by internal (solar wind) and external (LISM and LISMF) conditions.

The IBEX mission has already made numerous fundamental and sweeping discoveries (Figure 1), many unanticipated, about the local interstellar magnetic field (LISMF) and its influence, the boundaries of our solar system, their spatial structure, and the properties of the LISM. In the first extended mission, IBEX continued to discover the properties and fundamental physical processes (Figure 1) that regulate the global heliosphere, the LISM, and the IBEX ribbon while also revealing critical new information about global magnetospheric and lunar interactions with the solarwind. Outside the IBEX ribbon, IBEX observes the globally distributed flux (GDF) thought to be generated predominantly from the inner heliosheath. Observing the emissions of the ribbon, the GDF, and neutrals from the interstellar medium, IBEX has made sweeping discoveries allowing us to understand the global structure of the heliosphere, how it evolves and is influenced by internal (solar wind) and external (LISM and LISMF) conditions.

Figure 1:Overview of the physical processes that shape and control the global heliosphere imaged by IBEX.

Figure 2 Data from IBEX (panel c) shows the spectral slope of the particles looking down the solar system’s tail; the yellow and red colors represent areas of slow-moving particles, and the blue represents the fast-moving particles. Panel a) latitude structure of typical solar wind, with fast wind at high latitudes and slower wind at low latitudes. Panel b) corresponding solar wind structure looking down tail with faster wind in blue and slower wind in green. N & S refer to the North and South directions. St refers to the starboard direction and Pt refers to port (these are nautical terms)Panel on the right shows a cartoon of the structure of the heliotail being twisted by the LISMF. Adapted from [7]..

Discoveries and BreakthroughsDiscoveries and Breakthroughs

Figure 3 Mollweide projection of the ENA-inferred plasma pressure over the LOS integration (color coded) from [10]. The black dot indicates the location of Voyager 2 with the T and -N axes indicating flow directions in the RTN coordinate system and Voyager 2 observed flow direction in the NT plane (heavy back vector). Observed plasma motion flows away from the pressure max, and not with from the nose (blue). Taken from [11].

Figure 4. IBEX has begun to observe clear changes in the globally distributed flux associated with solar cycle changes, which will continue through the continued IBEX extended mission. These panels show the pressure of plasma protons from the inner heliosheath that form observed ENAs integrated over line-of-sight (LOS) as observed by IBEX from 0.7 to 4.3 keV. Note the stability of more distant tail compared to the marked reductions near the nose indicating clear changes associated with the evolving solar wind and inner heliosheath. Adapted from [10].

What is the Evolving Global Structure of the Heliosphere? The cycle 24 maximum did not show a large increase in solar wind pressure. In coming years, we expect that the decrease of the pressure of the globally distributed flux (GDF) will continue. Initial evidence of this trend is apparent in the solar cycle evolution of the globally distributed flux (Figure 4). We expect to first observe solar cycle changes near the nose and then observe changes in the tail over longer time scales The temporal variations in the GDF are striking, and provide the basis to begin to piece together the time variations in our global heliosphere in the continued IBEX mission.

Discoveries of Global Heliospheric StructureDiscoveries of Global Heliospheric Structure

The ribbon stretches across much of the sky and its origin remains an enigma despite its persistence for over five years and after more than a dozen ideas and theories that attempt to explain it. While each theory (Fig. 5 shows one example) that has been proposed has its strengths, each one also demonstrates potential flaws in internal consistency [see review, 16]. Observations generally show support for concepts involving creation of the ribbon from neutral (or secondary) solar wind. The ribbon emissions have solar wind-like latitude structure and energy ordering. Further, the IBEX team discovered different time variations in different portions of the ribbon, consistent with evolution of latitudinal structure toward solar maximum and secondary ENA ribbon source. Therefore both the general structure of the ribbon and its time dependence are consistent with a secondary ENA source, although collection of additional IBEX data, analysis, and modeling are clearly needed to finally resolve if this or any of the other possible ribbon source mechanisms reallyproduces the ribbon.

Figure 5. (Left) The structure outside the heliopause indicates the spatial retention region that, according to a model [38] holds higher concentrations of particles that form the IBEX ribbon. (Right) Panels show model results in comparison to observations of the ribbon in the left column.

Triangulating The LISM Magnetic FieldTriangulating The LISM Magnetic Field

The possibility that Voyager 1 has moved into the outer heliosheath now suggests that Voyager 1’s direct observations provide another independent determination of the LISMF. We show that LISMF direction measured by Voyager 1 is > 40◦ off from the IBEX ribbon center and the B-V plane. Taking into account the temporal gradient of the field direction measured by Voyager 1, we extrapolate to a field direction (Figure 7) that passes directly through the IBEX ribbon center (0.7-2.7 keV) and the B-V plane (Figure 6), allowing us to triangulate the LISMF direction and estimate the gradient scale size of the magnetic field.

The linear projection suggests that Voyager 1 could observe a field direction at the IBEX ribbon center by 2025 when the spacecraft is at 165 AU from the Sun.

This also indicates a draping region of ~ 45 AU in radial extent near Voyager 1

Figure 7. The interstellar magnetic field observed by Voyager 1 over a period from 2013/130.6 to 2014/232.3 when the magnetic field magnitude (upper panel) remained roughly constant and the field direction changed steadily. We show the field direction in heliocentric (J2000) ecliptic coordinates and find the best linear fit to the data to infer the change in direction over time.

Figure 6. We combine four different sets of observations to determine the direction of the interstellar magnetic field. The red line shows the linear fit to Voyager 1 observations of the interstellar magnetic field. This linear fit is projected forward in time (the red circles surrounding a “V” show discrete points in time along the Voyager trajectory). The H flow direction from SOHO/SWAN (Lallement et al. 2005, 2010) is shown with the He flow direction derived by Schwadron et al. (2015) in blue. The H inflow is more strongly deflected by secondary interactions in the heliosheath than the He inflow. Therefore, the BISM-VISM plane contains the deflection of H relative to He (Lallement et al. 2005). The region bounded by the dark blue dashed curves shows the limits of the BISM-VISM plane, which contains the orientation of the IBEX ribbon (Funsten et al. 2013) from 0.7-2.7 keV. The purple closed circle shows the He inflow direction based on the most recent analysis of Ulysses ISN flow observations (Wood et al. 2015). The purple line shows the corresponding B-V plane connecting the Ulysses He and SOHO/SWAN H observations. The center (closed black circles) of the IBEX ribbon is shown at separate energy steps observed by the Hi sensor on IBEX. The projected interstellar field direction from Voyager 1 converges with the 1.7 keV IBEX ribbon center on the date of 2024.7. The light blue region shows the extent of the intersections between the B-V plane and the Voyager 1 projection. This region of intersection contains the IBEX ribbon centers from 0.7-2.7 keV, demonstrating the successful triangulation of these distinct data sets.

Schwadron, N. A., Richardson, J. D., Burlaga, L. F., McComas, D. J., and Moebius, E., Triangulation of the Interstellar

Magnetic Field, The Astrophysical Journal, 813, L20, 2015

IMAPIMAPThe next quantum leap in imaging our global heliosphere will be enabled by IMAP. IMAP with 100 times the combined resolution and sensitivity of IBEX will discover the substructure of the IBEX ribbon and reveal in unprecedented resolution global maps of our heliosphere. The remarkable synergy between IMAP, Voyager 1 and Voyager 2 will remain for at least the next decade as Voyager 1 pushes further into the interstellar domain and Voyager 2 moves through the heliosheath. Voyager 2's plasma measurements will create singular opportunities for discovery in the context of IMAP's global measurements.

IMAP, like ACE before it, will be a keystone of the Heliophysics System Observatory by providing comprehensive cosmic ray, energetic particle, pickup ion, suprathermal ion, neutral atom, solar wind, solar wind heavy ion, and magnetic field observations to diagnose the changing space environment and understand the fundamental origins of particle acceleration. IMAP's comprehensive interplanetary monitoring suite is critical to support ongoing geospace interaction studies and space weather observations at the ideal location of the Lagrangian point L1. The high societal relevance of comprehensive solar wind, suprathermal, magnetic field and cosmic ray observations from L1 makes the IMAP mission an imperative as a successor to ACE.