Report on the activity in the field of heliospheric research in Poland 2008 – 2010
M. Bzowski, A. Czechowski, R. Ratkiewicz, ZFUSiA
August 4, 2010
The heliospheric research in Poland during the report period has been carried out in a broad international collaboration. Main topics included determination of the strength and direction of interstellar magnetic field in the Local Interstellar Cloud based on plasma observations from the Voyager spacecraft and MHD modeling, studies of hypothetical nano-dust particles in the solar wind based on observations from STEREO, and studies of the heliosheath thickness based on ENA observations from the HSTOF instrument on SOHO. Most effort, however, was put into the final preparations to the launch of the first dedicated heliospheric ENA space observatory IBEX, developed and operated in the United States under NASA contracts, and to the interpretation of the first observations obtained from this mission.
The strength and orientation of magnetic field in the immediate Galactic neighborhood of the Sun escapes direct detection and thus has to be established indirectly based on proxies and appropriate modeling. The important discovery of the role of the local interstellar magnetic field (LIMF) in altering the shape and modifying the structure of the heliosphere was first demonstrated by Ratkiewicz at the COSPAR Symposium in 1996.
Crossing the termination shock by the Voyager spacecraft enabled an attempt to determine the parameters of LIMF from an MHD modeling which used as guidelines the following desiderate:
1. The heliospheric model should place the termination shock at the distances where the Voyager spacecraft crossed it.
Based on this prerequisite, Ratkiewicz & Grygorczuk (2008, 2010), and Ratkiewicz et al. (2008) proposed a constraint on the strength and direction of LIMF which placed it close to the so-called Hydrogen Deflection Plane, postulated by Lallement et al. in 2005 based on the deviation of the flow vector of neutral interstellar H, perturbed in the heliospheric interface, from the flow of neutral interstellar He, which is presumably unperturbed.
Further measurements of the plasma flow and magnetic field in the inner heliosheath by Voyager 2 allowed to formulate another desiderate:
2. The solar wind flow obtained from the MHD simulation should follow the plasma flow measured by Voyager 2 in the inner heliosheath.
After the discovery of heliospheric ribbon by the first dedicated Energetic Neutral Atom spaceborne observatory IBEX (McComas et al. 2009b) an interpretation of the ribbon as coming up due to the enhanced production of ENA in locations in space ordered by LIMF was proposed by the team of discoverers (Schwadron et al, 2009b), which included also personnel from the ZFUSiA group in SRC PAS, another desiderate was put forward:
3. The geometric location of the modeled LIMF perpendicular to the line of sight in the immediate neighborhood of the heliosphere should reproduce the shape of the ribbon as seen from Earth.
Based on these three criteria and 3D-MHD numerical simulations a team of researchers from ZFUSiA (R. Ratkiewicz, J. Grygorczuk, M. Strumik) determined that a reasonable agreement of model results with measurements is obtained when the LIMF strength is between 3.3 and 4.5 μG and the LIMF vector points from the ecliptic coordinates close the center of ribbon (λ, β)= (221°, 39°) (work reported at the 9th Annual International Astrophysics Conference, Maui, Hawaii, March 14-19, 2010 in two presentations by Grygorczuk and Ratkiewicz). A team of researchers including J. Grygorczuk, A. Czechowski, R. Ratkiewicz, and M. Strumik using 3D-MHD simulations showed also the time evolution of the forms and shapes of the canonical heliospheric surfaces (termination shock, heliopause, and bow shock) resulting from such a magnetic field and measured variations in the solar wind (Grygorczuk et al. 2010).
From its launch in 1996 the instrument HSTOF on board SOHO was the only source of data about the energetic neutral atom fluxes (55-88 keV for hydrogen, 28-58 keV/n for helium) from the inner heliosheath. After Voyager 1 entered the heliosheath it was possible to compare these ENA fluxes with the parent ion fluxes measured in situ (the LECP instruments on Voyagers have the energy range overlapping with HSTOF). This permitted to make the first estimation of the thickness of the forward heliosheath (the distance between the termination shock and the heliopause), which was done by a group including A. Czechowski from ZFUSiA, M. Hilchenbach from the Max-Planck-Institut in Lindau, and K.C. Hsieh and J. Kóta from the University of Arizona in Tucson. In 2008 this study was extended to the first attempt to image the forward heliosheath, made by measuring the thickness of the heliosheath in three different directions (the nose and two flanks) by Czechowski et al. (2008a,b). These first studies were based on the HSTOF data together with the first post-shock measurements by Voyager 1. This work is now being continued by taking into account the ion spectra measured subsequently by Voyager 1 and 2 deeper in the heliosheath. Also, the new HSTOF data from the flank directions are included. To interpret the results for the flank sectors, it is necessary to understand the spatial distribution of the energetic ions in the heliosheath. A study of this problem was performed by A. Czechowski using the numerical model of the heliosphere developed by K. Scherer. The results were presented at a recent (2010) conference on Maui: they suggest that the assumption of uniform ion distribution is incorrect and that it is likely to lead to the underestimation of the thickness of the heliosheath. An illustration of the results can be seen in Figures 1 and 2.
Figure
1: Estimated thickness of the heliosheath in three 90 deg wide ecliptic
longitude
sectors near the ecliptic: the "nose" sector (
from
the ISM
inflow) and two flank sectors. The results including also the more
recent HSTOF
data for the flanks are shown in red. The heliosheath ion spectrum used
for the
estimation is the Voyager 1 
spectrum
up to year 2008
(see Krimigis et al., Science 2009). The results for the thickness L
depend on
the assumed neutral hydrogen density nH in the heliosheath (here nH=0.1
cm–l ) and the fraction of protons fp in the Voyager
spectrum
(here fp=1): the result for L scales as L/[(nH/0.1)*(fp/1.0)].
Figure 2: Calculated flux profiles of 53 keV protons in the heliosheath along two directions: the "nose" and the "crosswind" (the flank). The red curves show the final result including all transport processes considered in the model: the loss of ions to charge-exchange; the adiabatic acceleration; parallel diffusion; the escape across the heliopause by transverse diffusion. The dotted lines illustrate the results obtained neglecting some of these effects. The final result is clearly sensitive to the details of the model.
Preparation for the IBEX mission, where one of the Polish researches (M. Bzowski from ZFUSiA) is a Co-I, included both developing of the mission itself and doing supporting modeling/theory research. The mission development & support included modeling and interpretation of the signal from the IBEX-Lo Star Sensor (M. Bzowski, M. Hlond, both ZFUSiA) and development and interpretation of the theory of ENA propagation in the inner heliosphere and of their losses underway from their birth site to the detector (Bzowski 2008). It required development of a new model of the propagation and losses of neutral hydrogen in the heliospheric origin (published by Tarnopolski & Bzowski 2009), which apart from the IBEX mission was used by a team fostered by the International Space Science Institute in Bern (ISSI), lead by E. Moebius from the University of New Hampshire, including personnel from SRC PAS (M. Bzowski, supported by S. Tarnopolski), to determine the density of neutral interstellar hydrogen at the termination shock of the solar wind and in the Local Interstellar Cloud (Bzowski et al. 2008, 2009; Nakagawa et al. 2008, Pryor et al. 2008, apart from other papers by the team, not co-authored by Polish researchers). After successful determination of the parameters of interstellar He in 2003 by an earlier ISSI team, also lead by Moebius and including personnel from ZFUSiA (M. Bzowski, D. Rucinski), the series of papers published in a special section of Astronomy & Astrophysics provided the missing link of the parameters of dominant species in the Local Interstellar Cloud, which together with the measurements of the solar wind plasma performed at 1 AU and by Voyager spacecraft provide a reference for the interpretation of heliospheric ENA observations by IBEX.
Other activities in the IBEX preparations with a participation of researchers from ZFUSiA included: (1) a reconnaissance of the possibility of detection of interstellar deuterium by IBEX (Ph.D. awarded to S. Tarnopolski by SRC PAS, summarized in a paper by Tarnopolski & Bzowski 2008), (2) a discovery of neutralization of the solar wind heavy ions by charge exchange with neutral interstellar gas in the inner heliosheath, which results in a flux of heavy-species ENA that will be possible to detect at 1 AU with appropriate equipment and which possibly are a source of both ACR and the inner source pickup ions (Grzedzielski et al. 2010a), and (3) publication of the “IBEX Book”, being a collection of papers reprinted from an issue of Space Science Reviews, some of them co-authored by personnel from ZFUSiA (McComas et al. 2009a, Frisch et al. 2009, Moebius et al, 2009a, Schwadron et al. 2009a), where both scientific and technical aspects of the IBEX mission were detailed.
IBEX was launched in October 2008 and the first results were published in a series of 5 papers in the Science journal, three of them coauthored by personnel from ZFUSiA (three by M. Bzowski and one by M.A. Kubiak: McComas et al. 2009b, Schwadron et al. 2009b, Moebius et al. 2009b). The most notable discovery by IBEX was the heliospheric ribbon, an almost-circular sky area of enhanced ENA emissions not predicted by earlier heliospheric models (see Fig. 3). The series of discovery papers included a summary of attempts to explain the ribbon ( Schwadron et al. 2009b), where proposed 6 hypotheses involving mostly heliospheric processes and heliospheric and extraheliospheric magnetic field were discussed. As mentioned above, based on some of them Ratkiewicz & Grygorczuk (2008) suggested the strength and direction of the local interstellar magnetic field. Since, however, none of the hypotheses was able to explain the details of the observations and not all aspects of the proposed models were sufficiently developed, Grzedzielski, Bzowski, Czechowski from ZFUSiA together with 3 US scientists from the Los Alamos National Labs, Southwest Research Institute, and Boston University proposed another, totally different hypothesis: ribbon comes up from a very nearby boundary between the local interstellar cloud in which the Sun is embedded, and the hot, tenuous Local Bubble cloud. Details of this hypothesis can be found in the caption to Fig. 4.
In still another IBEX effort, an international team lead by E. Moebius and including M. Bzowski, M.A. Kubiak, and M. Hlond from ZFUSiA has been studying neutral interstellar gas, measured in situ by IBEX Lo. In the discovery paper in Science (Moebius et al. 2009b) the first direct detection of neutral interstellar oxygen, neon, and hydrogen was reported, along with a strong signal from interstellar helium. A meticulous, detailed analysis of these observations is still going on.
Figure 3: The almost-circular shape of the heliospheric ENA ribbon (upper panel) and its projection on equatorial coordinates (lower panel), with isocontours of the column densities of neutral gas in the solar neighborhood from Redfield & Linsky 2002, which suggests that the ribbon is located in the region of sky pretty much devoid of neutral gas, as required by the hypothesis of extra-heliospheric origin of the ribbon proposed by Grzedzielski et al (2010b).
Figure 4: Schematic diagram illustrating the idea of heliospheric ribbon as a geometric effect coming up because the Sun happens to be located within ~1000 AU from the boundary between the Local Cloud of interstellar gas and the hot (>106 K), rarefied, fully ionized and turbulent Local Bubble cloud. The boundary should be planar or better slightly extruded towards the Sun. The cool (104 K) neutral gas penetrates the boundary between the LB and LIC plasma and starts to exchange charge with the energetic protons from the Local Bubble. Some of the hot protons from LB take away electrons from the cool atoms originating from the LIC without momentum change and, suddenly free from the confines of electromagnetic forces, run away at straight-line trajectories from their neutralization sites, some of them towards IBEX. Despite some ionization losses in the intervening slabs of interstellar gas, they are able to reach the detectors. The ribbon is a geometric effect: it is seen in the areas of sky where the lines of sight are longest in the LIC/LB boundary layer. The model proposed by Grzedzielski et al. (2010b) suggests that the ENA arriving directly from the LIC/LB boundary layer should be seen as the ribbon and the area inside the ribbon, while some of the ENA that had originally run away from the LIC/LB boundary not directed towards the Sun might eventually be detected after a series of scattering and ionization/neutralization processes in the Local Cloud gas, forming an omnidirectional diffuse ENA background to the ENA signal of the genuine heliospheric origin.
Apart from the mainstream heliospheric research focused on establishing the heliospheric basics, a few other heliospheric topics of interest have been addressed.
The dust grains with size in the nanometer range (1-few tens nm) are expected to be present in the circumsolar dust cloud. In distinction to the larger grains, the nano-grains dynamics is dominated by the electromagnetic forces. Detailed understanding of the nano-dust production and behavior in the inner solar system became important recently because of new observations. In particular, the voltage bursts detected on the STEREO spacecraft were attributed to the impacts of fast nano-dust grains.
In ZFUSiA the dynamics of nano-dust is the topic of theoretical investigation by A. Czechowski, who works in cooperation with I. Mann, and also with N. Meyer-Vernet, who heads the WAVE experiment on board STEREO. The experimental results with a theoretical interpretation were published by Meyer-Vernet et al. (2009, 2010). It was found that the nano-dust grains produced in collisions between the larger grains in the circumsolar dust cloud can be accelerated to the velocity approaching that of the solar wind plasma by the interaction with the solar magnetic field and subsequently escape from the solar system (Czechowski & Mann 2010). These escaping grains may be responsible for the impacts observed by the STEREO spacecraft. However, the grains formed very close to the Sun (within about 0.15 AU) move in trapped non-Keplerian orbits characterized by low perihelion distance, leading to fast destruction of these grains by sublimation.
Figure 5: Grain velocity as a function of distance from the Sun for the nano-dust grains of different charge to mass ratios Q/m (equivalently, different sizes) produced at 0.2 AU from the Sun. The Q/m values are the following: Q/m = 10–4 e/mp (1); 10–5 e/mp (2); 7 10–6 e/mp (3); 5 10–6 e/mp (4); 3 10–6 e/mp (5); 10–6 e/mp (6); 10–7 e/mp (7). The solid lines show the results for the "focusing" magnetic field orientation, the dashed lines for "antifocusing" orientation. The "kinks" in the velocity curves occur at crossings of the current sheet.
The heliospheric current sheet (HCS) is a separating surface between the solar wind regions of opposite polarity of magnetic field. Despite numerous observations by spacecraft the global structure and the underlying physics of the heliospheric current sheet is badly understood. HCS is also very difficult to include in the numerical models of the heliosphere because of its thinness: it can be less than 10000 km across, that is much below the characteristic size of the heliosphere. However, HCS is likely to have an important role in the particle transport and in the physics of the outer heliosphere, where the magnetic field structure must somehow be rearranged to avoid unphysical singularities.
ZFUSiA commenced a pioneering study of the structure HCS. A. Czechowski, M. Strumik and S. Grzedzielski, using the input from the time-dependent 3-D MHD model of the heliosphere by R. Ratkiewicz and J. Grygorczuk (and also from another model by K. Scherer from the Bochum University in Germany) calculated the global structure of HCS and found new features resulting from the time-dependent structure of the flow, in particular the secondary folding. They also discussed the issues concerning the fate of HCS on approaching the heliopause, where the regions of opposite field orientation are pressed together by the slowing plasma flow.
Another pioneering investigation was based on the idea by S. Grzedzielski (in cooperation with A. Czechowski and M. Strumik), who proposed that the reconnection exhausts from the reconnection sites on the heliospheric current sheet can accelerate electrons. This process is efficient provided that the particle can interact with many reconnection exhausts. Such a situation is possible near the heliopause. This points to a possible connection with the 2-3 kHz heliospheric radio emissions, supposed to come from this region.
The Polish heliospheric research has been funded in Poland by 5 grants from the Ministry of Higher Education and Science (2 of them already completed and 3 still active) and by the Space Research Centre PAS from the basic research funding allocation, as well as by a number of national and international programs sponsoring international science cooperation and the international members of the science teams collaborating with the Polish scientists.
During the report period, the ZFUSiA researchers have been at the forefront of heliospheric research. They focused on the fundamental problems in heliospheric physics and provided answers or at least hypotheses to the pivotal questions on the physical state of the interstellar gas in front of the heliosphere, strength and direction of the interstellar magnetic field, and the thickness of the inner heliosheath. They proposed an explanation for the unexpected heliospheric ribbon, which they had helped to discover, participated in the discovery of the nano-dust particles picked up by the solar wind, and proposed a novel hypothesis on the structure of the heliospheric current sheet.
Literature
S. Grzêdzielski, M. Bzowski, A. Czechowski, H.O. Funsten, D.J. McComas, N.A. Schwadron – 2010b, A possible generation mechanism for the IBEX ribbon from outside the heliosphere, Astrophys. J. Lett. Vol 715, pp L84-L87
S. Grzêdzielski, M.E. Wachowicz, M. Bzowski, V. Izmodenov – 2010a, Heavy coronal ions in the heliosphere: I. Global distribution of charge-states of C, N, O, Mg, Si and S, Astronomy & Astrophysics Vol 715, pp L84-L87, A72 , doi: 10.1051/0004-6361/200809900
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D.J. McComas, F. Allegrini, P. Bochsler, M. Bzowski, M. Collier, H. Fahr, H. Fichtner, P. Frisch, H. Funsten, S. Fuselier, G. Gloeckler, M. Gruntman, V. Izmodenov, P. Knappenberger, M. Lee, S. Livi, D. Mitchell, E. Möbius, T. Moore, S. Pope, D. Reisenfeld, E. Roelof, J. Scherrer, N. Schwadron, R. Tyler, M. Weser, M. Witte, P. Wurz, G. Zank – 2009a, IBEX – Interstellar Boundary Explorer, Space Science Reviews, Vol 146 No 1, pp 11-33, 10.1007/s11214-009-9499-4
N.A. Schwadron, G. Crew, R. Vanderspek, F. Allegrini, M. Bzowski, R. DeMagistre, G. Dunn, H. Funsten, S. A. Fuselier, K. Goodrich, M. Gruntman, J. Hanley, J. Heerikhuisen, D. Heirtzler, P. Janzen, H. Kucharek, C. Loeffler, K. Mashburn, K. Maynard, D.J. McComas, E. Moebius, C. Prested, B. Randol, D. Reisenfeld, M. Reno, E. Roelof, P. Wu – 2009a, The Interstellar Boundary Explorer Science Operations Center, Space Science Reviews, Vol 146 No 1, pp 207-234, 10.1007/s11214-009-9513-x
P.C. Frisch, M. Bzowski, E. Grün, V. Izmodenov, H. Krüger, J.L. Linsky, D.J. McComas, E. Möbius, S. Redfield, N. Schwadron, R. Shelton, J.D. Slavin, B.E. Wood – 2009, The Galactic environment of the Sun: Interstellar material inside and outside of the heliosphere, Space Science Reviews, Vol 146 No 1, pp 235-273, 10.1007/s11214-009-9502-0
E. Möbius, H. Kucharek, G. Clark, M. O'Neill, L. Petersen, M. Bzowski, L. Saul, P. Wurz, S.A.Fuselier, V.A. Izmodenov, D.J. McComas, H.R. Müller, D.B. Alexashov – 2009a, Diagnosing the neutral interstellar gas flow at 1 AU with IBEX-Lo, Space Science Reviews, Vol 146 No 1, pp 149-172 0.1007/s11214-009-9498-5
S. Tarnopolski, M. Bzowski – 2009, Neutral interstellar hydrogen in the inner heliosphere under the influence of wavelength-dependent solar radiation pressure, Astronomy & Astrophysics Vol 493, pp 207-216, 10.1051/0004-6361:20077058 (astro-ph/0812.0835 v1)
M. Bzowski, E. Möbius, S. Tarnopolski, V. Izmodenov, G. Gloeckler – 2009, Neutral H density at the termination shock: a consolidation of recent results, Space Science Reviews Vol 143, pp 177-190, 10.1007/s11214-008-9479-0 (astro-ph/0812.0839 v1)
S. Tarnopolski, M. Bzowski – 2008, Detectability of neutral interstellar deuterium by a forthcoming SMEX mission IBEX, Astronomy & Astrophysics, Vol 483 No 3, pp L35-L38 (astro-ph/0803.0220 v2)
M. Bzowski – 2008, Survival probability and energy modification of hydrogen Energetic Neutral Atoms on their way from the termination shock to Earth orbit, Astronomy & Astrophysics Vol 488 No 3, pp 1155-1164 (astro-ph/0801.2190 v4)
H. Nakagawa, M. Bzowski, A. Yamazaki, H. Fukunishi, S. Watanabe, Y. Takahashi, M. Taguchi, I. Yoshikawa, K. Shiomi, M. Nakamura – 2008, UV optical measurements of the NOZOMI spacecraft interpreted with a two-component LIC-flow model, Astronomy & Astrophysics Vol 491 No 1, pp 29-41 (10.1051/0004-6361:20079241)
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Meyer-Vernet, M., Maksimovic, M., Czechowski, A., Mann, I., Zouganelis, I., Goetz, K., Kaiser, M.L., St. Cyr, O.C., Bougeret, J.-L., Bale, S.D. – 2009, Dust detection by the wave instrument on STEREO: nanoparticles picked up by the solar wind?, Solar Phys. 256, pp 463-474 (astro-ph/0903.4141 v1)
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IBEX is the latest in NASA's series of low-cost, rapidly developed Small Explorers space missions. Southwest Research Institute in San Antonio, TX, leads and developed the mission with a team of US and international partners. NASA's Goddard Space Flight Center in Greenbelt, Md., manages the Explorers Program for NASA's Science Mission Directorate in Washington DC.