The enigma on the nearest galactic neighborhood of the Sun resolved Print E-mail
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Wednesday, 21 October 2015 10:31

The November issue of the prestigious science journal Astrophysical Journal Supplement Series presents results of investigations of interstellar matter in the immediate galactic neighborhood of the Sun, carried out by the Interstellar Boundary Explorer (IBEX) space probe. An important contribution to the findings presented in a series of 14 papers published in a special topical issue of ApJS was brought by a team of scientists from the Laboratory for Solar System Physics and Astrophysics of the Space Research Centre of the Polish Academy of Sciences (Centrum Badań Kosmicznych PAN, CBK PAN), led by Dr. Maciej Bzowski.

The topical series of papers, reviewed by McComas et al. 2015, presents a complex analysis of observations of neutral interstellar helium from the immediate solar neighborhood, collected by IBEX. The IBEX Science Team, including scientists from the United States, Poland, Switzerland, Germany, and Russia studied the physical state of matter in the Local Interstellar Cloud and resolved the enigmatic difference between the temperature and velocity of neutral interstellar helium obtained from analyses of observations from two space missions: Ulysses and IBEX.

During its galactic journey, the Sun penetrates various clouds of interstellar matter. Presently, it traverses the so-called Local Interstellar Cloud. The interstellar gas within this cloud is composed of both ionized and neutral atoms of various species, dominated by hydrogen and helium. The neutral component of interstellar gas is able to enter the heliosphere and reach the Earth orbit. The heliosphere is a region in space dominated by the solar wind, which is a constant hypersonic outflow of ionized material from the solar corona. The most intense flux of interstellar atom reaching the Earth’s orbit is that of helium. IBEX detects those helium atoms and thus provides information needed to analyze the physical state of the interstellar medium from which they originate. Based on analysis of the intensity of the flux of those atoms and of the direction in space from which they reach IBEX, scientists determine the temperature of the gas in the Local Interstellar Cloud and the velocity of its flow past the Sun.

Direct sampling of interstellar neutral helium atoms have been carried out from 1990s, initially by the space probe Ulysses, which ceased operations in 2008, and subsequently by IBEX, which started collecting the data in 2009. The temperature and velocity of interstellar gas determined based on observations obtained from the first two solar orbits of Ulysses and from the first two years of IBEX operations differed. This difference was difficult to understand and became known as the Ulysses – IBEX enigma. Scientists wondered if it was due to an error in analysis of one of the data sets, to an unknown phenomenon, or perhaps due to a gradual change in these parameters with time, which implied the existence of a spatial shear in the flow vector and a gradient in the temperature within the Local Cloud. This latter hypothesis was puzzling because the existence of such gradients in the temperature and shears in the velocity on such a small spatial scale was challenging to understand on physical basis. The complex study undertaken by the IBEX Science Team was aimed at resolving this enigma.

The heliospheric research team from CBK PAN lead by Dr. M. Bzowski carried out a detailed study of the temperature and velocity of interstellar helium inflowing to the heliosphere. During the previous years, the team analyzed the complete data set of measurements of interstellar helium collected by Ulysses, including the portion previously not analyzed, and published the results in a paper by Bzowski et al. 2014. Presently, the team has completed a meticulous analysis of the data from the first six years of operations of IBEX. The final results are presented in a paper by Bzowski et al. 2015, being part of the Special Issue of ApJS. The results of these studies suggest that the IBEX – Ulysses enigma has been resolved. The temperature and velocity vector obtained from the recent Ulysses data analysis and from the present analysis of IBEX data agree with each other very well. The presently obtained velocity vector turned out to be very close to the vector obtained originally from Ulysses analysis, but the temperature is higher about 1000 K.

The sophisticated analysis of the six years of IBEX data, performed by the CBK PAN team, was based on a detailed statistical analysis of the measurements, meticulous reproduction of the observation details in the analysis process, and a novel method of parameter fitting in the four-dimensional parameter space, developed by a CBK PAN PhD student Mr. Paweł Swaczyna. Details of this work are presented in a paper by Swaczyna et al. 2015 in the special issue of ApJS. The analysis required using a very precise model of interstellar neutral helium in the heliosphere, able to exactly simulate the measurements carried out by the IBEX detector. This model, developed by Ms Justyna M. Sokół and Ms Marzena A. Kubiak from CBK PAN, is presented in a paper by Sokół et al. 2015a, also being part of the ApJS special issue.

The statistical study performed by Mr. P. Swaczyna showed that the difference between the result obtained from the analysis of the data from the first two years of observations by IBEX is on one hand due to statistical fluctuations of the flux of interstellar neutral atoms observed by IBEX and the measurement background, and on the other hand due to the presence of a new population of neutral helium atoms, discovered based on IBEX observations by Ms. Marzena A. Kubiak and her collaborators. The new population, described in Astronomy & Astrophysics in a paper by Kubiak et al. 2014, is so tenuous that Ulysses was not able to see it against its observational background. The IBEX instruments are sophisticated and sensitive enough to have seen it. The nature of the Warm Breeze is not fully understood yet, but the most likely hypothesis on its origin suggests that the Warm Breeze is the secondary component of interstellar neutral helium which is created due to processes operating in the interstellar gas just in front of the heliosphere. The presence of the Warm Breeze in the flux measured by IBEX must be appropriately taken into account when searching for the interstellar He velocity and temperature, and the measurement base must be sufficiently large to suppress the influence of the inevitable fluctuations on the derived parameters.

The Warm Breeze produces a relatively weak signal in the IBEX observations. Ms Justyna M. Sokół with her collaborators studied the expected distribution of this signal in the IBEX full-sky maps and verified the hypothesis that the Warm Breeze signal is due to departures of the distribution of interstellar neutral helium from the physical state of thermodynamic equilibrium. Such a departure could produce an excess of relatively fast atoms in the sample, described by the so-called kappa function, instead of the commonly assumed Maxwell-Boltzmann distribution, characteristic for the fully equilibrated gas. The study by Ms J.M. Sokół and her team, published in another paper in the special series in ApJS Sokół et al. 2015b suggests that the interstellar gas and the Warm Breeze should form a signal in the form of a well defined core and a characteristic, extended “haze” in the sky. The expected distribution of this tenuous haze is different in the case of a separate Warm Breeze and interstellar neutral helium populations on one hand and of just one population of interstellar gas described by the kappa function on the other hand. The study by Ms J.M. Sokół and her team also suggests the best portions of the sky to look for the signal signatures characteristic for different hypotheses of the interstellar haze origin.

The results of the study of interstellar neutral helium were obtained in a broad collaboration of research groups from SRC PAS, Department of Physics and Space Science Center of University of New Hampshire in Durham, NH, Physics Institute of the Bern University in Switzerland, and Southwest Research Institute from San Antonio, TX. The research was carried out in parallel by the groups of researchers from different institutions, supporting each other, but using different, independent analysis methods. The results obtained by those groups turned out to agree with each other very well. The importance of these results has been recognized by the Editors of The Astrophysical Journal Supplement Series, who decided to publish them in a special topical series of papers in the Journal. The SRC scientists are lead authors of four of those papers and contributing authors of other six of them.

IBEX is an Earth satellite from the NASA Small Explorers series. The IBEX mission was developed and is carried out by Southwest Research Institute, San Antonio TX (PI: Dr. David McComas), with an international team of researchers, including scientists from the Space Research Centre of the Polish Academy of Sciences. The lead of the Polish research team and Co-Investigator in the IBEX Science Team is Dr. Maciej Bzowski. The Small Explorers program is managed by the NASA Science Mission Directorate in Washington DC. The participation of Polish scientists in the IBEX program was supported by grants from the Polish Ministry of Higher Education and Science, and currently by a grant from the Polish National Science Centre and by the Space Research Centre of the Polish Academy of Sciences.

M. Bzowski, J.M. Sokół


Astrophysical Journal Supplement Series: Special Issue on Interstellar Neutrals investigated by IBEX

Papers authored and coauthored by personnel from the Laboratory for Solar System Physics and Astrophysics, Space Research Centre of the Polish Academy of Sciences

  1. D.J. McComas, M. Bzowski, S.A. Fuselier, P.C. Frisch, A. Galli, V.V. Izmodenov, O.A. Katushkina, M.A. Kubiak, M.A. Lee, T.W. Leonard, E. Möbius, N.A. Schwadron, J.M. Sokół, P. Swaczyna,  B.E. Wood, P. Wurz – 2015, Local interstellar medium: six years of direct sampling by IBEX, Astrophysical Journal Supplement Series Vol 220:22, doi:10.1088/0067-0049/220/2/22
  2. E. Möbius, M. Bzowski, P.C. Frisch, S.A. Fuselier, D. Heirtzler, M.A. Kubiak, H. Kucharek, M.A. Lee, T. Leonard, D.J. McComas, N.A. Schwadron, J.M. Sokół, P. Swaczyna, P. Wurz – 2015, Interstellar flow and temperature determination with IBEX: Robustness and sensitivity to systematic effects, Astrophysical Journal Supplement Series Vol 220:24, doi:10.1088/0067-0049/220/2/24
  3. N.A. Schwadron, E. Möbius, T. Leonard, S.A. Fuelier, D.J. McComas, D. Heirtzler, H. Kucharek, F. Rahmanifard, M. Bzowski, M.A. Kubiak, J.M. Sokół, P. Swaczyna, P. Frisch – 2015, Determination of interstellar He parameters using 5 years of data from IBEX: Beyond closed-form approximations, Astrophysical Journal Supplement SeriesVol 220:25, doi:10.1088/0067-0049/220/2/25
  4. P. Swaczyna, M. Bzowski, M.A. Kubiak, J.M. Sokół, S.A. Fuselier, D. Heirtzler, H. Kucharek, T.W. Leonard, D.J. McComas, E. Möbius, N.A. Schwadron – 2015, Interstellar neutral helium in the heliosphere from IBEX observations. I. Uncertainties and backgrounds in the data and parameter determination method, Astrophysical Journal Supplement Series Vol 220:26, doi:10.1088/0067-0049/220/2/26
  5. J.M. Sokół, M.A. Kubiak, M. Bzowski, P. Swaczyna – 2015, Interstellar neutral helium in the heliosphere from IBEX observations. II. The Warsaw Test Particle Model (WTPM), Astrophysical Journal Supplement Series, Vol 220:27, doi:10.1088/0067-0049/220/2/27,  (arXiv:1510.04869)

  6. M. Bzowski, P. Swaczyna, M.A. Kubiak. J.M. Sokół, S.A. Fuselier, A. Galli, D. Heirtzler, H. Kucharek, T.W. Leonard, D.J. McComas, E. Möbius, N.A. Schwadron, P. Wurz – 2015, Interstellar neutral helium in the heliosphere from IBEX observations. III. Mach number of the flow, velocity vector, and temperature from the first six years of measurements, Astrophysical Journal Supplement Series Vol  220:28, doi:10.1088/0067-0049/220/2/28 (arXiv:1510.04835)

  7. J.M. Sokół, M. Bzowski, M.A. Kubiak, P. Swaczyna, A. Galli, P. Wurz, E. Möbius, H. Kucharek, S.A. Fuselier, D.J. McComas – 2015, The interstellar neutral He haze in the heliosphere: what can we learn?, Astrophysical Journal Supplement Series Vol 220:29, doi:10.1088/0067-0049/220/2/29(arXiv:1510.04874)
  8. A. Galli, P. Wurz, J. Park, H. Kucharek, E. Möbius, N.A. Schwadron, J.M. Sokół, M. Bzowski, M.A. Kubiak, P. Swaczyna, S.A. Fuselier, D.J. McComas – 2015, Can IBEX detect interstellar neutral helium or oxygen from anti-ram directions?, Astrophysical Journal Supplement Series Vol 220:30, doi:10.1088/0067-0049/220/2/30

  9. B.E. Wood, H.-R.Müller, M. Bzowski, J.M. Sokół, E. Möbius, M. Witte, D.J. McComas – 2015, Exploring the possibility of O and Ne contamination in Ulysses observations of interstellar helium, Astrophysical Journal Supplement SeriesVol 220:31, doi:10.1088/0067-0049/220/2/31
  10. H. Kucharek, A. Galli, P. Wurz, E. Möbius, M.A. Lee, J. Park, S.A. Fuselier, M. Bzowski, N.A. Schwadron, D. McComas – 2015, Impact of planetary gravitation on high precision neutral atom measurements, Astrophysical Journal Supplement SeriesVol 220:35, doi:10.1088/0067-0049/220/2/35


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