Conclusions from ENA observations from HSTOF: The heliosphere has a tail and is asymmetrical! Print E-mail
Thursday, 25 October 2018 12:34

Magnetic field strongly disturbs the motion of electrically charged particles in space but neutral atoms are insensitive to magnetic forces and consequently they move freely in all directions. This is true also for energetic neutral atoms (ENAs) in the heliosphere.

These atoms are former ions (protons) from the hot plasma in the boundary region of the heliosphere. The original protons collided with interstellar hydrogen atoms and captured electron from them, becoming electrically de-charged while maintaining the original speed and direction of motion. Since the trajectories of ENAs are not modified by magnetic fields, these atoms travel freely at large distances throughout the heliosphere and reach detectors located close to the Earth’s orbit. Because the speed and direction of motion of these particles is not modified during the neutralization process, ENA observations offer good insight into the energy distribution of cosmic plasma located far away from the observations site.

The HSTOF instrument onboard the Solar and Heliospheric Observatory (SOHO) space probe has been carrying out ENA observations since 1996. The instrument is sensitive to ENAs with the energies from 55 to 88 kiloelectronvolts (keV), i.e., to atoms traveling at 3250 to 4100 km/s. Other instruments observing ENAs (in different energy bands) include INCA onboard the Cassini Saturn probe (observations between 2005-2013), the ASPERA instruments onboard the planetary probes Mars Express and Venus Express (also presently inactive), and the IBEX satellite (carrying out observations from 2009 until present).

Implications of the HSTOF observations carried out from 1996 to 2010 were studied by a team pf scientists from CBK PAN: professors Andrzej Czechowski, Maciej Bzowski, and Stanisław Grzędzielski, Dr. Justyna M. Sokół and M.Sc. Jolanta Grygorczuk, in collaboration with Professor K.C. Hsieh from the University of Alabama, who formulated the concept of the HSTOF experiment, and with Dr. Martin Hilchenbach from Max-Planck-Institut für Sonnensystemforschung in Göttingen, Germany, who processed the data. The researchers found a clear downward trend in the observed ENA intensities, which seems to be correlated with the secular systematic decrease in the solar wind flux observed in situ at the Earth orbit, but not with the periodic variation in the solar wind related to the solar activity cycle. A similar downward trend in ENA observations was also found by INCA and IBEX. The correlation of the ENA flux with the solar wind flux supports the hypothesis that the observed ENAs originate from the aforementioned charge exchange reactions between protons from the shocked solar wind plasma and interstellar hydrogen atoms in the boundary regions of the heliosphere.

Furthermore, the observations corroborate the hypothesis that the heliosphere is distorted from axial symmetry due to the action of interstellar magnetic field. This is because the observed ENA flux In the “starboard flank” (ecliptic longitudes from 120 to 210 degree) is larger than from the flux from the “portside flank” (ecliptic longitudes from 300 to 30 degree). The observed asymmetry is in agreement with recent estimates of the direction of interstellar magnetic field.

Finally, the HSTOF observations lend strong support to the existence of the heliospheric tail: the flux observed from the upwind region of the heliosphere, i.e., from the direction of Sun’s motion through the local interstellar matter, is much lower than the flux from the opposite direction. This behavior is expected if the heliosphere has a long tail. The upwind to downwind ratio of the ENA fluxes observed by HSTOF differs from the ratios observed by INCA and IBEX. The researchers from CBK PAN demonstrated that this difference is a natural consequence of the larger energies observed by IBEX: 55 to 88 keV in comparison with 5 to 55 keV observed by INCA and 0.7 to 4.3 keV observed by IBEX, and the resultant difference in the reaction cross sections. This result is another confirmation that “The heliosphere is not round!”, as recently pointed out Professor Nathan Schwadron from the University of New Hampshire and Professor Maciej Bzowski from CBK PAN.

In addition to the data from HSTOF, the research team used results of earlier determinations of the densities of interstellar hydrogen and helium at the termination shock of the solar wind, obtained based on data from the Voyager and Ulysses spacecraft with an important contribution from scientists from CBK PAN, as well as the WTPM model of the density distribution of interstellar H and He in the heliosphere, developed in CBK PAN, and the Warsaw MHD model of the heliosphere.

The results and conclusions were presented by the science team led by Professor A. Czechowski from the Laboratory for Solar System Physics and Astrophysics, CBK PAN, in a recently published paper in Astronomy & Astrophysics.

A, Czechowski

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