The cycle of solar activity responsible for modulation of interstellar neutral gas density and pick-up ions along the Earth’s orbit
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These density patterns vary during the solar cycle due to variation of the intensity of ionization of neutral atoms in the heliosphere. Details of this modulation depend on the evolution of the solar EUV radiation and the heliolatitudinal structure of solar wind, which both also varies during the solar cycle.   

Pick-up ions originate from the interstellar atoms that had penetrated near the Sun and were ionized subsequently and immediately picked up by the magnetic field of the solar wind. The intensity of the production of PUIs is proportional to the solar activity, which changes during the 11-year cycle. PUIs form a distinct ion population in the solar wind plasma, which can be measured by  detectors onboard spacecraft orbiting the Sun with Earth. The observed PUI flux features a characteristic, quasi-periodic yearly pattern, with a wide enhancement at the direction of inflow of interstellar gas into the heliosphere, the so-called PUI crescent, and a sharp peak at the opposite side, the so-called cone. By investigating the location of the maximum of these features the ecliptic longitude of inflow of interstellar gas into the heliosphere can be determined.

The direction of the inflow of the interstellar neutral gas (ISN) is needed to define the orientation of the plane of approximate symmetry of the heliosphere and better understand the heliospheric structure. In the past, this direction was a subject of controversy due to surprising results obtained from analysis of the first two years of direct sampling of interstellar neutral atoms observed by the Interstellar Boundary Explorer (IBEX) and an analysis of measurements of pick-up ions of helium, oxygen, and neon from the STEREO A/PLASTIC experiment (Drews et al. 2012). These measurements suggested surprisingly that the inflow direction of interstellar gas differed from that known previously and consequently the orientation of the heliospheric symmetry plane must have been significantly different than previously thought (McComas et al. 2013, Frisch et al. 2013). However, recent reanalysis of the Ulysses data made by the group from CBK PAN (Bzowski et al. 2014) and the newest analysis of the first six years of IBEX data by the IBEX Science Team with a prominent contribution from the researchers from CBK PAN (McComas et al 2015, Bzowski et al 2015) showed that the velocity vectors of the ISN gas derived from Ulysses and IBEX data sets are similar within uncertainties, with the temperature of the ISN gas higher than previously thought. Thus direction of the inflow of interstellar gas was determined quite accurately, more on this topic can be found here.

However, the result of the PUI analysis from the STEREO A/PLASTIC does not fit to the new picture. This mystery has been clarified by Justyna M. Sokół, Maciej Bzowski and Marzena A. Kubiak from CBK PAN and Eberhard Möbius from the University of New Hampshire in Durham NH, USA. In a paper published in Monthly Notices of the Royal Astronomical Society (Sokół et al. 2016),  the researchers looked into details of the production of the He, Ne, O PUIs studied its evolution during the solar cycle. They found that effects of time- and latitude-variation of the ionization rate on the distribution of ISN gas along the Earth's orbit affect the measured PUI flux so that they may masquerade as a few degree change in the inflow direction. They used to the study of the evolution of the structure of the ISN He, Ne, and O gas density and the derivative PUIs along the Earth's orbit during solar cycle the models of ionization losses of ISN gas inside heliosphere developed in CBK PAN (Bzowski et al. 2013, Sokół et al. 2013, Sokół & Bzowski 2014). They investigated the factors responsible for the modulation of the modeled quantities, with special attention paid to the modification of the observed features by the ionization rates close to the Sun.  They found that the main reason for the systematic shift of the determined ISN inflow direction observed via PUIs is the modulation of the parent ISN gas distribution along the Earth orbit. The short-term variations in the PUI production rates are responsible for additionalyearly scatter and tend to increase the observed differences. The directions of the departures agree among the three species studied (He, Ne, and O), they are towards larger longitudesfor the crescent, in agreement with the direction of the difference found by Drews et al. 2012 in their analysis of STEREO observations, and towards lower longitudes for the cone, oppositely to the difference found by Drews et al. 2012,  however,  the latter ones are within the error bar for He and Ne.

Thus, Sokół et al. 2016 concluded that the most likely explanation for the systematic difference between the actual inflow direction of the interstellar gas on the heliosphere and this obtained from analysis of pick-up ions is neglecting in the PUI analysis the solar cycle modulation of interstellar gas inside the heliosphere.

Justyna Sokół, Maciej Bzowski, Marzena Kubiak

editing: Joanna Pietrzak

Last Updated on Wednesday, 20 April 2016 09:59

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The interstellar gas in the Local Interstellar Cloud (LIC) surrounding the Sun is composed mainly of hydrogen and helium. But it contains also other chemical elements, among which the most abundant are oxygen and neon. The neutral atoms from the LIC flow into the heliosphere, where some of them are ionized and carried away with the solar wind as pick-up ions (PUIs), but some of them reach the Earth's orbit and distances even closer to the Sun. Underway, they are focused by solar gravity and form a characteristic, elongated patterns in density, with a cone of enhanced concentration at the downwind side of the Sun.


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