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Register a new userEnergetic neutral atoms from the heliosheath as an additional population of neutral hydrogen in the inner heliosphere
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Interstellar neutral hydrogen (ISN H) gas penetrates freely the heliopause. Inside the inner heliosheath, the charge-exchange interaction of this gas with the shocked solar wind and pickup ions creates energetic neutral atoms (ENAs). ISN H is strongly depleted inside the termination shock but a fraction reaches the Earths orbit. In these regions of the heliosphere, ISN H is the source population for interstellar pickup ions and for the heliospheric backscatter glow. The Globally Distributed Flux (GDF) of ENAs created in the inner heliosheath has been sampled directly by Interstellar Boundary Explorer (IBEX). Based on these measurements, we calculate the density of the GDF ENA population at the Earths orbit. We find that this number density is between $10^{-4}$ and $10^{-3}$ cm$^{-3}$, i.e., comparable in magnitude to the number density of ISN H in the downwind portion of the Earths orbit. Half of this atom population have energies less than $sim 80$ eV. This GDF population of neutral hydrogen is likely to provide a significant contribution to the intensity of heliospheric glow in the downwind hemisphere, may be the source of the inner source of hydrogen pickup ions, and may be responsible for the excess of production of pickup ions found in the analysis of magnetic wave events induced by the proton pickup process in the downwind region at 1 au from the Sun.
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Traditionally, the solar magnetic field has been considered to have a negligible effect in the outer regions of the heliosphere. Recent works have shown that the solar magnetic field may play a crucial role in collimating the plasma in the heliosheath. Interstellar Boundary Explorer (IBEX) observations of the heliotail indicated a latitudinal structure varying with energy in the energetic neutral atom (ENA) fluxes. At energies ~1 keV, the ENA fluxes show an enhancement at low latitudes and a deficit of ENAs near the poles. At energies >2.7 keV, ENA fluxes had a deficit within low latitudes, and lobes of higher ENA flux near the poles. This ENA structure was initially interpreted to be a result of the latitudinal profile of the solar wind during solar minimum. We extend the work of Kornbleuth et al. (2018) by using solar minimum-like conditions and the recently developed SHIELD model. The SHIELD model couples the magnetohydrodynamic (MHD) plasma solution with a kinetic description of neutral hydrogen. We show that while the latitudinal profile of the solar wind during solar minimum contributes to the lobes in ENA maps, the collimation by the solar magnetic field is important in creating and shaping the two high latitude lobes of enhanced ENA flux observed by IBEX. This is the first work to explore the effect of the changing solar magnetic field strength on ENA maps. Our findings suggest that IBEX is providing the first observational evidence of the collimation of the heliosheath plasma by the solar magnetic field.
In situ measurements of the heliospheric particle populations by the Voyager spacecraft can only be put in an appropriate context with remote-sensing observations of energetic and interstellar neutral atoms (ENAs and ISN, respectively) at 1 au when the time delay between the production and the observation times is taken into account. ENA times of flight from the production regions in the heliosheath are relatively easy to estimate because these atoms follow almost constant speed, force-free trajectories. For the ISN populations, dynamical and ballistic selection effects are important, and times of flight are much longer. We estimate these times for ISN He and H atoms observed by IBEX and in the future by IMAP using the WTPM model with synthesis method. We show that for the primary population atoms, the times of flight are on the order of three solar cycle periods, with a spread equivalent to one solar cycle. For the secondary populations, the times of flight are on the order of ten solar cycle periods, and during the past ten years of observations, IBEX has been collecting secondary He atoms produced in the OHS during almost entire 19th century. ISN atoms penetrating the heliopause at the time of Voyager crossing will become gradually visible about 2027, during the planned IMAP observations. Hypothetical variations in the ISN flow in the Local Interstellar Medium are currently not detectable. Nevertheless, we expect steady-state heliosphere models used with appropriately averaged solar wind parameters to be suitable for understanding the ISN observations.
With the plethora of detailed results from heliospheric missions and at the advent of the first mission dedicated IBEX, we have entered the era of precision heliospheric studies. Interpretation of these data require precision modeling, with second-order effects quantitatively taken into account. We study the influence of the non-flat shape of the solar Ly-alpha line on the distribution of neutral interstellar H in the inner heliosphere. Based on available data, we (i) construct a model of evolution for the solar Ly-alpha line profile with solar activity, (ii) modify an existing test-particle code used to calculate the distribution of neutral interstellar H in the inner heliosphere so that it takes the dependence of radiation pressure on radial velocity into account, and (iii) compare the results of the old and new version. Discrepancies between the classical and Doppler models appear between ~5 and ~3 AU and increase towards the Sun from a few percent to a factor of 1.5 at 1 AU. The classical model overestimates the density everywhere except for a ~60-degr cone around the downwind direction, where a density deficit appears. The magnitude of the discrepancies appreciably depends on the phase of the solar cycle, but only weakly on the parameters of the gas at the termination shock. For in situ measurements of neutral atoms performed at ~1 AU, the Doppler correction will need to be taken into account, because the modifications include both the magnitude and direction of the local flux by a few km/s and degrees, respectively, which, when unaccounted for, would introduce an error of a few km/s and degrees in determination of the magnitude and direction of the bulk velocity vector at the termination shock.
Solar Orbiter strives to unveil how the Sun controls and shapes the heliosphere and fills it with energetic particle radiation. To this end, its Energetic Particle Detector (EPD) has now been in operation, providing excellent data, for just over a year. EPD measures suprathermal and energetic particles in the energy range from a few keV up to (near-) relativistic energies (few MeV for electrons and about 500 MeV/nuc for ions). We present an overview of the initial results from the first year of operations and we provide a first assessment of issues and limitations. During this first year of operations of the Solar Orbiter mission, EPD has recorded several particle events at distances between 0.5 and 1 au from the Sun. We present dynamic and time-averaged energy spectra for ions that were measured with a combination of all four EPD sensors, namely: the SupraThermal Electron and Proton sensor (STEP), the Electron Proton Telescope (EPT), the Suprathermal Ion Spectrograph (SIS), and the High-Energy Telescope (HET) as well as the associated energy spectra for electrons measured with STEP and EPT. We illustrate the capabilities of the EPD suite using the 10-11 December 2020 solar particle event. This event showed an enrichment of heavy ions as well as $^3$He, for which we also present dynamic spectra measured with SIS. The high anisotropy of electrons at the onset of the event and its temporal evolution is also shown using data from these sensors. We discuss the ongoing in-flight calibration and a few open instrumental issues using data from the 21 July and the 10-11 December 2020 events and give guidelines and examples for the usage of the EPD data. We explain how spacecraft operations may affect EPD data and we present a list of such time periods in the appendix. A list of the most significant particle enhancements as observed by EPT during this first year is also provided.
We analyze magnetic field data from the first six encounters of PSP, three Helios fast streams and two Ulysses south polar passes covering heliocentric distances $0.1lesssim Rlesssim 3$ au. We use this data set to statistically determine the evolution of switchbacks of different periods and amplitudes with distance from the Sun. We compare the radial evolution of magnetic field variances with that of the mean square amplitudes of switchbacks, and quantify the radial evolution of the cumulative counts of switchbacks per km. We find that the amplitudes of switchbacks decrease faster than the overall turbulent fluctuations, in a way consistent with the radial decrease of the mean magnetic field. This could be the result of a saturation of amplitudes and may be a signature of decay processes of large amplitude Alfvenic fluctuations in the solar wind. We find that the evolution of switchback occurrence in the solar wind is scale-dependent: the fraction of longer duration switchbacks increases with radial distance whereas it decreases for shorter switchbacks. This implies that switchback dynamics is a complex process involving both decay and in-situ generation in the inner heliosphere. We confirm that switchbacks can be generated by the expansion although other type of switchbacks generated closer to the sun cannot be ruled out.
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